Subversion of Programed Cell Death by Poxviruses

Abstract

Poxviruses have been long regarded as potent inhibitors of apoptotic cell death. More recently, they have been shown to inhibit necroptotic cell death through two distinct strategies. These strategies involve either blocking virus sensing by the host pattern recognition receptor, ZBP1 (also called DAI) or by influencing receptor interacting protein kinase (RIPK)3 signal transduction by inhibition of activation of the executioner of necroptosis, mixed lineage kinase-like protein (MLKL). Vaccinia virus E3 specifically blocks ZBP1 → RIPK3 → MLKL necroptosis, leaving virus-infected cells susceptible to the TNF death-receptor signaling (e.g., TNFR1 → FADD → RIPK1 → RIPK3 → MLKL), and, potentially, TLR3 → TRIF → RIPK3 → MLKL necroptosis. While E3 restriction of necroptosis appears to be common to many poxviruses that infect vertebrate hosts, another modulatory strategy not observed in vaccinia or variola virus manifests through subversion of MLKL activation. Recently described viral mimics of MLKL in other chordopoxviruses inhibit all three modes of necroptotic cell death. As with inhibition of apoptosis, the evolution of potentially redundant viral mechanisms to inhibit programmed necroptotic cell death emphasizes the importance of this pathway in the arms race between pathogens and their hosts.

Keywords: Apoptosis; E3L; Interferon; Necroptosis; Poxviruses; Programed cell death; Z-nucleic acid-binding protein.

Figure. 1.

Inhibition of caspase-dependent cell death by VACV proteins. The apoptosome that contains pro-caspase-9 is formed in response to cytochrome c signal from the mitochondrion and subsequently activates the executioner caspases 3 and 7. Activation of the BH3 such as Bid and Bad proteins leads to a conformational change in the effectors Bax and Bak ultimately resulting in mitochondrial membrane permeability. VACV protein F1 blocks this by blocking Bim, Bak and caspase-9. The VACV N1 bocks the intrinsic death by blocking Bad and Bid. Intrinsic death can also be mediated by the mitochondrion independent of the BH3 proteins due to environmental changes such as increases in cytoplasmic Ca²⁺ release from the ER and Golgi apparatus. These changes lead to Bax/Bak activation. Some VACV strains regulate this by expressing a channel-like protein vGAAP that depletes intracellular stores of Ca²⁺. Extrinsic signals also trigger the activation of caspase-3 and −7 via the death receptor signaling leading to the activation of caspase-8 . VACV proteins B13 and B22 restrict caspase-8 by acting as a pan caspase inhibitor. Viral infections can also activate inflammasomes and activation of caspase-1. VACV can block pyroptosis by B13 that acts as a pan-caspase inhibitor and counter the action of caspase-1. F1 can also inhibit the the NLRP1 inflammasome blocking the upstream activation of caspase-1.

Figure 2.

Inhibition of necroptosis cell death by poxvirus proteins. Necroptosis is known to occur following extracellular signaling through death domain containing receptors or by intracellular sensing or PAMPS or DAMPS. Activation of either extracellular and intracellular necroptosis pathways restrict VACV. Under normal conditions necroptosis is kept in check by caspsase-8 cleavage of RIPK3. Cytokine-mediated necroptosis occurs in VACV infected cells that express the pan-caspase inhibitor B13. Signaling then leads to activation of RIPK1 which acts as a RHIM containing adaptor for the downstream activation of RIPK3 and subsequent activation of the necroptosis executer, MLKL. This activation then leads to oligomerization and transmigration of MLKL to the membrane forming pores. Necroptosis can also be unleashed by the pathogen sensor ZBP1. VACV transcription leads to the accumulation of cytoplasmic Z-RNA that is sensed by ZBP1. The virulence factor, E3 blocks this sensing when the VACV-E3Zα is present. In the absence of the VACV-E3Zα (E3ΔZα) exposes the Z-RNA. Sensing by ZBP1 results in the activation of RIPK3 and MLKL. Some poxvirus strains encode a truncated viral homologue of the host MLKL that competitively blocks that activation of MLKL and subsequently inhibits death.