Hearing loss is normally permanent as sensory cells responsible for transmitting frequency information from the world around us to the brain get damaged from excessive noise and lifestyle factors as we age. Up until now, it's been challenging to selectively regrow these sensory cells that play an important part in transmitting sound through the outer and inner ear to the brain, but that might be about to change.

In a study involving mice, scientists from Northwestern University have identified a single master gene that can program ear hair cells (known as cochlear hair cells) into becoming either outer or inner ear hair cells required for hearing. The breakthrough is reported in the journal Nature.

“Our finding gives us the first clear cell switch to make one type versus the other,” said lead study author Jaime García-Añoveros, PhD, in a statement. “It will provide a previously unavailable tool to make an inner or outer hair cell. We have overcome a major hurdle.”

The ear is a complex organ, it's required for balance and it allows us to translate the frequency of sounds from the world around us into sensible sensory information for our brain to interpret. In the ear, special sensory hair cells play an important role. Firstly, the outer hair cells of the ear change shape in response to changes in pressure from sound waves that enter the ear and then transmit those signals to the inner hair cells. The inner hair cells transmit those vibrations onto sensory brain cells which then send the signals to the brain's auditory cortex where we comprehend the sounds we hear. 

“It’s like a ballet,” García-Añoveros said with awe, describing the coordinated movement of the inner and outer cells. “The outers crouch and jump and lift the inners further into the ear. The ear is a beautiful organ. There is no other organ in a mammal where the cells are so precisely positioned. (I mean, with micrometric precision). Otherwise, hearing doesn’t occur.”

Although scientists have been able to create artificial cochlear hair cells in the lab before, they have not been able to accurately differentiate them into these specific outer and inner types required for hearing. 

The team at Northwestern discovered that when a gene called Tbx2 was selectively activated in mice, it programs cochlear hair cells to become inner ear cells, however, when this gene was switched off, those cells turned into outer ear cells. Furthermore, they found a protocol that could derive these specific ear air cells from non-hair cell types. If provided with a cocktail of two genes, ATOH1 and GF1, cochlear hair cells could be derived from non-hair cells the scientists say. By selectively turning the "master gene" Tbx2 on or off, they were then able to create the specific outer and inner ear hair cell types that are essential for hearing. 

In future, the goal is to reprogram other cell types found in the ear, like support cells, into functional outer and inner ear hair cells that might help restore hearing when it's lost due to damage to the hair cells.

The authors do stress however that these findings are still at an experimental stage and there's still a long road ahead before it could be used to restore hearing in humans, but the discovery certainly remains a promising avenue to keep a close eye on.