hat makes a living cell commit "suicide?" Cell death, or apoptosis, sounds ominous, but it’s actually a necessary part of normal cellular activity. Mohanish Deshmukh, assistant professor of cell and developmental biology, works to understand exactly what happens when nerve cells, or neurons, die by apoptosis.

If a cell is damaged, and the damage is too extensive, the cell can activate a self-destruct mechanism. This programmed cell death is vital to the survival of the organism because it prevents the damaged cell from producing other damaged cells. "Apoptosis is also important during early embryonic development," Deshmukh explains. "The developing embryo generates more neurons than are actually needed to ensure that, if any neurons become lost on the way to their target area or are damaged, the body still has enough to make the proper connections. The body uses apoptosis in this case to eradicate unnecessary neurons."

In his research, Deshmukh uses "sympathetic neurons," which are part of the peripheral nervous system. Sympathetic neurons are an excellent model system, he says, largely because of their dependence on a single protein called nerve growth factor (NGF). "If these neurons are placed in culture in the presence of NGF, virtually one hundred percent of these neurons will survive, but if the NGF is removed, one hundred percent will die," Deshmukh says.

At the molecular level, apoptosis is an intricate web of cellular communication. After a cell activates the programmed cell death pathway, cytochrome c, an important protein for all oxygen-breathing organisms, is released from the mitochondria, the "powerhouse of the cell." Once released, cytochrome c binds to a group of proteins, which eventually leads to the activation of caspases, which are the actual "executioners" of cell death. Caspases destroy the cell by "chopping up" the DNA and other cellular proteins.

ased on the accepted model, Deshmukh questioned whether the release of cytochrome c was sufficient to activate caspases in neurons. To examine this, he injected cytochrome c into cells and found that caspases became activated. But when he injected cytochrome c into sympathetic neurons, caspases were not activated and the neurons remained alive. "Something in these neurons made them completely resistant to cytochrome c-induced apoptosis," Deshmukh says. "Normally, when neurons reach the point of cytochrome c release, they have been deprived of NGF for a period of time, so some other event is occurring which makes them more susceptible or more competent to die." To test this idea, Deshmukh injected cytochrome c into sympathetic neurons after depriving them of NGF, and found that under these conditions, caspases were immediately activated and the neurons died rapidly. This finding indicates that in addition to the release of cytochrome c, neuronal apoptosis requires a novel pathway called the development of competence. "It makes perfect sense that neurons would have evolved a system that has additional checks and balances in place compared to other cell types because neurons have to last the entire life of the organism," Deshmukh says. "Our lab is interested in understanding the molecular events that make up the development-of-competence pathway and determining whether it is unique to neurons."

To study this pathway, the Deshmukh lab uses a variety of biochemical and molecular techniques such as intracellular injections, localization of certain proteins using antibodies, and mice that have genes deleted in the pathway that is required for activated caspases. Knowledge gleaned from these experiments may eventually lead to new therapeutic strategies that will prevent neuronal loss after injury or disease. For instance, a group of proteins called inhibitors of apoptotic proteins (IAPs) are the "brakes" that stop caspases. Recent findings from Deshmukh’s lab demonstrate that the development-of-competence pathway somehow removes IAPs and that this removal, along with the release of cytochrome c, leads to neuronal apoptosis. The work is exciting, Deshmukh says, because molecules such as IAPs may have the potential to prevent neurons from dying in stroke, spinal cord injury, and neurodegenerative diseases such as Alzheimer’s, Huntington’s chorea, and Parkinson’s.