The PerpleXing Germ Line: Why and How to Turn a Chromosome OFF and ON Again, Dr Bernhard Payer, Centre for Genomic Regulation (CRG), Barcelona, Spain

Abstract: The mammalian germline is characterised by extensive epigenetic reprogramming during development into eggs and sperm. Specifically, the epigenome requires resetting before parental marks can be established and transmitted to the next generation. In the female germline, X-chromosome inactivation and reactivation are among the most prominent epigenetic reprogramming events, yet little is known about their kinetics and biological function.

Here I present how we study X-inactivation and reactivation dynamics using an in vitro system of primordial germ cell-like cell (PGCLC) differentiation in ovarian organoids. We find that X-inactivation in PGCLCs is moderate compared to somatic cells, and characterised by many genes escaping X-inactivation. Subsequently we observe step-wise X-reactivation, which is mostly completed during meiotic prophase I. Importantly, we find that PGCLCs, which fail X-inactivation or reactivate too rapidly display impaired meiotic potential. Now we are testing, if X-dosage control is a direct functional requirement or rather a diagnostic mark for proper germ cell differentiation. It appears that active and inactive X-chromosome states present stage-specific constraints that are crucial for the normal development of female germ cells towards meiosis and oogenesis.

Biography: In my lab we study the importance of epigenetic reprogramming for pluripotency and germ cell development. A major focus has been the iPSC-reprogramming system, with which we identified molecular regulators and the role of 3D-genome structure for X-chromosome reactivation (Bauer, Nature Comm 2021; Generoso, PNAS 2023, Barrero, BioRxiv 2023).

The other main area of interest concerns female germ cell development, where we identified PRDM14 as important for X-reactivation in mice (Mallol, Epigenetics & Chromatin 2019). Using in vitro derived ovarian organoids from mouse ESCs, we found that the correct X-chromosome dynamics to be a critical indicator of meiotic and oogenic potential (Severino, EMBO J 2022). In collaboration with the clinic, we generated patient-specific hiPSCs for human in vitro germ cell modeling and studied human oocyte aging by single-cell RNA-Seq (Llonch, Aging Cell 2021)

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Transcriptional activation by pioneer transcription factors in mammalian pre-implantation development, Dr Wataru Kobayashi, MPI of Biochemistry, Bayern, Germany

Abstract: Following fertilization, the totipotency is gradually decreased during cleavage divisions until reaching a pluripotency or differentiated state. In this process, transcription factors (TFs) play crucial roles in successful pre-implantation development. Notably, specialized TFs, called pioneer transcription factors (pTFs) have unique abilities to modulate epigenetic and chromatin states by recruiting chromatin remodelers and histone modifiers. Mammalian embryos are initially awakened during zygotic genome activation (ZGA) and further develop into blastocysts including three cell lineages. However, how pTFs/TFs control transcriptional regulation to achieve proper pre-implantation development is largely unknown.

Orphan nuclear receptor Nr5a2 is expressed in murine oocytes and embryos, but its function during pre-implantation development is unknown. Leveraging the low-input genomic method, we discovered that Nr5a2 functions as a pTF that opens the chromatin during ZGA in mouse embryos (Gassler*, Kobayashi* et al., Science, 2022). We recently found that the chromatin binding of Nr5a2 is dynamically changed during the totipotency-to-pluripotency transition (Kobayashi et al., unpublished). We further determined the cryo-EM structure of the human NR5A2-nucleosome complex and revealed its pioneering activity at the atomic resolution (Kobayashi et al., bioRxiv, 2023). Taken together, my works opened new opportunities for understanding the function of TF in mammalian embryos.

Biography: My broad interest is how transcription factors control transcriptional networks to determine cell potentials and fate during mammalian development. I have gained expertise in biochemistry and structural biology through my Ph.D. and postdoctoral research in the laboratory of Dr. Hitoshi Kurumizaka at Waseda University. I am a postdoctoral fellow in Dr. Kikuë Tachibana’s group at the Max Planck Institute of Biochemistry. I expanded my expertise in embryology and genomics and combined them with biochemical & structural approaches to uncover the molecular mechanism of transcriptional regulations in mammalian pre-implantation embryos.