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

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.

Truncated brush border myosin I affects membrane traffic in polarized epithelial cells

We investigate, in this study, the potential involvement of an acto-myosin-driven mechanism in endocytosis of polarized cells. We observed that depolymerization of actin filaments using latrunculin A decreases the rate of transferrin recycling to the basolateral plasma membrane of Caco-2 cells, and increases its delivery to the apical plasma membrane. To analyze whether a myosin was involved in endocytosis, we produced, in this polarized cell line, truncated, non-functional, brush border, myosin I proteins (BBMI) that we have previously demonstrated to have a dominant negative effect on endocytosis of unpolarized cells. These non-functional proteins affect the rate of transferrin recycling and the rate of transepithelial transport of dipeptidyl-peptidase IV from the basolateral plasma membrane to the apical plasma membrane. They modify the distribution of internalized endocytic tracers in apical multivesicular endosomes that are accessible to fluid phase tracers internalized from apical and basolateral plasma membrane domains. Altogether, these observations suggest that an acto-myosin-driven mechanism is involved in the trafficking of basolaterally internalized molecules to the apical plasma membrane.

Bidirectional transcytosis of IgG by the rat neonatal Fc receptor expressed in a rat kidney cell line: a system to study protein transport across epithelia

The neonatal Fc receptor, FcRn, transports immunoglobulin G (IgG) across cellular barriers between mother and offspring. FcRn also protects circulating IgG from catabolism, probably during transport across the capillary endothelium. Only one cell culture model of transcytosis has been used extensively, the transport of IgA from the basolateral to the apical surface of Madin-Darby canine kidney cells by the polymeric immunoglobulin receptor (pIgR). We report that rat inner medullary collecting duct (IMCD) cells transfected with DNA encoding the (alpha) subunit of rat FcRn specifically and saturably transport Fc when grown as polarized monolayers. Using this system, we have found that transcytosis by FcRn, like transcytosis by the pIgR, depends upon an intact microtubule system. FcRn differs most strikingly from the pIgR in its ability to transport its ligand in both the apical to basolateral and basolateral to apical directions. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 inhibited basolateral to apical transport by FcRn more than apical to basolateral transport, suggesting that there are differences in the mechanisms of transport in the two directions. Lastly, we found that transcytosis by FcRn depends upon vesicular acidification. We anticipate that the IMCD cell culture model will allow further elucidation of the mechanism of IgG transport by FcRn.

Basolateral sorting signals differ in their ability to redirect apical proteins to the basolateral cell surface

Polarized sorting of membrane proteins in epithelial cells is mediated by cytoplasmic basolateral signals or by apical signals in the transmembrane or exoplasmic domains. Basolateral signals were generally found to be dominant over apical determinants. We have generated chimeric proteins with the cytoplasmic domain of either the asialoglycoprotein receptor H1 or the transferrin receptor, two basolateral proteins, fused to the transmembrane and exoplasmic segments of aminopeptidase N, an apical protein, and analyzed them in Madin-Darby canine kidney cells. Whereas both cytoplasmic sequences induced endocytosis of the chimeras, only that of the transferrin receptor mediated basolateral expression in steady state. The H1 fusion protein, although still largely sorted to the basolateral side in biosynthetic surface transport, was subsequently resorted to the apical cell surface. We tested whether the difference in sorting between trimeric wild-type H1 and the dimeric aminopeptidase chimera was caused by the number of sorting signals presented in the oligomers. Consistent with this hypothesis, the H1 signal was fully functional in a tetrameric fusion protein with the transmembrane and exoplasmic domains of influenza neuraminidase. The results suggest that basolateral signals per se need not be dominant over apical determinants for steady-state polarity and emphasize an important contribution of the valence of signals in polarized sorting.

Canalicular export pumps traffic with polymeric immunoglobulin A receptor on the same microtubule-associated vesicle in rat liver

Basolateral to apical vesicular transcytosis in the hepatocyte is an essential pathway for the delivery of compounds from the sinusoidal blood to the bile and to traffic newly synthesized resident apical membrane proteins to their site of function at the canalicular membrane front. To characterize this pathway better, microtubules in a hepatocyte homogenate were polymerized by addition of taxol, and associated membrane-bound vesicles were isolated. This fraction was enriched in polymeric immunoglobulin A receptor and contained apical membrane proteins. Immunoelectron microscopy demonstrated that polymeric immunoglobulin A receptor was localized predominantly on vesicles ranging from 100 to 160 nm and that the multidrug resistance protein 2 and the bile salt export pump co-localized on these vesicles. The minus-ended microtubule motor, dynein, was highly enriched in the fraction, and its intermediate chain could be released effectively by incubation with 1 mM ATP or GTP. However, the association of the transcytotic vesicles with the microtubules was not sensitive to hydrolyzable or non-hydrolyzable nucleotides. This study characterizes a fraction of microtubule-associated vesicles from rat hepatocytes and demonstrates that several resident apical membrane transport proteins and the polymeric immunoglobulin A receptor traffic on the same vesicle.

Targeting of aminopeptidase N to bile canaliculi correlates with secretory activities of the developing canalicular domain

We have used human hepatoma cell lines as an in vitro model to study the development of hepatic bile canaliculi (BC). Well-differentiated hepatoma cells cultured for 72 hours could develop characteristic spheroid structures at sites of cell-cell contact that contained tight junctions and various membrane protein markers, resembling BC found in vivo. Intact cytoskeleton was essential for this differentiation process. In the coculture experiments in which cells of different origins were populated together, BC only formed between hepatic cells and preferentially among well-differentiated cells. Poorly differentiated hepatoma cells never formed BC among themselves, but could be induced to undergo canalicular differentiation by interacting with well-differentiated cells. During BC morphogenesis, integral canalicular membrane proteins were gradually delivered and accumulated at the developing BC. Among them, targeting of aminopeptidase N (APN) seemed to correlate with activation of certain secretory functions. Specifically, only APN-positive BC supported excretion of fluorescein diacetate (FDA) and 70-kd dextran, but had no relationship with secretion of horseradish peroxidase (HRP). Targeting of another BC protein, dipeptidyl peptidase IV (DPPIV), on the other hand, bore no association with any secretory activity examined. In addition, inhibition of enzymatic activity of APN could perturb canalicular differentiation without affecting cell proliferation. Our results suggest that targeting of APN proteins may reflect or even play an important role in the development and functional maturation of the canalicular structures.

Biosynthesis and secretion of the mannose 6-phosphate receptor and its ligands in polarized Caco-2 cells

We have analyzed the transport of newly synthesized mannose 6-phosphate (Man-6-P)-bearing proteins (i.e., lysosomal enzymes) in the polarized human colon adenocarcinoma cell line, Caco-2, by subjecting filter-grown cells to a pulse-chase labeling protocol using [(35)S]methionine, and the resulting cell lysate, apical medium, and basolateral medium were immunoprecipitated with insulin-like growth factor II/Man-6-P receptor (IGF-II/MPR)-specific antisera. The results showed that the majority of secreted lysosomal enzymes accumulated in the apical medium at >2 h of chase and that this polarized distribution was facilitated by the IGF-II/MPR selectively endocytosing lysosomal enzymes from the basolateral surface. Treatment with various agents known to affect vesicular transport events demonstrated that incubations at 16 degrees C or incubations with brefeldin A inhibited the secretion of lysosomal enzymes from both the apical and basolateral surface, whereas treatment with nocodazole selectively blocked apical secretion. In contrast, incubation with NH4Cl or nocodazole had a stimulatory effect on basolateral secretion. Taken together, these results demonstrate that the sorting of Man-6-P-containing proteins into the apical and basolateral secretory pathways is regulated by distinct components of the intracellular trafficking machinery.

Temporal association of the N- and O-linked glycosylation events and their implication in the polarized sorting of intestinal brush border sucrase-isomaltase, aminopeptidase N, and dipeptidyl peptidase IV

The temporal association between O-glycosylation and processing of N-linked glycans in the Golgi apparatus as well as the implication of these events in the polarized sorting of three brush border proteins has been the subject of the current investigation. O-Glycosylation of pro-sucrase-isomaltase (pro-SI), aminopeptidase N (ApN), and dipeptidyl peptidase IV (DPPIV) is drastically reduced when processing of the mannose-rich N-linked glycans is blocked by deoxymannojirimycin, an inhibitor of the Golgi-located mannosidase I. By contrast, O-glycosylation is not affected in the presence of swainsonine, an inhibitor of Golgi mannosidase II. The results indicate that removal of the outermost mannose residues by mannosidase I from the mannose-rich N-linked glycans is required before O-glycosylation can ensue. On the other hand, subsequent mannose residues in the core chain impose no sterical constraints on the progression of O-glycosylation. Reduction or modification of N- and O-glycosylation do not affect the transport of pro-SI, ApN, or DPPIV to the cell surface per se. However, the polarized sorting of two of these proteins, pro-SI and DPPIV, to the apical membrane is substantially altered when O-glycans are not completely processed, while the sorting of ApN is not affected. The processing of N-linked glycans, on the other hand, has no influence on sorting of all three proteins. The results indicate that O-linked carbohydrates are at least a part of the sorting mechanism of pro-SI and DPPIV. The sorting of ApN implicates neither O-linked nor N-linked glycans and is driven most likely by carbohydrate-independent mechanisms.

O-linked glycans mediate apical sorting of human intestinal sucrase-isomaltase through association with lipid rafts

The plasma membrane of polarised epithelial cells is characterised by two structurally and functionally different domains, the apical and basolateral domains. These domains contain distinct protein and lipid constituents that are sorted by specific signals to the correct surface domain [1]. The best characterised apical sorting signal is that of glycophosphatidylinositol (GPI) membrane anchors [2], although N-linked glycans on some secreted proteins [3] and O-linked glycans [4] also function as apical sorting signals. In the latter cases, however, the underlying sorting mechanisms remain obscure. Here, we have analysed the role of O-glycosylation in the apical sorting of sucrase-isomaltase (SI), a highly polarised N- and O-glycosylated intestinal enzyme, and the mechanisms underlying this process. Inhibition of O-glycosylation by benzyl-N-acetyl-alpha-D-galactosaminide (benzyl-GalNAc) was accompanied by a dramatic shift in the sorting of SI from the apical membrane to both membranes. The sorting mechanism of SI involves its association with sphingolipid- and cholesterol-rich membrane rafts because this association was eliminated when O-glycosylation was inhibited by benzyl-GaINAc. The results demonstrate for the first time that O-linked glycans mediate apical sorting through association with lipid rafts.

Mechanisms of apical protein sorting in polarized thyroid epithelial cells

The process leading to thyroid hormone synthesis is vectorial and depends upon the polarized organization of the thyrocytes into the follicular unit. Thyrocyte membrane proteins are delivered to two distinct domains of the plasma membrane using apical (AP) and basolateral (BL) sorting signals. A recent hypothesis for AP sorting proposes that apically destined proteins cluster with glycosphingolipids (GSLs) and cholesterol, into microdomains (or rafts) of the Golgi membrane from which AP vesicles originate. In MDCK cells the human neurotrophin receptor, p75hNTR, is delivered to the AP surface through a sorting signal, rich in O-glycosylated sugars, identified in its ectodomain. We have investigated whether this signal is functional in the thyroid-derived FRT cell line and whether p75hNTR clusters into lipid rafts to be sorted to the AP membrane. We found that p75hNTR is apically delivered via a direct pathway and does not associate with rafts during its transport to the surface of FRT cells. Therefore, although the same signal could be recognized by different cell types thyroid cells may possess a tissue-specific sorting machinery.

Selective localization of the polytopic membrane protein prominin in microvilli of epithelial cells - a combination of apical sorting and retention in plasma membrane protrusions

Prominin is a recently identified polytopic membrane protein expressed in various epithelial cells, where it is selectively associated with microvilli. When expressed in non-epithelial cells, prominin is enriched in plasma membrane protrusions. This raises the question of whether the selective association of prominin with microvilli in epithelial cells is solely due to its preference for, and stabilization in, plasma membrane protrusions, or is due to both sorting to the apical plasma membrane domain and subsequent enrichment in plasma membrane protrusions. To investigate this question, we have generated stably transfected MDCK cells expressing either full-length or C-terminally truncated forms of mouse prominin. Confocal immunofluorescence and domain-selective cell surface biotinylation experiments on transfected MDCK cells grown on permeable supports demonstrated the virtually exclusive apical localization of prominin at steady state. Pulse-chase experiments in combination with domain-selective cell surface biotinylation showed that newly synthesized prominin was directly targeted to the apical plasma membrane domain. Immunoelectron microscopy revealed that prominin was confined to microvilli rather than the planar region of the apical plasma membrane. Truncation of the cytoplasmic C-terminal tail of prominin impaired neither its apical cell surface expression nor its selective retention in microvilli. Both the apical-specific localization of prominin and its selective retention in microvilli were maintained when MDCK cells were cultured in low-calcium medium, i.e. in the absence of tight junctions. Taken together, our results show that: (i) prominin contains dual targeting information, for direct delivery to the apical plasma membrane domain and for the enrichment in the microvillar subdomain; and (ii) this dual targeting does not require the cytoplasmic C-terminal tail of prominin and still occurs in the absence of tight junctions. The latter observation suggests that entry into, and retention in, plasma membrane protrusions may play an important role in the establishment and maintenance of the apical-basal polarity of epithelial cells.

Stimulation side-dependent asymmetrical secretion of poly I:poly C-induced interferon-beta from polarized epithelial cell lines

Mode of secretion of poly I:poly C-induced IFN was examined using epithelial cell lines in a bicameral culture system. Although the cell lines formed a tight cell sheet and produced IFN-beta following poly I:poly C treatment in spite of its application to the upper or lower compartment, IFN secretion differed between the apical and basolateral cell membranes. When poly I:poly C was applied to the upper compartment, IFN was secreted predominantly from the apical membrane. Inversely, poly I:poly C applied to the lower compartment caused preferential IFN secretion from the basolateral membrane. These results suggest that in epithelial cells poly I:poly C stimulation induces intracellular membrane traffic toward the stimulation side.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Mechanisms and functional features of polarized membrane traffic in epithelial and hepatic cells

Epithelial cells express plasma-membrane polarity in order to meet functional requirements that are imposed by their interaction with different extracellular environments. Thus apical and basolateral membrane domains are distinguished that are separated by tight junctions in order to maintain the specific lipid and protein composition of each domain. In hepatic cells, the plasma membrane is also polarized, containing a sinusoidal (basolateral) and a bile canalicular (apical)-membrane domain. Relevant to the biogenesis of these domains are issues concerning sorting, (co-)transport and regulation of transport of domain-specific membrane components. In epithelial cells, specific proteins and lipids, destined for the apical membrane, are sorted in the trans-Golgi network (TGN), which involves their sequestration into cholesterol/sphingolipid 'rafts', followed by 'direct' transport to the apical membrane. In hepatic cells, a direct apical transport pathway also exists, as revealed by transport of sphingolipids from TGN to the apical membrane. This is remarkable, since in these cells numerous apical membrane proteins are 'indirectly' sorted, i.e. they are first transferred to the basolateral membrane prior to their subsequent transcytosis to the apical membrane. This raises intriguing questions as to the existence of specific lipid rafts in hepatocytes. As demonstrated in studies with HepG2 cells, it has become evident that, in hepatic cells, apical transport pathways can be regulated by protein kinase activity, which in turn modulates cell polarity. Finally, an important physiological function of hepatic cells is their involvement in intracellular transport and secretion of bile-specific lipids. Mechanisms of these transport processes, including the role of multidrug-resistant proteins in lipid translocation, will be discussed in the context of intracellular vesicular transport. Taken together, hepatic cell systems provide an important asset to studies aimed at elucidating mechanisms of sorting and trafficking of lipids (and proteins) in polarized cells in general.

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.

Rotavirus infection reduces sucrase-isomaltase expression in human intestinal epithelial cells by perturbing protein targeting and organization of microvillar cytoskeleton

Rotavirus infection is the most common cause of severe infantile gastroenteritis worldwide. These viruses infect mature enterocytes of the small intestine and cause structural and functional damage, including a reduction in disaccharidase activity. It was previously hypothesized that reduced disaccharidase activity resulted from the destruction of rotavirus-infected enterocytes at the villus tips. However, this pathophysiological model cannot explain situations in which low disaccharidase activity is observed when rotavirus-infected intestine exhibits few, if any, histopathologic changes. In a previous study, we demonstrated that the simian rotavirus strain RRV replicated in and was released from human enterocyte-like Caco-2 cells without cell destruction (N. Jourdan, M. Maurice, D. Delautier, A. M. Quero, A. L. Servin, and G. Trugnan, J. Virol. 71:8268-8278, 1997). In the present study, to reinvestigate disaccharidase expression during rotavirus infection, we studied sucrase-isomaltase (SI) in RRV-infected Caco-2 cells. We showed that SI activity and apical expression were specifically and selectively decreased by RRV infection without apparent cell destruction. Using pulse-chase experiments and cell surface biotinylation, we demonstrated that RRV infection did not affect SI biosynthesis, maturation, or stability but induced the blockade of SI transport to the brush border. Using confocal laser scanning microscopy, we showed that RRV infection induces important alterations of the cytoskeleton that correlate with decreased SI apical surface expression. These results lead us to propose an alternate model to explain the pathophysiology associated with rotavirus infection.

TGN38 cycles via the basolateral membrane of polarized Caco-2 cells

TGN38 is a heavily glycosylated, type I integral membrane protein which is predominantly localized to the trans Golgi network (TGN), but which constitutively traffics between the TGN and the cell surface. The trafficking of TGN38 has been extensively studied in non-polarized cells, and a short, tyrosine-based, peptide motif within the cytosolic domain of the protein has been shown to be necessary and sufficient for its rapid internalization from the cell surface and efficient delivery to the TGN. Such tyrosine-based motifs have also been shown to act as basolateral targeting signals, whilst N-linked glycans (as occur on the extracytosolic domain of TGN38) can act as apical targeting signals. TGN38 has previously been shown to be sorted to the basolateral surface of polarized canine MDCK cells; a polarized cell line in which biosynthetic sorting decisions concerning the eventual destination of apical or basolateral targeted plasma membrane proteins are made at the TGN. We now show that TGN38 is targeted exclusively to the basolateral domain of polarized human Caco-2 cells, a cell line in which newly synthesized membrane proteins destined for either the apical or basolateral plasma membrane may be sorted for delivery to their final destination either at the TGN or at the cell surface. These data also demonstrate that the heavily glycosylated, extracytosolic domain of TGN38 does not contain a dominant apical targeting signal.

Targeting of CFTR protein is linked to the polarization of human pancreatic duct cells in culture

A relationship between targeting of the protein CFTR (Cystic Fibrosis Transmembrane conductance Regulator) and cellular polarization has been observed in various types of epithelial cells. However, there are no reports on this in human exocrine pancreatic cells, which are functionally altered in patients with cystic fibrosis. The expression of CFTR and its targeting to apical plasma membranes was investigated during growth and polarization of human ductal pancreatic cancerous Capan-1 cells. Despite their neoplastic origin, the cancerous pancreatic duct cells of the Capan-1 line secrete Cl- and HCO3- ions. We showed by electron microscopy, impregnation of cells with tannin and freeze-fracture that these cells become polarized during growth in culture, and are joined by tight junctions. The expression of CFTR and the various stages in its anchorage to membranes was followed using a specific polyclonal antibody, ECL-885, directed against a synthetic peptide mimicking one of the extracellular loops of CFTR. Qualitative and quantitative confocal microscopic studies showed that: (i) the expression of CFTR was constant during growth, irrespective of cellular conformation, (ii) the number of cells presenting CFTR anchored to membranes increased with time in culture, (iii) the rise in membrane-bound CFTR-immunoreactivity accompanied the polarization of the cells, (iv) CFTR anchored to plasma membranes was distributed regularly over the surface of non-polarized cells, but was localized only at the apical membranes of the polarized cells. Moreover, patch-clamp studies indicated the presence of few Cl- cAMP-dependent conductance CFTR channels on unpolarized cells, and a larger number of CFTR channels on the apical plasma membranes of polarized cells. These results indicated that the anchorage of a functional CFTR to the plasma membrane is progressive and occurs in step with polarization of these human pancreatic duct cells in culture. We suggest that the targeting of CFTR to the apical membranes is directly linked to the process of cellular polarization.

A novel tetanus neurotoxin-insensitive vesicle-associated membrane protein in SNARE complexes of the apical plasma membrane of epithelial cells

The importance of soluble N-ethyl maleimide (NEM)-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (SNAREs) in synaptic vesicle exocytosis is well established because it has been demonstrated that clostridial neurotoxins (NTs) proteolyze the vesicle SNAREs (v-SNAREs) vesicle-associated membrane protein (VAMP)/brevins and their partners, the target SNAREs (t-SNAREs) syntaxin 1 and SNAP25. Yet, several exocytotic events, including apical exocytosis in epithelial cells, are insensitive to numerous clostridial NTs, suggesting the presence of SNARE-independent mechanisms of exocytosis. In this study we found that syntaxin 3, SNAP23, and a newly identified VAMP/brevin, tetanus neurotoxin (TeNT)-insensitive VAMP (TI-VAMP), are insensitive to clostridial NTs. In epithelial cells, TI-VAMP-containing vesicles were concentrated in the apical domain, and the protein was detected at the apical plasma membrane by immunogold labeling on ultrathin cryosections. Syntaxin 3 and SNAP23 were codistributed at the apical plasma membrane where they formed NEM-dependent SNARE complexes with TI-VAMP and cellubrevin. We suggest that TI-VAMP, SNAP23, and syntaxin 3 can participate in exocytotic processes at the apical plasma membrane of epithelial cells and, more generally, domain-specific exocytosis in clostridial NT-resistant pathways.

Analysis of a naturally occurring mutation in sucrase-isomaltase: glutamine 1098 is not essential for transport to the surface of COS-1 cells

A glutamine for proline substitution at position 1098 was previously shown to result in accumulation of brush-border sucrase-isomaltase in the Golgi apparatus. The substitution is present in a highly homologous region of the protein, and results in a comparable accumulation when introduced into the same region in lysosomal alpha-glucosidase. To study the importance of the glutamine-1098, we analyzed the transport compatibility of two mutants in which glutamine-1098 is substituted by lysine or alanine. Both mutants were transported to the cell surface and processed comparable to wild type. We concluded that glutamine-1098 is not essential for transport to the cell surface.

Carbohydrate-mediated Golgi to cell surface transport and apical targeting of membrane proteins

Polarized expression of most epithelial plasma membrane proteins is achieved by selective transport from the Golgi apparatus or from endosomes to a specific cell surface domain. In Madin-Darby canine kidney (MDCK) cells, basolateral sorting generally depends on distinct cytoplasmic targeting determinants. Inactivation of these signals often resulted in apical expression, suggesting that apical transport of transmembrane proteins occurs either by default or is mediated by widely distributed characteristics of membrane glycoproteins. We tested the hypothesis of N-linked carbohydrates acting as apical targeting signals using three different membrane proteins. The first two are normally not glycosylated and the third one is a glycoprotein. In all three cases, N-linked carbohydrates were clearly able to mediate apical targeting and transport. Cell surface transport of proteins containing cytoplasmic basolateral targeting determinants was not significantly affected by N-linked sugars. In the absence of glycosylation and a basolateral sorting signal, the reporter proteins accumulated in the Golgi complex of MDCK as well as CHO cells, indicating that efficient transport from the Golgi apparatus to the cell surface is signal-mediated in polarized and non-polarized cells.

Specific interaction of food proteins with apical membranes of the human intestinal cell lines Caco-2 and T84

A comparison between the intestinal epithelial cell lines Caco-2 and T84 was made to assess the influence of enterocytic differentiation on food protein binding capacities of the brush border membrane. Cell morphology and expression of brush border-associated enzymes were studied as differentiation markers. Food protein binding to isolated brush border membranes was measured with a dot blot chemiluminescence assay. Early at confluence, Caco-2 cells exhibited a more differentiated state compared to T84 cells. Brush border membranes of both cell lines bound gliadin peptides, beta-lactoglobulin and ovalbumin specifically. Binding capacities increased from gliadin peptides to ovalbumin to beta-lactoglobulin. There was correlation of membrane binding capacity with degree of cell differentiation. Due to their similarity to small intestinal epithelial cells, the colon carcinoma cell lines Caco-2 and T84 represent models for studying food protein-enterocytic brush border membrane interactions in relation to varying degrees of cell differentiation.

Signals and Mechanisms of Sorting in Epithelial Polarity

This chapter discusses epithelial-membrane polarity, sorting pathways in polarized cells, and the sorting-signal paradigm. Polarized epithelial cells have long captured the attention of cell biologists and cell physiologists. At the electron-microscopic level, one of the most apparent and fundamental features of this cell type is its polarized organization of intracellular organelles and its structurally and compositionally distinct lumenal (apical) and serosal (basolateral) plasma-membrane domains. The polarized epithelial phenotype is an absolute necessity for organ-system function. In the most general sense, these cells organize to form a continuous, single layer of cells, or epithelium, which serves as a semi-permeable barrier between apposing and biologically distinct compartments. Within the tubules of the nephron, these cells orchestrate complex ion-transporting processes that ultimately control the overall fluid balance of the organism. At the surface of the gastrointestinal tract, specialized versions of this cell type control the digestion, absorption, and immuno-protection of the organism.

Neuroepithelial cells downregulate their plasma membrane polarity prior to neural tube closure and neurogenesis

Cell differentiation often involves changes in cell polarity. In this study we show that neuroepithelial cells, the progenitors of all neurons and macroglial cells of the vertebrate central nervous system, downregulate the polarized delivery to the apical and basolateral plasma membrane domains during development. Upon infection of the neuroepithelium of mouse embryos with fowl plague virus (FPV), polarized delivery of the viral envelope hemagglutinin, an apical marker, occurred at the neural plate stage (E8), but was downregulated at the open neural tube stage (E9). Upon infection with vesicular stomatitis virus, the viral envelope G protein, a basolateral marker, showed an unpolarized delivery not only at the open neural tube stage, but already at the neural plate stage. These results show that a progressive downregulation of plasma membrane polarity of neuroepithelial cells precedes neural tube closure and the onset of neurogenesis.

Apical targeting in polarized epithelial cells: There's more afloat than rafts

Most metazoan cells are 'polarized'. A crucial aspect of this polarization is that the plasma membrane is divided into two or more domains with different protein and lipid compositions or example, the apical and basolateral domains of epithelial cells or the axonal and somatodendritic domains of neurons. This polarity is established and maintained by highly specific vesicular membrane transport in the biosynthetic, endocytic and transcytotic pathways. Two important concepts, the 'SNARE' and the 'raft' hypotheses, have been developed that together promise at least a partial understanding of the underlying general mechanisms that ensure the necessary specificity of these pathways.

Regulation of the epithelial brush border Na+/H+ exchanger isoform 3 stably expressed in fibroblasts by fibroblast growth factor and phorbol esters is not through changes in phosphorylation of the exchanger

The epithelial brush border Na+/H+ exchanger isoform 3 (NHE3) is regulated by growth factors and protein kinases. When stably expressed in PS120 fibroblasts, NHE3 is stimulated by serum and fibroblast growth factor (FGF) and inhibited by phorbol esters. To examine the role of phosphorylation of NHE3 in growth factor/protein kinase regulation, NHE3 was C-terminally tagged with an 11-amino acid epitope of the vesicular stomatitis virus glycoprotein (VSVG) and stably expressed in Na+/H+ exchanger null PS120 fibroblasts (PS120/NHE3V). NHE3V was regulated by serum, FGF, and phorbol ester in a manner identical to wild type non-VSVG-tagged NHE3. Phosphorylation of NHE3V was evaluated via immunoprecipitation with anti-VSVG antibody after in vivo labeling of PS120/NHE3V cells with [32P]orthophosphate. NHE3V was phosphorylated under basal conditions. However, FGF and PMA, under conditions in which these agonists regulate NHE3V, altered neither the amount of phosphorylation of NHE3V as analyzed by one-dimensional SDS-polyacrylamide gel electrophoresis and autoradiography nor two-dimensional phosphopeptide maps of tryptic digests of NHE3V. In contrast, while changes in NHE3V phosphorylation were not observed with serum exposure by one-dimensional SDS-polyacrylamide gel electrophoresis, two-dimensional studies showed increases in two phosphopeptides. Under all these conditions, phosphoamino acid analysis showed that NHE3V was phosphorylated only on serine residues. By cell surface protein biotinylation studies under basal conditions, at least 27% of the NHE3V was expressed on the cell surface. To further analyze the phosphorylation status of the surface and intracellular forms of NHE3V under basal conditions and determine whether the amount of phosphorylation of the surface form changes upon serum, FGF, and PMA regulation, the surface form of NHE3V was separated from intracellular form by biotinylation/avidin-agarose precipitation. Under basal conditions, both intracellular and surface forms of NHE3V were phosphorylated. However, the amount of phosphorylation of the surface form of NHE3V did not change upon stimulation by serum and FGF and inhibition by PMA based on one-dimensional SDS-polyacrylamide gel electrophoresis and autoradiography. Thus, we conclude that when expressed in PS120 cells, while NHE3 is a phosphoprotein under basal conditions, its regulation by FGF and PMA is not by changes in the phosphorylation of NHE3, while regulation by serum may involve changes in its phosphorylation. Regulation of NHE3 probably involves intermediate associated regulatory proteins. The function of basal phosphorylation of NHE3 is not known.

Glutamate receptors are selectively targeted to postsynaptic sites in neurons

The objective of the present study was to determine if a neuron that expresses multiple glutamate receptors targets the same receptors to all glutamatergic postsynaptic populations, or if the receptors are differentially targeted to specific postsynaptic populations. As a model for this study, we chose the fusiform cell of the dorsal cochlear nucleus. This neuron expresses multiple glutamate receptors and receives two distinct glutamatergic inputs: parallel fibers synapse on apical dendrites, and auditory nerve fibers synapse on basal dendrites. Pre- and postembedding immunocytochemistry were combined with retrograde tracing to identify the receptors expressed on postsynaptic membranes of parallel fiber and auditory nerve synapses. Most receptors were found at both populations of synapses, but the AMPA receptor subunit, GluR4, and the metabotropic receptor, mGluR1 alpha, were found only at the auditory nerve synapse. These results demonstrate that glutamate receptors are targeted to specific postsynaptic populations of glutamatergic synapses.

The apical submembrane cytoskeleton participates in the organization of the apical pole in epithelial cells

In a previous publication (Rodriguez, M.L., M. Brignoni, and P.J.I. Salas. 1994. J. Cell Sci. 107: 3145-3151), we described the existence of a terminal web-like structure in nonbrush border cells, which comprises a specifically apical cytokeratin, presumably cytokeratin 19. In the present study we confirmed the apical distribution of cytokeratin 19 and expanded that observation to other epithelial cells in tissue culture and in vivo. In tissue culture, subconfluent cell stocks under continuous treatment with two different 21-mer phosphorothioate oligodeoxy nucleotides that targeted cytokeratin 19 mRNA enabled us to obtain confluent monolayers with a partial (40-70%) and transitory reduction in this protein. The expression of other cytoskeletal proteins was undisturbed. This downregulation of cytokeratin 19 resulted in (a) decrease in the number of microvilli; (b) disorganization of the apical (but not lateral or basal) filamentous actin and abnormal apical microtubules; and (c) depletion or redistribution of apical membrane proteins as determined by differential apical-basolateral biotinylation. In fact, a subset of detergent-insoluble proteins was not expressed on the cell surface in cells with lower levels of cytokeratin 19. Apical proteins purified in the detergent phase of Triton X-114 (typically integral membrane proteins) and those differentially extracted in Triton X-100 at 37 degrees C or in n-octyl-beta-D-glycoside at 4 degrees C (representative of GPI-anchored proteins), appeared partially redistributed to the basolateral domain. A transmembrane apical protein, sucrase isomaltase, was found mispolarized in a subpopulation of the cells treated with antisense oligonucleotides, while the basolateral polarity of Na+-K+ATPase was not affected. Both sucrase isomaltase and alkaline phosphatase (a GPI-anchored protein) appeared partially depolarized in A19 treated CACO-2 monolayers as determined by differential biotinylation, affinity purification, and immunoblot. These results suggest that an apical submembrane cytoskeleton of intermediate filaments is expressed in a number of epithelia, including those without a brush border, although it may not be universal. In addition, these data indicate that this structure is involved in the organization of the apical region of the cytoplasm and the apical membrane.

Colon carcinoma glycoproteins carrying alpha 2,6-linked sialic acid reactive with Sambucus nigra agglutinin are not constitutively expressed in normal human colon mucosa and are distinct from sialyl-Tn antigen

In human colon carcinoma, increased amounts of sialic acids have been found and correlated with tumor progression. Further, the degree of O-acetylation of sialic acid residues in normal mucosa is higher than in colon carcinoma. Thus, tumor-associated sialylated antigens may be constitutively expressed in O-acetylated form in normal mucosa unreactive with the respective monoclonal antibodies. We have earlier demonstrated a colon carcinoma-associated expression of alpha 2,6-linked sialic acid residues with the Sambucus nigra agglutinin (SNA). We report now that de-acetylation of normal and transitional colonic mucosa, in contrast to sialyl-Tn antigen, does not result in SNA binding. Further, the alpha 2,6-linked sialic acid recognized by SNA is distinct from that of sialyl-Tn antigen. This is confirmed by Northern blotting detecting transcripts for alpha 2,6 sialyltransferase of N-glycoproteins and measurement of activity for this sialyltransferase. Blot analysis by SNA of colon carcinoma cells revealed few reactive glycoproteins. Quantitative differences in lectin labeling and sialyltransferase activity were found in HCT116 colon carcinoma cell sub-lines. Our data suggest that SNA binding in human colon carcinoma is due to de novo expression of a specific sialic acid present on selected glycoproteins.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Basal nutrition promotes human intestinal epithelial (Caco-2) proliferation, brush border enzyme activity, and motility

OBJECTIVES

Provision of nutrients to the apical membrane of intestinal epithelial cells by the enteral route is critical for normal gut mucosal function and for the sheet migration required for mucosal healing. The present work attempts to determine whether supplemental nutrient delivery to the basal epithelial surface is important for intestinal epithelial biology. Since attempts to regulate intestinal epithelial cell biology by manipulation of parenteral nutrition solutions have met with some success, we hypothesized that basally delivered nutrients might also be important for intestinal epithelial biology.

DESIGN

To test this hypothesis, we compared the brush border enzyme activity, proliferation, and motility of human intestinal epithelial (Caco-2) cells cultured on a type I collagen substrate either on cell culture dishes with culture medium above the apical side of the cell monolayer or in culture inserts on 0.45-mu semipermeable membranes with culture medium beneath the monolayers as well as above them. Proliferation was assessed by serial hematocytometric counts over 13-day period. Doubling times were calculated by logarithmic transformation of cell counts 48 hrs apart. The specific activity of the brush border enzymes, dipeptidyl dipeptidase and alkaline phosphatase, was assayed by the digestion of synthetic chromogenic substrates in protein-matched aliquots of cell lysates. Sheet migration was quantitated by the expansion of Caco-2 monolayers across collagen. Motility was dissociated from the proliferative component of monolayer expansion by blocking proliferation with mitomycin C.

SETTING

Laboratory for gastrointestinal mucosal biology.

SUBJECTS

A well-differentiated subclone of cells derived from the established human Caco-2 colonic epithelial cell line.

MEASUREMENTS AND MAIN RESULTS

Basal nutrient delivery promoted Caco-2 proliferation, brush border enzyme activity, monolayer expansion, and cell motility. Proliferation was actually increased by 694 +/- 9.89% (n = 90, p < .0001) in cells nourished apically and basally compared with a 314 +/- 3.31% increase (n = 90, p < .0001) in those cells receiving only apical nutrition. The addition of basal nutrient delivery to the cell culture system augmented both alkaline phosphatase and dipeptidyl dipeptidase specific activity by 116 +/- 5.4% and 256 +/- 14.0%, respectively (p < .0001, n = 6 for each group). The effects of basal nutrient delivery were maintained after mitomycin blockade of proliferation for both alkaline phosphatase (392 +/- 89.8% of control, n = 3, p < .0005) and dipeptidyl dipeptidase (374 +/- 79.1% of control, n = 3, p < .005), suggesting that the increased digestive enzyme-specific activity reflected differentiation rather than indirect effects of slowing of proliferation. Epithelial sheet migration increased by 389 +/- 8.8% and proliferation-blocked cell motility also increased by 76.5 +/- 1.56% (p < .0005, n = 12 for each) compared with apical nutrient delivery only.

CONCLUSIONS

These results suggest that although apical nutrition may be critical for intestinal epithelial cell biology, nutrient delivery to the basal surface of intestinal epithelial cell membranes may also promote intestinal epithelial differentiation, proliferation, and mucosal healing.

The biogenesis, traffic, and function of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-activated chloride channel that is encoded by the gene that is defective in cystic fibrosis. This ion channel resides at the luminal surfaces and in endosomes of epithelial cells that line the airways, intestine, and a variety of exocrine glands. In this article we discuss current hypotheses regarding how CFTR functions as a regulated ion channel and how CF mutations lead to disease. We also evaluate the emerging notion that CFTR is a multifunctional protein that is capable of regulating epithelial physiology at several levels, including the modulation of other ion channels and the regulation of intracellular membrane traffic. Elucidating the various functions of CFTR should contribute to our understanding of the pathology in cystic fibrosis, the most common lethal genetic disorder among Caucasians.

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.

Two different cytoplasmic tails direct isoforms of the membrane cofactor protein (CD46) to the basolateral surface of Madin-Darby canine kidney cells

Membrane cofactor protein (MCP; CD46), a widely distributed regulatory protein of the complement system, was analyzed for expression in polarized epithelial cells. Both a human and a simian (Vero C1008) cell line were found to contain endogenous MCP mainly on the basolateral surface. Transfected Madin-Darby canine kidney cells stably expressing human MCP delivered this protein also predominantly to the basolateral surface. A deletion mutant lacking the cytoplasmic tail was transported in a nonpolarized fashion, indicating that the targeting signal for the basolateral transport is located in the cytoplasmic domain. A characteristic feature of MCP is the presence of various isoforms that contain either of two different cytoplasmic tails as a consequence of alternative splicing. Two isoforms differing only in the cytoplasmic tail (tail 1 or 2) were analyzed for polarized expression in Madin-Darby canine kidney cells. Surface biotinylation, as well as confocal immunofluorescence microscopy, indicated that both proteins were transported to the basolateral surface. Because no sequence similarity has been observed, the two tails contain different basolateral targeting signals. A deletion mutant lacking the only tyrosine residue in tail 1 retained the polarized expression indicating that, in contrast to most basolateral sorting signals, the transport signal of the tail 1 isoform is not dependent on tyrosine. The maintenance of a targeting motif in two distinct cytoplasmic tails suggests that the basolateral expression of MCP in polarized epithelial cells is of physiological importance.

Amplitude-dependent modulation of brush border enzymes and proliferation by cyclic strain in human intestinal Caco-2 monolayers

Little is known about the effects of repetitive deformation during peristaltic distension and contraction or repetitive villus shortening on the proliferation and differentiation of the intestinal epithelium. We sought to characterize the effects of repetitive deformation of a physiologically relevant magnitude and frequency on the proliferation and differentiation of human intestinal epithelial Caco-2 cells, a common cell culture model for intestinal epithelial biology. Human intestinal epithelial Caco-2 cells were cultured on collagen-coated membranes deformed by -20 kPa vacuum at 10 cycles/minute, producing an average 10% strain on the adherent cells. Proliferation was assessed by cell counting and 3H-thymidine incorporation. Alkaline phosphatase and dipeptidyl dipeptidase specific activity were measured in cell lysates. Since cells at the membrane periphery experience higher strain than cells in the center, the topography of brush border enzyme histochemical and immunohistochemical staining was analyzed for strain-dependence. Cyclic strain stimulated proliferation compared to static cells. Proliferation was highest in the membrane periphery where strain was maximal. Strain also modulated differentiation independently of its mitogenic effects, selectively stimulating dipeptidyl dipeptidase while inhibiting alkaline phosphatase. Strain-associated enzyme changes were also maximal in areas of greatest strain. The PKC inhibitors staurosporine and calphostin C ablated strain mitogenic effects while intracellular PKC activity was increased by strain. The strain-associated brush border enzyme changes were attenuated but not blocked by PKC inhibition. Thus, strain of a physiologically relevant frequency and magnitude promotes proliferation and modulates the differentiation of a well-differentiated human intestinal epithelial cell line in an amplitude-dependent fashion. PKC may be involved in coupling strain to increased proliferation.

Amplitude-dependent modulation of brush border enzymes and proliferation by cyclic strain in human intestinal Caco-2 monolayers

Little is known about the effects of repetitive deformation during peristaltic distension and contraction or repetitive villus shortening on the proliferation and differentiation of the intestinal epithelium. We sought to characterize the effects of repetitive deformation of a physiologically relevant magnitude and frequency on the proliferation and differentiation of human intestinal epithelial Caco-2 cells, a common cell culture model for intestinal epithelial biology. Human intestinal epithelial Caco-2 cells were cultured on collagen-coated membranes deformed by -20 kPa vacuum at 10 cycles/minute, producing an average 10% strain on the adherent cells. Proliferation was assessed by cell counting and 3H-thymidine incorporation. Alkaline phosphatase and dipeptidyl dipeptidase specific activity were measured in cell lysates. Since cells at the membrane periphery experience higher strain than cells in the center, the topography of brush border enzyme histochemical and immunohistochemical staining was analyzed for strain-dependence. Cyclic strain stimulated proliferation compared to static cells. Proliferation was highest in the membrane periphery where strain was maximal. Strain also modulated differentiation independently of its mitogenic effects, selectively stimulating dipeptidyl dipeptidase while inhibiting alkaline phosphatase. Strain-associated enzyme changes were also maximal in areas of greatest strain. The PKC inhibitors staurosporine and calphostin C ablated strain mitogenic effects while intracellular PKC activity was increased by strain. The strain-associated brush border enzyme changes were attenuated but not blocked by PKC inhibition. Thus, strain of a physiologically relevant frequency and magnitude promotes proliferation and modulates the differentiation of a well-differentiated human intestinal epithelial cell line in an amplitude-dependent fashion. PKC may be involved in coupling strain to increased proliferation.

Sphingolipids and glycoproteins are differentially trafficked to the Chlamydia trachomatis inclusion

Chlamydia trachomatis is an obligate intracellular pathogen that multiples within the confines of a membrane-bound vacuole called an inclusion. Approximately 40-50% of the sphingomyelin synthesized from exogenously added NBD-ceramide is specifically transported from the Golgi apparatus to the chlamydial inclusion (Hackstadt, T., M.A. Scidmore, and D.D. Rockey. 1995. Proc. Natl. Acad. Sci. USA. 92: 4877-4881). Given this major disruption of a cellular exocytic pathway and the similarities between glycolipid and glycoprotein exocytosis, we wished to determine whether the processing and trafficking of glycoproteins through the Golgi apparatus to the plasma membrane in chlamydia-infected cells was also disrupted. We analyzed the processing of several model glycoproteins including vesicular stomatitis virus G-protein, transferrin receptor, and human histocompatibility leukocyte class I antigen. In infected cells, the posttranslational processing and trafficking of these specific proteins through the Golgi apparatus and subsequent transport to the plasma membrane was not significantly impaired, nor were these glycoproteins found associated with the chlamydial inclusion membrane. Studies of receptor recycling from endocytic vesicles employing fluorescently and HRP-tagged transferrin and anti-transferrin receptor antibody revealed an increased local concentration of transferrin and transferrin receptor around but never within the chlamydial inclusion. However, Scatchard analysis failed to show either an increased intracellular accumulation of transferrin receptor or a decreased number of plasma membrane receptors in infected cells. Furthermore, the rate of exocytosis from the recycling endosomes to the plasma membrane was not altered in chlamydia-infected cells. Thus, although C. trachomatis disrupts the exocytosis of sphingolipids and the Golgi apparatus appears physically distorted, glycosylation and exocytosis of representative secreted and endocytosed proteins are not disrupted. These results suggest the existence of a previously unrecognized sorting of sphingolipids and glycoproteins in C. trachomatis-infected cells.

Carboxy-terminal vesicular stomatitis virus G protein-tagged intestinal Na+-dependent glucose cotransporter (SGLT1): maintenance of surface expression and global transport function with selective perturbation of transport kinetics and polarized expression

The Na+-dependent glucose transporter (SGLT1) mediates absorption of luminal glucose by the intestine. However, available intestinal cell lines that recapitulate a monolayer phenotype only express SGLT1 at low levels. Thus, to facilitate studies of the biology of SGLT1 function in epithelial monolayers, we engineered an epitope-tagged construct containing the YTDIEMNRLGK sequence (from the vesicular stomatitis virus G protein). The tag was placed at the carboxyl terminus since this is the least conserved portion of SGLT1. Transiently transfected COS-1 cells demonstrated surface expression of the immunoreactive protein and enhanced Na+-dependent glucose uptake that was phloridzin-sensitive (a specific competitive inhibitor of SGLT1). However, subsequent detailed analyses of epitope-tagged SGLT1 using stably transfected clones derived from the Caco-2 human intestinal epithelial cell line revealed substantial effects of the epitope on critical functions of SGLT1. When compared with native SGLT1 transfectants, the apparent Km for sugar transport was increased 23-fold (313 microM to 7.37 mM for native versus epitope-tagged SGLT1). In contrast, the apparent KNa for epitope-tagged SGLT1 was similar to that for native SGLT1. Permeabilization studies indicated that the C-terminal epitope tag was intracellular and thus could not directly disrupt extracellular ligand-binding sites. Immunolocalization and functional assays designed to detect polarized surface expression indicated that epitope tagging resulted in loss of apical targeting and enrichment of basolateral expression. Functional isolation of the small apical pool of epitope-tagged SGLT1 (by selective inhibition of basolateral epitope-tagged SGLT1) revealed that, despite the documented kinetic alterations in sugar transport, epitope-tagged SGLT1 could promote absorptive Na+ currents. These data show that 1) the C terminus of SGLT1 is intracellular; 2) disruption of protein structure by addition of a C-terminal tag leads to selective modifications of SGLT1 function; 3) the kinetics of sugar transport can be altered independently of influences on the Na+-binding site of SGLT1; and 4) the weak basolateral targeting sequence present within the epitope tag is dominant over endogenous SGLT1 apical targeting information and can direct polytopic membrane protein localization. The data also caution that subtle effects of foreign sequences must be considered when epitope tagging polytopic membrane proteins.

Expression of insulin-like growth factor (IGF)-I receptors, IGF-II/cation-independent mannose 6-phosphate receptors (CI-MPRs), and cation-dependent MPRs in polarized human intestinal Caco-2 cells

We have analyzed the surface distribution and functional expression of the insulin-like growth factor I (IGF-I) receptor and the IGF-II/cation-independent mannose 6-phosphate (IGF-II/CI-MPR) in the polarized human colon adenocarcinoma cell line, Caco 2. Domain-selective biotinylation of the apical and basolateral surfaces of Caco-2 cells grown on filter supports revealed a 3-4-fold enrichment of these receptors on basolateral membranes. In addition, the biotinylation studies revealed the presence of the cation-dependent MPR on both membrane surfaces, with a 3.4-fold enrichment on basolateral membranes. Binding of 125I-IGF-I at 4 degrees C confirmed similar higher levels of expression of the IGF-I receptor at the basolateral surface than at the apical surface. Cell surface-specific binding of the iodinated lysosomal enzyme beta-glucuronidase was detected at 4 degrees C on both plasma membrane domains. However, significant uptake of beta-glucuronidase at 37 degrees C was observed only from the basolateral surface. These results indicate that the MPRs and the IGF-I receptor are expressed in a polarized fashion in Caco-2 cells and that the IGF-II/CI-MPR present on apical membranes, unlike the IGF-II/CI-MPR expressed on the basolateral surface, is not functional in endocytosing lysosomal enzymes.

Congenital sucrase-isomaltase deficiency. Identification of a glutamine to proline substitution that leads to a transport block of sucrase-isomaltase in a pre-Golgi compartment

Congenital sucrase-isomaltase deficiency is an example of a disease in which mutant phenotypes generate transport-incompetent molecules. Here, we analyze at the molecular level a phenotype of congenital sucrase-isomaltase deficiency in which sucrase-isomaltase (SI) is not transported to the brush border membrane but accumulates as a mannose-rich precursor in the endoplasmic reticulum (ER), ER-Golgi intermediate compartment, and the cis-Golgi, where it is finally degraded. A 6-kb clone containing the full-length cDNA encoding SI was isolated from the patient's intestinal tissue and from normal controls. Sequencing of the cDNA revealed a single mutation, A/C at nucleotide 3298 in the coding region of the sucrase subunit of the enzyme complex. The mutation leads to a substitution of the glutamine residue by a proline at amino acid 1098 (Q1098P). The Q1098P mutation lies in a region that is highly conserved between sucrase and isomaltase from different species and several other structurally and functionally related proteins. This is the first report that characterizes a point mutation in the SI gene that is responsible for the transport incompetence of SI and for its retention between the ER and the Golgi.

Immunoadsorption of hepatic vesicles carrying newly synthesized dipeptidyl peptidase IV and polymeric IgA receptor

Hepatocytes must transport newly synthesized apical membrane proteins from the basolateral to the apical plasma membrane. Our earlier morphological study showed that the apical proteins share a late (subapical) part of the transcytotic pathway with the well characterized polymeric immunoglobulin A receptor (Barr, V. A., and Hubbard, A. L. (1993) Gastroenterology 105, 554-571). Starting with crude microsomes from the livers of [35S]methionine-labeled rats, we sequentially immunoadsorbed first vesicles containing the endocytic asialoglycoprotein receptor and then (from the depleted supernatant) vesicles containing the polymeric IgA receptor. Biochemical characterization indicated that early basolateral and late endosomes were present in the first population but not in the second. Neither Golgi-, apical plasma membrane (PM)-, nor basolateral PM-derived vesicles were significant contaminants of either population. Both vesicle populations contained 35S-labeled receptor and 35S-labeled-dipeptidyl peptidase IV. Importantly, the elevated relative specific activity of the dipeptidyl peptidase (% of 35S-labeled/% immunoblotted) in the second population indicated that these vesicles must transport newly synthesized dipeptidyl peptidase IV. A distinct kind of vesicle was immunoadsorbed from a "carrier-vesicle fraction"; surprisingly, these vesicles contained little 35S-receptor and virtually no dipeptidyl peptidase IV. These results, together with previous kinetic data from in vivo experiments, are consistent with a computer-generated model predicting that newly synthesized dipeptidyl peptidase IV is delivered to basolateral endosomes, which also contain newly synthesized polymeric immunoglobulin A receptor. The two proteins are then transcytosed together to the subapical region.

Localization and biosynthesis of aminopeptidase N in pig fetal small intestine

BACKGROUND & AIMS

Little is known about the expression of brush border enzymes in fetal enterocytes. The aim of this study was to describe the localization and biosynthesis of porcine fetal aminopeptidase N.

METHODS

This study was performed using histochemistry and immunoelectron microscopy and [35S]methionine labeling of cultured mucosal explants.

RESULTS

Enzyme activity was present in the brush border membrane and extended into the apical cytoplasm. The protein was colocalized with cationized ferritin at the surface of endocytic structures including coated pits, vesicles, tubules, and large vacuoles in the apical cytoplasm. The transient high mannose-glycosylated form of fetal aminopeptidase N was processed to the mature complex-glycosylated form at a markedly slower rate than the enzyme in adult intestine. Likewise, dimerization occurred slowly compared with the adult form of aminopeptidase N, and it took place mainly after the Golgi-associated complex glycosylation. The enzyme had a biphasic appearance in the Mg(2+)-precipitated and microvillar fractions, indicating that the bulk of newly made aminopeptidase N is transported to the brush border membrane before appearing in the apical endocytic structures.

CONCLUSIONS

In comparison with the adult enzyme, fetal aminopeptidase N has a more widespread subcellular distribution with substantial amounts present in apical endocytic compartments characteristic of the fetal enterocyte.

Transcytosis of aminopeptidase N in Caco-2 cells is mediated by a non-cytoplasmic signal

In Caco-2 cells, aminopeptidase N is transported to the apical membrane from the trans Golgi network by both the direct and the indirect pathway (Matter, K., Brauchbar, M., Bucher, K., and Hauri, H.-P. (1990) Cell 60, 429-437). The aim of this study was to determine the importance of the transmembrane or cytoplasmic domain of aminopeptidase N for transport of aminopeptidase N by the indirect pathway by analysis of mutated forms of aminopeptidase N recombinantly expressed in Caco-2 cells. A tail-less and two secretory forms of aminopeptidase N, all deprived of the cytoplasmic tail, were transported to the basolateral plasma membrane in proportions equivalent to the wild type enzyme. This shows that no cytoplasmic basolateral sorting signal is involved in directing aminopeptidase N to the basolateral plasma membrane. Both the wild type and the tail-less aminopeptidase N were transcytosed from the basolateral to the apical plasma membrane, whereas no transcytosis of two secretory forms could be detected, showing that the transmembrane domain is important for efficient transcytosis to take place. A significant difference in transcytosis kinetics of the human and the porcine wild type aminopeptidase N was observed. This indicates that transcytosis of aminopeptidase N from the basolateral to the apical membrane does not occur by default transport but involves an active sorting mechanism.

Two stages of enteropathogenic Escherichia coli intestinal pathogenicity are up and down-regulated by the epithelial cell differentiation

Pathogens and eucaryotic cells are active partners during the process of pathogenicity. To gain access to enterocytes and to cross the epithelial membrane, many enterovirulent microorganisms interact with the brush border membrane-associated components as receptors. Recent reports provide evidence that intestinal cell differentiation plays a role in microbial pathogenesis. Human enteropathogenic Escherichia coli (EPEC) develop their pathogenicity upon infecting enterocytes. To determine if intestinal epithelial cell differentiation influences EPEC pathogenicity, we examined the infection of human intestinal epithelial cells by JPN 15 (pMAR7) [EAF+ eae+] EPEC strain as a function of the cell differentiation. The human embryonic intestinal INT407 cells, the human colonic T84 cells, the human undifferentiated HT-29 cells (HT-29 Std) and two enterocytic cell lines, HT-29 glc-/+ and Caco-2 cells, were used as cellular models. Cells were infected apically with the EPEC strain and the cell-association and cell-entry were examined by quantitative determination using metabolically radiolabeled bacteria, as well as by light, scanning and transmission electron microscopy. [EAF+ eae+] EPEC bacteria efficiently colonized the cultured human intestinal cells. Diffuse bacterial adhesion occurred to undifferentiated HT-29 Std and INT407 cells, whereas characteristic EPEC cell clusters were observed on fully differentiated enterocytic HT-29 glc-/+ cells and on colonic crypt T84 cells. As shown using the Caco-2 cell line, which spontaneously differentiates in culture, the formation of EPEC clusters increased as a function of the epithelial cell differentiation. In contrast, efficient cell-entry of [EAF+ eae+] EPEC bacteria occurred in recently differentiated Caco-2 cells and decreased when the cells were fully differentiated.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of a novel protein (p137) which transcytoses bidirectionally in Caco-2 cells

Antisera raised against detergent-extracted membrane fractions from the human intestinal epithelial cell line Caco-2 were used to screen a human colon cDNA library in a bacteriophage expression vector. This led to the identification, molecular cloning, and sequencing of a novel plasma membrane protein (p137) which was present in approximately equal amounts on the basolateral and apical surfaces of the cell. The pattern of extraction of p137 from membranes by Triton X-114 and its release from membranes after incubation with phosphatidylinositol-specific phospholipase C were consistent with it being a glycosylphosphatidylinositol-anchored membrane protein. Using antibodies raised against bacterial fusion proteins, it was shown that p137 was present on the cell surface as a reducible homodimer of 137 kDa subunits. There was constitutive release of p137 into the culture medium as a non-reducible 280-kDa entity. Pulse-chase experiments showed that newly synthesized p137 appeared at the basolateral side of a Caco-2 cell layer before appearing at the apical domain. Domain-specific surface biotinylation of Caco-2 cells at 4 degrees C, followed by chasing at 37 degrees C, demonstrated that p137 is capable of transcytosing in both directions across Caco-2 cells. The unusual plasma membrane domain distribution of this glycosylphosphatidylinositol-linked protein and its transcytosis characteristics demonstrate the existence of a previously uncharacterized apical to basolateral transcytotic pathway in Caco-2 cells.