| Host cell death plays an important role in the pathogenesis of viral disease. Reovirus induces apoptotic cell death in cultured cells and in vivo and is a versatile experimental system for investigating virus-induced death signaling. Activation of host transcription factor, NF-kappaB, is required for cell death following reovirus infection. We found that the caspase-8 dependent cleavage of Bid, a proapoptotic BH3-only member of the Bcl-2 family, occurs downstream of NF-kappaB and functions to link the extrinsic and intrinsic apoptotic pathways. Our studies demonstrated a requirement for Bid not only in reovirus-induced cell death but also for efficient viral replication in the mouse central nervous system. We discovered that in some cell types, inhibition of NF-kappaB or caspases does not diminish cell death following reovirus infection. Instead, a reduction of reovirus-induced cell death occurred in the presence of an inhibitor to the kinase activity of receptor interacting protein 1 (RIP1) or knockdown of RIP3. Based on RIP1 and RIP3 requirements as well as leaking nuclear and plasma membranes, we conclude that reovirus is also capable of inducing an alternate form of cell death described as necroptosis, a regulated form of necrosis. Induction of necroptosis by reovirus requires synthesis of viral RNA, distinct from the sufficient function of the viral capsid to evoke apoptosis. Using mutant viruses and inhibitors, we have determined the extent of cell death correlates directly with the amount of viral RNA produced in the cells. Correspondingly, we found that diminishment of the levels of RIG-I and MDA5, two cellular sensors of viral RNA, reduced necroptosis. Our work indicates that distinct cell death pathways are induced following reovirus infection by sensing of proteinaceous or nucleic acid components. This ability to induce cell death by two pathways may be a fail-safe for the host to ensure viral replication is limited. |
