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In interphase nuclei, chromatin is organized into interspersed dense domains with characteristic sizes, both in the nuclear interior and periphery. However, the quantitative impact of transcription and histone modifications on the size and distribution of these domains remains unclear. Here, we introduce a mesoscale theoretical model that investigates the relationship between heterochromatic domain sizes and loop extrusion rates from these domains. The model considers chromatin-chromatin and chromatin-lamina interactions, methylation and acetylation kinetics, and diffusion of epigenetic marks and nucleoplasm. Our model generates testable predictions that help reveal the biophysics underlying chromatin organization in the presence of transcription-driven loop extrusion. This process is kinetically captured through the conversion of heterochromatin to euchromatin in response to RNAPII activity. We discovered that a balance between diffusive and reactive fluxes governs the steady-state sizes of heterochromatin domains. Using theory and simulations, we predicted that a loss of transcription results in increased chromatin compaction and larger heterochromatin domain sizes. To validate our predictions, we employed complementary super-resolution and nano-imaging techniques on five different cell lines with impaired transcription. We quantitatively assessed how domain sizes scale with loop extrusion rates at the hetero-euchromatin interfaces. Our analysis of previously obtained super-resolution images of nuclei revealed that excessive loop extrusion leads to smaller heterochromatin domains. The model successfully recapitulated these observations, explaining how transcription loss can counteract the effects of cohesin overloading. As the general biophysical mechanisms regulating heterochromatin domain sizes are independent of cell type, our findings have significant implications for understanding the role of transcription in global genome organization.

Chromocenters are established after the 2-cell (2C) stage during mouse embryonic development, but the factors that mediate chromocenter formation remain largely unknown. To identify regulators of 2C heterochromatin establishment, we generated an inducible system to convert embryonic stem cells (ESCs) to 2C-like cells. This conversion is marked by a global reorganization and dispersion of H3K9me3-heterochromatin foci, which are then reversibly formed upon re-entry into pluripotency. Profiling the chromatin-bound proteome (chromatome) by genome capture of ESCs transitioning to 2C-like cells, we uncover chromatin regulators involved inde novoheterochromatin formation. We identified TOPBP1 and investigated its binding partner SMARCAD1. SMARCAD1 and TOPBP1 associate with H3K9me3-heterochromatin in ESCs. Interestingly, the nuclear localization of SMARCAD1 is lost in 2C-like cells. SMARCAD1 or TOPBP1 depletion in mouse embryos lead to developmental arrest, reduction of H3K9me3 and remodeling of heterochromatin foci. Collectively, our findings contribute to comprehending the maintenance of chromocenters during early development.

Reactivation of the inactive X chromosome is a hallmark epigenetic event during reprogramming of mouse female somatic cells to induced pluripotent stem cells (iPSCs). This involves global structural remodeling from a condensed, heterochromatic into an open, euchromatic state, thereby changing a transcriptionally inactive into an active chromosome. Despite recent advances, very little is currently known about the molecular players mediating this process and how this relates to iPSC-reprogramming in general. To gain more insight, here we perform a RNAi-based knockdown screen during iPSC-reprogramming of mouse fibroblasts. We discover factors important for X chromosome reactivation (XCR) and iPSC-reprogramming. Among those, we identify the cohesin complex member SMC1a as a key molecule with a specific function in XCR, as its knockdown greatly affects XCR without interfering with iPSC-reprogramming. Using super-resolution microscopy, we find SMC1a to be preferentially enriched on the active compared with the inactive X chromosome and that SMC1a is critical for the decompacted state of the active X. Specifically, depletion of SMC1a leads to contraction of the active X both in differentiated and in pluripotent cells, where it normally is in its most open state. In summary, we reveal cohesin as a key factor for remodeling of the X chromosome from an inactive to an active structure and that this is a critical step for XCR during iPSC-reprogramming.

The linear sequence of DNA provides invaluable information about genes and their regulatory elements along chromosomes. However, to fully understand gene function and regulation, we need to dissect how genes physically fold in the three-dimensional nuclear space. Here we describe immuno-OligoSTORM, an imaging strategy that reveals the distribution of nucleosomes within specific genes in super-resolution, through the simultaneous visualization of DNA and histones. We combine immuno-OligoSTORM with restraint-based and coarse-grained modeling approaches to integrate super-resolution imaging data with Hi-C contact frequencies and deconvoluted micrococcal nuclease-sequencing information. The resulting method, called Modeling immuno-OligoSTORM, allows quantitative modeling of genes with nucleosome resolution and provides information about chromatin accessibility for regulatory factors, such as RNA polymerase II. With Modeling immuno-OligoSTORM, we explore intercellular variability, transcriptional-dependent gene conformation, and folding of housekeeping and pluripotency-related genes in human pluripotent and differentiated cells, thereby obtaining the highest degree of data integration achieved so far to our knowledge.

Data de publicació electrònica: 13-04-2023 Over the last decades, technological breakthroughs in super-resolution microscopy have allowed us to reach molecular resolution and design experiments of unprecedented complexity. Investigating how chromatin is folded in 3D, from the nucleosome level up to the entire genome, is becoming possible by "magic" (imaging genomic), i.e., the combination of imaging and genomic approaches. This offers endless opportunities to delve into the relationship between genome structure and function. Here, we review recently achieved objectives and the conceptual and technical challenges the field of genome architecture is currently undertaking. We discuss what we have learned so far and where we are heading. We elucidate how the different super-resolution microscopy approaches and, more specifically, live-cell imaging have contributed to the understanding of genome folding. Moreover, we discuss how future technical developments could address remaining open questions. We acknowledge the support from the European Union’s Horizon 2020 Research and Innovation Program (no. 964342 to M.P.C.); the Ministerio de Ciencia e Innovación (grant no. 008506-PID2020- 114080GB-I00 to M.P.C.); an AGAUR grant from Secretaria d’Universitats i Recerca del Departament d’Empresa iConeixement de la Generalitat de Catalunya (grant no. 006712 BFU2017-86760-P [AEI/FEDER, UE] to M.P.C.); the CERCA Program/Generalitat de Catalunya (to CRG); ICREA (Institucio Catalana de Recerca i Estudis Avançats) (to M.P.C.); the National Natural Science Foundation of China (31971177 and 32270577 to M.P.C.); Guangzhou Key Projects of Brain Science and Brain-Like Intelligence Technology (20200730009 to M.P.C.); Barcelona Institute of Science and Technology (BIST) Ignite Grants (Seeding Stage 2017 and Second Phase 2018, to M.V.N.); the People Program (Marie Curie Actions) FP7/2007–2013 under REA (grant no. 608959 to M.V.N.); and Juan de la Cierva-Incorporación 2017 (to M.V.N.) and the Spanish Ministry of Science and Innovation through the Centro de Excelencia Severo Ochoa (CEX2020-001049-S, MCIN/AEI/10.13039/501100011033 to CRG).

Early during preimplantation development and in heterogeneous mouse embryonic stem cells (mESC) culture, pluripotent cells are specified towards either the primed epiblast or the primitive endoderm (PE) lineage. Canonical Wnt signaling is crucial for safeguarding naive pluripotency and embryo implantation, yet the role and relevance of canonical Wnt inhibition during early mammalian development remains unknown. Here, we demonstrate that transcriptional repression exerted by Wnt/TCF7L1 promotes PE differentiation of mESCs and in preimplantation inner cell mass. Time-series RNA sequencing and promoter occupancy data reveal that TCF7L1 binds and represses genes encoding essential naive pluripotency factors and indispensable regulators of the formative pluripotency program, including Otx2 and Lef1. Consequently, TCF7L1 promotes pluripotency exit and suppresses epiblast lineage formation, thereby driving cells into PE specification. Conversely, deletion of Tcf7l1 abrogates PE differentiation without restraining epiblast priming. Taken together, our study underscores the importance of transcriptional Wnt inhibition in regulating lineage segregation in ESCs and preimplantation embryo development as well as identifies TCF7L1 as key regulator of this process.

The relationship between the 4D folding of the genome and its function is an outstanding question in biology. A range of methods that probe the folding of the genome in space and time with unprecedented resolution have been developed. These methods, including chromosome conformation capture and high-resolution light and electron microscopy, are shedding new light on genome architecture and function. Here, we review the emerging picture of genome organization revealed by super-resolution and live-cell imaging. We compare and contrast population-based chromosome conformation capture approaches and imaging-based approaches and highlight future challenges. This work was supported by the University of Pennsylvania Epigenetics Pilot Award (to M.L.), the Center for Engineering and Mechanobiology (CEMB), an NSF Science and Technology Center Pilot Award under grant agreement CMMI 15-48571 (to M.L.), a Linda Pechenik Montague Investigator Award (to M.L.), the European Union’s Horizon 2020 Research and Innovation Programme (CellViewer No 686637 to M.L. and M.P.C.), Ministerio de Ciencia e Innovación, grant BFU2017-86760-P (AEI/FEDER, UE), and an AGAUR grant from Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya (2017 SGR 689 to M.P.C.). We acknowledge the support of the Spanish Ministry of Science and Innovation to the EMBL partnership, the Centro de Excelencia Severo Ochoa and the CERCA Programme / Generalitat de Catalunya

Transcription and genome architecture are interdependent, but it is still unclear how nucleosomes in the chromatin fiber interact with nascent RNA, and which is the relative nuclear distribution of these RNAs and elongating RNA polymerase II (RNAP II). Using super-resolution (SR) microscopy, we visualized the nascent transcriptome, in both nucleoplasm and nucleolus, with nanoscale resolution. We found that nascent RNAs organize in structures we termed RNA nanodomains, whose characteristics are independent of the number of transcripts produced over time. Dual-color SR imaging of nascent RNAs, together with elongating RNAP II and H2B, shows the physical relation between nucleosome clutches, RNAP II, and RNA nanodomains. The distance between nucleosome clutches and RNA nanodomains is larger than the distance measured between elongating RNAP II and RNA nanodomains. Elongating RNAP II stands between nascent RNAs and the small, transcriptionally active, nucleosome clutches. Moreover, RNA factories are small and largely formed by few RNAP II. Finally, we describe a novel approach to quantify the transcriptional activity at an individual gene locus. By measuring local nascent RNA accumulation upon transcriptional activation at single alleles, we confirm the measurements made at the global nuclear level., Graphical Abstract Graphical AbstractSchematic representation depicting the association between nucleosome clutches, RNAP II clusters and RNA nanodomains. Chromatin (in green) is organized in clutches. Small active nucleosome clutches are associated with elongating RNAP II clusters (in purple) which generates small RNA nanodomains (in red).

Summary Here, we describe three complementary microscopy-based approaches to quantify morphological changes of chromatin organization in cultured adherent cells: the analysis of the coefficient of variation of DNA, the measurement of DNA-free nuclear areas, and the quantification of chromatin-associated proteins at the nuclear edge. These approaches rely on confocal imaging and stochastic optical reconstruction microscopy and allow a fast and robust quantification of chromatin compaction. These approaches circumvent inter-variability between imaging conditions and apply to every type of adherent cells. For complete details on the use and execution of this protocol, please refer to Neguembor et al. (2021)., Graphical abstract, Highlights • Protocol for three complementary techniques to measure chromatin compaction • Chromatin compaction is quantified from confocal and super-resolution images • CV and Black Space analyses score for heterogeneity of nuclear DNA distribution • Edge analysis measures the enrichment of a protein signal around the nuclear edge, Here, we describe three complementary microscopy-based approaches to quantify morphological changes of chromatin organization in cultured adherent cells: the analysis of the coefficient of variation of DNA, the measurement of DNA-free nuclear areas, and the quantification of chromatin-associated proteins at the nuclear edge. These approaches rely on confocal imaging and stochastic optical reconstruction microscopy and allow a fast and robust quantification of chromatin compaction. These approaches circumvent inter-variability between imaging conditions and apply to every type of adherent cells.

Most studies of cohesin function consider the Stromalin Antigen (STAG/SA) proteins as core complex members given their ubiquitous interaction with the cohesin ring. Here, we provide functional data to support the notion that the SA subunit is not a mere passenger in this structure, but instead plays a key role in the localization of cohesin to diverse biological processes and promotes loading of the complex at these sites. We show that in cells acutely depleted for RAD21, SA proteins remain bound to chromatin, cluster in 3D and interact with CTCF, as well as with a wide range of RNA binding proteins involved in multiple RNA processing mechanisms. Accordingly, SA proteins interact with RNA and are localised to R-loops where they contribute to R-loop regulation. Our results place SA1 within R-loop domains upstream of the cohesin complex and reveal a role for SA1 in cohesin loading which is independent of NIPBL, the canonical cohesin loader. We propose that SA1 takes advantage of structural R-loop platforms to link cohesin loading and chromatin structure with diverse functions. Since SA proteins are pan-cancer targets, and R-loops play an increasingly prevalent role in cancer biology, our results have important implications for the mechanistic understanding of SA proteins in cancer and disease.

SUMMARY Nucleosomes form heterogeneous groups in vivo, named clutches. Clutches are smaller and less dense in mouse embryonic stem cells (ESCs) compared to neural progenitor cells (NPCs). Using coarse-grained modeling of the pluripotency Pou5f1 gene, we show that the genome-wide clutch differences between ESCs and NPCs can be reproduced at a single gene locus. Larger clutch formation in NPCs is associated with changes in the compaction and internucleosome contact probability of the Pou5f1 fiber. Using single-molecule tracking (SMT), we further show that the core histone protein H2B is dynamic, and its local mobility relates to the structural features of the chromatin fiber. H2B is less stable and explores larger areas in ESCs compared to NPCs. The amount of linker histone H1 critically affects local H2B dynamics. Our results have important implications for how nucleosome organization and H2B dynamics contribute to regulate gene activity and cell identity., Graphical Abstract, In Brief Gómez-García et al. show that the Pou5f1 gene folds into nucleosome clutches, with larger clutches in differentiated cells than in stem cells. These clutch changes are accompanied by enhanced hierarchical looping in differentiated cells. H2B dynamics is cell-type specific, correlates with clutch patterns, and is regulated by linker histone H1.

Nucleosomes cluster together when chromatin folds in the cell to form heterogeneous groups termed "clutches". These structural units add another level of chromatin regulation, for example during cell differentiation. Yet, the mechanisms that regulate their size and compaction remain obscure. Here, using our chromatin mesoscale model, we dissect clutch patterns in fibers with different combinations of nucleosome positions, linker histone density, and acetylation levels to investigate their role in clutch regulation. First, we isolate the effect of each chromatin parameter by studying systems with regular nucleosome spacing; second, we design systems with naturally-occurring linker lengths that fold onto specific clutch patterns; third, we model gene-encoding fibers to understand how these combined factors contribute to gene structure. Our results show how these chromatin parameters act together to produce different-sized nucleosome clutches. The length of nucleosome free regions (NFRs) profoundly affects clutch size, while the length of linker DNA has a moderate effect. In general, higher linker histone densities produce larger clutches by a chromatin compaction mechanism, while higher acetylation levels produce smaller clutches by a chromatin unfolding mechanism. We also show that it is possible to design fibers with naturally-occurring DNA linkers and NFRs that fold onto specific clutch patterns. Finally, in gene-encoding systems, a complex combination of variables dictates a gene-specific clutch pattern. Together, these results shed light into the mechanisms that regulate nucleosome clutches and suggest a new epigenetic mechanism by which chromatin parameters regulate transcriptional activity via the three-dimensional folded state of the genome at a nucleosome level.

The chromatin fiber folds into loops, but the mechanisms controlling loop extrusion are still poorly understood. Using super-resolution microscopy, we visualize that loops in intact nuclei are formed by a scaffold of cohesin complexes from which the DNA protrudes. RNA polymerase II decorates the top of the loops and is physically segregated from cohesin. Augmented looping upon increased loading of cohesin on chromosomes causes disruption of Lamin at the nuclear rim and chromatin blending, a homogeneous distribution of chromatin within the nucleus. Altering supercoiling via either transcription or topoisomerase inhibition counteracts chromatin blending, increases chromatin condensation, disrupts loop formation, and leads to altered cohesin distribution and mobility on chromatin. Overall, negative supercoiling generated by transcription is an important regulator of loop formation in vivo., e acknowledge support from European Union’s Horizon 2020 research and innovation program (CellViewer 686637 to J.S., M.L., and M.P.C.); Ministerio de Ciencia e Innovación (grant BFU2017-86760-P [AEI/FEDER, UE] to M.P.C.), and an AGAUR grant from Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya (2017 SGR 689 to M.P.C.); Centro de Excelencia Severo Ochoa (2013–2017 to M.P.C.); CERCA Programme/Generalitat de Catalunya (to M.P.C.); the Spanish Ministry of Science and Innovation to the European Molecular Biology Laboratory (EMBL) partnership (to M.P.C.); the National Natural Science Foundation of China (31971177 to M.P.C.); the Innovative Team Program of Guangzhou Regenerative Medicine and Health Guangdong Laboratory (2018GZR110103001 to M.P.C.); a Linda Pechenik Montague Investigator Award (to M.L.); a University of Pennsylvania Epigenetics Pilot Award (to M.L.); the Center for Engineering and Mechanobiology (CEMB) a National Science Foundation (NSF) Science and Technology Center Pilot Award (Division of Civil, Mechanical and Manufacturing Innovation [CMMI]: 15-48571 to M.L.); NIH grants R01HD091797 and R01GM123289 (to C.t.W.); the People Program (Marie Skłodowska-Curie Actions) FP7/2007–2013 under an REA grant (608959 to M.V.N.); Juan de la Cierva-Incorporación 2017 (to M.V.N.); Grant for the recruitment of early-stage research staff FI-2020 (Operational Program of Catalonia 2014–2020 CCI 2014ES05SFOP007 of the European Social Fund to L.M.); a “la Caixa” Foundation Fellowship (ID 100010434, LCF/BQ/DR20/11790016) (to L.M.); “La Caixa-Severo Ochoa” pre-doctoral fellowship (to Á.C.-G.); and Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya and the co-financing of Fondo Social Europeo (2018FI_B_00637 and FSE to R.S.-P.), and a La Caixa international PhD fellowship (to F.S.).

Cell therapy approaches hold great potential for treating retinopathies, which are currently incurable. This study addresses the problem of inadequate migration and integration of transplanted cells into the host retina. To this end, we have identified the chemokines that were most upregulated during retinal degeneration and that could chemoattract mesenchymal stem cells (MSCs). The results were observed using a pharmacological model of ganglion/amacrine cell degeneration and a genetic model of retinitis pigmentosa, from both mice and human retinae. Remarkably, MSCs overexpressing Ccr5 and Cxcr6, which are receptors bound by a subset of the identified chemokines, displayed improved migration after transplantation in the degenerating retina. They also led to enhanced rescue of cell death and to preservation of electrophysiological function. Overall, we show that chemokines released from the degenerating retinae can drive migration of transplanted stem cells, and that overexpression of chemokine receptors can improve cell therapy-based regenerative approaches. This work was supported by Velux Stiftung (976 a to M.P.C.), La Caixa Health (HR17-00231), the Ministerio de Ciencia e Innovación, (BFU2017-86760-P to M.P.C.) (AEI/FEDER, UE), the AGAUR grant from Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya, (2017 SGR 689 to M.P.C.), the subprograma estatal de Formación del Ministerio de Economía y Competitividad ref. BES-2015-075802 (to M.P.) and ref. BES-2015-075805 (to S.A.B.-P.), the Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya ref. 2018FI_B_00637 (to R.S.-P.), and the co‐finance of Fondo Social Europeo (FSE to R.S.-P.). We acknowledge support of the Spanish Ministry of Science and Innovation to the EMBL partnership, the Centro de Excelencia Severo Ochoa and the CERCA Programme / Generalitat de Catalunya

β-Catenin recruits BRD4 and other coregulators to protect pluripotency gene transcription against network perturbation., 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 β-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 β-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.

Bi-species, fusion-mediated, somatic cell reprogramming allows precise, organism-specific tracking of unknown lineage drivers. The fusion of Tcf7l1-/- murine embryonic stem cells with EBV-transformed human B cell lymphocytes, leads to the generation of bi-species heterokaryons. Human mRNA transcript profiling at multiple time points permits the tracking of the reprogramming of B cell nuclei to a multipotent state. Interrogation of a human B cell regulatory network with gene expression signatures identifies 8 candidate master regulator proteins. Of these 8 candidates, ectopic expression of BAZ2B, from the bromodomain family, efficiently reprograms hematopoietic committed progenitors into a multipotent state and significantly enhances their long-term clonogenicity, stemness, and engraftment in immunocompromised mice. Unbiased systems biology approaches let us identify the early driving events of human B cell reprogramming. This work was supported by Human Frontier Science Program Grant 2010 (to M.P.C. and A.C.); the European Union’s Horizon 2020 Research and Innovation Programme (CellViewer no. 686637 to M.P.C); Ministerio de Ciencia e Innovación grant no. BFU2017-86760-P (AEI/FEDER, UE); AGAUR grant from Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya ( 2017 SGR 689 to M.P.C.); Guangzhou Key Projects of Brain Science and Brain-Like Intelligence Technology ( 20200730009 to M.P.C.); Juan de la Cierva Fellowship (to K.A.); BIST Master Fellowship (to X.T.); and the R35CA197745 outstanding NCI investigator award to A.C. We acknowledge the support of the Spanish Ministry of Science and Innovation to the EMBL partnership, the Centro de Excelencia Severo Ochoa , and the CERCA Programme (to M.P.C.), and the two instrumentation grants S10OD012351 and S10OD021764 to A.C. supporting the analytical work

Young mammals possess a limited regenerative capacity in some tissues, which is lost upon maturation. We investigated whether cellular senescence might play a role in such loss during liver regeneration. We found that following partial hepatectomy, the senescence-associated genes p21, p16Ink4a, and p19Arf become dynamically expressed in different cell types when regenerative capacity decreases, but without a full senescent response. However, we show that treatment with a senescence-inhibiting drug improves regeneration, by disrupting aberrantly prolonged p21 expression. This work suggests that senescence may initially develop from heterogeneous cellular responses, and that senotherapeutic drugs might be useful in promoting organ regeneration. Work in the Keyes lab was funded in part by grants from the Spanish Ministry for Economy and Competitiveness (SAF2013-49082-P), La Fondation Recherche Medicale (FRM) (AJE20160635985), Fondation ARC (PJA20181208104), IDEX Attractivité-University of Strasbourg (IDEX2017), and La Fondation Schlumberger pour l'Education et la Recherche (FSER) (FSER 19-Year 2018), and ANR (ANR-19-CE13-0023-03). Work was also supported by grant ANR-10-LABX-0030-INRT, a French State fund managed by the Agence Nationale de la Recherche under the frame program Investissements d'Avenir (ANR-10-IDEX-0002-02)

Regenerative capacities vary enormously across the animal kingdom. In contrast to most cold-blooded vertebrates, mammals, including humans, have very limited regenerative capacity when it comes to repairing damaged or degenerating tissues. Here, we review the main mechanisms of tissue regeneration, underlying the importance of cell dedifferentiation and reprogramming. We discuss the significance of cell fate and identity changes in the context of regenerative medicine, with a particular focus on strategies aiming at the promotion of the body's self-repairing mechanisms. We also introduce some of the most recent advances that have resulted in complete reprogramming of cell identity in vivo. Lastly, we discuss the main challenges that need to be addressed in the near future to develop in vivo reprogramming approaches with therapeutic potential.

Cellular systems have evolved numerous mechanisms to adapt to environmental stimuli, underpinned by dynamic patterns of gene expression. In addition to gene transcription regulation, modulation of protein levels, dynamics and localization are essential checkpoints governing cell functions. The introduction of inducible promoters has allowed gene expression control using orthogonal molecules, facilitating its rapid and reversible manipulation to study gene function. However, differing protein stabilities hinder the generation of protein temporal profiles seen in vivo. Here, we improve the Tet-On system integrating conditional destabilising elements at the post-translational level and permitting simultaneous control of gene expression and protein stability. We show, in mammalian cells, that adding protein stability control allows faster response times, fully tunable and enhanced dynamic range, and improved in silico feedback control of gene expression. Finally, we highlight the effectiveness of our dual-input system to modulate levels of signalling pathway components in mouse Embryonic Stem Cells., Cellular systems have numerous mechanisms to control gene expression. Here the authors build a Tet-On system with conditional destablising elements to regulate gene expression and protein stability, allowing fine modulation of mESC signalling pathways.

SUMMARYYoung mammals possess a limited regenerative capacity in tissues such as the liver, heart and limbs, but which is quickly lost upon maturation or transition to adulthood. Chronic cellular senescence is a known mediator of decreased tissue function in aging and disease. Here we investigated whether senescence plays a role in the progressive loss of liver regenerative capacity that develops in young adult mice. We find that following partial hepatectomy, the senescence markers p21, p16Ink4a and p19Arf become dynamically expressed at an age when regenerative capacity decreases. In addition, we demonstrate that treatment with a senescence-inhibiting drug improves regenerative capacity, through targeting of aberrant p21 expression. Surprisingly, we also find that the senescence marker p16Ink4a is expressed in a different cell-population to p21, and is unaffected by senescence targeting. This work suggests that senescence may initially develop as a heterogeneous cellular response, and that treatment with senolytic drugs may aid in promoting organ regeneration.

Mouse embryonic stem cells (mESCs) are pluripotent and can differentiate into cells belonging to the three germ layers of the embryo. However, mESC pluripotency and genome stability can be compromised in prolonged in vitro culture conditions. Several factors control mESC pluripotency, including Wnt/β-catenin signaling pathway, which is essential for mESC differentiation and proliferation. Here we show that the activity of the Wnt/β-catenin signaling pathway safeguards normal DNA methylation of mESCs. The activity of the pathway is progressively silenced during passages in culture and this results into a loss of the DNA methylation at many imprinting control regions (ICRs), loss of recruitment of chromatin repressors, and activation of retrotransposons, resulting into impaired mESC differentiation. Accordingly, sustained Wnt/β-catenin signaling maintains normal ICR methylation and mESC homeostasis and is a key regulator of genome stability. We acknowledge the support from Spanish Ministry of Economy and Competitiveness and FEDER funds (SAF2011-28580, BFU2014-54717-P, BFU2017-86760-P and BFU2015-71984-ERC to M.P.C.), as well as ‘Centro de Excelencia Severo Ochoa 2013–2017’, Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement de la Generalitat de Catalunya grant (2014 SGR1137 to M.P.C.), AGAUR (SGR 2017-2019 to M.P.C) and the CERCA Programme/Generalitat de Catalunya (to M.P.C.), the European Union’s Horizon 2020 research and innovation programme under grant agreement CellViewer No 686637 (to M.P.C.), People Programme Marie Curie Actions of the European Union’s Seventh Framework Programme (FP7/2007–2013/, n° 290123 to I.T.), La Caixa international PhD fellowship (to F.S.) and Ministerio de Ciencia e Innovació FPI (to F.A.) and People Program (Marie Curie Actions) FP7/2007–2013 under REA grant (608959 to M.V.N.).

Millions of people worldwide suffer from visual disabilities as a result of retinal degeneration. Due to the poor regenerative capability of the central nervous system, retinal cell loss is essentially irreversible. Currently available therapies can only decelerate the degenerative process at late stages and are largely ineffective. However, the possibility of using stem cell-based therapy as cell rescue or cell replacement therapy is broadly being explored. While cell rescue is based on the secretion of biologically active molecules by the transplanted cells, cell replacement refers to the possibility of injected stem cells replacing the defective ones either via direct differentiation, transdifferentiation of the transplanted cells, or via cell fusion-mediated reprogramming of retinal cells.

Mouse pluripotent tetraploid cells generate diploid cells during embryogenesis through nonrandom chromosome segregation., Cells with high ploidy content are common in mammalian extraembryonic and adult tissues. Cell-to-cell fusion generates polyploid cells during mammalian development and tissue regeneration. However, whether increased ploidy can be occasionally tolerated in embryonic lineages still remains largely unknown. Here, we show that pluripotent, fusion-derived tetraploid cells, when injected in a recipient mouse blastocyst, can generate diploid cells upon ploidy reduction. The generated diploid cells form part of the adult tissues in mouse chimeras. Parental chromosomes in pluripotent tetraploid cells are segregated through tripolar mitosis both randomly and nonrandomly and without aneuploidy. Tetraploid-derived diploid cells show a differentiated phenotype. Overall, we discovered an unexpected process of controlled genome reduction in pluripotent tetraploid cells. This mechanism can ultimately generate diploid cells during mouse embryo development and should also be considered for cell fusion–mediated tissue regeneration approaches.

Chromatin organization plays an important role in regulating gene expression. Previously, we showed that chromatin is organized in the form of nucleosome groups or clutches. The size and nucleosome packing density of clutches decreased in hyperacetylated cells having more open chromatin. While hyperacetylation is thought to disrupt histone-DNA and inter-nucleosome interactions, its impact on higher order DNA compaction by groups of nucleosomes in vivo is not clear. To elucidate this question, we carried out two-color super-resolution imaging of histones and DNA in cells treated with the Histone Deacetylase (HDAC) inhibitor Trichostatin A (TSA). We showed that a lower percentage of DNA was associated to clutches in hyperacetylated cells, suggesting a decrease in nucleosome occupancy. We further identified the presence of “clutch” DNA within a nanoscale distance around the clutches. Upon histone hyperacetylation, the radius of the clutch DNA decreased leading to DNA release from the clutches, consistent with disruption of DNA-histone interactions. Finally, the most dramatic decompaction was observed for groups of clutches in close spatial proximity, suggesting that neighboring clutches influence each other’s DNA compaction.SummarySuper-resolution imaging of histones and DNA reveals that DNA is compacted by groups of nucleosomes – clutches – at the nanoscale level and clutch compaction of DNA is affected by histone tail acetylation especially in highly folded regions containing several nearby clutches.

Cellular systems have evolved numerous mechanisms to finely control signalling pathway activation and properly respond to changing environmental stimuli. This is underpinned by dynamic spatiotemporal patterns of gene expression. Indeed, in addition to gene transcription and translation regulation, modulation of protein levels, dynamics and localization are also essential checkpoints that govern cell functions. The introduction of tetracycline-inducible promoters has allowed gene expression control using orthogonal small molecules, facilitating rapid and reversible manipulation to study gene function in biological systems. However, differing protein stabilities means this solely transcriptional regulation is insufficient to allow precise ON-OFF dynamics, thus hindering generation of temporal profiles of protein levels seenin vivo. We developed an improved Tet-On based system augmented with conditional destabilising elements at the post-translational level that permits simultaneous control of gene expression and protein stability. Integrating these properties to control expression of a fluorescent protein in mouse Embryonic Stem Cells (mESCs), we found that adding protein stability control allows faster response times to changes in small molecules, fully tunable and enhanced dynamic range, and vastly improved microfluidic-basedin-silicofeedback control of gene expression. Finally, we highlight the effectiveness of our dual-input system to finely modulate levels of signalling pathway components in stem cells.

The past few years have witnessed an exponential increase of interest in the reprogramming process. This has been motivated by the enthusiasm of unravelling key aspects not only of cell identity and dedifferentiation, but also of the endogenous regenerative capacities of mammalian organs. Here, we present the most recent advances in the field of reprogramming, stressing how they are re-defining the rules of cell fate and plasticity in vivo. Specifically, we focus on the emerging role of the tissue microenvironment, with particular emphasis on tissue damage, inflammation and senescence that can facilitate in vivo reprogramming and regeneration through cell-extrinsic mechanisms.

CRISPR/dCas9-based labeling has allowed direct visualization of genomic regions in living cells. However, poor labeling efficiency and signal-to-background ratio have limited its application to visualize genome organization using super-resolution microscopy. We developed (Po)STAC (Polycistronic SunTAg modified CRISPR) by combining CRISPR/dCas9 with SunTag labeling and polycistronic vectors. (Po)STAC enhances both labeling efficiency and fluorescence signal detected from labeled loci enabling live cell imaging as well as super-resolution fixed-cell imaging of multiple genes with high spatiotemporal resolution. European Union’s Horizon 2020 Research and Innovation Programme [CellViewer No 686637 to M.L., M.P.C.]; Ministerio de Economia y Competitividad [BFU2013–49867-EXP to M.L., M.P.C.]; Fundació Cellex Barcelona (to M.L); European Union Seventh Framework Programme under the European Research Council Grants [337191-MOTORS to M.L.]; ‘Severo Ochoa’ Programme for Centres of Excellence in R&D [SEV-2015- 0522 to M.L.]; Ministerio de Economia y Competitividad and FEDER Funds [BFU2014–54717-P, BFU2015–71984-ERC to M.P.C.]; AGAUR Grant [2014 SGR1137 to M.P.C.]; Spanish Ministry of Economy and Competitiveness (to M.P.C.); Centro de Excelencia Severo Ochoa [2013–2017 to M.P.C.]; CERCA Programme/Generalitat de Catalunya (to M.P.C); Ministerio de Ciencia e Innovacion FPI (to F.A.); People Program (Marie Curie Actions) FP7/2007–2013 under REA grant [608959 to M.V.N.]. Funding for open access charge: European Union’s Horizon 2020 Research and Innovation Programme [CellViewer No 686637].

Imprinted genes control several cellular and metabolic processes in embryonic and adult tissues. In particular, paternally expressed gene-3 (Peg3) is active in the adult stem cell population and during muscle and neuronal lineage development. Here we have investigated the role of Peg3 in mouse embryonic stem cells (ESCs) and during the process of somatic cell reprogramming towards pluripotency. Our data show that Peg3 knockdown increases expression of pluripotency genes in ESCs and enhances reprogramming efficiency of both mouse embryonic fibroblasts and neural stem cells. Interestingly, we observed that altered activity of Peg3 correlates with major perturbations of mitochondrial gene expression and mitochondrial function, which drive metabolic changes during somatic cell reprogramming. Overall, our study shows that Peg3 is a regulator of pluripotent stem cells and somatic cell reprogramming. We thank Marie Victoire Neguembor, Martina Pesaresi and Sergi Angel Bonilla Pons for suggestions on the manuscript and Umberto Di Vicino for technical support. We thank Angelique di Domenico and the group of Antonella Consiglio for providing us the OPA1, DRP1 and VDAC1 antibodies. We thank Mari Carmen Ortells and the group of Bill Keyes for providing us the OXPHOS antibody. The pInducer10-mir-RUP-PheS plasmid was a gift from Stephen Elledge (Addgene plasmid # 44011). We are grateful for support from Ministerio de Economia y Competitividad and FEDER funds (SAF2011-28580, BFU2014-54717-P, and BFU2015-71984-ERC to M.P.C.), Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat de Catalunya (2014 SGR1137 to M.P.C.), the European Union's Horizon 2020 research and innovation programme under grant agreement CellViewer No 686637 (to M.P.C.), Spanish Ministry of Economy and Competitiveness, 'Centro de Excelencia Severo Ochoa 2013-2017 and the CERCA Programme/Generalitat de Catalunya (to M.P.C.), People Programme Marie Curie Actions of the European Union's Seventh Framework Programme (FP7/2007-2013/, n° 290123 to I.T.), La Caixa international PhD fellowship (to F.S.), Ministerio de Ciencia e Innovació FPI (to F.A.) and Ministerio de Ciencia e Innovación FPI-Severo Ochoa (to A.C.G).

Understanding the mechanisms regulating cell cycle, proliferation and potency of pluripotent stem cells guarantees their safe use in the clinic. Embryonic stem cells (ESCs) present a fast cell cycle with a short G1 phase. This is due to the lack of expression of cell cycle inhibitors, which ultimately determines naïve pluripotency by holding back differentiation. The canonical Wnt/β-catenin pathway controls mESC pluripotency via the Wnt-effector Tcf3. However, if the activity of the Wnt/β-catenin controls the cell cycle of mESCs remains unknown. Here we show that the Wnt-effector Tcf1 is recruited to and triggers transcription of the Ink4/Arf tumor suppressor locus. Thereby, the activation of the Wnt pathway, a known mitogenic pathway in somatic tissues, restores G1 phase and drastically reduces proliferation of mESCs without perturbing pluripotency. Tcf1, but not Tcf3, is recruited to a palindromic motif enriched in the promoter of cell cycle repressor genes, such as p15Ink4b, p16Ink4a and p19Arf, which mediate the Wnt-dependent anti-proliferative effect in mESCs. Consistently, ablation of β-catenin or Tcf1 expression impairs Wnt-dependent cell cycle regulation. All together, here we showed that Wnt signaling controls mESC pluripotency and proliferation through non-overlapping functions of distinct Tcf factors., Author summary Studying how to safely expand stem cells in culture is essential for regenerative medicine applications. Hence there is a clear need to decode how the cell cycle of mouse embryonic stem cells (mESCs) is regulated. Tcf3 and Tcf1 belong to the Tcf family of proteins. Tcf/Lef are effectors of the Wnt/β-catenin pathway and Tcf3 controls mESC pluripotency. Here we identified a recruitment site for Tcf1 embedded into a number of cell cycle repressor genes such as p15Ink4b, p16Ink4a and p19Arf. Tcf1-mediated activation of these genes drastically slows down proliferation of mESCs. In conclusion, here we showed that the Wnt pathway, besides controlling mESC pluripotency via Tcf3, also regulates mESC cell cycle through the recruitment of Tcf1 to the regulatory sites of key cell cycle genes.

Summary In rodents and humans, the liver can efficiently restore its mass after hepatectomy. This is largely attributed to the proliferation and cell cycle re-entry of hepatocytes. On the other hand, bone marrow cells (BMCs) migrate into the liver after resection. Here, we find that a block of BMC recruitment into the liver severely impairs its regeneration after the surgery. Mobilized hematopoietic stem and progenitor cells (HSPCs) in the resected liver can fuse with hepatocytes, and the hybrids proliferate earlier than the hepatocytes. Genetic ablation of the hybrids severely impairs hepatocyte proliferation and liver mass regeneration. Mathematical modeling reveals a key role of bone marrow (BM)-derived hybrids to drive proliferation in the regeneration process, and predicts regeneration efficiency in experimentally non-testable conditions. In conclusion, BM-derived hybrids are essential to trigger efficient liver regeneration after hepatectomy., Graphical Abstract, Highlights • Bone marrow cell migration after liver hepatectomy is key for liver regeneration • Migrated bone marrow cells fuse with hepatocytes • Hybrids are essential for liver regeneration • Mathematical modeling unveils the hybrid function for liver regeneration, Hepatocyte replication is considered the main mechanism of liver regeneration after hepatectomy in mammals. Pedone et al. report that bone marrow cells can migrate into the liver upon resection and fuse with the hepatocytes. The derived hybrids are essential for efficient liver regeneration, which is also predicted by mathematical modeling.

Chromatin fiber organization is essential for gene function in all cell types. Moreover it helps to determine cell fate in embryonic/adult stem cells and in somatic cells undergoing reprogramming to pluripotency. Until now the diffraction limit of light has limited the inspection of the chromatin fiber organization to a level sufficient to understand how it impacts gene function. The development of advanced microscopy methods, such as single molecule localization microscopy, has largely opened a new field of research providing us with the tools to visualize and quantitatively analyze chromatin fiber organization and thus gene activity at nanoscale resolution in single cells. We are grateful for support from Ministerio de Economia y Competitividad and FEDER funds (SAF2011-28580, BFU2014-54717-P, and BFU2015-71984-ERC to MPC, FIS2015-63550-R to ML, BFU2013-49867-EXP to MPC and ML), ERC 337191-MOTORS to ML, AGAUR grant (2014 SGR1137 to MPC), the European Union's Horizon 2020 research and innovation programme under grant agreement CellViewer No. 686637 (to MPC and ML), Spanish Ministry of Economy and Competitiveness, ‘Centro de Excelencia Severo Ochoa 2013–2017 and the CERCA Programme/Generalitat de Catalunya (to MPC).

Sulphatases undergo a unique post-translational modification that converts a highly conserved cysteine located within their active site into formylglycine. This modification is necessary for the catalytic activities of the sulphatases, and it is generated by the protein product of sulphatase-modifying factor 1 (SUMF1), the gene mutated in multiple sulphatase deficiency (MSD). A paralogous gene, SUMF2, was discovered through its sequence similarity to SUMF1. We present evidence that SUMF2 colocalizes with SUMF1 within the endoplasmic reticulum and that the two proteins form heterodimers. SUMF1 and SUMF2 also form homodimers. In addition, SUMF2 is able to associate with the sulphatases with and without SUMF1. We have previously shown that co-transfection of SUMF1 with the sulphatase complementary DNAs greatly enhances the activities of the overexpressed sulphatases. Here, we show that SUMF2 inhibits the enhancing effects of SUMF1 on sulphatases, suggesting that the SUMF1–SUMF2 interaction represents a further level of control of these sulphatase activities.

SummaryCell-fusion-mediated somatic-cell reprogramming can be induced in culture; however, whether this process occurs in mammalian tissues remains enigmatic. Here, we show that upon activation of Wnt/β-catenin signaling, mouse retinal neurons can be transiently reprogrammed in vivo back to a precursor stage. This occurs after their spontaneous fusion with transplanted hematopoietic stem and progenitor cells (HSPCs). Moreover, we demonstrate that retinal damage is essential for cell-hybrid formation in vivo. Newly formed hybrids can proliferate, commit to differentiation toward a neuroectodermal lineage, and finally develop into terminally differentiated neurons. This results in partial regeneration of the damaged retinal tissue, with functional rescue. Following retinal damage and induction of Wnt/β-catenin signaling, cell-fusion-mediated reprogramming also occurs after endogenous recruitment of bone-marrow-derived cells in the eyes. Our data demonstrate that in vivo reprogramming of terminally differentiated retinal neurons after their fusion with HSPCs is a potential mechanism for tissue regeneration.

Vision impairments and blindness caused by retinitis pigmentosa result from severe neurodegeneration that leads to a loss of photoreceptors, the specialized light-sensitive neurons that enable vision. Although the mammalian nervous system is unable to replace neurons lost due to degeneration, therapeutic approaches to reprogram resident glial cells to replace retinal neurons have been proposed. Here, we demonstrate that retinal Müller glia can be reprogrammed in vivo into retinal precursors that then differentiate into photoreceptors. We transplanted hematopoietic stem and progenitor cells (HSPCs) into retinas affected by photoreceptor degeneration and observed spontaneous cell fusion events between Müller glia and the transplanted cells. Activation of Wnt signaling in the transplanted HSPCs enhanced survival and proliferation of Müller-HSPC hybrids as well as their reprogramming into intermediate photoreceptor precursors. This suggests that Wnt signaling drives the reprogrammed cells toward a photoreceptor progenitor fate. Finally, Müller-HSPC hybrids differentiated into photoreceptors. Transplantation of HSPCs with activated Wnt functionally rescued the retinal degeneration phenotype in rd10 mice, a model for inherited retinitis pigmentosa. Together, these results suggest that photoreceptors can be generated by reprogramming Müller glia and that this approach may have potential as a strategy for reversing retinal degeneration. This work was supported by an ERC grant (242630-RERE to M.P. Cosma), the Ministerio de Economia y Competitividad y FEDER (SAF2011-28580, and BFU2015-71984, BFU2015-71984-ERC to M.P. Cosma), an AGAUR grant from Secretaria d´Universitats i Investigació del Departament d´Economia i Coneixement de la Generalitat de Catalunya (2014SGR1137 to M.P. Cosma), 976_Velux Stiftung (to M.P. Cosma), TV3’12_120530_ FUNDACIO MARATO TV3, Ferrer International (to M.P. Cosma and D. Sanges), and Subprograma Juan de la Cierva - Ministerio Ciencia e Innovación (to D. Sanges).