Isolation of novel murine maternal mRNAs regulated by cytoplasmic polyadenylation

Hydroxyurea and inactivation of checkpoint kinase MEC1 inhibit transcription termination and pre-mRNA cleavage at polyadenylation sites in budding yeast

The DNA damage response (DDR) is an evolutionarily conserved process essential for cell survival. The transcription changes triggered by DDR depend on the nature of DNA damage, activation of checkpoint kinases, and the stage of cell cycle. The transcription changes can be localized and affect only damaged DNA, but they can be also global and affect genes that are not damaged. While the purpose of localized transcription inhibition is to avoid transcription of damaged genes and make DNA accessible for repair, the purpose and mechanisms of global transcription inhibition of undamaged genes are less well understood. We show here that a brief cell treatment with hydroxyurea (HU) globally inhibits RNA synthesis and transcription by RNA polymerase I, II, and III (RNAPI, RNAPII, and RNAPIII). HU reduces efficiency of transcription termination and inhibits pre-mRNA cleavage at the polyadenylation (pA) sites, destabilizes mRNAs, and shortens poly(A) tails of mRNAs, indicating defects in pre-mRNA 3' end processing. Inactivation of the checkpoint kinase Mec1p downregulates the efficiency of transcription termination and reduces the efficiency of pre-mRNAs clevage at the pA sites, suggesting the involvement of DNA damage checkpoint in transcription termination and pre-mRNA 3' end processing.

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.

mRNA 3' -UTR-mediate translational control through PAS and CPE in sheep oocyte

In oocytes, the cytoplasmic polyadenylation and maternal mRNAs translation is regulated by cis-elements, including polyadenylation signal (PAS) and cytoplasmic polyadenylation element (CPE) in 3'-UTR. Recent studies illustrate non-canonical polyadenylation mechanisms of translational regulation in mouse oocytes, which is different from that in Xenopus oocytes. However, it is still unclear if this regulation in rodent oocytes functions in the domestic animal oocyte. Here, by using sheep as an animal model, we cloned the 3'-UTRs of Cpeb1 or Btg4 and ligated it into the pRK5-Flag-Gfp vector. Variant numbers and positions of PASs and CPEs within the 3'-UTRs were constructed to detect their effects on translational control. After in vitro-transcription and microinjection into sheep fully grown germinal vesicle stage oocytes, the expression efficiency of mRNAs was detected by the GFP and flag expression. Our results show that: (i) PAS located at the proximal end of 3'-UTR can mediate the translation of the maternal mRNAs, as long as they locate far from CPEs; (ii) The proximal PAS has higher efficiency in regulating transcription than the distal one; (iii) increase of PAS number can promote the translational activity more efficiently; (iv) a single CPE located close to PAS (<50 bp) in 3'-UTRs of Cpeb1 or Btg4 could partially repress translation. In 3'-UTRs of Btg4, two CPEs have a higher inhibitory effect, and three CPEs can completely inhibit mRNA translation. These results confirm the existence of the non-canonical mechanism in domestic animal oocytes.

Measuring the tail: Methods for poly(A) tail profiling

The 3'-end poly(A) tail is an important and potent feature of most mRNA molecules that affects mRNA fate and translation efficiency. Polyadenylation is a posttranscriptional process that occurs in the nucleus by canonical poly(A) polymerases (PAPs). In some specific instances, the poly(A) tail can also be extended in the cytoplasm by noncanonical poly(A) polymerases (ncPAPs). This epitranscriptomic regulation of mRNA recently became one of the most interesting aspects in the field. Advances in RNA sequencing technologies and software development have allowed the precise measurement of poly(A) tails, identification of new ncPAPs, expansion of the function of known enzymes, discovery and a better understanding of the physiological role of tail heterogeneity, and recognition of a correlation between tail length and RNA translatability. Here, we summarize the development of polyadenylation research methods, including classic low-throughput approaches, Illumina-based genome-wide analysis, and advanced state-of-art techniques that utilize long-read third-generation sequencing with Pacific Biosciences and Oxford Nanopore Technologies platforms. A boost in technical opportunities over recent decades has allowed a better understanding of the regulation of gene expression at the mRNA level. This article is categorized under: RNA Methods > RNA Analyses In Vitro and In Silico.

Dynamic Variations of 3'UTR Length Reprogram the mRNA Regulatory Landscape

This paper concerns 3′-untranslated regions (3′UTRs) of mRNAs, which are non-coding regulatory platforms that control stability, fate and the correct spatiotemporal translation of mRNAs. Many mRNAs have polymorphic 3′UTR regions. Controlling 3′UTR length and sequence facilitates the regulation of the accessibility of functional effectors (RNA binding proteins, miRNAs or other ncRNAs) to 3′UTR functional boxes and motifs and the establishment of different regulatory landscapes for mRNA function. In this context, shortening of 3′UTRs would loosen miRNA or protein-based mechanisms of mRNA degradation, while 3′UTR lengthening would strengthen accessibility to these effectors. Alterations in the mechanisms regulating 3′UTR length would result in widespread deregulation of gene expression that could eventually lead to diseases likely linked to the loss (or acquisition) of specific miRNA binding sites. Here, we will review the mechanisms that control 3′UTR length dynamics and their alterations in human disorders. We will discuss, from a mechanistic point of view centered on the molecular machineries involved, the generation of 3′UTR variability by the use of alternative polyadenylation and cleavage sites, of mutually exclusive terminal alternative exons (exon skipping) as well as by the process of exonization of Alu cassettes to generate new 3′UTRs with differential functional features.

Exploring Translational Control of Maternal mRNAs in Zebrafish

The study of translational regulation requires reliable measurement of both mRNA levels and protein synthesis. Cytoplasmic polyadenylation is a prevalent mode of translational regulation during oogenesis and early embryogenesis. Here the length of the poly(A) tail of an mRNA is coupled to its translatability. We describe a protocol to identify translationally regulated genes and measure their translation rate in the early zebrafish embryo using genome-wide polysome profiling. This protocol relies on the isolation of mRNA by means of an rRNA depletion strategy, which avoids capture bias due to short poly(A) tail that can occur when using conventional oligo(dT)-based methods. We also present a simple PCR-based method to measure the poly(A) tail length of selected mRNAs.

Wwc2 Is a Novel Cell Division Regulator During Preimplantation Mouse Embryo Lineage Formation and Oogenesis

Formation of the hatching mouse blastocyst marks the end of preimplantation development, whereby previous cell cleavages culminate in the formation of three distinct cell lineages (trophectoderm, primitive endoderm and epiblast). We report that dysregulated expression of Wwc2, a genetic paralog of Kibra/Wwc1 (a known activator of Hippo-signaling, a key pathway during preimplantation development), is specifically associated with cell autonomous deficits in embryo cell number and cell division abnormalities. Division phenotypes are also observed during mouse oocyte meiotic maturation, as Wwc2 dysregulation blocks progression to the stage of meiosis II metaphase (MII) arrest and is associated with spindle defects and failed Aurora-A kinase (AURKA) activation. Oocyte and embryo cell division defects, each occurring in the absence of centrosomes, are fully reversible by expression of recombinant HA-epitope tagged WWC2, restoring activated oocyte AURKA levels. Additionally, clonal embryonic dysregulation implicates Wwc2 in maintaining the pluripotent epiblast lineage. Thus, Wwc2 is a novel regulator of meiotic and early mitotic cell divisions, and mouse blastocyst cell fate.

The translational regulation of maternal mRNAs in time and space

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

Cytoplasmic polyadenylation-mediated translational control of maternal mRNAs directs maternal-to-zygotic transition

In the earliest stages of animal development following fertilization, maternally deposited mRNAs direct biological processes to the point of zygotic genome activation (ZGA). These maternal mRNAs undergo cytoplasmic polyadenylation (CPA), suggesting translational control of their activation. To elucidate the biological role of CPA during embryogenesis, we performed genome-wide polysome profiling at several stages of zebrafish development. Our analysis revealed a correlation between CPA and polysome-association dynamics, demonstrating a coupling of translation to the CPA of maternal mRNAs. Pan-embryonic CPA inhibition disrupted the maternal-to-zygotic transition (MZT), causing a failure of developmental progression beyond the mid-blastula transition and changes in global gene expression that indicated a failure of ZGA and maternal mRNA clearance. Among the genes that were differentially expressed were those encoding chromatin modifiers and key transcription factors involved in ZGA, including nanog, pou5f3 and sox19b, which have distinct CPA dynamics. Our results establish the necessity of CPA for ensuring progression of the MZT. The RNA-seq data generated in this study represent a valuable zebrafish resource for the discovery of novel elements of the early embryonic transcriptome.

Analysis of circadian regulation of poly(A)-tail length

The poly(A) tail is found on the 3'-end of most eukaryotic mRNAs, and its length significantly contributes to the mRNAs half-life and translational competence. Circadian regulation of poly(A)-tail length is a powerful mechanism to confer rhythmicity in gene expression posttranscriptionally and provides a means to regulate protein levels independent of rhythmic transcription in the nucleus. Therefore, analysis of circadian poly(A)-tail length regulation is important for a complete understanding of rhythmic physiology, since rhythmically expressed proteins are the ultimate mediators of rhythmic function. Nevertheless, it has previously been challenging to measure changes in poly(A)-tail length, especially at a global level, due to technical constraints. However, new methodology depending on differential fractionation of mRNAs depending on the length of their tails has recently been developed. In this chapter, we describe these methods as used for examining the circadian regulation of poly(A)-tail length and provide detailed experimental procedures to measure poly(A)-tail length both at a the single mRNA level and the global level. Although this chapter concentrates on methods we used for analyzing poly(A)-tail length in the mammalian circadian system, the methods described here can be applicable to any organisms and any biological processes.

Efficient translation of Dnmt1 requires cytoplasmic polyadenylation and Musashi binding elements

Regulation of DNMT1 is critical for epigenetic control of many genes and for genome stability. Using phylogenetic analysis we characterized a block of 27 nucleotides in the 3′UTR of Dnmt1 mRNA identical between humans and Xenopus and investigated the role of the individual elements contained within it. This region contains a cytoplasmic polyadenylation element (CPE) and a Musashi binding element (MBE), with CPE binding protein 1 (CPEB1) known to bind to the former in mouse oocytes. The presence of these elements usually indicates translational control by elongation and shortening of the poly(A) tail in the cytoplasm of the oocyte and in some somatic cell types. We demonstrate for the first time cytoplasmic polyadenylation of Dnmt1 during periods of oocyte growth in mouse and during oocyte activation in Xenopus. Furthermore we show by RNA immunoprecipitation that Musashi1 (MSI1) binds to the MBE and that this element is required for polyadenylation in oocytes. As well as a role in oocytes, site-directed mutagenesis and reporter assays confirm that mutation of either the MBE or CPE reduce DNMT1 translation in somatic cells, but likely act in the same pathway: deletion of the whole conserved region has more severe effects on translation in both ES and differentiated cells. In adult cells lacking MSI1 there is a greater dependency on the CPE, with depletion of CPEB1 or CPEB4 by RNAi resulting in substantially reduced levels of endogenous DNMT1 protein and concurrent upregulation of the well characterised CPEB target mRNA cyclin B1. Our findings demonstrate that CPE- and MBE-mediated translation regulate DNMT1 expression, representing a novel mechanism of post-transcriptional control for this gene.

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.

Inhibition of tristetraprolin deadenylation by poly(A) binding protein

Tristetraprolin (TTP) is the prototype for a family of RNA binding proteins that bind the tumor necrosis factor (TNF) messenger RNA AU-rich element (ARE), causing deadenylation of the TNF poly(A) tail, RNA decay, and silencing of TNF protein production. Using mass spectrometry sequencing we identified poly(A) binding proteins-1 and -4 (PABP1 and PABP4) in high abundance and good protein coverage from TTP immunoprecipitates. PABP1 significantly enhanced TNF ARE binding by RNA EMSA and prevented TTP-initiated deadenylation in an in vitro macrophage assay of TNF poly(A) stability. Neomycin inhibited TTP-promoted deadenylation at concentrations shown to inhibit the deadenylases poly(A) ribonuclease and CCR4. Stably transfected RAW264.7 macrophages overexpressing PABP1 do not oversecrete TNF; instead they upregulate TTP protein without increasing TNF protein production. The PABP1 inhibition of deadenylation initiated by TTP does not require the poly(A) binding regions in RRM1 and RRM2, suggesting a more complicated interaction than simple masking of the poly(A) tail from a 3'-exonuclease. Like TTP, PABP1 is a substrate for p38 MAP kinase. Finally, PABP1 stabilizes cotransfected TTP in 293T cells and prevents the decrease in TTP levels seen with p38 MAP kinase inhibition. These findings suggest several levels of functional antagonism between TTP and PABP1 that have implications for regulation of unstable mRNAs like TNF.

Mos 3' UTR regulatory differences underlie species-specific temporal patterns of Mos mRNA cytoplasmic polyadenylation and translational recruitment during oocyte maturation

The Mos proto-oncogene is a critical regulator of vertebrate oocyte maturation. The maturation-dependent translation of Mos protein correlates with the cytoplasmic polyadenylation of the maternal Mos mRNA. However, the precise temporal requirements for Mos protein function differ between oocytes of model mammalian species and oocytes of the frog Xenopus laevis. Despite the advances in model organisms, it is not known if the translation of the human Mos mRNA is also regulated by cytoplasmic polyadenylation or what regulatory elements may be involved. We report that the human Mos 3' untranslated region (3' UTR) contains a functional cytoplasmic polyadenylation element (CPE) and demonstrate that the endogenous Mos mRNA undergoes maturation-dependent cytoplasmic polyadenylation in human oocytes. The human Mos 3' UTR interacts with the human CPE-binding protein and exerts translational control on a reporter mRNA in the heterologous Xenopus oocyte system. Unlike the Xenopus Mos mRNA, which is translationally activated by an early acting Musashi/polyadenylation response element (PRE)-directed control mechanism, the translational activation of the human Mos 3' UTR is dependent on a late acting CPE-dependent process. Taken together, our findings suggest a fundamental difference in the 3' UTR regulatory mechanisms controlling the temporal induction of maternal Mos mRNA polyadenylation and translational activation during Xenopus and mammalian oocyte maturation.

Efficient 5' cap-dependent RNA purification : use in identifying and studying subsets of RNA

Microarray-based screening technologies have revealed a larger than expected diversity of gene expression profiles for many cells, tissues, and organisms. The complexity of RNA species, defined by their molecular structure, represents a major new development in biology. RNA not only carries genetic information in the form of templates and components of the translational machinery for protein synthesis but also directly regulates gene expression as exemplified by micro-RNAs (miRNAs). Recent evidence has demonstrated that 5' capped and 3' polyadenylated ends are not restricted to mRNAs, but that they are also present in precursors of both miRNAs and some antisense RNA transcripts. In addition, as many as 40% of transcribed RNAs may lack 3' poly(A) ends. In concert with the presence of a 5' cap (m7 GpppN), the length of the 3' poly(A) end plays a critical role in determining the translational efficiency, stability, and the cellular distribution of a specific mRNA. RNAs with short or lacking 3' poly(A) ends, that escape isolation and amplification with oligo(dT)-based methods, provide a challenge in RNA biology and gene expression studies. To circumvent the limitations of 3' poly(A)-dependent RNA isolation methods, we developed an efficient RNA purification system that binds the 5' cap of RNA with a high-affinity variant of the cap-binding protein eIF4E. This system can be used in differential selection approaches to isolate subsets of RNAs, including those with short 3' poly(A) ends that are likely targets of post-transcriptional regulation of gene expression. The length of the 3' poly(A) ends can be defined using a rapid polymerase chain reaction (PCR)- based approach.

ERK is a novel regulatory kinase for poly(A) polymerase

Poly(A) polymerase (PAP), which adds poly(A) tails to the 3′ end of mRNA, can be phosphorylated at several sites in the C-terminal domain. Phosphorylation often mediates regulation by extracellular stimuli, suggesting PAP may be regulated by such stimuli. In this study, we found that phosphorylation of PAP was increased upon growth stimulation and that the mitogen-activated protein kinase ERK was responsible for the increase in phosphorylation. We identified serine 537 of PAP as a unique phosphorylation site by ERK. PAP phosphorylation of serine 537 by ERK increased its nonspecific polyadenylation activity in vitro. This PAP activity was also activated by stimulation of ERK with phorbol-12-myristate-13-acetate in vivo. These data suggest that ERK is a novel regulatory kinase for PAP and further, that PAP activity could be regulated by extracellular stimuli through an ERK-dependent signaling pathway(s).

Serotonin transport and serotonin transporter‐mediated antidepressant recognition are controlled by 5‐HT2Breceptor signaling in serotonergic neuronal cells

The plasma membrane 5-HT transporter (SERT) is the major protagonist in regulating extracellular 5-HT concentration and constitutes the target of drugs used to treat a host of metabolic and psychiatric disorders. The exact mechanisms sustaining SERT function still remain elusive. The present work exploits the properties of the 1C11 neuroectodermal progenitor, which acquires, upon 4 days of differentiation, a functional SERT within an integrated serotonergic phenotype to investigate regulatory mechanisms involved in SERT onset and functions. We show that poly(A) addition precedes SERT mRNA translation on day 2 of the serotonergic program. The newly translated transporter molecules immediately bind cocaine. Day 4 must be awaited to monitor antidepressant recognition and 5-HT uptake. Because external 5-HT reduces both 5-HT transport and SERT antidepressant binding, we identify 5-HT(2B) receptors as key players in controlling the overall 5-HT transport system. In the absence of external 5-HT, 5-HT(2B) receptor coupling to NO production ensures SERT phosphorylation to basal level and maximal 5-HT uptake. In the presence of 5-HT, the 5-HT(2B) receptor-PKC coupling promotes additional phosphorylations of both SERT and Na(+),K(+)-ATPase alpha-subunit, impairing the electrochemical gradient necessary to 5-HT uptake. SERT hyperphosphorylation also affects antidepressant recognition. Finally, such 5-HT(2B) receptor-mediated control of SERT activity operates in primary neurons from raphe nuclei. Altogether, our data shed new light on the 5-HT-driven post-translational modifications involved in the control of SERT activity.

Analysis of polysomal mRNA populations of mouse oocytes and zygotes: dynamic changes in maternal mRNA utilization and function

Transcriptional activation in mammalian embryos occurs in a stepwise manner. In mice, it begins at the late one-cell stage, followed by a minor wave of activation at the early two-cell stage, and then the major genome activation event (MGA) at the late two-cell stage. Cellular homeostasis, metabolism, cell cycle, and developmental events are orchestrated before MGA by time-dependent changes in the array of maternal transcripts being translated. Many elegant studies have documented the importance of maternal mRNA (MmRNA) and its correct recruitment for development. Many other studies have illuminated some of the molecular mechanisms regulating MmRNA utilization. However, neither the complete array of recruited mRNAs nor the regulatory mechanisms responsible for temporally different patterns of recruitment have been well characterized. We present a comprehensive analysis of changes in the maternal component of the zygotic polysomal mRNA population during the transition from oocyte to late one-cell stage embryo. We observe global transitions in the functional classes of translated MmRNAs and apparent changes in the underlying cis-regulatory mechanisms.

Over-expression of Aurora-A targets cytoplasmic polyadenylation element binding protein and promotes mRNA polyadenylation of Cdk1 and cyclin B1

Aurora-A is a centrosomal serine-threonine kinase that regulates mitosis. Over-expression of Aurora-A has been found in a wide range of tumors and has been implicated in oncogenic transformation. However, how Aurora-A over-expression contributes to promotion of carcinogenesis remains elusive. Immunohistochemical analysis of breast tumors revealed that over-expressed Aurora-A is not restricted to the centrosomes but is also found in the cytoplasm. This over-expressed Aurora-A appeared to be phosphorylated on Thr288, which is known to be required for its enzymatic activation. In analogy to Aurora-A's role in oocyte maturation and the early embryonic cell cycle, here we investigated whether ectopically over-expressed Aurora-A can similarly stimulate polyadenylation of mRNA in human somatic cultured cells by interacting with a human ortholog of cytoplasmic polyadenylation element binding protein, h-CPEB. In vitro experiments revealed that Aurora-A binds directly to, and phosphorylates, h-CPEB. We found that polyadenylation of mRNA tails of cyclin B1 and Cdk1 was synergistically stimulated when Aurora-A and h-CPEB were over-expressed, and they were further promoted in the presence of an Aurora-A activator Ajuba. Our results suggest a function of ectopically over-expressed Aurora-A that might be relevant for carcinogenesis.

RNA-binding proteins in early development

RNA-binding proteins play a major part in the control of gene expression during early development. At this stage, the majority of regulation occurs at the levels of translation and RNA localization. These processes are, in general, mediated by RNA-binding proteins interacting with specific sequence motifs in the 3'-untranslated regions of their target RNAs. Although initial work concentrated on the analysis of these sequences and their trans-acting factors, we are now beginning to gain an understanding of the mechanisms by which some of these proteins function. In this review, we will describe a number of different families of RNA-binding proteins, grouping them together on the basis of common regulatory strategies, and emphasizing the recurrent themes that occur, both across different species and as a response to different biological problems.

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

Metazoans rely on the regulated translation of select maternal mRNAs to control oocyte maturation and the initial stages of embryogenesis. These transcripts usually remain silent until their translation is temporally and spatially required during early development. Different translational regulatory mechanisms, varying from cytoplasmic polyadenylation to localization of maternal mRNAs, have evolved to assure coordinated initiation of development. A common feature of these mechanisms is that they share a few key trans-acting factors. Increasing evidence suggest that ubiquitous conserved mRNA-binding factors, including the eukaryotic translation initiation factor 4E (eIF4E) and the cytoplasmic polyadenylation element binding protein (CPEB), interact with cell-specific molecules to accomplish the correct level of translational activity necessary for normal development. Here we review how capping and polyadenylation of mRNAs modulate interaction with multiple regulatory factors, thus controlling translation during oogenesis and early development.

Rapid, activity-induced increase in tissue plasminogen activator is mediated by metabotropic glutamate receptor-dependent mRNA translation

Long-term synaptic plasticity is both protein synthesis-dependent and synapse-specific. Therefore, the identity of the newly synthesized proteins, their localization, and mechanism of regulation are critical to our understanding of this process. Tissue plasminogen activator (tPA) is a secreted protease required for some forms of long-term synaptic plasticity. Here, we show tPA activity is rapidly increased in hippocampal neurons after glutamate stimulation. This increase in tPA activity corresponds to an increase in tPA protein synthesis that results from the translational activation of mRNA present at the time of stimulation. Furthermore, the mRNA encoding tPA is present in dendrites and is rapidly polyadenylated after glutamate stimulation. Both the polyadenylation of tPA mRNA and the subsequent increase in tPA protein is dependent on metabotropic glutamate receptor (mGluR) activation. A similar mGluR-dependent increase in tPA activity was detected after stimulation of a synaptic fraction isolated from the hippocampus, suggesting tPA synthesis is occurring in the synaptodendritic region. Finally, we demonstrate that tPA mRNA is bound by the mRNA-binding protein CPEB (cytoplasmic polyadenylation element binding protein-1), a protein known to regulate mRNA translation via polyadenylation. These results indicate that neurons are capable of synthesizing a secreted protein in the synaptic region, that mGluR activation induces mRNA polyadenylation and translation of specific mRNA, and suggest a model for synaptic plasticity whereby translational regulation of an immediate early gene precedes the increase in gene transcription.

Expression of Cyclin B1 Messenger RNA Isoforms and Initiation of Cytoplasmic Polyadenylation in the Bovine Oocyte1

Oocytes can synthesize and store maternal mRNA in an inactive translational state until the start of in vitro maturation. Cytoplasmic polyadenylation, driven by 3'-untranslated region (UTR) cis-acting cytoplasmic polyadenylation element (CPE), is associated with translational activation of cyclin B1 mRNA during maturation. The main aim of the present study was to investigate if bovine oocyte cyclin B1 mRNA undergoes cytoplasmic polyadenylation/translation during in vitro maturation, as in other species. We have found that cyclin B1 mRNA is present in two isoforms, consisting of the same open reading frame but with different 3'-UTR lengths. Only the longest isoform (cyclin B1L) has a putative CPE sequence and other regulatory sequences, and its mRNA level decreases during early embryo development. The polyadenylation state of cyclin B1L during in vitro maturation was studied. Results demonstrated that cyclin B1L bears a relatively long poly(A) tail in germinal vesicle-stage oocytes, which is further lengthened at 10 h of maturation, before metaphase I. Interestingly, cyclin B1L bears a short poly(A) tail when the ovaries and the oocytes are transported and manipulated on ice to stop the polyadenylation process. Cytoplasmic polyadenylation most probably occurs during ovary transport in warm saline, when oocytes are still in their follicular environment. Our results also show a link between cytoplasmic polyadenylation of cyclin B1 and translation/appearance of cyclin B1 protein before in vitro maturation.

Posttranscriptional regulation of the immediate-early gene EGR1 by light in the mouse retina

Synaptic plasticity is modulated by differential regulation of transcription factors such as EGR1 which binds to DNA via a zinc finger binding domain. Inactivation of EGR1 has implicated this gene as a key regulator of memory formation and learning. However, it remains puzzling how synaptic input can lead to an up-regulation of the EGR-1 protein within only a few minutes. Here, we show by immunohistochemical staining that the EGR-1 protein is localized in synapses throughout the mouse retina. We demonstrate for the first time that two variants of Egr-1 mRNA are produced in the retina by alternative polyadenylation, with the longer version having an additional 293 base pairs at the end of the 3'UTR. Remarkably, the use of the alternative polyadenylation site is controlled by light. The additional 3'UTR sequence of the longer variant displays an even higher level of phylogenetic conservation than the coding region of this highly conserved gene. Additionally, it harbours a cytoplasmic polyadenylation element which is known to respond to NMDA receptor activation. The longer version of the Egr-1 mRNA could therefore rapidly respond to excitatory stimuli such as light or glutamate release whereas the short variant, which is predominantly expressed and contains the full coding sequence, lacks the regulatory elements for cytoplasmic polyadenylation in its 3'UTR.

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.

Developmental expression of 2489 gene clusters during pig embryogenesis: an expressed sequence tag project

Identification of mRNAs that are present at early stages of embryogenesis is critical for a better understanding of development. To this end, cDNA libraries were constructed from germinal vesicle-stage oocytes, in vivo-produced four-cell- and blastocyst-stage embryos, and from in vitro-produced four-cell- and blastocyst-stage embryos. Randomly picked clones (10 848) were sequenced from the 3' end and those of sufficient quality (8066, 74%) were clustered into groups of sequence similarity (>95% identity), resulting in 2489 clusters. The sequence of the longest representative expressed sequence tag (EST) of each cluster was compared with GenBank and TIGR. Scores below 200 were considered unique, and 1114 (44.8%) did not have a match in either database. Sequencing from the 5' end yielded 12 of 37 useful annotations, suggesting that one third of the 1114 might be identifiable, still leaving over 700 unique ESTs. Virtual Northerns compared between the stages identified numerous genes where expression appears to change from the germinal vesicle oocyte to the four-cell stage, from the four-cell to blastocyst stage, and between in vitro- and in vivo-derived four-cell- and blastocyst-stage embryos. This is the first large-scale sequencing project on early pig embryogenesis and has resulted in the discovery of a large number of genes as well as possible stage-specific expression. Because many of these ESTs appear to not be in the public databases, their addition will be useful for transcriptional profiling experiments conducted on early pig embryos.

Cytoplasmic polyadenylation element (CPE)- and CPE-binding protein (CPEB)-independent mechanisms regulate early class maternal mRNA translational activation in Xenopus oocytes

Meiotic cell cycle progression during vertebrate oocyte maturation requires the correct temporal translation of maternal mRNAs encoding key regulatory proteins. The mechanism by which specific mRNAs are temporally activated is unknown, although both cytoplasmic polyadenylation elements (CPE) within the 3'-untranslated region (3'-UTR) of mRNAs and the CPE-binding protein (CPEB) have been implicated. We report that in progesterone-stimulated Xenopus oocytes, the early cytoplasmic polyadenylation and translational activation of multiple maternal mRNAs occur in a CPE- and CPEB-independent manner. We demonstrate that polyadenylation response elements, originally identified in the 3'-UTR of the mRNA encoding the Mos proto-oncogene, direct CPE- and CPEB-independent polyadenylation of an early class of Xenopus maternal mRNAs. Our findings refute the hypothesis that CPE sequences alone account for the range of temporal inductions of maternal mRNAs observed during Xenopus oocyte maturation. Rather, our data indicate that the sequential action of distinct 3'-UTR-directed translational control mechanisms coordinates the complex temporal patterns and extent of protein synthesis during vertebrate meiotic cell cycle progression.

Molecular control of the oocyte to embryo transition

The elucidation of the molecular control of the initiation of mammalian embryogenesis is possible now that the transcriptomes of the full-grown oocyte and two-cell stage embryo have been prepared and analysed. Functional annotation of the transcriptomes using gene ontology vocabularies, allows comparison of the oocyte and two-cell stage embryo between themselves, and with all known mouse genes in the Mouse Genome Database. Using this methodology one can outline the general distinguishing features of the oocyte and the two-cell stage embryo. This, when combined with oocyte-specific targeted deletion of genes, allows us to dissect the molecular networks at play as the differentiated oocyte and sperm transit into blastomeres with unlimited developmental potential.

Purifying mRNAs with a high-affinity eIF4E mutant identifies the short 3' poly(A) end phenotype

The use of DNA microarrays has revolutionized the manner in which mRNA populations are analyzed. One limitation of the current technology is that mRNAs are often purified on the basis of their 3' poly(A) ends, which can be extremely short or absent in some mRNAs. To circumvent this limitation, we have developed a procedure for the purification of eukaryotic mRNAs using a mutant version of the mRNA 5' cap-binding protein (eIF4E) with increased affinity for the m7GTP moiety of the cap. By using this procedure, we have compared the populations of mammalian mRNAs purified by oligo(dT) and 5' cap selection with oligonucleotide microarrays. This analysis has identified a subpopulation of mRNAs that are present with short 3' poly(A) ends at steady state and are missed or underrepresented after purification by oligo(dT). These mRNAs may respond to specific posttranscriptional control mechanisms such as cytoplasmic polyadenylation.

Mouse oocytes and early embryos express multiple histone H1 subtypes

Oocytes and embryos of many species, including mammals, contain a unique linker (H1) histone, termed H1oo in mammals. It is uncertain, however, whether other H1 histones also contribute to the linker histone complement of these cells. Using immunofluorescence and radiolabeling, we have examined whether histone H10, which frequently accumulates in the chromatin of nondividing cells, and the somatic subtypes of H1 are present in mouse oocytes and early embryos. We report that oocytes and embryos contain mRNA encoding H10. A polymerase chain reaction-based test indicated that the poly(A) tail did not lengthen during meiotic maturation, although it did so beginning at the four-cell stage. Antibodies raised against histone H10 stained the nucleus of wild-type prophase-arrested oocytes but not of mice lacking the H10 gene. Following fertilization, H10 was detected in the nuclei of two-cell embryos and less strongly at the four-cell stage. No signal was detected in H10 -/- embryos. Radiolabeling revealed that species comigrating with the somatic H1 subtypes H1a and H1c were synthesized in maturing oocytes and in one- and two-cell embryos. Beginning at the four-cell stage in both wild-type and H10 -/- embryos, species comigrating with subtypes H1b, H1d, and H1e were additionally synthesized. These results establish that histone H10 constitutes a portion of the linker histone complement in oocytes and early embryos and that changes in the pattern of somatic H1 synthesis occur during early embryonic development. Taken together with previous results, these findings suggest that multiple H1 subtypes are present on oocyte chromatin and that following fertilization changes in the histone H1 complement accompany the establishment of regulated embryonic gene expression.

A novel regulatory element determines the timing of Mos mRNA translation during Xenopus oocyte maturation

Progression through vertebrate oocyte maturation requires that pre-existing, maternally derived mRNAs be translated in a strict temporal order. The mechanism that controls the timing of oocyte mRNA translation is unknown. In this study we show that the early translational induction of the mRNA encoding the Mos proto-oncogene is mediated through a novel regulatory element within the 3' untranslated region of the Mos mRNA. This novel element is responsive to the MAP kinase signaling pathway and is distinct from the late acting, cdc2-responsive, cytoplasmic polyadenylation element. Our findings suggest that the timing of maternal mRNA translation is controlled through signal transduction pathways targeting distinct 3' UTR mRNA elements.

Posttranscriptional regulation of cyclin A1 and cyclin A2 during mouse oocyte meiotic maturation and preimplantation development

A shift from a meiotic cell cycle to a mitotic cell cycle occurs following fertilization. The molecular basis for this transition, however, is poorly understood. Although cyclin A1 is proposed to regulate M phase in the meiotic cell cycle, and cyclin A2 is proposed to regulate S and M phases in the mitotic cell cycle, little is known about changes in the expression levels of cyclin A1 and A2 during meiotic and mitotic cell cycles in mammalian oocytes. We report that the mRNA levels of both cyclins A1 and A2 decrease during oocyte maturation. The amount of cyclin A1 mRNA then increases between the one-cell and blastocyst stages, whereas that of cyclin A2 remains relatively constant. The amount of cyclin A1 protein declines during maturation and is not readily detected from the two-cell to the blastocyst stage. In contrast, cyclin A2 is not readily detected in the oocyte and metaphase II-arrested egg but is detected following fertilization and throughout the subsequent stages of preimplantation development. The appearance of cyclin A2 protein following fertilization positively correlates with an increase in the size of the mRNA. This increase, as well as the increase in the amount of cyclin A2 protein, is prevented by 3'-deoxyadenosine (3'-dA), an inhibitor of polyadenylation. Consistent with a role for cyclin A2 in regulating the G1/S transition, 3'-dA also inhibits DNA replication in treated one-cell embryos. These results suggest that regulation of expression of cyclins A1 and A2 is under posttranscriptional regulation and that the observed changes in their expression may be involved in the transformation of a meiotic cell cycle to a mitotic cell cycle following fertilization.

Photoreceptor regulated expression of Ca(2+)/calmodulin-dependent protein kinase II in the mouse retina

The objective of this investigation is to determine mechanisms for regulation of retinal calmodulin kinase II (CaMKII). To this end, the expression and activity of CaMKII are examined in the retina of the rdta mouse, in which rod photoreceptors have been genetically ablated [47]. CaMKII levels are compared between rdta mice and the normal, littermate control mice. It is demonstrated that retinal CaMKII protein, enzyme activity and mRNA are significantly increased in response to the genetic ablation of rod photoreceptors. The data indicate that CaMKII expression/activity in amacrine and ganglion cells is negatively regulated by the rod photoreceptor-mediated visual input. The regulation appears to occur primarily at the transcriptional level. It is shown that the cytoplasmic polyadenylation element binding protein (CPEB), a regulatory factor for translation that is known to promote CaMKIIalpha translation in dendrites [83], is also present in the mouse retina. However, the polyadenylation-mediated translational control mechanism is not activated in this experimental paradigm.

CPEB proteins control two key steps in spermatogenesis in C. elegans

Cytoplasmic polyadenylation element binding (CPEB) proteins bind to and regulate the translation of specific mRNAs. CPEBs from Xenopus, Drosophila, and Spisula participate in oogenesis. In this report, we examine the biological roles of all identifiable CPEB homologs in a single organism, Caenorhabditis elegans. We find four homologs in the C. elegans genome: cbp-1, cpb-2, cpb-3, and fog-1. Surprisingly, two homologs, CPB-1 and FOG-1, have key functions in spermatogenesis and are dispensable for oogenesis. CPB-2 and CPB-3 also appear not to be required for oogenesis. CPB-1 is essential for progression through meiosis: cpb-1(RNAi) spermatocytes fail to undergo the meiotic cell divisions. CPB-1 protein is present in the germ line just prior to overt spermatogenesis; once sperm differentiation begins, CPB-1 disappears. CPB-1 physically interacts with FBF, another RNA-binding protein and 3' UTR regulator. In addition to its role in controlling the sperm/oocyte switch, we find that FBF also appears to be required for spermatogenesis, consistent with its interaction with CPEB. A second CPEB homolog, FOG-1, is required for specification of the sperm fate. The fog-1 gene produces fog-1(L) and fog-1(S) transcripts. The fog-1(L) RNA is enriched in animals making sperm and is predicted to encode a larger protein; fog-1(S) RNA is enriched in animals making oocytes and is predicted to encode a smaller protein. The relative abundance of the two mRNAs is controlled temporally during germ-line development and by the sex determination pathway in a fashion that suggests that the fog-1(L) species encodes the active form. In sum, our results demonstrate that, in C. elegans, two CPEB proteins have distinct functions in the germ line, both in spermatogenesis: FOG-1 specifies the sperm cell fate and CPB-1 executes that decision.

A frequent TG deletion near the polyadenylation signal of the human HEXB gene: occurrence of an irregular DNA structure and conserved nucleotide sequence motif in the 3' untranslated region

While screening for new mutations in the HEXB gene, which encodes the beta-subunit of beta-hexosaminidase, a TG deletion (deltaTG) was found in the 3' untranslated region (3'UTR) of the gene, 7 bp upstream from the polyadenylation signal. Examination of DNA samples of 145 unrelated Argentinean individuals from different racial backgrounds showed that the deltaTG allele was present with a frequency of approximately 0.1, compared with the wild-type (WT) allele. The deletion was not associated with infantile or variant forms of Sandhoff disease when present in combination with a deleterious allele. Total Hex and Hex B enzymatic activities measured in individuals heterozygous for deltaTG and a null allele, IVS-2 + 1G-->A (G-->A), were approximately 30% lower than the activities of G-->A/WT individuals. Analysis of the HEXB mRNA from leukocytes of deltaTG/WT individuals by RT-PCR of the 3'UTR showed that the deltaTG allele is present at lower level than the WT allele. By polyacrylamide gel electrophoresis, it was determined that a PCR fragment containing the +TG version of the 3'UTR of the HEXB gene had an irregular structure. On inspection of genes containing a TG dinucleotide upstream from the polyadenylation signal we found that this dinucleotide was part of a conserved sequence (TGTTTT) immersed in a A/T-rich region. This sequence arrangement was present in more than 40% analyzed eukaryotic mRNAs, including in the human, mouse and cat HEXB genes. The significance of the TG deletion in reference to Sandhoff disease as well as the possible functional role of the consensus sequence and the DNA structure of the 3'UTR are considered.

RNA-binding proteins in mouse oocytes and embryos: expression of genes encoding Y box, DEAD box RNA helicase, and polyA binding proteins

Growth and differentiation of early embryos depends almost entirely on information which is maternally inherited in the form of macromolecules accumulated by the female gamete during its growth phase. Most of the maternal mRNAs synthesized by growing oocytes are not immediately recruited onto polysomes but are stored as translationally dormant messenger ribonucleoprotein (mRNP) particles. mRNA binding proteins which have been associated with masked mRNP complexes in Xenopus oocytes fall into two main categories, those having affinity for a variety of RNA sequences (members of the Y box and DEAD box RNA helicase families) and those which interact more specifically with 3' polyA tails (the polyA binding proteins or PABPs). The objective of this study was to determine whether mouse oocytes and embryos express sequences encoding a Y box protein, (MSY1); on RNA helicase, (RCK/p54); and a universally expressed PABP and testis specific isoform (PABP1 and PABPt, respectively). RNAs were amplified by RT/PCR and the identities of targeted cDNAs were confirmed by restriction analysis and/or direct sequencing. Relative steady state levels and time courses of accumulation/decay were compared by Northern hybridization. All of the sequences are transcribed as maternal mRNAs. MSY1 transcripts accumulated during the growth phase appear to be degraded in parallel with the bulk of maternal mRNAs by the mid-late two-cell stage. RCK/p54 mRNAs are most abundant in growing oocytes; steady state levels decline in primary and secondary oocytes, and degradation appears to be complete by the mid-late two-cell stage. Zygotic transcription of MSY1 and RCK/p54 is evident in four-cell stage embryos. Most of the PABP1 message accumulated by growing oocytes decays during meiotic maturation with transcription resuming in two-cell embryos. PABPt is expressed at very low levels in oocytes and embryos. Based on the temporal patterns of expression and the reported activities of homologous sequences in other systems, we suggest that these RNA binding proteins may participate in the post-transcriptional regulation of gene expression during the period of maternal control of development in the mouse.

Effects of cyclin A2 noncoding regions on reporter gene translation during early development of Xenopus laevis

The repression of translation of Xenopus cyclin A2 transcripts during early development was examined by analyzing the effects of cyclin A2 noncoding regions using a CAT reporter system. On their own, the 5' and 3' UTRs (untranslated regions) were unable to inhibit reporter translation until approximately the time of the midblastula transition. Transcripts containing the 3' UTR were polyadenylated after fertilization and the midblastula transition. When both noncoding regions flanked a CAT reporter gene, translation was repressed at all stages of development examined in spite of their polyadenylation after fertilization. From these data, we conclude that the 5' and 3' UTRs interact synergically to prevent translation during early development and that the poly(A) tail is insufficient to promote their translation.

Differential regulation of the translation and the stability of two maternal transcripts in preimplantation rabbit embryos

In most species, transcription is essentially silent during the first mitotic cell cycles that follow fertilization. This means that the regulation of gene expression in early embryos heavily relies on the translational activation or inactivation of maternal mRNAs. In mammals, the mechanisms that control the translation of maternal mRNAs have been mainly studied in the mouse when maternal to zygotic transition occurs after the first mitotic division. In other mammalian species, however, this transition occurs later after several cell cycles, and little is known concerning the regulation of maternal information during this period. To address this question, we have used rabbit pre-implantation embryos to analyze the translational activation and stability of two maternal mRNAs, mm 41 and mm61. During the cleavage period, these mRNAs exhibit distinct kinetics for both their translational activation and degradation. In addition, these mRNAs both undergo cytoplasmic polyadenylation but with different efficiencies. This polyadenylation was functionally correlated with the translational activation of these mRNAs; inhibiting polyadenylation prevented translational activation. The differential efficiency of cytoplasmic polyadenylation, driven by cis-elements in the 3' untranslated region of these mRNAs, was also observed in Xenopus laevis embryos, which emphasizes the high conservation of this mechanism between species.

Caenorhabditis elegans homologue of the human azoospermia factor DAZ is required for oogenesis but not for spermatogenesis

DAZ (Deleted in Azoospermia), the putative azoospermia factor gene in human, encodes a ribonucleoprotein-type RNA-binding protein required for spermatogenesis. A Drosophila homologue of DAZ, called boule, is also essential for spermatogenesis. A mouse homologue, Dazla, is implicated in both spermatogenesis and oogenesis. Here, we report the identification and characterization of daz-1, the single DAZ homologue in the nematode Caenorhabditis elegans. Loss of daz-1 function caused sterility in hermaphrodites, by blocking oogenesis at the pachytene stage of meiosis I. Epistasis analysis suggested that this gene executes its function succeeding gld-1, which governs the early pachytene stage in the oogenic pathway. Spermatogenesis did not appear to be affected in daz-1 hermaphrodites. Males defective in daz-1 produced sperm fully competent in fertilization. Analysis employing sex-determination mutants indicated that the daz-1 function was required for meiosis of female germline regardless of the sex of the soma. Transcription of daz-1 was restricted to the germline, starting prior to the onset of meiosis and was most conspicuous in cells undergoing oogenesis. Thus, daz-1 in C. elegans is an essential factor for female meiosis but, unlike other DAZ family members so far reported, it is dispensable for male meiosis.

Translational regulation of cyclin B mRNA by 17alpha,20beta-dihydroxy-4-pregnen-3-one (maturation-inducing hormone) during oocyte maturation in a teleost fish, the goldfish (Carassius auratus)

17Alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-DP) was identified as maturation-inducing hormone (MIH) in several teleost fishes. In goldfish (Carassius auratus), 17alpha,20beta-DP induces oocyte maturation by stimulating the de novo synthesis of cyclin B, a regulatory subunit of maturation-promoting factor (MPF). In this study, we examined the control mechanisms of 17alpha,20beta-DP-induced de novo synthesis of cyclin B protein in oocytes, which is a prerequisite step for MPF activation during oocyte maturation in goldfish. Cycloheximide-treated oocytes failed to undergo meiotic maturation in response to 17alpha,20beta-DP; in this group neither cyclin B nor 34-kDa active cdc2 was detectable in oocytes. In contrast, oocytes exposed to actinomycin D plus 17alpha,20beta-DP or 17alpha,20beta-DP underwent maturation; in these groups both cyclin B and 34-kDa cdc2 were present. Northern blotting showed that cyclin B mRNA is present in both immature and mature oocytes. Sequence analysis revealed that goldfish cyclin B mRNA contains four copies of cytoplasmic polyadenylation element (CPE)-like motifs in the 3' noncoding region, suggesting that the initiation of cyclin B synthesis during oocyte maturation may be controlled by the elongation of poly (A) tail. We then examined the polyadenylation state of cyclin B mRNA during 17alpha,20beta-DP-induced oocyte maturation by means of a PCR poly (A) test, and found that cyclin B mRNA is polyadenylated during oocyte maturation. Polyadenylation of cyclin B mRNA occurred at the same time of germinal vesicle breakdown. Furthermore, cordycepin, an inhibitor of poly (A) addition of mRNA, prevented 17alpha,20beta-DP-induced oocyte maturation. These findings suggest that in goldfish oocytes, the synthesis of cyclin B protein is under translational control and that cytoplasmic 3' poly(A) elongation is involved in 17alpha,20beta-DP-induced translation of cyclin B mRNA.

LIS1 and platelet-activating factor acetylhydrolase (Ib) catalytic subunits, expression in the mouse oocyte and zygote

Platelet-activating factor is a phospholipid with several documented roles in the pre-implantation embryo. Enzymes that belong to the platelet-activating factor acetylhydrolases family inactivate platelet-activating factor. Cytosolic platelet-activating factor acetylhydrolase (Ib) is a heterotetramer composed of two catalytic subunits (alpha1/alpha2) and two regulatory LIS1 subunits. The expression of these components was monitored in the mouse oocytes and zygotes using reverse-transcribed PCR and Western blot analysis. Interestingly, these proteins are expressed in the oocyte and zygote and their expression increases after fertilization, probably due to stabilization of maternal RNA. Lis1 mRNA transcription also increases after fertilization. However, assaying for expression of a specific paternal LIS1 isoform detected no zygotic translation in the one cell stage. These findings suggest a potential role for platelet-activating factor acetylhydrolase (Ib) components in the early mouse embryo.

Assaying the polyadenylation state of mRNAs

The poly(A) tail present at the 3' end of most eukaryotic mRNAs can play a critical role in message translation and stability. Therefore, identifying alterations in poly(A) tail length can yield important insights into an mRNA's function and subsequent physiological impact. Here, we present three methods for assaying polyadenylation of a specific mRNA in the context of total cellular RNA. The first method described, oligo(dT)/RNase H-Northern analysis, is the classic labor-intensive assay for polyadenylation and is included for historical reference and as a potential experimental control for the poly(A) test (PAT) assays described subsequently. The PAT methods-rapid amplification of cDNA ends-PAT (RACE-PAT), and ligase-mediated PAT (LM-PAT)-are polymerase chain reaction-driven assays that allow speed, sensitivity, and length quantitation. The PAT assays can be conducted in a single day and can readily detect the poly(A) status of an mRNA present in subnanogram quantities of total cellular RNA.

CPEB-mediated cytoplasmic polyadenylation and the regulation of experience-dependent translation of alpha-CaMKII mRNA at synapses

Long-term changes in synaptic efficacy may require the regulated translation of dendritic mRNAs. While the basis of such regulation is unknown, it seemed possible that some features of translational control in development could be recapitulated in neurons. Polyadenylation-induced translation of oocyte mRNAs requires the cis-acting CPE sequence and the CPE-binding protein CPEB. CPEB is also present in the dendritic layers of the hippocampus, at synapses in cultured neurons, and in postsynaptic densities of adult brain. alpha-CaMKII mRNA, which is localized in dendrites and is necessary for synaptic plasticity and LTP, contains two CPEs. These CPEs are bound by CPEB and mediate polyadenylation-induced translation in injected Xenopus oocytes. In the intact brain, visual experience induces alpha-CaMKII mRNA polyadenylation and translation, suggesting that this process likely occurs at synapses.

Modifications of the 5' cap of mRNAs during Xenopus oocyte maturation: independence from changes in poly(A) length and impact on translation

The translation of specific maternal mRNAs is regulated during early development. For some mRNAs, an increase in translational activity is correlated with cytoplasmic extension of their poly(A) tails; for others, translational inactivation is correlated with removal of their poly(A) tails. Recent results in several systems suggest that events at the 3' end of the mRNA can affect the state of the 5' cap structure, m7G(5')ppp(5')G. We focus here on the potential role of cap modifications on translation during early development and on the question of whether any such modifications are dependent on cytoplasmic poly(A) addition or removal. To do so, we injected synthetic RNAs into Xenopus oocytes and examined their cap structures and translational activities during meiotic maturation. We draw four main conclusions. First, the activity of a cytoplasmic guanine-7-methyltransferase increases during oocyte maturation and stimulates translation of an injected mRNA bearing a nonmethylated GpppG cap. The importance of the cap for translation in oocytes is corroborated by the sensitivity of protein synthesis to cap analogs and by the inefficient translation of mRNAs bearing nonphysiologically capped 5' termini. Second, deadenylation during oocyte maturation does not cause decapping, in contrast to deadenylation-triggered decapping in Saccharomyces cerevisiae. Third, the poly(A) tail and the N-7 methyl group of the cap stimulate translation synergistically during oocyte maturation. Fourth, cap ribose methylation of certain mRNAs is very inefficient and is not required for their translational recruitment by poly(A). These results demonstrate that polyadenylation can cause translational recruitment independent of ribose methylation. We propose that polyadenylation enhances translation through at least two mechanisms that are distinguished by their dependence on ribose modification.

Gene expression of mouse M1 and M2 pyruvate kinase isoenzymes correlates with differential poly[A] tract extension of their mRNAs during the development of spermatogenesis

In eukaryotes, different isoenzymes for pyruvate kinase have been characterized. M2-type Pk cDNA from a mouse fetal ovary library was isolated and differential expression for M1 and M2-types during testis development was observed. While the presence of M2 mRNAs decreases throughout the development of spermatogenesis, we deduced that M1 type expression increases in adult testis coinciding with the presence of elongating spermatids in the seminiferous epithelium. Polyadenylation tests showed a concurrent increase in the length of the polyadenylation tail of transcribed M1-type pyruvate kinase mRNAs in prepuberal to adult seminiferous tubules. A similar relationship between poly[A] tail extension and differential increase of gene expression was detected for M1-type mRNA in adult brain and muscle. Length of poly[A] tail of M2-type transcripts is shown to decrease during the development of mouse testis. These results suggest that changes in the length of the poly[A] tail of transcripts are associated with differential expression of both regulated isoenzymes during testicular development.

GLUT-1 expression in the cerebra of patients with Alzheimer's disease

To investigate the molecular basis of reduced GLUT-1 concentration of the blood-brain barrier in patients with Alzheimer's disease (AD), the GLUT-1 mass, mRNA content, and structure were studied in eight patients with AD and seven age-matched controls. The results indicate that the 55-kDa GLUT-1 is significantly reduced in AD without a significant change in GLUT-1 mRNA concentrations. Because in some animal models changes in GLUT-1 expression is associated with changes in GLUT-1 mRNA structure, the length of the poly(A) tail of the GLUT-1 mRNA was estimated with a reverse transcription-polymerase chain reaction technique. The length of poly(A) tail of GLUT-1 mRNA in AD subjects was not significantly different from the controls. It is concluded that the AD-related change in GLUT-1 expression is not the result of altered poly(A) length of GLUT-1 mRNA.

A dependent pathway of cytoplasmic polyadenylation reactions linked to cell cycle control by c-mos and CDK1 activation

During oocyte maturation and early development, mRNAs receive poly(A) in the cytoplasm at distinct times relative to one another and to the cell cycle. These cytoplasmic polyadenylation reactions do not occur during oogenesis, but begin during oocyte maturation and continue throughout early development. In this report, we focus on the link between cytoplasmic polyadenylation and control of the cell cycle during meiotic maturation. Activation of maturation promoting factor, a complex of CDK1 and cyclin, is required for maturation and dependent on c-mos protein kinase. We demonstrate here that two classes of polyadenylation exist during oocyte maturation, defined by their dependence of c-mos and CDK1 protein kinases. Polyadenylation of the first class of mRNAs (class I) is independent of c-mos and CDK1 kinase activities, whereas polyadenylation of the second class (class II) requires both of these activities. Class I polyadenylation, through its effects on c-mos mRNA, is required for class II polyadenylation. cis-acting elements responsible for this distinction reside in the 3'-untranslated region, upstream of the polyadenylation signal AAUAAA. Cytoplasmic polyadenylation elements (CPEs) are sufficient to specify class I polyadenylation, and subtle changes in the CPE can substantially, though not entirely, shift an RNA from class I to class II. Activation of class I polyadenylation events is independent of hyperphosphorylation of CPE-binding protein or poly(A) polymerase, and requires cellular protein synthesis. The two classes of polyadenylation and of mRNA define a dependent pathway, in which polyadenylation of certain mRNAs requires the prior polyadenylation of another. We propose that this provides one method of regulating the temporal order of polyadenylation events, and links polyadenylation to the control of the meiotic cell cycle.