The maternal nucleolus is essential for early embryonic development in mammals

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.

High-resolution microscopy of active ribosomal genes and key members of the rRNA processing machinery inside nucleolus-like bodies of fully-grown mouse oocytes

Nucleolus-like bodies (NLBs) of fully-grown (germinal vesicle, GV) mammalian oocytes are traditionally considered as morphologically distinct entities, which, unlike normal nucleoli, contain transcribed ribosomal genes (rDNA) solely at their surface. In the current study, we for the first time showed that active ribosomal genes are present not only on the surface but also inside NLBs of the NSN-type oocytes. The "internal" rRNA synthesis was evidenced by cytoplasmic microinjections of BrUTP as precursor and by fluorescence in situ hybridization with a probe to the short-lived 5'ETS segment of the 47S pre-rRNA. We further showed that in the NLB mass of NSN-oocytes, distribution of active rDNA, RNA polymerase I (UBF) and rRNA processing (fibrillarin) protein factors, U3 snoRNA, pre-rRNAs and 18S/28S rRNAs is remarkably similar to that in somatic nucleoli capable to make pre-ribosomes. Overall, these observations support the occurrence of rDNA transcription, rRNA processing and pre-ribosome assembly in the NSN-type NLBs and so that their functional similarity to normal nucleoli. Unlike the NSN-type NLBs, the NLBs of more mature SN-oocytes do not contain transcribed rRNA genes, U3 snoRNA, pre-rRNAs, 18S and 28S rRNAs. These results favor the idea that in a process of transformation of NSN-oocytes to SN-oocytes, NLBs cease to produce pre-ribosomes and, moreover, lose their rRNAs. We also concluded that a denaturing fixative 70% ethanol used in the study to fix oocytes could be more appropriate for light microscopy analysis of nucleolar RNAs and proteins in mammalian fully-grown oocytes than a commonly used cross-linking aldehyde fixative, formalin.

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.

How cells build totipotency and pluripotency: nuclear, chromatin and transcriptional architecture

Totipotent and pluripotent cells display different degrees of cellular plasticity. After fertilization, embryonic cells transit naturally from a totipotent to a pluripotent state. Major changes in nuclear architecture, chromatin mobility and gene expression occur during this transition. In particular, nuclear architecture has recently emerged as a potential regulator of heterochromatin formation in the early embryo. Future research should address whether a causal, functional link between nuclear organization and gene regulation is a general theme during reprogramming and the formation of pluripotent cells.

Effects of high hydrostatic pressure on genomic expression profiling of porcine parthenogenetic activated and cloned embryos

Handmade cloning (HMC) has been used to generate transgenic pigs for biomedical research. Recently, we found that parthenogenetic activation (PA) of porcine oocytes and improved HMC efficiency could be achieved by treatment with sublethal high hydrostatic pressure (HHP). However, the molecular mechanism underlying the effects of HHP treatment on embryonic development is poorly understood and so was investigated in the present study. Thus, in the present study, we undertook genome-wide gene expression analysis in HHP-treated and untreated oocytes, as well as in 4-cell and blastocyst stage embryos derived by PA or HMC. Hierarchical clustering depicted stage-specific genomic expression profiling. At the 4-cell and blastocyst stages, 103 and 163 transcripts were differentially expressed between the HMC and PA embryos, respectively (P<0.05). These transcripts are predominantly involved in regulating cellular differentiation, gene expression and cell-to-cell signalling. We found that 44 transcripts were altered by HHP treatment, with most exhibiting lower expression in HHP-treated oocytes. Genes involved in embryonic development were prominent among the transcripts affected by HHP. Two of these genes (INHBB and ME3) were further validated by quantitative reverse transcription-polymerase chain reaction. We also observed that HHP treatment activated expression of the imprinting gene DLX5 in 4-cell PA embryos. In conclusion, our genomic expression profiling data suggest that HHP alters the RNA constitution in porcine oocytes and affects the expression of imprinting genes during embryonic development.

Analysis of nucleolar morphology and protein localization as an indicator of nuclear reprogramming

When a cell is reprogrammed to a new phenotype, the nucleolus undergoes more or less dramatic modulations, which can be used as a marker for the occurrence of the reprogramming. This phenomenon is most pronounced when differentiated cells are reprogrammed to totipotency when they are submitted to cloning by somatic cell nuclear transfer. However, when cells are reprogrammed by less fundamental means, as for example treatment by Xenopus extract or expression of pluripotency genes, more subtle nucleolar modulations can also be noted. The monitoring and understanding of the reprogramming-related nucleolar modulations are based upon detailed knowledge about the nucleolar changes that occur during normal development from the developing oocyte over oocyte maturation and fertilization to the activation of the embryonic genome in the early embryo. Below, the ultrastructural and molecular modulations of the nucleolus are summarized in this developmental context, but also as they occur in assisted reproductive technologies such as in vitro fertilization and somatic cell nuclear transfer. Moreover, detailed protocols for monitoring the nucleolar changes by transmission electron microscopy and immunocytochemistry are presented.

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.

TFIIB co-localizes and interacts with α-tubulin during oocyte meiosis in the mouse and depletion of TFIIB causes arrest of subsequent embryo development

TFIIB (transcription factor IIB) is a transcription factor that provides a bridge between promoter-bound TFIID and RNA polymerase II, and it is a target of various transcriptional activator proteins that stimulate the pre-initiation complex assembly. The localization and/or attachment matrix of TFIIB in the cytoplast is not well understood. This study focuses on the function of TFIIB and its interrelationship with α-tubulins in a mouse model. During oocyte maturation TFIIB distributes throughout the entire nucleus of the germinal vesicle (GV). After progression to GV breakdown (GVBD), TFIIB and α-tubulin co-localize and accumulate in the vicinity of the condensed chromosomes. During the MII stage, the TFIIB signals are more concentrated at the equatorial plate and the kinetochores. Colcemid treatment of oocytes disrupts the microtubule (MT) system, although the TFIIB signals are still present with the altered MT state. Injection of oocytes with TFIIB antibodies and siRNAs causes abnormal spindle formation and irregular chromosome alignment. These findings suggest that TFIIB dissociates from the condensed chromatids and then tightly binds to microtubules from GVBD to the MII phase. The assembly and disassembly of TFIIB may very well be associated with and driven by microtubules. TFIIB maintains its contact with the α-tubulins and its co-localization forms a unique distribution pattern. Depletion of Tf2b in oocytes results in a significant decrease in TFIIB expression, although polar body extrusion does not appear to be affected. Knockdown of Tf2b dramatically affects subsequent embryo development with more than 85% of the embryos arrested at the 2-cell stage. These arrested embryos still maintain apparently normal morphology for at least 96h without any obvious degeneration. Analysis of the effects of TFIIB in somatic cells by co-transfection of BiFC plasmids pHA-Tf2b and pFlag-Tuba1α further confirms a direct interaction between TFIIB and α-tubulins.

Reprogramming of round spermatids by the germinal vesicle cytoplasm in mice

The birthrate following round spermatid injection (ROSI) remains low in current and evidence suggests that factors in the germinal vesicle (GV) cytoplasm and certain substances in the GV such as the nucleolus might be responsible for genomic reprogramming and embryonic development. However, little is known whether the reprogramming factors in GV oocyte cytoplasm and/or nucleolus in GV are beneficial to the reprogramming of round spermatids and development of ROSI embryos. Here, round spermatids were treated with GV cytolysates and injected this round spermatid alone or co-injected with GV oocyte nucleolus into mature metaphase II oocytes. Subsequent embryonic development was assessed morphologically and by Oct4 expression in blastocysts. There was no significant difference between experimental groups at the zygote to four-cell development stages. Blastocysts derived from oocytes which were injected with cytolysate treated-round spermatid alone or co-injected with nucleoli injection yielded 63.6% and 70.3% high quality embryos, respectively; comparable to blastocysts derived by intracytoplasmic sperm injection (ICSI), but higher than these oocytes which were co-injected with lysis buffer-treated round spermatids and nucleoli or injected with the lysis buffer-treated round spermatids alone. Furthermore, the proportion of live offspring resulting from oocytes which were co-injected with cytolysate treated-round spermatids and nucleoli or injected with cytolysate treated-round spermatids alone was higher than those were injected with lysis buffer treated-round spermaids, but comparable with the ICSI group. Our results demonstrate that factors from the GV cytoplasm improve round spermatid reprogramming, and while injection of the extra nucleolus does not obviously improve reprogramming its potential contribution, although which cannot be definitively excluded. Thus, some reprogramming factors are evidently present in GV oocyte cytoplasm and could significantly facilitate ROSI technology, while the nucleolus in GV seems also having a potential to improve reprogramming of round spermatids.

Interspecies somatic cell nuclear transfer: advancements and problems

Embryologists working with livestock species were the pioneers in the field of reprogramming by somatic cell nuclear transfer (SCNT). Without the "Dolly experiment," the field of cellular reprogramming would have been slow and induced plutipotent cells (iPSCs) would not have been conceived. The major drive of the work in mammalian cloning was the interest of the breeding industry to propagate superior genotypes. Soon it was realized that the properties of oocytes could be used also to clone endangered mammalian species or to reprogram the genomes of unrelated species through what is known as interspecies (i) SCNT, using easily available oocytes of livestock species. iSCNT for cloning animals works only for species that can interbreed, and experiments with taxonomically distant species have not been successful in obtaining live births or deriving embryonic stem cell (ESC) lines to be used for regenerative medicine. There are controversial reports in the literature, but in most cases these experiments have underlined some of the cellular and molecular mechanisms that are incomplete during cell nucleus reprogramming, including the failure to organize nucleoli, silence somatic cell genes, activate the embryonic genome, and resume mitochondrial replication and function, thus indicating nucleus-cytoplasmic incompatibility.

Nuclear Wave1 is required for reprogramming transcription in oocytes and for normal development

Eggs and oocytes have a remarkable ability to induce transcription of sperm after normal fertilization and in somatic nuclei after somatic cell nuclear transfer. This ability of eggs and oocytes is essential for normal development. Nuclear actin and actin-binding proteins have been shown to contribute to transcription, although their mode of action is elusive. Here, we find that Xenopus Wave1, previously characterized as a protein involved in actin cytoskeleton organization, is present in the oocyte nucleus and is required for efficient transcriptional reprogramming. Moreover, Wave1 knockdown in embryos results in abnormal development and defective hox gene activation. Nuclear Wave1 binds by its WHD domain to active transcription components, and this binding contributes to the action of RNA polymerase II. We identify Wave1 as a maternal reprogramming factor that also has a necessary role in gene activation in development.

Hematoxylin staining reveals a decrease in nucleolar diameter of pig oocytes before germinal vesicle breakdown

During oocyte growth, the morphology of the nucleolus changes into a compact and homogenous structure. The compact nucleoli in full-grown oocytes are not stained by aceto-orcein staining or immunofluorescence staining. In this study, we developed a hematoxylin staining method for pig oocytes in whole-mount preparations to visualize the nucleoli. Nucleoli of growing and full-grown oocytes were stained blue with hematoxylin. Using this staining method, the changes in the oocyte nucleolus during maturation were examined. The nucleolar diameter gradually decreased in maturing oocytes (10.7 ± 0.1 μm to 9.0 ± 0.7 μm, P<0.05) before germinal vesicle breakdown (GVBD). The results suggest that the nucleolar volume of oocytes decreases before GVBD.

Nuclear reprogramming of sperm and somatic nuclei in eggs and oocytes

Eggs and oocytes have a prominent ability to reprogram sperm nuclei for ensuring embryonic development. The reprogramming activity that eggs/oocytes intrinsically have towards sperm is utilised to reprogram somatic nuclei injected into eggs/oocytes in nuclear transfer (NT) embryos. NT embryos of various species can give rise to cloned animals, demonstrating that eggs/oocytes can confer totipotency even to somatic nuclei. However, many studies indicate that reprogramming of somatic nuclei is not as efficient as that of sperm nuclei. In this review, we explain how and why sperm and somatic nuclei are differentially reprogrammed in eggs/oocytes. Recent studies have shown that sperm chromatin is epigenetically modified to be adequate for early embryonic development, while somatic nuclei do not have such modifications. Moreover, epigenetic memories encoded in sperm chromatin are transgenerationally inherited, implying unique roles of sperm. We also discuss whether somatic nuclei can be artificially modified to acquire sperm-like chromatin states in order to increase the efficiency of nuclear reprogramming.

Importance of the pluripotency factor LIN28 in the mammalian nucleolus during early embryonic development

The maternal nucleolus is required for proper activation of the embryonic genome (EGA) and early embryonic development. Nucleologenesis is characterized by the transformation of a nucleolar precursor body (NPB) to a mature nucleolus during preimplantation development. However, the function of NPBs and the involved molecular factors are unknown. We uncover a novel role for the pluripotency factor LIN28, the biological significance of which was previously demonstrated in the reprogramming of human somatic cells to induced pluripotent stem (iPS) cells. Here, we show that LIN28 accumulates at the NPB and the mature nucleolus in mouse preimplantation embryos and embryonic stem cells (ESCs), where it colocalizes with the nucleolar marker B23 (nucleophosmin 1). LIN28 has nucleolar localization in non-human primate (NHP) preimplantation embryos, but is cytoplasmic in NHP ESCs. Lin28 transcripts show a striking decline before mouse EGA, whereas LIN28 protein localizes to NPBs at the time of EGA. Following knockdown with a Lin28 morpholino, the majority of embryos arrest between the 2- and 4-cell stages and never develop to morula or blastocyst. Lin28 morpholino-injected embryos arrested at the 2-cell stage were not enriched with nucleophosmin at presumptive NPB sites, indicating that functional NPBs were not assembled. Based on these results, we propose that LIN28 is an essential factor of nucleologenesis during early embryonic development.

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.

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.

Vertebrate nucleoplasmin and NASP: egg histone storage proteins with multiple chaperone activities

Recent reviews have focused on the structure and function of histone chaperones involved in different aspects of somatic cell chromatin metabolism. One of the most dramatic chromatin remodeling processes takes place immediately after fertilization and is mediated by egg histone storage chaperones. These include members of the nucleoplasmin (NPM2/NPM3), which are preferentially associated with histones H2A-H2B in the egg and the nuclear autoantigenic sperm protein (NASP) families. Interestingly, in addition to binding and providing storage to H3/H4 in the egg and in somatic cells, NASP has been shown to be a unique genuine chaperone for histone H1. This review revolves around the structural and functional roles of these two families of chaperones whose activity is modulated by their own post-translational modifications (PTMs), particularly phosphorylation. Beyond their important role in the remodeling of paternal chromatin in the early stages of embryogenesis, NPM and NASP members can interact with a plethora of proteins in addition to histones in somatic cells and play a critical role in processes of functional cell alteration, such as in cancer. Despite their common presence in the egg, these two histone chaperones appear to be evolutionarily unrelated. In contrast to members of the NPM family, which share a common monophyletic evolutionary origin, the different types of NASP appear to have evolved recurrently within different taxa.

Early aberrations in chromatin dynamics in embryos produced under in vitro conditions

In vitro production of porcine embryos by means of in vitro fertilization (IVF) or somatic cell nuclear transfer (SCNT) is limited by great inefficienciy. The present study investigated chromatin and nucleolar dynamics in porcine embryos developed in vivo (IV) and compared this physiological standard to that of embryos produced by IVF, parthenogenetic activation (PA), or SCNT. In contrast to IV embryos, chromatin spatial and temporal dynamics in PA, IVF, and SCNT embryos were altered; starting with aberrant chromatin-nuclear envelope interactions at the two-cell stage, delayed chromatin decondensation and nucleolar development at the four-cell stage, and ultimately culminating in failure of proper first lineage segregation at the blastocyst stage, demonstrated by poorly defined inner cell mass. Interestingly, in vitro produced (IVP) embryos also lacked a heterochromatin halo around nucleolar precursors, indicating imperfections in global chromatin remodeling after fertilization/activation. Porcine IV-produced zygotes and embryos display a well-synchronized pattern of chromatin dynamics compatible with genome activation and regular nucleolar formation at the four-cell stage. Production of porcine embryos under in vitro conditions by IVF, PA, or SCNT is associated with altered chromatin remodeling, delayed nucleolar formation, and poorly defined lineage segregation at the blastocyst stage, which in turn may impair their developmental capacity.

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.

Deficient induction response in a Xenopus nucleocytoplasmic hybrid

Defects in induction signaling and response underlie the nucleocytoplasmic incompatibility between two evolutionarily distant frog species, while specific treatments partially restore this response in explants and whole embryos.

Incompatibilities between the nucleus and the cytoplasm of sufficiently distant species result in developmental arrest of hybrid and nucleocytoplasmic hybrid (cybrid) embryos. Several hypotheses have been proposed to explain their lethality, including problems in embryonic genome activation (EGA) and/or nucleo-mitochondrial interactions. However, conclusive identification of the causes underlying developmental defects of cybrid embryos is still lacking. We show here that while over 80% of both Xenopus laevis and Xenopus (Silurana) tropicalis same-species androgenetic haploids develop to the swimming tadpole stage, the androgenetic cybrids formed by the combination of X. laevis egg cytoplasm and X. tropicalis sperm nucleus invariably fail to gastrulate properly and never reach the swimming tadpole stage. In spite of this arrest, these cybrids show quantitatively normal EGA and energy levels at the stage where their initial gastrulation defects are manifested. The nucleocytoplasmic incompatibility between these two species instead results from a combination of factors, including a reduced emission of induction signal from the vegetal half, a decreased sensitivity of animal cells to induction signals, and differences in a key embryonic protein (Xbra) concentration between the two species, together leading to inefficient induction and defective convergence-extension during gastrulation. Indeed, increased exposure to induction signals and/or Xbra signalling partially rescues the induction response in animal explants and whole cybrid embryos. Altogether, our study demonstrates that the egg cytoplasm of one species may not support the development promoted by the nucleus of another species, even if this nucleus does not interfere with the cytoplasmic/maternal functions of the egg, while the egg cytoplasm is also capable of activating the genome of that nucleus. Instead, our results provide evidence that inefficient signalling and differences in the concentrations of key proteins between species lead to developmental defects in cybrids. Finally, they show that the incompatibilities of cybrids can be corrected by appropriate treatments.

When two species evolve separately for several million years, their respective genomes accumulate many small changes that together are responsible for the differences in their characters. Some of these affect the way eggs are prepared inside the germline, and/or how embryos develop, such that the egg cytoplasm of a given species can only support development promoted by its own genome or nucleus. Thus, developmental incompatibility arises between the cytoplasm and the nucleus of distant species during evolution and we don't know its mechanism. We have studied this phenomenon in an advantageous system using two evolutionarily distant frog species (Xenopus laevis and Xenopus tropicalis). We found that hybrid frog embryos with X. laevis cytoplasm and X. tropicalis nuclei are always defective in an important process that is necessary to generate morphogenetic cell movements during development. Through a series of experiments in which we dissect out and/or recombine parts of such hybrid embryos and observe their behaviour in culture, we show that this phenomenon occurs because of malfunctions in the signalling cascade that is responsible for generating these cell movements. Thus, we postulate that inefficient molecular signalling contributes to the death of such hybrids.

The effect of growth hormone (GH) and insulin-like growth factor-I (IGF-I) on in vitro maturation of equine oocytes

The objective of this study was to test the hypothesis that equine growth hormone (eGH), in combination with insulin growth factor-I (IGF-I), influences positively in vitro nuclear and cytoplasmic maturation of equine oocytes. Cumulus–oocyte complexes were recovered from follicles that were < 25 mm in diameter, characterized by morphology and were allocated randomly as follow: (a) control (no additives); (b) 400 ng/ml eGH; (c) 200 ng/ml IGF-I; (d) eGH + IGF-I; and (e) eGH + IGF-I + 400 ng/ml anti-IGF-I antibody. Oocytes were matured for 30 h at 38.5°C in air with 5% CO2 and then stained with 10 μg/ml propidium iodide (PI) to evaluate nuclear status and 10 μg/ml Lens culinaris agglutinin-fluorescein complex (FITC-LCA) to assess cortical granule migration by confocal microscopy. The proportion of immature oocytes that developed to the metaphase II (MII) stage in the eGH + IGF-I group (15 of 45) was greater than in the groups that were treated only with IGF-I (7 of 36, p = 0.03). Oocytes that reached MII in the control group (20 of 56; 35.7%) showed a tendency to be different when compared with eGH + IGF-I group (15 of 45; 33.3%, p = 0.08). The treated group that contained anti-IGF-I (15 of 33; 45.4%) decreased the number of oocytes reaching any stage of development when compared with eGH (47 of 72; 65.3%) and eGH + IGF-I (33 of 45; 73.3%) groups (p = 0.05) when data from MI and MII were combined. We concluded that the addition of eGH to in vitro maturation (IVM) medium influenced the in vitro nuclear and cytoplasmic maturation of equine oocytes. The use of GH and IGF-I in vitro may represent a potential alternative for IVM of equine oocytes.

Nucleoli from growing oocytes inhibit the maturation of enucleolated, full-grown oocytes in the pig

In mammals, the nucleolus of full-grown oocyte is essential for embryonic development but not for oocyte maturation. In our study, the role of the growing oocyte nucleolus in oocyte maturation was examined by nucleolus removal and/or transfer into previously enucleolated, growing (around 100 µm in diameter) or full-grown (120 µm) pig oocytes. In the first experiment, the nucleoli were aspirated from growing oocytes whose nucleoli had been compacted by actinomycin D treatment, and the enucleolated oocytes were matured in vitro. Most of non-treated or actinomycin D-treated oocytes did not undergo germinal vesicle breakdown (GVBD; 13% and 12%, respectively). However, the GVBD rate of enucleolated, growing oocytes significantly increased to 46%. The low GVBD rate of enucleolated, growing oocytes was restored again by the re-injection of nucleoli from growing oocytes (23%), but not when nucleoli from full-grown oocytes were re-injected into enucleolated, growing oocytes (49%). When enucleolated, full-grown oocytes were injected with nucleoli from growing or full-grown oocytes, the nucleolus in the germinal vesicle was reassembled (73% and 60%, respectively). After maturation, the enucleolated, full-grown oocytes injected with nucleoli from full-grown oocytes matured to metaphase II (56%), whereas injection with growing-oocyte nucleoli reduced this maturation to 21%. These results suggest that the growing-oocyte nucleolus is involved in the oocyte's meiotic arrest, and that the full-grown oocyte nucleolus has lost the ability.

Nuclear transfer of embryonic cell nuclei to non-enucleated eggs in zebrafish, Danio rerio

We previously established a novel method for nuclear transfer in medaka (Oryzias latipes) using non-enucleated, diploidized eggs as recipients for adult somatic cell nuclei. Here we report the first attempt to apply this method to another fish species. To examine suitability of using non-enucleated eggs as recipients for nuclear transfer in the zebrafish (Danio rerio), we transferred blastula cell nuclei from a wild-type donor strain to non-enucleated, unfertilized eggs from a golden recipient strain. As a result, 31 of 184 (16.8%) operated eggs developed normally and reached the adult stage. Twenty-eight (15.2%) of these transplants showed wild-type phenotype and the remaining three (1.6%) were golden. Except for one individual that exhibited diploid/tetraploid mosaicism, all of the wild-type nuclear transplants were either triploid or diploid. While all of 19 triploid transplants were infertile, a total of six transplants (21.4%) were fertile (five of the eight diploid transplants and one transplant exhibiting ploidy mosaicism). Except for one diploid individual, all of the fertile transplants transferred both the wild-type golden gene allele (slc24a5) as well as the phenotype, the wild-type body color, to their F₁ and F₂ progeny in a typical Mendelian fashion. PCR analysis of slc24a5 suggested that triploidy originated from a fused nucleus in the diploid donor and haploid recipient nuclei, and that the sole origin of diploidy was the diploid donor nucleus. The results of the present study demonstrated the suitability of using non-enucleated eggs as recipients for nuclear transfer experiments in zebrafish.

Formation of nucleoli in interspecies nuclear transfer embryos derived from bovine, porcine, and rabbit oocytes and nuclear donor cells of various species

The most successful development of interspecies somatic cell nuclear transfer (iSCNT) embryos has been achieved in closely related species. The analyses of embryonic gene activity in iSCNT embryos of different species combinations have revealed the existence of significant aberrations in expression of housekeeping genes and genes dependent on the major embryonic genome activation (EGA). However, there are many studies with successful blastocyst (BL) development of iSCNT embryos derived from donor cells and oocytes of animal species with distant taxonomical relations (inter-family/inter-class) that should indicate proper EGA at least in terms of RNA polymerase I activation, nucleoli formation, and activation of genes engaged in morula and BL formation. We investigated the ability of bovine, porcine, and rabbit oocytes to activate embryonic nucleoli formation in the nuclei of somatic cells of different mammalian species. In iSCNT embryos, nucleoli precursor bodies originate from the oocyte, while most proteins engaged in the formation of mature nucleoli should be transcribed from genes de novo in the donor nucleus at the time of EGA. Thus, the success of nucleoli formation depends on species compatibility of many components of this complex process. We demonstrate that the time and cell stage of nucleoli formation are under the control of recipient ooplasm. Oocytes of the studied species possess different abilities to support nucleoli formation. Formation of nucleoli, which is a complex but small part of the whole process of EGA, is essential but not absolutely sufficient for the development of iSCNT embryos to the morula and BL stages.

Possible involvement of Class III phosphatidylinositol-3-kinase in meiotic progression of porcine oocytes beyond germinal vesicle stage

Phosphatidylinositol-3-kinases (PI3Ks) play pivotal roles in meiotic progression of oocytes from metaphase I to metaphase II stage. Using a Class III-specific inhibitor of PI3K, 3-methyladenine (3MA), this study shows that Class III PI3K may be essential for meiotic progression of porcine oocytes beyond germinal vesicle (GV) stage. Treatment of immature porcine oocytes with 3MA for 22-42 h arrested them at the GV stage, irrespective of the presence or absence of cumulus cells. Furthermore, a significantly high proportion (60.9 ± 13.8%) of 3MA-treated oocytes acquired a nucleolus completely surrounded by a rim of highly condensed chromatin (GV-II stage). The GV-arresting effect of 3MA was, however, completely reversible upon their further culture in the absence of 3MA for 22 h. When cumulus-oophorus-complexes (COCs), arrested at the GV stage for 22 h by 3MA, were further cultured for 22 h in the absence of 3MA, 96.1 ± 1.5% of oocytes reached the MII stage at 42 h of IVM and did not differ from non-treated control oocytes with respect to their ability to fertilize, cleave and form blastocyst (P > 0.05) upon in vitro fertilization (IVF) or parthenogenetic activation (PA). These data suggest that 3MA efficiently blocks and synchronizes the meiotic progression of porcine oocytes at the GV stage without affecting their ooplasmic maturation in terms of post-fertilization/activation in vitro embryonic development. Our data also provide indirect evidence for the likely participation of Class III PI3K in meiotic maturation of porcine oocyte beyond the GV stage.

EMG1 is essential for mouse pre-implantation embryo development

Background

Essential for mitotic growth 1 (EMG1) is a highly conserved nucleolar protein identified in yeast to have a critical function in ribosome biogenesis. A mutation in the human EMG1 homolog causes Bowen-Conradi syndrome (BCS), a developmental disorder characterized by severe growth failure and psychomotor retardation leading to death in early childhood. To begin to understand the role of EMG1 in mammalian development, and how its deficiency could lead to Bowen-Conradi syndrome, we have used mouse as a model. The expression of Emg1 during mouse development was examined and mice carrying a null mutation for Emg1 were generated and characterized.

Results

Our studies indicated that Emg1 is broadly expressed during early mouse embryonic development. However, in late embryonic stages and during postnatal development, Emg1 exhibited specific expression patterns. To assess a developmental role for EMG1 in vivo, we exploited a mouse gene-targeting approach. Loss of EMG1 function in mice arrested embryonic development prior to the blastocyst stage. The arrested Emg1^-/- embryos exhibited defects in early cell lineage-specification as well as in nucleologenesis. Further, loss of p53, which has been shown to rescue some phenotypes resulting from defects in ribosome biogenesis, failed to rescue the Emg1^-/- pre-implantation lethality.

Conclusion

Our data demonstrate that Emg1 is highly expressed during mouse embryonic development, and essential for mouse pre-implantation development. The absolute requirement for EMG1 in early embryonic development is consistent with its essential role in yeast. Further, our findings also lend support to the previous study that showed Bowen-Conradi syndrome results from a partial EMG1 deficiency. A complete deficiency would not be expected to be compatible with a live birth.

Two-step imprinted X inactivation: repeat versus genic silencing in the mouse

Mammals compensate for unequal X-linked gene dosages between the sexes by inactivating one X chromosome in the female. In marsupials and in the early mouse embryo, X chromosome inactivation (XCI) is imprinted to occur selectively on the paternal X chromosome (X(P)). The mechanisms and events underlying X(P) imprinting remain unclear. Here, we find that the imprinted X(P) can be functionally divided into two domains, one comprising traditional coding genes (genic) and the other comprising intergenic repetitive elements. X(P) repetitive element silencing occurs by the two-cell stage, does not require Xist, and occurs several divisions prior to genic silencing. In contrast, genic silencing initiates at the morula-to-blastocyst stage and absolutely requires Xist. Genes translocate into the presilenced repeat region as they are inactivated, whereas active genes remain outside. Thus, during the gamete-embryo transition, imprinted XCI occurs in two steps, with repeat silencing preceding genic inactivation. Nucleolar association may underlie the epigenetic asymmetry of X(P) and X(M). We hypothesize that transgenerational information (the imprint) is carried by repeats from the paternal germ line or that, alternatively, repetitive elements are silenced at the two-cell stage in a parent-of-origin-specific manner. Our model incorporates aspects of the so-called classical, de novo, and preinactivation hypotheses and suggests that Xist RNA functions relatively late during preimplantation mouse development.

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

During oocyte growth in the ovary, the nucleolus is mainly responsible for ribosome biogenesis. However, in the fully-grown oocyte, all transcription ceases, including ribosomal RNA synthesis, and the nucleolus adopts a specific monotonous fibrillar morphology without chromatin. The function of this inactive nucleolus in oocytes and embryos is still unknown. We previously reported that the embryo lacking an inactive nucleolus failed to develop past the first few cleavages, indicating the requirement of a nucleolus for preimplantation development. Here, we reinjected the nucleolus into oocytes and zygotes without nucleoli at various time points to examine the timing of the nucleolus requirement during meiosis and early embryonic development. When we put the nucleolus back into oocytes lacking a nucleolus at the germinal vesicle (GV) stage and at second metaphase (MII), these oocytes were fertilized, formed pronuclei with nucleoli and developed to full term. When the nucleolus was reinjected at the pronucleus (PN) stage, most of the reconstructed zygotes cleaved and formed nuclei with nucleoli at the 2-cell stage, but the rate of blastocyst formation and the numbers of surviving pups were profoundly reduced. Moreover, the zygotes without nucleoli showed a disorder of higher chromatin organization not only in the female pronucleus but also, interestingly, in the male pronucleus. Thus, the critical time point when the nucleolus is required for progression of early embryonic development appears to be at the point of the early step of pronucleus organization.

How to improve the success rate of mouse cloning technology

It has now been 13 years since the first cloned mammal Dolly the sheep was generated from somatic cells using nuclear transfer (SCNT). Since then, this technique has been considered an important tool not only for animal reproduction but also for regenerative medicine. However, the success rate is still very low and the mechanisms involved in genomic reprogramming are not yet clear. Moreover, the NT technique requires donated fresh oocyte, which raises ethical problems for production of human cloned embryo. For this reason, the use of induced pluripotent stem cells for genomic reprogramming and for regenerative medicine is currently a hot topic in this field. However, we believe that the NT approach remains the only valid way for the study of reproduction and basic biology. For example, only the NT approach can reveal dynamic and global modifications in the epigenome without using genetic modification, and it can generate offspring from a single cell or even a frozen dead body. Thanks to much hard work by many groups, cloning success rates are increasing slightly year by year, and NT cloning is now becoming a more applicable method. This review describes how to improve the efficiency of cloning, the establishment of clone-derived embryonic stem cells and further applications.

Nucleoli from growing oocytes support the development of enucleolated full-grown oocytes in the pig

Recent research has shown that the maternal nucleolus is essential for embryonic development. The morphology of the nucleolus in growing oocytes differs from that in full-grown oocytes. We determined the ability of nucleoli from growing oocytes to substitute for nucleoli of full-grown oocytes in terms of supporting embryonic development in this study. Growing (around 100 microm in diameter) and full-grown porcine oocytes (120 microm) were collected from small (0.6-1.0 mm) and large antral follicles (4-5 mm), respectively. The nucleolus was aspirated from full-grown oocytes by micromanipulation, and the resulting enucleolated oocytes were matured to metaphase II; the nucleoli originating from full-grown and growing oocytes were then injected into the oocytes. The Chromatin of growing oocytes was aspirated with the nucleolus during the enucleolation process. Growing oocytes were thus treated with actinomycin D to release the chromatin from their nucleoli, and the nucleoli were collected and transferred to the enucleolated and matured full-grown oocytes. After activation by electro-stimulation, nucleoli were formed in pronuclei of sham-operated oocytes. Enucleolated oocytes that had been injected with nucleoli from either full-grown or growing, however, did not form any nucleoli in the pronuclei. No enucleolated oocytes developed to blastocysts, whereas enucleolated oocytes injected with nucleoli from full-grown oocytes (15%) or growing oocytes (18%) developed to blastocysts. These results indicate that the nucleoli from growing oocytes can substitute for nucleoli from full-grown oocytes during early embryonic development.

Pronuclear zygote score following intracytoplasmic injection of hyaluronan-bound spermatozoa: a prospective randomized study

The aim of this study was to determine whether the early stages of embryo development, as assessed by the zygote score (Z-score), could be influenced by the injection of spermatozoa that had been preselected on the basis of their binding to hyaluronic acid (HA). A total of 407 sibling metaphase II oocytes, belonging to 44 different patients, were injected in a prospective randomized way, with either hyaluronic acid bound (HA(+) ) or non-bound (HA(-)) spermatozoa. The fertilization rate (75-70%), the percentage of the different Z-scores (Z 1: 22-24%, Z 2: 22-22.5%, Z 3: 44- 45%, Z 4: 12-8.5%), the mean score of the transferred embryos (3.76 +/- 1.29, 3.78 +/- 1.1) and the number of embryos at the 4-cell stage 45 h after injection (77-76%) were not different between the two groups. The ongoing pregnancy rate in this study (>20 weeks of gestation) was 36.4% per replacement, the implantation rate 28% and the twin pregnancy rate 44% (7/16). Although binding to HA did not apparently influence the Z-score, this agent continues to be used for the immobilization of spermatozoa prior to injection, on the basis that it is a natural product that can easily be metabolized by the oocyte via normal biological mechanisms.

SUV39h-independent association of HP1 beta with fibrillarin-positive nucleolar regions

Heterochromatin protein 1 (HP1), which binds to sites of histone H3 lysine 9 (H3K9) methylation, is primarily responsible for gene silencing and the formation of heterochromatin. We observed that HP1 beta is located in both the chromocenters and fibrillarin-positive nucleoli interiors. However, HP1 alpha and HP1 gamma occupied fibrillarin-positive compartments to a lesser extent, corresponding to the distinct levels of HP1 subtypes at the promoter of rDNA genes. Deficiency of histone methyltransferases SUV39h and/or inhibition of histone deacetylases (HDACi) decreased HP1 beta and H3K9 trimethylation at chromocenters, but not in fibrillarin-positive regions that co-localized with RNA polymerase I. Similarly, SUV39h- and HDACi-dependent nucleolar rearrangement and inhibition of rDNA transcription did not affect the association between HP1 beta and fibrillarin. Moreover, the presence of HP1 beta in nucleoli is likely connected with transcription of ribosomal genes and with the role of fibrillarin in nucleolar processes.

Nucleolar transplantation in oocytes and zygotes: challenges for further research

In germinal vesicles of immature mammalian oocytes, including humans, as well as in pronuclei in 1-cell stage embryos, prominent nuclear organelles, nucleoli, can be easily detected even under a relatively low magnification. In humans, it has been clearly documented that their number, position and distribution in pronuclei can be used as an indicator of embryonic developmental potential. In the light of some recent experiments showing the feasibility of nucleolar manipulation we discuss here if these new approaches can be used to rescue those embryos with abnormal pronuclear nucleolar patterns.

Characterization of SoxB2 and SoxC genes in amphioxus (Branchiostoma belcheri): implications for their evolutionary conservation

Most Sox genes directly affect cell fate determination and differentiation. In this study, we isolated two Sox genes: SoxB2 and SoxC from amphioxus (Branchiostoma belcheri), the closest living invertebrate relative of the vertebrates. Alignments of SoxB2 and SoxC protein sequences and their vertebrate homologs show high conservation of their HMG domains. Phylogenic analysis shows that amphioxus SoxB2 and SoxC fall out of the vertebrate branches, suggesting that vertebrate homologs might arise from gene duplications during evolution. The two genes possess similar spatial and temporal expression patterns during embryogenesis and in adults. They are both maternally inherited. During neurulation, they are expressed in the neural ectoderm and archenterons. In adults, they are expressed not only in the nerve cord, but also in the gut, midgut diverticulum, gill and oocytes. These results suggest that amphioxus SoxB2 and SoxC might co-function and have conserved functions in the nervous system and gonads as their vertebrate homologs.

Role of the nucleoplasmin 2 C-terminal domain in the formation of nucleolus-like bodies in mouse oocytes

Nucleolus-like bodies (NLBs) are characteristic structures found in the germinal vesicles of mammalian oocytes. Although these structures are essential for embryonic development, their composition, precise function, and mechanism of formation have not been elucidated. Here, we used immunoblotting and EGFP fusion protein fluorescence to demonstrate that murine nucleoplasmin 2 (NPM2) is a component of mouse NLBs and that the targeting of NPM2 to NLBs is regulated by a lysine-rich, 16-aa C-terminal motif (K-rich motif). When the K-rich motif was fused to another nuclear protein, MafG, the resultant fusion protein accumulated in NLBs but not in the nucleoli of somatic cells, suggesting that the K-rich motif functions to target NPM2 specifically to NLBs. To investigate the role of the K-rich motif in NLB formation, we replaced the endogenous NPM2 in growing oocytes with a mutant NPM2 protein lacking the K-rich motif (NPM2(C16del)). Growing oocytes surrounded by granulosa layers were coinjected with NPM2(C16del) mRNA and with small-interfering RNA targeting NPM2 (siNpm2), which was used to degrade the endogenous NPM2 mRNA. After culture in vitro, the NLBs in the resulting full-grown oocytes were significantly smaller than those in control oocytes that had been coinjected with siNpm2 and NPM2 mRNA, indicating that the K-rich motif is necessary for NLB development. Together, these results suggest that NPM2 targeting of NLBs is regulated by the K-rich motif and is essential for the formation of NLBs.