The studies proposed here continue our work on the biophysical mechanism of sensory transduction by hair cells, and are designed to confirm and expand the current model of transduction. Recent evidence from several labs has converged to suggest a unified model for hair-cell transduction- channel gating. This """"""""string hypothesis"""""""" maintains that displacements of the stereociliary bundle cause the distension of fine, filamentous tip links, which are observed in electron micrographs stretching from the distal tip of each stereocilium to the side of the adjacent, taller stereocilium. Tension in the link is thought to decrease the relative energy of the open state of the channels, and thus to control open channel probability. While the model is extremely attractive, there is as yet no direct evidence to confirm it. One study will investigate the relationship between displacement and the open probability of the channels, to see if it is characteristic of a filamentous structure that could pull channels open but not push them close. Channels will be blocked by iontophoresis of streptomycin, to see if more channels can be closed with the blocker than with displacement steps that relax the filament. A second study will determine whether enzymes that might decrease the elasticity of the filaments change the displacement sensitivity of the cell. This would also help indicate how many filaments are attached to each channel.
The third aim i s to understand the interaction of blocking drugs such as streptomycin with the channel's gate, as a way of determining whether the gating portion of the channel protein is intra- or extracellular. A fourth project is to find where calcium acts to regulate the resting bias on the transduction channels. Laser-activated photorelease of calcium amy pinpoint the calcium site with millisecond and micron resolution; this presumably is the site of the mechanism itself. The fifth aim is to look for movement of the hair bundle. The calcium and voltage dependence of the resting bias, together with the strings model, predict a voltage-dependent displacement of the bundle with a certain timecourse. Finding it would be further evidence for the model. This understanding of the gating mechanism may illuminate certain pathological conditions of the auditory system. In particular, strong evidence for the strings hypothesis would implicate these filaments in the temporary threshold shift caused by noise trauma.
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