Knockdown of YY1 Inhibits XIST Expression and Enhances Cloned Pig Embryo Development

The technique of cloning has wide applications in animal husbandry and human biomedicine. However, the very low developmental efficiency of cloned embryos limits the application of cloning. Ectopic XIST-expression-induced abnormal X chromosome inactivation (XCI) is a primary cause of the low developmental competence of cloned mouse and pig embryos. Knockout or knockdown of XIST improves cloning efficiency in both pigs and mice. The transcription factor Yin yang 1(YY1) plays a critical role in XCI by triggering the transcription of X-inactive specific transcript (XIST) and facilitating the localization of XIST RNA on the X chromosome. This study aimed to investigate whether RNA interference to suppress the expression of YY1 can inhibit erroneous XIST expression, rescue abnormal XCI, and improve the developmental ability of cloned pig embryos. The results showed that YY1 binds to the 5' regulatory region of the porcine XIST gene in pig cells. The microinjection of YY1 siRNA into cloned pig embryos reduced the transcript abundance of XIST and upregulated the mRNA level of X-linked genes at the 4-cell and blastocyst stages. The siRNA-mediated knockdown of YY1 altered the transcriptome and enhanced the in vitro and in vivo developmental efficiency of cloned porcine embryos. These results suggested that YY1 participates in regulating XIST expression and XCI in cloned pig embryos and that the suppression of YY1 expression can increase the developmental rate of cloned pig embryos. The present study established a new method for improving the efficiency of pig cloning.

Parental methylome reprogramming in human uniparental blastocysts reveals germline memory transition

Uniparental embryos derived from only the mother (gynogenetic [GG]) or the father (androgenetic [AG]) are unique models for studying genomic imprinting and parental contributions to embryonic development. Human parthenogenetic embryos can be obtained following artificial activation of unfertilized oocytes, but the production of AG embryos by injection of two sperm into one denucleated oocyte leads to an extra centriole, resulting in multipolar spindles, abnormal cell division, and developmental defects. Here, we improved androgenote production by transferring the male pronucleus from one zygote into another haploid androgenote to prevent extra centrioles and successfully generated human diploid AG embryos capable of developing into blastocysts with an identifiable inner cell mass (ICM) and trophectoderm (TE). The GG embryos were also generated. The zygotic genome was successfully activated in both the AG and GG embryos. DNA methylome analysis showed that the GG blastocysts partially retain the oocyte transcription-dependent methylation pattern, whereas the AG blastocyst methylome showed more extensive demethylation. The methylation states of most known imprinted differentially methylated regions (DMRs) were recapitulated in the AG and GG blastocysts. Novel candidate imprinted DMRs were also identified. The production of uniparental human embryos followed by transcriptome and methylome analysis is valuable for identifying parental contributions and epigenome memory transitions during early human development.

Investigating Markers of Reprogramming Potential in Somatic Cell Lines Derived from Matched Donors

Somatic cell biobanking and related technologies, somatic cell nuclear transfer (SCNT), and induction of pluripotent stem cells offer significant promise for wildlife conservation, but have yet to achieve optimal success. Inefficiency and variability in outcome have been linked to incomplete nuclear reprogramming, highlighting the importance of donor cell contribution. Studies show significant differences in SCNT outcome in donor cell lines within and between individuals, highlighting the necessity for a standardized characterization method to evaluate cell line reprogramming potential. Stringently standardized bovine fibroblast cell lines were generated and assessed for inter- and intraindividual variability on cellular (morphology, chromosome number, apoptotic incidence; Experiment 1) and molecular (pluripotency and epigenetic-related gene expression; Experiment 2) levels encompassing putative biomarkers of reprogramming potential. Cellular parameters were similar across cell lines. While some statistically significant differences were observed in DNMT1, DNMT3B, and HAT1, but not HDAC1, their biological relevance could not be determined with the information at hand. This study lays the foundation for understanding cellular characteristics in cultured cell lines; however, further studies are required to determine any correlation with reprogramming potential.

Somatic cells, stem cells, and induced pluripotent stem cells: how do they now contribute to conservation?

More than a decade has now passed since the birth of the first endangered species produced from an adult somatic cell reprogrammed by somatic cell nuclear transfer. At that time, advances made in domestic and laboratory animal species provided the necessary foundation for attempting cutting-edge technologies on threatened and endangered species. In addition to nuclear transfer, spermatogonial stem cell transplantation and induction of pluripotent stem cells have also been explored. Although many basic scientific questions have been answered and more than 30 wild species have been investigated, very few successes have been reported. The majority of studies document numerous obstacles that still need to be overcome to produce viable gametes or embryos for healthy offspring production. This chapter provides an overview of somatic cell and stem cell technologies in different taxa (mammals, fishes, birds, reptiles and amphibians) and evaluates the potential and impact of these approaches for animal species conservation.

Reprogramming mammalian somatic cells

Somatic cell nuclear transfer (SCNT), the technique commonly known as cloning, permits transformation of a somatic cell into an undifferentiated zygote with the potential to develop into a newborn animal (i.e., a clone). In somatic cells, chromatin is programmed to repress most genes and express some, depending on the tissue. It is evident that the enucleated oocyte provides the environment in which embryonic genes in a somatic cell can be expressed. This process is controlled by a series of epigenetic modifications, generally referred to as "nuclear reprogramming," which are thought to involve the removal of reversible epigenetic changes acquired during cell differentiation. A similar process is thought to occur by overexpression of key transcription factors to generate induced pluripotent stem cells (iPSCs), bypassing the need for SCNT. Despite its obvious scientific and medical importance, and the great number of studies addressing the subject, the molecular basis of reprogramming in both reprogramming strategies is largely unknown. The present review focuses on the cellular and molecular events that occur during nuclear reprogramming in the context of SCNT and the various approaches currently being used to improve nuclear reprogramming. A better understanding of the reprogramming mechanism will have a direct impact on the efficiency of current SCNT procedures, as well as iPSC derivation.

Dynamic changes of histone H3 trimethylated at positions K4 and K27 in human oocytes and preimplantation embryos

OBJECTIVE

To investigate the distribution patterns of H3K4me3 and H3K27me3 in human oocytes and preimplantation embryos.

DESIGN

Experimental study.

SETTING

University reproductive medical center.

PATIENT(S)

Patients undergoing IVF cycles.

INTERVENTION(S)

Oocytes and embryos were collected from patients undergoing IVF cycles.

MAIN OUTCOME MEASURE(S)

The distribution patterns of H3K4me3 and H3K27me3 in oocytes and embryos were analyzed by indirect immunofluorescent staining and scanning confocal microscopy.

RESULT(S)

H3K4me3 and H3K27me3 signals were detectable at each stage of oocyte and embryonic development. However, only one of the pronuclei showed signal for H3K27me3 in each of the zygotes, whereas H3K4me3 staining was always uniform in all zygotes. The level of H3K4me3 decreased steadily from germinal vesicle to metaphase II stage, obviously increased from zygote stage to four-cell stage, and reached the lowest at eight-cell stage. A sharp increase was then observed at blastocyst stage. The level of H3K27me3 slightly changed from germinal vesicle stage to zygote stage, then decreased steadily and reached the lowest at eight-cell stage, followed by a significant increase at blastocyst stage.

CONCLUSION(S)

The levels of H3K4me3 and H3K27me3 show dynamic changes during human oocyte maturation and preimplantation embryonic development. Asymmetric distribution of H3K27me3 exists in human zygote pronuclei, whereas H3K4me3 is always uniform in all of the pronuclei.

Uncoupled embryonic and extra-embryonic tissues compromise blastocyst development after somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) is the most efficient cell reprogramming technique available, especially when working with bovine species. Although SCNT blastocysts performed equally well or better than controls in the weeks following embryo transfer at Day 7, elongation and gastrulation defects were observed prior to implantation. To understand the developmental implications of embryonic/extra-embryonic interactions, the morphological and molecular features of elongating and gastrulating tissues were analysed. At Day 18, 30 SCNT conceptuses were compared to 20 controls (AI and IVP: 10 conceptuses each); one-half of the SCNT conceptuses appeared normal while the other half showed signs of atypical elongation and gastrulation. SCNT was also associated with a high incidence of discordance in embryonic and extra-embryonic patterns, as evidenced by morphological and molecular “uncoupling”. Elongation appeared to be secondarily affected; only 3 of 30 conceptuses had abnormally elongated shapes and there were very few differences in gene expression when they were compared to the controls. However, some of these differences could be linked to defects in microvilli formation or extracellular matrix composition and could thus impact extra-embryonic functions. In contrast to elongation, gastrulation stages included embryonic defects that likely affected the hypoblast, the epiblast, or the early stages of their differentiation. When taking into account SCNT conceptus somatic origin, i.e. the reprogramming efficiency of each bovine ear fibroblast (Low: 0029, Med: 7711, High: 5538), we found that embryonic abnormalities or severe embryonic/extra-embryonic uncoupling were more tightly correlated to embryo loss at implantation than were elongation defects. Alternatively, extra-embryonic differences between SCNT and control conceptuses at Day 18 were related to molecular plasticity (high efficiency/high plasticity) and subsequent pregnancy loss. Finally, because it alters re-differentiation processes in vivo, SCNT reprogramming highlights temporally and spatially restricted interactions among cells and tissues in a unique way.

Nuclear reprogramming: kinetics of cell cycle and metabolic progression as determinants of success

Establishment of totipotency after somatic cell nuclear transfer (NT) requires not only reprogramming of gene expression, but also conversion of the cell cycle from quiescence to the precisely timed sequence of embryonic cleavage. Inadequate adaptation of the somatic nucleus to the embryonic cell cycle regime may lay the foundation for NT embryo failure and their reported lower cell counts. We combined bright field and fluorescence imaging of histone H_2b-GFP expressing mouse embryos, to record cell divisions up to the blastocyst stage. This allowed us to quantitatively analyze cleavage kinetics of cloned embryos and revealed an extended and inconstant duration of the second and third cell cycles compared to fertilized controls generated by intracytoplasmic sperm injection (ICSI). Compared to fertilized embryos, slow and fast cleaving NT embryos presented similar rates of errors in M phase, but were considerably less tolerant to mitotic errors and underwent cleavage arrest. Although NT embryos vary substantially in their speed of cell cycle progression, transcriptome analysis did not detect systematic differences between fast and slow NT embryos. Profiling of amino acid turnover during pre-implantation development revealed that NT embryos consume lower amounts of amino acids, in particular arginine, than fertilized embryos until morula stage. An increased arginine supplementation enhanced development to blastocyst and increased embryo cell numbers. We conclude that a cell cycle delay, which is independent of pluripotency marker reactivation, and metabolic restraints reduce cell counts of NT embryos and impede their development.

ES cell differentiation system recapitulates the establishment of imprinted gene expression in a cell-type-specific manner

Genomic imprinting is a phenomenon whereby monoallelic gene expression occurs in a parent-of-origin-specific manner. A subset of imprinted genes acquires a tissue-specific imprinted status during the course of tissue development, and this process can be analyzed by means of an in vitro differentiation system utilizing embryonic stem (ES) cells. In neurons, the gene Ube3a is expressed from the maternal allele only, and a paternally expressed non-coding, antisense RNA has been implicated in the imprinting process in mice and humans. Here, to study the genomic imprinting mechanism, we established F1 hybrid ES cells derived from two sub-species of Mus musculus and established an in vitro neuronal differentiation system in which neuron-specific imprinting of Ube3a was recapitulated. With this system, we revealed that the switch from biallelic expression to maternal, monoallelic expression of Ube3a occurs late in neuronal development, during the neurite outgrowth period, and that the expression of endogenous antisense transcript from the Ube3a locus is up-regulated several hundred-fold during the same period. Our results suggest that evaluation of the quality of ES cells by studying their differentiation in vitro should include evaluation of epigenetic aspects, such as a comparison with the genomic imprinting status found in tissues in vivo, in addition to the evaluation of differentiation gene markers and morphology. Our F1 hybrid ES cells and in vitro differentiation system will allow researchers to investigate complex end-points such as neuron-specific genomic imprinting, and our F1 hybrid ES cells are a useful resource for other tissue-specific genomic imprinting and epigenetic analyses.

Efficient production and cellular characterization of sheep androgenetic embryos

The production of androgenetic embryos in large animals is a complex procedure. Androgenetic embryos have been produced so far only in cattle and sheep using pronuclear transfer (PT) between zygotes derived from in vitro fertilization (IVF) of previously enucleated oocytes. PT is required due to the poor developmental potential of androgenotes derived from IVF of enucleated oocytes. Here we compare the developemt to blastocyst of androgenetic embryos produced by the standard pronuclear transfer and by fertilization of oocytes enucleated in Ca2+/Mg2+-free medium, without pronuclear transfer. The enucleation in Ca2+/Mg2+-free medium abolished almost completely the manipulation-induced activation, significantly improving the development to blastocyst of the androgenetic embryos (IVF followed by PT; 18.6%: IVF only; 17.7%, respectively). Karyotype analysis of IVF revealed a similar proportion of diploid embryos in androgenetic and control blastocysts (35% and 36%, respectively), although mixoploid blastocysts were frequently observed in both groups (64%). Androgenotes had lower total cell numbers than control and parthenogenetic embryos, but more cells in ICM cells comparing to parthenogenotes (30.42 vs. 17.15%). Higher expression of the pluripotency-associated gene NANOG, and trophoblastic-specific gene CDX2, were also observed in androgenotes compared to parthenogenotes and controls. The global methytion profile of androgenetic embryos was comparable to controls, but was lower than parthenogenetic embryos. The cell composition and methylation pattern we have detected in monoparental sheep monoparental embryos are unprecedented, and differ considerably from the standard reference mouse embryos. Altogether, these finding indicate significant differences across species in the molecular mechanisms regulating early development of monoparental embryos, and highlights the need to study postimplantation development of androgenetic embryos in sheep.

[Embryonic genome organization after fertilization in mammals]

In mammals, the embryonic genome is first transcriptionally inactive after fertilization. Embryonic development is then strictly dependent on the maternally inherited RNA and proteins accumulated before ovulation and present in the oocyte cytoplasm. The onset of embryonic gene expression is initiated later during development, i.e. during the "embryonic genome activation (EGA)". EGA takes place at various preimplantation stages according to species and is dependent on the presence of the basal transcriptional machinery components but also on parental genomes reorganizations after fertilization. Indeed, during the first embryonic cycles, nuclei undergo intense remodeling that could be a key regulator of embryonic development.

Dynamics of constitutive heterochromatin: two contrasted kinetics of genome restructuring in early cloned bovine embryos

Efficient reprograming of the donor cell genome in nuclear transfer (NT) embryos is linked to the ability of the embryos to sustain full-term development. As the nuclear architecture has recently emerged as a key factor in the regulation of gene expression, we questioned whether early bovine embryos obtained from transfer of cultured fibroblasts into enucleated oocytes would adopt an embryo-like nuclear organization. We studied the dynamics of constitutive heterochromatin in the stages prior to embryonic genome activation by distribution analysis of heterochromatin protein CBX1 (HP1), centromeric proteins CENPA and CENPB, and histone H3 three-methylated at lysine 9. Then we applied descriptive, quantitative, and co-localization analyses. A dramatic reorganization of heterochromatic blocks of somatic donor cells was first observed in the late one-cell stage NT embryos. Then at two- and four-cell stages, we found two types of NT embryos: one displaying noncondensed heterochromatin patches similar to IVF embryos, whereas the second type displayed condensed heterochromatin blocks, normally observed in IVF embryos only after the eight-cell stage. These analyses discriminate for the first time two contrasted types of nuclear organization in NT embryos, which may correspond to different functional states of the nuclei. The relationship with the somatic nucleus reprograming efficiency is discussed.

Nuclear reprogramming: what has been done and potential avenues for improvements

A major challenge for reproductive biologists is the development of novel strategies to improve cloning efficiency. Even in species for which cloning is relatively successful, like cattle, the efficiency is still unacceptably low. In this review article we critically analyse all approaches that have been suggested by different laboratories in the field so far. As will be discussed below, so far none of these gives rise to a dramatic increase in cloning efficiency. Possibly, a multi-step approach including a pre-treatment of donor cells to modify their chromatin, along with improved culture system for cloned embryos would be the most promising.

Freeze-dried somatic cells direct embryonic development after nuclear transfer

The natural capacity of simple organisms to survive in a dehydrated state has long been exploited by man, with lyophylization the method of choice for the long term storage of bacterial and yeast cells. More recently, attempts have been made to apply this procedure to the long term storage of blood cells. However, despite significant progress, practical application in a clinical setting is still some way off. Conversely, to date there are no reports of attempts to lyophilize nucleated somatic cells for possible downstream applications. Here we demonstrate that lyophilised somatic cells stored for 3 years at room temperature are able to direct embryonic development following injection into enucleated oocytes. These remarkable results demonstrate that alternative systems for the long-term storage of cell lines are now possible, and open unprecedented opportunities in the fields of biomedicine and for conservation strategies.

Epigenetic reprogramming of the male genome during gametogenesis and in the zygote

During post-meiotic maturation, male germ cells undergo a formidable reorganization and condensation of their genome. During this phase most histones are globally acetylated and then replaced by sperm-specific basic proteins, named protamines, which compact the genome into a very specific structure within the sperm nucleus. Several studies suggest that this sperm-specific genome packaging structure conveys an important epigenetic message to the embryo. This paper reviews what is known about this fundamental, yet poorly understood, process, which involves not only global changes of the structure of the haploid genome, but also localized specific modifications of particular genomic regions, including pericentric heterochromatin and sex chromosomes. After fertilization, the male genome undergoes a drastic decondensation, and rapidly incorporates new histones. However, it remains different from the maternal genome, bearing specific epigenetic marks, especially in the pericentric heterochromatin region. The functional implications of male post-meiotic and post-fertilization genome reprogramming are not well known, but there is experimental evidence to show that it affects early embryonic development.