Mouse Cochlea with hair cells shown in green and auditory nerves in red (Photo by Doetzlhofer Lab)
New research at Johns Hopkins may be able to restore hearing to people who have suffered what was previously thought to be irreversible hearing loss. This will give new hope to millions of people who are hearing impaired.
The study focused on the sound detecting hair cells in the inner ear. Once these hairs are damaged due to loud noises – yes loud music can cause hearing loss – or viral infections, they do not regenerate in humans; unlike other mammals.
We hear when sound vibrations travel through the cochlea that is lined with inner and outer hair cells which transmit sound information to our brains. Around 90 percent of hearing impairment is caused by the damage or loss of the hair cells or damage to the auditory nerve.
The researchers at the university in Baltimore, Maryland, used genetic tools in mice to identify a pair of proteins that control when the sound detecting hair cells develop in the ear canals of mammals. This discovery, which was published in the June 12 edition of eLife, could hold the key to restoring hearing in people who have suffered irreversible hearing impairment.
“Scientists in our field have long been looking for the molecular signals that trigger the formation of the hair cells that sense and transmit sound,” said Angelika Doetzlhofer, Ph.D., associate professor of neuroscience at the Johns Hopkins University School of Medicine in a news release.
“These hair cells are a major player in hearing loss and knowing more about how they develop will help us figure out ways to replace hair cells that are damaged,” she said.
Scientists already understand the first step in hair cell birth, which is when the cells develop into the hairs that transmit sound in the cochlea. Learning exactly where this formation begins, was the basis on Doetzlhofer and her teams search for what the cues are that causes this transformation.
Two of the proteins that they studied, Activin A and follistatin stood out. Activin A seemed to move in a wave-like motion inward, while follistatin moved in a wave-like motion outward.
“In nature, we knew that Activin A and follistatin work in opposite ways to regulate cells,” said Doetzlhofer. “And so, it seems, based on our findings like in the ear, the two proteins perform a balancing act on precursor cells to control the orderly formation of hair cells along the cochlear spiral.”
Then the team studied the development of the hair cells in mice, and they changed the levels of the proteins to see how they affected the process and found that, "the action of Activin A and follistatin is so precisely timed during development that any disturbance can negatively affect the organization of the cochlea,” said Doetzlhofer. “It’s like building a house — if the foundation is not laid correctly, anything built upon it is affected.”
The researchers said that while this research in the hair cell development is very fundamental, it could have far-reaching implications in treating the hearing impairment that is caused by damaged hair cells. Reversing something that has previously believed to be permanent could help millions of people worldwide to be able to better communicate with their loved ones, hear music again, and live fuller lives.
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