Expression of sucrase-isomaltase mRNA along the villus-crypt axis in the rat small intestine

Sucrase-isomaltase has been used as a marker enzyme to study cell differentiation along the intestinal villus-crypt axis. Previous studies are in agreement that sucrase activity is confined to villus epithelial cells. However, immunoreactivity data are at conflict, with some studies reporting sucrase antigen in crypts as well as villi. To resolve this discrepancy, our goal was to determine the distribution of sucrase-isomaltase mRNA. A cDNA clone representing 3.0 kb of rat sucrase-isomaltase, including the sucrase active site, was characterized. Northern analysis of 12 tissues demonstrated a 6 kb transcript only in the small intestine. Jejunal cell fractions prepared by a washing technique showed declining levels of both sucrase activity and sucrase-isomaltase mRNA as well as increasing levels of thymidine kinase activity from early to later fractions. Since later fractions did not yield pure crypt cells, in situ hybridization using an 35S-labeled sucrase-isomaltase riboprobe was performed. The transition from zero to intense signal at the crypt-villus junction leads us to conclude that in the adult rat, sucrase-isomaltase gene expression is initiated only after cells leave the proliferative cycle and migrate onto the villi.