Nuclear distribution of Oct-4 transcription factor in transcriptionally active and inactive mouse oocytes and its relation to RNA polymerase II and splicing factors

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.

Ribosomal RNA gene functioning in avian oogenesis

Despite long-term exploration into ribosomal RNA gene functioning during the oogenesis of various organisms, many intriguing problems remain unsolved. In this review, we describe nucleolus organizer region (NOR) activity in avian oocytes. Whereas oocytes from an adult avian ovary never reveal the formation of the nucleolus in the germinal vesicle (GV), an ovary from juvenile birds possesses both nucleolus-containing and non-nucleolus-containing oocytes. The evolutionary diversity of oocyte NOR functioning and the potential non-rRNA-related functions of the nucleolus in oocytes are also discussed.

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.

Nuclear Nox4 Role in Stemness Power of Human Amniotic Fluid Stem Cells

Human amniotic fluid stem cells (AFSC) are an attractive source for cell therapy due to their multilineage differentiation potential and accessibility advantages. However the clinical application of human stem cells largely depends on their capacity to expand in vitro, since there is an extensive donor-to-donor heterogeneity. Reactive oxygen species (ROS) and cellular oxidative stress are involved in many physiological and pathophysiological processes of stem cells, including pluripotency, proliferation, differentiation, and stress resistance. The mode of action of ROS is also dependent on the localization of their target molecules. Thus, the modifications induced by ROS can be separated depending on the cellular compartments they affect. NAD(P)H oxidase family, particularly Nox4, has been known to produce ROS in the nucleus. In the present study we show that Nox4 nuclear expression (nNox4) depends on the donor and it correlates with the expression of transcription factors involved in stemness regulation, such as Oct4, SSEA-4, and Sox2. Moreover nNox4 is linked with the nuclear localization of redox sensitive transcription factors, as Nrf2 and NF-κB, and with the differentiation potential. Taken together, these results suggest that nNox4 regulation may have important effects in stem cell capability through modulation of transcription factors and DNA damage.

Genomic characterization of the mouse ribosomal DNA locus

The transcription of rRNA is critical to all living cells and is tightly controlled at the level of chromatin structure. Although the widespread adoption of genomic technologies including chromatin immunoprecipitation with massively parallel short-read sequencing (ChIP-seq) has allowed for the interrogation of chromatin structure on a genome-wide scale, until recently rDNA has not been analyzed by this technique. We extended genomic analysis of rDNA to mouse (Mus musculus), in which rDNA is similar in structure but highly divergent in sequence compared with human rDNA. Comparison of rDNA histone marks between mouse embryonic stem cells (mESCs) and more differentiated mouse cell types revealed differences between pluripotent and differentiated states. We also observed substantial divergence in rDNA histone modification patterns between mESCs and human embryonic stem cells (hESCs). Surprisingly, we found that the pluripotency factor OCT4 was bound to rDNA in similar patterns in mESCs and hESCs. Extending this analysis, we found that an additional 17 pluripotency-associated factors were bound to rDNA in mESCs, suggesting novel modes of rDNA regulation in pluripotent cells. Taken together, our results provide a detailed view of rDNA chromatin structure in an important model system and enable high-resolution comparison of rDNA regulation between mouse and human.

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.

Periodic expression of Sm proteins parallels formation of nuclear Cajal bodies and cytoplasmic snRNP-rich bodies

Small nuclear ribonucleoproteins (snRNPs) play a fundamental role in pre-mRNA processing in the nucleus. The biogenesis of snRNPs involves a sequence of events that occurs in both the nucleus and cytoplasm. Despite the wealth of biochemical information about the cytoplasmic assembly of snRNPs, little is known about the spatial organization of snRNPs in the cytoplasm. In the cytoplasm of larch microsporocytes, a cyclic appearance of bodies containing small nuclear RNA (snRNA) and Sm proteins was observed during anther meiosis. We observed a correlation between the occurrence of cytoplasmic snRNP bodies, the levels of Sm proteins, and the dynamic formation of Cajal bodies. Larch microsporocytes were used for these studies. This model is characterized by natural fluctuations in the level of RNA metabolism, in which periods of high transcriptional activity are separated from periods of low transcriptional activity. In designing experiments, the authors considered the differences between the nuclear and cytoplasmic phases of snRNP maturation and generated a hypothesis about the direct participation of Sm proteins in a molecular switch triggering the formation of Cajal bodies.

No evidence for the presence of oogonia in the human ovary after their final clearance during the first two years of life

BACKGROUND

Conflicting results of studies on mouse and human have either verified or refuted the presence of oogonia/primordial germ cells in the post-natal ovary. The aim of this study was to trace whether oogonia recognized by immunohistochemical methods in the first trimester human ovary were present also in peri- and post-natal ovaries.

METHODS

For this study, 82 human ovaries were collected: 25 from embryos from 5 to 10 weeks post conception (wpc), 2 at 18 wpc, 32 from 32 wpc to 2 years and 23 from 2 to 32 years. Of these, 80 ovaries were fixed and paraffin-embedded and 2 (8 year-old) ovaries were processed for plastic sections. Serial sections were prepared for immunohistochemical detection of markers for oogonia: tyrosine kinase receptor for stem cell factor (SCF)(C-KIT), stage-specific embryonic antigen-4 (SSEA4), homeobox gene transcription factor (NANOG), octamer binding transcription factor 4 (OCT4) and melanoma antigen-4 (Mage-A4), while noting that C-KIT also stains diplotene oocytes.

RESULTS

Almost all oogonia exclusively stained for SSEA4, NANOG, OCT4 and C-KIT, whereas MAGE-A4 only stained a small fraction. At birth only a few oogonia were stained. These disappeared before 2 years, leaving only diplotene oocytes stained for C-KIT. From 18 wpc to 2 years, the medulla contained conglomerates of healthy and degenerating oogonia and small follicles, waste baskets (WBs) and oogonia enclosed in growing follicles (FWB). Medulla of older ovaries contained groups of primordial, healthy follicles.

CONCLUSIONS

We found no evidence for the presence of oogonia in the human ovary after their final clearing during the first 2 years. We suggest that perinatal medullary WB and FWB give rise to the groups of small, healthy follicles in the medulla.

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.

Awakening the oocyte: controlling primordial follicle development

Oocytes are sequestered in primordial follicles before birth and remain quiescent in the ovary, often for decades, until recruited into the growing pool throughout the reproductive years. Therefore, activation of follicle growth is a major biological checkpoint that controls female reproductive potential. However, we are only just beginning to elucidate the cellular mechanisms required for either maintenance of the quiescent primordial follicle pool or initiation of follicle growth. Understanding the intracellular signalling systems that control oocyte maintenance and activation has significant implications for improving female reproductive productivity and longevity in mammals, and has application in domestic animal husbandry, feral animal population control and infertility in women.

Maternal Oct-4 is a potential key regulator of the developmental competence of mouse oocytes

Background

The maternal contribution of transcripts and proteins supplied to the zygote is crucial for the progression from a gametic to an embryonic control of preimplantation development. Here we compared the transcriptional profiles of two types of mouse MII oocytes, one which is developmentally competent (MII^SN oocyte), the other that ceases development at the 2-cell stage (MII^NSN oocyte), with the aim of identifying genes and gene expression networks whose misregulated expression would contribute to a reduced developmental competence.

Results

We report that: 1) the transcription factor Oct-4 is absent in MII^NSN oocytes, accounting for 2) the down-regulation of Stella, a maternal-effect factor required for the oocyte-to-embryo transition and of which Oct-4 is a positive regulator; 3) eighteen Oct-4-regulated genes are up-regulated in MII^NSN oocytes and are part of gene expression networks implicated in the activation of adverse biochemical pathways such as oxidative phosphorylation, mitochondrial dysfunction and apoptosis.

Conclusion

The down-regulation of Oct-4 plays a crucial function in a sequence of molecular processes that leads to the developmental arrest of MII^NSN oocytes. The use of a model study in which the MII oocyte ceases development consistently at the 2-cell stage has allowed to attribute a role to the maternal Oct-4 that has never been described before. Oct-4 emerges as a key regulator of the molecular events that govern the establishment of the developmental competence of mouse oocytes.

UBE2I (UBC9), a SUMO-conjugating enzyme, localizes to nuclear speckles and stimulates transcription in mouse oocytes

Sumoylation is a posttranslational modification in which SUMO (small ubiquitin-related modifier) proteins are covalently attached to their substrates. In vertebrates, developmental roles for sumoylation have been studied, but the function of sumoylation during mammalian oocyte growth and maturation is not known. As a prelude to conducting studies on the role of sumoylation during oocyte development, we analyzed the temporal and spatial pattern of expression of UBE2I, a SUMO-conjugating E2 enzyme. Immunocytochemical analysis of UBE2I revealed a punctate nuclear staining pattern, with transcriptionally quiescent, fully grown, GV-intact oocytes having larger UBE2I-containing bodies than transcriptionally active, meiotically incompetent growing oocytes. Inhibiting transcription in incompetent oocytes resulted in an increase in the size of the UBE2I-containing bodies. Overexpression of either wild-type UBE2I or catalytically inactive UBE2I resulted in an increase in the size of the UBE2I-containing bodies but also an increase in BrUTP incorporation, suggesting that transcriptional activation by UBE2I is independent of its catalytic activity. Although UBE2I-containing bodies did not completely colocalize with SUMO1 or SUMO2 and SUMO3, which were localized mainly on the nuclear membrane and in the nucleoplasm, UBE2I strikingly colocalized with SFRS2, which is a component of nuclear speckles and critical for mRNA processing. These results suggest a novel function for UBE2I and therefore sumoylation in gene expression.

PIAS proteins as repressors of Oct4 function

The POU domain transcription factor Oct4 plays essential functions in the maintenance of pluripotent embryonic and germ cells of mammals. Molecular mechanisms of Oct4 action remain poorly understood. To isolate modulators of Oct4 activity, we performed a yeast two-hybrid screen with the Oct4 POU domain as a bait and isolated PIASy as an Oct4-interacting protein. Oct4 and PIASy interact in vivo via their POU domain and SAP-domain-containing N terminus, respectively. PIASy does not enhance Oct4 sumoylation but acts as a potent inhibitor of Oct4-mediated transcriptional activation, sequestering Oct4 protein from the vicinity of Cajal bodies and splicing speckles to the nuclear periphery. These modes of PIASy action are uncoupled from its sumoylation activity. Other PIAS family members, PIAS1 and PIAS3, can also interact with Oct4 in vivo and target Oct4 to the nuclear periphery, depending on cellular context. We propose that Oct4 inhibition, mediated by this new class of transcriptional partners, might be instrumental during mammalian development.

Localization of interchromatin granule cluster and Cajal body components in oocyte nuclear bodies of the hemipterans

An oocyte nucleus contains different extrachromosomal nuclear domains collectively called nuclear bodies (NBs). In the present work we revealed, using immunogold labeling electron microscopy, some marker components of interchromatin granule clusters (IGCs) and Cajal bodies (CBs) in morphologically heterogeneous oocyte NBs studied in three hemipteran species: Notostira elongata, Capsodes gothicus (Miridae) and Velia caprai (Veliidae). Both IGC and CB counterparts were revealed in oocyte nuclei of the studied species but morphological and biochemical criteria were found to be not sufficient to determine carefully the define type of oocyte NBs. We found that the molecular markers of the CBs (coilin and non-phosphorylated RNA polymerase II) and IGCs (SC35 protein) may be localized in the same NB. Anti-SC35 antibody may decorate not only a granular material representing "true" interchromatin granules but also masks some fibrillar parts of complex NBs. Our first observations on the hemipteran oocyte NBs confirm the high complexity and heterogeneity of insect oocyte IGCs and CBs in comparison with those in mammalian somatic cells and amphibian oocytes.

Nuclear reprogramming: the zygotic transcription program is established through an “erase-and-rebuild” strategy

Oocytes display a maternal-specific gene expression profile, which is switched to a zygotic profile when a haploid set of chromatin is passed on to the fertilized egg that develops into an embryo. The mechanism underlying this transcription reprogramming is currently unknown. Here we demonstrate that by the time when transcription is shut down in germinal vesicle oocytes, a range of general transcription factors and transcriptional regulators are dissociated from the chromatin. The global dissociation of chromatin factors (CFs) disrupts physical contacts between the chromatin and CFs and leads to erasure of the maternal transcription program at the functional level. Critical transcription factors and regulators remain separated from chromatin for a prolonged period, and become re-associated with chromatin shortly after pronuclear formation. This is followed temporally by the re-establishment of nuclear functions such as DNA replication and transcription. We propose that the maternal transcription program is erased during oogenesis to generate a relatively naïve chromatin and the zygotic transcription program is rebuilt de novo after fertilization. This process is termed as the "erase-and-rebuild" process, which is used to reset the transcription program, and most likely other nuclear processes as well, from a maternal one to that of the embryo. We further show in the accompanying paper (Gao T, et al., Cell Res 2007; 17: 135-150.) that the same strategy is also employed to reprogram transcriptional profiles in somatic cell nuclear transfer and parthenogenesis, suggesting that this model is universally applicable to all forms of transcriptional reprogramming during early embryogenesis. Displacement of CFs from chromatin also offers an explanation for the phenomenon of transcription silence during the maternal to zygotic transition.

Nuclear structure in early mouse embryos: A comparative ultrastructural and immunocytochemical study with special emphasis on the "2-cell block in vitro"

The embryos from many outbred and inbred strains of mice are arrested at the late 2-cell stage when cultured in vitro in simple culture media. This phenomenon is referred to as the "2-cell block in vitro". The ultrastructural morphology of the nuclei of the blocked embryos is not yet well described. In the present paper we documented the results of a comparative study on the nuclei of mouse embryos, both normally developing and arrested at the 2-cell stage. The blocked embryos maintain the morphological integrity of their nuclei. Main nuclear domains (nucleolus precursor bodies, interchromatin granule clusters, perichromatin granules, and perichromatin fibrils), typical for the control embryos, are observed in the blocked ones. A number and morphological characteristics of these nuclear substructures are not changed significantly in the blocked embryos. At the same time, RNA polymerase II and pre-mRNA splicing factors are redistributed in the nucleus of the blocked embryos. Although something goes to show that nuclear organization of the blocked embryos differ from that of the control, we could not reveal in the blocked embryos distinct signs of degeneration which might characterize aged or dying cells. Our data confirm a peculiar functional state of the 2-cell blocked embryos.

Chromatin modifications in the germinal vesicle (GV) of mammalian oocytes

The nucleus of eukaryotic cells is organized into functionally specialized compartments that are essential for the control of gene expression, chromosome architecture and cellular differentiation. The mouse oocyte nucleus or germinal vesicle (GV) exhibits a unique chromatin configuration that is subject to dynamic modifications during oogenesis. This process of 'epigenetic maturation' is critical to confer the female gamete with meiotic as well as developmental competence. In spite of its biological significance, little is known concerning the cellular and molecular mechanisms regulating large-scale chromatin structure in mammalian oocytes. Here, recent findings that provide mechanistic insight into the complex relationship between large-scale chromatin structure and global transcriptional repression in pre-ovulatory oocytes will be discussed. Post-translational modifications of histone proteins such as acetylation and methylation are crucial for heterochromatin formation and thus play a key role in remodeling the oocyte genome. This strategy involves multiple and hierarchical chromatin modifications that regulate nuclear dynamics in response to a developmentally programmed signal(s), presumably of paracrine origin, before the resumption of meiosis. Models for the experimental manipulation of large-scale chromatin structure in vivo and in vitro will be instrumental to determine the key cellular pathways and oocyte-derived factors involved in genome-wide chromatin modifications. Importantly, analysis of the functional differentiation of chromatin structure in the oocyte genome with high resolution and in real time will have wide-ranging implications to understand the role of nuclear organization in meiosis, the events of nuclear reprogramming and the spatio-temporal regulation of gene expression during development and differentiation.

Subnuclear compartmentalization of sequence-specific transcription factors and regulation of eukaryotic gene expression

To date, the majority of the research regarding eukaryotic transcription factors has focused on characterizing their function primarily through in vitro methods. These studies have revealed that transcription factors are essentially modular structures, containing separate regions that participate in such activities as DNA binding, protein-protein interaction, and transcriptional activation or repression. To fully comprehend the behavior of a given transcription factor, however, these domains must be analyzed in the context of the entire protein, and in certain cases the context of a multiprotein complex. Furthermore, it must be appreciated that transcription factors function in the nucleus, where they must contend with a variety of factors, including the nuclear architecture, chromatin domains, chromosome territories, and cell-cycle-associated processes. Recent examinations of transcription factors in the nucleus have clarified the behavior of these proteins in vivo and have increased our understanding of how gene expression is regulated in eukaryotes. Here, we review the current knowledge regarding sequence-specific transcription factor compartmentalization within the nucleus and discuss its impact on the regulation of such processes as activation or repression of gene expression and interaction with coregulatory factors.

Immortalization of mouse germ line stem cells

In the mammalian testis, the germ line stem cells are a small subpopulation of type A spermatogonia that proliferate and ultimately differentiate into sperm under the control of both endocrine and paracrine factors. To study the early phases of spermatogenesis at the molecular level, an in vitro system must be devised whereby germ line stem cells can be either cultured for a prolonged period of time or expanded as cell lines. In the study reported here, we chose to immortalize type A spermatogonia using the Simian virus large T-antigen gene (LTAg) under the control of an ecdysone-inducible promoter. While the cells escaped the hormonal control after a finite number of generations and expressed the LTAg constitutively, their growth remained slow and the cells exhibited morphological features typical of spermatogonia at the light microscopic level. Moreover, the cells expressed detectable levels of protein markers specific for germ cells such as Dazl, and specific for germ line stem cells such as Oct-4, a transcription factor, and GFRalpha-1, the receptor for glial cell line-derived neurotrophic factor (GDNF). Further analysis confirmed the spermatogonial phenotype and also revealed the expression of markers expressed in stem cells such as Piwi12 and Prame11. Since the cells respond to GDNF by a marked increase in their rate of proliferation, this cell line represents a good in vitro model for studying aspects of mouse germ line stem cell biology.

Identification and dynamics of Cajal bodies in relation to karyosphere formation in scorpionfly oocytes

In oocyte nuclei of the scorpionfly, Panorpa communis, we have recently defined a population of nuclear bodies (NBs) that contain some components of Cajal bodies (CBs). In the present study, we used several criteria [presence of coilin, U7 snRNA, RNA polymerase II (pol II) and specific ultrastructure] to identify these NBs as CBs. The essential evidence for CB identification came from experiments with microinjection of fluorescein-tagged U7 snRNA. Consistent with the U7 data, we found pol II and pre-mRNA splicing factor, SC35, in Panorpa oocyte CBs. We show here that the dynamics of CBs differs from that in somatic cells and correlates with the level of oocyte chromosome condensation. We also found that the significant increase of CB size is accompanied by condensation of the chromosomes in the karyosphere, which is indicative of a decline in transcription. Using immunogold microscopy we determined that pol II and coilin are shared by CBs and the granular material associated with condensed chromosomes in the Panorpa karyosphere. The colocalization of pol II, U7 snRNA and splicing factors with CBs at the inactive stage of late oogenesis suggests that the latter may serve as storage domains for components that were earlier engaged in RNA transcription and processing.