Fast Electromechanical Amplification in the Lateral Membrane of the Outer Hair Cell

Abstract

Outer hair cells provide amplification within the mammalian cochlea to enhance audition. The mechanism is believed to reside within the lateral membrane of the cell that houses an expansive array of molecular motors, identified as prestin, which drives somatic electromotility. By measuring nonlinear capacitance, the electrical signature of electromotility, at kilohertz rates we have uncovered new details of the early molecular events that arise from voltage perturbations of prestin. We show that dynamic changes in motor state probability occur within the kilohertz range, and signify an amplificatory event. Additionally, we show a lack of effect of Cl driving force, an absence of cell length effect (indicating that the kinetics does not vary across auditory frequency), and the first demonstration of the time dependence of tension induced amplificatory shifts. The process we have identified, where the stimulus-response function shifts in time along the stimulus axis in a multi-exponential manner, bears similarities to those components of adaptation found in the OHC stereociliar transducer identified recently. As with the forward transducer, the speed of the reverse transducer amplificatory event consequently impacts on high frequency peripheral auditory processing.

Disparities in voltage-sensor charge and electromotility imply slow chloride-driven state transitions in the solute carrier SLC26a5

“…It is important to note that the simple model we provide here has single exponential kinetics, whereas previously observed kinetic characteristics of NLC (e.g., see Fig. 1 NLC time course), which we can now ascribe to this gateway, clearly show that the transition actually has stretched exponential behavior providing control ranging from the submillisecond to the seconds timescale (20,23). Implementing this kinetic diversity will certainly increase the complexity of the model, but may improve its correspondence with biophysical data.…”

“…Furthermore, Fig. 1C illustrates that the time course of OHC length change follows the time course of NLC during step voltage stimulation, each showing stretched (multi) exponential behavior (23). This tight coupling between NLC and eM is observed when intracellular chloride is high, e.g., 140 mM, the typical intracellular level used for experimentation during the last few decades to study OHCs.…”

“…Coupled to this nonlinear charge movement is a mechanical response of the whole cell (a length change; eM) that follows the charge's voltage dependence. Shifts in NLC V h along the voltage axis, known to be induced by changing the long-term holding potential (prepulse) of the OHC or prestin-transfected cells (4,20,23), are accompanied by corresponding shifts in eM voltage dependence. In this example (Fig.…”

“…Here we find that the disparity between eM and NLC is susceptible to membrane tension, thus directly implicating molecular underpinnings. To be sure, the hallmark of the intermediate transition rate, the C m relaxation indicative of the shifting Q-V function, is impervious to intracellular pronase treatment (20), does not depend on OHC length (23), and is found in prestin transfected cells (4), as expected for a membrane-based phenomenon. Indeed, the minor effects of present cytoskeletal perturbations on the ΔV h disparity confirm those observations.…”

“…In any case, their model cannot reproduce our data, as chloride binding leads directly to fast voltage-dependent reactions. It should be noted, that we previously identified a voltage dependence of NLC relaxations, which could signal that the slow intermediate transition may possess some measure of voltage dependency (23). In subsequent model renditions we intend to evaluate this possibility, and the possible effects of electric field on anion binding.…”