Inducible and tissue-specific cell labeling in Cre-ER^T2 transgenic Xenopus lines

Investigating cell lineage requires genetic tools that label cells in a temporal and tissue-specific manner. The bacteriophage-derived Cre-ER^T2 /loxP system has been developed as a genetic tool for lineage tracing in many organisms. We recently reported a stable transgenic Xenopus line with a Cre-ER^T2 /loxP system driven by the mouse Prrx1 (mPrrx1) enhancer to trace limb fibroblasts during the regeneration process (Prrx1:CreER line). Here we describe the detailed technological development and characterization of such line. Transgenic lines carrying a CAG promoter-driven Cre-ER^T2 /loxP system showed conditional labeling of muscle, epidermal, and interstitial cells in both the tadpole tail and the froglet leg upon 4-hydroxytamoxifen (4OHT) treatment. We further improved the labeling efficiency in the Prrx1:CreER lines from 12.0% to 32.9% using the optimized 4OHT treatment regime. Careful histological examination showed that Prrx1:CreER lines also sparsely labeled cells in the brain, spinal cord, head dermis, and fibroblasts in the tail. This work provides the first demonstration of conditional, tissue-specific cell labeling with the Cre-ER^T2 /loxP system in stable transgenic Xenopus lines.