Separation of caveolae from associated microdomains of GPI-anchored proteins

Rapid precision targeting of nanoparticles to lung via caveolae pumping system in endothelium

Modern medicine seeks precision targeting, imaging and therapy to maximize efficacy and avoid toxicities. Nanoparticles (NPs) have tremendous yet unmet clinical potential to carry and deliver imaging and therapeutic agents systemically with tissue precision. But their size contributes to rapid scavenging by the reticuloendothelial system and poor penetration of key endothelial cell (EC) barriers, limiting target tissue uptake, safety and efficacy. Here we discover the ability of the EC caveolae pumping system to outpace scavenging and deliver NPs rapidly and specifically into the lungs. Gold and dendritic NPs are conjugated to antibodies targeting caveolae of the lung microvascular endothelium. SPECT-CT imaging and biodistribution analyses reveal that rat lungs extract most of the intravenous dose within minutes to achieve precision lung imaging and targeting with high lung concentrations exceeding peak blood levels. These results reveal how much ECs can both limit and promote tissue penetration of NPs and the power and size-dependent limitations of the caveolae pumping system. This study provides a new retargeting paradigm for NPs to avoid reticuloendothelial system uptake and achieve rapid precision nanodelivery for future diagnostic and therapeutic applications.

Caveolin-1 affects early mycobacterial infection and apoptosis in macrophages and mice

Tuberculosis, caused by Mycobacterium tuberculosis, remains one of the deadliest infections in humans. Because Mycobacterium bovis Bacillus Calmette-Guérin (BCG) share genetic similarities with Mycobacterium tuberculosis, it is often used as a model to elucidate the molecular mechanisms of more severe tuberculosis infection. Caveolin-1 has been implied in many physiological processes and diseases, but it's role in mycobacterial infections has barely been studied. We isolated macrophages from Wildtype or Caveolin-1 deficient mice and analyzed hallmarks of infection, such as internalization, induction of autophagy and apoptosis. For in vivo assays we intravenously injected mice with BCG and investigated tissues for bacterial load with colony-forming unit assays, bioactive lipids with mass spectrometry and changes of protein expressions by Western blotting. Our results revealed that Caveolin-1 was important for early killing of BCG infection in vivo and in vitro, controlled acid sphingomyelinase (Asm)-dependent ceramide formation, apoptosis and inflammatory cytokines upon infection with BCG. In accordance, Caveolin-1 deficient mice and macrophages showed higher bacterial burdens in the livers. The findings indicate that Caveolin-1 plays a role in infection of mice and murine macrophages with BCG, by controlling cellular apoptosis and inflammatory host response. These clues might be useful in the fight against tuberculosis.

Advances in the Physiology of Transvascular Exchange and A New Look At Rational Fluid Prescription

The Starling principle is a model that explains the transvascular distribution of fluids essentially governed by hydrostatic and oncotic forces, which dynamically allow vascular refilling according to the characteristics of the blood vessel. However, careful analysis of fluid physiology has shown that the principle, while correct, is not complete. The revised Starling principle (Michel-Weinbaum model) provides relevant information on fluid kinetics. Special emphasis has been placed on the endothelial glycocalyx, whose subendothelial area allows a restricted oncotic pressure that limits the reabsorption of fluid from the interstitial space, so that transvascular refilling occurs mainly from the lymphatic vessels. The close correlation between pathological states of the endothelium (eg: sepsis, acute inflammation, or chronic kidney disease) and the prescription of fluids forces the physician to understand the dynamics of fluids in the organism; this will allow rational fluid prescriptions. A theory that integrates the physiology of exchange and transvascular refilling is the "microconstant model", whose variables include dynamic mechanisms that can explain edematous states, management of acute resuscitation, and type of fluids for common clinical conditions. The clinical-physiological integration of the concepts will be the hinges that allow a rational and dynamic prescription of fluids.

Impact of lipid rafts on transient receptor potential channel activities

Members of the transient receptor potential (TRP) superfamily are cation channels that are expressed in nearly every mammalian cell type and respond as cellular sensors to various environmental stimuli. Light, pressure, osmolarity, temperature, and other stimuli can induce TRP calcium conductivity and correspondingly trigger many signaling processes in cells. Disruption of TRP channel activity, as a rule, harms cellular function. Despite numerous studies, the mechanisms of TRP channel regulation are not yet sufficiently clear, in part, because TRP channels are regulated by a broad set of ligands having diverse physical and chemical features. It is now known that some TRP members are located in membrane microdomains termed lipid rafts. Moreover, interaction between specific raft-associated lipids with channels may be a key regulation mechanism. This review examines recent findings related to the roles of lipid rafts in regulation of TRP channel activity. The mechanistic events of channel interactions with the main lipid raft constituent, cholesterol, are being clarified. Better understanding of mechanisms behind such interactions would help establish the key elements of TRP channel regulation and hence allow control of cellular responses to environmental stimuli.

Mechanisms underlying unidirectional laminar shear stress-mediated Nrf2 activation in endothelial cells: Amplification of low shear stress signaling by primary cilia

Endothelial cells are sensitive to mechanical stress and respond differently to oscillatory flow versus unidirectional flow. This review highlights the mechanisms by which a wide range of unidirectional laminar shear stress induces activation of the redox sensitive antioxidant transcription factor nuclear factor-E2-related factor 2 (Nrf2) in cultured endothelial cells. We propose that fibroblast growth factor-2 (FGF-2), brain-derived neurotrophic factor (BDNF) and 15-Deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) are potential Nrf2 activators induced by laminar shear stress. Shear stress-dependent secretion of FGF-2 and its receptor-mediated signaling is tightly controlled, requiring neutrophil elastase released by shear stress, αvβ3 integrin and the cell surface glycocalyx. We speculate that primary cilia respond to low laminar shear stress (<10 dyn/cm2), resulting in secretion of insulin-like growth factor 1 (IGF-1), which facilitates αvβ3 integrin-dependent FGF-2 secretion. Shear stress induces generation of heparan-binding epidermal growth factor-like growth factor (HB-EGF), which contributes to FGF-2 secretion and gene expression. Furthermore, HB-EGF signaling modulates FGF-2-mediated NADPH oxidase 1 activation that favors casein kinase 2 (CK2)-mediated phosphorylation/activation of Nrf2 associated with caveolin 1 in caveolae. Higher shear stress (>15 dyn/cm2) induces vesicular exocytosis of BDNF from endothelial cells, and we propose that BDNF via the p75NTR receptor could induce CK2-mediated Nrf2 activation. Unidirectional laminar shear stress upregulates gene expression of FGF-2 and BDNF and generation of 15d-PGJ2, which cooperate in sustaining Nrf2 activation to protect endothelial cells against oxidative damage.

Engineering nanoparticles to tackle tumor barriers

Engineering nanoparticles (NPs) as delivery systems of anticancer therapeutics has attracted tremendous attention in recent decades, and some nanoscale drug formulations have been approved for clinical use. However, their therapeutic efficacies are still limited by the presence of a series of biological barriers during the delivery process. Among these obstacles, tumor barriers are generally recognized as the bottleneck, because they dominate the NP extravasation from the tumor vasculature and penetration into the tumor parenchyma. Therefore, this review first discussed tumor barriers from two aspects: tumor vascular barriers and tumor stromal barriers. Pathological features of the two sets of barriers as well as their influence on the delivery efficacy were described. Then, we outlined strategies for engineering NPs to overcome these challenges: increasing extravasation through physical property optimization and tumor vascular targeting; and facilitating deep penetration through particle size manipulation, modulation of the tumor extracellular matrix, and some new mechanisms. This review will provide a critical perspective on engineering strategies for more efficient nanomedicine in oncology.

Heterogeneous Ganglioside-Enriched Nanoclusters with Different Densities in Membrane Rafts Detected by a Peptidyl Molecular Probe

The specific features of the lateral distribution of gangliosides play key roles in cell-cell communications and the onset of various diseases related to the plasma membrane. We herein demonstrated that an artificial peptide identified from a phage-displayed library is available as a molecular probe for specific ganglioside nanoclustering sites in caveolae/membrane rafts on the cell surface. Atomic force microscopy studies indicated that the peptide specifically binds to the highly enriched monosialoganglioside GM1 nanodomains of reconstituted lipid bilayers composed of GM1, sphingomyelin, cholesterol, and unsaturated phospholipids. The ganglioside-containing area recognized by the peptide on the surface of PC12 cells was part of the area recognized by the cholera toxin B subunit, which has high affinity for GM1. Furthermore, the peptide bound to the cell surface after a treatment with methyl-β-cyclodextrin (MβCD), which disrupts membrane rafts by removing cholesterol. The present results indicate that there are heterogeneous ganglioside clusters with different ganglioside densities in caveolae/membrane rafts, and the peptidyl probe selectively recognizes the high-density ganglioside nanodomain that resists the MβCD treatment. This peptidyl probe will be useful for obtaining information on the lipid organization of the cell membrane and will help clarify the mechanisms by which the lateral distribution of gangliosides affects biological functions and the onset of diseases.

Targeting drug delivery in the vascular system: Focus on endothelium

The bloodstream is the main transporting pathway for drug delivery systems (DDS) from the site of administration to the intended site of action. In many cases, components of the vascular system represent therapeutic targets. Endothelial cells, which line the luminal surface of the vasculature, play a tripartite role of the key target, barrier, or victim of nanomedicines in the bloodstream. Circulating DDS may accumulate in the vascular areas of interest and in off-target areas via mechanisms bypassing specific molecular recognition, but using ligands of specific vascular determinant molecules enables a degree of precision, efficacy, and specificity of delivery unattainable by non-affinity DDS. Three decades of research efforts have focused on specific vascular targeting, which have yielded a multitude of DDS, many of which are currently undergoing a translational phase of development for biomedical applications, including interventions in the cardiovascular, pulmonary, and central nervous systems, regulation of endothelial functions, host defense, and permeation of vascular barriers. We discuss the design of endothelial-targeted nanocarriers, factors underlying their interactions with cells and tissues, and describe examples of their investigational use in models of acute vascular inflammation with an eye on translational challenges.

Spatially controlled assembly of affinity ligand and enzyme cargo enables targeting ferritin nanocarriers to caveolae

One of the goals of nanomedicine is targeted delivery of therapeutic enzymes to the sub-cellular compartments where their action is needed. Endothelial caveolae-derived endosomes represent an important yet challenging destination for targeting, in part due to smaller size of the entry aperture of caveolae (ca. 30-50 nm). Here, we designed modular, multi-molecular, ferritin-based nanocarriers with uniform size (20 nm diameter) for easy drug-loading and targeted delivery of enzymatic cargo to these specific vesicles. These nanocarriers targeted to caveolar Plasmalemmal Vesicle-Associated Protein (Plvap) deliver superoxide dismutase (SOD) into endosomes in endothelial cells, the specific site of influx of superoxide mediating by such pro-inflammatory signaling as some cytokines and lipopolysaccharide (LPS). Cell studies showed efficient internalization of Plvap-targeted SOD-loaded nanocarriers followed by dissociation from caveolin-containing vesicles and intracellular transport to endosomes. The nanocarriers had a profound protective anti-inflammatory effect in an animal model of LPS-induced inflammation, in agreement with the characteristics of their endothelial uptake and intracellular transport, indicating that these novel, targeted nanocarriers provide an advantageous platform for caveolae-dependent delivery of biotherapeutics.

Quantifying spatial and temporal variations of the cell membrane ultra-structure by bimFCS

It has been long recognized that the cell membrane is heterogeneous on scales ranging from a couple of molecules to micrometers in size and hence diffusion of receptors is length scale dependent. This heterogeneity modulates many cell-membrane-associated processes requiring transient spatiotemporal separation of components. The transient increase in local concentration of interacting signal components enables robust signaling in an otherwise thermally noisy system. Understanding how lipids and proteins self-organize and interact with the cell cortex requires quantifying the motion of the components. Multi-length scale diffusion measurements by single particle tracking, fluorescence correlation spectroscopy (FCS) or related techniques are able to identify components being transiently trapped in nanodomains, from freely moving one and from ones with reduced long-scale diffusion due to interaction with the cell cortex. One particular implementation of multi-length scale diffusion measurements is the combination of FCS with a spatially resolved detector, such as a camera and two-dimensional extended excitation profile. The main advantages of this approach are that all length scales are interrogated simultaneously, uniquely permits quantifying changes to the membrane structure caused by extrenal or internal perturbations. Here, we review how combining total internal reflection microscopy (TIRF) with FC resolves the membrane organization in living cells. We show how to implement the method, which requires only a few seconds of data acquisition to quantify membrane nanodomains, or the spacing of membrane fences caused by the actin cortex. The choice of diffusing fluorescent probe determines which membrane heterogeneity is detected. We review the instrument, sample preparation, experimental and computational requirements to perform such measurements, and discuss the potential and limitations. The discussion includes examples of spatial and temporal comparisons of the membrane structure in response to perturbations demonstrating the complex cell physiology.

The Role of Endothelial Surface Glycocalyx in Mechanosensing and Transduction

The endothelial cells (ECs) forming the inner wall of every blood vessel are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. A variety of mechanosensors and transducers, locating on the EC surface, intra- and trans-EC membrane, and within the EC cytoskeleton, have thus been identified to ensure proper functions of ECs. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins and is close to other prominent EC mechanosensors and transducers. This chapter summarizes the ESG composition, thickness, and structure observed by different labeling and visualization techniques and in different types of vessels. It also presents the literature in determining the ESG mechanical properties by atomic force microscopy and optical tweezers. The molecular mechanisms by which the ESG plays the role in EC mechanosensing and transduction are described as well as the ESG remodeling by shear stress, the actin cytoskeleton, the membrane rafts, the angiogenic factors, and the sphingosine-1-phosphate.

Targeting superoxide dismutase to endothelial caveolae profoundly alleviates inflammation caused by endotoxin

Inflammatory mediators binding to Toll-Like receptors (TLR) induce an influx of superoxide anion in the ensuing endosomes. In endothelial cells, endosomal surplus of superoxide causes pro-inflammatory activation and TLR4 agonists act preferentially via caveolae-derived endosomes. To test the hypothesis that SOD delivery to caveolae may specifically inhibit this pathological pathway, we conjugated SOD with antibodies (Ab/SOD, size ~10nm) to plasmalemmal vesicle-associated protein (Plvap) that is specifically localized to endothelial caveolae in vivo and compared its effects to non-caveolar target CD31/PECAM-1. Plvap Ab/SOD bound to endothelial cells in culture with much lower efficacy than CD31 Ab/SOD, yet blocked the effects of LPS signaling with higher efficiency than CD31 Ab/SOD. Disruption of cholesterol-rich membrane domains by filipin inhibits Plvap Ab/SOD endocytosis and LPS signaling, implicating the caveolae-dependent pathway(s) in both processes. Both Ab/SOD conjugates targeted to Plvap and CD31 accumulated in the lungs after IV injection in mice, but the former more profoundly inhibited LPS-induced pulmonary inflammation and elevation of plasma level of interferon-beta and -gamma and interleukin-27. Taken together, these results indicate that targeted delivery of SOD to specific cellular compartments may offer effective, mechanistically precise interception of pro-inflammatory signaling mediated by reactive oxygen species.

The role of cholesterol and sphingolipids in the dopamine D1 receptor and G protein distribution in the plasma membrane

G proteins are peripheral membrane proteins which interact with the inner side of the plasma membrane and form part of the signalling cascade activated by G protein-coupled receptors (GPCRs). Since many signalling proteins do not appear to be homogeneously distributed on the cell surface, they associate in particular membrane regions containing specific lipids. Therefore, protein-lipid interactions play a pivotal role in cell signalling. Our previous results showed that although Gαs and Gαi₃ prefer different types of membrane domains they are both co-localized with the D₁ receptor. In the present report we characterize the role of cholesterol and sphingolipids in the membrane localization of Gαs, Gαi₃ and their heterotrimers, as well as the D₁ receptor. We measured the lateral diffusion and membrane localization of investigated proteins using fluorescence recovery after photobleaching (FRAP) microscopy and fluorescence resonance energy transfer (FRET) detected by lifetime imaging microscopy (FLIM). The treatment with either methyl-β-cyclodextrin or Fumonisin B₁ led to the disruption of cholesterol-sphingolipids containing domains and changed the diffusion of Gαi₃ and the D₁ receptor but not of Gαs. Our results imply a sequestration of Gαs into cholesterol-independent solid-like membrane domains. Gαi₃ prefers cholesterol-dependent lipid rafts so it does not bind to those domains and its diffusion is reduced. In turn, the D₁ receptor exists in several different membrane localizations, depending on the receptor's conformation. We conclude that the inactive G protein heterotrimers are localized in the low-density membrane phase, from where they displace upon dissociation into the membrane-anchor- and subclass-specific lipid domain.

Glycomics and glycoproteomics of membrane proteins and cell-surface receptors: Present trends and future opportunities

Membrane proteins mediate cell-cell interactions and adhesion, the transfer of ions and metabolites, and the transmission of signals from the extracellular environment to the cell interior. The extracellular domains of most cell membrane proteins are glycosylated, often at multiple sites. There is a growing awareness that glycosylation impacts the structure, interaction, and function of membrane proteins. The application of glycoproteomics and glycomics methods to membrane proteins has great potential. However, challenges also arise from the unique physical properties of membrane proteins. Successful analytical workflows must be developed and disseminated to advance functional glycoproteomics and glycomics studies of membrane proteins. This review explores the opportunities and challenges related to glycomic and glycoproteomic analysis of membrane proteins, including discussion of sample preparation, enrichment, and MS/MS analyses, with a focus on recent successful workflows for analysis of N- and O-linked glycosylation of mammalian membrane proteins.

Molecular Mechanism Linking BRCA1 Dysfunction to High Grade Serous Epithelial Ovarian Cancers with Peritoneal Permeability and Ascites

Ovarian cancer constitutes the second most common gynecological cancer with a five-year survival rate of 40%. Among the various histotypes associated with hereditary ovarian cancer, high-grade serous epithelial ovarian carcinoma (HGSEOC) is the most predominant and women with inherited mutations in BRCA1 have a lifetime risk of 40-60%. HGSEOC is a challenge for clinical oncologists, due to late presentation of patient, diagnosis and high rate of relapse. Ovarian tumors have a wide range of clinical presentations including development of ascites as a result of deregulated endothelial function thereby causing increased vascular permeability of peritoneal vessels. The molecular mechanisms remain elusive. Studies have shown that fallopian tube cancers develop in women with BRCA1 gene mutations more often than previously suspected. Recent studies suggest that many primary peritoneal cancers and some high-grade serous epithelial ovarian carcinomas actually start in the fallopian tubes. In this article we have addressed the molecular pathway of a recently identified potential biomarker Ubc9 whose deregulated expression due to BRCA1 dysfunction can result in HGSEOC with peritoneal permeability and formation of ascites. We also discuss the role of downstream targets Caveolin-1 and Vascular Endothelial Growth Factor (VEGF) in the pathogenesis of ascites in ovarian carcinomas. Finally we hypothesize a signaling axis between Ubc9 over expression, loss of Caveolin-1 and induction of VEGF in BRCA1 mutant HGSEOC cells. We suggest that Ubc9-mediated stimulation of VEGF as a novel mechanism underlying ovarian cancer aggressiveness and ascites formation. Agents that target Ubc9 and VEGF signaling may represent a novel therapeutic strategy to impede peritoneal growth and spread of HGSEOC.

Targeted endothelial nanomedicine for common acute pathological conditions

Endothelium, a thin monolayer of specialized cells lining the lumen of blood vessels is the key regulatory interface between blood and tissues. Endothelial abnormalities are implicated in many diseases, including common acute conditions with high morbidity and mortality lacking therapy, in part because drugs and drug carriers have no natural endothelial affinity. Precise endothelial drug delivery may improve management of these conditions. Using ligands of molecules exposed to the bloodstream on the endothelial surface enables design of diverse targeted endothelial nanomedicine agents. Target molecules and binding epitopes must be accessible to drug carriers, carriers must be free of harmful effects, and targeting should provide desirable sub-cellular addressing of the drug cargo. The roster of current candidate target molecules for endothelial nanomedicine includes peptidases and other enzymes, cell adhesion molecules and integrins, localized in different domains of the endothelial plasmalemma and differentially distributed throughout the vasculature. Endowing carriers with an affinity to specific endothelial epitopes enables an unprecedented level of precision of control of drug delivery: binding to selected endothelial cell phenotypes, cellular addressing and duration of therapeutic effects. Features of nanocarrier design such as choice of epitope and ligand control delivery and effect of targeted endothelial nanomedicine agents. Pathological factors modulate endothelial targeting and uptake of nanocarriers. Selection of optimal binding sites and design features of nanocarriers are key controllable factors that can be iteratively engineered based on their performance from in vitro to pre-clinical in vivo experimental models. Targeted endothelial nanomedicine agents provide antioxidant, anti-inflammatory and other therapeutic effects unattainable by non-targeted counterparts in animal models of common acute severe human disease conditions. The results of animal studies provide the basis for the challenging translation endothelial nanomedicine into the clinical domain.

Effect of receptor dimerization on membrane lipid raft structure continuously quantified on single cells by camera based fluorescence correlation spectroscopy

Membrane bound cell signaling is modulated by the membrane ultra-structure, which itself may be affected by signaling. However, measuring the interaction of membrane proteins with membrane structures in intact cells in real-time poses considerable challenges. In this paper we present a non-destructive fluorescence method that quantifies these interactions in single cells, and is able to monitor the same cell continuously to observe small changes. This approach combines total internal fluorescence microscopy with fluorescence correlation spectroscopy to measure the protein's diffusion and molecular concentration in different sized areas simultaneously. It correctly differentiates proteins interacting with membrane fences from proteins interacting with cholesterol-stabilized domains, or lipid rafts. This method detects small perturbations of the membrane ultra-structure or of a protein's tendency to dimerize. Through continuous monitoring of single cells, we demonstrate how dimerization of GPI-anchored proteins increases their association with the structural domains. Using a dual-color approach we study the effect of dimerization of one GPI-anchored protein on another type of GPI-anchored protein expressed in the same cell. Scans over the cell surface reveal a correlation between cholesterol stabilized domains and membrane cytoskeleton.

Controlled protein separation based on pressure–voltage (P–V) coupling effects in a nanopore based device

A U-type device containing two cells connected by nanopore arrays was designed for controlled protein separation. By finding P–V equilibrium points for BSA and Hb, the separation ratios can be achieved as BSA : Hb = 12.5 and Hb : BSA = 14.3.

Plasma membrane reorganization: A glycolipid gateway for microbes

Ligand-receptor interactions, which represent the core for cell signaling and internalization processes are largely affected by the spatial configuration of host cell receptors. There is a growing piece of evidence that receptors are not homogeneously distributed within the plasma membrane, but are rather pre-clustered in nanodomains, or clusters are formed upon ligand binding. Pathogens have evolved many strategies to evade the host immune system and to ensure their survival by hijacking plasma membrane receptors that are most often associated with lipid rafts. In this review, we discuss the early stage molecular and physiological events that occur following ligand binding to host cell glycolipids. The ability of various biological ligands (e.g. toxins, lectins, viruses or bacteria) that bind to glycolipids to induce their own uptake into mammalian cells by creating negative membrane curvature and membrane invaginations is explored. We highlight recent trends in understanding nanoscale plasma membrane (re-)organization and present the benefits of using synthetic membrane systems. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

In vivo proteomic imaging analysis of caveolae reveals pumping system to penetrate solid tumors

Technologies are needed to map and image biological barriers in vivo that limit solid tumor delivery and, ultimately, the effectiveness of imaging and therapeutic agents. Here we integrate proteomic and imaging analyses of caveolae at the blood-tumor interface to discover an active transendothelial portal to infiltrate tumors. A post-translationally modified form of annexin A1 (AnnA1) is selectively concentrated in human and rodent tumor caveolae. To follow trafficking, we generated a specific AnnA1 antibody that targets caveolae in the tumor endothelium. Intravital microscopy of caveolae-immunotargeted fluorophores even at low intravenous doses showed rapid and robust pumping across the endothelium to enter mammary, prostate and lung tumors. Within 1 h, the fluorescence signal concentrated throughout tumors to exceed the peak levels in blood. This transvascular pumping required the expression of caveolin 1 and annexin A1. Tumor uptake with other antibodies were >100-fold less. This proteomic imaging strategy reveals a unique target, antibody and caveolae pumping system for solid tumor penetration.

Protein-centric N-glycoproteomics analysis of membrane and plasma membrane proteins

The advent of proteomics technology has transformed our understanding of biological membranes. The challenges for studying membrane proteins have inspired the development of many analytical and bioanalytical tools, and the techniques of glycoproteomics have emerged as an effective means to enrich and characterize membrane and plasma-membrane proteomes. This Review summarizes the development of various glycoproteomics techniques to overcome the hurdles formed by the unique structures and behaviors of membrane proteins with a focus on N-glycoproteomics. Example contributions of N-glycoproteomics to the understanding of membrane biology are provided, and the areas that require future technical breakthroughs are discussed.

The adaptive remodeling of endothelial glycocalyx in response to fluid shear stress

The endothelial glycocalyx is vital for mechanotransduction and endothelial barrier integrity. We previously demonstrated the early changes in glycocalyx organization during the initial 30 min of shear exposure. In the present study, we tested the hypothesis that long-term shear stress induces further remodeling of the glycocalyx resulting in a robust layer, and explored the responses of membrane rafts and the actin cytoskeleton. After exposure to shear stress for 24 h, the glycocalyx components heparan sulfate, chondroitin sulfate, glypican-1 and syndecan-1, were enhanced on the apical surface, with nearly uniform spatial distributions close to baseline levels that differed greatly from the 30 min distributions. Heparan sulfate and glypican-1 still clustered near the cell boundaries after 24 h of shear, but caveolin-1/caveolae and actin were enhanced and concentrated across the apical aspects of the cell. Our findings also suggest the GM₁-labelled membrane rafts were associated with caveolae and glypican-1/heparan sulfate and varied in concert with these components. We conclude that remodeling of the glycocalyx to long-term shear stress is associated with the changes in membrane rafts and the actin cytoskeleton. This study reveals a space- and time- dependent reorganization of the glycocalyx that may underlie alterations in mechanotransduction mechanisms over the time course of shear exposure.

Raft-like membranes from the trans-Golgi network and endosomal compartments

This article describes a procedure to prepare a raft-like intracellular membrane fraction enriched for the trans-Golgi network (TGN) and endosomal compartments. The initial step in this technique involves cell disruption by homogenization, followed by clearance of the plasma membrane, late endosomes, mitochondria and the endoplasmic reticulum by differential sedimentation. Carbonate treatment, sonication and sucrose density-gradient ultracentrifugation are subsequently used to isolate the target membranes. The isolated subcellular fraction contains less than 1% of the total cellular proteins, but it is highly enriched for syntaxin-6 and Rab11. Typically, 40-60% of the cellular pool of GM1 glycosphingolipid and 10-20% of the total cellular cholesterol cofractionate with this buoyant membrane fraction. Given the role of GM1 as a cell-surface receptor for the cholera toxin and that levels of both GM1 and cholesterol in the TGN-endosomal compartment are upregulated in some inherited diseases, this protocol can potentially be applied to the analysis of disease-associated changes to GM1-enriched intracellular membranes. The isolated membranes are very well separated from caveolin-rich domains of the plasma membrane, the TGN and recycling endosomes. The entire protocol can be completed in as little as 1 d.

Estradiol dose-dependent regulation of membrane estrogen receptor-α, metabotropic glutamate receptor-1a, and their complexes in the arcuate nucleus of the hypothalamus in female rats

Sexual receptivity in the female rat is dependent on dose and duration of estradiol exposure. A 2 μg dose of estradiol benzoate (EB) primes reproductive behavior circuits without facilitating lordosis. However, 50 μg EB facilitates lordosis after 48 hours. Both EB doses activate membrane estrogen receptor-α (mERα) that complexes with and signals through metabotropic glutamate receptor-1a (mGluR1a). This mERα-mGluR1a signaling activates a multisynaptic lordosis-inhibiting circuit in the arcuate nucleus (ARH) that releases β-endorphin in the medial preoptic nucleus (MPN), activating μ-opioid receptors (MOP). MPN MOP activation is maintained, inhibiting lordosis for 48 hours by 2 μg EB, whereas 50 μg EB at 48 hours deactivates MPN MOP, facilitating lordosis. We hypothesized that 50 μg EB down-regulates ERα and mERα-mGluR1a complexes in the ARH to remove mERα-mGluR1a signaling. In experiment I, 48 hours after 2 μg or 50 μg EB, the number of ARH ERα-immunopositive cells was reduced compared with controls. In experiment II, compared with oil controls, total ARH ERα protein was decreased 48 hours after 50 μg EB, but the 2 μg dose was not. These results indicate that both EB doses reduced the total number of cells expressing ERα, but 2 μg EB may have maintained or increased ERα expressed per cell, whereas 50 μg EB appeared to reduce total ERα per cell. In experiment III, coimmunoprecipitation and Western blot revealed that total mERα and coimmunoprecipitated mERα with mGluR1a were greater 48 hours after 2 μg EB treatment vs rats receiving 50 μg EB. These results indicate 2 μg EB maintains but 50 μg EB down-regulates mERα-mGluR1a to regulate the lordosis circuit activity.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Fluid shear stress induces the clustering of heparan sulfate via mobility of glypican-1 in lipid rafts

The endothelial glycocalyx plays important roles in mechanotransduction. We recently investigated the distribution and interaction of glycocalyx components on statically cultured endothelial cells. In the present study, we further explored the unknown organization of the glycocalyx during early exposure (first 30 min) to shear stress and tested the hypothesis that proteoglycans with glycosaminoglycans, which are localized in different lipid microdomains, respond distinctly to shear stress. During the initial 30 min of exposure to shear stress, the very early responses of the glycocalyx and membrane rafts were detected using confocal microscopy. We observed that heparan sulfate (HS) and glypican-1 clustered in the cell junctions. In contrast, chondroitin sulfate (CS), bound albumin, and syndecan-1 did not move. The caveolae marker caveolin-1 did not move, indicating that caveolae are anchored sufficiently to resist shear stress during the 30 min of exposure. Shear stress induced significant changes in the distribution of ganglioside GM1 (a marker for membrane rafts labeled with cholera toxin B subunit). These data suggest that fluid shear stress induced the cell junctional clustering of lipid rafts with their anchored glypican-1 and associated HS. In contrast, the mobility of CS, transmembrane bound syndecan-1, and caveolae were constrained during exposure to shear stress. This study illuminates the role of changes in glycocalyx organization that underlie mechanisms of mechanotransduction.

Lipid raft integrity affects GABAA receptor, but not NMDA receptor modulation by psychopharmacological compounds

Lipid rafts have been shown to play an important role for G-protein mediated signal transduction and the function of ligand-gated ion channels including their modulation by psychopharmacological compounds. In this study, we investigated the functional significance of the membrane distribution of NMDA and GABAA receptor subunits in relation to the accumulation of the tricyclic antidepressant desipramine (DMI) and the benzodiazepine diazepam (Diaz). In the presence of Triton X-100, which allowed proper separation of the lipid raft marker proteins caveolin-1 and flotillin-1 from the transferrin receptor, all receptor subunits were shifted to the non-raft fractions. In contrast, under detergent-free conditions, NMDA and GABAA receptor subunits were detected both in raft and non-raft fractions. Diaz was enriched in non-raft fractions without Triton X-100 in contrast to DMI, which preferentially accumulated in lipid rafts. Impairment of lipid raft integrity by methyl-β-cyclodextrine (MβCD)-induced cholesterol depletion did not change the inhibitory effect of DMI at the NMDA receptor, whereas it enhanced the potentiating effect of Diaz at the GABAA receptor at non-saturating concentrations of GABA. These results support the hypothesis that the interaction of benzodiazepines with the GABAA receptor likely occurs outside of lipid rafts while the antidepressant DMI acts on ionotropic receptors both within and outside these membrane microdomains.

On protein abundance distributions in complex mixtures

Mass spectrometry, an analytical technique that measures the mass-to-charge ratio of ionized atoms or molecules, dates back more than 100 years, and has both qualitative and quantitative uses for determining chemical and structural information. Quantitative proteomic mass spectrometry on biological samples focuses on identifying the proteins present in the samples, and establishing the relative abundances of those proteins. Such protein inventories create the opportunity to discover novel biomarkers and disease targets. We have previously introduced a normalized, label-free method for quantification of protein abundances under a shotgun proteomics platform (Griffin et al., 2010). The introduction of this method for quantifying and comparing protein levels leads naturally to the issue of modeling protein abundances in individual samples. We here report that protein abundance levels from two recent proteomics experiments conducted by the authors can be adequately represented by Sichel distributions. Mathematically, Sichel distributions are mixtures of Poisson distributions with a rather complex mixing distribution, and have been previously and successfully applied to linguistics and species abundance data. The Sichel model can provide a direct measure of the heterogeneity of protein abundances, and can reveal protein abundance differences that simpler models fail to show.

Methods for the study of dopamine receptors within lipid rafts of kidney cells

There is increasing evidence that G protein-coupled receptor (GPCR) signaling is regulated in lipid raft microdomains. GPCRs and GPCR-signaling molecules, including G proteins and protein kinases, have been reported to compartmentalize in these microdomains. Dopamine D(1)-like receptors (D(1)R and D(5)R) belong to a family of GPCRs that are important in the regulation of renal function. These receptors are not only localized and regulated in caveolae that contains caveolin-1 but are also distributed in non--caveolar lipid rafts which do not contain caveolin-1. This chapter describes detergent- and non-detergent-based methods to obtain lipid raft fractions from renal proximal tubule cells.

Endothelin induces rapid, dynamin-mediated budding of endothelial caveolae rich in ET-B

Clathrin-independent trafficking pathways for internalizing G protein-coupled receptors (GPCRs) remain undefined. Clathrin-mediated endocytosis of receptors including ligand-engaged GPCRs can be very rapid and comprehensive (<10 min). Caveolae-mediated endocytosis of ligands and antibodies has been reported to be much slower in cell culture (≫10 min). Little is known about the role of physiological ligands and specific GPCRs in regulating caveolae trafficking. Here, we find that one receptor for endothelin, ET-B but not ET-A, resides on endothelial cell surfaces in both tissue and cell culture primarily concentrated within caveolae. Reconstituted cell-free budding assays show that endothelins (ETs) induce the fission of caveolae from endothelial plasma membranes purified from rat lungs. Electron microcopy of lung tissue sections and tissue subcellular fractionation both show that endothelin administered intravascularly in rats also induces a significant loss of caveolae at the luminal surface of lung vascular endothelium. Endothelial cells in culture show that ET stimulates very rapid internalization of caveolae and cargo including caveolin, caveolae-targeting antibody, and itself. The ET-B inhibitor BQ788, but not the ET-A inhibitor BQ123, blocks the ET-induced budding of caveolae. Both the pharmacological inhibitor Dynasore and the genetic dominant negative K44A mutant of dynamin prevent this induced budding and internalization of caveolae. Also shRNA lentivirus knockdown of caveolin-1 expression prevents rapid internalization of ET and ET-B. It appears that endothelin can engage ET-B already highly concentrated in caveolae of endothelial cells to induce very rapid caveolae fission and endocytosis. This transport requires active dynamin function. Caveolae trafficking may occur more rapidly than previously documented when it is stimulated by a specific ligand to signaling receptors already located in caveolae before ligand engagement.

Membrane microdomain sphingolipids are required for anti-CD20-induced death of chronic lymphocytic leukemia B cells

BACKGROUND

Chronic lymphocytic leukemia remains incurable, despite the addition of rituximab to chemotherapy as an available means of treatment. The resistance of certain patients to this monoclonal antibody prompted us to set up in vitro studies of another CD20-specific monoclonal antibody, B1 (later termed tositumomab). We hypothesized that the membrane lipid organization of leukemic B cells might be instrumental in the cells' sensitivity to the B1 monoclonal antibody.

DESIGN AND METHODS

B lymphocytes from 36 patients with chronic lymphocytic leukemia and 13 patients with non-Hodgkin's lymphoma were investigated for B1-triggered cell death. Membrane components, such as sphingomyelin and ganglioside M1, were investigated by flow cytometry, immunofluorescence and co-immunoprecipitation, together with the Csk-binding protein.

RESULTS

Chronic lymphocytic leukemia patients segregated into two groups: B cells from one group were sensitive to B1, whereas those from the second group were not. Further results ascribed the resistance of these latter cases to a defective recruitment of Csk-binding protein, resulting in a lack of sphingomyelin and ganglioside M1 at the outer leaflet of the plasma membrane of their malignant B cells. Sphingolipids were indeed retained in the cytoplasm, because of lowered activity of P-glycoprotein. Supporting this mechanism, rifampicin, an inducer of P-glycoprotein, improved the activity of this transmembrane efflux pump, normalized the quantity of sphingomyelin within the membrane, and thereby restored the efficacy of the B1 monoclonal antibody in the formerly B1-resistant cases of chronic lymphocytic leukemia.

CONCLUSIONS

The lipid organization of membranes of B cells from patients with chronic lymphocytic leukemia differs from one patient to another. In practice, given the relevance of the membrane lipid distribution to the efficacy of biotherapies, this observation is of potential importance.

Isolation of detergent-resistant membranes from plant photosynthetic and non-photosynthetic tissues

Microdomains, or lipid rafts, are transient membrane regions enriched in sphingolipids and sterols that have only recently, but intensively, been studied in plants. In this work, we report a detailed, easy-to-follow, and fast procedure to isolate detergent-resistant membranes (DRMs) from purified plasma membranes (PMs) that was used to obtain DRMs from Phaseolus vulgaris and Nicotiana tabacum leaves and germinating Zea mays embryos. Characterized according to yield, ultrastructure, and sterol composition, these DRM preparations showed similarities to analogous preparations from other eukaryotic cells. Isolation of DRMs from germinating maize embryos reveals the presence of microdomains at very early developmental stages of plants.

Systems analysis of endothelial cell plasma membrane proteome of rat lung microvasculature

BACKGROUND

Endothelial cells line all blood vessels to form the blood-tissue interface which is critical for maintaining organ homeostasis and facilitates molecular exchange. We recently used tissue subcellular fractionation combined with several multi-dimensional mass spectrometry-based techniques to enhance identification of lipid-embedded proteins for large-scale proteomic mapping of luminal endothelial cell plasma membranes isolated directly from rat lungs in vivo. The biological processes and functions of the proteins expressed at this important blood-tissue interface remain unexplored at a large scale.

RESULTS

We performed an unbiased systems analysis of the endothelial cell surface proteome containing over 1800 proteins to unravel the major functions and pathways apparent at this interface. As expected, many key functions of plasma membranes in general (i.e., cell surface signaling pathways, cytoskeletal organization, adhesion, membrane trafficking, metabolism, mechanotransduction, membrane fusion, and vesicle-mediated transport) and endothelial cells in particular (i.e., blood vessel development and maturation, angiogenesis, regulation of endothelial cell proliferation, protease activity, and endocytosis) were significantly overrepresented in this proteome. We found that endothelial cells express multiple proteins that mediate processes previously reported to be restricted to neuronal cells, such as neuronal survival and plasticity, axon growth and regeneration, synaptic vesicle trafficking and neurotransmitter metabolic process. Surprisingly, molecular machinery for protein synthesis was also detected as overrepresented, suggesting that endothelial cells, like neurons, can synthesize proteins locally at the cell surface.

CONCLUSION

Our unbiased systems analysis has led to the potential discovery of unexpected functions in normal endothelium. The discovery of the existence of protein synthesis at the plasma membrane in endothelial cells provides new insight into the blood-tissue interface and endothelial cell surface biology.

Tandem application of cationic colloidal silica and Triton X-114 for plasma membrane protein isolation and purification: towards developing an MDCK protein database

Plasma membrane (PM) proteins are attractive therapeutic targets because of their accessibility to drugs. Although genes encoding PM proteins represent 20-30% of eukaryotic genomes, a detailed characterisation of their encoded proteins is underrepresented, due, to their low copy number and the inherent difficulties in their isolation and purification as a consequence of their high hydrophobicity. We describe here a strategy that combines two orthogonal methods to isolate and purify PM proteins from Madin Darby canine kidney (MDCK) cells. In this two-step method, we first used cationic colloidal silica (CCS) to isolate adherent (Ad) and non-adherent (nAd) PM fractions, and then subjected each fraction to Triton X-114 (TX-114) phase partitioning to further enrich for hydrophobic proteins. While CCS alone identified 255/757 (34%) membrane proteins, CCS/TX-114 in combination yielded 453/745 (61%). Strikingly, of those proteins unique to CCS/TX-114, 277/393 (70%) had membrane annotation. Further characterisation of the CCS/TX-114 data set using Uniprot and transmembrane hidden Markov model revealed that 306/745 (41%) contained one or more transmembrane domains (TMDs), including proteins with 25 and 17 TMDs. Of the remaining proteins in the data set, 69/439 (16%) are known to contain lipid modifications. Of all membrane proteins identified, 93 had PM origin, including proteins that mediate cell adhesion, modulate transmembrane ion transport, and cell-cell communication. These studies reveal that the application of CCS to first isolate Ad and nAd PM fractions, followed by their detergent-phase TX-114 partitioning, to be a powerful method to isolate low-abundance PM proteins, and a useful adjunct for in-depth cell surface proteome analyses.

Perspectives on remorin proteins, membrane rafts, and their role during plant-microbe interactions

Invasion of host cells by pathogenic or mutualistic microbes requires complex molecular dialogues that often determine host survival. Although several components of the underlying signaling cascades have recently been identified and characterized, our understanding of proteins that facilitate signal transduction or assemble signaling complexes is rather sparse. Our knowledge of plant-specific remorin proteins, annotated as proteins with unknown function, has recently advanced with respect to their involvement in host-microbe interactions. Current data demonstrating that a remorin protein restricts viral movement in tomato leaves and the importance of a symbiosis-specific remorin for bacterial infection of root nodules suggest that these proteins may serve such regulatory functions. Direct interactions of other remorins with a resistance protein in Arabidopsis thaliana, and differential phosphorylation upon perception of microbial-associated molecular patterns and during expression of bacterial effector proteins, strongly underline their roles in plant defense. Furthermore, the specific subcellular localization of remorins in plasma membrane microdomains now provides the opportunity to visualize membrane rafts in living plants cells. There, remorins may oligomerize and act as scaffold proteins during early signaling events. This review summarizes current knowledge of this protein family and the potential roles of remorins in membrane rafts.

Caveolin-1 is required for vascular endothelial insulin uptake

As insulin's movement from plasma to muscle interstitium is rate limiting for its metabolic action, defining the regulation of this movement is critical. Here, we address whether caveolin-1 is required for the first step of insulin's transendothelial transport, its uptake by vascular endothelial cells (ECs), and whether IL-6 and TNFα affect insulin uptake or caveolin-1 expression. Uptake of FITC-labeled insulin was measured using confocal microscopy in control bovine aortic ECs (bAECs), in bAECs in which caveolin-1 was either knocked down or overexpressed, in murine ECs from caveolin-1(-/-) mice and in bAECs exposed to inflammatory cytokines. Knockdown of caveolin-1 expression in bAECs using specific caveolin-1 siRNA reduced caveolin-1 mRNA and protein expression by ∼ 70%, and reduced FITC-insulin uptake by 67% (P < 0.05 for each). Over-expression of caveolin-1 increased insulin uptake (P < 0.05). Caveolin-1-null mouse aortic ECs did not take up insulin and re-expression of caveolin-1 by transfecting these cells with FLAG-tagged caveolin-1 DNA rescued FITC-insulin uptake. Knockdown of caveolin-1 significantly reduced both insulin receptor protein level and insulin-stimulated Akt1 phosphorylation. Knockdown of caveolin-1 also inhibited insulin-induced caveolin-1 and IGF-1 receptor translocation to the plasma membrane. Compared with controls, IL-6 or TNFα (20 ng/ml for 24 h) inhibited FITC-insulin uptake as well as the expression of caveolin-1 mRNA and protein (P < 0.05 for each). IL-6 or TNFα also significantly reduced plasma membrane-associated caveolin-1. Thus, we conclude that insulin uptake by ECs requires expression of caveolin-1 supporting a role for caveolae mediating insulin uptake. Proinflammatory cytokines may inhibit insulin uptake, at least in part, by inhibiting caveolin-1 expression.

Nanomedicine Faces Barriers

Targeted nanoparticles have the potential to improve drug delivery efficiencies by more than two orders of magnitude, from the ~ 0.1% which is common today. Most pharmacologically agents on the market today are small drug molecules, which diffuse across the body’s blood-tissue barriers and distribute not only into the lesion, but into almost all organs. Drug actions in the non-lesion organs are an inescapable part of the drug delivery principle, causing “side-effects” which limit the maximally tolerable doses and result in inadequate therapy of many lesions. Nanoparticles only cross barriers by design, so side-effects are not built into their mode of operation. Delivery rates of almost 90% have been reported. This review examines the significance of these statements and checks how far they need qualification. What type of targeting is required? Is a single targeting sufficient? What new types of clinical challenge, such as immunogenicity, might attend the use of targeted nanoparticles?