Translational control during early development

Cytoplasmic Polyadenylation Is an Ancestral Hallmark of Early Development in Animals

Differential regulation of gene expression has produced the astonishing diversity of life on Earth. Understanding the origin and evolution of mechanistic innovations for control of gene expression is therefore integral to evolutionary and developmental biology. Cytoplasmic polyadenylation is the biochemical extension of polyadenosine at the 3'-end of cytoplasmic mRNAs. This process regulates the translation of specific maternal transcripts and is mediated by the Cytoplasmic Polyadenylation Element-Binding Protein family (CPEBs). Genes that code for CPEBs are amongst a very few that are present in animals but missing in nonanimal lineages. Whether cytoplasmic polyadenylation is present in non-bilaterian animals (i.e., sponges, ctenophores, placozoans, and cnidarians) remains unknown. We have conducted phylogenetic analyses of CPEBs, and our results show that CPEB1 and CPEB2 subfamilies originated in the animal stem lineage. Our assessment of expression in the sea anemone, Nematostella vectensis (Cnidaria), and the comb jelly, Mnemiopsis leidyi (Ctenophora), demonstrates that maternal expression of CPEB1 and the catalytic subunit of the cytoplasmic polyadenylation machinery (GLD2) is an ancient feature that is conserved across animals. Furthermore, our measurements of poly(A)-tail elongation reveal that key targets of cytoplasmic polyadenylation are shared between vertebrates, cnidarians, and ctenophores, indicating that this mechanism orchestrates a regulatory network that is conserved throughout animal evolution. We postulate that cytoplasmic polyadenylation through CPEBs was a fundamental innovation that contributed to animal evolution from unicellular life.

TMT-based quantitative proteomic analysis of cumulus cells derived from vitrified porcine immature oocytes following in vitro maturation

As the immature oocytes are submitted to cryopreservation, their surrounding cumulus cells (CCs) will inevitably suffer, which may have some adverse effects on subsequent oocyte maturation and development. So far, little is known about the molecular differences in CCs of immature oocytes after vitrification. The aim of this study therefore was to analyze the protein profile of CCs derived from vitrified porcine immature oocytes following in vitro maturation, using TMT-based quantitative proteomic approach. A total of 5910 proteins were identified, and 88 of them presented significant difference, with 46 up-regulated and 42 down-regulated proteins. Gene Ontology enrichment analysis revealed that cell cycle phase transition, mitotic cell cycle phase transition, positive regulation of cell differentiation and regulation of oogenesis were significantly down-regulated within the biological process. After Kyoto Encyclopedia of Genes and Genomes pathway analysis, some up-regulated proteins were significantly enriched in TGF-beta signaling pathway and 4 pathways related to steroid hormones. Furthermore, 10 selected proteins were quantified and verified by a parallel reaction monitoring technique, indicating a high reliability of the TMT results. In conclusion, vitrification affects protein profile of CCs as well as their biological functions, which will offer a new perspective to understand the reasons for decline in maturation quality of vitrified immature oocytes.

Musashi interaction with poly(A)-binding protein is required for activation of target mRNA translation

The Musashi family of mRNA translational regulators controls both physiological and pathological stem cell self-renewal primarily by repressing target mRNAs that promote differentiation. In response to differentiation cues, Musashi can switch from a repressor to an activator of target mRNA translation. However, the molecular events that distinguish Musashi-mediated translational activation from repression are not understood. We have previously reported that Musashi function is required for the maturation of Xenopus oocytes and specifically for translational activation of specific dormant maternal mRNAs. Here, we employed MS to identify cellular factors necessary for Musashi-dependent mRNA translational activation. We report that Musashi1 needs to associate with the embryonic poly(A)-binding protein (ePABP) or the canonical somatic cell poly(A)-binding protein PABPC1 for activation of Musashi target mRNA translation. Co-immunoprecipitation studies demonstrated an increased Musashi1 interaction with ePABP during oocyte maturation. Attenuation of endogenous ePABP activity severely compromised Musashi function, preventing downstream signaling and blocking oocyte maturation. Ectopic expression of either ePABP or PABPC1 restored Musashi-dependent mRNA translational activation and maturation of ePABP-attenuated oocytes. Consistent with these Xenopus findings, PABPC1 remained associated with Musashi under conditions of Musashi target mRNA de-repression and translation during mammalian stem cell differentiation. Because association of Musashi1 with poly(A)-binding proteins has previously been implicated only in repression of Musashi target mRNAs, our findings reveal novel context-dependent roles for the interaction of Musashi with poly(A)-binding protein family members in response to extracellular cues that control cell fate.

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

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

Granules harboring translationally active mRNAs provide a platform for P-body formation following stress

The localization of mRNA to defined cytoplasmic sites in eukaryotic cells not only allows localized protein production but also determines the fate of mRNAs. For instance, translationally repressed mRNAs localize to P-bodies and stress granules where their decay and storage, respectively, are directed. Here, we find that several mRNAs are localized to granules in unstressed, actively growing cells. These granules play a key role in the stress-dependent formation of P-bodies. Specific glycolytic mRNAs are colocalized in multiple granules per cell, which aggregate during P-body formation. Such aggregation is still observed under conditions or in mutants where P-bodies do not form. In unstressed cells, the mRNA granules appear associated with active translation; this might enable a coregulation of protein expression from the same pathways or complexes. Parallels can be drawn between this coregulation and the advantage of operons in prokaryotic systems.

Heck products of parthenolide and melampomagnolide-B as anticancer modulators that modify cell cycle progression

(E)-13-(Aryl/heteroaryl)parthenolides (5a-i and 6a-i) were synthesized and evaluated for their ability to modify cell cycle progression during progesterone-stimulated Xenopus oocyte maturation and screened for their anticancer activity against a panel of 60 human cancer cell lines. (E)-13-(4-aminophenyl) parthenolide (5b) caused a significant inhibition of progesterone-stimulated oocyte maturation, and was determined to function downstream of MAP kinase signaling, but upstream of the activation of the universal G2/M regulator, M-phase promoting factor (MPF), cyclin B/Cyclin-dependent kinase (CDK). The compound (E)-13-(2-bromo-phenyl)parthenolide (5c) activates oocyte maturation independently of progesterone stimulation. Compounds 5b and 5c displayed modest growth inhibition on select cancer cell lines at 10 μM dose when tested on the panel of 60 cancer cell lines. By contrast, compounds (5f and 7) did not modulate oocyte maturation but did exhibit micromolar level growth inhibition against most of the human cancer cell lines over a range of doses. Together, our findings indicate that screening of compounds in the oocyte maturation assay may identify additional effective cell cycle regulatory compounds that do not necessarily exert overt cytotoxicity as assessed in traditional drug screening assays.

C. elegans RNA-binding protein GLD-1 recognizes its multiple targets using sequence, context, and structural information to repress translation

Caenorhabditis elegans GLD-1, a maxi-KH motif containing RNA-binding protein, has various functions mainly during female germ cell development, suggesting that it likely controls the expression of a selective group of maternal mRNAs. To gain an insight into how GLD-1 specifically recognizes these mRNA targets, we identified 38 biochemically proven GLD-1 binding regions from multiple mRNA targets that are among over 100 putative targets co-immunoprecipitated with GLD-1. The sequence information of these regions revealed three over-represented and phylogenetically conserved sequence motifs. We found that two of the motifs, one of which is novel, are important for GLD-1 binding in several GLD-1 binding regions but not in other regions. Further analyses indicate that the importance of one of the sequence motifs is dependent on two aspects: (1) surrounding sequence information, likely acting as an accessory feature for GLD-1 to efficiently select the sequence motif and (2) RNA secondary structural environment where the sequence motif resides, which likely provides "binding-site accessibility" for GLD-1 to effectively recognize its targets. Our data suggest some mRNAs recruit GLD-1 by a distinct mechanism, which involves more than one sequence motif that needs to be embedded in the correct context and structural environment.

Heterocyclic aminoparthenolide derivatives modulate G(2)-M cell cycle progression during Xenopus oocyte maturation

Aminoparthenolide derivatives have been prepared by reaction of parthenolide with various heterocyclic amines to afford corresponding Michael addition products. These novel compounds were evaluated for their modulatory effects on Xenopus oocyte maturation. Two compounds, 6e and 6f, were identified that promote G2-M cell cycle progression.

Translation repressors, an RNA helicase, and developmental cues control RNP phase transitions during early development

Like membranous organelles, large-scale coassembly of macromolecules can organize functions in cells. Ribonucleoproteins (RNPs) can form liquid or solid aggregates, but control and consequences of these RNP states in living, developing tissue are poorly understood. Here, we show that regulated RNP factor interactions drive transitions among diffuse, semiliquid, or solid states to modulate RNP sorting and exchange in the Caenorhabditis elegans oocyte cytoplasm. Translation repressors induce an intrinsic capacity of RNP components to coassemble into either large semiliquids or solid lattices, whereas a conserved RNA helicase prevents polymerization into nondynamic solids. Developmental cues dramatically alter both fluidity and sorting within large RNP assemblies, inducing a transition from RNP segregation in quiescent oocytes to dynamic exchange in the early embryo. Therefore, large-scale organization of gene expression extends to the cytoplasm, where regulation of supramolecular states imparts specific patterns of RNP dynamics.

Taking a cellular road-trip: mRNA transport and anchoring

mRNA localization is a crucial mechanism for post-transcriptional control of gene expression used in numerous cellular contexts to generate asymmetric enrichment of an encoded protein. This process has emerged as a fundamental regulatory mechanism that operates in a wide range of organisms to control an array of cellular processes. Recently, significant advances have been made in our understanding of the mechanisms that regulate several steps in the mRNA localization pathway. Here we discuss the progress made in understanding localization element recognition, paying particular attention to the role of RNA structure. We also consider the function of mRNP granules in mRNA transport, as well as new results pointing to roles for the endocytic pathway in mRNA localization.

A network of PUF proteins and Ras signaling promote mRNA repression and oogenesis in C. elegans

Cell differentiation requires integration of gene expression controls with dynamic changes in cell morphology, function, and control. Post-transcriptional mRNA regulation and signaling systems are important to this process but their mechanisms and connections are unclear. During C. elegans oogenesis, we find that two groups of PUF RNA binding proteins (RNABPs), PUF-3/11 and PUF-5/6/7, control different specific aspects of oocyte formation. PUF-3/11 limits oocyte growth, while PUF-5/6/7 promotes oocyte organization and formation. These two PUF groups repress mRNA translation through overlapping but distinct sets of 3' untranslated regions (3'UTRs). Several PUF-dependent mRNAs encode other mRNA regulators suggesting both PUF groups control developmental patterning of mRNA regulation circuits. Furthermore, we find that the Ras-MapKinase/ERK pathway functions with PUF-5/6/7 to repress specific mRNAs and control oocyte organization and growth. These results suggest that diversification of PUF proteins and their integration with Ras-MAPK signaling modulates oocyte differentiation. Together with other studies, these findings suggest positive and negative interactions between the Ras-MAPK system and PUF RNA-binding proteins likely occur at multiple levels. Changes in these interactions over time can influence spatiotemporal patterning of tissue development.

Transcriptome analysis of the Arabidopsis megaspore mother cell uncovers the importance of RNA helicases for plant germline development

Germ line specification is a crucial step in the life cycle of all organisms. For sexual plant reproduction, the megaspore mother cell (MMC) is of crucial importance: it marks the first cell of the plant "germline" lineage that gets committed to undergo meiosis. One of the meiotic products, the functional megaspore, subsequently gives rise to the haploid, multicellular female gametophyte that harbours the female gametes. The MMC is formed by selection and differentiation of a single somatic, sub-epidermal cell in the ovule. The transcriptional network underlying MMC specification and differentiation is largely unknown. We provide the first transcriptome analysis of an MMC using the model plant Arabidopsis thaliana with a combination of laser-assisted microdissection and microarray hybridizations. Statistical analyses identified an over-representation of translational regulation control pathways and a significant enrichment of DEAD/DEAH-box helicases in the MMC transcriptome, paralleling important features of the animal germline. Analysis of two independent T-DNA insertion lines suggests an important role of an enriched helicase, MNEME (MEM), in MMC differentiation and the restriction of the germline fate to only one cell per ovule primordium. In heterozygous mem mutants, additional enlarged MMC-like cells, which sometimes initiate female gametophyte development, were observed at higher frequencies than in the wild type. This closely resembles the phenotype of mutants affected in the small RNA and DNA-methylation pathways important for epigenetic regulation. Importantly, the mem phenotype shows features of apospory, as female gametophytes initiate from two non-sister cells in these mutants. Moreover, in mem gametophytic nuclei, both higher order chromatin structure and the distribution of LIKE HETEROCHROMATIN PROTEIN1 were affected, indicating epigenetic perturbations. In summary, the MMC transcriptome sets the stage for future functional characterization as illustrated by the identification of MEM, a novel gene involved in the restriction of germline fate.

Posttranscriptional regulation in Drosophila oocytes and early embryos

Molecular asymmetries underlying embryonic axis patterning and germ cell specification are established in Drosophila largely by position-dependent translational regulation of maternally expressed messenger RNAs. Here, I review several mechanisms of posttranscriptional regulation in the Drosophila oocyte and syncytial embryo, and how they relate to embryonic patterning, with a strong emphasis on the most recent advances in the area. The review is not exhaustive, but focuses on examples that illustrate the interplay between specific regulatory events and the general metabolic machinery that governs translation and mRNA stability. Biophysical investigations into how the Bicoid gradient is formed are discussed, as are the elaborate mechanisms controlling how the Oskar and Nanos proteins become restricted to the posterior pole of the embryo. Work on Vasa, a translational activator of some germ line mRNAs and on 4EHP, a negative regulator that unproductively binds the 5' cap structure of target mRNAs, is also briefly reviewed. Finally, the emerging understanding of the role of microRNAs in regulating translation of germ line mRNAs is also discussed.

Regulation of protein synthesis and the role of eIF3 in cancer

Maintenance of cell homeostasis and regulation of cell proliferation depend importantly on regulating the process of protein synthesis. Many disease states arise when disregulation of protein synthesis occurs. This review focuses on mechanisms of translational control and how disregulation results in cell malignancy. Most translational controls occur during the initiation phase of protein synthesis, with the initiation factors being the major target of regulation through their phosphorylation. In particular, the recruitment of mRNAs through the m⁷G-cap structure and the binding of the initiator methionyl-tRNA(i) are frequent targets. However, translation, especially of specific mRNAs, may also be regulated by sequestration into processing bodies or stress granules, by trans-acting proteins or by microRNAs. When the process of protein synthesis is hyper-activated, weak mRNAs are translated relatively more efficiently, leading to an imbalance of cellular proteins that promotes cell proliferation and malignant transformation. This occurs, for example, when the cap-binding protein, eIF4E, is overexpressed, or when the methionyl-tRNA(i)-binding factor, eIF2, is too active. In addition, enhanced activity of eIF3 contributes to oncogenesis. The importance of the translation initiation factors as regulators of protein synthesis and cell proliferation makes them potential therapeutic targets for the treatment of cancer.