Sorting of plasma membrane proteins in epithelial cells

Cell polarity, auxin transport, and cytoskeleton-mediated division planes: who comes first?

In plants, cell polarity is an issue more recurring than in other systems, because plants, due to their adaptive and flexible development, often change cell polarity postembryonically according to intrinsic cues and demands of the environment. Recent findings on the directional movement of the plant signalling molecule auxin provide a unique connection between individual cell polarity and the establishment of polarity at the tissue, organ, and whole-plant levels. Decisions about the subcellular polar targeting of PIN auxin transport components determine the direction of auxin flow between cells and consequently mediate multiple developmental events. In addition, mutations or chemical interference with PIN-based auxin transport result in abnormal cell divisions. Thus, the complicated links between cell polarity establishment, auxin transport, cytoskeleton, and oriented cell divisions now begin to emerge. Here we review the available literature on the issues of cell polarity in both plants and animals to extend our understanding on the generation, maintenance, and transmission of cell polarity in plants.

The role of the Brambell receptor (FcRB) in liver: protection of endocytosed immunoglobulin G (IgG) from catabolism in hepatocytes rather than transport of IgG to bile

The Brambell receptor (FcRB) mediates functions of both immunoglobulin G (IgG) transport, transmitting immunity from mother to young, and IgG protection, making IgG the longest surviving of all plasma proteins. Reflecting its role as transport receptor (termed FcRn, for neonatal rat intestine, the tissue from which it was first cloned), FcRB is expressed antenatally in the rabbit, mouse and rat fetal yolk sac and in human placental syncytiotrophoblasts, and neonatally in the intestinal epithelium of mice and rats. Reflecting its role as protection receptor (FcRp), FcRB is expressed in the vascular endothelium throughout life, where it protects IgG from the on-going catabolic activities of this tissue. FcRB detected in hepatocytes was hypothesized to mediate transport of IgG from serum to bile, thus potentially extending the transport expression (FcRn) of this receptor beyond the perinatal period. Our results show serum-to-bile transport of IgG to be unaffected in mice functionally deleted for FcRB. Accordingly, the hypothesis is rejected that FcRB functions as transport receptor (FcRn) in liver. The default conclusion is that FcRB in hepatocytes functions as FcRp, serving to protect IgG from catabolism in hepatocytes that accompanies the endocytic activity of these cells. We conclude that there remains to date no evidence of an FcRn-like transport function of the Brambell receptor beyond the perinatal period, after which the FcRp function of the receptor predominates, paralleling the endocytic activities of the associated tissues.

Sorting mechanisms regulating membrane protein traffic in the apical transcytotic pathway of polarized MDCK cells

The transcytotic pathway followed by the polymeric IgA receptor (pIgR) carrying its bound ligand (dIgA) from the basolateral to the apical surface of polarized MDCK cells has been mapped using morphological tracers. At 20 degreesC dIgA-pIgR internalize to interconnected groups of vacuoles and tubules that comprise the endosomal compartment and in which they codistribute with internalized transferrin receptors (TR) and epidermal growth factor receptors (EGFR). Upon transfer to 37 degreesC the endosome vacuoles develop long tubules that give rise to a distinctive population of 100-nm-diam cup-shaped vesicles containing pIgR. At the same time, the endosome gives rise to multivesicular endosomes (MVB) enriched in EGFR and to 60-nm-diam basolateral vesicles. The cup-shaped vesicles carry the dIgA/pIgR complexes to the apical surface where they exocytose. Using video microscopy and correlative electron microscopy to study cells grown thin and flat we show that endosome vacuoles tubulate in response to dIgA/pIgR but that the tubules contain TR as well as pIgR. However, we show that TR are removed from these dIgA-induced tubules via clathrin-coated buds and, as a result, the cup-shaped vesicles to which the tubules give rise become enriched in dIgA/pIgR. Taken together with the published information available on pIgR trafficking signals, our observations suggest that the steady-state concentrations of TR and unoccupied pIgR on the basolateral surface of polarized MDCK cells are maintained by a signal-dependent, clathrin-based sorting mechanism that operates along the length of the transcytotic pathway. We propose that the differential sorting of occupied receptors within the MDCK endosome is achieved by this clathrin-based mechanism continuously retrieving receptors like TR from the pathways that deliver pIgR to the apical surface and EGFR to the lysosome.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Transcytosis of infectious human immunodeficiency virus across a tight human epithelial cell line barrier

Contact between various epithelial cell lines and HIV chronically infected mononuclear cell lines results in a massive and rapid budding of HIV virions toward the epithelium followed by their internalization into epithelial endosome-like structures. Here it is shown that as early as 30 minutes after apical contact, primary virus isolates generated from primary peripheral blood leukocytes from HIV-infected patients can cross an epithelial cell line barrier using transcytosis, the characteristic epithelial transcellular vesicular pathway. As the next step in the spread of infection, transcytosed HIV particles can productively infect mononuclear cells located at the basolateral side of the epithelial barrier. These observations suggest an alternative, rapid and efficient mechanism for transmission of HIV across an intact epithelial barrier.

Heparan sulfate expression in polarized epithelial cells: the apical sorting of glypican (GPI-anchored proteoglycan) is inversely related to its heparan sulfate content

Several processes that occur in the luminal compartments of the tissues are modulated by heparin-like polysaccharides. To identify proteins responsible for the expression of heparan sulfate at the apex of polarized cells, we investigated the polarity of the expression of the cell surface heparan sulfate proteoglycans in CaCo-2 cells. Domain-specific biotinylation of the apical and basolateral membranes of these cells identified glypican, a GPI-linked heparan sulfate proteoglycan, as the major source of apical heparan sulfate. Yet, most of this proteoglycan was expressed at the basolateral surface, an unexpected finding for a glypiated protein. Metabolic labeling and chase experiments indicated that sorting mechanisms, rather than differential turnover, accounted for this bipolar expression of glypican. Chlorate treatment did not affect the polarity of the expression of glypican in CaCo-2 cells, and transfectant MDCK cells expressed wild-type glypican and a syndecan-4/glypican chimera also in an essentially unpolarized fashion. Yet, complete removal of the heparan sulfate glycanation sites from the glypican core protein resulted in the nearly exclusive apical targeting of glypican in the transfectants, whereas two- and one-chain mutant forms had intermediate distributions. These results indicate that glypican accounts for the expression of apical heparan sulfate, but that glycanation of the core protein antagonizes the activity of the apical sorting signal conveyed by the GPI anchor of this proteoglycan. A possible implication of these findings is that heparan sulfate glycanation may be a determinant of the subcellular expression of glypican. Alternatively, inverse glycanation-apical sorting relationships in glypican may insure near constant deliveries of HS to the apical compartment, or "active" GPI-mediated entry of heparan sulfate into apical membrane compartments may require the overriding of this antagonizing effect of the heparan sulfate chains.

Immunolocalisation of nucleoside transporters in human placental trophoblast and endothelial cells: evidence for multiple transporter isoforms

Polyclonal antibodies raised against the human erythrocyte nucleoside transporter were used to investigate the distribution of the nucleoside transporters in the placenta. Immunoblots of brush-border membranes isolated from the human syncytiotrophoblast revealed a cross-reactive species that co-migrated with the erythrocyte nucleoside transporter as a broad band of apparent M(r) 55,000. In contrast, no labelling was detected in basal membranes containing a similar number of equilibrative nucleoside transporters as assessed by nitrobenzylthioinosine (NBMPR)-binding. The absence of cross-reactive epitopes in basal membranes and their presence in brush-border membranes was confirmed by confocal immunofluorescence microscopy. These results suggest that at least two isoforms of the NBMPR-sensitive nucleoside transporter are present in the human placenta. The lumenal surfaces of fetal capillaries, small placental vessels and umbilical vein were also strongly labelled by the antibody, a finding that suggests that the high fetal-placental adenosine uptake previously reported is due to endothelial transporters.

Biosynthesis and intracellular transport of a bile canalicular plasma membrane protein: studies in vivo and in the perfused rat liver

B10 is an integral glycoprotein of the plasma membrane that is exclusively localized to the canalicular (apical) domain in normal rat hepatocytes but may be expressed on the basolateral (sinusoidal and lateral) membrane in pathophysiological situations. To understand how B10 may be localized to the basolateral surface, we studied the biosynthesis and transport of this apical protein. In vivo pulse-chase experiments, followed by subcellular fractionation of the liver and immunoprecipitation, showed that B10 is first synthesized as a high-mannose form of 123 kD and then matured to a complex glycosylated form of 130 kD, which peaks in the Golgi apparatus after 15 min of chase and reaches the plasma membrane with a half-time of 30 to 45 min. Analysis of the protein in plasma membrane domain fractions showed that most of the newly synthesized molecule was localized in basolateral fractions after 30 min of chase and subsequently appeared in apical fractions. After 90 min of chase, most of the radiolabeled protein had reached its steady-state apical distribution. The same experiments performed in the perfused rat liver, in which the chase can be improved, gave similar results, except that the apical distribution of the radioactive molecule was attained more quickly. Thus B10, like all apical plasma membrane proteins studied so far in hepatocytes, is first transported to the basolateral surface and then reaches the membrane of the bile canaliculi. Alterations of the transcytotic step from the basolateral to the apical surfaces may result in abnormal basolateral localization.

Stimulation of transcytosis of the polymeric immunoglobulin receptor by dimeric IgA

The polymeric immunoglobulin receptor (pIgR) is transcytosed from the basolateral to the apical surface of polarized epithelial cells. We have previously shown that phosphorylation of Ser-664 in the cytoplasmic domain of the pIgR is a signal for its transcytosis. We now report that binding of a physiological ligand, dimeric IgA, to pIgR stimulates pIgR transcytosis. This stimulation occurs in both the presence or absence of Ser-664 phosphorylation. We have used three methods to measure transcytosis of the pIgR. (i) The pIgR was biosynthetically labeled and its cleavage to secretory component after transcytosis was measured. (ii) The pIgR was labeled with biotin at the basolateral surface. After transcytosis, release of the biotin-labeled secretory component into the apical medium was measured. (iii) Transcytosis of a ligand bound to the pIgR was measured. All three methods indicated that dimeric IgA stimulates transcytosis of the pIgR.

Regulation of pathways within cultured epithelial cells for the transcytosis of a basal membrane-bound peroxidase-polylysine conjugate

A conjugate of horseradish peroxidase (HRP) to poly(L-lysine) (PLL) was used as a non-specific adsorptive probe to study transcytosis in MDCK strain I and Caco-2 epithelial cells. As we have shown previously, HRP-PLL transcytosis proceeds via an intracellular, non-lysosomal proteolytic compartment in MDCK cells; yet, this compartment is utilized for transcytosis only in the basal-to-apical direction (Taub, M. E. and Shen, W.-C. J. Cell. Physiol., 150, 283–290, 1992). Using size exclusion chromatography, we demonstrate that the PLL moiety of the conjugate is effectively cleaved during transcytosis, thus releasing free HRP from the apical surface of the cells. Pulse-chase studies indicate that approximately 6% of basolateral surface-associated HRP-PLL conjugate in Transwell-grown cell monolayers enters the basal-to-apical transcytotic pathway. Brief (1 hour) treatment with 160 nM phorbol ester (PMA), a protein kinase C stimulator, elicits a 2-fold increase in the rate and amount of HRP-PLL transcytosis following a 1 hour lag time. Treatment with 1.6 micrograms/ml brefeldin A (BFA) inhibits HRP-PLL transcytosis by approximately 30%; additionally, BFA is able to abolish completely the PMA stimulatory effect. Removal of BFA causes a re-establishment of the normal rate of transcytosis within 2 hours, demonstrating the reversibility of BFA inhibition with respect to HRP-PLL transcytosis. Thus, PMA most likely elicits an increase in the amount of basally internalized conjugate delivered to BFA-sensitive transcytotic compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutational and secondary structural analysis of the basolateral sorting signal of the polymeric immunoglobulin receptor

The 17-juxtamembrane cytoplasmic residues of the polymeric immunoglobulin receptor contain an autonomous basolateral targeting signal that does not mediate rapid endocytosis (Casanova, J. E., G. Apodaca, and K. E. Mostov. Cell. 66:65-75). Alanine-scanning mutagenesis identifies three residues in this region, His656, Arg657, and Val660, that are most essential for basolateral sorting and two residues, Arg655 and Tyr668, that play a lesser role in this process. Progressive truncations suggested that Ser664 and Ile665 might also play a role in basolateral sorting. However, mutation of these residues to Ala or internal deletions of these residues did not affect basolateral sorting, indicating that these residues are probably not required for basolateral sorting. Two-dimensional NMR spectroscopy of a peptide corresponding to the 17-mer signal indicates that the sequence Arg658-Asn-Val-Asp661 has a propensity to adopt a beta-turn in solution. Residues COOH-terminal to the beta-turn (Arg662 to Arg669) seem to take up a nascent helix structure in solution. Substitution of Val660 with Ala destabilizes the turn, while mutation of Arg657 to Ala does not appear to affect the turn structure. Neither mutation detectably altered the stability of the nascent helix in the COOH-terminal portion of the peptide.

HT29-18-C1 intestinal cells: a new model for studying the epithelial transport of drugs

A polarized differentiated subclone of HT29, a human colon carcinoma cell line, was used to measure the passage of drugs across the intestinal epithelium. These cells, HT29-18-C1, when grown on permeable filters, formed tight monolayers of high electrical resistance (> 400 omega cm2). Electron micrographs revealed the presence of numerous apical microvilli and well developed junctional complexes. The transepithelial passage of various drugs was studied: the transepithelial permeability coefficients ranged from 0.8 x 10(-6) cm/s for fluorescein, to 4.8 x 10(-5) cm/s for the most lipophilic molecule, testosterone. For the five compounds tested, there was a coarse correlation between the apparent transepithelial permeability coefficient and the octanol/buffer distribution coefficient. The HT29-18-C1 subclone thus represents a new in-vitro model for studying the intestinal absorption of drugs.

Gastric parietal cell development: expression of the H+/K+ ATPase subunits coincides with the biogenesis of the secretory membranes

The early development of the parietal cell in the embryonic murine gastric mucosa was investigated with particular attention paid to the biogenesis of the secretory membranes and the localization of the gastric H+/K+ ATPase alpha and beta subunits. Gastric glands were recognized in the day 18 foetus. However, at this stage in development no parietal cells could be distinguished ultrastructurally in the glands, and no immunoreactivity was detected with monoclonal antibodies to either the alpha or beta subunits of the gastric H+/K+ ATPase. In the 19 day embryo, parietal cells were recognizable morphologically by the presence of slender microvilli on the apical (lumenal) surface and differentiating intracellular canaliculi in the apical cytoplasm. Both subunits of the proton pump were found to be specifically associated with the apical and canalicular membranes and with the membranes of relatively large vesicles distributed in the subapical cytoplasm and the cytoplasm surrounding the canaliculi. In the parietal cells of the day 1 neonate, the intracellular canaliculi had extended basally to form the extensive compartments typical of parietal cells in the adult animal. Again, profiles of vesicles showing H+/K+ ATPase immunoreactivity were present in the pericanalicular cytoplasm. These results indicate that the intracellular canaliculi are formed by expansion of the apical surface and suggest that the delivery of newly synthesized gastric H+/K+ ATPase alpha and beta subunits to the apical plasma membrane is mediated by typical Golgi transport vesicles. The large immunoreactive vesicles that occur in the apical and pericanalicular cytoplasm of the developing cells may represent artefacts generated by fixation-induced fragmentation of the differentiating canalicular membrane system during preparation.

Amplification of the Golgi complex in MDCK cells secreting human growth hormone

MDCK cells were transfected with pXGH5, a plasmid containing the human growth hormone (hGH) gene, and permanently expressing cell lines were selected. Clone 3A cells, which secrete quantities of hGH through both apical and basolateral surfaces, were examined in detail. Immunofluorescence analysis using anti-hGH antibody revealed bright perinuclear staining coinciding with the area delineated by anti-p52 kDa protein (a resident Golgi protein) antibody. There appeared to be less Golgi-specific fluorescence in untransfected cells. This difference correlated with an increased amount of 52 kDa in the clone 3A cells. Morphometric analysis was performed on electron micrographs of clone 3A and untransfected cells using the fractionator to estimate average number of Golgi stacks per cell, and values were statistically analyzed. It was found that clone 3A cells contained 3.3 and untransfected cells 1.6 stacks (P < or = 0.005), respectively. When clone 3A cells were placed into defined medium, the synthesis and secretion of hGH declined 4-fold, and the number of Golgi stacks also decreased to the untransfected level within seven days. The number of Golgi stacks per untransfected cell was not affected by the presence of exogenous hGH, indicating that Golgi amplification was directly related to secretory demand. Generation times and cell volumes were identical for both cell types under all growth conditions. In addition, the kinetics of protein secretion from radiolabelled cells demonstrated that clone 3A cells generally secrete lower amounts of endogenously synthesized apical proteins than do untransfected cells, while basolateral secretion remains the same. In both cases hGH comprised only about 10% of total secretory proteins, so that the increase in total protein secretion did not seem to warrant the two-fold elaboration of Golgi by 3A cells. But there might be significant amounts of hGH which traverse the Golgi to end up in lysosomes, rather than being secreted, leading to Golgi amplification.

Cell biology of viruses that assemble along the biosynthetic pathway

In this review we discuss five groups of viruses that bud into, or assemble from, different compartments along the biosynthetic pathway. These are herpes-, rota-, corona-, bunya- and pox-viruses. Our main emphasis will be on the virally-encoded membrane glycoproteins that are responsible for determining the site of virus assembly. In a number of cases these proteins have been well characterized and appear to serve as resident markers of the budding compartments. The assembly and dissemination of these viruses raises many questions of cell biological interest.

Selective inhibition of protein targeting to the apical domain of MDCK cells by brefeldin A

Dipeptidyl peptidase IV (DPPIV) is mainly vectorially targeted to the apical surface in MDCK cells. BFA was found to abolish the apical targeting of DPPIV. This BFA effect could be achieved under conditions where the ER to Golgi transport and the total surface expression of DPPIV were essentially unaffected. BFA executed its effect during the transport from the trans-Golgi network (TGN) to the surface. The inhibition of apical targeting resulted in enhanced mis-targeting to the basolateral surface. The mistargeted DPPIV was transcytosed back to the apical domain only after BFA withdrawal. In contrast, the basolateral targeting of uvomorulin was unaffected by BFA. These results established that the apical targeting of DPPIV was selectively abolished by BFA.

Cytoplasmic sequence required for basolateral targeting of LDL receptor in livers of transgenic mice

When expressed in livers of transgenic mice, the human low density lipoprotein (LDL) receptor is specifically targeted to the basolateral (sinusoidal) surface of hepatocytes as determined by immunofluorescence and immunoelectron microscopy. The COOH-terminal cytoplasmic domain of the receptor (residues 790-839) contains a signal for this targeting. A mutant receptor truncated at residue 812 was localized exclusively to the apical (bile canalicular) surface. A mutant receptor terminating at residue 829 showed the normal basolateral distribution, as did a receptor in which alanine was substituted for serine 833, which was previously shown to be a site for phosphorylation in vitro. These data localize the basolateral targeting signal to the 17-residue segment between residues 812 and 828. A 10-amino acid stretch within this segment shows a 4/10 match with a sequence within a previously identified basolateral sorting motif for the receptor for polymeric IgA/IgM in MDCK cells. The four shared residues are spaced at intervals of three, raising the possibility that they all face the same side of an alpha-helix. We conclude that this 10-amino acid stretch may contain a signal that directs certain proteins, including the LDL receptor and the polymeric IgG/IgM receptor, to the basolateral surface of polarized epithelia.