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

Abstract

Outer hair cells (OHCs) drive cochlear amplification that enhances our ability to detect and discriminate sounds. The motor protein, prestin, which evolved from the SLC26 anion transporter family, underlies the OHC's voltage-dependent mechanical activity (eM). Here we report on simultaneous measures of prestin's voltagesensor charge movement (nonlinear capacitance, NLC) and eM that evidence disparities in their voltage dependence and magnitude as a function of intracellular chloride, challenging decades' old dogma that NLC reports on eM steady-state behavior. A very simple kinetic model, possessing fast anion-binding transitions and fast voltage-dependent transitions, coupled together by a much slower transition recapitulates these disparities and other biophysical observations on the OHC. The intermediary slow transition probably relates to the transporter legacy of prestin, and this intermediary gateway, which shuttles anion-bound molecules into the voltage-enabled pool of motors, provides molecular delays that present as phase lags between membrane voltage and eM. Such phase lags may help to effectively inject energy at the appropriate moment to enhance basilar membrane motion.hearing | molecular motor O uter hair cells (OHC) foster mechanical feedback within the mammalian cochlear partition that enhances perception of auditory stimuli by 2-3 orders of magnitude; this is known as cochlear amplification (1, 2). Following the molecular identification of prestin (3), an SLC26 family member (a5) that recapitulates the electromechanical properties of the native OHC's voltage sensor/motor (4, 5), a preponderance of evidence pointed to prestin-based electromotility (eM) as the basis of this boost (6, 7). During the last couple of decades, measures of nonlinear charge movement or capacitance (NLC), an electrical correlate of voltage-dependent conformational changes within the motor protein prestin, have served as surrogate, indeed as signature, of voltage-evoked eM of the OHC (8-12). Intracellular chloride plays an important role in prestin function (13), and recent evidence indicates that anions serve a modulatory, allosteric-like role (14-17). For the most part, anion effects, as well as many other influences on prestin, have been limited to the study of NLC, on the assumption that NLC reports on steady-state characteristics of OHC eM. We now show that this assumption is wrong.Using simultaneous measures of charge movement and eM, we show disparity between deduced characteristics of prestin motor protein conformation. That is, a dissociation in magnitude and voltage dependence of NLC and eM is revealed by lowering intracellular chloride to physiological levels (≤ 10 mM; ref. 6), all previous comparisons having been made with abnormally high intracellular chloride levels. Furthermore, this chloride effect, which presents as a function of rate and polarity of voltage stimulation, uncovers a mechanism predicted to be frequency dependent in vivo, and likely plays an important role in frequency selectivity and efficienc...

Outer hair cell electromotility is low-pass filtered relative to the molecular conformational changes that produce nonlinear capacitance

“…S1 (guinea pig parameters) and Fig. S2 (mouse parameters) show that each method of R s correction gives the same results, revealing the intrinsic low pass eM response of the meno presto model that derives from its stretched-exponential intermediary transition kinetics (Song and Santos-Sacchi, 2013; Santos-Sacchi and Tan, 2018).…”

“…Models were implemented in MATLAB Simulink and Simscape, as detailed previously (Song and Santos-Sacchi, 2013; Santos-Sacchi and Song, 2014b). Model R s and R m were from the averages of guinea pig and mouse OHC parameters obtained (see below).…”

Prestin at year 14: Progress and prospect

“…They showed that prestin-associated charge movement and the resulting electromotility are fully coupled and that prestin has at least two voltage-dependent conformational transition steps. While tight coupling of motility and NLC was also demonstrated in a recent study (Song and Santos-Sacchi, 2013), intracellular Cl − concentration can significantly modulate the coupling.…”

Energy Output from a Single Outer Hair Cell

“…Considerable progress has been made in recent years in our understanding of the mechanism of prestin-based somatic motility, or "electromotility," of outer hair cells (OHCs) in the cochlea, both on cellular- [1][2][3] and the molecular levels [4][5][6], as well as clarifying its physical basis that it is based on electromechanical coupling [7][8][9]. For example, experiments on isolated OHCs have determined load-free displacement [1,2] and isometric force production [10,11].…”