RNA-Seq Providing More Clues to Understanding the Mechanism Between Zika Virus, Birth Defects

from HealthDay News – Scientists say they’ve discovered how the Zika virus might cause severe brain and eye birth defects.

The Zika outbreak in Brazil and other parts of Latin American and the Caribbean has coincided with a sharp increase in the number of babies born with microcephaly, which results in abnormally small heads and brains.

There has also been a rise in other brain and eye birth defects in countries affected by the Zika outbreak. But firm evidence of a link between the virus and these birth defects has been lacking.

The brain and eye birth defects occurring in areas with Zika outbreaks are “precisely the kind of damage we would expect to see from something that was destroying neural and retinal stem cells during development,” said study senior author Dr. Arnold Kriegstein. He is director of UCSF’s Center of Regeneration Medicine and Stem Cell Research.

“If we can understand how Zika may be causing birth defects, we can start looking for compounds to protect pregnant women who become infected,” Kriegstein said in a university news release.

The candidate viral entry receptor for Zika virus, AXL, is highly expressed by cells in the developing human cortex and retina, according to an experimental study published online March 30 in Cell Stem Cell.

Tomasz J. Nowakowski, Ph.D., from the University of California in San Francisco, and colleagues explored the expression of receptors implicated in cell entry of several enveloped viruses including Zika virus across diverse cell types.

The researchers found that the candidate viral entry receptor AXL was highly expressed by human radial glial cells, astrocytes, endothelial cells, and microglia in the developing human cortex, based on single-cell RNA-seq and immunohistochemistry. High expression was also seen on progenitor cells in the developing retina. AXL expression in the radial glia was conserved in the cortex of developing mouse and ferret and in stem cell-derived cerebral organoids from humans.

AXL Is Expressed in Human Radial Glia and Blood Vessels in the Developing Cortex at Mid-neurogenesis


(A) Heatmap showing expression levels of candidate flavivirus entry receptors in primary cells from developing human cortex. Genes directly implicated in ZIKV entry are indicated with dots. Cells are arranged based on inferred cell type identity.

(B) Violin plots showing distribution of expression levels of AXL across single cells of each respective cell type.

(C) Overview of AXL expression in the developing human brain. Images show immunostaining of a section through human cortex at GW18. Radial glia marker SOX2 expression is enriched in the germinal zones, the ventricular zone (VZ), and the outer subventricular zone (OSVZ), while the expression of neuronal marker SATB2 is enriched in the cortical plate (CP). LV, lateral ventricle; MZ, marginal zone. AXL expression is enriched in the germinal zones and at the pial and ventricular edges. Right schematic highlights cell types that strongly express AXL receptor, including the radial glia and brain vasculature.

(D) Immunostaining of the pial edge of the developing cerebral cortex. AXL expression is found in the pial end-feet and pia-contacting radial fibers of the radial glia, visualized by VIM immunostaining. Examples of fibers with double-immunoreactivity for VIM and AXL are highlighted with arrows.

(E) AXL immunostaining in the OSVZ, where AXL is expressed in the oRG cells, visualized by SOX2 nuclear immunoreactivity (arrows).

(F) Strong AXL immunostaining in the VZ can be detected at the edge of the lateral ventricle. Examples of abventricular radial glia with strong AXL expression are highlighted with arrows.

(G) Bar chart showing that AXL expression is highly correlated with a stem cell signature and anti-correlated with a neuronal signature across single cells collected from GW10 and GW12 primary human neural retina. Immunostaining for AXL protein shows strong expression in SOX2-expressing cells at the outer edge of the neural retina (NR), and in addition, very strong staining in the ciliary marginal zone (CMZ). Patches of strong AXL staining are indicated by arrows.

“The current manuscript constitutes an initial step toward the understanding of how Zika virus might cause developmental brain malformations,” the authors write.

Source – HealthDay News

Nowakowski TJ, Pollen AA, Di Lullo E, Sandoval-Espinosa C, Bershteyn M, Kriegstein AR. (2016) Expression Analysis Highlights AXL as a Candidate Zika Virus Entry Receptor in Neural Stem Cells. Cell Stem Cell. [Epub ahead of print]. [article]

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