Promotion of Reprogramming to Ground State Pluripotency by Signal Inhibition

Abstract

Induced pluripotent stem (iPS) cells are generated from somatic cells by genetic manipulation. Reprogramming entails multiple transgene integrations and occurs apparently stochastically in rare cells over many days. Tissue stem cells may be subject to less-stringent epigenetic restrictions than other cells and might therefore be more amenable to deprogramming. We report that brain-derived neural stem (NS) cells acquire undifferentiated morphology rapidly and at high frequency after a single round of transduction with reprogramming factors. However, critical attributes of true pluripotency—including stable expression of endogenous Oct4 and Nanog, epigenetic erasure of X chromosome silencing in female cells, and ability to colonise chimaeras—were not attained. We therefore applied molecularly defined conditions for the derivation and propagation of authentic pluripotent stem cells from embryos. We combined dual inhibition (2i) of mitogen-activated protein kinase signalling and glycogen synthase kinase-3 (GSK3) with the self-renewal cytokine leukaemia inhibitory factor (LIF). The 2i/LIF condition induced stable up-regulation of Oct4 and Nanog, reactivation of the X chromosome, transgene silencing, and competence for somatic and germline chimaerism. Using 2i /LIF, NS cell reprogramming required only 1–2 integrations of each transgene. Furthermore, transduction with Sox2 and c-Myc is dispensable, and Oct4 and Klf4 are sufficient to convert NS cells into chimaera-forming iPS cells. These findings demonstrate that somatic cell state influences requirements for reprogramming and delineate two phases in the process. The ability to capture pre-pluripotent cells that can advance to ground state pluripotency simply and with high efficiency opens a door to molecular dissection of this remarkable phenomenon. Development of an organism proceeds irreversibly from embryo to adult, with cells differentiating progressively towards specialised final phenotypes. Now, following the pioneering discovery of induced pluripotency by Shinya Yamanaka, it has become possible to reverse developmental time: we can reprogramme an adult cell back to the naïve state of pluripotency found in the early embryo. Induction of pluripotency is an extraordinary phenomenon but is currently poorly understood and inefficient. We investigated stem cells from the mouse brain and found that they reprogrammed faster than other cell types. However, the reprogrammed brain cells arrested on the verge of full pluripotency and did not gain some essential properties of induced pluripotency. Guided by the rationale of reversing a development process, we explored the effect of blocking the signal that initiates loss of pluripotency and entry into differentiation in the embryo. We used a chemical inhibitor of this signal in combination with stimulation of a second pathway known to promote maintenance of pluripotency. This simple treatment allowed the partly converted neural stem cells to complete the transition efficiently and become indistinguishable from embryonic stem cells. Therefore, incompletely reprogrammed cells, which have previously been dismissed as useless by-products of attempts to generate pluripotent stem cells, in fact provide the fastest, most reliable, and most efficient route to obtaining authentic induced pluripotent cells.