A membrane bending model of outer hair cell electromotility

Giant Iontronic Flexoelectricity in Soft Hydrogels Induced by Tunable Biomimetic Ion Polarization

Flexoelectricity features the strain gradient-induced mechanoelectric conversion using materials not limited by their crystalline symmetry, but state-of-the-art flexoelectric materials exhibit very small flexoelectric coefficients and are too brittle to withstand large deformations. Here, inspired by the ion polarization in living organisms, this paper reports the giant iontronic flexoelectricity of soft hydrogels where the ion polarization is attributed to the different transfer rates of cations and anions under bending deformations. The flexoelectricity is found to be easily regulated by the types of anion-cation pairs and polymer networks in the hydrogel. A polyacrylamide hydrogel with 1 m NaCl achieves a record-high flexoelectric coefficient of ≈1160 µC m-1, which can even be improved to ≈2340 µC m-1 by synergizing with the effects of ion pairs and extra polycation chains. Furthermore, the hydrogel as flexoelectric materials can withstand larger bending deformations to obtain higher polarization charges owing to its intrinsic low modulus and high elasticity. A soft flexoelectric sensor is then demonstrated for object recognition by robotic hands. The findings greatly broaden the flexoelectricity to soft, biomimetic, and biocompatible materials and applications.

A minimal physics-based model for musical perception

Some people, entirely untrained in music, can listen to a song and replicate it on a piano with unnerving accuracy. What enables some to "hear" music so much better than others? Long-standing research confirms that part of the answer is undoubtedly neurological and can be improved with training. However, are there structural, physical, or engineering attributes of the human hearing mechanism apparatus (i.e., the hair cells of the internal ear) that render one human innately superior to another in terms of propensity to listen to music? In this work, we investigate a physics-based model of the electromechanics of the hair cells in the inner ear to understand why a person might be physiologically better poised to distinguish musical sounds. A key feature of the model is that we avoid a "black-box" systems-type approach. All parameters are well-defined physical quantities, including membrane thickness, bending modulus, electromechanical properties, and geometrical features, among others. Using the two-tone interference problem as a proxy for musical perception, our model allows us to establish the basis for exploring the effect of external factors such as medicine or environment. As an example of the insights we obtain, we conclude that the reduction in bending modulus of the cell membranes (which for instance may be caused by the usage of a certain class of analgesic drugs) or an increase in the flexoelectricity of the hair cell membrane can interfere with the perception of two-tone excitation.

3D Ultrastructure of the Cochlear Outer Hair Cell Lateral Wall Revealed By Electron Tomography

Outer Hair Cells (OHCs) in the mammalian cochlea display a unique type of voltage-induced mechanical movement termed electromotility, which amplifies auditory signals and contributes to the sensitivity and frequency selectivity of mammalian hearing. Electromotility occurs in the OHC lateral wall, but it is not fully understood how the supramolecular architecture of the lateral wall enables this unique form of cellular motility. Employing electron tomography of high-pressure frozen and freeze-substituted OHCs, we visualized the 3D structure and organization of the membrane and cytoskeletal components of the OHC lateral wall. The subsurface cisterna (SSC) is a highly prominent feature, and we report that the SSC membranes and lumen possess hexagonally ordered arrays of particles. We also find the SSC is tightly connected to adjacent actin filaments by short filamentous protein connections. Pillar proteins that join the plasma membrane to the cytoskeleton appear as variable structures considerably thinner than actin filaments and significantly more flexible than actin-SSC links. The structurally rich organization and rigidity of the SSC coupled with apparently weaker mechanical connections between the plasma membrane (PM) and cytoskeleton reveal that the membrane-cytoskeletal architecture of the OHC lateral wall is more complex than previously appreciated. These observations are important for our understanding of OHC mechanics and need to be considered in computational models of OHC electromotility that incorporate subcellular features.

Electrophysiological-mechanical coupling in the neuronal membrane and its role in ultrasound neuromodulation and general anaesthesia

The current understanding of the role of the cell membrane is in a state of flux. Recent experiments show that conventional models, considering only electrophysiological properties of a passive membrane, are incomplete. The neuronal membrane is an active structure with mechanical properties that modulate electrophysiology. Protein transport, lipid bilayer phase, membrane pressure and stiffness can all influence membrane capacitance and action potential propagation. A mounting body of evidence indicates that neuronal mechanics and electrophysiology are coupled, and together shape the membrane potential in tight coordination with other physical properties. In this review, we summarise recent updates concerning electrophysiological-mechanical coupling in neuronal function. In particular, we aim at making the link with two relevant yet often disconnected fields with strong clinical potential: the use of mechanical vibrations-ultrasound-to alter the electrophysiogical state of neurons, e.g., in neuromodulation, and the theories attempting to explain the action of general anaesthetics. STATEMENT OF SIGNIFICANCE: General anaesthetics revolutionised medical practice; now an apparently unrelated technique, ultrasound neuromodulation-aimed at controlling neuronal activity by means of ultrasound-is poised to achieve a similar level of impact. While both technologies are known to alter the electrophysiology of neurons, the way they achieve it is still largely unknown. In this review, we argue that in order to explain their mechanisms/effects, the neuronal membrane must be considered as a coupled mechano-electrophysiological system that consists of multiple physical processes occurring concurrently and collaboratively, as opposed to sequentially and independently. In this framework the behaviour of the cell membrane is not the result of stereotypical mechanisms in isolation but instead emerges from the integrative behaviour of a complexly coupled multiphysics system.

Plasmonic imaging of subcellular electromechanical deformation in mammalian cells

A membrane potential change in cells is accompanied with mechanical deformation. This electromechanical response can play a significant role in regulating action potential in neurons and in controlling voltage-gated ion channels. However, measuring this subtle deformation in mammalian cells has been a difficult task. We show a plasmonic imaging method to image mechanical deformation in single cells upon a change in the membrane potential. Using this method, we have studied the electromechanical response in mammalian cells and have observed the local deformation within the cells that are associated with cell-substrate interactions. By analyzing frequency dependence of the response, we have further examined the electromechanical deformation in terms of mechanical properties of cytoplasm and cytoskeleton. We demonstrate a plasmonic imaging approach to quantify the electromechanical responses of single mammalian cells and determine local variability related to cell-substrate interactions.

Diflunisal inhibits prestin by chloride-dependent mechanism

The motor protein prestin is a member of the SLC26 family of anion antiporters and is essential to the electromotility of cochlear outer hair cells and for hearing. The only direct inhibitor of electromotility and the associated charge transfer is salicylate, possibly through direct interaction with an anion-binding site on prestin. In a screen to identify other inhibitors of prestin activity, we explored the effect of the non-steroid anti-inflammatory drug diflunisal, which is a derivative of salicylate. We recorded prestin activity by whole-cell patch clamping HEK cells transiently expressing prestin and mouse outer hair cells. We monitored the impact of diflunisal on the prestin-dependent non-linear capacitance and electromotility. We found that diflunisal triggers two prestin-associated effects: a chloride independent increase in the surface area and the specific capacitance of the membrane, and a chloride dependent inhibition of the charge transfer and the electromotility in outer hair cells. We conclude that diflunisal affects the cell membrane organization and inhibits prestin-associated charge transfer and electromotility at physiological chloride concentrations. The inhibitory effects on hair cell function are noteworthy given the proposed use of diflunisal to treat neurodegenerative diseases.

Flexoelectricity in two-dimensional crystalline and biological membranes

The ability of a material to convert electrical stimuli into mechanical deformation, i.e. piezoelectricity, is a remarkable property of a rather small subset of insulating materials. The phenomenon of flexoelectricity, on the other hand, is universal. All dielectrics exhibit the flexoelectric effect whereby non-uniform strain (or strain gradients) can polarize the material and conversely non-uniform electric fields may cause mechanical deformation. The flexoelectric effect is strongly enhanced at the nanoscale and accordingly, all two-dimensional membranes of atomistic scale thickness exhibit a strong two-way coupling between the curvature and electric field. In this review, we highlight the recent advances made in our understanding of flexoelectricity in two-dimensional (2D) membranes-whether the crystalline ones such as dielectric graphene nanoribbons or the soft lipid bilayer membranes that are ubiquitous in biology. Aside from the fundamental mechanisms, phenomenology, and recent findings, we focus on rapidly emerging directions in this field and discuss applications such as energy harvesting, understanding of the mammalian hearing mechanism and ion transport among others.

Voltage-morphology coupling in biomimetic membranes: dynamics of giant vesicles in applied electric fields

An electric potential difference across the plasma membrane is common to all living cells and is essential to physiological functions such as the generation of action potentials for cell-to-cell communication. While the basics of cell electrical activity are well established (e.g. the Hodgkin-Huxley model of the action potential), the reciprocal coupling of voltage and membrane deformation has received limited attention. In recent years, studies of biomimetic membranes in externally applied electric fields have revealed a plethora of intriguing dynamics (formation of edges, pearling, and phase separation) that challenge the current understanding of membrane electromechanics.

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.

Chloride and salicylate influence prestin-dependent specific membrane capacitance: support for the area motor model

The outer hair cell is electromotile, its membrane motor identified as the protein SLC26a5 (prestin). An area motor model, based on two-state Boltzmann statistics, was developed about two decades ago and derives from the observation that outer hair cell surface area is voltage-dependent. Indeed, aside from the nonlinear capacitance imparted by the voltage sensor charge movement of prestin, linear capacitance (Clin) also displays voltage dependence as motors move between expanded and compact states. Naturally, motor surface area changes alter membrane capacitance. Unit linear motor capacitance fluctuation (δCsa) is on the order of 140 zeptofarads. A recent three-state model of prestin provides an alternative view, suggesting that voltage-dependent linear capacitance changes are not real but only apparent because the two component Boltzmann functions shift their midpoint voltages (Vh) in opposite directions during treatment with salicylate, a known competitor of required chloride binding. We show here using manipulations of nonlinear capacitance with both salicylate and chloride that an enhanced area motor model, including augmented δCsa by salicylate, can accurately account for our novel findings. We also show that although the three-state model implicitly avoids measuring voltage-dependent motor capacitance, it registers δCsa effects as a byproduct of its assessment of Clin, which increases during salicylate treatment as motors are locked in the expanded state. The area motor model, in contrast, captures the characteristics of the voltage dependence of δCsa, leading to a better understanding of prestin.

Apparent flexoelectricity in lipid bilayer membranes due to external charge and dipolar distributions

In this Rapid Communication we show that the interplay between the deformation geometric-nonlinearity and distributions of external charges and dipoles lead to the renormalization of the membrane's native flexoelectric response. Our work provides a framework for a mesoscopic interpretation of flexoelectricity and if necessary, artificially "design" tailored flexoelectricity in membranes. Comparisons with experiments indicate reasonable quantitative agreement.

Filopodia as sensors

Filopodia are sensors on both excitable and non-excitable cells. The sensing function is well documented in neurons and blood vessels of adult animals and is obvious during dorsal closure in embryonic development. Nerve cells extend neurites in a bidirectional fashion with growth cones at the tips where filopodia are concentrated. Their sensing of environmental cues underpins the axon's ability to "guide," bypassing non-target cells and moving toward the target to be innervated. This review focuses on the role of filopodia structure and dynamics in the detection of environmental cues, including both the extracellular matrix (ECM) and the surfaces of neighboring cells. Other protrusions including the stereocilia of the inner ear and epididymus, the invertebrate Type I mechanosensors, and the elongated processes connecting osteocytes, share certain principles of organization with the filopodia. Actin bundles, which may be inside or outside of the excitable cell, function to transduce stress from physical perturbations into ion signals. There are different ways of detecting such perturbations. Osteocyte processes contain an actin core and are physically anchored on an extracellular structure by integrins. Some Type I mechanosensors have bridge proteins that anchor microtubules to the membrane, but bundles of actin in accessory cells exert stress on this complex. Hair cells of the inner ear rely on attachments between the actin-based protrusions to activate ion channels, which then transduce signals to afferent neurons. In adherent filopodia, the focal contacts (FCs) integrated with ECM proteins through integrins may regulate integrin-coupled ion channels to achieve signal transduction. Issues that are not understood include the role of Ca(2+) influx in filopodia dynamics and how integrins coordinate or gate signals arising from perturbation of channels by environmental cues.

The capacitance and electromechanical coupling of lipid membranes close to transitions: the effect of electrostriction

Biomembranes are thin capacitors with the unique feature of displaying phase transitions in a physiologically relevant regime. We investigate the voltage and lateral pressure dependence of their capacitance close to their chain melting transition. Because the gel and the fluid membrane have different area and thickness, the capacitance of the two membrane phases is different. In the presence of external fields, charges exert forces that can influence the state of the membrane, thereby influencing the transition temperature. This phenomenon is called "electrostriction". We show that this effect allows us to introduce a capacitive susceptibility that assumes a maximum in the melting transition with an associated excess charge. As a consequence, voltage regimes exist in which a small change in voltage can lead to a large uptake of charge and a large capacitive current. Furthermore, we consider electromechanical behavior such as pressure-induced changes in capacitance, and the application of such concepts in biology.

Evidence of interlinks between bioelectromagnetics and biomechanics: from biophysics to medical physics

A vast literature on electromagnetic and mechanical bioeffects at the bone and soft tissue level, as well as at the cellular level (osteoblasts, osteoclasts, keratinocytes, fibroblasts, chondrocytes, nerve cells, endothelial and muscle cells) has been reviewed and analysed in order to show the evident connections between both types of physical energies. Moreover, an intimate link between the two is suggested by transduction phenomena (electromagnetic-acoustic transduction and its reverse) occurring in living matter, as a sound biophysical literature has demonstrated. However, electromagnetic and mechanical signals are not always interchangeable, depending on their respective intensity. Calculations are reported in order to show in which cases (read: for which values of electric field in V/m and of mechanical pressure in Pa) a given electromagnetic or mechanical bioeffect is only due to the directly impinging energy or even to the indirect transductional energy. The relevance of the treated item for the applications of medical physics to regenerative medicine is stressed.

ON THE ACTION POTENTIAL AS A PROPAGATING DENSITY PULSE AND THE ROLE OF ANESTHETICS

The Hodgkin-Huxley model of nerve pulse propagation relies on ion currents through specific resistors called ion channels. We discuss a number of classical thermodynamic findings on nerves that are not contained within this classical theory. In particular striking is the finding of reversible heat changes, thickness and phase changes of the membrane during the action potential. Data on various nerves rather suggest that a reversible density pulse accompanies the action potential of nerves. Here, we attempted to explain these phenomena by propagating solitons that depend on the presence of cooperative phase transitions in the nerve membrane. The transitions, however, are strongly influenced by the presence of anesthetics. Therefore, the thermodynamic theory of nerve pulses suggests an explanation for the famous Meyer-Overton rule that states that the critical anesthetic dose is linearly related to the solubility of the drug in the membranes.

Piezo- and Flexoelectric Membrane Materials Underlie Fast Biological Motors in the Ear

The mammalian inner ear is remarkably sensitive to quiet sounds, exhibits over 100dB dynamic range, and has the exquisite ability to discriminate closely spaced tones even in the presence of noise. This performance is achieved, in part, through active mechanical amplification of vibrations by sensory hair cells within the inner ear. All hair cells are endowed with a bundle of motile microvilli, stereocilia, located at the apical end of the cell, and the more specialized outer hair cells (OHC's) are also endowed with somatic electromotility responsible for changes in cell length in response to perturbations in membrane potential. Both hair bundle and somatic motors are known to feed energy into the mechanical vibrations in the inner ear. The biophysical origin and relative significance of the motors remains a subject of intense research. Several biological motors have been identified in hair cells that might underlie the motor(s), including a cousin of the classical ATP driven actin-myosin motor found in skeletal muscle. Hydrolysis of ATP, however, is much too slow to be viable at audio frequencies on a cycle-by-cycle basis. Heuristically, the OHC somatic motor behaves as if the OHC lateral wall membrane were a piezoelectric material and the hair bundle motor behaves as if the plasma membrane were a flexoelectric material. We propose these observations from a continuum materials perspective are literally true. To examine this idea, we formulated mathematical models of the OHC lateral wall "piezoelectric" motor and the more ubiquitous "flexoelectric" hair bundle motor. Plausible biophysical mechanisms underlying piezo- and flexoelectricity were established. Model predictions were compared extensively to the available data. The models were then applied to study the power conversion efficiency of the motors. Results show that the material properties of the complex membranes in hair cells provide them with the ability to convert electrical power available in the inner ear cochlea into useful mechanical amplification of sound induced vibrations at auditory frequencies. We also examined how hair cell amplification might be controlled by the brain through efferent synaptic contacts on hair cells and found a simple mechanism to tune hearing to signals of interest to the listener by electrical control of these motors.

The remarkable cochlear amplifier

This composite article is intended to give the experts in the field of cochlear mechanics an opportunity to voice their personal opinion on the one mechanism they believe dominates cochlear amplification in mammals. A collection of these ideas are presented here for the auditory community and others interested in the cochlear amplifier. Each expert has given their own personal view on the topic and at the end of their commentary they have suggested several experiments that would be required for the decisive mechanism underlying the cochlear amplifier. These experiments are presently lacking but if successfully performed would have an enormous impact on our understanding of the cochlear amplifier.

Cell membrane tethers generate mechanical force in response to electrical stimulation

Living cells maintain a huge transmembrane electric field across their membranes. This electric field exerts a force on the membrane because the membrane surfaces are highly charged. We have measured electromechanical force generation by cell membranes using optically trapped beads to detach the plasma membrane from the cytoskeleton and form long thin cylinders (tethers). Hyperpolarizing potentials increased and depolarizing potentials decreased the force required to pull a tether. The membrane tether force in response to sinusoidal voltage signals was a function of holding potential, tether diameter, and tether length. Membrane electromechanical force production can occur at speeds exceeding those of ATP-based protein motors. By harnessing the energy in the transmembrane electric field, cell membranes may contribute to processes as diverse as outer hair cell electromotility, ion channel gating, and transport.

Voltage-induced bending and electromechanical coupling in lipid bilayers

The electrical properties of the cellular membrane are important for ion transport across cells and electrophysiology. Plasma membranes also resist bending and stretching, and mechanical properties of the membrane influence cell shape and forces in membrane tethers pulled from cells. There exists a coupling between the electrical and mechanical properties of the membrane. Previous work has shown that applied voltages can induce forces and movements in the lipid bilayer. We present a theory that computes membrane bending deformations and forces as the applied voltage is changed. We discover that electromechanical coupling in lipid bilayers depends on the voltage-dependent adsorption of ions into the region occupied by the phospholipid head groups. A simple model of counter-ion absorption is investigated. We show that electromechanical coupling can be measured using membrane tethers and we use our model to predict the membrane tether tension as a function of applied voltage. We also discuss how electromechanical coupling in membranes may influence transmembrane protein function.

WITHDRAWN: Membrane-based amplification in hearing

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Optical coherence tomography phase measurement of transient changes in squid giant axons during activity

Noncontact optical measurements reveal that transient changes in squid giant axons are associated with action potential propagation and altered under different environmental (i.e., temperature) and physiological (i.e., ionic concentrations) conditions. Using a spectral-domain optical coherence tomography system, which produces real-time cross-sectional images of the axon in a nerve chamber, axonal surfaces along a depth profile are monitored. Differential phase analyses show transient changes around the membrane on a millisecond timescale, and the response is coincident with the arrival of the action potential at the optical measurement area. Cooling the axon slows the electrical and optical responses and increases the magnitude of the transient signals. Increasing the NaCl concentration bathing the axon, whose diameter is decreased in the hypertonic solution, results in significantly larger transient signals during action potential propagation. While monophasic and biphasic behaviors are observed, biphasic behavior dominates the results. The initial phase detected was constant for a single location but alternated for different locations; therefore, these transient signals acquired around the membrane appear to have local characteristics.

Power efficiency of outer hair cell somatic electromotility

Cochlear outer hair cells (OHCs) are fast biological motors that serve to enhance the vibration of the organ of Corti and increase the sensitivity of the inner ear to sound. Exactly how OHCs produce useful mechanical power at auditory frequencies, given their intrinsic biophysical properties, has been a subject of considerable debate. To address this we formulated a mathematical model of the OHC based on first principles and analyzed the power conversion efficiency in the frequency domain. The model includes a mixture-composite constitutive model of the active lateral wall and spatially distributed electro-mechanical fields. The analysis predicts that: 1) the peak power efficiency is likely to be tuned to a specific frequency, dependent upon OHC length, and this tuning may contribute to the place principle and frequency selectivity in the cochlea; 2) the OHC power output can be detuned and attenuated by increasing the basal conductance of the cell, a parameter likely controlled by the brain via the efferent system; and 3) power output efficiency is limited by mechanical properties of the load, thus suggesting that impedance of the organ of Corti may be matched regionally to the OHC. The high power efficiency, tuning, and efferent control of outer hair cells are the direct result of biophysical properties of the cells, thus providing the physical basis for the remarkable sensitivity and selectivity of hearing.

Electrohydrodynamic model of vesicle deformation in alternating electric fields

We develop an analytical theory to explain the experimentally observed morphological transitions of quasispherical giant vesicles induced by alternating electric fields. The model treats the inner and suspending media as lossy dielectrics, and the membrane as an impermeable flexible incompressible-fluid sheet. The vesicle shape is obtained by balancing electric, hydrodynamic, bending, and tension stresses exerted on the membrane. Our approach, which is based on force balance, also allows us to describe the time evolution of the vesicle deformation, in contrast to earlier works based on energy minimization, which are able to predict only stationary shapes. Our theoretical predictions for vesicle deformation are consistent with experiment. If the inner fluid is more conducting than the suspending medium, the vesicle always adopts a prolate shape. In the opposite case, the vesicle undergoes a transition from a prolate to oblate ellipsoid at a critical frequency, which the theory identifies with the inverse membrane charging time. At frequencies higher than the inverse Maxwell-Wagner polarization time, the electrohydrodynamic stresses become too small to alter the vesicle's quasispherical rest shape. The model can be used to rationalize the transient and steady deformation of biological cells in electric fields.

Lipid lateral mobility in cochlear outer hair cells: regional differences and regulation by cholesterol

The outer hair cell (OHC) lateral plasma membrane houses the transmembrane protein prestin, a necessary component of the yet unknown molecular mechanism(s) underlying electromotility and the exquisite sensitivity and frequency selectivity of mammalian hearing. The importance of the plasma membrane environment in modulating OHC electromotility has been substantiated by recent studies demonstrating that membrane cholesterol alters prestin activity in a manner consistent with cholesterol-induced changes in auditory function. Cholesterol is known to affect membrane material properties, and measurements of lipid lateral mobility provide a method to asses these changes in living OHCs. Using fluorescence recovery after photobleaching (FRAP), we characterized regional differences in the lateral diffusion of the lipid analog di-8-ANEPPS in OHCs and investigated whether lipid mobility, which reflects membrane fluidity, is sensitive to membrane cholesterol. FRAP experiments revealed quantitative differences in lipid lateral mobility among the apical, lateral, and basal regions of the OHC and demonstrated that diffusion in individual regions is uniquely sensitive to cholesterol manipulations. Interestingly, in the lateral region, both cholesterol depletion and loading significantly reduced the effective diffusion coefficient from control values. Thus, the fluidity of the OHC lateral plasma membrane is regulated by cholesterol levels in a non-monotonic manner, suggesting that the overall material properties of the lateral plasma membrane are optimally tuned for OHC function in the native state. These results support the idea that the cholesterol-dependent regulation of prestin function and electromotility correlates with changes in the properties of the lipid environment that surrounds and supports prestin.

Hair cell bundles: flexoelectric motors of the inner ear

Microvilli (stereocilia) projecting from the apex of hair cells in the inner ear are actively motile structures that feed energy into the vibration of the inner ear and enhance sensitivity to sound. The biophysical mechanism underlying the hair bundle motor is unknown. In this study, we examined a membrane flexoelectric origin for active movements in stereocilia and conclude that it is likely to be an important contributor to mechanical power output by hair bundles. We formulated a realistic biophysical model of stereocilia incorporating stereocilia dimensions, the known flexoelectric coefficient of lipid membranes, mechanical compliance, and fluid drag. Electrical power enters the stereocilia through displacement sensitive ion channels and, due to the small diameter of stereocilia, is converted to useful mechanical power output by flexoelectricity. This motor augments molecular motors associated with the mechanosensitive apparatus itself that have been described previously. The model reveals stereocilia to be highly efficient and fast flexoelectric motors that capture the energy in the extracellular electro-chemical potential of the inner ear to generate mechanical power output. The power analysis provides an explanation for the correlation between stereocilia height and the tonotopic organization of hearing organs. Further, results suggest that flexoelectricity may be essential to the exquisite sensitivity and frequency selectivity of non-mammalian hearing organs at high auditory frequencies, and may contribute to the “cochlear amplifier” in mammals.

Amphipath-induced nanoscale changes in outer hair cell plasma membrane curvature

Outer hair cell (OHC) electromotility enables frequency selectivity and sensitivity in mammalian audition. Electromotility is generated by the transmembrane protein prestin and is sensitive to amphipathic compounds including salicylate, chlorpromazine (CPZ), and trinitrophenol (TNP). Although these compounds induce observable membrane curvature changes in erythrocytes, their effects on OHC membrane curvature are unknown. In this work, fluorescence polarization microscopy was applied to investigate the effects of salicylate, CPZ, and TNP on di-8-ANEPPS orientation in the OHC plasma membrane. Our results demonstrate the ability of fluorescence polarization microscopy to measure amphipath-induced changes in di-8-ANEPPS orientation, consistent with nanoscale changes in membrane curvature between regularly spaced proteins connecting the OHC plasma membrane and cytoskeleton. Simultaneous application of oppositely charged amphipaths generally results in no net membrane bending, consistent with predictions of the bilayer couple hypothesis; however, the application of salicylate (10 mM), which inhibits electromotility, is not reversed by the addition of CPZ. This result supports other findings that suggest salicylate primarily influences electromotiliy and OHC nonlinear capacitance via a direct interaction with prestin. In contrast, we find that CPZ and TNP influence the voltage sensitivity of prestin via membrane bending, demonstrating the mechanosensitivity of this unique membrane motor protein.

Electromagnetic effects - From cell biology to medicine

In this review we compile and discuss the published plethora of cell biological effects which are ascribed to electric fields (EF), magnetic fields (MF) and electromagnetic fields (EMF). In recent years, a change in paradigm took place concerning the endogenously produced static EF of cells and tissues. Here, modern molecular biology could link the action of ion transporters and ion channels to the "electric" action of cells and tissues. Also, sensing of these mainly EF could be demonstrated in studies of cell migration and wound healing. The triggers exerted by ion concentrations and concomitant electric field gradients have been traced along signaling cascades till gene expression changes in the nucleus. Far more enigmatic is the way of action of static MF which come in most cases from outside (e.g. earth magnetic field). All systems in an organism from the molecular to the organ level are more or less in motion. Thus, in living tissue we mostly find alternating fields as well as combination of EF and MF normally in the range of extremely low-frequency EMF. Because a bewildering array of model systems and clinical devices exits in the EMF field we concentrate on cell biological findings and look for basic principles in the EF, MF and EMF action. As an outlook for future research topics, this review tries to link areas of EF, MF and EMF research to thermodynamics and quantum physics, approaches that will produce novel insights into cell biology.

Anion control of voltage sensing by the motor protein prestin in outer hair cells

The outer hair cell from Corti's organ possesses voltage-dependent intramembranous molecular motors evolved from the SLC26 anion transporter family. The motor, identified as prestin (SLC26a5), is responsible for electromotility of outer hair cells and mammalian cochlear amplification, a process that heightens our auditory responsiveness. Here, we describe experiments designed to evaluate the effects of anions on the motor's voltage-sensor charge movement, focusing on prestin's voltage-dependent Boltzmann characteristics. We find that the nature of the anion, including species, valence, and structure, regulates characteristics of the charge movement, signifying that anions play a more complicated role than simple voltage sensing in cochlear amplification.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

Towards local electromechanical probing of cellular and biomolecular systems in a liquid environment

Electromechanical coupling is ubiquitous in biological systems, with examples ranging from simple piezoelectricity in calcified and connective tissues to voltage-gated ion channels, energy storage in mitochondria, and electromechanical activity in cardiac myocytes and outer hair cell stereocilia. Piezoresponse force microscopy (PFM) originally emerged as a technique to study electromechanical phenomena in ferroelectric materials, and in recent years has been employed to study a broad range of non-ferroelectric polar materials, including piezoelectric biomaterials. At the same time, the technique has been extended from ambient to liquid imaging on model ferroelectric systems. Here, we present results on local electromechanical probing of several model cellular and biomolecular systems, including insulin and lysozyme amyloid fibrils, breast adenocarcinoma cells, and bacteriorhodopsin in a liquid environment. The specific features of PFM operation in liquid are delineated and bottlenecks on the route towards nanometre-resolution electromechanical imaging of biological systems are identified.

Capillary models for liquid crystal fibers, membranes, films, and drops

This paper presents an overview of the capillary modeling science of nematic liquid crystals and its applications to the stability, structure, and shape of films, membranes, fibers, and drops. Liquid crystals are anisotropic viscoelastic materials possessing long range orientational order, and hence these models are relevant to the capillary science of anisotropic soft matter. A systematic multiscale approach is used to derive the equations that govern the shape of interfaces and contact lines. These shape equations generalize the surface Laplace and the contact line Neuman equations by introducing long range orientational order, gradient elasticity, surfactant adsorbants, magnetic and electric fields. The thermodynamics of capillary systems is used to reveal novel cross-effects such as adsorption-driven shape changes of surfaces and contact lines. The capillary models are used to analyze the structure and stability of films, membranes, fibers, and drops, of direct relevance to the processing and performance of structural and functional liquid crystals. Novel soft materials and mechanisms analyzed in this paper include: (1) stabilization of freely-suspended nematic films by orientation and molecular order heterogeneities, (2) orientational defects in polymer dispersed liquid crystals films, shown to originate from surface anchoring transitions, (3) electric field-induced curvature in membranes for sensor and actuator applications, (4) new helical morphologies of thin nematic filaments driven by strong interfacial anchoring, (5) tunable partial wetting through contact angle modification gradient and anchoring elasticity, (6) liquid crystal nanoemulsion shape control through anchoring effects, and (7) magnetic shaping in liquid crystal colloids. Readily accessible applications to biological liquid crystal materials and processes indicate that capillary modeling science will be a most active area of research in the very near future.

Tuning of the outer hair cell motor by membrane cholesterol

Cholesterol affects diverse biological processes, in many cases by modulating the function of integral membrane proteins. We observed that alterations of cochlear cholesterol modulate hearing in mice. Mammalian hearing is powered by outer hair cell (OHC) electromotility, a membrane-based motor mechanism that resides in the OHC lateral wall. We show that membrane cholesterol decreases during maturation of OHCs. To study the effects of cholesterol on hearing at the molecular level, we altered cholesterol levels in the OHC wall, which contains the membrane protein prestin. We show a dynamic and reversible relationship between membrane cholesterol levels and voltage dependence of prestin-associated charge movement in both OHCs and prestin-transfected HEK 293 cells. Cholesterol levels also modulate the distribution of prestin within plasma membrane microdomains and affect prestin self-association in HEK 293 cells. These findings indicate that alterations in membrane cholesterol affect prestin function and functionally tune the outer hair cell.

NSAID injury to the gastrointestinal tract: evidence that NSAIDs interact with phospholipids to weaken the hydrophobic surface barrier and induce the formation of unstable pores in membranes

In this review, we have discussed our current understanding of the barrier properties that are in place to protect the upper gastrointestinal mucosa from luminal acid, and the pathogenic mechanism by which nonsteroidal anti-inflammatory drugs (NSAIDs) induce injury to the gastrointestinal tract. The changes in our view of the importance of NSAID-induced cyclo-oxygenase (COX) inhibition on the pathogenesis and prevention of NSAID-induced gastrointestinal injury is presented. The focus of this paper has been placed on the effects of NSAIDs on the mucosal surface, and specifically the effect of these powerful drugs in inducing changes in the hydrophobicity, fluidity, biomechanical and permeability properties of extracellular and membrane phospholipids. Lastly, recent evidence is presented that salicylic acid and related NSAIDs may alter the stability of membranes, inducing the formation of unstable pores that may lead to back-diffusion of luminal acid and membrane rupture. This understanding of the interaction of NSAIDs with membrane phospholipids may prove valuable in the design of novel NSAID formulations with reduced gastrointestinal side-effects.

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.

Application of fluorescence recovery after photobleaching to study prestin lateral mobility in the human embryonic kidney cell

The transmembrane protein prestin is crucial to outer hair cell (OHC) electromotility and contributes to the sensitivity and frequency selectivity of mammalian hearing. The molecular mechanisms of electromotility remain unclear, but prestin is purported to function as both a voltage sensor and a molecular motor. Understanding the role of prestin requires characterizing its organization and behavior in the plasma membrane. Fluorescence recovery after photobleaching (FRAP) provides a powerful means to quantitatively study molecular diffusion. However, OHCs are inherently fragile ex vivo, and dynamic studies of prestin require model systems, such as human embryonic kidney (HEK) cells, expressing fluorescently labeled prestin. Utilizing this system, we provide the first direct, quantitative measurement of prestin lateral mobility. The results show remarkably different diffusion behavior for prestin-green fluorescent protein (GFP) as compared to a control protein, human somatostatin receptor 5 (SSTR5). Prestin-GFP FRAP experiments reveal immobile fractions approaching 50%, low effective diffusion coefficients, and recovery times slower than those of SSTR5. Secondary bleaching of a region reveals distinctly different diffusion parameters, which we propose reflect the transient confinement of prestin in the HEK cell. Although uncharacterized, intermolecular interactions between prestin and the membrane and/or cytoskeleton may be important for the unique properties of prestin in electromotile OHCs.

Spontaneous corrugation of dipolar membranes

We present a simple model for dipolar elastic membranes that gives lattice-bound point dipoles complete orientational freedom as well as translational freedom along one coordinate (out of the plane of the membrane). There is an additional harmonic term which binds each of the dipoles to the six nearest neighbors on either triangular or distorted lattices. The translational freedom of the dipoles allows triangular lattices to find states that break out of the normal orientational disorder of frustrated configurations and which are stabilized by long-range antiferroelectric ordering. In order to break out of the frustrated states, the dipolar membranes form corrugated or "rippled" phases that make the lattices effectively nontriangular. We observe three common features of the corrugated dipolar membranes: (1) the corrugated phases develop easily when hosted on triangular lattices, (2) the wave vectors for the surface ripples are always found to be perpendicular to the dipole director axis, and (3) on triangular lattices, the dipole director axis is found to be parallel to any of the three equivalent lattice directions.

Hypotonic swelling of salicylate-treated cochlear outer hair cells

The outer hair cell (OHC) is a hydrostat with a low hydraulic conductivity of Pf=3x10(-4) cm/s across the plasma membrane (PM) and subsurface cisterna that make up the OHC's lateral wall. The SSC is structurally and functionally a transport barrier in normal cells that is known to be disrupted by salicylate. The effect of sodium salicylate on Pf is determined from osmotic experiments in which isolated, control and salicylate-treated OHCs were exposed to hypotonic solutions in a constant flow chamber. The value of Pf=3.5+/-0.5x10(-4) cm/s (mean+/-s.e.m., n=34) for salicylate-treated OHCs was not significantly different from Pf=2.4+/-0.3x10(-4) cm/s (mean+/-s.e.m., n=31) for untreated OHCs (p=.3302). Thus Pf is determined by the PM and is unaffected by salicylate treatment. The ratio of longitudinal strain to radial strain epsilonz/epsilonc=-0.76 for salicylate-treated OHCs was significantly smaller (p=.0143) from -0.72 for untreated OHCs, and is also independent of the magnitude of the applied osmotic challenge. Salicylate-treated OHCs took longer to attain a steady-state volume which is larger than that for untreated OHCs and increased in volume by 8-15% prior to hypotonic perfusion unlike sodium alpha-ketoglutarate-treated OHCs. It is suggested that depolymerization of cytoskeletal proteins and/or glycogen may be responsible for the large volume increase in salicylate-treated OHCs as well as the different responses to different modes of application of the hypotonic solution.

Application of fluorescence polarization microscopy to measure fluorophore orientation in the outer hair cell plasma membrane

The biophysical properties and organization of cell membranes regulate many membrane-based processes, including electromotility in outer hair cells (OHCs) of the cochlea. Studies of the membrane environment can be carried out by measuring the orientation of membrane-bound fluorophores using fluorescence polarization microscopy (FPM). Due to the cylindrical shape of OHCs, existing FPM theory developed for spherical cells is not applicable. We develop a new method for analyzing FPM data suitable for the quasi-cylindrical OHC. We present the theory for this model, as well as a study of the orientation of the fluorescent probe pyridinium, 4-[2-[6-(dioctylamino)-2-naphthalenyl]ethenyl]-1-(3-sulfopropyl) (di-8-ANEPPS) in the OHC membrane. Our results indicate that the absorption transition dipole moment of di-8-ANEPPS orients symmetrically about the membrane normal at 27 deg with respect to the plane of the membrane. The observed agreement between theoretical predictions and experimental measurements establishes the applicability of FPM to study OHC plasma membrane properties.

High frequency electrically-induced force generation by cellular plasma membranes

Using a novel experimental technique that combines optical trapping with patch-clamp and fluorescence photometry, we provide preliminary evidence that native biological membranes are capable of electrically-induced piconewton level force generation in the absence of specialized transmembrane proteins. Force generation is dependent on membrane tension and the transmembrane electrical potential. Salicylate diminishes and presence of prestin, a transmembrane protein found in cochlear outer hair cells, enhances force generation.

A decade of piezoresponse force microscopy: progress, challenges, and opportunities

Coupling between electrical and mechanical phenomena is a near-universal characteristic of inorganic and biological systems alike, with examples ranging from piezoelectricity in ferroelectric perovskites to complex, electromechanical couplings in electromotor proteins in cellular membranes. Understanding electromechanical functionality in materials such as ferroelectric nanocrystals and thin films, relaxor ferroelectrics, and biosystems requires probing these properties on the nanometer level of individual grain, domain, or protein fibril. In the last decade, piezoresponse force microscopy (PFM) was established as a powerful tool for nanoscale imaging, spectroscopy, and manipulation of ferroelectric materials. Here, we present principles and recent advances in PFM, including vector and frequency-dependent imaging of piezoelectric materials, briefly review applications for ferroelectric materials, discuss prospects for electromechanical imaging of local crystallographic and molecular orientations and disorder, and summarize future challenges and opportunities for PFM emerging in the second decade since its invention.

Prestin and the cochlear amplifier

In non-mammalian, hair cell-bearing sense organs amplification is associated with mechano-electric transducer channels in the stereovilli (commonly called stereocilia). Because mammals possess differentiated outer hair cells (OHC), they also benefit from a novel electromotile process, powered by the motor protein, prestin. Here we consider new work pertaining to this protein and its potential role as the mammalian cochlear amplifier.

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.

Electricity and mechanics of biomembrane systems: Flexoelectricity in living membranes

Flexoelectricity provides a reciprocal relationship between electricity and mechanics in membranes, i.e., between membrane curvature and polarization. Experimental evidence of biomembrane flexoelectricity (including direct and converse flexoelectric effect) is reviewed. Biological implications of flexoelectricity in membrane transport, membrane contact, mechanosensitivity, electromotility and hearing are underlined. Flexoelectricity enables membrane structures to function like soft micro- and nano-machines, sensors and actuators, thus providing important input to molecular electronics applications.

Assessment of prestin self-association using fluorescence resonance energy transfer

An active process within the cochlea is necessary to obtain the sensitivity and frequency selectivity characteristic of mammalian hearing. This process is realized, at least in part, through the electromotile response of outer hair cells (OHCs). Electromotility requires the presence of prestin, a transmembrane protein highly expressed in the OHC lateral wall. Very little is known about how prestin functions at the molecular level to elicit electromotility, but theoretical models and recent experiments suggest that prestin-prestin interactions are required. To explore the extent of proposed prestin interactions, we employ fluorescence resonance energy transfer (FRET). FRET is a powerful optical technique capable of measuring inter-fluorophore distances less than 10 nm. Using human embryonic kidney cells (HEKs) as a model cell system and the standard FRET pair, cyan fluorescent protein (CFP) as the donor and yellow fluorescent protein (YFP) as the acceptor, we assay for the self-association of prestin under steady-state conditions using acceptor photobleach FRET (apFRET) and sensitized emission FRET (seFRET). Our findings from apFRET indicate the presence of prestin self-association when HEKs express both prestin-CFP and prestin-YFP in the membrane. The average FRET efficiency was approximately 9%, but values as high as 20% were measured. Notably, a higher efficiency of energy transfer ranging from 10-30% was obtained with seFRET. Additionally, we report an apFRET efficiency of approximately 10% for cells expressing a CFP-prestin-YFP double fusion protein. We discuss the significance of these measurements for establishing the presence of prestin-prestin interactions in transfected HEK cells.

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.