Fig. 2.

A schematic diagram illustrating the canonical and non-canonical inflammasome signaling pathways in the brain during chronic cerebral hypoperfusion. In the canonical inflammasome pathway, two signals – priming and activation are involved. The first signal is the priming step whereby endogenous extracellular ligands (DAMPs) are able to bind onto its respective pattern recognition receptors (i.e. TLR, RAGE, IFN-γR, IL-1R) on neighbouring cells, activating several downstream regulatory pathways (i.e. NF-κB, MAPK, P53 and JAK-STAT), leading to increased gene expression of inflammasome components (i.e. receptors, adaptor and effector proteins) and both precursor IL-1β and IL-18 in the cytosol. Following priming, a second signal is required to activate the inflammasome receptor(s) to form a macromolecular platform that recruits the adaptor protein (i.e. ASC) and effector proteins (i.e. total caspase-1 and -8) to form a multi-protein complex termed an inflammasome. In the inflammasome complex, total caspase-1 and -8 undergo proximity-induced activation to form active cleaved caspase-1 and -8 that initiates several catalytic functions. First, cleaved caspase-1 and -8 induces mature cytokine production by cleaving precursors IL-1β and IL-18 into active mature IL-1β and IL-18 proinflammatory cytokines. Second, cleaved caspase-1 and -8 are able to initiate an inflammatory form of cell death by cleaving GSDMD-FL into GSDMD-NT. As more GSDMD-NTs are produced in the cytosol, these fragments self-oligomerize onto the plasma membrane to form a pore to facilitate the influx of water molecules to induce a lytic form of cell death known as pyroptosis. Third, cleaved caspase-1 and -8 can trigger apoptosis by cleaving total caspase-3 into active cleaved caspase-3. Moreover, active cleaved caspase-3 can also initiate another form of cell death known as secondary necrosis by cleaving GSDME-FL into GSDME-NT. Similar to GSDMD-NT, GSDME-NT can self-oligomerize to form pores on the plasma membrane; in addition to forming pores on the mitochondrial membrane, which results in cytochrome c release, further exacerbating apoptosis. In the non-canonical inflammasome pathway, total caspase-11 can be activated by binding to an endogenous ligand (i.e. OxPAPC) that allows oligomerization of total caspase-11. Such oligomerization initiates the proximity-induced activation of total caspase-11 to form active cleaved caspase-11. The non-canonical effector protein, cleaved caspase-11, can also directly cleave GSDMD-FL into GSDMD-NT to cause pore formation. It has been shown that K⁺ efflux resulting from pore formation can serve as an activation signal for canonical NLRP3 receptor activation, indicating cross-talk between the canonical and non-canonical inflammasome signalling pathways. Abbreviations: DAMPs, damage associated molecular patterns; HMGB1, high mobility group box protein 1; IL, interleukin; IFN, interferon; TLR, toll-like receptor; RAGE, receptor for advanced glycation end products; NF-κB, nuclear factor kappa-light-chain enhancer of activated B cells; MAPK, mitogen activated protein kinase; JAK/STAT, janus kinase-signal transducer and activator of transcription; Pre, precursor; GSDMD, gasdermin D; GSDME, gasdermin E