Mitochondrial morphology, distribution and activity during oocyte development

Mitochondria have a crucial role in cellular function and exhibit remarkable plasticity, adjusting both their structure and activity to meet the changing energy demands of a cell. Oocytes, female germ cells that become eggs, undergo unique transformations: the extended dormancy period, followed by substantial increase in cell size and subsequent maturation involving the segregation of genetic material for the next generation, present distinct metabolic challenges necessitating varied mitochondrial adaptations. Recent findings in dormant oocytes challenged the established respiratory complex hierarchies and underscored the extent of mitochondrial plasticity in long-lived oocytes. In this review, we discuss mitochondrial adaptations observed during oocyte development across three vertebrate species (Xenopus, mouse, and human), emphasising current knowledge, acknowledging limitations, and outlining future research directions.

Gonadal morphogenesis in the South American toad Rhinella Arenarum (Anura, Bufonidae) unveils an extremely delayed rate of sex differentiation

Among anurans, Bufonids are recognized for their retarded sex differentiation. However, few studies have addressed gonadal morphogenesis in this family. Here, we analyzed the early gonadogenesis in laboratory-reared Rhinella arenarum. Few germ cells were identified in the genital ridge at Gosner stage 26. At metamorphosis, somatic cells and germ cells were observed in the outer region of the undifferentiated gonad, whereas the central region was occupied by stromal tissue. A cortico-medullary organization was first recognized on Day 7 postmetamorphosis. The cortex was composed of germ cells and encompassing epithelial cells, whereas the medulla contained cells presumptively derived from the coelomic epithelium. Medullary somatic cells formed metameric knots along the length of the undifferentiated gonad. Consequently, a series of 12-14 gonomeres became recognizable externally. The first sign of ovarian differentiation was observed on Day 15 postmetamorphosis, when a cavity was formed within each gonomere. In contrast, testes were recognized by a uniform distribution of germ cells and intermingled somatic cells, as the division into cortex and medulla was lost. By Day 50 postmetamorphosis, the gonadal metameric organization was still apparent both in the ovaries and testes. Follicles containing diplotene oocytes were observed within the ovary. In the testis, an incipient lobular architecture was recognized without initiation of meiosis within the seminiferous cords. These observations reveal an extremely delayed gonadal development in R. arenarum, not reported previously for other anuran species. In addition, the late differentiation of the gonads contrasted with the early appearance of follicles in the Bidder's organ. Lastly, we observed that delayed metamorphs exhibited an undifferentiated gonad, demonstrating that gonadogenesis in this species is more dependent on somatic development than on age.

The Embryonic Origin of Primordial Germ Cells in the Tardigrade Hypsibius exemplaris

Primordial germ cells (PGCs) give rise to gametes â€" cells necessary for the propagation and fertility of diverse organisms. Current understanding of PGC development is limited to the small number of organisms whose PGCs have been identified and studied. Expanding the field to include little-studied taxa and emerging model organisms is important to understand the full breadth of the evolution of PGC development. In the phylum Tardigrada, no early cell lineages have been identified to date using molecular markers. This includes the PGC lineage. Here, we describe PGC development in the model tardigrade Hypsibius exemplaris . The four earliest-internalizing cells (EICs) exhibit PGC-like behavior and nuclear morphology. The location of the EICs is enriched for mRNAs of conserved PGC markers wiwi1 (water bear piwi 1) and vasa . At early stages, both wiwi1 and vasa mRNAs are detectable uniformly in embryos, which suggests that these mRNAs do not serve as localized determinants for PGC specification. Only later are wiwi1 and vasa enriched in the EICs. Finally, we traced the cells that give rise to the four PGCs. Our results reveal the embryonic origin of the PGCs of H. exemplaris and provide the first molecular characterization of an early cell lineage in the tardigrade phylum. We anticipate that these observations will serve as a basis for characterizing the mechanisms of PGC development in this animal.

Generalized nuclear localization of retroelement transcripts

BACKGROUND

LINE-1s, Alus and SVAs are the only retrotransposition competent elements in humans. Their mobilization followed by insertional mutagenesis is often linked to disease. Apart from these rare integration events, accumulation of retrotransposition intermediates in the cytoplasm is potentially pathogenic due to induction of inflammatory response pathways. Although the retrotransposition of LINE-1 and Alu retroelements has been studied in considerable detail, there are mixed observations about the localization of their RNAs.

RESULTS

We undertook a comprehensive and unbiased approach to analyze retroelement RNA localization using common cell lines and publicly available datasets containing RNA-sequencing data from subcellular fractions. Using our customized analytic pipeline, we compared localization patterns of RNAs transcribed from retroelements and single-copy protein coding genes. Our results demonstrate a generalized characteristic pattern of retroelement RNA nuclear localization that is conserved across retroelement classes as well as evolutionarily young and ancient elements. Preferential nuclear enrichment of retroelement transcripts was consistently observed in cell lines, in vivo and across species. Moreover, retroelement RNA localization patterns were dynamic and subject to change during development, as seen in zebrafish embryos.

CONCLUSION

The pronounced nuclear localization of transcripts arising from ancient as well as de novo transcribed retroelements suggests that these transcripts are retained in the nucleus as opposed to being re-imported to the nucleus or degraded in the cytoplasm. This raises the possibility that there is adaptive value associated with this localization pattern to the host, the retroelements or possibly both.

The Cif proteins from Wolbachia prophage WO modify sperm genome integrity to establish cytoplasmic incompatibility

Inherited microorganisms can selfishly manipulate host reproduction to drive through populations. In Drosophila melanogaster, germline expression of the native Wolbachia prophage WO proteins CifA and CifB cause cytoplasmic incompatibility (CI) in which embryos from infected males and uninfected females suffer catastrophic mitotic defects and lethality; however, in infected females, CifA expression rescues the embryonic lethality and thus imparts a fitness advantage to the maternally transmitted Wolbachia. Despite widespread relevance to sex determination, evolution, and vector control, the mechanisms underlying when and how CI impairs male reproduction remain unknown and a topic of debate. Here, we use cytochemical, microscopic, and transgenic assays in D. melanogaster to demonstrate that CifA and CifB proteins of wMel localize to nuclear DNA throughout the process of spermatogenesis. Cif proteins cause abnormal histone retention in elongating spermatids and protamine deficiency in mature sperms that travel to the female reproductive tract with Cif proteins. Notably, protamine gene knockouts enhance wild-type CI. In ovaries, CifA localizes to germ cell nuclei and cytoplasm of early-stage egg chambers; however, Cifs are absent in late-stage oocytes and subsequently in fertilized embryos. Finally, CI and rescue are contingent upon a newly annotated CifA bipartite nuclear localization sequence. Together, our results strongly support the Host modification model of CI in which Cifs initially modify the paternal and maternal gametes to bestow CI-defining embryonic lethality and rescue.

A transgenic system for targeted ablation of reproductive and maternal-effect genes

Maternally provided gene products regulate the earliest events of embryonic life, including formation of the oocyte that will develop into an egg, and eventually into an embryo. Forward genetic screens have provided invaluable insights into the molecular regulation of embryonic development, including the essential contributions of some genes whose products must be provided to the transcriptionally silent early embryo for normal embryogenesis, called maternal-effect genes. However, other maternal-effect genes are not accessible due to their essential zygotic functions during embryonic development. Identifying these regulators is essential to fill the large gaps in our understanding of the mechanisms and molecular pathways contributing to fertility and to maternally regulated developmental processes. To identify these maternal factors, it is necessary to bypass the earlier requirement for these genes so that their potential later functions can be investigated. Here, we report reverse genetic systems to identify genes with essential roles in zebrafish reproductive and maternal-effect processes. As proof of principle and to assess the efficiency and robustness of mutagenesis, we used these transgenic systems to disrupt two genes with known maternal-effect functions: kif5ba and bucky ball.

Comparisons among rainbow trout, Oncorhynchus mykiss, populations of maternal transcript profile associated with egg viability

Background

Transcription is arrested in the late stage oocyte and therefore the maternal transcriptome stored in the oocyte provides nearly all the mRNA required for oocyte maturation, fertilization, and early cleavage of the embryo. The transcriptome of the unfertilized egg, therefore, has potential to provide markers for predictors of egg quality and diagnosing problems with embryo production encountered by fish hatcheries. Although levels of specific transcripts have been shown to associate with measures of egg quality, these differentially expressed genes (DEGs) have not been consistent among studies. The present study compares differences in select transcripts among unfertilized rainbow trout eggs of different quality based on eyeing rate, among 2 year classes of the same line (A1, A2) and a population from a different hatchery (B). The study compared 65 transcripts previously reported to be differentially expressed with egg quality in rainbow trout.

Results

There were 32 transcripts identified as DEGs among the three groups by regression analysis. Group A1 had the most DEGs, 26; A2 had 15, 14 of which were shared with A1; and B had 12, 7 of which overlapped with A1 or A2. Six transcripts were found in all three groups, dcaf11, impa2, mrpl39_like, senp7, tfip11 and uchl1.

Conclusions

Our results confirmed maternal transcripts found to be differentially expressed between low- and high-quality eggs in one population of rainbow trout can often be found to overlap with DEGs in other populations. The transcripts differentially expressed with egg quality remain consistent among year classes of the same line. Greater similarity in dysregulated transcripts within year classes of the same line than among lines suggests patterns of transcriptome dysregulation may provide insight into causes of decreased viability within a hatchery population. Although many DEGs were identified, for each of the genes there is considerable variability in transcript abundance among eggs of similar quality and low correlations between transcript abundance and eyeing rate, making it highly improbable to predict the quality of a single batch of eggs based on transcript abundance of just a few genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07773-1.

Dynamics of small non-coding RNAs in bovine scNT embryos through the maternal-to-embryonic transition

The efficiency of somatic cell nuclear transfer (scNT) for production of viable offspring is relatively low as compared to in vitro fertilization (IVF), presumably due to deficiencies in epigenetic reprogramming of the donor cell genome. Such defects may also involve the population of small non-coding RNAs (sncRNAs), which are important during early embryonic development. The objective of this study was to examine dynamic changes in relative abundance of sncRNAs during the maternal-to embryonic transition (MET) in bovine embryos produced by scNT as compared to IVF by using RNA sequencing. When comparing populations of miRNA in scNT versus IVF embryos, only miR-2340, miR-345, and miR34a were differentially expressed in morulae, though many more miRNAs were differentially expressed when comparing across developmental stages. Also of interest, distinct populations of piwi-interacting like RNAs (pilRNAs) were identified in bovine embryos prior to and during embryonic genome activation (EGA) as compared bovine embryos post EGA and differentiated cells. Overall, sncRNA sequencing analysis of preimplantation embryos revealed largely similar profiles of sncRNAs for IVF and scNT embryos at the 2-cell, 8-cell, morula and blastocyst stages of development. However, these sncRNA profiles, including miRNA, piRNA and tRNA fragments, were notably distinct prior to and after completion of the MET.

Human pericentromeric tandemly repeated DNA is transcribed at the end of oocyte maturation and is associated with membraneless mitochondria-associated structures

Most of the human genome is non-coding. However, some of the non-coding part is transcriptionally active. In humans, the tandemly repeated (TR) pericentromeric non-coding DNA-human satellites 2 and 3 (HS2, HS3)-are transcribed in somatic cells. These transcripts are also found in pre- and post-implantation embryos. The aim of this study was to analyze HS2/HS3 transcription and cellular localization of transcripts in human maturating oocytes. The maternal HS2/HS3 TR transcripts transcribed from both strands were accumulated in the ooplasm in GV-MI oocytes as shown by DNA-RNA FISH (fluorescence in-situ hybridization). The transcripts' content was higher in GV oocytes than in somatic cumulus cells according to real-time PCR. Using bioinformatics analysis, we demonstrated the presence of polyadenylated HS2 and HS3 RNAs in datasets of GV and MII oocyte transcriptomes. The transcripts shared a high degree of homology with HS2, HS3 transcripts previously observed in cancer cells. The HS2/HS3 transcripts were revealed by a combination of FISH and immunocytochemical staining within membraneless RNP structures that contained DEAD-box helicases DDX5 and DDX4. The RNP structures were closely associated with mitochondria, and are therefore similar to membraneless bodies described previously only in oogonia. These membraneless structures may be a site for spatial sequestration of RNAs and proteins in both maturating oocytes and cancer cells.

Maternal Larp6 controls oocyte development, chorion formation and elevation

La-related protein 6 (Larp6) is a conserved RNA-binding protein found across eukaryotes that has been suggested to regulate collagen biogenesis, muscle development, ciliogenesis, and various aspects of cell proliferation and migration. Zebrafish have two Larp6 family genes: larp6a and larp6b Viable and fertile single and double homozygous larp6a and larp6b zygotic mutants revealed no defects in muscle structure, and were indistinguishable from heterozygous or wild-type siblings. However, larp6a mutant females produced eggs with chorions that failed to elevate fully and were fragile. Eggs from larp6b single mutant females showed minor chorion defects, but chorions from eggs laid by larp6a;larp6b double mutant females were more defective than those from larp6a single mutants. Electron microscopy revealed defective chorionogenesis during oocyte development. Despite this, maternal zygotic single and double mutants were viable and fertile. Mass spectrometry analysis provided a description of chorion protein composition and revealed significant reductions in a subset of zona pellucida and lectin-type proteins between wild-type and mutant chorions that paralleled the severity of the phenotype. We conclude that Larp6 proteins are required for normal oocyte development, chorion formation and egg activation.

Biomolecular condensates in neurodegeneration and cancer

The intracellular environment is partitioned into functionally distinct compartments containing specific sets of molecules and reactions. Biomolecular condensates, also referred to as membrane-less organelles, are diverse and abundant cellular compartments that lack membranous enclosures. Molecules assemble into condensates by phase separation; multivalent weak interactions drive molecules to separate from their surroundings and concentrate in discrete locations. Biomolecular condensates exist in all eukaryotes and in some prokaryotes, and participate in various essential house-keeping, stress-response and cell type-specific processes. An increasing number of recent studies link abnormal condensate formation, composition and material properties to a number of disease states. In this review, we discuss current knowledge and models describing the regulation of condensates and how they become dysregulated in neurodegeneration and cancer. Further research on the regulation of biomolecular phase separation will help us to better understand their role in cell physiology and disease.

Survival of Polyploid hybrid salamander embryos

Background

Animals with polyploid, hybrid nuclei offer a challenge for models of gene expression and regulation during embryogenesis. To understand how such organisms proceed through development, we examined the timing and prevalence of mortality among embryos of unisexual salamanders in the genus Ambystoma.

Results

Our regional field surveys suggested that heightened rates of embryo mortality among unisexual salamanders begin in the earliest stages of embryogenesis. Although we expected elevated mortality after zygotic genome activation in the blastula stage, this is not what we found among embryos which we reared in the laboratory. Once embryos entered the first cleavage stage, we found no difference in mortality rates between unisexual salamanders and their bisexual hosts. Our results are consistent with previous studies showing high rates of unisexual mortality, but counter to reports that heightened embryo mortality continues throughout embryo development.

Conclusions

Possible causes of embryonic mortality in early embryogenesis suggested by our results include abnormal maternal loading of RNA during meiosis and barriers to insemination. The surprising survival rates of embryos post-cleavage invites further study of how genes are regulated during development in such polyploid hybrid organisms.

Transcriptome analysis of egg viability in rainbow trout, Oncorhynchus mykiss

Background

Maternal transcripts are accumulated in the oocyte during oogenesis to provide for protein synthesis from oocyte maturation through early embryonic development, when nuclear transcription is silenced. The maternal mRNAs have short poly(A) tails after undergoing post-transcriptional processing necessary for stabilizing them for storage. The transcripts undergo cytoplasmic polyadenylation when they are to be translated. Transcriptome analyses comparing total mRNA and elongated poly(A) mRNA content among eggs of different quality can provide insight into molecular mechanisms affecting egg developmental competence in rainbow trout. The present study used RNA-seq to compare transcriptomes of unfertilized eggs of rainbow trout females yielding different eyeing rates, following rRNA removal and poly(A) retention for construction of the libraries.

Results

The percentage of embryos to reach the 32-cell stage at 24 h post fertilization was significantly correlated to family eyeing rate, indicating that inviable embryos were developmentally compromised before zygotic genome activation. RNA sequencing identified 2 differentially expressed transcripts (DETs) from total mRNA sequencing comparing females with low-quality (< 5% eyeing), medium-quality (30–50% eyeing), and high-quality (> 80% eyeing) eggs. In contrast, RNA sequencing from poly(A) captured transcripts identified 945 DETs between low- and high-quality eggs, 1012 between low- and medium-quality eggs, and only 2 between medium- and high-quality eggs. The transcripts of mitochondrial genes were enriched with polyadenylated transcript sequencing and they were significantly reduced in low-quality eggs. Similarly, mitochondrial DNA was reduced in low-quality eggs compared with medium- and high-quality eggs. The functional gene analysis classified the 945 DETs between low- and high-quality eggs into 31 functional modules, many of which were related to ribosomal and mitochondrial functions. Other modules involved transcription, translation, cell division, apoptosis, and immune responses.

Conclusions

Our results indicate that differences in egg quality may be derived from differences in maternal nuclear transcript activation and cytoplasmic polyadenylation before ovulation, as opposed to accumulation and storage of maternal nuclear transcripts during oogenesis. Transcriptome comparisons suggest low-quality eggs suffered from impaired oxidative phosphorylation and translation. The DETs identified in this study provide insight into developmental competence in rainbow trout eggs.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5690-5) contains supplementary material, which is available to authorized users.

The model of "genetic compartments": a new insight into reproductive genetics

Currently, we are witnessing revolutionary advances in the analytical power of genetic tools. An enormous quantity of data can now be obtained from samples; however, the translation of genetic findings to the general status of individuals, or their offspring, should be done with caution. This is especially relevant in the reproductive context, where the concepts of "transmission" and "inheritability" of a trait are crucial. Against this background, we offer new insight based on a systemic view of genetic constitution in the compartmentalized organism, that is, the human body. This model considers the coexistence of "different" genomes in the same individual and the repercussion of this on reproductive efficacy and offspring. Herein, we review the major differences between somatic, germinal, embryonic, and fetal/placental genomes and their contribution to the next generation and its reproductive efficacy. The major novelty of our approach is the holistic interaction between microsystems within a macrosystem (i.e., the reproductive system). This panoramic model allows us to sketch the future implications of genetic results in function of the origin (compartment) of the sample: peripheral blood or other somatic tissues, gametes, zygotes, preimplantation embryos, fetus, or placenta. We believe this perspective can be of great use in the context of reproductive genetic counseling.

Visualizing the Balbiani Body in Zebrafish Oocytes

Approaches to visualize the Balbiani body of zebrafish primary oocytes using protein, RNA, and mitochondrial markers are described. The method involves isolation, histology, staining, and microscopic examination of early zebrafish oocytes. These techniques can be applied to visualize gene products that are localized to the Balbiani body, and when applied to mutants can be used to decipher molecular and genetic pathways acting in Balbiani body development in early oocytes.

The genetic program of oocytes can be modified in vivo in the zebrafish ovary

Oocytes, the irreplaceable gametes for generating a new organism, are matured in the ovary of living female animals. It is unknown whether any genetic manipulations can be applied to immature oocytes inside the living ovaries. As a proof-of-concept, we here demonstrate genetic amendments of zebrafish immature oocytes within the ovary. Oocyte microinjection in situ (OMIS) stimulates tissue repair responses, but some of the microinjected immature oocytes are matured, ovulated and fertilizable. By OMIS-mediated Cas9 approach, ntla and gata5 loci of oocytes arrested at prophase I of meiosis are successfully edited before fertilization. Through OMIS, high efficiency of biallelic mutations in single or multiple loci using Cas9/gRNAs allows immediate manifestation of mutant phenotypes in F0 embryos and multiple transgenes can co-express the reporters in F0 embryos with patterns similar to germline transgenic embryos. Furthermore, maternal knockdown of dnmt1 by antisense morpholino via OMIS results in a dramatic decrease of global DNA methylation level at the dome stage and causes embryonic lethality prior to segmentation period. Therefore, OMIS opens a door to efficiently modify the genome and provides a possibility to repair genetically abnormal oocytes in situ.

The Role of Maternal HP1a in Early Drosophila Embryogenesis via Regulation of Maternal Transcript Production

Heterochromatin protein 1a (HP1a) is a highly conserved and versatile epigenetic factor that can both silence and activate transcription. However, the function of HP1a in development has been underinvestigated. Here, we report the role of maternal HP1a in producing maternal transcripts that drive early Drosophila embryogenesis. Maternal HP1a upregulates genes involved in translation, mRNA splicing, and cell division, but downregulates genes involved in neurogenesis, organogenesis, and germline development, which all occur later in development. Our study reveals the earliest contribution of HP1a during oogenesis in regulating the production of maternal transcripts that drive early Drosophila embryogenesis.

Potential sperm contributions to the murine zygote predicted by in silico analysis

Paternal contributions to the zygote are thought to extend beyond delivery of the genome and paternal RNAs have been linked to epigenetic transgenerational inheritance in different species. In addition, sperm–egg fusion activates several downstream processes that contribute to zygote formation, including PLC zeta-mediated egg activation and maternal RNA clearance. Since a third of the preimplantation developmental period in the mouse occurs prior to the first cleavage stage, there is ample time for paternal RNAs or their encoded proteins potentially to interact and participate in early zygotic activities. To investigate this possibility, a bespoke next-generation RNA sequencing pipeline was employed for the first time to characterise and compare transcripts obtained from isolated murine sperm, MII eggs and pre-cleavage stage zygotes. Gene network analysis was then employed to identify potential interactions between paternally and maternally derived factors during the murine egg-to-zygote transition involving RNA clearance, protein clearance and post-transcriptional regulation of gene expression. Ourin silicoapproach looked for factors in sperm, eggs and zygotes that could potentially interact co-operatively and synergistically during zygote formation. At least five sperm RNAs (Hdac11,Fbxo2,Map1lc3a,Pcbp4andZfp821) met these requirements for a paternal contribution, which with complementary maternal co-factors suggest a wider potential for extra-genomic paternal involvement in the developing zygote.

Myosin-1 inhibition by PClP affects membrane shape, cortical actin distribution and lipid droplet dynamics in early Zebrafish embryos

Myosin-1 (Myo1) represents a mechanical link between the membrane and actin-cytoskeleton in animal cells. We have studied the effect of Myo1 inhibitor PClP in 1-8 cell Zebrafish embryos. Our results indicate a unique involvement of Myo1 in early development of Zebrafish embryos. Inhibition of Myo1 (by PClP) and Myo2 (by Blebbistatin) lead to arrest in cell division. While Myo1 isoforms appears to be important for both the formation and the maintenance of cleavage furrows, Myo2 is required only for the formation of furrows. We found that the blastodisc of the embryo, which contains a thick actin cortex (~13 μm), is loaded with cortical Myo1. Myo1 appears to be crucial for maintaining the blastodisc morphology and the actin cortex thickness. In addition to cell division and furrow formation, inhibition of Myo1 has a drastic effect on the dynamics and distribution of lipid droplets (LDs) in the blastodisc near the cleavage furrow. All these results above are effects of Myo1 inhibition exclusively; Myo2 inhibition by blebbistatin does not show such phenotypes. Therefore, our results demonstrate a potential role for Myo1 in the maintenance and formation of furrow, blastodisc morphology, cell-division and LD organization within the blastodisc during early embryogenesis.

The maternal control in the embryonic development of zebrafish

The maternal control directing the very first hours of life is of pivotal importance for ensuring proper development to the growing embryo. Thanks to the finely regulated inheritance of maternal factors including mRNAs and proteins produced during oogenesis and stored into the mature oocyte, the embryo is sustained throughout the so-called maternal-to-zygotic transition, a period in development characterized by a species-specific length in time, during which critical biological changes regarding cell cycle and zygotic transcriptional activation occur. In order not to provoke any kind of persistent damage, the process must be delicately balanced. Surprisingly, our knowledge as to the possible effects of beneficial bacteria regarding the modulation of the quality and/or quantity of both maternally-supplied and zygotically-transcribed mRNAs, is very limited. To date, only one group has investigated the consequences of the parentally-supplied Lactobacillus rhamnosus on the storage of mRNAs into mature oocytes, leading to an altered maternal control process in the F1 generation. Particular attention was called on the monitoring of several biomarkers involved in autophagy, apoptosis and axis patterning, while data on miRNA generation and pluripotency maintenance are herein presented for the first time, and can assist in laying the ground for further investigations in this field. In this review, the reader is supplied with the current knowledge on the above-mentioned biological process, first by drawing the general background and then by emphasizing the most important findings that have highlighted their focal role in normal animal development.

The Principal Forces of Oocyte Polarity Are Evolutionary Conserved but May Not Affect the Contribution of the First Two Blastomeres to the Blastocyst Development in Mammals

Oocyte polarity and embryonic patterning are well-established features of development in lower species. Whether a similar form of pre-patterning exists in mammals is currently under hot debate in mice. This study investigated this issue for the first time in ovine as a large mammal model. Microsurgical trisection of unfertilized MII-oocytes revealed that cortical cytoplasm around spindle (S) contained significant amounts of total maternal mRNAs and proteins compared to matched cytoplast hemispheres that were located either near (NS) or far (FS) -to-spindle. RT-qPCR provided striking examples of maternal mRNA localized to subcellular substructures S (NPM2, GMNN, H19, PCAF, DNMT3A, DNMT1, and STELLA), NS (SOX2, NANOG, POU5F1, and TET1), and FS (GCN) of MII oocyte. Immunoblotting revealed that specific maternal proteins DNMT3A and NANOG were asymmetrically enriched in MII-spindle-half of the oocytes. Topological analysis of sperm entry point (SEP) revealed that sperm preferentially entered via the MII-spindle-half of the oocytes. Even though, the topological position of first cleavage plane with regard to SEP was quite stochastic. Spatial comparison of lipid content revealed symmetrical distribution of lipids between 2-cell blastomeres. Lineage tracing using Dil, a fluorescent dye, revealed that while the progeny of leading blastomere of 2-cell embryos contributed to more cells in the developed blastocysts compared to lagging counterpart, the contributions of leading and lagging blastomeres to the embryonic-abembryonic parts of the developed blastocysts were almost unbiased. And finally, separated sister blastomeres of 2-cell embryos had an overall similar probability to arrest at any stage before the blastocyst (2-cell, 4-cell, 8-cell, and morula) or to achieve the blastocyst stage. It was concluded that the localization of maternal mRNAs and proteins at the spindle are evolutionarily conserved between mammals unfertilized ovine oocyte could be considered polar with respect to the spatial regionalization of maternal transcripts and proteins. Even though, the principal forces of this definitive oocyte polarity may not persist during embryonic cleavages.

Primordial Germ Cell Specification and Migration

Primordial germ cells are the progenitor cells that give rise to the gametes. In some animals, the germline is induced by zygotic transcription factors, whereas in others, primordial germ cell specification occurs via inheritance of maternally provided gene products known as germ plasm. Once specified, the primordial germ cells of some animals must acquire motility and migrate to the gonad in order to survive. In all animals examined, perinuclear structures called germ granules form within germ cells. This review focuses on some of the recent studies, conducted by several groups using diverse systems, from invertebrates to vertebrates, which have provided mechanistic insight into the molecular regulation of germ cell specification and migration.

Kinesin-1 interacts with Bucky ball to form germ cells and is required to pattern the zebrafish body axis

In animals, specification of the primordial germ cells (PGCs), the stem cells of the germ line, is required to transmit genetic information from one generation to the next. Bucky ball (Buc) is essential for germ plasm (GP) assembly in oocytes, and its overexpression results in excess PGCs in zebrafish embryos. However, the mechanistic basis for the excess PGCs in response to Buc overexpression, and whether endogenous Buc functions during embryogenesis, are unknown. Here, we show that endogenous Buc, like GP and overexpressed Buc-GFP, accumulates at embryonic cleavage furrows. Furthermore, we show that the maternally expressed zebrafish Kinesin-1 Kif5Ba is a binding partner of Buc and that maternal kif5Ba (Mkif5Ba) plays an essential role in germline specification in vivo. Specifically, Mkif5Ba is required to recruit GP to cleavage furrows and thereby specifies PGCs. Moreover, Mkif5Ba is required to enrich Buc at cleavage furrows and for the ability of Buc to promote excess PGCs, providing mechanistic insight into how Buc functions to assemble embryonic GP. In addition, we show that Mkif5Ba is also essential for dorsoventral (DV) patterning. Specifically, Mkif5Ba promotes formation of the parallel vegetal microtubule array required to asymmetrically position dorsal determinants (DDs) towards the prospective dorsal side. Interestingly, whereas Syntabulin and wnt8a translocation depend on kif5Ba, grip2a translocation does not, providing evidence for two distinct mechanisms by which DDs might be asymmetrically distributed. These studies identify essential roles for maternal Kif5Ba in PGC specification and DV patterning, and provide mechanistic insight into Buc functions during early embryogenesis.

New insights into the maternal to zygotic transition

The initial phases of embryonic development occur in the absence of de novo transcription and are instead controlled by maternally inherited mRNAs and proteins. During this initial period, cell cycles are synchronous and lack gap phases. Following this period of transcriptional silence, zygotic transcription begins, the maternal influence on development starts to decrease, and dramatic changes to the cell cycle take place. Here, we discuss recent work that is shedding light on the maternal to zygotic transition and the interrelated but distinct mechanisms regulating the onset of zygotic transcription and changes to the cell cycle during early embryonic development.

Oocyte polarity requires a Bucky ball-dependent feedback amplification loop

In vertebrates, the first asymmetries are established along the animal-vegetal axis during oogenesis, but the underlying molecular mechanisms are poorly understood. Bucky ball (Buc) was identified in zebrafish as a novel vertebrate-specific regulator of oocyte polarity, acting through unknown molecular interactions. Here we show that endogenous Buc protein localizes to the Balbiani body, a conserved, asymmetric structure in oocytes that requires Buc for its formation. Asymmetric distribution of Buc in oocytes precedes Balbiani body formation, defining Buc as the earliest marker of oocyte polarity in zebrafish. Through a transgenic strategy, we determined that excess Buc disrupts polarity and results in supernumerary Balbiani bodies in a 3'UTR-dependent manner, and we identified roles for the buc introns in regulating Buc activity. Analyses of mosaic ovaries indicate that oocyte pattern determines the number of animal pole-specific micropylar cells that are associated with an egg via a close-range signal or direct cell contact. We demonstrate interactions between Buc protein and buc mRNA with two conserved RNA-binding proteins (RNAbps) that are localized to the Balbiani body: RNA binding protein with multiple splice isoforms 2 (Rbpms2) and Deleted in azoospermia-like (Dazl). Buc protein and buc mRNA interact with Rbpms2; buc and dazl mRNAs interact with Dazl protein. Cumulatively, these studies indicate that oocyte polarization depends on tight regulation of buc: Buc establishes oocyte polarity through interactions with RNAbps, initiating a feedback amplification mechanism in which Buc protein recruits RNAbps that in turn recruit buc and other RNAs to the Balbiani body.

The roles of maternal Vangl2 and aPKC in Xenopus oocyte and embryo patterning

The Xenopus oocyte contains components of both the planar cell polarity and apical-basal polarity pathways, but their roles are not known. Here, we examine the distribution, interactions and functions of the maternal planar cell polarity core protein Vangl2 and the apical-basal complex component aPKC. We show that Vangl2 is distributed in animally enriched islands in the subcortical cytoplasm in full-grown oocytes, where it interacts with a post-Golgi v-SNARE protein, VAMP1, and acetylated microtubules. We find that Vangl2 is required for the stability of VAMP1 as well as for the maintenance of the stable microtubule architecture of the oocyte. We show that Vangl2 interacts with atypical PKC, and that both the acetylated microtubule cytoskeleton and the Vangl2-VAMP1 distribution are dependent on the presence of aPKC. We also demonstrate that aPKC and Vangl2 are required for the cell membrane asymmetry that is established during oocyte maturation, and for the asymmetrical distribution of maternal transcripts for the germ layer and dorsal/ventral determinants VegT and Wnt11. This study demonstrates the interaction and interdependence of Vangl2, VAMP1, aPKC and the stable microtubule cytoskeleton in the oocyte, shows that maternal Vangl2 and aPKC are required for specific oocyte asymmetries and vertebrate embryonic patterning, and points to the usefulness of the oocyte as a model to study the polarity problem.