RNA synthesis by villus and crypt cell nuclei of rat intestinal mucosa

In vitro drug absorption models. I. Brush border membrane vesicles, isolated mucosal cells and everted intestinal rings: characterization and salicylate accumulation

Brush border membrane vesicles, isolated mucosal cells and everted rings from rat intestine were compared for their suitability for drug uptake studies. Vesicles from brush border membranes were judged to be metabolically and morphologically functional on the basis of biochemical and microscopic criteria. With the use of a collagenase-vascular-perfusion method, populations of villus, mid villus and crypt cells were separated. An alternative approach that is based on an EDTA-dissociation procedure afforded fractions enriched in villus and crypt cells. Although several enzymatic and metabolic activities of these two cell preparations were comparable, cell viability based on the Trypan Blue dye exclusion test, ultrastructural appearance and glucose uptake more closely conformed to in vivo values for cells isolated according to the EDTA-dissociation method. These cells were chosen as a model for drug transport investigation. The morphological and functional integrity of everted rings was verified by histological examination, extracellular space estimation and assessment of glucose transport ability. Sodium salicylate uptake studies using brush border membrane vesicles and isolated mucosal cells were highly variable, whereas everted segments exhibited good reproducibility in uptake experiments. Time dependence of salicylate uptake was demonstrated with membrane vesicles and everted rings. Time dependence was not observed in mucosal cell uptake studies, probably because of the time required to separate the cells from the incubation solution. Based on ease of preparation, technical aspects of in vitro incubation and reproducibility of results, everted intestinal rings were considered to be a good potential model for in vivo drug absorption. Brush border membrane vesicles were generally regarded as unacceptable because of variations after storage and between experiments. Isolated cells offered certain advantages, but the utility of cells as an in vitro model remains equivocal.

A regulatory element is characterized by purine-mediated and cell-type-specific gene transcription

Purines and purine nucleotides were found to affect transcription of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in whole nuclei isolated from intestinal mucosa of adult rats fed a purine- and purine nucleotide-free diet. Nuclear run-on transcription assays, performed on whole nuclei from different tissues and cell types, identified an intestine-specific decrease in the overall incorporation of [alpha-32P]UTP in HPRT transcripts from intestinal epithelial cell nuclei when exogenous purines or purine nucleotides were omitted from either the diet or culture medium. Using a 990-base-pair genomic fragment that contains the 5'-flanking region from the HPRT gene, we generated plasmid constructs with deletions, transfected the DNA into various cell types, and assayed for chloramphenicol acetyltransferase (CAT) reporter activity in vitro. We determined that an element upstream from the putative transcriptional start site is necessary to maintain the regulatory response to purine and nucleotide levels in cultured intestinal epithelial cells. These results were tissue and cell type specific and suggest that in the absence of exogenous purines, the presence of specific factors influences transcriptional initiation of HPRT. This information provides evidence for a mechanism by which the intestinal epithelium, which has been reported to lack constitutive levels of de novo purine nucleotide biosynthetic activity, could maintain and regulate the salvage of purines and nucleotides necessary for its high rate of cell and protein turnover during fluctuating nutritional and physiological conditions. Furthermore, this information may provide more insight into regulation of the broad class of genes recognized by their lack of TATA and CCAAT box consensus sequences within the region proximal to the promoter.

A regulatory element is characterized by purine-mediated and cell-type-specific gene transcription

Purines and purine nucleotides were found to affect transcription of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in whole nuclei isolated from intestinal mucosa of adult rats fed a purine- and purine nucleotide-free diet. Nuclear run-on transcription assays, performed on whole nuclei from different tissues and cell types, identified an intestine-specific decrease in the overall incorporation of [alpha-32P]UTP in HPRT transcripts from intestinal epithelial cell nuclei when exogenous purines or purine nucleotides were omitted from either the diet or culture medium. Using a 990-base-pair genomic fragment that contains the 5'-flanking region from the HPRT gene, we generated plasmid constructs with deletions, transfected the DNA into various cell types, and assayed for chloramphenicol acetyltransferase (CAT) reporter activity in vitro. We determined that an element upstream from the putative transcriptional start site is necessary to maintain the regulatory response to purine and nucleotide levels in cultured intestinal epithelial cells. These results were tissue and cell type specific and suggest that in the absence of exogenous purines, the presence of specific factors influences transcriptional initiation of HPRT. This information provides evidence for a mechanism by which the intestinal epithelium, which has been reported to lack constitutive levels of de novo purine nucleotide biosynthetic activity, could maintain and regulate the salvage of purines and nucleotides necessary for its high rate of cell and protein turnover during fluctuating nutritional and physiological conditions. Furthermore, this information may provide more insight into regulation of the broad class of genes recognized by their lack of TATA and CCAAT box consensus sequences within the region proximal to the promoter.

Onset of transcription of the aminopeptidase N (leukemia antigen CD 13) gene at the crypt/villus transition zone during rabbit enterocyte differentiation

The sequence of a cDNA clone (2.82 kbp) of rabbit intestinal aminopeptidase N (CD 13) is reported. Using the corresponding anti-sense RNA probe, the distribution of aminopeptidase N mRNA along the crypt/villus axis of the rabbit small intestine was studied by in situ hybridization. The aminopeptidase N gene is expressed along the whole length of the villus with a maximum at its base. Expression was not detected in the crypt cells. The distribution of aminopeptidase N mRNA correlates with the presence of active enzyme as monitored by histochemical staining. The results are compatible with onset of transcription of the aminopeptidase N gene at the crypt/villus transition zone during the enterocyte differentiation.

Stability of enterocytes and certain enzymatic activities in suspensions of cells from the villous tip to the crypt of Lieberkühn of the mouse small intestine

A series of studies in this laboratory have focused on how an indigenous microbiota influences the activities of alkaline phosphatase, phosphodiesterase I, and thymidine kinase in the enterocytes of the upper small intestine of mice. To draw conclusions about the role of the microflora in determining levels of enzymatic activity, we found it necessary to develop a procedure by which cell suspensions could be obtained containing enterocytes isolated sequentially from the villous tip to the crypt of Lieberkühn. The procedure was modified from the one developed for rats by Weiser (J. Biol. Chem. 248:2536-2541, 1973), involved a minimum number of interfering factors (e.g., proteolytic enzymes and mechanical agitation), and worked reproducibly for mice. During development of the procedure, some variables affecting the assays of the enzymes known to be present in enterocytes were also explored. Rods to which were tied everted segments of gut were incubated in a series of tubes containing a solution of EDTA the concentration of which was changed from 1.5 to 5.0 mM, thus giving a greater yield of enterocytes at every step. The cells incubating in the chelating solution were most stable when 0.23 M sucrose was included in the EDTA solutions. Success in assaying enzymatic activities in the cell suspensions depended on (i) how the cells were isolated, (ii) the assay procedure for thymidine kinase, and (iii) whether cellular suspensions or extracts were assayed.