The research program in my laboratory focuses on two broad themes. We work on the molecular basis of a number of physiological phenomena related to the hearing and balance organs. In addition we also work on regeneration in the auditory and vestibular system.
Electrical tuning is a phenomenon by which certain vertebrates discriminate between different frequencies of sound. Electrical resonance results when the inherent oscillation in the membrane potential of hair cells corresponds to sound of a particular frequency. This gives rise to a resonance and amplification of signal with consequent transmitter release from these cells. The inherent oscillation in membrane potential in a hair cell is brought about by an inward calcium current and an outward potassium current (calcium dependent). The molecular identity of these currents has been established with the Ca current carried by a L type DHP sensitive Ca channel, and the K current carried by the large conductance Ca activated K channel (BK). The systematic variation in the frequency of membrane potential oscillation in hair cells that occurs along the tonotopic axis is brought about primarily by a variation in the kinetic properties of the BK current. BK channels are made up of the Slo protein and a variable number of associated proteins. Previously we had shown that some of the variation in kinetic properties along the tonotopic axis was brought about by alternative splicing of the Slo gene. However, a large part of this variation in BK kinetics cannot be explained by alternative splicing alone. Our present hypothesis is that this variation in current is brought about by association with other proteins and systematic changes in phosphorylation that intersects with alternative splicing. For instance gene expression differences along the tonotopic axis stongly suggest that PKA is more active in low frequency hair cells while PKC is more active in high frequency hair cells. Moreover, we have determined novel interactions between CDK5 and Slo and demonstrated that CDK5 is expressed in increasing amounts in high frequency hair cells.
In addition, we have also demonstrated by fluorescence immunohistochemistry that the BK channel and the calcium channel are in fact in close physical approximation in these hair cells. There is considerable theoretical reasons to support this observation. We have determined that while localization to the basolateral surface of hair cells is determined by signals within the protein, a number of BK channel associated proteins also play a critical role in its surface expression. Additionally we determine that phosphorylation both directly (CDK5) and indirectly (PKC) affects BK channel expression on the surface of hair cells.
A goal in our lab is to identify the molecular events that prevent the adult mammalian cochlea and vestibular epithelium from regenerating new hair cells. We have previously concentrated on the bird auditory epithelium since it shows mitotic quiescence, but is able to mount a robust proliferative response to hair cell damage. We reasoned that understanding the molecular events that lead to the conversion of supporting cells from mitotic quiescence to proliferation would give us insights into the mammal. However, we have been discouraged by this strategy. We have gained a lot of insights into the molecular events that underlies regeneration, but have not been able to convert these insights into the mammal. For instance we determined that microRNA 181 was responsible for a large number of gene expression changes in the regenerating chick auditory epithelium. Transfection of microRNA181 results in a proliferative response in the bird auditory epithelium. Such an effect is not observed in mammalian utricles.
Our present efforts are now directed at understanding the molecular basis of how the vestibular epithelium in mammals undergoes an age associated loss in mitotic ability.