Cholesterol depletion alters detergent-specific solubility profiles of selected tight junction proteins and the phosphorylation of occludin

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Occludin, caveolin-1, and Alix form a multi-protein complex and regulate HIV-1 infection of brain pericytes

HIV-1 enters the brain by altering properties of the blood-brain barrier (BBB). Recent evidence indicates that among cells of the BBB, pericytes are prone to HIV-1 infection. Occludin (ocln) and caveolin-1 (cav-1) are critical determinants of BBB integrity that can regulate barrier properties of the BBB in response to HIV-1 infection. Additionally, Alix is an early acting endosomal factor involved in HIV-1 budding from the cells. The aim of the present study was to evaluate the role of cav-1, ocln, and Alix in HIV-1 infection of brain pericytes. Our results indicated that cav-1, ocln, and Alix form a multi-protein complex in which they cross-regulate each other's expression. Importantly, the stability of this complex was affected by HIV-1 infection. Modifications of the complex resulted in diminished HIV-1 infection and alterations of the cytokine profile produced by brain pericytes. These results identify a novel mechanism involved in HIV-1 infection contributing to a better understanding of the HIV-1 pathology and the associated neuroinflammatory responses.

Tight Junction Structure and Function Revisited

Tight junctions (TJs) are intercellular junctions critical for building the epithelial barrier and maintaining epithelial polarity. The claudin family of membrane proteins play central roles in TJ structure and function. However, recent findings have uncovered claudin-independent aspects of TJ structure and function, and additional players including junctional adhesion molecules (JAMs), membrane lipids, phase separation of the zonula occludens (ZO) family of scaffolding proteins, and mechanical force have been shown to play important roles in TJ structure and function. In this review, we discuss how these new findings have the potential to transform our understanding of TJ structure and function, and how the intricate network of TJ proteins and membrane lipids dynamically interact to drive TJ assembly.

Post-translational modifications of tight junction transmembrane proteins and their direct effect on barrier function

Post-translational modifications (PTMs) such as phosphorylation, ubiquitination or glycosylation are processes affecting the conformation, stability, localization and function of proteins. There is clear evidence that PTMs can act upon tight junction (TJ) proteins, thus modulating epithelial barrier function. Compared to transcriptional or translational regulation, PTMs are rapid and more dynamic processes so in the context of barrier maintenance they might be essential for coping with changing environmental or external impacts. The aim of this review is to extract literature deciphering PTMs in TJ proteins directly contributing to epithelial barrier changes in permeability to ions and macromolecules. It is not intended to cover the entire scope of PTMs in TJ proteins and should rather be understood as a digest of TJ protein modifications directly resulting in the tightening or opening of the epithelial barrier.

Tight junction modulation at the blood-brain barrier: Current and future perspectives

The blood-brain barrier (BBB) is the one of the most robust physical barriers in the body, comprised of tight junction (TJ) proteins in brain microvascular endothelial cells. The need for drugs to treat central nervous systems diseases is ever increasing, however the presence of the BBB significantly hampers the uptake of drugs into the brain. To overcome or circumvent the barrier, many kinds of techniques are being developed. Modulating the paracellular route by disruption of the TJ complex has been proposed as a potential drug delivery system to treat brain diseases, however, it has several limitations and is still in a developmental stage. However, recent significant advance in medical equipment /tools such as targeted ultra-sound technologies may resolve these limitations. In this review, we introduce recent advances in site- or molecular size-selective BBB disruption/modulation technologies and we include details on pharmacological inhibitory molecules against intercellular TJ proteins to modulate the BBB.

Tight junction proteins at the blood-brain barrier: far more than claudin-5

At the blood-brain barrier (BBB), claudin (Cldn)-5 is thought to be the dominant tight junction (TJ) protein, with minor contributions from Cldn3 and -12, and occludin. However, the BBB appears ultrastructurally normal in Cldn5 knock-out mice, suggesting that further Cldns and/or TJ-associated marvel proteins (TAMPs) are involved. Microdissected human and murine brain capillaries, quickly frozen to recapitulate the in vivo situation, showed high transcript expression of Cldn5, -11, -12, and -25, and occludin, but also abundant levels of Cldn1 and -27 in man. Protein levels were quantified by a novel epitope dilution assay and confirmed the respective mRNA data. In contrast to the in vivo situation, Cldn5 dominates BBB expression in vitro, since all other TJ proteins are at comparably low levels or are not expressed. Cldn11 was highly abundant in vivo and contributed to paracellular tightness by homophilic oligomerization, but almost disappeared in vitro. Cldn25, also found at high levels, neither tightened the paracellular barrier nor interconnected opposing cells, but contributed to proper TJ strand morphology. Pathological conditions (in vivo ischemia and in vitro hypoxia) down-regulated Cldn1, -3, and -12, and occludin in cerebral capillaries, which was paralleled by up-regulation of Cldn5 after middle cerebral artery occlusion in rats. Cldn1 expression increased after Cldn5 knock-down. In conclusion, this complete Cldn/TAMP profile demonstrates the presence of up to a dozen TJ proteins in brain capillaries. Mouse and human share a similar and complex TJ profile in vivo, but this complexity is widely lost under in vitro conditions.

Cyclodextrins and Iatrogenic Hearing Loss: New Drugs with Significant Risk

Cyclodextrins are a family of cyclic oligosaccharides with widespread usage in medicine, industry and basic sciences owing to their ability to solubilize and stabilize guest compounds. In medicine, cyclodextrins primarily act as a complexing vehicle and consequently serve as powerful drug delivery agents. Recently, uncomplexed cyclodextrins have emerged as potent therapeutic compounds in their own right, based on their ability to sequester and mobilize cellular lipids. In particular, 2-hydroxypropyl-β-cyclodextrin (HPβCD) has garnered attention because of its cholesterol chelating properties, which appear to treat a rare neurodegenerative disorder and to promote atherosclerosis regression related to stroke and heart disease. Despite the potential health benefits, use of HPβCD has been linked to significant hearing loss in several species, including humans. Evidence in mice supports a rapid onset of hearing loss that is dose-dependent. Ototoxicity can occur following central or peripheral drug delivery, with either route resulting in the preferential loss of cochlear outer hair cells (OHCs) within hours of dosing. Inner hair cells and spiral ganglion cells are spared at doses that cause ~85% OHC loss; additionally, no other major organ systems appear adversely affected. Evidence from a first-to-human phase 1 clinical trial mirrors animal studies to a large extent, indicating rapid onset and involvement of OHCs. All patients in the trial experienced some permanent hearing loss, although a temporary loss of function can be observed acutely following drug delivery. The long-term impact of HPβCD use as a maintenance drug, and the mechanism(s) of ototoxicity, are unknown. β-cyclodextrins preferentially target membrane cholesterol, but other lipid species and proteins may be directly or indirectly involved. Moreover, as cholesterol is ubiquitous in cell membranes, it remains unclear why OHCs are preferentially susceptible to HPβCD. It is possible that HPβCD acts upon several targets—for example, ion channels, tight junctions (TJ), membrane integrity, and bioenergetics—that collectively increase the sensitivity of OHCs over other cell types.

Chloride channel ClC- 2 enhances intestinal epithelial tight junction barrier function via regulation of caveolin-1 and caveolar trafficking of occludin

Previous studies have demonstrated that the chloride channel ClC-2 plays a critical role in intestinal epithelial tight junction (TJ) barrier function via intracellular trafficking of TJ protein occludin. To study the mechanism of ClC-2-mediated TJ barrier function and intracellular trafficking of occludin, we established ClC-2 over-expressing Caco-2 cell line (Caco-2^CLCN2) by full length ClC-2 ORF transfection. ClC-2 over-expression (Caco-2^CLCN2) significantly enhanced TJ barrier (increased TER by ≥2 times and reduced inulin flux by 50%) compared to control Caco-2^pEZ cells. ClC-2 over-expression (Caco-2^CLCN2) increased occludin protein level compared to control Caco-2^pEZ cells. Surface biotinylation assay revealed reduced steady state endocytosis of occludin in Caco-2^CLCN2 cells. Furthermore, ClC-2 over-expression led to reduction in caveolin-1 protein level and diminishment of caveolae assembly. Caveolae disruption increased TJ permeability in control but not ClC-2 over-expressing Caco-2^CLCN2 cells. Selective ClC-2 channel blocker GaTx2 caused an increase in caveolin-1 protein level and reduced occludin level. Delivery of cell permeable caveolin-1 scaffolding domain reduced the occludin protein level. Over all, these results suggest that ClC- 2 enhances TJ barrier function in intestinal epithelial cells via regulation of caveolin-1 and caveolae-mediated trafficking of occludin.

Epithelial Cholesterol Deficiency Attenuates Human Antigen R-linked Pro-inflammatory Stimulation via an SREBP2-linked Circuit

Patients with chronic intestinal ulcerative diseases, such as inflammatory bowel disease, tend to exhibit abnormal lipid profiles, which may affect the gut epithelial integrity. We hypothesized that epithelial cholesterol depletion may trigger inflammation-checking machinery via cholesterol sentinel signaling molecules whose disruption in patients may aggravate inflammation and disease progression. In the present study, sterol regulatory element-binding protein 2 (SREBP2) as the cholesterol sentinel was assessed for its involvement in the epithelial inflammatory responses in cholesterol-depleted enterocytes. Patients and experimental animals with intestinal ulcerative injuries showed suppression in epithelial SREBP2. Moreover, SREBP2-deficient enterocytes showed enhanced pro-inflammatory signals in response to inflammatory insults, indicating regulatory roles of SREBP2 in gut epithelial inflammation. However, epithelial cholesterol depletion transiently induced pro-inflammatory chemokine expression regardless of the well known pro-inflammatory nuclear factor-κB signals. In contrast, cholesterol depletion also exerts regulatory actions to maintain epithelial homeostasis against excessive inflammation via SREBP2-associated signals in a negative feedback loop. Mechanistically, SREBP2 and its induced target EGR-1 were positively involved in induction of peroxisome proliferator-activated receptor γ (PPARγ), a representative anti-inflammatory transcription factor. As a crucial target of the SREBP2-EGR-1-PPARγ-associated signaling pathways, the mRNA stabilizer, human antigen R (HuR) was retained in nuclei, leading to reduced stability of pro-inflammatory chemokine transcripts. This mechanistic investigation provides clinical insights into protective roles of the epithelial cholesterol deficiency against excessive inflammatory responses via the SREBP2-HuR circuit, although the deficiency triggers transient pro-inflammatory signals.

Tight junctions: from simple barriers to multifunctional molecular gates

Epithelia and endothelia separate different tissue compartments and protect multicellular organisms from the outside world. This requires the formation of tight junctions, selective gates that control paracellular diffusion of ions and solutes. Tight junctions also form the border between the apical and basolateral plasma-membrane domains and are linked to the machinery that controls apicobasal polarization. Additionally, signalling networks that guide diverse cell behaviours and functions are connected to tight junctions, transmitting information to and from the cytoskeleton, nucleus and different cell adhesion complexes. Recent advances have broadened our understanding of the molecular architecture and cellular functions of tight junctions.

Systems Proteomics View of the Endogenous Human Claudin Protein Family

Claudins are the major transmembrane protein components of tight junctions in human endothelia and epithelia. Tissue-specific expression of claudin members suggests that this protein family is not only essential for sustaining the role of tight junctions in cell permeability control but also vital in organizing cell contact signaling by protein-protein interactions. How this protein family is collectively processed and regulated is key to understanding the role of junctional proteins in preserving cell identity and tissue integrity. The focus of this review is to first provide a brief overview of the functional context, on the basis of the extensive body of claudin biology research that has been thoroughly reviewed, for endogenous human claudin members and then ascertain existing and future proteomics techniques that may be applicable to systematically characterizing the chemical forms and interacting protein partners of this protein family in human. The ability to elucidate claudin-based signaling networks may provide new insight into cell development and differentiation programs that are crucial to tissue stability and manipulation.

Hearing Loss and Otopathology Following Systemic and Intracerebroventricular Delivery of 2-Hydroxypropyl-Beta-Cyclodextrin

Cyclodextrins are simple yet powerful molecules widely used in medicinal formulations and industry for their ability to stabilize and solubilize guest compounds. However, recent evidence shows that 2-hydroxypropyl-β-cyclodextrin (HPβCD) causes severe hearing loss in mice, selectively killing outer hair cells (OHC) within 1 week of subcutaneous drug treatment. In the current study, the impact of HPβCD on auditory physiology and pathology was explored further as a function of time and route of administration. When administered subcutaneously or directly into cerebrospinal fluid, single injections of HPβCD caused up to 60 dB threshold shifts and widespread OHC loss in a dose-dependent manner. Combined dosing caused no greater deficit, suggesting a common mode of action. After drug treatment, OHC loss progressed over time, beginning in the base and extending toward the apex, creating a sharp transition between normal and damaged regions of the cochlea. Administration into cerebrospinal fluid caused rapid ototoxicity when compared to subcutaneous delivery. Despite the devastating effect on the cochlea, HPβCD was relatively safe to other peripheral and central organ systems; specifically, it had no notable nephrotoxicity in contrast to other ototoxic compounds like aminoglycosides and platinum-based drugs. As cyclodextrins find expanding medicinal applications, caution should be exercised as these drugs possess a unique, poorly understood, ototoxic mechanism.

Conceptual barriers to understanding physical barriers

The members of the large family of claudin proteins regulate ion and water flux across the tight junction. Many claudins, e.g. claudins 2 and 15, accomplish this by forming size- and charge-selective paracellular channels. Claudins also appear to be essential for genesis of tight junction strands and recruitment of other proteins to these sites. What is less clear is whether claudins form the paracellular seal. While this seal is defective when claudins are disrupted, some results, including ultrastructural and biochemical data, suggest that lipid structures are an important component of tight junction strands and may be responsible for the paracellular seal. This review highlights current understanding of claudin contributions to barrier function and tight junction structure and suggests a model by which claudins and other tight junction proteins can drive assembly and stabilization of a lipid-based strand structure.

Transmembrane proteins of the tight junctions at the blood-brain barrier: structural and functional aspects

The blood-brain barrier (BBB) is formed by microvascular endothelial cells sealed by tetraspanning tight junction (TJ) proteins, such as claudins and TAMPs (TJ-associated marvel proteins, occludin and tricellulin). Claudins are the major components of the TJs. At the BBB, claudin-5 dominates the TJs by preventing the paracellular permeation of small molecules. On the other hand, TAMPs regulate the structure and function of the TJs; tricellulin may tighten the barrier for large molecules. This review aims at integrating and summarizing the most relevant and recent work on how the BBB is influenced by claudin-1, -3, -5, -12 and the TAMPs occludin and tricellulin, all of which are four-transmembrane TJ proteins. The exact functions of claudin-1, -3, -12 and TAMPs at this barrier still need to be elucidated.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Caveolin binds independently to claudin-2 and occludin

Treatment of epithelial and endothelial cells with proinflammatory cytokines can stimulate tight junction protein endocytosis, with associated loss of physiologic barrier function. In some instances, the endocytic scaffolding protein, caveolin-1, has been implicated in the cytokine-dependent retrieval of the tight junction proteins occludin and claudins. How caveolin-1 interacts with these proteins, however, remains undefined. Using co-immunoprecipitation assays, we found that caveolin-1 separately interacts with claudin-2 and occludin, but not with ZO-1, ZO-2, or claudin-4. Further, we found that the interactions of caveolin-1 with claudin-2 and occludin were not disrupted by cholesterol removal, suggesting that they were not dependent on co-localization to cholesterol-rich lipid rafts. Co-immunoprecipitation of caveolin-1 with chimeras between claudin-2 and -4 indicated that the C-terminal cytoplasmic domain of claudin-2 is required for association with caveolin-1; similar analysis showed that the ZO-1 binding region of occludin is not required for its interaction with caveolin-1. The finding that caveolin-1 interacts with claudin-2 and occludin, but not with claudin-4 or ZO-1, suggests a potential mechanism for selective retrieval of tight junction components.

Lipid rafts are disrupted in mildly inflamed intestinal microenvironments without overt disruption of the epithelial barrier

Intestinal epithelial barrier disruption is a feature of inflammatory bowel disease (IBD), but whether barrier disruption precedes or merely accompanies inflammation remains controversial. Tight junction (TJ) adhesion complexes control epithelial barrier integrity. Since some TJ proteins reside in cholesterol-enriched regions of the cell membrane termed lipid rafts, we sought to elucidate the relationship between rafts and intestinal epithelial barrier function. Lipid rafts were isolated from Caco-2 intestinal epithelial cells primed with the proinflammatory cytokine interferon-γ (IFN-γ) or treated with methyl-β-cyclodextrin as a positive control for raft disruption. Rafts were also isolated from the ilea of mice in which colitis had been induced in conjunction with in vivo intestinal permeability measurements, and lastly from intestinal biopsies of ulcerative colitis (UC) patients with predominantly mild or quiescent disease. Raft distribution was analyzed by measuring activity of the raft-associated enzyme alkaline phosphatase and by performing Western blot analysis for flotillin-1. Epithelial barrier integrity was estimated by measuring transepithelial resistance in cytokine-treated cells or in vivo permeability to fluorescent dextran in colitic mice. Raft and nonraft fractions were analyzed by Western blotting for the TJ proteins occludin and zonula occludens-1 (ZO-1). Our results revealed that lipid rafts were disrupted in IFN-γ-treated cells, in the ilea of mice with subclinical colitis, and in UC patients with quiescent inflammation. This was not associated with a clear pattern of occludin or ZO-1 relocalization from raft to nonraft fractions. Significantly, a time-course study in colitic mice revealed that disruption of lipid rafts preceded the onset of increased intestinal permeability. Our data suggest for the first time that lipid raft disruption occurs early in the inflammatory cascade in murine and human colitis and, we speculate, may contribute to subsequent disruption of epithelial barrier function.

Occludin: one protein, many forms

Intercellular tight junctions (TJs) exhibit a complex molecular architecture involving the regulated cointeraction of cytoplasmic adaptor proteins (e.g., zonula occludens) and integral membrane linker proteins (e.g., occludin and claudins). They provide structural integrity to epithelial and endothelial tissues and create highly polarized barriers essential to homeostatic maintenance within vertebrate physiological systems, while their dysregulation is an established pathophysiological hallmark of many diseases (e.g., cancer, stroke, and inflammatory lung disease). The junctional complex itself is a highly dynamic signaling entity wherein participant proteins constantly undergo a blend of regulatory modifications in response to diverse physiological and pathological cues, ultimately diversifying the overall adhesive properties of the TJ. Occludin, a 65-kDa tetraspan integral membrane protein, contributes to TJ stabilization and optimal barrier function. This paper reviews our current knowledge of how tissue occludin is specifically modified at the posttranscriptional and posttranslational levels in diverse circumstances, with associated consequences for TJ dynamics and epithelial/endothelial homeostasis. Mechanistic concepts such as splice variance and alternate promoter usage, proteolysis, phosphorylation, dimerization, and ubiquitination are comprehensively examined, and possible avenues for future investigation highlighted.

Absorption enhancement effect of acylcarnitines through changes in tight junction protein in Caco-2 cell monolayers

We investigated the effects of lauroylcarnitine and palmitoylcarnitine on major tight junction proteins such as claudins in Caco-2 cell monolayers and also examined the involvement of cholesterol in the effects induced by both acylcarnitines on these proteins. We investigated the effects of lauroylcarnitine and palmitoylcarnitine on the barrier function of tight junctions by measuring transepithelial electrical resistance (TEER) and fluorescein isothiocyanate dextran 40,000 (FD-40) flux. A decrease in the TEER value and an increase in FD-40 flux were observed after incubating Caco-2 cell monolayers with lauroylcarnitine and palmitoylcarnitine for 1 h, suggesting the loss of the barrier function of tight junctions. In addition, lauroylcarnitine and palmitoylcarnitine decreased the protein levels of claudin 1, 4, and 5 but not those of claudin 2, 3, 6, or 7. In addition, palmitoylcarnitine and methyl-β-cyclodextrin increased cholesterol release from the plasma membrane. It is suggested that the effects of palmitoylcarnitine and methyl-β-cyclodextrin on claudin 4 and 5 may be associated with cholesterol leakage from the plasma membrane into the apical side. These results indicate that the protein levels of claudin 4 and 5 are decreased by treatment with palmitoylcarnitine and lauroylcarnitine, and that this change is involved in the absorption-enhancing mechanism.

Occludin is required for cytokine-induced regulation of tight junction barriers

The function of occludin remains elusive. Proposed roles include maintenance of tight junction barriers, signaling and junction remodeling. To investigate a potential role in mediating cytokine-induced changes in barrier properties, we measured barrier responses to interferon-gamma plus TNFalpha in control, occludin-overexpressing and occludin knockdown MDCK II monolayers. MDCK cells show a complex response to cytokines characterized by a simultaneous increase in the transepithelial electrical resistance and a decrease in the barrier for large solutes. We observed that overexpression of occludin increased and occludin knockdown decreased sensitivity to cytokines as assessed by both these parameters. It is known that caveolin-1 interacts with occludin and is implicated in several models of cytokine-dependent barrier disruption; we found that occludin knockdown altered the subcellular distribution of caveolin-1 and that partitioning of caveolin into detergent-insoluble lipid rafts was influenced by changing occludin levels. Knockdown of caveolin decreased the cytokine-induced flux increase, whereas the increase in the electrical barrier was unaltered; the effect of double knockdown of occludin and caveolin was similar to that of occludin single knockdown, consistent with the possibility that they function in the same pathway. These results demonstrate that occludin is required for cells to transduce cytokine-mediated signals that either increase the electrical barrier or decrease the large solute barrier, possibly by coordinating the functions of caveolin-1.

Human eyelid meibomian glands and tarsal muscle are recognized by autoantibodies from patients affected by a new variant of endemic pemphigus foliaceus in El-Bagre, Colombia, South America

BACKGROUND

Previously, we described a new variant of endemic pemphigus foliaceus (EPF) in Colombia, South America (El Bagre-EPF).

OBJECTIVE

Continuing our characterization of this variant of EPF, we now focus on one of our previously reported clinical findings: the presence of ocular lesions. These ocular lesions are seen in patients having extensive skin involvement, as measured by the Lund and Browder scale, which is generally used for patients with skin burns.

METHODS

We specifically searched for evidence of autoreactivity to various eyelid structures in these patients and correlated our immunologic data with the clinical findings. We performed indirect immunofluorescence studies using normal-appearing human eyelid skin from routine blepharoplasties as substrate tissue. We tested sera from 12 patients with El Bagre-EPF and ocular lesions, 5 patients with sporadic (nonendemic) pemphigus foliaceus, and 20 healthy control subjects (10 from the El Bagre-EPF endemic area and 10 from nonendemic areas). We used fluorescein isothiocyanate conjugated goat antiserum to human total IgG/IgA/IgM as a secondary antibody. In addition, we used fluorescein isothiocyanate conjugated antibodies to human fibrinogen, albumin, IgG, IgE, C1q, and C3, Texas Red (Rockland Immunochemicals, Inc, Gilbertsville, PA), Alexa Fluor 555, or Alexa Fluor 594 (Invitrogen, Carlsbad, CA). Ki-67 (a cell proliferation marker) was used to determine the cell proliferation rate, and nuclear counterstaining was performed with either 4', 6-diamidino-2-phenylindole or Topro III (Invitrogen, Carlsbad, CA).

RESULTS

We observed autoreactivity to multiple eyelid structures, including meibomian glands and tarsal muscle bundles at different levels, and some areas of the epidermis and the dermis close to the isthmus of the eyelids. Tarsal plate autoreactivity was seen in 10 of 12 of the El Bagre-EPF sera and in one control with pemphigus erythematosus. Furthermore, immunoprecipitation using an eyelid sample as a substrate with 1 mmol/L of sodium orthovanodate showed autoreactivity to several antigens, including some of possible lipid origin.

LIMITATIONS

The main limitation of this study is the fact that the antigen or antigens remain unknown.

CONCLUSION

We identified for the first time to our knowledge autoantibodies to meibomian glands and tarsal muscle in El Bagre-EPF. Our findings suggest that the autoantibodies to the ocular structures cause the clinical and histopathological findings in the ocular lesions in El Bagre-EPF.

Cholesterol efflux stimulates metalloproteinase-mediated cleavage of occludin and release of extracellular membrane particles containing its C-terminal fragments

That changes in membrane lipid composition alter the barrier function of tight junctions illustrates the importance of the interactions between tetraspan integral tight junction proteins and lipids of the plasma membrane. Application of methyl-beta-cyclodextrin to both apical and basolateral surfaces of MDCK cell monolayers for 2 h, results in an approximately 80% decrease in cell cholesterol, a fall in transepithelial electrical resistance, and a 30% reduction in cell content of occludin, with a smaller reduction in levels of claudins-2, -3, and -7. There were negligible changes in levels of actin and the two non-tight junction membrane proteins GP-135 and caveolin-1. While in untreated control cells breakdown of occludin, and probably other tight junction proteins, is mediated by intracellular proteolysis, our current data suggest an alternative pathway whereby in a cholesterol-depleted membrane, levels of tight junction proteins are decreased via direct release into the intercellular space as components of membrane-bound particles. Occludin, along with two of its degradation products and several claudins, increases in the basolateral medium after incubation with methyl-beta-cyclodextrin for 30 min. In contrast caveolin-1 is detected only in the apical medium after adding methyl-beta-cyclodextrin. Release of occludin and its proteolytic fragments continues even after removal of methyl-beta-cyclodextrin. Sedimentation and ultrastructural studies indicate that the extracellular tight junction proteins are associated with the membrane-bound particles that accumulate between adjacent cells. Disruption of the actin filament network by cytochalasin D did not diminish methyl-beta-cyclodextrin-induced release of tight junction proteins into the medium, suggesting that the mechanism underlying their formation is not actin-dependent. The 41- and 48-kDa C-terminal occludin fragments formed during cholesterol depletion result from the action of a GM6001-sensitive metalloproteinase(s) at some point in the path leading to release of the membrane particles.

Functions of lipid raft membrane microdomains at the blood-brain barrier

The blood-brain barrier (BBB) is a highly specialized structural and functional component of the central nervous system that separates the circulating blood from the brain and spinal cord parenchyma. Brain endothelial cells (BECs) that primarily constitute the BBB are tightly interconnected by multiprotein complexes, the adherens junctions and the tight junctions, thereby creating a highly restrictive cellular barrier. Lipid-enriched membrane microdomain compartmentalization is an inherent property of BECs and allows for the apicobasal polarity of brain endothelium, temporal and spatial coordination of cell signaling events, and actin remodeling. In this manuscript, we review the role of membrane microdomains, in particular lipid rafts, in the BBB under physiological conditions and during leukocyte transmigration/diapedesis. Furthermore, we propose a classification of endothelial membrane microdomains based on their function, or at least on the function ascribed to the molecules included in such heterogeneous rafts: (1) rafts associated with interendothelial junctions and adhesion of BECs to basal lamina (scaffolding rafts); (2) rafts involved in immune cell adhesion and migration across brain endothelium (adhesion rafts); (3) rafts associated with transendothelial transport of nutrients and ions (transporter rafts).

Identification and chromosomal localization of one locus of Leishmania (L.) major related with resistance to itraconazole

Ergosterol is an important compound responsible to maintain integrity and fluidity of Leishmania spp. membranes. Starting from an overexpression/selection method, our group has isolated and mapped nine different loci of Leishmania (L.) major related to resistance against two inhibitors of the ergosterol biosynthesis pathway, terbinafine (TBF) and itraconazole (ITZ). Individual functional analysis after overexpression induction of these loci in the presence of TBF and/or ITZ [or the ITZ analog ketoconazole (CTZ)] have shown low but significant levels of resistance after transfection into L. major wild-type parasites. In this work, we have shown the insert mapping and chromosomal identification of one of these loci (cosItz2). Functional analysis experiments associated with chromosomal localization by comparison at genomic database allowed us to identify two prospective gene-protein systems not related to the ergosterol biosynthesis and capable to confer wild-type cells resistance to ITZ-CTZ after transfection. We expected that this approach can open new insights for a better understanding of mechanisms of ITZ-CTZ action and resistance in Leishmania resulting in new strategies for the leishmaniasis treatment.

A lipid-protein hybrid model for tight junction

The epithelial tight junction (TJ) was first described ultrastructurally as a fusion of the outer lipid leaflets of the adjoining cell membrane bilayers (hemifusion). The discovery of an increasing number of integral TJ and TJ-associated proteins has eclipsed the original lipid-based model with the wide acceptance of a protein-centric model for the TJ. In this review, we stress the importance of lipids in TJ structure and function. A lipid-protein hybrid model accommodates a large body of information supporting the lipidic characteristics of the TJ, harmonizes with the accumulating evidence supporting the TJ as an assembly of lipid rafts, and focuses on an important, but relatively unexplored, field of lipid-protein interactions in the morphology, physiology, and pathophysiology of the TJ.

Evidence that membrane rafts are not required for the action of Clostridium perfringens enterotoxin

The action of bacterial pore-forming toxins typically involves membrane rafts for binding, oligomerization, and/or cytotoxicity. Clostridium perfringens enterotoxin (CPE) is a pore-forming toxin with a unique, multistep mechanism of action that involves the formation of complexes containing tight junction proteins that include claudins and, sometimes, occludin. Using sucrose density gradient centrifugation, this study evaluated whether the CPE complexes reside in membrane rafts and what role raft microdomains play in complex formation and CPE-induced cytotoxicity. Western blot analysis revealed that the small CPE complex and the CPE hexamer 1 (CH-1) complex, which is sufficient for CPE-induced cytotoxicity, both localize outside of rafts. The CH-2 complex was also found mainly in nonraft fractions, although a small pool of raft-associated CH-2 complex that was sensitive to cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) was detected. Pretreatment of Caco-2 cells with MbetaCD had no appreciable effect on CPE-induced cytotoxicity. Claudin-4 was localized to Triton X-100-soluble gradient fractions of control or CPE-treated Caco-2 cells, indicating a raft-independent association for this CPE receptor. In contrast, occludin was present in raft fractions of control Caco-2 cells. Treatment with either MbetaCD or CPE caused most occludin molecules to shift out of lipid rafts, possibly due (at least in part) to the association of occludin with the CH-2 complex. Collectively, these results suggest that CPE is a unique pore-forming toxin for which membrane rafts are not required for binding, oligomerization/pore formation, or cytotoxicity.