Biophysical Study Of Sensory Transduction Mechanisms

National Institute On Deafness And Other Communication Disorders

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Abstract

Hearing of mammals depends on hair cells, both inner and outer, which convert mechanical signal into electrical signal to transmit to the brain. Outer hair cells are of particular significance in the ear's sensitivity and frequency selectivity, critical factors for communication. The role of outer hair cells stems from their property that these sensory cells are also motors to amplify weak signals. We have previously established that the hair cell motor uses electrical energy available at the plasma membrane in a manner similar to piezoelectricity, based on the coupling of electric charge transfer across the membrane with membrane area changes. Specifically, this motility can be reasonably explained by a simple two state model in which two states differ in charge and membrane area. The area difference is determined by tension dependence of the motor activity. It has been shown that the major component of this membrane motor is prestin, a member of the SLC26 superfamily. Thiol-reactive optical switch probes were used to experimentally examine conformational changes of prestin-based membrane motor. Because this motor is based on mechanoelectric coupling similar to piezoelectricity, the motile activity can be monitored by charge movements across the plasma membrane, which appears as nonlinear capacitance. When the plasma membrane is conjugated with the probes, optically induced spiro-merocyanine transition positively shifted nonlinear capacitance of outer hair cells and prestin-transfected cells by about 10 mV. These shifts were reversible and were eliminated by pretreatment with iodoacetamide. However, they were little affected by pretreatment with biotin maleiminde, which cannot reach the cytoplasmic surface. Our results showed that merocyanine states, with a larger dipole moment, interact with the motor's extended conformation stronger than with the compact conformation. The interaction sites are near the cytoplasmic side of the motor protein. We have also tried to address an important issue regarding the efficiency of outer hair cell motility in affecting hearing. Because outer hair cells'motility is driven by the receptor potential, the electric potential difference across the plasma membrane generated by the transducer current though the hair bundle, the frequency characteristics are determined by the low-pass resistance-capacitance coupled filter, which is intrinsic to the cell. The corner frequency of this filter is about 1/10-th of the operating frequency. This filter significantly decreases the efficiency of electromotility as an amplifier. We theoretically examined a proposal that the cochlear microphonic, the voltage drop across the extracellular medium by the receptor current, contributes to overcome this problem. It is found that this effect can improve the frequency dependence. However, this effect alone is too small to enhance the effectiveness of electromotility beyond 10 kHz in the mammalian ear. Another mechanism that performs reverse transduction is the fast component in the hair bundle's response to mechanical stimulation. In many experiments, this response, however, appears similar to common mechanical relaxation with a damping effect. This observation is puzzling because such a response is expected to have an amplifying role in the mechanoelectrical transduction process in hair cells. We showed that a release mechanism can indeed have a role in amplification, if it is associated with negative stiffness due to the gating of the mechanoelectric transducer channel. To examine the effectiveness of hair bundle motility in mammalian and avian ears, we examined energy balance for a small sinusoidal displacement of the hair bundle. The condition that the energy generated by a hair bundle must be greater than energy loss due to the shear in the subtectorial gap per hair bundle leads to a limiting frequency that can be supported by hair bundle motility. Limiting frequencies are obtained for two motile mechanisms for fast adaptation, the channel re-closure''model and a model that assumes that fast adaptation is an interplay between gating of the channel and the myosin motor. The limiting frequency obtained for each of these models is an increasing function of a factor that is determined by the morphology of hair bundles and the cochlea. Primarily due to the higher density of hair cells in the avian cochlea, this factor is about 10-fold greater for the avian ear than the mammalian ear, which has much higher auditory frequency limit. This result is consistent with a much greater significance of hair bundle motility in the avian ear than that in the mammalian ear.