Biosynthesis of brush border glycoproteins by human small intestinal mucosa in organ culture

Functional diversity and interactions between the repeat domains of rat intestinal lactase

Lactase-phlorizin hydrolase (LPH), a major digestive enzyme in the small intestine of newborns, is synthesized as a high-molecular-mass precursor comprising four tandemly repeated domains. Proteolytic cleavage of the precursor liberates the pro segment (LPHalpha) corresponding to domains I and II and devoid of known enzymic function. The mature enzyme (LPHbeta) comprises domains III and IV and is anchored in the brush border membrane via a C-terminal hydrophobic segment. To analyse the roles of the different domains of LPHalpha and LPHbeta, and the interactions between them, we have engineered a series of modified derivatives of the rat LPH precursor. These were expressed in cultured cells under the control of a cytomegalovirus promoter. The results show that recombinant LPHbeta harbouring both domains III and IV produces lactase activity. Neither domain III nor IV is alone sufficient to generate active enzyme, although the corresponding proteins are transport-competent. Tandem duplication of domains III or IV did not restore lactase activity, demonstrating the separate roles of both domains within LPHbeta. Further, the development of lactase activity did not require LPHalpha; however, LPHalpha potentiated the production of active LPHbeta but the individual LPHalpha subdomains I and II were unable to do so. Lactase activity and targeting required the C-terminal transmembrane anchor of LPH; this requirement was terminal transmembrane anchor or LPH; this requirement was not satisfied by the signal/anchor region of another digestive enzyme: sucrase-isomaltase. On the basis of this study we suggest that multiple levels of intramolecular interactions occur within the LPH precursor to produce the mature enzyme, and that the repeat domains of the precursor have distinct and specific functions in protein processing, substrate recognition and catalysis. We propose a functional model of LPHbeta in which substrate is channelled from an entry point located within domain II to the active site located in domain IV.

Serum free medium increases expression of markers of differentiation in human colonic crypt cells

In colitis, colonic epithelial cells have a shortened life span but show normal or increased expression of phenotypic markers of differentiation. This study examined the effect of differing culture conditions on the expression of such markers in colonic crypt cells. Crypt cells were enzymatically isolated from macroscopically normal large bowel mucosa resected because of neoplasia, inflammatory bowel disease or non-neoplastic non-inflammatory conditions. Cells cultured in the presence of serum exhibited a doubling of the rate of protein synthesis (measured by 14C-leucine uptake; p < 0.001) compared with autologous cells cultured in the absence of serum without evidence of loss of cell viability (assessed by 51Cr release from prelabelled cells) or of change in the rate of cell proliferation (assessed by total DNA content and 3H-thymidine uptake). Irrespective of the underlying colonic disease, crypt cells cultured in the absence of serum exhibited increased expression of phenotypic markers of differentiation compared with those cultured with serum: the rate of glycoprotein synthesis relative to that of protein synthesis increased by a mean of 59% and the cellular expression of brush border glycoproteins, alkaline phosphatase, and carcinoembryonic antigen significantly increased. The effects seen could not be mimicked by addition of dexamethasone or insulin to serum free medium. Thus, under less optimal (serum free) culture conditions, colonic crypt cells express phenotypic markers of differentiation at an accelerated rate suggesting that unfavourable microenvironmental conditions themselves are probably in part responsible for the normal or increased expression of such markers in colitis.

Contrasting effects of butyrate on the expression of phenotypic markers of differentiation in neoplastic and non-neoplastic colonic epithelial cells in vitro

The in vitro effect of butyrate on expression of differentiation markers in colonic epithelial cells was assessed in the colon cancer cell line, LIM1215 and in epithelial cells isolated from a surgically resected histologically normal colon. Markers used to assess cell differentiation were: net glycoprotein synthesis ([3H]-glucosamine uptake) expressed relative to net protein synthesis ([14C]-leucine uptake), and the expression of the brush border glycoproteins (alkaline phosphatase and carcino-embryonic antigen) in cell homogenates calculated relative to cellular protein content. In response to 24 h exposure to 1 mmol/L butyrate, all markers significantly increased in LIM1215 cells whereas they all significantly decreased in isolated colonic epithelial cells under identical culture conditions. Similar effects were seen at butyrate concentrations of up to 4 mmol/L. Butyrate suppressed proliferation of LIM1215 cells but had no consistent effect on [3H]-thymidine uptake by, or DNA content of, normal epithelial cells. Additional experiments found no evidence of a toxic effect of butyrate at those concentrations nor of an alteration of cell responsiveness to butyrate due to the isolation process itself. In contrast to its differentiative effect on neoplastic cells, butyrate reduces the expression of phenotypic markers of differentiation in vitro in colonic epithelial cells from non-neoplastic mucosa.

Biosynthesis of N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase: disulfide-linked dimers are formed at the site of synthesis in the rough endoplasmic reticulum

N-Benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase (PPH) is a metalloendopeptidase found in mucosal epithelial cells of the human small intestine. The purification and characterization of this enzyme was described in the preceding paper (E. E. Sterchi et al. (1988) Arch. Biochem. Biophys. 265, 105-118). In this paper, we report on the biosynthesis and posttranslational processing of PPH in organ cultures of human small intestinal mucosa. Continuous labeling for 6 h with L-[35S]methionine, immunoprecipitation with monoclonal antibody 3/716/36, and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a polypeptide with Mr 100,000. This molecule was highly glycosylated as treatment with endo-beta-N-acetylglucosaminidase F resulted in a reduction to Mr 70,000. This was also the size of the species isolated after culture in the presence of tunicamycin, an inhibitor of N-linked glycosylation. Pulse-chase labeling showed that the first detectable form of PPH had a Mr 90,000 which corresponded to the high-mannose precursor as assessed by its sensitivity to endo-beta-N-acetylglucosaminidase H. Within 15 min of chase and prior to complex glycosylation, dimerization due to the formation of interchain disulfide bonds occurred (Mr 180,000). Dimerization thus took place within the rough endoplasmic reticulum and might play an important role in the transport through to the cell surface. After 2 h of chase, PPH started to appear in the culture medium, indicating that the enzyme was secreted from the cells, a finding not observed with other microvillus membrane hydrolases.

Intestinal tissue and cell cultures

The culture of animal cells and tissues is a widely used technique in the field of cellular and molecular biology; one of the most interesting aspect being linked to the study of the mechanisms of cell differentiation. In the specific case of intestinal epithelial cells, various tissue culture technologies have proved to be important tools for the study of precise facets related to intestinal function, pathology and differentiation. Concerning this latter aspect, organ culture experiments have brought about interesting data on the hormonal or nutritional control of intestinal maturation. Nevertheless, the study of the precise mechanisms underlying epithelial proliferation and/or differentiation at the cellular level needs more adequate cell culture model systems. One of them has been described for two cell lines derived from human colonic adenocarcinomas, in which the cells can be induced to achieve enterocytic-like differentiation. Up to date, none of the continuous cell lines starting from normal undifferentiated cells have allowed generation of morphological or functional enterocytic polarity. In contrast, primary cell cultures which allow maintenance of a more physiological environment for the epithelial cells like contacts with their in vivo counterparts, mesenchymal cells or extracellular matrix molecules, have proved to be promising approaches.

Can villin be used to identify malignant and undifferentiated normal digestive epithelial cells?

We have investigated the presence of villin (a Ca2+-regulated actin binding protein) in various tissues (normal or malignant) and in established cell lines by using sensitive immunochemical techniques on cell extracts and immunofluorescence analysis on frozen sections. Our results show that villin is a marker that can be used to distinguish normal differentiated epithelial cells from the simple epithelia lining the gastrointestinal tract and renal tubules. Villin is found in the absorptive cells of the small and large intestines, in the duct cells of pancreas and biliary system, and in the cells of kidney proximal tubules. Furthermore, undifferentiated normal and tumoral cells of intestinal origin in vivo and in cell culture express villin. Therefore, expression of villin is seen in cells that do not necessarily display the morphological features characteristic of their terminally differentiated state, such as the microvilli-lined brush border. We suggest the possible clinical implications of using villin as a marker in the diagnosis of metastatic adenocarcinomas.

Early organogenesis of human small intestine: scanning electron microscopy and brush border enzymology

Human small bowel early organogenesis was studied by scanning electron microscopy and found to be correlated to brush border enzymology. The appearance of the brush border enzymes sucrase, lactase, and aminopeptidase (measured in a purified apical membrane fraction) coincides with the first outgrowth of villi (eight weeks). Alkaline phosphatase was detected at seven weeks. The content of these enzymes furthermore increased up to the 14th week when both sucrase and aminopeptidase activities were comparable with adult values.

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: III. Inhibition of intracellular migration of membrane glycoproteins in rat intestinal columnar cells and hepatocytes as visualized by light and electron-microscope radioautography after 3H-fucose injection

In the first paper of this series (Bennett et al., 1984), light-microscope radioautographic studies showed that colchicine or vinblastine inhibited intracellular migration of glycoproteins out of the Golgi region in a variety of cell types. In the present work, the effects of these drugs on migration of membrane glycoproteins have been examined at the ultrastructural level in duodenal villous columnar cells and hepatocytes. Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for radioautography. In duodenal villous columnar cells, 3H-fucose labeling of the apical plasma membrane was reduced by 51% after colchicine and by 67% after vinblastine treatment; but there was little change in labeling of the lateral plasma membrane. Labeling of the Golgi apparatus increased. This suggests that labeled glycoproteins destined for the apical plasma membrane were inhibited from leaving the Golgi region, while migration to the lateral plasma membrane was not impaired. In hepatocytes, labeling of the sinusoidal plasma membrane was reduced by 83% after colchicine and by 85% after vinblastine treatment. Labeling of the lateral plasma membrane also decreased, although not so dramatically. Labeling of the Golgi apparatus and neighboring secretory vesicles increased. This indicates that the drugs inhibited migration of membrane glycoproteins from the Golgi region to the various portions of the plasma membrane. Accumulation of secretory vesicles at the sinusoidal front suggests that exocytosis may also have been partially inhibited. In both cell types, microtubules almost completely disappeared after drug treatment. Microtubules may, therefore, be necessary for intracellular transport of membrane glycoproteins, although the possibility of a direct action of these drugs on Golgi or plasma membranes must also be considered.

Protein synthesis and secretion by hamster, rat and mouse jejunum

The synthesis and secretion of [35S]methionine-labelled proteins by hamster, rat and mouse jejunum has been measured in vitro after 60 min incubation in culture medium. Twenty-three major bands of radioactively labelled proteins were detected by PAGE analysis of enterocyte extracts. Three of these proteins having mol. wts of 28,000, 43,000 and 60,000 appeared to be synthesised preferentially by older enterocytes. A small number of radioactively labelled proteins also appeared in culture medium at the end of incubation. Three of these proteins, accounting for most of the recovered radioactivity, had mol. wts of 28,000, 43,000 and 60,000. Secretion of these proteins was inhibited by monensin. Further experiments showed these secreted proteins to be acidic and possibly glycoprotein in nature. Their rapid turnover, selective secretion and changing abundance in enterocytes of different ages all suggest that they may have important functions to perform in the intestine.

Sucrase-isomaltase: a marker of foetal and malignant epithelial cells of the human colon

The presence of sucrase-isomaltase (SI), a glycoprotein hydrolase normally restricted to the brush border membrane of the enterocytes of the small intestine, was investigated in tumours which developed in nude mice inoculated with six human colon carcinoma cell lines (HT-29, Caco-2, HRT-18, HCT-8R, SW-480, and CO-115). Foetal and normal adult human small intestines and colons were used as controls. SI was studied by (1) immunofluorescence with rabbit antibodies raised against purified human small intestine SI; (2) polyacrylamide gel electrophoresis and immunoblotting; and (3) determination of the enzyme activity. SI was antigenically present, and enzymatically active, in all the tumours derived from Caco-2 and HT-29 cells. The presence of the enzyme was associated with that of typical brush borders at transmission electron microscopy examination. SI was absent from the tumours developed with the other four cell lines, as well as from the normal adult colon mucosa. SI was also present and active in the colons of mid-gestation foetuses, ranging in ages between 20 and 28 weeks; it was absent from the colons of late-gestation foetuses. The presence of SI in tumours derived from two cell lines suggests that this enzyme is a marker, so far unsuspected, of certain human colon cancers, and that the differentiation pattern of these particular cancers closely resembles that of the foetal colon.

Brush border and lysosomal marker enzyme profiles in duodenal mucosa from coeliac patients before and after organ culture

Five brush border and 2 lysosomal enzymes were measured in duodenal tissue explants from 21 children and young adults (16 coeliac and 5 non-coeliac) before and after organ culture. Reduced activity of brush border enzymes and increased activity of lysosomal enzymes were recorded in flat mucosas from coeliac patients compared with remission coeliac explants and biopsy specimens from non-coeliac controls. Slightly increased activity of alkaline phosphate and sucrase was recorded during culture (24 h) of coeliac explants. Coeliac specimens in the exacerbation state showed increased activity of acid phosphatase after culture in the presence of gluten, whereas gluten did not provoke detectable alterations in brush border enzyme activities during culture unless the wet weight of material was 1.5 mg or more. In such explants lower activity of brush border enzymes was measured after in vitro gluten exposure than after culture on gluten-free medium. Mucus removed from the specimen surface after culture contained considerable amounts of brush border enzymes and reflected the variations in the tissue homogenates. Culture media contained smaller quantities of enzymes.

Biosynthesis of intestinal microvillar proteins. Characterization of intestinal explants in organ culture and evidence for the existence of pro-forms of the microvillar enzymes

Explants of pig small intestine were maintained at 37 degrees C in organ culture for periods up to 24 h in a system using Trowell T-8 medium supplemented with 10% foetal-calf serum. The mucosal morphology was well preserved during culture, as judged by light and electron microscopy. The explant contents of protein and two brush-border enzymes, microvillus aminopeptidase (EC 3.4.11.2) and dipeptidyl peptidase IV (EC 3.4.14.5), were not significantly modified during culture compared with controls, but a moderate, continuous release of both protein and enzyme activities into the medium was observed. Continuous labelling with [35S]methionine resulted in an even incorporation of radioactivity in the protein components, and the rate of labelling only moderately decreased over the 24 h period. The polypeptide compositions of sucrase (EC 3.2.1.48)--isomaltase (EC 3.2.1.10), maltase--glucoamylase (EC 3.2.1.20) lactase (EC 3.2.1.23)--phlorizin hydrolase (EC 3.2.1.62), microvillus aminopeptidase and aspartate aminopeptidase (EC 3.4.11.7) synthesized during culture were studied, and some were found to be similar to those of the pro-forms of the enzymes isolated from animals that had had their pancreatic duct disconnected 3 days before being killed. These results confirmed earlier findings of the existence of pro-forms of some of the microvillar enzymes and thus indicate a low activity of pancreatic proteinases in the culture system.

Thyroxine-stimulated synthesis of microvillus membrane glycoproteins during culture of chick embryonic duodenum

The effect of thyroxine on biosynthesis of microvillus membrane glycoproteins has been investigated in organ culture of 18-day-old chick embryonic duodenum. Explants incorporate [3H]leucine and [3H]glucosamine continuously, and overall incorporation is enhanced by 10 nM thyroxine during 48 h of labeling; this increase in radioactivity is associated with vesicles released from the microvilli. Light microscope autoradiography, pulse labeling of brush border fragments, and pulse chase experiments reveal that [3H]glucosamine is incorporated into brush border at an increasing rate during culture, and that newly synthesized glycoproteins are discharged into the medium along with brush border enzymes (alkaline phosphatase and maltase). These results suggest that thyroxine stimulates biosynthesis of microvillus membrane glycoproteins, in addition to stimulating vesiculation of the membrane. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 3H-labeled vesicles and brush border fragments show that [3H]leucine and [3H]glucosamine are incorporated into proteins of high molecular weight. Two protein bands are identified as alkaline phosphatase and maltase. Thyroxine stimulates glycosylation of these enzymes, but does not change protein patterns. Radioactivity assay of alkaline phosphatase- and maltase-active gel slices suggests that thyroxine stimulation of these enzyme activities during culture is not correlated with de novo synthesis of these proteins.

Endocytosis in absorptive cells of cultured human small-intestinal tissue: effect of cytochalasin B and D

The effect of cytochalasin B (CB) and cytochalasin D (CD) on the endocytotic uptake of horseradish peroxidase (HRP) by intestinal absorptive cells was investigated by morphometric methods. The results showed that CD inhibited endocytosis considerably, and without any detrimental side-effects. CB had hardly any effect on the endocytosis of HRP, but caused a significant decrease in the number of apical vesicles and tubules involved in the transport of cell-coat glycoproteins from the Golgi apparatus to the brush border. Electron-microscopic autoradiographic analysis of the effect of CD showed that although endocytosis is inhibited significantly by the drug, the amount of radiolabelled cell-coat material entering the lysosome-like bodies was unaltered compared with control cultures. These observations support our hypothesis that the cell-coat glycoproteins of the absorptive cells enter the lysosome-like bodies by a crinophagic rather than by an exocytotic-endocytotic mechanism.

Studies on the incorporation of radioactive amino acids into tissue and brush border membrane proteins of mouse jejunum in organ culture

1. The incorporation of radioactive amino acids into brush border membrane proteins of mouse jejunal mucosa has been investigated in organ culture and compared to similar data obtained in intestinal culture of other species. 2. Amino acid precurosor pools for protein synthesis remained almost unchanged during 24 hr culture. 3. By pulse experiments, it has been possible to show a constant rate of protein synthesis up to 24 hr culture. 4. Continuous labelling studies have shown a non-linear incorporation of amino acids into tissue proteins, brush border membrane proteins and total proteins (explants + media) synthesized during the culture. A turnover of tissue proteins has been assumed from the release of labelled proteins into the culture media. The analysis of radioelectrophoregrams of brush border membrane proteins and proteins from the particulate fraction of media showed that the same brush border membrane proteins were synthesized in the tissue and released in the media all along the culture. 6. However, some quantitative differences have been found in the relative labelling between protein bands, suggesting that some proteins would be synthesized or released at different rates depending upon the duration of the culture.

Turnover of mouse intestinal brush border membrane proteins and enzymes in organ culture. A direct evaluation from studies on the evolution of enzyme activities during the culture

The turnover of mouse intestinal brush border membrane enzymes has been studied by kinetic analysis of the evolution of enzyme activities during organ culture. By comparing the results obtained in these studies with the predictions from a mathematical model of enzyme synthesis and degradation in organ cultures, it has been possible to reach the following conclusions: (1) There is no degradation of brush border membrane enzymes during culture and the rate of synthesis of each enzyme is directly measurable from the kinetics of total enzyme accumulation (tissue + media). (2) Brush border membrane enzymes are released in culture media by two complementary processes. The first one involves a differential solubilization of enzymes but its exact nature cannot be exactly stated. The second one involves a microvesiculation of brush border membranes, the importance of which in vivo is seen in the possible conciliation between urinary membrane synthesis and heterogeneous turnover of membrane components.

The effect of chloroquine on the intralysosomal degradation of cell-coat glycoproteins in the absorptive cells of cultured human small-intestinal tissue as shown by silver proteinate staining

The effect of chloroquine on the intralysosomal degradation of cell-coat glycoproteins in cultured intestinal absorptive cell was investigated by silver proteinate staining. The results of this staining method, which is specific for carbohydrate containing macromolecules such as glycoproteins and mucopolysaccharides, showed that in the presence of the drug considerable amounts of silver proteinate-positive material accumulated in one type of lysosome-like body: the dense bodies. The staining pattern of other cell organelles was not affected by chloroquine. The presence of the drug in the culture medium also resulted in the occurrence of numerous small vesicular structures in the matrix of the dense bodies. These showed a similar size and structure to those present in the other type of lysosome-like body: the multivesicular bodies. This observation, together with earlier autoradiographical data, suggests that cell-coat material is transferred from multivesicular to dense bodies by fusion between these organelles. This study thus provides further evidence for a regulatory mechanism of cell-coat glycoprotein transport by the lysosome-like bodies in human intestinal absorptive cells.

DNA synthesis in human duodenal biopsies maintained in organ culture

Incorporation of 3H-thymidine during organ culture was studied in duodenal biopsies from 14 patients. Pulse-label at various intervals disclosed active incorporations during the first 2 h in culture. Labelling index declined to low levels at 3-4 h. Thereafter incorporation increased again and persisted throughout the rest of the culture period of 11 h. The DNA synthesis rate of crypt cells between 4 and 11 h in culture was calculated in 5 patients after pulse-label and continuous labelling of explants in parallel culture. The rate of entry into DNA synthesis was about 24 cells per 1,000 crypt cells per hour in flat, coeliac biopsies, versus 9-13 in controls, Gluten did not influence DNA synthesis rate, whereas wheat germ lectin inhibited DNA synthesis. Counting of the total number of mitoses and labelled fraction of mitoses disclosed active crypt cell renewal in flat, coeliac biopsies. In normal-appearing biopsies no mitoses were labelled, indicating delayed exit from S-phase or long duration of G2-phase in these explants.

The effect of chloroquine on lysosomal function and cell-coat glycoprotein transport in the absorptive cells of cultured human small-intestinal tissue

The effect of chloroquine, an inhibitor of intralysosomal catabolism, on the synthesis, transport, and degradation of cell-coat glycoproteins in absorptive cells of cultured human small-intestine tissue was investigated by morphometrical, autoradiographical, and biochemical methods. Neither synthesis nor transport of cell-coat material was affected by the drug, but culturing of the absorptive cells in the presence of chloroquine led to a dose- and time-dependent enlargement of the dense bodies; other cell structures showed no alterations. 3H-fucose-labelled material accumulated in the dense bodies of the absorptive cells of these cultures. Since no increase of beta-glucuronidase and acid phosphatase activity (both lysosomal enzymes of glycoprotein nature) was found, this accumulation of radiolabeled material can be explained as a chloroquine-mediated inhibition of the degradation of cell-coat glycoproteins. These macromolecules probably enter the lysosome-like bodies by a crino-phagic mechanism, i.e., fusion of these organelles with the apical vesicles and tubules involved in intracellular transport. These findings suggest that the lysosome-like bodies have a function in the regulating of cell-coat glycoprotein transport in human intestinal absorptive cell, i.e., the degradation of excess cell-coat material.

Characterization of brush borders purified in iso-osmotic medium and microvillar membranes subfractionated from mouse small intestine

Brush borders free of nuclei were isolated by repeated homogenization and centrifugation in iso-osmotic medium. They showed typical morphology under electron microscopy. The mean recovery and enrichment of alkaline phosphatase activity in the brush-border fraction were 50% and 17.5-fold respectively. gamma-Glutamyl transpeptidase showed a close parallelism with alkaline phosphatase and sucrase in subcellular distribution. Microvillar membranes were purified from isolated brush borders; they showed a further enrichment for alkaline phosphatase and were composed of homogeneous vesicles. Both brush-border and microvillar-membrane preparations were analysed for contamination by basolateral and endoplasmic-reticular membranes. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of the microvillar-membrane preparation in six different systems revealed approx. 40 components in the mol.wt. range 15 000-232 000. They were grouped into seven major classes on the basis of molecular weight and electrophoretic patterns.

Studies on the determination of extracellular galactosyltransferase in human intestinal tissue

The determination of extracellular galactosyl transferase (EC 2.4.1.38) activity in human intestinal tissue by assessment of the incorporation of label after incubation with UDP[3H]galactose was evaluated. Intestinal biopsy specimens were incubated with membrane-permeable L-[1-14C]fucose and non-permeable UDP-D-[6-3H]galactose (UDP[3H]Gal). Comparison of the amounts of 3H- and 14C-label incorporated into subcellular fractions showed uptake and incorporation of galactose formed by the hydrolysis of UDP[3H]Gal by brush-border enzymes. The results indicate that incorporation of galactose after incubation of the tissue with UDP[3H]Gal is not exclusively attributable to extracellular galactosyl transferase.

Labelling indices after 3H-thymidine incorporation during organ culture of duodenal mucosa in coeliac disease

Incorporation of 3H-thymidine during organ culture of duodenal biopsy specimens from 34 coeliac and 10 non-coeliac patients was studied by autoradiography. High labelling indices were found in flat, coeliac mucosas. Gluten fractions, which provoked histological deterioration during culture, induced labelling of a greater proportion of crypt cells and higher migration rate than parallelly cultured specimens on gluten-free medium. No influences on crypt cell kinetics could be observed after culture with gluten fractions incapable of producing histological damage or with alpha-lactalbumin. In coeliac remission mucosas, labelling indices were at the same level as in non-coeliac biopsies, and no significant effects of gluten were observed. Autoradiography seems to be a fairly sensitive and reliable determinant of gluten toxicity by organ culture in coeliac disease and should supplement the histological appraisal of the biopsies. The increment of labelling indices provoked by gluten exposure seemed not merely to be a consequence of increased desquamation of cells from the biopsy surface but could imply a direct influence of gluten on crypt cell kinetics in coeliac disease.

Morphological and morphometric assessment of human duodenal biopsies maintained in organ culture. In vitro influences of gluten in coeliac disease

Duodenal biopsy specimens from 51 coeliac patients (children and young adults) and 13 non-coeliac controls were maintained in organ culture for 24 h. Morphometric determinants were compared after culture in the presence of different gluten fractions, after culture on gluten-free medium, and in non-cultured specimens. Biopsies from patients with untreated coeliac disease were susceptible to gluten fractions, which provoked disorganization of crypt architecture, reduced height and irregularities of enterocytes and crypt cells, together with detrition of surface epithelial cells and even tissue necrosis. In latent coeliac disease (silent relapse) biopsies were more resistant to gluten, and higher concentrations were required to induce signs of morphological deterioration. Remission mucosa and biopsies from non-coeliac controls showed no signs of impairment after gluten exposure in vitro.

Characterization of glycopeptides labelled from D-[2-3H]mannose and L-[6-3H]fucose in intestinal epithelial cell membranes during differentiation

The labelled glycopeptides obtained by Pronase digestion of rat intestinal epithelial cell membranes were examined by gel filtration after injection of D-[2-3H]mannose and L-[6-3H]fucose. Three labelled fraction were eluted in the following order from Bio-Gel P-6, Fraction I, which was excluded from the gel, was labelled mostly with [3H]fucose and slightly with [3H]mannose. Fraction II contained "complex" asparagine-linked oligosaccharides since it was labelled with [3H]mannose and [3H]fucose, was stable to mild alkali treatment, and resistant to endo-beta-N-acetyl-glucosaminidase H. Fraction III contained "high-mannose" asparagine-linked oligosaccharides, which were labelled with [3H]mannose, but not with [3H]fucose; these were sensitive to endo-beta-N-acetylglucosaminidase H, and were adsorbed on concanavalin A-Sepharose and subsequently eluted with methyl alpha-D-mannopyranoside. The time course of incorporation of [3H]mannose into these glycopeptides in microsomal fractions showed that high-mannose oligosaccharides were precursors of complex oligosaccharides. The rate of this processing was faster in rapidly dividing crypt cells than in differentiated villus cells. The ratio of radioactively labelled complex oligosaccharides to high-mannose oligosaccharides, 3h after [3H]mannose injection, was greater in crypt than in villus-cell lateral membranes. Luminal membranes of both crypt and villus cells were greatly enriched in labelled complex oligosaccharides compared with the labelling in lateral-basal membranes. These studies show that intestinal epithelial cells are polarized with respect to the structure of the asparagine-linked oligosaccharides on their membrane glycoproteins. During differentiation of these cells quantitative differences in labelled membrane glycopeptides, But no major qualitative change, were observed.

Surface-membrane biogenesis in rat intestinal epithelial cells at different stages of maturation

The biosynthesis of membrane proteins and glycoproteins has been studied in rat intestinal crypt and villus cells by measuring the incorporation of L-[5,6-3H] fucose, D-[2-3H] mannose and L-[3,4,5-3H] leucine, given intraperitoneally, into Golgi, lateral-basal and luminal membranes. Incorporation of leucine and mannose was approximately equal in crypt and villus cells, whereas fucose incorporation was markedly higher (3-4 times) in the differentiated villus cells. As previously reported [Quaroni, Kirsch & Weiser (1979) Biochem J. 182. 203-212] most of the fucosylated glyco-proteins synthesized in the villus cells and initially present in the Golgi and lateral-basal membranes were found re-distributed, within 3-4h of label administration, in the luminal membrane. A similar process appeared to occur in the crypt cells, where, however, only few fucose-labelled glycoproteins were identified. In contrast, most of the leucine-labelled and many mannose-labelled membrane components found in the lateral-basal membrane of both crypt and villus cells did not seen to undergo a similar re-distribution process. The fucosylated glycoproteins of the intestinal epithelial cells represent, therefore, a special class of membrane components, most of which appear with differentiation, that are selectively localized in the luminal portion of the plasmalemma. In contrast with the marked differences in protein and glycoprotein patterns between the luminal membrane of villus and crypt cells, only minor differences were found between their lateral-basal membrane components: their protein patterns on sodium dodecyl sulphate/polyacrylamide slab gels, and the patterns of fucose-, mannose- and leucine-labelled components (analysed 3-4h after label administration) were very similar. Although the minor differences detected may be of importance, it appears that most of the surface-membrane changes accompanying cell differentiation in the intestinal epithelial cells are localized in the luminal portion of their surface membrane.

Isolation and characterization of the proximal and distal forms of lactase-phlorizin hydrolase from the small intestine of the suckling rat

The complex between lactase (beta-D-galactoside galactohydrolase, EC 3.2.1.23) and phlorizin hydrolase (glycosyl-N-acylsphingosine glycohydrolase, EC 3.2.1.62) has been purified from the proximal and distal regions of the small intestine of suckling rats. The two enzymes behaved differently on DEAE-cellulose ion-exchange chromatography and during electrophoresis in the presence and absence of sodium dodecyl sulphate (SDS), but they have very similar cyanoge bromide cleavage patterns. Kinetic studies on the proximal and distal enzymes showed the same pH optimum of 6.0 and the same heat stability at 45 degrees C, but a small difference in Km. Treatment of both enzymes with fucosidase, mannosidase or N-acetylhexosaminidase did not affect enzymic activity or electrophoretic mobility. Neuraminidase digestion abolished the electrophoretic differences and gave two active enzymes with similar isoelectric points.

Effect of triton X-100 on the electrophoretic mobility of solubilized intestinal brush border membrane dipeptidyl peptidase IV

Dipeptidyl peptidase IV (dipeptidylpeptide hydrolase, EC 3.4.14.-) was solubilized from a rat intestinal mucosal brush border membrane preparation in varying concentrations of Triton X-100. Samples of a solubilized supernatant fraction were subjected to polyacrylamide gel electrophoresis, and the activities of several brush border enzymes were measured in gel slices following elution of the enzymes from the gel. At low concentrations of Triton X-100 (0.5%), two peaks of dipeptidyl peptidase IV activity were observed suggesting the presence of two electrophoretically distinct enzymes. However with increasing Triton X-100 concentrations, the slower migrating species was converted to a faster migrating form and only one major band of activity was observed at 10% Triton. These results indicate that there is only one form of the enzyme and that care must be taken when interpreting the electrophoretic patterns of detergent solubilized membrane bound enzymes.

Effect of sucrose refeeding on disaccharidase and aminopeptidase activities of intestinal villus and crypt cells in adult rats. Evidence for a sucrose-dependent induction of sucrase in the crypt cells

In order to gain more insight into the adaptative mechanism of intestinal enzymes to dietary factors in rats, modifications in the activities of disaccharidases and aminopeptidase were measured after refeeding of a 70% solution of sucrose for 15 h following a 2-day fast. Mature epithelial cells from the villus and immature cells from the crypt were isolated after sequential removal of the cells along the villus-crypt axis. Synthesis of brush border disaccharidases was determined by measuring [3H]valine incorporation into proteins. 1. In the whole mucosa, a highly significant increase in sucrase and maltase activities and a significant drop in aminopeptidase activity was observed in the brush border membranes after sucrose refeeding. 2. Stimulation of sucrase and maltase activities in sucrose refed rats was produced mainly in the immature cells of the crypt and lower villus compartment. 3. After separation of the brush border proteins by SDS gel electrophoresis from villus and crypt cells of sucrose refed rats, major incorporation of the radioactive precursor occured in the protein bands corresponding to sucrase and maltase activities of the lower villus and crypt cell brush borders. These findings demonstrate that sucrase stimulation by sucrose occurs mainly in the immature epithelial cells and that the substrate induces de novo synthesis of sucrase molecules.

Transport of radiolabelled glycoprotein to cell surface and lysosome-like bodies of absorptive cells in clutured small-intestinal tissue from normal subjects and patients with a lysosomal storage disease

The transport of 3H-fucose- and 3H-glucosamine-labelled glycoproteins in the absorptive cells of cultured human small-intestinal tissue was investigated with light- and electron-microscopical autoradiography. The findings showed that these glycoproteins were completed in the Golgi apparatus and transported in small vesicular structures to the apical cytoplasm of these cells. Since this material arrived in the cell coat on the microvilli and in the lysosome-like bodies simultaneously, a crinophagic function of these organelles in the regulation of the transport or secretion of cell-coat material was supported. In the absorptive cells of patients with fucosidosis or Hunter's type of lysosomal storage disease, a smiliar transport of cell-coat material to the lysosome-like bodies and a congenital defect of a lysosomal hydrolase normally involved in the degradation of cell-coat material, can explain the accumulation of this material in the dense bodies.

Organ culture of adult mouse intestine. III. Behavior of the proteins, DNA content and brush border membrane enzymatic activities

The proteins, the DNA content and the brush border membrane enzyme activities of adult mouse intestinal explants have been measured during a 24-hr organ culture. These activities were not modified in comparison with the controls at the beginning of the culture. The activity of the enzymes secreted in the medium during the 24-hr culture was equal to the activity present in the explants at the start of the culture. These results show that several metabolic functions of the intestine are fully preserved in organ culture.

Re-evaluation of the techique of organ culture for studying gluten toxicity in coeliac disease

In vitro cytotoxicity of four different gluten fractions was tested in organ culture for up to 48 hours using flat intestinal biopsies from children with coeliac disease. The fractions were (1) a peptic-tryptic digest of gliadin containing a moderate amount of alpha-gliadin, (2) a peptic-tryptic digest of gluten (Frazer fraction III) froma strain of wheat with a high content of alpha-gliadin, (3) alpha-gliadin, and (4) alpha-GT-18,000, a tryptic fragment of alpha-gliadin. The latter three fractions were toxic to coeliac patients in vivo. In vitro, however, none of these fractions proved to be cytotoxic. When added to the culture medium they were not capable of inhibiting the regeneration of the surface epithelium as visualised by histology and electron microscopy. The only difference between cultures with and without gluten fractions was that the former produced slightly more mucus when maintained in vitro as observed in the dissecting microscope. Furthermore, for Frazer fraction III the absence of apparent toxicity was confirmed by the behaviour of brush border enzyme activities during culture. Our results are not in accordance with those reported in the literature. We believe that the criteria used at the present time for the assessment of gluten toxicity in vitro should be extended to include the process of enterocyte desquamation.

Organ culture of adult mouse intestine IV. Stimulation of glucose-6-phosphatase in vitro

Explants of adult mouse jejunum have been maintained in organ culture with or without fructose added to the medium in order to stimulate the intestinal glucose-6-phosphatase (G-6-Pase). When the fructose is added, at the beginning of the culture, a three-fold increase of G-6-Pase in measured during the first 24 h. If the fructose is added after 24 h of culture, no significant increase of the G-6-Pase is registered in comparison with the controls. Proteins, DNA content and dissacharidase activities are not modified during the culture. Alkaline phosphatase activity presents a twofold increase in the controls and stimulated explants. The ultrastructural localization of the G-6-Pase is not altered during the culture.

The identification of rat intestinal membrane enzymes after electrophoresis on polyacrylamide gels containing sodium dodecyl sulphate

Brush-border membranes were isolated from the rat small intestine and then treated with sodium dodecyl sulphate under non-reducing conditions at room temperature. Analysis of the solubilized components by polyacrylamide-gel electrophoresis identified three major glycoproteins that co-migrate with glucoamylase-maltase-sucrase, lactase and isomaltase-maltase-sucrase activities. High activities of alkaline phosphatase and trehalase were detectable, but they could not be attributed to distinct co-migrating protein bands. Analysis of mucosa from the distal small intestine by the same methods showed a pattern of bands different from that obtained with the proximal intestine, which appeared to correlate with the relative deficiency of some of the enzymes in the distal region.