The temporal control of Wee1 mRNA translation during Xenopus oocyte maturation is regulated by cytoplasmic polyadenylation elements within the 3'-untranslated region

The Wee1 protein tyrosine kinase is a key regulator of cell cycle progression. Wee1 activity is necessary for the control of the first embryonic cell cycle following the fertilization of meiotically mature Xenopus oocytes. Wee1 mRNA is present in immature oocytes, but Wee1 protein does not accumulate in immature oocytes or during the early stages of progesterone-stimulated maturation. This delay in Wee1 translation is critical since premature Wee1 protein accumulation has been shown to inhibit oocyte maturation. In this study we provide evidence that Wee1 protein accumulation is regulated at the level of mRNA translation. This translational control is directed by sequences within the Wee1 mRNA 3'-untranslated region (3' UTR). Specifically, cytoplasmic polyadenylation element (CPE) sequences within the Wee1 3' UTR are necessary for full translational repression in immature oocytes. Our data further indicate that while CPE-independent mechanisms may regulate the levels of Wee1 protein accumulation during progesterone-stimulated oocyte maturation, the timing of Wee1 mRNA translational induction is directed through a CPE-dependent mechanism.     

Xenopus laevis zygote arrest 2 (zar2) encodes a zinc finger RNA-binding protein that binds to the translational control sequence in the maternal Wee1 mRNA and regulates translation

Zygote arrest (Zar) proteins are crucial for early embryonic development, but their molecular mechanism of action is unknown. The Translational Control Sequence (TCS) in the 3' untranslated region (UTR) of the maternal mRNA, Wee1, mediates translational repression in immature Xenopus oocytes and translational activation in mature oocytes, but the protein that binds to the TCS and mediates translational control is not known. Here we show that Xenopus laevis Zar2 (encoded by zar2) binds to the TCS in maternal Wee1 mRNA and represses translation in immature oocytes. Using yeast 3 hybrid assays and electrophoretic mobility shift assays, Zar2 was shown to bind specifically to the TCS in the Wee1 3'UTR. RNA binding required the presence of Zn(2+) and conserved cysteines in the C-terminal domain, suggesting that Zar2 contains a zinc finger. Consistent with regulating maternal mRNAs, Zar2 was present throughout oogenesis, and endogenous Zar2 co-immunoprecipitated endogenous Wee1 mRNA from immature oocytes, demonstrating the physiological significance of the protein-RNA interaction. Interestingly, Zar2 levels decreased during oocyte maturation. Dual luciferase reporter tethered assays showed that Zar2 repressed translation in immature oocytes. Translational repression was relieved during oocyte maturation and this coincided with degradation of Zar2 during maturation. This is the first report of a molecular function of zygote arrest proteins. These data show that Zar2 contains a zinc finger and is a trans-acting factor for the TCS in maternal mRNAs in immature Xenopus oocytes.     

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.     

Cytoplasmic polyadenylation elements mediate masking and unmasking of cyclin B1 mRNA

During oocyte maturation, cyclin B1 mRNA is translationally activated by cytoplasmic polyadenylation. This process is dependent on cytoplasmic polyadenylation elements (CPEs) in the 3' untranslated region (UTR) of the mRNA. To determine whether a titratable factor might be involved in the initial translational repression (masking) of this mRNA, high levels of cyclin B1 3' UTR were injected into oocytes. While this treatment had no effect on the poly(A) tail length of endogenous cyclin B1 mRNA, it induced cyclin B1 synthesis. A mutational analysis revealed that the most efficient unmasking element in the cyclin 3' UTR was the CPE. However, other U-rich sequences that resemble the CPE in structure, but which do not bind the CPE-binding polyadenylation factor CPEB, failed to induce unmasking. When fused to the chloramphenical acetyl transferase (CAT) coding region, the cyclin B1 3' UTR inhibited CAT translation in injected oocytes. In addition, a synthetic 3' UTR containing multiple copies of the CPE also inhibited translation, and did so in a dose-dependent manner. Furthermore, efficient CPE-mediated masking required cap-dependent translation. During the normal course of progesterone-induced maturation, cytoplasmic polyadenylation was necessary for mRNA unmasking. A model to explain how cyclin B1 mRNA masking and unmasking could be regulated by the CPE is presented.     

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.     

Subdividing cell populations in the developing limbs of Drosophila: do wing veins and leg segments define units of growth control?

In maturing mouse oocytes, protein synthesis is required for meiotic maturation subsequent to germinal vesicle breakdown (GVBD). While the number of different proteins that must be synthesized for this progression to occur is unknown, at least one of them appears to be cyclin B1, the regulatory subunit of M-phase-promoting factor. Here, we investigate the mechanism of cyclin B1 mRNA translational control during mouse oocyte maturation. We show that the U-rich cytoplasmic polyadenylation element (CPE), a cis element in the 3' UTR of cyclin B1 mRNA, mediates translational repression in GV-stage oocytes. The CPE is also necessary for cytoplasmic polyadenylation, which stimulates translation during oocyte maturation. The injection of oocytes with a cyclin B1 antisense RNA, which probably precludes the binding of a factor to the CPE, delays cytoplasmic polyadenylation as well as the transition from GVBD to metaphase II. CPEB, which interacts with the cyclin B1 CPE and is present throughout meiotic maturation, becomes phosphorylated at metaphase I. These data indicate that CPEB is involved in both the repression and the stimulation of cyclin B1 mRNA and suggest that the phosphorylation of this protein could be involved in regulating its activity.     

Expression of cyclin B1 messenger RNA isoforms and initiation of cytoplasmic polyadenylation in the bovine oocyte

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.     

Translational control of cyclin B1 mRNA during meiotic maturation: coordinated repression and cytoplasmic polyadenylation

Translational control is prominent during meiotic maturation and early development. In this report, we investigate a mode of translational repression in Xenopus laevis oocytes, focusing on the mRNA encoding cyclin B1. Translation of cyclin B1 mRNA is relatively inactive in the oocyte and increases dramatically during meiotic maturation. We show, by injection of synthetic mRNAs, that the cis-acting sequences responsible for repression of cyclin B1 mRNA reside within its 3'UTR. Repression can be saturated by increasing the concentration of reporter mRNA injected, suggesting that the cyclin B1 3'UTR sequences provide a binding site for a trans-acting repressor. The sequences that direct repression overlap and include cytoplasmic polyadenylation elements (CPEs), sequences known to promote cytoplasmic polyadenylation. However, the presence of a CPE per se appears insufficient to cause repression, as other mRNAs that contain CPEs are not translationally repressed. We demonstrate that relief of repression and cytoplasmic polyadenylation are intimately linked. Repressing elements do not override the stimulatory effect of a long poly(A) tail, and polyadenylation of cyclin B1 mRNA is required for its translational recruitment. Our results suggest that translational recruitment of endogenous cyclin B1 mRNA is a collaborative effect of derepression and poly(A) addition. We discuss several molecular mechanisms that might underlie this collaboration.     

Possible involvement of insulin-like growth factor 2 mRNA-binding protein 3 in zebrafish oocyte maturation as a novel cyclin B1 mRNA-binding protein that represses the translation in immature oocytes

In immature zebrafish oocytes, dormant cyclin B1 mRNAs localize to the animal polar cytoplasm as aggregates. After hormonal stimulation, cyclin B1 mRNAs are dispersed and translationally activated, which are necessary and sufficient for the induction of zebrafish oocyte maturation. Besides cytoplasmic polyadenylation element-binding protein (CPEB) and cis-acting elements in the 3′ untranslated region (UTR), Pumilio1 and a cis-acting element in the coding region of cyclin B1 mRNA are important for the subcellular localization and timing of translational activation of the mRNA. However, mechanisms underlying the spatio-temporal control of cyclin B1 mRNA translation during oocyte maturation are not fully understood. We report that insulin-like growth factor 2 mRNA-binding protein 3 (IMP3), which was initially described as a protein bound to Vg1 mRNA localized to the vegetal pole of Xenopus oocytes, binds to the 3′ UTR of cyclin B1 mRNA that localizes to the animal pole of zebrafish oocytes. IMP3 and cyclin B1 mRNA co-localize to the animal polar cytoplasm of immature oocytes, but in mature oocytes, IMP3 dissociates from the mRNA despite the fact that its protein content and phosphorylation state are unchanged during oocyte maturation. IMP3 interacts with Pumilio1 and CPEB in an mRNA-dependent manner in immature oocytes but not in mature oocytes. Overexpression of IMP3 and injection of anti-IMP3 antibody delayed the progression of oocyte maturation. On the basis of these results, we propose that IMP3 represses the translation of cyclin B1 mRNA in immature zebrafish oocytes and that its release from the mRNA triggers the translational activation.     

CPEB and miR-15/16 Co-Regulate Translation of Cyclin E1 mRNA during Xenopus Oocyte Maturation

Cell cycle transitions spanning meiotic maturation of the Xenopus oocyte and early embryogenesis are tightly regulated at the level of stored inactive maternal mRNA. We investigated here the translational control of cyclin E1, required for metaphase II arrest of the unfertilised egg and the initiation of S phase in the early embryo. We show that the cyclin E1 mRNA is regulated by both cytoplasmic polyadenylation elements (CPEs) and two miR-15/16 target sites within its 3’UTR. Moreover, we provide evidence that maternal miR-15/16 microRNAs co-immunoprecipitate with CPE-binding protein (CPEB), and that CPEB interacts with the RISC component Ago2. Experiments using competitor RNA and mutated cyclin E1 3’UTRs suggest cooperation of the regulatory elements to sustain repression of the cyclin E1 mRNA during early stages of maturation when CPEB becomes limiting and cytoplasmic polyadenylation of repressed mRNAs begins. Importantly, injection of anti-miR-15/16 LNA results in the early polyadenylation of endogenous cyclin E1 mRNA during meiotic maturation, and an acceleration of GVBD, altogether strongly suggesting that the proximal CPEB and miRNP complexes act to mutually stabilise each other. We conclude that miR-15/16 and CPEB co-regulate cyclin E1 mRNA. This is the first demonstration of the co-operation of these two pathways.     

The Mos pathway regulates cytoplasmic polyadenylation in Xenopus oocytes.

Cytoplasmic polyadenylation controls the translation of several maternal mRNAs during Xenopus oocyte maturation and requires two sequences in the 3' untranslated region (UTR), the U-rich cytoplasmic polyadenylation element (CPE), and the hexanucleotide AAUAAA. c-mos mRNA is polyadenylated and translated soon after the induction of maturation, and this protein kinase is necessary for a kinase cascade culminating in cdc2 kinase (MPF) activation. Other mRNAs are polyadenylated later, around the time of cdc2 kinase activation. To determine whether there is a hierarchy in the cytoplasmic polyadenylation of maternal mRNAs, we ablated c-mos mRNA with an antisense oligonucleotide. This prevented histone B4 and cyclin A1 and B1 mRNA polyadenylation, indicating that the polyadenylation of these mRNAs is Mos dependent. To investigate a possible role of cdc2 kinase in this process, cyclin B was injected into oocytes lacking c-mos mRNA. cdc2 kinase was activated, but mitogen-activated protein kinase was not. However, polyadenylation of cyclin B1 and histone B4 mRNA was still observed. This demonstrates that cdc2 kinase can induce cytoplasmic polyadenylation in the absence of Mos. Our data further indicate that although phosphorylation of the CPE binding protein may be involved in the induction of Mos-dependent polyadenylation, it is not required for Mos-independent polyadenylation. We characterized the elements conferring Mos dependence (Mos response elements) in the histone B4 and cyclin B1 mRNAs by mutational analysis. For histone B4 mRNA, the Mos response elements were in the coding region or 5' UTR. For cyclin B1 mRNA, the main Mos response element was a CPE that overlaps with the AAUAAA hexanucleotide. This indicates that the position of the CPE can have a profound influence on the timing of cytoplasmic polyadenylation.     

Translational control during early development

Early development in many animals is programmed by maternally inherited messenger RNAs. Many of these mRNAs are translationally dormant in immature oocytes, but are recruited onto polysomes during meiotic maturation, fertilization, or early embryogenesis. In contrast, other mRNAs that are translated in oocytes are released from polysomes during these later stages of development. Recent studies have begun to define the cis and trans elements that regulate both translational repression and translational induction of maternal mRNA. The inhibition of translation of some mRNAs during early development is controlled by discrete sequences residing in the 3' and 5' untranslated regions, respectively. The translation of other RNAs is due to polyadenylation which, at least in oocytes of the frog Xenopus laevis, is regulated by a U-rich cytoplasmic polyadenylation element (CPE). Although similar, the CPE sequences of various mRNAs are sufficiently different to be bound by different proteins. Two of these proteins and their interactions are described here.     

Multiple sequence elements and a maternal mRNA product control cdk2 RNA polyadenylation and translation during early Xenopus development.

Cytoplasmic poly(A) elongation is one mechanism that regulates translational recruitment of maternal mRNA in early development. In Xenopus laevis, poly(A) elongation is controlled by two cis elements in the 3' untranslated regions of responsive mRNAs: the hexanucleotide AAUAAA and a U-rich structure with the general sequence UUUUUAAU, which is referred to as the cytoplasmic polyadenylation element (CPE). B4 RNA, which contains these sequences, is polyadenylated during oocyte maturation and maintains a poly(A) tail in early embryos. However, cdk2 RNA, which also contains these sequences, is polyadenylated during maturation but deadenylated after fertilization. This suggests that cis-acting elements in cdk2 RNA signal the removal of the poly(A) tail at this time. By using poly(A) RNA-injected eggs, we showed that two elements which reside 5' of the CPE and 3' of the hexanucleotide act synergistically to promote embryonic deadenylation of this RNA. When an identical RNA lacking a poly(A) tail was injected, these sequences also prevented poly(A) addition. When fused to CAT RNA, the cdk2 3' untranslated region, which contains these elements, as well as the CPE and the hexanucleotide, promoted poly(A) addition and enhanced chloramphenicol acetyltransferase activity during maturation, as well as repression of these events after fertilization. Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA. This suggests that a maternal mRNA, whose translation occurs only after fertilization, is necessary for the cdk2 deadenylation or inhibition of RNA polyadenylation. This was further suggested when poly(A)+ RNA isolated from two-cell embryos was injected into oocytes that were then allowed to mature. Such oocytes became deficient for cdk2 RNA polyadenylation but remained proficient for B4 RNA polyadenylation. These data show that CPE function is developmentally regulated by multiple sequences and factors.     

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.     

Involvement of Xenopus Pumilio in the translational regulation that is specific to cyclin B1 mRNA during oocyte maturation

Protein synthesis of cyclin B by translational activation of the dormant mRNA stored in oocytes is required for normal progression of maturation. In this study, we investigated the involvement of Xenopus Pumilio (XPum), a cyclin B1 mRNA-binding protein, in the mRNA-specific translational activation. XPum exhibits high homology to mammalian counterparts, with amino acid identity close to 90%, even if the conserved RNA-binding domain is excluded. XPum is bound to cytoplasmic polyadenylation element (CPE)-binding protein (CPEB) through the RNA-binding domain but not to its phosphorylated form in mature oocytes. In addition to the CPE, the XPum-binding sequence of cyclin B1 mRNA acts as a cis-element for translational repression. Injection of anti-XPum antibody accelerated oocyte maturation and synthesis of cyclin B1, and, conversely, over-expression of XPum retarded oocyte maturation and translation of cyclin B1 mRNA, which was accompanied by inhibition of poly(A) tail elongation. The injection of antibody and the over-expression of XPum, however, had no effect on translation of Mos mRNA, which also contains the CPE. These findings provide the first evidence that XPum is a translational repressor specific to cyclin B1 in vertebrates. We propose that in cooperation with the CPEB-maskin complex, the master regulator common to the CPE-containing mRNAs, XPum acts as a specific regulator that determines the timing of translational activation of cyclin B1 mRNA by its release from phosphorylated CPEB during oocyte maturation.     

Translational control by cytoplasmic polyadenylation of c-mos mRNA is necessary for oocyte maturation in the mouse.

The c-mos proto-oncogene product is a key element in the cascade of events leading to meiotic maturation of vertebrate oocytes. We have investigated the role of cytoplasmic polyadenylation in the translational control of mouse c-mos mRNA and its contribution to meiosis. Using an RNase protection assay we show that optimal cytoplasmic polyadenylation of c-mos mRNA requires three cis elements in the 3' UTR: the polyadenylation hexanucleotide AAUAAA and two U-rich cytoplasmic polyadenylation elements (CPEs) located 4 and 51 nucleotides upstream of the hexanucleotide. When fused to CAT coding sequences, the wild-type 3' UTR of c-mos mRNA, but not a 3' UTR containing mutations in both CPEs, confers translational recruitment during maturation. This recruitment coincides with maximum polyadenylation. To assess whether c-mos mRNA polyadenylation is necessary for maturation of mouse oocytes, we have ablated endogenous c-mos mRNA by injecting an antisense oligonucleotide, which results in a failure to progress to meiosis II after emission of the first polar body. Such antisense oligonucleotide-injected oocytes could be efficiently rescued by co-injection of a c-mos mRNA carrying a wild-type 3' UTR. However, co-injection of a c-mos mRNA lacking functional CPEs substantially lowered the rescue activity. These results demonstrate that translational control of c-mos mRNA by cytoplasmic polyadenylation is necessary for normal development.     

Maturation‐associated Dbf4 expression is essential for mouse zygotic DNA replication

Cdc7 is an S‐phase‐promoting kinase (SPK) that is required for the activation of replication initiation complex assembly because it phosphorylates the MCM protein complex serving as the replicative helicase in eukaryotic organisms. Cdc7 activity is undetectable in immature mouse GV oocytes, although Cdc7 protein is already expressed at the same level as in mature oocytes or early one‐cell embryos at zygotic S‐phase, in which Cdc7 kinase activity is clearly detectable. Dbf4 is a regulatory subunit of Cdc7 and is required for Cdc7 kinase activity. Dbf4 is not readily detectable in immature GV oocytes but accumulates to a level similar to that in one‐cell embryos during oocyte maturation, suggesting that Cdc7 is already activated in unfertilized eggs (metaphase II). RNAi‐mediated knockdown of maternal Dbf4 expression prevents the maturation‐associated increase in Dbf4 protein, abolishes the activation of Cdc7, and leads to the failure of DNA replication in one‐cell embryos, demonstrating that Dbf4 expression is the key regulator of Cdc7 activity in mouse oocytes. Dormant Dbf4 mRNA in immature GV oocytes is recruited by cytoplasmic polyadenylation during oocyte maturation and is dependent on MPF activity via its cytoplasmic polyadenylation element (CPE) upstream of the hexanucleotide (HEX) in the 3′ untranslated region (3′UTR). Our results suggest that Cdc7 is inactivated in immature oocytes, preventing it from the unwanted phosphorylation of MCM proteins, and the oocyte is qualified by proper maturation to proceed following embryogenesis after fertilization through zygotic DNA replication.     

Enforcing temporal control of maternal mRNA translation during oocyte cell‐cycle progression

Meiotic cell‐cycle progression in progesterone‐stimulated Xenopus oocytes requires that the translation of pre‐existing maternal mRNAs occur in a strict temporal order. Timing of translation is regulated through elements within the mRNA 3′ untranslated region (3′ UTR), which respond to cell cycle‐dependant signalling. One element that has been previously implicated in the temporal control of mRNA translation is the cytoplasmic polyadenylation element (CPE). In this study, we show that the CPE does not direct early mRNA translation. Rather, early translation is directed through specific early factors, including the Musashi‐binding element (MBE) and the MBE‐binding protein, Musashi. Our findings indicate that although the cyclin B5 3′ UTR contains both CPEs and an MBE, the MBE is the critical regulator of early translation. The cyclin B2 3′ UTR contains CPEs, but lacks an MBE and is translationally activated late in maturation. Finally, utilizing antisense oligonucleotides to attenuate endogenous Musashi synthesis, we show that Musashi is critical for the initiation of early class mRNA translation and for the subsequent activation of CPE‐dependant mRNA translation.     

Cytoplasmic polyadenylation element-like sequences are involved in dendritic targeting of BDNF mRNA in hippocampal neurons

Several mRNAs are known to be targeted to dendrites in hippocampal neurons. In this study, we show that brain-derived neurotrophic factor (BDNF) mRNA has two distinct cis-acting dendritic targeting elements in the short 3′ untranslated region (UTR): a constitutive element and an activity-dependent one. Moreover, deletion of serial cytoplasmic polyadenylation element (CPE)-like sequences in the short 3′UTR suppressed both constitutive and activity-dependent dendritic targeting. In addition to the interaction with cytoplasmic polyadenylation element binding protein-1 (CPEB-1), depolarization enhanced CPEB-1 recruitment to the activity-dependent targeting element. These results suggest that CPE-like sequences are involved in the activity-dependent as well as constitutive dendritic targeting of BDNF mRNA.