Structural and functional changes in acute liver injury.
Carbon tetrachloride produces liver cell injury in a variety of animal species. The first structurally recognizable changes occur in the endoplasmic reticulum, with alteration in ribosome-membrane interactions. Later there is an increase in intracellular fat, and the formation of tangled nets of the ergastoplasm. At no time are there changes in mitochondria or single membrane limited bodies in cells with intact plasmalemma, although a relative increase in cell sap may appear. In dead cells (those with plasmalemma discontinuties) crystalline deposits of calcium phosphatase may be noted. Functional changes are related to the endoplasmic reticulum and the plasma membrane. An early decrease in protein synthesis takes place; an accumulation of neutral lipid is related to this change. Later alterations in the ergastoplasmic functions (e.g., mixed function oxidation) occurs. Carbon tetrachloride is not the active agent; rather, a product of its metabolism, probably the CC1, free radical, is. The mechanisms of injury include macromolecular adduction and peroxide propagation. A third possibility includes a cascade effect with the production of secondary and tertiary products, also toxic in nature, with the ability to produce more widespread damage to intracellular structures. (+info)
The Saccharomyces cerevisiae CWH8 gene is required for full levels of dolichol-linked oligosaccharides in the endoplasmic reticulum and for efficient N-glycosylation.
The Saccharomyces cerevisiae mutant cwh8 was previously found to have an anomalous cell wall. Here we show that the cwh8 mutant has an N -glycosylation defect. We found that cwh8 cells were resistant to vanadate and sensitive to hygromycin B, and produced glycoforms of invertase and carboxypeptidase Y with a reduced number of N -chains. We have cloned the CWH8 gene. We found that it was nonessential and encoded a putative transmembrane protein of 239 amino acids. Comparison of the in vitro oligosaccharyl transferase activities of membrane preparations from wild type or cwh8 Delta cells revealed no differences in enzyme kinetic properties indicating that the oligosaccharyl transferase complex of mutant cells was not affected. cwh8 Delta cells also produced normal dolichols and dolichol-linked oligosaccharide intermediates including the full-length form Glc3Man9GlcNAc2. The level of dolichol-linked oligosaccharides in cwh8 Delta cells was, however, reduced to about 20% of the wild type. We propose that inefficient N -glycosylation of secretory proteins in cwh8 Delta cells is caused by an insufficient supply of dolichol-linked oligosaccharide substrate. (+info)
The role of oocyte transcription, the 5'UTR, and translation repression and derepression in Drosophila gurken mRNA and protein localization.
The establishment of the major body axes of the Drosophila egg and future embryo requires strict regulation of gurken mRNA and protein localization. Here, we show that grk mRNA and protein localization is dependent on synthesis of grk transcripts in the oocyte nucleus and on RNA localization elements in the 5' portion of the transcript. We also show that gurken mRNA and protein localization is dependent on region-specific translation of gurken transcripts and identify K10 as a probable negative regulator of gurken translation. (+info)
Re-entering the translocon from the lumenal side of the endoplasmic reticulum. Studies on mutated carboxypeptidase yscY species.
Misfolded or unassembled secretory proteins are retained in the endoplasmic reticulum (ER) and subsequently degraded by the cytosolic ubiquitin-proteasome system. This requires their retrograde transport from the ER lumen into the cytosol, which is mediated by the Sec61 translocon. It had remained a mystery whether ER-localised soluble proteins are at all capable of re-entering the Sec61 channel de novo or whether a permanent contact of the imported protein with the translocon is a prerequisite for retrograde transport. In this study we analysed two new variants of the mutated yeast carboxypeptidase yscY, CPY*: a carboxy-terminal fusion protein of CPY* and pig liver esterase and a CPY* species carrying an additional glycosylation site at its carboxy-terminus. With these constructs it can be demonstrated that the newly synthesised CPY* chain is not retained in the translocation channel but reaches its ER lumenal side completely. Our data indicate that the Sec61 channel provides the essential pore for protein transport through the ER membrane in either direction; persistent contact with the translocon after import seems not to be required for retrograde transport. (+info)
Cloning of the peroxiredoxin gene family in rats and characterization of the fourth member.
Peroxiredoxin (PRx) exhibits thioredoxin-dependent peroxidase activity and constitutes a family of proteins. Four members of genes from rat tissues were isolated by PCR using degenerated primers based on the sequences which encode a pair of highly conserved Cys-containing domains, and were then cloned to full-length cDNAs. These included two genes which have previously been isolated in rats, PRx I and PRx II, and two rat homologues of PRx III and PRx IV. We showed, for the first time, the simultaneous expression of all four genes in various rat tissues by Northern blotting. Since a discrepancy exists regarding cellular distribution, we further characterized PRx IV by expressing it in COS-1 cells. This clearly demonstrates that PRx IV is a secretory form and functions within the extracellular space. (+info)
Characterization of elementary Ca2+ release signals in NGF-differentiated PC12 cells and hippocampal neurons.
Elementary Ca2+ release signals in nerve growth factor- (NGF-) differentiated PC12 cells and hippocampal neurons, functionally analogous to the "Ca2+ sparks" and "Ca2+ puffs" identified in other cell types, were characterized by confocal microscopy. They either occurred spontaneously or could be activated by caffeine and metabotropic agonists. The release events were dissimilar to the sparks and puffs described so far, as many arose from clusters of both ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (InsP3Rs). Increasing either the stimulus strength or loading of the intracellular stores enhanced the frequency of and coupling between elementary release sites and evoked global Ca2+ signals. In the PC12 cells, the elementary Ca2+ release preferentially occurred around the branch points. Spatio-temporal recruitment of such elementary release events may regulate neuronal activities. (+info)
Lectin receptor sites on rat liver cell nuclear membranes.
The presence and localization of lectin receptor sites on rat liver cell nuclear and other endomembranes was studied by light and electron microscopy using fluorescein and ferritin-coupled lectin conjugates. Isolated nuclei labelled with fluorescein-conjugated Concanavalin A (Con A) or wheat germ agglutinin (WGA) often showed membrane staining, which sometimes was especially bright on small stretches of the nuclear surface. Unlabelled nuclei and nuclei with a complete ring fluorescence were also seen. The nuclear fluorescence corresponded in intensity to that seen on the surface of isolated rat liver cells. Con A-ferritin particles were seldom detected on the cytoplasmic surface of the intact nuclear envelope. However, at places where the 2 leaflets of the envelope were widely separated or where the outer nuclear membrane was partly torn away, heavy labelling was seen on the cisternal surface of both the inner and outer nuclear membranes. Labelling with Con A-ferritin was also found on the cisternal side of rough endoplasmic reticulum present in the specimens. No labelling was seen on the cytoplasmic surface of mitochondrial outer membrane. The results demonstrate the presence of binding sites for Con A and WGA in nuclei and an asymmetric localization of these sites on the cisternal side of ribosome-carrying endomembranes in rat liver cells. (+info)
Missense mutations in SGLT1 cause glucose-galactose malabsorption by trafficking defects.
Glucose-galactose malabsorption (GGM) is an autosomal recessive disorder caused by defects in the Na+/glucose cotransporter (SGLT1). Neonates present with severe diarrhea while on any diet containing glucose and/or galactose . This study focuses on a patient of Swiss and Dominican descent. All 15 exons of SGLT1 were screened using single stranded conformational polymorphism analyses, and aberrant PCR products were sequenced. Two missense mutations, Gly318Arg and Ala468Val, were identified. SGLT1 mutants were expressed in Xenopus laevis oocytes for radiotracer uptake, electrophysiological experiments, and Western blotting. Uptakes of [14C]alpha-methyl-d-glucoside by the mutants were 5% or less than that of wild-type. Two-electrode voltage-clamp experiments confirmed the transport defects, as no noticeable sugar-induced current could be elicited from either mutant . Western blots of cell protein showed levels of each SGLT1 mutant protein comparable to that of wild-type, and that both were core-glycosylated. Presteady-state current measurements indicated an absence of SGLT1 in the plasma membrane. We suggest that the compound heterozygote missense mutations G318R and A468V lead to GGM in this patient by defective trafficking of mutant proteins from the endoplasmic reticulum to the plasma membrane. (+info)