Two Steps Forward

By Eric Sauter

The study of human immunodeficiency virus (HIV), the cause of AIDS, has always been a case of two steps forward, one step back.

The virus's complex transmission profile, the swift virulence of its attack, and its unnerving ability to overwhelm the human immune system are some factors that contribute to the difficulty of tackling a pathogen that has developed "sophisticated strategies to enter and survive in human cells," in the words of Scripps Research Associate Professor Philippe Gallay, the senior author of a new study that sheds considerable light on a surprisingly effective HIV-1 inhibitor.

The new study, "Degradation of HIV-1 Capsid and TRIM5α Restriction," has just been published online by the Journal of Biological Chemistry and falls in the "two steps forward" category.

In the new study, the Scripps Research team examined cells to describe the activity of TRIM5α, a protein found in most mammalian cells that combats a variety of retrovirus infections. What TRIM5α does exceptionally well in several species of monkeys is to prevent infection by HIV-1, the most common form of the human immunodeficiency virus.

Normally, when HIV enters the cell cytoplasm, it immediately cycles through a number of infective processes including: uncoating (the breakdown of the protein shell called a capsid that surrounds the virus); reverse transcription (the conversion of viral RNA into DNA); and, finally, integration, when the viral DNA enters the cell nucleus and begins to reproduce itself ad infinitum. But when HIV-1 enters a monkey cell, it is blocked before the integration process even begins.

Within the cellular cytoplasm of monkey cells, TRIM5α somehow interferes with the highly selective attack of the HIV capsid core needed for a successful infection.

An Intriguing New Theory

The existence of TRIM5αis most likely the result of primate evolution, providing a block to cross-species infection. Interestingly enough, while monkey TRIM5α does recognize and block HIV, the human form of TRIM5α does not. However, Gallay said, human TRIM5α can be modified to recognize HIV-1 just like its simian relative.

While no one really knows the exact mechanism of how TRIM5α manages to block HIV-1 infection—only that it does and does it well—the authors of the Scripps Research study offer an intriguing new theory.

"We think there are two possible models for how TRIM5α works in the cell," Gallay said. "The one that has been proposed before is that as soon the HIV capsid core that surrounds the viral genome enters the cytoplasm, TRIM5α rapidly breaks down the core, and this premature destabilization perturbs the normally ordered uncoating of the viral core. The second model, the one that was suggested by our research, is that TRIM5α recognizes the HIV capsid core very rapidly, but rather than breaking down the core, TRIM5α drags it into vesicular or cytosolic compartments where the core is rapidly degraded. In contrast to the first theory, we found that uncoating of the HIV capsid core is already an extremely rapid event. In our experiments, the capsid core vanished in less than two hours. That was very surprising. We can't really see how TRIM5α can further accelerate that process."

What this means, Gallay says, is that probably more than one step is involved in the process of the TRIM5α restriction of HIV-1 in monkey cells and that the redirection of the capsid core into what the study calls an "abortive degradation pathway" makes very good sense as the most probable hypothesis.

In support of the re-direction hypothesis, the study describes how inhibitors of proteasome, a multi-protein complex that breaks down other proteins, protect TRIM5α from degradation in infected cells and redirect a subset of TRIM5α proteins into vesicular compartments, which leaves Gallay and his colleagues with the work of determining the exact contribution of the proteasome in the TRIM5α-mediated restriction of HIV-1.

One conclusion of the study was that, in contrast to the non-selective proteasome-mediated attack on all incoming viral proteins, the TRIM5α-mediated degradation targets the HIV-1 capsid in the cytoplasm and does so exclusively. This joint action of the non-selective degradation of all incoming viral proteins by the proteasome and the selective capsid degradation by TRIM5α may contribute to the accelerated capsid degradation the scientists observed under some conditions.

Unpacking the Black Box

The scientists were able to detect this accelerated capsid degradation in certain "restrictive" cells—cells which carry components specifically designed to thwart infection—but only in cases where inhibition of HIV was at its maximum. Why?

Once again, the innate difficulty of studying HIV virus comes into play.

"The step we're observing—after HIV-1 entry but before integration—is a huge black box in HIV research," Gallay said. "You have to go through sophisticated cellular and biochemical methodologies. So we don't know if our inability to distinguish between these two types of cells with either complete or partial inhibition is simply a technological limitation or whether it's something else. Inorder to better dissect the TRIM5α action,we're trying to generate HIV mutants with a slower uncoating process. The virus uncoats so fast right now, even in the absence of TRIM5α, that we can't see the viral core already after a couple of hours."

Interestingly, the study notes that HIV-1 capsid cores that reach the cytoplasm are at "a point of vulnerability in the HIV-1 life cycle that could be exploited" as a potential therapeutic target.

Whether or not TRIM5α might become the proverbial magic bullet in the fight against a sophisticated virus, Gallay does not yet know, although thanks to this new study he now knows where he wants to look, an advance of no small consequence.

"Is TRIM5α a potential treatment?" he says. "I don't know. However, if we could understand how TRIM5α uncoats or directs the capsid core to a specific compartment that would be very, very important. If we could identify the potential mechanism of how it does and what it does, then perhaps, yes. That's what we're attempting to do now. I believe we're getting very close to having an amazing story."

Other authors of the study include Udayan Chatterji, Michael D. Bobardt, Peter Gaskill, Dennis Sheeter, and Howard Fox, all of Scripps Research. The study was supported by the Center for AIDS Research (CFAR) and the National Institute of Allergy and Infectious Diseases (NIAID).

Send comments to: mikaono[at]scripps.edu

A study by Associate Professor Philippe Gallay and colleagues sheds light on a surprisingly effective HIV-1 inhibitor. Photo by BioMedical Graphics.