TRPM7 is a stretch- and swelling-activated cation channel involved in volume regulation in human epithelial cells

Calcium flickers steer cell migration

Directional movement is a property common to all cell types during development and is critical to tissue remodelling and regeneration after damage. In migrating cells, calcium has a multifunctional role in directional sensing, cytoskeleton redistribution, traction force generation, and relocation of focal adhesions. Here we visualize high-calcium microdomains ('calcium flickers') and their patterned activation in migrating human embryonic lung fibroblasts. Calcium flicker activity is dually coupled to membrane tension (by means of TRPM7, a stretch-activated Ca(2+)-permeant channel of the transient receptor potential superfamily) and chemoattractant signal transduction (by means of type 2 inositol-1,4,5-trisphosphate receptors). Interestingly, calcium flickers are most active at the leading lamella of migrating cells, displaying a 4:1 front-to-rear polarization opposite to the global calcium gradient. When exposed to a platelet-derived growth factor gradient perpendicular to cell movement, asymmetric calcium flicker activity develops across the lamella and promotes the turning of migrating fibroblasts. These findings show how the exquisite spatiotemporal organization of calcium microdomains can orchestrate complex cellular processes such as cell migration.

Gq-coupled receptors as mechanosensors mediating myogenic vasoconstriction

Despite the central physiological function of the myogenic response, the underlying signalling pathways and the identity of mechanosensors in vascular smooth muscle (VSM) are still elusive. In contrast to present thinking, we show that membrane stretch does not primarily gate mechanosensitive transient receptor potential (TRP) ion channels, but leads to agonist-independent activation of G(q/11)-coupled receptors, which subsequently signal to TRPC channels in a G protein- and phospholipase C-dependent manner. Mechanically activated receptors adopt an active conformation, allowing for productive G protein coupling and recruitment of beta-arrestin. Agonist-independent receptor activation by mechanical stimuli is blocked by specific antagonists and inverse agonists. Increasing the AT(1) angiotensin II receptor density in mechanically unresponsive rat aortic A7r5 cells resulted in mechanosensitivity. Myogenic tone of cerebral and renal arteries is profoundly diminished by the inverse angiotensin II AT(1) receptor agonist losartan independently of angiotensin II (AII) secretion. This inhibitory effect is enhanced in blood vessels of mice deficient in the regulator of G-protein signalling-2. These findings suggest that G(q/11)-coupled receptors function as sensors of membrane stretch in VSM cells.

Ion channel regulation of intracellular calcium and airway smooth muscle function

Airway hyper-responsiveness associated with asthma is mediated by airway smooth muscle cells (SMCs) and has a complicated etiology involving increases in cell contraction and proliferation and the secretion of inflammatory mediators. Although these pathological changes are diverse, a common feature associated with their regulation is a change in intracellular Ca(2+) concentration ([Ca(2+)](i)). Because the [Ca(2+)](i) itself is a function of the activity and expression of a variety of ion channels, in both the plasma membrane and sarcoplasmic reticulum of the SMC, the modification of this ion channel activity may predispose airway SMCs to hyper-responsiveness. Our objective is to review how ion channels determine the [Ca(2+)](i) and influence the function of airway SMCs and emphasize the potential of ion channels as sites for therapeutic approaches to asthma.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Proton conductivity through the human TRPM7 channel and its molecular determinants

TRPM7 is a divalent cation-permeable channel that is ubiquitously expressed. Recently, mouse TRPM7 has been shown to be sensitive to, and even permeable to, protons when heterologously expressed. Here we have demonstrated that human TRPM7 expressed either heterologously or endogenously also exhibits proton conductivity. The gene silencing of TRPM7 by small interfering RNA suppressed H+ currents in human cervical epithelial HeLa cells. In HEK293T cells transfected with human TRPM7, the inward proton conductance was suppressed by extracellular Mg2+ or Ca2+ with IC(50) values of 0.5 and 1.9 mm, respectively. Anomalous mole fraction behavior of H+ currents in the presence of Mg2+ or Ca2+ indicated that these divalent cations compete with protons for binding sites. Systematic mutation of negatively charged amino acid residues within the putative pore-forming region of human TRPM7 into the neutral amino acid alanine was tested. E1047A resulted in non-functional channels, and D1054A abolished proton conductance, whereas E1052A and D1059A only partially reduced proton conductivity. Thus, it is concluded that Asp-1054 is an essential determinant of the proton conductivity, whereas Glu-1047 might be required for channel formation, and the remaining negatively charged amino acids in the pore region (Glu-1052 and Asp-1059) may play a facilitating role in the proton conductivity of human TRPM7. It is suggested that proton conductivity of endogenous human TRPM7 plays a role in physiologically/pathologically acidic situations.

Exploring mechanisms involved in renal tubular sensing of mechanical stretch following ureteric obstruction

Tubular mechanical stretch is the key primary insult in obstructive nephropathy. This review addresses how the renal tubular epithelium senses and responds to mechanical stretch. Using data from renal and nonrenal systems, we describe how sensing of stretch initially occurs via the activation of ion channels and subsequent increases in intracellular calcium levels. Calcium influxes activate a number of adaptive and proinjury responses. Key among these are 1) the activation of Rho, consequent cytoskeletal rearrangements, and downstream increases in focal adhesion assembly; and 2) phospholipase activation and resultant mitogen-activated protein kinase activation. These early signaling events culminate in adaptive cellular coupling to the extracellular matrix, a process termed the cell strengthening response. Direct links can be made between increased expression of genes involved in the development of obstructive nephropathy and initial sensing of mechanical stretch. The review illustrates the repercussions of mechanical stretch as a renal stress stimulus, specific to ureteric obstruction, and provides an insight into how tubular responses to mechanical stretch are ultimately implicated in the development of obstructive nephropathy.

Massive autophosphorylation of the Ser/Thr-rich domain controls protein kinase activity of TRPM6 and TRPM7

TRPM6 and TRPM7 are bifunctional proteins expressing a TRP channel fused to an atypical α-kinase domain. While the gating properties of TRPM6 and TRPM7 channels have been studied in detail, little is known about the mechanisms regulating kinase activity. Recently, we found that TRPM7 associates with its substrate myosin II via a kinase-dependent mechanism suggesting a role for autophosphorylation in substrate recognition. Here, we demonstrate that the cytosolic C-terminus of TRPM7 undergoes massive autophosphorylation (32±4 mol/mol), which strongly increases the rate of substrate phosphorylation. Phosphomapping by mass spectrometry indicates that the majority of autophosphorylation sites (37 out of 46) map to a Ser/Thr-rich region immediately N-terminal of the catalytic domain. Deletion of this region prevents substrate phosphorylation without affecting intrinsic catalytic activity suggesting that the Ser/Thr-rich domain contributes to substrate recognition. Surprisingly, the TRPM6-kinase is regulated by an analogous mechanism despite a lack of sequence conservation with the TRPM7 Ser/Thr-rich domain. In conclusion, our findings support a model where massive autophosphorylation outside the catalytic domain of TRPM6 and TRPM7 may facilitate kinase-substrate interactions leading to enhanced phosphorylation of those substrates.

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.

Transient receptor potential melastatin 6 and 7 channels, magnesium transport, and vascular biology: implications in hypertension

Magnesium, an essential intracellular cation, is critically involved in many biochemical reactions involved in the regulation of vascular tone and integrity. Decreased magnesium concentration has been implicated in altered vascular reactivity, endothelial dysfunction, vascular inflammation, and structural remodeling, processes important in vascular changes and target organ damage associated with hypertension. Until recently, very little was known about mechanisms regulating cellular magnesium homeostasis, and processes controlling transmembrane magnesium transport had been demonstrated only at the functional level. Two cation channels of the transient receptor potential melastatin (TRPM) cation channel family have now been identified as magnesium transporters, TRPM6 and TRPM7. These unique proteins, termed chanzymes because they possess a channel and a kinase domain, are differentially expressed, with TRPM6 being found primarily in epithelial cells and TRPM7 occurring ubiquitously. Vascular TRPM7 is modulated by vasoactive agents, pressure, stretch, and osmotic changes and may be a novel mechanotransducer. In addition to its magnesium transporter function, TRPM7 has been implicated as a signaling kinase involved in vascular smooth muscle cell growth, apoptosis, adhesion, contraction, cytoskeletal organization, and migration, important processes involved in vascular remodeling associated with hypertension and other vascular diseases. Emerging evidence suggests that vascular TRPM7 function may be altered in hypertension. This review discusses the importance of magnesium in vascular biology and implications in hypertension and highlights the transport systems, particularly TRPM6 and TRPM7, which may play a role in the control of vascular magnesium homeostasis. Since the recent identification and characterization of Mg2+-selective transporters, there has been enormous interest in the field. However, there is still a paucity of information, and much research is needed to clarify the exact mechanisms of magnesium regulation in the cardiovascular system and the implications of aberrant transmembrane magnesium transport in the pathogenesis of hypertension and other vascular diseases.

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

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

Magnesium transport in hypertension

Epidemiological, clinical and experimental evidence indicates an inverse association between Mg(2+) levels (serum and tissue) and blood pressure. Magnesium may influence blood pressure by modulating vascular tone and structure through its effects on numerous biochemical reactions that control vascular contraction/dilation, growth/apoptosis, differentiation and inflammation. Magnesium acts as a calcium channel antagonist, it stimulates production of vasodilator prostacyclins and nitric oxide and it alters vascular responses to vasoactive agonists. Mammalian cells regulate Mg(2+) concentration through specialized influx and efflux transport systems that have only recently been characterized. Magnesium efflux occurs via Na(2+)-dependent and Na(2+)-independent pathways. Mg(2+) influx is controlled by recently cloned transporters including Mrs2p, SLC41A1, SLC41A1, ACDP2, MagT1, TRPM6 and TRPM7. Alterations in some of these systems may contribute to hypomagnesemia and intracellular Mg(2+) deficiency in hypertension. In particular increased Mg(2+) efflux through altered regulation of the vascular Na(+)/Mg(2+) exchanger and decreased Mg(2+) influx due to defective vascular and renal TRPM6/7 expression/activity may be important. This review discusses the role of Mg(2+) in vascular biology and implications in hypertension and focuses on the putative transport systems that control vascular magnesium homeostasis. Much research is still needed to clarify the exact mechanisms of Mg(2+) regulation in the cardiovascular system and the implications of aberrant transcellular Mg(2+) transport in the pathogenesis of cardiovascular disease.

Expression and functional characterization of transient receptor potential vanilloid-related channel 4 (TRPV4) in rat cortical astrocytes

Cell-cell communication in astroglial syncytia is mediated by intracellular Ca(2+) ([Ca(2+)](i)) responses elicited by extracellular signaling molecules as well as by diverse physical and chemical stimuli. Despite the evidence that astrocytic swelling promotes [Ca(2+)](i) elevation through Ca(2+) influx, the molecular identity of the channel protein underlying this response is still elusive. Here we report that primary cultured cortical astrocytes express the transient receptor potential vanilloid-related channel 4 (TRPV 4), a Ca(2+)-permeable cation channel gated by a variety of stimuli, including cell swelling. Immunoblot and confocal microscopy analyses confirmed the presence of the channel protein and its localization in the plasma membrane. TRPV4 was functional because the selective TRPV4 agonist 4-alpha-phorbol 12,13-didecanoate (4alphaPDD) activated an outwardly rectifying cation current with biophysical and pharmacological properties that overlapped those of recombinant human TRPV4 expressed in COS cells. Moreover, 4alphaPDD and hypotonic challenge promoted [Ca(2+)](i) elevation mediated by influx of extracellular Ca(2+). This effect was abolished by low micromolar concentration of the TRPV4 inhibitor Ruthenium Red. Immunofluorescence and immunogold electron microscopy of rat brain revealed that TRPV4 was enriched in astrocytic processes of the superficial layers of the neocortex and in astrocyte end feet facing pia and blood vessels. Collectively, these data indicate that cultured cortical astroglia express functional TRPV4 channels. They also demonstrate that TRPV4 is particularly abundant in astrocytic membranes at the interface between brain and extracerebral liquid spaces. Consistent with its roles in other tissues, these results support the view that TRPV4 might participate in astroglial osmosensation and thus play a key role in brain volume homeostasis.

TRP channels in mechanosensation: direct or indirect activation?

Ion channels of the transient receptor potential (TRP) superfamily are involved in a wide variety of neural signalling processes, most prominently in sensory receptor cells. They are essential for mechanosensation in systems ranging from fruitfly hearing, to nematode touch, to mouse mechanical pain. However, it is unclear in many instances whether a TRP channel directly transduces the mechanical stimulus or is part of a downstream signalling pathway. Here, we propose criteria for establishing direct mechanical activation of ion channels and review these criteria in a number of mechanosensory systems in which TRP channels are involved.

Membrane permeabilization and cell damage by ultrashort electric field shocks

Mammalian cells exposed to electric field pulses of nanosecond duration (nsPEF; 60-ns, 12 kV/cm) experienced a profound and long-lasting increase in passive electrical conductance (G(m)) of the cell membrane, probably caused by opening of stable conductance pores (CPs). The CPs were permeable to Cl(-) and alkali metal cations, but not to larger molecules such as propidium iodide (PI). CPs gradually resealed; the process took minutes and could be observed even in dialyzed cells and in ATP- and glucose-free solutions. Cells subjected to long nsPEF trains (up to 200 pulses) underwent severe and immediate necrotic transformation (cell swelling, blebbing, cytoplasm granulation), but remained impermeable to PI for at least 30-60 min after the exposure. Both G(m) increase after short nsPEF trains and necrotic changes after long nsPEF trains were cell type-dependent: they were much weaker in HeLa than in GH3 cells. La(3+) and Gd(3+) ions significantly inhibited the nsPEF-induced G(m) increase (probably by blocking the CPs), and effectively protected intensely exposed cells from developing necrosis. We conclude that plasma membrane permeabilization is the principal cause of necrotic transformation in nsPEF-exposed cells and probably contributes to other known nsPEF bioeffects.

TRPM7 channel is sensitive to osmotic gradients in human kidney cells

TRPM7 (transient-receptor-potential melastatin 7) is an ion channel with alpha-kinase function. TRPM7 is divalent-selective and regulated by a range of receptor-stimulated second messenger pathways, intracellular Mg-nucleotides, divalent and polyvalent cations and pH. TRPM7 is ubiquitously found in mammalian cells, including kidney, the responsible organ for osmolyte regulation, posing the question whether the channel is osmosensitive. Recent reports investigated the sensitivity of native TRPM7-like currents to cell swelling with contradictory results. Here, we assess the sensitivity of TRPM7 to both hypo- and hyperosmotic conditions and explored the involvement of the channel's kinase domain. We find that hypotonicity facilitates TRPM7 at elevated intracellular magnesium and Mg.ATP (3-4 mm), but has no effect in the absence of these solutes. Hypertonic conditions, in contrast, inhibit TRPM7 with an IC(50) of 430 mosmol l(-1). This inhibitory effect is maintained in the complete absence of intra- and extracellular divalent ions, although shifted to higher osmolarities (IC(50) = 510 mosmol l(-1)). TRPM7 senses osmotic gradients rather than ionic strength and this is independent of cAMP or not affected by cytochalasin D treatment. Furthermore, the kinase-domain deletion mutant of TRPM7 shows a similar behaviour to osmolarity as the wild-type protein, both in the presence and absence of divalent ions. This indicates that at least part of the osmosensitivity resides in the channel domain. Physiologically, TRPM7 channels do not seem to play an active role in regulatory volume changes, but rather those volume changes modulate TRPM7 activity through changes in the cytosolic concentrations of free Mg, Mg-nucleotides and a further unidentified factor. We conclude that TRPM7 senses osmotically induced changes primarily through molecular crowding of solutes that affect channel activity.

Myosin II and mechanotransduction: a balancing act

Adherent cells respond to mechanical properties of the surrounding extracellular matrix. Mechanical forces, sensed at specialized cell-matrix adhesion sites, promote actomyosin-based contraction within the cell. By manipulating matrix rigidity and adhesion strength, new roles for actomyosin contractility in the regulation of basic cellular functions, including cell proliferation, migration and stem cell differentiation, have recently been discovered. These investigations demonstrate that a balance of forces between cell adhesion on the outside and myosin II-based contractility on the inside of the cell controls many aspects of cell behavior. Disturbing this balance contributes to the pathogenesis of various human diseases. Therefore, elaborate signaling networks have evolved that modulate myosin II activity to maintain tensional homeostasis. These include signaling pathways that regulate myosin light chain phosphorylation as well as myosin II heavy chain interactions.

Transient Receptor Potential Channels in Mechanosensing and Cell Volume Regulation

Transient receptor potential (TRP) channels are unique cellular sensors responding to a wide variety of extra- and intracellular signals, including mechanical and osmotic stress. In recent years, TRP channels from multiple subfamilies have been added to the list of mechano- and/or osmosensitive channels, and it is becoming increasingly apparent that Ca(2+) influx via TRP channels plays a crucial role in the response to mechanical and osmotic perturbations in a wide range of cell types. Although the events translating mechanical and osmotic stimuli into regulation of TRP channels are still incompletely understood, the specific mechanisms employed vary between different TRP isoforms, and probably include changes in the tension and/or curvature of the lipid bilayer, changes in the cortical cytoskeleton, and signaling events such as lipid metabolism and protein phosphorylation/dephosphorylation. This chapter describes candidate mechanosensitive channels from mammalian TRP subfamilies, discusses inherent and technical issues potentially confounding evaluation of mechano- and/or osmosensitivity, and presents methods relevant to the study of TRP channel regulation by mechanical and osmotic stimuli and involvement in cell volume regulation.