Hair Cell Biophysics
Our laboratory is interested in identifying the cellular and molecular mechanisms responsible for the conversion of sound waves into signals recognized by the brain. The primary sensory cell, the hair cell, accomplishes this remarkable feat with unprecedented sensitivity and fidelity. As we only have one set of hair cells for our lifetime, loss of these sensory cells due to noise exposure, aging or exposure to toxic agents results in permanent hearing loss. Understanding the mechanisms by which these cells function will better enable us to develop treatment plans and strategies for preventing, alleviating and replacing hair cell loss. To this end there are four basic areas of research in the laboratory.
First is to characterize the multiple roles of mechanotransduction in hearing by delineating the mechanisms by which this process occurs. Figure 1A shows an inner hair cell bundle that we can record electrically and image optically, while Figure 1B demonstrates the high resolution we can obtain using two photon imaging of living hair cells. Figure 1C presents an example of a hair bundle imaged at high speeds (1000 fps) while recording electrically from the soma and stimulating mechanically (the probe can be seen in the image). Figure 1D is an example of an electrical recording obtained while mechanically stimulating a hair bundle. We are presently using electrophysiologic, imaging and molecular tools to manipulate and probe the sensory hair bundle. To date our work has provided information as to how frequency selectivity, sensitivity and the remarkable dynamic range of the cochlea are established. We have also shown how mechanotransduction sets the resting potential of the hair cell that is critical for maintaining continual synaptic activity. Identifying the molecules and mechanisms of mechanotransduction will better enable us to design preventive strategies and therapeutic interventions to alleviate hearing loss.
Secondly we are interested in synaptic transmission, how the sensory hair cell communicates with the brain. As the success of hearing aids, cochlear implants and even hair cell regenerative techniques requires an understanding of nerve activity and the ability to keep the nerve viable, we are investigating synaptic transmission and postsynaptic processing at its fundamental level. Figure 1E is a 3D reconstruction of a living afferent nerve fiber using two photon imaging. Figure 1F shows a schematic representation of the synaptic specialization, the ribbon synapse, whose function we are investigating. To date we have elucidated a possible role in vesicle trafficking for these ribbons and demonstrated a linearization of release properties that is critical for information transfer.
Our third direction is identifying factors that control hair cell fate, whether these be intrinsic to the hair cell or provided by external means. A clear developmental pathway to a mature hair cell including the mechanisms underlying frequency specializations is required if regenerative approaches to hair cell replacement are to be successful. To this end, in collaboration with John Brigande, we have recently been able to induce the overproduction of sensory hair cells and to demonstrate that these ‘extra’ cells had morphological and electrical properties similar to the native cells. In addition, we have begun mapping the developmental maturation of hair cells in vivo and in vitro to identify intrinsic and extrinsic factors required for development.
And finally we are embarking on a new direction, the development of nontoxic aminoglycosides. By identifying mechanisms by which these molecules enter and remain in hair cells we hope to develop new agents that maintain their antibiotic activity but lose their ototoxic nature. Although this approach is at its very beginning, we have obtained important proof of principal data, suggesting that mechanotransduction is the site of entry and hope to capitalize on the biophysical properties of the mechanotransducer channel (Figure 1G) in order to develop these new compounds.
