Overcoming Aging Related Hearing Loss- A Possibility In The Not So Distant Future

Hearing depends on a series of events that change sound waves in the air into electrical signals. A critical role in conversion of sound waves into electrical signals is carried out by Hair/sensory cells sitting on top of the basilar membrane in the cochlea. These hair cells can be differentiated into inner hair cells (IHCs) and outer hair cells (OHCs) in the cochlear canals of the inner ear. Once the vibrations cause the fluid inside the cochlea to ripple the hair cells move up and down, microscopic hair-like projections (known as stereocilia) that perch on top of the hair cells bump against an overlying structure and bend. Bending leads to opening of pore-like channels at the tips of the stereocilia. When that happens, chemicals rush into the cells, creating an electrical signal. The auditory nerve then carries these signals to the brain which interprets these signals into a sound that we recognize and understand.

Hearing Loss And The Lacunae in Overcoming It

According to the Centers for Disease Control, loss of hearing is a problem that affects about 8.5 percent of adults aged 55 to 64 in the U.S. This number increases to nearly 25 percent of those aged 65 to 74 and 50 percent of those who are 75 and older. Hearing loss due to aging, noise and certain cancer therapy drugs and antibiotics has been irreversible because scientists have not been able to reprogram existing cells to develop into the outer and inner ear sensory cells once they die. Earlier studies have implicated two genes in the specific differentiation of OHCs but no gene has yet been identified that triggers IHC differentiation. A transcription factor called INSM1 acts in the embryo to establish an OHC fate and prevent OHCs from turning into IHCs. When INSM1 is absent, embryonic OHCs mis-express genes that specify IHCs. 

A New Breakthrough Points To A Possible Route For Overcoming Hearing Loss

In a recent study published in Nature, scientists from the Northwestern University’s  Feinberg School of Medicine have discovered a single master gene (Tbx2) that programs ear hair cells into either outer or inner hair cells. Removal of this gene results in Inner Hair Cells (IHCs) developing into Outer Hair Cells (OHCs).  This interconversion between cell types shows that Tbx2 is needed to bifurcate and maintain the difference between the hair cell types. The study has also found the role of additional genes (ATOH1 and GF1) responsible for converting a non-hair cell to an ear hair cell. The study provides scientific evidence which helps in overcoming a lack of knowledge regarding  generation of specific ear hair cells.  which when lost lead to loss of hearing.

The Challenges And The Path Ahead

The study shows that there exists a high degree of plasticity to convert cells from one to another (IHCs to OHCs and vice versa); which can be used to generate or regenerate the specific type of cells needed artificially. This would help in overcoming the present inability to reprogram existing cells to develop into the outer and inner ear sensory cells once they die. However, whether and when this regeneration capability will be practical in terms of treating patients needs to be seen. Another difficulty that would need to be addressed is restoring/creating the other specialized supporting cells and properly innervated connections to the brain which would be essential for restoring hearing. The study however, is a giant leap in the positive direction.

Author

Arvind C. Shekhar