Functions of the viral attachment protein in reovirus neurovirulence

Abstract

Viral encephalitis is a serious and life-threatening inflammation of the central nervous system (CNS). However, mechanisms of viral neuroinvasion and disease pathogenesis in the CNS are often poorly understood. Mammalian orthoreoviruses spread systemically in newborn mice to cause serotype-specific disease in the central nervous system (CNS). Serotype 1 reoviruses (T1) infect ependymal cells to cause hydrocephalus, whereas serotype 3 reoviruses (T3) infect neurons to cause lethal encephalitis. These serotype-specific differences in tropism and subsequent disease are dictated by the viral S1 gene, which encodes the viral attachment protein, σ1. T1 and T3 σ1 viral attachment proteins are thought to engage different receptors on ependyma and neurons, however, the receptors used on these cell types have remained elusive. Nogo receptor 1 (NgR1) is a neuronally expressed receptor that was recently found to mediate reovirus infection in vitro. Using biochemical and structural techniques, we found that N-terminal NgR1 sequences bind the reovirus σ3 outer-capsid protein in a serotype-independent manner to promote infection. The function of NgR1 in serotype-dependent reovirus neuropathogenesis is not yet known, but these studies enhance our understanding of structural interactions between reovirus and NgR1. To identify the specific viral sequences that contribute to neuronal targeting, σ1-chimeric viruses were engineered that express the σ1 head, body, or tail domain of T1 reciprocally exchanged with coordinating sequences for T3, and vice versa. Mice were inoculated intracranially with wild-type or chimeric viruses and viral replication, tropism, and disease pathogenesis were monitored. Viruses expressing T1 σ1 head sequences target ependymal cells, replicate to relatively lower viral titers in the brain, and do not cause significant disease. In contrast, mice inoculated with viruses expressing T3 σ1 head sequences infect neurons, replicate to nearly 1,000-fold higher peak viral brain titers than T1 and succumb to a fulminant, lethal encephalitis. These results demonstrate that reovirus serotypes have selected two distinct mechanisms to use the σ1 head-domain to infect disparate sites in the CNS. Collectively, studies reported here improve an understanding of how receptors coordinate with viral proteins to promote infection and initiate disease.