Do you know that 30 million people in the US over the age of 12 suffer from hearing loss in both ears? Globally, about 20 percent (1.5 billion) of the human population is hard of hearing. Of course, there are different types of hearing aids available on the market that could help but these could never restore the natural hearing mechanism in a patient. With increasing age or further damage to ears, hearing aids also stop working for many people. However, a new study offers valuable insights into how hearing loss could be reversed in mammals. 

Five years ago, a team of researchers at the University of Rochester Medical Center (URMC) was able to regrow cochlear hair cells in mice for the first time. These hair cells are found in the cochlear region of ears in all mammals. They sense sound vibrations, convert those into brain signals, and eventually allow a person to hear and understand the different sounds around them. The new study from URMC researchers sheds light on the underlying mechanism that allowed the ear hairs to regrow in mice.

“We know from our previous work that expression of an active growth gene, called ERBB2, was able to activate the growth of new hair cells (in mammals), but we didn’t fully understand why. This new study tells us how that activation is happening—a significant advance toward the ultimate goal of generating new cochlear hair cells in mammals,” said Patricia White, one of the study authors and a neuroscience professor at URMC.

How can hearing loss be reversed by regrowing hair cells?

In reptiles, fish, and birds, cochlear hairs that are responsible for detecting sound can regrow. However, the number of ear hairs in mammals is finite and there is no way for them to regrow naturally. For instance, humans have 16,000 hair cells in each ear. Over time these cells get damaged due to ear infections, loud noises, and aging, leading to decreasing hearing ability. This is also why many older people find it difficult to hear clearly.

During their 2018 study, the researchers discovered that birds could regrow hair cells because of triggered epidermal growth factor (EGF) receptors in their ears. They noticed that by activating a specific EGF receptor known as ERBB2 in mice they were able to spark cellular pathways that eventually reversed hearing loss in the animals.

To find out why and how this happened. This time, the study author performed single-cell RNA sequencing to compare ear cells with activated ERBB2 receptors against normal cells (with no EGF activation). They discovered that ERBB2 genes actually make the ear cells behave like stem cells. It makes the cell produce a protein called SPP1. 

The protein then activates the CD44 receptor in cochlear-supporting cells (hair cells in the cochlea or internal ear that support auditory function) which is responsible for hair cell regeneration. No such activity takes place in normal cells.

“Our results suggest that ectopic activation of ERBB2 signaling in cochlear supporting cells can alter the microenvironment, promoting proliferation and cell rearrangements. Together these results suggest a novel mechanism for inducing stem cell-like activity in the adult mammalian cochlea.” the researchers note.

Reversing hearing loss is no longer impossible   

Many patients who suffer from permanent hearing loss often feel hopeless because as far as they know, there is no cure for their condition. Studies like this are important because they give people hope that no disease or medical condition can stay incurable forever. This is probably the first time scientists have been able to clearly understand the underlying mechanism for regrowing ear hairs in mammals. 

These are valuable insights, as they reveal what could possibly reverse hearing loss in humans as well. However, we should also not forget that the findings from the current study are just the first set of clues scientists have come across in their quest to reverse hearing loss. There is still a long way to go. 

Dr. White said, “We plan to further investigate this phenomenon from a mechanistic perspective to determine whether it can improve auditory function after damage in mammals. That is the ultimate goal.”

Hopefully, these developments will someday make hearing loss a 100 percent treatable medical condition in humans.

The study is published in the journal Frontiers in Cellular Neuroscience.