Production of porcine cloned embryos derived from cells conditionally expressing an exogenous gene using Cre-loxP

It is increasingly evident that conditional gene expression in pigs is necessary to make transgenic models. In this study, we investigated conditional expression in porcine fetal fibroblasts using Cre-loxP recombination, a system that has had limited application in large animals to date. Transformed fibroblasts were reprogrammed in enucleated oocytes to support further early embryonic development. Fetal fibroblasts from miniature pigs were used for transfection with a plasmid that contained a red fluorescent protein marker (pCALNL-DsRed) and a floxed neomycin-resistance gene. Cells were selected with 750 μ/ml neomycin for 2 weeks following transfection but did not express DsRed after visualization under a fluorescence microscope. Expression was achieved only after transient transfection with plasmid DNA that expressed the Cre recombinase enzyme. The cells that expressed DsRed were used for somatic cell nuclear transfer (SCNT). A total of 121 oocytes were used for SCNT and 76 cloned embryos (62.8%) were seen to have cleaved. Six blastocysts developed after SCNT and expressed DsRed. Deletion of the floxed neomycin-resistance gene was confirmed by reverse transcription polymerase chain reaction (RT-PCR) in cloned blastocysts. This study demonstrated that Cre-loxP recombination can be conducted successfully in miniature pig fibroblasts and that the sequentially transformed cells can develop to the pre-implantation embryo stage via SCNT.