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What a Mouse With ‘Golden Ears’ Can Tell Us About Older Brains
Most people lose some of their hearing as they get older. That’s a common part of aging. But for a small number of seniors—roughly 5 percent—it’s not the ears that are the problem, it’s the brain.
In new research published in the Neurobiology of Aging, NIDCD-supported scientists have created a mouse model that is comparable to an older adult who has the ears of a healthy 20-year-old but the brain of that 20-year-old’s great-grandmother. They created the model when they crossed two mouse strains commonly used to study age-related hearing loss: the CBA mouse, which loses its hearing gradually, like most people, and the C57 mouse, which loses its hearing rapidly in middle-age and becomes deaf later in life.
“The CBAs don’t breed very well. The C57s do. And so we thought, if we could get an offspring that breeds well but still has pretty good hearing, we would get an improved animal model for studying age-related hearing loss,” said Robert Frisina, University of Rochester Medical Center, and principal investigator on the study. “But we actually got one that had significantly better hearing than either mouse model. And that was a surprise.”
Using technologies that are commonly used to screen newborns for hearing loss, the researchers indeed found that the offspring of this cross, called the F1 generation—a.k.a. the golden ear mice—had significantly better hearing than the C57 mice and the CBA mice as they aged. On the other hand, they also found that the F1 mouse brain was less able to compensate for sound in background noise than the CBA mice. This is similar to an aging person whose ears are working fine but who still has trouble understanding speech when there’s a lot of background noise, such as at restaurants and dinner parties.
The research team’s next step is to figure out what molecular pathways are in place that may be protecting the F1 mouse’s hearing. “We definitely know why the F1’s hearing is better than the C57’s. It’s because it doesn’t have two copies of the recessive genes that the C57 has. So that’s a slam dunk,” said Frisina. “Why it has better hearing than the CBA, that’s still open.”
In addition, age-related hearing loss usually is the outcome of aging ears and an aging brain, and for this reason, it’s difficult for scientists to untangle the roles the two systems play. Because F1’s ears are still working well, the researchers plan to explore how the part of the brain that processes the sounds we hear ages on its own—without the added complication of processing distorted sounds from damaged ears. If researchers can pinpoint key changes occurring in the brain as a result of aging, they may be able to develop drug or gene therapies to ward off hearing loss in some older adults.