Yehoash Raphael, Ph.D.
Hearing loss is often caused by the loss of auditory hair cells, due to loud acoustic signals or medications that have side effects in the ear. The loss of hair cells reduces the sensitivity of the ear and makes sounds like speech blurry and harder to comprehend. It is also the case that ears with trauma are more susceptible to further lesions because the natural protective mechanisms are impaired. Once lost, hair cells in the mature mammalian cochlea are not naturally replaced, making them perhaps the only type of epithelial cell (epithelium is a layer of cells that lines cavities and covers surfaces in the body) that is not replaced/regenerated. Due to the lack of spontaneous regeneration, the hearing loss caused by hair cell loss is permanent. One way to envision future biological/medical repair of the hair cell-depleted inner ear is to medically induce the generation of new hair cells.
Gfi1 enhances Atoh1-induced hair cell regeneration in the adult mouse cochlea. Images show cochleae of adult mice four weeks after hair cells were killed and the two different gene combinations—Atoh1 by itself, and Atoh1 and Gfi1 together—were injected into the scala media.
Transdifferentiation: Identifying Molecular Pathways That Can “Reprogram” Supporting Cells
Hair cells are surrounded by other cells that are generally called supporting cells. Both originate from a common ancestor cell in the embryonic ear. One idea is to identify molecular pathways that can “reprogram” supporting cells in deaf ears to turn them into new hair cells. This process is called transdifferentiation and implies a change in the identity of one cell type into another, manifested in the shape, molecular composition, and function. Transdifferentiation is a relatively rare biological phenomenon. Among the few examples of naturally occurring transdifferentiation in mammals are the balance organs in the inner ear, where the sensory cells are also hair cells and the transdifferentiation of supporting cells can replenish lost hair cells. The therapeutic strategy guiding our efforts for inducing transdifferentiation in the cochlea is guided by combining knowledge of molecular pathways active during development and during natural transdifferentiation in the balance organs.
The huge task is to identify genes or small molecules that can trigger the reprogramming of supporting cells into hair cells. This requires the identification of genes that promote the development of hair cells versus supporting cells. Several labs have performed experiments to express or activate genes that induce generation of hair cells, in supporting cells. In mature living mammals, induced expression of genes can be accomplished by gene transfer using inactivated viruses or by using drugs that regulate the molecular pathways that control these genes.
Research in the Raphael Lab Focuses on Atoh1 Gene
Research on transdifferentiation has focused on a gene called Atoh1 that is necessary for hair cell formation during development. Earlier work in my lab and elsewhere has shown that the artificial activation of Atoh1 with viruses can induce the appearance of cells that exhibit hair cell features in mature mammalian ears. While the data were promising and exciting, the efficiency was typically too low and the variability too large, indicating that a better approach that yielded a more efficient and reliable regeneration of hair cells is needed.
The Experiment: Looking at Atoh1 and Gfi1 in Tandem
Together with the Groves lab, we tested whether two hair cell genes, Atoh1 and Gfi1, in tandem could enhance regeneration efficiency. We generated viruses that included both genes and injected them into the inner ears of mice deafened by killing their hair cells. This allowed us to evaluate the combination of Atoh1 and Gfi1 in promoting regeneration in the adult deaf cochlea. Our December 2020 paper in Scientific Reports showed that the combination of Atoh1 and Gfi1 produces six times more new cells resembling hair cells than did Atoh1 alone. Although these new cells have many characteristics of hair cells, they do not appear like normal mature hair cells. We now face the task of identifying the means to enhance completion of their differentiation into mature functional hair cells. Combinatorial Atoh1 and Gfi1 treatment is thus a promising strategy to promote hair cell regeneration in the mature mammalian cochlea.
Study coauthors and Emerging Research Grants alumni Andy Groves, Ph.D. (below) and Yehoash Raphael, Ph.D., are members of HHF’s Hearing Restoration Project.
What’s New in Hearing and Balance Research
Our results suggest that mature cochlear supporting cells can be reprogrammed into sensory hair cells, providing a possible target for hair cell regeneration in mammals.