Fig. 4

A schematic diagram illustrating potential stimuli involved in inflammasome receptor activation during CCH. The precise molecular and cellular mechanisms of inflammasome receptor activation during CCH are unknown. However, relevant studies suggest several plausible mechanisms including - decreased intracellular K⁺ concentration, increased intracellular Ca²⁺ concentration, ROS production, DNA fragmentation and oxidized mitochondrial DNA. During CCH, lower cerebral blood flow reduces ATP production, and impairs ATP-dependent transporters such as the Na⁺/K⁺-ATPase pump, leading to K⁺ accumulation in the extracellular space. Alternatively, ATP released by damaged cells can bind to the P2X4 and P2X7 receptors on neighbouring cells, leading to the receptor opening and efflux of K⁺. In addition, damaged cells can also passively release K⁺ into the extracellular environment. Extracellular K⁺ can activate Pannexin-1 channels on the plasma membrane through a mechanism independent of the membrane potential. This further promotes the release of ATP into the extracellular space, creating a positive feedback loop for K⁺ efflux. Consequently, the accumulation of extracellular K⁺ and decrease in intracellular K⁺ levels can activate the NLRP3 receptor by inducing a conformational change that promotes oligomerization. During CCH, severely damaged necrotic cells may also release Ca²⁺ into the extracellular space, activating calcium-sensing receptors (CaSRs) on neighbouring cells. Activated CaSRs inhibit the activity of adenylate cyclase, reducing the conversion of ATP to cAMP. As cAMP is an inhibitor for NLRP3, a reduction in cAMP levels in the cytosol can promote NLRP3 inflammasome activity. Ca²⁺ can also promote inflammasome activation through the TRPM2 Ca²⁺ channel during CCH. As Ca²⁺ enters the cell via the TRPM2 channel, it enables protein kinase R (PKR) in the cytoplasm to phosphorylate NLRP1 and NLRP3 receptors resulting in inflammasome activation. CCH also caused a substantial degree of oxidative stress and the production of ROS in the cell. ROS can interact with the TXNIP-TRX complex to release TXNIP from TRX, allowing it to bind to the NLRP3 receptor for subsequent inflammasome activation. CCH induces AIM2 inflammasome activation via the production and release of fragmented dsDNA. Severely damaged cells and mitochondria are the source of fragmented dsDNA during CCH. While intracellular mitochondrial dsDNA interacts directly with the AIM2 receptor in the cytosol, extracellular dsDNA enters the cell via the facilitation of RAGE. When RAGE detects the presence of dsDNA in extracellular space, it promotes endosomal DNA uptake of the cell. The dsDNA will then bind onto the HIN-domain of the AIM2 receptor, releasing the receptor from its autoinhibitory state. This allows the AIM2 receptor to oligomerize and initiate inflammasome activation. Abbreviations: CCH, chronic cerebral hypoperfusion; ROS, reactive oxygen species; ATP, adenosine triphosphate; P2X4, P2X purinoceptor 4; P2X7, P2X purinoceptor 7; NLR, nucleotide-binding oligomerization domain-like receptor; NLRP1, NLR family pyrin domain containing 1; NLRP3, NLR family pyrin domain containing 3; cAMP, cyclic adenosine monophosphate; TRPM2, transient receptor potential melastatin 2; TXNIP, thioredoxin-interacting protein; TRX, thioredoxin; AIM2, absent in melanoma 2; HIN200, hematopoietic interferon-inducible nuclear proteins; dsDNA, double-stranded DNA; RAGE, receptor for advanced glycation end-products