Specific detection and semi-quantitative analysis of TRPC4 protein expression by antibodies

Subunit composition, molecular environment, and activation of native TRPC channels encoded by their interactomes

In the mammalian brain TRPC channels, a family of Ca²⁺-permeable cation channels, are involved in a variety of processes from neuronal growth and synapse formation to transmitter release, synaptic transmission and plasticity. The molecular appearance and operation of native TRPC channels, however, remained poorly understood. Here, we used high-resolution proteomics to show that TRPC channels in the rodent brain are macro-molecular complexes of more than 1 MDa in size that result from the co-assembly of the tetrameric channel core with an ensemble of interacting proteins (interactome). The core(s) of TRPC1-, C4-, and C5-containing channels are mostly heteromers with defined stoichiometries for each subtype, whereas TRPC3, C6, and C7 preferentially form homomers. In addition, TRPC1/C4/C5 channels may co-assemble with the metabotropic glutamate receptor mGluR1, thus guaranteeing both specificity and reliability of channel activation via the phospholipase-Ca²⁺ pathway. Our results unveil the subunit composition of native TRPC channels and resolve the molecular details underlying their activation.

Qualitative and semiquantitative assessment of thyroid hormone binding proteins in greyhounds and other dog breeds

Total thyroxine (T4) concentrations are lower in healthy greyhounds compared to most other non-sighthound breeds. In humans, variations in the structure or concentration of the major thyroid hormone binding proteins are responsible for most reported differences between total T4 concentrations in healthy individuals from different ethnic groups or other subpopulations. The aim of this study was to determine if such variations are also responsible for the lower total T4 concentrations in greyhounds. The predicted protein sequences of thyroxine-binding globulin (TBG), transthyretin and albumin were determined in liver tissue from a euthyroid greyhound with decreased T4 concentration and a Jack Russell terrier using reverse-transcriptase PCR. Sequences were compared to each other and online reference sequences. Serum proteins from 21 greyhounds and 21 non-sighthound dogs were separated by denaturing electrophoresis and immunoblots probed with polyclonal antibodies to human TBG and transthyretin. Reactive bands were quantified by densitrometry, expressed relative to the mean of reference samples included in each gel. Serum albumin concentrations were measured using a commercially-available assay. Several SNPs were identified but none was thought likely to explain the lower total T4 concentrations in greyhounds. There was no significant difference between the quantity of any of the binding proteins in serum from greyhounds and non-sighthound dogs. However, total T4 and transthyretin concentrations were highly correlated in the greyhound group (r = 0.73, P = 0.0002). Variation in the sequence of thyroid hormone binding proteins is not responsible for low greyhound total T4 concentrations. Further evaluation of the role of transthyretin is warranted.

TRPC1 and TRPC4 channels functionally interact with STIM1L to promote myogenesis and maintain fast repetitive Ca2+ release in human myotubes

STIM1 and Orai1 are essential players of store-operated Ca²⁺ entry (SOCE) in human skeletal muscle cells and are required for adult muscle differentiation. Besides these two proteins, TRPC (transient receptor potential canonical) channels and STIM1L (a longer STIM1 isoform) are also present on muscle cells. In the present study, we assessed the role of TRPC1, TRPC4 and STIM1L in SOCE, in the maintenance of repetitive Ca²⁺ transients and in muscle differentiation. Knockdown of TRPC1 and TRPC4 reduced SOCE by about 50% and significantly delayed the onset of Ca²⁺ entry, both effects similar to STIM1L invalidation. Upon store depletion, TRPC1 and TRPC4 appeared to interact preferentially with STIM1L compared to STIM1. STIM1L invalidation affected myoblast differentiation, with the formation of smaller myotubes, an effect similar to what we reported for TRPC1 and TRPC4 knockdown. On the contrary, the overexpression of STIM1L leads to the formation of larger myotubes. All together, these data strongly suggest that STIM1L and TRPC1/4 are working together in myotubes to ensure efficient store refilling and a proper differentiation program.

The involvement of transient receptor potential canonical type 1 in skeletal muscle regrowth after unloading-induced atrophy

KEY POINTS

Decreased mechanical loading results in skeletal muscle atrophy. The transient receptor potential canonical type 1 (TRPC1) protein is implicated in this process. Investigation of the regulation of TRPC1 in vivo has rarely been reported. In the present study, we employ the mouse hindlimb unloading and reloading model to examine the involvement of TRPC1 in the regulation of muscle atrophy and regrowth, respectively. We establish the physiological relevance of the concept that manipulation of TRPC1 could interfere with muscle regrowth processes following an atrophy-inducing event. Specifically, we show that suppressing TRPC1 expression during reloading impairs the recovery of the muscle mass and slow myosin heavy chain profile. Calcineurin appears to be part of the signalling pathway involved in the regulation of TRPC1 expression during muscle regrowth. These results provide new insights concerning the function of TRPC1. Interventions targeting TRPC1 or its downstream or upstream pathways could be useful for promoting muscle regeneration.

ABSTRACT

Decreased mechanical loading, such as bed rest, results in skeletal muscle atrophy. The functional consequences of decreased mechanical loading include a loss of muscle mass and decreased muscle strength, particularly in anti-gravity muscles. The purpose of this investigation was to clarify the regulatory role of the transient receptor potential canonical type 1 (TRPC1) protein during muscle atrophy and regrowth. Mice were subjected to 14 days of hindlimb unloading followed by 3, 7, 14 and 28 days of reloading. Weight-bearing mice were used as controls. TRPC1 expression in the soleus muscle decreased significantly and persisted at 7 days of reloading. Small interfering RNA (siRNA)-mediated downregulation of TRPC1 in weight-bearing soleus muscles resulted in a reduced muscle mass and a reduced myofibre cross-sectional area (CSA). Microinjecting siRNA into soleus muscles in vivo after 7 days of reloading provided further evidence for the role of TRPC1 in regulating muscle regrowth. Myofibre CSA, as well as the percentage of slow myosin heavy chain-positive myofibres, was significantly lower in TRPC1-siRNA-expressing muscles than in control muscles after 14 days of reloading. Additionally, inhibition of calcineurin (CaN) activity downregulated TRPC1 expression in both weight-bearing and reloaded muscles, suggesting a possible association between CaN and TRPC1 during skeletal muscle regrowth.

The role of canonical transient receptor potential channels in seizure and excitotoxicity

Canonical transient receptor potential (TRPC) channels are a family of polymodal cation channels with some degree of Ca²⁺ permeability. Although initially thought to be channels mediating store-operated Ca²⁺ influx, TRPC channels can be activated by stimulation of Gq-coupled G-protein coupled receptors, or by an increase in intracellular free Ca²⁺ concentration. Thus, activation of TRPC channels could be a common downstream event of many signaling pathways that contribute to seizure and excitotoxicity, such as N-methyl-D-aspartate (NMDA) receptor-mediated Ca²⁺ influx, or metabotropic glutamate receptor activation. Recent studies with genetic ablation of various TRPC family members have demonstrated that TRPC channels, in particular heteromeric TRPC1/4 channels and homomeric TRPC5 channels, play a critical role in both pilocarpine-induced acute seizures and neuronal cell death. However, exact underlying mechanisms remain to be fully elucidated, and selective TRPC modulators and antibodies with better specificity are urgently needed for future research.

TRPM3

Like most other members of the TRP family, the Trpm3 gene encodes proteins that form cation-permeable ion channels on the plasma membrane. However, TRPM3 proteins have several unique features that set them apart from the other members of this diverse family. The Trpm3 gene encodes for a surprisingly large number of isoforms generated mainly by alternative splicing. Only for two of the (at least) eight sites at which sequence diversity is generated the functional consequences have been elucidated, one leading to nonfunctional channels, the other one profoundly affecting the ionic selectivity. In the Trpm3 gene an intronic microRNA (miR-204) is co-transcribed with Trpm3. By regulating the expression of a multitude of genes, miR-204 increases the functional complexity of the Trpm3 locus. Over the past years, important progress has been made in discovering pharmacological tools to manipulate TRPM3 channel activity. These substances have facilitated the identification of endogenously expressed functional TRPM3 channels in nociceptive neurons, pancreatic beta cells, and vascular smooth muscle cells, among others. TRPM3 channels, which themselves are temperature sensitive, thus have been implicated in sensing noxious heat, in modulating insulin release, and in secretion of inflammatory cytokines. However, in many tissues where TRPM3 proteins are known to be expressed, no functional role has been identified for these channels so far. Because of the availability of adequate pharmacological and genetic tools, it is expected that future investigations on TRPM3 channels will unravel important new aspects and functions of these channels.

What do we really know and what do we need to know: some controversies, perspectives, and surprises

TRP channels comprise one of the most rapid growing research topics in ion channel research, in fields related to ion channels including channelopathies and translational medicine. We provide here a critical survey on our current knowledge of TRP channels and highlight some of the still open or controversial questions. This comprises questions related to evolution of TRP channels; biophysics, i.e., permeation; pore properties and gating; modulation; the still-elusive 3D structure; and channel subunits but also their role as general sensory channels and in human diseases. We will conclude that our knowledge on TRP channels is still at the very beginning of an exciting research journey.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

Mechano-regulation of the beating heart at the cellular level--mechanosensitive channels in normal and diseased heart

The heart as a contractile hollow organ finely tunes mechanical parameters such as stroke volume, stroke pressure and cardiac output according to filling volumes, filling pressures via intrinsic and neuronal routes. At the cellular level, cardiomyocytes in beating hearts are exposed to large mechanical stress during successive heart beats. Although the mechanisms of excitation-contraction coupling are well established in mammalian heart cells, the putative contribution of mechanosensitive channels to Ca²⁺ homeostasis, Ca²⁺ signaling and force generation has been primarily investigated in relation to heart disease states. For instance, transient receptor potential channels (TRPs) are up-regulated in animal models of congestive heart failure or hypertension models and seem to play a vital role in pathological Ca²⁺ overload to cardiomyocytes, thus aggravating the pathology of disease at the cellular level. Apart from that, the contribution of mechanosensitive channels (MsC) in the normal beating heart to the downstream force activation cascade has not been addressed. We present an overview of the current literature and concepts of mechanosensitive channel involvement in failing hearts and cardiomyopathies and novel data showing a likely contribution of Ca²⁺ influx via mechanosensitive channels in beating normal cardiomyocytes during systolic shortening.

TRPC1 protein channel is major regulator of epidermal growth factor receptor signaling

TRP channels have been associated with cell proliferation and aggressiveness in several cancers. In particular, TRPC1 regulates cell proliferation and motility, two processes underlying cancer progression. We and others have described the mechanisms of TRPC1-dependent cell migration. However, the involvement of TRPC1 in cell proliferation remains unexplained. In this study, we show that siRNA-mediated TRPC1 depletion in non small cell lung carcinoma cell lines induced G(0)/G(1) cell cycle arrest resulting in dramatic decrease in cell growth. The expression of cyclins D1 and D3 was reduced after TRPC1 knockdown, pointing out the role of TRPC1 in G(1)/S transition. This was associated with a decreased phosphorylation and activation of EGFR and with a subsequent disruption of PI3K/Akt and MAPK downstream pathways. Stimulation of EGFR by its natural ligand, EGF, induced Ca(2+) release from the endoplasmic reticulum and Ca(2+) entry through TRPC1. Ca(2+) entry through TRPC1 conversely activated EGFR, suggesting that TRPC1 is a component of a Ca(2+)-dependent amplification of EGF-dependent cell proliferation.

TRP channels in normal and dystrophic skeletal muscle

TRP proteins constitute non-selective cation-permeable ion channels, most of which are permeable to Ca²⁺. In skeletal muscle, several isoforms of the TRPC (Canonical), TRPV (Vanilloid) and TRPM (Melastatin) subfamilies are expressed. In particular, TRPC1, C3 and C6, TRPV2 and V4, TRPM4 and TRPM7 have been consistently found in cultured myoblasts or in adult muscles. These channels seem to directly or indirectly respond to membrane stretch or to Ca²⁺ stores depletion; some isoforms might also constitute unregulated Ca²⁺ leak channels. Their function is largely unknown. TRPC1 and C3 have been involved in muscle development, in particular in myoblasts migration and differentiation. TRPC1 and V4 might allow a basal influx of Ca²⁺ at rest. Their lack has consequences on muscle fatigue. TRPV2 seems to be stretch-sensitive. It localizes mainly in intracellular pools at rest, and translocates to the plasma membrane upon IGF-1 stimulation. TRP channels seem to be involved in the pathophysiology of muscle disorders. In particular in Duchenne muscular dystrophy, the lack of the cytoskeletal protein dystrophin induces a disregulation of several ion channels leading to an abnormal influx of Ca²⁺. We discuss here, the possible involvement of TRP channels in this abnormal influx of Ca²⁺.

TRPC channels are involved in calcium-dependent migration and proliferation in immortalized GnRH neurons

Gonadotropin-releasing hormone (GnRH)-secreting neurons are key regulators of the reproductive behaviour in vertebrates. These neurons show a peculiar migratory pattern during embryonic development, and its perturbations have profound impact on fertility and other related functional aspects. Changes in the intracellular calcium concentration, [Ca(2+)](i), induced by different extracellular signals, play a central role in the control of neuronal migration, but the available knowledge regarding GnRH neurons is still limited. Our goal was to investigate mechanisms that may be involved in the Ca(2+) dependence of the migratory behaviour in these neurons. We focused on the "classical" Transient Receptor Potential (TRPC) subfamily of Ca(2+)-permeable cation channels, recently shown to be involved in other aspects of neuronal development. Using GN11 cells, immortalized early stage GnRH neurons, we set to investigate Ca(2+) signals under basal conditions and in the presence of a well-established motogen, fetal calf serum (FCS), and the effect of pharmacological TRPC agonists and antagonists on Ca(2+) oscillations, cell motility and proliferation. We have found that a subpopulation of GN11 cells shows spontaneous Ca(2+) transients and that this activity is increased in the presence of serum. Quantitative real-time PCR showed that transcripts of some TRPC members are expressed in GN11 cells. Interestingly, pharmacological experiments with inhibitors, SKF-96365, lanthanum, anti-TRPC1 antibody, and activators, 1-oleil 2-acetyl-sn-glycerol, of TRPCs suggested that the activation of these channels can account for both the basal Ca(2+) oscillations and the increased activity in the presence of FCS. Moreover, functional studies using the same pharmacological tools supported their involvement in the control of motility and proliferation. Thus, our data provide evidence for the involvement of Ca(2+)-permeable channels of the TRPC subfamily in the control of functional properties of neurosecretory cells and neuronal motility.

TRP channels in the cardiopulmonary vasculature

Transient receptor potential (TRP) channels are expressed in almost every human tissue, including the heart and the vasculature. They play unique roles not only in physiological functions but, if over-expressed, also in pathophysiological disease states. Cardiovascular diseases are the leading cause of death in the industrialized countries. Therefore, TRP channels are attractive drug targets for more effective pharmacological treatments of these diseases. This review focuses on three major cell types of the cardiovascular system: cardiomyocytes as well as smooth muscle cells and endothelial cells from the systemic and pulmonary circulation. TRP channels initiate multiple signals in all three cell types (e.g. contraction, migration) and are involved in gene transcription leading to cell proliferation or cell death. Identification of their genes has significantly improved our knowledge of multiple signal transduction pathways in these cells. Some TRP channels are important cellular sensors and are mostly permeable to Ca(2+), while most other TRP channels are receptor activated and allow for the entry of Na(+), Ca(2+) and Mg(2+). Physiological functions of TRPA, TRPC, TRPM, TRPP and TRPV channels in the cardiovascular system, dissected by down-regulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models, are reviewed. The involvement of TRPs as homomeric or heteromeric channels in pathophysiological processes in the cardiovascular system like heart failure, cardiac hypertrophy, hypertension as well as edema formation by increased endothelial permeability will be discussed.

TRP channels in skeletal muscle: gene expression, function and implications for disease

Besides the well known voltage-gated Ca(2+) channels skeletal muscle fibres contain several non-voltage gated Ca(2+) conducting cation channels. They have been physiologically characterized as stretch activated, store operated and Ca(2+) leak channels. TRP channels are good candidates to account for these sarcolemmal channels and Ca(2+) influx pathways or at least contribute to the responsible macromolecular complexes. Several members of the TRPC, TRPV and TRPM subfamilies of TRP channels are expressed in skeletal muscle as shown by RT-PCR, Western blot and immunohistochemistry. The most prominent and consistently found are TRPC1, C3, C4 and C6, TRPV2 and V4 as well as TRPM4 and M7. However, the precise function of individual channels is largely unknown. Linking physiologically characterized channels of the muscle fibre membrane to TRP channel proteins has been a major challenge during the last years. It has been successful only in a few cases and is complicated by the fact that some channels have dual functions in cultured, immature muscle cells and adult fibres. The best characterized TRP channel in skeletal muscle is TRPC1, a small-conductance channel of the sarcolemma. It is needed for Ca(2+) homeostasis during sustained contractile muscle activity. In addition to certain physiological functions TRP channels seem to be involved in the pathomechanisms of muscle disorders. There is a broad body of evidence that dysregulation of Ca(2+) conducting channels plays a key role in the pathomechanism of Duchenne muscular dystrophy. Lack of the cytoskeletal protein dystrophin or δ-sarcoglycan, seems to disturb the function of one or several Ca(2+) channels of the muscle fibre membrane, leading to pathological dystrophic changes. Almost 10 different TRP channels have been detected in skeletal muscle. They seem to be involved in muscle development, Ca(2+) homeostasis, Ca(2+) signalling and in disease progression of certain muscle disorders. However, we are still at the beginning of understanding the impact of TRP channel functions in skeletal muscle.

Cell-cell contact formation governs Ca2+ signaling by TRPC4 in the vascular endothelium: evidence for a regulatory TRPC4-beta-catenin interaction

TRPC4 is well recognized as a prominent cation channel in the vascular endothelium, but its contribution to agonist-induced endothelial Ca²⁺ entry is still a matter of controversy. Here we report that the cellular targeting and Ca²⁺ signaling function of TRPC4 is determined by the state of cell-cell adhesions during endothelial phenotype transitions. TRPC4 surface expression in human microvascular endothelial cells (HMEC-1) increased with the formation of cell-cell contacts. Epidermal growth factor recruited TRPC4 into the plasma membrane of proliferating cells but initiated retrieval of TRPC4 from the plasma membrane in quiescent, barrier-forming cells. Epidermal growth factor-induced Ca²⁺ entry was strongly promoted by the formation of cell-cell contacts, and both siRNA and dominant negative knockdown experiments revealed that TRPC4 mediates stimulated Ca²⁺ entry exclusively in proliferating clusters that form immature cell-cell contacts. TRPC4 co-precipitated with the junctional proteins β-catenin and VE-cadherin. Analysis of cellular localization of fluorescent fusion proteins provided further evidence for recruitment of TRPC4 into junctional complexes. Analysis of TRPC4 function in the HEK293 expression system identified β-catenin as a signaling molecule that enables cell-cell contact-dependent promotion of TRPC4 function. Our results place TRPC4 as a Ca²⁺ entry channel that is regulated by cell-cell contact formation and interaction with β-catenin. TRPC4 is suggested to serve stimulated Ca²⁺ entry in a specific endothelial state during the transition from a proliferating to a quiescent phenotype. Thus, TRPC4 may adopt divergent, as yet unappreciated functions in endothelial Ca²⁺ homeostasis and emerges as a potential key player in endothelial phenotype switching and tuning of cellular growth factor signaling.

Deletion of TRPC4 and TRPC6 in mice impairs smooth muscle contraction and intestinal motility in vivo

BACKGROUND & AIMS

Downstream effects of muscarinic receptor stimulation in intestinal smooth muscle include contraction and intestinal transit. We thought to determine whether classic transient receptor potential (TRPC) channels integrate the intracellular signaling cascades evoked by the stimulated receptors and thereby contribute to the control of the membrane potential, Ca-influx, and cell responses.

METHODS

We created trpc4-, trpc6-, and trpc4/trpc6-gene-deficient mice and analyzed them for intestinal smooth muscle function in vitro and in vivo.

RESULTS

In intestinal smooth muscle cells TRPC4 forms a 55 pS cation channel and underlies more than 80% of the muscarinic receptor-induced cation current (mI(CAT)). The residual mI(CAT) depends on the expression of TRPC6, indicating that TRPC6 and TRPC4 determine mI(CAT) channel activity independent of other channel subunits. In TRPC4-deficient ileal myocytes the carbachol-induced membrane depolarizations are diminished greatly and the atropine-sensitive contraction elicited by acetylcholine release from excitatory motor neurons is reduced greatly. Additional deletion of TRPC6 aggravates these effects. Intestinal transit is slowed down in mice lacking TRPC4 and TRPC6.

CONCLUSIONS

In intestinal smooth muscle cells TRPC4 and TRPC6 channels are gated by muscarinic receptors and are responsible for mI(CAT). They couple muscarinic receptors to depolarization of intestinal smooth muscle cells and voltage-activated Ca(2+)-influx and contraction, and thereby accelerate small intestinal motility in vivo.

TRPC4 expression determines sensitivity of the platelet-type capacitative Ca2+entry channel to intracellular alkalosis

The present study was designed to analyze the molecular basis of the intracellular pH-dependent capacitative Ca2+ entry (CCE) of human platelets and megakaryocytic cells, specifically to test the hypothesis that members of the classical transient receptor potential (TRPC) protein family are involved in the CCE pathway that is promoted by intracellular alkalosis. Human platelets as well as the tested megakaryocytic cell lines (CMK cells, MEG-01 cells) and HEK293 cells displayed thapsigargin-induced CCE and responded to monensin with comparable elevation in intracellular pH. Promotion of CCE by monensin-induced intracellular alkalosis, however, was profound in mature platelets, moderate in CMK cells and lacking in MEG-01 cells as well as in HEK293 cells. Analysis of the TRPC expression pattern by immunoblotting revealed that mature platelets and CMK cells express TRPC4 along with TRPC1 and TRPC3, while TRPC4 is lacking in MEG-01 cells. HEK293 cells displayed CCE characteristics as well as lack of TRPC4 expression similar to MEG-01 cells. Over-expression of TRPC4 in HEK293 cells was found to result in a gain of pH-sensitivity of CCE with clearly detectable promotion of CCE in response to monensin. These results suggest that platelet CCE channel complexes contain TRPC4 as a molecular component that determines sensitivity of CCE to intracellular alkalosis.

Store-operated Ca2+ channels formed by TRPC1, TRPC6 and Orai1 and non-store-operated channels formed by TRPC3 are involved in the regulation of NADPH oxidase in HL-60 granulocytes

Ca(2+) influx has been shown to be essential for NADPH oxidase activity which is involved in the inflammatory process. Ca(2+) conditions underlying the oxidative response are clearly delineated. Here, we show that store-operated Ca(2+) entry (SOCE) is required at the beginning of NADPH oxidase activation in response to fMLF (N-formyl-L-methionyl-L-leucyl-L-phenylalanine) in neutrophil-like HL-60 cells. When extracellular Ca(2+) is initially removed, early addition of Ca(2+) after stimulation causes a complete restoration of Ca(2+) entry and H(2)O(2) production. Both Ca(2+) entry and H(2)O(2) production are decreased by purported SOCE blockers, 2-aminoethoxydiphenyl borane (2-APB) and SK&F 96365. Endogenously expressed TRPC (transient receptor potential canonical) homologues and Orai1 were investigated for their role in supporting store-operated Ca(2+) channels activity. TRPC1, TRPC6 and Orai1 knock-out by siRNA resulted in the inhibition of Ca(2+) influx and H(2)O(2) production in response to fMLF and thapsigargin while suppression of TRPC3 had no effect on thapsigargin induced-SOCE. 2-APB and SK&F 96365 were able to amplify the reduction of fMLF-stimulated Ca(2+) entry and H(2)O(2) production observed in cells transfected by TRPC3 siRNA. In summary, Ca(2+) influx in HL-60 cells relies on different membrane TRPC channels and Orai1 for allowing NADPH oxidase activation. TRPC3 primarily mediates SOCE-independent pathways and TRPC1, TRPC6 and Orai1 exclusively contribute to SOCE.

Isoform-specific inhibition of TRPC4 channel by phosphatidylinositol 4,5-bisphosphate

Full-length transient receptor potential (TRP) cation channel TRPC4alpha and shorter TRPC4beta lacking 84 amino acids in the cytosolic C terminus are expressed in smooth muscle and endothelial cells where they regulate membrane potential and Ca(2+) influx. In common with other "classical" TRPCs, TRPC4 is activated by G(q)/phospholipase C-coupled receptors, but the underlying mechanism remains elusive. Little is also known about any isoform-specific channel regulation. Here we show that TRPC4alpha but not TRPC4beta was strongly inhibited by intracellularly applied phosphatidylinositol 4,5-bisphosphate (PIP(2)). In contrast, several other phosphoinositides (PI), including PI(3,4)P(2), PI(3,5)P(2), and PI(3,4,5)P(3), had no effect or even potentiated TRPC4alpha indicating that PIP(2) inhibits TRPC4alpha in a highly selective manner. We show that PIP(2) binds to the C terminus of TRPC4alpha but not that of TRPC4beta in vitro. Its inhibitory action was dependent on the association of TRPC4alpha with actin cytoskeleton as it was prevented by cytochalasin D treatment or by the deletion of the C-terminal PDZ-binding motif (Thr-Thr-Arg-Leu) that links TRPC4 to F-actin through the sodium-hydrogen exchanger regulatory factor and ezrin. PIP(2) breakdown appears to be a required step in TRPC4alpha channel activation as PIP(2) depletion alone was insufficient for channel opening, which additionally required Ca(2+) and pertussis toxin-sensitive G(i/o) proteins. Thus, TRPC4 channels integrate a variety of G-protein-dependent stimuli, including a PIP(2)/cytoskeleton dependence reminiscent of the TRPC4-like muscarinic agonist-activated cation channels in ileal myocytes.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

Genetic evidence supporting caveolae microdomain regulation of calcium entry in endothelial cells

Various cellular signals initiate calcium entry into cells, and there is evidence that lipid rafts and caveolae may concentrate proteins that regulate transmembrane calcium fluxes. Here, using mice deficient in caveolin-1 (Cav-1) and Cav-1 knock-out reconstituted with endothelium-specific Cav-1, we show that Cav-1 is essential for calcium entry in endothelial cells and governs the localization and protein-protein interactions between transient receptor channels C4 and C1. Thus, Cav-1 is required for calcium entry in vascular endothelial cells and perhaps other specialized cell types containing caveolae.

Non-selective cation channels, transient receptor potential channels and ischemic stroke

Several pathways to neural cell death are involved in ischemic stroke, and all require monovalent or divalent cation influx, implicating non-selective cation (NC) channels. NC channels are also likely to be involved in the dysfunction of vascular endothelial cells that leads to formation of edema following cerebral ischemia. Two newly described NC channels have emerged as potential participants in ischemic stroke, the acid sensing ion channel (ASIC), and the sulfonylurea receptor-1 (SUR1)-regulated NC(Ca-ATP) channel. Non-specific blockers of NC channels, including pinokalant (LOE 908 MS) and rimonabant (SR141716A), have beneficial effects in rodent models of ischemic stroke. Evidence is accumulating that NC channels formed by members of the transient receptor potential (TRP) family are also up-regulated in ischemic stroke and may play a direct role in calcium-mediated neuronal death. The nascent field of NC channels, including TRP channels, in ischemic stroke is poised to provide novel mechanistic insights and therapeutic strategies for this often devastating human condition.

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.

TRPC3 channels colocalize with Na+/Ca2+exchanger and Na+pump in axial component of transverse-axial tubular system of rat ventricle

Transient receptor potential canonical (TRPC) proteins form Ca2+-permeable, nonselective cation channels activated after stimulation of G protein-coupled membrane receptors linked to phospholipase C (PLC). Although the PLC/inositol phosphate signaling pathway is known to exist in heart, expression and subcellular distribution of TRPC channel proteins in ventricular myocardium have not been evaluated. Of the six members of the TRPC channel family examined here, only TRPC3 was found by Western blot analysis of membrane proteins from rodent or canine ventricle. Likewise, only TRPC3 was observed in immunofluorescence analysis of thin sections from rat ventricle. TRPC3 was also the only family member observed in neonatal rat ventricular myocytes in culture. In longitudinal sections of rat ventricle, TRPC3 was predominantly localized to the intercalated disk region of the myocyte. However, transverse sections through heart muscle or single isolated adult myocytes revealed TRPC3-specific labeling in a vast network of intracellular membranes, where it colocalized with the Na+-K+-ATPase (NKA) pump and the Na+/Ca2+exchanger (NCX) but not with the ryanodine receptor or the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pump. Reciprocal immunoprecipitation assays from rat or canine ventricle showed that TRPC3 associates with NKA and NCX but not with the plasmalemmal Ca2+-ATPase pump. Immunoprecipitations from Sf9 insect cells heterologously expressing TRPC3, NKA, and NCX in various combinations revealed that NKA and NCX interact and that TRPC3 and NCX interact, but that TRPC3 does not directly associate with NKA. Together, these results suggest that TRPC3 is localized in the ventricular myocyte to the axial component of the transverse-axial tubular system, where it exists in a signaling complex that includes NCX and NKA.

TRP channels in platelet function

Ca2+ entry forms an essential component of platelet activation; however, the mechanisms associated with this process are not understood. Ca2+ entry upon receptor activation occurs as a consequence of intracellular store depletion (referred to as store-operated Ca2+ entry or SOCE), a direct action of second messengers on cation entry channels or the direct occupancy of a ligand-gated P2(Xi) receptor. The molecular identity of the SOCE channel has yet to be established. Transient receptor potential (TRP) proteins are candidate cation entry channels and are classified into a number of closely related subfamilies including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin) and TRPML (mucolipins). From the TRPC family, platelets have been shown to express TRPC6 and TRPC1, and are likely to express other TRPC and other TRP members. TRPC6 is suggested to be involved with receptor-activated, diacyl-glycerol-mediated cation entry. TRPC1 has been suggested to be involved with SOCE, though many of the suggested mechanisms remain controversial. As no single TRP channel has the properties described for SOCE in platelets, it is likely that it is composed of a heteromeric association of TRP and related subunits, some of which may be present in intracellular compartments in the resting cell.

Characterization of the transient receptor potential channels mediating lysophosphatidic acid-stimulated calcium mobilization in B lymphoblasts

Altered 1-oleoyl-lysophosphatidic acid (LPA, 100 microM)-stimulated calcium responses occur in B-lymphoblast cell lines from bipolar disorder patients, but the mechanism(s) involved is uncertain. Lysophosphatidic acid shares a structurally similar fatty acid side chain with the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a known activator of subtypes 3, 6 and 7 of the canonical transient receptor potential (TRPC) cation channel subfamily. Accordingly, the objective of this study was to determine whether the LPA-stimulated calcium response in B-lymphoblasts is mediated, in part, through this TRPC channel subfamily. Divalent cation selectivity in response to thapsigargin, LPA and OAG were used to distinguish TRPC-like character of the responses to these agents in BLCLs. The sensitivity to gadolinium, an inhibitor of capacitative calcium channels, was used to determine the store-operated nature of the responses. The TRPC isoforms that are present in BLCLs as identified by immunoblotting and/or PCR include TRPC1, 3 and 5. Minimal barium influx in calcium-free buffer was observed following thapsigargin stimulation. However, LPA stimulated barium influx of a magnitude similar to that induced by OAG. Thapsigargin-provoked calcium influx was completely inhibited by gadolinium (10 microM), whereas LPA and OAG-stimulated responses were partially inhibited and potentiated, respectively. The results suggest that 100 microM LPA stimulates calcium entry through channels with characteristics similar to TRPC3, as TRPC6 and 7 are absent in B-lymphoblasts.

Recent developments in vascular endothelial cell transient receptor potential channels

Among the 28 identified and unique mammalian TRP (transient receptor potential) channel isoforms, at least 19 are expressed in vascular endothelial cells. These channels appear to participate in a diverse range of vascular functions, including control of vascular tone, regulation of vascular permeability, mechanosensing, secretion, angiogenesis, endothelial cell proliferation, and endothelial cell apoptosis and death. Malfunction of these channels may result in disorders of the human cardiovascular system. All TRP channels, except for TRPM4 and TRPM5, are cation channels that allow Ca2+ influx. However, there is a daunting diversity in the mode of activation and regulation in each case. Specific TRP channels may be activated by different stimuli such as vasoactive agents, oxidative stress, mechanical stimuli, and heat. TRP channels may then transform these stimuli into changes in the cytosolic Ca2+, which are eventually coupled to various vascular responses. Evidence has been provided to suggest the involvement of at least the following TRP channels in vascular function: TRPC1, TRPC4, TRPC6, and TRPV1 in the control of vascular permeability; TRPC4, TRPV1, and TRPV4 in the regulation of vascular tone; TRPC4 in hypoxia-induced vascular remodeling; and TRPC3, TRPC4, and TRPM2 in oxidative stress-induced responses. However, in spite of the large body of data available, the functional role of many endothelial TRP channels is still poorly understood. Elucidating the mechanisms regulating the different endothelial TRP channels, and the associated development of drugs selectively to target the different isoforms, as a means to treat cardiovascular disease should, therefore, be a high priority.

Identification and localization of TRPC channels in the rat kidney

It is well established that transient receptor potential (TRP) channels are activated following stimulation of G protein-coupled membrane receptors linked to PLC, but their differential expression in various cells of the renal nephron has not been described. In the present study, immunoprecipitations from rat kidney lysates followed by Western blot analysis using TRPC-specific, affinity-purified antibodies revealed the presence of TRPC1, -C3, and -C6. TRPC4, -C5, and -C7 were nondetectable. TRPC1 immunofluorescence was detected in glomeruli and specific tubular cells of the cortex and outer medulla. TRPC1 colocalized with aquaporin-1, a marker for proximal tubule and thin descending limb, but not with aquaporin-2, a marker for connecting tubule and collecting duct cells. TRPC3 and -C6 immunolabeling was predominantly confined to glomeruli and specific tubular cells of the cortex and both the outer and inner medulla. TRPC3 and -C6 colocalized with aquaporin-2, but not with the Na(+)/Ca(2+) exchanger or peanut lectin. Thus TRPC3 and -C6 proteins are expressed in principle cells of the collecting duct. In polarized cultures of M1 and IMCD-3 collecting duct cells, TRPC3 was localized exclusively to the apical domain, whereas TRPC6 was found in both the basolateral and apical membranes. TRPC3 and TRPC6 were also detected in primary podocyte cultures, whereas TRPC1 was exclusively expressed in mesangial cell cultures. Specific immunopositive labeling for TRPC4, -C5, or -C7 was not observed in kidney sections or cell lines. These results suggest that TRPC1, -C3, and -C6 may play a functional role in PLC-dependent signaling in specific regions of the nephron.

Functional role of TRPC proteins in native systems: implications from knockout and knock-down studies

Available data on transient receptor potential channel (TRPC) protein functions indicate that these proteins represent essential constituents of agonist-activated and phospholipase C-dependent cation entry pathways in primary cells which contribute to the elevation of cytosolic Ca2+. In addition, a striking number of biological functions have already been assigned to the various TRPC proteins, including mechanosensing activity (TRPC1), chemotropic axon guidance (TRPC1 and TRPC3), pheromone sensing and the regulation of sexual and social behaviour (TRPC2), endothelial-dependent regulation of vascular tone, endothelial permeability and neurotransmitter release (TRPC4), axonal growth (TRPC5), modulation of smooth muscle tone in blood vessels and lung and regulation of podocyte structure and function in the kidney (TRPC6). The lack of compounds which specifically block or activate TRPC proteins impairs the analysis of TRPC function in primary cells. We therefore concentrate in this contribution on (i) studies of TRPC-deficient mouse lines, (ii) data obtained by gene-silencing approaches using antisense oligonucleotides or RNA interference, (iii) expression experiments employing dominant negative TRPC constructs, and (iv) recent data correlating mutations of TRPC genes associated with human disease.