Novel insights could pave the way for biological treatments of human hearing loss.
By Nesrine Benkafadar, Ph.D., and Stefan Heller, Ph.D.
Over 30 years ago, researchers reported that birds regenerate auditory hair cells. They showed that the surrounding supporting cells start dividing and regenerating hair cells when they die due to acoustic trauma. These findings inspired a continuing quest to find biological cures for hearing loss.
When avian hair cells are exposed to aminoglycosides, they begin to die. This leads to the activation of proteases (enzymes that cut proteins at specific locations). The receptor protein F2RL1 is inactive but becomes activated by proteases. This sets in motion a cascade of signaling in supporting cells that involves the growth factor HBEGF, followed by cell division. The result is that the dead hair cell is replaced with a new hair cell and the supporting cell has made a copy of itself. Credit: Benkafadar et al./Developmental Cell
For many years, research in this area was hampered by an unsurmountable roadblock: the paucity of hair cells and supporting cells inside the inner ear. For comparison, a single eye contains ≈ 125 million photoreceptor cells, whereas a cochlea harbors roughly 3,600 hair cells and an equally low number of supporting cells. Until a few years ago, biochemistry and cell-based drug screens required millions of cells, making it impossible to systematically research the precise mechanisms that initiate and sustain avian hair cell regeneration.
Hearing Health Foundation’s Hearing Restoration Project (HRP) was inspired by the initial findings that birds naturally regenerate hair cells and by the emergence of novel technologies that promised to overcome the roadblock of low cell numbers. For more than five years now, the collaborative group of 15 laboratories has used so-called single-cell technology to determine gene activity in hair cells and supporting cells from humans and mice on one side and from zebrafish and chickens on the other side. Humans and mice do not regenerate hair cells, whereas fish and birds robustly regenerate hair cells.
Our laboratory at Stanford University began to inventorize gene activity in chicken inner ear cells, initially in the undamaged organ. We then determined how gene expression changed when hair cells died in response to aminoglycoside antibiotic treatments.
As a result of an HRP-funded project, we reported in 2021 that the dying hair cells in the avian model communicate with their surrounding supporting cells. Now we are focusing on the supporting cells in birds. We compared gene expression changes in supporting cells during the demise of hair cells and before the supporting cells start dividing to regenerate hair cells. This comparison revealed a novel signaling mechanism that turned out to be essential for supporting cell division and also for hair cell regeneration in avians. Our results were published in the journal Developmental Cell in December 2023.
At the core of this signaling mechanism is a novel receptor protein that senses changes in the fluid surrounding the dying hair cells in birds. In supporting cells, this leads to a response that results in gene activation. This analysis revealed previously known and unknown genes. We then determined the order by which these genes must be active to ultimately initiate hair cell regeneration in birds.
This work provides a new starting point to investigate the natural triggers of hair cell regeneration in birds. We also identified novel genes that drive hair cell regeneration in the chicken inner ear.
These novel genes will be key for investigating their potential role in triggering a change in supporting cells in mammals, in the damaged mouse or human cochlea. We still need to fill in many details to fully understand how chicken hair cell regeneration happens.
At the same time, we are now working on manipulating genes in the mouse cochlea that are the equivalents of the genes we identified in chickens. Our goal is to find subtle changes in supporting cells in the adult mouse cochlea that could become a starting point for investigating new therapies. This work will also involve similar single-cell technologies that allowed these initial findings.
We are grateful for the work of fellow HRP member Edwin Rubel, Ph.D., as well as Brenda Ryals, Ph.D., Douglas Cotanche, Ph.D., and Jeffrey Corwin, Ph.D., for providing inspiration. Their investigations triggered this research. Likewise, we are grateful to the HRP and all our colleagues for the continued support that made this work possible.
The research was supported by the National Institutes of Health (R01-DC019619 and K08-DC019683), the American Hearing Research Foundation, the Hearing Restoration Project of the Hearing Health Foundation, and the Stanford Initiative to Cure Hearing Loss.
Hearing Restoration Project member Stefan Heller, Ph.D., is a professor of otolaryngology–head & neck surgery at Stanford University. He is also a 2001–2002 Emerging Research Grants scientist.
Nesrine Benkafadar, Ph.D., who led this 2023 study published in Developmental Cell and the 2021 research, is an instructor in the Heller lab.
Douglas Cotanche, Ph.D., is an ERG scientist who received funding in the 1980s and in 2001.
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I wanted to create a story that not only celebrated the beauty of differences but also conveyed the importance of empathy and understanding. My heart was set on crafting a tale that could empower children with hearing loss while also educating their peers about the significance of inclusivity.