Zika virus harms adult brain cells in mice

The Zika virus might not just harm the brain of the developing foetus – it could also affect parts of the adult brain, a new study in mice suggests.

A published in Cell Stem Cell by US researchers found the virus targets stem cells in parts of a fully grown mouse brain responsible for learning and memory and stops them proliferating. The team suggests the virus should be monitored in every patient – not just pregnant women.

“Based on our findings, getting infected with Zika as an adult may not be as innocuous as people think,” says Joseph Gleeson, a neurologist at Rockefeller University in New York and an author of the paper.

The Zika virus is a mosquito-borne and sexually transmitted virus first reported in the 1950s. Back then, it was contained within a narrow area along the equator, but the virus spread in 2007 to the Americas, eventually creating an epidemic the World Health Organisation has called a “Public Health Emergency of International Concern”.

Most recent attention in Zika research to date has centred on pregnant women. This is because the virus can spread to foetuses, causing severe brain defects including microcephaly – a condition that causes babies to be born with an unusually small head.

Now, a new study by researchers at Rockefeller University and La Jolla Institute in San Diego suggests the virus might also put adult brains at serious risk.

Microcephaly is the result of interruption during a very important process in a foetus’ brain development: when stem cells divide to turn into fully functioning neurons.

Some of these pre-neuron stem cells stick around in the adult brain, too. In mice, they exist in two regions that are important for learning and memory: the subventricular zone of the anterior forebrain and the subgranular zone of the hippocampus.

Interruptions to the neuron development process in adult stem cells have been linked to Alzheimer’s disease and cognitive deficits similar to the symptoms of depression.

While it’s not known exactly how Zika leads to microcephaly, the research team hypothesised that this progression could also potentially play out in stem cells in the adult brain.

To test this theory, the team monitored the brains of mice infected with Zika, using glowing biomarkers to track the virus and measure affected areas of the brain.

According to their results, which Gleeson says were “dramatic”, the virus targeted the two areas containing stem cells and stopped the cells dividing.

“In the parts of the brain that lit up, it was like a Christmas tree,” Gleeson explains.

“It was very clear that the virus wasn’t affecting the whole brain evenly, like people are seeing in the foetus. In the adult, it’s only these two populations that are very specific to the stem cells that are affected by virus. These cells are special, and somehow very susceptible to the infection.”

The long-term effects of Zika on adult brains are still unknown, and the team says more research is needed to ascertain whether the mouse model echoes the virus’ effect on humans. Nevertheless, the researchers consider their results a warning sign.

“The virus seems to be travelling quite a bit as people move around the world,” says Gleeson.

“Given this study, I think the public health enterprise should consider monitoring for Zika infections in all groups, not just pregnant women.”