Caveolin-1 contributes to assembly of store-operated Ca2+ influx channels by regulating plasma membrane localization of TRPC1

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.

The role of caveolin-1 in cardiovascular regulation

Caveolae are omega-shaped membrane invaginations present in essentially all cell types in the cardiovascular system, and numerous functions have been ascribed to these structures. Caveolae formation depends on caveolins, cholesterol and polymerase I and transcript release factor-Cavin (PTRF-Cavin). The current review summarizes and critically discusses the cardiovascular phenotypes reported in caveolin-1-deficient mice. Major changes in the structure and function of heart, lung and blood vessels have been documented, suggesting that caveolae play a critical role at the interface between blood and surrounding tissue. According to an emerging paradigm, many of these changes are secondary to uncoupling of endothelial nitric oxide synthase. Thus, nitric oxide synthase not only synthesizes more nitric oxide in the absence of caveolin-1, but also more superoxide with potential pathogenic consequences. It is further argued that the vasodilating drive from increased nitric oxide production in caveolin-1-deficient mice is balanced by changes in the vascular media that favour increased dynamic resistance regulation. Harnessing the therapeutic opportunities buried in caveolae, while challenging, could expand the arsenal of treatment options in cancer, lung disease and atherosclerosis.

Altered central TRPV4 expression and lipid raft association related to inappropriate vasopressin secretion in cirrhotic rats

Inappropriate vasopressin (AVP) release causes dilutional hyponatremia in many pathophysiological states such as cirrhosis. The central molecular mechanisms that mediate inappropriate AVP release are unknown. We tested the hypothesis that changes in the expression or trafficking of TRPV4 in the central nervous system may contribute to inappropriate AVP release in the bile duct ligation (BDL) model of cirrhosis in the rat. Four weeks after surgery, BDL rats demonstrated significantly increased plasma vasopressin and plasma renin activity (PRA), hypervolemia, and decreased plasma osmolality. These effects were blocked by providing BDL rats with 2% saline to drink for 15 days. TRPV4 protein expression was significantly increased in brain punches from BDL rats containing the supraoptic nucleus (SON) of the hypothalamus (100% +/- 11 to 157% +/- 4.8), and this effect was blocked in BDL rats given saline. Immunohistochemistry demonstrated a significant increase in TRPV4-positive cells and the percentage of AVP neurons that also were TRPV4-positive in the SON of BDL rats. In the hypothalamus of BDL rats, TRPV4 lipid raft association increased compared with sham (from 100% +/- 2.1 to 326.1% +/- 16). This effect was significantly attenuated in BDL rats given 2% saline to drink (174% +/- 11). In the brain stem, TRPV4 lipid raft association was reduced by BDL and inversely related to plasma AVP and PRA. We speculate that changes in TRPV4 expression and compartmentalization within lipid rafts could contribute to a feed-forward mechanism related to AVP release in cirrhosis.

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.

Regulation of caveolin-1 membrane trafficking by the Na/K-ATPase

Here, we show that the Na/K-ATPase interacts with caveolin-1 (Cav1) and regulates Cav1 trafficking. Graded knockdown of Na/K-ATPase decreases the plasma membrane pool of Cav1, which results in a significant reduction in the number of caveolae on the cell surface. These effects are independent of the pumping function of Na/K-ATPase, and instead depend on interaction between Na/K-ATPase and Cav1 mediated by an N-terminal caveolin-binding motif within the ATPase α1 subunit. Moreover, knockdown of the Na/K-ATPase increases basal levels of active Src and stimulates endocytosis of Cav1 from the plasma membrane. Microtubule-dependent long-range directional trafficking in Na/K-ATPase–depleted cells results in perinuclear accumulation of Cav1-positive vesicles. Finally, Na/K-ATPase knockdown has no effect on processing or exit of Cav1 from the Golgi. Thus, the Na/K-ATPase regulates Cav1 endocytic trafficking and stabilizes the Cav1 plasma membrane pool.

TRPC1 binds to caveolin-3 and is regulated by Src kinase - role in Duchenne muscular dystrophy

Transient receptor potential canonical 1 (TRPC1), a widely expressed calcium (Ca(2+))-permeable channel, is potentially involved in the pathogenesis of Duchenne muscular dystrophy (DMD). Ca(2+) influx through stretch-activated channels, possibly formed by TRPC1, induces muscle-cell damage in the mdx mouse, an animal model of DMD. In this study, we showed that TRPC1, caveolin-3 and Src-kinase protein levels are increased in mdx muscle compared with wild type. TRPC1 and caveolin-3 colocalised and co-immunoprecipitated. Direct binding of TRPC1-CFP to caveolin-3-YFP was confirmed in C2 myoblasts by fluorescence energy resonance transfer (FRET). Caveolin-3-YFP targeted TRPC1-CFP to the plasma membrane. Hydrogen peroxide, a reactive oxygen species (ROS), increased Src activity and enhanced Ca(2+) influx, but only in C2 myoblasts co-expressing TRPC1 and caveolin-3. In mdx muscle, Tiron, a ROS scavenger, and PP2, a Src inhibitor, reduced stretch-induced Ca(2+) entry and increased force recovery. Because ROS production is increased in mdx/DMD, these results suggest that a ROS-Src-TRPC1/caveolin-3 pathway contributes to the pathogenesis of mdx/DMD.

Phorbol ester induced trafficking-independent regulation and enhanced phosphorylation of the dopamine transporter associated with membrane rafts and cholesterol

We examined the mechanisms involved in protein kinase C (PKC)-dependent down-regulation of dopamine transporter (DAT) activity and cell surface expression by treating heterologously expressing cells with the clathrin-mediated endocytosis inhibitor concanavalin A (Con A) or the cholesterol depleter/membrane raft disrupter methyl-beta-cyclodextrin (MbetaC) prior to treatment with the PKC activator phorbol 12-myristate, 13-acetate (PMA). Con A blocked PMA-induced surface reductions of DAT but only partially inhibited down-regulation, while MbetaC partially blocked down-regulation but did not inhibit loss of cell surface DAT, demonstrating that PKC-induced DAT down-regulation occurs by a combination of trafficking and non-trafficking processes. Using density-gradient centrifugation, we found that DATs are distributed approximately equally between Triton-insoluble, cholesterol-rich membrane rafts and Triton-soluble non-raft membranes. DATs in both populations are present at the cell surface and are active for dopamine and cocaine binding. PMA-induced loss of cell surface DAT occurred only from non-raft populations, demonstrating that non-raft DATs are regulated by trafficking events and indicating the likelihood that the cholesterol-dependent non-trafficking regulatory mechanism occurs in rafts. PMA did not affect the DAT raft-non-raft distribution but stimulated the phosphorylation of DAT to a substantially greater level in rafts than non-rafts. These findings reveal a previously unknown role for cholesterol in DAT function and demonstrate the presence of distinct subcellular DAT populations that possess multiple regulatory differences that may impact dopaminergic neurotransmission.

STIM1 converts TRPC1 from a receptor-operated to a store-operated channel: moving TRPC1 in and out of lipid rafts

While the role of members from the TRPC family of channels as receptor-operated channels (ROC) is well established and supported by numerous studies, the role of this family of channels as store-operated channels (SOC) has been the focus of a heated controversy over the last few years. In the present study, we have explored the modulation of STIM1 on human TRPC1 channel. We show that the association of STIM1 to TRPC1 favors the insertion of TRPC1 into lipid rafts, where TRPC1 functions as a SOC. In the absence of STIM1, TRPC1 associates to other members from the TRPC family of channels to form ROCs. A novel TIRFM-FRET method illustrates the relevance of the dynamic association between STIM1 and TRPC1 for the activation of SOC and the lipid raft localization of the STIM1-TRPC1 complex. This study provides new evidence about the dual activity of TRPC1 (forming ROC or SOC) and the partners needed to determine TRPC1 functional fate. It highlights also the role of plasma membrane microdomains and ER-PM junctions in modulating TRPC1 channel function and its association to STIM1.

Caveolae/caveolin-1 are important modulators of store-operated calcium entry in Hs578/T breast cancer cells

Caveolin-1 is a principal component of caveolae, invaginations of the plasma membrane that are enriched in cholesterol and sphingolipids. The expression of caveolin-1 has been shown to be tightly correlated to the progression of breast cancer tumors. However, the consequences of altered caveolin-1 expression during tumor progression still remain unclear. Modification of caveolin-1 expression modulates store-operated Ca(2+) entry (SOCE) in various cell types. SOCE is a ubiquitous Ca(2+) entry pathway that previous studies have linked to apoptosis and tumor progression in prostate cancer cells. In this study, we tested the effect of altering caveolin-1 expression on SOCE in Hs578/T breast cancer cells. Through overexpression of caveolin-1 and small hairpin RNA (shRNA) knockdown, we generated four stable cell lines that have 3 different caveolin-1 protein levels. Cav-1 overexpression could increase SOCE activity, while knockdown of caveolin-1 significantly reduced SOCE activity. These functional consequences were correlated with changes in caveolae number in Hs578/T cells. Our results suggest alteration of SOCE by caveolin-1 expression changes could be one of the mechanisms contributing to the progression of breast cancer.

Lipid rafts determine clustering of STIM1 in endoplasmic reticulum-plasma membrane junctions and regulation of store-operated Ca2+ entry (SOCE)

Store depletion induces STIM1 to aggregate and relocate into clusters at ER-plasma membrane junctions where it functionally interacts with and activates plasma membrane channels that mediate store-operated Ca(2+) entry (SOCE). Thus, the site of peripheral STIM1 clusters is critical for the regulation of SOCE. However, what determines the location of the STIM1 clusters in the ER-PM junctional regions, and whether these represent specific sites in the cell is not yet known. Here we report that clustering of STIM1 in the subplasma membrane region of the cell and activation of TRPC1-dependent SOCE are determined by lipid raft domains (LRD). We show that store depletion increased partitioning of TRPC1 and STIM1 into plasma membrane LRD. TRPC1 and STIM1 associated with each other within the LRD, and this association was dynamically regulated by the status of the ER Ca(2+) store. Peripheral STIM1 clustering was independent of TRPC1. However, sequestration of membrane cholesterol attenuated thapsigargin-induced clustering of STIM1 as well as SOCE in HSG and HEK293 cells. Recruitment and association of STIM1 and TRPC1 in LRD was also decreased. Additionally STIM1(D76A), which is peripherally localized and constitutively activates SOCE in unstimulated cells, displayed a relatively higher partitioning into LRD and interaction with TRPC1, as compared with STIM1. Disruption of membrane rafts decreased peripheral STIM1(D76A) puncta, its association with TRPC1 and the constitutive SOCE. Together, these data demonstrate that intact LRD determine targeting of STIM1 clusters to ER-plasma membrane junctions following store depletion. This facilitates the functional interaction of STIM1 with TRPC1 and activation of SOCE.

HNF4 alpha and the Ca-channel TRPC1 are novel disease candidate genes in diabetic nephropathy

OBJECTIVE

The nuclear receptor hepatic nuclear factor 4 alpha (HNF 4 alpha) is a master regulatory protein and an essential player in the control of a wide range of metabolic processes. Dysfunction of HNF 4 alpha is associated with metabolic disorders including diabetes. We were particularly interested in investigating molecular causes associated with diabetic nephropathy.

RESEARCH DESIGN AND METHODS

Novel disease candidate genes were identified by the chromatin immunoprecipitation-cloning assay and by sequencing of immunoprecipitated DNA. Expression of candidate genes was analyzed in kidney and liver of Zucker diabetic fatty (ZDF) and of streptozotocin (STZ)-administered rats and after siRNA-mediated silencing of HNF 4 alpha.

RESULTS

We identified the calcium-permeable nonselective transient receptor potential cation channel, subfamily C, member 1 (TRPC1) as a novel HNF 4 alpha gene target. Strikingly, TRPC1 is localized on human chromosome 3q22-24, i.e., a region considered to be a hotspot for diabetic nephropathy. We observed a significant reduction of TRPC1 gene expression in kidney and liver of diabetic ZDF and of STZ-administered rats as a result of HNF 4 alpha dysfunction. We found HNF 4 alpha and TRPC1 protein expression to be repressed in kidneys of diabetic patients diagnosed with nodular glomerulosceloris as evidenced by immunohistochemistry. Finally, siRNA-mediated functional knock down of HNF 4 alpha repressed TRPC1 gene expression in cell culture experiments.

CONCLUSIONS

Taken collectively, results obtained from animal studies could be translated to human diabetic nephropathy; there is evidence for a common regulation of HNF 4 alpha and TRPC1 in human and rat kidney pathologies. We propose dysregulation of HNF 4 alpha and TRPC1 as a possible molecular rationale in diabetic nephropathy.

Role of caveolar compartmentation in endothelium-derived hyperpolarizing factor-mediated relaxation: Ca2+ signals and gap junction function are regulated by caveolin in endothelial cells

BACKGROUND

In endothelial cells, caveolin-1, the structural protein of caveolae, acts as a scaffolding protein to cluster lipids and signaling molecules within caveolae and, in some instances, regulates the activity of proteins targeted to caveolae. Specifically, different putative mediators of the endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation are located in caveolae and/or regulated by the structural protein caveolin-1, such as potassium channels, calcium regulatory proteins, and connexin 43, a molecular component of gap junctions.

METHODS AND RESULTS

Comparing relaxation in vessels from caveolin-1 knockout mice and their wild-type littermates, we observed a complete absence of EDHF-mediated vasodilation in isolated mesenteric arteries from caveolin-1 knockout mice. The absence of caveolin-1 is associated with an impairment of calcium homeostasis in endothelial cells, notably, a decreased activity of Ca2+-permeable TRPV4 cation channels that participate in nitric oxide- and EDHF-mediated relaxation. Moreover, morphological characterization of caveolin-1 knockout and wild-type arteries showed fewer gap junctions in vessels from knockout animals associated with a lower expression of connexins 37, 40, and 43 and altered myoendothelial communication. Finally, we showed that TRPV4 channels and connexins colocalize with caveolin-1 in the caveolar compartment of the plasma membrane.

CONCLUSIONS

We demonstrated that expression of caveolin-1 is required for EDHF-related relaxation by modulating membrane location and activity of TRPV4 channels and connexins, which are both implicated at different steps in the EDHF-signaling pathway.

Caveolin-1 and -2 interact with connexin43 and regulate gap junctional intercellular communication in keratinocytes

Connexin43 (Cx43) has been reported to interact with caveolin (Cav)-1, but the role of this association and whether other members of the caveolin family bind Cx43 had yet to be established. In this study, we show that Cx43 coimmunoprecipitates and colocalizes with Cav-1 and Cav-2 in rat epidermal keratinocytes. The colocalization of Cx43 with Cav-1 was confirmed in keratinocytes from human epidermis in vivo. Our mutation and Far Western analyses revealed that the C-terminal tail of Cx43 is required for its association with Cavs and that the Cx43/Cav-1 interaction is direct. Our results indicate that newly synthesized Cx43 interacts with Cavs in the Golgi apparatus and that the Cx43/Cavs complex also exists at the plasma membrane in lipid rafts. Using overexpression and small interfering RNA approaches, we demonstrated that caveolins regulate gap junctional intercellular communication (GJIC) and that the presence of Cx43 in lipid raft domains may contribute to the mechanism modulating GJIC. Our results suggest that the Cx43/Cavs association occurs during exocytic transport, and they clearly indicate that caveolin regulates GJIC.

Caveolin regulates endocytosis of the muscle repair protein, dysferlin

Dysferlin and Caveolin-3 are plasma membrane proteins associated with muscular dystrophy. Patients with mutations in the CAV3 gene show dysferlin mislocalization in muscle cells. By utilizing caveolin-null cells, expression of caveolin mutants, and different mutants of dysferlin, we have dissected the site of action of caveolin with respect to dysferlin trafficking pathways. We now show that Caveolin-1 or -3 can facilitate exit of a dysferlin mutant that accumulates in the Golgi complex of Cav1(-/-) cells. In contrast, wild type dysferlin reaches the plasma membrane but is rapidly endocytosed in Cav1(-/-) cells. We demonstrate that the primary effect of caveolin is to cause surface retention of dysferlin. Caveolin-1 or Caveolin-3, but not specific caveolin mutants, inhibit endocytosis of dysferlin through a clathrin-independent pathway colocalizing with internalized glycosylphosphatidylinositol-anchored proteins. Our results provide new insights into the role of this endocytic pathway in surface remodeling of specific surface components. In addition, they highlight a novel mechanism of action of caveolins relevant to the pathogenic mechanisms underlying caveolin-associated disease.

Association of PepT1 with lipid rafts differently modulates its transport activity in polarized and nonpolarized cells

The transporter PepT1, apically expressed in intestinal epithelial cells, is responsible for the uptake of di/tripeptides. PepT1 is also expressed in nonpolarized immune cells. Here we investigated the localization of PepT1 in lipid rafts in small intestinal brush border membranes (BBMs) and polarized and nonpolarized cells, as well as functional consequences of the association of PepT1 with lipid rafts. Immunoblot analysis showed the presence of PepT1 in low-density fractions isolated from mouse intestinal BBMs, polarized intestinal Caco2-BBE cells, and nonpolarized Jurkat cells by solubilization in ice-cold 0.5% Triton X-100 and sucrose gradient fractionation. PepT1 colocalized with lipid raft markers GM1 and N-aminopeptidase in intestinal BBMs and Caco2-BBE cell membranes. Disruption of lipid rafts with methyl-beta-cyclodextrin (MbetaCD) shifted PepT1 from low- to high-density fractions. Remarkably, we found that MbetaCD treatment increased PepT1 transport activity in polarized intestinal epithelia but decreased that in intestinal BBM vesicles and nonpolarized immune cells. Mutational analysis showed that phenylalanine 293, phenylalanine 297, and threonine 281 in transmembrane segment 7 of the human di/tripeptide transporter, hPepT1, are important for the targeting to lipid rafts and transport activity of hPepT1. In conclusion, the association of PepT1 with lipid rafts differently modulates its transport activity in polarized and nonpolarized cells.

Caveolin regulates kv1.5 trafficking to cholesterol-rich membrane microdomains

The targeting of ion channels to cholesterol-rich membrane microdomains has emerged as a novel mechanism of ion channel localization. Previously, we reported that Kv1.5, a prominent cardiovascular K(+) channel alpha-subunit, localizes to caveolar microdomains. However, the mechanisms regulating Kv1.5 targeting and the functional significance of this localization are largely unknown. In this study, we demonstrate a role for caveolin in the trafficking of Kv1.5 to lipid raft microdomains where cholesterol modulates channel function. In cells lacking endogenous caveolin-1 or -3, the association of Kv1.5 with low-density, detergent-resistant membrane fractions requires coexpression with exogenous caveolin, which can form channel-caveolin complexes. Caveolin is not required for cell surface expression, however, and caveolin-trafficking mutants sequester Kv1.5, but not Kv2.1, in intracellular compartments, resulting in a loss of functional cell surface channel. Coexpression with wild type caveolin-1 does not alter Kv1.5 current density; rather, it induces depolarizing shifts in steady-state activation and inactivation. These shifts are analogous to those produced by elevation of membrane cholesterol. Together, these results show that caveolin modulates channel function by regulating trafficking to cholesterol-rich membrane microdomains.

Characterization of the alpha1-adrenoceptor subtype activating extracellular signal-regulated kinase in submandibular gland acinar cells

Alpha(1)-Adrenoceptors and extracellular signal-regulated kinases 1 and 2 (ERK1/2) regulate salivary secretion. However, whether alpha(1)-adrenoceptors couple to ERK1/2 activation and the specific alpha(1)-adrenoceptor subtypes involved in salivary glands is unknown. Western blotting of ERK1/2 phosphorylation showed phenylephrine activated ERK1/2 by 2-3-fold in submandibular gland slices and 3-4-fold in submandibular acinar (SMG-C10) cells with an EC(50) of 2.7+/-2 microM. ERK1/2 activation was blocked by either prazosin or HEAT, indicating alpha(1)-adrenoceptors stimulate ERK1/2 in native glands and SMG-C10 cells. Inhibition of [(125)I]HEAT binding by 5-methylurapidil (selective for alpha(1A) over alpha(1B/)alpha(1D)), but not BMY 7378 (selective for alpha(1D) over alpha(1A/)alpha(1B)), was biphasic and best-fit by a two-site binding model with K(i)(H) and K(i)(L) values for 5-methylurapidil of 0.64+/-0.3 and 91+/-7 nM, respectively, in SMG-C10 membranes. From these binding data, we obtained subtype-selective concentrations of 5-methylurapidil to determine the alpha(1)-adrenoceptor subtype/s activating ERK1/2 in SMG-C10 cells. 5-methylurapidil (20 nM) did not affect phenylephrine- or A-61603- (alpha(1A)-selective agonist) induced ERK1/2 activation; whereas, 30 microM chloroethylclonidine (alpha(1B)-selective antagonist) inhibited ERK1/2 activation by phenylephrine, indicating alpha(1B)-adrenoceptors, but not alpha(1A)-adrenoceptors, activate ERK1/2 in submandibular cells. We also examined alpha(1)-adrenoceptor location and dependence on cholesterol-rich microdomains for activating ERK1/2. Sucrose density gradient centrifugation showed 71+/-3% of alpha(1)-adrenoceptor binding sites were in plasma membranes. Cholesterol-disrupting agents filipin and methyl-beta-cyclodextrin inhibited phenylephrine-stimulated ERK1/2. These results show only alpha(1B)-adrenoceptors activate ERK1/2 and suggest subtype-specific ERK1/2 signaling by alpha(1B)-adrenoceptors may be determined by localization to cholesterol-rich microdomains in submandibular cells.

TRPC1: The link between functionally distinct store-operated calcium channels

Although store-operated calcium entry (SOCE) was identified more that two decades ago, understanding the molecular mechanisms that regulate and mediate this process continue to pose a major challenge to investigators in this field. Thus, there has been major focus on determining which of the models proposed for this mechanism is valid and conclusively establishing the components of the store-operated calcium (SOC) channel(s). The transient receptor potential canonical (TRPC) proteins have been suggested as candidate components of the elusive store-operated Ca(2+) entry channel. While all TRPCs are activated in response to agonist-stimulated phosphatidylinositol 4,5, bisphosphate (PIP(2)) hydrolysis, only some display store-dependent regulation. TRPC1 is currently the strongest candidate component of SOC and is shown to contribute to SOCE in many cell types. Heteromeric interactions of TRPC1 with other TRPCs generate diverse SOC channels. Recent studies have revealed novel components of SOCE, namely the stromal interacting molecule (STIM) and Orai proteins. While STIM1 has been suggested to be the ER-Ca(2+) sensor protein relaying the signal to the plasma membrane for activation of SOCE, Orai1 is reported to be the pore-forming component of CRAC channel that mediates SOCE in T-lymphocytes and other hematopoetic cells. Several studies now demonstrate that TRPC1 also associates with STIM1 suggesting that SOC and CRAC channels are regulated by similar molecular components. Interestingly, TRPC1 is also associated with Orai1 and a TRPC1-Orai1-STIM1 ternary complex contributes to SOC channel function. This review will focus on the diverse SOC channels formed by TRPC1 and the suggestion that TRPC1 might serve as a molecular link that determines their regulation by store-depletion.

TRP channels in hypertension

Pulmonary and systemic arterial hypertension are associated with profound alterations in Ca(2+) homeostasis and smooth muscle cell proliferation. A novel class of non-selective cation channels, the transient receptor potential (TRP) channels, have emerged at the forefront of research into hypertensive disease states. TRP channels are identified as molecular correlates for receptor-operated and store-operated cation channels in the vasculature. Over 10 TRP isoforms are identified at the mRNA and protein expression levels in the vasculature. Current research implicates upregulation of specific TRP isoforms to be associated with increased Ca(2+) influx, characteristic of vasoconstriction and vascular smooth muscle cell proliferation. TRP channels are implicated as Ca(2+) entry pathways in pulmonary hypertension and essential hypertension. Caveolae have recently emerged as membrane microdomains in which TRP channels may be co-localized with the endoplasmic reticulum in both smooth muscle and endothelial cells. Such enhanced expression and function of TRP channels and their localization in caveolae in pathophysiological hypertensive disease states highlights their importance as potential targets for pharmacological intervention.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

TRPC1: a core component of store-operated calcium channels

The TRPC (transient receptor potential canonical) proteins are activated in response to agonist-stimulated PIP(2) (phosphatidylinositol 4,5-bisphosphate) hydrolysis and have been suggested as candidate components of the elusive SOC (store-operated calcium channel). TRPC1 is currently the strongest candidate component of SOC. Endogenous TRPC1 has been shown to contribute to SOCE (store-operated calcium entry) in several different cell types. However, the mechanisms involved in the regulation of TRPC1 and its exact physiological function have yet to be established. Studies from our laboratory and several others have demonstrated that TRPC1 is assembled in a signalling complex with key calcium signalling proteins in functionally specific plasma membrane microdomains. Furthermore, critical interactions between TRPC1 monomers as well as interactions between TRPC1 and other proteins determine the surface expression and function of TRPC1-containing channels. Recent studies have revealed novel regulators of TRPC1-containing SOCs and have demonstrated a common molecular basis for the regulation of CRAC (calcium-release-activated calcium) and SOC channels. In the present paper, we will revisit the role of TRPC1 in SOCE and discuss how studies with TRPC1 provide an experimental basis for validating the mechanism of SOCE.

Hormonal signaling and signal pathway crosstalk in the control of myometrial calcium dynamics

Understanding the basis for the control of myometrial contractant and relaxant signaling pathways is important to understanding how to manage myometrial contractions. Signaling pathways are influenced by the level of expression of the signals and signal pathway components, the location of these components in the appropriate subcellular environment, and covalent modification. Crosstalk between these pathways regulates the effectiveness of signal transduction and represents an important way by which hormones can regulate phenotype. This review deals primarily with signaling pathways that control Ca2+ entry and intracellular release, as well as the interplay between these pathways.

Caveolin-1 influences P2X7 receptor expression and localization in mouse lung alveolar epithelial cells

The P2X7 receptor has recently been described as a marker for lung alveolar epithelial type I cells. Here, we demonstrate both the expression of P2X7 protein and its partition into lipid rafts in the mouse lung alveolar epithelial cell line E10. A significant degree of colocalization was observed between P2X7 and the raft marker protein Caveolin-1; also, P2X7 protein was associated with caveolae. A marked reduction in P2X7 immunoreactivity was observed in lung sections prepared from Caveolin-1-knockout mice, indicating that Caveolin-1 expression was required for full expression of P2X7 protein. Indeed, suppression of Caveolin-1 protein expression in E10 cells using short hairpin RNAs resulted in a large reduction in P2X7 protein expression. Our data demonstrate a potential interaction between P2X7 protein and Caveolin-1 in lipid rafts, and provide a basis for further functional and biochemical studies to probe the physiologic significance of this interaction.

Organization and function of TRPC channelosomes

TRPC proteins constitute a family of conserved Ca2+-permeable cation channels which are activated in response to agonist-stimulated PIP2 hydrolysis. These channels were initially proposed to be components of the store-operated calcium entry channel (SOC). Subsequent studies have provided substantial evidence that some TRPCs contribute to SOC activity. TRPC proteins have also been shown to form agonist-stimulated calcium entry channels that are not store-operated but are likely regulated by PIP2 or diacylglycerol. Further, and consistent with the presently available data, selective homomeric or heteromeric interactions between TRPC monomers generate distinct agonist-stimulated cation permeable channels. We suggest that interaction between TRPC monomers, as well as the association of these channels with accessory proteins, determines their mode of regulation as well as their cellular localization and function. Currently identified accessory proteins include key Ca2+ signaling proteins as well as proteins involved in vesicle trafficking, cytoskeletal interactions, and scaffolding. Studies reported until now demonstrate that TRPC proteins are segregated into specific Ca2+ signaling complexes which can generate spatially and temporally controlled [Ca2+]i signals. Thus, the functional organization of TRPC channelosomes dictates not only their regulation by extracellular stimuli but also serves as a platform to coordinate specific downstream cellular functions that are regulated as a consequence of Ca2+ entry. This review will focus on the accessory proteins of TRPC channels and discuss the functional implications of TRPC channelosomes and their assembly in microdomains.

Increased smooth muscle cell expression of caveolin-1 and caveolae contribute to the pathophysiology of idiopathic pulmonary arterial hypertension

Vasoconstriction and vascular medial hypertrophy, resulting from increased intracellular [Ca2+] in pulmonary artery smooth muscle cells (PASMC), contribute to elevated vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Caveolae, microdomains within the plasma membrane, contain the protein caveolin, which binds certain signaling molecules. We tested the hypothesis that PASMC from IPAH patients express more caveolin-1 (Cav-1) and caveolae, which contribute to increased capacitative Ca2+ entry (CCE) and DNA synthesis. Immunohistochemistry showed increased expression of Cav-1 in smooth muscle cells but not endothelial cells of pulmonary arteries from patients with IPAH. Subcellular fractionation and electron microscopy confirmed the increase in Cav-1 and caveolae expression in IPAH-PASMC. Treatment of IPAH-PASMC with agents that deplete membrane cholesterol (methyl-beta-cyclodextrin or lovastatin) disrupted caveolae, attenuated CCE, and inhibited DNA synthesis of IPAH-PASMC. Increasing Cav-1 expression of normal PASMC with a Cav-1-encoding adenovirus increased caveolae formation, CCE, and DNA synthesis; treatment of IPAH-PASMC with siRNA targeted to Cav-1 produced the opposite effects. Treatments that down-regulate caveolin/caveolae expression, including cholesterol-lowering drugs, reversed the increased CCE and DNA synthesis in IPAH-PASMC. Increased caveolin and caveolae expression thus contribute to IPAH-PASMC pathophysiology. The close relationship between caveolin/caveolae expression and altered cell physiology in IPAH contrast with previous results obtained in various animal models, including caveolin-knockout mice, thus emphasizing unique features of the human disease. The results imply that disruption of caveolae in PASMC may provide a novel therapeutic approach to attenuate disease manifestations of IPAH.

TRPCs as MS Channels

This chapter reviews recent evidence indicating that canonical or classical transient receptor potential (TRPC) channels are directly or indirectly mechanosensitive (MS) and can therefore be designated as mechano-operated channels (MOCs). The MS functions of TRPCs may be mechanistically related to their better known functions as store-operated and receptor-operated channels (SOCs and ROCs). Mechanical forces may be conveyed to TRPC channels through the "conformational coupling" mechanism that transmits information regarding the status of internal Ca(2+) stores. All TRPCs are regulated by receptors coupled to phospholipases that are themselves MS and can regulate channels via lipidic second messengers. Accordingly, there may be several nonexclusive mechanisms by which mechanical forces may regulate TRPC channels, including direct sensitivity to bilayer mechanics, physical coupling to internal membranes and/or cytoskeletal proteins, and sensitivity to lipidic second messengers generated by MS enzymes. Various strategies that can be used for separating out different MS-gating mechanisms and their possible role in specific TRPCs are discussed.

MscCa Regulation of Tumor Cell Migration and Metastasis

The acquisition of cell motility is a required step in order for a cancer cell to migrate from the primary tumor and spread to secondary sites (metastasize). For this reason, blocking tumor cell migration is considered a promising approach for preventing the spread of cancer. However, cancer cells just as normal cells can migrate by several different modes referred to as "amoeboid," "mesenchymal," and "collective cell." Under appropriate conditions, a single cell can switch between modes. A consequence of this plasticity is that a tumor cell may be able to avoid the effects of an agent that targets only one mode by switching modes. Therefore, a preferred strategy would be to target mechanisms that are shared by all modes. This chapter reviews the evidence that Ca(2+) influx via the mechanosensitive Ca(2+)-permeable channel (MscCa) is a critical regulator of all modes of cell migration and therefore represents a very good therapeutic target to block metastasis.

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.

The multiple faces of caveolae

Caveolae are a highly abundant but enigmatic feature of mammalian cells. They form remarkably stable membrane domains at the plasma membrane but can also function as carriers in the exocytic and endocytic pathways. The apparently diverse functions of caveolae, including mechanosensing and lipid regulation, might be linked to their ability to respond to plasma membrane changes, a property that is dependent on their specialized lipid composition and biophysical properties.

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.

Caveolin-1 and caveolin-3 regulate Ca2+ homeostasis of single smooth muscle cells from rat cerebral resistance arteries

The role of caveolins, signature proteins of caveolae, in arterial Ca(2+) regulation is unknown. We investigated modulation of Ca(2+) homeostasis by caveolin-1 and caveolin-3 using smooth muscle cells from rat cerebral resistance arteries. Membrane current and Ca(2+) transients were simultaneously measured with voltage-clamped single cells. Membrane depolarization triggered Ca(2+) current and increased intracellular Ca(2+) concentration ([Ca(2+)](i)). After repolarization, elevated [Ca(2+)](i) returned to the resting level. Ca(2+) removal rate was determined from the declining phase of the Ca(2+) transient. Application of caveolin-1 antibody or caveolin-1 scaffolding domain peptide, corresponding to amino acid residues 82-101 of caveolin-1, significantly slowed Ca(2+) removal rate at a measured [Ca(2+)](i) of 250 nM, with little effect at a measured [Ca(2+)](i) of 600 nM. Application of caveolin-3 antibody or caveolin-3 scaffolding domain peptide, corresponding to amino acid residues 55-74 of caveolin-3, also significantly slowed Ca(2+) removal rate at a measured [Ca(2+)](i) of 250 nM, with little effect at a measured [Ca(2+)](i) of 600 nM. Likewise, application of calmodulin inhibitory peptide, autocamtide-2-related inhibitory peptide, and cyclosporine A, inhibitors for calmodulin, Ca(2+)/calmodulin-dependent protein kinase II, and calcineurin, also significantly inhibited Ca(2+) removal rate at a measured [Ca(2+)](i) of 250 nM but not at 600 nM. Application of cyclopiazonic acid, a sarcoplasmic reticulum Ca(2+) ATPase inhibitor, also significantly inhibited Ca(2+) removal rate at a measured [Ca(2+)](i) of 250 nM but not at 600 nM. Our results suggest that caveolin-1 and caveolin-3 are important in Ca(2+) removal of resistance artery smooth muscle cells.

Loss of caveolin-3 induced by the dystrophy-associated P104L mutation impairs L-type calcium channel function in mouse skeletal muscle cells

Caveolins are membrane scaffolding proteins that associate with and regulate a variety of signalling proteins, including ion channels. A deficiency in caveolin-3 (Cav-3), the major striated muscle isoform, is responsible for skeletal muscle disorders, such as limb-girdle muscular dystrophy 1C (LGMD 1C). The molecular mechanisms leading to the muscle wasting that characterizes this pathology are poorly understood. Here we show that a loss of Cav-3 induced by the expression of the LGMD 1C-associated mutant P104L (Cav-3(P104L)) provokes a reduction by half of the maximal conductance of the voltage-dependent L-type Ca(2+) channel in mouse primary cultured myotubes and fetal skeletal muscle fibres. Confocal immunomiscrocopy indicated a colocalization of Cav-3 and Ca(v)1.1, the pore-forming subunit of the L-type Ca(2+) channel, at the surface membrane and in the developing T-tubule network in control myotubes and fetal fibres. In myotubes expressing Cav-3(P104L), the loss of Cav-3 was accompanied by a 66% reduction in Ca(v)1.1 mean labelling intensity. Our results suggest that Cav-3 is involved in L-type Ca(2+) channel membrane function and localization in skeletal muscle cells and that an alteration of L-type Ca(2+) channels could be involved in the physiopathological mechanisms of caveolinopathies.

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

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

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.

Mechanosensitive channel properties and membrane mechanics in mouse dystrophic myotubes

Muscular dystrophy is associated with increased activity of mechanosensitive channels (MSCs) and increased cell calcium levels. MSCs in patches from mdx mouse myotubes have higher levels of resting activity, compared to patches from wild-type mice, and a pronounced latency of activation and deactivation. Measurements of patch capacitance and geometry reveal that the differences are linked to cortical membrane mechanics rather than to differences in channel gating. We found unexpectedly that patches from mdx mice are strongly curved towards the pipette tip by actin pulling normal to the membrane. This force produces a substantial tension (approximately 5 mN m(-1)) that can activate MSCs in the absence of overt stimulation. The inward curvature of patches from mdx mice is eliminated by actin inhibitors. Applying moderate suction to the pipette flattens the membrane, reducing tension, and making the response appear to be stretch inactivated. The pronounced latency to activation in patches from mdx mice is caused by the mechanical relaxation time required to reorganize the cortex from inward to outward curvature. The increased latency is equivalent to a three-fold increase in cortical viscosity. Disruption of the cytoskeleton by chemical or mechanical means eliminates the differences in kinetics and curvature between patches from wild-type and mdx mice. The stretch-induced increase in specific capacitance of the patch, approximately 80 fF microm(-2), far exceeds the specific capacitance of bilayers, suggesting the presence of stress-sensitive access to large pools of membrane, possibly caveoli, T-tubules or portions of the gigaseal. In mdx mouse cells the intrinsic gating property of fast voltage-sensitive inactivation is lost. It is robust in wild-type mouse cells (observed in 50% of outside-out patches), but never observed in mdx cells. This link between dystrophin and inactivation may lead to increased background cation currents and Ca2+ influx. Spontaneous Ca2+ transients in mdx mouse cells are sensitive to depolarization and are inhibited by the specific MSC inhibitor GsMTx4, in both the D and L forms.

Dynamic assembly of TRPC1-STIM1-Orai1 ternary complex is involved in store-operated calcium influx. Evidence for similarities in store-operated and calcium release-activated calcium channel components

Store-operated calcium entry (SOCE) is a ubiquitous mechanism that is mediated by distinct SOC channels, ranging from the highly selective calcium release-activated Ca2+ (CRAC) channel in rat basophilic leukemia and other hematopoietic cells to relatively Ca2+-selective or non-selective SOC channels in other cells. Although the exact composition of these channels is not yet established, TRPC1 contributes to SOC channels and regulation of physiological function of a variety of cell types. Recently, Orai1 and STIM1 have been suggested to be sufficient for generating CRAC channels. Here we show that Orai1 and STIM1 are also required for TRPC1-SOC channels. Knockdown of TRPC1, Orai1, or STIM1 attenuated, whereas overexpression of TRPC1, but not Orai1 or STIM1, induced an increase in SOC entry and I(SOC) in human salivary gland cells. All three proteins were co-localized in the plasma membrane region of cells, and thapsigargin increased co-immunoprecipitation of TRPC1 with STIM1, and Orai1 in human salivary gland cells as well as dispersed mouse submandibular gland cells. In aggregate, the data presented here reveal that all three proteins are essential for generation of I(SOC) in these cells and that dynamic assembly of TRPC1-STIM1-Orai1 ternary complex is involved in activation of SOC channel in response to internal Ca2+ store depletion. Thus, these data suggest a common molecular basis for SOC and CRAC channels.

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.

Trafficking of TRP channels: determinants of channel function

Transient receptor potential (TRP) channels are members of a relatively newly described family of cation channels that display a wide range of properties and mechanisms of activation. The exact physiological function and regulation of most of these channels have not yet been conclusively determined. Studies over the past decade have revealed important features of the channels that contribute to their function. These include homomeric interactions between TRP monomers, selective heteromeric interactions within members of the same subfamily, interactions of TRPs with accessory proteins and assembly into macromolecular signaling complexes, and regulation within functionally distinct cellular microdomains. Further, distinct constitutive and regulated vesicular trafficking mechanisms have a critical role not only in controlling the surface expression of TRP channels but also their activation in response to stimuli. A number of cellular components such as cytoskeletal and scaffolding proteins also contribute to TRP channel trafficking. Thus, mechanisms involved in the assembly and trafficking of TRP channels control their plasma membrane expression and critically impact their function and 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.

Functional organization of TRPC-Ca2+ channels and regulation of calcium microdomains

TRP family of proteins are components of unique cation channels that are activated in response to diverse stimuli ranging from growth factor and neurotransmitter stimulation of plasma membrane receptors to a variety of chemical and sensory signals. This review will focus on members of the TRPC sub-family (TRPC1-TRPC7) which currently appear to be the strongest candidates for the enigmatic Ca(2+) influx channels that are activated in response to stimulation of plasma membrane receptors which result in phosphatidyl inositol-(4,5)-bisphosphate (PIP(2)) hydrolysis, generation of IP(3) and DAG, and IP(3)-induced Ca(2+) release from the intracellular Ca(2+) store via inositol trisphosphate receptor (IP(3)R). Homomeric or selective heteromeric interactions between TRPC monomers generate distinct channels that contribute to store-operated as well as store-independent Ca(2+) entry mechanisms. The former is regulated by the emptying/refilling of internal Ca(2+) store(s) while the latter depends on PIP(2) hydrolysis (due to changes in PIP(2) per se or an increase in diacylglycerol, DAG). Although the exact physiological function of TRPC channels and how they are regulated has not yet been conclusively established, it is clear that a variety of cellular functions are controlled by Ca(2+) entry via these channels. Thus, it is critical to understand how cells coordinate the regulation of diverse TRPC channels to elicit specific physiological functions. It is now well established that segregation of TRPC channels mediated by interactions with signaling and scaffolding proteins, determines their localization and regulation in functionally distinct cellular domains. Furthermore, both protein and lipid components of intracellular and plasma membranes contribute to the organization of these microdomains. Such organization serves as a platform for the generation of spatially and temporally dictated [Ca(2+)](i) signals which are critical for precise control of downstream cellular functions.

PAR-1-stimulated factor IXa binding to a small platelet subpopulation requires a pronounced and sustained increase of cytoplasmic calcium

We previously reported that only a subpopulation of PAR-1-stimulated platelets binds coagulation factor IXa, since confirmed by other laboratories. Since calcium changes have been implicated in exposure of procoagulant aminophospholipids, we have now examined calcium fluxes in this subpopulation by measuring fluorescence changes in Fura Red/AM-loaded platelets following PAR-1 stimulation. While fluorescence changes in all platelets indicated calcium release from internal stores and influx of external calcium, a subpopulation of platelets displayed a pronounced increase in calcium transients by 15 s and positive factor IXa binding by 2 min, with calcium transients sustained for 45 min. Pretreatment of platelets with Xestospongin C to inhibit IP3-mediated dense tubule calcium release, and the presence of impermeable calcium channel blockers nifedipine, SKF96365, or LaCl3, inhibited PAR-1-induced development of a subpopulation with pronounced calcium transients, factor IXa binding, and platelet support of FXa generation, suggesting the importance of both release of calcium from internal stores and influx of extracellular calcium. When platelets were stimulated in EDTA for 5-20 min before addition of calcium, factor IXa binding sites developed on a smaller subpopulation but with unchanged rate, indicating sustained opening of calcium channels and continued availability of signaling elements required for binding site exposure. While pretreatment of platelets with 100 microM BAPTA/AM (Kd 160 nM) had minimal effects, 100 microM 5,5'-dimethylBAPTA/AM (Kd 40 nM) completely inhibited the appearance and function of the platelet subpopulation, indicating the importance of minor increases of cytoplasmic calcium. We conclude that PAR-1-stimulated development of factor IXa binding sites in a subpopulation of platelets is dependent upon release of calcium from internal stores leading to sustained and pronounced calcium transients.

Transient receptor potential channels and caveolin-1: good friends in tight spaces

Caveolae formation has raised the concept of energy efficiency to new heights. The ultimate purpose of caveolae formation is to colocalize signaling proteins with membrane microdomains in order to facilitate their interaction and improve signal transduction efficiency. Although we know that the main structural protein of caveolae is caveolin, how caveolin interacts with membrane proteins to facilitate their integration into lipid raft domains is unclear. A caveolin-scaffolding domain (CSD) on caveolin itself can associate with membrane proteins such as G proteins and endothelial nitric oxide synthase. In this issue, Kwiatek et al. (p. 1174) report that the TRPC1 channel protein contains a C-terminal CSD-consensus binding sequence that allows for its physical and functional interaction with caveolin-1 in the caveolae of human pulmonary artery endothelial cells (PAEC). Competitive interaction with a CSD-conjugated peptide attenuates thrombin- and thapsigargin-induced Ca2+ influx via store-operated TRPC1 channels. Their data suggest that caveolin-1 can directly regulate TRPC1 function, extending its already ascribed role as a structural protein.

Caveolin-1 regulates store-operated Ca2+ influx by binding of its scaffolding domain to transient receptor potential channel-1 in endothelial cells

Caveolin-1 associates with store-operated cation channels (SOC) in endothelial cells. We examined the role of the caveolin-1 scaffolding domain (CSD) in regulating the SOC [i.e., transient receptor potential channel-1 (TRPC1)] in human pulmonary artery endothelial cells (HPAECs). We used the cell-permeant antennapedia (AP)-conjugated CSD peptide, which competes for protein binding partners with caveolin-1, to assess the interactions of caveolin-1 with TRPC1 and its consequences on thrombin-induced Ca2+ influx. We observed that AP-CSD peptide markedly reduced thrombin-induced Ca2+ influx via SOC in HPAECs in contrast to control peptide. AP-CSD also suppressed thapsigargin-induced Ca2+ influx. Streptavidin-bead pull-down assay indicated strong binding of biotin-labeled AP-CSD peptide to TRPC1. Immunoprecipitation studies demonstrated an interaction between endogenous TRPC1 and ectopically expressed hemagglutinin-tagged CSD. Analysis of the deduced TRPC1 amino acid sequence revealed the presence of CSD binding consensus sequence in the TRPC1 C terminus. We also observed that an AP-TRPC1 peptide containing the CSD binding sequence markedly reduced the thrombin-induced Ca2+ influx. We identified the interaction between biotin-labeled AP-TRPC1 C terminus peptide and caveolin-1. Thus, these results demonstrate a crucial role of caveolin-1 scaffolding domain interaction with TRPC1 in regulating Ca2+ influx via SOC.

TRPC1 protects human SH-SY5Y cells against salsolinol-induced cytotoxicity by inhibiting apoptosis

Salsolinol, an endogenous neurotoxin, may be involved in the pathogenesis of Parkinson's disease. In this study, we sought to determine whether salsolinol-induced cytotoxicity in SH-SY5Y human neuroblastoma cells, a cloned cell line which expresses dopaminergic activity, could be prevented by overexpressing a Ca(2+) channel, transient receptor potential (TRPC1) protein. Exposure of SH-SY5Y cells to 500 microM salsolinol for 12 h resulted in a significant decrease in thapsigargin or carbachol-mediated Ca(2+) influx. Consistent with these results, SH-SY5Y cells treated with salsolinol showed approximately 60% reduction in TRPC1 protein levels. Confocal microscopy also showed that SH-SY5Y cells treated with salsolinol had a significant decrease in the plasma membrane staining of the TRPC1 protein. Interestingly, overexpression of TRPC1 increases TRPC1 protein levels and also protected SH-SY5Y neuroblastoma cells against salsolinol-mediated cytotoxicity as determined by 3,[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. The protective effect of TRPC1 was blocked by the addition of TRPC1 blockers lanthanum, or 2APB. Activation of TRPC1 protein by either thapsigargin or carbachol further protected SH-SY5Y cells from salsolinol treatments. Staining of SH-SY5Y cells with an apoptotic marker (YO-PRO-1) showed that TRPC1 protein protects against apoptosis. Furthermore, TRPC1 overexpression also inhibited cytochrome c release and decreased BAX protein levels required for apoptosis. Taken together, these findings suggest that the reduction in cell surface TRPC1 protein expression in response to salsolinol may be a contributory factor in cellular toxicity of the dopaminergic neurons. Furthermore, overexpression of TRPC1 could inhibit apoptotic complex thereby increasing neuronal cell survivability in Parkinson's disease.

Cellular cholesterol controls TRPC3 function: evidence from a novel dominant-negative knockdown strategy

TRPC3 (canonical transient receptor potential protein 3) has been suggested to be a component of cation channel complexes that are targeted to cholesterol-rich lipid membrane microdomains. In the present study, we investigated the potential role of membrane cholesterol as a regulator of cellular TRPC3 conductances. Functional experiments demonstrated that cholesterol loading activates a non-selective cation conductance and a Ca2+ entry pathway in TRPC3-overexpressing cells but not in wild-type HEK-293 (human embryonic kidney 293) cells. The cholesterol-induced membrane conductance exhibited a current-to-voltage relationship similar to that observed upon PLC (phospholipase C)-dependent activation of TRPC3 channels. Nonetheless, the cholesterol-activated conductance lacked negative modulation by extracellular Ca2+, a typical feature of agonist-activated TRPC3 currents. Involvement of TRPC3 in the cholesterol-dependent membrane conductance was further corroborated by a novel dominant-negative strategy for selective blockade of TRPC3 channel activity. Expression of a TRPC3 mutant, which contained a haemagglutinin epitope tag in the second extracellular loop, conferred antibody sensitivity to both the classical PLC-activated as well as the cholesterol-activated conductance in TRPC3-expressing cells. Moreover, cholesterol loading as well as PLC stimulation was found to increase surface expression of TRPC3. Promotion of TRPC3 membrane expression by cholesterol was persistent over 30 min, while PLC-mediated enhancement of plasma membrane expression of TRPC3 was transient in nature. We suggest the cholesterol content of the plasma membrane as a determinant of cellular TRPC3 activity and provide evidence for cholesterol dependence of TRPC3 surface expression.

Role of O2-sensitive K+ and Ca2+ channels in the regulation of the pulmonary circulation: Potential role of caveolae and implications for high altitude pulmonary edema

High altitude pulmonary edema (HAPE) is a potentially fatal complication in response to exposure to low O(2) at high altitudes. Hypoxia, by causing pulmonary vasoconstriction, increases pulmonary vascular resistance and pulmonary arterial pressure, both of which are features in the pathogenesis of HAPE. Uneven hypoxic pulmonary vasoconstriction is thought to be responsible for increased capillary pressure and leakage, resulting in edema. O(2)-sensitive ion channels are known to play pivotal roles in determining vascular tone in response to hypoxia. K(+), Ca(2+) and Na(+) channels are ubiquitously expressed in both endothelial and smooth muscle cells of the pulmonary microvasculature, subfamilies of which are regulated by local changes in P(O(2)). Hypoxia reduces activity of voltage-gated K(+) channels and down-regulates their expression leading to membrane depolarization, Ca(2+) influx in pulmonary artery smooth muscle cells (by activating voltage-dependent Ca(2+) channels) and vasoconstriction. Hypoxia up-regulates transient receptor potential channels (TRPC) leading to enhanced Ca(2+) entry through receptor- and store-operated Ca(2+) channels. Altered enrichment of ion channels in membrane microdomains, in particular in caveolae, may play a role in excitation-contraction coupling and perhaps in O(2)-sensing in the pulmonary circulation and thereby may contribute to the development of HAPE. We review the role of ion channels, in particular those outlined above, in response to low O(2) on vascular tone and pulmonary edema. Advances in the understanding of ion channels involved in the physiological response to hypoxia should lead to a greater understanding of the pathogenesis of HAPE and perhaps in the identification of new therapies.

Caveolins and flotillin-2 are present in the blood stages of Plasmodium vivax

Blood stages of Plasmodium vivax induce the development of caveolae and caveola-vesicle complexes (CVC) in the membrane of their host erythrocyte. Caveolae are found in almost all types of cells and are involved in endogenous processes as calcium and cholesterol homeostasis, cell signalling, transporting, ligand internalization and transcytosis of serum components. Major structural components of caveolae are the proteins caveolins and flotillins. The functional role of caveolae in the P. vivax-infected erythrocyte is not properly understood. As these organelles have been shown to contain malaria antigens, it has been suggested that they are involved in the transport and release of specific parasite antigens from the infected erythrocyte and in the uptake of plasma proteins. Using specific antibodies to classical caveolae proteins and an immunolocalization approach, we found caveolin-2, caveolin-3, and flotillin-2 in the vesicle profiles and some CVC of P. vivax-infected erythrocytes. Caveolin-1-3 were not found in uninfected erythrocytes. This is the first report of identification and localization of caveolins in the CVC present in erythrocytes infected with P. vivax, thereby providing evidence of the role of this particular organelle in the protein-trafficking pathway that connect parasite-encoded proteins with the erythrocyte cytoplasm and the cell surface throughout the asexual blood cycle of vivax malaria parasite.