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Simian virus 40 (SV40) has been shown to enter mammalian cells via uncoated plasma membrane invaginations. Viral particles subsequently appear within the endoplasmic reticulum. In the present study, we have examined the surface binding and internalization of SV40 by immunoelectron microscopy. We show that SV40 associates with surface pits which have the characteristics of caveolae and are labeled with antibodies to the caveolar marker protein, caveolin-1. SV40 is believed to use major histocompatibility complex (MHC) class I molecules as cell surface receptors. Using a number of MHC class I-specific monoclonal antibodies, we found that both viral infection and association of virus with caveolae were strongly reduced by preincubation with anti-MHC class I antibodies. Because binding of SV40 to MHC class I molecules may induce clustering, we investigated whether antibody cross-linked class I molecules also redistributed to caveolae. Clusters of MHC class I molecules were indeed shown to be specifically associated with caveolin-labeled surface pits. Taken together, the results suggest that SV40 may make use of MHC class I molecule clustering and the caveolae pathway to enter mammalian cells.

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The filoviruses Ebola Zaire virus and Marburg virus are believed to infect target cells through endocytic vesicles, but the details of this pathway are unknown. We used a pseudotyping strategy to investigate the cell biology of filovirus entry. We observed that specific inhibitors of the caveola system, including cholesterol-sequestering drugs and phorbol esters, inhibited the entry of filovirus pseudotypes into human cells. We also measured slower cell entry kinetics for both filovirus pseudotypes than for pseudotypes of vesicular stomatitis virus (VSV), which has been recognized to exploit the clathrin-mediated entry pathway. Finally, visualization by immunofluorescence and confocal microscopy revealed that the filovirus pseudotypes colocalized with the caveola protein marker caveolin-1 but that VSV pseudotypes did not. Collectively, these results provide evidence suggesting that filoviruses use caveolae to gain entry into cells.

GPI-anchored surface proteins mediate many important functions, including transport, signal transduction, adhesion, and protection against complement. They cluster into glycolipid-based membrane domains and caveolae, plasmalemmal vesicles involved in the transcytosis and endocytosis of these surface proteins. However, in lymphocytes, neither the characteristic flask shaped caveolae nor caveolin, a transmembrane protein typical of caveolae, have been observed. Here, we show that the GPI-anchored CD59 molecule on Jurkat T cells is internalized after cross-linking, a process inhibited by nystatin, a sterol chelating agent. Clustered CD59 molecules mostly accumulate in non-coated invaginations of the lymphocyte membrane before endocytosis, in marked contrast with the pattern of CD3-TCR internalization. Cytochalasin H blocked CD59 internalization in lymphocytes, but neither CD3 internalization nor transferrin uptake. Confocal microscopy analysis of F-actin distribution within lymphocytes showed that CD59 clusters were associated with patches of polymerized actin. Also, we found that internalization of CD59 was prevented by the protein kinase C inhibitor staurosporine and by the protein kinase A activator forskolin. Thus, in lymphocytes, as in other cell types, glycolipid-based domains provide sites of integration of signaling pathways involved in GPI-anchored protein endocytosis. This process, which is regulated by both protein kinase C and A activity, is tightly controlled by the dynamic organization of actin cytoskeleton, and may be critical for polarized contacts of circulating cells.

Electron and confocal microscopy were used to observe the entry and the movement of polyomavirus virions and artificial virus-like particles (VP1 pseudocapsids) in mouse fibroblasts and epithelial cells. No visible differences in adsorption and internalization of virions and VP1 pseudocapsids (“empty” or containing DNA) were observed. Viral particles entered cells internalized in smooth monopinocytic vesicles, often in the proximity of larger, caveola-like invaginations. Both “empty” vesicles derived from caveolae and vesicles containing viral particles were stained with the anti-caveolin-1 antibody, and the two types of vesicles often fused in the cytoplasm. Colocalization of VP1 with caveolin-1 was observed during viral particle movement from the plasma membrane throughout the cytoplasm to the perinuclear area. Empty vesicles and vesicles with viral particles moved predominantly along microfilaments. Particle movement was accompanied by transient disorganization of actin stress fibers. Microfilaments decorated by the VP1 immunofluorescent signal could be seen as concentric curves, apparently along membrane structures that probably represent endoplasmic reticulum. Colocalization of VP1 with tubulin was mostly observed in areas close to the cell nuclei and on mitotic tubulin structures. By 3 h postinfection, a strong signal of the VP1 (but no viral particles) had accumulated in the proximity of nuclei, around the outer nuclear membrane. However, the vast majority of VP1 pseudocapsids did not enter the nuclei.

Glycosyl-phosphatidylinositol (GPI)-linked proteins are transported to the apical surface of epithelial cells where they undergo cholesterol- dependent clustering in membrane micro-invaginations, termed caveolae or plasmalemmal vesicles. However, the sorting machinery responsible for this caveolar-clustering mechanism remains unknown. Using transfected MDCK cells as a model system, we have identified a complex of cell surface molecules (80, 50, 40, 22-24, and 14 kD) that interact in a pH- and cholesterol-dependent fashion with an apical recombinant GPI-linked protein. A major component of this hetero-oligomeric protein complex is caveolin, a type II transmembrane protein. As this hetero- oligomeric caveolin complex is detectable almost immediately after caveolin synthesis, our results suggest that caveolae may assemble intracellularly during transport to the cell surface. As such, our studies have implications for understanding both the intracellular biogenesis of caveolae and their subsequent interactions with GPI- linked proteins in epithelia and other cell types.

Plasmalemmal vesicles (PVs) or caveolae are plasma membrane invaginations and associated vesicles of regular size and shape found in most mammalian cell types. They are particularly numerous in the continuous endothelium of certain microvascular beds (e.g., heart, lung, and muscles) in which they have been identified as transcytotic vesicular carriers. Their chemistry and function have been extensively studied in the last years by various means, including several attempts to isolate them by cell fractionation from different cell types. The methods so far used rely on nonspecific physical parameters of the caveolae and their membrane (e.g., size-specific gravity and solubility in detergents) which do not rule out contamination from other membrane sources, especially the plasmalemma proper. We report here a different method for the isolation of PVs from plasmalemmal fragments obtained by a silica-coating procedure from the rat lung vasculature. The method includes sonication and flotation of a mixed vesicle fraction, as the first step, followed by specific immunoisolation of PVs on anticaveolin-coated magnetic microspheres, as the second step. The mixed vesicle fraction, is thereby resolved into a bound subfraction (B), which consists primarily of PVs or caveolae, and a nonbound subfraction (NB) enriched in vesicles derived from the plasmalemma proper. The results so far obtained indicate that some specific endothelial membrane proteins (e.g., thrombomodulin, functional thrombin receptor) are distributed about evenly between the B and NB subfractions, whereas others are restricted to the NB subfraction (e.g., angiotensin converting enzyme, podocalyxin). Glycoproteins distribute unevenly between the two subfractions and antigens involved in signal transduction [e.g., annexin II, protein kinase C alpha, the G alpha subunits of heterotrimeric G proteins (alpha s, alpha q, alpha i2, alpha i3), small GTP-binding proteins, endothelial nitric oxide synthase, and nonreceptor protein kinase c-src] are concentrated in the NB (plasmalemma proper-enriched) subfraction rather than in the caveolae of the B subfraction. Additional work should show whether discrepancies between our findings and those already recorded in the literature represent inadequate fractionation techniques or are accounted for by chemical differentiation of caveolae from one cell type to another.

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Caveolae or noncoated plasmalemmal vesicles found in a variety of cells have been implicated in a number of important cellular functions including endocytosis, transcytosis, and potocytosis. Their function in transport across endothelium has been especially controversial, at least in part because there has not been any way to selectively inhibit this putative pathway. We now show that the ability of sterol binding agents such as filipin to disassemble endothelial noncoated but not coated plasmalemmal vesicles selectively inhibits caveolae-mediated intracellular and transcellular transport of select macromolecules in endothelium. Filipin significantly reduces the transcellular transport of insulin and albumin across cultured endothelial cell monolayers. Rat lung microvascular permeability to albumin in situ is significantly decreased after filipin perfusion. Conversely, paracellular transport of the small solute inulin is not inhibited in vitro or in situ. In addition, we show that caveolae mediate the scavenger endocytosis of conformationally modified albumins for delivery to endosomes and lysosomes for degradation. This intracellular transport is inhibited by filipin both in vitro and in situ. Other sterol binding agents including nystatin and digitonin also inhibit this degradative process. Conversely, the endocytosis and degradation of activated alpha 2- macroglobulin, a known ligand of the clathrin-dependent pathway, is not affected. Interestingly, filipin appears to inhibit insulin uptake by endothelium for transcytosis, a caveolae-mediated process, but not endocytosis for degradation, apparently mediated by the clathrin-coated pathway. Such selective inhibition of caveolae not only provides critical evidence for the role of caveolae in the intracellular and transcellular transport of select macromolecules in endothelium but also may be useful for distinguishing transport mediated by coated versus noncoated vesicles.

The mechanism by which murine polyomavirus penetrates cells and arrives at the nucleus, the site of viral replication, is not well understood. Simian virus 40 and JC virus, two closely related members of the polyomavirus subfamily, use caveola- and clathrin-mediated uptake pathways for entry, respectively. The data presented here indicate that compounds that block endocytosis of both caveola- and clathrin-derived vesicles have no effect on polyomavirus infectivity. Polyomavirus does not appear to colocalize with either clathrin light chain or caveolin-1 by immunofluorescence microscopy. Additionally, expression of a dominant-negative form of dynamin I has no effect on polyomavirus uptake and infectivity. Therefore, polyomavirus uptake occurs through a class of uncoated vesicles in a clathrin-, caveolin-1-, and dynamin I-independent manner.

Caveolae, also termed plasmalemmal vesicles, are small, flask-shaped, non-clathrin-coated invaginations of the plasma membrane. Caveolin is a principal component of the filaments that make up the striated coat of caveolae. Using caveolin as a marker protein for the organelle, we found that adipose tissue is the single most abundant source of caveolae identified thus far. Caveolin mRNA and protein are strongly induced during differentiation of 3T3-L1 fibroblasts to adipocytes; during adipogenesis there is also a dramatic increase in the complexity of the protein composition of caveolin-rich membrane domains. About 10- 15% of the insulin-responsive glucose transporter GLUT4 is found in this caveolin-rich fraction, and immuno-isolated vesicles containing GLUT4 also contain caveolin. However, in non-stimulated adipocytes the majority of caveolin fractionates with the plasma membrane, while most GLUT4 associates with low-density microsomes. Upon addition of insulin to 3T3-L1 adipocytes, there is a significant increase in the amount of GLUT4 associated with caveolin-rich membrane domains, an increase in the amount of caveolin associated with the plasma membrane, and a decrease in the amount of caveolin associated with low-density microsomes. Caveolin does not undergo a change in phosphorylation upon stimulation of 3T3-L1 adipocytes with insulin. However, after treatment with insulin it is associated with a 32-kD phosphorylated protein. Caveolae thus may play an important role in the vesicular transport of GLUT4 to or from the plasma membrane. 3T3-L1 adipocytes offer an attractive system to study the function of caveolae in several cellular trafficking and signaling events.

Present on the plasma membrane of most metazoans, caveolae are specialized microdomains implicated in several endocytic and trafficking mechanisms. Caveolins and the more recently discovered cavins are the major protein components of caveolae. Previous studies reported that caveolar invaginations can be induced de novo on the surface of caveolae-negative mammalian cells upon heterologous expression of caveolin-1. However, it remains undocumented whether other components in the transfected cells participate in caveolae formation. To address this issue, we have exploited the protozoan Toxoplasma as a heterologous expression system to provide insights into the minimal requirements for caveogenesis and caveolar endocytosis. Upon expression of caveolin-1, Toxoplasma accumulates prototypical exocytic caveolae ‘precursors’ in the cytoplasm. Toxoplasma expressing caveolin-1 alone, or in conjunction with cavin-1, neither develops surface-located caveolae nor internalizes caveolar ligands. These data suggest that the formation of functional caveolae at the plasma membrane in Toxoplasma and, by inference in all non-mammalian cells, requires effectors other than caveolin-1 and cavin-1. Interestingly, Toxoplasma co-expressing caveolin-1 and cavin-1 displays an impressive spiraled network of membranes containing the two proteins, in the cytoplasm. This suggests a synergistic activity of caveolin-1 and cavin-1 in the morphogenesis and remodeling of membranes, as illustrated for Toxoplasma.

Vaccinia virus is a large DNA virus that infects many cell cultures in vitro and animal species in vivo. Although it has been used widely as a vaccine, its cell entry pathway remains unclear. In this study, we showed that vaccinia virus intracellular mature virions bound to the filopodia of HeLa cells and moved toward the cell body and entered the cell through an endocytic route that required a dynamin-mediated pathway but not a clathrin- or caveola-mediated pathway. Moreover, virus penetration required a novel cellular protein, vaccinia virus penetration factor (VPEF). VPEF was detected on cell surface lipid rafts and on vesicle-like structures in the cytoplasm. Both vaccinia virus and dextran transiently colocalized with VPEF, and, importantly, knockdown of VPEF expression blocked vaccinia virus penetration as well as intracellular transport of dextran, suggesting that VPEF mediates vaccinia virus entry through a fluid uptake endocytosis process in HeLa cells. Intracellular VPEF-containing vesicles did not colocalize with Rab5a or caveolin but partially colocalized with Rab11, supporting the idea that VPEF plays a role in vesicle trafficking and recycling in HeLa cells. In summary, this study characterized the mechanism by which vaccinia virus enters HeLa cells and identified a cellular factor, VPEF, that is exploited by vaccinia virus for cell entry through fluid phase endocytosis.

Mouse polyomavirus (PyV) virions enter cells by internalization into smooth monopinocytic vesicles, which fuse under the cell membrane with larger endosomes. Caveolin-1 was detected on monopinocytic vesicles carrying PyV particles in mouse fibroblasts and epithelial cells (33). Here, we show that PyV can be efficiently internalized by Jurkat cells, which do not express caveolin-1 and lack caveolae, and that overexpression of a caveolin-1 dominant-negative mutant in mouse epithelial cells does not prevent their productive infection. Strong colocalization of VP1 with early endosome antigen 1 (EEA1) and of EEA1 with caveolin-1 in mouse fibroblasts and epithelial cells suggests that the monopinocytic vesicles carrying the virus (and vesicles containing caveolin-1) fuse with EEA1-positive early endosomes. In contrast to SV40, PyV infection is dependent on the acidic pH of endosomes. Bafilomycin A1 abolished PyV infection, and an increase in endosomal pH by NH4Cl markedly reduced its efficiency when drugs were applied during virion transport towards the cell nucleus. The block of acidification resulted in the retention of a fraction of virions in early endosomes. To monitor further trafficking of PyV, we used fluorescent resonance energy transfer (FRET) to determine mutual localization of PyV VP1 with transferrin and Rab11 GTPase at a 2- to 10-nm resolution. Positive FRET between PyV VP1 and transferrin cargo and between PyV VP1 and Rab11 suggests that during later times postinfection (1.5 to 3 h), the virus meets up with transferrin in the Rab11-positive recycling endosome. These results point to a convergence of the virus and the cargo internalized by different pathways in common transitional compartments.

Caveolae are plasma membrane domains involved in the uptake of certain pathogens and toxins. Internalization of some cell surface integrins occurs via caveolae suggesting caveolae may play a crucial role in modulating integrin-mediated adhesion and cell migration. Here we demonstrate a critical role for gangliosides (sialo-glycosphingolipids) in regulating caveolar endocytosis in human skin fibroblasts. Pretreatment of cells with endoglycoceramidase (cleaves glycosphingolipids) or sialidase (modifies cell surface gangliosides and glycoproteins) selectively inhibited caveolar endocytosis by >70%, inhibited the formation of plasma membrane domains enriched in sphingolipids and cholesterol (“lipid rafts”), reduced caveolae and caveolin-1 at the plasma membrane by ~80%, and blunted activation of β1-integrin, a protein required for caveolar endocytosis in these cells. These effects could be reversed by a brief incubation with gangliosides (but not with asialo-gangliosides or other sphingolipids) at 10°C, suggesting that sialo-lipids are critical in supporting caveolar endocytosis. Endoglycoceramidase treatment also caused a redistribution of focal adhesion kinase, paxillin, talin, and PIP Kinase Iγ away from focal adhesions. The effects of sialidase or endoglycoceramidase on membrane domains and the distribution of caveolin-1 could be recapitulated by β1-integrin knockdown. These results suggest that both gangliosides and β1-integrin are required for maintenance of caveolae and plasma membrane domains.

The pathway of entry of polyomavirus (Py) has been investigated with glycolipid-deficient C6 cells and added ganglioside GD1a as a specific virus receptor. Unsupplemented C6 cells show a low basal level of infection but become highly infectable by Py following preincubation with the sialic acid-containing ganglioside GD1a (38). Addition of GD1a has no effect on the overall level of virus binding but mediates the internalization and transit of virus to the endoplasmic reticulum (ER). This pathway of entry is cholesterol and caveola dependent and requires intact microtubules as well as a dynamic state of the microfilament system. In contrast to vesicular transport of other cargo via glycolipids, Py particles do not appear to pass through the Golgi apparatus. Colcemid and brefeldin A block transport of the virus to the ER in GD1a-supplemented cells and lead to accumulation of virus in a caveolin-1-containing environment. Several features distinguish the efficient GD1a-mediated pathway of virus uptake from the less-efficient pathway of basal infection in C6 cells.

The dynamins comprise an expanding family of ubiquitously expressed 100-kD GTPases that have been implicated in severing clathrin-coated pits during receptor-mediated endocytosis. Currently, it is unclear whether the different dynamin isoforms perform redundant functions or participate in distinct endocytic processes. To define the function of dynamin II in mammalian epithelial cells, we have generated and characterized peptide-specific antibodies to domains that either are unique to this isoform or conserved within the dynamin family. When microinjected into cultured hepatocytes these affinity-purified antibodies inhibited clathrin-mediated endocytosis and induced the formation of long plasmalemmal invaginations with attached clathrin-coated pits. In addition, clusters of distinct, nonclathrin-coated, flask-shaped invaginations resembling caveolae accumulated at the plasma membrane of antibody-injected cells. In support of this, caveola-mediated endocytosis of labeled cholera toxin B was inhibited in antibody-injected hepatocytes. Using immunoisolation techniques an anti-dynamin antibody isolated caveolar membranes directly from a hepatocyte postnuclear membrane fraction. Finally, double label immunofluorescence microscopy revealed a striking colocalization between dynamin and the caveolar coat protein caveolin. Thus, functional in vivo studies as well as ultrastructural and biochemical analyses indicate that dynamin mediates both clathrin-dependent endocytosis and the internalization of caveolae in mammalian cells.

Ebolavirus (EBOV) is an enveloped, single-stranded, negative-sense RNA virus that causes severe hemorrhagic fever with mortality rates of up to 90% in humans and nonhuman primates. Previous studies suggest roles for clathrin- or caveolae-mediated endocytosis in EBOV entry; however, ebolavirus virions are long, filamentous particles that are larger than the plasma membrane invaginations that characterize clathrin- or caveolae-mediated endocytosis. The mechanism of EBOV entry remains, therefore, poorly understood. To better understand Ebolavirus entry, we carried out internalization studies with fluorescently labeled, biologically contained Ebolavirus and Ebolavirus-like particles (Ebola VLPs), both of which resemble authentic Ebolavirus in their morphology. We examined the mechanism of Ebolavirus internalization by real-time analysis of these fluorescently labeled Ebolavirus particles and found that their internalization was independent of clathrin- or caveolae-mediated endocytosis, but that they co-localized with sorting nexin (SNX) 5, a marker of macropinocytosis-specific endosomes (macropinosomes). Moreover, the internalization of Ebolavirus virions accelerated the uptake of a macropinocytosis-specific cargo, was associated with plasma membrane ruffling, and was dependent on cellular GTPases and kinases involved in macropinocytosis. A pseudotyped vesicular stomatitis virus possessing the Ebolavirus glycoprotein (GP) also co-localized with SNX5 and its internalization and infectivity were affected by macropinocytosis inhibitors. Taken together, our data suggest that Ebolavirus is internalized into cells by stimulating macropinocytosis in a GP-dependent manner. These findings provide new insights into the lifecycle of Ebolavirus and may aid in the development of therapeutics for Ebolavirus infection.

Author Summary

Ebolavirus (EBOV) is an enveloped, single-stranded, negative-sense RNA virus that causes severe hemorrhagic fever with high mortality rates in humans and nonhuman primates. Previous studies suggest roles for clathrin- or caveolae-mediated endocytosis in EBOV entry; however, questions remain regarding the mechanism of EBOV entry. Here, we demonstrate that internalization of EBOV particles is independent of clathrin- or caveolae-mediated endocytosis. Specifically, we show that internalized EBOV particles co-localize with macropinocytosis-specific endosomes (macropinosomes) and that their entry is negatively affected by treatment with macropinocytosis inhibitors. Moreover, the internalization of Ebola virions accelerated the uptake of a macropinocytosis-specific cargo, was associated with plasma membrane ruffling, and was dependent on cellular GTPases and kinases involved in macropinocytosis. We further demonstrate that a pseudotyped vesicular stomatitis virus possessing the EBOV glycoprotein (GP) also co-localizes with macropinosomes and its internalization is similarly affected by macropinocytosis inhibitors. Our results indicate that EBOV uptake into cells involves the macropinocytic pathway and is GP-dependent. These findings provide new insights into the lifecycle of EBOV and may aid in the development of therapeutics for EBOV infection.

Caveolae, a class of cholesterol-rich lipid rafts, are smooth invaginations of the plasma membrane whose formation in nonmuscle cells requires caveolin-1 (Cav1). The recent demonstration that Cav1-associated cavin proteins, in particular PTRF/cavin-1, are also required for caveolae formation supports a functional role for Cav1 independently of caveolae. In tumor cells deficient for Golgi β-1,6N-acetylglucosaminyltransferase V (Mgat5), reduced Cav1 expression is associated not with caveolae but with oligomerized Cav1 domains, or scaffolds, that functionally regulate receptor signaling and raft-dependent endocytosis. Using subdiffraction-limit microscopy, we show that Cav1 scaffolds are homogenous subdiffraction-limit sized structures whose size distribution differs from that of Cav1 in caveolae expressing cells. These cell lines displaying differing Cav1/caveolae phenotypes are effective tools for probing the structure and composition of caveolae. Using stable isotope labeling by amino acids in cell culture, we are able to quantitatively distinguish the composition of caveolae from the background of detergent-resistant membrane proteins and show that the presence of caveolae enriches the protein composition of detergent-resistant membrane, including the recruitment of multiple heterotrimeric G-protein subunits. These data were further supported by analysis of immuno-isolated Cav1 domains and of methyl-β-cyclodextrin-disrupted detergent-resistant membrane. Our data show that loss of caveolae results in a dramatic change to the membrane raft proteome and that this change is independent of Cav1 expression. The proteomics data, in combination with subdiffraction-limit microscopy, indicates that noncaveolar Cav1 domains, or scaffolds are structurally and functionally distinct from caveolae and differentially impact on the molecular composition of lipid rafts.

Early results suggested that the amphotropic murine leukemia virus (A-MLV) does not enter cells via endocytosis through clathrin-coated pits and this gammaretrovirus has therefore been anticipated to fuse directly with the plasma membrane. However, here we present data implicating a caveola-mediated endocytic entry route for A-MLV via its receptor Pit2. Caveolae belong to the cholesterol-rich microdomains characterized by resistance to nonionic detergents such as Triton X-100. Extraction of murine fibroblastic NIH 3T3 cells in cold Triton X-100 showed the presence of the A-MLV receptor Pit2 in detergent-insoluble microdomains. Using coimmunoprecipitation of cell extracts, we were able to demonstrate direct association of Pit2 with caveolin-1, the structural protein of caveolae. Other investigations revealed that A-MLV infection in contrast to vesicular stomatitis virus infection is a slow process (t ≈5 h), which is dependent on plasma membrane cholesterol but independent of NH4Cl treatment of cells; NH4Cl impairs entry via clathrin-coated pits. Furthermore, expression of dominant-negative caveolin-1 decreased the susceptibility to infection via Pit2 by approximately 70%. These results show that A-MLV can enter cells via a caveola-dependent entry route. Moreover, increase in A-MLV infection by treatment with okadaic acid as well as entry of fusion-defective fluorescent A-MLV virions in NIH 3T3 cells further confirmed our findings and show that A-MLV can enter mouse fibroblasts via an endocytic entry route involving caveolae. Finally, we also found colocalization of fusion-defective fluorescent A-MLV virions with caveolin-1 in NIH 3T3 cells. This is the first time substantial evidence has been presented implicating the existence of a caveola-dependent endocytic entry pathway for a retrovirus.

Using total internal reflection fluorescence microscopy (TIR-FM), fluorescence recovery after photobleaching (FRAP), and other light microscopy techniques, we analyzed the dynamics, the activation, and the assembly of caveolae labeled with fluorescently tagged caveolin-1 (Cav1). We found that when activated by simian virus 40 (SV40), a nonenveloped DNA virus that uses caveolae for cell entry, the fraction of mobile caveolae was dramatically enhanced both in the plasma membrane (PM) and in the caveosome, an intracellular organelle that functions as an intermediate station in caveolar endocytosis. Activation also resulted in increased microtubule (MT)-dependent, long-range movement of caveolar vesicles. We generated heterokaryons that contained GFP- and RFP-tagged caveolae by fusing cells expressing Cav1-GFP and -RFP, respectively, and showed that even when activated, individual caveolar domains underwent little exchange of Cav1. Only when the cells were subjected to transient cholesterol depletion, did the caveolae domain exchange Cav1. Thus, in contrast to clathrin-, or other types of coated transport vesicles, caveolae constitute stable, cholesterol-dependent membrane domains that can serve as fixed containers through vesicle traffic. Finally, we identified the Golgi complex as the site where newly assembled caveolar domains appeared first.

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- β-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 = ∞) 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.

Arenaviruses are important causes of viral hemorrhagic fevers in humans. Arenavirus infection of cells occurs via a pH dependent endocytic route, but detailed studies of entry pathways have not been done. We investigated the role of cell membrane cholesterol, caveolae, and clathrin coated pits in infection by Lassa virus (LASV), which utilizes alpha-dystroglycan (α-DG) as a receptor, and Pichindé virus (PICV), which does not. Depletion of cellular cholesterol by treatment with methyl betacyclodextrin (MβCD) or nystatin/progesterone inhibited PICV replication and transfer of packaged marker gene by LASV or PICV pseudotyped retroviral particles. In cells lacking caveolae due to silencing of the caveolin-1 gene, no inhibition of PICV infection or LASV pseudotype transduction was observed. However, PICV infection and LASV and PICV pseudotype transduction was inhibited when an Eps15 dominant negative mutant was used to inhibit clathrin-mediated endocytosis. Altogether, the results indicate that diverse arenaviruses have a common requirement for cell membrane cholesterol and clathrin mediated endocytosis in establishing infection.

Transcytosis via caveolae is critical for maintaining vascular homeostasis by regulating the tissue delivery of macromolecules, hormones, and lipids. In the present study, we test the hypothesis that interactions between F-actin cross-linking protein filamin A and caveolin-1 facilitate the internalization and trafficking of caveolae. Small interfering RNA-mediated knockdown of filamin A, but not filamin B, reduced the uptake and transcytosis of albumin by ∼35 and 60%, respectively, without altering the actin cytoskeletal structure or cell–cell adherens junctions. Mobility of both intracellular caveolin-1–green fluorescent protein (GFP)-labeled vesicles measured by fluorescence recovery after photobleaching and membrane-associated vesicles measured by total internal reflection-fluorescence microscopy was decreased in cells with reduced filamin A expression. In addition, in melanoma cells that lack filamin A (M2 cells), the majority of caveolin-1-GFP was localized on the plasma membrane, whereas in cells in which filamin A expression was reconstituted (A7 cells and M2 cells transfected with filamin A-RFP), caveolin-1-GFP was concentrated in intracellular vesicles. Filamin A association with caveolin-1 in endothelial cells was confirmed by cofractionation of these proteins in density gradients, as well as by coimmunoprecipitation. Moreover, this interaction was enhanced by Src activation, associated with increased caveolin-1 phosphorylation, and blocked by Src inhibition. Taken together, these data suggest that filamin A association with caveolin-1 promotes caveolae-mediated transport by regulating vesicle internalization, clustering, and trafficking.

We tested the hypothesis that the albumin-docking protein gp60, which is localized in caveolae, couples to the heterotrimeric GTP binding protein Gi, and thereby activates plasmalemmal vesicle formation and the directed migration of vesicles in endothelial cells (ECs). We used the water-soluble styryl pyridinium dye N-(3-triethylaminopropyl)-4-(p-dibutylaminostyryl) pyridinium dibromide (FM 1-43) to quantify vesicle trafficking by confocal and digital fluorescence microscopy. FM 1-43 and fluorescently labeled anti-gp60 antibody (Ab) were colocalized in endocytic vesicles within 5 min of gp60 activation. Vesicles migrated to the basolateral surface where they released FM 1-43, the fluid phase styryl probe. FM 1-43 fluorescence disappeared from the basolateral EC surface without the loss of anti-gp60 Ab fluorescence. Activation of cell-surface gp60 by cross-linking (using anti-gp60 Ab and secondary Ab) in EC grown on microporous filters increased transendothelial 125I-albumin permeability without altering liquid permeability (hydraulic conductivity), thus, indicating the dissociation of hydraulic conductivity from the albumin permeability pathway. The findings that the sterol-binding agent, filipin, prevented gp60-activated vesicle formation and that caveolin-1 and gp60 were colocalized in vesicles suggest the caveolar origin of endocytic vesicles. Pertussis toxin pretreatment and expression of the dominant negative construct encoding an 11–amino acid Gαi carboxyl-terminal peptide inhibited endothelial 125I-albumin endocytosis and vesicle formation induced by gp60 activation. Expression of dominant negative Src (dn-Src) and overexpression of wild-type caveolin-1 also prevented gp60-activated endocytosis. Caveolin-1 overexpression resulted in the sequestration of Gαi with the caveolin-1, whereas dn-Src inhibited Gαi binding to caveolin-1. Thus, vesicle formation induced by gp60 and migration of vesicles to the basolateral membrane requires the interaction of gp60 with caveolin-1, followed by the activation of the downstream Gi-coupled Src kinase signaling pathway.

SUMMARY

Caveolae are specialized invaginations of the plasma membrane found in numerous cell types. They have been implicated as playing a role in a variety of physiological processes and are typically characterized by their association with the caveolin family of proteins. We show here by means of targeted gene disruption in mice, that a distinct caveolae-associated protein, Cavin/PTRF, is an essential component of caveolae. Animals lacking Cavin have no morphologically detectable caveolae in any cell type examined and have markedly diminished protein expression of all three caveolin isoforms whilst retaining normal or above normal caveolin mRNA expression. Cavin knockout mice are viable and of normal weight but have higher circulating triglyceride levels, significantly reduced adipose tissue mass, glucose intolerance and hyperinsulinemia, which characteristics constitute a lipodystrophic phenotype. Our results underscore the multi-organ role of caveolae in metabolic regulation and the obligate presence of Cavin for caveolae formation.

Plasmalemmal caveolae are a membrane specialization that mediates transcytosis across endothelial cells and the uptake of small molecules and ions by both epithelial and connective tissue cells. Recent findings suggest that caveolae may, in addition, be involved in signal transduction. To better understand the molecular composition of this membrane specialization, we have developed a biochemical method for purifying caveolae from chicken smooth muscle cells. Biochemical and morphological markers indicate that we can obtain approximately 1.5 mg of protein in the caveolae fraction from approximately 100 g of chicken gizzard. Gel electrophoresis shows that there are more than 30 proteins enriched in caveolae relative to the plasma membrane. Among these proteins are: caveolin, a structural molecule of the caveolae coat; multiple, glycosylphosphatidylinositol-anchored membrane proteins; both G alpha and G beta subunits of heterotrimeric GTP-binding protein; and the Ras-related GTP-binding protein, Rap1A/B. The method we have developed will facilitate future studies on the structure and function of caveolae.