Transcription of rRNA in early mouse embryos promotes chromatin reorganization and expression of major satellite repeats

During the first cell cycles of the early development, the chromatin of the embryo is highly reprogrammed alongside that embryonic genome starts its own transcription. The spatial organization of the genome is a major process that contributes to regulating gene transcription in time and space, however, it is poorly studied in the context of early embryos. To study the cause and effect link between transcription and spatial organization in embryos, we focused on the ribosomal genes, that are first silent and begin to transcribe during the 2-cell stage in the mouse. We demonstrated that ribosomal sequences and early unprocessed rRNAs are spatially organized in a very peculiar manner from the 2-cell to the 16-cell. Using drugs interfering with ribosomal DNA transcription, we show that this organization, totally different from somatic cells, depends on an active transcription of ribosomal genes and induces a unique chromatin environment that favors transcription of major satellite sequences after the 4-cell stage.

Transient Changes of Metabolism at the Pronuclear Stage in Mice Influences Skeletal Muscle Phenotype in Adulthood

Skeletal muscle has a remarkable plasticity, and its phenotype is strongly influenced by hormones, transcription factors, and physical activity. However, whether skeletal phenotype can be oriented or not during early embryonic stages has never been investigated. Here, we report that pyruvate as the only source of carbohydrate in the culture medium of mouse one cell stage embryo influenced the establishment of the muscular phenotype in adulthood. We found that pyruvate alone induced changes in the contractile phenotype of the skeletal muscle in a sexually dependent manner. For male mice, a switch to a more glycolytic phenotype was recorded, whereas, in females, the pyruvate induced a switch to a more oxidative phenotype. In addition, the influence of pyruvate on the contractile phenotypes was confirmed in two mouse models of muscle hypertrophy: the well-known myostatin deficient mouse (Mstn-/-) and a mouse carrying a specific deletion of p43, a mitochondrial triiodothyronine receptor. Finally, to understand the link between these adult phenotypes and the early embryonic period, we assessed the levels of two histone H3 post-translational modifications in presence of pyruvate alone just after the wave of chromatin reprogramming specific of the first cell cycle. We showed that H3K4 acetylation level was decreased in Mstn-/- 2-cell embryos, whereas no difference was found for H3K27 trimethylation level, whatever the genotype. These findings demonstrate for the first time that changes in the access of energy substrate during the very first embryonic stage can induce a precocious orientation of skeletal muscle phenotype in adulthood.

Three-dimensional analysis of nuclear heterochromatin distribution during early development in the rabbit

Changes to the spatial organization of specific chromatin domains such as constitutive heterochromatin have been studied extensively in somatic cells. During early embryonic development, drastic epigenetic reprogramming of both the maternal and paternal genomes, followed by chromatin remodeling at the time of embryonic genome activation (EGA), have been observed in the mouse. Very few studies have been performed in other mammalian species (human, bovine, or rabbit) and the data are far from complete. During this work, we studied the three-dimensional organization of pericentromeric regions during the preimplantation period in the rabbit using specific techniques (3D-FISH) and tools (semi-automated image analysis). We observed that the pericentromeric regions (identified with specific probes for Rsat I and Rsat II genomic sequences) changed their shapes (from pearl necklaces to clusters), their nuclear localizations (from central to peripheral), as from the 4-cell stage. This reorganization goes along with histone modification changes and reduced amount of interactions with nucleolar precursor body surface. Altogether, our results suggest that the 4-cell stage may be a crucial window for events necessary before major EGA, which occurs during the 8-cell stage in the rabbit.

Contrasting epigenetic states of heterochromatin in the different types of mouse pluripotent stem cells

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.

Antagonist Xist and Tsix co-transcription during mouse oogenesis and maternal Xist expression during pre-implantation development calls into question the nature of the maternal imprint on the X chromosome

During the first divisions of the female mouse embryo, the paternal X-chromosome is coated by Xist non-coding RNA and gradually silenced. This imprinted X-inactivation principally results from the apposition, during oocyte growth, of an imprint on the X-inactivation master control region: the X-inactivation center (Xic). This maternal imprint of yet unknown nature is thought to prevent Xist upregulation from the maternal X (X^M) during early female development. In order to provide further insight into the X^M imprinting mechanism, we applied single-cell approaches to oocytes and pre-implantation embryos at different stages of development to analyze the expression of candidate genes within the Xic. We show that, unlike the situation pertaining in most other cellular contexts, in early-growing oocytes, Xist and Tsix sense and antisense transcription occur simultaneously from the same chromosome. Additionally, during early development, Xist appears to be transiently transcribed from the X^M in some blastomeres of late 2-cell embryos concomitant with the general activation of the genome indicating that X^M imprinting does not completely suppress maternal Xist transcription during embryo cleavage stages. These unexpected transcriptional regulations of the Xist locus call for a re-evaluation of the early functioning of the maternal imprint on the X-chromosome and suggest that Xist/Tsix antagonist transcriptional activities may participate in imprinting the maternal locus as described at other loci subject to parental imprinting.

Three-Dimensional Distribution of UBF and Nopp140 in Relationship to Ribosomal DNA Transcription During Mouse Preimplantation Development

The nucleolus is a dynamic nuclear compartment that is mostly involved in ribosome subunit biogenesis; however, it may also play a role in many other biological processes, such as stress response and the cell cycle. Mainly using electron microscopy, several studies have tried to decipher how active nucleoli are set up during early development in mice. In this study, we analyzed nucleologenesis during mouse early embryonic development using 3D-immunofluorescent detection of UBF and Nopp140, two proteins associated with different nucleolar compartments. UBF is a transcription factor that helps maintain the euchromatic state of ribosomal genes; Nopp140 is a phosphoprotein that has been implicated in pre-rRNA processing. First, using detailed image analyses and the in situ proximity ligation assay technique, we demonstrate that UBF and Nopp140 dynamic redistribution between the two-cell and blastocyst stages (time of implantation) is correlated with morphological and structural modifications that occur in embryonic nucleolar compartments. Our results also support the hypothesis that nucleoli develop at the periphery of nucleolar precursor bodies. Finally, we show that the RNA polymerase I inhibitor CX-5461: 1) disrupts transcriptional activity, 2) alters preimplantation development, and 3) leads to a complete reorganization of UBF and Nopp140 distribution. Altogether, our results underscore that highly dynamic changes are occurring in the nucleoli of embryos and confirm a close link between ribosomal gene transcription and nucleologenesis during the early stages of development.

Sperm nuclear architecture is locally modified in presence of a Robertsonian translocation t(13;17)

In mammals, the non-random organization of the sperm nucleus supports an early function during embryonic development. Altering this organization may interfere with the zygote development and reduce fertility or prolificity. Thus, rare studies on sperm cells from infertile patients described an altered nuclear organization that may be a cause or a consequence of their respective pathologies. Thereby, chromosomal rearrangements and aneuploidy can be studied not only for their adverse effects on production of normal/balanced gametes at meiosis but also for their possible impact on sperm nuclear architecture and the epigenetic consequences of altered chromosome positioning. We decided to compare the global architecture of sperm nuclei from boars, either with a normal chromosome composition or with a Robertsonian translocation involving chromosomes 13 and 17. We hypothesized that the fusion between these chromosomes may change their spatial organization and we examined to what extend it could also modify the global sperm nuclear architecture. Analysis of telomeres, centromeres and gonosomes repartition does not support a global nuclear disorganization. But specific analysis of chromosomes 13 and 17 territories highlights an influence of chromosome 17 for the positioning of the fused chromosomes within the nucleus. We also observed a specific clustering of centromeres depending of the chromosome subtypes. Altogether our results showed that chromosome fusion does not significantly alter sperm nucleus architecture but suggest that centromere remodelling after chromosome fusion locally impacts chromosome positioning.

3D-FISH analysis of embryonic nuclei in mouse highlights several abrupt changes of nuclear organization during preimplantation development

Background

Embryonic 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).

Results

In 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.

Conclusions

The 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.

35 DYNAMICS OF PERICENTRIC REPETITIVE SEQUENCES IN PREIMPLANTATION RABBIT EMBRYOS UNDERLINES INADEQUATE SPATIO-TEMPORAL REORGANIZATION AFTER NUCLEAR TRANSFER

Nowadays, a critical question in the epigenetic field is how chromatin is organised within the cell nucleus and how it affects gene expression. In this study we hypothesise that nuclear structure could be involved in the control of gene expression during early rabbit embryonic development. We focused on pericentric/centric heterochromatin, a peculiar region within nuclei known to form higher-order chromatin structures. We therefore performed immunostaining of associated proteins (HP1 and CENP) as well as FISH (fluorescent in situ hybridization) with probes corresponding to these genomic regions. Fertilized embryos were collected from New Zealand white rabbit female, cultured in vitro and fixed at different developmental stages up to 16-cell. Nuclear transfer of rabbit fetal skin fibroblasts was performed by electrofusion and electroactivation (Chesne et al., 2002). Immunostaining experiments were performed as previously described (Martin et al., 2006) and FISH experiments were performed using DNA probes specific to Rsat I and Rsat II sequences (Ekes et al., 2004) according to our published protocol (Maalouf et al., 2010). Three-dimensional images were acquired by confocal laser microscopy (each group included 20 to 40 embryos) and then analysed through automated 3D image processes (according to Ballester et al., 2008; Pichugin et al., 2010). Briefly, after segmentation of the nuclei, immunostaining signals were analysed using an automatic threshold segmentation procedure and FISH spots distributions were analysed with the eroded volume fraction (EVF) method. In in vivo fertilized rabbit embryos, we observed that during the 1- and 2-cell cycles pericentric heterochromatin was dispersed throughout the embryonic nuclei. Large foci of pericentric heterochromatin then progressively appeared at the 4-cell stage and increased in frequency and size by the 8/16-cell stage [∼75% of the embryos, n = 40; i.e. the stage of major transcriptional activation also called MET (maternal to embryonic-transition)]. This suggested that dramatic aggregation of pericentric heterochromatin is associated to the onset of transcription. Interestingly, after nuclear transfer, the donor nuclear organisation was quickly reverted into an early embryonic dispersed form in most 1-cell embryos (80%, n = 35). Subsequently, a somatic-like nuclear reorganization with large heterochromatin foci was re-established in 86% of the 4-cell stage cloned embryos (n = 35). This pattern was similar to the one observed in fertilized embryos at MET; it also appeared one stage earlier (4-cell vs 8-cell). Coincidently, most cloned embryos stopped developing at that stage (85% of cleavage until the 4-cell but only 38% at morula, n = 173). Together, the results suggested that inadequate spatio-temporal reorganization of the donor nucleus could be deleterious during early development in rabbit clones. The present work was supported by INRA « Jeune Equipe » funding, the European CLONET (MRTN-CT-2006-035468) grant and the Ambassade de France en Chine.

[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.

Fluorescence in situ hybridization applied to domestic animal cytogenetics

The aim of this article is not to present an exhaustive review of molecular cytogenetics applications in domestic animal species, but more to illustrate the considerable contribution of these approaches in diagnostics and research in economically important species. A short presentation of the main applications of molecular cytogenetics in humans points out the domains in which it has become an essential tool and underlines the specificities attached to this species in comparison to farm animals. This article is devoted to outlining the current resources available in domestic species and to some examples of fluorescence in situ hybridization applications in the cattle, pig, horse and avian species. From a clinical point of view, these examples illustrate the advantages of FISH for the study of chromosomal abnormalities (identification, characterization and estimation of their effects). Other applications of molecular cytogenetics are also illustrated, particularly ZOO-FISH, an approach which allows the determination of chromosome homologies between species. Finally, a specific emphasis was placed on the usefulness of molecular cytogenetics for the analysis of species such as poultry, which harbour a complex karyotype.

Analysis using sperm-FISH of a putative interchromosomal effect in boars carrying reciprocal translocations

The occurrence of interchromosomal effects (ICE) in reciprocal translocation carriers still remains contradictory in the human literature. We used the pig as an animal model to investigate whether the structure of the reciprocal translocations as well as the size and/or type of the chromosomes not involved in the rearrangement may influence the occurrence and the extent of ICE. Analyses of chromosomal sperm content by fluorescence in situ hybridization (FISH) using whole-chromosome painting probes for 7 chromosomes (1, 10, 11, 13, 18, X and Y) were carried out on sperm samples of 2 boars with normal semen parameters carrying different balanced reciprocal translocations: 38, XY, t(3;15)(q27;q13) or 38, XY, t(12;14)(q13;q21). One fertile boar with normal karyotype was also studied as a control. Aneuploidy rates for the 7 chromosomes were estimated by scoring 10,000 to 20,000 spermatozoa for each probe combination. No significant ICE was found except for chromosome 1 in the case of the t(3;15) translocation. Even if statistically significant, this ICE remained very weak and should have very little impact on the reproductive performance of the carrier boar. The size and/or type of chromosomes not involved in the translocation do not seem to have a major influence on the occurrence of ICE. The structure of the translocation could play a role, but complementary studies should be carried out to confirm this assumption.

Cytogenetic screening of livestock populations in Europe: an overview

Clinical animal cytogenetics development began in the 1960's, almost at the same time as human cytogenetics. However, the development of the two disciplines has been very different during the last four decades. Clinical animal cytogenetics reached its 'Golden Age' at the end of the 1980's. The majority of the laboratories, as well as the main screening programs in farm animal species, presented in this review, were implemented during that period, under the guidance of some historical leaders, the first of whom was Ingemar Gustavsson. Over the past 40 years, hundreds of scientific publications reporting original chromosomal abnormalities generally associated with clinical disorders (mainly fertility impairment) have been published. Since the 1980's, the number of scientists involved in clinical animal cytogenetics has drastically decreased for different reasons and the activities in that field are now concentrated in only a few laboratories (10 to 15, mainly in Europe), some of which have become highly specialized. Currently between 8,000 and 10,000 chromosomal analyses are carried out each year worldwide, mainly in cattle, pigs, and horses. About half of these analyses are performed in one French laboratory. Accurate estimates of the prevalence of chromosomal abnormalities in some populations are now available. For instance, one phenotypically normal pig in 200 controlled in France carries a structural chromosomal rearrangement. The frequency of the widespread 1;29 Robertsonian translocation in cattle has greatly decreased in most countries, but remains rather high in certain breeds (up to 20-25% in large beef cattle populations, even higher in some local breeds). The continuation, and in some instances the development of the chromosomal screening programs in farm animal populations allowed the implementation of new and original scientific projects, aimed at exploring some basic questions in the fields of chromosome and/or cell biology, thanks to easier access to interesting biological materials (germ cells, gametes, embryos ...).

Sperm nuclei analysis of 1/29 Robertsonian translocation carrier bulls using fluorescence in situ hybridization

In 1964, Gustavsson and Rockborn first described the 1/29 Robertsonian translocation in cattle. Since then, several studies have demonstrated the negative effect of this particular chromosomal rearrangement on the fertility of carrier animals. During the last decade, meiotic segregation patterns have been studied on human males carrying balanced translocations using FISH on decondensed sperm nuclei. In this work, we have applied the 'Sperm-FISH' technique to determine the chromosomal content of spermatozoa from two bulls heterozygous for the 1/29 translocation and one normal bull (control). 5425 and 2702 sperm nuclei were scored, respectively, for the two heterozygous bulls, using whole chromosome painting probes of chromosomes 1 and 29. Very similar proportions of normal (or balanced) spermatozoa resulting from alternate segregation were observed (97.42% and 96.78%). For both heterozygous bulls, the proportions of nullisomic and disomic spermatozoa did not follow the theoretical 1:1 ratio. Indeed, proportions of nullisomic spermatozoa were higher than those of disomic sperma tozoa (1.40% vs 0.09% (bull 1) and 1.29% vs 0.15% (bull 2) for BTA1, and 0.65% vs 0.40% (bull 1) and 1.11% vs 0.63% (bull 2) for BTA29). The average frequencies of disomic and diploid spermatozoa in the normal bull were 0.11% and 0.05%, respectively.

Chromosome painting comparison of Leontopithecus chrysomelas (Callitrichine, Platyrrhini) with man and its phylogenetic position

Using human probes of whole chromosomes, the homoeologies between human and Leontopithecus chrysomelas (Platyrrhini) karyotypes were established. Thirty-three conserved segments were observed between the two species. Intrachromosomal rearrangements between the two species were identified using hybridization of chromosome arm probes of human chromosomes 1 and 3. We also used chromosomal data to investigate phylogenetic relationships of Callitrichines. These data were encoded using Cebus capucinus , a species which kept fairly ancestral chromosomes, as reference. Two equi-parsimonious trees, including reversion or convergence events, were obtained. The monophyly of Callitrichines is confirmed. They share nine chromosomal rearrangements at least. The Cebuella-Callithrix group forms a clade sharing five rearrangements at least. According to the tree considered, the Tamarins, Leontopithecus and Saguinus share two chromosomal rearrangements restricted to these two taxa or none. Callimico accumulated seven chromosomal rearrangements unshared with other taxa, at least. To avoid convergence and reversion events, we propose the hypothesis of a network (or populational) evolution. Six chromosomal rearrangements would have occurred during the period of this network evolution. Finally, the karyotype of the last common ancestor to all Callitrichines has been reconstructed. It possessed 48 chromosomes.

Identification by R-banding and FISH of chromosome arms involved in Robertsonian translocations in several deer species

We constructed and analyzed the RBG-banded karyotype of five deer species: Chital (Axis axis), White-lipped deer (Cervus albirostris), Rusa deer (Cervus timorensis russa), Sambar deer (Cervus unicolor) and Eld's deer (Cervus eldi siamensis). Among these five species, only Eld's deer had been previously karyotyped using R-banding. In order to identify all the chromosome correspondences with cattle and precisely which chromosome arms are involved in Robertsonian translocations, we compared the karyotypes of these five species with those of the closely related and well-characterized species, cattle (Bos taurus) and Vietnamese Sika deer (Cervus nippon pseudaxis). Among these six deer species (the five above plus the Vietnamese Sika deer), we found thirteen different Robertsonian translocations involving nineteen different chromosome arms. Thirteen chromosome arms were identified by comparison of R-banding patterns only and the remaining six were either confirmed or identified by FISH-mapping of bovine or caprine probes previously localized in cattle. Finally, we observed that five of the thirteen Robertsonian translocations are shared by at least two species and that some chromosome arms are more frequently involved in Robertsonian translocations than others.

Highly conserved chromosomes in an Asian squirrel (Menetes berdmorei, Rodentia: Sciuridae) as demonstrated by ZOO-FISH with human probes

The chromosomes of Menetes berdmorei (Rodentia, Sciuridae, Sciurinae) were studied by ZOO-FISH using whole human chromosome probes. All homoeologies between M. berdmorei and human chromosomes were determined, except for two small chromosome segments. Twelve human chromosomes are conserved in a unique block of synteny; ten are split into two and one into three blocks. Thus, a small number of interchromosomal rearrangements, about twenty, separates human from this squirrel karyotype. Homoeologies between human and the presumed ancestral chromosomes of Sciurinae could also be deduced, as well as those with the presumed ancestral chromosomes of eutherian mammals. Sciurinae chromosomes appear to be much closer to those of non-rodent mammals than those of Muridae and Cricetidae species studied so far. Thus, they provide an interesting tool to link the rodent genome to those of other mammals.