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PubMedDate not supplied

[Fatty-acid composition of lipids and physicochemical characteristics of membrane fractions and the membrane of endoplasmic reticulum of thymus and Pliss' lymphosarcoma].

The paper is concerned with composition of neutral lipids and phospholipids, the regularity of lipid bilayer and structural reorganization of plasma membranes, and membranes of smooth and rough cell reticulum of thymus and Pliss lymphosarcoma are studied at linear and stationary growth phase. No qualitative differences are found in the fatty-acid composition of lipid membranes in normal and tumour cells. In plasma membranes of phospholipids and in membranes of smooth reticulum of tumour cells the unsaturated lipid component increases in the process of growth, the cholesterin/phospholipids ratio decreases, fluidity of the lipid bilayer diminishes and structural heterogeneity of these membranes rises while in membranes of rough reticulum the saturation of lipids increases, but the cholesterin/phospholipids ratio does not change. The temperatures of structural reorganization also does not change, which evidences for a less structural lability of membranes of rough reticulum as compared with other membranes.

Animals↗
PubMed1978

[Membrane phospholipids of the rat liver endoplasmic reticulum normally and in the early stage of carcinogenesis].

There were found certain changes in the total content of phospholipids as well as in the content of their separate fractions in the membranes of endoplasmic reticulum in the rat liver at early stages of carcinogenesis. In the first week under the carcinogen DENA exposure a considerable reduction of the total phospholipids content was noted in the fraction of granular endoplasmic reticulum, while in agranular reticulum during this period of embryogenesis the total content of phospholipids is not changed, but subsequently the former increases as compared with normal values. During the whole period of carcinogenesis under study changes in the phospholipid content are observed in both fractions of endoplasmic reticulum membranes, the greatest changes being noted in the fractions of lysophosphatidicholines, phosphatidilcholines, and phosphatidilinosites.

Animals↗
PubMed1981

Calcium uptake by isolated sarcoplasmic reticulum: examination of halothane inhibition, pH dependence, and Ca2+ dependence of normal and malignant hyperthermic human muscle.

Ca2+ uptake and release in muscle homogenates and fragmented sarcoplasmic reticulum were examined in biopsy specimens from nonsusceptible and malignant hyperthermia (MH) susceptible patients. Ca2+ flux was examined by the filter binding assay technique using 45Ca. It was found that Ca2+ uptake and release were the same in both normal and MH muscle homogenates. Halothane inhibited the uptake of Ca2+ by the sarcoplasmic reticulum. The halothane inhibition of Ca2+ uptake in normal and MH sarcoplasmic reticulum was fitted to a single line with a correlation coefficient (r) of -0.958. The pH and Ca2+ dependence of Ca2+ uptake were the same for both normal and MH sarcoplasmic reticulum. The pK for Ca2+ uptake is approximately 5.9. It is concluded that the Ca2+ uptake function of the muscle from the five patients with MH examined is not abnormal and might not be the locus for the initiation of MH.

Calcium↗
PubMed1981

Kinetics of appearance of lipoprotein particles in perisinusoidal cisternae of smooth endoplasmic reticulum of isolated rat livers perfused with free fatty acid.

Vesicles containing single lipoprotein particles are found in the perisinusoidal cytoplasm of rat hepatocytes in the proximity of particle-containing rough-smooth endoplasmic reticulum transition elements. these lipoprotein particle-containing smooth endoplasmic reticulum elements provide an alternative to Golgi apparatus as a supplier of very low density lipoprotein particles to the circulation. To test this possibility, the kinetics of appearance of lipoprotein particles in these elements was determined for isolated rat livers first depleted of particles and then perfused with free fatty acid to restore particles. The results show that elements of the smooth endoplasmic reticulum in the perisinusoidal cytoplasm acquire lipoprotein particles in advance of elements of the conventional Golgi apparatus and that the particles contained within the perisinusoidal elements of smooth endoplasmic reticulum are not products of endocytotic uptake of particles from the circulation.

Animals↗
PubMed1995

Effect of ATP depletion and DTT on the transport of membrane proteins from the endoplasmic reticulum and the intermediate compartment to the Golgi complex.

Newly synthesized membrane proteins are exported from the endoplasmic reticulum to the Golgi complex through an intermediate compartment. Incubation at low temperature (15 degrees C) arrests the proteins in the intermediate compartment and prevents the entry into the Golgi complex. We have studied, in living cells, the effect of dithiothreitol (DTT) and ATP depletion on the transport to the Golgi complex of proteins accumulated either in the endoplasmic reticulum or in the intermediate compartment after a temperature block. The morphological results obtained with vesicular stomatitis virus ts-O45 G glycoprotein and the biochemical analysis performed with human CD8 protein, an O-glycosylated protein, showed that: 1) ATP depletion blocks the export to the Golgi complex of proteins located either in the endoplasmic reticulum or in the intermediate compartment and ii) DTT interferes with the folding and export of proteins located in the endoplasmic reticulum, but it does not prevent the transfer from the intermediate compartment to the Golgi complex.

Adenosine Triphosphate↗
PubMedDate not supplied

[The structural-functional organization of the endoplasmic reticulum in the neurons of the rat cerebellar cortex].

Electron microscopic investigation of neuronal elements in the cerebellar cortex of rats revealed the existence of septal reticulum in axonal collaterals of Purkinje cells, in axons of basket cells and in mossy fibers. It was shown that this reticulum acts as a calcium depot. The presence in neuroplasm of the developed system of evenly distributed reticulum, which forms relatively isolated compartments and acts as the calcium depot may account for local regulation of rearrangements in the cytoskeleton. Wide presentation of the septal reticulum in the nervous cells of both invertebrates and vertebrates indicates functional determination of this structure.

Animals↗
PubMed1993

The role of endoplasmic reticulum calcium pumps during cytosolic calcium spiking in pancreatic acinar cells.

Many cell types show repetitive short lasting cytosolic calcium spikes with long interspike periods when stimulated with submaximal concentrations of agonists linked to the phosphoinositide signaling pathway. In pancreatic acinar cells these spikes have been shown to be evoked by constant levels of inositol trisphosphate through a mechanism of calcium-induced calcium release and do not depend acutely on the presence of external calcium. However, the processes involved in the interspike period have remained unclear. Here we report that the endoplasmic reticulum Ca(2+)-ATPase play a significant role, not only in resequestering calcium after a spike, but also in regulating the long interspike period. Decreasing the activity of the endoplasmic reticulum calcium pumps leads to shorter interspike intervals and thus higher spiking frequencies, while the duration of each spike increases. The endoplasmic reticulum Ca(2+)-ATPases are able to entirely suppress a response that can subsequently be evoked by partial inhibition of the pumps. This suggests that during the interspike period there is a considerable amount of calcium released from intracellular stores, which is rapidly buffered by the endoplasmic reticulum calcium pumps and the cytosolic calcium-binding proteins. A calcium spike will be initiated by calcium-induced calcium release only when the buffering is saturated.

Animals↗
PubMed1993

Positioning of major tryptic fragments in the Ca2+ release channel (ryanodine receptor) resulting from partial digestion of rabbit skeletal muscle sarcoplasmic reticulum.

Site-specific antibodies against different regions of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum (ryanodine receptor) were developed and used as probes for immunoblotting of the major tryptic fragments resulting from partial digestion of the ryanodine receptor in sarcoplasmic reticulum membranes. Five major tryptic fragments, some of which migrated as doublets, with apparent masses of 150/140, 110/100, 55, 170/160, and 76 kDa were ordered so that they covered the bulk of the protein from the NH2 to the COOH terminus. Tryptic subfragments of 53, 63, and 115/95 kDa were also derived from the 150/140-, 110/100-, and 170/160-kDa fragments, respectively. All of these fragments and subfragments were detected only in the insoluble membrane fraction of the trypsinized sarcoplasmic reticulum. Upon Na2CO3 extraction, the 150/140-, 110/100-, and 55-kDa fragments could be solubilized, suggesting their origin in the cytoplasmic domain of the ryanodine receptor. The 170/160- and 76-kDa fragments and the 115/95-kDa subfragment remained insoluble, suggesting their origin in the transmembrane region of the ryanodine receptor. The 150/140-, 110/100-, 170/160-, and 76-kDa fragments and the 115/95 subfragment co-migrated near the bottom of a sucrose density gradient after CHAPS solubilization, suggesting that they were associated in an oligomeric complex. By contrast, the 53- and 63-kDa subfragments and the 55-kDa fragment were detected near the top of the sucrose gradient after CHAPS solubilization, suggesting that they were not involved in the formation of the core of the oligomeric complex. These studies identify 7 sites that are exposed to trypsin in the ryanodine receptor in sarcoplasmic reticulum, 3 of which are novel and 4 of which are in the same location as proteolytic cleavage sites identified previously (Marks, A. R., Fleischer, S., and Tempst, P. (1990) J. Biol. Chem. 265, 13143-13149).

Animals↗
PubMed1993

[Effect of cyproheptadine on Ca2+,Mg(2+)-ATPase activities and 45Ca2+ uptake function in canine myocardial sarcoplasmic reticulum].

The effects of cyproheptadine on Ca2+,Mg(2+)-ATPase activities and 45Ca2+ uptake function in isolated canine myocardial sarcoplasmic reticulum were studied. Cyproheptadine at concentrations of 8 x 10(-6) mol/L-2 x 10(-4) mol/L showed hardly any influence on Ca2+,Mg(2+)-ATPase activities of the myocardial sarcoplasmic reticulum in canine, but inhibited the enzyme activities (rate of inhibition: 39.85%, P < 0.01) at the concentration of 10(-3) mol/L. The myocardial sarcoplasmic reticulum was shown to take up 45Ca2+ significantly in a time-dependent manner and the 45Ca2+ uptake was 312.79 +/- 22.25 nmol/mg protein at 30 min. Cyproheptadine was found to inhibit the Ca2+ uptake of sarcoplasmic reticulum with an IC50 of 1.94 x 10(-4) mol/L.

Animals↗
PubMed1993

Amino-terminal topology of thromboxane synthase in the endoplasmic reticulum.

The membrane topology of the NH2-terminal portion of human thromboxane synthase (TXS), a member of the cytochrome P450 superfamily, has been investigated. By sequence alignment, the first 6 residues of the mature TXS polypeptide are likely to form a distinctive "tail" structure not found in many other mammalian cytochromes P450 in the endoplasmic reticulum membrane. Peptides with either the ultimate 10 or 15 residues of the NH2 terminus of TXS were synthesized and used to produce site-directed antibodies. The resulting peptide antibodies were highly specific and recognized human TXS, as shown by binding assays and Western blot analysis. Binding of the peptide antibodies to recombinant TXS in transfected COS-1 and to endogenous TXS in THP-1 cells was analyzed by immunocytochemistry. Selective permeabilization of the plasma membrane to immunoglobulin was achieved with streptolysin O; general permeabilization, including the endoplasmic reticulum membrane, was accomplished with Triton X-100. Permeabilization of the plasma membrane was sufficient to produce binding of both peptide antibodies to their epitopes, indicating that the epitopes for both of the peptide antibodies were exposed on the cytoplasmic side of the endoplasmic reticulum membrane. The results with the peptide antibodies provide direct experimental evidence supporting the topological model for membrane-bound cytochrome P450 proposed by Nelson and Strobel (Nelson, D. R., and Strobel, H. W. (1988) J. Biol. Chem. 263, 6038-6050), in which the NH2 terminus is oriented toward the cytoplasmic side of the endoplasmic reticulum membrane.

Amino Acid Sequence↗
PubMed1993

The secretory pathway is normal in dithiothreitol-treated cells, but disulfide-bonded proteins are reduced and reversibly retained in the endoplasmic reticulum.

Addition of 2 mM dithiothreitol (DTT) to HepG2 human hepatoma cells blocks secretion of newly made albumin and causes it to accumulate in the endoplasmic reticulum in the reduced state. Subsequent incubation of the cells in the absence of DTT allows the reduced albumin to form apparently normal disulfide bonds and to be secreted normally. Similarly, DTT treatment causes all newly made subunits of the H1 subunit of the asialoglycoprotein receptor to be retained in the endoplasmic reticulum in the reduced state; following removal of DTT H1 subunits form disulfide bonds via a normal endoplasmic reticulum folding intermediate and mature to the Golgi. Thus, apparently normal formation of disulfide bonds on these two proteins can occur post-translationally, as has been shown previously for the influenza HA hemagglutinin (Braakman, I., Helenius, J., and Helenius, A. (1992) EMBO J. 11, 1717-1722; Braakman, I., Helenius, J., and Helenius, A. (1992) Nature 356, 260-262). alpha 1-Antitrypsin contains no disulfide bonds; in the continuous presence of DTT it acquires a normal complement of complex oligosaccharides and is secreted at an only slightly reduced rate. Thus, the secretory pathway functions efficiently even when it is reduced by DTT. Endoplasmic reticulum retention of albumin and H1 during treatment with DTT presumably occurs because, without disulfide bonds, these proteins cannot fold properly, not because of any defect in the overall processes of vesicular transport and protein secretion.

Asialoglycoprotein Receptor↗
PubMed1993

Transit and sorting of apolipoprotein B within the endoplasmic reticulum and Golgi compartments of isolated hepatocytes from normal and orotic acid-fed rats.

Apolipoprotein B (apoB) secretion by isolated rat hepatocytes was dependent on addition of oleate to the incubation medium and inhibited in hepatocytes isolated from livers of orotic acid-fed rats (OA hepatocytes). To investigate the intracellular transit of newly synthesized apoB under different conditions, normal hepatocytes (with or without oleate) or OA hepatocytes (with oleate) were incubated with [35S]methionine, and subcellular fractions (rough endoplasmic reticulum, smooth endoplasmic reticulum, cis-Golgi, and trans-Golgi and membrane and lumenal contents from these) were isolated at intervals. The specific activities and pool sizes of apoB100 and apoB48 were determined. The observations indicate that there are several points at which intracellular transit of apoB is regulated. Newly synthesized apoB is either translocated to the lumen of the rough endoplasmic reticulum or remains membrane bound and is degraded. The lumenal apoB is either retained and degraded or transferred to the Golgi lumen and secreted. In OA hepatocytes degradation of the membrane-bound form of apoB is inhibited, and the protein accumulates in the trans-Golgi membranes. Although apoB is translocated to the lumen of the rough endoplasmic reticulum in OA hepatocytes, it is not packaged with lipid and is transferred to the Golgi lumen only slowly.

Animals↗
PubMed1993

An N-terminal glycosylation signal on cytochrome P450 is restricted to the endoplasmic reticulum in a luminal orientation.

The mechanism of retention of cytochrome P450 in the endoplasmic reticulum is unknown, and the membrane topology of the N-terminal region remains controversial. To address these problems, a sequence of 29 amino acids encoding an internal N-glycosylation site of rabbit cytochrome P450 2C2 was attached to the N terminus of cytochrome P450 2C1. This protein is glycosylated at a single site in a cell-free translation system containing microsomal membranes, as indicated by gel mobility and sensitivity to endoglycosidase H. When expressed in COS1 cells, an immunoreactive species with the same gel mobility as the in vitro synthesized glycosylated product was detected. Treatment with endoglycosidase H changed its mobility to that of unglycosylated hybrid cytochrome P450 2C1. These results indicate that in intact cells, as in the cell-free system, the N terminus of cytochrome P450 is luminally oriented which is not consistent with a hairpin loop conformation. Sensitivity of the glycosylated protein to endoglycosidase H suggests that the protein does not reach the Golgi compartments. When transfected cells were incubated at low temperatures to inhibit retrograde transport from the intermediate pre-Golgi compartment into the endoplasmic reticulum, localization of cytochrome P450 was not changed, as assayed by subcellular fractionation and immunofluorescent staining. These observations suggest that cytochrome P450 is restricted to the endoplasmic reticulum membrane by a mechanism different from recycling through the intermediate compartment, which is a pathway utilized by soluble endoplasmic reticulum proteins.

Amino Acid Sequence↗
PubMed1993

Assembly of rat hepatic very low density lipoproteins in the endoplasmic reticulum.

The intracellular site of assembly of hepatic very low density lipoproteins has been investigated. Two endoplasmic reticulum fractions and Golgi vesicles (relatively free from endosomal contamination) were isolated from rat liver and the luminal contents were released. The apoB-containing entities were separated from the lumen of the endoplasmic reticulum and Golgi vesicles by an immunoaffinity isolation procedure. The amount of each lipid moiety (triacylglycerols, cholesterol plus cholesteryl esters and phospholipids) associated with a unit mass of apoB was shown to be very similar in the luminal contents isolated from each of the three fractions. Moreover, the apoB-containing particles that were isolated from the endoplasmic reticulum and Golgi were mainly present in a fraction of density < 1.02 g/ml. The average diameter of these lipoprotein particles was shown by negative staining electron microscopy to be in the size range of very low density lipoproteins and low density lipoproteins. The size and composition of the intracellular lipoproteins were very similar to those of both very low density lipoproteins isolated from the culture medium from rat hepatocytes and plasma very low density lipoproteins. The conclusion from this study is that the full complement of lipids associate with apoB at an early stage during the secretory pathway, in the endoplasmic reticulum.

Animals↗
PubMed1996

The ontogeny of key endoplasmic reticulum proteins in human embryonic and fetal red blood cells.

Recently, using immunohistochemical methods, we surprisingly found that endoplasmic reticulum glucose-6-phosphatase is present in human embryonic and fetal red blood cells (RBCs) but not in adult RBCs. The fact that an endoplasmic reticulum enzyme, whose major site of expression in adults is the liver, is present in human embryonic and fetal RBCs, particularly nucleated cells, indicated that it would be sensible to determine whether these cells also contain other endoplasmic reticulum enzyme systems normally found in adult liver. Therefore, we have studied the expression of other endoplasmic reticulum proteins and found that human embryonic and fetal RBC precursors contain other protein components of the glucose-6-phosphatase system, ie, the phosphate and glucose transport proteins as well as other enzymes (eg, uridine diphosphate-glucuronosyltransferases, cytochrome P450 isozymes, nicotinamide adenine dinucleotide phosphate cytochrome P450 oxidoreductase, and prostaglandin H synthase). In addition, we also found the predominantly cytosolic markers 15-hydroxyprostaglandin dehydrogenase, prostaglandins PGE2 and 13,14-dihydro-15-keto-PGE2. The expression of key enzymes that control glucose production, detoxification of endobiotics and xenobiotics, and the regulation of prostaglandin levels in embryonic and early fetal RBCs means that these cells may have an important role in protecting the developing conceptus before it establishes an efficient circulation and before all tissues fully express their normal complement of these enzymes.

Adult↗
PubMed1995

[Association of rabbit skeletal muscle phosphorylase kinase with sarcoplasmic reticulum membranes].

The binding of phosphorylase kinase to sarcoplasmic reticulum has been studied using gel chromatography. The presence of Ca2+, Mg2+ and glycogen was found to be necessary for the maximal binding. The phosphorylase kinase adsorbed on sarcoplasmic reticulum is capable of phosphorylating exogenous phosphorylase b. Phosphorylase kinase was shown to take no part in the phosphorylation of sarcoplasmic reticulum proteins. Exogenous calmodulin initiates the incorporation of [gamma-32P] of ATP into sarcoplasmic reticulum proteins. The data obtained point to a possibility that another Ca(2+)-calmodulin-dependent protein kinase may participate in the phosphorylation of sarcoplasmic proteins.

Animals↗
PubMed1996

Electro-optical imaging of F-actin and endoplasmic reticulum in living and fixed plant cells.

Confocal and video micrographs of living and fixed alfalfa roots, onion epithelial and pear pollen cells illustrate the architecture of the cytoskeleton and endoplasmic reticulum in plant cells. Fixation of plant tissues to preserve cytoplasmic structure poses special problems. When possible, emphasis should be placed on the imaging of structures in stained living cells over time. The early events that occur when Nod factors or bacteria elicit nodule formation in alfalfa roots will illustrate several approaches to plant cell fixation, staining and imaging. The first observable events after Nod factor stimulation occur in root hairs and are changes in rates of cytoplasmic streaming, nuclear movements, and changes in the shape of the vacuole. Within ten minutes, the endoplasmic reticulum shifts position towards the tip of the root hair. For comparison, the endoplasmic reticulum localization in pollen tubes and onion epithelial cells will be illustrated. The actin cytoskeleton undergoes a series of changes over a twelve hour period. These changes in the cytoskeleton are spatially and temporally correlated with the observed growth changes of the root hairs. This dynamic change of the actin filament and endoplasmic reticulum and associated secretory vesicles in these root hairs suggests a mechanism for the observed root hair growth changes.

Actins↗
PubMed2006

Sarco/endoplasmic reticulum Ca2+ATPase type 3 isoforms (SERCA3b and SERCA3f): distinct roles in cell adhesion and ER stress.

Sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) pump free Ca(2+) from the cytosol into the endoplasmic reticulum. The human SERCA3 family counts six members named SERCA3a to 3f. However, the exact role of these different isoforms in cellular physiology remains undetermined. In this study, we compared some physiological consequences of SERCA3b and SERCA3f overexpression in HEK-293 cells. We observed that overexpression of SERCA3b affected cell adhesion capacity associated with a major disorganization of F-actin and a decrease in focal adhesion. Furthermore, we found that SERCA3f overexpression resulted in an increase in endoplasmic reticulum stress markers (including processing of X-box-binding protein-1 (XBP-1) mRNA and expression of chaperone glucose-regulated protein 78 (GRP78)). This was associated with the activation of caspase cascade and a higher spontaneous cell death. In conclusion, these data point for the first time to distinct physiological roles of SERCA3 isoforms in cell functions.

Apoptosis↗