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At least 811 records · Page 45Linked to original sources
PubMed1980

Three-dimensional electron microscopy of mitochondria and endoplasmic reticulum in the red muscle fiber of the rat diaphragm.

The three-dimensional arrangement of mitochondria and endoplasmic reticulum in the red muscle fiber was studied both in thick sections of the rat diaphragm fixed in glutaraldehyde and impregnated with uranyl acetate followed by lead and copper citrate, and in thin sections of glutaraldehyde fixed tissue treated with ferrocyanide-reduced osmium. The mitochondria were located either at the periphery of the fiber, where they were spherical, or between the myofibrils, where they formed longitudinal columns of rectangular, slightly flattened elements. From both types of mitochondria, thin, elongated branches arose at right angles that formed transversely oriented mitochondrial pairs at the I band level. At the periphery of the fiber, the endoplasmic reticulum took the appearance of a subsarcolemmal network of tubular cisternae oriented parallel to the cell surface. In the juxtanuclear region, it was made up of spherical masses composed of tightly knitted tubules that were interconnected by more loosely anastomosed tubules. In between the myofibrils, it was composed of longitudinally oriented repetitive units whose structure varied according to their position in from of the A or I bands of the myofibrils. In front of the A band, the endoplasmic reticulum appeared as a single sheet of anastomotic tubules compressed between the adjacent myofibrils, whereas at the I band level, its tubular elements passed in front and behind the transverse expansions of the mitochondria to form an intricate ultilayered network in from of the Z line.

Animals↗
PubMed1981

Three dimensional arrangement of mitochondria and endoplasmic reticulum in the heart muscle fiber of the rat.

The three-dimensional arrangement of mitochondria and endoplasmic reticulum was studied in thick sections of the heart left ventricle fixed in glutaraldehyde and impregnated with the Ur-Pb-Cu technique and in thin sections of glutaraldehyde-fixed tissue post-fixed in potassium ferrocyanide-reduced osmium. Squarish flattened mitochondria, approximately the size of a sarcomere, were arranged in longitudinal columns in the clefts between the myofibrils. At the periphery of the fiber, the endoplasmic reticulum took the appearance of a subsarcolemmal network of plate-like and tubular cisternae running parallel to the cell surface. Between the myofibrils, the ER network formed longitudinally oriented repetitive units whose structure varied according to their position in relation to the A- or I- bands of the myofibrils. In front of the A-band, the endoplasmic reticulum appeared as a single layered network of anastomotic tubules compressed between the adjacent myofibrils. In front of the I-band, it formed a multilayered network the three-dimensional arrangement of which was dependent upon the presence or absence of the T-tubule. In the absence of the T-tubule, the ER cisternae were loosely anastomosed and occasionally displayed bulbous terminal swellings. In the presence of T-tubules, tubular ER cisternae were seen running parallel on both sides of the T-tubules and were continuous with sheet-like cisternae sandwiched between the distended T-tubule and adjacent extremities of longitudinally arranged mitochondria. These tubular or flattened cisternae were connected to each other by numerous bridging cisternae around the T-tubules.

Animals↗
PubMed1984

Quantitative analysis of rough endoplasmic reticulum approaches to the cell membrane in the secretory ameloblast of the rat incisor.

The distribution of approaches of rough endoplasmic reticulum to the cell membrane within the supranuclear region of the secretory ameloblast of the rat incisor was quantitated using a Zeiss MOP-3. In ameloblasts cut in cross section, most of these approaches appear as circular profiles representing cross sections of elongated cisternae, which are aligned parallel to the long axis of the cell. Because of their position, orientation, and distribution of ribosomes, these approaches were consistent with the appearance of subsurface cisternae. Using cross-sectioned ameloblasts, the lengths of apposed plasma membranes either between or within rows of cells were measured from electron micrographs. Along these lengths, matched approaches of rough endoplasmic reticulum from opposite sides of the apposed plasma membranes were counted. Approaches from either side that were unmatched were also counted. Thirteen percent of the approaches were matched between rows of ameloblasts, and 13.5% of the approaches were matched within rows, demonstrating no significant difference between the two sites. Furthermore, mathematical analysis showed that the theoretical probability of two approaches coinciding is 17%. The experimental values are not statistically different from the theoretical probability, and it is concluded that the matching of rough endoplasmic reticulum approaches to the plasma membrane, or subsurface cisternae, occurs at random.

Ameloblasts↗
PubMed1987

Inhibition of beta-adrenergic stimulated calcium pump of rat cardiac sarcoplasmic reticulum by tricyclohexyltin hydroxide.

The effects of tricyclohexyltin hydroxide (Plictran), an organotin acaricide, on 45Ca2+ uptake and Ca2+ ATPase were studied in vitro and in vivo in rat heart ventricular membrane vesicles, primarily sarcoplasmic reticulum. There was a concentration dependent inhibition of both 45Ca2+ uptake and Ca2+ ATPase in vivo as well as in vitro. Isoproterenol, a beta-adrenergic agonist, stimulated 45Ca2+ uptake and Ca2+ ATPase of sarcoplasmic reticulum and this was also inhibited by Plictran. Since cardiac relaxation is mediated by beta-adrenergic stimulation via Ca+ uptake by sarcoplasmic reticulum, the inhibition of calcium pump activity by Plictran may result in alterations in cardiac Ca2+ fluxes leading to cardiac dysfunction.

Animals↗
PubMed1984

The neuronal endoplasmic reticulum: its cytochemistry and contribution to the endomembrane system. II. Axons and terminals.

The morphology and cytochemistry of the endoplasmic reticulum (ER) in axons and terminals of a number of different types of neurons in brains from mice were investigated ultrastructurally. The neurohypophysis received particular attention because the morphology and enzyme cytochemical activities of many of the preterminal swellings of hypothalamo-neurohypophysial axons are altered by chronic salt-stress. Membrane contrast and enzyme cytochemical staining techniques were employed to characterize the axonal reticulum and to determine if organelles representing the lysosomal system in the axon and the tubular profiles participating in the anterograde axonal transport of native horseradish peroxidase (HRP) are associated with the ER. Potential enzyme cytochemical markers for the axonal ER included glucose-6-phosphatase (G6Pase), thiamine pyrophosphatase, nucleoside diphosphatase, and acid hydroxylase activities. The anterograde transport of HRP was analyzed in undamaged hypothalamo-neurohypophysial neurons and in facial and hypoglossal motoneurons of mice receiving the protein in the lateral cerebral ventricle. The ER pervaded the axon and appeared as parallel, 20-40-nm-wide tubules interconnected by oblique anastomoses. Membrane thickness of the axonal reticulum measured 60-100 A, which is similar to that of the perikaryal ER. Enzyme cytochemical activities associated with the ER or lysosomes were not conspicuous in axons and terminals under normal conditions but became prominent in some axons and preterminal swellings manifesting an autophagic appearance within neurohypophyses from salt-stressed mice. Only G6Pase activity was a marker for the ER in these axons and preterminals. Many ER profiles in non-incubated sections and in G6Pase cytochemical preparations of salt-stressed neurohypophyses were wrapped around or interspersed among secretory granules, multilamellar bodies, and vacuoles that may represent forms of lysosomes involved in autophagy and crinophagy. Acid hydrolase activities were localized within the vacuoles as well as within 80-130-nm-wide, blunt-ended tubules in pituitary stalk axons; similar reactive tubules were confluent with large secondary lysosomes in neurosecretory cell bodies and may be derived from these lysosomes. Morphologically identical tubules transporting HRP in the anterograde direction were observed only in the salt-stressed hypothalamo-neurohypophysial neuron. The HRP-positive tubules very likely are affiliated with the lysosomal system.

Acid Anhydride Hydrolases↗
PubMed1985

Epoxide hydrolase is a marker for the smooth endoplasmic reticulum in rat liver.

Epoxide hydrolase (EH, EC 3.3.2.3) was chosen as a potential marker for smooth endoplasmic reticulum, because this enzyme is inducible by drugs such as phenobarbital. The hypothesis was verified in rat liver using immunochemical and immunocytochemical techniques. Antibodies were raised to the purified protein. These antibodies were affinity purified using the enzyme immobilized on Sepharose Ultrogel. The specificity of the antibodies was assayed by immunoelectrotransfer (Western blot). The labelling of rat liver thin frozen sections with protein A-gold particles demonstrated that the antibodies specifically recognised smooth endoplasmic reticulum membranes. Rough endoplasmic reticulum, other intracellular organelles and plasma membrane were unlabelled.

Animals↗
PubMed1995

In vivo reactivation of heat-denatured protein in the endoplasmic reticulum of yeast.

Saccharomyces cerevisiae cells grown at 24 degrees C acquire thermotolerance and survive exposure to 50 degrees C, but only if they are first incubated at 30 degrees C, the temperature where heat shock genes are activated. We show here that the enzymatic activity of a secretory beta-lactamase fusion protein, pre-accumulated at 37 degrees C in the endoplasmic reticulum, was abolished by exposure of the cells to 50 degrees C. When the cells were returned to 24 degrees C, beta-lactamase activity was resumed. Reactivation occurred in the endoplasmic reticulum, but not in the Golgi apparatus. It was dependent on metabolic energy, but did not require de novo protein synthesis. According to co-immunoprecipitation experiments, immuno-globulin-binding protein (BiP/Kar2p) was associated with the fusion protein. We suggest that recovery from thermal insult involves, in addition to cytoplasmic and nuclear events, refolding of heat-damaged proteins in the endoplasmic reticulum by a heat-resistant machinery, which forms part of a fundamental survival mechanism.

Adenosine Triphosphate↗
PubMed2003

Estradiol dependent anchoring of the goat uterine estrogen receptor activation factor (E-RAF) at the endoplasmic reticulum by a 55 kDa anchor protein (ap55).

The primary intracellular site of localization of the estrogen receptor activation factor (E-RAF) is shown here to be the endoplasmic reticulum where the protein remains anchored through an estrogen dependent mechanism. The retention of E-RAF by the endoplasmic reticulum is facilitated by two proteins: (1) a 55 kDa anchor protein (ap55) which is an integral membrane protein of the endoplasmic reticulum. ap55 is a high affinity estrogen binding protein. A conformational change induced by estrogen binding is thought to favor the anchoring process. (2) The anchoring of E-RAF by ap55 is mediated by yet another protein. This is the 66 kDa transport protein (tp66) which recognizes ap55 on the one hand and E-RAF on the other. The presence of estradiol that saturates the hormone binding sites on ap55 appears to favor the anchoring of tp66-E-RAF complex to ap55. This interaction appears to be weakened by levels of estradiol below 7 nM concentration leading to the dissociation of the tp66-E-RAF complex from ap55. The tp66-E-RAF complex moves towards the nucleus.

Active Transport, Cell Nucleus↗
PubMed1991

Regulation of the sarcoplasmic reticulum Ca(2+)-release channel requires intact annexin VI.

Annexin VI has eight highly conserved repeated domains; all other annexins have four. Díaz-Muñoz et al. (J Biol Chem 265:15894, 1990) reported that annexin VI alters the gating properties of the ryanodine-sensitive Ca(2+)-release channel isolated from sarcoplasmic reticulum. The investigate the domain structure of rat annexin VI (67 kDa calcimedin) required for this channel regulation, various proteolytic digestions were performed. In each case, protease-resistant core polypeptides were produced. Annexin VI was digested with V8 protease and two core polypeptides were purified by Ca(2+)-dependent phospholipid binding followed by HPLC. The purified fragments were shown to be derived from the N- and C-terminal halves of annexin VI, and demonstrated differential immunoreactivity with monoclonal antibodies to rat annexin VI. While both core polypeptides retained their ability to bind phospholipids in a Ca(2+)-dependent manner, they did not regulate the sarcoplasmic reticulum Ca(2+)-dependent manner, they did not regulate the sarcoplasmic reticulum Ca(2+)-release channel as did intact annexin VI.

Amino Acid Sequence↗
PubMed1991

Trans-Golgi reticulum.

The trans-Golgi apparatus reticulum is that portion of the Golgi apparatus located in the trans-most aspect of the stack exhibiting certain characteristic morphological and functional characteristics. The membranes of the trans-Golgi reticulum are reticular in form, thickened with plasma membrane-like characteristics and with a considerable portion of their surface covered by clathrin coats. The enzymes thiamine pyrophosphatase and sialyl- and galactosyl transferases are functional markers. Correlative studies show the trans-Golgi apparatus reticulum to be involved in glycoprotein, enzyme and receptor processing and sorting along multiple pathways. Sorting and transfer of constituents to lysosomes, to secretory granules, or to the plasma membrane emerge as dominant functions.

Animals↗
PubMed2003

Lithium inhibits Abeta-induced stress in endoplasmic reticulum of rabbit hippocampus but does not prevent oxidative damage and tau phosphorylation.

The goal of this study was to assess the in vivo effect of Abeta on apoptosis pathways involving the endoplasmic reticulum and mitochondria, and its relationship to the induction of tau phosphorylation and DNA oxidative damage. In rabbits treated intracisternally with aggregated Abeta(1-42), clear evidence of endoplasmic reticulum stress was observed by the activation of caspase-12 and cleavage of caspase-3 in the endoplasmic reticulum. Mitochondrial injury was evident from the release of cytochrome c into the cytosol and the induction of oxidized mitochondrial DNA. Tau phosphorylation and nuclear translocation of NF-kappaB and GSK-3beta were also observed. Treatment with lithium, an inhibitor of GSK-3beta, inhibited caspase activation but did not prevent mitochondrial DNA damage or tau hyperphosphorylation, suggesting that the translocation of GSK-3beta may represent an upstream event that leads to caspase activation but is unrelated to tau hyperphosphorylation or mitochondrial DNA oxidative damage. We propose that treatment by lithium alone is not sufficient to protect against the multiple adverse effects of Abeta, and the use of agents that prevent oxidative DNA damage and tau hyperphosphorylation, together with lithium, may provide better protection from the neurotoxic effect of Abeta.

Amyloid beta-Peptides↗
PubMed1983

Membrane crystals of Ca2+-ATPase in sarcoplasmic reticulum of normal and dystrophic muscle.

Two-dimensional crystalline arrays of the Ca2+ transport ATPase develop after treatment of sarcoplasmic reticulum vesicles with Na3VO4. The dimensions of the crystal lattice are similar in sarcoplasmic reticulum membranes isolated from normal and genetically dystrophic human, mouse, and chicken muscles. These observations indicate similar requirements for ATPase-ATPase interactions in sarcoplasmic reticulum of normal and dystrophic muscles and lessen the likelihood of a molecular defect of the Ca2+ pump in the various forms of genetic muscular dystrophies.

Animals↗
PubMed1994

Ascorbate/iron activates Ca(2+)-release channels of skeletal sarcoplasmic reticulum vesicles reconstituted in lipid bilayers.

Ca(2+)-release channel or ryanodine receptor is known to be involved in physiologic Ca(2+)-release from sarcoplasmic reticulum in skeletal and cardiac muscle. A variety of chemical oxidants and in particular SH-oxidizing reagents have been shown to activate Ca2+ release. However, the role of the oxidative modification of the channel in the physiologic mechanism(s) of Ca2+ release and in pathologic states of the muscle remains to be elucidated. Ascorbate/iron redox couple is known to be an efficient generator of oxygen radicals and semidehydroascorbyl radicals. Ascorbate/iron was shown to be released from cardiomyocytes during ischemia-reperfusion and was suggested to be involved in the ischemia-reperfusion injury and cardiomyocyte death. To understand the potential contribution of ascorbate/iron to Ca2+ release mechanism(s), calcium release channels from skeletal sarcoplasmic reticulum (SR) were reconstituted in artificial planar bilayers to examine the effects of this redox couple on the channel activity. Ascorbate elicited a transient (about 2 min) but dramatic increase of open-time probability of the channel. At pCa = 7.0, the presence of EGTA blocked ascorbate induced activation of release channels. However, when exogenous iron was added, ascorbate activated Ca2+ release channels, even in the presence of EGTA. ESR measurements demonstrated that semidehydroascorbyl radicals were generated from ascorbate in the absence of EGTA. The semidehydroascorbyl radical ESR signal was quenched by EGTA in the absence (but not in the presence) of exogenous iron. Thus, the production of ascorbyl radicals was associated with increased channel activity. In the presence of heparin, ascorbate plus iron elicited a long-lasting activation of the channel which had conductance gCa2+ = 100 pS characteristic for the ryanodine receptor and which could be blocked by the ryanodine channel inhibitor, ruthenium red. In conclusion the physiologically relevant redox couple--ascorbate/iron--at physiologic concentrations can activate Ca2+ channels in sarcoplasmic reticulum vesicles.

Animals↗
PubMed2000

Inositol polyphosphates regulate Ca2+ efflux in a cardiac membrane subtype distinct from junctional sarcoplasmic reticulum.

The membrane location and mechanism of inositol 1,3,4,5-tetrakisphosphate (InsP4)-regulated Ca2+ uptake in cardiac membrane vesicles was investigated. In canine and rat membranes separated by sucrose density gradient centrifugation, InsP4-regulated Ca2+ uptake was slightly more enriched in low density than in higher density membranes. Membranes supporting InsP4-regulated Ca2+ uptake were correspondingly enriched in type 1 InsP3 receptors. Junctional sarcoplasmic reticulum (J-SR), enriched in sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) and ryanodine receptors, separated predominantly with higher density membranes. In membranes supporting InsP4-regulated Ca2+ uptake, Ca2+ uptake was facilitated by a high Ca2+ affinity carrier that was insensitive to thapsigargin. Ca2+ uptake in J-SR was mediated by thapsigargin-sensitive SERCA2a. Net Ca accumulation was enhanced by oxalate in both SR subtypes. Although Ca2+-carrier-mediated Ca2+ uptake was ATP independent, ATP indirectly regulated net Ca2+ accumulation by modifying Ca2+ efflux via a Ca2+ channel with properties of type 1 InsP3 receptors. In the presence of < or = 0.1 mM ATP, InsP4 enhanced Ca2+ accumulation whereas InsP4 inhibited Ca2+ uptake at higher ATP concentrations. In the presence of 0.15 mM ATP, InsP4 stimulated Ca2+ efflux from vesicles preloaded with Ca. Several other InsP4 isomers and 1,3,4-InsP3 also stimulated Ca2+ efflux but with slightly less potency than 1,3,4,5-InsP4. Ruthenium red enhanced net Ca accumulation by the Ca2+ carrier and reduced the potency of ATP, InsP4, and InsP3 to stimulate Ca2+ efflux in vesicles. In summary, this investigation shows that a Ca2+ carrier facilitates Ca loading in a sarcoplasmic reticulum subtype distinct from J-SR. InsP4 and InsP3 are proposed to regulate Ca2+ efflux in low density SR by acting on an ATP-modulated Ca2+ channel with properties of type 1 InsP3 receptors.

Animals↗
PubMed1995

Identification of triadin and of histidine-rich Ca(2+)-binding protein as substrates of 60 kDa calmodulin-dependent protein kinase in junctional terminal cisternae of sarcoplasmic reticulum of rabbit fast muscle.

The endogenous calmodulin-protein kinase system of sarcoplasmic reticulum terminal cisternae of rabbit fast-twitch muscle was studied. Investigation of a single Ca(2+)-channel in terminal cisternae fused to planar lipid bilayers demonstrated that the endogenous kinase inhibits the channel, although it remained unclear whether the phosphorylation sites are on the channel protein or on other junctional sarcoplasmic reticulum specific proteins [Hain et al., (1994) Biophys. J. 67, 1823-1833]. Our results, which show that two junctional sarcoplasmic reticulum specific proteins, i.e., triadin and histidine-rich Ca(2+)-binding protein, but not the ryanodine receptor/Ca(2+)-channel protein, are phosphorylated by membrane-bound 60 kDa protein kinase, seem to be able to resolve this ambiguity. Furthermore, such a probably specific protein isoform of calmodulin-protein kinase, by its substrate specificity and exposure to the cytoplasmic side of terminal cisternae at the junctional membrane domain and based on protease sensitivity, also seems to possess some of the potential requirements for a regulatory role in the functional state of the Ca(2+)-channel.

Animals↗
PubMed1996

Phosphatidylserine synthesis in glioma C6 cells is inhibited by Ca2+ depletion from the endoplasmic reticulum: effects of 2,5-di-tert-butylhydroquinone and thimerosal.

The effects of 2,5-di-tert-butylhydroquinone (DBHQ) and thimerosal on phosphatidylserine synthesis by the base exchange reaction and on calcium mobilization in intact glioma C6 cells were compared with that of thapsigargin, a selective inhibitor of the endoplasmic reticulum Ca(2+)-ATPase. It has been found that all these agents inhibit phosphatidylserine synthesis by 70%, but their effectiveness are different. The data show that this inhibition is caused by Ca2+ depletion of the endoplasmic reticulum, indicating that phosphatidylserine synthesis requires high concentration of Ca2+ within this structure. On this basis and on literature data, a new model for the localization of the serine base exchange enzyme in the endoplasmic reticulum membrane is proposed.

Brain Neoplasms↗
PubMed1996

Phorbol myristate acetate-induced hypertrophy of neonatal rat cardiac myocytes is associated with decreased sarcoplasmic reticulum Ca2+ ATPase (SERCA2) gene expression and calcium reuptake.

The decreased expression of the sarcoplasmic reticulum Ca(2+)-ATPase associated with cardiac hypertrophy was investigated in cultured neonatal rat cardiac myocytes. Northern blot analysis indicated a significant 55-60% decrease in Ca(2+)-ATPase mRNA levels and after 12 and 24 h of treatment with the phorbol ester phorbol myristate acetate (PMA). Myocytes treated with the phorbol ester for 80 h showed a significant 34% decrease (relative to vehicle-treated control cells) in the levels of Ca(2+)-ATPase protein, and a significant 38% increase in the levels of alpha-sarcomeric actin, as assessed by Western blot analysis using specific antibodies. Immunocytochemistry of myocytes treated for 72 h with the phorbol ester revealed a hypertrophied cell morphology, and showed a marked decrease in Ca(2+)-ATPase staining intensity. Contractile calcium transients were evaluated through the use of indo-1. It was found that the t1/2 for the decline of calcium transient was significantly prolonged by PMA treatment (0.51 +/- 0.15) when compared to controls (0.38 +/- 0.17, P < 0.001). Treatment of myocytes with endothelin-1 also led to a 35% decrease in sarcoplasmic reticulum Ca(2+)-ATPase mRNA levels. It is concluded that phorbol ester treatment of neonatal rat cardiac myocytes induces similar changes in Ca(2+)-ATPase mRNA levels. It is concluded that phorbol ester treatment of neonatal rat cardiac myocytes induces similar changes in Ca(2+)-ATPase gene expression as observed in vivo in the hypertrophied and failing heart. The observed prolongation in t1/2 for [Ca2+]i decline might be due to the observed depressed levels for sarcoplasmic reticulum Ca(2+)-ATPase in PMA treated cells.

Animals↗
PubMed1999

Procollagen folding and assembly: the role of endoplasmic reticulum enzymes and molecular chaperones.

Procollagen assembly occurs within the endoplasmic reticulum, where the C-propeptide domains of three polypeptide alpha-chains fold individually, and then interact and trimerise to initiate folding of the triple helical region. This highly complex folding and assembly pathway requires the co-ordinated action of a large number of endoplasmic reticulum-resident enzymes and molecular chaperones. Disease-causing mutations in the procollagens disturb folding and assembly and lead to prolonged interactions with molecular chaperones, retention in the endoplasmic reticulum, and intracellular degradation. This review focuses predominantly on prolyl 1-hydroxylase, an essential collagen modifying enzyme, and HSP47, a collagen-specific binding protein, and their proposed roles as molecular chaperones involved in fibrillar procollagen folding and assembly, quality control, and secretion.

Collagen↗