The latest outbreak of Zika virus (ZIKV) in the Americas was associated with significant neurologic complications, including microcephaly of newborns. We evaluated mechanisms that regulate ZIKV entry into human fetal astrocytes (HFAs). Astrocytes are key players in maintaining brain homeostasis. We show that the central mediator of canonical Wnt signaling, β-catenin, regulates Axl, a receptor for ZIKV infection of HFAs, at the transcriptional level. In turn, ZIKV inhibited β-catenin, potentially as a mechanism to overcome its restriction of ZIKV internalization through regulation of Axl. This was evident with three ZIKV strains tested but not with a laboratory-adapted strain which has a large deletion in its envelope gene. Finally, we show that β-catenin-mediated Axl-dependent internalization of ZIKV may be of increased importance for brain cells, as it regulated ZIKV infection of astrocytes and human brain microvascular cells but not kidney epithelial (Vero) cells. Collectively, our studies reveal a role for β-catenin in ZIKV infection and highlight a dynamic interplay between ZIKV and β-catenin to modulate ZIKV entry into susceptible target cells. IMPORTANCE ZIKV is an emerging pathogen with sporadic outbreaks throughout the world. The most recent outbreak in North America was associated with small brains (microcephaly) in newborns. We studied the mechanism(s) that may regulate ZIKV entry into astrocytes. Astrocytes are a critical resident brain cell population with diverse functions that maintain brain homeostasis, including neurogenesis and neuronal survival. We show that three ZIKV strains (and not a heavily laboratory-adapted strain with a large deletion in its envelope gene) require Axl for internalization. Most importantly, we show that β-catenin, the central mediator of canonical Wnt signaling, negatively regulates Axl at the transcriptional level to prevent ZIKV internalization into human fetal astrocytes. To overcome this restriction, ZIKV downregulates β-catenin to facilitate Axl expression. This highlights a dynamic host-virus interaction whereby ZIKV inhibits β-catenin to promote its internalization into human fetal astrocytes through the induction of Axl.
Keywords: Axl; TCF4; Tyro3; Wnt signaling; ZIKV; Zika; astrocytes; brain; flavivirus; infection; internalization; β-catenin.