Co-option of endocytic functions of cellular caveolae by pathogens

Caveolin-Mediated Endocytosis: Bacterial Pathogen Exploitation and Host-Pathogen Interaction

Within mammalian cells, diverse endocytic mechanisms, including phagocytosis, pinocytosis, and receptor-mediated endocytosis, serve as gateways exploited by many bacterial pathogens and toxins. Among these, caveolae-mediated endocytosis is characterized by lipid-rich caveolae and dimeric caveolin proteins. Caveolae are specialized microdomains on cell surfaces that impact cell signaling. Caveolin proteins facilitate the creation of caveolae and have three members in vertebrates: caveolin-1, caveolin-2, and caveolin-3. Many bacterial pathogens hijack caveolin machinery to invade host cells. For example, the Gram-positive facultative model intracellular bacterial pathogen Listeria monocytogenes exploits caveolin-mediated endocytosis for efficient cellular entry, translocation across the intestinal barrier, and cell-cell spread. Caveolin facilitates the internalization of group A streptococci by promoting the formation of invaginations in the plasma membrane and avoiding fusion with lysosomes, thereby aiding intracellular survival. Caveolin plays a crucial role in internalizing and modulation of host immune responses by Gram-negative bacterial pathogens, such as Escherichia coli K1, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Salmonella enterica serovar Typhimurium. Here, we summarize how bacterial pathogens manipulate the host's caveolin system to facilitate bacterial entry and movement within and between host cells, to support intracellular survival, to evade immune responses, and to trigger inflammation. This knowledge enhances the intervention of new therapeutic targets against caveolin in microbial invasion and immune evasion processes.

Glypican-3-targeted macrophages delivering drug-loaded exosomes offer efficient cytotherapy in mouse models of solid tumours

Cytotherapy is a strategy to deliver modified cells to a diseased tissue, but targeting solid tumours remains challenging. Here we design macrophages, harbouring a surface glypican-3-targeting peptide and carrying a cargo to combat solid tumours. The anchored targeting peptide facilitates tumour cell recognition by the engineered macrophages, thus enhancing specific targeting and phagocytosis of tumour cells expressing glypican-3. These macrophages carry a cargo of the TLR7/TLR8 agonist R848 and INCB024360, a selective indoleamine 2,3-dioxygenase 1 (IDO1) inhibitor, wrapped in C16-ceramide-fused outer membrane vesicles (OMV) of Escherichia coli origin (RILO). The OMVs facilitate internalization through caveolin-mediated endocytosis, and to maintain a suitable nanostructure, C16-ceramide induces membrane invagination and exosome generation, leading to the release of cargo-packed RILOs through exosomes. RILO-loaded macrophages exert therapeutic efficacy in mice bearing H22 hepatocellular carcinomas, which express high levels of glypican-3. Overall, we lay down the proof of principle for a cytotherapeutic strategy to target solid tumours and could complement conventional treatment.

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.

Exploration of the Molecular Mechanism by Which Caveolin-1 Regulates Changes in Blood-Brain Barrier Permeability Leading to Eosinophilic Meningoencephalitis

Angiostrongylus cantonensis, a zoonotic parasite, can invade the human central nervous system (CNS) and cause acute eosinophilic meningitis or eosinophilic meningoencephalitis. Mice infected with A. cantonensis show elevated levels of pro-inflammatory cytokines, plasminogen activators, and matrix metalloproteinase-9, resulting in disruption of the blood-brain barrier (BBB) and immune cell infiltration into the CNS. Caveolin-1 (Cav-1) regulates the permeability of the BBB, which affects immune cells and cerebrospinal fluid. This intricate interaction ultimately fuels the progression of brain damage and edema. This study aims to investigate the regulatory role of Cav-1 in the pathogenesis of meningoencephalitis induced by A. cantonensis infection. We investigated pathological alterations by triphenyl-tetrazolium chloride, brain water content, BBB permeability, Western blot analysis, and gelatin zymography in BALB/c mice after A. cantonensis. The study evaluates the critical role of Cav-1 regulation through the TLR4/MyD88 signaling pathway, modulates tight junction proteins, influences BBB permeability, and contributes to brain damage in A. cantonensis-induced meningoencephalitis.

Lipids in Pathophysiology and Development of the Membrane Lipid Therapy: New Bioactive Lipids

Membranes are mainly composed of a lipid bilayer and proteins, constituting a checkpoint for the entry and passage of signals and other molecules. Their composition can be modulated by diet, pathophysiological processes, and nutritional/pharmaceutical interventions. In addition to their use as an energy source, lipids have important structural and functional roles, e.g., fatty acyl moieties in phospholipids have distinct impacts on human health depending on their saturation, carbon length, and isometry. These and other membrane lipids have quite specific effects on the lipid bilayer structure, which regulates the interaction with signaling proteins. Alterations to lipids have been associated with important diseases, and, consequently, normalization of these alterations or regulatory interventions that control membrane lipid composition have therapeutic potential. This approach, termed membrane lipid therapy or membrane lipid replacement, has emerged as a novel technology platform for nutraceutical interventions and drug discovery. Several clinical trials and therapeutic products have validated this technology based on the understanding of membrane structure and function. The present review analyzes the molecular basis of this innovative approach, describing how membrane lipid composition and structure affects protein-lipid interactions, cell signaling, disease, and therapy (e.g., fatigue and cardiovascular, neurodegenerative, tumor, infectious diseases).

Molecular Mechanisms of Lipid Metabolism Disorders in Infectious Exacerbations of Chronic Obstructive Pulmonary Disease

Exacerbations largely determine the character of the progression and prognosis of chronic obstructive pulmonary disease (COPD). Exacerbations are connected with changes in the microbiological landscape in the bronchi due to a violation of their immune homeostasis. Many metabolic and immune processes involved in COPD progression are associated with bacterial colonization of the bronchi. The objective of this review is the analysis of the molecular mechanisms of lipid metabolism and immune response disorders in the lungs in COPD exacerbations. The complex role of lipid metabolism disorders in the pathogenesis of some infections is only beginning to be understood, however, there are already fewer and fewer doubts even now about its significance both in the pathogenesis of infectious exacerbations of COPD and in general in the progression of the disease. It is shown that the lipid rafts of the plasma membranes of cells are involved in many processes related to the detection of pathogens, signal transduction, the penetration of pathogens into the cell. Smoking disrupts the normally proceeded processes of lipid metabolism in the lungs, which is a part of the COPD pathogenesis.

Responses of Mast Cells to Pathogens: Beneficial and Detrimental Roles

Mast cells (MCs) are strategically located in tissues close to the external environment, being one of the first immune cells to interact with invading pathogens. They are long living effector cells equipped with different receptors that allow microbial recognition. Once activated, MCs release numerous biologically active mediators in the site of pathogen contact, which induce vascular endothelium modification, inflammation development and extracellular matrix remodeling. Efficient and direct antimicrobial mechanisms of MCs involve phagocytosis with oxidative and non-oxidative microbial destruction, extracellular trap formation, and the release of antimicrobial substances. MCs also contribute to host defense through the attraction and activation of phagocytic and inflammatory cells, shaping the innate and adaptive immune responses. However, as part of their response to pathogens and under an impaired, sustained, or systemic activation, MCs may contribute to tissue damage. This review will focus on the current knowledge about direct and indirect contribution of MCs to pathogen clearance. Antimicrobial mechanisms of MCs are addressed with special attention to signaling pathways involved and molecular weapons implicated. The role of MCs in a dysregulated host response that can increase morbidity and mortality is also reviewed and discussed, highlighting the complexity of MCs biology in the context of host-pathogen interactions.

Caveolin-1 Controls Vesicular TLR2 Expression, p38 Signaling and T Cell Suppression in BCG Infected Murine Monocytic Myeloid-Derived Suppressor Cells

Monocytic myeloid-derived suppressor cells (M-MDSCs) and granulocytic MDSCs (G-MDSCs) have been found to be massively induced in TB patients as well in murine Mtb infection models. However, the interaction of mycobacteria with MDSCs and its role in TB infection is not well studied. Here, we investigated the role of Cav-1 for MDSCs infected with Mycobacterium bovis Bacille-Calmette-Guerín (BCG). MDSCs that were generated from murine bone marrow (MDSCs) of wild-type (WT) or Cav1^−/− mice upregulated Cav-1, TLR4 and TLR2 expression after BCG infection on the cell surface. However, Cav-1 deficiency resulted in a selective defect of intracellular TLR2 levels predominantly in the M-MDSC subset. Further analysis indicated no difference in the phagocytosis of BCG by M-MDSCs from WT and Cav1^−/− mice or caveosome formation, but a reduced capacity to up-regulate surface markers, to secrete various cytokines, to induce iNOS and NO production required for suppression of T cell proliferation, whereas Arg-1 was not affected. Among the signaling pathways affected by Cav-1 deficiency, we found lower phosphorylation of the p38 mitogen-activated protein kinase (MAPK). Together, our findings implicate that (i) Cav-1 is dispensable for the internalization of BCG, (ii) vesicular TLR2 signaling in M-MDSCs is a major signaling pathway induced by BCG, (iii) vesicular TLR2 signals are controlled by Cav-1, (iv) vesicular TLR2/Cav-1 signaling is required for T cell suppressor functions.

Distinctive expression profiles of Caveolin-1 and Notch-1 protein in patients with nasal polyps or sinonasal inverted papillomas

BACKGROUND

Nasal polyposis (NP) and sinonasal inverted papillomas (SIP) are considered benign lesions capable of recurrence or malignant transformation although not with the same prevalence. Since fluctuations of Caveolin-1 and Notch-1 proteins expression have been reported in many pathologies, the current study aimed to investigate their involvement in the epithelial transformation observed in SIPs compared to NP.

METHODS

Immunohistochemical expression of Caveolin-1 and Notch-1 proteins was assessed in 104 patients with sinonasal lesions (45 NP, 45 SIP and 14 NP with SIP), semiquantively (percentage times intensity). Proteins expression profiles were evaluated statistically for their correlation with patients demographic and clinicopathological variables (grade of dysplasia, inflammation, recurrence) as well as with markers of proliferation (Ki67) and apoptosis (7-AAD) as determined by flow cytometry analysis.

RESULTS

SIP lesions presented increased Caveolin-1 immunopositivity compared to NP (62.2%, vs 40.9%; p = 0.045). Cytoplasmic staining was observed only in epithelium's basal and suprabasal layers. Caveolin-1 positivity was not related to Ki67 expression, apoptosis, inflammation or dysplasia, eventhough 81.8% of highly immunopositive lesions were dysplastic (p = 0.03). Also, smokers presented significantly increased immunopositivy (p = 0.03). In contrast SIP lesions presented reduced Notch-1 expression compared to NP (68.9% vs 100%; p < 0.001). Dysplastic lesions presented low Notch-1 immunopositivity (p < 0.001). Enhancement of Notch-1 gene expression was also associated with inflammation.

CONCLUSIONS

The herein presented data suggest that the expression profiles of Caveolin-1 and Notch-1 proteins in sinonasal pathologies are distinctive and that could be explored as potential targets for the development of alternative therapeutic approaches.

Selective Surface PEGylation of UiO-66 Nanoparticles for Enhanced Stability, Cell Uptake, and pH-Responsive Drug Delivery

The high storage capacities and excellent biocompatibilities of metal-organic frameworks (MOFs) have made them emerging candidates as drug-delivery vectors. Incorporation of surface functionality is a route to enhanced properties, and here we report on a surface-modification procedure-click modulation-that controls their size and surface chemistry. The zirconium terephthalate MOF UiO-66 is (1) synthesized as ∼200 nm nanoparticles coated with functionalized modulators, (2) loaded with cargo, and (3) covalently surface modified with poly(ethylene glycol) (PEG) chains through mild bioconjugate reactions. At pH 7.4, the PEG chains endow the MOF with enhanced stability toward phosphates and overcome the "burst release" phenomenon by blocking interaction with the exterior of the nanoparticles, whereas at pH 5.5, stimuli-responsive drug release is achieved. The mode of cellular internalization is also tuned by nanoparticle surface chemistry, such that PEGylated UiO-66 potentially escapes lysosomal degradation through enhanced caveolae-mediated uptake. This makes it a highly promising vector, as demonstrated for dichloroacetic-acid-loaded materials, which exhibit enhanced cytotoxicity. The versatility of the click modulation protocol will allow a wide range of MOFs to be easily surface functionalized for a number of applications.

Role of histamine in the regulation of intestinal immunity in fish

In mammals, during the acute inflammatory response, the complex interrelationship and cross-talk among histamine and the immune system has been fairly well characterized. There is a substantial body of information on its structure, metabolism, receptors, signal transduction, physiologic and pathologic effects. However, for early vertebrates, there is little such knowledge. In the case of teleost fish, this lack of knowledge has been due to the widely held belief that histamine is not present in this phylogenetic group. However, it has been recently demonstrated, that granules of mast cells in perciforms contain biologically active histamine. More importantly, the inflammatory response was clearly demonstrated to be regulated by the direct action of histamine on professional phagocytes. Nevertheless, the molecular basis and exact role of this biogenic amine in perciforms is still a matter of speculation. Therefore, this review intends to summarize recent experimental evidence regarding fish mast cells and correlate the same with their mammalian counterparts to establish the possible role of histamine in the fish intestinal inflammatory response.

Innate immunity and its regulation by mast cells

Mast cells (MCs), which are granulated tissue-resident cells of hematopoietic lineage, constitute a major sensory arm of the innate immune system. In this review we discuss the evidence supporting the dual role of MCs, both as sentinels for invading pathogens and as regulatory cells throughout the course of acute inflammation, from its initiation to resolution. This versatility is dependent on the ability of MCs to detect pathogens and danger signals and release a unique panel of mediators to promote pathogen-specific clearance mechanisms, such as through cellular recruitment or vascular permeability. It is increasingly understood that MCs also contribute to the regulated contraction of immune activation that occurs within tissues as inflammation resolves. This overarching regulatory control over innate immune processes has made MCs successful targets to purposefully enhance or, alternatively, suppress MC responses in multiple therapeutic contexts.

Rift Valley fever virus strain MP-12 enters mammalian host cells via caveola-mediated endocytosis

Rift Valley fever virus (RVFV) is a zoonotic pathogen capable of causing serious morbidity and mortality in both humans and livestock. The lack of efficient countermeasure strategies, the potential for dispersion into new regions, and the pathogenesis in humans and livestock make RVFV a serious public health concern. The receptors, cellular factors, and entry pathways used by RVFV and other members of the family Bunyaviridae remain largely uncharacterized. Here we provide evidence that RVFV strain MP-12 uses dynamin-dependent caveola-mediated endocytosis for cell entry. Caveolae are lipid raft domains composed of caveolin (the main structural component), cholesterol, and sphingolipids. Caveola-mediated endocytosis is responsible for the uptake of a wide variety of host ligands, as well as bacteria, bacterial toxins, and a number of viruses. To determine the cellular entry mechanism of RVFV, we used small-molecule inhibitors, RNA interference (RNAi), and dominant negative (DN) protein expression to inhibit the major mammalian cell endocytic pathways. Inhibitors and RNAi specific for macropinocytosis and clathrin-mediated endocytosis had no effect on RVFV infection. In contrast, inhibitors of caveola-mediated endocytosis, and RNAi targeted to caveolin-1 and dynamin, drastically reduced RVFV infection in multiple cell lines. Expression of DN caveolin-1 also reduced RVFV infection significantly, while expression of DN EPS15, a protein required for the assembly of clathrin-coated pits, and DN PAK-1, an obligate mediator of macropinocytosis, had no significant impact on RVFV infection. These results together suggest that the primary mechanism of RVFV MP-12 uptake is dynamin-dependent, caveolin-1-mediated endocytosis.

Mast cells act as phagocytes against the periodontopathogen Aggregatibacter actinomycetemcomitans

BACKGROUND

Evidence to date shows that mast cells play a critical role in immune defenses against infectious agents, but there have been no reports about involvement of these cells in eliminating periodontopathogens. In this study, the phagocytic ability of mast cells against Aggregatibacter actinomycetemcomitans compared with macrophages is evaluated.

METHODS

In vitro phagocytic assays were conducted using murine mast cells and macrophages, incubated with A. actinomycetemcomitans, either opsonized or not, with different bacterial load ratios. After 1 hour, cells were stained with acridine orange and assessed by confocal laser-scanning electron microscopy.

RESULTS

Phagocytic ability of murine mast cells against A. actinomycetemcomitans was confirmed. In addition, the percentage of mast cells with internalized bacteria was higher in the absence of opsonization than in the presence of opsonization. Both cell types showed significant phagocytic activity against A. actinomycetemcomitans. However, the percentage of mast cells with non-opsonized bacteria was higher than that of macrophages with opsonized bacteria in one of the ratios (1:10).

CONCLUSIONS

This is the first report about the participation of murine mast cells as phagocytes against A. actinomycetemcomitans, mainly in the absence of opsonization with human serum. Our results may indicate that mast cells act as professional phagocytes in the pathogenesis of biofilm-associated periodontal disease.

LDL uptake by Leishmania amazonensis: involvement of membrane lipid microdomains

Leishmania amazonensis lacks a de novo mechanism for cholesterol synthesis and therefore must scavenge this lipid from the host environment. In this study we show that the L. amazonensis takes up and metabolizes human LDL(1) particles in both a time and dose-dependent manner. This mechanism implies the presence of a true LDL receptor because the uptake is blocked by both low temperature and by the excess of non-labelled LDL. This receptor is probably associated with specific microdomains in the membrane of the parasite, such as rafts, because this process is blocked by methyl-β-cyclodextrin (MCBD). Cholesteryl ester fluorescently-labeled LDL (BODIPY-cholesteryl-LDL) was used to follow the intracellular distribution of this lipid. After uptake it was localized in large compartments along the parasite body. The accumulation of LDL was analyzed by flow cytometry using FITC-labeled LDL particles. Together these data show for the first time that L. amazonensis is able to compensate for its lack of lipid synthesis through the use of a lipid importing machinery largely based on the uptake of LDL particles from the host. Understanding the details of the molecular events involved in this mechanism may lead to the identification of novel targets to block Leishmania infection in human hosts.

Chlamydia pneumoniae entry into epithelial cells by clathrin-independent endocytosis

A gram-negative obligate intracellular bacterium, Chlamydia pneumoniae, is a common respiratory pathogen. Here, we examined the invasion and attachment of C. pneumoniae K6 into nonphagocytic HL epithelial cell line by manipulating host plasma membranes by using cholesterol-depleting methyl-beta-cyclodextrin (MβCD) and cholesterol-loading MβCD complexed cholesterol (chol-MβCD). The invasion was attenuated by MβCD-treatment while chol-MβCD augmented the attachment and invasion. In addition, the invasion was inhibited by cholesterol sequestering reagents, nystatin and filipin. Furthermore, exposure of host cells to sphingomyelinase inhibited the invasion. RNA interference was used to assay the role of clathrin and human scavenger receptor B, type I (SR-BI) in the entry of C. pneumoniae into A549 lung epithelial adenocarcinoma cells. In contrast to Chlamydia trachomatis L2, the entry of C. pneumoniae was found to be independent of clathrin. In addition, the entry was found to be SR-BI-independent, but interestingly, the chlamydial growth was attenuated in the SR-BI-silenced cells. These findings suggest that the attachment and invasion of C. pneumoniae into nonphagocytic epithelial cells is dependent on the formation of cholesterol- and sphingomyelin-rich plasma membrane microdomains, and the entry is a clathrin-independent process. In addition, our data indicate that SR-BI supports the growth of C. pneumoniae in epithelial cells.

Cell delivery of therapeutic nanoparticles

Nanomedicine seeks to manufacture drugs and other biologically relevant molecules that are packaged into nanoscale systems for improved delivery. This includes known drugs, proteins, enzymes, and antibodies that have limited clinical efficacy based on delivery, circulating half-lives, or toxicity profiles. The <100 nm nanoscale physical properties afford them a unique biologic potential for biomedical applications. Hence they are attractive systems for treatment of cancer, heart and lung, blood, inflammatory, and infectious diseases. Proposed clinical applications include tissue regeneration, cochlear and retinal implants, cartilage and joint repair, skin regeneration, antimicrobial therapy, correction of metabolic disorders, and targeted drug delivery to diseased sites including the central nervous system. The potential for cell and immune side effects has necessitated new methods for determining formulation toxicities. To realize the potential of nanomedicine from the bench to the patient bedside, our laboratories have embarked on developing cell-based carriage of drug nanoparticles to improve clinical outcomes in infectious and degenerative diseases. The past half decade has seen the development and use of cells of mononuclear phagocyte lineage, including dendritic cells, monocytes, and macrophages, as Trojan horses for carriage of anti-inflammatory and anti-infective medicines. The promise of this new technology and the perils in translating it for clinical use are developed and discussed in this chapter.

Differences in the aromatic domain of homologous streptococcal fibronectin-binding proteins trigger different cell invasion mechanisms and survival rates

Group A streptococci (GAS, Streptococcus pyogenes) and Group G streptococci (GGS, Streptococcus dysgalactiae ssp. equisimilis) adhere to and invade host cells by binding to fibronectin. The fibronectin-binding protein SfbI from GAS acts as an invasin by using a caveolae-mediated mechanism. In the present study we have identified a fibronectin-binding protein, GfbA, from GGS, which functions as an adhesin and invasin. Although there is a high degree of similarity in the C-terminal sequence of SfbI and GfbA, the invasion mechanisms are different. Unlike caveolae-mediated invasion by SfbI-expressing GAS, the GfbA-expressing GGS isolate trigger cytoskeleton rearrangements. Heterologous expression of GfbA on the surface of a commensal Streptococcus gordonii and purified recombinant protein also triggered actin rearrangements. Expression of a truncated GfbA (lacking the aromatic domain) and chimeric GfbA/SfbI protein (replacing the aromatic domain of SfbI with the GfbA aromatic domain) on S. gordonii or recombinant proteins alone showed that the aromatic domain of GfbA is responsible for different invasion mechanisms. This is the first evidence for a biological function of the aromatic domain of fibronectin-binding proteins. Furthermore, we show that streptococci invading via cytoskeleton rearrangements and intracellular trafficking along the classical endocytic pathway are less persistence than streptococci entering via caveolae.

Medicago N2-fixing symbiosomes acquire the endocytic identity marker Rab7 but delay the acquisition of vacuolar identity

Rhizobium bacteria form N(2)-fixing organelles, called symbiosomes, inside the cells of legume root nodules. The bacteria are generally thought to enter the cells via an endocytosis-like process. To examine this, we studied the identity of symbiosomes in relation to the endocytic pathway. We show that in Medicago truncatula, the small GTPases Rab5 and Rab7 are endosomal membrane identity markers, marking different (partly overlapping) endosome populations. Although symbiosome formation is considered to be an endocytosis-like process, symbiosomes do not acquire Rab5 at any stage during their development, nor do they accept the trans-Golgi network identity marker SYP4, presumed to mark early endosomes in plants. By contrast, the endosomal marker Rab7 does occur on symbiosomes from an early stage of development when they have stopped dividing up to the senescence stage. However, the symbiosomes do not acquire vacuolar SNAREs (SYP22 and VTI11) until the onset of their senescence. By contrast, symbiosomes acquire the plasma membrane SNARE SYP132 from the start of symbiosome formation throughout their development. Therefore, symbiosomes appear to be locked in a unique SYP132- and Rab7-positive endosome stage and the delay in acquiring (lytic) vacuolar identity (e.g., vacuolar SNAREs) most likely ensures their survival and maintenance as individual units.

Francisella targets cholesterol-rich host cell membrane domains for entry into macrophages

Francisella tularensis is a pathogen optimally adapted to efficiently invade its respective host cell and to proliferate intracellularly. We investigated the role of host cell membrane microdomains in the entry of F. tularensis subspecies holarctica vaccine strain (F. tularensis live vaccine strain) into murine macrophages. F. tularensis live vaccine strain recruits cholesterol-rich lipid domains ("lipid rafts") with caveolin-1 for successful entry into macrophages. Interference with lipid rafts through the depletion of plasma membrane cholesterol, through induction of raft internalization with choleratoxin, or through removal of raft-associated GPI-anchored proteins by treatment with phosphatidylinositol phospholipase C significantly inhibited entry of Francisella and its intracellular proliferation. Lipid raft-associated components such as cholesterol and caveolin-1 were incorporated into Francisella-containing vesicles during entry and the initial phase of intracellular trafficking inside the host cell. These findings demonstrate that Francisella requires cholesterol-rich membrane domains for entry into and proliferation inside macrophages.

Caveolin-1: an ambiguous partner in cell signalling and cancer

Caveolae are small plasma membrane invaginations that have been implicated in a variety of functions including transcytosis, potocytosis and cholesterol transport and signal transduction. The major protein component of this compartment is a family of proteins called caveolins. Experimental data obtained in knockout mice have provided unequivocal evidence for a requirement of caveolins to generate morphologically detectable caveolae structures. However, expression of caveolins is not sufficient per seto assure the presence of these structures. With respect to other roles attributed to caveolins in the regulation of cellular function, insights are even less clear. Here we will consider, more specifically, the data concerning the ambiguous roles ascribed to caveolin-1 in signal transduction and cancer. In particular, evidence indicating that caveolin-1 function is cell context dependent will be discussed.

The Clinical and Cellular Basis of Contact Lens-related Corneal Infections: A Review

Microbial keratitis (MK) is the most visually devastating complication associated with contact lens wear. Pseudomonas aeruginosa (PA) is highly invasive in the corneal epithelium and is responsible for more than half of the reported cases of contact lens-related MK. To protect against Pseudomonas-mediated MK, the corneal epithelium has evolved overlapping defense mechanisms that function to protect the ocular surface from microbial invasion. Research has shown that contact lens wear disrupts these protective mechanisms through breakdown of normal homeostatic surface renewal as well as damaging the corneal surface, exposing underlying cell membrane receptors that bind and internalize PA through the formation of lipid rafts. Human clinical trials have shown that initial adherence of PA with resulting increased risk for microbial infection is mediated in part by contact lens oxygen transmissibility. Recently, chemical preserved multipurpose solutions (MPS) have been implicated in increasing PA adherence to corneal epithelial cells, in addition to inducing significant levels of toxic staining when used in conjunction with specific silicone hydrogel lenses. This review summarizes what is currently known about the relationship between contact lenses, the corneal epithelium, MPS, and infection.

Anaplasma phagocytophilum specifically induces tyrosine phosphorylation of ROCK1 during infection

Anaplasma phagocytophilum, an obligate intracellular pathogen that persists within polymorphonuclear leucocytes, is the second most common tick-borne agent in North America. We now show that infection of a promyelocytic cell line and neutrophils with A. phagocytophilum results in pathogen-specific tyrosine phosphorylation of ROCK1. Phosphorylation is associated with PSGL-1 and Syk, because PSGL-1 blocking antibodies and siRNA targeting Syk interfere with ROCK1 phosphorylation in A. phagocytophilum-infected cells. Knockdown of either Syk or ROCK1 also markedly impaired A. phagocytophilum infection. These data demonstrate a role for A. phagocytophilum-mediated ROCK1 phosphorylation in infection, and suggests that inhibiting this pathway may lead to new, non-antibiotic strategies to treat human granulocytic anaplasmosis.

Distinct pattern of lung gene expression in theCftr-KO mice developing spontaneous lung disease compared with their littermate controls

Cystic fibrosis (CF) is caused by a defect in the CF transmembrane conductance regulator (CFTR) protein that functions as a chloride channel. Dysfunction of the CFTR protein results in salty sweat, pancreatic insufficiency, intestinal obstruction, male infertility, and severe pulmonary disease. Most of the morbidity and mortality of CF patients results from pulmonary complications. Differences in susceptibility to bacterial infection and variable degree of CF lung disease among CF patients remain unexplained. Many phenotypic expressions of the disease do not directly correlate with the type of mutation in the Cftr gene. Using a unique CF mouse model that mimics aspects of human CF lung disease, we analyzed the differential gene expression pattern between the normal lungs of wild-type mice (WT) and the affected lungs of CFTR knockout mice (KO). Using microarray analysis followed by quantitation of candidate gene mRNA and protein expression, we identified many interesting genes involved in the development of CF lung disease in mice. These findings point to distinct mechanisms of gene expression regulation between mice with CF and control mice.

Surface Adhesins of Staphylococcus aureus

An important facet in the interaction between Staphylococcus aureus and its host is the ability of the bacterium to adhere to human extracellular matrix components and serum proteins. In order to colonise the host and disseminate, it uses a wide range of strategies, the molecular and genetic basis of which are multifactorial, with extensive functional overlap between adhesins. Here, we describe the current knowledge of the molecular features of the adhesive components of S. aureus, mechanisms of adhesion and the impact that these have on host-pathogen interaction.

The role of lipid rafts in the pathogenesis of bacterial infections

Numerous pathogens have evolved mechanisms of co-opting normal host endocytic machinery as a means of invading host cells. While numerous pathogens have been known to enter cells via traditional clathrin-coated pit endocytosis, a growing number of viral and bacterial pathogens have been recognized to invade host cells via clustered lipid rafts. This review focuses on several bacterial pathogens that have evolved several different mechanisms of co-opting clustered lipid rafts to invade host cells. Although these bacteria have diverse clinical presentations and many differences in their pathogenesis, they each depend on the integrity of clustered lipid rafts for their intracellular survival. Bacterial invasion via clustered lipid rafts has been recognized as an important virulence factor for a growing number of bacterial pathogens in their battle against host defenses.

Cytopathic and non-cytopathic bovine viral diarrhoea virus biotypes affect fluid phase uptake and mannose receptor-mediated endocytosis in bovine monocytes

We have used non-cytopathic (ncp) and cytopathic (cp) bovine viral diarrhoea viruses (BVDV) to determine how the two biotypes affect mannose receptor (MR)-mediated endocytosis and fluid phase uptake in bovine monocytes. We have demonstrated that endocytosis in uninfected monocytes after 1 h of culture was mediated by the MR and fluid phase uptake, and after 24 h of culture it was mediated via fluid phase uptake only. Both cp and ncp BVDV affected the mechanisms of antigen uptake in monocytes. Endocytosis in BVDV infected monocytes, unlike in uninfected cells, was MR-independent and mediated by fluid phase uptake after 1 h of infection. The 24-h-BVDV infection changed the antigen uptake mechanisms to become MR- and fluid phase uptake-dependent. We conclude that antigen uptake, an important antigen presenting cell (APC) function, is affected in the early stage of BVDV infection during the first 24 h, with both BVDV biotypes, cp and ncp, having similar effects on monocyte antigen uptake in cattle. By influencing the early antigen uptake function of APC, BVDV might disrupt the function of monocytes as professional APC and contribute to the specific immunotolerance to BVDV.

Pseudomonas invasion of type I pneumocytes is dependent on the expression and phosphorylation of caveolin-2

Pseudomonas aeruginosa is a major cause of pneumonia in patients with cystic fibrosis and other immuncompromising conditions. Here we showed that P. aeruginosa invades type I pneumocytes via a lipid raft-mediated mechanism. P. aeruginosa invasion of rat primary type I-like pneumocytes as well as a murine lung epithelial cell line 12 (MLE-12) is inhibited by drugs that remove membrane cholesterol and disrupt lipid rafts. Confocal microscopy demonstrated co-localization of intracellular P. aeruginosa with lipid raft components including caveolin-1 and -2. We generated caveolin-1 and -2 knockdowns in MLE-12 cells by using RNA interference techniques. Decreased expression of caveolin-2 significantly impaired the ability of P. aeruginosa to invade MLE-12 cells. In addition, the lipid raft-dependent tyrosine phosphorylation of caveolin-2 appeared to be a critical regulator of P. aeruginosa invasion.

Immunolocalization of caveolin-1 in rat and human mesothelium

Flask-shaped vesicles have been described as caveolae in mesothelial cells in a number of animal species based on morphological criteria only. Using an antibody against caveolin-1, said to be a biochemical marker of caveolae, immunoelectron microscopy suggests that many but not all such vesicles in mesothelial cells are caveolae. Mesothelial cells from different anatomical sites showed obvious variations in both the population density and distribution of these flask-shaped vesicles and in their density of immunostaining. Lung and pericardial sac had the highest staining density. In some sites (e.g., lung, bladder, colon) caveolae were equally distributed between apical and basolateral surfaces, whereas in others (e.g., spleen, liver), they were predominantly apical. Additional immunopositive sites in the peritoneal membrane were identified, including the epineurium of peripheral nerves and the endothelium of lymphatic vessels. We further suggest that variations in the number of mesothelial cell caveolae and the density of their immunolabeling may have implications for our understanding of certain diseases such as malignant mesothelioma, especially in view of the recent hypothesis that it may be caused by SV40, a virus that appears to enter cells via caveolae.

Novel program of macrophage gene expression induced by phagocytosis of Leishmania chagasi

Leishmania spp. are protozoans that survive and replicate intracellularly in mammalian macrophages. Antileishmanial immunity requires gamma interferon (IFN-gamma)-mediated macrophage activation and generation of microbicidal effector molecules. The presence of intracellular Leishmania sp. impairs macrophage responses to IFN-gamma, which has led to the description of macrophages as deactivated. It has recently become apparent that in addition to classical activation, macrophages can be activated by distinct triggers to express noninflammatory or anti-inflammatory genes. These nonclassical activation programs have been called alternative or type II pathways. We hypothesized that during initial contact with a phagocyte, leishmaniae activate one of these nonclassical pathways, resulting in expression of genes whose products suppress microbicidal responses. Using DNA microarrays, we studied gene expression in RNAs from BALB/c bone marrow macrophages with and without Leishmania chagasi infection. Some changes were verified by an RNase protection assay, reverse transcription-PCR, immunoblotting, or a bioassay. The pattern of genes activated by leishmania phagocytosis differed from the pattern of genes activated by bacteria or lipopolysaccharide and IFN-gamma. Genes encoding some proinflammatory cytokines, receptors, and Th1-type immune response genes were down-modulated, and some genes associated with anti-inflammatory or Th2-like immune responses were up-regulated. Nonetheless, some markers of alternative (arginase) or type II activation (interleukin-10, tumor necrosis factor alpha) were unchanged. These data suggest that macrophages infected with L. chagasi exhibit a hybrid activation profile that is more characteristic of alternative or type II activation than of classical activation but does not strictly fall into either of these categories. We speculate that the pattern of genes upregulated by leishmania phagocytosis optimizes the chance of parasite survival in this hostile environment.

Host but not parasite cholesterol controls Toxoplasma cell entry by modulating organelle discharge

Host cell cholesterol is implicated in the entry and replication of an increasing number of intracellular microbial pathogens. Although uptake of viral particles via cholesterol-enriched caveolae is increasingly well described, the requirement of cholesterol for internalization of eukaryotic pathogens is poorly understood and is likely to be partly organism specific. We examined the role of cholesterol in active host cell invasion by the protozoan parasite Toxoplasma gondii. The parasitophorous vacuole membrane (PVM) surrounding T. gondii contains cholesterol at the time of invasion. Although cholesterol-enriched parasite apical organelles termed rhoptries discharge at the time of cell entry and contribute to PVM formation, surprisingly, rhoptry cholesterol is not necessary for this process. In contrast, host plasma membrane cholesterol is incorporated into the forming PVM during invasion, through a caveolae-independent mechanism. Unexpectedly, depleting host cell plasma membrane cholesterol blocks parasite internalization by reducing the release of rhoptry proteins that are necessary for invasion. Cholesterol back-addition into host plasma membrane reverses this inhibitory effect of depletion on parasite secretion. These data define a new mechanism by which host cholesterol specifically controls entry of an intracellular pathogen.

Lipid rafts, caveolae, caveolin-1, and entry by Chlamydiae into host cells

Obligate intracellular bacterial pathogens of the genus Chlamydia are reported to enter host cells by both clathrin-dependent and clathrin-independent processes. C. trachomatis serovar K recently was shown to enter cells via caveolae-like lipid raft domains. We asked here how widespread raft-mediated entry might be among the Chlamydia. We show that C. pneumoniae, an important cause of respiratory infections in humans that additionally is associated with cardiovascular disease, and C. psittaci, an important pathogen in domestic mammals and birds that also infects humans, each enter host cells via cholesterol-rich lipid raft microdomains. Further, we show that C. trachomatis serovars E and F also use these domains to enter host cells. The involvement of these membrane domains in the entry of these organisms was indicated by the sensitivity of their entry to the raft-disrupting agents Nystatin and filipin, and by their intracellular association with caveolin-1, a 22-kDa protein associated with the formation of caveolae in rafts. In contrast, caveolin-marked lipid raft domains do not mediate entry of C. trachomatis serovars A, 36B, and C, nor of LGV serovar L2 and MoPn. Finally, we show that entry of each of these chlamydial strains is independent of cellular expression of caveolin-1. Thus, entry via the Nystatin and filipin-sensitive pathway is dependent on lipid rafts containing cholesterol, rather than invaginated caveolae per se.

Host cell caveolae act as an entry-port for group A streptococci

This study identified caveolae as an entry port for group A streptococci into epithelial and endothelial cells. Scanning electron microscopy as well as ultrathin sections of infected cells demonstrated accumulation of small omega-shaped cavities in the host cell membrane close to adherent streptococci. During invasion, invaginations were formed that subsequently revealed intracellular compartments surrounding streptococci. Caveolin-1 was shown to be present in the membrane of invaginations and the compartment membranes. These compartments were devoid of any classic endosomal/lysosomal marker proteins and can thus be described as caveosomes. Disruption of caveolae with methyl-beta-cyclodextrin and filipin abolished host cell invasion. Importantly, streptococci inside caveosomes avoid fusion with lysosomes. Expressing of SfbI protein on the surface of the non-invasive S. gordonii resulted in identical morphological alterations on the host cell as for S. pyogenes. Incubation of HUVEC cells with purified recombinant sole SfbI protein also triggered accumulation of cavity-like structures and formation of membrane invaginations. Tagged to colloidal gold-particles, SfbI protein was shown to cluster following membrane contact. Thus, our results demonstrate that host cell caveolae initiate the invasion process of group A streptococci and that the streptococcal invasin SfbI is the triggering factor that activates the caveolae-mediated endocytic pathway.

Role of the Brucella suis lipopolysaccharide O antigen in phagosomal genesis and in inhibition of phagosome-lysosome fusion in murine macrophages

Brucella species are gram-negative, facultative intracellular bacteria that infect humans and animals. These organisms can survive and replicate within a membrane-bound compartment inside professional and nonprofessional phagocytic cells. Inhibition of phagosome-lysosome fusion has been proposed as a mechanism for intracellular survival in both cell types. However, the molecular mechanisms and the microbial factors involved are poorly understood. Smooth lipopolysaccharide (LPS) of Brucella has been reported to be an important virulence factor, although its precise role in pathogenesis is not yet clear. In this study, we show that the LPS O side chain is involved in inhibition of the early fusion between Brucella suis-containing phagosomes and lysosomes in murine macrophages. In contrast, the phagosomes containing rough mutants, which fail to express the O antigen, rapidly fuse with lysosomes. In addition, we show that rough mutants do not enter host cells by using lipid rafts, contrary to smooth strains. Thus, we propose that the LPS O chain might be a major factor that governs the early behavior of bacteria inside macrophages.

Investigation of mechanisms involved in phagocytosis of Legionella pneumophila by human cells

Legionella pneumophila, the causative agent of Legionnaires' disease, is able to survive and multiply efficiently in a variety of mammalian cells. By using in vitro assays, the uptake of L. pneumophila into monocytes has shown to be mediated, at least in part, through attachment of complement-coated bacteria to complement receptors, but complement-independent phagocytosis could also be demonstrated. Since complement levels in the human lung are normally low, the role of complement-dependent phagocytosis in the pathogenesis of Legionnaires' disease is doubtful. However, the contribution of other potential phagocytosis-related host cell surface molecules to the phagocytosis of L. pneumophila has never been investigated. We therefore analyzed the role of complement receptors 1 (CD35) and 3 (CD11b/18), the lipopolysaccharide (LPS) receptor (CD14), the beta(1)-integrin chain of the fibronectin receptor (CD29), the intercellular adhesion molecule 1 (ICAM-1, CD54) and the transferrin receptor (CD71) in the complement-independent uptake of L. pneumophila. To exclude any influence of culture conditions onto phagocytosis rates, we compared a fresh clinical isolate with an agar-adapted isolate of L. pneumophila. In addition, we used three different host cell types (MM6, HeLa and Jurkat cells) expressing different rates of complement receptors. We could show that both strains of L. pneumophila were phagocytized by the three host cell lines to the same extent, but intracellular multiplication was only found in MM6 and, although to a much lesser degree, in Jurkat cells. Preincubation of MM6 cells with monoclonal antibodies directed against the above cited phagocytosis-related receptors did not result in inhibition of L. pneumophila uptake. We therefore conclude that typical phagocytosis-related cell surface receptors are not involved in the complement-independent phagocytosis of L. pneumophila.

Proton magnetic resonance spectroscopy of basal ganglia in chronic fatigue syndrome

Fatigue is a common symptom of neurological diseases that affect basal ganglia function. We used proton magnetic resonance spectroscopy ((1)H MRS) to study the metabolic functions of the basal ganglia in chronic fatigue syndrome (CFS) to test the hypothesis that fatigue in CFS may have a neurogenic component. (1)H MRS of left basal ganglia was carried out in eight non-psychiatric patients with CFS and their results were compared to age- and sex-matched healthy asymptomatic healthy controls. A highly significant increase in the spectra from choline-containing compounds was seen in the CFS patient group (p < 0.001). In the absence of regional structural or inflammatory pathology, increased choline resonance in CFS may be an indicator of higher cell membrane turnover due to gliosis or altered intramembrane signalling.

Caveolae--from ultrastructure to molecular mechanisms

Almost 50 years after the first sighting of small pits that covered the surface of mammalian cells, investigators are now getting to grips with the detailed workings of these enigmatic structures that we now know as caveolae.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Escherichia coli K1 internalization via caveolae requires caveolin-1 and protein kinase Calpha interaction in human brain microvascular endothelial cells

The morbidity and mortality associated with Escherichia coli K1 meningitis during the neonatal period have remained significant over the last decade and are once again on the rise. Transcytosis of brain microvascular endothelial cells (BMEC) by E. coli within an endosome to avoid lysosomal fusion is crucial for dissemination into the central nervous system. Central to E. coli internalization of BMEC is the expression of OmpA (outer membrane protein A), which interacts with its receptor for the actin reorganization that leads to invasion. However, nothing is known about the nature of the signaling events for the formation of endosomes containing E. coli K1. We show here that E. coli K1 infection of human BMEC (HBMEC) results in activation of caveolin-1 for bacterial uptake via caveolae. The interaction of caveolin-1 with phosphorylated protein kinase Calpha (PKCalpha) at the E. coli attachment site is critical for the invasion of HBMEC. Optical sectioning of confocal images of infected HBMEC indicates continuing association of caveolin-1 with E. coli during transcytosis. Overexpression of a dominant-negative form of caveolin-1 containing mutations in the scaffolding domain blocked the interaction of phospho-PKCalpha with caveolin-1 and the E. coli invasion of HBMEC, but not actin cytoskeleton rearrangement or the phosphorylation of PKCalpha. The interaction of caveolin-1 with phospho-PKCalpha was completely abrogated in HBMEC overexpressing dominant-negative forms of either focal adhesion kinase or PKCalpha. Treatment of HBMEC with a cell-permeable peptide that represents the scaffolding domain, which was coupled to an antennapedia motif of a Drosophila transcription factor significantly blocked the interaction of caveolin-1 with phospho-PKCalpha and E. coli invasion. These results show that E. coli K1 internalizes HBMEC via caveolae and that the scaffolding domain of caveolin-1 plays a significant role in the formation of endosomes.

The intramacrophagic environment of Brucella suis and bacterial response

Phagocytes have developed various antimicrobial defense mechanisms to eliminate pathogens. They comprise the oxidative burst, acidification of phagosomes, or fusion of phagosomes with lysosomes. Facultative intracellular bacteria, in return, have developed strategies counteracting the host cell defense, resulting in intramacrophagic survival. Until lately, only very little was known about the phagosomal compartment containing Brucella spp., the environmental conditions the bacteria encounter, and the pathogen's stress response. Recently, we have determined that the phagosomes acidify rapidly to a pH of 4.0-4.5 following infection, but this early acidification is crucial for intracellular replication as neutralization results in bacterial elimination. A vacuolar proton-ATPase is responsible for this phenomenon that is not linked to phagosome-lysosome fusion. On the contrary, in vitro reconstitution assays revealed association only between phagosomes containing killed B. suis and lysosomes, describing the absence of phagolysosome fusion due to specific recognition inhibition for live bacteria. Further evidence for the necessity of an intact, acidic phagosome as a predominant niche of brucellae in macrophages was obtained with a strain of B. suis secreting listeriolysin. It partially disrupts the phagosomal membranes and fails to multiply intracellularly. How does B. suis adapt to this environment? We have identified and studied a series of genes that are involved in this process of adaptation. The bacterial heat shock protein and chaperone DnaK is induced in phagocytes and it is essential for intracellular multiplication. A low-level, constitutive expression of dnaK following promoter exchange does not restore intramacrophagic survival. Another chaperone and heat shock protein, ClpB, belonging to the family of ClpATPases, is important for the resistance of B. suis to several in vitro stresses, but does not contribute to intramacrophagic survival of the pathogen. Additional bacterial genes specifically induced within the phagocyte were identified by an intramacrophagic screen of random promoter fusions to the reporter gene gfp. A large majority of these genes are encoding proteins involved in transport of nutrients (sugars, amino acids), or cofactors, such as nickel. Analysis of the intracellular gene activation reveals that low oxygen tension is encountered by B. suis. Altogether, these results suggest three major stress conditions encountered by brucellae in the phagosome: acid stress, starvation and low oxygen tension.

Blockade of human immunodeficiency virus type 1 expression by caveolin-1

Caveolin-1 (Cav-1) is a major protein constituent of caveolae, a type of plasma membrane raft. We observed that coexpression of human Cav-1 with human immunodeficiency virus type 1 (HIV-1) blocked virion production from cells that are ordinarily highly permissive. Further investigation showed that this effect is specific, occurs at low ratios of Cav-1 to HIV-1 DNA, depends on expression of Cav-1 protein, and involves severely impaired expression of HIV-1 proteins. Cav-1 also blocked HIV-2 expression. In contrast, Cav-1 did not inhibit protein expression by a paramyxovirus and did not induce apoptosis or affect cellular morphology, cell viability, or cell cycle progression. Although only small amounts of HIV-1 virions were released from Cav-1-transfected cells, these were fully infectious. Deletion mutagenesis showed that the C-terminal 78 residues were as active as the full-length (178-amino-acid) protein in producing the block. In contrast, the 100 most N-terminal amino acids of Cav-1, which include the previously identified oligomerization and scaffolding domains, were shown to be dispensable. Study of single-amino-acid-exchange mutants of Cav-1 established that palmitoylation was not required. Additional deletion mutants then identified the hydrophobic, membrane-associated domain (residues 101 to 135) as the main determinant. Cellular distribution of wild-type and mutant proteins correlated with ability to block HIV-1 expression. Finally, Cav-2 also blocked HIV-1 expression. These data show that coexpression of caveolins can markedly inhibit expression of HIV proviral DNA and establish that the inhibition is mediated by the hydrophobic, membrane-associated domain.

Plasma membrane cholesterol modulates cellular vacuolation induced by the Helicobacter pylori vacuolating cytotoxin

The Helicobacter pylori vacuolating cytotoxin (VacA) induces the degenerative vacuolation of mammalian cells both in vitro and in vivo. Here, we demonstrate that plasma membrane cholesterol is essential for vacuolation of mammalian cells by VacA. Vacuole biogenesis in multiple cell lines was completely blocked when cholesterol was extracted selectively from the plasma membrane by using beta-cyclodextrins. Moreover, increasing plasma membrane cholesterol levels strongly potentiated VacA-induced vacuolation. In contrast, inhibiting de novo biosynthesis of cholesterol with lovastatin or compactin had no detectable effect on vacuolation. While depletion of plasma membrane cholesterol has been shown to interfere with both clathrin-mediated endocytosis and caveola-dependent endocytosis, neither of these two internalization pathways was found to be essential for vacuolation of cells by VacA. Depleting plasma membrane cholesterol attenuated the entry of VacA into HeLa cells. In addition, beta-cyclodextrin reagents blocked vacuolation of cells that were either preloaded with VacA or had VacA directly expressed within the cytosol. Collectively, our results suggest that plasma membrane cholesterol is important for both the intoxication mechanism of VacA and subsequent vacuole biogenesis.

An efficient genetic screen in mammalian cultured cells

Genetic approaches in mammalian cultured cells had limited success because the isolation of mutants and the identification of the mutated genes were often difficult. In the present report, we describe the establishment of a novel genetic screen in Cos-7 cells that allows rapid identification of polypeptides whose overexpression inhibits a certain cellular process. We demonstrate that this approach can be used successfully to isolate partial cDNAs whose overexpression specifically interfered with the clathrin-mediated endocytosis of transferrin.

Modulation of Brucella-induced macropinocytosis by lipid rafts mediates intracellular replication

Intracellular replication of Brucella requires the VirB complex, which is highly similar to conjugative DNA transfer systems. In this study, we show that Brucella internalizes into macrophages by swimming on the cell surface with generalized membrane ruffling for several minutes, after which the bacteria are enclosed by macropinosomes. Lipid raft-associated molecules such as glycosylphosphatidylinositol (GPI)-anchored proteins, GM1 gangliosides and cholesterol were selectively incorporated into macropinosomes containing Brucella. In contrast, lysosomal glycoprotein LAMP-1 and host cell transmembrane protein CD44 were excluded from the macropinosomes. Removing GPI-anchored proteins from the macrophage surface and cholesterol sequestration markedly inhibited the VirB-dependent macropinocytosis and intracellular replication. Our results suggest that the entry route of Brucella into the macrophage determines the intracellular fate of the bacteria that is modulated by lipid raft microdomains.

Association of the caveola vesicular system with cellular entry by filoviruses

The filoviruses Ebola Zaire virus and Marburg virus are believed to infect target cells through endocytic vesicles, but the details of this pathway are unknown. We used a pseudotyping strategy to investigate the cell biology of filovirus entry. We observed that specific inhibitors of the caveola system, including cholesterol-sequestering drugs and phorbol esters, inhibited the entry of filovirus pseudotypes into human cells. We also measured slower cell entry kinetics for both filovirus pseudotypes than for pseudotypes of vesicular stomatitis virus (VSV), which has been recognized to exploit the clathrin-mediated entry pathway. Finally, visualization by immunofluorescence and confocal microscopy revealed that the filovirus pseudotypes colocalized with the caveola protein marker caveolin-1 but that VSV pseudotypes did not. Collectively, these results provide evidence suggesting that filoviruses use caveolae to gain entry into cells.

Role of cholesterol and the ganglioside GM(1) in entry and short-term survival of Brucella suis in murine macrophages

Brucella species are gram-negative, facultative intracellular bacteria that infect humans and animals. These organisms can survive and replicate within a membrane-bound compartment inside professional and nonprofessional phagocytic cells. Inhibition of phagosome-lysosome fusion has been proposed as a mechanism for intracellular survival in both types of cells. We have previously shown that the maturation inhibition of the Brucella-containing phagosome appears to be restricted at the phagosomal membrane, but the precise molecular mechanisms and factors involved in this inhibition have yet to be identified. Interestingly, recent studies have revealed that caveolae or lipid rafts are implicated in the entry of some microorganisms into host cells and mediate an endocytic pathway avoiding fusion with lysosomes. In this study, we investigated the role of cholesterol and the ganglioside GM(1), two components of lipid rafts, in entry and short-term survival of Brucella suis in murine macrophages, by using cholesterol-sequestering (filipin and beta-methyl cyclodextrin) and GM(1)-binding (cholera toxin B) molecules. Our results suggest that lipid rafts may provide a portal for entry of Brucella into murine macrophages under nonopsonic conditions, thus allowing phagosome-lysosome fusion inhibition, and provide further evidence to support the idea that the phagosome maturation inhibition is restricted at the phagosomal membrane.

The role of mast cells in host defense and their subversion by bacterial pathogens

Mast cells (MCs) play a prominent role in the early immune response to invading pathogenic bacteria. This newly discovered role for MCs involves the release of chemoattractants that recruit neutrophils and the direct phagocytosis and killing of opsonized bacteria. Whereas these activities are clearly beneficial to the host, certain pathogens have evolved mechanisms to evoke anomalous MC responses to the detriment of the host. These include evoking phagocytosis without killing of unopsonized bacteria and the production of toxins that corrupt the release of mediators by MCs. Elucidating how pathogens subvert the activities of MCs could provide clues to limiting the pathological activities of these cells during infectious diseases.