Reovirus apoptosis and virulence are regulated by host cell membrane penetration efficiency

Apoptosis plays an important role in the pathogenesis of reovirus encephalitis and myocarditis in infected animals. Differences in apoptosis efficiency displayed by reovirus strains are linked to the viral mu 1-encoding M2 gene segment. Studies using pharmacologic inhibitors of reovirus replication demonstrate that apoptosis induction by reovirus requires viral disassembly in cellular endosomes but not RNA synthesis. Since the mu 1 protein functions to pierce endosomal membranes during this temporal window, these findings point to an important role for mu 1 in activating signaling pathways that lead to apoptosis. To understand mechanisms used by mu 1 to induce apoptosis, a panel of mu 1 mutant viruses generated by reverse genetics was analyzed for the capacities to penetrate host cell membranes, activate proapoptotic signaling pathways, evoke cell death, and produce encephalitis in newborn mice. We found that single amino acid changes within the delta region of mu 1 reduce the efficiency of membrane penetration. These mutations also diminish the capacities of reovirus to activate proapoptotic transcription factors NF-kappa B and IRF-3 and elicit apoptosis. Additionally, we observed that following intracranial inoculation, an apoptosis-deficient mu 1 mutant is less virulent in newborn mice in comparison to the wild-type virus. These results indicate a critical function for the membrane penetration activity of mu 1 in evoking prodeath signaling pathways that regulate reovirus pathogenesis.