August 16 (HealthDay News) -- HIV, the virus that causes AIDS, often infiltrates the brains of infected patients, causing everything from cognitive decline to death.

Now, new research in mice suggests the virus doesn't just kill brain cells but also prevents replacement cells from developing.

This "double whammy" explains why HIV is so devastating to the brain, but it could also point the way to new treatments, said study co-author Dr. Stuart Lipton, a neurologist and researcher with the Burnham Institute for Medical Research and the University of California at San Diego.

According to Lipton, HIV infection has become the leading cause of dementia in people under the age of 40. In some cases, "you can't work, you can't concentrate or pay attention, you can't move properly," he said.

His team published its findings in the August issue of Cell Stem Cell.

Brain problems are on the rise as HIV-infected patients live long enough to develop related health problems, he said. Still, the brain illnesses that afflict these patients "may be the best-kept secret of AIDS in the world."

The challenge for doctors is figuring out how to treat brain symptoms when the most powerful antiretroviral drugs cannot pass the natural barrier that protects the brain. HIV, however, can make its way into the brain and remains there even when the level of virus in the blood approaches zero during treatment.

In the new study, researchers genetically engineered mice to have higher levels of a protein called HIV/gp120, which is thought to contribute to the deterioration of the brain in HIV-infected patients.

Lipton and his colleagues found that the protein disrupts stem cells in the brain that are supposed to turn into new brain cells when needed. As a result, it becomes difficult for HIV-infected brains to create new cells when old ones are injured or killed off by the virus.

"Most people would not think that AIDS has anything to do with stem cells," Lipton said. "But it has the ability to stop stem cells from dividing."

Paul Thompson, a professor of neurology at the University of California at Los Angeles, explained the findings this way: "As a brain cell is born, there are various checkpoints and phases of development that the cell has to go through. These researchers found the exact checkpoint where the birth of new brain cells is stopped. Now that they've identified the type of interference, it is a lot easier to focus on overcoming it."

The new research is in mice, and many medical discoveries don't translate from rodents to humans. But Lipton said he expect humans to be similar to mice when it comes to this particular kind of research.

According to Lipton, the next step is to figure out a way to develop a drug with a "double bang" that would stop HIV from attacking both existing brain cells and stem cells that could become brain cells. Scientists are already working on drugs that target an enzyme that's involved in that process, he noted.

SOURCES: Stuart Lipton, M.D., Ph.D., neurologist and director, Del E. Webb Center for Neuroscience and Aging, Burnham Institute for Medical Research, La Jolla, Calif., and University of California at San Diego; Paul Thompson, Ph.D., professor, neurology, University of California at Los Angeles; August 2007 Cell Stem Cell