Increase of sialyltransferase activity in the small intestine following thermal injury in rats

Injury-associated induction of two novel and replication-defective murine retroviral RNAs in the liver of mice

Injury can alter the expression of numerous genes in affected tissues as well as in distant organs. The mouse genome harbors numerous copies of endogenous murine leukemia virus (MuLV)-related retroviral sequences. Mouse liver tissues harvested after burn injury were subjected to RT-PCR analysis to investigate the regulation of MuLV-related sequences using a primer set capable of amplifying the full-length transcript. A doublet of approximately 5-kb was transiently up-regulated at 3 and 6 h after injury. Sequence analyses revealed that these are novel defective endogenous retroviral sequences (MuLV(LI-8) and MuLV(LI-12)), which are predominantly characterized by major deletions in pol and env genes. The MuLV(LI-8) clone is 4.85 kb long and the deduced gag polypeptide sequence was almost identical to a previously reported replication-defective retroviral sequence associated with immunesuppression. In the MuLV(LI-12) clone of 5.06 kb, there were two truncated gag open reading frames (ORFs) and 1 pol ORF fused to the C-terminus of the env p15E. Furthermore, the ORFs for the unique gag p12 presumed to be responsible for the immunesuppression were present in both clones. These novel replication-defective MuLVs may participate in the pathogenesis of distant organs after injury.

Coordinate expression of beta-galactoside alpha 2,6-sialyltransferase mRNA and enzyme activity in suckling rat jejunum cultured in different media: transcriptional induction by dexamethasone

In attempting to elucidate the molecular basis of the expression of alpha 2,6-sialyltransferase (alpha 2,6-ST) in jejunal explants of 7-day-old rats during cultivation, the total jejunal RNA was analysed by hybridization using a cDNA clone encoding rat liver alpha 2,6-ST. Under cultivation in both serum-free and serum-containing media jejunal alpha 2,6-ST mRNA closely paralleled the bound (100,000 g pellet) as well as the soluble (100,000 g supernatant) alpha 2,6-ST activity, the correlation coefficients being 0.976 and 0.816, respectively. Dexamethasone (Dx) treatment enhanced alpha 2,6-ST mRNA and membrane-bound alpha 2,6-ST activity in close correlation. Jejunal alpha 2,6-ST mRNA is sensitive to actinomycin D and is lost with apparently identical kinetics in Dx-stimulated and control explants, suggesting that regulation by Dx may be exerted by altering the rate of mRNA synthesis. Dx-dependent activation resulted in elevation of the 4.3-Kb mRNA and can be inhibited by the antiglucocorticoid onapristone, demonstrating the participation of the glucocorticoid receptor pathway.

Coordinated activation of corneal wound response genes in vivo as observed by in situ hybridization

We used subtractive screening of a cDNA library prepared from corneoscleral rims after cauterizing rat corneas. We identified 76 clones whose corresponding mRNA increased during the wound healing process in an in vivo model of injury which damages the corneal epithelium, stroma, and endothelium. Of these clones, 31 sequences encode known proteins. Another 45 clones are novel sequences based on comparison with the GenBank/EMBL databases. Changes in the level of expression of the novel genes, and a selected number of the known genes, were examined by in situ hybridization 22 and 72 hr after corneal injury. The majority produced a 'wound pattern' of expression such that the mRNAs were highly induced in all cell types adjacent to the wound site at 22 hr post injury. This signal decreased in intensity with distance from the wound site. In a subset of corneoslceral rims examined by in situ hybridization, the mRNAs for these genes were also highly induced in the limbal epithelium, where the progenitor corneal epithelial stem cells reside. By 72 hr, when acute tissue damage had been repaired, the induced mRNA was only faintly present in the thickened epithelium. Our results provide a useful framework for further studies defining the pathophysiological roles of the known and novel proteins encoded by the isolated cDNA clones.

Alpha 2,6-sialyltransferase predominates in cultured jejunum of suckling rats: it is up-regulated by dexamethasone and secreted during cultivation

The alpha 2,6-sialyltransferase (alpha 2,6-ST) acting on N-acetyllactosamine is the major sialyltransferase of suckling rat jejunum. Jejunal explants of 7-day-old rats maintained in serum-free or serum-containing organ culture secreted alpha 2,6-ST into the cultivation medium. Dexamethasone (80 nM) stimulates primarily the secreted pool of alpha 2,6-ST. Fetal calf serum promotes the stimulatory effect of dexamethasone also on the bound form of alpha 2,6-ST.

Role of antiproteolytic heparin-binding serum protein(s) in modulating the levels of sialyl- and galactosyltransferase activity released during the incubation of rat jejunal slices

1. Sialyltransferase released into the medium during the incubation of rat jejunal slices in serum-free buffer, was susceptible to proteolytic degradation. Heat inactivated horse serum or its antiproteolytic heparin-binding fraction was found to be necessary in determining the activity of sialyltransferase released (Nadkarni et al., 1991). 2. In the present study, we have shown that heat inactivated rat serum (HRS) or its antiproteolytic heparin-binding fraction (HBF) had a role in determining the sialyltransferase activity released during jejunal slice incubations. 3. Galactosyltransferase was also released during incubations, but was not proteolytically degraded and the presence of HRS or HBF in incubations did not alter the levels of galactosyltransferase activity released. 4. Trypsin activity in serum-free incubation medium was higher compared to medium containing HRS. 5. Addition of serum-free medium obtained from 4 hr incubations of the jejunal slices, to medium obtained from parallel incubations done in the presence of HRS, caused inhibition of sialyl- but not galactosyltransferase activity. 6. In jejunal homogenates stored at -20 degrees C, sialyltransferase activity was decreased during 0-45 days of storage, whereas galactosyltransferase activity remained fairly stable for upto 56 days. 7. Inclusion of HRS or HBF in homogenates resulted in higher sialyl- but not galactosyltransferase activity compared to serum-free homogenate samples. 8. The results suggest that HRS or its antiproteolytic heparin-binding proteins have a role in determining the sialyltransferase activity released from the jejunal slices. In contrast galactosyltransferase released was not susceptible to proteolysis, and HRS or HBF was not required to express its activity.

Glycosylation in intestinal epithelium

This chapter reviews the glycosylation reactions in the intestinal epithelium. The intestinal epithelium represents a good model system in which the glycosylation process can be studied. The intestinal epithelium is composed of two basic epithelial cell types: the absorptive enterocyte and the mucus-producing goblet cell. Gastrointestinal epithelial renewal ensues through the processes of cell proliferation, migration, and differentiation. This renewal occurs in discrete proliferative zones along the gastrointestinal tract. In the small intestine, this proliferative zone is restricted to the base of the crypts, whereas in the large intestine it is less restrictive, occurring in the basal two thirds of the crypt. A longitudinal section along the crypt-to-surface axis, cells in various degrees of differentiation is observed, providing a unique in vivo system in which to investigate differentiation-related glycosylation events. The glycoconjugate repertoire displayed by a given cell reflects its endogenous expression of glycosyltransferases. The role played by terminal oligosaccharide structures in cell–cell recognition phenomena and the expression of glycosyltransferases occupy a key position in the post-translational processing of glycoconjugates and thus influence cellular function.

Injury-induced inhibition of small intestinal protein and nucleic acid synthesis

Small intestinal mucosal weight and nutrient absorption are significantly diminished early after cutaneous thermal injuries. Because these intestinal properties are highly dependent on rates of nucleic acid and protein synthesis, in vivo incorporation of thymidine, uridine, and leucine into small intestinal deoxyribonucleic acid, ribonucleic acid, and proteins were measured. Deoxyribonucleic acid synthesis was markedly decreased with the lowest thymidine incorporation in the jejunum (p less than 0.01); these findings were confirmed by autoradiographic identification of radiolabeled nuclei in the intestinal crypts. Protein synthesis was decreased by 6 h postinjury (p less than 0.01) but had returned to normal by 48 h. Consistent with a decreased rate of protein synthesis, ribonucleic acid synthesis was also decreased 18 h postinjury (p less than 0.01). These decreased deoxyribonucleic acid, ribonucleic acid, and protein synthesis rates are not likely a result of ischemia because in other studies of this injury model, intestinal blood flow was not significantly changed by the burn injury. Potentially, factors initiating the acute inflammatory reaction may directly inhibit nucleic acid and protein synthesis and lead to alterations in nutrient absorption and intestinal barrier function after injury.