We have developed a method to produce live somatic clones in the rabbit, one of the mammalian species considered up to now as difficult to clone. To do so, we have modified current cloning protocols proven successful in other species by taking into account both the rapid kinetics of the cell cycle of rabbit embryos and the narrow window of time for their implantation after transfer into foster recipients. Although our method still has a low level of efficiency, it has produced several clones now proven to be fertile. Our work indicates that cloning can probably be carried out successfully in any mammalian species by taking into account physiological features of their oocytes and embryos. Our results will contribute to extending the use of rabbit models for biomedical research.
BackgroundEmbryonic development proceeds through finely tuned reprogramming of the parental genomes to form a totipotent embryo. Cells within this embryo will then differentiate and give rise to all the tissues of a new individual. Early embryonic development thus offers a particularly interesting system in which to analyze functional nuclear organization. When the organization of higher-order chromatin structures, such as pericentromeric heterochromatin, was first analyzed in mouse embryos, specific nuclear rearrangements were observed that correlated with embryonic genome activation at the 2-cell stage. However, most existing analyses have been conducted by visual observation of fluorescent images, in two dimensions or on z-stack sections/projections, but only rarely in three dimensions (3D).ResultsIn the present study, we used DNA fluorescent in situ hybridization (FISH) to localize centromeric (minor satellites), pericentromeric (major satellites), and telomeric genomic sequences throughout the preimplantation period in naturally fertilized mouse embryos (from the 1-cell to blastocyst stage). Their distribution was then analyzed in 3D on confocal image stacks, focusing on the nucleolar precursor bodies and nucleoli known to evolve rapidly throughout the first developmental stages. We used computational imaging to quantify various nuclear parameters in the 3D-FISH images, to analyze the organization of compartments of interest, and to measure physical distances between these compartments.ConclusionsThe results highlight differences in nuclear organization between the two parental inherited genomes at the 1-cell stage, i.e. just after fertilization. We also found that the reprogramming of the embryonic genome, which starts at the 2-cell stage, undergoes other remarkable changes during preimplantation development, particularly at the 4-cell stage.
In mammalian embryos, zygotic gene transcription initiates after a limited number of cell divisions through a two-step process termed the zygotic gene activation (ZGA). Here we report that RNA polymerase II undergoes major changes in mouse and rabbit preimplantation embryos during the ZGA. In transcriptionally inactive unfertilized oocytes, the RNA polymerase II largest subunit is predominantly hyperphosphorylated on its carboxy-terminal domain (CTD). The CTD is markedly dephosphorylated several hours after fertilization, before the onset of a period characterized by a weak transcriptional activity. The largest subunit of RNA polymerase II then lacks immunological and drug-sensitivity characteristics related to its phosphorylation by the TFIIH-associated kinase and gradually translocates into the nuclei independently of DNA replication and mitosis. A phosphorylation pattern of the largest subunit, close to that observed in somatic cells, is established in both mouse and rabbit embryos at the stage when transcription becomes a requirement for further development (respectively at the 2-and 8/16-cell stage). As these events occurred in the presence of actinomycin D, the nuclear translocation of RNA polymerase II and the phosphorylation of the CTD might be major determinants of ZGA.
Mice have recently been successfully cloned from embryonic stem (ES) cells. However, these fast dividing cells provide a heterogeneous population of donor nuclei, in terms of cell cycle stage. Here we used metaphases as a source of donor nuclei because they offer the advantage of being both unambiguously recognizable and synchronous with the recipient metaphase II oocyte. We showed that metaphases from ES cells can provide a significantly higher development rate to the morula or blastocyst stage (56--70%) than interphasic nuclei (up to 28%) following injection into a recipient oocyte. Selective detachment of mitotic cells after a demecolcin treatment greatly facilitates and accelerates the reconstruction of embryos by providing a nearly pure population of cells in metaphase and did not markedly affect the developmental rate. Most of the blastocysts obtained by this procedure were normal in terms of both morphology and ratio of inner cell mass and total cell number. After transfer into pseudopregnant recipients at the one- or two-cell stage, the ability of metaphase to be fully reprogrammed was demonstrated by the birth of two pups (1.5% of activated oocytes). Although the implantation rate was quite high (up to 32.9% of activated oocytes), the postimplantation development was characterized by a high and rapid mortality. Our data provide a clear situation to explore the long-lasting effects that can be induced by early reprogramming events.
The mouse HSP70.1 gene, which codes for a heat shock protein (hsp70), is highly transcribed at the onset of zygotic genome activation (ZGA). This expression, which occurs in the absence of stress, is then repressed. It has been claimed that this gene does not exhibit a stress response until the blastocyst stage. The promoter of HSP70.1 contains four heat shock element (HSE) boxes which are the binding sites of heat shock transcription factors (HSF). We have been studying the presence and localization of the mouse HSFs, mHSF1 and mHSF2, at different stages of embryo development. We show that mHSF1 is already present at the one-cell stage and concentrated in the nucleus. Moreover, by mutagenizing HSE sequences and performing competition experiments (in transgenic embryos with the HSP70.1 promoter inserted before a reporter gene), we show that, in contrast with previous findings, HSE boxes are involved in this spontaneous activation. Therefore, we suggest that HSF1 and HSE are important in this transient expression at the two-cell stage and that the absence of typical inducibility at this early stage of development results mainly from the high level of spontaneous transcription of this gene during the ZGA.All organisms respond to proteotoxic stress (heat shock or toxic agent exposure) by the synthesis of a group of proteins called heat shock proteins. Heat shock proteins are classified into different families on the basis of molecular mass (20, 70, and 90 kDa) and distinguished according to their inducibility: some members of heat shock families, such as heat shock cognate (hsc), are constitutively synthesized, whereas others (hsp) are expressed only following stress. Heat shock proteins interact with numerous other proteins, and their main function is the control of the accurate folding and translocation of polypeptides in the different cellular compartments (reviewed by Parsell and Lindquist [38].
Mouse embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) represent naive and primed pluripotency states, respectively, and are maintained in vitro by specific signalling pathways. Furthermore, ESCs cultured in serum-free medium with two kinase inhibitors (2i-ESCs) are thought to be the ground naïve pluripotent state. Here, we present a comparative study of the epigenetic and transcriptional states of pericentromeric heterochromatin satellite sequences found in these pluripotent states. We show that 2i-ESCs are distinguished from other pluripotent cells by a prominent enrichment in H3K27me3 and low levels of DNA methylation at pericentromeric heterochromatin. In contrast, serum-containing ESCs exhibit higher levels of major satellite repeat transcription, which is lower in 2i-ESCs and even more repressed in primed EpiSCs. Removal of either DNA methylation or H3K9me3 at PCH in 2i-ESCs leads to enhanced deposition of H3K27me3 with few changes in satellite transcript levels. In contrast, their removal in EpiSCs does not lead to deposition of H3K27me3 but rather removes transcriptional repression. Altogether, our data show that the epigenetic state of PCH is modified during transition from naive to primed pluripotency states towards a more repressive state, which tightly represses the transcription of satellite repeats.
Using video-enhanced fluorescence microscopy, we describe in live mouse zygotes the paternal chromatin changes undergone after fertilization. We focus on the sperm recondensation process and the formation of the paternal pronucleus, in relationship with the progression of maternal chromatin. Chromatin is labeled with the vital fluorophore Hoechst 33342. Our conditions of dye concentration and irradiation allow a continuous following of the dynamics of changes without major perturbation. We combine these observations with ultrastructural analysis performed by electron microscopy of the same eggs fixed at chosen stages. We show that the highly recondensed state corresponds to the appearance of the nuclear envelope and therefore the beginning of the pronuclear stage.
During the periovulatory period, the induction of prostaglandin G/H synthase-2 (PTGS2) expression in cumulus cells and associated prostaglandin E2 (PGE2) production are implicated in the terminal differentiation of the cumulus-oocyte complex. During the present study, the effects of the PTGS2/PGE2 pathway on the developmental competence of bovine oocytes were investigated using an in vitro model of maturation, fertilization, and early embryonic development. The specific inhibition of PTGS2 activity with NS-398 during in vitro maturation (IVM) significantly restricted mitogen-activated protein kinase (MAPK) activation in oocytes at the germinal vesicle breakdown stage and reduced both cumulus expansion and the maturation rate after 22 h of culture. In addition, significantly higher rates of abnormal meiotic spindle organization were observed after 26 h of culture. Periconceptional PTGS2 inhibition did not affect fertilization but significantly reduced the speed of embryo development. Embryo output rates were significantly decreased on Day 6 postfertilization but not on Day 7. However, total blastomere number was significantly lower in embryos obtained after PTGS2 inhibition. The addition of PGE2 to IVM and in vitro fertilization cultures containing NS-398 overrode oocyte maturation and early embryonic developmental defects. Protein and mRNA expression for the prostaglandin E receptor PTGER2 were found in oocytes, whereas the PTGER2, PTGER3, and PTGER4 subtypes were expressed in cumulus cells. This study is the first to report the involvement of PGE2 in oocyte MAPK activation during the maturation process. Taken together, these results indicate that PGE2-mediated interactions between somatic and germ cells during the periconceptional period promote both in vitro oocyte maturation and preimplantation embryonic development in cattle.
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