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

TRP channels in cardiac mechano-redox coupling and diseases

Reactive oxygen species (ROS) produced by mechanically stretching cardiomyocytes is a crucial mediator to increase contractile force in accordance with the Frank-Starling law. However, excessive ROS production leads to oxidative stress, contributing to myocardial atrophic remodeling and cellular damage. NADPH oxidase, the primary enzyme responsible for ROS production localized on the plasma membrane and organelle membranes, plays a key role in membrane-oriented ROS signaling. Two isoforms of NADPH oxidase, Nox2 (constitutive) and Nox4 (inducible), are predominantly expressed in cardiomyocytes, each playing unique roles in different contexts. Recent studies have revealed that Nox proteins form protein signaling complexes with transient receptor potential (TRP) channel proteins, amplifying ROS signaling in hearts. This review presents the putative mechanism of protein-protein interaction between TRP and Nox and their pathophysiological significance in hearts and discusses therapeutic strategies targeting TRP-Nox protein interactions for the treatment of heart failure.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Oxidative Regulation of Vascular Cav1.2 Channels Triggers Vascular Dysfunction in Hypertension-Related Disorders

Blood pressure is determined by cardiac output and peripheral vascular resistance. The L-type voltage-gated Ca²⁺ (Ca_v1.2) channel in small arteries and arterioles plays an essential role in regulating Ca²⁺ influx, vascular resistance, and blood pressure. Hypertension and preeclampsia are characterized by high blood pressure. In addition, diabetes has a high prevalence of hypertension. The etiology of these disorders remains elusive, involving the complex interplay of environmental and genetic factors. Common to these disorders are oxidative stress and vascular dysfunction. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) and mitochondria are primary sources of vascular oxidative stress, whereas dysfunction of the Ca_v1.2 channel confers increased vascular resistance in hypertension. This review will discuss the importance of ROS derived from NOXs and mitochondria in regulating vascular Ca_v1.2 and potential roles of ROS-mediated Ca_v1.2 dysfunction in aberrant vascular function in hypertension, diabetes, and preeclampsia.

The role of activation of two different sGC binding sites by NO-dependent and NO-independent mechanisms in the regulation of SACs in rat ventricular cardiomyocytes

The mechanoelectrical feedback (MEF) mechanism in the heart that plays a significant role in the occurrence of arrhythmias, involves cation flux through cation nonselective stretch-activated channels (SACs). It is well known that nitric oxide (NO) can act as a regulator of MEF. Here we addressed the possibility of SAC's regulation along NO-dependent and NO-independent pathways, as well as the possibility of S-nitrosylation of SACs. In freshly isolated rat ventricular cardiomyocytes, using the patch-clamp method in whole-cell configuration, inward nonselective stretch-activated cation current ISAC was recorded through SACs, which occurs during dosed cell stretching. NO donor SNAP, α1-subunit of sGC activator BAY41-2272, sGC blocker ODQ, PKG blocker KT5823, PKG activator 8Br-cGMP, and S-nitrosylation blocker ascorbic acid, were employed. We concluded that the physiological concentration of NO in the cell is a necessary condition for the functioning of SACs. An increase in NO due to SNAP in an unstretched cell causes the appearance of a Gd3+ -sensitive nonselective cation current, an analog of ISAC , while in a stretched cell it eliminates ISAC . The NO-independent pathway of sGC activation of α subunit, triggered by BAY41-2272, is also important for the regulation of SACs. Since S-nitrosylation inhibitor completely abolishes ISAC , this mechanism occurs. The application of BAY41-2272 cannot induce ISAC in a nonstretched cell; however, the addition of SNAP on its background activates SACs, rather due to S-nitrosylation. ODQ eliminates ISAC , but SNAP added on the background of stretch increases ISAC in addition to ODQ. This may be a result of the lack of NO as a result of inhibition of NOS by metabolically modified ODQ. KT5823 reduces PKG activity and reduces SACs phosphorylation, leading to an increase in ISAC . 8Br-cGMP reduces ISAC by activating PKG and its phosphorylation. These results demonstrate a significant contribution of S-nitrosylation to the regulation of SACs.

Baroreceptors in the Aortic Arch and Their Potential Role in Aortic Dissection and Aneurysms

The arterial baroreflex is a key autonomic regulator of blood pressure whose dysfunction has been related to several cardiovascular diseases. Changes in blood pressure are sensed by specific mechanosensory proteins, called baroreceptors, particularly located in the outer layer of the carotid sinus and the inner curvature of the aortic arch. The signal is propagated along the afferent nerves to the central nervous system and serves as negative feedback of the heart rate. Despite extensive research, the precise molecular nature of baroreceptors remains elusive. Current knowledge assumes that baroreceptors are ion channels at the nerve endings within the outer layer of the arteries. However, the evidence is based mainly on animal experiments, and the specific types of mechanosensitive receptors responsible for the signal transduction are still unknown. Only a few studies have investigated mechanosensory transmission in the aortic arch. In addition, although aortic dissection, and particularly type A involving the aortic arch, is one of the most life-threatening cardiovascular disorders, there is no knowledge about the impact of aortic dissection on baroreceptor function. In this review, we aim not to highlight the regulation of the heart rate but what mechanical stimuli and what possible ion channels transfer the corresponding signal within the aortic arch, summarizing and updating the current knowledge about baroreceptors, specifically in the aortic arch, and the impact of aortic pathologies on their function.

Mechanosensitivity in Pulmonary Circulation: Pathophysiological Relevance of Stretch-Activated Channels in Pulmonary Hypertension

A variety of cell types in pulmonary arteries (endothelial cells, fibroblasts, and smooth muscle cells) are continuously exposed to mechanical stimulations such as shear stress and pulsatile blood pressure, which are altered under conditions of pulmonary hypertension (PH). Most functions of such vascular cells (e.g., contraction, migration, proliferation, production of extracellular matrix proteins, etc.) depend on a key event, i.e., the increase in intracellular calcium concentration ([Ca²⁺]_i) which results from an influx of extracellular Ca²⁺ and/or a release of intracellular stored Ca²⁺. Calcium entry from the extracellular space is a major step in the elevation of [Ca²⁺]_i, involving a variety of plasmalemmal Ca²⁺ channels including the superfamily of stretch-activated channels (SAC). A common characteristic of SAC is that their gating depends on membrane stretch. In general, SAC are non-selective Ca²⁺-permeable cation channels, including proteins of the TRP (Transient Receptor Potential) and Piezo channel superfamily. As membrane mechano-transducers, SAC convert physical forces into biological signals and hence into a cell response. Consequently, SAC play a major role in pulmonary arterial calcium homeostasis and, thus, appear as potential novel drug targets for a better management of PH.

TRPV Protein Family-From Mechanosensing to Cancer Invasion

Biophysical cues from the cellular microenvironment are detected by mechanosensitive machineries that translate physical signals into biochemical signaling cascades. At the crossroads of extracellular space and cell interior are located several ion channel families, including TRP family proteins, that are triggered by mechanical stimuli and drive intracellular signaling pathways through spatio-temporally controlled Ca2+-influx. Mechanosensitive Ca2+-channels, therefore, act as critical components in the rapid transmission of physical signals into biologically compatible information to impact crucial processes during development, morphogenesis and regeneration. Given the mechanosensitive nature of many of the TRP family channels, they must also respond to the biophysical changes along the development of several pathophysiological conditions and have also been linked to cancer progression. In this review, we will focus on the TRPV, vanilloid family of TRP proteins, and their connection to cancer progression through their mechanosensitive nature.

Role of mechanosignaling on pathology of varicose vein

Varicose veins are the most common vascular disease in humans. Veins have valves that help the blood return gradually to the heart without leaking blood. When these valves become weak, blood and fluid collect and pool by pressing against the walls of the veins, causing varicose veins. In the cardiovascular system, mechanical forces are important determinants of vascular homeostasis and pathological processes. Blood vessels are constantly exposed to a variety of hemodynamic forces, including shear stress and environmental strains caused by the blood flow. In varicose veins within the leg, venous blood pressure rises in the vein of the lower extremities due to prolonged standing, creating a peripheral tension in the vessel wall thereby causing mechanical stimulation of endothelial cells and vascular smooth muscle. Studies have shown that long-term increased exposure to vascular wall tension is associated with the overexpression of HIF-1α and HIF-2α and increased levels of MMP-2 and MMP-9, thereby reducing venous contraction and progressive venous dilatation, which is involved in the development of varicose veins. Following the expression of metalloproteinase, the expression of type 1 collagen increases, and the amount of type 3 collagen decreases. Therefore, collagen imbalance will cause the varicose veins to not stretch. Loss of structural proteins (type 3 collagen and elastin) in the vessel wall causes the loss of the biophysical properties of the varicose vein wall. This review article tries to elaborate on the effect of mechanical forces and sensors of these forces on the vascular wall in creating the mechanism of mechanosignaling, as well as the role of the onset of molecular signaling cascades in the pathology of varicose veins.

Biophysical Principles of Ion-Channel-Mediated Mechanosensory Transduction

Recent rapid progress in the field of mechanobiology has been driven by novel emerging tools and methodologies and growing interest from different scientific disciplines. Specific progress has been made toward understanding how cell mechanics is linked to intracellular signaling and the regulation of gene expression in response to a variety of mechanical stimuli. There is a direct link between the mechanoreceptors at the cell surface and intracellular biochemical signaling, which in turn controls downstream effector molecules. Among the mechanoreceptors in the cell membrane, mechanosensitive (MS) ion channels are essential for the ultra-rapid (millisecond) transduction of mechanical stimuli into biologically relevant signals. The three decades of research on mechanosensitive channels resulted in the formulation of two basic principles of mechanosensitive channel gating: force-from-lipids and force-from-filament. In this review, we revisit the biophysical principles that underlie the innate force-sensing ability of mechanosensitive channels as contributors to the force-dependent evolution of life forms.

Transcriptional signatures regulated by TRPC1/C4-mediated Background Ca2+ entry after pressure-overload induced cardiac remodelling

AIMS

After summarizing current concepts for the role of TRPC cation channels in cardiac cells and in processes triggered by mechanical stimuli arising e.g. during pressure overload, we analysed the role of TRPC1 and TRPC4 for background Ca²⁺ entry (BGCE) and for cardiac pressure overload induced transcriptional remodelling.

METHODS AND RESULTS

Mn²⁺-quench analysis in cardiomyocytes from several Trpc-deficient mice revealed that both TRPC1 and TRPC4 are required for BGCE. Electrically-evoked cell shortening of cardiomyocytes from TRPC1/C4-DKO mice was reduced, whereas parameters of cardiac contractility and relaxation assessed in vivo were unaltered. As pathological cardiac remodelling in mice depends on their genetic background, and the development of cardiac remodelling was found to be reduced in TRPC1/C4-DKO mice on a mixed genetic background, we studied TRPC1/C4-DKO mice on a C57BL6/N genetic background. Cardiac hypertrophy was reduced in those mice after chronic isoproterenol infusion (-51.4%) or after one week of transverse aortic constriction (TAC; -73.0%). This last manoeuvre was preceded by changes in the pressure overload induced transcriptional program as analysed by RNA sequencing. Genes encoding specific collagens, the Mef2 target myomaxin and the gene encoding the mechanosensitive channel Piezo2 were up-regulated after TAC in wild type but not in TRPC1/C4-DKO hearts.

CONCLUSIONS

Deletion of the TRPC1 and TRPC4 channel proteins protects against development of pathological cardiac hypertrophy independently of the genetic background. To determine if the TRPC1/C4-dependent changes in the pressure overload induced alterations in the transcriptional program causally contribute to cardio-protection needs to be elaborated in future studies.

Post-Translational Modification and Natural Mutation of TRPC Channels

Transient Receptor Potential Canonical (TRPC) channels are homologues of Drosophila TRP channel first cloned in mammalian cells. TRPC family consists of seven members which are nonselective cation channels with a high Ca²⁺ permeability and are activated by a wide spectrum of stimuli. These channels are ubiquitously expressed in different tissues and organs in mammals and exert a variety of physiological functions. Post-translational modifications (PTMs) including phosphorylation, N-glycosylation, disulfide bond formation, ubiquitination, S-nitrosylation, S-glutathionylation, and acetylation play important roles in the modulation of channel gating, subcellular trafficking, protein-protein interaction, recycling, and protein architecture. PTMs also contribute to the polymodal activation of TRPCs and their subtle regulation in diverse physiological contexts and in pathological situations. Owing to their roles in the motor coordination and regulation of kidney podocyte structure, mutations of TRPCs have been implicated in diseases like cerebellar ataxia (moonwalker mice) and focal and segmental glomerulosclerosis (FSGS). The aim of this review is to comprehensively integrate all reported PTMs of TRPCs, to discuss their physiological/pathophysiological roles if available, and to summarize diseases linked to the natural mutations of TRPCs.

A TR(i)P to Cell Migration: New Roles of TRP Channels in Mechanotransduction and Cancer

Cell migration is a key process in cancer metastasis, allowing malignant cells to spread from the primary tumor to distant organs. At the molecular level, migration is the result of several coordinated events involving mechanical forces and cellular signaling, where the second messenger Ca²⁺ plays a pivotal role. Therefore, elucidating the regulation of intracellular Ca²⁺ levels is key for a complete understanding of the mechanisms controlling cellular migration. In this regard, understanding the function of Transient Receptor Potential (TRP) channels, which are fundamental determinants of Ca²⁺ signaling, is critical to uncovering mechanisms of mechanotransduction during cell migration and, consequently, in pathologies closely linked to it, such as cancer. Here, we review recent studies on the association between TRP channels and migration-related mechanotransduction events, as well as in the involvement of TRP channels in the migration-dependent pathophysiological process of metastasis.

Cellular and Molecular Mechanisms Underlying Arterial Baroreceptor Remodeling in Cardiovascular Diseases and Diabetes

Clinical trials and animal experimental studies have demonstrated an association of arterial baroreflex impairment with the prognosis and mortality of cardiovascular diseases and diabetes. As a primary part of the arterial baroreflex arc, the pressure sensitivity of arterial baroreceptors is blunted and involved in arterial baroreflex dysfunction in cardiovascular diseases and diabetes. Changes in the arterial vascular walls, mechanosensitive ion channels, and voltage-gated ion channels contribute to the attenuation of arterial baroreceptor sensitivity. Some endogenous substances (such as angiotensin II and superoxide anion) can modulate these morphological and functional alterations through intracellular signaling pathways in impaired arterial baroreceptors. Arterial baroreceptors can be considered as a potential therapeutic target to improve the prognosis of patients with cardiovascular diseases and diabetes.

Role of TRPC3 and TRPC6 channels in the myocardial response to stretch: Linking physiology and pathophysiology

Transient receptor potential (TRP) channels constitute a large family of versatile multi-signal transducers. In particular, TRP canonical (TRPC) channels are known as receptor-operated, non-selective cation channels. TRPC3 and TRPC6, two members in the TRPC family, are highly expressed in the heart, and participate in the pathogenesis of cardiac hypertrophy and heart failure as a pathological response to chronic mechanical stress. In the pathological response, myocardial stretch increases intracellular Ca²⁺ levels and activates nuclear factor of activated T cells to induce cardiac hypertrophy. Recent studies have revealed that TRPC3 and TRPC6 also contribute to the physiological stretch-induced slow force response (SFR), a slow increase in the Ca²⁺ transient and twitch force during stretch. In the physiological response, a stretch-induced increase in intracellular Ca²⁺ mediated by TRPC3 and TRPC6 causes the SFR. We here overview experimental evidence of the involvement of TRPC3 and TRPC6 in cardiac physiology and pathophysiology in response to stretch.

Regulation of glomerulotubular balance: flow-activated proximal tubule function

The purpose of this review is to summarize our knowledge and understanding of the physiological importance and the mechanisms underlying flow-activated proximal tubule transport. Since the earliest micropuncture studies of mammalian proximal tubule, it has been recognized that tubular flow is an important regulator of sodium, potassium, and acid-base transport in the kidney. Increased fluid flow stimulates Na⁺ and HCO₃^- absorption in the proximal tubule via stimulation of Na/H-exchanger isoform 3 (NHE3) and H⁺-ATPase. In the proximal tubule, brush border microvilli are the major flow sensors, which experience changes in hydrodynamic drag and bending moment as luminal flow velocity changes and which transmit the force of altered flow to cytoskeletal structures within the cell. The signal to NHE3 depends upon the integrity of the actin cytoskeleton; the signal to the H⁺-ATPase depends upon microtubules. We have demonstrated that alterations in fluid drag impact tubule function by modulating ion transporter availability within the brush border membrane of the proximal tubule. Beyond that, there is evidence that transporter activity within the peritubular membrane is also modulated by luminal flow. Secondary messengers that regulate the flow-mediated tubule function have also been delineated. Dopamine blunts the responsiveness of proximal tubule transporters to changes in luminal flow velocity, while a DA1 antagonist increases flow sensitivity of solute reabsorption. IP3 receptor-mediated intracellular Ca²⁺ signaling is critical to transduction of microvillus drag. In this review, we summarize our findings of the regulatory mechanism of flow-mediated Na⁺ and HCO₃^- transport in the proximal tubule and review available information about flow sensing and regulatory mechanism of glomerulotubular balance.

ROK and Arteriolar Myogenic Tone Generation: Molecular Evidence in Health and Disease

The myogenic response is an inherent property of resistance arteries that warrants a relatively constant blood flow in response to changes in perfusion pressure and protect delicate organs from vascular insufficiencies and excessive blood flow. This fundamental phenomenon has been extensively studied aiming to elucidate the underlying mechanisms triggering smooth muscle contraction in response to intraluminal pressure elevation, particularly, Rho-associated kinase (ROK)-mediated Ca2+-independent mechanisms. The size of the resistance arteries limits the capacity to examine changes in protein phosphorylation/expression levels associated with ROK signaling. A highly sensitive biochemical detection approach was beneficial in examining the role of ROK in different force generation mechanisms along the course of myogenic constriction. In this mini review, we summarize recent results showing direct evidence for the contribution of ROK in development of myogenic response at the level of mechanotransduction, myosin light chain phosphatase inhibition and dynamic actin cytoskeleton reorganization. We will also present evidence that alterations in ROK signaling could underlie the progressive loss in myogenic response in a rat model of type 2 diabetes.

Differences in TRPC3 and TRPC6 channels assembly in mesenteric vascular smooth muscle cells in essential hypertension

KEY POINTS

Canonical transient receptor potential (TRPC)3 and TRPC6 channels of vascular smooth muscle cells (VSMCs) mediate stretch- or agonist-induced cationic fluxes, contributing to membrane potential and vascular tone. Native TRPC3/C6 channels can form homo- or heterotetrameric complexes, which can hinder individual TRPC channel properties. The possibility that the differences in their association pattern may change their contribution to vascular tone in hypertension is unexplored. Functional characterization of heterologously expressed channels showed that TRPC6-containing complexes exhibited Pyr3/Pyr10-sensitive currents, whereas TRPC3-mediated currents were blocked by anti-TRPC3 antibodies. VSMCs from hypertensive (blood pressure high; BPH) mice have larger cationic basal currents insensitive to Pyr10 and sensitive to anti-TRPC3 antibodies. Consistently, myography studies showed a larger Pyr3/10-induced vasodilatation in BPN (blood pressure normal) mesenteric arteries. We conclude that the increased TRPC3 channel expression in BPH VSMCs leads to changes in TRPC3/C6 heteromultimeric assembly, with a higher TRPC3 channel contribution favouring depolarization of hypertensive VSMCs.

ABSTRACT

Increased vascular tone in essential hypertension involves a sustained rise in total peripheral resistance. A model has been proposed in which the combination of membrane depolarization and higher L-type Ca²⁺ channel activity generates augmented Ca²⁺ influx into vascular smooth muscle cells (VSMCs), contraction and vasoconstriction. The search for culprit ion channels responsible for membrane depolarization has provided several candidates, including members of the canonical transient receptor potential (TRPC) family. TRPC3 and TRPC6 are diacylglycerol-activated, non-selective cationic channels contributing to stretch- or agonist-induced depolarization. Conflicting information exists regarding changes in TRPC3/TRPC6 functional expression in hypertension. However, although TRPC3-TRPC6 channels can heteromultimerize, the possibility that differences in their association pattern may change their functional contribution to vascular tone is largely unexplored. We probe this hypothesis using a model of essential hypertension (BPH mice; blood pressure high) and its normotensive control (BPN mice; blood pressure normal). First, non-selective cationic currents through homo- and heterotetramers recorded from transfected Chinese hamster ovary cells indicated that TRPC currents were sensitive to the selective antagonist Pyr10 only when TRPC6 was present, whereas intracellular anti-TRPC3 antibody selectively blocked TRPC3-mediated currents. In mesenteric VSMCs, basal and agonist-induced currents were more sensitive to Pyr3 and Pyr10 in BPN cells. Consistently, myography studies showed a larger Pyr3/10-induced vasodilatation in BPN mesenteric arteries. mRNA and protein expression data supported changes in TRPC3 and TRPC6 proportions and assembly, with a higher TRPC3 channel contribution in BPH VSMCs that could favour cell depolarization. These differences in functional and pharmacological properties of TRPC3 and TRPC6 channels, depending on their assembly, could represent novel therapeutical opportunities.

Analysis of the Molecular Mechanisms of Reepithelialization in Drosophila Embryos

Significance: The epidermis provides the main barrier function of skin, and therefore its repair following wounding is an essential component of wound healing. Repair of the epidermis, also known as reepithelialization, occurs by collective migration of epithelial cells from around the wound edge across the wound until the advancing edges meet and fuse. Therapeutic manipulation of this process could potentially be used to accelerate wound healing.

Recent Advances: It is difficult to analyze the cellular and molecular mechanisms of reepithelialization in human tissue, so a variety of model organisms have been used to improve our understanding of the process. One model system that has been especially useful is the embryo of the fruit fly Drosophila, which provides a simple, accessible model of the epidermis and can be manipulated genetically, allowing detailed analysis of reepithelialization at the molecular level. This review will highlight the key insights that have been gained from studying reepithelialization in Drosophila embryos.

Critical Issues: Slow reepithelialization increases the risk of wounds becoming infected and ulcerous; therefore, the development of therapies to accelerate or enhance the process would be a great clinical advance. Improving our understanding of the molecular mechanisms that underlie reepithelialization will help in the development of such therapies.

Future Directions: Research in Drosophila embryos has identified a variety of genes and proteins involved in triggering and driving reepithelialization, many of which are conserved in humans. These novel reepithelialization proteins are potential therapeutic targets and therefore findings obtained in Drosophila may ultimately lead to significant clinical advances.

Polymodal Transient Receptor Potential Vanilloid (TRPV) Ion Channels in Chondrogenic Cells

Mature and developing chondrocytes exist in a microenvironment where mechanical load, changes of temperature, osmolarity and acidic pH may influence cellular metabolism. Polymodal Transient Receptor Potential Vanilloid (TRPV) receptors are environmental sensors mediating responses through activation of linked intracellular signalling pathways. In chondrogenic high density cultures established from limb buds of chicken and mouse embryos, we identified TRPV1, TRPV2, TRPV3, TRPV4 and TRPV6 mRNA expression with RT-PCR. In both cultures, a switch in the expression pattern of TRPVs was observed during cartilage formation. The inhibition of TRPVs with the non-selective calcium channel blocker ruthenium red diminished chondrogenesis and caused significant inhibition of proliferation. Incubating cell cultures at 41 °C elevated the expression of TRPV1, and increased cartilage matrix production. When chondrogenic cells were exposed to mechanical load at the time of their differentiation into matrix producing chondrocytes, we detected increased mRNA levels of TRPV3. Our results demonstrate that developing chondrocytes express a full palette of TRPV channels and the switch in the expression pattern suggests differentiation stage-dependent roles of TRPVs during cartilage formation. As TRPV1 and TRPV3 expression was altered by thermal and mechanical stimuli, respectively, these are candidate channels that contribute to the transduction of environmental stimuli in chondrogenic cells.

Functional similarities between heterogeneously and homogenously expressed MscL constructs

The mechanosensitive channel of large conductance MscL is a well-characterized mechanically gated non-selective ion channel, which often serves as a prototype mechanosensitive channel for mechanotransduction studies. However, there are some discrepancies between MscL constructs used in these studies, most notably unintended heterogeneous expression from some MscL expression constructs. In this study we investigate the possible cause of this expression pattern, and compare the original non-homogenously expressing constructs with our new homogeneously expressing one to confirm that there is little functional difference between them. In addition, a new MscL construct has been developed with an improved molar extinction coefficient at 280 nm, enabling more accurate protein quantification.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Contribution of mechanosensitive ion channels to somatosensation

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal, is a central mechanism to several physiological functions in mammals. It relies on the function of mechanosensitive ion channels (MSCs). Although the first single-channel recording from MSCs dates back to 30 years ago, the identity of the genes encoding MSCs has remained largely elusive. Because these channels have an important role in the development of mechanical hypersensitivity, a better understanding of their function may lead to the identification of selective inhibitors and generate novel therapeutic pathways in the treatment of chronic pain. Here, I will describe our current understanding of the role MSCs may play in somatosensation and the potential candidate genes proposed to encode them.

Role of TRP channels in the cardiovascular system

The transient receptor potential (TRP) superfamily consists of a large number of nonselective cation channels with variable degree of Ca(2+)-permeability. The 28 mammalian TRP channel proteins can be grouped into six subfamilies: canonical, vanilloid, melastatin, ankyrin, polycystic, and mucolipin TRPs. The majority of these TRP channels are expressed in different cell types including both excitable and nonexcitable cells of the cardiovascular system. Unlike voltage-gated ion channels, TRP channels do not have a typical voltage sensor, but instead can sense a variety of other stimuli including pressure, shear stress, mechanical stretch, oxidative stress, lipid environment alterations, hypertrophic signals, and inflammation products. By integrating multiple stimuli and transducing their activity to downstream cellular signal pathways via Ca(2+) entry and/or membrane depolarization, TRP channels play an essential role in regulating fundamental cell functions such as contraction, relaxation, proliferation, differentiation, and cell death. With the use of targeted deletion and transgenic mouse models, recent studies have revealed that TRP channels are involved in numerous cellular functions and play an important role in the pathophysiology of many diseases in the cardiovascular system. Moreover, several TRP channels are involved in inherited diseases of the cardiovascular system. This review presents an overview of current knowledge concerning the physiological functions of TRP channels in the cardiovascular system and their contributions to cardiovascular diseases. Ultimately, TRP channels may become potential therapeutic targets for cardiovascular diseases.

TRPV1 Antagonism: From Research to Clinic

The capsaicin receptor, TRPV1, has been one of the most extensively studied molecules in sensory research. Its contribution to the sensation of pain in numerous pre-clinical inflammatory and neuropathic paradigms has been well-established and expression analysis suggests a potential role clinically in pain and bladder conditions. The field has now reached an exciting point in time with the development of a number of high quality TRPV1 antagonist drug candidates and the release of clinical data. What has become apparent from this work is that inhibition of TRPV1 function brings with it the potential liabilities of increased body temperature and altered thermal perception. However, there is cause for optimism because it appears that not all antagonists have the same properties and compounds can be identified that lack significant on-target side-effects whilst retaining efficacy, at least pre-clinically. What is perhaps now more critical to address is the question of how effective the analgesia provided by a TRPV1 antagonist will be. Although tantalizing clinical data showing effects on experimentally-induced pain or pain following molar extraction have been reported, no clear efficacy in a chronic pain condition has yet been demonstrated making it difficult to perform an accurate risk-benefit analysis for TRPV1 antagonists. Here we provide an overview of some of the most advanced clinical candidates and discuss the approaches being taken to avoid the now well established on-target effects of TRPV1 antagonists.

Transient receptor potential channels as drug targets: from the science of basic research to the art of medicine

The large Trp gene family encodes transient receptor potential (TRP) proteins that form novel cation-selective ion channels. In mammals, 28 Trp channel genes have been identified. TRP proteins exhibit diverse permeation and gating properties and are involved in a plethora of physiologic functions with a strong impact on cellular sensing and signaling pathways. Indeed, mutations in human genes encoding TRP channels, the so-called "TRP channelopathies," are responsible for a number of hereditary diseases that affect the musculoskeletal, cardiovascular, genitourinary, and nervous systems. This review gives an overview of the functional properties of mammalian TRP channels, describes their roles in acquired and hereditary diseases, and discusses their potential as drug targets for therapeutic intervention.

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

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

TRPV2 is critical for the maintenance of cardiac structure and function in mice

The heart has a dynamic compensatory mechanism for haemodynamic stress. However, the molecular details of how mechanical forces are transduced in the heart are unclear. Here we show that the transient receptor potential, vanilloid family type 2 (TRPV2) cation channel is critical for the maintenance of cardiac structure and function. Within 4 days of eliminating TRPV2 from hearts of the adult mice, cardiac function declines severely, with disorganization of the intercalated discs that support mechanical coupling with neighbouring myocytes and myocardial conduction defects. After 9 days, cell shortening and Ca²⁺ handling by single myocytes are impaired in TRPV2-deficient hearts. TRPV2-deficient neonatal cardiomyocytes form no intercalated discs and show no extracellular Ca²⁺-dependent intracellular Ca²⁺ increase and insulin-like growth factor (IGF-1) secretion in response to stretch stimulation. We further demonstrate that IGF-1 receptor/PI3K/Akt pathway signalling is significantly downregulated in TRPV2-deficient hearts, and that IGF-1 administration partially prevents chamber dilation and impairment in cardiac pump function in these hearts. Our results improve our understanding of the molecular processes underlying the maintenance of cardiac structure and function.

The TRPV2 calcium channel can be activated by mechanical stretch and may act as a mechanoreceptor in tissues. Here the authors deplete the TRPV2 calcium channel from the hearts of adult mice, showing that TRPV2 is important for the maintenance of cardiac structure and function.

Functional role for Piezo1 in stretch-evoked Ca²⁺ influx and ATP release in urothelial cell cultures

The urothelium is a sensory structure that contributes to mechanosensation in the urinary bladder. Here, we provide evidence for a critical role for the Piezo1 channel, a newly identified mechanosensory molecule, in the mouse bladder urothelium. We performed a systematic analysis of the molecular and functional expression of Piezo1 channels in the urothelium. Immunofluorescence examination demonstrated abundant expression of Piezo1 in the mouse and human urothelium. Urothelial cells isolated from mice exhibited a Piezo1-dependent increase in cytosolic Ca(2+) concentrations in response to mechanical stretch stimuli, leading to potent ATP release; this response was suppressed in Piezo1-knockdown cells. In addition, Piezo1 and TRPV4 distinguished different intensities of mechanical stimulus. Moreover, GsMTx4, an inhibitor of stretch-activated channels, attenuated the Ca(2+) influx into urothelial cells and decreased ATP release from them upon stretch stimulation. These results suggest that Piezo1 senses extension of the bladder urothelium, leading to production of an ATP signal. Thus, inhibition of Piezo1 might provide a promising means of treating bladder dysfunction.

Polycystins and partners: proposed role in mechanosensitivity

Mutations of the two polycystins, PC1 and PC2, lead to polycystic kidney disease. Polycystins are able to form complexes with numerous families of proteins that have been suggested to participate in mechanical sensing. The proposed role of polycystins and their partners in the kidney primary cilium is to sense urine flow. A role for polycystins in mechanosensing has also been shown in other cell types such as vascular smooth muscle cells and cardiac myocytes. At the plasma membrane, polycystins interact with diverse ion channels of the TRP family and with stretch-activated channels (Piezos, TREKs). The actin cytoskeleton and its interacting proteins, such as filamin A, have been shown to be essential for these interactions. Numerous proteins involved in cell-cell and cell-extracellular matrix junctions interact with PC1 and/or PC2. These multimeric protein complexes are important for cell structure integrity, the transmission of force, as well as for mechanosensing and mechanotransduction. A group of polycystin partners are also involved in subcellular trafficking mechanisms. Finally, PC1 and especially PC2 interact with elements of the endoplasmic reticulum and are essential components of calcium homeostasis. In conclusion, we propose that both PC1 and PC2 act as conductors to tune the overall cellular mechanosensitivity.

Photosensitive TRPs

The Drosophila "transient receptor potential" channel is the prototypical TRP channel, belonging to and defining the TRPC subfamily. Together with a second TRPC channel, trp-like (TRPL), TRP mediates the transducer current in the fly's photoreceptors. TRP and TRPL are also implicated in olfaction and Malpighian tubule function. In photoreceptors, TRP and TRPL are localised in the ~30,000 packed microvilli that form the photosensitive "rhabdomere"-a light-guiding rod, housing rhodopsin and the rest of the phototransduction machinery. TRP (but not TRPL) is assembled into multimolecular signalling complexes by a PDZ-domain scaffolding protein (INAD). TRPL (but not TRP) undergoes light-regulated translocation between cell body and rhabdomere. TRP and TRPL are also found in photoreceptor synapses where they may play a role in synaptic transmission. Like other TRPC channels, TRP and TRPL are activated by a G protein-coupled phospholipase C (PLCβ4) cascade. Although still debated, recent evidence indicates the channels can be activated by a combination of PIP2 depletion and protons released by the PLC reaction. PIP2 depletion may act mechanically as membrane area is reduced by cleavage of PIP2's bulky inositol headgroup. TRP, which dominates the light-sensitive current, is Ca(2+) selective (P Ca:P Cs >50:1), whilst TRPL has a modest Ca(2+) permeability (P Ca:P Cs ~5:1). Ca(2+) influx via the channels has profound positive and negative feedback roles, required for the rapid response kinetics, with Ca(2+) rapidly facilitating TRP (but not TRPL) and also inhibiting both channels. In trp mutants, stimulation by light results in rapid depletion of microvillar PIP2 due to lack of Ca(2+) influx required to inhibit PLC. This accounts for the "transient receptor potential" phenotype that gives the family its name and, over a period of days, leads to light-dependent retinal degeneration. Gain-of-function trp mutants with uncontrolled Ca(2+) influx also undergo retinal degeneration due to Ca(2+) cytotoxicity. In vertebrate retina, mice knockout studies suggest that TRPC6 and TRPC7 mediate a PLCβ4-activated transducer current in intrinsically photosensitive retinal ganglion cells, expressing melanopsin. TRPA1 has been implicated as a "photo-sensing" TRP channel in human melanocytes and light-sensitive neurons in the body wall of Drosophila.

Evolutionarily conserved, multitasking TRP channels: lessons from worms and flies

The Transient Receptor Potential (TRP) channel family is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling. TRPs allow animals to detect chemicals, mechanical force, light, and changes in temperature. Consequently, these channels control a plethora of animal behaviors. Moreover, their functions are not limited to the classical senses, as they are cellular sensors, which are critical for ionic homeostasis and metabolism. Two genetically tractable invertebrate model organisms, Caenorhabditis elegans and Drosophila melanogaster, have led the way in revealing a wide array of sensory roles and behaviors that depend on TRP channels. Two overriding themes have emerged from these studies. First, TRPs are multitasking proteins, and second, many functions and modes of activation of these channels are evolutionarily conserved, including some that were formerly thought to be unique to invertebrates, such as phototransduction. Thus, worms and flies offer the potential to decipher roles for mammalian TRPs, which would otherwise not be suspected.

TRPs in mechanosensing and volume regulation

Mechanosensitive channels allow cells to respond to changes in membrane stretch that occur due to external stimuli like pressure or flow or that occur because of osmotically induced cell swelling or shrinkage. Ion fluxes through the channels change the membrane potential and ion concentrations and link the stretch to cellular signalling. Changes in cellular activity evoked by mechanical stimuli can be used to elicit local tissue responses or can be transmitted further to generate more widespread responses. Channels can respond directly to membrane stress, can be conferred mechanosensitive by interaction with structural proteins, or can be activated by mechanosensitive signalling pathways. Because mechanosensitive channels are often nonselective cation channels, and invertebrate TRP isoforms are involved in mechanosensation, many of the mammalian TRP isoforms have been investigated with regard to their mechanosensitivity. There is evidence that members of the TRPC, TRPV, TRPM, TRPA and TRPP subfamilies could be in some way mechanosensitive, and each of the activation mechanisms described above is used by a TRP channel. TRP channels may be involved in mechanosensitive processes ranging from flow and pressure sensing in the vasculature and other organs to mechanosensation in sensory neurones and sensory organs. There is also evidence for a role of mechano- or osmosensitive TRP isoforms in osmosensing and the regulation of cell volume. Often, a number of different TRP isoforms have been implicated in a single type of mechanosensitive response. In many cases, the involvement of the isoforms needs to be confirmed, and their exact role in the signalling process determined.

TRPC6: physiological function and pathophysiological relevance

TRPC6 is a non-selective cation channel 6 times more permeable to Ca(2+) than to Na(+). Channel homotetramers heterologously expressed have a characteristic doubly rectifying current-voltage relationship and are directly activated by the second messenger diacylglycerol (DAG). TRPC6 proteins are also regulated by specific tyrosine or serine phosphorylation and phosphoinositides. Given its specific expression pattern, TRPC6 is likely to play a number of physiological roles which are confirmed by the analysis of a Trpc6 (-/-) mouse model. In smooth muscle Na(+) influx through TRPC6 channels and activation of voltage-gated Ca(2+) channels by membrane depolarisation is the driving force for contraction. Permeability of pulmonary endothelial cells depends on TRPC6 and induces ischaemia-reperfusion oedema formation in the lungs. TRPC6 was also identified as an essential component of the slit diaphragm architecture of kidney podocytes and plays an important role in the protection of neurons after cerebral ischaemia. Other functions especially in immune and blood cells remain elusive. Recently identified TRPC6 blockers may be helpful for therapeutic approaches in diseases with highly activated TRPC6 channel activity.

Smooth muscle cell transient receptor potential polycystin-2 (TRPP2) channels contribute to the myogenic response in cerebral arteries

Intravascular pressure-induced vasoconstriction is a smooth muscle cell-specific mechanism that controls systemic blood pressure and organ regional blood flow. Smooth muscle cell polycystin-1 and -2 (TRPP1 and -2) proteins modulate the myogenic response in mesenteric arteries, but involvement in other vascular beds is unclear. Here, we examined TRPP2 expression, cellular distribution, cation currents (ICat), and physiological functions in smooth muscle cells of rat and human cerebral arteries. We demonstrate that TRPP2 is the major TRPP isoform expressed in cerebral artery smooth muscle cells, with message levels higher than those of TRPP1. Arterial biotinylation and immunofluorescence indicated that TRPP2 is located primarily (∼88%) in the smooth muscle cell plasma membrane. RNA interference reduced TRPP2 expression by ∼55% compared to control, but did not alter levels of TRPP1, TRPC1, TRPC3, TRPC6, TRPM4, ANO1/TMEM16A, or voltage-dependent Ca(2+) (CaV1.2) channels, other ion channel proteins that modulate myogenic tone. Cell swelling induced by hyposmotic (250 osmol (l solution)(-1)) bath solution stimulated Gd(3+)-sensitive ICat in smooth muscle cells that were reduced by selective TRPP2 knockdown. TRPP2 knockdown did not alter myogenic tone at 20 mmHg but reduced tone between ∼28 and 39% over an intravascular pressure range between 40 and 100 mmHg. In contrast, TRPP2 knockdown did not alter depolarization-induced (60 mmol l K(+)) vasoconstriction. In summary, we show that TRPP2 is expressed in smooth muscle cells of resistance-size cerebral arteries, resides primarily in the plasma membrane, and contributes to the myogenic response. Data also suggest that TRPP2 differentially regulates the myogenic response in cerebral and mesenteric arteries.

Novel role of mechanosensitive AT1B receptors in myogenic vasoconstriction

Myogenic vasoconstriction is an inherent property of vascular smooth muscle cells (VSMCs) of resistance arteries harboring ill-defined mechanosensing and mechanotransducing elements. G protein-coupled receptors (GPCRs) are discussed as mechanosensors in VSMCs. In this study, we sought to identify and characterize the role and impact of GPCRs on myogenic vasoconstriction. Thus, we analyzed mRNA expression levels of GPCRs in resistance versus preceding conduit arteries revealing a significant enrichment of several GPCRs in resistance vessels. Selective pharmacological blockade of the highly expressed GPCRs in isolated murine mesenteric arteries ex vivo decreased myogenic vasoconstriction. In particular, candesartan and losartan most prominently suppressed myogenic tone, suggesting that AT1 receptors play a central role in myogenic vasoconstriction. Analyzing angiotensinogen(-/-) mice, a relevant contribution of locally produced angiotensin II to myogenic tone could be excluded. Investigation of AT1A (-/-) and AT1B (-/-) murine mesenteric arteries revealed that AT1B, but not AT1A, receptors are key components of myogenic regulation. This notion was supported by examining fura-2-loaded isolated AT1A (-/-) and AT1B (-/-) VSMCs. Our results indicate that in VSMCs, AT1B receptors are more mechanosensitive than AT1A receptors even at comparable receptor expression levels. Furthermore, we demonstrate that the mechanosensitivity of GPCRs is agonist-independent and positively correlates with receptor expression levels.

The ion channels to cytoskeleton connection as potential mechanism of mechanosensitivity

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

Renal microvascular dysfunction, hypertension and CKD progression

PURPOSE OF REVIEW

Despite apparent blood pressure (BP) control and renin-angiotensin system (RAS) blockade, the chronic kidney disease (CKD) outcomes have been suboptimal. Accordingly, this review is addressed to renal microvascular and autoregulatory impairments that underlie the enhanced dynamic glomerular BP transmission in CKD progression.

RECENT FINDINGS

Clinical data suggest that failure to achieve adequate 24-h BP control is likely contributing to the suboptimal outcomes in CKD. Whereas evidence continues to accumulate regarding the importance of preglomerular autoregulatory impairment to the dynamic glomerular BP transmission, emerging data indicate that nitric oxide-mediated efferent vasodilation may play an important role in mitigating the consequences of glomerular hypertension. By contrast, the vasoconstrictor effects of angiotensin II are expected to potentially reduce glomerular barotrauma and possibly enhance ischemic injury. When adequate BP measurement methods are used, the evidence for BP-independent injury initiating mechanisms is considerably weaker and the renoprotection by RAS blockade largely parallels its antihypertensive effectiveness.

SUMMARY

Adequate 24-h BP control presently offers the most feasible intervention for reducing glomerular BP transmission and improving suboptimal outcomes in CKD. Investigations addressed to improving myogenic autoregulation and/or enhancing nitric oxide-mediated efferent dilation in addition to the more downstream mediators may provide additional future therapeutic targets.

Mechanism of stretch-induced activation of the mechanotransducer zyxin in vascular cells

Vascular cells respond to supraphysiological amounts of stretch with a characteristic phenotypic change that results in dysfunctional remodeling of the affected arteries. Although the pathophysiological consequences of stretch-induced signaling are well characterized, the mechanism of mechanotransduction is unclear. We focused on the mechanotransducer zyxin, which translocates to the nucleus to drive gene expression in response to stretch. In cultured human endothelial cells and perfused femoral arteries isolated from wild-type and several knockout mouse strains, we characterized a multistep signaling pathway whereby stretch led to a transient receptor potential channel 3-mediated release of the endothelial vasoconstrictor peptide endothelin-1 (ET-1). ET-1, through autocrine activation of its B-type receptor, elicited the release of pro-atrial natriuretic peptide (ANP), which caused the autocrine activation of the ANP receptor guanylyl cyclase A (GC-A). Activation of GC-A, in turn, led to protein kinase G-mediated phosphorylation of zyxin at serine 142, thereby triggering the translocation of zyxin to the nucleus, where it was required for stretch-induced gene expression. Thus, we have identified a stretch-induced signaling pathway in vascular cells that leads to the activation of zyxin, a cytoskeletal protein specifically involved in transducing mechanical stimuli.

Nitric oxide: orchestrator of endothelium-dependent responses

The present review first summarizes the complex chain of events, in endothelial and vascular smooth muscle cells, that leads to endothelium-dependent relaxations (vasodilatations) due to the generation of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS) and how therapeutic interventions may improve the bioavailability of NO and thus prevent/cure endothelial dysfunction. Then, the role of other endothelium-derived mediators (endothelium-derived hyperpolarizing (EDHF) and contracting (EDCF) factors, endothelin-1) and signals (myoendothelial coupling) is summarized also, with special emphasis on their interaction(s) with the NO pathway, which make the latter not only a major mediator but also a key regulator of endothelium-dependent responses.

Primary cilium-dependent sensing of urinary flow and paracrine purinergic signaling

During the last 10 years or so, the renal research community has set the primary cilium into the lime light. From being viewed as a possible evolutionary rudiment, today the primary cilium has achieved the noble status of a physiologically relevant and necessary cellular structure. Its prime function in renal epithelium appears to be its ability to sense urinary flow. Much is still lacking to understand how the primary cilium senses flow. Transducer proteins, such as specific mechano-sensory ion channels, have been identified and are necessary for flow-dependent increases of epithelial [Ca(2+)](i). Other ciliary receptor proteins have been suggested, which may open the field of primary cilia sensing to become an even more dynamic topic of research. A flow-induced increase of [Ca(2+)](i) has been observed in all renal and other ciliated epithelial cells. Work over the last 5 years has addressed the mechanism underlying the flow-induced increase of [Ca(2+)](i). It has become apparent that an initial Ca(2+) influx triggers a global increase of epithelial [Ca(2+)](i). Eventually, it also became clear that mechanical stimulation of the epithelial cells triggers the release of ATP. Intriguingly, ATP is an auto- and paracrine signaling molecule that regulates electrolyte and water transport in the nephron by binding to apical and basolateral purinergic receptors. ATP inhibits transport at almost all sites from the proximal to the distal tubule and thus elicits a diuretic response. In the perspective of this review, the primary cilium is a sensory structure and the adequate stimulus is the mechanical deflection. The output signal is the released ATP, a paracrine factor that ultimately modulates the main function of the kidney, i.e. the enormous task of absorbing some 180 L of filtrate every day.

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

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

TRPC6: an underlying target for human glaucoma

Glaucoma is one of the leading causes of visual impairment and blindness worldwide. Of known risk factors for glaucoma, an increased in intraocular pressure is most highly correlated with glaucomatous damage. Irrespective of the cause, apoptosis of the retinal ganglion cells is the eventual outcome. It is widely accepted that glaucoma is a neurodegenerative disease that is strongly correlated with central nervous system disorders, such as Alzheimer's disease. These two disorders also share some similarities in pathogenic mechanisms. Recent studies suggest that the transient receptor potential canonical 6 channel could work together with brain-derived neurotrophic factor to promote neuron survival in brain and retina. In this study, we propose that transient receptor potential canonical 6 may contribute to the pathogenesis of human glaucoma and become a potential therapeutic target.

On the role of the difference in surface tensions involved in the allosteric regulation of NHE-1 induced by low to mild osmotic pressure, membrane tension and lipid asymmetry

The sodium-proton exchanger 1 (NHE-1) is a membrane transporter that exchanges Na(+) for H(+) ion across the membrane of eukaryotic cells. It is cooperatively activated by intracellular protons, and this allosteric regulation is modulated by the biophysical properties of the plasma membrane and related lipid environment. Consequently, NHE-1 is a mechanosensitive transporter that responds to osmotic pressure, and changes in membrane composition. The purpose of this study was to develop the relationship between membrane surface tension, and the allosteric balance of a mechanosensitive transporter such as NHE-1. In eukaryotes, the asymmetric composition of membrane leaflets results in a difference in surface tensions that is involved in the creation of a reservoir of intracellular vesicles and membrane buds contributing to buffer mechanical constraints. Therefore, we took this phenomenon into account in this study and developed a set of relations between the mean surface tension, membrane asymmetry, fluid phase endocytosis and the allosteric equilibrium constant of the transporter. We then used the experimental data published on the effects of osmotic pressure and membrane modification on the NHE-1 allosteric constant to fit these equations. We show here that NHE-1 mechanosensitivity is more based on its high sensitivity towards the asymmetry between the bilayer leaflets compared to mean global membrane tension. This compliance to membrane asymmetry is physiologically relevant as with their slower transport rates than ion channels, transporters cannot respond as high pressure-high conductance fast-gating emergency valves.

Emerging role of G protein-coupled receptors in microvascular myogenic tone

Blood flow autoregulation results from the ability of resistance arteries to reduce or increase their diameters in response to changes in intravascular pressure. The mechanism by which arteries maintain a constant blood flow to organs over a range of pressures relies on this myogenic response, which defines the intrinsic property of the smooth muscle to contract in response to stretch. The resistance to flow created by myogenic tone (MT) prevents tissue damage and allows the maintenance of a constant perfusion, despite fluctuations in arterial pressure. Interventions targeting MT may provide a more rational therapeutic approach in vascular disorders, such as hypertension, vasospasm, chronic heart failure, or diabetes. Despite its early description by Bayliss in 1902, the cellular and molecular mechanisms underlying MT remain poorly understood. We now appreciate that MT requires a complex mechanotransduction converting a physical stimulus (pressure) into a biological response (change in vessel diameter). Although smooth muscle cell depolarization and a rise in intracellular calcium concentration are recognized as cornerstones of the myogenic response, the role of wall strain-induced formation of vasoactive mediators is less well established. The vascular system expresses a large variety of Class 1 G protein-coupled receptors (GPCR) activated by an eclectic range of chemical entities, including peptides, lipids, nucleotides, and amines. These messengers can function in blood vessels as vasoconstrictors. This review focuses on locally generated GPCR agonists and their proposed contributions to MT. Their interplay with pivotal G(q-11) and G(12-13) protein signalling is also discussed.

Sensing pressure with ion channels

Opening of stretch-activated ion channels (SACs) is the earliest event occurring in mechanosensory transduction. The molecular identity of mammalian SACs has long remained a mystery. Only very recently, Piezo1 and Piezo2 have been shown to be essential components of distinct SACs and moreover, purified Piezo1 forms cationic channels when reconstituted into artificial bilayers. In line with these findings, dPiezo was demonstrated to act in the Drosophila mechanical nociception pathway. Finally, the 3D structure of the two-pore domain potassium channel (K(2P)), TRAAK [weakly inward rectifying K⁺ channel (TWIK)-related arachidonic acid stimulated K⁺ channel], has recently been solved, providing valuable information about pharmacology, selectivity and gating mechanisms of stretch-activated K⁺ channels (SAKs). These recent findings allow a better understanding of the molecular basis of molecular and cellular mechanotransduction.

Mechanical stimulation of epithelial cells using polypyrrole microactuators

The importance of mechanotransduction for physiological systems is becoming increasingly recognized. The effect of mechanical stimulation is well studied in organs and tissues, for instance by using flexible tissue culture substrates that can be stretched by external means. However, on the cellular and subcellular level, dedicated technology to apply appropriate mechanical stimuli is limited. Here we report an organic electronic microactuator chip for mechanical stimulation of single cells. These chips are manufactured on silicon wafers using traditional microfabrication and photolithography techniques. The active unit of the chip consists of the electroactive polymer polypyrrole that expands upon the application of a low potential. The fact that polypyrrole can be activated in physiological electrolytes makes it well suited as the active material in a microactuator chip for biomedical applications. Renal epithelial cells, which are responsive to mechanical stimuli and relevant from a physiological perspective, are cultured on top of the microactuator chip. The cells exhibit good adhesion and spread along the surface of the chip. After culturing, individual cells are mechanically stimulated by electrical addressing of the microactuator chip and the response to this stimulation is monitored as an increase in intracellular Ca(2+). This Ca(2+) response is caused by an autocrine ATP signalling pathway associated with mechanical stimulation of the cells. In conclusion, the present work demonstrates a microactuator chip based on an organic conjugated polymer, for mechanical stimulation of biological systems at the cellular and sub-cellular level.