Transient Receptor Potential Melastatin 7 Ion Channels Regulate Magnesium Homeostasis in Vascular Smooth Muscle Cells

Vascular smooth muscle cell differentiation to an osteogenic phenotype involves TRPM7 modulation by magnesium

Arterial calcification, common in vascular diseases, involves vascular smooth muscle cell (VSMC) transformation to an osteoblast phenotype. Clinical studies suggest that magnesium may prevent this, but mechanisms are unclear. We assessed whether increasing magnesium levels reduce VSMC calcification and differentiation and questioned the role of the Mg(2+) transporter, transient receptor potential melastatin (TRPM)7 cation channels in this process. Rat VSMCs were exposed to calcification medium in the absence and presence of magnesium (2.0 to 3.0 mmol/L) or 2-aminoethoxy-diphenylborate (2-APB) (TRPM7 inhibitor). VSMCs from mice with genetically low (MgL) or high-normal (MgH) [Mg(2+)](i) were also studied. Calcification was assessed by von Kossa staining. Expression of osteocalcin, osteopontin, bone morphogenetic protein (BMP)-2, BMP-4, BMP-7, and matrix Gla protein and activity of TRPM7 (cytosol:membrane translocation) were determined by immunoblotting. Calcification medium induced osteogenic differentiation, reduced matrix Gla protein content, and increased expression of the sodium-dependent cotransporter Pit-1. Magnesium prevented calcification and decreased osteocalcin expression and BMP-2 activity and increased expression of calcification inhibitors, osteopontin and matrix Gla protein. TRPM 7 activation was decreased by calcification medium, an effect reversed by magnesium. 2-APB recapitulated the VSMC osteoblastic phenotype in VSMCs. Osteocalcin was increased by calcification medium in VSMCs and intact vessels from MgL but not MgH, whereas osteopontin was increased in MgH, but not in MgL mice. Magnesium negatively regulates vascular calcification and osteogenic differentiation through increased/restored TRPM7 activity and increased expression of anticalcification proteins, including osteopontin, BMP-7, and matrix Gla protein. New molecular insights are provided whereby magnesium could protect against VSMC calcification.

Cellular functions of transient receptor potential channels

Transient Receptor Potential channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by increasing cellular Ca(2+). In this review we focus on the roles of these channels in: (i) cell death (ii) proliferation and differentiation and (iii) transmitter release. Cell death: Ca(2+) influx participates in apoptotic and necrotic cell death. The Ca(2+) permeability and high sensitivity of part of these channels to oxidative/metabolic stress make them important participants in cell death. Several examples are given. Transient Receptor Potential Melastatin 2 is activated by H(2)O(2), inducing cell death through an increase in cellular Ca(2+) and activation of Poly ADP-Ribose Polymerase. Exposure of cultured cortical neurons to oxygen-glucose deprivation, in vitro, causes cell death via cation influx, mediated by Transient Receptor Potential Melastatin 7. Metabolic stress constitutively activates the Ca(2+) permeable Transient Receptor Potential channels of Drosophila photoreceptor in the dark, potentially leading to retinal degeneration. Similar sensitivity to metabolic stress characterizes several mammalian Transient Receptor Potential Canonical channels. Proliferation and differentiation: The rise in cytosolic Ca(2+) induces cell growth, differentiation and proliferation via activation of several transcription factors. Activating a variety of store operated and Transient Receptor Potential channels cause a rise in cytosolic Ca(2+), making these channels components involved in proliferation and differentiation. Transmitter release: Transient Receptor Potential Melastatin 7 channels reside in synaptic vesicles and regulate neurotransmitter release by a mechanism that is not entirely clear. All the above features of Transient Receptor Potential channels make them crucial components in important, sometimes conflicting, cellular processes that still need to be explored.

Transient receptor potential channels and vascular function

TRP (transient receptor potential) channels play important roles in the regulation of normal and pathological cellular function. In the vasculature, TRP channels are present both in ECs (endothelial cells) and vascular SMCs (smooth muscle cells) and contribute to vasomotor control mechanisms in most vascular beds. Vascular TRP channels are activated by various stimuli, such as mechanical perturbation, receptor activation and dietary molecules. Some of the specific roles of these channels in normal and impaired vascular function have emerged in recent years and include participation in vascular signalling processes, such as neurotransmission, hormonal signalling, NO production, myogenic tone and autoregulation of blood flow, thermoregulation, responses to oxidative stress and cellular proliferative activity. Current research is aimed at understanding the interactions of TRP channels with other vascular proteins and signalling mechanisms. These studies should reveal new targets for pharmacological therapy of vascular diseases, such as hypertension, ischaemia and vasospasm, and vascular proliferative states.

Pharmacology of transient receptor potential melastatin channels in the vasculature

Mammalian transient receptor potential melastatin (TRPM) non-selective cation channels, the largest TRP subfamily, are widely expressed in excitable and non-excitable cells where they perform diverse functions ranging from detection of cold, taste, osmolarity, redox state and pH to control of Mg(2+) homeostasis and cell proliferation or death. Recently, TRPM gene expression has been identified in vascular smooth muscles with dominance of the TRPM8 channel. There has been in parallel considerable progress in decoding the functional roles of several TRPMs in the vasculature. This research on native cells is aided by the knowledge of the activation mechanisms and pharmacological properties of heterologously expressed TRPM subtypes. This paper summarizes the present state of knowledge of vascular TRPM channels and outlines several anticipated directions of future research in this area.

Molecular identification of ancient and modern mammalian magnesium transporters

A large number of mammalian Mg2+ transporters have been hypothesized on the basis of physiological data, but few have been investigated at the molecular level. The recent identification of a number of novel proteins that mediate Mg2+ transport has enhanced our understanding of how Mg2+ is translocated across mammalian membranes. Some of these transporters have some similarity to those found in prokaryocytes and yeast cells. Human Mrs2, a mitochondrial Mg2+ channel, shares many of the properties of the bacterial CorA and yeast Alr1 proteins. The SLC41 family of mammalian Mg2+ transporters has a similarity with some regions of the bacterial MgtE transporters. The mammalian ancient conserved domain protein (ACDP) Mg2+ transporters are found in prokaryotes, suggesting an ancient origin. However, other newly identified mammalian transporters, including TRPM6/7, MagT, NIPA, MMgT, and HIP14 families, are not represented in prokaryotic genomes, suggesting more recent development. MagT, NIPA, MMgT, and HIP14 transporters were identified by differential gene expression using microarray analysis. These proteins, which are found in many different tissues and subcellular organelles, demonstrate a diversity of structural properties and biophysical functions. The mammalian Mg2+ transporters have no obvious amino acid similarities, indicating that there are many ways to transport Mg2+ across membranes. Most of these proteins transport a number of divalent cations across membranes. Only MagT1 and NIPA2 are selective for Mg2+. Many of the identified mammalian Mg2+ transporters are associated with a number of congenital disorders encompassing a wide range of tissues, including intestine, kidney, brain, nervous system, and skin. It is anticipated that future research will identify other novel Mg2+ transporters and reveal other diseases.

Mechanosensitive channels in striated muscle and the cardiovascular system: not quite a stretch anymore

Stretch-activated or mechanosensitive channels transduce mechanical forces into ion fluxes across the cell membrane. These channels have been implicated in several aspects of cardiovascular physiology including regulation of blood pressure, vasoreactivity, and cardiac arrhythmias, as well as the adverse remodeling associated with cardiac hypertrophy and heart failure. This review discusses mechanosensitive channels in skeletal muscle and the cardiovascular system and their role in disease pathogenesis. We describe the regulation of gating of mechanosensitive channels including direct mechanisms and indirect activation by signaling pathways, as well as the influence on activation of these channels by the underlying cytoskeleton and scaffolding proteins. We then focus on the role of transient receptor potential channels, several of which have been implicated as mechanosensitive channels, in the pathogenesis of adverse cardiac remodeling and as potential therapeutic targets in the treatment of heart failure.

Silencing TRPM7 promotes growth/proliferation and nitric oxide production of vascular endothelial cells via the ERK pathway

AIMS

The presence and potential function of transient receptor potential melastatin 7 (TRPM7), a Ca2+-permeable non-selective cation channel of the TRP channel superfamily in human vascular endothelial cells, were examined.

METHODS AND RESULTS

Whole-cell patch-clamp recordings showed outward-rectifying currents in human umbilical vein endothelial cells (HUVECs), which was potentiated by removing the extracellular Ca2+ and Mg2+, but inhibited by non-specific TRPM7 blocker Gd3+ or 2-aminoethoxydiphenyl borate (2-APB). TRPM7 mRNA was detected in HUVECs by RT-PCR, but TRPM6, its closest homologue, was not. Silencing TRPM7 by small interfering RNA (siRNA) decreased the level of TRPM7 mRNA and the TRPM7-like current. Interestingly, knockdown of TRPM7 with siRNA or inhibition of TRPM7 function with 2-APB increased the phosphorylation of extracellular signal-regulated kinase (ERK) and enhanced growth/proliferation of HUVECs. This enhanced cell growth/proliferation was abolished by an inhibitor of the ERK signalling pathway. In addition to cell growth/proliferation, silencing TRPM7 also increased expression of nitric oxide synthase and nitric oxide production in an ERK pathway-dependent manner.

CONCLUSION

These observations suggest that TRPM7 channels may play an important role in the function of vascular endothelial cells.

Distribution profiles of transient receptor potential melastatin- and vanilloid-related channels in rat spermatogenic cells and sperm

In the present study, we aimed to investigate the expression and distribution of transient receptor potential melastatin (TRPM)- and vanilloid (TRPV)- related channels in rat spermatogenic cells and spermatozoa. Spermatogenic cells and spermatozoa were obtained from male Sprague-Dawley rats. Reverse transcription polymerase chain reaction (RT-PCR) were used to detect the expression of all TRPM and TRPV channel members with specific primers. Western blot analysis was applied for detecting the expression of TRPM and TRPV channel proteins. Immunohistochemistry staining for TRPM4, TRPM7 and TRPV5 was also performed in rat testis. The mRNAs of TRPM3, TRPM4, TRPM7 and TRPV5 were detected in the spermatogenic cells and spermatozoa in rat. Western blot analysis verified the expression of TRPM4, TRPM7 and TRPV5 in the rat spermatogenic cells and spermatozoa. Immunocytochemistry staining for TRPM and TRPV channel families indicated that TRPM4 and TRPM7 proteins were highly expressed in different stages of spermatogenic cells and spermatozoa, while TRPV5 protein was lowly expressed in these cells. Our results demonstrate that mRNAs or proteins for TRPM3, TRPM4, TRPM7 and TRPV5 exist in rat spermatogenic cells and spermatozoa. These data presented here may assist in elucidating the possible physiological function of TRPM and TRPV channels in spermatogenic cells and spermatozoa.

Bradykinin regulates calpain and proinflammatory signaling through TRPM7-sensitive pathways in vascular smooth muscle cells

Transient receptor potential melastatin-7 (TRPM7) channels have recently been identified to be regulated by vasoactive agents acting through G protein-coupled receptors in vascular smooth muscle cells (VSMC). However, downstream targets and functional responses remain unclear. We investigated the subcellular localization of TRPM7 in VSMCs and questioned the role of TRPM7 in proinflammatory signaling by bradykinin. VSMCs from Wistar-Kyoto rats were studied. Cell fractionation by sucrose gradient and differential centrifugation demonstrated that in bradykinin-stimulated cells, TRPM7 localized in fractions corresponding to caveolae. Immunofluorescence confocal microscopy revealed that TRPM7 distributes along the cell membrane, that it has a reticular-type intracellular distribution, and that it colocalizes with flotillin-2, a marker of lipid rafts. Bradykinin increased expression of calpain, a TRPM7 target, and stimulated its cytosol/membrane translocation, an effect blocked by 2-APB (TRPM7 inhibitor) and U-73122 (phospholipase C inhibitor), but not by chelerythrine (PKC inhibitor). Expression of proinflammatory mediators VCAM-1 and cyclooxygenase-2 (COX-2) was time-dependently increased by bradykinin. This effect was blocked by Hoe-140 (B2 receptor blocker) and 2-APB. Our data demonstrate that in bradykinin-stimulated VSMCs: 1) TRPM7 is upregulated, 2) TRPM7 associates with cholesterol-rich microdomains, and 3) calpain and proinflammatory mediators VCAM-1 and COX2 are regulated, in part, via TRPM7- and phospholipase C-dependent pathways through B2 receptors. These findings identify a novel signaling pathway for bradykinin, which involves TRPM7. Such phenomena may play a role in bradykinin/B2 receptor-mediated inflammatory responses in vascular cells.

Regulation of the novel Mg2+ transporter transient receptor potential melastatin 7 (TRPM7) cation channel by bradykinin in vascular smooth muscle cells

BACKGROUND

Transient receptor potential melastatin 7 (TRPM7) channels have been identified in the vasculature. However, their regulation and function remain unclear.

METHODS

Here, we tested the hypothesis that bradykinin and its second messenger cAMP upregulate TRPM7, which stimulates activation of annexin-1 (TRPM7 substrate) and increases transmembrane Mg2+ transport and vascular smooth muscle cell (VSMC) migration. Human and rat VSMCs were studied. TRPM7 phosphorylation was assessed by immunoprecipitation:immunoblotting using antiphospho-serine/threonine and anti-TRPM7 antibodies. [Mg2+]i was measured by mag-fura-2. TRPM7 was downregulated by small interfering RNA and 2-aminoethoxydiphenyl borate. Annexin-1 activity was assessed by cytosol-to-membrane translocation. Cell migration and invasion, functional responses to bradykinin, were assessed in transwell chambers.

RESULTS

Bradykinin increased expression of TRPM7 and annexin-1. TRPM7 was rapidly (5 min) phosphorylated on serine/threonine but not on tyrosine residues by bradykinin. [Mg2+]i was increased in bradykinin-stimulated cells (0.53 versus 0.68 mmol/l, basal versus bradykinin, P < 0.01). Annexin-1 activation was increased by bradykinin and inhibited by 2-aminoethoxydiphenyl borate. Although Hoe 140 (B2 receptor antagonist), U-73122 (phospholipase C inhibitor), 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (c-Src inhibitor) and chelerythrine (protein kinase C inhibitor) blocked bradykinin actions, dibutyryl-c-AMP was without effect. In small interfering RNA-transfected and in 2-aminoethoxydiphenyl borate-treated cells, bradykinin-induced Mg2+ influx and VSMC migration were reduced.

CONCLUSION

Our results demonstrate that bradykinin regulates TRPM7 and its downstream target annexin-1 through phospholipase C-dependent, protein kinase C-dependent and c-Src-dependent and cAMP-independent pathways; effects are mediated through bradykinin type 2 receptor; and bradykinin regulates VSMC [Mg2+]i and migration through TRPM7. These data identify TRPM7/annexin-1/Mg2+ as a novel pathway in bradykinin signaling.

A double TRPtych: six views of transient receptor potential channels in disease and health

At the 2008 Annual Meeting of the Society for Neuroscience, a Mini-Symposium entitled "Contributions to TRP Channels to Neurological Disease" included talks from six heads of newly established laboratories, each with a unique research focus, model system, and set of experimental tools. Some of the questions addressed in these talks include the following. What is the role of transient receptor potential (TRP) channels in pain perception? How do normally functioning TRP channels contribute to cell death pathways? What are the characteristics of TRPpathies, disease states that result from overactive or underactive TRP channels? How are TRP channels regulated by signal transduction cascades? This review summarizes recent results from those laboratories and provides six perspectives on the subject of TRP channels and disease.

Dysregulation of vascular TRPM7 and annexin-1 is associated with endothelial dysfunction in inherited hypomagnesemia

Inadequate magnesium intake and hypomagnesemia may contribute to chronic diseases, such as hypertension. The novel magnesium transporter TRPM7 is a critical regulator of magnesium homeostasis in vascular cells, but its role in pathophysiology is unclear. In a model of hypomagnesemia, we examined microvascular structure and function, TRPM7 expression, and vascular inflammatory status using inbred mice selected for normal-high intracellular magnesium levels or low intracellular magnesium levels (MgLs). Blood pressure was significantly increased in MgLs compared with normal-high intracellular magnesium levels. Pressurized myography of mesenteric resistance arteries showed that MgLs had significantly impaired endothelial function together with decreased plasma nitrate levels and endothelial NO synthase expression when compared with normal-high intracellular magnesium levels. Significant differences in vascular structure were also evident in both mesenteric arteries and aortas from MgLs. Aortas from MgLs had increased medial cross-sectional areas, whereas mesenteric arteries from MgLs had increased lumen diameters with increased medial cross-sectional areas, indicating outward hypertrophic remodeling. Expression of the magnesium transporter TRPM7 was significantly elevated in the vasculature of MgLs, whereas expression of a TRPM7 downstream target, the anti-inflammatory molecule annexin-1, was reduced. MgLs had increased expression of vascular cell adhesion molecule-1 and plasminogen activator inhibitor-1, indicating vascular inflammation. Taken together, these data demonstrate that the inherited magnesium status of MgLs and normal-high intracellular magnesium levels mice affects magnesium transporter expression, endothelial function, vascular structure, and inflammation. Our findings suggest a potential regulatory role for TRPM7 signaling in the maintenance of vascular integrity. Alterations in magnesium status and/or TRPM7 signaling may contribute to vascular injury in conditions associated with hypomagnesemia.

Suppression of transient receptor potential melastatin 7 channel induces cell death in gastric cancer

Ca2+ and Mg2+ have a fundamental role in many cellular processes and ion channels are involved in normal physiologic processes and in the pathology of various diseases. The aim here was to show that the presence and potential role of transient receptor potential melastatin 7 (TRPM7) channels in the growth and survival of AGS cells, the most common human gastric adenocarcinoma cell line. The patch-clamp technique for whole-cell recording was used in AGS cells. TRPM7-specific small interfering RNAs were used for specific inhibition of TRPM7. Whole-cell voltage-clamp recordings revealed the TRPM7-like currents that activated spontaneously following loss of intracellular Mg2+. The current had a non-linear current-voltage relationship with the characteristic steep outward rectification associated with TRPM7 channels. Reverse transcription-polymerase chain reaction, western blotting, and immunoreactivity all showed abundant expression of TRPM7 messenger RNA and protein in AGS cells. Transfection of AGS cells with TRPM7 siRNA significantly reduced the expression of TRPM7 mRNA and protein as well as the amplitude of the TRPM7-like currents. Furthermore, we found that Mg2+ is critical for the growth and survival in AGS cells. Blockade of TRPM7 channels by La3+ and 2-APB or suppression of TRPM7 expression by siRNA inhibited the growth and survival of these cells. Human gastric adenocarcinoma cells express TRPM7 channel whose presence is essential for cell survival. The protein is a likely potential target for the pharmacological treatment of gastric cancer.

X-ray crystal structure of a TRPM assembly domain reveals an antiparallel four-stranded coiled-coil

Transient receptor potential (TRP) channels comprise a large family of tetrameric cation-selective ion channels that respond to diverse forms of sensory input. Earlier studies showed that members of the TRPM subclass possess a self-assembling tetrameric C-terminal cytoplasmic coiled-coil domain that underlies channel assembly and trafficking. Here, we present the high-resolution crystal structure of the coiled-coil domain of the channel enzyme TRPM7. The crystal structure, together with biochemical experiments, reveals an unexpected four-stranded antiparallel coiled-coil architecture that bears unique features relative to other antiparallel coiled-coils. Structural analysis indicates that a limited set of interactions encode assembly specificity determinants and uncovers a previously unnoticed segregation of TRPM assembly domains into two families that correspond with the phylogenetic divisions seen for the complete subunits. Together, the data provide a framework for understanding the mechanism of TRPM channel assembly and highlight the diversity of forms found in the coiled-coil fold.

Redox-sensitive signaling by angiotensin II involves oxidative inactivation and blunted phosphorylation of protein tyrosine phosphatase SHP-2 in vascular smooth muscle cells from SHR

Angiotensin II (Ang II) signaling in vascular smooth muscle cells (VSMCs) involves reactive oxygen species (ROS) through unknown mechanisms. We propose that Ang II induces phosphorylation of growth signaling kinases by redox-sensitive regulation of protein tyrosine phosphatases (PTP) in VSMCs and that augmented Ang II signaling in spontaneously hypertensive rats (SHRs) involves oxidation/inactivation and blunted phosphorylation of the PTP, SHP-2. PTP oxidation was assessed by the in-gel PTP method. SHP-2 expression and activity were evaluated by immunoblotting and by a PTP activity assay, respectively. SHP-2 and Nox1 were downregulated by siRNA. Ang II induced oxidation of multiple PTPs, including SHP-2. Basal SHP-2 content was lower in SHRs versus WKY. Ang II increased SHP-2 phosphorylation and activity with blunted responses in SHRs. Ang II-induced SHP-2 effects were inhibited by valsartan (AT(1)R blocker), apocynin (NAD(P)H oxidase inhibitor), and Nox1 siRNA. Ang II stimulation increased activation of ERK1/2, p38MAPK, and AKT, with enhanced effects in SHR. SHP-2 knockdown resulted in increased AKT phosphorylation, without effect on ERK1/2 or p38MAPK. Nox1 downregulation attenuated Ang II-mediated AKT activation in SHRs. Hence, Ang II regulates PTP/SHP-2 in VSMCs through AT(1)R and Nox1-based NAD(P)H oxidase via two mechanisms, oxidation and phosphorylation. In SHR Ang II-stimulated PTP oxidation/inactivation is enhanced, basal SHP-2 expression is reduced, and Ang II-induced PTP/SHP-2 phosphorylation is blunted. These SHP-2 actions are associated with augmented AKT signaling. We identify a novel redox-sensitive SHP-2-dependent pathway for Ang II in VSMCs. SHP-2 dysregulation by increased Nox1-derived ROS in SHR is associated with altered Ang II-AKT signaling.

SLC41A1 is a novel mammalian Mg2+ carrier

The molecular biology of mammalian magnesium transporters and their interrelations in cellular magnesium homeostasis are largely unknown. Recently, the mouse SLC41A1 protein was suggested to be a candidate magnesium transporter with channel-like properties when overexpressed in Xenopus laevis oocytes. Here, we demonstrate that human SLC41A1 overexpressed in HEK293 cells forms protein complexes and locates to the plasma membrane without, however, giving rise to any detectable magnesium currents during whole cell patch clamp experiments. Nevertheless, in a strain of Salmonella enterica exhibiting disruption of all three distinct magnesium transport systems (CorA, MgtA, and MgtB), overexpression of human SLC41A1 functionally substitutes these transporters and restores the growth of the mutant bacteria at magnesium concentrations otherwise non-permissive for growth. Thus, we have identified human SLC41A1 as being a bona fide magnesium transporter. Most importantly, overexpressed SLC41A1 provide HEK293 cells with an increased magnesium efflux capacity. With outwardly directed Mg(2+) gradients, a SLC41A1-dependent reduction of the free intracellular magnesium concentration accompanied by a significant net decrease of the total cellular magnesium concentration could be observed in such cells. SLC41A1 activity is temperature-sensitive but not sensitive to the only known magnesium channel blocker, cobalt(III) hexaammine. Taken together, these data functionally identify SLC41A1 as a mammalian carrier mediating magnesium efflux.

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.

Relationship between low magnesium status and TRPM6 expression in the kidney and large intestine

The body maintains Mg(2+) homeostasis by renal and intestinal (re)absorption. However, the molecular mechanisms that mediate transepithelial Mg(2+) transport are largely unknown. Transient receptor potential melastatin 6 (TRPM6) was recently identified and shown to function in active epithelial Mg(2+) transport in intestine and kidney. To define the relationship between Mg(2+) status and TRPM6 expression, we used two models of hypomagnesemia: 1) C57BL/6J mice fed a mildly or severely Mg(2+)-deficient diet, and 2) mice selected for either low (MgL) or high (MgH) erythrocyte and plasma Mg(2+) status. In addition, the mice were subjected to a severely Mg(2+)-deficient diet. Our results show that C57BL/6J mice fed a severely Mg(2+)-deficient diet developed hypomagnesemia and hypomagnesuria and showed increased TRPM6 expression in kidney and intestine. When fed a Mg(2+)-adequate diet, MgL mice presented hypomagnesemia and hypermagnesuria, and lower kidney and intestinal TRPM6 expression, compared with MgH mice. A severely Mg(2+)-deficient diet led to hypomagnesemia and hypomagnesuria in both strains. Furthermore, this diet induced kidney TRPM6 expression in MgL mice, but not in MgH mice. In conclusion, as shown in C57BL/6J mice, dietary Mg(2+)-restriction results in increased Mg(2+) (re)absorption, which is correlated with increased TRPM6 expression. In MgL and MgH mice, the inherited Mg(2+) status is linked to different TRPM6 expression. The MgL and MgH mice respond differently to a low-Mg(2+) diet with regard to TRPM6 expression in the kidney, consistent with genetic factors contributing to the regulation of cellular Mg(2+) levels. Further studies of these mice strains could improve our understanding of the genetics of Mg(2+) homeostasis.

Molecular basis of the mammalian pressure-sensitive ion channels: focus on vascular mechanotransduction

Mechano-gated ion channels are implicated in a variety of neurosensory functions ranging from touch sensitivity to hearing. In the heart, rhythm disturbance subsequent to mechanical effects is also associated with the activation of stretch-sensitive ion channels. Arterial autoregulation in response to hemodynamic stimuli, a vital process required for protection against hypertension-induced injury, is similarly dependent on the activity of force-sensitive ion channels. Seminal work in prokaryotes and invertebrates, including the nematode Caenorhabditis elegans and the fruit fly drosophila, greatly helped to identify the molecular basis of volume regulation, hearing and touch sensitivity. In mammals, more recent findings have indicated that members of several structural family of ion channels, namely the transient receptor potential (TRP) channels, the amiloride-sensitive ENaC/ASIC channels and the potassium channels K2P and Kir are involved in cellular mechanotransduction. In the present review, we will focus on the molecular and functional properties of these channel subunits and will emphasize on their role in the pressure-dependent arterial myogenic constriction and the flow-mediated vasodilation.

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.

Calcium-associated mechanisms in gut pacemaker activity

A considerable body of evidence has revealed that interstitial cells of Cajal (ICC), identified with c-Kit-immunoreactivity, act as gut pacemaker cells, with spontaneous Ca²⁺ activity in ICC as the probable primary mechanism. Namely, intracellular (cytosolic) Ca²⁺ oscillations in ICC periodically activate plasmalemmal Ca²⁺-dependent ion channels and thereby generate pacemaker potentials. This review will, thus, focus on Ca²⁺-associated mechanisms in ICC in the gastrointestinal (GI) tract, including auxiliary organs.

Importance of melastatin-like transient receptor potential 7 and cations (magnesium, calcium) in human osteoblast-like cell proliferation

Bone tissue in the adult is continuously being remodelled, and overall bone mass is maintained constant by the balance between osteoclastic bone resorption and osteoblastic bone formation. Adequate osteoblastic proliferation is essential for both appropriate formation and for regulation of resorption, and thereby the maintenance of bone remodelling equilibrium.

OBJECTIVES

Here, we have investigated the roles of melastatin-like transient receptor potential 6 and 7 (TRPM6, TRPM7), which are calcium (Ca2+) and magnesium (Mg2+) conducting channels, during proliferation of human osteoblasts.

RESULTS

Genetic expression of TRPM6 and TRPM7 was shown in human osteoblast-like MG-63, SaOS and U2-OS cells, and reduction of extracellular Mg2+ or Ca2+ led to a decrease of cell proliferation. Concomitant reduction of both ions further accentuated reduction of cell proliferation. Expression of TRPM7 channels was increased under conditions of reduced extracellular Mg2+ and Ca2+ levels whereas expression of TRPM6 was not modified, suggesting compensatory mechanisms afforded by TRPM7 in order to maintain intracellular ion homeostasis. Pre-incubation of cells in reduced extracellular Mg2+ conditions led to activation of Ca2+ and Mg2+ influx. Reduction of TRPM7 expression by specific siRNA prevented latter influx and inhibited cell proliferation.

CONCLUSIONS

Our results indicate that extracellular Mg2+ and Ca2+ deficiency reduces the proliferation of human osteoblastic cells. Expression and activity of TRPM7 is modulated by extracellular Mg2+ and Ca2+ availability, indicating that TRPM7 channels are involved in intracellular ion homeostasis and proliferation of osteoblasts.

Transient receptor potential melastatin 7-like current in human head and neck carcinoma cells: role in cell proliferation

Ion channels are involved in normal physiologic processes and in the pathology of various diseases. In this study, we investigated the presence and potential function of transient receptor potential melastatin 7 (TRPM7) channels in the growth and proliferation of FaDu and SCC25 cells, two common human head and neck squamous carcinoma cell lines, using a combination of patch-clamp recording, Western blotting, immunocytochemistry, small interfering RNA (siRNA), fluorescent Ca(2+) imaging, and cell counting techniques. Although voltage-gated K(+) currents were recorded in all cells, none of FaDu cells express voltage-gated Na(+) or Ca(2+) currents. Perfusion of cells with NMDA or acidic solution did not activate inward currents, indicating a lack of NMDA receptor and acid-sensing channels. Lowering extracellular Ca(2+), however, induced a large nondesensitizing current reminiscent of Ca(2+)-sensing cation current or TRPM7 current previously described in other cells. This Ca(2+)-sensing current can be inhibited by Gd(3+), 2-aminoethoxydiphenyl borate (2-APB), or intracellular Mg(2+), consistent with the TRPM7 current being activated. Immunocytochemistry, Western blot, and reverse transcription-PCR detected the expression of TRPM7 protein and mRNA in these cells. Transfection of FaDu cells with TRPM7 siRNA significantly reduced the expression of TRPM7 mRNA and protein as well as the amplitude of the Ca(2+)-sensing current. Furthermore, we found that Ca(2+) is critical for the growth and proliferation of FaDu cells. Blockade of TRPM7 channels by Gd(3+) and 2-APB or suppression of TRPM7 expression by siRNA inhibited the growth and proliferation of these cells. Similar to FaDu cells, SCC25 cells also express TRPM7-like channels. Suppressing the function of these channels inhibited the proliferation of SCC25 cells.

Angiotensin-(1-7) counterregulates angiotensin II signaling in human endothelial cells

Angiotensin (Ang)-(1-7), acting through the Mas receptor, opposes the actions of Ang II. Molecular mechanisms for this are unclear. Here we sought to determine whether Ang-(1-7) influences Ang II signaling in human endothelial cells, focusing specifically on Src homology 2-containing inositol phosphatase 2 (SHP-2) and its interaction with c-Src. Ang II-induced phosphorylation of c-Src, extracellular signal regulated kinase (ERK)1/2, and SHP-2 and activation of NAD(P)H oxidase were assessed in the absence and presence of Ang-(1-7) (10(-6) mol/L, 15 minutes) by immunoblotting and lucigenin-enhanced chemiluminescence, respectively. (D-Ala(7))-Ang I/II (1-7) (Ang fragment 1-7 receptor antagonist) was used to block Ang-(1-7) effects. Association between SHP-2 and c-Src was assessed by immunoprecipitation/immunoblotting studies. Ang II significantly increased activation of c-Src, ERK1/2, and NAD(P)H oxidase and reduced phosphorylation of SHP-2 (P<0.05) in human endothelial cells. These effects were abrogated in cells pre-exposed to Ang-(1-7). Ang fragment 1-7 receptor antagonist pretreatment blocked the negative modulatory actions of Ang-(1-7) on Ang II-induced signaling. Ang-(1-7) alone did not significantly alter phosphorylation of c-Src, ERK1/2, and SHP-2 and had no effect on basal activity of NAD(P)H oxidase. SHP-2 and c-Src were physically associated in the basal state. This association was increased by Ang-(1-7) and blocked by Ang fragment 1-7 receptor antagonist. Our findings demonstrate that, in human endothelial cells, Ang-(1-7) negatively modulates Ang II/Ang II type 1 receptor-activated c-Src and its downstream targets ERK1/2 and NAD(P)H oxidase. We also show that SHP-2-c-Src interaction is enhanced by Ang-(1-7). These phenomena may represent a protective mechanism in the endothelium whereby potentially deleterious effects of Ang II are counterregulated by Ang-(1-7).

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.

Enhancement of receptor-operated cation current and TRPC6 expression in arterial smooth muscle cells of deoxycorticosterone acetate-salt hypertensive rats

OBJECTIVES

In deoxycorticosterone acetate (DOCA)-salt hypertensive rats, altered reactivity of blood vessels to vasoactive agonists is frequently associated with an elevation in blood pressure. Canonical transient receptor potential (TRPC) channels are believed to encode receptor-operated cation channels (ROC), the activation of which is involved in smooth muscle depolarization and vasoconstriction. The aims of the present study were to investigate whether the ROC current is increased in DOCA-hypertensive rats and determine whether aldosterone directly enhances the expression of TRPC.

METHODS

The nystatin-perforated patch-clamp technique was used for the recording of receptor-stimulated ion currents in mesenteric arterial smooth muscle cells, which were enzymatically dispersed from sham-operated and DOCA-salt hypertensive rats. Expressions of TRPCs were evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR) and by Western blot analysis.

RESULTS

Receptor-stimulated currents activated by 5-hydroxytryptamine (serotonin) and norepinephrine were increased significantly in the mesenteric arterial smooth muscle cells of DOCA-salt hypertensive rats compared to sham-operated rats. Ion-substitution experiments revealed that the enhanced currents were cation currents (ROC currents). Enhanced expression of TRPC6 in mesenteric arteries from DOCA-salt hypertensive rats was demonstrated by real-time RT-PCR. Up-regulation of TRPC6 by aldosterone treatment in vitro was also observed in A7r5 cells by RT-PCR and in western blots.

CONCLUSION

These results suggest that aldosterone enhances TRPC6 expression and ROC currents in vascular smooth muscle cells, and that this may in turn contribute to altered vascular reactivity and to hypertension.

TRP channels in hypertension

Pulmonary and systemic arterial hypertension are associated with profound alterations in Ca(2+) homeostasis and smooth muscle cell proliferation. A novel class of non-selective cation channels, the transient receptor potential (TRP) channels, have emerged at the forefront of research into hypertensive disease states. TRP channels are identified as molecular correlates for receptor-operated and store-operated cation channels in the vasculature. Over 10 TRP isoforms are identified at the mRNA and protein expression levels in the vasculature. Current research implicates upregulation of specific TRP isoforms to be associated with increased Ca(2+) influx, characteristic of vasoconstriction and vascular smooth muscle cell proliferation. TRP channels are implicated as Ca(2+) entry pathways in pulmonary hypertension and essential hypertension. Caveolae have recently emerged as membrane microdomains in which TRP channels may be co-localized with the endoplasmic reticulum in both smooth muscle and endothelial cells. Such enhanced expression and function of TRP channels and their localization in caveolae in pathophysiological hypertensive disease states highlights their importance as potential targets for pharmacological intervention.

TRPM7 and TRPM2-Candidate susceptibility genes for Western Pacific ALS and PD?

Recent findings implicating TRPM7 and TRPM2 in oxidative stress-induced neuronal death thrust these channels into the spotlight as possible therapeutic targets for neurodegenerative diseases. In this review, we describe how the functional properties of TRPM7 and TRPM2 are interconnected with calcium (Ca(2+)) and magnesium (Mg(2+)) homeostasis, oxidative stress, mitochondrial dysfunction, and immune mechanisms, all principal suspects in neurodegeneration. We focus our discussion on Western Pacific Amyotrophic Lateral Sclerosis (ALS) and Parkinsonism Dementia (PD) because extensive studies conducted over the years strongly suggest that these diseases are ideal candidates for a gene-environment model of etiology. The unique mineral environment identified in connection with Western Pacific ALS and PD, low Mg(2+) and Ca(2+), yet high in transition metals, creates a condition that could affect the proper function of these two channels.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

Cotransporters, WNKs and hypertension: important leads from the study of monogenetic disorders of blood pressure regulation

Major advances are being made in identifying the structure and behaviour of regulatory cascades that control the activity of cation-Cl(-) cotransporters and certain Na(+), K(+) and Cl(-) channels. These transporters play key roles in regulating arterial blood pressure as they are not only responsible for NaCl reabsorption in the thick ascending limb and distal tubule of the kidney, but are also involved in regulating smooth muscle Ca(2+) levels. It is now apparent that defects in these transporters, and particularly in the regulatory cascades, cause some monogenetic forms of hypertension and may contribute to essential hypertension and problems with K(+) homoeostasis. Two families of kinases are prominent in these processes: the Ste-20-related kinases [OSR1 (oxidative stress-responsive kinase 1) and SPAK (Ste20/SPS1-related proline/alanine-rich kinase)] and the WNKs [with no lysine kinases]. These kinases affect the behaviour of their targets through both phosphorylation and by acting as scaffolding proteins, bringing together regulatory complexes. This review analyses how these kinases affect transport by activating or inhibiting individual transporters at the cell surface, or by changing the surface density of transporters by altering the rate of insertion or removal of transporters from the cell surface, and perhaps through controlling the rate of transporter degradation. This new knowledge should not only help us target antihypertensive therapy more appropriately, but could also provide the basis for developing new therapeutic approaches to essential hypertension.

Cell death of melanophores in zebrafish trpm7 mutant embryos depends on melanin synthesis

Transient receptor potential melastatin 7 (TRPM7) is a broadly expressed, non-selective cation channel. Studies in cultured cells implicate TRPM7 in regulation of cell growth, spreading, and survival. However, zebrafish trpm7 homozygous mutants display death of melanophores and temporary paralysis, but no gross morphological defects during embryonic stages. This phenotype implies that melanophores are unusually sensitive to decreases in Trpm7 levels, a hypothesis we investigate here. We find that pharmacological inhibition of caspases does not rescue melanophore viability in trpm7 mutants, implying that melanophores die by a mechanism other than apoptosis. Consistent with this possibility, ultrastructural analysis of dying melanophores in trpm7 mutants reveals abnormal melanosomes and evidence of a ruptured plasma membrane, indicating that cell death occurs by necrosis. Interestingly, inhibition of melanin synthesis largely prevents melanophore cell death in trpm7 mutants. These results suggest that melanophores require Trpm7 in order to detoxify intermediates of melanin synthesis. We find that unlike TRPM1, TRPM7 is expressed in human melanoma cell lines, indicating that these cells may also be sensitized to reduction of TRPM7 levels.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

TRP channels: molecular diversity and physiological function

Calcium ions (Ca(2+)) are particularly important in cellular homeostasis and activity. To elicit physiologically relevant timing and spatial patterns of Ca(2+) signaling, ion channels in the surface of each cell precisely control Ca(2+) influx across the plasma membrane. A group of surface membrane ion channels called receptor-activated cation/Ca(2+) channels (RACCs) are activated by diverse cellular stimuli from the surrounding extracellular environment via receptors and other pathways such as heat, osmotic pressure, and mechanical and oxidative stress. An important clue to understanding the molecular mechanisms underlying the functional diversity of RACCs was first attained by molecular identification of the transient receptor potential (trp) protein (TRP), which mediates light-induced depolarization in Drosophila photoreceptor cells, and its homologues from various biological species. Recent studies have revealed that respective TRP channels are indeed activated by characteristic cellular stimuli. Furthermore, the involvement of TRP channels has been demonstrated in the signaling pathways essential for tissue-specific functions as well as ubiquitous biological responses, such as cell proliferation, differentiation, and death. These findings encourage the usage of TRP channels and their signalplexes as powerful tools for developing novel pharmaceutical targets.

TRPC3 and TRPC6 are essential for angiotensin II-induced cardiac hypertrophy

Angiotensin (Ang) II participates in the pathogenesis of heart failure through induction of cardiac hypertrophy. Ang II-induced hypertrophic growth of cardiomyocytes is mediated by nuclear factor of activated T cells (NFAT), a Ca(2+)-responsive transcriptional factor. It is believed that phospholipase C (PLC)-mediated production of inositol-1,4,5-trisphosphate (IP(3)) is responsible for Ca(2+) increase that is necessary for NFAT activation. However, we demonstrate that PLC-mediated production of diacylglycerol (DAG) but not IP(3) is essential for Ang II-induced NFAT activation in rat cardiac myocytes. NFAT activation and hypertrophic responses by Ang II stimulation required the enhanced frequency of Ca(2+) oscillation triggered by membrane depolarization through activation of DAG-sensitive TRPC channels, which leads to activation of L-type Ca(2+) channel. Patch clamp recordings from single myocytes revealed that Ang II activated DAG-sensitive TRPC-like currents. Among DAG-activating TRPC channels (TRPC3, TRPC6, and TRPC7), the activities of TRPC3 and TRPC6 channels correlated with Ang II-induced NFAT activation and hypertrophic responses. These data suggest that DAG-induced Ca(2+) signaling pathway through TRPC3 and TRPC6 is essential for Ang II-induced NFAT activation and cardiac hypertrophy.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.

Cation channels of the transient receptor potential superfamily: their role in physiological and pathophysiological processes of smooth muscle cells

Smooth muscle cells (SMC) are essential components of many tissues of the body. Ion channels regulate their membrane potential, the intracellular Ca(2+) concentration ([Ca(2+)](i)) and their contractility. Among the ion channels expressed in SMC cation channels of the transient receptor potential (TRP) superfamily allow the entry of Na(+), Ca(2+) and Mg(2+). Members of the TRP superfamily are essential constituents of tonically active channels (TAC), receptor-operated channels (ROC), store-operated channels (SOC) and stretch-activated channels (SAC). This review focusses on TRP channels (TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, TRPV2, TRPV4, TRPM4, TRPM7, TRPP2) whose physiological functions in SMC were dissected by downregulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models. Their possible functional role and physiological regulation as homomeric or heteromeric channels in SMC are discussed. Moreover, TRP channels may also be responsible for pathophysiological processes involving SMC-like airway hyperresponsiveness and pulmonary hypertension. Therefore, they present important drug targets for future pharmacological interventions.

Loading rat heart myocytes with Mg2+ using low-[Na+] solutions

The objective of our study was to investigate how Mg2+ enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg2+ entry, and now provide a preliminary account of its properties. Changes in Mg2+ influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg2+ concentration ([fMg2+]i) measured from the fluorescence of mag-fura-2 loaded into isolated cells. Superfusion of myocytes at 37 degrees C with Ca2+-free solutions with both reduced [Na+] and raised [Mg2+] caused myocytes to load with Mg2+. Uptake was seen with solutions containing 5 mm Mg2+ and 95 mm Na+, and increased linearly with increasing extracellular [Mg2+] or decreasing extracellular [Na+]. It was very sensitive to temperature (Q(10) > 9, 25--37 degrees C), was observed even in myocytes with very low Na+ contents, and stopped abruptly when external [Na+] was returned to normal. Uptake was greatly reduced by imipramine or KB-R7943 if these were added when [fMg2+]i was close to the physiological level, but was unaffected if they were applied when [fMg2+]i was above 2 mm. Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K+] or voltage clamp to 0 mV. We suggest that initial Mg2+ uptake may involve several transporters, including reversed Na+-Mg2+ antiport and, depending on the exact conditions, reversed Na+-Ca2+ antiport. The ensuing rise of [fMg2+]i, in conjunction with reduced [Na+], may then activate a new Mg2+ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB-R7943, but is inactivated by depolarization.

Role of magnesium in hypertension

Magnesium affects blood pressure by modulating vascular tone and reactivity. It acts as a calcium channel antagonist, it stimulates production of vasodilator prostacyclins and nitric oxide and it alters vascular responses to vasoactive agonists. Magnesium deficiency has been implicated in the pathogenesis of hypertension with epidemiological and experimental studies demonstrating an inverse correlation between blood pressure and serum magnesium levels. Magnesium also influences glucose and insulin homeostasis, and hypomagnesemia is associated with metabolic syndrome. Although most epidemiological and experimental studies support a role for low magnesium in the pathophysiology of hypertension, data from clinical studies have been less convincing. Furthermore, the therapeutic value of magnesium in the management of hypertension is unclear. The present review addresses the role of magnesium in the regulation of vascular function and blood pressure and discusses the implications of magnesium deficiency in experimental and clinical hypertension, in metabolic syndrome and in pre-eclampsia.

ATP and PIP2 dependence of the magnesium-inhibited, TRPM7-like cation channel in cardiac myocytes

The Mg(2+)-inhibited cation (MIC) current (I(MIC)) in cardiac myocytes biophysically resembles currents of heterologously expressed transient receptor potential (TRP) channels, particularly TRPM6 and TRPM7, known to be important in Mg(2+) homeostasis. To understand the regulation of MIC channels in cardiac cells, we used the whole cell voltage-clamp technique to investigate the role of intracellular ATP in pig, rat, and guinea pig isolated ventricular myocytes. I(MIC), studied in the presence or absence of extracellular divalent cations, was sustained for >or=50 min after patch rupture in ATP-dialyzed cells, whereas in ATP-depleted cells I(MIC) exhibited complete rundown. Equimolar substitution of internal ATP by its nonhydrolyzable analog adenosine 5'-(beta,gamma-imido)triphosphate failed to prevent rundown. In ATP-depleted cells, inhibition of lipid phosphatases by fluoride + vanadate + pyrophosphate prevented I(MIC) rundown. In contrast, under similar conditions neither the inhibition of protein phosphatases 1, 2A, 2B or of protein tyrosine phosphatase nor the activation of protein kinase A (forskolin, 20 microM) or protein kinase C (phorbol myristate acetate, 100 nM) could prevent rundown. In ATP-loaded cells, depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) by prevention of its resynthesis (10 microM wortmannin or 15 microM phenylarsine oxide) induced rundown of I(MIC). Finally, loading ATP-depleted cells with exogenous PIP(2) (10 microM) prevented rundown. These results suggest that PIP(2), likely generated by ATP-utilizing lipid kinases, is necessary for maintaining cardiac MIC channel activity.

A sphingosine-1-phosphate-activated calcium channel controlling vascular smooth muscle cell motility

In a screen of potential lipid regulators of transient receptor potential (TRP) channels, we identified sphingosine-1-phosphate (S1P) as an activator of TRPC5. We explored the relevance to vascular biology because S1P is a key cardiovascular signaling molecule. TRPC5 is expressed in smooth muscle cells of human vein along with TRPC1, which forms a complex with TRPC5. Importantly, S1P also activates the TRPC5-TRPC1 heteromultimeric channel. Because TRPC channels are linked to neuronal growth cone extension, we considered a related concept for smooth muscle. We find S1P stimulates smooth muscle cell motility, and that this is inhibited by E3-targeted anti-TRPC5 antibody. Ion permeation involving TRPC5 is crucial because S1P-evoked motility is also suppressed by the channel blocker 2-aminoethoxydiphenyl borate or a TRPC5 ion-pore mutant. S1P acts on TRPC5 via two mechanisms, one extracellular and one intracellular, consistent with its bipolar signaling functions. The extracellular effect appears to have a primary role in S1P-evoked cell motility. The data suggest S1P sensing by TRPC5 calcium channel is a mechanism contributing to vascular smooth muscle adaptation.

The role of TRP channels in oxidative stress-induced cell death

The transient receptor potential (TRP) protein superfamily is a diverse group of voltage-independent calcium-permeable cation channels expressed in mammalian cells. These channels have been divided into six subfamilies, and two of them, TRPC and TRPM, have members that are widely expressed and activated by oxidative stress. TRPC3 and TRPC4 are activated by oxidants, which induce Na(+) and Ca(2+) entry into cells through mechanisms that are dependent on phospholipase C. TRPM2 is activated by oxidative stress or TNFalpha, and the mechanism involves production of ADP-ribose, which binds to an ADP-ribose binding cleft in the TRPM2 C-terminus. Treatment of HEK 293T cells expressing TRPM2 with H(2)O(2) resulted in Ca(2+) influx and increased susceptibility to cell death, whereas coexpression of the dominant negative isoform TRPM2-S suppressed H(2)O(2)-induced Ca(2+) influx, the increase in [Ca(2+)](i), and onset of apoptosis. U937-ecoR monocytic cells expressing increased levels of TRPM2 also exhibited significantly increased [Ca(2+)](i) and increased apoptosis after treatment with H(2)O(2) or TNFalpha. A dramatic increase in caspase 8, 9, 3, 7, and PARP cleavage was observed in TRPM2-expressing cells, demonstrating a downstream mechanism through which cell death is mediated. Inhibition of endogenous TRPM2 function through three approaches, depletion of TRPM2 by RNA interference, blockade of the increase in [Ca(2+)](i) through TRPM2 by calcium chelation, or expression of the dominant negative splice variant TRPM2-S protected cell viability. H(2)O(2) and amyloid beta-peptide also induced cell death in primary cultures of rat striatal cells, which endogenously express TRPM2. TRPM7 is activated by reactive oxygen species/nitrogen species, resulting in cation conductance and anoxic neuronal cell death, which is rescued by suppression of TRPM7 expression. TRPM2 and TRPM7 channels are physiologically important in oxidative stress-induced cell death.

Functional expression of transient receptor potential melastatin- and vanilloid-related channels in pulmonary arterial and aortic smooth muscle

Transient receptor potential melastatin- (TRPM) and vanilloid-related (TRPV) channels are nonselective cation channels pertinent to diverse physiological functions. Multiple TRPM and TRPV channel subtypes have been identified and cloned in different tissues. However, their information in vascular tissue is scant. In this study, we sought to identify TRPM and TRPV channel subtypes expressed in rat deendothelialized intralobar pulmonary arteries (PAs) and aorta. With RT-PCR, mRNA of TRPM2, TRPM3, TRPM4, TRPM7, and TRPM8 of TRPM family and TRPV1, TRPV2, TRPV3, and TRPV4 of TRPV family were detected in both PAs and aorta. Quantitative real-time RT-PCR showed that TRPM8 and TRPV4 were the most abundantly expressed TRPM and TRPV subtypes, respectively. Moreover, Western blot analysis verified expression of TRPM2, TRPM8, TRPV1, and TRPV4 proteins in both types of vascular tissue. To examine the functional activities of these channels, we monitored intracellular Ca(2+) transients ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMCs) and aortic smooth muscle cells (ASMCs). The TRPM8 agonist menthol (300 muM) and the TRPV4 agonist 4alpha-phorbol 12,13-didecanoate (1 muM) evoked significant increases in [Ca(2+)](i) in PASMCs and ASMCs. These Ca(2+) responses were abolished in the absence of extracellular Ca(2+) or the presence of 300 muM Ni(2+) but were unaffected by 1 muM nifedipine, suggesting Ca(2+) influx via nonselective cation channels. Hence, for the first time, our results indicate that multiple functional TRPM and TRPV channels are coexpressed in rat intralobar PAs and aorta. These novel Ca(2+) entry pathways may play important roles in the regulation of pulmonary and systemic circulation.