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

Birch pollen allergen Bet v 1 binds to and is transported through conjunctival epithelium in allergic patients

BACKGROUND

Previous work in type-I pollen allergies has mainly focused on lymphocytes and immune responses. Here, we begin to analyse with a systems biology view the differences in conjunctival epithelium obtained from healthy and allergic subjects.

METHODS

Transcriptomics analysis combined with light and electron microscopic analysis of birch pollen allergen Bet v 1 located within conjunctival epithelial cells and tissues from birch allergic subjects and healthy controls was carried out.

RESULTS

Bet v 1 pollen allergen bound to conjunctival epithelial cells within minutes after the exposure even during the nonsymptomatic winter season only in allergic, but not in healthy individuals. Light- and electron microscopy showed that Bet v 1 was transported through the epithelium within lipid rafts/caveolae and reached mast cells only in allergic patients, but not in healthy individuals. Transcriptomics yielded 22 putative receptors expressed at higher levels in allergic epithelium compared with healthy specimens. A literature search indicated that out of these receptors, eight (i.e. 37%) were associated with lipid rafts/caveolae, which suggested again that Bet v 1 transport is lipid raft/caveola-dependent.

CONCLUSIONS

We show a clear difference in the binding and uptake of Bet v 1 allergen by conjunctival epithelial cells in allergic vs healthy subjects and several putative lipid raft/caveolar receptors were identified, which could mediate or be co-transported with this entry. The application of discovery driven methodologies on human conjunctival epithelial cells and tissues can provide new hypotheses worth a further analysis to the molecular mechanisms of a complex multifactorial disease such as type-I birch pollen allergy.

Membrane-targeted strategies for modulating APP and Abeta-mediated toxicity

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by numerous pathological features including the accumulation of neurotoxic amyloid-β (Aβ) peptide. There is currently no effective therapy for AD, but the development of therapeutic strategies that target the cell membrane is gaining increased interest. The amyloid precursor protein (APP) from which Aβ is formed is a membrane-bound protein, and Aβ production and toxicity are both membrane mediated events. This review describes the critical role of cell membranes in AD with particular emphasis on how the composition and structure of the membrane and its specialized regions may influence toxic or benign Aβ/APP pathways in AD. The putative role of copper (Cu) in AD is also discussed, and we highlight how targeting the cell membrane with Cu complexes has therapeutic potential in AD.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

Microglia mediate the clearance of soluble Abeta through fluid phase macropinocytosis

Alzheimer's disease is characterized by the progressive deposition of beta-amyloid (Abeta) within the brain parenchyma and its subsequent accumulation into senile plaques. Pathogenesis of the disease is associated with perturbations in Abeta homeostasis and the inefficient clearance of these soluble and insoluble peptides from the brain. Microglia have been reported to mediate the clearance of fibrillar Abeta (fAbeta) through receptor-mediated phagocytosis; however, their participation in clearance of soluble Abeta peptides (sAbeta) is largely unknown. We report that microglia internalize sAbeta from the extracellular milieu through a nonsaturable, fluid phase macropinocytic mechanism that is distinct from phagocytosis and receptor-mediated endocytosis both in vitro and in vivo. The uptake of sAbeta is dependent on both actin and tubulin dynamics and does not involve clathrin assembly, coated vesicles or membrane cholesterol. Upon internalization, fluorescently labeled sAbeta colocalizes to pinocytic vesicles. Microglia rapidly traffic these soluble peptides into late endolysosomal compartments where they are subject to degradation. Additionally, we demonstrate that the uptake of sAbeta and fAbeta occurs largely through distinct mechanisms and upon internalization are segregated into separate subcellular vesicular compartments. Significantly, we found that upon proteolytic degradation of fluorescently labeled sAbeta, the fluorescent chromophore is retained by the microglial cell. These studies identify an important mechanism through which microglial cells participate in the maintenance of Abeta homeostasis, through their capacity to constitutively clear sAbeta peptides from the brain.

Lipids and membrane microdomains in HIV-1 replication

Several critical steps in the replication cycle of human immunodeficiency virus type 1 (HIV-1) - entry, assembly and budding - are complex processes that take place at the plasma membrane of the host cell. A growing body of data indicates that these early and late steps in HIV-1 replication take place in specialized plasma membrane microdomains, and that many of the viral and cellular components required for entry, assembly, and budding are concentrated in these microdomains. In particular, a number of studies have shown that cholesterol- and sphingolipid-enriched microdomains known as lipid rafts play important roles in multiple steps in the virus replication cycle. In this review, we provide an overview of what is currently known about the involvement of lipids and membrane microdomains in HIV-1 replication.

Caveolar transport through nasal epithelium of birch pollen allergen Bet v 1 in allergic patients

BACKGROUND

Previous work in type I pollen allergies has focused on aberrant immunoresponses.

OBJECTIVE

Our systems-level analyses explore the role of epithelium in early pathogenesis of type I allergic reactions.

METHODS

We began top-down analyses of differences in human nasal epithelial cells and biopsy specimens obtained from patients with birch allergy and healthy control subjects in the resting state and after intranasal in vivo birch pollen challenges. Immunohistochemistry, immunotransmission electron microscopy, mass spectrometry, transcriptomics, and integration of data to a pathway were conducted.

RESULTS

Bet v 1 allergen bound to epithelium immediately after in vivo birch pollen challenge during winter only in allergic individuals. It also travelled through epithelium with caveolae to mast cells. Sixteen unique proteins were found to bind to the Bet v 1 column only in lysates from allergic epithelial cells; 6 of these were caveolar and 6 were cytoskeletal proteins. The nasal epithelial transcriptome analysis from allergic and healthy subjects differed during the winter season, and these subjects also responded differentially to birch pollen challenge. Within this pollen-induced response, the gene ontology categories of cytoskeleton and actin cytoskeleton were decreased in allergic patients, whereas the actin-binding category was enriched in healthy subjects. Integration of microscopic, mass spectrometric, and transcriptomic data to a common protein-protein binding network showed how these were connected to each other.

CONCLUSION

We propose a hypothesis of caveolae-dependent uptake and transport of birch pollen allergen in the epithelium of allergic patients only. Application of discovery-driven methodologies can provide new hypotheses worth further analysis of complex multifactorial diseases, such as type I allergy.

Endocytosis via caveolae: alternative pathway with distinct cellular compartments to avoid lysosomal degradation?

Endocytosis – the uptake of extracellular ligands, soluble molecules, protein and lipids from the extracellular surface – is a vital process, comprising multiple mechanisms, including phagocytosis, macropinocytosis, clathrin-dependent and clathrin-independent uptake such as caveolae-mediated and non-caveolar raft-dependent endocytosis. The best-studied endocytotic pathway for internalizing both bulk membrane and specific proteins is the clathrin-mediated endocytosis. Although many papers were published about the caveolar endocytosis, it is still not known whether it represents an alternative pathway with distinct cellular compartments to avoid lysosomal degradation or ligands taken up by caveolae can also be targeted to late endosomes/lysosomes. In this paper, we summarize data available about caveolar endocytosis. We are especially focussing on the intracellular route of caveolae and providing data supporting that caveolar endocytosis can join to the classical endocytotic pathway.

CtBP1/BARS is an activator of phospholipase D1 necessary for agonist-induced macropinocytosis

Vesicular trafficking such as macropinocytosis is a dynamic process that requires coordinated interactions between specialized proteins and lipids. A recent report suggests the involvement of CtBP1/BARS in epidermal growth factor (EGF)-induced macropinocytosis. Detailed mechanisms as to how lipid remodelling is regulated during macropinocytosis are still undefined. Here, we show that CtBP1/BARS is a physiological activator of PLD1 required in agonist-induced macropinocytosis. EGF-induced macropinocytosis was specifically blocked by 1-butanol but not by 2-butanol. In addition, stimulation of cells by serum or EGF resulted in the association of CtBP1/BARS with PLD1. Finally, CtBP1/BARS activated PLD1 in a synergistic manner with other PLD activators, including ADP-ribosylation factors as demonstrated by in vitro and intact cell systems. The present results shed light on the molecular basis of how the ‘fission protein' CtBP1/BARS controls vesicular trafficking events including macropinocytosis.

Adeno-associated virus type 5 exploits two different entry pathways in human embryo fibroblasts

The helper-dependent adeno-associated viruses (AAVs) have attracted great interest as vectors for gene therapy. Uptake and intracellular trafficking pathways of AAV are of importance, since they are often rate-limiting steps in infection. Here, we have investigated the entry of AAV type 5 (AAV5) in primary human embryo fibroblasts. At low binding temperatures, numerous virions are concentrated between cells, at contact points between cells and cellular protrusions, and at filopodia. When the temperature is raised to 37 degrees C, uptake of AAV5 takes place but up to 80 % of the bound virions dissociate from the cells. Uptake is achieved by cellular structures that are part of at least two different entry pathways. In addition to the common clathrin-dependent route, caveolar endocytosis and caveosome-like organelles are involved in a second pathway not yet described for parvoviruses. Both pathways can be used in parallel to enter an individual cell.

The activity of the epithelial sodium channels is regulated by caveolin-1 via a Nedd4-2-dependent mechanism

It has recently been shown that the epithelial Na(+) channel (ENaC) is compartmentalized in caveolin-rich lipid rafts and that pharmacological depletion of membrane cholesterol, which disrupts lipid raft formation, decreases the activity of ENaC. Here we show, for the first time, that a signature protein of caveolae, caveolin-1 (Cav-1), down-regulates the activity and membrane surface expression of ENaC. Physical interaction between ENaC and Cav-1 was also confirmed in a coimmunoprecipitation assay. We found that the effect of Cav-1 on ENaC requires the activity of Nedd4-2, a ubiquitin protein ligase of the Nedd4 family, which is known to induce ubiquitination and internalization of ENaC. The effect of Cav-1 on ENaC requires the proline-rich motifs at the C termini of the beta- and gamma-subunits of ENaC, the binding motifs that mediate interaction with Nedd4-2. Taken together, our data suggest that Cav-1 inhibits the activity of ENaC by decreasing expression of ENaC at the cell membrane via a mechanism that involves the promotion of Nedd4-2-dependent internalization of the channel.

Caveolin-2 is required for apical lipid trafficking and suppresses basolateral recycling defects in the intestine of Caenorhabditis elegans

Caveolins are plasma membrane-associated proteins that colocalize with, and stabilize caveolae. Their functions remain unclear although they are known to be involved in specific events in cell signaling and endocytosis. Caenorhabditis elegans encodes two caveolin genes, cav-1 and cav-2. We show that cav-2 is expressed in the intestine where it is localized to the apical membrane and in intracellular bodies. Using the styryl dye FM4-64 and BODIPY-labeled lactosylceramide, we show that the intestinal cells of cav-2 animals are defective in the apical uptake of lipid markers. These results suggest parallels with the function of caveolins in lipid homeostasis in mammals. We also show that CAV-2 depletion suppresses the abnormal accumulation of vacuoles that result from defective basolateral recycling in rme-1 and rab-10 mutants. Analysis of fluorescent markers of basolateral endocytosis and recycling suggest that endocytosis is normal in cav-2 mutants and thus, that the suppression of basolateral recycling defects in cav-2 mutants is due to changes in intracellular trafficking pathways. Finally, cav-2 mutants also have abnormal trafficking of yolk proteins. Taken together, these data indicate that caveolin-2 is an integral component of the trafficking network in the intestinal cells of C. elegans.

Counteracting signaling activities in lipid rafts associated with the invasion of lung epithelial cells by Pseudomonas aeruginosa

Pseudomonas aeruginosa has the capacity to invade lung epithelial cells by co-opting the intrinsic endocytic properties of lipid rafts, which are rich in cholesterol, sphingolipids, and proteins, such as caveolin-1 and -2. We compared intratracheal Pseudomonas infection in wild type and caveolin-deficient mice to investigate the role of caveolin proteins in the pathogenesis of Pseudomonas pneumonia. Unlike wild type mice, which succumb to pneumonia, caveolin-deficient mice are resistant to Pseudomonas. We observed that Pseudomonas invasion of lung epithelial cells is dependent on caveolin-2 but not caveolin-1. Phosphorylation of caveolin-2 by Src family kinases is an essential event for Pseudomonas invasion. Our studies also reveal the existence of a distinct signaling mechanism in lung epithelial cells mediated by COOH-terminal Src kinase (Csk) that negatively regulates Pseudomonas invasion. Csk migrates to lipid raft domains, where it decreases phosphorylation of caveolin-2 by inactivating c-Src. Whereas Pseudomonas co-opts the endocytic properties of caveolin-2 for invasion, there also exists in these cells an intrinsic Csk-dependent cellular defense mechanism aimed at impairing this activity. The success of Pseudomonas in co-opting lipid raft-mediated endocytosis to invade lung epithelial cells may depend on the relative strengths of these counteracting signaling activities.

Bacterial-mucosal interactions in inflammatory bowel disease: an alliance gone bad

The complex interaction of genetic, microbial, and environmental factors may result in continuous activation of the mucosal immune system leading to inflammatory bowel disease (IBD). Most present treatments for IBD involve altering or suppressing the aberrant immune response; however, the role of the intestinal microbiota in the pathophysiology of IBD is becoming more evident. The epithelial layer is essential for the proper functioning of the gastrointestinal tract, and its increased permeability to the luminal antigens may lead to the inflammatory processes and mucosal damage observed in IBD. Factors affecting the efficacy of the epithelial barrier include presence of pathogenic bacteria (e.g., Helicobacter spp.), presence of probiotic bacteria, availability of selected nutrients, and others. Defective function of the mucosal barrier might facilitate the contact of bacterial antigens and adjuvants with innate and adaptive immune cells to generate prolonged inflammatory responses. This review will briefly describe the complex structure of the epithelial barrier in the context of bacterial-mucosal interactions observed in human IBD and mouse models of colitis.

Aptamer-based endocytosis of a lysosomal enzyme

Enzyme replacement therapy for lysosomal storage diseases is currently based on endocytosis of lysosomal enzymes via the mannose or mannose 6-phosphate receptors. We are developing a technology for endocytosis of lysosomal enzymes that depends on generic, chemically conjugated reagents. These reagents are aptamers (single-stranded nucleic acid molecules) selected to bind to the extracellular domain of the mouse transferrin receptor. After selection, an RNA aptamer and a DNA aptamer were modified with biotin and linked to dye-labeled streptavidin for detection by confocal microscopy. Aptamer-streptavidin conjugates showed saturable uptake into mouse fibroblasts (Ltk(-) cells), which could be inhibited by an excess of free aptamer but not by tRNA, calf thymus DNA, or transferrin. The RNA aptamer-streptavidin conjugate was mouse-specific, as human cells (293T) did not take it up unless first transfected with the mouse transferrin receptor. Some streptavidin separated from the recycling pathway of transferrin and colocalized with lysosomes. After characterization in the model system, the DNA aptamer was conjugated to a lysosomal enzyme, alpha-l-iduronidase, from which mannose 6-phosphate had been removed. The aptamer had been modified by attachment of terminal glycerol for oxidation by periodate and reaction of the resulting aldehyde with amino groups on the protein. Dephospho-alpha-L-iduronidase-aptamer conjugate was taken up in saturable manner by alpha-L-iduronidase-deficient mouse fibroblasts, with half-maximal uptake estimated as 1.6 nM. Endocytosed enzyme-aptamer conjugate corrected glycosaminoglycan accumulation, indicating that it reached lysosomes and was functional in those organelles. Both uptake and correction were inhibited by unconjugated aptamer, confirming the role of the aptamer in receptor-mediated endocytosis.

Interfacial approach to polyaromatic hydrocarbon toxicity: phosphoglyceride and cholesterol monolayer response to phenantrene, anthracene, pyrene, chrysene, and benzo[a]pyrene

Interactions of phenantrene, anthracene, pyrene, chrysene, and benzo[a]pyrene (polyaromatic hydrocarbons) with model phospholipid membranes were probed using the Langmuir technique. The lipid monolayers were prepared using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol, 1,2-dipalmitoyl-sn-glycero-3-phosphoserine, 1,2-myristoyl-sn-glycero-3-phosphoethanolamine, 1,2-dilauroyl-sn-glycero-3-phosphocholine, and cholesterol. Surface pressure and electrical surface potential were measured on mixed phospholipid/PAH monolayers spread on a pure water subphase. The morphology of the mixed monolayers was followed with Brewster angle microscopy. Polarization-modulation infrared reflection-absorption spectroscopy spectra obtained on DPPE/benzo[a]pyrene showed that the latter interacts with the carbonyl groups of the phospholipid. On the other hand, the activity of phospholipase A2 toward DLPC used as a probe to locate benzo[a]pyrene in the monolayers indicates that the polyaromatic hydrocarbons are not accessible to the enzyme. The results obtained show that all PAHs studied affect the properties of the pure lipid, albeit in different ways. The most notable effects, namely, film fluidization and morphology changes, were observed with benzo[a]pyrene. In contrast, the complexity of mixed lipid monolayers makes the effect of PAHs difficult to detect. It can be assumed that the differences observed between PAHs in monolayers correlate with their toxicity.

The small-molecule iron transport inhibitor ferristatin/NSC306711 promotes degradation of the transferrin receptor

Iron delivery by transferrin (Tf) is accomplished through clathrin-mediated endocytosis of Tf receptors. The small molecule NSC306711 inhibits iron uptake from the Tf-TfR pathway. Here we show that the drug's mechanism of action is to induce internalization and degradation of unoccupied Tf receptors through an unexpected endocytic pathway. Unlike classical clathrin-mediated Tf receptor endocytosis, internalization promoted by NSC306711 is independent of clathrin and dynamin, and is sensitive to the cholesterol-depleting agents filipin and nystatin. The finding of this cholesterol-dependent Tf receptor internalization pathway through use of the small-molecule inhibitor sheds light on the pleiotropic nature of membrane trafficking dynamics and adds a complex dimension to our understanding of receptor regulation. Because of its unusual properties to inhibit iron uptake, we refer to NSC306711 as "ferristatin."

Transport and sorting of the solanum tuberosum sucrose transporter SUT1 is affected by posttranslational modification

The plant sucrose transporter SUT1 from Solanum tuberosum revealed a dramatic redox-dependent increase in sucrose transport activity when heterologously expressed in Saccharomyces cerevisiae. Plant plasma membrane vesicles do not show any change in proton flux across the plasma membrane in the presence of redox reagents, indicating a SUT1-specific effect of redox reagents. Redox-dependent sucrose transport activity was confirmed electrophysiologically in Xenopus laevis oocytes with SUT1 from maize (Zea mays). Localization studies of green fluorescent protein fusion constructs showed that an oxidative environment increased the targeting of SUT1 to the plasma membrane where the protein concentrates in 200- to 300-nm raft-like microdomains. Using plant plasma membranes, St SUT1 can be detected in the detergent-resistant membrane fraction. Importantly, in yeast and in plants, oxidative reagents induced a shift in the monomer to dimer equilibrium of the St SUT1 protein and increased the fraction of dimer. Biochemical methods confirmed the capacity of SUT1 to form a dimer in plants and yeast cells in a redox-dependent manner. Blue native PAGE, chemical cross-linking, and immunoprecipitation, as well as the analysis of transgenic plants with reduced expression of St SUT1, confirmed the dimerization of St SUT1 and Sl SUT1 (from Solanum lycopersicum) in planta. The ability to form homodimers in plant cells was analyzed by the split yellow fluorescent protein technique in transiently transformed tobacco (Nicotiana tabacum) leaves and protoplasts. Oligomerization seems to be cell type specific since under native-like conditions, a phloem-specific reduction of the dimeric form of the St SUT1 protein was detectable in SUT1 antisense plants, whereas constitutively inhibited antisense plants showed reduction only of the monomeric form. The role of redox control of sucrose transport in plants is discussed.

Physiological pathway for low-density lipoproteins across the blood-brain barrier: transcytosis through brain capillary endothelial cells in vitro

Although an immense knowledge has accumulated concerning regulation of cholesterol homeostasis in the body, this does not include the brain, where details are just emerging. Using an in vitro blood-brain barrier model, the authors have demonstrated that low-density lipoprotein (LDL) underwent transcytosis through the endothelial cells (ECs) by a receptor-mediated process, bypassing the lysosomal compartment. Moreover, caveolae might be involved in these blood-borne molecule transports from the blood to the brain. Although several ligands are known to be internalized through cell surface caveolae, the subsequent intracellular pathways have remained elusive. By cell fractionation experiment and Western blot, the authors have demonstrated that the LDL receptor is located in the caveolae membrane fraction. Then, LDLs internalized were detected by electron microscopy in multivesicular bodies. The authors identified in brain capillary ECs a novel endosomal compartment, mildly acidic, positive for marker Lamp-1 but devoid of any degradative capability. From the point of view of pH, cellular location, and caveolae-derived formation, the multivesicular organelles described here can be related to the caveosome structure. These results could provide clues to physiological functions of caveolae-caveosome transcellular pathway in brain capillary ECs and may help in the rational design of more effective therapeutic drugs to the brain.

Vascular endothelial growth factor and semaphorin induce neuropilin-1 endocytosis via separate pathways

The neuropilin (Nrp)1 receptor is essential for both nervous and vascular system development. Nrp1 is unusually versatile, because it transmits both chemoattractive and repulsive signals in response to vascular endothelial growth factor (VEGF)-A and class 3 semaphorins, respectively. Both Nrp1 and VEGF receptor 2 undergo ligand-dependent endocytosis. We sought to establish the endocytic pathway of Nrp1 and to determine whether uptake is required for its signaling. Whereas Nrp1 underwent clathrin-dependent endocytosis in response to VEGFA(165) treatment, semaphorin 3C (sema3C) induced lipid raft-dependent endocytosis. The myosin VI PDZ (postsynaptic density 95, Disk large, Zona occludens-1) adaptor protein synectin was essential for Nrp1 trafficking. Sema3C failed to inhibit migration of synectin(-/-) endothelial cells, mirroring the lower migratory response of these cells to VEGFA(165). These results show that the endocytic pathway of Nrp1 is determined by its ligand and that the trafficking of Nrp1 is essential for its signaling.

Beta2-integrin-induced p38 MAPK activation is a key mediator in the CD14/TLR4/MD2-dependent uptake of lipopolysaccharide by hepatocytes

The liver is the main organ that clears circulating lipopolysaccharide (LPS), and hepatocytes are a major cell type involved in LPS uptake. Little is known about the mechanisms for LPS internalization in hepatocytes and what signaling pathways are involved. We show here that LPS uptake is initiated after formation of a multi-receptor complex within lipid rafts. We find that essential components for LPS uptake are CD14, TLR4, MD2, and the beta2-integrin CD11b/CD18. Activation of p38 MAPK is also essential for the initiation of LPS uptake, and interestingly, we show that this activation is not through TLR4 signaling by MyD88 but through activation of TIRAP via CD11b/CD18. However, TLR4/MD2 remain essential components at the cell surface as part of the LPS receptor complex. We therefore suggest novel roles for TLR4/MD2, CD11b/CD18, TIRAP, and p38 MAPK in LPS uptake by hepatocytes.

pH-sensitive non-phospholipid vesicle and macrophage-like cells: binding, uptake and endocytotic pathway

Phospholipid and non-phospholipid vesicles are extensively studied as drug delivery systems to modify pharmacokinetics of drugs and to improve their action in target cells. It is believed that the major barrier to efficient drug delivery is entrapment of drugs in the endosomal compartment, since this eventually leads to its degradation in lysosomes. For these reasons, the knowledge of internalization pathway plays a fundamental role in optimizing drug targeting. The aim of this work is to characterize pH-sensitive Tween 20 vesicles, their interaction with macrophage-like cells and their comparison with pH-sensitive liposomes. The effect of different amounts of cholesteryl hemissucinate on surfactant vesicle formation and pH-sensitivity was studied. To evaluate the initial mode of internalization in Raw 264.7 and the intracellular fate of neutral and pH-sensitive formulations, flow cytometry in presence and in absence of selected inhibitors and fluorescence microscopy in absence and presence of specific fluorescent endocytotic markers were used. The obtained results showed that the surfactant vesicle pH-sensitivity was about two or three fold higher than that obtained with pH-sensitive liposomes in the presence of serum in vitro. The uptake mechanism of surfactant vesicles, after incubation with macrophage-like cells, is comparable to that of liposomes (clathrin-mediated endocytosis).

Exosomes from bronchoalveolar fluid of tolerized mice prevent allergic reaction

Exosomes are nanovesicles originating from multivesicular bodies that are secreted by a variety of cell types. The dual capability of exosomes to promote immunity or to induce tolerance has prompted their clinical use as vehicles for vaccination against different human diseases. In the present study, the effect of allergen-specific exosomes from tolerized mice on the development of allergen-induced allergic response was determined using a mouse model. Mice were tolerized by respiratory exposure to the olive pollen allergen Ole e 1. Exosome-like vesicles were isolated from bronchoalveolar lavage fluid of the animals by the well-established filtration and ultracentrifugation procedure, characterized by electron microscopy, Western blot, and FACS analyses, and assessed in a prophylactic protocol. To this end, BALB/c mice were intranasally treated with tolerogenic exosomes or naive exosomes as control, 1 wk before sensitization/challenge to Ole e 1. Blood, lungs, and spleen were collected and analyzed for immune responses. Intranasal administration of tolerogenic exosomes inhibited the development of IgE response, Th2 cytokine production, and airway inflammation--cardinal features of allergy--and maintained specific long-term protection in vivo. This protective effect was associated with a concomitant increase in the expression of the regulatory cytokine TGF-beta. These observations demonstrate that exosomes can induce tolerance and protection against allergic sensitization in mice. Thus, exosome-based vaccines could represent an alternative to conventional therapy for allergic diseases in humans.

Actin filaments and microtubules regulate endocytosis in marginal cells of the stria vascularis

CONCLUSION

Cationized ferritin (CF) was internalized via clathrin-mediated endocytosis. This process depends on clathrin, actin filaments, and microtubules. Microperoxidase (MPO) was internalized via a clathrin- and caveolin-independent endocytic pathway, which was partially dependent on microtubules but independent of clathrin and actin filaments.

OBJECTIVE

We investigated the role of actin filaments and microtubules in the transport of endocytic carrier vesicles (ECVs) from the plasma membrane to the early sorting endosomes, using CF and MPO as tracers.

MATERIALS AND METHODS

Fifty-five guinea pigs were used. The animals were divided into a CF endocytosis group and an MPO endocytosis group. These groups consisted of control, nocodazole-treated, cytochalasin (Cyt D)-treated, Cyt D + nocodazole-treated, and geldanamycin-treated subgroups.

RESULTS

For CF endocytosis, the following results were obtained. In the nocodazole experiment, in which microtubules were disrupted to form monomeric tubulin, the number of ECVs loaded with CF was greatly decreased. In the Cyt D experiment, in which the actin filaments were disrupted to form monomers, the number of ECVs labeled with CF was also greatly decreased. In the geldanamycin experiment, in which clathrin-mediated endocytosis was regulated and actin stress fibers were dissolved, the endocytosis of CF was severely inhibited. For MPO endocytosis, in the nocodazole experiment, the endocytosis of MPO was markedly suppressed.

Caveolae in fibroblast-like synoviocytes: static structures associated with vimentin-based intermediate filaments

The fibroblast-like synoviocyte is a CD13-positive cell-type containing numerous caveolae, both single and interconnected clusters. In unstimulated cells, all single caveolae at the cell surface and the majority of those localized deeper into the cytoplasm were freely accessible from the medium, as judged from electron microscopy of synoviocytes exposed to the membrane impermeable marker Ruthenium Red. Caveolar internalization could be induced by a CD13 antibody or by cholera toxin B subunit (CTB). Thus, in experiments using sequential labeling with Alexa 488- and 594-conjugated CTB, about 50% of CTB-positive caveolae were internalized by 5 min of chase, and these remained inaccessible from the cell surface for periods up to 24 h. No colocalization with an endosomal marker, EEA1, or Lysotracker was observed, indicating that internalized caveolae clusters represent a static compartment. Vimentin was identified as the most abundant protein in detergent resistant membranes (DRM's), and by immunogold electron microscopy caveolae were seen in intimate contact with intermediate-size filaments. These observations indicate that vimentin-based filaments are responsible for the spatio-temporal fixation of caveolae clusters. RECK, a glycosylphosphatidylinositol-anchored protein acting as a negative regulator of cell surface metalloproteinases, was also localized to the caveolae clusters. We propose that these clusters function as static reservoirs of specialized lipid raft domains where proteins involved in cell-cell interactions, such as CD13, can be sequestered by binding to RECK in a regulatory manner.

The neural adhesion molecule TAG-1 modulates responses of sensory axons to diffusible guidance signals

When the axons of primary sensory neurons project into the embryonic mammalian spinal cord, they bifurcate and extend rostrocaudally before sending collaterals to specific laminae according to neuronal subclass. The specificity of this innervation has been suggested to be the result both of differential sensitivity to chemorepellants expressed in the ventral spinal cord and of the function of Ig-like neural cell adhesion molecules in the dorsal horn. The relationship between these mechanisms has not been addressed. Focussing on the pathfinding of TrkA+ NGF-dependent axons, we demonstrate for the first time that their axons project prematurely into the dorsal horn of both L1 and TAG-1 knockout mice. We show that axons lacking TAG-1, similar to those lacking L1, are insensitive to wild-type ventral spinal cord(VSC)-derived chemorepellants, indicating that adhesion molecule function is required in the axons, and that this loss of response is explained in part by loss of response to Sema3A. We present evidence that TAG-1 affects sensitivity to Sema3A by binding to L1 and modulating the endocytosis of the L1/neuropilin 1 Sema3A receptor complex. However, TAG-1 appears to affect sensitivity to other VSC-derived chemorepellants via an L1-independent mechanism. We suggest that this dependence of chemorepellant sensitivity on the functions of combinations of adhesion molecules is important to ensure that axons project via specific pathways before extending to their final targets.

Nicotinic acetylcholine receptor is internalized via a Rac-dependent, dynamin-independent endocytic pathway

Endocytosis of the nicotinic acetylcholine receptor (AChR) is a proposed major mechanism of neuromodulation at neuromuscular junctions and in the pathology of synapses in the central nervous system. We show that binding of the competitive antagonist alpha-bungarotoxin (alphaBTX) or antibody-mediated cross-linking induces the internalization of cell surface AChR to late endosomes when expressed heterologously in Chinese hamster ovary cells or endogenously in C2C12 myocytes. Internalization occurs via sequestration of AChR-alphaBTX complexes in narrow, tubular, surface-connected compartments, which are indicated by differential surface accessibility of fluorescently tagged alphaBTX-AChR complexes to small and large molecules and real-time total internal reflection fluorescence imaging. Internalization occurs in the absence of clathrin, caveolin, or dynamin but requires actin polymerization. alphaBTX binding triggers c-Src phosphorylation and subsequently activates the Rho guanosine triphosphatase Rac1. Consequently, inhibition of c-Src kinase activity, Rac1 activity, or actin polymerization inhibits internalization via this unusual endocytic mechanism. This pathway may regulate AChR levels at ligand-gated synapses and in pathological conditions such as the autoimmune disease myasthenia gravis.

Lymphocytic choriomeningitis virus uses a novel endocytic pathway for infectious entry via late endosomes

The endocytic entry of lymphocytic choriomeningitis virus (LCMV) into host cells was compared to the entry of viruses known to exploit clathrin or caveolae/raft-dependent pathways. Pharmacological inhibitors, expression of pathway-specific dominant-negative constructs, and siRNA silencing of clathrin together with electron and light microscopy provided evidence that although a minority population followed a classical clathrin-mediated mechanism of entry, the majority of these enveloped RNA viruses used a novel endocytic route to late endosomes. The pathway was clathrin, dynamin-2, actin, Arf6, flotillin-1, caveolae, and lipid raft independent but required membrane cholesterol. Unaffected by perturbation of Rab5 or Rab7 and apparently without passing through Rab5/EEA1-positive early endosomes, the viruses reached late endosomes and underwent acid-induced penetration. This membrane trafficking route between the plasma membrane and late endosomes may function in the turnover of a select group of surface glycoproteins such as the dystroglycan complex, which serves as the receptor of LCMV.

Phorbol ester induced trafficking-independent regulation and enhanced phosphorylation of the dopamine transporter associated with membrane rafts and cholesterol

We examined the mechanisms involved in protein kinase C (PKC)-dependent down-regulation of dopamine transporter (DAT) activity and cell surface expression by treating heterologously expressing cells with the clathrin-mediated endocytosis inhibitor concanavalin A (Con A) or the cholesterol depleter/membrane raft disrupter methyl-beta-cyclodextrin (MbetaC) prior to treatment with the PKC activator phorbol 12-myristate, 13-acetate (PMA). Con A blocked PMA-induced surface reductions of DAT but only partially inhibited down-regulation, while MbetaC partially blocked down-regulation but did not inhibit loss of cell surface DAT, demonstrating that PKC-induced DAT down-regulation occurs by a combination of trafficking and non-trafficking processes. Using density-gradient centrifugation, we found that DATs are distributed approximately equally between Triton-insoluble, cholesterol-rich membrane rafts and Triton-soluble non-raft membranes. DATs in both populations are present at the cell surface and are active for dopamine and cocaine binding. PMA-induced loss of cell surface DAT occurred only from non-raft populations, demonstrating that non-raft DATs are regulated by trafficking events and indicating the likelihood that the cholesterol-dependent non-trafficking regulatory mechanism occurs in rafts. PMA did not affect the DAT raft-non-raft distribution but stimulated the phosphorylation of DAT to a substantially greater level in rafts than non-rafts. These findings reveal a previously unknown role for cholesterol in DAT function and demonstrate the presence of distinct subcellular DAT populations that possess multiple regulatory differences that may impact dopaminergic neurotransmission.

Separate endocytic pathways regulate IL-5 receptor internalization and signaling

Eosinophils are critically dependent on IL-5 for their activation, differentiation, survival, and augmentation of cytotoxic activity. We previously showed that the cytoplasmic domain of the hematopoietic receptor, betac, which is shared by IL-5, IL-3, and GM-CSF, is directly ubiquitinated and degraded by the proteasomes in a JAK2-dependent manner. However, studies describing the spatial distribution, endocytic regulation, and trafficking of betac-sharing receptors in human eosinophils are currently lacking. Using deconvolution microscopy and biochemical methods, we clearly demonstrate that IL-5Rs reside in and are internalized by clathrin- and lipid raft-dependent endocytic pathways. Microscopy analyses in TF1 cells and human eosinophils revealed significant colocalization of betac, IL-5Ralpha, and Cy3-labeled IL-5 with transferrin- (clathrin) and cholera toxin-B- (lipid raft) positive vesicles. Moreover, whereas internalized IL-5Rs were detected in both clathrin- and lipid raft-positive vesicles, biochemical data revealed that tyrosine phosphorylated, ubiquitinated, and proteasome-degraded IL-5Rs partitioned to the soluble, nonraft fractions (clathrin-containing). Lastly, we show that optimal IL-5-induced signaling requires entry of activated IL-5Rs into the intracellular compartment, as coimmunoprecipitation of key signaling molecules with the IL-5R was completely blocked when either endocytic pathway was inhibited. These data provide the first evidence that IL-5Rs segregate and traffic into two distinct plasma membrane compartments, and they further establish that IL-5R endocytosis regulates signaling both positively and negatively.

Expression of the muscular dystrophy-associated caveolin-3(P104L) mutant in adult mouse skeletal muscle specifically alters the Ca(2+) channel function of the dihydropyridine receptor

Caveolins are plasma-membrane-associated proteins potentially involved in a variety of signalling pathways. Different mutations in CAV3, the gene encoding for the muscle-specific isoform caveolin-3 (Cav-3), lead to muscle diseases, but the underlying molecular mechanisms remain largely unknown. Here, we explored the functional consequences of a Cav-3 mutation (P104L) inducing the 1C type limb-girdle muscular dystrophy (LGMD 1C) in human on intracellular Ca(2+) regulation of adult skeletal muscle fibres. A YFP-tagged human Cav-3(P104L) mutant was expressed in vivo in muscle fibres from mouse. Western blot analysis revealed that expression of this mutant led to an approximately 80% drop of the level of endogenous Cav-3. The L-type Ca(2+) current density was found largely reduced in fibres expressing the Cav-3(P104L) mutant, with no change in the voltage dependence of activation and inactivation. Interestingly, the maximal density of intramembrane charge movement was unaltered in the Cav-3(P104L)-expressing fibres, suggesting no change in the total amount of functional voltage-sensing dihydropyridine receptors (DHPRs). Also, there was no obvious alteration in the properties of voltage-activated Ca(2+) transients in the Cav-3(P104L)-expressing fibres. Although the actual role of the Ca(2+) channel function of the DHPR is not clearly established in adult skeletal muscle, its specific alteration by the Cav-3(P104L) mutant suggests that it may be involved in the physiopathology of LGMD 1C.

Protein kinase C inhibits caveolae-mediated endocytosis of TRPV5

Transient receptor potential vanilloid 5 (TRPV5) constitutes the apical entry pathway for transepithelial Ca2+reabsorption in kidney. Many hormones alter renal Ca2+reabsorption at least partly by regulating TRPV5. The mechanism for acute regulation of TRPV5 by phospholipase C-coupled hormones is largely unknown. Here, we found that protein kinase C (PKC) activator 1-oleoyl-acetyl-sn-glycerol (OAG) increased TRPV5 current density and surface abundance in cultured cells. The OAG-mediated increase of TRPV5 was prevented by preincubation with specific PKC inhibitors. Coexpression with a dominant-negative dynamin increased the basal TRPV5 current density and prevented the increase by OAG. Knockdown of caveolin-1 by small interference RNA (siRNA) prevented the increase of TRPV5 by OAG. In contrast, knockdown of clathrin heavy chain had no effects. OAG had no effect on TRPV5 expressed in caveolin-1 null cells derived from caveolin-1 knockout mice. Forced expression of recombinant caveolin-1 restored the regulation of TRPV5 by OAG in caveolin-1 knockout cells. Mutations of serine-299 and/or serine-654 of TRPV5 (consensus residues for phosphorylation by PKC) abolished the regulation by OAG. Parathyroid hormone (PTH) increased TRPV5 current density in cells coexpressing TRPV5 and type 1 PTH receptor. The increase caused by PTH was prevented by PKC inhibitor, mutation of serine-299/serine-654, or by knockdown of caveolin-1. Thus, TRPV5 undergoes constitutive caveolae-mediated endocytosis. Activation of PKC increases cell surface abundance of TRPV5 by inhibiting the endocytosis. This mechanism of regulation by PKC may contribute to the acute stimulation of TRPV5 and renal Ca2+reabsorption by PTH.

Pemphigus vulgaris IgG-induced desmoglein-3 endocytosis and desmosomal disassembly are mediated by a clathrin- and dynamin-independent mechanism

Pemphigus vulgaris (PV) is a life-threatening autoimmune disease characterized by oral mucosal erosions and epidermal blistering. The autoantibodies generated target the desmosomal cadherin desmoglein-3 (Dsg3). Previous studies demonstrate that upon PV IgG binding, Dsg3 is internalized and enters an endo-lysosomal pathway where it is degraded. To define the endocytic machinery involved in PV IgG-induced Dsg3 internalization, human keratinocytes were incubated with PV IgG, and various tools were used to perturb distinct endocytic pathways. The PV IgG.Dsg3 complex failed to colocalize with clathrin, and inhibitors of clathrin- and dynamin-dependent pathways had little or no effect on Dsg3 internalization. In contrast, cholesterol binding agents such as filipin and nystatin and the tyrosine kinase inhibitor genistein dramatically inhibited Dsg3 internalization. Furthermore, the Dsg3 cytoplasmic tail specified sensitivity to these inhibitors. Moreover, inhibition of Dsg3 endocytosis with genistein prevented disruption of desmosomes and loss of adhesion in the presence of PV IgG. Altogether, these results suggest that PV IgG-induced Dsg3 internalization is mediated through a clathrin- and dynamin-independent pathway and that Dsg3 endocytosis is tightly coupled to the pathogenic activity of PV IgG.

Cholesterol-rich membrane rafts and Lyn are involved in phagocytosis during Pseudomonas aeruginosa infection

The mechanism of phagocytosis of pathogens remains to be fully characterized. We report a novel phagocytosis pathway for Pseudomonas aeruginosa, which is initiated by cholesterol-rich membrane rafts and is dependent on Lyn, primarily an immune regulator with both positive and negative roles. Blocking of Lyn or blocking of cholesterol synthesis significantly inhibited phagocytosis by alveolar macrophages. We found that Lyn, via Src homology 2 and 3 domains, bound to and then activated PI3K and Akt to regulate intracellular routing of the engulfed P. aeruginosa. Further analysis indicates that Lyn and raft components entered in phagosomes and late lysosomes. Finally, respiratory burst was dependent on Lyn and membrane rafts, as confirmed by small interfering RNA and dominant-negative strategies. Our investigations demonstrate that Lyn along with membrane rafts plays a fundamental role in phagocytosis by alveolar macrophages during infection.

A two-pronged mechanism for HIV-1 Nef-mediated endocytosis of immune costimulatory molecules CD80 and CD86

The Nef protein of HIV-1 mediates immune evasion by relocating major histocompatibility complex (MHC) molecules and the immune costimulatory molecules CD80 and CD86 away from the monocytic cell surface. We describe a two-pronged mechanism by which Nef removes CD80 and CD86 from the cell surface. While MHCI, CD80, and CD86 are all internalized via a dynamin-independent pathway, the endocytic mechanism used for costimulatory molecules is distinct from MHCI relocation. Nef expression results in the activation and actin-dependent translocation of Src kinase to the cell periphery. At the cell surface, Src promotes Rac activation via TIAM, a guanine nucleotide exchange factor for Rac. Nef also binds to the cytosolic tails of CD80 and CD86, triggering their endocytosis via Rac-based actin polymerization. These data reveal the use of an unusual molecular mechanism triggered in the host cell by HIV to affect its viral immune evasion strategy and suggest approaches for its subversion.

Ethanol mimics ligand-mediated activation and endocytosis of IL-1RI/TLR4 receptors via lipid rafts caveolae in astroglial cells

We have recently reported that ethanol-induced inflammatory processes in the brain and glial cells are mediated via the activation of interleukin-1 beta receptor type I (IL-1RI)/toll-like receptor type 4 (TLR4) signalling. The mechanism(s) by which ethanol activates these receptors in astroglial cells remains unknown. Recently, plasma membrane microdomains, lipid rafts, have been identified as platforms for receptor signalling and, in astrocytes, rafts/caveolae constitute an important integrators of signal events and trafficking. Here we show that stimulation of astrocytes with IL-1beta, lipopolysaccharide or ethanol (10 and 50 mM), triggers the translocation of IL-1RI and/or TLR4 into lipid rafts caveolae-enriched fractions, promoting the recruitment of signalling molecules (phospho-IL-1R-associated kinase and phospho-extracellular regulated-kinase) into these microdomains. With confocal microscopy, we further demonstrate that IL-1RI is internalized by caveolar endocytosis via enlarged caveosomes organelles upon IL-1beta or ethanol treatment, which sorted their IL-1RI cargo into the endoplasmic reticulum-Golgi compartment and into the nucleus of astrocytes. In short, our findings demonstrate that rafts/caveolae are critical for IL-1RI and TLR4 signalling in astrocytes, and reveal a novel mechanism by which ethanol, by interacting with lipid rafts caveolae, promotes IL-1RI and TLR4 receptors recruitment, triggering their endocytosis via caveosomes and downstream signalling stimulation. These results suggest that TLRs receptors are important targets of ethanol-induced inflammatory damage in the brain.

The blood-brain barrier in health and chronic neurodegenerative disorders

The blood-brain barrier (BBB) is a highly specialized brain endothelial structure of the fully differentiated neurovascular system. In concert with pericytes, astrocytes, and microglia, the BBB separates components of the circulating blood from neurons. Moreover, the BBB maintains the chemical composition of the neuronal "milieu," which is required for proper functioning of neuronal circuits, synaptic transmission, synaptic remodeling, angiogenesis, and neurogenesis in the adult brain. BBB breakdown, due to disruption of the tight junctions, altered transport of molecules between blood and brain and brain and blood, aberrant angiogenesis, vessel regression, brain hypoperfusion, and inflammatory responses, may initiate and/or contribute to a "vicious circle" of the disease process, resulting in progressive synaptic and neuronal dysfunction and loss in disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, and others. These findings support developments of new therapeutic approaches for chronic neurodegenerative disorders directed at the BBB and other nonneuronal cells of the neurovascular unit.

The plant defense elicitor cryptogein stimulates clathrin-mediated endocytosis correlated with reactive oxygen species production in bright yellow-2 tobacco cells

The plant defense elicitor cryptogein triggers well-known biochemical events of early signal transduction at the plasma membrane of tobacco (Nicotiana tabacum) cells, but microscopic observations of cell responses related to these early events were lacking. We determined that internalization of the lipophilic dye FM4-64, which is a marker of endocytosis, is stimulated a few minutes after addition of cryptogein to tobacco Bright Yellow-2 (BY-2) cells. This stimulation is specific to the signal transduction pathway elicited by cryptogein because a lipid transfer protein, which binds to the same receptor as cryptogein but without triggering signaling, does not increase endocytosis. To define the nature of the stimulated endocytosis, we quantified clathrin-coated pits (CCPs) forming on the plasma membrane of BY-2 cells. A transitory stimulation of this morphological event by cryptogein occurs within the first 15 min. In the presence of cryptogein, increases in both FM4-64 internalization and clathrin-mediated endocytosis are specifically blocked upon treatment with 5 microm tyrphostin A23, a receptor-mediated endocytosis inhibitor. The kinetics of the transient increase in CCPs at the plasma membrane coincides with that of transitory reactive oxygen species (ROS) production occurring within the first 15 min after elicitation. Moreover, in BY-2 cells expressing NtrbohD antisense cDNA, which are unable to produce ROS when treated with cryptogein, the CCP stimulation is inhibited. These results indicate that the very early endocytic process induced by cryptogein in tobacco is due, at least partly, to clathrin-mediated endocytosis and is dependent on ROS production by the NADPH oxidase NtrbohD.

Translocation of glycosylphosphatidylinositol-anchored proteins from plasma membrane microdomains to lipid droplets in rat adipocytes is induced by palmitate, H2O2, and the sulfonylurea drug glimepiride

Inhibition of lipolysis by palmitate, H(2)O(2), and the antidiabetic sulfonylurea drug, glimepiride, in rat adipocytes has been shown previously to rely on the concerted degradation of cAMP by the glycosylphosphatidylinositol (GPI)-anchored phosphodiesterase Gce1 and 5'-nucleotidase CD73, which both gain access to the lipid droplets (LDs). The present report demonstrates the translocation of Gce1 and CD73, harboring the intact GPI anchor, from detergent-insoluble glycolipid-enriched plasma membrane domains (DIGs) to the LDs in response to palmitate, H(2)O(2), and glimepiride by analysis of their steady-state distribution using photoaffinity labeling and activity determination as well as of their redistribution after pulse or equilibrium metabolic labeling. We were surprised to find that palmitate, H(2)O(2), and glimepiride induced the activation of the GPI-specific phospholipase C (GPI-PLC) at DIGs of rat adipocytes, leading to anchorless Gce1 and CD73. Inhibition of the GPI-PLC or the presence of nonhydrolyzable substrate analogs of Gce1 and CD73 interfered with the palmitate-, H(2)O(2)-, and glimepiride-induced 1) lipolytic cleavage of Gce1 and CD73, 2) translocation of their GPI-anchored versions from DIGs to LDs, 3) up-regulation of cAMP degradation, and 4) inhibition of lipolysis. These data suggest a novel insulin-independent antilipolytic mechanism in rat adipocytes, which relies on the palmitate-, H(2)O(2)-, and glimepiride-induced and GPI-PLC-dependent translocation of (c)AMP-degrading GPI-anchored proteins from the adipocyte plasma membrane to LDs. The findings may shed new light on the biogenesis and degradation of LDs in response to physiological and pharmacological stimuli.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.