Dewey Receives NIH Research Grant to Expand Studies on Mechanisms of Hearing in Mammals

The hair cells in your ears are different than those on your head, but arguably far more important. There are two types of hair cells in your ear, the inner and the outer cells, said James Dewey, Ph.D. The inner hair cells are connected to 95% of the auditory nerve neurons, and are primarily responsible for communicating information about sounds in our environment to the brain. But that’s not what he’s interested in. He’s interested in the outer hair cells, of which humans have almost three times as many cells as the inner hair cells. But just 5% of our auditory neurons connect to these cells, despite their abundance.

“The outer hair cells’ function has been speculated about for a long time,” Dr. Dewey said. “We’ve known that they’re important for hearing sensitivity.”

By that he means that the quietest sounds we can hear are determined by whether or not the outer hair cells are functioning properly.

“It’s long been thought that they are serving as a mechanical amplifier,” he said.

The NIH has awarded him an R01 grant to work on figuring out just this question. His previous research at the USC Caruso Department of Otolaryngology – Head and Neck Surgery has involved using optical coherence tomography (OCT) to image the structures of the inner ear in mice. These structures are usually impossible to see without invasive surgery, which can also have unintended consequences on the way the structure is working. OCT works by shining a laser on something to get an image back, and is usually used to evaluate a person’s retinas in the clinic.

“It’s kind of like a sonogram, except it’s with light instead of sound,” Dewey said. “You can use this technique to image the structures of the inner ear non-invasively, at least in rodents where the bone is thin enough to see through optically.”

Recently this technique has revolutionized the ability for scientists to see what is happening in the inner ear mechanically, without being limited to the motion of a single membrane.

“We can actually image the whole cochlea in 3D and make measurements of the vibrations at different depths in the organ and from the tops and bottoms of the hair cells. So we can really see what’s happening in a living animal,” he said. “For the first time, we can start to put together a more clear picture of what’s happening in a living animal and not just in a dish.”

His earlier R21 research was focused on providing comprehensive evidence that the outer hair cells do, in fact, respond to sound by changing length, characterizing their movements in detail and determining whether these changes affect how the organ vibrates as a whole to influence hearing in mammals.

The research hopes to work toward determining how exactly these cells contribute to hearing so that therapies can eventually target these cells to restore hearing in people with damaged outer hair cells.

“We know that outer hair cell damage is actually one of the most common causes of hearing loss… especially age-related hearing loss,” Dewey said. “We know that they’re important, but we don’t know what we need to actually restore in order to ameliorate the hearing loss.”

Other researchers are working on figuring out how to regenerate these outer hair cells. But Dewey’s lab is focused on the mechanistic side of things.

“These hair cells, you’re born with them and you’re stuck with them. So if they die, you lose them; they don’t regenerate in mammals,” he said. “Even if we can regenerate the cells, whether or not they’re working properly or they’re working in the way that we need them to, to restore hearing remains an open question. So this research is providing the basic knowledge that we need to move toward that in the future.”