Regulation of germ layer formation by pluripotency factors during embryogenesis

Dynamic methylation pattern of H19DMR and KvDMR1 in bovine oocytes and preimplantation embryos

PURPOSE

This study aimed to evaluate the epigenetic reprogramming of ICR1 (KvDMR1) and ICR2 (H19DMR) and expression of genes controlled by them as well as those involved in methylation, demethylation, and pluripotency.

METHODS

We collected germinal vesicle (GV) and metaphase II (MII) oocytes, and preimplantation embryos at five stages [zygote, 4-8 cells, 8-16 cells, morula, and expanded blastocysts (ExB)]. DNA methylation was assessed by BiSeq, and the gene expression was evaluated using qPCR.

RESULTS

H19DMR showed an increased DNA methylation from GV to MII oocytes (68.04% and 98.05%, respectively), decreasing in zygotes (85.83%) until morula (61.65%), and ExB (63.63%). H19 and IGF2 showed increased expression in zygotes, which decreased in further stages. KvDMR1 was hypermethylated in both GV (71.82%) and MII (69.43%) and in zygotes (73.70%) up to morula (77.84%), with a loss of methylation at the ExB (36.64%). The zygote had higher expression of most genes, except for CDKN1C and PHLDA2, which were highly expressed in MII and GV oocytes, respectively. DNMTs showed increased expression in oocytes, followed by a reduction in the earliest stages of embryo development. TET1 was downregulated until 4-8-cell and upregulated in 8-16-cell embryos. TET2 and TET3 showed higher expression in oocytes, and a downregulation in MII oocytes and 4-8-cell embryo.

CONCLUSION

We highlighted the heterogeneity in the DNA methylation of H19DMR and KvDMR1 and a dynamic expression pattern of genes controlled by them. The expression of DNMTs and TETs genes was also dynamic owing to epigenetic reprogramming.

Exosomes from bovine endometrial epithelial cells ensure trophoblast cell development by miR-218 targeting secreted frizzled related protein 2

Endometritis is a common disease affecting fertility in cows during the perinatal period, which disturbs the molecular milieu of the uterine environment and impairs embryo development and implantation. Exosomes are important extracellular components that transmit a variety of micro RNAs (miRNAs), which perform key regulatory functions. In this study, we investigated plasma exosomal miRNAs from cows with endometritis and from cultured endometrial epithelial cells (EECs) challenged with lipopolysaccharide (LPS) to explore the role of EEC-derived exosomes and their miRNAs in bovine endometritis. Plasma exosomes were collected from nine healthy dairy cows and nine dairy cows with endometritis, and culture supernatant exosomes were isolated from EECs challenged with or without LPS. Exosomal RNA was extracted using commercial kits and miRNA profiles were generated using RNA-seq. We found that miR-218 was differentially expressed in EECs under conditions of endometrial inflammation. Inhibition studies suggested that reduced levels of miR-218 in EEC-derived exosomes when transferred into placental trophoblast cells impaired embryonic development and decreased placental trophoblast cell migration by targeting secreted frizzled related protein 2. We propose that exosomal miR-218 secreted from EECs acts as a driver of embryonic development and differentiation. In addition, exosomal miR-218 may provide a valuable diagnostic marker for bovine endometritis.

NANOG is required to form the epiblast and maintain pluripotency in the bovine embryo

During preimplantation development, the embryo undergoes two consecutive lineages specifications. The first cell fate decision determines which cells give rise to the trophectoderm (TE) and the inner cell mass (ICM). Subsequently, the ICM differentiates into hypoblast and epiblast, the latter giving rise to the embryo proper. The transcription factors that govern these cell fate decisions have been extensively studied in the mouse, but are still poorly understood in other mammalian species. In the present study, the role of NANOG in the formation of the epiblast and maintenance of pluripotency in the bovine embryo was investigated. Using a CRISPR-Cas9 approach, guide RNAs were designed to target exon 2, resulting in a functional deletion of bovine NANOG at the zygote stage. Disruption of NANOG resulted in the embryos that form a blastocoel and an ICM composed of hypoblast cells. Furthermore, NANOG-null embryos showed lower expression of epiblast cell markers SOX2 and HA2AFZ, and hypoblast marker GATA6; without affecting the expression of TE markers CDX2 and KRT8. Results indicate that NANOG, has no apparent role in segregation or maintenance of the TE, but it is required to derive and maintain the pluripotent epiblast and during the second lineage commitment in the bovine embryo.

Tcf7l1 promotes transcription of Kruppel-likefactor 4 during Xenopus embryogenesis

Kruppel-like factor 4 (Klf4) is a zinc finger transcriptionfactor and plays crucial roles in Xenopus embryogenesis. However, its regulation during embryogenesis is stillunclear. Here, we report that Tcf7l1, a key downstream transducerof the Wnt signaling pathway, could promote Klf4 transcription and stimulate Klf4 promoter activity in early Xenopus embryos. Furthermore, cycloheximide treatmentshowed a direct effect on Klf4 transcriptionfacilitated by Tcf7l1. Moreover, the dominant negative form of Tcf7l1(dnTcf7l1), which lacks N-terminusof the β-catenin binding motif, could still activate Klf4 transcription, suggesting that thisregulation is Wnt/β-catenin independent. Taken together, ourresults demonstrate that Tcf7l1 lies upstream of Klf4 to maintainits expression level during Xenopus embryogenesis.

Ascl1 represses the mesendoderm induction in Xenopus

Ascl1 is a multi-functional regulator of neural development in invertebrates and vertebrates. Ectopic expression of Ascl1 can generate functional neurons from non-neural somatic cells. The abnormal expression of ASCL1 has been reported in several types of carcinomas. We have previously identified Ascl1 as a crucial maternal regulator of the germ layer pattern formation in Xenopus Functional studies have indicated that the maternally-supplied Ascl1 renders embryonic cells a propensity to adopt neural fates on one hand, and represses the mesendoderm formation on the other. However, it remains unclear how Ascl1 achieves its repressor function during the activation of mesendoderm genes by VegT. Here, we performed series of gain- and loss-of-function experiments and found that: (i) VegT, the maternal mesendoderm determinant in Xenopus, is required for the deposition of H3K27ac and H3K9ac at its target gene loci during mesendoderm induction; (ii) Ascl1 and VegT antagonistically modulate the deposition of acetylated histone marks at mesendoderm gene loci; (iii) Ascl1 overexpression reduces the VegT-occupancy at mesendoderm gene loci; (iv) Ascl1 but not Neurog2 possesses a repressive activity during mesendoderm induction. These findings reveal a novel repressive function for Ascl1 in inhibiting non-neural fates during early Xenopus embryogenesis.

A novel role for Ascl1 in the regulation of mesendoderm formation via HDAC-dependent antagonism of VegT

Maternally expressed proteins function in vertebrates to establish the major body axes of the embryo and to establish a pre-pattern that sets the stage for later-acting zygotic signals. This pre-patterning drives the propensity of Xenopus animal cap cells to adopt neural fates under various experimental conditions. Previous studies found that the maternally expressed transcription factor, encoded by the Xenopus achaete scute-like gene ascl1, is enriched at the animal pole. Asc1l is a bHLH protein involved in neural development, but its maternal function has not been studied. Here, we performed a series of gain- and loss-of-function experiments on maternal ascl1, and present three novel findings. First, Ascl1 is a repressor of mesendoderm induced by VegT, but not of Nodal-induced mesendoderm. Second, a previously uncharacterized N-terminal domain of Ascl1 interacts with HDAC1 to inhibit mesendoderm gene expression. This N-terminal domain is dispensable for its neurogenic function, indicating that Ascl1 acts by different mechanisms at different times. Ascl1-mediated repression of mesendoderm genes was dependent on HDAC activity and accompanied by histone deacetylation in the promoter regions of VegT targets. Finally, maternal Ascl1 is required for animal cap cells to retain their competence to adopt neural fates. These results establish maternal Asc1l as a key factor in establishing pre-patterning of the early embryo, acting in opposition to VegT and biasing the animal pole to adopt neural fates. The data presented here significantly extend our understanding of early embryonic pattern formation.

Summary: The proneural factor ASCL1 recruits HDAC1 to repress VegT-induced, but not Nodal-induced, mesendoderm formation via a previously uncharacterized N-terminal domain.

[Pluripotency and induced nuclear reprogramming in vertebrates: new perspectives]

Pluripotency is a transitory state during vertebrate development. A pluripotent cell can theoretically acquire all cell fates of the organism. During ontogenetic dynamics, loss of pluripotency is associated with a progressive acquisition of a specific genetic program, which is determined both by instructions received and by cell position in the whole organism. Pluripotent embryonic stem cells can be isolated and cultured in vitro indefinitely. Using mammalian embryonic stem cells (ESCs), it has been possible to identify the factors involved in the establishment and maintenance of pluripotency state. In this review, we will describe recent scientific advances in the understanding of pluripotency, the molecular actors involved in such a regulation and their functional conservation during evolution. We shall focus on new concepts, obtained from the study of vertebrate model organisms, to shed light on the cell transition from pluripotency to differentiated state, and shall recapitulate fundamental and clinical applications of pluripotent cells, of "somatic cell nuclear transfer" (SCNT), of induced nuclear reprogramming in vitro and future perspectives of in vivo applications. Our results, in the xenopus, concerning the first in vivo induced nuclear reprogramming might open new perspectives about the understanding of cell plasticity in an integrated context. Our analyses sought to encourage new and alternative clinical approaches to achieve in situ tissue regeneration.

Embryonic stem cells shed new light on the developmental roles of p53

The viability and subtle developmental defects of p53 knockout mice suggest that p53 does not play major role in development. However, contradictory evidence also exists. This discrepancy mainly results from the lack of molecular and cellular mechanisms and the general fact that p53 activation requires stresses. Recent studies of p53 in mouse and human ES cells and induced pluripotent stem (iPS) cells shed new light on the mechanisms of the developmental roles of p53. This review summarizes these new studies that support the developmental roles of p53, highlights the possible underlying molecular mechanisms, and discusses the potential relationship between the developmental roles and the tumor suppressive function of p53. In summary, the molecular mechanisms underlying the developmental roles of p53 are emerging, and the developmental roles and tumor suppressive function of p53 may be closely related.