WASHINGTON, D.C.--Whether you're a body builder or 98-pound weakling, by the time you turn 45 your muscles will naturally begin to wither. Now, scientists have a provocative new theory that might explain in part this universal human decline: Dying cells secrete a protein that could trigger others to die as well, accelerating the body's deterioration. Experts say the idea of a cell-death domino effect, described here this week at the American Society for Cell Biology's annual meeting, merits further research.
Age-related muscle loss has puzzled many researchers, among them cell biologist Eugenia Wang of McGill University in Montreal. She and others have noted that muscle tissue from older rats can be grafted into young rat muscle and thrive, but that young muscle tissue doesn't do so well in older rats. Something in the older tissue, it seemed, was making cells in the younger tissue die.
Wang's latest work offers some preliminary evidence to support this theory. In her experiment, Wang starved cultured rat muscle cells in order to trigger apoptosis, a genetic program that instructs cells to self-destruct. She then bathed a separate dish of healthy cells with fluid from around the dying ones. The healthy cells self-destructed too. Since the deadly brew could be inactivated by boiling or adding protein-chopping enzymes--both of which destroy the function of proteins--Wang believes the "killing factor" released by the dying cells is a protein. Her previous work showed that as nerve cells attached to muscles begin to die with age, they cause apoptosis in some nearby muscle cells. She suspects that the dying muscle cells release the killer protein, triggering a slow domino killing effect that picks up speed over decades of life.
Wang's theory is "intriguing," says Charlotte Peterson, a molecular biologist at the University of Arkansas Medical School in Little Rock. But she and others want to see a link between cell suicide and age-related muscle loss as further evidence that a "killing factor" is at large in the body, not just the petri dish. Peterson predicts that pinning down the precise protein is likely to be "notoriously difficult."