Aug 2019 Tech Times

Researchers have identified a pair of proteins that can precisely control when sound-detecting cells known as hair cells develop in the inner ear. Mammals hear when sound vibrations travel through the cochlea. Lining inside this snail shell-looking structure are the sound detecting inner and outer hair cells, which transmit sound information to the brain.

About 90 percent of genetic hearing loss is caused by hair cell problems or damage to the auditory nerves that connect the hair cells to the brain. Deafness caused by exposure to viral infections or loud noises is caused by damage to hair cells. The problem with human hair cells is they cannot regenerate unlike those of other mammals and birds. This means that once the hair cells are damaged, hearing loss tend to be permanent.

Hair cells first develop at the outermost part of the cochlea, where precursor cells start to transform into hair cells. They undergo a wave of transformation that stops once they reach the inner part of the cochlea. Angelika Doetzlhofer, from the Johns Hopkins University School of Medicine, and colleagues, observed interesting behaviours of proteins Activin A and follistatin associated with the transformation of hair cells. They observed that the levels of follistatin were low in the area where the precursor cells were starting to transform into hair cells, and high at the innermost part of the cochlea's spiral where the precursor cells had not yet begun their transformation. Follistatin exhibited behaviour that is opposite of Activin A’s. "The two proteins perform a balancing act on precursor cells to control the orderly formation of hair cells along the cochlear spiral," Doetzlhofer said.

The researchers wanted to find out how the two proteins coordinate hair cell development, so they increased the levels of Activin A in the cochleas of normal mice. Precursor cells in the animals transformed the hair cells too early. In mice that overproduced follistatin or not produce any Activin A, researchers found that the hair cells formed late and appeared disorganised. These observations mean that the action of the two proteins are so precisely timed during the development of the hair cells that any disturbance can have a significant impact.

The discovery could pave the way for future treatment that can help restore hearing in people with irreversible deafness. The researchers said this research in hair cell development may have potential applications in treating deafness caused by damaged hair cells. "We are interested in how hair cells evolved because it's an interesting biological question," she said. "But we also want to use that knowledge to improve or develop new treatment strategies for hearing loss.”

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