Suppression of RIP3-dependent necroptosis by human cytomegalovirus
Shinya Omoto1, Hongyan Guo1, Ganesh R Talekar1
1From the Department of Microbiology and Immunology, Emory Vaccine Center, Emory University School of Medicine, Atlanta, Georgia 30322.
Abstract:
Necroptosis is an alternate programmed cell death pathway that is unleashed by caspase-8 compromise and mediated by receptor-interacting protein kinase 3 (RIP3). Murine cytomegalovirus (CMV) and herpes simplex virus (HSV) encode caspase-8 inhibitors that prevent apoptosis together with competitors of RIP homotypic interaction motif (RHIM)-dependent signal transduction to interrupt the necroptosis. Here, we show that pro-necrotic murine CMV M45 mutant virus drives virus-induced necroptosis during nonproductive infection of RIP3-expressing human fibroblasts, whereas WT virus does not. Thus, M45-encoded RHIM competitor, viral inhibitor of RIP activation, sustains viability of human cells like it is known to function in infected mouse cells. Importantly, human CMV is shown to block necroptosis induced by either TNF or M45 mutant murine CMV in RIP3-expressing human cells. Human CMV blocks TNF-induced necroptosis after RIP3 activation and phosphorylation of the mixed lineage kinase domain-like (MLKL) pseudokinase. An early, IE1-regulated viral gene product acts on a necroptosis step that follows MLKL phosphorylation prior to membrane leakage. This suppression strategy is distinct from RHIM signaling competition by murine CMV or HSV and interrupts an execution process that has not yet been fully elaborated.
Insights
Murine cytomegalovirus (CMV) M45 mutant induces necroptosis in human cells, but wild-type virus does not. Human CMV blocks this cell death pathway, distinct from other viruses, by targeting a later step.
Area of Science:
- Virology
- Immunology
- Cell Biology
Background:
- Necroptosis is programmed cell death triggered by caspase-8 inhibition and RIP3.
- Viruses like murine CMV (MCMV) and HSV encode proteins to evade apoptosis and necroptosis.
- Viral inhibitors of RIP activation (vIRAs) target RIP homotypic interaction motif (RHIM)-dependent signaling.
Purpose of the Study:
- To investigate the role of MCMV M45 in inducing necroptosis in human cells.
- To determine how human CMV (HCMV) inhibits virus-induced and TNF-induced necroptosis.
- To elucidate the mechanism by which HCMV suppresses necroptosis.
Main Methods:
- Infection of RIP3-expressing human fibroblasts with MCMV M45 mutant and wild-type viruses.
- Analysis of necroptosis induction and inhibition by MCMV and HCMV.
- Investigation of HCMV's effect on RIP3 activation, MLKL phosphorylation, and membrane leakage.
Main Results:
- MCMV M45 mutant virus drives necroptosis in human cells, while wild-type MCMV does not.
- The M45-encoded RHIM competitor sustains human cell viability.
- HCMV blocks TNF- and MCMV M45-induced necroptosis after RIP3 activation and MLKL phosphorylation.
- HCMV utilizes an early, IE1-regulated gene product to inhibit necroptosis before membrane leakage.
Conclusions:
- MCMV M45 is a key viral factor inducing necroptosis in human cells.
- HCMV employs a novel strategy to suppress necroptosis, distinct from MCMV and HSV.
- HCMV targets a late execution step of necroptosis, downstream of MLKL phosphorylation.
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