Mechanisms of neuronal injury during virus infection of the CNS

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Foodborne and waterborne picornaviruses such as enterovirus 71 are a global health issue. Neurologic complications associated with neurovirulent non-polio picornavirus infection are a serious ongoing health problem, especially in children. Unfortunately, the mechanisms of picornavirus-induced injury to the central nervous system (CNS) are unclear. We propose that the innate immune response is an important cause of neuron death during acute infection. This is in contrast to the prevailing hypothesis that neuron loss is mediated solely by virus. While we do not doubt that some neurons die directly as the result of viral infection, our preliminary findings suggest that certain populations, such as CA1 pyramidal neurons in the hippocampus, are killed by the innate immune response rather than by the virus. We have established a mouse model of picornavirus infection of the CNS using the Theiler's murine encephalomyelitis virus to directly test the role of neutrophils in the initiation of neuronal apoptosis. Our preliminary evidence indicates that during acute picornaviral infection of the CNS, many uninfected CA1 pyramidal neurons undergo apoptotic death associated with oxidative injury, calpain activity, and caspase activity; this injury severely reduces cognitive performance in a spatial memory test. We have further observed that neutrophils infiltrate the hippocampus within hours of infection. Reduced neutrophil infiltration is neuroprotective, while adoptive transfer of activated neutrophils into mice with a defective neutrophil response induces hippocampal injury. Finally, treatment with calpain inhibitors protects hippocampal neurons from death and preserves cognitive function without constraining the inflammatory response that is necessary to mediating host defense and viral clearance. On the basis of these observations we hypothesize that neutrophils kill hippocampal neurons via a calpain-dependent mechanism during acute picornaviral infections of the CNS. We intend to address the following experimental questions: 1) are neutrophils necessary and sufficient to kill hippocampal neurons?; 2) is calpain the key executioner of hippocampal neurons during death induced by the neutrophil response to acute CNS infection? We propose several innovations, including the use of live animal imaging and adoptive transfer of neutrophils, to address these questions. The key concept of our proposal is that while inflammation critically mediates host defense to virus infection, the inflammatory response may indirectly kill neurons, and therefore therapeutic interventions aimed at preventing neuronal death without thwarting inflammatory control of virus may preserve host function.