The AIDS-causing virus, HIV, must attach to cholesterol-rich regions of a cell's membrane before it can do its destructive work, researchers at the National Institute of Allergy and Infectious Diseases (NIAID) have discovered. When the investigators removed cholesterol from the cells, HIV lost much of its ability to produce new virus particles and infect additional cells. These findings provide a more detailed picture of how HIV travels into and out of cells as well as possible ways to block that travel.
"Our research raises the intriguing possibility that widely used cholesterol-lowering drugs might have an effect in humans similar to what we have found in these initial laboratory studies," says Eric O. Freed, Ph.D., an investigator in NIAID's Laboratory of Molecular Microbiology, and the senior author of a paper published in the November 20 issue of Proceedings of the National Academy of Sciences.
HIV, like everything else passing into or out of an animal cell, must navigate the cell's complex double-walled outer membrane. Much like a bog, a cell membrane contains some areas of relatively solid material while other regions are more fluid. Among the features of this constantly shifting landscape are small, cholesterol-rich patches called rafts, which are more solid than the surrounding membrane and are able to move about like a raft on water.
Rafts are believed to be most concentrated at points of cell-to-cell contact in the immune cells that HIV targets for infection. Any mechanism that helps HIV find and attach to rafts would help the virus spread. Conversely, even a modest degree of disruption could slow the virus' spread because it would hinder the virus' ability to enter and leave its host cells.
Scientists have known for some time that an HIV protein called Gag must attach to the inner surface of the cell membrane before new viruses can be produced. Scientists also suspected that Gag's attachment to the membrane is not random, but rather targets specific regions of the cell surface. The new research shows their suspicions were correct.
Dr. Freed and his co-author Akira Ono, Ph.D., first established that Gag does indeed attach to rafts. They then created several mutant forms of Gag and learned that two pieces of the protein are required for successful attachment. Finally, the investigators turned to the key question of what happens when HIV is kept off the rafts.
To determine this, they depleted cholesterol from rafts using two compounds, one of which removes cholesterol rapidly from the cell surface and one that inhibits cholesterol synthesis. When used alone, each compound significantly reduced HIV's ability to form particles that could infect new cells. Applied simultaneously to virus-producing cells, the compounds almost completely abolished HIV's power to replicate.
"Our findings are clear evidence that Gag-raft association is a critical step in HIV replication," says Dr. Freed. "Additional experiments are needed to determine whether this interaction can be interrupted therapeutically to treat HIV-infected people."
NIAID is a component of the National Institutes of Health (NIH). NIAID supports basic and applied research to prevent, diagnose, and treat infectious and immune-mediated illnesses, including HIV/AIDS and other sexually transmitted diseases, tuberculosis, malaria, autoimmune disorders, asthma and allergies.
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Reference: A Ono and EO Freed. Plasma membrane rafts play a critical role in HIV-1 assembly and release. Proceedings of the National Academy of Sciences 98:13925-13930 (2001). DOI: 10.1073/pnas.241320298.