Noise-induced hearing loss has affected millions of Americans and is currently an irreversible condition. A team of audiologists have successfully restored hearing in noise-deafened mice by increasing production of a certain protein within the inner ear. The researchers believe this could lead to new human therapies. Gabriel Corfas of the University of Michigan Medical School’s Kresge Hearing Research Institute was the senior author of the paper, published in the journal eLife.

When sound waves enter the inner ear, they bend over specialized hair cells called stereocilia. The bent hairs use ribbon synapses to quickly communicate with nerve cells, sending the signal to the brain where it is interpreted as sound. This pathway can become damaged through normal aging, overexposure to loud noises, or through acute trauma called a blast injury. Sometimes this damage and hearing loss is only temporary, but it can also be permanent.

“It has become apparent that hearing loss due to damaged ribbon synapses is a very common and challenging problem, whether it’s due to noise or normal aging,” Corfas said in a press release. “We began this work 15 years ago to answer very basic questions about the inner ear, and now we have been able to restore hearing after partial deafening with noise, a common problem for people. It’s very exciting.”

Image: Hair cells (blue) are connected to nerve cells (green) via specialized ribbon synapses (red). Credit: Corfas Lab, University of Michigan

Corfas’s team used mice to study Neurotrophin-3 (Ntf3), a protein that is essential for formation and maintenance of ribbon synapses. Some of the mice were genetically engineered to have inner ear Ntf3 production bolstered on command. After the mice had been partially deafened through exposure to loud noises, the researchers turned on the additional Ntf3 production. Hearing tests administered two weeks later revealed a reversal of deafness in the mice expressing larger amounts of Ntf3, and had recovered much better than control mice without the extra protein.

The team also investigated brain derived neurotrophic factor (Bdnf), another protein integral to to ribbon synapses. However, boosting production of Bdnf in mice did not restore hearing.

Following the success of this study, Corfas is hoping to find industrial partners to target Neurotrophin-3 in humans. He believes that there could be medication capable of turning up production of the protein in hopes that it could also reverse partial deafness. If he is successful in restoring human synapses for hearing, the approach could even be used to treat neurodegenerative diseases by improving neuronal activity through repaired synapses.

“This brings supporting cells into the spotlight, and starts to show how much they contribute to plasticity, development and maintenance of neural connections,” Corfas concluded.

[Header Image: Paul Townsend via flickr, CC BY-ND 2.0]