| Through the discovery of somatic cell reprogramming by Takahashi and Yamanaka, pluripotency can now be induced in somatic cells by expression of Oct4, Sox2, Klf4, and cMyc (OKSM) transcription factors. While the technology is being increasingly utilized to produce iPSCs, its mechanistic understanding remains incomplete. In particular, the process of epigenetic remodeling, shown to be a major rate-limiting step of the reprogramming process, remains poorly characterized. Recent work in the field identified a set of predictive markers (Utfl, Esrrb, Lin28 and Dppa2) whose expression labels rare cells, which have higher probability of becoming iPSCs. We show that Dppa2 and its close homolog Dppa4 are the essential components of the chromatin-remodeling network and govern the transition to pluripotency. Dppa2/4 are induced in pre-iPSCs and are required for efficient transition to pluripotency. Ectopic expression of single Dppa2/4 transgene in OKSM mouse embryonic fibroblasts significantly increases the number of iPSC colonies compared to OKSM alone. Expression of both Dppa2 and Dppa4 (Dppa2/4-OE) results in additional and drastic increase in colony numbers. In a single-cell assay, more that 80% of Dppa2/4-OE MEFs give rise to Nanog+ iPSCs compared to less than 2% in the OKSM cultures. The emergence of reprogramming intermediates is also greatly accelerated, resulting in formation of fully reprogrammed iPSCs in 2-4 days. Dppa2/4-OE iPSCs exhibit correct pattern of pluripotency marker expression and robustly generate germline-competent chimeric mice by blastocyst injection and tetraploid complementation. Importantly, overexpression of DPPA2/4 in human fibroblasts also drastically improves the efficiency of human iPSC generation. Mechanistically, Dppa2 and Dppa4 function as a dimer, which interacts with chromatin at promoters and intergenic regions located within large H3K9me3 heterochromatin domains. Biochemical and bioinformatics analyses revealed that Dppa2/4 binding leads to the recruitment of several chromatin remodeling complexes, increases in yH2AX deposition, gain of H3K4me3 marks, decompaction of H3K9me3 marks, and rapid activation of endogenous pluripotency network. These findings reveal a novel mechanism responsible for generation of accessible chromatin during reprogramming and provides a useful strategy to enhance iPSC generation for clinical and research use. |
