Translational regulation during oogenesis and early development: the cap-poly(A) tail relationship

Effect of pre-treatment of melatonin on superovulation response, circulatory hormones, and miRNAs in goats during environmental heat stress conditions

Environmental heat stress has a deleterious impact on farm animal reproductive performance. The purpose of this study was to see how the addition of melatonin affected the efficacy of the superovulation regimen in goats in hot climatic conditions. Sixteen Shiba goats were synchronized and divided into two equal groups (n = 8, each): the melatonin group, which received a single S/C dose of melatonin, and a control group, treated with one ml of corn oil only. Ultrasonographic assessment of ovarian structures (Graafian follicles; GFs and corpus lutea; CLs) morphometry and hemodynamics were performed during the estrous phase of the superovulation (D0) and at day7 after ovulation (D7) of the superovulation regimen. The peripheral reproductive hormones were measured, and microRNAs were characterized. The mean diameter and the total-colored area of GFs during the D0 were significantly (P˂0.05) higher in the melatonin group (5.42 ± 0.11 mm and 1592.20 ± 45.26 pixels, respectively) compared to the control group (4.62 ± 0.12 mm and 1052.55 ± 29.47 pixels, respectively). Concentrations of LH and E2 increased significantly (P˂0.05) in the melatonin group (1.06 ± 0.06 ng/ml and 46.34 ± 2.77 pg/ml, respectively) compared to the control group (0.75 ± 0.12 ng/ml and 29.33 ± 1.89 pg/ml, respectively). At D7, the melatonin-received goats attained greater values in the mean count (6.75 ± 0.33, P˂0.005), diameters (6.08 ± 0.12 mm, P˂0.01), and total-colored area (17137.30 ± 128.53 pixels, P˂0.01) of detected CLs and progesterone concentrations (4.08 ± 0.24 ng/ml) compared to control goats (4.00 ± 0.28, 4.50 ± 0.19 mm, 11156.87 ± 117.90 pixels, and 2.90 ± 0.18 ng/ml respectively). MiRNA expression analysis was identified during both stages denoting several up and downregulated miRNA candidates among the studied groups. In conclusion, incorporating melatonin enhanced the efficiency of the superovulation regimen in goats under hot climatic conditions.

Review of the Heat Stress-Induced Responses in Dairy Cattle

In the dairy cattle sector, the evaluation of the effects induced by heat stress is still one of the most impactful and investigated aspects as it is strongly connected to both sustainability of the production and animal welfare. On the other hand, more recently, the possibility of collecting a large dataset made available by the increasing technology diffusion is paving the way for the application of advanced numerical techniques based on machine learning or big data approaches. In this scenario, driven by rapid change, there could be the risk of dispersing the relevant information represented by the physiological animal component, which should maintain the central role in the development of numerical models and tools. In light of this, the present literature review aims to consolidate and synthesize existing research on the physiological consequences of heat stress in dairy cattle. The present review provides, in a single document, an overview, as complete as possible, of the heat stress-induced responses in dairy cattle with the intent of filling the existing research gap for extracting the veterinary knowledge present in the literature and make it available for future applications also in different research fields.

Cup is essential for oskar mRNA translational repression during early Drosophila oogenesis

Study of the timing and location for mRNA translation across model systems has begun to shed light on molecular events fundamental to such processes as intercellular communication, morphogenesis, and body pattern formation. In D. melanogaster, the posterior mRNA determinant, oskar, is transcribed maternally but translated only when properly localized at the oocyte's posterior cortex. Two effector proteins, Bruno1 and Cup, mediate steps of oskar mRNA regulation. The current model in the field identifies Bruno1 as necessary for Cup's recruitment to oskar mRNA and indispensable for oskar's translational repression. We now report that this Bruno1-Cup interaction leads to precise oskar mRNA regulation during early oogenesis and, importantly, the two proteins mutually influence each other's mRNA expression and protein distribution in the egg chamber. We show that these factors stably associate with oskar mRNA in vivo. Cup associates with oskar mRNA without Bruno1, while surprisingly Bruno1's stable association with oskar mRNA depends on Cup. We demonstrate that the essential factor for oskar mRNA repression in early oogenesis is Cup, not Bruno1. Furthermore, we find that Cup is a key P-body component that maintains functional P-body morphology during oogenesis and is necessary for oskar mRNA's association with P-bodies. Therefore, Cup drives the translational repression and stability of oskar mRNA. These experimental results point to a regulatory feedback loop between Bruno 1 and Cup in early oogenesis that appears crucial for oskar mRNA to reach the posterior pole and its expression in the egg chamber for accurate embryo development.

Single-cell quantitative proteomic analysis of human oocyte maturation revealed high heterogeneity in in vitro matured oocytes

Oocyte maturation is pertinent to the success of in vitro maturation (IVM), which is used to overcome female infertility, and produced over 5000 live births worldwide. However, the quality of human IVM oocytes has not been investigated at single-cell proteome level. Here, we quantified 2094 proteins in human oocytes during in vitro and in vivo maturation (IVO) by single-cell proteomic analysis and identified 176 differential proteins between IVO and germinal vesicle oocytes and 45 between IVM and IVO oocytes including maternal effect proteins, with potential contribution to the clinically observed decreased fertilization, implantation, and birth rates using human IVM oocytes. IVM and IVO oocytes showed separate clusters in principal component analysis, with higher inter-cell variability among IVM oocytes, and have little correlation between mRNA and protein changes during maturation. The patients with the most aberrantly expressed proteins in IVM oocytes had the lowest level of estradiol per mature follicle on trigger day. Our data provide a rich resource to evaluate effect of IVM on oocyte quality and study mechanism of oocyte maturation.

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Single-cell proteomic profiling of human oocytes matured in vitro and in vivo.

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Low correlation between protein and mRNA levels during human oocyte maturation.

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In vitro matured (IVM) oocytes exhibit higher heterogeneity at the proteome level.

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45 differentially expressed proteins between IVM and in vivo matured (IVO) oocytes.

The Melatonin Treatment Improves the Ovarian Responses After Superstimulation in Thai-Holstein Crossbreeds Under Heat Stress Conditions

The effect of heat stress with melatonin treatment on the superovulatory responses and embryo characteristics in Thai-Holstein crossbreeds under heat stress conditions was examined. Six non-lactating cows (replication = 4; n = 24) were assigned to one of 2 treatments in double cross-over design. All cows were superstimulated with traditional treatment. Melatonin group (n = 12): cows received intramuscularly injection 18 mg/50 kg. simultaneously with GnRH injection, while those in the control group (n = 12) received none. Bloods samples were taken to determine lipid peroxidation (MDA) and the activity of the antioxidant enzymes (superoxide dismutase; SOD). The experiment was conducted from April to September, which determined severe heat stress (the mean temperature-humidity index above 77). The results revealed that numbers of large follicles and corpora lutea were higher in the melatonin group than in the control group (p < 0.01). Numbers of recovered ova/embryos, fertilized ova, and transferable embryos were higher in the melatonin group (p < 0.01); meanwhile, more degenerated embryos were found in the control group (p < 0.01). Increased activity of the antioxidant enzymes SOD after melatonin administration decreased MDA levels (p < 0.05). In summary, melatonin administration benefited the ovarian response and embryo quality in superstimulated Thai-Holstein crossbreed affected by heat stress.

Aurora kinase B inhibits aurora kinase A to control maternal mRNA translation in mouse oocytes

Mammalian oocytes are transcriptionally quiescent, and meiosis and early embryonic divisions rely on translation of stored maternal mRNAs. Activation of these mRNAs is mediated by polyadenylation. Cytoplasmic polyadenylation binding element 1 (CPEB1) regulates mRNA polyadenylation. One message is aurora kinase C (Aurkc), encoding a protein that regulates chromosome segregation. We previously demonstrated that AURKC levels are upregulated in oocytes lacking aurora kinase B (AURKB), and this upregulation caused increased aneuploidy rates, a role we investigate here. Using genetic and pharmacologic approaches, we found that AURKB negatively regulates CPEB1-dependent translation of many messages. To determine why translation is increased, we evaluated aurora kinase A (AURKA), a kinase that activates CPEB1 in other organisms. We find that AURKA activity is increased in Aurkb knockout mouse oocytes and demonstrate that this increase drives the excess translation. Importantly, removal of one copy of Aurka from the Aurkb knockout strain background reduces aneuploidy rates. This study demonstrates that AURKA is required for CPEB1-dependent translation, and it describes a new AURKB requirement to maintain translation levels through AURKA, a function crucial to generating euploid eggs.

Mechanisms of Oocyte Maturation and Related Epigenetic Regulation

Meiosis is the basis of sexual reproduction. In female mammals, meiosis of oocytes starts before birth and sustains at the dictyate stage of meiotic prophase I before gonadotropins-induced ovulation happens. Once meiosis gets started, the oocytes undergo the leptotene, zygotene, and pachytene stages, and then arrest at the dictyate stage. During each estrus cycle in mammals, or menstrual cycle in humans, a small portion of oocytes within preovulatory follicles may resume meiosis. It is crucial for females to supply high quality mature oocytes for sustaining fertility, which is generally achieved by fine-tuning oocyte meiotic arrest and resumption progression. Anything that disturbs the process may result in failure of oogenesis and seriously affect both the fertility and the health of females. Therefore, uncovering the regulatory network of oocyte meiosis progression illuminates not only how the foundations of mammalian reproduction are laid, but how mis-regulation of these steps result in infertility. In order to provide an overview of the recently uncovered cellular and molecular mechanism during oocyte maturation, especially epigenetic modification, the progress of the regulatory network of oocyte meiosis progression including meiosis arrest and meiosis resumption induced by gonadotropins is summarized. Then, advances in the epigenetic aspects, such as histone acetylation, phosphorylation, methylation, glycosylation, ubiquitination, and SUMOylation related to the quality of oocyte maturation are reviewed.

The Recurrent Mutation in PATL2 Inhibits Its Degradation Thus Causing Female Infertility Characterized by Oocyte Maturation Defect Through Regulation of the Mos-MAPK Pathway

PAT1 homolog 2 (PATL2), encoding an RNA-binding protein, is a repressor involved in the translational regulation of maternal mRNAs during oocyte maturation. Previous studies have reported mutations in PATL2 those led to female infertility with oocyte maturation arrest; however, the mechanisms by which mutations affected meiotic maturation remained unclear. Here, we identified several novel and recurrent mutations of PATL2 in patients with similar phenotype, and chose the missense mutation c.649 T>A p.Tyr217Asn in PATL2 (PATL2^Y217N) as a typical to investigate the underlying mechanisms. We confirmed that this mutation disturbed oocyte maturation and observed morphological defects of large polar body, symmetrical division and abnormal spindle after microinjection of corresponding mutated mRNA. We further evaluated the effect of the PATL2^Y217N mutation in 293T cells, and found this mutation decreased the ubiquitination level and degradation of PATL2. Then, abnormally increased PATL2 bound mRNAs of Mos, an upstream activator of mitogen activated protein kinase (MAPK), to regulate its translational activity and subsequently impaired MAPK signaling pathway and oocyte meiosis. These results dissented from the previous view that PATL2 mutations reduced their expression and highlight the role of PATL2 in translational regulation of Mos and its association with MAPK signaling pathway during oocyte meiotic maturation.

Heat stress reduces maturation and developmental capacity in bovine oocytes

The ovarian pool of follicles, and their enclosed oocytes, is highly sensitive to hyperthermia. Heat-induced changes in small antral follicles can later manifest as impaired follicle development and compromised competence of the enclosed oocytes to undergo maturation, fertilisation and further development into an embryo. This review describes the main changes documented so far that underlie the oocyte damage. The review discusses some cellular and molecular mechanisms by which heat stress compromises oocyte developmental competence, such as impairment of nuclear and cytoplasmic maturation and mitochondrial function, changes in the expression of both nuclear and mitochondrial transcripts and the induction of apoptosis. The review emphasises that although the oocyte is exposed to heat stress, changes are also evident in the developed embryo. Moreover, the effect of heat stress is not limited to the summer; it carries over to the cold autumn, as manifest by impaired steroid production, low oocyte competence and reduced fertility. The spontaneous recovery of oocytes from the end of the summer through the autumn until the beginning of winter suggests that only subpopulations of follicles, rather than the entire ovarian reserve, are damaged upon heat exposure.

Granule regulation by phase separation during Drosophila oogenesis

Drosophila eggs are highly polarised cells that use RNA-protein complexes to regulate storage and translational control of maternal RNAs. Ribonucleoprotein granules are a class of biological condensates that form predominantly by intracellular phase separation. Despite extensive in vitro studies testing the physical principles regulating condensates, how phase separation translates to biological function remains largely unanswered. In this perspective, we discuss granules in Drosophila oogenesis as a model system for investigating the physiological role of phase separation. We review key maternal granules and their properties while highlighting ribonucleoprotein phase separation behaviours observed during development. Finally, we discuss how concepts and models from liquid-liquid phase separation could be used to test mechanisms underlying granule assembly, regulation and function in Drosophila oogenesis.

Dairy cow reproduction under the influence of heat stress

Dairy farming is vulnerable to global warming and climate change. Improving and maintaining conception rates (CRs) have a paramount importance for the profitability of any dairy enterprise. There is an antagonistic relationship between fertility and milk yield, and intensive selection for milk yield has severely deteriorated reproductive efficiency. Irrespective of geography and husbandry, modern dairy cows experience heat stress (HS) effects leading to fertility declines, but it worsens in tropical climates. The threshold of HS experience among modern dairy cow has lowered, leading to decreased thermal comfort zone. Studies show that this threshold is lower for fertility than for lactation. HS abatement and robustness response to lactation yield lead to negative energy balance, and cow's reproductive requirements remain unfulfilled. The adverse effects of HS commence from developing oocyte throughout later stages and its fertilization competence; the oestrus cycle and oestrus behaviour; the embryo development and implantation; on uterine environment; and even extend towards foetal calf. Even cows can become acyclic under the influence of HS. These harmful effects of HS arise due to hyperthermia, oxidative stress and physiological modifications in the body of dairy cows. Proper assessment of HS and efficient cooling of dairy animals irrespective of their stage of life at farm is the immediate strategy to reduce fertility declines. Other long- and short-term mitigation strategies to reduce fertility declines during HS include feeding care, reducing disease and mastitis rates, using semen from cooled bulls, timed artificial inseminations (AI), allied hormonal interventions and use of embryo transfer technology. Ultimate long-term solution should be well-planned breeding for fertility improvement and HS tolerance.

Differential gene expression between in vivo and in vitro maturation: a comparative study with bovine oocytes derived from the same donor pool

Objective:

In vitro maturation has been shown to influence gene expression in oocytes, but a common shortcoming in reports on the matter has been the use of different donors in each experimental group thus disregarding donor effects. This study aimed to investigate the abundance of mRNA in oocytes matured in vivo and in vitro obtained from the same group of donors.

Methods:

A bovine model was used to assess the relative abundance of specific transcripts in in vitro-matured (IN VITRO-OPU) and in vivo-matured (IN VIVO-OPU) oocytes collected from the same donors by transvaginal ovum pick-up (OPU). Transcript abundance in oocytes from the IN VIVO-OPU group and oocytes matured in vitro but retrieved from different cows slaughtered at a commercial abattoir (IN VITRO-Abattoir group) was also compared. Total RNA was extracted from denuded oocytes and cDNA was produced via reverse transcription using an oligo(dT) primer for relative quantification of eight target transcripts by real-time PCR.

Results:

Oocytes in the IN VITRO-OPU group had lower (p<0.05) abundance of peroxiredoxin 1 (Prdx1), heat shock protein 70.1 (Hsp70.1), growth and differentiation factor 9 (Gdf9), and maternal antigen that embryo requires (Mater) transcripts than the oocytes in the IN VIVO-OPU group, all obtained from the same pool of donor cows. Similar results were seen in the comparisons involving the IN VIVO-OPU and IN VITRO-Abattoir groups (p<0.05).

Conclusion:

In vitro maturation affected the abundance of polyadenylated transcripts in the oocyte cytoplasm when compared to in vivo maturation induced by exogenous hormones in oocytes collected from the same donor pool.

Ovarian extracellular MicroRNAs as the potential non-invasive biomarkers: An update

Through the reproductive system, it has been realized that the microRNAs (miRNAs) have emerged as one of the principal post-transcriptional gene regulators of the diverse developmental processes. The ovary, as a dynamic organ, co-ordinates follicle recruitment, selection, and ovulation, in which miRNAs play the central role almost in its all functions. Deregulation of these developmental procedures in ovary could lead to the ovarian dysfunction, infertility, decrease in the assisted reproductive treatment (ART) outcome, and death in some patients with ovarian cancer. In recent years, detection of ovarian extracellular miRNAs in body fluids such as follicular fluid and serum/plasma has opened a new era in the biomarker discovery field. Here through the present review, different aspects of the potential and proposed involvement of the extracellular miRNAs in both physiologic and pathologic contexts of the ovary have been discussed. Moreover, the researchers have addressed the relevant findings, challenges, and issues which associated with the extracellular miRNAs in the ovarian microenvironments to provide the better insight into understanding the molecular mechanisms which were involved in the pathophysiologic conditions. Finally, a comprehensive survey of the gaps has been discussed to hopefully shed new light and perspective on the development of the novel diagnostic and therapeutic platforms in the clinic.

Poly(A)-binding proteins are required for translational regulation in vertebrate oocytes and early embryos

Poly(A)-binding proteins (PABPs) function in the timely regulation of gene expression during oocyte maturation, fertilisation and early embryo development in vertebrates. To this end, PABPs bind to poly(A) tails or specific sequences of maternally stored mRNAs to protect them from degradation and to promote their translational activities. To date, two structurally different PABP groups have been identified: (1) cytoplasmic PABPs, including poly(A)-binding protein, cytoplasmic 1 (PABPC1), embryonic poly(A)-binding protein (EPAB), induced PABP and poly(A)-binding protein, cytoplasmic 3; and (2) nuclear PABPs, namely embryonic poly(A)-binding protein 2 and nuclear poly(A)-binding protein 1. Many studies have been undertaken to characterise the spatial and temporal expression patterns and subcellular localisations of PABPC1 and EPAB in vertebrate oocytes and early embryos. In the present review, we comprehensively evaluate and discuss the expression patterns and particular functions of the EPAB and PABPC1 genes, especially in mouse and human oocytes and early embryos.

Potential roles of the poly(A)-binding proteins in translational regulation during spermatogenesis

Spermatogenesis is briefly defined as the production of mature spermatozoa from spermatogonial stem cells at the end of a strictly regulated process. It is well known that, to a large extent, transcriptional activity ceases at mid-spermiogenesis. Several mRNAs transcribed during early stages of spermatogenesis are stored as ribonucleoproteins (RNPs). During the later stages, translational control of these mRNAs is mainly carried out in a time dependent-manner by poly(A)-binding proteins (PABPs) in cooperation with other RNA-binding proteins and translation-related factors. Conserved PABPs specifically bind to poly(A) tails at the 3′ ends of mRNAs to regulate their translational activity in spermatogenic cells. Studies in this field have revealed that PABPs, particularly poly(A)-binding protein cytoplasmic 1 (Pabpc1), Pabpc2, and the embryonic poly(A)-binding protein (Epab), play roles in the translational regulation of mRNAs required at later stages of spermatogenesis. In this review article, we evaluated the spatial and temporal expression patterns and potential functions of these PABPs in spermatogenic cells during spermatogenesis. The probable relationship between alterations in PABP expression and the development of male infertility is also reviewed.

Importance of ERK1/2 in Regulation of Protein Translation during Oocyte Meiosis

Although the involvement of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway in the regulation of cytostatic factor (CSF) activity; as well as in microtubules organization during meiotic maturation of oocytes; has already been described in detail; rather less attention has been paid to the role of ERK1/2 in the regulation of mRNA translation. However; important data on the role of ERK1/2 in translation during oocyte meiosis have been documented. This review focuses on recent findings regarding the regulation of translation and the role of ERK1/2 in this process in the meiotic cycle of mammalian oocytes. The specific role of ERK1/2 in the regulation of mammalian target of rapamycin (mTOR); eukaryotic translation initiation factor 4E (eIF4E) and cytoplasmic polyadenylation element binding protein 1 (CPEB1) activity is addressed along with additional focus on the other key players involved in protein translation.

Comparative Proteomic Profiling Reveals Molecular Characteristics Associated with Oogenesis and Oocyte Maturation during Ovarian Development of Bactrocera dorsalis (Hendel)

Time-dependent expression of proteins in ovary is important to understand oogenesis in insects. Here, we profiled the proteomes of developing ovaries from Bactrocera dorsalis (Hendel) to obtain information about ovarian development with particular emphasis on differentially expressed proteins (DEPs) involved in oogenesis. A total of 4838 proteins were identified with an average peptide number of 8.15 and sequence coverage of 20.79%. Quantitative proteomic analysis showed that a total of 612 and 196 proteins were differentially expressed in developing and mature ovaries, respectively. Furthermore, 153, 196 and 59 potential target proteins were highly expressed in early, vitellogenic and mature ovaries and most tested DEPs had the similar trends consistent with the respective transcriptional profiles. These proteins were abundantly expressed in pre-vitellogenic and vitellogenic stages, including tropomyosin, vitellogenin, eukaryotic translation initiation factor, heat shock protein, importin protein, vitelline membrane protein, and chorion protein. Several hormone and signal pathway related proteins were also identified during ovarian development including piRNA, notch, insulin, juvenile, and ecdysone hormone signal pathways. This is the first report of a global ovary proteome of a tephritid fruit fly, and may contribute to understanding the complicate processes of ovarian development and exploring the potentially novel pest control targets.

A MAPK cascade couples maternal mRNA translation and degradation to meiotic cell cycle progression in mouse oocytes

Mammalian oocyte maturation depends on the translational activation of stored maternal mRNAs upon meiotic resumption. Cytoplasmic polyadenylation element binding protein 1 (CPEB1) is a key oocyte factor that regulates maternal mRNA translation. However, the signal that triggers CPEB1 activation at the onset of mammalian oocyte maturation is not known. We provide evidence that a mitogen-activated protein kinase (MAPK) cascade couples maternal mRNA translation to meiotic cell cycle progression in mouse oocytes by triggering CPEB1 phosphorylation and degradation. Mutations of the phosphorylation sites or ubiquitin E3 ligase binding sites in CPEB1 have a dominant-negative effect in oocytes, and mimic the phenotype of ERK1/2 knockout, by impairing spindle assembly and mRNA translation. Overexpression of the CPEB1 downstream translation activator DAZL in ERK1/2-deficient oocytes partially rescued the meiotic defects, indicating that ERK1/2 is essential for spindle assembly, metaphase II arrest and maternal-zygotic transition (MZT) primarily by triggering the translation of key maternal mRNAs. Taken together, ERK1/2-mediated CPEB1 phosphorylation/degradation is a major mechanism of maternal mRNA translational activation, and is crucial for mouse oocyte maturation and MZT.

Polyadenylated tail length variation pattern in ultra-rapid vitrified bovine oocytes

Aim:

Thecurrent study aims at investigating the polyadenylated (poly[A]) tail length of morphologically high and low competent oocytes at different developmental stages. Furthermore, effect of ultra-rapid vitrification on the poly(A) tail length was studied.

Materials and Methods:

Fresh bovine cumulus oocyte complexes from abattoir originated ovaries were graded based on morphological characters and matured in vitro. Cryopreservation was done by ultra-rapid vitrification method. mRNA was isolated from different categories of oocyte and subjected to ligation-mediated poly(A) test followed by polymerase chain reaction for determining the poly(A) tail length of β actin, gap junction protein alpha 1 (GJA1), poly(A) polymerase alpha (PAPOLA), and heat shock 70 kDa protein (HSP70) transcripts.

Results:

GJA1, PAPOLA, and HSP70 showed significantly higher poly(A) in immature oocytes of higher competence irrespective of vitrification effects as compared to mature oocytes of higher competence.

Conclusion:

mRNA poly(A) tail size increases in developmentally high competent immature bovine oocytes. There was limited effect of ultra-rapid vitrification of bovine oocytes on poly(A).

Effect of Heat Stress on Reproduction in Dairy Cows: Insights into the Cellular and Molecular Responses of the Oocyte

Among the components of the female reproductive tract, the ovarian pool of follicles and their enclosed oocytes are highly sensitive to hyperthermia. Heat-induced alterations in small antral follicles can be expressed later as compromised maturation and developmental capacity of the ovulating oocyte. This review summarizes the most up-to-date information on the effects of heat stress on the oocyte with an emphasis on unclear points and open questions, some of which might involve new research directions, for instance, whether preantral follicles are heat resistant. The review focuses on the follicle-enclosed oocytes, provides new insights into the cellular and molecular responses of the oocyte to elevated temperature, points out the role of the follicle microenvironment, and discusses some mechanisms that might underlie oocyte impairment. Mechanisms include nuclear and cytoplasmic maturation, mitochondrial function, apoptotic pathways, and oxidative stress. Understanding the mechanism by which heat stress compromises fertility might enable development of new strategies to mitigate its effects.

Regulation of coronaviral poly(A) tail length during infection

The positive-strand coronavirus genome of ~30 kilobase in length and subgenomic (sg) mRNAs of shorter lengths, are 5’ and 3’-co-terminal by virtue of a common 5’-capped leader and a common 3’-polyadenylated untranslated region. Here, by ligating head-to-tail viral RNAs from bovine coronavirus-infected cells and sequencing across the ligated junctions, it was learned that at the time of peak viral RNA synthesis [6 hours postinfection (hpi)] the 3’ poly(A) tail on genomic and sgmRNAs is ~65 nucleotides (nt) in length. Surprisingly, this length was found to vary throughout infection from ~45 nt immediately after virus entry (at 0 to 4 hpi) to ~65 nt later on (at 6 h to 9 hpi) and from ~65 nt (at 6 h to 9 hpi) to ~30 nt (at 120-144 hpi). With the same method, poly(U) sequences of the same lengths were simultaneously found on the ligated viral negative-strand RNAs. Functional analyses of poly(A) tail length on specific viral RNA species, furthermore, revealed that translation, in vivo, of RNAs with the longer poly(A) tail was enhanced over those with the shorter poly(A). Although the mechanisms by which the tail lengths vary is unknown, experimental results together suggest that the length of the poly(A) and poly(U) tails is regulated. One potential function of regulated poly(A) tail length might be that for the coronavirus genome a longer poly(A) favors translation. The regulation of coronavirus translation by poly(A) tail length resembles that during embryonal development suggesting there may be mechanistic parallels.

Embryonic poly(A)-binding protein (EPAB) is required for oocyte maturation and female fertility in mice

Gene expression during oocyte maturation and early embryogenesis up to zygotic genome activation requires translational activation of maternally-derived mRNAs. EPAB [embryonic poly(A)-binding protein] is the predominant poly(A)-binding protein during this period in Xenopus, mouse and human. In Xenopus oocytes, ePAB stabilizes maternal mRNAs and promotes their translation. To assess the role of EPAB in mammalian reproduction, we generated Epab-knockout mice. Although Epab(-/-) males and Epab(+/-) of both sexes were fertile, Epab(-/-) female mice were infertile, and could not generate embryos or mature oocytes in vivo or in vitro. Epab(-/-) oocytes failed to achieve translational activation of maternally-stored mRNAs upon stimulation of oocyte maturation, including Ccnb1 (cyclin B1) and Dazl (deleted in azoospermia-like) mRNAs. Microinjection of Epab mRNA into Epab(-/-) germinal vesicle stage oocytes did not rescue maturation, suggesting that EPAB is also required for earlier stages of oogenesis. In addition, late antral follicles in the ovaries of Epab(-/-) mice exhibited impaired cumulus expansion, and a 8-fold decrease in ovulation, associated with a significant down-regulation of mRNAs encoding the EGF (epidermal growth factor)-like growth factors Areg (amphiregulin), Ereg (epiregulin) and Btc (betacellulin), and their downstream regulators, Ptgs2 (prostaglandin synthase 2), Has2 (hyaluronan synthase 2) and Tnfaip6 (tumour necrosis factor α-induced protein 6). The findings from the present study indicate that EPAB is necessary for oogenesis, folliculogenesis and female fertility in mice.

Disrupted redox homeostasis and aberrant redox gene expression in porcine oocytes contribute to decreased developmental competence

The objective of this study was to identify specific redox-related genes whose function contributes to oocyte quality and to characterize the role of redox homeostasis in oocyte development. We determined the redox genes glutaredoxin 2 (GLRX2), protein disulfide isomerase family A, members 4 and 6 (PDIA4, PDIA6), and thioredoxin reductase 1 (TXNRD1) were differentially expressed between adult (more competent) and prepubertal (less competent) porcine in vitro-matured (IVM) oocytes. The association between these genes and oocyte quality was further validated by comparing transcript abundance in IVM with that in in vivo-matured (VVM) prepubertal and adult oocytes. By maturing oocytes in variable redox environments, we demonstrated that a balanced redox environment is important for oocyte quality, and over-reduction of the environment is as detrimental as excess oxidation. Critical levels of reactive oxygen species (ROS) and glutathione (GSH) are required for oocyte competence. Elevated GSH and lower ROS in prepubertal oocytes suggest disrupted redox homeostasis exists in these cells. By further comparing GLRX2, PDIA4, PDIA6, and TXNRD1 expression levels in oocytes matured under these different redox environments, we found aberrant expression patterns in prepubertal oocytes but not in adult oocytes when the maturation medium contained high concentrations of antioxidants. These results suggest that prepubertal oocytes are less competent in regulating redox balance than adult oocytes, contributing to lower oocyte quality. In conclusion, aberrant redox gene expression patterns and disrupted redox homeostasis contribute to decreased developmental competence in prepubertal and IVM porcine oocytes. The balance between ROS and GSH plays an important role in oocyte quality.

Kinetics of GDF9 expression in buffalo oocytes during in vitro maturation and their associated development ability

The capacity of fully grown oocytes to regulate their own microenvironment by secreted paracrine factors contribute to their developmental competence. In spite of growing evidence about the vital role of Growth Differentiation Factor 9 (GDF9) in determination of oocyte developmental competence, there is insufficient information about time dependent behavior of its expression during in vitro maturation (IVM) to have definite understanding about at what time point during IVM it plays most crucial role. The study reports the kinetics of GDF9 expression under four different IVM supplement conditions in buffalo oocytes and their concomitant development rate up to blastocyst. Oocytes matured under an ideal media condition with all supplements and those cultured with only FSH resulted in significantly higher cumulus expansion, nuclear maturation, cleavage and blastocyst rates. GDF9 expression at both mRNA and protein levels at different time points of IVM revealed that magnitude of mRNA abundance at 8h of IVM was most important towards imparting development competence to buffalo oocytes. Appearance of GDF9 protein in maturing oocytes was found asynchronous with mRNA appearance in the time course of IVM suggesting possible posttranscriptional regulation of this gene under dynamic oocyte cumulus cell communication process. Abundance of mature GDF9 protein at 16 h was most consistently related with all oocyte development parameters.

The CPEB-family of proteins, translational control in senescence and cancer

Cytoplasmic elongation of the poly(A) tail was originally identified as a mechanism to activate maternal mRNAs, stored as silent transcripts with short poly(A) tails, during meiotic progression. A family of RNA-binding proteins named CPEBs, which recruit the translational repression or cytoplasmic polyadenylation machineries to their target mRNAs, directly mediates cytoplasmic polyadenylation. Recent years have witnessed an explosion of studies showing that CPEBs are not only expressed in a variety of somatic tissues, but have essential functions controlling gene expression in time and space in the adult organism. These "new" functions of the CPEBs include regulating the balance between senescence and proliferation and its pathological manifestation, tumor development. In this review, we summarize current knowledge on the functions of the CPEB-family of proteins in the regulation of cell proliferation, their target mRNAs and the mechanism controlling their activities.

The "closed loop model" in controlling mRNA translation during development

Translational control is particularly important in situations where the correlation of a distinct mRNA and the abundance of the corresponding protein might be low. This is the case for instance during oocyte maturation, shortly before the GVBD when the chromatin is condensed, until the embryonic genome is activated. In these situations, gene expression relies on the activation of maternal mRNAs which were stored stably in a dormant form. The most sophisticated model for translational initiation at present is the so-called "closed loop" model, where a circularization of the mRNA is mediated by associated 5'-cap- and 3'-poly(A) binding proteins. Depending on differential interactions, this event can result in translational stimulation or repression. Several studies describe correlated regulation mechanisms in model organisms like mouse or Xenopus, but data addressing translational regulation in farm animals are rare. Cytoplasmic mRNA activating or repressing factors, however, might contribute to achieve developmental competence in bovine or porcine oocytes. Recently we showed that, in the pig, embryonic signals can modify essential components of the mRNA-5'-translation initiation complex in the uterine luminal epithelium at the time of implantation. In accordance with the closed loop model of translational initiation, this review focuses on the regulatory impact of 5'-mRNA end associated proteins (components of the mRNA-cap binding complex) and 3'-end associated proteins (components of the poly(A) binding complex) during in vitro maturation of cattle and pig oocytes, early embryonic development and in the pig uterine epithelia.

Fractionation of mRNA based on the length of the poly(A) tail

Poly(A) tail length plays an important role in mRNA stability and translational control. Poly(A) fractionation is a very powerful technique to separate mRNAs according to the length of the poly(A) tail. Poly(A) fractionation can be used to detect small changes in poly(A) tail length or to prepare samples for microarray analysis. RNA or crude lysate is mixed with biotinylated oligo(dT), which is then bound to paramagnetic streptavidin beads. Oligoadenylated mRNA is eluted first with a high salt buffer, followed by a low salt elution for polyadenylated mRNA. Elution of the RNA in two fractions can be used as a preparation of samples for microarray analysis while elution of the mRNA in several fractions can be used to analyse (changes in) poly(A) tail length. This method allows for accurate quantification of the amount of oligoadenylated/polyadenylated RNA in each fraction because it is not dependent on visualising the smears representing the variations in poly(A) tail length. The method is technically easy, fast, highly reproducible and can be performed on almost any sample containing RNA.

Diminution of eIF4E activity suppresses parkin mutant phenotypes

Mutations in the human parkin (PARK2) gene cause autosomal recessive-juvenile Parkinson's disease (AR-JP). In Drosophila melanogaster, mutant parkin alleles display a broad range of phenotypic alterations, including female infertility. Here we report that reducing the level of eukaryotic translation initiation factor 4E (eIF4E) activity specifically rescues the female sterile phenotypes associated with the parkin(P23) mutant allele. Additional defects, including reduction of pupal viability and body size, are also entirely recovered in both male and female flies of the abovementioned genotype. We further show that a null eIF4E-binding protein (4E-BP) allele counteracts the in vivo effects produced, in a parkin(P23) mutant background, by the reduction of functional eIF4E copy number. Moreover, Parkin and eIF4E interact in vitro and co-localize at the posterior end of developing oocytes. Finally, we show that eIF4E is over-expressed in parkin(P23) mutant ovaries as compared to wild-types. Taken together, our data are consistent with the idea that Parkin and eIF4E act in a common pathway, likely modulating cap-dependent translation initiation events.

Multiple sequences and factors are involved in stability/degradation of Awnt-1, Awnt-5A and Awnt-5B mRNAs during axolotl development

Following fertilization in amphibian, early cleavage stages are maternally controlled at a post-transcriptional level before initiation of zygotic transcriptions at the mid blastula transition (MBT). We document the expression levels of the axolotl Awnt-1, Awnt-5A and Awnt-5B genes as well as the adenylation states of their corresponding mRNAs from the end of oogenesis until the tailbud stages. Awnt-1/-5A RNAs are stable until MBT then degraded before gastrulation. Awnt-5B RNAs are degraded at fertilization and zygotically expressed after MBT with high level expression from gastrulation. Estimation of the poly(A) tail lengths reveals no direct link between deadenylation and degradation periods for each Awnt transcript. To investigate the molecular mechanisms involved in Awnt-1/-5A/-5B RNAs stability, synthetic full-length or 3' untranslated region (UTR) Awnt RNAs progressively deleted from their 3' end were microinjected in axolotl oocytes, unfertilized and fertilized eggs. We identified degrading and stabilizing sequences in the 3'UTR whose activities depend on the cellular context and are also modulated by the 5'UTR and coding sequence within each RNA. Using axolotl nuclear extracts from stage VI oocytes, we further produced evidence of destabilizing factors targeting the Awnt-5B RNAs. Altogether, these results show that oocyte maturation and late cleavages following MBT are two important periods when axolotl Wnt RNAs are highly regulated.

Identification and characterization of miRNAs expressed in the bovine ovary

Background

MicroRNAs are the major class of gene-regulating molecules playing diverse roles through sequence complementarity to target mRNAs at post-transcriptional level. Tightly regulated expression and interaction of a multitude of genes for ovarian folliculogenesis could be regulated by these miRNAs. Identification of them is the first step towards understanding miRNA-guided gene regulation in different biological functions. Despite increasing efforts in miRNAs identification across various species and diverse tissue types, little is known about bovine ovarian miRNAs. Here, we report the identification and characterization of miRNAs expressed in the bovine ovary through cloning, expression analysis and target prediction.

Results

The miRNA library (5'-independent ligation cloning method), which was constructed from bovine ovary in this study, revealed cloning of 50 known and 24 novel miRNAs. Among all identified miRNAs, 38 were found to be new for bovine and were derived from 43 distinct loci showing characteristic secondary structure. While 22 miRNAs precursor loci were found to be well conserved in more than one species, 16 were found to be bovine specific. Most of the miRNAs were cloned multiple times, in which let-7a, let-7b, let-7c, miR-21, miR-23b, miR-24, miR-27a, miR-126 and miR-143 were cloned 10, 28, 13, 4, 11, 7, 6, 4 and 11 times, respectively. Expression analysis of all new and some annotated miRNAs in different intra-ovarian structures and in other multiple tissues showed that some were present ubiquitously while others were differentially expressed among different tissue types. Bta-miR-29a was localized in the follicular cells at different developmental stages in the cyclic ovary. Bio-informatics prediction, screening and Gene Ontology analysis of miRNAs targets identified several biological processes and pathways underlying the ovarian function.

Conclusion

Results of this study suggest the presence of miRNAs in the bovine ovary, thereby elucidate their potential role in regulating diverse molecular and physiological pathways underlying the ovarian functionality. This information will give insights into bovine ovarian miRNAs, which can be further characterized for their role in follicular development and female fertility as well.

The DAZL and PABP families: RNA-binding proteins with interrelated roles in translational control in oocytes

Gametogenesis is a highly complex process that requires the exquisite temporal, spatial and amplitudinal regulation of gene expression at multiple levels. Translational regulation is important in a wide variety of cell types but may be even more prevalent in germ cells, where periods of transcriptional quiescence necessitate the use of post-transcriptional mechanisms to effect changes in gene expression. Consistent with this, studies in multiple animal models have revealed an essential role for mRNA translation in the establishment and maintenance of reproductive competence. While studies in humans are less advanced, emerging evidence suggests that translational regulation plays a similarly important role in human germ cells and fertility. This review highlights specific mechanisms of translational regulation that play critical roles in oogenesis by activating subsets of mRNAs. These mRNAs are activated in a strictly determined temporal manner via elements located within their 3′UTR, which serve as binding sites fortrans-acting factors. While we concentrate on oogenesis, these regulatory events also play important roles during spermatogenesis. In particular, we focus on the deleted in azoospermia-like (DAZL) family of proteins, recently implicated in the translational control of specific mRNAs in germ cells; their relationship with the general translation initiation factor poly(A)-binding protein (PABP) and the process of cytoplasmic mRNA polyadenylation.

Regulation of cap-dependent translation initiation in the early stage porcine parthenotes

The binding of mRNAs to ribosomes is mediated by the protein complex eIF4F in conjunction with eIF4B (eukaryotic initiation factor 4F and 4B). EIF4F is a three subunit complex consisting of eIF4A (RNA helicase), eIF4E (mRNA cap binding protein), and eIF4G (bridging protein). The crucial role is played by eIF4E, which directly binds the 5'-cap structure of the mRNA and facilitates the recruitment to the mRNA of other translation factors and the 40S ribosomal subunit. EIF4E binding to mRNA and to other initiation factors is regulated on several levels, including its phosphorylation on Ser-209, and association with its regulatory protein 4E-binding protein (4E-BP1). In this study we document that both the translation initiation factor eIF4E and its regulator 4E-BP1 become dephosphorylated in the early stage porcine zygotes already 8 hr post-activation. Similarly, the activities of ERK1/2 MAP and Mnk1 kinases, which are both involved in eIF4E phosphorylation, gradually decrease during this period with the timing similar to that of eIF4E dephosphorylation. The formation of an active eIF4F complex is also diminished after 9-15 hr post-activation, although substantial amounts of this complex have been detected also 24 hr post-activation (2-cell stage). The overall protein synthesis in the parthenotes decreases gradually from 12 hr post-activation reaching a minimum after 48 hr (4-cell stage). Although the translation is gradually decreasing during early preimplantation development, the eIF4F complex, which is temporarily formed, might be a premise for the translation of a small subset of mRNAs at this period of development.

Genomic RNA profiling and the programme controlling preimplantation mammalian development

Preimplantation development shifts from a maternal to embryonic programme rapidly after fertilization. Although the majority of oogenetic products are lost during the maternal to embryonic transition (MET), several do survive this interval to contribute directly to supporting preimplantation development. Embryonic genome activation (EGA) is characterized by the transient expression of several genes that are necessary for MET, and while EGA represents the first major wave of gene expression, a second mid-preimplantation wave of transcription that supports development to the blastocyst stage has been discovered. The application of genomic approaches has greatly assisted in the discovery of stage specific gene expression patterns and the challenge now is to largely define gene function and regulation during preimplantation development. The basic mechanisms controlling compaction, lineage specification and blastocyst formation are defined. The requirement for embryo culture has revealed plasticity in the developmental programme that may exceed the adaptive capacity of the embryo and has fostered important research directions aimed at alleviating culture-induced changes in embryonic programming. New levels of regulation are emerging and greater insight into the roles played by RNA-binding proteins and miRNAs is required. All of this research is relevant due to the necessity to produce healthy preimplantation embryos for embryo transfer, to ensure that assisted reproductive technologies are applied in the most efficient and safest way possible.

Characterization of two cytoplasmic poly(A)-binding proteins, PABPC1 and PABPC2, in mouse spermatogenic cells

Mouse spermatogenic cells are known to contain at least two isoforms of cytoplasmic poly(A)-binding proteins, PABPC1 and PABPC2 (previously known as PABPT). In this study, we have characterized PABPC1 and PABPC2. PABPC2 was present in pachytene spermatocytes and round spermatids, whereas elongating spermatids still included PABPC1. These two proteins are capable of binding mRNA poly(A) tails nonspecifically and of directly associating with each other and with several translational regulators, including EIF4G1, PAIP1, PAIP2, and PIWIL1 (previously known as MIWI). Moreover, both PABPC1 and PABPC2 exhibited the ability to enhance translation of a reporter mRNA in vitro. Despite these similarities, PABPC2 was distinguished from PABPC1 by the absence of PABPC2 in actively translating polyribosomes of testicular cells. PABPC1 was distributed in polyribosomes and in translationally inactive messenger ribonucleoprotein particles. Most importantly, PABPC2 and PIWIL1 were noticeably enriched in the chromatoid body of round spermatids. These results suggest that PABPC2 may function in translational repression during spermatogenesis.

Trading translation with RNA-binding proteins

RNA-binding proteins regulate every aspect of RNA metabolism, including pre-mRNA splicing, mRNA trafficking, stability, and translation. This review summarizes the available information on molecular mechanisms of translational repression by RNA-binding proteins. By using a specific set of well-defined examples, we also describe how regulation can be reversed.

Global poly(A) mRNA expression profile measured in individual bovine oocytes and cleavage embryos

The objective of this article was to estimate quantitative differences for GAPDH transcripts and poly(A) mRNA: (i) between oocytes collected from cumulus–oocyte complexes (COCs) qualified morphologically as grades A and B; (ii) between grade A oocytes before and afterin vitromaturation (IVM); and (iii) amongin vitro-produced embryos at different developmental stages. To achieve this objective a new approach was developed to estimate differences between poly(A) mRNA when using small samples. The approach consisted of full-length cDNA amplification (acDNA) monitored by real-time PCR, in which the cDNA from half of an oocyte or embryo was used as a template. The GAPDH gene was amplified as a reverse transcription control and samples that were not positive for GAPDH transcripts were discarded. The fold differences between two samples were estimated using delta Ct and statistical analysis and were obtained using the pairwise fixed reallocation randomization test. It was found that the oocytes recovered from grade B COCs had quantitatively less poly(A) mRNA (p< 0.01) transcripts compared with grade A COCs (1 arbitrary unit expression rate). In the comparison with immature oocytes (1 arbitrary unit expression rate), the quantity of poly(A) mRNA did not change during IVM, but declined following IVF and varied with embryo culture (p< 0.05). Amplification of cDNA by real-time PCR was an efficient method to estimate differences in the amount of poly(A) mRNA between oocytes and embryos. The results obtained from individual oocytes suggested an association between poly(A) mRNA abundance and different morphological qualities of oocytes from COCs. In addition, a poly(A) mRNA profile was characterized from oocytes undergoing IVM, fertilization and blastocyst heating.

Analysis of selected transcript levels in porcine spermatozoa, oocytes, zygotes and two-cell stage embryos

It has been suggested that spermatozoa can deliver mRNAs to the oocyte during fertilisation. Using reverse transcription and real-time quantitative polymerase chain reaction analysis (RQ-PCR), we evaluated the presence of clusterin (CLU), protamine 2 (PRM2), calmegin (CLGN), cAMP-response element modulator protein (CREM), methyltransferase 1 (DNMT1), linker histone 1 (H1), protamine 1 (PRM1), TATA box-binding protein associated factor 1 (TAF1) and TATA box-binding protein (TBP) in porcine mature oocytes, zygotes and two-cell stage embryos. Spermatozoa isolated from semen samples of boars contained all transcripts investigated, whereas oocytes contained only CREM, H1, TAF1, and TBP mRNAs. The zygote and two-cell stage embryos contained CLU, CREM, H1, PRM1, PRM2, TAF1 and TBP transcripts. Our observations suggest that porcine spermatozoa may delivery CLU, PRM1 and PRM2 mRNAs to the oocyte, which may contribute to zygotic and early embryonic development.

A conserved U-rich RNA region implicated in regulation of translation in Plasmodium female gametocytes

Translational repression (TR) plays an important role in post-transcriptional regulation of gene expression and embryonic development in metazoans. TR also regulates the expression of a subset of the cytoplasmic mRNA population during development of fertilized female gametes of the unicellular malaria parasite, Plasmodium spp. which results in the formation of a polar and motile form, the ookinete. We report the conserved and sex-specific regulatory role of either the 3'- or 5'-UTR of a subset of translationally repressed mRNA species as shown by almost complete inhibition of expression of a GFP reporter protein in the female gametocyte. A U-rich, TR-associated element, identified previously in the 3'-UTR of TR-associated transcripts, played an essential role in mediating TR and a similar region could be found in the 5'-UTR shown in this study to be active in TR. The silencing effect of this 5'-UTR was shown to be independent of its position relative to its ORF, as transposition to a location 3' of the ORF did not affect TR. These results demonstrate for the first time in a unicellular organism that the 5' or the 3'-UTR of TR-associated transcripts play an important and conserved role in mediating TR in female gametocytes.

Control of protein translation by phosphorylation of the mRNA 5′-cap-binding complex

Initiation of mRNA translation is a key regulatory step in the control of gene expression. Microarray analysis indicates that total mRNA levels do not always reflect protein levels, since mRNA association with polyribosomes is necessary for protein synthesis. Phosphorylation of translation initiation factors offers a cost-effective and rapid way to adapt to physiological and environmental changes, and there is increasing evidence that many of these factors are subject to multiple regulatory phosphorylation events. The present article focuses on the nature of reversible phosphorylation and the function of the 5′-cap-binding complex in plants.

Glucocorticoid regulation of human pulmonary surfactant protein-B mRNA stability involves the 3'-untranslated region

Expression of pulmonary surfactant, a complex mixture of lipids and proteins that acts to reduce alveolar surface tension, is developmentally regulated and restricted to lung alveolar type II cells. The hydrophobic protein surfactant protein-B (SP-B) is essential in surfactant function, and insufficient levels of SP-B result in severe respiratory dysfunction. Glucocorticoids accelerate fetal lung maturity and surfactant synthesis both experimentally and clinically. Glucocorticoids act transcriptionally and post-transcriptionally to increase steady-state levels of human SP-B mRNA; however, the mechanism(s) by which glucocorticoids act post-transcriptionally is unknown. We hypothesized that glucocorticoids act post-transcriptionally to increase SP-B mRNA stability via sequence-specific mRNA-protein interactions. We found that glucocorticoids increase SP-B mRNA stability in isolated human type II cells and in nonpulmonary cells, but do not alter mouse SP-B mRNA stability in a mouse type II cell line. Deletion analysis of an artificially-expressed SP-B mRNA indicates that the SP-B mRNA 3'-untranslated region (UTR) is necessary for stabilization, and the region involved can be restricted to a 126-nucleotide-long region near the SP-B coding sequence. RNA electrophoretic mobility shift assays indicate that cytosolic proteins bind to this region in the absence or presence of glucocorticoids. The formation of mRNA:protein complexes is not seen in other regions of the SP-B mRNA 3'-UTR. These results indicate that a specific 126-nucleotide region of human SP-B 3'-UTR is necessary for increased SP-B mRNA stability by glucocorticoids by a mechanism that is not lung cell specific and may involve mRNA-protein interactions.

Regulating translation of maternal messages: multiple repression mechanisms

The dowry of mRNAs and proteins that mothers provide their progeny as part of a common developmental strategy to permit rapid embryogenesis necessitates precise translational regulation of the deposited mRNAs. Recent studies with Drosophila uncovered diverse mechanisms to control translation of the transcripts for genes that control the cell cycle and embryonic patterning. The newly delineated mechanisms include: alternative ways to disrupt eIF4E action and the formation of the preinitiation complex b y the eIF4E homologous protein, d4EHP; recruitment of the deadenylase complex by the SMAUG and PUMILIO proteins; both poly(A)-dependent and -independent promotion of translation by the PNG kinase complex; and 5' cap-independent translational regulation b y BRUNO.

A novel method for poly(A) fractionation reveals a large population of mRNAs with a short poly(A) tail in mammalian cells

The length of the poly(A) tail of an mRNA plays an important role in translational efficiency, mRNA stability and mRNA degradation. Regulated polyadenylation and deadenylation of specific mRNAs is involved in oogenesis, embryonic development, spermatogenesis, cell cycle progression and synaptic plasticity. Here we report a new technique to analyse the length of poly(A) tails and to separate a mixed population of mRNAs into fractions dependent on the length of their poly(A) tails. The method can be performed on crude lysate or total RNA, is fast, highly reproducible and minor changes in poly(A) tail length distribution are easily detected. We validated the method by analysing mRNAs known to undergo cytoplasmic polyadenylation during Xenopus laevis oocyte maturation. We then separated RNA from NIH3T3 cells into two fractions with short and long poly(A) tails and compared them by microarray analysis. In combination with the validation experiments, the results indicate that ∼25% of the expressed genes have a poly(A) tail of less than 30 residues in a significant percentage of their transcripts.

Global gene expression analysis during bovine oocyte in vitro maturation

It is well known that factors such as nutrition, hormonal alterations and environmental compounds can have a profound effect on oocyte quality and subsequently embryo development. Global mRNA expression analysis of immature and mature bovine oocytes was carried out to identify key pathways associated with oocyte meiotic maturation and developmental potential. We have carried out a global mRNA gene expression analysis of bovine oocytes pre- and post-resumption of meiotic maturation using the Affymetrix GeneChip Bovine Genome Array. Approximately 54% of the probe sets representing 23,000 transcripts were detected in bovine oocytes. Of which, 821 transcripts were differentially expressed (>or=2-fold) between the two groups (p<0.05), corresponding to 209 upregulated and 612 down regulated transcripts in the in vitro matured oocytes compared to their immature counterparts. In addition, transcripts uniquely detected in germinal vesicle stage or metaphase II were identified. The gene expression data was classified according to gene ontology; in terms of biological processing, the majority of these genes were associated with regulation activities, including the regulation of: MAPK activity, translation initiation and transcription. Our findings are in agreement with similar data from both mouse and human oocytes. Taken together this data provides a molecular transcriptome blueprint of immature and mature mammalian oocytes. This resource will be invaluable to our long-term objective which is to target identified pathways to elucidate the sensitivity of key regulatory genes and checkpoints of oocyte maturation to internal and external environmental influences.

Oocyte cytoplasmic maturation: a key mediator of oocyte and embryo developmental competence

Efforts have intensified to successfully mature and inseminate oocytes in vitro and then culture ensuing embryos to transferable stages from a large number of mammalian species. Success varies, but generally even for the most successful species it is only possible to obtain a maximum of a 40 to 50% development of zygotes to the blastocyst stage. Reduced oocyte developmental competence is suggested as a primary reason for the reduced potential of in vitro-produced embryos. The vast majority of in vitro-matured oocytes are meiotically competent; however, many do not attain an optimal oocyte diameter before insemination. Variations in oocyte in vitro maturation media can influence embryo development, blastocyst cell number, and apoptosis. In addition, studies have indicated that cytoplasmic donation from so-called competent to incompetent oocytes can improve developmental outcomes. Oocyte cytoplasmic maturation includes those events that instill upon the oocyte a capacity to complete nuclear maturation, insemination, early embryogenesis and thus provide a foundation for implantation, initiation of pregnancy, and normal fetal development. Although we can define oocyte cytoplasmic maturation, we are only now beginning to understand the molecular steps that underlie this process. In general terms, oocyte cytoplasmic maturation involves the accumulation of mRNA, proteins, substrates, and nutrients that are required to achieve the oocyte developmental competence that fosters embryonic developmental competence. Collectively we are beginning to specify oocyte cytoplasmic maturation, and eventually a coherent understanding of this critical event in oocyte biology will emerge.

Translation of the synaptonemal complex component Sycp3 is enhanced in vivo by the germ cell specific regulator Dazl

DAZ-related genes are essential for gametogenesis in diverse metazoa: in human males, a loss of DAZ genes is associated with infertility. These genes, expressed only in germ cells, regulate the translation of a yet undefined set of specific transcripts, and loss of function results in numerous defects throughout the mitotic and meiotic process of germ cell development. In a mouse model, absence of the autosomal Dazl gene results in a final block at zygotene of meiotic prophase. Sycp3 is also essential for meiosis, specifically for the formation of the synaptonemal complex lateral element with a mouse knockout model displaying a block in meiotic prophase similar to the Dazl knock out. Sycp3 was identified as a potential target for translational regulation by Dazl in male mouse germ cells. This was confirmed by both RNA binding and translation assays. In the Dazl knockout mouse model, Sycp3 protein levels were decreased, indicating that Dazl is required for efficient translation of the Sycp3 mRNA in vivo. Taken together these data support Sycp3 as a biologically relevant target of Dazl-mediated translation in mammals. This suggests that azoospermia associated with a decrease in DAZ gene function in humans may in part be a consequence of failure at synapsis caused by reduced levels of SYCP3 protein.

Lin-28 binds IGF-2 mRNA and participates in skeletal myogenesis by increasing translation efficiency

Lin-28 is a highly conserved, RNA-binding, microRNA-regulated protein that is involved in regulation of developmental timing in Caenorhabditis elegans. In mammals, Lin-28 is stage-specifically expressed in embryonic muscle, neurons, and epithelia, as well as in embryonic carcinoma cells, but is suppressed in most adult tissues, with the notable exception of skeletal and cardiac muscle. The specific function and mechanism of action of Lin-28 are not well understood. Here we used loss-of-function and gain-of-function assays in cultured myoblasts to show that expression of Lin-28 is essential for skeletal muscle differentiation in mice. In order to elucidate the specific function of Lin-28, we used a combination of biochemical and functional assays, which revealed that, in differentiating myoblasts, Lin-28 binds to the polysomes and increases the efficiency of protein synthesis. An important target of Lin-28 is IGF-2, a crucial growth and differentiation factor for muscle tissue. Interaction of Lin-28 with translation initiation complexes in skeletal myoblasts and in the embryonic carcinoma cell line P19 was confirmed by localization of Lin-28 to the stress granules, temporary structures that contain stalled mRNA-protein translation complexes. Our results unravel novel mechanisms of translational regulation in skeletal muscle and suggest that Lin-28 performs the role of "translational enhancer" in embryonic and adult cells and tissues.

Maternally derived transcripts: identification and characterisation during oocyte maturation and early cleavage

The identification and characterisation of differentially regulated genes in oocytes and early embryos are required to understand the mechanisms involved in maturation, fertilisation, early cleavage and even long-term development. Several methods, including reverse transcription-polymerase chain reaction-based suppression subtractive hybridisation, differential display and cDNA microarray, have been applied to identify maternally derived genes in mammalian oocytes. However, conventional gene-knockout experiments to determine specific gene functions are labour intensive and inefficient. Recent developments include the use of RNA interference techniques to establish specific gene functions in mammalian oocytes and early embryos. Regulation of the poly(A) tail length is a major factor in controlling the activities of maternal transcripts in mammals. Further studies are required to clarify the mechanisms by which expression levels of maternally derived transcripts are regulated. In the present review, we focus on the identification and functions of the differentially expressed transcripts during oocyte maturation, fertilisation and early cleavage.