Elucidation of a TRPC6-TRPC5 channel cascade that restricts endothelial cell movement

Dynamic remodeling of TRPC5 channel-caveolin-1-eNOS protein assembly potentiates the positive feedback interaction between Ca2+ and NO signals

The cell signaling molecules nitric oxide (NO) and Ca2+ regulate diverse biological processes through their closely coordinated activities directed by signaling protein complexes. However, it remains unclear how dynamically the multi-component protein assemblies behave within the signaling complexes upon the interplay between NO and Ca2+ signals. Here we demonstrate that TRPC5 channels activated by stimulation of G-protein-coupled ATP receptors mediate Ca2+ influx, that triggers NO production from endothelial NO synthase (eNOS), inducing secondary activation of TRPC5 via cysteine S-nitrosylation and eNOS in vascular endothelial cells. Mutations in the caveolin-1-binding domains of TRPC5 disrupt its association with caveolin-1 and impair Ca2+ influx and NO production, suggesting that caveolin-1 serves primarily as the scaffold for TRPC5 and eNOS to assemble into the signal complex. Interestingly, during ATP receptor activation, eNOS is dissociated from caveolin-1 and in turn directly associates with TRPC5, which accumulates at the plasma membrane dependently on Ca2+ influx and calmodulin (CaM). This protein reassembly likely results in a relief of eNOS from the inhibitory action of caveolin-1 and an enhanced TRPC5 S-nitrosylation by eNOS localized in the proximity, thereby facilitating the secondary activation of Ca2+ influx and NO production. In isolated rat aorta, vasodilation induced by acetylcholine was significantly suppressed by the TRPC5 inhibitor AC1903. Thus, our study provides evidence that dynamic remodeling of the protein assemblies among TRPC5, eNOS, caveolin-1, and CaM determines the ensemble of Ca2+ mobilization and NO production in vascular endothelial cells.

iPLA2 inhibition blocks LysoPC-induced TRPC6 externalization and promotes Re-endothelialization of carotid injuries in hypercholesterolemic mice

Lipid oxidation products, including lysophosphatidylcholine (lysoPC), accumulate at the site of arterial injury after vascular interventions and hinder re-endothelization. LysoPC activates calcium-permeable channels, specifically canonical transient receptor potential 6 (TRPC6) channels that induce a sustained increase in intracellular calcium ion concentration [Ca2+]i and contribute to dysregulation of the endothelial cell (EC) cytoskeleton. Activation of TRPC6 leads to inhibition of EC migration in vitro and delayed re-endothelization of arterial injuries in vivo. Previously, we demonstrated the role of phospholipase A2 (PLA2), specifically calcium-independent PLA2 (iPLA2), in lysoPC-induced TRPC6 externalization and inhibition of EC migration in vitro. The ability of FKGK11, an iPLA2-specific pharmacological inhibitor, to block TRPC6 externalization and preserve EC migration was assessed in vitro and in a mouse model of carotid injury. Our data suggest that FKGK11 prevents lysoPC-induced PLA2 activity, blocks TRPC6 externalization, attenuates calcium influx, and partially preserves EC migration in vitro. Furthermore, FKGK11 promotes re-endothelization of an electrocautery carotid injury in hypercholesterolemic mice. FKGK11 has similar arterial healing effects in male and female mice on a high-fat diet. This study suggests that iPLA2 is a potential therapeutic target to attenuate calcium influx through TRPC6 channels and promote EC healing in cardiovascular patients undergoing angioplasty.

p38 MAPK activation and STIM1-Orai3 association mediate TRPC6 externalization

Endothelial cell (EC) migration is critical for the repair of monolayer disruption following angioplasties, but migration is inhibited by lipid oxidation products, including lysophosphatidylcholine (lysoPC), which open canonical transient receptor potential 6 (TRPC6) channels. TRPC6 activation requires an increase in intracellular Ca2+ concentration ([Ca2+]i), the source of which is unknown. LysoPC can activate phospholipase A2 to release arachidonic acid (ArA). ArA can activate arachidonic acid-regulated calcium (ARC) channels that are formed by stromal interaction molecule 1 (STIM1) and Orai1 and Orai3 proteins. Both lysoPC and ArA can activate p38 mitogen-activated protein kinase (MAPK) that induces the phosphorylation required for STIM1-Orai3 association. This is accompanied by an increase in [Ca2+]i and TRPC6 externalization. The effect of lysoPC and ArA is not additive, suggesting activation of the same pathway. The increase in [Ca2+]i activates an Src kinase that leads to TRPC6 activation. Downregulation of Orai3 using siRNA blocks the lysoPC- or ArA-induced increase in [Ca2+]i and TRPC6 externalization and preserves EC migration. These data show that lysoPC induces activation of p38 MAPK, which leads to STIM1-Orai3 association and increased [Ca2+]i. This increase in [Ca2+]i activates an Src kinase leading to TRPC6 externalization, which initiates a cascade of events ending in cytoskeletal changes that disrupt EC migration. Blocking this pathway preserves EC migration in the presence of lipid oxidation products.NEW & NOTEWORTHY The major lysophospholipid component in oxidized LDL, lysophosphatidylcholine (lysoPC), can activate p38 MAP kinase, which in turn promotes externalization of Orai3 and STIM1-Orai3 association, suggesting involvement of arachidonic acid-regulated calcium (ARC) channels. The subsequent increase in intracellular calcium activates an Src kinase required for TRPC6 externalization. TRPC6 activation, which has been shown to inhibit endothelial cell migration, is blocked by p38 MAP kinase or Orai3 downregulation, and this partially preserves endothelial migration in lysoPC.

p85α regulatory subunit isoform controls PI3-kinase and TRPC6 membrane translocation

Activation of phosphatidylinositol 3-kinase (PI3K) by lipid oxidation products, including lysophosphatidylcholine (lysoPC), increases the externalization of canonical transient receptor potential 6 (TRPC6) channels leading to a subsequent increase in intracellular calcium that contributes to cytoskeletal changes which inhibit endothelial cell (EC) migration in vitro and impair EC healing of arterial injuries in vivo. The PI3K p110α and p110δ catalytic subunit isoforms regulate lysoPC-induced TRPC6 externalization in vitro, but have many other functions. The goal of the current study is to identify the PI3K regulatory subunit isoform involved in TRPC6 externalization to potentially identify a more specific treatment regimen to improve EC migration and arterial healing, while minimizing off-target effects. Decreasing the p85α regulatory subunit isoform protein levels, but not the p85β and p55γ regulatory subunit isoforms, with small interfering RNA inhibits lysoPC-induced translocation of the PI3K catalytic subunit to the plasma membrane, dramatically decreased phosphatidylinositol (3,4,5)-trisphosphate (PIP3) production and TRPC6 externalization, and significantly improves EC migration in the presence of lysoPC. These results identify the important and specific role of p85α in controlling translocation of PI3K from the cytosol to the plasma membrane and PI3K-mediated TRPC externalization by oxidized lipids. Current PI3K inhibitors block the catalytic subunit, but our data suggest that the regulatory subunit is a novel therapeutic target to promote EC migration and healing after arterial injuries that occur with angioplasty.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Activation of the cytosolic calcium-independent phospholipase A2 β isoform contributes to TRPC6 externalization via release of arachidonic acid

During vascular interventions, oxidized low-density lipoprotein and lysophosphatidylcholine (lysoPC) accumulate at the site of arterial injury, inhibiting endothelial cell (EC) migration and arterial healing. LysoPC activates canonical transient receptor potential 6 (TRPC6) channels, leading to a prolonged increase in intracellular calcium ion concentration that inhibits EC migration. However, an initial increase in intracellular calcium ion concentration is required to activate TRPC6, and this mechanism remains elusive. We hypothesized that lysoPC activates the lipid-cleaving enzyme phospholipase A₂ (PLA₂), which releases arachidonic acid (AA) from the cellular membrane to open arachidonate-regulated calcium channels, allowing calcium influx that promotes externalization and activation of TRPC6 channels. The focus of this study was to identify the roles of calcium-dependent and/or calcium-independent PLA₂ in lysoPC-induced TRPC6 externalization. We show that lysoPC induced PLA₂ enzymatic activity and caused AA release in bovine aortic ECs. To identify the specific subgroup and the isoform(s) of PLA₂ involved in lysoPC-induced TRPC6 activation, transient knockdown studies were performed in the human endothelial cell line EA.hy926 using siRNA to inhibit the expression of genes encoding cPLA₂α, cPLA₂γ, iPLA₂β, or iPLA₂γ. Downregulation of the β isoform of iPLA₂ blocked lysoPC-induced release of AA from EC membranes and TRPC6 externalization, as well as preserved EC migration in the presence of lysoPC. We propose that blocking TRPC6 activation and promoting endothelial healing could improve the outcomes for patients undergoing cardiovascular interventions.

Transient receptor potential canonical 5 mediates inflammatory mechanical and spontaneous pain in mice

Tactile and spontaneous pains are poorly managed symptoms of inflammatory and neuropathic injury. Here, we found that transient receptor potential canonical 5 (TRPC5) is a chief contributor to both of these sensations in multiple rodent pain models. Use of TRPC5 knockout mice and inhibitors revealed that TRPC5 selectively contributes to the mechanical hypersensitivity associated with CFA injection, skin incision, chemotherapy induced peripheral neuropathy, sickle cell disease, and migraine, all of which were characterized by elevated concentrations of lysophosphatidylcholine (LPC). Accordingly, exogenous application of LPC induced TRPC5-dependent behavioral mechanical allodynia, neuronal mechanical hypersensitivity, and spontaneous pain in naïve mice. Lastly, we found that 75% of human sensory neurons express TRPC5, the activity of which is directly modulated by LPC. On the basis of these results, TRPC5 inhibitors might effectively treat spontaneous and tactile pain in conditions characterized by elevated LPC.

Inhibition of P110α and P110δ catalytic subunits of PI3 kinase reverses impaired arterial healing after injury in hypercholesterolemic male mice

Endothelial cell (EC) migration is critical for healing arterial injuries, such as those that occur with angioplasty. Impaired re-endothelialization following arterial injury contributes to vessel thrombogenicity, intimal hyperplasia, and restenosis. Oxidized lipid products, including lysophosphatidylcholine (lysoPC), induce canonical transient receptor potential 6 (TRPC6) externalization leading to increased [Ca²⁺]_i, activation of calpains, and alterations of the EC cytoskeletal structure that inhibit migration. The p110α and p110δ catalytic subunit isoforms of phosphatidylinositol 3-kinase (PI3K) regulate lysoPC-induced TRPC6 externalization in vitro. The goal of this study was to assess the in vivo relevance of those in vitro findings to arterial healing following a denuding injury in hypercholesterolemic mice treated with pharmacologic inhibitors of the p110α and p110δ isoforms of PI3K and a general PI3K inhibitor. Pharmacologic inhibition of the p110α or the p110δ isoform of PI3K partially preserves healing in hypercholesterolemic male mice, similar to a general PI3K inhibitor. Interestingly, the p110α, p110δ, and the general PI3K inhibitor do not improve arterial healing after injury in hypercholesterolemic female mice. These results indicate a potential new role for isoform-specific PI3K inhibitors in male patients following arterial injury/intervention. The results also identify significant sex differences in the response to PI3K inhibition in the cardiovascular system, where female sex generally has a cardioprotective effect. This study provides a foundation to investigate the mechanism for the sex differences in response to PI3K inhibition to develop a more generally applicable treatment option.

P110α and P110δ catalytic subunits of PI3 kinase regulate lysophosphatidylcholine-induced TRPC6 externalization

Lipid oxidation products, including lysophosphatidylcholine (lysoPC) inhibit endothelial cell (EC) migration in vitro and impair EC healing of arterial injuries in vivo, in part by activating phosphatidylinositol 3-kinase (PI3K), which increases the externalization of canonical transient receptor potential 6 (TRPC6) channels and the subsequent increase in intracellular calcium. Inhibition of PI3K is a potential method to decrease TRPC6 activation and restore migration, but PI3K is involved in multiple intracellular signaling pathways and has multiple downstream effectors. The goal of this study is to identify the specific p110 catalytic subunit isoforms responsible for lysoPC-induced TRPC6 externalization to identify a target for intervention while minimizing impact on alternative signaling pathways. Down-regulation of the p110α and p110δ isoforms, but not the p110β or p110γ isoforms, with small interfering RNA significantly decreased phosphatidylinositol (3,4,5)-trisphosphate production and TRPC6 externalization, and significantly improved EC migration in the presence of lysoPC. These results identify an additional role of p110α in EC and reveal for the first time a specific role of p110δ in EC, providing a foundation for subsequent in vivo studies to investigate the impact of p110 isoform inhibition on arterial healing after injury.

Canonical transient receptor potential 6 channel deficiency promotes smooth muscle cells dedifferentiation and increased proliferation after arterial injury

Objective

Previous studies showed the benefit of canonical transient receptor potential 6 (TRPC6) channel deficiency in promoting endothelial healing of arterial injuries in hypercholesterolemic animals. Long-term studies utilizing a carotid wire-injury model were undertaken in wild-type (WT) and TRPC6^-/- mice to determine the effects of TRPC6 on phenotypic modulation of vascular smooth muscle cells (SMC) and neointimal hyperplasia. We hypothesized that TRPC6 was essential in the maintenance or reexpression of a differentiated SMC phenotype and minimized luminal stenosis following arterial injury.

Methods

The common carotid arteries (CCA) of WT and TRPC6^-/- mice were evaluated at baseline and 4 weeks after wire injury. At baseline, CCA of TRPC6^-/- mice had reduced staining of MYH11 and SM22, fewer elastin lamina, luminal dilation, and wall thinning. After carotid wire injury, TRPC6^-/- mice developed significantly more pronounced luminal stenosis compared with WT mice. Injured TRPC6^-/- CCA demonstrated increased medial/intimal cell number and active cell proliferation when compared with WT CCA. Immunohistochemistry suggested that expression of contractile biomarkers in medial SMC were essentially at baseline levels in WT CCA at 28 days after wire injury. By contrast, at 28 days after injury medial SMC from TRPC6^-/- CCA showed a significant decrease in the expression of contractile biomarkers relative to baseline levels. To assess the role of TRPC6 in systemic arterial SMC phenotype modulation, SMC were harvested from thoracic aortae of WT and TRPC6^-/- mice and were characterized. TRPC6^-/- SMC showed enhanced proliferation and migration in response to serum stimulation. Expression of contractile phenotype biomarkers, MYH11 and SM22, was attenuated in TRPC6^-/- SMC. siRNA-mediated TRPC6 deficiency inhibited contractile biomarker expression in a mouse SMC line.

Conclusions

These results suggest that TRPC6 contributes to the restoration or maintenance of arterial SMC contractile phenotype following injury. Understanding the role of TRPC6 in phenotypic modulation may lead to mechanism-based therapies for attenuation of IH.

After endovascular intervention and open vascular surgery, vascular smooth muscle cells (VSMC) undergo a coordinated reprogramming of gene expression to facilitate arterial healing. Down regulation of VSMC-specific contractile biomarkers (eg, SM22 and MYH11) and induction of pathways that promote cell proliferation, migration, and matrix synthesis are hallmarks of this phenotypic switch. Dysregulated phenotypic switching leads to the development of neointimal hyperplasia and vascular restenosis. Identifying pathways that regulate or constrain VSMC phenotypic modulation, therefore, has the potential to decrease neointimal hyperplasia and improve outcomes after vascular intervention. In this study, we demonstrate that depletion of the non-voltage-gated cation channel TRPC6 promotes phenotypic switching and loss of contractile biomarkers in systemic arterial VSMC. TRPC6^-/- mice developed significantly more pronounced luminal stenosis compared with wild-type mice after carotid wire injury. These results suggest that TRPC6 contributes to the restoration or maintenance of contractile phenotype in VSMC after injury. Understanding the role of TRPC6 in phenotypic switching may lead to mechanism-based therapies to mitigate restenosis.

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

Endothelial Transient Receptor Potential Channels and Vascular Remodeling: Extracellular Ca2 + Entry for Angiogenesis, Arteriogenesis and Vasculogenesis

Vasculogenesis, angiogenesis and arteriogenesis represent three crucial mechanisms involved in the formation and maintenance of the vascular network in embryonal and post-natal life. It has long been known that endothelial Ca²⁺ signals are key players in vascular remodeling; indeed, multiple pro-angiogenic factors, including vascular endothelial growth factor, regulate endothelial cell fate through an increase in intracellular Ca²⁺ concentration. Transient Receptor Potential (TRP) channel consist in a superfamily of non-selective cation channels that are widely expressed within vascular endothelial cells. In addition, TRP channels are present in the two main endothelial progenitor cell (EPC) populations, i.e., myeloid angiogenic cells (MACs) and endothelial colony forming cells (ECFCs). TRP channels are polymodal channels that can assemble in homo- and heteromeric complexes and may be sensitive to both pro-angiogenic cues and subtle changes in local microenvironment. These features render TRP channels the most versatile Ca²⁺ entry pathway in vascular endothelial cells and in EPCs. Herein, we describe how endothelial TRP channels stimulate vascular remodeling by promoting angiogenesis, arteriogenesis and vasculogenesis through the integration of multiple environmental, e.g., extracellular growth factors and chemokines, and intracellular, e.g., reactive oxygen species, a decrease in Mg²⁺ levels, or hypercholesterolemia, stimuli. In addition, we illustrate how endothelial TRP channels induce neovascularization in response to synthetic agonists and small molecule drugs. We focus the attention on TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPV1, TRPV4, TRPM2, TRPM4, TRPM7, TRPA1, that were shown to be involved in angiogenesis, arteriogenesis and vasculogenesis. Finally, we discuss the role of endothelial TRP channels in aberrant tumor vascularization by focusing on TRPC1, TRPC3, TRPV2, TRPV4, TRPM8, and TRPA1. These observations suggest that endothelial TRP channels represent potential therapeutic targets in multiple disorders featured by abnormal vascularization, including cancer, ischemic disorders, retinal degeneration and neurodegeneration.

Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein-lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

TRPC channels: Regulation, dysregulation and contributions to chronic kidney disease

Mutations in the gene encoding canonical transient receptor potential-6 (TRPC6) channels result in severe nephrotic syndromes that typically lead to end-stage renal disease. Many but not all of these mutations result in a gain in the function of the resulting channel protein. Since those observations were first made, substantial work has supported the hypothesis that TRPC6 channels can also contribute to progression of acquired (non-genetic) glomerular diseases, including primary and secondary FSGS, glomerulosclerosis during autoimmune glomerulonephritis, and possibly in type-1 diabetes. Their regulation has been extensively studied, especially in podocytes, but also in mesangial cells and other cell types present in the kidney. More recent evidence has implicated TRPC6 in renal fibrosis and tubulointerstitial disease caused by urinary obstruction. Consequently TRPC6 is being extensively investigated as a target for drug discovery. Other TRPC family members are present in kidney. TRPC6 can form a functional heteromultimer with TRPC3, and it has been suggested that TRPC5 may also play a role in glomerular disease progression, although the evidence on this is contradictory. Here we review literature on the expression and regulation of TRPC6, TRPC3 and TRPC5 in various cell types of the vertebrate kidney, the evidence that these channels are dysregulated in disease models, and research showing that knock-out or pharmacological inhibition of these channels can reduce the severity of kidney disease. We also summarize several areas that remain controversial, and some of the large gaps of knowledge concerning the fundamental role of these proteins in regulation of renal function.

The TRPC5 channel regulates angiogenesis and promotes recovery from ischemic injury in mice

Ischemia-related diseases are a leading cause of death worldwide, and promoting therapeutic angiogenesis is key for effective recovery from hypoxia-ischemia. Given the limited success of angiogenic factors, such as vascular endothelial growth factor, in clinical trials, it is important to find more promising angiogenic targets. Here, using both cell- and tissue-based assays and a mouse model of injury-induced ischemia, we investigated the involvement of the transient receptor potential canonical 5 (TRPC5) ion channel in angiogenesis and the effects of a TRPC5 activator, the Food and Drug Administration-approved drug riluzole, on recovery from ischemic injury. We demonstrate that TRPC5 is involved in endothelial cell sprouting, angiogenesis, and blood perfusion in an oxygen-induced retinopathy model and a hind limb ischemia model. We found a potential regulatory link between nuclear factor of activated T cell isoform c3 and angiopoietin-1 that could provide the mechanistic basis for the angiogenic function of TRPC5. Importantly, treatment with riluzole, which can activate TRPC5 in endothelial cells, improved recovery from ischemia in mice. Our study reveals TRPC5 as a potential angiogenic target and suggests riluzole as a promising drug for managing ischemic diseases.

Redox regulation of transient receptor potential channels in the endothelium

ROS and RNS are important mediators of signaling pathways in the endothelium. Specific members of the TRP superfamily of cation channels act as important Ca²⁺ influx pathways in endothelial cells and are involved in endothelium-dependent vasodilation, regulation of barrier permeability, and angiogenesis. ROS and RNS can modulate the activity of certain TRP channels mainly by modifying specific cysteine residues or by stimulating the production of second messengers. In this review, we highlight the recent literature describing redox regulation of TRP channel activity in endothelial cells as well as the physiological importance of these pathways and implication for cardiovascular diseases.

TRPC3 participates in angiotensin II type 1 receptor-dependent stress-induced slow increase in intracellular Ca2+ concentration in mouse cardiomyocytes

When a cardiac muscle is held in a stretched position, its [Ca2+] transient increases slowly over several minutes in a process known as stress-induced slow increase in intracellular Ca2+ concentration ([Ca2+]i) (SSC). Transient receptor potential canonical (TRPC) 3 forms a non-selective cation channel regulated by the angiotensin II type 1 receptor (AT1R). In this study, we investigated the role of TRPC3 in the SSC. Isolated mouse ventricular myocytes were electrically stimulated and subjected to sustained stretch. An AT1R blocker, a phospholipase C inhibitor, and a TRPC3 inhibitor suppressed the SSC. These inhibitors also abolished the observed SSC-like slow increase in [Ca2+]i induced by angiotensin II, instead of stretch. Furthermore, the SSC was not observed in TRPC3 knockout mice. Simulation and immunohistochemical studies suggest that sarcolemmal TRPC3 is responsible for the SSC. These results indicate that sarcolemmal TRPC3, regulated by AT1R, causes the SSC.

Endothelial Ca2+ Signaling and the Resistance to Anticancer Treatments: Partners in Crime

Intracellular Ca²⁺ signaling drives angiogenesis and vasculogenesis by stimulating proliferation, migration, and tube formation in both vascular endothelial cells and endothelial colony forming cells (ECFCs), which represent the only endothelial precursor truly belonging to the endothelial phenotype. In addition, local Ca²⁺ signals at the endoplasmic reticulum (ER)-mitochondria interface regulate endothelial cell fate by stimulating survival or apoptosis depending on the extent of the mitochondrial Ca²⁺ increase. The present article aims at describing how remodeling of the endothelial Ca²⁺ toolkit contributes to establish intrinsic or acquired resistance to standard anti-cancer therapies. The endothelial Ca²⁺ toolkit undergoes a major alteration in tumor endothelial cells and tumor-associated ECFCs. These include changes in TRPV4 expression and increase in the expression of P2X7 receptors, Piezo2, Stim1, Orai1, TRPC1, TRPC5, Connexin 40 and dysregulation of the ER Ca²⁺ handling machinery. Additionally, remodeling of the endothelial Ca²⁺ toolkit could involve nicotinic acetylcholine receptors, gasotransmitters-gated channels, two-pore channels and Na⁺/H⁺ exchanger. Targeting the endothelial Ca²⁺ toolkit could represent an alternative adjuvant therapy to circumvent patients' resistance to current anti-cancer treatments.

TRPC Channels and Stroke

TRPC channels play important roles in neuronal death/survival in ischemic stroke, vasospasm in hemorrhagic stroke, thrombin-induced astrocyte pathological changes, and also in the initiation of stroke by affecting blood pressure and atherogenesis. TRPCs' unique channel characters and downstream pathways make them possible new targets for stroke therapy. TRPC proteins have different functions in different cell types. Considering TRPCs' extensive distribution in various tissues and cell types, drugs targeting them could induce more complicated effects. More specific agonists/antagonists and antibodies are required for future study of TRPCs as potential targets for stroke therapy.

Integration of TRPC6 and NADPH oxidase activation in lysophosphatidylcholine-induced TRPC5 externalization

Lipid oxidation products, including lysophosphatidylcholine (lysoPC), activate canonical transient receptor potential 6 (TRPC6) channels, and the subsequent increase in intracellular Ca²⁺ leads to TRPC5 activation. The goal of this study is to elucidate the steps in the pathway between TRPC6 activation and TRPC5 externalization. Following TRPC6 activation by lysoPC, extracellular regulated kinase (ERK) is phosphorylated. This leads to phosphorylation of p47^phox and subsequent NADPH oxidase activation with increased production of reactive oxygen species. ERK activation requires TRPC6 opening and influx of Ca²⁺ as evidenced by the failure of lysoPC to induce ERK phosphorylation in TRPC6^-/- endothelial cells. ERK siRNA blocks the lysoPC-induced activation of NADPH oxidase, demonstrating that ERK activation is upstream of NADPH oxidase. The reactive oxygen species produced by NADPH oxidase promote myosin light chain kinase (MLCK) activation with phosphorylation of MLC and TRPC5 externalization. Downregulation of ERK, NADPH oxidase, or MLCK with the relevant siRNA prevents TRPC5 externalization. Blocking MLCK activation prevents the prolonged rise in intracellular calcium levels and preserves endothelial migration in the presence of lysoPC.

Muscling in on TRP channels in vascular smooth muscle cells and cardiomyocytes

The human TRP protein family comprises a family of 27 cation channels with diverse permeation and gating properties. The common theme is that they are very important regulators of intracellular Ca²⁺ signaling in diverse cell types, either by providing a Ca²⁺ influx pathway, or by depolarising the membrane potential, which on one hand triggers the activation of voltage-gated Ca²⁺ channels, and on the other limits the driving force for Ca²⁺ entry. Here we focus on the role of these TRP channels in vascular smooth muscle and cardiac striated muscle. We give an overview of highlights from the recent literature, and highlight the important and diverse roles of TRP channels in the pathophysiology of the cardiovascular system. The discovery of the superfamily of Transient Receptor Potential (TRP) channels has significantly enhanced our knowledge of multiple signal transduction mechanisms in cardiac muscle and vascular smooth muscle cells (VSMC). In recent years, multiple studies have provided evidence for the involvement of these channels, not only in the regulation of contraction, but also in cell proliferation and remodeling in pathological conditions. The mammalian family of TRP cation channels is composed by 28 genes which can be divided into 6 subfamilies groups based on sequence similarity: TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipins), TRPV (Vanilloid), TRPP (Policystin) and TRPA (Ankyrin-rich protein). Functional TRP channels are believed to form four-unit complexes in the plasma, each of them expressed with six transmembrane domain and intracellular N and C termini. Here we review the current knowledge on the expression of TRP channels in both muscle types, and discuss their functional properties and role in physiological and pathophysiological processes.

Stretch-activated TRPV2 channels: Role in mediating cardiopathies

Transient receptor potential vanilloid type 2, TRPV2, is a calcium-permeable cation channel belonging to the TRPV channel family. Although this channel has been first characterized as a noxious heat sensor, its mechanosensor property recently gained importance in various physiological functions. TRPV2 has been described as a stretch-mediated channel and a regulator of calcium homeostasis in several cell types and has been shown to be involved in the stretch-dependent responses in cardiomyocytes. Hence, several studies in the last years support the idea that TRPV2 play a key role in the function and structure of the heart, being involved in the cardiac compensatory mechanisms in response to pathologic or exercise-induced stress. We present here an overview of the current literature and concepts of TRPV2 channels involvement (i) in the mechanical coupling mechanisms in heart and (ii) in the mechanisms that lead to cardiomyopathies. All these studies lead us to think that TRPV2 may also be an important cardiac drug target based on its major physiological roles in heart.

TRP channels in brown and white adipogenesis from human progenitors: new therapeutic targets and the caveats associated with the common antibiotic, streptomycin

Transient receptor potential (TRP) channels are polymodal cell sensors responding to diverse stimuli and widely implicated in the developmental programs of numerous tissues. The evidence for an involvement of TRP family members in adipogenesis, however, is scant. We present the first comprehensive expression profile of all known 27 human TRP genes in mesenchymal progenitors cells during white or brown adipogenesis. Using positive trilineage differentiation as an exclusion criterion, TRP polycystic (P)3, and TPR melastatin (M)8 were found to be uniquely adipospecific. Knockdown of TRPP3 repressed the expression of the brown fat signature genes uncoupling protein (UCP)-1 and peroxisome proliferator-activated receptor γ coactivator (PGC)-1α as well as attenuated forskolin-stimulated uncoupled respiration. However, indices of generalized adipogenesis, such as lipid droplet morphology and fatty acid binding protein (FAPB)-4 expression, were not affected, indicating a principal mitochondrial role of TRPP3. Conversely, activating TRPM8 with menthol up-regulated UCP-1 expression and augmented uncoupled respiration predominantly in white adipocytes (browning), whereas streptomycin antagonized TRPM8-mediated calcium entry, downregulated UCP-1 expression, and mitigated uncoupled respiration; menthol was less capable of augmenting uncoupled respiration (thermogenesis) in brown adipocytes. TRPP3 and TRPM8 hence appear to be involved in the priming of mitochondria to perform uncoupled respiration downstream of adenylate cyclase. Our results also underscore the developmental caveats of using antibiotics in adipogenic studies.-Goralczyk, A., van Vijven, M., Koch, M., Badowski, C., Yassin, M. S., Toh, S.-A., Shabbir, A., Franco-Obregón, A., Raghunath, M. TRP channels in brown and white adipogenesis from human progenitors: new therapeutic targets and the caveats associated with the common antibiotic, streptomycin.

Canonical Transient Receptor Potential 6 Channel: A New Target of Reactive Oxygen Species in Renal Physiology and Pathology

SIGNIFICANCE

Regulation of Ca2+ signaling cascade by reactive oxygen species (ROS) is becoming increasingly evident and this regulation represents a key mechanism for control of many fundamental cellular functions. Canonical transient receptor potential (TRPC) 6, a member of Ca2+-conductive channel in the TRPC family, is widely expressed in kidney cells, including glomerular mesangial cells, podocytes, tubular epithelial cells, and vascular myocytes in renal microvasculature. Both overproduction of ROS and dysfunction of TRPC6 channel are involved in renal injury in animal models and human subjects. Although regulation of TRPC channel function by ROS has been well described in other tissues and cell types, such as vascular smooth muscle, this important cell regulatory mechanism has not been fully reviewed in kidney cells. Recent Advances: Accumulating evidence has shown that TRPC6 is a redox-sensitive channel, and modulation of TRPC6 Ca2+ signaling by altering TRPC6 protein expression or TRPC6 channel activity in kidney cells is a downstream mechanism by which ROS induce renal damage.

CRITICAL ISSUES

This review highlights how recent studies analyzing function and expression of TRPC6 channels in the kidney and their response to ROS improve our mechanistic understanding of oxidative stress-related kidney diseases.

FUTURE DIRECTIONS

Although it is evident that ROS regulate TRPC6-mediated Ca2+ signaling in several types of kidney cells, further study is needed to identify the underlying molecular mechanism. We hope that the newly identified ROS/TRPC6 pathway will pave the way to new, promising therapeutic strategies to target kidney diseases such as diabetic nephropathy. Antioxid. Redox Signal. 25, 732-748.

Membrane translocation of TRPC6 channels and endothelial migration are regulated by calmodulin and PI3 kinase activation

Lipid oxidation products, including lysophosphatidylcholine (lysoPC), activate canonical transient receptor potential 6 (TRPC6) channels leading to inhibition of endothelial cell (EC) migration in vitro and delayed EC healing of arterial injuries in vivo. The precise mechanism through which lysoPC activates TRPC6 channels is not known, but calmodulin (CaM) contributes to the regulation of TRPC channels. Using site-directed mutagenesis, cDNAs were generated in which Tyr(99) or Tyr(138) of CaM was replaced with Phe, generating mutant CaM, Phe(99)-CaM, or Phe(138)-CaM, respectively. In ECs transiently transfected with pcDNA3.1-myc-His-Phe(99)-CaM, but not in ECs transfected with pcDNA3.1-myc-His-Phe(138)-CaM, the lysoPC-induced TRPC6-CaM dissociation and TRPC6 externalization was disrupted. Also, the lysoPC-induced increase in intracellular calcium concentration was inhibited in ECs transiently transfected with pcDNA3.1-myc-His-Phe(99)-CaM. Blocking phosphorylation of CaM at Tyr(99) also reduced CaM association with the p85 subunit and subsequent activation of phosphatidylinositol 3-kinase (PI3K). This prevented the increase in phosphatidylinositol (3,4,5)-trisphosphate (PIP3) and the translocation of TRPC6 to the cell membrane and reduced the inhibition of EC migration by lysoPC. These findings suggest that lysoPC induces CaM phosphorylation at Tyr(99) by a Src family kinase and that phosphorylated CaM activates PI3K to produce PIP3, which promotes TRPC6 translocation to the cell membrane.

Second Messenger-Operated Calcium Entry Through TRPC6

Canonical transient receptor potential 6 (TRPC6) proteins assemble into heteromultimeric structures forming non-selective cation channels. In addition, many TRPC6-interacting proteins have been identified like some enzymes, channels, pumps, cytoskeleton-associated proteins, immunophilins, or cholesterol-binding proteins, indicating that TRPC6 are engaged into macromolecular complexes. Depending on the cell type and the experimental conditions used, TRPC6 activity has been reported to be controlled by diverse modalities. For instance, the second messenger diacylglycerol, store-depletion, the plant extract hyperforin or H2O2 have all been shown to trigger the opening of TRPC6 channels. A well-characterized consequence of TRPC6 activation is the elevation of the cytosolic concentration of Ca(2+). This latter response can reflect the entry of Ca(2+) through open TRPC6 channels but it can also be due to the Na(+)/Ca(2+) exchanger (operating in its reverse mode) or voltage-gated Ca(2+) channels (recruited in response to a TRPC6-mediated depolarization). Although TRPC6 controls a diverse array of biological functions in many tissues and cell types, its pathophysiological functions are far from being fully understood. This chapter covers some key features of TRPC6, with a special emphasis on their biological significance in kidney and blood cells.

TRPC6 channel translocation into phagosomal membrane augments phagosomal function

Defects in the innate immune system in the lung with attendant bacterial infections contribute to lung tissue damage, respiratory insufficiency, and ultimately death in the pathogenesis of cystic fibrosis (CF). Professional phagocytes, including alveolar macrophages (AMs), have specialized pathways that ensure efficient killing of pathogens in phagosomes. Phagosomal acidification facilitates the optimal functioning of degradative enzymes, ultimately contributing to bacterial killing. Generation of low organellar pH is primarily driven by the V-ATPases, proton pumps that use cytoplasmic ATP to load H(+) into the organelle. Critical to phagosomal acidification are various channels derived from the plasma membrane, including the anion channel cystic fibrosis transmembrane conductance regulator, which shunt the transmembrane potential generated by movement of protons. Here we show that the transient receptor potential canonical-6 (TRPC6) calcium-permeable channel in the AM also functions to shunt the transmembrane potential generated by proton pumping and is capable of restoring microbicidal function to compromised AMs in CF and enhancement of function in non-CF cells. TRPC6 channel activity is enhanced via translocation to the cell surface (and then ultimately to the phagosome during phagocytosis) in response to G-protein signaling activated by the small molecule (R)-roscovitine and its derivatives. These data show that enhancing vesicular insertion of the TRPC6 channel to the plasma membrane may represent a general mechanism for restoring phagosome activity in conditions, where it is lost or impaired.

"TRP inflammation" relationship in cardiovascular system

Despite considerable advances in the research and treatment, the precise relationship between inflammation and cardiovascular (CV) disease remains incompletely understood. Therefore, understanding the immunoinflammatory processes underlying the initiation, progression, and exacerbation of many cardiovascular diseases is of prime importance. The innate immune system has an ancient origin and is well conserved across species. Its activation occurs in response to pathogens or tissue injury. Recent studies suggest that altered ionic balance, and production of noxious gaseous mediators link to immune and inflammatory responses with altered ion channel expression and function. Among plausible candidates for this are transient receptor potential (TRP) channels that function as polymodal sensors and scaffolding proteins involved in many physiological and pathological processes. In this review, we will first focus on the relevance of TRP channel to both exogenous and endogenous factors related to innate immune response and transcription factors related to sustained inflammatory status. The emerging role of inflammasome to regulate innate immunity and its possible connection to TRP channels will also be discussed. Secondly, we will discuss about the linkage of TRP channels to inflammatory CV diseases, from a viewpoint of inflammation in a general sense which is not restricted to the innate immunity. These knowledge may serve to provide new insights into the pathogenesis of various inflammatory CV diseases and their novel therapeutic strategies.

TRPC6 is the endothelial calcium channel that regulates leukocyte transendothelial migration during the inflammatory response

Weber et al. identify TRPC6 as the calcium channel mediating the transient increase in endothelial cytosolic free calcium concentration required for transendothelial migration of leukocytes during the inflammatory response.

Leukocyte transendothelial migration (TEM) is a tightly regulated, multistep process that is critical to the inflammatory response. A transient increase in endothelial cytosolic free calcium ion concentration (↑[Ca²⁺]_i) is required for TEM. However, the mechanism by which endothelial ↑[Ca²⁺]_i regulates TEM and the channels mediating this ↑[Ca²⁺]_i are unknown. Buffering ↑[Ca²⁺]_i in endothelial cells does not affect leukocyte adhesion or locomotion but selectively blocks TEM, suggesting a role for ↑[Ca²⁺]_i specifically for this step. Transient receptor potential canonical 6 (TRPC6), a Ca²⁺ channel expressed in endothelial cells, colocalizes with platelet/endothelial cell adhesion molecule-1 (PECAM) to surround leukocytes during TEM and clusters when endothelial PECAM is engaged. Expression of dominant-negative TRPC6 or shRNA knockdown in endothelial cells arrests neutrophils apically over the junction, similar to when PECAM is blocked. Selectively activating endothelial TRPC6 rescues TEM during an ongoing PECAM blockade, indicating that TRPC6 functions downstream of PECAM. Furthermore, endothelial TRPC6 is required for trafficking of lateral border recycling compartment membrane, which facilitates TEM. Finally, mice lacking TRPC6 in the nonmyeloid compartment (i.e., endothelium) exhibit a profound defect in neutrophil TEM with no effect on leukocyte trafficking. Our findings identify endothelial TRPC6 as the calcium channel mediating the ↑[Ca²⁺]_i required for TEM at a step downstream of PECAM homophilic interactions.

The Na+/K+-ATPase and the amyloid-beta peptide aβ1-40 control the cellular distribution, abundance and activity of TRPC6 channels

The Na(+)/K(+)-ATPase interacts with the non-selective cation channels TRPC6 but the functional consequences of this association are unknown. Experiments performed with HEK cells over-expressing TRPC6 channels showed that inhibiting the activity of the Na(+)/K(+)-ATPase with ouabain reduced the amount of TRPC6 proteins and depressed Ca(2+) entry through TRPC6. This effect, not mimicked by membrane depolarization with KCl, was abolished by sucrose and bafilomycin-A, and was partially sensitive to the intracellular Ca(2+) chelator BAPTA/AM. Biotinylation and subcellular fractionation experiments showed that ouabain caused a multifaceted redistribution of TRPC6 to the plasma membrane and to an endo/lysosomal compartment where they were degraded. The amyloid beta peptide Aβ(1-40), another inhibitor of the Na(+)/K(+)-ATPase, but not the shorter peptide Aβ1-16, reduced TRPC6 protein levels and depressed TRPC6-mediated responses. In cortical neurons from embryonic mice, ouabain, veratridine (an opener of voltage-gated Na(+) channel), and Aβ(1-40) reduced TRPC6-mediated Ca(2+) responses whereas Aβ(1-16) was ineffective. Furthermore, when Aβ(1-40) was co-added together with zinc acetate it could no longer control TRPC6 activity. Altogether, this work shows the existence of a functional coupling between the Na(+)/K(+)-ATPase and TRPC6. It also suggests that the abundance, distribution and activity of TRPC6 can be regulated by cardiotonic steroids like ouabain and the naturally occurring peptide Aβ(1-40) which underlines the pathophysiological significance of these processes.

Increased size and cellularity of advanced atherosclerotic lesions in mice with endothelial overexpression of the human TRPC3 channel

In previous in vitro studies, we showed that Transient Receptor Potential Canonical 3 (TRPC3), a calcium-permeable, nonselective cation channel endowed with high constitutive function, is an obligatory component of the inflammatory signaling that controls expression of the vascular cell adhesion molecule-1 (VCAM-1) and monocyte adhesion to coronary artery endothelial cells. Also, TRPC3 expression in these cells was found to be up-regulated by proatherogenic factors, which enhanced inflammation and VCAM-1 expression. However, it remained to be determined whether these in vitro findings were of relevance to atherosclerotic lesion development in vivo. To answer this important question in the present work, we generated mice with endothelial-specific overexpression of human TRPC3 in an Apoe knockout background (TgEST3ApoeKO) and examined lesions in the aortic sinus following 10 and 16 wk on a high-fat diet. No significant differences were found in size or complexity of early stage lesions (10 wk). However, advanced plaques (16 wk) from TgEST3ApoeKO mice exhibited a significant increase in size and macrophage content compared with nontransgenic littermate controls. Remarkably, this change was correlated with increased VCAM-1 and phospho-IkBα immunoreactivity along the endothelial lining of lesions from transgenic animals compared with controls. These findings validate the in vivo relevance of previous in vitro findings and represent, to our knowledge, the first in vivo evidence for a proatherogenic role of endothelial TRPC3.

TRPV4 participates in the establishment of trailing adhesions and directional persistence of migrating cells

Calcium signaling participates in different cellular processes leading to cell migration. TRPV4, a non-selective cation channel that responds to mechano-osmotic stimulation and heat, is also involved in cell migration. However, the mechanistic involvement of TRPV4 in cell migration is currently unknown. We now report that expression of the mutant channel TRPV4-(121)AAWAA (lacking the phosphoinositide-binding site (121)KRWRK(125) and the response to physiological stimuli) altered HEK293 cell migration. Altered migration patterns included periods of fast and persistent motion followed by periods of stalling and turning, and the extension of multiple long cellular protrusions. TRPV4-WT overexpressing cells showed almost complete loss of directionality with frequent turns, no progression, and absence of long protrusions. Traction microscopy revealed higher tractions forces in the tail of TRPV4-(121)AAWAA than in TRPV4-WT expressing cells. These results are consistent with a defective and augmented tail retraction in TRPV4-(121)AAWAA- and TRPV4-WT-expressing cells, respectively. The activity of calpain, a protease implicated in focal adhesion (FA) disassembly, was decreased in TRPV4-(121)AAWAA compared with TRPV4-WT-expressing cells. Consistently, larger focal adhesions were seen in TRPV4-(121)AAWAA compared with TRPV4-WT-expressing HEK293 cells, a result that was also reproduced in T47D and U87 cells. Similarly, overexpression of the pore-dead mutant TRPV4-M680D resumed the TRPV4-(121)AAWAA phenotype presenting larger FA. The migratory phenotype obtained in HEK293 cells overexpressing TRPV4-(121)AAWAA was mimicked by knocking-down TRPC1, a cationic channel that participates in cell migration. Together, our results point to the participation of TRPV4 in the dynamics of trailing adhesions, a function that may require the interplay of TRPV4 with other cation channels or proteins present at the FA sites.

Hypercholesterolemia inhibits re-endothelialization of arterial injuries by TRPC channel activation

OBJECTIVE

After arterial injury, endothelial cell (EC) migration is essential for healing, but lipid oxidation products activate TRPC6 and TRPC5 ion channels, leading to increased intracellular calcium and inhibition of EC migration in vitro. The objective of this study was to further evaluate the role of TRPC channels in EC migration in vitro and to validate in vitro findings in an in vivo model.

METHODS

Mouse aortic ECs were cultured, and the effect of lysophosphatidylcholine, the major lysophospholipid in oxidized low-density lipoprotein, on migration was assessed in a razor-scrape assay. EC healing after a carotid injury with electrocautery was evaluated in wild-type (WT), TRPC6(-/-), and TRPC5(-/-) mice receiving either a chow or high-cholesterol (HC) diet.

RESULTS

Lysophosphatidylcholine inhibited EC migration of WT ECs to 22% of baseline and of TRPC5(-/-) ECs to 53% of baseline but had minimal effect on TRPC6(-/-) EC migration. Hypercholesterolemia severely impaired EC healing in vivo, with 51.4% ± 1.8% and 24.9% ± 2.0% of the injury resurfaced with ECs at 5 days in chow-fed and HC-fed WT mice, respectively (P < .001). Hypercholesterolemia did not impair healing in TRPC6(-/-) mice, with coverage of 48.4% ± 3.4% and 46.8% ± 1.6% in chow-fed and HC-fed TRPC6(-/-) mice, respectively. Hypercholesterolemia had a reduced inhibitory effect in TRPC5(-/-) mice, with EC coverage of 51.7% ± 3.0% and 37.% ± 1.4% in chow-fed and HC-fed TRPC5(-/-) mice, respectively.

CONCLUSIONS

Results suggest that activation of TRPC6 and TRPC5 channels is the key contributor to impaired endothelial healing of arterial injuries in hypercholesterolemic mice.

Lysophosphatidylcholine-induced cytotoxicity in osteoblast-like MG-63 cells: involvement of transient receptor potential vanilloid 2 (TRPV2) channels

Epidemiological studies indicate that patients suffering from atherosclerosis are predisposed to develop osteoporosis. Accordingly, atherogenic determinants such as oxidized low density lipoprotein (OxLDL) particles have been shown to alter bone cell functions. In this work, we investigated the cytotoxicity of lysophosphatidylcholine (lysoPC), a major phospholipid component generated upon LDL oxidation, on bone-forming MG-63 osteoblast-like cells. Cell viability was reduced by lysoPC in a concentration-dependent manner with a LC50 of 18.7±0.7 μM. LysoPC-induced cell death was attributed to induction of both apoptosis and necrosis. Since impairment of intracellular calcium homeostasis is often involved in mechanism of cell death, we determined the involvement of calcium in lysoPC-induced cytotoxicity. LysoPC promoted a rapid and transient increase in intracellular calcium attributed to mobilization from calcium stores, followed by a sustained influx. Intracellular calcium mobilization was associated to phospholipase C (PLC)-dependent mobilization of calcium from the endoplasmic reticulum since inhibition of PLC or calcium depletion of reticulum endoplasmic with thapsigargin prevented the calcium mobilization. The calcium influx induced by lysoPC was abolished by inhibition of transient receptor potential vanilloid (TRPV) channels with ruthenium red whereas gadolinium, which inhibits canonical TRP (TRPC) channels, was without effect. Accordingly, expression of TRPV2 and TRPV4 were shown in MG-63 cells. The addition of TRPV2 inhibitor Tranilast in the incubation medium prevent the calcium influx triggered by lysoPC and reduced lysoPC-induced cytotoxicity whereas TRPV4 inhibitor RN 1734 was without effect, which confirms the involvement of TRPV2 activation in lysoPC-induced cell death.

Do canonical transient receptor potential channels mediate cholinergic excitation of cortical pyramidal neurons?

Activation of M1-type muscarinic acetylcholine receptors excites neocortical pyramidal neurons, in part by gating a nonselective cation conductance that produces calcium-dependent 'afterdepolarizing potentials' (ADPs) following short trains of action potentials. Although the identity of the cation conductance mediating the ADP is not known, previous work has implicated canonical transient receptor potential (TRPC) channels, specifically the TRPC5 and TRPC6 subtypes. Using pharmacological and genetic approaches, we tested the role of TRPC channels in generating cholinergic ADPs in layer 5 pyramidal neurons in the mouse medial prefrontal cortex (mPFC). A variety of compounds that block TRPC channels, including 2-aminoethoxydiphenyl borate, flufenamic acid, lanthanum, SKF-96365, and Pyr-3, had little, if any, impact on cholinergic ADPs. Similarly, genetic deletion of several TRPC subunits, including TPRC1, TRPC5, and TRPC6 (single knockouts), or both TRPC5 and TRPC6 together (double knockout), failed to reduce the amplitude of cholinergic ADPs. These data suggest that TRPC5 and TRPC6 subunits are not required for cholinergic excitation of layer 5 pyramidal neurons in the mouse mPFC and that the focus of future work should be expanded to test the involvement of other potential ionic effectors.

The TRPC family of TRP channels: roles inferred (mostly) from knockout mice and relationship to ORAI proteins

Aside from entering into cells through voltage gated Ca channels and Na/Ca exchangers in those cells that express these proteins, for all cells be they excitable or non-excitable, Ca(2+) enters through channels that are activated downstream of phosphoinositide mobilization (activation of phospholipase C, PLC) and through channels that are activated secondary to depletion of internal stores. Depletion of internal stores activates plasma membrane channels known as ORAIs. Activation of PLCs activates the canonical class of transient receptor potential channels (TRPCs), and, because this activation also causes depletion of Ca(2+) stores, also ORAI based channels. Whereas the activation of ORAI is a well-accepted phenomenon, it appears that TRPC channels also participate in Ca(2+) entry triggered by store depletion with or without participation of ORAI molecules. Regardless of molecular makeup of TRPC containing channels, a plethora of studies have shown TRPCs to be important both in physiologic systems as well as in pathophysiologic phenomena. Particularly important in defining roles of TRPCs, have been studies with mice with targeted disruption of their genes, i.e., with TRPC KO mice. In this chapter we first focus on TRPCs as regulators of body functions in health and disease, and then focus on the possible make-up of the channels of which they participate. A hypothesis is set forth, whereby ORAI dimers are proposed to be regulatory subunits of tetrameric TRPC channels and serve as structural units that form ORAI channels either as dimers of dimers or trimers of dimers.

TRPC5

Human canonical transient receptor potential channel 5 (TRPC5) has been cloned from the Xq23 region on chromosome X as a suspect in nonsyndromic mental retardation. TRPC5 is a Ca(2+)-permeable cation channel predominantly expressed in the CNS, including the hippocampus, cerebellum, amygdala, sensory neurons, and retina. It also shows more restricted expression in the periphery, notably in the kidney and cardiovascular system. Homotetrameric TRPC5 channels are primarily activated by receptors coupled to Gq and phospholipase C and/or Gi proteins, but TRPC5 channels may also gate in a store-dependent manner, which requires other partner proteins such TRPC1, STIM1, and Orai1. There is an impressive array of other activators of TRPC5 channels, such as nitric oxide, lysophospholipids, sphingosine-1-phosphate, reduced thioredoxin, protons, lanthanides, and calcium, and many can cause its direct activation. Moreover, TRPC5 shows constitutive activity, and it is responsive to membrane stretch and cold. Thus, TRPC5 channels have significant potential for synergistic activation and may serve as an important focal point in Ca(2+) signalling and electrogenesis. Moreover, TRPC5 functions in partnership with about 60 proteins, including TRPC1, TRPC4, calmodulin, IP3 receptors, NHERF, NCS-1, junctate, stathmin 2, Ca(2+)-binding protein 1, caveolin, and SESTD1, while its desensitisation is mediated by both protein kinases A and C. TRPC5 has a distinct voltage dependence shared only with its closest relative, TRPC4. Its unique N-shaped activation curve underlined by intracellular Mg(2+) block seems to be perfectly "shaped" to trigger action potential discharge, but not to grossly interfere with the action potential shape. The range of biological functions of TRPC5 channels is also impressive, from neurotransmission to control of axon guidance and vascular smooth muscle cell migration and contractility. Recent studies of Trpc5 gene knockouts begin to uncover its roles in fear, anxiety, seizures, and cold sensing.

TRPC6: physiological function and pathophysiological relevance

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

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Inhibition of the TRPC5 ion channel protects the kidney filter

An intact kidney filter is vital to retention of essential proteins in the blood and removal of waste from the body. Damage to the filtration barrier results in albumin loss in the urine, a hallmark of cardiovascular disease and kidney failure. Here we found that the ion channel TRPC5 mediates filtration barrier injury. Using Trpc5-KO mice, a small-molecule inhibitor of TRPC5, Ca2+ imaging in isolated kidney glomeruli, and live imagining of podocyte actin dynamics, we determined that loss of TRPC5 or its inhibition abrogates podocyte cytoskeletal remodeling. Inhibition or loss of TRPC5 prevented activation of the small GTP-binding protein Rac1 and stabilized synaptopodin. Importantly, genetic deletion or pharmacologic inhibition of TRPC5 protected mice from albuminuria. These data reveal that the Ca2+-permeable channel TRPC5 is an important determinant of albuminuria and identify TRPC5 inhibition as a therapeutic strategy for the prevention or treatment of proteinuric kidney disease.

Emerging role of TRP channels in cell migration: from tumor vascularization to metastasis

Transient Receptor Potential (TRP) channels modulate intracellular Ca(2+) concentrations, controlling critical cytosolic and nuclear events that are involved in the initiation and progression of cancer. It is not, therefore, surprising that the expression of some TRP channels is altered during tumor growth and metastasis. Cell migration of both epithelial and endothelial cells is an essential step of the so-called metastatic cascade that leads to the spread of the disease within the body. It is in fact required for both tumor vascularization as well as for tumor cell invasion into adjacent tissues and intravasation into blood/lymphatic vessels. Studies from the last 15 years have unequivocally shown that the ion channles and the transport proteins also play important roles in cell migration. On the other hand, recent literature underlies a critical role for TRP channels in the migration process both in cancer cells as well as in tumor vascularization. This will be the main focus of our review. We will provide an overview of recent advances in this field describing TRP channels contribution to the vascular and cancer cell migration process, and we will systematically discuss relevant molecular mechanism involved.

Significance of the transient receptor potential canonical 2 (TRPC2) channel in the regulation of rat thyroid FRTL-5 cell proliferation, migration, adhesion and invasion

Mammalian transient receptor potential (TRP) channels are involved in many physiologically important processes. Here, we have studied the significance of the TRPC2 channel in the regulation of rat thyroid FRTL-5 cell proliferation, migration, adhesion and invasion, using stable TRPC2 (shTRPC2) knock-down cells. In the shTRPC2 cells, proliferation was decreased due to a prolonged G1/S cell cycle phase. The tumor suppressor p53 and the cyclin-dependant kinase inhibitors p27 and p21 were upregulated. Cell invasion, adhesion and migration were also attenuated in shTRPC2 cells, probably due to decreased activity of both Rac and calpain, and a decreased secretion and activity of matrix metalloproteinase 2. The attenuated proliferation, migration, invasion and ATP-evoked calcium entry was mimicked by overexpressing a non-conducting, truncated TRPC2 (TRPC2-DN) in wild type cells, and was reversed by overexpression of TRPC2-GFP in shTRPC2 cells. In conclusion, TRPC2 is an important regulator of rat thyroid cell function.

TRPC6 regulates CXCR2-mediated chemotaxis of murine neutrophils

Unraveling the mechanisms involved in chemotactic navigation of immune cells is of particular interest for the development of new immunoregulatory therapies. It is generally agreed upon that members of the classical transient receptor potential channel family (TRPC) are involved in chemotaxis. However, the regulatory role of TRPC channels in chemoattractant receptor-mediated signaling has not yet been clarified in detail. In this study, we demonstrate that the TRPC6 channels play a pronounced role in CXCR2-mediated intermediary chemotaxis, whereas N-formyl-methionine-leucine-phenylalanine receptor-mediated end-target chemotaxis is TRPC6 independent. The knockout of TRPC6 channels in murine neutrophils led to a strongly impaired intermediary chemotaxis after CXCR2 activation which is not further reinforced by CXCR2, PI3K, or p38 MAPK inhibition. Furthermore, CXCR2-mediated Ca(2+) influx but not Ca(2+) store release was attenuated in TRPC6(-/-) neutrophils. We demonstrate that the TRPC6 deficiency affected phosphorylation of AKT and MAPK downstream of CXCR2 receptor activation and led to altered remodeling of actin. The relevance of this TRPC6-depending defect in neutrophil chemotaxis is underscored by our in vivo findings. A nonseptic peritoneal inflammation revealed an attenuated recruitment of neutrophils in the peritoneal cavity of TRPC6(-/-) mice. In summary, this paper defines a specific role of TRPC6 channels in CXCR2-induced intermediary chemotaxis. In particular, TRPC6-mediated supply of calcium appears to be critical for activation of downstream signaling components.

Calcium regulates podocyte actin dynamics

Ca(2+)-mediated remodeling of the actin cytoskeleton is a dynamic process that regulates cell motility through the modulation of rho guanosine triphosphatase (GTPase) signaling. Kidney podocytes are unique, pericyte-like cells with a complex cellular organization consisting of a cell body, major processes, and foot processes (FPs). The FPs form a characteristic interdigitating pattern with FPs of neighboring podocytes, leaving in between filtration slits that are covered by the slit diaphragm (SD). The actin-based FP and the SD form the final barrier to proteinuria. Mutations affecting several podocyte proteins cause disruption of the filtration barrier and rearrangement of the highly dynamic podocyte actin cytoskeleton. Proteins regulating the plasticity of the podocyte actin cytoskeleton are therefore of critical importance for sustained kidney barrier function. Dynamic regulation of the actin-based contractile apparatus in podocyte FPs is essential for sustained kidney filter function. Thus, the podocyte represents an excellent model system to study calcium signaling and actin dynamics in a physiologic context. Here, we discuss the regulation of podocyte actin dynamics by angiotensin or bradykinin-mediated calcium influx and downstream Rho GTPase signaling pathways and how these pathways are operative in other cells including fibroblasts and cancer cells.

Inhibition of endothelial cell Ca²⁺ entry and transient receptor potential channels by Sigma-1 receptor ligands

BACKGROUND AND PURPOSE

The Sigma-1 receptor (Sig1R) impacts on calcium ion signalling and has a plethora of ligands. This study investigated Sig1R and its ligands in relation to endogenous calcium events of endothelial cells and transient receptor potential (TRP) channels.

EXPERIMENTAL APPROACH

Intracellular calcium and patch clamp measurements were made from human saphenous vein endothelial cells and HEK 293 cells expressing exogenous human TRPC5, TRPM2 or TRPM3. Sig1R ligands were applied and short interfering RNA was used to deplete Sig1R. TRP channels tagged with fluorescent proteins were used for subcellular localization studies.

KEY RESULTS

In endothelial cells, 10-100 μM of the Sig1R antagonist BD1063 inhibited sustained but not transient calcium responses evoked by histamine. The Sig1R agonist 4-IBP and related antagonist BD1047 were also inhibitory. The Sig1R agonist SKF10047 had no effect. Sustained calcium entry evoked by VEGF or hydrogen peroxide was also inhibited by BD1063, BD1047 or 4-IBP, but not SKF10047. 4-IBP, BD1047 and BD1063 inhibited TRPC5 or TRPM3, but not TRPM2. Inhibitory effects of BD1047 were rapid in onset and readily reversed on washout. SKF10047 inhibited TRPC5 but not TRPM3 or TRPM2. Depletion of Sig1R did not prevent the inhibitory actions of BD1063 or BD1047 and Sig1R did not co-localize with TRPC5 or TRPM3.

CONCLUSIONS AND IMPLICATIONS

The data suggest that two types of Sig1R ligand (BD1047/BD1063 and 4-IBP) are inhibitors of receptor- or chemically activated calcium entry channels, acting relatively directly and independently of the Sig1R. Chemical foundations for TRP channel inhibitors are suggested.

Role of ion channels and transporters in cell migration

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

Stimulus-dependent phosphorylation of profilin-1 in angiogenesis

Angiogenesis, the formation of new blood vessels, is fundamental to development and post-injury tissue repair. Vascular endothelial growth factor (VEGF)-A guides and enhances endothelial cell migration to initiate angiogenesis. Profilin-1 (Pfn-1) is an actin-binding protein that enhances actin filament formation and cell migration, but stimulus-dependent regulation of Pfn-1 has not been observed. Here, we show that VEGF-A-inducible phosphorylation of Pfn-1 at Tyr 129 is critical for endothelial cell migration and angiogenesis. Chemotactic activation of VEGF receptor kinase-2 (VEGFR2) and Src induces Pfn-1 phosphorylation in the cell leading edge, promoting Pfn-1 binding to actin and actin polymerization. Conditional endothelial knock-in of phosphorylation-deficient Pfn1(Y129F) in mice reveals that Pfn-1 phosphorylation is critical for angiogenesis in response to wounding and ischaemic injury, but not for developmental angiogenesis. Thus, VEGFR2/Src-mediated phosphorylation of Pfn-1 bypasses canonical, multistep intracellular signalling events to initiate endothelial cell migration and angiogenesis, and might serve as a selective therapeutic target for anti-angiogenic therapy.

On the role of endothelial TRPC3 channels in endothelial dysfunction and cardiovascular disease

In endothelium, calcium (Ca(2+)) influx through plasma membrane Ca(2+)-permeable channels plays a fundamental role in several physiological functions and in the pathogenesis of cardiovascular disease. Current knowledge on the influence of Ca(2+) influx in signaling events associated to endothelial dysfunction has grown significantly over recent years, particularly after identification of members of the Transient Receptor Potential Canonical (TRPC) family of channel forming proteins as prominent mediators of Ca(2+) entry in endothelial cells. Among TRPC members TRPC3 has been at the center of many of these physiopathological processes. Progress in elucidating the mechanism/s underlying regulation of endothelial TRPC3 and characterization of signaling events downstream TRPC3 activation are of most importance to fully appreciate the role of this peculiar cation channel in cardiovascular disease and its potential use as a therapeutic target. In this updated review we focus on TRPC3 channels, revising and discussing current knowledge on channel expression and regulation in endothelium and the roles of TRPC3 in cardiovascular disease in relation to endothelial dysfunction.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.