ADAPTATION OF THE GASTROINTESTINAL TRACT TO EXTRAUTERINE LIFE*

Epidermal growth factor receptor-stimulated intestinal epithelial cell migration requires phospholipase C activity

BACKGROUND & AIMS

Ulceration of intestinal mucosa is rapidly followed by enterocyte migration via restitution. The aim of this study was to investigate signaling mechanisms of epidermal growth factor (EGF) receptor-stimulated monolayer restitution in a mouse intestinal epithelial cell line.

METHODS

EGF-stimulated cell migration was determined using a wound model in the presence of agonists and/or antagonists of tyrosine kinase, phospholipase C, phosphatidylinositol 3-kinase, or protein kinase C. The tyrosine phosphorylation state of the EGF receptor, phosphatidylinositol phospholipase C gamma1 (PLCgamma1), focal adhesion kinase, and cellular lysates was determined by immunodetection.

RESULTS

EGF stimulated cell migration twofold at 4, 8, and 24 hours. Inhibition of EGF receptor tyrosine kinase activity, phospholipase C, or phosphatidylinositol 3-kinase attenuated EGF-induced intestinal cell migration. Pretreatment of cells with phorbol 12-myristate 13-acetate, known to down-regulate protein kinase C expression, blocked EGF-induced cell migration. Increased tyrosine phosphorylation of the EGF receptor and PLCgamma1 was detected within 5 minutes after wounding.

CONCLUSIONS

EGF-stimulated intestinal cell migration requires intact EGF receptor tyrosine kinase, phospholipase, and protein kinase C activities. PLCgamma1 may be a key regulatory molecule in the initial EGF receptor signal transduction pathway of EGF-stimulated cell migration.

Microflora persistence on duodenojejunal flat or normal mucosa in time after a meal in children

A pathogenic role for high numbers of bacteria in the small intestine had been suggested previously by bacterial counts on luminal aspirates, but these investigations were flawed by the sampling device "contamination" in the mouth and the changing nature of fluent intestinal content. A procedure was developed to sterilize the Watson biopsy capsule with HCl in the upper portion of the duodenum. Bacteria were counted in the mucosal homogenate of the first (diagnostic) duodenojejunal biopsy in 80 untreated celiac children, and in 46 children with irritable bowel syndrome (IBS) in a four-cell, controlled, randomized investigation. Persistence of bacteria on the mucosa for 20 h after the last meal was investigated in 62 subjects, and for 26 h after the last meal in 64 subjects. Bacteria, mainly streptococci and staphylococci, persisted at a concentration of 10(6) per gram of mucosa 20 h after the last meal. The number of bacteria per gram of mucosa was 24 times higher in all 62 children of the 20-h fast groups than in all 64 children of the 26-h fast groups (p < 0.001). The bacteria count in celiac children was 39 times higher in the 20-h fast group than in the 26-h one. This difference was significantly higher than the 11 times difference that was found on the normal mucosa between the 20- and 26-h fast IBS groups (p < 0.001), which was still significant. The number of bacteria on duodenojejunal mucosa depends on nutrient absorption and persists longer than the intermeal interval in these subjects.

Enhanced expression of neurotensin/neuromedin N mRNA and products of NT/NMN precursor processing in neonatal rats

Intestinal levels of immunoreactive neurotensin (iNT) and neuromedin N (iNMN), as well as mRNA for the NT/NMN precursor, were elevated during the suckling period in rats. While transient expression of NT/NMN was observed at 1-5 days of age in the proximal small intestine and colon, NT/NMN levels in the ileum increased to peak at 10-20 days of age and then decreased to adult levels. The levels of these peptides were not elevated in the central nervous system and pituitary over this time period. Chromatographic analyses of jejunoileal extracts indicated that large molecular forms of iNT and iNMN were present, constituting approximately 1.3% of total iNT and approximately 56% of total iNMN, respectively. Treatment of the large forms with pepsin, which is known to generate the fully immunoreactive peptides, NT(3-13), NT(4-13), and NMN, increased immunoreactivity tenfold (iNT) and 1.2-fold (iNMN). Thus, large forms actually constituted approximately 13% (iNT) and approximately 60% (iNMN). Based upon its physicochemical properties, large molecular iNMN was tentatively identified as a 125 residue peptide with NMN at its C-terminus [i.e., rat prepro-NT/NMN(23-147)]. The properties of large molecular iNT were most similar to those predicted for the entire precursor [i.e., rat prepro-NT/NMN(23-169)]. These results indicate a) that enhanced expression of NT/NMN occurs in a tissue-specific manner in rats during the suckling period; b) that the pattern of precursor processing in intestine yields primarily NT and a large molecular form of NMN.

Positional specificity of gastric hydrolysis of long-chain n-3 polyunsaturated fatty acids of seal milk triglycerides

Long-chain n-3 polyunsaturated fatty acids (n-3 PUFA) of marine oils are important dietary components for both infants and adults, and are incorporated into milks following maternal dietary intake. However, little is known about the hydrolysis of these PUFA from milk triglycerides (TG) by lipases in suckling young. Seals, like humans, possess gastric lipase; however, the milk lipids of seals and sea lions are almost devoid of the readily hydrolyzable medium-chain fatty acids, and are characterized by a large percentage (10-30%) of n-3 PUFA. Gastric hydrolysis of milk lipids was studied in vivo in suckling pups of three species (the California sea lion, the harp seal and the hooded seal) in order to elucidate the actions and specificity of gastric lipases on milk TG in relation to fatty acid composition and TG structure. Regardless of milk fat content (31-61% fat) or extent of gastric hydrolysis (10-56%), the same fatty acids were preferentially released in all three species, as determined by their relative enrichment in the free fatty acid (FFA) fraction. In addition to 16:1 and 18:0, these were the PUFA of 18 carbons and longer, except for 22:6n-3. Levels of 20:5n-3 were most notably enriched in FFA, at up to five times that found in the TG. Although 22:6n-3 was apparently also released from the TG (reduced in the diglyceride), it was also notably reduced in FFA.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C----U, replacing codon CAA (glutamine 2,153) with the inframe stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcription-polymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from approximately 1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from approximately 8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to approximately 80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue- and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism.

Milk lipid digestion in the neonatal dog: the combined actions of gastric and bile salt stimulated lipases

Intragastric lipolysis may be particularly important for the digestion of milk lipid since milk fat globules are resistant to pancreatic lipase without prior disruption; milk bile salt stimulated lipase (BSSL) may supplement further intestinal hydrolysis. Previous information on gastric lipolysis has been based primarily on in vitro studies using artificial lipid emulsions containing a single component fatty acid and have focused on the preferential release of medium-chain fatty acids. The actual contribution of these enzymes to overall fat digestion in vivo on natural substrates has rarely been studied, however. The neonatal dog is an excellent model in the study of lipid digestion because, like the human, milk lipids are high in long-chain unsaturated fatty acids, milk contains BSSL and gastric lipase is the predominant lipolytic enzyme acting in the stomach. We used a combination of in vivo studies with in vitro incubations to investigate digestion of milk lipid by gastric and milk (BSSL) lipases in the suckling dog. In the first 4 weeks postpartum, 14-41% and 42-60% of milk triacylglycerol was hydrolyzed to primarily diacylglycerol and free fatty acid (FFA) in the first 30 and 60 min in the stomach, respectively. Milk lipid contained high levels (63%) of long-chain unsaturated fatty acids, which were preferentially released as FFA during in vivo gastric lipolysis, consistent with the actions and stereospecificity of gastric lipase. While levels of hydrolysis in gastric aspirates were significantly different (by age and time in stomach) at the start of in vitro studies, total hydrolysis in all incubation systems plateaued at about 65%, suggesting product inhibition by the long-chain FFA, but to a much lesser degree than previously expected from in vitro studies. The magnitude of in vivo intragastric lipolysis was 3- to 6-times greater than that predicted by in vitro assays using either milk lipid or labeled emulsion as substrate, respectively. Prior exposure to intragastric lipolysis resulted in 30% hydrolysis by BSSL compared to 5% hydrolysis without prior exposure. We suggest that previous in vitro studies have largely underestimated the actual degree of intragastric lipolysis that can occur and its activity on long-chain fatty acids; this study indicates the importance of the combined mechanisms of gastric lipase and BSSL to fat digestion in the suckling neonate.

Accumulation of abundant messenger ribonucleic acids during postnatal development of mouse small intestine

To describe the differentiation of the small bowel at the molecular level, intestinal messenger ribonucleic acids (mRNAs) from mice at different stages of fetal and postnatal development were investigated. On the basis of cell-free translation and complementary deoxyribonucleic acid cloning experiments, abundant mRNAs coding for small polypeptides of 6-12 kilodaltons (low-molecular-weight mRNAs) were found in adult small intestine but not in the fetal gut. These developmentally regulated low-molecular-weight mRNAs are uniquely abundant in jejunum and ileum of adult mice, but they are absent or occur only at low levels in the duodenum, colon, stomach, and all other mouse organs examined. Low-molecular-weight mRNAs begin accumulating in the small bowel at approximately 3 wk of age, coinciding with weaning and with profound changes in intestinal differentiation. One complementary deoxyribonucleic acid clone of a low-molecular-weight mRNA (asb4/134) is specific to the distal small bowel, specifically accumulates at weaning, and hybridizes to RNA from mouse testis and brain at approximately 2%-5% of the intestinal level. Low-molecular-weight mRNA sequences may provide important markers of intestinal differentiation at the genetic level, leading to a better understanding of the factors that contribute to its postnatal maturation.

Hormonal regulation of epithelial cell proliferation in the fetal mouse duodenum in vitro

In adult rodents, radioautographic studies have shown that cell proliferation in the small intestinal epithelium is confined to the crypts. However, before birth DNA synthesis occurs also on villi. The role of extrinsic factors in the establishment of the adult pattern of enterocyte proliferation was studied in 17-day fetal mouse duodenal explants cultured with a completely synthetic medium. The effect of dexamethasone (30 and 300 ng/ml), thyroxine (10 nM), and insulin (125 mU/ml) on labelling indices was determined after 48 hours of culture. At the onset of the culture period close to 60% of the cells located in the first five cell positions in the intervillous areas became labelled after 4 hours of culture in presence of 3H-thymidine; 8% of the other epithelial cells found higher in the intervillous areas and on villi were also labelled. After 48 hours of culture, well differentiated crypts have developed in all the explants cultured with or without the hormones. The addition of thyroxine or insulin did not modify the labelling indices. However, after 48 hours of culture, the presence of dexamethasone (both concentrations) significantly reduced the labelling index of the cells found over cell position 5 but had no effect on the labelling index of cells located at the bottom of the crypts. When labelling index was determined for each cell position in the crypts at the end of the culture period, it appeared that dexamethasone (300 ng/ml) had no effect thereon for the first five cell positions but had a significant influence on the transitional zone from cell position 6 through 15.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hydrocortisone on carbohydrase concentrations, de novo synthesis and turnover patterns in immature rat intestine

Hydrocortisone administration to infant rats enhanced cellobiase and maltase activities and induced precocious expression of sucrase and trehalase activities along the length of the small intestine. These activity changes reflected proportional concentration increases in the enzymes lactase (EC 3.2.1.23), maltase/glucoamylase (EC 3.2.1.20) and sucrase-isomaltase (EC 3.2.1.48/10). Administration of an equivalent tracer dose of [3H]leucine (by body weight) to control and hydrocortisone-treated infant rats resulted in greater accumulation of label in the carbohydrase pools of the treated rats, suggesting their increased de novo synthesis. The increased concentrations of lactase and maltase/glucoamylase induced by exogenous hydrocortisone were matched by the presence of corresponding greater amounts of label in their brush border pools. Accumulation of label in each of the lactase, maltase/glucoamylase and sucrase-isomaltase pools was generally similar in the hydrocortisone-treated rats, suggesting equivalent stimulation of their synthesis as a group by the humoral agent. The turnover rates of the carbohydrases as a group were found to be similar and did not appear to differ in control and hydrocortisone-treated rats. Total protein synthesis rates were slightly greater in the intestine of the hydrocortisone-treated group of rats.

Jejunal mucosal DNA content and maturation. Inverse relation to serum gastrin levels in suckling and weanling rats

Although pentagastrin has a tropic action on intestinal mucosa in suckling rat pups, and at weaning a rise in gastrin levels coincides with maturation of the intestinal mucosa, direct correlations of serum gastrin levels and intestinal maturation have yet to be made. Ten-day-old rats were subjected either to antrectomy to produce a 43% decrease in serum gastrin levels or to fundectomy to produce a 319% increase over gastrin levels in rumenectomized or normal animals. These changes were not associated with tropic or adaptive changes in jejunal or colonic mucosa as determined by jejunal and colonic DNA content, jejunal sucrase activity, jejunal villous height, or crypt depths in jejunum and colon at the beginning (day 15), middle (day 21), or end (day 27) of the weaning period. To the contrary, an inverse relation was found between serum gastrin levels and both jejunal mucosal DNA content and sucrase activity as an index of maturation.

Effect of orally administered duodenal fluid susceptibility of newborn calves to an Escherichia coli challenge

This study was to determine if duodenal contents inoculated orally could alter symptomatic diarrhea produced by Escherichia coli [09:K(A)] in neonatal calves. Inoculum was obtained 2 h postprandially from duodenal-cannulated milk-fed calves. Thirty Holstein bull calves obtained within 3 h of birth were assigned alternately to a 3 X 2 factorial experiment. Calves received either 1) no E. coli challenge, 2) challenge at 12 h, or 3) challenge at 24 h each with or without 200 ml duodenal fluid orally with the first feeding of colostrum. Rectal temperature, urine output, milk intake, and fecal dry matter were recorded daily, and body weight and hematocrit every 3rd day for 1 wk. Gamma globulin of serum was determined at 2 h of age. Inocula and duodenal tissue and fluid of the last 12 calves were assayed for total anaerobic bacterica, anaerobic and aerobic lactobacilli, coliforms, and total aerobic bacteria. Inoculated calves exhibited lower incidence of diarrhea, greater urine output, lower hematocrit, and superior average daily gain as compared to uninoculated calves. Concentration of gamma globulin in serum of inoculated calves was subnormal. Bacterial populations of tissue and fluid at 7 days were not influenced by treatment.