Stretch-sensitive Ion Channels Research Articles

Nuclear Deformation Lets Cells Gauge Their Physical Confinement.

How cells sense their physical microenvironment remains incompletely understood. In two recent Science articles, Lomakin etal. (2020) and Venturini etal. (2020) demonstrate that progressive nuclear deformation associated with cellular confinement triggers intracellular events that promote cell contractility and migration, revealing the nucleus to serve as a central mechanosensor.

Hearing at threshold intensities: by slow mechanical traveling waves or by fast cochlear fluid pressure waves.

The three modes of auditory stimulation (air, bone and soft tissue conduction) at threshold intensities are thought to share a common excitation mechanism: the stimuli induce passive displacements of the basilar membrane propagating from the base to the apex (slow mechanical traveling wave), which activate the outer hair cells, producing active displacements, which sum with the passive displacements. However, theoretical analyses and modeling of cochlear mechanics provide indications that the slow mechanical basilar membrane traveling wave may not be able to excite the cochlea at threshold intensities with the frequency discrimination observed. These analyses are complemented by several independent lines of research results supporting the notion that cochlear excitation at threshold may not involve a passive traveling wave, and the fast cochlear fluid pressures may directly activate the outer hair cells: opening of the sealed inner ear in patients undergoing cochlear implantation is not accompanied by threshold elevations to low frequency stimulation which would be expected to result from opening the cochlea, reducing cochlear impedance, altering hydrodynamics. The magnitude of the passive displacements at threshold is negligible. Isolated outer hair cells in fluid display tuned mechanical motility to fluid pressures which likely act on stretch sensitive ion channels in the walls of the cells. Vibrations delivered to soft tissue body sites elicit hearing. Thus, based on theoretical and experimental evidence, the common mechanism eliciting hearing during threshold stimulation by air, bone and soft tissue conduction may involve the fast-cochlear fluid pressures which directly activate the outer hair cells.

Stretch-Gated Ion Channels in Neuronal Mechanoreceptors

The sense of touch, or mechanosensitivity, is crucial for virtually all aspects of everyday life, yet among all the senses possessed by vertebrates, it remains least explored at the cellular and molecular levels. Key to touch sensitivity are neuronal mechanoreceptors, which constitute a minor group of neurons in the somatosensory ganglia of most vertebrates. The scarcity of neuronal mechanoreceptors coupled with the absence of unequivocal markers for their identification impedes progress in understanding the molecular mechanism of touch sensitivity. Towards this end, we developed a novel animal model - the trigeminal system of duck embryos. Ducks are tactile specialists capable of foraging by means of acute mechanosensitivity of the glabrous skin covering their bill. We found that in contrast to rodents and other vertebrates, the trigeminal ganglia innervating duck head are dominated by mechanoreceptors instead of thermoreceptors and nociceptors. Electrophysiological analysis showed that duck trigeminal neurons have an augmented ability to convert touch into excitation, presumably due to the properties of the resident stretch-sensitive ion channels. We determined that a majority of duck trigeminal neurons express mechano-activated ion channels of the Piezo and Nav classes. We explored functional properties of these channels in vitro and inquired into their contribution to mechanically activated excitatory current and action potential generation in rapidly-adapting neuronal mechanoreceptors. Our data suggest a molecular mechanism underlying the sense of touch in the glabrous skin of vertebrates.

'Shooting gallery' for membrane proteins provides new insights into complexities of their function and structural dynamics.

'Shooting gallery' for membrane proteins provides new insights into complexities of their function and structural dynamics.

Ca2+ influx and ATP release mediated by mechanical stretch in human lung fibroblasts

One cause of progressive pulmonary fibrosis is dysregulated wound healing after lung inflammation or damage in patients with idiopathic pulmonary fibrosis and severe acute respiratory distress syndrome. The mechanical forces are considered to regulate pulmonary fibrosis via activation of lung fibroblasts. In this study, the effects of mechanical stretch on the intracellular Ca2+ concentration ([Ca2+]i) and ATP release were investigated in primary human lung fibroblasts. Uniaxial stretch (10–30% in strain) was applied to fibroblasts cultured in a silicone chamber coated with type I collagen using a stretching apparatus. Following stretching and subsequent unloading, [Ca2+]i transiently increased in a strain-dependent manner. Hypotonic stress, which causes plasma membrane stretching, also transiently increased the [Ca2+]i. The stretch-induced [Ca2+]i elevation was attenuated in Ca2+-free solution. In contrast, the increase of [Ca2+]i by a 20% stretch was not inhibited by the inhibitor of stretch-activated channels GsMTx-4, Gd3+, ruthenium red, or cytochalasin D. Cyclic stretching induced significant ATP releases from fibroblasts. However, the stretch-induced [Ca2+]i elevation was not inhibited by ATP diphosphohydrolase apyrase or a purinergic receptor antagonist suramin. Taken together, mechanical stretch induces Ca2+ influx independently of conventional stretch-sensitive ion channels, the actin cytoskeleton, and released ATP.

A role for membrane shape and information processing in cardiac physiology.

While the heart is a dynamic organ and one of its major functions is to provide the organism with sufficient blood supply, the regulatory feedback systems, which allow adaptation to hemodynamic changes, remain not well understood. Our current description of mechanosensation focuses on stretch-sensitive ion channels, cytoskeletal components, structures such as the sarcomeric Z-disc, costameres, caveolae, or the concept of tensegrity, but these models appear incomplete as the remarkable plasticity of the myocardium in response to biomechanical stress and heart rate variations remains unexplained. Signaling activity at membranes depends on their geometric parameters such as surface area and curvature, which links shape to information processing. In the heart, continuous cycles of contraction and relaxation reshape membrane morphology and hence affect cardio-mechanic signaling. This article provides a brief review on current models of mechanosensation and focuses on how signaling, cardiac myocyte dynamics, and membrane shape interact and potentially give rise to a self-organized system that uses shape to sense the extra- and intracellular environment. This novel concept may help to explain how changes in frequency, and thus membrane shape, affect cardiac plasticity. One of the conclusions is that hypertrophy and associated fibrosis, which have been considered as necessary to cope with increased wall stress, can also be seen as part of complex feedback systems which use local membrane inhomogeneity in different cardiac cell types to influence whole organphysiology and which are predicted to fine-tune and thus regulate membrane-mediated signaling.

Fibroblasts, electrophysiological changes, and atrial fibrillation: Focus on the arrhythmogenic properties of fibroblasts

Atrial fibrosis is one of the fundamental mechanisms for the pathogenesis of atrial fibrillation (AF), but the underlying electrophysiological changes involved are not completely understood. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate various signaling pathways that lead to cellular hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix (ECM), such as tissue fibrosis; these may lead to disruption of the electrical side-toside junctions between muscle bundles [1], resulting in electrical dissociation. Cardiomyocytes and cardiac fibroblasts are 2 major myocardial cell types, which constitute o50% and 40–60% of the total cell population, respectively [2,3]. In normal adult hearts, quiescent fibroblasts substantially outnumber myocytes, and in response to hemodynamic stress or injury, these fibroblasts differentiate into myofibroblasts that proliferate, secrete collagen, and synthesize new proteins such as α-smooth muscle actin (α-SMA), stretch-sensitive ion channels, and connexins [4]. Fibroblasts have arrhythmogenic properties such as those involved in blocking impulse propagation and differentiation into myofibroblasts, which have contractility, have electrical connectivity and stimulate electrical impulse. The alteration of fibroblast-induced electrical impulses depends on the number of fibroblasts: small numbers of fibroblasts lead to slow impulse conduction whereas large numbers of fibroblasts lead to conduction block [5]. Recently,

Assessment of the retinal pigment epithelial functions‐ modelling approach

AbstractPurpose Our purpose is to construct mathematical models of functions of retinal pigment epithelium (RPE). Here we show the development and results of two models: 1) a compartmental model of Ca2+ dynamics 2) finite element model of epithelial transport and trans‐epithelial resistance.Methods A model of Ca2+ dynamics of APRE‐19 cells based on an experimental data and literature was constructed. Ca+2 dynamics of APRE‐19 cells were recorded using fluorescent microscope after mechanical stimulation. Various Ca+2 functional conditions were tested. The computational model constructed (Matlab SimBiology ) comprises over 40 cell function parameters and twelve variables ranging from stretch‐sensitive ion channels to ryanodine receptor dynamics and sarco/endoplasmic reticulum ATPases. Further, a model of RPE epithelia barrier was developed based on finite element modelling (FEM) of the epithelia and trans‐epithelial resistance simulating the spatial distribution of epithelial properties originating of the tight junction distribution on epithelia.Results The Ca+2 kinetics model of RPE was able to reproduce the Ca+2 dynamics of APRE cells with high accuracy with above 0.97 cross‐correlation. Further, including stretch‐sensitive ion channels explained the Ca+2 dynamics of the cells close to the mechanically stimulated cell. The epithelial model showed that epithelial inhomogeneity may play a crucial role in epithelial tightness and even very small cellular changes produce large variation on the measures of the epithelial properties.Conclusion The models constructed provide new insight of the functions of the RPE with applications from drug transport studies to assessment of the functionality of stem cell derived RPE.

HIGHLIGHTS FROM THE LITERATURE

HIGHLIGHTS FROM THE LITERATURE

GENE EXPRESSION OF STRETCH‐ACTIVATED CHANNELS AND MECHANOELECTRIC FEEDBACK IN THE HEART

1. Mechanoelectric feedback (MEF) in the heart is the process by which mechanical forces on the myocardium can change its electrical properties. Mechanoelectric feedback has been demonstrated in many animal models, ranging from isolated cells, through isolated hearts to whole animals. In humans, MEF has been demonstrated directly in both the atria and the ventricles. It seems likely that MEF provides either the trigger or the substrate for some types of clinically important arrhythmias. 2. Mechanoelectric feedback may arise because of the presence of stretch-sensitive (or mechano-sensitive) ion channels in the cell membrane of the cardiac myocytes. Two types have been demonstrated: (i) a non-specific cation channel (stretch-activated channel (SAC); conductance of approximately 25 pS); and (ii) a potassium channel with a conductance of approximately 100 pS. The gene coding for the SAC has not yet been identified. The gene for the potassium channel is likely to be TREK, a member of the tandem pore potassium channel gene family. We have recorded stretch-sensitive potassium channels in rat isolated myocytes that have the properties of TREK channels expressed in heterologous systems. 3. It has been shown that TREK mRNA is expressed heterogeneously in the rat ventricular wall, with 17-fold more expression in endocardial compared with epicardial cells. This difference is reflected in the TREK currents recorded from endocardial and epicardial cells using whole-cell patch-clamp techniques, although the difference in current density was less pronounced (approximately threefold). Consistent with this, we show here that when the ventricle is stretched by inflation of an intraventricular balloon in a Langendorff perfused rat isolated heart, action potential shortening was more pronounced in the endocardium (30% shortening at 40 mmHg) compared with that in the epicardium (10% shortening at the same pressure). 4. Computer models of the mechanics of the (pig) heart show pronounced spatial variations in strain in the myocardium with large transmural differences (in the left ventricle in particular) and also large differences between the base and apex of the ventricle. 5. The importance of MEF and the non-homogeneous gene expression and strain distribution for arrhythmias is discussed.

Ultrastructure and functional organization of mouthpart sensory setae of the spiny lobster Panulirus argus: New features of putative mechanoreceptors

In comparison with other decapods, the Caribbean spiny lobster Panulirus argus has little diversity in the external morphology of the setae on the mouth apparatus. In mouthpart areas that frequently touch food items only two types of setae can be distinguished: simple setae and cuspidate setae. Simple setae are by far more numerous. The ultrastructural data presented here show that both types of seta are bimodal, in that they both contain mechano- and chemosensory cells as indicated by morphological features. The morphological features divide the sensory cells into three types: type 1, which has a mechanosensory appearance; type 2, which has a chemosensory appearance; and type 3, which is believed to be a mechanoreceptor due to desmosomal connections to a scolopale. All three cell types were found in all examined setae. In an earlier study the simple setae were found to contain two types of mechanosensors: bend-sensitive cells and displacement-sensitive cells. The morphological arrangement of the outer dendritic segment described in the present study cannot explain this division. Instead, it is suggested that the difference in sensitivity is caused by a differential arrangement of their stretch-sensitive ion channels. This hypothesis also provides an explanation for the earlier observation that only bend cells respond to changes in osmolarity.

Differential expression of the mechanosensitive potassium channel TREK‐1 in epicardial and endocardial myocytes in rat ventricle

Mechanoelectric feedback (MEF) is the process by which mechanical forces on the myocardium induce electrical responses. It is thought that MEF is important in controlling the beat to beat force of contraction in the ventricle, in response to fluctuations in load, and it may also play a role in controlling the dispersion of repolarization. The transduction mechanism for MEF is via stretch sensitive ion channels in the surface membrane of myocytes. Two types of stretch sensitive channels have been described; a non-selective cation channel, and a potassium selective channel. TREK-1 is a member of the recently cloned tandem pore potassium channels that has been shown to be mechanosensitive and to be expressed in rat heart. Here we report that the gene expression level of TREK-1, quantified using real-time RT-PCR against glyceraldehyde phosphate dehydrogenase (GAPDH) as a comparator gene, was found to be 0.34 +/- 0.14 in endocardial cells compared to 0.02 +/- 0.02 in epicardial cells (P < 0.05). To confirm that this is reflected in a different current density, whole cell TREK-1 currents, activated by chloroform, were recorded with patch clamp techniques in epicardial and endocardial cells. TREK-1 current density in epicardial and endocardial cells was 0.21 +/- 0.06 pA/pF and 0.8 +/- 0.27 pA/pF, respectively (P</= 0.05). We discuss the implications of this differential expression of TREK-1 for controlling action potential repolarization when the myocardium is stretched. We hypothesize that the gene expression of TREK-1 is controlled by the different amounts of stretch experienced by muscle cells across the ventricular wall.

Effect of the gadolinium ion on body fluid regulation

Both osmoreception and baroreception are thought to involve ion channels that are sensitive to changes in membrane stretch. We investigated the effect of a blocker of stretch-activated ion channels, the Gd 3+ ion, on osmoregulatory and cardiovascular responses in the intact rat. Intracerebroventricular injection of 50–100 nmol Gd 3+ reduced thirst induced by various treatments. Similar doses also reduced intake of saline induced by various treatments. Intracerebroventricular injection of 100 nmol Gd 3+ transiently increased arterial pressure and reduced the pressor response to intracerebroventricular angiotensin II (Ang II). Systemic administration of Gd 3+ failed to alter thirst, except for a high dose (270 μmol/kg) that induced illness. This high dose failed to prevent urinary hypertonicity and excretion of a load of hypertonic NaCl. Intravenous infusion of 270 μmol/kg of Gd 3+ reduced blood pressure and pressure responses to intravenous phenylephrine, but did not reduce the baroreceptor reflex control of heart rate. We conclude that the effects of Gd 3+ on thirst and on the cardiovascular system are probably not due to a direct effect of the drug on stretch-sensitive ion channels. Instead, many of the effects of Gd 3+ were compatible with blockade of voltage-gated Ca 2+ channels.

Endothelial responses to mechanical stress: where is the mechanosensor?

The endothelium is normally subjected to mechanical deformation resulting from shear stress and from strain associated with stretch of the vessel wall. These stimuli are detected by a mechanosensor that initiates a variety of signaling systems responsible for triggering the functional responses. The identity of the mechanosensor has not been established. This article discusses the different mechanisms of mechanosensing that have been proposed and reviews the literature with respect to signaling systems that are activated in response to stress and strain in endothelium. Published literature related to mechanotransduction, signal transduction pathways initiated by strain in endothelium, and pathophysiologic effects of abnormal shear forces in diseases. Proposed mechanisms of mechanosensing include stretch-sensitive ion channels, protein kinases associated with the cytoskeleton, integrin-cytoskeletal interactions, cytoskeletal-nuclear interactions, and oxidase systems capable of generating reactive oxygen species. However, the molecular identity of the mechanosensor is not known, nor is it clear whether multiple sensing mechanisms exist. Many responses are initiated in cells subjected to mechanical deformation, including alterations in ion channel conductance, activation of signal transduction pathways, and altered expression of specific genes. Future progress in this field will require a critical distinction between cell systems that become activated during mechanical strain and the identity of the cellular mechanosensor that triggers subsequent responses.

Physical Characterization of Gravitaxis in Euglena gracilis

Gravitaxis in unicellular microorganisms like Euglena gracilis has been known for more than 100 years. The current model explains this phenomenon on the basis of a specific density difference between cell body and surrounding medium. In order to test the feasibility of the current model in terms of physical considerations the specific density of different Euglena gracilis cultures was determined. Depending on the culture conditions the specific density was in a range between 1.046 g mL-1 and 1.054 g mL-1. Size and gravitaxis measurements were performed in parallel, which allowed to relate the force applied to the lower membrane to the kinetic properties of gravitactic reorientation. A linear relationship between force and gravitaxis kinetics was found. A comparison between estimated activation energy of the proposed stretch-sensitive ion channels and energy supplied by the displacement of the lower membrane by the sedimentation of the cell body revealed that a focusing, an amplification and/or an integration period over time must be involved in the gravitactic signal transduction chain. Analysis of stimulus-response curves revealed an integration period of about 5 seconds before a gravitactic reorientation starts. The kinetics of gravitaxis at 1 x gn, and 0.12 x gn, was found to be similar. A hypothesis is presented that explains this finding on the basis of a combination of an integration period and an all-or-none reaction during gravitactic reorientation.

Mechanisms of Na+-K+ pump regulation in cardiac myocytes during hyposmolar swelling.

We have previously demonstrated that the sarcolemmal Na+-K+ pump current (Ip) in cardiac myocytes is stimulated by cell swelling induced by exposure to hyposmolar solutions. However, the underlying mechanism has not been examined. Because cell swelling activates stretch-sensitive ion channels and intracellular messenger pathways, we examined their role in mediating Ip stimulation during exposure of rabbit ventricular myocytes to a hyposmolar solution. Ip was measured by the whole cell patch-clamp technique. Swelling-induced pump stimulation altered the voltage dependence of Ip. Pump stimulation persisted in the absence of extracellular Na+ and under conditions designed to minimize changes in intracellular Ca2+, excluding an indirect influence on Ip mediated via fluxes through stretch-activated channels. Pump stimulation was protein kinase C independent. The tyrosine kinase inhibitor tyrphostin A25, the phosphatidylinositol 3-kinase inhibitor LY-294002, and the protein phosphatase-1 and -2A inhibitor okadaic acid abolished Ip stimulation. Our findings suggest that swelling-induced pump stimulation involves the activation of tyrosine kinase, phosphatidylinositol 3-kinase, and a serine/threonine protein phosphatase. Activation of this messenger cascade may cause activation by the dephosphorylation of pump units.

Tensegrity: the architectural basis of cellular mechanotransduction.

Physical forces of gravity, hemodynamic stresses, and movement play a critical role in tissue development. Yet, little is known about how cells convert these mechanical signals into a chemical response. This review attempts to place the potential molecular mediators of mechanotransduction (e.g. stretch-sensitive ion channels, signaling molecules, cytoskeleton, integrins) within the context of the structural complexity of living cells. The model presented relies on recent experimental findings, which suggests that cells use tensegrity architecture for their organization. Tensegrity predicts that cells are hard-wired to respond immediately to mechanical stresses transmitted over cell surface receptors that physically couple the cytoskeleton to extracellular matrix (e.g. integrins) or to other cells (cadherins, selectins, CAMs). Many signal transducing molecules that are activated by cell binding to growth factors and extracellular matrix associate with cytoskeletal scaffolds within focal adhesion complexes. Mechanical signals, therefore, may be integrated with other environmental signals and transduced into a biochemical response through force-dependent changes in scaffold geometry or molecular mechanics. Tensegrity also provides a mechanism to focus mechanical energy on molecular transducers and to orchestrate and tune the cellular response.

REVIEW ■ : Axonal Hyperexcitability: Mechanisms and Role in Symptom Production in Demyelinating Diseases

Some of the symptoms associated with demyelinating disorders are believed to originate from spurious impulses arising ectopically in axons at the site of demyelination. This review describes such "positive" symptoms and the patterns of impulses that may be associated with them, including continuous trains of impulses, as well as spontaneous and triggered impulse bursts. The mechanisms underlying the generation of such trains by individual axons are described, including the roles of sodium and potassium currents, the composition of the extracellular fluid, impulse "reflection" at demyelinated sites, and stretch-sensitive ion channels. The contribution of ephaptic transmission to symptom production and its potential role in the generation of ectopic impulses are discussed. The factors involved in the generation of massed discharges are also examined as a basis for certain paroxysmal clinical phenomena. NEUROSCIENTIST 3:237-246, 1997

Stretch-sensitive ion channels of neuronal growth cones.

This paper unifies and outlines some important points made by recent experimental work, dealing with MS ion channels and growth cone motility. The considerable work carried out in cell biology, neurochemistry and membrane biophysics in the past decade allows us to imagine the neurite growth in term of known molecules and chains of simple mechanisms. On the other hand, it is becoming increasingly clear the real complexity of the growth cone behavior. There remain important gaps in our understanding of the role of MS channels in growth cone motility. At present, a major limitation for physiological work is the lack of specific blockers. An encouraging development of molecular genetic studies of Drosophila SAK channels (60) will probably provide valuable tools for straightforward experiments.

Signal perception and transduction of gravitaxis in the flagellate Euglena gracilis

Summary The motile flagellate Euglena gracilis orients itself in its natural environment by means of gravitaxis and phototaxis. Until recently the mechanism of gravitaxis was not understood. Current results indicate that the density difference between cell body and surrounding medium causes sedimentation of the cell body, resulting in a stretching of the lower membrane. This activates stretch-sensitive ion channels, which leads to a modulation of the membrane potential. The modulation triggers a flagellar reorientation. In order to test this model we performed a series of experiments regarding the role of different ions in signal transduction. Increasing the external calcium concentration (but not decreasing), addition of a calcium ionophore and changes in the calcium fluxes had a drastic effect on gravitaxis. Changes in the potassium concentration resulted in an even higher inhibition. In addition, direct changes in the membrane potential affected gravitaxis. The obtained results indicate that both potassium and calcium play an important role in gravitaxis.