Brefeldin A arrests the intracellular transport of a precursor of complement C3 before its conversion site in rat hepatocytes

The Utility of Fluorescence Recovery after Photobleaching (FRAP) to Study the Plasma Membrane

The plasma membrane of mammalian cells is involved in a wide variety of cellular processes, including, but not limited to, endocytosis and exocytosis, adhesion and migration, and signaling. The regulation of these processes requires the plasma membrane to be highly organized and dynamic. Much of the plasma membrane organization exists at temporal and spatial scales that cannot be directly observed with fluorescence microscopy. Therefore, approaches that report on the membrane's physical parameters must often be utilized to infer membrane organization. As discussed here, diffusion measurements are one such approach that has allowed researchers to understand the subresolution organization of the plasma membrane. Fluorescence recovery after photobleaching (or FRAP) is the most widely accessible method for measuring diffusion in a living cell and has proven to be a powerful tool in cell biology research. Here, we discuss the theoretical underpinnings that allow diffusion measurements to be used in elucidating the organization of the plasma membrane. We also discuss the basic FRAP methodology and the mathematical approaches for deriving quantitative measurements from FRAP recovery curves. FRAP is one of many methods used to measure diffusion in live cell membranes; thus, we compare FRAP with two other popular methods: fluorescence correlation microscopy and single-particle tracking. Lastly, we discuss various plasma membrane organization models developed and tested using diffusion measurements.

The Erns Carboxyterminus: Much More Than a Membrane Anchor

Pestiviruses express the unique essential envelope protein E^rns, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E^rns is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host's innate immune response to pestivirus infection. The unusual membrane anchor of E^rns raises questions with regard to proteolytic processing of the viral polyprotein at the E^rns carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E^rns including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E^rns membrane anchor.

Oxidative Cyclization in Natural Product Biosynthesis

Oxidative cyclizations are important transformations that occur widely during natural product biosynthesis. The transformations from acyclic precursors to cyclized products can afford morphed scaffolds, structural rigidity, and biological activities. Some of the most dramatic structural alterations in natural product biosynthesis occur through oxidative cyclization. In this Review, we examine the different strategies used by nature to create new intra(inter)molecular bonds via redox chemistry. This Review will cover both oxidation- and reduction-enabled cyclization mechanisms, with an emphasis on the former. Radical cyclizations catalyzed by P450, nonheme iron, α-KG-dependent oxygenases, and radical SAM enzymes are discussed to illustrate the use of molecular oxygen and S-adenosylmethionine to forge new bonds at unactivated sites via one-electron manifolds. Nonradical cyclizations catalyzed by flavin-dependent monooxygenases and NAD(P)H-dependent reductases are covered to show the use of two-electron manifolds in initiating cyclization reactions. The oxidative installations of epoxides and halogens into acyclic scaffolds to drive subsequent cyclizations are separately discussed as examples of "disappearing" reactive handles. Last, oxidative rearrangement of rings systems, including contractions and expansions, will be covered.

Fission Yeast SCYL1/2 Homologue Ppk32: A Novel Regulator of TOR Signalling That Governs Survival during Brefeldin A Induced Stress to Protein Trafficking

Target of Rapamycin (TOR) signalling allows eukaryotic cells to adjust cell growth in response to changes in their nutritional and environmental context. The two distinct TOR complexes (TORC1/2) localise to the cell’s internal membrane compartments; the endoplasmic reticulum (ER), Golgi apparatus and lysosomes/vacuoles. Here, we show that Ppk32, a SCYL family pseudo-kinase, is a novel regulator of TOR signalling. The absence of ppk32 expression confers resistance to TOR inhibition. Ppk32 inhibition of TORC1 is critical for cell survival following Brefeldin A (BFA) induced stress. Treatment of wild type cells with either the TORC1 specific inhibitor rapamycin or the general TOR inhibitor Torin1 confirmed that a reduction in TORC1 activity promoted recovery from BFA induced stress. Phosphorylation of Ppk32 on two residues that are conserved within the SCYL pseudo-kinase family are required for this TOR inhibition. Phosphorylation on these sites controls Ppk32 protein levels and sensitivity to BFA. BFA induced ER stress does not account for the response to BFA that we report here, however BFA is also known to induce Golgi stress and impair traffic to lysosomes. In summary, Ppk32 reduce TOR signalling in response to BFA induced stress to support cell survival.

The Target of Rapamycin (TOR) pathway plays a central role coordinating cell growth and cell division in response to the different cellular environments. This is achieved by TOR controlling various metabolic processes, cell growth and cell division, and in part by the localisation of TOR protein complexes to specific internal endomembranes and compartments. Here, we report a novel role for the SCYL family pseudo-kinase, Ppk32 in restraining TOR signalling when cells experience stresses, which specifically affect endomembranes and compartments where TOR complexes are localised. Cells exposed to endomembrane stress (induced by Brefeldin A), displayed increased cell survival when simultaneously treated with the TOR complex 1 (TORC1) inhibitor, rapamycin, presumably because the reduction in TORC1 signalling slows cellular processes to allow cells sufficient time to recover and adapt to this stress. Importantly cancer, neuro-degeneration and neurological diseases are all associated with stress to the endomembrane protein trafficking system. Our findings suggest that therapeutic rapamycin treatment to reduce TOR signalling and block proliferation of cancer cells, which are inherently experiencing such stress, may have the unintended consequence of enhancing cell survival. It is notable, therefore, that our reported mechanisms to reduce Ppk32 protein levels, likely to be conserved in humans, may represent a way to increase TOR signalling and thus increase cell death of cancer types with inherent stress to these internal membrane systems.

Neural stem cells sustain natural killer cells that dictate recovery from brain inflammation

Recovery from organ-specific autoimmune diseases largely relies on the mobilization of endogenous repair mechanisms and local factors that control them. Natural killer (NK) cells are swiftly mobilized to organs targeted by autoimmunity and typically undergo numerical contraction when inflammation wanes. We report the unexpected finding that NK cells are retained in the brain subventricular zone (SVZ) during the chronic phase of multiple sclerosis in humans and its animal model in mice. These NK cells were found preferentially in close proximity to SVZ neural stem cells (NSCs) that produce interleukin-15 and sustain functionally competent NK cells. Moreover, NK cells limited the reparative capacity of NSCs following brain inflammation. These findings reveal that reciprocal interactions between NSCs and NK cells regulate neurorepair.

The neuroactive steroid pregnenolone sulfate stimulates trafficking of functional N-methyl D-aspartate receptors to the cell surface via a noncanonical, G protein, and Ca2+-dependent mechanism

N-methyl D-aspartate (NMDA) receptors (NMDARs) mediate fast excitatory synaptic transmission and play a critical role in synaptic plasticity associated with learning and memory. NMDAR hypoactivity has been implicated in the pathophysiology of schizophrenia, and clinical studies have revealed reduced negative symptoms of schizophrenia with a dose of pregnenolone that elevates serum levels of the neuroactive steroid pregnenolone sulfate (PregS). This report describes a novel process of delayed-onset potentiation whereby PregS approximately doubles the cell's response to NMDA via a mechanism that is pharmacologically and kinetically distinct from rapid positive allosteric modulation by PregS. The number of functional cell-surface NMDARs in cortical neurons increases 60-100% within 10 minutes of exposure to PregS, as shown by surface biotinylation and affinity purification. Delayed-onset potentiation is reversible and selective for expressed receptors containing the NMDAR subunit subtype 2A (NR2A) or NR2B, but not the NR2C or NR2D, subunits. Moreover, substitution of NR2B J/K helices and M4 domain with the corresponding region of NR2D ablates rapid allosteric potentiation of the NMDA response by PregS but not delayed-onset potentiation. This demonstrates that the initial phase of rapid positive allosteric modulation is not a first step in NMDAR upregulation. Delayed-onset potentiation by PregS occurs via a noncanonical, pertussis toxin-sensitive, G protein-coupled, and Ca(2+)-dependent mechanism that is independent of NMDAR ion channel activation. Further investigation into the sequelae for PregS-stimulated trafficking of NMDARs to the neuronal cell surface may uncover a new target for the pharmacological treatment of disorders in which NMDAR hypofunction has been implicated.

A novel nicotinic mechanism underlies β-amyloid-induced neuronal hyperexcitation

There is a significantly elevated incidence of epilepsy in Alzheimer's disease (AD). Moreover, there is neural hyperexcitation/synchronization in transgenic mice expressing abnormal levels or forms of amyloid precursor protein and its presumed, etiopathogenic product, amyloid-β1-42 (Aβ). However, the underlying mechanisms of how Aβ causes neuronal hyperexcitation remain unclear. Here, we report that exposure to pathologically relevant levels of Aβ induces Aβ form-dependent, concentration-dependent, and time-dependent neuronal hyperexcitation in primary cultures of mouse hippocampal neurons. Similarly, Aβ exposure increases levels of nicotinic acetylcholine receptor (nAChR) α7 subunit protein on the cell surface and α7-nAChR function, but not α7 subunit mRNA, suggesting post-translational upregulation of functional α7-nAChRs. These effects are prevented upon coexposure to brefeldin A, an inhibitor of endoplasmic reticulum-to-Golgi protein transport, consistent with an effect on trafficking of α7 subunits and assembled α7-nAChRs to the cell surface. Aβ exposure-induced α7-nAChR functional upregulation occurs before there is expression of neuronal hyperexcitation. Pharmacological inhibition using an α7-nAChR antagonist or genetic deletion of nAChR α7 subunits prevents induction and expression of neuronal hyperexcitation. Collectively, these results, confirmed in studies using slice cultures, indicate that functional activity and perhaps functional upregulation of α7-nAChRs are necessary for production of Aβ-induced neuronal hyperexcitation and possibly AD pathogenesis. This novel mechanism involving α7-nAChRs in mediation of Aβ effects provides potentially new therapeutic targets for treatment of AD.

Cell biology of the endoplasmic reticulum and the Golgi apparatus through proteomics

Enriched endoplasmic reticulum (ER) and Golgi membranes subjected to mass spectrometry have uncovered over a thousand different proteins assigned to the ER and Golgi apparatus of rat liver. This, in turn, led to the uncovering of several hundred proteins of poorly understood function and, through hierarchical clustering, showed that proteins distributed in patterns suggestive of microdomains in cognate organelles. This has led to new insights with respect to their intracellular localization and function. Another outcome has been the critical testing of the cisternal maturation hypothesis showing overwhelming support for a predominant role of COPI vesicles in the transport of resident proteins of the ER and Golgi apparatus (as opposed to biosynthetic cargo). Here we will discuss new insights gained and also highlight new avenues undertaken to further explore the cell biology of the ER and the Golgi apparatus through tandem mass spectrometry.

Phospholipase A2 mediates apolipoprotein-independent uptake of chylomicron remnant-like particles by human macrophages

Apolipoprotein E-receptor-mediated pathways are the main routes by which macrophages take up chylomicron remnants, but uptake may also be mediated by receptor-independent routes. To investigate these mechanisms, triacylglycerol (TG) accumulation induced by apolipoprotein-free chylomicron remnant-like particles (CRLPw/o) in human monocyte-derived macrophages was evaluated. Macrophage TG content increased about 5-fold after incubation with CRLPw/o, and this effect was not reduced by the inhibition of phagocytosis, macropinocytosis, apolipoprotein E function, or proteoglycan bridging. The role of lipases, including lipoprotein lipase, cholesteryl ester hydrolase, and secretory (sPLA2) and cytosolic phospholipase A2, was studied using [³H]TG-labelled CRLPw/o. Total cell radioactivity after incubation with [³H]TG CRLPw/o was reduced by 15–30% by inhibitors of lipoprotein lipase and cholesteryl ester hydrolase and by about 45% by inhibitors of sPLA2 and cytosolic PLA₂ . These results suggest that macrophage lipolytic enzymes mediate the internalization of postprandial TG-rich lipoproteins and that sPLA₂ and cytosolic PLA2, play a more important role than extracellular lipoprotein lipase-mediated TG hydrolysis.

BODIPY probes to study peroxisome dynamics in vivo

There is a continuing need for bioprobes that are target-specific and combine speed of delivery with maintenance of normal cell behaviour. Towards this end, we are developing small pro-fluorescent molecules that provide such specificity through chemical activation by biomolecules. We have generated a set of BODIPY (boron dipyrromethane) fluorophores, including one that is intrinsically non-fluorescent but on incubation with cells becomes fluorescent at its target site. Addition of these BODIPY probes to plant cells identifies peroxisomes, as verified by co-localization with an SKL-FP construct. Interestingly, in mammalian cells, co-localization with the mammalian peroxisomal marker SelectFX(TM) was not observed. These data suggest fundamental differences in peroxisome composition, development or function between plant and animal cells.

A new type of signal peptidase cleavage site identified in an RNA virus polyprotein

Pestiviruses, a group of enveloped positive strand RNA viruses belonging to the family Flaviviridae, express their genes via a polyprotein that is subsequently processed by proteases. The structural protein region contains typical signal peptidase cleavage sites. Only the site at the C terminus of the glycoprotein E(rns) is different because it does not contain a hydrophobic transmembrane region but an amphipathic helix functioning as the E(rns) membrane anchor. Despite the absence of a hydrophobic region, the site between the C terminus of E(rns) and E1, the protein located downstream in the polyprotein, is cleaved by signal peptidase, as demonstrated by mutagenesis and inhibitor studies. Thus, E(rns)E1 is processed at a novel type of signal peptidase cleavage site showing a different membrane topology. Prevention of glycosylation or introduction of mutations into the C-terminal region of E(rns) severely impairs processing, presumably by preventing proper membrane interaction or disturbing a conformation critical for the protein to be accepted as a substrate by signal peptidase.

Intracellular processing, glycosylation, and cell surface expression of human metapneumovirus attachment glycoprotein

The biosynthesis and posttranslational processing of human metapneumovirus attachment G glycoprotein were investigated. After pulse-labeling, the G protein accumulated as three species with molecular weights of 45,000, 50,000, and 53,000 (45K, 50K, and 53K, respectively). N-Glycosidase digestion indicated that these forms represent the unglycosylated precursor and N-glycosylated intermediate products, respectively. After an appropriate chase, these three naive forms were further processed to a mature 97K form. The presence of O-linked sugars in mature G protein was confirmed by O-glycanase digestion and lectin-binding assay using Arachis hypogaea (peanut agglutinin), an O-glycan-specific lectin. In addition, in the O-glycosylation-deficient cell line (CHO ldlD cell), the G protein could not be processed to the mature form unless the exogenous Gal and GalNAc were supplemented, which provided added evidence supporting the O-linked glycosylation of G protein. The maturation of G was completely blocked by monensin but was partially sensitive to brefeldin A (BFA), suggesting the O-linked glycosylation of G initiated in the trans-Golgi compartment and terminated in the trans-Golgi network. Enzymatic deglycosylation analysis confirmed that the BFA-G was a partial mature form containing N-linked oligosaccharides and various amounts of O-linked carbohydrate side chains. The expression of G protein at the cell surface could be detected by indirect immunofluorescence staining assay. Furthermore, cell surface immunoprecipitation displayed an efficient intracellular transport of G protein.

Charged bipolar suramin derivatives induce aggregation of the prion protein at the cell surface and inhibit PrPSc replication

The conversion of the cellular prion protein (PrPc) into a pathogenic isoform (PrP(Sc)) is one of the underlying events in the pathogenesis of the fatal transmissible spongiform encephalopathies (TSEs). Numerous compounds have been described to inhibit prion replication and PrP(Sc) accumulation in cell culture. Among these, the drug suramin induces aggregation and re-targeting of PrPc to endocytic compartments. Plasma membrane and sites of conversion into PrP(Sc) are thereby bypassed. In the present study, a library of suramin analogues was tested as a potential class of new anti-prion compounds and the molecular mechanisms underlying these effects were analysed. Treatment of prion-infected neuroblastoma cells with compounds containing symmetrical aromatic sulfonic acid substitutions inhibited de novo synthesis of PrP(Sc) and induced aggregation and reduction of the half-life of PrPc without downregulating PrPc cell surface expression. Half-molecule compounds lacking the symmetrical bipolar structure or the anionic groups had no effect on PrP(Sc) synthesis or PrPc solubility. Cell surface expression of PrPc was necessary for the activity of effective compounds. Suramin derivatives did not induce aggregation of PrPc when transport along the secretory pathway was compromised, suggesting that their effects occur at a post trans-Golgi network (TGN) site, possibly close to the compartment of conversion into PrP(Sc). In vitro studies with recombinant PrP demonstrated that the inhibitory effect correlated with direct binding to PrP and induction of insoluble PrP aggregates. Our data reveal an anti-prion effect that differs from those characterising other sulphated polyanions and is dependent on the presence of the symmetrical anionic structure of these molecules.

HIP/PAP, a C-type lectin overexpressed in hepatocellular carcinoma, binds the RII alpha regulatory subunit of cAMP-dependent protein kinase and alters the cAMP-dependent protein kinase signalling

HIP/PAP is a C-type lectin overexpressed in hepatocellular carcinoma (HCC). Pleiotropic biological activities have been ascribed to this protein, but little is known about the function of HIP/PAP in the liver. In this study, therefore, we searched for proteins interacting with HIP/PAP by screening a HCC cDNA expression library. We have identified the RII alpha regulatory subunit of cAMP-dependent protein kinase (PKA) as a partner of HIP/PAP. HIP/PAP and RII alpha were coimmunoprecipitated in HIP/PAP expressing cells. The biological relevance of the interaction between these proteins was established by demonstrating, using fractionation methods, that they are located in a same subcellular compartment. Indeed, though HIP/PAP is a protein secreted via the Golgi apparatus we showed that a fraction of HIP/PAP escaped the secretory apparatus and was recovered in the cytosol. Basal PKA activity was increased in HIP/PAP expressing cells, suggesting that HIP/PAP may alter PKA signalling. Indeed, we showed, using a thymidine kinase-luciferase reporter plasmid in which a cAMP responsive element was inserted upstream of the thymidine kinase promoter, that luciferase activity was enhanced in HIP/PAP expressing cells. Thus our findings suggest a novel mechanism for the biological activity of the HIP/PAP lectin.

Mechanisms of TfR-mediated transcytosis and sorting in epithelial cells and applications toward drug delivery

Transferrin receptor has been an important protein for many of the advances made in understanding the intricacies of the intramolecular sorting pathways of endocytosed molecules. The unique internalization and recycling functions of transferrin receptor have also made it an attractive choice for drug targeting and delivery of large protein-based therapeutics and toxins. Recent advances in elucidating the role of the intracellular controllers of transferrin recycling and sorting, such as Rab proteins and their effectors, have led to enhancement of transferrin receptor as a drug delivery vehicle. This review focuses on the use of transferrin receptor as an agent for facilitating drug delivery and targeting, and the role that mechanisms of transferrin receptor sorting and transcytosis play in these events.

Proteasomes and ubiquitin are involved in the turnover of the wild-type prion protein

Prion diseases propagate by converting a normal glycoprotein of the host, PrP(C), into a pathogenic "prion" conformation. Several misfolding mutants of PrP(C) are degraded through the ER-associated degradation (ERAD)-proteasome pathway. In their infectious form, prion diseases such as bovine spongiform encephalopathy involve PrP(C) of wild-type sequence. In contrast to mutant PrP, wild-type PrP(C) was hitherto thought to be stable in the ER and thus immune to ERAD. Using proteasome inhibitors, we now show that approximately 10% of nascent PrP(C) molecules are diverted into the ERAD pathway. Cells incubated with N-acetyl-leucinal-leucinal-norleucinal (ALLN), lactacystin or MG132 accumulated both detergent-soluble and insoluble PrP species. The insoluble fraction included an unglycosylated 26 kDa PrP species with a protease-resistant core, and a M(r) "ladder" that contained ubiquitylated PrP. Our results show for the first time that wild-type PrP(C) molecules are subjected to ERAD, in the course of which they are dislocated into the cytosol and ubiquitylated. The presence of wild-type PrP molecules in the cytosol may have potential pathogenic implications.

Enhancement of diphtheria toxin-induced apoptosis in Vero cells by combination treatment with brefeldin A and okadaic acid

In the present study, we compared the abilities of ricin and diphtheria toxin to induce apoptosis in Vero cells. The cytolysis and DNA fragmentation by ricin paralleled its protein synthesis inhibitory activity. However, unlike ricin, diphtheria toxin could induce neither cytolysis nor DNA fragmentation in Vero cells up to very high concentration, in spite of the fact that Vero cells were even more sensitive to protein synthesis inhibition by diphtheria toxin than ricin. Interestingly, coexistence of brefeldin A (BFA) and okadaic acid (OA) significantly enhanced diphtheria toxin-mediated cytolysis and DNA fragmentation without affecting the activity of protein synthesis inhibition. Ammonium chloride almost completely abolished the ability of diphtheria toxin to induce apoptosis in the presence of BFA and OA as well as the protein synthesis inhibitory activity. The mutant CRM 197, which does not catalyze the ADP ribosylation of elongation factor-2 (EF-2), failed to induce apoptosis in Vero cells even in the presence of BFA and OA. Thus, translocation of diphtheria toxin into the cytosol and subsequent enzymatic inactivation of EF-2 may be necessary steps to induce apoptosis. Taken together our results suggest that protein synthesis inhibition by toxins is not sufficient to induce apoptosis, and underlying mechanisms of apoptosis induction may be distinct between ricin and diphtheria toxin. Since a morphological change in the Golgi complex was observed in Vero cells treated with BFA and OA, modulation of the Golgi complex by these reagents may be partly responsible for enhanced apoptosis induction by diphtheria toxin.

Inhibitory effects of brefeldin A, a membrane transport blocker, on the bradykinin-induced hyperpolarization-mediated relaxation in the porcine coronary artery

1. To elucidate the mechanism of the relaxation mediated by endothelium-derived hyperpolarizing factors (EDHFs), the effect of brefeldin A, a membrane transport blocker, on cytosolic Ca(2+) concentration ([Ca(2+)]i) and tension was determined in the porcine coronary arterial strips. We also examined the effect of brefeldin A on [Ca(2+)]i in the endothelial cells of the porcine aortic valve. 2. In the presence of 10 microM indomethacin and 30 microM N(G)-nitro-L-arginine (L-NOARG), both bradykinin and substance P induced a transient decrease in [Ca(2+)]i and tension in arterial strips contracted with 100 nM U46619 (thromboxane A2 analogue). A 6 h pre-treatment with 20 microg ml(-1) brefeldin A abolished the bradykinin-induced relaxation, while it had no effect on the substance P-induced relaxation. 3. In the absence of indomethacin and L-NOARG, brefeldin A had no effect on the bradykinin-induced relaxation during the contraction induced by U46619 or 118 mM K(+). 4. The indomethacin/L-NOARG-resistant relaxation induced by bradykinin was completely inhibited by 3 mM tetrabutylammonium (non-specific Ca(2+)-activated K(+) channel blocker), while that induced by substance P was not inhibited by 3 mM tetrabutylammonium or 1 mM 4-aminopyridine (voltage-dependent K(+) channels blocker) alone, but completely inhibited by their combination. 5. Brefeldin A had no effect on the [Ca(2+)]i elevation in endothelial cells induced by bradykinin or substance P. 6. In conclusion, bradykinin produce EDHF in a brefeldin A-sensitive mechanism in the porcine coronary artery. However, this mechanism is not active in a substance P-induced production of EDHF, which thus suggests EDHF to be more than a single entity.

Involvement of the secretory pathway for AMPA receptors in NMDA-induced potentiation in hippocampus

A chemical form of synaptic potentiation was produced with a brief bath application of NMDA to rat hippocampal slices. Two methods were used to assess changes in membrane-bound AMPA receptors. Traditional subcellular fractionation was used to isolate synaptic membranes; alternatively, membrane receptors were cross-linked with the membrane-impermeable reagent bis(sulfosuccinimidyl) suberate, and levels of nonmembrane receptors were determined. In both cases, Western blots were used to determine the content of receptor subunits in various subcellular fractions. NMDA-induced potentiation was associated with increased levels of glutamate receptor 1 (GluR1) and GluR2/3 subunits of AMPA receptors in synaptic membrane preparations, whereas no change was observed in whole homogenates. Both KN-62, an inhibitor of calcium/calmodulin kinase, and calpain inhibitor III, a calpain inhibitor, inhibited NMDA-induced potentiation and changes in GluR1 and GluR2/3 subunits of AMPA receptors. Brefeldin A (BFA) inhibits protein trafficking between the Golgi apparatus and cell membranes. Pretreatment of hippocampal slices with BFA significantly decreased NMDA-induced potentiation and completely prevented an NMDA-induced increase in GluR1 levels in membrane fractions. Thus, the levels of GluR1 and GluR2/3 subunits of AMPA receptors are rapidly upregulated in synaptic membranes under conditions associated with potentiation of synaptic responses, and this upregulation requires a functional secretory pathway.

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

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

Effects of cadmium, copper, and zinc and beta APP processing and turnover in COS-7 and PC12 cells. Relationship to Alzheimer disease pathology

The effects of cadmium, copper, and zinc on beta APP metabolism were investigated in COS-7 and PC12 cells. Cadmium chloride (CdCl2) increased beta APP steady-state protein levels and decreased beta APP posttranslational processing. These changes were not accompanied by alterations in beta APP mRNA levels or splicing. In addition, cytosolic alpha-actin and G3PDH levels were not affected. Further, neither zinc (ZnCl2) nor copper (CuSO4) altered beta APP levels or affected its normal processing. Pulse-chase studies revealed that the rate of beta APP maturation decreased twofold in the presence of 25 microM CdCl2 compared to untreated controls. beta APP secretion from the cell also dramatically slowed. These two factors result in the accumulation of partially processed beta APP inside cells. The presence of CdCl2 also decreased the amount of an 8-kDa beta APP C-terminal fragment, indicating that the cellular compartment in which beta APP accumulates is not accessible to alpha-secretase. Studies using Brefeldin A suggest that this compartment may be the cis or medial Golgi. However, A beta production was proportionately increased. These data show that CdCl2 can modulate the beta APP cleavage to favor A beta. Finally, beta APP mis- metabolism was shown to be unrelated to the hsp70 induction elicited by CdCl2; both heat shock and CuSO4 induced hsp70 but had no effect on steady-state levels of beta APP, although heat shock did slow beta APP maturation. These data indicate that hsp70 alone cannot chaperone beta APP through an alternate processing pathway leading to A beta production.

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

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

Selective translocation of different markers in the ante- and retrograde pathways between the Golgi apparatus and the rough endoplasmic reticulum in a hybridoma cell line

We examined the effects of brefeldin A (BFA, 10 micrograms/ml), an inhibitor of protein transport, on the redistribution of different markers of the Golgi apparatus (GA) in hybridoma H35 cells to examine selective transport of marker molecules between the rough endoplasmic reticulum (RER) and the GA. In H35 cells, the GAs had several cisternae with cis and trans faces as deduced by morphology such as relationship with RER and secretory granules. Thiamin pyrophosphatase (TPPase) was distributed in the trans elements, mannosidase II (man II) was in the cis-medial elements, and deposits of Zinc-Iodide-Osmium (ZIO) staining were localized in the cis/intermediate compartment. Upon BFA treatment for 5 min, man II and TPPase were redistributed in all cisternae. After 10 min of BFA treatment, TPPase activity was observed only in the RER, while the cis/intermediate compartment as evidenced by ZIO staining and man II remained. Upon clearance of BFA from the medium, cisternal structures with man II and ZIO staining reappeared at 30 min. TPPase activity was detected in the GA only after 120 min. Thus, in the retrograde pathway, the trans marker, TPPase moves earlier than the cismedial markers, man II and ZIO staining, whereas in the antegrade pathway, the cis-medial markers move earlier than the trans marker. These results suggest that BFA first alters the characteristic enzyme localization before the GA vanishes into the RER, and that selective transport mechanisms may exist for components of different stacks of the GA.

Bip/GRP78 but not calnexin associates with a precursor of glycosylphosphatidylinositol-anchored protein

When fused in-frame with a C-terminal propeptide of placental alkaline phosphatase (PLAP), rat alpha 2u-globulin (alpha GL), a nonglycosylated secretory protein, was expressed on the cell surface as a glycosylphosphatidylinositol (GPI)-linked chimaeric protein (alpha GL-PLAP). In contrast with the wild-type alpha GL-PLAP, a mutant, in which Asp at the cleavage/attachment site of GPI was replaced by Trp, failed to become a GPI-linked mature form and was retained as a precursor form within the cell [Oda, Cheng, Saku, Takami, Sohda, Misumi, Ikehara and Millán (1994) Biochem. J. 301, 577-583]. To elucidate the molecular interactions involved in the retention of the proform within the cell, we examined the association of the proform with molecular chaperones in the endoplasmic reticulum (ER). Antibody against the ER retrieval motif KDEL coimmunoprecipitated a 25 kDa proform, but not a 22 kDa GPI-linked mature form. Pulse-chase experiments showed that the wild-type alpha GL-PLAP with a cleavable propeptide was converted into the mature form, whereas the mutant alpha GL-PLAP with an uncleavable propeptide remained associated with ER-resident proteins with a KDEL motif and underwent rapid degradation in a pre-Golgi compartment. Chemical cross-linking studies showed that, of the several ER-resident proteins immunoreactive with the anti-KDEL antibody, a 78 kDa protein was the only protein associated with the proform. Furthermore this 78 kDa protein was dissociated from the precursor molecule on incubation with ATP, allowing us tentatively to assign it as Bip/GRP78. Anticalnexin antibody, however, failed to coprecipitate any form of the chimaeric protein. Immunoelectron microscopy showed that the proform with the uncleavable propeptide was localized in the ER, but not detected in the Golgi apparatus or plasma membranes. Taken together, these results suggest that Bip/GRP78 is associated with pro alpha GL-PLAP and retains it within the ER until pro alpha GL-PLAP is either modified by GPI or degraded, thereby participating in the quality control of this GPI-linked chimaeric protein.

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.

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.

Effects of brefeldin-A on Golgi morphology in human cultured fibroblasts observed in three-dimensional stereo scanning electron microscopy

Brefeldin A (BFA) has been reported to cause disassembly of the Golgi. We have used three-dimensional (3-D) high-resolution scanning electron microscopy (HRSEM) to investigate these effects in human skin fibroblast cells. The spontaneous reassembly during prolonged exposure to BFA and some effects of forskolin were observed. A BFA concentration of 5 micrograms/ml caused Golgi complexes to become vesicular, resulting in a progressive decrease in the size of the Golgi. Morphologic changes were visible within 2 min of BFA incubation, and by 30 min no identifiable Golgi could be found. Spontaneous reassembly of the Golgi apparatus upon the removal of the BFA or with continued long-term exposure with BFA could not be confirmed. Preliminary experiments with forskolin were not effective in reversing or inhibiting the effects of BFA in human fibroblast cells grown in culture. This inability for spontaneous reassembly and nonreversal by forskolin may reflect a differential effect of BFA in various cell types. HRSEM has proven to be useful for observing 3-D morphologic effects of BFA in Golgi.

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.

Influence of compounds affecting synthesis, modification and transport of proteins on the expression and release of interleukin-2 receptor

One of the most central events during lymphocyte activation is the synthesis and release of IL-2. IL-2 induces the synthesis and expression of the IL-2 receptor alpha-chain (IL-2R) on the lymphocyte as well as the release of a truncated form of IL-2R (sIL-2R). The two proteins are identical except for the absence of the transmembrane and intracellular part on sIL-2R. We have in an in vitro model investigated the influence of certain compounds, affecting different parts of the proliferative response, on the release and expression of IL-2R. We found the generation of the two molecules to have different sensitivity to blocking of protein synthesis, glycosylation, microtubular assembly and proteolytic activity. However, blocking of intracellular transport from Golgi to the endoplasmic reticulum, disassembly of actin filaments and disturbances in the intracellular sodium/calcium balance had identical effects on expression and release of the respective IL-2R. These findings indicate a more complex and specific mechanism behind the generation of sIL-2R than simply by shedding through the action of proteases on the membrane-bound IL-2R.

Brefeldin A inhibits protein synthesis through the phosphorylation of the alpha-subunit of eukaryotic initiation factor-2

Brefeldin A is a fungal metabolite which disrupts protein traffic through the Golgi apparatus and thereby inhibits protein secretion. Recently, it has been shown that Brefeldin A also causes a marked decrease in the rate of protein synthesis in cells in culture [1992, FEBS Lett. 314, 371-374]. We show here that treatment of rat GH3 pituitary cells with Brefeldin A leads to an inhibition of protein synthesis at the level of peptide-chain initiation through a mechanism involving the phosphorylation of the alpha-subunit of eukaryotic initiation factor-2 (eIF-2 alpha).

Rapid cleavage of the endogenous PC3 prosegment and slow conversion to 74 kDa and 66 kDa proteins in AtT-20 cells

AtT-20 cells synthesize 87 kDa and 66 kDa forms of the prohormone convertase PC3 (also known as PC1). In the present work, using biosynthetic labeling experiments (performed both at 20 degrees C and at 37 degrees C), followed by immunoprecipitation with aminoterminally and carboxyterminally-directed antisera, we have found that the first PC3 translational product was a 94 kDa protein that was then converted to an 84 kDa form. This processing was extremely rapid, occurring with a half-life of less than 2 min at 20 degrees C. The 84 kDa form was endoglycosidase H-sensitive, indicating a lack of acquisition of sugar transferred in the medial golgi. Dithiothreitol, a reducing agent that prevents the disulfide bond formation of newly synthesized proteins in the endoplasmic reticulum (ER), inhibited the processing of the 94 kDa to the 84 kDa form. However, brefeldin A (BFA), an inhibitor of ER/golgi transport, and monensin, an inhibitor of the medial/trans-golgi transport, did not affect the cleavage of the 94 kDa to the 84 kDa protein. The 84 kDa protein was converted to an endoglycosidase H-resistant form of 87 kDa that was sequentially processed to 74 kDa and 66 kDa proteins. The 87 kDa protein was immunoprecipitated by the PC3 aminoterminally and carboxyterminally-directed antisera, while the 74 kDa and 66 kDa protein were only detected with the aminoterminally-directed antibody. Radiosequencing of the 87 kDa and 66 kDa proteins indicated that the biosynthesis of the 87 kDa proteins involves the removal of the 83 amino acid prosegment, and that the processing of the 87 kDa to 66 kDa form occurred by cleavage at the carboxyterminal portion. BFA and monensin effectively interrupted the processing of the 84-87 kDa protein to the 74 and 66 kDa species. In addition, while the 84-87 kDa protein produced in monensin-treated cells was still sensitive to endoglycosidase H, the 66 kDa protein was resistant to this enzyme. These results indicate that the post-translational processing of PC3 occurs in three steps: (1) rapid conversion, probably in the ER, of the 94 kDa precursor to the 84 kDa protein by removal of the aminoterminus prosegment; (2) cleavage of the 87 kDa protein to an intermediate product of 74 kDa; and (3) production of the 66 kDa protein. The second and third steps occur in late cellular compartments such as the trans-golgi network or secretory granules and involve sequential cleavages at the carboxyterminus.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition by brefeldin A of NADH oxidation activity of rat liver Golgi apparatus accelerated by GDP

Reduced pyridine nucleotide has been reported to enhance cell-free transfer of membrane material from a radiolabeled Golgi apparatus donor fraction from rat liver to an acceptor fraction consisting of inside-out plasma vesicles immobilized on nitrocellulose [(1992) Biochim. Biophys. Acta 1107, 131]. As part of a continuing effort to identify NADH-requiring enzymes in the Golgi apparatus which may be important to membrane trafficking, highly purified fractions of Golgi apparatus from rat liver were tested for their ability to oxidize NADH and the inhibition of the oxidation of NADH by brefeldin A. The isolated Golgi apparatus fractions were found to oxidize NADH with a specific activity comparable to that of the plasma membrane of rat liver. The activity was inhibited by brefeldin A and this inhibition was augmented by GDP. At near optimal concentrations of 7 microM brefeldin A and 1 microM GDP, the activity was > 90% inhibited. Brefeldin A inhibition of NADH oxidation by the Golgi apparatus was time-dependent and GDP appeared to accelerate the inhibition by brefeldin A.

Polarization-dependent apical membrane CFTR targeting underlies cAMP-stimulated Cl- secretion in epithelial cells

The relationship between adenosine 3',5'-cyclic monophosphate (cAMP)-mediated Cl- secretion and the cellular location of the cystic fibrosis transmembrane conductance regulator (CFTR) was determined in both polarized (Cl.19A) and unpolarized (parental) HT-29 colonocytes expressing similar levels of CFTR mRNA and protein. CFTR immunolocalized to the apical membrane domain of polarized colonocytes exhibiting cAMP-responsive Cl- secretion. In contrast, CFTR staining was perinuclear in unpolarized colonocytes, which gave little or no cAMP-stimulated Cl- conductance responses. Thus cAMP-stimulated Cl- secretion coincided with an apical localization of CFTR. Brefeldin A (BFA) was used to perturb glycoprotein targeting in these cells. In polarized colonocytes, BFA caused a reversible, time-dependent decrease in the Cl-conductance response to cAMP but not Ca2+. Apical CFTR redistributed into large coalesced intracellular vesicles, located within the same plane as the microtubule organizing center, a marker for the trans-Golgi network (TGN). In preconfluent monolayers or unpolarized HT-29 cells, BFA had no effect on CFTR staining, which remained perinuclear. Mature, Golgi-processed CFTR protein was isolated from both polarized and unpolarized colonocytes. Thus the mechanism for polarization-dependent apical membrane CFTR targeting and the acquisition of cAMP-dependent Cl- secretion lies at or beyond the late Golgi-TGN in epithelial cells.

"BFA bodies": a subcompartment of the endoplasmic reticulum

A specialized region of the endoplasmic reticulum--the BFA body--is defined by the site of accumulation of coatomer when nonclathrin coat protein (COP)-coated vesicle assembly is prevented by the drug brefeldin A (BFA). BFA bodies are formed by part smooth, part rough domains of endoplasmic reticulum that are cis to the classical transitional endoplasmic reticulum and to BFA-induced Golgi remnants.

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.

In vitro reactions of vacuole inheritance in Saccharomyces cerevisiae

Vacuole inheritance is temporally coordinated with the cell cycle and is restricted spatially to an axis between the maternal vacuole and the bud. The new bud vacuole is founded by a stream of vacuole-derived membranous vesicles and tubules which are transported from the mother cell into the bud to form the daughter organelle. We now report in vitro formation of vacuole-derived tubules and vesicles. In semi-intact cells, formation of tubulovesicular structures requires ATP and the proteins encoded by VAC1 and VAC2, two genes which are required for vacuole inheritance in vivo. Isolation of vacuoles from cell lysates before in vitro incubation reveals that formation of tubulovesicular structures requires cytosol as well as ATP. After forming tubulovesicular structures, isolated vacuoles subsequently increase in size. Biochemical assays reveal that this increase results from vacuole to vacuole fusion, leading to mixing of organellar contents. Intervacuolar fusion is sensitive to the phosphatase inhibitors microcystin-LR and okadaic acid, suggesting that protein phosphorylation/dephosphorylation reactions play a role in this event.

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.

Bacterial ADP-ribosyltransferase with a substrate specificity of the rho protein disassembles the Golgi apparatus in Vero cells and mimics the action of brefeldin A

Epidermal-cell differentiation inhibitor (EDIN) is an exoenzyme produced by Staphylococcus aureus that catalyzes the ADP-ribosylation of rho proteins, members of the small GTP-binding protein family. In this study we demonstrate that EDIN induces a rapid morphological change in the Golgi structure of monkey kidney Vero cells that is similar to the changes elicited by brefeldin A (BFA). Treatment of Vero cells with EDIN resulted in a rapid disappearance of N-7-(4-nitrobenzo-2-oxa-1,3-diazole)-6-aminocaproylsphingosine, a 110-kDa protein (beta-COP, coat protein), and mannosidase II from the Golgi structure. Lower doses of EDIN and BFA had a synergistic effect on the redistribution of the Golgi markers. The similarities in the effects of EDIN and BFA in Vero cells also include the EDIN- or BFA-mediated protection of Vero cells from ricin cytotoxicity and prevention of the effects of EDIN or BFA on the distribution of Golgi markers by the pretreatment of Vero cells with guanosine 5'-[gamma-thio]triphosphate or forskolin. Incubation of a Vero-cell homogenate with [32P]NAD+ and EDIN in vitro resulted in the appearance of a labeled band with an apparent molecular mass of 22 kDa. The morphological change of the Golgi structure induced by EDIN was inhibited by nicotinamide, an inhibitor of EDIN-catalyzed ADP-ribosylation. Thus these data suggest that a rho protein is involved in the membrane trafficking between the Golgi and the endoplasmic reticulum of Vero cells and that this rho protein may be a target shared by EDIN and BFA.

Brefeldin A, thapsigargin, and AIF4- stimulate the accumulation of GRP78 mRNA in a cycloheximide dependent manner, whilst induction by hypoxia is independent of protein synthesis

The glucose regulated proteins (GRPs) are major structural components of the endoplasmic reticulum (ER) and are involved in the import, folding, and processing of ER proteins. Expression of the glucose regulated proteins (GRP78 and GRP94) is greatly increased after cells are exposed to stress agents (including A23187 and tunicamycin) which inhibit ER function. Here, we demonstrate that three novel inhibitors of ER function, thapsigargin (which inhibits the ER Ca(2+)-ATPase), brefeldin A (an inhibitor of vesicle transport between the ER and Golgi) and AIF4-, (which inhibits trimeric G-proteins), can increase the expression of both GRP78 and 94. The common characteristic shared by activators of GRP expression is that they disrupt some function of the ER. The increased levels of GRPs may be a response to the accumulation of aberrant proteins in the ER or they may be increased in response to structural/functional damage to the ER. The increased accumulation of GRP78 mRNA after exposure of cells to either thapsigargin, brefeldin A, AIF4-, A23187, or tunicamycin can be blocked by pre-incubation in cycloheximide. In contrast, accumulation of GRPs after exposure to hypoxia was independent of cycloheximide. In addition, the protein kinase inhibitor genistein blocked the thapsigargin induced accumulation of GRP78 mRNA, whereas the protein phosphatase inhibitor okadaic acid caused increased accumulation of GRP78 mRNA. The data indicates that there are at least 2 mechanisms for induced expression of GRPs, one of which involves a phosphorylation step and requires new protein synthesis (e.g., thapsigargin, A23187) and one which is independent of both these steps (hypoxia).

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.

Post-Golgi membrane traffic: brefeldin A inhibits export from distal Golgi compartments to the cell surface but not recycling

Recent studies using the fungal metabolite brefeldin A (BFA) have provided important insights into the dynamics and the organization of the ER/Golgi membrane system. Here we examined the effect of BFA on the functional integrity of the distal part of the secretory pathway, i.e., transport between trans-Golgi cisternae and the cell surface. To assay export via the constitutive pathway, we followed the movement of vesicular stomatitis virus (VSV) G glycoprotein that had been accumulated in the trans-Golgi network (TGN) by incubation of infected BHK-21 cells at 20 degrees C. Addition of BFA rapidly and reversibly inhibited cell surface transport of G protein. The block to secretion was not due to redistribution of externalized G protein to internal pools. It was also not due to collapse of TGN to the ER, since VSV G protein blocked in treated cells resided in compartments that were distinct from the ER/Golgi system. Similar effects were found with a bulk-flow marker: BFA blocked constitutive secretion of glycosaminoglycan chains that had been synthesized and sulfated in the trans-Golgi cisternae. To examine export via the regulated secretory pathway, we assayed secretion of [35S]SO4 labeled secretogranin II from PC12 cells, a marker that has been used to study secretory granule budding from the TGN (Tooze, S. A., U. Weiss, and W. B. Huttner. 1990. Nature [Lond.]. 347:207-208). BFA potently inhibited secretion of sulfated secretogranin II induced by K+ depolarization. Inhibition was at the level of granule formation, since BFA had no effect on regulated secretion from preformed granules. Taken together, the results suggest that BFA blocks export via both the constitutive and the regulated pathways. In contrast, endocytosis and recycling of VSV G protein were not blocked by BFA, consistent with previous studies that endocytosis is unaffected (Misumi, Y., Y. Misumi, K. Miki, A Takatsuki, G. Tamura, and Y. Ikehara. 1986. J. Biol. Chem. 261:11398-11403). These and earlier results suggest that the exo/endocytic pathway of mammalian cells consist of two similar but distinct endomembrane systems: an ER/Golgi system and a post-Golgi system. BFA prevents forward transport without affecting return traffic in both systems.