A prestin motor in chicken auditory hair cells: active force generation in a nonmammalian species

Abstract

Active force generation by outer hair cells (OHCs) underlies amplification and frequency tuning in the mammalian cochlea but whether such a process exists in nonmammals is unclear. Here, we demonstrate that hair cells of the chicken auditory papilla possess an electromechanical force generator in addition to active hair bundle motion due to mechanotransducer channel gating. The properties of the force generator, its voltage dependence and susceptibility to salicylate, as well as an associated chloride-sensitive nonlinear capacitance, suggest involvement of the chicken homolog of prestin, the OHC motor protein. The presence of chicken prestin in the hair cell lateral membrane was confirmed by immunolabeling studies. The hair bundle and prestin motors together create sufficient force to produce fast lateral displacements of the tectorial membrane. Our results imply that the first use of prestin as a motor protein occurred early in amniote evolution and was not a mammalian invention as is usually supposed.

Figure 1

Mechanotransduction and hair bundle movements in chicken SHCs. (A) Surface view of chicken basilar papilla showing the hair bundles of SHCs, orientation indicated by Ab (abneural) and N (neural) sides. Note the bundles are eccentrically located towards the abneural edge of papilla. (B) MT currents in response to saturating bundle motion elicited by sinusoidal fluid jet stimuli. In the stimulus monitor (top) the thin line is the calibrated photodiode current superimposed on the driving voltage to the fluid jet piezoelectric disk. The MT current throughout the first cycle is plotted against bundle displacement, ΔX (bottom) and is fitted with a Boltzmann equation (dashed line) with I_MAX of −0.64 nA and 10-90 per cent working range (see Methods) of 37 nm. In this and subsequent current-displacement relations, the absolute amplitude of the current is plotted. (C) Depolarizing voltage step (top) from −84 to +56 mV generates a partially inactivating membrane current (middle) and a deflection of a free-standing bundle (bottom) which is predominantly negative (away from the tallest edge of the bundle). (D) Set of depolarizing voltage steps in another SHC generates negative bundle movements graded with the depolarization. In this case, a flexible glass fiber, stiffness 1.2 mN/m, was attached to the hair bundle allowing calculation of forces produced (bottom, right hand axis).

Figure 2

Two components of the voltage-induced hair bundle motion. (A) Depolarization from −84 to +56 mV evoked biphasic bundle motion. The MT channel blocker FM1-43 (7.5 μM) blocked a positive component of the motion thereby increasing the negative component. The difference motion (control – FM1-43), reflecting the MT channel component is entirely positive (towards abneural edge). (B) The same depolarization in another SHC elicited a biphasic bundle motion that became positive and sustained with application of 10 mM Na⁺ salicylate. The difference motion (control - salicylate) is entirely negative (away from bundle’s tallest edge). (C) MT currents in these same two SHCs. Top is SHC in (A) without (black trace) and with (red trace) FM1-43 which blocks 90 per cent of the current. Bottom is SHC in (B) without (black trace) and with (red trace) salicylate.

Figure 3

Action of FM1-43 on voltage-induced bundle motion in a THC. (A) Depolarizing voltage steps from −84 mV to +60 mV evokes biphasic bundle movements which with 7.5 μM FM1-43 become largely negative. The difference motion, (control – FM1-43) is sustained and positive. (B) Voltage dependence of peak displacements for the two processes: owing to block of the positive component by FM1-43, the negative component (crossed circles) increased with application of the MT channel blocker; the difference motion (filled triangles) was positive and both increase with depolarization. Results with FM1-43 fit with a Boltzmann equation ΔX = ΔX_max /(1 + exp((V_0.5-V)/ α)) with V_0.5 = 10 mV, α = 37 mV and ΔX_max = −70 nm, where V is membrane potential. (C) Effect of 7.5 μM FM1-43 in a SHC showing recovery of the control amplitude of the voltage-evoked bundle movement on washing.

Figure 4

MT currents in SHCs are unaffected by salicylate. (A) MT currents before (control) and 18 minutes after application of 10 mM Na⁺ salicylate to the bath. (B) Current displacement relationships for the responses in (A), each fit with a single Boltzmann (dashed line) with I_MAX and 10 – 90 percent working range of 0.6 nA and 39 nm (control) and 0.6 nA and 36 nm (salicylate). (C). Peak MT current versus recording time, 10 mM Na⁺ salicylate was bath applied between 5 and 20 minutes. Records in (A) obtained during the initial control and 13 minutes after salicylate application. (D). Onset of MT current in a SHC in response to step deflections of the hair bundle with a stiff glass probe; low-level stimuli evoke fast adaptation that is unaffected by application of 10 mM Na⁺ salicylate; currents fit (dashed line) with an adaptation time constant of 0.23 ms, time course of bundle deflection being shown at top. (E) Voltage-evoked hair bundle movements for the cell in (A – C) showing initial control, reduction in 10 mM Na⁺ salicylate and full recovery. (F) Voltage-induced bundle motion in another SHC recorded with pipette solution at pH 6.5, before and after application of 10 mM Na⁺ salicylate.

Figure 5

SHC properties linked to voltage-evoked bundle motion. (A) Non-linear capacitance in a SHC determined (see Methods) as the difference, ΔC_m, between membrane capacitance without and with perfusion of 10 mM Na⁺ salicylate. Pre- and post-controls were obtained by using the control prior to salicylate application (filled circles) and the wash after salicylate (open circles) for the subtraction and indicate full recovery from the salicylate. The ordinate is scaled by the linear capacitance, 6.4 pF. Each set of points was fit with equation 1, with V_0.5 = 7 mV, z = 0.71 (filled circles) and V_0.5 = 25 mV, z = 0.63 (open circles) and ΔC_m = 22 fF/pF for both. (B) Non-linear capacitance in two SHCs recorded with normal intracellular saline containing 161 mM Cl⁻ (filled circles) and low Cl⁻ in which all but 20 mM CsCl was replaced by Cs⁺ aspartate (open circles). Points fit with equation 1, with V_0.5 = 4 mV, z = 0.57, linear capacitance = 5.7 pF (normal) and V_0.5 = 60 mV, z = 0.62, linear capacitance = 5.5 pF (low Cl⁻). Note positive voltage shift with low Cl⁻ intracellular. (C). Voltage-induced hair bundle motion in patch clamped cell, SHC1, and in adjacent cell, SHC2 (not clamped), with return control from original cell SHC1. Bundle motion of nearby SHC suggests force generation in cell body or cuticular plate. Polarity and magnitude of bundle motion obtained by sequential calibration of bundle-1 and bundle-2 images. (D). Schematic of experiments in (C), polarity, Ab (abneural) and N (neural) sides of papilla.

Figure 6

SHC hair bundle deflection with a flexible fiber. (A) MT currents for step deflections of a flexible fiber: top, motion of proximal end of fiber attached to piezoactuator; middle, MT currents recorded under voltage clamp at −84 mV holding potential; bottom, motion of distal end of fiber which contacted the neural edge of the hair bundle. Note the initial notch on the bundle displacement, appearing as an apparent time-dependent increase in stiffness. (B) Receptor potentials in the same SHC recorded under current-clamp from a resting potential of −48 mV. Top and bottom traces as in (A); note the larger size of the notch or recoil in the bundle displacement. The recoil was quantified by the displacement difference, ΔX1, between the initial peak and the steady state. (C) Size of recoil, ΔX1, in the voltage-clamp (filled circles) and the current-clamp experiments (open circles). ΔX1 is plotted against the MT current or receptor potential, S, normalized to their respective maximum values, S_MAX = 0.71 nA or 32 mV. (D) Current-displacement relationship from records in (A), peak current plotted against peak displacement. Points fitted with a single Boltzmann equation (dashed line) with I_MAX = 0.65 nA, 10 – 90 percent working range = 33 nm. Improved fit was obtained with a double Boltzmann equation (continuous line), I = I_MAX/(1 + {exp(−(ΔX-3)/3)}{1 + exp(−(ΔX −14)/12)}) where I is the MT current, I_MAX = 0.71 nA, ΔX is bundle displacement and numbers in exponents are in nm.

Figure 7

Voltage-induced motion of tectorial membrane. (A) Images at different focal planes from SHC hair bundles (top) to surface of tectorial membrane (bottom) with 3-μm silica beads attached. Middle image shows transverse fibers of the tectorial membrane and out of focus beads. (B) Displacement of hair bundle and bead on tectorial membrane in response to extracellular current injection. (C) Displacement of bead on tectorial membrane in absence (control and recovery) and presence of 10 mM Na⁺ salicylate. (D) Tectorial membrane movements in control, 5 mM and 20 mM Na⁺ salicylate and recovery; each trace generated by subtraction of the MT current component determined by application of 8 μM FM1-43. (E) Mean fractional block (± 1 standard deviation) of tectorial membrane motion versus salicylate concentration, number of measurements beside points; dose-response relationship fit with a Hill equation with half-blocking concentration K_B = 3.6 mM and Hill coefficient 1.4. (F) Calibration of extracellular current stimuli (60 and 100 A; top) by simultaneous recording of SHC membrane potential (middle) and hair bundle motion (bottom) in a preparation without a tectorial membrane.

Figure 8

Prestin in the chicken papilla. (A) RT-PCR products from amplification with chicken prestin primers: lane 1, E21 papilla; lane 2, E16 papillae; lane 3, no cDNA; lane 4, pEGFP-N1 plasmid containing chicken prestin. The predicted product size is 382 base pairs. (B) Immunoblot of chicken basilar papillar and mouse cochlear extracts labeled with the N-terminal prestin antibody used on tissue shows for both animals a principal band ~80 kDa as expected for prestin. Lanes loaded with 140 μg of chicken protein extract and 84 μg of mouse protein (C) Confocal section (0.8 μm full-width at half-maximum) from SHC stack at the level of the nucleus; z-projections, denoted by white cross lines, shown at the right and bottom; hair bundle actin (phalloidin, blue), otoferlin (HCS-1, red), prestin (green); d = 0.8. (D) Confocal section of THCs, evident by higher packing density; d = 0.55. (E) Confocal sections at SHC nucleus level for locations d = 0.2, 0.5 and 0.8; all images from the same preparation under identical acquisition settings. Labeling increased from apex to base; section orientations, N (neural), Ab (abneural).