Force generation in the outer hair cell of the cochlea.

The outer hair cell of the mammalian cochlea has a unique motility directly dependent on the membrane potential. Examination of the force generated by the cell is an important step in clarifying the detailed mechanism as well as the biological importance of this motility. We performed a series of experiments to measure force in which an elastic probe was attached to the cell near the cuticular plate and the cell was driven with voltage pulses delivered from a patch pipette under whole-cell voltage clamp. The axial stiffness was also determined with the same cell by stretching it with the patch pipette. The isometric force generated by the cell is around 0.1 nN/mV, somewhat smaller than 0.15 nN/mV, predicted by an area motor model based on mechanical isotropy, but larger than in earlier reports in which the membrane potential was not controlled. The axial stiffness obtained, however, was, on average, 510 nN per unit strain, about half of the value expected from the mechanical isotropy of the membrane. We extended the area motor theory incorporating mechanical orthotropy to accommodate the axial stiffness determined. The force expected from the orthotropic model was within experimental uncertainties.     

Piezoelectric reciprocal relationship of the membrane motor in the cochlear outer hair cell

It has been shown that the membrane motor in the outer hair cell is driven by the membrane potential. Here we examine whether the motility satisfies the reciprocal relationship, the characteristic of piezoelectricity, by measuring charge displacement induced by stretching the cell with known force. The efficiency of inducing charge displacement was membrane potential dependent. The maximum efficiency of inducing charge displacement by force was approximately 20 fC/nN for 50-microm-long lateral membrane. The efficiency per cell stretching was 0.1 pC/microm. We found that these values are consistent with the reciprocal relationship based on the voltage sensitivity of approximately 20 nm/mV for 50-microm-long cell and force production of 0.1 nN/mV by the cell. We can thus conclude that the membrane motor in the outer hair cell satisfies a necessary condition for piezoelectricity and that the hair cell's piezoelectric coefficient of 20 fC/nN is four orders of magnitude greater than the best man-made material.     

Two-state model for outer hair cell stiffness and motility

With discovery of the protein prestin and the gathering evidence linking it to outer hair cell electromotility, the working mechanism of outer hair cells is becoming clearer. Recent experiments have established the voltage-dependent stiffness of outer hair cells and given an insight into the nature of variation of stiffness with respect to voltage. These and earlier experiments are used to analyze and develop models of outer hair cell response. In this article, recent modeling efforts have been reconciled and placed into a common mechanics-based framework. The constitutive models are analyzed with regard to their capability to replicate experimental results. We extend the area motor model to include elastic constants dependent on motor state. The modified model successfully captures stiffness variations of outer hair cells and capacitance changes with respect to voltage.     

Consequences of Location-Dependent Organ of Corti Micro-Mechanics

The cochlea performs frequency analysis and amplification of sounds. The graded stiffness of the basilar membrane along the cochlear length underlies the frequency-location relationship of the mammalian cochlea. The somatic motility of outer hair cell is central for cochlear amplification. Despite two to three orders of magnitude change in the basilar membrane stiffness, the force capacity of the outer hair cell’s somatic motility, is nearly invariant over the cochlear length. It is puzzling how actuators with a constant force capacity can operate under such a wide stiffness range. We hypothesize that the organ of Corti sets the mechanical conditions so that the outer hair cell’s somatic motility effectively interacts with the media of traveling waves—the basilar membrane and the tectorial membrane. To test this hypothesis, a computational model of the gerbil cochlea was developed that incorporates organ of Corti structural mechanics, cochlear fluid dynamics, and hair cell electro-physiology. The model simulations showed that the micro-mechanical responses of the organ of Corti are different along the cochlear length. For example, the top surface of the organ of Corti vibrated more than the bottom surface at the basal (high frequency) location, but the amplitude ratio was reversed at the apical (low frequency) location. Unlike the basilar membrane stiffness varying by a factor of 1700 along the cochlear length, the stiffness of the organ of Corti complex felt by the outer hair cell remained between 1.5 and 0.4 times the outer hair cell stiffness. The Y-shaped structure in the organ of Corti formed by outer hair cell, Deiters cell and its phalange was the primary determinant of the elastic reactance imposed on the outer hair cells. The stiffness and geometry of the Deiters cell and its phalange affected cochlear amplification differently depending on the location.     

Kinesin force generation measured using a centrifuge microscope sperm-gliding motility assay.

To measure force generation and characterize the relationship between force and velocity in kinesin-driven motility we have developed a centrifuge microscope sperm-gliding motility assay. The average (extrapolated) value of maximum isometric force at low kinesin density was 0.90 +/- 0.14 pN. Furthermore, in the experiments at low kinesin density, sperm pulled off before stall at forces between 0.40 and 0.75 pN. To further characterize our kinesin-demembranated sperm assay we estimated maximum isometric force using a laser trap-based assay. At low kinesin density, 4.34 +/- 1.5 pN was the maximum force. Using values of axoneme stiffness available from other studies, we concluded that, in our centrifuge microscope-based assay, a sperm axoneme functions as a lever arm, magnifying the centrifugal force and leading to pull-off before stall. In addition, drag of the distal portion of the axoneme is increased by the centrifugal force (because the axoneme is rotated into closer proximity to the glass surface) and represents an additional force that the kinesin motor must overcome.     

Effects of chlorpromazine and trinitrophenol on the membrane motor of outer hair cells

The motile activity of outer hair cells' cell body is associated with large nonlinear capacitance due to a membrane motor that couples electric displacement with changes in the membrane area, analogous to piezoelectricity. This motor is based on prestin, a member of the SLC26 family of anion transporters and utilizes the electric energy available at the plasma membrane associated with the sensory function of these cells. To understand detailed mechanism of this motile activity, we examined the effect of amphipathic ions, cationic chlorpromazine and anionic trinitrophenol, which are thought to change the curvature of the membrane in opposite directions. We found that both chemicals reduced cell length at the holding potential of -75 mV and induced positive shifts in the cells' voltage dependence. The shift observed was approximately 10 mV for 500 microM trinitrophenol and 20 mV for 100 microM cationic chlorpromazine. Length reduction at the holding potential and voltage shifts of the motile activity were well correlated. The voltage shifts of nonlinear capacitance were not diminished by eliminating the cells' turgor pressure or by digesting the cortical cytoskeleton. These observations suggest that the membrane motor undergoes conformational transitions that involve changes not only in membrane area but also in bending stiffness.     

Rho GTPases mediate the regulation of cochlear outer hair cell motility by acetylcholine

Outer hair cells are the mechanical effectors of the cochlear amplifier, an active process that improves the sensitivity and frequency discrimination of the mammalian ear. In vivo, the gain of the cochlear amplifier is regulated by the efferent neurotransmitter acetylcholine through the modulation of outer hair cell motility. Little is known, however, regarding the molecular mechanisms activated by acetylcholine. In this study, intracellular signaling pathways involving the small GTPases RhoA, Rac1, and Cdc42 have been identified as regulators of outer hair cell motility. Changes in cell length (slow motility) and in the amplitude of electrically induced movement (fast motility) were measured in isolated outer hair cells patch clamped in whole-cell mode, internally perfused through the patch pipette with different inhibitors and activators of these small GTPases while being externally stimulated with acetylcholine. We found that acetylcholine induces outer hair cell shortening and a simultaneous increase in the amplitude of fast motility through Rac1 and Cdc42 activation. In contrast, a RhoA- and Rac1-mediated signaling pathway induces outer hair cell elongation and decreases fast motility amplitude. These two opposing processes provide the basis for a regulatory mechanism of outer hair cell motility.     

Force Transmission in the Organ of Corti Micromachine

Auditory discrimination is limited by the performance of the cochlea whose acute sensitivity and frequency tuning are underpinned by electromechanical feedback from the outer hair cells. Two processes may underlie this feedback: voltage-driven contractility of the outer hair cell body and active motion of the hair bundle. Either process must exert its mechanical effect via deformation of the organ of Corti, a complex assembly of sensory and supporting cells riding on the basilar membrane. Using finite element analysis, we present a three-dimensional model to illustrate deformation of the organ of Corti by the two active processes. The model used available measurements of the properties of structural components in low-frequency and high-frequency regions of the rodent cochlea. The simulations agreed well with measurements of the cochlear partition stiffness, the longitudinal space constant for point deflection, and the deformation of the organ of Corti for current injection, as well as displaying a 20-fold increase in passive resonant frequency from apex to base. The radial stiffness of the tectorial membrane attachment was found to be a crucial element in the mechanical feedback. Despite a substantial difference in the maximum force generated by hair bundle and somatic motility, the two mechanisms induced comparable amplitudes of motion of the basilar membrane but differed in the polarity of their feedback on hair bundle position. Compared to the hair bundle motor, the somatic motor was more effective in deforming the organ of Corti than in displacing the basilar membrane.     

Characterization of the adhesive mucilages secreted by live diatom cells using atomic force microscopy

Atomic Force Microscopy (AFM) resolved the topography and mechanical properties of two distinct adhesive mucilages secreted by the marine, fouling diatom Craspedostauros australis. Tapping mode images of live cells revealed a soft and cohesive outer mucilage layer that encased most of the diatom's siliceous wall, and force curves revealed an adhesive force of 3.58 nN. High loading force, contact mode imaging resulted in cantilever 'cleaned' cell walls, which enabled the first direct observation of the active secretion of soft mucilage via pore openings. A second adhesive mucilage consisted of strands secreted at the raphe, a distinct slit in the silica wall involved in cell-substratum attachment and motility. Force measurements revealed a raphe adhesive strand(s) resistant to breaking forces up to 60 nN, and these strands could only be detached from the AFM cantilever probe using the manual stepper motor.     

Membrane tension directly shifts voltage dependence of outer hair cell motility and associated gating charge.

The unique electromotility of the outer hair cell (OHC) is believed to promote sharpening of the passive mechanical vibration of the mammalian basilar membrane. The cell also presents a voltage-dependent capacitance, or equivalently, a nonlinear gating current, which correlates well with its mechanical activity, suggesting that membrane-bound voltage sensor-motor elements control OHC length. We report that the voltage dependence of the gating charge and motility are directly related to membrane stress induced by intracellular pressure. A tracking procedure was devised to continuously monitor the voltage at peak capacitance (VpkCm) after obtaining whole cell voltage clamp configuration. In addition, nonlinear capacitance was more fully evaluated with a stair step voltage protocol. Upon whole cell configuration, VpkCm was typically near -20 mV. Negative patch pipette pressure caused a negative shift in VpkCm, which obtained a limiting value near the normal resting potential of the OHC (approximately -70 mV) at the point of cell collapse. Positive pressure in the pipette caused a positive shift that could reach values greater than 0 mV. Measures of the mechanical activity of the OHC mirrored those of charge movement. Similar membrane-tension dependent peak shifts were observed after the cortical cytoskeletal network was disrupted by intracellular dialysis of trypsin from the patch pipette. We conclude that unlike stretch receptors, which may sense tension through elastic cytoskeletal elements, the OHC motor senses tension directly. Furthermore, since the voltage dependence of the OHC nonlinear capacitance and motility is directly regulated by intracellular turgor pressure, we speculate that modification of intracellular pressure in vivo provides a mechanism for controlling the gain of the mammalian "cochlear amplifier".     

On the effect of prestin on the electrical breakdown of cell membranes

The voltage-dependent activity of prestin, the outer hair cell (OHC) motor protein essential for its electromotility, enhances the mammalian inner ear's auditory sensitivity. We investigated the effect of prestin's activity on the plasma membrane's (PM) susceptibility to electroporation (EP) via cell-attached patch-clamping. Guinea pig OHCs, TSA201 cells, and prestin-transfected TSA cells were subjected to incremental 50 mus and/or 50 ms voltage pulse trains, or ramps, at rates from 10 V/s to 1 kV/s, to a maximum transmembrane potential of +/-1000 mV. EP was determined by an increase in capacitance to whole-cell levels. OHCs were probed at the prestin-rich lateral PM or prestin-devoid basal portion; TSA cells were patched at random points. OHCs were consistently electroporated with 50 ms pulses, with significant resistance to depolarizing pulses. Although EP rarely occurred with 50 mus pulses, prior stimulation with this protocol had a significant effect on the sensitivity to EP with 50 ms pulses, regardless of polarity or PM domain. Consistent with these results, resistance to EP with depolarizing 10-V/s ramps was also found. Our findings with TSA cells were comparable, showing resistance to EP with both depolarizing 50-ms pulses and 10 V/s ramps. We conclude prestin significantly affects susceptibility to EP, possibly via known biophysical influences on specific membrane capacitance and/or membrane stiffness.     

On the mechanism of a high-frequency force generator in outer hair cells isolated from the guinea pig cochlea

Isolated mammalian outer hair cells elongate or shorten respectively by several micrometres when electrically hyperpolarized or depolarized. The experiments in this paper were designed to locate the force-generating mechanism that drives length changes in outer hair cells, and to determine some of its basic properties. The whole-cell mode of the patch-clamp technique was used to stimulate cells electrically and to perfuse them with specific drugs. The pattern of displacement of cellular organelles, and the relative displacements of the cell base and apex during electrical stimulation with the cell mechanically anchored at various points along its length, suggest that the force-generating mechanism is distributed throughout the length of the cell. Further experiments altering the shape, volume and intracellular pressure of outer hair cells suggest that the mechanism is closely associated with the plasma membrane. These experiments also demonstrate that the characteristic tubular shape of outer hair cells is maintained by membrane-associated structures with elastic properties that enable the cell to return to its original shape after deformation. The mechanism controlling length changes may, therefore, be composed of two elements in parallel, namely a force generating element and a passive elastic element. Inhibitors of ATP synthesis, or the presence of the non-hydrolysable ATP analogue AMP.PNP, perfused into outer hair cells, failed to inhibit length changes. Drugs against actin, including phalloidin, cytochalasin B and cytochalasin D, and against tubulin, including colchicine, nocodazole and colcemid, also failed to inhibit length changes. We conclude that the force-generating mechanism is, therefore, unlike most other forms of cell motility, and possible alternative hypotheses are briefly discussed.     

Effectiveness, active energy produced by molecular motors, and nonlinear capacitance of the cochlear outer hair cell

Cochlear outer hair cells are crucial for active hearing. These cells have a unique form of motility, named electromotility, whose main features are the cell's length changes, active force production, and nonlinear capacitance. The molecular motor, prestin, that drives outer hair cell electromotility has recently been identified. We reveal relationships between the active energy produced by the outer hair cell molecular motors, motor effectiveness, and the capacitive properties of the cell membrane. We quantitatively characterize these relationships by introducing three characteristics: effective capacitance, zero-strain capacitance, and zero-resultant capacitance. We show that zero-strain capacitance is smaller than zero-resultant capacitance, and that the effective capacitance is between the two. It was also found that the differences between the introduced capacitive characteristics can be expressed in terms of the active energy produced by the cell's molecular motors. The effectiveness of the cell and its molecular motors is introduced as the ratio of the motors'active energy to the energy of the externally applied electric field. It is shown that the effectiveness is proportional to the difference between zero-strain and zero-resultant capacitance. We analyze the cell and motor's effectiveness within a broad range of cellular parameters and estimate it to be within a range of 12%-30%.     

Membrane tether formation from outer hair cells with optical tweezers

Optical tweezers were used to characterize the mechanical properties of the outer hair cell (OHC) plasma membrane by pulling tethers with 4.5-microm polystyrene beads. Tether formation force and tether force were measured in static and dynamic conditions. A greater force was required for tether formations from OHC lateral wall (499 +/- 152 pN) than from OHC basal end (142 +/- 49 pN). The difference in the force required to pull tethers is consistent with an extensive cytoskeletal framework associated with the lateral wall known as the cortical lattice. The apparent plasma membrane stiffness, estimated under the static conditions by measuring tether force at different tether length, was 3.71 pN/microm for OHC lateral wall and 4.57 pN/microm for OHC basal end. The effective membrane viscosity was measured by pulling tethers at different rates while continuously recording the tether force, and estimated in the range of 2.39 to 5.25 pN x s/microm. The viscous force most likely results from the viscous interactions between plasma membrane lipids and the OHC cortical lattice and/or integral membrane proteins. The information these studies provide on the mechanical properties of the OHC lateral wall is important for understanding the mechanism of OHC electromotility.     

Modeling electrically active viscoelastic membranes

The membrane protein prestin is native to the cochlear outer hair cell that is crucial to the ear's amplification and frequency selectivity throughout the whole acoustic frequency range. The outer hair cell exhibits interrelated dimensional changes, force generation, and electric charge transfer. Cells transfected with prestin acquire unique active properties similar to those in the native cell that have also been useful in understanding the process. Here we propose a model describing the major electromechanical features of such active membranes. The model derived from thermodynamic principles is in the form of integral relationships between the history of voltage and membrane resultants as independent variables and the charge density and strains as dependent variables. The proposed model is applied to the analysis of an active force produced by the outer hair cell in response to a harmonic electric field. Our analysis reveals the mechanism of the outer hair cell active (isometric) force having an almost constant amplitude and phase up to 80 kHz. We found that the frequency-invariance of the force is a result of interplay between the electrical filtering associated with prestin and power law viscoelasticity of the surrounding membrane. Paradoxically, the membrane viscoelasticity boosts the force balancing the electrical filtering effect. We also consider various modes of electromechanical coupling in membrane with prestin associated with mechanical perturbations in the cell. We consider pressure or strains applied step-wise or at a constant rate and compute the time course of the resulting electric charge. The results obtained here are important for the analysis of electromechanical properties of membranes, cells, and biological materials as well as for a better understanding of the mechanism of hearing and the role of the protein prestin in this mechanism.     

Sensitivity of Prestin-Based Membrane Motor to Membrane Thickness

Prestin is the membrane protein in outer hair cells that harnesses electrical energy by changing its membrane area in response to changes in the membrane potential. To examine the effect of membrane thickness on this protein, phosphatidylcholine (PC) with various acyl-chain lengths were incorporated into the plasma membrane by using γ-cyclodextrin. Incorporation of short chain PCs increased the linear capacitance and positively shifted the voltage dependence of prestin, up to 120 mV, in cultured cells. PCs with long acyl chains had the opposite effects. Because the linear capacitance is inversely related to the membrane thickness, these voltage shifts are attributable to membrane thickness. The corresponding voltage shifts of electromotility were observed in outer hair cells. These results demonstrate that electromotility is extremely sensitive to the thickness of the plasma membrane, presumably involving hydrophobic mismatch. These observations indicate that the extended state of the motor molecule, which is associated with the elongation of outer hair cells, has a conformation with a shorter hydrophobic height in the lipid bilayer.     

Mechanical properties of the lateral cortex of mammalian auditory outer hair cells.

Mammalian auditory outer hair cells generate high-frequency mechanical forces that enhance sound-induced displacements of the basilar membrane within the inner ear. It has been proposed that the resulting cell deformation is directed along the longitudinal axis of the cell by the cortical cytoskeleton. We have tested this proposal by making direct mechanical measurements on outer hair cells. The resultant stiffness modulus along the axis of whole dissociated cells was 3 x 10(-3) N/m, consistent with previously published values. The resultant axial and circumferential stiffness moduli for the cortical lattice were 5 x 10(-4) N/m and 3 x 10(-3) N/m, respectively. Thus the cortical lattice is a highly orthotropic structure. Its axial stiffness is small compared with that of the intact cell, but its circumferential stiffness is within the same order of magnitude. These measurements support the theory that the cortical cytoskeleton directs electrically driven length changes along the longitudinal axis of the cell. The Young's modulus of the circumferential filamentous components of the lattice were calculated to be 1 x 10(7) N/m2. The axial cross-links, believed to be a form of spectrin, were calculated to have a Young's modulus of 3 x 10(6) N/m2. Based on the measured values for the lattice and intact cell cortex, an estimate for the resultant stiffness modulus of the plasma membrane was estimated to be on the order of 10(-3) N/m. Thus, the plasma membrane appears to be relatively stiff and may be the dominant contributor to the axial stiffness of the intact cell.     

Simulation of motor-driven cochlear outer hair cell electromotility.

We propose a three-dimensional (3D) model to simulate outer hair cell electromotility. In our model, the major components of the composite cell wall are explicitly represented. We simulate the activity of the particles/motor complexes in the plasma membrane by generating active strains inside them and compute the overall response of the cell. We also consider the constrained wall and compute the generated active force. We estimate the parameters of our model by matching the predicted longitudinal and circumferential electromotile strains with those observed in the microchamber experiment. In addition, we match the earlier estimated values of the active force and cell wall stiffness. The computed electromotile strains in the plasma membrane and other components of the wall are in agreement with experimental observations in trypsinized cells and in nonmotile cells transfected with Prestin. We discover several features of the 3D mechanism of outer hair cell electromotilty. Because of the constraints under which the motors operate, the motor-related strains have to be 2-3 times larger than the observable strains. The motor density has a strong effect on the electromotile strain. Such effect on the active force is significantly lower because of the interplay between the active and passive properties of the cell wall.     

Prestin-based outer hair cell motility is necessary for mammalian cochlear amplification

It is a central tenet of cochlear neurobiology that mammalian ears rely on a local, mechanical amplification process for their high sensitivity and sharp frequency selectivity. While it is generally agreed that outer hair cells provide the amplification, two mechanisms have been proposed: stereociliary motility and somatic motility. The latter is driven by the motor protein prestin. Electrophysiological phenotyping of a prestin knockout mouse intimated that somatic motility is the amplifier. However, outer hair cells of knockout mice have significantly altered mechanical properties, making this mouse model unsatisfactory. Here, we study a mouse model without alteration to outer hair cell and organ of Corti mechanics or to mechanoelectric transduction, but with diminished prestin function. These animals have knockout-like behavior, demonstrating that prestin-based electromotility is required for cochlear amplification.     

Heterogeneous response of traction force at focal adhesions of vascular smooth muscle cells subjected to macroscopic stretch on a micropillar substrate

Traction force generated at focal adhesions (FAs) of cells plays an essential role in regulating cellular functions. However, little is known about how the traction force at each FA changes during cell stretching. Here we investigated dynamic changes in traction force at FAs during macroscopic stretching of porcine aortic smooth muscle cells (SMCs) cultured on elastic micropillar substrates. SMCs were cultured on polydimethylsiloxane (PDMS)-based substrates with a micropillar array, and stretched approximately in the direction of their major axis and then released by stretching and relaxing the substrates. This stretch-release cycle was repeated twice with cell strain rates of 0.3%/15s up to a 3% strain, and the deflection of the PDMS micropillars was measured simultaneously to obtain the traction force at each FA F, total force in the cell's major axis direction F(all), and whole-cell strain ε(cell). Traction forces of SMCs during stretching varied widely with location: their changes at some pillars synchronized well with the applied strain ε(cell), but others did not synchronized. Whole-cell stiffness estimated as the slope of the loading limb of the F(all)-ε(cell) curves was ～10nN/%, which was the same order of magnitude of the reported stiffness of cultured SMCs obtained in a tensile test. Interestingly, F(all) at a zero-strain state (pretension at the whole-cell level) actively increased in some cells following the loading/unloading process, as did whole-cell stiffness. Such a change did not occur in cultured SMCs in the tensile test in which cells were held with a pair of micropipettes coated with nonspecific adhesive. These results indicate that SMCs showed a myogenic response when stretched through their multiple FAs, but not through nonspecific adhesions on their membrane. SMCs may behave differently depending on the sites through which they are stretched.