Selective inhibition of transcytosis by brefeldin A in MDCK 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.

Transport through the yeast endocytic pathway occurs through morphologically distinct compartments and requires an active secretory pathway and Sec18p/N-ethylmaleimide-sensitive fusion protein

Molecules travel through the yeast endocytic pathway from the cell surface to the lysosome-like vacuole by passing through two sequential intermediates. Immunofluorescent detection of an endocytosed pheromone receptor was used to morphologically identify these intermediates, the early and late endosomes. The early endosome is a peripheral organelle that is heterogeneous in appearance, whereas the late endosome is a large perivacuolar compartment that corresponds to the prevacuolar compartment previously shown to be an endocytic intermediate. We demonstrate that inhibiting transport through the early secretory pathway in sec mutants quickly impedes transport from the early endosome. Treatment of sensitive cells with brefeldin A also blocks transport from this compartment. We provide evidence that Sec18p/N-ethylmaleimide-sensitive fusion protein, a protein required for membrane fusion, is directly required in vivo for forward transport early in the endocytic pathway. Inhibiting protein synthesis does not affect transport from the early endosome but causes endocytosed proteins to accumulate in the late endosome. As newly synthesized proteins and the late steps of secretion are not required for early to late endosome transport, but endoplasmic reticulum through Golgi traffic is, we propose that efficient forward transport in the early endocytic pathway requires delivery of lipid from secretory organelles to endosomes.

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.

Characterization of the endogenous mono-ADP-ribosylation stimulated by brefeldin A

We have recently described a novel enzymatic mono-ADP-ribosyl transfer reaction induced by brefeldin A, a well characterized inhibitor of vesicular traffic, which selectively modifies two cytosolic proteins of 38 kDa (p38) and 50 kDa (BARS-50). p38 was identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme and a multifunctional protein involved in several cellular processes; BARS-50 might be a novel G protein, since it is able to bind GTP and the beta gamma subunit of G proteins. We have characterized this enzymatic activity and screened in vitro the effects of different drugs belonging to the coumarine (dicumarol, coumermicin A1 and novobiocin) and quinone (ilimaquinones, benzoquinones and naphtoquinones) class. These drugs blocked the BFA-dependent mono-ADP-ribosylation, showed remarkable effects on Golgi morphology in control cells, and antagonized the tubular reticular redistribution of the Golgi complex in brefeldin A treated cells (see papers of Corda and Colanzi in this issue) suggesting a possible role for ADP-ribosylation in both the cellular effects of brefeldin A and the maintenance of the structure/function of the Golgi complex.

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.

Intracellular distribution of Arf proteins in mammalian cells. Arf6 is uniquely localized to the plasma membrane

Subcellular distributions of the five human Arf proteins were examined, using a set of isoform-specific polyclonal and a pan-Arf monoclonal antibodies. Subcellular fractionation of cultured mammalian cells allowed the demonstration that Arf6 is uniquely localized to the plasma membranes of Chinese hamster ovary cells. The plasma membrane distrubution was unaffected by either GTPgammaS (guanosine 5'-O-(3-thio)triphosphate) or brefeldin A, an activator and inhibitor of Arf activities, respectively. In contrast, Arf proteins 1, 3, 4, and 5 were predominantly cytosolic but could be recruited to a variety of intracellular membranes, but not plasma membranes, upon incubation in the presence of GTPgammaS. The GTPgammaS-promoted binding of the cytosolic Arf proteins to membranes was blocked by brefeldin A. The stable association of Arf6 with plasma membranes and the insensitivity of its localization to either GTPgammaS or brefeldin A revealed a clear distinction between Arf6 and the other Arf isoforms. Localization of Arf6 to the plasma membrane suggests a unique cellular role for this isoform at the plasma membrane, but failure to find endogenous Arf6 on endocytic structures, including clathrin-coated vesicles, appears inconsistent with the proposed role of Arf6 in assembly of coat structures or endosomes in transfected fibroblasts (1,2).

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.

Functional dissociation of paracellular permeability and transepithelial electrical resistance and disruption of the apical-basolateral intramembrane diffusion barrier by expression of a mutant tight junction membrane protein

Tight junctions, the most apical of the intercellular junctions that connect individual cells in a epithelial sheet, are thought to form a seal that restricts paracellular and intramembrane diffusion. To analyze the functioning of tight junctions, we generated stable MDCK strain 2 cell lines expressing either full-length or COOH-terminally truncated chicken occludin, the only known transmembrane component of tight junctions. Confocal immunofluorescence and immunoelectron microscopy demonstrated that mutant occludin was incorporated into tight junctions but, in contrast to full-length chicken occludin, exhibited a discontinuous junctional staining pattern and also disrupted the continuous junctional ring formed by endogenous occludin. This rearrangement of occludin was not paralleled by apparent changes in the junctional morphology as seen by thin section electron microscopy nor apparent discontinuities of the junctional strands observed by freeze-fracture. Nevertheless, expression of both wild-type and mutant occludin induced increased transepithelial electrical resistance (TER). In contrast to TER, particularly the expression of COOH-terminally truncated occludin led to a severalfold increase in paracellular flux of small molecular weight tracers. Since the selectivity for size or different types of cations was unchanged, expression of wild-type and mutant occludin appears to have activated an existing mechanism that allows selective paracellular flux in the presence of electrically sealed tight junctions. Occludin is also involved in the formation of the apical/basolateral intramembrane diffusion barrier, since expression of the COOH-terminally truncated occludin was found to render MDCK cells incapable of maintaining a fluorescent lipid in a specifically labeled cell surface domain.

Analysis of the role of p200-containing vesicles in post-Golgi traffic

p200 is a cytoplasmic protein that associates with vesicles budding from the trans-golgi network (TGN). The protein was identified by a monoclonal antibody AD7. We have used this antibody to analyze whether p200 functions in exocytic transport from the TGN to the apical or basolateral plasma membrane in Madin-Darby canine kidney cells. We found that transport of the viral marker proteins, influenza hemagglutinin (HA) to the apical surface or vesicular stomatitis virus glycoprotein (VSV G) to the basolateral surface in streptolysin O-permeabilized cells was not affected when p200 was depleted from both the membranes and the cytosol. When vesicles isolated from perforated cells were analyzed by equilibrium density gradient centrifugation, the p200 immunoreactive membranes did not comigrate with either the apical vesicle marker HA or the basolateral vesicle marker VSV G. Immunoelectron microscopy of perforated and double-labeled cells showed that the p200 positive vesicular profiles were not labeled by antibodies to HA or VSV G when the viral proteins were accumulated in the TGN. Furthermore, the p200-decorated vesicles were more electron dense than those labeled with the viral antibodies. Together, these results suggest that p200 does not function in the transport pathways that carry HA from the TGN to the apical surface or VSV G from the TGN to the basolateral surface.

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.

Cytoplasmic coat proteins involved in endosome function

Endosomes are intermediates for a complex series of sorting and transport events that occur during receptor-mediated endocytosis. These involve the recognition of targeting determinants on the cytoplasmic domains of many membrane proteins as well as the formations of specific transport vesicles. Accordingly, endosome function is likely to be governed by the regulated assembly of cytoplasmic coat complexes. We have found that, in vitro, endosomes recruit a characteristic set of cytoplasmic proteins in a GTP gamma S-stimulated and brefeldin A-sensitive fashion. Among these are members of the COP-I and ARF families of coat proteins. In addition, endosomes were also found to assemble distinct, clathrin-like coats. Since microinjection of antibodies to beta-COP inhibits the entry of enveloped viruses via the endocytic pathway, it is apparent that the recruitment of COP-I or COP-I-related proteins plays an important role in the function of endosomes in intact cells.

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits transcytosis in polarized epithelial cells

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits both basolateral to apical and apical to basolateral transcytosis of ricin in Fisher rat thyroid (FRT) cells by 50% at 100 nM in a continuous transcytosis assay. In MDCK cells, a similar effect of wortmannin on basolateral to apical transcytosis of ricin was found, whereas apical to basolateral transcytosis was inhibited to a lesser degree. Transcytosis of dimeric IgA in MDCK cells expressing the polymeric immunoglobulin receptor was also reduced to 50% of controls, suggesting that wortmannin inhibits membrane translocation rather than sorting of specific proteins in the transcytotic pathway. This effect of wortmannin is selective, however, in that endocytosis at the basolateral domain and recycling at both the basolateral and apical membrane domains are unaffected, and apical endocytosis and apical secretion are only moderately reduced. We have shown previously that cAMP stimulates a late stage in basolateral to apical transcytosis in MDCK cells through activation of protein kinase A (Hansen, S. H., and Casanova, J.E. (1994) J. Cell Biol. 126, 677-687). Elevation of cellular cAMP still induced a 100% increase in transcytosis in wortmannin-treated cells, but transcytosis was no longer increased when compared to cells which received no drugs. In contrast, in experiments using a 17 degrees C block to accumulate ricin internalized from the basolateral surface in the apical compartment of MDCK cells, wortmannin had little effect on the stimulation of transcytosis by activators of protein kinase A observed under these conditions. The data thus suggest the existence of a wortmannin-sensitive step in the transcytotic pathway, positioned after endocytosis but prior to translocation into the protein kinase A-sensitive apical compartment, implying a role for phosphoinositide 3-kinase in an intermediate step in transcytosis in polarized epithelial cells.

Sorting and intracellular trafficking of a glycosylphosphatidylinositol-anchored protein and two hybrid transmembrane proteins with the same ectodomain in Madin-Darby canine kidney epithelial cells

We compared the trafficking of the glycosylphosphatidylinositol (GPI)-anchored placental alkaline phosphatase (PLAP) and two chimeric transmembrane proteins containing the PLAP ectodomain in stably transfected Madin-Darby canine kidney epithelial cells to determine whether different mechanisms might be used in apical sorting of GPI-anchored and transmembrane proteins. PLAP-G, which contained the transmembrane and cytoplasmic domains of the vesicular stomatitis virus glycoprotein, was delivered directly to the basolateral surface. PLAP-HA contained the transmembrane and cytoplasmic domains of influenza hemagglutinin. Both PLAP and PLAP-HA were delivered directly to the apical membrane. PLAP becomes insoluble in Triton X-100 during biosynthetic transport, as it associates with detergent-resistant membranes. Neither hybrid protein was detergent insoluble, though the small amount of PLAP that was missorted to the basolateral surface was insoluble. We examined the effects of three drugs known to interfere with membrane trafficking on sorting and delivery of PLAP and the hybrid proteins. Monensin had no effect on sorting or surface expression of any of the proteins. Nocodazole affected the sorting of both PLAP and PLAP-HA but not of PLAP-G. Brefeldin A appeared to disrupt the sorting of PLAP and PLAP-HA but not of PLAP-G. This conclusion was tempered by the observation that this drug affected the distribution of proteins at the cell surface. Thus, sorting and transport of GPI-anchored and apical transmembrane proteins are similar in a number of respects.

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.

A role for calmodulin in organelle membrane tubulation

Membrane tubules of uniform diameter (60-80 nm) and variable lengths have been seen to extend from the main bodies of the Golgi complex, trans Golgi network (TGN), and endosomes. In the case of endosomes, these tubules appear to mediate membrane and receptor recycling events. Brefeldin A (BFA) is a potent drug that completely blocks coated vesicle formation from the Golgi complex and TGN, but at the same time causes the enhanced formation of membrane tubules from these same organelles. Recently, experiments have shown that calmodulin antagonists inhibit the transport of receptors out of endosomes, perhaps by inhibiting the formation of recycling tubules. Using the potent calmodulin-specific antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), N-(4-aminobutyl)-5-chloro-2-naphthalenesulfonamide (W-13), and N-(4-aminobutyl)-5-chloro-1-naphthalenesulfonamide (C-1), we found that the recycling of transferrin from endosomes to the cell surface was significantly inhibited, resulting in the formation of enlarged endosomal vacuoles. In addition, these same calmodulin antagonists also potently inhibited the formation of BFA-stimulated membrane tubules from the Golgi complex, TGN, and endosomes. In the case of the Golgi complex, failure to form tubules resulted in the inhibition of BFA-stimulated retrograde transport to the endoplasmic reticulum. These results suggest that calmodulin is a general regulator of membrane tubulation and is capable of influencing the morphology of several organelles.

Cytotoxicity of brefeldin A correlates with its inhibitory effect on membrane binding of COP coat proteins

The fungal metabolite brefeldin A (BFA) causes the inhibition of protein secretion and the disruption of the structure and function of organelles along the exocytic and endocytic pathways including the Golgi complex. Such effects of BFA have been ascribed in large part to its ability to prevent recruitment of cytosolic coat proteins onto organelle membranes. Here we show that mammalian cell lines differ from one another with respect to sensitivity to this drug. The BFA sensitivity of a given cell line appears to be dependent on the species or the order from which the cell line originates, rather than on the cell line itself. In each cell line, the dose of BFA required for inhibition of cell growth and of protein secretion correlates with the dose required for inhibition of binding of beta-COP, a coat protein of COP-coated vesicles, but not that for inhibition of binding of gamma-adaptin, a component of HA-I/AP-1 adaptor of clathrin-coated vesicles. These observations suggest that: (i) there are at least two targets for BFA that differ from each other in sensitivity to this drug, (ii) the difference in the sensitivity to BFA of the beta-COP binding is determined by the difference in the structure of a target protein for this drug, and (iii) the cytotoxicity of BFA is ascribed mainly to its inhibitory effect on the membrane binding of COP-coat proteins.

Trafficking of cell surface beta-amyloid precursor protein: retrograde and transcytotic transport in cultured neurons

Amyloid beta-protein (A beta), the principal constituent of senile plaques seen in Alzheimer's disease (AD), is derived by proteolysis from the beta-amyloid precursor protein (beta PP). The mechanism of A beta production in neurons, which are hypothesized to be a rich source of A beta in brain, remains to be defined. In this study, we describe a detailed localization of cell surface beta PP and its subsequent trafficking in primary cultured neurons. Full-length cell surface beta PP was present primarily on perikarya and axons, the latter with a characteristic discontinuous pattern. At growth cones, cell surface beta PP was inconsistently detected. By visualizing the distribution of beta PP monoclonal antibodies added to intact cultures, beta PP was shown to be internalized from distal axons or terminals and retrogradely transported back to perikarya in organelles which colocalized with fluid-phase endocytic markers. Retrograde transport of beta PP was shown in both hippocampal and peripheral sympathetic neurons, the latter using a compartment culture system that isolated cell bodies from distal axons and terminals. In addition, we demonstrated that beta PP from distal axons was transcytotically transported to the surface of perikarya from distal axons in sympathetic neurons. Indirect evidence of this transcytotic pathway was obtained in hippocampal neurons using antisense oligonucleotide to the kinesin heavy chain to inhibit anterograde beta PP transport. Taken together, these results demonstrate novel aspects of beta PP trafficking in neurons, including retrograde axonal transport and transcytosis. Moreover, the axonal predominance of cell surface beta PP is unexpected in view of the recent report of polarized sorting of beta PP to the basolateral domain of MDCK cells.

Ceramide reverses brefeldin A (BFA) resistance in BFA-resistant cell lines

We have found that C6 ceramide, a cell-permeable ceramide analog, partially restored the brefeldin A (BFA) sensitivity in a BFA-resistant mutant of Vero cells (BER-40) and in the naturally BFA-resistant Madin-Darby canine kidney (MDCK) cells. Incubation of BER-40 and MDCK cells with low concentrations of C6 ceramide resulted in (i) a pronounced increase in BFA cytotoxicity as measured by the inhibition of [3H]thymidine incorporation and the inhibition of colony formation by BFA, (ii) a significant protection by BFA against ricin cytotoxicity, and (iii) an inhibition of bulk protein secretion by BFA in BER-40 and MDCK cells. Related sphingolipids including sphingosine, sphingomyelin, and lactosylceramide and other unrelated lipid second messengers such as arachidonic acid and 1,2-diacylglycerol did not elicit the protection of BER-40 and MDCK cells against ricin cytotoxicity by BFA. C6 ceramide was the most effective among the ceramides with different acyl chain lengths. Interestingly, dihydro-C6 ceramide, which lacks the trans double bond in the sphingoid base, had no effect. On the other hand, C6 ceramide did not enhance BFA sensitivity in BFA-sensitive Vero cells. The LD50 of C6 ceramide were similar in Vero and BER-40 cells. Fluorescence microscopic studies revealed that C6 ceramide induced the redistribution of beta-COP from the Golgi membranes to a more dispersed localization in both BFA-sensitive and BFA-resistant cell lines, mimicking the effect of BFA. Suboptimal concentration of C6 ceramide also restored the effect of BFA on the beta-COP distribution in BER-40 and MDCK cells. These results indicate that C6 ceramide restores the BFA sensitivity in BFA-resistant BER-40 and MDCK cells.

A fluorescent lipid analogue can be used to monitor secretory activity and for isolation of mammalian secretion mutants

The use of reporter proteins to study the regulation of secretion has often been complicated by posttranslational processing events that influence the secretion of certain proteins, but are not part of the cellular mechanisms that specifically regulate secretion. This has been a particular limitation for the isolation of mammalian secretion mutants, which has typically been a slow process. To provide a reporter of secretory activity independent of protein processing events, cells were labeled with the fluorescent lipid analogue C5-DMB-ceramide (ceramide coupled to the fluorophore boron dipyrromethene difluoride) and its secretion was followed by fluorescence microscopy and fluorescence-activated cell sorting. Brefeldin A, which severely inhibits secretion in Chinese hamster ovary cells, blocked secretion of C5-DMB-ceramide. At high temperature, export of C5-DMB-ceramide was inhibited in HRP-1 cells, which have a conditional defect in secretion. Using C5-DMB-ceramide as a reporter of secretory activity, several different pulse-chase protocols were designed that selected mutant Chinese hamster ovary cells that were resistant to the drug brefeldin A and others that were defective in the transport of glycoproteins to the cell surface. Mutant cells of either type were identified in a mutagenized population at a frequency of 10(-6). Thus, the fluorescent lipid C5-DMB-ceramide can be used as a specific marker of secretory activity, providing an efficient, general approach for isolating mammalian cells with defects in the secretory pathway.

Role of clathrin-coated vesicles in glycoprotein transport from the cell surface to the Golgi complex

Plasma membrane glycoproteins recycle to the Golgi complex, but the route followed by these proteins is not known. To elucidate the pathway of transport, the involvement of clathrin-coated vesicles was tested. This was accomplished by comparing the traffic of wild type low density lipoprotein receptor (LDLR) and FH 683, a mutant receptor whose endocytosis from the cell surface in coated vesicles is reduced by 90-95%. Wild type LDLR traveled from the cell surface to the sialyltransferase compartment of the Golgi with a half-time of 2.5 h in K562 human leukemia cells expressing receptor from a transfected cDNA. In contrast, FH 683 LDLR recycled to the Golgi at 33% of the wild type rate, suggesting that wild type LDLR is largely transported to the Golgi by a pathway that involves clathrin-coated vesicles. Moreover, because clathrin-coated vesicles that bud from the plasma membrane are transported to endosomes, surface-to-Golgi transport probably involves an endosomal intermediate. Finally, because there was substantial transport of mutant LDLR to the Golgi even though its endocytosis in coated vesicles was greatly reduced, there may be a second pathway of surface-to-Golgi traffic. Our results suggest that wild type LDLR may move from plasma membrane to Golgi by two routes. Two-thirds of the traffic proceeds via a coated vesicle-mediated pathway while the remainder may follow a clathrin-independent pathway.

Brefeldin A inhibits insulin-dependent receptor redistribution in HIRcB cells

Brefeldin A (BFA) is a potent inhibitor of intracellular vesicle traffic. We have investigated the effects of BFA on the traffic of the insulin receptor in HIRcB cells, a cell line derived from Rat-1 fibroblasts that over-expresses a normal human insulin receptor. We report here that insulin-dependent receptor redistribution is inhibited by BFA and that this drug has no effects on the insulin-dependent redistribution of the receptor. Auto-phosphorylation of the insulin receptor and the stimulation of mitogen-activated protein kinase (MAPK) by insulin were not affected by treatment with the drug. The effects of BFA were further shown to require addition of the drug prior to the addition of insulin. BFA added 10 min after stimulation with insulin had no effects on the redistribution of the receptor. Dose-response studies demonstrated that the effects of BFA were half-maximal at a dose of 1 microgram/ml and maximal at about 10 micrograms/ml. These findings suggest that BFA blocks an early step in the chain of events that lead to insulin receptor internalization without affecting the interactions of the receptor with insulin, the stimulation of the tyrosine kinase activity of the receptor by the hormone, or other insulin-regulated signalling pathways, such as the activation of MAPK.

Analysis of transport and targeting of syndecan-1: effect of cytoplasmic tail deletions

Madin-Darby canine kidney (MDCK) cells and Chinese hamster ovary (CHO) cells were transfected with wild-type and cytoplasmic deletion mutants of mouse syndecan-1 to study the requirements for transport and polarized expression of this proteoglycan. Expression in MDCK cells revealed that wild-type syndecan-1 is directed to the basolateral surface via a brefeldin A-insensitive route. A deletion of the last 12 amino acids of the syndecan-1 cytoplasmic tail (CT22) was sufficient to result in the appearance of mutant proteoglycans at both the basolateral and apical cell surfaces. Treatment with brefeldin A was able to prevent apical transport of the mutants. We thus propose that the C-terminal part of the cytoplasmic tail is required for steady-state basolateral distribution of syndecan-1. In CHO cells a deletion of the last 25 or 33 amino acids of the 34-residue cytoplasmic domain (CT9 and CT1, respectively) resulted in partial retention of the mutants in the endoplasmic reticulum (ER). A deletion mutant lacking the last 12 amino acids (CT22) was not retained. Interestingly, the unglycosylated core proteins of the CT9 and CT1 mutants showed a significantly lower apparent molecular weight when analyzed by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis than wild-type syndecan-1. However, when CHO transfectants expressing the CT1 mutant were incubated with brefeldin A, causing fusion of the ER and Golgi, CT1 ran with an almost equally high apparent molecular weight as the wild-type molecule. This would suggest that syndecan-1 undergoes extensive posttranslational modifications or forms an SDS-resistant dimer/complex after transit from the ER.

Golgi spectrin: identification of an erythroid beta-spectrin homolog associated with the Golgi complex

Spectrin is a major component of a membrane-associated cytoskeleton involved in the maintenance of membrane structural integrity and the generation of functionally distinct membrane protein domains. Here, we show that a homolog of erythrocyte beta-spectrin (beta I sigma*) co-localizes with markers of the Golgi complex in a variety of cell types, and that microinjected beta-spectrin codistributes with elements of the Golgi complex. Significantly, we show a dynamic relationship between beta-spectrin and the structural and functional organization of the Golgi complex. Disruption of both Golgi structure and function, either in mitotic cells or following addition of brefeldin A, is accompanied by loss of beta-spectrin from Golgi membranes and dispersal in the cytoplasm. In contrast, perturbation of Golgi structure without a loss of function, by the addition of nocodazole, results in retention of beta-spectrin with the dispersed Golgi elements. These results indicate that the association of beta-spectrin with Golgi membranes is coupled to Golgi organization and function.

Fluorescent choleretic and cholestatic bile salts take different paths across the hepatocyte: transcytosis of glycolithocholate leads to an extensive redistribution of annexin II

We have used fluorescent derivatives of the choleretic bile salts cholate and chenodeoxycholate, the cholestatic salt lithocholate, and the therapeutic agent ursodeoxycholate to visualize distinct routes of transport across the hepatocyte and delivery to the canalicular vacuole of isolated hepatocyte couplets. The cholate and chenodeoxycholate derivatives produced homogeneous intracellular fluorescence and were rapidly transported to the vacuole, while the lithocholate analogue accumulated more slowly in the canalicular vacuole and gave rise to punctate fluorescence within the cell. Fluorescent ursodeoxycholate showed punctate intracellular fluorescence against a high uniform background indicating use of both pathways. Inhibition of vesicular transport by treatment with colchicine and Brefeldin A had no effect on the uptake of any of the compounds used, but it dramatically impaired delivery of both the lithocholate and the ursodeoxycholate derivatives to the canalicular vacuole. We conclude that while the chenodeoxycholate and cholate analogues traverse the hepatocyte by a cytoplasmic route, lithocholate and ursodeoxycholate analogues are transported by vesicle-mediated transcytosis. Treatment of couplets with glycine derivatives of lithocholate and ursodeoxycholate, but not cholate or chenodeoxycholate, led to a marked relocalization of annexin II, which initially became concentrated at the basolateral membrane, then moved to a perinuclear distribution and finally to the apical membrane as the incubation progressed. This suggests that lithocholate and ursodeoxycholate treatment leads to a rapid induction of transcytosis and that annexin II exchange occurs upon membrane fusion at all stages of the hepatocyte transcytotic pathway. These results indicate that isolated hepatocyte couplets may provide an inducible model system for the study of vesicle-mediated transcytosis.

Mechanisms of cell polarity: sorting and transport in epithelial cells

The recent discovery of widely distributed targeting determinants, which govern the polarized cell-surface distribution of plasma membrane proteins in epithelial cells, has significantly changed our view of how polarized cells generate functionally distinct membrane domains. Together with the surprising finding that the same determinants are recognized on both the biosynthetic and the endocytic pathways, it now appears likely that a common epigenetic code may exist that controls molecular sorting of membrane proteins in a wide variety of polarized, and perhaps even non-polarized, cell types.

Gs alpha stimulates transcytosis and apical secretion in MDCK cells through cAMP and protein kinase A

Recent evidence suggests a role for heterotrimeric G proteins in vesicular transport. Cholera toxin, which activates Gs alpha by ADP-ribosylation, has been reported to stimulate both apical secretion (Pimplikar, S.W., and K. Simons. 1993. Nature (Lond.). 352:456-458) and apically directed transcytosis (Bomsel, M., and K.E. Mostov. 1993. J. Biol. Chem. 268:25824-25835) in MDCK cells, via a cAMP-independent mechanism. Here, we demonstrate that apical secretion and apically directed transcytosis are significantly stimulated by agents that elevate cellular cAMP. Forskolin, which activates adenylyl cyclase directly, and 8BrcAMP augment both transport processes in MDCK cells. The increase is not limited to receptor-mediated transport (polymeric Ig receptor), since transcytosis of ricin, a galactose-binding lectin, is similarly stimulated. The effects of elevated cellular cAMP on apical secretion and transcytosis are apparently mediated via protein kinase A (PKA), as they are inhibited by H-89, a selective PKA inhibitor. Experiments employing a 17 degrees C temperature block indicate that cAMP/PKA acts at a late, possibly rate-limiting stage in the transcytotic pathway, after translocation of internalized markers into the apical cytoplasm. However, no significant stimulus of apical recycling was observed in the presence of FSK, suggesting that cAMP/PKA either affects transcytosis at a level proximal to apical early endosomes and/or specifically increases the efficiency by which transcytosing molecules are delivered to the apical plasma membrane. Finally, we overexpressed wild-type Gs alpha and a mutant, Q227L, which constitutively activates adenylyl cyclase, in MDCK cells. Although Q227L increased transcytosis more than wild-type Gs alpha, neither construct was as effective as FSK in stimulating transcytosis, arguing against a significant role of Gs alpha in transcytosis independent of cAMP and PKA.

Isolation and characterization of mutant CHO cell lines with compartment-specific resistance to brefeldin A

22 CHOBFY (BFY) cell lines were isolated at a frequency 2-30 x 10(-7) from mutagenized populations on the basis of their ability to grow in the presence of 1 microgram/ml brefeldin A (BFA). Four of the five mutant lines tested are genetically stable and none of the mutant lines characterized degrade this drug. Immunofluorescence studies reveal that whereas early endosomes and the Golgi complex have nearly identical BFA sensitivities in the parent CHO line, the relative sensitivities of these two organelles were dramatically altered in all six mutant lines tested. Four cell lines maintain normal Golgi appearance at a BFA concentration as high as 10 micrograms/ml. Mutant lines show wide variation in the level of resistance to growth inhibition by BFA, but none of the mutant lines characterized grow above 2 micrograms/ml BFA. This specific growth inhibition is observed under conditions where Golgi morphology and function remain unaffected, suggesting that some factor(s) unrelated to Golgi function remains sensitive to BFA in BFY mutant lines. These observations provide strong evidence for the presence of multiple, organelle-specific targets for BFA. Cell-free measurements with membrane extracts establish that resistance to BFA in BFY-1 cells involves a membrane-associated factor distinct from ARFs and coatomers. This collection of mutant lines may prove valuable for the identification of intracellular target(s) for BFA and/or of effectors that interact upstream or downstream with these targets, thereby uncovering the cascade which regulates assembly of organelle-specific coats.

Brefeldin A down-regulates the transferrin receptor in K562 cells

The fungal metabolite brefeldin A (BFA) induces profound alterations in the morphology of intracellular organelles. Although BFA promotes the formation of extensive tubular endosomal domains, our understanding of the effects of the antibiotic on vesicle traffic events associated with endocytosis is limited. Thus, alterations in the transferrin (Tf) receptor's endocytic/recycling pathway upon treatment of human erythroleukemia K562 cells with BFA were studied as a pharmacological response. Treatment of K562 cells with BFA caused a down-regulation in the number of cell surface Tf receptors. This effect is highly reminiscent of the well-known action of phorbol 12-myristate 13-acetate (PMA) on Tf receptor traffic in K562 cells. However, our results demonstrate that these two agents down-regulate the Tf receptor via different mechanisms. The effects of BFA and PMA were additive when K562 cells were incubated with both together. Using the In/Sur method, the endocytic rate constant for Tf internalization was determined and PMA was found to greatly enhance ke, from 0.28 min-1 to 0.43 min-1, while BFA had little effect (Ke = 0.20 min-1). In contrast, BFA-treatment alters the exocytic rate constant for return of internalized receptors to the cell surface, with the largest effect exerted on a 'slow-release', monensin-sensitive, compartment. The sum of the endocytic and exocytic kinetic data support a model in which BFA and PMA down-regulate the Tf receptor in K562 cells by mechanistically distinct actions, with BFA targeting exocytic monensin-sensitive intracellular compartments and PMA acting to exert a profound influence on elements of receptor internalization.

Polarity of processes with Golgi apparatus in a subpopulation of type I astrocytes

The Golgi apparatus-complex (GA), is a key organelle involved in several posttranslational modifications of polypeptides destined for lysosomes, plasma membranes and secretion. As reported from this laboratory, certain astrocytes in rat brain contain cisternae of the GA not only in perikarya, but also in processes. In order to further investigate which type of astrocytes contain GA in processes we conducted the present study using primary cultures of rat astrocytes and organelle specific antibodies against the GA and the rough endoplasmic reticulum (RER). While the perikarya of all cells contained elements of the GA, only a single process of a subset of type I astrocytes, negative to antibodies A2B5 and HNK-1, contained GA. In contrast, elements of the RER were found within perikarya and all processes. In order to confirm that the immunostained structures in processes indeed represent the GA, we exposed cultures to Brefeldin A (BFA), a secretion blocker which disperses the GA and redistributes it to the RER. We observed that BFA disrupted the GA of both perikarya and processes. However, astrocytes were resistant to prolonged incubations with BFA, while a similar treatment killed cultured fibroblasts and PC-12 cells. Furthermore, in astrocytes exposed to BFA for several days, the delicate network of glial fibrillary acidic protein (GFAP), was replaced by large perinuclear masses of the protein. These observations demonstrate that a subset of type I astrocytes have a single process with elements of the GA. We suggest that this specialization of the GA may be related to yet unrecognized secretory or protein processing functions of these cells. The resistance of astrocytes to BFA and the striking changes in their cytoskeleton induced by the drug, may contribute to studies on the mechanism(s) of action of BFA.

Brefeldin A causes structural and functional alterations of the trans-Golgi network of MDCK cells

The trans-Golgi network (TGN) of MDCK cells is exquisitely sensitive to the fungal metabolite brefeldin A (BFA), in contrast to the refractory Golgi stack of these cells. At a concentration of 1 microgram/ml, BFA promoted extensive tubulation of the TGN while the medical Golgi marker alpha-mannosidase II was not affected. Tubules emerging minutes after addition of the drug contained both the apical marker influenza hemagglutinin (HA), previously accumulated at 20 degrees C, and the fusion protein interleukin receptor/TGN38 (TGG), a TGN marker that recycles basolaterally, indicating that, in contrast to TGN vesicles, TGN-derived tubules cannot sort apical and basolateral proteins. After 60 minutes treatment with BFA, HA and TGG tubules formed extensive networks widely spread throughout the cell, different from the focused centrosomal localization previously described in non-polarized cells. The TGG network partially codistributed with an early endosomal tubular network loaded with transferrin, suggesting that the TGG and endosomal networks had fused or that TGG had entered the endosomal network via surface recycling and endocytosis. The extensive structural alterations of the TGN were accompanied by functional disruptions, such as the extensive mis-sorting of influenza HA, and by the release of the TGN marker gamma-adaptin. Our results suggest the involvement of BFA-sensitive adaptor proteins in TGN-->surface transport.

Differential effects of brefeldin A on hormonally regulated Na+ transport in a model renal epithelial cell line

Na+ transport in renal epithelia is regulated by a wide variety of endogenous and exogenous cellular factors. Although most natriferic agents have an action on the amiloride-sensitive Na+ channel, the biochemical pathways which precede activation of the channel remain incompletely defined. One approach to dissecting such intricate pathways is to perturb a specific cellular process and determine its importance in the postulated mechanism. The current studies examine the effect of brefeldin A (BFA), an inhibitor of the central vacuolar system, on basal as well as aldosterone-, insulin-, and forskolin-stimulated Na+ transport. In the A6 cell line, BFA had a time-dependent effect on basal transport. Aldosterone-induced Na+ transport was sensitive to BFA while insulin's action was only partially blocked and forskolin-stimulated Na+ transport was relatively resistant to the action of the inhibitor. These studies highlight differences as well as points of convergence in the natriferic pathways.

Phorbol myristate acetate-mediated stimulation of transcytosis and apical recycling in MDCK cells

We observed that phorbol myristate acetate (PMA) stimulates transcytosis of the polymeric immunoglobulin receptor (pIgR) in MDCK cells. Apical release of pre-endocytosed ligand (dimeric IgA) bound to the pIgR can be stimulated twofold within 7 min of addition of PMA while recycling of the ligand from the basal surface is not affected. In addition, apical surface delivery of pIgR and cleavage of its ectodomain to secretory component (SC) is also stimulated by PMA. The recycling of apically internalized ligand back to the apical surface is similarly stimulated. These results suggest that the stimulation of apical delivery is from an apical recycling compartment. The effect of PMA suggests that protein kinase C (PKC) is involved in the regulation of pIgR trafficking in MDCK cells. To test this we down regulated PKC activity by pre-treating cells with PMA for 16 h and observed that transcytosis could no longer be stimulated by PMA. Western blots show that the PKC isozymes alpha and to a lesser extent epsilon, are depleted from MDCK cells which have been pre-treated with PMA for 16 h and that treatment of MDCK cells with PMA for 5 min causes a dramatic translocation of the PKC alpha isozyme and a partial translocation of the epsilon isozyme from the cytosol to the membrane fraction of cell homogenates. This translocation suggests that the alpha and/or epsilon isozymes may be involved in PMA mediated stimulation of transcytosis. A mutant pIgR in which serines 664 and 726, the major sites of phosphorylation, are replaced by alanine is stimulated to transcytose by PMA, suggesting that phosphorylation of pIgR at these sites is not required for the effect of PMA. These results suggest that PMA-mediated stimulation of pIgR transcytosis may involve the activation of PKC alpha and/or epsilon, and that this stimulation occurs independently of the major phosphorylation sites on the pIgR. Finally, PMA stimulates transcytosis of basolaterally internalized transferrin, suggesting that PMA acts to generally stimulate delivery of endocytosed proteins to the apical surface.

The coated pit and macropinocytic pathways serve distinct endosome populations

Clathrin-coated vesicle endocytosis and macropinocytosis are distinct endocytic pathways demonstrable in several cell types including human epidermoid A431 cells (West, M.A., M.S. Bretscher, and C. Watts. 1989. J. Cell Biol. 109:2731-2739). Here we analyze the extent of mixing of macropinocytic endosome (macropinosome) content with that of conventional endosomes served by coated vesicle endocytosis. Using laser scanning confocal fluorescence microscopy we detected very little delivery of macropinosome content to either early or late endosomes-lysosomes as defined by labeling with transferrin or with LDL. Mixing of the contents of the macropinosomes and conventional endosomes was not induced by the addition of brefeldin A. Moreover, the morphology of macropinosomes was not grossly altered in the presence of brefeldin A, whilst in the same cells there were dramatic tubulation effects on conventional endosomes as reported by others. Although refractory to fusion with conventional endosomes, macropinosomes were nonetheless dynamic structures which sometimes exhibited vesiculo-tubular morphology in living cells and were capable of fusing with each other. We suggest that different endocytic mechanisms can give rise to distinct endosome populations.

ADP-ribosylation factor and coatomer couple fusion to vesicle budding

The coat proteins required for budding COP-coated vesicles from Golgi membranes, coatomer and ADP-ribosylation factor (ARF) protein, are shown to be required to reconstitute the orderly process of transport between Golgi cisternae in which fusion of transport vesicles begins only after budding ends. When either coat protein is omitted, fusion is uncoupled from budding-donor and acceptor compartments pair directly without an intervening vesicle. Coupling may therefore results from the sequestration of fusogenic membrane proteins into assembling coated vesicles that are only exposed when the coat is removed after budding is complete. This mechanism of coupling explains the phenomenon of "retrograde transport" triggered by uncouplers such as the drug brefeldin A.

Expression of a dominant allele of human ARF1 inhibits membrane traffic in vivo

ADP-ribosylation factor (ARF) proteins and inhibitory peptides derived from ARFs have demonstrated activities in a number of in vitro assays that measure ER-to-Golgi and intra-Golgi transport and endosome fusion. To better understand the roles of ARF proteins in vivo, stable cell lines were obtained from normal rat kidney (NRK) cells transfected with either wild-type or a dominant activating allele ([Q71L]) of the human ARF1 gene under the control of the interferon-inducible mouse Mx1 promoter. Upon addition of interferon, expression of ARF1 proteins increased with a half-time of 7-8 h, as determined by immunoblot analysis. Induction of mutant ARF1, but not wild-type ARF1, led to an inhibition of protein secretion with kinetics similar to that observed for induction of protein expression. Examination of the Golgi apparatus and the ER by indirect immunofluorescence or transmission electron microscopy revealed that expression of low levels of mutant ARF1 protein correlated with a dramatic increase in vesiculation of the Golgi apparatus and expansion of the ER lumen, while expression of substantially higher levels of wild-type ARF1 had no discernible effect. Endocytosis was also inhibited by expression of mutant ARF1, but not by the wild-type protein. Finally, the expression of [Q71L]ARF1, but not wild-type ARF1, antagonized the actions of brefeldin A, as determined by the delayed loss of ARF and beta-COP from Golgi membranes and disruption of the Golgi apparatus. General models for the actions of ARF1 in membrane traffic events are discussed.

Basolateral to apical transcytosis in polarized cells is indirect and involves BFA and trimeric G protein sensitive passage through the apical endosome

We have used temperature and nocodazole blocks in an in vivo basolateral to apical transcytosis assay to dissociate the early transcytotic steps occurring during the formation of transcytotic vesicles and their microtubule-dependent translocation into the apical region, from the late steps when transcytotic cargo is delivered into the apical media. We found that polarized MDCK cells transfected with rabbit polymeric IgA receptor (pIgA-R) internalize basolaterally added pIgA-R ligand ([Fab]2 fragment of IgG against the receptor's ectodomain) at 17 degrees C but do not deliver it to the apical PM. Instead, the ligand accumulates in an apically localized transcytotic compartment, distal to the basolateral endosome and the microtubule-requiring translocation step. We have characterized this compartment and show that it is distinct from basolateral transferrin recycling endosomes, basolateral early endosomes or late endosomes or lysosomes. The apical transcytotic compartment colocalizes with the compartment containing apically recycling membrane markers (ricin and apically internalized pIgA-R ligand) but is distinct from the compartment receiving apically internalized fluid phase marker (BSA). This compartment is an intermediate station of the overall pathway since transcytotic ligand can exit the compartment and be released into the apical medium when cells preloaded at 17 degrees C are subsequently incubated at 37 degrees C. We have used this system to examine the effect of Brefeldin A (BFA) and the involvement of trimeric GTPases in the late (post apical transcytotic compartment) steps of the transcytotic pathway. We found that addition of BFA or cholera toxin, a known activator of Gs alpha, to cells preloaded with transcytotic ligand at 17 degrees C significantly inhibits the exit of ligand from the apical transcytotic compartment. General structure and function of the apical endosome are not affected since neither BFA nor cholera toxin inhibit the recycling of apically internalized membrane markers (ricin and pIgA-R ligand) from the same compartment. The data suggest that transcytosis connects the "membrane-sorting" sub-domain of the basolateral endosome with a homologous sub-domain of the apical endosome and that exit of transcytosing cargo from the apical endosome is controlled by a BFA and trimeric G protein sensitive mechanism, distinct from that used for recycling of apically internalized proteins (ricin or pIgA-R).

Regulation of pathways within cultured epithelial cells for the transcytosis of a basal membrane-bound peroxidase-polylysine conjugate

A conjugate of horseradish peroxidase (HRP) to poly(L-lysine) (PLL) was used as a non-specific adsorptive probe to study transcytosis in MDCK strain I and Caco-2 epithelial cells. As we have shown previously, HRP-PLL transcytosis proceeds via an intracellular, non-lysosomal proteolytic compartment in MDCK cells; yet, this compartment is utilized for transcytosis only in the basal-to-apical direction (Taub, M. E. and Shen, W.-C. J. Cell. Physiol., 150, 283–290, 1992). Using size exclusion chromatography, we demonstrate that the PLL moiety of the conjugate is effectively cleaved during transcytosis, thus releasing free HRP from the apical surface of the cells. Pulse-chase studies indicate that approximately 6% of basolateral surface-associated HRP-PLL conjugate in Transwell-grown cell monolayers enters the basal-to-apical transcytotic pathway. Brief (1 hour) treatment with 160 nM phorbol ester (PMA), a protein kinase C stimulator, elicits a 2-fold increase in the rate and amount of HRP-PLL transcytosis following a 1 hour lag time. Treatment with 1.6 micrograms/ml brefeldin A (BFA) inhibits HRP-PLL transcytosis by approximately 30%; additionally, BFA is able to abolish completely the PMA stimulatory effect. Removal of BFA causes a re-establishment of the normal rate of transcytosis within 2 hours, demonstrating the reversibility of BFA inhibition with respect to HRP-PLL transcytosis. Thus, PMA most likely elicits an increase in the amount of basally internalized conjugate delivered to BFA-sensitive transcytotic compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

Entry of cholera toxin into polarized human intestinal epithelial cells. Identification of an early brefeldin A sensitive event required for A1-peptide generation

The effect of brefeldin-A (BFA), a reversible inhibitor of vesicular transport, on cholera toxin (CT)-induced Cl- secretion (Isc) was examined in the polarized human intestinal cell line, T84. Pretreatment of T84 monolayers with 5 microM BFA reversibly inhibited Isc in response to apical or basolateral addition of 120 nM CT (2.4 +/- 0.5 vs. 68 +/- 3 microA/cm2, n = 5). In contrast, BFA did not inhibit Isc responses to the cAMP agonist VIP (63 +/- 7 microA/cm2). BFA had no effect on cell surface binding and endocytosis of a functional fluorescent CT analog or on the dose dependency of CT induced 32P-NAD ribosylation of Gs alpha in vitro. In contrast, BFA completely inhibited (> 95%) the ability of T84 cells to reduce CT to the enzymatically active A1-peptide. BFA had to be added within the first 10 min of CT exposure to inhibit CT-elicited Isc. The early BFA-sensitive step occurred before a temperature-sensitive step essential for apical CT action. These studies show that sequential steps are required for a biological response to apical CT: (a) binding to cell surfaces and rapid endocytosis; (b) early, BFA-sensitive vesicular transport essential for reduction of the A1-peptide; and (c) subsequent temperature-sensitive translocation of a signal (the A1-peptide or possibly ADP-ribose-Gs alpha) to the basolateral domain.

The MHC class II-associated invariant chain contains two endosomal targeting signals within its cytoplasmic tail

The oligomeric complex formed by major histocompatibility complex (MHC) class II alpha and beta chains and invariant chain (Ii) assembles in the endoplasmic reticulum and is then transported via the Golgi complex to compartments of the endocytic pathway. When Ii alone is expressed in CV1 cells it is sorted to endosomes. The Ii cytoplasmic tail has been found to be essential for targeting to these compartments. In order to characterize further the signals responsible for endosomal targeting, we have deleted various segments of the cytoplasmic tail. The Ii mutants were transiently expressed and the cellular location of the proteins was analyzed biochemically and morphologically. The cytoplasmic tail of Ii was found to contain two endosomal targeting sequences within its cytoplasmic tail; one targeting sequence was present within amino acid residues 12–29 and deletion of this segment revealed the presence of a second endosomal targeting sequence, located within the first 11 amino acid residues. The presence of a leucine-isoleucine pair at positions 7 and 8 within this sequence was found to be essential for endosomal targeting. In addition, the presence of this L-I motif lead to accumulation of Ii molecules in large endosomal vacuoles containing lysosomal marker proteins. Both wild type Ii and Ii mutant molecules containing only one endosomal targeting sequence were rapidly internalized from the plasma membrane. When the Ii cytoplasmic tail was fused to the membrane-spanning region of neuraminidase, a resident plasma membrane protein, the resulting chimera (INA) was found in endocytic compartments containing lysosomal marker proteins. Thus the cytoplasmic tail of Ii is sufficient for targeting to the endocytic/lysosomal pathway.

Transcytosis of the polymeric immunoglobulin receptor in cultured hippocampal neurons

BACKGROUND

A wide variety of proteins are transported across epithelial cells by vesicular carriers. This process, transcytosis, is used to generate cell surface polarity and to transport macromolecules between the luminal and serosal sides of the epithelial layer. The polymeric immunoglobulin receptor is a well-characterized transcytotic molecule in epithelia. It binds to its ligand, polymeric immunoglobulin, at the basolateral surface, and the receptor-ligand complex is transcytosed to the apical surface, where the ligand is released. Our previous studies have shown that hippocampal neurons may employ mechanisms similar to those of epithelial cells to sort proteins to two plasma membrane domains. The machinery used for axonal delivery recognizes proteins that are targeted apically in epithelia, whereas basolaterally destined proteins are delivered to the dendrites. It has not been clear, however, whether transcytosis occurs in neurons.

RESULTS

We report expression of the polymeric immunoglobulin receptor in cultured hippocampal neurons, using a Semliki Forest Virus expression system, and show by immunofluorescence microscopy that the newly synthesized receptor is targeted from the Golgi complex predominantly to the dendrites - only about 20% of the infected neurons display axonal immunofluorescence. Addition of ligand leads to significant redistribution of the receptor to the axons, shown by an approximately three-fold increase in axonal immunoreactivity with the anti-receptor antibodies.

CONCLUSIONS

Our results suggest that a transcytotic route, analogous to that in epithelia, exists in neurons, where it transports proteins from the somatodendritic to the axonal domain. Cultured neurons expressing the polymeric immunoglobulin receptor offer an experimental system that should be useful for further characterization of this novel neuronal pathway at the molecular and functional level.

Common signals control low density lipoprotein receptor sorting in endosomes and the Golgi complex of MDCK cells

The cytoplasmic domain of the LDL receptor bears two tyrosine-containing determinants that can independently target receptors from the Golgi to the basolateral plasma membrane of MDCK cells. We found that these determinants, localized to the membrane-proximal and -distal regions of the receptor's cytoplasmic domain, also control polarized sorting in endosomes. Inactivation of the distal determinant reduced receptors' ability to return to the basolateral domain following endocytosis, resulting instead in receptor transcytosis from basolateral endosomes to the apical plasma membrane. Similarly, receptors internalized from the apical surface were transported from apical endosomes to the basolateral surface, owing to the proximal basolateral targeting determinant. Thus, receptor recycling in endosomes is directed by the same signals as polarized sorting in the Golgi, indicating that sorting on the endocytic and biosynthetic pathways involves similar mechanisms. The observation that brefeldin A interfered with sorting but not transport in both endosomes and the Golgi further supports this.

Synaptic vesicle proteins and early endosomes in cultured hippocampal neurons: differential effects of Brefeldin A in axon and dendrites

The pathways of synaptic vesicle (SV) biogenesis and recycling are still poorly understood. We have studied the effects of Brefeldin A (BFA) on the distribution of several SV membrane proteins (synaptophysin, synaptotagmin, synaptobrevin, p29, SV2 and rab3A) and on endosomal markers to investigate the relationship between SVs and the membranes with which they interact in cultured hippocampal neurons developing in isolation. In these neurons, SV proteins are detected as punctate immunoreactivity that is concentrated in axons but is also present in perikarya and dendrites. In the same neurons, the transferrin receptor, a well established marker of early endosomes, is selectively concentrated in perikarya and dendrites. In the perikaryal-dendritic region, BFA induced a dramatic tubulation of transferrin receptors as well as a cotubulation of the bulk of synaptophysin. Synaptotagmin, synaptobrevin, p29 and SV2 immunoreactivities retained a primarily punctate distribution. No tubulation of rab3A was observed. In axons, BFA did not produce any obvious alteration of the distribution of SV proteins, nor of peroxidase- or Lucifer yellow-labeled early endosomes. The selective effect of BFA on dendritic membranes suggests the existence of functional differences between the endocytic systems in dendrites and axons. Cotubulation of transferrin receptors and synaptophysin in the perikaryal-dendritic region is consistent with a functional interconnection between the traffic of SV proteins and early endosomes. The heterogeneous effects of BFA on SV proteins in this cell region indicates that SV proteins are differentially sorted upon exit from the TGN and are coassembled into SVs at the cell periphery.

Receptor and protein kinase C-mediated regulation of ARF binding to the Golgi complex

The formation of constitutive transport vesicles involves the association of non-clathrin coat proteins to transport organelles. Here we report that IgE receptors and protein kinase C (PKC) regulate the GTP-dependent binding of the two coat proteins ADP-ribosylation factor (ARF) and beta-COP to Golgi membranes in rat basophilic leukaemia cells. Activation of IgE receptors and PKC prevented the ARF and beta-COP dissociation from Golgi membranes that occurs in permeabilized cells in the absence of GTP and potentiated the association-promoting effects of GTP and the G protein activator fluoroaluminate. In contrast, PKC downregulation and PKC inhibition abolished the activity of GTP and fluoroaluminae in promoting ARF binding to the Golgi complex. Studies of ARF binding to isolated Golgi membranes gave similar results. These findings suggest that coat assembly on Golgi membranes, and thus possibly constitutive secretory traffic, is modulated by membrane receptors and second messengers.