Experts Hopeful New Discovery May Lead to Pathway for Hearing Loss Reversal

A study led by investigators in the Eaton-Peabody Laboratories (EPL) demonstrates an improved way of studying hair cell regeneration in the inner ear, moving scientists closer in their quest to developing new treatments for permanent hearing loss.

Over 400 million people worldwide live with sensorineural hearing loss, which occurs most often when hair cells, the cells in the inner ear responsible for hearing, deteriorate. Aging, infections, medications or exposure to loud noises can damage these cells. There are no built-in replacements for the damaged hair cells – they become permanently lost, resulting in irreversible hearing loss.

Scientists have sought new ways to regenerate hair cells in humans by looking toward other species for clues. Non-mammals like birds and fish, for example, can regenerate damaged hair cells. Identifying which molecular pathways allow hair cells to regenerate could help researchers better understand the mechanisms needed to reverse hearing loss. However, studying these pathways has been a challenge for researchers who have long relied on complex mouse models and a limited set of relevant tools in a dish.

“One of the biggest challenges has been the limited number of cells in the cochlea and the lack of high throughput models to study hair cell regeneration,” said Dunia Abdul-Aziz, MD, an EPL investigator and ENT surgeon at Mass Eye and Ear. “With the advent of inner ear organoids, we can now robustly study this process.”

Dr. Abdul-Aziz and Albert Edge, PhD, director of the Tillotson Cell Biology Unit and principal investigator in the EPL, have recently reported a novel approach that combines genetic tools with organoid technology to uncover a promising pathway involved in hair cell regeneration. In their study published in Stem Cell Reports, they reveal a new molecular player called “hypermethylated in cancer 1,” or “HIC1,” that hinders the regeneration of hair cells in mammals.

Their finding could change the way researchers study hair loss restoration for years to come.

A new way of testing molecular pathways

Scientists studying the brain often run into the same issue facing scientists studying the inner ear; brains and inner ears can only be closely examined upon autopsy, therefore limiting how much researchers can learn about the function of their tissues. The emergence of organoids, or artificially grown tissues and cell masses, has changed how scientists have approached translational research in recent years.

“Organoids are being used to study cells from pretty much every organ in the body; for the brain, skin, intestinal epithelium,” explained Dr. Edge. “Our identification of cells in the cochlea that could multiply and turn into hair cells has made it possible to grow cochlear organoids in a culture system and manipulate different factors to study their effects.”

Testing hair cell development in a mammal’s inner ear typically requires an abundance of cells called progenitor cells, or stem cells, that are capable of being differentiated, or turned into, hair cells. In need of a sufficient amount of progenitor cells for testing, Dr. Abdul-Aziz and Dr. Edge turned to their new organoid system. They built the organoid by first isolating and expanding progenitor cells from newborn mouse cochlea. Then, they genetically modified the organoids in a way that allowed the team to specifically target specific proteins. After 10 days, the cochlear organoid was ready for testing.

Targeting the gene responsible for hair cell development

Understanding why hair cells naturally regenerate in some species and not others begins with the gene ATOH1. Across species, ATOH1 is an essential gene required for hair cell development. Progenitor cells differentiate into hair cells in the inner ear through expression of ATOH1. And, while non-mammals, such as a bird, can replace damaged hair cells throughout their lives, mammals lose this function after their first week of life.

Through a series of previous studies, scientists have learned that hair cells cannot form without ATOH1 and that more hair cells can be made by overexpressing it. Understanding what factors repress ATOH1 holds significant promise for hair cell restoration. Previous studies indicated that the protein HIC1 repressed ATOH1 in the brain and in the intestine. These studies encouraged Drs. Abdul-Aziz and Edge to hypothesize that HIC1 may contribute to silencing of ATOH1 and the differentiation of hair cells in the inner ear. Their study would be the first time HIC1 had been tested to modify hair cell regeneration.

In the new study, the researchers used genetic tools to limit the expression of HIC1 in cochlear organoids and observed changes in ATOH1 levels. They saw that ATOH1 expression increased, and moreover, that there were markedly more hair cells generated from progenitor cells. Conversely, when the researchers overexpressed HIC1 in the organoid model, they witnessed ATOH1 inhibition and a subsequent decrease in hair cells. They went on to study the mechanism of HIC1’s repression, which is caused in part by modulating a key developmental pathway known as “Wnt signaling.”

“Fundamentally, we seek to understand how ATOH1 and other hair cell genes are regulated,” Dr. Abdul-Aziz explained. “And by targeting these pathways in future studies, we might be able to help spur hair cell regeneration in the inner ear.”

Future expectations for hair cell regeneration research

Drs. Abdul-Aziz and Edge believe that finding ways to promote and control ATOH1 expression will be crucial to reversing hearing loss. According to Dr. Abdul-Aziz, HIC1 is one of many factors playing a role in regulating hair cell development. Their findings will provide researchers with a logical approach to test other candidate regulators in addition to HIC1.

The findings verified a novel system by which other molecular pathways can be studied. The organoid model, and the genetic toolkit, can be adjusted to test countless other genes and regulators, all of which could help move researchers a step closer to restoring hearing.

“This is a big step forward in the exciting work being done in the lab to understand what prevents mammalian hair cells from regenerating,” Dr. Edge said. “The sky is the limit with these organoid models.”

Funding for the study performed by Dr. Abdul-Aziz and Dr. Edge was made possible by grants from the National Institute of Health, Hearing Health Foundation, American Academy of Otolaryngology, American Neurotology Society.