Brefeldin A causes a microtubule-mediated fusion of the trans-Golgi network and early endosomes

Evidence that phospholipase A2 activity is required for Golgi complex and trans Golgi network membrane tubulation

Membrane tubules of uniform diameter (60-80 nm) and various lengths (up to several micrometers) emanate from elements of the Golgi stack and trans Golgi network (TGN). These organelle membrane tubules are thought to be involved in membrane trafficking and maintenance of Golgi/TGN architecture. The number of these tubules, and their frequency of formation, can be greatly enhanced by the fungal metabolite brefeldin A (BFA), an inhibitor of Golgi/TGN-associated coated vesicle formation. We show here that BFA stimulation of Golgi and TGN membrane tubulation, and the resultant retrograde transport of resident Golgi enzymes to the endoplasmic reticulum, was potently inhibited by a number of membrane-permeant antagonists of phospholipase A2 (PLA2; EC 3.1.1.4) activity. In addition, PLA2 inhibitors on their own caused a reversible fragmentation of the Golgi complex into juxtanuclear, stacked cisternal elements. We conclude from these observations that tubulation of Golgi complex and TGN membranes requires a PLA2 activity, and that this activity may participate not only in Golgi tubule-mediated retrograde trafficking to the endoplasmic reticulum, but also in the maintenance of Golgi complex architecture.

Brefeldin A (BFA) inhibits basolateral membrane (BLM) delivery and dimerization of transcobalamin II receptor in human intestinal epithelial Caco-2 cells. BFA effects on BLM cholesterol content

Brefeldin A (BFA) treatment of Caco-2 cells (5 microg/ml for 12 h) reduced by 90% the cholesterol, but not the phospholipid (PL), levels of the basolateral membrane (BLM), thus altering its PL/cholesterol molar ratio from 2.6 to 22.0, and decreasing its steady state fluorescent anisotropy (rs) from 0.27 to 0.15. BFA treatment for 12 h also resulted in complete loss of transcobalamin II receptor (TC II-R) activity/protein levels in the BLM and the disappearance of trans-Golgi network (TGN) morphology as revealed by confocal immunofluorescence microscopy using antibody to TGN 38. However, BFA treatment had no effect on either total cellular cholesterol, TC II-R activity, or PL levels. When cells treated with BFA for 12 h were exposed to BFA-free medium for 0-24 h, all of the effects were reversed, including reappearance of normal TGN morphology. TC II-R delivered to the BLM during this period was progressively sialylated and changed its physical state from a monomer (8 h) to a dimer (12 h), coinciding with increased delivery (11-53 pmol) of cholesterol to the BLM and an increase in the BLM rs from 0.15 to 0.21. These results indicate that cholesterol, but not PL, delivery to the BLM of Caco-2 cells is BFA-sensitive, and cholesterol, by influencing the higher order of the BLM, is essential for TC II-R dimerization.

Endobrevin, a novel synaptobrevin/VAMP-like protein preferentially associated with the early endosome

Synaptobrevins/vesicle-associated membrane proteins (VAMPs) together with syntaxins and a synaptosome-associated protein of 25 kDa (SNAP-25) are the main components of a protein complex involved in the docking and/or fusion of synaptic vesicles with the presynaptic membrane. We report here the molecular, biochemical, and cell biological characterization of a novel member of the synaptobrevin/VAMP family. The amino acid sequence of endobrevin has 32, 33, and 31% identity to those of synaptobrevin/VAMP-1, synaptobrevin/VAMP-2, and cellubrevin, respectively. Membrane fractionation studies demonstrate that endobrevin is enriched in membrane fractions that are also enriched in the asialoglycoprotein receptor. Indirect immunofluorescence microscopy establishes that endobrevin is primarily associated with the perinuclear vesicular structures of the early endocytic compartment. The preferential association of endobrevin with the early endosome was further established by electron microscopy (EM) immunogold labeling. In vitro binding assays show that endobrevin interacts with immobilized recombinant alpha-SNAP fused to glutathione S-transferase (GST). Our results highlight the general importance of members of the synaptobrevin/VAMP protein family in membrane traffic and provide new avenues for future functional and mechanistic studies of this protein as well as the endocytotic pathway.

In polarized MDCK cells basolateral vesicles arise from clathrin-gamma-adaptin-coated domains on endosomal tubules

Human transferrin receptors (TR) and receptors for polymeric immunoglobulins (pIgR) expressed in polarized MDCK cells maintain steady-state, asymmetric distributions on the separate basolateral and apical surfaces even though they are trafficking continuously into and across these cells. The intracellular mechanisms required to maintain these asymmetric distributions have not been located. Here we show that TR and pIgR internalize from both surfaces to a common interconnected endosome compartment that includes tubules with buds coated with clathrin lattices. These buds generate vesicles that carry TR to the basolateral border. The lattices contain gamma-adaptin and are dispersed by treatment with brefeldin A (BFA). Since BFA treatment abrogates the vectorial trafficking of TR in polarized MDCK cells, we propose that the clathrin-coated domains of the endosome tubules contain the polarized sorting mechanism responsible for their preferential basolateral distribution.

Lumenal and transmembrane domains play a role in sorting type I membrane proteins on endocytic pathways

Previous studies have shown that when the cytosolic domains of the type I membrane proteins TGN38 and lysosomal glycoprotein 120 (lgp120) are added to a variety of reporter molecules, the resultant chimeric molecules are localized to the trans-Golgi network (TGN) and to lysosomes, respectively. In the present study we expressed chimeric constructs of rat TGN38 and rat lgp120 in HeLa cells. We found that targeting information in the cytosolic domain of TGN38 could be overridden by the presence of the lumenal and transmembrane domains of lgp120. In contrast, the presence of the transmembrane and cytosolic domains of TGN38 was sufficient to deliver the lumenal domain of lgp120 to the trans-Golgi network. On the basis of steady-state localization of the various chimeras and antibody uptake experiments, we propose that there is a hierarchy of targeting information in each molecule contributing to sorting within the endocytic pathway. The lumenal and cytosolic domains of lgp120 contribute to sorting and delivery to lysosomes, whereas the transmembrane and cytosolic domains of TGN38 contribute to sorting and delivery to the trans-Golgi network.

Intracellular trafficking of metallocarboxypeptidase D in AtT-20 cells: localization to the trans-Golgi network and recycling from the cell surface

Carboxypeptidase D (CPD) is a recently discovered membrane-bound metallocarboxypeptidase that has been proposed to be involved in the post-translational processing of peptides and proteins that transit the secretory pathway. In the present study, the intracellular distribution of CPD was examined in AtT-20 cells, a mouse anterior pituitary-derived corticotroph. Antisera to CPD stain the same intracellular structures as those labeled with furin and wheat germ agglutinin. This distribution is distinct from carboxypeptidase E, which is localized to the secretory vesicles in the cell processes. The perinuclear distribution of CPD is detected even when the AtT-20 cells are treated with brefeldin A for 1–30 minutes, suggesting that CPD is present in the trans-Golgi network (TGN). Although CPD is predominantly found in the TGN, an antiserum to the full length protein is internalized within 15–30 minutes of incubation at 37 degrees C. In contrast, an antiserum raised against the C-terminal region of CPD does not become internalized, suggesting that this domain is cytosolic. The antiserum to the full length CPD is internalized to a structure that co-stains with furin and wheat germ agglutinin, but is distinct from transferrin recycling endosomes. The internalization of CPD is not substantially affected by treatment of the AtT-20 cells with brefeldin A. These data are consistent with the cycling of CPD to the cell surface and back to the TGN. The TGN localization of CPD raises the possibility of a role for this enzyme in the processing of proteins that transit the secretory pathway.

Retrograde trafficking of both Golgi complex and TGN markers to the ER induced by nordihydroguaiaretic acid and cyclofenil diphenol

Previous studies have shown that the Golgi stack and the trans-Golgi network (TGN) may play a role in capturing escaped resident endoplasmic reticulum (ER) proteins, and directing their retrograde transport back to that organelle. Whether this retrograde movement represents a highly specific or more generalized membrane trafficking pathway is unclear. To better understand both the retrograde and anterograde trafficking pathways of the secretory apparatus, we examined more closely the in vivo effects of two structurally unrelated compounds, the potent lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA), and the non-steroidal estrogen cyclofenil diphenol (CFD), both of which are known to inhibit secretion. In the presence of these compounds, transport of vesicular stomatitis virus G membrane glycoprotein from the ER to the Golgi complex, and from the TGN to the cell surface, was inhibited potently and rapidly. Surprisingly, we found that NDGA and CFD stimulated the rapid, but not concomitant, retrograde movement of both Golgi stack and TGN membrane proteins back to the ER until both organelles were morphologically absent from cells. Both NDGA- and CFD-stimulated TGN and Golgi retrograde membrane trafficking were inhibited by microtubule depolymerizing agents and energy poisons. Removal of NDGA and CFD resulted in the complete, but not concomitant, reformation of both Golgi stacks and their closely associated TGN compartments. These studies suggest that NDGA and CFD unmask a generalized bulk recycling pathway to the ER for both Golgi and TGN membranes and, further, that NDGA and CFD are useful for investigating the molecular mechanisms that control the formation and maintenance of both the Golgi stack proper and the TGN.

Transport of glutathione conjugates into secretory vesicles is mediated by the multidrug-resistance protein 1

Intracellular glutathione-conjugate transport was evaluated in the human small cell lung carcinoma cell line GLC4 with low multidrug resistance protein (MRP1) expression and its 300x doxorubicin-resistant, MRP1-over-expressing, GLC4-Adr subline. Transport of non-toxic concentrations of monochlorobimane and 5-chloro-methyl fluorescein diacetate was evaluated using fluorescence microscopy. After exposure to these compounds, fluorescence was observed especially in intracellular vesicles in GLC4-Adr. Immunotransmission electron microscopy showed that MRP1 was present in the vesicle membranes and plasma membrane, while inside the vesicles the glutathione conjugate of 1-chloro-2,4-dinitrobenzene could be detected. Experiments with brefeldin A, which induces arrest in vesicle release from the Golgi complex, indicated that these vesicles may originate from the trans-Golgi network. In GLC4-Adr cells, doxorubicin also was transported in vesicles, with an arrest in vesicle release from the Golgi complex. Our results indicate that MRP1 functions as a glutathione-conjugate transporter not only at the plasma membrane but also in intracellular secretory vesicles.

Syntaxin 12, a member of the syntaxin family localized to the endosome

We have cloned a new member of the syntaxin family of proteins. The open reading frame encodes a polypeptide of 272 amino acids with potential coiled-coil domains and a C-terminal hydrophobic tail. Northern blot analysis showed that the transcript is fairly ubiquitous. A soluble recombinant form of the polypeptide without the hydrophobic region binds to alpha-SNAP (soluble N-ethylmaleimide-sensitive factor attachment protein) and syndet/SNAP-23 in vitro. Polyclonal antibody raised against the recombinant protein recognized a 39-kDa protein in the membrane fraction of cell lysates. Indirect immunofluorescence studies using the polyclonal antibody showed that the protein is localized to intracellular membrane structures. Selective permeabilization studies with digitonin and saponin indicate that the epitope(s) recognized by the antibody is expose to the cytoplasm, consistent with the predicted orientation characteristic of SNAP receptor molecules. Morphological alterations of the staining pattern of the protein with brefeldin A and wortmannin treatment indicate that the protein is localize to the endosome. The cDNA we have cloned apparently corresponded to three previously described expressed sequence tags named as syntaxins 12, 13, and 14, respectively. We therefore propose to retain the name syntaxin 12 for this protein.

Nordihydroguaiaretic acid blocks protein transport in the secretory pathway causing redistribution of Golgi proteins into the endoplasmic reticulum

We have investigated the effect of nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, on the intracellular protein transport and the structure of the Golgi complex. Pulse-chase experiments and immunoelectron microscopy showed that NDGA strongly inhibits the transport of newly synthesized secretory proteins to the Golgi complex resulting in their accumulation in the endoplasmic reticulum (ER). Despite their retention in the ER, oligosaccharides of secretory and ER-resident proteins were processed to endoglycosidase H-resistant forms, raising the possibility that oligosaccharide-processing enzymes are redistributed from the Golgi to the ER. Morphological observations further revealed that alpha-mannosidase II (a cis/medial-Golgi marker), but not TGN38 (a trans-Golgi network marker), rapidly redistributes to the ER in the presence of NDGA, resulting in the disappearance of the characteristic Golgi structure. Upon removal of the drug, the Golgi complex was reassembled into the normal structure as judged by perinuclear staining of alpha-mannosidase II and by restoration of the secretion. These effects of NDGA are quite similar to those of brefeldin A. However, unlike brefeldin A, NDGA did not cause a dissociation of beta-coatomer protein, a subunit of coatomer, from the Golgi membrane. On the contrary, NDGA exerted the stabilizing effect on beta-coatomer protein/membrane interaction against the dissociation caused by brefeldin A and ATP depletion. Taken together, these results indicate that NDGA is a potent agent disrupting the structure and function of the Golgi complex with a mechanism different from those known for other drugs reported so far.

Mechanism of acidification of the trans-Golgi network (TGN). In situ measurements of pH using retrieval of TGN38 and furin from the cell surface

Sorting of secretory cargo and retrieval of components of the biosynthetic pathway occur at the trans-Golgi network (TGN). The pH within the TGN is thought to be an important determinant of these functions. However, studies of the magnitude and regulation of the pH of the TGN have been hampered by the lack of appropriate detection methods. This report describes a noninvasive strategy to measure the luminal pH of the TGN in intact cells. We took advantage of endogenous cellular mechanisms for the specific retrieval of TGN resident proteins, such as TGN38 and furin, that transit briefly to the plasma membrane. Cells were transfected with chimeric constructs that contained the internalization and retrieval signals of TGN resident proteins, and a luminal (extracellular) epitope (CD25). Like TGN38 and furin, the chimeras were shown by fluorescence microscopy to accumulate within the TGN. During their transient exposure at the cell surface, the chimeras were labeled with extracellular anti-CD25 antibodies conjugated with a pH-sensitive fluorophore. Subsequent endocytosis and retrograde transport resulted in preferential labeling of the TGN with the pH-sensitive probe. Continuous, quantitative measurements of the pH of the TGN were obtained by ratio fluorescence imaging. The resting pH, calibrated using either ionophores or the "null point" technique, averaged 5.95 in Chinese hamster ovary cells and 5.91 in HeLa cells. The acidification was dissipated upon addition of concanamycin, a selective blocker of vacuolar-type ATPases. The counterion conductance was found to be much greater than the rate of H+ pumping at the steady state, suggesting that the acidification is not limited by an electrogenic potential. Both Cl- and K+ were found to contribute to the overall counterion permeability of the TGN. No evidence was found for the presence of active Na+/H+ or Ca2+/H+ exchangers on the TGN membrane. In conclusion, selective retrieval of recombinant proteins can be exploited to target ion-sensitive fluorescent probes to specific organelles. The technique provides real-time, noninvasive, and quantitative determinations of the pH, allowing the study of pH regulation within the TGN in intact cells. The acidic pH of the TGN reflects active H+ pumping into an organelle with a low intrinsic H+ permeability and a high conductance to monovalent ions.

Syntaxin 7, a novel syntaxin member associated with the early endosomal compartment

Members of the syntaxin family are key molecules involved in diverse vesicle docking/fusion events. We report here the molecular, biochemical, and cell biological characterizations of a novel member (syntaxin 7) of the syntaxin family. Syntaxin 7 is structurally related to all known syntaxins. Within a 79-residue region preceding the C-terminal hydrophobic tail, syntaxin 7 is 35, 34, 34, 34, 25, and 19% identical to syntaxins 1, 2, 3, 4, 5, and 6, respectively. Northern blot analysis showed that syntaxin 7 is widely expressed. Indirect immunofluorescence microscopy revealed that syntaxin 7 is primarily associated with the early endosome. In vitro binding assays established that syntaxin 7 in membrane extracts interacts with immobilized recombinant alpha-soluble N-ethylmaleimide-sensitive factor attachment proteins fused to glutathione S-transferase. Our results highlight the general importance of members of the syntaxin family in protein trafficking and provide new avenues for future functional and mechanistic studies of this first endosomal syntaxin as well as the endocytotic pathway.

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.

Inhibition of endosome function in CHO cells bearing a temperature-sensitive defect in the coatomer (COPI) component epsilon-COP

Recent evidence has suggested that subunits of the coatomer protein (COPI) complexes are functionally associated with endosomes in mammalian cells. We now provide genetic evidence that COPI plays a role in endocytosis in intact cells. The ldlF mutant CHO cell line bears a temperature-sensitive defect in the COPI subunit epsilon-COP. In addition to exhibiting conditional defects in the secretory pathway, we find that the cells are also defective at mediating endosome-associated functions. As found for cells microinjected with anti-COPI antibodies, ldlF cells at the restrictive temperature could not be infected by vesicular stomatitis (VSV) or Semliki Forest virus (SFV) that require delivery to acidic endosomes to penetrate into the cytosol. Although there was no temperature-sensitive defect in the internalization of receptor-bound transferrin (Tfn), Tfn recycling and accumulation of HRP were markedly inhibited at the restrictive temperature. Sorting of receptor-bound markers such as EGF to lysosomes was also reduced, although delivery of fluid-phase markers was only partially inhibited. In addition, lysosomes redistributed from their typical perinuclear location to the tips of the ldlF cells. Mutant phenotypes began to emerge within 2 h of temperature shift, the time required for the loss of detectable epsilon-COP, suggesting that the endocytic defects were not secondary to a block in the secretory pathway. Importantly, the mutant phenotypes were also corrected by transfection of wild-type epsilon-COP cDNA demonstrating that they directly or indirectly reflected the epsilon-COP defect. Taken together, the results suggest that epsilon-COP acts early in the endocytic pathway, most likely inhibiting the normal sorting and recycling functions of early endosomes.

Degradation of connexin43 gap junctions involves both the proteasome and the lysosome

Intercellular communication may be modulated by the rather rapid turnover and degradation of gap junction proteins, since many connexins have half-lives of 1-3 h. While several morphological studies have suggested that gap junction degradation occurs after endocytosis, our recent biochemical studies have demonstrated involvement of the ubiquitin-proteasome pathway in proteolysis of the connexin43 polypeptide. The present study was designed to reconcile these observations by examining the degradation of connexin43-containing gap junctions in rat heart-derived BWEM cells. After treatment of BWEM cells with Brefeldin A to prevent transport of newly synthesized connexin43 polypeptides to the plasma membrane, quantitative confocal microscopy showed the disappearance of immunoreactive connexin43 from the cell surface with a half-life of approximately 1 h. This loss of connexin43 immunoreactivity was inhibited by cotreatment with proteasomal inhibitors (ALLN, MG132, or lactacystin) or lysosomal inhibitors (leupeptin or E-64). Similar results were seen when connexin43 export was blocked with monensin. After treatment of BWEM cells with either proteasomal or lysosomal inhibitors alone, immunoblots showed accumulation of connexin43 in both whole cell lysates and in a 1% Triton X-100-insoluble fraction. Immunofluorescence studies showed that connexin43 accumulated at the cell surface in lactacystin-treated cells, but in vesicles in BWEM cells treated with lysosomal inhibitors. These results implicate both the proteasome and the lysosome in the degradation of connexin43-containing gap junctions.

The activity of the epithelial sodium channel is regulated by clathrin-mediated endocytosis

Activity of the epithelial sodium channel (ENaC) is a key determinant of sodium homeostasis and blood pressure. Liddle's syndrome, an inherited form of hypertension, is caused by mutations that delete or alter PY domains in the carboxyl termini of beta or gamma ENaC subunits, leading to increased channel activity. In this study we investigated the mechanism of this effect by analysis of wild-type and mutant ENaC activity in Xenopus oocytes. By inhibiting insertion of new channels into the plasma membrane with brefeldin A, we demonstrate that the half-life of the activity of channels containing Liddle's mutations is markedly prolonged compared with wild-type channels (t1/2 of 30 h in mutant versus 3.6 in wild-type, p < 0.001). We investigated the involvement of clathrin-coated pit-mediated endocytosis by co-expressing a dominant-negative dynamin mutant with wild-type ENaC in oocytes. Expression of this specific inhibitor of endocytosis leads to a large increase in the activity of wild-type channels, demonstrating that normal turnover of this channel is through the clathrin-coated pit pathway. In contrast, co-expression of Liddle's mutations and dynamin mutants leads to no further increase in channel activity, consistent with one of the effects of Liddle's mutations being the loss of endocytosis of these channels. These findings demonstrate the normal mechanism of turnover of ENaC from the cell surface and demonstrate a mechanism that can account for the increased number of channels in the plasma membrane seen in Liddle's syndrome.

ADP-ribosylation factor 6 regulates a novel plasma membrane recycling pathway

ADP-ribosylation factor (ARF) 6 localizes to the plasma membrane (PM) in its GTP state and to a tubulovesicular compartment in its GDP state in HeLa cells that express wild-type or mutant forms of this GTPase. Aluminum fluoride (AlF) treatment of ARF6-transfected cells redistributes ARF6 to the PM and stimulates the formation of actin-rich surface protrusions. Here we show that cytochalasin D (CD) treatment inhibited formation of the AlF-induced protrusions and shifted the distribution of ARF6 to a tubular membrane compartment emanating from the juxtanuclear region of cells, which resembled the compartment where the GTP-binding defective mutant of ARF6 localized. This membrane compartment was distinct from transferrin-positive endosomes, could be detected in the absence of ARF6 overexpression or CD treatment, and was accessible to loading by PM proteins lacking clathrin/AP-2 cytoplasmic targeting sequences, such as the IL-2 receptor alpha subunit Tac. ARF6 and surface Tac moved into this compartment and back out to the PM in the absence of pharmacologic treatment. Whereas AlF treatment blocked internalization, CD treatment blocked the recycling of wild-type ARF6 and Tac back to the PM; these blocks were mimicked by expression of ARF6 mutants Q67L and T27N, which were predicted to be in either the GTP- or GDP-bound state, respectively. Thus, the ARF6 GTP cycle regulates this membrane traffic pathway. The delivery of ARF6 and membrane to defined sites along the PM may provide components necessary for remodeling the cell surface and the underlying actin cytoskeleton.

Inhibition of lipid absorption as an approach to the treatment of obesity

A reduction in fat intake may be achieved by making educated choices to reduce total calorie intake, to consume a lower quantity of total fats, or to modify the ratio of saturated-to-polyunsaturated lipids. Leptin agonists or NPY or CCK antagonists may prove to be useful to diminish appetite and thereby reduce the total intake of food. But eating has such cultural, social, and hedonistic attributes that such a single-pronged approach is unlikely to be successful. The use of fat substitutes may prove to be popular to provide a wide range of snack food options, but these are likely to be of minimal use in weight reduction programs because of their distribution of additives in only a limited number of foods. The inhibitors of lipid digestion will be modestly successful in the short term; their long-term success will be influenced by gastrointestinal adverse effects and the need to consume fat-soluble vitamin supplements to prevent the development of fat-soluble vitamin deficiencies. The inhibition of lipid absorption is an attractive targeted approach for the treatment of obesity, since this would reduce the uptake of visible as well as invisible fats, which would potentially offer convenient dosing, and could also be a means to inhibit secondarily the uptake of carbohydrate calories.

ADP ribosylation factor 1 is required for synaptic vesicle budding in PC12 cells

Carrier vesicle generation from donor membranes typically progresses through a GTP-dependent recruitment of coats to membranes. Here we explore the role of ADP ribosylation factor (ARF) 1, one of the GTP-binding proteins that recruit coats, in the production of neuroendocrine synaptic vesicles (SVs) from PC12 cell membranes. Brefeldin A (BFA) strongly and reversibly inhibited SV formation in vivo in three different PC12 cell lines expressing vesicle-associated membrane protein-T Antigen derivatives. Other membrane traffic events remained unaffected by the drug, and the BFA effects were not mimicked by drugs known to interfere with formation of other classes of vesicles. The involvement of ARF proteins in the budding of SVs was addressed in a cell-free reconstitution system (Desnos, C., L. Clift-O'Grady, and R.B. Kelly. 1995. J. Cell Biol. 130:1041-1049). A peptide spanning the effector domain of human ARF1 (2-17) and recombinant ARF1 mutated in its GTPase activity, both inhibited the formation of SVs of the correct size. During in vitro incubation in the presence of the mutant ARFs, the labeled precursor membranes acquired different densities, suggesting that the two ARF mutations block at different biosynthetic steps. Cell-free SV formation in the presence of a high molecular weight, ARF-depleted fraction from brain cytosol was significantly enhanced by the addition of recombinant myristoylated native ARF1. Thus, the generation of SVs from PC12 cell membranes requires ARF and uses its GTPase activity, probably to regulate coating phenomena.

Localization of human heparan glucosaminyl N-deacetylase/N-sulphotransferase to the trans-Golgi network

In order to determine the intracellular location of heparan N-deacetylase/N-sulphotransferase, cDNAs encoding human heparan glucosaminyl N-deacetylase/N-sulphotransferase were cloned from human umbilical vein endothelial cells. The deduced amino acid sequence was identical to that of the human heparan N-sulphotransferase cloned previously [Dixon, Loftus, Gladwin, Scambler, Wasmuth and Dixon (1995) Genomics 26, 239-244]. RNA blot analysis indicated that two heparan N-sulphotransferase transcripts of approx. 8.5 and 4 kb were produced in all tissues. Expression was most abundant in heart, liver and pancreas. A cDNA encoding a Flag-tagged human heparan N-sulphotransferase (where Flag is an epitope with the sequence DYKDDDDK) was transfected into mouse LTA cells. Immunofluorescence detection using anti-Flag monoclonal antibodies demonstrated that the enzyme was localized to the trans-Golgi network. A truncated Flag-tagged heparan N-sulphotransferase was also retained in the Golgi, indicating that, as for many other Golgi enzymes, the N-terminal region of heparan N-sulphotransferase is sufficient for retention in the Golgi apparatus.

Characterization of chemical inhibitors of brefeldin A-activated mono-ADP-ribosylation

Brefeldin A, a toxin inhibitor of vesicular traffic, induces the selective mono-ADP-ribosylation of two cytosolic proteins, glyceraldehyde-3-phosphate dehydrogenase and the novel GTP-binding protein BARS-50. Here, we have used a new quantitative assay for the characterization of this reaction and the development of specific pharmacological inhibitors. Mono-ADP-ribosylation is activated by brefeldin A with an EC50 of 17.0 +/- 3.1 microg/ml, but not by biologically inactive analogs including a brefeldin A stereoisomer. Brefeldin A acts by increasing the Vmax of the reaction, whereas it does not influence the Km of the enzyme for NAD+ (154 +/- 13 microM). The enzyme is an integral membrane protein present in most tissues and is modulated by Zn2+, Cu2+, ATP (but not by other nucleotides), pH, temperature, and ionic strength. To identify inhibitors of the reaction, a large number of drugs previously tested as blockers of bacterial ADP-ribosyltransferases were screened. Two classes of molecules, one belonging to the coumarin group (dicumarol, coumermycin A1, and novobiocin) and the other to the quinone group (ilimaquinone, benzoquinone, and naphthoquinone), rather potently and specifically inhibited brefeldin A-dependent mono-ADP-ribosylation. When tested in living cells, these molecules antagonized the tubular reticular redistribution of the Golgi complex caused by brefeldin A at concentrations similar to those active in the mono-ADP-ribosylation assay in vitro, suggesting a role for mono-ADP-ribosylation in the cellular actions of brefeldin A.

Dissociation of coatomer from membranes is required for brefeldin A-induced transfer of Golgi enzymes to the endoplasmic reticulum

Addition of brefeldin A (BFA) to mammalian cells rapidly results in the removal of coatomer from membranes and subsequent delivery of Golgi enzymes to the endoplasmic reticulum (ER). Microinjected anti-EAGE (intact IgG or Fab-fragments), antibodies against the "EAGE"-peptide of beta-COP, inhibit BFA-induced redistribution of beta-COP in vivo and block transfer of resident proteins of the Golgi complex to the ER; tubulo-vesicular clusters accumulate and Golgi membrane proteins concentrate in cytoplasmic patches containing beta-COP. These patches are devoid of marker proteins of the ER, the intermediate compartment (IC), and do not contain KDEL receptor. Interestingly, relocation of KDEL receptor to the IC, where it colocalizes with ERGIC53 and ts-O45-G, is not inhibited under these conditions. While no stacked Golgi cisternae remain in these injected cells, reassembly of stacks of Golgi cisternae following BFA wash-out is inhibited to only approximately 50%. Mono- or divalent anti-EAGE stabilize binding of coatomer to membranes in vitro, at least as efficiently as GTP(gamma)S. Taken together these results suggest that enhanced binding of coatomer to membranes completely inhibits the BFA-induced retrograde transport of Golgi resident proteins to the ER, probably by inhibiting fusion of Golgi with ER membranes, but does not interfere with the disassembly of the stacked Golgi cisternae and recycling of KDEL receptor to the IC. These results confirm our previous results suggesting that COPI is involved in anterograde membrane transport from the ER/IC to the Golgi complex (Pepperkok et al., 1993), and corroborate that COPI regulates retrograde membrane transport between the Golgi complex and ER in mammalian cells.

Targeting of an intestinal apical endosomal protein to endosomes in nonpolarized cells

Polarized cells such as epithelial cells and neurons have distinct endosomal compartments associated with different plasma membrane domains. The endosomes of the neuronal cell body and the basolateral cytoplasm of epithelial cells are thought to perform cellular "housekeeping" functions such as the uptake of nutrients and metabolites, while the endosomes in the apical cytoplasm or axons are thought to be specialized for the sorting and transcytosis of cell type-specific ligands and receptors. However, it is not known if nonpolarized cells such as fibroblasts contain a specialized endosomal compartment analogous to the specialized endosomes found in neurons and epithelia. We have expressed a protein that is normally found in the apical early endosomes of developing intestinal epithelial cells in normal rat kidney fibroblasts. This apical endosomal marker, called endotubin, is targeted to early endosomes in transfected fibroblasts, and is present in peripheral as well as perinuclear endosomes. The peripheral endosomes that contain endotubin appear to exclude transferrin, fluid phase markers, and the mannose-6-phosphate receptor, although in the perinuclear region colocalization of endotubin and these markers is present. In addition, endotubin positive structures do not tubulate in response to brefeldin A and instead redistribute to a diffuse perinuclear location. Since this endosomal compartment has many of the characteristics of an apical or axonal endosomal compartment, our results indicate that nonpolarized cells also contain a specialized early endosomal compartment.

Transcytosis of albumin in endothelial cells is brefeldin A--independent

To determine whether in endothelial cells (EC) the pathways of endocytosis and transcytosis of macromolecules interconnect, the effect of Brefeldin A (BFA) on these processes was tested. To this purpose EC were grown to confluence on plastic culture dishes or on cell culture chamber inserts placed into corresponding wells, so as to obtain a dual chamber system. The cells maintained the typical characteristics of EC and had an electrical resistance in the range of 30-60 Ohm.cm2. Transendothelial transport of albumin conjugated to the fluorochrom Texas Red (Alb-TR) and of horseradish peroxidase (HRP) added to the upper compartment, in the absence or presence of BFA (0-25 micrograms/ml), was evaluated in aliquots collected from the lower compartment. At different time intervals, quantitative data were obtained by fluorimetry and spectrophotometry. In other experiments transcytosis of Alb-TR was examined in the presence of 100 microM forskolin (an inhibitor of BFA effect). The endocytosis of Alb-TR and HRP was evaluated by incubating EC with the probes, and the internalized tracers determined in the cell lysate using the methods described above. The results showed that BFA has no significant effect on transcytosis of albumin and HRP. In contradistinction, BFA (5 micrograms/ml) reduced markedly endocytosis of HRP (by 47%). Forskolin has no effect on transcytosis. The data indicate that the BFA-induced perturbance in the endocytic route does not affect the transcytotic pathway of albumin, and suggest that in EC, transcytosis of macromolecules may represent a shortcut for rapid and direct transport of some plasma molecules across the cell.

The MAL proteolipid is a component of the detergent-insoluble membrane subdomains of human T-lymphocytes

The human mal gene, identified during a search for cDNAs selectively expressed during T-cell development, encodes a highly hydrophobic protein belonging to a group of proteins, termed proteolipids, characterized by their unusual property of being soluble in organic solvents used to extract cell lipids. To study the localization of the MAL protein we have prepared stable transfectants expressing the MAL protein tagged with a c-myc epitope (MAL/c-myc) using human epithelial A-498 cells. Immunofluorescence analysis suggested that MAL/c-myc is localized mainly to cholesterol-enriched structures with a post-Golgi location and, at low levels, in early endosomes. Moreover, extraction of A-498 cell membranes with Triton X-100 (TX100) and fractionation by centrifugation to equilibrium in sucrose gradients demonstrated the presence of MAL/c-myc in the detergent-insoluble buoyant fraction, known to be enriched in glycolipids and cholesterol. To compare the behaviour of MAL in T-cells with that in epithelial A-498 cells, we prepared stably transfected cells expressing MAL/c-myc using human Jurkat T-cells. When TX100 extracts from Jurkat cells were subjected to centrifugation to equilibrium in sucrose gradients we found MAL exclusively in the floating fractions, together with molecules characteristic of the T-cell insoluble complexes, such as the tyrosine kinase p56lck, the glycosylphosphatidylinositol-anchored protein CD59 and the ganglioside GM1. These results, taken together, indicate that the MAL proteolipid is a component of the detergent-resistant membrane microdomains present in T-lymphocytes, and suggest that MAL might play a role in modulating the function of these microdomains during T-cell differentiation.

Ceramide inhibits antigen uptake and presentation by dendritic cells

Ceramides are intramembrane diffusible mediators involved in transducing signals originated from a variety of cell surface receptors. Different adaptive and differentiative cellular responses, including apoptotic cell death, use ceramide-mediated pathways as an essential part of the program. Here, we show that human dendritic cells respond to CD40 ligand, as well as to tumor necrosis factor-alpha and IL-1 beta, with intracellular ceramide accumulation, as they are induced to differentiate. Dendritic cells down-modulate their capacity to take up soluble antigens in response to exogenously added or endogenously produced ceramides. This is followed by an impairment in presenting soluble antigens to specific T cell clones, while cell viability and the capacity to stimulate allogeneic responses or to present immunogenic peptides is fully preserved. Thus, ceramide-mediated pathways initiated by different cytokines can actively modulate professional antigen-presenting cell function and antigen-specific immune responses.

Endocytosis and molecular sorting

Endocytosis in eukaryotic cells is characterized by the continuous and regulated formation of prolific numbers of membrane vesicles at the plasma membrane. These vesicles come in several different varieties, ranging from the actin-dependent formation of phagosomes involved in particle uptake, to smaller clathrin-coated vesicles responsible for the internalization of extracellular fluid and receptor-bound ligands. In general, each of these vesicle types results in the delivery of their contents to lysosomes for degradation. The membrane components of endocytic vesicles, on the other hand, are subject to a series of highly complex and iterative molecular sorting events resulting in their targeting to specific destinations. In recent years, much has been learned about the function of the endocytic pathway and the mechanisms responsible for the molecular sorting of proteins and lipids. This review attempts to integrate these new concepts with long-established views of endocytosis to present a more coherent picture of how the endocytic pathway is organized and how the intracellular transport of internalized membrane components is controlled. Of particular importance are emerging concepts concerning the protein-based signals responsible for molecular sorting and the cytosolic complexes responsible for the decoding of these signals.

Mammalian Cdc42 is a brefeldin A-sensitive component of the Golgi apparatus

In this study, we have used immunocytochemical and fractionation approaches to provide a description of the localization of the mammalian Cdc42 protein (designated Cdc42Hs) in vivo. A specific anti-peptide antibody was generated against the C-terminal region of Cdc42Hs. Using affinity-purified preparations of this antibody in indirect immunofluorescence experiments, Cdc42Hs was found to be localized to the Golgi apparatus. Similar to the well-characterized non-clathrin coat proteins ADP-ribosylation factor (ARF) and beta-COP, the perinuclear clustering of Cdc42Hs is rapidly dispersed upon exposure of the cells to the drug brefeldin A, suggesting that it too may play a role in the processes of intracellular lipid and protein transport. Employing cell lines possessing inducible forms of ARF, we demonstrate here a tight coupling of the nucleotide-bound state of ARF and the subcellular localization of Cdc42Hs. Specifically, the expression of wild-type ARF had no effect on the brefeldin A sensitivity of Cdc42Hs while, as is the case for ARF and beta-COP, expression of a GTPase-deficient form of ARF (ARF(Q71L)) renders these Golgi-localized proteins resistant to brefeldin A treatment (; ). Moreover, the induced expression of a mutant form of ARF with a low affinity for nucleotide resulted in constitutive redistribution of Cdc42Hs in the absence of brefeldin A treatment. These results suggest that Cdc42Hs may play a role in cell morphogenesis by acting on targets in the Golgi that direct polarized growth at the plasma membrane.

Rab4 and cellubrevin define different early endosome populations on the pathway of transferrin receptor recycling

During receptor mediated endocytosis, at least a fraction of recycling cargo typically accumulates in a pericentriolar cluster of tubules and vesicles. However, it is not clear if these endosomal structures are biochemically distinct from the early endosomes from which they are derived. To better characterize this pericentriolar endosome population, we determined the distribution of two endogenous proteins known to be functionally involved in receptor recycling [Rab4, cellubrevin (Cbvn)] relative to the distribution of a recycling ligand [transferrin (Tfn)] as it traversed the endocytic pathway. Shortly after internalization, Tfn entered a population of early endosomes that contained both Rab4 and Cbvn, demonstrated by triple label immunofluorescence confocal microscopy. Tfn then accumulated in the pericentriolar cluster of recycling vesicles (RVs). However, although these pericentriolar endosomes contained Cbvn, they were strikingly depleted of Rab4. The ability of internalized Tfn to reach the Rab4-negative population was not blocked by nocodazole, although the characteristic pericentriolar location of the population was not maintained in the absence of microtubules. Similarly, Rab4-positive and -negative populations remained distinct in cells treated with brefeldin A, with only Rab4-positive elements exhibiting the extended tubular morphology induced by the drug. Thus, at least with respect to Rab4 distribution, the pathway of Tfn receptor recycling consists of at least two biochemically and functionally distinct populations of endosomes, a Rab4-positive population of early endosomes to which incoming Tfn is initially delivered and a Rab4-negative population of recycling vesicles that transiently accumulates Tfn on its route back to the plasma membrane.

Dynamic measurement of the pH of the Golgi complex in living cells using retrograde transport of the verotoxin receptor

The B subunit of verotoxin (VT1B) from enterohemorrhagic Escherichia coli is responsible for the attachment of the holotoxin to the cell surface, by binding to the glycolipid, globotriaosyl ceramide. After receptor-mediated endocytosis, the toxin is targeted to the Golgi complex by a process of retrograde transport. We took advantage of this unique property of VT1B to measure the pH of the Golgi complex in intact live cells. Purified recombinant VT1B was labeled with either rhodamine or fluorescein for subcellular localization by confocal microscopy. After 1 h at 37 degrees C, VT1B accumulated in a juxtanuclear structure that colocalized with several Golgi markers, including alpha-mannosidase II, beta-COP, and NBD-ceramide. Moreover, colchicine and brefeldin A induced dispersal of the juxtanuclear staining, consistent with accumulation of VT1B in the Golgi complex. Imaging of the emission of fluorescein-labeled VT1B was used to measure intra-Golgi pH (pHG), which was calibrated in situ with ionophores. In intact Vero cells, pHG averaged 6.45 +/- 0.03 (standard error). The acidity of the Golgi lumen dissipated rapidly upon addition of bafilomycin A1, a blocker of vacuolar-type ATPases, pHG remained constant despite acidification of the cytosol by reversal of the plasmalemmal Na+/H+ antiport. Similarly, pHG was unaffected by acute changes in cytosolic calcium. Furthermore, pHG recovered quickly toward the basal level after departures imposed with weak bases. These findings suggest that pHG is actively regulated, despite the presence of a sizable H+ "leak" pathway. The ability of VT1B to target the Golgi complex should facilitate not only studies of acid-base regulation, but also analysis of other ionic species.

The synaptic vesicle cycle: a single vesicle budding step involving clathrin and dynamin

Strong evidence implicates clathrin-coated vesicles and endosome-like vacuoles in the reformation of synaptic vesicles after exocytosis, and it is generally assumed that these vacuoles represent a traffic station downstream from clathrin-coated vesicles. To gain insight into the mechanisms of synaptic vesicle budding from endosome-like intermediates, lysed nerve terminals and nerve terminal membrane subfractions were examined by EM after incubations with GTP gamma S. Numerous clathrin-coated budding intermediates that were positive for AP2 and AP180 immunoreactivity and often collared by a dynamin ring were seen. These were present not only on the plasma membrane (Takei, K., P.S. McPherson, S.L.Schmid, and P. De Camilli. 1995. Nature (Lond.). 374:186-190), but also on internal vacuoles. The lumen of these vacuoles retained extracellular tracers and was therefore functionally segregated from the extracellular medium, although narrow connections between their membranes and the plasmalemma were sometimes visible by serial sectioning. Similar observations were made in intact cultured hippocampal neurons exposed to high K+ stimulation. Coated vesicle buds were generally in the same size range of synaptic vesicles and positive for the synaptic vesicle protein synaptotagmin. Based on these results, we suggest that endosome-like intermediates of nerve terminals originate by bulk uptake of the plasma membrane and that clathrin- and dynamin-mediated budding takes place in parallel from the plasmalemma and from these internal membranes. We propose a synaptic vesicle recycling model that involves a single vesicle budding step mediated by clathrin and dynamin.

pH-sensitive, cationic liposomes: a new synthetic virus-like vector

We describe the use of cationic, pH-sensitive liposomes to mediate the efficient transfer of DNA into a variety of cells in culture. Cationic lipids, containing an amine with a pK within the physiologic range of 4.5 to 8, were synthesized and incorporated with dioleoylphosphatidylethanolamine into liposomes. Acid conditions promoted DNA-binding, DNA-incorporation, and DNA-induced fusion by these cationic, pH-sensitive liposomes. Transfection efficiency in cultured cells was dependent on endosomal acidification in a manner akin to acidic-induced endosomal release of viruses. These liposomes constitute a promising new class of reagents for gene therapy.

Association of a dynamin-like protein with the Golgi apparatus in mammalian cells

Dynamins are a family of 100-kD GTPases comprised of at least three distinct gene products and multiple alternatively spliced variants. Homologies with the shibire gene product in Drosophila melanogaster and with Vps1p and Dnm1p in Saccharomyces cerevisiae suggest that dynamins play an important role in vesicular transport. Morphological studies have localized brain dynamin to coated pits and tubular invaginations at the plasma membrane, where it is believed to facilitate the formation of endocytic vesicles. Because similar membrane-budding events occur at the Golgi apparatus and multiple dynamin isoforms exist, we have studied the distribution of dynamins in mammalian cells. To this end, we generated and characterized peptide-specific antibodies directed against conserved regions of the dynamin family. By immunoblot analysis, these antibodies reacted specifically with a 100-kD protein in fibroblasts that sedimented with membranes and microtubules in vitro in a manner similar to brain dynamin. By immunofluorescence microscopy, these antibodies strongly labeled the Golgi complex in cultured fibroblasts and melanocytes, as confirmed by double labeling with a Golgi-specific antibody. Furthermore, Western blot analysis showed significant enrichment of a 100-kD dynamin band in Golgi fractions isolated from the liver. To substantiate these findings, we use a specific antidynamin antibody to immunoisolate Golgi membranes from subcellular Golgi fractions, as determined by EM and immunoblot analysis. This study provides the first morphological and biochemical evidence that a dynamin-like protein associates with the Golgi apparatus in mammalian cells, and suggests that dynamin-related proteins may have multiple cytoplasmic distributions. The potential contributions of dynamin to the secretory and endocytic pathways are discussed.

Ceramides are transported through the Golgi apparatus in human keratinocytes in vitro

The intercellular lipid sheets of the stratum corneum constitute the epidermal permeability barrier that permits terrestrial life. Although lamellar granules are known to deliver the precursors of the stratum corneum lipids into the intercellular spaces, their site of origin remains unknown. Lamellar granules have characteristics of both secretory granules and lysosomes, which are known to originate from the Golgi apparatus in other cell types. Glucosylceramides, a major component of lamellar granule contents and the precursors of stratum corneum ceramides, have been found to be synthesized primarily in the early compartments of the Golgi apparatus in other cell types. We have investigated the transport and metabolism of a fluorescently labeled ceramide in human keratinocyte cultures using laser-scanning confocal microscopy and lipid analysis. We found that ceramide is metabolized to glucosylceramide and sphingomyelin as it passes through the Golgi apparatus and the metabolites are then delivered to the plasma membrane. Cold temperature, Brefeldin A, and monensin, all known to inhibit transport from the Golgi to the plasma membrane, prevented ceramide metabolites from appearing at the plasma membrane. Because glucosylceramides are one of the most important lipid constituents of lamellar granules, these results support the hypothesis that the Golgi is the origin of lamellar granules.

The oligomeric state of 46-kDa mannose 6-phosphate receptor does not change upon intracellular recycling and binding of ligands

46-kDa mannose-6-phosphate receptor forms homooligomers in cell membranes and in detergent solution. The quaternary structure of detergent-solubilized 46-kDa mannose 6-phosphate receptor is regulated by the presence of ligands, pH and receptor concentration [Waheed, A. & von Figura, K. (1990) Eur. J. Biochem. 193, 47-54). To find out whether the intracellular recycling of 46-kDa mannose 6-phosphate receptor is accompanied by changes in its quaternary structure, we have performed chemical cross-linking in membranes of intact cells. In all conditions tested, the dimer was the predominating form (more than 67% of total 46-kDa mannose 6-phosphate receptor). The amount of trimeric and tetrameric forms varied among cell lines and contributed up to 20% of total endogenous 46-kDa mannose 6-phosphate receptor in human and mouse fibroblasts. Within a given cell line, the ratio of the oligomers was not significantly changed upon elevating endosomal pH by bafilomycin A1, upon changes in receptor occupancy (treatment of cells with tunicamycin or use of mouse fibroblasts deficient in 300-kDa mannose 6-phosphate receptor), nor upon depletion of adaptors from clathrin-coated vesicles of the trans Golgi network by brefeldin A. At the cell surface, where 46-kDa mannose 6-phosphate receptor does not bind ligands, the percentage of dimer was similar to that observed intracellularly. Thus, the oligomeric state of 46-kDa mannose 6-phosphate receptor apparently does not change during recycling as well as binding and dissociation of ligands. In view of the abundance of the dimer of 46-kDa mannose 6-phosphate receptor in situ, our data suggest that it represents the main physiologically active form of the receptor, and therefore present indirect evidence that binding of ligands to 46-kDa mannose 6-phosphate receptor is probably regulated by conformational changes of receptor or ligand rather than by changes in the quaternary structure.

Mammalian vesicle trafficking proteins of the endoplasmic reticulum and Golgi apparatus

Vesicle traffic propagates and maintains distinct subcellular compartments and routes secretory products from their site of synthesis to their final destinations. As a basis for the specificity of vesicular transport reactions, each step in the secretory pathway appears to be handled by a distinct set of evolutionarily conserved proteins. Mammalian proteins responsible for vesicle trafficking at early steps in the secretory pathway are not well understood. In this report, we describe rat sec22 (rsec22) and rat bet1 (rbet1), mammalian sequence homologs of yeast proteins identified as mediators of endoplasmic reticulum-to-Golgi protein transport. rsec22 and rbet1 were expressed widely in mammalian tissues, as anticipated for proteins involved in fundamental membrane trafficking reactions. Recombinant rsec22 and rbet1 proteins behaved as integral membrane components of 28 and 18 kDa, respectively, consistent with their primary structures, which contain a predicted transmembrane domain at or near the carboxyl terminus. Recombinant rsec22 and rbet1 had distinct subcellular localizations, with rsec22 residing on endoplasmic reticulum membranes and rbet1 found on Golgi membranes. Studies with brefeldin A and nocodazole indicated that rbet1 function might involve interaction with or retention in the intermediate compartment. The distinct localizations of rsec22 and rbet1 may reflect their participation in opposite directions of membrane flow between the endoplasmic reticulum and Golgi apparatus.

Retrieval of human cytomegalovirus glycoprotein B from the infected cell surface for virus envelopment

Surface biotinylation of human cytomegalovirus (HCMV)-infected fibroblasts under pulse-chase conditions was used to define the cellular route of the dominant viral envelope glycoprotein gB into the cytoplasmic compartment of viral maturational envelopment. The results showed that a major fraction of gB was re-internalized from the infected cell surface prior to incorporation into the viral envelope. Viral particles carrying biotinylated gB were subsequently released into the culture medium. Viral release appeared to be inhibited in the presence of gB-specific antibody or when infected cultures were incubated at room temperature, but was not reduced by inhibitors of cellular glycoprotein transport. To our knowledge this is the first report describing that HCMV gB is retrieved from the infected cell surface prior to viral envelopment.

Processing of DR1-restricted determinants from the fusion protein of measles virus following two distinct pathways

A panel of human T cell clones specific for measles virus was characterized and among them fusion protein-specific, DR1-and DP-restricted T cell clones were selected to study the processing and presentation of determinants borne by a viral membrane protein. Using two independent methods to assess the activation of T cells when they encounter antigen-presenting cells, proliferation assay and Ca2+ flux measure by flow cytometry, we show that determinants from the fusion protein of measles virus presented to two DR1-restricted T cell clones have strikingly different processing requirements. While treatment with chloroquine, leupeptin and brefeldin A of antigen-presenting cells infected with the measles virus inhibits presentation of the first determinant, presentation of the second is prevented only by leupeptin but not by chloroquine and brefeldin A. The major histocompatibility complex deletion mutant cell line T2 was transfected with DR alpha and DR1 beta genes to be tested as antigen-presenting cells with the measles virus-specific T cell clones. DR1-transfected T2 cells infected with the measles virus presented the fusion protein determinant whose processing was sensitive to chloroquine and brefeldin A but failed to display insensitivity to these two drugs, further indicating that the two determinants are generated following two distinct pathways. The first is likely to be independent of the expression of the class II major histocompatibility complex-like molecule DM, whereas the other requires it. In conclusion, determinants on the same polypeptide can have profoundly dissimilar processing requirements. Due to transport to successive compartments with different processing capabilities, more determinants are successfully released from antigens and/or captured by class II major histocompatibility complex molecules, thereby increasing the repertoire of determinants displayed by class II major histocompatibility complex molecules.

Lysosomal alpha-glucosidase: cell-specific processing and altered maturation in HT-29 colon cancer cells

We have previously described the abnormal localization of resident Golgi proteins and O-glycans in the rough endoplasmic reticulum of mucin-secreting HT-29 M6 colon cancer cells, suggesting altered protein trafficking in these cells [Egea, Francí, Gambús, Lesuffleur, Zweibaum and Real (1993) J. Cell Sci. 105, 819-830]. In the present work, we have chosen lysosomal alpha-glucosidase as a reporter to examine the intracellular traffic of glycoproteins in M6 cells. We have compared the synthesis and processing of alpha-glucosidase in mucin-secreting M6 cells and in Caco-2 colon cancer cells, the latter resembling normal absorptive intestinal epithelium. Our results show that alpha-glucosidase processing and secretion is markedly delayed in M6 cells as compared to Caco-2 cells or normal fibroblasts, and this delay is caused by an accumulation of alpha-glucosidase precursor form in the trans-Golgi network. Furthermore, treatment in Caco-2 cells with brefeldin A led to changes in alpha-glucosidase maturation similar to those observed in untreated M6 cells. To determine whether altered processing occurs in other cultured cells, a panel of cancer cell lines and cultures from normal exocrine pancreas were examined. In pancreas-derived cultures, alpha-glucosidase showed a processing pattern different from that described until now. Only HT-29 cells and HT-29-derived subpopulations displayed a defect in alpha-glucosidase maturation. In conclusion, alpha-glucosidase processing is more diverse than has previously been described; this finding may have tissue-specific functional implications.

Brefeldin A inhibits the antiviral action of interferon against encephalomyocarditis virus

Brefeldin A (BFA), a unique fungal metabolite of a 13-membered lactone ring, exhibits various biological actions, including antitumor, antifungal and antiviral activities. In the present study, mouse LB cells were treated with various concentrations of interferon (IFN) and/or BFA overnight and infected with encephalomyocarditis virus (EMCV) after removal of IFN and BFA. Doses of BFA which neither inhibit the metabolism of the cell nor the infectivity of EMCV, decreased the IFN-induced antiviral activity against EMCV as demonstrated by virus titer from supernatants. Since 2-5A synthetase and double-stranded RNA (dsRNA)-dependent protein kinase (PKR) have been suggested to be involved in the antiviral action of IFN against EMCV, their activities were investigated in LB cells after BFA treatment. Northern blot analysis and in situ hybridization showed a decrease (2-3-fold) in the mRNA of 2'-5' oligoadenylate (2-5A) synthetase after BFA treatment. BFA also inhibited the activity of 2-5A synthetase, 2-5A dependent RNase and phosphorylation of PKR in cellular extracts, indicating that BFA may be exerting its inhibitory effect both at the transcriptional and post-transcriptional levels. This study reports a new biological action of BFA, demonstrating that BFA antagonized the antiviral action of IFN by inhibiting IFN-induced enzymatic pathways. These studies also suggest that both 2-5A and PKR are important in the antiviral activity of IFN against EMCV.

Potential sites of PI-3 kinase function in the endocytic pathway revealed by the PI-3 kinase inhibitor, wortmannin

Previously we have shown that PDGF receptor mutants that do not bind PI-3 kinase internalize after ligand binding, but fail to downregulate and degrade. To define further the role of PI-3 kinase in trafficking processes in mammalian cells, we have investigated the effects of a potent inhibitor of PI-3 kinase activity, wortmannin. At nanomolar concentrations, wortmannin inhibited both the transfer of PDGF receptors from peripheral compartments to juxtanuclear vesicles, and their subsequent degradation. In contrast, the delivery of soluble phase markers to lysosomes, assessed by the accumulation of Lucifer yellow (LY) in perinuclear vesicles after 120 min of incubation, was not blocked by wortmannin. Furthermore, wortmannin did not affect the rate of transferrin uptake, and caused only a small decrease in its rate of recycling. Thus, the effects of wortmannin on PDGFr trafficking are much more pronounced than its effects on other endocytic events. Unexpectedly, wortmannin also caused a striking effect on the morphology of endosomal compartments, marked by tubulation and enlargement of endosomes containing transferrin or LY. This effect was somewhat similar to that produced by brefeldin A, and was also blocked by pre-treatment of cells with aluminum fluoride (AlF4-). These results suggest two sites in the endocytic pathway where PI-3 kinase activity may be required: (a) to sort PDGF receptors from peripheral compartments to the lysosomal degradative pathway; and (b) to regulate the structure of endosomes containing lysosomally directed and recycling molecules. This latter function could be mediated through the activation of AlFt4-)-sensitive GTP-binding proteins downstream of PI-3 kinase.

A novel class of clathrin-coated vesicles budding from endosomes

Clathrin-coated vesicles transport selective integral membrane proteins from the plasma membrane to endosomes and from the TGN to endosomes. Recycling of proteins from endosomes to the plasma membrane occurs via unidentified vesicles. To study this pathway, we used a novel technique that allows for the immunoelectron microscopic examination of transferrin receptor-containing endosomes in nonsectioned cells. Endosomes were identified as separate discontinuous tubular-vesicular entities. Each endosome was decorated, mainly on the tubules, with many clathrin-coated buds. Endosome-associated clathrin-coated buds were discerned from plasma membrane-derived clathrin-coated vesicles by three criteria: size (60 nm and 100 nm, respectively), continuity with endosomes, and the lack of labeling for alpha-adaptin. They were also distinguished from TGN-derived clathrin-coated vesicles by their location at the periphery of the cell, size, and the lack of labeling for gamma-adaptin. In the presence of brefeldin A, a large continuous endosomal network was formed. Transferrin receptor recycling as well as the formation of clathrin-coated pits at endosomes was inhibited in the presence of brefeldin A. Together with the localization of transferrin receptors at endosome-associated buds, this indicates that a novel class of clathrin-coated vesicles serves an exit pathway from endosomes. The target organelles for endosome-derived clathrin-coated vesicles remain, however, to be identified.

Low-efficiency (macro-)pinocytic internalization of non-pathogenic Escherichia coli into HEp-2 cells

HEp-2 cells internalize non-pathogenic Escherichia coli bacteria by a low-efficiency internalization mechanism which is upregulated in Pho-derepressed strains (as shown by Sinai and Bavoil in 1993), and is independent of microfilament integrity but requires functional microtubules. Here, we further characterize the microtubule requirement of this pathway using various effectors of microtubule integrity and function. Furthermore, we show that internalization is enhanced upon treatment with monodansylcadaverine, a specific inhibitor of receptor mediated endocytosis, and is insensitive to brefeldin A, which promotes the microtubule-dependent reorganization of the endosome. An assay system is also described to directly evaluate the contribution of pinocytosis to this pathway based on the ability of the bacteria to cointernalize and consequently colocalize with the fluid-phase marker, Texas-red-conjugated dextran (TRD). Using this assay, Hoescht-stained bacteria were observed in TRD-containing vesicles in numbers that are consistent with their observed internalization rate. Overall, these data are strongly supportive of the existence of a low-efficiency macropinocytic mechanism of entry for these non-pathogenic bacteria. Moreover, the observed requirements for host tyrosine kinase and protein kinase C activities suggest that it is inducible.

Ionic milieu controls the compartment-specific activation of pro-opiomelanocortin processing in AtT-20 cells

Newly synthesized prohormones and their processing enzymes transit through the same compartments before being packaged into regulated secretory granules. Despite this coordinated intracellular transport, prohormone processing does not occur until late in the secretory pathway. In the mouse pituitary AtT-20 cell line, conversion of pro-opiomelanocortin (POMC) to mature adrenocorticotropic hormone involves the prohormone convertase PC1. The mechanism by which this proteolytic processing is restricted to late secretory compartments is unknown; PC1 activity could be regulated by compartment-specific activators/inhibitors, or through changes in the ionic milieu that influence its activity. By arresting transport in a semi-intact cell system, we have addressed whether metabolically labeled POMC trapped in early secretory compartments can be induced to undergo conversion if the ionic milieu in these compartments is experimentally manipulated. Prolonged incubation of labeled POMC trapped in the endoplasmic reticulum or Golgi/trans-Golgi network did not result in processing, thereby supporting the theory that processing is normally a post-Golgi/trans-Golgi network event. However, acidification of these compartments allowed effective processing of POMC to the intermediate and mature forms. The observed processing increased sharply at a pH below 6.0 and required millimolar calcium, regardless of the compartment in which labeled POMC resided. These conditions also resulted in the coordinate conversion of PC1 from the 84/87 kDa into the 74-kDa and 66-kDa forms. We propose that POMC processing is predominantly restricted to acidifying secretory granules, and that a change in pH within these granules is both necessary and sufficient to activate POMC processing.

Brefeldin A inhibits the endocytosis of plasma-membrane-associated heparan sulphate proteoglycans of cultured rat ovarian granulosa cells

Rat ovarian granulosa cells were labelled with [35S]sulphate for 0.5-20 h and chased in the presence or absence of 1-2 micrograms/ml of brefeldin A (BFA) for up to 21 h. Heparan [35S]sulphate (HS) proteoglycans from the culture medium, plasma membrane and intracellular fractions were then analysed by gel chromatography. In the absence of BFA, about 85% of the plasma membrane-associated HS proteoglycans were endocytosed and subsequently degraded intracellularly. Recirculation of the HS proteoglycans between the intracellular pool and the cell surface was not observed. Exposing the cells to BFA for less than 1 h did not influence the turnover of the HS proteoglycans, whereas the effect of the drug on the Golgi functions reached a maximum in approx. 10 min. When the cells were treated with BFA for more than 1-2 h, the rate of endocytosis of HS proteoglycans was reduced to about 50% of the control. The delivery of endocytosed HS proteoglycans to lysosomes were not affected by the drug. Cycloheximide also reduced the endocytosis of HS proteoglycans, but not as much as BFA, indicating that the inhibitory effect of BFA can be only partly accounted for by a block of protein transport from the endoplasmic reticulum to the plasma membrane. In contrast with the endocytosis of HS proteoglycans, neither that of 125I-transferrin, known to be mediated by clathrin-coated vesicles, nor that of 125I-ricin, a marker molecule for bulk endocytosis, was affected by BFA. The half-life of 125I-transferrin and 125I-ricin in the plasma membrane was about 10 and 25 min respectively compared with about 5 h for the HS proteoglycans. Altogether, these results indicate that the endocytosis of plasma-membrane-associated HS proteoglycans is mediated by different mechanisms than the endocytosis of most other cell-surface proteins. Further, the mechanisms involved in the endocytosis of HS proteoglycans are sensitive to BFA.

Role for phosphatidylinositol 3-kinase in the sorting and transport of newly synthesized lysosomal enzymes in mammalian cells

Previous work with the yeast Saccharomyces cerevisiae has demonstrated a role for a phosphatidylinositol-specific PI 3-kinase, the product of the VPS34 gene, in the targeting of newly synthesized proteins to the vacuole, an organelle functionally equivalent to mammalian lysosomes (Schu, P. V., K. Takegawa, M. J. Fry, J. H. Stack, M. D. Waterfield, and S. D. Emr. 1993. Science [Wash. DC]. 260:88-91). The activity of Vps34p kinase is significantly reduced by the PI 3-kinase inhibitors wortmannin, a fungal metabolite, and LY294002, a quercetin analog (Stack, J. H., and S. D. Emr. 1994. J. Biol. Chem. 269:31552-31562). We show here that at concentrations which inhibit VPS34-encoded PI 3-kinase activity, wortmannin also inhibits the processing and delivery of newly synthesized cathepsin D to lysosomes in mammalian cells with half-maximal inhibition of delivery occurring at 100 nM wortmannin. As a result of wortmannin action, newly synthesized, unprocessed cathepsin D is secreted into the media. Moreover, after accumulation in the trans-Golgi network (TGN) at 20 degrees C, cathepsin D was rapidly missorted to the secretory pathway after addition of wortmannin and shifting to 37 degrees C. At concentrations that inhibited lysosomal enzyme delivery, both wortmannin and LY294002 caused a highly specific dilation of mannose 6-phosphate receptor (M6PR)-enriched vesicles of the prelysosome compartment (PLC), which swelled to approximately 1 micron within 15 min after treatment. With increasing time, the inhibitors caused a significant yet reversible change in M6PR distribution. By 3 h of treatment, the swollen PLC vacuoles were essentially depleted of receptors and, in addition, there was a fourfold loss of receptors from the cell surface. However, M6PRs were still abundant in the TGN. These results are most consistent with the interpretation that PI 3-kinase regulates the trafficking of lysosomal enzymes by interfering with a M6PR-dependent sorting event in the TGN. Moreover, they provide evidence that trafficking of soluble hydrolases to mammalian lysosomes and yeast vacuoles rely on similar regulatory mechanisms.

Endoproteolytic cleavage of HIV-1 gp160 envelope precursor occurs after exit from the trans-Golgi network (TGN)

Endoproteolytic processing of human immunodeficiency virus type 1 (HIV-1) gp160 membrane glycoprotein precursor into gp 120 and gp41 is necessary for formation of infectious HIV particles [1]. We have studied the intracellular site of this processing using inhibition of transport at reduced temperature (20 degrees C). That reduced temperature (20 degrees C) inhibits the intracellular transport also in Jurkat-tat cells was demonstrated using the Semliki Forest virus p62 precursor processing as model. In HIV-1 infected Jurkat-tat cells the proteolytic processing of gp 160 precursor did not occur when the protein was accumulated in the TGN at 20 degrees C temperature. When the temperature was shifted to 37 degrees C the HIV-1 gp 160, which had accumulated in the TGN at the reduced temperature, was proteolytically processed. The processing of gp 160 was inhibited when the temperature reversion was carried out in the presence of brefeldin A (BFA) or aluminium fluoride (ALFn) indicating that the exit from the TGN is required for the proteolytic cleavage of HIV-1 gp160 precursor. The results suggest that the processing of gp 160 takes place at a yet unidentified transport step which is distal to the TGN/20 degrees C block site.