TRPC1 forms the stretch-activated cation channel in vertebrate cells

TRPC1-mediated Ca2+ and Na+ signalling in astroglia: differential filtering of extracellular cations

Canonical transient receptor potential 1 (TRPC1) plasmalemmal cation channels mediate Ca2+ and Na+ fluxes and control respective cytoplasmic ion signals in rat cortical astrocytes. Mechanical stimulation of astrocytes results in an increase in the levels of cytosolic Ca2+ and Na+ that are in part due to entry of extracellular cations through TRPC1 containing channels. Inhibition of the TRPC1 pore with an antibody against its selective filter reduced cytosolic Ca2+ accumulation caused by mechanical stimulation. In contrast, this immunological treatment increased the cytosolic Na+ peak accumulation induced by mechanical stimulation. We propose that TRPC channels are amenable to changes in selective filtering, as mutations in previous studies and antibody binding in our present study differentially affect the flux of Ca2) and Na+. TRPC1 containing channels might represent focal points for co-ordination of Ca2+ and Na+ signalling in astroglia and this can have consequences on Ca(2+)- and Na(+)-dependent processes such as regulated exocytosis and lactate production, respectively, which in turn can modulate neuronal synaptic transmission.

Cardiac and respiratory dysfunction in Duchenne muscular dystrophy and the role of second messengers

Duchenne muscular dystrophy (DMD) affects young boys and is characterized by the absence of dystrophin, a large cytoskeletal protein present in skeletal and cardiac muscle cells and neurons. The heart and diaphragm become necrotic in DMD patients and animal models of DMD, resulting in cardiorespiratory failure as the leading cause of death. The major consequences of the absence of dystrophin are high levels of intracellular Ca(2+) and the unbalanced production of NO that can finally trigger protein degradation and cell death. Cytoplasmic increase in Ca(2+) concentration directly and indirectly triggers different processes such as necrosis, fibrosis, and activation of macrophages. The absence of the neuronal isoform of nitric oxide synthase (nNOS) and the overproduction of NO by the inducible isoform (iNOS) further increase the intracellular Ca(2+) via a hypernitrosylation of the ryanodine receptor. NO overproduction, which further induces the expression of iNOS but decreases the expression of the endothelial isoform (eNOS), deregulates the muscle tissue blood flow creating an ischemic situation. The high levels of Ca(2+) in dystrophic muscles and the ischemic state of the muscle tissue would culminate in a positive feedback loop. While efforts continue toward optimizing cardiac and respiratory care of DMD patients, both Ca(2+) and NO in cardiac and respiratory muscle pathways have been shown to be important to the etiology of the disease. Understanding the mechanisms behind the fine regulation of Ca(2+) -NO may be important for a noninterventional and noninvasive supportive approach to treat DMD patients, improving the quality of life and natural history of DMD patients.

Microfluidics study of intracellular calcium response to mechanical stimulation on single suspension cells

A microfluidic microdevice was developed to exert mechanical stimulation on an individual suspension cell for mechanosensation research. In this microfluidic chip, an individual cell was isolated from a population of cells, and trapped in a microchannel with a compressive component made of a deflectable membrane. The mechanosensation of HL60 cells (leukemic cells) was studied using this chip, and the results showed that mechanical stimulations could trigger extracellular calcium to flow into HL60 cells through ion channels on cell membranes. The tension on individual HL60 cells exerted by the microdevice was showed large variations in the threshold of mechanosensation activation. In contrast to previous reports using patch clamp technique, there was little influence of cytoskeleton interruption on HL60 cell mechanosensation triggered by whole-cell compression. Additionally, two functional units were integrated in one chip for carrying out mechanosensation study in parallel, where HL60 cells (leukemic cells) and Jurkat cells (lymphocytes) were shown to respond to mechanical stimulation with different kinetics. The results demonstrated that the microfluidic device provides a novel approach to investigating the mechanosensation of single suspension cells in high-throughput.

Mechanosensitive TRPC1 channels promote calpain proteolysis of talin to regulate spinal axon outgrowth

Intracellular Ca(2+) signals control the development and regeneration of spinal axons downstream of chemical guidance cues, but little is known about the roles of mechanical cues in axon guidance. Here we show that transient receptor potential canonical 1 (TRPC1) subunits assemble mechanosensitive (MS) channels on Xenopus neuronal growth cones that regulate the extension and direction of axon outgrowth on rigid, but not compliant, substrata. Reducing expression of TRPC1 by antisense morpholinos inhibits the effects of MS channel blockers on axon outgrowth and local Ca(2+) transients. Ca(2+) influx through MS TRPC1 activates the protease calpain, which cleaves the integrin adaptor protein talin to reduce Src-dependent axon outgrowth, likely through altered adhesion turnover. We found that talin accumulates at the tips of dynamic filopodia, which is lost upon cleavage of talin by active calpain. This pathway may also be important in axon guidance decisions since asymmetric inhibition of MS TRPC1 is sufficient to induce growth cone turning. Together our results suggest that Ca(2+) influx through MS TRPC1 on filopodia activates calpain to control growth cone turning during development.

Organellar mechanosensitive channels in fission yeast regulate the hypo-osmotic shock response

A key molecule of sensing machineries essential for survival upon hypo-osmotic shock is the mechanosensitive channel. The bacterial mechanosensitive channel MscS functions directly for this purpose by releasing cytoplasmic solutes out of the cell, whereas plant MscS homologues are found to function in chloroplast organization. Here we show that the fission yeast MscS homologues, designated Msy1 and Msy2, participate in the hypo-osmotic shock response by a mechanism different from that operated by the bacterial MscS. Upon hypo-osmotic shock, msy2(-) and msy1(-) msy2(-) cells display greater cell swelling than wild-type cells and undergo cell death. Cell swelling precedes an intracellular Ca(2+) increase, which was greater in msy1(-) and msy1(-) msy2(-) cells than in wild-type cells. Fluorescent microscopy showed that Msy1 and Msy2 localize mainly to the endoplasmic reticulum. These observations suggest that organellar Msy1 and Msy2 regulate intracellular Ca(2+) and cell volume for survival upon hypo-osmotic shock.

Mechanobiology in cardiac physiology and diseases

Mechanosensitivity is essential for heart function just as for all other cells and organs in the body, and it is involved in both normal physiology and diseases processes of the cardiovascular system. In this review, we have outlined the relationship between mechanosensitivity and heart physiology, including the Frank-Starling law of the heart and mechanoelectric feedback. We then focused on molecules involved in mechanotransduction, particularly mechanosensitive ion channels. We have also discussed the involvement of mechanosensitivity in heart diseases, such as arrhythmias, hypertrophy and ischaemic heart disease. Finally, mechanobiology in cardiogenesis is described with regard to regenerative medicine.

Calcium-dependent deceleration of the cell cycle in muscle cells by simulated microgravity

Of all our mechanosensitive tissues, skeletal muscle is the most developmentally responsive to physical activity. Conversely, restricted mobility due to injury or disease results in muscle atrophy. Gravitational force is another form of mechanical input with profound developmental consequences. The mechanical unloading resulting from the reduced gravitational force experienced during spaceflight results in oxidative muscle loss. We examined the early stages of myogenesis under conditions of simulated microgravity (SM). C2C12 mouse myoblasts in SM proliferated more slowly (2.23× less) as a result of their being retained longer within the G2/M phase of the cell cycle (2.10× more) relative to control myoblasts at terrestrial gravity. Blocking calcium entry via TRP channels with SKF-96365 (10-20 μM) accumulated myoblasts within the G2/M phase of the cell cycle and retarded their proliferation. On the genetic level, SM resulted in the reduced expression of TRPC1 and IGF-1 isoforms, transcriptional events regulated by calcium downstream of mechanical input. A decrease in TRPC1-mediated calcium entry thus appears to be a pivotal event in the muscle atrophy brought on by gravitational mechanical unloading. Hence, relieving the constant force of gravity on cells might prove one valid experimental approach to expose the underlying mechanisms modulating mechanically regulated developmental programs.

The mechanobiology of brain function

All cells are influenced by mechanical forces. In the brain, force-generating and load-bearing proteins twist, turn, ratchet, flex, compress, expand and bend to mediate neuronal signalling and plasticity. Although the functions of mechanosensitive proteins have been thoroughly described in classical sensory systems, the effects of endogenous mechanical energy on cellular function in the brain have received less attention, and many working models in neuroscience do not currently integrate principles of cellular mechanics. An understanding of cellular-mechanical concepts is essential to allow the integration of mechanobiology into ongoing studies of brain structure and function.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

TRP channels coordinate ion signalling in astroglia

Astroglial excitability is based on highly spatio-temporally coordinated fluctuations of intracellular ion concentrations, among which changes in Ca(2+) and Na(+) take the leading role. Intracellular signals mediated by Ca(2+) and Na(+) target numerous molecular cascades that control gene expression, energy production and numerous homeostatic functions of astrocytes. Initiation of Ca(2+) and Na(+) signals relies upon plasmalemmal and intracellular channels that allow fluxes of respective ions down their concentration gradients. Astrocytes express several types of TRP channels of which TRPA1 channels are linked to regulation of functional expression of GABA transporters, whereas TRPV4 channels are activated following osmotic challenges and are up-regulated in ischaemic conditions. Astrocytes also ubiquitously express several isoforms of TRPC channels of which heteromers assembled from TRPC1, 4 and/or 5 subunits that likely act as stretch-activated channels and are linked to store-operated Ca(2+) entry. The TRPC channels mediate large Na(+) fluxes that are associated with the endoplasmic reticulum Ca(2+) signalling machinery and hence coordinate Na(+) and Ca(2+) signalling in astroglia.

Molecular basis of physiological heart growth: fundamental concepts and new players

The heart hypertrophies in response to developmental signals as well as increased workload. Although adult-onset hypertrophy can ultimately lead to disease, cardiac hypertrophy is not necessarily maladaptive and can even be beneficial. Progress has been made in our understanding of the structural and molecular characteristics of physiological cardiac hypertrophy, as well as of the endocrine effectors and associated signalling pathways that regulate it. Physiological hypertrophy is initiated by finite signals, which include growth hormones (such as thyroid hormone, insulin, insulin-like growth factor 1 and vascular endothelial growth factor) and mechanical forces that converge on a limited number of intracellular signalling pathways (such as PI3K, AKT, AMP-activated protein kinase and mTOR) to affect gene transcription, protein translation and metabolism. Harnessing adaptive signalling mediators to reinvigorate the diseased heart could have important medical ramifications.

Cell mechanosensitivity: mechanical properties and interaction with gravitational field

This paper addressed the possible mechanisms of primary reception of a mechanical stimulus by different cells. Data concerning the stiffness of muscle and nonmuscle cells as measured by atomic force microscopy are provided. The changes in the mechanical properties of cells that occur under changed external mechanical tension are presented, and the initial stages of mechanical signal transduction are considered. The possible mechanism of perception of different external mechanical signals by cells is suggested.

Cardiac phenotype of Duchenne Muscular Dystrophy: insights from cellular studies

Dilated cardiomyopathy is a serious and almost inevitable complication of Duchenne Muscular Dystrophy, a devastating and fatal disease of skeletal muscle resulting from the lack of functional dystrophin, a protein linking the cytoskeleton to the extracellular matrix. Ultimately, it leads to congestive heart failure and arrhythmias resulting from both cardiac muscle fibrosis and impaired function of the remaining cardiomyocytes. Here we summarize findings obtained in several laboratories, focusing on cellular mechanisms that result in degradation of cardiac functions in dystrophy.

The Anrep effect: 100 years later

Myocardial stretch elicits a rapid increase in developed force, which is mainly caused by an increase in myofilament calcium sensitivity (Frank-Starling mechanism). Over the ensuing 10-15 min, a second gradual increase in force takes place. This slow force response to stretch is known to be the result of an increase in the calcium transient amplitude and constitutes the in vitro equivalent of the Anrep effect described 100 years ago in the intact heart. In the present review, we will update and discuss what is known about the Anrep effect as the mechanical counterpart of autocrine/paracrine mechanisms involved in its genesis. The chain of events triggered by myocardial stretch comprises 1) release of angiotensin II, 2) release of endothelin, 3) activation of the mineralocorticoid receptor, 4) transactivation of the epidermal growth factor receptor, 5) increased formation of mitochondria reactive oxygen species, 6) activation of redox-sensitive kinases upstream myocardial Na(+)/H(+) exchanger (NHE1), 7) NHE1 activation, 8) increase in intracellular Na(+) concentration, and 9) increase in Ca(2+) transient amplitude through the Na(+)/Ca(2+) exchanger. We will present the experimental evidence supporting each of the signaling steps leading to the Anrep effect and its blunting by silencing NHE1 expression with a specific small hairpin interference RNA injected into the ventricular wall.

Quantitative analysis of TRP channel genes in mouse organs

The transient receptor potential (TRP) channel superfamily is a set of channel genes that mediate numerous physiological functions such as sensing irritants or detecting temperature changes. Despite their functions, expressional information on TRP channels in various organs is largely elusive. Therefore, we conducted a systematic quantitative comparison of each mRNA expression level of 22 mouse TRP channels in various organs. As a result, we found that average levels of TRP channel transcripts were very low reaching ∼3% of the GAPDH transcript level. Among 22 TRP channels, TRPC1 and TRPM7 were most abundant in the majority of organs. In contrast, TRPV3, TRPV5, TRPV6, TRPC7, TRPM1, and TRPM5 elicited very low message profiles throughout the major organs. Consistent with their functions as molecular sensors for irritants and temperature changes, TRPV1, TRPM8 and TRPA1 showed exclusive expression in sensory ganglia. TRPC3 and TRPM3 were abundant in the sensory ganglia and brain. High levels of transcripts of TRPV2, TRPC6, TRPM4, and TRPM6 were observed in the lung. In addition, channel transcript levels were very low except TRPM7 in the liver. In summary, the expression profile of TRP channels in major tissues provides insight to their physiological functions and therefore application to new drug development.

The functions of TRPA1 and TRPV1: moving away from sensory nerves

The transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) channels are members of the TRP superfamily of structurally related, non-selective cation channels. It is rapidly becoming clear that the functions of TRPV1 and TRPA1 interlink with each other to a considerable extent. This is especially clear in relation to pain and neurogenic inflammation where TRPV1 is coexpressed on the vast majority of TRPA1-expressing sensory nerves and both integrate a variety of noxious stimuli. The more recent discovery that both TRPV1 and TRPA1 are expressed on a multitude of non-neuronal sites has led to a plethora of research into possible functions of these receptors. Non-neuronal cells on which TRPV1 and TRPA1 are expressed vary from vascular smooth muscle to keratinocytes and endothelium. This review will discuss the expression, functionality and roles of these non-neuronal TRP channels away from sensory nerves to demonstrate the diverse nature of TRPV1 and TRPA1 in addition to a direct role in pain and neurogenic inflammation.

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.

Mechano-sensitivity of cardiac pacemaker function: pathophysiological relevance, experimental implications, and conceptual integration with other mechanisms of rhythmicity

Cardiac pacemaker cells exhibit spontaneous, rhythmic electrical excitation, termed automaticity. This automatic initiation of action potentials requires spontaneous diastolic depolarisation, whose rate determines normal rhythm generation in the heart. Pacemaker mechanisms have been split recently into: (i) cyclic changes in trans-sarcolemmal ion flows (termed the ‘membrane-clock’), and (ii) rhythmic intracellular calcium cycling (the ‘calcium-clock’). These two ‘clocks’ undoubtedly interact, as trans-sarcolemmal currents involved in pacemaking include calcium-carrying mechanisms, while intracellular calcium cycling requires trans-sarcolemmal ion flux as the mechanism by which it affects membrane potential. The split into separate ‘clocks’ is, therefore, somewhat arbitrary. Nonetheless, the ‘clock’ metaphor has been conceptually stimulating, in particular since there is evidence to support the view that either ‘clock’ could be sufficient in principle to set the rate of pacemaker activation.

Of course, the same has also been shown for sub-sets of ‘membrane-clock’ ion currents, illustrating the redundancy of mechanisms involved in maintaining such basic functionality as the heartbeat, a theme that is common for vital physiological systems.

Following the conceptual path of identifying individual groups of sub-mechanisms, it is important to remember that the heart is able to adapt pacemaker rate to changes in haemodynamic load, even after isolation or transplantation, and on a beat-by-beat basis. Neither the ‘membrane-’ nor the ‘calcium-clock’ do, as such, inherently account for this rapid adaptation to circulatory demand (cellular Ca²⁺ balance changes over multiple beats, while variation of sarcolemmal ion channel presence takes even longer). This suggests that a third set of mechanisms must be involved in setting the pace. These mechanisms are characterised by their sensitivity to the cyclically changing mechanical environment, and – in analogy to the above terminology – this might be considered a ‘mechanics-clock’.

In this review, we discuss possible roles of mechano-sensitive mechanisms for the entrainment of membrane current dynamics and calcium-handling. This can occur directly via stretch-activation of mechano-sensitive ion channels in the sarcolemma and/or in intracellular membrane compartments, as well as by modulation of ‘standard’ components of the ‘membrane-’ or ‘calcium-clock’. Together, these mechanisms allow rapid adaptation to changes in haemodynamic load, on a beat-by-beat basis.

Additional relevance arises from the fact that mechano-sensitivity of pacemaking may help to explain pacemaker dysfunction in mechanically over- or under-loaded tissue. As the combined contributions of the various underlying oscillatory mechanisms are integrated at the pacemaker cell level into a single output – a train of pacemaker action potentials – we will not adhere to a metaphor that implies separate time-keeping units (‘clocks’), and rather focus on cardiac pacemaking as the result of interactions of a set of coupled oscillators, whose individual contributions vary depending on the pathophysiological context. We conclude by considering the utility and limitations of viewing the pacemaker as a coupled system of voltage-, calcium-, and mechanics-modulated oscillators that, by integrating a multitude of inputs, offers the high level of functional redundancy that is vitally important for cardiac automaticity.

Piezo1: properties of a cation selective mechanical channel

Piezo ion channels have been found to be essential for mechanical responses in cells. These channels were first shown to exist in Neuro2A cells, and the gene was identified by siRNAs that diminished the mechanical response. Piezo channels are approximately 2500 amino acids long, have between 24-32 transmembrane regions, and appear to assemble into tetramers and require no other proteins for activity. They have a reversal potential around 0 mV and show voltage dependent inactivation. The channel is constitutively active in liposomes, indicating that no cytoskeletal elements are required. Heterologous expression of the Piezo protein can create mechanical sensitivity in otherwise insensitive cells.   Piezo1 currents in outside-out patches were blocked by the extracellular MSC inhibitor peptide GsMTx4. Both enantiomeric forms of GsMTx4 inhibited channel activity in a manner similar to endogenous mechanical channels. Piezo1 can adopt a tonic (non-inactivating) form with repeated stimulation. The transition to the non-inactivating form generally occurs in large groups of channels, indicating that the channels exist in domains, and once the domain is compromised, the members simultaneously adopt new properties. Piezo proteins are associated with physiological responses in cells, such as the reaction to noxious stimulus of Drosophila larvae. Recent work measuring cell crowding, shows that Piezo1 is essential for the removal of extra cells without apoptosis. Piezo1 mutations have also been linked to the pathological response of red blood cells in a genetic disease called Xerocytosis. These finding suggest that Piezo1 is a key player in cells' responses to mechanical stimuli.

Evidence TRPV4 contributes to mechanosensitive ion channels in mouse skeletal muscle fibers

We recorded the activity of single mechanosensitive (MS) ion channels from membrane patches on single muscle fibers isolated from mice. We investigated the actions of various TRP (transient receptor potential) channel blockers on MS channel activity. 2-aminoethoxydiphenyl borate (2-APB) neither inhibited nor facilitated single channel activity at submillimolar concentrations. The absence of an effect of 2-APB indicates MS channels are not composed purely of TRPC or TRPV1, 2 or 3 proteins. Exposing patches to 1-oleolyl-2-acetyl-sn-glycerol (OAG), a potent activator of TRPC channels, also had no effect on MS channel activity. In addition, flufenamic acid and spermidine had no effect on the activity of single MS channels. By contrast, SKF-96365 and ruthenium red blocked single-channel currents at micromolar concentrations. SKF-96365 produced a rapid block of the open channel current. The blocking rate depended linearly on blocker concentration, while the unblocking rate was independent of concentration, consistent with a simple model of open channel block. A fit to the concentration-dependence of block gave k(on) = 13 x 10 ( 6) M (-1) s (-1) and k(off) = 1609 sec (-1) with K(D) = ~124 µM. Block by ruthenium red was complex, involving both reduction of the amplitude of the single-channel current and increased occupancy of subconductance levels. The reduction in current amplitude with increasing concentration of ruthenium red gave a K(D) = ~49 µM. The high sensitivity of MS channels to block by ruthenium red suggests MS channels in skeletal muscle contain TRPV subunits. Recordings from skeletal muscle isolated from TRPV4 knockout mice failed to show MS channel activity, consistent with a contribution of TRPV4. In addition, exposure to hypo-osmotic solutions increases opening of MS channels in muscle. Our results provide evidence TRPV4 contributes to MS channels in skeletal muscle.

Mechanosensitive Ca(2+) permeant cation channels in human prostate tumor cells

The acquisition of cell motility plays a critical role in the spread of prostate cancer (PC), therefore, identifying a sensitive step that regulates PC cell migration should provide a promising target to block PC metastasis. Here, we report that a mechanosensitive Ca(2+)-permeable cation channel (MscCa) is expressed in the highly migratory/invasive human PC cell line, PC-3 and that inhibition of MscCa by Gd(3+) or GsMTx-4 blocks PC-3 cell migration and associated elevations in [Ca(2+)](i). Genetic suppression or overexpression of specific members of the canonical transient receptor potential Ca(2+) channel family (TRPC1 and TRPC3) also inhibit PC-3 cell migration, but they do so by mechanisms other that altering MscCa activity. Although LNCaP cells are nonmigratory, they also express relatively large MscCa currents, indicating that MscCa expression alone cannot confer motility on PC cells. MscCa in both cell lines show similar conductance and ion selectivity and both are functionally coupled via Ca(2+) influx to a small Ca(2+)-activated K(+) channel. However, MscCa in PC-3 and LNCaP cell patches show markedly different gating dynamics--while PC-3 cells typically express a sustained, non-inactivating MscCa current, LNCaP cells express a mechanically-fragile, rapidly inactivating MscCa current. Moreover, mechanical forces applied to the patch, can induce an irreversible transition from the transient to the sustained MscCa gating mode. Given that cancer cells experience increasing compressive and shear forces within a growing tumor, a similar shift in channel gating in situ would have significant effects on Ca(2+) signaling that may play a role in tumor progression.

On the role of TRPC1 in control of Ca2+ influx, cell volume, and cell cycle

Ca(+) signaling plays a crucial role in control of cell cycle progression, but the understanding of the dynamics of Ca(2+) influx and release of Ca(2+) from intracellular stores during the cell cycle is far from complete. The aim of the present study was to investigate the role of the free extracellular Ca(2+) concentration ([Ca(2+)](o)) in cell proliferation, the pattern of changes in the free intracellular Ca(2+) concentration ([Ca(2+)](i)) during cell cycle progression, and the role of the transient receptor potential (TRP)C1 in these changes as well as in cell cycle progression and cell volume regulation. In Ehrlich Lettré Ascites (ELA) cells, [Ca(2+)](i) decreased significantly, and the thapsigargin-releasable Ca(2+) pool in the intracellular stores increased in G(1) as compared with G(0). Store-depletion-operated Ca(2+) entry (SOCE) and TRPC1 protein expression level were both higher in G(1) than in G(0) and S phase, in parallel with a more effective volume regulation after swelling [regulatory volume decrease (RVD)] in G(1) as compared with S phase. Furthermore, reduction of [Ca(2+)](o), as well as two unspecific SOCE inhibitors, 2-APB (2-aminoethyldiphenyl borinate) and SKF96365 (1-(β-[3-(4-methoxy-phenyl)propoxyl-4-methoxyphenethyl)1H-imidazole-hydrochloride), inhibited ELA cell proliferation. Finally, Madin-Darby canine kidney cells in which TRPC1 was stably silenced [TRPC1 knockdown (TRPC1-KD) MDCK] exhibited reduced SOCE, slower RVD, and reduced cell proliferation compared with mock controls. In conclusion, in ELA cells, SOCE and TRPC1 both seem to be upregulated in G(1) as compared with S phase, concomitant with an increased rate of RVD. Furthermore, TRPC1-KD MDCK cells exhibit decreased SOCE, decreased RVD, and decreased proliferation, suggesting that, at least in certain cell types, TRPC1 is regulated during cell cycle progression and is involved in SOCE, RVD, and cell proliferation.

Fluid flows and forces in development: functions, features and biophysical principles

Throughout morphogenesis, cells experience intracellular tensile and contractile forces on microscopic scales. Cells also experience extracellular forces, such as static forces mediated by the extracellular matrix and forces resulting from microscopic fluid flow. Although the biological ramifications of static forces have received much attention, little is known about the roles of fluid flows and forces during embryogenesis. Here, we focus on the microfluidic forces generated by cilia-driven fluid flow and heart-driven hemodynamics, as well as on the signaling pathways involved in flow sensing. We discuss recent studies that describe the functions and the biomechanical features of these fluid flows. These insights suggest that biological flow determines many aspects of cell behavior and identity through a specific set of physical stimuli and signaling pathways.

Pathways of Ca²⁺ entry and cytoskeletal damage following eccentric contractions in mouse skeletal muscle

Muscles that are stretched during contraction (eccentric contractions) show deficits in force production and a variety of structural changes, including loss of antibody staining of cytoskeletal proteins. Extracellular Ca(2+) entry and activation of calpains have been proposed as mechanisms involved in these changes. The present study used isolated mouse extensor digitorum longus (EDL) muscles subjected to 10 eccentric contractions and monitored force production, immunostaining of cytoskeletal proteins, and resting stiffness. Possible pathways for Ca(2+) entry were tested with streptomycin (200 μM), a blocker of stretch-activated channels, and with muscles from mice deficient in the transient receptor potential canonical 1 gene (TRPC1 KO), a candidate gene for stretch-activated channels. At 30 min after the eccentric contractions, the isometric force was decreased to 75 ± 3% of initial control and this force loss was reduced by streptomycin but not in the TRPC1 KO. Desmin, titin, and dystrophin all showed patchy loss of immunostaining 30 min after the eccentric contractions, which was substantially reduced by streptomycin and in the TRPC1 KO muscles. Muscles showed a reduction of resting stiffness following eccentric contractions, and this reduction was eliminated by streptomycin and absent in the TRPC1 KO muscles. Calpain activation was determined by the appearance of a lower molecular weight autolysis product and μ-calpain was activated at 30 min, whereas the muscle-specific calpain-3 was not. To test whether the loss of stiffness was caused by titin cleavage, protein gels were used but no significant titin cleavage was detected. These results suggest that Ca(2+) entry following eccentric contractions is through a stretch-activated channel that is blocked by streptomycin and encoded or modulated by TRPC1.

Calcium influx through a possible coupling of cation channels impacts skeletal muscle satellite cell activation in response to mechanical stretch

When skeletal muscle is stretched or injured, satellite cells, resident myogenic stem cells positioned beneath the basal lamina of mature muscle fibers, are activated to enter the cell cycle. This signaling pathway is a cascade of events including calcium-calmodulin formation, nitric oxide (NO) radical production by NO synthase, matrix metalloproteinase activation, release of hepatocyte growth factor (HGF) from the extracellular matrix, and presentation of HGF to the receptor c-met, as demonstrated by assays of primary cultures and in vivo experiments. Here, we add evidence that two ion channels, the mechanosensitive cation channel (MS channel) and the long-lasting-type voltage-gated calcium-ion channel (L-VGC channel), mediate the influx of extracellular calcium ions in response to cyclic stretch in satellite cell cultures. When applied to 1-h stretch cultures with individual inhibitors for MS and L-VGC channels (GsMTx-4 and nifedipine, respectively) or with a less specific inhibitor (gadolinium chloride, Gd), satellite cell activation and upstream HGF release were abolished, as revealed by bromodeoxyuridine-incorporation assays and Western blotting of conditioned media, respectively. The inhibition was dose dependent with a maximum at 0.1 μM (GsMTx-4), 10 μM (nifedipine), or 100 μM (Gd) and canceled by addition of HGF to the culture media; a potent inhibitor for transient-type VGC channels (NNC55-0396, 100 μM) did not show any significant inhibitory effect. The stretch response was also abolished when calcium-chelator EGTA (1.8 mM) was added to the medium, indicating the significance of extracellular free calcium ions in our present activation model. Finally, cation/calcium channel dependencies were further documented by calcium-imaging analyses on stretched cells; results clearly demonstrated that calcium ion influx was abolished by GsMTx-4, nifedipine, and EGTA. Therefore, these results provide an additional insight that calcium ions may flow in through L-VGC channels by possible coupling with adjacent MS channel gating that promotes the local depolarization of cell membranes to initiate the satellite cell activation cascade.

TRPC1 expression and distribution in rat hearts

Transient receptor potential canonical (TRPC) proteins have been identified as a family of plasma membrane calcium-permeable channels. TRPC proteins can be activated by various stimuli and act as cellular sensors in mammals. Stretch-activated ion channels (SACs) have been proposed to underlie cardiac mechano-electric feedback (MEF), although the molecular entity of SAC remains unknown. There is evidence suggesting that transient receptor potential canonical 1 (TRPC1) is a stretch-activated ion channel. As a non-selective cation channel, TRPC1 may cause stretch-induced depolarization and arrhythmia and thus may contribute to the MEF of the heart. In this study, we examined the expression patterns of TRPC1 in detail at both the mRNA and protein levels in rat hearts. We isolated total RNA from the left and right atria, and the left and right ventricles, and detected TRPC1 mRNA in these tissues using reverse-transcriptase polymerase chain reaction (RT-PCR). To study the protein localization and targeting, we performed immunohistochemistry and immunofluorescence labeling with the antibody against TRPC1. TRPC1 was detected in the cardiomyocytes of the ventricle and atrium at both the mRNA and protein levels. The cell membrane and T-tubule showed strong fluorescence labeling in the ventricular myocytes. Purkinje cells, the endothelial cells and smooth muscle cells of the coronary arterioles also displayed TRPC1 labeling. No TRPC1 was detected in fibroblasts. In conclusion, TRPC1 is widely expressed in the rat heart, including in working cells, Purkinje cells and vascular cells, suggesting that it plays an important role in the heart. The specific distribution pattern offered a useful insight into its function in adult rat ventricular cells. Further investigations are needed to clarify the role of TRPC1 in regulating cardiac activity, including cardiac MEF.

Transient receptor potential channel activation and endothelium-dependent dilation in the systemic circulation

The endothelium plays a crucial role in the regulation of vascular tone by releasing a number of vasodilator mediators, including nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor(s). The production of these mediators is typically initiated by an increase in intracellular Ca(2+) concentration ([Ca(2+)]i) in endothelial cells. An essential component of this Ca(2+) signal is the entry of Ca(2+) from the extracellular space through plasma membrane Ca(2+)-permeable channels. Although the molecular identification of the potential Ca(2+) entry channel(s) responsible for the release of endothelial relaxing factors is still evolving, accumulating evidence indicates that the transient receptor potential (TRP) channels, a superfamily of Ca(2+)-permeable cation channels, serve as an important mechanism of Ca(2+) entry in endothelial cells and other nonexcitable cells. The activation of these channels has been implicated in diverse endothelial functions ranging from control of vascular tone and regulation of vascular permeability to angiogenesis and vascular remodeling. This review summarizes recent evidence concerning TRP channels and endothelium-dependent dilation in several systemic vascular beds. In particular, we highlight the emerging roles of several TRP channels from the canonical and vanilloid subfamilies, including TRPV4, TRPC4, and TRPC6, in vasodilatory responses to shear stress and receptor agonists and discuss potential signaling mechanisms linking the TRP channel activation and the initiation of endothelium-derived hyperpolarizing factor-mediated responses in endothelial cells.

TRP channels in normal and dystrophic skeletal muscle

TRP proteins constitute non-selective cation-permeable ion channels, most of which are permeable to Ca²⁺. In skeletal muscle, several isoforms of the TRPC (Canonical), TRPV (Vanilloid) and TRPM (Melastatin) subfamilies are expressed. In particular, TRPC1, C3 and C6, TRPV2 and V4, TRPM4 and TRPM7 have been consistently found in cultured myoblasts or in adult muscles. These channels seem to directly or indirectly respond to membrane stretch or to Ca²⁺ stores depletion; some isoforms might also constitute unregulated Ca²⁺ leak channels. Their function is largely unknown. TRPC1 and C3 have been involved in muscle development, in particular in myoblasts migration and differentiation. TRPC1 and V4 might allow a basal influx of Ca²⁺ at rest. Their lack has consequences on muscle fatigue. TRPV2 seems to be stretch-sensitive. It localizes mainly in intracellular pools at rest, and translocates to the plasma membrane upon IGF-1 stimulation. TRP channels seem to be involved in the pathophysiology of muscle disorders. In particular in Duchenne muscular dystrophy, the lack of the cytoskeletal protein dystrophin induces a disregulation of several ion channels leading to an abnormal influx of Ca²⁺. We discuss here, the possible involvement of TRP channels in this abnormal influx of Ca²⁺.

Rapid aquaporin translocation regulates cellular water flow: mechanism of hypotonicity-induced subcellular localization of aquaporin 1 water channel

The control of cellular water flow is mediated by the aquaporin (AQP) family of membrane proteins. The structural features of the family and the mechanism of selective water passage through the AQP pore are established, but there remains a gap in our knowledge of how water transport is regulated. Two broad possibilities exist. One is controlling the passage of water through the AQP pore, but this only has been observed as a phenomenon in some plant and microbial AQPs. An alternative is controlling the number of AQPs in the cell membrane. Here, we describe a novel pathway in mammalian cells whereby a hypotonic stimulus directly induces intracellular calcium elevations through transient receptor potential channels, which trigger AQP1 translocation. This translocation, which has a direct role in cell volume regulation, occurs within 30 s and is dependent on calmodulin activation and phosphorylation of AQP1 at two threonine residues by protein kinase C. This direct mechanism provides a rationale for the changes in water transport that are required in response to constantly changing local cellular water availability. Moreover, because calcium is a pluripotent and ubiquitous second messenger in biological systems, the discovery of its role in the regulation of AQP translocation has ramifications for diverse physiological and pathophysiological processes, as well as providing an explanation for the rapid regulation of water flow that is necessary for cell homeostasis.

Integration of transient receptor potential canonical channels with lipids

Transient receptor potential canonical (TRPC) channels are the canonical (C) subset of the TRP proteins, which are widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium and sodium entry and have wide-ranging functions that include regulation of cell proliferation, motility and contraction. The channels are modulated by a multiplicity of factors, putatively existing as integrators in the plasma membrane. This review considers the sensitivities of TRPC channels to lipids that include diacylglycerols, phosphatidylinositol bisphosphate, lysophospholipids, oxidized phospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate, cholesterol and some steroidal derivatives and other lipid factors such as gangliosides. Promiscuous and selective lipid sensing have been detected. There appear to be close working relationships with lipids of the phospholipase C and A₂ enzyme systems, which may enable integration with receptor signalling and membrane stretch. There are differences in the properties of each TRPC channel that are further complicated by TRPC heteromultimerization. The lipids modulate activity of the channels or insertion in the plasma membrane. Lipid microenvironments and intermediate sensing proteins have been described that include caveolae, G protein signalling, SEC14-like and spectrin-type domains 1 (SESTD1) and podocin. The data suggest that lipid sensing is an important aspect of TRPC channel biology enabling integration with other signalling systems.

Plasmalemmal Na+/Ca2+ exchanger modulates Ca2+-dependent exocytotic release of glutamate from rat cortical astrocytes

Astroglial excitability operates through increases in Ca2+cyt (cytosolic Ca2+), which can lead to glutamatergic gliotransmission. In parallel fluctuations in astrocytic Na+cyt (cytosolic Na+) control metabolic neuronal-glial signalling, most notably through stimulation of lactate production, which on release from astrocytes can be taken up and utilized by nearby neurons, a process referred to as lactate shuttle. Both gliotransmission and lactate shuttle play a role in modulation of synaptic transmission and plasticity. Consequently, we studied the role of the PMCA (plasma membrane Ca2+-ATPase), NCX (plasma membrane Na+/Ca2+ exchanger) and NKA (Na+/K+-ATPase) in complex and coordinated regulation of Ca2+cyt and Na+cyt in astrocytes at rest and upon mechanical stimulation. Our data support the notion that NKA and PMCA are the major Na+ and Ca2+ extruders in resting astrocytes. Surprisingly, the blockade of NKA or PMCA appeared less important during times of Ca2+ and Na+ cytosolic loads caused by mechanical stimulation. Unexpectedly, NCX in reverse mode appeared as a major contributor to overall Ca2+ and Na+ homoeostasis in astrocytes both at rest and when these glial cells were mechanically stimulated. In addition, NCX facilitated mechanically induced Ca2+-dependent exocytotic release of glutamate from astrocytes. These findings help better understanding of astrocyte-neuron bidirectional signalling at the tripartite synapse and/or microvasculature. We propose that NCX operating in reverse mode could be involved in fast and spatially localized Ca2+-dependent gliotransmission, that would operate in parallel to a slower and more widely distributed gliotransmission pathway that requires metabotropically controlled Ca2+ release from the ER (endoplasmic reticulum).

G protein-mediated stretch reception

Mechanosensation and -transduction are important for physiological processes like the senses of touch, hearing, and balance. The mechanisms underlying the translation of mechanical stimuli into biochemical information by activating various signaling pathways play a fundamental role in physiology and pathophysiology but are only poorly understood. Recently, G protein-coupled receptors (GPCRs), which are essential for the conversion of light, olfactory and gustatory stimuli, as well as of primary messengers like hormones and neurotransmitters into cellular signals and which play distinct roles in inflammation, cell growth, and differentiation, have emerged as potential mechanosensors. The first candidate for a mechanosensitive GPCR was the angiotensin-II type-1 (AT(1)) receptor. Agonist-independent mechanical receptor activation of AT(1) receptors induces an active receptor conformation that appears to differ from agonist-induced receptor conformations and entails the activation of G proteins. Mechanically induced AT(1) receptor activation plays an important role for myogenic vasoconstriction and for the initiation of cardiac hypertrophy. A growing body of evidence suggests that other GPCRs are involved in mechanosensation as well. These findings highlight physiologically relevant, ligand-independent functions of GPCRs and add yet another facet to the polymodal activation spectrum of this ubiquitous protein family.

Upregulation of osmo-mechanosensitive TRPV4 channel facilitates chronic hypoxia-induced myogenic tone and pulmonary hypertension

Chronic hypoxia causes pulmonary hypertension with vascular remodeling, increase in vascular tone, and altered reactivity to agonists. These changes involve alterations in multiple Ca(2+) pathways in pulmonary arterial smooth muscle cells (PASMCs). We have previously shown that vanilloid (TRPV)- and melastatin-related transient receptor potential (TRPM) channels are expressed in pulmonary arteries (PAs). Here we found that TRPV4 was the only member of the TRPV and TRPM subfamilies upregulated in PAs of chronic hypoxic rats. The increase in TRPV4 expression occurred within 1 day of hypoxia exposure, indicative of an early hypoxic response. TRPV4 in PASMCs were found to be mechanosensitive. Osmo-mechanical stress imposed by hypotonic solution activated Ca(2+) transients; they were inhibited by TRPV4 specific short interfering RNA, the TRPV blocker ruthenium red, and the cytochrome P450 epoxygenase inhibitor N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide. Consistent with TRPV4 upregulation, the Ca(2+) response induced by the TRPV4 agonist 4α-phorbol 12,13-didecanoate and hypotonicity was potentiated in hypoxic PASMCs. Moreover, a significant myogenic tone, sensitive to ruthenium red, was observed in pressurized endothelium denuded small PAs of hypoxic but not normoxic rats. The elevated basal intracellular Ca(2+) concentration in hypoxic PASMCs was also reduced by ruthenium red. In extension of these results, the development of pulmonary hypertension, right heart hypertrophy, and vascular remodeling was significantly delayed and suppressed in hypoxic trpv4(-/-) mice. These results suggest the novel concept that TRPV4 serves as a signal pathway crucial for the development of hypoxia-induced pulmonary hypertension. Its upregulation may provide a pathogenic feed-forward mechanism that promotes pulmonary hypertension via facilitated Ca(2+) influx, subsequently enhanced myogenic tone and vascular remodeling.

Transient receptor potential channel 1 (TRPC1) reduces calcium permeability in heteromeric channel complexes

Specific biological roles of the classical transient receptor potential channel 1 (TRPC1) are still largely elusive. To investigate the function of TRPC1 proteins in cell physiology, we studied heterologously expressed TRPC1 channels and found that recombinant TRPC1 subunits do not form functional homomeric channels. Instead, by electrophysiological analysis TRPC1 was shown to form functional heteromeric, receptor-operated channel complexes with TRPC3, -4, -5, -6, and -7 indicating that TRPC1 proteins can co-assemble with all members of the TRPC subfamily. In all TRPC1-containing heteromers, TRPC1 subunits significantly decreased calcium permeation. The exchange of select amino acids in the putative pore-forming region of TRPC1 further reduced calcium permeability, suggesting that TRPC1 subunits contribute to the channel pore. In immortalized immature gonadotropin-releasing hormone neurons endogenously expressing TRPC1, -2, -5, and -6, down-regulation of TRPC1 resulted in increased calcium permeability and elevated basal cytosolic calcium concentrations. We did not observe any involvement of TRPC1 in store-operated cation influx. Notably, TRPC1 suppressed the migration of gonadotropin-releasing hormone neurons without affecting cell proliferation. Conversely, in TRPC1 knockdown neurons, specific migratory properties like distance covered, locomotion speed, and directionality were increased. These findings suggest a novel regulatory mechanism relying on the expression of TRPC1 and the subsequent formation of heteromeric TRPC channel complexes with reduced calcium permeability, thereby fine-tuning neuronal migration.

Stretch-activated calcium channel protein TRPC1 is correlated with the different degrees of the dystrophic phenotype in mdx mice

In Duchenne muscular dystrophy (DMD) and in the mdx mouse model of DMD, the lack of dystrophin is related to enhanced calcium influx and muscle degeneration. Stretch-activated channels (SACs) might be directly involved in the pathology of DMD, and transient receptor potential cation channels have been proposed as likely candidates of SACs. We investigated the levels of transient receptor potential canonical channel 1 (TRPC1) and the effects of streptomycin, a SAC blocker, in muscles showing different degrees of the dystrophic phenotype. Mdx mice (18 days old, n = 16) received daily intraperitoneal injections of streptomycin (182 mg/kg body wt) for 18 days, followed by removal of the diaphragm, sternomastoid (STN), biceps brachii, and tibialis anterior muscles. Control mdx mice ( n = 37) were injected with saline. Western blot analysis showed higher levels of TRPC1 in diaphragm muscle compared with STN and limb muscles. Streptomycin reduced creatine kinase and prevented exercise-induced increases of total calcium and Evans blue dye uptake in diaphragm and in STN muscles. It is suggested that different levels of the stretch-activated calcium channel protein TRPC1 may contribute to the different degrees of the dystrophic phenotype seen in mdx mice. Early treatment designed to regulate the activity of these channels may ameliorate the progression of dystrophy in the most affected muscle, the diaphragm.

Critical role of TRPP2 and TRPC1 channels in stretch-induced injury of blood-brain barrier endothelial cells

The microvessels of the brain are very sensitive to mechanical stresses such as observed in traumatic brain injury (TBI). Such stresses can quickly lead to dysfunction of the microvessel endothelial cells, including disruption of blood-brain barrier (BBB). It is now evident that elevation of cytosolic calcium levels ([Ca2+]i) can compromise the BBB integrity, however the mechanism by which mechanical injury can produce a [Ca2+]i increase in brain endothelial cells is unclear. To assess the effects of mechanical/stretch injury on [Ca2+]i signaling, mouse brain microvessel endothelial cells (bEnd3) were grown to confluency on elasticized membranes and [Ca2+]i monitored using fura 2 fluorescence imaging. Application of an injury, using a pressure/stretch pulse of 50 ms, induced a rapid transient increase in [Ca2+]i. In the absence of extracellular Ca2+, the injury-induced [Ca2+]i transient was greatly reduced, but not fully eliminated, while unloading of Ca2+ stores by thapsigargin treatment in the absence of extracellular Ca2+ abolished the injury transient. Application of LOE-908 and amiloride, TRPC and TRPP2 channel blockers, respectively, both reduced the transient [Ca2+]i increase. Further, siRNA knockdown assays directed at TRPC1 and TRPP2 expression also resulted in a reduction of the injury-induced [Ca2+]i response. In addition, stretch injury induced increases of NO production and actin stress fiber formation, both of which were markedly reduced upon treatment with LOE908 and/or amiloride. We conclude that mechanical injury of brain endothelial cells induces a rapid influx of calcium, mediated by TRPC1 and TRPP2 channels, which leads to NO synthesis and actin cytoskeletal rearrangement.

Expression and characterization of the bacterial mechanosensitive channel MscS in Xenopus laevis oocytes

We have successfully expressed and characterized mechanosensitive channel of small conductance (MscS) from Escherichia coli in oocytes of the African clawed frog, Xenopus laevis. MscS expressed in oocytes has the same single-channel conductance and voltage dependence as the channel in its native environment. Two hallmarks of MscS activity, the presence of conducting substates at high potentials and reversible adaptation to a sustained stimulus, are also exhibited by oocyte-expressed MscS. In addition to its ease of use, the oocyte system allows the user to work with relatively large patches, which could be an advantage for the visualization of membrane deformation. Furthermore, MscS can now be compared directly to its eukaryotic homologues or to other mechanosensitive channels that are not easily studied in E. coli.

TRPC1 contributes to light-touch sensation and mechanical responses in low-threshold cutaneous sensory neurons

The cellular proteins that underlie mechanosensation remain largely enigmatic in mammalian systems. Mechanically sensitive ion channels are thought to distinguish pressure, stretch, and other types of tactile signals in skin. Transient receptor potential canonical 1 (TRPC1) is a candidate mechanically sensitive channel that is expressed in primary afferent sensory neurons. However, its role in the mechanical sensitivity of these neurons is unclear. Here, we investigated TRPC1-dependent responses to both innocuous and noxious mechanical force. Mechanically evoked action potentials in cutaneous myelinated A-fiber and unmyelinated C-fiber neurons were quantified using the ex vivo skin-nerve preparation to record from the saphenous nerve, which terminates in the dorsal hairy skin of the hindpaw. Our data reveal that in TRPC1-deficient mice, mechanically evoked action potentials were decreased by nearly 50% in slowly adapting Aβ-fibers, which largely innervate Merkel cells, and in rapidly adapting Aδ-Down-hair afferent fibers compared with wild-type controls. In contrast, differences were not found in slowly adapting Aδ-mechanoreceptors or unmyelinated C-fibers, which primarily respond to nociceptive stimuli. These results suggest that TRPC1 may be important in the detection of innocuous mechanical force. We concurrently investigated the role of TRPC1 in behavioral responses to mechanical force to the plantar hindpaw skin. For innocuous stimuli, we developed a novel light stroke assay using a "puffed out" cotton swab. Additionally, we used repeated light, presumably innocuous punctate stimuli with a low threshold von Frey filament (0.68 mN). In agreement with our electrophysiological data in light-touch afferents, TRPC1-deficient mice exhibited nearly a 50% decrease in behavioral responses to both the light-stroke and light punctate mechanical assays when compared with wild-type controls. In contrast, TRPC1-deficient mice exhibited normal paw withdrawal response to more intense mechanical stimuli that are typically considered measures of nociceptive behavior.

Calcium signaling in live cells on elastic gels under mechanical vibration at subcellular levels

A new device was designed to generate a localized mechanical vibration of flexible gels where human umbilical vein endothelial cells (HUVECs) were cultured to mechanically stimulate these cells at subcellular locations. A Fluorescence Resonance Energy Transfer (FRET)-based calcium biosensor (an improved Cameleon) was used to monitor the spatiotemporal distribution of intracellular calcium concentrations in the cells upon this mechanical stimulation. A clear increase in intracellular calcium concentrations over the whole cell body (global) can be observed in the majority of cells under mechanical stimulation. The chelation of extracellular calcium with EGTA or the blockage of stretch-activated calcium channels on the plasma membrane with streptomycin or gadolinium chloride significantly inhibited the calcium responses upon mechanical stimulation. Thapsigargin, an endoplasmic reticulum (ER) calcium pump inhibitor, or U73122, a phospholipase C (PLC) inhibitor, resulted in mainly local calcium responses occurring at regions close to the stimulation site. The disruption of actin filaments with cytochalasin D or inhibition of actomyosin contractility with ML-7 also inhibited the global calcium responses. Therefore, the global calcium response in HUVEC depends on the influx of calcium through membrane stretch-activated channels, followed by the release of inositol trisphosphate (IP3) via PLC activation to trigger the ER calcium release. Our newly developed mechanical stimulation device can also provide a powerful tool for the study of molecular mechanism by which cells perceive the mechanical cues at subcellular levels.

Role of reactive oxygen species and redox in regulating the function of transient receptor potential channels

Cellular redox status, regulated by production of reactive oxygen species (ROS), greatly contributes to the regulation of vascular smooth muscle cell contraction, migration, proliferation, and apoptosis by modulating the function of transient receptor potential (TRP) channels in the plasma membrane. ROS functionally interact with the channel protein via oxidizing the redox-sensitive residues, whereas nitric oxide (NO) regulates TRP channel function by cyclic GMP/protein kinase G-dependent and -independent pathways. Based on the structural differences among different TRP isoforms, the effects of ROS and NO are also different. In addition to regulating TRP channels in the plasma membrane, ROS and NO also modulate Ca(2+) release channels (e.g., IP(3) and ryanodine receptors) on the sarcoplasmic/endoplasmic reticulum membrane. This review aims at briefly describing (a) the role of TRP channels in receptor-operated and store-operated Ca(2+) entry, and (b) the role of ROS and redox status in regulating the function and structure of TRP channels.

Muscular effects of orexin A on the mouse duodenum: mechanical and electrophysiological studies

Orexin A (OXA) has been reported to influence gastrointestinal motility, acting at both central and peripheral neural levels. The aim of the present study was to evaluate whether OXA also exerts direct effects on the duodenal smooth muscle. The possible mechanism of action involved was investigated by employing a combined mechanical and electrophysiological approach. Duodenal segments were mounted in organ baths for isometric recording of the mechanical activity. Ionic channel activity was recorded in current- and voltage-clamp conditions by a single microelectrode inserted in a duodenal longitudinal muscle cell. In the duodenal preparations, OXA (0.3 μM) caused a TTX-insensitive transient contraction. Nifedipine (1 μM), as well as 2-aminoethyl diphenyl borate (10 μM), reduced the amplitude and shortened the duration of the response to OXA, which was abolished by Ni(2+) (50 μM) or TEA (1 mM). Electrophysiological studies in current-clamp conditions showed that OXA caused an early depolarization, which paralleled in time the contractile response, followed by a long-lasting depolarization. Such a depolarization was triggered by activation of receptor-operated Ca(2+) channels and enhanced by activation of T- and L-type Ca(2+) channels and store-operated Ca(2+) channels and by inhibition of K(+) channels. Experiments in voltage-clamp conditions demonstrated that OXA affects not only receptor-operated Ca(2+) channels, but also the maximal conductance and kinetics of activation and inactivation of Na(+), T- and L-type Ca(2+) voltage-gated channels. The results demonstrate, for the first time, that OXA exerts direct excitatory effects on the mouse duodenal smooth muscle. Finally, this work demonstrates new findings related to the expression and kinetics of the voltage-gated channel types, as well as store-operated Ca(2+) channels.

Physiological validation of cell health upon probing with carbon nanotube endoscope and its benefit for single-cell interrogation

New-generation nanoscale devices for single-cell study are intensively being developed. As has been shown, nanodevices are minimally invasive because of their order-of-magnitude smaller size in comparison to conventional glass pipettes. However, in most studies the evaluation of the nanodevice impact on cell health has not extended to their effects on cell metabolic integrity. In this work we evaluated the degree to which the insertion of a carbon-based nanotube endoscope into a cell induces mechanical and biochemical stress, and affects cellular key metabolic systems. The effects of insertion of the nanotube endoscope on cell morphological and physiological modulations were monitored and compared to those of glass micropipettes. We report that nanotube endoscope insertion does not significantly modulate the plasma membrane and actin network. The cell metabolic mechanisms such as energy production and inositol 1,4,5-trisphosphate-dependent calcium signaling remain preserved for prolonged endoscope presence within a cell.

FROM THE CLINICAL EDITOR

In this basic science study, the effects of insertion of carbon nanotube endoscope on cell morphological and physiological modulations were monitored and compared to those of glass micropipettes. Nanotube endoscope insertion is truly minimally invasive: it does not significantly modulate the plasma membrane and actin network; the energy production and inositol 1,4,5-trisphosphate-dependent calcium signaling also remain preserved during prolonged endoscope presence within a cell.

Osmosensory mechanisms in cellular and systemic volume regulation

Perturbations of cellular and systemic osmolarity severely challenge the function of all organisms and are consequently regulated very tightly. Here we outline current evidence on how cells sense volume perturbations, with particular focus on mechanisms relevant to the kidneys and to extracellular osmolarity and whole body volume homeostasis. There are a variety of molecular signals that respond to perturbations in cell volume and osmosensors or volume sensors responding to these signals. The early signals of volume perturbation include integrins, the cytoskeleton, receptor tyrosine kinases, and transient receptor potential channels. We also present current evidence on the localization and function of central and peripheral systemic osmosensors and conclude with a brief look at the still limited evidence on pathophysiological conditions associated with deranged sensing of cell volume.

A key role of TRPC channels in the regulation of electromechanical activity of the developing heart

AIMS

It is well established that dysfunction of voltage-dependent ion channels results in arrhythmias and conduction disturbances in the foetal and adult heart. However, the involvement of voltage-insensitive cationic TRPC (transient receptor potential canonical) channels remains unclear. We assessed the hypothesis that TRPC channels play a crucial role in the spontaneous activity of the developing heart.

METHODS AND RESULTS

TRPC isoforms were investigated in isolated hearts obtained from 4-day-old chick embryos. Using RT-PCR, western blotting and co-immunoprecipitation, we report for the first time that TRPC1, 3, 4, 5, 6, and 7 isoforms are expressed at the mRNA and protein levels and that they can form a macromolecular complex with the α1C subunit of the L-type voltage-gated calcium channel (Cav1.2) in atria and ventricle. Using ex vivo electrocardiograms, electrograms of isolated atria and ventricle and ventricular mechanograms, we found that inhibition of TRPC channels by SKF-96365 leads to negative chrono-, dromo-, and inotropic effects, prolongs the QT interval, and provokes first- and second-degree atrioventricular blocks. Pyr3, a specific antagonist of TRPC3, affected essentially atrioventricular conduction. On the other hand, specific blockade of the L-type calcium channel with nifedipine rapidly stopped ventricular contractile activity without affecting rhythmic electrical activity.

CONCLUSIONS

These results give new insights into the key role that TRPC channels, via interaction with the Cav1.2 channel, play in regulation of cardiac pacemaking, conduction, ventricular activity, and contractility during cardiogenesis.