Translational Control: A Cup Half Full

The Dynamic Regulation of mRNA Translation and Ribosome Biogenesis During Germ Cell Development and Reproductive Aging

The regulation of mRNA translation, both globally and at the level of individual transcripts, plays a central role in the development and function of germ cells across species. Genetic studies using flies, worms, zebrafish and mice have highlighted the importance of specific RNA binding proteins in driving various aspects of germ cell formation and function. Many of these mRNA binding proteins, including Pumilio, Nanos, Vasa and Dazl have been conserved through evolution, specifically mark germ cells, and carry out similar functions across species. These proteins typically influence mRNA translation by binding to specific elements within the 3′ untranslated region (UTR) of target messages. Emerging evidence indicates that the global regulation of mRNA translation also plays an important role in germ cell development. For example, ribosome biogenesis is often regulated in a stage specific manner during gametogenesis. Moreover, oocytes need to produce and store a sufficient number of ribosomes to support the development of the early embryo until the initiation of zygotic transcription. Accumulating evidence indicates that disruption of mRNA translation regulatory mechanisms likely contributes to infertility and reproductive aging in humans. These findings highlight the importance of gaining further insights into the mechanisms that control mRNA translation within germ cells. Future work in this area will likely have important impacts beyond germ cell biology.

Monocarboxylate Transporter 1 in Brain Diseases and Cancers

Background:

Monocarboxylate Transporter 1 (MCT1), an important membrane transport protein, mediates the translocation of monocarboxylates together with protons across biological membranes. Due to its pathological significance, MCT1 plays an important role in the progression of some diseases, such as brain diseases and cancers.

Methods:

We summarize the general description of MCT1 and provide a comprehensive understanding of the role of MCT1 in brain diseases and cancers. Furthermore, this review discusses the opportunities and challenges of MCT1- targeting drug-delivery systems in the treatment of brain diseases and cancers.

Results:

In the brain, loss of MCT1 function is associated with pathologies of degeneration and injury of the nervous system. In tumors, MCT1 regulates the activity of signaling pathways and controls the exchange of monocarboxylates in aerobic glycolysis to affect tumor metabolism, proliferation and invasion. Meanwhile, MCT1 also acts as a good biomarker for the prediction and diagnosis of cancer progressions.

Conclusion:

MCT1 is an attractive transporter in brain diseases and cancers. Moreover, the development of MCT1- based small molecule drugs and MCT1 inhibitors in the clinic is promising. This review systematically summarizes the basic characteristics of MCT1 and its role in brain diseases and cancers, laying the foundation for further research on MCT1.

Structural basis for LeishIF4E-1 modulation by an interacting protein in the human parasite Leishmania major

Leishmania parasites are unicellular pathogens that are transmitted to humans through the bite of infected sandflies. Most of the regulation of their gene expression occurs post-transcriptionally, and the different patterns of gene expression required throughout the parasites’ life cycle are regulated at the level of translation. Here, we report the X-ray crystal structure of the Leishmania cap-binding isoform 1, LeishIF4E-1, bound to a protein fragment of previously unknown function, Leish4E-IP1, that binds tightly to LeishIF4E-1. The molecular structure, coupled to NMR spectroscopy experiments and in vitro cap-binding assays, reveal that Leish4E-IP1 allosterically destabilizes the binding of LeishIF4E-1 to the 5′ mRNA cap. We propose mechanisms through which Leish4E-IP1-mediated LeishIF4E-1 inhibition could regulate translation initiation in the human parasite.

Bruno-like proteins modulate flowering time via 3' UTR-dependent decay of SOC1 mRNA

The Bruno RNA-binding protein (RBP) has been shown to initially repress the translation of oskar mRNA during Drosophila oogenesis and later to be involved in a broad range of RNA regulation. Here, we show that homologous constitutive overexpression of each of two Arabidopsis thaliana Bruno-like genes, AtBRN1 and AtBRN2, delayed the flowering time, while the atbrn1 atbrn2-3 double mutant flowered early and exhibited increased expression of APETALA1 (AP1) and LEAFY (LFY) transcripts. Crossing of 35S::AtBRNs with SOC1 101-D plants demonstrated that 35S::AtBRNs suppress an early-flowering phenotype of SOC1 101-D in which the coding sequence (CDS) with the 3' UTR of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) gene is overexpressed. However, this early-flowering phenotype by SOC1 overexpression was maintained in the plants coexpressing 35S::AtBRNs and 35S::SOC1 without the 3' UTR (-3' UTR). Using yeast three-hybrid, electrophoretic mobility shift, RNA immunoprecipitation, and protoplast transient assays, we found that AtBRNs bind to the 3' UTR of SOC1 RNA and participate in mRNA decay, which was mediated by the distal region of the SOC1 3' UTR. Overall, AtBRNs repress SOC1 activity in a 3' UTR-dependent manner, thereby controlling the flowering time in Arabidopsis.

Microtubule-based motor-mediated mRNA localization in Drosophila oocytes and embryos

RNA localization coupled to translational repression is a general mechanism for creating structural and functional asymmetry within the cell. While there are many possible ways to target an mRNA to its destination, a large fraction of the studied transcripts undertake active transport mediated by cytoskeletal elements (microtubules and actin filaments) and associated mechanoenzymes. Among the best-studied model systems of RNA localization are the oocyte and the early embryo of Drosophila melanogaster, for which many well-characterized tools have been developed to study this cell biological phenomenon in a dynamic, developing system in its in vivo context. In the present paper, we review the current evidence and models explaining the different modes of RNA localization that depend on active transport within cells.

Mechanisms of translational regulation in Drosophila

Translational regulation plays an essential role in many phases of the Drosophila life cycle. During embryogenesis, specification of the developing body pattern requires co-ordination of the translation of oskar, gurken and nanos mRNAs with their subcellular localization. In addition, dosage compensation is controlled by Sex-lethal-mediated translational regulation while dFMR1 (the Drosophila homologue of the fragile X mental retardation protein) controls translation of various mRNAs which function in the nervous system. Here we describe some of the mechanisms that are utilized to regulate these various processes. Our review highlights the complexity that can be involved with multiple factors employing different mechanisms to control the translation of a single mRNA.

The monocarboxylate transporter family--role and regulation

Monocarboxylate transporter (MCT) isoforms 1-4 catalyze the proton-linked transport of monocarboxylates such as L-lactate across the plasma membrane, whereas MCT8 and MCT10 are thyroid hormone and aromatic amino acid transporters, respectively. The importance of MCTs is becoming increasingly evident as their extensive physiological and pathological roles are revealed. MCTs 1-4 play essential metabolic roles in most tissues with their distinct properties, expression profile, and subcellular localization matching the particular metabolic needs of a tissue. Important metabolic roles include energy metabolism in the brain, skeletal muscle, heart, tumor cells, and T-lymphocyte activation, gluconeogenesis in the liver and kidney, spermatogenesis, bowel metabolism of short-chain fatty acids, and drug transport. MCT8 is essential for thyroid hormone transport across the blood-brain barrier. Genetic perturbation of MCT function may be involved in disease states such as pancreatic β-cell malfunction (inappropriate MCT1 expression), chronic fatigue syndromes (impairment of muscle MCT function), and psychomotor retardation (MCT8 mutation). MCT expression can be regulated at both the transcriptional and post-transcriptional levels. Of particular importance is the upregulation of muscle MCT1 expression in response to training and MCT4 expression in response to hypoxia. The latter is mediated by hypoxia inducible factor 1α and often observed in tumor cells that rely almost entirely on glycolysis for their energy provision. The recent discovery of potent and specific MCT1 inhibitors that prevent proliferation of T-lymphocytes confirms that MCTs may be promising pharmacological targets including for cancer chemotherapy.

NANOS3 function in human germ cell development

Human infertility is common and frequently linked to poor germ cell development. Yet, human germ cell development is poorly understood, at least in part due to the inaccessibility of germ cells to study especially during fetal development. Here, we explored the function of a highly conserved family of genes, the NANOS genes, in the differentiation of human germ cells from human embryonic stem cells. We observed that NANOS-1, -2 and -3 mRNAs and proteins were expressed in human gonads. We also noted that NANOS3 was expressed in germ cells throughout spermatogenesis and oogenesis and thus, focused further efforts on this family member. NANOS3 expression was highest in human germ cell nuclei where the protein co-localized with chromosomal DNA during mitosis/meiosis. Reduced expression of NANOS3 (via morpholinos or short hairpin RNA) resulted in a reduction in germ cell numbers and decreased expression of germ cell-intrinsic genes required for the maintenance of pluripotency and meiotic initiation and progression. These data provide the first direct experimental evidence that NANOS3 functions in human germ cell development; indeed, NANOS3 is now one of just two genes that has been directly shown to function in germ cell development across diverse species from flies, worms, frogs and mice to humans [the other is BOULE, a member of the Deleted in Azoospermia (DAZ) gene family]. Findings may contribute to our understanding of the basic biology of human germ cell development and may provide clinical insights regarding infertility.

A collection of caged compounds for probing roles of local translation in neurobiology

Spatially localized translation plays a vital role in the normal functioning of neuronal systems and is widely believed to be involved in both learning and memory formation. It is of central interest to understand both the phenomenon and molecular mechanisms of local translation using new tools and approaches. Caged compounds can, in principle, be used as tools to investigate local translation since optical activation of bioactive molecules can achieve both spatial and temporal resolution on the micron scale and on the order of seconds or less, respectively. Successful caging of bioactive molecules requires the identification of key functional groups in appropriate molecules and the introduction of a suitable caging moiety. Here we present the design, synthesis and testing of a collection of three caged compounds: anisomycin caged with a diethylaminocoumarin moiety and dimethoxynitrobenzyl caged versions of 4E-BP and rapamycin. Whereas caged anisomycin can be used to control general translation, caged 4E-BP serves as a probe of cap-dependent translation initiation and caged rapamycin serves a probe of the role of mTORC1 in translation initiation. In vitro translation assays demonstrate that these caging strategies, in combination with the aforementioned compounds, are effective for optical control making it likely that such strategies can successfully employed in the study of local translation in living systems.

Drosophila female sterile mutation spoonbill interferes with multiple pathways in oogenesis

spoonbill is a Drosophila female-sterile mutation, which displays a range of eggshell and egg chamber patterning defects. Previous analysis has shown that the mutation interfered with the function of two major signaling pathways, GRK/EGFR and DPP. In this report, the nature of spoonbill was further investigated to examine whether it was associated with additional pathways in oogenesis. Clonal analysis, presented here, demonstrated that most of the aberrant phenotypes associated with spoonbill were dependent on a mutant germline. Nevertheless, SPOONBILL may function also in the soma to ensure proper polarization and migration of the border-cell-cluster. Further, genetic interaction studies implicated spoonbill in additional unrelated pathways such as the one(s) involved in actin polymerization/depolymerization. Based on the previous data and the results presented here, it is anticipated that spoonbill may encode a multifunctional protein that perhaps coordinately regulated the activity of multiple signaling pathways during oogenesis.

The role of PIWI and the miRNA machinery in Drosophila germline determination

BACKGROUND

The germ plasm has long been demonstrated to be necessary and sufficient for germline determination, with translational regulation playing a key role in the process. Beyond this, little is known about molecular activities underlying germline determination.

RESULTS

We report the function of Drosophila PIWI, DICER-1, and dFMRP (Fragile X Mental Retardation Protein) in germline determination. PIWI is a maternal component of the polar granule, a germ-plasm-specific organelle essential for germline specification. Depleting maternal PIWI does not affect OSK or VASA expression or abdominal patterning but leads to failure in pole-plasm maintenance and primordial-germ-cell (PGC) formation, whereas doubling and tripling the maternal piwi dose increases OSK and VASA levels correspondingly and doubles and triples the number of PGCs, respectively. Moreover, PIWI forms a complex with dFMRP and DICER-1, but not with DICER-2, in polar-granule-enriched fractions. Depleting DICER-1, but not DICER-2, also leads to a severe pole-plasm defect and a reduced PGC number. These effects are also seen, albeit to a lesser extent, for dFMRP, another component of the miRISC complex.

CONCLUSIONS

Because DICER-1 is required for the miRNA pathway and DICER-2 is required for the siRNA pathway yet neither is required for the rasiRNA pathway, our data implicate a crucial role of the PIWI-mediated miRNA pathway in regulating the levels of OSK, VASA, and possibly other genes involved in germline determination in Drosophila.

Quantitative proteomics identifies Gemin5, a scaffolding protein involved in ribonucleoprotein assembly, as a novel partner for eukaryotic initiation factor 4E

Protein complexes are dynamic entities; identification and quantitation of their components is critical in elucidating functional roles under specific cellular conditions. We report the first quantitative proteomic analysis of the human cap-binding protein complex. Components and proteins associated with the translation initiation eIF4F complex that may affect complex formation were identified and quantitated under distinct growth conditions. Site-specific phosphorylation of eIF4E and eIF4G and elevated levels of eIF4G:eIF4E complexes in phorbol ester treated HEK293 cells, and in serum-starved tumorigenic human mesenchymal stromal cells, attested to their activated translational states. The WD-repeat, scaffolding-protein Gemin5 was identified as a novel eIF4E binding partner, which interacted directly with eIF4E through a motif (YXXXXLPhi) present in a number of eIF4E-interacting partners. Elevated levels of Gemin5:eIF4E complexes were found in phorbol ester treated HEK293 cells. Gemin5 and eIF4E co-localized to cytoplasmic P-bodies in human osteosarcoma U2OS cells. Interaction between eIF4E and Gemin5 and their co-localization to the P-bodies, may serve to recruit capped mRNAs to these RNP complexes, for functions related to RNP assembly, remodeling and/or transition from active translation to mRNA degradation. Our results demonstrate that our quantitative proteomic strategy can be applied to the identification and quantitation of protein complex components in human cells grown under different conditions.

Translational regulation during oogenesis and early development: the cap-poly(A) tail relationship

Metazoans rely on the regulated translation of select maternal mRNAs to control oocyte maturation and the initial stages of embryogenesis. These transcripts usually remain silent until their translation is temporally and spatially required during early development. Different translational regulatory mechanisms, varying from cytoplasmic polyadenylation to localization of maternal mRNAs, have evolved to assure coordinated initiation of development. A common feature of these mechanisms is that they share a few key trans-acting factors. Increasing evidence suggest that ubiquitous conserved mRNA-binding factors, including the eukaryotic translation initiation factor 4E (eIF4E) and the cytoplasmic polyadenylation element binding protein (CPEB), interact with cell-specific molecules to accomplish the correct level of translational activity necessary for normal development. Here we review how capping and polyadenylation of mRNAs modulate interaction with multiple regulatory factors, thus controlling translation during oogenesis and early development.

Mechanisms of translational control by the 3' UTR in development and differentiation

Translational control plays a major role in early development, differentiation and the cell cycle. In this review, we focus on the four main mechanisms of translational control by 3' untranslated regions: 1. Cytoplasmic polyadenylation and deadenylation; 2. Recruitment of 4E binding proteins; 3. Regulation of ribosomal subunit binding; 4. Post-initiation repression by microRNAs. Proteins with conserved functions in translational control during development include cytoplasmic polyadenylation element binding proteins (CPEB/Orb), Pumilio, Bruno, Fragile X mental retardation protein and RNA helicases. The translational regulation of the mRNAs encoding cyclin B1, Oskar, Nanos, Male specific lethal 2 (Msl-2), lipoxygenase and Lin-14 is discussed.

Cup is a nucleocytoplasmic shuttling protein that interacts with the eukaryotic translation initiation factor 4E to modulate Drosophila ovary development

In Drosophila, the product of the fs (2)cup gene (Cup) is known to be crucial for diverse aspects of female germ-line development. Its functions at the molecular level, however, have remained mainly unexplored. Cup was found to directly associate with eukaryotic translation initiation factor 4E (eIF4E). In this report, we show that Cup is a nucleocytoplasmic shuttling protein and that the interaction with eIF4E promotes retention of the Cup protein in the cytoplasm. Cup is required for the correct accumulation and localization of eIF4E within the posterior cytoplasm of developing oocytes. We furthermore show that cup and eIF4E interact genetically, because a reduction in the level of eIF4E activity deteriorates the development and growth of ovaries bearing homozygous cup mutant alleles. Our results reveal a crucial role for the Cup-eIF4E complex in ovary-specific developmental programs.

Germ Cell Specification and Migration in Drosophila and beyond

The passage of an individual's genome to future generations is essential for the maintenance of species and is mediated by highly specialized cells, the germ cells. Genetic studies in a number of model organisms have provided insight into the molecular mechanisms that control specification, migration and survival of early germ cells. Focusing on Drosophila, we will discuss the mechanisms by which germ cells initially form and remain transcriptionally silent while somatic cells are transcriptionally active. We will further discuss three separate attractive and repellent guidance pathways, mediated by a G-protein coupled receptor, two lipid phosphate phosphohydrolases, and isoprenylation. We will compare and contrast these findings with those obtained in other organisms, in particular zebrafish and mice. While aspects of germ cell specification are strikingly different between these species, germ cell specific gene functions have been conserved. In particular, mechanisms that sense directional cues during germ cell migration seem to be shared between invertebrates and vertebrates.