Abstract
Authentic or naïve embryonic stem cells (ESC) have probably never been derived from the inner cell mass (ICM) of pig blastocysts, despite over 25 years of effort. Recently, several groups, including ours, have reported induced pluripotent stem cells (iPSC) from swine by reprogramming somatic cells with a combination of four factors, OCT4 (POU5F1)/SOX2/KLF4/c-MYC delivered by retroviral transduction. The porcine (p) iPSC resembled human (h) ESC and the mouse "Epiblast stem cells" (EpiSC) in their colony morphology and expression of pluripotent genes, and are likely dependent on FGF2/ACTIVIN/NODAL signaling, therefore representing a primed ESC state. These cells are likely to advance swine as a model in biomedical research, since grafts could potentially be matched to the animal that donated the cells for re-programming. The objective of the present work has been to develop naïve piPSC. Employing a combination of seven reprogramming factors assembled on episomal vectors, we successfully reprogrammed porcine embryonic fibroblasts on a modified LIF-medium supplemented with two kinase inhibitors; CHIR99021, which inhibits GSK-3beta, and PD0325901, a MEK inhibitor. The derived piPSC bear a striking resemblance to naïve mESC in colony morphology, are dependent on LIF to maintain an undifferentiated phenotype, and express markers consistent with pluripotency. They exhibit high telomerase activity, a short cell cycle interval, and a normal karyotype, and are able to generate teratomas. Currently, the competence of these lines for contributing to germ-line chimeras is being tested.