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.     

The role of adipose TRP channels in the pathogenesis of obesity

The prevalence of obesity is continuously increasing worldwide. Transient receptor potential (TRP) channels constitute a family of nonselective cation channels that are ubiquitously expressed in mammalian tissues, including adipose tissue. Although TRP channels might be regarded as therapeutic targets for obesity due to the inhibitory effects of their agonists on body weight and adiposity, the exact role of TRP channels in the development of obesity by modulating the function of adipose tissue has not been systemically reviewed. Multiple TRP channels are present in adipocytes and are involved in diverse aspects of cellular function, including differentiation and maturation of white adipose tissue (WAT), browning of WAT and thermogenesis of brown adipose tissue (BAT). Most of these functions are mediated by alterations in intracellular Ca²⁺ levels or subcellular Ca²⁺ signaling pathway. TRP channels influence intracellular Ca²⁺ dynamics through directly mediating Ca²⁺ entry (TRPVs and others) or store-operated mechanisms (TRPCs). Intracellular Ca²⁺ displays a biphasic effect on regulation adipocyte behaviors depending on the differentiation stage, which may account for the different roles of individual TRP channels in regulation of adiposity. This review emphasizes the contribution of TRP channels to obesity and provide an in-depth discussion on the complexity of their mechanism of actions.     

Methods for monitoring Ca2+ and ion channels in the lysosome

Lysosomes and lysosome-related organelles are emerging as intracellular Ca²⁺ stores and play important roles in a variety of membrane trafficking processes, including endocytosis, exocytosis, phagocytosis and autophagy. Impairment of lysosomal Ca²⁺ homeostasis and membrane trafficking has been implicated in many human diseases such as lysosomal storage diseases (LSDs), neurodegeneration, myopathy and cancer. Lysosomal membrane proteins, in particular ion channels, are crucial for lysosomal Ca²⁺ signaling. Compared with ion channels in the plasma membrane, lysosomal ion channels and their roles in lysosomal Ca²⁺ signaling are less understood, largely due to their intracellular localization and the lack of feasible functional assays directly applied to the native environment. Recent advances in biomedical methodology have made it possible to directly investigate ion channels in the lysosomal membrane. In this review, we provide a summary of the newly developed methods for monitoring lysosomal Ca²⁺ and ion channels, as well as the recent discovery of lysosomal ion channels and their significances in intracellular Ca²⁺ signaling. These new techniques will expand our research scope and our understanding of the nature of lysosomes and lysosome-related diseases.     

Focus on TRP channels in cystic fibrosis

The Transient Receptor Potential (TRP) protein superfamily is a group of cation channels expressed in various cell types and involved in respiratory diseases such as cystic fibrosis (CF), the genetic disease caused by CF Transmembrane conductance Regulator (CFTR) mutations. In human airway epithelial cells, there is growing evidence for a functional link between CFTR and TRP channels. TRP channels contribute to transmitting extracellular signals into the cells and, in an indirect manner, to CFTR activity via a Ca²⁺ rise signaling. Indeed, mutated CFTR-epithelial cells are characterized by an increased Ca²⁺ influx and, on the opposite, by a decreased of magnesium influx, both being mediated by TRP channels. This increasing cellular Ca²⁺ triggers the activation of calcium-activated chloride channels (CaCC) or CFTR itself, via adenylyl cyclase, PKA and tyrosine kinases activation, but also leads to an exaltation of the inflammatory response. Another shortcoming in mutated CFTR-epithelial cells is a [Mg²⁺]_i decrease, associated with impaired TRPM7 functioning. This deregulation has to be taken into consideration in CF physiopathology, as Mg²⁺ is required for ATP hydrolysis and CFTR activity. The modulation of druggable TRP channels could supplement CF therapy either an anti-inflammatory drug or for CFTR potentiation, according to the balance between exacerbation and respite phases. The present paper focus on TRPA1, TRPC6, TRPM7, TRPV2, TRPV4, TRPV6 and ORAI 1, the proteins identified, for now, as dysfunctional channels, in CF cells.     

TRPs and Ca2+ in cell death and survival

Transient Receptor Potential (TRP) family mediate the influx of monovalent and/or divalent cations into cells in response to environmental stimuli. Pharmacological or genetic manipulations of TRP channels demonstrate that TRP channels influence cell death rates, prolonging or shortening of cell survival. Due to their diverse cellular localization, TRP channels mediated Ca²⁺ influx generates distinct intracellular Ca²⁺ signals that regulate downstream pathways converging to apoptosis or survival. In this review, we summarize the accumulated knowledge focused on how TRP channel regulate cell fate and may affect different pathologies including cardiovascular, neurological, metabolic or neoplastic disorders.     

X-ray crystallography of TRP channels

Transient receptor potential (TRP) ion channels are molecular sensors of a large variety of stimuli including temperature, mechanical stress, voltage, small molecules including capsaicin and menthol, and lipids such as phosphatidylinositol 4,5-bisphosphate (PIP₂). Since the same TRP channels may respond to different physical and chemical stimuli, they can serve as signal integrators. Many TRP channels are calcium permeable and contribute to Ca²⁺ homeostasis and signaling. Although the TRP channel family was discovered decades ago, only recently have the structures of many of these channels been solved, largely by cryo-electron microscopy (cryo-EM). Complimentary to cryo-EM, X-ray crystallography provides unique tools to unambiguously identify specific atoms and can be used to study ion binding in channel pores. In this review we describe crystallographic studies of the TRP channel TRPV6. The methodology used in these studies may serve as a template for future structural analyses of different types of TRP and other ion channels.     

Physiological roles of NAADP-mediated Ca<Superscript>2+</Superscript> signaling

Nicotinic acid dinucleotide phosphate (NAADP) is unique amongst Ca²⁺ mobilizing messengers in that its principal function is to mobilize Ca²⁺ from acidic organelles. Early studies indicated that it was likely that NAADP activates a novel Ca²⁺ release channel distinct from the well characterized Ca²⁺ release channels on the (sarco)-endoplasmic reticulum (ER), inositol trisphosphate and ryanodine receptors. In this review, we discuss the emergence of a novel family of endolysosomal channels, the two-pore channels (TPCs), as likely targets for NAADP, and how molecular and pharmacological manipulation of these channels is enhancing our understanding of the physiological roles of NAADP as an intracellular Ca²⁺ mobilizing messenger.     

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) and Ca2+ Mobilization

Many physiological processes are controlled by a great diversity of Ca^{2 +} signals that depend on Ca^{2 +} entry into the cell and/or Ca^{2 +} release from internal Ca^{2 +} stores. Ca^{2 +} mobilization from intracellular stores is gated by a family of messengers including inositol-1,4,5-trisphosphate (InsP₃), cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). There is increasing evidence for a novel intracellular Ca^{2 +} release channel that may be targeted by NAADP and that displays properties distinctly different from the well-characterized InsP₃ and ryanodine receptors. These channels appear to localize on a wider range of intracellular organelles, including the acidic Ca^{2 +} stores. Activation of the NAADP-sensitive Ca^{2 +} channels evokes complex changes in cytoplasmic Ca^{2 +} levels by means of channel chatter with other intracellular Ca^{2 +} channels. The recent demonstration of changes in intracellular NAADP levels in response to physiologically relevant extracellular stimuli highlights the significance of NAADP as an important regulator of intracellular Ca^{2 +} signaling.     

The oncogenic roles of TRPM ion channels in cancer

Transient receptor potential (TRP) proteins are a diverse family of ion channels present in multiple types of tissues. They function as gatekeepers for responses to sensory stimuli including temperature, vision, taste, and pain through their activities in conducting ion fluxes. The TRPM (melastatin) subfamily consists of eight members (i.e., TRPM1–8), which collectively regulate fluxes of various types of cations such as K⁺, Na⁺, Ca²⁺, and Mg²⁺. Growing evidence in the past two decades indicates that TRPM ion channels, their isoforms, or long noncoding RNAs encoded within the locus may be oncogenes involved in the regulation of cancer cell growth, proliferation, autophagy, invasion, and epithelial–mesenchymal transition, and their significant association with poor clinical outcomes of cancer patients. In this review, we describe and discuss recent findings implicating TRPM channels in different malignancies, their functions, mechanisms, and signaling pathways involved in cancers, as well as summarizing their normal physiological functions and the availability of ion channel pharmacological inhibitors.     

Imbalance and dysfunction of transient receptor potential channels contribute to the pathogenesis of hypertension

Intracellular Ca²⁺ homeostasis is essential for vascular function and blood pressure regulation. Because of their unique roles in regulating intracellular Ca²⁺ concentration and vascular function, a novel class of non-selective cation channels, called transient receptor potential (TRP) channels, have emerged at the frontier of hypertension research. Based on their role in vasculature function regulation, TRP channels can be divided into two functional subtypes: one that participates in vasoconstriction and one that participates in vasodilatation. A functional imbalance of these two subtypes of TRP channels may disturb intracellular calcium ([Ca²⁺]i) homeostasis, and the consequent vascular dysfunction may contribute to the development of hypertension. The potential of these TRP channels as novel pharmacological targets for the treatment of human hypertension is of great interest.     

Mechanism of Ca2+-dependent desensitization in TRP channels

The 30+ members of the family of TRP channels are diverse in their physiological roles, yet the mechanisms that regulate their gating may be conserved. In particular, all TRP channels show an activity-dependent inhibition which is mediated by Ca²⁺. The mechanism by which Ca²⁺ inhibits TRP channels is currently a matter of intense debate, with Ca²⁺-regulated kinases, phosphatases, phospholipases, and calmodulin all proposed to be involved. In this review, we will discuss different mechanisms for Ca²⁺-dependent desensitization in TRP channels. We will conclude with a model that focuses on Ca²⁺-dependent activation of phospholipase C and Ca²⁺ binding to calmodulin and propose that the phospholipase C and calmodulin pathways are structurally and functionally coupled.     

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

Abstract. 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.     

The Xenopus TRPV6 homolog encodes a Mg2+‐permeant channel that is inhibited by interaction with TRPC1

The TRP gene family encodes primarily cation non‐selective, Ca²⁺ permeant channels that are involved in a dizzying array of sensory mechanisms. Two channels in this large family TRPV5 and TRPV6 are highly Ca²⁺ selective and are expressed in epithelia where they are important in Ca²⁺ uptake. TRPV5/6 are constitutively active, yet the mechanisms regulating their activation in native tissue remains elusive. Here we functionally characterize the Xenopus TRPV6 homolog. xTRPV6 is expressed in the oocyte and encodes a channel that is permeant to divalents including Ca²⁺, and displays a high permeability to Mg²⁺. The oocyte does not exhibit functional TRPV6‐like current at rest, showing that the endogenous channel is somehow maintained in an inactive state. We show that endogenous as well as overexpressed xTRPV6 interacts with xTRPC1 and that this interaction inhibits xTRPV6 currents. As such TRPC1 is likely to regulate the activity of TRPV6 under physiological conditions. J. Cell. Physiol. 228: 2386–2398, 2013. © 2013 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc.     

TRPP2 channels regulate apoptosis through the Ca2+ concentration in the endoplasmic reticulum

Ca²⁺ is an important signalling molecule that regulates multiple cellular processes, including apoptosis. Although Ca²⁺ influx through transient receptor potential (TRP) channels in the plasma membrane is known to trigger cell death, the function of intracellular TRP proteins in the regulation of Ca²⁺‐dependent signalling pathways and apoptosis has remained elusive. Here, we show that TRPP2, the ion channel mutated in autosomal dominant polycystic kidney disease (ADPKD), protects cells from apoptosis by lowering the Ca²⁺ concentration in the endoplasmic reticulum (ER). ER‐resident TRPP2 counteracts the activity of the sarcoendoplasmic Ca²⁺ ATPase by increasing the ER Ca²⁺ permeability. This results in diminished cytosolic and mitochondrial Ca²⁺ signals upon stimulation of inositol 1,4,5‐trisphosphate receptors and reduces Ca²⁺ release from the ER in response to apoptotic stimuli. Conversely, knockdown of TRPP2 in renal epithelial cells increases ER Ca²⁺ release and augments sensitivity to apoptosis. Our findings indicate an important function of ER‐resident TRPP2 in the modulation of intracellular Ca²⁺ signalling, and provide a molecular mechanism for the increased apoptosis rates in ADPKD upon loss of TRPP2 channel function.     

Acidic Ca(2+) stores come to the fore

Changes in the concentration of cytosolic Ca²⁺ form the basis of a ubiquitous signal transduction pathway. Accumulating evidence implicates acidic organelles in the control of Ca²⁺ dynamics in organisms across phyla. In this special issue, we discuss Ca²⁺ signalling by these “acidic Ca²⁺ stores” which include acidocalcisomes, vacuoles, the endo-lysosomal system, lysosome-related organelles, secretory vesicles and the Golgi complex. Ca²⁺ release from these morphologically very different organelles is mediated by members of the TRP channel superfamily and two-pore channels. Inositol trisphosphate and ryanodine receptors which are traditionally viewed as endoplasmic reticulum Ca²⁺ release channels can also mobilize acidic Ca²⁺ stores. Ca²⁺ uptake into acidic Ca²⁺ stores is driven by Ca²⁺ ATPases and Ca²⁺/H⁺ exchangers. In animal cells, the Ca²⁺-mobilizing messenger NAADP plays a central role in mediating Ca²⁺ signals from acidic Ca²⁺ stores through activation of two-pore channels. These signals are important for several physiological processes including muscle contraction and differentiation. Dysfunctional acidic Ca²⁺ stores have been implicated in diseases such as acute pancreatitis and lysosomal storage disorders. Acidic Ca²⁺ stores are therefore emerging as essential components of the Ca²⁺ signalling network and merit extensive further study.     

Role of TRPC3 in the formation of receptor-and store-operated calcium channels in carcinoma A431 cells

Activation of phospholipase C (PLC)-linked signaling cascades in nonexcitable cells stimulates Ca²⁺ release from inositol-1,4,5-trisphosphate (IP₃)-sensitive intracellular Ca²⁺ stores and activation of Ca²⁺ entry via plasma membrane Ca²⁺ channels. The attention of investigators is currently focused on the properties and molecular basis of channels involved in Ca²⁺ entry into nonexcitable cells. According to current views, mammalian TRP proteins are involved in receptor-and store-dependent influx of Ca²⁺; however, little is known about the linkage between specific TRP proteins and endogenous channels responsible for Ca²⁺ entry. The aim of the present study was to elucidate the role of TRPC3 in the formation of store-dependent or receptor-operated pathways of Ca²⁺ entry into A431 cells. Registration of Ca²⁺ influx based on fluorescence measurements of intracellular Ca²⁺ concentrations and analysis of integral membrane currents revealed that partial inhibition of TRPC3 expression by small interfering RNA (siRNA) results in suppression of store-dependent Ca²⁺ entry without any effect on receptor-operated Ca²⁺ influx. In-depth studies of single channels revealed that TRPC3 suppression in A431 cells results in the disappearance of one type of store-operated channels and formation of a novel type of store-independent Ca²⁺-permeable channels. This, in turn, testifies to the crucial role of TRPC3 in normal functioning of store-operated Ca²⁺ channels in A431 cells.     

Magnetic field-induced Ca2+ intake by mesenchymal stem cells is mediated by intracellular Zn2+ and accompanied by a Zn2+ influx

Chronic exposure to magnetic fields (MFs) has a diverse range of effects on biological systems but definitive molecular mechanisms of the interaction remain largely unknown. One of the most frequently reported effects of MF exposure is an elevated concentration of intracellular Ca²⁺ through disputed pathways. Other prominent effects include increased oxidative stress and upregulation of neural markers through EGFR activation in stem cells. Further characterization of cascades triggered by MF exposure is hindered by the phenotype diversity of biological models used in the literature. In an attempt to reveal more mechanistic data in this field, we combined the most commonly used biological model and MF parameters with the most commonly reported effects of MFs.Based on clues from the pathways previously defined as sensitive to MFs (EGFR and Zn²⁺-binding enzymes), the roles of different types of channels (voltage gated Ca²⁺ channels, NMDA receptors, TRP channels) were inquired in the effects of 50 Hz MFs on bone marrow-derived mesenchymal stem cells. We report that, an influx of Zn²⁺ accompanies MF-induced Ca²⁺ intake, which is only attenuated by the broad-range inhibitor of TRP channels and store-operated Ca²⁺ entry (SOCE), 2-Aminoethoxydiphenyl borate (2-APB) among other blockers (memantine, nifedipine, ethosuximide and gabapentin). Interestingly, cation influx completely disappears when intracellular Zn²⁺ is chelated. Our results rule out voltage gated Ca²⁺ channels as a gateway to MF-induced Ca²⁺ intake and suggest Zn²⁺-related channels as a new focus in the field.     

Role of Cl− channels in primary brain tumour

There is tight interplay between Ca²⁺ and Cl⁻ flux that can influence brain tumour proliferation, migration and invasion. Glioma is the predominant malignant primary brain tumour, accounting for ˜80% of all cases. Voltage-gated Cl⁻ channel family (ClC) proteins and Cl⁻ intracellular channel (CLIC) proteins are drastically overexpressed in glioma, and are associated with enhanced cell proliferation, migration and invasion. Ca²⁺ also plays fundamental roles in the phenomenon. Ca²⁺-activated Cl⁻ channels (CaCC) such as TMEM16A and bestrophin-1 are involved in glioma formation and assist Ca²⁺ movement from intracellular stores to the plasma membrane. Additionally, the transient receptor protein (TRP) channel TRPC1 can induce activation of ClC-3 by increasing intracellular Ca²⁺concentrations and activating Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). Therefore, Ca²⁺ and Cl⁻currents can concurrently mediate brain tumour cellular functions. Glioma also expresses volume regulated anion channels (VRACs), which are responsible for the swelling-induced Cl⁻ current, I_Cl,swell. This current enables glioma cells to perform regulatory volume decrease (RVD) as a survivability mechanism in response to hypoxic conditions within the tumour microenvironment. RVD can also be exploited by glioma for invasion and migration. Effective treatment for glioma is challenging, which can be in part due to prolonged chemotherapy leading to mutations in genes associated with multi-drug resistances (MRP1, Bcl-2, and ABC family). Thus, a potential therapeutic strategy for treatment of glioma can be through the inhibition of selected Cl⁻ channels.