The AXL Receptor: An Incomplete Explanation of how Zika Virus Infects the CNS

Braden Danbury

 Introduction. The ongoing Zika epidemic has drawn attention to the neurological consequences of congenital ZikV infection1-3.  Studies have established a causal link between ZikV and abnormal brain development in murine models and models using cerebral organoids3, 6.  Preventing this irreparable damage caused by ZikV has fueled research to understand the mechanisms behind vertical transmission of the virus.  A primary target for vaccine development is the viral entry receptor.  The AXL receptor fell under suspicion early and has been shown to act as a ZikV entry receptor for some neural cell types4, 5.  However, other studies have shown that productive ZikV infection of neural cells and organoids is possible even without the AXL receptor6Methods.  First, the Wells study used human induced pluripotent stem cells (iPSCs) and maintained one set without AXL mutations (AXLWT).  CRISPR-Cas9 technology was used to create two other sets of iPSCs with mutations in their AXL transmembrane or extracellular regions (AXLKO-Tm and AXLKO-Ec).  The iPSCs underwent neuronal differentiation to early neural progenitor cells (NPCs).  The NPCs were infected with a strain of ZikV from Uganda (ZikV-Ug) or Puerto Rico (ZikV-PR).  Cell viability was compared between mock-infected control, ZikV-UG and ZikV-PR infected groups of NPCs grown from the AXLWT, AXLKO-Tm and AXLKO-Ec iPSCs.  Second, they cultivated cerebral organoids from the AXLWT and AXLKO-Tm iPSCs and infected them with ZikV-PR.  The virus distribution was studied via immunohistochemistry staining of organoid slices, and organoid growth after infection was compared between the two groups and a control.  Results.  ZikV infection with either strain produced comparable decreases in cell viability in all three groups of NPCs.  In fact, viral-mediated cell death was slightly higher in the AXLKO-Tm and AXLKO-Ec groups than in the AXLWT NPCs at the lowest MOI (0.1).  In the cerebral organoids, ZikV infection was detected in radial glial-like cells in both AXLWT and AXLKO-Tm organoids.  Both AXLWT and AXLKO-Tm organoids infected with ZikV-PR showed significant reductions in organoid size.  Conclusions. Though the AXL receptor may play an important role in ZikV entry for some cell types, this study shows that it is not the sole determinant of ZikV neurotropism and pathology.  Efforts to design treatments which prevent entry of the virus into neural cells and mitigate the neurological consequences of congenital ZikV infection will be premature without better understanding of other ZikV entry receptors.

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