Hair cells (HCs) convert sound and balance signals into electrical impulses in the inner ear, including the cochlea and the vestibular endorgans, with remarkable precision and sensitivity. Our long-term goal is to stimulate HC regeneration in human inner ears and to enable the functional innervation of HCs by neurons. Hair cells are terminally-differentiated cells. Once hair cells are lost due to noise, ototoxic drugs or aging, there is no effective way to stimulate HC regeneration in mature inner ears. However, recent studies from our group and others have demonstrated very encouraging results: new HCs may be formed from stem cells.
We know very little about how to induce HC regeneration in a mature sensory epithelia in the auditory and vestibular organs. Indeed, determination of the mechanisms of induction of HCs and the assembly of the functional machinery of HCs in the mature cochlea has direct relevance to our understanding of how a HC may be derived from specific human embryonic stem cells (hESCs). Strong evidence from data in developmental cell biology and neurophysiology motivates our hypothesis that the specific factors regulating HC differentiation interact to confer their functions and that specific hESC-types have the potential to differentiate into HCs and their innervating neurons. We further predicted that newly differentiated HCs assemble their transduction apparatus and ionic currents in a coordinated fashion to achieve the cell’s sensitivity.
The proposed research identify some of the candidate factors and their mechanisms of interactions that are required to induce HC differentiation from hESCs. The project determined the hESC-types, which have the competence to transform into HCs. We are assessing whether a HC assembles its entire transduction apparatus and ionic conductance simultaneously, at a specific stage in the process of differentiation, or whether the assembly of the final apparatus entails multiple steps during maturation. Moreover, these studies should reveal how HC coordinates and regulates the mechano-electrical apparatus, information that might be exploited to induce regeneration and functional transduction apparatus assembly after damage.
To increase our chances of inducing hESCs to differentiate into HC and spiral ganglia neurons (SGNs), we used a variety of hESCs (these include; NIH code: WA01, WA07, WA09, WA14) from the UC Davis repository under Dr. Ronald Li, our collaborator. Using a cocktail culture media these hESCs, WA07 were induced to differentiate into Myo 7A positive cells, HC marker. Additionally, we have been able to identify cells that have differentiated into neurons
Of particular importance to auditory and vestibular science is the possibility that a rational design of a cocktail of factors may be assembled for ‘biological implants’, as our understanding of the mechanisms of regeneration of HCs becomes more refined. Since the mechanisms used by the internal ear may be expressed in different forms by other signal transduction systems, these studies are providing novel insights into such areas as protein-protein interaction, cell proliferation, developmental processes, and hESC signaling in general.