Cytoskeletal sheets of mammalian eggs and embryos: a lattice-like network of intermediate filaments

Crystal structure of human peptidylarginine deiminase type VI (PAD6) provides insights into its inactivity

Human peptidylarginine deiminase isoform VI (PAD6), which is predominantly limited to cytoplasmic lattices in the mammalian oocytes in ovarian tissue, is essential for female fertility. It belongs to the peptidylarginine deiminase (PAD) enzyme family that catalyzes the conversion of arginine residues to citrulline in proteins. In contrast to other members of the family, recombinant PAD6 was previously found to be catalytically inactive. We sought to provide structural insight into the human homologue to shed light on this observation. We report here the first crystal structure of PAD6, determined at 1.7 Å resolution. PAD6 follows the same domain organization as other structurally known PAD isoenzymes. Further structural analysis and size-exclusion chromatography show that PAD6 behaves as a homodimer similar to PAD4. Differential scanning fluorimetry suggests that PAD6 does not coordinate Ca2+ which agrees with acidic residues found to coordinate Ca2+ in other PAD homologs not being conserved in PAD6. The crystal structure of PAD6 shows similarities with the inactive state of apo PAD2, in which the active site conformation is unsuitable for catalytic citrullination. The putative active site of PAD6 adopts a non-productive conformation that would not allow protein-substrate binding due to steric hindrance with rigid secondary structure elements. This observation is further supported by the lack of activity on the histone H3 and cytokeratin 5 substrates. These findings suggest a different mechanism for enzymatic activation compared with other PADs; alternatively, PAD6 may exert a non-enzymatic function in the cytoplasmic lattice of oocytes and early embryos.

A genome-wide perspective of the maternal mRNA translation program during oocyte development

Transcriptional and post-transcriptional regulations control gene expression in most cells. However, critical transitions during the development of the female gamete relies exclusively on regulation of mRNA translation in the absence of de novo mRNA synthesis. Specific temporal patterns of maternal mRNA translation are essential for the oocyte progression through meiosis, for generation of a haploid gamete ready for fertilization and for embryo development. In this review, we will discuss how mRNAs are translated during oocyte growth and maturation using mostly a genome-wide perspective. This broad view on how translation is regulated reveals multiple divergent translational control mechanisms required to coordinate protein synthesis with progression through the meiotic cell cycle and with development of a totipotent zygote.

Mammalian oocytes store proteins for the early embryo on cytoplasmic lattices

Mammalian oocytes are filled with poorly understood structures called cytoplasmic lattices. First discovered in the 1960s and speculated to correspond to mammalian yolk, ribosomal arrays, or intermediate filaments, their function has remained enigmatic to date. Here, we show that cytoplasmic lattices are sites where oocytes store essential proteins for early embryonic development. Using super-resolution light microscopy and cryoelectron tomography, we show that cytoplasmic lattices are composed of filaments with a high surface area, which contain PADI6 and subcortical maternal complex proteins. The lattices associate with many proteins critical for embryonic development, including proteins that control epigenetic reprogramming of the preimplantation embryo. Loss of cytoplasmic lattices by knocking out PADI6 or the subcortical maternal complex prevents the accumulation of these proteins and results in early embryonic arrest. Our work suggests that cytoplasmic lattices enrich maternally provided proteins to prevent their premature degradation and cellular activity, thereby enabling early mammalian development.

PADI6: What we know about the elusive fifth member of the peptidyl arginine deiminase family

Peptidyl arginine deiminase 6 (PADI6) is a maternal factor that is vital for early embryonic development. Deletion and mutations of its encoding gene in female mice or women lead to early embryonic developmental arrest, female infertility, maternal imprinting defects and hyperproliferation of the trophoblast. PADI6 is the fifth and least well-characterized member of the peptidyl arginine deiminases (PADIs), which catalyse the post-translational conversion of arginine to citrulline. It is less conserved than the other PADIs, and currently has no reported catalytic activity. While there are many suggested functions of PADI6 in the early mouse embryo, including in embryonic genome activation, cytoplasmic lattice formation, maternal mRNA and ribosome regulation, and organelle distribution, the molecular mechanisms of its function remain unknown. In this review, we discuss what is known about the function of PADI6 and highlight key outstanding questions that must be answered if we are to understand the crucial role it plays in early embryo development and female fertility. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

The subcortical maternal complex: emerging roles and novel perspectives

Since its recent discovery, the subcortical maternal complex (SCMC) is emerging as a maternally inherited and crucial biological structure for the initial stages of embryogenesis in mammals. Uniquely expressed in oocytes and preimplantation embryos, where it localizes to the cell subcortex, this multiprotein complex is essential for early embryo development in the mouse and is functionally conserved across mammalian species, including humans. The complex has been linked to key processes leading the transition from oocyte to embryo, including meiotic spindle formation and positioning, regulation of translation, organelle redistribution, and epigenetic reprogramming. Yet, the underlying molecular mechanisms for these diverse functions are just beginning to be understood, hindered by unresolved interplay of SCMC components and variations in early lethal phenotypes. Here we review recent advances confirming involvement of the SCMC in human infertility, revealing an unexpected relationship with offspring health. Moreover, SCMC organization is being further revealed in terms of novel components and interactions with additional cell constituents. Collectively, this evidence prompts new avenues of investigation into possible roles during the process of oogenesis and the regulation of maternal transcript turnover during the oocyte to embryo transition.

Large lamellar bodies and their role in the growing oocytes of the armadillo Chaetophractus villosus

Oogenesis in the armadillo Chaetophractus villosus, a representative species of a mammalian basal clade, was investigated by light microscopy, transmission electron microscopy, and immunohistochemical localization of keratin. At the beginning of the growth phase, oocyte follicles showed one, and sometimes several, large bodies composed of lamellae (multilamellar bodies [MLBs]), which entrap other cytoplasmic organelles at more advanced stages. Lamellae diameter is described in cross-section (37 nm) and tangential sections (50 nm). The MLB of early oocytes is most frequently located close to the nucleus. In large oocytes, both, this body and the free organelles are relocated at the oocyte periphery. The MLB grows from the primary follicle up to its full development at the follicular phase characterized by tall granulosa cells. Mitochondria, smooth small vesicles, and lipofuscin granules are trapped between lamellae. MLBs engage in the formation of different sets of organelles, both trapped and free ones. When oocytes are well developed and the zona pellucida is formed, the MLB is reduced to small remnants detected only by transmission electron microscopy. The MLB disintegrates when an antrum develops. Immunohistochemical localization techniques showed the presence of cytokeratin in the MLBs. This cytokeratin pool may be involved in the filament and desmosome formation found in the periphery of late oocytes.

Identifying the Translatome of Mouse NEBD-Stage Oocytes via SSP-Profiling; A Novel Polysome Fractionation Method

Meiotic maturation of oocyte relies on pre-synthesised maternal mRNA, the translation of which is highly coordinated in space and time. Here, we provide a detailed polysome profiling protocol that demonstrates a combination of the sucrose gradient ultracentrifugation in small SW55Ti tubes with the qRT-PCR-based quantification of 18S and 28S rRNAs in fractionated polysome profile. This newly optimised method, named Scarce Sample Polysome Profiling (SSP-profiling), is suitable for both scarce and conventional sample sizes and is compatible with downstream RNA-seq to identify polysome associated transcripts. Utilising SSP-profiling we have assayed the translatome of mouse oocytes at the onset of nuclear envelope breakdown (NEBD)—a developmental point, the study of which is important for furthering our understanding of the molecular mechanisms leading to oocyte aneuploidy. Our analyses identified 1847 transcripts with moderate to strong polysome occupancy, including abundantly represented mRNAs encoding mitochondrial and ribosomal proteins, proteasomal components, glycolytic and amino acids synthetic enzymes, proteins involved in cytoskeleton organization plus RNA-binding and translation initiation factors. In addition to transcripts encoding known players of meiotic progression, we also identified several mRNAs encoding proteins of unknown function. Polysome profiles generated using SSP-profiling were more than comparable to those developed using existing conventional approaches, being demonstrably superior in their resolution, reproducibility, versatility, speed of derivation and downstream protocol applicability.

Widespread Enhancer Dememorization and Promoter Priming during Parental-to-Zygotic Transition

The epigenome plays critical roles in controlling gene expression and development. However, how the parental epigenomes transit to the zygotic epigenome in early development remains elusive. Here we show that parental-to-zygotic transition in zebrafish involves extensive erasure of parental epigenetic memory, starting with methylating gametic enhancers. Surprisingly, this occurs even prior to fertilization for sperm. Both parental enhancers lose histone marks by the 4-cell stage, and zygotic enhancers are not activated until around zygotic genome activation (ZGA). By contrast, many promoters remain hypomethylated and, unexpectedly, acquire histone acetylation before ZGA at as early as the 4-cell stage. They then resolve into either activated or repressed promoters upon ZGA. Maternal depletion of histone acetyltransferases results in aberrant ZGA and early embryonic lethality. Finally, such reprogramming is largely driven by maternal factors, with zygotic products mainly contributing to embryonic enhancer activation. These data reveal widespread enhancer dememorization and promoter priming during parental-to-zygotic transition.

The subcortical maternal complex protein Nlrp4f is involved in cytoplasmic lattice formation and organelle distribution

In mammalian oocytes and embryos, the subcortical maternal complex (SCMC) and cytoplasmic lattices (CPLs) are two closely related structures. Their detailed compositions and functions remain largely unclear. Here, we characterized Nlrp4f as a novel component associated with the SCMC and CPLs. Disruption of maternal Nlrp4f leads to decreased fecundity and delayed preimplantation development in the mouse. Lack of Nlrp4f affects organelle distribution in mouse oocytes and early embryos. Depletion of Nlrp4f disrupts CPL formation but does not affect the interactions of other SCMC proteins. Interestingly, the loss of Filia or Tle6, two other SCMC proteins, also disrupts CPL formation in mouse oocytes. Thus, the absence of CPLs and aberrant distribution of organelles in the oocytes disrupted the examined SCMC genes, including previously reported Zbed3, Mater, Floped and Padi6, indicate that the SCMC is required for CPL formation and organelle distribution. Consistent with the SCMC's role in CPL formation, the SCMC forms before CPLs during oogenesis. Together, our results suggest that SCMC protein Nlrp4f is involved in CPL formation and organelle distribution in mouse oocytes.

Re-Defining Stem Cell-Cardiomyocyte Interactions: Focusing on the Paracrine Effector Approach

Stem cell research for treating or curing ischemic heart disease has, till date, culminated in three basic approaches: the use of induced pluripotent stem cell (iPSC) technology; reprogramming cardiac fibroblasts; and cardiovascular progenitor cell regeneration. As each approach has been shown to have its advantages and disadvantages, exploiting the advantages while minimizing the disadvantages has been a challenge. Using human germline pluripotent stem cells (hgPSCs) along with a modified version of a relatively novel cell-expansion culture methodology to induce quick, indefinite expansion of normally slow growing hgPSCs, it was possible to emphasize the advantages of all three approaches. We consistently found that unipotent germline stem cells, when removed from their niche and cultured in the correct medium, expressed endogenously, pluripotency genes, which induced them to become hgPSCs. These cells are then capable of producing cell types from all three germ layers. Upon differentiation into cardiac lineages, our data consistently showed that they not only expressed cardiac genes, but also expressed cardiac-promoting paracrine factors. Taking these data a step further, we found that hgPSC-derived cardiac cells could integrate into cardiac tissue in vivo. Note, while the work presented here was based on testes-derived hgPSCs, data from other laboratories have shown that ovaries contain very similar types of stem cells that can give rise to hgPSCs. As a result, hgPSCs should be considered a viable option for eventual use in patients, male or female, with ischemic heart disease

Acquisition of oocyte competence to develop as an embryo: integrated nuclear and cytoplasmic events

Infertility affects ~7% of couples of reproductive age with little change in incidence in the last two decades. ART, as well as other interventions, have made major strides in correcting this condition. However, and in spite of advancements in the field, the age of the female partner remains a main factor for a successful outcome. A better understanding of the final stages of gamete maturation yielding an egg that can sustain embryo development and a pregnancy to term remains a major area for improvement in the field. This review will summarize the major cellular and molecular events unfolding at the oocyte-to-embryo transition. We will provide an update on the most important processes/pathways currently understood as the basis of developmental competence, including the molecular processes involved in mRNA storage, its recruitment to the translational machinery, and its degradation. We will discuss the hypothesis that the translational programme of maternal mRNAs plays a key role in establishing developmental competence. These regulations are essential to assemble the machinery that is used to establish a totipotent zygote. This hypothesis further supports the view that embryogenesis begins during oogenesis. A better understanding of the events required for developmental competence will guide the development of novel strategies to monitor and improve the success rate of IVF. Using this information, it will be possible to develop new biomarkers that may be used to better predict oocyte quality and in selection of the best egg for IVF.

Translational Regulation in the Mammalian Oocyte

Fully grown oocytes arrest meiosis at prophase I and deposit maternal RNAs. A subset of maternal transcripts is stored in a dormant state in the oocyte, and the timely driven translation of specific mRNAs guides meiotic progression, the oocyte-embryo transition, and early embryo development. In the absence of transcription, the regulation of gene expression in oocytes is controlled almost exclusively at the level of transcriptome and proteome stabilization and at the level of protein synthesis.This chapter focuses on the recent findings on RNA distribution related to the temporal and spatial translational control of the meiotic cycle progression in mammalian oocytes. We discuss the most relevant mechanisms involved in the organization of the oocyte's maternal transcriptome storage and localization, and the regulation of translation, in correlation with the regulation of oocyte meiotic progression.

Role for PADI6 in securing the mRNA-MSY2 complex to the oocyte cytoplasmic lattices

The oocyte cytoplasmic lattices (CPLs) have long been predicted to function as a storage form for the maternal contribution of ribosomes to the early embryo. Our previous studies have demonstrated that ribosomal component S6 is stored in the oocyte CPLs and peptidylarginine deiminase 6 (PADI6) is critical for CPLs formation. Additionally, we found that depletion of PADI6 reduced de novo protein synthesis prior to the maternal-to-embryonic transition, therefore causing embryos to arrest at the 2-cell stage. Here, we present evidence further supporting the association of ribosomes with the CPLs by demonstrating that rRNAs are dramatically decreased in Padi6 KO oocytes. We also show that the abundance and localization of mRNAs is affected upon PADI6 depletion, suggesting that mRNAs are very possibly associated with CPLs. Consistent with this observation, the amount of the major RNA binding protein, MSY2, that is associated with the insoluble fraction of the oocytes after Triton X-100 extraction is also markedly decreased in the Padi6 KO oocytes. Furthermore, treatment of the oocytes with RNase A followed by Triton X-100 extraction severely impairs the localization of PADI6 and MSY2 in oocytes. These results indicate that mRNAs, possibly in a complex with MSY2 and PADI6, are bound in the CPLs and may play a role in securing the mRNA-MSY2 complex to the CPLs.

The subcortical maternal complex: multiple functions for one biological structure?

The subcortical maternal complex (SCMC) is a multiprotein complex uniquely expressed in mammalian oocytes and early embryos, essential for zygote progression beyond the first embryonic cell divisions. Similiar to other factors encoded by maternal effect genes, the physiological role of SCMC remains unclear, although recent evidence has provided important molecular insights into different possible functions. Its potential involvement in human fertility is attracting increasing attention; however, the complete story is far from being told. The present mini review provides an overview of recent findings related to the SCMC and discusses its potential physiological role/s with the aim of inspiring new directions for future research.

NLRP7 and KHDC3L, the two maternal-effect proteins responsible for recurrent hydatidiform moles, co-localize to the oocyte cytoskeleton

STUDY QUESTION

What is the subcellular localization in human oocytes and preimplantation embryos, of the two maternal-effect proteins, NLRP7 and KHDC3L, responsible for recurrent hydatidiform moles (RHMs)?

SUMMARY ANSWER

NLRP7 and KHDC3L localize to the oocyte cytoskeleton and are polar and absent from the cell-to-cell contact region in early preimplantation embryos.

WHAT IS KNOWN ALREADY

NLRP7 and KHDC3L expression has been described at the RNA level in some stages of human oocytes and preimplantation embryos and at the protein level by immunohistochemistry in human and bovine ovaries. NLRP7 and KHDC3L co-localize to the microtubule organizing center and/or the Golgi apparatus in human hematopoietic cells.

STUDY DESIGN, SIZE, DURATION

A total of 164 spare human oocytes and embryos from patients undergoing in vitro fertilization were used.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Oocytes and early cleavage-stage embryos were fixed, immunostained with NLRP7 and/or KHDC3L antibodies, and analyzed using high-resolution confocal immunofluorescence and electron microscopies.

MAIN RESULTS AND THE ROLE OF CHANCE

NLRP7 and KHDC3L localize to the cytoskeleton and are predominant at the cortical region in growing oocytes. After the first cellular division, these two maternal-effect proteins become asymmetrically confined to the outer cortical region and excluded from the cell-to-cell contact region until the blastocyst stage where NLRP7 and KHDC3L homogeneously redistribute to the cytoplasm and the nucleus, respectively.

LIMITATIONS, REASONS FOR CAUTION

We could not analyze fresh human oocytes and embryos. The analyzed materials were donated by patients undergoing assisted reproductive technologies and released for research 1-3 days after their collection and the transfer of embryos to the patients.

WIDER IMPLICATIONS OF THE FINDINGS

Our study is the first comprehensive and high-resolution localization of the only two known maternal-effect proteins, NLRP7 and KHDC3L, in human oocytes and preimplantation embryos. Our data contribute to a better understanding of the roles of these two proteins in the integrity of the oocytes, post-zygotic divisions, and cell-lineage differentiation.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by the Canadian Institute of Health Research (86546 to R.S.); E.A. was supported by fellowships from the Research Institute of the McGill University Health Centre and a CREATE award from the Réseau Québécois en Reproduction. All authors declare no conflict of interest.

Post-transcriptional control of gene expression during mouse oogenesis

Post-transcriptional mechanisms play a central role in regulating gene expression during oogenesis and early embryogenesis. Growing oocytes accumulate an enormous quantity of messenger RNAs (mRNAs), but transcription decreases dramatically near the end of growth and is undetectable during meiotic maturation. Following fertilization, the embryo is initially transcriptionally inactive and then becomes active at a species-specific stage of early cleavage. Meanwhile, beginning during maturation and continuing after fertilization, the oocyte mRNAs are eliminated, allowing the embryonic genome to assume control of development. How the mammalian oocyte manages the storage, translation, and degradation of the huge quantity and diversity of mRNAs that it harbours has been the focus of enormous research effort and is the subject of this review. We discuss the roles of sequences within the 3'-untranslated region of certain mRNAs and the proteins that bind to them, sequence-non-specific RNA-binding proteins, and recent studies implicating ribonucleoprotein processing (P-) bodies and cytoplasmic lattices. We also discuss mechanisms that may control the temporally regulated translational activation of different mRNAs during meiotic maturation, as well as the signals that trigger silencing and degradation of the oocyte mRNAs. We close by highlighting areas for future research including the potential key role of small RNAs in regulating gene expression in oocytes.

Polarity and asymmetry during mouse oogenesis and oocyte maturation

Cell polarity and asymmetry play a fundamental role in embryo development. The unequal segregation of determinants, cues, and activities is the major event in the differentiation of cell fate and function in all multicellular organisms. In oocytes, polarity and asymmetry in the distribution of different molecules are prerequisites for the progression and proper outcome of embryonic development. The mouse oocyte, like the oocytes of other mammals, seems to apply a less stringent strategy of polarization than other vertebrates. The mouse embryo undergoes a regulative type of development, which permits the full rectification of development even if the embryo loses up to half of its cells or its size is experimentally doubled during the early stages of embryogenesis. Such pliability is strongly related to the proper oocyte polarization before fertilization. Thus, the molecular mechanisms leading to the development and maintenance of oocyte polarity must be included in any fundamental understanding of the principles of embryo development. In this chapter, we provide an overview of current knowledge regarding the development and maintenance of polarity and asymmetry in the distribution of organelles and molecules in the mouse oocyte. Curiously, the mouse oocyte becomes polarized at least twice during ontogenesis; the question of how this phenomenon is achieved and what role it might play is addressed in this chapter.

Potential role for MATER in cytoplasmic lattice formation in murine oocytes

BACKGROUND

Mater and Padi6 are maternal effect genes that are first expressed during oocyte growth and are required for embryonic development beyond the two-cell stage in the mouse. We have recently found that PADI6 localizes to, and is required for the formation of, abundant fibrillar Triton X-100 (Triton) insoluble structures termed the oocyte cytoplasmic lattices (CPLs). Given their similar expression profiles and mutant mouse phenotypes, we have been testing the hypothesis that MATER also plays a role in CPL formation and/or function.

METHODOLOGY/FINDINGS

Herein, we show that PADI6 and MATER co-localize throughout the oocyte cytoplasm following Triton extraction, suggesting that MATER co-localizes with PADI6 at the CPLs. Additionally, the solubility of PADI6 was dramatically increased in Mater(tm/tm) oocytes following Triton extraction, suggesting that MATER is involved in CPL nucleation. This prediction is supported by transmission electron microscopic analysis of Mater(+/+) and Mater(tm/tm) germinal vesicle stage oocytes which illustrated that volume fraction of CPLs was reduced by 90% in Mater(tm/tm) oocytes compared to Mater(+/+) oocytes.

CONCLUSIONS

Taken together, these results suggest that, similar to PADI6, MATER is also required for CPL formation. Given that PADI6 and MATER are essential for female fertility, these results not only strengthen the hypothesis that the lattices play a critical role in mediating events during the oocyte-to-embryo transition but also increase our understanding of the molecular nature of the CPLs.

Phosphorylation statuses at different residues of lamin B2, B1, and A/C dynamically and independently change throughout the cell cycle

Lamins, major components of the nuclear lamina, undergo phosphorylation at multiple residues during cell cycle progression, but their detailed phosphorylation kinetics remain largely undetermined. Here, we examined changes in the phosphorylation of major phosphorylation residues (Thr14, Ser17, Ser385, Ser387, and Ser401) of lamin B2 and the homologous residues of lamin B1, A/C during the cell cycle using novel antibodies to the site-specific phosphorylation. The phosphorylation levels of these residues independently changed during the cell cycle. Thr14 and Ser17 were phosphorylated during G(2)/M phase to anaphase/telophase. Ser385 was persistently phosphorylated during mitosis to G(1) phase, whereas Ser387 was phosphorylated discontinuously in prophase and G(1) phase. Ser401 phosphorylation was enhanced in the G(1)/S boundary. Immunoprecipitation using the phospho-antibodies suggested that metaphase-phosphorylation at Thr14, Ser17, and Ser385 of lamins occurred simultaneously, whereas G(1)-phase phosphorylation at Ser385 and Ser387 occurred in distinct pools or with different timings. Additionally, we showed that lamin B2 phosphorylated at Ser17, but not Ser385, Ser387 and Ser401, was exclusively non-ionic detergent soluble, depolymerized forms in growing cells, implicating specific involvement of Ser17 phosphorylation in lamin depolymerization and nuclear envelope breakdown. These results suggest that the phosphorylations at different residues of lamins might play specific roles throughout the cell cycle.

Use of proteomics to identify highly abundant maternal factors that drive the egg-to-embryo transition

As IVF becomes an increasingly popular method for human reproduction, it is more critical than ever to understand the unique molecular composition of the mammalian oocyte. DNA microarray studies have successfully provided valuable information regarding the identity and dynamics of factors at the transcriptional level. However, the oocyte transcribes and stores a large amount of material that plays no obvious role in oogenesis, but instead is required to regulate embryogenesis. Therefore, an accurate picture of the functional state of the oocyte requires both transcriptional profiling and proteomics. Here, we summarize our previous studies of the oocyte proteome, and present new panels of oocyte proteins that we recently identified in screens of metaphase II-arrested mouse oocytes. Importantly, our studies indicate that several abundant oocyte proteins are not, as one might predict, ubiquitous housekeeping proteins, but instead are unique to the oocyte. Furthermore, mouse studies indicate that a number of these factors arise from maternal effect genes (MEGs). One of the identified MEG proteins, peptidylarginine deiminase 6, localizes to and is required for the formation of a poorly characterized, highly abundant cytoplasmic structure: the oocyte cytoplasmic lattices. Additionally, a number of other MEG-derived abundant proteins identified in our proteomic screens have been found by others to localize to another unique oocyte feature: the subcortical maternal complex. Based on these observations, we put forth the hypothesis that the mammalian oocyte contains several unique storage structures, which we have named maternal effect structures, that facilitate the oocyte-to-embryo transition.

Actin-driven chromosomal motility leads to symmetry breaking in mammalian meiotic oocytes

Movement of meiosis I (MI) chromosomes from the oocyte centre to a subcortical location is the first step in the establishment of cortical polarity. This is required for two consecutive rounds of asymmetric meiotic cell divisions, which generate a mature egg and two polar bodies. Here we use live-cell imaging and genetic and pharmacological manipulations to determine the force-generating mechanism underlying this chromosome movement. Chromosomes were observed to move toward the cortex in a pulsatile manner along a meandering path. This movement is not propelled by myosin-II-driven cortical flow but is associated with a cloud of dynamic actin filaments trailing behind the chromosomes/spindle. Formation of these filaments depends on the actin nucleation activity of Fmn2, a formin-family protein that concentrates around chromosomes through its amino-terminal region. Symmetry breaking of the actin cloud relative to chromosomes, and net chromosome translocation toward the cortex require actin turnover.

Role for PADI6 and the cytoplasmic lattices in ribosomal storage in oocytes and translational control in the early mouse embryo

The mechanisms that mediate the establishment of totipotency during the egg-to-embryo transition in mammals remain poorly understood. However, it is clear that unique factors stored in the oocyte cytoplasm are crucial for orchestrating this complex cellular transition. The oocyte cytoplasmic lattices (CPLs) have long been predicted to function as a storage form for the maternal contribution of ribosomes to the early embryo. We recently demonstrated that the CPLs cannot be visualized in Padi6-/- oocytes and that Padi6-/- embryos arrest at the two-cell stage. Here, we present evidence further supporting the association of ribosomes with the CPLs by demonstrating that the sedimentation properties of the small ribosomal subunit protein, S6, are dramatically altered in Padi6-/- oocytes. We also show that the abundance and localization of ribosomal components is dramatically affected in Padi6-/- two-cell embryos and that de novo protein synthesis is also dysregulated in these embryos. Finally, we demonstrate that embryonic genome activation (EGA) is defective in Padi6-/- two-cell embryos. These results suggest that, in mammals, ribosomal components are stored in the oocyte CPLs and are required for protein translation during early development.

Dynamics and unexpected localization of the plakin binding protein, kazrin, in mouse eggs and early embryos

The cell uses the cytoskeleton in virtually every aspect of cell survival and function. One primary function of the cytoskeleton is to connect to and stabilize intercellular junctions. To accomplish this task, microtubules, actin filaments, and intermediate filaments utilize cytolinker proteins, which physically bind the cytoskeletal filament to the core proteins of the adhesion junction. The plakin family of linker proteins have been in the spotlight recently as critical components for embryo survival and, when mutated, the cause of diseases such as muscular dystrophy and cardiomyopathies. Here, we reveal the dynamics of a recently discovered plakin binding protein, kazrin (kaz), during early mouse development. Kaz was originally found in adult tissues, primarily epidermis, linking periplakin to the plasma membrane and colocalizing with desmoplakin in desmosomes. Using reverse transcriptase-polymerase chain reaction, Western blots, and confocal microscopy, we found kaz in unfertilized eggs associated with the spindle apparatus and cytoskeletal sheets. As quickly as 5 min after egg activation, kaz relocates to a diffuse peri-spindle position, followed 20-30 min later by clear localization to the presumptive cytokinetic ring. Before the blastocyst stage of development, kaz associates with the nuclear matrix in a cell cycle-dependent manner, and also associates with the cytoplasmic actin cytoskeleton. After blastocyst formation, kaz localization and potential function(s) become highly complex as it is found associating with cell-cell junctions, the cytoskeleton, and nucleus. Postimplantation stages of development reveal that kaz retains a multifunctional, tissue-specific role as it is detected at diverse locations in various embryonic tissue types.

Keratin 14 Cre transgenic mice authenticate keratin 14 as an oocyte-expressed protein

Three mouse lines expressing Cre recombinase under the control of the human K14 promoter induced specific deletion of loxP flanked target sequences in the epidermis, in tongue, and thymic epithelium of the offspring where the Cre allele was inherited from the father. Where the mother carried the Cre allele, loxP flanked sequences were completely deleted in all tissues of the offspring, even in littermates that did not inherit the Cre allele. This maternally inherited phenotype indicates that the human K14 promoter is transcriptionally active in murine oocytes and that the enzyme remains active until after fertilization, even when the Cre allele becomes transmitted to the polar bodies during meiosis. Detection of K14 mRNA by RT-PCR in murine ovaries and immunohistochemical identification of the K14 protein in oocytes demonstrates that the human K14 promoter behaves like its murine homolog, thus identifying K14 as an authentic oocytic protein.

ePAD, an oocyte and early embryo-abundant peptidylarginine deiminase-like protein that localizes to egg cytoplasmic sheets

Selected for its high relative abundance, a protein spot of MW approximately 75 kDa, pI 5.5 was cored from a Coomassie-stained two-dimensional gel of proteins from 2850 zona-free metaphase II mouse eggs and analyzed by tandem mass spectrometry (TMS), and novel microsequences were identified that indicated a previously uncharacterized egg protein. A 2.4-kb cDNA was then amplified from a mouse ovarian adapter-ligated cDNA library by RACE-PCR, and a unique 2043-bp open reading frame was defined encoding a 681-amino-acid protein. Comparison of the deduced amino acid sequence with the nonredundant database demonstrated that the protein was approximately 40% identical to the calcium-dependent peptidylarginine deiminase (PAD) enzyme family. Northern blotting, RT-PCR, and in situ hybridization analyses indicated that the protein was abundantly expressed in the ovary, weakly expressed in the testis, and absent from other tissues. Based on the homology with PADs and its oocyte-abundant expression pattern, the protein was designated ePAD, for egg and embryo-abundant peptidylarginine deiminase-like protein. Anti-recombinant ePAD monospecific antibodies localized the molecule to the cytoplasm of oocytes in primordial, primary, secondary, and Graafian follicles in ovarian sections, while no other ovarian cell type was stained. ePAD was also expressed in the immature oocyte, mature egg, and through the blastocyst stage of embryonic development, where expression levels began to decrease. Immunoelectron microscopy localized ePAD to egg cytoplasmic sheets, a unique keratin-containing intermediate filament structure found only in mammalian eggs and in early embryos, and known to undergo reorganization at critical stages of development. Previous reports that PAD-mediated deimination of epithelial cell keratin results in cytoskeletal remodeling suggest a possible role for ePAD in cytoskeletal reorganization in the egg and early embryo.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.

Long-term two-photon fluorescence imaging of mammalian embryos without compromising viability

A major challenge for fluorescence imaging of living mammalian cells is maintaining viability following prolonged exposure to excitation illumination. We have monitored the dynamics of mitochondrial distribution in hamster embryos at frequent intervals over 24 h using two-photon microscopy (1,047 nm) while maintaining blastocyst, and even fetal, developmental competence. In contrast, confocal imaging for only 8 h inhibits development, even without fluorophore excitation. Photo-induced production of H2O2 may account, in part, for this inhibition. Thus, two-photon microscopy, but not confocal microscopy, has permitted long-term fluorescence observations of the dynamics of three-dimensional cytoarchitecture in highly photosensitive specimens such as mammalian embryos.

Growth and development of the mammalian oocyte

The oocyte is not only the rarest and the largest cell in the body, but it also has one of the most remarkable life histories. Formed in the fetal ovary and suspended at diplotene of meiosis, it may wait for years before beginning to grow, and not until this process is complete can it resume meiosis and undergo fertilisation. Major changes in the number, morphology and distribution of cytoplasmic organelles occur during growth, and a molecular program for embryogenesis is formed. Specific yolk proteins are absent and much of the RNA and some of the protein are degraded by the cleavage stage. The zona pellucida has been intensively studied, but knowledge of oocyte-specific genes is otherwise surprisingly patchy given the significance of this cell type and the expansion of reproductive technology. Finally, it is now clear that oocytes are not mere passengers which depend on granulosa cells for nutrition and regulation but actively promote the growth and differentiation of their follicles.

Cytokeratin dynamics during oocyte maturation in the hamster requires reaching of metaphase I

Cytoskeletal components like microfilaments and microtubules are known to play important roles during the processes of oocyte maturation, fertilization and early embryonic development in mammals. However, the roles of other components such as cytoplasmic intermediate filaments, during these critical events remain largely unknown. Oocyte maturation is the final step of oogenesis, immediately before ovulation. Several cytological changes involving the cytoskeleton take place during the maturation process, including meiotic spindle formation, redistribution of cell organelles, membrane polarization and first polar body emission. In this study we determined the organization and rearrangements of cytokeratins during hamster oocyte maturation. Fully grown oocytes were cultured and then visualised using microscopic immunolabelling techniques to monitor the cytokeratin dynamics at specific meiotic stages of the maturation process. In prophase-I-arrested fully grown hamster oocytes, cytokeratins are confined to 4-10 large cortical aggregates, corresponding to extensive meshworks of intermediate filaments. These large aggregates disperse into multiple small spots starting at metaphase I until the end of the maturation period at metaphase II, where cytokeratin exhibits a homogeneously distributed spotted pattern. However, meiotic progression to metaphase II is not necessary for cytokeratin redistribution to occur, since precociously arrested metaphase I oocytes also exhibit dispersed cytoplasmic foci at the end of the culture period. The redistribution of cytokeratins is insensitive to nocodazole and cytochalasin D suggesting it occurs independent of microtubules and microfilaments. In contrast, both cumulus cells and protein synthesis are required for cytokeratin modifications to take place during oocyte maturation. These results show that cytokeratin intermediate filaments are present in the fully grown hamster oocyte, and that a striking reorganization of cytokeratins, triggered by attainment of the metaphase I stage, occurs during maturation.

Remodeling of the specialized intermediate filament network in mammalian eggs and embryos during development: regulation by protein kinase C and protein kinase M

The sheets serve as an maternal supply of assembled, cytokeratin, intermediate filaments. They are remodeled at each major developmental transition in mammalian early development, that is fertilization, embryonic compaction, blastocyst formation, and formation of the primitive ectoderm and primitive endoderm during implantation into the uterine wall. Our results indicate that the sheets exist as specialization for placental development as they have a major role in the maintenance of epithelial integrity at the time the embryo is implanting into the uterine wall. They also contribute intermediate filaments to the junctional complexes required for embryonic compaction. Our analyses demonstrate the they are regulated at the time of fertilization by the action of PKC/PKM, a kinase that acts as a cellular chronometer with both temporal and spatial precision that remodels the egg into the zygote.

A role for intermediate filaments in the establishment of the primitive epithelia during mammalian embryogenesis

Investigations of the cytoskeleton in mammalian eggs and embryos have revealed the existence of an unusual array of crosslinked intermediate filaments composed of cytokeratins 5, 6, 16, and 'Z' that are referred to as cytoskeletal sheets. We have been investigating the function of these cytoskeletal sheets during embryogenesis. In this investigation we report the rapid appearance of extensive arrays of tonofilaments extending across blastomeres and in association with intercellular desmosomal junctions appearing at the time the embryo hatches from its zona pellucida, through the time of implantation of the embryo into the uterine wall. Just prior to the time of gastrulation these tonofilaments disappear. Electron microscopy and immunoconfocal microscopy demonstrate that the tonofilaments are composed of cytokeratins characteristic of the type found earlier in development, that is types 5 and 6; whereas, cytokeratin type 8 which has been shown to be synthesized in blastocysts is localized primarily at perinuclear regions. Cytokeratins 8 and 18 are synthesized to about the same extent as actin at the time the tonofilaments appear whereas the synthesis of cytokeratins 5 and 6 is greatly reduced. Our results suggest that cytokeratins 5 and 6 in the tonofilaments may arise from the stored form of cytokeratins in the cytoskeletal sheets. Consequently, our results suggest that the sheets may serve as a maternal reserve of cytokeratin employed by the embryo at the time of implantation to form extensive arrays of tonofilaments in the embryo that likely provide structural integrity to the embryo as it is subjected to mechanical stress during invasion and implantation into the uterine wall.

Changes in the expression of intermediate filaments and desmoplakins during development of human notochord

Indirect immunofluorescence was used to study the expression of desmosomal and intermediate filament (IF) proteins in the human notochord between the 4th and 12th weeks of embryonic development. Towards the end of this period, the development of the notochord is characterized by its gradual physiological atrophy and disappearance inside the vertebral bodies. In all of our embryos, the notochord cells expressed cytokeratin and vimentin but not desmin, neurofilament protein or glial fibrillary acidic protein. Throughout the stages studied, the expression of cytokeratin was strong. Vimentin expression, on the other hand, changed during the stages studied. In our youngest embryos, vimentin could be detected only in the peripheral cells of the notochord. During development, a distinct increase occurred in vimentin expression, and in the oldest embryos, all notochord cells showed bright vimentin-specific fluorescence. Simultaneously with this modification, a change occurred in the expression of desmosomal proteins: The notochord cells expressed desmoplakins abundantly during early stages, but weakly or not at all during later stages. Correspondingly, electron microscopy of the same stages showed a striking decrease in the number of desmosomes between notochord cells. Our results confirm that, during early development, the notochord displays features specific for epithelial cells. This accords with the view that notochord is of epithelial origin. The modifications observed in the expression of IF and desmosomal proteins were temporally correlated with developmentally regulated atrophy of the notochord. The programmed regression of the notochord cells is thus associated with a switch from a predominantly epithelial phenotype to a more mesenchymal one.

Novel cytoskeletal elements in mammalian eggs are composed of a unique arrangement of intermediate filaments

Mammalian eggs and embryos contain a major network of specialized cytoskeletal components known as 'sheets' that have not been identified in any other cell type. Although eggs from at least seven different mammalian species have been shown to contain these cytoskeletal structures, embedment-free electron microscopic analysis of these eggs revealed that two basic forms of cytoskeletal sheets exist, a solid, planar type of sheet typical of hamster and rat eggs and a fibrous sheet typical of mouse, porcine, bovine, canine, and human eggs. In this study we have investigated the structural composition of the fibrous type of sheet in mouse eggs by employing biochemical approaches as well as two forms of ultrastructural analyses including: (1) analysis of thick, resin-embedded specimens using an intermediate voltage electron microscope (IVEM); (2) analysis of replicas from quick-frozen, deep-etched specimens. Our results indicate that the sheets of mouse eggs and preimplantation embryos are composed of cylindrical bundles of 10-11 nm filaments, with each of these filaments held in register by periodically arranged crossbridges spaced 23-25 nm apart. This sheet substructure of filaments and crossbridges is covered by a particulate material which can be removed by non-ionic detergent. Immunoelectron microscopic analysis of mouse eggs demonstrates that sheets bind antibodies to keratin and to a small extent, actin, but do not bind antibodies to vimentin or tubulin. Confirmation that keratin exists in these eggs was obtained by electrophoretic separation and one- and two-dimensional Western blot analysis demonstrating the existence of keratin types 5, 6, 8, 16, and type Z. The low abundancy of keratin type 8 compared to other keratin types explains the difficulties other investigators have had identifying intermediate filaments in mammalian embryos since most investigators have used antibodies directed specifically against keratin type 8 or its pair keratin type 18. Examination of compacted mouse embryos reveals that the filamentous framework of sheets disassembled and established close contact with the basolateral plasma membrane and the nucleus. However, sheets at the apical plasma membrane of blastomeres attach to the membrane but remain intact. Based on our biochemical and ultrastructural data, the fibrous sheets of mouse eggs appear to be cytoskeletal structures comparable to the solid, planar sheets of the Syrian hamster egg and probably serve similar function(s) in eggs and embryos of several mammalian species.

Ontogeny of the cytoskeleton during mammalian oogenesis

Mammalian oogenesis is a process which requires a variety of changes in the structure and function of the specialized female germ cell. Evidence suggests that the cytoskeleton may mediate several of these structural and functional changes. In this review we evaluate what is known of cytoskeletal function during oogenesis, with emphasis on specialized cytoskeletal features in mammals. Existing investigations suggest that the oocyte, as a highly specialized cell, contains unique cytoskeletal elements which exhibit functions restricted to the process of early development.