Musashi regulates the temporal order of mRNA translation during Xenopus oocyte maturation

Structure of Musashi1 in a complex with target RNA: the role of aromatic stacking interactions

Mammalian Musashi1 (Msi1) is an RNA-binding protein that regulates the translation of target mRNAs, and participates in the maintenance of cell ‘stemness’ and tumorigenesis. Msi1 reportedly binds to the 3′-untranslated region of mRNA of Numb, which encodes Notch inhibitor, and impedes initiation of its translation by competing with eIF4G for PABP binding, resulting in triggering of Notch signaling. Here, the mechanism by which Msi1 recognizes the target RNA sequence using its Ribonucleoprotein (RNP)-type RNA-binding domains (RBDs), RBD1 and RBD2 has been revealed on identification of the minimal binding RNA for each RBD and determination of the three-dimensional structure of the RBD1:RNA complex. Unique interactions were found for the recognition of the target sequence by Msi1 RBD1: adenine is sandwiched by two phenylalanines and guanine is stacked on the tryptophan in the loop between β1 and α1. The minimal recognition sequences that we have defined for Msi1 RBD1 and RBD2 have actually been found in many Msi1 target mRNAs reported to date. The present study provides molecular clues for understanding the biology involving Musashi family proteins.

Translational control in oocyte development

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

Translational control in germ cell development: A role for the RNA-binding proteins Musashi-1 and Musashi-2

Mammalian gametogenesis is a complex process involving specialised cell cycle progression and differentiation. As part of their differentiation, germ cells experience periods of transcriptional inactivation and chromatin inaccessibility whilst continuing to coordinate the correct temporal and spatial expression of genes required for continued development. To overcome these obstacles, mammalian germ cells express a wide variety of sequence-specific RNA-binding proteins, which assist in the translational control of many mRNA transcripts which are produced and stored during periods of high mRNA synthesis. In this review we focus on the Musashi family of RNA-binding proteins, a highly conserved family of translational regulatory proteins whose recent identification in germ cells of Drosophila and Xenopus, as well as their well described role in processes such as cell cycle progression and stem cell identity, has led us to investigate the role of these proteins in mammalian germ cell development.

DAZAP1, an RNA-binding protein required for development and spermatogenesis, can regulate mRNA translation

DAZ-associated protein 1 (DAZAP1) is an RNA-binding protein required for normal growth, development, and fertility in mice. However, its molecular functions have not been elucidated. Here we find that Xenopus laevis and human DAZAP1, which are each expressed as short and long forms, act as mRNA-specific activators of translation in a manner that is sensitive to the number of binding sites present within the 3' UTR. Domain mapping suggests that this conserved function is mainly associated with C-terminal regions of DAZAP1. Interestingly, we find that the expression of xDAZAP1 and its polysome association are developmentally controlled, the latter suggesting that the translational activator function of DAZAP1 is regulated. However, ERK phosphorylation of DAZAP1, which can alter protein interactions with its C terminus, does not play a role in regulating its ability to participate in translational complexes. Since relatively few mRNA-specific activators have been identified, we explored the mechanism by which DAZAP1 activates translation. By utilizing reporter mRNAs with internal ribosome entry sites, we establish that DAZAP1 stimulates translation initiation. Importantly, this activity is not dependent on the recognition of the 5' cap by initiation factors, showing that it functions downstream from this frequently regulated event, but is modulated by changes in the adenylation status of mRNAs. This suggests a function in the formation of "end-to-end" complexes, which are important for efficient initiation, which we show to be independent of a direct interaction with the bridging protein eIF4G.

The poly(rC)-binding protein alphaCP2 is a noncanonical factor in X. laevis cytoplasmic polyadenylation

Post-transcriptional control of mRNA stability and translation is central to multiple developmental pathways. This control can be linked to cytoplasmic polyadenylation in certain settings. In maturing Xenopus oocytes, specific mRNAs are targeted for polyadenylation via recruitment of the Cytoplasmic Polyadenylation Element (CPE) binding protein (CPEB) to CPE(s) within the 3' UTR. Cytoplasmic polyadenylation is also critical to early embryonic events, although corresponding determinants are less defined. Here, we demonstrate that the Xenopus ortholog of the poly(rC) binding protein αCP2 can recruit cytoplasmic poly(A) polymerase activity to mRNAs in Xenopus post-fertilization embryos, and that this recruitment relies on cis sequences recognized by αCP2. We find that the hα-globin 3' UTR, a validated mammalian αCP2 target, constitutes an effective target for cytoplasmic polyadenylation in Xenopus embryos, but not during Xenopus oocyte maturation. We further demonstrate that the cytoplasmic polyadenylation activity is dependent on the action of the C-rich αCP-binding site in conjunction with the adjacent AAUAAA. Consistent with its ability to target mRNA for poly(A) addition, we find that XαCP2 associates with core components of the Xenopus cytoplasmic polyadenylation complex, including the cytoplasmic poly(A) polymerase XGLD2. Furthermore, we observe that the C-rich αCP-binding site can robustly enhance the activity of a weak canonical oocyte maturation CPE in early embryos, possibly via a direct interaction between XαCP2 and CPEB1. These studies establish XαCP2 as a novel cytoplasmic polyadenylation trans factor, indicate that C-rich sequences can function as noncanonical cytoplasmic polyadenylation elements, and expand our understanding of the complexities underlying cytoplasmic polyadenylation in specific developmental settings.

Identification of a novel intronic enhancer responsible for the transcriptional regulation of musashi1 in neural stem/progenitor cells

Background

The specific genetic regulation of neural primordial cell determination is of great interest in stem cell biology. The Musashi1 (Msi1) protein, which belongs to an evolutionarily conserved family of RNA-binding proteins, is a marker for neural stem/progenitor cells (NS/PCs) in the embryonic and post-natal central nervous system (CNS). Msi1 regulates the translation of its downstream targets, including m-Numb and p21 mRNAs. In vitro experiments using knockout mice have shown that Msi1 and its isoform Musashi2 (Msi2) keep NS/PCs in an undifferentiated and proliferative state. Msi1 is expressed not only in NS/PCs, but also in other somatic stem cells and in tumours. Based on previous findings, Msi1 is likely to be a key regulator for maintaining the characteristics of self-renewing stem cells. However, the mechanisms regulating Msi1 expression are not yet clear.

Results

To identify the DNA region affecting Msi1 transcription, we inserted the fusion gene ffLuc, comprised of the fluorescent Venus protein and firefly Luciferase, at the translation initiation site of the mouse Msi1 gene locus contained in a 184-kb bacterial artificial chromosome (BAC). Fluorescence and Luciferase activity, reflecting the Msi1 transcriptional activity, were observed in a stable BAC-carrying embryonic stem cell line when it was induced toward neural lineage differentiation by retinoic acid treatment. When neuronal differentiation was induced in embryoid body (EB)-derived neurosphere cells, reporter signals were detected in Msi1-positive NSCs and GFAP-positive astrocytes, but not in MAP2-positive neurons. By introducing deletions into the BAC reporter gene and conducting further reporter experiments using a minimized enhancer region, we identified a region, "D5E2," that is responsible for Msi1 transcription in NS/PCs.

Conclusions

A regulatory element for Msi1 transcription in NS/PCs is located in the sixth intron of the Msi1 gene. The 595-bp D5E2 intronic enhancer can transactivate Msi1 gene expression with cell-type specificity markedly similar to the endogenous Msi1 expression patterns.

Context-dependent regulation of Musashi-mediated mRNA translation and cell cycle regulation

Musashi-mediated mRNA translational control has been implicated in the promotion of physiological and pathological stem cell proliferation. During self-renewal of mammalian stem cells, Musashi has been proposed to act to repress the translation of mRNAs encoding inhibitors of cell cycle progression. By contrast, in maturing Xenopus oocytes Musashi activates translation of target mRNAs that encode proteins promoting cell cycle progression. The mechanisms directing Musashi to differentially control mRNA translation in mammalian stem cells and Xenopus oocytes is unknown. In this study, we demonstrate that the mechanisms defining Musashi function lie within the cellular context. Specifically, we show that murine Musashi acts as an activator of translation in maturing Xenopus oocytes while Xenopus Musashi functions as a repressor of target mRNA translation in mammalian cells. We further demonstrate that within the context of a primary mammalian neural stem/progenitor cell, Musashi can be converted from a repressor of mRNA translation to an activator of translation in response to extracellular stimuli. We present current models of Musashi-mediated mRNA translational control and discuss possible mechanisms for regulating Musashi function. An understanding of these mechanisms presents exciting possibilities for development of therapeutic targets to control physiological and pathological stem cell proliferation.

Mos in the oocyte: how to use MAPK independently of growth factors and transcription to control meiotic divisions

In many cell types, the mitogen-activated protein kinase (MAPK) also named extracellular signal-regulated kinase (ERK) is activated in response to a variety of extracellular growth factor-receptor interactions and leads to the transcriptional activation of immediate early genes, hereby influencing a number of tissue-specific biological activities, as cell proliferation, survival and differentiation. In one specific cell type however, the female germ cell, MAPK does not follow this canonical scheme. In oocytes, MAPK is activated independently of growth factors and tyrosine kinase receptors, acts independently of transcriptional regulation, plays a crucial role in controlling meiotic divisions, and is under the control of a peculiar upstream regulator, the kinase Mos. Mos was originally identified as the transforming gene of Moloney murine sarcoma virus and its cellular homologue was the first proto-oncogene to be molecularly cloned. What could be the specific roles of Mos that render it necessary for meiosis? Which unique functions could explain the evolutionary cost to have selected one gene to only serve for few hours in one very specific cell type? This review discusses the original features of MAPK activation by Mos and the roles of this module in oocytes.

Developmental timing of mRNA translation--integration of distinct regulatory elements

Targeted mRNA translation is emerging as a critical mechanism to control gene expression during developmental processes. Exciting new findings have revealed a critical role for regulatory elements within the mRNA untranslated regions to direct the timing of mRNA translation. Regulatory elements can be targeted by sequence-specific binding proteins to direct either repression or activation of mRNA translation in response to developmental signals. As new regulatory elements continue to be identified it has become clear that targeted mRNAs can contain multiple regulatory elements, directing apparently contradictory translational patterns. How is this complex regulatory input integrated? In this review, we focus on a new challenge area-how sequence-specific RNA binding proteins respond to developmental signals and functionally integrate to regulate the extent and timing of target mRNA translation. We discuss current understanding with a particular emphasis on the control of cell cycle progression that is mediated through a complex interplay of distinct mRNA regulatory elements during Xenopus oocyte maturation.

To polyadenylate or to deadenylate: that is the question

mRNA polyadenylation and deadenylation are important processes that allow rapid regulation of gene expression in response to different cellular conditions. Almost all eukaryotic mRNA precursors undergo a co-transcriptional cleavage followed by polyadenylation at the 3' end. After the signals are selected, polyadenylation occurs to full extent, suggesting that this first round of polyadenylation is a default modification for most mRNAs. However, the length of these poly(A) tails changes by the activation of deadenylation, which might regulate gene expression by affecting mRNA stability, mRNA transport, or translation initiation. The mechanisms behind deadenylation activation are highly regulated and associated with cellular conditions such as development, mRNA surveillance, DNA damage response, cell differentiation and cancer. After deadenylation, depending on the cellular response, some mRNAs might undergo an extension of the poly(A) tail or degradation. The polyadenylation/deadenylation machinery itself, miRNAs, or RNA binding factors are involved in the regulation of polyadenylation/deadenylation. Here, we review the mechanistic connections between polyadenylation and deadenylation and how the two processes are regulated in different cellular conditions. It is our conviction that further studies of the interplay between polyadenylation and deadenylation will provide critical information required for a mechanistic understanding of several diseases, including cancer development.

Neural RNA-binding protein Musashi1 controls midline crossing of precerebellar neurons through posttranscriptional regulation of Robo3/Rig-1 expression

Precisely regulated spatiotemporal gene expression is essential for the establishment of neural circuits. In contrast to the increasing evidence for transcriptional regulation of axon guidance cues and receptors, the role of posttranscriptional regulation in axon guidance, especially in vivo, remains poorly characterized. Here, we demonstrate that the expression of Slit receptor Robo3/Rig-1, which plays crucial roles in axonal midline crossing, is regulated by a neural RNA-binding protein Musashi1 (Msi1). Msi1 binds to Robo3 mRNA through RNA recognition motifs and increases the protein level of Robo3 without affecting its mRNA level. In Msi1-deficient precerebellar neurons, Robo3 protein, but not its mRNA, is dramatically reduced. Moreover, similar to defects in Robo3-deficient mice, axonal midline crossing and neuronal migration of precerebellar neurons are severely impaired in Msi1-deficient mice. Together, these findings indicate that Msi1-mediated posttranscriptional regulation of Robo3 controls midline crossing of precerebellar neurons.

Disease-associated mutations that alter the RNA structural ensemble

Genome-wide association studies (GWAS) often identify disease-associated mutations in intergenic and non-coding regions of the genome. Given the high percentage of the human genome that is transcribed, we postulate that for some observed associations the disease phenotype is caused by a structural rearrangement in a regulatory region of the RNA transcript. To identify such mutations, we have performed a genome-wide analysis of all known disease-associated Single Nucleotide Polymorphisms (SNPs) from the Human Gene Mutation Database (HGMD) that map to the untranslated regions (UTRs) of a gene. Rather than using minimum free energy approaches (e.g. mFold), we use a partition function calculation that takes into consideration the ensemble of possible RNA conformations for a given sequence. We identified in the human genome disease-associated SNPs that significantly alter the global conformation of the UTR to which they map. For six disease-states (Hyperferritinemia Cataract Syndrome, β-Thalassemia, Cartilage-Hair Hypoplasia, Retinoblastoma, Chronic Obstructive Pulmonary Disease (COPD), and Hypertension), we identified multiple SNPs in UTRs that alter the mRNA structural ensemble of the associated genes. Using a Boltzmann sampling procedure for sub-optimal RNA structures, we are able to characterize and visualize the nature of the conformational changes induced by the disease-associated mutations in the structural ensemble. We observe in several cases (specifically the 5′ UTRs of FTL and RB1) SNP–induced conformational changes analogous to those observed in bacterial regulatory Riboswitches when specific ligands bind. We propose that the UTR and SNP combinations we identify constitute a “RiboSNitch,” that is a regulatory RNA in which a specific SNP has a structural consequence that results in a disease phenotype. Our SNPfold algorithm can help identify RiboSNitches by leveraging GWAS data and an analysis of the mRNA structural ensemble.

Genome-wide association studies identify mutations in the human genome that correlate with a particular disease. It is common to find mutations associated with disease in the non-coding region of the genome. These non-coding mutations are more difficult to interpret at a molecular level, because they do not affect the protein sequence. In this study, we analyze disease-associated mutations in non-coding regions of our genome in the context of their structural effect on the message of genetic information in our cells, Ribonucleic Acid (RNA). We focus in particular on the regulatory parts of our genes known as untranslated regions. We find that certain disease-associated mutations in these regulatory untranslated regions have a significant effect on the structure of the RNA message. We call these elements “RiboSNitches,” because they act like switches turning on and off genes, but are caused by Single Nucleotide Polymorphisms (SNPs), which are single point mutations in our genome. The RiboSNitches we identify are potentially a new class of pharmaceutical targets, as it is possible to change the structure of RNA with small drug-like molecules.

Translational repression by the oocyte-specific protein P100 in Xenopus

The translational regulation of maternal mRNAs is one of the most important steps in the control of temporal-spatial gene expression during oocyte maturation and early embryogenesis in various species. Recently, it has become clear that protein components of mRNPs play essential roles in the translational regulation of maternal mRNAs. In the present study, we investigated the function of P100 in Xenopus oocytes. P100 exhibits sequence conservation with budding yeast Pat1 and is likely the orthologue of human Pat1a (also called PatL2). P100 is maternally expressed in immature oocytes, but disappears during oocyte maturation. In oocytes, P100 is an RNA binding component of ribosome-free mRNPs, associating with other mRNP components such as Xp54, xRAP55 and CPEB. Translational repression by overexpression of P100 occurred when reporter mRNAs were injected into oocytes. Intriguingly, we found that when P100 was overexpressed in the oocytes, the kinetics of oocyte maturation was considerably retarded. In addition, overexpression of P100 in oocytes significantly affected the accumulation of c-Mos and cyclin B1 during oocyte maturation. These results suggest that P100 plays a role in regulating the translation of specific maternal mRNAs required for the progression of Xenopus oocyte maturation.

Oocyte triplet pairing for electrophysiological investigation of gap junctional coupling

Gap junctions formed by expressing connexin subunits in Xenopus oocytes provide a valuable tool for revealing the gating properties of intercellular gap junctions in electrically coupled cells. We describe a new method that consists of simultaneous triple recordings from 3 apposed oocytes expressing exogenous connexins. The advantages of this method are that in one single experiment, 1 oocyte serves as control while a pair of oocytes, which have been manipulated differently, may be tested for different gap junctional properties. Moreover, we can study simultaneously the gap junctional coupling of 3 different pairs of oocytes in the same preparation. If the experiment consists of testing the effect of a single drug, this approach will reduce the time required, as background coupling in control pairs of oocytes does not need to be measured separately as with the conventional 2 oocyte pairing. The triplet approach also increases confidence that any changes seen in junctional communication are due to the experimental treatment and not variation in the preparation of oocytes or execution of the experiment. In this study, we show the example of testing the gap junctional properties among 3 oocytes, 2 of which are expressing rat connexin36.

Role of oocyte quality in meiotic maturation and embryonic development

The quality of oocytes plays a key role in a proper embryo development. In humans, oocytes of poor quality may be the cause of women infertility and an important obstacle in successful in vitro fertilization (IVF). The competence of oocytes depends on numerous processes taking place during the whole oogenesis, but its final steps such as oocyte maturation, seem to be of key importance. In this paper, we overview factors involved in the development of a fully functional female gamete with Xenopus laevis as a major experimental model. Modern approaches, e.g. proteomic analysis, enable the identification of novel proteins involved in oocyte development. EP45, called also Seryp or pNiXa, which belongs to the serpin (serine protease inhibitors) super-family is one of such recently analyzed proteins. This protein seems to be involved in the stimulation of meiotic maturation and embryo development. EP45 is potentially a key factor in correct oocyte development and determining the quality of oocytes.

A novel, noncanonical mechanism of cytoplasmic polyadenylation operates in Drosophila embryogenesis

Cytoplasmic polyadenylation is a widespread mechanism to regulate mRNA translation that requires two sequences in the 3' untranslated region (UTR) of vertebrate substrates: the polyadenylation hexanucleotide, and the cytoplasmic polyadenylation element (CPE). Using a cell-free Drosophila system, we show that these signals are not relevant for Toll polyadenylation but, instead, a "polyadenylation region" (PR) is necessary. Competition experiments indicate that PR-mediated polyadenylation is required for viability and is mechanistically distinct from the CPE/hexanucleotide-mediated process. These data indicate that Toll mRNA is polyadenylated by a noncanonical mechanism, and suggest that a novel machinery functions for cytoplasmic polyadenylation during Drosophila embryogenesis.

Kicked by Mos and tuned by MPF-the initiation of the MAPK cascade in Xenopus oocytes

The mitogen-activated protein kinase (MAPK) cascade is a paradigmatic signaling cascade, which plays a crucial role in many aspects of cellular events. The main initiator of the cascade in Xenopus oocytes is the oncoprotein Mos. After activation of the cascade, Mos activity is stabilized by MAPK via a feedback loop. Mos concentration levels are, however, not controlled by MAPK alone. In this paper we show, by imposing either a sustained or a peaked activity of M-phase promoting factor (MPF) (Cdc2-cyclin B), how the latter regulates the dynamics of Mos. Our experiments are supported by a detailed kinetic model for the Mos-MPF-MAPK network, which takes into account the three different phosphorylation states of Mos and, as a consequence, allows us to determine the time evolution of Mos under control of MPF. Our work opens a path toward a more complete and biologically realistic quantitative understanding of the dynamic interdependence of Mos and MPF in Xenopus oocytes.

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.

Genomic analyses of musashi1 downstream targets show a strong association with cancer-related processes

Musashi1 (Msi1) is a highly conserved RNA-binding protein with pivotal functions in stem cell maintenance, nervous system development, and tumorigenesis. Despite its importance, only three direct mRNA targets have been characterized so far: m-numb, CDKN1A, and c-mos. Msi1 has been shown to affect their translation by binding to short elements located in the 3'-untranslated region. To better understand Msi1 functions, we initially performed an RIP-Chip analysis in HEK293T cells; this method consists of isolation of specific RNA-protein complexes followed by identification of the RNA component via microarrays. A group of 64 mRNAs was found to be enriched in the Msi1-associated population compared with controls. These genes belong to two main functional categories pertinent to tumorigenesis: 1) cell cycle, cell proliferation, cell differentiation, and apoptosis and 2) protein modification (including ubiquitination and ubiquitin cycle). To corroborate our findings, we examined the impact of Msi1 expression on both mRNA (transcriptomic) and protein (proteomic) expression levels. Genes whose mRNA levels were affected by Msi1 expression have a Gene Ontology distribution similar to RIP-Chip results, reinforcing Msi1 participation in cancer-related processes. The proteomics study revealed that Msi1 can have either positive or negative effects on gene expression of its direct targets. In summary, our results indicate that Msi1 affects a network of genes and could function as a master regulator during development and tumor formation.

Cytoplasmic polyadenylation and cytoplasmic polyadenylation element-dependent mRNA regulation are involved in Xenopus retinal axon development

BACKGROUND

Translation in axons is required for growth cone chemotropic responses to many guidance cues. Although locally synthesized proteins are beginning to be identified, how specific mRNAs are selected for translation remains unclear. Control of poly(A) tail length by cytoplasmic polyadenylation element (CPE) binding protein 1 (CPEB1) is a conserved mechanism for mRNA-specific translational regulation that could be involved in regulating translation in axons.

RESULTS

We show that cytoplasmic polyadenylation is required in Xenopus retinal ganglion cell (RGC) growth cones for translation-dependent, but not translation-independent, chemotropic responses in vitro, and that inhibition of CPE binding through dominant-negative interference severely reduces axon outgrowth in vivo. CPEB1 mRNA transcripts are present at low levels in RGCs but, surprisingly, CPEB1 protein was not detected in eye or brain tissue, and CPEB1 loss-of-function does not affect chemotropic responses or pathfinding in vivo. UV cross-linking experiments suggest that CPE-binding proteins other than CPEB1 in the retina regulate retinal axon development.

CONCLUSION

These results indicate that cytoplasmic polyadenylation and CPE-mediated translational regulation are involved in retinal axon development, but that CPEB1 may not be the key regulator of polyadenylation in the developing retina.

Function and regulation of the mammalian Musashi mRNA translational regulator

The evolutionarily conserved RNA-binding protein, Musashi, regulates neural stem cell self-renewal. Musashi expression is also indicative of stem cell populations in breast and intestinal tissues and is linked to cell overproliferation in cancers of these tissues. Musashi has been primarily implicated as a repressor of target mRNAs in stem cell populations. However, little is known about the mechanism by which Musashi exerts mRNA translational control or how Musashi function is regulated. Recent findings in oocytes of the frog, Xenopus, indicate an unexpected role for Musashi as an activator of a number of maternal mRNAs during meiotic cell cycle progression. Given the importance of Musashi function in stem cell biology and the implications of aberrant Musashi expression in cancer, it is critical that we understand the molecular processes that regulate Musashi function.

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.

Sequential waves of polyadenylation and deadenylation define a translation circuit that drives meiotic progression

The maternal mRNAs that drive meiotic progression in oocytes contain short poly(A) tails and it is only when these tails are elongated that translation takes place. Cytoplasmic polyadenylation requires two elements in the 3′-UTR (3′-untranslated region), the hexanucleotide AAUAAA and the CPE (cytoplasmic polyadenylation element), which also participates in the transport and localization, in a quiescent state, of its targets. However, not all CPE-containing mRNAs are activated at the same time during the cell cycle, and polyadenylation is temporally and spatially regulated during meiosis. We have recently deciphered a combinatorial code that can be used to qualitatively and quantitatively predict the translational behaviour of CPE-containing mRNAs. This code defines positive and negative feedback loops that generate waves of polyadenylation and deadenylation, creating a circuit of mRNA-specific translational regulation that drives meiotic progression.

Neural RNA-binding protein Musashi1 inhibits translation initiation by competing with eIF4G for PABP

Musashi1 (Msi1) is an RNA-binding protein that is highly expressed in neural stem cells. We previously reported that Msi1 contributes to the maintenance of the immature state and self-renewal activity of neural stem cells through translational repression of m-Numb. However, its translation repression mechanism has remained unclear. Here, we identify poly(A) binding protein (PABP) as an Msi1-binding protein, and find Msi1 competes with eIF4G for PABP binding. This competition inhibits translation initiation of Msi1's target mRNA. Indeed, deletion of the PABP-interacting domain in Msi1 abolishes its function. We demonstrate that Msi1 inhibits the assembly of the 80S, but not the 48S, ribosome complex. Consistent with these conclusions, Msi1 colocalizes with PABP and is recruited into stress granules, which contain the stalled preinitiation complex. However, Msi1 with mutations in two RNA recognition motifs fails to accumulate into stress granules. These results provide insight into the mechanism by which sequence-specific translational repression occurs in stem cells through the control of translation initiation.

A conserved U-rich RNA region implicated in regulation of translation in Plasmodium female gametocytes

Translational repression (TR) plays an important role in post-transcriptional regulation of gene expression and embryonic development in metazoans. TR also regulates the expression of a subset of the cytoplasmic mRNA population during development of fertilized female gametes of the unicellular malaria parasite, Plasmodium spp. which results in the formation of a polar and motile form, the ookinete. We report the conserved and sex-specific regulatory role of either the 3'- or 5'-UTR of a subset of translationally repressed mRNA species as shown by almost complete inhibition of expression of a GFP reporter protein in the female gametocyte. A U-rich, TR-associated element, identified previously in the 3'-UTR of TR-associated transcripts, played an essential role in mediating TR and a similar region could be found in the 5'-UTR shown in this study to be active in TR. The silencing effect of this 5'-UTR was shown to be independent of its position relative to its ORF, as transposition to a location 3' of the ORF did not affect TR. These results demonstrate for the first time in a unicellular organism that the 5' or the 3'-UTR of TR-associated transcripts play an important and conserved role in mediating TR in female gametocytes.

A novel mRNA 3' untranslated region translational control sequence regulates Xenopus Wee1 mRNA translation

Cell cycle progression during oocyte maturation requires the strict temporal regulation of maternal mRNA translation. The intrinsic basis of this temporal control has not been fully elucidated but appears to involve distinct mRNA 3' UTR regulatory elements. In this study, we identify a novel translational control sequence (TCS) that exerts repression of target mRNAs in immature oocytes of the frog, Xenopus laevis, and can direct early cytoplasmic polyadenylation and translational activation during oocyte maturation. The TCS is functionally distinct from the previously characterized Musashi/polyadenylation response element (PRE) and the cytoplasmic polyadenylation element (CPE). We report that TCS elements exert translational repression in both the Wee1 mRNA 3' UTR and the pericentriolar material-1 (Pcm-1) mRNA 3' UTR in immature oocytes. During oocyte maturation, TCS function directs the early translational activation of the Pcm-1 mRNA. By contrast, we demonstrate that CPE sequences flanking the TCS elements in the Wee1 3' UTR suppress the ability of the TCS to direct early translational activation. Our results indicate that a functional hierarchy exists between these distinct 3' UTR regulatory elements to control the timing of maternal mRNA translational activation during oocyte maturation.

Translational control by cytoplasmic polyadenylation in Xenopus oocytes

Elongation of the poly(A) tails of specific mRNAs in the cytoplasm is a crucial regulatory step in oogenesis and early development of many animal species. The best studied example is the regulation of translation by cytoplasmic polyadenylation elements (CPEs) in the 3′ untranslated region of mRNAs involved in Xenopus oocyte maturation. In this review we discuss the mechanism of translational control by the CPE binding protein (CPEB) in Xenopus oocytes as follows:1.

The cytoplasmic polyadenylation machinery such as CPEB, the subunits of cleavage and polyadenylation specificity factor (CPSF), symplekin, Gld-2 and poly(A) polymerase (PAP).

2.

The signal transduction that leads to the activation of CPE-mediated polyadenylation during oocyte maturation, including the potential roles of kinases such as MAPK, Aurora A, CamKII, cdk1/Ringo and cdk1/cyclin B.

3.

The role of deadenylation and translational repression, including the potential involvement of PARN, CCR4/NOT, maskin, pumilio, Xp54 (Ddx6, Rck), other P-body components and isoforms of the cap binding initiation factor eIF4E.

Finally we discuss some of the remaining questions regarding the mechanisms of translational regulation by cytoplasmic polyadenylation and give our view on where our knowledge is likely to be expanded in the near future.

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

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

Oocyte quality and maternal control of development

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

Ca2+ homeostasis regulates Xenopus oocyte maturation

In contrast to the well-defined role of Ca2+ signals during mitosis, the contribution of Ca2+ signaling to meiosis progression is controversial, despite several decades of investigating the role of Ca2+ and its effectors in vertebrate oocyte maturation. We have previously shown that during Xenopus oocyte maturation, Ca2+ signals are dispensable for entry into meiosis and for germinal vesicle breakdown. However, normal Ca2+ homeostasis is essential for completion of meiosis I and extrusion of the first polar body. In this study, we test the contribution of several downstream effectors in mediating the Ca2+ effects during oocyte maturation. We show that calmodulin and calcium-calmodulin-dependent protein kinase II (CAMK2) are not critical downstream Ca2+ effectors during meiotic maturation. In contrast, accumulation of Aurora kinase A (AURKA) protein is disrupted in cells deprived of Ca2+ signals. Since AURKA is required for bipolar spindle formation, failure to accumulate AURKA may contribute to the defective spindle phenotype following Ca2+ deprivation. These findings argue that Ca2+ homeostasis is important in establishing the oocyte's competence to undergo maturation in preparation for fertilization and embryonic development.

Musashi-1 expression in postnatal mouse olfactory epithelium

We investigated the age-related change in the distribution of a molecular marker for neural stem and precursor cells, Musashi-1, in the olfactory epithelium of mice from 1 day up to 16 months of age using immunohistochemistry. We also compared the distribution pattern of Musashi-1 with that of growth-associated protein 43, the olfactory marker protein, and Notch-1. Musashi-1 was expressed in the globose basal cell layer and the lower part of the growth-associated protein 43-positive layer, with immunoreactivity decreasing with aging. Notch-1 was observed only in the early postnatal period. These findings are consistent with the fact that globose basal cells are proliferating olfactory precursor cells and that their ability to generate new neurons decreases with aging.

MAPK interacts with XGef and is required for CPEB activation during meiosis in Xenopus oocytes

Meiotic progression in Xenopus oocytes, and all other oocytes investigated, is dependent on polyadenylation-induced translation of stockpiled maternal mRNAs. Early during meiotic resumption, phosphorylation of CPE-binding protein (CPEB) is required for polyadenylation-induced translation of mRNAs encoding cell cycle regulators. Xenopus Gef (XGef), a Rho-family guanine-exchange factor, influences the activating phosphorylation of CPEB. An exchange-deficient version of XGef does not, therefore implicating Rho-family GTPase function in early meiosis. We show here that Clostridium difficile Toxin B, a Rho-family GTPase inhibitor, does not impair early CPEB phosphorylation or progression to germinal vesicle breakdown, indicating that XGef does not influence these events through activation of a Toxin-B-sensitive GTPase. Using the inhibitors U0126 for mitogen-activated protein kinase (MAPK), and ZM447439 for Aurora kinase A and Aurora kinase B, we found that MAPK is required for phosphorylation of CPEB, whereas Aurora kinases are not. Furthermore, we do not detect active Aurora kinase A in early meiosis. By contrast, we observe an early, transient activation of MAPK, independent of Mos protein expression. MAPK directly phosphorylates CPEB on four residues (T22, T164, S184, S248), but not on S174, a key residue for activating CPEB function. Notably, XGef immunoprecipitates contain MAPK, and this complex can phosphorylate CPEB. MAPK may prime CPEB for phosphorylation on S174 by an as-yet-unidentified kinase or may activate this kinase.