Poly(A) polymerase and the regulation of cytoplasmic polyadenylation

Translational activation in oocytes and embryos is often regulated via increases in poly(A) length. Cleavage and polyadenylation specificity factor (CPSF), cytoplasmic polyadenylation element binding protein (CPEB), and poly(A) polymerase (PAP) have each been implicated in cytoplasmic polyadenylation in Xenopus laevis oocytes. Cytoplasmic polyadenylation activity first appears in vertebrate oocytes during meiotic maturation. Data presented here shows that complexes containing both CPSF and CPEB are present in extracts of X. laevis oocytes prepared before or after meiotic maturation. Assessment of a variety of RNA sequences as polyadenylation substrates indicates that the sequence specificity of polyadenylation in egg extracts is comparable to that observed with highly purified mammalian CPSF and recombinant PAP. The two in vitro systems exhibit a sequence specificity that is similar, but not identical, to that observed in vivo, as assessed by injection of the same RNAs into the oocyte. These findings imply that CPSFs intrinsic RNA sequence preferences are sufficient to account for the specificity of cytoplasmic polyadenylation of some mRNAs. We discuss the hypothesis that CPSF is required for all polyadenylation reactions, but that the polyadenylation of some mRNAs may require additional factors such as CPEB. To test the consequences of PAP binding to mRNAs in vivo, PAP was tethered to a reporter mRNA in resting oocytes using MS2 coat protein. Tethered PAP catalyzed polyadenylation and stimulated translation approximately 40-fold; stimulation was exclusively cis-acting, but was independent of a CPE and AAUAAA. Both polyadenylation and translational stimulation required PAPs catalytic core, but did not require the putative CPSF interaction domain of PAP. These results demonstrate that premature recruitment of PAP can cause precocious polyadenylation and translational stimulation in the resting oocyte, and can be interpreted to suggest that the role of other factors is to deliver PAP to the mRNA.     

CPEB phosphorylation and cytoplasmic polyadenylation are catalyzed by the kinase IAK1/Eg2 in maturing mouse oocytes

In both vertebrates and invertebrates, the expression of several maternal mRNAs is regulated by cytoplasmic polyadenylation. In Xenopus oocytes, where most of the biochemical details of this process have been examined, polyadenylation is controlled by CPEB, a sequence-specific RNA binding protein. The activity of CPEB, which is to recruit cleavage and polyadenylation specificity factor (CPSF) and poly(A) polymerase (PAP) into an active cytoplasmic polyadenylation complex, is controlled by Eg2-catalyzed phosphorylation. Soon after CPEB phosphorylation and resulting polyadenylation take place, the interaction between maskin, a CPEB-associated factor, and eIF4E, the cap-binding protein, is destroyed, which results in the recruitment of mRNA into polysomes. Polyadenylation also occurs in maturing mouse oocytes, although the biochemical events that govern the reaction in these cells are not known. In this study, we have examined the phosphorylation of CPEB and have assessed the necessity of this protein for polyadenylation in maturing mouse oocytes. Immunohistochemistry has revealed that all the factors that control polyadenylation and translation in Xenopus oocytes (CPEB, CPSF, PAP, maskin, and IAK1, the murine homologue of Eg2) are also present in the cytoplasm of mouse oocytes. After the induction of maturation, a kinase is activated that phosphorylates CPEB on a critical regulatory residue, an event that is essential for CPEB activity. A peptide that competitively inhibits the activity of IAK1/Eg2 blocks the progression of meiosis in injected oocytes. Finally, a CPEB protein that acts as a dominant negative mutation because it cannot be phosphorylated by IAK1/Eg2, prevents cytoplasmic polyadenylation. These data indicate that cytoplasmic polyadenylation in mouse oocytes is mediated by IAK1/Eg2-catalyzed phosphorylation of CPEB.     

Amyloid precursor proteins anchor CPEB to membranes and promote polyadenylation-induced translation

The cytoplasmic polyadenylation element (CPE) binding factor, CPEB, is a sequence-specific RNA binding protein that controls polyadenylation-induced translation in germ cells and at postsynaptic sites of neurons. A yeast two-hybrid screen with a mouse brain cDNA library identified the transmembrane amyloid precursor-like protein 1 (APLP1) as a CPEB-interacting factor. CPEB binds the small intracellular domain (ICD) of APLP1 and the related proteins APLP2 and APP. These proteins promote polyadenylation and translation by stimulating Aurora A catalyzed CPEB serine 174 phosphorylation. Surprisingly, CPEB, Maskin, CPSF, and several other factors involved in polyadenylation and translation and CPE-containing RNA are all detected on membranes by cell fractionation and immunoelectron microscopy. Moreover, most of the RNA that undergoes polyadenylation does so in membrane-containing fractions. These data demonstrate a link between cytoplasmic polyadenylation and membrane association and implicate APP family member proteins as anchors for localized mRNA polyadenylation and translation.     

Phosphorylation of CPEB by Eg2 mediates the recruitment of CPSF into an active cytoplasmic polyadenylation complex

The release of Xenopus oocytes from prophase I arrest is largely driven by the cytoplasmic polyadenylation-induced translation of dormant maternal mRNAs. Two cis elements, the CPE and the hexanucleotide AAUAAA, and their respective binding factors, CPEB and a cytoplasmic form of CPSF, control polyadenylation. The most proximal stimulus for polyadenylation is Eg2-catalyzed phosphorylation of CPEB serine 174. Here, we show that this phosphorylation event stimulates an interaction between CPEB and CPSF. This interaction is direct, does not require RNA tethering, and occurs through the 160 kDa subunit of CPSF. Eg2-stimulated and CPE-dependent polyadenylation is reconstituted in vitro using purified components. These results demonstrate that the molecular function of Eg2-phosphorylated CPEB is to recruit CPSF into an active cytoplasmic polyadenylation complex.     

Opposing polymerase-deadenylase activities regulate cytoplasmic polyadenylation

Cytoplasmic polyadenylation is one mechanism that regulates translation in early animal development. In Xenopus oocytes, polyadenylation of dormant mRNAs, including cyclin B1, is controlled by the cis-acting cytoplasmic polyadenylation element (CPE) and hexanucleotide AAUAAA through associations with CPEB and CPSF, respectively. Previously, we demonstrated that the scaffold protein symplekin contacts CPEB and CPSF and helps them interact with Gld2, a poly(A) polymerase. Here, we report the mechanism by which poly(A) tail length is regulated. Cyclin B1 pre-mRNA acquires a long poly(A) tail in the nucleus that is subsequently shortened in the cytoplasm. The shortening is controlled by CPEB and PARN, a poly(A)-specific ribonuclease. Gld2 and PARN both reside in the CPEB-containing complex. However, because PARN is more active than Gld2, the poly(A) tail is short. When oocytes mature, CPEB phosphorylation causes PARN to be expelled from the ribonucleoprotein complex, which allows Gld2 to elongate poly(A) by default.     

N-methyl-D-aspartate receptor signaling results in Aurora kinase-catalyzed CPEB phosphorylation and alpha CaMKII mRNA polyadenylation at synapses

Activity-dependent local translation of dendritic mRNAs is one process that underlies synaptic plasticity. Here, we demonstrate that several of the factors known to control polyadenylation-induced translation in early vertebrate development [cytoplasmic polyadenylation element-binding protein (CPEB), maskin, poly(A) polymerase, cleavage and polyadenylation specificity factor (CPSF) and Aurora] also reside at synaptic sites of rat hippocampal neurons. The induction of polyadenylation at synapses is mediated by the N-methyl-D-aspartate (NMDA) receptor, which transduces a signal that results in the activation of Aurora kinase. This kinase in turn phosphorylates CPEB, an essential RNA-binding protein, on a critical residue that is necessary for polyadenylation-induced translation. These data demonstrate a remarkable conservation of the regulatory machinery that controls signal-induced mRNA translation, and elucidates an axis connecting the NMDA receptor to localized protein synthesis at synapses.     

Xenopus Rbm9 is a novel interactor of XGld2 in the cytoplasmic polyadenylation complex

During early development, control of the poly(A) tail length by cytoplasmic polyadenylation is critical for the regulation of specific mRNA expression. Gld2, an atypical poly(A) polymerase, is involved in cytoplasmic polyadenylation in Xenopus oocytes. In this study, a new XGld2-interacting protein was identified: Xenopus RNA-binding motif protein 9 (XRbm9). This RNA-binding protein is exclusively expressed in the cytoplasm of Xenopus oocytes and interacts directly with XGld2. It is shown that XRbm9 belongs to the cytoplasmic polyadenylation complex, together with cytoplasmic polyadenylation element-binding protein (CPEB), cleavage and polyadenylation specificity factor (CPSF) and XGld2. In addition, tethered XRbm9 stimulates the translation of a reporter mRNA. The function of XGld2 in stage VI oocytes was also analysed. The injection of XGld2 antibody into oocytes inhibited polyadenylation, showing that endogenous XGld2 is required for cytoplasmic polyadenylation. Unexpectedly, XGld2 and CPEB antibody injections also led to an acceleration of meiotic maturation, suggesting that XGld2 is part of a masking complex with CPEB and is associated with repressed mRNAs in oocytes.     

CPEB controls the cytoplasmic polyadenylation of cyclin, Cdk2 and c-mos mRNAs and is necessary for oocyte maturation in Xenopus.

Cytoplasmic polyadenylation is a key mechanism controlling maternal mRNA translation in early development. In most cases, mRNAs that undergo poly(A) elongation are translationally activated; those that undergo poly(A) shortening are deactivated. Poly(A) elongation is regulated by two cis-acting sequences in the 3'-untranslated region (UTR) of responding mRNAs, the polyadenylation hexanucleotide AAUAAA and the U-rich cytoplasmic polyadenylation element (CPE). Previously, we cloned and characterized the Xenopus oocyte CPE binding protein (CPEB), showing that it was essential for the cytoplasmic polyadenylation of B4 RNA. Here, we show that CPEB also binds the CPEs of G10, c-mos, cdk2, cyclins A1, B1 and B2 mRNAs. We find that CPEB is necessary for polyadenylation of these RNAs in egg extracts, suggesting that this protein is required for polyadenylation of most RNAs during oocyte maturation. Our data demonstrate that the complex timing and extent of polyadenylation are partially controlled by CPEB binding to multiple target sites in the 3' UTRs of responsive mRNAs. Finally, injection of CPEB antibody into oocytes not only inhibits polyadenylation in vivo, but also blocks progesterone-induced maturation. This is due to inhibition of polyadenylation and translation of c-mos mRNA, suggesting that CPEB is critical for early development.     

Aurora Kinase A Is Not Involved in CPEB1 Phosphorylation and cyclin B1 mRNA Polyadenylation during Meiotic Maturation of Porcine Oocytes

Regulation of mRNA translation by cytoplasmic polyadenylation is known to be important for oocyte maturation and further development. This process is generally controlled by phosphorylation of cytoplasmic polyadenylation element binding protein 1 (CPEB1). The aim of this study is to determine the role of Aurora kinase A in CPEB1 phosphorylation and the consequent CPEB1-dependent polyadenylation of maternal mRNAs during mammalian oocyte meiosis. For this purpose, we specifically inhibited Aurora kinase A with MLN8237 during meiotic maturation of porcine oocytes. Using poly(A)-test PCR method, we monitored the effect of Aurora kinase A inhibition on poly(A)-tail extension of long and short cyclin B1 encoding mRNAs as markers of CPEB1-dependent cytoplasmic polyadenylation. Our results show that inhibition of Aurora kinase A activity impairs neither cyclin B1 mRNA polyadenylation nor its translation and that Aurora kinase A is unlikely to be involved in CPEB1 activating phosphorylation.     

CPEB: a life in translation

Nearly two decades ago, Xenopus oocytes were found to contain mRNAs harboring a small sequence in their 3' untranslated regions that control cytoplasmic polyadenylation and translational activation during development. This cytoplasmic polyadenylation element (CPE) is the binding platform for CPE-binding protein (CPEB), which promotes polyadenylation-induced translation. Since then, the biochemistry and biology of CPEB has grown rather substantially: mechanistically, CPEB nucleates a complex of factors that regulates poly(A) elongation through, of all things, a deadenylating enzyme; biologically, CPEB mediates many processes including germ-cell development, cell division and cellular senescence, and synaptic plasticity and learning and memory. These observations underscore the growing complexities of CPEB involvement in cell function.     

An Unusual Two-Step Control of CPEB Destruction by Pin1

Cytoplasmic polyadenylation is a conserved mechanism that controls mRNA translation and stability. A key protein that promotes polyadenylation-induced translation of mRNAs in maturing Xenopus oocytes is the cytoplasmic polyadenylation element binding protein (CPEB). During this meiotic transition, CPEB is subjected to phosphorylation-dependent ubiquitination and partial destruction, which is necessary for successive waves of polyadenylation of distinct mRNAs. Here we identify the peptidyl-prolyl cis-trans isomerase Pin1 as an important factor mediating CPEB destruction. Pin1 interacts with CPEB in an unusual manner in which it occurs prior to CPEB phosphorylation and prior to Pin1 activation by serine 71 dephosphorylation. Upon induction of maturation, CPEB becomes phosphorylated, which occurs simultaneously with Pin1 dephosphorylation. At this time, the CPEB-Pin1 interaction requires cdk1-catalyzed CPEB phosphorylation on S/T-P motifs. Subsequent CPEB ubiquitination and destruction are mediated by a conformational change induced by Pin1 isomerization of CPEB. Similar to M phase progression in maturing Xenopus oocytes, the destruction of CPEB during the mammalian cell cycle requires Pin1 as well. These data identify Pin1 as a new and essential factor regulating CPEB degradation.     

Activity-dependent polyadenylation in neurons

Activity-dependent changes in protein synthesis modify synaptic efficacy. One mechanism that regulates mRNA translation in the synapto-dendritic compartment is cytoplasmic polyadenylation, a process controlled by CPEB, the cytoplasmic polyadenylation element (CPE)-specific RNA binding protein. In neurons, very few mRNAs are known CPEB substrates, and none appear to be responsible for the effects on plasticity that are found in the CPEB knockout mouse. These results suggest that the translation of other mRNAs is regulated by CPEB. To identify them, we have developed a functional assay based on the polyadenylation of brain-derived mRNAs injected into Xenopus oocytes, a surrogate system that carries out this 3' end processing event in an efficient manner. The polyadenylated RNAs were isolated by binding to and thermal elution from poly(U) agarose and identified by microarray analysis. Selected sequences that were positive for polyadenylation were cloned and retested for polyadenylation by injection into oocytes. These sequences were then examined for activity-dependent polyadenylation in cultured hippocampal neurons. Finally, the levels of two proteins encoded by polyadenylated mRNAs were examined in glutamate-stimulated synaptoneurosomes. These studies show that many mRNAs undergo activity-dependent polyadenylation in neurons and that this process coincides with increased translation in the synapto-dendritic compartment.     

Meiosis requires a translational positive loop where CPEB1 ensues its replacement by CPEB4

Meiotic progression is driven by the sequential translational activation of maternal messenger RNAs stored in the cytoplasm. This activation is mainly induced by the cytoplasmic elongation of their poly(A) tails, which is mediated by the cytoplasmic polyadenylation element (CPE) present in their 3′ untranslated regions. Although polyadenylation in prophase I and metaphase I is mediated by the CPE‐binding protein 1 (CPEB1), this protein is degraded during the first meiotic division. Thus, raising the question of how the cytoplasmic polyadenylation required for the second meiotic division is achieved. In this work, we show that CPEB1 generates a positive loop by activating the translation of CPEB4 mRNA, which, in turn, replaces CPEB1 and drives the transition from metaphase I to metaphase II. We further show that CPEB1 and CPEB4 are differentially regulated by phase‐specific kinases, generating the need of two sequential CPEB activities to sustain cytoplasmic polyadenylation during all the meiotic phases. Altogether, this work defines a new element in the translational circuit that support an autonomous transition between the two meiotic divisions in the absence of DNA replication.     

Transient CPEB dimerization and translational control

During oocyte development, the cytoplasmic polyadenylation element-binding protein (CPEB) nucleates a set of factors on mRNA that controls cytoplasmic polyadenylation and translation. The regulation of polyadenylation is mediated in part through serial phosphorylations of CPEB, which control both the dynamic integrity of the cytoplasmic polyadenylation apparatus and CPEB stability, events necessary for meiotic progression. Because the precise stoichiometry between CPEB and CPE-containing RNA is responsible for the temporal order of mRNA polyadenylation during meiosis, we hypothesized that, if CPEB production exceeded the amount required to bind mRNA, the excess would be sequestered in an inactive form. One attractive possibility for the sequestration is protein dimerization. We demonstrate that not only does CPEB form a dimer, but dimerization requires its RNA-binding domains. Dimer formation prevents CPEB from being UV cross-linked to RNA, which establishes a second pool of CPEB that is inert for polyadenylation and translational control. During oocyte maturation, the dimers are degraded much more rapidly than the CPEB monomers, due to their greater affinity for polo-like kinase 1 (plx1) and the ubiquitin E3 ligase β-TrCP. Because dimeric CPEB also binds cytoplasmic polyadenylation factors with greater affinity than monomeric CPEB, it may act as a hub or reservoir for the polyadenylation machinery. We propose that the balance between CPEB and its target mRNAs is maintained by CPEB dimerization, which inactivates spare proteins and prevents them from inducing polyadenylation of RNAs with low affinity binding sites. In addition, the dimers might serve as molecular hubs that release polyadenylation factors for translational activation upon CPEB dimer destruction.     

Differential phosphorylation controls Maskin association with eukaryotic translation initiation factor 4E and localization on the mitotic apparatus

Several cytoplasmic polyadenylation element (CPE)-containing mRNAs that are repressed in Xenopus oocytes become active during meiotic maturation. A group of factors that are anchored to the CPE are responsible for this repression and activation. Two of the most important are CPEB, which binds directly to the CPE, and Maskin, which associates with CPEB. In oocytes, Maskin also binds eukaryotic translation initiation factor 4E (eIF4E), an interaction that excludes eIF4G and prevents formation of the eIF4F initiation complex. When the oocytes are stimulated to reenter the meiotic divisions (maturation), CPEB promotes cytoplasmic polyadenylation. The newly elongated poly(A) tail becomes bound by poly(A) binding protein (PABP), which in turn binds eIF4G and helps it displace Maskin from eIF4E, thereby inducing translation. Here we show that Maskin undergoes several phosphorylation events during oocyte maturation, some of which are important for its dissociation from eIF4E and translational activation of CPE-containing mRNA. These sites are T58, S152, S311, S343, S453, and S638 and are phosphorylated by cdk1. Mutation of these sites to alanine alleviates the cdk1-induced dissociation of Maskin from eIF4E. Prior to maturation, Maskin is phosphorylated on S626 by protein kinase A. While this modification has no detectable effect on translation during oocyte maturation, it is critical for this protein to localize on the mitotic apparatus in somatic cells. These results show that Maskin activity and localization is controlled by differential phosphorylation.     

CPEB, maskin, and cyclin B1 mRNA at the mitotic apparatus: implications for local translational control of cell division

In Xenopus development, the expression of several maternal mRNAs is regulated by cytoplasmic polyadenylation. CPEB and maskin, two factors that control polyadenylation-induced translation are present on the mitotic apparatus of animal pole blastomeres in embryos. Cyclin B1 protein and mRNA, whose translation is regulated by polyadenylation, are colocalized with CPEB and maskin. CPEB interacts with microtubules and is involved in the localization of cyclin B1 mRNA to the mitotic apparatus. Agents that disrupt polyadenylation-induced translation inhibit cell division and promote spindle and centrosome defects in injected embryos. Two of these agents inhibit the synthesis of cyclin B1 protein and one, which has little effect on this process, disrupts the localization of cyclin B1 mRNA and protein. These data suggest that CPEB-regulated mRNA translation is important for the integrity of the mitotic apparatus and for cell division.     

Assembly of a processive messenger RNA polyadenylation complex.

Polyadenylation of mRNA precursors by poly(A) polymerase depends on two specificity factors and their recognition sequences. These are cleavage and polyadenylation specificity factor (CPSF), recognizing the polyadenylation signal AAUAAA, and poly(A) binding protein II (PAB II), interacting with the growing poly(A) tail. Their effects are independent of ATP and an RNA 5'-cap. Analysis of RNA-protein interactions by non-denaturing gel electrophoresis shows that CPSF, PAB II and poly(A) polymerase form a quaternary complex with the substrate RNA that transiently stabilizes the binding of poly(A) polymerase to the RNA 3'-end. Only the complex formed from all three proteins is competent for the processive synthesis of a full-length poly(A) tail.     

Cytoplasmic CstF-77 protein belongs to a masking complex with cytoplasmic polyadenylation element-binding protein in Xenopus oocytes

Regulated mRNA translation is a hallmark of oocytes and early embryos, of which cytoplasmic polyadenylation is a major mechanism. This process involves multiple protein components, including the CPSF (cleavage and polyadenylation specificity factor), which is also required for nuclear polyadenylation. The CstF (cleavage stimulatory factor), with CPSF, is required for the pre-mRNA cleavage before nuclear polyadenylation. However, some evidence suggests that the CstF-77 subunit might have a function independent of nuclear polyadenylation, which could be related to the cell cycle. As such, we addressed the question whether CstF-77 might have a role in cytoplasmic polyadenylation. We investigated the function of the CstF-77 protein in Xenopus oocytes, and show that CstF-77 has indeed a role in the cytoplasm. The Xenopus CstF-77 protein (X77K) localizes mainly to the nucleus, but also in punctuate cytoplasmic foci. We show that X77K resides in a cytoplasmic complex with eIF4E, CPEB (cytoplasmic polyadenylation element-binding protein), CPSF-100 and XGLD2, but is not required for cytoplasmic polyadenylation per se. Impairment of X77K function in ovo leads to an acceleration of the G(2)/M transition, with a premature synthesis of Mos and AuroraA proteins. However, the kinetic of Mos mRNA polyadenylation is not modified. Furthermore, X77K represses mRNA translation in vitro. These results suggest that X77K could be involved in masking of mRNA prior to polyadenylation.     

A Role of CPEB1 in the Modulation of Proliferation and Neuronal Maturation of Rat Primary Neural Progenitor Cells

Cytoplasmic polyadenylation binding protein 1 (CPEB1) is a RNA binding protein, which regulates translation of target mRNAs by regulating polyadenylation status. CPEB1 plays important roles in the regulation of germline cell development by modulating cell cycle progression through the polyadenylation of target mRNAs such as cyclin B1. Similar mechanism is reported in proliferating astrocytes by us, although CPEB1 is involved in the transport of target mRNAs as well as local translation at dendritic spines. In this study, we found the expression of CPEB1 in cultured rat primary neural progenitor cells (NPCs). EGF stimulation of cultured NPCs induced rapid phosphorylation of CPEB1, a hallmark of CPEB1-dependent translational control along with cyclin B1 polyadenylation and translation. EGF-induced activation of ERK1/2 and Aurora A kinase was responsible for CPEB1 phosphorylation. Pharmacological inhibition studies suggested that ERK1/2 is involved in the activation of Aurora A kinase and regulation of CPEB1 phosphorylation in cultured NPCs. Long-term incubation in EGF resulted in the down-regulation of CPEB1 expression, which further increased expression of cyclin B1 and cell cycle progression. When we down-regulated the expression of CPEB1 in NPCs by siRNA transfection, the proliferation of NPCs was increased. Increased NPCs proliferation by down-regulation of CPEB1 resulted in eventual up-regulation of neuronal differentiation with increase in both pre- and post-synaptic proteins. The results from the present study may suggest the importance of translational control in the regulation of neuronal development, an emerging concept in many neurodevelopmental and psychiatric disorders such as autism spectrum disorder.