Developmental expression of the sucrase-isomaltase (SI) gene in the mouse intestine involves two major transitions that correspond to critical developmental events. Low levels of SI mRNA were first identified in day 16.5 fetal mouse intestine, immediately after the transition from stratified endoderm to a columnar epithelium organized in nascent villi. Low levels were maintained until the third week of life, when induction of SI mRNA to adult levels was observed coincident with the time of weaning. The mechanism of this pattern of SI gene expression was studied in transgenic mice using a reporter gene construct containing an SI gene promoter that is evolutionarily conserved between mouse and human (nucleotides -201 to +54 of the mouse SI gene). This promoter included the necessary regulatory information to direct transcription to enterocytes in developmental and differentiation-dependent patterns that recapitulated the expression of the endogenous SI gene. However, transgenes lacked the ability to direct induction of precocious expression in suckling animals after administration of corticosteroids. These findings define a short SI gene promoter that contains cis-acting elements that are responsible for developmental and differentiation-dependent transcriptional regulation.

Sucrase-isomaltase (SI) is an enterocyte-specific gene that is expressed in complex developmental and spatial patterns. In this study, we examine the ability of regulatory elements within the human SI (hSI) gene to direct appropriate cell lineage and spatial patterns of expression in transgenic mice. Transgenic mouse lines were established using a construct containing bases -3424 to +54 of the hSI gene linked to the human growth hormone (hGH) structural gene. In each transgenic line, hGH mRNA and protein were expressed only in the small intestine and colon. In contrast to the endogenous mouse SI (mSI) gene, which was expressed along the entire length of the small intestine, hGH mRNA expression was predominantly found in the distal jejunum and ileum, with very low levels in more proximal portions of the small intestine. However, the pattern of transgene expression along the small intestinal crypt-villus axis was identical to the pattern of the endogenous mSI gene. These results suggest that regulatory elements necessary for intestine-specific transcription and differential expression along the intestinal crypt-villus axis are included in the 5'-flanking region of the hSI gene. Furthermore, these data suggest that different DNA regulatory regions regulate transcription along the horizontal intestinal axis. In the colon, there was aberrant expression of hGH in a subpopulation of enteroendocrine cells that contained peptide tyrosine tyrosine (PYY). This suggests that there are DNA regulatory elements, missing in the transgene construct, which normally suppress expression of the endogenous mSI gene in these cells. Taken together, these findings define the SI gene as a useful model for studies of differentiation, cell lineage determination, and mechanisms of complex spatial gene expression in the intestine.

Sucrase-isomaltase (SI), a gene expressed exclusively in absorptive enterocytes, was used to examine the molecular mechanisms that regulate cell-specific gene expression in the intestinal epithelium. Transgenic mice were made with a construct containing nucleotides -8,500 to +54 of the mouse SI gene linked to a human growth hormone reporter gene. In adult transgenic animals, high-level transgene expression was limited to the small intestine, with low levels of ectopic expression in the colon. In contrast to the endogenous gene that is expressed only in enterocytes, the transgene was expressed in all four cell lineages, including enterocytes, enteroendocrine, goblet, and Paneth cells. To examine this process of lineage-specific expression further we studied Caco-2 and COLO DM cell lines, which model enterocytes and enteroendocrine cells, respectively. Reminiscent of results in transgenic animals, only Caco-2 cells transcribed the endogenous SI gene, whereas both Caco-2 and COLO DM cells supported transcription from chimeric SI reporter gene constructs. Taken together, these data suggest that each intestinal cell lineage has the cellular machinery to transcribe the SI gene. Moreover, these findings imply that transcription is normally repressed in nonenterocytic cells, possibly via a transcriptional silencer residing outside of the region of the SI gene examined in these studies.

This study was directed to determine the extent of variability in structure or expression of intestinal disaccharidase [gamma-glucoamylase (gamma-GA), sucrase-isomaltase (SI), and lactase] between different strains of mice. Reduced levels of sucrase activity (approximately 20 U/g of protein) were observed in three strains of mice belonging to the CBA/Ca lineage. Four other strains of mice analyzed exhibited higher levels of sucrase activity (approximately 50 U/g of protein). Decreased levels of sucrase in CBA/Ca mice were not associated with decreased levels of activity associated with the isomaltase subunit or with decreased levels of SI mRNA expression. High-performance liquid chromatographic gel filtration, heat inactivation, and kinetic analysis indicated that the differences between strains in sucrase activity might be attributed to structural differences in the sucrase subunit of the SI complex, thus rendering it more susceptible to cleavage and inactivation. However, no differences in kinetic properties of the sucrase subunit were observed between strains. Murine gamma-GA was found to account for a greater proportion of maltase activity (approximately 70%) than that observed in other species (i.e., approximately 20%). In addition, CBA/Ca mice were found to be deficient in intestinal maltase activity (approximately 60 U/g) compared with the other strains studied (approximately 300 U/g).