Shu-Heh W. Chu

Developmental changes in the activities of sialyl- and fucosyltransferases in rat small intestine.

To study an enzymatic basis for the postnatal changes in intestinal glycosylation, the activities of sialyl- and fucosyltransferases were determined in the particulate fraction of mucosal cells prepared from rat small intestine of various ages. The results show that sialyltransferase activity was present in increased levels compared to adults during the preweaning period (1-2 weeks) and subsequently declined 5-fold to adult levels after weaning, while fucosyltransferase activity was decreased compared to adults in the first 3 weeks of life, rapidly increased at 4 weeks, and reached adult levels (10-fold) by 5 weeks. The changes in both sialyl- and fucosyltransferase activities were reflected by the membranous content of glycosidic-bound sialic acid and fucose, respectively. Cortisone injection precociously induced a decreased sialyltransferase activity and an increased fucosyltransferase activity in 2-week-old suckling rats. This study indicates that the activities of sialyl- and fucosyltransferases were reciprocally related and modulated by cortisone action in the developing intestine. These enzyme changes may be responsible for the previously noted shift from sialylation to fucosylation of the intestinal mucosa during maturation.

Bacterial toxin interaction with the developing intestine

An important approach to the major health problem of bacterial infection in young children has been to examine bacterial toxin binding to microvillus membrane receptors, the signal transduction produced by that interaction and the mechanisms of fluid secretion in the developing intestine as a basis for toxigenic diarrhea in the infant population. These studies indicate that receptor binding and effector responses may be subjected to developmental regulation. This regulation process of toxin interaction with the developing intestine may have an enhanced or harmful effect or, under some circumstances, may have a beneficial effect and be protective to the vulnerable child. Specific mechanisms for the developmental control of receptor expression may involve the regulation of individual glycosyltransferases responsible for the addition of receptor sugar sequences to glycolipids and/or glycoproteins, presumably at the transcriptional level. Furthermore, although highly speculative at this point, the differential expression of signal transducers (e.g., guanine nucleotide-regulatory proteins or G proteins) and ion transporters (e.g., Na+,K(+)-stimulated adenosine triphosphatase, the Cl- channels, etc.) during development may also alter the neonatal hosts responsiveness. Therefore, the developmental control of microvillus membrane receptors, signal transduction mechanisms, and ion transport systems in the gastro-intestinal tract may in part contribute to the altered host sensitivity in toxigenic diarrhea of infancy.

Developmental changes in galactosyltransferase activity in the rat small intestine

During postnatal development, UDP-Gal: GlcNAc(beta 1-4)-galactosyltransferase (4 beta-GT) and UDP-Gal:GalNAc(beta 1-3)-galactosyltransferase (3 beta-GT) activities were increased by 17- and 24-fold, respectively, in the rat small intestine. The injection of cortisone into suckling rats resulted in precocious induction of distal 4 beta- and 3 beta-GT activities by 2.7- and 1.8-fold, respectively. Injection of phorbol-12-myristate-13-acetate (PMA) resulted in precocious induction of distal 3 beta-GT by 2.7-fold. These results suggest that intestinal galactosyltransferase activities are under developmental regulation and can be modified by cortisone and PMA.

myo-Inositol action on gerbil intestine: Association of phosphatidylinositol metabolism with lipid clearance

The synthesis and turnover of phosphatidylinositol as well as clearance were studied in the intestines of lipodystrophic gerbils treated with or without an intraperitoneal dose of myo-inositol by monitoring the incorporation of 32Pi and the retention of absorbed [1-14C] palmitic acid. 1. myo-Inositol deficiency produced an intestinal lipodystrophy with a large lipid accumulation and a decreased level of phosphatidylinositol. Upon myo-inositol repletion, the intestinal phosphatidylinositol rapidly returned to the control level by h, at which time the removal of excess lipid still remained in a lag phase. 2. myo-Inositol injection caused an increase in the incorporation of 32Pi into phosphatidylinositol mainly due to an increased phosphatidylinositol synthesis de novo. As a result, the turnover of phosphatidylinositol molecules might increase because of an expanded pool size. 3. The stimulation of phosphatidylinositol synthesis was then followed by an enhanced clearance of absorbed [14C] palmitate and by an intestinal recovery which was monitored by the loss of accumulated triacylglycerol. 4. This study indicates that myo-inositol availability appears to regulate the in vivo biosynthesis of phosphatidylinositol which, in turn, may play a crucial role in normal lipid transport across gerbil intestine.

Growth Factor Signal Transduction in Human Intestinal Cells

One of the basic biological problems in the area of developmental gastroenterology is to understand the factors and mechanisms controlling the intestinal epithelial cell growth and differentiation. Enterocytes are the type of cells undergoing a rapid turnover during development and mature life. Cells in the crypt are rapidly dividing and less differentiated, whereas cells at the villus tip are terminally differentiated. Furthermore, there is a differential gene expression in both crypt cells and villus cells between the neonatal and adult intestine. Increasing evidence indicates that the control of enterocyte proliferation and differentiation appears to be mediated not only by endocrine or paracrine but also by autocrine mechanisms1,2. Table 1 lists several growth factors that may potentially target at the human intestinal cells. No matter how the production of growth factors are regulated, they must interact with their specific cell-surface receptors. Ligand-receptor interaction on the cell surface is then translated into activation of intracellular signaling pathways, triggering a sequence of events that eventually leads to cell division or differentiation.

Myo-inositol action on gerbil intestine: reversal of a diet-induced lipodystrophy and change in microsomal lipase activity.

An intestinal lipodystrophy induced by dietary fat in female Mongolian gerbils (Meriones unguiculatus) was reversed to normal by myo-inositol given in the diet or by injection within 1-4 days. An increase in plasma chylomicron and lipid concentrations was observed before the occurrence of rapid disappearance of accumulated lipids from the intestine. Dietary myo-inositol also caused an increase in triacylglycerol release from everted gut sacs. Thus, myo-inositol might act on the intestine to stimulate the production and secretion of chylomicrons, travelling via the lymphatic pathway into the bloodstream. The activity of pH 9.0 microsomal lipase (EC 3.1.1.3) of gerbil intestine was decreased due to myo-inositol deficiency. The lowered activity could be restored to high levels by feeding of injecting myo-inositol in vivo. A time-course study during intestinal recovery indicates that the increase in microsomal lipase activity correlated with the rapid lipid removal phase of the intestine, but not the initial increase in plasma chylomicron and triacylglycerol concentrations.

myo-Inositol action on gerbil intestine: Alterations in alkaline phosphatase activity upon phosphatidylinositol depletion and repletion in vivo

The influence of phosphatidylinositol (PI) on intestinal alkaline phosphatase activity was studied in myo-inositol deficient gerbils. A reduction of membrane PI in intestinal mucosa to 30-40% of the control was produced by feeding female gerbils a myo-inositol-deficient diet containing coconut oil for 2 weeks. As expected, the animals developed typical intestinal lipodystrophy with abnormal fat accumulation. In the PI-depleted animal, intestinal alkaline phosphatase activity was reduced to 20-30% of the control group. The levels of both membranous and soluble enzymes in intestinal mucosa were affected, but there were no changes in liver, kidney and plasma levels. When the lipodystrophic gerbils were given dietary myo-inositol, the complete repletion of intestinal membrane PI to the control level occurred 36 h later, whereas membrane-bound alkaline phosphatase activity in intestine was not restored to the control level until 72 h later. Administration of cycloheximide or actinomycin D did not block this enzyme induction. Lymphatic output of triacylglycerol into the bloodstream was stimulated 10-fold at 18 h of myo-inositol repletion, but there was no parallel increase in the activity of alkaline phosphatase in plasma during this early phase of intestinal recovery. Thus, these data suggest a possible regulatory role of PI in the processing and/or turnover of alkaline phosphatase in vivo, but a negative role of alkaline phosphatase in lipid transport across gerbil intestine.

IN UTERO CORTISONE ALTERS THE INTESTINAL SIALYL-AND FUCOSYLTRANSFERASE ACTIVIES IN NEWBORN RATS

Changes in intestinal surface carbohydrates, such as sialic acid and fucose, may influence the binding of membrane receptors to bacteria/toxins or antigens. We have recently reported (Biochim. Biophys. Acta 883, 496, 1986) that the activities of intestinal sialyltransferase (ST) and fucosyltransferase (FT) are reciprocally related and modulated by cortisone during postnatal development. These glycosyl-transferase changes may provide an enzyme basis for the shift from sialylatipn to fucosylation of the intestinal surface during maturation. To further study whether prenatal treatment of cortisone may affect the activities of these two enzymes, cortisone (20 mg/100 g body weight) was injected i.p. in pregnant rats daily beginning at 17 days of gestation. The newborn rats (<24 hr old) were sacrificed and ST and FT were assayed in the membranous (105,000 × g pellet) fraction of small intestine, using asialofetuin as an exogenous acceptor. The results show that the activity of ST was decreased by 50% in the newborn rat exposured to cortisone in utero, while the activity of FT in the cortisone-treated group was induced 2-3 fold compared to the control. This study indicates that cortisone seems able to act as a signal in utero to modulate the expression of intestinal ST and FT and suggests that the cortisone-induced changes in intestinal glycosyltransferases may play a role in the maturation of the mucosal barrier in neonates. (Support by NIH grant DK37521).