Zika Virus Pathogenesis in the Developing Brain and the Inner Ear
Zika virus (ZIKV) is a mosquito-borne pathogen that stayed unnoticed for over half a century. Only after the 2015-16 Brazilian outbreak did the severity of the infectious outcome, particularly the Congenital Zika Syndrome, become apparent. ZIKV is associated with severe neurodevelopmental impairments in human fetuses, including microencephaly, ventriculomegaly, retinopathy, and sensorineural hearing loss. Though the pandemic is now under control in the Latin American countries, several tropical countries could still be at risk of widespread infection. This warrants a better understanding of the congenital Zika syndrome; this project attempts to contribute towards this goal.
Previous reports examining neural progenitor tropism of ZIKV in organoid and animal models did not address whether the virus infects all neural progenitors uniformly. To explore this, ZIKV was injected into the neural tube of 2-day-old chicken embryos, resulting in non-uniform periventricular infection 3 days later. Recurrent foci of intense infection were present at specific signaling centers that influence neuroepithelial patterning at a distance through secretion of morphogens. ZIKV infection reduced transcript levels for 3 morphogens, SHH, BMP7, and FGF8, expressed at the midbrain basal plate, hypothalamic floor plate, and isthmus, respectively. Levels of Patched1, a SHH-pathway downstream gene, were also reduced and a SHH-dependent cell population in the ventral midbrain was shifted in position. Thus, the diminishment of signaling centers through ZIKV-mediated apoptosis may yield broader, non-cell autonomous changes in brain patterning.
Sensorineural hearing loss is a relatively understudied consequence of congenital Zika syndrome, and balance disorders are essentially unreported to date. ZIKV pathogenesis was explored in the developing inner ear using the accessible chicken embryo model system. One goal was to assess the spatiotemporal susceptibility of otic epithelial-derived structures to ZIKV infectivity. Direct injections of the inner ear or the inner ear primordium were performed in ovowith subsequent harvests at 2 to 8 days-post-infection. The degree of infection in sensory/prosensory organs was evaluated histologically to determine the susceptibility of one auditory and five vestibular organs. ZIKV infection of the sensory as well as non-sensory epithelia was observed at most stages of analysis, with no apparent preference for one over the other. The lagena, the ventral most tip of the chicken inner ear, and the endolymphatic sac/duct were least frequently infected. In this report, two novel findings in sequela of ZIKV infection are presented: the vestibular labyrinth can present with stalled canal morphogenesis, and the auditory ganglion can be severely shrunken, perhaps due to an increased cell death upon early ZIKV infection of the inner ear.
Additional methods of peripheral infection in the chicken embryos were tested to examine ZIKV transmission to the central nervous system: E3 blood vessel, E4 limb bud, and E10 chorioallantoic membrane infections. Although none of these methods resulted in a histologically significant infection of the developing brain 3 to 6 days-post-infection, evidence of ZIKV genome replication and viremia was detected in several tissue types.