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PubMed1998

Caveolae: from a morphological point of view.

Caveolae in the plasma membrane have been a focus of intensive research during the past several years. There has been confusion concerning caveolae and caveola-like membrane domains, but it is now generally thought that the latter is a region distinct from caveolae. However, due to similar buoyancy of caveolae and caveola-like membranes, whether caveolae in situ are enriched with a given molecule is often difficult to be concluded by biochemical techniques alone. Furthermore, relatively shallow caveolae may be detected by some techniques, but not by others. Thus whether a molecule is enriched in caveolae should be confirmed by methods based on different principles. Among many putative caveolar molecules, those related to Ca2+ influx and extrusion were shown to be concentrated in caveolae by both immunocytochemical and biochemical techniques. In conjunction with other characteristics, the result implies that caveolae may function as a mobile compartment for Ca2+ signalling.

Animals↗
PubMed2004

Caveolae localization and caveolin expressions in Schwann cells of mature rat spinal nerves.

Caveolae are a specific plasmalemmal microdomain which appear as flask-like invaginations and/or vesicles attaching just beneath the plasmalemma. Caveolae are present in many cell types and are found in the Schwann cells that ensheath myelinated and unmyelinated nerve fibers of the peripheral nervous tissues. However, the precise distribution in the Schwann cell plasmalemma and the functional properties of these caveolae remains obscure. The present study revealed: (1) Caveolae are most commonly observed in the Schwann cell plasmalemma of myelinated nerve fibers. (2) Caveolae are principally located in the perikaryal plasmalemma of Schwann cells of the myelinated nerves. (3) Caveolin-1 is expressed strongly in Schwann cell caveolae. (4) Caveolin 2 and 3 are also immunohistochemically detected in Schwann cell caveolae, although the immunoreactivities are slight. The results suggested that caveolae of the peripheral nervous system are involved in cellular activities specific to Schwann cells in myelinated nerve fibers. These caveolae may contain receptors for signaling molecules that could affect the myelinated nerve fibers, and/or proteins related to ion transfer activity. Further studies are necessary in order to clarify the types of proteins associated with Schwann cell caveolae.

Animals↗
PubMed1997

Bradykinin sequesters B2 bradykinin receptors and the receptor-coupled Galpha subunits Galphaq and Galphai in caveolae in DDT1 MF-2 smooth muscle cells.

In this report, we show that the vasoactive peptide agonist bradykinin (BK) when bound to B2 BK receptors on DDT1 MF-2 smooth muscle cells promotes the recruitment and sequestration of the occupied receptors and the receptor-coupled G-protein alpha subunits Galphaq and Galphai in caveolae. Association of ligand receptor complexes and Galpha subunits with caveolae was indicated by their co-enrichment on density gradients with caveolin, a marker protein for caveolae. Caveolin and Galpha subunits were monitored by immunoblotting, whereas receptors were monitored as ligand receptor complexes formed by labeling receptors with the agonist BK or the antagonist NPC17731 prior to cell disruption and caveolae enrichment. These complexes were detected with radioligand and by immunoblotting with BK antibodies. A direct interaction of Galpha subunits with caveolin was also indicated by their co-immunoprecipitation. Immunoelectron microscopy revealed that the enriched caveolin, Galpha subunits, and BK receptor complexes were present in structures of 0.1-0.2 microm. At 4 degrees C, BK and NPC17731 receptor complexes were detected in caveolae, and both complexes were sensitive to acid washing prior to cell disruption and caveolae enrichment. Elevation of the temperature to 37 degrees C increased the amount of BK receptor complexes in caveolae with a maximal response at 10 min (continuous labeling) or 20 min (single-round labeling), and the complexes became acid-resistant. These conditions also increased the amount of Galphaq and Galphai in caveolae with a maximal response at 5-10 min. In contrast, the NPC17731 receptor complexes remained acid-sensitive and dissociated at this temperature, and antagonists did not increase the amount of Galpha subunits in caveolae. These results show that some agonists that act through G-protein-coupled receptors promote the association of their receptors and receptor-coupled Galpha subunits with caveolae.

Animals↗
PubMed1999

Oxidized low density lipoprotein displaces endothelial nitric-oxide synthase (eNOS) from plasmalemmal caveolae and impairs eNOS activation.

Hypercholesterolemia-induced vascular disease and atherosclerosis are characterized by a decrease in the bioavailability of endothelium-derived nitric oxide. Endothelial nitric-oxide synthase (eNOS) associates with caveolae and is directly regulated by the caveola protein, caveolin. In the present study, we examined the effects of oxidized low density lipoprotein (oxLDL) on the subcellular location of eNOS, on eNOS activation, and on caveola cholesterol in endothelial cells. We found that treatment with 10 microgram/ml oxLDL for 60 min caused greater than 90% of eNOS and caveolin to leave caveolae. Treatment with oxLDL also inhibited acetylcholine-induced activation of eNOS but not prostacyclin production. oxLDL did not affect total cellular eNOS abundance. Oxidized LDL also did not affect the palmitoylation, myristoylation or phosphorylation of eNOS. Oxidized LDL, but not native LDL, or HDL depleted caveolae of cholesterol by serving as an acceptor for cholesterol. Cyclodextrin also depleted caveolae of cholesterol and caused eNOS and caveolin to translocate from caveolae. Furthermore, removal of oxLDL allowed eNOS and caveolin to return to caveolae. We conclude that oxLDL-induced depletion of caveola cholesterol causes eNOS to leave caveolae and inhibits acetylcholine-induced activation of the enzyme. This process may be an important mechanism in the early pathogenesis of atherosclerosis.

Animals↗
PubMed1975

The relative contributions of the folds and caveolae to the surface membrane of frog skeletal muscle fibres at different sarcomere lengths.

The plasmalemmal area of striated muscle fibres is greater than the apparent surface area (A = circumference x length) because of variable folds and the invaginations of the caveolae and T-tubules. Freeze-fracture replicas of the surface membrane of sartorius and semitendinosus muscles from Rana pipiens have been used to determine the numbers and distribution of folds and caveolae at different sarcomere lengths. (1) The plasmalemma folds are variable in size and shape, but are always oriented perpendicular to the long axis of the fibre. The folds vary with stretch, being more prominent at short sarcomere lengths. The caveolae are elliptical invaginations of the plasmalemma which open to the outside by a narrow "neck" of approximately 20 nm. The caveolar lumen has an average long dimension of 81.6 +/- 11.7 nm and an average short dimension of 66.9 +/- 7.9 nm. The caveolar "necks" only can be seen in freeze-fracture replicas and these are distributed in two circumferential bands on either side of the Z-line, and in longitudinal bands separated by distances of 1-5 mum. In the sartorius muscle, at a sarcomere length of 2.8 mum, there is an average number of thirty-seven caveolae per square micrometer of fibre surface. (2) During passive stretch the opening of folds provides membrane for the necessary increase in surface area up to a sarcomere length of about 3.0 mum. This length is defined as the critical sarcomere length (Sc). The number of caveolae remains constant at all sarcomere lengths less than Sc and thus their "necks" have been used as membrane markers to determine the amount of folding at different sarcomere lengths. The membrane area contained in folds and caveolae is expressed as a fraction of the apparent surface area (A). For example, in the sartorius muscle, at a sarcomere length of 2.4 mum, the membrane area, excluding the T-tubules, is: A + 0.1A (folding) + 0.7A (caveolae) = 1.8A. (3) For stretch beyond Sc membrane is provided by the opening of caveolae. At a sarcomere length of about 8 mum all the caveolae are open and the fibres rupture with further stretch. (4) The relative contributions of folds and caveolae vary with sarcomere length in a way that is consistent with assumptions of constant volume and plasmalemma area. The maintenance of constant plasmalemma area, even after excessive stretch, suggests that the plasmalemma is relatively inelastic in this situation.

Animals↗
PubMed1999

Activated protein kinase C isoforms target to cardiomyocyte caveolae : stimulation of local protein phosphorylation.

Protein kinase C (PKC) isoforms constitute an important component of the signal transduction pathway used by cardiomyocytes to respond to a variety of extracellular stimuli. Translocation to distinct intracellular sites represents an essential step in the activation of PKC isoforms, presumably as a prerequisite for stable access to substrate. Caveolae are specialized subdomains of the plasma membrane that are reported to concentrate key signaling proteins and may represent a locus for PKC action, given that PKC activators have been reported to dramatically alter caveolae morphology. Accordingly, this study examines whether PKC isoforms initiate signaling in cardiomyocyte caveolae. Phorbol ester-sensitive PKC isoforms were detected at very low levels in caveolae fractions prepared from unstimulated cardiomyocytes; phorbol 12-myristate 13-acetate (PMA) (but not 4alpha-PMA, which does not activate PKC) recruited calcium-sensitive PKCalpha and novel PKCdelta and PKCepsilon to this compartment. The subcellular localization of the phorbol ester-insensitive PKClambda isoform was not influenced by PMA. Endothelin also induced the selective translocation of PKCalpha and PKCepsilon (but not PKCdelta or PKClambda) to caveolae. Multiple components of the extracellular signal-regulated protein kinase (ERK) cascade, including A-Raf, c-Raf-1, mitogen-activated protein kinase kinase, and ERK, were detected in caveolae under resting conditions. Although levels of these proteins were not altered by PMA, translocation of phorbol ester-sensitive PKC isoforms to caveolae was associated with the activation of a local ERK cascade as well as the phosphorylation of a approximately 36-kDa substrate protein in this fraction. Finally, a minor fraction of a protein that has been designated as a receptor for activated protein kinase C resides in caveolae and (along with caveolin-3) could represent a mechanism to target PKC isoforms to cardiomyocyte caveolae. These studies identify cardiomyocyte caveolae as a meeting place for activated PKC isoforms and their downstream target substrates.

Animals↗
PubMed2005

Caveolae, lipid rafts, and vascular disease.

Caveolae and lipid rafts are discrete regions within the plasma membrane that coordinate and regulate a variety of signaling processes. The exact relationship between caveolae and lipid rafts is unclear. However, caveolae contain a protein called caveolin that serves as a biochemical marker for caveolae. In addition, caveolin plays a role in maintaining the lipid composition of caveolae, the morphology of caveolae, and the signals that emanate from caveolae. The physiologic importance of caveolae is evidenced by recent studies using caveolin knockout mice that show dramatic abnormalities in the cardiovascular system, such as pulmonary hypertension and cardiac hypertrophy. In this review, we will focus on the role of caveolae in the cardiovascular system.

Animals↗
PubMed2003

Caveolae, caveolin and cav-p60 in smooth muscle and renin-producing cells in the rat kidney.

AIMS: In vascular smooth muscle cells caveolae are important for signalling mechanisms regulating vascular contraction. In smooth muscle layer of the renal afferent arteriole juxtaglomerular cells (JG cells) are non-contractile renin producing cells that have the capacity to change their phenotype into smooth muscle cells and back again by metaplastic transformation. Signalling mechanisms in JG cells are not fully understood and we therefore investigated if caveolae were present, and thereby could be involved as integrators of cellular signalling in both of these phenotypes of smooth muscle cells. METHODS: Using electron microscopy we compared the number of caveolae in JG cells and smooth muscle cells in the afferent arteriole of the rat kidney. The expression of caveolin and cav-p60 was examined using a combination of immunogold electron microscopy and immunofluorescence microscopy. RESULTS: We found that JG cells have sixfold less caveolae per cell surface sectional length than smooth muscle cells. The expression of cavolin-1 and cav-p60 correlated with the number of caveolae. An examination of the general distribution of caveolae, cav-p60 and caveolins in the rat kidney showed that cav-p60, like caveolin-1, is a specific maker of caveolae. CONCLUSION: The number of caveolae in JG cells is very low, and this makes it unlikely that caveolae are of major importance for the renin secretion specific for JG cells.

Animals↗
PubMed2000

Differential targeting of beta -adrenergic receptor subtypes and adenylyl cyclase to cardiomyocyte caveolae. A mechanism to functionally regulate the cAMP signaling pathway.

Differential modes for beta(1)- and beta(2)-adrenergic receptor (AR) regulation of adenylyl cyclase in cardiomyocytes is most consistent with spatial regulation in microdomains of the plasma membrane. This study examines whether caveolae represent specialized subdomains that concentrate and organize these moieties in cardiomyocytes. Caveolae from quiescent rat ventricular cardiomyocytes are highly enriched in beta(2)-ARs, Galpha(i), protein kinase A RIIalpha subunits, caveolin-3, and flotillins (caveolin functional homologues); beta(1)-ARs, m(2)-muscarinic cholinergic receptors, Galpha(s), and cardiac types V/VI adenylyl cyclase distribute between caveolae and other cell fractions, whereas protein kinase A RIalpha subunits, G protein-coupled receptor kinase-2, and clathrin are largely excluded from caveolae. Cell surface beta(2)-ARs localize to caveolae in cardiomyocytes and cardiac fibroblasts (with markedly different beta(2)-AR expression levels), indicating that the fidelity of beta(2)-AR targeting to caveolae is maintained over a physiologic range of beta(2)-AR expression. In cardiomyocytes, agonist stimulation leads to a marked decline in the abundance of beta(2)-ARs (but not beta(1)-ARs) in caveolae. Other studies show co-immunoprecipitation of cardiomyocytes adenylyl cyclase V/VI and caveolin-3, suggesting their in vivo association. However, caveolin is not required for adenylyl cyclase targeting to low density membranes, since adenylyl cyclase targets to low buoyant density membrane fractions of HEK cells that lack prototypical caveolins. Nevertheless, cholesterol depletion with cyclodextrin augments agonist-stimulated cAMP accumulation, indicating that caveolae function as negative regulators of cAMP accumulation. The inhibitory interaction between caveolae and the cAMP signaling pathway as well as domain-specific differences in the stoichiometry of individual elements in the beta-AR signaling cascade represent important modifiers of cAMP-dependent signaling in the heart.

Adenylyl Cyclases↗
PubMed2002

Caveolae are highly immobile plasma membrane microdomains, which are not involved in constitutive endocytic trafficking.

To investigate whether caveolae are involved in constitutive endocytic trafficking, we expressed N- and C- terminally green fluorescent protein (GFP)-tagged caveolin- 1 fusion proteins in HeLa, A431, and Madin-Darby canine kidney cells. The fusion proteins were shown by immunogold labeling to be sorted correctly to caveolae. By using confocal microscopy and photobleaching techniques, it was found that although intracellular structures labeled with GFP-tagged caveolin were dynamic, GFP-labeled caveolae were very immobile. However, after incubation with methyl- beta-cyclodextrin, distinct caveolae disappeared and the mobility of GFP-tagged caveolin in the plasma membrane increased. Treatment of cells with cytochalasin D caused lateral movement and aggregation of GFP-labeled caveolae. Therefore, both cholesterol and an intact actin cytoskeleton are required for the integrity of GFP-labeled caveolae. Moreover, stimulation with okadaic acid caused increased mobility and internalization of the labeled caveolae. Although the calculated mobile fraction (for t = infinity) of intracellular, GFP-tagged caveolin- associated structures was 70-90%, GFP-labeled caveolae in unstimulated cells had a mobile fraction of <20%, a value comparable to that previously reported for E-cadherin in junctional complexes. We therefore conclude that caveolae are not involved in constitutive endocytosis but represent a highly stable plasma membrane compartment anchored by the actin cytoskeleton.

Animals↗
PubMed2003

Intersectin regulates fission and internalization of caveolae in endothelial cells.

Intersectin, a multiple Eps15 homology and Src homology 3 (SH3) domain-containing protein, is a component of the endocytic machinery in neurons and nonneuronal cells. However, its role in endocytosis via caveolae in endothelial cells (ECs) is unclear. We demonstrate herein by coimmunoprecipitation, velocity sedimentation on glycerol gradients, and cross-linking that intersectin is present in ECs in a membrane-associated protein complex containing dynamin and SNAP-23. Electron microscopy (EM) immunogold labeling studies indicated that intersectin associated preferentially with the caveolar necks, and it remained associated with caveolae after their fission from the plasmalemma. A cell-free system depleted of intersectin failed to support caveolae fission from the plasma membrane. A biotin assay used to quantify caveolae internalization and extensive EM morphological analysis of ECs overexpressing wt-intersectin indicated a wide range of morphological changes (i.e., large caveolae clusters marginated at cell periphery and pleiomorphic caveolar necks) as well as impaired caveolae internalization. Biochemical evaluation of caveolae-mediated uptake by ELISA showed a 68.4% inhibition by reference to control. We also showed that intersectin interaction with dynamin was important in regulating the fission and internalization of caveolae. Taken together, the results indicate the crucial role of intersectin in the mechanism of caveolae fission in endothelial cells.

Adaptor Proteins, Vesicular Transport↗
PubMed2001

Insulin induces translocation of glucose transporter GLUT4 to plasma membrane caveolae in adipocytes.

Insulin-stimulated glucose uptake in muscle and adipose tissue is the result of translocation of insulin-regulated glucose transporters (GLUT4) from intracellular vesicles to the plasma membrane. Here we report that GLUT4 in the plasma membrane of 3T3-L1 adipocytes were located predominantly in caveolae invaginations: by immunogold electron microscopy of plasma membranes, 88% of GLUT4 were localized to caveolae structures and this distribution within the plasma membrane was not affected by insulin. By immunofluorescence microscopy, a major part of GLUT 4 was colocalized with caveolin. The total amount of GLUT4 in the plasma membrane increased 2.2-fold in response to insulin as determined by immunogold electron or immunofluorescence microscopy. GLUT4 were enriched in caveolae fractions isolated without detergents from plasma membranes of rat adipocytes. In these fractions, GLUT4 were largely confined to caveolin-containing membranes of the caveolae preparation isolated from insulin-stimulated cells, determined by electron microscopy. Insulin increased the amount of GLUT4 2.7-fold in this caveolae fraction. Caveolae were purified further by immunoisolation with antibodies against caveolin. The amount of GLUT4 increased to the same extent in the immunopurified caveolae as in the cruder caveolae fractions from insulin-stimulated cells. We conclude that insulin induces translocation of GLUT4 to caveolae.

3T3 Cells↗
PubMed2004

n-3 PUFA alter caveolae lipid composition and resident protein localization in mouse colon.

Caveolae, by virtue of their unique lipid environment, serve as signaling platforms that regulate cellular events. Perturbations in caveolae lipid composition have been shown in vitro to displace proteins from lipid microdomains, thereby altering their functionality and subsequent downstream signaling. Because membrane remodeling may not be accurately represented by using pharmacological treatments and in vitro models, we investigated the in vivo ability of dietary n-3 polyunsaturated fatty acids (PUFA) to alter caveolae lipid environment and the compartmentalization of resident proteins in mouse colonic mucosa. n-3 PUFA were examined for their chemoprotective, membrane lipid-modifying properties. Colonic caveolae in mice fed n-6 or n-3 PUFA enriched diets were characteristically enriched in cholesterol, sphingomyelin, and caveolin-1. n-3 PUFA feeding, compared with n-6 PUFA, significantly altered colonic caveolae microenvironment by increasing phospholipid n-3 fatty acyl content and reducing both cholesterol (by 46%) and caveolin-1 (by 53%), without altering total cellular levels. Concomitantly, localization of caveolae-resident signaling proteins H-Ras and eNOS in colonic caveolae was decreased by n-3 PUFA, by 45 and 56%, respectively. The distribution of non-caveolae proteins K-Ras and clathrin was unaffected. Moreover, EGF-stimulated H-Ras, but not K-Ras activation was significantly suppressed following n-3 PUFA feeding, in parallel with the selective alterations in their microlocalization. These findings reveal a novel modality by which n-3 PUFA remodel membrane microdomains in vivo and thereby alter caveolae protein localization and functionality.

Animals↗
PubMed2004

Caveolae-associated signalling in smooth muscle.

Caveolae are flask-shaped invaginations in the membrane that depend on the contents of cholesterol and on the structural protein caveolin. The organisation of caveolae in parallel strands between dense bands in smooth muscle is arguably unique. It is increasingly recognised, bolstered in large part by recent studies in caveolae deficient animals, that caveolae sequester and regulate a variety of signalling intermediaries. The role of caveolae in smooth muscle signal transduction, as inferred from studies on transgenic animals and in vitro approaches, is the topic of the current review. Both G-protein coupled receptors and tyrosine kinase receptors are believed to cluster in caveolae, and the exciting possibility that caveolae provide a platform for interactions between the sarcoplasmic reticulum and plasmalemmal ion channels is emerging. Moreover, messengers involved in Ca2+ sensitization of myosin phosphorylation and contraction may depend on caveolae or caveolin. Caveolae thus appear to constitute an important signalling domain that plays a role not only in regulation of smooth muscle tone, but also in proliferation, such as seen in neointima formation and atherosclerosis.

Animals↗
PubMed2006

Regulation of growth plate chondrocytes by 1,25-dihydroxyvitamin D3 requires caveolae and caveolin-1.

UNLABELLED: We examined the role of caveolae and caveolin-1 in the mechanism of 1alpha,25(OH)(2)D(3) action in growth plate chondrocytes. We found that caveolae are required for rapid 1alpha,25(OH)(2)D(3)-dependent PKC signaling, and caveolin-1 must be present based on studies using chondrocytes from Cav-1(-/-) mice. INTRODUCTION: 1,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] regulates endochondral ossification in part through membrane-associated mechanisms, including protein kinase C (PKC) signaling activated by a membrane-associated 1alpha,25(OH)(2)D(3)-binding protein, ERp60. We tested the hypothesis that caveolae are required for 1alpha,25(OH)(2)D(3) action and play an important role in regulating chondrocyte biology and growth plate physiology. MATERIALS AND METHODS: Rat costochondral chondrocytes were examined for caveolae by transmission electron microscopy of cultured cells and of cells in situ. Western blots and confocal microscopy were used to detect caveolae proteins including caveolin-1 (Cav-1) and 1alpha,25(OH)(2)D(3) receptors. Caveolae cholesterol was depleted with beta-cyclodextrin (CD) and effects of 1alpha,25(OH)(2)D(3) on PKC, DNA synthesis, alkaline phosphatase, and proteoglycan production determined. Chondrocytes from Cav-1(-/-) and C57BL/6 wildtype mice were also treated with 1alpha,25(OH)(2)D(3). Epiphyses and costochondral junctions of 8-week-old male Cav-1(-/-) and wildtype mice (N = 8) were compared by histomorphometry and microCT. Data were analyzed by ANOVA and Bonferroni for posthoc comparisons. RESULTS: Growth zone chondrocytes had caveolae and Cav-1, -2, and -3. Resting zone chondrocytes, which do not exhibit a rapid 1alpha,25(OH)(2)D(3)-dependent increase in PKC activity, also had these caveolins, but caveolae were larger and fewer in number. ERp60 but not VDR co-localized with Cav-1 in plasma membranes and in lipid rafts. CD-treatment blocked 1alpha,25(OH)(2)D(3) effects on all parameters tested. The Cav-1(-/-) cells did not respond to 1alpha,25(OH)(2)D(3), although 1alpha,25(OH)(2)D(3) increased PKC, alkaline phosphatase, and [(35)S]-sulfate incorporation in wildtype C57BL/6 cells. Histology and microCT showed that Cav-1(-/-) growth plates were longer and had more hypertrophic cells in each column. Growth plate changes were reflected in the metaphysis. CONCLUSIONS: The membrane-mediated effects of 1alpha,25(OH)(2)D(3) require caveolae and Cav-1, and Cav-1 deficiency results in altered growth plate physiology.

Animals↗
PubMed1982

Plasmalemmal vesicles (caveolae) of fibrous astrocytes of the cat optic nerve.

Plasmalemmal vesicles (caveolae) are described in fibrous astrocytes of the cat optic nerve. In thin sections, astrocytic caveolae appear as flask-shaped invaginations of the plasma membrane with a 60-nm lumen that communicates with the extracellular space through a 35-nm stoma. Occasional bilobed caveolae occur. The caveolae extend approximately 85 nm into the astrocytic cytoplasm and are often embedded within a granular or filamentous ectoplasmic substance connected by 4-8-nm filament bridges to underlying bundles of 10-nm glial filaments. In freeze-fracture replicas, the caveolar stomata appear as dimples on the P face and as craters on the E face, often arranged in hexagonal or linear arrays and spaced at a center-to-center distance of 110-130 nm. The caveolar membrane is apparently particle-free. Fibrous astrocytes related to the connective-tissue septa of the optic nerve show different densities of caveolae on different areas of their plasma membranes. Plasma membranes apposing a basal lamina have few caveolae, whereas membranes not apposed to the basal lamina but to other astrocytic membranes have up to 17 caveolae/micrometers 2. Caveolae also occur on astrocytic plasma membranes apposed to myelin sheaths. Possible functions of the astrocytic caveolae are discussed in the light of plasmalemmal properties of other types of caveolae-bearing cells.

Animals↗
PubMed1995

Caveolae from luminal plasmalemma of rat lung endothelium: microdomains enriched in caveolin, Ca(2+)-ATPase, and inositol trisphosphate receptor.

A distinctive feature of many endothelia is an abundant population of noncoated plasmalemmal vesicles, or caveolae. Caveolae have been implicated in many important cellular processes, including transcytosis, endocytosis, potocytosis, and even signal transduction. Because caveolae have not been purified from endothelial cell surfaces, little is known directly about their structure and function in the endothelium. To delineate the transport role of these caveolae, we purified them from isolated luminal endothelial plasma membranes of rat lung. The rat lung luminal endothelial cell surfaces were isolated after coating them, in situ, with positively charged colloidal silica. The caveolae were then separated from these coated membranes and purified to yield a homogeneous population of morphologically distinct vesicles enriched in the structural protein caveolin. As with caveolae found on the endothelial cell surface in vivo, these highly purified caveolae contained the plasmalemmal Ca(2+)-ATPase and inositol 1,4,5-trisphosphate surface receptors. By contrast, other plasma membrane proteins were excluded from the caveolae, including angiotensin-converting enzyme, beta-actin, and band 4.1. The purified caveolae appeared to represent specific microdomains of the cell surface with their own unique molecular topography.

Animals↗
PubMed1996

Localization of platelet-derived growth factor-stimulated phosphorylation cascade to caveolae.

Previously we showed that interleukin 1 beta stimulates the conversion of sphingomyelin to ceramide in the caveolae fraction of normal human fibroblasts. The ceramide, in turn, blocked platelet-derived growth factor (PDGF) stimulated DNA synthesis. We now present evidence that the PDGF receptor initiates signal transduction from caveolae. Cell fractionation and immunocytochemistry show caveolae to be the principal location of PDGF receptors at the cell surface. Multiple caveolae proteins acquire phosphotyrosine when PDGF binds to its receptor, but the hormone appears to have little effect on the tyrosine phosphorylation of non-caveolae membrane proteins. Five proteins known to interact with the phosphorylated receptor were found to be highly enriched in caveolae membrane. PDGF caused the concentration of three of these proteins to significantly increase in the caveolae fraction. Finally, PDGF stimulated the association of a 190-kDa phosphoprotein with the caveolae marker protein, caveolin. Therefore, ceramide may modulate PDGF receptor function directly in caveolae.

Caveolin 1↗