New research on the Zika virus contradicts the theory that high rates of microcephaly and other neural defects seen in the 2015 Brazilian Zika outbreak were due to newly acquired mutations in the virus.
In a study published this week, scientists at Columbia University Medical Center discovered that older strains of the mosquito-borne virus can infect and damage developing brains just as well as recently isolated ones. The new work, published in the Proceedings of the National Academy of Sciences, also provides a platform for studying other neurotropic viruses.
Zika virus was identified in 1947, however human infections were rare and generally considered mild, attracting little scientific attention. That changed in 2015, when authorities in Brazil noted a strong correlation between Zika virus infection in pregnant women and microcephaly in their babies.
The news caught the attention of Amy Rosenfeld, PhD, associate research scientist in microbiology & immunology. At the time, Dr. Rosenfeld was working with Vincent Racaniello, PhD, the Higgins Professor of Microbiology & Immunology, on enterovirus D68, which can cause sudden paralysis in children.
“Zika is similar to enterovirus D68 in that they both affect the central nervous system,” Dr. Rosenfeld says. “We thought, ‘We have all these tools for studying neurotropic viruses like enterovirus D68, why don’t we apply them to Zika?’.” With assistance from laboratory technician Audrey Warren, the group quickly pivoted to examine how Zika virus might infect the nervous system.
After acquiring several strains of Zika virus, the team soon developed methods for growing it in cultured cells and measuring its infectivity. While awaiting approval to infect mice with the virus, “members of Richard Vallee’s lab heard through the grapevine that we were working on Zika and came to talk to us about collaborating,” Dr. Rosenfeld says.
Dr. Vallee, director of the Division of Cell and Molecular Biology in the Department of Pathology & Cell Biology, had developed methods for keeping slices of fetal rat brains alive in culture to study genetic microcephaly, a condition in which embryos develop abnormally small brains and heads.
David Doobin, PhD, an MD/PhD student in the Vallee lab, had recently published a seminal paper identifying defects in neuronal progenitor cell behavior responsible for microcephaly in the developing rat brain. Drs. Rosenfeld and Doobin teamed up to combine their expertise in virology and in microcephaly to attack the Zika virus question.
They quickly discovered that the rat brain cells wouldn’t support Zika virus replication, but mouse brain slices would.
Crucially, all Zika strains tested by the team seemed capable of replicating in brain tissue. That undermined one of the most alarming—and heavily promoted—theories in the field: that the previously benign Zika virus had suddenly gained the ability to cause neurological disease. “The idea that the virus had evolved into this neurotropic pathogen was very aggressively put forward, everybody picked up on it,” says Dr. Rosenfeld. However, this new study found that the 1947 isolate of the virus infects brain slices just as strains from Brazil’s 2015 outbreak and those isolated elsewhere can; Zika virus has been able to infect brain tissue all along.
By studying the virus in slices of actual animal brains, the Columbia team was also able to see that the virus isn’t just killing cells, it’s causing widespread chaos.
Other groups have studied Zika virus infection in brain organoids, miniature structures of cultured cells that self-assemble to mimic brain tissue; in using actual brain tissue, the Columbia team can dissect Zika’s developmental effects in much greater detail.
As the new paper illustrates, Zika virus infection derails neuronal migration. During brain development, scaffolds composed of vimentin protein guide neuronal cells to their final positions. Zika disrupts the scaffold. Most previous work has shown that the virus causes microcephaly by inducing apoptosis, or programmed cell death, in brain cells; the new results point to a far more complex disease process.
Indeed, the pathogenesis of Zika virus infection in the mouse brain slices may be similar to the way genetic microcephaly develops in Dr. Vallee’s rat brain system. That makes Zika virus and the mouse brain slice system powerful tools for studying brain development in general. Zika virus’s ability to disrupt brain development on demand provides numerous options for tightly controlled experiments in any genetically manipulated mouse background, Dr. Rosenfeld says.
As to why Zika virus was linked to microcephaly in the 2015 Brazilian outbreak but not previous (or subsequent) outbreaks, multiple theories have been offered. Possibilities include genetic differences in the affected people, prior or concurrent infection with other microbes, or environmental factors. The virus certainly can damage fetal brains, but it does not do it consistently. With the small number of cases in the early East African outbreak and the lack of demographic infrastructure at that time, an association with brain developmental defects might have been overlooked.
The Zika-mediated brain developmental defects identified by the Columbia team are also consistent with other forms of brain abnormality, such as lissencephaly (smooth brain), which have been observed in Zika-infected babies.