Recent Insights on Inflammasomes, Gasdermin Pores, and Pyroptosis

Abstract

Inflammasomes assemble in the cytosol of myeloid and epithelial cells on sensing of cellular stress and pathogen-associated molecular patterns and serve as scaffolds for recruitment and activation of inflammatory caspases. Inflammasomes play beneficial roles in host and immune responses against diverse pathogens but may also promote inflammatory tissue damage if uncontrolled. Gasdermin D (GSDMD) is a recently identified substrate of murine caspase-1 and caspase-11, and human caspases-1, -4, and -5 that mediates a regulated lytic cell death mode termed pyroptosis. Recent studies have identified pyroptosis as a critical inflammasome effector mechanism that controls inflammasome-dependent cytokine secretion and contributes to antimicrobial defense and inflammasome-mediated autoinflammatory diseases. Here, we review recent developments on inflammasome-associated effector functions with an emphasis on the emerging roles of gasdermin pores and pyroptosis.

Figure 1.

Inflammasome platforms activate caspase-1, -11, and -8 for multiple downstream effector functions. Intracellular lipopolysaccharide (LPS) precipitates the oligomerization of caspase-11, in the so-called noncanonical inflammasome (right), and its automaturation. Cytosolic presence of danger- and pathogen-associated molecular patterns (DAMPs or PAMPs) can also trigger the canonical inflammasome pathway, which recruits procaspase-1 for proximity-induced autoactivation (left). Caspase-1 and caspase-11 cleave the common substrate, gasdermin D (GSDMD), releasing an amino-terminal fragment (GSDMD_N), which initiates cell death by pyroptosis. This lytic cell death mode is associated with alarmin release to the extracellular space and discharge of tissue factor–containing microvesicles. Inflammasomes also mediate the activation and secretion of the proinflammatory cytokines interleukin (IL)-1β and IL-18, either through caspase-1-directed cleavage of the procytokines or through indirect NLRP3 activation. At the canonical inflammasome, ASC also recruits FADD and caspase-8, through a still-undefined mechanism. Caspase-8 activation at inflammasomes leads to apoptosis in caspase-1-deficient cells.

Figure 2.

The gasdermin family encompasses pore-forming proteins. All gasdermin family members are composed of a pore-forming amino-terminal domain, which is kept autoinhibited by the carboxy-terminal domain, both connected by an unstructured flexible linker. Caspase-1 (Casp1), caspase-8 (Casp8), caspase-11 (Casp11), and neutrophil elastase (NE) cleave gasdermin D (GSDMD) in the linker region (the caspase recognition sequences are marked in red), releasing the pore forming amino-terminal fragment (GSDMD_N). GSDMD_N is able to perforate liposomes containing complex mixtures, which include phosphatidylserine (PS), phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and cardiolipin-containing preparations. Conversely, caspase-3 (Casp3) cleavage of deafness-associated tumor suppressor/gasdermin E (DFNA5/GSDME) in the linker region releases an amino-terminal fragment that perforates liposomes containing PI(4,5)P₂. Caspase-3 (Casp3) can also cleave GSDMD, generating an amino-terminal fragment that is unable to perforate plasma membranes. Although amino-terminal fragments of human gasdermin A (GSDMA) and murine gasdermin A3 (GSDMA3) forms pores in lipososomes containing cardiolipin and/or PI(4,5)P₂, their activation mechanism remains unknown. It is also unclear how gasdermin B (GSDMB) and gasdermin C (GSDMC) could be activated and, although overexpression of their amino-terminal fragments triggers a lytic form of cell death, pore formation by these proteins has not yet been confirmed.

Figure 3.

Amino-terminal gasdermin D (GSDMD) determines several morphological and biochemical features during pyroptosis. Presence of intracellular danger- and pathogen-associated molecular patterns (DAMPs and PAMPs) initiates signaling through canonical and noncanonical inflammasomes. Both caspase-1 and caspase-11 direct the cleavage of GSDMD, releasing its pore-forming amino-terminal fragment (GSDMD_N). Evidence suggests that GSDMD_N inserts into the cellular plasma membrane in lower magnitude structures and assembles the pore through oligomerization within membranes until a higher magnitude final pore is formed. Finally, pyroptotic cellular swelling and plasma membrane rupture release soluble intracellular contents, including the proinflammatory cytokines IL-1β and IL-18, matured by caspase-1. During the noncanonical signaling, pyroptotic-mediated K⁺ efflux causes cell-intrinsic NLRP3 inflammasome activation, accounting for cytokine maturation. GSDMD_N also damages mitochondria during execution of cell death, and might be responsible for the organellar damage seen in lysosomes and nuclei. The pyroptotic cell corpse further maintains intracellular pathogens, in the pore-induced intracellular trap (PIT), facilitating their clearance by infiltrating immune cells.