Viral infection and the evolution of caspase 8-regulated apoptotic and necrotic death pathways

Abstract

Pathogens specifically target both the caspase 8-dependent apoptotic cell death pathway and the necrotic cell death pathway that is dependent on receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3). The fundamental co-regulation of these two cell death pathways emerged when the midgestational death of mice deficient in FAS-associated death domain protein (FADD) or caspase 8 was reversed by elimination of RIP1 or RIP3, indicating a far more entwined relationship than previously appreciated. Thus, mammals require caspase 8 activity during embryogenesis to suppress the kinases RIP1 and RIP3 as part of the dialogue between two distinct cell death processes that together fulfil reinforcing roles in the host defence against intracellular pathogens such as herpesviruses.

Figure 1. Caspase 8-mediated regulation of RIP1–RIP3 signalling pathways

a ∣ Following tumour necrosis factor (TNF) binding, TNF receptor 1 (TNFR1) recruits receptor-interacting protein 1 (RIP1) via its death domain. When polyubiquitylated by the E3 ligases cellular inhibitor of apoptosis 1 (cIAP1), cIAP2 and linear ubiquitin chain assembly complex (LUBAC), RIP1 promotes the activation of nuclear factor-κB (NF- κB), which enhances cell survival by inducing the expression of cIAP1, cIAP2 and cellular FLICE-like inhibitory protein (cFLIP) (not shown). In the absence of RIP1 polyubiquitylation (for example, owing to insufficient cIAP levels or deubiquitylation by proteins such as cylindromatosis (CYLD)), RIP1 kinase activity downstream of TNFR1 facilitates the assembly of alternative signalling platforms comprising RIP1, FAS-associated death domain protein (FADD) and caspase 8. In an alternative scenario, the Toll-like receptor 3 (TLR3) and TLR4 adaptor protein TIR domain-containing adaptor protein inducing IFNβ (TRIF) binds to both RIP1 and RIP3 via RIP homotypic interaction motif (RHIM)-mediated interactions, bridging TLR3 signalling to the RIP1–FADD–caspase 8 complex (ripoptosome). Genotoxic damage or cell stress leads to the degradation of cIAP1 and cIAP2 and the formation of a ripoptosome. The cytosolic DNA sensor DAI (DNA-dependent activator of interferon regulatory factors) directly engages RIP1 and RIP3 in RHIM-dependent complexes to potentially drive the assembly and/or recruitment of a ripoptosome, in a similar manner to TRIF. The levels of cFLIP, the balance of cFLIP isoforms and the levels of caspase 8 activity determine whether apoptosis, necroptosis or cell survival ensues following the formation of the ripoptosome. b ∣ When the levels of cFLIP long (cFLIP_L) or cFLIP short (cFLIP_S) are limiting, caspase 8 homodimerization promotes enzymatic activity, and this leads to caspase 8 autoprocessing and the execution of apoptosis through BID and/or caspase 3. c,d ∣ In the presence of sufficient cFLIP_L or cFLIP_S, a caspase 8–cFLIP_L or caspase 8–cFLIP_S heterodimer forms, and this supports cell survival. Importantly, the caspase 8–cFLIP_L heterodimer retains sufficient proteolytic activity to cleave substrates such as RIP1 and CYLD to prevent necroptosis without allowing caspase 8 to induce apoptosis. e ∣ T cell survival and proliferation following stimulation of the T cell receptor (TCR) and CD28 requires the inactivation of RIP1- and RIP3-mediated necroptosis by FADD–caspase 8–cFLIP_L, and this probably occurs downstream of the CARMA1–BCL-10–MALT1 signalling complex. f ∣ When caspase 8 activity is blocked — under conditions of elevated cFLIP_S levels or in the presence of a caspase 8 inhibitor — RIP1 binds to RIP3 to form a kinase-active necrosome to initiate necroptosis. RIP1 kinase activity drives the assembly of the cytosolic RIP1–FADD–caspase 8 signalling platform, as well as the necrosome. The RIP1 kinase inhibitor necrostatin 1 (Nec1) blocks both RIP1-dependent apoptosis and necroptosis. Stars indicate catalytically active caspase 8. DED, death effector domain; dsDNA, double-stranded DNA; dsRNA, double-stranded RNA; PKCθ, protein kinase Cθ; TRADD, TNFR1-associated death domain protein; TRAF2, TNFR-associated factor 2; Ub, ubiquitin.

Figure 2. Viral modulation of cell death signals mediated by caspase 8 activation and RIP1–RIP3 pathways

a ∣ Viral proteins that directly target receptor-interacting protein 1 (RIP1), FAS-associated death domain protein (FADD) and/or caspase 8 suppress signalling pathways that are activated by death receptors, pattern recognition receptors (PRRs; such as Toll-like receptor 3 (TLR3)) or cell stress (such as DNA damage). Many virus-encoded proteins block caspase 8-dependent apoptosis by interfering with caspase 8, FADD and/or RIP1 (see also TABLE 1). Several poxvirus proteins of the serpin family bind directly to fully processed caspase 8 to prevent apoptosis, whereas other viral inhibitors of caspase 8 — such as vICA (viral inhibitor of caspase 8 activation), CrmA and B13R (all highlighted in blue) — sensitize cells to death receptor-induced necroptosis by disrupting caspase 8-mediated suppression of RIP1–RIP3 activity. A subset of viral FLICE-like inhibitory proteins (vFLIPs) — including MC159, E8 and the murine cytomegalovirus (MCMV) protein vIRA (viral inhibitor of RIP activation) — block both apoptosis and RIP1- and RIP3-mediated necrosis. b ∣ The MCMV protein vICA prevents caspase 8 activation, and this sensitizes cells to death receptor-induced necroptosis. In addition, the MCMV protein vIRA blocks necroptosis and MCMV-induced programmed necrosis by inhibiting RHIM (RIP homotypic interaction motif)-dependent interactions. vIRA also inhibits the activation of caspase 8 by RIP1 or TIR domain-containing adaptor protein inducing IFNβ (TRIF). c ∣ A mutant MCMV that encodes vIRA with a mutant RHIM domain triggers programmed necrosis in cells with sufficient levels of RIP3. MCMV-induced programmed necrosis is independent of RIP1. The cellular RHIM-containing cytosolic sensor of double-stranded DNA DAI (DNA-dependent activator of interferon regulatory factors) may promote RIP3-dependent programmed necrosis during infection with mutant MCMV. Stars indicate catalytically active caspase 8. BHV-4, bovine herpesvirus 4; DED, death effector domain; EHV-1, equine herpesvirus 1; HCMV, human cytomegalovirus; HPV-16, human papillomavirus 16; HSV, herpes simplex virus; HVS, herpesvirus saimiri; KSHV, Kaposi’s sarcoma-associated herpesvirus; MCV, molluscum contagiosum virus.