Na(+) entry and modulation of Na(+)/Ca(2+) exchange as a key mechanism of TRPC signaling

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

S-adenosyl methionine regulates calcium channels and inhibits uterine smooth muscle contraction in rats with infectious premature delivery through the transient receptor protein 3/protein kinase Cβ/C-kinase-activated protein phosphatase-1 inhibitor of 17 kDa signaling pathway

The aim of the present study was to investigate the effects of S-adenosyl methionine (SAMe) on infectious premature inflammatory factors and uterine contraction, and to further explore its mechanism of action via the transient receptor protein 3 (TRPC3)/protein kinase Cβ (PKCβ)/C-kinase-activated protein phosphatase-1 inhibitor of 17 kDa (CPI-17) signaling pathway, following intervention by a TRPC3 inhibitor. A rat model of premature delivery induced by lipopolysaccharide (LPS) was established. Following treatment with SAMe and inhibiting TRPC3 expression, rat serum and uterus were isolated. Hematoxylin and eosin staining was used to observe the histopathological changes in the uterus. Uterine muscle strips in vitro were selected to measure the changes in muscle tension. ELISA was utilized to measure the changes in serum inflammatory factor and oxidative stress indexes. Immunohistochemistry, western blot assay and reverse transcription-quantitative polymerase chain reaction were applied to detect calcium channel protein expression in the uterus. Western blot analysis was employed to measure the expression of TRPC3/PKCβ/CPI-17 signaling pathway-related proteins. TRPC3 was highly expressed in the uterus of rat models of premature delivery induced by LPS. Following treatment with SAMe, inflammatory cell infiltration markedly reduced in the uterus and the tension of in vitro uterine muscle strips significantly decreased. SAMe treatment suppressed inflammatory reaction and oxidative stress, and diminished L-type and T-type calcium channel protein expression. TRPC3/PKCβ/CPI-17 signaling pathway-related protein expression was also reduced. When TRPC3 expression was suppressed, the effects of SAMe against inflammation and oxidative stress were diminished. TRPC3/PKCβ/CPI-17 signaling pathway-related protein expression significantly increased. SAMe was able to reduce inflammatory reaction and oxidative stress in the uterus of rat model of infectious premature delivery induced by LPS, prolong delivery time, reduce the mortality rate of offspring rats, and serve a therapeutic role. This effect is likely achieved via the regulation of uterine contractions and childbirth through the TRPC3/PKCβ/CPI-17 signaling pathway.

TRPC3 is required for the survival, pluripotency and neural differentiation of mouse embryonic stem cells (mESCs)

Transient receptor potential canonical subfamily member 3 (TRPC3) is known to be important for neural development and the formation of neuronal networks. Here, we investigated the role of TRPC3 in undifferentiated mouse embryonic stem cells (mESCs) and during the differentiation of mESCs into neurons. CRISPR/Cas9-mediated knockout (KO) of TRPC3 induced apoptosis and the disruption of mitochondrial membrane potential both in undifferentiated mESCs and in those undergoing neural differentiation. In addition, TRPC3 KO impaired the pluripotency of mESCs. TRPC3 KO also dramatically repressed the neural differentiation of mESCs by inhibiting the expression of markers for neural progenitors, neurons, astrocytes and oligodendrocytes. Taken together, our new data demonstrate an important function of TRPC3 with regards to the survival, pluripotency and neural differentiation of mESCs.

Cellular Na+ handling mechanisms involved in airway smooth muscle contraction (Review)

A decrease in bronchial diameter is designated as bronchoconstriction (BC) and impedes the flow of air through the airway. Asthma is characterized by inflammation of the airways, reversible BC and nonspecific hyperreactivity. These last two symptoms are dependent on airway smooth muscle. Stimuli that trigger contraction can be characterized as chemical (neurotransmitters, cytokines and terpenoids) and physical (volume inspired, air pressure). Both stimuli activate signaling pathways by acting on membrane proteins and facilitating the passage of ions through the membrane, generating a voltage change and a subsequent depolarization. Na⁺ plays an important role in preserving the resting membrane potential; this ion is extracted from the cells by the Na⁺/K⁺ ATPase (NKA) or introduced into the cytoplasm by the Na⁺/Ca²⁺ exchanger (NCX). During depolarization, Na⁺ appears to accumulate in specific regions beneath the plasma membrane, generating local concentration gradients which determine the handling of Ca²⁺. At rest, the smooth muscle has a basal tone that is preserved by the continuous adjustment of intracytoplasmic concentrations of Ca²⁺ and Na⁺. At homeostasis, the Na⁺ concentration is primarily dependent on three structures: the NKA, the NCX and non-specific cation channels (NSCC). These three structures, their functions and the available evidence of the probable role of Na⁺ in asthma are described in the present review.

Deletion of TRPC6 Attenuates NMDA Receptor-Mediated Ca2+ Entry and Ca2+-Induced Neurotoxicity Following Cerebral Ischemia and Oxygen-Glucose Deprivation

Transient receptor potential canonical 6 (TRPC6) channels are permeable to Na⁺ and Ca²⁺ and are widely expressed in the brain. In this study, the role of TRPC6 was investigated following ischemia/reperfusion (I/R) and oxygen-glucose deprivation (OGD). We found that TRPC6 expression was increased in wild-type (WT) mice cortical neurons following I/R and in primary neurons with OGD, and that deletion of TRPC6 reduced the I/R-induced brain infarct in mice and the OGD- /neurotoxin-induced neuronal death. Using live-cell imaging to examine intracellular Ca²⁺ levels ([Ca²⁺]_i), we found that OGD induced a significant higher increase in glutamate-evoked Ca²⁺ influx compared to untreated control and such an increase was reduced by TRPC6 deletion. Enhancement of TRPC6 expression using AdCMV-TRPC6-GFP infection in WT neurons increased [Ca²⁺]_i in response to glutamate application compared to AdCMV-GFP control. Inhibition of N-methyl-d-aspartic acid receptor (NMDAR) with MK801 decreased TRPC6-dependent increase of [Ca²⁺]_i in TRPC6 infected cells, indicating that such a Ca²⁺ influx was NMDAR dependent. Furthermore, TRPC6-dependent Ca²⁺ influx was blunted by blockade of Na⁺ entry in TRPC6 infected cells. Finally, OGD-enhanced Ca²⁺ influx was reduced, but not completely blocked, in the presence of voltage-dependent Na⁺ channel blocker tetrodotoxin (TTX) and dl-α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) blocker CNQX. Altogether, we concluded that I/R-induced brain damage was, in part, due to upregulation of TRPC6 in cortical neurons. We postulate that overexpression of TRPC6 following I/R may induce neuronal death partially through TRPC6-dependent Na⁺ entry which activated NMDAR, thus leading to a damaging Ca²⁺ overload. These findings may provide a potential target for future intervention in stroke-induced brain damage.

Unraveling the Rat Intestine, Spleen and Liver Genome-Wide Transcriptome after the Oral Administration of Lavender Oil by a Two-Color Dye-Swap DNA Microarray Approach

The use of lavender oil (LO)--a commonly, used oil in aromatherapy, with well-defined volatile components linalool and linalyl acetate--in non-traditional medicine is increasing globally. To understand and demonstrate the potential positive effects of LO on the body, we have established an animal model in this current study, investigating the orally administered LO effects genome wide in the rat small intestine, spleen, and liver. The rats were administered LO at 5 mg/kg (usual therapeutic dose in humans) followed by the screening of differentially expressed genes in the tissues, using a 4×44-K whole-genome rat chip (Agilent microarray platform; Agilent Technologies, Palo Alto, CA, USA) in conjunction with a dye-swap approach, a novelty of this study. Fourteen days after LO treatment and compared with a control group (sham), a total of 156 and 154 up (≧ 1.5-fold)- and down (≦ 0.75-fold)-regulated genes, 174 and 66 up- (≧ 1.5-fold)- and down (≦ 0.75-fold)-regulated genes, and 222 and 322 up- (≧ 1.5-fold)- and down (≦ 0.75-fold)-regulated genes showed differential expression at the mRNA level in the small intestine, spleen and liver, respectively. The reverse transcription-polymerase chain reaction (RT-PCR) validation of highly up- and down-regulated genes confirmed the regulation of the Papd4, Lrp1b, Alb, Cyr61, Cyp2c, and Cxcl1 genes by LO as examples in these tissues. Using bioinformatics, including Ingenuity Pathway Analysis (IPA), differentially expressed genes were functionally categorized by their Gene Ontology (GO) and biological function and network analysis, revealing their diverse functions and potential roles in LO-mediated effects in rat. Further IPA analysis in particular unraveled the presence of novel genes, such as Papd4, Or8k5, Gprc5b, Taar5, Trpc6, Pld2 and Onecut3 (up-regulated top molecules) and Tnf, Slc45a4, Slc25a23 and Samt4 (down-regulated top molecules), to be influenced by LO treatment in the small intestine, spleen and liver, respectively. These results are the first such inventory of genes that are affected by lavender essential oil (LO) in an animal model, forming the basis for further in-depth bioinformatics and functional analyses and investigation.

Transient receptor potential channels in the vasculature

The mammalian genome encodes 28 distinct members of the transient receptor potential (TRP) superfamily of cation channels, which exhibit varying degrees of selectivity for different ionic species. Multiple TRP channels are present in all cells and are involved in diverse aspects of cellular function, including sensory perception and signal transduction. Notably, TRP channels are involved in regulating vascular function and pathophysiology, the focus of this review. TRP channels in vascular smooth muscle cells participate in regulating contractility and proliferation, whereas endothelial TRP channel activity is an important contributor to endothelium-dependent vasodilation, vascular wall permeability, and angiogenesis. TRP channels are also present in perivascular sensory neurons and astrocytic endfeet proximal to cerebral arterioles, where they participate in the regulation of vascular tone. Almost all of these functions are mediated by changes in global intracellular Ca(2+) levels or subcellular Ca(2+) signaling events. In addition to directly mediating Ca(2+) entry, TRP channels influence intracellular Ca(2+) dynamics through membrane depolarization associated with the influx of cations or through receptor- or store-operated mechanisms. Dysregulation of TRP channels is associated with vascular-related pathologies, including hypertension, neointimal injury, ischemia-reperfusion injury, pulmonary edema, and neurogenic inflammation. In this review, we briefly consider general aspects of TRP channel biology and provide an in-depth discussion of the functions of TRP channels in vascular smooth muscle cells, endothelial cells, and perivascular cells under normal and pathophysiological conditions.

Identification and optimization of 2-aminobenzimidazole derivatives as novel inhibitors of TRPC4 and TRPC5 channels

BACKGROUND AND PURPOSE

Transient receptor potential canonical (TRPC) channels play important roles in a broad array of physiological functions and are involved in various diseases. However, due to a lack of potent subtype-specific inhibitors the exact roles of TRPC channels in physiological and pathophysiological conditions have not been elucidated.

EXPERIMENTAL APPROACH

Using fluorescence membrane potential and Ca(2+) assays and electrophysiological recordings, we characterized new 2-aminobenzimidazole-based small molecule inhibitors of TRPC4 and TRPC5 channels identified from cell-based fluorescence high-throughput screening.

KEY RESULTS

The original compound, M084, was a potent inhibitor of both TRPC4 and TRPC5, but was also a weak inhibitor of TRPC3. Structural modifications of the lead compound resulted in the identification of analogues with improved potency and selectivity for TRPC4 and TRPC5 channels. The aminobenzimidazole derivatives rapidly inhibited the TRPC4- and TRPC5-mediated currents when applied from the extracellular side and this inhibition was independent of the mode of activation of these channels. The compounds effectively blocked the plateau potential mediated by TRPC4-containing channels in mouse lateral septal neurons, but did not affect the activity of heterologously expressed TRPA1, TRPM8, TRPV1 or TRPV3 channels or that of the native voltage-gated Na(+) , K(+) and Ca(2) (+) channels in dissociated neurons.

CONCLUSIONS AND IMPLICATIONS

The TRPC4/C5-selective inhibitors developed here represent novel and useful pharmaceutical tools for investigation of physiological and pathophysiological functions of TRPC4/C5 channels.

Sodium entry through endothelial store-operated calcium entry channels: regulation by Orai1

Orai1 interacts with transient receptor potential protein of the canonical subfamily (TRPC4) and contributes to calcium selectivity of the endothelial cell store-operated calcium entry current (ISOC). Orai1 silencing increases sodium permeability and decreases membrane-associated calcium, although it is not known whether Orai1 is an important determinant of cytosolic sodium transitions. We test the hypothesis that, upon activation of store-operated calcium entry channels, Orai1 is a critical determinant of cytosolic sodium transitions. Activation of store-operated calcium entry channels transiently increased cytosolic calcium and sodium, characteristic of release from an intracellular store. The sodium response occurred more abruptly and returned to baseline more rapidly than did the transient calcium rise. Extracellular choline substitution for sodium did not inhibit the response, although 2-aminoethoxydiphenyl borate and YM-58483 reduced it by ∼50%. After this transient response, cytosolic sodium continued to increase due to influx through activated store-operated calcium entry channels. The magnitude of this sustained increase in cytosolic sodium was greater when experiments were conducted in low extracellular calcium and when Orai1 expression was silenced; these two interventions were not additive, suggesting a common mechanism. 2-Aminoethoxydiphenyl borate and YM-58483 inhibited the sustained increase in cytosolic sodium, only in the presence of Orai1. These studies demonstrate that sodium permeates activated store-operated calcium entry channels, resulting in an increase in cytosolic sodium; the magnitude of this response is determined by Orai1.

A novel homology model of TRPC3 reveals allosteric coupling between gate and selectivity filter

Utilizing a novel molecular model of TRPC3, based on the voltage-gated sodium channel from Arcobacter butzleri (Na(V)AB) as template, we performed structure-guided mutagenesis experiments to identify amino acid residues involved in divalent permeation and gating. Substituted cysteine accessibility screening within the predicted selectivity filter uncovered amino acids 629-631 as the narrowest part of the permeation pathway with an estimated pore diameter of < 5.8Å. E630 was found to govern not only divalent permeability but also sensitivity of the channel to block by ruthenium red. Mutations in a hydrophobic cluster at the cytosolic termini of transmembrane segment 6, corresponding to the S6 bundle crossing structure in Na(V)AB, distorted channel gating. Removal of a large hydrophobic residue (I667A or I667E) generated channels with approximately 60% constitutive activity, suggesting I667 as part of the dynamic structure occluding the permeation path. Destabilization of the gate was associated with reduced Ca2+ permeability, altered cysteine cross-linking in the selectivity filter and promoted channel block by ruthenium red. Collectively, we present a structural model of the TRPC3 permeation pathway and localize the channel's selectivity filter and the occluding gate. Moreover, we provide evidence for allosteric coupling between the gate and the selectivity filter in TRPC3.

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

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

TRP channels coordinate ion signalling in astroglia

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

Livin' with NCX and lovin' it: a 45 year romance

This conference commemorates, almost to the day, the 45th anniversary of the discovery of the Na(+)/Ca(2+) exchanger (NCX). The discovery was serendipitous, as is so often the case with scientific breakthroughs. Indeed, that is what is so fascinating and romantic about scientific research. I will describe the discovery of NCX, but will begin by explaining how I got there, and will then discuss how the discovery influenced my career path.

Cross talk between plasma membrane Na(+)/Ca (2+) exchanger-1 and TRPC/Orai-containing channels: key players in arterial hypertension

Arterial smooth muscle (ASM) Na(+)/Ca(2+) exchanger type 1 (NCX1) and TRPC/Orai-containing receptor/store-operated cation channels (ROC/SOC) are clustered with α2 Na(+) pumps in plasma membrane microdomains adjacent to the underlying junctional sarcoplasmic reticulum. This arrangement enables these transport proteins to function as integrated units to help regulate local Na(+) metabolism, Ca(2+) signaling, and arterial tone. They thus influence vascular resistance and blood pressure (BP). For instance, upregulation of NCX1 and TRPC6 has been implicated in the pathogenesis of high BP in several models of essential hypertension. The models include ouabain-induced hypertensive rats, Milan hypertensive rats, and Dahl salt-sensitive hypertensive rats, all of which exhibit elevated plasma ouabain levels. We suggest that these molecular mechanisms are key contributors to the increased vascular resistance ("whole body autoregulation") that elevates BP in essential hypertension. Enhanced expression and function of ASM NCX1 and TRPC/Orai1-containing channels in hypertension implies that these proteins are potential targets for pharmacological intervention.

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.

Increased arterial smooth muscle Ca2+ signaling, vasoconstriction, and myogenic reactivity in Milan hypertensive rats

The Milan hypertensive strain (MHS) rats are a genetic model of hypertension with adducin gene polymorphisms linked to enhanced renal tubular Na(+) reabsorption. Recently we demonstrated that Ca(2+) signaling is augmented in freshly isolated mesenteric artery myocytes from MHS rats. This is associated with greatly enhanced expression of Na(+)/Ca(2+) exchanger-1 (NCX1), C-type transient receptor potential (TRPC6) protein, and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) compared with arteries from Milan normotensive strain (MNS) rats. Here, we test the hypothesis that the enhanced Ca(2+) signaling in MHS arterial smooth muscle is directly reflected in augmented vasoconstriction [myogenic and phenylephrine (PE)-evoked responses] in isolated mesenteric small arteries. Systolic blood pressure was higher in MHS (145 ± 1 mmHg) than in MNS (112 ± 1 mmHg; P < 0.001; n = 16 each) rats. Pressurized mesenteric resistance arteries from MHS rats had significantly augmented myogenic tone and reactivity and enhanced constriction to low-dose (1-100 nM) PE. Isolated MHS arterial myocytes exhibited approximately twofold increased peak Ca(2+) signals in response to 5 μM PE or ATP in the absence and presence of extracellular Ca(2+). These augmented responses are consistent with increased vasoconstrictor-evoked sarcoplasmic reticulum (SR) Ca(2+) release and increased Ca(2+) entry, respectively. The increased SR Ca(2+) release correlates with a doubling of inositol 1,4,5-trisphosphate receptor type 1 and tripling of SERCA2 expression. Pressurized MHS arteries also exhibited a ∼70% increase in 100 nM ouabain-induced vasoconstriction compared with MNS arteries. These functional alterations reveal that, in a genetic model of hypertension linked to renal dysfunction, multiple mechanisms within the arterial myocytes contribute to enhanced Ca(2+) signaling and myogenic and vasoconstrictor-induced arterial constriction. MHS rats have elevated plasma levels of endogenous ouabain, which may initiate the protein upregulation and enhanced Ca(2+) signaling. These molecular and functional changes provide a mechanism for the increased peripheral vascular resistance (whole body autoregulation) that underlies the sustained hypertension.

GVI phospholipase A2 role in the stimulatory effect of sphingosine-1-phosphate on TRPC5 cationic channels

The Transient Receptor Potential Canonical 5 (TRPC5) protein forms calcium-permeable cationic channels that are stimulated by G protein-coupled receptor agonists. The signaling pathways of such agonist effects are poorly understood. Here we investigated the potential for involvement of lysophosphatidylcholine (LPC) and arachidonic acid generated by group 6 (GVI) phospholipase A2 (PLA2) enzymes, focusing on stimulation of TRPC5 by sphingosine-1-phosphate (S1P) which acts via a pertussis toxin-sensitive (Gi/o protein) pathway without Ca²⁺-release. Experiments were on HEK 293 cells containing conditional expression of human TRPC5. Channel activity was recorded using an intracellular calcium indicator or whole-cell patch-clamp and PLA2 activity was detected using ³H-arachidonic acid. S1P stimulated PLA2 and TRPC5 activities. Both effects were suppressed by the GVI PLA2 inhibitor bromoenol lactone. Knock-down of GVI PLA2 by RNA interference suppressed channel activity evoked by S1P whereas activity evoked by the direct channel stimulator LPC was unaffected. Arachidonic acid did not stimulate the channels. Prior exposure of channels to LPC but not arachidonic acid suppressed channel activity evoked by S1P but not gadolinium, a putative direct stimulator of the channels. The data suggest roles of LPC and GVI PLA2 in S1P-evoked TRPC5 activity.

Podocyte-specific overexpression of wild type or mutant trpc6 in mice is sufficient to cause glomerular disease

Mutations in the TRPC6 calcium channel (Transient receptor potential channel 6) gene have been associated with familiar forms of Focal and Segmental Glomerulosclerosis (FSGS) affecting children and adults. In addition, acquired glomerular diseases are associated with increased expression levels of TRPC6. However, the exact role of TRPC6 in the pathogenesis of FSGS remains to be elucidated. In this work we describe the generation and phenotypic characterization of three different transgenic mouse lines with podocyte-specific overexpression of the wild type or any of two mutant forms of Trpc6 (P111Q and E896K) previously related to FSGS. Consistent with the human phenotype a non-nephrotic range of albuminuria was detectable in almost all transgenic lines. The histological analysis demonstrated that the transgenic mice developed a kidney disease similar to human FSGS. Differences of 2-3 folds in the presence of glomerular lesions were found between the non transgenic and transgenic mice expressing Trpc6 in its wild type or mutant forms specifically in podocytes. Electron microscopy of glomerulus from transgenic mice showed extensive podocyte foot process effacement. We conclude that overexpression of Trpc6 (wild type or mutated) in podocytes is sufficient to cause a kidney disease consistent with FSGS. Our results contribute to reinforce the central role of podocytes in the etiology of FSGS. These mice constitute an important new model in which to study future therapies and outcomes of this complex disease.

TRPC channels in smooth muscle cells

Transient receptor potential canonical (TRPC) proteins constitute a family of seven (TRPC1-7) nonselective cation channels within the wider TRP superfamily. TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 channels are expressed in vascular smooth muscle cells from human vessels of all calibers and in smooth muscle from organs such as the uterus and the gastrointestinal tract. TRPC channels have recently emerged as important players in the control of smooth muscle function. This review will focus on the retrospective analysis of studies proposing contributions of TRPC channels to native calcium entry pathways in smooth muscle and to physiological and pathophysiological responses with emphasis on the vascular system.

Na+-Ca2+ exchanger contributes to Ca2+ extrusion in ATP-stimulated endothelium of intact rat aorta

The role of Na(+)-Ca(2+) exchanger (NCX) in vascular endothelium is still matter of debate. Depending on both the endothelial cell (EC) type and the extracellular ligand, NCX has been shown to operate in either the forward (Ca(2+) out)- or the reverse (Ca(2+) in)-mode. In particular, acetylcholine (Ach) has been shown to promote Ca(2+) inflow in the intact endothelium of excised rat aorta. Herein, we assessed the involvement of NCX into the Ca(2+) signals elicited by ATP in such preparation. Removal of extracellular Na(+) (0Na(+)) causes the NCX to switch into the reverse-mode and induced an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), which disappeared in the absence of extracellular Ca(2+), and in the presence of benzamil, which blocks both modes of NCX, and KB-R 7943, a selective inhibitor of the reverse-mode. ATP induced a transient Ca(2+) signal, whose decay was significantly prolonged by 0Na(+), benzamil, DCB, and monensin while it was unaffected by KB-R 7943. Notably, lowering extracellular Na(+) concentration increased the sensibility to lower doses of ATP. These date suggest that, unlike Ach-stimulated ECs, NCX promotes Ca(2+) extrusion when the stimulus is provided by ATP in intact endothelium of rat aorta. These data show that, within the same preparation, NCX operates in both modes, depending on the chemical nature of the extracellular stimulus.

Cerebral ischemia-reperfusion injury in rats--a 3 T MRI study on biphasic blood-brain barrier opening and the dynamics of edema formation

Serial magnetic resonance imaging (MRI) was performed to investigate the temporal and spatial relationship between the biphasic nature of blood-brain barrier (BBB) opening and, in parallel, edema formation after ischemia-reperfusion (I/R) injury in rats. T(2)-weighted imaging combined with T(2)-relaxometry, mainly for edema assessment, was performed at 1 h after ischemia, after reperfusion, and at 4, 24 and 48 h after reperfusion. T(1)-weighted imaging was performed before and after gadolinium contrast at the last three time points to assess BBB integrity. The biphasic course of BBB opening with a significant reduction in BBB permeability at 24 h after reperfusion, associated with a progressive expansion of leaky BBB volume, was accompanied by a peak ipsilateral edema formation. In addition, at 4 h after reperfusion, edema formation could also be detected at the contralateral striatum as determined by the elevated T(2)-values that persisted to varying degrees, indicative of widespread effects of I/R injury. The observations of this study may indicate a dynamic temporal shift in the mechanisms responsible for biphasic BBB permeability changes, with complex relations to edema formation. Stroke therapy aimed at vasogenic edema and drug delivery for neuroprotection may also be guided according to the functional status of the BBB, and these findings have to be confirmed in human stroke.

The puzzling role of TRPC3 channels in motor coordination

Transient receptor potential canonical 3 (TRPC3) proteins are nonselective cation channels activated downstream of phospholipase-C-coupled receptors. TRPC3 channels have emerged as major players in the function of the central nervous system. They have been described as important contributors to brain-derived neurotrophic factor mediated survival and growth-cone guidance of cerebellar granule neurons. TRPC3 were also identified as postsynaptic cation channels essential for metabotropic glutamate receptor1-dependent synaptic transmission in cerebellar Purkinje neurons. A recent report described motor coordination defects in TRPC3 knockout mice while a subsequent study reported a similar phenotype in so-called moonwalker mice, harboring a TRPC3 gain-of-function mutation. How can opposing aspects of TRPC3 channel activation lead to the same phenotype? Here we discuss the salient features of TRPC3 knockout mice and moonwalker mice and attempt to reconcile the apparently conflicting findings from these two animal models.

A model for the generation of localized transient [Na+] elevations in vascular smooth muscle

We present a stochastic computational model to study the mechanism of signaling between a source and a target ionic transporter, both localized on the plasma membrane (PM). In general this requires a nanometer-scale cytoplasmic space, or nanodomain, between the PM and a peripheral organelle to reflect ions back towards the PM. Specifically we investigate the coupling between Na(+) entry via the transient receptor potential canonical channel 6 (TRPC6) and the Na(+)/Ca(2+) exchanger (NCX), a process which is essential for reloading the sarcoplasmic reticulum (SR) via the sarco/endoplasmic reticulum Ca(2+)ATPase (SERCA) and maintaining Ca(2+) oscillations in activated vascular smooth muscle. Having previously modeled the flow of Ca(2+) between reverse NCX and SERCA during SR refilling, this quantitative approach now allows us to model the upstream linkage of Na(+) entry through TRPC6 to reversal of NCX. We have implemented a random walk (RW) Monte Carlo (MC) model with simulations mimicking a diffusion process originating at the TRPC6 within PM-SR junctions. The model calculates the average Na(+) in the nanospace and also produces profiles as a function of distance from the source. Our results highlight the necessity of a strategic juxtaposition of the relevant ion translocators as well as other physical structures within the nanospaces to permit adequate Na(+) build-up to initiate NCX reversal and Ca(2+) influx to refill the SR.

Orai1, a critical component of store-operated Ca2+ entry, is functionally associated with Na+/Ca2+ exchanger and plasma membrane Ca2+ pump in proliferating human arterial myocytes

Ca(2+) entry through store-operated channels (SOCs) in the plasma membrane plays an important role in regulation of vascular smooth muscle contraction, tone, and cell proliferation. The C-type transient receptor potential (TRPC) channels have been proposed as major candidates for SOCs in vascular smooth muscle. Recently, two families of transmembrane proteins, Orai [also known as Ca(2+) release-activated Ca(2+) channel modulator (CRACM)] and stromal interacting molecule 1 (STIM1), were shown to be essential for the activation of SOCs mainly in nonexcitable cells. Here, using small interfering RNA, we show that Orai1 plays an essential role in activating store-operated Ca(2+) entry (SOCE) in primary cultured proliferating human aortic smooth muscle cells (hASMCs), whereas Orai2 and Orai3 do not contribute to SOCE. Knockdown of Orai1 protein expression significantly attenuated SOCE. Moreover, inhibition of Orai1 downregulated expression of Na(+)/Ca(2+) exchanger type 1 (NCX1) and plasma membrane Ca(2+) pump isoform 1 (PMCA1). The rate of cytosolic free Ca(2+) concentration decay after Ca(2+) transients in Ca(2+)-free medium was also greatly decreased under these conditions. This reduction of Ca(2+) extrusion, presumably via NCX1 and PMCA1, may be a compensation for the reduced SOCE. Immunocytochemical observations indicate that Orai1 and NCX1 are clustered in plasma membrane microdomains. Cell proliferation was attenuated in hASMCs with disrupted Orai1 expression and reduced SOCE. Thus Orai1 appears to be a critical component of SOCE in proliferating vascular smooth muscle cells, and may therefore be a key player during vascular growth and remodeling.

Post-transcriptional silencing of TRPC1 ion channel gene by RNA interference upregulates TRPC6 expression and store-operated Ca2+ entry in A7r5 vascular smooth muscle cells

This study investigates functional consequences of TRPC1 ion channel downregulation observed in aging rat aorta by employing RNA interference in cultured vascular smooth muscle cells. For this purpose, A7r5 aortic smooth muscle cells were used in quantitative gene and protein expression as well as in functional analyses. According to quantitative RT-PCR results, TRPC3, TRPC4 and TRPC5 mRNAs were not at detectable levels. In siTRPC1-transfected cells, TRPC1 mRNA and protein levels were decreased by 40% and 64%; however, those of TRPC6 were drastically increased by 100% and 200%, respectively. In fura-2-loaded TRPC1 knockdown cells, despite the decreased TRPC1 levels, cyclopiazonic acid-induced Ca2+ entry and store-operated Ca2+ entry following Ca2+ addition were elevated by 77% and 135%, respectively. Results suggest that decrease in TRPC1 may be compensated by upregulated TRPC6 that possibly takes part in store-operated Ca2+ entry in vascular smooth muscle cells.

PKC and PLA2: probing the complexities of the calcium network

Lipid signaling and phosphorylation cascades are fundamental to calcium signaling networks. In this review, we will discuss the recent laboratory findings for the phospholipase A(2) (PLA(2))/protein kinase C (PKC) pathway within cellular calcium networks. The complexity and connectivity of these ubiquitous cellular signals make interpretation of experimental results extremely challenging. We present here computational methods which have been developed to conquer such complex data, and how they can be used to make models capable of accurately predicting cellular responses within multiple calcium signaling pathways. We propose that information obtained from network analysis and computational techniques provides a rich source of knowledge which can be directly translated to the laboratory benchtop.

VEGF-mediated elevated intracellular calcium and angiogenesis in human microvascular endothelial cells in vitro are inhibited by dominant negative TRPC6

OBJECTIVE

Vascular endothelial growth factor (VEGF)-induced vascular permeability has been shown to be dependent on calcium influx, possibly through a transient receptor potential cation channel (TRPC)-mediated cation channel with properties of the TRPC3/6/7 subfamily. To investigate further the involvement of this subfamily, we determined the effects of dominant negative TRPC6 overexpression on VEGF-mediated changes of human microvascular endothelial cell (HMVEC) calcium, proliferation, migration, and sprouting.

METHODS

Cytoplasmic calcium concentration was estimated by fura-2 fluorescence spectrophotometry, migration by Boyden chamber assay, sprouting by immunofluorescence imaging of stimulated endothelial cells, and proliferation by flow cytometry.

RESULTS

Overexpression of a dominant negative TRPC6 construct in HMVECs inhibited the VEGF-mediated increases in cytosolic calcium, migration, sprouting, and proliferation. In contrast, overexpression of a wild-type TRPC6 construct increased the proliferation and migration of HMVECs.

CONCLUSIONS

TRPC6 is an obligatory component of cation channels required for the VEGF-mediated increase in cytosolic calcium and subsequent downstream signaling that leads to processes associated with angiogenesis.

Transient receptor potential channel C3 contributes to the progression of human ovarian cancer

Ovarian cancer (OC) is the leading cause of death from gynecological malignancy. However, the mechanism by which OC develops remains largely unknown. Increases in cytosolic free Ca(2+) ([Ca(2+)](i)) can result in different physiological changes including cell growth, differentiation and death. The transient receptor potential (TRP) C channels are nonselective cation channels with permeability to Ca(2+). Here we report that TRPC3 channels promote human OC growth. The TRPC3 protein levels in human OC specimens were greatly increased than those in normal ovarian specimens. Downregulating TRPC3 expression in SKOV3 cells, a human OC cell line, led to reduction of proliferation, suppression in epidermal growth factor-induced Ca(2+) influx, dephosphorylation of Cdc2 and CaMKIIalpha and prolonged progression through M phase of these cells. Further, decreased the expression of TRPC3 suppressed the tumor formation generated by injecting SKOV3 cells in nude mice. Together, our results suggest that increased activity of TRPC3 channels is necessary for the development of OCs.

The Na+/Ca2+ exchanger inhibitor, KB-R7943, blocks a nonselective cation channel implicated in chemosensory transduction

The mechanism(s) of olfactory transduction in invertebrates remains to be fully understood. In lobster olfactory receptor neurons (ORNs), a nonselective sodium-gated cation (SGC) channel, a presumptive transient receptor potential (TRP)C channel homolog, plays a crucial role in olfactory transduction, at least in part by amplifying the primary transduction current. To better determine the functional role of the channel, it is important to selectively block the channel independently of other elements of the transduction cascade, causing us to search for specific pharmacological blockers of the SGC channel. Given evidence that the Na(+)/Ca(2+) exchange inhibitor, KB-R7943, blocks mammalian TRPC channels, we studied this probe as a potential blocker of the lobster SGC channel. KB-R7943 reversibly blocked the SGC current in both inside- and outside-out patch recordings in a dose- and voltage-dependent manner. KB-R7943 decreased the channel open probability without changing single channel amplitude. KB-R7943 also reversibly and in a dose-dependent manner inhibited both the odorant-evoked discharge of lobster ORNs and the odorant-evoked whole cell current. Our findings strongly imply that KB-R7943 potently blocks the lobster SGC channel and likely does so directly and not through its ability to block the Na(+)/Ca(2+) exchanger.

Role of TRP channels and NCX in mediating hypoxia-induced [Ca(2+)](i) elevation in PC12 cells

Mammalian cells require a constant O2 supply to produce adequate energy, and sustained hypoxia can kill cells. Mammals therefore have evolved sophisticated mechanisms to allow their cells to adapt to hypoxia. In this study, we investigated the role of TRP channels and the Na+-Ca2+ exchanger (NCX) in mediating hypoxia-induced [Ca2+]i elevation in a model of the O2-sensing rat pheochromocytoma (PC12) cell line by using Ca2+ imaging and molecular biological approaches. Non-selective cation channels, such as TRPC1, 3 and 6, were found to be functionally expressed in PC12 cells. They mediated Ca2+ entry when cells were exposed to acute hypoxia (PO2 of 15 mmHg), in addition to Ca2+ entry via VGCCs. Blockage of TRPCs by 2APB and SKF96365 could significantly reduce hypoxia-mediated [Ca2+]i elevation. Suramin and U73122 attenuated the hypoxia-induced [Ca2+]i elevation, implying the involvement of the G-protein and PLC pathways in the hypoxic response. In addition to TRPCs and VGCCs, NCX also contributed to the hypoxia-induced [Ca2+]i elevation, and blockade of NCX by KBR7943 could significantly decrease the hypoxia-induced [Ca2+]i elevation. Our results suggest that the activation of TRP by hypoxia could lead to NCX reversal; furthermore, membrane depolarization and TRPCs may play a primary role in mediating the hypoxic response in PC12 cells.

A mathematical model of Ca2+ dynamics in rat mesenteric smooth muscle cell: agonist and NO stimulation

A mathematical model of calcium dynamics in vascular smooth muscle cell (SMC) was developed based on data mostly from rat mesenteric arterioles. The model focuses on (a) the plasma membrane electrophysiology; (b) Ca2+ uptake and release from the sarcoplasmic reticulum (SR); (c) cytosolic balance of Ca2+, Na+, K+, and Cl ions; and (d) IP3 and cGMP formation in response to norepinephrine(NE) and nitric oxide (NO) stimulation. Stimulation with NE induced membrane depolarization and an intracellular Ca2+ ([Ca2+]i) transient followed by a plateau. The plateau concentrations were mostly determined by the activation of voltage-operated Ca2+ channels. NE causes a greater increase in [Ca2+]i than stimulation with KCl to equivalent depolarization. Model simulations suggest that the effect of[Na+]i accumulation on the Na+/Ca2+ exchanger (NCX) can potentially account for this difference.Elevation of [Ca2+]i within a concentration window (150-300 nM) by NE or KCl initiated [Ca2+]i oscillations with a concentration-dependent period. The oscillations were generated by the nonlinear dynamics of Ca2+ release and refilling in the SR. NO repolarized the NE-stimulated SMC and restored low [Ca2+]i mainly through its effect on Ca2+-activated K+ channels. Under certain conditions, Na+-K+-ATPase inhibition can result in the elevation of [Na+]i and the reversal of NCX, increasing resting cytosolic and SR Ca2+ content, as well as reactivity to NE. Blockade of the NCX's reverse mode could eliminate these effects. We conclude that the integration of the selected cellular components yields a mathematical model that reproduces, satisfactorily, some of the established features of SMC physiology. Simulations suggest a potential role of intracellular Na+ in modulating Ca2+ dynamics and provide insights into the mechanisms of SMC constriction, relaxation, and the phenomenon of vasomotion. The model will provide the basis for the development of multi-cellular mathematical models that will investigate microcirculatory function in health and disease.

Mechanical deformation of ventricular myocytes modulates both TRPC6 and Kir2.3 channels

Cardiomyocytes respond to mechanical stretch with an increase [Ca2+]i. Here, we analyzed which ion channels could mediate this effect. Murine ventricular myocytes were attached to a glass coverslip and a cell-attached glass stylus sheared the upper cell part versus the attached cell bottom. At negative clamp potentials, stretch induced inward currents that increased with the extent of stretch and reversed within 2 min after relaxation from stretch. Stretch activated a nearly voltage-independent GsMTx-4-sensitive non-selective cation conductance Gns, antibodies against TRPC6 prevented Gns activation. In addition, stretch deactivated a Cs+-sensitive inwardly rectifying potassium conductance GK1, antibodies against Kir2.3 inhibited this effect. Immunolabeling localized TRPC6 and Kir2.3 in T-tubular membranes, and stretch-induced changes in membrane currents were absent in cells whose T-tubules had been removed. In absence of stretch, we could activate Gns and deactivate GK1 by 1-oleoyl-2-acetyl-sn-glycerol (OAG) and other amphipaths. We interpret that the function of TRPC6 and Kir2.3 channels is controlled by both tension and curvature of the surrounding lipid bilayer that are changed by incorporation of amphipaths. Stretch-activation of TRPC6 channels may increase Ca2+ influx directly and indirectly, by membrane depolarization (activation of voltage-gated Ca2+ channels) and by elevated [Na+]i (augmented Na+,Ca2+-exchange).

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

Calcium signalling and cell-fate choice in B cells

Alterations in the cytosolic concentration of calcium ions (Ca2+) transmit information that is crucial for the development and function of B cells. Cytosolic Ca2+ concentration is determined by a balance of active transport and gradient-driven Ca2+ fluxes, both of which are subject to the influence of multiple receptors and environmental sensing pathways. Recent advances in genomics have allowed for the compilation of an increasingly comprehensive list of Ca2+ transporters and channels expressed by B cells. The increasing understanding of the function and regulation of these proteins has begun to shift the frontier of Ca2+ physiology in B cells from molecular analysis to determining how diverse inputs to cytosolic Ca2+ concentration are integrated in specific immunological contexts.

Transient receptor potential channel 6-mediated, localized cytosolic [Na+] transients drive Na+/Ca2+ exchanger-mediated Ca2+ entry in purinergically stimulated aorta smooth muscle cells

The Na+/Ca2+ exchanger (NCX) is increasingly recognized as a physiological mediator of Ca2+ influx and significantly contributes to salt-sensitive hypertension. We recently reported that Ca2+ influx by the NCX (1) is the primary mechanism of Ca2+ entry in purinergically stimulated rat aorta smooth muscle cells and (2) requires functional coupling with transient receptor potential channel 6 nonselective cation channels. Using the Na+ indicator CoroNa Green, we now directly observed and characterized the localized cytosolic [Na+] ([Na+]i) elevations that have long been hypothesized to underlie physiological NCX reversal but that have never been directly shown. Stimulation of rat aorta smooth muscle cells caused both global and monotonic [Na+]i elevations and localized [Na+]i transients (LNats) at the cell periphery. Inhibition of nonselective cation channels with SKF-96365 (50 micromol/L) and 2-amino-4-phosphonobutyrate (75 micromol/L) reduced both global and localized [Na+]i elevations in response to ATP (1 mmol/L). This effect was mimicked by expression of a dominant negative construct of transient receptor potential channel 6. Selective inhibition of NCX-mediated Ca2+ entry with KB-R7943 (10 micromol/L) enhanced the LNats, whereas the global cytosolic [Na+] signal was unaffected. Inhibition of mitochondrial Na+ uptake with CGP-37157 (10 micromol/L) increased both LNats and global cytosolic [Na+] elevations. These findings directly demonstrate NCX regulation by LNats, which are restricted to subsarcolemmal, cytoplasmic microdomains. Analysis of the LNats, which facilitate Ca2+ entry via NCX, suggests that mitochondria limit the cytosolic diffusion of LNats generated by agonist-mediated activation of transient receptor potential channel 6-containing channels.

Mitochondria and Ca(2+) signaling: old guests, new functions

Mitochondria are ancient endosymbiotic guests that joined the cells in the evolution of complex life. While the unique ability of mitochondria to produce adenosine triphosphate (ATP) and their contribution to cellular nutrition metabolism received condign attention, our understanding of the organelle's contribution to Ca(2+) homeostasis was restricted to serve as passive Ca(2+) sinks that accumulate Ca(2+) along the organelle's negative membrane potential. This paradigm has changed radically. Nowadays, mitochondria are known to respond to environmental Ca(2+) and to contribute actively to the regulation of spatial and temporal patterns of intracellular Ca(2+) signaling. Accordingly, mitochondria contribute to many signal transduction pathways and are actively involved in the maintenance of capacitative Ca(2+) entry, the accomplishment of Ca(2+) refilling of the endoplasmic reticulum and Ca(2+)-dependent protein folding. Mitochondrial Ca(2+) homeostasis is complex and regulated by numerous, so far, genetically unidentified Ca(2+) channels, pumps and exchangers that concertedly accomplish the organelle's Ca(2+) demand. Notably, mitochondrial Ca(2+) homeostasis and functions are crucially influenced by the organelle's structural organization and motility that, in turn, is controlled by matrix/cytosolic Ca(2+). This review intends to provide a condensed overview on the molecular mechanisms of mitochondrial Ca(2+) homeostasis (uptake, buffering and storage, extrusion), its modulation by other ions, kinases and small molecules, and its contribution to cellular processes as fundamental basis for the organelle's contribution to signaling pathways. Hence, emphasis is given to the structure-to-function and mobility-to-function relationship of the mitochondria and, thereby, bridging our most recent knowledge on mitochondria with the best-established mitochondrial function: metabolism and ATP production.

Transient receptor potential cation channels in disease

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

TRP channels in disease

"Transient receptor potential" cation channels (TRP channels) play a unique role as cell sensors, are involved in a plethora of Ca(2+)-mediated cell functions, and play a role as "gate-keepers" in many homeostatic processes such as Ca(2+) and Mg(2+) reabsorption. The variety of functions to which TRP channels contribute and the polymodal character of their activation predict that failures in correct channel gating or permeation will likely contribute to complex pathophysiological mechanisms. Dysfunctions of TRPs cause human diseases but are also involved in a complex manner to contribute and determine the progress of several diseases. Contributions to this special issue discuss channelopathias for which mutations in TRP channels that induce "loss-" or "gain-of-function" of the channel and can be considered "disease-causing" have been identified. The role of TRPs will be further elucidated in complex diseases of the intestinal, renal, urogenital, respiratory, and cardiovascular systems. Finally, the role of TRPs will be discussed in neuronal diseases and neurodegenerative disorders.

TRPC1 Ca(2+)-permeable channels in animal cells

The full-length transient receptor (TRPC)1 polypeptide is composed of about 790 amino acids, and several splice variants are known. The predicted structure and topology is of an integral membrane protein composed of six transmembrane domains, and a cytoplasmic C- and N-terminal domain. The N-terminal domain includes three ankyrin repeat motifs. Antibodies which recognise TRPC1 have been developed, but it has been difficult to obtain antibodies which have high affinity and specificity for TRPC1. This has made studies of the cellular functions of TRPC1 somewhat difficult. The TRPC1 protein is widely expressed in different types of animal cells, and within a given cell is found at the plasma membrane and at intracellular sites. TRPC1 interacts with calmodulin, caveolin-1, the InsP3 receptor, Homer, phospholipase C and several other proteins. Investigations of the biological roles and mechanisms of action of TRPC1 have employed ectopic (over-expression or heterologous expression) of the polypeptide in addition to studies of endogenous TRPC1. Both approaches have encountered difficulties. TRPC1 forms heterotetramers with other TRPC polypeptides resulting in cation channels which are non-selective. TRPC1 may be: a component of the pore of store-operated Ca2+ channels (SOCs); a subsidiary protein in the pathway of activation of SOCs; activated by interaction with InsP3R; and/or activated by stretch. Further experiments are required to resolve the exact roles and mechanisms of activation of TRPC1. Cation entry through the TRPC1 channel is feed-back inhibited by Ca2+ through interaction with calmodulin, and is inhibited by Gd3+, La3+, SKF96365 and 2-APB, and by antibodies targeted to the external mouth of the TRPC1 pore. Activation of TRPC1 leads to the entry to the cytoplasmic space of substantial amounts of Na+ as well as Ca2+. A requirement for TRPC1 is implicated in numerous downstream cellular pathways. The most clearly described roles are in the regulation of growth cone turning in neurons. It is concluded that TRPC1 is a most interesting protein because of the apparent wide variety of its roles and functions and the challenges posed to those attempting to elucidate its primary intracellular functions and mechanisms of action.

TRPC3: a multifunctional, pore-forming signalling molecule

TRPC3 represents one of the first identified mammalian relatives of the Drosophila trp gene product. Despite intensive biochemical and biophysical characterization as well as numerous attempts to uncover its physiological role in native cell systems, this channel protein still represents one of the most enigmatic members of the transient receptor potential (TRP) superfamily. TRPC3 is significantly expressed in brain and heart and likely to play a role in both non-excitable as well as excitable cells, being potentially involved in a wide spectrum of Ca2+ signalling mechanisms. Its ability to associate with a variety of partner proteins apparently enables TRPC3 to form different cation channels in native cells. TRPC3 cation channels display unique gating and regulatory properties that allow for recognition and integration of multiple input stimuli including lipid mediators and cellular Ca2+ gradients as well as redox signals. The physiological/pathophysiological functions of this highly versatile cation channel protein are as yet barely delineated. Here we summarize current knowledge on properties and possible signalling functions of TRPC3 and discuss the potential biological relevance of this signalling molecule.

Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca(2+)](cyt) in human PASMC. The resting [Ca(2+)](cyt) and Ca(2+) entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na(+)/Ca(2+) exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na(+)/Ca(2+) exchanger operates in a forward (Ca(2+) exit) and reverse (Ca(2+) entry) mode. By activating the reverse mode of Na(+)/Ca(2+) exchange, removal of extracellular Na(+) caused a rapid increase in [Ca(2+)](cyt), which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca(2+) stores using cyclopiazonic acid (10 microM) not only caused a rise in [Ca(2+)](cyt) due to Ca(2+) influx through store-operated Ca(2+) channels but also mediated a rise in [Ca(2+)](cyt) via the reverse mode of Na(+)/Ca(2+) exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange, but did not accelerate Ca(2+) extrusion via the forward mode of Na(+)/Ca(2+) exchange. These observations indicate that the upregulated NCX1 and enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange are an additional mechanism responsible for the elevated [Ca(2+)](cyt) in PASMC from IPAH patients.

TRPC6 in glomerular health and disease: what we know and what we believe

Mutations in TRPC6, a member of the transient receptor potential (TRP) superfamily of non-selective cation channels, have been identified as causing a familial form of focal segmental glomerulosclerosis, a disease characterized by proteinuria and progressive renal failure. Here we review the effect of disease-associated mutations on TRPC6 function and place TRPC6 within the context of other proteins central to glomerular and podocyte function. Finally, the known roles of TRPC6 in the kidney and other organ systems are used as a framework to discuss possible signaling pathways that TRPC6 may modulate during normal glomerular function and in disease states.

TRPC6 silencing in primary airway smooth muscle cells inhibits protein expression without affecting OAG-induced calcium entry

TRPC proteins have been described as non-selective cation channels and are thought to be involved in the regulation of Ca(2+) movement in various cells, including airway smooth muscle (ASM) cells. In order to study the role of these channels in ASM cells, transfection of a small interfering RNA (siRNA) designed against the TRPC6 channel was performed in guinea pig primary ASM cells. This specific siRNA was complexed with the new X-TremeGene (X-TG) chemical transfection reagent, whose efficiency and low cytotoxicity were determined by the use of a non-silencing rhodamine-tagged siRNA. It was found that more than 95% of cells were transfected by an optimized protocol. Verification of TRPC6 transcript down-regulation was determined by RT-PCR while Western blot analysis attested to lower protein content in the microsomal fraction. Micro-spectrofluorimetry measurements of control and siRNA-treated cells revealed that lower TRPC6 expression did not affect OAG-induced intracellular Ca(2+) movement. Thus, TRPC6 channels cannot be defined as simple Ca(2+) transporters but more likely as protein complexes supporting monovalent cation conductance in ASM cells. These conductances would in turn facilitate membrane depolarization of high input resistance cells, Ca(2+) channel activation and tone increase. In conclusion, this study defines a valuable model of RNA interference study in primary cultures of ASM cells, eventually allowing for silencing of other target proteins for which no pharmacological modulators are currently available.

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

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

Ca2+ signaling microdomains:platforms for the assembly and regulation of TRPC channels

The transient receptor potential canonical family (TRPC1-TRPC7) of ion channel proteins, which are activated in response to agonist-stimulated phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] hydrolysis, are proposed components of the elusive store-operated Ca(2+) (SOC) channel. TRPC channels display distinct properties and interact to form homomeric or heteromeric channels that differ in their function and regulation. Although the exact function of TRPC channels and how they are regulated has not been established, increasing data suggest that they are localized and regulated within Ca(2+) signaling microdomains. TRPC channels contribute to store-operated and store-independent Ca(2+) entry mechanisms, both of which are activated by agonist-stimulated PtdIns(4,5)P(2) hydrolysis. Elucidation of how cells achieve specificity and precise temporal and spatial coordination of channel activation is crucial for understanding the molecular basis of agonist-mediated stimulation of Ca(2+) entry and identifying downstream physiological functions. This review will address the assembly and localization of TRPC channels and how these processes impact their function.

TRP channels: a TR(I)P through a world of multifunctional cation channels

The "transient receptor potential" (TRP) family of ion channels comprises more than 50 cation-permeable channels expressed from yeast to man. On the basis of structural homology, the TRP family can be subdivided in to seven main subfamilies: the TRPC ('Canonical') group, the TRPV ('Vanilloid') group, the TRPM ('Melastatin') group, the TRPP ('Polycystin'), the TRPML ('Mucolipin'), the TRPA ('Ankyrin') and the TRPN ('NOMP') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data concerning TRPs in a variety of cell types, tissues and species. This paper briefly reviews the TRP superfamily and the basic properties of its many members as a reader's guide in this Special Issue. Hopefully, a better understanding of TRP channel physiology will provide important insight into the relationship between TRP channel dysfunction and human diseases.