328 NONVIRAL REPROGRAMMING OF mCHERRY-EXPRESSING PORCINE FIBROBLASTS INTO INDUCED PLURIPOTENT STEM CELLS BY piggyBac TRANSPOSONS

Abstract

The generation of induced pluripotent stem (iPS) cells is a promising approach for innovative cell therapies, as well as for animal biotechnology. The original method requires viral transduction of several reprogramming factors, which may be associated with an increased risk of tumorigenicity due to the preferential integration into active genes. The domestic pig is an attractive large animal model for preclinical testing of safety and efficacy of cell-based therapies. Porcine organs are similar in size and physiology to their human counterparts, and a suitable model for cardiovascular disease, muscular dystrophies, atherosclerosis, wound repair, diabetes, and ophthalmological diseases. Therefore, the present study was carried out to derive porcine iPS cells from transgenic fetuses systemically expressing mCherry (Garrels et al. 2011 PLOS ONE 6) through a nonviral piggyBac transposon. The piggyBac transposon system has several advantages: (i) piggyBac has no bias to integrate in expressed gene-like lenti- or retroviral vectors, (ii) the cargo capacity is >100 kb, (iii) seamless removal is possible, and (iv) the production of transposon plasmid is cost-efficient and does not require S2 safety cabinets. Porcine fetal fibroblasts isolated from CAGGS-mCherry founder porcine line fetuses (passage 2), were co-electroporated with a PB transposon carrying a multigene cassette consisting of human cDNA for OCT4, SOX2, KLF4, c-MYc, NANOG, and LIN28 separated by self-cleaving 2A peptide sequences, driven by a CAGGS promoter and a helper plasmid expressing the pCMV-PB transposase. On Day 6 postelectroporation, morphology of fibroblasts started change to round structure, and on Day 9 loose aggregates of cells developed. Putative iPS cell colonies were cultured, propagated, and characterised through morphology and expression of pluripotency markers, such as AP, OCT4, SSEA-1, and SSEA-4, through immunostaining. Further, various stemness genes, including OCT4, SOX2, NANOG, and UTF, were detected by porcine-specific primers through endpoint RT-PCR. In vitro differentiation potential was assessed by embryoid body (EB) formation. The formed EB exhibited the expression of mCherry in their cells and expressed differentiation markers, such as NESTIN, TUJI, GATA4 and AFP. To test their tumorigenic potential, 1 × 10⁶ iPS cells were injected under the skin of nude mice. An mCherry-positive tumour was recovered 6 weeks later. Presently the tumour is being prepared for histological analysis. This study indicates that piggyBac transposon containing 6 transcription factors is able to reprogram porcine fetal fibroblasts into iPS cells. These cells could be cultured and maintained in vitro for a prolonged period, exhibit characteristics of stem cells, and offer a potential source for future blastocyst complementation experiments.