Marking transgenic Xenopus froglets with passive micro transponders

Forward genetic screens in Xenopus using transposon-mediated insertional mutagenesis

The class II DNA "cut-and-paste" transposons have been used to efficiently modify the Xenopus genome for transgenesis applications. Once integrated, the transposon is an effective substrate for excision and re-integration (remobilization) elsewhere in the genome by simply supplying the transposase enzyme in trans. We have used two methods to remobilize transposons resident in the frog genome: micro-injection of transposase mRNA at the one-cell stage and expression of the enzyme in the germline from a transgene. Double-transgenic frogs (hoppers) that harbor transgenes for both the substrate transposon and the transposase enzyme are outcrossed to wild-type animals and the progeny are scored for changes in reporter gene expression. Although both methods work effectively to remobilize transposons, the breeding-mediated strategy eliminates the time-consuming micro-injection step; novel integration events are produced by simply outcrossing the hopper frogs. As each outcross of Xenopus tropicalis typically produces 2,000, or more, progeny, this method can be used to perform large-scale insertional mutagenesis screens in this highly tractable developmental model system.

Transposon transgenesis in Xenopus

Transposon-mediated integration strategies in Xenopus offer simple and robust methods for the generation of germline transgenic animals. Co-injection of fertilized one-cell embryos with plasmid DNA harboring a transposon transgene and synthetic mRNA encoding the cognate transposase enzyme results in mosaic integration of the transposon at early cleavage stages that are frequently passed through the germline in the adult animal. Micro-injection of fertilized embryos is a routine procedure used by many laboratories that use Xenopus as a developmental model and, as such, the transposon transgenesis method can be performed without additional equipment or specialized methodologies. The methods for injecting Xenopus embryos are well documented in the literature so here we provide a step-by-step guide to other aspects of transposon transgenesis, including screening mosaic founders for germline transmission of the transgene and general husbandry considerations related to management of populations of transgenic frogs.

Red fluorescent Xenopus laevis: a new tool for grafting analysis

BACKGROUND

Fluorescent proteins such as the green fluorescent protein (GFP) have widely been used in transgenic animals as reporter genes. Their use in transgenic Xenopus tadpoles is especially of interest, because large numbers of living animals can easily be screened. To track more than one event in the same animal, fluorescent markers that clearly differ in their emission spectrum are needed.

RESULTS

We established the transgenic Xenopus laevis strain tom3 that expresses ubiquitously red fluorescence from the tdTomato gene through all larval stages and in the adult animal. This new tool was applied to track transplanted blastemas obtained after tail amputation. The blastema can regenerate ectopic tails marked by red fluorescence in the host animal. Surprisingly, we also found contribution of the host animal to form the regenerate.

CONCLUSION

We have established a useful new tool to label grafts in Xenopus transplantation experiments.