Inducible gene knockouts in the small intestinal and colonic epithelium

We have developed two systems for performing Cre-mediated recombination of target genes in the rapidly self-renewing mouse small intestinal and coloni c epithelium, When expression of Cre recombinase is placed directly under t he control of transcriptional regulatory elements from a fatty acid-binding protein gene (Fabp), deletion of loxP flanked (floxed) DNA sequences is in itiated as early as embryonic day 13.5, well before completion of intestina l morphogenesis. By embryonic day 16.5, Fabp-Cre also directs recombination in all cell layers of the transitional epithelium that lines the renal cal yces and pelvis, meters, and bladder, Fabp-Cre expression and recombination are maintained in both epithelia throughout adulthood. The second system a llows recombination to be induced only in the gut and at any period during adulthood. This system uses Fabp regulatory elements to direct expression o f a reverse tetracycline-regulated transactivator (rtTA), Another transgene encodes Cre under the control of tet operator sequences and a minimal prom oter from human cytomegalovirus (tetO-P-hCMV-Cre). In the absence of a doxy cycline inducer, no basal recombination is detectable in the gut of adult t ri-transgenic mice containing Fabp-rtTA, tetO-P-hCMV-Cre, plus a floxed rep orter gene. After 4 days of oral administration of doxycycline, recombinati on of the reporter is apparent in the small intestinal, cecal, and colonic epithelium, After doxycycline is withdrawn, the recombined locus persists f or at least 60 days, indicating that recombination has occurred in epitheli al cell progenitors that have long residency times in the proliferative uni ts of the intestine (crypts of Lieberkuhn), This inducible system should ha ve a number of applications for examining gene function at selected times i n postnatal life, under selected physiologic or pathophysiologic conditions .