Tamoxifen-Induced Cre-loxP Recombination Is Prolonged in Pancreatic Islets of Adult Mice

Tamoxifen (Tm)-inducible Cre recombinases are widely used to perform gene inactivation and lineage tracing studies in mice. Although the efficiency of inducible Cre-loxP recombination can be easily evaluated with reporter strains, the precise length of time that Tm induces nuclear translocation of CreER(Tm) and subsequent recombination of a target allele is not well defined, and difficult to assess. To better understand the timeline of Tm activity in vivo, we developed a bioassay in which pancreatic islets with a Tm-inducible reporter (from Pdx1(PB)-CreER(Tm);R26R(lacZ) mice) were transplanted beneath the renal capsule of adult mice previously treated with three doses of 1 mg Tm, 8 mg Tm, or corn oil vehicle. Surprisingly, recombination in islet grafts, as assessed by expression of the beta-galactosidase (beta-gal) reporter, was observed days or weeks after Tm treatment, in a dose-dependent manner. Substantial recombination occurred in islet grafts long after administration of 3x8 mg Tm: in grafts transplanted 48 hours after the last Tm injection, 77.9 +/- 0.4% of beta-cells were beta-gal+; in beta-cells placed after 1 week, 46.2 +/- 5.0% were beta-gal+; after 2 weeks, 26.3 +/- 7.0% were beta-gal+; and after 4 weeks, 1.9 +/- 0.9% were beta-gal+. Islet grafts from mice given 361 mg Tm showed lower, but notable, recombination 48 hours (4.9 +/- 1.7%) and 1 week (4.5 +/- 1.9%) after Tm administration. These results show that Tm doses commonly used to induce Cre-loxP recombination may continue to label significant numbers of cells for weeks after Tm treatment, possibly confounding the interpretation of time-sensitive studies using Tm-dependent models. Therefore, investigators developing experimental approaches using Tm-inducible systems should consider both maximal recombination efficiency and the length of time that Tm-induced Cre-loxP recombination occurs.