Carm1-arginine methylation of the transcription factor C/EBPα regulates transdifferentiation velocity.

Here, we describe how the speed of C/EBPα-induced B cell to macrophage transdifferentiation (BMT) can be regulated, using both mouse and human models. The identification of a mutant of C/EBPα (C/EBPαR35A) that greatly accelerates BMT helped to illuminate the mechanism. Thus, incoming C/EBPα binds to PU.1, an obligate partner expressed in B cells, leading to the release of PU.1 from B cell enhancers, chromatin closing and silencing of the B cell program. Released PU.1 redistributes to macrophage enhancers newly occupied by C/EBPα, causing chromatin opening and activation of macrophage genes. All these steps are accelerated by C/EBPαR35A, initiated by its increased affinity for PU.1. Wild-type C/EBPα is methylated by Carm1 at arginine 35 and the enzyme's perturbations modulate BMT velocity as predicted from the observations with the mutant. Increasing the proportion of unmethylated C/EBPα in granulocyte/macrophage progenitors by inhibiting Carm1 biases the cell's differentiation toward macrophages, suggesting that cell fate decision velocity and lineage directionality are closely linked processes.

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.

The transcription factor code: a beacon for histone methyltransferase docking.

Histone methylation is required for the establishment and maintenance of gene expression patterns that determine cellular identity, and its perturbation often leads to aberrant development and disease. Recruitment of histone methyltransferases (HMTs) to gene regulatory elements (GREs) of developmental genes is important for the correct activation and silencing of these genes, but the drivers of this recruitment are largely unknown. Here we propose that lineage-instructive transcription factors (Lin-TFs) act as general recruiters of HMT complexes to cell type-specific GREs through protein-protein interactions. We also postulate that the specificity of these interactions is dictated by Lin-TF post-translational modifications (PTMs), which act as a 'transcription factor code' that can determine the directionality of cell fate decisions during differentiation and development.

From research to rapid response: mass COVID-19 testing by volunteers at the Centre for Genomic Regulation.

The COVID-19 pandemic has posed and is continuously posing enormous societal and health challenges worldwide. The research community has mobilized to develop novel projects to find a cure or a vaccine, as well as to contribute to mass testing, which has been a critical measure to contain the infection in several countries. Through this article, we share our experiences and learnings as a group of volunteers at the Centre for Genomic Regulation (CRG) in Barcelona, Spain. As members of the ORFEU project, an initiative by the Government of Catalonia to achieve mass testing of people at risk and contain the epidemic in Spain, we share our motivations, challenges and the key lessons learnt, which we feel will help better prepare the global society to address similar situations in the future.