Investigations into Zika virus-host interactions: A neurological perspective

Zika virus (ZIKV) is a member of the Flavivirus genus of the family Flaviviridae. Members of this genus possess a single-stranded positive-sense RNA genome which are flanked by 5’ and 3’ untranslated regions. Previously, ZIKV infection was thought to cause symptomatic infection in 20% of patients, characterised by a non-purulent rash and a mild fever. However, recent ZIKV outbreaks have seen the emergence of novel neurological sequelae associated with ZIKV infection. These symptoms affected the central and peripheral nervous systems (CNS and PNS, respectively) of neonates and adults (congenital ZIKV syndrome and Guillain-Barré syndrome, respectively). Consequently, prior to commencement of this thesis, the knowledge of ZIKV interactions within the CNS and PNS was limited. Therefore, the primary aim of this thesis was to expand the current knowledgebase of ZIKV infection within cells of the CNS and PNS. The work presented herein assessed available tools to study ZIKV infection in vitro, including testing previously uncharacterised commercial antibodies targeting the ZIKV envelope (ZIKV E) and NS1 proteins. Subsequently, a suitable antibody raised against the ZIKV E was identified and used in downstream analyses. A model system of ZIKV infection of the CNS and PNS using mixed-cell co-cultures derived from mouse spinal cord (CNS) or dorsal root ganglion (PNS) was used to determined cell-type specific susceptibility to ZIKV infection. It was found that cells of the PNS are refractory to ZIKV infection, whereas cells of the CNS are permissive to ZIKV infection. In the CNS, oligodendrocytes and their precursors were the most susceptible cell type to ZIKV infection. Moreover, when infecting CNS co-cultures at a time point which reflects prenatal life through to post-natal life, white matter structures (myelin sheath and axons) are injured. However, neuronal cell bodies remained healthy. Using purified primary neuronal cultures derived from mouse spinal cord, it was determined that neurons were refractory to ZIKV infection in the absence of accompanying glial cells. This suggests axonal damage may not be a result of direct infection of neurons themselves and may be a consequence of oligodendrocyte infection and injury. Transcriptomic analyses of ZIKV-infected CNS co-cultures revealed that ZIKV infection induces the upregulation of genes involved in antiviral responses and inflammatory pathways, including TNF and ROS/NO pathway components; these pathways may be involved in exacerbating injury to white matter structures within ZIKV-infected CNS-co-cultures. In summary, the results described in this thesis show that ZIKV can infect the early post-natal CNS. Furthermore, ZIKV-infection of this murine model demonstrates that infection induces the injury and depletion of myelin and axons. This is likely to be due to a combination of effects such as direct viral infection, cell death of oligodendrocytes, and secreted inflammatory factors. Myelination occurs late in foetal development and carries on into early adulthood. These data, in combination with precedent pathological findings, suggest that ZIKV-infected children born absent of microcephaly may develop other, more subtle, neurological sequelae later in life