Identification of CD146 as a component of the endothelial junction involved in the control of cell-cell cohesion.

CD146 is a cell-surface molecule belonging to the immunoglobulin superfamily and expressed in all types of human endothelial cells. Confocal and electron microscopic analysis of confluent human umbilical vein endothelial cells (HUVECs) were used to demonstrate that CD146 is a component of the endothelial junction. Double immunolabeling with vascular endothelial cadherin showed that CD146 is localized outside the adherens junction. Moreover, CD146 expression is not restricted to the junction, since part of the labeling was detectable at the apical side of the HUVECs. Interestingly, cell-surface expression of CD146 increased when HUVECs reached confluence. In addition, the paracellular permeability of CD146-transfected fibroblast cells was decreased compared with that of control cells. Finally, CD146 colocalized with actin, was partly resistant to Triton X-100 extraction, and had its expression altered by actin-disrupting agents, indicating that CD146 is associated with the actin cytoskeleton. These results show the regulated expression of CD146 at areas of cell-cell junction and strongly suggest involvement of CD146 as a mediator of cell-cell interaction.

Lano, a novel LAP protein directly connected to MAGUK proteins in epithelial cells.

Protein networks asymetrically distributed to basolateral and apical epithelial membranes maintain cell polarity and homeostasis of epithelial tissues. Genetic studies in non-vertebrates assigned two families of basolateral proteins, MAGUK (membrane-associated and guanylate kinase) and LAP (leucine-rich repeats and PDZ) proteins, to a common pathway crucial for the epithelial architecture and acting as a gatekeeper to malignancy. In mammals, three LAP proteins have been described, Densin-180, Erbin, and hScribble. Here, we identify a protein called Lano (LAP and no PDZ) only present in vertebrates and presenting strong identities with LAP proteins. Despite the lack of PDZ domain, Lano is located at the basolateral side of epithelial cells in a similar manner to Erbin and hScribble. Using in vitro and in vivo experiments, we demonstrate that Lano directly interacts with the PDZ domains of MAGUK proteins, including hDLG (human disc large), in epithelial cells. A second pool of Lano is complexed to Erbin. These LAP-MAGUK protein complexes coexist at the basolateral side of epithelial cells. We provide evidence for a direct interaction between LAP and MAGUK proteins, and we propose that various LAP-MAGUK networks targeted to the basolateral side of epithelial cells participate to homeostasis of epithelial tissues and tumor growth.

The ERBB2/HER2 receptor differentially interacts with ERBIN and PICK1 PSD-95/DLG/ZO-1 domain proteins.

Identification of protein complexes associated with the ERBB2/HER2 receptor may help unravel the mechanisms of its activation and regulation in normal and pathological situations. Interactions between ERBB2/HER2 and Src homology 2 or phosphotyrosine binding domain signaling proteins have been extensively studied. We have identified ERBIN and PICK1 as new binding partners for ERBB2/HER2 that associate with its carboxyl-terminal sequence through a PDZ (PSD-95/DLG/ZO-1) domain. This peptide sequence acts as a dominant retention or targeting basolateral signal for receptors in epithelial cells. ERBIN belongs to the newly described LAP (LRR and PDZ) protein family, whose function is crucial in non vertebrates for epithelial homeostasis. Whereas ERBIN appears to locate ERBB2/HER2 to the basolateral epithelium, PICK1 is thought to be involved in the clustering of receptors. We show here that ERBIN and PICK1 bind to ERBB2/HER2 with different mechanisms, and we propose that these interactions are regulated in cells. Since ERBIN and PICK1 tend to oligomerize, further complexity of protein networks may participate in ERBB2/HER2 functions and specificity.

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.

ERBIN: a basolateral PDZ protein that interacts with the mammalian ERBB2/HER2 receptor.

The ERBB receptors have a crucial role in morphogenesis and oncogenesis. We have identified a new PDZ protein we named ERBIN (ERBB2 interacting protein) that acts as an adaptor for the receptor ERBB2/HER2 in epithelia. ERBIN contains 16 leucine-rich repeats (LRRs) in its amino terminus and a PDZ (PSD-95/DLG/ZO-1) domain at its carboxy terminus, and belongs to a new PDZ protein family. The PDZ domain directly and specifically interacts with ERBB2/HER2. ERBIN and ERBB2/HER2 colocalize to the lateral membrane of human intestinal epithelial cells. The ERBIN-binding site in ERBB2/HER2 has a critical role in restricting this receptor to the basolateral membrane of epithelial cells, as mutation of the ERBIN-binding site leads to the mislocalization of the receptor in these cells. We suggest that ERBIN acts in the localization and signalling of ERBB2/HER2 in epithelia.

Different functional recognition of basolateral signals in Caco-2 and MDCK cells.

Using the basolateral mutant PS of the normally apical neurotrophin receptor p75 (p75NTR) we have identified two cytoplasmic determinants responsible for this reversed localization in the human intestinal cell line, Caco2. These signals are based on two consecutive leucines (322-323) and a tyrosine (Y308). Truncation of the cytoplasmic tail removing the two leucines or their replacement by alanines led to a nonpolarized expression of the resulting mutants in Caco2 cells. To our surprise, the same mutations had no effect on the basolateral localization of the mutant PS in MDCK cells. In MDCK cells, the basolateral localization was entirely dependent on a cytoplasmic tyrosine Y308, while in Caco-2 cells this tyrosine signal was functional as a basolateral signal only when the cytoplasmic domain of PS was truncated shortly after it. These data indicate for the first time that there is a differential recognition of basolateral signals between MDCK and Caco-2 cells.

The glycosylphosphatidyl inositol-anchored adhesion molecule F3/contactin is required for surface transport of paranodin/contactin-associated protein (caspr).

Paranodin/contactin-associated protein (caspr) is a transmembrane glycoprotein of the neurexin superfamily that is highly enriched in the paranodal regions of myelinated axons. We have investigated the role of its association with F3/contactin, a glycosylphosphatidyl inositol (GPI)-anchored neuronal adhesion molecule of the Ig superfamily. Paranodin was not expressed at the cell surface when transfected alone in CHO or neuroblastoma cells. Cotransfection with F3 resulted in plasma membrane delivery of paranodin, as analyzed by confocal microscopy and cell surface biotinylation. The region that mediates association with paranodin was mapped to the Ig domains of F3 by coimmunoprecipitation experiments. The association of paranodin with F3 allowed its recruitment to Triton X-100-insoluble microdomains. The GPI anchor of F3 was necessary, but not sufficient for surface expression of paranodin. F3-Ig, a form of F3 deleted of the fibronectin type III (FNIII) repeats, although GPI-linked and expressed at the cell surface, was not recovered in the microdomain fraction and was unable to promote cell surface targeting of paranodin. Thus, a cooperative effect between the GPI anchor, the FNIII repeats, and the Ig regions of F3 is required for recruitment of paranodin into lipid rafts and its sorting to the plasma membrane.

[Identification of signals and mechanisms of sorting of plasma membrane proteins in intestinal epithelial cells]. / Identification de signaux et de mécanismes de tri des protéines de la membrane plasmique dans les cellules épithéliales intestinales.

In epithelial cells the plasma membrane is divided into domains that are biochemically and functionally different. In intestinal cells for example the apical domain is facing the intestinal lumen and is involved in the uptake of nutriments while the basolateral domain is mediating cell-cell adhesion and signalisation. We are interested in deciphering the mechanisms underlying the creation and maintenance of such specialized domains. As an epithelial model we have used the intestinal cell line Caco-2 and we have studied the transport and sorting of the human neurotrophin receptor (p75 NTR) in these cells. Newly synthesized p75 NTR is first transported to the basolateral membrane and then is accumulated on the apical membrane after transcytosis. This final apical localization is controlled by the presence of a membrane anchor and a cluster of O-glycosylation sites located in the part of the ectodomain close to the membrane. Among the mechanisms likely to be involved in the sorting of apical components we have looked for a role of lipid-protein microdomain formation in the Golgi apparatus. These membrane microdomains are highly enriched in glycosylphosphatidyl inositol (GPI) anchored proteins, glycosphingolipids and apical proteins such as sucrase isomaltase (SI). Such a composition is also found for endocytic structures called caveolae which are made of caveolin 1. We have expressed caveolin 1 in Caco-2 cells which do not express it and also caveolin 2, a related protein of unknown function. Expression of caveolin 1 led to formation of caveolae indicating that this protein is necessary for caveolae formation while caveolin 2 is restricted to the Golgi apparatus and has no effect on caveolae formation. However Caveolin 2 increased the amount of SI incorporated in microdomains suggesting a role in recruitment into the apical pathway. The choice for a site of fusion for transport vesicles is the last step of control during exocytosis. To identify proteins involved in that step we have cloned and characterized two members of the t-SNARE family, namely syntaxin 3 and SNAP23. Syntaxin 3 is present on the apical membrane and forms a complex with SNAP23 which is also localized on the basolateral membrane where it forms a complex with syntaxin 4. Overexpression of syntaxin 3 in Caco-2 led to a decrease of SI exocytosis towards the apical membrane confirming that syntaxin 3 is involved in targeting the fusion of apical transport vesicles to the apical pole of the cells.

[Mechanisms of polarized targeting of the FSH receptor]. / Mécanismes de l'adressage polarisé du récepteur de la FSH.

Gonadotropin and TSH receptors belong to a subgroup of G protein-coupled receptors. TSH and FSH receptor present a particular intracellular traffic: they present a polarized basolateral expression in thyroid follicular cells and in Sertoli cells respectively. By contrast, the LH receptor is expressed circumferentially in target gonadic cells. We expressed these receptors in MDCK cells (a well characterized model of polarized epithelial cells) to understand this difference of properties. We show that the three receptors have a polarized basolateral expression in these cells. All contain a basolateral targeting signal. Furthermore, gonadotropin receptors undergo a partial transcytosis which is not observed for the TSH receptor. We show that heterotrimeric G proteins play a role in this mechanism of transcytosis. This effect is not mediated by adenylate cyclase activation and involves a population of G proteins different from that involved in signal transduction. We thus used in vitro mutagenesis to delineate the basolateral localization signal of the FSH receptor. Surprisingly, the signal is localized in the C-terminal tail of the intracellular domain which is not conserved between the three receptors. It contains 14 amino-acids and its activity is mainly dependent on a tyrosine and a leucine residue. The basolateral localization signal of the FSHR is not colinear with its internalization signal. This signal is autonomous and dominant because, when transferred to an apically targeted membrane protein, the neurotrophin receptor, it redirects the chimeric construct to the basolateral domain of MDCK cells. The basolateral localization signal of the FSH receptor is thus the first signal identified for a G protein-coupled receptor and more generally for a hormone receptor.

Putative O-glycosylation sites and a membrane anchor are necessary for apical delivery of the human neurotrophin receptor in Caco-2 cells.

We have expressed the human neurotrophin receptor p75 (p75(NTR)) in the intestinal epithelial cell line Caco-2 as a model to study intracellular transport and subcellular sorting signals in intestinal cells. p75(NTR) was localized at the apical membrane of Caco-2 cells and reached this membrane mainly via an indirect pathway. Apical localization, intracellular routing, and basolateral to apical transcytosis were not affected by truncation of the cytoplasmic domain or replacement of the transmembrane domain by a glycosyl phosphatidylinositol anchor. Removal of membrane anchoring resulted in basolateral secretion of the ectodomain of p75(NTR) in Caco-2 cells but in apical secretion in Madin-Darby canine kidney (MDCK) cells. Substitution of potential O-glycosylation sites present in the stalk of p75(NTR) led to intracellular cleavage and secretion of the ectodomain into the basolateral medium both in Caco-2 and MDCK cells. These results suggest that the stalk of p75(NTR) carries an apical sorting information that is recognized efficiently by Caco-2 cells only when attached to the membrane. This apical sorting information is linked to the presence of predicted O-glycosylation sites in that region. These putative O-glycosylation sites also play a role in the regulation of p75(NTR) transport to the cell surface and in the prevention of rapid degradation by cleavage of the stalk domain.

The basolateral localization signal of the follicle-stimulating hormone receptor.

The follicle-stimulating hormone receptor (FSHR) is physiologically localized in the basolateral compartment of the membrane of Sertoli cells. This localization is also observed when the receptor is experimentally expressed in Madin-Darby canine kidney cells. We thus used in vitro mutagenesis and transfection into these polarized cells to delineate the basolateral localization signal of the receptor. The signal was localized in the C-terminal tail of the intracellular domain (amino acids 678-691) at a marked distance of the membrane. Mutation of individual amino acids highlighted the importance of Tyr684 and Leu689. The 14-amino acid sequence was grafted onto the p75 neurotrophin receptor and redirected this apical protein to the basolateral cell membrane compartment. Deletion of amino acids 677-695 did not modify the internalization of the FSHR, showing that the basolateral localization signal of the FSHR is not colinear with its internalization signal.

The O-glycosylated stalk domain is required for apical sorting of neurotrophin receptors in polarized MDCK cells.

Delivery of newly synthesized membrane-spanning proteins to the apical plasma membrane domain of polarized MDCK epithelial cells is dependent on yet unidentified sorting signals present in the luminal domains of these proteins. In this report we show that structural information for apical sorting of transmembrane neurotrophin receptors (p75(NTR)) is localized to a juxtamembrane region of the extracellular domain that is rich in O-glycosylated serine/threonine residues. An internal deletion of 50 amino acids that removes this stalk domain from p75(NTR) causes the protein to be sorted exclusively of the basolateral plasma membrane. Basolateral sorting stalk-minus p75(NTR) does not occur by default, but requires sequences present in the cytoplasmic domain. The stalk domain is also required for apical secretion of a soluble form of p75(NTR), providing the first demonstration that the same domain can mediate apical sorting of both a membrane-anchored as well as secreted protein. However, the single N-glycan present on p75(NTR) is not required for apical sorting of either transmembrane or secreted forms.

Human syntaxin 3 is localized apically in human intestinal cells.

To understand the molecular mechanisms underlying sorting of apical and basolateral membrane components in human intestinal epithelial cells, we have cloned the human homolog of rat syntaxin 3 and looked for its subcellular localization. Endogenous human syntaxin 3 was found to be localized at the apical membrane of colon epithelial and Caco-2 cells. This apical localization was confirmed by confocal microscopy after transfection of the cDNA coding for either full length or N-terminally truncated human syntaxin 3 in Caco-2 cells. Furthermore the signal(s) and machinery targeting human syntaxin 3 to the apical membrane of epithelial cells are conserved between species since human syntaxin 3 was also localized at the apical membrane of canine MDCK cells and of epithelial cells in transgenic Drosophila melanogaster.

Detergent-resistant membrane microdomains from Caco-2 cells do not contain caveolin.

In this study we analyzed the relationship between detergent-resistant microdomains and caveolae in Caco-2 cells. Caveolin was not detected on Western blots or Northern blots or by immunoprecipitation in these cells, in contrast to A 431 cells. Triton X-100-resistant membranes from Caco-2 and A 431 cells showed the same morphological aspect by electron microscopy and peaked at the same isopycnic density on sucrose gradients. Detergent-resistant microdomains from Caco-2 cells were enriched in glycosyl phosphatidylinositol (GPI)-anchored proteins, in sucrase-isomaltase, an apical marker, and in most of the proteins found in caveolin-rich membranes such as src-like proteins, fimbrin, ezrin, and Gi alpha. Caveolae-like structures were present in A 431 but absent from Caco-2 cells at the electron microscopic level. Detergent-resistant microdomains from Caco-2 cells resemble caveolin-rich microdomains in their molecular composition but do not seem to derive from morphologically identified caveolae. Our results also indicate that caveolin is not necessary for sorting of GPI-linked proteins to the apical membrane of Caco-2 cells.

A cytoplasmic tyrosine is essential for the basolateral localization of mutants of the human nerve growth factor receptor in Madin-Darby canine kidney cells.

Deletion of 58 internal amino acids from the C-terminal cytoplasmic domain of p75 human nerve growth factor receptor (hNGFR) changes its localization from apical to basolateral in transfected Madin-Darby Canine Kidney (MDCK) cells (Le Bivic, A., Sambuy, Y., Patzak, A., Patil, N., Chao, M., and Rodriguez-Boulan, E. (1991) J. Cell Biol. 115, 607-618). The mutant protein, PS-NGFR, also shows a dramatic increase in its ability to endocytose NGF and to recycle through basolateral endosomes. We report here the site-directed mutagenesis analysis of PS-NGFR to localize and characterize its basolateral and endocytic sorting signals. Both signals reside in the proximal part of the PS cytoplasmic tail, between positions 306 and 314. Transferring the cytoplasmic tail (19 residues) and transmembrane domain of a truncated PS mutant to the ectodomain of the placental alkaline phosphatase, an apical glypiated ectoenzyme, redirected it to the basolateral membrane and the endocytic compartments. A tyrosine at position 308, present in this short cytoplasmic segment, was mutated into phenylalanine or alanine. The resulting mutants were expressed predominantly on the apical membrane of MDCK cells. Their ability to endocytose NGF was reduced with the alanine mutant showing the stronger diminution. The PS mutant contains a short cytoplasmic sequence necessary both for basolateral targeting and endocytosis, and the requirement for tyrosine at position 308 is crucial for basolateral targeting.

TGN38 recycles basolaterally in polarized Madin-Darby canine kidney cells.

Sorting of newly synthesized plasma membrane proteins to the apical or basolateral surface domains of polarized cells is currently thought to take place within the trans-Golgi network (TGN). To explore the relationship between protein localization to the TGN and sorting to the plasma membrane in polarized epithelial cells, we have expressed constructs encoding the TGN marker, TGN38, in Madin-Darby canine kidney (MDCK) cells. We report that TGN38 is predominantly localized to the TGN of these cells and recycles via the basolateral membrane. Analyses of the distribution of Tac-TGN38 chimeric proteins in MDCK cells suggest that the cytoplasmic domain of TGN38 has information leading to both TGN localization and cycling through the basolateral surface. Mutations of the cytoplasmic domain that disrupt TGN localization also lead to nonpolarized delivery of the chimeric proteins to both surface domains. These results demonstrate an apparent equivalence of basolateral and TGN localization determinants and support an evolutionary relationship between TGN and plasma membrane sorting processes.

Ricin-resistant Madin-Darby canine kidney cells missort a major endogenous apical sialoglycoprotein.

gp114 is a major sialoglycoprotein expressed on the apical membrane of Madin-Darby canine kidney (MDCK) II cells. We investigated its distribution in two lectin-resistant mutant cell lines derived from MDCKII cells, MDCKII-RCAr and MDCKII-ConAr cells. gp114 was present on the apical membrane of MDCKII-ConAr cells but was predominantly basolateral in MDCKII-RCAr cells. No change of polarity was observed for several apical and basolateral markers in this cell line. Reversal of polarity of gp114 mainly resulted from a modification of its intracellular sorting. gp114 showed altered endocytosis in MDCKII-RCAr cells. In MDCKII cells gp114 was slowly endocytosed, whereas in MDCKII-RCAr cells endocytosis of gp114 was highly increased. Using mannosidase I and II inhibitors we found that N-glycosylation only slightly affects gp114 sorting and endocytosis. Our results suggest that gp114 or an associated component in MDCK-RCAr fails to express apical information or that a mutation creates a basolateral sorting signal which could be related to endocytic signals.

GPI-anchored proteins associate to form microdomains during their intracellular transport in Caco-2 cells.

In this study, we have investigated the possibility that glycosyl-phosphatidylinositol (GPI)-anchored proteins form insoluble membrane complexes in Caco-2 cells and that transmembrane proteins are associated with these complexes. GPI-anchored proteins were mainly resistant to Triton X-100 (TX-100) extraction at 4 degrees C but fully soluble in n-octyl-glucoside. Resistance to Triton X-100 extraction was not observed in the endoplasmic reticulum but appeared during transport through the Golgi complex. It was not dependent upon N-glycosylation processing, or pH variation from 6.5 to 8.5, and was not affected by sterol-binding agents. Other apical or basolateral transmembrane proteins were well solubilized in TX-100, with the exception of sucrase-isomaltase, which was partly insoluble. We isolated a membrane fraction from Caco-2 cells that contained GPI-anchored proteins and sucrase-isomaltase but no antigen 525, a basolateral marker, or dipeptidylpeptidase IV, an apical one. These data suggest that GPI-anchored proteins cluster to form membrane microdomains together with an apical transmembrane protein, providing a possible apical sorting mechanism for intestinal cells in vitro that might be related to apical sorting in MDCK cells, and that other mechanisms might exist to sort proteins to the apical membrane.

The internalization signal and the phosphorylation site of transferrin receptor are distinct from the main basolateral sorting information.

Wild-type human transferrin receptor (hTfR), like endogenous canine receptor, is expressed almost exclusively (97%) at the basolateral membrane of transfected Madin-Darbey canine kidney (MDCK) cells. We investigated the role of two distinct features of the hTfR cytoplasmic domain, namely the endocytic signal and the unique phosphorylation site, in polarized cell surface delivery. Basolateral location was not altered by point mutation of Ser24-->Ala24, indicating that phosphorylation is not involved in vectorial sorting of hTfR. The steady state distribution of hTfR was partially affected by a deletion of 36 cytoplasmic residues encompassing the internalization sequence. However, 80% of the receptors were still basolateral. As assessed by pulse-chase experiments in combination with biotinylation, newly synthesized wild-type and deletion mutant receptors were directly sorted to the domain of their steady state residency. Although both receptors could bind human transferrin, endocytosis of the deletion mutant was strongly impaired at either surface. These data indicate that the predominant basolateral targeting signal of hTfR is independent of the internalization sequence.