Activation of Trp3 by inositol 1,4,5-trisphosphate receptors through displacement of inhibitory calmodulin from a common binding domain

Ca(2+) signalling by IP(3) receptors

The Ca(2) (+) signals evoked by inositol 1,4,5-trisphosphate (IP(3)) are built from elementary Ca(2) (+) release events involving progressive recruitment of IP(3) receptors (IP(3)R), intracellular Ca(2) (+) channels that are expressed in almost all animal cells. The smallest events ('blips') result from opening of single IP(3)R. Larger events ('puffs') reflect the near-synchronous opening of a small cluster of IP(3)R. These puffs become more frequent as the stimulus intensity increases and they eventually trigger regenerative Ca(2) (+) waves that propagate across the cell. This hierarchical recruitment of IP(3)R is important in allowing Ca(2) (+) signals to be delivered locally to specific target proteins or more globally to the entire cell. Co-regulation of IP(3)R by Ca(2) (+) and IP(3), the ability of a single IP(3)R rapidly to mediate a large efflux of Ca(2) (+) from the endoplasmic reticulum, and the assembly of IP(3)R into clusters are key features that allow IP(3)R to propagate Ca(2) (+) signals regeneratively. We review these properties of IP(3)R and the structural basis of IP(3)R behavior.

Function and regulation of endothelin type A receptor-operated transient receptor potential canonical channels

The purpose of this study is to identify transient receptor potential canonical (TRPC) channels responsible for receptor-operated Ca(2+) entry (ROCE) triggered by activation of endothelin type A receptor (ET(A)R) and to clarify the importance of calmodulin (CaM) / inositol 1,4,5-trisphosphate (IP(3)) receptor binding (CIRB) domain at the C terminus of TRPC channels in ET(A)R-activated channel regulation. In HEK293 cells coexpressing ET(A)R and one of seven TRPC isoforms, ET(A)R stimulation induced ROCE through TRPC3, TRPC5, TRPC6, and TRPC7. The TRPC3- and TRPC6-mediated ROCE was inhibited by selective inhibitors of G(q) protein, phospholipase C (PLC), and CaM. The CIRB domain deletion mutants of TRPC3 and TRPC6 failed to induce ET(A)R-mediated ROCE. Either deletion of the CIRB domain or pharmacological inhibition of CaM did not inhibit the targeting of these channels to the plasma membrane. These results suggest that 1) TRPC3, TRPC5, TRPC6, and TRPC7 can function as ET(A)R-operated Ca(2+) channels; 2) G(q) protein, PLC, and CaM are involved in TRPC3- and TRPC6-mediated ROCE; 3) ET(A)R-mediated activation of TRPC3 and TRPC6 requires the CIRB domain; and 4) abolition of ET(A)R-induced ROCE by CIRB domain deletion and CaM inhibition is due to loss of CaM binding to the channels but not loss of cell surface TRPC3 and TRPC6.

Molecular determinants of PKA-dependent inhibition of TRPC5 channel

Canonical transient receptor potential (TRPC) channels are Ca(2+)-permeable, nonselective cation channels that are widely expressed in numerous cell types. Here, we demonstrate a new mechanism of TPRC isofom 5 (TRPC5) regulation, via cAMP signaling via Gα(s). Monovalent cation currents in human embryonic kidney-293 cells transfected with TRPC5 were induced by G protein activation with intracellular perfusion of GTPγS or by muscarinic stimulation. This current could be inhibited by a membrane-permeable analog of cAMP, 8-bromo-cAMP, by isoproterenol, by a constitutively active form of Gα(s) [Gα(s) (Q227L)], and by forskolin. These inhibitory effects were blocked by the protein kinase A (PKA) inhibitors, KT-5720 and H-89, as well as by two point mutations at consensus PKA phosphorylation sites on TRPC5 (S794A and S796A). Surface expression of several mutated versions of TRPC5, quantified using surface biotinylation, were not affected by Gα(s) (Q227L), suggesting that trafficking of this channel does not underlie the regulation we report. This mechanism of inhibition was also found to be important for the closely related channel, TRPC4, in particular for TRPC4α, although TRPC4β was also affected. However, this form of regulation was not found to be involved in TRPC6 and transient receptor potential vanilloid 6 function. In murine intestinal smooth muscle cells, muscarinic stimulation-induced cation currents were mediated by TRPC4 (>80%) and TRPC6. In murine intestinal smooth muscle cells, 8-bromo-cAMP, adrenaline, and isoproterenol decreased nonselective cation currents activated by muscarinic stimulation or GTPγS. Together, these results suggest that TRPC5 is directly phosphorylated by G(s)/cAMP/PKA at positions S794 and S796. This mechanism may be physiologically important in visceral tissues, where muscarinic receptor and β(2)-adrenergic receptor are involved in the relaxation and contraction of smooth muscles.

In vitro maturation of the cisternal organelle in the hippocampal neuron's axon initial segment

Regulation of Ca(2+) concentrations is essential to maintain the structure and function of the axon initial segment (AIS). The so-called cisternal organelle of the AIS is a structure involved in this regulation, although little is known as to how this organelle matures and is stabilized. Here we describe how the cisternal organelle develops in cultured hippocampal neurons and the interactions that facilitate its stabilization in the AIS. We also characterize the developmental expression of molecules involved in Ca(2+) regulation in the AIS. Our results indicate that synaptopodin (synpo) positive elements considered to be associated to the cisternal organelle are present in the AIS after six days in vitro. There are largely overlapping microdomains containing the inositol 1,4,5-triphosphate receptor 1 (IP(3)R1) and the Ca(2+) binding protein annexin 6, suggesting that the regulation of Ca(2+) concentrations in the AIS is sensitive to IP(3) and subject to regulation by annexin 6. The expression of synpo, IP(3)R1 and annexin 6 in the AIS is independent of the neuron activity, as it was unaffected by tetrodotoxin blockage of action potentials and it was resistant to detergent extraction, indicating that these proteins interact with scaffolding and/or cytoskeleton proteins. The presence of ankyrin G seems to be required for the acquisition and maintenance of the cisternal organelle, while the integrity of the actin cytoskeleton must be maintained for the expression IP(3)R1 and annexin 6 to persist in the AIS.

Activation and regulation of store-operated calcium entry

The process of store-operated Ca²⁺ entry (SOCE), whereby Ca²⁺ influx across the plasma membrane is activated in response to depletion of intracellular Ca²⁺ stores in the endoplasmic reticulum (ER), has been under investigation for greater than 25 years; however, only in the past 5 years have we come to understand this mechanism at the molecular level. A surge of recent experimentation indicates that STIM molecules function as Ca²⁺ sensors within the ER that, upon Ca²⁺ store depletion, rearrange to sites very near to the plasma membrane. At these plasma membrane-ER junctions, STIM interacts with and activates SOCE channels of the Orai family. The molecular and biophysical data that have led to these findings are discussed in this review, as are several controversies within this rapidly expanding field.

TRP channels and neural persistent activity

One of the integrative properties of the nervous system is its capability to, by transient motor commands or brief sensory stimuli, evoke persistent neuronal changes, mainly as a sustained, tonic action potential firing. This neural activity, named persistent activity, is found in a good number of brain regions and is thought to be a neural substrate for short-term storage and accumulation of sensory or motor information [1]. Examples of this persistent neural activity have been reported in prefrontal [2] and entorhinal [3] cortices, as part of the neural mechanisms involved in short-term working memory [4]. Interestingly, the general organization of the motor systems assumes the presence of bursts of short-lasting motor commands encoding movement characteristics such as velocity, duration, and amplitude, followed by a maintained tonic firing encoding the position at which the moving appendage should be maintained [5, 6]. Generation of qualitatively similar sustained discharges have also been found in spinal and supraspinal regions in relation to pain processing [7, 8]. Thus, persistent neural activity seems to be necessary for both behavioral (positions of fixation) and cognitive (working memory) processes. Persistent firing mechanisms have been proposed to involve the participation of a non-specific cationic current (CAN current) mainly mediated by activation of TRPC channels. Because the function and generation of persistent activity is still poorly understood, here we aimed to review and discuss the putative role of TRP-like channels on its generation and/or maintenance.

C. elegans TRP channels

Transient receptor potential (TRP) channels represent a superfamily of cation channels found in all eukaryotes. The C. elegans genome encodes seventeen TRP channels covering all of the seven TRP subfamilies. Genetic analyses in C. elegans have implicated TRP channels in a wide spectrum of behavioral and physiological processes, ranging from sensory transduction (e.g. chemosensation, touch sensation, proprioception and osmosensation) to fertilization, drug dependence, organelle biogenesis, apoptosis, gene expression, and neurotransmitter/hormone release. Many C. elegans TRP channels share similar activation and regulatory mechanisms with their vertebrate counterparts. Studies in C. elegans have also revealed some previously unrecognized functions and regulatory mechanisms of TRP channels. C. elegans represents an excellent genetic model organism for the study of function and regulation of TRP channels in vivo.

Ca2+-dependent desensitization of TRPV2 channels is mediated by hydrolysis of phosphatidylinositol 4,5-bisphosphate

TRPV2 is a member of the transient receptor potential family of ion channels involved in chemical and thermal pain transduction. Unlike the related TRPV1 channel, TRPV2 does not appear to bind either calmodulin or ATP in its N-terminal ankyrin repeat domain. In addition, it does not contain a calmodulin-binding site in the distal C-terminal region, as has been proposed for TRPV1. We have found that TRPV2 channels transiently expressed in F-11 cells undergo Ca(2+)-dependent desensitization, similar to the other TRPVs, suggesting that the mechanism of desensitization may be conserved in the subfamily of TRPV channels. TRPV2 desensitization was not altered in whole-cell recordings in the presence of calmodulin inhibitors or on coexpression of mutant calmodulin but was sensitive to changes in membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)), suggesting a role of membrane PIP(2) in TRPV2 desensitization. Simultaneous confocal imaging and electrophysiological recording of cells expressing TRPV2 and a fluorescent PIP(2)-binding probe demonstrated that TRPV2 desensitization was concomitant with depletion of PIP(2). We conclude that the decrease in PIP(2) levels on channel activation underlies a major component of Ca(2+)-dependent desensitization of TRPV2 and may play a similar role in other TRP channels.

TRP-ing up heart and vessels: canonical transient receptor potential channels and cardiovascular disease

Transient receptor potential channels are a large superfamily of non-selective and non-voltage-gated ion channels that convey signaling information linked to a broad range of sensory inputs. In the cardiovascular system, the canonical transient receptor potential (TRPC) family has been particularly found to play a role in vascular and cardiac disease, responding to neurohormonal and mechanical load stimulation. TRPC1, TRPC3, and TRPC6 are often upregulated in models of cardiovascular disease, and their inhibition ameliorates the associated pathophysiology. Studies in gene deletion models and overexpression models of wild-type and dominant-negative proteins supports a direct role of these channels, which likely act together as heterotetramers to influence signaling. Recent evidence has further revealed the importance of protein kinase G phosphorylation as a mechanism to suppress TRPC6 channel current and dependent signaling in vascular and cardiac myocytes. This suggests a novel mechanism underlying benefits of drugs such as sildenafil, a phosphodiesterase type 5 inhibitor, nitrates, and atrial natriuretic peptides. This review describes new evidence supporting a pathophysiologic role of these three TRPC channels, and the potential utility of inhibition strategies to treat cardiovascular disease.

The properties of spontaneous transient inward currents of interstitial cells in rabbit portal vein

The present study was designed to investigate the properties of spontaneous transient inward currents generated by interstitial cells (ICs) in the rabbit portal vein. Single ICs were freshly isolated from smooth muscle of the rabbit portal vein enzymetically. Using whole-cell patch clamp techniques, the spontaneous transient inward currents (STICs) were recorded at -60 mV of holding potential in freshly dispersed ICs. Both gadolinium, a non-selective cation channel inhibitor, and niflumic acid, a calcium-activated chloride channel blocker, abolished the inward currents. Replacement of external Na(+) with N-methyl-d-glucamine (NMDG(+)) also blocked the inward currents. The inward currents were abolished by caffeine, carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP), thapsigargin and ryanodine, but were partly inhibited by 2-aminoethoxydiphenyl borate (2-APB). W-7, a calmodulin inhibitor, increased the amplitude of the inward currents. These results suggest that non-selective cation channels are involved in the generation of the spontaneous transient inward currents recorded from ICs. The currents are regulated by intracellular calcium and calmodulin. But in the present study, the involvement of the calcium-activated chloride channels in the generation of the currents cannot be excluded.

Spontaneous rhythmic inward currents recorded in interstitial cells of rabbit portal vein

It is now well established that smooth muscle of the portal vein exhibits spontaneous rhythmic contraction in vitro. The present study was designed to investigate the pacemaking mechanism(s) underlying the spontaneous rhythmic contractions in the rabbit portal vein (RPV). Using whole-cell patch clamp techniques, spontaneous inward currents were recorded at -60 mV of holding potential in freshly dispersed c-Kit immunopositive interstitial cells (ICs) isolated from sections of RPV. The inward currents were abolished by caffeine, FCCP, thapsigargin, and ryanodine, but were partially inhibited by 2-APB. Both gadolinium, a non-selective cation channel inhibitor, and niflumic acid, a chloride channel blocker, inhibited the inward currents completely. Replacement of external Na(+) with NMDG(+) also blocked the inward currents. W-7, a calmodulin inhibitor, increased both the amplitude and frequency of the inward currents. Taken together, these results indicate that non-selective cationic channels are involved in the generation of spontaneous inward currents recorded from ICs. Intracellular calcium concentration and calmodulin regulate the spontaneous inward currents, which may account for spontaneous rhythmic contraction in the RPV, but a role of chloride channels may not be excluded in the present study.

Interdomain interactions control Ca2+-dependent potentiation in the cation channel TRPV4

Several Ca²⁺-permeable channels, including the non-selective cation channel TRPV4, are subject to Ca²⁺-dependent facilitation. Although it has been clearly demonstrated in functional experiments that calmodulin (CaM) binding to intracellular domains of TRP channels is involved in this process, the molecular mechanism remains elusive. In this study, we provide experimental evidence for a comprehensive molecular model that explains Ca²⁺-dependent facilitation of TRPV4. In the resting state, an intracellular domain from the channel N terminus forms an autoinhibitory complex with a C-terminal domain that includes a high-affinity CaM binding site. CaM binding, secondary to rises in intracellular Ca²⁺, displaces the N-terminal domain which may then form a homologous interaction with an identical domain from a second subunit. This represents a novel potentiation mechanism that may also be relevant in other Ca²⁺-permeable channels.

Isoform-selective physical coupling of TRPC3 channels to IP3 receptors in smooth muscle cells regulates arterial contractility

RATIONALE

Inositol 1,4,5-trisphosphate (IP(3))-induced vasoconstriction can occur independently of intracellular Ca(2+) release and via IP(3) receptor (IP(3)R) and canonical transient receptor potential (TRPC) channel activation, but functional signaling mechanisms mediating this effect are unclear.

OBJECTIVES

Study mechanisms by which IP(3)Rs stimulate TRPC channels in myocytes of resistance-size cerebral arteries.

METHODS AND RESULTS

Immunofluorescence resonance energy transfer (immuno-FRET) microscopy using isoform-selective antibodies indicated that endogenous type 1 IP(3)Rs (IP(3)R1) are in close spatial proximity to TRPC3, but distant from TRPC6 or TRPM4 channels in arterial myocytes. Endothelin-1 (ET-1), a phospholipase C-coupled receptor agonist, elevated immuno-FRET between IP(3)R1 and TRPC3, but not between IP(3)R1 and TRPC6 or TRPM4. TRPC3, but not TRPC6, coimmunoprecipitated with IP(3)R1. TRPC3 and TRPC6 antibodies selectively inhibited recombinant channels, but only the TRPC3 antibody blocked IP(3)-induced nonselective cation current (I(Cat)) in myocytes. TRPC3 knockdown attenuated immuno-FRET between IP(3)R1 and TRPC3, IP(3)-induced I(Cat) activation, and ET-1 and IP(3)-induced vasoconstriction, whereas TRPC6 channel knockdown had no effect. ET-1 did not alter total or plasma membrane-localized TRPC3, as determined using surface biotinylation. RT-PCR demonstrated that C-terminal calmodulin and IP(3)R binding (CIRB) domains are present in myocyte TRPC3 and TRPC6 channels. A peptide corresponding to the IP(3)R N-terminal region that can interact with TRPC channels activated I(Cat). A TRPC3 CIRB domain peptide attenuated IP(3)- and ET-1-induced I(Cat) activation and vasoconstriction.

CONCLUSIONS

IP(3) stimulates direct coupling between IP(3)R1 and membrane-resident TRPC3 channels in arterial myocytes, leading to I(Cat) activation and vasoconstriction. Close spatial proximity between IP(3)R1 and TRPC3 establishes this isoform-selective functional interaction.

TRPC3 ion channel subunit immunolocalization in the cochlea

Canonical transient receptor potential (TRPC) subunits assemble as tetramers to form ion channels with high calcium (Ca(2+)) permeability. Here, we investigated the possibility that TRPC3 ion channels are broadly expressed in the adult guinea pig and mouse cochleae. Using immunofluorescence, pronounced labeling occurred in the spiral ganglion (SG) neurons, inner hair cells (IHC), outer hair cells (OHC) and epithelial cells lining scala media. TRPC3 expression was homogeneous in the SG throughout the cochlea. In contrast, there was marked spatial variation in the immunolabeling in the cochlear hair cells with respect to location. This likely relates to the tonotopy of these cells. TRPC3 immunolabeling was more pronounced in the IHC than OHC. Both basal region IHC and OHC had higher TRPC3 expression levels than the corresponding cells from the apical region of the cochlea. These data suggest that TRPC3 ion channels contribute to Ca(2+) homeostasis associated with the hair cells, with higher ion fluxes in more basal regions of the cochlea, and may also be a significant pathway for Ca(2+) entry associated with auditory neurotransmission via the SG neurons. TRPC3 expression was also identified within the spiral limbus region, inner and outer sulcus, but without evidence for spatial variation in expression level. Expression in these gap junction-coupled epithelial cells lining scala media is indicative of a contribution of TRPC3 channels to cochlear electrochemical homeostasis.

Caffeine inhibits nonselective cationic currents in interstitial cells of Cajal from the murine jejunum

Interstitial cells of Cajal (ICC) discharge unitary potentials in gastrointestinal muscles that constitute the basis for pacemaker activity. Caffeine has been used to block unitary potentials, but the ionic conductance responsible for unitary potentials is controversial. We investigated currents in cultured ICC from murine jejunum that may underlie unitary potentials and studied the effects of caffeine. Networks of ICC generated slow wave events under current clamp, and these events were blocked by caffeine in a concentration-dependent manner. Single ICC generated spontaneous transient inward currents (STICs) under voltage clamp at -60 mV and noisy voltage fluctuations in current clamp. STICs were unaffected when the equilibrium potential for Cl- (ECl) was set to -60 mV (excluding Cl- currents) and reversed at 0 mV, demonstrating that a nonselective cationic conductance, and not a Cl- conductance, is responsible for STICs in ICC. Caffeine inhibited STICs in a concentration-dependent manner. Reduced intracellular Ca2+ and calmidazolium (CMZ; 1 microM) activated persistent inward, nonselective cation currents in ICC. Currents activated by CMZ and by dialysis of cells with 10 mM BAPTA were also inhibited by caffeine. Excised inside-out patches contained channels that exhibited spontaneous openings, and resulting currents reversed at 0 mV. Channel openings were increased by reducing Ca2+ concentration from 10(-6) M to 10(-8) M. CMZ (1 microM) also increased openings of nonselective cation channels. Spontaneous currents and channels activated by CMZ were inhibited by caffeine (5 mM). The findings demonstrate that the Ca2+-inhibited nonselective cation channels that generate STICs in ICC are blocked directly by caffeine. STICs are responsible for unitary potentials in intact muscles, and the block of these events by caffeine is consistent with the idea that a nonselective cation conductance underlies unitary potentials in ICC.

Regulation of cation channel voltage and Ca2+ dependence by multiple modulators

Ion channel regulation is key to controlling neuronal excitability. However, the extent that modulators and gating factors interact to regulate channels is less clear. For Aplysia, a nonselective cation channel plays an essential role in reproduction by driving an afterdischarge in the bag cell neurons to elicit egg-laying hormone secretion. We examined the regulation of cation channel voltage and Ca2+ dependence by protein kinase C (PKC) and inositol trisphosphate (IP3)-two prominent afterdischarge signals. In excised, inside-out patches, the channel remained open longer and reopened more often with depolarization from -90 to +30 mV. As previously reported, PKC could closely associate with the channel and increase activity at -60 mV. We now show that, following the effects of PKC, voltage dependence was shifted to the left (essentially enhanced), particularly at more negative voltages. Conversely, the voltage dependence of channels lacking PKC was shifted to the right (essentially suppressed). Predictably, activity was increased at all Ca2+ concentrations following the effects of PKC; nevertheless, Ca2+ dependence was actually shifted to the right. Moreover, whereas IP3 did not alter activity at -60 mV, it drastically shifted Ca2+ dependence to the right-an outcome largely reversed by PKC. With respect to the afterdischarge, these data suggest PKC initially upregulates the channel by direct gating and shifting voltage dependence to the left. Subsequently, PKC and IP3 attenuate the channel by suppressing Ca2+ dependence. This ensures hormone delivery by allowing afterdischarge initiation and maintenance but also prevents interminable bursting. Similar regulatory interactions may be used by other neurons to achieve diverse outputs.

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.

Glutamate at position 227 of junctophilin-2 is involved in binding to TRPC3

Canonical-type transient receptor potential cation channel type 3 (TRPC3) allows the entry of extracellular Ca(2+) and Na(+) into various cells. In mouse skeletal myotubes, functional interaction between TRPC3 and RyR1 (ryanodine receptor type 1/Ca(2+)-release channel on sarcoplasmic reticulum membrane) regulates the gain of excitation-contraction coupling. Junctophilin-2 (JP2) is a TRPC3-interacting protein in mouse skeletal myotubes. Based on these knowledge from bona-fide TRPC3-expressing cells, to identify critical binding region(s) of JP2 that participate in binding to TRPC3, various JP2 portions were subjected to co-immunoprecipitation assay with intact TRPC3 from rabbit skeletal muscle. A region covering 143 to 234 amino acids of JP2 (F1-2) was the most efficient portion binding to TRPC3. Through mutational studies, we found that the binding ability of JP2 to TRPC3 was mainly due to glutamate in the F1-2 region (E227). This substantial binding between JP2 and TRPC3 suggests that JP2 can be a regulatory protein of TRPC3 and/or TRPC3-mediated Ca(2+) homeostasis in skeletal muscle.

TRPC3 activation by erythropoietin is modulated by TRPC6

Regulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an essential part of signaling pathways controlling proliferation and differentiation of erythroid progenitors, but regulatory mechanisms are largely unknown. TRPC3 and the homologous TRPC6 are two members of the transient receptor potential channel (TRPC) superfamily that are expressed on normal human erythroid precursors. Here we show that TRPC3 expression increases but TRPC6 decreases during erythroid differentiation. This is associated with a significantly greater increase in [Ca(2+)](i) in response to Epo stimulation, suggesting that the ratio of TRPC3/TRPC6 is physiologically important. In HEK 293T cells heterologously expressing TRPC and erythropoietin receptor (Epo-R), Epo stimulated an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Replacement of the C terminus of TRPC3 with the TRPC6 C terminus (TRPC3-C6C) resulted in loss of activation by Epo. In contrast, substitution of the C terminus of TRPC6 with that of TRPC3 (TRPC6-C3C) resulted in an increase in [Ca(2+)](i) in response to Epo. Substitution of the N termini had no effect. Domains in the TRPC3 C terminus between amino acids 671 and 746 are critical for the response to Epo. Epo-R and phospholipase Cgamma associated with TRPC3, and these interactions were significantly reduced with TRPC6 and TRPC3-C6C chimeras. TRPC3 and TRPC6 form heterotetramers. Coexpression of TRPC6 or C3/C6 chimeras with TRPC3 and Epo-R inhibited the Epo-stimulated increase in [Ca(2+)](i). In a heterologous expression system, Epo stimulation increased cell surface expression of TRPC3, which was inhibited by TRPC6. However, in primary erythroblasts, an increase in TRPC3 cell surface expression was not observed in erythroblasts in which Epo stimulated an increase in [Ca(2+)](i), demonstrating that increased membrane insertion of TRPC3 is not required. These data demonstrate that TRPC6 regulates TRPC3 activation by Epo. Endogenously, regulation of TRPC3 by TRPC6 may primarily be through modulation of signaling mechanisms, including reduced interaction of TRPC6 with phospholipase Cgamma and Epo-R.

A role for Orai in TRPC-mediated Ca2+ entry suggests that a TRPC:Orai complex may mediate store and receptor operated Ca2+ entry

TRPC and Orai proteins have both been proposed to form Ca(2+)-selective, store-operated calcium entry (SOCE) channels that are activated by store-depletion with Ca(2+) chelators or calcium pump inhibitors. In contrast, only TRPC proteins have been proposed to form nonselective receptor-operated calcium entry (ROCE) cation channels that are activated by Gq/Gi-PLCbeta signaling, which is the physiological stimulus for store depletion. We reported previously that a dominant negative Orai1 mutant, R91W, inhibits Ca(2+) entry through both SOCE and ROCE channels, implicating Orai participation in both channel complexes. However, the argument for Orai participating in ROCE independently of store depletion is tenuous because store depletion is an integral component of the ROCE response, which includes formation of IP3, a store-depleting agent. Here we show that the R91W mutant also blocks diacylglycerol (DAG)-activated Ca(2+) entry into cells that stably, or transiently, express DAG-responsive TRPC proteins. This strongly suggests that Orai and TRPC proteins form complexes that participate in Ca(2+) entry with or without activation of store depletion. To integrate these results with recent data linking SOCE with recruitment of Orai and TRPCs to lipid rafts by STIM, we develop the hypothesis that Orai:TRPC complexes recruited to lipid rafts mediate SOCE, whereas the same complexes mediate ROCE when they are outside of lipid rafts. It remains to be determined whether the molecules forming the permeation pathway are the same when Orai:TRPC complexes mediate ROCE or SOCE.

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.

A calmodulin antagonist reveals a calmodulin-independent interdomain interaction essential for activation of inositol 1,4,5-trisphosphate receptors

CaM (calmodulin) has been implicated in the regulation of IP3R [IP3 (inositol 1,4,5-trisphosphate) receptors] and a recent report suggested that CaM tightly tethered to IP3R was essential for IP3R activation [Nadif Kasri, Torok, Galione, Garnham, Callewaert, Missiaen, Parys and De Smedt (2006) J. Biol. Chem. 281, 8332-8338]. In the present study, we confirm that a CaM-binding peptide derived from MLCK (myosin light chain kinase) inhibits IP3-evoked Ca2+ release via all three IP3R subtypes. However,inhibition by MLCK peptide is not mimicked by other CaM antagonists that effectively block regulation of IP3R by CaM. Inhibition by MLCK peptide is rapid, fully reversible and occurs under conditions where there is no CaM associated with IP3R. MLCK peptide stimulates IP3 binding to IP3R1 and to its bacterially expressed N-terminal, but not after removal of the suppressor domain (residues 1-224).We suggest that MLCK peptide mimics a sequence within the suppressor domain that is similar to a1-8-14 CaM-binding motif. The peptide may thereby unzip an interdomain interaction that is essential for IP3R activation. We conclude that CaM is not essential for IP3R activation, and that MLCK peptide is a selective antagonist of the IP3R that binds directly to the N-terminal to uncouple IP3 binding from channel gating. The results of the present study highlight the importance of the suppressor domain in IP3R activation and suggest that MLCK peptide may provide a route to novel non-competitive antagonists of IP3R.

Caveolin-1 scaffold domain interacts with TRPC1 and IP3R3 to regulate Ca2+ store release-induced Ca2+ entry in endothelial cells

Caveolin-1 (Cav-1) regulates agonist-induced Ca(2+) entry in endothelial cells; however, how Cav-1 regulates this process is poorly understood. Here, we describe that Cav-1 scaffold domain (NH(2)-terminal residues 82-101; CSD) interacts with transient receptor potential canonical channel 1 (TRPC1) and inositol 1,4,5-trisphosphate receptor 3 (IP(3)R3) to regulate Ca(2+) entry. We have shown previously that the TRPC1 COOH-terminal residues 781-789 bind to CSD. In the present study, we show that the TRPC1 COOH-terminal residues 781-789 truncated (TRPC1-CDelta781-789) mutant expression abolished Ca(2+) store release-induced Ca(2+) influx in human dermal microvascular endothelial cell line (HMEC) and human embryonic kidney (HEK-293) cells. To understand the basis of loss of Ca(2+) influx, we determined TRPC1 binding to IP(3)R3. We observed that the wild-type (WT)-TRPC1 but not TRPC1-CDelta781-789 effectively interacted with IP(3)R3. Similarly, WT-TRPC1 interacted with Cav-1, whereas TRPC1-CDelta781-789 binding to Cav-1 was markedly suppressed. We also assessed the direct binding of Cav-1 with TRPC1 and observed that the WT-Cav-1 but not the Cav-1DeltaCSD effectively interacted with TRPC1. Since the interaction between TRPC1 and Cav-1DeltaCSD was reduced, we measured Ca(2+) store release-induced Ca(2+) influx in Cav-1DeltaCSD-transfected cells. Surprisingly, Cav-1DeltaCSD expression showed a gain-of-function in Ca(2+) entry in HMEC and HEK-293 cells. We observed a similar gain-of-function in Ca(2+) entry when Cav-1DeltaCSD was expressed in lung endothelial cells of Cav-1 knockout mice. Immunoprecipitation results revealed that WT-Cav-1 but not Cav-1DeltaCSD interacted with IP(3)R3. Furthermore, we observed using confocal imaging the colocalization of IP(3)R3 with WT-Cav-1 but not with Cav-1DeltaCSD on Ca(2+) store release in endothelial cells. These findings suggest that CSD interacts with TRPC1 and IP(3)R3 and thereby regulates Ca(2+) store release-induced Ca(2+) entry in endothelial cells.

Functional roles of TRPC channels in the developing brain

Transient receptor potential canonical (TRPC) channels are Ca(2+)-permeable, nonselective cation channels formed by homomeric or heteromeric complexes of TRPC proteins that contain six transmembrane domains. These channels can be activated through a phospholipase-C-dependent mechanism, making them sensors for environmental cues. Their expression begins early in embryonic days and remains in adulthood. These channels have important roles in the processes of neuronal development, including neural stem cell proliferation, cerebellar granule cell survival, axon path finding, neuronal morphogenesis, and synaptogenesis. In this review, we will discuss functional implications of TRPC channels during brain development.

IP3 receptor binds to and sensitizes TRPV4 channel to osmotic stimuli via a calmodulin-binding site

Activation of the non-selective cation channel TRPV4 by mechanical and osmotic stimuli requires the involvement of phospholipase A2 and the subsequent production of the arachidonic acid metabolites, epoxieicosatrienoic acids (EET). Previous studies have shown that inositol trisphosphate (IP3) sensitizes TRPV4 to mechanical, osmotic, and direct EET stimulation. We now search for the IP3 receptor-binding site on TRPV4 and its relevance to IP3-mediated sensitization. Three putative sites involved in protein-protein interactions were evaluated: a proline-rich domain (PRD), a calmodulin (CaM)-binding site, and the last four amino acids (DAPL) that show a PDZ-binding motif-like. TRPV4-DeltaCaM-(Delta812-831) channels preserved activation by hypotonicity, 4alpha-phorbol 12,13-didecanoate, and EET but lost their physical interaction with IP3 receptor 3 and IP3-mediated sensitization. Deletion of a PDZ-binding motif-like (TRPV4-DeltaDAPL) did not affect channel activity or IP3-mediated sensitization, whereas TRPV4-DeltaPRD-(Delta132-144) resulted in loss of channel function despite correct trafficking. We conclude that IP3-mediated sensitization requires IP3 receptor binding to a TRPV4 C-terminal domain that overlaps with a previously described calmodulin-binding site.

Type 1 inositol 1,4,5-trisphosphate receptors mediate UTP-induced cation currents, Ca2+ signals, and vasoconstriction in cerebral arteries

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) regulate diverse physiological functions, including contraction and proliferation. There are three IP(3)R isoforms, but their functional significance in arterial smooth muscle cells is unclear. Here, we investigated relative expression and physiological functions of IP(3)R isoforms in cerebral artery smooth muscle cells. We show that 2-aminoethoxydiphenyl borate and xestospongin C, membrane-permeant IP(3)R blockers, reduced Ca(2+) wave activation and global intracellular Ca(2+) ([Ca(2+)](i)) elevation stimulated by UTP, a phospholipase C-coupled purinergic receptor agonist. Quantitative PCR, Western blotting, and immunofluorescence indicated that all three IP(3)R isoforms were expressed in acutely isolated cerebral artery smooth muscle cells, with IP(3)R1 being the most abundant isoform at 82% of total IP(3)R message. IP(3)R1 knockdown with short hairpin RNA (shRNA) did not alter baseline Ca(2+) wave frequency and global [Ca(2+)](i) but abolished UTP-induced Ca(2+) wave activation and reduced the UTP-induced global [Ca(2+)](i) elevation by approximately 61%. Antibodies targeting IP(3)R1 and IP(3)R1 knockdown reduced UTP-induced nonselective cation current (I(cat)) activation. IP(3)R1 knockdown also reduced UTP-induced vasoconstriction in pressurized arteries with both intact and depleted sarcoplasmic reticulum (SR) Ca(2+) by approximately 45%. These data indicate that IP(3)R1 is the predominant IP(3)R isoform expressed in rat cerebral artery smooth muscle cells. IP(3)R1 stimulation contributes to UTP-induced I(cat) activation, Ca(2+) wave generation, global [Ca(2+)](i) elevation, and vasoconstriction. In addition, IP(3)R1 activation constricts cerebral arteries in the absence of SR Ca(2+) release by stimulating plasma membrane I(cat).

TRPC3 controls agonist-stimulated intracellular Ca2+ release by mediating the interaction between inositol 1,4,5-trisphosphate receptor and RACK1

Activation of TRPC3 channels is concurrent with inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-mediated intracellular Ca(2+) release and associated with phosphatidylinositol 4,5-bisphosphate hydrolysis and recruitment to the plasma membrane. Here we report that interaction of TRPC3 with receptor for activated C-kinase-1 (RACK1) not only determines plasma membrane localization of the channel but also the interaction of IP(3)R with RACK1 and IP(3)-dependent intracellular Ca(2+) release. We show that TRPC3 interacts with RACK1 via N-terminal residues Glu-232, Asp-233, Glu-240, and Glu-244. Carbachol (CCh) stimulation of HEK293 cells expressing wild type TRPC3 induced recruitment of a ternary TRPC3-RACK1-IP(3)R complex and increased surface expression of TRPC3 and Ca(2+) entry. Mutation of the putative RACK1 binding sequence in TRPC3 disrupted plasma membrane localization of the channel. CCh-stimulated recruitment of TRPC3-RACK1-IP(3)R complex as well as increased surface expression of TRPC3 and receptor-operated Ca(2+) entry were also attenuated. Importantly, CCh-induced intracellular Ca(2+) release was significantly reduced as was RACK1-IP(3)R association without any change in thapsigargin-stimulated Ca(2+) release and entry. Knockdown of endogenous TRPC3 also decreased RACK1-IP(3)R association and decreased CCh-stimulated Ca(2+) entry. Furthermore, an oscillatory pattern of CCh-stimulated intracellular Ca(2+) release was seen in these cells compared with the more sustained pattern seen in control cells. Similar oscillatory pattern of Ca(2+) release was seen after CCh stimulation of cells expressing the TRPC3 mutant. Together these data demonstrate a novel role for TRPC3 in regulation of IP(3)R function. We suggest TRPC3 controls agonist-stimulated intracellular Ca(2+) release by mediating interaction between IP(3)R and RACK1.

TRP channels: an overview

The TRP ("transient receptor potential") family of ion channels now comprises more than 30 cation channels, most of which are permeable for Ca2+, and some also for Mg2+. On the basis of sequence homology, the TRP family can be divided in seven main subfamilies: the TRPC ('Canonical') family, the TRPV ('Vanilloid') family, the TRPM ('Melastatin') family, the TRPP ('Polycystin') family, the TRPML ('Mucolipin') family, the TRPA ('Ankyrin') family, and the TRPN ('NOMPC') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data on the roles of TRPs in a variety of tissues and species, including mammals, insects, and yeast. The present review summarizes the most pertinent recent evidence regarding the structural and functional properties of TRP channels, focusing on the regulation and physiology of mammalian TRPs.

TRPC3-interacting triadic proteins in skeletal muscle

The expression of TRPC3 (canonical-type transient receptor potential cation channel type 3) is tightly regulated during skeletal muscle cell differentiation, and a functional interaction between TRPC3 and RyR1 [(ryanodine receptor type 1), an SR (sarcoplasmic reticulum) Ca2+-release channel] regulates the gain of SR Ca2+ release during EC (excitation–contraction) coupling. However, it has not been possible to demonstrate direct protein–protein interactions between TRPC3 and RyR1. To identify possible candidate(s) for a linker protein(s) between TRPC3 and RyR1 in skeletal muscle, in the present study we performed MALDI–TOF (matrix-assisted laser-desorption ionization–time-of-flight) MS analysis of a cross-linked triadic protein complex from rabbit skeletal triad vesicles and co-immunoprecipitation assays using primary mouse skeletal myotubes. From these studies, we found that six triadic proteins, that are known to regulate RyR1 function and/or EC coupling [TRPC1, JP2 (junctophilin 2), homer, mitsugumin 29, calreticulin and calmodulin], interacted directly with TRPC3 in a Ca2+-independent manner. However we again found no direct interaction between TRPC3 and RyR1. TRPC1 was identified as a potential physical link between TRPC3 and RyR1, as it interacted with both TRPC3 and RyR1, and JPs showed subtype-specific interactions with both RyR1 and TRPC3 (JP1–RyR1 and JP2–TRPC3). These results support the hypothesis that TRPC3 and RyR1 are functionally engaged via linker proteins in skeletal muscle.

Calcium plays a central role in the sensitization of TRPV3 channel to repetitive stimulations

Transient receptor potential channels are involved in sensing chemical and physical changes inside and outside of cells. TRPV3 is highly expressed in skin keratinocytes, where it forms a nonselective cation channel activated by hot temperatures in the innocuous and noxious range. The channel has also been implicated in flavor sensation in oral and nasal cavities as well as being a molecular target of some allergens and skin sensitizers. TRPV3 is unique in that its activity is sensitized upon repetitive stimulations. Here we investigated the role of calcium ions in the sensitization of TRPV3 to repetitive stimulations. We show that the sensitization is accompanied by a decrease of Ca(2+)-dependent channel inhibition mediated by calmodulin acting at an N-terminal site (amino acids 108-130) and by an acidic residue (Asp(641)) at the pore loop of TRPV3. These sites also contribute to the voltage dependence of TRPV3. During sensitization, the channel displayed a gradual shift of the voltage dependence to more negative potentials as well as uncoupling from voltage sensing. The initial response to ligand stimulation was increased and sensitization to repetitive stimulations was decreased by increasing the intracellular Ca(2+)-buffering strength, inhibiting calmodulin, or disrupting the calmodulin-binding site. Mutation of Asp(641) to Asn abolished the high affinity extracellular Ca(2+)-mediated inhibition and greatly facilitated the activation of TRPV3. We conclude that Ca(2+) inhibits TRPV3 from both the extracellular and intracellular sides. The inhibition is sequentially reduced, appearing as sensitization to repetitive stimulations.

Synergistic actions of diacylglycerol and inositol 1,4,5 trisphosphate for Ca2+-dependent inactivation of TRPC7 channel

AIM

The aim of the present study was to explore the mechanism for the Ca2+- dependent inactivation of the canonical transient receptor potential (TRPC) 7 channel expressed in human embryonic kidney 293 cells.

METHOD

The whole-cell patch-clamp technique was used in the study.

RESULTS

With Ca2+-free external solution, the perfusion of 100 micromol/L carbachol to, or dialysis of the cell with 100 micromol/L guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), induced large inward currents, respectively. These currents were rapidly inhibited by the addition of 1 mmol/L Ca2+ into the bath, and recovery from this inhibition was only partial after the Ca2+ removal, unless vigorous intracellular Ca2+ buffering with 10 mmol/L 1,2 bis(2- aminophenoxy)ethane-N,N,No,No-tetraacetic acid (BAPTA) (plus 4 mmol/L Ca2+) was employed. In contrast, the current induced by a membrane-permeable analog of diacylglycerol (DAG), 1-oleoyl-2-acetyl-sn-glycerol (OAG; 100 micromol/L) did not undergo the inhibition persisting after Ca2+ removal. Interestingly, the inclusion of inositol 1,4,5 trisphosphate (IP3; 100 micromol/L) in the patch pipette rendered the OAG-induced current susceptible to the persistent Ca2+-mediated inhibition independent of the IP3 receptor in the majority of the tested cells, as evidenced by the inability of heparin and thapsigargin in reversing the effect of IP3.

CONCLUSION

The present results suggest that Ca2+ entry via the activated TRPC7 channel plays a critical role in inactivating the channel where the cooperative actions of DAG and IP3 are essentially involved.

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.

Anti-calmodulins and tricyclic adjuvants in pain therapy block the TRPV1 channel

Ca(2+)-loaded calmodulin normally inhibits multiple Ca(2+)-channels upon dangerous elevation of intracellular Ca(2+) and protects cells from Ca(2+)-cytotoxicity, so blocking of calmodulin should theoretically lead to uncontrolled elevation of intracellular Ca(2+). Paradoxically, classical anti-psychotic, anti-calmodulin drugs were noted here to inhibit Ca(2+)-uptake via the vanilloid inducible Ca(2+)-channel/inflamatory pain receptor 1 (TRPV1), which suggests that calmodulin inhibitors may block pore formation and Ca(2+) entry. Functional assays on TRPV1 expressing cells support direct, dose-dependent inhibition of vanilloid-induced (45)Ca(2+)-uptake at microM concentrations: calmidazolium (broad range) > or = trifluoperazine (narrow range) chlorpromazine/amitriptyline>fluphenazine>>W-7 and W-13 (only partially). Most likely a short acidic domain at the pore loop of the channel orifice functions as binding site either for Ca(2+) or anti-calmodulin drugs. Camstatin, a selective peptide blocker of calmodulin, inhibits vanilloid-induced Ca(2+)-uptake in intact TRPV1(+) cells, and suggests an extracellular site of inhibition. TRPV1(+), inflammatory pain-conferring nociceptive neurons from sensory ganglia, were blocked by various anti-psychotic and anti-calmodulin drugs. Among them, calmidazolium, the most effective calmodulin agonist, blocked Ca(2+)-entry by a non-competitive kinetics, affecting the TRPV1 at a different site than the vanilloid binding pocket. Data suggest that various calmodulin antagonists dock to an extracellular site, not found in other Ca(2+)-channels. Calmodulin antagonist-evoked inhibition of TRPV1 and NMDA receptors/Ca(2+)-channels was validated by microiontophoresis of calmidazolium to laminectomised rat monitored with extracellular single unit recordings in vivo. These unexpected findings may explain empirically noted efficacy of clinical pain adjuvant therapy that justify efforts to develop hits into painkillers, selective to sensory Ca(2+)-channels but not affecting motoneurons.

Intracellular trafficking of TRP channels

Thirteen years ago, it was suggested that exocytotic insertion of store-operated channels into the plasma membrane lead to increased Ca(2+) entry in non-excitable cells upon G protein-coupled or tyrosine kinase receptor stimulation. Since the discovery of the TRP channel superfamily and their involvement in receptor-induced Ca(2+) entry, many studies have shown that different members of the TRP superfamily translocate into the plasma membrane upon stimulation. While the exact molecular mechanism by which TRP channels insert into the plasma membrane is unknown, TRP-binding proteins have been shown to directly regulate this trafficking. This review summarizes recent advances related to the mechanism of TRP channel trafficking, focusing on the role of TRP-binding proteins.

Transient receptor potential channel A1 is directly gated by calcium ions

Members of the superfamily of transient receptor potential (TRP) channels are proposed to play important roles in sensory physiology. As an excitatory ion channel TRPA1 is robustly activated by pungent irritants in mustard and garlic and is suggested to mediate the inflammatory actions of environmental irritants and proalgesic agents. Here, we demonstrate that, in addition to pungent natural compounds, Ca(2+) directly gates heterologously expressed TRPA1 in whole-cell and excised-patch recordings with an apparent EC(50) of 905 nm. Pharmacological experiments and site-directed mutagenesis indicate that the N-terminal EF-hand calcium-binding domain of the channel is involved in Ca(2+)-dependent activation. Furthermore, we determine Ca(2+) as prerequisite for icilin activity on TRPA1.

TRPC channels: interacting proteins

TRP channels, in particular the TRPC and TRPV subfamilies, have emerged as important constituents of the receptor-activated Ca2+ influx mechanism triggered by hormones, growth factors, and neurotransmitters through activation ofphospholipase C (PLC). Several TRPC channels are also activated by passive depletion of endoplasmic reticulum (ER) Ca2+. Although in several studies the native TRP channels faithfully reproduce the respective recombinant channels, more often the properties of Ca2+ entry and/or the store-operated current are strikingly different from that of the TRP channels expressed in the same cells. The present review aims to discuss this disparity in the context of interaction of TRPC channels with auxiliary proteins that may alter the permeation and regulation of TRPC channels.

Ionic channels formed by TRPC4

TRPC4 (transient receptor potential canonical 4) is a member of the TRPC sub-family and, within this sub-family, TRPC4 is most closely related to TRPC5. A number of splice variants of TRPC4 have been identified, whereby TRPC4alpha and TRPC4beta appear to be the most abundant isoforms in various species. TRPC4alpha comprises six transmembrane segments and the N- and C-termini are located intracellularly. Additionally, TRPC4alpha shares other structural features with members of the TRPC sub-group, including ankyrin-like repeats, coiled-coil regions and binding sites for calmodulin and IP3 receptors. Three calmodulin-binding domains have been identified in the C-terminus of TRPC4alpha. TRPC4beta lack 84 amino acids in the C-terminus, which correspond to the last two calmodulin-binding sites of TRPCalpha. The first and last calmodulin-binding domains of TRPC4alpha overlap with binding sites for the N- and C-termini of IP3 receptors. The ionic channels formed by TRPC4 appear to be Ca(2+)-permeable, although there is a considerably discrepancy in the degree of Ca2+ selectivity. Studies with mice lacking TRPC4 (TRPC4(-/-)) suggest an important role for TRPC4 in supporting Ca2+ entry. The defect in Ca2+ entry in TRPC4(-/-) mice appears to be associated with a reduction of the vasorelaxation of arteries, vascular permeability in the lung and neurotransmitter release from thalamic dendrites.

Calcium signaling in non-excitable cells: Ca2+ release and influx are independent events linked to two plasma membrane Ca2+ entry channels

The regulatory mechanism of Ca2+ influx into the cytosol from the extracellular space in non-excitable cells is not clear. The "capacitative calcium entry" (CCE) hypothesis suggested that Ca2+ influx is triggered by the IP(3)-mediated emptying of the intracellular Ca2+ stores. However, there is no clear evidence for CCE and its mechanism remains elusive. In the present work, we have provided the reported evidences to show that inhibition of IP(3)-dependent Ca2+ release does not affect Ca2+ influx, and the experimental protocols used to demonstrate CCE can stimulate Ca2+ influx by means other than emptying of the Ca2+ stores. In addition, we have presented the reports showing that IP(3)-mediated Ca2+ release is linked to a Ca2+ entry from the extracellular space, which does not increase cytosolic [Ca2+] prior to Ca2+ release. Based on these and other reports, we have provided a model of Ca2+ signaling in non-excitable cells, in which IP(3)-mediated emptying of the intracellular Ca2+ store triggers entry of Ca2+ directly into the store, through a plasma membrane TRPC channel. Thus, emptying and direct refilling of the Ca2+ stores are repeated in the presence of IP(3), giving rise to the transient phase of oscillatory Ca2+ release. Direct Ca2+ entry into the store is regulated by its filling status in a negative and positive manner through a Ca2+ -binding protein and Stim1/Orai complex, respectively. The sustained phase of Ca2+ influx is triggered by diacylglycerol (DAG) through the activation of another TRPC channel, independent of Ca2+ release. The plasma membrane IP(3) receptor (IP(3)R) plays an essential role in Ca2+ influx, by interacting with the DAG-activated TRPC, without the requirement of binding to IP(3).

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.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Transient Receptor Potential Channels and Intracellular Signaling

The transient receptor potential (TRP) family of ion channels is composed of more than 50 functionally versatile cation-permeant ion channels expressed in most mammalian cell types. Considerable research has been brought to bear on the members of this family, especially with regard to their possible role as store-operated calcium channels, although studies have provided evidence that TRP channels exhibit a number of regulatory and functional aspects. Endogenous and transiently expressed TRP channels can be activated by different mechanisms grouped into four main categories: receptor-operated activation, store depletion-mediated activation, ligand-induced activation, and direct activation. This article reviews the biochemical characteristics of the different members of the TRP family and summarizes their involvement in a number of physiological events ranging from sensory transduction to development, which might help in understanding the relationship between TRP channel dysfunction and the development of several diseases.

TRPC6

TRPC6 is a Ca(2+)-permeable non-selective cation channel expressed in brain, smooth muscle containing tissues and kidney, as well as in immune and blood cells. Channel homomers heterologously expressed have a characteristic doubly rectifying current-voltage relationship and are six times more permeable for Ca2+ than for Na+. In smooth muscle tissues, however, Na+ influx and activation of voltage-gated calcium channels by membrane depolarization rather than Ca2+ elevation by TRPC6 channels is the driving force for contraction. TRPC6 channels are directly activated by the second messenger diacylglycerol (DAG) and regulated by specific tyrosine or serine phosphorylation. Extracellular Ca2+ has inhibitory effects, while Ca2+/calmodulin acting from the intracellular side has potentiator effects on channel activity. Given its specific expression, TRPC6 is likely to play a number of physiological roles. Studies with TRPC6(-/-) mice suggest a role for the channel in the regulation of vascular and pulmonary smooth muscle contraction. TRPC6 was identified as an essential component of the slit diaphragm architecture of kidney podocytes. Other functions in immune and blood cells, as well as in brain and in smooth muscle-containing tissues such as stomach, colon and myometrium, remain elusive.

TRPC channels: integrators of multiple cellular signals

TRPC channels are ubiquitously expressed among cell types and mediate signals in response to phospholipase C (PLC)-coupled receptors. TRPC channels function as integrators of multiple signals resulting from receptor-induced PLC activation, which catalyzes the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG). InsP3 depletes Ca2+ stores and TRPC3 channels can be activated by store-depletion. InsP3 also activates the InsP3 receptor, which may undergo direct interactions with the TRPC3 channel, perhaps mediating store-dependence. The other PLC product, DAG, has a direct non-PKC-dependent activating role on TRPC3 channels likely by direct binding. DAG also has profound effects on the TRPC3 channel through PKC. Thus PKC is a powerful inhibitor of most TRPC channels and DAG is a dual regulator of the TRPC3 channel. PLC-mediated DAG results in rapid channel opening followed later by a slower DAG-induced PKC-mediated deactivation of the channel. The decreased level of PIP2 from PLC activation also has an important modifying action on TRPC3 channels. Thus, the TRPC3 channel and PLCgamma form an intermolecular PH domain that has high specificity for binding PIP2. This interaction allows the channel to be retained within the plasma membrane, a further operational control factor for TRPC3. As nonselective cation channels, TRPC channel opening results in the entry of both Na+ and Ca2+ ions. Thus, while they may mediate Ca2+ entry signals, TRPC channels are also powerful modifiers of membrane potential.

The TRPC3 channel has a large internal chamber surrounded by signal sensing antennas

Transient receptor potential (TRP) channels are intrinsic sensors adapted for response to all manner of stimuli both from inside and from outside the cell. Within the TRP superfamily, the canonical TRP-3 (TRPC3) has been widely studied and is involved in various biological processes such as neuronal differentiation, blood vessel constriction, and immune cell maturation. Upon stimulation of surface membrane receptors linked to phospholipase C, TRPC3 mediates transmembrane Ca(2+) influx from outside the cell to control Ca(2+) signaling, in concert with the Ca(2+) release from internal stores. The structural basis of TRP superfamily has, however, been poorly understood. Here we present a structure of the TRPC3 at 15 A resolution. This first 3D depiction of TRP superfamily was reconstructed from 135,909 particle images obtained with cryo-electron microscopy. The large intracellular domain represents a "nested-box" structure: a wireframe outer shell is functionable as sensors for activators and modulators, and a globular inner chamber may modulate ion flow, since it is aligned tandem along the central axis with the dense membrane-spanning core. The transmembrane domain demonstrates a pore-forming property. This structure implies that the TRP superfamily has diversely evolved as sensors specialized for various signals, rather than as simple ion-conducting apparatuses.

TRPC-like conductance mediates restoration of intracellular Ca2+ in cochlear outer hair cells in the guinea pig and rat

Ca2+ signalling is central to cochlear sensory hair cell physiology through its influence on sound transduction, membrane filter properties and neurotransmission. However, the mechanism for establishing Ca2+ homeostasis in these cells remains unresolved. Canonical transient receptor potential (TRPC) Ca2+ entry channels provide an important pathway for maintaining intracellular Ca2+ levels. TRPC3 subunit expression was detected in guinea pig and rat organ of Corti by RT-PCR, and localized to the sensory and neural poles of the inner and outer hair cells (OHCs) by confocal immunofluorescence imaging. A cation entry current with a TRPC-like phenotype was identified in guinea pig and rat OHCs by whole-cell voltage clamp. This slowly activating current was induced by the lowering of cytosolic Ca2+ levels ([Ca2+]i) following a period in nominally Ca2+-free solution. Activation was dependent upon the [Ca2+]o and was sustained until [Ca(2+)]i was restored. Ca2+ entry was confirmed by confocal fluorescence imaging, and rapidly recruited secondary charybdotoxin- and apamin-sensitive K(Ca) currents. Dual activation by the G protein-coupled receptor (GPCR)-phospholipase C-diacylglycerol (DAG) second messenger pathway was confirmed using the analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Ion substitution experiments showed that the putative TRPC Ca2+ entry current was selective for Na+ > K+ with a ratio of 1: 0.6. The Ca2+ entry current was inhibited by the TRPC channel blocker 2-aminoethyl diphenylborate (2APB) and the tyrosine kinase inhibitor, erbstatin analogue. We conclude that TRPC Ca2+ entry channels, most likely incorporating TRPC3 subunits, support cochlear hair cell Ca2+ homeostasis and GPCR signalling.