The nucleolus in the mouse oocyte is required for the early step of both female and male pronucleus organization

Essential roles of the nucleolus during early embryonic development: a regulatory hub for chromatin organization

The nucleolus is the most prominent liquid droplet-like membrane-less organelle in mammalian cells. Unlike the nucleolus in terminally differentiated somatic cells, those in totipotent cells, such as murine zygotes or two-cell embryos, have a unique nucleolar structure known as nucleolus precursor bodies (NPBs). Previously, it was widely accepted that NPBs in zygotes are simply passive repositories of materials that will be gradually used to construct a fully functional nucleolus after zygotic genome activation (ZGA). However, recent research studies have challenged this simplistic view and demonstrated that functions of the NPBs go beyond ribosome biogenesis. In this review, we provide a snapshot of the functions of NPBs in zygotes and early two-cell embryos in mice. We propose that these membrane-less organelles function as a regulatory hub for chromatin organization. On the one hand, NPBs provide the structural platform for centric and pericentric chromatin remodelling. On the other hand, the dynamic changes in nucleolar structure control the release of the pioneer factors (i.e. double homeobox (Dux)). It appears that during transition from totipotency to pluripotency, decline of totipotency and initiation of fully functional nucleolus formation are not independent events but are interconnected. Consequently, it is reasonable to hypothesize that dissecting more unknown functions of NPBs may shed more light on the enigmas of early embryonic development and may ultimately provide novel approaches to improve reprogramming efficiency.

Heterogeneity of nucleolar morphology in four-cell mouse embryos after IVF: association with developmental potential

In mammals, around fertilization, the nucleolus of embryos transforms into the nucleolus precursor bodies (NPBs), which continue to mature until the blastocyst stage, leading to distinct morphological changes. In our study, we observed two types of nucleolar morphology in mouse in vitro fertilized embryos at the four-cell stage, which we refer to single nucleolus (SN) and multiple nucleoli (MN). To visualize nucleolar morphology, four-cell embryos were immunostained with anti-NOPP140 antibody. These embryos were categorized into five types based on the number of blastomeres carrying SN: SN4/MN0, SN3/MN1, SN2/MN2, SN1/MN3, and SN0/MN4, with percentages of 13, 27, 21, 23 and 9, respectively. Next, using a light microscope, we divided the four-cell in vitro fertilized embryos without fixation into two groups: those with at least two blastomeres displaying SN (SN embryos) and those without (MN embryos). Notably, significantly more SN embryos developed into blastocysts and offspring at 18.5 dpc compared with MN embryos. Furthermore, SN embryos displayed a higher NANOG-positive cell number at the blastocyst stage, significantly lower body and placental weights, resulting in a higher fetal/placental ratio. These findings suggest a close association between nucleolar state at the four-cell stage and subsequent developmental potential.

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.

J. Nucleus reprogramming/remodeling through selective enucleation (SE) of immature oocytes and zygotes: a nucleolus point of view

It is now approximately 25 years since the sheep Dolly, the first cloned mammal where the somatic cell nucleus from an adult donor was used for transfer, was born. So far, somatic cell nucleus transfer, where G1-phase nuclei are transferred into cytoplasts obtained by enucleation of mature metaphase II (MII) oocytes followed by the activation of the reconstructed cells, is the most efficient approach to reprogram/remodel the differentiated nucleus. In general, in an enucleated oocyte (cytoplast), the nuclear envelope (NE, membrane) of an injected somatic cell nucleus breaks down and chromosomes condense. This condensation phase is followed, after subsequent activation, by chromatin decondensation and formation of a pseudo-pronucleus (i) whose morphology should resemble the natural postfertilization pronuclei (PNs). Thus, the volume of the transferred nuclei increases considerably by incorporating the content released from the germinal vesicles (GVs). In parallel, the transferred nucleus genes must be reset and function similarly as the relevant genes in normal embryo reprogramming. This, among others, covers the relevant epigenetic modifications and the appropriate organization of chromatin in pseudo-pronuclei. While reprogramming in SCNT is often discussed, the remodeling of transferred nuclei is much less studied, particularly in the context of the developmental potential of SCNT embryos. It is now evident that correct reprogramming mirrors appropriate remodeling. At the same time, it is widely accepted that the process of rebuilding the nucleus following SCNT is instrumental to the overall success of this procedure. Thus, in our contribution, we will mostly focus on the remodeling of transferred nuclei. In particular, we discuss the oocyte organelles that are essential for the development of SCNT embryos.

Migration speed of nucleolus precursor bodies in human male pronuclei: a novel parameter for predicting live birth

PURPOSE

To study the relationship between the migration speed of nucleolus precursor bodies (NPBs) in male and female pronuclei (mPN; fPN) and human embryo development during assisted reproduction.

METHODS

The migration speed of 263 NPBs from 47 zygotes was quantitated, and embryonic development was observed until the blastocyst stage. The central coordinates of mPN, fPN, and NPBs were noted at multiple timepoints. Then, the distance traveled by the NPBs between two sequential images was measured, and migration speed was calculated. Additionally, we investigated the relationship between NPB migration speed and ploidy status (N = 33) or live birth/ongoing pregnancy (LB/OP) (N = 60) after assisted reproduction.

RESULTS

The NPB migration speed in both mPN and fPN was significantly faster in the zygotes that developed into blastocysts (N = 25) than that in the zygotes that arrested (N = 22). The timing of blastulation was negatively correlated with NPB migration speed in the mPN. Faster NPB migration was significantly correlated with LB/OP. In multivariate logistic analysis, NPB migration speed in the mPN was the only morphokinetic parameter associated with LB/OP. In a receiver-operating characteristic curve analysis of LB/OP by the NPB migration speed in the mPN, the cut-off value was 4.56 μm/h. When this cut-off value was applied to blastocysts with preimplantation genetic testing for aneuploidy, 100% of the blastocysts faster than or equal to the cut-off value were euploid.

CONCLUSION

The NPBs migrated faster in zygotes having the potential to develop into a blastocyst, and eventually into a baby. This predictor could be an attractive marker for non-invasive embryo selection.

Improving the Quality of Oocytes with the Help of Nucleolotransfer Therapy

The nucleolus is an important nucleus sub-organelle found in almost all eukaryotic cells. On the one hand, it is known as a differentiated active site of ribosome biogenesis in somatic cells, but on the other hand, in fully grown oocytes, zygotes, and early embryos (up to the major embryonic genome activation), it is in the form of a particular homogenous and compact structure called a fibrillar sphere. Nowadays, thanks to recent studies, we know many important functions of this, no doubt, interesting membraneless nucleus sub-organelle involved in oocyte maturation, embryonic genome activation, rRNA synthesis, etc. However, many questions are still unexplained and remain a mystery. Our aim is to create a comprehensive overview of the recent knowledge on the fibrillar sphere and envision how this knowledge could be utilized in further research in the field of biotechnology and nucleolotransfer therapy.

The nucleolus-like and precursor bodies of mammalian oocytes and embryos and their possible role in post-fertilization centromere remodelling

In nearly all somatic cells, the ribosome biosynthesis is a key activity. The same is true also for mammalian oocytes and early embryos. This activity is intimately linked to the most prominent nuclear organelles - the nucleoli. Interestingly, during a short period around fertilization, the nucleoli in oocytes and embryos transform into ribosome-biosynthesis-inactive structures termed nucleolus-like or nucleolus precursor bodies (NPBs). For decades, researchers considered these structures to be passive repositories of nucleolar proteins used by the developing embryo to rebuild fully functional, ribosome-synthesis competent nucleoli when required. Recent evidence, however, indicates that while these structures are unquestionably essential for development, the material is largely dispensable for the formation of active embryonic nucleoli. In this mini-review, we will describe some unique features of oocytes and embryos with respect to ribosome biogenesis and the changes in the structure of oocyte and embryonic nucleoli that reflect this. We will also describe some of the different approaches that can be used to study nucleoli and NPBs in embryos and discuss the different results that might be expected. Finally, we ask whether the main function of nucleolar precursor bodies might lie in the genome organization and remodelling and what the involved components might be.

The nucleolar protein NOP2 is required for nucleolar maturation and ribosome biogenesis during preimplantation development in mammals

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 Transmission 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 In 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 conservation of NOP2 in mammals. Taken together, these results suggest that NOP2 is required for mammalian preimplantation development, presumably by regulating nucleolar maturation and ribosome biogenesis.

Nucleolus and rRNA Gene Chromatin in Early Embryo Development

The nucleolus is the largest substructure in the nucleus and forms around the nucleolar organizer regions (NORs), which comprise hundreds of rRNA genes. Recent evidence highlights further functions of the nucleolus that go beyond ribosome biogenesis. Data indicate that the nucleolus acts as a compartment for the location and regulation of repressive genomic domains and, together with the nuclear lamina, represents the hub for the organization of the inactive heterochromatin. In this review, we discuss recent findings that have revealed how nucleolar structure and rRNA gene chromatin states are regulated during early mammalian development and their contribution to the higher-order spatial organization of the genome.

Nucleoli in embryos: a central structural platform for embryonic chromatin remodeling?

Nucleoli are the site of ribosomal RNA production and subunit assembly. In contrast to active nucleoli in somatic cells, where three basic sub-compartments can be observed, mammalian oocytes and early embryos contain atypical nucleoli termed "nucleolus-like bodies" or "nucleolus precursor bodies", respectively. Unlike their somatic counterparts, these structures are composed of dense homogenous fibrillar material and exhibit no polymerase activity. Irrespective of these unusual properties, they have been shown to be absolutely essential for embryonic development, as their microsurgical removal results in developmental arrest. Historically, nucleolus-like and nucleolus precursor bodies have been perceived as passive storage sites of nucleolar material, which is gradually utilized by embryos to construct fully functional nucleoli once they have activated their genome and have started to produce ribosomes. For decades, researchers have been trying to elucidate the composition of these organelles and provide the evidence for their repository role. However, only recently has it become clear that the function of these atypical nucleoli is altogether different, and rather than being involved in ribosome biogenesis, they participate in parental chromatin remodeling, and strikingly, the artificial introduction of a single NPB component is sufficient to rescue the developmental arrest elicited by the NPB removal. In this review, we will describe and summarize the experiments that led to the change in our understanding of these unique structures.

Single nucleolus precursor body formation in the pronucleus of mouse zygotes and SCNT embryos

Mammalian oocytes and zygotes have nucleoli that are transcriptionally inactive and structurally distinct from nucleoli in somatic cells. These nucleoli have been termed nucleolus precursor bodies (NPBs). Recent research has shown that NPBs are important for embryonic development, but they are only required during pronuclear formation. After fertilization, multiple small NPBs are transiently formed in male and female pronuclei and then fuse into a single large NPB in zygotes. In cloned embryos produced by somatic cell nuclear transfer (SCNT), multiple NPBs are formed and maintained in the pseudo-pronucleus, and this is considered an abnormality of the cloned embryos. Despite this difference between SCNT and normal embryos, it is unclear how the size and number of NPBs in pronuclei is determined. Here, we show that in mouse embryos, the volume of NPB materials plays a major role in the NPB scaling through a limiting component mechanism and determines whether a single or multiple NPBs will form in the pronucleus. Extra NPB- and extra MII spindle-injection experiments demonstrated that the total volume of NPBs was maintained regardless of the pronucleus number and the ratio of pronucleus/NPB is important for fusion into a single NPB. Based on these results, we examined whether extra-NPB injection rescued multiple NPB maintenance in SCNT embryos. When extra-NPBs were injected into enucleated-MII oocytes before SCNT, the number of NPBs in pseudo-pronuclei of SCNT embryos was reduced. These results indicate that multiple NPB maintenance in SCNT embryos is caused by insufficient volume of NPB.

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.

Mouse oocytes nucleoli rescue embryonic development of porcine enucleolated oocytes

It is well known that nucleoli of fully grown mammalian oocytes are indispensable for embryonic development. Therefore, the embryos originated from previously enucleolated (ENL) oocytes undergo only one or two cleavages and then their development ceases. In our study the interspecies (mouse/pig) nucleolus transferred embryos (NuTE) were produced and their embryonic development was analyzed by autoradiography, transmission electron microscopy (TEM) and immunofluorescence (C23 and upstream binding factor (UBF)). Our results show that the re-injection of isolated oocyte nucleoli, either from the pig (P + P) or mouse (P + M), into previously enucleolated and subsequently matured porcine oocytes rescues their development after parthenogenetic activation and some of these develop up to the blastocyst stage (P + P, 11.8%; P + M, 13.5%). In nucleolus re-injected 8-cell and blastocyst stage embryos the number of nucleoli labeled with C23 in P + P and P + M groups was lower than in control (non-manipulated) group. UBF was localized in small foci within the nucleoli of blastocysts in control and P + P embryos, however, in P + M embryos the labeling was evenly distributed in the nucleoplasm. The TEM and autoradiographic evaluations showed the formation of functional nucleoli and de novo rRNA synthesis at the 8-cell stage in both, control and P + P group. In the P + M group the formation of comparable nucleoli was delayed. In conclusion, our results indicate that the mouse nucleolus can rescue embryonic development of enucleolated porcine oocytes, but the localization of selected nucleolar proteins, the timing of transcription activation and the formation of the functional nucleoli in NuTE compared with control group show evident aberrations.

Nucleologenesis and Nucleolotransfer in Mammalian Oocytes: A Review

An effort to improve development potential of early embryos is one of the main goals of biotechnology in the area of reproductive biology with application in veterinary or human medicine. Recent observations of the function of nucleolus or rather its forms before, during and after the fertilisation or parthenogenetic activation show the key role(s) of nucleolus in the processes of early genome activation. The nucleolus is a subnuclear structure (organelle) mainly involved in regulation of transcription and translation. This organelle has been characterized in detail by immunofluorescence, cell transfection and proteomics. This data was, however, mostly obtained in nucleoli of differentiated eukaryotic cells. Much less is known about the nucleolar structural changes and related functional processes in growing and fully grown mammalian oocytes, zygotes and early cleavage stage embryos, especially in the context of embryonic genome activation. It has been shown, that nucleoli in mammalian oocytes and early embryos have several forms and functions, which vary during the oocyte growth and embryonic development. Certain functions have not been fully described or explained, yet. The method of enucleolation, which allows to remove nucleoli from the oocytes or to exchange nucleoli between oocytes or zygotes, together with their proteomic and structural analyses brought new information about functions of nucleoli in oocytes and early cleavage-stage embryos and allowed to explain some new key roles of nucleoli during oocyte maturation and early embryonic development.

Reconstitution of the oocyte nucleolus in mice through a single nucleolar protein, NPM2

The mammalian oocyte nucleolus, the most prominent subcellular organelle in the oocyte, is vital in early development, yet its key functions and constituents remain unclear. We show here that the parthenotes/zygotes derived from enucleolated oocytes exhibited abnormal heterochromatin formation around parental pericentromeric DNAs, which led to a significant mitotic delay and frequent chromosome mis-segregation upon the first mitotic division. A proteomic analysis identified nucleoplasmin 2 (NPM2) as a dominant component of the oocyte nucleolus. Consistently, Npm2-deficient oocytes, which lack a normal nucleolar structure, showed chromosome segregation defects similar to those in enucleolated oocytes, suggesting that nucleolar loss, rather than micromanipulation-related damage to the genome, leads to a disorganization of higher-order chromatin structure in pronuclei and frequent chromosome mis-segregation during the first mitosis. Strikingly, expression of NPM2 alone sufficed to reconstitute the nucleolar structure in enucleolated embryos, and rescued their first mitotic division and full-term development. The nucleolus rescue through NPM2 required the pentamer formation and both the N- and C-terminal domains. Our findings demonstrate that the NPM2-based oocyte nucleolus is an essential platform for parental chromatin organization in early embryonic development.

Localisation of RNAs and proteins in nucleolar precursor bodies of early mouse embryos

Early embryos of all mammalian species contain morphologically distinct but transcriptionally silent nucleoli called the nucleolar precursor bodies (NPBs), which, unlike normal nucleoli, have been poorly studied at the biochemical level. To bridge this gap, here we examined the occurrence of RNA and proteins in early mouse embryos with two fluorochromes - an RNA-binding dye pyronin Y (PY) and the protein-binding dye fluorescein-5'-isothiocyanate (FITC). The staining patterns of zygotic NPBs were then compared with those of nucleolus-like bodies (NLBs) in fully grown surrounded nucleolus (SN)-type oocytes, which are morphologically similar to NPBs. We show that both entities contain proteins, but unlike NLBs, NPBs are significantly impoverished for RNA. Detectable amounts of RNA appear on the NPB surface only after resumption of rDNA transcription and includes pre-rRNAs and 28S rRNA as evidenced by fluorescence in situ hybridisation with specific oligonucleotide probes. Immunocytochemical assays demonstrate that zygotic NPBs contain rRNA processing factors fibrillarin, nucleophosmin and nucleolin, while UBF (the RNA polymerase I transcription factor) and ribosomal proteins RPL26 and RPS10 are not detectable. Based on the results obtained and data in the contemporary literature, we suggest a scheme of NPB assembly and maturation to normal nucleoli that assumes utilisation of maternally derived nucleolar proteins but of nascent rRNAs.

Building up the nucleus: nuclear organization in the establishment of totipotency and pluripotency during mammalian development

In mammals, epigenetic reprogramming, the acquisition and loss of totipotency, and the first cell fate decision all occur within a 3-d window after fertilization from the one-cell zygote to the formation of the blastocyst. These processes are poorly understood in molecular detail, yet this is an essential prerequisite to uncover principles of stem cells, chromatin biology, and thus regenerative medicine. A unique feature of preimplantation development is the drastic genome-wide changes occurring to nuclear architecture. From studying somatic and in vitro cultured embryonic stem cells (ESCs) it is becoming increasingly established that the three-dimensional (3D) positions of genomic loci relative to each other and to specific compartments of the nucleus can act on the regulation of gene expression, potentially driving cell fate. However, the functionality, mechanisms, and molecular characteristics of the changes in nuclear organization during preimplantation development are only now beginning to be unraveled. Here, we discuss the peculiarities of nuclear compartments and chromatin organization during mammalian preimplantation development in the context of the transition from totipotency to pluripotency.

Nucleolus Precursor Bodies and Ribosome Biogenesis in Early Mammalian Embryos: Old Theories and New Discoveries

In mammals, mature oocytes and early preimplantation embryos contain transcriptionally inactive structures termed nucleolus precursor bodies instead of the typical fibrillo-granular nucleoli. These nuclear organelles are essential and strictly of maternal origin. If they are removed from oocytes, the resulting embryos are unable to replace them and consequently fail to develop. Historically, nucleolus precursor bodies have been perceived as a passive repository site of nucleolar proteins that are required for embryos to form fully functional nucleoli. Recent results, however, contradict this long-standing dogma and show that these organelles are dispensable for nucleologenesis and ribosome biogenesis. In this article, we discuss the possible roles of nucleolus precursor bodies and propose how they might be involved in embryogenesis. Furthermore, we argue that these organelles are essential only shortly after fertilization and suggest that they might actively participate in centromeric chromatin establishment.

Nucleolus precursor body (NPB): a distinct structure in mammalian oocytes and zygotes

Nucleoli in mammalian oocytes and zygotes, sometimes referred to as nucleolus precursor bodies (NPBs), are compact and morphologically different from nucleoli in somatic cells. We applied a unique NPB analyzing method "enucleolation" technique to zygotes to remove the NPBs. It has been reported that oocyte NPBs are essential for embryonic development; in their absence, the oocytes complete maturation and can be fertilized, but no nucleoli are formed in the zygotes and embryos, leading to developmental failure. However, we found that when NPBs were removed from zygotes, the zygotes developed successfully to live-born pups. These results indicated that oocyte NPBs are essential for embryonic development, but zygote NPBs are not. In addition, the enucleolated zygotes formed somatic-type nucleoli during early embryonic development, demonstrating that somatic-type nucleoli do not originate from zygote NPBs. We summarize our recent investigation on NPBs, and provide additional comments and findings.

Expression Pattern and Localization Dynamics of Guanine Nucleotide Exchange Factor RIC8 during Mouse Oogenesis

Targeting of G proteins to the cell cortex and their activation is one of the triggers of both asymmetric and symmetric cell division. Resistance to inhibitors of cholinesterase 8 (RIC8), a guanine nucleotide exchange factor, activates a certain subgroup of G protein α-subunits in a receptor independent manner. RIC8 controls the asymmetric cell division in Caenorhabditis elegans and Drosophila melanogaster, and symmetric cell division in cultured mammalian cells, where it regulates the mitotic spindle orientation. Although intensely studied in mitosis, the function of RIC8 in mammalian meiosis has remained unknown. Here we demonstrate that the expression and subcellular localization of RIC8 changes profoundly during mouse oogenesis. Immunofluorescence studies revealed that RIC8 expression is dependent on oocyte growth and cell cycle phase. During oocyte growth, RIC8 is abundantly present in cytoplasm of oocytes at primordial, primary and secondary preantral follicle stages. Later, upon oocyte maturation RIC8 also populates the germinal vesicle, its localization becomes cell cycle dependent, and it associates with chromatin and the meiotic spindle. After fertilization, RIC8 protein converges to the pronuclei and is also detectable at high levels in the nucleolus precursor bodies of both maternal and paternal pronucleus. During first cleavage of zygote RIC8 localizes in the mitotic spindle and cell cortex of forming blastomeres. In addition, we demonstrate that RIC8 co-localizes with its interaction partners Gα_i1/2:GDP and LGN in meiotic/mitotic spindle, cell cortex and polar bodies of maturing oocytes and zygotes. Downregulation of Ric8 by siRNA leads to interferred translocation of Gα_i1/2 to cortical region of maturing oocytes and reduction of its levels. RIC8 is also expressed at high level in female reproductive organs e.g. oviduct. Therefore we suggest a regulatory function for RIC8 in mammalian gametogenesis and fertility.

Nucleolus-like bodies of fully-grown mouse oocytes contain key nucleolar proteins but are impoverished for rRNA

It is well known that fully-grown mammalian oocytes, rather than typical nucleoli, contain prominent but structurally homogenous bodies called "nucleolus-like bodies" (NLBs). NLBs accumulate a vast amount of material, but their biochemical composition and functions remain uncertain. To clarify the composition of the NLB material in mouse GV oocytes, we devised an assay to detect internal oocyte proteins with fluorescein-5-isothiocyanate (FITC) and applied the fluorescent RNA-binding dye acridine orange to examine whether NLBs contain RNA. Our results unequivocally show that, similarly to typical nucleoli, proteins and RNA are major constituents of transcriptionally active (or non-surrounded) NLBs as well as of transcriptionally silent (or surrounded) NLBs. We also show, by exposing fixed oocytes to a mild proteinase K treatment, that the NLB mass in oocytes of both types contains nucleolar proteins that are involved in all major steps of ribosome biogenesis, including rDNA transcription (UBF), early rRNA processing (fibrillarin), and late rRNA processing (NPM1/nucleophosmin/B23, nucleolin/C23), but none of the nuclear proteins tested, including SC35, NOBOX, topoisomerase II beta, HP1α, and H3. The ribosomal RPL26 protein was detected within the NLBs of NSN-type oocytes but is virtually absent from NLBs of SN-type oocytes. Taking into account that the major class of nucleolar RNA is ribosomal RNA (rRNA), we applied fluorescence in situ hybridization with oligonucleotide probes targeting 18S and 28S rRNAs. The results show that, in contrast to active nucleoli, NLBs of fully-grown oocytes are impoverished for the rRNAs, which is consistent with the absence of transcribed ribosomal genes in the NLB mass. Overall, the results of this study suggest that NLBs of fully-grown mammalian oocytes serve for storing major nucleolar proteins but not rRNA.

The maternal nucleolus plays a key role in centromere satellite maintenance during the oocyte to embryo transition

The oocyte (maternal) nucleolus is essential for early embryonic development and embryos originating from enucleolated oocytes arrest at the 2-cell stage. The reason for this is unclear. Surprisingly, RNA polymerase I activity in nucleolus-less mouse embryos, as manifested by pre-rRNA synthesis, and pre-rRNA processing are not affected, indicating an unusual role of the nucleolus. We report here that the maternal nucleolus is indispensable for the regulation of major and minor satellite repeats soon after fertilisation. During the first embryonic cell cycle, absence of the nucleolus causes a significant reduction in major and minor satellite DNA by 12% and 18%, respectively. The expression of satellite transcripts is also affected, being reduced by more than half. Moreover, extensive chromosome bridging of the major and minor satellite sequences was observed during the first mitosis. Finally, we show that the absence of the maternal nucleolus alters S-phase dynamics and causes abnormal deposition of the H3.3 histone chaperone DAXX in pronuclei of nucleolus-less zygotes.

De novo formation of nucleoli in developing mouse embryos originating from enucleolated zygotes

The large, compact oocyte nucleoli, sometimes referred to as nucleolus precursor bodies (NPBs), are essential for embryonic development in mammals; in their absence, the oocytes complete maturation and can be fertilized, but no nucleoli are formed in the zygote or embryo, leading to developmental failure. It has been convincingly documented that zygotes inherit the oocyte nucleolar material and form NPBs again in pronuclei. It is commonly accepted that during early embryonic development, the original compact zygote NPBs gradually transform into reticulated nucleoli of somatic cells. Here, we show that zygote NPBs are not required for embryonic and full-term development in the mouse. When NPBs were removed from late-stage zygotes by micromanipulation, the enucleolated zygotes developed to the blastocyst stage and, after transfer to recipients, live pups were obtained. We also describe de novo formation of nucleoli in developing embryos. After removal of NPBs from zygotes, they formed new nucleoli after several divisions. These results indicate that the zygote NPBs are not used in embryonic development and that the nucleoli in developing embryos originate from de novo synthesized materials.

Cytoplasmic inheritance redux

Since the early twentieth century, inheritance was seen as the inheritance of genes. Concurrent with the acceptance of the genetic theory of inheritance was the rejection of the idea that the cytoplasm of the oocyte could also play a role in inheritance and a corresponding devaluation of embryology as a discipline critical for understanding human development. Development, and variation in development, came to be viewed solely as matters of genetic inheritance and genetic variation. We now know that inheritance is a matter of both genetic and cytoplasmic inheritance. A growing awareness of the centrality of the cytoplasm in explaining both human development and phenotypic variation has been promoted by two contemporaneous developments: the continuing elaboration of the molecular mechanisms of epigenetics and the global rise of artificial reproductive technologies. I review recent developments in the ongoing elaboration of the role of the cytoplasm in human inheritance and development.

Nucleoli from two-cell embryos support the development of enucleolated germinal vesicle oocytes in the pig

Recent research has shown that nucleoli of oocytes at the germinal vesicle (GV) stage (GV nucleoli) are not necessary for oocyte maturation but are essential for early embryonic development. Nucleoli of 2-cell embryos (2-cell nucleoli) have morphology similar to that of nucleoli in oocytes at the GV stage. In this study, we examined the ability of 2-cell nucleoli to substitute for GV nucleoli in terms of supporting early embryonic development by nucleolus aspiration (enucleolation) and transfer into metaphase II (MII) oocytes or 2-cell embryos that were derived from enucleolated oocytes at the GV stage in the pig. When 2-cell embryos were centrifuged to move the lipid droplets to one side of the blastomere, multiple nucleoli in the nucleus fused into a single nucleolus. The nucleoli were then aspirated from the 2-cell embryos by micromanipulation. The injection of 2-cell nucleoli to GV enucleolated oocytes at the MII stage rescued the embryos from the early embryonic arrest, and the resulting oocytes developed to blastocysts. However, the injection of 2-cell and GV nucleoli to 2-cell embryos derived from GV enucleolated oocytes rarely restored the development to blastocysts. These results indicate that 2-cell nucleoli support early embryonic development as GV nucleoli and that the presence of nucleoli is essential for pig embryos before the 2-cell stage.

Production of giant mouse oocyte nucleoli and assessment of their protein content

Compared with advanced developmental stage embryos and somatic cells, fully grown mammalian oocytes contain specific nucleolus-like structures (NPB - nucleolus precursor bodies). It is commonly accepted that they serve as a store of material(s) from which typical nucleoli are gradually formed. Whilst nucleoli from somatic cells can be collected relatively easily for further biochemical analyses, a sufficient number of oocyte nucleoli is very difficult to obtain. We have found that isolated oocytes nucleoli fuse very efficiently when contact is established between them. Thus, well visible giant nucleoli can be obtained, relatively easily handled and then used for further biochemical analyses. With the use of colloidal gold staining, we estimated that a single fully grown mouse oocyte nucleolus contains approximately 1.6 ng of protein. We do believe that this approach will accelerate further research aiming at analyzing the composition of oocyte nucleoli in more detail.