Researchers at Johns Hopkins Medicine revealed that they have discovered a pair of proteins that exhibit potential to help restore hearing by boosting the growth and health of cells that facilitate hearing.
The ability of an individual to hear depends significantly on cells in the ear known as hair cells. These are sensory receptors of the auditory and vestibular systems present in all vertebrates. They detect sound and motion to initiate necessary processes for hearing and balance.
Scientists have been trying to figure out ways they can promote the formation of these cells. The new research offers a direction for further work targeted at achieving this aim, with the identification of proteins that could be helpful.
“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,” said Angelika Doetzlhofer, Ph.D., of Johns Hopkins University School of Medicine.
The researchers were able to identify these beneficial proteins in mice with the aid of genetic tools. The findings appeared in the journal eLife.
Role of Hair Cells in Hearing
Sound vibrations pass through the cochlea for hearing to be possible. Within this structure, there are two types of cells that help to detect sound, namely: inner hair cells and outer hair cells. These, in turn, transmit detected sound to the brain.
Most cases of hearing loss are as a result of damage to hair cells or to the auditory nerves connecting them to the brain. Certain viral infections and prolonged exposure to loud noises can damage these sound-detecting cells, thereby leading to loss of hearing.
What makes damage to hair cells more worrisome is that humans lack the ability to regenerate them, unlike what obtains in birds and other mammals. Their loss can potentially lead to permanent hearing loss.
The development of the cells begins in the outermost region of the cochlea, where precursor cells start to transform into hair cells. The transformation completes in the inner part of the cochlea.
Researchers in this current study wanted to find out what facilitates this process at the molecular level.
Effects of Activin A and Follistatin
Doetzlhofer and her team evaluated the roles of different proteins and observed that two, Activin A and follistatin, appeared to contribute the most to hearing.
Amounts of the protein Activin A were highest in the outermost portion of the cochlea, where precursor cells were transforming into hair cells, while those of follistatin were low there. However, the levels of the latter were higher in the innermost part of the cochlea.
The behaviors of the two proteins were essentially opposite. While Activin A appeared to move in a wave-like fashion inward, follistatin moved in similar manner outward.
The researchers went further to study the effects of the proteins individually. They observed premature appearance of hair cells along the cochlea spiral when they increased the amounts of Activin A in the cochlea of healthy mice. Increased levels of follistatin, on the other hand, led to late appearance of hair cells, which looked disordered and scattered.
Doetzlhofer said the proteins seemed to perform a “balancing act” on precursor cells to promote proper formation of hair cells in the cochlea.
“The action of Activin A and follistatin is so precisely timed during development that any disturbance can negatively affect the organization of the cochlea,” the associate professor of neuroscience said. “It’s like building a house – if the foundation is not laid correctly, anything built upon it is affected.”
The researchers hope their findings will prove useful for developing therapies that can help restore hearing in individuals with irreversible deafness.