Signal transducing molecules and glycosyl-phosphatidylinositol-linked proteins form a caveolin-rich insoluble complex in MDCK cells

Muscle-specific interaction of caveolin isoforms: differential complex formation between caveolins in fibroblastic vs. muscle cells

It is generally well accepted that caveolin-3 expression is muscle specific, whereas caveolin-1 and -2 are coexpressed in a variety of cell types, including adipocytes, endothelial cells, epithelial cells, and fibroblasts. Caveolin-1 and -2 are known to form functional hetero-oligomeric complexes in cells where they are coexpressed, whereas caveolin-3 forms homo-oligomeric high molecular mass complexes. Although caveolin-2 might be expected to interact in a similar manner with caveolin-3, most studies indicate that this is not the case. However, this view has recently been challenged as it has been demonstrated that caveolin-2 and -3 are coexpressed in primary cultures of cardiac myocytes, where these two proteins can be coimmunoprecipitated. Thus it remains controversial whether caveolin-2 interacts with caveolin-3. Here, we directly address the issue of caveolin isoform protein-protein interactions by means of three distinct molecular genetic approaches. First, using caveolin-1-deficient mouse embryonic fibroblasts, in which we have stably expressed caveolin-1, -2, or -3, we find that caveolin-1 interacts with caveolin-2 in this setting, whereas caveolin-3 does not, in agreement with most published observations. Next, we used a transfected L6 myoblast cell system expressing all three caveolin proteins. Surprisingly, we found that caveolin-1, -2, and -3 all coimmunoprecipitate in this cell type, suggesting that this interaction is muscle cell specific. Similar results were obtained when the skeletal muscle of caveolin-1 transgenic animals was analyzed for caveolin-1 and caveolin-3 coimmunoprecipitation. Thus we conclude that all three caveolins can interact to form a discrete hetero-oligomeric complex, but that such complex formation is clearly muscle specific.

A simplified method for the preparation of detergent-free lipid rafts

Lipid rafts are small plasma membrane domains that contain high levels of cholesterol and sphingolipids. Traditional methods for the biochemical isolation of lipid rafts involve the extraction of cells with nonionic detergents followed by the separation of a low-density, detergent-resistant membrane fraction on density gradients. Because of concerns regarding the possible introduction of artifacts through the use of detergents, it is important to develop procedures for the isolation of lipid rafts that do not involve detergent extraction. We report here a simplified method for the purification of detergent-free lipid rafts that requires only one short density gradient centrifugation, but yields a membrane fraction that is highly enriched in cholesterol and protein markers of lipid rafts, with no contamination from nonraft plasma membrane or intracellular membranes.

TRAF interactions with raft-like buoyant complexes, better than TRAF rates of degradation, differentiate signaling by CD40 and EBV latent membrane protein 1

The CD40 receptor and the Epstein-Barr virus oncoprotein LMP1 are both members of the TNF-receptor family and share several signaling mediators, including TRAF2 and TRAF3. Depending on the cell lineage and stage of maturation, LMP1 and CD40 can have synergistic, antagonist or unrelated effects. Previous publications have suggested that both TRAF2 and TRAF3 move into lipid rafts upon LMP1 expression or CD40 activation, whereas their proteolysis is only enhanced by CD40. However CD40-induced proteolysis of TRAF2 has only been reported in murine cells, and there are conflicting data regarding translocation of TRAF2 into lipid rafts. We therefore investigated TRAF2 and TRAF3 modifications induced by CD40 and LMP1 signaling in a panel of human cell lines of lymphoid and epithelial origins. Upon CD40 stimulation, a marked redistribution of TRAF2 into the buoyant raft fraction was observed in all cell lines and was often associated with a similar redistribution of TRAF3. In contrast, only TRAF3 was redistributed into the raft fraction upon LMP1 expression. Moreover parallel changes in subcellular distribution of TRAF2 and TRAF3 were recorded by electron microscopy. A significant decrease in TRAF2 and TRAF3 concentrations triggered by CD40 ligation was observed in only 1 cell line and there was no evidence that this decrease was required for the negative feed-back on JNK activation. TRAF2 redistribution into raft-like complexes thus appears as the most significant event distinctive of CD40 and LMP1 signaling. On the other hand, the parallel influence of CD40 and LMP1 on TRAF3 redistribution is consistent with functional similarities between the CD40-TRAF3 and LMP1-TRAF3 axes.

Viral entry, lipid rafts and caveosomes

Lipid rafts and caveolae are detergent-insoluble plasma membrane microdomains, involved in cellular endocytic processes and signalling. Several viruses, including a human pathogen, echovirus 1, and an extensively studied simian virus 40 utilize these domains for internalization into the host cells. Interaction of viruses with receptors on the cell surface triggers specific conformational changes of the virus particle and can give rise to signalling events, which determine the mechanisms of virus entry. After internalization via cell surface lipid rafts or caveolae, virus-containing vesicles can fuse with caveosomes, pre-existing cytoplasmic organelles, or dock on other intracellular organelles. These pathways may deliver viruses further to different cellular destinations, where the viral replication cycle then takes place. The information concerning the viral entry processes is important for understanding the details of the infections, for finding new targets for antiviral therapy and for elucidating the cellular internalization pathways in general.

Oligomerization of amyloid ?-protein occurs during the isolation of lipid rafts

Cholesterol- and glycosphingolipid-rich microdomains, called "lipid rafts," are suggested to initiate and promote the pathophysiology of Alzheimer's disease by serving as a platform for generation, aggregation, or degradation of amyloid-beta protein (Abeta). However, methods for biochemical isolation of these microdomains may produce artifacts. In this study, when synthetic Abeta1- 40 monomers were added to the brain fragment at a final concentration of 2.1 microM, followed by homogenization and isolation of lipid rafts by an established method, Abeta1- 40 accumulated as oligomers in the lipid raft fraction. However, in the absence of a brain homogenate, synthetic Abeta1- 40 did not accumulate in the lipid raft fraction. When fractionation was performed in the absence of synthetic Abeta1-40 and synthetic Abeta1-40 was incubated in an aliquot of each fraction, a marked oligomerization of Abeta1- 40 was observed in the lipid raft aliquot. These results indicate that exogenous Abeta associates with lipid rafts, and Abeta bound to rafts forms oligomers during the isolation of lipid rafts. In addition, endogenous Abeta1-40 in a Triton X-100-insoluble fraction of a brain homogenate of the Tg2576 transgenic mouse model of Alzheimer's disease formed oligomers when the fraction was incubated at 4 degrees C for 20 hr. Thus, one should be careful when one discusses the role of lipid rafts in amyloid precursor protein processing and in the generation, aggregation, and degradation of Abeta.

Loss of caveolin-1 expression is associated with disruption of muscarinic cholinergic activities in the urinary bladder

Caveolin-1 (Cav1), a structural protein of caveolae, plays cell- and context-dependent roles in signal transduction pathway regulation. We have generated a knockout mouse homozygous for a null mutation of the Cav1 gene. Cav1 knockout mice exhibited impaired urinary bladder contractions in vivo during cystometry. Contractions of male bladder strips were evoked with electric and pharmacologic stimulation (5-40 Hz, 1-10 microM carbachol, 10 mM alpha,beta-methylene ATP, 100 mM KCl). Acetylcholine (ACh) and norepinephrine (NE) release from bladder strips were measured with a radiochemical method by incubating the strips with 14C-choline and 3H-NE prior to electric stimulation, whereas ATP release was measured using the luciferin-luciferase assay with a luminometer. A 60-75% decline in contractility was observed when Cav1 knockout muscle strips were stimulated with electric current or carbachol, compared to wildtype muscle strips. No difference in contractility was noted when contractions were evoked either by the purinergic agonist alpha,beta-methylene ATP, or by extracellular potassium. To investigate the relative contribution of non-cholinergic activity to bladder contractility, the amplitude of the electric stimulation-evoked contractions was compared in the presence of the muscarinic antagonist atropine (1 microM). While the non-muscarinic (purinergic) response was unaltered, muscarinic cholinergic response was principally disrupted in Cav1 knockout mice. The loss of Cav1 gene expression was also associated with a 70% reduction in ACh release. NE and ATP release was not altered. It is concluded that the loss of caveolin-1 is associated with disruption of M3 muscarinic cholinergic activity in the bladder. Both pre-junctional (acetylcholine neurotransmitter release from neuromuscular junctions) and post-junctional (M3 receptor-mediated signal transduction in bladder smooth muscles) mechanisms are disrupted, resulting in impaired bladder contraction.

Tyrosine phosphorylation of caveolin-2 at residue 27: differences in the spatial and temporal behavior of phospho-Cav-2 (pY19 and pY27)

Caveolin-2 is an accessory molecule and the binding partner of caveolin-1. Previously, we showed that c-Src expression leads to the tyrosine phosphorylation of Cav-2 at position 19. To further investigate the tyrosine phosphorylation of Cav-2, we have now generated a novel phospho-specific antibody directed against phospho-Cav-2 (pY27). Here, we show that Cav-2 is phosphorylated at both tyrosines 19 and 27. We reconstituted this phosphorylation event by recombinantly coexpressing c-Src and Cav-2. We generated a series of Cav-2 constructs harboring the mutation of each tyrosine to alanine, singly or in combination, i.e., Cav-2 Y19A, Y27A, and Y19A/Y27A. Recombinant expression of these mutants in Cos-7 cells demonstrated that neither tyrosine is the unique phosphorylation site, and that double mutation of tyrosines 19 and 27 to alanine abrogates Cav-2 tyrosine phosphorylation. Immunofluorescence analysis of NIH 3T3 cells revealed that the two tyrosine-phosphorylated forms of Cav-2 exhibited some distinct properties. Phospho-Cav-2 (pY19) is concentrated at cell edges and at cell-cell contacts, whereas phospho-Cav-2 (pY27) is distributed in a dotlike pattern throughout the cell surface and cytoplasm. Further functional analysis revealed that tyrosine phosphorylation of Cav-2 has no effect on its targeting to lipid rafts, but clearly disrupts the hetero-oligomerization of Cav-2 with Cav-1. In an attempt to identify upstream mediators, we investigated Cav-2 tyrosine phosphorylation in an endogenous setting. We found that in A431 cells, EGF stimulation is sufficient to induce Cav-2 phosphorylation at tyrosines 19 and 27. However, the behavior of the two phosphorylated forms of Cav-2 diverges upon EGF stimulation. First, phospho-Cav-2 (pY19) and phospho-Cav-2 (pY27) display different localization patterns. In addition, the temporal response to EGF stimulation appears to be different. Cav-2 is phosphorylated at tyrosine 19 in a rapid and transient fashion, whereas phosphorylation at tyrosine 27 is sustained over time. Three SH2 domain-containing proteins, c-Src, Nck, and Ras-GAP, were found to associate with Cav-2 in a phosphorylation-dependent manner. However, phosphorylation at tyrosine 27 appears to be more critical than phosphorylation at tyrosine 19 for this binding to occur. Taken together, these results suggest that, in addition to the common characteristics that these two sites appear to share, phospho-Cav-2 (pY19) and phospho-Cav-2 (pY27) may each possess a set of unique functional roles.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

Gβγ Activation of Src Induces Caveolae-mediated Endocytosis in Endothelial Cells

Caveolae-mediated endocytosis in endothelial cells is stimulated by the binding of albumin to gp60, a specific albumin-binding protein localized in caveolae. The activation of gp60 induces its cell surface clustering and association with caveolin-1, the caveolar-scaffolding protein. This interaction leads to G(i)-induced Src kinase activation, which in turn signals dynamin-2-mediated fission and directed migration of caveolae-derived vesicles from apical to basal membrane. In this study, we investigated the possible role of the Gbetagamma heterodimer in signaling G(i)-induced Src activation and subsequent caveolae-mediated endocytosis. We observed using rat lung microvascular endothelial cells that expression of the C terminus of beta-adrenergic receptor kinase (ct-betaARK), an inhibitor Gbetagamma signaling, prevented gp60-dependent Src activation as well as caveolae-mediated endocytosis and transcellular transport of albumin and uptake of cholera toxin subunit B, a specific marker of caveolae internalization. Expression of ct-betaARK also prevented Src-mediated tyrosine phosphorylation of caveolin-1 and dynamin-2 and the resultant phosphorylation-dependent association of dynamin-2 and caveolin-1. Also, the direct activation of Gbetagamma using a specific cell-permeant activating peptide (myristoylated-SIRKALNILGYPDYD) simulated the effects of gp60 in inducing Src activation, caveolin-1, and dynamin-2 phosphorylation as well as caveolae-mediated endocytosis of cholera toxin subunit B. The myristoylated-SIRKALNILGYPDYD peptide-induced responses were inhibited by the expression of ct-betaARK. Taken together, our results demonstrate that Gbetagamma activation of Src signals caveolae-mediated endocytosis and transendothelial albumin transport via transcytosis.

Analysis of the interaction between respiratory syncytial virus and lipid-rafts in Hep2 cells during infection

The assembly of respiratory syncytial virus (RSV) in lipid-rafts was examined in Hep2 cells. Confocal and electron microscopy showed that during RSV assembly, the cellular distribution of the complement regulatory proteins, decay accelerating factor (CD55) and CD59, changes and high levels of these cellular proteins are incorporated into mature virus filaments. The detergent-solubility properties of CD55, CD59, and the RSV fusion (F) protein were found to be consistent with each protein being located predominantly within lipid-raft structures. The levels of these proteins in cell-released virus were examined by immunoelectronmicroscopy and found to account for between 5% and 15% of the virus attachment (G) glycoprotein levels. Collectively, our findings suggest that an intimate association exists between RSV and lipid-raft membranes and that significant levels of these host-derived raft proteins, such as those regulating complement activation, are subsequently incorporated into the envelope of mature virus particles.

Endothelial cells as early sensors of pulmonary interstitial edema

We studied responses of endothelial and epithelial cells in the thin portion of the air-blood barrier to a rise in interstitial pressure caused by an increase in extravascular water (interstitial edema) obtained in anesthetized rabbits receiving saline infusion (0.5 ml·kg−1·min−1 for 3 h). We obtained morphometric analyses of the cells and of their microenvironment (electron microscopy); furthermore, we also studied in lung tissue extracts the biochemical alterations of proteins responsible for signal transduction (PKC, caveolin-1) and cell-cell adhesion (CD31) and of proteins involved in membrane-to-cytoskeleton linkage (α-tubulin and β-tubulin). In endothelial cells, we observed a folding of the plasma membrane with an increase in cell surface area, a doubling of plasmalemma vesicular density, and an increase in cell volume. Minor morphological changes were observed in epithelial cells. Edema did not affect the total plasmalemma amount of PKC, β-tubulin, and caveolin-1, but α-tubulin and CD-31 increased. In edema, the distribution of these proteins changed between the detergent-resistant fraction of the plasma membrane (DRF, lipid microdomains) and the rest of the plasma membrane [high-density fractions (HDFs)]. PKC and tubulin isoforms shifted from the DRF to HDFs in edema, whereas caveolin-1 increased in DRF at the expense of a decrease in phosphorylated caveolin-1. The changes in cellular morphology and in plasma membrane composition suggest an early endothelial response to mechanical stimuli arising at the interstitial level subsequently to a modest (∼5%) increase in extravascular water.

Role of Caveolae and Caveolins in Health and Disease

Although they were discovered more than 50 years ago, caveolae have remained enigmatic plasmalemmal organelles. With their characteristic “flasklike” shape and virtually ubiquitous tissue distribution, these interesting structures have been implicated in a wide range of cellular functions. Similar to clathrin-coated pits, caveolae function as macromolecular vesicular transporters, while their unique lipid composition classifies them as plasma membrane lipid rafts, structures enriched in a variety of signaling molecules. The caveolin proteins (caveolin-1, -2, and -3) serve as the structural components of caveolae, while also functioning as scaffolding proteins, capable of recruiting numerous signaling molecules to caveolae, as well as regulating their activity. That so many signaling molecules and signaling cascades are regulated by an interaction with the caveolins provides a paradigm by which numerous disease processes may be affected by ablation or mutation of these proteins. Indeed, studies in caveolin-deficient mice have implicated these structures in a host of human diseases, including diabetes, cancer, cardiovascular disease, atherosclerosis, pulmonary fibrosis, and a variety of degenerative muscular dystrophies. In this review, we provide an in depth summary regarding the mechanisms by which caveolae and caveolins participate in human disease processes.

Localization of ABCG5 and ABCG8 proteins in human liver, gall bladder and intestine

Background

The molecular mechanisms that regulate the entry of dietary sterols into the body and their removal via hepatobiliary secretion are now beginning to be defined. These processes are specifically disrupted in the rare autosomal recessive disease, Sitosterolemia (MIM 210250). Mutations in either, but not both, of two genes ABCG5 or ABCG8, comprising the STSL locus, are now known to cause this disease and their protein products are proposed to function as heterodimers. Under normal circumstances cholesterol, but not non-cholesterol sterols, is preferentially absorbed from the diet. Additionally, any small amounts of non-cholesterol sterols that are absorbed are rapidly taken up by the liver and preferentially excreted into bile. Based upon the defects in sitosterolemia, ABCG5 and ABCG8 serve specifically to exclude non-cholesterol sterol entry at the intestinal level and are involved in sterol excretion at the hepatobiliary level.

Methods

Here we report the biochemical and immuno-localization of ABCG5 and ABCG8 in human liver, gallbladder and intestine using cell fractionation and immunohistochemical analyses.

Results

We raised peptide antibodies against ABCG5 and ABCG8 proteins. Using human liver samples, cell fractionation studies showed both proteins are found in membrane fractions, but they did not co-localize with caveolin-rafts, ER, Golgi or mitochondrial markers. Although their distribution in the sub-fractions was similar, they were not completely contiguous. Immunohistochemical analyses showed that while both proteins were readily detectable in the liver, ABCG5 was found predominately lining canalicular membranes, whereas ABCG8 was found in association with bile duct epithelia. At the cellular level, ABCG5 appeared to be apically expressed, whereas ABCG8 had a more diffuse expression pattern. Both ABCG5 and ABCG8 appeared to localize apically as shown by co-localization with MRP2. The distribution patterns of ABCG5 and ABCG8 in the gallbladder were very similar to each other. In the small intestine both ABCG5 and ABCG8 appear to line the brush border. However, at the level of the enterocyte, the cellular distribution patterns of ABCG5 and ABCG8 differed, such that ABCG5 was more diffuse, but ABCG8 was principally apical. Using standard deglycosylation methods, ABCG5 and ABCG8 do not appear to be glycosylated, suggesting a difference between human and mouse proteins.

Conclusion

We report the distribution patterns of ABCG5 and ABCG8 in human tissues. Cell fractionation studies showed that both proteins co-fractionated in general, but could also be found independent of each other. As predicted, they are expressed apically in both intestine and liver, although their intracellular expression patterns are not completely congruent. These studies support the concept of heterodimerization of ABCG5 and ABCG8, but also support the notion that these proteins may have an independent function.

The plasma membrane Ca2+pump from proximal kidney tubules is exclusively localized and active in caveolae

Plasma membrane Ca2+-ATPase is involved in the fine-tuned regulation of intracellular Ca2+. In this study, the presence of Ca2+-ATPase in caveolae from kidney basolateral membranes was investigated. With the use of a discontinuous sucrose gradient, we show that Ca2+-ATPase is exclusively located and fully active in caveolin-containing microdomains. Treatment with methyl-beta-cyclodextrin--a cholesterol chelator--leads to a spreading of both caveolin and completely inactive Ca2+-ATPase toward high-density fractions. These data support the view that Ca2+ fluxes mediated by Ca2+-ATPase in kidney epithelial cells occur only in caveolae, being strictly dependent on the integrity of these microdomains.

AIP1/DAB2IP, a novel member of the Ras-GAP family, transduces TRAF2-induced ASK1-JNK activation

Previously we have shown that ASK-interacting protein 1 (AIP1, also known as DAB2IP), a novel member of the Ras-GAP protein family, mediates TNF-induced activation of ASK1-JNK signaling pathway. However, the mechanism by which TNF signaling is coupled to AIP1 is not known. Here we show that AIP1 is localized on the plasma membrane in resting endothelial cells (EC) in a complex with TNFR1. TNF binding induces release of AIP1 from TNFR1, resulting in cytoplasmic translocation and concomitant formation of an intracellular signaling complex comprised of TRADD, RIP1, TRAF2, and AIPl. A proline-rich region (amino acids 796-807) is critical for maintaining AIP1 in a closed form, which associates with a region of TNFR1 distinct from the death domain, the site of TNFR1 association with TRADD. An AIP1 mutant with deletion of this proline-rich region constitutively binds to TRAF2 and ASK1. A PERIOD-like domain (amino acids 591-719) of AIP1 binds to the intact RING finger of TRAF2, and specifically enhances TRAF2-induced ASK1 activation. At the same time, the binding of AIP1 to TRAF2 inhibits TNF-induced IKK-NF-kappaB signaling. Taken together, our data suggest that AIP1 is a novel transducer in TNF-induced TRAF2-dependent activation of ASK1 that mediates a balance between JNK versus NF-kappaB signaling.

Caveolin-2 associates with intracellular chlamydial inclusions independently of caveolin-1

BACKGROUND

Lipid raft domains form in plasma membranes of eukaryotic cells by the tight packing of glycosphingolipids and cholesterol. Caveolae are invaginated structures that form in lipid raft domains when the protein caveolin-1 is expressed. The Chlamydiaceae are obligate intracellular bacterial pathogens that replicate entirely within inclusions that develop from the phagocytic vacuoles in which they enter. We recently found that host cell caveolin-1 is associated with the intracellular vacuoles and inclusions of some chlamydial strains and species, and that entry of those strains depends on intact lipid raft domains. Caveolin-2 is another member of the caveolin family of proteins that is present in caveolae, but of unknown function.

METHODS

We utilized a caveolin-1 negative/caveolin-2 positive FRT cell line and laser confocal immunofluorescence techniques to visualize the colocalization of caveolin-2 with the chlamydial inclusions.

RESULTS

We show here that in infected HeLa cells, caveolin-2, as well as caveolin-1, colocalizes with inclusions of C. pneumoniae (Cp), C. caviae (GPIC), and C. trachomatis serovars E, F and K. In addition, caveolin-2 also associates with C. trachomatis serovars A, B and C, although caveolin-1 did not colocalize with these organisms. Moreover, caveolin-2 appears to be specifically, or indirectly, associated with the pathogens at the inclusion membranes. Using caveolin-1 deficient FRT cells, we show that although caveolin-2 normally is not transported out of the Golgi in the absence of caveolin-1, it nevertheless colocalizes with chlamydial inclusions in these cells. However, our results also show that caveolin-2 did not colocalize with UV-irradiated Chlamydia in FRT cells, suggesting that in these caveolin-1 negative cells, pathogen viability and very likely pathogen gene expression are necessary for the acquisition of caveolin-2 from the Golgi.

CONCLUSION

Caveolin-2 associates with the chlamydial inclusion independently of caveolin-1. The function of caveolin-2, either in the uninfected cell or in the chlamydial developmental cycle, remains to be elucidated. Nevertheless, this second caveolin protein can now be added to the small number of host proteins that are associated with the inclusions of this obligate intracellular pathogen.

Dimeric amyloid beta protein rapidly accumulates in lipid rafts followed by apolipoprotein E and phosphorylated tau accumulation in the Tg2576 mouse model of Alzheimer's disease

To investigate lipid rafts as a site where amyloid beta protein (Abeta) oligomers might accumulate and cause toxicity in Alzheimer's disease (AD), we analyzed Abeta in the Tg2576 transgenic mouse model of AD. Abeta was highly concentrated in lipid rafts, which comprise a small fraction of brain volume but contain 27% of brain Abeta42 and 24% of Abeta40 in young mice. In the Tg2576 model, memory impairment begins at 6 months before amyloid plaques are visible. Here we show that Abeta dimers appear in lipid rafts at 6 months and that raft Abeta, which is primarily dimeric, rapidly accumulates reaching levels >500x those in young mice by 24-28 months. A similar large accumulation of dimeric Abeta was observed in lipid rafts from AD brain. In contrast to extracellular amyloid fibrils, which are SDS-insoluble, virtually all Abeta in lipid rafts is SDS soluble. Coupled with recent studies showing that synthetic and naturally occurring Abeta oligomers can inhibit hippocampal long-term potentiation, the in vivo age-dependent accumulation of SDS-soluble Abeta dimers in lipid rafts at the time when memory impairment begins in Tg2576 mice provides strong evidence linking Abeta oligomers to memory impairment. After dimeric Abeta began to accumulate in lipid rafts of the Tg2576 brain, apolipoprotein E (ApoE) and then phosphorylated tau accumulated. A similar increase in ApoE and a large increase in phosphorylated tau was observed in lipid rafts from AD brain. These findings suggest that lipid rafts may be an important site for interaction between dimeric Abeta, ApoE, and tau.

The effect of raft lipid depletion on microvilli formation in MDCK cells, visualized by atomic force microscopy

We have investigated whether raft lipids of Madin-Darby canine kidney (MDCK) cells play any role in microvilli maintenance using a combination of atomic force microscopy (AFM) and laser scanning confocal microscopy. MDCK cells were treated to reduce the amount of sphingolipids, cholesterol, or both and subsequently imaged, in buffer solution, using AFM. It was observed that inhibition of either sphingolipid or cholesterol biosynthesis led to a reduction in the number of microvilli on the surface of MDCK cells. However, this effect was not uniform across the monolayer, with some cells resembling those in untreated controls. The subsequent extraction of cholesterol from cells grown in the presence of inhibitors led to a further reduction in microvilli on the surface of the cells and, in some cases, resulted in monolayers devoid of full length microvilli. Significantly, smaller spikes were observed on the surface of the smoother cells.

Caveolae and Caveolins in the Cardiovascular System

Caveolae and the caveolae coat proteins, caveolins, are putatively implicated in many cellular processes, including transcytosis of macromolecules, cholesterol transport, and signal transduction. Recent insights into the physiological and pathophysiological roles of these organelles and the caveolins from genetically modified mice suggest that they may be profoundly important for postnatal cardiovascular function, including endothelial barrier function, regulation of nitric oxide synthesis, cholesterol metabolism, and cardiac function.

Combined Loss of INK4a and Caveolin-1 Synergistically Enhances Cell Proliferation and Oncogene-induced Tumorigenesis

Tumorigenesis is a multistep process that involves a series of genetic changes or "multiple hits," leading to alterations in signaling, proliferation, immortalization, and transformation. Many of the molecular factors that govern tumor initiation and progression remain unknown. Here, we evaluate the transformation suppressor potential of caveolin-1 (Cav-1) and its ability to cooperate with a well established tumor suppressor, the INK4a locus. To study the effects of loss of caveolin-1 on cellular transformation, we established immortalized primary mouse embryonic fibroblasts (MEFs) expressing and lacking caveolin-1 by interbreeding Cav-1 (+/+) and Cav-1 (-/-) mice with INK4a (-/-) mice. Analysis of these cells reveals that loss of caveolin-1 confers a significant growth advantage, as measured via cellular proliferation and cell cycle analysis. Loss of caveolin-1 in the INK4a (-/-) genetic background results in constitutive hyperactivation of the p42/44 MAP kinase cascade, decreased expression of p21(Cip1), as well as cyclin D1 and PCNA overexpression, consistent with their hyperproliferative phenotype. Importantly, in cells lacking Cav-1 expression, transformation by activated oncogenes (H-Ras(G12V) or v-Src) results in increased tumor growth in vivo (up to >40-fold). Finally, INK4a (-/-)/Cav-1 (-/-) mice demonstrate disturbed mammary epithelial ductal morphology, with hyperplasia, increased side-branching, and fibrosis. Our results provide important new evidence for the transformation suppressor properties of Cav-1 and the first molecular genetic evidence that Cav-1 cooperates with a tumor suppressor, namely the INK4a genetic locus.

CEACAM6 cross-linking induces caveolin-1-dependent, Src-mediated focal adhesion kinase phosphorylation in BxPC3 pancreatic adenocarcinoma cells

Despite lacking transmembrane or intracellular domains, glycosylphosphatidylinositol-anchored proteins can modulate intracellular signaling events, in many cases through aggregation within membrane "lipid raft" microdomains. CEACAM6 is a glycosylphosphatidylinositol-linked cell surface protein of importance in the anchorage-independent survival and metastasis of pancreatic adenocarcinoma cells. We examined the effects of antibody-mediated cross-linking of CEACAM6 on intracellular signaling events and anchorage-independent survival of the CEACAM6-overexpressing pancreatic ductal adenocarcinoma cell line, BxPC3. CEACAM6 cross-linking increased c-Src activation and induced tyrosine phosphorylation of p125(FAK) focal adhesion kinase. Focal adhesion kinase phosphorylation was dependent on c-Src kinase activation, for which caveolin-1 was required. CEACAM6 cross-linking induced a significant increase in cellular resistance to anoikis. These observations represent the first characterization of the mechanism through which this important cell surface oncoprotein influences intracellular signaling events and hence malignant cellular behavior.

Long-term agonist stimulation of IP prostanoid receptor depletes the cognate G(s)alpha protein in membrane domains but does not change the receptor level

Iloprost (IP) stimulation (1 microM, 2 h) of Flag-epitope-tagged human IP prostanoid receptor (FhIPR) expressed in HEK293 cells resulted in specific decrease of endogenous G(s)alpha protein in detergent-insensitive, caveolin-enriched, membrane domains (DIMs). Receptor protein FhIPR, caveolin, G(i)alpha and GPI-linked, domain markers CD55 and CD59 were unchanged. The same result was obtained in HEK293 cells expressing FhIPR-G(s)alpha fusion protein. The endogenous G(s)alpha decreased, but the level of Flag-hIPR-G(s)alpha protein did not change. The specific depletion of domain-bound pool of G(s)alpha as consequence of iloprost stimulation was also demonstrated in membrane domains prepared according to alkaline treatment plus sonication protocol (detergent-free procedure of Song et al.). Our data further indicated that in control, quiescent cells only a very small amount of IP prostanoid receptor was present in DIMs together with large amount of its cognate G(s)alpha protein. Expressed in quantitative terms, DIMs contained 30-40% of the total cellular amount of G proteins whereas the content of IP prostanoid receptors was 1-3%. The dominant portion (>95%) of FhIPR as well as FhIPR-G(s)alpha was localised in high-density area of the gradient containing detergent-solubilised proteins. FhIPR and FhIPR-G(s)alpha distribution was similar to that of transmembrane plasma membrane (PM) markers (CD147, MHCI, CD29, Tapa1, the alpha subunit of Na,K-ATPase, transmembrane form of CD58 and CD44). All these proteins are known to be fully solubilised by detergent and thus unable to float in density gradient. Our data indicate that (i) long-term agonist stimulation of IP prostanoid receptor is associated with preferential decrease of its cognate G protein G(s)alpha from membrane domains; receptor level is unchanged. (ii) Very small fraction (1-3%) of total cellular amount of receptors is recovered in DIMs together with roughly 40% of G proteins. These data suggest a "supra-stoichiometric" arrangement of G proteins and corresponding receptors in DIMs.

Lipid rafts: heterogeneity on the high seas

Lipid rafts are membrane microdomains that are enriched in cholesterol and glycosphingolipids. They have been implicated in processes as diverse as signal transduction, endocytosis and cholesterol trafficking. Recent evidence suggests that this diversity of function is accompanied by a diversity in the composition of lipid rafts. The rafts in cells appear to be heterogeneous both in terms of their protein and their lipid content, and can be localized to different regions of the cell. This review summarizes the data supporting the concept of heterogeneity among lipid rafts and outlines the evidence for cross-talk between raft components. Based on differences in the ways in which proteins interact with rafts, the Induced-Fit Model of Raft Heterogeneity is proposed to explain the establishment and maintenance of heterogeneity within raft populations.

Plasma membrane microdomains: organization, function and trafficking

The plasma membrane consists of a mosaic of functional microdomains facilitating a variety of physiological processes associated with the cell surface. In most cells, the majority of the cell surface is morphologically featureless, leading to difficulties in characterizing its organization and microdomain composition. The reliance on indirect and perturbing techniques has led to vigorous debate concerning the nature and even existence of some microdomains. Recently, increasing technical sophistication has been applied to study cell surface compartmentalization providing evidence for small, short-lived clusters that may be much less than 50 nm in diameter. Lipid rafts and caveolae are cholesterol-dependent, highly ordered microdomains that have received most attention in recent years, yet their precise roles in regulating functions such as cell signalling remain to be determined. Endocytosis of lipid rafts/caveolae follows a clathrin-independent route to both early endosomes and non-classical caveosomes. The observation that a variety of cellular pathogens localize to and internalize with these microdomains provides an additional incentive to characterize the organization, dynamics and functions of these domains.

Accumulation of glycosphingolipids in Niemann-Pick C disease disrupts endosomal transport

Glycosphingolipids are endocytosed and targeted to the Golgi apparatus but are mistargeted to lysosomes in sphingolipid storage disorders. Substrate reduction therapy utilizes imino sugars to inhibit glucosylceramide synthase and potentially abrogate the effects of storage. Niemann-Pick type C (NPC) disease is a disorder of intracellular transport where glycosphingolipids (GSLs) and cholesterol accumulate in endosomal compartments. The mechanisms of altered intracellular trafficking are not known but may involve the mistargeting and disrupted function of proteins associated with GSL membrane microdomains. Membrane microdomains were isolated by Triton X-100 and sucrose density gradient ultracentrifugation. High pressure liquid chromatography and mass spectrometric analysis of NPC1(-/-) mouse brain revealed large increases in GSL. Sphingosine was also found to be a component of membrane microdomains, and in NPC liver and spleen, large increases in cholesterol and sphingosine were found. GSL and cholesterol levels were increased in mutant NPC1-null Chinese hamster ovary cells as well as U18666A and progesterone induced NPC cell culture models. However, inhibition of GSL synthesis in NPC cells with N-butyldeoxygalactonojirimycin led to marked decreases in GSL but only small decreases in cholesterol levels. Both annexin 2 and 6, membrane-associated proteins that are important in endocytic trafficking, show distorted distributions in NPC cells. Altered BODIPY lactosylceramide targeting, decreased endocytic uptake of a fluid phase marker, and mistargeting of annexin 2 (phenotypes associated with NPC) are reversed by inhibition of GSL synthesis. It is suggested that accumulating GSL is part of a mislocalized membrane microdomain and is responsible for the deficit in endocytic trafficking found in NPC disease.

Mutant K-ras regulates cathepsin B localization on the surface of human colorectal carcinoma cells

Cathepsin B protein and activity are known to localize to the basal plasma membrane of colon carcinoma cells following the appearance of K-ras mutations. Using immunofluorescence and subcellular fractionation techniques and two human colon carcinoma cell lines - one with a mutated K-ras allele (HCT 116) and a daughter line in which the mutated allele has been disrupted (HKh-2)-we demonstrate that the localization of cathepsin B to caveolae on the surface of these carcinoma cells is regulated by mutant K-ras. In HCT 116 cells, a greater percentage of cathepsin B was distributed to the caveolae, and the secretion of cathepsin B and pericellular (membrane-associated and secreted) cathepsin B activity were greater than observed in HKh-2 cells. Previous studies established the light chain of annexin II tetramer, p11, as a binding site for cathepsin B on the surface of tumor cells. The deletion of active K-ras in HKh-2 cells reduced the steady-state levels of p11 and caveolin-1 and the distribution of p11 to caveolae. Based upon these results, we speculate that cathepsin B, a protease implicated in tumor progression, plays a functional role in initiating proteolytic cascades in caveolae as downstream components of this cascade (e.g., urokinase plasminogen activator and urokinase plasminogen activator receptor) are also present in HCT 116 caveolae.

Obligatory intracellular parasitism by Ehrlichia chaffeensis and Anaplasma phagocytophilum involves caveolae and glycosylphosphatidylinositol-anchored proteins

Obligatory intracellular, human ehrlichiosis agents Ehrlichia chaffeensis and Anaplasma phagocytophilum create unique replicative compartments devoid of lysosomal markers in monocytes/macrophages and granulocytes respectively. The entry of these bacteria requires host phospholipase C (PLC)-gamma2 and protein tyrosine kinases, but their entry route is still unclear. Here, using specific inhibitors, double immunofluorescence labelling and the fractionation of lipid rafts, we demonstrate that bacterial entry and intracellular infection involve cholesterol-rich lipid rafts or caveolae and glycosylphosphatidylinositol (GPI)-anchored proteins. By fluorescence microscopy, caveolar marker protein caveolin-1 was co-localized with both early and replicative bacterial inclusions. Additionally, tyrosine-phosphorylated proteins and PLC-gamma2 were found in bacterial early inclusions. In contrast, clathrin was not found in any inclusions from either bacterium. An early endosomal marker, transferrin receptor, was not present in the early inclusions of E. chaffeensis, but was found in replicative inclusions of E. chaffeensis. Furthermore, several bacterial proteins from E. chaffeensis and A. phagocytophilum were co-fractionated with Triton X-100-insoluble raft fractions. The formation of bacteria-encapsulating caveolae, which assemble and retain signalling molecules essential for bacterial entry and interact with the recycling endosome pathway, may ensure the survival of these obligatory intracellular bacteria in primary host defensive cells.

The caveolin proteins

Caveolins are markers of caveolae, invaginations in the plasma membrane, and there are three members of the family in vertebrates. Caveolins participate in many important cellular processes, including vesicular transport, cholesterol homeostasis, signal transduction, and tumor suppression.

The caveolin gene family has three members in vertebrates: caveolin-1, caveolin-2, and caveolin-3. So far, most caveolin-related research has been conducted in mammals, but the proteins have also been found in other animals, including Xenopus laevis, Fugu rubripes, and Caenorhabditis elegans. Caveolins can serve as protein markers of caveolae ('little caves'), invaginations in the plasma membrane 50-100 nanometers in diameter. Caveolins are found predominantly at the plasma membrane but also in the Golgi, the endoplasmic reticulum, in vesicles, and at cytosolic locations. They are expressed ubiquitously in mammals, but their expression levels vary considerably between tissues. The highest levels of caveolin-1 (also called caveolin, Cav-1 and VIP2I) are found in terminally-differentiated cell types, such as adipocytes, endothelia, smooth muscle cells, and type I pneumocytes. Caveolin-2 (Cav-2) is colocalized and coexpressed with Cav-1 and requires Cav-1 for proper membrane targeting; the Cav-2 gene also maps to the same chromosomal region as Cav-1 (7q31.1 in humans). Caveolin-3 (Cav-3) has greater protein-sequence similarity to Cav-1 than to Cav-2, but it is expressed mainly in muscle cells, including smooth, skeletal, and cardiac myocytes. Caveolins participate in many important cellular processes, including vesicular transport, cholesterol homeostasis, signal transduction, and tumor suppression.

P-glycoprotein in blood-brain barrier endothelial cells: interaction and oligomerization with caveolins

P-glycoprotein (P-gp), an adenosine triphosphate (ATP)-binding cassette transporter which acts as a drug efflux pump, is highly expressed at the blood-brain barrier (BBB) where it plays an important role in brain protection. Recently, P-gp has been reported to be located in the caveolae of multidrug-resistant cells. In this study, we investigated the localization and the activity of P-gp in the caveolae of endothelial cells of the BBB. We used an in vitro model of the BBB which is formed by co-culture of bovine brain capillary endothelial cells (BBCEC) with astrocytes. Caveolar microdomains isolated from BBCEC are enriched in P-gp, cholesterol, caveolin-1, and caveolin-2. Moreover, P-gp interacts with caveolin-1 and caveolin-2; together, they form a high molecular mass complex. P-gp in isolated caveolae is able to bind its substrates, and the caveolae-disrupting agents filipin III and nystatin decrease P-gp transport activity. In addition, mutations in the caveolin-binding motif present in P-gp reduced the interaction of P-gp with caveolin-1 and increased the transport activity of P-gp. Thus, P-gp expressed at the BBB is mainly localized in caveolae and its activity may be modulated by interaction with caveolin-1.

Characterization of membrane rafts isolated from rat sertoli cell cultures: caveolin and flotillin-1 content

Membrane rafts from Sertoli cell cultures were isolated as detergent-insoluble glycosphingolipid-enriched (DIG) fractions on the basis of their enriched content of glycosphingolipids and cholesterol and the resulting insolubility in 1% Triton X-100 and their low buoyant density. Because lipid rafts have been implicated in numerous cell functions, including cell signaling and sites for actin/membrane attachment, studies were initiated to characterize Sertoli cell rafts. This study reports the distribution of the raft structural proteins, caveolin and flotillin-1, implicated in raft microdomain organization. Methods employed included the immunoblotting of cell lysates and detergent-insoluble glycosphingolipid-enriched (DIG) fractions, the immunofluorescent microscopy of peritubular myoid cell (PMC) cultures and cryostat-sectioned testis, and the immunohistochemical staining of paraffin-embedded sections following microwave antigen retrieval techniques. Sertoli cells and Sertoli DIG fractions were found to lack the common raft-associated protein, caveolin, a marker protein for caveolae, but they are enriched in the 48-kd protein, flotillin-1, a protein also implicated in raft formation, cell signaling, and cell motility. Since the primary cell contaminant of Sertoli cell cultures is the PMC, these cells, along with spermatogenic cell fraction (SPGC), were also examined for caveolin and flotillin-1 content. The PMCs contained significant concentrations of both caveolin and flotillin-1. PMCs in culture exhibited a punctate caveolin staining pattern at the cell surface characteristic of a caveolar location. These data support the idea that the pinocytotic vesicles observed in PMCs are caveolae. PMCs also show a perinuclear location for caveolin characteristic of a Golgi location. Cryostat sections of rat testis showed a marked concentration of caveolin in the PMCs. The PMC location of caveolin was also confirmed by the immunohistochemical staining of sections from paraffin-embedded rat testis following microwave antigen retrieval techniques. Similar experiments showed a more ubiquitous, stage-specific distribution of flotillin-1 among testicular cell types.

Phosphofructokinase muscle-specific isoform requires caveolin-3 expression for plasma membrane recruitment and caveolar targeting: implications for the pathogenesis of caveolin-related muscle diseases

Previous co-immunoprecipitation studies have shown that endogenous PFK-M (phosphofructokinase, muscle-specific isoform) associates with caveolin (Cav)-3 under certain metabolic conditions. However, it remains unknown whether Cav-3 expression is required for the plasma membrane recruitment and caveolar targeting of PFK-M. Here, we demonstrate that recombinant expression of Cav-3 dramatically affects the subcellular localization of PFK-M, by targeting PFK-M to the plasma membrane, and by trans-locating PFK-M to caveolae-enriched membrane domains. In addition, we show that the membrane recruitment and caveolar targeting of PFK-M appears to be strictly dependent on the concentration of extracellular glucose. Interestingly, recombinant expression of PFK-M with three Cav-3 mutants [DeltaTFT (63 to 65), P104L, and R26Q], which harbor the same mutations as seen in the human patients with Cav-3-related muscle diseases, causes a substantial reduction in PFK-M expression levels, and impedes the membrane recruitment of PFK-M. Analysis of skeletal muscle tissue samples from Cav-3(-/-) mice directly demonstrates that Cav-3 expression regulates the phenotypic behavior of PFK-M. More specifically, in Cav-3-null mice, PFK-M is no longer targeted to the plasma membrane, and is excluded from caveolar membrane domains. As such, our current results may be important in understanding the pathogenesis of Cav-3-related muscle diseases, such as limb-girdle muscular dystrophy-1C, distal myopathy, and rippling muscle disease, that are caused by mutations within the human Cav-3 gene.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Important role of raft aggregation in the signaling events of cold-induced platelet activation

When human platelets are chilled below 20 degrees C, they undergo cold-induced activation. We have previously shown that cold activation correlates with the main phospholipid phase transition (10-20 degrees C) and induces the formation of large raft aggregates. In addition, we found that the glycoprotein CD36 is selectively enriched within detergent-resistant membranes (DRMs) of cold-activated platelets and is extremely sensitive to treatment with methyl-beta-cyclodextrin (MbetaCD). Here, we further studied the partitioning of downstream signaling molecules within the DRMs. We found that the phospholipase Cgamma2 (PLCgamma2) and the protein tyrosine kinase Syk do not partition exclusively within the DRMs, but their distribution is perturbed by cholesterol extraction. In addition, PLCgamma2 activity increases in cold-activated cells compared to resting platelets and is entirely inhibited after treatment with MbetaCD. The Src-family protein tyrosine kinases Src and Lyn preferentially partition within the DRMs and are profoundly affected by removal of cholesterol. These kinases are non-redundant in cold-activation. CD36, active Lyn, along with inactive Src and PLCgamma2 co-localize in small raft complexes in resting platelets. Cold-activation induces raft aggregation, resulting in changes in the activity of these proteins. These data suggest a crucial role of raft aggregation in the early events of cold-induced platelet activation.

Identification and characterization of differentially active pools of type IIalpha phosphatidylinositol 4-kinase activity in unstimulated A431 cells

The seven known polyphosphoinositides have been implicated in a wide range of regulated and constitutive cell functions, including cell-surface signalling, vesicle trafficking and cytoskeletal reorganization. In order to understand the spatial and temporal control of these diverse cell functions it is necessary to characterize the subcellular distribution of a wide variety of polyphosphoinositide synthesis and signalling events. The predominant phosphatidylinositol kinase activity in many mammalian cell types involves the synthesis of the signalling precursor, phosphatidylinositol 4-phosphate, in a reaction catalysed by the recently cloned PI4KIIalpha (type IIalpha phosphatidylinositol 4-kinase). However the regulation of this enzyme and the cellular distribution of its product in different organelles are very poorly understood. This report identifies the existence, in unstimulated cells, of two major subcellular membrane fractions, which contain PI4KIIalpha possessing different levels of intrinsic activity. Separation of these membranes from each other and from contaminating activities was achieved by density gradient ultracentrifugation at pH 11 in a specific detergent mixture in which both membrane fractions, but not other membranes, were insoluble. Kinetic comparison of the purified membrane fractions revealed a 4-fold difference in K (m) for phosphatidylinositol and a 3.5-fold difference in V (max), thereby indicating a different mechanism of regulation to that described previously for agonist-stimulated cells. These marked differences in basal activity and the occurrence of this isozyme in multiple organelles emphasize the need to investigate cell signalling via PI4KIIalpha at the level of individual organelles rather than whole-cell lysates.

Caveosomes and endocytosis of lipid rafts

Endocytosis of various endogenous plasma membrane molecules, including signalling receptors, glycosphingolipids and glycosylphosphatidylinositol (GPI)-linked proteins, occurs in the absence of functional clathrin-coated pits. Most of these molecules are found in biochemically defined lipid rafts, which suggests that at least some clathrin-independent endocytosis may be raft specific or raft mediated. However, recent studies of the uptake of raft markers have revealed a diversity of internalization methods. Although lipid rafts may somehow be recognized by endocytic machinery, at this stage the data do not readily fit with the idea of a single raft-specific or raft-dependent endocytic pathway. Many studies report uptake of raft molecules by caveolar endocytosis (defined by sensitivity to cholesterol depletion and to overexpression of a specific mutant of dynamin 2). It is now apparent that this is a highly regulated process, and caveolin 1, one of the characteristic protein components of caveolae, might in fact act to slow or inhibit endocytosis. The molecular details of caveolar endocytosis have yet to be elucidated. Several sources indicate that clathrin-independent uptake to a distinct class of caveolin-1-containing endosome, termed the caveosome, allows different types of endocytic mechanisms to have different functional consequences for the cell. It is likely that there are mechanisms that allow recruitment and targeting of specific molecules to caveosomes.

Glycosylphosphatidylinositol-anchored Proteins Regulate Transforming Growth Factor-β Signaling in Human Keratinocytes

Glycosylphosphatidylinositol (GPI)-anchored proteins have been demonstrated to bind transforming growth factor-beta (TGF-beta) in certain cell lines. However, the identity of these GPI-anchored proteins and the role they may play in TGF-beta signaling remain unknown. We have previously reported the presence of GPI-anchored TGF-beta-binding proteins on human skin fibroblasts and keratinocytes (Tam, B. Y. Y., and Philip, A. (1998) J. Cell. Physiol. 176, 553-564; Tam, B. Y. Y., Germain, L., and Philip, A. (1998) J. Cell. Biochem. 70, 573-586). On human keratinocytes, we identified a 150-kDa GPI-anchored TGF-beta1-binding protein (r150) and demonstrated that it can form a heteromeric complex with the type I and II TGF-beta signaling receptors. To explore whether GPI-anchored proteins modulate TGF-beta signaling in keratinocytes, we created keratinocytes defective in GPI anchor biosynthesis (GPI mutant cells) by chemical mutagenesis of HaCaT cells. Mutant clones were selected by fluorescence-activated cell sorting analysis based on the loss of a CD59 marker. In comparison with parental HaCaT cells, GPI mutant cells demonstrated a significant loss of r150 expression. In contrast, the levels of the type I and II TGF-beta receptors and their ligand affinities, cell morphology, and doubling time remained unchanged. Importantly, GPI mutant cells displayed enhanced gene transcriptional activity and Smad2 and Smad3 activation in response to TGF-beta1 treatment in a dose-dependent manner. Taken together, our results indicate that GPI-anchored protein(s) inhibit TGF-beta signaling and implicate r150 as the GPI-anchored protein responsible for this inhibition in human keratinocytes.

Identification of a stomatin orthologue in vacuoles induced in human erythrocytes by malaria parasites. A role for microbial raft proteins in apicomplexan vacuole biogenesis

When the human malaria parasite Plasmodium falciparum infects erythrocytes, proteins associated with host-derived detergent-resistant membrane (DRM) rafts are selectively recruited into the newly formed vacuole, but parasite proteins that contribute to raft-based vacuole development are unknown. In mammalian cells, DRM-associated integral membrane proteins such as caveolin-1 and flotillin-1 that form oligomers have been linked to the formation of DRM-based invaginations called caveolae. Here we show that the P. falciparum genome does not encode caveolins or flotillins but does contain an orthologue of human band 7 stomatin, a protein known to oligomerize, associate with non-caveolar DRMs and is distantly related to flotillins. Stomatins are members of a large protein family conserved in evolution and P. falciparum (Pf) stomatin appears to be a prokaryotic-like molecule. Evidence is presented that it associates with DRMs and may oligomerize, suggesting that these features are conserved in the stomatin family. Further, Pfstomatin is an integral membrane protein concentrated at the apical end of extracellular parasites, where it co-localizes with invasion-associated rhoptry organelles. A resident rhoptry protein, RhopH2 also resides in DRMs. This provides the first evidence that rhoptries of an apicomplexan parasite contain DRM rafts. Further, when the parasite invades erythrocytes, rhoptry Pfstomatin and RhopH2 are inserted into the newly formed vacuole. Thus, like caveolin-1 and flotillin-1, a stomatin may also associate with non-clathrin coated, DRM-enriched vacuoles. We propose a new model of invasion and vacuole formation involving DRM-based interactions of both host and parasite molecules.

Distinct ligand binding sites in integrin alpha3beta1 regulate matrix adhesion and cell-cell contact

The integrin alpha3beta1 mediates cellular adhesion to the matrix ligand laminin-5. A second integrin ligand, the urokinase receptor (uPAR), associates with alpha3beta1 via a surface loop within the alpha3 beta-propeller (residues 242-246) but outside the laminin binding region, suggesting that uPAR-integrin interactions could signal differently from matrix engagement. To explore this, alpha3-/- epithelial cells were reconstituted with wild-type (wt) alpha3 or alpha3 with Ala mutations within the uPAR-interacting loop (H245A or R244A). Wt or mutant-bearing cells showed comparable expression and adhesion to laminin-5. Cells expressing wt alpha3 and uPAR dissociated in culture, with increased Src activity, up-regulation of SLUG, and down-regulation of E-cadherin and gamma-catenin. Src kinase inhibition or expression of Src 1-251 restored the epithelial phenotype. The H245A and R244A mutants were unaffected by coexpression of uPAR. We conclude that alpha3beta1 regulates both cell-cell contact and matrix adhesion, but through distinct protein interaction sites within its beta-propeller. These studies reveal an integrin- and Src-dependent pathway for SLUG expression and mesenchymal transition.

Endocytosis of oxidized low density lipoprotein through scavenger receptor CD36 utilizes a lipid raft pathway that does not require caveolin-1

The scavenger receptor CD36 binds a diverse array of ligands, including thrombospondin-1, oxidized low density lipoprotein (OxLDL), fatty acids, anionic phospholipids, and apoptotic cells. CD36 has been reported to be present in lipid rafts/caveolae, but little is known about the membrane trafficking of this protein at baseline or following ligand binding. Here, we determined that expression of CD36 in Chinese hamster ovary (CHO) cells and endogenous expression of CD36 in C32 cells led to a homogeneous distribution of the protein on the plasma membrane, as judged by confocal fluorescence microscopy. This homogeneous pattern was observed both by anti-CD36 antibody staining and by live cell imaging of CHO cells expressing a chimeric CD36-green fluorescent protein construct. In contrast, caveolin-1 displayed its usual punctate surface distribution. Correspondingly, dual labeling of CD36 and caveolin-1 showed essentially no overlap, neither by immunofluorescence light microscopy nor by immunogold electron microscopy. Furthermore, isolation of lipid rafts by sucrose gradient ultracentrifugation of cold Triton X-100 cell lysates yielded both CD36 and caveolin-1, but immunoprecipitates of caveolin-1 did not contain CD36. Binding of Ox-LDL led to internalization of CD36 and OxLDL into endosomal structures that did not contain caveolin-1 or transferrin but that co-internalized the glycosyl-phosphatidylinositol-anchored protein decay accelerating factor, a lipid raft protein. Furthermore, expression of CD36 in the caveolin-1-negative KB cell line is sufficient for OxLDL-induced internalization of CD36, indicating that caveolin-1 is not required for this endocytic process. Taken together, these data demonstrate that at steady state, CD36 is localized in lipid rafts but not in caveolae, and that binding of OxLDL to CD36 leads to endocytosis through a lipid raft pathway that is distinct from the clathrin-mediated or caveolin internalization pathways.

Phenotypic behavior of caveolin-3 R26Q, a mutant associated with hyperCKemia, distal myopathy, and rippling muscle disease

Four different phenotypes have been associated with CAV3 mutations: limb girdle muscular dystrophy-1C (LGMD-1C), rippling muscle disease (RMD), and distal myopathy (DM), as well as idiopathic and familial hyperCKemia (HCK). Detailed molecular characterization of two caveolin-3 mutations (P104L and DeltaTFT), associated with LGMD-1C, shows them to impart a dominant-negative effect on wild-type caveolin-3, rendering it dysfunctional through sequestration in the Golgi complex. Interestingly, substitution of glutamine for arginine at amino acid position 26 (R26Q) of caveolin-3 is associated not only with RMD but also with DM and HCK. However, the phenotypic behavior of the caveolin-3 R26Q mutation has never been evaluated in cultured cells. Thus we characterized the cellular and molecular properties of the R26Q mutant protein to better understand how this mutation can manifest as such distinct disease phenotypes. Here, we show that the caveolin-3 R26Q mutant is mostly retained at the level of the Golgi complex. The caveolin-3 R26Q mutant formed oligomers of a much larger size than wild-type caveolin-3 and was excluded from caveolae-enriched membranes. However, caveolin-3 R26Q did not behave in a dominant-negative fashion when coexpressed with wild-type caveolin-3. Thus the R26Q mutation behaves differently from other caveolin-3 mutations (P104L and DeltaTFT) that have been previously characterized. These data provide a possible explanation for the scope of the various disease phenotypes associated with the caveolin-3 R26Q mutation. We propose a haploinsufficiency model in which reduced levels of wild-type caveolin-3, although not rendered dysfunctional due to the caveolin-3 R26Q mutant protein, are insufficient for normal muscle cell function.

Localization of low density lipoprotein receptor-related protein 1 to caveolae in 3T3-L1 adipocytes in response to insulin treatment

The insulin-induced translocation of low density lipoprotein receptor-related protein 1 (LRP1) from intracellular membranes to the cell surface in 3T3-L1 adipocytes was differentiation-dependent and did not occur in 3T3-L1 fibroblasts. Prompted by findings that the plasma membrane of 3T3-L1 adipocytes was rich in caveolae, we determined whether LRP1 became caveolae-associated upon insulin stimulation. The caveolae domain was isolated by the well characterized detergent solubilization and sucrose density ultracentrifugation methodology. Under basal conditions, only a trace amount of LRP1 was caveolae-associated despite the markedly elevated caveolin-1 and caveolae after adipocytic cell differentiation. Upon insulin treatment, the amount of LRP1 associated with caveolae was increased by 4-fold within 10 min, which was blocked completely by pretreatment with wortmannin prior to insulin. The caveolar localization of LRP1 in adipocytes was specific to insulin; treatment with platelet-derived growth factor-bb isoform did not promote but rather decreased caveolar localization of LRP1 below basal levels. The insulin-induced caveolar localization of LRP1 was also observed in 3T3-L1 fibroblasts where translocation of LRP1 from intracellular membranes to the cell surface was absent, suggesting that association of LRP1 with caveolae was achieved, at least in part, through lateral transmigration along the plane of plasma membranes. Immunocytochemistry studies revealed partial co-localization of LRP1 (either endogenous LRP1 or an epitope-tagged minireceptor) with caveolin-1 in cells treated with insulin, which was confirmed by co-immunoprecipitation of LRP1 with caveolin-1 in cells treated with insulin but not platelet-derived growth factor-bb. These results suggest that the localization of LRP1 to caveolae responds selectively to extracellular signals.

Differential recruitment of Kv1.4 and Kv4.2 to lipid rafts by PSD-95

The activity of voltage-gated potassium (Kv) channels, and consequently their influence on cellular functions, can be substantially altered by phosphorylation. Several protein kinases that modulate Kv channel activity are found in membrane subdomains known as lipid rafts, which are thought to organize signaling complexes in the cell. Thus, we asked whether Kv1.4 and Kv4.2, two channels with critical roles in excitable cells, are found in lipid rafts. Acylation can target proteins to raft regions; however, Kv channels are not acylated, and therefore, a different mechanism must exist to bring them into these membrane subdomains. Because both Kv1.4 and Kv4.2 interact with postsynaptic density protein 95 (PSD-95), which is acylated (specifically, palmitoylated), we examined whether PSD-95 can recruit these channels to lipid rafts. We found that a portion of Kv1.4 and Kv4.2 protein in rat brain membranes is raft-associated. Lipid raft patching and immunostaining confirmed that some Kv4.2 is in Thy-1-containing rafts in rat hippocampal neurons. Using a heterologous expression system, we determined that palmitoylation of PSD-95 was crucial to its localization to lipid rafts. We then assessed the contribution of PSD-95 to the raft association of these channels. Co-expression of PSD-95 increased the amount of Kv1.4, but not Kv4.2, in lipid rafts. Deleting the PSD-95 binding motif of Kv1.4 eliminated this recruitment, as did substituting a palmitoylation-deficient PSD-95 mutant. This work represents the first evidence that PSD-95 binding can recruit Kv channels into lipid rafts, a process that could facilitate interactions with the protein kinases that affect channel activity.

Lipid Rafts and Caveolae as Portals for Endocytosis: New Insights and Common Mechanisms

Clathrin-coated pits and caveolae are two of the most recognizable features of the plasma membrane of mammalian cells. While our understanding of the machinery regulating and driving clathrin-coated pit-mediated endocytosis has progressed dramatically, including the elucidation of the structure of individual components and partial in vitro reconstitution, the role of caveolae as alternative endocytic carriers still remains elusive 50 years after their discovery. However, recent work has started to provide new insights into endocytosis by caveolae and into apparently related pathways involving lipid raft domains. These pathways, distinguished by their exquisite sensitivity to cholesterol-sequestering agents, can involve caveolae but also exist in cells devoid of caveolins and caveolae. This review examines the current evidence for the involvement of rafts and caveolae in endocytosis and the molecular players involved in their regulation.

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.

Endostatin associates with lipid rafts and induces reorganization of the actin cytoskeleton via down-regulation of RhoA activity

Endostatin, the C-terminal fragment of collagen XVIII, is a potent inhibitor of angiogenesis. Observations that endostatin inhibits endothelial cell migration and induces disassembly of the actin cytoskeleton provide putative cellular mechanisms for this effect. To understand the mechanisms of endostatin-induced intracellular signaling, we analyzed the association of recombinant endostatin with endothelial cell lipid rafts and the roles of its heparin- and integrin-binding properties in this interaction. We observed that a fraction of cell surface-bound endostatin partitioned in low density membrane raft fractions together with caveolin-1. Heparinase treatment of cells prevented the recruitment of endostatin to the lipid rafts but did not affect the association of endostatin with the non-raft fraction, whereas preincubation of endostatin with soluble alpha5beta1 integrin prevented the association of endostatin with the endothelial cell membrane. Endostatin treatment induced recruitment of alpha5beta1 integrin into the raft fraction via a heparan sulfate proteoglycan-dependent mechanism. Subsequently, through alpha5beta1 integrin, heparan sulfate, and lipid raft-mediated interactions, endostatin induced Src-dependent activation of p190RhoGAP with concomitant decrease in RhoA activity and disassembly of actin stress fibers and focal adhesions. These observations provide a cell biological mechanism, which plausibly explains the anti-angiogenic mechanisms of endostatin in vivo.

Multiple determinants for rapid agonist-induced internalization of a nonmammalian gonadotropin-releasing hormone receptor: a putative palmitoylation site and threonine doublet within the carboxyl-terminal tail Are critical

The chicken GnRH receptor (cGnRH-R) differs from all mammalian GnRH-Rs in possessing a cytoplasmic carboxyl-terminal tail. We have previously demonstrated that the cGnRH-R undergoes more rapid agonist-induced internalization than the mammalian GnRH-Rs and requires the carboxyl-terminal tail for this process. To investigate the structural determinants mediating this rapid internalization, a series of mutant receptors was generated, including progressive truncations of the tail and substitution of serine and threonine residues with alanine. Truncation of the carboxyl-terminal tail to position 366 and then to position 356 resulted in a progressive attenuation of the rate and total extent of receptor internalization. However, truncation between positions 356 and 346 did not alter the kinetics of internalization further, whereas a further truncation to position 337 resulted in an additional marked reduction of internalization. We show that the membrane-proximal Cys(328) and the Thr(369)Thr(370) doublet located in the distal carboxyl terminus play a critical role in mediating rapid internalization. We demonstrate that the cGnRH-R, when expressed in both COS-7 and HEK 293 cells, preferentially undergoes rapid agonist-induced internalization in a caveolae-like, dynamin-dependent manner. These conclusions are based on our observation that pretreatments with filipin and methyl-beta-cyclodextrin, agents that disrupt lipid rafts such as caveolae, and coexpression of dominant-negative dynamin-1 (K44A) and caveolin-1 (Delta 1-81) mutants, effectively inhibited rapid agonist-induced internalization. Furthermore, cGnRH-Rs appeared to be mobilized to the beta-arrestin- and clathrin-coated, vesicle-mediated endocytic pathway upon beta-arrestin overexpression.

Specific association of glycoprotein B with lipid rafts during herpes simplex virus entry

Herpes simplex virus (HSV) entry requires the interaction of glycoprotein D (gD) with a cellular receptor such as herpesvirus entry mediator (HVEM or HveA) or nectin-1 (HveC). However, the fusion mechanism is still not understood. Since cholesterol-enriched cell membrane lipid rafts are involved in the entry of other enveloped viruses such as human immunodeficiency virus and Ebola virus, we tested whether HSV entry proceeds similarly. Vero cells and cells expressing either HVEM or nectin-1 were treated with cholesterol-sequestering drugs such as methyl-beta-cyclodextrin or nystatin and then exposed to virus. In all cases, virus entry was inhibited in a dose-dependent manner, and the inhibitory effect was fully reversible by replenishment of cholesterol. To examine the association of HVEM and nectin-1 with lipid rafts, we analyzed whether they partitioned into nonionic detergent-insoluble glycolipid-enriched membranes (DIG). There was no constitutive association of either receptor with DIG. Binding of soluble gD or virus to cells did not result in association of nectin-1 with the raft-containing fractions. However, during infection, a fraction of gB but not gC, gD, or gH associated with DIG. Similarly, when cells were incubated with truncated soluble glycoproteins, soluble gB but not gC was found associated with DIG. Together, these data favor a model in which HSV uses gB to rapidly mobilize lipid rafts that may serve as a platform for entry and cell signaling. It also suggests that gB may interact with a cellular molecule associated with lipid rafts.

Trafficking of prion proteins through a caveolae-mediated endosomal pathway

To understand the posttranslational conversion of the cellular prion protein (PrPC) to its pathologic conformation, it is important to define the intracellular trafficking pathway of PrPC within the endomembrane system. We studied the localization and internalization of PrPC in CHO cells using cryoimmunogold electron microscopy. At steady state, PrPC was enriched in caveolae both at the TGN and plasma membrane and in interconnecting chains of endocytic caveolae. Protein A-gold particles bound specifically to PrPC on live cells. These complexes were delivered via caveolae to the pericentriolar region and via nonclassical, caveolae-containing early endocytic structures to late endosomes/lysosomes, thereby bypassing the internalization pathway mediated by clathrin-coated vesicles. Endocytosed PrPC-containing caveolae were not directed to the ER and Golgi complex. Uptake of caveolae and degradation of PrPC was slow and sensitive to filipin. This caveolae-dependent endocytic pathway was not observed for several other glycosylphosphatidyl inositol (GPI)-anchored proteins. We propose that this nonclassical endocytic pathway is likely to determine the subcellular location of PrPC conversion.