Mechanisms Underlying Memory Deficits Following West Nile Virus Neuroinvasive Disease

Neurocognitive sequelae are observed in \u3e50% of patients who survive neuroinvasive infections with encephalitic arboviruses, such as the mosquito-borne West Nile virus (WNV). Early diagnosis and high survival rates from WNV neuroinvasive disease (WNND) (\u3e90%) have thus led cumulatively to approximately ten thousand patients living with neurocognitive impairments, with 1-3000 cases accruing yearly, yet underlying mechanisms responsible for these deficits have not been investigated. Within the last 15 years, studies have begun uncover many pathways which are utilized both by the developing CNS as well as the immune system, including the use of cytokines in the regulation of progenitor cell proliferation and synaptic refinement via the classical complement cascade. Under healthy conditions, an intact blood-brain-barrier limits potential crosstalk between these domains. But amidst settings of CNS infection or damage, invocation of CNS developmental programs could be initiated by proinflammatory factors. With this in mind, the chapters of this thesis are centered around a theme of studying pathways utilized by both the immune system and the developing CNS and determining what the consequences are for cognition and memory during CNS West Nile virus infection. First we have established a novel murine model of recovery from WNND in which intracranial inoculation of the attenuated mutant WNV-NS5-E218A leads to similar CNS viral loads and inflammation as peripheral inoculation of its parent strain, WNV-NY99, with rates of survival and cognitive dysfunction that mirror human WNND. WNV-NS5-E218A-recovered mice exhibit impaired spatial learning without significant alterations in cortical and hippocampal volume or total neuron numbers, but exhibit persistently activated microglia. Whole transcriptome analysis of hippocampi from WNV-NS5-E218A-recovered mice with poor spatial learning revealed increased expression of genes known to drive microglial effects on synaptic pruning, including the classical complement pathway and phagocytosis. Indeed, the classical complement cascade initiation factor, C1qA, was found to be produced primarily by microglia and localized to infected neurons and synapses during WNND. Electron and confocal microscopy revealed a loss of hippocampal mossy fiber synapses while synaptophysin-positive puncta and phagosomes containing synaptic vesicles were observed within microglia. This loss of mossy fiber synapses was also observed in human WNND post-mortem samples. Importantly, mice with fewer microglia (IL34 -/-) or mice deficient in complement (C3 -/-) were protected from WNV-induced synapse loss. This study provides a novel murine model of WNV-induced spatial memory impairment, provides evidence that viral infection of adult neurons may induce complement-mediated elimination of synapses, and identifies a potential mechanism underlying neurocognitive impairments experienced by patients recovering from WNND. Next we report that during WNND recovery, significantly fewer new granule cell neurons are born within the hippocampal dentate gyrus, as revealed by BrdU-labeling. And although neuronal progenitor cells (NPCs) are not direct targets of WNV infection, their homeostatic levels of proliferation, differentiation, and migration were significantly altered. Using immunohistochemistry and flow cytometry coupled with in vivo BrdU labeling during acute WNV infection, we found NPCs within the subventricular (SVZ) and subgranular (SGZ) neurogenic zones generated fewer new neuroblasts, but greater numbers of astrocytes, an effect which slowly recovers over about 45 days. This effect was dependent upon Interleukin-1 receptor (IL-1R) signaling, which we have previously shown to be necessary for promotion of viral clearance by T lymphocytes. Lastly, mice deficient in IL-1R were resistant to the WNV-mediated impairment in spatial learning and memory via the Barnes maze task at 45 days post infection, suggesting that alterations to neuronal progenitor cell homeostasis could also underlie long term cognitive consequences of WNND