The trophectoderm acts as a niche for the inner cell mass through C/EBPα-regulated IL-6 signaling.

IL-6 has been shown to be required for somatic cell reprogramming into induced pluripotent stem cells (iPSCs). However, how Il6 expression is regulated and whether it plays a role during embryo development remains unknown. Here, we describe that IL-6 is necessary for C/EBPα-enhanced reprogramming of B cells into iPSCs but not for B cell to macrophage transdifferentiation. C/EBPα overexpression activates both Il6 and Il6ra genes in B cells and in PSCs. In embryo development, Cebpa is enriched in the trophectoderm of blastocysts together with Il6, while Il6ra is mostly expressed in the inner cell mass (ICM). In addition, Il6 expression in blastocysts requires Cebpa. Blastocysts secrete IL-6 and neutralization of the cytokine delays the morula to blastocyst transition. The observed requirement of C/EBPα-regulated IL-6 signaling for pluripotency during somatic cell reprogramming thus recapitulates a physiologic mechanism in which the trophectoderm acts as niche for the ICM through the secretion of IL-6.

RANK links senescence to stemness in the mammary epithelia, delaying tumor onset but increasing tumor aggressiveness.

Rank signaling enhances stemness in mouse and human mammary epithelial cells (MECs) and mediates mammary tumor initiation. Mammary tumors initiated by oncogenes or carcinogen exposure display high levels of Rank and Rank pathway inhibitors have emerged as a new strategy for breast cancer prevention and treatment. Here, we show that ectopic Rank expression in the mammary epithelia unexpectedly delays tumor onset and reduces tumor incidence in the oncogene-driven Neu and PyMT models. Mechanistically, we have found that ectopic expression of Rank or exposure to Rankl induces senescence, even in the absence of other oncogenic mutations. Rank leads to DNA damage and senescence through p16/p19. Moreover, RANK-induced senescence is essential for Rank-driven stemness, and although initially translates into delayed tumor growth, eventually promotes tumor progression and metastasis. We uncover a dual role for Rank in the mammary epithelia: Rank induces senescence and stemness, delaying tumor initiation but increasing tumor aggressiveness.

In Vivo Reprogramming Ameliorates Aging Features in Dentate Gyrus Cells and Improves Memory in Mice.

Post-translational epigenetic modifications take place in mouse neurons of the dentate gyrus (DG) with age. Here, we report that age-dependent reduction in H3K9 trimethylation (H3K9me3) is prevented by cyclic induction of the Yamanaka factors used for cell reprogramming. Interestingly, Yamanaka factors elevated the levels of migrating cells containing the neurogenic markers doublecortin and calretinin, and the levels of the NMDA receptor subunit GluN2B. These changes could result in an increase in the survival of newborn DG neurons during their maturation and higher synaptic plasticity in mature neurons. Importantly, these cellular changes were accompanied by an improvement in mouse performance in the object recognition test over long time. We conclude that transient cyclic reprogramming in vivo in the central nervous system could be an effective strategy to ameliorate aging of the central nervous system and neurodegenerative diseases.

Global hyperactivation of enhancers stabilizes human and mouse naive pluripotency through inhibition of CDK8/19 Mediator kinases.

Pluripotent stem cells (PSCs) transition between cell states in vitro, reflecting developmental changes in the early embryo. PSCs can be stabilized in the naive state by blocking extracellular differentiation stimuli, particularly FGF-MEK signalling. Here, we report that multiple features of the naive state in human and mouse PSCs can be recapitulated without affecting FGF-MEK signalling or global DNA methylation. Mechanistically, chemical inhibition of CDK8 and CDK19 (hereafter CDK8/19) kinases removes their ability to repress the Mediator complex at enhancers. CDK8/19 inhibition therefore increases Mediator-driven recruitment of RNA polymerase II (RNA Pol II) to promoters and enhancers. This efficiently stabilizes the naive transcriptional program and confers resistance to enhancer perturbation by BRD4 inhibition. Moreover, naive pluripotency during embryonic development coincides with a reduction in CDK8/19. We conclude that global hyperactivation of enhancers drives naive pluripotency, and this can be achieved in vitro by inhibiting CDK8/19 kinase activity. These principles may apply to other contexts of cellular plasticity.

Tissue damage and senescence provide critical signals for cellular reprogramming in vivo.

Reprogramming of differentiated cells into pluripotent cells can occur in vivo, but the mechanisms involved remain to be elucidated. Senescence is a cellular response to damage, characterized by abundant production of cytokines and other secreted factors that, together with the recruitment of inflammatory cells, result in tissue remodeling. Here, we show that in vivo expression of the reprogramming factors OCT4, SOX2, KLF4, and cMYC (OSKM) in mice leads to senescence and reprogramming, both coexisting in close proximity. Genetic and pharmacological analyses indicate that OSKM-induced senescence requires the Ink4a/Arf locus and, through the production of the cytokine interleukin-6, creates a permissive tissue environment for in vivo reprogramming. Biological conditions linked to senescence, such as tissue injury or aging, favor in vivo reprogramming by OSKM. These observations may be relevant for tissue repair.

The pluripotency factor NANOG promotes the formation of squamous cell carcinomas.

NANOG is a key pluripotency factor in embryonic stem cells that is frequently expressed in squamous cell carcinomas (SCCs). However, a direct link between NANOG and SCCs remains to be established. Here, we show that inducible overexpression of NANOG in mouse skin epithelia favours the malignant conversion of skin papillomas induced by chemical carcinogenesis, leading to increased SCC formation. Gene expression analyses in pre-malignant skin indicate that NANOG induces genes associated to epithelial-mesenchymal transition (EMT). Some of these genes are directly activated by NANOG, including EMT-associated genes Zeb1, Zeb2, Twist1, Prrx1 and miR-21. Finally, endogenous NANOG binds to the promoters of theses genes in human SCC cells and, moreover, NANOG induces EMT features in primary keratinocytes. These results provide in vivo evidence for the oncogenic role of NANOG in squamous cell carcinomas.