Viscoelastic relaxation in the membrane of the auditory outer hair cell

Determining the Time Window for Dynamic Nanowire Cell Penetration Processes

Nanowire (NW) arrays offer opportunities for parallel, nondestructive intracellular access for biomolecule delivery, intracellular recording, and sensing. Spontaneous cell membrane penetration by vertical nanowires is essential for these applications, yet the time- and geometry-dependent penetration process is still poorly understood. In this work, the dynamic NW-cell interface during cell spreading was examined through experimental cell penetration measurements combined with two mechanical models based on substrate adhesion force or cell traction forces. Penetration was determined by comparing the induced tension at a series of given membrane configurations to the critical membrane failure tension. The adhesion model predicts that penetration occurs within a finite window shortly after initial cell contact and adhesion, while the traction model predicts increasing penetration over a longer period. NW penetration rates determined from a cobalt ion delivery assay are compared to the predicted results from the two models. In addition, the effects of NW geometry and cell properties are systematically evaluated to identify the key factors for penetration.

Actuation of flexoelectric membranes in viscoelastic fluids with applications to outer hair cells

Liquid crystal flexoelectric actuation uses an imposed electric field to create membrane bending, and it is used by the outer hair cells (OHCs) located in the inner ear, whose role is to amplify sound through generation of mechanical power. Oscillations in the OHC membranes create periodic viscoelastic flows in the contacting fluid media. A key objective of this work on flexoelectric actuation relevant to OHCs is to find the relations and impact of the electromechanical properties of the membrane, the rheological properties of the viscoelastic media, and the frequency response of the generated mechanical power output. The model developed and used in this work is based on the integration of: (i) the flexoelectric membrane shape equation applied to a circular membrane attached to the inner surface of a circular capillary and (ii) the coupled capillary flow of contacting viscoelastic phases, such that the membrane flexoelectric oscillations drive periodic viscoelastic capillary flows, as in OHCs. By applying the Fourier transform formalism to the governing equation, analytical expressions for the transfer function associated with the curvature and electrical field and for the power dissipation of elastic storage energy were found.

Fast electromechanical amplification in the lateral membrane of the outer hair cell

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.

Mechanokinetic model of cell membrane: theoretical analysis of plasmalemma homeostasis, growth and division

A theoretical model dealing with endocytosis, exocytosis and caveolae invagination, describing plasmalemma homeostasis during cell growth and division, is proposed. It considers transmembrane pressure, membrane tension and mechanosensitivity of membrane processes. Membrane hydraulic conductivity and the flux of transmembrane nonvesicular transport are taken into account. The developed mathematical analysis operates with a formulated set of constitutive equations describing the mechanical state and kinetics of changes in an open dynamic membrane system. The standard version of a model with adjusted parameters was implemented, and predictions including a discussion on the effect of possible parameter modifications were presented. Computer simulations indicate big changes in the magnitude of membrane tension and elasticity, and in the number of membrane buddings in young cells and during mitosis. They also show the extent of cell growth inhibition resulting from a decrease in transmembrane transport or an increase in the exerted difference in osmotic pressure. Moreover, the simulations reveal that exocytosis regulated during mitosis may not be as important for cell growth, as sometimes presumed. Finally, practical application and possible extension of the model are discussed.

Electromechanical models of the outer hair cell composite membrane

The outer hair cell (OHC) is an extremely specialized cell and its proper functioning is essential for normal mammalian hearing. This article reviews recent developments in theoretical modeling that have increased our knowledge of the operation of this fascinating cell. The earliest models aimed at capturing experimental observations on voltage-induced cellular length changes and capacitance were based on isotropic elasticity and a two-state Boltzmann function. Recent advances in modeling based on the thermodynamics of orthotropic electroelastic materials better capture the cell's voltage-dependent stiffness, capacitance, interaction with its environment and ability to generate force at high frequencies. While complete models are crucial, simpler continuum models can be derived that retain fidelity over small changes in transmembrane voltage and strains occurring in vivo. By its function in the cochlea, the OHC behaves like a piezoelectric-like actuator, and the main cellular features can be described by piezoelectric models. However, a finer characterization of the cell's composite wall requires understanding the local mechanical and electrical fields. One of the key questions is the relative contribution of the in-plane and bending modes of electromechanical strains and forces (moments). The latter mode is associated with the flexoelectric effect in curved membranes. New data, including a novel experiment with tethers pulled from the cell membrane, can help in estimating the role of different modes of electromechanical coupling. Despite considerable progress, many problems still confound modelers. Thus, this article will conclude with a discussion of unanswered questions and highlight directions for future research.

Evidence for a highly elastic shell-core organization of cochlear outer hair cells by local membrane indentation

Cochlear outer hair cells (OHCs) are thought to play an essential role in the high sensitivity and sharp frequency selectivity of the hearing organ by generating forces that amplify the vibrations of this organ at frequencies up to several tens of kHz. This tuning process depends on the mechanical properties of the cochlear partition, which OHC activity has been proposed to modulate on a cycle-by-cycle basis. OHCs have a specialized shell-core ultrastructure believed to be important for the mechanics of these cells and for their unique electromotility properties. Here we use atomic force microscopy to investigate the mechanical properties of isolated living OHCs and to show that indentation mechanics of their membrane is consistent with a shell-core organization. Indentations of OHCs are also found to be highly nonhysteretic at deformation rates of more than 40 microm/s, which suggests the OHC lateral wall is a highly elastic structure, with little viscous dissipation, as would appear to be required in view of the very rapid changes in shape and mechanics OHCs are believed to undergo in vivo.

Modeling high-frequency electromotility of cochlear outer hair cell in microchamber experiment

Cochlear outer hair cells (OHC) are critically important for the amplification and sharp frequency selectivity of the mammalian ear. The microchamber experiment has been an effective tool to analyze the OHC high-frequency performance. In this study, the OHC electrical stimulation in the microchamber is simulated. The model takes into account the inertial and viscous properties of fluids inside and outside the cell as well as the viscoelastic and piezoelectric properties of the cell composite membrane (wall). The closed ends of the cylindrical cell were considered as oscillatory rigid plates. The final solution was obtained in terms of Fourier series, and it was checked against the available results of the microchamber experiment. The conditions of the interaction between the cell and pipette was analyzed, and it was found that the amount of slip along the contact surface has a significant effect on the cell electromotile response. The cell's length changes were computed as a function of frequency, and their dependence on the viscosities of both fluids and the cell wall was analyzed. The distribution of the viscous losses inside the fluids was also estimated. The proposed approach can help in a better understanding of the high-frequency OHC electromotility under experimental and physiological conditions.

Regulation of the lateral wall stiffness by acetylcholine and GABA in the outer hair cells of the guinea pig

Acetylcholine (ACh) and GABA, the main neurotransmitters of the efferent innervation of the outer hair cells (OHCs), are assumed to regulate the efficacy of the cochlear amplifier through a variety of mechanisms. The recently described stretch-induced changes of the lateral wall stiffness (regulatory stiffness response) and the stretch-induced slow cell motility of OHCs may be important regulatory mechanisms in this process. We found that ACh in cochleobasal OHCs significantly reduces the stiffness of the lateral wall but increases the regulatory stiffness response and stretch-induced slow cell motility. Qualitatively similar cellular responses were evoked by GABA in cochleoapical OHCs. The effects of ACh could be inhibited by strychnine, the specific inhibitor of the alpha(9) ACh receptors expressed in OHCs, whereas the effects of GABA could be blocked by bicuculline, a specific GABA(A) receptor antagonist. In the absence of extracellular Ca(2+) the effects of ACh and GABA on the regulatory stiffness response were reduced, indicating the involvement of Ca(2+) in the control of this process. Based on our results we suggest that efferent innervation protects the organ of Corti against high sound intensities and supports adaptation by modification of the micromechanical properties of OHCs. This could be governed by ACh and GABA indirectly, via the potentiation of stretch-induced cell shortening in a Ca(2+)-dependent manner, rather than by a direct stiffness regulation-related mechanism.

Effect of turgor pressure on outer hair cell motility

The motor in the outer hair cell converts electrical energy directly into mechanical energy. There are two possible mechanisms for such a motor: one depends on changes in the membrane area ("area motor") and the other on changes in the stiffness ("stiffness motor"). These two mechanisms, which are not mutually exclusive, give different predictions on turgor pressure dependence of the amplitude. It was found that an increased pressure shifts the voltage dependence but does not change the amplitude of length changes. This observation is incompatible with a pure stiffness motor model. It was also confirmed that length changes are closely related to charge movements by monitoring the membrane capacitance. It can be concluded that hair cell motility is primarily based on area changes, and not changes in the elastic moduli.

Physical and physiological components of the graviresponses of wild-type and mutant Paramecium Tetraurelia

Wild-type and the morphological mutant kin 241 of Paramecium tetraurelia showed improved orientation away from the centre of gravity (negative gravitaxis) when accelerations were increased from 1 to 7 g. Gravitaxis was more pronounced in the mutant. A correlation between the efficiency of orientation and the applied g value suggests a physical basis for gravitaxis. Transiently enhanced rates of reversal of the swimming direction coincided with transiently enhanced gravitaxis because reversals occurred more often in downward swimmers than in upward swimmers. The results provide evidence of a physiological modulation of gravitaxis by means of the randomizing effect of depolarization-dependent swimming reversals. Gravity bimodally altered propulsion rates of wild-type P. tetraurelia so that sedimentation was partly antagonized in upward and downward swimmers (negative gravikinesis). In the mutant, only increases in propulsion were observed, although the orientation-dependent sensitivity of the gravikinetic response was the same as in the wild-type population. Observed swimming speed and sedimentation rates in the wild-type and mutant cells were linearly related to acceleration, allowing the determination of gravikinesis as a linear (and so far non-saturating) function of gravity.

Mechanical properties of L929 cells measured by atomic force microscopy: effects of anticytoskeletal drugs and membrane crosslinking

To shed light on the architecture of the cytoskeleton, we used the atomic force microscope (AFM) to measure the elasticity, viscoelasticity, and plasticity of L929 cells. The initial elastic response (Young's modulus approximately 4,000 Pa) of the cells to an applied force was followed by a slow compression of the cytoskeleton (tau 1/2 approximately equal to 10 s). When force application was terminated, the cytoskeleton underwent a sudden partial decompression and a subsequent slow, incomplete recovery. The role of the cytoskeletal elements in cell mechanics was accessed in AFM measurements carried out on cells treated with cytochalasin D, nocodazole, or colcemid. Cytochalasin D treatment reduced both elasticity (approximately 45%) and cytoplasmic viscosity (approximately 65%), whereas cells treated with nocodazole or colcemid exhibited a marked increase in elasticity (approximately 100%) and a slight increase in viscosity (approximately 15%). The AFM force measurements also provided evidence that the cell membrane and the cytoskeleton are mechanically coupled. Tightly adherent cells were stiffer than cells that were loosely attached. Moreover, cells crosslinked with either glutaraldehyde, 3, 3'-dithiobis [sulfosuccinimidylpropionate] (DTSSP), or Concanavalin A were more rigid than untreated cells. It is of interest that cells crosslinked with Concanavalin A, but not DTSSP, displayed plastic behaviors that may reflect the induction of cytoskeletal reorganization by Concanavalin A.

Effects of membrane potential on the voltage dependence of motility-related charge in outer hair cells of the guinea-pig

1. Isolated outer hair cells (OHCs) from the guinea-pig were whole-cell voltage clamped to study the influence of initial voltage on the voltage dependence of motility-related gating current or, equivalently, on the voltage dependence of membrane capacitance. 2. Prepulse delivery caused changes in the magnitude of motility-related gating currents, which are due predominantly to shifts in the voltage at peak capacitance (VpkCmm). Depolarization shifts VpkCm in the hyperpolarizing direction, and hyperpolarization does the opposite. The mean shift between -120 and +40 mV prepulse states with long-term holding potentials (> 2 min) at -80 mV was 14. 67 +/- 0.95 mV (n = 10; mean +/- s.e.m.). 3. The effect of initial membrane potential is sigmoidal, with a voltage dependence of 23 mV per e-fold change in VpkCm, and maximum slope within the physiological range of OHC resting potentials. This indicates that the cell is poised to respond maximally to changes in resting potential. 4. The kinetics of prepulse effects are slow compared with motility-related gating current kinetics. High-resolution measurement of membrane capacitance (Cm) using two voltage sinusoids indicates that shifts in VpkCm induce Cm changes with time courses fitted by two exponentials (tau0, 0.070 +/- 0.003 s; tau1, 1.28 +/- 0.07 s; A0, 1.54 +/- 0.13 pF; A1, 1.51 +/- 0.13 pF; means +/- s.e.m. ; n = 22; step from +50 to -80 mV). Recovery of prepulse effects exhibits a similar time course. 5. Prepulse effects are resistant to intracellular enzymatic digestion, to fast intracellular calcium buffers, and to intracellular pressure. Through modelling, we indicate how the effect may be explained by an intrinsic voltage-induced tension generated by the molecular motors residing in the lateral membrane.

Effect of diamide on force generation and axial stiffness of the cochlear outer hair cell

We found that diamide, which affects spectrin, reduces the axial stiffness of the cochlear outer hair cell, the cylindrically shaped mechanoreceptor cell with a unique voltage-sensitive motility. This effect thus provides a means of examining the relationship between the stiffness and the motility of the cell. For measuring axial stiffness and force production, we used an experimental configuration in which an elastic probe was attached to the cell near the cuticular plate and the other end of the cell was held with a patch pipette in the whole-cell recording mode. Diamide at concentrations of up to 5 mM reduced the axial stiffness in a dose-dependent manner to 165 nN per unit strain from 502 nN for untreated cells. The isometric force elicited by voltage pulses under whole-cell voltage clamp was also reduced to 35 pN/mV from 105 pN/mV for untreated cells. Thus the isometric force was approximately proportional to the axial stiffness. Our observations suggest a series connection between the motor and cytoskeletal elements and can be explained by the area motor model previously proposed for the outer hair cell.

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.