The Role of NLRP3 Inflammasome in Cerebrovascular Diseases Pathology and Possible Therapeutic Targets

Abstract

Cerebrovascular diseases are pathological conditions involving impaired blood flow in the brain, primarily including ischaemic stroke, intracranial haemorrhage, and subarachnoid haemorrhage. The nucleotide-binding and oligomerisation (NOD) domain-like receptor (NLR) family pyrin domain (PYD)-containing 3 (NLRP3) inflammasome is a protein complex and a vital component of the immune system. Emerging evidence has indicated that the NLRP3 inflammasome plays an important role in cerebrovascular diseases. The function of the NLRP3 inflammasome in the pathogenesis of cerebrovascular diseases remains an interesting field of research. In this review, we first summarised the pathological mechanism of cerebrovascular diseases and the pathological mechanism of the NLRP3 inflammasome in aggravating atherosclerosis and cerebrovascular diseases. Second, we outlined signalling pathways through which the NLRP3 inflammasome participates in aggravating or mitigating cerebrovascular diseases. Reactive oxygen species (ROS)/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), ROS/thioredoxin-interacting protein (TXNIP) and purinergic receptor-7 (P2X7R) signalling pathways can activate the NLRP3 inflammasome; activation of the NLRP3 inflammasome can aggravate cerebrovascular diseases by mediating apoptosis and pyroptosis. Autophagy/mitochondrial autophagy, nuclear factor E2-related factor-2 (Nrf2), interferon (IFN)-β, sirtuin (SIRT), and phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) reportedly alleviate cerebrovascular diseases by inhibiting NLRP3 inflammasome activation. Finally, we explored specific inhibitors of the NLRP3 inflammasome based on the two-step activation of the NLRP3 inflammasome, which can be developed as new drugs to treat cerebrovascular diseases.

Keywords: NLRP3 inflammasome; atherosclerosis; intracerebral haemorrhage; ischaemia stroke; subarachnoid haemorrhage.

Figure 1.

Structure of NLRs and NLRP3 Inflammasome. NLRs consist of three components, the C-terminal LRRs, the central NACHT, which consists of NBD, HD1, WHD, and HD2, and the N-terminal domain. The N-terminal domain can be Pyrin, CARD, or BIR. When the N-terminal part is CARD, it is called NLRC inflammasome, and when the N-terminal part is Pyrin, it is called NLRP inflammasome. The NLRP3 inflammasome is composed of three parts, the first part is NLRP3 composed of LRR-NACHT-Pyrin/PYD, the second part is PYCARD composed of PYD-CARD, also known as ASC, and the third part is pro-caspase-1 composed of CARD-Caspase. These three parts form NLRP3 inflammasome through the interaction of PYD-PYD and CARD-CARD. NEK7 is a newly discovered inflammasome component of NLRP3 in recent years, which is related to ROS-induced priming. Part of NEK7 binds to LRRs of NLRP3, and part of NEK7 binds to NBD and HD2 of NACHT. Abbreviations: NLRs: NOD domain like receptor, LRRs: leucine-rich repeats, NACHT: nucleotide binding and oligomerization domain, NBD: nucleotide-binding domain, HD1: helical domain 1, WHD: winged helix domain, HD2: helical domain 2, CARD: caspase activation and recruitment domain, BIR: baculoviral inhibitor of apoptosis protein repeat, NLRC: NOD-like receptor subfamily C, NLRP: NOD domain like receptor family contain pyrin domain, PYD: pyrin domain, PYCARD/ASC: apoptosis-associated speck-like protein containing a CARD, NEK7: NIMA-related kinase 7.

Figure 2.

Main Mechanism of NLRP3 Inflammasome in Development of Atherosclerosis. The phagocytosis of ox-LDL generates ROS through the cathepsin B pathway to activate the NLRP3 inflammasome. Activation of the NLRP3 inflammasome accelerates neutrophil and macrophage recruitment, increases the susceptibility of macrophages to lipid deposition, promotes foam cell formation, induces macrophages secreting IL-1β, and impairs plaque stability. Rupture of atherosclerotic plaques can cause stroke. Ox-LDL also upregulates the expression of the pro-IL-1β. IL-1β inhibits cholesterol efflux through a negative feedback, results in accumulation of intracellular cholesterol and foam cell formation. CD36 converts intracellular soluble ligands into crystals or fibrils, while UDCA increases cholesterol solubility, decreases cholesterol crystals-depositions, and inhibits NLRP3 inflammasome dependent inflammation. HDL play a protective role in atherosclerosis by suppressing monocyte cell recruitment and IL-1β secretion. Abbreviations: ox-LDL: oxidized low-density lipoprotein, ROS: reactive oxygen species, UDCA: ursodeoxycholic acid, HDL: high-density lipoproteins.

Figure 3.

The Mechanism Underlying NLRP3 Inflammasome Aggravation of Cerebrovascular Diseases. NLRP3 inflammasomes are activated after brain injury. Activation of NLRP3 inflammasome aggravates brain edema by increasing inflammatory cell infiltration, destroying tight junction proteins, and enhancing permeability of brain microvessel endothelial cell. Inhibition of NLRP3 inflammasome with inhibitors or RNAi reduces proinflammatory cytokines levels, promotes the transformation of M1-type microglia cells to M2-type, inhibits the aggregation and activation of inflammatory cells, and reduces myeloperoxidase level. M2-type microglia have anti-inflammatory effects, and the level of anti-inflammatory cytokines is up-regulated after phenotypic transformation of microglia. Inhibition of NLRP3 inflammasome also increases tight junction proteins expression, reduces cerebral edema and blood-brain barrier permeability, reduces neuronal degeneration and apoptosis, and reduces microthrombosis. All of these lead to improvement of neurological functions.

Figure 4.

Signalling Pathways of NLRP3 Inflammasome Activation in Cerebrovascular Diseases. Hypoxia and membrane damage induce mtROS production. ROS leads to NLRP3 inflammasome activation and IL-1β secretion through inducing the binding of TXNIP to NLRP3. ROS also activates NLRP3 inflammasome via NF-κB and apoptosis. The mitophagy/autophagy system removes mtROS and suppresses NLRP3 inflammasome activation. NF-κB regulates both pyroptosis and oxidative stress, it also influences macrophage differentiation into M1 or M2 cells. IFN-β inhibits NLRP3 in three pathways: (A) represses the activity of the NLRP3 inflammasome via the signal transducer and activator of transcription (STAT)1 transcription factor; (B) induces IL-10 in a STAT1-dependent manner, while IL-10 reduces pro-IL-1β production via STAT3 signalling; (C) suppresses ROS generation in antigen presenting cells. The opening of P2X7R allows potassium ions to efflux from cells and sodium and calcium ions to influx into cells. The NLRP3 inflammasome is activated when the intracellular potassium ion level is below the threshold of 90 Mm. ATP leads to neuroinflammation through activating the P2X7R/cryopyrin inflammasome axis. Ox-LDL upregulates the expression levels of P2X7R, NLRP3 inflammasome and IL-1β via phosphorylation of protein kinase R. Abbreviations: mtROS: mitochondrial ROS, ROS: reactive oxygen species, TXNIP: thioredoxin-interacting protein, IFN-β: interferon-β, STAT: signal transducer and activator of transcription, P2X7R: purinergic receptor-7.