In experiments on mice, researchers at John Hopkins University have identified two proteins responsible for growing cells related to hearing. These proteins are responsible for growing the hair cells in the inner ear of mammals that detect sound. (1) This new discovery may hold the key to restoring hearing loss.
How Does Hearing Work?

Sound waves travel through the ear canal to the eardrum. The eardrum then sends vibrations to the bones in the middle ear. The bones increase the vibrations and send them to the cochlea in the inner ear, which is filled with fluid. The vibrations cause the fluid in the cochlea to ripple. Hair cells line the cochlea in inner and outer rows, and they respond to the wave by moving up and down. (2)

At the wide end of the cochlea, high-pitched sounds are detected whereas lower-pitched sounds are heard closer to the center. As the hair cells move up and down, projections on top of the hair cells bend and create channels. Chemicals then flood into the cells and create an electrical signal. Finally, the signal travels to the brain via the auditory nerve where it converts into a sound we can actually use and make sense of. (2)

What researchers haven’t understood, is the molecular signals that differentiate the auditory sensory information and the graded pattern.

Hearing
What Causes Hearing Loss?

Damage to hair cells or auditory nerves cause hearing loss. This damage can occur from genetic factors, prolonged exposure to loud noises, infections, diabetes, high blood pressure, and toxic medications like chemotherapy. (2) Once the damage occurs, the hearing loss is irreversible, and unlike other mammals and birds, human hearing hair cells cannot regenerate.
Can We Restore Hearing Loss?

The short answer is: not yet. Hopkins researchers discovered the protein called Activin A and its antagonist called follistatin in mice. (3) The study shows these proteins are key regulators of hair cell differentiation and work in opposition to each other. (3) If too much Activin A is present, hair cells show up prematurely. (1) If follistatin is overproduced, or no Activin A at all, hair cells formed too late or were disorganized and scattered. (1) So it seems that Activin A and follistatin work intricately together, and any disturbance during development can negatively affect the organization of the cochlea, researchers said. (1)

It also seems that Activin A moves in a wave inward in the cochlea, while follistatin moves in a wave outward. (1) Additionally, the researchers discovered that the number of inner hairs being produced is limited and regulated by these signaling proteins. (3)

Reversing hearing loss is one of the goals of this research, they said. (1) Learning how hair cells grow and evolve helps understand how damaged hair cells can be replaced.
Reverse Hearing Loss

Current therapies for hearing loss include various types of hearing aids, cochlear implants, hearing systems anchored to the bone, listening devices, and medications that control other conditions. (2) Technology has advanced substantially over the years and created better aids and assisted hearing devices, but hearing loss treatments are limited. Real reversal of hearing loss has not been something scientists have been able to remedy. These new discoveries may lead to a better understanding of how to reverse or restore hearing loss. Though this research in hair cell development is rudimentary and fundamental, all great advancements in understanding the human body had to start somewhere.

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