RNA-binding proteins, RNA granules, and gametes: is unity strength?

Changes in mRNA translation and degradation represent post-transcriptional processes operating during gametogenesis and early embryogenesis to ensure regulated protein synthesis. Numerous mRNA-binding proteins (RBPs) have been described in multiple animal models that contribute to the control of mRNA translation and decay during oogenesis and spermatogenesis. An emerging view from studies performed in germ cells and somatic cells is that RBPs associate with their target mRNAs in RNA-protein (or ribonucleoprotein) complexes (mRNPs) that assemble in various cytoplasmic RNA granules that communicate with the translation machinery and control mRNA storage, triage, and degradation. In comparison with Xenopus, Caenorhabditis elegans, or Drosophila, the composition and role of cytoplasmic RNA-containing granules in mammalian germ cells are still poorly understood. However, regained interest for these structures has emerged with the recent discovery of their role in small RNA synthesis and transposon silencing through DNA methylation. In this review, we will briefly summarize our current knowledge on cytoplasmic RNA granules in murine germ cells and describe the role of some of the RBPs they contain in regulating mRNA metabolism and small RNA processing during gametogenesis.

zeta-Crystallin is a bcl-2 mRNA binding protein involved in bcl-2 overexpression in T-cell acute lymphocytic leukemia.

The human antiapoptotic bcl-2 gene has been discovered in t(14;18) B-cell leukemias/lymphomas because of its overexpression caused at a transcriptional control level by the bcl-2/IgH fusion gene. We were the first to disclose the post-transcriptional control of bcl-2 expression mediated by interactions of an adenine + uracil (AU)-rich element (ARE) in the 3'-UTR of bcl-2 mRNA with AU-binding proteins (AUBPs). Here, we identify and characterize zeta-crystallin as a new bcl-2 AUBP, whose silencing or overexpression has impact on bcl-2 mRNA stability. An increased Bcl-2 level observed in normal phytohemagglutinin (PHA)-activated T lymphocytes, acute lymphatic leukemia (ALL) T-cell lines, and T cells of patients with leukemia in comparison with normal non-PHA-activated T lymphocytes was concomitant with an increase in zeta-crystallin level. The specific association of zeta-crystallin with the bcl-2 ARE was significantly enhanced in T cells of patients with ALL, which accounts for the higher stability of bcl-2 mRNA and suggests a possible contribution of zeta-crystallin to bcl-2 overexpression occurring in this leukemia.

Low dietary inorganic phosphate affects the brain by controlling apoptosis, cell cycle and protein translation.

Inorganic phosphate (Pi) plays a key role in diverse physiologic functions. In a previous study, we showed that high dietary Pi perturbs brain growth through Akt/ERK signaling in developing mice. However, no study has investigated the response of the brain to low dietary Pi. In this study, we addressed this question by studying the effects of low dietary Pi on the cerebrum of developing mice. Two-week-old weaned mice were fed with a low phosphate diet for 4 weeks. At the end of the study, their cerebrum was dissected and signals important for protein translation, apoptosis and cell cycle were examined. The low phosphate diet did not cause physiologically significant changes; it increased the protein expression of phosphatase and tensin homolog deleted on chromosome 10 but decreased Akt activity. In addition, expression of eukaryotic translation initiation factor binding protein coupled with increased complex formation of eukaryotic translation initiation factor 4E/eukaryotic translation initiation factor binding protein 1 was induced in the cerebrum by low phosphate, leading to reduced cap-dependent protein translation. Finally, low phosphate facilitated apoptosis and suppressed signals important for the cell cycle in the cerebrum of dual-luciferase reporter mice. In summary, our results showed that a low phosphate diet affects the brain by controlling protein translation, apoptosis and cell cycle in developing mice. Our results support the hypothesis that Pi works as a stimulus capable of increasing or decreasing several pivotal genes for normal development and suggest that regulation of Pi consumption is important in maintaining a healthy life.

[Aberrant regulation of mRNA 3' untranslated region in cancers and inflammation]. / Contribution de la régulation post-transcriptionnelle à l'émergence de maladies: l'enfance de l'ARE.

Almost 10% of mammalian coding mRNAs contain in their 3' untranslated region a sequence rich in adenine and uridine residues known as AU-rich element (ARE). Many of them encode oncogenes (for instance c-Myc and c-Fos), cell cycle regulators (cyclin D1, A1, B1), cytokines (TNFalpha, IL2) and growth factors (GM-CSF) which are overexpressed in cancer or inflammatory diseases due to increased mRNA stability and/or translation. AREs are recognized by a group of proteins, collectively called AUBPs which display various functions. For instance, HuR/ELAV is mainly known to protect ARE-containing mRNAs from degradation, while AUF1, TTP and KSRP act to destabilize their bound target mRNAs and TIA/TIAR to inhibit their translation. Alterations in ARE sequences or AUBP abundance, cellular localization or activity due to post-translational modifications such as phosphorylation can promote or enhance malignancy or perturb immune homeostasis. Here, c-myc and TNFalpha are chosen as examples to illustrate how altered 3' UTR gene regulation impacts on pathologies.

A high inorganic phosphate diet perturbs brain growth, alters Akt-ERK signaling, and results in changes in cap-dependent translation.

Inorganic phosphate (Pi) plays a key role in diverse physiological functions. Recently, considerable progress has been made in our understanding of the function and regulation of the brain-specific sodium-dependent inorganic phosphate transporter 1 (NPT1), which is found to exist principally in cerebrum and cerebellum. The potential importance of Pi as a novel signaling molecule and the poor prognosis of diverse neurodegenerative diseases that involve brain-specific NPT1 have prompted us to define the pathways by which Pi affects mouse brain growth. A high phosphate diet caused an increase in serum Pi accompanied by a decrease in calcium, and a decrease in body weight coupled with a decreased relative weight of cerebellum. A high phosphate diet caused a significant increase in protein expression of NPT1, both in cerebrum and cerebellum. Additionally, the high phosphate diet increased Homo sapiens v-akt murine thymoma viral oncogene homolog 1 (Akt) phosphorylation at Ser473 in cerebrum and cerebellum, whereas suppression of Akt phosphorylation at Thr308 was observed only in cerebellum. Selective suppression of eukaryotic translation initiation factor-binding protein (eIF4E-BP1) in cerebrum was induced by high levels of Pi, which induced cap-dependent and cap-independent protein translation in cerebrum and cerebellum, respectively. Phosphorylation of extracellular regulated kinase 1 (ERK1) in comparison with that of ERK2 was significantly reduced in both cerebrum and cerebellum. High levels of Pi reduced protein expressions of proliferating cell nuclear antigen (PCNA) and cyclin D1 in cerebrum and cerebellum. In conclusion, the results indicate that high dietary Pi can perturb normal brain growth, possibly through Akt-ERK signaling in developing mice.

A new player in oncogenesis: AUF1/hnRNPD overexpression leads to tumorigenesis in transgenic mice.

AUF1/heterogeneous nuclear ribonucleoprotein D (hnRNPD) binds to adenylate uridylate-rich elements contained in the 3' untranslated region of many short-lived mRNAs. This binding has been shown in vitro to control the stability of adenylate uridylate-rich element-containing mRNAs, including mRNAs encoding proto-oncogenes, cytokines, or other signaling molecules. However, no studies have yet been undertaken to identify the mRNAs subject to AUF1-mediated regulation in vivo. The purpose of our study was to investigate the biological functions of AUF1. Thus, we derived transgenic (Tg) mice, which overexpress one isoform of AUF1, the p37(AUF1). Mice of the three Tg lines analyzed exhibit altered levels of expression of several target mRNAs, such as c-myc, c-jun, c-fos, granulocyte macrophage colony-stimulating factor, and tumor necrosis factor alpha. The Tg line with the highest amount of Tg p37(AUF1) protein developed sarcomas. The tumors strongly expressed AUF1 Tg protein and Cyclin D1. Taken together, our data show that: (a) AUF1 is a key regulatory factor of gene expression in vivo; and (b) the deregulation of this heterogeneous nuclear ribonucleoprotein leads to tumorigenesis.

Modulo is a target of Myc selectively required for growth of proliferative cells in Drosophila.

In Drosophila, the homologue of the proto-oncogene Myc is a key regulator of both cell size and cell growth. The identities and roles of dMyc target genes in these processes, however, remain largely unexplored. Here, we investigate the function of the modulo (mod) gene, which encodes a nucleolus localized protein. In gain of function or loss of function experiments, we demonstrate that mod is directly controlled by dMyc. Strikingly, in proliferative imaginal cells, mod loss-of-function impairs both cell growth and cell size, whereas larval endoreplicative tissues grow normally. In contrast to dMyc, over-expressing Mod in wing imaginal discs is not sufficient to induce cell growth. Taken together, our results indicate that mod does not possess the full spectrum of dMyc activities, but is required selectively in proliferative cells to sustain their growth and to maintain their specific size.

ALK-mediated post-transcriptional regulation: focus on RNA-binding proteins.

Extensive research has been carried out in the past two decades to provide insights into the molecular mechanisms by which the Nucleophosmin-Anaplastic Lymphoma Kinase (NPM-ALK) exerts its oncogenic effects. These studies led to the concept that NPM-ALK acts at the transcriptional level through the activation of several transcription factors downstream of many different signaling pathways including JAK3/STAT3, PI3K/AKT and RAS/ERK. Nevertheless, the discovery of several RNA-binding proteins (RBPs) within ALK interactome suggested an additional and complementary role of this oncogenic kinase at the post-transcriptional level. This review gives emerging views in ALK-mediated post-transcriptional regulation with a focus on RBPs that are associated with ALK. We will summarize the capacity of NPM-ALK in modulating the biological properties of RBPs and then discuss the role of cytoplasmic aggregates, called AGs for "ALK granules", which are observed in anaplastic large cell lymphoma (ALCL) expressing the ALK kinase. AGs contain polyadenylated mRNAs and numerous RBPs but are distinct from processing bodies (PBs) and stress granules (SGs), two well-known discrete cytoplasmic sites involved in mRNA fate.

Evidence of finely tuned expression of DNA polymerase beta in vivo using transgenic mice.

DNA polymerase (Pol) is an error-prone repair DNA polymerase that has been shown to create genetic instability and tumorigenesis when overexpressed by only 2-fold in cells, suggesting that a rigorous regulation of its expression may be essential in vivo. To address this question, we have generated mice which express a transgene (Tg) bearing the Pol cDNA under the control of the ubiquitous promoter of the mouse H-2K gene from the major histocompatibility complex. These mice express the Tg only in thymus, an organ which normally contains the most abundant endogenous Pol mRNA and protein, supporting the idea of a tight regulation of Pol in vivo. Furthermore, we found no tumor incidence, suggesting that the single Pol overexpression event is not sufficient to initiate tumorigenesis in vivo.

Expression patterns of the coe/ebf transcription factor genes during chicken and mouse limb development.

The COE (Collier/Olf/EBF) family of transcription factors comprises a single member in Drosophila and four members in human and mice. We have examined by in situ hybridization the expression patterns of each ebf/coe gene during limb development in mouse and chicken embryos. Expression of mouse ebf1, 2 and 3 is detected in mesenchymal cells from stages E10.5-11, expression of ebf2 being restricted to the presumptive zeugopod. Cross sections of mouse and chicken limb buds at several stages reveal that ebfs are specifically expressed in the connective tissues surrounding chondrogenic condensations and forming tendons. They thus represent useful new markers for studying vertebrate limb development, particularly formation of ligaments.

Looking for the functions of RNA granules in ALK-transformed cells.

Numerous cytoplasmic foci containing mRNA s and their associated proteins have been described in mammalian somatic and germ cells. The best studied examples are given by the processing bodies (PBs) that are present in all cell types, and the stress granules (SGs) that are transiently formed following stress stimuli. Those foci are non-membranous dynamic structures that, through the continuous exchange of their content with the cytoplasm, are believed to control mRNA storage, translation and degradation. However, due in part to the fact that their composition has not been fully characterized, their relevance to mRNA regulation and cell survival remains a matter of debate. In a recent study, we described new cytoplasmic foci that form specifically in transformed cells expressing the constitutively active ALK tyrosine kinase. Those granules, further called AGs for ALK granules, contain polyadenylated mRNAs but are distinct from PBs and SGs. Using a method based on sucrose density gradient fractionation, we further purified AGs and identified their mRNA content. We discuss our findings in relation to other granules containing untranslated mRNAs and speculate on the possible contribution of AGs to the oncogenic properties of ALK-expressing cells.

Novel mRNA-containing cytoplasmic granules in ALK-transformed cells.

In mammalian cells, nontranslating messenger RNAs (mRNAs) are concentrated in different cytoplasmic foci, such as processing bodies (PBs) and stress granules (SGs), where they are either degraded or stored. In the present study, we have thoroughly characterized cytoplasmic foci, hereafter called AGs for ALK granules that form in transformed cells expressing the constitutively active anaplastic lymphoma kinase (ALK). AGs contain polyadenylated mRNAs and a unique combination of several RNA binding proteins that so far has not been described in mammalian foci, including AUF1, HuR, and the poly (A(+)) binding protein PABP. AGs shelter neither components of the mRNA degradation machinery present in PBs nor known markers of SGs, such as translation initiation factors or TIA/TIAR, showing that they are distinct from PBs or SGs. AGs and PBs, however, both move on microtubules with similar dynamics and frequently establish close contacts. In addition, in conditions in which mRNA metabolism is perturbed, AGs concentrate PB components with the noticeable exception of the 5' to 3' exonuclease XRN1. Altogether, we show that AGs constitute novel mRNA-containing cytoplasmic foci and we propose that they could protect translatable mRNAs from degradation, contributing thus to ALK-mediated oncogenicity.

The RNA-binding protein ELAVL1/HuR is essential for mouse spermatogenesis, acting both at meiotic and postmeiotic stages.

Posttranscriptional mechanisms are crucial to regulate spermatogenesis. Accurate protein synthesis during germ cell development relies on RNA binding proteins that control the storage, stability, and translation of mRNAs in a tightly and temporally regulated manner. Here, we focused on the RNA binding protein Embryonic Lethal Abnormal Vision (ELAV) L1/Human antigen R (HuR) known to be a key regulator of posttranscriptional regulation in somatic cells but the function of which during gametogenesis has never been investigated. In this study, we have used conditional loss- and gain-of-function approaches to address this issue in mice. We show that targeted deletion of HuR specifically in germ cells leads to male but not female sterility. Mutant males are azoospermic because of the extensive death of spermatocytes at meiotic divisions and failure of spermatid elongation. The latter defect is also observed upon HuR overexpression. To elucidate further the molecular mechanisms underlying spermatogenesis defects in HuR-deleted and -overexpressing testes, we undertook a target gene approach and discovered that heat shock protein (HSP)A2/HSP70-2, a crucial regulator of spermatogenesis, was down-regulated in both situations. HuR specifically binds hspa2 mRNA and controls its expression at the translational level in germ cells. Our study provides the first genetic evidence of HuR involvement during spermatogenesis and reveals Hspa2 as a target for HuR.

Stabilization of Dll1 mRNA by Elavl1/HuR in neuroepithelial cells undergoing mitosis.

In the vertebrate neuroepithelium, the decision to differentiate is made by neural precursors soon after mitosis, when they are apically located. This process is controlled by lateral inhibitory signals triggered by the Delta/Notch pathway. During mitosis, the capacity of neural precursors to express the neurogenic genes Dll1 and Notch1 is maximal due to mRNA stabilization, but the mechanism controlling this process remains unknown. Here we show that Elav-like (Elavl1)/HuR becomes enriched in the cytoplasm of neuroepithelial cells undergoing mitosis and that this ribonucleoprotein interacts with the 3' untranslated region (UTR) of Dll1 mRNA. This interaction is functionally relevant because RNAi against Elavl1 reduces the stability of Dll1-3'UTR-containing transcripts in mitosis-arrested neuroepithelial cells, and Elavl1 null-mutant heterozygous mice show decreased Dll1 expression in the developing brain in vivo. We also show that RNAi against Elavl1 diminishes the capacity of brain precursors to trigger lateral inhibitory signals to their neighbors, an observation consistent with the increase in the rate of neurogenesis which can be detected in vivo in the developing retina of Elavl1 heterozygous mice. We conclude that Elavl1/HuR facilitates the exposure of vertebrate neuronal precursors to apically located Delta/Notch signals.

HuR-mediated control of C/EBPbeta mRNA stability and translation in ALK-positive anaplastic large cell lymphomas.

The CCAAT/enhancer-binding protein ß (C/EBPß) plays a major role in the pathogenesis of anaplastic large cell lymphomas (ALCL) that express the nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) tyrosine kinase (ALK(+)). Although ALK-mediated C/EBPß transcriptional activation has been reported, C/EBPß mRNA possesses U- and AU-rich domains in its 3'-untranslated region (3'-UTR) that might be privileged targets for posttranscriptional control in ALK(+) ALCLs. The purpose of this study was to explore this possibility. By using human ALCL-derived cells and a murine model of ALK-transformed cells, we show that the AU-binding protein HuR binds to the 3'-UTR of C/EBPß mRNA, as previously reported in adipocytes, and that NPM-ALK enhances this interaction. Interaction between HuR and C/EBPß mRNA impacts on C/EBPß gene expression at both the mRNA and protein levels. Indeed, C/EBPß mRNA stability following HuR silencing is reduced and reaches the value observed in ALK-inactivated cells. Remarkably, HuR expression is not modified by NPM-ALK, but its association with actively translating polysomes is dramatically increased in ALK(+) cells. HuR/polysomes association diminishes when NPM-ALK activity is inhibited and is accompanied by a concomitant decrease of C/EBPß mRNA translation. Finally, we show that HuR and NPM-ALK colocalized in cytoplasmic granules and HuR is phosphroylated on tyrosine residues in ALK(+) ALCL cells. Our study thus demonstrates that C/EBPß is indeed regulated at the posttranscriptional level by HuR in ALK(+) cells, leading us to propose that part of NPM-ALK oncogenic properties relies on its ability to modify HuR properties in the cytoplasm and hence to alter expression of key actors of transformation.

Impaired embryonic development in mice overexpressing the RNA-binding protein TIAR.

BACKGROUND: TIA-1-related (TIAR) protein is a shuttling RNA-binding protein involved in several steps of RNA metabolism. While in the nucleus TIAR participates to alternative splicing events, in the cytoplasm TIAR acts as a translational repressor on specific transcripts such as those containing AU-Rich Elements (AREs). Due to its ability to assemble abortive pre-initiation complexes coalescing into cytoplasmic granules called stress granules, TIAR is also involved in the general translational arrest observed in cells exposed to environmental stress. However, the in vivo role of this protein has not been studied so far mainly due to severe embryonic lethality upon tiar invalidation. METHODOLOGY/PRINCIPAL FINDINGS: To examine potential TIAR tissue-specificity in various cellular contexts, either embryonic or adult, we constructed a TIAR transgenic allele (loxPGFPloxPTIAR) allowing the conditional expression of TIAR protein upon Cre recombinase activity. Here, we report the role of TIAR during mouse embryogenesis. We observed that early TIAR overexpression led to low transgene transmission associated with embryonic lethality starting at early post-implantation stages. Interestingly, while pre-implantation steps evolved correctly in utero, in vitro cultured embryos were very sensitive to culture medium. Control and transgenic embryos developed equally well in the G2 medium, whereas culture in M16 medium led to the phosphorylation of eIF2alpha that accumulated in cytoplasmic granules precluding transgenic blastocyst hatching. Our results thus reveal a differential TIAR-mediated embryonic response following artificial or natural growth environment. CONCLUSIONS/SIGNIFICANCE: This study reports the importance of the tightly balanced expression of the RNA-binding protein TIAR for normal embryonic development, thereby emphasizing the role of post-transcriptional regulations in early embryonic programming.

AU-rich elements regulate Drosophila gene expression.

In mammals, AU-rich elements (AREs) are critical regulators of mRNA turnover. They recruit ARE-binding proteins that inhibit or stimulate rapid mRNA degradation in response to stress or developmental cues. Using a bioinformatics approach, we have identified AREs in Drosophila melanogaster 3' untranslated regions and validated their cross-species conservation in distant Drosophila genomes. We have generated a Drosophila ARE database (D-ARED) and established that about 16% of D. melanogaster genes contain the mammalian ARE signature, an AUUUA pentamer in an A/U-rich context. Using candidate ARE genes, we show that Drosophila AREs stimulate reporter mRNA decay in cultured cells and in the physiological context of the immune response in D. melanogaster. In addition, we found that the conserved ARE-binding protein Tis11 regulates temporal gene expression through ARE-mediated decay (AMD) in D. melanogaster. Our work reveals that AREs are conserved and functional cis regulators of mRNA decay in Drosophila and highlights this organism as a novel model system to unravel in vivo the contribution of AMD to various processes.

Temporally regulated traffic of HuR and its associated ARE-containing mRNAs from the chromatoid body to polysomes during mouse spermatogenesis.

BACKGROUND: In mammals, a temporal disconnection between mRNA transcription and protein synthesis occurs during late steps of germ cell differentiation, in contrast to most somatic tissues where transcription and translation are closely linked. Indeed, during late stages of spermatogenesis, protein synthesis relies on the appropriate storage of translationally inactive mRNAs in transcriptionally silent spermatids. The factors and cellular compartments regulating mRNA storage and the timing of their translation are still poorly understood. The chromatoid body (CB), that shares components with the P. bodies found in somatic cells, has recently been proposed to be a site of mRNA processing. Here, we describe a new component of the CB, the RNA binding protein HuR, known in somatic cells to control the stability/translation of AU-rich containing mRNAs (ARE-mRNAs). METHODOLOGY/PRINCIPAL FINDINGS: Using a combination of cell imagery and sucrose gradient fractionation, we show that HuR localization is highly dynamic during spermatid differentiation. First, in early round spermatids, HuR colocalizes with the Mouse Vasa Homolog, MVH, a marker of the CB. As spermatids differentiate, HuR exits the CB and concomitantly associates with polysomes. Using computational analyses, we identified two testis ARE-containing mRNAs, Brd2 and GCNF that are bound by HuR and MVH. We show that these target ARE-mRNAs follow HuR trafficking, accumulating successively in the CB, where they are translationally silent, and in polysomes during spermatid differentiation. CONCLUSIONS/SIGNIFICANCE: Our results reveal a temporal regulation of HuR trafficking together with its target mRNAs from the CB to polysomes as spermatids differentiate. They strongly suggest that through the transport of ARE-mRNAs from the CB to polysomes, HuR controls the appropriate timing of ARE-mRNA translation. HuR might represent a major post-transcriptional regulator, by promoting mRNA storage and then translation, during male germ cell differentiation.

A "liaison dangereuse" between AUF1/hnRNPD and the oncogenic tyrosine kinase NPM-ALK.

Nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) is a chimeric protein expressed in a subset of cases of anaplastic large cell lymphoma (ALCL) for which constitutive expression represents a key oncogenic event. The ALK signaling pathway is complex and probably involves functional redundancy between various signaling substrates of ALK. Despite numerous studies on signaling mediators, the molecular mechanisms contributing to the distinct oncogenic features of NPM-ALK remain incompletely understood. The search for additional interacting partners of NPM-ALK led to the discovery of AUF1/hnRNPD, a protein implicated in AU-rich element (ARE)-directed mRNA decay. AUF1 was immunoprecipitated with ALK both in ALCL-derived cells and in NIH3T3 cells stably expressing NPM-ALK or other X-ALK fusion proteins. AUF1 and NPM-ALK were found concentrated in the same cytoplasmic foci, whose formation required NPM-ALK tyrosine kinase activity. AUF1 was phosphorylated by ALK in vitro and was hyperphosphorylated in NPM-ALK-expressing cells. Its hyperphosphorylation was correlated with increased stability of several AUF1 target mRNAs encoding key regulators of cell proliferation and with increased cell survival after transcriptional arrest. Thus, AUF1 could function in a novel pathway mediating the oncogenic effects of NPM-ALK. Our data establish an important link between oncogenic kinases and mRNA turnover, which could constitute a critical aspect of tumorigenesis.

Impaired gametogenesis in mice that overexpress the RNA-binding protein HuR.

A series of experiments, using cell culture models or in vitro assays, has shown that the RNA-binding protein HuR increases the half-life of some messenger RNAs that contain adenylate/uridylate-rich decay elements. However, its function in an integrated system has not yet been investigated. Here, using a mouse model, we report that misregulation of HuR, due to expression of an HuR transgene, prevents the production of fully functional gametes. This work provides the first evidence for a physiological function of HuR, and highlights its involvement in spermatogenesis.