When Zika virus hit the Americas in 2016, it hit hard, leading to intense interest in the disease and the mechanism by which the virus infects cells. Two years later, researchers still don’t know much, but Stanford chemists have now taken a step toward understanding one aspect of Zika’s function.
In a paper recently published in ACS Central Science, the researchers showed that a key step on the way to viruses infecting cells, called membrane fusion, is significantly more complicated in Zika compared to other related viruses.
“Understanding the process of membrane fusion is not going to cure or prevent Zika,” but the results could nonetheless help researchers eventually identify drug targets and potential treatments, said Elizabeth Webster, a graduate student in Stanford ChEM-H’s Chemistry/Biology Interface Training Program and co-lead author on the new paper with former postdoctoral fellow Bob Rawle, PhD, and Peter Kasson, MD, PhD, a collaborator at the University of Virginia Medical School.
Like HIV, influenza, and many other viruses, Zika travels around enveloped in a membrane (recall that viruses are not cells). To invade a cell, Zika’s membrane first interacts with receptor molecules on the cell’s surface and is then taken inside the cell in a kind of bubble called an endosome.
To actually infect the cell, however, the virus needs to break out of both its own membrane and the endosome’s, and here it makes use of a key feature of endosomes: as they mature, they become more acidic. As the acidity increases, the virus undergoes a structural shift and its membrane merges with the endosome’s and opens a hole through which the virus can enter the main part of the cell.
At least that’s the cartoon version — even in relatively well studied enveloped viruses like influenza, HIV, and dengue, the details of membrane fusion are not particularly well understood.
To address that problem, a team led by Steven Boxer, PhD, the Camille Dreyfus Professor of Chemistry, had been developing special tools to study membrane assemblies and fusion, including flexible tethers that gently hold synthetic target membranes in place long enough to watch how viruses invade them.
The lab had been using those tethers to study influenza virus when the 2016 Zika outbreak struck. “The news was filled with terrible stories about Zika, which continues to be a serious problem though it has dropped from the news,” Boxer said.
The team soon decided to shift gears to study Zika, but there was an extra challenge. In order to study membrane fusion, the researchers needed a means of binding the virus onto their synthetic membranes — and unlike flu and other viruses, no one knew which receptor molecules were key for Zika to enter a cell.
The key to solving that problem, Webster, Boxer and colleagues realized, was that they didn’t actually need to identify Zika’s receptor. In the course of their work on influenza, the team showed they could bind influenza virus to synthetic membranes using the special tethers they had developed in place of the receptors flu would ordinarily use. What’s more, they showed in a 2016 paper, those tethers do not affect membrane fusion dynamics. With that in mind, the team reasoned they could tether Zika virus to a synthetic target membrane and study its membrane fusion without knowing what Zika binds to out in the wild.
The resulting experiments revealed a number of surprises. First, Zika’s membrane fusion is less sensitive to acidity than other viruses, so that Zika virus membrane fusion occurs at a higher pH — that is, a lower acidity — compared to other related viruses and that membrane fusion’s efficiency increased with acidity.
Second, the dynamics of Zika membrane fusion seem to work differently. By carefully observing and analyzing the rate of membrane fusion, the researchers inferred that Zika can take on a state that does not undergo membrane fusion and, as a result, never enters cells. Although they do not know exactly what that state looks like — they did not directly observe it, after all — the team showed that it must nonetheless be taken into account to understand Zika’s infection dynamics.
Those findings — and the tools the team deployed to uncover them — could help reveal more about Zika and other viruses and shed light on membrane-membrane interactions in general, all things he and his lab will be looking at in the near future, Boxer said.
Image by CDC Global