JMJD3 acts in tandem with KLF4 to facilitate reprogramming to pluripotency.

The interplay between the Yamanaka factors (OCT4, SOX2, KLF4 and c-MYC) and transcriptional/epigenetic co-regulators in somatic cell reprogramming is incompletely understood. Here, we demonstrate that the histone H3 lysine 27 trimethylation (H3K27me3) demethylase JMJD3 plays conflicting roles in mouse reprogramming. On one side, JMJD3 induces the pro-senescence factor Ink4a and degrades the pluripotency regulator PHF20 in a reprogramming factor-independent manner. On the other side, JMJD3 is specifically recruited by KLF4 to reduce H3K27me3 at both enhancers and promoters of epithelial and pluripotency genes. JMJD3 also promotes enhancer-promoter looping through the cohesin loading factor NIPBL and ultimately transcriptional elongation. This competition of forces can be shifted towards improved reprogramming by using early passage fibroblasts or boosting JMJD3's catalytic activity with vitamin C. Our work, thus, establishes a multifaceted role for JMJD3, placing it as a key partner of KLF4 and a scaffold that assists chromatin interactions and activates gene transcription.

ß-Catenin safeguards the ground state of mousepluripotency by strengthening the robustness of the transcriptional apparatus.

Mouse embryonic stem cells cultured with MEK (mitogen-activated protein kinase kinase) and GSK3 (glycogen synthase kinase 3) inhibitors (2i) more closely resemble the inner cell mass of preimplantation blastocysts than those cultured with SL [serum/leukemia inhibitory factor (LIF)]. The transcriptional mechanisms governing this pluripotent ground state are unresolved. Release of promoter-proximal paused RNA polymerase II (Pol2) is a multistep process necessary for pluripotency and cell cycle gene transcription in SL. We show that ß-catenin, stabilized by GSK3 inhibition in medium with 2i, supplies transcriptional coregulators at pluripotency loci. This selectively strengthens pluripotency loci and renders them addicted to transcription initiation for productive gene body elongation in detriment to Pol2 pause release. By contrast, cell cycle genes are not bound by ß-catenin, and proliferation/self-renewal remains tightly controlled by Pol2 pause release under 2i conditions. Our findings explain how pluripotency is reinforced in the ground state and also provide a general model for transcriptional resilience/adaptation upon network perturbation in other contexts.

mTORC1-PGC1 axis regulates mitochondrial remodeling during reprogramming.

Metabolic reprogramming, hallmarked by enhanced glycolysis and reduced mitochondrial activity, is a key event in the early phase of somatic cell reprogramming. Although extensive work has been conducted to identify the mechanisms of mitochondrial remodeling in reprogramming, many questions remain. In this regard, different laboratories have proposed a role in this process for either canonical (ATG5-dependent) autophagy-mediated mitochondrial degradation (mitophagy), noncanonical (ULK1-dependent, ATG5-independent) mitophagy, mitochondrial fission or reduced biogenesis due to mTORC1 suppression. Clarifying these discrepancies is important for providing a comprehensive picture of metabolic changes in reprogramming. Yet, the comparison among these studies is difficult because they use different reprogramming conditions and mitophagy detection/quantification methods. Here, we have systematically explored mitochondrial remodeling in reprogramming using different culture media and reprogramming factor cocktails, together with appropriate quantification methods and thorough statistical analysis. Our experiments show lack of evidence for mitophagy in mitochondrial remodeling in reprogramming, and further confirm that the suppression of the mTORC1-PGC1 pathway drives this process. Our work helps to clarify the complex interplay between metabolic changes and nutrient sensing pathways in reprogramming, which may also shed light on other contexts such as development, aging and cancer.