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J. Biol. Chem., Vol. 282, Issue 22, 16631-16643, June 1, 2007
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1
From the
Department of Pharmacology and Program in Vascular Cell Signaling and Therapeutics, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, Connecticut 06536 and the Department of Pathology, Dartmouth Medical School, Lebanon, New Hampshire 03756
Received for publication, August 18, 2006 , and in revised form, April 4, 2007.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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q/11 (3, 4)) activating calcium influx pathways is complex and controversial and may depend on the channel composition and cell type being studied.
There is growing evidence that members of the transient receptor potential (TRP) cation channel family can assemble in the plasma membrane to participate in cation influx pathways. TRPs are members of a large superfamily of mammalian TRP channels (TRPCs), and to date, seven TRPCs (TRPC1-7) have been identified (5). Heterologous expressions of the highly homologous TRPC3, -6, and -7 channels are clearly activated by phospholipase C-derived DAG (6, 7), whereas activations of TRPC1, -4, and -5 are less clear. Although TRPC1 was characterized as a component of SOC in nerve cells (8), subsequent studies indicated that TRPC3, TRPC4, and TRPC5 might also contribute to calcium entry mechanisms after agonist stimulation (9-11). In vascular endothelial cells, sustained calcium entry into the cytosol is necessary to release vasoactive agents such as nitric oxide or prostaglandins (12, 13), and mice deficient in TRPC4 (10) show markedly reduced acetylcholine (ACh) agonist-induced calcium entry and endothelium-dependent, nitric oxide-mediated vasorelaxation. The compositions of the native TRPC channels in endothelial cells are not known and are likely complex and species-specific. However, there is recent evidence that TRPC3/C4 heteromers are necessary for cation influx activated by redox signaling in porcine endothelial cells (14).
Caveolae are 50-100 nm cholesterol-rich invaginations of the plasma membrane that have been implicated in a variety of cellular functions, including endocytosis and signal transduction events (15). There are three family members of caveolins (Cav-1, -2, and -3) (16-18). Cav-1 is predominantly expressed in endothelial cells, smooth muscle cells, fibroblasts, and adipocytes, and Cav-3, which is specifically expressed in striated muscle, is sufficient for formation of caveolae in these cell types or tissues. Recent studies have demonstrated that isolated caveolae contain complex sets of functionally organized signaling molecules, including Ca2+ signaling molecules (19-21), and have been suggested to regulate Ca2+ entry in living cells (22).
Recently, we have developed a mouse model to study the effects Cav-1 on endothelial function in vivo by breeding the Cav-1 knock-out mice to endothelium-specific Cav-1 transgenic mice, thus rescuing caveolae in the endothelium only (23). Here we show using genetic criterion in intact blood vessels and isolated cells that Cav-1 is necessary for the second phase of calcium entry in endothelial cells. Mechanistically, Cav-1 is associated with a dynamic protein complex consisting of TRPC4, TRPC1, and IP3Rs, and the loss of Cav-1 impairs the localization of TRPC4 and agonist-simulated complex formation. These data favor the idea that Cav-1 in caveolae of specialized cells, such as endothelia, may serve as a scaffold to integrate TRPC regulation of calcium influx pathways.
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EXPERIMENTAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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Isolation of Murine Lung Endothelial Cells
Lungs were dissected from the mice, minced in 0.1% collagenase, and homogenized by passing through a needle (14-gauge). After centrifugation (1000 rpm, 5 min), cells were plated into flasks and were cultured for 2 days. Cell were then immortalized by infection with a retrovirus expressing the polyoma middle T antigen, and cultured again for 2 days. Magnetic beads (Dynal) coated with anti-ICAM-2 antibodies (Pharmingen) were added to cells and incubated at 4 °C for 1 h, and cells were detached by trypsin. Then the endothelial cells were collected using a magnetic holder. After washing, cells were plated and passaged for use. After 48 h serum-starved murine lung endothelial cells (MLEC) (passage 7-15) were used for experiments.
Immunostaining
Whole-mount Staining—The mouse intrapulmonary arteries were fixed with 4% paraformaldehyde (4 °C; 10 min), permeabilized with 0.3% Triton X-100/PBS (room temperature, 30 min), and blocked by 3% normal goat serum containing PBS (room temperature, 30 min). Then the arteries were probed with rabbit anti-Cav-1 polyclonal antibody (BD Transduction Laboratories) and rat anti-PECAM (BD Biosciences) polyclonal anti-body (1:200 dilution; 4 °C; 12 h). Alexa 568 anti-rabbit IgG and 488 anti-rat IgG (Molecular Probe) were used as secondary antibodies (1:500 dilution at room temperature, 1 h).
Cross-section Staining—After dissection, intrapulmonary arteries were fixed with 4% paraformaldehyde for 10 min at 4 °C and then dehydrated in 15% sucrose overnight at 4 °C. The vessels were then embedded in OCT (Sakura) and flash-frozen in a liquid nitrogen-cooled isopentane bath. Sections (5 µm) were blocked with 3% normal goat serum and probed with anti-Cav-1 polyclonal antibody (BD Transduction Laboratories) and anti-smooth muscle -actin (Sigma) monoclonal antibody Cy3 conjugate (1:500 dilution, 4 °C, 12 h). Alexa 488 anti-rabbit IgG (Molecular Probe) was used as secondary antibody (1:500 dilution, room temperature, 1 h).
MLEC Staining—Confluent MLEC were fixed with 4% paraformaldehyde (4 °C, 10 min), permeabilized with 0.1% Triton X-100/PBS (room temperature, 5-15 min), and blocked with 3% normal goat serum containing PBS (room temperature, 30 min). Then the cells were probed with anti-Cav-1 monoclonal antibody (BD Transduction Laboratories), rabbit anti-TRPC1 (Alomone Labs and Pharmingen) and anti-TRPC4 (Alomone Labs) polyclonal antibodies, and rabbit anti-IP3Rs polyclonal antibody (kindly donated by Barbara Ehrlich; 1:500 dilution at room temperature for 3 h). Alexa 568 anti-rabbit IgG and 488 anti-rat IgG (Molecular Probe) were used as secondary antibodies. Alexa 488 anti-rabbit IgG and 568 anti-mouse IgG (Molecular Probe) were used as secondary antibody (1:500 dilution, room temperature, 1 h). For actin staining, we applied Alexa Fluor 488 phalloidin (Molecular Probe) to samples (1:200 dilution, room temperature,10 min) after the paraformaldehyde fixation.
After mounting of 4,6-diamidino-2-phenylindole, all of the images were captured using a Carl Zeiss scanning microscope Axiovert 200M imaging system (x400-630), and images were digitized under constant exposure time, gain, and offset. After capturing images, protein co-localization and cell area were measured by the OpenLab program (Improvision).
Electron Microscopy
Cav-1 WT, KO, and RC mice were perfused for 10 min at room temperature under anesthesia with oxygenated Dulbecco's modified Eagle's medium through the left ventricle, followed by fixation by perfusion (10 min at room temperature) with 2.5% glutaraldehyde and 3% paraformaldehyde in 0.1 M sodium cacodylate buffer. Specimens were taken from lung and trimmed into small blocks. The blocks were immersed into fresh fixative (1 h at room temperature), washed twice (15 min at room temperature) in 0.1 M cacodylate, postfixed in Palade's OsO4 (1 h on ice), stained en bloc in Kellemberger's uranyl acetate (overnight at room temperature), dehydrated in graded ethanol, and embedded in LX112 resin (Ladd Research Industries). Thin sections (50 nm) were cut, stained with lead citrate, and examined and photographed under an electron microscope (JEOL 1200EX or CM10, Phillips Electronic Instruments).
Prostacyclin Release
After 12 h of incubation, pulmonary arterial segments (4 mm) were placed in sterile 48-well tissue culture plates with 300 µl of HEPES solution at 37 °C and kept in the incubator for 30 min. Aliquots of 100 µl were then taken to measure the basal release of prostacyclin. ACh (3 µM) and ionomycin (3 µM) were added, and the plate was incubated for an additional 30 min, after which the aliquots were again taken to measure the ACh-stimulated release of prostacyclin. This protocol was carried out both in the absence and in the presence of ibuprofen (10 µM) and La3+ (300 µM). The aliquots were frozen, and prostacyclin release was measured as the stable breakdown product, 6-keto-PGF1, by immunoassay (Amersham Biosciences).
IP3 Accumulation
Serum-starved MLECs were stimulated by ACh (3 µM) for 3 min, homogenized in 7.5% trichloroacetic acid solution, and centrifuged at 2,000 x g for 15 min at 4 °C, as described previously. The cell suspensions were applied to an IP3 (25) Biotrak Assay System (GE Healthcare), and pellets were used to determine protein content. IP3 contents are expressed as pmol/mg protein content.
Tissue and Cell [Ca2+]i Measurements—Measurement of the [Ca2+]i in endothelial cells in situ was performed using fluo-4AM (Molecular Probe) according to the method described previously (26). The artery (0.6 mm wide) and confluent MLEC were incubated in physiological salt solution (136.9 mM NaCl, 5.4 mM KCl, 2.0 mM CaCl2, 1.0 mM MgCl2, 23.8 mM NaHCO3, and 5.5 mM glucose) with 5-µM fluo-4AM and 0.02% cremophor EL for 30 min at room temperature. The opened strip of pulmonary artery or MLEC was mounted on the stage of a microscope, and the images were captured by a Carl Zeiss scanning microscope Axiovert 200M imaging system (x100-400) using 488-nm (excitation) and 540-nm (emission) filters. Images of vascular endothelial cells stained with fluo-4 were taken every 1.8-10 s at 37 °C, and data were expressed as the change in fluorescence after agonist challenge relative to base line (F/Fo) as a function of time.
Western Blotting
Confluent MLEC were washed twice with phosphate-buffered saline and lysed on ice in 50 mM Tris-HCl, pH 7.5, 1% Nonidet P-40 (v/v), sodium orthovanadate, 20 mM NaF, 1 mM Na4P2O7, 1 mM Pefabloc SC, and protease inhibitor mixture (Roche Diagnostics) and rotated for 1 h at 4 °C. Lysates were Dounce-homogenized (30 strokes), and insoluble material was removed by centrifugation at 13,000 x g for 10 min at 4 °C, and 20-50 µg of each protein was subjected to regular Western blot analysis.
Knockdown of TRPC1 and 4 by siRNA
Small inhibitory RNA (siRNA) duplexes for mouse TRPC1 (GenBankTM accession number NM_011643
[GenBank]
) and TRPC4 (GenBankTM accession number NM_016984
[GenBank]
) were designed by Qiagen. Individual TRPC1 siRNA duplexes (sense sequences) were as follows: TRPC1, r(CGAGGACUAGAAGAAA9)dTdT and r(GCCGUAAGCCCACCUGUAA)dTdT, and TRPC4, r(CGUUAUUCGAGACACUACA)dTdT and r(GGAGCACUUGAGCGAAAUA)dTdT. MLEC maintained in serum-free EBM-2 were transfected with siRNAs (40 nM) using Lipo-fectamine 2000 following the manufacturer's protocol. Fresh medium was added 6 h post-transfection, and experiments were conducted 72 h after transfection to assess mRNA levels using each specific primers against TRPC1 and -4 and protein using Western blotting.
Cellular Fractionation
Confluent MLEC (3 x 150-mm dish) were serum-starved for 48 h and were washed twice with phosphate-buffered saline, scraped in 2 ml of ice-cold hypotonic buffer (500 mM Na2CO3, pH 11), and sonicated (3 times, 20-s bursts). The homogenate was then adjusted to 42.5% sucrose by the addition of 2 ml of 85% sucrose prepared in MBS (25 mM MES, 150 mM NaCl, pH 6.5) and placed at the bottom of an ultracentrifuge tube. A 5-30% discontinuous sucrose gradient was formed and centrifuged at 35,000 rpm for 17 h in an SW40 rotor (Beckman Instruments). Gradient fractions (1 ml) were collected from the top of the tube to yield a total of 12 fractions, and each fraction (fractions 1-12) was used for Western blot analysis. Blots were probed with the anti-Cav-1 monoclonal (1:500), anti-TRPC1 polyclonal (1:200), and anti-TRPC4 polyclonal (1:200) antibodies (as described above), rabbit anti-IP3R1 polyclonal antibody (1:500; Calbiochem), anti-IP3R3 monoclonal antibody (1:1000; BD Transduction Laboratories), anti-flotillin-2 monoclonal (1:5000; BD Transduction Laboratories) and anti--actin monoclonal anti-bodies (1:5000; Sigma).
Immunoprecipitation Studies
Tissues and MLECs for Western blot analysis were resuspended in lysis buffer (for TRPC1 and -4, and IP3Rs immunoprecipitation: 50 mM Tris-HCl, pH 7.4, 125 mM NaCl, 0.1 mM EGTA, 0.1 mM EDTA, 0.75% Nonidet P-40, 1 mM sodium orthovanadate, 20 mM NaF, 1 mM Na4P2O7, 1 mM Pefabloc SC, and protease inhibitor mixture (mentioned above); for Cav-1 immunoprecipitation: 10 mM Tris, pH 8.0, 60 mM n-octyl-D-glucopyranoside, 150 mM NaCl, 1 mM sodium orthovanadate, 20 mM NaF, 1 mM Na4P2O7, 1 mM Pefabloc, and protease inhibitor mixture (Roche Diagnostics)). Insoluble material was removed by centrifugation at 13,000 x g for 10 min at 4 °C. One thousand (for TRPC1 and -4, and IP3Rs) or 500 µg (Cav-1) of protein from lysates was subjected to immunoprecipitation. Anti-TRPC1 and -4, IP3Rs, and 50 µl of protein A-agarose (Sigma) were added to the supernatant, which was then rotated at 4 °C for 3 h. After washing 3 times with lysis buffer, binding protein was eluted with SDS sample buffer and analyzed by Western blot.
Purification of GST-Caveolin-1 Fusion Proteins and in Vitro Interactions
Construction of the Cav fusion proteins was performed as follows. Full-length Cav-1 (residues 1-178, Cav-FL) and Cav subdomains (the N-terminal domains, residues 1-61 and 61-101) and the C-terminal domain (residues 135-178) were separately amplified by PCR and subcloned into the vector pGEX-4T-1 (XmaI-XhoI sites) as described previously (27). All fusions in Escherichia coli strain BL-21, were induced with 0.5 mM isopropyl 1-thio-
-D-galactopyranoside and lysed by sonication (three 20-s bursts) after addition of purification buffer (7.5 mM Tris-Cl, pH 8.0, 150 mM NaCl, 1.5% N-lauryl Sarkosyl (v/v), 2% Triton X-100, 100 µM phenylmethylsulfonyl fluoride, 5 mM dithiothreitol, 3 mM EDTA). Debris was removed by centrifugation. Purified GST-Cavs were immobilized on glutathione-agarose beads (GE Healthcare). Cells were lysed in a modified RIPA buffer (50 mM Tris-Cl, pH 7.4, 1% Nonidet P-40, 0.1 mM EDTA, 0.1 mM EGTA, 0.1% SDS, 0.1% deoxycholic acid, 1 mM Pefabloc, 1 µg/ml aprotinin, 1 µg/ml leupeptin, and 2 µg/ml pepstatin) and centrifuged for 10 min at 13,000 x g at 4 °C. Lysates (500 µg of proteins) were incubated for 2 h at 4 °C with beads containing either GST alone or GST-Cav. After binding, the beads were washed (3 times) with wash buffer containing 50 mM Tris-Cl, pH 7.4, 125 mM NaCl, 1 mM EDTA, and 1 mM EGTA. Binding protein was eluted with SDS sample buffer, and analyzed by Western blot.
Statistical Analysis
The results of the experiments were expressed as the means ± S.E. Statistical evaluation of the data was performed by analysis of variance, followed by the Tukey post-test for comparison between groups using Prism 3.0. A value of p < 0.05 was regarded as statistically significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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-actin, and bottom panel show the merged images), whereas as Cav-1 KO (middle column) lacks Cav-1 immunostaining in both layers (n = 9). Reconstitution of Cav-1 into the endothelium generated by crossing Cav-1 KO to EC-specific Cav-1 TG mice reintroduces Cav-1 into the endothelium but not into smooth muscle layers marked by smooth muscle -actin staining (n = 22; Fig. 1B, right columns). Next we used whole-mount staining of the endothelium to examine the extent of Cav-1 reconstitution in these strains. As seen in Fig. 1C, WT and Cav-1 RC have relatively equal levels of immunoreactive Cav-1 in endothelial cells (n = 6 and 10, respectively; top row, left and right columns) defined by the endothelial cell marker, PECAM-1 (n = 6 each; bottom row). Cav-1 KO vessels lack endothelial cell Cav-1 immunoreactivity without changing the levels of PECAM-1 (n = 10 and 6, respectively). In multiple experiments, we estimate 
85-90% of the endothelial cells from intrapulmonary arteries are Cav-1-positive. Cav-1 KO mice lack caveolae organelles in several tissues examined, including blood vessels, adipocytes, and fibroblasts (18, 28). As shown in Fig. 1D, transmission electron micrographs document the presence of ample caveolae in EC lining pulmonary vessels, the loss of EC caveolae in Cav-1 KO vessels, and the presence of the organelle in the Cav-1 RC vessels. Thus, we have generated a model of Cav-1 re-expression in the endothelium only, resulting in formation of endothelial caveolae.
Next, we examined the changes in endothelial [Ca2+]i induced by the calcium-mobilizing agonist ACh (3 µM)by imaging fields of endothelial cells lining the intrapulmonary arteries loaded with the calcium indicator fluo-4AM. As shown in Fig. 1E, ACh induced a transient increase in [Ca2+]i followed by a sustained increase in all of the arteries (n = 5 each). However, the sustained phases of [Ca2+]i stimulated by ACh was reduced in the endothelium of Cav-1 KO pulmonary arteries compared with that seen in WT and Cav-1 RC mice. Thus, Cav-1 reconstitution in the endothelium completely restored the impairment in [Ca2+]i fluxes during the tonic phase of the response. Next we sought to examine if the reduction in stimulated changes in calcium entry may impair endothelial function. To this end, we examined ACh-stimulated increases in prostacyclin production (an index of endothelial function; quantified by the stable breakdown product 6-keto-PGF1), which is known to be calcium-dependent (13). The loss of Cav-1 reduced resting and ACh but not ionomycin-induced 6-keto-PGF1 production in vessels compared with vessels from WT and Cav-1 RC mice (n = 5 each, Fig. 1F). The cyclooxgenase inhibitor ibuprofen completely inhibited 6-keto-PGF1 production in all of the arteries (n = 5 each). To confirm the dependence of 6-keto-PGF1 release on extracellular calcium, vessels were treated with the nonspecific blocker lanthanum (La3+; 300 µM). La3+ inhibited ACh-induced prostacyclin release in the pulmonary arteries from WT mice (n = 4). These results demonstrate that the impairment in endothelial cell calcium handling in Cav-1 KO mice results in reduced endothelial function quantified by the release of prostacyclin, and both calcium fluxes and prostacyclin production could be genetically rescued by the endothelium-specific transgenic Cav-1.
Ca2+ Fluxes and Entry, but Not IP3 Production, Are Impaired in Isolated Endothelial Cells of Cav-1 KO Mice and Rescued by Reintroduction of Endothelial Cav-1—To examine potential mechanisms of how the loss of Cav-1 and caveolae impaired the tonic phase of agonist-stimulated changes in calcium, MLEC were isolated from WT, Cav-1 KO, and Cav-1 RC mice. ACh-, but not ionomycin, stimulated increases in [Ca2+]i were decreased in Cav-1 KO MLEC compared with cells isolated from WT and Cav-1 RC mice (n = 5 individual experiments; five individual cells each; see Fig. 2A). These data are consistent with the data collected monitoring intracellular calcium in the endothelium of intact blood vessels.
It is well accepted that Ca2+ enters the cytosol from two general sources, namely Ca2+ release from mainly the ER and Ca2+ entry across the plasma membrane. Next, we investigated whether Cav-1 deficiency would affect Ca2+ release from the intracellular calcium stores. Fig. 2B shows typical traces of calcium signal intensities, and C and D show the summary from individual experiments quantifying phasic (C) and tonic (D) calcium levels after ACh (n = 5 experiments per cell type; three individual cells shown). In the absence of external Ca2+ (plus 0.5 mM EGTA), stimulation with ACh (Fig. 2B, top, left panel) induced a transient increase in [Ca2+]i due to primarily Ca2+ release from intracellular stores. Cav-1 deficiency slightly decreased the amount of ACh-induced peak Ca2+ release, an effect rescued by Cav-1 (Fig. 2C).
Next, we investigated the effects of Cav-1 on Ca2+ entry. Re-addition of external Ca2+ after ACh stimulation induced sustained Ca2+ entry (Fig. 2B), and the loss of Cav-1 markedly attenuated the entry of Ca2+ by at least 50% (see top left panel after re-addition of 2 mM Ca2+ after stimulation by ACh and summary data in Fig. 2D). We further investigated calcium entry by examining the effects of three nonspecific inhibitors of cation entry, gadolinium (Gd3+), 2-aminoethoxydiphenyl borate (2-APB), and lanthanum (La3+; n = 4 experiments) in WT endothelial cells. Gd3+ (100-300 µM), 2-APB (100 µM), and La3+(100-300 µM) all decreased ACh-induced Ca2+ entry (see Fig. 2B and summary of data in Fig. 2D).
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activation and the generation of DAG and IP3. To examine if Cav-1 was important for IP3 generation, we measured basal and stimulated IP3 accumulation in WT, Cav-1 KO, and Cav-1 RC MLEC. As seen in Fig. 2E, basal and ACh-stimulated IP3 accumulations were identical from these cells suggesting that defective calcium influx was not attributable to defective IP3 production. Caveolin-1 Is Crucial for the Subcellular Distribution of TRPC1 and -4 in Endothelial Cells—Recently, both the N and C termini of TRPC1 have been shown to bind to Cav-1, and perhaps binding to Cav-1 may regulate the trafficking of TRPC and calcium influx pathways (29). Initially the protein levels of TRPCs (1 and 4) and Cav-1 were determined in MLEC lysates using isoform-specific antisera. To examine Ab specificity, siRNA and peptide blocking experiments were performed. Transfection of specific siRNAs (but not control siRNAs) for TRPC1 and TRPC4 into MLEC reduced the mRNA expression (by PCR; Fig. 3A) and protein levels (Fig. 3B) of TRPC1 (93 kDa) and TRPC4 (95 kDa). Commercially available Abs to TRPC3 detected a 160-kDa band, which was inconsistent with the predicted molecular weight of TRPC3; therefore, we could not study this TRPC further. In addition, preincubation of the peptide immunogen reduced the immunoreactivity of the TRPC1 and TRPC4 Abs (data not shown). These data are consistent with recent publications using these Abs for the respective TRPC (30-32) and are in contrast with other studies using these Abs (33, 34).
In the three lines of MLEC used (WT, Cav-1 KO, and Cav-1 RC), there were no significant changes in the levels of TRPC1 (93 kDa) and TRPC4 (95 kDa; n = 4-7; Fig. 3C, far right panel, the levels of -actin as a loading control were identical, and Cav-1 was present at equal levels in WT and Cav-1 RC cells). Because TRPCs are known to have a tissue-specific variation of subtype expression, we compared the protein expression of TRPCs (Fig. 3D, left panel) in lysates prepared from MLEC or the cerebral cortex (from 3-week-old mice). MLEC expressed primarily TRPC1 and -4, whereas TRPC1 and -4 were equally expressed in cerebral cortex; thus for further analysis of TRPCs in MLEC we focused on TRPC1 and -4.
Because caveolae have been implicated as microdomains that segregate signal transduction mechanisms, we next determined if the presence or absence of Cav-1 (and caveolae) could influence the subcellular distributions of TRPC1 and -4. Thus, cholesterol- and caveolin-1-enriched microdomains from WT, Cav-1 KO, and Cav-1 RC MLEC were subfractionated using a sucrose density gradient that separates the lipid rafts/caveolae from other intracellular cellular membranes and cytosolic proteins. As demonstrated in Fig. 3E, Cav-1 (left panel, rows 1-3) and lipid raft marker flotillin-2 (Flot-2, left panel, rows 4-6) were correctly targeted to low density, cholesterol-enriched membranes (fractions 1-4), and the loss of Cav-1 did not impair the distribution of the raft marker Flot-2 (n = 4 experiments). Conversely, -actin (Fig. 3E, left panel, rows 7-9) was localized in heavy membrane fractions (fraction 8-12). Using this methodology, TRPC1 and TRPC4 (Fig. 3E, left panel, rows 1-6) were found enriched with Cav-1/Flot-2 representing caveolae/lipid rafts (n = 4 each). In Cav-1 KO MLEC, TRPC1 and TRPC4 were mislocalized away from fractions that co-sediment with Cav-1/Flot-2 and were relocated into heavy fractions (n = 4). Interestingly, the reintroduction of Cav-1 (Cav-1 RC) rescued the mislocalization of TRPCs (n = 4 each). These data suggest that in MLEC, Cav-1 and presumably caveolae were required for TRPC1 and -4 sedimentation into cholesterol-rich caveolae/rafts.
To examine this possibility at the cellular level, we determined the localization of TRPC1 and -4 by immunofluorescence microscopy. As shown in Fig. 3F, immunofluorescent staining of MLECs with anti-TRPC1 and anti-Cav-1 antibodies (left panels) revealed a diffuse cytoplasmic pattern for TRPC1 in WT MLEC with a punctate, dot-like, monocillia pattern near the nucleus (top panel, n = 6 experiments). Cytosolic localization of TRPC1 was confirmed with the use of two other antibodies, and the staining was eliminated by preincubating with the immunogen peptide (data not shown). Cav-1 had a diffuse staining pattern throughout the membranes and co-localized with TRPC1 in WT and Cav-1 RC cells (see merged images); however, the loss of Cav-1 did not affect TRPC1 localization (n = 6 each). TRPC4 had a distinct junctional, plasmalemmal pattern (Fig. 3G, see arrowheads) that partially co-localized with Cav-1 in WT or Cav-1 RC MLEC (n = 6 each); however, the loss of Cav-1 reduced the plasmalemmal staining pattern of TRPC4 (n = 6).
Cav-1 Is Necessary for a Regulated Channel Complex Consisting of TRPC1, TRPC4, and IP3R in Endothelial Cells—There is evidence that IP3R may bind directly or indirectly via the adaptor protein junctate (35) to the TRPC family of channels; however, the functional role of this interaction is not completely understood. As shown Fig. 4A, upon immunoprecipitation of TRPC4, a protein complex consisting of IP3R3, TRPC1, and Cav-1 was detected in WT and Cav-1 RC cell lines (n = 5 each). Stimulation of WT or Cav-1 RC MLEC with ACh increased the association of TRPC4 with IP3R1, IP3R3, and TRPC1 (n = 5 each). Interestingly, ACh-stimulated complex formation and recruitment of IP3R1 and IP3R3 were markedly reduced in cells lacking Cav-1 and rescued by reintroducing Cav-1 (n = 5 each). In all lines of MLEC under conditions where we could detect a TRPC4-TRPC1-IP3Rs-Cav-1 complex, we could not detect any interaction of IP3Rs with TRPC1 and -3 when these TRPCs were immunoprecipitated (n = 5 each, data not shown). Using a pan-IP3R antibody for immunoprecipitation, a complex of IP3R3, TRPC4, and Cav-1 existed (Fig. 4B; n = 5 experiments). ACh increased the interaction of IP3R3 with TRPC4 (n = 5), and the levels of IP3R3 and Cav-1 were constant. TRPC1 was undetectable in the IP3R immunoprecipitate under basal conditions. Again, the loss of Cav-1 reduced the ACh-stimulated assembly of the Cav-1-IP3R3-TRPC4 complex (n = 5 each). In contrast to results immunoprecipitating TRPC4 where TRPC1 is associated with the complex in IP3R3 immunoisolates, TRPC1 did not co-precipitate (n = 5). Finally, immunoprecipitation of Cav-1 recovered a basal complex of IP3R3 and TRPC4 (Fig. 4C). Cav-1 is required for the interaction because the loss of Cav-1 obviates any interaction, and the subsequent reconstitution of Cav-1 permits recovery of the complex.
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The Presence of Cav-1 in Endothelial Cells Specializes the Interaction between IP3Rs and TRPCs—Cav-1 is expressed in vasculature and adipose tissue but not in neural cells, whereas non-neural and neural cells both exhibited calcium influx pathways that have been linked to TRPC expression. To specify the role of Cav-1 in regulating calcium entry in endothelial cells and to examine the assembly of TRPCs in a native tissue lacking Cav-1, we examined IP3R and TRPC interactions in lysates prepared from the cerebral cortex of 3-4-week-old mice (see Fig. 3D and Fig. 4E). First, we confirmed that the cerebral cortex did not express Cav-1 protein (Fig. 3D, n = 4 experiments). Compared with MLEC, the murine cerebral cortex expressed less TRPC1 and TRPC4. In addition, IP3R1, not IP3R3, was the primary subtype of the IP3R family in the cerebral cortex as mentioned before (n = 4 experiments). In co-immunoprecipitation experiments from cortical extracts, precipitation of both TRPC1 and TRPC4 did not result in the significant recovery of IP3Rs or additional TRPCs in the complex. However, precipitation of IP3Rs with pan-IP3Rs antibody did not result in co-precipitation of TRPC1 or TRPC4 in cortical extracts (n = 4 each), suggesting that Cav-1 may uniquely regulate the assembly of IP3R-TRPC interactions in endothelial cells.
Binding of Recombinant Cav-1 to TRPC4 and IP3Rs Isolated from Cav-1 KO MLEC—Because both IP3Rs and TRPC4 co-immunoprecipitate with Cav-1 and vice versa in cellular extracts, we sought to examine if either protein could co-purify and interact with recombinant Cav-1 using GST-Cav-1 as bait and Cav-1 KO lysates (Fig. 4, F and G). Fig. 4F depicts the expression of several distinct modules of the Cav-1 GST proteins as follows: GST alone, GST-Cav-(1-61), GST-Cav-(61-101), GST-Cav-(135-178), as well as full-length Cav-1, GST-Cav-(1-178). Incubation of GST alone (Fig. 4F, lane 1) or Cav (1-61, lane 2) with Cav-1 KO lysates did not permit recovery of immunoreactive IP3Rs or TRPCs from the lysates. On the other hand, incubation of equal amounts of GST-Cav-(61-101, see lane 3), GST-Cav-(135-178, lane 4), or full-length Cav-(1-178, lane 5) with Cav-1 KO lysates resulted in the recovery of an IP3R1-IP3R3-TRPC4 complex (Fig. 4G; n = 4 each). No detectable interactions between the GST-Cav constructs and TRPC1 were found (n = 4 experiments), although TRPC1 was present in the starting lysate (Fig. 4G, last lane labeled WT total).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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The lack of embryonic lethality in Cav-1-specific knock-out mice has allowed investigators to test the importance of this protein and corresponding organelle in several post-natal functions. By breeding this KO strain to a newly generated strain of mice expressing Cav-1 specifically in the endothelium, we were able to reconstitute (Cav-1 RC) caveolae in endothelia confirming the idea that Cav-1 is responsible for caveolae biogenesis in the endothelium in vivo. Recently, using this model, we have shown that Cav-1 is necessary for flow-dependent remodeling and mechanotransduction in endothelial cells (23). In intact endothelia lining intrapulmonary arteries, Cav-1 deficiency impairs endothelial Ca2+ metabolism, primarily agonist-mediated calcium entry, which is required for production of the vasodilator, anti-thrombotic lipid prostacyclin (Fig. 2B). Whether the defects in mechanotransduction in carotid arteries as described relate to impaired calcium influx in intrapulmonary arteries in this paper are not clear. Furthermore, qualitatively similar results were seen in endothelial cells isolated and cultured from Cav-1 WT, Cav-1 KO, and Cav-1 RC mice thus demonstrating the necessary role of Cav-1 in regulating the tonic increase in [Ca2+]i after ACh challenge.
Cav-1 deficiency did not influence basal or ACh-stimulated IP3 production (Fig. 2E), but significantly reduced endothelial cell calcium influx (Fig. 2B), prostacyclin release (Fig. 1F), and TRPC1/4-IP3Rs interactions (Fig. 5, A and B) primarily after stimulation with ACh. Interestingly, ACh-induced Ca2+ release from intracellular Ca2+ stores was slightly less in Cav-1 KO MLEC and rescued by the re-addition of Cav-1 (Fig. 2C), suggesting partial Cav-1 regulation of calcium loading perhaps by its effect in reducing influx mechanisms because Ca2+ entry is required for replenishment of Ca2+ into intracellular Ca2+ stores. As a result, the changes in Ca2+ metabolism may reduce both basal and agonist-stimulated increases in prostacyclin release.
Subsequently, quenching followed by the re-addition of extracellular Ca2+ to endothelial cells (Fig. 2) demonstrates that Cav-1 deficiency mainly impairs Ca2+ entry across the plasma membrane and, to a lesser degree, the above-mentioned Ca2+ release from internal stores. This phenotype is most evident in the cultured MLEC when ACh responses were examined in the absence of extracellular calcium, followed by the re-addition of calcium. Previous work has shown that endothelial cells express the mRNAs for several TRPC subtypes 1, 3, 4, 6, and 7 (43), and in porcine endothelial cells, TRPC3 and TRPC4 form a redox-sensitive cation channel (14). The idea that TRPC4 is necessary for calcium influx in murine endothelial cells is supported by data showing that loss of TRPC4 in knock-out mice reduces calcium influx into endothelial cells in vivo (10, 44).
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Using co-precipitation techniques (detergent-soluble, native complexes) in resting endothelial cells, TRPC4 can interact with TRPC1, IP3R3, and Cav-1. The loss of Cav-1 does not markedly influence the basal interaction of the TRPCs with the IP3R3. However, upon challenge with ACh, the interactions of TRPC4 with IP3Rs and Cav-1 are enhanced with no changes in the levels of co-associated TRPC1. In contrast, following immunoprecipitation of either IP3R3 or Cav-1, there is an increase in TRPC4 recovered in the complex suggesting that TRPC4 and IP3R3 are regulated in the Cav-1 complex. The lack of TRPC1 in these complexes appears to be at odds with fractionation studies that demonstrate the loss of both TRPC1 and TRPC4 from buoyant membranes isolated from Cav-1 KO cells (shown by fractionation studies). However, the co-association of TRPC1 with TRPC4 under these differing biochemical conditions and the interactions of TRPC1 with other components may stabilize/destabilize TRPC1 in these complexes. Alternatively, during immunoprecipitation of soluble, native complexes, perhaps only a small fraction of the total TRPC1/4 interactions was recovered. Regardless of these technical differences, there is evidence supporting TRPC1 and TRPC4 interactions in regulating TRPC function because co-expression of TRPC4 with TRPC1 results in heteromultimer assembly and proper trafficking of TRPC1 to the cell membrane (48). Also, Cav-1, via its scaffolding domain, can interact with TRPC1 (46); however, we did see a direct interaction of TRPC1 with GST-Cav-1 in our experiments (Fig. 4, F and G). In this study, the loss of Cav-1 reduces ACh-mediated calcium entry and the co-precipitation of TRPCs with IP3Rs, phenotypes rescued in Cav-1 RC cells, suggesting that Cav-1 is responsible for the assembly of an activated calcium-influx protein complex consisting of TRPCs and IP3Rs in endothelial cells.
Three subtypes of IP3Rs (IP3R1, -2, and -3) are expressed in a tissue-specific manner and form heterotetramic channels (49, 50), and endothelial cells express mainly IP3R3, whereas the brain mainly expresses IP3R1 (Figs. 3B and 4E). In endothelial cells, an endogenous TRPC4-IP3R3 interaction occurs, and while in cerebral cortical extracts lacking caveolins, a TRPC3-IP3Rs interaction occurs (Fig. 4E) suggesting that the expression of Cav-1 regulates specialized calcium-dependent processes in endothelial cells. Previous studies also reported TRPC1- and TRPC3-IP3R interactions when overexpressed in HEK293 cells, and the N-terminal fragment of the IP3R directly gates TRPC3 in patch clamp studies (9); however, a definitive link between the ER or ER-derived calcium and activation of native TRPCs has not been established unequivocally. Thus, interactions between TRPCs and Cav-1 may regulate TRPC localization, or the proximity of TRPCs in invaginated caveolae juxtaposed to the IP3Rs in the ER may explain our data. Clearly, when Cav-1 and caveolae are lacking, ACh-stimulated calcium entry and TRPC4-IP3R complex formation are reduced. Our data with Cav-1 in endothelial cells are similar to recent study showing that junctate, IP3R-associated protein, serves as a scaffold for TRPC4 and TRPC5 in sperm (35). In neurons, other molecules may subserve a similar adaptor function for TRPCs such as Homer, NHERF-2, and the immunophillin FKBP59 (36, 51, 52). Collectively, specialized cells may have different combinations of TRPC subtype expression, heterologous channel multimerization, TRPC-IP3R interaction, and adaptors to control the multitude of cellular functions in a tissue-dependent manner.
Control of calcium entry via Cav-1 defines a new, clear in vivo function for this protein and caveolae in endothelial cells. The expression of Cav-1 in other specialized cells such as adipocytes, epithelial cells, and vascular smooth muscle cells suggests Cav-1 may influence additional pathways of calcium regulation and calcium-mediated signal transduction in these cell types. The role of caveolins and lipid rafts as plasma membrane microdomains in signal transduction is a attractive hypothesis to integrate disparate pathways into concentrated lipid domains; however, this concept has been met with skepticism despite clear genetic evidence showing impaired mechanosignaling, nitric oxide synthesis, transcytosis, pulmonary and cardiac abnormalities, and abnormal adipogenesis in mice deficient in Cav-1 (18, 23, 53, 54). The cell-specific reconstitution of Cav-1 or perhaps Cav-3, in the respective deficient backgrounds, provides another level of support for the vital role of this protein and organelle in physiological processes.
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FOOTNOTES |
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1 To whom correspondence should be addressed. E-mail: william.sessa{at}yale.edu.
2 The abbreviations used are: ER, endoplasmic reticulum; DAG, diacylglycerol; IP3, inositol 1,4,5-trisphosphate; IP3R, IP3 receptor; TRP, transient receptor potential; TRPC, transient receptor potential channel; WT, wild type; KO, knock-out; GST, glutathione S-transferase; PBS, phosphate-buffered saline; siRNA, small inhibitory RNA; Cav-1, caveolin-1; MLEC, murine lung endothelial cells; [Ca2+]i, intracellular Ca2+; ACh, acetylcholine; MES, 4-morpho-lineethanesulfonic acid; PG, prostaglandin; Ab, antibody; TG, transgenic; EC, endothelial cells; RC, reconstituted; 2-APB, 2-aminoethoxydiphenyl borate.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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