Cyclin B1 mRNA translation is temporally controlled through formation and disassembly of RNA granules

Post-transcriptional regulation of cyclin A and cyclin B mRNAs is mediated by Bruno 1 and Cup, and further fine-tuned within P-bodies

Cell cycle progression is tightly controlled by the regulated synthesis and degradation of Cyclins, such as Cyclin A and Cyclin B, which activate CDK1 to trigger mitosis. Mutations affecting Cyclin regulation are often linked to tumorigenesis, making the study of cyclin mRNA regulation critical for identifying new cancer therapies. In this study, we demonstrate via super-resolution microscopy that cyclin A and cyclin B mRNAs associate with Bruno 1 and Cup in nurse cells. Depletion of either protein leads to abnormal Cyclin A and Cyclin B protein expression and a reduction in mRNA levels for both Cyclins. We further reveal that both cyclin A and cyclin B mRNAs accumulate in P-bodies marked by Me31B. Interestingly, Me31B is not involved in regulating cyclin A mRNA, as no changes in cyclin A mRNA levels or repression were observed upon Me31B depletion. However, cyclin B mRNA shows stage-specific derepression and reduced levels when Me31B is absent. Notably, the association between cyclin B and Cup is strengthened in the absence of Me31B, indicating that this interaction occurs independently of P-bodies. These results highlight the nuanced, mRNA-specific roles of P-body condensates in post-transcriptional regulation, challenging the idea of a uniform, binary mechanism of mRNA repression in P-bodies.

Interplay of RNA-binding proteins controls germ cell development in zebrafish

The specification of germ cells in zebrafish mostly relies on an inherited mechanism by which localized maternal determinants, called germ plasm, confer germline fate in the early embryo. Extensive studies have partially allowed the identification of key regulators governing germ plasm formation and subsequent germ cell development. RNA-binding proteins, acting in concert with other germ plasm components, play essential roles in the organization of the germ plasm and the specification, migration, maintenance, and differentiation of primordial germ cells. The loss of their functions impairs germ cell formation and causes sterility or sexual conversion. Evidence is emerging that they instruct germline development through differential regulation of mRNA fates in somatic and germ cells. However, the challenge remains to decipher the complex interplay of maternal germ plasm components in germ plasm compartmentalization and germ cell specification. Because failure to control the developmental outcome of germ cells disrupts the formation of gametes, it is important to gain a complete picture of regulatory mechanisms operating in the germ cell lineage. This review sheds light on the contributions of RNA-binding proteins to germ cell development in zebrafish and highlights intriguing questions that remain open for future investigation.

High-sensitivity whole-mount in situ Hybridization of Mouse Oocytes and Embryos Visualizes the Super-resolution Structures and Distributions of mRNA Molecules

Mammalian oocytes accumulate more than ten thousand mRNAs, of which three to four thousand mRNAs are translationally repressed. The timings and sites of translational activation of these dormant mRNAs are crucial for promoting oocyte maturation and embryonic development. How these mRNAs are accumulated and distributed in oocytes is therefore a fundamental issue to be explored. A method that enables visualization of mRNA molecules with high resolution in a simple manner would be valuable for understanding how oocytes accumulate and regulate the dormant mRNAs. We have developed a highly sensitive whole-mount in situ hybridization method using in vitro-synthesized RNA probes and the tyramide signal amplification (TSA) system optimized for mouse oocytes and embryos. By using this method, Pou5f1/Oct4, Emi2, and cyclin B1 mRNAs were detected in immature oocytes and 2-cell stage embryos. Confocal microscopy showed that these mRNAs formed granular structures in the oocyte cytoplasm. The structures of Pou5f1/Oct4 and cyclin B1 mRNAs persisted in 2-cell stage embryos. Pou5f1/Oct4 RNA granules exhibited a solid-like property in immature oocytes and became liquid-like droplets in 2-cell stage embryos. Double-staining of cyclin B1 mRNA with Emi2 or Pou5f1/Oct4 mRNA revealed that these mRNAs were distributed as different RNA granules without overlapping each other and that the size of cyclin B1 RNA granules tended to be larger than that of Emi2 RNA granules. The structures and distribution patterns of these mRNAs were further analyzed by N-SIM super-resolution microscopy. This analysis revealed that the large-sized RNA granules consist of many small-sized granules, suggesting the accumulation and regulation of dormant mRNAs as basal-sized RNA granules. The method established in this study can easily visualize the structure and distribution of mRNAs accumulated in mammalian oocytes and embryos with high sensitivity and super-resolution. This method is useful for investigating the cellular and molecular mechanisms of translational control of mRNAs by which maturation and early developmental processes are promoted.

Nano-graphene oxide particles induce inheritable anomalies through altered gene expressions involved in oocyte maturation

The inheritable impact of exposure to graphene oxide nanoparticles (GO NPs) on vertebrate germline during critical windows of gamete development remain undetermined to date. Here, we analyzed the transgenerational effects of exposure to nano-graphene oxide particles (nGO) synthesized in house with lateral dimensions 300-600 nm and surface charge of -36.8 mV on different developmental stages of germ cells (GCs): (1) during GCs undergoing early development and differentiation, and (2) during GCs undergoing gametogenesis and maturation in adulthood. Biocompatibility analyses in Japanese medaka embryos showed lethality above 1 µg/ml and also an aberrant increase in germ cell count of both males and females at doses below the lethal dose. However, no lethality or anomalies were evident in adults up to 45 µg/ml. Long term exposure of embryos and adults for 21 days resulted in reduced fecundity. This effect was transmitted to subsequent generations, F1 and F2. Importantly, the inheritable effects of nGO in adults were pronounced at a high dose of 10 µg/ml, while 1 µg/ml showed no impact on the germline indicating lower doses used in this study to be safe. Further, expressions of selected genes that adversely affected oocyte maturation were enhanced in F1 and F2 individuals. Interestingly, the inheritance patterns differed corresponding to the stage at which the fish received the exposure.

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.

Alternative cleavage and polyadenylation of the Ccnb1 mRNA defines accumulation of cyclin protein during the meiotic cell cycle

Progression through the mitotic and meiotic cell cycle is driven by fluctuations in the levels of cyclins, the regulatory subunits controlling the localization and activity of CDK1 kinases. Cyclin levels are regulated through a precise balance of synthesis and degradation. Here we demonstrate that the synthesis of Cyclin B1 during the oocyte meiotic cell cycle is defined by the selective translation of mRNA variants generated through alternative cleavage and polyadenylation (APA). Using gene editing in mice, we introduced mutations into the proximal and distal polyadenylation elements of the 3' untranslated region (UTR) of the Ccnb1 mRNA. Through in vivo loss-of-function experiments, we demonstrate that the translation of mRNA with a short 3' UTR specifies Cyclin B1 protein levels that set the timing of meiotic re-entry. In contrast, translation directed by a long 3' UTR is necessary to direct Cyclin B1 protein accumulation during the MI/MII transition. These findings establish that the progression through the cell cycle is dependent on the selective translation of multiple mRNA variants generated by APA.

Mature mRNA processing that deletes 3' end sequences directs translational activation and embryonic development

Eggs accumulate thousands of translationally repressed mRNAs that are translated into proteins after fertilization to direct diverse developmental processes. However, molecular mechanisms underlying the translation of stored mRNAs after fertilization remain unclear. Here, we report a previously unknown RNA processing of 3' end sequences of mature mRNAs that activates the translation of stored mRNAs. Specifically, 9 to 72 nucleotides at the 3' ends of zebrafish pou5f3 and mouse Pou5f1 mRNAs were deleted in the early stages of development. Reporter assays illustrated the effective translation of the truncated forms of mRNAs. Moreover, promotion and inhibition of the shortening of 3' ends accelerated and attenuated Pou5f3 accumulation, respectively, resulting in defective development. Identification of proteins binding to unprocessed and/or processed mRNAs revealed that mRNA shortening acts as molecular switches. Comprehensive analysis revealed that >250 mRNAs underwent this processing. Therefore, our results provide a molecular principle that triggers the translational activation and directs development.

Meiotic Cell Cycle Progression in Mouse Oocytes: Role of Cyclins

All eukaryotic cells, including oocytes, utilize an engine called cyclin-dependent kinase (Cdk) to drive the cell cycle. Cdks are activated by a co-factor called cyclin, which regulates their activity. The key Cdk-cyclin complex that regulates the oocyte cell cycle is known as Cdk1-cyclin B1. Recent studies have elucidated the roles of other cyclins, such as B2, B3, A2, and O, in oocyte cell cycle regulation. This review aims to discuss the recently discovered roles of various cyclins in mouse oocyte cell cycle regulation in accordance with the sequential progression of the cell cycle. In addition, this review addresses the translation and degradation of cyclins to modulate the activity of Cdks. Overall, the literature indicates that each cyclin performs unique and redundant functions at various stages of the cell cycle, while their expression and degradation are tightly regulated. Taken together, this review provides new insights into the regulatory role and function of cyclins in oocyte cell cycle progression.

RNA-binding proteins in autoimmunity: From genetics to molecular biology

Autoimmune diseases (ADs) are chronic pathologies generated by the loss of immune tolerance to the body's own cells and tissues. There is growing recognition that RNA-binding proteins (RBPs) critically govern immunity in healthy and pathological conditions by modulating gene expression post-transcriptionally at all levels: nuclear mRNA splicing and modification, export to the cytoplasm, as well as cytoplasmic mRNA transport, storage, editing, stability, and translation. Despite enormous efforts to identify new therapies for ADs, definitive solutions are not yet available in many instances. Recognizing that many ADs have a strong genetic component, we have explored connections between the molecular biology and the genetics of RBPs in ADs. Here, we review the genetics and molecular biology of RBPs in four major ADs, multiple sclerosis (MS), type 1 diabetes mellitus (T1D), systemic lupus erythematosus (SLE), and rheumatoid arthritis (RA). We anticipate that gaining insights into the genetics and biology of ADs can facilitate the discovery of new therapies. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Sex blind: bridging the gap between drug exposure and sex-related gene expression in Danio rerio using next-generation sequencing (NGS) data and a literature review to find the missing links in pharmaceutical and environmental toxicology studies

The sex of both humans and Danio rerio has previously been shown to affect the way individuals respond to drug exposure. Genes which allow identification of sex in juvenile zebrafish show potential to reveal these confounding variables between sex in toxicological and preclinical trials but the link between these is so far missing. These sex-specific, early expressed genes where expression is not altered by drug exposure must be carefully selected for this purpose. We aimed to discover genes which can be used in pharmaceutical trials and environmental toxicology studies to uncover sex-related variations in gene expression with drug application using the model organism Danio rerio. Previously published early sex determining genes from King et al. were evaluated as well as additional genes selected from our zebrafish Next-generation sequencing (NGS) data which are known from previously published works not to be susceptible to changes in expression with drug exposure. NGS revealed a further ten female-specific genes (vtg1, cyp17a1, cyp19a1a, igf3, ftz-f1, gdf9, foxl2a, Nr0b1, ipo4, lhcgr) and five male related candidate genes (FKBP5, apobb1, hbaa1, dmrt1, spata6) which are also expressed in juvenile zebrafish, 28 days post fertilisation (dpf). Following this, a literature review was performed to classify which of these early-expressed sex specific genes are already known to be affected by drug exposure in order to determine candidate genes to be used in pharmaceutical trials or environmental toxicology testing studies. Discovery of these early sex-determining genes in Danio rerio will allow identification of sex-related responses to drug testing to improve sex-specific healthcare and the medical treatment of human patients.

Spatiotemporal regulation of maternal mRNAs during vertebrate oocyte meiotic maturation

Vertebrate oocytes face a particular challenge concerning the regulation of gene expression during meiotic maturation. Global transcription becomes quiescent in fully grown oocytes, remains halted throughout maturation and fertilization, and only resumes upon embryonic genome activation. Hence, the oocyte meiotic maturation process is largely regulated by protein synthesis from pre-existing maternal messenger RNAs (mRNAs) that are transcribed and stored during oocyte growth. Rapidly developing genome-wide techniques have greatly expanded our insights into the global translation changes and possible regulatory mechanisms during oocyte maturation. The storage, translation, and processing of maternal mRNAs are thought to be regulated by factors interacting with elements in the mRNA molecules. Additionally, posttranscriptional modifications of mRNAs, such as methylation and uridylation, have recently been demonstrated to play crucial roles in maternal mRNA destabilization. However, a comprehensive understanding of the machineries that regulate maternal mRNA fate during oocyte maturation is still lacking. In particular, how the transcripts of important cell cycle components are stabilized, recruited at the appropriate time for translation, and eliminated to modulate oocyte meiotic progression remains unclear. A better understanding of these mechanisms will provide invaluable insights for the preconditions of developmental competence acquisition, with important implications for the treatment of infertility. This review discusses how the storage, localization, translation, and processing of oocyte mRNAs are regulated, and how these contribute to oocyte maturation progression.

DIS3L2 ribonuclease degrades terminal-uridylated RNA to ensure oocyte maturation and female fertility

During oocyte development in mice, transcripts accumulate in the growth phase and are subsequently degraded during maturation. At the transition point between growth and maturation, oocytes have an intact nucleus or germinal vesicle (GV), and terminal uridylation labels RNA for degradation in meiosis I. By profiling the transcriptome using single-oocyte long-read PacBio RNA sequencing, we document that a small cohort of mRNAs are polyadenylated after terminal uridylation in GV oocytes [designated uridylated-poly(A) RNA]. Because DIS3L2 ribonuclease is known to degrade uridylated transcripts, we established oocyte-specific Dis3l2 knockout mice (Dis3l2cKO). Upon DIS3L2 depletion, uridylated-poly(A) RNAs remain intact which increases their abundance, and they predominate in the transcriptome of Dis3l2cKO oocytes. The abundance of uridylated-poly(A) RNA in Dis3l2cKO oocytes arises not only from insufficient degradation, but also from the stabilizing effect of subsequent polyadenylation. Uridylated-poly(A) RNAs have shorter poly(A) tails and their translation activity decreases in Dis3l2cKO oocytes. Almost all Dis3l2cKO oocytes arrest at the GV stage, and female mice are infertile. Our study demonstrates multiple fates for RNA after terminal uridylation and highlights the role of DIS3L2 ribonuclease in safeguarding the transcriptome and ensuring female fertility.

cAMP signaling in ovarian physiology in teleosts: A review

Ovarian function in teleosts, like in other vertebrates, is regulated by two distinct gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin effects are mediated by membrane-bound G protein-coupled receptors localized on the surface of follicle cells. Gonadotropin receptor activation results in increased intracellular cAMP, the most important second cellular signaling molecule. FSH stimulation induces the production of 17β-estradiol in the cells of growing follicles to promote vitellogenesis in oocytes. In contrast, in response to LH, fully grown post-vitellogenic follicles gain the ability to synthesize maturation-inducing steroids, which induce meiotic resumption and ovulation. All these events were induced downstream of cAMP. In this review, we summarize studies addressing the role of the cAMP pathway in gonadotropin-induced processes in teleost ovarian follicles. Furthermore, we discuss future problems concerning cAMP signaling in relation to teleost ovarian function and the differences and similarities in the gonadotropin-induced cAMP signaling pathways between mammals and teleosts.

Translational control by maternal Nanog promotes oogenesis and early embryonic development

Many maternal mRNAs are translationally repressed during oocyte development and spatio-temporally activated during early embryogenesis, which is crucial for oocyte and early embryo development. By analyzing maternal mutants of nanog (Mnanog) in zebrafish, we demonstrated that Nanog tightly controls translation of maternal mRNA during oogenesis via transcriptional repression of eukaryotic translation elongation factor 1 alpha 1, like 2 (eef1a1l2). Loss of maternal Nanog led to defects of egg maturation, increased endoplasmic reticulum stress, and an activated unfold protein response, which was caused by elevated translational activity. We further demonstrated that Nanog, as a transcriptional repressor, represses the transcription of eefl1a1l2 by directly binding to the eef1a1l2 promoter in oocytes. More importantly, depletion of eef1a1l2 in nanog mutant females effectively rescued the elevated translational activity in oocytes, oogenesis defects and embryonic defects of Mnanog embryos. Thus, our study demonstrates that maternal Nanog regulates oogenesis and early embryogenesis through translational control of maternal mRNA via a mechanism whereby Nanog acts as a transcriptional repressor to suppress transcription of eef1a1l2.

A mitochondrial niche protects oocyte RNPs

Preserving maternal RNA transmitted by the oocyte to its progeny is an essential aspect of oogenesis, yet not much is known about how this is achieved in mammalian species. In a recent issue of Science, Cheng et al. uncover a novel structure involved in this fundamental aspect.

Zearalenone induces apoptosis and autophagy by regulating endoplasmic reticulum stress signalling in porcine trophectoderm cells

Zearalenone (ZEA), a mycotoxin produced mainly by fungi belonging to Fusarium species in foods and feeds, causes a serious hazard to humans and animals. Numerous studies have revealed that ingesting ZEA can disrupt the reproductive function and impair the reproductive process in animals. This experiment was to investigate the toxicological effect and the mechanism of ZEA exposure on reproduction in pigs during early stages of pregnancy. In the present study, we treated with 0 to 80 μmol/L ZEA for 12 or 24 h in trophoblast ectoderm (pTr) cells. The results showed that ZEA had significantly decreased cell proliferation (P < 0.05), which was accompanied by DNA damage-related cell cycle arrest at G2/M phase, activation of the apoptosis and endoplasmic reticulum (ER) stress, as well as impairment of barrier function (P < 0.05). Western blot analysis and transmission electron microscopy (TEM) showed that exposure to ZEA can activation of autophagy in pTr cells. Importantly, pretreatment with chloroquine (CQ) or 3-methyladenine (3-MA) led to increased apoptosis in pTr cells. Interestingly, pTr cells pretreated with 4-phenylbutyric acid (4-PBA), an inhibitor of ER stress, resulted in reduced cell death in pTr cells, indicating a critical role for ER stress in the activation of autophagy. In conclusion, these results reveal that ZEA-triggered ER stress is critical for the cell fate decision of pTr cells during early porcine embryonic development. Application of small molecules with ability of blocking ER stress might be therapeutic option to reduce the deleterious effect of ZEA in pregnant animals.

The RNA-binding protein Puf5 and the HMGB protein Ixr1 contribute to cell cycle progression through the regulation of cell cycle-specific expression of CLB1 in Saccharomyces cerevisiae

Puf5, a Puf-family RNA-binding protein, binds to 3´ untranslated region of target mRNAs and negatively regulates their expression in Saccharomyces cerevisiae. The puf5Δ mutant shows pleiotropic phenotypes including a weakened cell wall, a temperature-sensitive growth, and a shorter lifespan. To further analyze a role of Puf5 in cell growth, we searched for a multicopy suppressor of the temperature-sensitive growth of the puf5Δ mutant in this study. We found that overexpression of CLB2 encoding B-type cyclin suppressed the temperature-sensitive growth of the puf5Δ mutant. The puf5Δ clb2Δ double mutant displayed a severe growth defect, suggesting that Puf5 positively regulates the expression of a redundant factor with Clb2 in cell cycle progression. We found that expression of CLB1 encoding a redundant B-type cyclin was decreased in the puf5Δ mutant, and that this decrease of the CLB1 expression contributed to the growth defect of the puf5Δ clb2Δ double mutant. Since Puf5 is a negative regulator of the gene expression, we hypothesized that Puf5 negatively regulates the expression of a factor that represses CLB1 expression. We found such a repressor, Ixr1, which is an HMGB (High Mobility Group box B) protein. Deletion of IXR1 restored the decreased expression of CLB1 caused by the puf5Δ mutation and suppressed the growth defect of the puf5Δ clb2Δ double mutant. The expression of IXR1 was negatively regulated by Puf5 in an IXR1 3´ UTR-dependent manner. Our results suggest that IXR1 mRNA is a physiologically important target of Puf5, and that Puf5 and Ixr1 contribute to the cell cycle progression through the regulation of the cell cycle-specific expression of CLB1.

Identification of embryonic RNA granules that act as sites of mRNA translation after changing their physical properties

Fertilized eggs begin to translate mRNAs at appropriate times and placements to control development, but how the translation is regulated remains unclear. Here, we found that pou5f3 mRNA encoding a transcriptional factor essential for development formed granules in a dormant state in zebrafish oocytes. Although the number of pou5f3 granules remained constant, Pou5f3 protein accumulated after fertilization. Intriguingly, signals of newly synthesized peptides and a ribosomal protein became colocalized with pou5f3 granules after fertilization and, moreover, nascent Pou5f3 was shown to be synthesized in the granules. This functional change was accompanied by changes in the state and internal structure of granules. Dissolution of the granules reduced the rate of protein synthesis. Similarly, nanog and sox19b mRNAs in zebrafish and Pou5f1/Oct4 mRNA in mouse assembled into granules. Our results reveal that subcellular compartments, termed embryonic RNA granules, function as activation sites of translation after changing physical properties for directing vertebrate development.

Vasa nucleates asymmetric translation along the mitotic spindle during unequal cell divisions

mRNA translation on the spindle is hypothesized to be an essential strategy for the localized production of cell regulators. This mechanism may be important particularly in early embryonic cells, which have a large diffusion volume and that undergo rapid cell divisions. Evidence to test such a hypothesis has been, however, limited. Here, we use an embryo with both symmetric and asymmetric cell divisions and manipulate Vasa protein, an RNA-helicase, on the spindle in live sea urchin embryos. We learned that the spindle serves as a major site of translation and that protein synthesis within a single spindle can be unequal and help drive asymmetric cell divisions during embryogenesis. Recruiting Vasa to the ectopic sub-cellular region induced a new site of translation, disturbed asymmetric translation on the spindle, and changed the cell fate. Based on these observations, we conclude that Vasa functions in localized translation, which provides a spatiotemporal control in protein synthesis and is essential for rapidly developing embryonic cells.

Association of mRNA translation with the mitotic spindle is thought to be involved in localized production of cell fate determinants. Here, the authors show Vasa facilitates asymmetric translation, which contributes to differential regulation during sea urchin embryogenesis.

A proteomics approach identifies novel resident zebrafish Balbiani body proteins Cirbpa and Cirbpb

The Balbiani body (Bb) is the first marker of polarity in vertebrate oocytes. The Bb is a conserved structure found in diverse animals including insects, fish, amphibians, and mammals. During early zebrafish oogenesis, the Bb assembles as a transient aggregate of mRNA, proteins, and membrane-bound organelles at the presumptive vegetal side of the oocyte. As the early oocyte develops, the Bb appears to grow slowly, until at the end of stage I of oogenesis it disassembles and deposits its cargo of localized mRNAs and proteins. In fish and frogs, this cargo includes the germ plasm as well as gene products required to specify dorsal tissues of the future embryo. We demonstrate that the Bb is a stable, solid structure that forms a size exclusion barrier similar to other biological hydrogels. Despite its central role in oocyte polarity, little is known about the mechanism behind the Bb's action. Analysis of the few known protein components of the Bb is insufficient to explain how the Bb assembles, translocates, and disassembles. We isolated Bbs from zebrafish oocytes and performed mass spectrometry to define the Bb proteome. We successfully identified 77 proteins associated with the Bb sample, including known Bb proteins and novel RNA-binding proteins. In particular, we identified Cirbpa and Cirbpb, which have both an RNA-binding domain and a predicted self-aggregation domain. In stage I oocytes, Cirbpa and Cirbpb localize to the Bb rather than the nucleus (as in somatic cells), indicating that they may have a specialized function in the germ line. Both the RNA-binding domain and the self-aggregation domain are sufficient to localize to the Bb, suggesting that Cirbpa and Cirbpb interact with more than just their mRNA targets within the Bb. We propose that Cirbp proteins crosslink mRNA cargo and proteinaceous components of the Bb as it grows. Beyond Cirbpa and Cirbpb, our proteomics dataset presents many candidates for further study, making it a valuable resource for building a comprehensive mechanism for Bb function at a protein level.

Glucosamine alleviates zearalenone damage to porcine trophectoderm cells by activating PI3K/AKT signaling pathway

As one of the mycotoxins commonly found in feed and food, zearalenone (ZEA) mainly harms the reproductive functions of humans and animals. In our study, we investigated the protective effects...

Single Molecule RNA Localization and Translation in the Mammalian Oocyte and Embryo

During oocyte growth the cell accumulates RNAs to contribute to oocyte and embryo development which progresses with ceased transcription. To investigate the subcellular distribution of specific RNAs and their translation we developed a technique revealing several instances of localized translation with distinctive regulatory implications. We analyzed the localization and expression of candidate non-coding and mRNAs in the mouse oocyte and embryo. Furthermore, we established simultaneous visualization of mRNA and in situ translation events validated with polysomal occupancy. We discovered that translationally dormant and abundant mRNAs CyclinB1 and Mos are localized in the cytoplasm of the fully grown GV oocyte forming cloud-like structures with consequent abundant translation at the center of the MII oocyte. Coupling detection of the localization of specific single mRNA molecules with their translation at the subcellular context is a valuable tool to quantitatively study temporal and spatial translation of specific target mRNAs to understand molecular processes in the developing cell.

Intracellular mRNA transport and localized translation

Fine-tuning cellular physiology in response to intracellular and environmental cues requires precise temporal and spatial control of gene expression. High-resolution imaging technologies to detect mRNAs and their translation state have revealed that all living organisms localize mRNAs in subcellular compartments and create translation hotspots, enabling cells to tune gene expression locally. Therefore, mRNA localization is a conserved and integral part of gene expression regulation from prokaryotic to eukaryotic cells. In this Review, we discuss the mechanisms of mRNA transport and local mRNA translation across the kingdoms of life and at organellar, subcellular and multicellular resolution. We also discuss the properties of messenger ribonucleoprotein and higher order RNA granules and how they may influence mRNA transport and local protein synthesis. Finally, we summarize the technological developments that allow us to study mRNA localization and local translation through the simultaneous detection of mRNAs and proteins in single cells, mRNA and nascent protein single-molecule imaging, and bulk RNA and protein detection methods.

Reverse transcription priming methods affect normalisation choices for gene expression levels in oocytes and early embryos

Mammalian oocytes and embryos rely exclusively on maternal mRNAs to accomplish early developmental processes. Since oocytes and early embryos are transcriptionally silent after meiotic resumption, most of the synthesised maternal mRNA does not undergo immediate translation but is instead stored in the oocyte. Quantitative RT-PCR is commonly used to quantify mRNA levels, and correct quantification relies on reverse transcription and the choice of reference genes. Different methods for reverse transcription may affect gene expression determination in oocytes. In this study, we examined the suitability of either random or oligo(dT) primers for reverse transcription to be used for quantitative RT-PCR. We further looked for changes in poly(A) length of the maternal mRNAs during oocyte maturation. Our data indicate that depending on the method of reverse transcription, the optimal combination of reference genes for normalisation differed. Surprisingly, we observed a shortening of the poly(A) tail lengths of maternal mRNA as oocytes progressed from germinal vesicle to metaphase II. Overall, our findings suggest dynamic maternal regulation of mRNA structure and gene expression during oocyte maturation and early embryo development.

Changes in subcellular structures and states of pumilio 1 regulate the translation of target Mad2 and cyclin B1 mRNAs

Temporal and spatial control of mRNA translation has emerged as a major mechanism for promoting diverse biological processes. However, the molecular nature of temporal and spatial control of translation remains unclear. In oocytes, many mRNAs are deposited as a translationally repressed form and are translated at appropriate times to promote the progression of meiosis and development. Here, we show that changes in subcellular structures and states of the RNA-binding protein pumilio 1 (Pum1) regulate the translation of target mRNAs and progression of oocyte maturation. Pum1 was shown to bind to Mad2 (also known as Mad2l1) and cyclin B1 mRNAs, assemble highly clustered aggregates, and surround Mad2 and cyclin B1 RNA granules in mouse oocytes. These Pum1 aggregates were dissolved prior to the translational activation of target mRNAs, possibly through phosphorylation. Stabilization of Pum1 aggregates prevented the translational activation of target mRNAs and progression of oocyte maturation. Together, our results provide an aggregation-dissolution model for the temporal and spatial control of translation.

Shedding new light on early sex determination in zebrafish

In contrast to established zebrafish gene annotations, the question of sex determination has still not been conclusively clarified for developing zebrafish, Danio rerio, larvae, 28 dpf or earlier. Recent studies indicate polygenic sex determination (PSD), with the genes being distributed throughout the genome. Early genetic markers of sex in zebrafish help unravel co-founding sex-related differences to apply to human health and environmental toxicity studies. A qPCR-based method was developed for six genes: cytochrome P450, family 17, subfamily A, polypeptide 1 (cyp17a1); cytochrome P450, family 19, subfamily A, polypeptide 1a (cyp19a1a); cytochrome P450, family 19, subfamily A, polypeptides 1b (cyp19a1b); vitellogenin 1 (vtg1); nuclear receptor subfamily 0, group B, member 1 (nr0b1), sry (sex-determining region Y)-box 9b (sox9b) and actin, beta 1 (actb1), the reference gene. Sry-box 9a (Sox9a), insulin-like growth factor 3 (igf3) and double sex and mab-3 related transcription factor 1 (dmrt1), which are also known to be associated with sex determination, were used in gene expression tests. Additionally, Next-Generation-Sequencing (NGS) sequenced the genome of two adult female and male and two juveniles. PCR analysis of adult zebrafish revealed sex-specific expression of cyp17a1, cyp19a1a, vtg1, igf3 and dmrt1, the first four strongly expressed in female zebrafish and the last one highly expressed in male conspecifics. From NGS, nine female and four male-fated genes were selected as novel for assessing zebrafish sex, 28 dpf. Differences in transcriptomes allowed allocation of sex-specific genes also expressed in juvenile zebrafish.

Knockout of the Glucocorticoid Receptor Impairs Reproduction in Female Zebrafish

The pleiotropic effects of glucocorticoids in metabolic, developmental, immune and stress response processes have been extensively investigated; conversely, their roles in reproduction are still less documented. It is well known that stress or long-lasting therapies can cause a strong increase in these hormones, negatively affecting reproduction. Moreover, the need of glucocorticoid (GC) homeostatic levels is highlighted by the reduced fertility reported in the zebrafish glucocorticoid receptor mutant (nr3c1ia30/ia30) line (hereafter named gr-/-). Starting from such evidence, in this study, we have investigated the role of glucocorticoid receptor (Gr) in the reproduction of female zebrafish. Key signals orchestrating the reproductive process at the brain, liver, and ovarian levels were analyzed using a multidisciplinary approach. An impairment of the kiss-GnRH system was observed at the central level in (gr-/-) mutants as compared to wild-type (wt) females while, in the liver, vitellogenin (vtg) mRNA transcription was not affected. Changes were instead observed in the ovary, particularly in maturing and fully grown follicles (classes III and IV), as documented by the mRNA levels of signals involved in oocyte maturation and ovulation. Follicles isolated from gr-/- females displayed a decreased level of signals involved in the acquisition of competence and maturation, causing a reduction in ovulation with respect to wt females. Fourier transform infrared imaging (FTIRI) analysis of gr-/- follicle cytoplasm showed major changes in macromolecule abundance and distribution with a clear alteration of oocyte composition. Finally, differences in the molecular structure of the zona radiata layer of gr-/- follicles are likely to contribute to the reduced fertilization rate observed in mutants.

Posttranscriptional regulation of maternal Pou5f1/Oct4 during mouse oogenesis and early embryogenesis

Protein syntheses at appropriate timings are important for promoting diverse biological processes and are controlled at the levels of transcription and translation. Pou5f1/Oct4 is a transcription factor that is essential for vertebrate embryonic development. However, the precise timings when the mRNA and protein of Pou5f1/Oct4 are expressed during oogenesis and early stages of embryogenesis remain unclear. We analyzed the expression patterns of mRNA and protein of Pou5f1/Oct4 in mouse oocytes and embryos by using a highly sensitive in situ hybridization method and a monoclonal antibody specific to Pou5f1/Oct4, respectively. Pou5f1/Oct4 mRNA was detected in growing oocytes from the primary follicle stage to the fully grown GV stage during oogenesis. In contrast, Pou5f1/Oct4 protein was undetectable during oogenesis, oocyte maturation and the first cleavage stage but subsequently became detectable in the nuclei of early 2-cell-stage embryos. Pou5f1/Oct4 protein at this stage was synthesized from maternal mRNAs stored in oocytes. The amount of Pou5f1/Oct4 mRNA in the polysomal fraction was small in GV-stage oocytes but was significantly increased in fertilized eggs. Taken together, our results indicate that the synthesis of Pou5f1/Oct4 protein during oogenesis and early stages of embryogenesis is controlled at the level of translation and suggest that precise control of the amount of this protein by translational regulation is important for oocyte development and early embryonic development.

Antagonistic control of Caenorhabditis elegans germline stem cell proliferation and differentiation by PUF proteins FBF-1 and FBF-2

Stem cells support tissue maintenance, but the mechanisms that coordinate the rate of stem cell self-renewal with differentiation at a population level remain uncharacterized. We find that two PUF family RNA-binding proteins FBF-1 and FBF-2 have opposite effects on Caenorhabditis elegans germline stem cell dynamics: FBF-1 restricts the rate of meiotic entry, while FBF-2 promotes both cell division and meiotic entry rates. Antagonistic effects of FBFs are mediated by their distinct activities toward the shared set of target mRNAs, where FBF-1-mediated post-transcriptional control requires the activity of CCR4-NOT deadenylase, while FBF-2 is deadenylase-independent and might protect the targets from deadenylation. These regulatory differences depend on protein sequences outside of the conserved PUF family RNA-binding domain. We propose that the opposing FBF-1 and FBF-2 activities serve to modulate stem cell division rate simultaneously with the rate of meiotic entry.

The Tudor-domain protein TDRD7, mutated in congenital cataract, controls the heat shock protein HSPB1 (HSP27) and lens fiber cell morphology

Mutations of the RNA granule component TDRD7 (OMIM: 611258) cause pediatric cataract. We applied an integrated approach to uncover the molecular pathology of cataract in Tdrd7-/- mice. Early postnatal Tdrd7-/- animals precipitously develop cataract suggesting a global-level breakdown/misregulation of key cellular processes. High-throughput RNA sequencing integrated with iSyTE-bioinformatics analysis identified the molecular chaperone and cytoskeletal modulator, HSPB1, among high-priority downregulated candidates in Tdrd7-/- lens. A protein fluorescence two-dimensional difference in-gel electrophoresis (2D-DIGE)-coupled mass spectrometry screen also identified HSPB1 downregulation, offering independent support for its importance to Tdrd7-/- cataractogenesis. Lens fiber cells normally undergo nuclear degradation for transparency, posing a challenge: how is their cell morphology, also critical for transparency, controlled post-nuclear degradation? HSPB1 functions in cytoskeletal maintenance, and its reduction in Tdrd7-/- lens precedes cataract, suggesting cytoskeletal defects may contribute to Tdrd7-/- cataract. In agreement, scanning electron microscopy (SEM) revealed abnormal fiber cell morphology in Tdrd7-/- lenses. Further, abnormal phalloidin and wheat germ agglutinin (WGA) staining of Tdrd7-/- fiber cells, particularly those exhibiting nuclear degradation, reveals distinct regulatory mechanisms control F-actin cytoskeletal and/or membrane maintenance in post-organelle degradation maturation stage fiber cells. Indeed, RNA immunoprecipitation identified Hspb1 mRNA in wild-type lens lysate TDRD7-pulldowns, and single-molecule RNA imaging showed co-localization of TDRD7 protein with cytoplasmic Hspb1 mRNA in differentiating fiber cells, suggesting that TDRD7-ribonucleoprotein complexes may be involved in optimal buildup of key factors. Finally, Hspb1 knockdown in Xenopus causes eye/lens defects. Together, these data uncover TDRD7's novel upstream role in elevation of stress-responsive chaperones for cytoskeletal maintenance in post-nuclear degradation lens fiber cells, perturbation of which causes early-onset cataracts.

A novel testis-specific long noncoding RNA, Tesra, activates the Prss42/Tessp-2 gene during mouse spermatogenesis†

The progression of spermatogenesis is precisely controlled by meiotic stage-specific genes, but the molecular mechanism for activation of such genes is still elusive. Here we found a novel testis-specific long noncoding RNA (lncRNA), Tesra, that was specifically expressed in the mouse testis at the Prss/Tessp gene cluster on chromosome 9. Tesra was transcribed downstream of Prss44/Tessp-4, starting within the gene, as a 4435-nucleotide transcript and developmentally activated at a stage similar to that for Prss/Tessp genes. By in situ hybridization, Tesra was found to be localized in and around germ cells and Leydig cells, being consistent with biochemical data showing its existence in cytoplasmic, nuclear, and extracellular fractions. Based on the finding of more signals in nuclei of pachytene spermatocytes, we explored the possibility that Tesra plays a role in transcriptional activation of Prss/Tessp genes. By a ChIRP assay, the Tesra transcript was found to bind to the Prss42/Tessp-2 promoter region in testicular germ cells, and transient overexpression of Tesra significantly activated endogenous Prss42/Tessp-2 expression and increased Prss42/Tessp-2 promoter activity in a reporter construct. These findings suggest that Tesra activates the Prss42/Tessp-2 gene by binding to the promoter. Finally, we investigated whether Tesra co-functioned with enhancers adjacent to another lncRNA, lncRNA-HSVIII. In the Tet-on system, Tesra transcription significantly increased activity of one enhancer, but Tesra and the enhancer were not interdependent. Collectively, our results proposed a potential function of an lncRNA, Tesra, in transcriptional activation and suggest a novel relationship between an lncRNA and an enhancer.

Cytoskeletal proteins in the cell nucleus: a special nuclear actin perspective

The emerging role of cytoskeletal proteins in the cell nucleus has become a new frontier in cell biology. Actin and actin-binding proteins regulate chromatin and gene expression, but importantly they are beginning to be essential players in genome organization. These actin-based functions contribute to genome stability and integrity while affecting DNA replication and global transcription patterns. This is likely to occur through interactions of actin with nuclear components including nuclear lamina and subnuclear organelles. An exciting future challenge is to understand how these actin-based genome-wide mechanisms may regulate development and differentiation by interfering with the mechanical properties of the cell nucleus and how regulated actin polymerization plays a role in maintaining nuclear architecture. With a special focus on actin, here we summarize how cytoskeletal proteins operate in the nucleus and how they may be important to consolidate nuclear architecture for sustained gene expression or silencing.

Adaptive Protein Translation by the Integrated Stress Response Maintains the Proliferative and Migratory Capacity of Lung Adenocarcinoma Cells

The integrated stress response (ISR) is a conserved pathway that is activated by cells that are exposed to stress. In lung adenocarcinoma, activation of the ATF4 branch of the ISR by certain oncogenic mutations has been linked to the regulation of amino acid metabolism. In the present study, we provide evidence for ATF4 activation across multiple stages and molecular subtypes of human lung adenocarcinoma. In response to extracellular amino acid limitation, lung adenocarcinoma cells with diverse genotypes commonly induce ATF4 in an eIF2α-dependent manner, which can be blocked pharmacologically using an ISR inhibitor. Although suppressing eIF2α or ATF4 can trigger different biological consequences, adaptive cell-cycle progression and cell migration are particularly sensitive to inhibition of the ISR. These phenotypes require the ATF4 target gene asparagine synthetase (ASNS), which maintains protein translation independently of the mTOR/PI3K pathway. Moreover, NRF2 protein levels and oxidative stress can be modulated by the ISR downstream of ASNS. Finally, we demonstrate that ASNS controls the biosynthesis of select proteins, including the cell-cycle regulator cyclin B1, which are associated with poor lung adenocarcinoma patient outcome. Our findings uncover new regulatory layers of the ISR pathway and its control of proteostasis in lung cancer cells. IMPLICATIONS: We reveal novel regulatory mechanisms by which the ISR controls selective protein translation and is required for cell-cycle progression and migration of lung cancer cells.

Pumilio directs deadenylation-associated translational repression of the cyclin-dependent kinase 1 activator RGC-32

Response gene to complement-32 (RGC-32) activates cyclin-dependent kinase 1, regulates the cell cycle and is deregulated in many human tumours. We previously showed that RGC-32 expression is upregulated by the cancer-associated Epstein-Barr virus (EBV) in latently infected B cells through the relief of translational repression. We now show that EBV infection of naïve primary B cells also induces RGC-32 protein translation. In EBV-immortalised cell lines, we found that RGC-32 depletion resulted in cell death, indicating a key role in B cell survival. Studying RGC-32 translational control in EBV-infected cells, we found that the RGC-32 3′untranslated region (3′UTR) mediates translational repression. Repression was dependent on a single Pumilio binding element (PBE) adjacent to the polyadenylation signal. Mutation of this PBE did not affect mRNA cleavage, but resulted in increased polyA tail length. Consistent with Pumilio-dependent recruitment of deadenylases, we found that depletion of Pumilio in EBV-infected cells increased RGC-32 protein expression and polyA tail length. The extent of Pumilio binding to the endogenous RGC-32 mRNA in EBV-infected cell lines also correlated with RGC-32 protein expression. Our data demonstrate the importance of RGC-32 for the survival of EBV-immortalised B cells and identify Pumilio as a key regulator of RGC-32 translation.

Translational regulation of protrusion-localized RNAs involves silencing and clustering after transport

Localization of RNAs to various subcellular destinations is a widely used mechanism that regulates a large proportion of transcripts in polarized cells. In many cases, such localized transcripts mediate spatial control of gene expression by being translationally silent while in transit and locally activated at their destination. Here, we investigate the translation of RNAs localized at dynamic cellular protrusions of human and mouse, migrating, mesenchymal cells. In contrast to the model described above, we find that protrusion-localized RNAs are not locally activated solely at protrusions, but can be translated with similar efficiency in both internal and peripheral locations. Interestingly, protrusion-localized RNAs are translated at extending protrusions, they become translationally silenced in retracting protrusions and this silencing is accompanied by coalescence of single RNAs into larger heterogeneous RNA clusters. This work describes a distinct mode of translational regulation of localized RNAs, which we propose is used to regulate protein activities during dynamic cellular responses.

Cycling through mammalian meiosis: B-type cyclins in oocytes

B-type cyclins in association with Cdk1 mediate key steps of mitosis and meiosis, by phosphorylating a plethora of substrates. Progression through the meiotic cell cycle requires the execution of two cell divisions named meiosis I and II without intervening S-phase, to obtain haploid gametes. These two divisions are highly asymmetric in the large oocyte. Chromosome segregation in meiosis I and sister chromatid segregation in meiosis II requires the sharp, switch-like inactivation of Cdk1 activity, which is brought about by degradation of B-type cyclins and counteracting phosphatases. Importantly and contrary to mitosis, inactivation of Cdk1 must not allow S-phase to take place at exit from meiosis I. Here, we describe recent studies on the regulation of translation and degradation of B-type cyclins in mouse oocytes, and how far their roles are redundant or specific, with a special focus on the recently discovered oocyte-specific role of cyclin B3.

Regulation of Translationally Repressed mRNAs in Zebrafish and Mouse Oocytes

From the beginning of oogenesis, oocytes accumulate tens of thousands of mRNAs for promoting oocyte growth and development. A large number of these mRNAs are translationally repressed and localized within the oocyte cytoplasm. Translational activation of these dormant mRNAs at specific sites and timings plays central roles in driving progression of the meiotic cell cycle, axis formation, mitotic cleavages, transcriptional initiation, and morphogenesis. Regulation of the localization and temporal translation of these mRNAs has been shown to rely on cis-acting elements in the mRNAs and trans-acting factors recognizing and binding to the elements. Recently, using model vertebrate zebrafish, localization itself and formation of physiological structures such as RNA granules have been shown to coordinate the accurate timings of translational activation of dormant mRNAs. This subcellular regulation of mRNAs is also utilized in other animals including mouse. In this chapter, we review fundamental roles of temporal regulation of mRNA translation in oogenesis and early development and then focus on the mechanisms of mRNA regulation in the oocyte cytoplasm by which the activation of dormant mRNAs at specific timings is achieved.

Spatiotemporal Patterning of Zygotic Genome Activation in a Model Vertebrate Embryo

A defining feature of early embryogenesis is the transition from maternal to zygotic control. This transition requires embryo-wide zygotic genome activation (ZGA), but the extent of spatiotemporal coordination of ZGA between individual cells is unknown. Multiple interrelated parameters, including elapsed time, completed cycles of cell division, and cell size may impact ZGA onset; however, the principal determinant of ZGA during vertebrate embryogenesis is debated. Here, we perform single-cell imaging of large-scale ZGA in whole-mount Xenopus embryos. We find a striking new spatiotemporal pattern of ZGA whose onset tightly correlates with cell size but not with elapsed time or number of cell divisions. Further, reducing cell size induces premature ZGA, dose dependently. We conclude that large-scale ZGA is not spatially uniform and that its onset is determined at the single-cell level, primarily by cell size. Our study suggests that spatial patterns of ZGA onset may be a common feature of embryonic systems.

Mysteries in embryonic development: How can errors arise so frequently at the beginning of mammalian life?

Chromosome segregation errors occur frequently during female meiosis but also in the first mitoses of mammalian preimplantation development. Such errors can lead to aneuploidy, spontaneous abortions, and birth defects. Some of the mechanisms underlying these errors in meiosis have been deciphered but which mechanisms could cause chromosome missegregation in the first embryonic cleavage divisions is mostly a “mystery”. In this article, we describe the starting conditions and challenges of these preimplantation divisions, which might impair faithful chromosome segregation. We also highlight the pending research to provide detailed insight into the mechanisms and regulation of preimplantation mitoses.

Starting a new life is a challenging business. This Essay explores the changes at the oocyte-to-embryo transition to highlight the circumstances under which the very first and decisive — but ‘mysteriously’ error-prone — mitotic divisions occur.

Pumilio1 phosphorylation precedes translational activation of its target mRNA in zebrafish oocytes

Translational regulation of mRNAs is crucial for promoting various cellular and developmental processes. Pumilio1 (Pum1) has been shown to play key roles in translational regulation of target mRNAs in many systems of diverse organisms. In zebrafish immature oocytes, Pum1 was shown to bind to cyclin B1 mRNA and promote the formation of cyclin B1 RNA granules. This Pum1-mediated RNA granule formation seemed critical to determine the timing of translational activation of cyclin B1 mRNA during oocyte maturation, leading to activation of maturation/M-phase-promoting factor (MPF) at the appropriate timing. Despite its fundamental importance, the mechanisms of translational regulation by Pum1 remain elusive. In this study, we examined the phosphorylation of Pum1 as a first step to understand the mechanisms of Pum1-mediated translation. SDS-PAGE analyses and phosphatase treatments showed that Pum1 was phosphorylated at multiple sites during oocyte maturation. This phosphorylation began in an early period after induction of oocyte maturation, which preceded the polyadenylation of cyclin B1 mRNA. Interestingly, depolymerization of actin filaments in immature oocytes caused phosphorylation of Pum1, disassembly of cyclin B1 RNA granules, and polyadenylation of cyclin B1 mRNA but not translational activation of the mRNA. Overexpression of the Pum1 N-terminus prevented the phosphorylation of Pum1, disassembly of cyclin B1 RNA granules, and translational activation of the mRNA even after induction of oocyte maturation. These results suggest that Pum1 phosphorylation in the early period of oocyte maturation is one of the key processes for promoting the disassembly of cyclin B1 RNA granules and translational activation of target mRNA.

Staufen1, Kinesin1 and microtubule function in cyclin B1 mRNA transport to the animal polar cytoplasm of zebrafish oocytes

In zebrafish oocytes, cyclin B1 mRNAs are transported to the animal polar cytoplasm. To elucidate the molecular basis of cyclin B1 mRNA transport, we analyzed zebrafish Staufen1, a protein known to play a central role in mRNA transport to the vegetal pole of Xenopus oocytes. Zebrafish Staufen1 interacts with cyclin B1 mRNA throughout oocyte growth. Both cyclin B1 mRNA and Staufen1 are evenly distributed in the cytoplasm of young oocytes but are co-localized to the animal polar cytoplasm in later stages. Real-time imaging showed that the plus ends of oocyte microtubules are free in the cytoplasm in early stages but anchored to the animal polar cytoplasm in later stages. Transport of cyclin B1 reporter mRNA to the animal polar cytoplasm was inhibited by disruption of microtubules and injection of antibodies against Staufen1 or Kinesin1, a plus-end-directed microtubule motor that interacts with Staufen1, indicating that the transport depends on movement along microtubules toward the plus ends. Reporter mRNAs with an element required for the vegetal localization of vg1 mRNA in Xenopus oocytes were localized to the animal polar cytoplasm in zebrafish oocytes, indicating that the element is functional for animal polar localization in zebrafish oocytes. Our findings suggest that cyclin B1 mRNA-Staufen1 protein complexes are transported toward the animal pole of zebrafish oocytes by the plus-end-directed motor protein Kinesin1 along microtubules and that a common mRNA transport machinery functions in zebrafish and Xenopus oocytes, although its transport direction is opposite due to different organizations of microtubules.

The translational regulation of maternal mRNAs in time and space

Since their discovery, the study of maternal mRNAs has led to the identification of mechanisms underlying their spatiotemporal regulation within the context of oogenesis and early embryogenesis. Following synthesis in the oocyte, maternal mRNAs are translationally silenced and sequestered into storage in cytoplasmic granules. At the same time, their unique distribution patterns throughout the oocyte and embryo are tightly controlled and connected to their functions in downstream embryonic processes. At certain points in oogenesis and early embryogenesis, maternal mRNAs are translationally activated to perform their functions in a timely manner. The cytoplasmic polyadenylation machinery is responsible for the translational activation of maternal mRNAs, and its role in initiating the maternal to zygotic transition events has recently come to light. Here, we summarize the current knowledge on maternal mRNA regulation, with particular focus on cytoplasmic polyadenylation as a mechanism for translational regulation.

Uncovering the Role of RNA-Binding Proteins in Gene Expression in the Immune System

Fighting external pathogens requires an ever-changing immune system that relies on tight regulation of gene expression. Transcriptional control is the first step to build efficient responses while preventing immunodeficiencies and autoimmunity. Post-transcriptional regulation of RNA editing, location, stability, and translation are the other key steps for final gene expression, and they are all controlled by RNA-binding proteins (RBPs). Nowadays we have a deep understanding of how transcription factors control the immune system but recent evidences suggest that post-transcriptional regulation by RBPs is equally important for both development and activation of immune responses. Here, we review current knowledge about how post-transcriptional control by RBPs shapes our immune system and discuss the perspective of RBPs being the key players of a hidden immune cell epitranscriptome.

CNOT6 regulates a novel pattern of mRNA deadenylation during oocyte meiotic maturation

In many cell types, the length of the poly(A) tail of an mRNA is closely linked to its fate - a long tail is associated with active translation, a short tail with silencing and degradation. During mammalian oocyte development, two contrasting patterns of polyadenylation have been identified. Some mRNAs carry a long poly(A) tail during the growth stage and are actively translated, then become deadenylated and down-regulated during the subsequent stage, termed meiotic maturation. Other mRNAs carry a short tail poly(A) tail and are translationally repressed during growth, and their poly(A) tail lengthens and they become translationally activated during maturation. As well, a program of elimination of this ‘maternal’ mRNA is initiated during oocyte maturation. Here we describe a third pattern of polyadenylation: mRNAs are deadenylated in growing oocytes, become polyadenylated during early maturation and then deadenylated during late maturation. We show that the deadenylase, CNOT6, is present in cortical foci of oocytes and regulates deadenylation of these mRNAs, and that PUF-binding elements (PBEs) regulate deadenylation in mature oocytes. Unexpectedly, maintaining a long poly(A) tail neither enhances translation nor inhibits degradation of these mRNAs. Our findings implicate multiple machineries, more complex than previously thought, in regulating mRNA activity in oocytes.

High-Sensitivity and High-Resolution In Situ Hybridization of Coding and Long Non-coding RNAs in Vertebrate Ovaries and Testes

Background

Subcellular localization of coding and non-coding RNAs has emerged as major regulatory mechanisms of gene expression in various cell types and many organisms. However, techniques that enable detection of the subcellular distribution of these RNAs with high sensitivity and high resolution remain limited, particularly in vertebrate adult tissues and organs. In this study, we examined the expression and localization of mRNAs encoding Pou5f1/Oct4, Mos, Cyclin B1 and Deleted in Azoospermia-like (Dazl) in zebrafish and mouse ovaries by combining tyramide signal amplification (TSA)-based in situ hybridization with paraffin sections which can preserve cell morphology of tissues and organs at subcellular levels. In addition, the distribution of a long non-coding RNA (lncRNA), lncRNA-HSVIII, in mouse testes was examined by the same method.

Results

The mRNAs encoding Mos, Cyclin B1 and Dazl were found to assemble into distinct granules that were distributed in different subcellular regions of zebrafish and mouse oocytes, suggesting conserved and specific regulations of these mRNAs. The lncRNA-HSVIII was first detected in the nucleus of spermatocytes at prophase I of the meiotic cell cycle and was then found in the cytoplasm of round spermatids, revealing expression patterns of lncRNA during germ cell development. Collectively, the in situ hybridization method demonstrated in this study achieved the detection and comparison of precise distribution patterns of coding and non-coding RNAs at subcellular levels in single cells of adult tissues and organs.

Conclusions

This high-sensitivity and high-resolution in situ hybridization is applicable to many vertebrate species and to various tissues and organs and will be useful for studies on the subcellular regulation of gene expression at the level of RNA localization.

Electronic supplementary material

The online version of this article (10.1186/s12575-018-0071-z) contains supplementary material, which is available to authorized users.

Tia1 dependent regulation of mRNA subcellular location and translation controls p53 expression in B cells

Post-transcriptional regulation of cellular mRNA is essential for protein synthesis. Here we describe the importance of mRNA translational repression and mRNA subcellular location for protein expression during B lymphocyte activation and the DNA damage response. Cytoplasmic RNA granules are formed upon cell activation with mitogens, including stress granules that contain the RNA binding protein Tia1. Tia1 binds to a subset of transcripts involved in cell stress, including p53 mRNA, and controls translational silencing and RNA granule localization. DNA damage promotes mRNA relocation and translation in part due to dissociation of Tia1 from its mRNA targets. Upon DNA damage, p53 mRNA is released from stress granules and associates with polyribosomes to increase protein synthesis in a CAP-independent manner. Global analysis of cellular mRNA abundance and translation indicates that this is an extended ATM-dependent mechanism to increase protein expression of key modulators of the DNA damage response.Sequestering mRNA in cytoplasmic stress granules is a mechanism for translational repression. Here the authors find that p53 mRNA, present in stress granules in activated B lymphocytes, is released upon DNA damage and is translated in a CAP-independent manner.

Maternal mRNAs with distinct 3' UTRs define the temporal pattern of Ccnb1 synthesis during mouse oocyte meiotic maturation

In this study, Yang et al. find that the timing of Ccnb1 mRNA translation in mouse oocytes is dependent on the presence of transcripts with different 3′ UTRs. Their results reveal an additional layer of translation control through alternative polyadenylation usage required to fine-tune the timing of meiosis progression.

The final stages of female gamete maturation occur in the virtual absence of transcription, with gene expression driven by a program of selective unmasking, translation, and degradation of maternal mRNAs. Here we demonstrate that the timing of Ccnb1 mRNA translation in mouse oocytes is dependent on the presence of transcripts with different 3′ untranslated regions (UTRs). This 3′ UTR heterogeneity directs distinct temporal patterns of translational activation or repression. Inclusion or exclusion of cis-acting elements is responsible for these divergent regulations. Our findings reveal an additional layer of translation control through alternative polyadenylation usage required to fine-tune the timing of meiosis progression.