Kawamura, Y. K. | bioRxiv Science

bioRxiv2023

Comprehensive comparison of female germ cell development in vitro and in vivo identifies epigenetic gene regulation crucial for oocyte development and embryonic competence

Germ cells are the origin of new individuals. Hence, specifying germ cell identity is crucial for reproduction. The recent establishment of in vitro culture systems for generating oocytes from mouse pluripotent stem cells provides a basis for progress in studies of oogenesis and reproductive technology. However, currently the developmental competence of in vitro generated oocytes is low compared to in vivo grown oocytes. The causes underlying poor oocyte quality remain to be determined. By reconstituting germ cell development in culture from different developmental starting points within gametogenesis, we show that the differentiation of primordial germ cells (PGCs) and primordial germ cell-like cells (PGCLCs) to growing oocytes (GROs), as well as the subsequent growth of follicles are critical culture steps for specifying competence of fully-grown oocytes (FGOs) for preimplantation development. A systematic comparison of transcriptomes of single oocytes having undergone different in vitro culture trajectories identifies genes normally upregulated during oocyte growth to be susceptible for mis-regulation during in vitro oogenesis. Many of such genes have been described as targets of Polycomb repressive complexes (PRCs). Deregulation of Polycomb repression therefore likely perturbs the accumulation of cytoplasmic factors and/or setting of chromatin states in FGOs that are required for embryonic development after fertilization. Conversely, in vitro derived oocytes often displayed failure of zygotic genome activation (ZGA) and abnormal acquisition of 5-hydroxymethylcytosine (5hmC) on maternal chromosomes after activation. In addition, subcellular delocalization of pyruvate dehydrogenase (PDH) and of STELLA were observed suggesting new molecular markers for defective oocyte development. Our study identifies epigenetic regulation at an early stage of oogenesis as crucial for developmental competence and suggests specific in vitro culture steps as targets for improving oocyte quality. HighlightsO_LISingle cell transcriptomics and functional assessment of oocyte development from pluripotent stem cells in culture in a stage-specific manner provides a comprehensive resource for comparisons to oogenesis in vivo. C_LIO_LICulture steps for growth and differentiation of reconstituted follicles are critical for defining embryonic competence of in vitro generated oocytes. C_LIO_LIZygotic genome activation failure and epigenetic impairment are hallmarks of in vitro-generated oocytes that fail to develop after activation or fertilization. C_LIO_LIComputational analysis of gene expression changes and chromatin modification patterns identifies specific gene sets that indicate that Polycomb mediated repression is vulnerable during in vitro folliculogenesis. C_LI

developmental biology↗

bioRxiv2023

H3K27me3 dictates atypical genome-nuclear lamina interactions and allelic asymmetry during early embryogenesis

The very first days of mammalian embryonic development are accompanied by epigenetic reprogramming and extensive changes in nuclear organization. In particular, genomic regions located at the periphery of the nucleus, termed lamina-associated domains (LADs), undergo major rearrangements after fertilization. However, the role of LADs in regulating gene expression as well as the interplay with various chromatin marks during preimplantation development remains elusive. In this study, we obtained single-cell LAD profiles coupled with the corresponding gene expression readout throughout the first days of mouse development. We detect extensive cell-cell LAD variability at the 2-cell stage, which surprisingly does not seem to functionally affect gene expression. This suggests an unusual uncoupling between 3D-nuclear genome organization and gene expression during totipotent developmental stages. By analyzing LAD dynamics and chromatin states across early developmental stages in an allelic-specific manner, we identify genomic regions that transiently detach from the nuclear lamina and are enriched by non-canonical H3K27me3. Upon maternal knock-out of a component of the Polycomb repressive complex 2 and concomitant loss of H3K27me3 during early embryogenesis, these regions relocate to the lamina at the 2-cell stage. Our results suggest that H3K27me3 is the prime determinant in establishing the atypical distribution of the genome at the nuclear periphery during the first days of embryonic development. This study provides insight into the molecular mechanisms regulating nuclear organization of parental genomes during very early mammalian development.

genomics↗