Mechanosensitive ion channels in nonspecialized cells

Mechanosensation and endothelin in astrocytes--hypothetical roles in CNS pathophysiology

Endothelin (ET) is a potent autocrine mitogen produced by reactive and neoplastic astrocytes. ET has been implicated in the induction of astrocyte proliferation and other transformations engendered by brain pathology, and in promoting the malignant behavior of astrocytomas. Reactive astrocytes containing ET are found in the periphery/penumbra of a wide array of CNS pathologies. Virtually all brain pathology deforms the surrounding parenchyma, either by direct mass effect or edema. Mechanical stress is a well established stimulus for ET production and release by other cell types, but has not been well studied in the brain. However, numerous studies have illustrated that astrocytes can sense mechanical stress and translate it into chemical messages. Furthermore, the ubiquitous reticular meshwork formed by interconnected astrocytes provides an ideal morphology for sensing and responding to mechanical disturbances. We have recently demonstrated stretch-induced ET production by astrocytes in vitro. Inspired by this finding, the purpose of this article is to review the literature on (1) astrocyte mechanosensation, and (2) the endothelin system in astrocytes, and to consider the hypothesis that mechanical induction of the ET system may influence astrocyte functioning in CNS pathophysiology. We conclude by discussing evidence supporting future investigations to determine whether specific inhibition of stretch-activated ion channels may represent a novel strategy for treating or preventing CNS disturbances, as well as the relevance to astrocyte-derived tumors.

STRETCH-ACTIVATED CHANNELS IN STRETCH-INDUCED MUSCLE DAMAGE: ROLE IN MUSCULAR DYSTROPHY

1. Stretch-induced muscle injury results in the damage that causes reduced force and increased membrane permeability. This muscle damage is caused, in part, by ionic entry through stretch-activated channels and blocking these channels with Gd3+ or streptomycin reduces the force deficit associated with damage. 2. Dystrophin-deficient muscles are more susceptible to stretch-induced muscle injury and the recovery from injury can be incomplete. We have found that Na+ entry associated with stretch-induced injury is enhanced in dystrophin-deficient muscles and that blockers of stretch-activated channels are capable of preventing ionic entry and reducing muscle damage. 3. A model is presented that proposes links between stretch-induced injury, opening of stretch-activated channels, increased levels of intracellular ions and various forms of muscle damage. Although changes in Na+ accompany stretch-induced muscle injury, we believe that changes in Ca2+ probably have a more central role in the damage process.

Physiological mechanisms of lysophosphatidylcholine-induced de-ramification of murine microglia

Activation of microglial cells, the resident macrophages of the brain, occurs rapidly following brain injury. De-ramification, i.e. transformation from ramified into amoeboid morphology is one of the earliest manifestations of microglial activation. In the present study, we identified the physiological mechanisms underlying microglial de-ramification induced by lysophosphatidylcholine (LPC). Patch-clamp experiments revealed activation of non-selective cation currents and Ca(2+)-dependent K(+) currents by extracellular LPC. LPC-activated non-selective cation channels were permeable for monovalent and divalent cations. They were inhibited by Gd(3+), La(3+), Zn(2+) and Grammostola spatulata venom, but were unaffected by diltiazem, LOE908MS, amiloride and DIDS. Ca(2+) influx through non-selective cation channels caused sustained increases in intracellular Ca(2+) concentration. These Ca(2+) increases were sufficient to elicit charybdotoxin-sensitive Ca(2+)-dependent K(+) currents. However, increased [Ca(2+)](i) was not required for LPC-induced morphological changes. In LPC-stimulated microglial cells, non-selective cation currents caused transient membrane depolarization, which was followed by sustained membrane hyperpolarization induced by Ca(2+)-dependent K(+) currents. Furthermore, LPC elicited K(+) efflux by stimulating electroneutral K(+)-Cl(-) cotransporters, which were inhibited by furosemide and DIOA. LPC-induced microglial de-ramification was prevented by simultaneous inhibition of non-selective cation channels and K(+)-Cl(-) cotransporters, suggesting their functional importance for microglial activation.

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.

Distribution of the intensity-characteristic parameters of cat rapidly adapting mechanoreceptive fibers

Modeling population responses of nerve fibers requires statistical characterization of fiber-response properties. The rate/intensity characteristics of cat rapidly adapting (RA) fibers were fitted by four-parameter, piece-wise linear functions using nonlinear regression (n = 14; R2 > 0.958). The parameters were tested against the null hypothesis that they are log normally distributed. The test fail to reject this hypothesis (Kolmogorov-Smirnov p>0.477). However, a significant statistical difference was found between the specific lognormal distributions obtained from monkey (Johnson, J Neurophysiol 37: 48-72, 1974) and cat for all four parameters (Kolmogorov-Smirnov, p<0.0075, p<0.05, p<0.0001, p<0.00007). Although the stimulus contactor size was not the same in monkey and cat studies, the differences between monkey and cat fibers are attributed to anatomical differences in the glabrous sin of both species. Modeling studies suggest that the absolute firing thresholds of RA fibers have a right-skewed distribution because of the anatomical constraints present in both species' skin. Meissner corpuscles, which are the sensory end-organs of RA fibers, are likely to be found deeper in the skin within dermal papilla, therefore, the thresholds can be elevated. However, the thresholds are bounded at lower end, probably due to the epidermal junction that acts as a superficial mechanical barrier for these corpuscles.

Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells

Decrease in extracellular osmolarity ([Os]e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (IL) and T-type (IT) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in [Os]e) and strong (31-32% decrease in [Os]e). Exposure to moderate hyposmotic stimuli resulted in three response types in IL (a decrease, a biphasic effect, and an increase in IL) and in increase in IT. Exposure to strong hyposmotic stimuli resulted only in increases in both IL and IT. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both IL and IT. Thus it appears that the main effect of decrease in [Os]e is increase in calcium channel currents. This increase was differential (IL were more sensitive than IT) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of IL and IT may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed.

Articular cartilage functional histomorphology and mechanobiology: a research perspective

The histomorphogenesis of articular cartilage is regulated during skeletal development by the intermittent forces and motions imposed at diarthrodial joints. A key feature in this development is the formation of the superficial, transitional, radial, and calcified cartilage zones through the cartilage thickness. The histomorphological, biological, and mechanical characteristics of these zones can be correlated with the distributions of pressures, deformations, and pressure-induced fluid flow that are created in vivo. In a mature joint, cyclic loads produce cyclic hydrostatic fluid pressure through the entire cartilage thickness that is comparable in magnitude to the applied joint pressure. Prolonged physical activity can cause the total cartilage thickness to decrease about 5%, although the consolidation strains vary tremendously in the different zones. The superficial zone can experience significant fluid exudation and consolidation (compressive strains) in the range of 60% while the radial zone experiences relatively little fluid flow and consolidation. The topological variation in the histomorphologic appearance of articular cartilage is influenced by the local mechanical loading of chondrocytes in the different zones. Patterns of stress, strain, and fluid flow created in the joint result in spatial and temporal changes in the rates of synthesis and degradation of matrix proteins. When viewed over the course of a lifetime, even subtle difference in these cellular processes can affect the micro- and macro-morphology of articular cartilage. This hypothesis is supported by in vivo and ex vivo experiments where load-induced changes in matrix synthesis and catabolism, gene expression, and signal transduction pathways have been observed.

Cardiac fibroblasts and the mechano-electric feedback mechanism in healthy and diseased hearts

Cardiac arrhythmia is a serious clinical condition, which is frequently associated with abnormalities of mechanical loading and changes in wall tension of the heart. Recent novel findings suggest that fibroblasts may function as mechano-electric transducers in healthy and diseased hearts. Cardiac fibroblasts are electrically non-excitable cells that respond to spontaneous contractions of the myocardium with rhythmical changes of their resting membrane potential. This phenomenon is referred to as mechanically induced potential (MIP) and has been implicated in the mechano-electric feedback mechanism of the heart. Mechano-electric feedback is thought to adjust the frequency of spontaneous myocardial contractions to changes in wall tension, which may result from variable filling pressure. Electrophysiological recordings of single atrial fibroblasts indicate that mechanical compression of the cells may activate a non-selective cation conductance leading to depolarisation of the membrane potential. Reduced amplitudes of MIPs due to pharmacological disruption of F-actin and tubulin suggest a role for the cytoskeleton in the mechano-electric signal transduction process. Enhanced sensitivity of the membrane potential of the fibroblasts to mechanical stretch after myocardial infarction correlates with depression of heart rates. It is assumed that altered electrical function of cardiac fibroblasts may contribute to the increased risk of post-infarct arrhythmia.

Mechano-electric feedback and arrhythmias

The mechanical state of the heart feeds back to modify cardiac rate and rhythm. Mechanical stretch of myocardial tissue causes immediate and chronic responses that lead to the common end point of arrhythmia. This review provides a brief summary of the author's personal choice of contributions that she considers have fostered our understanding of the role of mechano-electric feedback in arrhythmogenesis. Acute mechanical stretch reversibly depolarises the cell membrane and shortens the action potential duration. These electrophysiological changes are related to the activation of mechano-sensitive ion channels. Several different ion channels are involved in the sensing of stretch, among them K(+)-selective, Cl(-)-selective, non-selective, and ATP-sensitive K(+) channels. Sodium and Ca(2+) entering the cells via non-selective ion channels are thought to contribute to the genesis of stretch-induced arrhythmia. Mechano-sensitive channels have been cloned from non-vertebrate and vertebrate species. Chronic stress on the heart activates gene expression in cardiomyocytes and non-myocytes. The signal transduction involves atrial natriuretic peptides and growth factors that initiate remodelling processes leading to hypertrophy which in turn may contribute to the electrical instability of the heart by increasing the responsiveness of mechano-sensitive channels. Selective block of these channels could provide some new form of treatment of mechanically induced arrhythmias, although at present there are no drugs available with sufficient selectivity. Detailed understanding of how mechanical strain on myocardial cells is translated into channel activation will allow to identify new targets for putative antiarrhythmic drugs.

Dynamic properties of stretch-activated K+ channels in adult rat atrial myocytes

The effect of mechanical stress on the heart's electrical activity has been termed mechanoelectric feedback. The response to stretch depends upon the magnitude and the waveform of the stimulus, and upon the timing relative to the cardiac cycle. Stretch-activated ion channels (SACs) have been regarded as the most likely candidates for serving as the primary transducers of mechanical stress. We explored the steady state and dynamic responses of single channels in adult rat atrial cells using the patch clamp with a pressure clamp. Surprisingly, we only observed K(+)-selective SACs, probably of the 2P domain family. The channels were weakly outward rectifying with flickery bursts. In cell attached mode, the mean conductance was 74+/-14 and 65+/-16 pS for +60 and -60 mV, respectively (140 mM [K(+)](out), 2mM [Mg(2+)](out) and 0mM [Ca(2+)](out)). The latency of the response to pressure steps was 50-100 ms and the time to peak approximately 400 ms. About half of the channels in cell-attached patches showed adaptation/inactivation where channel activity declined to a plateau of 20-30% of peak in approximately 1s. The time dependent behavior of these SACs is generally consistent with whole-cell currents observed in chick and rat ventricular cells, although the net current was outward rather than inward.

Ion selectivity of stretch-activated cation currents in mouse ventricular myocytes

Stretch-activated non-selective cation currents ( I(SAC)) constitute a mechanism that can induce cardiac arrhythmias. We studied I(SAC) in mouse ventricular myocytes by stretching part of the cell surface between the patch-pipette and a motor-driven glass stylus. In non-clamped cells, local stretch depolarised and induced after-depolarisations and extrasystoles. In voltage-clamped cells (K(+) currents suppressed) I(SAC) activated by local stretch had a nearly linear voltage dependence and reversed polarity between -12 and 0 mV. Conductance G(SAC) increased with the extent of local stretch. I(SAC) was not a Cl(-) current (insensitivity to replacement of Cl(-) by aspartate(-)). I(SAC) was not a Ca(2+)-activated current (insensitivity to 5 mM intracellular BAPTA). G(SAC) was blocked by 5 micro M GdCl(3) or by 75 mM extracellular (e.c.) CaCl(2). Removal of e.c. CaCl(2) increased G(SAC) 2.5-fold, as if G(SAC) were sensitive to Ca(2+) and Gd(3+). Replacement of 150 mM e.c. Na(+) by 150 mM Cs(+), Li(+), tetraethylammonium (TEA(+)) or N-methyl d-glucosamine (NMDG(+)) yielded currents that suggested for the conductance a selectivity G(Cs)> G(Na)> G(Li)> G(TEA)> G(NMDG). I(SAC) was suppressed by cytochalasin D, as if an intact F-actin cytoskeleton were necessary for activation of I(SAC).

Syntrophin gamma 2 regulates SCN5A gating by a PDZ domain-mediated interaction

SCN5A encodes the alpha subunit of the cardiac muscle and intestinal smooth muscle mechanosensitive Na(+) channel. Mechanosensitivity in the intestine requires an intact cytoskeleton. We report, using laser capture microdissection, single cell PCR, and immunohistochemistry, that syntrophins, scaffolding proteins, were expressed in human intestinal smooth muscle cells. The distribution of syntrophin gamma 2 was similar to that of SCN5A. Yeast two-hybrid and glutathione S-transferase pull-down experiments show that SCN5A and syntrophin gamma 2 co-express and that the PDZ domain of syntrophin gamma 2 directly interacts with the C terminus of SCN5A. In native cells, disruption of the C terminus-syntrophin gamma 2 PDZ domain interaction using peptides directed against either region result in loss of mechanosensitivity. Co-transfection of syntrophin gamma 2 with SCN5A in HEK293 cells markedly shifts the activation kinetics of SCN5A and reduces the availability of Na(+) current. We propose that syntrophin gamma 2 is an essential Na(+) channel-interacting protein required for the full expression of the Na(+) current and that the SCN5A-syntrophin gamma 2 interaction determines mechanosensitivity and current availability.

Maxi K+ channel mediates regulatory volume decrease response in a human bronchial epithelial cell line

The cell regulatory volume decrease (RVD) response triggered by hypotonic solutions is mainly achieved by the coordinated activity of Cl- and K+ channels. We now describe the molecular nature of the K(+) channels involved in the RVD response of the human bronchial epithelial (HBE) cell line 16HBE14o-. These cells, under isotonic conditions, present a K+ current consistent with the activity of maxi K+ channels, confirmed by RT-PCR and Western blot. Single-channel and whole cell maxi K+ currents were readily and reversibly activated following the exposure of HBE cells to a 28% hypotonic solution. Both maxi K+ current activation and RVD response showed calcium dependency, inhibition by TEA, Ba2+, iberiotoxin, and the cationic channel blocker Gd3+ but were insensitive to clofilium, clotrimazole, and apamin. The presence of the recently cloned swelling-activated, Gd3+-sensitive cation channels (TRPV4, also known as OTRPC4, TRP12, or VR-OAC) was detected by RT-PCR in HBE cells. This channel, TRPV4, which senses changes in volume, might provide the pathway for Ca2+ influx under hypotonic solutions and, consequently, for the activation of maxi K+ channels.

How did cells get their size?

Cells exercise size homeostasis, and the origins of their ability to do so is the topic of this essay. Before there were cells, there were protocells. The most basic questions about protocells as objects are: What were they made of, and how big were they? Asking how big they were implies that the answer to the first part includes a boundary. The best candidate for that boundary is a self-assembling lipid bilayer. Therefore, protocells are defined here as Darwinian liposomes-bilayer vesicles with mutable on-board replicases linked to phenotypes. Because liposomes undergo spontaneous fission and fusion, and are subject to osmotic forces, size regulation in the earliest protocells would essentially have been liposome physics. For successful protocells, averting osmotic lysis would have been the first order of business. However, from the outset size mattered too, because of sex and reproduction (i.e., genome mixing and genome copying in entities with phenotypes). Protocell fission and fusion would have blended seamlessly into protocell sex and reproduction, making any gene product that furnished control over protocell size changes doubly adaptive. A recurrent theme is the feedback role of bilayer tension in protocell size control. Ways in which primitive peptides and their aggregates (e.g., channels) might have allowed liposomes to gain improved volume and surface area homeostasis are suggested. Domain-swapped proteins that polymerize as filaments are discussed as the origin of cytoskeleton structures that diversify and stabilize liposome shapes and sizes. Throughout, attention is paid to the question of set points for cell size.

Coronary perfusion and muscle lengthening increase cardiac contraction: different stretch-triggered mechanisms

An increase in coronary perfusion, transversal stretch of the myocardium, increases developed force (F(dev)) (Gregg effect) through activation of stretch-activated ion channels (SACs). Lengthening of the muscle, longitudinal stretch of the myocardium, causes an immediate increase in F(dev) followed by a slow F(dev) increase (Anrep effect). In isometrically contracting perfused papillary muscles of Wistar rats, we investigated whether both effects were based on similar stretch-induced mechanisms by measuring F(dev) and intracellular Ca(2+) concentration ([Ca(2+)](i)) after a muscle length increase from 85% to 95% L(max) (length at which maximal isometric force develops) at low and high coronary perfusion before and after inhibition of SACs with gadolinium (10 micromol/l Gd(3+)). The increase of F(dev) and peak [Ca(2+)](i) by the Gregg effect was of similar magnitude as the Anrep effect (from 3.5 +/- 0.8 to 3.9 +/- 1.2 mN/mm(2) and from 3.0 +/- 0.7% to 3.8 +/- 0.9% normalized [Ca(2+)](i), means +/- SE). SAC blockade completely blunted the increase of F(dev) and peak [Ca(2+)](i) by the Gregg effect; however, it did not affect the Anrep effect. The slow force response, but not the calcium response, was augmented by an increase in coronary perfusion. Therefore, increased coronary perfusion, transversal stretch of the myocardium, and muscle lengthening, longitudinal stretch of the myocardium, increase myocardial contraction in the rat through different stretch-triggered mechanisms.

alpha(1C) (Ca(V)1.2) L-type calcium channel mediates mechanosensitive calcium regulation

Smooth muscle exhibits mechanosensitivity independent of neural input, suggesting that mechanosensitive pathways reside within smooth muscle cells. The native L-type calcium current recorded from human intestinal smooth muscle is modulated by stretch. To define mechanosensitive mechanisms involved in the regulation of smooth muscle calcium entry, we cloned the alpha(1C) L-type calcium channel subunit (Ca(V)1.2) from human intestinal smooth muscle and expressed the channel in a heterologous system. This channel subunit retained mechanosensitivity when expressed alone or coexpressed with a beta(2) calcium channel subunit in HEK-293 or Chinese hamster ovary cells. The heterologously expressed human cardiac alpha(1C) splice form also demonstrated mechanosensitivity. Inhibition of kinase signaling did not affect mechanosensitivity of the native channel. Truncation of the alpha(1C) COOH terminus, which contains an inhibitory domain and a proline-rich domain thought to mediate mechanosensitive signaling from integrins, did not disrupt mechanosensitivity of the expressed channel. These data demonstrate mechanical regulation of calcium entry through molecularly identified L-type calcium channels in mammalian cells and suggest that the mechanosensitivity resides within the pore forming alpha(1C)-subunit.

Determination of cellular strains by combined atomic force microscopy and finite element modeling

Many organs adapt to their mechanical environment as a result of physiological change or disease. Cells are both the detectors and effectors of this process. Though many studies have been performed in vitro to investigate the mechanisms of detection and adaptation to mechanical strains, the cellular strains remain unknown and results from different stimulation techniques cannot be compared. By combining experimental determination of cell profiles and elasticities by atomic force microscopy with finite element modeling and computational fluid dynamics, we report the cellular strain distributions exerted by common whole-cell straining techniques and from micromanipulation techniques, hence enabling their comparison. Using data from our own analyses and experiments performed by others, we examine the threshold of activation for different signal transduction processes and the strain components that they may detect. We show that modulating cell elasticity, by increasing the F-actin content of the cytoskeleton, or cellular Poisson ratio are good strategies to resist fluid shear or hydrostatic pressure. We report that stray fluid flow in some substrate-stretch systems elicits significant cellular strains. In conclusion, this technique shows promise in furthering our understanding of the interplay among mechanical forces, strain detection, gene expression, and cellular adaptation in physiology and disease.

A mechanogated nonselective cation channel in proximal tubule that is ATP sensitive

Ion channels that are gated in response to membrane deformation or "stretch" are empirically designated stretch-activated channels. Here we describe a stretch-activated nonselective cation channel in the basolateral membrane (BLM) of the proximal tubule (PT) that is nucleotide sensitive. Single channels were studied in cell-intact and cell-free patches from the BLM of PT cells that maintain their epithelial polarity. The limiting inward Cs+ conductance is ~28 pS, and channel activity persists after excision into a Ca2+- and ATP-free bath. The stretch-dose response is sigmoidal, with half-maximal activation of about -19 mmHg at -40 mV, and the channel is activated by depolarization. The inward conductance sequence is: NH ~ Cs+ ~ Rb+ > K+ ~ Na+ ~ Li+ > Ca2+ ~ Ba2+ > N-methyl-D-glucamine ~ tetraethylammonium. The venom of the common Chilean tarantula, Grammostola spatulata, completely blocks channel activity in cell-attached patches. Hypotonic swelling reversibly activates the channel. Intracellular ATP concentration ([ATP]i) reversibly blocks the channel (inhibitory constant approximately 0.48 mM), suggesting that channel function is coupled to the metabolic state of the cell. We conclude that this channel may function as a Ca2+ entry pathway and/or be involved in regulation of cell volume. We speculate this channel may be important when [ATP]i is depleted, as occurs during periods of increased transepithelial transport or with ischemic injury.

An intracellular proton sensor commands lipid- and mechano-gating of the K(+) channel TREK-1

The 2P domain K(+) channel TREK-1 is widely expres sed in the nervous system. It is opened by a variety of physical and chemical stimuli including membrane stretch, intracellular acidosis and polyunsaturated fatty acids. This activation can be reversed by PKA-mediated phosphorylation. The C-terminal domain of TREK-1 is critical for its polymodal function. We demonstrate that the conversion of a specific glutamate residue (E306) to an alanine in this region locks TREK-1 in the open configuration and abolishes the cAMP/PKA down-modulation. The E306A substitution mimics intracellular acidosis and rescues both lipid- and mechano-sensitivity of a loss-of-function truncated TREK-1 mutant. We conclude that protonation of E306 tunes the TREK-1 mechanical setpoint and thus sets lipid sensitivity.

Membrane stretch accelerates activation and slow inactivation in Shaker channels with S3-S4 linker deletions

At low P(open)(V) Shaker exhibits pronounced stretch-activation. Possible explanations for Shaker's sensitivity to tension include 1) Shaker channels are sufficiently distensible that stretch produces novel channel states and 2) Shaker channels expand in the plane of the membrane during voltage gating. For channels expressed in oocytes, we compared effects of patch stretch on Shaker and mutants that retain their voltage-gating ability but activate sluggishly because all or most of the S3-S4 linker has been deleted. Deletants had 10, 5, or 0 amino acid (aa) linkers, whereas wild-type is 31 aa. In deletants, though activation is exceptionally slow, slow inactivation is exceptionally quick; the resulting kinetic match was a bonus that allowed effects of stretch to be followed simultaneously in both processes. With the intact linker, an approximately 3 orders of magnitude mismatch in the two processes makes this impracticable. Standard stretch stimuli increased the rates and extent of activation by about the same degree in wild type and deletants, with effects especially pronounced near the foot of G(V). In deletants (where slow inactivation is strongly coupled to activation) stretch also accelerated slow inactivation. Maximum conductances were unaffected by stretch in all variants. In ramp clamp dose experiments, near-lytic patch stretch acted, for all variants, like a approximately 10 mV hyperpolarizing shift. These results suggested that, whether basal rates were high (wild type) or low (deletants), stretch acted by facilitating voltage-dependent activation. Channel activity was therefore simulated with/without "tension," tension being simulated via rate changes at voltage-dependent closed-closed transitions that might involve in-plane expansion (explanation 2). Simulated Delta P(open) arising from approximately 2 kT of "mechanical gating energy" mimicked experimental effects seen with comfortably sub-lytic stretch.

Single cell mechanotransduction and its modulation analyzed by atomic force microscope indentation

The skeleton adapts to its mechanical usage, although at the cellular level, the distribution and magnitude of strains generated and their detection are ill-understood. The magnitude and nature of the strains to which cells respond were investigated using an atomic force microscope (AFM) as a microindentor. A confocal microscope linked to the setup enabled analysis of cellular responses. Two different cell response pathways were identified: one, consequent upon contact, depended on activation of stretch-activated ion channels; the second, following stress relaxation, required an intact microtubular cytoskeleton. The cellular responses could be modulated by selectively disrupting cytoskeletal components thought to be involved in the transduction of mechanical stimuli. The F-actin cytoskeleton was not required for responses to mechanical strain, whereas the microtubular and vimentin networks were. Treatments that reduced membrane tension, or its transmission, selectively reduced contact reactions. Immunostaining of the cell cytoskeleton was used to interpret the results of the cytoskeletal disruption studies. We provide an estimate of the cellular strain magnitude needed to elicit intracellular calcium responses and propose a model that links single cell responses to whole bone adaptation. This technique may help to understand adaptation to mechanical usage in other organs.

Mechanosensitive cation channels in human leukaemia cells: calcium permeation and blocking effect

Cell-attached and inside-out patch-clamp methods were employed to identify and characterize mechanosensitive (MS) ionic channels in the plasma membrane of human myeloid leukaemia K562 cells. A reversible activation of gadolinium-blockable mechanogated currents in response to negative pressure application was found in 58 % of stable patches (n = 317). I-V relationships measured with a sodium-containing pipette solution showed slight inward rectification. Data analysis revealed the presence of two different populations of channels that were distinguishable by their conductance properties (17.2 +/- 0.3 pS and 24.5 +/- 0.5 pS), but were indistinguishable with regard to their selective and pharmacological properties. Ion-substitution experiments indicated that MS channels in leukaemia cells were permeable to cations but not to anions and do not discriminate between Na(+) and K(+). The channels were fully impermeable to large organic cations such as Tris(+) and N-methyl-D-glucamine ions (NMDG(+)). Ca(2+) permeation and blockade of MS channels were examined using pipettes containing different concentrations of Ca(2+). In the presence of 2 mM CaCl(2), when other cations were impermeant, both outward and inward single-channel currents were observed; the I-V relationship showed a unitary conductance of 7.7 +/- 1.0 pS. The relative permeability value, P(Ca)/P(K), was equal to 0.75, as estimated at physiological Ca(2+) concentrations. Partial or full inhibition of inward Ca(2+) currents through MS channels was observed at higher concentrations of external Ca(2+) (10 or 20 mM). No MS channels were activated when using a pipette containing 90 mM CaCl(2). Monovalent mechanogated currents were not significantly affected by extracellular Ca(2+) at concentrations within the physiological range (0-2 mM), and at some higher Ca(2+) concentrations.

Development of a 'mechano-active' scaffold for tissue engineering

In this study. we investigate the potential for manipulating bone cell mechanotransducers in tissue engineering. Membrane ion channels such as voltage operated calcium channels (VOCC) have been shown to be a critical component of the bone cell transduction pathway with agonists and inhibitors of this pathway having profound effects on the load signal. By encapsulating a calcium channel agonist with slow release within a poly(L-lactide) (PLLA) scaffold, we can generate a 'mechano-active' scaffold for use in skeletal tissue engineering. PLLA scaffolds with and without a calcium channel agonist, BAY K8644, were seeded with primary human bone cells or the human MG63 bone cell line and cultured for 13 weeks followed by mechanical stimulation with a four-point bending model. Our results show that addition of the agonist for slow release is sufficient to enhance the load-related responses in bone cells within the scaffolds. Specifically, collagen type I expression and the ratio of alkaline phosphatase to protein are elevated in response to cyclical mechanical stimulation of approximately 1000 microstr which is then further enhanced in the mechano-active' scaffolds. As the agonists only act when the calcium channels are open by attenuating the calcium flux, the stimulation is specifically targeted to scaffolds subjected to load either in vitro or ultimately in vivo. Our results suggest that manipulating the VOCC and attenuating the opening of the calcium channels may be an effective technique to amplify matrix production via mechanical stimulation which may be applied to bone tissue engineering and potentially engineering of other load-bearing connective tissues.

Gramicidin A channels switch between stretch activation and stretch inactivation depending on bilayer thickness

The patch clamp-liposome technique was used to examine the stretch sensitivity of a model membrane ion channel, gramicidin A, in membrane patches of different bilayer thickness. We found that small changes in phospholipid acyl chain length (i.e., PC-20 to PC-18) can switch gramicidin A from a stretch-activated to a stretch-inactivated channel. The demonstration that subnanometer changes in bilayer thickness can reverse the response polarity of a model channel has implications for other signaling proteins that may experience local changes in bilayer thickness as a consequence of dynamic targeting to lipid microdomains, electrocompression, or chemical modification of the bilayer.

Actin cytoskeletal modulation of pressure-induced depolarization and Ca(2+) influx in cerebral arteries

The objective of this study was to examine the role of the actin cytoskeleton in the development of pressure-induced membrane depolarization and Ca(2+) influx underlying myogenic constriction in cerebral arteries. Elevating intraluminal pressure from 10 to 60 mmHg induced membrane depolarization, increased intracellular cytosolic Ca(2+) concentration ([Ca(2+)](i)) and elicited myogenic constriction in both intact and denuded rat posterior cerebral arteries. Pretreatment with cytochalasin D (5 microM) or latrunculin A (3 microM) abolished constriction but enhanced the [Ca(2+)](i) response; similarly, acute application of cytochalasin D to vessels with tone, or in the presence of 60 mM K(+), elicited relaxation accompanied by an increase in [Ca(2+)](i). The effects of cytochalasin D were inhibited by nifedipine (3 microM), demonstrating that actin cytoskeletal disruption augments Ca(2+) influx through voltage-sensitive L-type Ca(2+) channels. Finally, pressure-induced depolarization was enhanced in the presence of cytochalasin D, further substantiating a role for the actin cytoskeleton in the modulation of ion channel function. Together, these results implicate vascular smooth muscle actin cytoskeletal dynamics in the control of cerebral artery diameter through their influence on membrane potential as well as via a direct effect on L-type Ca(2+) channels.

Increased coronary perfusion augments cardiac contractility in the rat through stretch-activated ion channels

The role of stretch-activated ion channels (SACs) in coronary perfusion-induced increase in cardiac contractility was investigated in isolated isometrically contracting perfused papillary muscles from Wistar rats. A brief increase in perfusion pressure (3-4 s, perfusion pulse, n = 7), 10 repetitive perfusion pulses (n = 4), or a sustained increase in perfusion pressure (150-200 s, perfusion step, n = 7) increase developed force by 2.7 +/- 1.1, 7.7 +/- 2.2, and 8.3 +/- 2.5 mN/mm(2) (means +/- SE, P < 0.05), respectively. The increase in developed force after a perfusion pulse is transient, whereas developed force during a perfusion step remains increased by 5.1 +/- 2.5 mN/mm(2) (P < 0.05) in the steady state. Inhibition of SACs by addition of gadolinium (10 micromol/l) or streptomycin (40 and 100 micromol/l) blunts the perfusion-induced increase in developed force. Incubation with 100 micromol/l N(omega)-nitro-L-arginine [nitric oxide (NO) synthase inhibition], 10 micromol/l sodium nitroprusside (NO donation) and 0.1 micromol/l verapamil (L-type Ca(2+) channel blockade) are without effect on the perfusion-induced increase of developed force. We conclude that brief, repetitive, or sustained increases in coronary perfusion augment cardiac contractility through activation of stretch-activated ion channels, whereas endothelial NO release and L-type Ca(2+) channels are not involved.

A possible role for atrial fibroblasts in postinfarction bradycardia

Atrial fibroblasts are considered to modulate the contractile activity of the heart in response to mechanical stretch. In this study we examined whether atrial fibroblasts are possibly involved in bradyarrhythmia, which is a severe complication after myocardial infarction. For this purpose, transmembrane electrical potentials were recorded in cardiac fibroblasts near the sinoatrial node from sham-operated rats and from rats with myocardial infarction. Twenty days after infarction due to coronary artery ligation, the right atrial tissue weights and the sensitivity of the fibroblast membrane potential to mechanical stretch correlated positively with the infarct size. Cardiac growth was enhanced, but the stretch sensitivity and the resting membrane potential of the atrial fibroblasts declined between 8 and 30 days after infarction. The frequency of spontaneous atrial contractions was significantly reduced 8 days after myocardial infarction and recovered in parallel with the membrane potential of the fibroblasts. These findings suggest that changes in the susceptibility of atrial fibroblasts to mechanical stretch may contribute to bradyarrhythmia during postinfarct remodeling of the heart.

Changes in mechanosensitive channel gating following mechanical stimulation in skeletal muscle myotubes from the mdx mouse

We studied the effects of membrane stretch and voltage on the gating of single mechanosensitive (MS) channels in myotubes from dystrophin-deficient mdx mice. In earlier studies of MS channels in mdx myotubes, we found a novel class of stretch-inactivated channels. In the present experiments, we used a gentle suction protocol to determine whether seal formation damaged the membrane and altered MS channel gating, since dystrophin-deficiency is known to be associated with an increased susceptibility to mechanically induced damage. In some recordings from mdx myotubes, MS channel open probability gradually increased to levels approaching unity following seal formation. In these recordings, channels remained open for the duration of the recording. In other recordings, MS channel open probability remained low after seal formation and applying weak suction evoked conventional stretch-activated gating. Applying strong suction or very positive voltages, however, caused some channels to enter a high open probability gating mode. The shift to a high open probability gating mode coincided with the appearance of stretch-inactivated gating. These findings suggested that mechanical stimulation altered the mechanical properties of the patch causing some MS channels to enter a novel gating mode. In support of this idea, stretch-activated and stretch-inactivated channels were not detected in the same membrane patch and channel inactivation occurred at lower pressures than activation (P(1/2,) = -13 and -26.5 mmHg, respectively). Other experiments showed that stretch-inactivated gating was not due to a simple loss of MS channel activity from a non-random process such as vesiculation or bleb formation: channel inactivation by suction was readily reversible, stable over tens of minutes, and followed the predictions of the binomial theorem for independent, randomly gating channels. In addition, the voltage-dependent gating of stretch-inactivated channels was similar to that of stretch-activated channels. The results show that MS channels in dystrophin-deficient muscle exist in two distinct gating modes and that mechanical stimuli cause an irreversible conversion between modes. We discuss possible mechanisms for the changes in MS channel gating in relation to the known cytoskeletal abnormalities of mdx muscle and its possible implications for the pathogenesis of Duchenne dystrophy.

Micro- and nanomechanics of the cochlear outer hair cell

Outer hair cell electromotility is crucial for the amplification, sharp frequency selectivity, and nonlinearities of the mammalian cochlea. Current modeling efforts based on morphological, physiological, and biophysical observations reveal transmembrane potential gradients and membrane tension as key independent variables controlling the passive and active mechanics of the cell. The cell's mechanics has been modeled on the microscale using a continuum approach formulated in terms of effective (cellular level) mechanical and electric properties. Another modeling approach is nanostructural and is based on the molecular organization of the cell's membranes and cytoskeleton. It considers interactions between the components of the composite cell wall and the molecular elements within each of its components. The methods and techniques utilized to increase our understanding of the central role outer hair cell mechanics plays in hearing are also relevant to broader research questions in cell mechanics, cell motility, and cell transduction.

Physiological and pharmacological role of lysophosphatidic acid as modulator in mechanotransduction

The mechanotransduction mechanism is believed to play an important role in maintenance of cellular homeostasis in a wide variety of cell types. In particular, the mechanotransduction system in vascular endothelial cells may be an essential mechanism for local hemodynamic control. Elevations in intracellular free Ca2+ concentration ([Ca2]i) are an important signal in the initial step of mechanotransduction and mechanosensitive (MS) cation channels are thought to be a putative pathway; however, the molecular mechanisms remain unclear. We found that lysophosphatidic acid (LPA), a bioactive phospholipid, sensitizes the response of [Ca2+]i to mechanical stress in several cell types. Employing real-time confocal microscopy, local increases in [Ca2+]i in several regions within the cell during application of mechanical stress were clearly visualized in bovine lens epithelial and endothelial cells in the presence of LPA. The phenomenon was termed "Ca2+ spots". Pharmacological studies revealed that Ca2+ spots arise due to influx through MS channels. In this report, our data indicating the possible significance of LPA as an endogenous factor involved in regulation of mechanotransduction is reviewed. Furthermore, our findings suggest that the Ca2+ spot is a novel phenomenon occurring as an elementary Ca2+-influx event through MS channels directly coupled with the initial step in mechanotransduction.

Two-dimensional kinetic analysis suggests nonsequential gating of mechanosensitive channels in Xenopus oocytes

Xenopus oocytes express mechanosensitive (MS(XO)) channels that can be studied in excised patches of membrane with the patch-clamp technique. This study examines the steady-state kinetic gating properties of MS(XO) channels using detailed single-channel analysis. The open and closed one-dimensional dwell-time distributions were described by the sums of 2-3 open and 5-7 closed exponential components, respectively, indicating that the channels enter at least 2-3 open and 5-7 closed kinetic states during gating. Dependency plots revealed that the durations of adjacent open and closed intervals were correlated, indicating two or more gateway states in the gating mechanism for MS channels. Maximum likelihood fitting of two-dimensional dwell-time distributions to both generic and specific models was used to examine gating mechanism and rank models. A kinetic scheme with five closed and five open states, in which each closed state could make a direct transition to an open state (two-tiered model) could account for the major features of the single-channel data. Two-tiered models that allowed direct transitions to subconductance open states in addition to the fully open state were also consistent with multiple gateway states. Thus, the gating mechanism of MS(XO) channels differs from the sequential (linear) gating mechanisms considered for MS channels in bacteria, chick skeletal muscle, and Necturus proximal tubule.

Stretch-activation and stretch-inactivation of Shaker-IR, a voltage-gated K+ channel

Mechanosensitive (MS) ion channels are ubiquitous in eukaryotic cell types but baffling because of their contentious physiologies and diverse molecular identities. In some cellular contexts mechanically responsive ion channels are undoubtedly mechanosensory transducers, but it does not follow that all MS channels are mechanotransducers. Here we demonstrate, for an archetypical voltage-gated channel (Shaker-IR; inactivation-removed), robust MS channel behavior. In oocyte patches subjected to stretch, Shaker-IR exhibits both stretch-activation (SA) and stretch-inactivation (SI). SA is seen when prestretch P(open) (set by voltage) is low, and SI is seen when it is high. The stretch effects occur in cell-attached and excised patches at both macroscopic and single-channel levels. Were one ignorant of this particular MS channel's identity, one might propose it had been designed as a sophisticated reporter of bilayer tension. Knowing Shaker-IR's provenance and biology, however, such a suggestion would be absurd. We argue that the MS responses of Shaker-IR reflect not overlooked "mechano-gating" specializations of Shaker, but a common property of multiconformation membrane proteins: inherent susceptibility to bilayer tension. The molecular diversity of MS channels indicates that susceptibility to bilayer tension is hard to design out of dynamic membrane proteins. Presumably the cost of being insusceptible to bilayer tension often outweighs the benefits, especially where the in situ milieu of channels can provide mechanoprotection.

Calcium is involved in the gravitactic orientation in colorless flagellates

The colorless flagellate Astasia longa shows a pronounced negative gravitaxis. The calcium fluorescence indicator Calcium Crimson was used to detect changes of the intracellular calcium concentration during gravitactical orientation. Astasia shows an increase of the fluorescence after a lag phase of about 10 s, a maximum after about 30 s and a decrease to the basic level within 60 s during gravitactic reorientation. The observed change in fluorescence corresponds to an almost doubling of the initial free calcium concentration. The influence of inhibitors, known to impair gravitaxis, on the calcium concentration of Astasia longa was tested. Addition of caffeine, an inhibitor of phosphodiesterase, increases, while addition of gadolinium, an inhibitor of mechanosensitive ion channels decreases the fluorescence signal. While gravitactic stimulation of caffeine-treated cells resulted in a kinetics of fluorescence intensity changes comparable to control cells the addition of gadolinium inhibited any calcium concentration change. Dynamic fluorescence imaging was used during a sounding rocket experiment (MAXUS 3 campaign). Different accelerations interrupted by microgravity intervals were applied to Astasia cells. The cells show an increase in the calcium signal upon acceleration and a decrease during the microgravity state. The results strongly reemphasize the working model of gravitaxis which is based on the activation of mechano-sensitive ion channels as one of the primary events in signal perception.

Molecular cloning and functional expression of Xenopus laevis oocyte ATP-activated P2X4 channels

All cells contain mechanosensitive ion channels, yet the molecular identities of most are unknown. The purpose of our study was to determine what encodes the Xenopus oocyte's mechanosensitive cation channel. Based on the idea that homologues to known channels might contribute to the stretch channels, we screened a Xenopus oocyte cDNA library with cation channel probes. Whereas other screens were negative, P2X probes identified six isoforms of the P2X4 subtype of ATP-gated channels. From RNase protection assays and RT-PCR, we demonstrated that Xenopus oocytes express P2X4 mRNA. In expression studies, four isoforms produced functional ATP-gated ion channels; however, one, xP2X4c, had a conserved cysteine replaced by a tyrosine and failed to give rise to functional channels. By changing the tyrosine to a cysteine, we showed that this cysteine was crucial for function. We raised antibodies against a Xenopus P2X4 C-terminal peptide to investigate xP2X4 protein expression. This affinity purified anti-xP2X4 antibody recognized a 56 kDa glycosylated Xenopus P2X4 protein expressed in stably transfected HEK-293 cells and in P2X4 cDNA injected oocytes overexpressing the cloned P2X4 channels; however, it failed to recognize proteins in control, uninjected oocytes. This suggests that P2X4 channels and mechanosensitive cation channels are not linked. Instead, oocyte P2X4 mRNA may be part of the stored pool of stable maternal mRNA that remains untranslated until later developmental stages.

Chemically charging the pore constriction opens the mechanosensitive channel MscL

MscL is a bacterial mechanosensitive channel that protects the cell from osmotic downshock. We have previously shown that substitution of a residue that resides within the channel pore constriction, MscL's Gly-22, with all other 19 amino acids affects channel gating according to the hydrophobicity of the substitution (). Here, we first make a mild substitution, G22C, and then attach methanethiosulfonate (MTS) reagents to the cysteine under patch clamp. Binding MTS reagents that are positively charged ([2-(trimethylammonium)ethyl] methanethiosulfonate and 2-aminoethyl methanethiosulfonate) or negatively charged (sodium (2-sulfonatoethyl)methanethiosulfonate) causes MscL to gate spontaneously, even when no tension is applied. In contrast, the polar 2-hydroxyethyl methanethiosulfonate halves the threshold, and the hydrophobic methyl methanethiolsulfonate increases the threshold. These observations indicate that residue 22 is in a hydrophobic environment before gating and in a hydrophilic environment during opening to a substate, a finding consistent with our previous study. In addition, we have found that cysteine 22 is accessible to reagents from the cytoplasmic side only when the channel is opened whereas it is accessible from the periplasmic side even in the closed state. These results support the view that exposure of hydrophobic surfaces to a hydrophilic environment during channel opening serves as the barrier to gating.

Structural and functional differences between two homologous mechanosensitive channels of Methanococcus jannaschii

We report the molecular cloning and characterization of MscMJLR, a second type of mechanosensitive (MS) channel found in the archaeon Methanococcus jannaschii. MscMJLR is structurally very similar to MscMJ, the MS channel of M.jannaschii that was identified and cloned first by using the TM1 domain of Escherichia coli MscL as a genetic probe. Although it shares 44% amino acid sequence identity and similar cation selectivity with MscMJ, MscMJLR exhibits other major functional differences. The conductance of MscMJLR of approximately 2 nS is approximately 7-fold larger than the conductance of MscMJ and rectifies with voltage. The channel requires approximately 18 kT for activation, which is three times the amount of energy required to activate MscMJ, but is comparable to the activation energy of Eco-MSCL: Our study indicates that a multiplicity of conductance-wise and energetically well-tuned MS channels in microbial cell membranes may provide for cell survival by the sequential opening of the channels upon challenge with different osmotic cues.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Visualization of elementary mechanosensitive Ca2+-influx events, Ca2+ spots, in bovine lens epithelial cells

Local increases in the intracellular Ca2+ concentration ([Ca2+]i) in several regions within the bovine lens epithelial cell during application of mechanical stress were clearly visualized in the presence of lysophosphatidic acid (LPA), a bioactive lysophospholipid, using real-time confocal microscopy. We called the phenomenon 'Ca2+ spots'. Ca2+ spots started in a circular area with a radius of about 1.5 m. These Ca2+ spots spread concentrically, resulting in a mean global increase in [Ca2+]i. The local increase often occurred in a stepwise manner or repetitively at the same region. The spatiotemporal properties of the Ca2+ spots were completely different from those of the Ca2+ wave induced by ATP, a Ca2+-mobilizing agonist. Ca2+ spots were inhibited by decreasing the extracellular Ca2+ concentration or by the presence of Gd3+, an inhibitor of mechanosensitive (MS) channels, but not by thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+ pump, suggesting that Ca2+ spots arise from Ca2+ influx through Gd3+-sensitive MS channels. On the assumption that, in lens epithelial cells, the open probability of the MS channel is 0.4, the membrane potential is 56 mV and the channel conductance is 50 pS, the estimated maximum flux of Ca2+ in a Ca2+ spot (0.4 x 10-17 to 4.7 x 10-17 mol x s(-1)) was comparable to currents of one or a few MS channels. On real-time three-dimensional confocal imaging analysis, which permitted simultaneous imaging of basal and apical planes of cells at 37.6 ms intervals, Ca2+ spots on the apical plane were more clearly visualized than those on the basal plane. From these results, we propose that the Ca2+ spot is an elementary Ca2+-influx event through MS channels directly coupled with the first step in mechanoreception In addition, our results strongly suggest that LPA functions as an endogenous factor affecting mechanotransduction systems.

Stretch-activated cation channels of leech neurons exhibit two activity modes

Single-channel recordings were used to characterize two activity modes of stretch activated channels (SACs) in identified neurons of the leech. Clear-cut differences in the activity pattern of SACs from freshly desheathed cell bodies and from cultured AP cells were observed. SACs of inside-out patches, made by 'gentle' sealing and excised from cell bodies of freshly desheathed ganglia exhibited spike-like (SL) activity, with a mean channel open time (MCOT) shorter than 10 ms. Fitting of dwell open-time distributions revealed time constants shorter than 2 and 10 ms. This activity was characterized by a chord conductance of about 115 pS. SACs from cultured cells often displayed activity just after excision. MCOT exceeded 200 ms and the time constants of open-time interval distributions were longer than 10 and 100 ms. Furthermore, this activity pattern was characterized by both sub- (about 80 and 40 pS) and super-conductance (150 pS) levels, hence denoted as multiconductance (MC) mode. The percentage of open time spent at the main subconductance level (80 pS) was significantly higher in patches isolated from growth cones than in those from cell bodies of cultured neurons. The two activity modes (SL and MC) should belong to the same channel because both modes have a common main conductance value and exhibit outward rectification, stretch sensitivity and blockage by Gd3+ and gentamicin. Cytochalasin D applied to the cytoplasmic side induced activation of SACs or increased their ongoing activity. Thus, the observed differences in the expression of the two activity modes of SACs might be associated with different arrangements of the cortical cytoskeleton.

Graviperception and gravitaxis in algae

Photosynthetic flagellates are among the most intensely studied unicellular organisms in the field of graviperception and gravitaxis. While the phenomenon of graviorientation has been known for many decades, only recently was the molecular mechanism unveiled. Earlier hypotheses tried to explain the precise orientation by a passive buoy mechanism assuming the tail end to be heavier than the front. In the photosynthetic flagellate Euglena gracilis, the whole cell body is denser than the surrounding medium, pressing onto the lower cell membrane where it seems to activate mechanosensitive ion channels specific for calcium. The calcium entering the cells during reorientation can be visualized by the fluorescence probe, Calcium Crimson. Cyclic AMP is likewise involved in the molecular pathway. Inhibitors of calcium channels and ionophores impair gravitaxis while caffeine, a blocker of the phosphodiesterase, enhances the precision of orientation.

Whole cell current and membrane potential regulation by a human smooth muscle mechanosensitive calcium channel

Mechanotransduction is required for a wide variety of biological functions. The aim of this study was to determine the effect of activation of a mechanosensitive Ca(2+) channel, present in human jejunal circular smooth muscle cells, on whole cell currents and on membrane potential. Currents were recorded using patch-clamp techniques, and perfusion of the bath (10 ml/min, 30 s) was used to mechanoactivate the L-type Ca(2+) channel. Perfusion resulted in activation of L-type Ca(2+) channels and an increase in outward current from 664 +/- 57 to 773 +/- 72 pA at +60 mV. Membrane potential hyperpolarized from -42 +/- 4 to -50 +/- 5 mV. In the presence of nifedipine (10 microM), there was no increase in outward current or change in membrane potential with perfusion. In the presence of charybdotoxin or iberiotoxin, perfusion of the bath did not increase outward current or change membrane potential. A model is proposed in which mechanoactivation of an L-type Ca(2+) channel current in human jejunal circular smooth muscle cells results in increased Ca(2+) entry and cell contraction. Ca(2+) entry activates large-conductance Ca(2+)-activated K(+) channels, resulting in membrane hyperpolarization and relaxation.

Selected contribution: regulatory pathways involved in mechanical induction of c-fos gene expression in bone cells

The regulatory pathways involved in the rapid response of the AP-1 transcription factor, c-fos, to mechanical load in human primary osteoblast-like (HOB) cells and the human MG-63 bone cell line were investigated using a four-point bending model. HOB and MG-63 cells showed upregulation of c-fos expression on fibronectin and collagen type I substrates; however, MG-63 cells did not respond on laminin YIGSR substrates. Addition of cytochalasin D and Arg-Gly-Asp peptides during loading did not inhibit the response, whereas addition of beta(1)-integrin antibodies inhibited the load response. The role of Ca(2+) signaling has been demonstrated by blocking upregulation with addition of 2 mM EGTA, which chelates extracellular Ca(2+), and gadolinium (10 microM), which inhibits stretch-activated channels. Addition of the Ca(2+) ionophore A-23187 induced upregulation without loading; however, addition of nifedipine (10 microM), the L-type channel blocker, failed to prevent the load response. Inhibitors of downstream pathways indicated the involvement of protein kinase C. Our results demonstrate a key involvement of Ca(2+) signaling pathways and integrin binding in the c-fos response to mechanical strain.

Microvillar ion channels: cytoskeletal modulation of ion fluxes

The recently presented theory of microvillar Ca(2+)signaling [Lange, K. (1999) J. Cell. Physiol.180, 19-35], combined with Manning's theory of "condensed counterions" in linear polyelectrolytes [Manning, G. S. (1969). J. Chem. Phys.51, 924-931] and the finding of cable-like ion conductance in actin filaments [Lin, E. C. & Cantiello, H. F. (1993). Biophys. J.65, 1371-1378], allows a systematic interpretation of the role of the actin cytoskeleton in ion channel regulation. Ion conduction through actin filament bundles of microvilli exhibits unique nonlinear transmission properties some of which closely resemble that of electronic semiconductors: (1) bundles of microfilaments display significant resistance to cation conduction and (2) this resistance is decreased by supply of additional energy either as thermal, mechanical or electromagnetic field energy. Other transmission properties, however, are unique for ionic conduction in polyelectrolytes. (1) Current pulses injected into the filaments were transformed into oscillating currents or even into several discrete charge pulses closely resembling that of single-channel recordings. Discontinuous transmission is due to the existence of counterion clouds along the fixed anionic charge centers of the polymer, each acting as an "ionic capacitor". (2) The conductivity of linear polyelectrolytes strongly decreases with the charge number of the counterions; thus, Ca(2+)and Mg(2+)are effective modulator of charge transfer through linear polyelectrolytes. Field-dependent formation of divalent cation plugs on either side of the microvillar conduction line may generate the characteristic gating behavior of cation channels. (3) Mechanical movement of actin filament bundles, e.g. bending of hair cell microvilli, generates charge translocations along the filament structure (mechano-electrical coupling). (4) Energy of external fields, by inducing molecular dipoles within the polyelectrolyte matrix, can be transformed into mechanical movement of the system (electro-mechanical coupling). Because ionic transmission through linear polyelectrolytes is very slow compared with electronic conduction, only low-frequency electromagnetic fields can interact with the condensed counterion systems of linear polyelectrolytes. The delineated characteristics of microvillar ion conduction are strongly supported by the phenomenon of electro-mechanical coupling (reverse transduction) in microvilli of the audioreceptor (hair) cells and the recently reported dynamics of Ca(2+)signaling in microvilli of audio- and photoreceptor cells. Due to the cell-specific expression of different types and combinations of ion channels and transporters in the microvillar tip membrane of differentiated cells, the functional properties of this cell surface organelle are highly variable serving a multitude of different cellular functions including receptor-mediated effects such as Ca(2+)signaling, regulation of glucose and amino acid transport, as well as modulation of membrane potential. Even mechanical channel activation involved in cell volume regulation can be deduced from the systematic properties of the microvillar channel concept. In addition, the specific ion conduction properties of microfilaments combined with their proposed role in Ca(2+)signaling make microvilli the most likely cellular site for the interaction with external electric and magnetic fields.

Regulation of cell volume via microvillar ion channels

A novel mechanism of cellular volume regulation is presented, which ensues from the recently introduced concept of transport and ion channel regulation via microvillar structures (Lange K, 1999, J Cell Physiol 180:19-35). According to this notion, the activity of ion channels and transporter proteins located on microvilli of differentiated cells is regulated by changes in the structural organization of the bundle of actin filaments in the microvillar shaft region. Cells with microvillar surfaces represent two-compartment systems consisting of the cytoplasm on the one side and the sum of the microvillar tip (or, entrance) compartments on the other side. The two compartments are separated by the microvillar actin filament bundle acting as diffusion barrier ions and other solutes. The specific organization of ion and water channels on the surface of microvillar cell types enables this two-compartment system to respond to hypo- and hyperosmotic conditions by activation of ionic fluxes along electrochemical gradients. Hypotonic exposure results in swelling of the cytoplasmic compartment accompanied by a corresponding reduction in the length of the microvillar diffusion barrier, allowing osmolyte efflux and regulatory volume decrease (RVD). Hypertonic conditions, which cause shortening of the diffusion barrier via swelling of the entrance compartment, allow osmolyte influx for regulatory volume increase (RVI). Swelling of either the cytoplasmic or the entrance compartment, by using membrane portions of the microvillar shafts for surface enlargement, activates ion fluxes between the cytoplasm and the entrance compartment by shortening of microvilli. The pool of available membrane lipids used for cell swelling, which is proportional to length and number of microvilli per cell, represents the sensor system that directly translates surface enlargements into activation of ion channels. Thus, the use of additional membrane components for osmotic swelling or other types of surface-expanding shape changes (such as the volume-invariant cell spreading or stretching) directly regulates influx and efflux activities of microvillar ion channels. The proposed mechanism of ion flux regulation also applies to the physiological main functions of epithelial cells and the auxiliary action of swelling-induced ATP release. Furthermore, the microvillar entrance compartment, as a finely dispersed ion-accessible peripheral space, represents a cellular sensor for environmental ionic/osmotic conditions able to detect concentration gradients with high lateral resolution. Volume regulation via microvillar surfaces is only one special aspect of the general property of mechanosensitivity of microvillar ionic pathways.

Multiple triggers of oocyte maturation in nemertean worms: the roles of calcium and serotonin

To analyze the process of oocyte maturation in nemertean worms, oocytes with a large nucleus (=germinal vesicle, or GV) were removed from gravid ovaries of Cerebratulus lacteus and Micrura alaskensis. Following transfer to natural seawater (NSW), fully grown oocytes spontaneously matured as indicated by their completion of germinal vesicle breakdown (GVBD), whereas GVBD was reversibly blocked if the oocytes were initially placed in calcium-free seawater (CaFSW). Similarly, calcium ionophore treatments triggered GVBD in calcium-containing artificial seawater (ASW) but not in CaFSW, suggesting that external calcium influx may facilitate maturation. However, compared to the overall levels of maturation elicited by ASW, significantly higher percentages of GVBD were achieved with NSW or with ASW that had been conditioned with marine sediment. Moreover, calcium channel blockers decreased GVBD rates in ASW but not in NSW, which is consistent with the view that substances other than external calcium ions can trigger maturation. Accordingly, oocytes underwent equally high levels of GVBD when treated with serotonin (=5-hydroxytryptamine, or 5-HT) in ASW or CaFSW. The 5-HT-induced maturation was blocked by inhibitors of 5-HT receptors but continued to occur in the presence of calcium channel blockers or the calcium chelator BAPTA. In addition, oocytes microinjected with fluorescent calcium indicators underwent GVBD in response to 5-HT without displaying marked calcium transients during confocal imaging runs. Collectively, such findings suggest that nemertean oocytes can mature via multiple pathways that may include external calcium influx or a 5-HT-induced signaling cascade that lacks prominent calcium fluctuations. J. Exp. Zool. 287:243-261, 2000.

The soma and neurites of primary afferent neurons in the guinea-pig intestine respond differentially to deformation

1. Intrinsic primary afferent neurons in the small intestine are exposed to distortion of their processes and of their cell bodies. Recordings of mechanosensitivity have previously been made from these neurons using intracellular microelectrodes, but this form of recording has not permitted detection of generator potentials from the processes, or of responses to cell body distortion. 2. We have developed a technique to record from enteric neurons in situ using patch electrodes. The mechanical stability of the patch recordings has allowed recording in cell-attached and whole cell configuration during imposed movement of the neurons. 3. Pressing with a fine probe initiated generator potentials (14 +/- 9 mV) from circumscribed regions of the neuron processes within the same myenteric ganglion, at distances from 100 to 500 microm from the cell body that was patched. Generator potentials persisted when synaptic transmission was blocked with high Mg2+, low Ca2+ solution. 4. Soma distortion, by pressing down with the whole cell recording electrode, inhibited action potential firing. Consistent with this, moderate intra-electrode pressure (10 mbar; 1 kPa) increased the opening probability of large-conductance (BK) potassium channels, recorded in cell-attached mode, but suction was not effective. In outside-out patches, suction, but not pressure, increased channel opening probability. Mechanosensitive BK channels have not been identified on other neurons. 5. The BK channels had conductances of 195 +/- 25 pS. Open probability was increased by depolarization, with a half-maximum activation at a patch potential of 20 mV and a slope factor of 10 mV. Channel activity was blocked by charybdotoxin (20 nM). 6. Stretch that increased membrane area under the electrode by 15 % was sufficient to double open probability. Similar changes in membrane area occur when the intestine changes diameter and wall tension under physiological conditions. Thus, the intestinal intrinsic primary afferent neurons are detectors of neurite distortion and of compression of the soma, these stimuli having opposite effects on neuron excitability.

Stretch-activated and background non-selective cation channels in rat atrial myocytes

1. Stretch-activated channels (SACs) were studied in isolated rat atrial myocytes using the whole-cell and single-channel patch clamp techniques. Longitudinal stretch was applied by using two patch electrodes. 2. In current clamp configuration, mechanical stretch of 20 % of resting cell length depolarised the resting membrane potential (RMP) from -63.6 +/- 0.58 mV (n = 19) to -54.6 +/- 2.4 mV (n = 13) and prolonged the action potential duration (APD) by 32.2 +/- 8.8 ms (n = 7). Depolarisation, if strong enough, triggered spontaneous APs. In the voltage clamp configuration, stretch increased membrane conductance in a progressive manner. The current-voltage (I-V ) relationship of the stretch-activated current (ISAC) was linear and reversed at -6.1 +/- 3.7 mV (n = 7). 3. The inward component of ISAC was abolished by the replacement of Na+ with NMDG+, but ISAC was hardly altered by the Cl- channel blocker DIDS or removal of external Cl-. The permeability ratio for various cations (PCs:PNa:PLi = 1.05:1:0.98) indicated that the SAC current was a non-selective cation current (ISAC,NC). The background current was also found to be non-selective to cations (INSC,b); the permeability ratio (PCs:PNa:PLi = 1.49:1:0.70) was different from that of ISAC,NC. 4. Gadolinium (Gd3+) acted on INSC,b and ISAC,NC differently. Gd3+ inhibited INSC,b in a concentration-dependent manner with an IC50 value of 46.2 +/- 0.8 microM (n = 5). Consistent with this effect, Gd3+ hyperpolarised the resting membrane potential (-71.1 +/- 0.26 mV, n = 9). In the presence of Gd3+ (0.1 mM), stretch still induced ISAC,NC and diastolic depolarisation. 5. Single-channel activities were recorded in isotonic Na+ and Cs+ solutions using the inside-out configuration. In NMDG+ solution, outward currents were abolished. Gd3+ (100 microM) strongly inhibited channel opening both from the inside and outside. In the presence of Gd3+ (100 microM) in the pipette solution, an increase in pipette pressure induced an increase in channel opening (21.27 +/- 0.24 pS; n = 7), which was distinct from background activity. 6. We concluded from the above results that longitudinal stret in rat atrial myocytes induces the activation of non-selective cation channels that can be distinguished from background channels by their different electrophysiology and pharmacology.

Stretch-induced activation of Ca(2+)-activated K(+) channels in mouse skeletal muscle fibers

High-conductance Ca(2+)-activated K(+) (K(Ca)) channels were studied in mouse skeletal muscle fibers using the patch-clamp technique. In inside-out patches, application of negative pressure to the patch induced a dose-dependent and reversible activation of K(Ca) channels. Stretch-induced increase in channel activity was found to be of the same magnitude in the presence and in the absence of Ca(2+) in the pipette. The dose-response relationships between K(Ca) channel activity and intracellular Ca(2+) and between K(Ca) channel activity and membrane potential revealed that voltage and Ca(2+) sensitivity were not altered by membrane stretch. In cell-attached patches, in the presence of high external Ca(2+) concentration, stretch-induced activation was also observed. We conclude that membrane stretch is a potential mode of regulation of skeletal muscle K(Ca) channel activity and could be involved in the regulation of muscle excitability during contraction-relaxation cycles.