June 13, 2008

Factor blocks release of HIV from human cells

Featured Image

Vasundhara Varthakavi, D.V.M., Ph.D., center, in the lab with research colleagues Rita Smith, left, and Ellen Heimann-Nichols. (photo by Neil Brake)

Factor blocks release of HIV from human cells

A newly identified factor that locks HIV inside human cells could prove to be a valuable therapeutic target in the fight against the virus that causes AIDS.

Vanderbilt Medical Center investigators, with collaborators at Emory University and the Mayo Clinic, report in the June issue of Nature Medicine that a protein called CAML blocks the release of HIV from human cells.

The search for such a factor goes back to an observation made two decades ago. Scientists found that an HIV protein called Vpu (viral protein u) is essential for the release of new virus particles from infected human cells. Without the Vpu gene, HIV stays imprisoned inside the cell.

“HIV without Vpu replicates poorly in T-lymphocytes and macrophages (immune cells infected by HIV),” said Vasundhara Varthakavi, D.V.M., Ph.D., research assistant professor in Pediatric Infectious Diseases and lead investigator of the current study.

But the requirement for Vpu is unique to human cells. HIV without the Vpu gene can exit from African green monkey cells just fine, Varthakavi said, suggesting that there is something in human cells that normally hinders virus release and that Vpu overcomes this block.

Varthakavi and colleagues took advantage of the difference between monkey and human cells several years ago to investigate the mysterious factor that blocks HIV release.

They showed by fusing the two cell types that the factor in human cells could be transferred to the monkey cells, causing them to require Vpu for virus release.

In the recent studies, the team used Vpu as “bait” to fish for human host proteins that interact with it. They identified CAML (calcium-modulating cyclophilin ligand), a protein known to be critical for T cell survival and activation.

The investigators showed that CAML and Vpu are localized to the same places inside cells, suggesting that the two proteins may function in the same processes. They knocked down CAML expression in human cells and found that HIV without Vpu could now exit the cells.

And they showed that expressing CAML in monkey cells, which normally do not require Vpu for HIV release, blocks virus release in the absence of Vpu.

The findings support the idea that CAML is a factor in human cells that restricts HIV release.

“We're really excited about the therapeutic potential of this finding,” Varthakavi said.

“We believe that the interaction of Vpu with CAML inside the cells is what allows HIV to overcome the cellular block, so we are thinking of strategies to disrupt this interaction as a way to trap HIV in the cells.”

Aiming therapies at host molecules like CAML, instead of viral proteins — the targets of all current HIV therapies — may be a way to combat drug-resistant HIV, Varthakavi added.

She and her team are pursuing questions of how CAML blocks HIV release and what Vpu does to get around it.

Vanderbilt authors of the Nature Medicine paper include Ellen Heimann-Nichols, Rita Smith and Yuehui Sun. Varthakavi is grateful for support she received as a Hazinski Scholar of the Turner Fund for Pediatric Research, as a faculty scholar of the Building Interdisciplinary Research Careers in Women's Health (BIRCWH) program at Vanderbilt, and from the Vanderbilt-Meharry Center for AIDS Research.

“We are very proud of Dr. Varthakavi,” said Peter Wright, M.D., Shedd Professor of Pediatric Infectious Diseases.

“Her paper in Nature Medicine highlights the opportunities that Vanderbilt training programs and support of young investigators create for outstanding research.”