Viral glycoproteins destined for apical or basolateral plasma membrane domains traverse the same Golgi apparatus during their intracellular transport in doubly infected Madin-Darby canine kidney cells

Membrane trafficking of the cystic fibrosis gene product, cystic fibrosis transmembrane conductance regulator, tagged with green fluorescent protein in madin-darby canine kidney cells

The mechanism by which cAMP stimulates cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride (Cl-) secretion is cell type-specific. By using Madin-Darby canine kidney (MDCK) type I epithelial cells as a model, we tested the hypothesis that cAMP stimulates Cl- secretion by stimulating CFTR Cl- channel trafficking from an intracellular pool to the apical plasma membrane. To this end, we generated a green fluorescent protein (GFP)-CFTR expression vector in which GFP was linked to the N terminus of CFTR. GFP did not alter CFTR function in whole cell patch-clamp or planar lipid bilayer experiments. In stably transfected MDCK type I cells, GFP-CFTR localization was substratum-dependent. In cells grown on glass coverslips, GFP-CFTR was polarized to the basolateral membrane, whereas in cells grown on permeable supports, GFP-CFTR was polarized to the apical membrane. Quantitative confocal fluorescence microscopy and surface biotinylation experiments demonstrated that cAMP did not stimulate detectable GFP-CFTR translocation from an intracellular pool to the apical membrane or regulate GFP-CFTR endocytosis. Disruption of the microtubular cytoskeleton with colchicine did not affect cAMP-stimulated Cl- secretion or GFP-CFTR expression in the apical membrane. We conclude that cAMP stimulates CFTR-mediated Cl- secretion in MDCK type I cells by activating channels resident in the apical plasma membrane.

The polarized sorting of membrane proteins expressed in cultured hippocampal neurons using viral vectors

One model of neuronal polarity (Dotti and Simons, 1990) proposes that neurons and polarized epithelia use similar mechanisms to sort membrane proteins. To explore this hypothesis, we used viral vectors to express proteins in cultured neurons and assessed their distribution using quantitative immunofluorescence microscopy. Basolateral epithelial proteins were polarized to dendrites; more significantly, mutations of sequences required for their basolateral targeting in epithelia also disrupted dendritic targeting. Unexpectedly, apical proteins were not polarized to axons but were expressed at roughly equal amounts in dendrites and axons. These data provide strong evidence that targeting of basolateral and dendritic proteins depends on common mechanisms. In contrast, the sorting of proteins to the axon requires signals that are not present in apical proteins.

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.

Four distinct secretory pathways serve protein secretion, cell surface growth, and peroxisome biogenesis in the yeast Yarrowia lipolytica

We have identified and characterized mutants of the yeast Yarrowia lipolytica that are deficient in protein secretion, in the ability to undergo dimorphic transition from the yeast to the mycelial form, and in peroxisome biogenesis. Mutations in the SEC238, SRP54, PEX1, PEX2, PEX6, and PEX9 genes affect protein secretion, prevent the exit of the precursor form of alkaline extracellular protease from the endoplasmic reticulum, and compromise peroxisome biogenesis. The mutants sec238A, srp54KO, pex2KO, pex6KO, and pex9KO are also deficient in the dimorphic transition from the yeast to the mycelial form and are affected in the export of only plasma membrane and cell wall-associated proteins specific for the mycelial form. Mutations in the SEC238, SRP54, PEX1, and PEX6 genes prevent or significantly delay the exit of two peroxisomal membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en route to the peroxisomal membrane. Mutations in the PEX5, PEX16, and PEX17 genes, which have previously been shown to be essential for peroxisome biogenesis, affect the export of plasma membrane and cell wall-associated proteins specific for the mycelial form but do not impair exit from the endoplasmic reticulum of either Pex2p and Pex16p or of proteins destined for secretion. Biochemical analyses of these mutants provide evidence for the existence of four distinct secretory pathways that serve to deliver proteins for secretion, plasma membrane and cell wall synthesis during yeast and mycelial modes of growth, and peroxisome biogenesis. At least two of these secretory pathways, which are involved in the export of proteins to the external medium and in the delivery of proteins for assembly of the peroxisomal membrane, diverge at the level of the endoplasmic reticulum.

Membrane cofactor protein (CD46) is a basolateral protein that is not endocytosed. Importance of the tetrapeptide FTSL at the carboxyl terminus

Membrane cofactor protein (MCP) is a widely distributed complement regulatory protein that is expressed on the basolateral surface of polarized epithelial cells. The basolateral targeting of the BC1 isoform of MCP was analyzed by generating deletion mutants and point mutants within the cytoplasmic tail of 16 amino acids. A sequence of four amino acids, FTSL, was found to be indispensable for the basolateral transport of MCP. This tetrapeptide has two unique features compared with the targeting motifs of other basolateral proteins: (i) it contains a phenylalanine rather than a tyrosine at position 1; (ii) it is located at the very COOH-terminal end. Replacement of the phenylalanine or the leucine by an alanine resulted in a nonpolarized delivery to the cell surface. On the other hand, substitution of a tyrosine for the phenylalanine did not affect the basolateral transport of MCP. The latter mutant, however, was efficiently internalized, whereas the wild type protein was not subject to endocytosis. Our results indicate that the targeting signal YXX-large aliphatic that is involved in various sorting events has been modulated in MCP in such a way that it allows basolateral transport but not endocytosis.

Structural requirements for major histocompatibility complex class II invariant chain trafficking in polarized Madin-Darby canine kidney cells

The invariant chain (Ii) targets major histocompatibility complex class II molecules to an endocytic processing compartment where they encounter antigenic peptides. Analysis of Ii-transferrin receptor chimeras expressed in polarized Madin-Darby canine kidney (MDCK) cells shows that the Ii cytoplasmic tail contains a dihydrophobic basolateral sorting signal, Met16-Leu17, which is recognized in both the biosynthetic and endocytic pathways. Pro15-Met16-Leu17 has previously been identified as one of two dihydrophobic Ii internalization signals active in non-polarized cells. Pro15 is also required for endocytosis in MDCK cells but not for basolateral sorting, indicating that the internalization signal recognized at the plasma membrane is distinct from the sorting signal recognized by basolateral sorting machinery. Another dihydrophobic sequence, Leu7-Ile8, is required for rapid internalization of the chimeric receptors in MDCK cells but not for basolateral sorting, providing further evidence that the structural requirements for basolateral sorting and internalization differ. Deletion analysis suggests that basolateral sorting of newly synthesized Ii-TR chimeras is also mediated by the membrane-proximal region of the Ii cytoplasmic tail. However, this region does not promote polarized basolateral recycling, indicating that the structural requirements for polarized sorting in the biosynthetic and endocytic pathways are not identical.

Dendritic and postsynaptic localizations of glycine receptor alpha subunit mRNAs

Some synaptic neurotransmitter receptors, such as those for glycine, have somato-dendritic distributions. Although the machinery for protein synthesis and several mRNAs are present in dendrites and close to synapses in central neurons, so far the mRNAs for neurotransmitter receptors have not been found unequivocally in dendrites. The glycine receptor (GlyR), a ligand-gated channel mediating a chloride-dependent inhibition, is composed of transmembrane alpha and beta subunits. GlyRs are only present at glycinergic postsynaptic differentiation, where they are stabilized by the associated protein gephyrin. With light nonradioactive in situ hybridization (ISH), we observe that GlyR alpha subunit mRNAs are present in both somata and dendrites of most neurons of the ventral horn of rat spinal cord, whereas the beta subunit and gephyrin mRNAs are predominantly in somata. Interestingly, within dendrites GlyR alpha subunit mRNAs form aggregates that are mostly localized peripherally to the dendritic axial core. Electron microscopic ISH shows that GlyR alpha subunit mRNAs are associated with postsynaptic differentiations. At these sites, the GlyR alpha subunit mRNAs are detected in close association with subsynaptic cisternae. This targeting of alpha subunit mRNAs to postsynaptic domains could provide a means of dynamically modulating synaptic efficacy by changing the composition and the density of receptors at glycinergic synapses.

Apical sorting of hepatitis B surface antigen (HBsAg) is independent of N-glycosylation and glycosylphosphatidylinositol-anchored protein segregation

We have used the hepatitis B surface antigen (HBsAg) as a tool to explore mechanisms by which polarized epithelial cells address specific proteins to their apical domain. It recently has been proposed that N-glycans can serve as apical signals recognized by lectin-like sorting receptors in the trans-Golgi network. We found, however, conclusive evidence that the HBsAg follows an apical pathway not mediated by N-glycan signaling. Neither tunicamycin treatment nor replacement of its single glycosylated residue, Asn-146, altered its predominant (>85%) apical secretion from transfected Madin-Darby canine kidney cells (MDCK). Although HBsAg is known to be secreted as a lipoprotein particle, our results suggest that the exocytic machinery involved in its N-glycan-independent pathway overlaps, at least partially, with that of other apically targeted proteins, including the endogenous gp80, as judged by the effects of brefeldin A. We also tested whether its sorting behavior could be ascribed to association with glycosylphosphatidylinositol (GPI)-anchored proteins, which, together with glycosphingolipids, primarily are targeted to the apical domain of MDCK cells. HBsAg was preferentially secreted from the apices of transfected Fisher rat thyroid cells, which, in contrast to MDCK cells, address GPI-proteins and glycosphingolipids to their basal domain. Moreover, complete inhibition of GPI biogenesis by mannosamine treatment did not impair the HBsAg apical secretion, discarding the possibility that HBsAg could be "hitchhiking" with a newly synthesized GPI-protein. Thus, the HBsAg provides a unique model system to search for yet-unknown apical sorting mechanisms that could depend on proteinaceous targeting signals interacting with cognate trans-Golgi network receptors that are at present unidentified.

Dendritic and postsynaptic localizations of glycine receptor alpha subunit mRNAs

Some synaptic neurotransmitter receptors, such as those for glycine, have somato-dendritic distributions. Although the machinery for protein synthesis and several mRNAs are present in dendrites and close to synapses in central neurons, so far the mRNAs for neurotransmitter receptors have not been found unequivocally in dendrites. The glycine receptor (GlyR), a ligand-gated channel mediating a chloride-dependent inhibition, is composed of transmembrane alpha and beta subunits. GlyRs are only present at glycinergic postsynaptic differentiation, where they are stabilized by the associated protein gephyrin. With light nonradioactive in situ hybridization (ISH), we observe that GlyR alpha subunit mRNAs are present in both somata and dendrites of most neurons of the ventral horn of rat spinal cord, whereas the beta subunit and gephyrin mRNAs are predominantly in somata. Interestingly, within dendrites GlyR alpha subunit mRNAs form aggregates that are mostly localized peripherally to the dendritic axial core. Electron microscopic ISH shows that GlyR alpha subunit mRNAs are associated with postsynaptic differentiations. At these sites, the GlyR alpha subunit mRNAs are detected in close association with subsynaptic cisternae. This targeting of alpha subunit mRNAs to postsynaptic domains could provide a means of dynamically modulating synaptic efficacy by changing the composition and the density of receptors at glycinergic synapses.

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.

Cell-free reconstitution of the transport of viral glycoproteins from the TGN to the basolateral plasma membrane of MDCK cells

An in vitro system to study the transport of plasma membrane proteins from the TGN to the basolateral plasma membrane of polarized MDCK cells has been developed in which purified cell fractions are combined and transport between them is studied under controlled conditions. In this system, a donor Golgi fraction derived from VSV or influenza virus-infected MDCK cells, in which 35S-labeled viral glycoproteins were allowed to accumulate in the TGN during a low temperature block, is incubated with purified immobilized basolateral plasma membranes that have their cytoplasmic face exposed and are obtained by shearing-lysis of MDCK monolayers grown on cytodex beads. Approximately 15–30% of the labeled glycoprotein molecules are transferred from the Golgi fraction to the acceptor plasma membranes and are recovered with the sedimentable (1 g) beads. Transport is temperature, energy and cytosol dependent, and is abolished by alkylation of SH groups and inhibited by the presence of GTP-gamma-S, which implicates GTP-binding proteins and the requirement for GTP hydrolysis in one or more stages of the transport process. Endo H-resistant glycoprotein molecules that had traversed the medial region of the Golgi apparatus are preferentially transported and their luminal domains become accessible to proteases, indicating that membrane fusion with the plasma membrane takes place in the in vitro system. Mild proteolysis of the donor or acceptor membranes abolishes transport, suggesting that protein molecules exposed on the surface of these membranes are involved in the formation and consumption of transport intermediates, possibly as addressing and docking proteins, respectively. Surprisingly, both VSV-G and influenza HA were transported with equal efficiencies to the basolateral acceptor membranes. However, low concentrations of a microtubular protein fraction preferentially inhibited the transport of HA, although this effect was not abolished by microtubule depolymerizing agents. This system shows great promise for elucidating the mechanisms that effect the proper sorting of plasma membrane proteins in the TGN and their subsequent targeting to the appropriate acceptor membrane.

Transport of vesicular stomatitis virus G protein to the cell surface is signal mediated in polarized and nonpolarized cells

Current model propose that in nonpolarized cells, transport of plasma membrane proteins to the surface occurs by default. In contrast, compelling evidence indicates that in polarized epithelial cells, plasma membrane proteins are sorted in the TGN into at least two vectorial routes to apical and basolateral surface domains. Since both apical and basolateral proteins are also normally expressed by both polarized and nonpolarized cells, we explored here whether recently described basolateral sorting signals in the cytoplasmic domain of basolateral proteins are recognized and used for post TGN transport by nonpolarized cells. To this end, we compared the inhibitory effect of basolateral signal peptides on the cytosol-stimulated release of two basolateral and one apical marker in semi-intact fibroblasts (3T3), pituitary (GH3), and epithelial (MDCK) cells. A basolateral signal peptide (VSVGp) corresponding to the 29-amino acid cytoplasmic tail of vesicular stomatitis virus G protein (VSVG) inhibited with identical potency the vesicular release of VSVG from the TGN of all three cell lines. On the other hand, the VSVG peptide did not inhibit the vesicular release of HA in MDCK cells not of two polypeptide hormones (growth hormone and prolactin) in GH3 cells, whereas in 3T3 cells (influenza) hemagglutinin was inhibited, albeit with a 3x lower potency than VSVG. The results support the existence of a basolateral-like, signal-mediated constitutive pathway from TGN to plasma membrane in all three cell types, and suggest that an apical-like pathway may be present in fibroblast. The data support cargo protein involvement, not bulk flow, in the formation of post-TGN vesicles and predict the involvement of distinct cytosolic factors in the assembly of apical and basolateral transport vesicles.

Uptake of human immunodeficiency virus envelope protein gp120 by human trophoblast in culture

OBJECTIVE

Our purpose was to determine whether human trophoblast has a cell surface CD4 antigen that will bind to gp120, the envelope protein of human immunodeficiency virus.

STUDY DESIGN

Uptake of iodine 125-labeled gp120 by trophoblast in culture was measured. Particular attention was paid to technical details that may have caused the contradictory results reported by previous investigators: the source of the recombinant gp120, the method of radioiodination, and the isolation procedure of trophoblast to ensure elimination of contaminating cells, particularly macrophages.

RESULTS

Uptake of transferrin-free iodine 125-labeled gp120 to trophoblast was unaffected by adding a 200 molar excess of gp120, by preincubating gp120 with soluble CD4 to block the CD4 binding sites on gp120 and by preincubation of trophoblast with a blocking antibody to CD4 (OKT4a). In contrast, uptake of gp120 by CD4-positive H9 human lymphocytes was reduced 79% by a 200 molar excess of gp120 and > 50% by a CD4-blocking antibody.

CONCLUSIONS

Uptake of gp120 to trophoblast is by a high capacity, CD4-independent mechanism that is probably nonspecific and may be related to the mechanism for binding other circulating glycoproteins in maternal blood.

Parallel secretory pathways to the cell surface in yeast

Saccharomyces cerevisiae mutants that have a post-Golgi block in the exocytic pathway accumulate 100-nm vesicles carrying secretory enzymes as well as plasma membrane and cell-wall components. We have separated the vesicle markers into two groups by equilibrium isodensity centrifugation. The major population of vesicles contains Bg12p, an endoglucanase destined to be a cell-wall component, as well as Pma1p, the major plasma membrane ATPase. In addition, Snc1p, a synaptobrevin homologue, copurifies with these vesicles. Another vesicle population contains the periplasmic enzymes invertase and acid phosphatase. Both vesicle populations also contain exoglucanase activity; the major exoglucanase normally secreted from the cell, encoded by EXG1, is carried in the population containing periplasmic enzymes. Electron microscopy shows that both vesicle groups have an average diameter of 100 nm. The late secretory mutants sec1, sec4, and sec6 accumulate both vesicle populations, while neither is detected in wild-type cells, early sec mutants, or a sec13 sec6 double mutant. Moreover, a block in endocytosis does not prevent the accumulation of either vesicle species in an end4 sec6 double mutant, further indicating that both populations are of exocytic origin. The accumulation of two populations of late secretory vesicles indicates the existence of two parallel routes from the Golgi to the plasma membrane.

Expression of parathyroid hormone receptors in MDCK and LLC-PK1 cells

Parathyroid hormone (PTH) inhibits renal proximal tubular phosphate (Pi) and bicarbonate reabsorption by regulating the activity of apical Na/Pi cotransport and Na/H exchange. Two renal epithelial cell lines ["proximal tubular", LLC-PK1; "distal tubular", Madin-Darby canine kidney, (MDCK) cells] were stably transfected with complementary deoxyribonucleic acids (cDNAs) encoding a cloned PTH receptor in order to examine the polarity of transfected receptor function and whether or not intrinsic Pi transport is regulated by the transfected PTH receptor. The receptors are functionally coupled to the stimulation of adenosine 3':5' cyclic monophosphate (cAMP) production at both cell surfaces in LLC-PK1 cells, whereas this response is primarily limited to the basolateral surface in MDCK cells. Immunocytochemistry suggests an apical and basolateral localization of the transfected PTH receptor in LLC-PK1 cells and only a basolateral localization in MDCK cells. PTH activation of the transfected receptors is not coupled to the regulation of intrinsic Pi transport in either LLC-PK1 or MDCK cells.

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

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

Virus-like particles in MDCK cells persistently infected with Borna disease virus

A line of Madin Darby canine kidney (MDCK) cells persistently infected with Borna disease virus was examined by electron microscopy. Thin sections revealed the presence of intracytoplasmic virus-like particles ranging from 50-100 nm in diameter. Nuclei of the infected cells exhibited accumulation of electron-dense granular structures 15-18 nm in diameter. The intracytoplasmic particles were roughly spherical with a limiting membrane, suggesting the presence of a lipid-containing envelope. The internal structure consisted of strand-like material which in some cases was condensed underneath the envelope. The possible relationship of these particles to Borna disease virions is discussed.

Apical plasma membrane proteins are not obligatorily stored in secretory granules in exocrine cells

Exocrine cells are epithelial cells in which secretory granules undergo fusion with the apical plasma membrane upon secretagogue stimulation. Several apical plasma membrane proteins have been found in secretory granules in cells from pancreas and salivary glands raising the possibility that incorporation into secretory granules followed by exocytosis of the granules accounts for their insertion into the apical plasma membrane. To test this hypothesis, we have expressed the influenza hemagglutinin (HA) in pancreatic AR42J cells, which make zymogen-like granules upon incubation with dexamethasone. The influenza virus HA is known to be specifically targeted to the apical plasma membrane of epithelial cells that lack a regulated pathway and is also known to be excluded from secretory granules in virally-infected pituitary AtT20 cells. Localization of the protein by immunofluorescence microscopy revealed that it accumulated at the plasma membrane of the transfected AR42J cells. HA was not observed in the amylase-rich secretory granules. By immunolabeling of ultrathin cryosections of the transfected cells, HA was also found exclusively on the cell surface, with label over secretory granules not exceeding that seen in control, untransfected cells. In addition, in cell fractionation experiments performed on radiolabeled AR42J cell transformants, HA was not detectable in the secretory granule fractions. These results indicate that HA is not efficiently stored in mature secretory granules and is likely to reach the cell surface via constitutive transport pathways.

Basolateral protein transport in streptolysin O-permeabilized MDCK cells

We have reconstituted polarized protein transport in streptolysin O-permeabilized MDCK cells from the TGN to the basolateral surface and to the apical surface. These transport steps are dependent on temperature, energy and exogenously supplied cytosol. Using this in vitro system we show that a whole tail peptide (WT peptide) corresponding to the cytoplasmic tail of a basolaterally sorted protein, the vesicular stomatitis virus glycoprotein (VSV G) inhibits the TGN to basolateral transport but does not affect any other transport step. Inhibition of VSV G transport to basolateral surface by WT peptide did not result in missorting of the protein to the apical surface. Mutation of the single tyrosine residue in the WT peptide reduced its inhibitory potency four- to fivefold. These results suggest that the VSV G tail physically interacts with a component of the sorting machinery. Using a cross-linking approach, we have identified proteins that associate with the cytoplasmic tail domain of VSV G. One of these polypeptides, Tin-2 (Tail interacting protein-2), associates with VSV G in the TGN, the site of protein sorting, but not in the ER nor at the cell surface. Tin-2 does not associate with apically targeted hemagglutinin. WT peptide that inhibited the basolateral transport of VSV G also inhibited the association of Tin-2 with VSV G. Together, these properties make Tin-2 a candidate basolateral sorter. The results demonstrate the usefulness of the SLO-permeabilized cell system in dissecting the sorting machinery.

Analysis of the signals for polarized transport of influenza virus (A/WSN/33) neuraminidase and human transferrin receptor, type II transmembrane proteins

In polarized MDCK cells influenza virus (A/WSN/33) neuraminidase (NA) and human transferrin receptor (TR), type II glycoproteins, when expressed from cloned cDNAs, were transported and accumulated preferentially on the apical and basolateral surfaces, respectively. We have investigated the signals for polarized sorting by constructing chimeras between NA and TR and by making deletion mutants. NATR delta 90, which contains the cytoplasmic tail and transmembrane domain of NA and the ectodomain of TR, was found to be localized predominantly on the apical membrane, whereas TRNA delta 35, containing the cytoplasmic and transmembrane domains of TR and the ectodomain of NA, was expressed preferentially on the basolateral membrane. TR delta 57, a TR deletion mutant lacking 57 amino acids in the TR cytoplasmic tail, did not exhibit any polarized expression and was present on both apical and basolateral surfaces, whereas a deletion mutant (NA delta 28-35) lacking amino acid residues from 28 to 35 in the transmembrane domain of NA resulted in secretion of the NA ectodomain predominantly from the apical side. These results taken together indicate that the cytoplasmic tail of TR was sufficient for basolateral transport, but influenza virus NA possesses two sorting signals, one in the cytoplasmic or transmembrane domain and the other within the ectodomain, both of which are independently able to transport the protein to the apical plasma membrane.

Viruses as model systems in cell biology

This chapter focuses on the contributions that studies with viruses have made to current concepts in cell biology. Among the important advantages that viruses provide in such studies is their structural and genetic simplicity. The chapter describes the methods for growth, assay, and purification of viruses and infection of cells by several viruses that have been widely utilized for studies of cellular processes. Most investigations of virus replication at the cellular level are carried out using animal cells in culture. For the events in individual cells to occur with a high level of synchrony, single cycle growth conditions are used. Cells are infected using a high multiplicity of infectious virus particles in a low volume of medium to enhance the efficiency of virus adsorption to cell surfaces. After the adsorption period, the residual inoculum is removed and replaced with an appropriate culture medium. During further incubation, each individual cell in the culture is at a similar temporal stage in the viral replication process. Therefore, experimental procedures carried out on the entire culture reflect the replicative events occurring within an individual cell. The length of a single cycle of virus growth can range from a few hours to several days, depending on the virus type.

Regulation of cell surface polarity in renal epithelia

In the kidney, polarized epithelial cells play critical roles in ion, fluid and solute reabsorption from the ultrafiltrate to the blood supply. Detailed analysis of protein distributions has revealed that ion channels, transporters and pumps are restricted to distinct domains of the plasma membrane that face either the ultrafiltrate (apical membrane) or the blood supply (basal-lateral membrane). The importance of the development and maintenance of the polarized distributions of these proteins in renal epithelia for normal cell function is demonstrated by the fact that several disease states are characterized by abnormal distributions of proteins; for example in polycystic kidney disease, Na+/K(+)-ATPase has been detected in the apical and lateral membranes, compared with normal cells where Na+/K(+)-ATPase is localized in the basal-lateral membrane domain. Recent studies indicate that the development of restricted distributions of proteins at the cell surface of Madin Darby canine kidney epithelial cells is determined by direct sorting of proteins in the trans Golgi network into vesicles that are delivered vectorially to either the apical or basal-lateral membrane. Upon arrival at the plasma membrane, some proteins, such as Na+/K(+)-ATPase, may be selectively retained by binding to the membrane cytoskeleton.

Regulation of apical transport in epithelial cells by a Gs class of heterotrimeric G protein

The role of heterotrimeric GTP-binding proteins in signal transduction is well established. They might also be involved in vesicular transport. Here we show that in the epithelial cell line Madin-Darby Canine Kidney, transport of influenza haemagglutinin protein to the apical surface is stimulated and that of vesicular stomatitis virus glycoprotein to the basolateral surface is retarded by AlF(3-5) treatment. Treatment of cells with the reagents known to influence the Gi class of G proteins affected only the basolateral pathway whereas reagents acting on the Gs class of G proteins specifically affected the apical pathway. In permeabilized cells, antibodies raised against the N-terminal domain of the alpha-subunit of Gs inhibited the transport of haemagglutinin from the trans-Golgi network to apical surface but not between the endoplasmic reticulum and Golgi complex. These observations demonstrate involvement of a Gs class of heterotrimeric G proteins, besides that of the Gi, in vesicular transport. Moreover, the apical and the basolateral pathways in epithelial cells seem to be regulated by Gs and Gi proteins, respectively, in the trans-Golgi network.

Actin microfilaments play a critical role in endocytosis at the apical but not the basolateral surface of polarized epithelial cells

Treatment with cytochalasin D, a drug that acts by inducing the depolymerization of the actin cytoskeleton, selectively blocked endocytosis of membrane bound and fluid phase markers from the apical surface of polarized MDCK cells without affecting the uptake from the basolateral surface. Thus, in MDCK cell transformants that express the VSV G protein, cytochalasin blocked the internalization of an anti-G mAb bound to apical G molecules, but did not reduce the uptake of antibody bound to the basolateral surface. The selective effect of cytochalasin D on apical endocytosis was also demonstrated by the failure of the drug to reduce the uptake of 125I-labeled transferrin, which occurs by receptor-mediated endocytosis, via clathrin-coated pits, almost exclusively from the basolateral surface. The actin cytoskeleton appears to play a critical role in adsorptive as well as fluid phase apical endocytic events, since treatment with cytochalasin D prevented the apical uptake of cationized ferritin, that occurs after the marker binds to the cell surface, as well as uptake of Lucifer yellow, a fluorescent soluble dye. Moreover, the drug efficiently blocked infection of the cells with influenza virus, when the viral inoculum was applied to the apical surface. On the other hand, it did not inhibit the basolateral uptake of Lucifer yellow, nor did it prevent infection with VSV from the basolateral surface, or with influenza when this virus was applied to monolayers in which the formation of tight junctions had been prevented by depletion of calcium ions. EM demonstrated that cytochalasin D leads to an increase in the number of coated pits in the apical surface where it suppresses the pinching off of coated vesicles. In addition, in drug-treated cells cationized ferritin molecules that were bound to microvilli were not cleared from the microvillar surface, as is observed in untreated cells. These findings indicate that there is a fundamental difference in the process by which endocytic vesicles are formed at the two surfaces of polarized epithelial cells and that the integrity and/or the polymerization of actin filaments are required at the apical surface. Actin filaments in microvilli may be part of a mechanochemical motor that moves membrane components along the microvillar surface towards intermicrovillar spaces, or provides the force required for converting a membrane invagination or pit into an endocytic vesicle within the cytoplasm.

Virus infection of polarized epithelial cells

This chapter focuses on the interaction of viruses with epithelial cells. The role of specific pathways of virus entry and release in the pathogenesis of viral infection is examined together with the mechanisms utilized by viruses to circumvent the epithelial barrier. Polarized epithelial cells in culture, which can be grown on permeable supports, provide excellent systems for investigating the events in virus entry and release at the cellular level, and much information is being obtained using such systems. Much remains to be learned about the precise routes by which many viruses traverse the epithelial barrier to initiate their natural infection processes, although important information has been obtained in some systems. Another area of great interest for future investigation is the process of virus entry and release from other polarized cell types, including neuronal cells.

The apical sorting signal on human aminopeptidase N is not located in the stalk but in the catalytic head group

Human aminopeptidase N carries an apical sorting signal on its ectodomain necessary for its correct transport to the apical membrane in Madin-Darby canine kidney cells. To determine whether the apical sorting signal is localized in the serine/threonine rich stalk or in the catalytic head group, anchor/stalk-minus aminopeptidase N, consisting of the hemagglutinin signal peptide and the catalytic head group of human aminopeptidase N, was expressed in MDCK cells. Anchor/stalk-minus aminopeptidase N was secreted mainly to the apical side. The catalytic head group of human aminopeptidase N thus carries an apical sorting signal.

Opposite polarity of virus budding and of viral envelope glycoprotein distribution in epithelial cells derived from different tissues

We compared the surface envelope glycoprotein distribution and the budding polarity of four RNA viruses in Fischer rat thyroid (FRT) cells and in CaCo-2 cells derived from a human colon carcinoma. Whereas both FRT and CaCo-2 cells sort similarly influenza hemagglutinin and vesicular stomatitis virus (VSV) G protein, respectively, to apical and basolateral membrane domains, they differ in their handling of two togaviruses, Sindbis and Semliki Forest virus (SFV). By conventional EM Sindbis virus and SFV were shown to bud apically in FRT cells and basolaterally in CaCo-2 cells. Consistent with this finding, the distribution of the p62/E2 envelope glycoprotein of SFV, assayed by immunoelectronmicroscopy and by domain-selective surface biotinylation was predominantly apical on FRT cells and basolateral on CaCo-2 cells. We conclude that a given virus and its envelope glycoprotein can be delivered to opposite membrane domains in epithelial cells derived from different tissues. The tissue specificity in the polarity of virus budding and viral envelope glycoprotein distribution indicate that the sorting machinery varies considerably between different epithelial cell types.

Uncoupling of chondroitin sulfate glycosaminoglycan synthesis by brefeldin A

Brefeldin A has dramatic, well-documented, effects on the structural and functional organization of the Golgi complex. We have examined the effects of brefeldin A (BFA) on the Golgi-localized synthesis and addition of chondroitin sulfate glycosaminoglycan carbohydrate side chains. BFA caused a dose-dependent inhibition of chondroitin sulfate glycosaminoglycan elongation and sulfation onto the core proteins of the melanoma-associated proteoglycan and the major histocompatibility complex class II-associated invariant chain. In the presence of BFA, the melanoma proteoglycan core protein was retained in the ER but still acquired complex, sialylated, N-linked oligosaccharides, as measured by digestion with endoglycosidase H and neuraminidase. The initiation of glycosaminoglycan synthesis was not affected by BFA, as shown by the incorporation of [6-3H]galactose into a protein-carbohydrate linkage region that was sensitive to beta-elimination. The ability of cells to use an exogenous acceptor, p-nitrophenyl-beta-D-xyloside, to elongate and sulfate core protein-free glycosaminoglycans, was completely inhibited by BFA. The effects of BFA were completely reversible in the absence of new protein synthesis. These experiments indicate that BFA effectively uncouples chondroitin sulfate glycosaminoglycan synthesis by segregating initiation reactions from elongation and sulfation events. Our findings support the proposal that glycosaminoglycan elongation and sulfation reactions are associated with the trans-Golgi network, a BFA-resistant, Golgi subcompartment.

Basolateral sorting in MDCK cells requires a distinct cytoplasmic domain determinant

In MDCK cells, Golgi to basolateral transport of several membrane proteins has been found to involve a cytoplasmic domain determinant. In some cases (Fc receptor, lysosomal glycoprotein Igp120), the determinant appears similar to that required for endocytosis via clathrin-coated pits; for Igp120, elimination of a single cytoplasmic domain tyrosine both blocks internalization and results in apical transport. In other cases (LDL receptor), the determinant does not involve the cytoplasmic domain tyrosine required for endocytosis. Thus, contrary to current models, basolateral transport in MCDK cells occurs not by default but depends on one or more cytoplasmic domain determinants, the precise nature of which is unknown. For some proteins, it is closely related to coated pit determinants. The fact that many membrane proteins can reach the apical surface in the absence of this determinant suggests that signals for apical transport are widely distributed.

An autonomous signal for basolateral sorting in the cytoplasmic domain of the polymeric immunoglobulin receptor

The polymeric immunoglobulin receptor is normally delivered from the Golgi to the basolateral surface of epithelial cells and then transports polymeric IgA and IgM to the apical surface. We now report that a 14 residue segment of the 103 amino acid cytoplasmic domain, proximal to the plasma membrane, directs the receptor to the basolateral surface. A mutant receptor lacking these 14 amino acids is sorted directly to the apical surface from the Golgi. Furthermore, this sequence is sufficient to redirect an apical membrane protein, placental alkaline phosphatase, to the basolateral plasma membrane. We conclude that this sequence contains an autonomous signal, which specifies sorting from the Golgi to the basolateral surface, a process previously postulated to occur by default.

Disruption of microtubules alters polarity of basement membrane proteoglycan secretion in epithelial cells

Basement membrane proteins such as the heparan sulfate proteoglycan (HSPG) are secreted in a polarized fashion from the basolateral membrane of epithelial cells. We have used the microtubule-disrupting drug colchicine to study the role of the microtubule network in directing constitutive secretion to the basolateral membrane of LLC-PK1 renal epithelial cells. Microtubule depolymerization induced by colchicine resulted in fragmentation and redistribution of fluorescently labeled trans-Golgi membranes. Increased immunofluorescent staining of HSPG was associated with these dispersed Golgi cisternae. The biosynthetic processing of HSPG was not significantly altered by the loss of microtubules or by the dispersal of the Golgi elements. The most striking effect of microtubule disruption was the loss of polarity of HSPG secretion. Immunoprecipitation studies showed that HSPG was secreted from both apical and basolateral surfaces of LLC-PK1 cells treated with colchicine, and a similar result was found for the delivery of laminin, another basement membrane protein. In contrast, there was no change in the distribution of an integral basolateral membrane protein, Na(+)-K(+)-ATPase, following colchicine treatment. Our results provide the first demonstration that microtubules are involved in the constitutive trafficking of basolateral secretory proteins. These data also suggest that there may be an inherent difference in the targeting or delivery of membrane and secretory proteins to the basolateral cell surface.

Polarized intestinal hybrid cell lines derived from primary culture: establishment and characterization

A cell culture system has been developed that produces stable gastrointestinal (GI) polarized cell lines capable of maintaining hormone secretion. A spontaneously transformed rat mucosal epithelial cell was selected for hypoxanthine/guanine phosphoribosyltransferase deficiency and transfected with a plasmid conferring hygromycin resistance (BRIE 291 cells). Fusion of these cells with dispersed small intestinal epithelia cells resulted in hybrid cell lines that retained characteristic properties of the native GI cell more effectively than the transformed tumorigenic parental cell line. Hybrid hBRIE 380 cells are uniformly cuboidal with microvilli, contain villin, are contact inhibited, are anchorage dependent, require serum supplementation for growth, and are more sensitive to virus infection than the parental BRIE 291 cells. Fusion of BRIE 291 with dispersed pancreatic islet cells has resulted in a variety of pancreatic-hormone-producing cell lines. One of these, hybrid hBRIE 291-i2, forms confluent monolayers capable of synthesizing insulin-like immunoreactivity. These studies demonstrate that functionally polarized GI cells can be generated from primary cultures of nondividing committed epithelial cells by somatic cell hybridization and make feasible the selection and maintenance of specific GI epithelial cell types in confluent monolayer cultures.

Microtubular organization and its involvement in the biogenetic pathways of plasma membrane proteins in Caco-2 intestinal epithelial cells

We characterized the three-dimensional organization of microtubules in the human intestinal epithelial cell line Caco-2 by laser scanning confocal microscopy. Microtubules formed a dense network approximately 4-microns thick parallel to the cell surface in the apical pole and a loose network 1-micron thick in the basal pole. Between the apical and the basal bundles, microtubules run parallel to the major cell axis, concentrated in the vicinity of the lateral membrane. Colchicine treatment for 4 h depolymerized 99.4% of microtubular tubulin. Metabolic pulse chase, in combination with domain-selective biotinylation, immune and streptavidin precipitation was used to study the role of microtubules in the sorting and targeting of four apical and one basolateral markers. Apical proteins have been recently shown to use both direct and transcytotic (via the basolateral membrane) routes to the apical surface of Caco-2 cells. Colchicine treatment slowed down the transport to the cell surface of apical and basolateral proteins, but the effect on the apical proteins was much more drastic and affected both direct and indirect pathways. The final effect of microtubular disruption on the distribution of apical proteins depended on the degree of steady-state polarization of the individual markers in control cells. Aminopeptidase N (APN) and sucrase-isomaltase (SI), which normally reach a highly polarized distribution (110 and 75 times higher on the apical than on the basolateral side) were still relatively polarized (9 times) after colchicine treatment. The decrease in the polarity of APN and SI was mostly due to an increase in the residual basolateral expression (10% of control total surface expression) since 80% of the newly synthesized APN was still transported, although at a slower rate, to the apical surface in the absence of microtubules. Alkaline phosphatase and dipeptidylpeptidase IV, which normally reach only low levels of apical polarity (four times and six times after 20 h chase, nine times and eight times at steady state) did not polarize at all in the presence of colchicine due to slower delivery to the apical surface and increased residence time in the basolateral surface. Colchicine-treated cells displayed an ectopic localization of microvilli or other apical markers in the basolateral surface and large intracellular vacuoles. Polarized secretion into apical and basolateral media was also affected by microtubular disruption. Thus, an intact microtubular network facilitates apical protein transport to the cell surface of Caco-2 cells via direct and indirect routes; this role appears to be crucial for the final polarity of some apical plasma membrane proteins but only an enhancement factor for others.

Disruption of microtubules alters polarity of basement membrane proteoglycan secretion in epithelial cells

Basement membrane proteins such as the heparan sulfate proteoglycan (HSPG) are secreted in a polarized fashion from the basolateral membrane of epithelial cells. We have used the microtubule-disrupting drug colchicine to study the role of the microtubule network in directing constitutive secretion to the basolateral membrane of LLC-PK1 renal epithelial cells. Microtubule depolymerization induced by colchicine resulted in fragmentation and redistribution of fluorescently labeled trans-Golgi membranes. Increased immunofluorescent staining of HSPG was associated with these dispersed Golgi cisternae. The biosynthetic processing of HSPG was not significantly altered by the loss of microtubules or by the dispersal of the Golgi elements. The most striking effect of microtubule disruption was the loss of polarity of HSPG secretion. Immunoprecipitation studies showed that HSPG was secreted from both apical and basolateral surfaces of LLC-PK1 cells treated with colchicine, and a similar result was found for the delivery of laminin, another basement membrane protein. In contrast, there was no change in the distribution of an integral basolateral membrane protein, Na+-K+-ATPase, following colchicine treatment. Our results provide the first demonstration that microtubules are involved in the constitutive trafficking of basolateral secretory proteins. These data also suggest that there may be an inherent difference in the targeting or delivery of membrane and secretory proteins to the basolateral cell surface.

polarized secretion; heparan sulfate proteoglycans; sorting; Golgi processing

Submitted on July 10, 1990

Accepted on November 12, 1990

Protein traffic between distinct plasma membrane domains: isolation and characterization of vesicular carriers involved in transcytosis

We have isolated a population of vesicular carriers involved in the transport (transcytosis) of proteins from the basolateral to the apical plasma membrane of hepatocytes. The obtained fraction was enriched in compartments containing known transcytosed proteins and depleted in elements of the secretory pathway, Golgi elements, basolateral plasma membrane, as well as early endosomal components. The fraction was analyzed by biochemical and immunological procedures. Antibodies raised against the proteins in the fraction recognized a single 108K antigen. Based on its subcellular distribution, the 108K antigen may represent a novel marker for transcytotic vesicular carriers.

Chapter 2 Biogenesis and Sorting of Plasma Membrane Proteins

The cell surface membrane is the boundary between a cell and its environment. In case of polarized epithelial cells, the apical plasma membrane is frequently the boundary between an organism and its environment. The plasmalemma possesses the elements that endow a cell with the capacity to converse with its environment. Plasmalemmal receptor and transducer proteins allow the cell to recognize and respond to various external influences. Membrane-associated proteins anchor cells to their substrata and mediate their integration into tissues. Many properties of a given cell type may be attributed to the protein composition of its plasma membrane. Most cells go to large lengths to control the nature and distribution of polypeptides that populate their plasmalemmas. Cells regulate the expression of genes encoding plasma membrane proteins. Proteins destined for the insertion into the plasma membrane pass through a complex system of processing organelles prior to arriving at their site of ultimate functional residence. Each of these organelles makes a unique contribution to the maturation of these proteins as they transit through them. This chapter discusses the postsynthetic steps involved in the biogenesis of plasma membrane proteins. The chapter discusses some of the events common to all plasmalemmal polypeptides, with special emphasis on those that contribute directly to the character of the cell surface. The chapter then discusses the specializations, associated with cell types, possessing differentiated cell surface sub-domains. The chapter highlights some of the important and fascinating questions confronting investigators interested in the cell biology of the plasma membrane.