Hepatocyte adhesion to carbohydrate-derivatized surfaces. II. Regulation of cytoskeletal organization and cell morphology.

Rat hepatic lectins mediate adhesion of isolated rat hepatocytes to synthetic surfaces derivatized with galactosides. Initial weak adhesion is followed by rapid adhesion strengthening. After hepatocytes contact galactose-derivatized gels, the hepatic lectins move rapidly into an inaccessible patch at the adhesive surface (Weisz, O. A., and R. L. Schnaar. 1991. J. Cell Biol. 115:485-493). Hepatic lectin patching, which occurs both at 37 degrees C and 4 degrees C, is not responsible for adhesion strengthening, which does not occur at 4 degrees C. Of various cytoskeletal and metabolic perturbants tested, only a combination of hyperosmotic medium, colchicine, and cytochalasin caused a marked (72%) reduction of adhesion strengthening (without reducing weak cell adhesion). Clathrin and actin were readily detected in the adhesive patch by immunofluorescence microscopy. Rat hepatocytes also adhered avidly to surfaces derivatized with asialofetuin, a high-affinity ligand for the rat hepatic lectins. However, hepatic lectin molecules did not migrate into a patch on the asialofetuin-derivatized surface, suggesting that hepatic lectin-asialofetuin binding may have resulted in the rapid formation of a ring of essentially irreversibly adherent receptors that prevented diffusion of additional lectin molecules into the contact site. The cells were unable to increase their adhesive contact area by flattening onto the derivatized surface. Treatment of cells with cytochalasin, however, did result in an increase in the size of the contact area. Cells adhering to surfaces derivatized with an adhesion-promoting peptide (containing an arg-gly-asp sequence) had larger contact areas than those adhering to galactoside-derivatized surfaces. A model is proposed in which carbohydrate-mediated adhesion causes specific reorganization of cytoskeletal components, leading to strengthened adhesion and a characteristic spherical cell morphology.

Hepatocyte adhesion to carbohydrate-derivatized surfaces. I. Surface topography of the rat hepatic lectin.

The rat hepatic lectins, galactose- and N-acetylgalactosamine-binding proteins found on the hepatocyte cell surface, mediate adhesion of isolated primary rat hepatocytes to artificial galactose-derivatized polyacrylamide gels. Biochemical and immunohistochemical techniques were used to examine the topographical redistribution of the rat hepatic lectins in response to galactose-mediated cell adhesion. Hepatocytes isolated from rat liver by collagenase perfusion had an average of 7 x 10(5) cell surface lectin molecules per cell, representing 30-50% of the total lectin molecules per cell, the remainder residing in intracellular pools. Hepatocytes incubated on galactose-derivatized surfaces, whether at 0-4 degrees C or 37 degrees C, rapidly lost greater than 80% of their accessible cell surface lectin binding sites into an adhesive patch of characteristic morphology. The kinetics of rat hepatic lectin disappearance were used to estimate a lateral diffusion coefficient greater than 9 x 10(-9) cm2/s at 37 degrees C, suggesting rapid and unimpeded lectin diffusion in the plane of the membrane. Indirect immunofluorescence labeling of adherent cells using antihepatic lectin antibody revealed a structured ring of receptors surrounding an area of exclusion (patch) of reproducible size and shape which represented approximately 8% of the hepatocyte cell surface. Notably, adherent cells, which had lost greater than 80% of their accessible surface binding sites, still endocytosed soluble galactose-terminated radioligand at greater than 50% of the rate of nonadherent control cells. No net movement of rat hepatic lectin from intracellular pools to the cell surface was found on cells recovered after adhesion to galactose-derivatized surfaces at 37 degrees C, suggesting that the physical size and/or lectin density of the patch was restricted by kinetic or topological constraints.

Oligomerization of a membrane protein correlates with its retention in the Golgi complex.

The first membrane-spanning domain (m1) of the M glycoprotein of avian coronavirus (formerly called E1) is sufficient to retain this protein in the cis-Golgi. When the membrane-spanning domain of a protein which is efficiently delivered to the plasma membrane (VSV G protein) is replaced with m1, the resulting chimera (Gm1) is retained in the Golgi (Swift, A. M., and C. E. Machamer. 1991. J. Cell Biol. 115:19-30). When assayed in sucrose gradients, we observed that Gm1 formed a large oligomer, and that much of this oligomer was SDS resistant and stayed near the top of the stacking gel of an SDS-polyacrylamide gel. The unusual stability of the oligomer allowed it to be detected easily. Gm1 mutants with single amino acid substitutions in the m1 domain that were retained in the Golgi complex formed SDS-resistant oligomers, whereas mutants that were rapidly released to the plasma membrane did not. Oligomerization was not detected immediately after synthesis of Gm1, but occurred gradually with a lag of approximately 10 min, suggesting that it is not merely aggregation of misfolded proteins. Furthermore, oligomerization did not occur under several conditions that block ER to Golgi transport. The lumenal domain was not required for oligomerization since another chimera (alpha m1G), where the lumenal domain of Gm1 was replaced by the alpha subunit of human chorionic gonadotropin, also formed an SDS-resistant oligomer, and was able to form hetero-oligomers with Gm1 as revealed by coprecipitation experiments. SDS resistance was conferred by the cytoplasmic tail of VSV G, because proteolytic digestion of the tail in microsomes containing Gm1 oligomers resulted in loss of SDS resistance, although the protease-treated material continued to migrate as a large oligomer on sucrose gradients. Interestingly, treatment of cells with cytochalasin D blocked formation of SDS-resistant (but not SDS-sensitive) oligomers. Our data suggest that SDS-resistant oligomers form as newly synthesized molecules of Gm1 arrive at the Golgi complex and may interact (directly or indirectly) with an actin-based cytoskeletal matrix. The oligomerization of Gm1 and other resident proteins could serve as a mechanism for their retention in the Golgi complex.

Influenza M2 proton channel activity selectively inhibits trans-Golgi network release of apical membrane and secreted proteins in polarized Madin-Darby canine kidney cells.

The function of acidification in protein sorting along the biosynthetic pathway has been difficult to elucidate, in part because reagents used to alter organelle pH affect all acidified compartments and are poorly reversible. We have used a novel approach to examine the role of acidification in protein sorting in polarized Madin-Darby canine kidney (MDCK) cells. We expressed the influenza virus M2 protein, an acid-activated ion channel that equilibrates lumenal and cytosolic pH, in polarized MDCK cells and examined the consequences on the targeting and delivery of apical and basolateral proteins. M2 activity affects the pH of only a subset of acidified organelles, and its activity can be rapidly reversed using ion channel blockers (Henkel, J.R., G. Apodaca, Y. Altschuler, S. Hardy, and O.A. Weisz. 1998. Mol. Biol. Cell. 8:2477-2490; Henkel, J.R., J.L. Popovich, G.A. Gibson, S.C. Watkins, and O.A. Weisz. 1999. J. Biol. Chem. 274:9854-9860). M2 expression significantly decreased the kinetics of cell surface delivery of the apical membrane protein influenza hemagglutinin, but not of the basolaterally delivered polymeric immunoglobulin receptor. Similarly, the kinetics of apical secretion of a soluble form of gamma-glutamyltranspeptidase were reduced with no effect on the basolaterally secreted fraction. Interestingly, M2 activity had no effect on the rate of secretion of a nonglycosylated protein (human growth hormone [hGH]) that was secreted equally from both surfaces. However, M2 slowed apical secretion of a glycosylated mutant of hGH that was secreted predominantly apically. Our results suggest a role for acidic trans-Golgi network pH in signal-mediated loading of apical cargo into forming vesicles.

Efficient export of the vesicular stomatitis virus G protein from the endoplasmic reticulum requires a signal in the cytoplasmic tail that includes both tyrosine-based and di-acidic motifs.

The vesicular stomatitis virus (VSV) G protein is a model transmembrane glycoprotein that has been extensively used to study the exocytotic pathway. A signal in the cytoplasmic tail of VSV G (DxE or Asp-x-Glu, where x is any amino acid) was recently proposed to mediate efficient export of the protein from the endoplasmic reticulum (ER). In this study, we show that the DxE motif only partially accounts for efficient ER exit of VSV G. We have identified a six-amino-acid signal, which includes the previously identified Asp and Glu residues, that is required for efficient exit of VSV G from the ER. This six-residue signal also includes the targeting sequence YxxO (where x is any amino acid and O is a bulky, hydrophobic residue) implicated in several different sorting pathways. The only defect in VSV G proteins with mutations in the six-residue signal is slow exit from the ER; folding and oligomerization in the ER are normal, and the mutants eventually reach the plasma membrane. Addition of this six-residue motif to an inefficiently transported reporter protein is sufficient to confer an enhanced ER export rate. The signal we have identified is highly conserved among divergent VSV G proteins, and we suggest this reflects the importance of this motif in the evolution of VSV G as a proficient exocytic protein.

Selective perturbation of apical membrane traffic by expression of influenza M2, an acid-activated ion channel, in polarized madin-darby canine kidney cells.

The function of acidification along the endocytic pathway is not well understood, in part because the perturbants used to modify compartmental pH have global effects and in some cases alter cytoplasmic pH. We have used a new approach to study the effect of pH perturbation on postendocytic traffic in polarized Madin-Darby canine kidney (MDCK) cells. Influenza M2 is a small membrane protein that functions as an acid-activated ion channel and can elevate the pH of the trans-Golgi network and endosomes. We used recombinant adenoviruses to express the M2 protein of influenza virus in polarized MDCK cells stably transfected with the polymeric immunoglobulin (Ig) receptor. Using indirect immunofluorescence and immunoelectron microscopy, M2 was found to be concentrated at the apical plasma membrane and in subapical vesicles; intracellular M2 colocalized partly with internalized IgA in apical recycling endosomes as well as with the trans-Golgi network marker TGN-38. Expression of M2 slowed the rate of IgA transcytosis across polarized MDCK monolayers. The delay in transport occurred after IgA reached the apical recycling endosome, consistent with the localization of intracellular M2. Apical recycling of IgA was also slowed in the presence of M2, whereas basolateral recycling of transferrin and degradation of IgA were unaffected. By contrast, ammonium chloride affected both apical IgA and basolateral transferrin release. Together, our data suggest that M2 expression selectively perturbs acidification in compartments involved in apical delivery without disrupting other postendocytic transport steps.

Clathrin-mediated endocytosis of MUC1 is modulated by its glycosylation state.

MUC1 is a mucin-like type 1 transmembrane protein associated with the apical surface of epithelial cells. In human tumors of epithelial origin MUC1 is overexpressed in an underglycosylated form with truncated O-glycans and accumulates in intracellular compartments. To understand the basis for this altered subcellular localization, we compared the synthesis and trafficking of various glycosylated forms of MUC1 in normal (Chinese hamster ovary) cells and glycosylation-defective (ldlD) cells that lack the epimerase to make UDP-Gal/GalNAc from UDP-Glc/GlcNAc. Although the MUC1 synthesized in ldlD cells was rapidly degraded, addition of GalNAc alone to the culture media resulted in stabilization and near normal surface expression of MUC1 with truncated but sialylated O-glycans. Interestingly, the initial rate of endocytosis of this underglycosylated MUC1 was stimulated by twofold compared with fully glycosylated MUC1. However, the half-lives of the two forms were not different, indicating that trafficking to lysosomes was not affected. Both the normal and stimulated internalization of MUC1 could be blocked by hypertonic media, a hallmark of clathrin-mediated endocytosis. MUC1 endocytosis was also blocked by expression of a dominant-negative mutant of dynamin-1 (K44A), and MUC1 was observed in both clathrin-coated pits and vesicles by immunoelectron microscopy of ultrathin cryosections. Our data suggest that the subcellular redistribution of MUC1 in tumor cells could be a direct result of altered endocytic trafficking induced by its aberrant glycosylation; potential models are discussed. These results also implicate a new role for O-glycans on mucin-like membrane proteins entering the endocytic pathway through clathrin-coated pits.

Influenza virus M2 protein slows traffic along the secretory pathway. pH perturbation of acidified compartments affects early Golgi transport steps.

M2, an acid-activated ion channel, is an influenza A virus membrane protein required for efficient nucleocapsid release after viral fusion with the endosomal membrane. Recombinant M2 slows protein traffic through the Golgi complex (Sakaguchi, T., Leser, G. P)., and Lamb, R. A. (1996) J. Cell Biol. 133, 733-47). Despite its critical role in viral infection, little is known about the subcellular distribution of M2 or its fate following delivery to the plasma membrane (PM). We measured the kinetics of M2 transport in HeLa cells, and found that active M2 reached the PM considerably more slowly than inactive M2. In addition, M2 delayed intra-Golgi transport and cell surface delivery of influenza hemagglutinin (HA). We hypothesized that the effects of M2 on transport through non-acidified compartments are due to inefficient retrieval from the trans-Golgi of machinery required for intra-Golgi transport. In support of this, acutely activated M2 had no effect on intra-Golgi transport of HA, but still slowed HA delivery to the PM. Thus, M2 has an indirect effect on early transport steps, but a direct effect on late steps in PM delivery. These findings may help explain the conflicting and unexplained effects on protein traffic observed with other perturbants of intraorganelle pH such as weak bases and inhibitors of V-type ATPases.

Hepatocytes mediate coenzyme A transfer to specific carbohydrate-derivatized surfaces.

Freshly isolated chicken and rat hepatocytes adhere with carbohydrate specificity to surfaces derivatized with N-acetylglucosamine and galactose respectively. Previously (Brandley, B.K. and Schnaar, R.L. (1985) J. Biol. Chem. 260, 12474-12483) we reported that metabolically radiolabeled chicken hepatocytes covalently transferred phosphate radiolabel specifically to N-acetylglucosamine-derivatized surfaces. We now report that rat hepatocytes transfer phosphate radiolabel specifically to galactose-derivatized surfaces. Transferred radiolabel from both species to their appropriate carbohydrate-derivatized surface was identified as CoASH. After specific adhesion via the appropriate carbohydrate, CoASH is apparently released from cells and undergoes disulfide exchange with the cleavable immobilization linker we used to tether the sugars to the artificial surfaces. Although CoASH from lysed cells can undergo similar disulfide exchange, the data suggest that other, perhaps physiological mechanisms may be responsible for the carbohydrate-specific radiolabel transfer.

Evidence against the acidification hypothesis in cystic fibrosis.

The pleiotropic effects of cystic fibrosis (CF) result from the mislocalization or inactivity of an apical membrane chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR may also modulate intracellular chloride conductances and thus affect organelle pH. To test the role of CFTR in organelle pH regulation, we developed a model system to selectively perturb the pH of a subset of acidified compartments in polarized cells and determined the effects on various protein trafficking steps. We then tested whether these effects were observed in cells lacking wild-type CFTR and whether reintroduction of CFTR affected trafficking in these cells. Our model system involves adenovirus-mediated expression of the influenza virus M2 protein, an acid-activated ion channel. M2 expression selectively slows traffic through the trans-Golgi network (TGN) and apical endocytic compartments in polarized Madin-Darby canine kidney (MDCK) cells. Expression of M2 or treatment with other pH perturbants also slowed protein traffic in the CF cell line CFPAC, suggesting that the TGN in this cell line is normally acidified. Expression of functional CFTR had no effect on traffic and failed to rescue the effect of M2. Our results argue against a role for CFTR in the regulation of organelle pH and protein trafficking in epithelial cells.

Cell attachment and long-term growth on derivatizable polyacrylamide surfaces.

Important cellular characteristics, including selective adhesion, growth rate, motility, and differentiation, are controlled, in part, by signals received at the cell surface. The molecular mechanisms for the cell surface control of these cell behaviors are largely unknown. In order to probe the role of specific extracellular molecules in controlling cell function, we report the development of synthetic surfaces which generally support the long-term growth of cells yet can be readily derivatized with a wide variety of molecules of biological interest. Polyacrylamide gels containing a gradient of active ester groups were prepared and then the esters were displaced with ligands to generate a gradient of carboxylic acid, tertiary amine, or hydroxyl groups. When untransformed mouse fibroblasts (BALB/3T3) were seeded on the various surfaces, they attached and grew only on those derivatized with carboxylic acids or hydroxyl groups within narrow concentration ranges. Cell growth rate, density, and morphology on polyacrylamide gels containing the optimal concentration of carboxylic acid groups (approximately 30 mumol/ml) were comparable to those on tissue culture plastic, whereas growth on hydroxyl group-derivatized gels was less extensive. In contrast, short-term (90-min) adhesion to hydroxyl group-derivatized gels was greater than that to carboxylic acid-derivatized gels. Both short-term adhesion and long-term growth required serum. Growth-supportive polyacrylamide gels were readily derivatized with molecules of biological interest. The techniques reported here are applicable to other types of cell in culture since the nature and concentration of substratum functional groups can be easily varied and tested for support of long-term cell growth.

Rat liver dipeptidylpeptidase IV contains competing apical and basolateral targeting information.

Madin-Darby canine kidney (MDCK) cells deliver endogenous apical and basolateral proteins directly to the appropriate domains. We are investigating the molecular signals on a model plasma membrane hydrolase, dipeptidylpeptidase IV (DPPIV). Most newly synthesized rat liver DPPIV is delivered directly to the apical surface of transfected MDCK cells; however, about 20% is delivered first to the basolateral surface and reaches the apical surface via transcytosis (Casanova, J. E., Mishumi, Y., Ikehara, Y., Hubbard, A. L., and Mostov, K. E. (1991) J. Biol. Chem. 266, 24428-24432). A soluble form of DPPIV (solDPPIV) containing only the lumenal domain of the protein was efficiently transported and secreted by stably transfected MDCK cells. If this domain contains apical sorting information, we would expect 80% of the soluble protein to be secreted apically. Surprisingly, 95% of the secreted solDPPIV was found in the apical medium. The high efficiency of apical secretion suggested that the transmembrane domain and cytoplasmic tail of DPPIV might contain competing basolateral targeting information. To test this hypothesis, we investigated the trafficking of a chimera in which the cytoplasmic tail and transmembrane domains of DPPIV were joined to lysozyme, an exogenous protein which should not contain sorting information. This protein was delivered predominantly to the basolateral surface. Our results suggest that the lumenal domain of DPPIV carries dominant apical sorting information while the transmembrane domain and cytoplasmic tail of the molecule contains competing basolateral sorting information.

Overexpression of frequenin, a modulator of phosphatidylinositol 4-kinase, inhibits biosynthetic delivery of an apical protein in polarized madin-darby canine kidney cells.

Polyphosphoinositides regulate numerous steps in membrane transport. The levels of individual phosphatidylinositols are controlled by specific lipid kinases, whose activities and localization are in turn regulated by a variety of effectors. Here we have examined the effect of overexpression of frequenin, a modulator of phosphatidylinositol 4-kinase activity, on biosynthetic and postendocytic traffic in polarized Madin-Darby canine kidney cells. Endogenous frequenin was identified in these cells by polymerase chain reaction, Western blotting, and indirect immunofluorescence. Adenoviral-mediated overexpression of frequenin had no effect on early Golgi transport of membrane proteins, as assessed by acquisition of resistance to endoglycosidase H. However, delivery of newly synthesized influenza hemagglutinin from the trans-Golgi network to the apical cell surface was severely inhibited in cells overexpressing frequenin, whereas basolateral delivery of the polymeric immunoglobulin receptor was unaffected. Overexpression of frequenin did not affect postendocytic trafficking steps including apical and basolateral recycling and basal-to-apical transcytosis. We conclude that frequenin, and by inference, phosphatidylinositol 4-kinase, plays an important and selective role in apical delivery in polarized cells.

Non-coordinate regulation of endogenous epithelial sodium channel (ENaC) subunit expression at the apical membrane of A6 cells in response to various transporting conditions.

In many epithelial tissues in the body (e.g. kidney distal nephron, colon, airways) the rate of Na(+) reabsorption is governed by the activity of the epithelial Na(+) channel (ENaC). ENaC activity in turn is regulated by a number of factors including hormones, physiological conditions, and other ion channels. To begin to understand the mechanisms by which ENaC is regulated, we have examined the trafficking and turnover of ENaC subunits in A6 cells, a polarized, hormonally responsive Xenopus kidney cell line. As previously observed by others, the half-life of newly synthesized ENaC subunits was universally short ( approximately 2 h). However, the half-lives of alpha- and gamma-ENaC subunits that reached the apical cell surface were considerably longer (t(12) > 24 h), whereas intriguingly, the half-life of cell surface beta-ENaC was only approximately 6 h. We then examined the effects of various modulators of sodium transport on cell surface levels of individual ENaC subunits. Up-regulation of ENaC-mediated sodium conductance by overnight treatment with aldosterone or by short term incubation with vasopressin dramatically increased cell surface levels of beta-ENaC without affecting alpha- or gamma-ENaC levels. Conversely, treatment with brefeldin A selectively decreased the amount of beta-ENaC at the apical membrane. Short term treatment with aldosterone or insulin had no effect on cell surface amounts of any subunits. Subcellular fractionation revealed a selective loss of beta-ENaC from early endosomal pools in response to vasopressin. Our data suggest the possibility that trafficking and turnover of individual ENaC subunits at the apical membrane of A6 cells is non-coordinately regulated. The selective trafficking of beta-ENaC may provide a mechanism for regulating sodium conductance in response to physiological stimuli.

Glycosylation differences between a cystic fibrosis and rescued airway cell line are not CFTR dependent.

Altered glycosylation of mucus and membrane glycoconjugates could explain reported differences in binding of bacterial pathogens to cystic fibrosis (CF) versus normal tissue. However, because bacteria can alter cell surface glycoconjugates, it is not possible to assess the role of cystic fibrosis transmembrane conductance regulators (CFTR) in glycosylation in these studies. To address this issue, we have developed quantitative lectin binding assays to compare cell surface glycosylation in well-matched immortalized CF cells and rescued cell lines. The CF airway bronchial epithelial cell line IB3-1 consistently bound more peanut agglutinin (PNA) than its clonal derivative S9, which stably expresses functional wild-type CFTR. Pretreatment with neuraminidase increased PNA binding and abolished the difference between the two cell lines. However, infection of the IB3-1 cells with a replication-deficient recombinant adenovirus encoding CFTR restored CFTR function but did not alter PNA binding to cells. In contrast, treatment with the weak base ammonium chloride increased PNA binding to both cell lines as expected. Our data show that even clonally related CF and rescued cells can exhibit significant differences in carbohydrate processing. Although the differences that we found are consistent with the proposed role for CFTR in modulating intraorganellar pH, our data strongly suggest that they are CFTR independent. These studies add a cautionary note to the interpretation of differences in glycosylation between CF and normal primary tissues and immortalized cells.

Defined geometry of binding between triantennary glycopeptide and the asialoglycoprotein receptor of rat heptocytes.

Three derivatives of a triantennary glycopeptide, each containing a single uniquely located 6-amino-galactose residue at either position 6', 6, or 8, were modified at the 6-amino group by attachment of a photolyzable reagent and radiolabeled by iodination of tyrosine. These were allowed to bind to the asialoglycoprotein receptor of isolated rat hepatocytes and photolyzed for affinity labeling. (formula; see text) Each probe specifically labeled either the major (RHL1) or minor (RHL2/3) subunits which comprise the receptor. A photolyzable group attached to galactose residue 6 6' specifically radiolabeled RHL1, whereas a photolyzable group attached to galactose 8 specifically labeled RHL2/3. Photoaffinity labeling of a soluble rat hepatic lectin preparation demonstrated that the minor subunits (RHL2/3) were no longer labeled by the triantennary probe with a photolyzable group at galactose 8. The inhibitory potency of a variety of complex glycopeptides against radiolabeled ligand binding to both rat hepatocytes and soluble lectin are in agreement with photoaffinity results that galactose 8 of triantennary glycopeptide is of unique importance by binding solely to the minor subunits (RHL2/3) of the asialoglycoprotein receptor on hepatocytes. Conversely, galactose residues 6 and 6' bind specifically to the major subunit (RHL1), indicating a precise binding geometry between the trivalent ligand and lectin.

Competing sorting signals guide endolyn along a novel route to lysosomes in MDCK cells.

We have examined the trafficking of the mucin-like protein endolyn in transfected, polarized MDCK cells using biochemical approaches and immunofluorescence microscopy. Although endolyn contains a lysosomal targeting motif of the type YXXPhi and was localized primarily to lysosomes at steady state, significant amounts of newly synthesized endolyn were delivered to the apical cell surface. Antibodies to endolyn, but not lamp-2, were preferentially internalized from the apical plasma membrane and efficiently transported to lysosomes. Analysis of endolyn-CD8 chimeras showed that the lumenal domain of endolyn contains apical targeting information that predominates over basolateral information in its cytoplasmic tail. Interestingly, surface polarity of endolyn was independent of O-glycosylation processing, but was reversed by disruption of N-glycosylation using tunicamycin. At all times, endolyn was soluble in cold Triton X-100, suggesting that apical sorting was independent of sphingolipid rafts. Our data indicate that a strong, N-glycan-dependent apical targeting signal in the lumenal domain directs endolyn into a novel biosynthetic pathway to lysosomes, which occurs via the apical surface of polarized epithelial cells.

Selective perturbation of early endosome and/or trans-Golgi network pH but not lysosome pH by dose-dependent expression of influenza M2 protein.

Many sorting stations along the biosynthetic and endocytic pathways are acidified, suggesting a role for pH regulation in protein traffic. However, the function of acidification in individual compartments has been difficult to examine because global pH perturbants affect all acidified organelles in the cell and also have numerous side effects. To circumvent this problem, we have developed a method to selectively perturb the pH of a subset of acidified compartments. We infected HeLa cells with a recombinant adenovirus encoding influenza virus M2 protein (an acid-activated ion channel that dissipates proton gradients across membranes) and measured the effects on various steps in protein transport. At low multiplicity of infection (m.o.i.), delivery of influenza hemagglutinin from the trans-Golgi network to the cell surface was blocked, but there was almost no effect on the rate of recycling of internalized transferrin. At higher m.o.i., transferrin recycling was inhibited, suggesting increased accumulation of M2 in endosomes. Interestingly, even at the higher m.o.i., M2 expression had no effect on lysosome morphology or on EGF degradation, suggesting that lysosomal pH was not compromised by M2 expression. However, delivery of newly synthesized cathepsin D to lysosomes was slowed in cells expressing active M2, suggesting that acidification of the TGN and endosomes is important for efficient delivery of lysosomal hydrolases. Fluorescence labeling using a pH-sensitive dye confirmed the reversible effect of M2 on the pH of a subset of acidified compartments in the cell. The ability to dissect the role of acidification in individual steps of a complex pathway should be useful for numerous other studies on protein processing and transport.

Forskolin-induced apical membrane insertion of virally expressed, epitope-tagged CFTR in polarized MDCK cells.

Channel gating of the cystic fibrosis transmembrane conductance regulator (CFTR) is activated in response to cAMP stimulation. In addition, CFTR activation may also involve rapid insertion of a subapical pool of CFTR into the plasma membrane (PM). However, this issue has been controversial, in part because of the difficulty in distinguishing cell surface vs. intracellular CFTR. Recently, a fully functional, epitope-tagged form of CFTR (M2-901/CFTR) that can be detected immunologically in nonpermeabilized cells was characterized (Howard M, Duvall MD, Devor DC, Dong J-Y, Henze K, and Frizzell RA. Am J Physiol Cell Physiol 269: C1565-C1576, 1995; and Schultz BD, Takahashi A, Liu C, Frizzell RA, and Howard M. Am J Physiol Cell Physiol 273: C2080-C2089, 1997). We have developed replication-defective recombinant adenoviruses that express M2-901/CFTR and used them to probe cell surface CFTR in forskolin (FSK)-stimulated polarized Madin-Darby canine kidney (MDCK) cells. Virally expressed M2-901/CFTR was functional and was readily detected on the apical surface of FSK-stimulated polarized MDCK cells. Interestingly, at low multiplicity of infection, we observed FSK-stimulated insertion of M2901/CFTR into the apical PM, whereas at higher M2-901/CFTR expression levels, no increase in surface expression was detected using indirect immunofluorescence. Immunoelectron microscopy of unstimulated and FSK-stimulated cells confirmed the M2-901/CFTR redistribution to the PM upon FSK stimulation and demonstrates that the apically inserted M2-901/CFTR originates from a population of subapical vesicles. Our observations may reconcile previous conflicting reports regarding the effect of cAMP stimulation on CFTR trafficking.

Carboxylmethylation of the beta subunit of xENaC regulates channel activity.

The action of aldosterone to increase apical membrane permeability in responsive epithelia is thought to be due to activation of sodium channels. Aldosterone stimulates methylation of a 95-kDa protein in apical membrane of A6 cells, and we have previously shown that methylation of a 95-kDa protein in the immunopurified Na+ channel complex increases open probability of these channels in planar lipid bilayers. We report here that aldosterone stimulates carboxylmethylation of the beta subunit of xENaC in A6 cells. In vitro translated beta subunit, but not alpha or gamma, serves as a substrate for carboxylmethylation. Carboxylmethylation of ENaC reconstituted in planar lipid bilayers leads to an increase in open probability only when beta subunit is present. When the channel complex is immunoprecipitated from A6 cells and analyzed by Western blot with antibodies to the three subunits of xENaC, all three subunits are recognized as constituents of the complex. The results suggest that Na+ channel activity in A6 cells is regulated, in part, by carboxylmethylation of the beta subunit of xENaC.