Although AIDS is not usually considered a neurological disorder, the human immunodeficiency virus (HIV) can and does attack the brain—resulting in tremors, memory impairments, even dementia. Researchers have now identified a route through which the virus wreaks havoc on brain cells. The finding, appearing online in the Nov. 14 issue of the open-access journal PLoS One, may point to new approaches for treating a phase of the disease that is ominously on the increase.
“It may be that more people with AIDS are living long enough to have neurological symptoms,” says Harris Gelbard, a neuroscientist at the University of Rochester and lead author of the study. “It’s also possible that the problems were there all along, but when so many patients were dying from more readily apparent diseases, like pneumonia, no one thought to test them for brain disorders.”
The death toll from AIDS has been sharply decreasing since the 1990s, thanks to the advent of powerful drug combinations known as highly active antiretroviral therapy. These drug “cocktails” can reduce HIV in patients’ blood to nearly undetectable levels while improving overall health. But the drugs have been less successful at eliminating the virus within the brain. According to current estimates, one out of three people living with HIV may have an associated neurological disease, and ongoing studies suggest the rate may be as high as one out of two.
”Even patients with a viral load close to zero can suffer from AIDS-related brain disorders,” Gelbard says. “Antiretroviral drugs stop HIV from making copies of itself. But a virus like this, which is still around after two decades of efforts to eradicate it, obviously has other defenses.”
One such defense is a protein known by its abbreviation, Tat (Trans-Activator of Transcription), which helps the virus cripple infection-fighting immune cells, leaving patients unable to fend off disease. In the brain, HIV hijacks the immune cells and forces them to produce viral proteins, including Tat. These, in turn, make their way into neurons, effectively throwing a wrench into the cells’ internal machinery.
In earlier research, Gelbard’s lab and others demonstrated that when cultured neurons are exposed to Tat, the cells’ energy generators (called mitochondria) are thrown into overdrive. The ramped-up energy output stresses the neurons until they begin the process of cell death known as apoptosis.
One step in this self-destruct sequence is a rapid loss of calcium. Because calcium is stored and released in another part of the neuron, the endoplasmic reticulum (ER), Gelbard and colleagues zeroed in on this area as the probable site of Tat activity.
The new study shows, for the first time, both the specific effect of Tat and the receptor where the protein acts in both mitochondria and the ER, thus unveiling a target for future medications. Working with cultured neurons taken from the mouse cortex—a part of the brain that, in humans, handles thinking, analysis and memory—the investigators confirmed that Tat triggers the “unfolded protein response.” Assembling or folding proteins into the shape needed to construct enzymes and other structures is the ER’s main job.
A temporary halt in protein folding can help the cell withstand sudden stress, such as infection. But this emergency measure, if prolonged, can lead to plummeting calcium, excessive energy output from the mitochondria and, ultimately, the death of the cell.
Cortical neurons exposed to Tat demonstrated this process in striking, visible detail, Gelbard says. “In healthy neurons the ER is barely visible, but in neurons exposed to Tat for as little as 15 minutes, the ER looks like an ant farm as it acutely undergoes the unfolded protein response.” (See accompanying photos.)
|This is an electron micrograph of a normal cortical neuron where the rough endoplasmic reticulum is hardly visible. (Image courtesy of Harris A. Gelbard)||This is an electron micrograph of a cortical neuron that has been exposed to Tat for 15 minutes. The rough endoplasmic reticulum is grossly dilated, distended and looks like an "ant farm." because it is acutely undergoing an unfolded protein response. This type of change is completely reversed by exposure to dantrolene (as shown in the PLoS One paper). (Image courtesy of Harris A. Gelbard)|
New target: reversing the damage
These changes, though profound, were completely reversed when the researchers used the drug dantrolene to block the ryanodine receptor, which studs the ER and plays a key role in calcium storage. “Our study offers proof of principle and validates a new target for drug development efforts, such as small molecules or genetically modified compounds that could block the interaction between Tat and this receptor,” Gelbard says, though he cautions that blockading these receptors throughout the brain is not currently a feasible treatment approach.
Even at sub-lethal levels, Tat can still impair brain function, forcing neurons to work harder while becoming less efficient at their jobs. Studies of brain tissue from AIDS patients show a decrease in synapses, or points of connection where neurons relay signals—another of the brain’s responses to stress that can lead to damage if the emergency is not resolved.
To find out whether Tat’s effects within neurons might explain the observed decrease in synapses and the devastating neurological symptoms of AIDS, the researchers studied cortical tissue taken after death from six AIDS patients, three with no signs of brain disease and three with HIV encephalitis or dementia. Again, the patients with brain disorders showed abnormalities in both the mitochondria and the ER, similar to the results in cultured neurons.
“The study is a standout in showing how the viral protein Tat can cause neuronal injury,” says Howard Gendelman of the University of Nebraska Medical Center, who was not involved in the study. “The team uses impressive experimental rigor to bring forward an older question in the field of neuroAIDS with fresh new ideas in regards to receptor pharmacology.”
On a separate project Gelbard and Gendelman are using several imaging technologies along with biochemical analysis to study the effects of HIV in mice with a condition similar to AIDS-related neurological impairment. “Our preliminary data suggest that measurable changes occur in several chemical messengers and molecules involved in inflammation as the disease progresses in these mice,” Gelbard says. “We are continuing to use imaging to measure brain function and correlate it with our molecular models of disease progression to help develop the next generation of therapeutics to restore synaptic function.”
Gelbard adds that the implications may extend beyond brain conditions directly related to AIDS. “Unfolded or misfolded neuronal proteins are a feature of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, as well,” he says. “As patients living with AIDS grow older along with the rest of the U.S. population, they may be doubly at risk for brain disease, making the need to treat this aspect of AIDS more urgent than ever.”