Generation of intermediate porcine iPS cells under culture condition favorable for mesenchymal-to-epithelial transition.

It has been demonstrated that naïve and primed pluripotency are determined by different extracellular signals. In this study, we investigated whether intermediate pluripotent states could be available by manipulating the culture condition during the process of generating pig induced pluripotent stem cells (piPSCs). By optimizing the culture condition that efficiently promotes mesenchymal-to-epithelial transition (MET), we found that combination of three growth factors (LIF, FGF2 and BMP4) and two inhibitors (2i: CHIR99021 and SB431542) could generate an intermediate pluripotent state of piPSCs, which were named as LFB2i-piPSCs. The LFB2i-piPSCs are stable and fulfill all the criteria of pluripotency, including expression of pluripotent genes, differentiation into three germ layers via embryoid bodies in vitro and teratoma in vivo. More importantly, the mRNA-sequencing data showed that LFB2i-piPSCs had a mixed transcriptome of naïve and primed pluripotency, which featured by expressing high levels of SOX2, L-MYC and ESRRB and relatively low levels of POU5F1, KLF4 and NANOG. Small RNA sequencing also demonstrated that LFB2i-piPSCs had a mixed microRNA profile of naïve and primed pluripotency, which featured by expressing high levels of miR-302b/367 cluster and miR-106a/363 cluster, and low levels of most let-7 family members and miR-17/92 cluster. Altogether, the LFB2i-piPSCs represent a stable intermediate pluripotent state with unique transcriptome and microRNA signatures. The LFB2i-piPSCs will provide a new tool to explore the mechanisms of pluripotency and reprogramming on pig species.