Separation of caveolae from associated microdomains of GPI-anchored proteins

Overcoming key technological challenges in using mass spectrometry for mapping cell surfaces in tissues

Plasma membranes form a critical biological interface between the inside of every cell and its external environment. Their roles in multiple key cellular functions make them important drug targets. However the protein composition of plasma membranes in general is poorly defined as the inherent properties of lipid embedded proteins, such as their hydrophobicity, low abundance, poor solubility and resistance to digestion and extraction makes them difficult to isolate, solubilize, and identify on a large scale by traditional mass spectrometry methods. Here we describe some of the significant advances that have occurred over the past ten years to address these challenges including: i) the development of new and improved membrane isolation techniques via either subfractionation or direct labeling and isolation of plasma membranes from cells and tissues; ii) modification of mass spectrometry methods to adapt to the hydrophobic nature of membrane proteins and peptides; iii) improvements to digestion protocols to compensate for the shortage of trypsin cleavage sites in lipid-embedded proteins, particularly multi-spanning proteins, and iv) the development of numerous bioinformatics tools which allow not only the identification and quantification of proteins, but also the prediction of membrane protein topology, membrane post-translational modifications and subcellular localization. This review emphasis the importance and difficulty of defining cells in proper patho- and physiological context to maintain the in vivo reality. We focus on how key technological challenges associated with the isolation and identification of cell surface proteins in tissues using mass spectrometry are being addressed in order to identify and quantify a comprehensive plasma membrane for drug and target discovery efforts.

Methods for the identification of vascular markers in health and disease: from the bench to the clinic

Several diseases are characterized by changes in the molecular composition of vascular structures, thus offering the opportunity to use specific ligands (e.g., monoclonal antibodies) for imaging and therapy application. This novel pharmaceutical strategy, often referred to as "vascular targeting", promises to facilitate the discovery and development of selective biopharmaceuticals for the management of angiogenesis-related diseases. This article reviews novel biomedical applications based on vascular targeting strategies, as well as methodologies which have been used for the discovery of vascular markers of pathology.

Lung vascular targeting using antibody to aminopeptidase P: CT-SPECT imaging, biodistribution and pharmacokinetic analysis

BACKGROUND/AIMS

Aminopeptidase P (APP) is specifically enriched in caveolae on the luminal surface of pulmonary vascular endothelium. APP antibodies bind lung endothelium in vivo and are rapidly and actively pumped across the endothelium into lung tissue. Here we characterize the immunotargeting properties and pharmacokinetics of the APP-specific recombinant antibody 833c.

METHODS

We used in situ binding, biodistribution analysis and in vivo imaging to assess the lung targeting of 833c.

RESULTS

More than 80% of 833c bound during the first pass through isolated perfused lungs. Dynamic SPECT acquisition showed that 833c rapidly and specifically targeted the lungs in vivo, reaching maximum levels within 2 min after intravenous injection. CT-SPECT imaging revealed specific targeting of 833c to the thoracic cavity and co-localization with a lung perfusion marker, Tc99m-labeled macroaggregated albumin. Biodistribution analysis confirmed lung-specific uptake of 833c which declined by first-order kinetics (t(½) = 110 h) with significant levels of 833c still present 30 days after injection.

CONCLUSION

These data show that APP expressed in endothelial caveolae appears to be readily accessible to circulating antibody rather specifically in lung. Targeting lung-specific caveolar APP provides an extraordinarily rapid and specific means to target pulmonary vasculature and potentially deliver therapeutic agents into the lung tissue.

Targeting acetylcholinesterase to membrane rafts: a function mediated by the proline-rich membrane anchor (PRiMA) in neurons

In the mammalian brain, acetylcholinesterase (AChE) is anchored in cell membranes by a transmembrane protein PRiMA (proline-rich membrane anchor). We present evidence that at least part of the PRiMA-linked AChE is integrated in membrane microdomains called rafts. A significant proportion of PRiMA-linked AChE tetramers from rat brain was recovered in raft fractions; this proportion was markedly higher at low rather than at high concentrations of cold Triton X-100. The detergent-resistant fraction increased during brain development. In NG108-15 neuroblastoma cells transfected with cDNAs encoding AChE(T) and PRiMA, PRiMA-linked G(4) AChE was found in membrane rafts and showed the same sensitivity to cold Triton X-100 extraction as in the brain. The association of PRiMA-linked AChE with rafts was weaker than that of glycosylphosphatidylinositol-anchored G(2) AChE or G(4) Q(N)-H(C)-linked AChE. It was found to depend on the presence of a cholesterol-binding motif, called CRAC (cholesterol recognition/interaction amino acid consensus), located at the junction of transmembrane and cytoplasmic domains of both PRiMA I and II isoforms. The cytoplasmic domain of PRiMA, which differs between PRiMA I and PRiMA II, appeared to play some role in stabilizing the raft localization of G(4) AChE, because the Triton X-100-resistant fraction was smaller with the shorter PRiMA II isoform than that with the longer PRiMA I isoform.

Caveolae and cancer

All blood vessels are lined by a layer of endothelial cells that help to control vascular permeability. The luminal surface of vascular endothelial cells is studded with transport vesicles called caveolae that are directly in contact with the blood and can transport molecules into and across the endothelium. The vasculature within distinct tissue types expresses a unique array of proteins that can be used to target intravenously injected antibodies directly to that tissue. When the tissue-specific proteins are concentrated in caveolae, the antibodies can be rapidly pumped out of the blood and into the tissue. Tumors appear to be a distinct tissue type with their own unique marker proteins. Targeting accessible proteins at the surface of tumor vasculature with radiolabeled antibodies destroys tumors and drastically increases animal survival. One day, it may be possible to specifically pump targeted molecules into tumors. This could increase therapeutic efficacy and decrease side effects because most of the drug would accumulate specifically in the tumor. Thus, targeting caveolae may provide a universal portal to pump drugs, imaging agents, and gene vectors out of the blood and into underlying tissue.

Targeting and imaging signature caveolar molecules in lungs

A major goal of molecular medicine is to target imaging agents or therapeutic compounds to a single organ. Targeting imaging agents to a single organ could facilitate the high-resolution, in vivo imaging of molecular events. In addition, genetic and acquired diseases primary to a single organ, such as cystic fibrosis, tuberculosis, lung cancer, pulmonary fibrosis, pulmonary hypertension, and acute respiratory distress syndrome, could be specifically targeted in the lung. By targeting and concentrating imaging agents or therapeutics to the lungs, deleterious side effects can be avoided with greater efficacy at much lower dosages. Pathologic changes can be identified earlier and followed over time. In addition, therapeutics that have been abandoned due to toxicities may find renewed utility when coupled with specific targeting agents such as antibodies. To achieve these goals, distinct molecular signatures must be found for each organ or disease-state.

Immunotargeting and cloning of two CD34 variants exhibiting restricted expression in adult rat endothelia in vivo

Mapping protein expression of endothelial cells (EC) in vivo is fundamental to understanding cellular function and may yield new tissue-selective targets. We have developed a monoclonal antibody, MAb J120, to a protein expressed primarily in rat lung and heart endothelium. The antigen was identified as CD34, a marker of hematopoietic stem cells and global marker of endothelial cells in human and mouse tissues. PCR-based cloning identified two CD34 variant proteins, full length and truncated, both of which are expressed on luminal endothelial cell plasma membranes (P) isolated from lung. Truncated CD34 predominated in heart P, and neither variant was detected in P from kidney or liver. CD34 in lung was readily accessible to (125)I-J120 inoculated intravenously, and immunohistochemistry showed strong CD34 expression in lung EC. Few microvessels stained in heart and kidney, and no CD34 was detected in vessels of other organs or in lymphatics. We present herein the first complete sequence of a rat CD34 variant and show for the first time that the encoded truncated variant is endogenously expressed on EC in vivo. We also demonstrate that CD34 expression in rat EC, unlike mouse and human, is restricted in its distribution enabling quite specific lung targeting in vivo.

Dynamic partitioning of a glycosyl-phosphatidylinositol-anchored protein in glycosphingolipid-rich microdomains imaged by single-quantum dot tracking

Recent experimental developments have led to a revision of the classical fluid mosaic model proposed by Singer and Nicholson more than 35 years ago. In particular, it is now well established that lipids and proteins diffuse heterogeneously in cell plasma membranes. Their complex motion patterns reflect the dynamic structure and composition of the membrane itself, as well as the presence of the underlying cytoskeleton scaffold and that of the extracellular matrix. How the structural organization of plasma membranes influences the diffusion of individual proteins remains a challenging, yet central, question for cell signaling and its regulation. Here we have developed a raft-associated glycosyl-phosphatidyl-inositol-anchored avidin test probe (Av-GPI), whose diffusion patterns indirectly report on the structure and dynamics of putative raft microdomains in the membrane of HeLa cells. Labeling with quantum dots (qdots) allowed high-resolution and long-term tracking of individual Av-GPI and the classification of their various diffusive behaviors. Using dual-color total internal reflection fluorescence (TIRF) microscopy, we studied the correlation between the diffusion of individual Av-GPI and the location of glycosphingolipid GM1-rich microdomains and caveolae. We show that Av-GPI exhibit a fast and a slow diffusion regime in different membrane regions, and that slowing down of their diffusion is correlated with entry in GM1-rich microdomains located in close proximity to, but distinct, from caveolae. We further show that Av-GPI dynamically partition in and out of these microdomains in a cholesterol-dependent manner. Our results provide direct evidence that cholesterol-/sphingolipid-rich microdomains can compartmentalize the diffusion of GPI-anchored proteins in living cells and that the dynamic partitioning raft model appropriately describes the diffusive behavior of some raft-associated proteins across the plasma membrane.

Enhancing identifications of lipid-embedded proteins by mass spectrometry for improved mapping of endothelial plasma membranes in vivo

Lipid membranes structurally define the outer surface and internal organelles of cells. The multitude of proteins embedded in lipid bilayers are clearly functionally important, yet they remain poorly defined. Even today, integral membrane proteins represent a special challenge for current large scale shotgun proteomics methods. Here we used endothelial cell plasma membranes isolated directly from lung tissue to test the effectiveness of four different mass spectrometry-based methods, each with multiple replicate measurements, to identify membrane proteins. In doing so, we substantially expanded this membranome to 1,833 proteins, including >500 lipid-embedded proteins. The best method combined SDS-PAGE prefractionation with trypsin digestion of gel slices to generate peptides for seamless and continuous two-dimensional LC/MS/MS analysis. This three-dimensional separation method outperformed current widely used two-dimensional methods by significantly enhancing protein identifications including single and multiple pass transmembrane proteins; >30% are lipid-embedded proteins. It also profoundly improved protein coverage, sensitivity, and dynamic range of detection and substantially reduced the amount of sample and the number of replicate mass spectrometry measurements required to achieve 95% analytical completeness. Such expansion in comprehensiveness requires a trade-off in heavy instrument time but bodes well for future advancements in truly defining the ever important membranome with its potential in network-based systems analysis and the discovery of disease biomarkers and therapeutic targets. This analytical strategy can be applied to other subcellular fractions and should extend the comprehensiveness of many future organellar proteomics pursuits.

Ubiquitous yet distinct expression of podocalyxin on vascular surfaces in normal and tumor tissues in the rat

BACKGROUND/AIMS

The vasculature has become an important target in the development of therapies for increasing numbers of human diseases, yet there are few reliable markers available to identify the endothelium in rodent models. We have characterized the expression, subcellular localization and accessibility of the rat pan-endothelial marker podocalyxin (podxl) using a newly developed monoclonal antibody (mAb), G278.

METHODS

podxl expression and accessibility to binding by G278 were determined in the rat by a variety of experimental approaches.

RESULTS

mAb G278 reliably immunostained blood vessels of all types and of every size in fresh-frozen, fixed-frozen and paraffin-embedded sections of all tissues, but did not stain lymphatic vessels. Western blotting, in vivo imaging and biodistribution analyses demonstrated that the highest levels of endothelial podxl were found in the lung and heart. We also determined that podxl is not enriched in caveolae and that its expression can be modulated in the tumor microenvironment.

CONCLUSION

Our study shows that podxl is a better identifier of rat endothelia than are some of the more commonly used markers and that mAb G278 is a robust antibody for use not only in identifying rat blood vessels but also as a tool to elucidate podxl function.

Purification of basolateral integral membrane proteins by cationic colloidal silica-based apical membrane subtraction

Epithelial cell polarity mediates many essential biological functions and perturbation of the apical/basolateral divide is a hallmark of epithelial to mesenchymal transition in carcinoma. Therefore, correct targeting of proteins to the apical and basolateral surfaces is essential to proper epithelial cell function. However, proteomic characterisation of apical/basolateral sorting has been largely ignored, due to ineffectual separation techniques and contamination of plasma-membrane preparations with housekeeping proteins. Here we describe a method that strips the apical membrane from the adherent cells and releases the intracellular contents, thereby leaving the basolateral membrane available for stringent washes and collection. Analysis of the basolateral membrane of an adherent colon adenocarcinoma cell line resulted in 66% of identified proteins being integral membrane proteins, which possessed either a transmembrane domain or lipid modification, including 35 CD antigens. Based on the abundance of peptides from basolateral marker proteins, this method efficiently captures basolateral integral membrane proteins, with minimal contamination from other membranes and basic proteins.

Use of Bodipy-labeled sphingolipid and cholesterol analogs to examine membrane microdomains in cells

Much evidence has accumulated to show that cellular membranes such as the plasma membrane, contain multiple "microdomains" of differing lipid and protein composition and function. These domains are sometimes enriched in cholesterol and sphingolipids and are believed to be important structures for the regulation of many biological and pathological processes. This review focuses on the use of fluorescent (Bodipy) labeled analogs of sphingolipids and cholesterol to study such domains. We discuss the similarities between the behavior of Bodipy-cholesterol and natural cholesterol in artificial bilayers and in cultured cells, and the use of Bodipy-sphingolipid analogs to visualize membrane domains in living cells based on the concentration-dependent monomer-excimer fluorescence properties of the Bodipy-fluorophore. The use of Bodipy-D-erythro-lactosylceramide is highlighted for detection of domains on the plasma membrane and endosome membranes, and the importance of the sphingolipid stereochemistry in modulating domain formation is discussed. Finally, we suggest that Bodipy-sphingolipids may be useful in future studies to examine the relationship between membrane domains at the cell surface and domains enriched in other lipids and proteins on the inner leaflet of the plasma membrane.

Bluetongue virus outer capsid protein VP5 interacts with membrane lipid rafts via a SNARE domain

Bluetongue virus (BTV) is a nonenveloped double-stranded RNA virus belonging to the family Reoviridae. The two outer capsid proteins, VP2 and VP5, are responsible for virus entry. However, little is known about the roles of these two proteins, particularly VP5, in virus trafficking and assembly. In this study, we used density gradient fractionation and methyl beta cyclodextrin, a cholesterol-sequestering drug, to demonstrate not only that VP5 copurifies with lipid raft domains in both transfected and infected cells, but also that raft domain integrity is required for BTV assembly. Previously, we showed that BTV nonstructural protein 3 (NS3) interacts with VP2 and also with cellular exocytosis and ESCRT pathway proteins, indicating its involvement in virus egress (A. R. Beaton, J. Rodriguez, Y. K. Reddy, and P. Roy, Proc. Natl. Acad. Sci. USA 99:13154-13159, 2002; C. Wirblich, B. Bhattacharya, and P. Roy J. Virol. 80:460-473, 2006). Here, we show by pull-down and confocal analysis that NS3 also interacts with VP5. Further, a conserved membrane-docking domain similar to the motif in synaptotagmin, a protein belonging to the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) family was identified in the VP5 sequence. By site-directed mutagenesis, followed by flotation and confocal analyses, we demonstrated that raft association of VP5 depends on this domain. Together, these results indicate that VP5 possesses an autonomous signal for its membrane targeting and that the interaction of VP5 with membrane-associated NS3 might play an important role in virus assembly.

Triglyceride-rich lipoprotein lipolysis increases aggregation of endothelial cell membrane microdomains and produces reactive oxygen species

Triglyceride-rich lipoprotein (TGRL) lipolysis may provide a proinflammatory stimulus to endothelium. Detergent-resistant plasma membrane microdomains (lipid rafts) have a number of functions in endothelial cell inflammation. The mechanisms of TGRL lipolysis-induced endothelial cell injury were investigated by examining endothelial cell lipid rafts and production of reactive oxygen species (ROS). Lipid raft microdomains in human aortic endothelial cells were visualized by confocal microscopy with fluorescein isothiocyanate-labeled cholera toxin B as a lipid raft marker. Incubation of Atto565-labeled TGRL with lipid raft-labeled endothelial cells showed that TGRL colocalized with the lipid rafts, TGRL lipolysis caused clustering and aggregation of lipid rafts, and colocalization of TGRL remnant particles on the endothelial cells aggregated lipid rafts. Furthermore, TGRL lipolysis caused translocation of low-density lipoprotein receptor-related protein, endothelial nitric oxide synthase, and caveolin-1 from raft regions to nonraft regions of the membrane 3 h after treatment with TGRL lipolysis. TGRL lipolysis significantly increased the production of ROS in endothelial cells, and both NADPH oxidase and cytochrome P-450 inhibitors reduced production of ROS. Our studies suggest that alteration of lipid raft morphology and composition and ROS production could contribute to TGRL lipolysis-mediated endothelial cell injury.

Membrane partitioning of various delta-opioid receptor forms before and after agonist activations: the effect of cholesterol

Lipid rafts depicted as densely packed and thicker membrane microdomains, based on the dynamic clustering of cholesterol and sphingolipids, may help as platforms involved in a wide variety of cellular processes. The reasons why proteins segregate into rafts are yet to be clarified. The human delta opioid receptor (hDOR) reconstituted in a model system has been characterised after ligand binding by an elongation of its transmembrane part, inducing rearrangement of its lipid microenvironment [Alves, Salamon, Hruby, and Tollin (2005) Biochemistry 44, 9168-9178]. We used hDOR to understand better the correlation between its function and its membrane microdomain localisation. A fusion protein of hDOR with the Green Fluorescent Protein (DOR*) allows precise receptor membrane quantification. Here we report that (i) a fraction of the total receptor pool requires cholesterol for binding activity, (ii) G-proteins stabilize a high affinity state conformation which does not seem modulated by cholesterol. In relation to its distribution, and (iii) a fraction of DOR* is constitutively associated with detergent-resistant membranes (DRM) characterised by an enrichment in lipids and proteins raft markers. (iv) An increase in the quantity of DOR* was observed upon agonist addition. (v) This DRM relocation is prevented by uncoupling the receptor-G-protein interaction.

Caveolae structure and function

Studies on the structure and function of caveolae have revealed how this versatile subcellular organelle can influence numerous signalling pathways. This brief review will discuss a few of the key features of caveolae as it relates to signalling and disease processes.

Internalization of caveolae and their relationship with endosomes in cultured human and mouse endothelial cells

Treatment of cells with pervanadate or vanadate induces the phosphorylation of caveolin-1 and its internalization from the cell surface, but the intracellular fate of caveolae has not been fully elucidated. In the present study, we examined the fate of endocytosed caveolae in human umbilical vein endothelial cells and mouse endothelial KOP2.16 cells. The localization of internalized caveolae and their relationship with the endosomes were examined by immunofluorescence microscopy as well as by immunoprecipitation and chasing of biotinylated transferrin. In untreated cells, caveolin-1 was mostly confined to the cell surface. When cells were treated with either pervanadate for 30 min or vanadate for 3 h, many caveolin-1-labeled vesicles were formed inside the cells, some of which were colocalized with Rab5 or Rab4. The internalized caveolin-1 was colocalized with the endocytosed transferrin in the Rab5-, Rab4- or early endosome antigen-1-labeled compartment where caveolin-1 was phosphorylated. It then moved to the Rabl 1-associated compartment. Immunogold electron microscopy revealed that internalized caveolin-1 colocalized with Rab5 or Rab4 in vesicles larger than caveolae. These results suggest that the internalized caveolae interact with early endosomes.

Lipid raft heterogeneity: an enigma

Glycosphingolipids (GSLs) and glycoproteins are ubiquitous components of mammalian cell membranes. GSLs are especially enriched in the nervous system and significantly contribute to membrane organization and a variety of cellular functions. Current body of evidence suggests that GSLs along with cholesterol are enriched in discrete membrane domains that associate specific proteins. Current notion of membrane organization is that, the GSL-cholesterol-enriched membrane domains known as 'lipid rafts' float in the phospholipid-enriched bulk of the membrane and regulate the cell signaling by facilitating the lipid-protein/protein-protein interactions. The sizeable literature accumulated during the last decade has provided some insight into the organization and function of rafts; however, they still remain perplexing. In recent years, an appealing concept of lipid raft heterogeneity has emerged. GSL- and glycosylphosphatidylinositol-anchored proteins are considered as the crucial pivots of heterogeneous rafts. This review deals with the enigma of organizational and functional heterogeneity of lipid rafts and discusses the dynamic coalescence of heterogeneous rafts during signaling that can explain the specificity of raft-regulated cellular signaling events.

Mammalian plasma membrane proteomics

Plasma membrane proteins serve essential functions for cells, interacting with both cellular and extracellular components, structures and signaling molecules. Additionally, plasma membrane proteins comprise more than two-thirds of the known protein targets for existing drugs. Consequently, defining membrane proteomes is crucial to understanding the role of plasma membranes in fundamental biological processes and for finding new targets for action in drug development. MS-based identification methods combined with chromatographic and traditional cell-biology techniques are powerful tools for proteomic mapping of proteins from organelles. However, the separation and identification of plasma membrane proteins remains a challenge for proteomic technology because of their hydrophobicity and microheterogeneity. Creative approaches to solve these problems and potential pitfalls will be discussed. Finally, a representative overview of the impressive achievements in this field will also be given.

Lipid rafts and caveolae in signaling by growth factor receptors

Lipid rafts and caveolae are microdomains of the plasma membrane enriched in sphingolipids and cholesterol, and hence are less fluid than the remainder of the membrane. Caveolae have an invaginated structure, while lipid rafts are flat regions of the membrane. The two types of microdomains have different protein compositions (growth factor receptors and their downstream molecules) suggesting that lipid rafts and caveolae have a role in the regulation of signaling by these receptors. The purpose of this review is to discuss this model, and the implications that it might have regarding a potential role for lipid rafts and caveolae in human cancer. Particular attention will be paid to the epidermal growth factor receptor, for which the largest amount of information is available. It has been proposed that caveolins act as tumor suppressors. The role of lipid rafts is less clear, but they seem to be capable of acting as 'signaling platforms', in which signal initiation and propagation can occur efficiently.

Regulation of tumor phenotypes by caveolin-1 and sphingolipid-controlled membrane signaling complexes

Aberrant (glyco)sphingolipid expression deeply affects several properties of tumor cells that are involved in tumor progression and metastasis formation: cell adhesion (to the extracellular matrix or to the endothelium of blood vessels), motility, recognition and invasion of host tissues. In particular, (glyco)sphingolipids might contribute to the modulation of integrin-dependent interactions of tumor cells (determining their adhesion, motility and invasiveness) with the extracellular matrix as well as with host cells present in the stromal compartment of the tumor. A model based on solid experimental evidence has been proposed: (glyco)sphingolipids at the cell surface interact with plasma membrane receptors (e.g., integrin receptors and growth factor receptors) and adapter molecules (including tetraspanins) forming signaling complexes that are able to influence the activity of signal transduction molecules oriented at the cytosolic surface of the plasma membrane (mainly the Src kinases pathway members). The function of these signaling complexes appears to be strictly dependent on their (glyco)sphingolipid composition, and likely on specific sphingolipid-protein interactions. From this point of view, particularly intriguing is the connection between (glyco)sphingolipids and caveolin-1, a membrane protein that plays multiple roles as a suppressor of tumor growth and metastasis in ovarian, breast and colon human carcinomas.

Vascular proteomic mapping in vivo

Molecular targeting of drugs and imaging agents remain important yet elusive goals in modern medicine. Technological advancements in genomics and proteomics methods have detected differentially expressed genes and proteins, uncovering many new candidate targets in a wide array of diseases and tissues. However, methods to validate potential targets in vivo tend to be quite laborious so that the validation and testing phase has become rate-limiting in bringing treatments to the clinic. There is a critical need for integrated approaches combining state-of-the-art methodologies in proteomics and in vivo imaging to accelerate validation of newly discovered vascular targets for nanomedicines, drugs, imaging agents, and gene vectors. This paper is a review of vascular targeting and proteomics, and will present recent developments in proteomic imaging. A new in vivo organellar proteomic imaging platform will be discussed, which combines subcellular fractionation, mass spectrometry, bioinformatic database interrogation, monoclonal antibody technology and a battery of imaging modalities to rapidly discover and validate tissue-specific endothelial protein targets in vivo. Technological advancements are permitting large-scale proteomic mapping to be performed. New targets have been discovered that permit organ-specific targeting in vivo. Improvements in imaging are creating standards for validation of targets in vivo. Tumor imaging and radioimmunotherapy have also been improved through these efforts. Although we are moving towards a comprehensive mapping of the protein expression by the endothelium, much more needs to be done.

Full-length, glycosylated NSP4 is localized to plasma membrane caveolae by a novel raft isolation technique

Rotavirus NSP4, initially characterized as an endoplasmic reticulum intracellular receptor, is a multifunctional viral enterotoxin that induces diarrhea in murine pups. There have been recent reports of the secretion of a cleaved NSP4 fragment (residues 112 to 175) and of the association of NSP4 with LC3-positive autophagosomes, raft membranes, and microtubules. To determine if NSP4 traffics to a specific subset of rafts at the plasma membrane, we isolated caveolae from plasma membrane-enriched material that yielded caveola membranes free of endoplasmic reticulum and nonraft plasma membrane markers. Analyses of the newly isolated caveolae from rotavirus-infected MDCK cells revealed full-length, high-mannose glycosylated NSP4. The lack of Golgi network-specific processing of the caveolar NSP4 glycans supports studies showing that NSP4 bypasses the Golgi apparatus. Confocal imaging showed the colocalization of NSP4 with caveolin-1 early and late in infection, elucidating the temporal and spatial NSP4-caveolin-1 association during infection. These data were extended with fluorescent resonance energy transfer analyses that confirmed the NSP4 and caveolin-1 interaction in that the specific fluorescently tagged antibodies were within 10 nm of each other during infection. Cells transfected with NSP4 showed patterns of staining and colocalization with caveolin-1 similar to those of infected cells. This study presents an endoplasmic reticulum contaminant-free caveola isolation protocol; describes the presence of full-length, endoglycosidase H-sensitive NSP4 in plasma membrane caveolae; provides confirmation of the NSP4-caveolin interaction in the presence and absence of other viral proteins; and provides a final plasma membrane destination for Golgi network-bypassing NSP4 transport.

Gangliosides GM1 and GM3 in the living cell membrane form clusters susceptible to cholesterol depletion and chilling

Presence of microdomains has been postulated in the cell membrane, but two-dimensional distribution of lipid molecules has been difficult to determine in the submicrometer scale. In the present paper, we examined the distribution of gangliosides GM1 and GM3, putative raft molecules in the cell membrane, by immunoelectron microscopy using quick-frozen and freeze-fractured specimens. This method physically immobilized molecules in situ and thus minimized the possibility of artifactual perturbation. By point pattern analysis of immunogold labeling, GM1 was shown to make clusters of <100 nm in diameter in normal mouse fibroblasts. GM1-null fibroblasts were not labeled, but developed a similar clustered pattern when GM1 was administered. On cholesterol depletion or chilling, the clustering of both endogenous and exogenously-loaded GM1 decreased significantly, but the distribution showed marked regional heterogeneity in the cells. GM3 also showed cholesterol-dependent clustering, and although clusters of GM1 and GM3 were found to occasionally coincide, these aggregates were separated in most cases, suggesting the presence of heterogeneous microdomains. The present method enabled to capture the molecular distribution of lipids in the cell membrane, and demonstrated that GM1 and GM3 form clusters that are susceptible to cholesterol depletion and chilling.

[Role of lipid rafts in trimeric G protein-mediated signal transduction]

Lipid rafts and caveolae are microdomains in the cell membranes, which contain cholesterol, glycolipids, and sphingomyelin. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are considered to be unstable, being dynamically produced and degraded. Recent studies have reported that lipid rafts contain many signaling molecules, such as glycosylphosphatidylinositol-anchored proteins, acylated proteins, G-protein-coupled receptors (GPCRs), trimeric and small G-proteins and their effectors, suggesting that the lipid rafts have an important role in receptor-mediated signal transduction. Therefore drugs that modify the composition of lipid rafts might influence the efficacy of cellular signal transduction. In this review, we demonstrate the role of lipid rafts in GPCR-G-protein signaling and also present our recent results showing that the wasp toxin mastoparan modifies G(q/11)-mediated phospholipase C activation through the interaction with gangliosides in lipid rafts.

Endothelial cytoskeletal reorganization in response to PAR1 stimulation is mediated by membrane rafts but not caveolae

The vasoactive protease thrombin is a known activator of the protease-activated receptor-1 (PAR1) via cleavage of its NH(2) terminus. PAR1 activation stimulates the RhoA/Rho kinase signaling cascade, leading to myosin light chain (MLC) phosphorylation, actin stress fiber formation, and changes in endothelial monolayer integrity. Previous studies suggest that some elements of this signaling pathway are localized to caveolin-containing cholesterol-rich membrane domains. Here we show that PAR1 and key components of the PAR-associated signaling cascade localize to membrane rafts and caveolae in bovine aortic endothelial cells (BAEC). To investigate the functional significance of this localization, BAEC were pretreated with filipin (5 mug/ml, 5 min) to ablate lipid rafts before thrombin (100 nM) or PAR agonist stimulation. We found that diphosphorylation of MLC and the actin stress fiber formation normally induced by PAR activation were attenuated after lipid raft disruption. To target caveolae specifically, we used a small interferring RNA approach to knockdown caveolin-1 expression. Thrombin-induced MLC phosphorylation and stress fiber formation were not altered in caveolin-1-depleted cells, suggesting that lipid rafts, but not necessarily caveolae, modulate thrombin-activated signaling pathways leading to alteration of the actin cytoskeleton in endothelial cells.

Live dynamic imaging of caveolae pumping targeted antibody rapidly and specifically across endothelium in the lung

How effectively and quickly endothelial caveolae can transcytose in vivo is unknown, yet critical for understanding their function and potential clinical utility. Here we use quantitative proteomics to identify aminopeptidase P (APP) concentrated in caveolae of lung endothelium. Electron microscopy confirms this and shows that APP antibody targets nanoparticles to caveolae. Dynamic intravital fluorescence microscopy reveals that targeted caveolae operate effectively as pumps, moving antibody within seconds from blood across endothelium into lung tissue, even against a concentration gradient. This active transcytosis requires normal caveolin-1 expression. Whole body gamma-scintigraphic imaging shows rapid, specific delivery into lung well beyond that achieved by standard vascular targeting. This caveolar trafficking in vivo may underscore a key physiological mechanism for selective transvascular exchange and may provide an enhanced delivery system for imaging agents, drugs, gene-therapy vectors and nanomedicines. 'In vivo proteomic imaging' as described here integrates organellar proteomics with multiple imaging techniques to identify an accessible target space that includes the transvascular pumping space of the caveola.

Aqueous polymer two-phase systems: effective tools for plasma membrane proteomics

Plasma membranes (PMs) are of particular importance for all living cells. They form a selectively permeable barrier to the environment. Many essential tasks of PMs are carried out by their proteinaceous components, including molecular transport, cell-cell interactions, and signal transduction. Due to the key role of these proteins for cellular function, they take center-stage in basic and applied research. A major problem towards in-depth identification and characterization of PM proteins by modern proteomic approaches is their low abundance and immense heterogeneity in different cells. Highly selective and efficient purification protocols are hence essential to any PM proteome analysis. An effective tool for preparative isolation of PMs is partitioning in aqueous polymer two-phase systems. In two-phase systems, membranes are separated according to differences in surface properties rather than size and density. Despite their rare application to the fractionation of animal tissues and cells, they represent an attractive alternative to conventional fractionation protocols. Here, we review the principles of partitioning using aqueous polymer two-phase systems and compare aqueous polymer two-phase systems with other methods currently used for the isolation of PMs.

Polymer genomics: an insight into pharmacology and toxicology of nanomedicines

Synthetic polymers and nanomaterials display selective phenotypic effects in cells and in the body signal transduction mechanisms involved in inflammation, differentiation, proliferation, and apoptosis. When physically mixed or covalently conjugated with cytotoxic agents, bacterial DNA or antigens, polymers can drastically alter specific genetically controlled responses to these agents. These effects, in part, result from cooperative interactions of polymers and nanomaterials with plasma cell membranes and trafficking of polymers and nanomaterials to intracellular organelles. Cells and whole organism responses to these materials can be phenotype or genotype dependent. In selected cases, polymer agents can bypass limitations to biological responses imposed by the genotype, for example, phenotypic correction of immune response by polyelectrolytes. Overall, these effects are relatively benign as they do not result in cytotoxicity or major toxicities in the body. Collectively, however, these studies support the need for assessing pharmacogenomic effects of polymer materials to maximize clinical outcomes and understand the pharmacological and toxicological effects of polymer formulations of biological agents, i.e. polymer genomics.

Spatial segregation of transport and signalling functions between human endothelial caveolae and lipid raft proteomes

Lipid rafts and caveolae are biochemically similar, specialized domains of the PM (plasma membrane) that cluster specific proteins. However, they are morphologically distinct, implying different, possibly complementary functions. Two-dimensional gel electrophoresis preceding identification of proteins by MS was used to compare the relative abundance of proteins in DRMs (detergent-resistant membranes) isolated from HUVEC (human umbilical-vein endothelial cells), and caveolae immunopurified from DRM fractions. Various signalling and transport proteins were identified and additional cell-surface biotinylation revealed the majority to be exposed, demonstrating their presence at the PM. In resting endothelial cells, the scaffold of immunoisolated caveolae consists of only few resident proteins, related to structure [CAV1 (caveolin-1), vimentin] and transport (V-ATPase), as well as the GPI (glycosylphosphatidylinositol)-linked, surface-exposed protein CD59. Further quantitative characterization by immunoblotting and confocal microscopy of well-known [eNOS (endothelial nitric oxide synthase) and CAV1], less known [SNAP-23 (23 kDa synaptosome-associated protein) and BASP1 (brain acid soluble protein 1)] and novel [C8ORF2 (chromosome 8 open reading frame 2)] proteins showed different subcellular distributions with none of these proteins being exclusive to either caveolae or DRM. However, the DRM-associated fraction of the novel protein C8ORF2 (approximately 5% of total protein) associated with immunoseparated caveolae, in contrast with the raft protein SNAP-23. The segregation of caveolae from lipid rafts was visually confirmed in proliferating cells, where CAV1 was spatially separated from eNOS, SNAP-23 and BASP1. These results provide direct evidence for the previously suggested segregation of transport and signalling functions between specialized domains of the endothelial plasma membrane.

TNF-alpha potentiates protein-tyrosine nitration through activation of NADPH oxidase and eNOS localized in membrane rafts and caveolae of bovine aortic endothelial cells

A major source of reactive oxygen species (ROS) in endothelial cells is the NADPH oxidase enzyme complex. The selective distributions of any enzyme within cells have important implications in regulating enzyme effectiveness through facilitation of access to local substrates and/or product targets. Because membrane rafts provide a spatially preferable environment for a variety of enzyme systems, we sought to determine whether NADPH oxidase is present and functional in this plasma membrane compartment in endothelial cells. We found that, in resting endothelial cells, NADPH oxidase subunits were preassembled and the enzyme functional in membrane rafts, specifically in caveolae. Stimulation with TNF-alpha induced additional recruitment of the p47(phox) regulatory subunit to raft-localized NADPH oxidase and enhanced ROS production within raft domains. TNF-alpha also induced nitric oxide production through activation of endothelial nitric oxide synthase (eNOS) present in the same membrane compartment. The dual activation of superoxide and nitric oxide-generating systems provided a spatially favorable environment for nitration of tyrosine-containing proteins localized to rafts. Perturbation of membrane raft structural integrity with cholesterol-sequestering compounds caused the delocalization of NADPH oxidase subunits and eNOS from the rafts and inhibited TNF-alpha-induced ROS production and protein tyrosine nitration. Together, these data provide evidence that membrane rafts and caveolae play a role in the spatial regulation of NADPH oxidase and subsequent ROS/reactive nitrogen species in endothelial cells.

Reorganization of mouse sperm lipid rafts by capacitation

One of the hallmarks of mammalian sperm capacitation is the loss of cholesterol from the plasma membrane. Cholesterol has been associated with the formation of detergent insoluble membrane microdomains in many cell types, and sperm from several mammalian species have been shown to contain detergent-resistant membranes (DRMs). The change in cholesterol composition of the sperm plasma membrane during capacitation raises the question of whether the contents of DRMs are altered during this process. In this study, we investigated changes in protein composition of DRMs isolated from uncapacitated or capacitated mouse sperm. TX-100 insoluble membranes were fractionated by sucrose flotation gradient centrifugation and analyzed by Western and lectin blotting, and capacitation-related differences in protein composition were identified. Following capacitation, the detergent insoluble fractions moved to lighter positions on the sucrose gradients, reflecting a global change in density or composition. We identified several individual proteins that either became enriched or depleted in DRM fractions following capacitation. These data suggest that the physiological changes in sperm motility, ability to penetrate the zona pellucida (ZP), ZP responsiveness, and other capacitation-dependent changes, may be due in part to a functional reorganization of plasma membrane microdomains.

Cellular Uptake of Exogenous Human PDCD5 Protein

PDCD5 (human programmed cell death 5) plays a significant role in apoptotic and paraptotic cell deaths. However, it was found that recombinant PDCD5 added exogenously to culture medium could also enhance programmed cell death triggered by certain stimuli. Here we show that PDCD5 has a remarkable role in intercellular transport in various cells (endogenous caveolin-1-positive and -negative cells) through a clathrin-independent endocytic pathway that originates from heparan sulfate proteoglycan binding and lipid rafts. These conclusions are supported by the studies of slow internalization kinetics of PDCD5 endosomes, by the resistance of endosomes to nonionic detergents, by the overexpression of the clathrin dominant negative mutant form, which did not block PDCD5-fluorescein isothiocyanate uptake, and by PDCD5 localization in lipid rafts by immunofluorescence, electron microscopy techniques, and sucrose density centrifugation. This is further supported by the findings that certain drugs that disrupt lipid rafts, compete with cell membrane heparan sulfate proteoglycans, or block the caveolae pathway, impair the PDCD5 internalization process. The translocation activity of PDCD5 may possess physiological significance and be a potential mechanism for its programmed cell death-promoting activity. PDCD5 protein also has the ability to drive the internalization of large protein cargo, depending on the residues 109-115 mapped by deletion mutagenesis, and can introduce the Mdm-2 binding domain of human p53 into living cells to induce cell death in human cancer cells, indicating that PDCD5 may serve as a vehicle and thus have potential in the field of protein delivery to the cells. This is the first evidence of such findings.

Probing lipid rafts with proximity imaging: actions of proatherogenic stimuli

Glycosylphosphatidylinositol (GPI)-anchored proteins have been shown to cluster in microdomains enriched in glycosphingolipids and cholesterol and represent a relatively selective marker of lipid rafts. In recent years, several attempts have been made to use fluorescent probes to nondisruptively label these domains in living cells. Here, we have transfected endothelial cells with a GPI-anchored thermotolerant green fluorescent protein (ttGFP) to show colocalization of this fluoroprobe with another marker of lipid rafts, urokinase-type plasminogen activator receptor-1. ttGFP was used to quantify the cell surface area occupied by lipid rafts and to examine the effect of various proatherogenic signals on lipid rafts. Exposure of endothelial cells to asymmetric dimethylarginine and oxidized LDL (oxLDL), as well as oxidant stress, reduced the cell surface area occupied by lipid rafts. Next, the property of ttGFP to undergo a shift in absorbance depending on the clustering of these molecules was utilized to perform proximity imaging (PRIM). PRIM showed that nitric oxide (NO) increased the distance between GPI-anchored ttGFP molecules clustered in lipid-rich microdomains. This “unclustering” of GPI-anchored ttGFP was not reproduced by prooxidant signals and was due to reduction in membrane-cytoskeletal constraints on the lipid rafts. These findings suggested that two fundamentally different mechanisms modulate lipid rafts: 1) substance regulation of lipid rafts involving modification of cholesterol and sphingolipids and 2) structural regulation of lipid rafts through disruption of membrane-cytoskeletal interactions, switching off the spatial confinement of lipid rafts.

Membrane microdomains, caveolae, and caveolar endocytosis of sphingolipids

Caveolae are flask-shape membrane invaginations of the plasma membrane that have been implicated in endocytosis, transcytosis, and cell signaling. Recent years have witnessed the resurgence of studies on caveolae because they have been found to be involved in the uptake of some membrane components such as glycosphingolipids and integrins, as well as viruses, bacteria, and bacterial toxins. Accumulating evidence shows that endocytosis mediated by caveolae requires unique structural and signaling machinery (caveolin-1, src kinase), which indicates that caveolar endocytosis occurs through a mechanism which is distinct from other forms of lipid microdomain-associated, clathrin-independent endocytosis. Furthermore, a balance of glycosphingolipids, cholesterol, and caveolin-1 has been shown to be important in regulating caveolae endocytosis.

Signaling by the angiotensin-converting enzyme

Inhibition of the angiotensin-converting enzyme (ACE) protects against the progression of several cardiovascular diseases. Because of its dual role in regulating angiotensin II and bradykinin levels, the positive clinical effects of ACE inhibitors were thought to be the consequence of concomitant reductions in the production of angiotensin II and the degradation of bradykinin. Recent evidence suggests that some of the beneficial effects of ACE inhibitors on cardiovascular function and homeostasis can be attributed to novel mechanisms. These include the accumulation of the ACE substrate N-acetyl-seryl-aspartyl-lysyl-proline, which blocks collagen deposition in the injured heart, as well as the activation of an ACE signaling cascade that involves the activation of the kinase CK2 and the c-Jun N-terminal kinase in endothelial cells and leads to changes in gene expression. Moreover, at least one other ACE homologue (ACE2) is proposed to counteract the detrimental effects associated with the activation of the classical renin-angiotensin system. These data reveal hitherto unexpected levels of internal regulation of the renin-angiotensin system.

Antidepressants and antipsychotic drugs colocalize with 5-HT3 receptors in raft-like domains

Despite different chemical structure and pharmacodynamic signaling pathways, a variety of antidepressants and antipsychotics inhibit ion fluxes through 5-HT3 receptors in a noncompetitive manner with the exception of the known competitive antagonists mirtazapine and clozapine. To further investigate the mechanisms underlying the noncompetitive inhibition of the serotonin-evoked cation current, we quantified the concentrations of different types of antidepressants and antipsychotics in fractions of sucrose flotation gradients isolated from HEK293 (human embryonic kidney 293) cells stably transfected with the 5-HT3A receptor and of N1E-115 neuroblastoma cells in relation to the localization of the 5-HT3 receptor protein within the cell membrane. Western blots revealed a localization of the 5-HT3 receptor protein exclusively in the low buoyant density (LBD) fractions compatible with a localization within raft-like domains. Also, the antidepressants desipramine, fluoxetine, and reboxetine and the antipsychotics fluphenazine, haloperidol, and clozapine were markedly enriched in LBD fractions, whereas no accumulation occurs for mirtazapine, carbamazepine, moclobemide, and risperidone. The concentrations of psychopharmacological drugs within LBD fractions was strongly associated with their inhibitory potency against serotonin-induced cation currents. The noncompetitive antagonism of antidepressants at the 5-HT3 receptor was not conferred by an enhancement of receptor internalization as shown by immunofluorescence studies, assessment of receptor density in clathrin-coated vesicles, and electrophysiological recordings after coexpression of a dominant-negative mutant of dynamin I, which inhibits receptor internalization. In conclusion, enrichment of antidepressants and antipsychotics in raft-like domains within the cell membrane appears to be crucial for their antagonistic effects at ligand-gated ion channels such as 5-HT3 receptors.

Oligodendrocyte-myelin glycoprotein is present in lipid rafts and caveolin-1-enriched membranes

The oligodendrocyte-myelin glycoprotein is a ligand of the neuronal Nogo receptor and a potent inhibitor of neurite outgrowth, but its physiological function remains to be elucidated. The oligodendrocyte-myelin glycoprotein is anchored solely in the outer leaflet of the plasma membrane via its glycosylphosphatidylinositol anchor, and through its leucine-rich repeat domain, it likely interacts with other proteins. In the present study, we compare its buoyancy and detergent solubility characteristics with those of other myelin proteins. Based on its detergent solubility profile and membrane fractionation using established ultracentrifugation procedures, we conclude that the oligodendrocyte-myelin glycoprotein is a lipid raft component that is closely associated with the axolemma. Moreover, it associates with caveolin-1 and caveolin-1-enriched membranes. We postulate that, by virtue of its concentration in lipid rafts and perhaps through interactions with caveolin-1, the oligodendrocyte-myelin glycoprotein may influence signaling pathways.

Expression of a secreted form of Dally, a Drosophila glypican, induces overgrowth phenotype by affecting action range of Hedgehog

Glypicans, a family of heparan sulfate proteoglycans attached to the cell surface via a glycosylphosphatidylinositol (GPI)-anchor, play essential roles in morphogen signaling and distributions. A Drosophila glypican, Dally, regulates the gradient formation of Decapentaplegic (Dpp) in the developing wing. To gain insights into the function of glypicans in morphogen signaling, we examined the activities of two mutant forms of Dally: a transmembrane form (TM-Dally) and a secreted form (Sec-Dally). Misexpression of tm-dally in the wing disc had a similar yet weaker effect in enhancing Dpp signaling compared to that of wild-type dally. In contrast, Sec-Dally shows a weak dominant negative activity on Dpp signal transduction. Furthermore, sec-dally expression led to patterning defects as well as a substantial overgrowth of tissues and animals through the expansion of the action range of Hh. These findings support the recently proposed model that secreted glypicans have opposing and/or distinct effects on morphogen signaling from the membrane-tethered forms.