The emergence of the first three lineages during development is orchestrated by a network of transcription factors, which are best characterized in mice. However, the role and regulation of these factors are not completely conserved in other mammals, including human and cattle. Here, we establish a gene inactivation system with a robust efficiency by introducing premature codon with cytosine base editors in bovine early embryos. By using this approach, we have determined the functional consequences of three critical lineage-specific genes (SOX2, OCT4 and CDX2) in bovine embryos. In particular, SOX2 knockout results in a failure of the establishment of pluripotency in blastocysts. Indeed, OCT4 level is significantly reduced and NANOG barely detectable. Furthermore, the formation of primitive endoderm is compromised with few SOX17 positive cells. RNA-seq analysis of single blastocysts (day 7.5) reveals dysregulation of 2074 genes, among which 90% are up-regulated in SOX2-null blastocysts. Intriguingly, more than a dozen lineage-specific genes, including OCT4 and NANOG, are down-regulated. Moreover, SOX2 level is sustained in the trophectoderm in absence of CDX2. However, OCT4 knockout does not affect the expression of SOX2. Overall, we propose that SOX2 is indispensable for OCT4 and NANOG expression and CDX2 represses the expression of SOX2 in the trophectoderm in cattle, which are all in sharp contrast with results in mice.

The maternal nucleolus plays an indispensable role in zygotic genome activation (ZGA) and early embryonic development in mice. During oocyte-to-embryo transition, the nucleolus is subject to substantial transformation. Despite the primary role of the nucleolus is ribosome biogenesis, accumulating evidence has uncovered its functions in various other cell processes. However, the regulation of nucleolar maturation and ribosome biogenesis and the molecules involved remain unclear during early embryonic development. In this study, we observed that nucleolar protein 2 (NOP2) is restrictedly localized within the nucleolus, first detected in the late two-cell embryos, and increases to a peak level at the eight-cell stage in mice. RNAi-mediated NOP2 depletion leads to a developmental arrest during the morula-to-blastocyst transition. RNA-seq analyses reveal that 208 genes are differentially expressed, including multiple lineage-specific genes and several genes encoding ribosome proteins.Indeed, we observe a failure of the first lineage specification with reduced TEA domain transcription factor 4(TEAD4) (trophectoderm-specific), tir na nog (NANOG), and kruppel-like factor 4 (KLF4) (inner cell mass-specific). Importantly, by Transm ission Electron Microscopy (TEM), we noted a decrease in the ratio of the nucleolus size and an increase in the ratio of the size of the nucleolus precursor body, suggesting the nucleolar maturation is disrupted. Moreover, both qPCR and Fluorescence I n Situ Hybridization (FISH) data showcase a significant decrease in the abundance of ribosome RNAs. Similarly, NOP2 depletion causes reduced developmental potential and decreased rRNA level in bovine early embryos, suggesting a functional 2716 |

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Current understandings of the initiation of the trophectoderm (TE) program during mammalian embryonic development lack evidence of how TE-associated factors such as GATA3 and CDX2 participate in bovine lineage specification. In this study, we describe effects of TE-associated factors on the expression of lineage specification marker genes such as SOX2, OCT4, NANOG, GATA6 and SOX17, by using cytosine base editor system. We successfully knockout GATA3 or CDX2 in bovine embryos with robust efficiency. However, GATA3 or CDX2 deletion does not affect the developmental potential of embryos to reach blastocyst stage. Interestingly, GATA3 deletion downregulates the NANOG expression in bovine blastocysts. Further analysis of the mosaic embryos shows that GATA3 is required for NANOG in the TE of bovine blastocysts. Single blastocyst RNA-seq analysis reveals that GATA3 deletion disrupts the transcriptome in bovine blastocysts. Altogether, we propose that GATA3 plays an important role in maintaining TE lineage program in bovine embryos and the functional role of GATA3 is species-specific.

The newly identified liver-enriched gene 1 (LEG1) encodes a protein with a characteristic domain of unknown function 781 (DUF781/LEG1), constituting a protein family with only one member in mammals. A functional study in zebrafish suggested that LEG1 genes are involved in liver development, while the platypus LEG1 homolog, Monotreme Lactation Protein (MLP), which is enriched in the mammary gland and milk, acts as an antibacterial substance. However, no functional studies on eutherian LEG1s have been published to date. Thus, we here report the first functional prediction study at the cellular level. As previously reported, eutherian LEG1s can be classified into three paralogous groups. Pigs have all three LEG1 genes (pLEG1s), while humans and mice have retained only LEG1a. Hence, pLEG1s might represent an ideal model for studying LEG1 gene functions. RNA-seq was performed by the overexpression of pLEG1s and platypus MLP in HepG2 cells. Enrichment analysis showed that pLEG1a and pLEG1b might exhibit little function in liver cells; however, pLEG1c is probably involved in the endoplasmic reticulum (ER) stress response and protein folding. Additionally, gene set enrichment analysis revealed that platypus MLP shows antibacterial activity, confirming the functional study in platypus. Therefore, our study showed from the transcriptomic perspective that mammalian LEG1s have different functions in liver cells due to the subfunctionalization of paralogous genes.

The emergence of the first three lineages during development are orchestrated by a network of transcription factors, which are best characterized in mice. However, the role and regulation of these factors are not completely conserved in other mammals, including human and cattle. Here, we establish a gene inactivation system by introducing premature codon with cytosine base editor in bovine embryos with a robust efficiency. Of interest, SOX2 is universally localized in early blastocysts but gradually restricted into the inner cell mass in cattle. SOX2 knockout results in a failure of the establishment of pluripotency. Indeed, OCT4 level is significantly reduced and NANOG was barely detectable. Furthermore, the formation of primitive endoderm is compromised with few SOX17 positive cells. Single embryo RNA-seq reveals a dysregulation of 2074 genes, among which 90% are up-regulated in SOX2-null blastocysts. Intriguingly, more than a dozen lineage-specific genes, including OCT4 and NANOG, are down-regulated. Moreover, SOX2 expression is sustained in the trophectoderm in absence of CDX2 in bovine late blastocysts. Overall, we propose that SOX2 is dispensable for OCT4 and NANOG expression and disappearance of SOX2 in the trophectoderm depends on CDX2 in cattle, which are all in sharp contrast with results in mice.SignificanceThe first and second cell fate decisions of a new life are important for subsequent embryonic and plancental development. These events are finely controlled by a network of transcriptional factors, which are extensively characterized in mice. Species-specific roles of these proteins are emerging in mammals. Here, we develop a gene loss-of-function system by using cytosine base editors in bovine embryos. We find that expression pattern, functional roles, and regulation of SOX2 are all different between mouse and bovine embryos. Remarkbly, SOX2 is required for OCT4 and NANOG, two well established pluripoteny genes. Furthermore, CDX2 is required to shut down SOX2 in the trophectoderm. Given similar expression pattern of SOX2 between human and bovine blastocysts, bovine embryos represents a putative model to investigate human pluripotency regulation in vivo.