HIV is double trouble for brain.

People who live a long time while infected with HIV sometimes develop dementia. The virus that causes AIDS is known to damage brain cells, and it now appears that the virus halts the creation of new neurons as well.

A single protein in the virus' outer shell triggers both dementia-inducing effects, new research shows. Brains normally generate a steady stream of new nerve cells in the hippocampus, a region associated with learning. But the HIV protein called gp120 inhibits the stem cells in the brain from producing new nerve cells.

However, "there is a silver lining to this," says research leader Stuart A. Lipton of the University of California, San Diego. That's because gp120's ability both to kill mature brain cells and to hinder the birth of new ones depends on a single brain-cell enzyme. This enzyme, called p38 mitogen-activated protein kinase (MAPK), could provide a target for drugs against the dementia.

"If you can inhibit that enzyme, you might be able to reduce the brain damage," Lipton says. Drugs that block MAPK are currently in late-stage clinical trials for treating arthritis.

Lipton's team had previously demonstrated gp120's lethal effect on mature neurons by injecting it into mice. In the new research, reported in the August Cell Stem Cell, mice getting such injections ended up with few stem cells, which appeared not to be dividing.--P.B.