Meiotic maturation of mouse oocytes triggers the translation and polyadenylation of dormant tissue-type plasminogen activator mRNA

Degradation and translation of maternal mRNA for embryogenesis

Maternal mRNAs accumulate during egg growth and must be judiciously degraded or translated to ensure successful development of mammalian embryos. In this review we integrate recent investigations into pathways controlling rapid degradation of maternal mRNAs during the maternal-to-zygotic transition. Degradation is not indiscriminate, and some mRNAs are selectively protected and rapidly translated after fertilization for reprogramming the zygotic genome during early embryogenesis. Oocyte specific cofactors and pathways have been illustrated to control different futures of maternal mRNAs. We discuss mechanisms that control the fate of maternal mRNAs during late oogenesis and after fertilization. Issues to be resolved in current maternal mRNA research are described, and future research directions are proposed.

An extended wave of global mRNA deadenylation sets up a switch in translation regulation across the mammalian oocyte-to-embryo transition

Without new transcription, gene expression across the oocyte-to-embryo transition (OET) relies instead on regulation of mRNA poly(A) tails to control translation. However, how tail dynamics shape translation across the OET in mammals remains unclear. We perform long-read RNA sequencing to uncover poly(A) tail lengths across the mouse OET and, incorporating published ribosome profiling data, provide an integrated, transcriptome-wide analysis of poly(A) tails and translation across the entire transition. We uncover an extended wave of global deadenylation during fertilization in which short-tailed, oocyte-deposited mRNAs are translationally activated without polyadenylation through resistance to deadenylation. Subsequently, in the embryo, mRNAs are readenylated and translated in a surge of global polyadenylation. We further identify regulation of poly(A) tail length at the isoform level and stage-specific enrichment of mRNA sequence motifs among regulated transcripts. These data provide insight into the stage-specific mechanisms of poly(A) tail regulation that orchestrate gene expression from oocyte to embryo in mammals.

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.

A unique poly(A) tail profile uncovers the stability and translational activation of TOP transcripts during neuronal differentiation

Cell differentiation is associated with global changes in translational activity. Here, we characterize how mRNA poly(A) tail processing supports this dynamic. We observe that decreased translation during neuronal differentiation of P19 cells correlates with the downregulation of 5'-terminal oligopyrimidine (TOP) transcripts which encode the translational machinery. Despite their downregulation, TOP transcripts remain highly stable and show increased translation as cells differentiate. Changes in TOP mRNA metabolism are reflected by their accumulation with poly(A) tails ∼60-nucleotide (nt) long. The dynamic changes in poly(A) processing can be partially recapitulated by depleting LARP1 or activating the mTOR pathway in undifferentiated cells. Although mTOR-induced accumulation of TOP mRNAs with tails ∼60-nt long does not trigger differentiation, it is associated with reduced proliferation of neuronal progenitors. We propose that while TOP mRNAs are transcriptionally silenced, their post-transcriptional regulation mediated by a specific poly(A) processing ensures an adequate supply of ribosomes to complete differentiation.

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.

Plasminogen activator, tissue type regulates germinal vesicle breakdown and cumulus expansion of bovine cumulus-oocyte complex in vitro†

Plasminogen activator, tissue type (PLAT) and its inhibitor serpin family E member 1 (SERPINE1) cooperatively regulate PLAT activity in various reproductive processes. However, it is unknown whether this includes bovine oocyte maturation. We addressed this question in the present study by evaluating PLAT and SERPINE1 protein localization in immature cumulus-oocyte complexes (COCs), as well as PLAT mRNA and protein expression in cultured COCs after 0, 8, 16, and 24 h of in vitro maturation (IVM). We also examined the effects of PLAT and SERPINE1 on germinal vesicle breakdown (GVBD) and oocyte cyclic 3' 5' adenosine monophosphate (cAMP) levels, cumulus expansion index, and expansion-related gene expression in oocytes derived from bovine COCs cultured for 4, 8, and 12 h and in COCs cultured for 16 h. Both PLAT and SERPINE1 localized in cumulus cells but only the latter was detected in oocytes. PLAT and SERPINE1 transcript levels increased during IVM; however, from 8 to 16 h, the levels of PLAT remained stable whereas those of SERPINE1 increased, resulting in a decline in PLAT concentration. Additionally, PLAT delayed GVBD, increased oocyte cAMP levels, and blocked cumulus expansion and associated gene expression, which was reversed by SERPINE1 supplemented. Thus, PLAT delays bovine oocyte GVBD by enhancing oocyte cAMP levels during the first 8 h of IVM; suppression of PLAT activity via accumulation of SERPINE1 in COCs results in cumulus expansion from 8 to 16 h of IVM. These findings provide novel insights into the molecular mechanisms underlying in vitro bovine oocyte maturation.

Long non-coding RNA exchange during the oocyte-to-embryo transition in mice

The oocyte-to-embryo transition (OET) transforms a differentiated gamete into pluripotent blastomeres. The accompanying maternal-zygotic RNA exchange involves remodeling of the long non-coding RNA (lncRNA) pool. Here, we used next generation sequencing and de novo transcript assembly to define the core population of 1,600 lncRNAs expressed during the OET (lncRNAs). Relative to mRNAs, OET lncRNAs were less expressed and had shorter transcripts, mainly due to fewer exons and shorter 5′ terminal exons. Approximately half of OET lncRNA promoters originated in retrotransposons suggesting their recent emergence. Except for a small group of ubiquitous lncRNAs, maternal and zygotic lncRNAs formed two distinct populations. The bulk of maternal lncRNAs was degraded before the zygotic genome activation. Interestingly, maternal lncRNAs seemed to undergo cytoplasmic polyadenylation observed for dormant mRNAs. We also identified lncRNAs giving rise to trans-acting short interfering RNAs, which represent a novel lncRNA category. Altogether, we defined the core OET lncRNA transcriptome and characterized its remodeling during early development. Our results are consistent with the notion that rapidly evolving lncRNAs constitute signatures of cells-of-origin while a minority plays an active role in control of gene expression across OET. Our data presented here provide an excellent source for further OET lncRNA studies.

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

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

mRNA 3' uridylation and poly(A) tail length sculpt the mammalian maternal transcriptome

A fundamental principle in biology is that the program for early development is established during oogenesis in the form of the maternal transcriptome. How the maternal transcriptome acquires the appropriate content and dosage of transcripts is not fully understood. Here we show that 3' terminal uridylation of mRNA mediated by TUT4 and TUT7 sculpts the mouse maternal transcriptome by eliminating transcripts during oocyte growth. Uridylation mediated by TUT4 and TUT7 is essential for both oocyte maturation and fertility. In comparison to somatic cells, the oocyte transcriptome has a shorter poly(A) tail and a higher relative proportion of terminal oligo-uridylation. Deletion of TUT4 and TUT7 leads to the accumulation of a cohort of transcripts with a high frequency of very short poly(A) tails, and a loss of 3' oligo-uridylation. By contrast, deficiency of TUT4 and TUT7 does not alter gene expression in a variety of somatic cells. In summary, we show that poly(A) tail length and 3' terminal uridylation have essential and specific functions in shaping a functional maternal transcriptome.

Noninheritable Maternal Factors Useful for Genetic Manipulation in Mammals

Mammalian early embryogenesis is supported by maternal factors, such as messenger RNA (mRNA) and proteins, produced and accumulated during oogenesis at least up to the stage when zygotic activation commences. These maternal factors are involved in biologically important events such as epigenetic activation, reprogramming, and mitochondrial growth. Most of these maternal mRNAs are degraded by the 2-cell to 4 ~ 8-cell stages. Maternal proteins, which are produced during oogenesis or by the maternal mRNAs, are degraded by the 4 ~ 8-cell stage. In other words, the maternal factors exist during specific stages of early embryogenesis. In this chapter, we will briefly summarize the property of these maternal factors and mention possible applications of these factors for developing new reproduction engineering-related technologies and producing genetically modified animals. More specifically, we will show the usefulness of maternally accumulated Cas9 protein as a promising tool for CRISPR-/Cas9-based simultaneous genetic modification of multiple loci in mammals.

Sculpting the Transcriptome During the Oocyte-to-Embryo Transition in Mouse

In mouse, the oocyte-to-embryo transition entails converting a highly differentiated oocyte to totipotent blastomeres. This transition is driven by degradation of maternal mRNAs, which results in loss of oocyte identity, and reprogramming of gene expression during the course of zygotic gene activation, which occurs primarily during the two-cell stage and confers blastomere totipotency. Full-grown oocytes are transcriptionally quiescent and mRNAs are remarkably stable in oocytes due to the RNA-binding protein MSY2, which stabilizes mRNAs, and low activity of the 5' and 3' RNA degradation machinery. Oocyte maturation initiates a transition from mRNA stability to instability due to phosphorylation of MSY2, which makes mRNAs more susceptible to the RNA degradation machinery, and recruitment of dormant maternal mRNAs that encode for critical components of the 5' and 3' RNA degradation machinery. Small RNAs (miRNA, siRNA, and piRNA) play little, if any, role in mRNA degradation that occurs during maturation. Many mRNAs are totally degraded but a substantial fraction is only partially degraded, their degradation completed by the end of the two-cell stage. Genome activation initiates during the one-cell stage, is promiscuous, low level, and genome wide (and includes both inter- and intragenic regions) and produces transcripts that are inefficiently spliced and polyadenylated. The major wave of genome activation in two-cell embryos involves expression of thousands of new genes. This unique pattern of gene expression is the product of maternal mRNAs recruited during maturation that encode for transcription factors and chromatin remodelers, as well as dramatic changes in chromatin structure due to incorporation of histone variants and modified histones.

Mobilization of Dormant Cnot7 mRNA Promotes Deadenylation of Maternal Transcripts During Mouse Oocyte Maturation

Maternal mRNAs in oocytes are remarkably stable. In mouse, oocyte maturation triggers a transition from mRNA stability to instability. This transition is a critical event in the oocyte-to-embryo transition in which a differentiated oocyte loses its identity as it is transformed into totipotent blastomeres. We previously demonstrated that phosphorylation of MSY2, an RNA-binding protein, and mobilization of mRNAs encoding the DCP1A-DCP2 decapping complex contribute to maternal mRNA destruction during meiotic maturation. We report here that Cnot7, Cnot6l, and Pan2, key components of deadenylation machinery, are also dormant maternal mRNAs that are recruited during oocyte maturation. Inhibiting the maturation-associated increase in CNOT7 (or CNOT6L) using a small interference RNA approach inhibits mRNA deadenylation, whereas inhibiting the increase in PAN2 has little effect. Reciprocally, expressing CNOT7 (or CNOT6L) in oocytes prevented from resuming meiosis initiates deadenylation of mRNAs. These effects on deadenylation are also observed when the total amount of poly (A) is quantified. Last, inhibiting the increase in CNOT7 protein results in an ~70% decrease in transcription in 2-cell embryos.

Resveratrol down-regulates a glutamate-induced tissue plasminogen activator via Erk and AMPK/mTOR pathways in rat primary cortical neurons

Resveratrol (3,5,4'-trihydroxy-trans-stilbene, RSV) is a polyphenolic compound present in a variety of plant species (including grapes) that produces a myriad of biological activities including anti-inflammatory, antioxidant and neuroprotective effects. In this study, we investigate the effects of resveratrol on the basal and glutamate-stimulated expression and activity of a tissue plasminogen activator (tPA) that plays neuromodulatory or neurotoxic roles in many different neurological situations. Under basal conditions, resveratrol decreased the tPA expression and activity without affecting the tPA mRNA level in rat primary cortical neurons. RSV induced AMPK phosphorylation and inhibited mTOR phosphorylation. Inhibition of AMPK phosphorylation using compound C prevented resveratrol-induced down-regulation of tPA activity. This suggested that AMPK/mTOR-dependent translational inhibition contributes to the down-regulation of the tPA. Under glutamate-stimulated conditions of rat primary cortical neurons, tPA activity and expression were increased along with increased tPA mRNA expression but afterward treatment of RSV inhibited the glutamate-induced increase in tPA activity and expression and tPA mRNA expression. Glutamate stimulation induced activation of Akt and MAPK pathways as well as mTOR which were inhibited by RSV. Interestingly, the Erk pathway inhibitor U0126, but neither PI3K-Akt inhibitor LY294002 nor p38 inhibitor SB203580, mimicked the inhibitory action of RSV on glutamate-induced tPA up-regulation. This suggested the essential role of Erk in the transcriptional up-regulation of tPA expression, which is targeted by RSV. Glutamate stimulation induced neuronal cell death as determined by PI staining and MTT assay. However, RSV protected the cultured rat primary cortical neurons from glutamate-induced cell death as paralleled with the changes in tPA expression. These results suggested that RSV can modulate tPA activity under basal and stimulated conditions by both translational and transcriptional mechanisms. The regulation of the tPA by RSV provides additional therapeutic targets on top of the growing number of molecular substrates of RSV's action in the brain.

Activators and inhibitors of the plasminogen system in Alzheimer's disease

Accumulation and deposition of Aβ is one of the main neuropathological hallmarks of Alzheimer's disease (AD) and impaired Aβ degradation may be one mechanism of accumulation. Plasmin is the key protease of the plasminogen system and can cleave Aβ. Plasmin is activated from plasminogen by tissue plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). The activators are regulated by inhibitors which include plasminogen activator inhibitor-1 (PAI-1) and neuroserpin. Plasmin is also regulated by inhibitors including α2-antiplasmin and α2-macroglobulin. Here, we investigate the mRNA levels of the activators and inhibitors of the plasminogen system and the protein levels of tPA, neuroserpin and α2-antiplasmin in post-mortem AD and control brain tissue. Distribution of the activators and inhibitors in human brain sections was assessed by immunoperoxidase staining. mRNA measurements were made in 20 AD and 20 control brains by real-time PCR. In an expanded cohort of 38 AD and 38 control brains tPA, neuroserpin and α2-antiplasmin protein levels were measured by ELISA. The activators and inhibitors were present mainly in neurons and α2-antiplasmin was also associated with Aβ plaques in AD brain tissue. tPA, uPA, PAI-1 and α2-antiplasmin mRNA were all significantly increased in AD compared to controls, as were tPA and α2-antiplasmin protein, whereas neuroserpin mRNA and protein were significantly reduced. α2-macroglobulin mRNA was not significantly altered in AD. The increases in tPA, uPA, PAI-1 and α2-antiplasmin may counteract each other so that plasmin activity is not significantly altered in AD, but increased tPA may also affect synaptic plasticity, excitotoxic neuronal death and apoptosis.

Post-transcriptional control of gene expression during mouse oogenesis

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

Fertilization outcome could be regulated by binding of oviductal plasminogen to oocytes and by releasing of plasminogen activators during interplay between gametes

OBJECTIVE

To detect plasminogen and plasminogen activators (PA) in oviduct and oocytes and to clarify the role of the plasminogen/plasmin system on mammalian fertilization.

DESIGN

Experimental prospective study.

SETTING

Mammalian reproduction research laboratory.

ANIMAL(S)

Oviducts and ovaries from porcine and bovine females were collected at slaughterhouse. A total of 52 oviducts and 2,292 oocytes were used. Boar and bull ejaculated spermatozoa were also used.

INTERVENTION(S)

Plasminogen concentration in oviductal fluid (OF) through the cycle was measured. Immunolocalization of plasminogen and PAs in oocytes was carried out before and after fertilization. Porcine and bovine oocytes were in vitro fertilized, with plasminogen and plasmin added to the culture medium at different concentrations.

MAIN OUTCOME MEASURE(S)

Plasminogen concentration in OF. Plasminogen and PAs immunolocalization in oocytes. Penetration and monospermy rates, number of spermatozoa in the ooplasma and on the zona pellucida (ZP) after IVF.

RESULT(S)

Oviductal fluid contains about 92 μg/mL of plasminogen. The mature oocyte shows immunoreactivity toward plasminogen and toward PAs on its oolemma and ZP. After fertilization, plasminogen and PAs immunolabeling decreases in the oocyte, suggesting its conversion into plasmin. When exogenous plasminogen is added to the IVF medium, sperm entry into the oocyte is hampered, suggesting that the role of plasminogen activation during fertilization is to reduce the number of (or to select) penetrating spermatozoa.

CONCLUSION(S)

The plasminogen/plasmin system is activated during gamete interaction and regulates the sperm entry into the oocyte.

Key contribution of CPEB4-mediated translational control to cancer progression

Malignant transformation, invasion and angiogenesis rely on the coordinated reprogramming of gene expression in the cells from which the tumor originated. Although deregulated gene expression has been extensively studied at genomic and epigenetic scales, the contribution of the regulation of mRNA-specific translation to this reprogramming is not well understood. Here we show that cytoplasmic polyadenylation element binding protein 4 (CPEB4), an RNA binding protein that mediates meiotic mRNA cytoplasmic polyadenylation and translation, is overexpressed in pancreatic ductal adenocarcinomas and glioblastomas, where it supports tumor growth, vascularization and invasion. We also show that, in pancreatic tumors, the pro-oncogenic functions of CPEB4 originate in the translational activation of mRNAs that are silenced in normal tissue, including the mRNA of tissue plasminogen activator, a key contributor to pancreatic ductal adenocarcinoma malignancy. Taken together, our results document a key role for post-transcriptional gene regulation in tumor development and describe a detailed mechanism for gene expression reprogramming underlying malignant tumor progression.

Translational control in oocyte development

Translational control of specific mRNAs is a widespread mechanism of gene regulation, and it is especially important in pattern formation in the oocytes of organisms in which the embryonic axes are established maternally. Drosophila and Xenopus have been especially valuable in elucidating the relevant molecular mechanisms. Here, we comprehensively review what is known about translational control in these two systems, focusing on examples that illustrate key concepts that have emerged. We focus on protein-mediated translational control, rather than regulation mediated by small RNAs, as the former appears to be predominant in controlling these developmental events. Mechanisms that modulate the ability of the specific mRNAs to be recruited to the ribosome, that regulate polyadenylation of specific mRNAs, or that control the association of particular mRNAs into translationally inert ribonucleoprotein complexes will all be discussed.

Zebrafish mRNA sequencing deciphers novelties in transcriptome dynamics during maternal to zygotic transition

Maternally deposited mRNAs direct early development before the initiation of zygotic transcription during mid-blastula transition (MBT). To study mechanisms regulating this developmental event in zebrafish, we applied mRNA deep sequencing technology and generated comprehensive information and valuable resources on transcriptome dynamics during early embryonic (egg to early gastrulation) stages. Genome-wide transcriptome analysis documented at least 8000 maternal genes and identified the earliest cohort of zygotic transcripts. We determined expression levels of maternal and zygotic transcripts with the highest resolution possible using mRNA-seq and clustered them based on their expression pattern. We unravel delayed polyadenylation in a large cohort of maternal transcripts prior to the MBT for the first time in zebrafish. Blocking polyadenylation of these transcripts confirms their role in regulating development from the MBT onward. Our study also identified a large number of novel transcribed regions in annotated and unannotated regions of the genome, which will facilitate reannotation of the zebrafish genome. We also identified splice variants with an estimated frequency of 50%-60%. Taken together, our data constitute a useful genomic information and valuable transcriptome resource for gene discovery and for understanding the mechanisms of early embryogenesis in zebrafish.

Proteomic analysis of mouse oocytes reveals 28 candidate factors of the "reprogrammome"

The oocyte is the only cell of the body that can reprogram transplanted somatic nuclei and sets the gold standard for all reprogramming methods. Therefore, an in-depth characterization of its proteome holds promise to advance our understanding of reprogramming and germ cell biology. To date, limitations on oocyte numbers and proteomic technology have impeded this task, and the search for reprogramming factors has been conducted in embryonic stem (ES) cells instead. Here, we present the proteome of metaphase II mouse oocytes to a depth of 3699 proteins, which substantially extends the number of proteins identified until now in mouse oocytes and is comparable by size to the proteome of undifferentiated mouse ES cells. Twenty-eight oocyte proteins, also detected in ES cells, match the criteria of our multilevel approach to screen for reprogramming factors, namely nuclear localization, chromatin modification, and catalytic activity. Our oocyte proteome catalog thus advances the definition of the "reprogrammome", the set of molecules--proteins, RNAs, lipids, and small molecules--that enable reprogramming.

Sequential analysis of global gene expression profiles in immature and in vitro matured bovine oocytes: potential molecular markers of oocyte maturation

Background

Without intensive selection, the majority of bovine oocytes submitted to in vitro embryo production (IVP) fail to develop to the blastocyst stage. This is attributed partly to their maturation status and competences. Using the Affymetrix GeneChip Bovine Genome Array, global mRNA expression analysis of immature (GV) and in vitro matured (IVM) bovine oocytes was carried out to characterize the transcriptome of bovine oocytes and then use a variety of approaches to determine whether the observed transcriptional changes during IVM was real or an artifact of the techniques used during analysis.

Results

8489 transcripts were detected across the two oocyte groups, of which ~25.0% (2117 transcripts) were differentially expressed (p < 0.001); corresponding to 589 over-expressed and 1528 under-expressed transcripts in the IVM oocytes compared to their immature counterparts. Over expression of transcripts by IVM oocytes is particularly interesting, therefore, a variety of approaches were employed to determine whether the observed transcriptional changes during IVM were real or an artifact of the techniques used during analysis, including the analysis of transcript abundance in oocytes in vitro matured in the presence of α-amanitin. Subsets of the differentially expressed genes were also validated by quantitative real-time PCR (qPCR) and the gene expression data was classified according to gene ontology and pathway enrichment. Numerous cell cycle linked (CDC2, CDK5, CDK8, HSPA2, MAPK14, TXNL4B), molecular transport (STX5, STX17, SEC22A, SEC22B), and differentiation (NACA) related genes were found to be among the several over-expressed transcripts in GV oocytes compared to the matured counterparts, while ANXA1, PLAU, STC1and LUM were among the over-expressed genes after oocyte maturation.

Conclusion

Using sequential experiments, we have shown and confirmed transcriptional changes during oocyte maturation. This dataset provides a unique reference resource for studies concerned with the molecular mechanisms controlling oocyte meiotic maturation in cattle, addresses the existing conflicting issue of transcription during meiotic maturation and contributes to the global goal of improving assisted reproductive technology.

Interrogating the transcriptome of oocytes and preimplantation embryos

During its growth phase, a mouse oocyte accumulates RNA that is the sole template for new protein synthesis in the transcriptionally silent interval between growth completion and transcriptional activation of the embryonic genome. Over this transcriptionally silent interval, almost half the quantity of RNA accumulated in the full-grown oocyte is degraded, while stable messages undergo major transcript-specific polyadenylation fluctuations associated with timely translation of new proteins. These processes, in the background of substantial RNA degradation, create unique pitfalls for transcriptome analysis. Three particular challenges are discussed herein. (1) Systematic errors of relative quantification occur if standard approaches are used, wherein samples are normalized to a constant quantity of RNA, or when computational analyses are normalized to an apparent "constant" endogenous to the sample. We show that use of a fixed quantity of exogenous RNA per oocyte or embryo alleviates this problem. (2) Comparison of large-scale expression analyses from widely disparate platforms highlights how the differing protocols produce correspondingly different lists of genes with significant changes in transcript abundance. Only with careful attention to the differences among experiments can such discrepancies be understood. (3) The complete assessment of changes in expression requires correspondingly comprehensive assessment of the role of isoform-specific changes.

Dcp1-bodies in mouse oocytes

Processing bodies (P-bodies) are cytoplasmic granules involved in the storage and degradation of mRNAs. In somatic cells, their formation involves miRNA-mediated mRNA silencing. Many P-body protein components are also found in germ cell granules, such as in mammalian spermatocytes. In fully grown mammalian oocytes, where changes in gene expression depend entirely on translational control, RNA granules have not as yet been characterized. Here we show the presence of P-body-like foci in mouse oocytes, as revealed by the presence of Dcp1a and the colocalization of RNA-associated protein 55 (RAP55) and the DEAD box RNA helicase Rck/p54, two proteins associated with P-bodies and translational control. These P-body-like structures have been called Dcp1-bodies and in meiotically arrested primary oocytes, two types can be distinguished based on their size. They also have different protein partners and sensitivities to the depletion of endogenous siRNA/miRNA and translational inhibitors. However, both type progressively disappear during in vitro meiotic maturation and are virtually absent in metaphase II-arrested secondary oocytes. Moreover, this disassembly of hDcp1a-bodies is concomitant with the posttranslational modification of EGFP-hDcp1a.

Proteasomal activity is required to initiate and to sustain translational activation of messenger RNA encoding the stem-loop-binding protein during meiotic maturation in mice

Developmentally regulated translation plays a key role in controlling gene expression during oogenesis. In particular, numerous mRNA species are translationally repressed in growing oocytes and become translationally activated during meiotic maturation. While many studies have focused on a U-rich sequence, termed the cytoplasmic polyadenylation element (CPE), located in the 3'-untranslated region (UTR) and the CPE-binding protein (CPEB) 1, multiple mechanisms likely contribute to translational control in oocytes. The stem-loop-binding protein (SLBP) is expressed in growing oocytes, where it is required for the accumulation of nonpolyadenylated histone mRNAs, and then accumulates substantially during meiotic maturation. We report that, in immature oocytes, Slbp mRNA carries a short poly(A) tail, and is weakly translated, and that a CPE-like sequence in the 3'-UTR is required to maintain this low activity. During maturation, Slbp mRNA becomes polyadenylated and translationally activated. Unexpectedly, proteasomal activity is required both to initiate and to sustain translational activation. This proteasomal activity is not required for the polyadenylation of Slbp mRNA during early maturation; however, it is required for a subsequent deadenylation of the mRNA that occurs during late maturation. Moreover, although CPEB1 is degraded during maturation, inhibiting its degradation by blocking mitogen-activated protein kinase 1/3 activity does not prevent the accumulation of SLBP, indicating that CPEB1 is not the protein whose degradation is required for translational activation of Slbp mRNA. These results identify a new role for proteasomal activity in initiating and sustaining translational activation during meiotic maturation.

Cytoplasmic Prep1 interacts with 4EHP inhibiting Hoxb4 translation

Background

Homeobox genes are essential for embryonic patterning and cell fate determination. They are regulated mostly at the transcriptional level. In particular, Prep1 regulates Hox transcription in association with Pbx proteins. Despite its nuclear role as a transcription factor, Prep1 is located in the cytosol of mouse oocytes from primary to antral follicles. The homeodomain factor Bicoid (Bcd) has been shown to interact with 4EHP (eukaryotic translation initiation factor 4E homolog protein) to repress translation of Caudal mRNA and to drive Drosophila embryo development. Interestingly, Prep1 contains a putative binding motif for 4EHP, which may reflect a novel unknown function.

Methodology/Principal Findings

In this paper we show by confocal microscopy and deconvolution analysis that Prep1 and 4EHP co-localize in the cytosol of growing mouse oocytes, demonstrating their interaction by co-immunoprecipitation and pull-down experiments. A functional 4EHP-binding motif present in Prep1 has been also identified by mutagenesis analysis. Moreover, Prep1 inhibits (>95%) the in vitro translation of a luciferase reporter mRNA fused to the Hoxb4 3′UTR, in the presence of 4EHP. RNA electrophoretic mobility shift assay was used to demonstrate that Prep1 binds the Hoxb4 3′UTR. Furthermore, conventional histology and immunohistochemistry has shown a dramatic oocyte growth failure in hypomorphic mouse Prep1^i/i females, accompanied by an increased production of Hoxb4. Finally, Hoxb4 overexpression in mouse zygotes showed a slow in vitro development effect.

Conclusions

Prep1 has a novel cytoplasmic, 4EHP-dependent, function in the regulation of translation. Mechanistically, the Prep1-4EHP interaction might bridge the 3′UTR of Hoxb4 mRNA to the 5′ cap structure. This is the first demonstration that a mammalian homeodomain transcription factor regulates translation, and that this function can be possibly essential for the development of female germ cells and involved in mammalian zygote 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.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Fibrinolysis, inflammation, and regulation of the plasminogen activating system

The maintenance of a given physiological process demands a coordinated and spatially regulated pattern of gene regulation. This applies to genes encoding components of enzyme cascades, including those of the plasminogen activating system. This family of proteases is vital to fibrinolysis and dysregulation of the expression pattern of one or more of these proteins in response to inflammatory events can impact on hemostasis. Gene regulation occurs on many levels, and it is apparent that the genes encoding the plasminogen activator (fibrinolytic) proteins are subject to both direct transcriptional control and significant post-transcriptional mechanisms. It is now clear that perturbation of these genes at either of these levels can dramatically alter expression levels and have a direct impact on the host's response to a variety of physiological and pharmacological challenges. Inflammatory processes are well known to impact on the fibrinolytic system and to promote thrombosis, cancer and diabetes. This review discusses how inflammatory and other signals affect the transcriptional and post-transcriptional expression patterns of this system, and how this modulates fibrinolysis in vivo.

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.

Epigenetic modifications by Trithorax group proteins during early embryogenesis: do members of Trx-G function as maternal effect genes?

Maternal effect genes encode transcripts that are expressed during oogenesis. These gene products are stored in the oocyte and become functional during resumption of meiosis and zygote genome activation, and in embryonic stem cells. To date, a few maternal effect genes have been identified in mammals. Epigenetic modifications have been shown to be important during early embryonic development and involve DNA methylation and post-translational modification of core histones. During development, two families of proteins have been shown to be involved in epigenetic changes: Trithorax group (Trx-G) and Polycomb group (Pc-G) proteins. Trx-G proteins function as transcriptional activators and have been shown to accumulate in the oocyte. Deletion of Trx-G members using conventional knockout technology results in embryonic lethality in the majority of the cases analysed to date. Recent studies using conditional knockout mice have revealed that at least one family member is necessary for zygote genome activation. We propose that other Trx-G members may also regulate embryonic genome activation and that the use of oocyte-specific deletor mouse lines will help clarify their roles in this process.

Cytoplasmic polyadenylation controls cdc25B mRNA translation in rat oocytes resuming meiosis

Resumption of meiosis in oocytes represents the entry into M-phase of the cell cycle and is regulated by the maturation-promoting factor (MPF). Activation of MPF is catalyzed by the dual specificity phosphatase, cdc25. In mammals, cdc25 is represented by a multigene family consisting of three isoforms: A, B and C. A recent report that female mice lacking cdc25B exhibit impaired fertility suggests a role for this isoform in regulating the G2- to M-transition in mammalian oocytes. Supporting the above-mentioned observation, we demonstrate herein that microinjection of neutralizing antibodies against cdc25B interfered with the ability of rat oocytes to undergo germinal vesicle breakdown (GVB). We also show accumulation of cdc25B in GVB oocytes and a transient reduction in its amount at metaphase I of meiosis. The accumulation of cdc25B was associated with its mRNA cytoplasmatic polyadenylation and was prevented by the protein synthesis inhibitor cyclohexamide as well as by the polyadenylation inhibitor cordycepin. Immunofluorescence staining revealed translocation of cdc25B to the metaphase II spindle apparatus. Taken together, our findings provide evidence that cdc25B is involved in resumption of meiosis in rat oocytes. We further demonstrate for the first time, a periodic accumulation of cdc25B throughout meiosis that is translationally regulated and involves cdc25B mRNA polyadenylation.

Expression of serpinb6 serpins in germ and somatic cells of mouse gonads

The serpin superfamily of serine protease inhibitors is implicated in the regulation of numerous physiological processes. In mice, Spi3/Serpinb6 has a broad tissue distribution. We have investigated the expression of Serpinb6 family members in embryonic and adult gonads. In male and female mice, Spi3/Serpinb6 and NK13/Serpinb6b were expressed in developing gonads and in both somatic and germ cells of adult gonads. By contrast, gonadal expression of Spi3C/Serpinb6c was sexually dimorphic and restricted to male germ cells and female somatic cells. These observations raise the question of the possible role(s) of the Serpinb6 family members in gonad development, gametogenesis, and/or fertilization.

Selective degradation of transcripts during meiotic maturation of mouse oocytes

There is massive destruction of transcripts during the maturation of mouse oocytes. The objective of this project was to identify and characterize the transcripts that are degraded versus those that are stable during the transcriptionally silent germinal vesicle (GV)-stage to metaphase II (MII)-stage transition using a microarray approach. A system for oocyte transcript amplification using both internal and 3'-poly(A) priming was utilized to minimize the impact of complex variations in transcript polyadenylation prevalent during this transition. Transcripts were identified and quantified using the Affymetrix Mouse Genome 430 v2.0 GeneChip. The significantly changed and stable transcripts were analyzed using Ingenuity Pathways Analysis and GenMAPP/MAPPFinder to characterize the biological themes underlying global changes in oocyte transcripts during maturation. It was concluded that the destruction of transcripts during the GV to MII transition is a selective rather than promiscuous process in mouse oocytes. In general, transcripts involved in processes that are associated with meiotic arrest at the GV-stage and the progression of oocyte maturation, such as oxidative phosphorylation, energy production, and protein synthesis and metabolism, were dramatically degraded. In contrast, transcripts encoding participants in signaling pathways essential for maintaining the unique characteristics of the MII-arrested oocyte, such as those involved in protein kinase pathways, were the most prominent among the stable transcripts.

A novel maternally transcribed homeobox gene,Eso-1, is preferentially expressed in oocytes and regulated by cytoplasmic polyadenylation

The homeobox gene families play important roles in the transcriptional regulation of gene expression prior to and during embryo development. To identify novel homeobox genes expressed in early embryonic development, we conducted a degenerated oligonucleotide polymerase chain reaction (PCR) to screen a mouse embryonic stem (ES) cell cDNA library. A novel homeobox-containing gene, Eso-1, which is preferentially expressed in ES cells and ovaries, was identified. The full-length Eso-1 cDNA was found to be 1,710 bp with a predicted homeodomain that has no significant homology to previously reported homeodomain proteins. Eso-1 was mapped to chromosome 14A3. Reverse transcription-polymerase chain reaction (RT-PCR) analyses showed that Eso-1 was expressed through oogenesis and continuing to be expressed through to the blastocyst stage. De novo expression of Eso-1 started at 13.5 days postcoitum in the ovaries, which coincides with the initiation of oogenesis. Northern blot analyses demonstrated that Eso-1 is preferentially expressed in both ovaries and ES cells as a 1.7-kb transcript. Results from whole mount in situ hybridization revealed that Eso-1 in oocytes showed increased expression from primordial to antral follicles. The 3'-untranslated region of Eso-1 transcripts contained cytoplasmic polyadenylation sequences while the length of poly (A) tails changed during oocyte maturation, indicating that Eso-1 expression is controlled by time-dependent translational activation. We suggest that the novel homeodomain protein, Eso-1, plays a role during oocyte maturation and early embryonic development.

Diverse patterns of poly(A) tail elongation and shortening of murine maternal mRNAs from fully grown oocyte to 2-cell embryo stages

We previously succeeded in constructing a cDNA library, CPF7, enriched with cDNA derived from maternal RNAs with the extended poly(A) tail in mouse fertilized eggs. In this study, we performed RNA blot analysis to examine the elongation in maternal RNAs using 20 representative clones isolated from CPF7 as probes. Various patterns of elongation, shortening, and/or degradation of maternal RNAs were observed from fully grown oocytes to early 2-cell embryos and could be roughly classified into three types and seven subtypes. These findings indicate that poly(A) elongation and shortening of maternal RNAs are not restricted to certain types of maternal RNAs but occur in many of them, and suggest a complex mechanism governing modification of the 3' end of maternal RNAs during the oocyte-to-embryo transition.

The stem-loop binding protein regulates translation of histone mRNA during mammalian oogenesis

Although messenger RNAs encoding the histone proteins are among the most abundant in mammalian oocytes, the mechanism regulating their translation has not been identified. The stem-loop binding protein (SLBP) binds to a highly conserved sequence in the 3'-untranslated region (utr) of the non-polyadenylated histone mRNAs in somatic cells and mediates their stabilization and translation. We previously showed that SLBP, which is expressed only during S-phase of proliferating cells, is expressed in growing oocytes at G2 of the cell cycle and accumulates substantially during meiotic maturation. We report here that elevating the amount of SLBP in immature (G2) oocytes is sufficient to increase translation of a reporter mRNA bearing the histone 3'-utr and endogenous histone synthesis and that this effect is not mediated through increased stability of the encoding mRNAs. We further report that translation of the reporter mRNA increases dramatically during meiotic maturation coincident with the accumulation of SLBP. Conversely, when SLBP accumulation during maturation is prevented using RNA interference, both translation of the reporter mRNA and synthesis of endogenous histones are significantly reduced. This effect is not mediated by a loss of the encoding mRNAs. Moreover, following fertilization, SLBP-depleted oocytes also show a significant decrease in pronuclear size and in the amount of acetylated histone detectable on the chromatin. These results demonstrate that histone synthesis in immature and maturing oocytes is governed by a translational control mechanism that is directly regulated by changes in the amount of SLBP.

Expression of plasminogen activators in preimplantation rat embryos developed in vivo and in vitro

BACKGROUND

Embryo implantation plays a major role in embryogenesis and the outcome of pregnancy. Plasminogen activators (PAs) have been implicated in mammalian fertilization, early stages of development and embryo implantation. The invasion of trophoblast cells into the endometrium during the implantation process can be blocked by inhibitors of serine proteases, illustrating the role of these enzymes in the invasion process. As in vitro developing embryos resulted in lower implantation rate than those developed in vivo we assume that a reduced PAs activity may lead to it. There is hardly any information regarding qualitative or quantitative differences in expression of PAs in preimplantation embryos, or comparisons between in vivo and in vitro developed embryos. The purpose of this study was to assess the expression of urokinase type (uPA) and tissue type (tPA) plasminogen activators in in vivo and in vitro preimplantation development in rat embryos using immunofluorescence confocal microscopy and computerized image analysis.

METHODS

Zygotes, 2-cell, 4-cell, 8-cell, morula and blastocyst stages of development were flushed from the reproductive tract (control groups) of Wistar rats. Zygotes were flushed and grown in vitro to the above mentioned developmental stages and comprised the experimental groups. Immunofluorescence microscopy and computerized image analysis were used to evaluate both qualitative (localization) and quantitative expression of plasminogen activators.

RESULTS

uPA and tPA were found to be expressed in rat embryos throughout their preimplantation development, both in vivo and in vitro. While uPA was localized mainly in the cell cytoplasm, the tPA was detected mainly on cell surface and in the perivitelline space. In blastocysts, both in vivo and in vitro, uPA and tPA were localized in the trophectoderm cells. Total uPA content per embryo was higher in the in vivo as compared with the in vitro developed embryos at all stages measured. Blastocyst uPA content was significantly low as compared with the four-cell, eight-cell, and morula stages. Total tPA content was higher in embryos developed in vivo than those developed in vitro except for the 4-cell and 8-cell stages.

CONCLUSION

In vitro embryo development leads to lower PAs expression in a stage dependent manner as compared with in vivo developing controls. The enzymes studied vary probably in the ratio of their active and inactive forms as there is no correlation between their content and the activity observed in our previous study. The localization of both PAs in the blastocysts' trophectoderm supports the assumption that PAs plays a role in the implantation process in rats.

A novel method for constructing murine cDNA library enriched with maternal mRNAs exhibiting de novo independent post-fertilization polyadenylation

Recently, mouse maternal mRNAs such as SSEC-D, Spin, beta-catenin, Ptp4a1, and Maid have been found to exhibit de novo independent polyadenylation after fertilization. To obtain an overall picture of post-fertilization polyadenylation events, we developed a novel method for constructing murine fertilized egg cDNA library enriched with cDNAs exhibiting de novo independent polyadenylation. As a pilot study, we isolated at least four new maternal mRNAs exhibiting extension of poly(A) tail in fertilized 1-cell eggs. Moreover, various types of polyadenylation of maternal RNAs were observed at this stage, suggesting the presence of novel mechanisms for regulating the length of poly(A) tails of maternal mRNA. This is the first report of successful construction of a cDNA library enriched with newly polyadenylated maternal mRNAs derived from post-fertilized mouse eggs. This cDNA library will be useful for molecular analysis of the mechanisms underlying post-fertilization polyadenylation of mammalian maternal RNAs.

Maternal beta-catenin and E-cadherin in mouse development

The oocyte to embryo transition in metazoans depends on maternal proteins and transcripts to ensure the successful initiation of development, and the correct and timely activation of the embryonic genome. We conditionally eliminated the maternal gene encoding the cell adhesion molecule E-cadherin and partially eliminated the beta-catenin gene from the mouse oocyte. Oocytes lacking E-cadherin, or expressing a truncated allele of beta-catenin without the N-terminal part of the protein, give rise to embryos whose blastomeres do not adhere. Blastomere adhesion is restored after translation of protein from the wild-type paternal alleles: at the morula stage in embryos lacking maternal E-cadherin, and at the late four-cell stage in embryos expressing truncated beta-catenin. This suggests that adhesion per se is not essential in the early cleavage stage embryos, that embryos develop normally if compaction does not occur until the morula stage, and that the zona pellucida suffices to maintain blastomere proximity. Although maternal E-cadherin is not essential for the completion of the oocyte-to-embryo transition, absence of wild-type beta-catenin in oocytes does statistically compromise developmental success rates. This developmental deficit is alleviated by the simultaneous absence of maternal E-cadherin, suggesting that E-cadherin regulates nuclear beta-catenin availability during embryonic genome activation.

Differences in the implantation rates of rat embryos developed in vivo and in vitro: possible role for plasminogen activators

OBJECTIVE

To assess the participation of urokinase-type (uPA) and tissue-type (tPA) plasminogen activators in embryo development and implantation.

DESIGN

The study was set to compare the activities of PAs in embryos developed in vivo and in vitro and their implantation rates. Endometrial PA activity was studied as well.

SETTING

University laboratories.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Plasminogen activator activity in embryos.

RESULT(S)

Whereas tPA activity in in vivo-developed embryos decreased from the zygote to the blastocyst stages, it did not change in embryos developed in vitro. The activity of uPA was lowest in two-cell-stage embryos, both in vivo and in vitro, and increased as embryos developed into blastocysts. The activity of uPA in the in vitro-developed embroys was lower in all developmental stages as compared with those developed in vivo. Whereas endometrial tPA activity did not change during the preimplantation period, uPA activity increased gradually toward the time of implantation. The implantation rate of the in vitro-developed embryos was much lower than that of those developed in vivo.

CONCLUSION(S)

Taken together, these data indicate that in vitro embryo development leads to lower PA activities as compared with in vivo development. The increased activity of uPA toward the blastocyst stage supports uPA's role in the implantation process. Moreover, the rise in endometrial uPA might also indicate its importance in this process.

Induction of enhanced green fluorescent protein expression in response to lesions in the nervous system

We have generated a mouse strain carrying a transgene driven by a strong and ubiquitous promoter (human cytomegalovirus hCMV/beta-actin) and containing an enhanced green fluorescent protein (eGFP) coding sequence upstream of the 3' untranslated region (3'UTR) of tissue-type plasminogen activator (t-PA) mRNA. The 3'UTR of t-PA mRNA is known to be involved in the reversible deadenylation and translational repression of transcripts in mouse oocytes. hCMV/beta-actin-eGFP-3'UTR t-PA transgenic mice express eGFP mRNA in all brain structures analyzed but lack eGFP fluorescence, with the exception of blood vessels, choroid plexus, and Purkinje cells. Taking advantage of these features, we tested whether certain pathological conditions, in particular injuries of the nervous system, might trigger eGFP fluorescence in traumatized cells or neurons. From this perspective, we analyzed eGFP mRNA expression and eGFP fluorescence in experimental models of nervous system lesions, such as motoneuron axotomy and cerebral stroke induced by middle cerebral artery occlusion. We found an increase in eGFP fluorescence in specific brain areas in cells suffering or reacting to these injuries. This increased fluorescence is correlated with an increased transcription of eGFP in lesioned cells, presumably enhanced by a release of the translational silencing mediated by the 3'UTR region of the t-PA mRNA. This transgenic mouse model may prove useful to study the development of neurodegenerative lesions.

Emotions are building up in the field of extracellular proteolysis

Activity-dependent remodeling of neural connections might require localized extracellular proteolysis. The tissue-type plasminogen activator (tPA)-plasmin proteolytic system is expressed in different regions of the central nervous system, in the context of a variety of physiological and pathological processes. Accumulating evidence regarding the expression and role of tPA and its inhibitors suggests that extracellular proteolysis is a key player in the biology of memory, emotions and neurodegeneration.

A history of poly A sequences: from formation to factors to function

Biological polyadenylation, first recognized as an enzymatic activity, remained an orphan enzyme until poly A sequences were found on the 3' ends of eukarvotic mRNAs. Their presence in bacteria viruses and later in archeae (ref. 338) established their universality. The lack of compelling evidence for a specific function limited attention to their cellular formation. Eventually the newer techniques of molecular biology and development of accurate nuclear processing extracts showed 3' end formation to be a two-step process. Pre-mRNA was first cleaved endonucleolytically at a specific site that was followed by sequential addition of AMPs from ATP to the 3' hydroxyl group at the end of mRNA. The site of cleavage was specified by a conserved hexanucleotide, AAUAAA, from 10 to 30 nt upstream of this 3' end. Extensive purification of these two activities showed that more than 10 polypeptides were needed for mRNA 3' end formation. Most of these were in complexes involved in the cleavage step. Two of the best characterized are CstF and CPSF, while two other remain partially purified but essential. Oddly, the specific proteins involved in phosphodiester bond hydrolysis have yet to be identified. The polyadenylation step occurs within the complex of poly A polymerase and poly A-binding protein, PABII, that controls poly A length. That the cleavage complex, CPSF, is also required for this step attests to a tight coupling of the two steps of 3' and formation. The reaction reconstituted from these RNA-free purified factors correctly processes pre-mRNAs. Meaningful analysis of the role of poly A in mRNA metabolism or function was possible once quantities of these proteins most often over-expressed from cDNA clones became available. The large number needed for two simple reactions of an endonuclease, a polymerase and a sequence recognition factor, pointed to 3' end formation as a regulated process. Polyadenylation itself had appeared to require regulation in cases where two poly A sites were alternatively processed to produce mRNA coding for two different proteins. The 64-KDa subunit of CstF is now known to be a regulator of poly A site choice between two sites in the immunoglobulin heavy chain of B cells. In resting cells the site used favors the mRNA for a membrane-bound protein. Upon differentiation to plasma cells, an upstream site is used the produce a secreted form of the heavy chain. Poly A site choice in the calcitonin pre-mRNA involves splicing factors at a pseudo splice site in an intron downstream of the active poly site that interacts with cleavage factors for most tissues. The molecular basis for choice of the alternate site in neuronal tissue is unknown. Proteins needed for mRNA 3' end formation also participate in other RNA-processing reactions: cleavage factors bind to the C-terminal domain of RNA polymerase during transcription; splicing of 3' terminal exons is stimulated port of by cleavage factors that bind to splicing factors at 3' splice sites. nuclear ex mRNAs is linked to cleavage factors and requires the poly A II-binding protein. Most striking is the long-sought evidence for a role for poly A in translation in yeast where it provides the surface on which the poly A-binding protein assembles the factors needed for the initiation of translation. This adaptability of eukaryotic cells to use a sequence of low information content extends to bacteria where poly A serves as a site for assembly of an mRNA degradation complex in E. coli. Vaccinia virus creates mRNA poly A tails by a streamlined mechanism independent of cleavage that requires only two proteins that recognize unique poly A signals. Thus, in spite of 40 years of study of poly A sequences, this growing multiplicity of uses and even mechanisms of formation seem destined to continue.

Stem-loop binding protein accumulates during oocyte maturation and is not cell-cycle-regulated in the early mouse embryo

The stem-loop binding protein (SLBP) binds to the 3' end of histone mRNA and participates in 3'-processing of the newly synthesized transcripts, which protects them from degradation, and probably also promotes their translation. In proliferating cells, translation of SLBP mRNA begins at G1/S and the protein is degraded following DNA replication. These post-transcriptional mechanisms closely couple SLBP expression to S-phase of the cell cycle, and play a key role in restricting synthesis of replication-dependent histones to S-phase. In contrast to somatic cells, replication-dependent histone mRNAs accumulate and are translated independently of DNA replication in oocytes and early embryos. We report here that SLBP expression and activity also differ in mouse oocytes and early embryos compared with somatic cells. SLBP is present in oocytes that are arrested at prophase of G2/M, where it is concentrated in the nucleus. Upon entry into M-phase of meiotic maturation, SLBP begins to accumulate rapidly, reaching a very high level in mature oocytes arrested at metaphase II. Following fertilization, SLBP remains abundant in the nucleus and the cytoplasm throughout the first cell cycle, including both G1 and G2 phases. It declines during the second and third cell cycles, reaching a relatively low level by the late 4-cell stage. SLBP can bind the histone mRNA-stem-loop at all stages of the cell cycle in oocytes and early embryos, and it is the only stem-loop binding activity detectable in these cells. We also report that SLBP becomes phosphorylated rapidly following entry into M-phase of meiotic maturation through a mechanism that is sensitive to roscovitine, an inhibitor of cyclin-dependent kinases. SLBP is rapidly dephosphorylated following fertilization or parthenogenetic activation, and becomes newly phosphorylated at M-phase of mitosis. Phosphorylation does not affect its stem-loop binding activity. These results establish that, in contrast to Xenopus, mouse oocytes and embryos contain a single SLBP. Expression of SLBP is uncoupled from S-phase in oocytes and early embryos, which indicates that the mechanisms that impose cell-cycle-regulated expression of SLBP in somatic cells do not operate in oocytes or during the first embryonic cell cycle. This distinctive pattern of SLBP expression may be required for accumulation of histone proteins required for sperm chromatin remodelling and assembly of newly synthesized embryonic DNA into chromatin.