'Viral' Video Shows HIV RNAs Bursting From Host
Madison, Wisconsin - Even after decades of research, there is much left to be learned about the mechanics of how HIV, the human immunodeficiency virus, does its deadly work.
Researchers at the University of Wisconsin-Madison have leveraged some helpful technology to make HIV and its components fluoresce, revealing what their new study calls "striking, unexpected features" of retroviral activity that could lead to antiviral strategies for HIV and beyond.
Fluorescence microscopy — tagging parts of the virus and host cell with fluorescent molecules — allowed researchers to track different viral components and see how they change over the course of infection to understand the viral lifecycle.
Current HIV/AIDS treatments target only virus particles in the bloodstream, but not the formation of new particles within infected cells. When retroviruses like HIV infect a host cell, the viral DNA integrates with the host cell’s DNA within the nucleus and hijacks the cell’s own machinery, tricking the cell to produce viral messenger RNA (mRNA) that make specific proteins. Those proteins ultimately assemble new virus particles ready to infect another cell.
But virus to virus, each step of this basic process occurs with some slight variation. To shine some light on some of those differences, Nate Sherer, assistant professor of oncology and molecular virology at the UW School of Medicine and Public Health with affiliations with the McArdle Laboratory for Cancer Research and Institute for Molecular Virology, led a comparative study of HIV-1 and Mason-Pfizer Monkey Virus (MPMV), a distant viral relative.
“We still don’t understand how HIV really works during gene expression; the where and the when of how viral RNAs are trafficked in the nucleus and cytoplasm are still a quagmire. Our study set out to visualize several integrated events in the virus’s productive stages simultaneously using multicolor live cell imaging. We show that the strategy works, and we think that we can now use movies to understand many of the nitty-gritty details that underpin HIV gene regulation and virus particle production that we previously overlooked,” said Sherer.
“We did a comparative study between HIV and MPMV, and what we found out was extraordinary; the dynamics of the gene expression profile — the ‘central dogma’ for HIV — look dramatically different compared to other retroviruses.”
With HIV, the viral RNA is made in the nucleus and then, as researchers captured, explodes into the cytoplasm, an event researchers called “burst nuclear export.”
“Nobody has seen that before,” said Sherer. “We think the reason the virus uses ‘burst’ export is it’s trying to stage the timing of when virus particles are made. It wants to get virus particles out of the cell very quickly, so it builds mRNA transcripts up in the nucleus, blasts them into the cytoplasm and makes protein and new viruses very quickly. There must be a core signaling event that initiates the burst, and we’re currently trying to identify this trigger because inactivating it could be a new antiviral strategy.”
MPMV was chosen for this comparative study because it’s fairly well understood compared to some other retroviruses. For instance, HIV-specific mRNAs use a different protein to exit the nucleus than MPMV-specific mRNAs. Instead of a burst strategy, MPMV lets its mRNA leak out into the cytoplasm as they’re ready, but once released they accumulate in tight proximity to the centrosome, the structure that provides organization to the cell.
Why? Researchers think being in one location may help coordinate assembly of new virus particles.
“This is one of these black boxes in terms of viral gene expression and virion biogenesis. There’s a general belief that all mRNAs have to go somewhere in the cell to be efficiently translated and for proteins to work properly,” said Sherer. “In these retroviral systems we think that’s incredibly important because if the virus isn’t able to make the virus particle in the right place in the cell at the right time with all the right pieces, then it won’t be capable of spreading infection. We’d like to find ways to perturb the timing of these events. ”
The imaging-based approach should have broad applicability among viruses because there is more and more evidence that these viruses cluster their viral mRNAs and capsid elements at places within the cell at different stages. Understanding how and where viral components are clustering within a host cell could have broad-spectrum appeal in terms of thinking about antiviral approaches to herpes virus, influenza and other retroviruses.
The research, published in PLOS Pathogens this week (April 12), was carried out by researchers Ginger Pocock and Jordan Becker in collaboration with Paul Ahlquist and UW-Madison’s Morgridge Institute for Research.
Date Published: 04/15/2016