George Bailey

Introduction: Zika virus (ZIKV) is a single stranded positive RNA flavivirus originally thought only to cause mild illness that rarely needed treatment; however, in recent outbreaks it has been associated with severe neurological conditions such as Guillan-Barre and fetal microcephaly. Studies show that ZIKV utilizes different cell-surface receptors (AXL and TIM-1) to mediate entry into placental cells and human neural progenitor cells (hNPCs) while evading the host IFN system.^3,4,5,6 Infection of hNPCs is shown to result in apoptosis, cell-cycle arrest, and decreased growth of cerebral organoids.^2,7 Understanding the mechanisms behind ZIKV’s neuropathogenesis will help identify potential prevention and treatment strategies. Methods: Organoids mirroring first trimester development of the human fetus were infected with ZIKV, grown and transcriptional profiles compared. hNPC viability was determined through differences in caspase-3 activation and flow-cytommetry. Neuospheres grown with infected NCCs were visualized using immunohistochemical stains and compared to controls. Human 63-plex Luminex assay was used to determine differences in cytokine production. Immunofluorescent imaging and immunoblot of ZIKV infected Vero and 293T cells was used to determine the virus’ effect on STAT2 levels. AXL-antibody was used to block ZIKV infection of human fetal endothelial cells. Results: ZIKV infected cerebral organoids were 45.9% smaller in size on average and had a reduced total number of viable hNPCS when compared to controls 5-days post infection.² Viral activation of TLR3 induced apoptosis and cell-cycle arrest in these cells.⁷ Neurospheres incubated with infected NCCs showed sixty-fold increase in LIF and ten-fold increase in VEGF cytokines without generalized cytokine elevation resulting in early differentiation and decreased neurogenesis.¹ Infection of different cell types is mediated by different cell-surface receptors; however, AXL (also found in NPCs) and TIM1 receptors have been shown to mediate the infection of human placental cells that are critical in ZIKV vertical transmission.^4,5,6 Infection results in lower levels of STAT2 in a dose-dependent manner.³ Conclusion: ZIKV is able to infect fetal cells secondary to infection of maternal placental cells using TIM-1 and AXL receptors while evading host IFN systems by degrading STAT2 of the JAK-STAT pathway. Once in the CNS it infects hNPCS and NCCs triggering the TLR3 pathway and altering cytokine production. This results in aberrant apoptosis, early differentiation, and inhibited neurogenesis that may result in severe neurological conditions.  Elucidating the mechanism of ZIKV pathogenesis will help produce effective treatment and prevention.

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3. Richard, Audrey S, et al. AXL-dependent infection of human fetal endothelial cells distinguishes Zika virus from other pathogenic flaviviruses. Proceedings of the National Academy of Sciences of the United States of America8 (2017):2024-2029.
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8. , C., Fang-Hoover, J., et al. Zika virus targets different primary human placental cells, suggesting two routes for vertical transmission. Cell Host & Microbe, 20(2), 155-166. Retrieved from http://dx.doi.org/10.1016/j.chom.2016.07.002
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