Roles of Caspases in Necrotic Cell Death

Abstract

Caspases were originally identified as important mediators of inflammatory response and apoptosis. Recent discoveries, however, have unveiled their roles in mediating and suppressing two regulated forms of necrotic cell death, termed pyroptosis and necroptosis, respectively. These recent advances have significantly expanded our understanding of the roles of caspases in regulating development, adult homeostasis, and host defense response.

Figure 1. Pyroptosis induction by noncanonical and canonical inflammasomes

Toll-like receptors (TLRs) and/or interferons (IFNs)-mediated priming upregulate the expression of guanylate binding proteins (GBPs) critical for bacterial vacuole lysis and/or pattern-associated molecular patterns (PAMPs, including LPS and bacterial DNA) exposure, sensors (NLRP3, caspase-11) and cytokine precursor (pro-IL-1β). Of note, MyD88 and TRIF-dependent signaling downstream of TLRs can alternatively prime NLRP3 in a transcription-independent manner. Upon detection of their respective agonists, NLRP3, NLRC4, NLRP1b, AIM2 and pyrin assemble canonical inflammasomes containing adaptor ASC and leading to the activation of caspase-1 that controls the maturation of pro-cytokines (pro-IL-1β and pro-IL-18) and pyroptosis through the cleavage of GSDMD among other mechanisms. LPS of some Gram-negative bacteria enter the cytosol through outer membrane vesicles (OMVs) or at the surface of cytosolic bacteria. Cytosolic LPS binds caspase-11 and triggers the oligomerization of the noncanonical inflammasome; Caspase-11 controls pyroptosis through the cleavage of GSDMD and pannexin-1. Pannexin-1-mediated release of ATP triggers the opening of P2X7-dependent pores, while GSDMD N-terminal fragments directly assemble to form pores. Membrane disruption leads to pyroptosis lytic cell death releasing alarmins and proinflammatory cytokines. It should be noted that IL-1β secretion can occur independently of cell lysis. Non-canonical inflammasome also controls caspase-1 activation mediated by the NLRP3 canonical inflammasome through pannexin-1 and GSDMD cleavages in a cell intrinsic manner involving K⁺ efflux. (Green: priming; blue: activation; purple: effector mechanisms.)

Figure 2. Caspase-8 regulates cell survival, apoptosis, and necroptosis

TNF ligation with the TNFR1 receptor induces pro-caspase-8 recruitment to the activated TNFR1 signaling complex-I via FADD. Pro-caspase-8 cleavage and activation results in an active caspase-8, which limits complex-I activity by cleaving RIPK1. c-FLIP_S inhibits caspase-8 activity, whereas c-FLIP_L can partially promote it. In complex-I, RIPK1 is ubiquitinated by cIAP1/2, TRAF2 and the LUBAC complex, resulting in K63-linked and M1-linked chains, respectively. IKKα/β/NEMO and TAK1/TAB2/3 kinase complexes are recruited to ubiquitinated RIPK1 and activated, inducing degradation of IκB and subsequent NFκB pathway activation, which results in expression of cytokines and A20. The latter inhibits RIPK1 and forms a negative feedback. RIPK1 is deubiquitinated by CYLD and A20 in complex-I to promote formation of complex-II. Caspase-8 cleaves CYLD and RIPK1 to inhibit complex-II formation. Canonical complex-IIa is formed independent of RIPK1 kinase activity, where FADD and caspase-8 are recruited to RIPK1. RIPK1-dependent apoptosis (RDA) complex-IIa forms downstream of RIPK1 kinase activity and is negatively influenced by IKKα/β/NEMO and TAK1/TAB2/3 kinase complexes. Both the canonical and RDA complex-IIa promote activation of caspase-8/caspase3/7 cascade and execution of apoptotic cell death. Under the conditions of caspase-8 inhibition, complex-IIb/necrosome is formed, to which RIPK3 is recruited downstream of RIPK1 kinase activity. Complex-IIb/necrosome results in phosphorylation of MLKL and execution of necrotic cell death. RIPK1 inhibitor Nec-1 inhibits both RDA complex-IIa and complex-IIb/necrosome. Activated RIPK1 is marked with *.