Tunnel-building virus: How Zika transmits from mother to fetus

UNIVERSITY PARK, Pa. — In 2015, an outbreak of Zika virus triggered an epidemic in the Americas. People infected with Zika, typically via a mosquito bite, may have mild symptoms, but if the person is pregnant, it can lead to severe birth defects. How does the virus pass from mother to fetus?

A team of researchers from Penn State and Baylor College of Medicine found that the Zika virus builds tiny tunnels, called tunneling nanotubes, to stealthily transport material needed to infect nearby cells, including in placental cells. It’s one way the virus crosses the placental barrier, transmitting from mother to fetus during pregnancy without raising alarm in the immune system. The team also demonstrated, for the first time, that one specific Zika protein — non-structural protein 1 (NS1) — is responsible for the formation of the nanotubes.

The findings, published in Nature Communications, are a first step toward identifying measures to prevent infection and potential targets for antiviral therapies. This study was funded by a grant from the U.S. National Institute of Allergy and Infectious Diseases, totaling nearly $4 million, received in 2024.

“For Zika to infect newborn babies, it means the virus has to pass from the mother across the placental barrier, which usually only allows select molecules to pass through, like nutrients and antibodies,” said Anoop Narayanan, research professor of biochemistry and molecular biology at Penn State and senior author of the paper. “With this new discovery, we may be able to find a way in which we can stop the viruses from going from mother to fetus and control its spread.”

Zika is an Orthoflavivirus member of the Flaviviridae family, which includes West Nile, dengue and yellow fever viruses, and is typically transmitted by vectors, or living organisms that carry the pathogen and infect another organism. In this case, mosquitoes are the vectors. Unlike other flaviviruses, Zika can also be transmitted between people in the absence of a vector and is the only flavivirus capable of crossing the placental barrier from infected mother to fetus.

Zika infections among adults usually aren’t serious, the researchers explained. However, if a pregnant woman is infected, there’s a chance the virus can affect the development of the fetus, resulting in neurological disorders and other abnormalities. Currently, there is no vaccine or antiviral medication for Zika.

“Having something to prevent this infection from proceeding to a fetus is important,” said Joyce Jose, associate professor of biochemistry and molecular biology at Penn State and co-author of the study. While human infections of Zika virus have declined, Jose said the threat of future epidemic remains, especially as mosquitoes that harbor Zika virus could spread to new and different regions due to changes in climate and weather.

It was happenstance that the team discovered Zika’s ability to build tiny tunnels. At Penn State, researchers were examining live cells infected with the Zika virus under a fluorescent microscope when they noticed long tube-like structures connecting neighboring cells, plasma membrane to plasma membrane. When they looked at cells infected with other viruses, like dengue and yellow fever, they didn’t see any tubes. Meanwhile, researchers at Baylor also noticed that Zika induced the formation of nanotubes in placental cells. When the two groups shared their findings with each other, they conducted more tests and found that the tiny tunnels were, in fact, more prominent in placental cells.

Viruses like HIV, herpes and SARS-CoV-2 — the virus that causes COVID-19 — also build tiny tunnels and use them to spread to uninfected cells, but these viruses don’t cross the placenta. With experiments in human placenta cells in vitro, meaning in a culture dish, the team found that placental cells infected with Zika create tunnels to uninfected cells. The cells were procured from a commercially available cell line.

The cell-to-cell connections act as conduits, allowing viral particles, proteins and RNA to be transported from infected cells to neighboring, uninfected cells.

“If a virus is outside of the cell, it can be caught by antibodies in the bloodstream. But the tubes act like an extension of the cell so the virus is protected and not neutralized by antibodies,” Jose said. In other words, the virus covertly bypasses the immune system. When the team examined placental cells infected with Zika virus that can’t construct nanotubes, the growth and spread of the virus was reduced.

But material doesn’t just flow in one direction through the tiny tunnels. Mitochondria, the cell’s main source of energy, are harvested from the uninfected cell and siphoned through these tubes to the infected cell.

“It’s a two-way street,” Narayanan said. “The virus is reprogramming the whole cell to enhance its growth. It collects mitochondria so it has energy to survive and spread to the uninfected cells around it.”

The team also discovered that the protein NS1 is responsible for the development of the tiny tunnels. While NS1 is an important protein for flaviviruses and plays an essential role in viral replication, it does not spur the development of nanotubes in the other viruses. The researchers identified the specific area of the Zika NS1 involved in building the tubes. Shay Toner, co-author of the study and doctoral student at the Massachusetts Institute of Technology, identified the region as part of his undergraduate honors thesis while at Penn State. His research was a key part of unraveling NS1’s role in nanotubes formation, according to Joyce.

“The Zika outbreak in 2015 got me interested in virology as a high schooler,” Toner said. “Getting to work on Zika in the Jose lab at Penn State and to play a small part in a project as impactful as this while I was an undergraduate student was an amazing opportunity.”

Next, the team will work to identify the specific signaling pathway activated by NS1 that leads to the creation of the tunnels. By doing so, they said they hope to identify potential drug targets for antiviral medication. They will also begin studies in a mouse model.

“This is like a detective story. We don’t understand the mechanism for how these tubes are formed yet, so we are continuing to ask more questions,” Jose said.

Authors on the paper from Baylor College of Medicine include Indira Mysorekar, E.I. Wagner Endowed, M.D., Chair of Internal Medicine II and professor of medicine; Rafael Michita, postdoctoral research associate; Long Tran, graduate student; Steven Bark, bioinformatics analyst; and Deepak Kumar, postdoctoral associate.

This work was supported in part by grants from the NIH’s National Institute of Allergy and Infectious Diseases (R01AI176505), the NIH’s National Institute of Child Health and Human Development (R01HD091218) and Penn State.