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

Multiple intersecting pathways are involved in CPEB1 phosphorylation and regulation of translation during mouse oocyte meiosis

The RNA-binding protein cytoplasmic polyadenylation element binding 1 (CPEB1) plays a fundamental role in regulating mRNA translation in oocytes. However, the specifics of how and which protein kinase cascades modulate CPEB1 activity are still controversial. Using genetic and pharmacological tools, and detailed time courses, we have re-evaluated the relationship between CPEB1 phosphorylation and translation activation during mouse oocyte maturation. We show that both the CDK1/MAPK and AURKA/PLK1 pathways converge on CPEB1 phosphorylation during prometaphase of meiosis I. Only inactivation of the CDK1/MAPK pathway disrupts translation, whereas inactivation of either pathway alone leads to CPEB1 stabilization. However, CPEB1 stabilization induced by inactivation of the AURKA/PLK1 pathway does not affect translation, indicating that destabilization and/or degradation is not linked to translational activation. The accumulation of endogenous CCNB1 protein closely recapitulates the translation data that use an exogenous template. These findings support the overarching hypothesis that the activation of translation during prometaphase in mouse oocytes relies on a CDK1/MAPK-dependent CPEB1 phosphorylation, and that translational activation precedes CPEB1 destabilization.

Control of poly(A)-tail length and translation in vertebrate oocytes and early embryos

During oocyte maturation and early embryogenesis, changes in mRNA poly(A)-tail lengths strongly influence translation, but how these tail-length changes are orchestrated has been unclear. Here, we performed tail-length and translational profiling of mRNA reporter libraries (each with millions of 3' UTR sequence variants) in frog oocytes and embryos and in fish embryos. Contrasting to previously proposed cytoplasmic polyadenylation elements (CPEs), we found that a shorter element, UUUUA, together with the polyadenylation signal (PAS), specify cytoplasmic polyadenylation, and we identified contextual features that modulate the activity of both elements. In maturing oocytes, this tail lengthening occurs against a backdrop of global deadenylation and the action of C-rich elements that specify tail-length-independent translational repression. In embryos, cytoplasmic polyadenylation becomes more permissive, and additional elements specify waves of stage-specific deadenylation. Together, these findings largely explain the complex tapestry of tail-length changes observed in early frog and fish development, with strong evidence of conservation in both mice and humans.

FUS reads histone H3K36me3 to regulate alternative polyadenylation

Complex organisms generate differential gene expression through the same set of DNA sequences in distinct cells. The communication between chromatin and RNA regulates cellular behavior in tissues. However, little is known about how chromatin, especially histone modifications, regulates RNA polyadenylation. In this study, we found that FUS was recruited to chromatin by H3K36me3 at gene bodies. The H3K36me3 recognition of FUS was mediated by the proline residues in the ZNF domain. After these proline residues were mutated or H3K36me3 was abolished, FUS dissociated from chromatin and bound more to RNA, resulting in an increase in polyadenylation sites far from stop codons genome-wide. A proline mutation corresponding to a mutation in amyotrophic lateral sclerosis contributed to the hyperactivation of mitochondria and hyperdifferentiation in mouse embryonic stem cells. These findings reveal that FUS is an H3K36me3 reader protein that links chromatin-mediated alternative polyadenylation to human disease.

A genome-wide perspective of the maternal mRNA translation program during oocyte development

Transcriptional and post-transcriptional regulations control gene expression in most cells. However, critical transitions during the development of the female gamete relies exclusively on regulation of mRNA translation in the absence of de novo mRNA synthesis. Specific temporal patterns of maternal mRNA translation are essential for the oocyte progression through meiosis, for generation of a haploid gamete ready for fertilization and for embryo development. In this review, we will discuss how mRNAs are translated during oocyte growth and maturation using mostly a genome-wide perspective. This broad view on how translation is regulated reveals multiple divergent translational control mechanisms required to coordinate protein synthesis with progression through the meiotic cell cycle and with development of a totipotent zygote.

Alternative cleavage and polyadenylation of the Ccnb1 mRNA defines accumulation of cyclin protein during the meiotic cell cycle

Progression through the mitotic and meiotic cell cycle is driven by fluctuations in the levels of cyclins, the regulatory subunits controlling the localization and activity of CDK1 kinases. Cyclin levels are regulated through a precise balance of synthesis and degradation. Here we demonstrate that the synthesis of Cyclin B1 during the oocyte meiotic cell cycle is defined by the selective translation of mRNA variants generated through alternative cleavage and polyadenylation (APA). Using gene editing in mice, we introduced mutations into the proximal and distal polyadenylation elements of the 3' untranslated region (UTR) of the Ccnb1 mRNA. Through in vivo loss-of-function experiments, we demonstrate that the translation of mRNA with a short 3' UTR specifies Cyclin B1 protein levels that set the timing of meiotic re-entry. In contrast, translation directed by a long 3' UTR is necessary to direct Cyclin B1 protein accumulation during the MI/MII transition. These findings establish that the progression through the cell cycle is dependent on the selective translation of multiple mRNA variants generated by APA.

Multiple intersecting pathways are involved in the phosphorylation of CPEB1 to activate translation during mouse oocyte meiosis

The RNA-binding protein cytoplasmic polyadenylation element binding 1 (CPEB1) plays a fundamental role in the regulation of mRNA translation in oocytes. However, the nature of protein kinase cascades modulating the activity of CPEB1 is still a matter of controversy. Using genetic and pharmacological tools and detailed time courses, here we have reevaluated the relationship between CPEB1 phosphorylation and the activation of translation during mouse oocyte maturation. We show that both the CDK1/MAPK and AURKA/PLK1 pathways converge on the phosphorylation of CPEB1 during prometaphase. Only inactivation of the CDK1/MAPK pathway disrupts translation, while inactivation of either pathway leads to CPEB1 stabilization. However, stabilization of CPEB1 induced by inactivation of the AURKA/PLK1 does not affect translation, indicating that destabilization/degradation can be dissociated from translational activation. The accumulation of the endogenous CCNB1 protein closely recapitulates the translation data. These findings support the overarching hypothesis that the activation of translation in prometaphase in mouse oocytes relies on a CDK1-dependent CPEB1 phosphorylation, and this translational activation precedes CPEB1 destabilization.

Meiotic Cell Cycle Progression in Mouse Oocytes: Role of Cyclins

All eukaryotic cells, including oocytes, utilize an engine called cyclin-dependent kinase (Cdk) to drive the cell cycle. Cdks are activated by a co-factor called cyclin, which regulates their activity. The key Cdk-cyclin complex that regulates the oocyte cell cycle is known as Cdk1-cyclin B1. Recent studies have elucidated the roles of other cyclins, such as B2, B3, A2, and O, in oocyte cell cycle regulation. This review aims to discuss the recently discovered roles of various cyclins in mouse oocyte cell cycle regulation in accordance with the sequential progression of the cell cycle. In addition, this review addresses the translation and degradation of cyclins to modulate the activity of Cdks. Overall, the literature indicates that each cyclin performs unique and redundant functions at various stages of the cell cycle, while their expression and degradation are tightly regulated. Taken together, this review provides new insights into the regulatory role and function of cyclins in oocyte cell cycle progression.

CPEB and translational control by cytoplasmic polyadenylation: impact on synaptic plasticity, learning, and memory

The late 1990s were banner years in molecular neuroscience; seminal studies demonstrated that local protein synthesis, at or near synapses, was necessary for synaptic plasticity, the underlying cellular basis of learning and memory [1, 2]. The newly made proteins were proposed to "tag" the stimulated synapse, distinguishing it from naive synapses, thereby forming a cellular memory [3]. Subsequent studies demonstrated that the transport of mRNAs from soma to dendrite was linked with translational unmasking at synapses upon synaptic stimulation. It soon became apparent that one prevalent mechanism governing these events is cytoplasmic polyadenylation, and that among the proteins that control this process, CPEB, plays a central role in synaptic plasticity, and learning and memory. In vertebrates, CPEB is a family of four proteins, all of which regulate translation in the brain, that have partially overlapping functions, but also have unique characteristics and RNA binding properties that make them control different aspects of higher cognitive function. Biochemical analysis of the vertebrate CPEBs demonstrate them to respond to different signaling pathways whose output leads to specific cellular responses. In addition, the different CPEBs, when their functions go awry, result in pathophysiological phenotypes resembling specific human neurological disorders. In this essay, we review key aspects of the vertebrate CPEB proteins and cytoplasmic polyadenylation within the context of brain function.

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.

The translation regulator Zar1l controls timing of meiosis in Xenopus oocytes

Oocyte maturation and early embryo development occur in vertebrates in the near absence of transcription. Thus, sexual reproduction of vertebrates critically depends on the timely translation of mRNAs already stockpiled in the oocyte. Yet how translational activation of specific mRNAs is temporally coordinated is still incompletely understood. Here, we elucidate the function of Zar1l, a yet uncharacterized member of the Zar RNA-binding protein family, in Xenopus oocytes. Employing TRIM-Away, we demonstrate that loss of Zar1l accelerates hormone-induced meiotic resumption of Xenopus oocytes due to premature accumulation of the M-phase-promoting kinase cMos. We show that Zar1l is a constituent of a large ribonucleoparticle containing the translation repressor 4E-T and the central polyadenylation regulator CPEB1, and that it binds directly to the cMos mRNA. Partial, hormone-induced degradation of Zar1l liberates 4E-T from CPEB1, which weakens translational repression of mRNAs encoding cMos and likely additional M-phase-promoting factors. Thus, our study provides fundamental insights into the mechanisms that ensure temporally regulated translation of key cell cycle regulators during oocyte maturation, which is essential for sexual reproductivity.

Control of immediate early gene expression by CPEB4-repressor complex-mediated mRNA degradation

Background

Cytoplasmic polyadenylation element-binding protein 4 (CPEB4) is known to associate with cytoplasmic polyadenylation elements (CPEs) located in the 3′ untranslated region (UTR) of specific mRNAs and assemble an activator complex promoting the translation of target mRNAs through cytoplasmic polyadenylation.

Results

Here, we find that CPEB4 is part of an alternative repressor complex that mediates mRNA degradation by associating with the evolutionarily conserved CCR4-NOT deadenylase complex. We identify human CPEB4 as an RNA-binding protein (RBP) with enhanced association to poly(A) RNA upon inhibition of class I histone deacetylases (HDACs), a condition known to cause widespread degradation of poly(A)-containing mRNA. Photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation (PAR-CLIP) analysis using endogenously tagged CPEB4 in HeLa cells reveals that CPEB4 preferentially binds to the 3′UTR of immediate early gene mRNAs, at G-containing variants of the canonical U- and A-rich CPE located in close proximity to poly(A) sites. By transcriptome-wide mRNA decay measurements, we find that the strength of CPEB4 binding correlates with short mRNA half-lives and that loss of CPEB4 expression leads to the stabilization of immediate early gene mRNAs. Akin to CPEB4, we demonstrate that CPEB1 and CPEB2 also confer mRNA instability by recruitment of the CCR4-NOT complex.

Conclusions

While CPEB4 was previously known for its ability to stimulate cytoplasmic polyadenylation, our findings establish an additional function for CPEB4 as the RNA adaptor of a repressor complex that enhances the degradation of short-lived immediate early gene mRNAs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02760-5.

Comparative analyses of vertebrate CPEB proteins define two subfamilies with coordinated yet distinct functions in post-transcriptional gene regulation

Background

Vertebrate CPEB proteins bind mRNAs at cytoplasmic polyadenylation elements (CPEs) in their 3′ UTRs, leading to cytoplasmic changes in their poly(A) tail lengths; this can promote translational repression or activation of the mRNA. However, neither the regulation nor the mechanisms of action of the CPEB family per se have been systematically addressed to date.

Results

Based on a comparative analysis of the four vertebrate CPEBs, we determine their differential regulation by phosphorylation, the composition and properties of their supramolecular assemblies, and their target mRNAs. We show that all four CPEBs are able to recruit the CCR4-NOT deadenylation complex to repress the translation. However, their regulation, mechanism of action, and target mRNAs define two subfamilies. Thus, CPEB1 forms ribonucleoprotein complexes that are remodeled upon a single phosphorylation event and are associated with mRNAs containing canonical CPEs. CPEB2–4 are regulated by multiple proline-directed phosphorylations that control their liquid–liquid phase separation. CPEB2–4 mRNA targets include CPEB1-bound transcripts, with canonical CPEs, but also a specific subset of mRNAs with non-canonical CPEs.

Conclusions

Altogether, these results show how, globally, the CPEB family of proteins is able to integrate cellular cues to generate a fine-tuned adaptive response in gene expression regulation through the coordinated actions of all four members.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02759-y.

Orb-dependent polyadenylation contributes to PLP expression and centrosome scaffold assembly

As the microtubule-organizing centers of most cells, centrosomes engineer the bipolar mitotic spindle required for error-free mitosis. Drosophila Pericentrin-like protein (PLP) directs formation of a pericentriolar material (PCM) scaffold required for PCM organization and microtubule-organizing center function. Here, we investigate the post-transcriptional regulation of Plp mRNA. We identify conserved binding sites for cytoplasmic polyadenylation element binding (CPEB) proteins within the Plp 3′-untranslated region and examine the role of the CPEB ortholog Oo18 RNA-binding protein (Orb) in Plp mRNA regulation. Our data show that Orb interacts biochemically with Plp mRNA to promote polyadenylation and PLP protein expression. Loss of orb, but not orb2, diminishes PLP levels in embryonic extracts. Consequently, PLP localization to centrosomes and its function in PCM scaffolding are compromised in orb mutant embryos, resulting in genomic instability and embryonic lethality. Moreover, we find that PLP overexpression restores centrosome scaffolding and rescues the cell division defects caused by orb depletion. Our data suggest that Orb modulates PLP expression at the level of Plp mRNA polyadenylation and demonstrates that the post-transcriptional regulation of core, conserved centrosomal mRNAs is crucial for centrosome function.

Single‐cell profiling of transcriptomic changes during in vitro maturation of human oocytes

Purpose

In vitro maturation (IVM) of human oocytes offers an invaluable opportunity for infertility treatment. However, in vitro matured oocytes often show lower developmental abilities than their in vivo counterparts, and molecular mechanisms underlying successful maturation remain unclear. In this study, we investigated gene expression profiles of in vitro matured oocytes at the single‐cell level to gain mechanistic insight into IVM of human oocytes.

Methods

Human oocytes were retrieved by follicular puncture and in vitro matured. In total, 19 oocytes from 11 patients were collected and subjected to single‐cell RNA‐seq analyses.

Results

Global gene expression profiles were similar among oocytes at the same maturation stage, while a small number of oocytes showed distinct transcriptomes from those at the corresponding maturation stage. Differential gene expression analysis identified hundreds of transcripts that dynamically altered their expression during IVM, and we revealed molecular pathways and upstream regulators that may govern oocyte maturation. Furthermore, oocytes that were delayed in their maturation showed distinct transcriptomes. Finally, we identified genes whose transcripts were enriched in each stage of oocyte maturation.

Conclusions

Our work uncovers transcriptomic changes during human oocyte IVM and the differential gene expression profile of each oocyte.

CPEB1 directs muscle stem cell activation by reprogramming the translational landscape

Skeletal muscle stem cells, also called Satellite Cells (SCs), are actively maintained in quiescence but can activate quickly upon extrinsic stimuli. However, the mechanisms of how quiescent SCs (QSCs) activate swiftly remain elusive. Here, using a whole mouse perfusion fixation approach to obtain bona fide QSCs, we identify massive proteomic changes during the quiescence-to-activation transition in pathways such as chromatin maintenance, metabolism, transcription, and translation. Discordant correlation of transcriptomic and proteomic changes reveals potential translational regulation upon SC activation. Importantly, we show Cytoplasmic Polyadenylation Element Binding protein 1 (CPEB1), post-transcriptionally affects protein translation during SC activation by binding to the 3' UTRs of different transcripts. We demonstrate phosphorylation-dependent CPEB1 promoted Myod1 protein synthesis by binding to the cytoplasmic polyadenylation elements (CPEs) within its 3' UTRs to regulate SC activation and muscle regeneration. Our study characterizes CPEB1 as a key regulator to reprogram the translational landscape directing SC activation and subsequent proliferation.

Translational Control in Liver Disease

Chronic liver disease is one of the biggest threats to public health worldwide. Worryingly, the incidence of liver disease is dramatically rising due to the aging of the population and the global epidemics of obesity. Both are major risk factors for chronic liver disease and adverse prognostic factors, causing an increase in mortality rate. It is of great concern that 80–95% of obese people have non-alcoholic fatty liver disease, the major precursor for liver failure and a global health challenge. Currently, the only curative treatment for advanced chronic liver disease is liver transplantation, which is, however, hampered by high treatment costs and the scarcity of donor organs. New strategies are therefore urgently needed to prevent and reverse chronic liver disease. And for that it is essential to understand better the molecular mechanisms underlying human disease. This review focuses on the abnormalities in the regulation of translation by RNA-binding proteins during chronic liver disease and their pathological impact on portal hypertension, fibrosis, steatosis, neovascularization, and cancer development.

The role of CPEB family proteins in the nervous system function in the norm and pathology

Posttranscriptional gene regulation includes mRNA transport, localization, translation, and regulation of mRNA stability. CPEB (cytoplasmic polyadenylation element binding) family proteins bind to specific sites within the 3′-untranslated region and mediate poly- and deadenylation of transcripts, activating or repressing protein synthesis. As part of ribonucleoprotein complexes, the CPEB proteins participate in mRNA transport and localization to different sub-cellular compartments. The CPEB proteins are evolutionarily conserved and have similar functions in vertebrates and invertebrates. In the nervous system, the CPEB proteins are involved in cell division, neural development, learning, and memory. Here we consider the functional features of these proteins in the nervous system of phylogenetically distant organisms: Drosophila, a well-studied model, and mammals. Disruption of the CPEB proteins functioning is associated with various pathologies, such as autism spectrum disorder and brain cancer. At the same time, CPEB gene regulation can provide for a recovery of the brain function in patients with fragile X syndrome and Huntington's disease, making the CPEB genes promising targets for gene therapy.

mRNA spindle localization and mitotic translational regulation by CPEB1 and CPEB4

Transition through cell cycle phases requires temporal and spatial regulation of gene expression to ensure accurate chromosome duplication and segregation. This regulation involves dynamic reprogramming of gene expression at multiple transcriptional and posttranscriptional levels. In transcriptionally silent oocytes, the CPEB-family of RNAbinding proteins coordinates temporal and spatial translation regulation of stored maternal mRNAs to drive meiotic progression. CPEB1 mediates mRNA localization to the meiotic spindle, which is required to ensure proper chromosome segregation. Temporal translational regulation also takes place in mitosis, where a large repertoire of transcripts are activated or repressed in specific cell cycle phases. However, whether control of localized translation at the spindle is required for mitosis is unclear, as mitotic and acentriolar-meiotic spindles are functionally and structurally different. Furthermore, the large differences in scale-ratio between cell volume and spindle size in oocytes compared to somatic mitotic cells may generate distinct requirements for gene expression compartmentalization in meiosis and mitosis. Here we show that mitotic spindles contain CPE-localized mRNAs and translating ribosomes. Moreover, CPEB1 and CPEB4 localize in the spindles and they may function sequentially in promoting mitotic stage transitions and correct chromosome segregation. Thus, CPEB1 and CPEB4 bind to specific spindle-associated transcripts controlling the expression and/or localization of their encoded factors that, respectively, drive metaphase and anaphase/cytokinesis.

The RNA-binding protein DAZL functions as repressor and activator of mRNA translation during oocyte maturation

Deleted in azoospermia-like (DAZL) is an RNA-binding protein critical for gamete development. In full-grown oocytes, the DAZL protein increases 4-fold during reentry into the meiotic cell cycle. Here, we have investigated the functional significance of this accumulation at a genome-wide level. Depletion of DAZL causes a block in maturation and widespread disruption in the pattern of ribosome loading on maternal transcripts. In addition to decreased translation, DAZL depletion also causes translational activation of a distinct subset of mRNAs both in quiescent and maturing oocytes, a function recapitulated with YFP-3′UTR reporters. DAZL binds to mRNAs whose translation is both repressed and activated during maturation. Injection of recombinant DAZL protein in DAZL-depleted oocytes rescues the translation and maturation to MII. Mutagenesis of putative DAZL-binding sites in these mRNAs mimics the effect of DAZL depletion. These findings demonstrate that DAZL regulates translation of maternal mRNAs, functioning both as the translational repressor and activator during oocyte maturation.

The RNA binding protein DAZL plays a critical role during germ cell development. Here the authors provide evidence that DAZL functions both as activator and repressor of translation during oocyte maturation in mouse.

tailfindr: alignment-free poly(A) length measurement for Oxford Nanopore RNA and DNA sequencing

Polyadenylation at the 3'-end is a major regulator of messenger RNA and its length is known to affect nuclear export, stability, and translation, among others. Only recently have strategies emerged that allow for genome-wide poly(A) length assessment. These methods identify genes connected to poly(A) tail measurements indirectly by short-read alignment to genetic 3'-ends. Concurrently, Oxford Nanopore Technologies (ONT) established full-length isoform-specific RNA sequencing containing the entire poly(A) tail. However, assessing poly(A) length through base-calling has so far not been possible due to the inability to resolve long homopolymeric stretches in ONT sequencing. Here we present tailfindr, an R package to estimate poly(A) tail length on ONT long-read sequencing data. tailfindr operates on unaligned, base-called data. It measures poly(A) tail length from both native RNA and DNA sequencing, which makes poly(A) tail studies by full-length cDNA approaches possible for the first time. We assess tailfindr's performance across different poly(A) lengths, demonstrating that tailfindr is a versatile tool providing poly(A) tail estimates across a wide range of sequencing conditions.

Pericytes in the Gut

This review chapter describes the current knowledge about the nature of pericytes in the gut, their interaction with endothelial cells in blood vessels, and their pathophysiological functions in the setting of chronic liver disease. In particular, it focuses on the role of these vascular cell types and related molecular signaling pathways in pathological angiogenesis associated with liver disease and in the establishment of the gut-vascular barrier and the potential implications in liver disease through the gut-liver axis.

Regulation of Translationally Repressed mRNAs in Zebrafish and Mouse Oocytes

From the beginning of oogenesis, oocytes accumulate tens of thousands of mRNAs for promoting oocyte growth and development. A large number of these mRNAs are translationally repressed and localized within the oocyte cytoplasm. Translational activation of these dormant mRNAs at specific sites and timings plays central roles in driving progression of the meiotic cell cycle, axis formation, mitotic cleavages, transcriptional initiation, and morphogenesis. Regulation of the localization and temporal translation of these mRNAs has been shown to rely on cis-acting elements in the mRNAs and trans-acting factors recognizing and binding to the elements. Recently, using model vertebrate zebrafish, localization itself and formation of physiological structures such as RNA granules have been shown to coordinate the accurate timings of translational activation of dormant mRNAs. This subcellular regulation of mRNAs is also utilized in other animals including mouse. In this chapter, we review fundamental roles of temporal regulation of mRNA translation in oogenesis and early development and then focus on the mechanisms of mRNA regulation in the oocyte cytoplasm by which the activation of dormant mRNAs at specific timings is achieved.

Terminal nucleotidyl transferases (TENTs) in mammalian RNA metabolism

In eukaryotes, almost all RNA species are processed at their 3′ ends and most mRNAs are polyadenylated in the nucleus by canonical poly(A) polymerases. In recent years, several terminal nucleotidyl transferases (TENTs) including non-canonical poly(A) polymerases (ncPAPs) and terminal uridyl transferases (TUTases) have been discovered. In contrast to canonical polymerases, TENTs' functions are more diverse; some, especially TUTases, induce RNA decay while others, such as cytoplasmic ncPAPs, activate translationally dormant deadenylated mRNAs. The mammalian genome encodes 11 different TENTs. This review summarizes the current knowledge about the functions and mechanisms of action of these enzymes.

This article is part of the theme issue ‘5′ and 3′ modifications controlling RNA degradation’.

CPEB2-dependent translation of long 3'-UTR Ucp1 mRNA promotes thermogenesis in brown adipose tissue

Expression of mitochondrial proton transporter uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) is essential for mammalian thermogenesis. While human UCP1 mRNA exists in a long form only, alternative polyadenylation creates two different isoforms in mice with 10% of UCP1 mRNA found in the long form (Ucp1L) and ~90% in the short form (Ucp1S). We generated a mouse model expressing only Ucp1S and found that it showed impaired thermogenesis due to a 60% drop in UCP1 protein levels, suggesting that Ucp1L is more efficiently translated than Ucp1S. In addition, we found that β3 adrenergic receptor signaling promoted the translation of mouse Ucp1L and human Ucp1 in a manner dependent on cytoplasmic polyadenylation element binding protein 2 (CPEB2). CPEB2-knockout mice showed reduced UCP1 levels and impaired thermogenesis in BAT, which was rescued by ectopic expression of CPEB2. Hence, long 3'-UTR Ucp1 mRNA translation activated by CPEB2 is likely conserved and important in humans to produce UCP1 for thermogenesis.

Post-translational regulation of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is essential for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stages of embryogenesis, including maternal component (mRNAs and proteins) degradation and zygotic genome activation (ZGA). Various protein post-translational modifications have been identified during the MZT, such as phosphorylation, methylation and ubiquitination. Precise post-translational regulation mechanisms are essential for the timely transition of early embryonic development. In this review, we summarize recent progress regarding the molecular mechanisms underlying post-translational regulation of maternal component degradation and ZGA during the MZT and discuss some important issues in the field.

Trans-acting translational regulatory RNA binding proteins

The canonical molecular machinery required for global mRNA translation and its control has been well defined, with distinct sets of proteins involved in the processes of translation initiation, elongation and termination. Additionally, noncanonical, trans-acting regulatory RNA-binding proteins (RBPs) are necessary to provide mRNA-specific translation, and these interact with 5' and 3' untranslated regions and coding regions of mRNA to regulate ribosome recruitment and transit. Recently it has also been demonstrated that trans-acting ribosomal proteins direct the translation of specific mRNAs. Importantly, it has been shown that subsets of RBPs often work in concert, forming distinct regulatory complexes upon different cellular perturbation, creating an RBP combinatorial code, which through the translation of specific subsets of mRNAs, dictate cell fate. With the development of new methodologies, a plethora of novel RNA binding proteins have recently been identified, although the function of many of these proteins within mRNA translation is unknown. In this review we will discuss these methodologies and their shortcomings when applied to the study of translation, which need to be addressed to enable a better understanding of trans-acting translational regulatory proteins. Moreover, we discuss the protein domains that are responsible for RNA binding as well as the RNA motifs to which they bind, and the role of trans-acting ribosomal proteins in directing the translation of specific mRNAs. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Translation > Translation Regulation Translation > Translation Mechanisms.

Phosphorylation Dynamics Dominate the Regulated Proteome during Early Xenopus Development

The earliest stages of animal development are largely controlled by changes in protein phosphorylation mediated by signaling pathways and cyclin-dependent kinases. In order to decipher these complex networks and to discover new aspects of regulation by this post-translational modification, we undertook an analysis of the X. laevis phosphoproteome at seven developmental stages beginning with stage VI oocytes and ending with two-cell embryos. Concurrent measurement of the proteome and phosphoproteome enabled measurement of phosphosite occupancy as a function of developmental stage. We observed little change in protein expression levels during this period. We detected the expected phosphorylation of MAP kinases, translational regulatory proteins, and subunits of APC/C that validate the accuracy of our measurements. We find that more than half the identified proteins possess multiple sites of phosphorylation that are often clustered, where kinases work together in a hierarchical manner to create stretches of phosphorylated residues, which may be a means to amplify signals or stabilize a particular protein conformation. Conversely, other proteins have opposing sites of phosphorylation that seemingly reflect distinct changes in activity during this developmental timeline.

Essential role for non-canonical poly(A) polymerase GLD4 in cytoplasmic polyadenylation and carbohydrate metabolism

Regulation of gene expression at the level of cytoplasmic polyadenylation is important for many biological phenomena including cell cycle progression, mitochondrial respiration, and learning and memory. GLD4 is one of the non-canonical poly(A) polymerases that regulates cytoplasmic polyadenylation-induced translation, but its target mRNAs and role in cellular physiology is not well known. To assess the full panoply of mRNAs whose polyadenylation is controlled by GLD4, we performed an unbiased whole genome-wide screen using poy(U) chromatography and thermal elution. We identified hundreds of mRNAs regulated by GLD4, several of which are involved in carbohydrate metabolism including GLUT1, a major glucose transporter. Depletion of GLD4 not only reduced GLUT1 poly(A) tail length, but also GLUT1 protein. GLD4-mediated translational control of GLUT1 mRNA is dependent of an RNA binding protein, CPEB1, and its binding elements in the 3΄ UTR. Through regulating GLUT1 level, GLD4 affects glucose uptake into cells and lactate levels. Moreover, GLD4 depletion impairs glucose deprivation-induced GLUT1 up-regulation. In addition, we found that GLD4 affects glucose-dependent cellular phenotypes such as migration and invasion in glioblastoma cells. Our observations delineate a novel post-transcriptional regulatory network involving carbohydrate metabolism and glucose homeostasis mediated by GLD4.

The Translation of Cyclin B1 and B2 is Differentially Regulated during Mouse Oocyte Reentry into the Meiotic Cell Cycle

Control of protein turnover is critical for meiotic progression. Using RiboTag immunoprecipitation, RNA binding protein immunoprecipitation, and luciferase reporter assay, we investigated how rates of mRNA translation, protein synthesis and degradation contribute to the steady state level of Cyclin B1 and B2 in mouse oocytes. Ribosome loading onto Ccnb1 and Mos mRNAs increases during cell cycle reentry, well after germinal vesicle breakdown (GVBD). This is followed by the translation of reporters containing 3′ untranslated region of Mos or Ccnb1 and the accumulation of Mos and Cyclin B1 proteins. Conversely, ribosome loading onto Ccnb2 mRNA and Cyclin B2 protein level undergo minimal changes during meiotic reentry. Degradation rates of Cyclin B1 or B2 protein at the GV stage are comparable. The translational activation of Mos and Ccnb1, but not Ccnb2, mRNAs is dependent on the RNA binding protein CPEB1. Inhibition of Cdk1 activity, but not Aurora A kinase activity, prevents the translation of Mos or Ccnb1 reporters, suggesting that MPF is required for their translation in mouse oocytes. Conversely, Ccnb2 translation is insensitive to Cdk1 inhibition. Thus, the poised state that allows rapid meiotic reentry in mouse GV oocytes may be determined by the differential translational control of two Cyclins.

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

Role and therapeutic potential of vascular stem/progenitor cells in pathological neovascularisation during chronic portal hypertension

OBJECTIVE

Pathological neovascularisation is intimately involved in portal hypertension (PH). Here, we determined the contribution of vascular stem/progenitor cells (VSPCs) to neovessel growth in PH and whether the RNA-binding protein cytoplasmic polyadenylation element binding protein-4 (CPEB4) was behind the mechanism controlling VSPC function.

DESIGN

To identify and monitor VSPCs in PH rats (portal vein-ligated), we used a combinatorial approach, including sphere-forming assay, assessment of self-renewal, 5-bromo-2'-desoxyuridine label retention technique, in vitro and in vivo stem/progenitor cell (SPC) differentiation and vasculogenic capability, cell sorting, as well as immunohistochemistry, immunofluorescence and confocal microscopy expression analysis. We also determined the role of CPEB4 on VSPC proliferation using genetically engineered mouse models.

RESULTS

We demonstrated the existence in the mesenteric vascular bed of VSPCs displaying capability to form cellular spheres in suspension culture, self-renewal ability, expression of molecules commonly found in SPCs, slow-cycling features, in addition to other cardinal properties exhibited by SPCs, like capacity to differentiate into endothelial cells and pericytes with remarkable vasculogenic activity. Such VSPCs showed, after PH induction, an early switch in proliferation, and differentiated in vivo into endothelial cells and pericytes, contributing, structurally and functionally, to abnormal neovessel formation. Quantification of VSPC-dependent neovessel formation in PH further illustrated the key role played by VSPCs. We also demonstrated that CPEB4 regulates the proliferation of the activated VSPC progeny upon PH induction.

CONCLUSIONS

These findings demonstrate that VSPC-derived neovessel growth (ie, vasculogenesis) and angiogenesis cooperatively stimulate mesenteric neovascularisation in PH and identify VSPC and CPEB4 as potential therapeutic targets.

CPEB4 is regulated during cell cycle by ERK2/Cdk1-mediated phosphorylation and its assembly into liquid-like droplets

The four members of the vertebrate CPEB family of RNA-binding proteins share similar RNA-binding domains by which they regulate the translation of CPE-containing mRNAs, thereby controlling cell cycle and differentiation or synaptic plasticity. However, the N-terminal domains of CPEBs are distinct and contain specific regulatory post-translational modifications that presumably differentially integrate extracellular signals. Here we show that CPEB4 activity is regulated by ERK2- and Cdk1-mediated hyperphosphorylation. These phosphorylation events additively activate CPEB4 in M-phase by maintaining it in its monomeric state. In contrast, unphosphorylated CPEB4 phase separates into inactive, liquid-like droplets through its intrinsically disordered regions in the N-terminal domain. This dynamic and reversible regulation of CPEB4 is coordinated with that of CPEB1 through Cdk1, which inactivates CPEB1 while activating CPEB4, thereby integrating phase-specific signal transduction pathways to regulate cell cycle progression.

ApCPEB4, a non-prion domain containing homolog of ApCPEB, is involved in the initiation of long-term facilitation

Two pharmacologically distinct types of local protein synthesis are required for synapse- specific long-term synaptic facilitation (LTF) in Aplysia: one for initiation and the other for maintenance. ApCPEB, a rapamycin sensitive prion-like molecule regulates a form of local protein synthesis that is specifically required for the maintenance of the LTF. However, the molecular component of the local protein synthesis that is required for the initiation of LTF and that is sensitive to emetine is not known. Here, we identify a homolog of ApCPEB responsible for the initiation of LTF. ApCPEB4 which we have named after its mammalian CPEB4-like homolog lacks a prion-like domain, is responsive to 5-hydroxytryptamine, and is translated (but not transcribed) in an emetine-sensitive, rapamycin-insensitive, and PKA-dependent manner. The ApCPEB4 binds to different target RNAs than does ApCPEB. Knock-down of ApCPEB4 blocked the induction of LTF, whereas overexpression of ApCPEB4 reduces the threshold of the formation of LTF. Thus, our findings suggest that the two different forms of CPEBs play distinct roles in LTF; ApCPEB is required for maintenance of LTF, whereas the ApCPEB4, which lacks a prion-like domain, is required for the initiation of LTF.

N6-Methyladenosine Sequencing Highlights the Involvement of mRNA Methylation in Oocyte Meiotic Maturation and Embryo Development by Regulating Translation in Xenopus laevis

During the oogenesis of Xenopus laevis, oocytes accumulate maternal materials for early embryo development. As the transcription activity of the oocyte is silenced at the fully grown stage and the global genome is reactivated only by the mid-blastula embryo stage, the translation of maternal mRNAs accumulated during oocyte growth should be accurately regulated. Previous evidence has illustrated that the poly(A) tail length and RNA binding elements mediate RNA translation regulation in the oocyte. Recently, RNA methylation has been found to exist in various systems. In this study, we sequenced the N⁶-methyladenosine (m⁶A) modified mRNAs in fully grown germinal vesicle-stage and metaphase II-stage oocytes. As a result, we identified 4207 mRNAs with m⁶A peaks in germinal vesicle-stage or metaphase II-stage oocytes. When we integrated the mRNA methylation data with transcriptome and proteome data, we found that the highly methylated mRNAs showed significantly lower protein levels than those of the hypomethylated mRNAs, although the RNA levels showed no significant difference. We also found that the hypomethylated mRNAs were mainly enriched in the cell cycle and translation pathways, whereas the highly methylated mRNAs were mainly associated with protein phosphorylation. Our results suggest that oocyte mRNA methylation can regulate cellular translation and cell division during oocyte meiotic maturation and early embryo development.

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.

Hormonal stimulation of starfish oocytes induces partial degradation of the 3' termini of cyclin B mRNAs with oligo(U) tails, followed by poly(A) elongation

In yeast, plant, and mammalian somatic cells, short poly(A) tails on mRNAs are subject to uridylation, which mediates mRNA decay. Although mRNA uridylation has never been reported in animal oocytes, maternal mRNAs with short poly(A) tails are believed to be translationally repressed. In this study, we found that 96% of cyclin B mRNAs with short poly(A) tails were uridylated in starfish oocytes. Hormonal stimulation induced poly(A) elongation of cyclin B mRNA, and 62% of long adenine repeats did not contain uridine residues. To determine whether uridylated short poly(A) tails destabilize cyclin B mRNA, we developed a method for producing RNAs with the strict 3' terminal sequences of cyclin B, with or without oligo(U) tails. When we injected these synthetic RNAs into starfish oocytes prior to hormonal stimulation, we found that uridylated RNAs were as stable as nonuridylated RNAs. Following hormonal stimulation, the 3' termini of short poly(A) tails of synthesized RNAs containing oligo(U) tails were trimmed, and their poly(A) tails were subsequently elongated. These results indicate that uridylation of short poly(A) tails in cyclin B mRNA of starfish oocytes does not mediate mRNA decay; instead, hormonal stimulation induces partial degradation of uridylated short poly(A) tails in the 3'-5' direction, followed by poly(A) elongation. Oligo(U) tails may be involved in translational inactivation of mRNAs.

Amyloidogenic Oligomerization Transforms Drosophila Orb2 from a Translation Repressor to an Activator

Memories are thought to be formed in response to transient experiences, in part through changes in local protein synthesis at synapses. In Drosophila, the amyloidogenic (prion-like) state of the RNA binding protein Orb2 has been implicated in long-term memory, but how conformational conversion of Orb2 promotes memory formation is unclear. Combining in vitro and in vivo studies, we find that the monomeric form of Orb2 represses translation and removes mRNA poly(A) tails, while the oligomeric form enhances translation and elongates the poly(A) tails and imparts its translational state to the monomer. The CG13928 protein, which binds only to monomeric Orb2, promotes deadenylation, whereas the putative poly(A) binding protein CG4612 promotes oligomeric Orb2-dependent translation. Our data support a model in which monomeric Orb2 keeps target mRNA in a translationally dormant state and experience-dependent conversion to the amyloidogenic state activates translation, resulting in persistent alteration of synaptic activity and stabilization of memory.

Sequential Functions of CPEB1 and CPEB4 Regulate Pathologic Expression of Vascular Endothelial Growth Factor and Angiogenesis in Chronic Liver Disease

BACKGROUND & AIMS

Vascular endothelial growth factor (VEGF) regulates angiogenesis, yet therapeutic strategies to disrupt VEGF signaling can interfere with physiologic angiogenesis. In a search for ways to inhibit pathologic production or activities of VEGF without affecting its normal production or functions, we investigated the post-transcriptional regulation of VEGF by the cytoplasmic polyadenylation element-binding proteins CPEB1 and CPEB4 during development of portal hypertension and liver disease.

METHODS

We obtained transjugular liver biopsies from patients with hepatitis C virus-associated cirrhosis or liver tissues removed during transplantation; healthy human liver tissue was obtained from a commercial source (control). We also performed experiments with male Sprague-Dawley rats and CPEB-deficient mice (C57BL6 or mixed C57BL6/129 background) and their wild-type littermates. Secondary biliary cirrhosis was induced in rats by bile duct ligation, and portal hypertension was induced by partial portal vein ligation. Liver and mesenteric tissues were collected and analyzed in angiogenesis, reverse transcription polymerase chain reaction, polyA tail, 3' rapid amplification of complementary DNA ends, Southern blot, immunoblot, histologic, immunohistochemical, immunofluorescence, and confocal microscopy assays. CPEB was knocked down with small interfering RNAs in H5V endothelial cells, and translation of luciferase reporters constructs was assessed.

RESULTS

Activation of CPEB1 promoted alternative nuclear processing within noncoding 3'-untranslated regions of VEGF and CPEB4 messenger RNAs in H5V cells, resulting in deletion of translation repressor elements. The subsequent overexpression of CPEB4 promoted cytoplasmic polyadenylation of VEGF messenger RNA, increasing its translation; the high levels of VEGF produced by these cells led to their formation of tubular structures in Matrigel assays. We observed increased levels of CPEB1 and CPEB4 in cirrhotic liver tissues from patients, compared with control tissue, as well as in livers and mesenteries of rats and mice with cirrhosis or/and portal hypertension. Mice with knockdown of CPEB1 or CPEB4 did not overexpress VEGF or have signs of mesenteric neovascularization, and developed less-severe forms of portal hypertension after portal vein ligation.

CONCLUSIONS

We identified a mechanism of VEGF overexpression in liver and mesentery that promotes pathologic, but not physiologic, angiogenesis, via sequential and nonredundant functions of CPEB1 and CPEB4. Regulation of CPEB4 by CPEB1 and the CPEB4 autoamplification loop induces pathologic angiogenesis. Strategies to block the activities of CPEBs might be developed to treat chronic liver and other angiogenesis-dependent diseases.

New Phosphospecific Antibody Reveals Isoform-Specific Phosphorylation of CPEB3 Protein

Cytoplasmic Polyadenylation Element Binding proteins (CPEBs) are a family of polyadenylation factors interacting with 3’UTRs of mRNA and thereby regulating gene expression. Various functions of CPEBs in development, synaptic plasticity, and cellular senescence have been reported. Four CPEB family members of partially overlapping functions have been described to date, each containing a distinct alternatively spliced region. This region is highly conserved between CPEBs-2-4 and contains a putative phosphorylation consensus, overlapping with the exon seven of CPEB3. We previously found CPEBs-2-4 splice isoforms containing exon seven to be predominantly present in neurons, and the isoform expression pattern to be cell type-specific. Here, focusing on the alternatively spliced region of CPEB3, we determined that putative neuronal isoforms of CPEB3 are phosphorylated. Using a new phosphospecific antibody directed to the phosphorylation consensus we found Protein Kinase A and Calcium/Calmodulin-dependent Protein Kinase II to robustly phosphorylate CPEB3 in vitro and in primary hippocampal neurons. Interestingly, status epilepticus induced by systemic kainate injection in mice led to specific upregulation of the CPEB3 isoforms containing exon seven. Extensive analysis of CPEB3 phosphorylation in vitro revealed two other phosphorylation sites. In addition, we found plethora of potential kinases that might be targeting the alternatively spliced kinase consensus site of CPEB3. As this site is highly conserved between the CPEB family members, we suggest the existence of a splicing-based regulatory mechanism of CPEB function, and describe a robust phosphospecific antibody to study it in future.

Post-Translational Modifications and RNA-Binding Proteins

RNA-binding proteins affect cellular metabolic programs through development and in response to cellular stimuli. Though much work has been done to elucidate the roles of a handful of RNA-binding proteins and their effect on RNA metabolism, the progress of studies to understand the effects of post-translational modifications of this class of proteins is far from complete. This chapter summarizes the work that has been done to identify the consequence of post-translational modifications to some RNA-binding proteins. The effects of these modifications have been shown to increase the panoply of functions that a given RNA-binding protein can assume. We will survey the experimental methods that are used to identify the presence of several protein modifications and methods that attempt to discern the consequence of these modifications.

Roles of RNA-binding Proteins and Post-transcriptional Regulation in Driving Male Germ Cell Development in the Mouse

Tissue development and homeostasis are dependent on highly regulated gene expression programs in which cell-specific combinations of regulatory factors determine which genes are expressed and the post-transcriptional fate of the resulting RNA transcripts. Post-transcriptional regulation of gene expression by RNA-binding proteins has critical roles in tissue development-allowing individual genes to generate multiple RNA and protein products, and the timing, location, and abundance of protein synthesis to be finely controlled. Extensive post-transcriptional regulation occurs during mammalian gametogenesis, including high levels of alternative mRNA expression, stage-specific expression of mRNA variants, broad translational repression, and stage-specific activation of mRNA translation. In this chapter, an overview of the roles of RNA-binding proteins and the importance of post-transcriptional regulation in male germ cell development in the mouse is presented.

Block of CDK1-dependent polyadenosine elongation of Cyclin B mRNA in metaphase-i-arrested starfish oocytes is released by intracellular pH elevation upon spawning

Meiotic progression requires the translation of maternal mRNAs in a strict temporal order. In isolated animal oocytes, translation of maternal mRNAs containing a cytoplasmic polyadenylation element (CPE), such as cyclin B, is activated by in vitro stimulation of meiotic resumption which induces phosphorylation of CPEB (CPE-binding protein) and elongation of their polyadenosine (poly(A)) tails; whether or not this model can be applied in vivo to oocytes arrested at metaphase of meiosis I in ovaries is unknown. In this study, we found that active CDK1 (cyclin-dependent kinase 1) phosphorylated CPEB in ovarian oocytes arrested at metphase I in the starfish body cavity, but phosphorylation of CPEB was not sufficient for elongation of cyclin B poly(A) tails. Immediately after spawning, however, mRNA was polyadenylated, suggesting that an increase in intracellular pH (pHi ) upon spawning triggers the elongation of poly(A) tails. Using a cell-free system made from maturing oocytes at metaphase I, we demonstrated that polyadenylation was indeed suppressed at pH below 7.0. These results suggest that a pH-sensitive process, functioning after CPEB phosphorylation, is blocked under physiologically low pHi (<7.0) in metaphase-I-arrested oocytes. The increase in pHi (>7.0) that occurs after spawning triggers polyadenylation of cyclin B mRNA and progression into meiosis II.

Cytoplasmic polyadenylation in mammalian oocyte maturation

Oocyte developmental competence is the ability of the mature oocyte to be fertilized and subsequently drive early embryo development. Developmental competence is acquired by completion of oocyte maturation, a process that includes nuclear (meiotic) and cytoplasmic (molecular) changes. Given that maturing oocytes are transcriptionally quiescent (as are early embryos), they depend on post-transcriptional regulation of stored transcripts for protein synthesis, which is largely mediated by translational repression and deadenylation of transcripts within the cytoplasm, followed by recruitment of specific transcripts in a spatiotemporal manner for translation during oocyte maturation and early development. Motifs within the 3' untranslated region (UTR) of messenger RNA (mRNA) are thought to mediate repression and downstream activation by their association with binding partners that form dynamic protein complexes that elicit differing effects on translation depending on cell stage and interacting proteins. The cytoplasmic polyadenylation (CP) element, Pumilio binding element, and hexanucleotide polyadenylation signal are among the best understood motifs involved in CP, and translational regulation of stored transcripts as their binding partners have been relatively well-characterized. Knowledge of CP in mammalian oocytes is discussed as well as novel approaches that can be used to enhance our understanding of the functional and contributing features to transcript CP and translational regulation during mammalian oocyte maturation. WIREs RNA 2016, 7:71-89. doi: 10.1002/wrna.1316 For further resources related to this article, please visit the WIREs website.

CPEB1 modulates differentiation of glioma stem cells via downregulation of HES1 and SIRT1 expression

Glioma stemness has been recognized as the most important reason for glioma relapse and drug resistance. Differentiation of glioma stem cells (GSCs) has been implicated as a novel approach to target recurrent glioma. However, the detailed molecular mechanism involved in the differentiation of GSCs has not yet been elucidated. This study identified CPEB1 as the key modulator that induces the differentiation of GSCs at the post-transcriptional level. Gain and loss of function experiments showed that CPEB1 expression reduced sphere formation ability and the expression of stemness markers such as Nestin and Notch. To elucidate the detailed molecular mechanism underlying the action of CPEB1, we investigated the interacting ribonome of the CPEB1 complex using a Ribonomics approach. CPEB1 specifically suppressed the translation of HES1 and SIRT1 by interacting with a cytoplasmic polyadenylation element. The expression profile of CPEB1 negatively correlated with overall survival in glioma patients. Overexpression of CPEB1 decreased the number of GSCs in an orthotopically implanted glioma animal model. These results suggest that CPEB1-mediated translational control is essential for the differentiation of GSCs and provides novel therapeutic concepts for differentiation therapy.

p53 Mutation Directs AURKA Overexpression via miR-25 and FBXW7 in Prostatic Small Cell Neuroendocrine Carcinoma

Prostatic small cell neuroendocrine carcinoma (SCNC) is a rare but aggressive form of prostate cancer that is negative for androgen receptor (AR) and not responsive to hormonal therapy. The molecular etiology of this prostate cancer variant is not well understood; however, mutation of the p53 (TP53) tumor suppressor in prostate neuroendocrine cells inactivates the IL8-CXCR2-p53 pathway that normally inhibits cellular proliferation, leading to the development of SCNC. SCNC also overexpresses Aurora kinase A (AURKA) which is considered to be a viable therapeutic target. Therefore, the relationship of these two molecular events was studied, and we show that p53 mutation leads to increased expression of miR-25 and downregulation of the E3 ubiquitin ligase FBXW7, resulting in elevated levels of Aurora kinase A. This study demonstrates an intracellular pathway by which p53 mutation leads to Aurora kinase A expression, which is critically important for the rapid proliferation and aggressive behavior of prostatic SCNC.

IMPLICATIONS

The pathogenesis of prostatic SCNC involves a p53 and Aurora Kinase A signaling mechanism, both potentially targetable pathways.

RNA-binding proteins, multifaceted translational regulators in cancer

RNA-binding proteins (RBPs) orchestrate transcript fate and function. Even though alterations in post-transcriptional events contribute to key steps of tumor initiation and progression, RBP-mediated control has remained relatively unexplored in cancer. Here, we discuss examples of this promising field focusing on translation regulation, and highlight the variety of molecular mechanisms by which RBPs impinge on translation with consequences for tumorigenesis. This article is part of a Special Issue entitled: Translation and Cancer.

A fly trap mechanism provides sequence-specific RNA recognition by CPEB proteins

How CPEB RNA-binding proteins regulate cytoplasmic polyadenylation and translation is poorly understood. Allain and colleagues report the structures of the tandem RNA recognition motifs (RRMs) of two human paralogs (CPEB1 and CPEB4) in their free and RNA-bound states. Structural and functional studies reveal how RNA binding by CPEB proteins leads to an optimal positioning of the N-terminal and zinc-binding domains at the 3′ UTR, which favors the nucleation of ribonucleoprotein complexes for translation regulation. This study provides the molecular basis for the translational regulatory circuit established by CPEB proteins.

Cytoplasmic changes in polyA tail length is a key mechanism of translational control and is implicated in germline development, synaptic plasticity, cellular proliferation, senescence, and cancer progression. The presence of a U-rich cytoplasmic polyadenylation element (CPE) in the 3′ untranslated regions (UTRs) of the responding mRNAs gives them the selectivity to be regulated by the CPE-binding (CPEB) family of proteins, which recognizes RNA via the tandem RNA recognition motifs (RRMs). Here we report the solution structures of the tandem RRMs of two human paralogs (CPEB1 and CPEB4) in their free and RNA-bound states. The structures reveal an unprecedented arrangement of RRMs in the free state that undergo an original closure motion upon RNA binding that ensures high fidelity. Structural and functional characterization of the ZZ domain (zinc-binding domain) of CPEB1 suggests a role in both protein–protein and protein–RNA interactions. Together with functional studies, the structures reveal how RNA binding by CPEB proteins leads to an optimal positioning of the N-terminal and ZZ domains at the 3′ UTR, which favors the nucleation of the functional ribonucleoprotein complexes for translation regulation.

Cytoplasmic polyadenylation element binding proteins in development, health, and disease

The cytoplasmic polyadenylation element binding (CPEB) proteins are sequence-specific mRNA binding proteins that control translation in development, health, and disease. CPEB1, the founding member of this family, has become an important model for illustrating general principles of translational control by cytoplasmic polyadenylation in gametogenesis, cancer etiology, synaptic plasticity, learning, and memory. Although the biological functions of the other members of this protein family in vertebrates are just beginning to emerge, it is already evident that they, too, mediate important processes, such as cancer etiology and higher cognitive function. In Drosophila, the CPEB proteins Orb and Orb2 play key roles in oogenesis and in neuronal function, as do related proteins in Caenorhabditis elegans and Aplysia. We review the biochemical features of the CPEB proteins, discuss their activities in several biological systems, and illustrate how understanding CPEB activity in model organisms has an important impact on neurological disease.

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.