Abstract
Ectopic transcription factor expression enables reprogramming of somatic cells to pluripotency, albeit with generally low efficiency. Despite steady progress in the field, the exact molecular mechanisms that coordinate this remarkable transition still remain largely elusive. To better characterize the final steps of pluripotency induction, we optimized an experimental system where pluripotent stem cells are differentiated for set intervals before being reintroduced to pluripotency-supporting conditions. Using this approach, we identify a transient period of high-efficiency reprogramming where ectopic transcription factors, but not serum/LIF alone, rapidly revert cells to pluripotency with near 100% efficiency. After this period, cells reprogram with somatic-like kinetics and efficiencies. We identify a set of OCT4 bound cis-regulatory elements that are dynamically regulated during this transient phase and appear central to facilitating reprogramming. Interestingly, these regions remain hypomethylated during in vitro and in vivo differentiation, which may allow them to act as primary targets of ectopically induced factors during somatic cell reprogramming.
Highlights
Ectopic transcription factor expression enables reprogramming of somatic cells to pluripotency, albeit with generally low efficiency
After the second transition point, the efficiency and kinetics of induced pluripotent stem cells (iPSCs) colony formation resembles those observed when reprogramming from somatic cells (Fig. 1e and Supplementary Fig. 1i, j)
We show that cells differentiated beyond the transient phase can still reprogram, but the efficiency and time required resembles those observed for mouse embryonic fibroblasts (MEFs)
Summary
Ectopic transcription factor expression enables reprogramming of somatic cells to pluripotency, albeit with generally low efficiency. This high-efficiency reversion is facilitated by a set of OCT4 bound cis-regulatory elements that display a unique silencing behavior during differentiation. These regions appear to be preferentially targeted by ectopically induced TFs in somatic reprogramming systems and retain reduced methylation levels throughout development and even into adult cells as an epigenetic remnant of the pluripotent state
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