Sorting of endogenous plasma membrane proteins occurs from two sites in cultured human intestinal epithelial cells (Caco-2)

Low Rho activity in hepatocytes prevents apical from basolateral cargo separation during trans-Golgi network to surface transport

Hepatocytes, the main epithelial cells of the liver, organize their polarized membrane domains differently from ductal epithelia. They also differ in their biosynthetic delivery of single-membrane-spanning and glycophosphatidylinositol-anchored proteins to the apical domain. While ductal epithelia target apical proteins to varying degrees from the trans-Golgi network (TGN) to the apical surface directly, hepatocytes target them first to the basolateral domain, from where they undergo basolateral-to-apical transcytosis. How TGN-to-surface transport differs in both scenarios is unknown. Here, we report that the basolateral detour of a hepatocyte apical protein is due, in part, to low RhoA activity at the TGN, which prevents its segregation from basolateral transport carriers. Activating Rho in hepatocytic cells, which switches their polarity from hepatocytic to ductal, also led to apical-basolateral cargo segregation at the TGN as is typical for ductal cells, affirming a central role for Rho-signaling in different aspects of the hepatocytic polarity phenotype. Nevertheless, Rho-induced cargo segregation was not sufficient to target the apical protein directly; thus, failure to recruit apical targeting machinery also contributes to its indirect itinerary.

Transcriptional regulation of cell shape during organ morphogenesis

The emerging field of transcriptional regulation of cell shape changes aims to address the critical question of how gene expression programs produce a change in cell shape. Together with cell growth, division, and death, changes in cell shape are essential for organ morphogenesis. Whereas most studies of cell shape focus on posttranslational events involved in protein organization and distribution, cell shape changes can be genetically programmed. This review highlights the essential role of transcriptional regulation of cell shape during morphogenesis of the heart, lungs, gastrointestinal tract, and kidneys. We emphasize the evolutionary conservation of these processes across different model organisms and discuss perspectives on open questions and research avenues that may provide mechanistic insights toward understanding birth defects.

Intestinal epithelial cell polarity defects in disease: lessons from microvillus inclusion disease

The intestinal epithelium is a highly organized tissue. The establishment of epithelial cell polarity, with distinct apical and basolateral plasma membrane domains, is pivotal for both barrier formation and for the uptake and vectorial transport of nutrients. The establishment of cell polarity requires a specialized subcellular machinery to transport and recycle proteins to their appropriate location. In order to understand and treat polarity-associated diseases, it is necessary to understand epithelial cell-specific trafficking mechanisms. In this Review, we focus on cell polarity in the adult mammalian intestine. We discuss how intestinal epithelial polarity is established and maintained, and how disturbances in the trafficking machinery can lead to a polarity-associated disorder, microvillus inclusion disease (MVID). Furthermore, we discuss the recent developments in studying MVID, including the creation of genetically manipulated cell lines, mouse models and intestinal organoids, and their uses in basic and applied research.

Summary: Microvillus inclusion disease serves as a useful model to enhance our understanding of the intestinal trafficking and polarity machinery in health and disease.

An apical MRCK-driven morphogenetic pathway controls epithelial polarity

Polarized epithelia develop distinct cell surface domains, with the apical membrane acquiring characteristic morphological features such as microvilli. Cell polarization is driven by polarity determinants including the evolutionarily conserved partitioning-defective (PAR) proteins that are separated into distinct cortical domains. PAR protein segregation is thought to be a consequence of asymmetric actomyosin contractions. The mechanism of activation of apically polarized actomyosin contractility is unknown. Here we show that the Cdc42 effector MRCK activates myosin-II at the apical pole to segregate aPKC-Par6 from junctional Par3, defining the apical domain. Apically polarized MRCK-activated actomyosin contractility is reinforced by cooperation with aPKC-Par6 downregulating antagonistic RhoA-driven junctional actomyosin contractility, and drives polarization of cytosolic brush border determinants and apical morphogenesis. MRCK-activated polarized actomyosin contractility is required for apical differentiation and morphogenesis in vertebrate epithelia and Drosophila photoreceptors. Our results identify an apical origin of actomyosin-driven morphogenesis that couples cytoskeletal reorganization to PAR polarity signalling.

Myosin Vb mediates Cu+ export in polarized hepatocytes

The cellular machinery responsible for Cu(+)-stimulated delivery of the Wilson-disease-associated protein ATP7B to the apical domain of hepatocytes is poorly understood. We demonstrate that myosin Vb regulates the Cu(+)-stimulated delivery of ATP7B to the apical domain of polarized hepatic cells, and that disruption of the ATP7B-myosin Vb interaction reduces the apical surface expression of ATP7B. Overexpression of the myosin Vb tail, which competes for binding of subapical cargos to myosin Vb bound to subapical actin, disrupted the surface expression of ATP7B, leading to reduced cellular Cu(+) export. The myosin-Vb-dependent targeting step occurred in parallel with hepatocyte-like polarity. If the myosin Vb tail was expressed acutely in cells just prior to the establishment of polarity, it appeared as part of an intracellular apical compartment, centered on γ-tubulin. ATP7B became selectively arrested in this compartment at high [Cu(+)] in the presence of myosin Vb tail, suggesting that these compartments are precursors of donor-acceptor transfer stations for apically targeted cargos of myosin Vb. Our data suggest that reduced hepatic Cu(+) clearance in idiopathic non-Wilsonian types of disease might be associated with the loss of function of myosin Vb.

Phloridzin-sensitive transport of echinacoside and acteoside and altered intestinal absorption route after application of Cistanche tubulosa extract

Objectives

The objective of this study was to address the beneficial effects of Cistanche tubulosa extract on improving the low intestinal permeability of echinacoside (ECH) and acteoside (ACT).

Methods

Absorption of ECH and ACT in C. tubulosa extract was characterized using human intestinal Caco-2 cell monolayers with intact compounds. Glucose transporter-dependent absorption of ECH and ACT was confirmed by an in-situ intestinal perfusion technique.

Key findings

The apparent permeability (Papp) was not significantly different between intact ECH and intact ACT. In the presence of phloridzin, the Papp of the ECH and ACT at a high dose was reduced to 20% of the respective non-treatment, but was not altered by phloretin and verapamil. C. tubulosa extract at low and high doses enhanced the Papp of ECH and ACT (both by threefold), resulting in their large participation in sodium-dependent glucose transporter-independent absorption. At a low concentration, concomitant ECH and ACT levels in portal blood were significantly suppressed by phloridzin.

Conclusion

The dietary and medicinal C. tubulosa extract enhancing the intestinal absorption of ECH and ACT may serve to better manage human health, although the involvement of phloridzin-sensitive transport should be reduced.

Intestinal cell targeting of a stable recombinant Cu-Zn SOD from Cucumis melo fused to a gliadin peptide

The mRNA encoding full length chloroplastic Cu-Zn SOD (superoxide dismutase) of Cucumis melo (Cantaloupe melon) was cloned. This sequence was then used to generate a mature recombinant SOD by deleting the first 64 codons expected to encode a chloroplastic peptide signal. A second hybrid SOD was created by inserting ten codons to encode a gliadin peptide at the N-terminal end of the mature SOD. Taking account of codon bias, both recombinant proteins were successfully expressed and produced in Escherichia coli. Both recombinant SODs display an enzymatic activity of ~5000U mg(-1) and were shown to be stable for at least 4h at 37°C in biological fluids mimicking the conditions of intestinal transit. These recombinant proteins were capable in vitro, albeit at different levels, of reducing ROS-induced-apoptosis of human epithelial cells. They also stimulated production and release in a time-dependent manner of an autologous SOD activity from cells located into jejunum biopsies. Nevertheless, the fused gliadin peptide enable the recombinant Cu-Zn SOD to maintain a sufficiently sustained interaction with the intestinal cells membrane in vivo rather than being eliminated with the flow. According to these observations, the new hybrid Cu-Zn SOD should show promise in applications for managing inflammatory bowel diseases.

Translocation of transfected GLUT2 to the apical membrane in rat intestinal IEC-6 cells

AIM

In this study, we transfected the full length cDNA of glucose transporter 2 (GLUT2) into IEC-6 cells (which lack GLUT2 expression) to investigate GLUT2 translocation in enterocytes. The purpose of this study was to investigate cellular mechanisms of GLUT2 translocation and its signaling pathway.

METHODS

Rat GLUT2 cDNA was transfected into IEC-6 cells. Glucose uptake was measured by incubating cell monolayers with glucose (0.5-50 mM), containing (14)C-D-glucose and (3)H-L-glucose, to measure stereospecific, carrier-mediated and passive uptake. We imaged GLUT2 immunoreactivity by confocal fluorescence microscopy. We evaluated the GLUT2 inhibitor (1 mM phloretin), SGLT1 inhibitor (0.5 mM phlorizin), disrupting microtubular integrity (2 μM nocodazole and 0.5 μM cytochalasin B), protein kinase C (PKC) inhibitors (50 nM calphostin C and 10 μM chelerythrine), and PKC activator (50 nM phorbol 12-myristate 13-acetate: PMA).

RESULTS

In GLUT2-IEC cells, the K(m) (54.5 mM) increased compared with non-transfected IEC-6 cells (7.8 mM); phloretin (GLUT2 inhibitor) inhibited glucose uptake to that of non-transfected IEC-6 cells (P < 0.05). Nocodazole and cytochalasin B (microtubule disrupters) inhibited uptake by 43-58% only at glucose concentrations ≥25 and 50 mM and the 10-min incubations. Calphostin C (PKC inhibitor) reproduced the inhibition of nocodazole; PMA (a PKC activator) enhanced glucose uptake by 69%. Exposure to glucose increased the GFP signal at the apical membrane of GLUT-1EC cells.

CONCLUSION

IEC-6 cells lacking GLUT2 translocate GLUT2 apically when transfected to express GLUT2. Translocation of GLUT2 occurs through glucose stimulation via a PKC-dependent signaling pathway and requires integrity of the microtubular skeletal structure.

Mechanisms of glucose uptake in intestinal cell lines: role of GLUT2

BACKGROUND

GLUT2 is translocated to the apical membrane of enterocytes exposed to glucose concentrations >∼50 mM. Mechanisms of GLUT2-mediated glucose uptake in cell culture models of enterocytes have not been studied.

AIM

To explore mechanism(s) of glucose uptake in 3 enterocyte-like cell lines.

METHODS

Glucose uptake was measured in Caco-2, RIE-1, and IEC-6 cell lines using varying concentrations of glucose (0.5-50 mM). Effects of phlorizin (SGLT1 inhibitor), phloretin (GLUT2 inhibitor), nocodazole and cytochalasin B (disrupters of cytoskeleton), calphostin C and chelerythrine (PKC inhibitors), and phorbol 12-myristate 13-acetate (PKC activator) were evaluated.

RESULTS

Phlorizin inhibited glucose uptake in all 3 cell lines. Phloretin inhibited glucose uptake in Caco-2 and RIE-1 cells. Starving cells decreased glucose uptake in Caco-2 and RIE-1 cells. Glucose uptake was saturated at >10 mM glucose in all 3 cell lines when exposed briefly (<1 min) to glucose. After exposure for >5 min in Caco-2 and RIE-1 cells, glucose uptake did not saturate and K(m) and V(max) increased. This increase in glucose uptake was inhibited by phloretin, nocodazole, cytochalasin B, calphostin C, and chelerythrine. PMA enhanced glucose uptake by 20%. Inhibitors and PMA had little or no effect in the IEC-6 cells.

CONCLUSION

Constitutive expression of GLUT2 in the apical membrane along with additional translocation of cytoplasmic GLUT2 to the apical membrane via an intact cytoskeleton and activated PKC appears responsible for enhanced carrier-mediated glucose uptake at greater glucose concentrations (>20 mM) in Caco-2 and RIE-1 cells. IEC-6 cells do not appear to express functional GLUT2.

The dual role of annexin II in targeting of brush border proteins and in intestinal cell polarity

Functional intestinal epithelium relies on complete polarization of enterocytes marked by the formation of microvilli and the accurate trafficking of glycoproteins to relevant membrane domains. Numerous transport pathways warrant the unique structural identity and protein/lipid composition of the brush border membrane. Annexin II (Ca(2+)-dependent lipid-binding protein) is an important component of one of the apical protein transport machineries, which involves detergent-resistant membranes and the actin cytoskeleton. Here, we investigate in intestinal Caco-2 cells the contribution of annexin II to the sorting and transport of brush border hydrolases and role in intestinal cell polarity. Downregulation of annexin II in Caco-2-A4 cell line results in a severe reduction of the levels of the brush border membrane resident enzyme sucrase isomaltase (SI) as well as structural components such as ezrin. This reduction is accompanied by a redistribution of these proteins to intracellular compartments and a striking morphological transition of Caco-2 cells to rudimentary epithelial cells that are characterized by an almost flat apical membrane with sparse and short microvilli. Concomitant with this alteration is the redistribution of the intermediate filament protein keratin 19 to the intracellular membranes in Caco-2-A4 cells. Interestingly, keratin 19 interacts with annexin II in wild type Caco-2 cells and this interaction occurs exclusively in lipid rafts. Our findings suggest a role for annexin II and K19 in differentiation and polarization of intestinal cells.

Apical trafficking in epithelial cells: signals, clusters and motors

In the early days of epithelial cell biology, researchers working with kidney and/or intestinal epithelial cell lines and with hepatocytes described the biosynthetic and recycling routes followed by apical and basolateral plasma membrane (PM) proteins. They identified the trans-Golgi network and recycling endosomes as the compartments that carried out apical-basolateral sorting. They described complex apical sorting signals that promoted association with lipid rafts, and simpler basolateral sorting signals resembling clathrin-coated-pit endocytic motifs. They also noticed that different epithelial cell types routed their apical PM proteins very differently, using either a vectorial (direct) route or a transcytotic (indirect) route. Although these original observations have generally held up, recent studies have revealed interesting complexities in the routes taken by apically destined proteins and have extended our understanding of the machinery required to sustain these elaborate sorting pathways. Here, we critically review the current status of apical trafficking mechanisms and discuss a model in which clustering is required to recruit apical trafficking machineries. Uncovering the mechanisms responsible for polarized trafficking and their epithelial-specific variations will help understand how epithelial functional diversity is generated and the pathogenesis of many human diseases.

Compound heterozygous mutations affect protein folding and function in patients with congenital sucrase-isomaltase deficiency

BACKGROUND & AIMS

Congenital sucrase-isomaltase (SI) deficiency is an autosomal-recessive intestinal disorder characterized by a drastic reduction or absence of sucrase and isomaltase activities. Previous studies have indicated that single mutations underlie individual phenotypes of the disease. We investigated whether compound heterozygous mutations, observed in some patients, have a role in disease pathogenesis.

METHODS

We introduced mutations into the SI complementary DNA that resulted in the amino acid substitutions V577G and G1073D (heterozygous mutations found in one group of patients) or C1229Y and F1745C (heterozygous mutations found in another group). The mutant genes were expressed transiently, alone or in combination, in COS cells and the effects were assessed at the protein, structural, and subcellular levels.

RESULTS

The mutants SI-V577G, SI-G1073D, and SI-F1745C were misfolded and could not exit the endoplasmic reticulum, whereas SI-C1229Y was transported only to the Golgi apparatus. Co-expression of mutants found on each SI allele in patients did not alter the protein's biosynthetic features or improve its enzymatic activity. Importantly, the mutations C1229Y and F1745C, which lie in the sucrase domains of SI, prevented its targeting to the cell's apical membrane but did not affect protein folding or isomaltase activity.

CONCLUSIONS

Compound heterozygosity is a novel pathogenic mechanism of congenital SI deficiency. The effects of mutations in the sucrase domain of SIC1229Y and SIF1745C indicate the importance of a direct interaction between isomaltase and sucrose and the role of sucrose as an intermolecular chaperone in the intracellular transport of SI.

Loss of galectin-3 impairs membrane polarisation of mouse enterocytes in vivo

Epithelial cells are characterised by distinct apical and basolateral membrane domains that are separated by tight junctions. Establishment and maintenance of this polarity depend on specific gene expression and protein targeting to their correct location. Our former studies, performed with renal epithelial MDCK cells, revealed a new function for galectin-3, a member of a conserved family of lectins. There, galectin-3 is required for intracellular sorting and correct targeting of non-raft-associated glycoproteins to the apical plasma membrane. In the present study, we found transport defects of the intestinal brush border hydrolases lactase-phlorizin hydrolase (LPH) and dipeptidylpeptidase IV (DPPIV) in galectin-3-null mutant mice. We could show that, in enterocytes of wild-type mice, both glycoproteins directly interact with galectin-3 and transit through non-raft-dependent apical transport platforms. Therefore, this genetic analysis provides definitive evidence for the involvement of galectin-3 in protein intracellular trafficking in vivo. Further investigations revealed that gal3-null enterocytes also exhibit striking cytoarchitecture defects, with the presence of numerous and regular protrusions located along basolateral membranes. Moreover, β-actin and villin, two characteristic markers of brush borders, become abnormally distributed along these atypical basolateral membranes in gal3–/– mice. Taken together, our results demonstrate that, in addition to a pivotal role in apical trafficking, galectin-3 also participates in epithelial morphogenesis in mouse enterocytes.

Parallels between global transcriptional programs of polarizing Caco-2 intestinal epithelial cells in vitro and gene expression programs in normal colon and colon cancer

Posttranslational mechanisms are implicated in the development of epithelial cell polarity, but little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized temporal patterns of gene expression during cell-cell adhesion-initiated polarization of cultured human Caco-2 cells, which develop structural and functional polarity resembling enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts. Comparison to gene expression patterns in normal human colon and colon tumors revealed that the pattern in proliferating, nonpolarized Caco-2 cells paralleled patterns seen in human colon cancer in vivo, including expression of genes involved in cell proliferation. The pattern switched in polarized Caco-2 cells to one more closely resembling that in normal colon tissue, indicating that regulation of transcription underlying Caco-2 cell polarization is similar to that during enterocyte differentiation in vivo. Surprisingly, the temporal program of gene expression in polarizing Caco-2 cells involved changes in signaling pathways (e.g., Wnt, Hh, BMP, FGF) in patterns similar to those during migration and differentiation of intestinal epithelial cells in vivo, despite the absence of morphogen gradients and interactions with stromal cells characteristic of enterocyte differentiation in situ. The full data set is available at http://microarray-pubs.stanford.edu/CACO2.

Transcriptional modulation of genes encoding structural characteristics of differentiating enterocytes during development of a polarized epithelium in vitro

Although there is considerable evidence implicating posttranslational mechanisms in the development of epithelial cell polarity, little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized the temporal program of gene expression during cell-cell adhesion-initiated polarization of human Caco-2 cells in tissue culture, which develop structural and functional polarity similar to that of enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts between neighboring cells. Expression of genes involved in cell proliferation was down-regulated concomitant with induction of genes necessary for functional specialization of polarized epithelial cells. Transcriptional up-regulation of these latter genes correlated with formation of important structural and functional features in enterocyte differentiation and establishment of structural and functional cell polarity; components of the apical microvilli were induced as the brush border formed during polarization; as barrier function was established, expression of tight junction transmembrane proteins peaked; transcripts encoding components of the apical, but not the basal-lateral trafficking machinery were increased during polarization. Coordinated expression of genes encoding components of functional cell structures were often observed indicating temporal control of expression and assembly of multiprotein complexes.

Influence of surface modification on vitality and differentiation of Caco-2 cells

It is widely accepted that the functional and morphological differentiation of cells is initiated and determined by the interaction of molecules of the extracellular matrix and adhesion molecules of the cell membrane. To assess the influence of the underlying matrix on the characteristics of cells, enterocyte-like Caco-2 cells were cultivated on substrates commonly used for cell culture as well as on glass coated with hydrophobic layers. Providing the same starting conditions for growth, the parameters investigated on preconfluent Caco-2 cells were the number of adhering cells, the proliferative activity and the degree of differentiation indicated by the expression of three brush border enzymes. Whereas tissue culture treated polystyrene elicited highest rates of adhesion, proliferation, and differentiation, even glass altered the pattern of brush border enzyme expression. The hydrophobic surfaces strongly decreased the adhesion and the proliferation but the surviving cells exhibited a pronounced higher degree of differentiation. Interestingly, each sub-type of hydrophobic matrix triggered a different pattern of brush border enzyme expression. Thus, the development of a certain phenotype of a cell can not only be triggered by certain components of the extracellular matrix but also by artificially prepared surface coatings of the underlying matrix. In the future it seems to be feasible that cells can be programmed by tailoring the surface of the underlying substrate.

Plasma membrane delivery, endocytosis and turnover of transcobalamin receptor in polarized human intestinal epithelial cells

Cells that are metabolically active and in a high degree of differentiation and proliferation require cobalamin (Cbl: vitamin B(12)) and they obtain it from the circulation bound to transcobalamin (TC) via the transcobalamin receptor (TC-R). This study has investigated the plasma membrane dynamics of TC-R expression in polarized human intestinal epithelial Caco-2 cells using techniques of pulse-chase labelling, domain-specific biotinylation and cell fractionation. Endogenously synthesized TC-R turned over with a half-life (T(1/2)) of 8 h following its delivery to the basolateral plasma membrane (BLM). The T(1/2) of BLM delivery was 15 min and TC-R delivered to the BLM was endocytosed and subsequently degraded by leupeptin-sensitive proteases. However, about 15% of TC-R endocytosed from the BLM was transcytosed (T(1/2), 45 min) to the apical membranes (BBM) where it underwent endocytosis and was degraded. TC-R delivery to both BLM and BBM was inhibited by Brefeldin A and tunicamycin, but not by wortmannin or leupeptin. Colchicine inhibited TC-R delivery to BBM, but not BLM. At steady state, apical TC-R was associated with megalin and both these proteins were enriched in an intracellular compartment which also contained Rab5 and transferrin receptor. These results indicate that following rapid delivery to both plasma membrane domains of Caco-2 cells, TC-R undergoes constitutive endocytosis and degradation by leupeptin-sensitive proteases. TC-R expressed in apical BBM complexes with megalin during its transcytosis from the BLM.

Rotavirus impairs the biosynthesis of brush-border-associated dipeptidyl peptidase IV in human enterocyte-like Caco-2/TC7 cells

Rotavirus is the leading cause of severe dehydrating diarrhoea in infants and young children worldwide. This virus infects mature enterocytes in the small intestine, and induces structural and functional damage. In the present study, we have identified a new mechanism by which rotavirus impairs a brush border-associated intestinal protein. We show that infection of enterocyte-like Caco-2/TC7 cells by rhesus monkey rotavirus (RRV) impairs the biosynthesis of dipeptidyl peptidase IV (DPP IV), an important hydrolase in the digestion of dietary proline-rich proteins. We show that the enzyme activity of DPP IV was reduced, and that rearrangements of the protein occurred at the apical domain of the RRV-infected cells. Using pulse-chase experiments and cell surface immunoprecipitation, we have demonstrated that RRV infection did not affect the stability or apical targeting of DPP IV, but did induce a dramatic decrease in its biosynthesis. Using quantitative RT-PCR, we showed that RRV had no effect on the level of expression of DPP IV mRNA, suggesting that the observed decrease in the biosynthesis of the protein is related to an effect of the virus at the translational level.

Vectorial insertion of apical and basolateral membrane proteins in polarized epithelial cells revealed by quantitative 3D live cell imaging

Although epithelial cells are known to exhibit a polarized distribution of membrane components, the pathways responsible for delivering membrane proteins to their appropriate domains remain unclear. Using an optimized approach to three-dimensional live cell imaging, we have visualized the transport of newly synthesized apical and basolateral membrane proteins in fully polarized filter-grown Madin–Darby canine kidney cells. We performed a detailed quantitative kinetic analysis of trans-Golgi network (TGN) exit, passage through transport intermediates, and arrival at the plasma membrane using cyan/yellow fluorescent protein–tagged glycosylphosphatidylinositol-anchored protein and vesicular stomatitis virus glycoprotein as apical and basolateral reporters, respectively. For both pathways, exit from the TGN was rate limiting. Furthermore, apical and basolateral proteins were targeted directly to their respective membranes, resolving current confusion as to whether sorting occurs on the secretory pathway or only after endocytosis. However, a transcytotic protein did reach the apical surface after a prior appearance basolaterally. Finally, newly synthesized proteins appeared to be delivered to the entire lateral or apical surface, suggesting—contrary to expectations—that there is not a restricted site for vesicle docking or fusion adjacent to the junctional complex.

Altered Folding, Turnover, and Polarized Sorting Act in Concert to Define a Novel Pathomechanism of Congenital Sucrase-Isomaltase Deficiency

Naturally occurring mutants of membrane and secretory proteins are often associated with the pathogenesis of human diseases. Here, we describe the molecular basis of a novel phenotype of congenital sucrase-isomaltase deficiency (CSID), a disaccharide malabsorption disorder of the human intestine in which several structural features and functional capacities of the brush-border enzyme complex sucrase-isomaltase (SI) are affected. The cDNA encoding SI from a patient with CSID reveals a mutation in the isomaltase subunit of SI that results in the substitution of a cysteine by an arginine at amino acid residue 635 (C635R). When this mutation is introduced into the wild type cDNA of SI a mutant enzyme, SI(C635R), is generated that shows a predominant localization in the endoplasmic reticulum. Nevertheless, a definite localization of SI(C635R) in the Golgi apparatus and at the cell surface could be also observed. Epitope mapping with conformation-specific mAbs protease sensitivity assays, and enzymatic activity measurements demonstrate an altered folding pattern of SI(C635R) that is responsible for a substantially increased turnover rate and an aberrant sorting profile. Thus, SI(C635R) becomes distributed also at the basolateral membrane in contrast to wild type SI. Concomitant with the altered sorting pattern, the partial detergent extractability of wild type SI shifts to a complete detergent solubility with Triton X-100. The mutation has therefore affected an epitope responsible for the apical targeting fidelity of SI. Altogether, the combined effects of the C635R mutation on the turnover rate, function, polarized sorting, and detergent solubility of SI constitute a unique and novel pathomechanism of CSID.

Insulin-dependent phosphorylation of DPP IV in liver. Evidence for a role of compartmentalized c-Src

Dipeptidyl peptidase IV (DPP IV, CD26, EC 3.4.14.5) serves as a model aimed at elucidating protein sorting signals. We identify here, by MS, several tyrosine-phosphorylated proteins in a rat liver Golgi/endosome (G/E) fraction including DPP IV. We show that a pool of DPP IV is tyrosine-phosphorylated. Maximal phosphorylation was observed after 2 min following intravenous insulin injection. DPP IV coimmunoprecipitated with the cellular tyrosine kinase Src (c-Src) with maximal association also observed after 2 min following insulin injection. DPP IV was found phosphorylated after incubation of nonsolubilized G/E membranes with [gamma-32P]ATP. The c-Src inhibitor PP2 inhibited DPP IV phosphorylation. Oriented proteolysis experiments indicate that a large pool of c-Src is protected in G/E fractions. Following injection of the protein-tyrosine phosphatase inhibitor bpV(phen), DPP IV levels markedly decreased by 40% both in plasma membrane and G/E fractions. In the fraction designated Lh, DPP IV levels decreased by 50% 15 min following insulin injection. Therefore, a pool of DPP IV is tyrosine-phosphorylated in an insulin-dependent manner. The results suggest the presence of a yet to be characterized signalling mechanism whereby DPP IV has access to c-Src-containing signalling platforms.

A phenylalanine-based folding determinant in intestinal sucrase-isomaltase that functions in the context of a quality control mechanism beyond the endoplasmic reticulum

Phenotype II of congenital sucrase-isomaltase deficiency in man is characterized by a retention of the brush border protein sucrase-isomaltase (SI) in the ER/cis-Golgi intermediate compartment (ERGIC) and the cis-Golgi. The transport block is due to the substitution of a glutamine by a proline at amino acid residue 1098 that generates a temperature-sensitive mutant enzyme, SIQ1098P, the transport of which is regulated by several cycles of anterograde and retrograde transport between the ER and the cis-Golgi (Propsting, M. J., Jacob, R. and Naim, H. Y. (2003). J. Biol. Chem. 278, 16310-16314). A quality control beyond the ER has been proposed that implicates a retention signal or a folding determinant elicited by the Q1098P mutation. We have used alanine-scanning mutagenesis to screen upstream and downstream regions flanking Q1098 and identified a putative motif, F1093-x-F1095-x-x-x-F1099 that is likely to be implicated in sensing the folding and subsequent trafficking of SI from the ER to the Golgi. The characteristics of this motif are three phenylalanine residues that upon substitution by alanine generate the temperature-sensitive SIQ1098P phenotype. This mutant protein undergoes transport arrest in the ERGIC and cis-Golgi compartments and acquires correct folding and functional activity at reduced temperatures as a consequence of cycles of anterograde and retrograde transport between the ER and cis-Golgi. Other amino acid residues in this motif are not significant in the context of phenotype II. We propose that the phenylalanine cluster is required for shielding a folding determinant in the extracellular domain of SI; substitution of a Q by a P at residue 1098 of sucrase disrupts this determinant and elicits retention of SIQ1098P in ERGIC and cis-Golgi in phenotype II of CSID.

Orally active antiviral tripeptide glycyl-prolyl-glycinamide is activated by CD26 (dipeptidyl peptidase IV) before transport across the intestinal epithelium

The tripeptide amide glycyl-prolyl-glycinamide (GPG-amide) is a new antiretroviral drug candidate, but its absorption mechanism is unknown. In this investigation, the transport and metabolism of GPG-amide were studied in a model of the human intestinal epithelium, Caco-2 cell monolayers. The results show that when the tripeptide amide came into contact with the apical enterocyte membrane, it was degraded by CD26 (dipeptidyl peptidase IV) to glycylproline and the antiretrovirally active metabolite glycinamide. Glycinamide retained antiretroviral activity in vitro after transport through the Caco-2 cell monolayers. The transport of glycinamide across Caco-2 cell monolayers occurred via passive diffusion with an apparent permeability coefficient of about 2 x 10(-6) cm s(-1), which suggests that it is absorbed by the oral route in sufficient amounts to be considered for oral administration. In conclusion, the tripeptide GPG-amide acts as a prodrug that is activated by CD26 to release the orally active antiretroviral compound glycinamide.

The FAM deubiquitylating enzyme localizes to multiple points of protein trafficking in epithelia, where it associates with E-cadherin and beta-catenin

Ubiquitylation is a necessary step in the endocytosis and lysosomal trafficking of many plasma membrane proteins and can also influence protein trafficking in the biosynthetic pathway. Although a molecular understanding of ubiquitylation in these processes is beginning to emerge, very little is known about the role deubiquitylation may play. Fat Facets in mouse (FAM) is substrate-specific deubiquitylating enzyme highly expressed in epithelia where it interacts with its substrate, beta-catenin. Here we show, in the polarized intestinal epithelial cell line T84, FAM localized to multiple points of protein trafficking. FAM interacted with beta-catenin and E-cadherin in T84 cells but only in subconfluent cultures. FAM extensively colocalized with beta-catenin in cytoplasmic puncta but not at sites of cell-cell contact as well as immunoprecipitating with beta-catenin and E-cadherin from a higher molecular weight complex ( approximately 500 kDa). At confluence FAM neither colocalized with, nor immunoprecipitated, beta-catenin or E-cadherin, which were predominantly in a larger molecular weight complex ( approximately 2 MDa) at the cell surface. Overexpression of FAM in MCF-7 epithelial cells resulted in increased beta-catenin levels, which localized to the plasma membrane. Expression of E-cadherin in L-cell fibroblasts resulted in the relocalization of FAM from the Golgi to cytoplasmic puncta. These data strongly suggest that FAM associates with E-cadherin and beta-catenin during trafficking to the plasma membrane.

Cell polarity and epithelial oncogenesis

Pathologists have long recognized that tumour formation in epithelia leads to disruption of normal epithelial cell polarity. Despite this, few studies have taken advantage of new information on the biogenesis of cell polarity to analyse the process of epithelial oncogenesis. Recent studies of epithelial cell lines now indicate that the pattern of breakdown of polarity during oncogenesis may reflect the way in which normal epithelial cells achieve polarity. These results suggest not only a novel way to study the development of polarity in vitro, but also new ideas for the early detection of cancer.

Raft-mediated trafficking of apical resident proteins occurs in both direct and transcytotic pathways in polarized hepatic cells: role of distinct lipid microdomains

In polarized hepatic cells, pathways and molecular principles mediating the flow of resident apical bile canalicular proteins have not yet been resolved. Herein, we have investigated apical trafficking of a glycosylphosphatidylinositol-linked and two single transmembrane domain proteins on the one hand, and two polytopic proteins on the other in polarized HepG2 cells. We demonstrate that the former arrive at the bile canalicular membrane via the indirect transcytotic pathway, whereas the polytopic proteins reach the apical membrane directly, after Golgi exit. Most importantly, cholesterol-based lipid microdomains ("rafts") are operating in either pathway, and protein sorting into such domains occurs in the biosynthetic pathway, largely in the Golgi. Interestingly, rafts involved in the direct pathway are Lubrol WX insoluble but Triton X-100 soluble, whereas rafts in the indirect pathway are both Lubrol WX and Triton X-100 insoluble. Moreover, whereas cholesterol depletion alters raft-detergent insolubility in the indirect pathway without affecting apical sorting, protein missorting occurs in the direct pathway without affecting raft insolubility. The data implicate cholesterol as a traffic direction-determining parameter in the direct apical pathway. Furthermore, raft-cargo likely distinguishing single vs. multispanning membrane anchors, rather than rafts per se (co)determine the sorting pathway.

PrPC is sorted to the basolateral membrane of epithelial cells independently of its association with rafts

PrP(C) is a glycosylphosphatidylinositol-anchored protein expressed in neurons as well as in the cells of several peripheral tissues. Although the normal function of PrP(C) remains unknown, a conformational isoform called PrP(Sc) (scrapie) has been proposed to be the infectious agent of transmissible spongiform encephalopathies in animals and humans. Where and how the PrP(C) to PrP(Sc) conversion occurs in the cells is not yet known. Therefore, dissecting the intracellular trafficking of the wild-type prion protein, as well as of the scrapie isoform, can be of major relevance to the pathogenesis of the diseases. In this report we have analyzed the exocytic pathway of transfected mouse PrP(C) in thyroid and kidney polarized epithelial cells. In contrast to the majority of glycosylphosphatidylinositol-anchored proteins, we found that PrP(C) is localized mainly on the basolateral domain of the plasma membrane of both cell lines. This is reminiscent of the predominant somatodendritic localization found in neurons. However, similarly to apical glycosylphosphatidylinositol-proteins, PrP(C) associates with detergent-resistant microdomains, which have been suggested to have a role in apical sorting of glycosylphosphatidylinositol-proteins, as well as in the conversion process of PrP(C) to PrP(Sc). In order to discriminate whether detergent-resistant microdomains have a direct role in PrP(Sc) conversion, or whether they are involved in the transport of the protein to the site of its conversion, we have examined the effect of disruption of detergent-resistant microdomain association on PrP(C) intracellular traffic. Consistent with the unusual basolateral localization of this glycosylphosphatidylinositol-linked protein, our data exclude a classical role for detergent-resistant microdomains in the post-trans-Golgi network sorting and transport of PrP(C) to the plasma membrane.

Activation of human meprin-alpha in a cell culture model of colorectal cancer is triggered by the plasminogen-activating system

The activation of latent proenzymes is an important mechanism for the regulation of localized proteolytic activity. Human meprin-alpha, an astacin-like zinc metalloprotease expressed in normal colon epithelial cells, is secreted as a zymogen into the intestinal lumen. Here, meprin is activated after propeptide cleavage by trypsin. In contrast, colorectal cancer cells secrete meprin-alpha in a non-polarized way, leading to accumulation and increased activity of meprin-alpha in the tumor stroma. We have analyzed the activation mechanism of promeprin-alpha in colorectal cancer using a co-culture model of the intestinal mucosa composed of colorectal adenocarcinoma cells (Caco-2) cultivated on filter supports and intestinal fibroblasts grown in the companion dish. We provide evidence that meprin-alpha is activated by plasmin and show that the presence of plasminogen in the basolateral compartment of the co-cultures is sufficient for promeprin-alpha activation. Analysis of the plasminogen-activating system in the co-cultures revealed that plasminogen activators produced and secreted by fibroblasts converted plasminogen to active plasmin, which in turn generated active meprin-alpha. This activation mechanism offers an explanation for the observed meprin-alpha activity in the tumor stroma, a prerequisite for a potential role of this protease in colorectal cancer.

Intestinal dipeptidyl peptidase IV is efficiently sorted to the apical membrane through the concerted action of N- and O-glycans as well as association with lipid microdomains

The apical sorting of human intestinal dipeptidyl peptidase IV (DPPIV) occurs through complex N-linked and O-linked carbohydrates. Inhibition of O-linked glycosylation by benzyl-N-acetyl-alpha-d-galactosaminide affects significantly the sorting behavior of DPPIV in intestinal Caco-2 and HT-29 cells. However, random delivery to the apical and basolateral membranes and hence a more drastic effect on the sorting of DPPIV in both cell types is only observed when, in addition to O-glycans, the processing of N-glycans is affected by swainsonine, an inhibitor of mannosidase II. Together the data indicate that both types of glycosylation are critical components of the apical sorting signal of DPPIV. The sorting mechanism of DPPIV implicates its association with detergent-insoluble membrane microdomains containing cholesterol and sphingolipids, whereas an efficient association largely depends on the presence of a fully complex N- and O-linked glycosylated DPPIV. Interestingly, cholesterol is a more critical component in this context than sphingolipids, because cholesterol depletion by beta-cyclodextrin affects the detergent solubility and the sorting behavior of DPPIV more strongly than fumonisin, an inhibitor of sphingolipid synthesis.

The cytoplasmic/transmembrane domain of dipeptidyl peptidase IV, a type II glycoprotein, contains an apical targeting signal that does not specifically interact with lipid rafts

We investigated the signals involved in the apical targeting of dipeptidyl peptidase IV (DPP IV/CD26), an archetypal type II transmembrane glycoprotein. A secretory construct, corresponding to the DPP IV ectodomain, was first stably expressed in both the enterocytic-like cell line Caco-2 and the epithelial kidney MDCK cells. Most of the secretory form of the protein was delivered apically in MDCK cells, whereas secretion was 60% basolateral in Caco-2 cells, indicating that DPP IV ectodomain targeting is cell-type-dependent. A chimera (CTM-GFP) containing only the cytoplasmic and transmembrane domains of mouse DPP IV plus the green fluorescent protein was then studied. In both cell lines, this chimera was preferentially expressed at the apical membrane. By contrast, a secretory form of GFP was randomly secreted, indicating that GFP by itself does not contain cryptic targeting information. Comparison of the sequence of the transmembrane domain of DPP IV and several other apically targeted proteins does not show any consensus, suggesting that the apical targeting signal may be conformational. Neither the DPP IV nor the CTM-GFP chimera was enriched in lipid rafts. Together these results indicate that, besides the well-known raft-dependent apical targeting pathway, the fate of the CTM domain of DPP IV may reveal a new raft-independent apical pathway.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Dynamics of glycine receptor insertion in the neuronal plasma membrane

The exocytosis site of newly synthesized glycine receptor was defined by means of a morphological assay to characterize its export from the trans-Golgi Network to the plasma membrane. This was achieved by expressing in transfected neurons an alpha1 subunit bearing an N-terminal tag selectively cleavable from outside the cell by thrombin. This was combined with a transient temperature-induced block of exocytic transport that creates a synchronized exocytic wave. Immunofluorescence microscopy analysis of the cell surface appearance of newly synthesized receptor revealed that exocytosis mainly occurred at nonsynaptic sites in the cell body and the initial portion of dendrites. At the time of cell surface insertion, the receptors existed as discrete clusters. Quantitative analysis showed that glycine receptor clusters are stable in size and subsequently appeared in more distal dendritic regions. This localization resulted from diffusion in the plasma membrane and not from exocytosis of transport vesicles directed to dendrites. Kinetic analysis established a direct substrate-product relationship between pools of somatic and dendritic receptors. This indicated that clusters represent intermediates between newly synthesized and synaptic receptors. These results support a diffusion-retention model for the formation of receptor-enriched postsynaptic domains and not that of a vectorial intracellular targeting to synapses.

Impairments in enzyme activity and biosynthesis of brush border-associated hydrolases in human intestinal Caco-2/TC7 cells infected by members of the Afa/Dr family of diffusely adhering Escherichia coli

Wild-type diffusely adhering Escherichia coli (DAEC) harbouring afimbrial adhesin (Afa) or fimbrial Dr and F1845 adhesins (Afa/Dr DAEC) apically infecting the human intestinal epithelial cells promote injuries in the brush border of the cells. We report here that infection by Afa/Dr DAEC wild-type strains C1845 and IH11128 in polarized human fully differentiated Caco-2/TC7 cells dramatically impaired the enzyme activity of functional brush border-associated proteins sucrase-isomaltase (SI) and dipeptidylpeptidase IV (DPP IV). Blockers of the transduction signal molecules, previously found to be active against the Afa/Dr DAEC-induced cytoskeleton injury, were inactive against the Afa/Dr-induced decrease in sucrase enzyme activity. In parallel, Afa/Dr DAEC infection promotes the blockade of the biosynthesis of SI and DPP IV without affection enzyme stability. The observation that no changes occurred in mRNA levels of SI and DPP IV upon infection suggested that the decrease in biosynthesis probably resulted from a decrease in the translation rate. When the cells were infected with recombinant E. coli strains expressing homologous adhesins of the wild-type strains, neither a decrease in sucrase and DPP IV enzyme activities nor an inhibition of enzyme biosynthesis were observed. In conclusion, taken together, these data give new insights into the mechanisms by which the wild-type Afa/Dr DAEC strains induce functional injuries in polarized fully differentiated human intestinal cells. Moreover, the results revealed that other pathogenic factor(s) distinct from the Afa/Dr adhesins may play(s) a crucial role in this mechanism of pathogenicity.

Novel, Aberrantly Truncated Isoform of Serum CD13 in a Family with High Serum Aminopeptidase N (CD13) Activity

Background: We previously reported a family in which the propositus and both her father and paternal grandmother had high serum aminopeptidase N (CD13; EC 3.4.11.2) activity (autosomal dominant). The molecular mass of the serum CD13 polypeptide of the propositus was larger than that of normal CD13, suggesting either a mutation in the CD13 gene or an abnormality in posttranslational modification of CD13 polypeptide in this family.

Methods: Reverse transcription-PCR and direct sequencing were performed with leukocyte CD13 mRNA from the propositus. Two-dimensional electrophoresis and N-terminal amino acid sequencing were performed with serum CD13 from the propositus, the father of the propositus, and healthy volunteers.

Results: The sequence of the CD13 cDNA of the propositus was essentially identical with that reported previously. However, the CD13 polypeptide of the propositus and the father of the propositus was truncated, lacking amino acids 1–43 of intact CD13 (43-truncated CD13), whereas CD13 lacking residues 1–58 (58-truncated CD13) and 43-truncated CD13 were detected in serum from healthy volunteers.

Conclusions: In serum from healthy volunteers, we found both 58-truncated CD13, a major isoform reported previously, and 43-truncated CD13, a novel, minor isoform with a larger polypeptide. In serum of the family, 43-truncated CD13 was extremely concentrated, suggesting that proteolytic cleavage of CD13 amino acids 43 and 44 (43-truncation) is abnormally promoted. Because no mutation was found in the CD13 cDNA from the propositus, increased serum CD13 in this family seems to be caused by a mutation in a gene that regulates 43-truncation protease activity.

Intracellular traffic of the ecto-nucleotide pyrophosphatase/phosphodiesterase NPP3 to the apical plasma membrane of MDCK and Caco-2 cells: apical targeting occurs in the absence of N-glycosylation

Glycosylation was considered the major signal candidate for apical targeting of transmembrane proteins in polarized epithelial cells. However, direct demonstration of the role of glycosylation has proved difficult because non-glycosylated apical transmembrane proteins usually do not reach the cell surface. Here we were able to follow the targeting of the apical transmembrane glycoprotein NPP3 both when glycosylated and non-glycosylated. Transfected in polarized MDCK and Caco-2 cells, NPP3 was exclusively expressed at the apical membrane. The transport kinetics of the protein to the cell surface were studied after metabolic (35)S-labeling and surface immunoprecipitation. The newly synthesized protein was mainly targeted directly to the apical surface in MDCK cells, whereas 50% transited through the basolateral surface in Caco-2 cells. In both cell types, the basolaterally targeted pool was effectively transcytosed to the apical surface. In the presence of tunicamycin, NPP3 was not N-glycosylated. The non-glycosylated protein was partially retained intracellularly but the fraction that reached the cell surface was nevertheless predominantly targeted apically. However, transcytosis of the non-glycosylated protein was partially impaired in MDCK cells. These results provide direct evidence that glycosylation cannot be considered an apical targeting signal for NPP3, although glycosylation is necessary for correct trafficking of the protein to the cell surface.

Transport and function of syntaxin 3 in human epithelial intestinal cells

To follow the transport of human syntaxin (Syn) 3 to the apical surface of intestinal cells, we produced and expressed in Caco-2 cells a chimera made of the entire Syn3 coding sequence and the extracellular domain of the human transferrin receptor (TfR). This chimera (Syn3TfR) was localized to the apical membrane and was transported along the direct apical pathway, suggesting that this is also the case for endogenous Syn3. To test the potential role of Syn3 in apical transport, we overexpressed it in Caco-2 cells and measured the efficiency of apical and basolateral delivery of several endogenous markers. We observed a strong inhibition of apical delivery of sucrase-isomaltase (SI), an apical transmembrane protein, and of alpha-glucosidase, an apically secreted protein. No effect was observed on the basolateral delivery of Ag525, a basolateral antigen, strongly suggesting that Syn3 is necessary for efficient delivery of proteins to the apical surface of intestinal cells.

Detergent insoluble microdomains are not involved in transcytosis of polymeric Ig receptor in FRT and MDCK cells

In polarized epithelial cells, sorting of proteins and lipids to the apical or basolateral domain of the plasma membrane can occur via direct or indirect (transcytotic) pathways from the trans Golgi network (TGN). The 'rafts' hypothesis postulates that the key event for direct apical sorting of some transmembrane proteins and the majority of GPI-anchored proteins depends on their association with glycosphingolipid and cholesterol enriched microdomains (rafts). However, the mechanism of indirect sorting to the apical membrane is not clear. The polyimmunoglobulin receptor (pIgR) is one of the best studied proteins that follow the transcytotic pathway. It is normally delivered from the TGN to the basolateral surface of polarized Madin-Darby Canine Kidney (MDCK) cells from where it transports dIgA or dIgM to the apical surface. We have studied the intracellular trafficking of pIgR in Fischer rat thyroid cells (FRT), and have investigated the sorting machinery involved in transcytosis of this receptor in both FRT and MDCK cells. We found that, in contrast with MDCK cells, a significant amount (approximately 30%) of pIgR reaches the apical surface by a direct pathway. Furthermore, in both cell lines it does not associate with Triton X-100 insoluble microdomains, suggesting that at least in these cells 'rafts' are not involved in basolateral to apical transcytosis.

Measles virus matrix protein specifies apical virus release and glycoprotein sorting in epithelial cells

In polarized epithelial cells measles virus (MV) is predominantly released at the apical cell surface, irrespective of the sorting of its two envelope glycoproteins F and H. It has been reported previously that the viral matrix (M) protein modulates the fusogenic capacity of the viral envelope glycoproteins. Here, extant MV mutants and chimeras were used to determine the role of M protein in the transport of viral glycoproteins and release of progeny virions in polarized epithelial CaCo2 cells. In the absence of M, envelope glycoproteins are sorted to the basolateral surface, suggesting that they possess intrinsic basolateral sorting signals. However, interactions of M with the glycoprotein cytoplasmic tails allow M-glycoprotein co-segregation to the apical surface, suggesting a vectorial function of M to retarget the glycoproteins for apical virion release. Whereas this may allow virus airway shedding, the intrinsic sorting of the glycoproteins to the basolateral surface may account for systemic host infection by allowing efficient cell-cell fusion.

Apical secretion and sialylation of soluble dipeptidyl peptidase IV are two related events

The role of glycans in the apical targeting of proteins in epithelial cells remains a debated question. We have expressed the mouse soluble dipeptidyl peptidase IV (DPP IV ectodomain) in kidney (MDCK) and in intestinal (Caco-2) epithelial cell lines, as a model to study the role of glycosylation in apical targeting. The mouse DPP IV ectodomain was secreted mainly into the apical medium by MDCK cells. Exposure of MDCK cells to GalNac-alpha-O-benzyl, a drug previously described as an inhibitor of mucin O-glycosylation, produced a protein with a lower molecular weight. In addition this treatment resulted in a decreased apical secretion and an increased basolateral secretion of mouse DPP IV ectodomain. When expressed in Caco-2 cells, the mouse DPP IV ectodomain was secreted mainly into the basolateral medium. However, BGN was still able to decrease the amount of apically secreted protein and to increase its basolateral secretion. Neuraminidase digestion showed that the most striking effect of BGN was a blockade of DPP IV sialylation in both MDCK and Caco-2 cells. These results indicate that a specific glycosylation step, namely, sialylation, plays a key role in the control of the apical targeting of a secreted DPP IV both in MDCK and Caco-2 cells.

Analysis of the transmembrane domain of influenza virus neuraminidase, a type II transmembrane glycoprotein, for apical sorting and raft association

Influenza virus neuraminidase (NA), a type II transmembrane protein, is directly transported to the apical plasma membrane in polarized MDCK cells. Previously, it was shown that the transmembrane domain (TMD) of NA provides a determinant(s) for apical sorting and raft association (A. Kundu, R. T. Avalos, C. M. Sanderson, and D. P. Nayak, J. Virol. 70:6508-6515, 1996). In this report, we have analyzed the sequences in the NA TMD involved in apical transport and raft association by making chimeric TMDs from NA and human transferring receptor (TR) TMDs and by mutating the NA TMD sequences. Our results show that the COOH-terminal half of the NA TMD (amino acids [aa] 19 to 35) was significantly involved in raft association, as determined by Triton X-100 (TX-100) resistance. However, in addition, the highly conserved residues at the extreme NH(2) terminus of the NA TMD were also critical for TX-100 resistance. On the other hand, 19 residues (aa 9 to 27) at the NH(2) terminus of the NA TMD were sufficient for apical sorting. Amino acid residues 14 to 18 and 27 to 31 had the least effect on apical transport, whereas mutations in the amino acid residues 11 to 13, 23 to 26, and 32 to 35 resulted in altered polarity for the mutant proteins. These results indicated that multiple regions in the NA TMD were involved in apical transport. Furthermore, these results support the idea that the signals for apical sorting and raft association, although residing in the NA TMD, are not identical and vary independently and that the NA TMD also possesses an apical determinant(s) which can interact with apical sorting machineries outside the lipid raft.

Newly synthesized canalicular ABC transporters are directly targeted from the Golgi to the hepatocyte apical domain in rat liver

Newly synthesized canalicular ectoenzymes and a cell adhesion molecule (cCAM105) have been shown to traffic from the Golgi to the basolateral plasma membrane, from where they transcytose to the apical bile canalicular domain. It has been proposed that all canalicular proteins are targeted via this indirect route in hepatocytes. We studied the membrane targeting of rat canalicular proteins by in vivo [(35)S]methionine metabolic labeling followed by preparation of highly purified Golgi membranes and canalicular (CMVs) and sinusoidal/basolateral (SMVs) membrane vesicles and subsequent immunoprecipitation. In particular, we compared membrane targeting of newly synthesized canalicular ABC (ATP-binding cassette) transporters MDR1, MDR2, and SPGP (sister of P-glycoprotein) with that of cCAM105. Significant differences were observed in metabolic pulse-chase labeling experiments with regard to membrane targeting of these apical proteins. After a chase time of 15 min, cCAM105 appeared exclusively in SMVs, peaked at 1 h, and progressively declined thereafter. In CMVs, cCAM105 was first detected after 1 h and subsequently increased for 3 h. This findings confirm the transcytotic targeting of cCAM105 reported in earlier studies. In contrast, at no time point investigated were MDR1, MDR2, and SPGP detected in SMVs. In CMVs, MDR1 and MDR2 appeared after 30 min, whereas SPGP appeared after 2 h of labeling. In Golgi membranes, each of the ABC transporters peaked at 30 min and was virtually absent thereafter. These data suggest rapid, direct targeting of newly synthesized MDR1 and MDR2 from the Golgi to the bile canaliculus and transient sequestering of SPGP in an intracellular pool en route from the Golgi to the apical plasma membrane. This study provides biochemical evidence for direct targeting of newly synthesized apical ABC transporters from the Golgi to the bile canaliculus in vivo.

Epithelial cell polarity as reflected in enterocytes

Absorptive cells are the main cells present in the intestinal epithelium. The plasma membrane of these tall columnar cells reflects their high degree of polarization, by dividing into apical and basolateral domains with different compositions. The most characteristic structure of these cells consists of closely packed apical microvilli with the same height, looking like a brush, which is why they were named the brush border. The concentrated pattern of some apical markers observed in a restricted brush border domain shows that mature enterocytes are hyperpolarized epithelial cells: the filamentous brush border glycocalyx is anchored at the top of the microvilli and the annexin XIII is concentrated in the lower three fourths. Many studies have been carried out on the biosynthesis and intracellular pathway of domain markers. The results show clearly that the basolateral markers take a direct pathway from the trans-Golgi network to the basolateral membrane. However, the two apical pathways, one direct and one indirect pathway via the basolateral membrane, are used, depending on the apical protein involved. Efficient protein sorting and addressing are essential to the establishment and maintenance of cell polarity, on which the integrity of the epithelial barrier depends.

Munc18-2, a functional partner of syntaxin 3, controls apical membrane trafficking in epithelial cells

The Sec1-related proteins bind to syntaxin family t-SNAREs with high affinity, thus controlling the interaction of syntaxins with their cognate SNARE partners. Munc18-2 is a Sec1 homologue enriched in epithelial cells and forms a complex with syntaxin 3, a t-SNARE localized to the apical plasma membrane. We generated here a set of Munc18-2 point mutants with substitutions in conserved amino acid residues. The mutants displayed a spectrum of different syntaxin binding efficiencies. The in vitro and in vivo binding patterns were highly similar, and the association of the Munc18-2 variants with syntaxin 3 correlated well with their ability to displace SNAP-23 from syntaxin 3 complexes when overexpressed in Caco-2 cells. Even the Munc18-2 mutants that do not detectably bind syntaxin 3 were membrane associated in Caco-2 cells, suggesting that the syntaxin interaction is not the sole determinant of Sec1 protein membrane attachment. Overexpression of the wild-type Munc18-2 was shown to inhibit the apical delivery of influenza virus hemagglutinin (HA). Interestingly, mutants unable to bind syntaxin 3 behaved differently in the HA transport assay. While one of the mutants tested had no effect, one inhibited and one enhanced the apical transport of HA. This implies that Munc18-2 function in apical membrane trafficking involves aspects independent of the syntaxin 3 interaction.

Expression of Rab small GTPases in epithelial Caco-2 cells: Rab21 is an apically located GTP-binding protein in polarised intestinal epithelial cells

Rab proteins belong to a subfamily of small GTP-binding protein genes of the Ras superfamily and play an important role in intracellular vesicular targeting. The presence of members of this protein family was examined in Caco-2 cells by a PCR-based strategy. Twenty-five different partial cDNA sequences were isolated, including 18 Rab protein family members. Seven novel human sequences, representing Rab2B, Rab6A', Rab6B, Rab10, Rab19B, Rab21 and Rab22A, were identified. For one clone, encoding Rab21, full-length cDNA was isolated from a Caco-2 cDNA library. Northern blot analysis showed a ubiquitous expression pattern of Rab21. To study Rab21 protein expression in Caco-2 cells, polyclonal antibodies were raised against GST-Rab21 fusion protein and characterised. The antibodies recognised Rab21 as a protein of approximately 25 kDa. Interestingly, the protein shows a general ER-like staining in nonpolarised Caco-2 cells in contrast to an apically located vesicle-like staining in polarised Caco-2 cells. Furthermore, immunohistochemical staining on human jejunal tissue showed a predominant expression of Rab21 in the epithelial cell layer with high expression levels in the apical region, whereas stem cells in the crypts were negative. We therefore suggest an alternative role for Rab21 in the regulation of vesicular transport in polarised intestinal epithelial cells.