By Melissa McGovern, Ph.D.
Sensory cells in the cochlea, called hair cells, detect sound from the environment and allow us to hear. These cells are susceptible to loud noises, some medications, and the natural aging process. When hair cells are damaged by these environmental factors, they are often killed and not naturally replaced or regenerated, so the accumulation of this damage leads to loss of hearing that is currently irreversible.
While hearing aids and cochlear implants can offer some relief from hearing loss, the hearing these devices offer do not accurately recapitulate natural hearing. As a result, the development of therapeutics that convert nonsensory cells or supporting cells into hair cells is one advancing strategy for hearing restoration.
Historically, a significant hurdle to hair cell regeneration has been the age of the organ. Nonsensory cells in very young animals are more permissive and naturally convert into hair cells after damage. Unfortunately, as animals age, cells in their inner ears mature into distinct cell types that are no longer permissive to natural regeneration.
One reason that this occurs is that the systems controlling the expression of the genes important for cellular identity become stronger as the organ matures. This means that the genes that activate a hair cell identity are less accessible for expression in non-hair cells. Therefore, as we describe in the journal Proceedings of the National Academy of Sciences in January 2024, we have artificially expressed three key hair cell fate promoting proteins in nonsensory cells of adult mice and found that a significant number of these cells will convert into cells resembling hair cells (hair cell-like cells). This offers a potential strategy for hair cell regeneration.
Reprogrammed hair cells were added over a period of 9 weeks after expression of Atoh1, Gfi1, and Pou4f3 in the adult mouse model. From left to right: a typical complement of hair cells in the control cochlea at 5 weeks of age; the expression of Atoh1, Gfi1, and Pou4f3 reprogrammed nonsensory cells into hair cell-like cells in the apical and middle turns of the cochlea at 5 weeks of age; additional reprogrammed hair cells appear throughout the cochlea and increasing in number at 8 and 12 weeks of age. Credit: McGovern et al./PNAS
The genes that we expressed in nonsensory cells are transcription factors. Transcription factors are proteins that interact with the DNA in a cell and increase or decrease the expression of other genes. The three transcription factors that we used are Atoh1, Gfi1, and Pou4f3. These three factors work in combination with one another to turn on genes that are critical for the formation of hair cells during the development of the inner ear, and we found that this combination will also reprogram a non-hair cell into a hair cell in the adult mouse cochlea.
In addition, we found that as these cells are being reprogrammed into hair cells, they activate a well-known cell to cell signaling pathway. This signaling pathway, called Notch signaling, is used during inner ear development to form both hair cells and supporting cells. During reprogramming, as hair cells form, they signal through Notch to their neighbors, activating the expression of genes that induce the neighboring cells to become supporting cells. By expressing Atoh1, Gfi1, and Pou4f3 in mature cochlear nonsensory cells, we can create both hair cells and their neighboring supporting cells that are also necessary for hearing.
A previous Hearing Research study from the labs of Yehoash Raphael, Ph.D., and Andy Groves, Ph.D., used a similar technique to convert non-hair cells in to hair cell-like cells. They investigated reprogramming of the flat epithelium, which is created following severe damage. They used a virus to artificially express Atoh1, Gfi1, and Pou4f3 as well as a fourth hair cell transcription factor Six1, and they were able to reprogram cells into hair cell-like cells, but these cells were below the membrane on which normal hair cells and supporting cells live.
In our study, we have used a genetically modified mouse model to artificially express Atoh1, Gfi1, and Pou4f3, in cells that are surrounding the original hair cells without killing the original hair cells. The cells that lie medial and lateral to the original hair cells are thought to be the source of the flat epithelium, and we have found that a large number of these cells can be reprogrammed into hair cells in the undamaged cochlea. Further work will be required to better understand whether the cells of the flat epithelium can be reprogrammed into hair cell-like cells.
Our results suggest that mature cochlear nonsensory cells can be reprogrammed into sensory hair cells in the adult mouse model, providing a possible target for hair cell regeneration in mammals. Further work is ongoing in the Groves and McGovern labs to understand how to refine and improve hair cell reprogramming in order to best achieve hearing restoration.
Melissa McGovern, Ph.D., is a 2024 Emerging Research Grants scientist and the recipient of the Neil Segil Memorial Award in Hair Cell Regeneration. She is the lead author on the PNAS paper with Hearing Restoration Project member Andy Groves, Ph.D., and team at the Baylor College of Medicine. She is now an assistant professor in the department of otolaryngology at the University of Pittsburgh.
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