Okadaic acid induces Golgi apparatus fragmentation and arrest of intracellular transport

The PRMT5/HURP axis retards Golgi repositioning by stabilizing acetyl-tubulin and Golgi apparatus during cell migration

The Golgi apparatus (GA) translocates to the cell leading end during directional migration, thereby determining cell polarity and transporting essential factors to the migration apparatus. The study provides mechanistic insights into how GA repositioning (GR) is regulated. We show that the methyltransferase PRMT5 methylates the microtubule regulator HURP at R122. The HURP methylation mimicking mutant 122F impairs GR and cell migration. Mechanistic studies revealed that HURP 122F or endogenous methylated HURP, that is, HURP m122, interacts with acetyl-tubulin. Overexpression of HURP 122F stabilizes the bundling pattern of acetyl-tubulin by decreasing the sensitivity of the latter to a microtubule disrupting agent nocodazole. HURP 122F also rigidifies GA via desensitizing the organelle to several GA disrupting chemicals. Similarly, the acetyl-tubulin mimicking mutant 40Q or tubulin acetyltransferase αTAT1 can rigidify GA, impair GR, and retard cell migration. Reversal of HURP 122F-induced GA rigidification, by knocking down GA assembly factors such as GRASP65 or GM130, attenuates 122F-triggered GR and cell migration. Remarkably, PRMT5 is found downregulated and the level of HURP m122 is decreased during the early hours of wound healing-based cell migration, collectively implying that the PRMT5-HURP-acetyl-tubulin axis plays the role of brake, preventing GR and cell migration before cells reach empty space.

Rab-dependent cellular trafficking and amyotrophic lateral sclerosis

Rab GTPases are becoming increasingly implicated in neurodegenerative disorders, although their role in amyotrophic lateral sclerosis (ALS) has been somewhat overlooked. However, dysfunction of intracellular transport is gaining increasing attention as a pathogenic mechanism in ALS. Many previous studies have focused axonal trafficking, and the extreme length of axons in motor neurons may contribute to their unique susceptibility in this disorder. In contrast, the role of transport defects within the cell body has been relatively neglected. Similarly, whilst Rab GTPases control all intracellular membrane trafficking events, their role in ALS is poorly understood. Emerging evidence now highlights this family of proteins in ALS, particularly the discovery that C9orf72 functions in intra transport in conjunction with several Rab GTPases. Here, we summarize recent updates on cellular transport defects in ALS, with a focus on Rab GTPases and how their dysfunction may specifically target neurons and contribute to pathophysiology. We discuss the molecular mechanisms associated with dysfunction of Rab proteins in ALS. Finally, we also discuss dysfunction in other modes of transport recently implicated in ALS, including nucleocytoplasmic transport and the ER-mitochondrial contact regions (MAM compartment), and speculate whether these may also involve Rab GTPases.

GADD34 Function in Protein Trafficking Promotes Adaptation to Hyperosmotic Stress in Human Corneal Cells

GADD34, a stress-induced regulatory subunit of the phosphatase PP1, is known to function in hyperosmotic stress through its well-known role in the integrated stress response (ISR) pathway. Adaptation to hyperosmotic stress is important for the health of corneal epithelial cells exposed to changes in extracellular osmolarity, with maladaptation leading to dry eye syndrome. This adaptation includes induction of SNAT2, an endoplasmic reticulum (ER)-Golgi-processed protein, which helps to reverse the stress-induced loss of cell volume and promote homeostasis through amino acid uptake. Here, we show that GADD34 promotes the processing of proteins synthesized on the ER during hyperosmotic stress independent of its action in the ISR. We show that GADD34/PP1 phosphatase activity reverses hyperosmotic-stress-induced Golgi fragmentation and is important for cis- to trans-Golgi trafficking of SNAT2, thereby promoting SNAT2 plasma membrane localization and function. These results suggest that GADD34 is a protective molecule for ocular diseases such as dry eye syndrome.

Extracellular wildtype and mutant SOD1 induces ER-Golgi pathology characteristic of amyotrophic lateral sclerosis in neuronal cells

Amyotrophic lateral sclerosis (ALS) is a fatal and rapidly progressing neurodegenerative disorder and the majority of ALS is sporadic, where misfolding and aggregation of Cu/Zn-superoxide dismutase (SOD1) is a feature shared with familial mutant-SOD1 cases. ALS is characterized by progressive neurospatial spread of pathology among motor neurons, and recently the transfer of extracellular, aggregated mutant SOD1 between cells was demonstrated in culture. However, there is currently no evidence that uptake of SOD1 into cells initiates neurodegenerative pathways reminiscent of ALS pathology. Similarly, whilst dysfunction to the ER-Golgi compartments is increasingly implicated in the pathogenesis of both sporadic and familial ALS, it remains unclear whether misfolded, wildtype SOD1 triggers ER-Golgi dysfunction. In this study we show that both extracellular, native wildtype and mutant SOD1 are taken up by macropinocytosis into neuronal cells. Hence uptake does not depend on SOD1 mutation or misfolding. We also demonstrate that purified mutant SOD1 added exogenously to neuronal cells inhibits protein transport between the ER-Golgi apparatus, leading to Golgi fragmentation, induction of ER stress and apoptotic cell death. Furthermore, we show that extracellular, aggregated, wildtype SOD1 also induces ER-Golgi pathology similar to mutant SOD1, leading to apoptotic cell death. Hence extracellular misfolded wildtype or mutant SOD1 induce dysfunction to ER-Golgi compartments characteristic of ALS in neuronal cells, implicating extracellular SOD1 in the spread of pathology among motor neurons in both sporadic and familial ALS.

Rift Valley fever virus strain MP-12 enters mammalian host cells via caveola-mediated endocytosis

Rift Valley fever virus (RVFV) is a zoonotic pathogen capable of causing serious morbidity and mortality in both humans and livestock. The lack of efficient countermeasure strategies, the potential for dispersion into new regions, and the pathogenesis in humans and livestock make RVFV a serious public health concern. The receptors, cellular factors, and entry pathways used by RVFV and other members of the family Bunyaviridae remain largely uncharacterized. Here we provide evidence that RVFV strain MP-12 uses dynamin-dependent caveola-mediated endocytosis for cell entry. Caveolae are lipid raft domains composed of caveolin (the main structural component), cholesterol, and sphingolipids. Caveola-mediated endocytosis is responsible for the uptake of a wide variety of host ligands, as well as bacteria, bacterial toxins, and a number of viruses. To determine the cellular entry mechanism of RVFV, we used small-molecule inhibitors, RNA interference (RNAi), and dominant negative (DN) protein expression to inhibit the major mammalian cell endocytic pathways. Inhibitors and RNAi specific for macropinocytosis and clathrin-mediated endocytosis had no effect on RVFV infection. In contrast, inhibitors of caveola-mediated endocytosis, and RNAi targeted to caveolin-1 and dynamin, drastically reduced RVFV infection in multiple cell lines. Expression of DN caveolin-1 also reduced RVFV infection significantly, while expression of DN EPS15, a protein required for the assembly of clathrin-coated pits, and DN PAK-1, an obligate mediator of macropinocytosis, had no significant impact on RVFV infection. These results together suggest that the primary mechanism of RVFV MP-12 uptake is dynamin-dependent, caveolin-1-mediated endocytosis.

BBF2H7-mediated Sec23A pathway is required for endoplasmic reticulum-to-Golgi trafficking in dermal fibroblasts to promote collagen synthesis

Collagen fibers, structural elements responsible for mechanical strength in skin, are synthesized constitutively in response to cytokines such as IGF-I. However, little is known about their intracellular trafficking from the endoplasmic reticulum (ER) to the Golgi apparatus during synthesis. We demonstrate herein that the BBF2 human homolog on chromosome 7 (BBF2H7)-mediated Sec23A pathway is involved in regulation of intracellular procollagen trafficking. The mRNA and protein expression of BBF2H7, Sec23A, and type I and III collagen (COL1 and COL3) was induced by IGF-I stimulation. In addition, the cleaved form of BBF2H7 was detected in IGF-I-treated cultures, indicating that activation occurs concurrently with its expression. Knockdown with small interfering RNAs targeting BBF2H7 caused a significant reduction in the expression of COL1 and COL3, regardless of IGF-I treatment. Both mitogen-activated protein kinase and phosphatidylinositol-3 kinase pathways via IGF-I receptor activation were required for BBF2H7 induction. Using immunofluorescence microscopy, we showed that Golgi apparatus dysmorphology is due to coat protein complex II vehicle hypoplasia caused by the absence of BBF2H7 and Sec23A. The BBF2H7-mediated Sec23A pathway was required for ER-to-Golgi procollagen trafficking to promote collagen synthesis. This role of growth factors such as IGF-I, which to our knowledge is previously unreported, suggests antiaging strategies.

Protein phosphatases and the regulation of mitosis

Dynamic control of protein phosphorylation is necessary for the regulation of many cellular processes, including mitosis and cytokinesis. Indeed, although the central role of protein kinases is widely appreciated and intensely studied, the importance of protein phosphatases is often overlooked. Recent studies, however, have highlighted the considerable role of protein phosphatases in both the spatial and temporal control of protein kinase activity, and the modulation of substrate phosphorylation. Here, we will focus on recent advances in our understanding of phosphatase structure, and the importance of phosphatase function in the control of mitotic spindle formation, chromosome architecture and cohesion, and cell division.

The subcellular distribution of small molecules: from pharmacokinetics to synthetic biology

The systemic pharmacokinetics and pharmacodynamics of small molecules are determined by subcellular transport phenomena. Although approaches used to study the subcellular distribution of small molecules have gradually evolved over the past several decades, experimental analysis and prediction of cellular pharmacokinetics remains a challenge. In this review, we survey the progress of subcellular distribution research since the 1960s, with a focus on the advantages, disadvantages and limitations of the various experimental techniques. Critical review of the existing body of knowledge points to many opportunities to advance the rational design of organelle-targeted chemical agents. These opportunities include (1) development of quantitative, non-fluorescence-based, whole cell methods and techniques to measure the subcellular distribution of chemical agents in multiple compartments; (2) exploratory experimentation with nonspecific transport probes that have not been enriched with putative, organelle-targeting features; (3) elaboration of hypothesis-driven, mechanistic and modeling-based approaches to guide experiments aimed at elucidating subcellular distribution and transport; and (4) introduction of revolutionary conceptual approaches borrowed from the field of synthetic biology combined with cutting edge experimental strategies. In our laboratory, state-of-the-art subcellular transport studies are now being aimed at understanding the formation of new intracellular membrane structures in response to drug therapy, exploring the function of drug-membrane complexes as intracellular drug depots, and synthesizing new organelles with extraordinary physical and chemical properties.

Receptor-Mediated Transcytosis of Leptin through Human Intestinal Cells In Vitro

Gastric Leptin is absorbed by duodenal enterocytes and released on the basolateral side towards the bloodstream. We investigated in vitro some of the mechanisms of this transport. Caco-2/15 cells internalize leptin from the apical medium and release it through transcytosis in the basal medium in a time- temperature-dependent and saturable fashion. Leptin receptors are revealed on the apical brush-border membrane of the Caco-2 cells. RNA-mediated silencing of the receptor led to decreases in the uptake and basolateral release. Leptin in the basal medium was found bound to the soluble form of its receptor. An inhibitor of clathrin-dependent endocytosis (chlorpromazine) decreased leptin uptake. Confocal immunocytochemistry and the use of brefeldin A and okadaic acid revealed the passage of leptin through the Golgi apparatus. We propose that leptin transcytosis by intestinal cells depends on its receptor, on clathrin-coated vesicles and transits through the Golgi apparatus.

Regulation of cytosolic phospholipase A2alpha by hsp90 and a p54 kinase in okadaic acid-stimulated macrophages

In resident mouse peritoneal macrophages, group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) mediates arachidonic acid (AA) release and eicosanoid production in response to diverse agonists such as A23187, phorbol myristate acetate, zymosan, and the enterotoxin, okadaic acid (OA). cPLA(2)alpha is regulated by phosphorylation and by calcium that binds to the C2 domain and induces translocation from the cytosol to membranes. In contrast, OA activates cPLA(2)alpha-induced AA release and translocation to the Golgi in macrophages without an apparent increase in calcium. Inhibitors of heat shock protein 90 (hsp90), geldanamycin, and herbimycin blocked AA release in response to OA but not to A23187, PMA, or zymosan. OA, but not the other agonists, induced activation of a cytosolic serine/threonine 54-kDa kinase (p54), which phosphorylated cPLA(2)alpha in in-gel kinase assays and was associated with cPLA(2)alpha in immunoprecipitates. Activation of the p54 kinase was inhibited by geldanamycin. The kinase coimmunoprecipitated with hsp90 in unstimulated macrophages, and OA induced its loss from hsp90, concomitant with its association with cPLA(2)alpha. The results demonstrate a role for hsp90 in regulating cPLA(2)alpha-mediated AA release that involves association of a p54 kinase with cPLA(2)alpha upon OA stimulation.

Golgi apparatus and neurodegenerative diseases

Neurodegenerative disorders are typically characterized by progressive and extensive neuronal loss in specific populations of neurons and brain areas which lead to the observed clinical manifestations. Despite the recent advances in molecular neuroscience, the subcellular bases such as Golgi apparatus (GA) for most neurodegenerative diseases are poorly understood. This review gives a brief overview of the contribution of the neuronal GA in the pathogeneses of neurodegeneration, summarizes what is known of the GA machinery in these diseases, and present the relationship between GA fragmentation and the aggregation and accumulation of misfolded or aberrant proteins including mutant SOD1, a-synuclein, tau, which is considered to be a key event in the pathogenic process, and perturbating in calcium homeostasis, regulation of hormones, lipid metabolism are also linkage to the function of the GA thought to underlie neurodegeneration. Although these precise diseases mechanisms remain to be clarified, more research is needed to better understand how GA function for it and to enable physicians to use this knowledge for the benefit of the patients.

Expression of a wild-type CFTR maintains the integrity of the biosynthetic/secretory pathway in human cystic fibrosis pancreatic duct cells

The structural integrity of the Golgi complex is essential to its functions in the maturation, sorting, and transport of plasma membrane proteins. Previously, we demonstrated that in pancreatic duct CFPAC-1 cells, which express DeltaF508 CFTR (cystic fibrosis transmembrane conductance regulator), the intracellular trafficking of carbonic anhydrase IV (CA IV), a membrane protein involved in HCO(3)(-) secretion, was impaired. To determine whether these abnormalities were related to changes in the Golgi complex, we examined the ultrastructure and distribution of Golgi compartments with regard to the microtubule cytoskeleton in CFPAC-1 cells transfected or not with the wild-type CFTR. Ultrastructural and immunocytochemical analysis showed that in polarized CFPAC-1 cells, Golgi stacks were disconnected from one another and scattered throughout the cytoplasm. The colocalization of CA IV with markers of Golgi compartments indicated the ability of stacks to transfer this enzyme. This Golgi dispersal was associated with abnormal microtubule distribution and multiplicity of the microtubule-organizing centers (MTOCs). In reverted cells, the normalization of Golgi structure, microtubule distribution, and MTOC number was observed. These observations suggest that the entire biosynthetic/secretory pathway is disrupted in CFPAC-1 cells, which might explain the abnormal intracellular transport of CA IV. Taken together, these results point to the fact that the expression of DeltaF508 CFTR affects the integrity of the secretory pathway.

Genome-wide analysis of human kinases in clathrin- and caveolae/raft-mediated endocytosis

Endocytosis is a key cellular process, encompassing different entry routes and endocytic compartments. To what extent endocytosis is subjected to high-order regulation by the cellular signalling machinery remains unclear. Using high-throughput RNA interference and automated image analysis, we explored the function of human kinases in two principal types of endocytosis: clathrin- and caveolae/raft-mediated endocytosis. We monitored this through infection of vesicular stomatitis virus, simian virus 40 and transferrin trafficking, and also through cell proliferation and apoptosis assays. Here we show that a high number of kinases are involved in endocytosis, and that each endocytic route is regulated by a specific kinase subset. Notably, one group of kinases exerted opposite effects on the two endocytic routes, suggesting coordinate regulation. Our analysis demonstrates that signalling functions such as those controlling cell adhesion, growth and proliferation, are built into the machinery of endocytosis to a much higher degree than previously recognized.

MEK1-induced Golgi dynamics during cell cycle progression is partly mediated by Polo-like kinase-3

MEK1, a gene product that regulates cell growth and differentiation, also plays an important role in Golgi breakdown during the cell cycle. We have recently shown that polo-like kinase (Plk3) is Golgi localized and involved in Golgi dynamics during the cell cycle. To study the mode of action of Plk3 in the Golgi fragmentation cascade, we examined functional as well as physical interactions between Plk3 and MEK1/ERKs. In HeLa cells, although a significant amount of Plk3 signals dispersed in a manner similar to those of Golgi during mitosis concentrated Plk3 was detected at spindle poles, which colocalized with phospho-MEKs and phospho-ERKs. Pull-down assays showed that Plk3 physically interacted with MEK1 and ERK2. Nocodazole activated Plk3 and its activation was blocked by MEK-specific inhibitors (PD98059 or U0126). Moreover, transfection of activated MEK1 resulted in an enhanced kinase activity of Plk3; Plk3-induced fragmentation of Golgi stacks was significantly reduced after treatment with MEK inhibitors. Consistently, ectopic expression of activated MEK1, but not kinase-dead MEK1(K97R), stimulated Plk3 to induce Golgi breakdown and the stimulation was not observed in cells expressing Plk3(K52R). Furthermore, PLK3(-/-) murine embryonic fibroblast cells exhibited a significantly less fragmentation of the Golgi complex than that in wild-type cells after exposed to nocodazole. Thus, our studies strongly suggest that Plk3 may be a key protein kinase mediating MEK1 function in the Golgi fragmentation pathway during cell division.

Polo-like kinase 3 is Golgi localized and involved in regulating Golgi fragmentation during the cell cycle

The Golgi apparatus undergoes extensive fragmentation during mitosis in animal cells. Protein kinases play a critical role during fragmentation of the Golgi apparatus. We reported here that Polo-like kinase 3 (Plk3) may be an important mediator during Golgi breakdown. Specifically, Plk3 was concentrated at the Golgi apparatus in HeLa and A549 cells during interphase. At the onset of mitosis, Plk3 signals disintegrated and redistributed in a manner similar to those of Golgi stacks. Nocodazole activated Plk3 kinase activity, correlating with redistribution of Plk3 signals and Golgi fragmentation. In addition, treatment with brefeldin A (BFA), a Golgi-specific poison, also resulted in disappearance of concentrated Plk3 signals. Plk3 interacted with giantin, a Golgi-specific protein. Expression of Plk3, but not the kinase-defective Plk3 (Plk3(K52R)), resulted in significant Golgi breakdown. Given its role in cell cycle progression, Plk3 may be a protein kinase involved in regulation of Golgi fragmentation during the cell cycle.

Fluoride causes reversible dispersal of Golgi cisternae and matrix in neuroendocrine cells

A role for heterotrimeric G proteins in the regulation of Golgi function and formation of secretory granules is generally accepted. We set out to study the effect of activation of heterotrimeric G proteins by aluminum fluoride on secretory granule formation in AtT-20 corticotropic tumor cells and in melanotrophs from the rat pituitary. In AtT-20 cells, treatment with aluminum fluoride or fluoride alone for 60 min induced complete dispersal of Golgi, ER-Golgi intermediate compartment and Golgi matrix markers, while betaCOP immunoreactiviy retained a juxtanuclear position and TGN38 was unaffected. Electron microscopy showed compression of Golgi cisternae followed by conversion of the Golgi stacks into clusters of tubular and vesicular elements. In the melanotroph of the rat pituitary a similar compression of Golgi cisternae was observed, followed by a progressive loss of cisternae from the stacks. As shown in other cells, brefeldin A induced redistribution of the Golgi matrix protein GM130 to punctate structures in the cytoplasm in AtT-20 cells, while mannosidase II immunoreactivity was completely dispersed. Fluoride induced a complete dispersal of mannosidase II and GM130 immunoreactivity. The effect of fluoride was fully reversible with reestablishment of normal mannosidase II and GM130 immunoreactivity within 2 h. After 1 h of recovery, showing varying stages of reassembly, the patterns of mannosidase II and GM130 immunoreactivity were identical in individual cells, indicating that Golgi matrix and cisternae reassemble with similar kinetics during recovery from fluoride treatment. Instead of a specific aluminum fluoride effect on secretory granule formation in the trans-Golgi network, we thus observe a unique form of Golgi dispersal induced by fluoride alone, possibly via its action as a phosphatase inhibitor.

Actin-rich spherical extrusion induced in okadaic acid-treated K562 cells by crosslinking of membrane microdomains

Interconnection between surface microdomains and the actin cytoskeleton is vital to various cellular activities. We studied the responses of okadaic acid (OKA)-treated K562 leukemia cells to crosslinking of membrane microdomains. Although OKA alone induced clustering of surface-bound F-actin, addition of a biotinylated poly(ethylene glycol) derivative of cholesterol (bPEG-Chol) and subsequent binding of streptavidin (SA) further induced accumulation of the clusters, resulting in the formation of a spherical cell extrusion. This extrusion was also induced by direct crosslinking of a raft marker, CD59, and ganglioside GM1. In addition, we found that knockout of the gene encoding Fyn kinase inhibited formation of the spherical extrusion in murine T-cells. In bPEG-Chol/SA-treated cells, CD59, ganglioside GM1, and clathrin/AP-2 were all accumulated on the surface of the actin-rich extrusion, whereas dynamin and transferrin receptors were unaffected. Intermediate filaments, mitochondria, and other vesicles also accumulated. These results suggest that crosslinking of membrane domains exaggerates the linkage between actin and a defined set of membrane proteins in OKA-treated cells.

Fragmentation of the Golgi apparatus. A role for beta III spectrin and synthesis of phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) synthesis has been implicated in maintaining the function of the Golgi apparatus. Here we demonstrate that the inhibition of PtdIns(4,5)P(2) synthesis in vitro in response to primary alcohol treatment and the kinetics of Golgi fragmentation in vivo were very rapid and tightly coupled. Preloading Golgi membranes with short chain phosphatidic acid abrogated the alcohol-mediated inhibition of PtdIns(4,5)P(2) synthesis in vitro. We also show that fragmentation of the Golgi apparatus in response to diminished PtdIns(4,5)P(2) synthesis correlated with both the phosphorylation of a Golgi form of beta III spectrin, a PtdIns(4,5)P(2)-interacting protein, and changes in its intracellular redistribution. The data are consistent with a model suggesting that the decreased PtdIns(4,5)P(2) synthesis and the phosphorylation state of beta III spectrin modulate the structural integrity of the Golgi apparatus.

In vitro transport on cis and trans sides of the Golgi involves two distinct types of coatomer and ADP-ribosylation factor-independent transport intermediates

The cisternal maturation model proposes that secretory proteins transit the Golgi in cisternae that mature by the continuous retrograde transport of Golgi enzymes in vesicles. We have tested the hypothesis that de novo generation of transport intermediates containing medial, trans, and trans Golgi network (TGN) enzymes is reconstituted in vitro. Our analysis shows that the majority of transport is mediated by a steady state of transport intermediate production and consumption by Golgi cisternae, with only a minor contribution of pre-existing transport intermediates. Transport in the medial and trans regions of the stack involved intermediates containing Golgi enzymes, apparently moving in a retrograde direction. In contrast, transport between the trans Golgi and TGN was exclusively mediated by intermediates containing secretory protein, as expected for anterograde transport. These intermediates may be physiologically relevant, because only these two specific types of intermediates can be detected in cell homogenates. By analogy to the coatomer (COPI)-independent transport of Golgi enzymes to the endoplasmic reticulum, the steady-state production of intra-Golgi transport intermediates was not impaired by inhibition of COPI vesicle formation. These data suggest a model for COPI-independent intra-Golgi transport by cisternal maturation with a shift in mechanism to anterograde transport at the trans Golgi and TGN boundary.

Structural integrity of the Golgi stack is essential for normal secretory functions of rat parotid acinar cells: effects of brefeldin A and okadaic acid

We examined the effects of specific inhibitors, brefeldin A (BFA) and okadaic acid (OA), on the ultrastructural organization of the Golgi apparatus and distributions of amylase, Golgi-associated proteins, and cathepsin D in the rat parotid acinar cells. BFA induced a rapid regression of the Golgi stack into rudimentary Golgi clusters composed of tubulovesicules, in parallel with a redistribution of the Golgi-resident proteins and a coat protein (beta-COP) into the region of the rough endoplasmic reticulum (rER) or cytosol. The rapid disruption of the Golgi stack could also be induced by the effect of OA. However, redistribution of the Golgi proteins in rER or cytosol could not be observed and beta-COP was not dispersed but was retained on the rudimentary Golgi apparatus. These findings suggested that the mechanism of OA in inducing degeneration of the Golgi stack was markedly different from that of BFA. In addition, missorting of amylase, a Golgi protein, and cathepsin D into incorrect transport pathways is apparent in the course of the disruption of the Golgi stack by OA. These Golgi-disrupting effects are reversible and the reconstruction of the stacked structure of the Golgi apparatus started immediately after the removal of inhibitors. In the recovery processes, missorting was also observed until the integrated structure of the Golgi apparatus was completely reconstructed. This suggested that the integrated structure of the Golgi apparatus was quite necessary for the occurrence of normal secretory events, including proper sorting of molecules.

The stack of the golgi apparatus

One hundred years have passed since the discovery of "the internal reticular apparatus" by Camillo GOLGI. Investigations into the structure and function of the "Golgi apparatus" have raised more and more challenging issues for cell biologists. After long debate, many new findings have accumulated in the last 10 years as a result of the availability of elegant new genetic, biochemical and morphological tools. This, in turn, has raised many new questions to be solved. In addition, numerous new findings have led to some confusion on the understanding of the Golgi apparatus. This review article deals with several modern aspects of vesicular transport versus cisternal maturation. Disruption of the stacked structure in mitotic and drug-induced conditions is also discussed to demonstrate the importance of structural integrity in the Golgi apparatus.

Glyceraldehyde-3-phosphate dehydrogenase is phosphorylated by protein kinase Ciota /lambda and plays a role in microtubule dynamics in the early secretory pathway

The small GTPase Rab2 immunolocalizes to vesicular tubular clusters (VTCs) that function as transport complexes carrying cargo between the endoplasmic reticulum and the Golgi complex. Our previous studies showed that Rab2 promotes vesicle formation from VTCs and that the released vesicles are enriched in beta-coat protein, protein kinase C iota/lambda (PKCiota/lambda), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the recycling protein p53/gp58. Because PKCiota/lambda kinase activity was necessary for vesicle formation, a search was initiated to identify the substrate(s) that potentiate Rab2 function within VTCs. In this study, we found that PKCiota/lambda phosphorylates GAPDH. Moreover, GAPDH interacts directly with the PKCiota/lambda regulatory domain. Based on numerous observations that show (beta-COP) GAPDH associates with cytoskeletal elements, we examined the role of phospho-GAPDH in promoting microtubule (MT) binding to membrane. Using a quantitative microsomal binding assay, we found that membrane association of beta-tubulin was dependent on phospho-GAPDH and was blocked by reagents that interfere with Rab2-dependent GAPDH membrane recruitment or with PKCiota/lambda kinase activity. Furthermore, normal rat kidney cells transfected with a constitutively activated form of Rab2 (Q65L) or with our anti-GAPDH polyclonal antibody displayed a dramatic change in MT organization. These combined results suggest that Rab2 stimulated PKCiota/lambda and GAPDH recruitment to VTCs, and the subsequent PKCiota/lambda phosphorylation of GAPDH ultimately influences MT dynamics in the early secretory pathway.

Evidence for prebudding arrest of ER export in animal cell mitosis and its role in generating Golgi partitioning intermediates

During mitosis the interconnected Golgi complex of animal cells breaks down to produce both finely dispersed elements and discrete vesiculotubular structures. The endoplasmic reticulum (ER) plays a controversial role in generating these partitioning intermediates and here we highlight the importance of mitotic ER export arrest in this process. We show that experimental inhibition of ER export (by microinjecting dominant negative Sar1 mutant proteins) is sufficient to induce and maintain transformation of Golgi cisternae to vesiculotubular remnants during interphase and telophase, respectively. We also show that buds on the ER, ER exit sites and COPII vesicles are markedly depleted in mitotic cells and COPII components Sec23p, Sec24p, Sec13p and Sec31p redistribute into the cytosol, indicating ER export is inhibited at an early stage. Finally, we find a markedly uneven distribution of Golgi residents over residual exit sites of metaphase cells, consistent with tubulovesicular Golgi remnants arising by fragmentation rather than redistribution via the ER. Together, these results suggest selective recycling of Golgi residents, combined with prebudding cessation of ER export, induces transformation of Golgi cisternae to vesiculotubular remnants in mitotic cells. The vesiculotubular Golgi remnants, containing populations of slow or nonrecycling Golgi components, arise by fragmentation of a depleted Golgi ribbon independently from the ER.

The manganese cation disrupts membrane dynamics along the secretory pathway

The endoplasmic reticulum and Golgi apparatus play key roles in regulating the folding, assembly, and transport of newly synthesized proteins along the secretory pathway. We find that the divalent cation manganese disrupts the Golgi apparatus and endoplasmic reticulum (ER). The Golgi apparatus is fragmented into smaller dispersed structures upon manganese treatment. Golgi residents, such as TGN46, beta1,4-galactosyltransferase, giantin, and GM130, are still segregated and partitioned correctly into smaller stacked fragments in manganese-treated cells. The mesh-like ER network is substantially affected and peripheral ER elements are collapsed. These effects are consistent with manganese-mediated inhibition of motor proteins that link membrane organelles along the secretory pathway to the cytoskeleton. This divalent cation thus represents a new tool for studying protein secretion and membrane dynamics along the secretory pathway.

The mitotic phosphorylation cycle of the cis-Golgi matrix protein GM130

The cis-Golgi matrix protein GM130 is phosphorylated in mitosis on serine 25. Phosphorylation inhibits binding to p115, a vesicle-tethering protein, and has been implicated as an important step in the mitotic Golgi fragmentation process. We have generated an antibody that specifically recognizes GM130 phosphorylated on serine 25, and used this antibody to study the temporal regulation of phosphorylation in vivo. GM130 is phosphorylated in prophase as the Golgi complex starts to break down, and remains phosphorylated during further breakdown and partitioning of the Golgi fragments in metaphase and anaphase. In telophase, GM130 is dephosphorylated as the Golgi fragments start to reassemble. The timing of phosphorylation and dephosphorylation correlates with the dissociation and reassociation of p115 with Golgi membranes. GM130 phosphorylation and p115 dissociation appear specific to mitosis, since they are not induced by several drugs that trigger nonmitotic Golgi fragmentation. The phosphatase responsible for dephosphorylation of mitotic GM130 was identified as PP2A. The active species was identified as heterotrimeric phosphatase containing the Balpha regulatory subunit, suggesting a role for this isoform in the reassembly of mitotic Golgi membranes at the end of mitosis.

Coatomer vesicles are not required for inhibition of Golgi transport by G-protein activators

The G-protein activators guanosine 5'-O-(3-thiodiphosphate) (GTP gamma S) and aluminum fluoride (AIF) are thought to inhibit transport between Golgi cisternae by causing the accumulation of nonfunctional coatomer-coated transport vesicles on the Golgi. Although GTP gamma S and AIF inhibit transport in cell-free intra-Golgi transport systems, blocking coatomer vesicle formation does not. We therefore determined whether inhibition of in vitro Golgi transport by these agents requires coatomer vesicle formation. Depletion of coatomer was found to completely block coated vesicle formation on Golgi cisternae without affecting inhibition of in vitro transport by either GTP gamma S or AIF. Depletion of ADP-ribosylation factor (ARF) prevented inhibition of transport by GTP gamma S, but not by AIF, suggesting that the AIF-sensitive component in transport may not be a GTP-binding protein. Surprisingly, depletion of cytosolic ARF did not prevent the GTP gamma S-induced formation of Golgi-coated vesicles, whereas ARF was required for AIF-induced vesicle formation. Although ARF or coatomer depletion caused an increase in the fenestration of cisternae, no other ultrastructural changes were observed that might explain the inhibition of transport by GTP gamma S or AIF. These findings suggest that ARF-GTP gamma S and AIF act by distinct and coatomer-independent mechanisms to inhibit membrane fusion in cell-free intra-Golgi transport.

The role of microtubules in the regulation of proteoglycan synthesis in chondrocytes under hydrostatic pressure

Chondrocytes of the articular cartilage sense mechanical factors associated with joint loading, such as hydrostatic pressure, and maintain the homeostasis of the extracellular matrix by regulating the metabolism of proteoglycans (PGs) and collagens. Intermittent hydrostatic pressure stimulates, while continuous high hydrostatic pressure inhibits, the biosynthesis of PGs. High continuous hydrostatic pressure also changes the structure of cytoskeleton and Golgi complex in cultured chondrocytes. Using microtubule (MT)-affecting drugs nocodazole and taxol as tools we examined whether MTs are involved in the regulation of PG synthesis in pressurized primary chondrocyte monolayer cultures. Disruption of the microtubular array by nocodazole inhibited [(35)S]sulfate incorporation by 39-48%, while MT stabilization by taxol caused maximally a 17% inhibition. Continuous hydrostatic pressure further decreased the synthesis by 34-42% in nocodazole-treated cultures. This suggests that high pressure exerts its inhibitory effect through mechanisms independent of MTs. On the other hand, nocodazole and taxol both prevented the stimulation of PG synthesis by cyclic 0. 5 Hz, 5 MPa hydrostatic pressure. The drugs did not affect the structural and functional properties of the PGs, and none of the treatments significantly affected cell viability, as indicated by the high level of PG synthesis 24-48 h after the release of drugs and/or high hydrostatic pressure. Our data on two-dimensional chondrocyte cultures indicate that inhibition of PG synthesis by continuous high hydrostatic pressure does not interfere with the MT-dependent vesicle traffic, while the stimulation of synthesis by cyclic pressure does not occur if the dynamic nature of MTs is disturbed by nocodazole. Similar phenomena may operate in cartilage matrix embedded chondrocytes.

Concentration of caveolin-1 in the cleavage furrow as revealed by time-lapse analysis

Caveolin-1 is a major component of caveolae. Recent studies have suggested a possible role of caveolin-1 in cell transformation and normal cell proliferation. To observe the behavior of caveolin-1 in living mitotic cells, we prepared cDNA constructs encoding the chimeric protein of alpha- or beta-caveolin-1 and green fluorescent protein (GFP) and transfected culture cells with them. Correct targeting of the chimera to the caveolae was confirmed by colocalization with the caveolar inositol 1,4,5-trisphosphate receptor-like protein. By time-lapse observation of mitotic MDCKII cells, the GFP-caveolin-1 chimeras were seen throughout the plasma membrane before cell division, but became markedly concentrated at the cleavage furrow during cytokinesis. Accumulation around the spindle poles was also observed at late telophase. The result showed that caveolin-1 undergoes a drastic distributional change during cell division and suggested that the protein may be involved in the cytokinetic process.

Relationship between phosphatidic acid level and regulation of protein transit in colonic epithelial cell line HT29-cl19A

Colonic epithelial HT29-cl19A cells are polarized and secrete proteins among which alpha(1)-antitrypsin represents about 95%. Secretion occurs via a constitutive pathway, so that the rates of secretion directly reflect the rates of protein transit. In this paper we have demonstrated that: 1) in resting cells phospholipase D (PLD) is implicated in the control of apical protein transit; 2) phorbol esters stimulate apical protein transit (stimulation factor 2.2), which is correlated with a PLD-catalyzed production of phosphatidic acid (PA) (2.45-fold increase); 3) the stimulation of cholinergic receptors by carbachol results in an increase (stimulation factor 1.45) of apical protein transit which is independent of protein kinase C and PLD activities, but related to PA formation (1.7-fold increase) via phospholipase(s) C and diacylglycerol kinase activation; 4) an elevation of the cAMP level enhances apical protein transit by a PA-independent mechanism; 5) a trans-Golgi network or post-trans-Golgi network step of the transit is the target for the regulatory events. In conclusion, we have shown that PA can be produced by two independent signaling pathways; whatever the pathway followed, a close relationship between the amount of PA and the level of secretion was observed.

Coalescence of Golgi fragments in microtubule-deprived living cells

The process of stack coalescence, an important mechanism of Golgi recovery from mitosis, was examined using novel experimental paradigms. In living cells with disrupted (by nocodazole) microtubules, galactosyl transferase-GFP-labelled Golgi fragments constantly appeared, grew, sometimes moved with a speed of 1-2 microns/min, coalesced or gradually diminished and disappeared. The rate of Golgi fragment turnover and coalescence was highly balanced to maintain a constant number of Golgi units per cell. Moreover some Golgi islands appear and some received new GalTase-GFP after photobleaching of cell cytoplasm. Short tubules extending from the rims of scattered Golgi fragments frequently formed bridges between ministacks, inducing their coalescence. The frequency of coalescence could also be inhibited by disruption of actin microfilaments. After the Golgi redistribution into endoplasmic reticulum induced by brefeldin A, either the growth of small Golgi fragments or their coalescence leads to compartmentalized stack formation without the participation of microtubules. These results demonstrate that this coalescence between isolated Golgi stacks is microtubule-independent and could thus be mediated by membranous tubules.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

Low cytoplasmic pH causes fragmentation and dispersal of the Golgi apparatus in human hepatoma cells

The centrosomal localization of the Golgi apparatus in interphase cells is thought to be maintained by retrograde microtubule-based motility. It is well established that, when intracellular pH is lowered, lysosomes and endosomes, also showing pericentrosomal localization, translocate towards the plus ends of microtubules within 15 min. In this study, we found that prolonged incubation in low pH medium (pH 6.6) with 20 mM Na acetate induced the fragmentation and dispersal of the Golgi apparatus in the human hepatoma cell line PLC/PRF/5. The fraction of Golgi-dispersed cells increased in a time-dependent manner, and reached over 60% after the 16-h incubation. The cytoplasmic pH was dropped to approximately 7.10. Replacement with normal pH medium restored the structure and localization of the apparatus within 30 min. In the low pH condition, the microtubular network and endoplasmic reticulum appeared normal, and cytoplasmic dynein was still bound to the fragmented Golgi membranes. These findings suggest that low cytoplasmic pH suppresses the retrograde movement of the Golgi apparatus as well as that of lysosomes and endosomes.

Aphidicolin induces alterations in Golgi complex and disorganization of microtubules of HeLa cells upon long-term administration

Treatment of HeLa cells with aphidicolin at 5 or 0.5 microg/ml induced cell cycle arrest at G1/S or G2/M phase, respectively, and was accompanied by unbalanced cell growth. Long-term administration of aphidicolin (more than 48 h) resulted in noticeable loss of reproductive capacity though cells were viable at the time of treatment. Immunofluorescence with anti-Golgi membrane protein monoclonal antibody (mAbG3A5) showed disfigurement of the characteristic mesh-like configuration when cells were treated for more than 48 h. Interestingly, we found that the fragmented Golgi complex formed a ring around the nucleus in more than 20% of the cells. Immunoelectron microscopy using mAbG3A5 antibody demonstrated that the stack structure of the fragmented Golgi complex in aphidicolin-arrested cells appeared partially broken up and seemed to have converted to a vesicle-like structure. Analysis using an antibody to tubulin and anticentrosome human autoimmune serum showed that alterations in the Golgi complex were induced even by the lower 0.5 microg/ml dose. These alterations were accompanied by both changes in the distribution of microtubules and an increase in the number of centrosomes. These cells lost their distinct perinuclear microtubule organizing center (MTOC). On the other hand, treatment with aphidicolin at 5 microg/ml did not induce multiplication of the centrosome although the loss of distinct MTOC was still evident. No changes took place in the Golgi complex, microtubule, or centrosome of cells treated with 0.5 microg/ml aphidicolin when cycloheximide was added simultaneously to the culture.

In vitro synthesis of sulfated glycosaminoglycans coupled to inter-compartmental Golgi transport

We have used isolated rat liver Golgi membranes to reconstitute the synthesis of sulfated glycosaminoglycans (GAGs) onto the membrane-permeable, external acceptor xyloside. Biosynthetic labeling of GAGs with [35S]sulfate in vitro is shown to have an absolute requirement for ATP and cytosolic proteins and is inhibited by dismantling the Golgi apparatus with okadaic acid or under mitotic conditions suggesting that inter-compartmental transport between Golgi cisternae is a prerequisite for the successful completion of the initiation, polymerization, and sulfation of GAGs. Accordingly, we show that in vitro synthesis of 35S-GAGs utilizes the same machinery employed in Golgi transport events in terms of vesicle budding (ADP-ribosylation factor and coatomer), docking (Rabs), targeting (SNAREs), and fusion (N-ethylmaleimide-sensitive factor). This provides compelling evidence that GAGs synthesis is linked to Golgi membrane traffic and suggests that it can be used as a complementation-independent method to study membrane transport in Golgi preparations from any source. We have applied this system to show that intra-Golgi traffic requires the function of the Golgi target-SNARE, syntaxin 5.

Calphostin C induces selective disassembly of the Golgi complex by a protein kinase C-independent mechanism

Intact cells incubated with calphostin C, an inhibitor of the regulatory domain of protein kinase C, showed fragmentation and dispersal of the Golgi complex by a light-dependent mechanism. At the ultrastructural level Golgi stacks were replaced by clusters of vesicles and short tubules that resembled the Golgi remnants present in control mitotic cells. Vesicle-mediated transport processes along both the exocytic and endocytic routes were also inhibited by calphostin C treatment. Golgi disassembly, however, was not due to protein kinase C inhibition since several inhibitors of the catalytic domain did not cause a similar effect. In contrast, pretreatment with phorbol 12-myristate 13-acetate partly protected the Golgi complex from disassembly by calphostin C. The in vitro effect was shown to be reversible, required both cytosol and ATP and it was inhibited by pretreatment of the Golgi membranes with trypsin but not with high salt. These results suggest the interaction of calphostin C with a structural Golgi protein containing a phorbol ester-binding domain and necessary for the stability of this organelle during interphase.

Signalling molecules and the regulation of intracellular transport

A variety of signalling molecules has been implicated over the past 8 years in the regulation of intracellular transport pathways. Those molecules include heterotrimeric GTP binding proteins, members of the protein kinase C family, and members of the Rho subfamily of small GTPases. Until recently, no common theme among the three classes of regulators was apparent. The finding that all three can influence the activity of phospholipase D (PLD), and the fact that members of the Arf subfamily of GTPases (with established roles in intracellular transport) are potent activators of PLD suggests the hypothesis that PLD is a focal point for integration of cellular responses to hormone signalling and for membrane homeostasis. Work during the past 2 years is beginning to uncover some transport pathways where PLD involvement is inferred. It is proposed that, if signalling is required to monitor and adjust transport rates to and from the various membrane organelles, the most economical way to achieve this would be to regulate recycling and allow the concentration of cargo receptors to determine forward transport.

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

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

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

Okadaic acid induces selective arrest of protein transport in the rough endoplasmic reticulum and prevents export into COPII-coated structures

Quantitative immunoelectron microscopy and subcellular fractionation established the site of endoplasmic reticulum (ER)-Golgi transport arrest induced by the phosphatase inhibitor okadaic acid (OA). OA induced the disappearance of transitional element tubules and accumulation of the anterograde-transported Chandipura (CHP) virus G protein only in the rough ER (RER) and not at more distal sites. The block was specific to the early part of the anterograde pathway, because CHP virus G protein that accumulated in the intermediate compartment (IC) at 15 degrees C could gain access to Golgi stack enzymes. OA also induced RER accumulation of the IC protein p53/p58 via an IC-RER recycling pathway which was resistant to OA and inhibited by the G protein activator aluminium fluoride. The role of COPII coats in OA transport block was investigated by using immunofluorescence and cell fractionation. In untreated cells the COPII coat protein sec 13p colocalized with p53/p58 in Golgi-IC structures of the juxtanuclear region and peripheral cytoplasm. During OA treatment, p53/p58 accumulated in the RER but was excluded from sec 13p-containing membrane structures. Taken together our data indicate that OA induces an early defect in RER export which acts to prevent entry into COPII-coated structures of the IC region.

Selective modulation of the major histocompatibility complex class II antigen presentation pathway following B cell receptor ligation and protein kinase C activation

We noticed that B cell receptor ligation or phorbol 12-myristate 13-acetate treatment induced intracellular vesicles containing major histocompatibility complex (MHC) class II and invariant chain (Ii), and increased the amount of transmembrane p12 Ii fragments coimmunoprecipitated with class II molecules. To determine the influence of protein kinase C activation on the MHC class II presentation pathway, we analyzed the subcellular distribution of Ii, the induction of SDS-stable forms of class II molecules, and their ability to present different antigens. Ii chains visualized with luminal and cytoplasmic directed antibodies appeared in early endosomal compartments accessible to transferrin in response to phorbol 12-myristate 13-acetate treatment, whereas transmembrane Ii degradation products equivalent to the p12 Ii fragments were colocalized with the B cell receptors internalized after cross-linking. Protein kinase C activation delayed in parallel the formation of SDS-stable forms of class II molecules and reduced the presentation of antigenic determinants requiring newly synthesized class II alphabeta-Ii complexes. These data indicate that B cell activation affects Ii processing and MHC class II peptide loading in endosomal compartments intersecting the biosynthetic pathway.

Feedback control of milk secretion from milk

Extracellular storage allows biologically-active substances in milk to influence mammary function. Among these factors is one which regulates the rate of milk secretion acutely according to frequency or completeness of milk removal in each mammary gland. The active factor in goat's milk has been identified by screening milk constituents for their ability to inhibit milk constituent secretion in tissue and cell culture bioassays, and found to be a novel milk protein. The proteins identified by bioassy in vitro, also inhibited milk secretion in lactating goats in a reversible, concentration-dependent manner. This protein, termed FIL (feedback inhibitor of lactation), acts by reversible blockade of constitutive secretion in the mammary epithelial cell. As the inhibitor is synthesized in the same epithelial cells, feedback inhibition is, therefore, an autocrine mechanism. FIL's unusual mechanism of action also influences other aspects of mammary function. Acute disruption of mammary membrane trafficking is associated with downregulation of prolactin receptors and followed by a decrease in epithelial cell differentiation. Thus, in addition to acutely-regulating milk secretion, FIL may induce the adaptation in mammary cell differentiation which acts in vivo to sustain the secretory response to a sustained change in milk removal. In the long term, matching of milk output to demand is achieved by a change in mammary cell number. This developmental response is also local in nature. Whether it too is due to autocrine modulation by FIL of mechanisms influencing cell proliferation or survival, or elicited by another milk-borne factor, remains to be determined.

Okadaic acid gives concentration-dependent reciprocal effects on the fluid phase endocytosis activated by Ca2+ and phorbol 12-myristate 13-acetate

Incubation of a human fibrosarcoma cell line HT-1080 in increasing concentration of Ca2+ was found to enhance endocytic internalization of a fluid phase marker, horseradish peroxidase. At 16.8 mM Ca2+, generation of the effect required incubation for more than 45 min. The effect was reversed by removal of the excess ion for 30 min. Monitoring the intracellular concentration showed that the incubation induced a transient large Ca2+ influx followed by a recovery to 230 +/- 50 nM instead of the normal level of 83 +/- 5 nM. The activation was not inhibited by inhibitors of protein kinases nor a cAMP antagonist. In contrast, the effect was prevented by okadaic acid (OKA) at 100 nM without detectable effect on the basal activity. Fluid phase uptake by HT-1080 cells was also enhanced by phorbol 12-myristate 13-acetate (PMA). In contrast to the case with Ca2+, OKA at 100 nM did not prevent the PMA effect but further enhanced the endocytosis. The effect of OKA was concentration-dependent, as the reagent at 1 microM inhibited not only both the activation but also the basal activity. In Ca(2+)- or PMA-stimulated cells, FITC-dextran was delivered to endosomes that had been labeled with TRITC-transferrin. In contrast, following treatment with a combination of PMA and 100 mM OKA, fluid phase was internalized in vesicular compartments devoid of transferrin labeling. These results suggest that, through differential modifications of protein phosphorylation, endocytosis can be enhanced distinctively either by employing conventional receptor-bearing compartments or generating a new endosomal population.

Evidence for recycling of the resident medial/trans Golgi enzyme, N-acetylglucosaminyltransferase I, in ldlD cells

ldlD cells, which lack the UDP-Gal/UDP-GalNAc 4-epimerase, were stably transfected with a Myc-tagged version of N-acetylglucosaminyltransferase I (Myc-Glc-NAc-T I). In the absence of GalNAc and Gal, newly synthesized GlcNAc-T I did not acquire O-linked oligosaccharides but was catalytically active and was transported to the Golgi region as defined using both immunofluorescence and immunoelectron microscopy. After addition of cycloheximide to prevent further synthesis, GalNAc and Gal were added, and the unglycosylated GlcNAc-T I was found to acquire mature, O-linked oligosaccharides with a half-time of about 150 min. The addition of these sugars was sensitive to N-ethylmaleimide and okadaic acid, both inhibitors of vesicle-mediated traffic. Together, these results suggest that Myc-Glc-NAc-T I undergoes retrograde transport to the early part of the Golgi apparatus where the first O-linked sugar, GalNAc, is added followed by anterograde transport back to the Golgi stack, where addition of Gal and sialic acid occurs.

Okadaic acid potentiates heat-induced activation of erk2

Subjecting exponentially growing HeLa cells to heat shock at 45 degrees C for 30 min leads to retarded migration of erk1 and erk2, as revealed on immunoblots indicating their activation. Renaturation gels confirmed activation of erk2 but not erk1. Treatment of cells with okadaic acid (OA) alone marginally upregulated erk1 and erk2, whereas simultaneous exposure to heat shock and OA led to a considerably augmented response for erk2 which was approximately 3-fold higher than the sum of heat- and OA-induced activation. Chronic treatment of cells with 12-O-tetradecanoyl-phorbol 13-acetate marginally diminished the extent of erk2 stimulation, but had no influence on the OA-induced potentiation of heat-induced erk2 activity.

Mechanism of transferrin receptor down-regulation in K562 cells in response to protein kinase C activation

Treatment with phorbol esters increases endocytosis of the transferrin receptor in K562 cells (Klausner, R. D., Harford, J., and van Renswoude, J. (1984) Proc. Natl. Acad. Sci. U. S. A. 81, 3005-3009). In this report, we demonstrate that this effect is reversible within early times of protein kinase C activation (< 2 h) but that prolonged exposure to phorbol esters results in a net loss of receptors. These effects are not due to the differentiation response of K562 cells to phorbol esters since bryostatin-1 also down-regulates the endocytosis of the transferrin receptor and shut downs receptor synthesis, but does not induce differentiation (Hocevar, B. A., Morrow, D. M., Tykocinski, M. L., and Fields, A. P. (1992) J. Cell Sci. 101, 671-679). We have characterized the early stages of receptor down-regulation which occur due to stimulation of receptor internalization from the cell surface. The fact that fluid-phase pinocytosis is also enhanced upon protein kinase C activation indicates that this effect is not specific for the transferrin receptor itself, but is a rather general cellular response to tumor-promoting phorbol esters. The fate of down-regulated transferrin receptors was followed in morphological and subcellular fractionation studies that demonstrate localization of this pool of receptors in early endocytic and recycling compartments. Our results exclude the possibility that transferrin receptor down-regulation results in trafficking of the receptor to lysosomal compartments for degradation. This idea is consistent with the observations that the time course of transferrin receptor degradation is not enhanced in stimulated K562 cells, while transferrin receptor synthesis is shut down. Our results rigorously demonstrate that activation of protein kinase C down-regulates the K562 cell transferrin receptor in two stages: acute regulation of early steps in endocytosis that results in an immediate reduction of approximately 40% in cell surface number of receptors and a more chronic reduction in transferrin receptor synthesis upon prolonged exposure to phorbol esters (> 15 h).

Polarized exocytosis in MDCK cells is regulated by phosphorylation

Protein phosphorylation and dephosphorylation systems modulate many cellular activities and have recently been implicated in the in vitro transport of newly synthesized proteins. Here we show that polarized transport from the Golgi to the plasma membrane in intact MDCK cells is regulated by phosphorylation-dephosphorylation. Transport is inhibited by the phosphatase inhibitor okadaic acid and is stimulated by the kinase inhibitor staurosporine. Stimulation of apical transport exceeds stimulation of basolateral transport by up to 5-fold. We also find that the G protein activator aluminum fluoride, which stimulates transport to the surface at low fluoride concentrations as previously reported, inhibits transport at higher concentrations. In the nonpolarized fibroblast cell line CV-1, neither staurosporine nor aluminum fluoride stimulates transport to the cell surface. Our results suggest that the phosphorylation-dephosphorylation system, like the G protein, may be involved in the specialized sorting process characteristic of polarized cells. We show some evidence that these two mechanisms of regulation may act through common intermediates.

A group of integral membrane proteins of the rat liver Golgi contains a conserved protein of 100 kDa

Rat liver Golgi membranes were washed with KCl and urea, and a polyclonal antiserum that stained the Golgi complex by immunofluorescence microscopy was raised. A group of proteins of apparent molecular mass 500 kDa, 200 kDa and 100 kDa were identified by immunoblotting with the antiserum, and were enriched in the Golgi membrane fraction. These proteins were also localised to the Golgi by immunofluorescence microscopy with affinity-purified antibodies. They are integral membrane proteins, and protease digestion experiments suggested that they are not exposed on the cytoplasmic face of the Golgi membrane. Immunofluorescence microscopy showed that staining of the Golgi complex by antibodies to the 100 kDa Golgi protein can be demonstrated among a wide range of mammalian species. This conservation may point to an important structural or functional role for the molecule. When the 100 kDa protein was reduced with dithiothreitol it was no longer recognised by the anti-Golgi antiserum. During phase separation in Triton X-114 the 100 kDa protein partitioned into the aqueous phase, rather than into the detergent phase, suggesting that it has a large luminal domain of hydrophilic amino acids.