State dependent effects on the frequency response of prestin’s real and imaginary components of nonlinear capacitance

Abstract

The outer hair cell (OHC) membrane harbors a voltage-dependent protein, prestin (SLC26a5), in high density, whose charge movement is evidenced as a nonlinear capacitance (NLC). NLC is bell-shaped, with its peak occurring at a voltage, Vh, where sensor charge is equally distributed across the plasma membrane. Thus, Vh provides information on the conformational state of prestin. Vh is sensitive to membrane tension, shifting to positive voltage as tension increases and is the basis for considering prestin piezoelectric (PZE). NLC can be deconstructed into real and imaginary components that report on charge movements in phase or 90 degrees out of phase with AC voltage. Here we show in membrane macro-patches of the OHC that there is a partial trade-off in the magnitude of real and imaginary components as interrogation frequency increases, as predicted by a recent PZE model (Rabbitt in Proc Natl Acad Sci USA 17:21880–21888, 2020). However, we find similar behavior in a simple 2-state voltage-dependent kinetic model of prestin that lacks piezoelectric coupling. At a particular frequency, Fis, the complex component magnitudes intersect. Using this metric, Fis, which depends on the frequency response of each complex component, we find that initial Vh influences Fis; thus, by categorizing patches into groups of different Vh, (above and below − 30 mV) we find that Fis is lower for the negative Vh group. We also find that the effect of membrane tension on complex NLC is dependent, but differentially so, on initial Vh. Whereas the negative group exhibits shifts to higher frequencies for increasing tension, the opposite occurs for the positive group. Despite complex component trade-offs, the low-pass roll-off in absolute magnitude of NLC, which varies little with our perturbations and is indicative of diminishing total charge movement, poses a challenge for a role of voltage-driven prestin in cochlear amplification at very high frequencies.

Single particle cryo-EM structure of the outer hair cell motor protein prestin

“…Thus, we might expect that direct voltage-driven conformational changes in prestin could differ from unphysiological (e.g., salicylate or SO 4 2− ) anion-induced steady state cryo-EM structures. Interestingly, we have recently shown that the ΔC sa component of membrane capacitance exists only in the real (capacitive) part of complex NLC, not in the imaginary (conductive) part, ostensibly linking it directly to membrane bilayer influence rather than prestin charge movement 56 .…”

Outer hair cell electro-mechanical power conversion

“…To determine the speed of the prestin-motor function in membrane patches, frequency-dependent electrical power consumption was determined directly from macro-patch NLC data reported and by Santos-Sacchi et al [21], replotted in Fig. 1 as the real Re ( C N ) (A) and imaginary Im ( C N ) (B) components.…”

“…2A, B and C show theoretical predictions for Re ( C N ), Im ( C N ) and P P respectively. For parameters approximated from the Santos-Sacchi et al data [21], PZT theory predicts band-pass power consumption to peak near 37 kHz. Results support the hypothesis that electrical power consumed by the prestin motor complex is band-pass and operates at frequencies much higher than would be implied by the low pass character of | C W |.…”

“…For the same reason, peak power output by OHCs in the cochlea is predicted by PZT to align with characteristic frequency [8–10, 30]. In contrast, if the mechanical load is neglected, Im ( C n ) arise in transition state theory rate constants, which means the conformation transition itself dissipates all of the power as heat, analogous to dielectric loss [21], with no possibility of power delivery to cochlear amplifier.…”

“…Low-pass characteristics are also present in nonlinear capacitance (NLC) recorded in isolated membrane patches [20][21][22], and in voltage-driven displacement of isolated OHCs measured using the µ-chamber approach [23,24]. Precisely how the prestin motor complex amplifies vibrations in the cochlea given these low-pass features remains unknow.…”