Caveolae: anchored, multifunctional platforms in the lipid ocean

Ganglioside GM1 levels are a determinant of the extent of caveolae/raft-dependent endocytosis of cholera toxin to the Golgi apparatus

Cholera toxin is associated with caveolae and raft domains in various cell types and previous studies have shown that cholera toxin can be internalized by caveolae/raft-dependent endocytosis as well as by other pathways. We undertook the study of cholera toxin endocytosis in CaCo-2 and HeLa cells. CaCo-2 cells do not express detectable levels of caveolin and, relative to HeLa cells, also present significantly reduced expression of ganglioside GM1, the cholera toxin receptor, that remains Triton X-100 insoluble. Amongst the HeLa cell population, caveolin expression is constant, however, GM1 expression is highly variable. Cholera toxin is internalized to the Golgi apparatus via a caveolae/raft-dependent pathway sensitive to methyl-β-cyclodextrin and genistein in high-GM1-expressing HeLa cells but not in low-GM1 HeLa cells or in CaCo-2 cells. Limited cholera toxin endocytosis to endosomes sensitive to neither methyl-β-cyclodextrin nor genistein is also observed in all cells and corresponds to a non-caveolae/raft endocytic pathway. Increasing cell-associated GM1 by adding GM1 to the cell media of both HeLa and CaCo-2 cells selectively enhances the methyl-β-cyclodextrin-, genistein-sensitive delivery of cholera toxin to the Golgi apparatus but not to endosomes. GM1 expression levels are therefore a selective determinant of caveolae/raft-dependent endocytosis of cholera toxin to the Golgi apparatus and variable expression of GM1 between cells can impact on the endocytosis and choice of pathway followed by cholera toxin.

Cholesterol depletion induces PKA-mediated basolateral-to-apical transcytosis of the scavenger receptor class B type I in MDCK cells

Cholesterol-based membrane microdomains, or lipid rafts, are believed to play important, yet poorly defined, roles in protein trafficking and signal transduction. In polarized epithelial cells, the current view is that rafts are involved in apical but not in basolateral protein transport from the trans-Golgi network (TGN). We report here that cholesterol is required in a post-TGN mechanism of basolateral regionalization. Permanently transfected Madin-Darby canine kidney cells segregated the caveolae/raft-associated high-density lipoprotein scavenger receptor class B type I (SR-BI) predominantly to the basolateral domain where it was constitutively internalized and recycled basolaterally. Acute cholesterol depletion did not significantly alter SR-BI internalization, implying a cholesterol depletion-insensitive endocytic process but instead induced its transcytosis through a protein kinase A (PKA)- and microtubule-dependent mechanism. Forskolin also elicited SR-BI transcytosis. The basolateral distribution of endogenous epidermal growth factor receptor remained unaffected. Strikingly, cholesterol depletion induced PKA activity without increasing the cAMP levels. Thus, our results are consistent with a scenario in which cholesterol-based rafts promote internalization and basolateral recycling of internalized SR-BI whereas a PKA pool sensitive to cholesterol depletion mediates SR-BI transcytosis. Regulated transcytosis of SR-BI may provide an additional mechanism to control cholesterol homeostasis. These results disclose relationships between cholesterol-based rafts and PKA activity operating in a post-TGN mechanism of regulated apical-to-basolateral cell surface protein distribution.

Caveolar and lipid raft localization of the growth hormone receptor and its signaling elements: impact on growth hormone signaling

The growth hormone receptor (GHR) is a cell surface receptor that mediates the somatogenic and metabolic effects of the growth hormone (GH). GHR signaling is transduced via the receptor-associated cytoplasmic tyrosine kinase called Janus protein kinase 2 (JAK2). The major intracellular signaling systems activated by JAK2 in response to GH include the signal transducer and activator of transcription (STAT) 5 and extracellular signal-regulated kinase (ERK)-1 and -2 pathways. In this report, we investigate the role of cholesterol-rich plasma membrane microdomains (caveolae and lipid rafts) in GH signaling. By subcellular fractionation of the GH-responsive 3T3-F442A murine preadipocyte, we found dramatic enrichment (6.7-fold) of plasma membrane GHR in the caveolae membranes (CM). JAK2 was also represented in the CM fraction, but was less enriched (2.5-fold) than GHR. ERK1/2 and the important ERK pathway upstream small adaptor protein, Grb2 (growth factor receptor-bound protein 2), were also enriched in caveolae (2.3- and 8.3-fold, respectively), but STAT5 was barely detected in the same fraction. Correspondingly, GH-induced tyrosine-phosphorylated GHR, JAK2, and ERK1/2 were highly represented in the CM fraction, whereas tyrosine-phosphorylated STAT5 was enriched in the non-membranous fraction of the post-nuclear supernatant. Additionally, GH induced further accumulation of GHR, Grb2, and SHC proteins in the CM fraction. Interestingly, treatment of the cells with the caveolae-disrupting agent, methyl-beta-cyclodextrin (mbetaCD), selectively inhibited GH-induced ERK1/2 activation but not STAT5 phosphorylation; repletion of cholesterol in mbetaCD-treated cells restored GH-induced ERK activation. Comparison of 3T3-F442A cells with the GHR-expressing human IM-9 lymphoblasts revealed similar enrichment of GHR in the lipid raft fraction of IM-9 as in the CM fraction of 3T3-F442A, but there were dramatic differences in the ERKs and Grb2. The IM-9 cell, in which ERKs are not activated by GH, displayed no enrichment of ERKs and Grb2 in the lipid raft fraction. Our results suggest that localization of GHRs in the CM fraction of the plasma membrane plays important roles in signaling.

Lipids partition caveolin‐1 from ER membranes into lipid droplets: updating the model of lipid droplet biogenesis

Caveolin-1, a putative mediator of intracellular cholesterol transport, is generally assumed to be integrated into the cytoplasmic leaflets of all cellular membranes. Lipid droplets form by budding at the endoplasmic reticulum (ER), and caveolin-1 is thought to be transferred to the droplet surface along with the cytoplasmic leaflet of ER membranes and not to enter the droplet core. We explored how caveolin-1 accesses lipid droplets from the ER by localizing caveolin-1 in ER membranes and in lipid droplets in cultured smooth muscle cells using freeze-fracture immunocytochemistry. We detected caveolin-1 in endoplasmic leaflets of ER membranes but never in cytoplasmic leaflets. Caveolin-1 was also present in lipid droplet cores. These findings are incompatible with the current hypothesis of lipid droplet biogenesis. We suggest that the inherent high affinity of caveolin-1 for neutral lipids causes caveolin-1 molecules to be extracted from the endoplasmic leaflets of ER membranes and to be transferred into the droplet core by inundating lipids during droplet formation.

Sarcolemmal FAT/CD36 in human skeletal muscle colocalizes with caveolin-3 and is more abundant in type 1 than in type 2 fibers

FAT/CD36 is a transmembrane protein that is thought to facilitate cellular long-chain fatty acid uptake. However, surprisingly little is known about the localization of FAT/CD36 in human skeletal muscle. By confocal immunofluorescence microscopy, we demonstrate high FAT/CD36 expression in endothelial cells and weaker but significant FAT/CD36 expression in sarcolemma in human skeletal muscle. No apparent intracellular staining was observed in the muscle cells. There are indications in the literature that caveolae may be involved in the uptake of fatty acids, possibly as regulators of FAT/CD36 or other fatty acid transporters. We show that in sarcolemma, FAT/CD36 colocalizes with the muscle-specific caveolae marker protein caveolin-3, suggesting that caveolae may regulate cellular fatty acid uptake by FAT/CD36. Furthermore, we provide evidence that FAT/CD36 expression is significantly higher in type 1 compared with type 2 fibers, whereas caveolin-3 expression is significantly higher in type 2 fibers than in type 1 fibers.

Do the signalling proteins for angiogenesis exist as a modular complex? The case for the angosome

The vasculature remains quiescent during much of adult life, but new blood vessels can be rapidly produced when required by a process referred to as angiogenesis. Angiogenesis involves a complex series of events including the proliferation, migration, differentiation and apoptosis of capillary endothelial cells, as well as changes in vascular permeability. This hypothesis argues that in the quiescent vasculature the many factors that regulate angiogenesis are normally held together as part of an inactive modular unit, and that when angiogenesis is stimulated the modular unit dissociates thus enabling angiogenic regulators to become active. I have termed this modular unit the "angosome". It is proposed that the angosome is present in the caveolae of capillary endothelial cells. Caveolae are flask-shaped invaginations in the plasmalemma that compartmentalise signalling molecules. Endothelial cells are particularly rich in caveolae. Many of the structural and functional aspects of caveolae are controlled by the protein caveolin, one form of which, caveolin-1, interacts directly or indirectly with most of the regulatory molecules involved in angiogenesis. Caveolin-1 forms oligomers of 14-16 sub-units and I propose that oligomers of caveolin-1 form the scaffold that holds together the angosome. There is evidence that caveolin-1 is up-regulated in the differentiated, quiescent vasculature and down-regulated in proliferating endothelial cells. Since the presence of caveolin-1 can inhibit pro-angiogenic factors, it may act as a "master-switch" co-ordinating events during angiogenesis. Thus when the vasculature is quiescent the angosome may hold angiogenic factors in an inactive state and when angiogenesis is required, the angosome must disassociate to enable angiogenic factors to become active.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Compositional changes in lipid microdomains of air-blood barrier plasma membranes in pulmonary interstitial edema

We evaluated in anesthetized rabbits the compositional changes of plasmalemmal lipid microdomains from lung tissue samples after inducing pulmonary interstitial edema (0.5 ml/kg for 3 h, leading to approximately 5% increase in extravascular water). Lipid microdomains (lipid rafts and caveolae) were present in the detergent-resistant fraction (DRF) obtained after discontinuous sucrose density gradient. DRF was enriched in caveolin-1, flotillin, aquaporin-1, GM1, cholesterol, sphingomyelin, and phosphatidylserine, and their contents significantly increased in interstitial edema. The higher DRF content in caveolin, flotillin, and aquaporin-1 and of the ganglioside GM1 suggests an increase both in caveolar domains and in lipid rafts, respectively. Compositional changes could be ascribed to endothelial and epithelial cells that provide most of plasma membrane surface area in the air-blood barrier. Alterations in lipid components in the plasma membrane may reflect rearrangement of floating lipid platforms within the membrane and/or lipid translocation from intracellular stores. Lipid traffic could be stimulated by the marked increase in hydraulic interstitial pressure after initial water accumulation, from approximately -10 to 5 cmH2O, due to the low compliance of the pulmonary tissue, in particular in the basement membranes and in the interfibrillar substance. Compositional changes in lipid microdomains represent a sign of cellular activation and suggest the potential role of mechanotransduction in response to developing interstitial edema.

Binding and entry of respiratory syncytial virus into host cells and initiation of the innate immune response

Respiratory syncytial virus (RSV) is the most common cause of severe lower respiratory tract infection in infants and the elderly. There is currently no effective antiviral treatment for the infection, but advances in our understanding of RSV uptake, especially the role of surfactant proteins, the attachment protein G and the fusion protein F, as well as the post-binding events, have revealed potential targets for new therapies and vaccine development. RSV infection triggers an intense inflammatory response, mediated initially by the infected airway epithelial cells and antigen-presenting cells. Humoral and cell-mediated immune responses are important in controlling the extent of infection and promoting viral clearance. The initial innate immune response may play a critical role by influencing the subsequent adaptive response generated. This review summarizes our current understanding of RSV binding and uptake in mammalian cells and how these initial interactions influence the subsequent innate immune response generated.

Cholesterol transporter caveolin-1 transits the lipid bilayer during intracellular cycling

Caveolin-1, a major protein of cell surface invaginations called caveolae, is currently believed to cycle between the plasma membrane and intracellular compartments via the endocytotic pathway, at least for part of its itinerary. We studied the distribution of caveolin-1 in cell membranes, using ultrathin cryosections and freeze-fracture immunolabeling and found this protein not only in the cytoplasmic leaflet of the plasma membrane, but also in the exoplasmic leaflet of all intracellular membranes. This sidedness implies that caveolin-1 switches from one membrane leaflet to the other somewhere on its way through the cell and rules out the classic mechanism of endocytotic membrane budding and fusion for caveolin-1 intracellular trafficking. Underlying the sidedness of caveolin-1 may be a fundamental, hitherto unrecognized, mechanism by which proteins transit membranes.

Caveolae/raft-dependent endocytosis

Although caveolae are well-characterized subdomains of glycolipid rafts, their distinctive morphology and association with caveolins has led to their internalization being considered different from that of rafts. In this review, we propose that caveolae and rafts are internalized via a common pathway, caveolae/raft-dependent endocytosis, defined by its clathrin independence, dynamin dependence, and sensitivity to cholesterol depletion. The regulatory role of caveolin-1 and ligand sorting in this complex endocytic pathway are specifically addressed.