Project description:MicroRNAs (miRNAs) have essential functions during embryonic development, and their dysregulation causes cancer. Altered global miRNA abundance is found in different tissues and tumours, which implies that precise control of miRNA dosage is important[134], but the underlying mechanism(s) of this control remain unknown. The protein complex Microprocessor, which comprises one DROSHA and two DGCR8 proteins, is essential for miRNA biogenesis. Here we identify a developmentally regulated miRNA dosage control mechanism that involves alternative transcription initiation (ATI) of DGCR8. ATI occurs downstream of a stem-loop in DGCR8 mRNA to bypass an autoregulatory feedback loop during mouse embryonic stem cell (mES cell) differentiation. Deletion of the stem-loop causes imbalanced DGCR8:DROSHA protein stoichiometry that drives irreversible Microprocessor aggregation, reduced primary miRNA processing, decreased mature miRNA abundance, and widespread de-repression of lipid metabolic mRNA targets. Although global miRNA dosage control is not essential for mES cells to exit from pluripotency, its dysregulation alters lipid metabolic pathways and interferes with embryonic development by disrupting germ layer specification in vitro and in vivo. This miRNA dosage control mechanism is conserved in humans. Our results identify a promoter switch that balances Microprocessor autoregulation and aggregation to precisely control global miRNA dosage and govern stem cell fate decisions during early embryonic development.
Project description:Differentiation of mouse embryonic stem cells (mESCs) is accompanied by global changes in replication timing. To elucidate this reorganization process and explore its potential impact on mouse development, we constructed genome-wide replication-timing profiles of 15 independent mouse cell types representing nine different stages of early mouse development, including all three germ layers. Overall, 45% of the genome exhibits significant changes in replication timing between cell types, indicating that replication-timing regulation is more extensive than previously estimated from neural differentiation. Intriguingly, analysis of early and late epiblast cell culture models suggest that the earliest changes in development include extensive lineage-independent early-to-late replication switches that are completed at a stage equivalent to the post-implantation epiblast, prior to germ layer specification and down-regulation of key pluripotency transcription factors (Oct4/Nanog/Sox2). These changes were stable in all subsequent lineages and involved a class of irreversibly silenced genes that were re-positioned closer to the nuclear periphery. Lineage-specific, late-to-early and early-to-late replication switches followed, which created cell-type specific replication profiles. Importantly, partially reprogrammed induced pluripotent stem cells (piPSCs) failed to restore ESC-specific replication timing and transcription programs particularly within regions of lineage-independent early-to-late replication changes, as well as the inactive X-chromosome. We conclude that lineage-independent, early-to-late replication-timing switches that occur in the post-implantation epiblast embody an epigenetic commitment to differentiation prior to germ layer specification. 22 cell lines, with a total of 36 individual replicates (i.e. 14 in duplicates, 8 in single replicates)
Project description:In gastrulation, distinct progenitor cell populations are induced and sorted into the three germ layers ectoderm, mesoderm and endoderm. In order to identify genes involved in germ layer specification and morphogenesis, we identified genes differentially expressed between ectodermal and mesendodermal progenitor cells. To do so, we first generated highly enriched pools of ectodermal and mesendodermal progenitor cells. Mesendodermal cells were generated by over-expressing the Nodal signal Cyclops in wild type embryos and ectodermal cells were taken from mz-one-eyed-pinhead (oep) mutant embryos. We then compared the transcriptome of ectodermal versus mesendodermal cells taken from embryos at 7 hours post fertilization (hpf). In wild type embryos at this stage (70% epiboly), the first ectodermal and mesendodermal progenitor cells have already been sorted into their respective germ layers and ingression of mesendodermal progenitors is still ongoing.
Project description:Differentiation of mouse embryonic stem cells (mESCs) is accompanied by global changes in replication timing. To elucidate this reorganization process and explore its potential impact on mouse development, we constructed genome-wide replication-timing profiles of 15 independent mouse cell types representing nine different stages of early mouse development, including all three germ layers. Overall, 45% of the genome exhibits significant changes in replication timing between cell types, indicating that replication-timing regulation is more extensive than previously estimated from neural differentiation. Intriguingly, analysis of early and late epiblast cell culture models suggest that the earliest changes in development include extensive lineage-independent early-to-late replication switches that are completed at a stage equivalent to the post-implantation epiblast, prior to germ layer specification and down-regulation of key pluripotency transcription factors (Oct4/Nanog/Sox2). These changes were stable in all subsequent lineages and involved a class of irreversibly silenced genes that were re-positioned closer to the nuclear periphery. Lineage-specific, late-to-early and early-to-late replication switches followed, which created cell-type specific replication profiles. Importantly, partially reprogrammed induced pluripotent stem cells (piPSCs) failed to restore ESC-specific replication timing and transcription programs particularly within regions of lineage-independent early-to-late replication changes, as well as the inactive X-chromosome. We conclude that lineage-independent, early-to-late replication-timing switches that occur in the post-implantation epiblast embody an epigenetic commitment to differentiation prior to germ layer specification.
Project description:The mechanism for human germ-cell specification, which sets out a complex program generating spermatozoa or oocytes, remains largely unknown. We established a system for inducing human primordial germ cell-like cells (hPGCLCs) from induced pluripotent stem cells (hiPSCs) via incipient mesoderm-like cells (iMeLCs). Here, we show that EOMESODERMIN (EOMES) elevates in iMeLCs through WNT signaling and is essential for activating SOX17, a key driver for hPGC(LC) specification, with the duration/dosage of the WNT signaling/EOMES expression dictating the germ-cell competence. Upon hPGCLC induction, BMP signaling activates TFAP2C independently from SOX17, and SOX17 and TFAP2C instate the hPGCLC program, including BLIMP1 expression, in an interdependent fashion. The hPGC(LC) specification program is highly consistent with the monkey program, but is divergent from the mouse one regarding key transcription factors and their hierarchy of actions. These findings delineate evolutionary divergence of mammalian germ-cell specification, providing a foundation for further human germ-cell development in vitro.
Project description:In recent years, it has become clear that membrane-less structures formed through phase separation represent an important class of subcellular compartmentalization. However, little is known about how the formation or disassembly of such compartments is regulated. The Balbiani body (Bb) is a phase-separated structure essential for germ cell specification and home to many germ cell-specific mRNAs and proteins. Throughout oogenesis, the structure assembles all critical components required for germ cell induction upon fertilization, then referred to as germ plasm (Gp). In zebrafish, formation of the Bb requires Bucky ball (Buc), a protein with prion-like properties. We have found that Tdrd6a interacts directly with Buc and regulates its aggregation into a functional Bb. Without Tdrd6a, the morphology and mobility of the Bb and the Gp are severely affected, ultimately resulting in PGC depletion in the progeny. Since germ cells are rich in phase-separated structures, many more of such regulatory interactions exist.
Project description:The germ layer concept has been one of the foremost organizing principles in developmental biology, classification, systematics and evolution for 150 years. Of the three germ layers, the mesoderm is found in bilaterian animals but is absent in species in the phyla Cnidaria and Ctenophora, which has been taken as evidence that the mesoderm was the final germ layer to evolve. The origin of the ectoderm and endoderm layers, however, remains unclear with models supporting the antecedence of each as well as a simultaneous origin. We hypothesized that global analysis of gene expression in each layer throughout development may resolve the early events of animal evolution. Here, we determine the temporal and spatial components of gene expression spanning embryonic development for all C. elegans genes and use it to determine the evolutionary age of the germ layers. The gene expression programs of the mesoderm is generally induced after the ectoderm and endoderm germ layers, thus making it the last germ layer to both evolve and develop. Strikingly, the C. elegans endoderm and ectoderm expression programs do not co-induce; rather the endoderm activates earlier. We also observed early expression of endoderm orthologs during the embryology of Xenopus tropicalis, Nematostella vectensis, and the sponge Amphimedon queenslandica. Querying for the phylogenetic ages of specifically expressed genes revealed that the endoderm is comprised of older genes, supporting its antecedence among the germ layers. Taken together, we propose that the endoderm program has retained the feeding functions of the last common ancestor with the choanoflagellates, thus allowing for the specialization of an ectoderm germ layer. Our work reveals that the evolutionary appearance of the germ layers continues to constrain regulatory networks in metazoans. Two temporal assays of C. elegans embryonic development, starting at the zygote: (a) Embryos collected at fixed (~10 minute) time intervals. (b) Embryo segregates, up to five lines of blastomeres, isolated in reference to mitotic events. There were 184 samples in total, representing 100 distnict data points.