The Role of Apolipoprotein E Isoforms in Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD), the most common type of dementia worldwide, is characterized by high levels of amyloid-β (Aβ) peptide and hyperphosphorylated tau protein. Genetically, the ɛ4 allele of apolipoprotein E (ApoE) has been established as the major risk factor for developing late-onset AD (LOAD), the most common form of the disease. Although the role ApoE plays in AD is still not completely understood, a differential role of its isoforms has long been known. The current review compiles the involvement of ApoE isoforms in amyloid-β protein precursor transcription, Aβ aggregation and clearance, synaptic plasticity, neuroinflammation, lipid metabolism, mitochondrial function, and tau hyperphosphorylation. Due to the complexity of LOAD, an accurate description of the interdependence among all the related molecular mechanisms involved in the disease is needed for developing successful therapies.

Keywords

Aβ clearance Aβ aggregation AβPP transcription AICD generation Alzheimer’s disease amyloid-β apolipoprotein E lipid metabolism mitochondrial damage neuroinflammation

INTRODUCTION

Alzheimer’s disease (AD) is the most common neurodegenerative disorder in the world, which affects up to 50% of individuals over the age of 85 [1]. AD is clinically characterized by progressive mental decline and histopathologically defined by highly abundant amyloid-β (Aβ) deposits and neurofibrillary tangles (composed of hyperphosphorylated tau) in the brain parenchyma [2]. Although the exact pathological mechanism of AD is still under discussion, according to the amyloid hypothesis, abnormal aggregation of Aβ in the brain would be the early event that triggers the pathogenic cascade leading to the disease development [3]. However, since the discovery by Holtzman’s group that Aβ is a normal, highly turned-over protein in the brain, including young individuals [4], the occurrence of a pathogenic cascade leading to dementia has been questioned [5, 6]. Apart from the fact that other factors such as oxidative stress could be also implicated in AD development [7], the recent observation that tau hyperphosphorylation may precede Aβ depositions [8, 9], leads to a new approach for therapeutic research. Accordingly, although Aβ plaques may play a key role in AD pathogenesis, the severity of cognitive impairment correlates better with the burden of neocortical neurofibrillary tangles and cerebrospinal fluid (CSF) tau levels [10].

Familial AD accounts for less than 1% of AD cases and is caused by rare, fully-penetrant genetic mutations that enhance Aβ production and amyloid-β protein precursor (AβPP) intracellular domain (AICD) generation. In comparison, the sporadic form of AD, mostly corresponding to late-onset AD (LOAD), is considered to be affected by highly-prevalent genetic variants with low penetrance, the heritability of which is predicted to be as high as 80% based on studies on twins [11]. The ɛ4 allele of the APOE gene, which encodes the lipid/cholesterol carrier apolipoprotein E (ApoE), is established as the major risk factor for developing LOAD, whereas the presence of the most common ɛ3 allele appears to be protective, and the ɛ2 allele seems to be more protective [12 –14]. The three isoforms of ApoE (ApoE2, ApoE3, and ApoE4) differ only at positions 112 and/or 158. ApoE3 contains a cysteine at position 112 and an arginine at 158, while ApoE2 contains a cysteine at both positions and ApoE4 contains arginine at both sites [15]. Human ApoE has two main regions: the receptor-binding region at the N-terminal domain and the lipid-binding region at the C-terminal domain, which are joined by a flexible hinge (Fig. 1) [16]. Despite differences of just one or two amino acids can be found in the receptor-binding domain, they significantly alter the folding structure of ApoE and change its ability to bind both lipids and receptors [17]. Interestingly, the ɛ4 allele was the first gene in evolution, being the ɛ3 and the ɛ2 the result of mutations produced at a CpG island 300000 and 80000 years ago, respectively [18].

Fig.1

ApoE3 NMR 3D-structure. A) Two different orientations of the first conformer in PDB 2L7B. The receptor binging site (pink), the lipid binding region (blue and orange), and the hinge region (yellow and green) are shown. The secondary structure is colored differently for each helix. B) Scheme of secondary structures. Secondary structures and domains are represented from N-terminal (left) to C-terminal (right). Only two positions differ among the three isoforms (labeled with a star at positions 112 and 158).

Although ApoE is a key player in the pathology of AD, little is known about its role, specifically how the different isoforms of ApoE promote the disease progression or even protect against it. The aim of this review is to examine the influence of ApoE in many mechanisms related to AD to help to understand the relevance of this molecule in the onset of the disease.

APOE, AβPP TRANSCRIPTION, Aβ SECRETION, AND AICD GENERATION

In LOAD, the most prevalent form of AD, decreased Aβ clearance, rather than increased production, might initiate the disease process [19], as no mutations that directly enhance Aβ generation are found in these patients. Whether ApoE isoforms enhance, reduce, or have no effect on Aβ production is still controversial. On the one hand, γ-secretase modulation by ApoE seems to reduce Aβ production. Irizarry et al. demonstrated that ApoE could impair the amyloidogenic pathway by blocking γ-secretase activity independently of ApoE isoform, not mediated by LRP (LDL receptor–related protein) or other LDL receptor family members, and comparable to γ-secretase inhibitors [20]. On the other hand, ApoE4 enhances Aβ production via LRP signaling cascade by affecting the endosomal recycling of AβPP [21]. Interestingly, a recent work by Huang et al. has shown that ApoE induces AβPP transcription and Aβ production in an isoform-dependent manner (ApoE4>ApoE3>ApoE2). Not only a relationship between ApoE internalization and Aβ production in cultured human neurons was found, but also the signaling pathway that controls the process was elucidated. Thus, ApoE binding to ApoE receptors activates dual leucine-zipper kinase (DLK), a MAP-kinase kinase kinase. Increased levels of DLK induce phosphorylation of MMK7 which, in turn, phosphorylates ERK1/2 triggering AP-1 transcription factor activation by c-Fos phosphorylation, ultimately promoting AβPP transcription [22]. In spite of the relevance of these results, we must be careful on extrapolating the relevance of the AβPP transcription modulation by ApoE to AD since prior experiments showed a non-relationship between AβPP mRNA levels and ApoE in human AD brains [23].

Furthermore, it has also been shown that ApoE has transcriptional effects on proteins related with AD. Theendakara et al. demonstrated that ApoE4 reduces the expression of Sirtuin 1 (SirT1) [24], which belongs to the Sirtuin family of NAD-dependent protein deacetylases and activates the transcription of ADAM10, thus increasing the levels of the neuroprotective sAβPPα [25, 26]. ApoE4, but not ApoE3, redistributes SirT1 from the nucleus to the cytosol, precluding its action [24]. Interestingly, not only SirT1 transcription is modulated by ApoE; several genes, some of them related to AD pathology, could be regulated by ApoE, as shown by a genome-wide mapping of ApoE binding carried out recently. The most striking finding of this work is the demonstration that ApoE4 binds DNA with high affinity. The number of these binding sites reaches the figure of 1,700 promoter regions, including genes associated with trophic support, programmed cell death, microtubule disassembly, synaptic function, aging, and insulin resistance [27, 28].

It is worth noting that an increase in AβPP transcription does not only imply high Aβ production, but also high AICD generation as a result of the presenilin-mediated cleavage [26]. Several studies state that AICD could play an important role in AD pathogenesis. A proteomic-based approach shows that some AICD-interactors are differentially expressed in the AD individuals and are involved in diverse cellular functions [29]. AICD-transgenic mice (FeCγ25) were able to mimic some AD-related characteristics as an increase in phosphorylated and non-phosphorylated tau levels, a high susceptibility of neurons to stressful insults, and working memory impairment [30]. Although the exact mechanism whereby AICD induces AD-like pathological features remains unknown, a research carried out with tau KO AICD-Tg mice suggested that tau protein could mediate the detrimental effects of AICD [31]. Taking into account these results, new pharmacological therapies targeting the AICD are currently under research [32].

APOE, Aβ AGGREGATION, AND CLEARANCE

Histological analyses of AD brains reveal that ApoE is co-deposited with Aβ amyloid plaques in an isoform-dependent manner (ApoE4>ApoE3>ApoE2) [33], suggesting a relevant role for ApoE in modulating Aβ metabolism, aggregation, and deposition. Epitope mapping shows that Aβ can interact with both the lipid-binding site and the receptor-binding site within ApoE. The lipidation status of ApoE may dictate both the Aβ-binding and the receptors’ affinities [34].

Although their interaction is demonstrated, the effect of ApoE on Aβ aggregation, whether it is facilitated or inhibited, is controversial [35]. On the one hand, ApoE can interact with Aβ and prevent its fibrillogenesis by capturing Aβ peptides and blocking their seeding properties [36, 37]. In this case, it seems that ApoE4 is less effective in the inhibition of Aβ fibril formation because it shows a lower capacity for interacting with Aβ. In vivo experiments have demonstrated that ApoE3, but not ApoE4 isoform attenuates Aβ deposition induced by protofibrils by stabilizing complexes with Aβ, slowing down fibril formation [38]. On the other hand, it is also known that ApoE self-aggregates in an irregular protofilament-like morphology where the aggregates are formed at substantially different rates depending on the isoform [39]. This may produce co-aggregates with Aβ through its own aggregation propensity, in that ApoE4 is able to promote Aβ aggregation to a higher extent than ApoE3 [38, 39]. Despite that ApoE’s effect on fibrillogenesis could depend on cell origin [40], the AβPP transgenic model Tg2576 ApoE knockout (KO) mice manifesting a decreased number of amyloid plaques in comparison with the wild-type Tg2576 suggests that ApoE enhances Aβ accumulation [41]. Even though the exact role ApoE plays in Aβ aggregation has not been revealed, it seems that the different isoforms may have different effects. ApoE4 is the one that, either being less protective against aggregation or more prone to induce it, potentiates the toxic effects of Aβ and, consequently, could trigger the disease process. A recent study showed that targeting ApoE/Aβ binding ameliorates AD-related cognitive decline, reduces Aβ levels, and significantly diminishes gliosis in the AβPP_Swe/PSEN1_dE9 AD mouse model [42]. In humans, it has been observed that APOE ɛ4 allele-associated AD risk is consistent with increased lifetime exposure to a neurotoxic process [43]. APOE ɛ4 carriage and older age were predictors of longitudinal Aβ accumulation within the Aβ- group, whereas APOE ɛ2 carriage was protective against longitudinal Aβ accumulation [44].

On the other hand, ApoE does not only affect Aβ metabolism by influencing its deposition and aggregation, but also its clearance. If one of the factors that could explain LOAD development is a decrease in Aβ clearance, and ApoE4 is the only well-established locus and the major risk factor for developing LOAD, it might be that ApoE influences Aβ elimination pathways. What is not well known, and remains an important question to answer, is if ApoE helps or inhibits Aβ clearance. ApoE and Aβ share common receptors including LRP1 (LDL receptor-related protein 1), LDLR (low-density lipoprotein receptor), and HSPG (heparansulfate proteoglycans) on the cell surface [45]. It is not known if having common receptors supposes an advantage for receptor-mediated Aβ clearance at the blood-brain barrier (BBB) or is an obstacle. ApoE could facilitate cellular Aβ uptake by forming ApoE/Aβ complexes. While ApoE2 and ApoE3 use both LRP1 and VLDLR to cross the BBB, ApoE4 shifts Aβ transport from LRP1 to the VLDL receptor [46], which may have a slower rate of transport [45]. Although some discrepancies about whether LRP1-mediated Aβ reduction correlates with a decrease in amyloid deposition have been found [47], the importance of LRP1 in amyloid clearance mediated by smooth cells [48], neurons [49, 50], and astrocytes [51] has been largely demonstrated. In contrast, it could also be possible that ApoE would compete with Aβ for their receptor binding. In this sense, Fu et al. have demonstrated that ApoE competes with Aβ in an isoform-independent manner inhibiting its HSPG-mediated uptake [52]. Although the effect of ApoE on Aβ clearance is considered to be influenced by several conditions such as concentration, ApoE isoform, lipidation status, Aβ aggregation extent, and receptor distribution patterns [39, 53], it is unequivocal that ApoE4 isoform impairs the clearance more than ApoE2 or ApoE3 [54].

APOE IN SYNAPTIC PLASTICITY

The main role of ApoE is to coordinate the mobilization and redistribution of cholesterol in repair, growth, and maintenance of myelin and neuronal membranes during development or after injury in the peripheral nervous system. In the central nervous system (CNS), ApoE, in partnership with ApoJ and ApoC1, plays a pivotal role in cholesterol delivery during the membrane remodeling associated with synaptic turnover and dendritic reorganization [55, 56]. Mature neurons appear not to produce sufficient cholesterol for membrane synthesis and repair, so lipid transport may be critical to maintaining neuronal activity [57]. Neuronal injuries are known to induce ApoE expression, and such an increase in ApoE levels may help to repair the nervous system by delivering cholesterol and lipids to neurons [17].

It has been reported that APOE ɛ4 AD carriers exhibit decreased levels of glutamate and postsynaptic proteins in the entorhinal cortex [58] and a reduced dendritic spine density in the hippocampus [59]. APO-E4-TR mice also have lower dendritic spine density and length along with reduced levels of glutamate-related proteins compared to APO-E3-TR mice, which could explain the deficits appreciated in contextual memory [59 –62]. Interestingly, restoration of plasma ApoE levels in ApoE KO mice, which develop cognitive impairment and synaptic dysfunction, partially reverses the learning and memory deficits [63], indicating that ApoE is needed for normal synaptic function, but ApoE4 isoform being detrimental in cognitive terms compared to ApoE3 or ApoE2. Although the mechanisms involved in synaptic impairment induced by ApoE4 are not completely elucidated, it has been recently shown how ApoE4 increases C1q levels, a protein involved in the classical complement cascade associated with neurodegenerative processes. The alteration of the complement cascade leads to an inefficient synaptic pruning by astrocytes, which results in the accumulation of senescent synapses that impair connectivity [64].

Moreover, as mentioned before, ApoE4 increases Aβ and AICD production, both synaptotoxic molecules [65]. How Aβ mediates synaptic loss is not fully understood but it seems that tau-dependent and tau-independent pathways could be involved. It has been reported that both Aβ and AICD are able to induce tau phosphorylation that destabilize synapses [66, 67]. Also, it has been shown that Aβ promotes AMP-activated protein kinase (AMPK) phosphorylation leading to an aberrant protein synthesis in response to stimuli, impairing the memory consolidation process [68].

Current data suggest that a decreased synaptic and dendritic plasticity could induce changes on neuronal activity [69]. A failure in the firing of the neurons would lead to dysregulation of the neural circuits, which would result in the loss of homeostasis and eventually in dementia [70]. Thus, taking into account that synaptic dysfunction is an early pathological feature that occurs prior to neurodegeneration and memory loss, carrying the APOE ɛ4 allele might be an accelerating factor for AD progression.

APOE AND NEUROINFLAMMATION

As it has been previously mentioned, ApoE colocalizes with plaque-associated amyloid and microglia. Apart from the role ApoE plays in Aβ aggregation, this observation could suggest a role in the innate immune response in AD. It has been reported that neuronal apoptotic signals are detected by microglial triggering receptor expressed on myeloid cells 2 (TREM2). TREM2 signaling through ApoE downstream regulates the transcriptional and post-transcriptional program of microglia. ApoE increases pro-inflammatory cytokines expression and reduces homeostatic gene expression. Thus, microglia switch from a homeostatic to neurodegenerative-associated phenotype [71]. Among the different ApoE isoforms, ApoE4 seems to have increased pro-inflammatory and/or reduced anti-inflammatory functions, which could further promote AD pathology. It has been demonstrated that ApoE4 enhances TNF-α and impairs IL-10 production in the AβPP_Swe/PSEN1_dE9 mouse model of AD [72]. Moreover, it has been suggested that Aβ and tau accumulation could be a result, at least in part, of complement factor C3 accumulation modulated by ApoE4 [73]. In humans, young ApoE4 carriers show an increased inflammatory response that may relate to AD later in life, and anti-inflammatory interventions showed greater benefit in ApoE4 carriers [74]. In this sense, an association of chronic low-grade inflammation with risk of AD in ApoE4 carriers has been found after a long-term study (2,656 members of the Framingham Heart Study offspring cohort (Generation 2; August 13, 1971-November 27, 2017) [75]).

On the other hand, cellular uptake of Aβ by astrocytes and microglia is a potential pathway for Aβ clearance [51]. In microglia, soluble Aβ is internalized into lysosomes for degradation [76]. However, different microglia phenotypes have different effects. In general, it can be assumed that M1 is pro-inflammatory, and M2 is anti-inflammatory. It is worth noting that microglial cells are able to express intermediate phenotypes that combine gene expression of M1 and M2 [77]. Although it has been reported that M2 is more protective than M1, it is the combination between both phenotypes, M1-M2, which is effective in reducing Aβ burden. Interestingly, ApoE4 carriers present difficulties for switching the microglial phenotype to M1-M2 compared to ApoE3 carriers [78].

In addition to this evidence, recent genetic studies have also identified several new AD risk genes that are potentially involved in regulating neuroinflammation-related functions including TREM2, CLU, CR1, CD33, and ABCA7 [79]. Although the exact correlation between this list of genes and AD pathology is not well-established, it shows that inflammation has a crucial role in AD development. It is worth noting from the therapeutic point of view that a subset of nonsteroidal anti-inflammatory drugs (NSAIDs) lower Aβ₄₂ independently of cyclooxygenase activity [80] likely by modulating the action of γ-secretase [81].

APOE AND LIPID METABOLISM

The brain is rich in cholesterol, containing 23% of the body’s total cholesterol [82]. Cholesterol is an essential component of cell membranes and plays a crucial role in the development and maintenance of neuronal plasticity and function, which are deeply compromised in AD [83]. Experiments performed in cell cultures and animal models have consistently demonstrated that hypercholesterolemia is associated with increased deposition of cerebral Aβ peptides [84]. Besides, a recent study associated high LDL and total cholesterol levels with an increased risk of dementia in humans [85]. The amyloidogenic processing of AβPP is believed to occur in, or in close proximity to, lipid rafts-cholesterol-rich membrane microdomains where α-secretase activity is negatively regulated, while both β- and γ-secretase activities are stimulated, resulting in increased Aβ and AICD production. In addition, among the list of new AD risk genes, CLU and ABCA7 are directly related to lipid metabolism [86, 87]. Although the exact mechanisms by which any of these lipid-related risk factors affect the pathophysiology of AD remain to be elucidated, it is well-established that there is a correlation between cholesterol metabolism and AD. Interestingly, statins seem to have an important role in AD prevention [88], which could be related to cellular membrane thickness having the same effect on γ-secretase action as NSAIDs (mentioned in the preceding paragraph).

Moreover, the differences among ApoE isoforms also play an important role in cholesterol levels. Neurons obtain lipids and cholesterol mainly from lipidated ApoE [89]. In the periphery, ApoE3 and ApoE2 display preference for HDL binding, whereas ApoE4 is mainly associated with VLDL and LDL [90]. In the brain, HDL-like particles are the most abundant lipoprotein [91], and it has been reported that the size of ApoE particles in humans is reduced in ApoE4 carriers compared to ApoE2/3 carriers [92], which could influence lipid supply to neurons. Altogether, an imbalanced cholesterol metabolism results in a poor synaptic remodeling capacity, increasing the risk for developing AD [93].

Finally, it has been demonstrated that the lipidation status of ApoE has a great importance in its interaction with Aβ. In vitro experiments have shown that lipid-free recombinant ApoE interacts with immobilized Aβ with higher affinity than lipidated recombinant ApoE particles [37] although contrary results have also been reported [94]. The association of poor-lipidated ApoE and Aβ interaction could give another explanation for the detrimental role of the ApoE4 isoform, which is less lipidated than ApoE2/3 isoforms [95]. In this sense, it has been shown that AD patients have 30% less efficiency in ABCA1-mediated cholesterol efflux capacity, a transporter that is also involved in ApoE lipidation [96]. The overlap of the Aβ-binding site with the lipid-binding region within the ApoE C-terminal domain suggests that Aβ and lipids might compete with one another for ApoE binding [39].

APOE AND IMPAIRED MITOCHONDRIAL FUNCTION

Another remarkable structural difference among ApoE isoforms is the distinctive stability of their N-terminal domains. ApoE4 is the least resistant isoform to thermal and chemical denaturation, ApoE2 the most, and ApoE3 shows an intermediate resistance [97]. Carboxy-terminal truncated fragments of ApoE4 enter the cytosol and cause neurotoxicity related to cytoskeletal disruption [98]. Biophysical studies also suggest that the lipid binding domain within the C-terminal truncated ApoE4 has a less organized structure and a greater exposure to the solvent of the hydrophobic residues than the full-length ApoE4, which may increase the interaction with mitochondrial membranes [99]. Time-lapsed recordings of cultured neuronal cells demonstrated that ApoE decreases mitochondrial mobility in an isoform specific manner (ApoE4 fragment>ApoE4>ApoE3) [100].

C-terminal truncated ApoE4 binds to mitochondrial proteins in complex III and IV, affecting the energy balance. Although whether this affects ATP synthesis remains under discussion [101], studies in ApoE4 carriers showed an altered mitochondrial activity [102] and a reduction in energy metabolism [103] of the posterior cingulate gyrus. Other genes related to mitochondrial function, as TOMM40, encoding the dimmer that builds the pore in the outer mitochondrial membrane [104], have been reported to increase the risk of AD [105 –107]. This highlights the important role that energy balance plays in AD pathogenesis. In addition, it has been recently demonstrated that altered energetic balance and mitochondrial damage could be the link between type II diabetes and AD [108]. Interestingly, ApoE4 inhibits insulin signaling by promoting endocytosis of its receptor, and consequently, impairing mitochondrial respiration and glycolysis [109]. Thus, recent data suggests a connection between insulin, mitochondrial damage, ApoE genotype, and AD.

The effect of mitochondrial damage in AD progression leads to the idea of mitochondrial therapy as a new possible attractive pharmacological approach to treat AD. Supporting this, a recent publication lead by Auwerx has demonstrated that mitochondrial function reestablishment reduces Aβ load and ameliorates cognitive impairment in the AβPP_Swe/PSEN1_dE9 mice [110].

APOE AND TAU HYPERPHOSPHORYLATION

The exact role tau hyperphosphorylation plays in AD remains unclear. Although tau hyperphosphorylation has been described to occur downstream of Aβ accumulation and AICD generation, high levels of hyperphosphorylated tau correlate much better with cognitive decline than Aβ plaques [111], and might generate the most toxic effect [112]. Remarkably, as mentioned before, recent studies claim that tau hyperphosphorylation occurs earlier than thought and that these species are the building blocks of early AD [8 , 114]. Not only Aβ, but also tau interacts with ApoE [115]. It has been proved that ApoE4 increases tau hyperphosphorylation in vivo [116]. However, it is worth noting that enhanced tau hyperphosphorylation by ApoE4 only occurs in humans if Aβ is present, supporting the idea that both are linked [117]. Besides tau hyperphosphorylation, a recent study also shows that ApoE4 exacerbates tau-mediated neurodegeneration in the P301S mouse model by increasing brain atrophy, gliosis, and neuronal death [118]. Supporting this, it has been reported that elevated tau levels in CSF in humans are associated with decreased cortical plasticity and impaired cognition in ApoE4 but not in ApoE3 carriers [119].

CONCLUSIONS

AD is a devastating disease that affects more than 47 million people worldwide, but the underlying causes of the disease are not completely understood. On the one hand, the ɛ4 allele of ApoE is the only well-established genetic risk factor for LOAD, the most common form of the disease. On the other hand, the accumulation of the Aβ peptide and AICD on prodromal stages has long been thought could be the seed initiating the disease progression but recent studies claim that tau hyperphosphorylation also occurs at early stages of the disease and that these species are the ones correlating with metal decline. What is not revealed are the interactions these two facts may have. As shown in this review, ApoE does not have a unique way of action: it does not only contribute to AD pathogenesis in an AβPP-dependent manner (Fig. 2) but also in AβPP-independent manner (Fig. 3) (Table 1). Rather than ApoE itself, is the ApoE isoform that influences the disease progression. ApoE4 could be detrimental by promoting the pathology or by being less efficient in preventing it than ApoE3 and ApoE2. The different ApoE isoforms have been shown here to differently modulate AβPP transcription, AICD production, Aβ aggregation and clearance, synaptic plasticity, neuroinflammation, lipid metabolism, mitochondrial function, and tau hyperphosphorylation.

Fig.2

ApoE’s effects on Aβ and AICD production, clearance, and aggregation. ApoE2 and ApoE4 effects are represented taking ApoE3 as the reference isoform. Green bars represent a direct relation. Red bars represent an inverse relation. In summary, ApoE4 increases Aβ production and aggregation and impairs its clearance. The opposite effect for ApoE2 can be appreciated.

Fig.3

ApoE AβPP–independent mechanisms on related-dementia hallmarks. ApoE2 and ApoE4 effects are represented taking ApoE3 as the reference isoform. Green bars represent a direct relation. Red bars represent an inverse relation. ApoE4, but not ApoE2/3, increases pro-inflammatory response, tau aggregation, cellular stress, and impairs synaptic connectivity.

Table 1

Differences among ApoE isoforms in several mechanisms in which ApoE has been reported to be involved

		Influence of ApoE isoform
Aβ METABOLISM	Aβ aggregation	ApoE4 shows a lower capacity to inhibit Aβ fibril formation than ApoE3. At the same time, its tendency to self-aggregation, and concomitantly co-aggregate Aβ, is higher.
	Aβ deposition	ApoE is co-deposited with Aβ amyloid plaques in an isoform dependent manner (ApoE4>ApoE3>ApoE2)
	Aβ uptake	It is unclear whether ApoE forms complexes with Aβ to cross the BBB, helping in Aβ clearance, or whether they compete for the same receptors, hindering Aβ clearance. While ApoE2 and ApoE3 use both LRP1 and VLDLR to cross the BBB, ApoE4 shifts this preference from LRP1 to the VLDL receptor, which may have a lower rate of clearance.
SYNAPTIC PLASTICITY	Dendritic spine density	APO-E4-TR mice have lower dendritic spine density and length compared to APO-E3-TR mice
	Remodeling capacity	ApoE3, but not ApoE4, prevents loss of synaptic networks induced by Aβ oligomers.
LIPID METABOLISM	Lipoprotein preference	ApoE3 and ApoE2 display preference for HDL binding, whereas ApoE4 is mainly associated with VLDL and LDL. Therefore, ApoE4 leads to an increase in plasma cholesterol levels.
	Lipidation status	The lipidation status of ApoE also affects its capacity for Aβ binding. The fact that the Aβ-binding site overlaps with the lipid-binding region within the ApoE C-terminal domain suggests that Aβ and lipids might compete with one another for ApoE binding.
MITOCHONDRIAL DYSFUNCTION	Stability	ApoE4 is the least resistant to thermal and chemical denaturation, ApoE2 the most, and ApoE3 shows an intermediate resistance.
		ApoE decreases mitochondrial mobility in an isoform specific manner (ApoE4 fragment>ApoE4>ApoE3).
NEUROINFLAMMATION	Microglial activation	ApoE promotes Aβ clearance by activating microglia phagocytosis and migration, while ApoE4 has a reduced capacity for inducing these phenotypes compared to ApoE3.
	Inflammatory response	ApoE4 seems to have pro-inflammatory and/or reduced anti-inflammatory functions. Increased inflammatory response occurs in young ApoE4 carriers, and anti-inflammatory interventions showed greater benefit in them than in non-carriers.

This multifactorial influence and the crucial role ApoE plays in AD have woken the interest of researchers, and many data have been published in recent years, including controversial results. It is worth noting that several factors influence ApoE performance, such as the lipidation status and aggregation state, which could explain, at least in part, the discrepancies observed in the literature. That might be also the reason why so many therapies designed to treat AD by targeting ApoE have not been successful [120]. As long as we do not understand how ApoE can modulate the onset/development of the pathology, the search for a cure will be a hard task with unproductive results.

Footnotes

ACKNOWLEDGMENTS

This study was funded by the Ministerio de Economía, Industria y Competitividad (MINECO, Spain) / FEDER, [SAF2017-89613R].

Authors’ disclosures available online ().

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