Leukocyte arrest: Biomechanics and molecular mechanisms of β 2 integrin activation

Abstract

Integrins are a group of heterodimeric transmembrane receptors that play essential roles in cell–cell and cell–matrix interaction. Integrins are important in many physiological processes and diseases. Integrins acquire affinity to their ligand by undergoing molecular conformational changes called activation. Here we review the molecular biomechanics during conformational changes of integrins, integrin functions in leukocyte biorheology (adhesive functions during rolling and arrest) and molecules involved in integrin activation.

Keywords

Talin RIAM kindlin-3 PSGL-1 GPCR Rap-1

1. Integrins

Integrins are $α β$ heterodimers that function as activation-dependent adhesion molecules at the interface between cells and immobilized ligands in the extracellular matrix or other cell surfaces [1–3]. The interactions of integrins with their ligands are broadly relevant to a multitude of physiological and disease situations, such as inflammation [4–7], immune responses [8–11], thrombosis and hemostasis [12–15], extracellular matrix assembly [1–3,16–18], tumor metastasis [15,19–22] and other cellular processes. This review is focused on the four $β_{2}$ integrins (Table 1) among the known 24 integrins [9,23]: $α_{L} β_{2}$ (LFA-1, lymphocyte function-associated antigen 1), $α_{M} β_{2}$ (Mac-1, macrophage-1 antigen), $α_{X} β_{2}$ (p150,95), $α_{D} β_{2}$ .

Table 1
$β_{2}$ integrins expressed on leukocytes

α subunit β subunit Alternative name Expressed in cells Main ligands

$α_{L}$ (CD11a) $β_{2}$ (CD18) LFA-1 Neutrophils [24–26], T-lymphocytes [11,27,28], B-lymphocytes [28,29], Monocyte [26], Macrophage [30], Dendritic cells [31], Natural killer cells [32] ICAM-1 [28,33–36], ICAM-2 [25,37], ICAM-3 [38], ICAM-4 [39,40], ICAM-5 [41,42], Collagen [34]

$α_{M}$ (CD11b) $β_{2}$ (CD18) Mac-1 Neutrophils [24,26,43], T-lymphocytes [27], B-lymphocytes [44], Monocytes [26,45], Macrophages [46], Dendritic cells [47], Natural killer cells [32] ICAM-1 [48–50], ICAM-2 [51], ICAM-4 [39], Fibrinogen [43,45,52], Collagen [34], iC3b [34,53], Heparin [54], GPIbα [55], JAM-3 [56], Thy-1 [57], Plasminogen [58]

$α_{X}$ (CD11c) $β_{2}$ (CD18) p150,95 Neutrophils [59,60], T-lymphocytes [27], B-lymphocytes [61,62], Monocytes [59,63], Macrophages [59], Dendritic cells [46], Natural killer cells [32] ICAM-1 [64,65], ICAM-2 [63], ICAM-4 [66], VCAM-1 [63], Fibrinogen [60,62,65], Collagen [34], iC3b [34,59,65,67], Heparin [68], GPIbα [69], Thy-1 [70], Plasminogen [71]

$α_{D}$ (CD11d) $β_{2}$ (CD18) Neutrophils [72,73], T-lymphocytes [27], Monocytes [72,73], Macrophages [72–74], Dendritic cells [73] ICAM-3 [72], VCAM-1 [75,76], Fibrinogen [77], Vitronectin [77], Cyr61 [77], Plasminogen [77]

α subunit	β subunit	Alternative name	Expressed in cells	Main ligands
$α_{L}$ (CD11a)	$β_{2}$ (CD18)	LFA-1	Neutrophils [24–26], T-lymphocytes [11,27,28], B-lymphocytes [28,29], Monocyte [26], Macrophage [30], Dendritic cells [31], Natural killer cells [32]	ICAM-1 [28,33–36], ICAM-2 [25,37], ICAM-3 [38], ICAM-4 [39,40], ICAM-5 [41,42], Collagen [34]
$α_{M}$ (CD11b)	$β_{2}$ (CD18)	Mac-1	Neutrophils [24,26,43], T-lymphocytes [27], B-lymphocytes [44], Monocytes [26,45], Macrophages [46], Dendritic cells [47], Natural killer cells [32]	ICAM-1 [48–50], ICAM-2 [51], ICAM-4 [39], Fibrinogen [43,45,52], Collagen [34], iC3b [34,53], Heparin [54], GPIbα [55], JAM-3 [56], Thy-1 [57], Plasminogen [58]
$α_{X}$ (CD11c)	$β_{2}$ (CD18)	p150,95	Neutrophils [59,60], T-lymphocytes [27], B-lymphocytes [61,62], Monocytes [59,63], Macrophages [59], Dendritic cells [46], Natural killer cells [32]	ICAM-1 [64,65], ICAM-2 [63], ICAM-4 [66], VCAM-1 [63], Fibrinogen [60,62,65], Collagen [34], iC3b [34,59,65,67], Heparin [68], GPIbα [69], Thy-1 [70], Plasminogen [71]
$α_{D}$ (CD11d)	$β_{2}$ (CD18)		Neutrophils [72,73], T-lymphocytes [27], Monocytes [72,73], Macrophages [72–74], Dendritic cells [73]	ICAM-3 [72], VCAM-1 [75,76], Fibrinogen [77], Vitronectin [77], Cyr61 [77], Plasminogen [77]

Excellent reviews cover the structures of integrins [9,78–81]. Both α and β subunits of integrins have large ectodomains, a single membrane-spanning helix (transmembrane, TM) and, usually, a short unstructured cytoplasmic tail (Fig. 1). Typically the α and β subunits contain around 1000 and 750 amino acids, respectively [78]. Specifically, human $α_{L}$ has 1170 amino acids, $α_{M}$ has 1152 amino acids, $α_{X}$ has 1163 amino acids, $α_{D}$ has 1161 amino acids and $β_{2}$ has 769 amino acids. All α chains of the $β_{2}$ integrins contain an I domain with homology to von Willebrand factor (VWF) A domain. The ectodomains are composed of several domains with flexible linkers between them. Each α ectodomain has (from C to N terminus) calf-1 and 2 domains, a thigh domain, a β propeller domain, and an α I domain. The $β_{2}$ ectodomains have a β-tail domain, integrin epidermal growth factor like repeat domains (I-EGF-1 to 4), a plexin-semaphorin-integrin (PSI) domain, a hybrid domain, and a β I-like domain. The ectodomains can be divided into headpiece and tailpiece as shown in Fig. 1. The α and β cytoplasmic tails of integrins are extended and flexible and can directly bind several adapter proteins with different functional effects [82–88] (Table 2).

Fig. 1.

Structural schematic of the extended $β_{2}$ integrin. α chain red, β chain blue. Subdomains and headpiece/tailpiece portions labeled.

Table 2

Cytoplasmic tail binding proteins of integrins^*

Tail of subunits	Activating	Inactivating	Signaling	Recycling
α	RapL [89–92]	SHARPIN [21], Nischarin [95,96], MDGI [97], Paxillin [98,99]	PP2A [93]	Rab21 [94], p120RasGAP [94]
β	Talin [100–104], Kindlin [100,109–111], Cytohesin [117,118]	Dok1 [105,106], Filamin [112–114], ICAP1 [119]	α-actinin [103,107], 14-3-3 [115], Arg [120], Src [122,123]	Numb [108], SNX17 [116], DAB2 [121]

Not necessarily shown for $β_{2}$ integrins.

$β_{2}$ integrins, also known as leukocyte integrins [4,27], are the most important molecules in recruiting leukocytes, especially neutrophils and naïve lymphocytes, from the blood stream to sites of immune responses and inflammation. $β_{2}$ integrins are involved in slowing down rolling [24,100,124–128], promoting arrest [100,129–137], supporting spreading and migration [128,136–143], homotypic and heterotypic cell–cell interactions [8,10,11,144–148] and phagocytosis [6,137,149–153]. Unlike other integrins, leukocyte integrins have few extracellular matrix ligands.

2. Conformational activation

Integrins regulate their adhesiveness through changes in the conformation of their ectodomain [154], which can increase ligand affinity by ∼10,000 fold [155]. The first integrin structure was solved for $α_{V} β_{3}$ integrin [156,157] starting a rich stream of publications with information about integrin ectodomain structures. It is widely accepted that $β_{2}$ integrins have at least three conformations with different ligand binding affinities [8,9,88,158–160] (Fig. 2(A)–(C)): bent ectodomain with closed headpiece (E⁻H⁻, resting state, low affinity), extended ectodomain with closed headpiece (E⁺H⁻, intermediate [88,100,135,161–163] or low [159] affinity), and extended ectodomain with open headpiece (E⁺H⁺, high affinity). Furthermore, a structure of bent ectodomain with open headpiece (E⁻H⁺, Fig. 2(D)), which can bind small soluble ligands [164,165], was found in structures of $α_{v} β_{3}$ [164] (Electron Microscopy, EM) and $α_{x} β_{2}$ [67] (X-ray crystallography), but it is not clear whether this structure exists on the cell surface and whether it has any function.

Fig. 2.

The existing conformations of the $β_{2}$ integrins and the proposed consequences of conformational changes. (A) The resting state of $β_{2}$ integrin has bent ectodomain with closed headpiece. (B) The resting $β_{2}$ integrin can extend its ectodomain. This conformation may have intermediate affinity to the ligand and mediate leukocyte slow rolling. (C) Further conformational changes can induce headpiece-opening and acquire fully activated $β_{2}$ integrin. This conformation has high affinity to the ligands and is thought to support arrest and leukocyte adhesion. (D) The structure of bent ectodomain with open headpiece was also crystallized in $β_{2}$ integrin.

2.1. Models of integrin activation

Most studies on conformational integrin activation are limited to the truncated ectodomains, often modified by site-directed mutagenesis to stabilize conformations. In these studies, amino acids thought to be in close proximity in one of the conformations are replaced by cysteines to form disulfide bonds. By necessity, such stabilized integrins must be studied outside their natural cellular context. These experiments suggested two different models of integrin conformational activation. The “switchblade” model proposes that integrin extension is required for headpiece opening and the bent integrin first undergoes extension followed by rearrangements in the metal ion-dependent adhesion site (MIDAS) motif leading to headpiece-opening and high affinity ligand binding [8,9,154,161,162,166,167] (Fig. 2, (A) to (B) to (C)). An alternative model is the “deadbolt” model, where interactions between the headpiece and the legs keep the integrin in the closed, bent conformation and extension can only occur after the “deadbolt” is released. Two studies have shown that E⁻H⁺ integrin can exist, suggesting that integrins can open their headpiece prior to extending their ectodomain (Fig. 2, (A) to (D) to (C)). It is not known whether this conformation exists on living cells. A third model is extension first, then force-facilitated headpiece opening [10,11,135].

2.2. Extension

The resting state of $β_{2}$ integrins shows a bent structure shaped like an inverted “V” with the low affinity headpiece closely approaching the plasma membrane [154,168], experimentally verified in live leukocytes by Förster resonance energy transfer (FRET) [169,170], in which FRET from α I domain (FITC-conjugated antibodies) to plasma membrane (Octadecyl rhodamine B, ORB) was observed in resting leukocytes and disappeared when the cells were activated. The bent ectodomain of $β_{2}$ integrins is about 11 nm above the plasma membrane, whereas the extended ectodomain is about 23 nm (with α I domain) [78]. To allow the headpiece to bind ligands on other cells or surfaces in trans, the ectodomain needs to be extended. Integrin extension is initiated by inside-out signaling [9]. EM and FRET studies show that the α and β feet of extended integrins are more separated than those of bent integrins [154,160]. This could be achieved by lateral displacement of the cytoplasmic tails or by a change of the angle between the α and β transmembrane domains, or both. Such molecular rearrangements could conceivably provide the force necessary to extend the ectodomain. There is good evidence that $β_{2}$ integrin extension is mediated by talin binding in the β cytoplasmic tail of integrin [88,100], thus causing the conformational changes of cytoplasmic tail and transmembrane domain [171].

2.3. Headpiece opening

The integrin headpiece includes the α I domain, the β propeller domain and the thigh domain of the α subunit and the β I-like domain, the hybrid domain, the PSI domain and the I-EGF-1 domain of the β subunit [9]. In $β_{2}$ integrins, the ligand-binding pocket is located in the α I domain. During integrin activation, the headpiece undergoes conformational changes allowing two ligand binding sites to be exposed, one for the external ligand like ICAM-1 and one for an internal ligand formed by the α I domain, binding to the β I-like domain. On $α_{M}$ or $α_{L}$ , the MIDAS is formed by the metal ion (such as Mg²⁺) and the residues T209, D242 and the D140XSXS motif of the $α_{M}$ I domain [9,172], or the residues T206, D239 and the D137XSXS motif of $α_{L}$ I domain [155], respectively. It has been demonstrated by introducing disulfide bonds that wild-type isolated $α_{L}$ I-domain has low affinity for ICAM-1, whereas pulling down the α7 helix of the I-domain partially or completely will include the stabilized intermediate or high affinity $α_{L}$ I-domain [155,173]. The α I-domain sits on top of the β propeller domain, in close proximity to the β I-like domain. In natural integrin without disulfide bonds, it is thought that upon integrin activation, the β I-like domain binds an internal ligand (amino acid residue G310) of the $α_{L}$ I domain. This binding pulls down the α7 helix and stabilizes the high affinity conformation of α I [9,67,154,174]. The internal ligand binding requires that the MIDAS in the β I-like domain is open, which is thought to be induced by hybrid domain swing-out [175,176]. In the “switchblade” model, it is suggested that integrin extension enables hybrid domain swing-out [175,176], thus inducing further conformational changes of the α and β I and I-like domains and acquiring high affinity for ligand [9,174]. However, in cell-free systems it has also been observed that bent integrin can have swung-out hybrid domain and open headpiece [154,174]. This bent conformation with open headpiece [67,164,177] (E⁻H⁺) can bind (small) soluble ligands [164,165,177] prior to extension, suggesting that integrin extension is not necessary for headpiece-opening. These observations are difficult to reconcile with the switchblade model. Kindlin-3 (another important adapter protein) deficient murine neutrophils or kindlin-3 knock down HL-60 cells show a defect in headpiece-opening as reported by conformation-specific antibodies [100]. A mutant talin-1 (L325R) [178] was also demonstrated to prevent headpiece-opening of $β_{2}$ integrin on neutrophils, which exhibit a similar phenotype as kindlin-3 knock out neutrophils [100]: both show normal slow rolling but deficient arrest.

Fig. 3.

The movements of $β_{2}$ integrins’ TMD induced by talin binding. The α and β TMDs of $β_{2}$ integrins cross at an angle at rest. The two stars indicate the talin binding sites (MP and MD) of the β cytoplasmic domain. (A) Upon binding to the talin F3 domain through the MD binding site of the β cytoplasmic domain, the attractions between talin F3 domain and the MP binding site of β cytoplasmic domain, as well as the attractions between the talin F2 domain and the plasma membrane, force the β TMD to move, resulting in an angle change between α and β TMDs, along with the pistoning of β TMD and the dissociation of α and β TMDs. (B) A more recent study suggested that talin first interacts with the plasma membrane through its F2 domain, then the attractions between talin F3 domain and the two β cytoplasmic binding sites force the β TMD to move and change their crossing angle, along with the pistoning of β TMD and the dissociation of α and β TMDs.

2.4. Cytoplasmic tail separation

In live cells, the cytoplasmic domains of the integrin α and β subunits are close to one another [171] in the resting (bent) state, close enough so FRET occurs between fluorescent proteins fused to the α and β cytoplasmic domains [171]. Replacement of the α and β cytoplasmic domains with acidic and basic amino acids that form a heterodimeric α-helical coiled-coil forces the two cytoplasmic domain together and stabilizes integrins in their inactive state [179]. The natural integrin cytoplasmic tails have flexible structures and several binding sites for different regulatory adaptor proteins (Table 2). Important regions of the cytoplasmic tails are the NPxY motifs in the $β_{2}$ tail, which bind talin (membrane-proximal NPxY) [83,101] and kindlin (membrane-distal NPxY) [109,110], respectively. The integrin “off” state is stabilized by binding of filamin or other phosphotyrosine-binding (PTB) domain-containing proteins. It is thought that filamin competes with talin. Tyrosine phosphorylation of the β-tail by some Src family kinases (SFK) has also been suggested to be involved in stabilizing the bent conformation through inhibiting talin binding [180]. β tail phosphorylation promotes the binding of PTB-containing proteins such as Dok1 (docking protein 1), thus preventing the binding of talin. The binding of other PTB-containing talin competitors, such as ICAP1 (integrin cytoplasmic domain associated protein 1), is not dependent on integrin tyrosine phosphorylation [84]. Upon talin binding, cytoplasmic tails separation occurred, resulting in diminished FRET between fluorescent proteins fused to the α and β cytoplasmic domains [171]. This separation of α and β cytoplasmic tails is thought to be critical for integrin extension and, in the switchblade model, also for headpiece opening [179].

2.5. Transmembrane domain (TMD) structure changes

The TMD of α and β are both α helices. The α and β helices cross at specific angles relative to the plane of the plasma membrane. This is stabilized by in-register alignment of side-chain arrays (Fig. 3) [181]. Nuclear magnetic resonance (NMR) spectroscopy showed the structures of the $α_{IIb} β_{3}$ TMD complex – the $α_{IIb}$ TMD helix is roughly perpendicular to the membrane while the $β_{3}$ TMD helix is tilted [182,183]. A specific α-helical interface (salt-bridge) between the α and β transmembrane domains stabilizes integrin in the resting state [181,184,185]. Three models have been proposed for the movement of the transmembrane domains during integrin activation: moving apart [184], pistoning up and down [181], or an angle change [181] between α and β. The pistoning model is related to the angle change model, because it suggests that intracellular activating signals could shorten the portion of helix which is buried within the lipid bilayer, thereby changing membrane tilt angle and register with the neighboring helix to avoid hydrophobic mismatch with the fixed width of the membrane bilayer. This change in tilt angle could be the critical event in disrupting transmembrane interactions that stabilize the low-affinity conformation, leading to integrin activation [181,186]. Structural studies of a complex between talin and the integrin $β_{3}$ cytoplasmic domain [102] suggested two talin binding sites on the $β_{3}$ cytoplasmic domain – membrane-proximal (MP) and membrane-distal (MD) sites. Talin binding to the MD part of the β cytoplasmic domain subsequently engages the MP binding site, resulting in reorientation of the TMD (Fig. 3(A)). The interaction of the MP part with talin results in the position and angle change of MP part, thus leading to the pistoning [102] and moving apart [187] of the transmembrane helices. A recent study showed that phosphatidylinositol 4,5-bisphosphate (PIP2) can interact with residue Arg995 within the TMD and break the salt-bridge when talin is bound, thus helping integrin activation [188].

3. Adaptor proteins and integrin activation

The main adaptor molecules regulating integrin activation include talin, Rap1-GTP interacting adapter molecule (RIAM) and kindlin (kindlin-3 in leukocytes). Talin and kindlin are reported to bind the integrin β cytoplasmic tail directly, whereas RIAM is thought to bind and recruit talin to integrin.

3.1. Talin

Talin is a high-molecular-weight cytoskeletal protein concentrated at regions of cell-substratum contact [189]. There are two genes, talin-1 and talin-2. Talin-1 contains an N-terminal 47-kD head domain and a ∼220-kD C-terminal flexible rod domain [190]. Talin-1 interacts with both actin and integrin [103]. The head domain consists of an N-terminal FERM domain (protein 4.1, ezrin, radixin and moesin) with three subdomains (F1, F2, F3) and an F0 subdomain with no homology to known domains [191]. It is suggested that the talin head domain has a unique extended structure different from the typical cloverleaf structure seen in other FERM domains [192]. The F3 subdomain contains a PTB domain that binds the integrin β subunit tail at the membrane-proximal NxxY site and promotes integrin activation [101–104]. The talin rod domain is composed of a series of helical bundles (R1 to 13) and a C-terminal single helix dimerization domain (DD). The rod domain contains multiple binding sites for RIAM [193–195], vinculin [195,196], deleted in liver cancer 1 (DLC1) [197], synemin [198], a second binding site for integrin [199,200], binding sites for actin [87,201,202] and internal binding sites for the talin F3 subdomain [203]. It is not clear whether and how all these binding sites are occupied when the attendant integrins are resting or activated.

Talin binding to integrin β cytoplasmic tail leads to spatial changes of the tail and TMD. Two models have been proposed for these changes: the “moving apart” model, in which talin binding leads to the moving apart of the α/β “legs” as demonstrated by FRET assay [171]; and the “piston and angle change” model, in which talin binding leads to angle change, pistoning up and down along with moving apart of the α and β TMDs (Fig. 3), supported by NMR structure studies using short peptides including the TMDs [102,181,187,204,205]. It is thought that the change of the position of the α and β chains relative to each other induces the conformation changes in the ectodomain, but it is not known how the forces are transduced.

The interaction of the talin F3 subdomain with the $β_{2}$ cytoplasmic tail is necessary but not sufficient for integrin activation. The interaction of talin and the membrane is also important [87]. Some basic residues in the F2 subdomain (Fig. 3), which is located close to the plasma membrane during talin-integrin binding, can bind acidic phospholipids in the plasma membrane [187]. This interaction is also thought to be necessary for integrin activation [204], because mutations of the membrane binding residues on F2 domain significantly decrease the binding of PAC-1 antibody, which is specific for activated $α_{IIb} β_{3}$ integrin. A recent molecular dynamics study using atomistic molecular stimulation demonstrated that PIP2 and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) are important for talin-mediated salt bridge breaking, which leads to spatial changes of the β TMD and leads to integrin activation [188].

3.2. RIAM

RIAM is a broadly expressed adapter protein that contains an RA (Ras association)-like domain, a pleckstrin homology (PH) domain, and several proline-rich sequences. RIAM is important for talin recruitment and integrin activation in leukocytes [206,207], but not platelets [208]. A short amphipathic helix (residues 6–30) in the N-terminal region of RIAM binds talin. A fusion protein containing this helix fused to the membrane-targeting sequence of Rap1A mimics recruitment of talin to the plasma membrane by the RIAM-Rap1 complex and supports integrin activation [193]. This result shows that RIAM is sufficient to recruit talin, but does not address the question whether it is required. RIAM and vinculin compete for the same binding site on talin-1 and their binding to talin is mutually exclusive [194]. RIAM is located in the newly formed protrusions of adherent cells, whereas vinculin is located in mature focal adhesions [194]. This suggests a model where RIAM binding to talin initializes the talin recruitment and integrin activation, then RIAM is replaced by vinculin to further stabilize the activated state of integrin. Interestingly, RIAM was also shown to immunoprecipitate with kindlin-3, even before it immunoprecipitates with talin [209]. However, whether RIAM directly interact with kindlin-3 is unknown.

3.3. Kindlin-3

Kindlin is a family of focal adhesion proteins including three subtypes – kindlin-1, 2, and 3, which are also known as fermitin family homolog 1, 2 and 3 (FERMT1, FERMT2, FERMT3) respectively [210]. Kindlin-3 is expressed in leukocytes and platelets and is another essential player that binds to the integrin $β_{2}$ subunit tail at the membrane-distal NxxY site [109,110]. Kindlin binding to $β_{2}$ does not compete with talin binding. Kindlin shows high levels of sequence similarity to talin FERM domain [211]. The main difference is that kindlin contains a PH domain that may be involved in membrane recruitment by phosphoinositide binding [190]. The subdomain 3 in kindlin binds the distal NPxY motif of integrin β tails [109,212,213]. Kindlin-3 is expressed exclusively in cells of haematopoietic origin. Mutations in kindlin-3 are found in leukocyte adhesion deficiency III (LAD-III) patients, which have severe bleeding and immune deficiency caused by defective integrin activation in leukocytes and platelets [109,214–217]. These findings demonstrate a critical role for kindlin-3 in leukocyte integrin activation, but the specific mechanisms remain controversial. Kindlin-3 knockout mice show a defect in neutrophil arrest but not slow rolling, which suggests that kindlin-3 is required for headpiece-opening but not extension of $β_{2}$ integrin [100]. Kindlin-3 knockdown in the HL-60 promyeloid cell line results in a binding defect of mAb24 [218–221], which is specific for open $β_{2}$ headpiece, but not KIM127 [222,223], which reports $β_{2}$ integrin extension. These results suggest essential roles of kindlin-3 in headpiece-opening of $β_{2}$ integrin [100]. An alternative hypothesis suggests that kindlin can promote talin binding to integrin cytoplasmic tail [224]. However, although kindlin, talin and integrin cytoplasmic tail can form a ternary complex, the binding of kindlin to integrins neither enhances talin–integrin binding nor increases talin targeting to the membrane [225,226], and no direct talin–kindlin interaction has been reported. Another hypothesis suggests that kindlin-binding may induce integrin clustering to enhance binding of multivalent ligands via increased avidity, rather than through conformational changes that lead to increased affinity for monovalent ligand [190,227], which is experimentally supported [228].

4. Leukocyte arrest and signaling of integrin activation

During infection or inflammation, leukocytes in blood, including granulocytes, monocytes and lymphocytes are recruited to the site of immune responses in a cascade-like fashion [17,27,136,229–234] that includes rolling, arrest, crawling, transendothelial migration and migration in tissues. Integrins play vital roles in all these processes. From rolling to arrest, leukocytes need to adhere with sufficient force to balance the force imposed by the drag and torque exerted by the flowing blood. Arrest is thought to be triggered by rapid changes in $β_{2}$ integrin conformation through inside-out signaling, which may be triggered from surface receptors P-selectin glycoprotein ligand-1 (PSGL-1) or chemokine receptors. This requires regulation of several kinases and assembly and disassembly of multiprotein complexes that form around the cytoplasmic tails of integrins.

4.1. Rolling

The dominant molecules involved in leukocyte rolling are selectins and their ligands, whose biomechanics has been reviewed before [24,25,125,127,235–238]. The interaction of endothelial selectins with leukocyte PSGL-1 primes integrin activation, specifically promoting extension of $β_{2}$ integrins [24,88,125,126,239]. Assays using conformation-specific antibodies have confirmed that rolling on P- or E-selectins can induce integrin extension on leukocytes within human blood [24]. Thus, $β_{2}$ integrins also serve as “rolling receptors” [27]: they transiently engage their ligand ICAM-1, thereby reducing rolling velocity (slow rolling). Slow rolling has been proposed to be mediated by extended integrin with intermediate affinity [24,100,125–127,135,161–163,240]. This mechanism of integrin-dependent slow rolling was confirmed by using an allosteric inhibitor [125,163] that allows integrin to assume the extended, but not high affinity conformation, and by using kindlin-3 knocked out mice, in which integrin can be extended but the headpiece cannot be opened [100].

Fig. 4.

Schematics of the inside-out signaling of integrin activation. The integrin activation through inside-out signaling can be divided into PSGL-1 signaling (green) and chemokine receptor (GPCR) signaling (blue). Molecules involved in both two signaling pathway are shown in both colors. The signaling source of kindlin-3 (black) is unknown.

4.2. PSGL-1 signaling (Fig. 4)

During leukocyte rolling, interactions of endothelial E- and P-selectins with PSGL-1 activate a key signaling pathway. PSGL-1 is the major ligand for P-selectin [239,241,242] and also binds E-selectin [236,243] and L-selectin [237] under flow conditions. PSGL-1 is preferentially located on the tips of leukocyte microvilli [25,244], which enables interactions with selectins during rolling. Granulocytes rolling on P- or E-selectin partially activate $β_{2}$ integrin through PSGL-1 induced inside-out signaling, which was demonstrated by observation of slow rolling on ICAM-1 [24,125,126] accompanied by increased staining with the $β_{2}$ extension reporter antibody KIM127 [24]. A mutational study of PSGL-1 showed that the cytoplasmic tail was crucial for this signal transduction, whereas the extracellular domain is responsible for leukocyte rolling [238].

Under no-flow conditions, soluble P-selectin-Fc chimeric protein was shown to induce SFK-dependent phosphorylation of Naf-1, followed by recruitment of the phosphoinositide-3-OH kinase (PI3K) p85-p110δ (PI3Kδ) heterodimer and priming of integrin activation [245]. This pathway may be important when leukocytes bind platelets, which have much higher density of P-selectin than endothelial cells. This pathway is not operative in rolling leukocytes, because PI3Kδ deficient leukocytes showed normal slow rolling on P-selectin and ICAM-1 under flow conditions [246].

On the neutrophil surface, PSGL-1 is closely associated with L-selectin and this does not require interaction of the L-selectin lectin domain with the known selectin-binding domain of PSGL-1 [127]. Engagement of P- or E-selectin with PSGL-1 triggers signaling through lateral interaction with L-selectin [127], followed by phosphorylation of the SFKs Fgr, Hck and Lyn [126,127,246,247]. Further studies indicated that Fgr is the main SFK required for slow rolling [126]. The phosphorylation of Fgr induces phosphorylation of the ITAM-containing adaptor proteins DAP12 or FcRγ [126], which subsequently interact with and phosphorylate the tyrosine kinase Syk [126,246]. Bruton tyrosine kinase (Btk) [246,248], which is one of Tec family kinase, acts downstream from Syk, where the signaling appears to divide into two pathways: one through phospholipase C-γ2 (PLCγ2); the other through PI3Kγ [248]. The downstream molecules of PLCγ2 are CalDAG-GEFI and p38 MAPK, which are known to activate the small GTPase Rap1a [240]. Rap-1a activation is associated with LFA-1 activation, but the details are unknown. Rap-1 has been proposed to bring talin-1 to the $β_{2}$ subunit, or it may be involved in kindlin-3 recruitment. In the PI3Kγ pathway, Akt is phosphorylated [127,248], which was reported to inhibit the integrin negative regulator GSK3α/β in platelets [249]. Again, the details are unknown. The PI3Kγ pathway could also be involved in integrin clustering.

4.3. Chemokine receptor signaling (Fig. 4)

Although PSGL-1 signaling can prime integrin activation by promoting integrin extension and cause slow rolling of leukocytes [24,100,125,126,250], chemokine receptor signaling is thought to be required for full integrin activation (probably associated with headpiece opening) and leukocyte arrest [125,130,135,251]. Chemokine receptors are G-protein coupled receptors (GPCRs), which are seven-transmembrane proteins that are identified by the coupling with the heterotrimeric G protein containing a particular α-subunit ( $G_{α i}$ ) paired with a $β γ$ -complex ( $G_{β γ}$ ) [252]. Several chemokine receptors are expressed on leukocytes, and many of them can trigger arrest [253]. The best-studied chemokine receptor in this context is the C-X-C chemokine receptor type 2 (CXCR2), which binds the chemokine (C-X-C motif) ligands (CXCL) 1, 2, 3, 5, 6 and 7.

The chemokine receptor signaling pathway triggering integrin activation is incompletely understood [88,234]. In neutrophils, the most relevant chemokine receptor responsible for arrest is CXCR2 [254]. In vitro flow chamber assays show that immobilized CXCL1 stimulating CXCR2 leads to LFA-1 activation and neutrophil arrest on an E-selectin/intercellular adhesion molecule 1 (ICAM-1) substrate through $G_{α i}$ depended signaling [125]. Specifically, $G_{α i 2}$ but not $G_{α i 3}$ is required. $G_{α i}$ -dependent Ras activation leads to the activation of phosphoinositide 3-kinase (PI3K) by binding to its catalytic subunit [255]. The dissociated $G_{β γ}$ subunit has been shown to interact with a number of molecules, including PI3K isoforms [256,257], PDZ proteins [258], guanine nucleotide exchange factors (GEFs) such as PIP3-dependent Rac exchanger (P-Rex) [259] and protein kinase D [260]. A recent study showed that the β subunits 1, 2, 4, and 5 in $G_{β γ}$ are indispensable for GPCR-mediated integrin activation and leukocyte recruitment, in which Ras-related C3 botulinum toxin substrate 1 (Rac-1) and phosphoinositide phospholipase C (PLC) $β_{2}$ and $β_{3}$ are involved in downstream signaling pathway [261]. In vivo, PI3Kγ [262,263] and protein kinase C (PKC) θ [264] deficient neutrophils show no defect in arrest, but they revert to rolling again, suggesting that there is a defect in integrin bond maturation. Blockade of p44/42 and p38 mitogen-activated protein kinases (MAPK), PI3K, or PKC signaling does not affect chemokine triggered integrin activation on monocytes. The role of inositol triphosphate (IP3) and intracellular free calcium (Ca²⁺), the calcium-binding messenger protein calmodulin, and inositol-1,4,5-triphosphate receptors as downstream events of PLC activation is controversial [265,266]. Ca²⁺/diacylglycerol (DAG)-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) is required for activation of Ras-related protein 1 (Rap1) and integrin in neutrophils [267]. In contrast, when the neutrophils were stimulated by fMLP, integrins were activated by activation of Rac through proto-oncogene vav (Vav1) and P-Rex1 [268]. Two (incomplete) chemokine receptor pathways leading to activation of different integrins were demonstrated in T-lymphocytes stimulated by CXCL12 or PMA: PLCγ → Ca²⁺/DAG → CalDAG-GEFI → Rap1 → LFA-1 and PLCγ → Ca²⁺/diacylglycerol → PKC → very late antigen-4 (VLA-4) [269] respectively. Another GTPase, cell division control protein 42 (Cdc42), may be a negative regulator of integrin activation upon chemokine stimulation [270]. Phospholipase D1 (PLD-1) was demonstrated as the downstream of Rac1 and RhoA [270]. Phosphatidylinositol-4-phosphate 5-kinase type I gamma (PIP5KC) is downstream of PLD-1 and found to specifically control the transition of LFA-1 from an extended low-intermediate state to a high-affinity state [270]. In a recent study [271] CXCL12 was shown to induce Janus kinase (JAK) 2 and 3 activation in a $G_{α i}$ -independent manner. JAK2 and 3 were upstream of Vav1, which then activates Ras homolog gene family member A (RhoA) and Rac-1. Blockade of RhoA and PLD-1 inhibits the activation of Rap1a. Another study on $α_{IIb} β_{3}$ suggests that Rap1 is downstream of PKC [272].

In conclusion, the proximal (GPCR to $G_{α i 2}$ to PLCβ) and distal (GALDAG-GEFI to Rap1 to talin-1 to integrin) parts of the pathway are clear, but the middle parts are not clear. It is also unknown whether integrin activation is local, i.e. in the microvillus where the leukocyte is in touch with chemokine, or global (whole cell).

4.4. Rap-1

As reviewed, the pathways of PSGL-1 signaling and chemokine receptor signaling may converge on Rap1. However, PSGL-1 ligation triggers only extension and chemokine receptor ligation triggers high affinity (open headpiece). This discrepancy requires that molecules other than Rap-1 are involved. During integrin activation, activated Rap1 needs to be recruited to the plasma membrane by a signaling module comprising the cytosolic adapter proteins ADAP (adhesion and degranulation promoting adapter protein) and SKAP55 (src kinase-associated protein of 55 kDa) [273]. RapL is one of the Rap1 downstream effectors, which was identified by a yeast two-hybrid screen [89,274]. RapL binds to a site consisting of two lysine residues (K1097/K1099) following the GFFKR motif, which is found only in the $α_{L}$ [90] subunit. RapL forms a complex with the serine/threonine kinase Mst1. Rap1 activation recruits both RapL and Mst1 to LFA-1 and actives Mst1 kinase activity. RapL-deficiency impairs Mst1 activation in cells. Knocking down Mst1 expression reduces integrin activation in response to chemokine stimulation [91], but it is not known how Mst1 is involved in extension or high affinity.

RIAM is another important Rap1 binding protein, which is very important in integrin activation [275]. RIAM has been reported to interact with SKAP-55 and play key roles in the recruitment of Rap1/RIAM complex to the plasma membrane [276]. The ability of Rap1 to activate integrins depends on talin binding to integrin cytoplasmic tail [272]. Rap1 induces the formation of an integrin-activation complex containing talin in combination with RIAM [272,277]. RIAM deficient mice have no defect in platelet arrest [206,208], but a severe defect in neutrophil arrest [207], macrophage adhesion [207], and lymphocyte adhesion [206,207]. A recent study shows that SLAT (SWAP-70-like adaptor of T cells) can interact with Rap-1 through its PH domain and promotes TCR-mediated, Rap1-dependent LFA-1 activation and adhesion [278]. Since SLAT is activated by T cell receptor engagement, this mechanism is unlikely to be involved in triggering arrest.

5. Conclusions and open questions

$β_{2}$ integrins are fascinating molecular machines that translate intracellular adaptor binding (kindlin-3, talin-1, RIAM) into conformation changes of the ectodomains. This may be transmitted through moving the cytoplasmic and transmembrane domains apart, or changing their crossing angle. The molecular mechanics by which these movements are translated to ectodomain changes are unknown. $β_{2}$ integrin activation is triggered by PSGL-1 ligation or chemokine receptor ligation or both. Under physiologic conditions, PSGL-1 ligation alone reduces the rolling velocity in a $β_{2}$ integrin-dependent fashion. When (soluble or immobilized) chemokines are available, full integrin activation (E⁺H⁺) ensues and leads to arrest of the rolling cell. Both the PSGL-1and the chemokine receptor signaling pathways to $β_{2}$ integrins are only partially understood. The initial hypothesis that all integrins are activated in the same way was refuted: not only are different integrins activated in different ways, but activation mechanisms even appear to be cell type-specific. The methods used to study integrin activation include crystallography of conformationally stabilized integrins, rotary shadowing EM, NMR, reporter antibodies, and functional studies of rolling and arrest. It is very challenging to integrate findings obtained with these fundamentally different and mutually exclusive methods.

Footnotes

Acknowledgements

This research was supported by funding from National Institutes of Health, USA (NIH, HL078784).

References

[1] Hynes

. Integrins: Bidirectional, allosteric signaling machines. Cell. 2002;110(6):673–687. PubMed PMID: 12297042.

[2] Ruoslahti

. Integrins. The Journal of Clinical Investigation. 1991;87(1):1–5. PubMed PMID: 1985087. PubMed Central PMCID: 294975.

[3] Schwartz

. Transmembrane signalling by integrins. Trends in Cell Biology. 1992;2(10):304–308. PubMed PMID: 14731926.

[4] Gahmberg

Valmu

Fagerholm

Kotovuori

Ihanus

Tian

, et al. Leukocyte integrins and inflammation. Cellular and Molecular Life Sciences: CMLS. 1998;54(6):549–555. PubMed PMID: 9676574.

[5] Savinko

Morrison

Uotila

Wolff

Alenius

Fagerholm

. Functional Beta2-integrins restrict skin inflammation in vivo. The Journal of Investigative Dermatology. 2015;135(9):2249–2257. PubMed PMID: 25918984.

[6] El Kebir

Filep

. Modulation of neutrophil apoptosis and the resolution of inflammation through

β_{2}

integrins. Frontiers in Immunology. 2013;4:60. PubMed PMID: 23508943. PubMed Central PMCID: 3589696.

[7] Zarbock

Kempf

Wollert

Vestweber

. Leukocyte integrin activation and deactivation: Novel mechanisms of balancing inflammation. Journal of Molecular Medicine. 2012;90(4):353–359. PubMed PMID: 22101734.

[8] Springer

Dustin

. Integrin inside-out signaling and the immunological synapse. Current Opinion in Cell Biology. 2012;24(1):107–115. PubMed PMID: 22129583. PubMed Central PMCID: 3294052.

[9] Luo

Carman

Springer

. Structural basis of integrin regulation and signaling. Annual Review of Immunology. 2007;25:619–647. PubMed PMID: 17201681. PubMed Central PMCID: 1952532.

10.

[10] Comrie

Babich

Burkhardt

. F-actin flow drives affinity maturation and spatial organization of LFA-1 at the immunological synapse. The Journal of Cell Biology. 2015;208(4):475–491. PubMed PMID: 25666810. PubMed Central PMCID: 4332248.

11.

[11] Comrie

Boyle

Burkhardt

. The dendritic cell cytoskeleton promotes T cell adhesion and activation by constraining ICAM-1 mobility. The Journal of Cell Biology. 2015;208(4):457–473. PubMed PMID: 25666808.

12.

[12] Shattil

Newman

. Integrins: Dynamic scaffolds for adhesion and signaling in platelets. Blood. 2004;104(6):1606–1615. PubMed PMID: 15205259.

13.

[13] Coller

Shattil

. The GPIIb/IIIa (integrin

α_{IIb} β_{3}

) odyssey: A technology-driven saga of a receptor with twists, turns, and even a bend. Blood. 2008;112(8):3011–3025. PubMed PMID: 18840725. PubMed Central PMCID: 2569161.

14.

[14] Chico

Chamberlain

Gunn

Arnold

Bullens

Gadek

, et al. Effect of selective or combined inhibition of integrins

α_{IIb} β_{3}

and

α_{v} β_{3}

on thrombosis and neointima after oversized porcine coronary angioplasty. Circulation. 2001;103(8):1135–1141. PubMed PMID: 11222478.

15.

[15] Cheresh

. Integrins in thrombosis, wound healing and cancer. Biochemical Society Transactions. 1991;19(4):835–838. PubMed PMID: 1794568.

16.

[16] Phillipson

Kubes

. The neutrophil in vascular inflammation. Nature Medicine. 2011;17(11):1381–1390. PubMed PMID: 22064428.

17.

[17] Kolaczkowska

Kubes

. Neutrophil recruitment and function in health and inflammation. Nature Reviews Immunology. 2013;13(3):159–175. PubMed PMID: 23435331.

18.

[18] Vecino

Heller

Veiga-Crespo

Martin

Fawcett

. Influence of extracellular matrix components on the expression of integrins and regeneration of adult retinal ganglion cells. PLoS ONE. 2015;10(5):e0125250. PubMed PMID: 26018803.

19.

[19] Foubert

Varner

. Integrins in tumor angiogenesis and lymphangiogenesis. Methods in Molecular Biology. 2012;757:471–486. PubMed PMID: 21909928.

20.

[20] Ruoslahti

. Specialization of tumour vasculature. Nature Reviews Cancer. 2002;2(2):83–90. PubMed PMID: 12635171.

21.

[21] Rantala

Pouwels

Pellinen

Veltel

Laasola

Mattila

, et al. SHARPIN is an endogenous inhibitor of

β_{1}

-integrin activation. Nature Cell Biology. 2011;13(11):1315–1324. PubMed PMID: 21947080. PubMed Central PMCID: 3257806.

22.

[22] Desgrosellier

Cheresh

. Integrins in cancer: Biological implications and therapeutic opportunities. Nature Reviews Cancer. 2010;10(1):9–22. PubMed PMID: 20029421. PubMed Central PMCID: 4383089.

23.

[23] Byron

Humphries

Askari

Craig

Mould

Humphries

. Anti-integrin monoclonal antibodies. Journal of Cell Science. 2009;122(22):4009–4011.

24.

[24] Kuwano

Spelten

Zhang

Ley

Zarbock

. Rolling on E- or P-selectin induces the extended but not high-affinity conformation of LFA-1 in neutrophils. Blood. 2010;116(4):617–624. PubMed PMID: 20445017. PubMed Central PMCID: 3324292.

25.

[25] Sundd

Gutierrez

Koltsova

Kuwano

Fukuda

Pospieszalska

, et al. ‘Slings’ enable neutrophil rolling at high shear. Nature. 2012;488(7411):399–403. PubMed PMID: 22763437. PubMed Central PMCID: 3433404.

26.

[26] Sumagin

Prizant

Lomakina

Waugh

Sarelius

. LFA-1 and Mac-1 define characteristically different intralumenal crawling and emigration patterns for monocytes and neutrophils in situ. Journal of Immunology. 2010;185(11):7057–7066. PubMed PMID: 21037096. PubMed Central PMCID: 3004223.

27.

[27] Hogg

Laschinger

Giles

McDowall

. T-cell integrins: More than just sticking points. J Cell Sci. 2003;116(Pt 23):4695–4705. PubMed PMID: 14600256.

28.

[28] Makgoba

Sanders

Ginther Luce

Dustin

Springer

Clark

, et al. ICAM-1 a ligand for LFA-1-dependent adhesion of B, T and myeloid cells. Nature. 1988;331(6151):86–88. PubMed PMID: 3277059.

29.

[29] Arana

Harwood

Batista

. Regulation of integrin activation through the B-cell receptor. Journal of Cell Science. 2008;121(14):2279–2286.

30.

[30] Shrivastava

Shishodia

Sodhi

. Expression of LFA-1 adhesion molecules on cisplatin-treated macrophages. Biochimica et Biophysica Acta (BBA) – Molecular Cell Research. 1998;1402(3):269–276.

31.

[31] Balkow

Heinz

Schmidbauer

Kolanus

Holzmann

Grabbe

, et al. LFA-1 activity state on dendritic cells regulates contact duration with T cells and promotes T-cell priming. Blood. 2010;116(11):1885–1894. PubMed PMID: 20530790.

32.

[32] Morris

Ley

. Trafficking of natural killer cells. Current Molecular Medicine. 2004;4(4):431–438. PubMed PMID: 15354873.

33.

[33] Fisher

Riddle

Kim

Presta

Bodary

. Identification of the binding site in intercellular adhesion molecule 1 for its receptor, leukocyte function-associated antigen 1. Molecular Biology of the Cell. 1997;8(3):501–515. PubMed PMID: 9188101. PubMed Central PMCID: 276100.

34.

[34] Lahti

Heino

Kapyla

. Leukocyte integrins

α_{L} β_{2}

α_{M} β_{2}

and

α_{X} β_{2}

as collagen receptors–receptor activation and recognition of GFOGER motif. The International Journal of Biochemistry & Cell Biology. 2013;45(7):1204–1211. PubMed PMID: 23542015.

35.

[35] Rothlein

Dustin

Marlin

Springer

. A human intercellular adhesion molecule (ICAM-1) distinct from LFA-1. Journal of Immunology. 1986;137(4):1270–1274. PubMed PMID: 3525675.

36.

[36] Stanley

Hogg

. The I domain of integrin LFA-1 interacts with ICAM-1 domain 1 at residue Glu-34 but not Gln-73. Journal of Biological Chemistry. 1998;273(6):3358–3362.

37.

[37] Staunton

Dustin

Springer

. Functional cloning of ICAM-2, a cell adhesion ligand for LFA-1 homologous to ICAM-1. Nature. 1989;339(6219):61–64. PubMed PMID: 2497351.

38.

[38] Klickstein

York

Fougerolles

Springer

. Localization of the binding site on intercellular adhesion molecule-3 (ICAM-3) for lymphocyte function-associated antigen 1 (LFA-1). The Journal of Biological Chemistry. 1996;271(39):23920–23927. PubMed PMID: 8798624.

39.

[39] Ihanus

Uotila

Toivanen

Stefanidakis

Bailly

Cartron

, et al. Characterization of ICAM-4 binding to the I domains of the CD11a/CD18 and CD11b/CD18 leukocyte integrins. European Journal of Biochemistry/FEBS. 2003;270(8):1710–1723. PubMed PMID: 12694184.

40.

[40] Bailly

Tontti

Hermand

Cartron

J-P

Gahmberg

. The red cell LW blood group protein is an intercellular adhesion molecule which binds to CD11/CD18 leukocyte integrins. European Journal of Immunology. 1995;25(12):3316–3320.

41.

[41] Tian

Kilgannon

Yoshihara

Mori

Gallatin

Carpén

, et al. Binding of T lymphocytes to hippocampal neurons through ICAM-5 (telencephalin) and characterization of its interaction with the leukocyte integrin CD11a/CD18. European Journal of Immunology. 2000;30(3):810–818.

42.

[42] Tian

Lappalainen

Autero

Hanninen

Rauvala

Gahmberg

. Shedded neuronal ICAM-5 suppresses T-cell activation. Blood. 2008;111(7):3615–3625. PubMed PMID: 18223167.

43.

[43] Kim

Skokos

Myer

Agaba

Gonzalez

α_{V} β_{3}

integrin regulation of respiratory burst in fibrinogen adherent human neutrophils. Cellular and Molecular Bioengineering. 2014;7(2):231–242. PubMed PMID: 25632307. PubMed Central PMCID: 4306468.

44.

[44] Ding

Chen

Liu

Cai

, et al. Integrin CD11b negatively regulates BCR signalling to maintain autoreactive B cell tolerance. Nature Communications. 2013;4:2813. PubMed PMID: 24264377.

45.

[45] Altieri

Bader

Mannucci

Edgington

. Oligospecificity of the cellular adhesion receptor Mac-1 encompasses an inducible recognition specificity for fibrinogen. The Journal of Cell Biology. 1988;107(5):1893–1900. PubMed PMID: 3053736. PubMed Central PMCID: 2115324.

46.

[46] Metlay

Witmer-Pack

Agger

Crowley

Lawless

Steinman

. The distinct leukocyte integrins of mouse spleen dendritic cells as identified with new hamster monoclonal antibodies. The Journal of Experimental Medicine. 1990;171(5):1753–1771.

47.

[47] Varga

Balkow

Wild

Stadtbaeumer

Krummen

Rothoeft

, et al. Active MAC-1 (CD11b/CD18) on DCs inhibits full T-cell activation. Blood. 2007;109(2):661–669.

48.

[48] Parkos

Colgan

Diamond

Nusrat

Liang

Springer

, et al. Expression and polarization of intercellular adhesion molecule-1 on human intestinal epithelia: Consequences for CD11b/CD18-mediated interactions with neutrophils. Molecular Medicine. 1996;2(4):489–505. PubMed PMID: 8827719. PubMed Central PMCID: 2230166.

49.

[49] Diamond

Staunton

de Fougerolles

Stacker

Garcia-Aguilar

Hibbs

, et al. ICAM-1 (CD54): A counter-receptor for Mac-1 (CD11b/CD18). The Journal of Cell Biology. 1990;111(6 Pt 2):3129–3139. PubMed PMID: 1980124. PubMed Central PMCID: 2116396.

50.

[50] Diamond

Staunton

Marlin

Springer

. Binding of the integrin Mac-1 (CD11b/CD18) to the third immunoglobulin-like domain of ICAM-1 (CD54) and its regulation by glycosylation. Cell. 1991;65(6):961–971. PubMed PMID: 1675157.

51.

[51] Xie

Kotovuori

Vermot-Desroches

Wijdenes

Arnaout

, et al. Intercellular adhesion molecule-2 (CD102) binds to the leukocyte integrin CD11b/CD18 through the A domain. Journal of Immunology. 1995;155(7):3619–3628. PubMed PMID: 7561061.

52.

[52] Wright

Weitz

Huang

Levin

Silverstein

Loike

. Complement receptor type three (CD11b/CD18) of human polymorphonuclear leukocytes recognizes fibrinogen. Proceedings of the National Academy of Sciences of the United States of America. 1988;85(20):7734–7738. PubMed PMID: 2971974. PubMed Central PMCID: 282267.

53.

[53] Beller

Springer

Schreiber

. Anti-Mac-1 selectively inhibits the mouse and human type three complement receptor. The Journal of Experimental Medicine. 1982;156(4):1000–1009. PubMed PMID: 7153706. PubMed Central PMCID: 2186827.

54.

[54] Diamond

Alon

Parkos

Quinn

Springer

. Heparin is an adhesive ligand for the leukocyte integrin Mac-1 (CD11b/CD18). The Journal of Cell Biology. 1995;130(6):1473–1482. PubMed PMID: 7559767. PubMed Central PMCID: 2120570.

55.

[55] Simon

Chen

Dong

McIntire

, et al. Platelet glycoprotein ibα is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18). The Journal of Experimental Medicine. 2000;192(2):193–204. PubMed PMID: 10899906. PubMed Central PMCID: 2193258.

56.

[56] Santoso

Sachs

Kroll

Linder

Ruf

Preissner

, et al. The junctional adhesion molecule 3 (JAM-3) on human platelets is a counterreceptor for the leukocyte integrin Mac-1. The Journal of Experimental Medicine. 2002;196(5):679–691. PubMed PMID: 12208882. PubMed Central PMCID: 2194005.

57.

[57] Wetzel

Chavakis

Preissner

Sticherling

Haustein

Anderegg

, et al. Human Thy-1 (CD90) on activated endothelial cells is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18). Journal of Immunology. 2004;172(6):3850–3859. PubMed PMID: 15004192.

58.

[58] Lishko

Novokhatny

Yakubenko

Skomorovska-Prokvolit

Ugarova

. Characterization of plasminogen as an adhesive ligand for integrins

α_{M} β_{2}

(Mac-1) and

α_{5} β_{1}

(VLA-5). Blood. 2004;104(3):719–726.

59.

[59] Myones

Dalzell

Hogg

Ross

. Neutrophil and monocyte cell surface p150,95 has iC3b-receptor (CR4) activity resembling CR3. The Journal of Clinical Investigation. 1988;82(2):640–651.

60.

[60] Loike

Sodeik

Cao

Leucona

Weitz

Detmers

, et al. CD11c/CD18 on neutrophils recognizes a domain at the N terminus of the Aα chain of fibrinogen. Proceedings of the National Academy of Sciences. 1991;88(3):1044–1048.

61.

[61] Rubtsov

Rubtsova

Fischer

Meehan

Gillis

Kappler

, et al. Toll-like receptor 7 (TLR7)-driven accumulation of a novel CD11c⁺ B-cell population is important for the development of autoimmunity. Blood. 2011;118(5):1305–1315.

62.

[62] Postigo

Corbí

Sánchez-Madrid

de Landázuri

. Regulated expression and function of CD11c/CD18 integrin on human B lymphocytes. Relation between attachment to fibrinogen and triggering of proliferation through CD11c/CD18. The Journal of Experimental Medicine. 1991;174(6):1313–1322.

63.

[63] Sadhu

Ting

Lipsky

Hensley

Garcia-Martinez

Simon

, et al. CD11c/CD18: Novel ligands and a role in delayed-type hypersensitivity. Journal of Leukocyte Biology. 2007;81(6):1395–1403. PubMed PMID: 17389580.

64.

[64] Frick

Odermatt

Zen

Mandell

Edens

Portmann

, et al. Interaction of ICAM-1 with

β_{2}

-integrin CD11c/CD18: Characterization of a peptide ligand that mimics a putative binding site on domain D4 of ICAM-1. European Journal of Immunology. 2005;35(12):3610–3621.

65.

[65] Choi

Choi

Nham

S-U

. Characterization of the residues of

α_{X}

I-domain and ICAM-1 mediating their interactions. Mol Cells. 2010;30(3):227–234 (in English).

66.

[66] Ihanus

Uotila

Toivanen

Varis

Gahmberg

. Red-cell ICAM-4 is a ligand for the monocyte/macrophage integrin CD11c/CD18: Characterization of the binding sites on ICAM-4. Blood. 2007;109(2):802–810.

67.

[67] Sen

Yuki

Springer

. An internal ligand-bound, metastable state of a leukocyte integrin,

α_{X} β_{2}

. The Journal of Cell Biology. 2013;203(4):629–642. PubMed PMID: 24385486. PubMed Central PMCID: 3840939.

68.

[68] Vorup-Jensen

Chi

Gjelstrup

Jensen

Jewett

Xie

, et al. Binding between the integrin

α_{X} β_{2}

(CD11c/CD18) and heparin. Journal of Biological Chemistry. 2007;282(42):30869–30877.

69.

[69] Yakubenko

Ustinov

Pluskota

López

Wang

Simon

. Platelet-leukocyte conjugation mediated by GPIbα–

α_{X} β_{2}

(CD11c/CD18) interaction. Blood. 2013;122(21):1029.

70.

[70] Choi

Leyton

Nham

S-U

. Characterization of

α_{X}

I-domain binding to Thy-1. Biochemical and Biophysical Research Communications. 2005;331(2):557–561.

71.

[71] Jongyun

Jeongsuk

Joo Hee

Sang-Uk

. Identification of critical residues for plasminogen binding by the

α_{X}

I-domain of the

β_{2}

integrin,

α_{X} β_{2}

. Mol Cells. 2007;24(2):240–246.

72.

[72] Van der Vieren

Le Trong

Wood

Moore

John

Staunton

, et al. A novel leukointegrin,

α_{d} β_{2}

, binds preferentially to ICAM-3. Immunity. 1995;3(6):683–690.

73.

[73] Miyazaki

Vieira-de-Abreu

Harris

Shah

Weyrich

Castro-Faria-Neto

, et al. Integrin

α_{D} β_{2}

(CD11d/CD18) is expressed by human circulating and tissue myeloid leukocytes and mediates inflammatory signaling. PLoS ONE. 2014;9(11):e112770.

74.

[74] Miyazaki

Bunting

Stafforini

Harris

McIntyre

Prescott

, et al. Integrin

α_{D} β_{2}

is dynamically expressed by inflamed macrophages and alters the natural history of lethal systemic infections. Journal of Immunology. 2008;180(1):590–600. PubMed PMID: 18097061. PubMed Central PMCID: 2275910.

75.

[75] Grayson

Van der Vieren

Sterbinsky

Michael Gallatin

Hoffman

Staunton

, et al.

α_{d} β_{2}

integrin is expressed on human eosinophils and functions as an alternative ligand for vascular cell adhesion molecule 1 (VCAM-1). The Journal of Experimental Medicine. 1998;188(11):2187–2191. PubMed PMID: 9841932. PubMed Central PMCID: 2212388.

76.

[76] Van der Vieren

Crowe

Hoekstra

Vazeux

Hoffman

Grayson

, et al. The leukocyte integrin

α_{D} β_{2}

binds VCAM-1: Evidence for a binding interface between I domain and VCAM-1. Journal of Immunology. 1999;163(4):1984–1990. PubMed PMID: 10438935.

77.

[77] Yakubenko

Yadav

Ugarova

. Integrin

α_{D} β_{2}

, an adhesion receptor up-regulated on macrophage foam cells, exhibits multiligand-binding properties. Blood. 2006;107(4):1643–1650. PubMed PMID: 16239428. PubMed Central PMCID: 1367263.

78.

[78] Campbell

Humphries

. Integrin structure, activation, and interactions. Cold Spring Harbor Perspectives in Biology. 2011;3(3):a004994. PubMed PMID: 21421922. PubMed Central PMCID: 3039929.

79.

[79] Arnaout

Goodman

Xiong

. Structure and mechanics of integrin-based cell adhesion. Current Opinion in Cell Biology. 2007;19(5):495–507. PubMed PMID: 17928215. PubMed Central PMCID: 2443699.

80.

[80] Askari

Buckley

Mould

Humphries

. Linking integrin conformation to function. J Cell Sci. 2009;122(Pt 2):165–170. PubMed PMID: 19118208. PubMed Central PMCID: 2714414.

81.

[81] Bennett

Berger

Billings

. The structure and function of platelet integrins. Journal of Thrombosis and Haemostasis: JTH. 2009;7(Suppl 1):200–205. PubMed PMID: 19630800.

82.

[82] Morse

Brahme

Calderwood

. Integrin cytoplasmic tail interactions. Biochemistry. 2014;53(5):810–820. PubMed PMID: 24467163. PubMed Central PMCID: 3985435.

83.

[83] Legate

Fässler

. Mechanisms that regulate adaptor binding to β-integrin cytoplasmic tails. Journal of Cell Science. 2009;122(2):187–198.

84.

[84] Anthis

Campbell

. The tail of integrin activation. Trends in Biochemical Sciences. 2011;36(4):191–198. PubMed PMID: 21216149. PubMed Central PMCID: 3078336.

85.

[85] Abram

Lowell

. The ins and outs of leukocyte integrin signaling. Annual Review of Immunology. 2009;27:339–362. PubMed PMID: 19302044. PubMed Central PMCID: 3248397.

86.

[86] Pouwels

Nevo

Pellinen

Ylanne

Ivaska

. Negative regulators of integrin activity. J Cell Sci. 2012;125(Pt 14):3271–3280. PubMed PMID: 22822081.

87.

[87] Calderwood

Campbell

Critchley

. Talins and kindlins: Partners in integrin-mediated adhesion. Nature Reviews Molecular Cell Biology. 2013;14(8):503–517. PubMed PMID: 23860236. PubMed Central PMCID: 4116690.

88.

[88] Lefort

Ley

. Neutrophil arrest by LFA-1 activation. Frontiers in Immunology. 2012;3:157. PubMed PMID: 22701459. PubMed Central PMCID: 3373145.

89.

[89] Katagiri

Maeda

Shimonaka

Kinashi

. RAPL, a Rap1-binding molecule that mediates Rap1-induced adhesion through spatial regulation of LFA-1. Nature Immunology. 2003;4(8):741–748. PubMed PMID: 12845325.

90.

[90] Tohyama

Katagiri

Pardi

Springer

Kinashi

. The critical cytoplasmic regions of the

α_{L}

β_{2}

integrin in Rap1-induced adhesion and migration. Molecular Biology of the Cell. 2003;14(6):2570–2582. PubMed PMID: 12808052. PubMed Central PMCID: 194904.

91.

[91] Katagiri

Imamura

Kinashi

. Spatiotemporal regulation of the kinase Mst1 by binding protein RAPL is critical for lymphocyte polarity and adhesion. Nature Immunology. 2006;7(9):919–928. PubMed PMID: 16892067.

92.

[92] Katagiri

Ohnishi

Kabashima

Iyoda

Takeda

Shinkai

, et al. Crucial functions of the Rap1 effector molecule RAPL in lymphocyte and dendritic cell trafficking. Nature Immunology. 2004;5(10):1045–1051. PubMed PMID: 15361866.

93.

[93] Nho

Kahm

β_{1}

-integrin-collagen interaction suppresses FoxO3a by the coordination of Akt and PP2A. The Journal of Biological Chemistry. 2010;285(19):14195–14209. PubMed PMID: 20223831. PubMed Central PMCID: 2863165.

94.

[94] Mai

Veltel

Pellinen

Padzik

Coffey

Marjomaki

, et al. Competitive binding of Rab21 and p120RasGAP to integrins regulates receptor traffic and migration. The Journal of Cell Biology. 2011;194(2):291–306. PubMed PMID: 21768288. PubMed Central PMCID: 3144408.

95.

[95] Alahari

Reddig

Juliano

. The integrin-binding protein Nischarin regulates cell migration by inhibiting PAK. The EMBO Journal. 2004;23(14):2777–2788. PubMed PMID: 15229651. PubMed Central PMCID: 514951.

96.

[96] Alahari

Lee

Juliano

. Nischarin, a novel protein that interacts with the integrin

α_{5}

subunit and inhibits cell migration. The Journal of Cell Biology. 2000;151(6):1141–1154. PubMed PMID: 11121431. PubMed Central PMCID: 2190593.

97.

[97] Nevo

Mai

Tuomi

Pellinen

Pentikainen

Heikkila

, et al. Mammary-derived growth inhibitor (MDGI) interacts with integrin α-subunits and suppresses integrin activity and invasion. Oncogene. 2010;29(49):6452–6463. PubMed PMID: 20802519.

98.

[98] Han

Rose

Woodside

Goldfinger

Ginsberg

. Integrin

α_{4} β_{1}

-dependent T cell migration requires both phosphorylation and dephosphorylation of the

α_{4}

cytoplasmic domain to regulate the reversible binding of paxillin. The Journal of Biological Chemistry. 2003;278(37):34845–34853. PubMed PMID: 12837751.

99.

[99] Feral

Rose

Han

Fox

Silverman

Kaushansky

, et al. Blocking the

α_{4}

integrin-paxillin interaction selectively impairs mononuclear leukocyte recruitment to an inflammatory site. The Journal of Clinical Investigation. 2006;116(3):715–723. PubMed PMID: 16470243. PubMed Central PMCID: 1361348.

100.

[100] Lefort

Rossaint

Moser

Petrich

Zarbock

Monkley

, et al. Distinct roles for talin-1 and kindlin-3 in LFA-1 extension and affinity regulation. Blood. 2012;119(18):4275–4282. PubMed PMID: 22431571. PubMed Central PMCID: 3359742.

101.

[101] Calderwood

Zent

Grant

Rees

Hynes

Ginsberg

. The talin head domain binds to integrin β subunit cytoplasmic tails and regulates integrin activation. The Journal of Biological Chemistry. 1999;274(40):28071–28074. PubMed PMID: 10497155.

102.

[102] Wegener

Partridge

Han

Pickford

Liddington

Ginsberg

, et al. Structural basis of integrin activation by talin. Cell. 2007;128(1):171–182. PubMed PMID: 17218263.

103.

[103] Sampath

Gallagher

Pavalko

. Cytoskeletal interactions with the leukocyte integrin

β_{2}

cytoplasmic tail. Activation-dependent regulation of associations with talin and α-actinin. The Journal of Biological Chemistry. 1998;273(50):33588–33594. PubMed PMID: 9837942. PubMed Central PMCID: 2823626.

104.

[104] Garcia-Alvarez

de Pereda

Calderwood

Ulmer

Critchley

Campbell

, et al. Structural determinants of integrin recognition by talin. Molecular Cell. 2003;11(1):49–58. PubMed PMID: 12535520.

105.

[105] Geier

Fengos

Iber

. A computational analysis of the dynamic roles of talin, Dok1, and PIPKI for integrin activation. PLoS ONE. 2011;6(11):e24808. PubMed PMID: 22110576. PubMed Central PMCID: 3217926.

106.

[106] Gupta

Chit

JC-Y

Feng

Bhunia

Tan

S-M

Bhattacharjya

. An alternative phosphorylation switch in integrin

β_{2}

(CD18) tail for Dok1 binding. Scientific Reports. 2015;5:11630.

107.

[107] Stanley

Smith

McDowall

Nicol

Zicha

Hogg

. Intermediate-affinity LFA-1 binds α-actinin-1 to control migration at the leading edge of the T cell. The EMBO Journal. 2008;27(1):62–75. PubMed PMID: 18079697. PubMed Central PMCID: 2147999.

108.

[108] Bogdanovic

Delfino-Machin

Nicolas-Perez

Gavilan

Gago-Rodrigues

Fernandez-Minan

, et al. Numb/Numbl-Opo antagonism controls retinal epithelium morphogenesis by regulating integrin endocytosis. Developmental Cell. 2012;23(4):782–795. PubMed PMID: 23041384.

109.

[109] Moser

Nieswandt

Ussar

Pozgajova

Fassler

. Kindlin-3 is essential for integrin activation and platelet aggregation. Nature Medicine. 2008;14(3):325–330. PubMed PMID: 18278053.

110.

[110] Montanez

Ussar

Schifferer

Bosl

Zent

Moser

, et al. Kindlin-2 controls bidirectional signaling of integrins. Genes & Development. 2008;22(10):1325–1330. PubMed PMID: 18483218. PubMed Central PMCID: 2377186.

111.

[111] Ma

Qin

Plow

. Kindlin-2 (Mig-2): A co-activator of

β_{3}

integrins. The Journal of Cell Biology. 2008;181(3):439–446. PubMed PMID: 18458155. PubMed Central PMCID: 2364684.

112.

[112] Liu

Das

Yang

Ithychanda

Yakubenko

Plow

, et al. Structural mechanism of integrin inactivation by filamin. Nature Structural & Molecular Biology. 2015;22(5):383–389. PubMed PMID: 25849143. PubMed Central PMCID: 4424056.

113.

[113] Truong

Shams

Mofrad

. Mechanisms of integrin and filamin binding and their interplay with talin during early focal adhesion formation. Integrative Biology. 2015;7(10):1285–1296. PubMed PMID: 26156744.

114.

[114] Sharma

Ezzell

Arnaout

. Direct interaction of filamin (ABP-280) with the beta 2-integrin subunit CD18. Journal of Immunology. 1995;154(7):3461–3470. PubMed PMID: 7534799.

115.

[115] Bonet

Vakonakis

Campbell

. Characterization of 14-3-3-zeta interactions with integrin tails. Journal of Molecular Biology. 2013;425(17):3060–3072. PubMed PMID: 23763993. PubMed Central PMCID: 4068353.

116.

[116] Bottcher

Stremmel

Meves

Meyer

Widmaier

Tseng

, et al. Sorting nexin 17 prevents lysosomal degradation of

β_{1}

integrins by binding to the

β_{1}

-integrin tail. Nature Cell Biology. 2012;14(6):584–592. PubMed PMID: 22561348.

117.

[117] Geiger

Nagel

Boehm

van Kooyk

Figdor

Kremmer

, et al. Cytohesin-1 regulates β-2 integrin-mediated adhesion through both ARF-GEF function and interaction with LFA-1. The EMBO Journal. 2000;19(11):2525–2536. PubMed PMID: 10835351. PubMed Central PMCID: 212768.

118.

[118] Kolanus

Nagel

Schiller

Zeitlmann

Godar

Stockinger

, et al.

α_{L} β_{2}

integrin/LFA-1 binding to ICAM-1 induced by cytohesin-1, a cytoplasmic regulatory molecule. Cell. 1996;86(2):233–242. PubMed PMID: 8706128.

119.

[119] Chang

Hoang

Liu

Springer

. Molecular basis for interaction between Icap1α PTB domain and

β_{1}

integrin. The Journal of Biological Chemistry. 2002;277(10):8140–8145. PubMed PMID: 11741908.

120.

[120] Simpson

Bradley

Harburger

Parsons

Calderwood

Koleske

. Direct interactions with the integrin

β_{1}

cytoplasmic tail activate the Abl2/Arg kinase. The Journal of Biological Chemistry. 2015;290(13):8360–8372. PubMed PMID: 25694433. PubMed Central PMCID: 4375489.

121.

[121] Teckchandani

Mulkearns

Randolph

Toida

Cooper

. The clathrin adaptor Dab2 recruits EH domain scaffold proteins to regulate integrin

β_{1}

endocytosis. Molecular Biology of the Cell. 2012;23(15):2905–2916. PubMed PMID: 22648170. PubMed Central PMCID: 3408417.

122.

[122] Arias-Salgado

Lizano

Sarkar

Brugge

Ginsberg

Shattil

. Src kinase activation by direct interaction with the integrin β cytoplasmic domain. Proceedings of the National Academy of Sciences of the United States of America. 2003;100(23):13298–13302. PubMed PMID: 14593208. PubMed Central PMCID: 263791.

123.

[123] Klinghoffer

Sachsenmaier

Cooper

Soriano

. Src family kinases are required for integrin but not PDGFR signal transduction. The EMBO Journal. 1999;18(9):2459–2471. PubMed PMID: 10228160. PubMed Central PMCID: 1171328.

124.

[124] Dunne

Ballantyne

Beaudet

Ley

. Control of leukocyte rolling velocity in TNF-α-induced inflammation by LFA-1 and Mac-1. Blood. 2002;99(1):336–341. PubMed PMID: 11756189.

125.

[125] Zarbock

Lowell

Ley

. Spleen tyrosine kinase Syk is necessary for E-selectin-induced

α_{L} β_{2}

integrin-mediated rolling on intercellular adhesion molecule-1. Immunity. 2007;26(6):773–783. PubMed PMID: 17543554. PubMed Central PMCID: 2600878.

126.

[126] Zarbock

Abram

Hundt

Altman

Lowell

Ley

. PSGL-1 engagement by E-selectin signals through Src kinase Fgr and ITAM adapters DAP12 and FcRγ to induce slow leukocyte rolling. The Journal of Experimental Medicine. 2008;205(10):2339–2347. PubMed PMID: 18794338. PubMed Central PMCID: 2556779.

127.

[127] Stadtmann

Germena

Block

Boras

Rossaint

Sundd

, et al. The PSGL-1-L-selectin signaling complex regulates neutrophil adhesion under flow. The Journal of Experimental Medicine. 2013;210(11):2171–2180. PubMed PMID: 24127491. PubMed Central PMCID: 3804951.

128.

[128] Herter

Rossaint

Block

Welch

Zarbock

. Integrin activation by P-Rex1 is required for selectin-mediated slow leukocyte rolling and intravascular crawling. Blood. 2013;121(12):2301–2310. PubMed PMID: 23343834.

129.

[129] Giagulli

Scarpini

Ottoboni

Narumiya

Butcher

Constantin

, et al. RhoA and zeta PKC control distinct modalities of LFA-1 activation by chemokines: Critical role of LFA-1 affinity triggering in lymphocyte in vivo homing. Immunity. 2004;20(1):25–35. PubMed PMID: 14738762.

130.

[130] Lum

Green

Lee

Staunton

Simon

. Dynamic regulation of LFA-1 activation and neutrophil arrest on intercellular adhesion molecule 1 (ICAM-1) in shear flow. The Journal of Biological Chemistry. 2002;277(23):20660–20670. PubMed PMID: 11929876.

131.

[131] Ley

. Arrest chemokines. Microcirculation. 2003;10(3-4):289–295. PubMed PMID: 12851646.

132.

[132] D’Ambrosio

Albanesi

Lang

Girolomoni

Sinigaglia

Laudanna

. Quantitative differences in chemokine receptor engagement generate diversity in integrin-dependent lymphocyte adhesion. Journal of Immunology. 2002;169(5):2303–2312. PubMed PMID: 12193695.

133.

[133] Constantin

Majeed

Giagulli

Piccio

Kim

Butcher

, et al. Chemokines trigger immediate

β_{2}

integrin affinity and mobility changes: Differential regulation and roles in lymphocyte arrest under flow. Immunity. 2000;13(6):759–769. PubMed PMID: 11163192.

134.

[134] DiVietro

Smith

Petruzzelli

Larson

Lawrence

. Immobilized IL-8 triggers progressive activation of neutrophils rolling in vitro on P-selectin and intercellular adhesion molecule-1. Journal of Immunology. 2001;167(7):4017–4025. PubMed PMID: 11564821.

135.

[135] Alon

Feigelson

. Chemokine-triggered leukocyte arrest: Force-regulated bi-directional integrin activation in quantal adhesive contacts. Current Opinion in Cell Biology. 2012;24(5):670–676. PubMed PMID: 22770729.

136.

[136] Green

Schaff

Sarantos

Lum

Staunton

Simon

. Dynamic shifts in LFA-1 affinity regulate neutrophil rolling, arrest, and transmigration on inflamed endothelium. Blood. 2006;107(5):2101–2111. PubMed PMID: 16269618. PubMed Central PMCID: 1895714.

137.

[137] Liu

Guo

Bai

Wang

Bian

, et al. Signal regulatory protein α negatively regulates

β_{2}

integrin-mediated monocyte adhesion, transendothelial migration and phagocytosis. PLoS ONE. 2008;3(9):e3291. PubMed PMID: 18820737. PubMed Central PMCID: 2553263.

138.

[138] Morrison

MacPherson

Savinko

San Lek

Prescott

Fagerholm

. The

β_{2}

integrin-kindlin-3 interaction is essential for T-cell homing but dispensable for T-cell activation in vivo. Blood. 2013;122(8):1428–1436.

139.

[139] Constantin

Laudanna

. A deadly migration. Immunity. 2010;32(2):147–149.

140.

[140] Schneider

Issekutz

. The role of

α_{4}

(CD49d) and

β_{2}

(CD18) integrins in eosinophil and neutrophil migration to allergic lung inflammation in the Brown Norway rat. American Journal of Respiratory Cell and Molecular Biology. 1999;20(3):448–457. PubMed PMID: 10030843.

141.

[141] Thatte

Dabak

Williams

Braciale

Ley

. LFA-1 is required for retention of effector CD8 T cells in mouse lungs. Blood. 2003;101(12):4916–4922. PubMed PMID: 12623847.

142.

[142] Kim

. Crawling of effector T cells on extracellular matrix: Role of integrins in interstitial migration in inflamed tissues. Cellular & Molecular Immunology. 2014;11(1):1–4. PubMed PMID: 24056781. PubMed Central PMCID: 4002148.

143.

[143] Xu

Jaeger

Wang

Lagler-Ferrez

Batalov

Mathis

, et al. Mst1 directs Myosin IIa partitioning of low and higher affinity integrins during T cell migration. PLoS ONE. 2014;9(8):e105561. PubMed PMID: 25133611. PubMed Central PMCID: 4136924.

144.

[144] Dustin

Shaw

. Costimulation: Building an immunological synapse. Science. 1999;283(5402):649–650. PubMed PMID: 9988658.

145.

[145] Lin

Fan

Suo

Deng

Zhang

Wang

, et al. The bullseye synapse formed between CD4⁺ T-cell and staphylococcal enterotoxin B-pulsed dendritic cell is a suppressive synapse in T-cell response. Immunology and Cell Biology. 2015;93(1):99–110. PubMed PMID: 25287444.

146.

[146] Lin

Suo

Deng

Fan

Zheng

Wei

, et al. Morphological change of CD4⁺ T cell during contact with DC modulates T-cell activation by accumulation of F-actin in the immunology synapse. BMC Immunology. 2015;16(1):49. PubMed PMID: 26306899. PubMed Central PMCID: 4549951.

147.

[147] Nolz

Medeiros

Mitchell

Zhu

Freedman

Shimizu

, et al. WAVE2 regulates high-affinity integrin binding by recruiting vinculin and talin to the immunological synapse. Molecular and Cellular Biology. 2007;27(17):5986–6000. PubMed PMID: 17591693. PubMed Central PMCID: 1952166.

148.

[148] Sumen

Dustin

Davis

. T cell receptor antagonism interferes with MHC clustering and integrin patterning during immunological synapse formation. The Journal of Cell Biology. 2004;166(4):579–590. PubMed PMID: 15314068. PubMed Central PMCID: 2172210.

149.

[149] Schnitzler

Haase

Podbielski

Lutticken

Schweizer

. A co-stimulatory signal through ICAM-

β_{2}

integrin-binding potentiates neutrophil phagocytosis. Nature Medicine. 1999;5(2):231–235. PubMed PMID: 9930874.

150.

[150] Jongstra-Bilen

Harrison

Grinstein

. Fcγ-receptors induce Mac-1 (CD11b/CD18) mobilization and accumulation in the phagocytic cup for optimal phagocytosis. The Journal of Biological Chemistry. 2003;278(46):45720–45729. PubMed PMID: 12941957.

151.

[151] Dewitt

Laffafian

Hallett

. Phagosomal oxidative activity during

β_{2}

integrin (CR3)-mediated phagocytosis by neutrophils is triggered by a non-restricted Ca²⁺ signal: Ca²⁺ controls time not space. J Cell Sci. 2003;116(Pt 14):2857–2865. PubMed PMID: 12771186.

152.

[152] Wiedemann

Patel

Lim

Tsun

van Kooyk

Caron

. Two distinct cytoplasmic regions of the

β_{2}

integrin chain regulate RhoA function during phagocytosis. The Journal of Cell Biology. 2006;172(7):1069–1079. PubMed PMID: 16567504. PubMed Central PMCID: 2063764.

153.

[153] MacPherson

Lek

Prescott

Fagerholm

. A systemic lupus erythematosus-associated R77H substitution in the CD11b chain of the Mac-1 integrin compromises leukocyte adhesion and phagocytosis. The Journal of Biological Chemistry. 2011;286(19):17303–17310. PubMed PMID: 21454473. PubMed Central PMCID: 3089572.

154.

[154] Nishida

Xie

Shimaoka

Cheng

Walz

Springer

. Activation of leukocyte

β_{2}

integrins by conversion from bent to extended conformations. Immunity. 2006;25(4):583–594. PubMed PMID: 17045822.

155.

[155] Shimaoka

Xiao

Liu

J-H

Yang

Dong

Jun

C-D

, et al. Structures of the

α_{L}

I domain and its complex with ICAM-1 reveal a shape-shifting pathway for integrin regulation. Cell. 2003;112(1):99–111.

156.

[156] Xiong

Stehle

Diefenbach

Zhang

Dunker

Scott

, et al. Crystal structure of the extracellular segment of integrin

α_{V} β_{3}

. Science. 2001;294(5541):339–345. PubMed PMID: 11546839. PubMed Central PMCID: 2885948.

157.

[157] Xiong

Stehle

Zhang

Joachimiak

Frech

Goodman

, et al. Crystal structure of the extracellular segment of integrin

α_{V} β_{3}

in complex with an Arg-Gly-Asp ligand. Science. 2002;296(5565):151–155. PubMed PMID: 11884718.

158.

[158] Chen

Lou

Zhu

. Forcing switch from short- to intermediate- and long-lived states of the αA domain generates LFA-1/ICAM-1 catch bonds. The Journal of Biological Chemistry. 2010;285(46):35967–35978. PubMed PMID: 20819952. PubMed Central PMCID: 2975219.

159.

[159] Chen

Xie

Nishida

Walz

Springer

. Requirement of open headpiece conformation for activation of leukocyte integrin

α_{X} β_{2}

. Proceedings of the National Academy of Sciences of the United States of America. 2010;107(33):14727–14732. PubMed PMID: 20679211. PubMed Central PMCID: 2930457.

160.

[160] Takagi

Petre

Walz

Springer

. Global conformational rearrangements in integrin extracellular domains in outside-in and inside-out signaling. Cell. 2002;110(5):599–611.

161.

[161] Schurpf

Springer

. Regulation of integrin affinity on cell surfaces. The EMBO Journal. 2011;30(23):4712–4727. PubMed PMID: 21946563. PubMed Central PMCID: 3243613.

162.

[162] Zhu

Luo

Xiao

Zhang

Nishida

Springer

. Structure of a complete integrin ectodomain in a physiologic resting state and activation and deactivation by applied forces. Molecular Cell. 2008;32(6):849–861. PubMed PMID: 19111664. PubMed Central PMCID: 2758073.

163.

[163] Salas

Shimaoka

Kogan

Harwood

von Andrian

Springer

. Rolling adhesion through an extended conformation of integrin

α_{L} β_{2}

and relation to α I and β I-like domain interaction. Immunity. 2004;20(4):393–406. PubMed PMID: 15084269.

164.

[164] Adair

Xiong

Maddock

Goodman

Arnaout

Yeager

. Three-dimensional EM structure of the ectodomain of integrin

α_{V} β_{3}

in a complex with fibronectin. The Journal of Cell Biology. 2005;168(7):1109–1118. PubMed PMID: 15795319. PubMed Central PMCID: 2171847.

165.

[165] Zhu

Boylan

Luo

Newman

Springer

. Tests of the extension and deadbolt models of integrin activation. The Journal of Biological Chemistry. 2007;282(16):11914–11920. PubMed PMID: 17301049. PubMed Central PMCID: 1952534.

166.

[166] Shimaoka

Takagi

Springer

. Conformational regulation of integrin structure and function. Annual Review of Biophysics and Biomolecular Structure. 2002;31:485–516. PubMed PMID: 11988479.

167.

[167] Ye

Kim

Ginsberg

. Molecular mechanism of inside-out integrin regulation. Journal of Thrombosis and Haemostasis: JTH. 2011;9(Suppl 1):20–25. PubMed PMID: 21781238. PubMed Central PMCID: 3979599.

168.

[168] Larson

Davis

Bologa

Semenuk

Vijayan

, et al. Dissociation of I domain and global conformational changes in LFA-1: Refinement of small molecule-I domain structure-activity relationships. Biochemistry. 2005;44(11):4322–4331. PubMed PMID: 15766261.

169.

[169] Lefort

Hyun

Kim

. Monitoring integrin activation by fluorescence resonance energy transfer. Methods in Molecular Biology. 2012;757:205–214. PubMed PMID: 21909915.

170.

[170] Lefort

Hyun

Y-M

Schultz

Law

F-Y

Waugh

Knauf

, et al. Outside-in signal transmission by conformational changes in integrin Mac-1. The Journal of Immunology. 2009;183(10):6460–6468.

171.

[171] Kim

Carman

Springer

. Bidirectional transmembrane signaling by cytoplasmic domain separation in integrins. Science. 2003;301(5640):1720–1725. PubMed PMID: 14500982.

172.

[172] Arnaout

Mahalingam

Xiong

. Integrin structure, allostery, and bidirectional signaling. Annual Review of Cell and Developmental Biology. 2005;21:381–410. PubMed PMID: 16212500.

173.

[173] Shimaoka

Palframan

von Andrian

McCormack

Takagi

, et al. Reversibly locking a protein fold in an active conformation with a disulfide bond: Integrin

α_{L}

I domains with high affinity and antagonist activity in vivo. Proceedings of the National Academy of Sciences of the United States of America. 2001;98(11):6009–6014. PubMed PMID: 11353828. PubMed Central PMCID: 33413.

174.

[174] Xie

Zhu

Chen

Nishida

Springer

. Structure of an integrin with an α I domain, complement receptor type 4. The EMBO Journal. 2010;29(3):666–679. PubMed PMID: 20033057. PubMed Central PMCID: 2830704.

175.

[175] Takagi

Strokovich

Springer

Walz

. Structure of integrin

α_{5} β_{1}

in complex with fibronectin. The EMBO Journal. 2003;22(18):4607–4615. PubMed PMID: 12970173. PubMed Central PMCID: 212714.

176.

[176] Xiao

Takagi

Coller

Wang

J-H

Springer

. Structural basis for allostery in integrins and binding to fibrinogen-mimetic therapeutics. Nature. 2004;432(7013):59–67.

177.

[177] Gupta

Gylling

Alonso

Sugimori

Ianakiev

Xiong

, et al. The β-tail domain (βTD) regulates physiologic ligand binding to integrin CD11b/CD18. Blood. 2007;109(8):3513–3520. PubMed PMID: 17170130. PubMed Central PMCID: 1852245.

178.

[178] Yago

Petrich

Zhang

Liu

Shao

Ginsberg

, et al. Blocking neutrophil integrin activation prevents ischemia-reperfusion injury. The Journal of Experimental Medicine. 2015;212(8):1267–1281. PubMed PMID: 26169939. PubMed Central PMCID: 4516797.

179.

[179] Lu

Takagi

Springer

. Association of the membrane proximal regions of the α and β subunit cytoplasmic domains constrains an integrin in the inactive state. The Journal of Biological Chemistry. 2001;276(18):14642–14648. PubMed PMID: 11279101.

180.

[180] Anthis

Haling

Oxley

Memo

Wegener

Lim

, et al. β integrin tyrosine phosphorylation is a conserved mechanism for regulating talin-induced integrin activation. The Journal of Biological Chemistry. 2009;284(52):36700–36710. PubMed PMID: 19843520. PubMed Central PMCID: 2794784.

181.

[181] Partridge

Liu

Kim

Bowie

Ginsberg

. Transmembrane domain helix packing stabilizes integrin

α_{IIb} β_{3}

in the low affinity state. The Journal of Biological Chemistry. 2005;280(8):7294–7300. PubMed PMID: 15591321.

182.

[182] Lau

Kim

Ginsberg

Ulmer

. The structure of the integrin

α_{IIb} β_{3}

transmembrane complex explains integrin transmembrane signalling. The EMBO Journal. 2009;28(9):1351–1361. PubMed PMID: 19279667. PubMed Central PMCID: 2683045.

183.

[183] Zhu

Luo

Barth

Schonbrun

Baker

Springer

. The structure of a receptor with two associating transmembrane domains on the cell surface: Integrin

α_{IIb} β_{3}

. Molecular Cell. 2009;34(2):234–249. PubMed PMID: 19394300. PubMed Central PMCID: 2694939.

184.

[184] Luo

Springer

Takagi

. A specific interface between integrin transmembrane helices and affinity for ligand. PLoS Biology. 2004;2(6):e153. PubMed PMID: 15208712. PubMed Central PMCID: 423134.

185.

[185] Luo

Carman

Takagi

Springer

. Disrupting integrin transmembrane domain heterodimerization increases ligand binding affinity, not valency or clustering. Proceedings of the National Academy of Sciences of the United States of America. 2005;102(10):3679–3684. PubMed PMID: 15738420. PubMed Central PMCID: 553322.

186.

[186] Liddington

Ginsberg

. Integrin activation takes shape. The Journal of Cell Biology. 2002;158(5):833–839. PubMed PMID: 12213832. PubMed Central PMCID: 2173150.

187.

[187] Kalli

Campbell

Sansom

. Multiscale simulations suggest a mechanism for integrin inside-out activation. Proceedings of the National Academy of Sciences of the United States of America. 2011;108(29):11890–11895. PubMed PMID: 21730166. PubMed Central PMCID: 3141943.

188.

[188] Orłowski

Kukkurainen

Pöyry

Rissanen

Vattulainen

Hytönen

, et al. PIP2 and talin join forces to activate integrin. The Journal of Physical Chemistry B. 2015;119(38):12381–12389. PubMed PMID: 26309152.

189.

[189] Burridge

Connell

. A new protein of adhesion plaques and ruffling membranes. The Journal of Cell Biology. 1983;97(2):359–367. PubMed PMID: 6684120. PubMed Central PMCID: 2112532.

190.

[190] Moser

Legate

Zent

Fassler

. The tail of integrins, talin, and kindlins. Science. 2009;324(5929):895–899. PubMed PMID: 19443776.

191.

[191] Goult

Bouaouina

Elliott

Bate

Patel

Gingras

, et al. Structure of a double ubiquitin-like domain in the talin head: A role in integrin activation. The EMBO Journal. 2010;29(6):1069–1080. PubMed PMID: 20150896. PubMed Central PMCID: 2845276.

192.

[192] Elliott

Goult

Kopp

Bate

Grossmann

Roberts

, et al. The structure of the talin head reveals a novel extended conformation of the FERM domain. Structure. 2010;18(10):1289–1299. PubMed PMID: 20947018. PubMed Central PMCID: 2977851.

193.

[193] Lee

Lim

Puzon-McLaughlin

Shattil

Ginsberg

. RIAM activates integrins by linking talin to Ras GTPase membrane-targeting sequences. The Journal of Biological Chemistry. 2009;284(8):5119–5127. PubMed PMID: 19098287. PubMed Central PMCID: 2643525.

194.

[194] Lee

Anekal

Lim

Liu

Ginsberg

. Two modes of integrin activation form a binary molecular switch in adhesion maturation. Molecular Biology of the Cell. 2013;24(9):1354–1362. PubMed PMID: 23468527. PubMed Central PMCID: 3639047.

195.

[195] Goult

Zacharchenko

Bate

Tsang

Hey

Gingras

, et al. RIAM and vinculin binding to talin are mutually exclusive and regulate adhesion assembly and turnover. The Journal of Biological Chemistry. 2013;288(12):8238–8249. PubMed PMID: 23389036. PubMed Central PMCID: 3605642.

196.

[196] Gingras

Ziegler

Frank

Barsukov

Roberts

Critchley

, et al. Mapping and consensus sequence identification for multiple vinculin binding sites within the talin rod. The Journal of Biological Chemistry. 2005;280(44):37217–37224. PubMed PMID: 16135522.

197.

[197] Li

Vass

Papageorge

Lowy

Qian

. Full activity of the deleted in liver cancer 1 (DLC1) tumor suppressor depends on an LD-like motif that binds talin and focal adhesion kinase (FAK). Proceedings of the National Academy of Sciences. 2011;108(41):17129–17134.

198.

[198] Sun

Critchley

Paulin

Robson

. Identification of a repeated domain within mammalian α-synemin that interacts directly with talin. Exp Cell Res. 2008;314(8):1839–1849. PubMed PMID: 18342854.

199.

[199] Gingras

Ziegler

Bobkov

Joyce

Fasci

Himmel

, et al. Structural determinants of integrin binding to the talin rod. The Journal of Biological Chemistry. 2009;284(13):8866–8876. PubMed PMID: 19176533. PubMed Central PMCID: 2659244.

200.

[200] Rodius

Chaloin

Moes

Schaffner-Reckinger

Landrieu

Lippens

, et al. The talin rod IBS2 α-helix interacts with the

β_{3}

integrin cytoplasmic tail membrane-proximal helix by establishing charge complementary salt bridges. Journal of Biological Chemistry. 2008;283(35):24212–24223.

201.

[201] Critchley

. Biochemical and structural properties of the integrin-associated cytoskeletal protein talin. Annual Review of Biophysics. 2009;38:235–254. PubMed PMID: 19416068.

202.

[202] Gingras

Bate

Goult

Hazelwood

Canestrelli

Grossmann

, et al. The structure of the C-terminal actin-binding domain of talin. The EMBO Journal. 2008;27(2):458–469. PubMed PMID: 18157087. PubMed Central PMCID: 2168396.

203.

[203] Song

Yang

Hirbawi

Perera

Goksoy

, et al. A novel membrane-dependent on/off switch mechanism of talin FERM domain at sites of cell adhesion. Cell Research. 2012;22(11):1533–1545.

204.

[204] Anthis

Wegener

Kim

Goult

Lowe

, et al. The structure of an integrin/talin complex reveals the basis of inside-out signal transduction. The EMBO Journal. 2009;28(22):3623–3632. PubMed PMID: 19798053. PubMed Central PMCID: 2782098.

205.

[205] Wegener

Campbell

. Transmembrane and cytoplasmic domains in integrin activation and protein–protein interactions (review). Molecular Membrane Biology. 2008;25(5):376–387. PubMed PMID: 18654929. PubMed Central PMCID: 3000922.

206.

[206] Su

Wynne

Pinheiro

Strazza

Mor

Montenont

, et al. Rap1 and its effector RIAM are required for lymphocyte trafficking. Blood. 2015;126(25):2695–2703. PubMed PMID: 26324702. PubMed Central PMCID: 4683330.

207.

[207] Sarah Klapproth

Klapproth

Sperandio

Fässler

Moser

. RIAM controls bidirectional signaling of leukocyte integrins. Blood. Forthcoming.

208.

[208] Stritt

Wolf

Lorenz

Vogtle

Gupta

Bosl

, et al. Rap1-GTP-interacting adaptor molecule (RIAM) is dispensable for platelet integrin activation and function in mice. Blood. 2015;125(2):219–222. PubMed PMID: 25336629. PubMed Central PMCID: 4347307.

209.

[209] Kliche

Worbs

Wang

Degen

Patzak

Meineke

, et al. CCR7-mediated LFA-1 functions in T cells are regulated by 2 independent ADAP/SKAP55 modules. Blood. 2012;119(3):777–785. PubMed PMID: 22117043.

210.

[210] Karakose

Schiller

Fassler

. The kindlins at a glance. J Cell Sci. 2010;123(Pt 14):2353–2356. PubMed PMID: 20592181.

211.

[211] Kloeker

Major

Calderwood

Ginsberg

Jones

Beckerle

. The kindler syndrome protein is regulated by transforming growth factor-β and involved in integrin-mediated adhesion. Journal of Biological Chemistry. 2004;279(8):6824–6833.

212.

[212] Schmidt

Nakchbandi

Ruppert

Kawelke

Hess

Pfaller

, et al. Kindlin-3-mediated signaling from multiple integrin classes is required for osteoclast-mediated bone resorption. The Journal of Cell Biology. 2011;192(5):883–897. PubMed PMID: 21357746. PubMed Central PMCID: 3051823.

213.

[213] Moser

Bauer

Schmid

Ruppert

Schmidt

Sixt

, et al. Kindlin-3 is required for

β_{2}

integrin-mediated leukocyte adhesion to endothelial cells. Nature Medicine. 2009;15(3):300–305.

214.

[214] McDowall

Svensson

Stanley

Patzak

Chakravarty

Howarth

, et al. Two mutations in the KINDLIN3 gene of a new leukocyte adhesion deficiency III patient reveal distinct effects on leukocyte function in vitro. Blood. 2010;115(23):4834–4842. PubMed PMID: 20357244.

215.

[215] Crazzolara

Maurer

Schulze

Zieger

Zustin

Schulz

. A new mutation in the KINDLIN-3 gene ablates integrin-dependent leukocyte, platelet, and osteoclast function in a patient with leukocyte adhesion deficiency-III. Pediatric Blood & Cancer. 2015;62(9):1677–1679. PubMed PMID: 25854317.

216.

[216] Gruda

Brown

Grabovsky

Mizrahi

Gur

Feigelson

, et al. Loss of kindlin-3 alters the threshold for NK cell activation in human leukocyte adhesion deficiency-III. Blood. 2012;120(19):3915–3924. PubMed PMID: 22983444.

217.

[217] Manevich-Mendelson

Feigelson

Pasvolsky

Aker

Grabovsky

Shulman

, et al. Loss of Kindlin-3 in LAD-III eliminates LFA-1 but not VLA-4 adhesiveness developed under shear flow conditions. Blood. 2009;114(11):2344–2353. PubMed PMID: 19617577.

218.

[218] Kamata

Tieu

Tarui

Puzon-McLaughlin

Hogg

Takada

. The role of the CPNKEKEC sequence in the

β_{2}

subunit I domain in regulation of integrin

α_{L} β_{2}

(LFA-1). Journal of Immunology. 2002;168(5):2296–2301. PubMed PMID: 11859118.

219.

[219] Lu

Shimaoka

Zang

Takagi

Springer

. Locking in alternate conformations of the integrin

α_{L} β_{2}

I domain with disulfide bonds reveals functional relationships among integrin domains. Proceedings of the National Academy of Sciences of the United States of America. 2001;98(5):2393–2398. PubMed PMID: 11226250. PubMed Central PMCID: 30149.

220.

[220] Yang

Shimaoka

Chen

Springer

. Activation of integrin β-subunit I-like domains by one-turn C-terminal α-helix deletions. Proceedings of the National Academy of Sciences of the United States of America. 2004;101(8):2333–2338. PubMed PMID: 14983010. PubMed Central PMCID: 356951.

221.

[221] Dransfield

Hogg

. Regulated expression of Mg²⁺ binding epitope on leukocyte integrin α subunits. The EMBO Journal. 1989;8(12):3759–3765. PubMed PMID: 2479549. PubMed Central PMCID: 402061.

222.

[222] Lu

Ferzly

Takagi

Springer

. Epitope mapping of antibodies to the C-terminal region of the integrin

β_{2}

subunit reveals regions that become exposed upon receptor activation. Journal of Immunology. 2001;166(9):5629–5637. PubMed PMID: 11313403.

223.

[223] Robinson

Andrew

Rosen

Brown

Ortlepp

Stephens

, et al. Antibody against the Leu-CAM β-chain (CD18) promotes both LFA-1- and CR3-dependent adhesion events. Journal of Immunology. 1992;148(4):1080–1085. PubMed PMID: 1371129.

224.

[224] Zhang

Wang

. Integrin signalling and function in immune cells. Immunology. 2012;135(4):268–275. PubMed PMID: 22211918. PubMed Central PMCID: 3372743.

225.

[225] Bledzka

Liu

Perera

Yadav

Bialkowska

, et al. Spatial coordination of kindlin-2 with talin head domain in interaction with integrin β cytoplasmic tails. The Journal of Biological Chemistry. 2012;287(29):24585–24594. PubMed PMID: 22648415. PubMed Central PMCID: 3397883.

226.

[226] Kahner

Kato

Banno

Ginsberg

Shattil

. Kindlins, integrin activation and the regulation of talin recruitment to

α_{IIb} β_{3}

. PLoS ONE. 2012;7(3):e34056. PubMed PMID: 22457811. PubMed Central PMCID: 3311585.

227.

[227] Ye

Snider

Ginsberg

. Talin and kindlin: The one-two punch in integrin activation. Frontiers of Medicine. 2014;8(1):6–16. PubMed PMID: 24477625.

228.

[228] Ye

Petrich

Anekal

Lefort

Kasirer-Friede

Shattil

, et al. The mechanism of kindlin-mediated activation of integrin

α_{IIb} β_{3}

. Current Biology: CB. 2013;23(22):2288–2295. PubMed PMID: 24210614. PubMed Central PMCID: 3912999.

229.

[229] McEver

Moore

Cummings

. Leukocyte trafficking mediated by selectin-carbohydrate interactions. The Journal of Biological Chemistry. 1995;270(19):11025–11028. PubMed PMID: 7538108.

230.

[230] Butcher

. Leukocyte-endothelial cell recognition: Three (or more) steps to specificity and diversity. Cell. 1991;67(6):1033–1036. PubMed PMID: 1760836.

231.

[231] Springer

. Traffic signals for lymphocyte recirculation and leukocyte emigration: The multistep paradigm. Cell. 1994;76(2):301–314. PubMed PMID: 7507411.

232.

[232] Ley

. Leukocyte adhesion to vascular endothelium. Journal of Reconstructive Microsurgery. 1992;8(6):495–503. PubMed PMID: 1453376.

233.

[233] Ley

Laudanna

Cybulsky

Nourshargh

. Getting to the site of inflammation: The leukocyte adhesion cascade updated. Nature Reviews Immunology. 2007;7(9):678–689. PubMed PMID: 17717539.

234.

[234] Herter

Zarbock

. Integrin regulation during leukocyte recruitment. Journal of Immunology. 2013;190(9):4451–4457. PubMed PMID: 23606722.

235.

[235] Sundd

Pospieszalska

Cheung

Konstantopoulos

Ley

. Biomechanics of leukocyte rolling. Biorheology. 2011;48(1):1–35. PubMed PMID: 21515934. PubMed Central PMCID: 3103268.

236.

[236] Xia

Sperandio

Yago

McDaniel

Cummings

Pearson-White

, et al. P-selectin glycoprotein ligand-1-deficient mice have impaired leukocyte tethering to E-selectin under flow. The Journal of Clinical Investigation. 2002;109(7):939–950. PubMed PMID: 11927621. PubMed Central PMCID: 150926.

237.

[237] Sperandio

Smith

Forlow

Olson

Xia

McEver

, et al. P-selectin glycoprotein ligand-1 mediates L-selectin-dependent leukocyte rolling in venules. The Journal of Experimental Medicine. 2003;197(10):1355–1363. PubMed PMID: 12756271. PubMed Central PMCID: 2193782.

238.

[238] Miner

Xia

Yago

Kappelmayer

Liu

Klopocki

, et al. Separable requirements for cytoplasmic domain of PSGL-1 in leukocyte rolling and signaling under flow. Blood. 2008;112(5):2035–2045. PubMed PMID: 18550846. PubMed Central PMCID: 2518905.

239.

[239] Zarbock

Ley

McEver

Hidalgo

. Leukocyte ligands for endothelial selectins: Specialized glycoconjugates that mediate rolling and signaling under flow. Blood. 2011;118(26):6743–6751. PubMed PMID: 22021370. PubMed Central PMCID: 3245201.

240.

[240] Stadtmann

Brinkhaus

Mueller

Rossaint

Bolomini-Vittori

Bergmeier

, et al. Rap1a activation by CalDAG-GEFI and p38 MAPK is involved in E-selectin-dependent slow leukocyte rolling. European Journal of Immunology. 2011;41(7):2074–2085. PubMed PMID: 21480213. PubMed Central PMCID: 3124568.

241.

[241] Norman

Moore

McEver

Ley

. Leukocyte rolling in vivo is mediated by P-selectin glycoprotein ligand-1. Blood. 1995;86(12):4417–4421. PubMed PMID: 8541529.

242.

[242] Xia

Ramachandran

McDaniel

Nguyen

Cummings

McEver

. N-terminal residues in murine P-selectin glycoprotein ligand-1 required for binding to murine P-selectin. Blood. 2003;101(2):552–559. PubMed PMID: 12393631.

243.

[243] Hirata

Merrill-Skoloff

Aab

Yang

Furie

. P-selectin glycoprotein ligand 1 (PSGL-1) is a physiological ligand for E-selectin in mediating T helper 1 lymphocyte migration. The Journal of Experimental Medicine. 2000;192(11):1669–1676. PubMed PMID: 11104809. PubMed Central PMCID: 2193099.

244.

[244] Moore

Patel

Bruehl

Johnson

Lichenstein

, et al. P-selectin glycoprotein ligand-1 mediates rolling of human neutrophils on P-selectin. The Journal of Cell Biology. 1995;128(4):661–671. PubMed PMID: 7532174. PubMed Central PMCID: 2199883.

245.

[245] Wang

Wang

Zhang

Geng

, et al. P-selectin primes leukocyte integrin activation during inflammation. Nature Immunology. 2007;8(8):882–892. PubMed PMID: 17632516.

246.

[246] Yago

Shao

Miner

Yao

Klopocki

Maeda

, et al. E-selectin engages PSGL-1 and CD44 through a common signaling pathway to induce integrin

α_{L} β_{2}

-mediated slow leukocyte rolling. Blood. 2010;116(3):485–494. PubMed PMID: 20299514. PubMed Central PMCID: 2913455.

247.

[247] Piccardoni

Sideri

Manarini

Piccoli

Martelli

de Gaetano

, et al. Platelet/polymorphonuclear leukocyte adhesion: A new role for SRC kinases in Mac-1 adhesive function triggered by P-selectin. Blood. 2001;98(1):108–116. PubMed PMID: 11418469.

248.

[248] Mueller

Stadtmann

Van Aken

Hirsch

Wang

Ley

, et al. Tyrosine kinase Btk regulates E-selectin-mediated integrin activation and neutrophil recruitment by controlling phospholipase C (PLC) γ2 and PI3Kγ pathways. Blood. 2010;115(15):3118–3127. PubMed PMID: 20167705. PubMed Central PMCID: 2858472.

249.

[249] Moore

van den Bosch

Hunter

Sakamoto

Poole

Hers

. Dual regulation of glycogen synthase kinase 3 (GSK3)α/β by protein kinase C (PKC)α and Akt promotes thrombin-mediated integrin

α_{IIb} β_{3}

activation and granule secretion in platelets. The Journal of Biological Chemistry. 2013;288(6):3918–3928. PubMed PMID: 23239877. PubMed Central PMCID: 3567645.

250.

[250] Chesnutt

Smith

Raffler

Smith

White

Ley

. Induction of LFA-1-dependent neutrophil rolling on ICAM-1 by engagement of E-selectin. Microcirculation. 2006;13(2):99–109. PubMed PMID: 16459323.

251.

[251] Rainger

Fisher

Nash

. Endothelial-borne platelet-activating factor and interleukin-8 rapidly immobilize rolling neutrophils. The American Journal of Physiology. 1997;272(1 Pt 2):H114–H122. PubMed PMID: 9038929.

252.

[252] Qin

Dong

Lambert

. Inactive-state preassembly of

G_{q}

-coupled receptors and

G_{q}

heterotrimers. Nature Chemical Biology. 2011;7(10):740–747. PubMed PMID: 21873996. PubMed Central PMCID: 3177959.

253.

[253] Ley

. Arrest chemokines. Frontiers in Immunology. 2014;5:150. PubMed PMID: 24772112. PubMed Central PMCID: 3983478.

254.

[254] Smith

Olson

Ley

. CXCR2- and E-selectin-induced neutrophil arrest during inflammation in vivo. The Journal of Experimental Medicine. 2004;200(7):935–939. PubMed PMID: 15466624. PubMed Central PMCID: 2213284.

255.

[255] Rodriguez-Viciana

Warne

Dhand

Vanhaesebroeck

Gout

Fry

, et al. Phosphatidylinositol-3-OH kinase as a direct target of Ras. Nature. 1994;370(6490):527–532. PubMed PMID: 8052307.

256.

[256] Kerchner

Clay

McCleery

Watson

McIntire

Myung

, et al. Differential sensitivity of phosphatidylinositol 3-kinase p110γ to isoforms of G protein

β γ

dimers. The Journal of Biological Chemistry. 2004;279(43):44554–44562. PubMed PMID: 15322106.

257.

[257] Dbouk

Vadas

Shymanets

Burke

Salamon

Khalil

, et al. G protein-coupled receptor-mediated activation of p110β by

G β γ

is required for cellular transformation and invasiveness. Science Signaling. 2012;5(253):ra89. PubMed PMID: 23211529. PubMed Central PMCID: 3979326.

258.

[258] Li

Benard

Margolskee

G γ 13

interacts with PDZ domain-containing proteins. The Journal of Biological Chemistry. 2006;281(16):11066–11073. PubMed PMID: 16473877.

259.

[259] Mayeenuddin

Garrison

. Phosphorylation of P-Rex1 by the cyclic AMP-dependent protein kinase inhibits the phosphatidylinositiol (3,4,5)-trisphosphate and

G β γ

-mediated regulation of its activity. The Journal of Biological Chemistry. 2006;281(4):1921–1928. PubMed PMID: 16301320.

260.

[260] Jamora

Yamanouye

Van Lint

Laudenslager

Vandenheede

Faulkner

, et al.

G β γ

-mediated regulation of Golgi organization is through the direct activation of protein kinase D. Cell. 1999;98(1):59–68. PubMed PMID: 10412981.

261.

[261] Block

Stadtmann

Riad

Rossaint

Sohlbach

Germena

, et al. Gnb isoforms orchestrate a signaling pathway comprising Rac1 and Plcβ2/3 leading to LFA-1 activation and neutrophil arrest in vivo. Blood. 2016;127(3):314–324. PubMed PMID: 26468229.

262.

[262] Smith

Deem

Bruce

Reutershan

Ley

. Leukocyte phosphoinositide-3 kinase {gamma} is required for chemokine-induced, sustained adhesion under flow in vivo. Journal of Leukocyte Biology. 2006;80(6):1491–1499. PubMed PMID: 16997858.

263.

[263] Liu

Puri

Penninger

Kubes

. Leukocyte PI3Kγ and PI3Kδ have temporally distinct roles for leukocyte recruitment in vivo. Blood. 2007;110(4):1191–1198. PubMed PMID: 17488877.

264.

[264] Bertram

Zhang

von Vietinghoff

de Pablo

Haller

Shushakova

, et al. Protein kinase C-theta is required for murine neutrophil recruitment and adhesion strengthening under flow. Journal of Immunology. 2012;188(8):4043–4051. PubMed PMID: 22403440.

265.

[265] Hyduk

Chan

Duffy

Chen

Peterson

Waddell

, et al. Phospholipase C, calcium, and calmodulin are critical for

α_{4} β_{1}

integrin affinity up-regulation and monocyte arrest triggered by chemoattractants. Blood. 2007;109(1):176–184. PubMed PMID: 16960156.

266.

[266] Huo

Schober

Forlow

Smith

Hyman

Jung

, et al. Circulating activated platelets exacerbate atherosclerosis in mice deficient in apolipoprotein E. Nature Medicine. 2003;9(1):61–67. PubMed PMID: 12483207.

267.

[267] Bergmeier

Goerge

Wang

Crittenden

Baldwin

Cifuni

, et al. Mice lacking the signaling molecule CalDAG-GEFI represent a model for leukocyte adhesion deficiency type III. The Journal of Clinical Investigation. 2007;117(6):1699–1707. PubMed PMID: 17492052. PubMed Central PMCID: 1865026.

268.

[268] Lawson

Donald

Anderson

Patton

Welch

. P-Rex1 and Vav1 cooperate in the regulation of formyl-methionyl-leucyl-phenylalanine-dependent neutrophil responses. Journal of Immunology. 2011;186(3):1467–1476. PubMed PMID: 21178006.

269.

[269] Ghandour

Cullere

Alvarez

Luscinskas

Mayadas

. Essential role for Rap1 GTPase and its guanine exchange factor CalDAG-GEFI in LFA-1 but not VLA-4 integrin mediated human T-cell adhesion. Blood. 2007;110(10):3682–3690. PubMed PMID: 17702895. PubMed Central PMCID: 2077316.

270.

[270] Bolomini-Vittori

Montresor

Giagulli

Staunton

Rossi

Martinello

, et al. Regulation of conformer-specific activation of the integrin LFA-1 by a chemokine-triggered Rho signaling module. Nature Immunology. 2009;10(2):185–194. PubMed PMID: 19136961.

271.

[271] Montresor

Bolomini-Vittori

Toffali

Rossi

Constantin

Laudanna

. JAK tyrosine kinases promote hierarchical activation of Rho and Rap modules of integrin activation. The Journal of Cell Biology. 2013;203(6):1003–1019. PubMed PMID: 24368807. PubMed Central PMCID: 3871442.

272.

[272] Han

Lim

Watanabe

Soriani

Ratnikov

Calderwood

, et al. Reconstructing and deconstructing agonist-induced activation of integrin

α_{IIb} β_{3}

. Current Biology: CB. 2006;16(18):1796–1806. PubMed PMID: 16979556.

273.

[273] Kliche

Breitling

Togni

Pusch

Heuer

Wang

, et al. The ADAP/SKAP55 signaling module regulates T-cell receptor-mediated integrin activation through plasma membrane targeting of Rap1. Molecular and Cellular Biology. 2006;26(19):7130–7144. PubMed PMID: 16980616. PubMed Central PMCID: 1592884.

274.

[274] Carmona

Gottig

Orlandi

Scheele

Bauerle

Jugold

, et al. Role of the small GTPase Rap1 for integrin activity regulation in endothelial cells and angiogenesis. Blood. 2009;113(2):488–497. PubMed PMID: 18805968.

275.

[275] Lafuente

van Puijenbroek

Krause

Carman

Freeman

Berezovskaya

, et al. RIAM, an Ena/VASP and Profilin ligand, interacts with Rap1-GTP and mediates Rap1-induced adhesion. Developmental Cell. 2004;7(4):585–595. PubMed PMID: 15469846.

276.

[276] Menasche

Kliche

Chen

Stradal

Schraven

Koretzky

. RIAM links the ADAP/SKAP-55 signaling module to Rap1, facilitating T-cell-receptor-mediated integrin activation. Molecular and Cellular Biology. 2007;27(11):4070–4081. PubMed PMID: 17403904. PubMed Central PMCID: 1900018.

277.

[277] Watanabe

Bodin

Pandey

Krause

Coughlin

Boussiotis

, et al. Mechanisms and consequences of agonist-induced talin recruitment to platelet integrin

α_{IIb} β_{3}

. The Journal of Cell Biology. 2008;181(7):1211–1222. PubMed PMID: 18573917. PubMed Central PMCID: 2442211.

278.

[278] Côte

Fos

Canonigo-Balancio

Ley

Bécart

Altman

. SLAT promotes TCR-mediated, Rap1-dependent LFA-1 activation and adhesion through interaction of its PH domain with Rap1. Journal of Cell Science. 2015;128(23):4341–4352. PubMed PMID: 26483383.