Mechanically gated channel activity in cytoskeleton-deficient plasma membrane blebs and vesicles from Xenopus oocytes

TRPV4 and chloride channels mediate volume sensing in trabecular meshwork cells

Aqueous humor drainage from the anterior eye determines intraocular pressure (IOP) under homeostatic and pathological conditions. Swelling of the trabecular meshwork (TM) alters its flow resistance but the mechanisms that sense and transduce osmotic gradients remain poorly understood. We investigated TM osmotransduction and its role in calcium and chloride homeostasis using molecular analyses, optical imaging, and electrophysiology. Anisosmotic conditions elicited proportional changes in TM cell volume, with swelling, but not shrinking, evoking elevations in intracellular calcium concentration [Ca2+]TM. Hypotonicity-evoked calcium signals were sensitive to HC067047, a selective blocker of TRPV4 channels, whereas the agonist GSK1016790A promoted swelling under isotonic conditions. TRPV4 inhibition partially suppressed hypotonicity-induced volume increases and reduced the magnitude of the swelling-induced membrane current, with a substantial fraction of the swelling-evoked current abrogated by Cl- channel antagonists 4,4'-diisothiocyanato-2,2'-stilbenedisulfonic acid (DIDS) and niflumic acid. The transcriptome of volume-sensing chloride channel candidates in primary human was dominated by ANO6 transcripts, with moderate expression of ANO3, ANO7, and ANO10 transcripts and low expression of LTTRC genes that encode constituents of the volume-activated anion channel. Imposition of 190 mosM but not 285 mosM hypotonic gradients increased conventional outflow in mouse eyes. TRPV4-mediated cation influx thus works with Cl- efflux to sense and respond to osmotic stress, potentially contributing to pathological swelling, calcium overload, and intracellular signaling that could exacerbate functional disturbances in inflammatory disease and glaucoma.NEW & NOTEWORTHY Intraocular pressure is dynamically regulated by the flow of aqueous humor through paracellular passages within the trabecular meshwork (TM). This study shows hypotonic gradients that expand the TM cell volume and reduce the outflow facility in mouse eyes. The swelling-induced current consists of TRPV4 and chloride components, with TRPV4 as a driver of swelling-induced calcium signaling. TRPV4 inhibition reduced swelling, suggesting a novel treatment for trabeculitis and glaucoma.

Communication in Plants: Comparison of Multiple Action Potential and Mechanosensitive Signals With Experiments

Both action potentials and mechanosensitive signalling are an important communication mechanisms in plants. Considering an information-theoretic framework, this paper explores the effective range of multiple action potentials for a long chain of cells (i.e., up to 100) in different configurations, and introduces the study of multiple mechanosensitive activation signals (generated due to a mechanical stimulus) in plants. For both these signals, we find that the mutual information per cell and information propagation speed tends to increase up to a certain number of receiver cells. However, as the number of cells increase beyond 10 to 12, the mutual information per cell starts to decrease. To validate our model and results, we include an experimental verification of the theoretical model, using a PhytlSigns biosignal amplifier, allowing us to measure the magnitude of the voltage associated with the multiple AP's and mechanosensitive activation signals induced by different stimulus in plants. Experimental data is used to calculate the mutual information and information propagation speed, which is compared with corresponding numerical results. Since these signals are used for a variety of important tasks within the plant, understanding them may lead to new bioengineering methods for plants.

Removal of the mechanoprotective influence of the cytoskeleton reveals PIEZO1 is gated by bilayer tension

Mechanosensitive ion channels are force-transducing enzymes that couple mechanical stimuli to ion flux. Understanding the gating mechanism of mechanosensitive channels is challenging because the stimulus seen by the channel reflects forces shared between the membrane, cytoskeleton and extracellular matrix. Here we examine whether the mechanosensitive channel PIEZO1 is activated by force-transmission through the bilayer. To achieve this, we generate HEK293 cell membrane blebs largely free of cytoskeleton. Using the bacterial channel MscL, we calibrate the bilayer tension demonstrating that activation of MscL in blebs is identical to that in reconstituted bilayers. Utilizing a novel PIEZO1-GFP fusion, we then show PIEZO1 is activated by bilayer tension in bleb membranes, gating at lower pressures indicative of removal of the cortical cytoskeleton and the mechanoprotection it provides. Thus, PIEZO1 channels must sense force directly transmitted through the bilayer.

Lipid-protein interactions: Lessons learned from stress

Biological membranes are essential for normal function and regulation of cells, forming a physical barrier between extracellular and intracellular space and cellular compartments. These physical barriers are subject to mechanical stresses. As a consequence, nature has developed proteins that are able to transpose mechanical stimuli into meaningful intracellular signals. These proteins, termed Mechanosensitive (MS) proteins provide a variety of roles in response to these stimuli. In prokaryotes these proteins form transmembrane spanning channels that function as osmotically activated nanovalves to prevent cell lysis by hypoosmotic shock. In eukaryotes, the function of MS proteins is more diverse and includes physiological processes such as touch, pain and hearing. The transmembrane portion of these channels is influenced by the physical properties such as charge, shape, thickness and stiffness of the lipid bilayer surrounding it, as well as the bilayer pressure profile. In this review we provide an overview of the progress to date on advances in our understanding of the intimate biophysical and chemical interactions between the lipid bilayer and mechanosensitive membrane channels, focusing on current progress in both eukaryotic and prokaryotic systems. These advances are of importance due to the increasing evidence of the role the MS channels play in disease, such as xerocytosis, muscular dystrophy and cardiac hypertrophy. Moreover, insights gained from lipid-protein interactions of MS channels are likely relevant not only to this class of membrane proteins, but other bilayer embedded proteins as well. This article is part of a Special Issue entitled: Lipid-protein interactions.

Expressing and characterizing mechanosensitive channels in Xenopus oocytes

The oocytes of the African clawed frog (Xenopus laevis) comprise one of the most widely used membrane protein expression systems. While frequently used for studies of transporters and ion channels, the application of this system to the study of mechanosensitive ion channels has been overlooked, perhaps due to a relative abundance of native expression systems. Recent advances, however, have illustrated the advantages of the oocyte system for studying plant and bacterial mechanosensitive channels. Here we describe in detail the methods used for heterologous expression and characterization of bacterial and plant mechanosensitive channels in Xenopus oocytes.

The force-from-lipid (FFL) principle of mechanosensitivity, at large and in elements

Focus on touch and hearing distracts attention from numerous subconscious force sensors, such as the vital control of blood pressure and systemic osmolarity, and sensors in nonanimals. Multifarious manifestations should not obscure invariant and fundamental physicochemical principles. We advocate that force from lipid (FFL) is one such principle. It is based on the fact that the self-assembled bilayer necessitates inherent forces that are large and anisotropic, even at life's origin. Functional response of membrane proteins is governed by bilayer force changes. Added stress can redirect these forces, leading to geometric changes of embedded proteins such as ion channels. The FFL principle was first demonstrated when purified bacterial mechanosensitive channel of large conductance (MscL) remained mechanosensitive (MS) after reconstituting into bilayers. This key experiment has recently been unequivocally replicated with two vertebrate MS K2p channels. Even the canonical Kv and the Drosophila canonical transient receptor potentials (TRPCs) have now been shown to be MS in biophysical and in physiological contexts, supporting the universality of the FFL paradigm. We also review the deterministic role of mechanical force during stem cell differentiation as well as the cell-cell and cell-matrix tethers that provide force communications. In both the ear hair cell and the worm's touch neuron, deleting the cadherin or microtubule tethers reduces but does not eliminate MS channel activities. We found no evidence to distinguish whether these tethers directly pull on the channel protein or a surrounding lipid platform. Regardless of the implementation, pulling tether tenses up the bilayer. Membrane tenting is directly visible at the apexes of the stereocilia.

Influence of mechanical stress on fibroblast-myocyte interactions in mammalian heart

Cardiac fibroblasts are an essential component of cardiac tissue. These cells not only produce the extracellular matrix, but also are electrically and mechanically coupled with cardiomyocytes. In this way, fibroblasts can influence the electrical activity of cardiomyocytes. Cardiac fibroblasts cannot generate action potentials, but their membrane potential is controlled by mechanical stretch or compression of the surrounding myocardium which in turn affects their interaction with myocytes and the way myocytes respond to mechanical stress. This review discusses the electrical properties of cardiac fibroblasts, the present evidence of fibroblast-myocyte coupling and the way in which these cells respond to mechanical stress. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Mechanosensitive channels are activated by stress in the actin stress fibres, and could be involved in gravity sensing in plants

Mechanosensitive (MS) channels are expressed in a variety of cells. The molecular and biophysical mechanism involved in the regulation of MS channel activities is a central interest in basic biology. MS channels are thought to play crucial roles in gravity sensing in plant cells. To date, two mechanisms have been proposed for MS channel activation. One is that tension development in the lipid bilayer directly activates MS channels. The second mechanism proposes that the cytoskeleton is involved in the channel activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. We tested whether tension in the cytoskeleton activates MS channels. Mammalian endothelial cells were microinjected with phalloidin-conjugated beads, which bound to stress fibres, and a traction force to the actin cytoskeleton was applied by dragging the beads with optical tweezers. MS channels were activated when the force was applied, demonstrating that a sub-pN force to the actin filaments activates a single MS channel. Plants may use a similar molecular mechanism in gravity sensing, since the cytoplasmic Ca(2+) concentration increase induced by changes in the gravity vector was attenuated by potential MS channel inhibitors, and by actin-disrupting drugs. These results support the idea that the tension increase in actin filaments by gravity-dependent sedimentation of amyloplasts activates MS Ca(2+) -permeable channels, which can be the molecular mechanism of a Ca(2+) concentration increase through gravistimulation. We review recent progress in the study of tension sensing by actin filaments and MS channels using advanced biophysical methods, and discuss their possible roles in gravisensing.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Force spectroscopy measurements show that cortical neurons exposed to excitotoxic agonists stiffen before showing evidence of bleb damage

In ischemic and traumatic brain injury, hyperactivated glutamate (N-methyl-D-aspartic acid, NMDA) and sodium (Nav) channels trigger excitotoxic neuron death. Na⁺, Ca⁺⁺ and H₂O influx into affected neurons elicits swelling (increased cell volume) and pathological blebbing (disassociation of the plasma membrane’s bilayer from its spectrin-actomyosin matrix). Though usually conflated in injured tissue, cell swelling and blebbing are distinct processes. Around an injury core, salvageable neurons could be mildly swollen without yet having suffered the bleb-type membrane damage that, by rendering channels leaky and pumps dysfunctional, exacerbates the excitotoxic positive feedback spiral. Recognizing when neuronal inflation signifies non-lethal osmotic swelling versus blebbing should further efforts to salvage injury-penumbra neurons. To assess whether the mechanical properties of osmotically-swollen versus excitotoxically-blebbing neurons might be cytomechanically distinguishable, we measured cortical neuron elasticity (gauged via atomic force microscopy (AFM)-based force spectroscopy) upon brief exposure to hypotonicity or to excitotoxic agonists (glutamate and Nav channel activators, NMDA and veratridine). Though unperturbed by solution exchange per se, elasticity increased abruptly with hypotonicity, with NMDA and with veratridine. Neurons then invariably softened towards or below the pre-treatment level, sometimes starting before the washout. The initial channel-mediated stiffening bespeaks an abrupt elevation of hydrostatic pressure linked to NMDA or Nav channel-mediated ion/H₂O fluxes, together with increased [Ca⁺⁺]_int-mediated submembrane actomyosin contractility. The subsequent softening to below-control levels is consistent with the onset of a lethal level of bleb damage. These findings indicate that dissection/identification of molecular events during the excitotoxic transition from stiff/swollen to soft/blebbing is warranted and should be feasible.

The ion channels to cytoskeleton connection as potential mechanism of mechanosensitivity

As biological force-sensing systems mechanosensitive (MS) ion channels present the best example of coupling molecular dynamics of membrane proteins to the mechanics of the surrounding cell membrane. In animal cells MS channels have over the past two decades been very much in focus of mechanotransduction research. In recent years this helped to raise awareness of basic and medical researchers about the role that abnormal MS channels may play in the pathophysiology of diseases, such as cardiac hypertrophy, atrial fibrillation, muscular dystrophy or polycystic kidney disease. To date a large number of MS channels from organisms of diverse phylogenetic origins have been identified at the molecular level; however, the structure of only few of them has been determined. Although their function has extensively been studied in a great variety of cells and tissues by different experimental approaches it is, with exception of bacterial MS channels, very little known about how these channels sense mechanical force and which cellular components may contribute to their function. By focusing on MS channels found in animal cells this article discusses the ways in which the connections between cytoskeleton and ion channels may contribute to mechanosensory transduction in these cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Spontaneous excitation patterns computed for axons with injury-like impairments of sodium channels and Na/K pumps

In injured neurons, “leaky” voltage-gated sodium channels (Nav) underlie dysfunctional excitability that ranges from spontaneous subthreshold oscillations (STO), to ectopic (sometimes paroxysmal) excitation, to depolarizing block. In recombinant systems, mechanical injury to Nav1.6-rich membranes causes cytoplasmic Na⁺-loading and “Nav-CLS”, i.e., coupled left-(hyperpolarizing)-shift of Nav activation and availability. Metabolic injury of hippocampal neurons (epileptic discharge) results in comparable impairment: left-shifted activation and availability and hence left-shifted I_Na-window. A recent computation study revealed that CLS-based I_Na-window left-shift dissipates ion gradients and impairs excitability. Here, via dynamical analyses, we focus on sustained excitability patterns in mildly damaged nodes, in particular with more realistic Gaussian-distributed Nav-CLS to mimic “smeared” injury intensity. Since our interest is axons that might survive injury, pumps (sine qua non for live axons) are included. In some simulations, pump efficacy and system volumes are varied. Impacts of current noise inputs are also characterized. The diverse modes of spontaneous rhythmic activity evident in these scenarios are studied using bifurcation analysis. For “mild CLS injury”, a prominent feature is slow pump/leak-mediated E_Ion oscillations. These slow oscillations yield dynamic firing thresholds that underlie complex voltage STO and bursting behaviors. Thus, Nav-CLS, a biophysically justified mode of injury, in parallel with functioning pumps, robustly engenders an emergent slow process that triggers a plethora of pathological excitability patterns. This minimalist “device” could have physiological analogs. At first nodes of Ranvier and at nociceptors, e.g., localized lipid-tuning that modulated Nav midpoints could produce Nav-CLS, as could co-expression of appropriately differing Nav isoforms.

Nerve cells damaged by trauma, stroke, epilepsy, inflammatory conditions etc, have chronically leaky sodium channels that eventually kill. The usual job of sodium channels is to make brief voltage signals –action potentials– for long distance propagation. After sodium channels open to generate action potentials, sodium pumps work harder to re-establish the intracellular/extracellular sodium imbalance that is, literally, the neuron's battery for firing action potentials. Wherever tissue damage renders membranes overly fluid, we hypothesize, sodium channels become chronically leaky. Our experimental findings justify this. In fluidized membranes, sodium channel voltage sensors respond too easily, letting channels spend too much time open. Channels leak, pumps respond. By mathematical modeling, we show that in damaged channel-rich membranes the continual pump/leak counterplay would trigger the kinds of bizarre intermittent action potential bursts typical of injured neurons. Arising ectopically from injury regions, such neuropathic firing is unrelated to events in the external world. Drugs that can silence these deleterious electrical barrages without blocking healthy action potentials are needed. If fluidized membranes house the problematic leaky sodium channels, then drug side effects could be diminished by using drugs that accumulate most avidly into fluidized membranes, and that bind their targets with highest affinity there.

Mechano-regulation of the beating heart at the cellular level--mechanosensitive channels in normal and diseased heart

The heart as a contractile hollow organ finely tunes mechanical parameters such as stroke volume, stroke pressure and cardiac output according to filling volumes, filling pressures via intrinsic and neuronal routes. At the cellular level, cardiomyocytes in beating hearts are exposed to large mechanical stress during successive heart beats. Although the mechanisms of excitation-contraction coupling are well established in mammalian heart cells, the putative contribution of mechanosensitive channels to Ca²⁺ homeostasis, Ca²⁺ signaling and force generation has been primarily investigated in relation to heart disease states. For instance, transient receptor potential channels (TRPs) are up-regulated in animal models of congestive heart failure or hypertension models and seem to play a vital role in pathological Ca²⁺ overload to cardiomyocytes, thus aggravating the pathology of disease at the cellular level. Apart from that, the contribution of mechanosensitive channels (MsC) in the normal beating heart to the downstream force activation cascade has not been addressed. We present an overview of the current literature and concepts of mechanosensitive channel involvement in failing hearts and cardiomyopathies and novel data showing a likely contribution of Ca²⁺ influx via mechanosensitive channels in beating normal cardiomyocytes during systolic shortening.

Perturbed voltage-gated channel activity in perturbed bilayers: implications for ectopic arrhythmias arising from damaged membrane

The ceaseless opening and closing of the voltage-gated channels (VGCs) underlying cardiac rhythmicity is controlled, in each VGC, by four mobile voltage sensors embedded in bilayer. Every action potential necessitates extensive packing/repacking of voltage sensor domains with adjacent interacting lipid molecules. This renders VGC activity mechanosensitive (MS), i.e., energetically sensitive to the bilayer's mechanical state. Irreversible perturbations of sarcolemmal bilayer such as those associated with ischemia, reperfusion, inflammation, cortical-cytoskeleton abnormalities, bilayer-disrupting toxins, diet aberrations, etc, should therefore perturb VGC activity. Disordered/fluidized bilayer states that facilitate voltage sensor repacking, and thus make VGC opening too easy could, therefore, explain VGC-leakiness in these conditions. To study this in membrane patches we impose mechanical blebbing injury during pipette aspiration-induced membrane stretch, a process that modulates VGC activity irreversibly (plastic regime) and then, eventually, reversibly (elastic regime). Because of differences in sensor-to-gate coupling among different VGCs, their responses to stretch fall into two major categories, MS-Speed, MS-Number, exemplified by Nav and Cav channels. For particular VGCs in perturbed bilayers, leak mechanisms depend on whether or not the rate-limiting voltage-dependent step is MS. Mode-switch transitions might also be mechanosensitive and thus play a role. Incorporated mathematically in axon models, plastic-regime Nav responses elicit ectopic firing behaviors typical of peripheral neuropathies. In cardiomyocytes with mild bleb damage, Nav and/or Cav leaks from irreversible MS modulation (MS-Speed, MS-Number, respectively) could, similarly, foster ectopic arrhythmias. Where pathologically leaky VGCs reside in damaged bilayer, peri-channel bilayer disorder/fluidity conditions could be an important "target feature" for anti-arrhythmic VGC drugs.

Molecular force transduction by ion channels: diversity and unifying principles

Cells perceive force through a variety of molecular sensors, of which the mechanosensitive ion channels are the most efficient and act the fastest. These channels apparently evolved to prevent osmotic lysis of the cell as a result of metabolite accumulation and/or external changes in osmolarity. From this simple beginning, nature developed specific mechanosensitive enzymes that allow us to hear, maintain balance, feel touch and regulate many systemic variables, such as blood pressure. For a channel to be mechanosensitive it needs to respond to mechanical stresses by changing its shape between the closed and open states. In that way, forces within the lipid bilayer or within a protein link can do work on the channel and stabilize its state. Ion channels have the highest turnover rates of all enzymes, and they can act as both sensors and effectors, providing the necessary fluxes to relieve osmotic pressure, shift the membrane potential or initiate chemical signaling. In this Commentary, we focus on the common mechanisms by which mechanical forces and the local environment can regulate membrane protein structure, and more specifically, mechanosensitive ion channels.

Chemical-induced pH-mediated molecular switch

The transmembrane protein α-hemolysin pore has been used to develop ultrasensitive biosensors, study biomolecular folding and unfolding, investigate covalent and noncovalent bonding interactions, and probe enzyme kinetics. Here, we report that, by addition of ionic liquid tetrakis(hydroxymethyl)phosphonium chloride solution to the α-hemolysin pore, the α-hemolysin channel can be controlled open or closed by adjusting the pH of the solution. This approach can be employed to develop a novel molecular switch to regulate molecular transport and should find potential applications as a "smart" drug delivery method.

Analyses of gating thermodynamics and effects of deletions in the mechanosensitive channel TREK-1: comparisons with structural models

TREK-1, a mechanosensitive K channel from the two-pore family (K(2)P), is involved in protective regulation of the resting potential in CNS neurons and other tissues. The structure of TREK-1 and the basis of its sensitivity to stretch and variety of lipid-soluble factors remain unknown. Using existing K channel structures as modeling templates, TREK-1 was envisioned as a two-fold symmetrical complex with the gate formed primarily by the centrally positioned TM2b helices of the second homologous repeat. Opening was modeled as a conical expansion of the barrel separating TM2b's accompanied by extension of TM2a helices with the cytoplasmic TM2a-TM1b connector. Seeking first experimental support to the models we have accomplished thermodynamic analysis of mouse TREK-1 gating and functional testing of several deletion mutants. The predicted increase of the channel in-plane area (ΔA) of ~5 nm(2) in models was supported by the experimental ΔA of ~4 nm(2) derived from the slope of open probability versus membrane tension in HEK-293T cells and their cytoskeleton-depleted blebs. In response to steps of suction, wild-type channel produced transient currents in cell-attached patches and mostly sustained currents upon patch excision. TREK-1 motifs not present in canonical K channels include divergent cytoplasmic N- and C-termini, and a characteristic 50-residue extracellular loop in the first homologous repeat. Deletion of the extracellular loop (Δ76-124) reduced the average current density in patches, increased spontaneous activity and generated a larger sub-population of high-conductance channels, while activation by tension augmented by arachidonic acid was fully retained. Further deletion of the C-terminal end (Δ76-124/Δ334-411) removed voltage dependency but otherwise produced no additional effect. In an attempt to generate a cysteine-free version of the channel, we mutated two remaining cysteines 159 and 219 in the transmembrane region. C219A did not compromise channel activity, whereas the C159A/S mutants were essentially inactive. Treatment with β-mercaptoethanol suggested that none of these cysteines form functionally-important disulfides.

Adaptive behavior of bacterial mechanosensitive channels is coupled to membrane mechanics

Mechanosensitive channel of small conductance (MscS), a tension-driven osmolyte release valve residing in the inner membrane of Escherichia coli, exhibits a complex adaptive behavior, whereas its functional counterpart, mechanosensitive channel of large conductance (MscL), was generally considered nonadaptive. In this study, we show that both channels exhibit similar adaptation in excised patches, a process that is completely separable from inactivation prominent only in MscS. When a membrane patch is held under constant pressure, adaptation of both channels is manifested as a reversible current decline. Their dose–response curves recorded with 1–10-s ramps of pressure are shifted toward higher tension relative to the curves measured with series of pulses, indicating decreased tension sensitivity. Prolonged exposure of excised patches to subthreshold tensions further shifts activation curves for both MscS and MscL toward higher tension with similar magnitude and time course. Whole spheroplast MscS recordings performed with simultaneous imaging reveal activation curves with a midpoint tension of 7.8 mN/m and the slope corresponding to ∼15-nm² in-plane expansion. Inactivation was retained in whole spheroplast mode, but no adaptation was observed. Similarly, whole spheroplast recordings of MscL (V23T mutant) indicated no adaptation, which was present in excised patches. MscS activities tried in spheroplast-attached mode showed no adaptation when the spheroplasts were intact, but permeabilized spheroplasts showed delayed adaptation, suggesting that the presence of membrane breaks or edges causes adaptation. We interpret this in the framework of the mechanics of the bilayer couple linking adaptation of channels in excised patches to the relaxation of the inner leaflet that is not in contact with the glass pipette. Relaxation of one leaflet results in asymmetric redistribution of tension in the bilayer that is less favorable for channel opening.

Wild-type and brachyolmia-causing mutant TRPV4 channels respond directly to stretch force

Whether animal ion channels functioning as mechanosensors are directly activated by stretch force or indirectly by ligands produced by the stretch is a crucial question. TRPV4, a key molecular model, can be activated by hypotonicity, but the mechanism of activation is unclear. One model has this channel being activated by a downstream product of phospholipase A(2), relegating mechanosensitivity to the enzymes or their regulators. We expressed rat TRPV4 in Xenopus oocytes and repeatedly examined >200 excised patches bathed in a simple buffer. We found that TRPV4 can be activated by tens of mm Hg pipette suctions with open probability rising with suction even in the presence of relevant enzyme inhibitors. Mechanosensitivity of TRPV4 provides the simplest explanation of its various force-related physiological roles, one of which is in the sensing of weight load during bone development. Gain-of-function mutants cause heritable skeletal dysplasias in human. We therefore examined the brachyolmia-causing R616Q gain-of-function channel and found increased whole-cell current densities compared with wild-type channels. Single-channel analysis revealed that R616Q channels maintain mechanosensitivity but have greater constitutive activity and no change in unitary conductance or rectification.

Stretch-activated ion channels: what are they?

Mechanosensitive ion channels (MSCs) exist in all cells, but mechanosensitivity is a phenotype not a genotype. Specialized mechanoreceptors such as the hair cells of the cochlea require elaborate mechanical impedance matching to couple the channels to the external stress. In contrast, MSCs in nonspecialized cells appear activated by stress in the bilayer local to the channel--within about three lipids. Local mechanical stress can be produced by far-field tension, amphipaths, phase separations, the cytoskeleton, the extracellular matrix, and the adhesion energy between the membrane and a patch pipette. Understanding MSC function requires under standing the stimulus.

Studying mechanosensitive ion channels using liposomes

Mechanosensitive (MS) ion channels are the primary molecular transducers of mechanical force into electrical and/or chemical intracellular signals in living cells. They have been implicated in innumerable mechanosensory physiological processes including touch and pain sensation, hearing, blood pressure control, micturition, cell volume regulation, tissue growth, or cellular turgor control. Much of what we know about the basic physical principles underlying the conversion of mechanical force acting upon membranes of living cells into conformational changes of MS channels comes from studies of MS channels reconstituted into artificial liposomes. Using bacterial MS channels as a model, we have shown by reconstituting these channels into liposomes that there is a close relationship between the physico-chemical properties of the lipid bilayer and structural dynamics bringing about the function of these channels.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Osmosensation in vasopressin neurons: changing actin density to optimize function

The proportional relation between circulating vasopressin concentration and plasma osmolality is fundamental for body fluid homeostasis. Although changes in the sensitivity of this relation are associated with pathophysiological conditions, central mechanisms modulating osmoregulatory gain are unknown. Here, we review recent data that sheds important light on this process. The cell autonomous osmosensitivity of vasopressin neurons depends on cation channels comprising a variant of the transient receptor potential vanilloid 1 (TRPV1) channel. Hyperosmotic activation is mediated by a mechanical process where sensitivity increases in proportion with actin filament density. Moreover, angiotensin II amplifies osmotic activation by a rapid stimulation of actin polymerization, suggesting that neurotransmitter-induced changes in cytoskeletal organization in osmosensory neurons can mediate central changes in osmoregulatory gain.

Membrane trauma and Na+ leak from Nav1.6 channels

During brain trauma, white matter experiences shear and stretch forces that, without severing axons, nevertheless trigger their secondary degeneration. In central nervous system (CNS) trauma models, voltage-gated sodium channel (Nav) blockers are neuroprotective. This, plus the rapid tetrodotoxin-sensitive Ca2+ overload of stretch-traumatized axons, points to "leaky" Nav channels as a pivotal early lesion in brain trauma. Direct effects of mechanical trauma on neuronal Nav channels have not, however, been tested. Here, we monitor immediate responses of recombinant neuronal Nav channels to stretch, using patch-clamp and Na+-dye approaches. Trauma constituted either bleb-inducing aspiration of cell-attached oocyte patches or abrupt uniaxial stretch of cells on an extensible substrate. Nav1.6 channel transient current displayed irreversible hyperpolarizing shifts of steady-state inactivation [availability(V)] and of activation [g(V)] and, thus, of window current. Left shift increased progressively with trauma intensity. For moderately intense patch trauma, a approximately 20-mV hyperpolarizing shift was registered. Nav1.6 voltage sensors evidently see lower energy barriers posttrauma, probably because of the different bilayer mechanics of blebbed versus intact membrane. Na+ dye-loaded human embryonic kidney (HEK) cells stably transfected with alphaNav1.6 were subjected to traumatic brain injury-like stretch. Cytoplasmic Na+ levels abruptly increased and the trauma-induced influx had a significant tetrodotoxin-sensitive component. Nav1.6 channel responses to cell and membrane trauma are therefore consistent with the hypothesis that mechanically induced Nav channel leak is a primary lesion in traumatic brain injury. Nav1.6 is the CNS node of Ranvier Nav isoform. When, during head trauma, nodes experienced bleb-inducing membrane damage of varying intensities, nodal Nav1.6 channels should immediately "leak" over a broadly left-smeared window current range.

Actin cytoskeleton regulates stretch-activated Ca2+ influx in human pulmonary microvascular endothelial cells

During high tidal volume mechanical ventilation in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), regions of the lung are exposed to excessive stretch, causing inflammatory responses and further lung damage. In this study, the effects of mechanical stretch on intracellular Ca(2+) concentration ([Ca(2+)](i)), which regulates a variety of endothelial properties, were investigated in human pulmonary microvascular endothelial cells (HPMVECs). HPMVECs grown on fibronectin-coated silicon chambers were exposed to uniaxial stretching, using a cell-stretching apparatus. After stretching and subsequent unloading, [Ca(2+)](i), as measured by fura-2 fluorescence, was transiently increased in a strain amplitude-dependent manner. The elevation of [Ca(2+)](i) induced by stretch was not evident in the Ca(2+)-free solution and was blocked by Gd(3+), a stretch-activated channel inhibitor, or ruthenium red, a transient receptor potential vanilloid inhibitor. The disruption of actin polymerization with cytochalasin D inhibited the stretch-induced elevation of [Ca(2+)](i). In contrast, increases in [Ca(2+)](i) induced by thapsigargin or thrombin were not affected by cytochalasin D. Increased actin polymerization with sphingosine-1-phosphate or jasplakinolide enhanced the stretch-induced elevation of [Ca(2+)](i). A simple network model of the cytoskeleton was also developed in support of the notion that actin stress fibers are required for efficient force transmission to open stretch-activated Ca(2+) channels. In conclusion, mechanical stretch activates Ca(2+) influx via stretch-activated channels which are tightly regulated by the actin cytoskeleton different from other Ca(2+) influx pathways such as receptor-operated and store-operated Ca(2+) entries in HPMVECs. These results suggest that abnormal Ca(2+) homeostasis because of excessive mechanical stretch during mechanical ventilation may play a role in the progression of ALI/ARDS.

Attachment of osteocyte cell processes to the bone matrix

In order for osteocytes to perceive mechanical information and regulate bone remodeling accordingly they must be anchored to their extracellular matrix (ECM). To date the nature of this attachment is not understood. Osteocytes are embedded in mineralized bone matrix, but maintain a pericellular space (50-80 nm) to facilitate fluid flow and transport of metabolites. This provides a spatial limit for their attachment to bone matrix. Integrins are cell adhesion proteins that may play a role in osteocyte attachment. However, integrin attachments require proximity between the ECM, cell membrane, and cytoskeleton, which conflicts with the osteocytes requirement for a pericellular fluid space. In this study, we hypothesize that the challenge for osteocytes to attach to surrounding bone matrix, while also maintaining fluid-filled pericellular space, requires different "engineering" solutions than in other tissues that are not similarly constrained. Using novel rapid fixation techniques, to improve cell membrane and matrix protein preservation, and transmission electron microscopy, the attachment of osteocyte processes to their canalicular boundaries are quantified. We report that the canalicular wall is wave-like with periodic conical protrusions extending into the pericellular space. By immunohistochemistry we identify that the integrin alphavbeta3 may play a role in attachment at these complexes; a punctate pattern of staining of beta3 along the canalicular wall was consistent with observations of periodic protrusions extending into the pericellular space. We propose that during osteocyte attachment the pericellular space is periodically interrupted by underlying collagen fibrils that attach directly to the cell process membrane via integrin-attachments.

Stretch-activated potassium channels in hypotonically induced blebs of atrial myocytes

Stress in the lipids of the cell membrane may be responsible for activating stretch-activated channels (SACs) in nonspecialized sensory cells such as cardiac myocytes, where they are likely to play a role in cardiac mechanoelectric feedback. We examined the influence of the mechanical microenvironment on the gating of stretch-activated potassium channels (SAKCs) in rat atrial myocytes. The goal was to examine the role of the cytoskeleton in the gating process. We recorded from blebs that have minimal cytoskeleton and cells treated with cytochalasin B (cyto-B) to disrupt filamentous actin. Histochemical and electron microscopic techniques confirmed that the bleb membrane was largely free of F-actin. Channel currents showed mechanosensitivity and potassium selectivity and were activated by low pH and arachidonic acid, similar to properties of TREK-1. Some patches showed a time-dependent decrease in current that may be adaptation or inactivation, and since this decrease appeared in control cells and blebs, it is probably not the result of adaptation in the cytoskeleton. Cyto-B treatment and blebbing caused an increase in background channel activity, suggesting a transfer of stress from actin to bilayer and then to the channel. The slope sensitivity of gating before and after cyto-B treatment was similar to that of blebs, implying the characteristic change of dimensions associated with channel gating was the same in the three mechanical environments. The mechanosensitivity of SAKCs appears to be the result of interaction with membrane lipids and not of direct involvement of the cytoskeleton.

Actin stress fibers transmit and focus force to activate mechanosensitive channels

Mechanosensitive (MS) channels are expressed in various cells in a wide range of phylogenetic lineages from bacteria to humans. Understanding the molecular and biophysical mechanisms of their activation is an important research pursuit. It is controversial whether eukaryotic MS channels need accessory proteins – typically cytoskeletal structures – for activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. Here we demonstrate that direct mechanical stimulation (stretching) of an actin stress fiber using optical tweezers can activate MS channels in cultured human umbilical vein endothelial cells (HUVECs). Furthermore, by using high-speed total internal reflection microscopy, we visualized spots of Ca2+ influx across individual MS channels distributed near focal adhesions in the basal surface of HUVECs. This study provides the first direct evidence that the cytoskeleton works as a force-transmitting and force-focusing molecular device to activate MS channels in eukaryotic cells.

TRP channels and mechanosensory transduction: insights into the arterial myogenic response

Mechano-gated ion channels are implicated in a variety of key physiological functions ranging from touch sensitivity to arterial pressure regulation. Seminal work in prokaryotes and invertebrates provided strong evidence for the role of specific ion channels in volume regulation, touch sensitivity, or hearing, specifically the mechanosensitive channel subunits of large and small conductances (MscL and MscS), the mechanosensory channel subunits (MEC) and the transient receptor potential channel subunits (TRP). In mammals, recent studies further indicate that members of the TRP channel family may also be considered as possible candidate mechanosensors responding to either tension, flow, or changes in cell volume. However, contradictory results have challenged whether these TRP channels, including TRPC1 and TRPC6, are directly activated by mechanical stimulation. In the present review, we will focus on the mechanosensory function of TRP channels, discuss whether a direct or indirect mechanism is at play, and focus on the proposed role for these channels in the arterial myogenic response to changes in intraluminal pressure.

Revisiting TRPC1 and TRPC6 mechanosensitivity

This article addresses whether TRPC1 or TRPC6 is an essential component of a mammalian stretch-activated mechano-sensitive Ca(2+) permeable cation channel (MscCa). We have transiently expressed TRPC1 and TRPC6 in African green monkey kidney (COS) or Chinese hamster ovary (CHO) cells and monitored the activity of the stretch-activated channels using a fast pressure clamp system. Although both TRPC1 and TRPC6 are highly expressed at the protein level, the amplitude of the mechano-sensitive current is not significantly altered by overexpression of these subunits. In conclusion, although several TRPC channel members, including TRPC1 and TRPC6, have been recently proposed to form MscCa in vertebrate cells, the functional expression of these TRPC subunits in heterologous systems remains problematic.

TRPCs as MS Channels

This chapter reviews recent evidence indicating that canonical or classical transient receptor potential (TRPC) channels are directly or indirectly mechanosensitive (MS) and can therefore be designated as mechano-operated channels (MOCs). The MS functions of TRPCs may be mechanistically related to their better known functions as store-operated and receptor-operated channels (SOCs and ROCs). Mechanical forces may be conveyed to TRPC channels through the "conformational coupling" mechanism that transmits information regarding the status of internal Ca(2+) stores. All TRPCs are regulated by receptors coupled to phospholipases that are themselves MS and can regulate channels via lipidic second messengers. Accordingly, there may be several nonexclusive mechanisms by which mechanical forces may regulate TRPC channels, including direct sensitivity to bilayer mechanics, physical coupling to internal membranes and/or cytoskeletal proteins, and sensitivity to lipidic second messengers generated by MS enzymes. Various strategies that can be used for separating out different MS-gating mechanisms and their possible role in specific TRPCs are discussed.

Mechanosensitive ion channels and the peptide inhibitor GsMTx-4: history, properties, mechanisms and pharmacology

Sensing the energy from mechanical inputs is ubiquitous--and perhaps the oldest form of biological energy transduction. However, the tools available to probe the mechanisms of transduction are far fewer than for the chemical and electric field sensitive transducers. The one pharmacological tool available for mechansensitive ion channels (MSCs) is a peptide (GsMTx-4) isolated from venom of the tarantula, Grammostola spatulata, that blocks cationic MSCs found in non-specialized eukaryotic tissues. In this review, we summarize the current knowledge of GsMTx-4, and discuss the inevitable crosstalk between the MSC behavior and the mechanical properties of the cell cortex.

Liposome reconstitution and modulation of recombinant N-methyl-D-aspartate receptor channels by membrane stretch

In this study, the heteromeric N-methyl-D-aspartate (NMDA) receptor channels composed of NR1a and NR2A subunits were expressed, purified, reconstituted into liposomes, and characterized by using the patch clamp technique. The protein exhibited the expected electrophysiological profile of activation by glutamate and glycine and internal Mg2+ blockade. We demonstrated that the mechanical energy transmitted to membrane-bound NMDA receptor channels can be exerted directly by tension developed in the lipid bilayer. Membrane stretch and application of arachidonic acid potentiated currents through NMDA receptor channels in the presence of intracellular Mg2+. The correlation of membrane tension induced by either mechanical or chemical stimuli with the physiological Mg2+ block of the channel suggests that the synaptic transmission can be altered if NMDA receptor complexes experience local changes in bilayer thickness caused by dynamic targeting to lipid microdomains, electrocompression, or chemical modification of the cell membranes. The ability to study gating properties of NMDA receptor channels in artificial bilayers should prove useful in further study of structure-function relationships and facilitate discoveries of new therapeutic agents for treatment of glutamate-mediated excitotoxicity or analgesic therapies.

Twenty odd years of stretch-sensitive channels

After formation of the giga-seal, the membrane patch can be stimulated by hydrostatic or osmotic pressure gradients applied across the patch. This feature led to the discovery of stretch-sensitive or mechanosensitive (MS) channels, which are now known to be ubiquitously expressed in cells representative of all the living kingdoms. In addition to mechanosensation, MS channels have been implicated in many basic cell functions, including regulation of cell volume, shape, and motility. The successful cloning, overexpression, and crystallization of bacterial MS channel proteins combined with patch clamp and modeling studies have provided atomic insight into the working of these nanomachines. In particular, studies of MS channels have revealed new understanding of how the lipid bilayer modulates membrane protein function. Three major membrane protein families, transient receptor potential, 2 pore domain K(+), and the epithelial Na(+) channels, have been shown to form MS channels in animal cells, and their polymodal activation embrace fields far beyond mechanosensitivity. The discovery of new drugs highly selective for MS channels ("mechanopharmaceutics") and the demonstration of MS channel involvement in several major human diseases ("mechanochannelopathies") provide added motivation for devising new techniques and approaches for studying MS channels.

Cellular mechanotransduction: putting all the pieces together again

Analysis of cellular mechanotransduction, the mechanism by which cells convert mechanical signals into biochemical responses, has focused on identification of critical mechanosensitive molecules and cellular components. Stretch-activated ion channels, caveolae, integrins, cadherins, growth factor receptors, myosin motors, cytoskeletal filaments, nuclei, extracellular matrix, and numerous other structures and signaling molecules have all been shown to contribute to the mechanotransduction response. However, little is known about how these different molecules function within the structural context of living cells, tissues, and organs to produce the orchestrated cellular behaviors required for mechanosensation, embryogenesis, and physiological control. Recent work from a wide range of fields reveals that organ, tissue, and cell anatomy are as important for mechanotransduction as individual mechanosensitive proteins and that our bodies use structural hierarchies (systems within systems) composed of interconnected networks that span from the macroscale to the nanoscale in order to focus stresses on specific mechanotransducer molecules. The presence of isometric tension (prestress) at all levels of these multiscale networks ensures that various molecular scale mechanochemical transduction mechanisms proceed simultaneously and produce a concerted response. Future research in this area will therefore require analysis, understanding, and modeling of tensionally integrated (tensegrity) systems of mechanochemical control.

Desensitization of mechano-gated K2P channels

The neuronal mechano-gated K2P channels TREK-1 and TRAAK show pronounced desensitization within 100 ms of membrane stretch. Desensitization persists in the presence of cytoskeleton disrupting agents, upon patch excision, and when channels are expressed in membrane blebs. Mechanosensitive currents evoked with a variety of complex stimulus protocols were globally fit to a four-state cyclic kinetic model in detailed balance, without the need to introduce adaptation of the stimulus. However, we show that patch stress can be a complex function of time and stimulation history. The kinetic model couples desensitization to activation, so that gentle conditioning stimuli do not cause desensitization. Prestressing the channels with pressure, amphipaths, intracellular acidosis, or the E306A mutation reduces the peak-to-steady-state ratio by changing the preexponential terms of the rate constants, increasing the steady-state current amplitude. The mechanical responsivity can be accounted for by a change of in-plane area of approximately 2 nm2 between the closed and open conformations. Desensitization and its regulation by chemical messengers is predicted to condition the physiological role of K2P channels.

Formation and fate of platelet microparticles

There is increasing evidence that platelet microparticles participate in thrombus formation. Yet the origin of platelet microparticles and their fate in the circulation remain poorly defined. It is unknown, for example, whether circulating platelet microparticles found in healthy individuals are derived from activated platelets or generated during megakaryopoiesis. The life span of platelet microparticles and the mechanism of their clearance have also not been determined. This article addresses these fundamental aspects of the physiology of platelet microparticles.

Effects of osmotic shrinkage on voltage-gated Ca2+ channel currents in rat anterior pituitary cells

Increased extracellular osmolarity ([Os]e) suppresses stimulated hormone secretion from anterior pituitary cells. Ca2+ influx may mediate this effect. We show that increase in [Os]e (by 18–125%) differentially suppresses L-type and T-type Ca2+ channel currents ( IL and IT, respectively); IL was more sensitive than IT. Hyperosmotic suppression of IL depended on the magnitude of increase in [Os]e and was correlated with the percent decrease in pituitary cell volume, suggesting that pituitary cell shrinkage can modulate L-type currents. The hyperosmotic suppression of IL and IT persisted after incubation of pituitary cells either with the actin-disrupter cytochalasin D or with the actin stabilizer phalloidin, suggesting that the actin cytoskeleton is not involved in this modulation. The hyperosmotic suppression of Ca2+ influx was not correlated with changes in reversal potential, membrane capacitance, and access resistance. Together, these results suggest that the hyperosmotic suppression of Ca2+ influx involves Ca2+ channel proteins. We therefore recorded the activity of L-type Ca2+ channels from cell-attached patches while exposing the cell outside the patch pipette to hyperosmotic media. Increased [Os]e reduced the activity of Ca2+ channels but did not change single-channel conductance. This hyperosmotic suppression of Ca2+ currents may therefore contribute to the previously reported hyperosmotic suppression of hormone secretion.

A possible unifying principle for mechanosensation

Of Aristotle's five senses, we know that sight, smell and much of taste are initiated by ligands binding to G-protein-coupled receptors; however, the mechanical sensations of touch and hearing remain without a clear understanding of their molecular basis. Recently, the relevant force-transducing molecules--the mechanosensitive ion channels--have been identified. Such channel proteins purified from bacteria sense forces from the lipid bilayer in the absence of other proteins. Recent evidence has shown that lipids are also intimately involved in opening and closing the mechanosensitive channels of fungal, plant and animal species.

Common evolutionary origins of mechanosensitive ion channels in Archaea, Bacteria and cell-walled Eukarya

The ubiquity of mechanosensitive (MS) channels triggered a search for their functional homologs in Archaea. Archaeal MS channels were found to share a common ancestral origin with bacterial MS channels of large and small conductance, and sequence homology with several proteins that most likely function as MS ion channels in prokaryotic and eukaryotic cell-walled organisms. Although bacterial and archaeal MS channels differ in conductive and mechanosensitive properties, they share similar gating mechanisms triggered by mechanical force transmitted via the lipid bilayer. In this review, we suggest that MS channels of Archaea can bridge the evolutionary gap between bacterial and eukaryotic MS channels, and that MS channels of Bacteria, Archaea and cell-walled Eukarya may serve similar physiological functions and may have evolved to protect the fragile cellular membranes in these organisms from excessive dilation and rupture upon osmotic challenge.

TRPC1 forms the stretch-activated cation channel in vertebrate cells

The mechanosensitive cation channel (MscCa) transduces membrane stretch into cation (Na(+), K(+), Ca(2+) and Mg(2+)) flux across the cell membrane, and is implicated in cell-volume regulation, cell locomotion, muscle dystrophy and cardiac arrhythmias. However, the membrane protein(s) that form the MscCa in vertebrates remain unknown. Here, we use an identification strategy that is based on detergent solubilization of frog oocyte membrane proteins, followed by liposome reconstitution and evaluation by patch-clamp. The oocyte was chosen because it expresses the prototypical MscCa (>or=10(7)MscCa/oocyte) that is preserved in cytoskeleton-deficient membrane vesicles. We identified a membrane-protein fraction that reconstituted high MscCa activity and showed an abundance of a protein that had a relative molecular mass of 80,000 (M(r) 80K). This protein was identified, by immunological techniques, as the canonical transient receptor potential channel 1 (TRPC1). Heterologous expression of the human TRPC1 resulted in a >1,000% increase in MscCa patch density, whereas injection of a TRPC1-specific antisense RNA abolished endogenous MscCa activity. Transfection of human TRPC1 into CHO-K1 cells also significantly increased MscCa expression. These observations indicate that TRPC1 is a component of the vertebrate MscCa, which is gated by tension developed in the lipid bilayer, as is the case in various prokaryotic mechanosensitive (Ms) channels.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Mechanosensitive channels: multiplicity of families and gating paradigms

Mechanosensitive ion channels are the primary transducers that convert mechanical force into an electrical or chemical signal in hearing, touch, and other mechanical senses. Unlike vision, olfaction, and some types of taste, which all use similar kinds of primary heterotrimeric GTP-binding protein-coupled receptors, mechanosensation relies on diverse types of transducer molecules. Unrelated types of channels can be used for the perception of various mechanical stimuli, not only in distant groups of organisms, but also in separate locations of the same organism. The extreme sensitivity of the transduction mechanism in the auditory system, which relies on an elaborate structure of rigid cilia, filamentous links, and molecular motors to focus force on transduction channels, contrasts with that of the bacterial channel MscL, which is opened by high lateral tension in the membrane and fulfills a safety-valve rather than a sensory function. The spatial scales of conformational movement and force in these two systems are described, and are shown to be consistent with a general physical description of mechanical channel gating. We outline the characteristics of several types of mechanosensitive channels and the functional contexts in which they participate in signaling and cellular regulation in sensory and nonsensory cells.

Electrophysiological, mechanosensitive responses of Xenopus laevis oocytes to direct, isotonic increase in intracellular volume

An intra-oocyte injection method for obtaining the electrophysiological response of follicle-enclosed Xenopus laevis oocytes to an increase in intracellular volume (i.e. stretch) without changing the extracellular medium is described. The response comprised a 'stretch-activated' (SA) current which was evoked by injection of an isotonic 14-70 nl droplet and had a transient, smooth profile. Ionic substitution experiments revealed that the current was carried mainly by Na(+), K(+) and Cl(-) and had a reversal potential of about -2 mV. A similar result was obtained from experiments in which the holding potential was varied between -40 and +10 mV whilst repeatedly inducing the SA current. On average, the channel was blocked 60% by 10 microM gadolinium chloride, 58% by 50 microM amiloride, 11% by 50 microM 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid and 63% by 50 microM 4-acetamido-4'-isothiocyanato-stilbene-2-2'-disulfonic acid. Maturation of the oocytes with 100 microM progesterone reduced the mechanosensitivity 12-fold. This injection technique is compared with other methods of eliciting mechanosensitive (MS) currents in X. laevis oocytes. These observed characteristics of the SA current are discussed in relation to the oocytes' endogenous MS cation and anion channels.