Abstract
Alzheimer’s disease (AD) is currently the most prevalent neurological disease, and no effective and practical treatments and therapies exist. The nucleotide-binding oligomerization domain-, leucine-rich repeat-, and pyrin domain- containing receptor 3 (NLRP3) inflammasome is vital in the human innate immune response. However, when the NLRP3 inflammasome is overactivated by persistent stimulation, several immune-related diseases, including AD, atherosclerosis, and obesity, result. This review will focus on the composition and activation mechanism of the NLRP3 inflammasome, the relevant mechanisms of involvement in the inflammatory response to AD, and AD treatment targeting NLRP3 inflammasome. This review aims to reveal the pathophysiological mechanism of AD from a new perspective and provide the possibility of more effective and novel strategies for preventing and treating AD.
INTRODUCTION
Alzheimer’s disease (AD), also known as senile dementia, was identified and described for the first time by Alois Alzheimer [1]. AD is the most widespread progressive neurodegenerative disease with complicated and diverse symptoms in the central nervous system (CNS). The major clinical symptoms of the disease include severe memory loss, cognitive and behavioral disorders, personality abnormalities, aphasia, posterior cortical atrophy, visual-spatial-perceptual abnormalities, executive dysfunction, and neuropsychiatric disorders [2, 3]. According to the data reported from the World Alzheimer’s Report, the number of patients suffering from AD worldwide is showing a significant growth trend, which reveals that AD is a worldwide challenge that needs to be urgently tackled. The typical neuropathological injuries within the brains of AD patients include senile plaque attributable to the amyloid-β (Aβ) deposition formed by irreversible loss and abnormal accumulation of extracellular protein aggregates in neurons, the intraneuronal formation of neurofibrillary tangles formed by the hyperphosphorylated tau proteins, the synaptic plasticity loss and selective loss of postsynaptic receptors, neuroinflammation, oxidative stress, and neuronal death [1, 5]. The etiology and pathogenesis of AD have not been conclusively established. However, the amyloid-β hypothesis, the hyperphosphorylation hypothesis of tau protein, and the cholinergic hypothesis are commonly accepted pathophysiological mechanisms related to AD [6].
Neuroinflammation, leading to cognitive dysfunction and neuronal apoptosis, was determined to be a crucial element at the onset of AD based on researchers’ extensive experimental studies. Moreover, recent studies have demonstrated that a prolonged inflammatory response is significant in the pathological pathogenesis of AD [7]. This was supported by the elevated levels of various inflammatory markers that the researchers detected in the brain tissues of AD patients [8]. Among the numerous organismal inflammasomes reported, the nucleotide-binding oligomerization domain-, leucine-rich repeat-, and pyrin domain- containing receptor 3 (NLRP3) inflammasome has attracted extensive attention as a critical pattern recognition receptor in the human innate immune system. Moreover, the NLRP3 inflammasome has been confirmed to be the most characterized within the CNS [9], and the NLRP3 inflammasome is also a crucial prerequisite in AD neuroinflammatory transduction pathways. Some exogenous or endogenous agonists induce the NLRP3 inflammasome activation, resulting in the secretion of inflammasome components and pro-inflammatory cytokines such as caspase-1, interleukin (IL)-1β, and interleukin (IL)-18 in microglia. Then these pro-inflammatory products continue to contribute to the major lesions in AD pathological process through various signaling pathways in the organism, involving Aβ deposition, tau protein hyperphosphorylation, and neuroinflammatory response. Eventually, the clinical signs of AD developed.
Further experimental studies revealed that when the NLRP3 inflammasome or its related signal transduction pathway was inhibited or stopped in AD model mice, the levels of AD-related markers were downregulated, and the disease’s pathological process was significantly improved [10]. Accordingly, the NLRP3 inflammasome may have the potential to be a viable therapeutic target for AD. In vitro tests have revealed that NLRP3 inflammasomes have excellent therapeutic effects on AD, which makes the drug development targeting the NLRP3 inflammasome and its signaling pathways a possible therapeutic method for AD.
COMPOSITION AND ACTIVATION MECHANISM OF NLRP3 INFLAMMASOME
The inflammasomes are an essential component of the innate immune system, as they can initiate the innate immune response and maintain homeostasis. The inflammasome, whose nature is a multimeric cytoplasmic protein complex mediating the activation of inflammatory cysteine aspartate proteases, assembles by activating intracellular pattern-recognition receptors to identify pathogen-associated molecular patterns (PAMPs, e.g., pore toxins, RNA viruses, and fungal cytosolic proteins) and damage-associated molecular patterns (DAMPs, e.g., ATP and urate) (Fig. 1) [11, 12]. To date, we understand that multiple inflammasomes are involved in the host response against pathogen invasion, while pathogens have also evolved various mechanisms to restrain the activation of the inflammasome [13].
NLRP3 inflammasome activators. Yellow represents PAMPs, red represents DAMPs, and blue in the cross dashed line represents either PAMPs or DAMPs under certain circumstances. NLRP3, NACHT, leucine-rich repeat (LRR), and pyrin domain (PYD)-containing protein 3; DAMP, damage- associated molecular pattern; PAMP, pathogen- associated molecular pattern; LPS, lipopolysaccharide; ROS, reactive oxygen species; ATP, adenosine triphosphate.
The much-proposed NLRP3 inflammasome consists of receptor protein NLRP3, apoptosis-associated speck-like protein (ASC), and inflammatory protease cystatin-1 (pro-caspase-1) [14]. NLRP3 proteins, upon stimulation by PAMPs or DAMPs, abolish autoinhibition and oligomerization, recruits ASC and effector protein precursor caspase-1 to promote the formation of NLRP3-ASC-pro-caspase-1 complex, also known as the NLRP3 inflammasome [15]. Pro-caspase-1 is cleaved to active caspase-1 through proximity-induced autocatalytic activation. As an inflammasome effector protein, activated caspase-1 can process and cleave upstream pro-inflammatory cytokine precursors such as inactive pro-interleukin-1 (IL-1β) and pro-interleukin-18 (IL-18) into biologically active mature forms and regulating their secretion [16, 17], promote the cleavage of protein Gasdermin D (GSDMD), which triggering pyroptosis [18]. The cleavage of GSDMD leads to the formation of pores that facilitate the transportation of secreted cytokines from the cell to the extracellular environment. This process serves as the trigger for the initiation of the inflammatory response within the body, subsequently driving the downstream cascade of inflammatory events.
Baseline NLRP3 concentrations in quiescent cells are inadequate to activate the NLRP3 inflammasome. In line with this, activation of the canonical NLRP3 inflammasome is generally accepted to require two separate but parallel steps, priming, and activation, a two-signal model (Fig. 2). First, some microbial components or endogenous factors bind to Toll like receptors (TLRs), activate the nuclear factor-κB (NF-κB) and subsequently upregulate the NLRP3 transcriptional expression and the precursor IL-1β by the MyD88- NF-κB pathway [18]. The second step is defined as the activation signal. Numerous NLRP3 inflammasome agonists, such as PAMPs and DAMPs, activate the NLRP3 inflammasome, resulting in its oligomerization and assembly activation [19]. In AD, Aβ and tau proteins act as second signaling agonists to induce the assembly and activation of NLRP3 inflammasome. Currently, several mechanisms for triggering NLRP3 inflammasome activation have been demonstrated, including potassium ion efflux, changes in calcium ion levels, mitochondrial dysfunction, reactive oxygen species (ROS) production, lysosomal destabilization, and specific disintegration of Golgi anticlinal network structures [19–21]. Not all NLRP3 inflammasome agonists are known to engage in inflammasome activation by the signaling pathways. In addition to transcription and translation, priming signals regulate several post-translational modifications of NLRP3 inflammasome. It has been many experiments reported that phosphorylation and ubiquitination can affect the initiation and activation of the NLRP3 inflammasome [22].
Mechanisms of activation of the NLRP3 inflammasome. The activation of the NLRP3 inflammasome is divided into the classical pathway and the non-canonical pathway. The classical pathway includes the initiation phase and the activation phase, that is, signal 1 and signal 2. Signal 1: After some microbial components bind to TLRs receptors, they activate NF-κB through the TLR4/MYD88 signaling pathway, and promote the transcription and expression of NLRP3 and pro-IL-1β. Signal 2: Stimulation by NLRP3 inflammasome agonists such as PAMPs and DAMPs, causing potassium ion efflux, changes in calcium ion levels, mitochondrial dysfunction, reactive oxygen species generation and lysosome destabilization, which in turn lead to the assembly of inflammasomes with activation. The activated NLRP3 inflammasome activates Pro-caspase-1 into active caspase-1, which can cleave the biologically inactive pro-IL-1β and pro-IL-18 into active forms and release them outside the cell. The active caspase-1 also can cleave GSDMD and trigger pyroptosis. The non-canonical pathway induced by LPS is the conversion of caspase-4/5/11 to active caspase-4/5/11, which mediates the activation of NLRP3 inflammasome. In addition, microbial molecules or other danger signals induce NLRP3 inflammasome activation only through TLR4 ligand activation of the TLR4–RIPK1–FADD–caspase-8 signaling pathway. NLRP3, NACHT, leucine-rich repeat (LRR), and pyrin domain (PYD)-containing protein 3; ASC, apoptosis-associated speck-like protein; pro-caspase-1, inflammatory protease cystatin-1; DAMP, damage- associated molecular pattern; PAMP, pathogen- associated molecular pattern; LPS, lipopolysaccharide; ROS, reactive oxygen species; pro-IL-1β, precursor interleukin-1β; pro-IL-18, precursor interleukin-18; TXNIP, thioredoxin-interacting protein; ER, endoplasmic reticulum; TLRs, Toll like receptors; NF-κB, the nuclear factor-κB; GSDMD, Gasdermin D.
In addition to the NLRP3 inflammasome activation by the canonical pathway described above, the NLRP3 inflammasome can also be activated by non-canonical activation pathways (Fig. 2) [23]. The experiment showed that after LPS reached the cytoplasm, it bound to human caspase-4/5 or mouse caspase-11 to trigger its activation, and the activated caspase-4/5/11 lead to the NLRP3 inflammasome activation in the same way as the canonical activation pathway [24]. Therefore, these two pathways may interact regarding the outcome. Alternative NLRP3 activation refers to discovering a third activation mechanism for the NLRP3 inflammasome, distinct from the two described above. This activation process has occurred in human and porcine monocytes, where microbial molecules or other danger signals induce NLRP3 inflammasome activation only through activation of the TLR4-TRIF-RIPK1-FADD-caspase-8 signaling pathway by TLR4 ligands [25].
NLRP3 INFLAMMASOME AND AD
AD patients’ brains exhibit neuroinflammation induced by activated NLRP3 inflammasomes, which induce higher levels of inflammatory cytokines in the body. Besides, numerous experimental findings suggest that the neuroinflammation driven by NLRP3 inflammasome activation is inextricably linked to the pathological mechanism of AD, which may be another novel pathogenesis of AD-associated inflammation (Fig. 3).
Schematic diagram of the mechanism of NLRP3 inflammasome involved in the pathological development of AD. When neurons are pathologically damaged, Aβ fibrils, oligomers, and tau proteins are transported outside the neurons. When Aβ and tau are absorbed by microglia, complex mechanism processes will also occur inside the microglia. When stimulated by Aβ, it activates the NLRP3 inflammasome in a variety of ways including: (1) Aβ binds to TLR4, acts on the initiation stage of NLRP3 inflammasome, and activates NLRP3 inflammasome; (2) Stimulate the upregulation of P2X7 receptors, directly acting on the NLRP3 inflammasome; (3) Lead to lysosomal damage, release cathepsin B into the cell, and then stimulate NLRP3 inflammasome; (4) Cause mitochondrial dysfunction, increase intracellular ROS content, and induce NLRP3 inflammasome activation; (5) Mitochondria secrete Drp1 through the RAGE-TXNIP axis, and this factor feeds back to the mitochondria to aggravate mitochondrial dysfunction; (6) Result in impair autophagy and activates NLRP3 inflammasome; (7) Cause ER stress, stimulate TXNIP to act on NLRP3 inflammasome or directly act on NLRP3 inflammasome, and induce tau protein phosphorylation. The NLRP3 inflammasome is also activated when stimulated by tau, in part in the same way as Aβ. The activated NLRP3 inflammasome secretes cytotoxic factors and produces neurotoxins; leads to increased deposition of Aβ plaques; leads to hyperphosphorylation and aggregation of tau protein, and finally forms NFT. The result of NLRP3 inflammasome activation will continue to aggravate neuron damage, causing it to continue to release Aβ and tau, and even infect healthy neurons. Thus a toxic closed loop is formed. NLRP3, NACHT, leucine-rich repeat (LRR), and pyrin domain (PYD)-containing protein 3; ASC, apoptosis-associated speck-like protein; pro-caspase-1,inflammatory protease cystatin-1; Aβ, amyloid-β; ROS, reactive oxygen species; NF-κB, the nuclear factor-κB; TLRs, Toll like receptors; RAGE, receptor for advanced glycosylation end products; TXNIP, thioredoxin-interacting protein; Drp 1, triggering abnormal activation of dynamin-related protein 1; TRPM2, transient receptor potential melastatin 2; ER, endoplasmic reticulum; NFT, neurofibrillary tangles; AD, Alzheimer’s disease.
NLRP3 inflammasome and Aβ
Activated NLRP3 inflammasomes influence AD development in two ways. First, several clinical studies and animal experiments in transgenic APP/PS1 mice demonstrated that Aβ as a neurotoxic protein in AD patients, which activates the NLRP3 inflammasome in microglia to release significant amounts of inflammatory cytokines including IL-1β, ultimately intensifying the inflammatory response in the brain and synthesizing neurotoxins, leading to neuroinflammation or neurotoxicity [26–30]. In addition, Halle et al. indicate that as microglia continue to phagocytose Aβ, it may cause intracellular lysosome damage and release cathepsin B to stimulate the activation of NLRP3 inflammasome [26]. A recent study indicated that only 5 nm of soluble low molecular weight Aβ oligomers and protofibrils could activate the NLRP3 inflammasome in microglia before Aβ deposition [31]. Intriguingly, research on the mechanism of NLRP3 inflammasome activation by Aβ indicates that Aβ activates NLRP3 inflammasomes by TLR4 receptors in microglia of AD model mice. When mice were treated with NLRP3-SiRNA or TLR4 inhibitors, the investigators found significant inhibition of Aβ-induced elevation of inflammatory cytokines, which contributed to the repair of neural damage [29, 32]. Alternatively, NLRP3 inflammasome activation impairs the clearance of Aβ plaques mediated by phagocytic microglia and decreases microglia phagocytosis of Aβ plaques, which causes the Aβ deposition and promotes the development of AD lesions [31, 33]. Therefore, once neuroinflammation occurs within the brain, NLRP3 inflammasomes will be continuously activated, and the Aβ plaques cannot be effectively cleared and deposited continuously, forming a closed-loop toxic feedback pathway in the brain, continuously inducing neuroinflammation and neurotoxic factors release, ultimately leading to neuronal loss and the clinical manifestations of AD. Halle and colleagues [26] demonstrated that protofibrils Aβ in wild-type (WT) microglia triggered the multiple inflammatory factors and chemokines secretion by microglia and induced ASC speck formation and caspase-1 activation. The Aβ oligomer complex formed by the combination of ASC specks and exposed Aβ can intensify the pro-inflammatory response leading to promote the pathological development of Aβ [34]. Moreover, several interesting experiments include: Aβ deposition induces significant levels of ROS production by mitochondria and NADPH oxidase, activating the NLRP3 inflammasome by the transient receptor potential melastatin 2 (TRPM2) signaling channel [35]. Soluble Aβ1-42 (sAβ1-42) activates the NLRP3 inflammasome but simultaneously inhibits TNF-α secretion [36]. The previous experimental results provide direct and strong evidence for the involvement of microglial NLRP3 inflammasome activation in the pathogenesis of AD.
NLRP3 inflammasome and tau
The Aβ plaque formation precedes the hyperphosphorylation of tau protein in AD. Is hyperphosphorylated tau protein in some way associated with NLRP3 inflammasome and Aβ deposition? To investigate this process, a major experiment conducted by Ising et al. [37] showed that loss of NLRP3 inflammasome function reduced tau hyperphosphorylation and aggregation by regulating tau kinases and phosphatases. It also demonstrated that Aβ deposition can drive tau pathology by inducing NLRP3 inflammasome activation, which supported the Aβ cascade hypothesis in AD. Similarly, Stancu and his team [38] discovered that NLRP3 inflammasome knockout accompanied by improved hippocampal atrophy and neurodegeneration, reduced prion-like inoculation, and downregulated tau pathological transmission. Thus, several studies have proved an extraordinary relationship between NLRP3 inflammasome activation and tau protein pathology in AD. Notably, Stancu and his team [39] discovered that ASC deficiency in tau transgenic mice greatly reduced mouse non-exogenous tau pathology and demonstrated that tau aggregation could activate the ASC inflammasome by the NLRP3-ASC signaling axis. Furthermore, lysosomes phagocytose tau seeds and secrete cathepsin B, which then activate the NLRP3 inflammasome. It has also been shown that tau-derived PHF6 peptide (VQIVYK) aggregation can upregulate NLRP3 protein expression levels and activate other inflammatory indicators and microglia [40].
NLRP3 inflammasome and oxidative stress
One of the histological characteristics of the AD brain is oxidative stress, which is intrinsically associated with neuroinflammation. A key protein, thioredoxin-interacting protein (TXNIP), has been identified in studies of oxidative stress [41]. NLRP3 inflammasome agonists can promote the TXNIP dissociation from thioredoxin (TRX) in a ROS sensitive manner and make it bind to NLRP3, thereby directly activating NLRP3 inflammasome, triggering the release of caspase-1 and IL-1β. This process is very important in the pathophysiological changes of AD. Initially, Li and his team [42] observed that the levels of TXNIP protein and mRNA in the cerebral cortex of AD patients were significantly higher than in the control group. The inflammasome-associated cytokines secretion was also significantly increased. Simultaneously, immunofluorescence showed that TXNIP and IL-1β co-localized near the Aβ plaque and tau protein. The same conclusion was obtained in Ismael and his colleagues’ research [43]. The investigation revealed that TXNIP is co-localized in neurons and microglia. They also found that endoplasmic reticulum (ER) stress can activate TXNIP. A recent study reveals a new pathway by which TXNIP mediates NLRP3 inflammasome activation in AD. Sbai et al. showed that Aβ on the cell surface is transported to mitochondria via the receptor for advanced glycosylation end products (RAGE)-TXNIP axis, triggering abnormal activation of dynamin-related protein 1 (Drp 1) in mitochondria, which in turn feeds back into mitochondrial dysfunction and is subsequently involved in the activation of NLRP3 inflammasome [44].
NLRP3 inflammasome and other clues
The purinergic receptor P2X7 is recognized to be abundant near Aβ plaques in AD patients [45], and it participates in the pathological development of AD through the P2X7/NLRP3 inflammasome pathway. In addition, Di Lauro and his team recently revealed the conclusion that overexpression of P2X7 can aggravate tau-induced toxic pathology [46]. Similar to P2X7, ER stress is associated with both Aβ and tau. HSPA 5, an ER stress marker, was detected to be elevated upon Aβ accumulation [47]. Additionally, recent study highlighted that kainic acid could induce ER stress and activate NLRP 3 inflammasomes. Its neurotoxicity, in turn, enhanced the hyperphosphorylation of tau [48]. It is worth noting that some scholars believed that the autophagy function of microglia could reduce the content of Aβ [49]. Subsequently, in-depth study indicated that autophagy dysfunction enhances the activation of NLRP3 inflammasome, which undoubtedly suggests that autophagy negatively regulates the activation of NLRP3 inflammasome induced by Aβ [50].
RESEARCH RELATED TO TARGETING NLRP3 INFLAMMASOME FOR THE TREATMENT OF AD
In recent years, investigators have looked at the NLRP3 inflammasome as a new focus for AD drug development [10]. Researchers have explored and produced numerous NLRP3 inflammasome inhibitors and drugs that block NLRP3 inflammatory signaling pathways by investigating NLRP3 inflammasome involvement in the inflammatory mechanism of AD. However, there are no NLRP3 inhibitors and related inhibitory pathway drugs in clinical application, and they are still in the clinical trial stage.
NLRP3 inflammasome inhibitor
MCC950
MCC950 is already well-known as the first demonstrated specific NLRP3 inflammasome inhibitor. MCC950 inhibits NLRP3 inflammasome activation and down-regulates inflammatory factor secretion by preventing ASC oligomerization [51]. Later, some scholars showed that treating APP/PS1 mice with NLRP3 inflammasome inhibitor MCC950 inhibited neuroinflammation activity, inflammasome reduction, microglial activation, and Aβ accumulation and improved behavioral abnormalities in mice [52]. Moreover, MCC950 exhibits anti-inflammatory activity and can be used to improve frontotemporal dementia [53].
JC124
Previous studies have shown that a lead compound, NLRP3 inflammasome inhibitor JC124, has therapeutic effects in AD mice models. Chronic treatment of TgCRND8 mice with JC124 inhibited caspase-1 cleavage while reducing Aβ accumulation and microglial activation [54]. This result was recently demonstrated again by researchers. Targeted inhibition of NLRP3 inflammasome by JC124 in APP/PS1 transgenic mice improved AD pathology and behavior, reducing astrocyte proliferation and neuronal cell cycle reentry, and concomitant preservation of synaptic plasticity and high production of pre- or postsynaptic proteins and enhanced hippocampal neurogenesis and improvement in cognitive function [11].
OLT1177
Initially, Dapansutrile (OLT1177) was a drug for arthritis. Marchetti and colleagues [55] have shown that OLT1177 can act as an orally active NLRP3 inflammasome inhibitor, resulting in reduced microglia activation and a return to normal levels of plasma metabolic markers in APP/PS1 mice. A recent study compared APP/PS1 mice fed the OLT177 diet to APP/PS1 mice and WT mice fed a regular diet and discovered that a three-month OLT177 diet might repair the synaptic plasticity of this AD mouse model. Additionally, OLT1177 administration to inhibit NLRP3 reduced the activity of microglia and decreased the number of cortical plaques. This research suggests the therapeutic potential of oral NLRP3 inflammasome inhibitors to treat neuroinflammatory disorders [56].
CY-09
CY-09 consistently acts as a recognized direct NLRP3 inflammasome inhibitor. Jiang and his team [57] found that CY-09 was independent of initiation signaling and post-translational modifications and could directly and specifically inhibit NLRP3-ATPas activity, blocking NLRP3 inflammasome assembly and activation.
Oridonin
Oridonin, a traditional Chinese herbal medicine, is widely recognized as a specific covalent inhibitor of NLRP3 inflammasome. It inhibits the association of NLRP3 and NEK7 by combining with the NACHT domain in the NLRP3 inflammasome [58]. It was reported that Oridonin could inhibit the activation of NF-κB or MAPK and decrease the non-inflammasome-independent pro-inflammatory cytokines secretion [59]. Targeting NLRP3 inflammasomes to treat AD has a significant potential for evaluation as it is primarily intended for treating inflammatory diseases.
Herbal ingredients
Traditional Chinese medicine plays a pivotal role in the treatment of diseases. The main reason is that Chinese herbal medicine has a long history, rich varieties, low toxicity, and can be taken for a long time. Using herbal medicines to treat the pathophysiology of AD has been the subject of ongoing research, and several herbal medicine constituents are significantly effective in reducing lesions in AD model mice. Recently, it has been discovered that several herbal ingredients can alleviate neurotoxicity and improve neuronal degeneration during AD by targeting NLRP3 inflammasome suppression (Table 1).
Natural substance extract and its target of NLRP3 inflammasome
This table summarizes the extracts of the natural products mentioned in the article: traditional Chinese herbal extracts and food extracts. NLRP3, NACHT, leucine-rich repeat (LRR), and pyrin domain (PYD)-containing protein 3; NF-κB, the nuclear factor-κB; IL-1β, interleukin-1; Nrf2, nuclear factor erythroid 2-related factor 2; Aβ, amyloid-β; ROS, reactive oxygen species; TLR4, Toll like receptors 4; TXNIP, thioredoxin-interacting protein; HSP90, the heat shock protein 90; MyD88, myeloid differentiation factor 88; GSDMD, Gasdermin D.
Dihydromyricetin
Dihydromyricetin (DHM) belongs to the same class of flavonoids as myricetin and are widely found in plants. Among them, the content of dihydromyricetin in vine tea is very high, and myricetin mainly comes from the extraction of bayberry [60]. Both of them exert anti-inflammatory ability in the treatment of AD. It was demonstrated that DHM treatment could promote clearance of Aβ by increasing levels of neprilysin, induce microglia polarization to the M2 phenotype, enhance Aβ phagocytosis, reduce the activation of NLRP3 inflammasome, significantly improve spatial memory and cognitive impairments in APP/PS1 transgenic mice [61]. Additionally, DHM was able to exert anti-inflammatory activity by upregulating the AMPK/SIRT1 pathway in AD rats [62]. The latest study demonstrated that DHM can reduce TLR4 protein through myeloid differentiation protein (MD2), thereby alleviating inflammatory damage in AD [63]. Similarly, study has found that AD mouse models treated with myricetin have greatly improved cognitive and memory deficits [64]. But there are currently some clinical issues that need to be further resolved. Such as the fact that due to the limited solubility of DHM, the body is unable to fully absorb it to maximize its effect when taken orally into the gut.
Salidroside
Salidroside (Sal) is isolated from Rhodiola rosea L. A number of experiments have proved that salidroside play a neuroprotective role and has a significant therapeutic effect on AD through different pathways [65, 66]. Notably, in vivo and in vitro experiments Cai et al. reported that salidroside could significantly inhibit NLRP3 inflammasome-mediated pyroptosis through inhibiting TLR4/NF-κB/NLRP3/Caspase-1 signaling pathway, and significantly downregulate various indicators of AD pathology [67].
Ginkgolide B
Ginkgo biloba has a long history of use in Chinese herbal medicine. Ginkgolide B (GB) is the main extract product and is broadly used in the treatment of various diseases. Of these, the researchers found that treating Aβ1-42-induced microglia with GB-containing cell cultures can inhibit the NLRP3 inflammasome activation pathways, promote the microglia polarization to the M2 type, and prevent AD neurotoxicity [68]. Furthermore, Shao et al. emphasized that GB can enhance autophagy and reduce the expression of NLRP3 inflammasomes and inflammatory factors, while improving cognitive function in SAMP8 mice [69].
Baicalin
Baicalin (BAI) is a kind of flavonoid compound extracted from Scutellaria baicalensis Georgi. As early as 2015, Chen and his team showed that BAI could play a neuroprotective role to improve cognitive dysfunction in AD mice [70]. By 2019, researchers had discovered a new mechanism of BAI in treating AD. In APP/PS1 transgenic mice, the flavonoid compound baicalin could inhibit microglia activation by the TLR4/NF-κB signal pathway, thereby reducing the NLRP3 inflammasome activation and downregulating the pro-inflammatory factors secretion, hence exerting a significant anti-inflammatory response and neuroprotective function during the progression of AD [71].
Curcumin
Curcumin, a phenolic compound isolated from the rhizome of turmeric, has been extensively and intensively studied as a medicinal component of AD, and is a commonly used herb in the treatment of AD. Dating back as far as 2005, Yang et al. pointed out that curcumin could prevent the formation of Aβ protofibrils and oligomers and degrade Aβ deposits in vitro experiments [72]. As continuous research has found, curcumin has a certain relieving effect on Aβ plaques and tau hyperphosphorylation formed in the pathological process of AD [73]. Recently, Nguyen et al. reveals that curcumin inhibits 1,2-diacetylbenzene (DAB)-induced neuroinflammation, reduces inflammatory cytokines expression and tau hyperphosphorylation through NF-κB/NLRP3/caspase-1/IL1β pathway [74].
Epigallocatechin-3-gallate
Epigallocatechin-3-gallate (EGCG), the main polyphenolic active ingredient of green tea, has long been known for its anti-inflammatory and antioxidant activity. Various studies have demonstrated the therapeutic potential of EGCG in the pathological progression of AD by inhibiting Aβ deposition and tau protein aggregation [75]. Of note, EGCG can repress the TLR4/NF-κB-related pathway activated by NLRP3 inflammasome in APP/PS1 transgenic mice and down-regulate levels of inflammatory factors, therefore restraining neuronal apoptosis and improving cognitive memory dysfunction [76]. Therefore, the subsequent treatment of EGCG on AD can focus on the NLRP3 inflammasome.
Epimedii Folium
Epimedii Folium is a Chinese herbal medicine that has been around for almost a thousand years, and Curculiginis Rhizoma is the working ingredient obtained from the rhizomes of the herb. Epimedii Folium has long been known as an anti-inflammatory drug and is gradually being shown to have neuroprotective effects, which is the expected drug action in neurodegenerative diseases. So on top of that, the researchers adjusted the way of medication. The combination of Epimedii Folium and Curculiginis Rhizoma has been used to detect the decreased expression of inflammatory factors in experiments and clinical trials, which can effectively inhibit the activation of NF-κB/NLRP3 inflammasome pathway and has neuroprotective effect on AD [77, 78]. In order to solve the low oral bioavailability of Epimedii Folium, clinical research has been committed to overcoming this problem.
Artemisinin
Artemisinin is a well-known drug which comes from the plant Artemisia annua and has weak toxicity. Artemisinin has been extensively used clinically as a drug against malaria. Gradually, some experiments revealed its neuroprotective effects by modulating microglia-induced neuroinflammation. A few studies have discovered that the pathological changes of AD were alleviated by administering artemisinin. Further exploration revealed that artemisinin relieved the deposition of Aβ plaques through the NF-κB/NLRP3 signaling pathway and inhibited the activation of NLRP3 inflammasomes, while demonstrating amelioration of neuronal damage [79, 80].
DL-3-N-butylphthalide
Dl-3-n-butylphthalide (Dl-NBP) is a characteristic component isolated from celery, which has been used as a traditional Chinese herbal medicine component in diseases. As a commonly used antioxidant, Wang et al. found that Dl-NBP treatment could suppress TXNIP-NLRP3 interaction and inhibit NLRP3 inflammasome activation via upregulating nuclear factor erythroid 2-related factor 2 (Nrf2), and ameliorate neuronal apoptosis in the APP/PS1 mouse brains [81].
Other herbal medicine
Echinachinol, a traditional herbal medicine licorice component, inhibits NLRP3 inflammasome by combining with heat shock protein 90 (HSP90) and attenuates the body inflammatory response; therefore, its use may be developed as a therapeutic approach for the treatment of NLRP3-driven diseases [82]. Significant downregulation of NLRP3 protein, Aβ deposition, and tau protein aggregation were detected in AD mice treated with traditional Chinese herbal medicine Shaoyao Gancao Decoction. While clues to alleviate neuroinflammatory reactions and spatial memory deficits in mice were also observed [83]. Several phytochemicals like morin (MOR), thymol (TML), and thymoquinone (TMQ) regulate Nrf2/HO-1, TLR4/NLRP3, APOE4/LRP1, and Wnt3/β-catenin/GSK-3β signaling pathways in an aluminum chloride (AlCl3)-induced AD rat model, which leads to a series of results including blocking the inflammasome activation, reducing the Aβ protein generation and decreasing tau protein phosphorylation [84]. Sodium Houttuyfonate (SH) ameliorates Aβ1-42-caused hippocampal neuronal damage, memory deficits, neuroinflammation, and pyrolysis by suppressing the NLRP3/GSDMD pathway in AD mice [85]. There are also some well-known traditional herbal ingredients like resveratrol, pterostilbene and rosemary, all of which have been proven to improve the neurological damage lesions in AD mice through various channels, and we anticipate these Chinese herbal medicines to exert their maximum clinical value [86–88].
Food extract
In recent years, a growing number of studies have found that various food extracts in daily life, including fruits and vegetables, are beneficial to AD within certain limits. Perhaps we can prevent the onset of neurodegenerative diseases in more mundane ways (Table 1).
The plant extract mangiferin (1,3,6,7-tetrahydroxyxanthone-2-β-D glucoside) is found to modulate microglia-macrophage polarization from pro-inflammatory to anti-inflammatory and also exert its effect on attenuating the neuroinflammation through inhibiting NF-κB/NLRP3 signaling pathway [89]. Extracts of Lion’s Mane Mushroom and Hericium erinaceus are found to attenuate oxidative and anti-inflammatory effects caused by damage to neurons and activated microglia [90]. When Lychee seed polyphenol was used to treat APP/PS1 mice, it is detected that the levels of NLRP3 inflammasome-related proteins and cytokines were significantly downregulated, hence Lychee seed polyphenol is regarded as a new drug to alleviate the pathological process of AD [91]. Soybean vegetable oil extract stigmasterol exhibits properties of reducing Aβ deposition in cerebral cortex and hippocampus, improving cognitive dysfunction and neuroinflammation in APP/PS1 mice, through activation of NF-κB and NLRP 3 signaling pathways via AMPK [92]. In vivo and in vitro studies, sulforaphane is found to be a more effective agent for AD treatment, which can activate Nrf2 to increase the expression of antioxidant enzymes and enhance its antioxidant capacity [93].
P2X7 receptor antagonist
In recent years, experts indicate that P2X7 receptor antagonists could represent a novel therapeutic approach for AD [94]. Some P2X7 receptor antagonists, such as BBG, can improve cognitive ability and dendritic spine development in Aβ42 injected rats, further exerting its neuroprotective and anti-inflammatory effects [95]. The amide GSK1370319A prevents neurodegeneration and the formation of cell death induced by inflammasomes [96]. Numerous novel P2X7R antagonists have recently been developed in the laboratory, which bodes well for their clinical application.
Other aspects
As an antioxidant, idebenone has been proved to up-regulate Nrf2 to exert neuroprotection effect and enhance mitochondrial function by inhibiting the secretion of NLRP3 inflammasome-related proteins and cytokines, which is a good choice for AD treatment [97]. Donepezil is a common acetylcholinesterase inhibitor. 5xFAD model mice and cell experiments have demonstrated that the drug dramatically downregulated the neuroinflammatory response induced by LPS and Aβ in vitro and in vivo.
Regarding the longevity gene Klotho, it was found that increased expression of Klotho in the brain and serum could enhance microglia-mediated Aβ plaque phagocytosis, significantly improve Aβ deposition and neurological function in APP/PS1 model mice [27]. Cui et al found that TMEM16F knockdown enhanced the expression of the M2 phenotype biomarkers, reduced the release of proinflammatory factors IL-1, IL-6, and TNF-α, and inhibited NLRP3 inflammasome activation through reducing downstream proinflammatory factors IL-1β and IL-18 [99].
Moreover, fresh coconut oil, RS, and progesterone, which antagonize the release of neurotoxic molecules induced by Aβ deposition, simultaneously reduce the expression of NLRP3 inflammatory component proteins, protecting the nervous system from toxic damage. Besides, autophagy inducers targeting microglial autophagy can also efficiently suppress the excessive activation of NLRP3 inflammasome and inflammatory response in neurodegenerative diseases [100].
SUMMARY AND OUTLOOK
Till now, AD remains a medical issue that is being investigated globally. Nevertheless, the clinical treatment of AD is still in the experimental phase, and no effective treatment strategy has yet been applied to clinical patients. Recently, research has focused on the contribution of the inflammasome in the pathological development of AD and understood that the NLRP3 inflammasome activation is closely linked to the pathological mechanism and progression of AD. The NLRP3 inflammasomes recognize PAMPs or DAMPs, and the activated NLRP3 inflammasomes induce the release of pro-inflammatory molecules, including IL-1β from microglia, which participate in the pathogenesis of AD by mediating Aβ and tau pathology and other aspects. Together with the studies on NLRP3 inflammasome and the ongoing exploration of its function and potential processes in AD, this makes targeted therapy against NLRP3 inflammasome a prospective treatment for AD. Until now, treatments targeting the NLRP3 inflammasome have shown promising therapeutic efficacy in vitro and in numerous mouse models of AD. However, several crucial concerns must be resolved to adapt these experimental data to clinical treatment, representing the largest challenge for future experimental investigations. In conclusion, we still need to investigate further and improve the mechanism of NLRP3 inflammasome about involving in the progression of AD and the influence of NLRP3 inflammasome inhibitors on the pathological process of AD.
Footnotes
ACKNOWLEDGMENTS
The authors have no acknowledgments to report.
FUNDING
This work was supported by the National Natural Science Foundation of China (Grant No. 82271483), the Shandong Province Natural Science Foundation of China (Grant No. ZR2019BH060, ZR2020MH150 and ZR2020MH149) and Shandong Medical and Health Science and Technology Development Plan project (Grant No.2019WS606). Support Program for Youth Innovation Technology in Colleges and Universities of Shandong Province of China (Grant No. 2019KJK004), Weifang Science and Technology Development Plan project (Grant No. 2022YX043).
CONFLICT OF INTEREST
The authors declare no conflict of interest.