07-06-2006

A protein associated with a disorder that causes deafness and blindness in people may be a key to unraveling one of the foremost mysteries of how we hear, says a study in the June 28 issue of the Journal of Neuroscience.

Scientists with the National Institute on Deafness and Other Communication Disorders (NIDCD), one of the National Institutes of Health (NIH), and the University of Sussex, Brighton, United Kingdom, have identified protocadherin-15 as a likely player in the moment-of-truth reaction in which sound is converted into electrical signals. Protocadherin-15 is a protein made by a gene that causes one form of type 1 Usher syndrome, the most common cause of deaf-blindness in humans.

The findings will not only provide insight into how hearing takes place at the molecular level, but also may help scientists figure out why some people temporarily lose their hearing after being exposed to loud noise, only to regain it a day or two later.

“These findings offer a more precise picture of the complicated processes involved with our sense of hearing,” says Elias A. Zerhouni, MD, director of the NIH. “With roughly 15% of American adults reporting some degree of hearing loss, it is increasingly vital that we continue making inroads into our understanding of these processes, helping us seek new and better treatments, and opening the doors to better hearing health for Americans.”

Researchers have long known that hair cells, small sensory cells in the inner ear, convert sound energy into electrical signals that travel to the brain. When fluid in the inner ear is set into motion by vibrations emanating from the bones of the middle ear, the rippling effect causes bristly structures atop the hair cells to bump up against an overlying membrane and to deflect. The bristles, called stereocilia, are arranged in tiers and connected by minute links. As the stereocilia are deflected, pore-like channels on the surface of the stereocilia open up, allowing potassium to rush in, and generate an electrical signal.

Because the “tip link”—the link that connects the tip of the shorter stereocilium to the side of the adjacent, taller stereocilium—must be present for the channel to function, scientists believe that this structure may be responsible for opening and closing the channel gate. Researchers suggest that if they can learn the makeup of the tip link, they’ll be that much closer to understanding how the gate mechanism operates.

“This research identifies protocadherin-15 to be one of the proteins associated with the tip link, thus finally answering a question that has been baffling researchers for years,” says James F. Battey, Jr, MD, PhD, director of the NIDCD. “Thanks to the collaborative effort among these researchers, we are now at the closest point we have ever been to understanding the mechanism by which the ear converts mechanical energy—or energy of motion—into a form of energy that the brain can recognize as sound.”