The Zika virus is causing global concern, and for good reason: Not only does it seem likely that 2.2 billion people are at risk from this infection, but there is mounting evidence that pregnant women suffering from the disease are at considerable risk from their offspring having microcephaly – a dramatically shrunken brain – along with other neurodegenerative traits.
Although the evidence connecting these ailments to the Zika virus was long considered to be circumstantial, albeit strong, the U.S.-based Centers for Disease Control and Prevention (CDC) declared this April that it was enough to conclude that the link was genuine. Now, a new Nature study has provided direct experimental evidence that the Brazilian strain of the Zika virus, without a doubt, causes neurological birth defects.
Brazil has been the center of the current Zika epidemic. There have been 1.5 million cases there to date, and at least 1,000 examples of microcephaly. Taking the regional viral strain, ZIKVBR, the researchers first showed that it can cross the placenta and cause damage to both the brain and the central nervous system in the fetuses of mice. Then, they used “mini-brains,” collections of human neurons resembling a basic brain structure, to show that ZIKVBR directly causes neurological cell death.
“The earlier media reports linking the Zika virus with microcephaly were getting ahead of themselves,” Alysson Muotri, an assistant professor of neurobiology at the University of California San Diego and one of the coordinators of the study, said in a press briefing. “This is the first experimental proof that the Zika virus causes birth defects. Zika alone is sufficient to cause birth defects; other factors may increase or decrease this impact, but it is in itself sufficient.”
The Zika virus had previously been found in the brains of babies, including one that almost entirely lacked the presence of a brain at all, but researchers could not conclusively say that it was this virus that was causing the damage – perhaps it just preferred to hide out in the brain without causing any cell death. For this study, the international team of scientists wanted to show direct evidence of cell death caused by Zika.
Firstly, they isolated a sample of ZIKVBR from a clinical case in northeastern Brazil, and infected two different types of pregnant mice bred in the laboratory. After evaluating the pups immediately after they were born, they found that those from one particular strain, known as SJL mice, showed evidence of both microcephaly and brain stem malformations; additionally, their entire bodies were smaller than they should be at birth.
Genetic analysis of the fetal brains revealed that they contained an exceedingly high presence of ZIKVBR, as expected, thus confirming the virus' propensity towards infecting neurons – a characteristic known as neurotropic behavior. Analysis of various neurons showed cell-based nuclear debris, suggestive of ongoing cell death.
Intriguingly, those from the second strain – C57BL/6 – didn’t show any of these afflictions, which the researchers put down to the robust anti-viral immune system bred into these mice over hundreds of generations.
Neurospheres – clusters of human neural stem cells – are far smaller and malformed when infected with the Zika virus, compared to their healthy equivalents. Scale bar is 200 micrometers. Cugola et al./Nature
In any case, all of this would still be circumstantial if it wasn’t for the results of the follow-up experiments. Small “mini-brains” of human neurons and neural stem cells were infected with both the African and Brazilian strains of the Zika virus, and their progress was observed in real-time.
The infiltration of the cells by both strains – but perhaps slightly more significantly in the ZIKVBR infections – caused cell shrinkage, deformation, and death. This seems to confirm beyond all doubt that the virus isn’t just neurotropic, but actually destroys brain and central nervous system cells.
Image in text: Comparing a regular mouse fetus to a Zika-infected one. Scale bar is 1 centimeter. Cugola et al./Nature
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