The sorting of proteins to the plasma membrane in epithelial cells

Self-organization of apical membrane protein sorting in epithelial cells

Polarized epithelial cells are characterized by the asymmetric distribution of proteins between apical and basolateral domains of the plasma membrane. This asymmetry is highly conserved and is fundamental to epithelial cell physiology, development, and homeostasis. How proteins are segregated for apical or basolateral delivery, a process known as sorting, has been the subject of considerable investigation for decades. Despite these efforts, the rules guiding apical sorting are poorly understood and remain controversial. Here, we consider mechanisms of apical membrane protein sorting and argue that they are largely driven by self-organization and biophysical principles. The preponderance of data to date is consistent with the idea that apical sorting is not ruled by a dedicated protein-based sorting machinery and relies instead on the concerted effects of oligomerization, phase separation of lipids and proteins in membranes, and pH-dependent glycan interactions.

Distinct roles for luminal acidification in apical protein sorting and trafficking in zebrafish

Epithelial cell physiology critically depends on the asymmetric distribution of channels and transporters. However, the mechanisms targeting membrane proteins to the apical surface are still poorly understood. Here, we performed a visual forward genetic screen in the zebrafish intestine and identified mutants with defective apical targeting of membrane proteins. One of these mutants, affecting the vacuolar H⁺-ATPase gene atp6ap1b, revealed specific requirements for luminal acidification in apical, but not basolateral, membrane protein sorting and transport. Using a low temperature block assay combined with genetic and pharmacologic perturbation of luminal pH, we monitored transport of newly synthesized membrane proteins from the TGN to apical membrane in live zebrafish. We show that vacuolar H⁺-ATPase activity regulates sorting of O-glycosylated proteins at the TGN, as well as Rab8-dependent post-Golgi trafficking of different classes of apical membrane proteins. Thus, luminal acidification plays distinct and specific roles in apical membrane biogenesis.

Impaired surface membrane insertion of homo- and heterodimeric human muscle chloride channels carrying amino-terminal myotonia-causing mutations

Mutations in the muscle chloride channel gene (CLCN1) cause myotonia congenita, an inherited condition characterized by muscle stiffness upon sudden forceful movement. We here studied the functional consequences of four disease-causing mutations that predict amino acid substitutions Q43R, S70L, Y137D and Q160H. Wild-type (WT) and mutant hClC-1 channels were heterologously expressed as YFP or CFP fusion protein in HEK293T cells and analyzed by whole-cell patch clamp and fluorescence recordings on individual cells. Q43R, Y137D and Q160H, but not S70L reduced macroscopic current amplitudes, but left channel gating and unitary current amplitudes unaffected. We developed a novel assay combining electrophysiological and fluorescence measurements at the single-cell level in order to measure the probability of ion channel surface membrane insertion. With the exception of S70L, all tested mutations significantly reduced the relative number of homodimeric hClC-1 channels in the surface membrane. The strongest effect was seen for Q43R that reduced the surface insertion probability by more than 99% in Q43R homodimeric channels and by 92 ± 3% in heterodimeric WT/Q43R channels compared to homodimeric WT channels. The new method offers a sensitive approach to investigate mutations that were reported to cause channelopathies, but display only minor changes in ion channel function.

A polarized cell model for Chikungunya virus infection: entry and egress of virus occurs at the apical domain of polarized cells

Chikungunya virus (CHIKV) has resulted in several outbreaks in the past six decades. The clinical symptoms of Chikungunya infection include fever, skin rash, arthralgia, and an increasing incidence of encephalitis. The re-emergence of CHIKV with more severe pathogenesis highlights its potential threat on our human health. In this study, polarized HBMEC, polarized Vero C1008 and non-polarized Vero cells grown on cell culture inserts were infected with CHIKV apically or basolaterally. Plaque assays, viral binding assays and immunofluorescence assays demonstrated apical entry and release of CHIKV in polarized HBMEC and Vero C1008. Drug treatment studies were performed to elucidate both host cell and viral factors involved in the sorting and release of CHIKV at the apical domain of polarized cells. Disruption of host cell myosin II, microtubule and microfilament networks did not disrupt the polarized release of CHIKV. However, treatment with tunicamycin resulted in a bi-directional release of CHIKV, suggesting that N-glycans of CHIKV envelope glycoproteins could serve as apical sorting signals.

Polarized cells are found in many parts of the human body and are characterized by the presence of two distinct plasma membrane domains: the apical domain facing the lumen and the basolateral domain facing the underlying tissues. Polarized epithelial cells line the major cavities of our body, while polarized endothelial cells line the blood-tissue interface, both of which protect our body against the invasion of biological pathogens. Thus, many pathogens have to invade the monolayer of epithelial or endothelial cells in order to establish infection. During infection with Chikungunya virus, a mosquito vector bites a human host and inoculates the virus into the host's bloodstream. In recent epidemics of Chikungunya infection, more severe clinical manifestations such as neurological complications were observed. As such, we studied the infection of Chikungunya virus in polarized cells in an aim to provide explanations for the more severe pathogenesis observed.

Influenza virus inhibits ENaC and lung fluid clearance

Fluid-free alveolar space is critical for normal gas exchange. Influenza virus alters fluid transport across respiratory epithelia producing rhinorrhea, middle ear effusions, and alveolar flooding. However, the mechanism of fluid retention remains unclear. We investigated influenza virus strain A/PR/8/34, which can attach and enter mammalian cells but is incapable of viral replication and productive infection in mammalian epithelia, on epithelial sodium channels (ENaC) in rat alveolar type II (ATII) cells. In parallel, we determined the effects of virus on amiloride-sensitive (i.e., ENaC-mediated) fluid clearance in rat lungs in vivo. Although influenza virus did not change the inulin permeability of ATII monolayers, it rapidly reduced the net volume transport across monolayers. Virus reduced the open probability of single ENaC channels in apical cell-attached patches. U-73122, a phospholipase C (PLC) inhibitor, and PP2, a Src inhibitor, blocked the effect of virus on ENaC. GF-109203X, a protein kinase C (PKC) inhibitor, also blocked the effect, suggesting a PKC-mediated mechanism. In parallel, intratracheal administration of influenza virus produced a rapid inhibition of amiloride-sensitive (i.e., ENaC-dependent) lung fluid transport. Together, these results show that influenza virus rapidly inhibits ENaC in ATII cells via a PLC- and Src-mediated activation of PKC but does not increase epithelial permeability in this same rapid time course. We speculate that this rapid inhibition of ENaC and formation of edema when the virus first attaches to the alveolar epithelium might facilitate subsequent influenza infection and may exacerbate influenza-mediated alveolar flooding that can lead to acute respiratory failure and death.

Polarized secretion of transgene products from salivary glands in vivo

Previously (Kagami et al. Hum. Gene Ther. 1996;7:2177-2184) we have shown that salivary glands are able to secrete a transgene-encoded protein into serum as well as saliva. This result and other published data suggest that salivary glands may be a useful target site for vectors encoding therapeutic proteins for systemic delivery. The aim of the present study was to assess in vivo if transgene-encoded secretory proteins follow distinct, polarized sorting pathways as has been shown to occur "classically" in cell biological studies in vitro. Four first-generation, E1-, type 5 recombinant adenoviruses were used to deliver different transgenes to a rat submandibular cell line in vitro or to rat submandibular glands in vivo. Subsequently, the secretory distribution of the encoded proteins was determined. Luciferase, which has no signal peptide, served as a cell-associated, negative control and was used to correct for any nonspecific secretory protein release from cells. The three remaining transgene products tested, human tissue kallikrein (hK1), human growth hormone (hGH), and human alpha1-antitrypsin (halpha1AT), were predominantly secreted (>96%) in vitro. Most importantly, in vivo, after a parasympathomimetic secretory stimulus, both hK1 and hGH were secreted primarily in an exocrine manner into saliva. Conversely, halpha1AT was predominantly secreted into the bloodstream, i.e., in an endocrine manner. The aggregate results are consistent with the recognition of signals encoded within the transgenes that result in specific patterns of polarized protein secretion from rat submandibular gland cells in vivo.

N-glycans are not a universal signal for apical sorting of secretory proteins

In MDCK cells, N-glycans have been shown to determine the sorting of secretory proteins and membrane proteins to the apical domain in the absence of a dominant basolateral targeting signal. We have examined the sorting of endogenous proteins in ECV304 cells in the presence and absence of tunicamycin, an inhibitor of N-linked glycosylation. A prominent apically secreted protein of 71 kDa was not N-glycosylated and continued to be secreted apically in the presence of tunicamycin. In contrast, other endogenous proteins that were N-glycosylated were secreted preferentially into the basolateral medium or without polarity. When rat growth hormone was expressed in MDCK and ECV304 cells, we observed 65 and 94% of the secretion to the basolateral medium, respectively. Introduction of a single N-glycan caused 83% of the growth hormone to be secreted at the apical surface in MDCK cells but had no significant effect on the polarity of secretion of growth hormone in ECV304 cells. These results indicate that not all cell lines recognise N-glycans as a signal for apical sorting and raises the possibility of using ECV304 cells as a model system for analysis of apical sorting molecules.

Transmembrane domain of influenza virus neuraminidase, a type II protein, possesses an apical sorting signal in polarized MDCK cells

The influenza virus neuraminidase (NA), a type II transmembrane protein, is directly transported to the apical plasma membrane in polarized MDCK cells. By using deletion mutants and chimeric constructs of influenza virus NA with the human transferrin receptor, a type II basolateral transmembrane protein, we investigated the location of the apical sorting signal of influenza virus NA. When these mutant and chimeric proteins were expressed in stably transfected polarized MDCK cells, the transmembrane domain of NA, and not the cytoplasmic tail, provided a determinant for apical targeting in polarized MDCK cells and this transmembrane signal was sufficient for sorting and transport of the ectodomain of a reporter protein (transferrin receptor) directly to the apical plasma membrane of polarized MDCK cells. In addition, by using differential detergent extraction, we demonstrated that influenza virus NA and the chimeras which were transported to the apical plasma membrane also became insoluble in Triton X-100 but soluble in octylglucoside after extraction from MDCK cells during exocytic transport. These data indicate that the transmembrane domain of NA provides the determinant(s) both for apical transport and for association with Triton X-100-insoluble lipids.

The hemispheric functional expression of the thyrotropin-releasing-hormone receptor is not determined by the receptors' physical distribution

The thyrotropin-releasing-hormone receptor (TRH-R) is a member of a family of the G-protein-coupled receptors that share structural similarities and exert their physiological action via the inositol lipid signal-transduction pathway. The TRH-R when expressed in Xenopus oocytes exhibits marked preference of the response (increased chloride conductance) for the animal hemisphere. Whereas the rat TRH-R functional distribution was strongly asymmetric (animal/vegetal ratio = 9.5), the mouse TRH-R exhibited a significantly lower ratio (3.9). Truncation of the last 59 amino acids of the C-terminal region of the mouse TRH-R did not lead to any changes in the functional hemispheric distribution. Despite the polarization of response, receptor number was similar on both hemispheres. Moreover, the apparent half-life of the functional expression of the TRH-R was approx. 4 h on both hemispheres when the expression was inhibited by a specific antisense oligonucleotide. Inhibition of total protein synthesis with cycloheximide affected hemispheric responses mediated by each of the three TRH-Rs tested in a qualitatively different way. These results suggest that an additional, rapidly degraded, protein modulates the functional hemispheric expression of the TRH-Rs.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Differential secretion of alpha 1-acid glycoprotein occurs in the Golgi complex of isolated rat hepatocytes. Evidence of partial retention in the Golgi

Using weakly basic amines, we investigated the step at which the secretion kinetics of concanavalin-A-retained and nonretained alpha 1-acid glycoprotein glycoforms diverge in isolated rat hepatocytes. Both chloroquine and primaquine, whose action on protein secretion is targeted to terminal domains of the Golgi apparatus, cancelled the kinetic difference without influencing carbohydrate chain sialylation. To test for a possible interaction of alpha 1-acid glycoprotein with Golgi membranes, we also permeabilized control and primaquine-treated hepatocytes, as well as purified Golgi preparations, with saponin. In each case, we found that alpha 1-acid glycoprotein was associated with Golgi membranes, the association being more marked in primaquine-treated cells than in control cells. Membrane-bound alpha 1-acid glycoprotein appeared to be preferentially retained on concanavalin A. Such retention could account for the divergent secretion kinetics of alpha 1-acid glycoprotein glycoforms.

Effects of NH4Cl and nocodazole on polarized fibronectin secretion vary amongst different epithelial cell types

The extracellular matrix protein fibronectin was found to be secreted by three polarized epithelial cell lines Madin-Darby canine kidney (MDCK), Caco-2 and LLC-PK1. About 54 and 46% of fibronectin was secreted from the apical and basolateral cell surfaces, respectively, in MDCK cells. In Caco-2 and LLC-PK1 cells, the majority (about 92-93%) of fibronectin secretion occurs from the basolateral cell surface, with the remaining 7-8% from the apical surface. In all three cell types, NH4Cl was found to inhibit basolateral secretion (resulting in enhanced apical secretion), while total fibronectin secretion was not significantly affected (although a delay in secretion was observed). Nocodazole reduced total fibronectin secretion to about 70% of control levels in MDCK and Caco-2 cells, with significant inhibition on secretion from both surfaces. In contrast, total fibronectin secretion was enhanced by nocodazole in LLC-PK1 cells. Furthermore, the majority of fibronectin secretion was redirected to the apical cell surface in LLC-PK1 cells. These observations demonstrate that the nature as well as the extent of the effects of NH4-Cl and nocodazole on polarized fibronectin secretion varies amongst different epithelial cell types.

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.

Possible involvement of microtubule disruption in bipolar budding of a Sendai virus mutant, F1-R, in epithelial MDCK cells

Envelope glycoproteins F and HN of wild-type Sendai virus are transported to the apical plasma membrane domain of polarized epithelial MDCK cells, where budding of progeny virus occurs. On the other hand, a pantropic mutant, F1-R, buds bipolarly at both the apical and basolateral domains, and the viral glycoproteins have also been shown to be transported to both of these domains (M. Tashiro, M. Yamakawa, K. Tobita, H.-D. Klenk, R. Rott, and J.T. Seto, J. Virol. 64:4672-4677, 1990). MDCK cells were infected with wild-type virus and treated with the microtubule-depolymerizing drugs colchicine and nocodazole. Budding of the virus and surface expression of the glycoproteins were found to occur in a nonpolarized fashion similar to that found in cells infected with F1-R. In uninfected cells, the drugs were shown to interfere with apical transport of a secretory cellular glycoprotein, gp80, and basolateral uptake of [35S]methionine as well as to disrupt microtubule structure, indicating that cellular polarity of MDCK cells depends on the presence of intact microtubules. Infection by the F1-R mutant partially affected the transport of gp80, uptake of [35S]methionine, and the microtubule network, whereas wild-type virus had a marginal effect. These results suggest that apical transport of the glycoproteins of wild-type Sendai virus in MDCK cells depends on intact microtubules and that bipolar budding by F1-R is possibly due, at least in part, to the disruption of microtubules. Nucleotide sequence analyses of the viral genes suggest that the mutated M protein of F1-R might be involved in the alteration of microtubules.

Newly synthesized hepatocyte plasma membrane proteins are transported in transcytotic vesicles in the bile duct-ligated rat

BACKGROUND

Newly synthesized apical membrane proteins in hepatocytes go first to the basolateral membrane, from which they are retrieved and delivered to the apical domain. The goal of the present study was to identify the vesicular carriers of these molecules.

METHODS

The common bile duct of rats was ligated for 10-72 hours, and then various plasma membrane proteins were localized using immunofluorescence and quantitative immuno-electron microscopy of fixed liver tissue.

RESULTS

By immunofluorescence, we found intracellular punctate staining near the bile canalicular membrane of polymeric immunoglobulin A (IgA) receptor and several apical membrane proteins, but not basolateral proteins. This compartment was membrane bounded and pleiomorphic by immunoelectron microscopy. Colocalization at the electron microscopic level showed that the apical protein, dipeptidyl peptidase IV, was in the same structures as aminopeptidase N, polymeric IgA receptor, or intravenously injected horseradish peroxidase. This intracellular immunolabeling decreased after cycloheximide treatment (t1/2 = 2-2.5 hours) or reversal of the ligation for 1 hour. In the latter case, bile canalicular labeling increased. Furthermore, polymeric IgA receptor was delivered to the bile canaliculi.

CONCLUSIONS

Bile duct ligation leads to an intracellular accumulation of vesicles carrying polymeric IgA receptor, several apical membrane proteins, and a fluid phase marker. These vesicles continue to fuse with the apical membrane, even during ligation.

Stimulated release of arachidonate and prostaglandins is vectorial in MDCK epithelial cells

The receptor mediated activation of phospholipase A2 by appropriate ligands results in the synthesis and release of eicosanoids, a class of potent bioregulatory molecules. Madin-Darby canine kidney cells (MDCK) are polarized epithelial cells, with structurally and functionally distinct plasma membrane domains separated by tight junctions. Using MDCK cells grown in dual sided chambers, we show in this report, that a) the receptor mediated release of prostaglandins and arachidonate into the extracellular medium is predominantly unidirectional, b) the direction of release is agonist specific, and c) the magnitude of the response due to a given agonist is cell-domain specific. These characteristics, if operative in vivo, would contribute towards the optimal function of trans-cellular metabolism of eicosanoids already demonstrated.

W(h)ither default? Sorting and polarization in epithelial cells

The discovery of basolateral sorting signals in the past year may leave the default pathway with nowhere to go. With new results suggesting that even more GTP-binding proteins and coatamers might be involved in transport and targeting, it is clear that the age of mapmaking in polarization research is nearly over.

Differential rates of glycoprotein secretion by isolated rat hepatocytes studied in terms of concanavalin A binding

Using a concanavalin-A-based method which respects cell function, we have shown that the kinetics of glycoprotein secretion appear to depend on the nature of the oligosaccharide moiety. In 37 degrees C pulse/chase experiments using freshly isolated normal rat hepatocytes, we found that except for transferrin, whose rate of secretion was independent of its concanavalin A reactivity, the secretion of the concanavalin-A-retained forms of alpha 1 acid glycoprotein, T-kininogen, alpha 1 protease inhibitor and alpha 1 inhibitor III was slower than that of the concanavalin-A-non-retained forms. When hepatocytes were incubated at 20 degrees C, secretion was blocked with the accumulation of mainly endoglycosidase-H-sensitive forms. The secretion kinetics of the concanavalin-A-differentiated forms were still different when the temperature was shifted back to 37 degrees C. The divergence between the secretion rates of the concanavalin-A-differentiated forms would appear to be due to a late event in intracellular protein trafficking, which may depend on the sugar content and/or the number of carbohydrate chains of the glycoproteins.

Desmosome assembly in MDCK epithelial cells does not require the presence of functional microtubules

Desmosomes, complex multisubunit structures that assemble at sites of cell-cell contact, are important components of the epithelial junctional complex. Desmosome assembly requires the coordinated interaction at the plasma membrane of at least 8 cytoplasmic and integral membrane proteins organized into two structurally and functionally distinct domains, the cytoplasmic plaque and membrane core. Previous studies (Pasdar et al., J. Cell Biol., 113:645-655) provided evidence that cytokeratin filaments and microtubules may regulate transfer and assembly of cytoplasmic plaque and membrane core proteins, respectively. To determine directly the role of microtubules in these processes, Madin-Darby canine kidney (MDCK) cells were treated with nocodazole or colchicine to disrupt the microtubular network. Biochemical analysis of the different components of the cytoplasmic plaque and membrane core domains revealed little or no effect of nocodazole or colchicine on the kinetics of synthesis, post-translational modifications, transfer of proteins to the plasma membrane or their metabolic stability in the presence or absence of cell-cell contact. Likewise, immunofluorescence analysis of desmosome formation demonstrated an apparently normal desmosome assembly in the presence of nocodazole or colchicine upon induction of cell-cell contact. These results indicate that an intact microtubular network is not necessary for the processing or transport of the desmosomal membrane core glycoproteins to the plasma membrane in the absence or presence of cell-cell contact. Furthermore, the integration of the cytoplasmic plaque and membrane core domains induced by cell-cell contact at the plasma membranes of adjacent cells does not require the presence of functional microtubules.

Retarded processing of influenza virus hemagglutinin in insect cells

When expressed in Spodoptera frugiperda cells by a baculovirus vector, the hemagglutinin of fowl plague virus has been found to contain palmitic acid in covalent hydroxylamine-sensitive linkage, indicating that these cells have the capacity to acylate foreign proteins at cysteine residues. Centrifugation on sucrose density gradients and immune precipitation with conformation-specific antibodies were used to compare trimerization of the hemagglutinin in insect cells and in fowl plague virus-infected MDCK cells. Trimerization of the hemagglutinin was incomplete in insect cells, and the kinetics of this reaction were about three times slower than in vertebrate cells. Similarly, post-translational proteolytic cleavage occurred in insect cells with a half-time of 90 min, and a substantial fraction of the hemagglutinin persisted in uncleaved form. In contrast, hemagglutinin was almost completely cleaved in MDCK cells, and the half-time of cleavage was only 30 min. The data indicate that in insect cells trimerization and, as a result, the subsequent processing steps of the hemagglutinin, are retarded and less efficient. The possible roles of aberrant glycosylation, acidic milieu, and lack of other influenza virus proteins in hemagglutinin trimerization are discussed.

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.

Polarized membrane expression of brush-border hydrolases in primary cultures of kidney proximal tubular cells depends on cell differentiation and is induced by dexamethasone

To analyze the influence of cell differentiation and the effects of hormones on the subcellular distribution of apical antigens in polarized epithelial cells, we have compared the localization of three brush border (BB) hydrolases [neutral endopeptidase (ENDO), aminopeptidase N (APN), and dipeptidylpeptidase IV (DPPIV)] in primary cultures of renal proximal tubule cells grown in various culture media. The degree of cell differentiation modulated by medium composition was estimated by measuring proximal functions, including glucose transport, specific enzymatic activities, and PTH responsiveness. In the dedifferentiated state observed in cells grown in 1% fetal calf serum (FCS)-supplemented medium, the three hydrolases are abnormally concentrated in a cytoplasmic vesicle compartment with weak expression on both membrane domains. By contrast, in serum-free hormonally defined medium (DM: insulin, 5 microgram/ml; dexamethasone, 5 x 10(-8) M), which markedly enhances morphological and functional cell differentiation, the distribution of hydrolases parallels that observed in the normal tubule. When added to the DM devoid of hormones, insulin has little polarizing effect, whereas dexamethasone dramatically increases the apical expression of the hydrolases, which then almost disappear from the basolateral membrane and cytoplasmic vesicular compartments. This glucocorticoid hormone augments the amount of immunoreactive antigen detectable on the apical domain in paraformaldehyde-fixed cells but does not change the total enzymatic activity. This suggests the presence in tubular cells of a dexamethasone-dependent polarizing machinery that requires de novo RNA and protein synthesis, and probably acts mainly by targeting a storage cytoplasmic pool of enzyme to the apical domain.

Microtubule perturbation inhibits intracellular transport of an apical membrane glycoprotein in a substrate-dependent manner in polarized Madin-Darby canine kidney epithelial cells

The effects of microtubule perturbation on the transport of two different viral glycoproteins were examined in infected Madin-Darby canine kidney (MDCK) cells grown on both permeable and solid substrata. Quantitative biochemical analysis showed that the microtubule-depolymerizing drug nocodazole inhibited arrival of influenza hemagglutinin on the apical plasma membrane in MDCK cells grown on both substrata. In contrast, the microtubule-stabilizing drug taxol inhibited apical appearance of hemagglutinin only when MDCK cells were grown on permeable substrata. On the basis of hemagglutinin mobility on sodium dodecyl sulfate gels and its sensitivity to endo H, it was evident that nocodazole and taxol arrested hemagglutinin at different intracellular sites. Neither drug caused a significant increase in the amount of hemagglutinin detected on the basolateral plasma membrane domain. In addition, neither drug had any noticeable effect on the transport of the vesicular stomatitis virus (VSV)-G protein to the basolateral surface. These results shed light on previous conflicting reports using this model system and support the hypothesis that microtubules play a role in the delivery of membrane glycoproteins to the apical, but not the basolateral, domain of epithelial cells.

Ultrastructural localization of milk fat globule membrane antigens in human breast carcinomas

The localization of milk fat globule membrane components has been assessed using post-fixation immunoelectron microscopy with three different antibodies for a group of breast carcinomas of different type and histological differentiation. For well differentiated carcinomas localization was in relation to the cell membrane, with polarization being evident in a proportion of cases. Moderately differentiated carcinomas showed a combined picture of cell membrane, vesicular, and intracytoplasmic luminal localization. The latter is a feature of infiltrating lobular carcinomas. Poorly differentiated carcinomas exhibited vesicular labelling throughout the cytoplasm, with no cell membrane localization. No labelling was seen over endoplasmic reticulum. It is proposed that carcinomas exhibit defects in intracellular transport of milk fat globule membrane components resulting in failure of expression at the cell surface and accumulation of vesicles within the cytoplasm, the extent of change relating to tumour differentiation.

Vectorial targeting of an endogenous apical membrane sialoglycoprotein and uvomorulin in MDCK cells

We studied the cell-surface delivery pathways of newly synthesized membrane glycoproteins in MDCK cells and for this purpose we characterized an endogenous apical integral membrane glycoprotein. By combining a pulse-chase protocol with domain-selective cell-surface biotinylation, immune precipitation, and streptavidin-agarose precipitation (Le Bivic et al. 1989. Proc. Natl. Acad. Sci USA. 86:9313-9317), we followed the appearance at the cell surface of a major apical sialoglycoprotein, gp114, a basolateral protein, uvomorulin, and a transcytosing protein, the polyimmunoglobulin receptor (pIg-R). We determined that both gp114 and uvomorulin appeared to be delivered directly to their respective surface, with mistargeting levels of 8 and 2%, respectively. Using the same technique, the pIg-R was first detected on the basolateral domain and then on the apical domain, to be finally released into the apical medium, as described (Mostov, K. E., and D. L. Deitcher. 1986. Cell. 46:613-621). To directly determine whether the gp114 pool present on the basolateral surface was a precursor of the apical gp114, we compared it with the equivalent pIg-R pool, by labeling with sulfo-NHS-SS-biotin, a cleavable, tight junction-impermeable probe, and by following the fraction of this probe that became resistant to basal glutathione and accessible to apical glutathione during incubation at 37 degrees C. We found that, contrary to pIg-R, basolateral gp114 was poorly endocytosed and was not transcytosed to the apical side. These results demonstrate that an endogenous apical integral membrane glycoprotein of Madin-Darby canine kidney cells is sorted intracellularly and is vectorially targeted to the apical surface.

Identification of a membrane-cytoskeletal complex containing the cell adhesion molecule uvomorulin (E-cadherin), ankyrin, and fodrin in Madin-Darby canine kidney epithelial cells

Cell-cell contact is an important determinant in the formation of functionally distinct plasma membrane domains during the development of epithelial cell polarity. In cultures of Madin-Darby canine kidney (MDCK) epithelial cells, cell-cell contact induces the assembly and accumulation of the Na+,K+-ATPase and elements of the membrane-cytoskeleton (ankyrin and fodrin) at the regions of cell-cell contact. Epithelial cell-cell contact appears to be regulated by the cell adhesion molecule uvomorulin (E-cadherin) which also becomes localized at the lateral plasma membrane of polarized cells. We have sought to determine whether the colocalization of these proteins reflects direct molecular interactions which may play roles in coordinating cell-cell contact and the assembly of the basal-lateral domain of the plasma membrane. Recently, we identified a complex of proteins containing the Na+,K+-ATPase, ankyrin, and fodrin in extracts of whole MDCK cells (Nelson, W.J., and R. W. Hammerton. 1989. J. Cell Biol. 108:893-902). We have now examined cell extracts for protein complexes containing the cell adhesion molecule uvomorulin. Proteins were solubilized from whole MDCK cells and fractionated in sucrose gradients. The sedimentation profile of solubilized uvomorulin is well separated from the majority of cell surface proteins, suggesting that uvomorulin occurs in a protein complex. A distinct portion of uvomorulin (30%) cosediments with ankyrin and fodrin (approximately 10.5S). Further fractionation of cosedimenting proteins in nondenaturing polyacrylamide gels reveals a discrete band of proteins that binds antibodies specific for uvomorulin, Na+,K+-ATPase, ankyrin, and fodrin. Significantly, ankyrin and fodrin, but not Na+K+-ATPase, coimmunoprecipitate in a complex with uvomorulin using uvomorulin antibodies. This result indicates that separate complexes exist containing ankyrin and fodrin with either uvomorulin or Na+,K+-ATPase. These results are discussed in the context of the possible roles of uvomorulin-induced cell-cell contact in the assembly of the membrane-cytoskeleton and associated membrane proteins (e.g., Na+,K+-ATPase) at the contact zone and in the development of cell polarity.

Polarized endocytosis by Madin-Darby canine kidney cells transfected with functional chicken liver glycoprotein receptor

We have studied the expression of the chicken hepatic glycoprotein receptor (chicken hepatic lectin [CHL]) in Madin-Darby canine kidney (MDCK) cells, by transfection of its cDNA under the control of a retroviral promotor. Transfected cell lines stably express 87,000 surface receptors/cell with a kd = 13 nM. In confluent monolayers, approximately 40% of CHL is localized at the plasma membrane. 98% of the surface CHL is expressed at the basolateral surface where it performs polarized endocytosis and degradation of glycoproteins carrying terminal N-acetylglucosamine at a rate of 50,000 ligand molecules/h. Studies of the half-life of metabolically labeled receptor and of the stability of biotinylated cell surface receptor after internalization indicate that transfected CHL performs several rounds of uptake and recycling before it gets degraded. The successful expression of a functional basolateral receptor in MDCK cells opens the way for the characterization of the mechanisms that control targeting and recycling of proteins to the basolateral membrane of epithelial cells.

Vectorial targeting of apical and basolateral plasma membrane proteins in a human adenocarcinoma epithelial cell line

We studied the surface delivery pathways followed by newly synthesized plasma membrane proteins in intestinal cells. To this end, we developed an assay and characterized an epithelial cell line (SK-CO-15) derived from human colon adenocarcinoma. Polarized confluent monolayers (2000 omega.cm2), grown on polycarbonate filter chambers, were pulsed with radioactive methionine/cysteine and, at different times of chase, the protein fraction reaching the apical or basolateral surface was recovered by domain-selective biotinylation, immunoprecipitation, and immobilized streptavidin precipitation. Both an apical and a basolateral marker were found to be delivered vectorially to the respective surface, with a sorting efficiency of 50:1 for the basolateral marker and 14:1 for the apical marker.

Protein transport from endoplasmic reticulum to the Golgi complex can occur during meiotic metaphase in Xenopus oocytes

We have previously shown that Xenopus oocytes arrested at second meiotic metaphase lost their characteristic multicisternal Golgi apparati and cannot secrete proteins into the surrounding medium. In this paper, we extend these studies to ask whether intracellular transport events affecting the movement of secretory proteins from the endoplasmic reticulum to the Golgi apparatus are also similarly inhibited in such oocytes. Using the acquisition of resistance to endoglycosidase H (endo H) as an assay for movement to the Golgi, we find that within 6 h, up to 66% of the influenza virus membrane protein, hemagglutinin (HA), synthesized from injected synthetic RNA, can move to the Golgi apparati in nonmatured oocytes; indeed after longer periods some correctly folded HA can be detected at the cell surface where it distributes in a nonpolarized fashion. In matured oocytes, up to 49% of the HA becomes endo H resistant in the same 6-h period. We conclude that movement from the endoplasmic reticulum to the Golgi can occur in matured oocytes despite the dramatic fragmentation of the Golgi apparati that we observe to occur on maturation. This observation of residual protein movement during meiotic metaphase contrasts with the situation at mitotic metabphase in cultured mammalian cells where all movement ceases, but resembles that in the budding yeast Saccharomyces cerevisiae where transport is unaffected.

Mechanism of membrane anchoring affects polarized expression of two proteins in MDCK cells

The signals that direct membrane proteins to the apical or basolateral plasma membrane domains of polarized epithelial cells are not known. Several of the class of proteins anchored in the membrane by glycosyl-phosphatidylinositol (GPI) are expressed on the apical surface of such cells. However, it is not known whether the mechanism of membrane anchorage or the polypeptide sequence provides the sorting information. The conversion of the normally basolateral vesicular stomatitis virus glycoprotein (VSV G) to a GPI-anchored protein led to its apical expression. Conversely, replacement of the GPI anchor of placental alkaline phosphatase with the transmembrane and cytoplasmic domains of VSV G shifted its expression from the apical to the basolateral surface. Thus, the mechanism of membrane anchorage can determine the sorting of proteins to the apical or basolateral surface, and the GPI anchor itself may provide an apical transport signal.

A membrane-targeting signal in the amino terminus of the neuronal protein GAP-43

Neurons and other cells, such as those of epithelia, accumulate particular proteins in spatially discrete domains of the plasma membrane. This enrichment is probably important for localization of function, but it is not clear how it is accomplished. One proposal for epithelial cells is that proteins contain targeting signals which guide preferential accumulation in basal or apical membranes. The growth-cone membrane of a neuron serves as a specialized transduction system, which helps to convert cues from its environment into regulated growth. Because it can be physically separated from the cell soma, it has been possible to show that the growth-cone membrane contains a restricted set of total cellular proteins, although, to our knowledge, no proteins are limited to that structure. One of the most prominent proteins in the growth-cone membrane is GAP-43. Basi et al. have suggested that the N-terminus of GAP-43 might be important for the binding of GAP-43 to the growth-cone membrane. Skene and Virag recently found that the cysteines in the N-terminus are fatty-acylated and that this post-translational modification correlates with membrane-binding ability. We investigated the binding of GAP-43 to the growth-cone membrane by mutational analysis and by laser-scanning confocal microscopy of fusion proteins that included regions of GAP-43 and chloramphenicol acetyltransferase (CAT). We found that a short stretch of the GAP-43 N-terminus suffices to direct accumulation in growth-cone membranes, especially in the filopodia. This supports a previous proposal for the importance of this region of GAP-43 in determining the membrane distribution of GAP-43.

Morphogenesis of the polarized epithelial cell phenotype

Polarized epithelial cells play fundamental roles in the ontogeny and function of a variety of tissues and organs in mammals. The morphogenesis of a sheet of polarized epithelial cells (the trophectoderm) is the first overt sign of cellular differentiation in early embryonic development. In the adult, polarized epithelial cells line all body cavities and occur in tissues that carry out specialized vectorial transport functions of absorption and secretion. The generation of this phenotype is a multistage process requiring extracellular cues and the reorganization of proteins in the cytoplasm and on the plasma membrane; once established, the phenotype is maintained by the segregation and retention of specific proteins and lipids in distinct apical and basal-lateral plasma membrane domains.

Spatial segregation of the regulated and constitutive secretory pathways

Recent experiments using DNA transfection have shown that secretory proteins in AtT-20 cells are sorted into two biochemically distinct secretory pathways. These two pathways differ in the temporal regulation of exocytosis. Proteins secreted by the regulated pathway are stored in dense-core granules until release is stimulated by secretagogues. In contrast, proteins secreted by the constitutive pathway are exported continuously, without storage. It is not known whether there are mechanisms to segregate regulated and constitutive secretory vesicles spatially. In this study, we examined the site of insertion of constitutive vesicles and compared it with that of regulated secretory granules. Regulated granules accumulate at tips of processes in these cells. To determine whether constitutively externalized membrane proteins are inserted into plasma membrane at the cell body or at process tips, AtT-20 cells were infected with ts-O45, a temperature-sensitive mutant of vesicular stomatitis virus in which transport of the surface glycoprotein G is conditionally blocked in the ER. After switching to the permissive temperature, insertion of G protein was detected at the cell body, not at process tips. Targeting of constitutive and regulated secretory vesicles to distinct areas of the plasma membrane appears to be mediated by microtubules. We found that while disruption of microtubules by colchicine had no effect on constitutive secretion, it completely blocked the accumulation of regulated granules at special release sites. Colchicine also affected the proper packaging of regulated secretory proteins. We conclude that regulated and constitutive secretory vesicles are targeted to different areas of the plasma membrane, most probably by differential interactions with microtubules. These results imply that regulated secretory granules may have unique membrane receptors for selective attachment to microtubules.

Permeability and mechanism of albumin, cationized albumin, and glycosylated albumin transcellular transport across monolayers of cultured bovine brain capillary endothelial cells

We have measured the permeability and binding characteristics of bovine serum albumin (BSA), cationized BSA (cBSA), and glycosylated BSA (gBSA) to primary cultures of bovine brain capillary endothelial cells (BBCEC). These endothelial cells serve as an in vitro model to study the binding, uptake, and transcellular transport of small and large molecule flux across the blood-brain barrier. The rate of [3H]BSA flux across the cultured BBCEC monolayers grown onto polycarbonate membranes (5-microns pore size) was linear with increasing BSA concentration and the flux could be inhibited by temperature reduction to 0-4 degrees C. The maximal binding of [3H]BSA was 0.04 fmol/mg total cell protein and could not be inhibited by nonradiolabeled BSA. The binding of cBSA and gBSA was rapid and could be inhibited by nonradiolabeled cBSA or gBSA, respectively. The maximal amount bound was 1.8 fmol/mg total cell protein for cBSA and 17.4 fmol/mg total cell protein for gBSA. The dissociation constants (Kd's) were 27 +/- 13 and 3.7 +/- 1.1 nM for cBSA and gBSA, respectively. The flux rates of cBSA and gBSA across the endothelial cell monolayers were linear with respect to concentration and they were approximately seven times greater than those observed for BSA. Each of the proteins appeared on the antiluminal side of the endothelial cell monolayers primarily (90%) as intact protein as determined by trichloroacetic acid (TCA) precipitations and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). The results for BSA are similar to those observed for lucifer yellow, a fluid-phase endocytic marker.(ABSTRACT TRUNCATED AT 250 WORDS)

Differing polarity of the constitutive and regulated secretory pathways for von Willebrand factor in endothelial cells

von Willebrand factor (vWf) is secreted from endothelial cells by one of two pathways-a constitutive pathway and a regulated pathway originating from the Weibel-Palade bodies. The molecular form of vWf from each of these pathways differs, with the most biologically potent molecules being released from Weibel-Palade bodies (Loesberg, C., M. D. Gonsalves, J. Zandbergen, C. Willems, W. G. Van Aken, H. V. Stel, J. A. Van Mourik, and P. G. DeGroot. 1983. Biochim. Biophys. Acta. 763:160-168; Sporn, L. A., V. J. Marder, and D. D. Wagner. 1987. Cell. 46:185-190). We investigated the polarity of the two secretory pathways using human umbilical vein endothelial cells cultured on polycarbonate membrane filters which allowed sampling of media from both the apical and basolateral compartments. After metabolic labeling of cells, vWf (constitutively secreted during a 10-min period or released during a 10-min treatment with a secretagogue) was purified from the apical and basolateral chambers and subjected to gel analysis. Approximately equal amounts of vWf were constitutively secreted into both chambers, and therefore this secretory pathway appeared to be nonpolarized. On the contrary, an average of 90% of vWf released from Weibel-Palade bodies after treatment with the calcium ionophore A23187 or PMA appeared in the basolateral chamber, indicating that the regulated pathway of secretion is highly polarized. Thrombin, a secretagogue which promotes disruption of the endothelial monolayer, led to release of vWf from cells with no apparent polarity. The presence of microtubule-depolymerizing agents nocodazol and colchicine inhibited the polarized release of vWf. Ammonium chloride treatment did not disrupt the polarity of the regulated secretory pathway, indicating that maintenance of low pH in intracellular compartments was not required for the polarized delivery of preformed Weibel-Palade bodies to the plasma membrane.

A membrane-cytoskeletal complex containing Na+,K+-ATPase, ankyrin, and fodrin in Madin-Darby canine kidney (MDCK) cells: implications for the biogenesis of epithelial cell polarity

In polarized Madin-Darby canine kidney (MDCK) epithelial cells, ankyrin, and the alpha- and beta-subunits of fodrin are components of the basolateral membrane-cytoskeleton and are colocalized with the Na+,K+-ATPase, a marker protein of the basolateral plasma membrane. Recently, we showed with purified proteins that the Na+,K+-ATPase is competent to bind ankyrin with high affinity and specificity (Nelson, W. J., and P. J. Veshnock. 1987. Nature (Lond.). 328:533-536). In the present study we have sought biochemical evidence for interactions between these proteins in MDCK cells. Proteins were solubilized from MDCK cells with an isotonic buffer containing Triton X-100 and fractionated rapidly in sucrose density gradients. Complexes of cosedimenting proteins were detected by analysis of sucrose gradient fractions in nondenaturing polyacrylamide gels. The results showed that ankyrin and fodrin cosedimented in sucrose gradient. Analysis of the proteins from the sucrose gradient in nondenaturing polyacrylamide gels revealed two distinct ankyrin:fodrin complexes that differed in their relative electrophoretic mobilities; both complexes had electrophoretic mobilities slower than that of purified spectrin heterotetramers. Parallel analysis of the distribution of solubilized Na+,K+-ATPase in sucrose gradients showed that there was a significant overlap with the distribution of ankyrin and fodrin. Analysis by nondenaturing polyacrylamide gel electrophoresis showed that the alpha- and beta-subunits of the Na+,K+-ATPase colocalized with the slower migrating of the two ankyrin:fodrin complexes. The faster migrating ankyrin:fodrin complex did not contain Na+,K+-ATPase. These results indicate strongly that the Na+,K+-ATPase, ankyrin, and fodrin are coextracted from whole MDCK cells as a protein complex. We suggest that the solubilized complex containing these proteins reflects the interaction of the Na+,K+-ATPase, ankyrin, and fodrin in the cell. This interaction may play an important role in the spatial organization of the Na+,K+-ATPase to the basolateral plasma membrane in polarized epithelial cells.

Candidacidal activities of proteins partially purified from rat epidermis

Proteins with approximate molecular weights of greater than 300,000 (EP greater than 300K) and 49,000 (EP 49K) were partially purified from terminally differentiated cells of 2-day-old rat epidermis. They were extracted in 0.34 M sucrose containing 0.01 M citric acid and purified by Sephacryl S-300 chromatography followed by reverse-phase column chromatography. The major constituents of EP greater than 300K and EP 49K were focused around pH 10 to 11 by sucrose gradient isoelectric focusing. Both proteins were effective at inhibiting colony formation of Candida albicans and C. tropicalis, but neither inhibited the growth of C. parapsilosis. The effect was maximum below pH 5.0 and reduced considerably above pH 5.0. The activity of EP greater than 300K on C. albicans TIMM 1623 (group A) was much stronger than that of EP 49K, whereas both proteins similarly inhibited C. albicans TIMM 1604 (group B). Their effects against C. albicans TIMM 1623 were dose dependent and were activated after a longer preincubation time, and NaCl concentration influenced their potency. At a low salt concentration and a 60-min preincubation at pH 4.5, the 50% effective dose for EP greater than 300K was calculated to be 1.7 x 10(-9) M, whereas that for EP 49K was 1.8 x 10(-7) M.