Reovirus Activates Transforming Growth Factor β and Bone Morphogenetic Protein Signaling Pathways in the Central Nervous System That Contribute to Neuronal Survival following Infection

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Abstract
Viral infections of the central nervous system (CNS) are important causes of worldwide morbidity and mortality, and understanding how viruses perturb host cell signaling pathways will facilitate identification of novel antiviral therapies. We now show that reovirus infection activates transforming growth factor β (TGF-β) and bone morphogenetic protein (BMP) signaling in a murine model of encephalitis in vivo. TGF-β receptor I (TGF-βRI) expression is increased and its downstream signaling factor, SMAD3, is activated in the brains of reovirus-infected mice. TGF-β signaling is neuroprotective, as inhibition with a TGF-βRI inhibitor increases death of infected neurons. Similarly, BMP receptor I expression is increased and its downstream signaling factor, SMAD1, is activated in reovirus-infected neurons in the brains of infected mice in vivo. Activated SMAD1 and SMAD3 were both detected in regions of brain infected by reovirus, but activated SMAD1 was found predominantly in uninfected neurons in close proximity to infected neurons. Treatment of reovirus-infected primary mouse cortical neurons with a BMP agonist reduced apoptosis. These data provide the first evidence for the activation of TGF-β and BMP signaling pathways following neurotropic viral infection and suggest that these signaling pathways normally function as part of the host's protective innate immune response against CNS viral infection.