Golgi fractions from livers of control and ethanol-intoxicated rats. Enzymic and morphologic properties following rapid isolation

The lipid fluidity of rat liver membrane subfractions

1. The lipid fluidity of three major rat liver plasma-membrane subfractions, as well as Golgi apparatus and endocytic fractions, was assessed with a fatty acid spin probe by using e.s.r. techniques. 2. The sinusoidal (blood-facing) plasma-membrane subfraction was the most fluid of the three plasma-membrane regions. Fractions originating from the bile-canalicular and contiguous (lateral) regions were most rigid. Endocytic fractions isolated (endosomes and diacytosomes) were of a similar fluidity to fractions originating from the sinusoidal plasma-membrane region. By far the most fluid fractions examined were derived from the Golgi-apparatus complex. 3. The three plasma-membrane subfractions each showed a different response to the bilayer-fluidizing effect of benzyl alcohol. 4. Arrhenius-type plots of the order parameter S and outer hyperfine splitting, 2T( parallel), identified lipid-phase separations in the plasma-membrane subfractions.

Identification and characterization of associated with lipid droplet protein 1: A novel membrane-associated protein that resides on hepatic lipid droplets

Alcoholic and nonalcoholic liver steatosis and steatohepatitis are characterized by the massive accumulation of lipid droplets (LDs) in the cytosol of hepatocytes. Although LDs are ubiquitous and dynamic organelles found in the cells of a wide range of organisms, little is known about the mechanisms and sites of LD biogenesis. To examine the participation of these organelles in the pathophysiological disorders of steatotic livers, we used a combination of mass spectrometry (matrix-assisted laser desorption ionization-time of flight and LC-MS electrospray) and Western blot analysis to study the composition of LDs purified from rat liver after a partial hepatectomy. Fifty proteins were identified. Adipose differentiation-related protein was the most abundant, but other proteins such as calreticulin, TIP47, Sar1, Rab GTPases, Rho and actin were also found. In addition, we identified protein associated with lipid droplets I ALDI (tentatively named Associated with LD protein 1), a novel protein widely expressed in liver and kidney corresponding to the product of 0610006F02Rik (GI:27229118). Our results show that, upon lipid loading of the cells, ALDI translocates from the endoplasmic reticulum into nascent LDs and indicate that ALDI may be targeted to the initial lipid deposits that eventually form these droplets. Moreover, we used ALDI expression studies to view other processes related to these droplets, such as LD biogenesis, and to analyze LD dynamics. In conclusion, here we report the composition of hepatic LDs and describe a novel bona fide LD-associated protein that may provide new insights into the mechanisms and sites of LD biogenesis.

The "early-sorting" endocytic compartment of rat hepatocytes is involved in the intracellular pathway of caveolin-1 (VIP-21)

The sinusoidal plasma membrane of the hepatocyte is organized into functional and structural microdomains whose origin, maintenance, and functioning are closely related with the endocytic compartment. Three different subcellular fractions, from rat liver, containing caveolin-1, the structural protein of caveolae, were morphologically and biochemically characterized. A caveolae-enriched plasma membrane fraction (CEF), contains large membrane structures surrounding attached internal plasmalemmal vesicles; the receptor-recycling compartment (RRC), contains tubules and vesicles with similar morphology to the internal vesicles observed by electron microscopy in CEF; and finally, caveolin-1 was also detected in early-sorting endosomes (CURL, compartment of uncoupling receptors and ligands). In this study, we show that following an intravenous administration of retinol-binding protein (RBP), there was a redistribution of caveolin-1 from the plasma membrane (CEF) to intracellular endocytic compartments (RRC and early-sorting endosomes). Thus, these results indicate that, in the hepatocyte, caveolae are dynamic structures actively interacting with the endocytic compartment.

Fusogenic domains of golgi membranes are sequestered into specialized regions of the stack that can be released by mechanical fragmentation

A well-characterized cell-free assay that reconstitutes Golgi transport is shown to require physically fragmented Golgi fractions for maximal activity. A Golgi fraction containing large, highly stacked flattened cisternae associated with coatomer-rich components was inactive in the intra-Golgi transport assay. In contrast, more fragmented hepatic Golgi fractions of lower purity were highly active in this assay. Control experiments ruled out defects in glycosylation, the presence of excess coatomer or inhibitory factors, as well as the lack or consumption of limiting diffusible factors as responsible for the lower activity of intact Golgi fractions. Neither Brefeldin A treatment, preincubation with KCl (that completely removed associated coatomer) or preincubation with imidazole buffers that caused unstacking, activated stacked fractions for transport. Only physical fragmentation promoted recovery of Golgi fractions active for transport in vitro. Rate-zonal centrifugation partially separated smaller transport-active Golgi fragments with a unique v-SNARE pattern, away from the bulk of Golgi-derived elements identified by their morphology and content of Golgi marker enzymes (N-acetyl glucosaminyl and galactosyl transferase activities). These fragments released during activation likely represent intra-Golgi continuities involved in maintaining the dynamic redistribution of resident enzymes during rapid anterograde transport of secretory cargo through the Golgi in vivo.

Assembly of heteromeric connexons in guinea-pig liver en route to the Golgi apparatus, plasma membrane and gap junctions

Guinea-pig liver gap junctions are constructed from approximately equal amounts of connexins 26 and 32. The assembly of these connexins into connexon hemichannels and gap junctions was studied using antibodies specific to each connexin. Intracellular membranes were shown to contain low amounts of connexin 26 relative to connexin 32 in contrast to the equal connexin ratios detected in lateral plasma membranes and gap junctions. Assembly of gap junctions requires oligomerization of connexins into connexons that may be homomeric or heteromeric. Immunoprecipitation using antibodies to connexins 26 and 32 showed that liver gap junctions were heteromeric. A chemical cross-linking procedure showed that connexons solubilized from guinea-pig liver gap junctions were constructed of hexameric assemblies of connexin subunits. The intracellular site of oligomerization of connexins was investigated by velocity sedimentation in sucrose-detergent gradients. Oligomers of connexins 26 and 32 were extensively present in Golgi membranes and oligomeric intermediates, especially of connexin 26, were detected in the endoplasmic reticulum-Golgi intermediate subcellular fraction. Two intracellular trafficking pathways that may account for the delivery of connexin 26 to the plasma membrane and explain the heteromeric nature of liver gap junctions are discussed.

Isolation of plasma membranes and Golgi apparatus from a single chicken liver homogenate

Chicken liver plasma membranes, minimally contaminated with Golgi apparatus-derived vesicles, were prepared from a low-speed (400 g) pellet by means of flotation in isotonic Percoll solution, followed by a hypotonic wash and flotation in a discontinuous sucrose gradient. Based on the analysis of suitable marker enzymes, alkaline phosphatase and alkaline phosphodiesterase, two plasma membrane fractions were isolated with enrichments, depending on the equilibrium density and marker of 28-97 and with a total yield of 4-5%. Golgi apparatus fractions were prepared by flotation of microsomes, obtained from the same homogenate as the low-speed pellet, in a discontinuous sucrose gradient. The trans-Golgi marker galactosyltransferase was 27-fold enriched in a fraction of intermediate density (d=1.077-1.116 g/ml). Approximately 12% of galactosyltransferase was recovered in the membranes equilibrating d=1.031-1.148 g/ml. Contamination with plasma membrane fragments was low in the light (d=1.031-1.077 g/ml) and intermediate density Golgi vesicles. The isolation of purified plasma membranes and Golgi vesicles from one liver homogenate will enable future studies on receptor cycling between these cell organelles.

Characterization of the Golgi complex cleared of proteins in transit and examination of calcium uptake activities

To characterize endogenous molecules and activities of the Golgi complex, proteins in transit were > 99% cleared from rat hepatocytes by using cycloheximide (CHX) treatment. The loss of proteins in transit resulted in condensation of the Golgi cisternae and stacks. Isolation of a stacked Golgi fraction is equally efficient with or without proteins in transit [control (CTL SGF1) and cycloheximide (CHX SGF1)]. Electron microscopy and morphometric analysis showed that > 90% of the elements could be positively identified as Golgi stacks or cisternae. Biochemical analysis showed that the cis-, medial-, trans-, and TGN Golgi markers were enriched over the postnuclear supernatant 200- to 400-fold with and 400- to 700-fold without proteins in transit. To provide information on a mechanism for import of calcium required at the later stages of the secretory pathway, calcium uptake into CTL SGF1 and CHX SGF1 was examined. All calcium uptake into CTL SGF1 was dependent on a thapsigargin-resistant pump not resident to the Golgi complex and a thapsigargin-sensitive pump resident to the Golgi. Experiments using CHX SGF1 showed that the thapsigargin-resistant activity was a plasma membrane calcium ATPase isoform in transit to the plasma membrane and the thapsigargin-sensitive pump was a sarcoplasmic/endoplasmic reticulum calcium ATPase isoform. In vivo both of these calcium ATPases function to maintain millimolar levels of calcium within the Golgi lumen.

Impairment of glucagon-induced hepatic system A activity by short-term ethanol administration in the rat

BACKGROUND/AIMS

System A is a membrane-bound, hormonally regulated carrier of amino acids that is induced by liver regeneration and impaired by ethanol. The mechanism of ethanol inhibition of system A is unknown; this study examines the effects of ethanol on the subcellular expression of system A activity following hormonal induction.

METHODS

Following hormonal treatment and short-term ethanol administration to rats, isolated liver Golgi and plasma membrane vesicles were examined for system A transport, and the kinetic parameters were determined.

RESULTS

Four hours after ethanol administration, the initial rate of system A activity was depressed 30% +/- 9% and 19% +/- 7% into Golgi and plasma membrane vesicles, respectively. The affinity constant of 2-(methylamino)-isobutyric acid uptake was unchanged between control and ethanol-treated vesicles, regardless of their subcellular origin. However, the maximal velocity of system A transport decreased from 1030 to 850 pmol.mg-1 protein.10 s-1 in Golgi vesicles and from 740 to 355 pmol.mg-1 protein.10 s-1 in plasma membrane vesicles.

CONCLUSIONS

Ethanol impairs hormonally induced system A activity in Golgi as well as in the plasma membrane vesicles. Ethanol potentially reduces glucagon induction of system A activity through an impairment of carrier biosynthesis or expression.

Determination of the intracellular distribution and pool sizes of apolipoprotein B in rabbit liver

We have investigated the intracellular distribution of apolipoprotein B (apo B) in rabbit liver by immunoblotting, radioimmunoassay (r.i.a.) and enzyme-linked immunoassay (e.l.i.s.a.). Apo B100 was detected in total microsomes, rough microsomes, smooth microsomes, trans-enriched Golgi and cis-enriched Golgi and membrane and cisternal-content subfractions prepared from these fractions. There was also evidence of degradation of apo B100 in the Golgi membrane fractions. The amount of apo B in the subcellular fractions detected by competitive r.i.a. or e.l.i.s.a. ranged from 1.5 micrograms/mg of protein in the rough endoplasmic reticulum to 13 micrograms/mg of protein in the trans-Golgi fraction. Using internal standards (NADPH-cytochrome c reductase for the endoplasmic reticulum and galactosyltransferase for the Golgi membranes) it was calculated that all the apo B of liver is recovered within the secretory compartment, with 63% of the total apo B in the endoplasmic reticulum and the remainder in the Golgi. When the subcellular fractions were separated into membranes and cisternal contents, 60%, 50%, 60% and 30% of the total apo B was recovered in the membrane of the rough microsomes, smooth microsomes, cis-Golgi and trans-Golgi respectively. Using competitive e.l.i.s.a. we found that the membrane-bound form of the apo B was exposed at the cytosolic surface of the intact subcellular fractions. These observations are consistent with a model for assembly of very-low-density lipoproteins (VLDL) in which newly synthesized apo B is incorporated into a membrane-bound pool and a lumenal pool. The membrane-bound pool not used for VLDL assembly may be degraded, possibly in the Golgi region.

Quantification of apolipoprotein B-48 and B-100 in rat liver endoplasmic reticulum and Golgi fractions

We have developed a method for measurement of apolipoprotein (apo) B-48 and apo B-100 in blood and subcellular fractions of rat liver based on SDS/PAGE followed by quantitative immunoblotting using 125I-Protein A. Standard curves were prepared in each assay using apo B prepared from total rat lipoproteins by extraction with tetramethylurea. Subcellular fractions (rough and smooth endoplasmic reticulum and Golgi fractions) were prepared from rat liver and separated into membrane and cisternal-content fractions. For quantification, membrane fractions were solubilized in Triton X-100, and the apo B was immunoprecipitated before separation by SDS/PAGE and immunoblotting. Content fractions were concentrated by ultrafiltration and separated by SDS/PAGE without immunoprecipitation. Quantification of apo B in subcellular fractions and detection of apo B by immunoblotting yielded consistent results. In all fractions apo B-48 was the major form, accounting for approximately three-quarters of the total apo B. By using marker enzymes as internal standards, it was calculated that all of the apo B was recovered in the endoplasmic reticulum and Golgi fractions, with approximately 80% of each form of apo B in the endoplasmic reticulum. More than 90% of the apo B of the rough- and smooth-endoplasmic-reticulum fractions was membrane-bound, whereas approx. 33 and 15% of the apo B of the cis-enriched Golgi fractions and trans-enriched Golgi fractions respectively were membrane-bound.

Differential distribution of a glucuronyltransferase, involved in glucuronoxylan synthesis, within the Golgi apparatus of pea (Pisum sativum var. Alaska)

The subcellular location of a glucuronyltransferase (GT) involved in glucuronoxylan synthesis in pea (Pisum sativum) has been investigated. Most of the GT activity was found in the Golgi fraction, but activity was also detected in the plasma-membrane fraction. Separation of Golgi membranes on a shallow continuous sucrose density gradient resulted in three distinct subfractions, with GT activity being confined to Golgi membranes of a density similar to that of smooth endoplasmic reticulum. The differential distribution of GT within the Golgi stack indicates that glucuronoxylan synthesis occurs in specific cisternae and that there is functional compartmentalization of the Golgi with respect to hemicellulose biosynthesis.

Distribution of polypeptides binding guanosine 5'-[gamma-[35S]thio]triphosphate and anti-(ras protein) antibodies in liver subcellular fractions. Evidence for endosome-specific components

The subcellular distribution in rat liver of polypeptides binding guanosine 5'-[gamma-[35S]thio]triphosphate [( 35S]GTP[S]) and seven antibodies against ras oncoproteins was evaluated. Multiple low-Mr (21,000-28,000) GTP-binding proteins were detected, but their relative distribution among the membrane fractions varied. A more specific compartmentation of polypeptides which bind antibodies generated against ras proteins was evident, with an Mr-28,000 polypeptide and a probable Mr-56,000 dimer, identified by six of the antibodies tested, being confined mainly to endosomes. An Mr-23,000 polypeptide was detected by some of the antibodies in all of the membrane fractions, but especially in the plasma membranes.

Lipid peroxidation and covalent binding in the early functional impairment of liver Golgi apparatus by carbon tetrachloride

The onset of the lipoprotein secretory block provoked by CCl4 in the whole animal was monitored after purification of liver Golgi membranes. Both lipid transit through the apparatus and hexosylation of the lipoprotein are markedly inhibited 5-15 min after poisoning. Pre-treating the animal with alpha-tocopherol, shown to prevent lipid peroxidation without modifying the covalent binding due to CCl4 metabolites, affords little protection against lipid accumulation in the Golgi, but total preservation of galactosyl transferase activity. While haloalkylation therefore appears to be the major mechanism of damage in the early phases of CCl4-induced derangement of lipid secretion, lipid peroxidation is probably more involved later; this is indicated by the marked, though never complete, protection against fatty liver afforded at 24 h after CCl4 poisoning by supplementation of the membrane with alpha-tocopherol.

Phosphatidylinositol-specific phospholipase C of Bacillus thuringiensis as a probe for the distribution of phosphatidylinositol in hepatocyte membranes

Phosphatidylinositol-specific phospholipase C (PI-PLC) produced by Bacillus thuringiensis has been used as a probe for the distribution of phosphatidylinositol in hepatocyte membranes. Approx. 50% of this phospholipid was hydrolysed in microsomal vesicles (endoplasmic reticulum) with no significant hydrolysis of the remaining membrane phospholipids. Latency of mannose-6-phosphatase was retained during treatment indicating that the vesicles remained impermeable. Stripping of the ribosomes did not increase hydrolysis of phosphatidylinositol; however, when the vesicles were opened using dilute sodium carbonate, hydrolysis increased to greater than 90%. Hydrolysis of phosphatidylinositol of Golgi membranes was 35% and of plasma membranes was 50%. After treatment with PI-PLC, radiolabelled secretory proteins were retained in Golgi membranes and trapped lactate dehydrogenase was retained in plasma-membrane preparations indicating that the vesicles remained closed. Hydrolysis of phosphatidylinositol increased to greater than 90% when the membranes were opened by treatment with dilute sodium carbonate. These observations indicate that PI-PLC of Bacillus thuringiensis is a suitable probe for the distribution of phosphatidylinositol in membranes, and that in liver membranes this phospholipid occurs on each side of the bilayer, a topography consistent with its diverse roles.

Reconstitution of the Golgi apparatus after microinjection of rat liver Golgi fragments into Xenopus oocytes

We have studied the reconstitution of the Golgi apparatus in vivo using an heterologous membrane transplant system. Endogenous glycopeptides of rat hepatic Golgi fragments were radiolabeled in vitro with [3H]sialic acid using detergent-free conditions. The Golgi fragments consisting of dispersed vesicles and tubules with intraluminal lipoprotein-like particles were then microinjected into Xenopus oocytes and their fate studied by light (LM) and electron microscope (EM) radioautography. 3 h after microinjection, radiolabel was observed by LM radioautography over yolk platelet-free cytoplasmic regions near the injection site. EM radioautography revealed label over Golgi stacked saccules containing the hepatic marker of intraluminal lipoprotein-like particles. At 14 h after injection, LM radioautographs revealed label in the superficial cortex of the oocytes between the yolk platelets and at the oocyte surface. EM radioautography identified the labeled structures as the stacked saccules of the Golgi apparatus, the oocyte cortical granules, and the plasmalemma, indicating that a proportion of microinjected material was transferred to the surface via the secretion pathway of the oocyte. The efficiency of transport was low, however, as biochemical studies failed to show extensive secretion of radiolabel into the extracellular medium by 14 h with approximately half the microinjected radiolabeled constituents degraded. Vinblastine (50 microM) administered to oocytes led to the formation of tubulin paracrystals. Although microinjected Golgi fragments were able to effect the formation of stacked saccules in vinblastine-treated oocytes, negligible transfer of heterologous material to the oocyte surface could be detected by radioautography. The data demonstrate that dispersed fragments of the rat liver Golgi complex (i.e., unstacked vesicles and tubules) reconstitute into stacked saccules when microinjected into Xenopus cytoplasm. After the formation of stacked saccules, reconstituted Golgi fragments transport constituents into a portion of the exocytic pathway of the host cell by a microtubule-regulated process.

Studies on the submicrosomal fractions of bovine oligodendroglia: lipid composition and glycolipid biosynthesis

Oligodendroglia were isolated from bovine brain, and a "crude" microsomal fraction obtained from cell homogenates was subfractionated into myelin (MP), plasma membranes (PM), Golgi (GF), smooth (SER) and rough (RER) endoplasmic membranes using discontinuous-sucrose gradient centrifugation. The submicrosomal fractions were characterized by ultrastructural examination and analysis of the specific organelle markers. The myelin and plasma membrane rich fractions contained characteristically the highest amounts of the lipid with lower mole percentages of total phospholipids and phosphatidylcholine, and higher concentrations of phosphatidylethanolamine (+ plasmalogens), cholesterol and galactolipids. Considerable amounts of the typical myelin galactolipids (galacto-cerebrosides, sulfatides and monogalactosyl diglycerides) were also found in the Golgi fraction (GF). The GF fraction had the greatest enrichment of glycolipid-forming galactosyltransferases, and the distribution of these enzymes correlated well with that of the Golgi marker enzymes. The results give evidence that intracellular Golgi apparatus of oligodendroglia is rich in the myelin-specific lipids, and suggest its involvement in the synthesis and processing of myelin lipids.

Evidence for a role of the hepatic endocytic compartment in the modulation of the extracellular matrix

The presence of fibronectin in rat liver endocytic compartment was investigated using biochemical and immunological approaches. Three endosome subfractions were separated from postmitochondrial lysosome supernatants using sucrose and isoosmotic Nycodenz gradients. Using these endosomes the presence of fibronectin in the "early" and "late" endosome subfractions and in a receptor-enriched fraction was demonstrated by Western blot analysis. Furthermore, immunofluorescence studies using an anti-rat fibronectin antiserum and an anti-rat endosome antiserum showed similar patterns of staining in frozen liver sections. The results indicate that the components of the extracellular matrix extend into the endocytic compartment and suggest that fibronectin is internalised in a manner similar to that of some plasma membrane proteins.

Determination of monoamine oxidase concentrations in rat liver by inhibitor binding

The concentrations of monoamine oxidase-A and -B have been determined in mitochondria, mitochondrial outer membranes and microsomes from Sprague-Dawley and Wistar rats by determining the binding of tritium-labelled pargyline. Although the amounts of each form present depended on the source and the preparation method, this was paralleled by the specific activity such that the molecular turnover number was found to remain constant. The catalytic constants, kcat/Km, which represents the apparent second-order rate constant for the combination of enzyme and substrate, were about 0.13 and 2.1 sec-1 X microM-1 for 5-hydroxytryptamine and 2-phenethylamine, respectively, regardless of the source. Estimations of the amounts of the two forms by determining the concentrations of the inhibitors clorgyline, (-)-deprenyl, J-508 or pargyline necessary to give complete inhibition were shown to give overestimates of the true values because of the non-specific binding of these inhibitors to sites other than the monoamine oxidase active site.

Morphology of endoplasmic reticulum and Golgi elements following microinjection of rat liver microsomes into Xenopus laevis oocyte cytoplasm

Fragments of rough endoplasmic reticulum and Golgi complex purified from rat liver homogenates were injected into Xenopus oocytes and the sites of microinjection analysed by electron microscopy at different times post-injection. The in vivo incubated fragments were located by their proximity to a microinjection vacuole, and identified by their association with specific morphological markers (peroxisomal cores associated with rough microsomes and lipoprotein particles with Golgi derivatives). Typical endoplasmic reticulum microsomes disappeared with time post-injection and seemed to be replaced by flattened cisternae of endoplasmic reticulum. Golgi fragments as defined by their content of lipoprotein particles became modified. Many were found associated with coated vesicles and some displayed membrane-coated regions. Furthermore lipoprotein particles were observed as integral components of Golgi stacks and were found within dilated rims in direct continuity with fenestrated Golgi saccules. The results suggest that the injected organelle fragments underwent transformation in vivo as a consequence of reconstitution.

Characterization of glucagon receptors in Golgi fractions of rat liver: evidence for receptors that are uncoupled from adenylyl cyclase

Glucagon receptors have been identified and characterized in intermediate (Gi) and heavy (Gh) Golgi fractions from rat liver. At saturation, plasma membranes bound 3500 fmol of hormone/mg of membrane protein, while Gi and Gh bound 24 and 60 fmol of 125I-glucagon/mg of protein, respectively. Half-maximal saturation of binding to plasma membranes, Gi, and Gh occurred at approximately 4, 10, and 20 nM 125I-glucagon, respectively. Trichloroacetic acid precipitation of intact, but not degraded, glucagon was used to correct binding isotherms for hormone degradation. After such correction, half-maximal saturation of binding to plasma membranes, Gi, and Gh was observed in the presence of approximately 2, 7, and 14 nM hormone, respectively. After 90 min of dissociation in the absence of guanosine 5'-triphosphate (GTP), 86% of 125I-glucagon remained bound to plasma membranes, whereas only 42% remained bound to Golgi membranes. GTP significantly increased the fraction of 125I-glucagon released from plasma membranes but only slightly augmented the dissociation of hormone from Golgi fractions. 125I-Glucagon/receptor complexes solubilized from plasma membranes fractionated by gel filtration as high molecular weight (Kav = 0.16), GTP-sensitive complexes and lower molecular weight (Kav = 0.46), GTP-insensitive complexes. 125I-Glucagon complexes solubilized from Golgi membranes fractionated almost exclusively as the lower molecular weight species. The lower affinity of Golgi than plasma membrane receptors for hormone, the ability of glucagon to stimulate plasma membrane, but not Golgi membrane, adenylyl cyclase, and the near absence of high molecular weight, GTP-sensitive complexes in solubilized Golgi membranes demonstrate that plasma membrane contamination of Golgi fractions cannot account for the 125I-glucagon binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Terminal glycosylation in rat hepatic Golgi fractions: heterogeneous locations for sialic acid and galactose acceptors and their transferases

Endogenous acceptors for N-acetylglucosamine (GlcNAc), galactose (Gal) or sialic acid (NeuAc) transfer were labeled to high activities when purified hepatic Golgi fractions were incubated with the corresponding radiolabeled nucleotide sugar in the absence of detergent. The in vitro conditions which were optimal for the endogenous glycosylation of GlcNAc and Gal acceptors (Mn2+, ATP) also promoted fusion within a subset of Golgi membranes. Electron microscope radioautography revealed that the majority of NeuAc acceptors were associated with unfused Golgi membranes, whereas the majority of Gal acceptors were localized to fused membranes. GlcNAc acceptors were approximately equally distributed between fused and unfused membranes. Under conditions in which Golgi membrane fusion was absent (-Mn2+), only NeuAc transfer was active. The majority of endogenous NeuAc acceptors were consequently assigned to the more trans regions of the hepatic Golgi apparatus as concluded from a combination of radioautography (NeuAc transfer) and acid NADPase cytochemistry (reactive medial and trans Golgi saccules). The distribution of NeuAc and Gal transferases was assessed after Percoll gradient centrifugation of disrupted Golgi fractions. The median density of NeuAc transferase was lower than that of Gal transferase. The studies are indicative of distinct Golgi components harboring the majority of acceptors and enzymes for terminal glycosylation.

Phospholipid, cholesterol, polypeptide and glycoprotein composition of hepatic endosome subfractions

The composition of hepatic endosome subfractions was compared directly with that of plasma membranes and Golgi-apparatus fractions. The neutral lipid and phospholipid composition of two endosome subfractions separated on Nycodenz gradients from a parent endosome fraction was similar to that of plasma membranes. The phospholipid/cholesterol ratios and the sphingomyelin contents were high, as in plasma membranes. However, the phosphatidylserine content was low. Endosomal subfractions contained a simpler polypeptide profile than did plasma membranes. However, a large number of glycoproteins were common to both fractions. Two endosome-specific glycoproteins, of Mr 59 000 and 38 000, were identified. Sialic acid was present at concentrations higher than in plasma membranes. The results indicate that endosomal membranes have a similar composition to plasma membranes that probably reflects their functional interaction during endocytosis and receptor recycling.

Subcellular localisation of inositol lipid kinases in rat liver

The subcellular distribution of the enzymes which phosphorylate phosphatidylinositol sequentially to form phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was investigated in rat liver. We demonstrate that whilst phosphatidylinositol kinase is present in Golgi, lysosomes and plasma membranes, the kinase that forms phosphatidylinositol 4,5-bisphosphate is localised predominantly at the plasma membrane. The role of the inositol lipid kinases in cell function is discussed.

Topology of UDP-galactose cleavage in relation to N-acetyl-lactosamine formation in Golgi vesicles. Translocation of activated galactose

UDP-galactose appears to be produced on one side of a membrane barrier, opposite the galactosyltransferases that use it as a sugar donor. The translocation of activated galactose across membranes was studied in rat submaxillary-gland microsomal vesicles and in rat liver Golgi vesicles. When these intact vesicles containing the acceptor, N-acetylglucosamine, were incubated in the presence of UDP-galactose and two inhibitors of galactosyltransferase activity, the product, N-acetyl-lactosamine, formed within the vesicles. Thus at least the galactose moiety of UDP-galactose crossed the membranes. When intact Golgi vesicles were incubated with UDP-galactose labelled in both the uridine and the galactose moieties, labelled N-acetyllactosamine was again produced in the vesicles, but less than stoichiometric amounts of the uridine label was found there. Calculation of internal and external concentrations of UMP, a major product released from the cleaved uridine moiety, showed that the vesicles were actually enriched in UMP. When free UMP was incubated with the vesicles, this enrichment did not occur. This result was direct evidence for facilitated transport of UDP-galactose into the Golgi for use by galactosyltransferase.

Hepatic endosome fractions contain an ATP-driven proton pump

Endosome fractions were isolated from rat liver homogenates on the basis of the subcellular distribution of circulating ligands, e.g. 125I-asialotransferrin internalized by hepatocytes by a receptor-mediated process. The distribution of endocytosed 125I-asialotransferrin 1-2 min and 15 min after uptake by liver and a monensin-activated Mg2+-dependent ATPase activity coincided on linear gradients of sucrose and Nycodenz. The monensin-activated Mg2+-ATPase was enriched relative to the liver homogenates up to 60-fold in specific activity in the endosome fractions. Contamination of the endosome fractions by lysosomes, endoplasmic reticulum, mitochondria, plasma membranes and Golgi-apparatus components was low. By use of 9-aminoacridine, a probe for pH gradients, the endosome vesicles were shown to acidify on addition of ATP. Acidification was reversed by addition of monensin. The results indicate that endosome fractions contain an ATP-driven proton pump. The ionophore-activated Mg2+-ATPase in combination with the presence of undegraded ligands in the endosome fractions emerge as linked markers for this new subcellular organelle.

Androgen regulation of specific mRNAs, endoplasmic reticulum and Golgi-system

The seminal vesicles of male rat secrete tissue-specific proteins under androgenic control. The effects of testosterone on the rough endoplasmic reticulum (RER) and Golgi-system of this tissue have been quantified using specific antibodies. Castration was followed within 2 weeks by a 10-fold reduction in RER-specific membrane protein. This was reversed by testosterone commencing about 4 h after exposure to the hormone. Five individual major RER antigens were separately quantified; these changed coordinately in response to androgen. No hormone-induced changes were seen in Golgi-specific membrane protein. Hormonal effects on mRNAs for two major secretory proteins were also measured using hybridisation to specific cDNA probes. The cellular concentrations of the two mRNAs changed by at least 1000-fold during hormonal treatment. A detailed examination of the time-course of induction by testosterone failed to show any temporal distinction between effects on mRNA and RER.

Microinjection of mRNA coding for an anti-Golgi antibody inhibits intracellular transport of a viral membrane protein

Messenger RNA was prepared from a hybridoma cell line secreting a monoclonal antibody (53FC3) directed against a luminal epitope of a Golgi membrane protein (Mr = 135 kd) found in rodent cells. When this mRNA was microinjected into the cytoplasm of BHK cells, mouse IgG was seen to accumulate in the Golgi complex after 5-6 hr of incubation. No accumulation was seen in 3T3 cells which lack the epitope recognized by 53FC3. When microinjected BHK cells were infected with vesicular stomatitis virus, surface expression of the viral G protein was considerably reduced when compared with neighboring noninjected cells.

The transverse distribution of phospholipids in the membranes of Golgi subfractions of rat hepatocytes

The transverse distribution of phospholipids in the membranes of subfractions of the Golgi complex was investigated by using phospholipase C and 2,4,6-trinitrobenzenesulphonic acid as probes. In trans-enriched Golgi membranes, 26% of the phosphatidylethanolamine is available for reaction with trinitrobenzenesulphonate or for hydrolysis by phospholipase C, and 72% of the phosphatidylcholine is hydrolysed by phospholipase C. In cis-enriched Golgi membranes, 45% of the phosphatidylethanolamine is available for reaction with trinitrobenzenesulphonate and for hydrolysis by phospholipase C, and 95% of the phosphatidylcholine is hydrolysed by phospholipase C. Under the conditions used with either probe the contents of the Golgi vesicles labelled with either [3H]palmitic acid or [14C]leucine were retained. Galactosyltransferase activity of the membrane vesicles was partially inhibited by the experimental procedures used to investigate the transverse distribution of phospholipids. However, the residual activity was latent, suggesting that the vesicles remained closed. Trinitrobenzenesulphonic acid caused no detectable morphological change in either Golgi fraction. Phospholipase C treatment caused morphological changes, including fusion of vesicles and the appearance of 'signet-ring' profiles in some vesicles; however, the vesicles remained closed and the bilayer was retained. It appears, therefore, that neither probe causes major disruption of the Golgi vesicles nor gains access to the inner surface of the membrane bilayer. These observations suggest that phospholipids have a transverse asymmetry in Golgi membranes, that this distribution differs in trans and cis membranes, and that the phospholipid structure of Golgi membranes is inconsistent with a simple flow of membrane bilayer from endoplasmic reticulum to Golgi membranes to plasma membrane.

Isolation of highly purified Golgi membranes from rat liver. Use of cycloheximide in vivo to remove Golgi contents

Following administration of cycloheximide to rats in order to deplete the liver of secretory products, Golgi membranes have been isolated largely free of internal contents. These membranes have a high specific activity of galactosyltransferase (400 times that of the homogenate) and are thought to be derived from the trans Golgi. Their phospholipid and polypeptide composition resembles that of Golgi membranes prepared by other procedures but their triacylglycerol and cholesterol contents are greatly reduced. These results conflict with previous reports that trans Golgi membranes are rich in cholesterol.

Analysis and purification of the blood-sinusoidal domain of rat liver plasma membrane

Methods are described for the analysis and purification of the blood-sinusoidal domains of rat liver plasma membranes using a combination of sucrose and Ficoll density gradient centrifugation. Use has been made of 125I-labelled wheat-germ agglutinin and hormone-stimulated adenylate cyclase to identify the blood sinusoidal fraction, which may be resolved from Golgi and endoplasmic reticulum markers on Ficoll gradients.

The rough endoplasmic reticulum and the Golgi apparatus visualized using specific antibodies in normal and tumoral prolactin cells in culture

Antibodies directed against membrane components of dog pancreas rough endoplasmic reticulum (A-RER) and rat liver Golgi apparatus (A-Golgi) (Louvard, D., H. Reggio, and G. Warren, 1982, J. Cell Biol. 92:92-107) have been applied to cultured rat prolactin (PRL) cells, either normal cells in primary cultures, or clonal GH3 cells. In normal PRL cells, the A-RER stained the membranes of the perinuclear cisternae as well as those of many parallel RER cisternae. The A-Golgi stained part of the Golgi membranes. In the stacks it stained the medial saccules and, with a decreasing intensity, the saccules of the trans side, as well as, in some cells, a linear cisterna in the center of the Golgi zone. It also stained the membrane of many small vesicles as well as that of lysosomelike structures in all cells. In contrast, it never stained the secretory granule membrane, except at the level of very few segregating granules on the trans face of the Golgi zone. In GH3 cells the A-RER stained the membrane of the perinuclear cisternae, as well as that of short discontinuous flat cisternae. The A-Golgi stained the same components of the Golgi zone as in normal PRL cells. In some cells of both types the A-Golgi also stained discontinuous patches on the plasma membrane and small vesicles fusing with the plasma membrane. Immunostaining of Golgi membranes revealed modifications of membrane flow in relation to either acute stimulation of PRL release by thyroliberin or inhibition of basal secretion by monensin.

Receptor-rich intracellular membrane vesicles transporting asialotransferrin and insulin in liver

A wide range of receptors are located at the blood sinusoidal aspect of the hepatocyte plasma membrane. Many circulating ligands that bind to receptors on the cell surfaces are interiorized along two pathways. Asialoglycoproteins are transferred from the plasma membrane to lysosomes and degraded, whereas immunoglobulin A and bile acids are transported across the hepatocyte interior and released into bile. Asialotransferrin type 3 (ref. 6) follows a further pathway termed diacytosis. After binding to the asialoglycoprotein receptor, asialotransferrin is endocytosed and then returned to blood with a proportion of its carbohydrate side chains resialylated. We now describe in liver the properties of intracellular asialotransferrin-enclosing vesicles (diacytosomes) and show that they differ from Golgi, lysosome and plasma membrane fractions. Furthermore, we show that the asialoglycoprotein binding sites are located on the cytoplasmic (outer) surface of diacytosomes.

Inhibition of pinocytosis in rat embryo fibroblasts treated with monensin

Rat embryo fibroblasts cultured in the presence of monensin exhibited an inhibited uptake of horseradish peroxidase. The inhibition was detected after 3 h, after which time the cells became increasingly vacuolated; the concentration of monensin required to inhibit pinocytosis (0.4 microM for half-maximum inhibition at 18 h) was similar to that found by others to inhibit secretion. Both the exchange of 5'-nucleotidase between the membranes of cytoplasmic organelles and the cell surface and the internalization of anti-5'-nucleotidase bound to the cell surface were inhibited by approximately 90% in monensin-treated cells. The effects of monensin were reversible: cells cultured first with monensin, and then in fresh medium, exhibited control levels of horseradish peroxidase uptake, exchange of 5'-nucleotidase, and internalization of anti-5'-nucleotidase bound to the cell surface. After monensin treatment, the median density of both galactosyl transferase and 5'-nucleotidase increased from 1.128 to 1.148, and the median density of both N-acetyl-beta-glucosaminidase and horseradish peroxidase taken up by endocytosis decreased from 1.194 to 1.160. The results indicate that monensin is a reversible inhibitor of pinocytosis and, presumably, therefore, of membrane recycling. They suggest that the inhibition of membrane recycling occurs at a step other than the fusion of pinocytic vesicles with lysosomes and is perhaps a consequence of an effect of the ionophore on the Golgi complex.

Galactose transfer to endogenous acceptors within Golgi fractions of rat liver

The distribution of galactosyl transferase was studied using trans and cis Golgi fractions isolated by a modification of the Ehrenreich et al. procedure (1973. J. Cell Biol. 59:45-72) as well as an intact Golgi fraction isolated by a new one-step procedure. Two methods of assay were used. The first method analyzed the ability of Golgi fractions to transfer galactose (from uridine diphosphogalactose [UDP-gal] substrate) to the defined exogenous acceptor ovomucoid. The second method assessed the transfer of galactose from UDP-gal substrate to endogenous acceptors (endogenous glycosylation). The trans Golgi fraction (Golgi light) was highly active by the first method but revealed only low activity by the second method. Golgi fractions enriched in central and cis elements (the Golgi intermediate, heavy and especially the intact Golgi fraction) were highly active in both methods of assay. The endogenous glycosylation approach was validated by gel fluorography of the endogenous acceptors. For all Golgi fractions, transfer of galactose was revealed to secretory glycopeptides. It is concluded that galactosyl transferase activity in vivo occurs primarily in central and cis Golgi elements but not trans Golgi vesicles.

Antibodies to the Golgi complex and the rough endoplasmic reticulum

Rabbits were immunized with membrane fractions from either the Golgi complex or the rough endoplasmic reticulum (RER) by injection into the popliteal lymph nodes. The antisera were then tested by indirect immunofluorescence on tissue culture cells or frozen, thin sections of tissue. There were may unwanted antibodies to cell components other than the RER or the Golgi complex, and these were removed by suitable absorption steps. These steps were carried out until the pattern of fluorescent labeling was that expected for the Golgi complex or RER. Electron microscopic studies, using immunoperoxidase labeling of normal rat kidney (NRK) cells, showed that the anti-Golgi antibodies labeled the stacks of flattened cisternae that comprise the central feature of the Golgi complex, many of the smooth vesicles around the stacks, and a few coated vesicles. These antibodies were directed, almost entirely, against a single polypeptide with an apparent molecular weight of 135,000. The endoplasmic reticulum (ER) in NRK cells is an extensive, reticular network that pervades the entire cell cytoplasm and includes the nuclear membrane. The anit-RER antibodies labeled this structure alone at the light and electron microscopic levels. They were largely directed against four polypeptides with apparent molecular weights of 29,000, 58,000, 66,000, and 91,000. Some examples are presented, using immunofluorescence microscopy, where these antibodies have been used to study the Golgi complex and RER under a variety of physiological and experimental condition . For biochemical studies, these antibodies should prove useful in identifying the origin of isolated membranes, particularly those from organelles such as the Golgi complex, which tend to lose their characteristic morphology during isolation.

Analytical study of microsomes and isolated subcellular membranes from rat liver VIII. Subfractionation of preparations enriched with plasma membranes, outer mitochondrial membranes, or Golgi complex membranes

Preparations enriched with plasmalemmal, outer mitochondrial, or Golgi complex membranes from rat liver were subfractionated by isopycnic centrifugation, without or after treatment with digitonin, to establish the subcellular distribution of a variety of enzymes. The typical plasmalemmal enzymes 5'-nucleotidase, alkaline phosphodiesterase I, and alkaline phosphatase were markedly shifted by digitonin toward higher densities in all three preparations. Three glycosyltransferases, highly purified in the Golgi fraction, were moderately shifted by digitonin in both this Golgi complex preparation and the microsomal fraction. The outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the out mitochondrial membrane preparation, in agreement wit its behavior in microsomes. With the exception of NADH cytochrome c reductase (which was concentrated in the outer mitochondrial membrane preparation), typical microsomal enzymes (glucose-6-phosphatase, esterase, and NADPH cytochrome c reductase) displayed low specific activities in the three preparations; except for part of the glucose-6-phosphatase activity in the plasma membrane preparation, their density distributions were insensitive to digitonin, as they were in microsomes. The influence of digitonin on equilibrium densities was correlated with its morphological effects. Digitonin induced pseudofenestrations in plasma membranes. In Golgi and outer mitochondrial membrane preparations, a few similarly altered membranes were detected in subfractions enriched with 5'-nucleotidase and alkaline phosphodiesterase I. The alterations of Golgi membranes were less obvious and seemingly restricted to some elements in the Golgi preparation. No morphological modification was detected in digitonin-treated outer mitochondrial membranes. These results indicate that each enzyme is associated with the same membrane entity in all membrane preparations and support the view that there is little overlap in the enzymatic equipment of the various types of cytomembranes.