Caspases in Cell Death, Inflammation, and Pyroptosis

Abstract

Caspases are a family of conserved cysteine proteases that play key roles in programmed cell death and inflammation. In multicellular organisms, caspases are activated via macromolecular signaling complexes that bring inactive procaspases together and promote their proximity-induced autoactivation and proteolytic processing. Activation of caspases ultimately results in programmed execution of cell death, and the nature of this cell death is determined by the specific caspases involved. Pioneering new research has unraveled distinct roles and cross talk of caspases in the regulation of programmed cell death, inflammation, and innate immune responses. In-depth understanding of these mechanisms is essential to foster the development of precise therapeutic targets to treat autoinflammatory disorders, infectious diseases, and cancer. This review focuses on mechanisms governing caspase activation and programmed cell death with special emphasis on the recent progress in caspase cross talk and caspase-driven gasdermin D-induced pyroptosis.

Keywords: PANoptosis; gasdermin; inflammasome; inflammatory caspase; innate immunity; pyroptosis.

Figure 1

Mechanism of caspase activation and execution of cell death. Caspases trigger activation of apoptosis, a noninflammatory form of cell death. In extrinsic apoptosis, death receptor activation facilitates recruitment of FADD and caspase-8 to form the death-inducing signaling complex (DISC). Activated caspase-8 in the DISC further processes executioner caspases (caspase-3, -7) to engage apoptosis. In intrinsic apoptosis, intracellular stress stimuli induce mitochondrial outer membrane permeabilization (MOMP). This leads to the release of cytochrome c into the cytosol. Binding of cytochrome c to APAF1 triggers assembly of the apoptosome complex, which facilitates activation of caspase-9. This in turn promotes activation of caspase-3 and -7 to execute apoptosis. Complex-I assembly is initiated by TNF binding to TNFR on the membrane. This promotes prosurvival signaling via receptor-interacting serine/threonine protein kinase 1 (RIPK1), TAK-TAB, and NEMO proteins. Loss of RIPK1 ubiquitination promotes formation of the intracellular ripoptosome complex. Inactivation of the catalytic activity of caspase-8 in the ripoptosome complex induces RIPK1-RIPK3 association and phosphorylation of MLKL to form pores on the membrane and engage necroptosis. ZBP1 also activates necroptosis via RIPK3. Activation of inflammatory caspases induces pyroptosis. Inflammasome assembly activates caspase-1 enzymatic function. ASC in the inflammasome complex recruits caspase-1. Some inflammasome-forming innate immune receptors recruit caspase-1 without the involvement of an ASC protein. ZBP1 activates the NLRP3 inflammasome in response to influenza infection. LPS binds to caspase-11 (or human caspase-4/-5) and triggers its activation through oligomerization and cleavage. Activation of caspase-1 and -11 proteolytically processes gasdermin D (GSDMD) to release the N-terminal domain, which forms pores on the membrane to further induce pyroptosis. Caspase-1 also cleaves pro-IL-1β and pro-IL-18 into IL-1β and IL-18, which are released through GSDMD pores. Caspase-3-mediated gasdermin E (GSDME) cleavage also drives pyroptosis.

Figure 2

Caspase-mediated processing of gasdermin proteins and execution of pyroptosis. (a) Gasdermin D (GSDMD) is cleaved by several caspases. Cleavage of GSDMD at D275 of the linker region by caspase-1, caspase-4/-5/-11, and caspase-8 or at C268 by ELANE liberates the functionally active N terminus of GSDMD. Caspase-3 cleavage of GSDMD at D87 in the N-terminal domain inactivates its pyroptotic function. (b) Caspase-3 cleaves gasdermin E (GSDME) (DFNA5) at D270 of the linker region to release its active N-terminal domain. (c) Inflammatory caspases cleave at the linker region connecting the N and C termini of GSDMD to facilitate the induction of pyroptosis. Cleavage of GSDMD liberates the pore-forming N-terminal domain from the autoinhibitory C-terminal domain. The N-terminal domain of GSDMD is transported to the membrane to assemble into a pore structure. The inner region of the pore is hollow with a large diameter that is sufficient to enable passage of small ions and proteins such as mature IL-1β and IL-18. The pore formation by GSDMD is essential for the induction of pyroptosis. IRF2 transcriptionally upregulates GSDMD expression. Once the GSDMD pore is formed on the cell membrane, it facilitates ion and water flux from the extracellular space into the cell. GSDMD pore formation, ion and water flux, and loss of cytoskeletal integrity facilitate plasma membrane rupture and pyroptosis of cells. Pyroptosis releases intracellular components, which are immunostimulatory and cause inflammation. Abbreviations: N-term, N-terminal domain; C-term, C-terminal domain.

Figure 3

Intracellular regulation of caspase function to modulate inflammatory cell death. cIAPs inhibit caspase-8-dependent cell death functions. Degradation of cIAPs activates caspase-8 in the ripoptosome, which promotes maturation and release of IL-1β and activation of gasdermin D (GSDMD)-independent inflammatory cell death. TAK1 is necessary to inhibit the spontaneous induction of pyroptosis in immune cells. The absence of TAK1 or Yersinia YopJ-mediated inhibition of TAK1 leads to the assembly of the receptor-interacting serine/threonine protein kinase (RIPK1)–FADD–caspase-8 complex. This complex induces GSDMD activation either directly or through the activation of NLRP3 inflammasome assembly to trigger pyroptosis. Caspase-11-induced GSDMD pore formation facilitates K⁺ efflux, which activates the assembly of the noncanonical NLRP3 inflammasome complex. The GSDMD pores mediate release of matured IL-1β and IL-18. Necrosulfonamide (NSA), disulfiram, and Bay 11–7082 directly bind to GSDMD and inhibit inflammatory cell death. N-acetyl-Phe-Leu-Thr-Asp-chloromethylketone (Ac-FLTD-CMK) is a GSDMD-derived peptide inhibitor that inhibits cleavage of GSDMD by inflammatory caspases. Canakinumab is a human IL-1β-specific antibody that inhibits activation of IL-1R signaling. Anakinra is an antagonist of IL-1R that counteracts IL-1 cytokine signaling. VX-740 (Pralnacasan) and VX-765 (Belnacasan) are therapeutic inhibitors of caspase-1.