Lipid rafts in epithelial brush borders: atypical membrane microdomains with specialized functions

Celiac anti-tissue transglutaminase antibodies interfere with the uptake of alpha gliadin peptide 31-43 but not of peptide 57-68 by epithelial cells

Celiac disease is characterized by the secretion of IgA-class autoantibodies that target tissue transglutaminase (tTG). It is now recognized that anti-tTG antibodies are functional and not mere bystanders in the pathogenesis of celiac disease. Here we report that interaction between anti-tTG antibodies and extracellular membrane-bound tTG inhibits peptide 31-43 (but not peptide 57-68) uptake by cells, thereby impairing the ability of p31-43 to drive Caco-2 cells into S-phase. This effect did not involve tTG catalytic activity. Because anti-tTG antibodies interfered with epidermal growth factor endocytosis, we assume that they exert their effect by reducing peptide 31-43 endocytosis. Our results suggest that cell-surface tTG plays a hitherto unknown role in the regulation of gliadin peptide uptake and endocytosis.

Molecular model of the microvillar cytoskeleton and organization of the brush border

Background

Brush border microvilli are ∼1-µm long finger-like projections emanating from the apical surfaces of certain, specialized absorptive epithelial cells. A highly symmetric hexagonal array of thousands of these uniformly sized structures form the brush border, which in addition to aiding in nutrient absorption also defends the large surface area against pathogens. Here, we present a molecular model of the protein cytoskeleton responsible for this dramatic cellular morphology.

Methodology/Principal Findings

The model is constructed from published crystallographic and microscopic structures reported by several groups over the last 30+ years. Our efforts resulted in a single, unique, self-consistent arrangement of actin, fimbrin, villin, brush border myosin (Myo1A), calmodulin, and brush border spectrin. The central actin core bundle that supports the microvillus is nearly saturated with fimbrin and villin cross-linkers and has a density similar to that found in protein crystals. The proposed model accounts for all major proteinaceous components, reproduces the experimentally determined stoichiometry, and is consistent with the size and morphology of the biological brush border membrane.

Conclusions/Significance

The model presented here will serve as a structural framework to explain many of the dynamic cellular processes occurring over several time scales, such as protein diffusion, association, and turnover, lipid raft sorting, membrane deformation, cytoskeletal-membrane interactions, and even effacement of the brush border by invading pathogens. In addition, this model provides a structural basis for evaluating the equilibrium processes that result in the uniform size and structure of the highly dynamic microvilli.

Prominin-1: a distinct cholesterol-binding membrane protein and the organisation of the apical plasma membrane of epithelial cells

The apical plasma membrane of polarized epithelial cells is composed of distinct subdomains, that is, planar regions and protrusions (microvilli, primary cilium), each of which are constructed from specific membrane microdomains. Assemblies containing the pentaspan glycoprotein prominin-1 and certain membrane lipids, notably cholesterol, are characteristic features of these microdomains in apical membrane protrusions. Here we highlight the recent findings concerning the molecular architecture of the apical plasma membrane of epithelial cells and its dynamics. The latter is illustrated by the budding and fission of prominin-1-containing membrane vesicles from apical plasma membrane protrusions, which is controlled, at least in part, by the level of membrane cholesterol and the cholesterol-dependent organization of membrane microdomains.

The apical plasma membrane of Drosophila embryonic epithelia

The apical plasma membrane of epithelia presents the interface between organs and the external environment. It has biochemical activities distinct from those of the basal and lateral plasma membranes, as it accommodates the production and assembly of ordered apical matrices involved in organ protection and physiology and determines the microenvironment in the apical extracellular milieu. Here, we emphasise the importance of the apical plasma membrane in tissue differentiation, by mainly focussing on the embryo of the fruit fly Drosophila melanogaster, and discuss the principal organisation of the apical plasma membrane into repetitive subdomains of specific topologies and activities essential for epithelial function.

Lovastatin-induced cholesterol depletion affects both apical sorting and endocytosis of aquaporin-2 in renal cells

Vasopressin causes the redistribution of the water channel aquaporin-2 (AQP2) from cytoplasmic storage vesicles to the apical plasma membrane of collecting duct principal cells, leading to urine concentration. The molecular mechanisms regulating the selective apical sorting of AQP2 are only partially uncovered. In this work, we investigate whether AQP2 sorting/trafficking is regulated by its association with membrane rafts. In both MCD4 cells and rat kidney, AQP2 preferentially associated with Lubrol WX-insoluble membranes regardless of its presence in the storage compartment or at the apical membrane. Block-and-release experiments indicate that 1) AQP2 associates with detergent-resistant membranes early in the biosynthetic pathway; 2) strong cholesterol depletion delays the exit of AQP2 from the trans-Golgi network. Interestingly, mild cholesterol depletion promoted a dramatic accumulation of AQP2 at the apical plasma membrane in MCD4 cells in the absence of forskolin stimulation. An internalization assay showed that AQP2 endocytosis was clearly reduced under this experimental condition. Taken together, these data suggest that association with membrane rafts may regulate both AQP2 apical sorting and endocytosis.

Isolation and biochemical characterisation of lipid rafts from Atlantic cod (Gadus morhua) intestinal enterocytes

Lipid rafts are glycosphingolipid/cholesterol-enriched membrane microdomains that have been extensively studied during the past two decades. Our aim was to isolate and perform biochemical characterization of lipid rafts from the intestinal brush border membrane (BBM) of Atlantic cod (Gadus morhua) to confirm their existence in a cold-water species and compare their characteristics with lipid rafts from other species in terms of lipid and protein content. To validate the isolation process, we assayed marker enzymes for subcellular organelles, including alkaline phosphatase (AP) and leucine aminopeptidase (LAP), both well-known marker enzymes for BBM and lipid rafts. All biochemical methods showed enrichment of AP in both the BBM and lipid raft fractions. Proteomic studies were performed by MALDI-TOF mass spectrometry using trypsin digested SDS-PAGE samples. Various proteins were associated with the cod intestinal lipid raft preparation such as aminopeptidase-N, prohibitin, and beta-actin. Lipid analysis with (31)P NMR and thin layer chromatography on BBMs and lipid rafts samples gave higher content of sphingomyelin than previously reported in the BBM and lower content of phosphatidylcholine. Furthermore, sphingomyelin was highly dominant in the lipid rafts together with cholesterol. The existence of lipid rafts containing previously reported lipid raft characteristics from the cod intestine has, therefore, been confirmed in a ray-finned fish for the first time to the best of our knowledge.

Sphingolipid/cholesterol regulation of neurotransmitter receptor conformation and function

Like all other monomeric or multimeric transmembrane proteins, receptors for neurotransmitters are surrounded by a shell of lipids which form an interfacial boundary between the protein and the bulk membrane. Among these lipids, cholesterol and sphingolipids have attracted much attention because of their well-known propensity to segregate into ordered platform domains commonly referred to as lipid rafts. In this review we present a critical analysis of the molecular mechanisms involved in the interaction of cholesterol/sphingolipids with neurotransmitter receptors, in particular acetylcholine and serotonin receptors, chosen as representative members of ligand-gated ion channels and G protein-coupled receptors. Cholesterol and sphingolipids interact with these receptors through typical binding sites located in both the transmembrane helices and the extracellular loops. By altering the conformation of the receptors ("chaperone-like" effect), these lipids can regulate neurotransmitter binding, signal transducing functions, and, in the case of multimeric receptors, subunit assembly and subsequent receptor trafficking to the cell surface. Several sphingolipids (especially gangliosides) also exhibit low/moderate affinity for neurotransmitters. We suggest that such lipids could facilitate (i) the attachment of neurotransmitters to the post-synaptic membrane and in some cases (ii) their subsequent delivery to specific protein receptors. Overall, various experimental approaches provide converging evidence that the biological functions of neurotransmitters and their receptors are highly dependent upon sphingolipids and cholesterol, which are active partners of synaptic transmission. Several decades of research have been necessary to untangle the skein of a complex network of molecular interactions between neurotransmitters, their receptors, cholesterol and sphingolipids. This sophisticated crosstalk between all four distinctive partners may allow a fine biochemical tuning of synaptic transmission.

Galectin-2 at the enterocyte brush border of the small intestine

The brush border of pig small intestine is a local hotspot for beta-galactoside-recognizing lectins, as evidenced by its prominent labeling with fluorescent lectin PNA. Previously, galectins 3-4, intelectin, and lectin-like anti-glycosyl antibodies have been localized to this important body boundary. Together with the membrane glycolipids these lectins form stable lipid raft microdomains that also harbour several of the major digestive microvillar enzymes. In the present work, we identified a lactose-sensitive 14-kDa protein enriched in a microvillar detergent resistant fraction as galectin-2. Its release from closed, right-side-out microvillar membrane vesicles shows that at least some of the galectin-2 resides at the lumenal surface of the brush border, indicating that it plays a role in the organization/stabilization of the lipid raft domains. Galectin-2 was released more effectively from the membrane by lactose than was galectin-4, and surprisingly, it was also released by the noncanonical disaccharides sucrose and maltose. Furthermore, unlike galectin-4, galectin-2 was preferentially co-immunoisolated with sucrase-isomaltase rather than with aminopeptidase N. Together, these results show that the galectins are not simply redundant proteins competing for the same ligands but rather act in concert to ensure an optimal cross-linking of membrane glycolipids and glycoproteins. In this way, they offer a maximal protection of the brush border against exposure to bile, pancreatic enzymes and pathogens.

A possible role of lysophospholipids produced by calcium-independent phospholipase A(2) in membrane-raft budding and fission

Phospholipase A(2) (PLA(2)) not only plays a role in the membrane vesiculation system but also mediates membrane-raft budding and fission in artificial giant liposomes. This study aimed to demonstrate the same effects in living cells. Differentiated Caco-2 cells were cultured on filter membranes. MDCK cells were challenged with Influenza virus. The MDCK cultures were harvested for virus titration with a plaque assay. Alkaline phosphatase (ALP), a membrane-raft associated glycosylphosphatidylinositol (GPI)-anchored protein, was 70% released by adding 0.2 mmol/l lysophosphatidylcholine, which was abolished by treatment with a membrane-raft disrupter, methyl-beta-cyclodextrin. Activation of calcium-independent PLA(2) (iPLA(2)) by brefeldin A increased the apical release of ALP by approximately 1.5-fold (p<0.01), which was blocked by PLA(2) inhibitor bromoenol lactone (BEL). BEL also reduced Influenza virus production into the media (<10%) in the MDCK culture. These results suggest that cells utilize inverted corn-shaped lysophospholipids generated by PLA(2) to modulate plasma membrane structure and assist the budding of raft-associated plasma membrane particles, which virus utilizes for its budding. Brush borders are enriched with membrane-rafts and undergo rapid turnover; thus, PLA(2) may be involved in the regulatory mechanism in membrane dynamism. Further, iPLA(2) may provide a therapeutic target for viral infections.

A Caenorhabditis elegans glycolipid-binding galectin functions in host defense against bacterial infection

Galectins are a family of beta-galactoside-binding proteins that are widely found among animal species and that regulate diverse biological phenomena. To study the biological function of glycolipid-binding galectins, we purified recombinant Caenorhabditis elegans galectins (LEC-1-11) and studied their binding to C. elegans glycolipids. We found that LEC-8 binds to glycolipids in C. elegans through carbohydrate recognition. It has been reported that Cry5B-producing Bacillus thuringiensis strains can infect C. elegans and that the C. elegans Cry5B receptor molecules are glycolipids. We found that Cry5B and LEC-8 bound to C. elegans glycolipid-coated plates in a dose-dependent manner and that Cry5B binding to glycolipids was inhibited by the addition of LEC-8. LEC-8 is usually expressed strongly in the pharyngeal-intestinal valve and intestinal-rectal valve and is expressed weakly in intestine. However, when C. elegans were fed Escherichia coli expressing Cry5B, intestinal LEC-8::EGFP protein levels increased markedly. In contrast, LEC-8::EGFP expression triggered by Cry5B was reduced in toxin-resistant C. elegans mutants, which had mutations in genes involved in biosynthesis of glycolipids. Moreover, the LEC-8-deficient mutant was more susceptible to Cry5B than wild-type worms. These results suggest that the glycolipid-binding lectin LEC-8 contributes to host defense against bacterial infection by competitive binding to target glycolipid molecules.

Steroidogenic acute regulatory-related lipid transfer domain protein 5 localization and regulation in renal tubules

STARD5 is a cytosolic sterol transport protein that is predominantly expressed in liver and kidney. This study provides the first report on STARD5 protein expression and distribution in mouse kidney. Immunohistochemical analysis of C57BL/6J mouse kidney sections revealed that STARD5 is expressed in tubular cells within the renal cortex and medullar regions with no detectable staining within the glomeruli. Within the epithelial cells of proximal renal tubules, STARD5 is present in the cytoplasm with high staining intensity along the apical brush-border membrane. Transmission electron microscopy of a renal proximal tubule revealed STARD5 is abundant at the basal domain of the microvilli and localizes mainly in the rough endoplasmic reticulum (ER) with undetectable staining in the Golgi apparatus and mitochondria. Confocal microscopy of STARD5 distribution in HK-2 human proximal tubule cells showed a diffuse punctuate pattern that is distinct from the early endosome marker EEA1 but similar to the ER membrane marker GRP78. Treatment of HK-2 cells with inducers of ER stress increased STARD5 mRNA expression and resulted in redistribution of STARD5 protein to the perinuclear and cell periphery regions. Since recent reports show elevated ER stress response gene expression and increased lipid levels in kidneys from diabetic rodent models, we tested STARD5 and cholesterol levels in kidneys from the OVE26 type I diabetic mouse model. Stard5 mRNA and protein levels are increased 2.8- and 1.5-fold, respectively, in OVE26 diabetic kidneys relative to FVB control kidneys. Renal free cholesterol levels are 44% elevated in the OVE26 mice. Together, our data support STARD5 functioning in kidney, specifically within proximal tubule cells, and suggest a role in ER-associated cholesterol transport.

Lipid raft organization and function in the small intestinal brush border

The enterocyte brush border of the small intestine is a highly specialized membrane designed to function both as a high capacity digestive/absorptive surface of dietary nutrients and a permeability barrier towards lumenal pathogens. It is characterized by an unusually high content of glycolipids (approximately 30% of the total microvillar membrane lipid), enabling the formation of liquid ordered microdomains, better known as lipid rafts. The glycolipid rafts are stabilized by galectin-4, a 36 kDa divalent lectin that cross-links galactosyl (and other carbohydrate) residues present on membrane lipids and several brush border proteins, including some of the major hydrolases. These supramolecular complexes are further stabilized by intelectin, a 35 kDa trimeric lectin that also functions as an intestinal lactoferrin receptor. As a result, brush border hydrolases, otherwise sensitive to pancreatic proteinases, are protected from untimely release into the gut lumen. Finally, anti-glycosyl antibodies, synthesized by plasma cells locally in the gut, are deposited on the brush border glycolipid rafts, protecting the epithelium from lumenal pathogens that exploit lipid rafts as portals for entry to the organism.

Depletion of phytosterols from the plant plasma membrane provides evidence for disruption of lipid rafts

Involvement of sterols in membrane structural properties has been extensively studied in model systems but rarely assessed in natural membranes and never investigated for the plant plasma membrane (PM). Here, we address the question of the role of phytosterols in the organization of the plant PM. The sterol composition of tobacco BY-2 cell PM was determined by gas chromatography. The cyclic oligosaccharide methyl-beta-cyclodextrin, commonly used in animal cells to decrease cholesterol levels, caused a drastic reduction (50%) in the PM total free sterol content of the plant material, without modification in amounts of steryl-conjugates. Fluorescence spectroscopy experiments using DPH, TMA-DPH, Laurdan, and di-4-ANEPPDHQ indicated that such a depletion in sterol content increased lipid acyl chain disorder and reduced the overall liquid-phase heterogeneity in correlation with the disruption of phytosterol-rich domains. Methyl-beta-cyclodextrin also prevented isolation of a PM fraction resistant to solubilization by nonionic detergents, previously characterized in tobacco, and induced redistribution of the proteic marker of this fraction, NtrbohD, within the membrane. Altogether, our results support the role of phytosterols in the lateral structuring of the PM of higher plant cells and suggest that they are key compounds for the formation of plant PM microdomains.

Ca2+ responses to thyrotropin-releasing hormone and angiotensin II: the role of plasma membrane integrity and effect of G11alpha protein overexpression on homologous and heterologous desensitization

The molecular mechanisms involved in GPCR-initiated signaling cascades where the two receptors share the same signaling cascade, such as thyrotropin-releasing hormone (TRH) and angiotensin II (ANG II), are still far from being understood. Here, we analyzed hormone-induced Ca(2+) responses and the process of desensitization in HEK-293 cells, which express endogenous ANG II receptors. These cells were transfected to express exogenously high levels of TRH receptors (clone E2) or both TRH receptors and G(11)alpha protein (clone E2M11). We observed that the characteristics of the Ca(2+) response, as well as the process of desensitization, were both strongly dependent on receptor number and G(11)alpha protein level. Whereas treatment of E2 cells with TRH or ANG II led to significant desensitization of the Ca(2+) response to subsequent addition of either hormone, the response was not desensitized in E2M11 cells expressing high levels of G(11)alpha. In addition, stimulation of both cell lines with THR elicited a clear heterologous desensitization to subsequent stimulation with ANG II. On the other hand, ANG II did not affect a subsequent response to TRH. ANG II-mediated signal transduction was strongly dependent on plasma membrane integrity modified by cholesterol depletion, but signaling through TRH receptors was altered only slightly under these conditions. It may be concluded that the level of expression of G-protein-coupled receptors and their cognate G-proteins strongly influences not only the magnitude of the Ca(2+) response but also the process of desensitization and resistance to subsequent hormone addition.

Intestinal epithelial CD98: an oligomeric and multifunctional protein

The intestinal epithelial cell-surface molecule, CD98 is a type II membrane glycoprotein. Molecular orientation studies have demonstrated that the C-terminal tail of human CD98 (hCD98), which contains a PDZ-binding domain, is extracellular. In intestinal epithelial cells, CD98 is covalently linked to an amino-acid transporter with which it forms a heterodimer. This heterodimer associates with beta(1)-integrin and intercellular adhesion molecular 1 (ICAM-1) to form a macromolecular complex in the basolateral membranes of polarized intestinal epithelial cells. This review focuses on the multifunctional roles of CD98, including involvement in extracellular signaling, adhesion/polarity, and amino-acid transporter expression in intestinal epithelia. A role for CD98 in intestinal inflammation, such as Intestinal Bowel Disease (IBD), is also proposed.

Renal Na+-K+-Cl- cotransporter activity and vasopressin-induced trafficking are lipid raft-dependent

Apical bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter (NKCC2), the kidney-specific member of a cation-chloride cotransporter superfamily, is an integral membrane protein responsible for the transepithelial reabsorption of NaCl. The role of NKCC2 is essential for renal volume regulation. Vasopressin (AVP) controls NKCC2 surface expression in cells of the thick ascending limb of the loop of Henle (TAL). We found that 40-70% of Triton X-100-insoluble NKCC2 was present in cholesterol-enriched lipid rafts (LR) in rat kidney and cultured TAL cells. The related Na(+)-Cl(-) cotransporter (NCC) from rat kidney was distributed in LR as well. NKCC2-containing LR were detected both intracellularly and in the plasma membrane. Bumetanide-sensitive transport of NKCC2 as analyzed by (86)Rb(+) influx in Xenopus laevis oocytes was markedly reduced by methyl-beta-cyclodextrin (MbetaCD)-induced cholesterol depletion. In TAL, short-term AVP application induced apical vesicular trafficking along with a shift of NKCC2 from non-raft to LR fractions. In parallel, increased colocalization of NKCC2 with the LR ganglioside GM1 and their polar translocation were assessed by confocal analysis. Apical biotinylation showed twofold increases in NKCC2 surface expression. These effects were blunted by mevalonate-lovastatin/MbetaCD-induced cholesterol deprivation. Collectively, these findings demonstrate that a pool of NKCC2 distributes in rafts. Results are consistent with a model in which LR mediate polar insertion, activity, and AVP-induced trafficking of NKCC2 in the control of transepithelial NaCl transport.

Renal Na+-K+-Cl- cotransporter activity and vasopressin-induced trafficking are lipid raft-dependent

Apical bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter (NKCC2), the kidney-specific member of a cation-chloride cotransporter superfamily, is an integral membrane protein responsible for the transepithelial reabsorption of NaCl. The role of NKCC2 is essential for renal volume regulation. Vasopressin (AVP) controls NKCC2 surface expression in cells of the thick ascending limb of the loop of Henle (TAL). We found that 40-70% of Triton X-100-insoluble NKCC2 was present in cholesterol-enriched lipid rafts (LR) in rat kidney and cultured TAL cells. The related Na(+)-Cl(-) cotransporter (NCC) from rat kidney was distributed in LR as well. NKCC2-containing LR were detected both intracellularly and in the plasma membrane. Bumetanide-sensitive transport of NKCC2 as analyzed by (86)Rb(+) influx in Xenopus laevis oocytes was markedly reduced by methyl-beta-cyclodextrin (MbetaCD)-induced cholesterol depletion. In TAL, short-term AVP application induced apical vesicular trafficking along with a shift of NKCC2 from non-raft to LR fractions. In parallel, increased colocalization of NKCC2 with the LR ganglioside GM1 and their polar translocation were assessed by confocal analysis. Apical biotinylation showed twofold increases in NKCC2 surface expression. These effects were blunted by mevalonate-lovastatin/MbetaCD-induced cholesterol deprivation. Collectively, these findings demonstrate that a pool of NKCC2 distributes in rafts. Results are consistent with a model in which LR mediate polar insertion, activity, and AVP-induced trafficking of NKCC2 in the control of transepithelial NaCl transport.

Cellular cross talk in the small intestinal mucosa: postnatal lymphocytic immigration elicits a specific epithelial transcriptional response

During the early postnatal period lymphocytes migrate into the mouse small intestine. Migrating infiltrative lymphocytes have the potential to affect the epithelial cells via secreted cytokines. Such cross talk can result in the elicitation of an epithelial transcriptional response. Knowledge about such physiological cross talk between the immune system and the epithelium in the postnatal small intestinal mucosa is lacking. We have investigated the transcriptome changes occurring in the postnatal mouse small intestine using DNA microarray technology, immunocytochemistry, and quantitative real-time RT-PCR analysis. The DNA microarray data were analyzed bioinformatically by using a combination of projections to latent structures analysis and functional annotation analysis. The results show that infiltrating lymphocytes appear in the mouse small intestine in the late postweaning period and give rise to distinct changes in the epithelial transcriptome. Of particular interest is the expression of three genes encoding a mucin (Muc4), a mucinlike protein (16000D21Rik), and ATP citrate lyase (Acly). All three genes were shown to be expressed by the epithelium and to be upregulated in response to lymphocytic migration into the small intestinal mucosa.

Protein kinase C inhibits caveolae-mediated endocytosis of TRPV5

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

Leptin and the obesity receptor (OB-R) in the small intestine and colon: a colocalization study

Leptin is a hormone that plays an important role in overall body energy homeostasis, and the obesity receptor, OB-R, is widely distributed in the organism. In the intestine, a multitude of leptin actions have been reported, but it is currently unclear to what extent the hormone affects the intestinal epithelial cells by an endocrine or exocrine signaling pathway. To elucidate this, the localization of endogenous porcine leptin and OB-R in enterocytes and colonocytes was studied. By immunofluorescence microscopy, both leptin and OB-R were mainly observed in the basolateral membrane of enterocytes and colonocytes but also in the apical microvillar membrane of the cells. By electron microscopy, coclustering of hormone and receptor in the plasma membrane and localization in endosomes was frequently detected at the basolateral surface of the epithelial cells, indicative of leptin signaling activity. In contrast, coclustering occurred less frequently at the apical cell surface, and subapical endosomal localization was hardly detectable. We conclude that leptin action in intestinal epithelial cells takes place at the basolateral plasma membrane, indicating that the hormone uses an endocrine pathway both in the jejunum and colon. In contrast, the data obtained did not provide evidence for an exocrine, lumenal action of the hormone in the intestine.

Prominin-2 is a cholesterol-binding protein associated with apical and basolateral plasmalemmal protrusions in polarized epithelial cells and released into urine

Prominin-2 is a pentaspan membrane glycoprotein structurally related to the cholesterol-binding protein prominin-1, which is expressed in epithelial and non-epithelial cells. Although prominin-1 expression is widespread throughout the organism, the loss of its function solely causes retinal degeneration. The finding that prominin-2 appears to be restricted to epithelial cells, such as those found in kidney tubules, raises the possibility that prominin-2 functionally substitutes prominin-1 in tissues other than the retina and provokes a search for a definition of its morphological and biochemical characteristics. Here, we have investigated, by using MDCK cells as an epithelial cell model, whether prominin-2 shares the biochemical and morphological properties of prominin-1. Interestingly, we have found that, whereas prominin-2 is not restricted to the apical domain like prominin-1 but is distributed in a non-polarized fashion between the apical and basolateral plasma membranes, it retains the main feature of prominin-1, i.e. its selective concentration in plasmalemmal protrusions; prominin-2 is confined to microvilli, cilia and other acetylated tubulin-positive protruding structures. Similar to prominin-1, prominin-2 is partly associated with detergent-resistant membranes in a cholesterol-dependent manner, suggesting its incorporation into membrane microdomains, and binds directly to plasma membrane cholesterol. Finally, prominin-2 is also associated with small membrane particles that are released into the culture media and found in a physiological fluid, i.e. urine. Together, these data show that all the characteristics of prominin-1 are shared by prominin-2, which is in agreement with a possible redundancy in their role as potential organizers of plasma membrane protrusions.

Cyclodextrin-induced apoptosis in human keratinocytes is caspase-8 dependent and accompanied by mitochondrial cytochromecrelease

Cyclodextrins (CDs) are cyclic oligosaccharides that are able to form inclusion complexes with a variety of substances. For pharmaceutical applications, CD-based drug formulations offer important advantages compared with uncomplexed drugs. These include improved water solubility of lipophilic drug molecules, increased chemical stability, as well as enhanced bioavailability and absorption rate. Also, a number of topical formulations for dermal and transdermal drug delivery contain CDs. However, the most frequently used CDs - beta-CD and MbetaCD - are known to extract cholesterol from plasma membranes and thus to cause cellular damage and cell death. In the present study, the influence of various CDs and CD derivatives on the human keratinocyte cell line HaCaT was assessed. We found that beta-CD and MbetaCD induce apoptosis via the activator caspase-8, which subsequently activates the effector caspases-3/-7. Furthermore, beta-CD-induced apoptosis is accompanied by mitochondrial cytochrome c release. A significant shift from mitochondria into the cytosol was found. These findings may provide further rationale to the use of CDs in topical formulations for dermal and transdermal drug delivery or as raw material in order to functionalize textiles for medical applications.

The Bacteroides fragilis toxin binds to a specific intestinal epithelial cell receptor

The Bacteroides fragilis toxin (BFT) is the only known virulence factor of enterotoxigenic B. fragilis. BFT has previously been shown to act, at least in part, through cleavage of the intercellular adhesion protein E-cadherin. A specific cellular receptor for BFT has not been identified. The goal of this study was to determine if the initial interaction of BFT with intestinal epithelial cells was consistent with binding to a specific cellular receptor. Purified BFT was labeled with a fluorophore or iodide to assess specific cellular binding and the properties of BFT cellular binding. BFT binds specifically to intestinal epithelial cell lines in vitro in a polarized manner. However, specific binding occurs only at 37 degrees C and requires BFT metalloprotease activity. The BFT receptor is predicted to be a membrane protein other than E-cadherin or a known protease-activated receptor (PAR1 to PAR4). BFT binding is resistant to acid washing, suggesting an irreversible interaction. Sugar or lipid residues do not appear to be involved in the mechanism of BFT cellular binding, but binding is sensitive to membrane cholesterol depletion. We conclude that intestinal epithelial cells in vitro possess a specific membrane BFT receptor that is distinct from E-cadherin. The data favor a model in which the metalloprotease domain of BFT processes its receptor protein, initiating cellular signal transduction that mediates the biological activity of BFT. However, activation of recognized protease-activated receptors does not mimic or block BFT biological activity or binding, suggesting that additional protease-activated receptors on intestinal epithelial cells remain to be identified.

Antibodies in the small intestine: mucosal synthesis and deposition of anti-glycosyl IgA, IgM, and IgG in the enterocyte brush border

Synthesis and deposition of immunoglobulins in the brush border was studied in organ-cultured pig small intestinal mucosal explants. Surprisingly, comparable amounts of IgM and IgA were synthesized during a 6-h pulse, and also newly made IgG was detected in media and explants, including the microvillar fraction. For IgA and IgM, this subcellular distribution is consistent with basolateral-to-apical transcytosis, mediated by the polymeric immunoglobulin receptor. IgG is a ligand for the Fc receptor FcRn, and beta2-microglobulin, the light chain of FcRn, coclustered in immunogold double labeling with IgG in subapical endosomes and in the basolateral membrane of enterocytes. In addition, beta2-microglobulin was copurified with IgG on protein G-Sepharose. Apical endocytosis of IgG, as judged by internalization of fluorescent protein G, was not detectable except in a few isolated cells. This suggests that IgG in the adult small intestine is transported across the enterocyte mainly in the basolateral to apical direction. Significant fractions of all immunoglobulins bound to lactoseagarose, indicating that "anti-glycosyl" antibodies, raised against commensal gut bacteria, are synthesized locally in the small intestine. By partial deposition in the brush border, these antibodies therefore may have a protective function by preventing lectin-like pathogens from gaining access to the brush border surface.

Lipid raft organization and function in brush borders of epithelial cells

Polarized epithelial cells of multicellular organisms confront the environment with a highly specialized apical cell membrane that differs in composition and function from that facing the internal milieu. In the case of absorptive cells, such as the small intestinal enterocyte and the kidney proximal tubule cell, the apical cell membrane is formed as a brush border, composed of regular, dense arrays of microvilli. Hydrolytic ectoenzymes make up the bulk of the microvillar membrane proteins, endowing the brush border with a huge digestive capacity. Several of the major enzymes are localized in lipid rafts, which, for the enterocyte in particular, are organized in a unique fashion. Glycolipids, rather than cholesterol, together with the divalent lectin galectin-4, define these rafts, which are stable and probably quite large. The architecture of these rafts supports a digestive/absorptive strategy for nutrient assimilation, but also serves as a portal for a large number of pathogens. Caveolae are well-known vehicles for internalization of lipid rafts, but in the enterocyte brush border, binding of cholera toxin is followed by uptake via a clathrin-dependent mechanism. Recently, 'anti-glycosyl' antibodies were shown to be deposited in the enterocyte brush border. When the antibodies were removed from the membrane, other carbohydrate-binding proteins, including cholera toxin, increased their binding to the brush border. Thus, anti-glycosyl antibodies may serve as guardians of glycolipid-based rafts, protecting them from lumenal pathogens and in this way be part of an ongoing 'cross-talk' between indigenous bacteria and the host.

Proteomic characterization of lipid rafts markers from the rat intestinal brush border

To assess intestinal lipid rafts functions through the characterization of their protein markers, we have isolated lipid rafts of rat mucosa either from the total membrane or purified brush-border membrane (BBM) by sucrose gradient fractionation after detergent treatment. In both membrane preparations, the floating fractions (4-5) were enriched in cholesterol, ganglioside GM1, and N aminopeptidase (NAP) known as intestinal lipid rafts markers. Based on MALDI-TOF/MS identification and simultaneous detection by immunoblotting, 12 proteins from BBM cleared from contaminants were selected as rafts markers. These proteins include several signaling/trafficking proteins belonging to the G protein family and the annexins as well as GPI-anchored proteins. Remarkably GP2, previously described as the pancreatic granule GPI-anchored protein, was found in intestinal lipid rafts. The proteomic strategy assayed on the intestine leads to the characterization of known (NAP, alkaline phosphatase, dipeptidyl aminopeptidase, annexin II, and galectin-4) and new (GP2, annexin IV, XIIIb, Galpha(q), Galpha(11), glutamate receptor, and GPCR 7) lipid rafts markers. Together our results indicate that some digestive enzymes, trafficking and signaling proteins may be functionally distributed in the intestine lipid rafts.

Rotavirus assembly: an alternative model that utilizes an atypical trafficking pathway

We review here recent advances in our knowledge on trafficking and assembly of rotavirus and rotaviral proteins in intestinal cells. Assembly of rotavirus has been extensively studied in nonpolarized kidney epithelial MA104 cells, where several data indicate that most if not all the steps of rotavirus assembly take place within the endoplasmic reticulum (ER) and that rotavirus is release upon cell lysis. We focus here on data obtained in intestinal cells that argue for another scheme of rotavirus assembly, where the final steps seem to take place outside the ER with an apically polarized release of rotavirus without significant cell lysis. One of the key observations made by different groups is that VP4 and other structural proteins interact substantially with specialized membrane microdomains enriched in cholesterol and sphingolipids termed rafts. In addition, recent data point to the fact that VP4 does not localize within the ER or the Golgi apparatus in infected intestinal cells. The mechanisms by which VP4, a cytosolic protein, may be targeted to the apical membrane in these cells and assembles with the other structural proteins are discussed. The identification of cellular proteins such as Hsp70, flotillin, rab5, PRA1 and cytoskeletal components that interact with VP4 may help to define an atypical polarized trafficking pathway to the apical membrane of intestinal cells that will be raft-dependent and by-pass the classical exocytic route.

Anti-glycosyl antibodies in lipid rafts of the enterocyte brush border: a possible host defense against pathogens

The pig small intestinal brush border is a glycoprotein- and glycolipid-rich membrane that functions as a digestive/absorptive surface for dietary nutrients as well as a permeability barrier for pathogens. The present work was performed to identify carbohydrate-binding (lectinlike) proteins associated with the brush border. Chromatography on lactose-agarose was used to isolate such proteins, and their localization was studied biochemically and by immunofluorescence microscopy and immunogold electron microscopy. IgG and IgM were the two major proteins isolated, indicating that naturally occurring anti-glycosyl antibodies are among the major lectinlike proteins in the gut. IgG and IgM as well as IgA were localized to the enterocyte brush border, and a brief lactose wash partially released all three immunoglobulins from the membrane, indicating that anti-glycosyl antibodies constitute a major part of the immunoglobulins at the lumenal surface of the gut. The antibodies were associated with lipid rafts at the brush border, and they frequently (52%) coclustered with the raft marker galectin 4. A lactose wash increased the susceptibility of the brush border toward lectin peanut agglutin and cholera toxin B, suggesting that anti-glycosyl antibodies compete with other carbohydrate-binding proteins at the lumenal surface of the gut. Thus anti-glycosyl antibodies constitute a major group of proteins associated with the enterocyte brush border membrane. We propose they function by protecting the lipid raft microdomains of the brush border against pathogens.

Specific binding of Autographa californica M nucleopolyhedrovirus occlusion-derived virus to midgut cells of Heliothis virescens larvae is mediated by products of pif genes Ac119 and Ac022 but not by Ac115

Per os infectivity factors PIF1 (Ac119) and PIF2 (Ac022), like P74, are essential for oral infection of lepidopteran larval hosts of Autographa californica M nucleopolyhedrovirus (AcMNPV). Here we show that Ac115 also is a PIF (PIF3) and that, unlike PIF1 and PIF2, it does not mediate specific binding of AcMNPV occlusion-derived virus (ODV) to midgut target cells. We used an improved in vivo fluorescence dequenching assay to compare binding, fusion, and competition among control AcMNPV ODV and the ODVs of AcMNPV PIF1, PIF2, and PIF3 deletion mutants. Our results showed that binding and fusion of PIF1 and PIF2 mutants, but not the PIF3 mutant, were both qualitatively and quantitatively different from those of control ODV. Unlike control and PIF3-deficient ODV, an excess of PIF1- or PIF2-deficient ODV failed to compete effectively with control ODV's binding to specific receptors on midgut epithelial cells. Moreover, the levels of PIF1- and PIF2-deficient ODV binding were depressed threefold compared to control levels. Binding, fusion, and competition by PIF3-deficient ODV, however, were all indistinguishable from those of control ODV. These results implicated PIF1 and PIF2 as ODV envelope attachment proteins that mediate specific binding to primary target cells within the midgut. In contrast, PIF3 mediates another unidentified, but critical, early event during primary infection.

Complex N-glycans are the major ligands for galectin-1, -3, and -8 on Chinese hamster ovary cells

Galectins are implicated in a large variety of biological functions, many of which depend on their carbohydrate-binding ability. Fifteen members of the family have been identified in vertebrates based on binding to galactose (Gal) that is mediated by one or two, evolutionarily conserved, carbohydrate-recognition domains (CRDs). Variations in glycan structures expressed on glycoconjugates at the cell surface may, therefore, affect galectin binding and functions. To identify roles for different glycans in the binding of the three types of mammalian galectins to cells, we performed fluorescence cytometry at 4 degrees C with recombinant rat galectin-1, human galectin-3, and three forms of human galectin-8, to Chinese hamster ovary (CHO) cells and 12 different CHO glycosylation mutants. All galectin species bound to parent CHO cells and binding was inhibited >90% by 0.2 M lactose. Galectin-8 isoforms with either a long or a short inter-CRD linker bound similarly to CHO cells. However, a truncated form of galectin-8 containing only the N-terminal CRD bound only weakly to CHO cells and the C-terminal galectin-8 CRD exhibited extremely low binding. Binding of the galectins to the different CHO glycosylation mutants revealed that complex N-glycans are the major ligands for each galectin except the N-terminal CRD of galectins-8, and also identified some fine differences in glycan recognition. Interestingly, increased binding of galectin-1 at 4 degrees C correlated with increased propidium iodide (PI) uptake, whereas galectin-3 or -8 binding did not induce permeability to PI. The CHO glycosylation mutants with various repertoires of cell surface glycans are a useful tool for investigating galectin-cell interactions as they present complex and simple glycans in a natural mixture of multivalent protein and lipid glycoconjugates anchored in a cell membrane.

The increase in mucin exocytosis and the upregulation of MUC genes encoding for membrane-bound mucins induced by the thiol-activated exotoxin listeriolysin O is a host cell defence response that inhibits the cell-entry of Listeria monocytogenes

In vivo Listeria monocytogenes infection results in the massive release of mucus by goblet cells into the lumen of the intestine. We have previously reported that apical infection by L. monocytogenes is followed by listeriolysin O (LLO)-dependent stimulation of mucus exocytosis, and the upregulation of the MUC genes. Here, we report that L. monocytogenes EGD wild-type bacteria enter cultured human polarized, mucin-secreting, HT29-MTX cells apically by an InlA-dependent mechanism. The LLO-induced increase in mucin secretion together with an increase in transcription of the MCU4 and MUC12 genes encoding for membrane-bound mucins, results in the inhibition of the cell-entry of L. monocytogenes into mucin-secreting, HT29-MTX cells. Moreover, we report that sialic acid residues in mucins are crucial for the inhibition of L. monocytogenes internalization. Based on these findings, we suggest that the LLO-induced mucin exocytosis and upregulation of the MUC genes encoding for membrane-bound mucins constitute a host cell defence response that inhibits the cell-entry of L. monocytogenes.

Depletion of Caco-2 cell cholesterol disrupts barrier function by altering the detergent solubility and distribution of specific tight-junction proteins

In the present study, we have investigated the role of cholesterol in maintaining the barrier properties of the model intestinal cell line Caco-2. We have extracted membrane cholesterol using methyl-beta-cyclodextrin and demonstrated that maximally, methyl-beta-cyclodextrin lowered cell cholesterol levels by 40-45%. Depletion of cell cholesterol was accompanied by an 80-90% decrease in monolayer transepithelial electrical resistance and a significant increase in the paracellular permeability of dextrans of 4, 10 and 40 kDa. The increase in dextran permeability was most pronounced for the two lower molecular mass species. In addition to the decline in the barrier properties of the monolayers, extraction of cell cholesterol produced an increase in the Triton X-100 solubility of claudin 3, claudin 4 and occludin, and the loss of all three proteins from the plasma membrane (tight junctions). In contrast, removal of cholesterol had no detectable influence on the detergent solubility or morphological distribution of claudin 1. These results indicate that membrane cholesterol is a critical factor in maintaining the barrier property of epithelial monolayers. More specifically, cholesterol appears to stabilize the association of certain proteins with the tight junctions.

Role of membrane microdomains in PTH-mediated down-regulation of NaPi-IIa in opossum kidney cells

BACKGROUND

Parathyroid hormone (PTH) rapidly down-regulates type IIa sodium-dependent phosphate transporter (NaPi-IIa) via an endocytic pathway. Since the relationship between PTH signaling and NaPi-IIa endocytosis has not been explored, we investigated the role of membrane microdomains in this process.

METHODS

We examined the submembrane localization of NaPi-IIa in opossum kidney (OK-N2) cells that stably expressed human NaPi-IIa, and searched for a PTH-induced specific phosphorylating substrate on their membrane microdomains by immunoblotting with specific antibody against phospho substrates of protein kinases.

RESULTS

We found that NaPi-IIa was primarily localized in low-density membrane (LDM) domains of the plasma membrane; PTH reduced the levels of immunoreactive NaPi-IIa in these domains. Furthermore, PTH activated both protein kinase A (PKA) and protein kinase Calpha (PKCa) and increased the phosphorylation of 250 kD and 80 kD substrates; this latter substrate was identified as ezrin, which a member of the ezrin-radixin-moesin (ERM) protein family. In response to PTH, ezrin was phosphorylated by both PKA and PKC. Dominant negative ezrin blocked the reduction in NaPi-IIa expression in the LDM domains that was induced by PTH.

CONCLUSION

These data suggest that NaPi-IIa and PTH-induced phosphorylated proteins that include ezrin are compartmentalized in LDM microdomains. This compartmentalization may play an important role in the down-regulation of NaPi-IIa via endocytosis.

Lipid raft localization of GABAA receptor and Na+, K+-ATPase in discrete microdomain clusters in rat cerebellar granule cells

The microdomain localization of the GABA(A) receptor in rat cerebellar granule cells was studied by subcellular fractionation and fluorescence- and immunogold electron microscopy. The receptor resided in lipid rafts, prepared at 37 degrees C by extraction with the nonionic detergent Brij 98, but the raft fraction, defined by the marker ganglioside GM(1) in the floating fractions following density gradient centrifugation, was heterogeneous in density and protein composition. Thus, another major raft-associated membrane protein, the Na(+), K(+)-ATPase, was found in discrete rafts of lower density, reflecting clustering of the two proteins in separate membrane microdomains. Both proteins were observed in patchy "hot spots" at the cell surface as well as in isolated lipid rafts. Their insolubility in Brij 98 was only marginally affected by methyl-beta-cyclodextrin. In contrast, both the GABA(A) receptor and Na(+), K(+)-ATPase were largely soluble in ice cold Triton X-100. This indicates that Brij 98 extraction defines an unusual type of cholesterol-independent lipid rafts that harbour membrane proteins also associated with underlying scaffolding/cytoskeletal proteins such as gephyrin (GABA(A) receptor) and ankyrin G (Na(+), K(+)-ATPase). By providing an ordered membrane microenvironment, lipid rafts may contribute to the clustering of the GABA(A) receptor and the Na(+), K(+)-ATPase at distinct functional locations on the cell surface.

Possible role of deep tubular invaginations of the plasma membrane in MHC-I trafficking

Tubules and vesicles are membrane carriers involved in traffic along the endocytic and secretory routes. The small GTPase Arf6 regulates a recycling branch of short dynamic tubular intermediates used by major histocompatibility class I (MHC-I) molecules to traffic through vesicles between endosomes and the plasma membrane. We observed that Arf6 also affects a second network of very long and stable tubules containing MHC-I, many of which correspond to deep invaginations of the plasma membrane. Treatment with wortmannin, an inhibitor of phosphatidylinositol-3-phosphate kinase, prevents formation of the short dynamic tubules while increasing the number of the long and very stable ones. Expression of NefAAAA, a mutant form of HIV Nef, increases the number of cells containing the stable tubules, and is used here as a tool to facilitate their study. Photoactivation of NefAAAA-PA-GFP demonstrates that this molecule traffics from endosomes to the tubules. Finally, live-cell imaging also shows internalization of MHC-I molecules into these tubules, suggesting that this is an additional route for MHC-I traffic.

Domain-specific lipid distribution in macrophage plasma membranes

Lipid rafts, defined as cholesterol- and sphingolipid-rich domains, provide specialized lipid environments understood to regulate the organization and function of many plasma membrane proteins. Growing evidence of their existence, protein cargo, and regulation is based largely on the study of isolated lipid rafts; however, the consistency and validity of common isolation methods is controversial. Here, we provide a detailed and direct comparison of the lipid and protein composition of plasma membrane "rafts" prepared from human macrophages by different methods, including several detergent-based isolations and a detergent-free method. We find that detergent-based and detergent-free methods can generate raft fractions with similar lipid contents and a biophysical structure close to that previously found on living cells, even in cells not expressing caveolin-1, such as primary human macrophages. However, important differences between isolation methods are demonstrated. Triton X-100-resistant rafts are less sensitive to cholesterol or sphingomyelin depletion than those prepared by detergent-free methods. Moreover, we show that detergent-based methods can scramble membrane lipids during the isolation process, reorganizing lipids previously in sonication-derived nonraft domains to generate new detergent-resistant rafts. The role of rafts in regulating the biological activities of macrophage plasma membrane proteins may require careful reevaluation using multiple isolation procedures, analyses of lipids, and microscopic techniques.

Interferon gamma induces translocation of commensal Escherichia coli across gut epithelial cells via a lipid raft-mediated process

BACKGROUND & AIMS

The "leaky gut" hypothesis proposes that leakage of enteric bacteria into the body resulting from disruption of the epithelial barrier is a critical step in the pathophysiology of various disorders such as inflammatory bowel disease and sepsis. However, the pathways and underlying mechanisms by which commensal bacteria cross the epithelial barrier in inflammatory conditions remain unclear. This study investigated the mechanisms of interferon gamma-mediated bacterial translocation across human colonic epithelial monolayers.

METHODS

Caco-2 and T84 monolayers were exposed to interferon gamma. Barrier function was assessed by transepithelial electrical resistance and lucifer yellow permeability. Internalization and translocation of Escherichia coli strain C25 were measured by quantitative bacterial culture. Expression and distribution of junctional proteins were assessed by immunoblotting and confocal imaging.

RESULTS

Minimal apical to basolateral translocation of C25 was observed in untreated T84 and Caco-2 monolayers. Interferon gamma caused a dramatic, dose-dependent increase in C25 translocation, which was uncoupled from cytokine-induced increases in paracellular permeability and disruption of tight junction proteins at low interferon gamma concentrations. These effects were associated with increased internalization of viable bacteria into, but not adherence to, Caco-2 cells. Interferon gamma-mediated bacterial translocation was abolished by pretreatment with the cholesterol-disrupting drugs filipin and methyl-beta-cyclodextrin, whereas these agents had no effect on infection of Caco-2 by the enteric pathogen Shigella sonnei.

CONCLUSIONS

Normally poorly invasive enteric bacteria may, in situations of inflammatory stress, exploit lipid raft-mediated transcytotic pathways to cross the intestinal epithelium, and these effects may precede cytokine-induced disruption of tight junctions.

Myosin-1a is critical for normal brush border structure and composition

To develop our understanding of myosin-1a function in vivo, we have created a mouse line null for the myosin-1a gene. Myosin-1a knockout mice demonstrate no overt phenotypes at the whole animal level but exhibit significant perturbations and signs of stress at the cellular level. Among these are defects in microvillar membrane morphology, distinct changes in brush-border organization, loss of numerous cytoskeletal and membrane components from the brush border, and redistribution of intermediate filament proteins into the brush border. We also observed significant ectopic recruitment of another short-tailed class I motor, myosin-1c, into the brush border of knockout enterocytes. This latter finding, a clear demonstration of functional redundancy among vertebrate myosins-I, may account for the lack of a whole animal phenotype. Nevertheless, these results indicate that myosin-1a is a critical multifunctional component of the enterocyte, required for maintaining the normal composition and highly ordered structure of the brush border.

Testisin, a Glycosyl-Phosphatidylinositol–Linked Serine Protease, Promotes Malignant Transformation In vitro and In vivo

Human testisin, a serine protease, is highly expressed in ovarian cancer and premeiotic spermatocytes with relatively little expression in other normal tissues. We first showed that testisin was localized on the surface of cultured tumor cells as a glycosyl-phosphatidylinositol–linked protein. We next explored the biological function of testisin in malignant transformation through manipulation of testisin expression in cell culture model systems. Small interfering RNA–mediated knockdown of endogenous testisin mRNA and protein expression in tumor cell lines led to increased apoptosis and diminished growth in soft agar. Conversely, overexpression of testisin in an epithelial cell line induced colony formation in soft agar as well as s.c. tumor growth in severe combined immunodeficient mice. A catalytic domain mutant was unable to induce soft-agar growth indicating that testisin protease activity is required for transformation. Ectopic expression of testisin in a human ovarian cancer cell line without endogenous testisin expression, led to the formation of larger tumors in severe combined immunodeficient mice. Data presented here provide the first demonstration that testisin can promote cellular processes that drive malignant transformation. Our functional data coupled with the restricted normal tissue distribution of testisin and its overexpression in a majority of ovarian cancers validates this cell surface protein as a target for therapeutic intervention.

Membrane cholesterol content modulates activation of BK channels in colonic epithelia

Changes in the level of membrane cholesterol regulate a variety of signaling processes including those mediated by acylated signaling molecules that localize to lipid rafts. Recently several types of ion channels have been shown to have cholesterol-dependent activity and to localize to lipid rafts. In this study, we have investigated the role of cholesterol in the regulation of ion transport in colonic epithelial cells. We observed that methyl-beta-cyclodextrin (MbetaCD), a cholesterol-sequestering molecule, activated transepithelial short circuit current (Isc), but only from the basolateral side. Similar results were obtained with a cholesterol-binding agent, filipin, and with the sphingomyelin-degrading enzyme, sphingomyelinase. Experiments with DeltaF508CFTR mutant mice indicated that raft disruption affected CFTR-mediated anion secretion, while pharmacological studies showed that this effect was due to activation of basolateral large conductance Ca2+-activated K+ (BK) channels. Sucrose density gradient centrifugation studies demonstrated that BK channels were normally present in the high-density fraction containing the detergent-insoluble cytoskeleton, and that following treatment with MbetaCD, BK channels redistributed into detergent-soluble fractions. Our evidence therefore implicates novel high-density cholesterol-enriched plasma membrane microdomains in the modulation of BK channel activation and anion secretion in colonic epithelia.

Enterotoxigenic Escherichia coli vesicles target toxin delivery into mammalian cells

Enterotoxigenic Escherichia coli (ETEC) is a prevalent cause of traveler's diarrhea and infant mortality in third-world countries. Heat-labile enterotoxin (LT) is secreted from ETEC via vesicles composed of outer membrane and periplasm. We investigated the role of ETEC vesicles in pathogenesis by analyzing vesicle association and entry into eukaryotic cells. Fluorescently labeled vesicles from LT-producing and LT-nonproducing strains were compared in their ability to bind adrenal and intestinal epithelial cells. ETEC-derived vesicles, but not control nonpathogen-derived vesicles, associated with cells in a time-, temperature-, and receptor-dependent manner. Vesicles were visualized on the cell surface at 4 degrees C and detected intracellularly at 37 degrees C. ETEC vesicle endocytosis depended on cholesterol-rich lipid rafts. Entering vesicles partially colocalized with caveolin, and the internalized vesicles accumulated in a nonacidified compartment. We conclude that ETEC vesicles serve as specifically targeted transport vehicles that mediate entry of active enterotoxin and other bacterial envelope components into host cells. These data demonstrate a role in virulence for ETEC vesicles.

Aminopeptidase N (CD13) is a molecular target of the cholesterol absorption inhibitor ezetimibe in the enterocyte brush border membrane

Intestinal cholesterol absorption is an important regulator of serum cholesterol levels. Ezetimibe is a specific inhibitor of intestinal cholesterol absorption recently introduced into medical practice; its mechanism of action, however, is still unknown. Ezetimibe neither influences the release of cholesterol from mixed micelles in the gut lumen nor the transfer of cholesterol to the enterocyte brush border membrane. With membrane-impermeable Ezetimibe analogues we could demonstrate that binding of cholesterol absorption inhibitors to the brush border membrane of small intestinal enterocytes from the gut lumen is sufficient for inhibition of cholesterol absorption. A 145-kDa integral membrane protein was identified as the molecular target for cholesterol absorption inhibitors in the enterocyte brush border membrane by photoaffinity labeling with photoreactive Ezetimibe analogues (Kramer, W., Glombik, H., Petry, S., Heuer, H., Schafer, H. L., Wendler, W., Corsiero, D., Girbig, F., and Weyland, C. (2000) FEBS Lett. 487, 293-297). The 145-kDa Ezetimibe-binding protein was purified by three different methods and sequencing revealed its identity with the membrane-bound ectoenzyme aminopeptidase N ((alanyl)aminopeptidase; EC 3.4.11.2; APN; leukemia antigen CD13). The enzymatic activity of APN was not influenced by Ezetimibe (analogues). The uptake of cholesterol delivered by mixed micelles by confluent CaCo-2 cells was partially inhibited by Ezetimibe and nonabsorbable Ezetimibe analogues. Preincubation of confluent CaCo-2 cells with Ezetimibe led to a strong decrease of fluorescent APN staining with a monoclonal antibody in the plasma membrane. Independent on its enzymatic activity, aminopeptidase N is involved in endocytotic processes like the uptake of viruses. Our findings suggest that binding of Ezetimibe to APN from the lumen of the small intestine blocks endocytosis of cholesterol-rich membrane microdomains, thereby limiting intestinal cholesterol absorption.

A role for myosin-1A in the localization of a brush border disaccharidase

To gain insight regarding myosin-1A (M1A) function, we expressed a dominant negative fragment of this motor in the intestinal epithelial cell line, CACO-2_BBE. Sucrase isomaltase (SI), a transmembrane disaccharidase found in microvillar lipid rafts, was missing from the brush border (BB) in cells expressing this fragment. Density gradient centrifugation, affinity purification, and immunopurification of detergent-resistant membranes isolated from CACO-2_BBE cells and rat microvilli (MV) all indicate that M1A and SI reside on the same population of low density (∼1.12 g/ml) membranes. Chemical cross-linking of detergent-resistant membranes from rat MV indicates that SI and M1A may interact in a lipid raft complex. The functional significance of such a complex is highlighted by expression of the cytoplasmic domain of SI, which results in lower levels of M1A and a loss of SI from the BB. Together, these studies are the first to assign a specific role to M1A and suggest that this motor is involved in the retention of SI within the BB.