Selective inhibition of transcytosis by brefeldin A in MDCK cells

A cytosolic complex of p62 and rab6 associates with TGN38/41 and is involved in budding of exocytic vesicles from the trans-Golgi network

TGN38/41, an integral membrane protein predominantly localized to the trans-Golgi network, has been shown to cycle to the plasma membrane and return to the TGN within 30 min. (Ladinsky, M. S., and K. E. Howell. 1992. Eur. J. Cell Biol. 59:92-105). In characterizing the proteins which associate with TGN38/41, a peripheral 62-kD protein, two forms of rab6 and two other small GTP-binding proteins were identified by coimmunoprecipitation. However, approximately 90% of the 62-kD protein is cytosolic and is associated with the same subset of small GTP-binding proteins. Both the membrane and cytoplasmic complexes were characterized by sizing column fractionation and velocity sedimentation. The membrane complex was approximately 250 kD (11.6 S) consisting of the cytosolic complex and a heterodimer of TGN38/41 (160 kD). The cytosolic complex was approximately 86 kD (6.1 S) consisting of p62 and one small GTP-binding protein. Preliminary evidence indicates that phosphorylation of the p62 molecule regulates the dissociation of the cytosolic complex from TGN38/41. Functionally the cytosolic p62 complex must bind to TGN38/41 for the budding of exocytic transport vesicles from the TGN as assayed in a cell-free system (Salamero, J., E. S. Sztul, and K. E. Howell. 1990. Proc. Natl. Acad. Sci. USA. 87:7717-7721). Interference with p62, rab6 or TGN38, and TGN41 cytoplasmic domains by immunodepletion or competing peptides completely inhibited the budding of exocytic transport vesicles. These results support an essential role for interaction of the cytosolic p62/rab6 complex with TGN38/41 in budding of exocytic vesicles from the TGN.

Vectorial release of poliovirus from polarized human intestinal epithelial cells

Polarized epithelial cells represent the primary barrier to virus infection of the host, which must also be traversed prior to virus dissemination from the infected organism. Although there is considerable information available concerning the release of enveloped viruses from such cells, relatively little is known about the processes involved in the dissemination of nonenveloped viruses. We have used two polarized epithelial cell lines, Vero C1008 (African green monkey kidney epithelial cells) and Caco-2 (human intestinal epithelial cells), infected with poliovirus and investigated the process of virus release. Release of poliovirus was observed to occur almost exclusively from the apical cell surface in Caco-2 cells, whereas infected Vero C1008 cells exhibited nondirectional release. Structures consistent with the vectorial transport of virus contained within vesicles or viral aggregates were observed by electron microscopy. Treatment with monensin or ammonium chloride partially inhibited virus release from Caco-2 cells. No significant cell lysis was observed at the times postinfection when extracellular virus was initially detected, and transepithelial resistance and vital dye uptake measurements showed only a moderate decrease. Brefeldin A was found to significantly and specifically inhibit poliovirus biosynthetic processes by an as yet uncharacterized mechanism. The vectorial release of poliovirus from the apical (or luminal) surface of human intestinal epithelial cells has significant implications for viral pathogenesis in the human gut.

Complete vesiculation of Golgi membranes and inhibition of protein transport by a novel sea sponge metabolite, ilimaquinone

We have identified a novel natural metabolite, ilimaquinone (IQ), from sea sponges that causes Golgi membranes to break down completely in vivo into small vesicular structures (called vesiculated Golgi membranes [VGMs]). Under these conditions, transport of newly synthesized proteins from endoplasmic reticulum (ER) to the cis-Golgi-derived VGMs is unaffected; however, further transport along the secretory pathway is blocked. Upon removal of the drug, VGMs reassemble rapidly into a Golgi complex, and protein transport is restored. By employing a cell-free system that reconstitutes vesicular transport between successive Golgi cisternae, we provide evidence that the inhibition of protein transport by IQ is specifically due to an inhibition of transport vesicle formation. In addition, like brefeldin A (BFA), IQ treatment prevents the association of beta-COP and ADP-ribosylation factor to the Golgi membranes; however, unlike BFA treatment, there is no retrograde transport of Golgi enzymes into ER.

Release of stress proteins from Mesocestoides corti is a brefeldin A-inhibitable process: evidence for active export of stress proteins

Substantial evidence indicates that molecules released by infectious organisms affect virulence and influence immunity to infection. The characterization of extracellular molecules and their mechanism of release is therefore critical to understanding host-parasite relationships. The cestode parasite Mesocestoides corti is known to release at the larval stage several molecules including the heat shock proteins hsp70 and hsp60. In this report, it is shown that several molecules released by M. corti, including 70- and 60-kDa proteins, are induced by a temperature shift from room temperature to 37 degrees C. Such a shift is comparable to the thermal stress of parasites transmitted from insect vector to vertebrate host. By drug inhibition studies and Western blot (immunoblot) analyses, it is shown that M. corti hsp70 and hsp60 as well as other released molecules are actively exported. The active release of stress proteins by parasites has not been described and may play a critical role in the disease process.

Budding from Golgi membranes requires the coatomer complex of non-clathrin coat proteins

Do the coats on vesicles budded from the Golgi apparatus actually cause the budding, or do they simply coat buds (Fig. 1)? One view (the membrane-mediated budding hypothesis) is that budding is an intrinsic property of Golgi membranes not requiring extrinsic coat proteins. Assembly of coats from dispersed subunits is super-imposed upon the intrinsic budding process and is proposed to convert the tips of tubules into vesicles. The alternative view (the coat-mediated budding hypothesis) is that coat formation provides the essential driving force for budding. The membrane-mediated budding hypothesis was inspired by the microtubule-dependent extension of apparently uncoated, 90-nm-diameter membrane tubules from the Golgi apparatus and other organelles in vivo after treatment with brefeldin A, a drug that inhibits the assembly of coat proteins onto Golgi membranes. This hypothesis predicts that tubules will be extended when coat proteins are unavailable to convert tubule-derived membrane into vesicles. Here we use a cell-free system in which coated vesicles are formed from Golgi cisternae to show that, on the contrary, when budding diminishes as a result of immunodepletion of coat protein pools, tubules are not formed at the expense of vesicles. We conclude that coat proteins are required for budding from Golgi membranes.

Protein trafficking along the exocytotic pathway

Proteins of the exocytotic (secretory) pathway are initially targeted to the endoplasmic reticulum (ER) and then translocated across and/or inserted into the membrane of the ER. During their anterograde transport with the bulk of the membrane flow along the exocytotic pathway, some proteins are selectively retained in various intracellular compartments, while others are sorted to different branches of the pathway. The signals or structural motifs that are involved in these selective targeting processes are being revealed and investigations into the mechanistic nature of these processes are actively underway.

Connexin trafficking and the control of gap junction assembly in mouse preimplantation embryos

Gap junction assembly in the preimplantation mouse embryo is a temporally regulated event, beginning a few hours after the third cleavage during the morphogenetic event known as compaction. Recently, we demonstrated that both mRNA and protein corresponding to connexin43, a gap junction protein, accumulate through preimplantation development beginning at least as early as the 4-cell stage. Using an antibody raised against a synthetic C-terminal peptide of connexin43, this protein was shown to assemble into gap junction-like plaques beginning at compaction (G. Valdimarsson, P. A. De Sousa, E. C. Beyer, D. L. Paul and G. M. Kidder (1991). Molec. Reprod. Dev. 30, 18–26). The purpose of the present study was to follow the fate of nascent connexin43 during preimplantation development, from synthesis to plaque insertion, and to learn more about the control of gap junction assembly during compaction. Cell fractionation and reverse transcription-polymerase chain reaction were employed to show that connexin43 mRNA is in polyribosomes at the 4-cell stage, suggesting that synthesis of connexin43 begins at least one cell cycle in advance of when gap junctions first form. The fate of nascent connexin43 was then followed throughout preimplantation development by means of laser confocal microscopy, using two other peptide (C-terminal)-specific antibodies. As was reported previously, connexin43 could first be detected in gap junction-like plaques beginning in the 8-cell stage, at which time considerable intracellular immunoreactivity could be seen as well. Later, connexin43 becomes differentially distributed in the apposed plasma membranes of morulae and blastocysts: a zonular distribution predominates between outside blastomeres and trophectoderm cells whereas plaque-like localizations predominate between inside blastomeres and cells of the inner cell mass. The cytoplasmic immunoreactivity in morulae was deemed to be nascent connexin en route to the plasma membrane since it could be abolished by treatment with cycloheximide, and redistributed by treatment with monensin or brefeldin-A, known inhibitors of protein trafficking. Treatment of uncompacted 8-cell embryos with either monensin or brefeldin-A inhibited the appearance of gap junction-like structures and the onset of gap junctional coupling in a reversible manner. These data demonstrate that the regulated step in the onset of gap junction assembly during compaction is downstream of transcription and translation and involves mobilization of connexin43 through trafficking organelles to plasma membranes.

Effects of brefeldin A on cleavage of invariant chain to p21 and p10 and the appearance of Ii-freed class II MHC dimers

Intracellular cleavage of class II MHC-associated Ii to p21 and p10 and the appearance of Ii-freed alpha, beta dimers were concurrent events lasting from 1 to 6 hr after synthesis of alpha, beta, Ii trimers, possibly related to charging of foreign peptides to the class II MHC antigen-binding site. Sequential immunoprecipitations of pulse-chase radiolabeled cells were made four times with anti-Ii monoclonal antibody to remove Ii and alpha, beta, Ii trimers and then with anti-class II antibody to detect the time-dependent appearance of Ii-freed alpha, beta dimers. The cleavage of Ii to p21 and p10 was revealed in leupeptin-treated cells. Cell treatment with Brefeldin A (BFA) was associated with a decrease in Ii-freed alpha, beta dimers, with inhibition of leupeptin-revealed cleavage of Ii to p21 and p10, and with persistence of endoglycosidase H susceptibility of Ii and class II alpha, beta chains. We conclude that in untreated cells, cleavage and release of Ii from class II MHC alpha and beta chains occur after those complexes traverse a BFA-sensitive step in the Golgi apparatus.

Regulation of acetylcholinesterase secretion from perfused bovine adrenal gland and isolated bovine chromaffin cells

The secretion of acetylcholinesterase (AChE) was studied in an isolated perfused bovine adrenal gland preparation and in cultured bovine adrenal medullary chromaffin cells. Electrical field stimulation (10 Hz) of splanchnic nerve terminals in the isolated perfused gland resulted in a two-fold increase in AChE secretion from the gland. Perfusion with the cholinergic receptor antagonists mecamylamine (5 microM) and atropine (1 microM) inhibited 70% of the stimulated secretion of AChE, demonstrating that most of the stimulated secretion was derived from chromaffin cells. The effect of nicotine stimulation on the secretion of AChE from isolated bovine chromaffin cells was compared with that produced by other compounds (histamine, angiotensin II) which are known to stimulate secretion of catecholamines. Incubation with nicotine (1-25 microM) stimulated the secretion of catecholamines and AChE. Histamine (1 nM-10 microM) and angiotensin II (10 pM-10 microM) did not stimulate AChE secretion. Time-course studies of AChE resynthesis after irreversible inhibition with the esterase inhibitor diisopropylfluorophosphate (DFP) demonstrated that AChE is stored within chromaffin cells for at least 11 h before being secreted. AChE secretion was inhibited within 2-3 h by 10 micrograms/ml brefeldin A (BFA), a compound known to block protein translocation from the endoplasmic reticulum (ER) to the Golgi apparatus (GA). The results suggest that AChE may reside for 8-9 h within the lumen of the ER before being actively secreted by processing through the GA.

Involvement of the Golgi region in the intracellular trafficking of cholera toxin

The intracellular pathway following receptor-mediated endocytosis of cholera toxin was studied using brefeldin A (BFA), which inhibited protein secretion and induced dramatic morphological changes in the Golgi region. In both mouse Y1 adrenal cells and CHO cells, BFA at 1 micrograms/ml caused a 80-90% inhibition of the cholera toxin (CT)-induced elevation of intracellular cAMP. The inhibition of the cytotoxicity of CT by BFA was also observed in a rounding assay of Y1 adrenal cells. The inhibition of CT cytotoxicity by BFA was dose dependent, with the ID50 value similar to the LD50 of BFA in Y1 adrenal cells. Binding and internalization of [125I]-labeled cholera toxin in Y1 adrenal cells was not affected by BFA. Unlike the BFA-sensitive cell lines such as Y1 adrenal and CHO cells, BFA at 1 micrograms/ml did not inhibit the cytotoxicity of CT in PtK1 cells, of which the Golgi structure was BFA-resistant. These results strongly suggest that a BFA-sensitive Golgi is required for the protection of CT cytotoxicity by BFA. In contrast, elevation of the intracellular cAMP by forskolin, which acts directly on the plasma membrane adenylate cyclase, was not affected by BFA. These observations indicate that the intoxication of target cells by CT requires an intact Golgi region for its intracellular trafficking and/or processing. In this respect, CT shares a common intracellular pathway with ricin, Pseudomonas toxin, and modeccin, even though their structures and modes of action are very different.

Molecular cloning, characterization, subcellular localization and dynamics of p23, the mammalian KDEL receptor

We have isolated a cDNA clone (mERD2) for the mammalian (bovine) homologue of the yeast ERD2 gene, which codes for the yeast HDEL receptor. The deduced amino acid sequence bears extensive homology to its yeast counterpart and is almost identical to a previously described human sequence. The sequence predicts a very hydrophobic protein with multiple membrane spanning domains, as confirmed by analysis of the in vitro translation product. The protein encoded by mERD2 (p23) has widespread occurrence, being present in all the cell types examined. p23 was localized to the cis-side of the Golgi apparatus and to a spotty intermediate compartment which mediates ER to Golgi transport. A majority of the intracellular staining could be accumulated in the intermediate compartment by a low temperature (15 degrees C) or brefeldin A. During recovery from these treatments, the spotty intermediate compartment staining of p23 was shifted to the perinuclear staining of the Golgi apparatus and tubular structures marked by p23 were observed. These tubular structures may serve to mediate transport between the intermediate compartment and the Golgi apparatus.

Multiple GTP-binding proteins participate in clathrin-coated vesicle-mediated endocytosis

We have examined the effects of various agonists and antagonists of GTP-binding proteins on receptor-mediated endocytosis in vitro. Stage-specific assays which distinguish coated pit assembly, invagination, and coat vesicle budding have been used to demonstrate requirements for GTP-binding protein(s) in each of these events. Coated pit invagination and coated vesicle budding are both stimulated by addition of GTP and inhibited by GDP beta S. Although coated pit invagination is resistant to GTP gamma S, A1F4-, and mastoparan, late events involved in coated vesicle budding are inhibited by these antagonists of G protein function. Earlier events involved in coated pit assembly are also inhibited by GTP gamma S, A1F4-, and mastoparan. These results demonstrate that multiple GTP-binding proteins, including heterotrimeric G proteins, participate at discrete stages in receptor-mediated endocytosis via clathrin-coated pits.

Intestinal tissue distribution and epithelial transport of the oral immunogen LTB, the B subunit of E. coli heat-labile enterotoxin

LTB provokes a systemic immune response and exerts adjuvant effects on mucosal immune responses to unrelated antigens. The binding and uptake of fluorescein-labelled LTB in the normal villus epithelium was compared to that in Peyer's patch dome epithelium in mouse intestine. LTB was bound by the GM1-receptor and taken up extensively by both tissues, indicating that not only the Peyer's patches but also the normal villus epithelium play a significant role in the transport of orally administered antigens. These results were supported by transport studies in the human intestinal epithelial cell line Caco-2 using 125I-LTB. After 2 h incubation, 5.1 +/- 0.1% and 5.9 +/- 0.1% of the added radioactivity was transported in the apical to basolateral and basolateral to apical direction, respectively. Less than 1% of the transported radioactivity was immunoprecipitated with anti-LTB antiserum indicating that LTB was extensively degraded during the transport. The results suggest that normal enterocytes play a significant role in the binding, uptake and transport of orally administered LTB.

Tubulation of Golgi membranes in vivo and in vitro in the absence of brefeldin A

Recent in vivo studies with the fungal metabolite, brefeldin A (BFA), have shown that in the absence of vesicle formation, membranes of the Golgi complex and the trans-Golgi network (TGN) are nevertheless able to extend long tubules which fuse with selected target organelles. We report here that the ability to form tubules (> 7 microns long) could be reproduced in vitro by treatment of isolated, intact Golgi membranes with BFA under certain conditions. Surprisingly, an even more impressive degree of tubulation could be achieved by incubating Golgi stacks with an ATP-reduced cytosolic fraction, without any BFA at all. Similarly, tubulation of Golgi membranes in vivo occurred after treatment of cells with intermediate levels of NaN3 and 2-deoxyglucose. The formation of tubules in vitro, either by BFA treatment or low-ATP cytosol, correlated precisely with a loss of the vesicle-associated coat protein beta-COP from Golgi membranes. After removal of BFA or addition of ATP, membrane tubules served as substrates for the rebinding of beta-COP and for the formation of vesicles in vitro. These results provide support for the idea that a reciprocal relationship exists between tubulation and vesiculation (Klausner, R. D., J. G. Donaldson, and J. Lippincott-Schwartz. 1992. J. Cell Biol. 116:1071-1080). Moreover, they show that tubulation is an inherent property of Golgi membranes, since it occurs without the aid of microtubules or BFA treatment. Finally the results indicate the presence of cytosolic factors, independent of vesicle-associated coat proteins, that mediate the budding/tubulation of Golgi membranes.

Redistribution of a Golgi glycoprotein in plant cells treated with Brefeldin A

The fungal fatty acid derivative Brefeldin A (BFA), has been used to study the reversible distribution of a Golgi glycoprotein, the JIM 84 epitope, into the cytosol of higher plant cells. Treatment of both maize and onion root tip cells resulted in a rearrangement of the Golgi stacks into either circular formations or a perinuclear distribution. The Golgi cisternae became curved and vesiculated and in cells where the Golgi apparatus was totally dispersed the JIM 84 epitope was associated with large areas in the cytosol which were also vesiculated. On removal of the BFA the Golgi apparatus reformed and the JIM 84 epitope was again located in the cisternal stacks. This mode of BFA action is compared with that so far described for animal cells.

Isolation and characterization of a brefeldin A-resistant mutant of monkey kidney Vero cells

Brefeldin A (BFA) is a fungal antibiotic which disrupts protein transport between the endoplasmic reticulum and the Golgi. A BFA-resistant mutant of monkey kidney Vero cells, BER-40, which exhibited about a 90-fold increase in the LD50 of BFA (5.2 ng/ml for Vero cells versus 460 ng/ml for BER-40 cells), has been isolated. The increased resistance of BER-40 cells toward BFA was also manifested in a greatly reduced inhibition of protein secretion by BFA in the mutant and a lack of protection by BFA of the mutant cells from ricin cytotoxicity. Somatic cell hybridization between the Vero and BER-40 cells showed that the BFA-resistance in BER-40 behaved as a codominant trait. The structure of the Golgi region, as examined by immunofluorescence microscopy with antibodies against Golgi markers (the 110-kDa protein and mannosidase II) or with fluorescent lipid NBD-ceramide, was unchanged in the mutant cells as compared to that in the wild-type cells. Treatment of Vero cells with BFA (1 micrograms/ml) or with 2-deoxyglucose plus sodium azide resulted in a rapid release of the 110-kDa protein, mannosidase II, and NBD-ceramide from the Golgi membrane to a more diffuse distribution in the cytosol. In contrast, these three Golgi markers remained to be Golgi-associated following treatment of BER-40 cells with BFA or with 2-deoxyglucose plus sodium azide. Immunoblotting of cell extracts from Vero and BER-40 cells with monoclonal antibody against the 110-kDa protein did not reveal any significant difference in the level of this Golgi marker in the mutant cells. These data suggest that the BFA-resistance mutation in BER-40 has rendered the cyclic pathway of the 110-kDa protein assembly to the Golgi membrane resistant to both BFA and 2-deoxyglucose plus sodium azide.

Epitope mapping of two isoforms of a trans Golgi network specific integral membrane protein TGN38/41

TGN38/41 is an integral membrane protein predominantly located in the trans Golgi network (TGN) of rat (NRK) cells. We have used a cDNA expression system to map the epitopes recognised by a panel of antibodies raised to TGN38/41 as a preliminary step in the accurate identification of the region(s) of the molecule responsible for its correct intracellular location. These studies have confirmed the predicted topology of the molecule, and have identified a region in the cytoplasmic domain which is immunologically (and hence potentially functionally) conserved between species.

Inhibition by brefeldin A of a Golgi membrane enzyme that catalyses exchange of guanine nucleotide bound to ARF

A wide variety of membrane transformations important in intracellular transport are inhibited by the fungal metabolite brefeldin A (refs 1-4), implying that the target for this drug is central to the formation and maintenance of subcellular compartments. Brefeldin A added to cells causes the rapid and reversible dissociation of a Golgi-associated peripheral membrane protein (M(r) 110,000) which was found to be identical to one of the subunits of the coat of Golgi-derived (non-clathrin) coated vesicles, beta-COP, implying that brefeldin A prevents transport by blocking the assembly of coats and thus the budding of enclosed vesicles. In addition to the coatomer (a cytosol-derived complex of seven polypeptide chains, one of which is beta-COP), the non-clathrin (COP) coat of Golgi-derived vesicles contains stoichiometric amounts of a small (M(r) approximately 20,000) GTP-binding protein, the ADP-ribosylation factor (ARF). Binding of ARF to Golgi membranes is necessary before coatomer/beta-COP can bind these membranes (ref. 12; and D. J. Palmer et al., manuscript submitted), so the primary effect of brefeldin A seems to be on the reaction responsible for ARF binding. Indeed, like beta-COP, ARF is dissociated from the Golgi complex by treatment with brefeldin A and brefeldin A prevents ARF from associating in vitro, but the mechanism of this action by brefeldin A has been unclear. Here we report the discovery of an enzyme in a Golgi-enriched fraction that catalyses guanine nucleotide (GDP-GTP) exchange on ARF-1 protein, and which is inhibited by brefeldin A. We suggest that activation of ARF proteins for membrane localization by compartmentalized exchange enzymes is in general the first committed step in membrane transformation pathways.

Synthesis and trafficking of prion proteins in cultured cells

Scrapie prions are composed largely, if not entirely, of the scrapie prion protein (PrPSc) that is encoded by a chromosomal gene. Scrapie-infected mouse neuroblastoma (ScN2a) and hamster brain (ScHaB) cells synthesize PrPSc from the normal PrP isoform (PrPC) or a precursor through a posttranslational process. In pulse-chase radiolabeling experiments, we found that presence of brefeldin A (BFA) during both the pulse and the chase periods prevented the synthesis of PrPSc. Removal of BFA after the chase permitted synthesis of PrPSc to resume. BFA also blocked the export of nascent PrPC to the cell surface but did not alter the distribution of intracellular deposits of PrPSc. Under the same conditions, BFA caused the redistribution of the Golgi marker MG160 into the endoplasmic reticulum (ER). Using monensin as an inhibitor of mid-Golgi glycosylation, we determined that PrP traverses the mid-Golgi stack before acquiring protease resistance. About 1 h after the formation of PrPSc, its N-terminus was removed by a proteolytic process that was inhibited by ammonium chloride, chloroquine, and monensin, arguing that this is a lysosomal event. These results suggest that the ER is not competent for the synthesis of PrPSc and that the synthesis of PrPSc occurs during the transit of PrP between the mid-Golgi stack and lysosomes. Presumably, the endocytic pathway features in the synthesis of PrPSc.

Involvement of membrane traffic in the replication of poliovirus genomes: effects of brefeldin A

Brefeldin A (BFA) is a macrolide antibiotic that has multiple targets in vesicular transport and blocks membrane traffic between the cis- and trans-Golgi compartments, leading to the disruption of the trans-Golgi apparatus (for a review see Pelham, 1991, Cell 67, 449-451). Consequently, BFA interferes with the maturation of viral glycoproteins and suppresses the formation of infectious viruses that contain a lipid envelope. We report that this antibiotic strongly inhibits poliovirus replication even though this virus lacks a lipid envelope and does not encode any glycoproteins. Addition of BFA from the beginning of poliovirus infection blocks the synthesis of late proteins but has no effect on p220 cleavage, indicating that the input viral RNA is translated to produce active 2Apro. The presence of BFA at later times has no effect on poliovirus protein synthesis, indicating that this step is not a direct target for the antibiotic. Indeed, the target of BFA is viral RNA synthesis, because addition of the antibiotic at any time after poliovirus infection drastically reduces the incorporation of labeled uridine into poliovirus RNA. Both plus- and minus-stranded RNA syntheses are diminished when BFA is present from the beginning of infection, but plus-stranded RNA synthesis is more affected when the inhibitor is added at later times. The replication of poliovirus RNA takes place in close association with membrane vesicles that fill the cytoplasm of the infected cells. Little is known about the origin and function of these vesicles that form part of the viral replication complexes. Our findings suggest that the replication of poliovirus genomes may require the maturation of membranous vesicles from a vesicular compartment that is affected by BFA. The effects of BFA on late protein synthesis by other animal viruses varies according to the virus species examined. Among picornaviruses, rhinoviruses are sensitive to the antibiotic, whereas encephalomyocarditis virus is resistant. A negative-stranded RNA virus such as vesicular stomatitis is blocked by BFA, whereas vaccinia virus, a cytoplasmic DNA virus, is resistant.

Imaging of luciferase secretion from transformed Chinese hamster ovary cells

The blue luminescence characteristic of the marine ostracod crustacean Vargula hilgendorfii is from a simple, but highly specific, enzyme-substrate reaction. Light is emitted by the oxidation of Vargula luciferin (substrate) by molecular oxygen, a reaction catalyzed by luciferase. Stable transformants of Chinese hamster ovary cells carrying the Vargula luciferase gene secreted luciferase from discrete sites on the cell surface, and this secretion could be monitored in real time by the bioluminescence produced by the secreted luciferase in the presence of Vargula luciferin by using an image-intensifying technique. Addition of anti-Vargula luciferase IgG to the luminescing cells almost completely extinguished the luminescence, confirming that Vargula luciferase caused the luminescence.

Effects of brefeldin A on endocytosis, transcytosis and transport to the Golgi complex in polarized MDCK cells

We have studied the effects of brefeldin A (BFA) on endocytosis and intracellular traffic in polarized MDCK cells by using the galactose-binding protein toxin ricin as a membrane marker and HRP as a marker of fluid phase transport. We found that BFA treatment rapidly increased apical endocytosis of both ricin and HRP, whereas basolateral endocytosis was unaffected, as was endocytosis of HRP in the poorly polarized carcinoma cell lines HEp-2 and T47D. Tubular endosomes were induced by BFA both apically and basolaterally in some MDCK cells, comparable with those seen in HEp-2 and T47D cells. In addition, in MDCK cells, BFA induced formation of small (< 300 nm) vesicles, labeled both after apical and basolateral uptake of HRP, as well as some very large (> 700 nm) vacuoles, which were only labeled when HRP was present in the apical medium. In contrast, neither in MDCK nor in HEp-2 or T47D cells, did BFA have any effect on lysosomal morphology. Moreover, transcytosis in the basolateral-apical direction was stimulated both for HRP and ricin. Other vesicular transport routes were less affected or unaffected by BFA treatment. Two closely related structural analogues of BFA (B16 and B21), unable to produce the changes in Golgi and endosomal morphology seen after BFA treatment in a number of different cell lines, were also unable to mimic the effects of BFA on MDCK cells.

The morphology but not the function of endosomes and lysosomes is altered by brefeldin A

Brefeldin A (BFA) induces the formation of an extensively fused network of membranes derived from the trans-Golgi network (TGN) and early endosomes (EE). We describe in detail here the unaffected passage of endocytosed material through the fused TGN/EE compartments to lysosomes in BFA-treated cells. We also confirmed that BFA caused the formation of tubular lysosomes, although the kinetics and extent of tubulation varied greatly between different cell types. The BFA-induced tubular lysosomes were often seen to form simple networks. Formation of tubular lysosomes was microtubule-mediated and energy-dependent; interestingly, however, maintenance of the tubulated lysosomes only required microtubules and was insensitive to energy poisons. Upon removal of BFA, the tubular lysosomes rapidly recovered in an energy-dependent process. In most cell types examined, the extensive TGN/EE network is ephemeral, eventually collapsing into a compact cluster of tubulo-vesicular membranes in a process that precedes the formation of tubular lysosomes. However, in primary bovine testicular cells, the BFA-induced TGN/EE network was remarkably stable (for > 12 h). During this time, the TGN/EE network coexisted with tubular lysosomes, however, the two compartments remained completely separate. These results show that BFA has multiple, profound effects on the morphology of various compartments of the endosome-lysosome system. In spite of these changes, endocytic traffic can continue through the altered compartments suggesting that transport occurs through noncoated vesicles or through vesicles that are insensitive to BFA.

Plasticity of early endosomes

We observed that the structural organization of early endosomes was significantly modified after cell surface biotinylation followed by incubation in the presence of low concentrations of avidin. Under these conditions early endosomes increased in size to form structures which extended over several micrometers and which had an intra-luminal content with a characteristic electron-dense appearance. The modified early endosomes were not formed when either avidin or biotinylation was omitted, suggesting that they resulted from the cross-linking of internalized biotinylated proteins by avidin. Accumulation of a fluid-phase tracer was increased after the avidin-biotin treatment (145% after 45 min). Both recycling and transport to the late endosomes still occurred, albeit to a somewhat lower extent than in control cells. Quantitative electron microscopy showed that the volume of the endosomal compartment was increased approximately 1.5-fold but that the surface area of the compartment decreased relative to its volume after avidin-biotin treatment. Finally, overexpression of a rab5 mutant, which is known to inhibit early endosome fusion in vitro, prevented the formation of these structures in vivo and caused early endosome fragmentation. Altogether, our data suggest that early endosomes exhibit a high plasticity in vivo. Cross-linking appears to interfere with this dynamic process but does not arrest membrane traffic to/from early endosomes.

The small GTP-binding protein rab4 controls an early sorting event on the endocytic pathway

rab4 is a ras-like GTP-binding protein that associates with early endosomes in a cell cycle-dependent fashion. To determine its role during endocytosis, we generated stable cell lines that overexpressed mutant or wild-type rab4. By measuring endocytosis, transport to lysosomes, and recycling, we found that overexpression of wild-type rab4 had differential effects on the endocytic pathway. Although initial rates of internalization and degradation were not inhibited, the transfectants exhibited a 3-fold decrease in fluid phase endocytosis as well as an alteration in transferrin receptor (Tfn-R) recycling. Wild-type rab4 caused a redistribution of Tfn-R's from endosomes to the plasma membrane. It also blocked iron discharge by preventing the delivery of Tfn to acidic early endosomes, instead causing Tfn accumulation in a population of nonacidic vesicles and tubules. rab4 thus appears to control the function or formation of endosomes involved in recycling.

CLIP-170 links endocytic vesicles to microtubules

Binding of endocytic carrier vesicles to microtubules depends on the microtubule-binding protein CLIP-170 in vitro. In vivo, CLIP-170 colocalizes with a subset of transferrin receptor-positive endocytic structures and, more extensively, with endosomal tubules induced by brefeldin A. The structure of CLIP-170 has been analyzed by cloning its cDNA. The predicted non-helical C- and N-terminal domains of the homodimeric protein are connected by a long coiled-coil domain. We have identified a novel motif present in a tandem repeat in the N-terminal domain of CLIP-170 that is involved in binding to microtubules. This motif is also found in the Drosophila Glued and yeast BIK1 proteins. These features, together with its very elongated structure, suggest that CLIP-170 belongs to a novel class of proteins, cytoplasmic linker proteins (CLIPs), mediating interactions of organelles with microtubules.

In AtT20 and HeLa cells brefeldin A induces the fusion of tubular endosomes and changes their distribution and some of their endocytic properties

We have studied the effects of brefeldin A (BFA) on the tubular endosomes in AtT20 and HeLa cells (Tooze, J., and M. Hollinshead. 1991. J. Cell Biol. 115:635-653) by electron microscopy of cells labeled with three endocytic tracers, HRP, BSA-gold, and transferrin conjugated to HRP, and by immunofluorescence microscopy. For the latter we used antibodies specific for transferrin receptor, and, in the case of AtT20 cells, also antibodies specific for synaptophysin. In HeLa cells BFA at concentrations ranging from 1 micrograms to 10 micrograms/ml causes the dispersed patches of network of preexisting tubular early endosomes to be incorporated within 5 min into tubules approximately 50 nm in diameter but up to 40-50 microns long. These long, straight tubular endosomes are aligned along microtubules; they branch relatively infrequently to form an open network or reticulum extending from the cell periphery to the microtubule organizing center (MTOC). As the incubation with BFA is prolonged beyond 5 min, a steady state is reached in which many tubules are located in a dense network enclosing the centrioles, with branches extending in a more open network to the periphery. This effect of BFA, which is fully reversed within 15-30 min of washing out, is inhibited by pre-incubating the cells with sodium azide and 2-deoxy-D-glucose. In AtT20 cells BFA at 5 micrograms/ml or above causes the same sorts of changes, preexisting tubular endosomes are recruited into a more continuous endosomal network, and there is a massive accumulation of this network around the MTOC. Maintenance of the BFA-induced endosomal reticulum in both cell types is dependent upon the integrity of microtubules. In AtT20 cells BFA at 1 microgram/ml has no detectable effect on the early endosomal system but the Golgi stacks are converted to clusters of tubules and vesicles that remain in the region of the MTOC during prolonged incubations. Therefore, the Golgi apparatus in these cells is more sensitive to BFA than the early endosomes. The morphological evidence suggests that all the tubular early endosomes in BFA-treated HeLa and AtT20 cells are linked together in a single reticulum. Consistent with this, incubations as short as 1-3 min with 10 or 20 mg/ml HRP in the medium result in the entire endosomal reticulum in most of the BFA-treated cells being filled with HRP reaction product.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of vesicular and tubular membrane traffic of the Golgi complex by coat proteins

Transport of cargo through and from the Golgi complex is mediated by vesicular carriers and transient tubular connections. Two classes of vesicle have been implicated in the biosynthetic or anterograde membrane traffic of this organelle. Both classes of vesicle are coated on the cytoplasmic surface with proteins, of which at least one component is related. Tubular connections also enable exchange of material between membrane-bounded compartments associated with the Golgi complex, most obviously in cells that have been treated with the drug, brefeldin A. Coat proteins appear to be involved in the regulation of these transport processes. Their putative functions include sorting of cargo, as well as regulation of budding, fusion or targeting of the membrane carriers.

Brefeldin A affects early events but does not affect late events along the exocytic pathway in pancreatic acinar cells

Brefeldin A (BFA) blocks protein export from the endoplasmic reticulum (ER) to Golgi complex and causes dismantling of the Golgi complex with relocation of resident Golgi proteins to the ER in some cultured cells. It is not known whether later steps in the secretory process are affected. We previously have shown that in BFA-treated rat pancreatic lobules, there is no detectable relocation of Golgi proteins to the ER and, although Golgi cisternae are rapidly dismantled, clusters of small smooth vesicles consisting of both bona fide Golgi remnants and associated vesicular carriers persist even with prolonged BFA exposure. We now report the effects of BFA on transport of proteins through the secretory pathway in exocrine pancreatic cells; we pulse-labeled pancreatic lobules with [35S]methionine and then chased for various times before adding BFA. When BFA was added at pulse, treated lobules released less than 10% of radioactive protein in comparison with controls, regardless of whether or not the lobule cultures were stimulated with carbamoylcholine. However, when lobules were pulsed and then chased for 30, 45, or 60 min before BFA addition, the amount of labeled protein released was comparable in both BFA-treated and untreated cultures. Furthermore, the kinetics and amounts of basal and carbamoylcholine-stimulated release of unlabeled alpha-amylase from storage in zymogen granules were similar in both control and BFA-treated lobules. Therefore, in the rat pancreas, BFA blocks ER to Golgi transport but does not affect later stages along the secretory pathway, including intra-Golgi transport, exit from the Golgi complex, formation and concentration of secretory granules, and exocytosis.

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

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

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

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

Brefeldin A and the endocytic pathway. Possible implications for membrane traffic and sorting

A number of recent observations have suggested that the endocytic and biosynthetic pathways may share fundamentally similar transport mechanisms at the molecular level. Some of the more striking of these suggestions have come from a comparison of the effects of the macrocyclic lactone brefeldin A (BFA) on endosomes and the Golgi complex. BFA is thought to affect Golgi-specific coat proteins that may be involved in maintaining the structural integrity of the organelle and in regulating membrane transport in the secretory pathway. Many of the effects of BFA on the endocytic system, such as the guanine nucleotide and aluminum fluoride (AlF4-)-regulated induction of microtubule-dependent endosomal tubules, are strikingly reminiscent of the action of the drug on the Golgi complex. Therefore, the similar mechanisms of action of the drug on endosomes suggest that organelles of the endocytic pathway may be associated with similar cytoplasmic coats that could regulate endosome function and integrity.

Brefeldin A affects the cellular distribution of endocytic receptors differentially

The cell surface expression of three endocytic receptors was studied in human hepatoma Hep G2 cells treated with brefeldin A (BFA). Ligand binding and cell surface iodination revealed that BFA increased the number of mannose 6-phosphate/insulin-like growth factor II receptors twofold and decreased the amount of asialoglycoprotein and transferrin receptors by 40-60%. The altered expression of receptors at the cell surface was paralleled by changes in the respective ligand uptake. The implications of this finding on our understanding of intracellular trafficking are discussed.

100-kD proteins of Golgi- and trans-Golgi network-associated coated vesicles have related but distinct membrane binding properties

The 100-110-kD proteins (alpha-, beta-, beta'-, and gamma-adaptins) of clathrin-coated vesicles and the 110-kD protein (beta-COP) of the nonclathrin-coated vesicles that mediate constitutive transport through the Golgi have homologous protein sequences. To determine whether homologous processes are involved in assembly of the two types of coated vesicles, the membrane binding properties of their coat proteins were compared. After treatment of MDBK cells with the fungal metabolite Brefeldin A (BFA), beta-COP was redistributed to the cytoplasm within 15 s, gamma-adaptin and clathrin in the trans-Golgi network (TGN) dispersed within 30 s, but the alpha-adaptin and clathrin present on coated pits and vesicles derived from the plasma membrane remained membrane associated even after a 15-min exposure to BFA. In PtK1 cells and MDCK cells, BFA did not affect beta-COP binding or Golgi morphology but still induced redistribution of gamma-adaptin and clathrin from TGN membranes to the cytoplasm. Thus BFA affects the binding of coat proteins to membranes in the Golgi region (Golgi apparatus and TGN) but not plasma membranes. However, the Golgi binding interactions of beta-COP and gamma-adaptin are distinct and differentially sensitive to BFA. BFA treatment did not release gamma-adaptin or clathrin from purified clathrin-coated vesicles, suggesting that their distribution to the cytoplasm after BFA treatment of cells was due to interference with their rebinding to TGN membranes after a normal cycle of disassembly. This was confirmed using an in vitro assay in which gamma-adaptin binding to TGN membranes was blocked by BFA and enhanced by GTP gamma S, similar to the binding of beta-COP to Golgi membranes. These results suggest the involvement of GTP-dependent proteins in the association of the 100-kD coat proteins with membranes in the Golgi region of the cell.

A brefeldin A-like phenotype is induced by the overexpression of a human ERD-2-like protein, ELP-1

Brefeldin A (BFA) is a unique drug affecting the molecular mechanisms that regulate membrane traffic and organelle structure. BFA's ability to alter retrograde traffic from the Golgi to the endoplasmic reticulum (ER) led us to ask whether the ERD-2 retrieval receptor, proposed to return escaped ER resident proteins from the Golgi, might either interfere with or mimic the effects of the drug. When either human ERD-2 or a novel human homolog (referred to as ELP-1) is overexpressed in a variety of cell types, the effects are phenotypically indistinguishable from the addition of BFA. These include the redistribution of the Golgi coat protein, beta-COP, to the cytosol, the loss of the Golgi apparatus as a distinct organelle, the mixing of this organelle with the ER, the addition of complex oligosaccharides to resident ER glycoproteins, and the block of anterograde traffic. Thus, these receptors may provide signals that regulate retrograde traffic between the Golgi and the ER.

Identification, molecular characterization and immunolocalization of an isoform of the trans-Golgi-network (TGN)-specific integral membrane protein TGN38

TGN38 is an integral membrane protein previously shown to be predominantly localized to the trans-Golgi network (TGN) of cells by virtue of a signal contained within its cytoplasmic 'tail' [Luzio, Brake, Banting, Howell, Braghetta & Stanley (1990) Biochem. J. 270, 97-102]. We now (i) describe the isolation of cDNA clones encoding an isoform of TGN38, (ii) present the sequence of that isoform and (iii) describe the production and use of antibodies which specifically recognize the new isoform. This isoform, designated TGN41, is also predominantly localized to the TGN. The only sequence differences between the protein coding regions of cDNA clones encoding TGN38 and those encoding TGN41 occur within the region specifying the cytoplasmic tails of the two proteins. The TGN localization signal is shown to be within the sequence common to both proteins.

Recruitment of coat proteins onto Golgi membranes in intact and permeabilized cells: effects of brefeldin A and G protein activators

Brefeldin A (BFA) causes a rapid redistribution of coat proteins (e.g., gamma-adaptin) associated with the clathrin-coated vesicles that bud from the trans-Golgi network (TGN), while the clathrin-coated vesicles that bud from the plasma membrane are unaffected. gamma-Adaptin redistributes with the same kinetics as beta-COP, a coat protein associated with the non-clathrin-coated vesicles that bud from the Golgi complex. Upon removal of BFA, however, gamma-adaptin recovers its perinuclear distribution more rapidly. Redistribution of both proteins can be prevented by pretreating cells with AlF4-. Recruitment of adaptors from the cytosol onto the TGN membrane has been reconstituted in a permeabilized cell system and is increased by addition of GTP gamma S and blocked by addition of BFA. These results suggest a role for G proteins in the control of the clathrin-coated vesicle cycle at the TGN and further extend the similarities between clathrin-coated vesicles and non-clathrin-coated vesicles.

Effects of brefeldin A on the synthesis of chondroitin 4-sulfate by cultures of mouse mastocytoma cells

Mouse mastocytoma cells were cultured with brefeldin A in medium containing [35S]sulfate and [3H]glucosamine in order to determine the effects of this fungal metabolite on the formation of chondroitin 4-sulfate by these cells. There was a marked reduction in the incorporation of [35S]sulfate into the glycosaminoglycan which was approximately equal to the reduction in the incorporation of [3H]hexosamine into the same molecule. The chondroitin 4-sulfate chain size was greatly diminished, while the number of chains appeared to remain relatively constant, indicating that the brefeldin A partially disrupted the polymerizing system, but had little effect upon movement of the nascent proteochondroitin to the site for chondroitin polymerization and sulfation.

Brefeldin A's effects on endosomes, lysosomes, and the TGN suggest a general mechanism for regulating organelle structure and membrane traffic

Addition of brefeldin A (BFA) to most cells results in both the formation of extensive, uncoated membrane tubules through which Golgi components redistribute into the ER and the failure to transport molecules out of this mixed ER/Golgi system. In this study we provide evidence that suggests BFA's effects are not limited to the Golgi apparatus but are reiterated throughout the central vacuolar system. Addition of BFA to cells resulted in the tubulation of the endosomal system, the trans-Golgi network (TGN), and lysosomes. Tubule formation of these organelles was specific to BFA, shared near identical pharmacologic characteristics as Golgi tubules and resulted in targeted membrane fusion. Analogous to the mixing of the Golgi with the ER during BFA treatment, the TGN mixed with the recycling endosomal system. This mixed system remained functional with normal cycling between plasma membrane and endosomes, but traffic between endosomes and lysosomes was impaired.