In vivo effect of tunicamycin on the expression of rat small intestinal brush border membrane glycoproteins and glycoenzymes

N-linked glycosylation is critical for the plasma membrane localization of nephrin

The expression pattern, subcellular localization, and the role of glycosylation of the human nephrin was examined in transfected cells. Stable cell lines, constitutively expressing a full-length human nephrin cDNA construct, were generated from transfected immortalized mouse podocytes (IMP) and a human embryonic kidney cell line (HEK-293). Immunofluorescence confocal microscopy of transfected cells showed plasma membrane localization of the recombinant nephrin. Immunoblotting showed that the recombinant nephrin expressed in transfected cell lines migrated as a double band with a molecular weight of 185 kD. When cells were treated with the N-glycosylation inhibitor, tunicamycin, the molecular weight of nephrin was decreased to a single immunoband of 150 kD, indicating that the shift in the electrophoretic migration of nephrin is due to N-linked carbohydrate moieties. It was further shown that this glycosylation process is highly sensitive to inhibition by tunicamycin, which is a naturally occurring antibiotic, leading to retention of nonglycosylated nephrin molecules in the endoplasmic reticulum. It was concluded that N-glycosylation of nephrin is crucial for its proper folding and thereby plasma membrane localization; therefore, inhibition of this process might be an important factor in the onset of pathogenesis of some acquired glomerular diseases.

Changes in intestinal absorption of nutrients and brush border glycoproteins after total parenteral nutrition in rats

The effect of total parenteral nutrition on nutrients absorption and glycoprotein changes of brush border membrane was examined in rat small intestine. In total parenteral nutrition rats, a marked decrease in activity of brush border enzymes was observed mainly in the proximal and middle segments of the intestine. Galactose perfusion of jejunal segment showed that hexose absorption was significantly inhibited, while intestinal absorption of glycine or dipeptide, glycylglycine was not significantly affected by total parenteral nutrition treatment. When brush border membrane glycoprotein profile was examined by [3H]-glucosamine or [3H]-fucose incorporation into jejunal loops, significant changes were observed in the glycoprotein pattern of brush border membrane especially in the high molecular weight range over 120 kDa after total parenteral nutrition treatment, suggesting strong dependency of glycoprotein synthesis on luminal substances. Molecular weight of sucrase isomaltase in brush border membrane detected by specific antibody showed no significant difference, however, in total parenteral nutrition and control rats. Also, molecular weight of specific sodium glucose cotransporter of intestinal brush border membrane detected by selective photoaffinity labelling was not altered in total parenteral nutrition rats. It may be that prolonged absence of oral food intake may produce significant biochemical changes in brush border membrane glycoprotein and absorptive capacity of small intestine, but these changes were not observed in all brush border membrane glycoproteins.

Functional and morphological changes induced by tunicamycin in dividing and confluent endothelial cells

Cultured bovine aortic endothelial cells treated with tunicamycin, an inhibitor of glycoprotein synthesis, developed a concentration-dependent inhibition of N-acetylglucosamine-1-phosphate transferase activity, and this inhibition was correlated with a substantial decrease in [3H]mannose incorporation by the cells. Endothelial cells were very sensitive to tunicamycin, and changes in their morphology occurred as a result of the inhibition of glycoprotein synthesis. The cells became elongated, the surface irregular, roughened, and granular, and there was an increase in the interstitial space between the cells. Electron dense material was accumulated within and dilated the rough endoplasmic reticulum, and the distribution of the glycoproteins laminin and fibronectin throughout the endothelial cell monolayer was modified. These morphological changes coincided with functional impairment with the permeability of endothelial cell monolayers to both 125I-albumin and [3H]inulin being increased by treatment with tunicamycin (10(-6) M) for 24 h. These results indicate that the synthesis of glycoproteins is crucial for cell-cell adhesion and the functional properties of the endothelial lining of blood vessels.

Nutritional and developmental regulation of glycosylation processes in digestive organs

We review the nutritional and developmental variations of the glycosylation processes in digestive organs, since glycoproteins play a prominent part as mucins or digestive enzymes in these tissues. The biosynthesis of the glycannic chains is demonstrated to be largely sensitive to various exogenous (such as nutritional) or endogenous (such as developmental) factors. Although the metabolic regulation by dietary variations appears as rather complex, according to the variety of experimental conditions and the diversity of the organs studied, available data demonstrate that this regulation does exist, depending on the quantity or sometimes the quality of the major or minor components of the diet, which induce significant variations in the glycosylation processes. The synthesis of the internal core of N-glycans is essentially regulated by diet-induced variations of the phosphoryl-dolichol level, whereas the modulation of the biosynthesis of the external part of N-glycans or the biosynthesis of O-glycans is controlled by diet-induced variations in the systems transferring fucose, galactose, sialic acid or hexosamines. Modifications in intestinal glycosylation during post-natal development in the rat control the quality of the glycannic chains of mucins and brush-border enzymes. The post-natal maturation of the intestinal rat tissue is characterized by a shift from sialylation to fucosylation, depending on coordinate changes in glycosyltransferase activities, in sugar-nucleotide breakdown or synthesis or in the activity of regulatory proteins. These activities are largely sensitive to dietary manipulations at weaning and to hormonal stimulations before weaning. However, glucocorticoid hormones do not appear as the triggering signal for the induction of these changes.

Disaccharidases activity in the intestinal mucosa after methotrexate therapy

Disaccharidases activity in the intestinal mucosa samples of rats was examined after intragastric and intramuscular methotrexate therapy. Methotrexate was given on a twice a week schedule (1 mg/kg body weight). The animals were sacrificed after 2 weeks, 1 month, and 2 months of this therapy. A statistically significant but transient decrease of the lactase and maltase activity was found. The authors suggest studying the beneficial effect of low disaccharidase diet in the first period of methotrexate therapy in children treated with methotrexate.