Mechanisms of translational regulation in Drosophila

Impact of tRNA-induced proline-to-serine mistranslation on the transcriptome of Drosophila melanogaster

Mistranslation is the misincorporation of an amino acid into a polypeptide. Mistranslation has diverse effects on multicellular eukaryotes and is implicated in several human diseases. In Drosophila melanogaster, a serine transfer RNA (tRNA) that misincorporates serine at proline codons (P→S) affects male and female flies differently. The mechanisms behind this discrepancy are currently unknown. Here, we compare the transcriptional response of male and female flies to P→S mistranslation to identify genes and cellular processes that underlie sex-specific differences. Both males and females downregulate genes associated with various metabolic processes in response to P→S mistranslation. Males downregulate genes associated with extracellular matrix organization and response to negative stimuli such as wounding, whereas females downregulate aerobic respiration and ATP synthesis genes. Both sexes upregulate genes associated with gametogenesis, but females also upregulate cell cycle and DNA repair genes. These observed differences in the transcriptional response of male and female flies to P→S mistranslation have important implications for the sex-specific impact of mistranslation on disease and tRNA therapeutics.

Forgotten memory storage and retrieval in Drosophila

Inaccessibility of stored memory in ensemble cells through the forgetting process causes animals to be unable to respond to natural recalling cues. While accumulating evidence has demonstrated that reactivating memory-stored cells can switch cells from an inaccessible state to an accessible form and lead to recall of previously learned information, the underlying cellular and molecular mechanisms remain elusive. The current study used Drosophila as a model to demonstrate that the memory of one-trial aversive olfactory conditioning, although inaccessible within a few hours after learning, is stored in KCαβ and retrievable after mild retraining. One-trial aversive conditioning triggers protein synthesis to form a long-lasting cellular memory trace, approximately 20 days, via creb in KCαβ, and a transient cellular memory trace, approximately one day, via orb in MBON-α3. PPL1-α3 negatively regulates forgotten one-trial conditioning memory retrieval. The current study demonstrated that KCαβ, PPL1-α3, and MBON-α3 collaboratively regulate the formation of forgotten one-cycle aversive conditioning memory formation and retrieval.

Ribosome biogenesis and function in development and disease

Although differential transcription drives the development of multicellular organisms, the ultimate readout of a protein-coding gene is ribosome-dependent mRNA translation. Ribosomes were once thought of as uniform molecular machines, but emerging evidence indicates that the complexity and diversity of ribosome biogenesis and function should be given a fresh look in the context of development. This Review begins with a discussion of different developmental disorders that have been linked with perturbations in ribosome production and function. We then highlight recent studies that reveal how different cells and tissues exhibit variable levels of ribosome production and protein synthesis, and how changes in protein synthesis capacity can influence specific cell fate decisions. We finish by touching upon ribosome heterogeneity in stress responses and development. These discussions highlight the importance of considering both ribosome levels and functional specialization in the context of development and disease.

Minimal effects of spargel (PGC-1) overexpression in a Drosophila mitochondrial disease model

PGC-1α and its homologues have been proposed to act as master regulators of mitochondrial biogenesis in animals. Most relevant studies have been conducted in mammals, where interpretation is complicated by the fact that there are three partially redundant members of the gene family. In Drosophila, only a single PGC-1 homologue, spargel (srl), is present in the genome. Here, we analyzed the effects of srl overexpression on phenotype and on gene expression in tko^25t, a recessive bang-sensitive mutant with a global defect in oxidative phosphorylation, resulting from a deficiency of mitochondrial protein synthesis. In contrast to previous reports, we found that substantial overexpression of srl throughout development had only minimal effects on the tko^25t mutant phenotype. Copy number of mtDNA was unaltered and srl overexpression produced no systematic effects on a representative set of transcripts related to mitochondrial OXPHOS and other metabolic enzymes, although these were influenced by sex and genetic background. This study provides no support to the concept of Spargel as a global regulator of mitochondrial biogenesis, at least in the context of the tko^25t model.

Summary: Overexpression of spargel, the fly PGC-1 homologue proposed as a mitochondrial biogenesis regulator, has minimal effects on the phenotype of tko^25t, considered a fly model for mitochondrial disease.

Shep interacts with posttranscriptional regulators to control dendrite morphogenesis in sensory neurons

RNA binding proteins (RBPs) mediate posttranscriptional gene regulatory events throughout development. During neurogenesis, many RBPs are required for proper dendrite morphogenesis within Drosophila sensory neurons. Despite their fundamental role in neuronal morphogenesis, little is known about the molecular mechanisms in which most RBPs participate during neurogenesis. In Drosophila, alan shepard (shep) encodes a highly conserved RBP that regulates dendrite morphogenesis in sensory neurons. Moreover, the C. elegans ortholog sup-26 has also been implicated in sensory neuron dendrite morphogenesis. Nonetheless, the molecular mechanism by which Shep/SUP-26 regulate dendrite development is not understood. Here we show that Shep interacts with the RBPs Trailer Hitch (Tral), Ypsilon schachtel (Yps), Belle (Bel), and Poly(A)-Binding Protein (PABP), to direct dendrite morphogenesis in Drosophila sensory neurons. Moreover, we identify a conserved set of Shep/SUP-26 target RNAs that include regulators of cell signaling, posttranscriptional gene regulators, and known regulators of dendrite development.

Translational repression of the Drosophila nanos mRNA involves the RNA helicase Belle and RNA coating by Me31B and Trailer hitch

Translational repression of maternal mRNAs is an essential regulatory mechanism during early embryonic development. Repression of the Drosophila nanos mRNA, required for the formation of the anterior-posterior body axis, depends on the protein Smaug binding to two Smaug recognition elements (SREs) in the nanos 3' UTR. In a comprehensive mass spectrometric analysis of the SRE-dependent repressor complex, we identified Smaug, Cup, Me31B, Trailer hitch, eIF4E, and PABPC, in agreement with earlier data. As a novel component, the RNA-dependent ATPase Belle (DDX3) was found, and its involvement in deadenylation and repression of nanos was confirmed in vivo. Smaug, Cup, and Belle bound stoichiometrically to the SREs, independently of RNA length. Binding of Me31B and Tral was also SRE-dependent, but their amounts were proportional to the length of the RNA and equimolar to each other. We suggest that "coating" of the RNA by a Me31B•Tral complex may be at the core of repression.

Systems Biology-Derived Discoveries of Intrinsic Clocks

A systems approach to studying biology uses a variety of mathematical, computational, and engineering tools to holistically understand and model properties of cells, tissues, and organisms. Building from early biochemical, genetic, and physiological studies, systems biology became established through the development of genome-wide methods, high-throughput procedures, modern computational processing power, and bioinformatics. Here, we highlight a variety of systems approaches to the study of biological rhythms that occur with a 24-h period-circadian rhythms. We review how systems methods have helped to elucidate complex behaviors of the circadian clock including temperature compensation, rhythmicity, and robustness. Finally, we explain the contribution of systems biology to the transcription-translation feedback loop and posttranslational oscillator models of circadian rhythms and describe new technologies and "-omics" approaches to understand circadian timekeeping and neurophysiology.

Depletion of juvenile hormone esterase extends larval growth in Bombyx mori

Two major hormones, juvenile hormone (JH) and 20-hydroxyecdysone (20E), regulate insect growth and development according to their precisely coordinated titres, which are controlled by both biosynthesis and degradation pathways. Juvenile hormone esterase (JHE) is the primary JH-specific degradation enzyme that plays a key role in regulating JH titers, along with JH epoxide hydrolase (JHEH) and JH diol kinase (JHDK). In the current study, a loss-of-function analysis of JHE in the silkworm, Bombyx mori, was performed by targeted gene disruption using the transgenic CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/RNA-guided Cas9 nucleases) system. Depletion of B. mori JHE (BmJHE) resulted in the extension of larval stages, especially the penultimate and ultimate larval stages, without deleterious effects to silkworm physiology. The expression of JHEH and JHDK was upregulated in mutant animals, indicating the existence of complementary routes in the JH metabolism pathway in which inactivation of one enzyme will activate other enzymes. RNA-Seq analysis of mutant animals revealed that genes involved in protein processing in the endoplasmic reticulum and in amino acid metabolism were affected by BmJHE depletion. Depletion of JHE and subsequent delayed JH metabolism activated genes in the TOR pathway, which are ultimately responsible for extending larval growth. The transgenic Cas9 system used in the current study provides a promising approach for analysing the actions of JH, especially in nondrosophilid insects. Furthermore, prolonging larval stages produced larger larvae and cocoons, which is greatly beneficial to silk production.

Multiple Roles for Egalitarian in Polarization of the Drosophila Egg Chamber

The Drosophila egg chamber provides a useful model for examining mechanisms by which cell fates are specified and maintained in the context of a complex tissue. The egg chamber is also an excellent model for understanding the mechanism by which cytoskeletal filaments are organized and the critical interplay between cytoskeletal organization, polarity establishment, and cell fate specification. Previous work has shown that Egalitarian (Egl) is required for specification and maintenance of oocyte fate. Mutants in egl either completely fail to specify an oocyte, or if specified, the oocyte eventually reverts back to nurse cell fate. Due to this very early role for Egl in egg chamber maturation, it is unclear whether later stages of egg chamber development also require Egl function. In this report, we have depleted Egl at specific stages of egg chamber development. We demonstrate that in early-stage egg chambers, Egl has an additional role in organization of oocyte microtubules. In the absence of Egl function, oocyte microtubules completely fail to reorganize. As such, the localization of microtubule motors and their cargo is disrupted. In addition, Egl also appears to function in regulating the translation of critical polarity determining messenger RNAs (mRNAs). Finally, we demonstrate that in midstage egg chambers, Egl does not appear to be required for microtubule organization, but rather for the correct spatial localization of oskar, bicoid, and gurken mRNAs.

The translational regulator Cup controls NMJ presynaptic terminal morphology

During oogenesis and early embryonic development in Drosophila, translation of proteins from maternally deposited mRNAs is tightly controlled. We and others have previously shown that translational regulatory proteins that function during oogenesis also have essential roles in the nervous system. Here we examine the role of Cup in neuromuscular system development. Maternal Cup controls translation of localized mRNAs encoding the Oskar and Nanos proteins and binds to the general translation initiation factor eIF4E. In this paper, we show that zygotic Cup protein is localized to presynaptic terminals at larval neuromuscular junctions (NMJs). cup mutant NMJs have strong phenotypes characterized by the presence of small clustered boutons called satellite boutons. They also exhibit an increase in the frequency of spontaneous glutamate release events (mEPSPs). Reduction of eIF4E expression synergizes with partial loss of Cup expression to produce satellite bouton phenotypes. The presence of satellite boutons is often associated with increases in retrograde bone morphogenetic protein (BMP) signaling, and we show that synaptic BMP signaling is elevated in cup mutants. cup genetically interacts with two genes, EndoA and Dap160, that encode proteins involved in endocytosis that are also neuronal modulators of the BMP pathway. Endophilin protein, encoded by the EndoA gene, is downregulated in a cup mutant. Our results are consistent with a model in which Cup and eIF4E work together to ensure efficient localization and translation of endocytosis proteins in motor neurons and control the strength of the retrograde BMP signal.

Studying RNA-Protein Interactions of Pre-mRNA Complexes by Mass Spectrometry

RNA-protein interactions play a crucial role in gene expression. These interactions take place in so-called ribonucleoprotein (RNP) complexes. To investigate which proteins interact with RNA in these complexes, and how they do so, UV-light-induced cross-linking has proven to be a valuable, yet straightforward technique. UV irradiation induces a covalent bond between the RNA and the proteins, whereafter cross-linked proteins can be identified by mass spectrometric (MS) approaches. Moreover, the cross-linked region of the protein, and often the actual cross-linked amino acid, can be identified by state-of-the-art MS, as can the cross-linked RNA moiety. This protocol describes in detail how to isolate peptide-RNA oligonucleotide cross-links from UV-irradiated human pre-mRNA RNPs and to perform the subsequent MS investigation of these peptide-RNA conjugates in combination with a dedicated computational analysis, in order to obtain sequence information about the cross-linked peptide and oligoribonucleotide. The described workflow can be applied to any RNP, irrespective of its origin, e.g., RNPs assembled in vitro (as described here) or RNPs isolated from UV-irradiated cells, either ex vivo or in vivo.

RNA metabolism in the pathogenesis of Parkinson׳s disease

Neurodegenerative diseases such as Parkinson׳s disease are progressive disorders of the nervous system that affect the function and maintenance of specific neuronal populations. While most disease cases are sporadic with no known cause, a small percentage of disease cases are caused by inherited genetic mutations. The identification of genes associated with the familial forms of the diseases and subsequent studies of proteins encoded by the disease genes in cellular or animal models have offered much-needed insights into the molecular and cellular mechanisms underlying disease pathogenesis. Recent studies of the familial Parkinson׳s disease genes have emphasized the importance of RNA metabolism, particularly mRNA translation, in the disease process. It is anticipated that continued studies on the role of RNA metabolism in Parkinson׳s disease will offer unifying mechanisms for understanding the cause of neuronal dysfunction and degeneration and facilitate the development of novel and rational strategies for treating this debilitating disease.

Dynein associates with oskar mRNPs and is required for their efficient net plus-end localization in Drosophila oocytes

In order for eukaryotic cells to function properly, they must establish polarity. The Drosophila oocyte uses mRNA localization to establish polarity and hence provides a genetically tractable model in which to study this process. The spatial restriction of oskar mRNA and its subsequent protein product is necessary for embryonic patterning. The localization of oskar mRNA requires microtubules and microtubule-based motor proteins. Null mutants in Kinesin heavy chain (Khc), the motor subunit of the plus end-directed Kinesin-1, result in oskar mRNA delocalization. Although the majority of oskar particles are non-motile in khc nulls, a small fraction of particles display active motility. Thus, a motor other than Kinesin-1 could conceivably also participate in oskar mRNA localization. Here we show that Dynein heavy chain (Dhc), the motor subunit of the minus end-directed Dynein complex, extensively co-localizes with Khc and oskar mRNA. In addition, immunoprecipitation of the Dynein complex specifically co-precipitated oskar mRNA and Khc. Lastly, germline-specific depletion of Dhc resulted in oskar mRNA and Khc delocalization. Our results therefore suggest that efficient posterior localization of oskar mRNA requires the concerted activities of both Dynein and Kinesin-1.

Fragile hearts: new insights into translational control in cardiac muscle

Current investigations focused on RNA-binding proteins in striated muscle, which provide a scenario whereby muscle function and development are governed by the interplay of post-transcriptional RNA regulation, including transcript localization, splicing, stability, and translational control. New data have recently emerged, linking the RNA-binding protein FXR1 to the translation of key cytoskeletal components such as talin and desmoplakin in heart muscle. These findings, together with a plethora of recent reports implicating RNA-binding proteins and their RNA targets in both basic aspects of muscle development and differentiation as well as heart disease and muscular dystrophies, point to a critical role of RNA-based regulatory mechanisms in muscle biology. Here we focus on FXR1, the striated muscle-specific member of the Fragile X family of RNA-binding proteins and discuss its newly reported cytoskeletal targets as well as potential implications for heart disease.

CPEB1 regulates the expression of MTDH/AEG-1 and glioblastoma cell migration

Cytoplasmic polyadenylation element-binding protein 1 (CPEB1) is an mRNA-binding protein present in both neurons and glia. CPEB1 is capable of both repressing mRNA translation and activating it depending upon its phosphorylation state. CPEB1-bound mRNAs are held in translational dormancy until CPEB1 is phosphorylated, leading to the cytoplasmic polyadenylation of the bound mRNA that triggers translation. Here, we show that CPEB1 can bind to and regulate translation of the mRNA-encoding metadherin (MTDH, also known as AEG-1 and Lyric) in the rat glioblastoma cell line CNS1. MTDH/AEG-1 is being revealed as a critical signaling molecule in tumor progression, playing roles in invasion, metastasis, and chemoresistance. By using a mutant of CPEB1 that cannot be phosphorylated (thereby holding target mRNAs in translational arrest), we show that inhibiting CPEB1-mediated translation blocks MTDH/AEG-1 expression in vitro and inhibits glioblastomas tumor growth in vivo. CPEB1-mediated translation is likely to impact several signaling pathways that may promote tumor progression, but we present evidence suggesting a role in directed cell migration in glioblastoma cells. In addition, reporter mRNA containing CPEB1-binding sites is transported to the leading edge of migrating cells and translated, whereas the same mRNA with point mutations in the binding sites is synthesized perinuclearly. Our findings show that CPEB1 is hyperactive in rat glioblastoma cells and is regulating an important cohort of mRNAs whose increased translation is fueling the progression of tumor proliferation and dispersal in the brain. Thus, targeting CPEB1-mediated mRNA translation might be a sound therapeutic approach.

Post-transcriptional stimulation of gene expression by microRNAs

MicroRNAs are small noncoding RNA regulatory molecules that control gene expression by guiding associated effector complexes to other RNAs via sequence-specific recognition of target sites. Misregulation of microRNAs leads to a wide range of diseases including cancers, inflammatory and developmental disorders. MicroRNAs were found to mediate deadenylation-dependent decay and translational repression of messages through partially complementary microRNA target sites in the 3'-UTR (untranslated region). A growing series of studies has demonstrated that microRNAs and their associated complexes (microRNPs) elicit alternate functions that enable stimulation of gene expression in addition to their assigned repressive roles. These reports, discussed in this chapter, indicate that microRNA-mediated effects via natural 3' and 5'-UTRs can be selective and controlled, dictated by the RNA sequence context, associated complex, and cellular conditions. Similar to the effects of repression, upregulated gene expression by microRNAs varies from small refinements to significant amplifications in expression. An emerging theme from this literature is that microRNAs have a versatile range of abilities to manipulate post-transcriptional control mechanisms leading to controlled gene expression. These studies reveal new potentials for microRNPs in gene expression control that develop as responses to specific cellular conditions.

microRNA-independent recruitment of Argonaute 1 to nanos mRNA through the Smaug RNA-binding protein

Argonaute (Ago) proteins are typically recruited to target messenger RNAs via an associated small RNA such as a microRNA (miRNA). Here, we describe a new mechanism of Ago recruitment through the Drosophila Smaug RNA-binding protein. We show that Smaug interacts with the Ago1 protein, and that Ago1 interacts with and is required for the translational repression of the Smaug target, nanos mRNA. The Ago1/nanos mRNA interaction does not require a miRNA, but it does require Smaug. Taken together, our data suggest a model whereby Smaug directly recruits Ago1 to nanos mRNA in a miRNA-independent manner, thereby repressing translation.

Spatial regulation of translation through RNA localization

RNA localization is a mechanism to post-transcriptionally regulate gene expression. Eukaryotic organisms ranging from fungi to mammals localize mRNAs to spatially restrict synthesis of specific proteins to distinct regions of the cytoplasm. In this review, we provide a general summary of RNA localization pathways in Saccharomyces cerevisiae, Xenopus, Drosophila and mammalian neurons.

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.

Synthetic antibodies as tools to probe RNA-binding protein function

RNA-binding proteins (RBPs) have essential roles in post-transcriptional regulation of gene expression. They bind sequence elements in specific mRNAs and control their splicing, transport, localization, translation, and stability. A complete understanding of RBP function requires identification of the target RNAs that an RBP regulates, the mechanisms by which the RBP regulates these targets, and the biological consequences for the cell in which these transactions occur. Antibodies are key tools in such studies: first, mRNA targets of RBPs can be identified by co-immunoprecipitation of RBPs with their associated RNAs followed by microarray analysis or sequencing; second, partner proteins can be identified by immunoprecipitation of the RBP followed by mass spectrometry; third, the mechanisms and functions of RBPs can be inferred from loss-of-function studies employing antibodies that block RBP-RNA interactions. One potentially powerful approach to making antibodies for such studies is the generation of synthetic antibodies using phage display, which involves in vitro selection using a human-designed antibody library to generate antibodies that recognize a target protein. Using two well-characterized Drosophila RNA-binding proteins, Staufen and Smaug, for proof-of-principle, we demonstrate that synthetic antibodies can be generated and used either to perform RNA-coimmunoprecipitations (RIPs) to identify RBP-bound mRNAs, or to block RBP-RNA interactions. Given that synthetic antibody selection protocols are amenable to high-throughput antibody production, these results demonstrate that synthetic antibodies can be powerful tools for genome-wide studies of RBP function.

New insights into the regulation of RNP granule assembly in oocytes

In a variety of cell types in plants, animals, and fungi, ribonucleoprotein (RNP) complexes play critical roles in regulating RNA metabolism. These RNP granules include processing bodies and stress granules that are found broadly across cell types, as well as RNP granules unique to the germline, such as P granules, polar granules, sponge bodies, and germinal granules. This review focuses on RNP granules localized in oocytes of the major model systems, Caenorhabditis elegans, Drosophila, Xenopus, mouse, and zebrafish. The signature families of proteins within oocyte RNPs include Vasa and other RNA-binding proteins, decapping activators and enzymes, Argonaute family proteins, and translation initiation complex proteins. This review describes the many recent insights into the dynamics and functions of RNP granules, including their roles in mRNA degradation, mRNA localization, translational regulation, and fertility. The roles of the cytoskeleton and cell organelles in regulating RNP granule assembly are also discussed.

Protein phosphatase 2A (PP2A)-specific ubiquitin ligase MID1 is a sequence-dependent regulator of translation efficiency controlling 3-phosphoinositide-dependent protein kinase-1 (PDPK-1)

We have shown previously that the ubiquitin ligase MID1, mutations of which cause the midline malformation Opitz BBB/G syndrome (OS), serves as scaffold for a microtubule-associated protein complex that regulates protein phosphatase 2A (PP2A) activity in a ubiquitin-dependent manner. Here, we show that the MID1 protein complex associates with mRNAs via a purine-rich sequence motif called MIDAS (MID1 association sequence) and thereby increases stability and translational efficiency of these mRNAs. Strikingly, inclusion of multiple copies of the MIDAS motif into mammalian mRNAs increases production of the encoded proteins up to 20-fold. Mutated MID1, as found in OS patients, loses its influence on MIDAS-containing mRNAs, suggesting that the malformations in OS patients could be caused by failures in the regulation of cytoskeleton-bound protein translation. This is supported by the observation that the majority of mRNAs that carry MIDAS motifs is involved in developmental processes and/or energy homeostasis. Further analysis of one of the proteins encoded by a MIDAS-containing mRNA, namely PDPK-1 (3-phosphoinositide dependent protein kinase-1), which is an important regulator of mammalian target of rapamycin/PP2A signaling, showed that PDPK-1 protein synthesis is significantly reduced in cells from an OS patient compared with an age-matched control and can be rescued by functional MID1. Together, our data uncover a novel messenger ribonucleoprotein complex that regulates microtubule-associated protein translation. They suggest a novel mechanism underlying OS and point at an enormous potential of the MIDAS motif to increase the efficiency of biotechnological protein production in mammalian cells.

Posttranscriptional upregulation by microRNAs

MicroRNAs are small non-coding RNA guide molecules that regulate gene expression via association with effector complexes and sequence-specific recognition of target sites on other RNAs; misregulated microRNA expression and functions are linked to a variety of tumors, developmental disorders, and immune disease. MicroRNAs have primarily been demonstrated to mediate posttranscriptional downregulation of expression; translational repression, and deadenylation-dependent decay of messages through partially complementary microRNA target sites in mRNA untranslated regions (UTRs). However, an emerging assortment of studies, discussed in this review, reveal that microRNAs and their associated protein complexes (microribonucleoproteins or microRNPs) can additionally function to posttranscriptionally stimulate gene expression by direct and indirect mechanisms. These reports indicate that microRNA-mediated effects can be selective, regulated by the RNA sequence context, and associated with RNP factors and cellular conditions. Like repression, translation upregulation by microRNAs has been observed to range from fine-tuning effects to significant alterations in expression. These studies uncover remarkable, new abilities of microRNAs and associated microRNPs in gene expression control and underscore the importance of regulation, in cis and trans, in directing appropriate microRNP responses.

CUP promotes deadenylation and inhibits decapping of mRNA targets

CUP is an eIF4E-binding protein (4E-BP) that represses the expression of specific maternal mRNAs prior to their posterior localization. Here, we show that CUP employs multiple mechanisms to repress the expression of target mRNAs. In addition to inducing translational repression, CUP maintains mRNA targets in a repressed state by promoting their deadenylation and protects deadenylated mRNAs from further degradation. Translational repression and deadenylation are independent of eIF4E binding and require both the middle and C-terminal regions of CUP, which collectively we termed the effector domain. This domain associates with the deadenylase complex CAF1-CCR4-NOT and decapping activators. Accordingly, in isolation, the effector domain is a potent trigger of mRNA degradation and promotes deadenylation, decapping and decay. However, in the context of the full-length CUP protein, the decapping and decay mediated by the effector domain are inhibited, and target mRNAs are maintained in a deadenylated, repressed form. Remarkably, an N-terminal regulatory domain containing a noncanonical eIF4E-binding motif is required to protect CUP-associated mRNAs from decapping and further degradation, suggesting that this domain counteracts the activity of the effector domain. Our findings indicate that the mode of action of CUP is more complex than previously thought and provide mechanistic insight into the regulation of mRNA expression by 4E-BPs.

The functioning of the Drosophila CPEB protein Orb is regulated by phosphorylation and requires casein kinase 2 activity

The Orb CPEB protein regulates translation of localized mRNAs in Drosophila ovaries. While there are multiple hypo- and hyperphosphorylated Orb isoforms in wild type ovaries, most are missing in orb^F303, which has an amino acid substitution in a buried region of the second RRM domain. Using a proteomics approach we identified a candidate Orb kinase, Casein Kinase 2 (CK2). In addition to being associated with Orb in vivo, we show that ck2 is required for orb functioning in gurken signaling and in the autoregulation of orb mRNA localization and translation. Supporting a role for ck2 in Orb phosphorylation, we find that the phosphorylation pattern is altered when ck2 activity is partially compromised. Finally, we show that the Orb hypophosphorylated isoforms are in slowly sedimenting complexes that contain the translational repressor Bruno, while the hyperphosphorylated isoforms assemble into large complexes that co-sediment with polysomes and contain the Wisp poly(A) polymerase.

Pabp binds to the osk 3'UTR and specifically contributes to osk mRNA stability and oocyte accumulation

RNA localization is tightly coordinated with RNA stability and translation control. Bicaudal-D (Bic-D), Egalitarian (Egl), microtubules and their motors are part of a Drosophila transport machinery that localizes mRNAs to specific cellular regions during oogenesis and embryogenesis. We identified the Poly(A)-binding protein (Pabp) as a protein that forms an RNA-dependent complex with Bic-D in embryos and ovaries. pabp also interacts genetically with Bic-D and, similar to Bic-D, pabp is essential in the germline for oocyte growth and accumulation of osk mRNA in the oocyte. In the absence of pabp, reduced stability of osk mRNA and possibly also defects in osk mRNA transport prevent normal oocyte localization of osk mRNA. pabp also interacts genetically with osk and lack of one copy of pabp(+) causes osk to become haploinsufficient. Moreover, pointing to a poly(A)-independent role, Pabp binds to A-rich sequences (ARS) in the osk 3'UTR and these turned out to be required in vivo for osk function during early oogenesis. This effect of pabp on osk mRNA is specific for this RNA and other tested mRNAs localizing to the oocyte are less and more indirectly affected by the lack of pabp.

New aminoacyl-tRNA synthetase-like protein in insecta with an essential mitochondrial function

Aminoacyl-tRNA synthetases (ARS) are modular enzymes that aminoacylate transfer RNAs (tRNA) for their use by the ribosome during protein synthesis. ARS are essential and universal components of the genetic code that were almost completely established before the appearance of the last common ancestor of all living species. This long evolutionary history explains the growing number of functions being discovered for ARS, and for ARS homologues, beyond their canonical role in gene translation. Here we present a previously uncharacterized paralogue of seryl-tRNA synthetase named SLIMP (seryl-tRNA synthetase-like insect mitochondrial protein). SLIMP is the result of a duplication of a mitochondrial seryl-tRNA synthetase (SRS) gene that took place in early metazoans and was fixed in Insecta. Here we show that SLIMP is localized in the mitochondria, where it carries out an essential function that is unrelated to the aminoacylation of tRNA. The knockdown of SLIMP by RNA interference (RNAi) causes a decrease in respiration capacity and an increase in mitochondrial mass in the form of aberrant mitochondria.

Smaug assembles an ATP-dependent stable complex repressing nanos mRNA translation at multiple levels

The nanos (nos) mRNA encodes the posterior determinant of the Drosophila embryo. Translation of the RNA is repressed throughout most of the embryo by the protein Smaug binding to Smaug recognition elements (SREs) in the 3' UTR. Translation is locally activated at the posterior pole by Oskar. This paper reports that the SREs govern the time- and ATP-dependent assembly of an exceedingly stable repressed ribonucleoprotein particle (RNP) in embryo extract. Repression can be virtually complete. Smaug and its co-repressor Cup as well as Trailer hitch and the DEAD box protein Me31B are part of the repressed RNP. The initiation factor eIF4G is specifically displaced, and 48S pre-initiation complex formation is inhibited. However, later steps in translation initiation are also sensitive to SRE-dependent inhibition. These data confirm several previously untested predictions of a current model for Cup-dependent repression but also suggest that the Cup model by itself is insufficient to explain translational repression of the nos RNA. In the embryo extract, recombinant Oskar relieves translational repression and deadenylation by preventing Smaug's binding to the SREs.

Posttranscriptional regulation of the p85α adapter subunit of phosphatidylinositol 3-kinase in human leukemia cells

Constitutive activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling has been observed in up to 70% of acute myeloid leukemia. Class I(A) PI3K consists of a catalytic subunit (p110α, p110β, p110δ) and an adapter subunit (p85α, p55α, p50α, p85β, p55γ). The p85α adapter subunit stabilizes the catalytic p110 subunit and recruits p110 to the plasma membrane. In addition, p85α inhibits the basal activity of p110α and can negatively regulate signal transduction, as shown for insulin and GM-CSF receptor signaling. Here, we describe that the expression of p85α is posttranscriptionally regulated in several human and murine leukemia cell lines and in a Hodgkin lymphoma cell line (CO) by translational repression. A detailed analysis of CO cells revealed that both wild type and a mutated p85α mRNA are detectable at similar ratios in the nucleus and polysomes. However, while the mutated p85α protein is expressed in CO cells, translation of the wild type p85α mRNA is completely inhibited. Ectopic expression of wild type p85α from a retroviral vector is suppressed in CO cells and in five out of six leukemia cell lines. Our data indicate that leukemia cells can regulate the expression of p85α by posttranscriptional regulation.

Sm proteins specify germ cell fate by facilitating oskar mRNA localization

Sm and Sm-like proteins are RNA-binding factors found in all three domains of life. Eukaryotic Sm proteins play essential roles in pre-mRNA splicing, forming the cores of spliceosomal small nuclear ribonucleoproteins (snRNPs). Recently, Sm proteins have been implicated in the specification of germ cells. However, a mechanistic understanding of their involvement in germline specification is lacking and a germline-specific RNA target has not been identified. We demonstrate that Drosophila SmB and SmD3 are specific components of the oskar messenger ribonucleoprotein (mRNP), proper localization of which is required for establishing germline fate and embryonic patterning. Importantly, oskar mRNA is delocalized in females harboring a hypomorphic mutation in SmD3, and embryos from mutant mothers are defective in germline specification. We conclude that Sm proteins function to establish the germline in Drosophila, at least in part by mediating oskar mRNA localization.

The realm of microRNAs in cancers

MicroRNAs (miRNAs) are members of the non-protein coding RNA family. miRNAs, which can regulate genes on transcriptomic level through either degrading the target messenger RNA (mRNA) or suppressing the protein synthesis, also take part in a number of biological functions that involve development, differentiation, proliferation and apoptosis. The mutations and polymorphisms in the expression levels of miRNA genes or alterations in their epigenetic mechanisms may play their roles in the formation of malignancies. Increasing evidence shows that aberrant miRNA expression profiles are present in a variety of cancers. Therefore, it has been suggested that these profiles could be useful for diagnosis and classification of different tumor types and that these small RNAs might provide significant opportunities for the development of future miRNA-based therapies. In this review, we aimed to look into the realm of miRNAs, which is a recent area of research, appraise their biological activities on molecular level and their probable benefits on clinical practice.

The SXL-UNR corepressor complex uses a PABP-mediated mechanism to inhibit ribosome recruitment to msl-2 mRNA

Drosophila female viability requires translational repression of msl-2 mRNA by the SXL-UNR 3' UTR corepressor complex, which inhibits ribosome recruitment by an unknown mechanism. Here, we reveal a key role for the poly(A)-binding protein (PABP), a translational activator, in this inhibitory mechanism. Efficient msl-2 mRNA silencing via the 3' UTR requires both a poly(A) tail and PABP function, and we find that UNR directly interacts with PABP. To investigate how the repressor complex and PABP affect RNP composition during early steps in translation initiation, we established direct biochemical assays for synergistic recruitment of eIF4F and ribosomes by the cap and poly(A) tail. We find that the repressor complex targets ribosome binding after PABP-mediated recruitment of eIF4E/G. Our results uncover an important regulatory mechanism of Drosophila dosage compensation and provide insight into PABP-dependent translational control by 3' UTR-bound regulatory proteins.

Translational control during early development

Translational control of specific messenger RNAs, which themselves are often asymmetrically localized within the cytoplasm of a cell, underlies many events in germline development, and in embryonic axis specification. This comprehensive, but by no means exhaustive, review attempts to present a picture of the present state of knowledge about mechanisms underlying mRNA localization and translational control of specific mRNAs that are mediated by trans-acting protein factors. While RNA localization and translational control are widespread in evolution and have been studied in many experimental systems, this article will focus mainly on three particularly well-characterized systems: Drosophila, Caenorhabditis elegans, and Xenopus. In keeping with the overall theme of this volume, instances in which translational control factors have been linked to human disease states will also be discussed.

Centrocortin cooperates with centrosomin to organize Drosophila embryonic cleavage furrows

In the Drosophila early embryo, the centrosome coordinates assembly of cleavage furrows. Currently, the molecular pathway that links the centrosome and the cortical microfilaments is unknown. In centrosomin (cnn) mutants, in which the centriole forms but the centrosome pericentriolar material (PCM) fails to assemble, actin microfilaments are not organized into furrows at the syncytial cortex [6]. Although CNN is required for centrosome assembly and function, little is known of its molecular activities. Here, we show the novel protein Centrocortin (CEN), which associates with centrosomes and also with cleavage furrows in early embryos, is required for cleavage furrow assembly. CEN binds to CNN within CNN Motif 2 (CM2), a conserved 60 amino acid domain at CNN's C terminus. The cnn(B4) allele, which contains a missense mutation at a highly conserved residue within CM2, blocks the binding of CEN and disrupts cleavage furrow assembly. Together, these findings show that the C terminus of CNN coordinates cleavage furrow formation through binding to CEN, thereby providing a molecular link between the centrosome and cleavage furrow assembly.

Bruno negatively regulates germ cell-less expression in a BRE-independent manner

Mechanisms of post-transcriptional control are essential during Drosophila oogenesis and embryogenesis to sequester gene products in discrete regions and ultimately achieve embryonic asymmetry. Maternal germ cell-less (gcl) mRNA accumulates in the pole plasm of the embryo before Gcl protein is detectable. gcl mRNA, but not Gcl protein, can also be detected in somatic regions of the embryo, suggesting that gcl RNA is subject to translational control. We find that Gcl is expressed during oogenesis, and that it is regulated by the translational repressor Bruno (Bru). Increased levels of Gcl are observed in the oocyte when Bru level is reduced, and overexpression of Bru reduces Gcl expression. Consistently, reduction of the maternal dosage of Bruno leads to ectopic Gcl expression in the embryo, which, in turn, represses anterior hückebein (hkb) expression. Bru binds directly to the gcl 3'UTR in vitro, but, surprisingly, this binding is independent of a BRE (Bruno response element)-like motif. This motif is also not required for in vivo repression of Gcl expression during oogenesis or early embryogenesis. Bru binds the gcl 3'UTR via its C-terminal domain, which includes RNA recognition motif 3 (RRM3), with little or no contribution from the remainder of the protein. We conclude that repression by Bruno during oogenesis is required to restrict Gcl expression in the early embryo and that Bru represses gcl expression in a manner that involves RRM3 and a sequence unrelated to the BRE.

A germline-specific isoform of eIF4E (IFE-1) is required for efficient translation of stored mRNAs and maturation of both oocytes and sperm

Fertility and embryonic viability are measures of efficient germ cell growth and development. During oogenesis and spermatogenesis, new proteins are required for both mitotic expansion and differentiation. Qualitative and quantitative changes in protein synthesis occur by translational control of mRNAs, mediated in part by eIF4E, which binds the mRNAs 5' cap. IFE-1 is one of five eIF4E isoforms identified in C. elegans. IFE-1 is expressed primarily in the germ line and associates with P granules, large mRNPs that store mRNAs. We isolated a strain that lacks IFE-1 [ife-1(bn127)] and demonstrated that the translation of several maternal mRNAs (pos-1, pal-1, mex-1 and oma-1) was inefficient relative to that in wild-type worms. At 25 degrees C, ife-1(bn127) spermatocytes failed in cytokinesis, prematurely expressed the pro-apoptotic protein CED-4/Apaf-1, and accumulated as multinucleate cells unable to mature to spermatids. A modest defect in oocyte development was also observed. Oocytes progressed normally through mitosis and meiosis, but subsequent production of competent oocytes became limiting, even in the presence of wild-type sperm. Combined gametogenesis defects decreased worm fertility by 80% at 20 degrees C; ife-1 worms were completely sterile at 25 degrees C. Thus, IFE-1 plays independent roles in late oogenesis and spermatogenesis through selective translation of germline-specific mRNAs.

Translational repression of gurken mRNA in the Drosophila oocyte requires the hnRNP Squid in the nurse cells

Establishment of the Drosophila dorsal-ventral axis depends upon the correct localization of gurken mRNA and protein within the oocyte. gurken mRNA becomes localized to the presumptive dorsal anterior region of the oocyte, but is synthesized in the adjoining nurse cells. Normal gurken localization requires the heterogeneous nuclear ribonucleoprotein Squid, which binds to the gurken 3' untranslated region. However, whether Squid functions in the nurse cells or the oocyte is unknown. To address this question, we generated genetic mosaics in which half of the nurse cells attached to a given oocyte are unable to produce Squid. In these mosaics, gurken mRNA is localized normally but ectopically translated during the dorsal anterior localization process, even though the oocyte contains abundant Squid produced by the wild type nurse cells. These data indicate that translational repression of gurken mRNA requires Squid function in the nurse cells. We propose that Squid interacts with gurken mRNA in the nurse cell nuclei and, together with other factors, maintains gurken in a translationally silent state during its transport to the dorsal anterior region of the oocyte. This translational repression is not required for gurken mRNA localization, indicating that the information repressing translation is separable from that regulating localization.

Maternal mRNAs are regulated by diverse P body-related mRNP granules during early Caenorhabditis elegans development

Processing bodies (P bodies) are conserved mRNA-protein (mRNP) granules that are thought to be cytoplasmic centers for mRNA repression and degradation. However, their specific functions in vivo remain poorly understood. We find that repressed maternal mRNAs and their regulators localize to P body-like mRNP granules in the Caenorhabditis elegans germ line. Surprisingly, several distinct types of regulated granules form during oocyte and embryo development. 3' untranslated region elements direct mRNA targeting to one of these granule classes. The P body factor CAR-1/Rap55 promotes association of repressed mRNA with granules and contributes to repression of Notch/glp-1 mRNA. However, CAR-1 controls Notch/glp-1 only during late oogenesis, where it functions with the RNA-binding regulators PUF-5, PUF-6, and PUF-7. The P body protein CGH-1/Rck/Dhh1 differs from CAR-1 in control of granule morphology and promotes mRNP stability in arrested oocytes. Therefore, a system of diverse and regulated RNP granules elicits stage-specific functions that ensure proper mRNA control during early development.

Trading translation with RNA-binding proteins

RNA-binding proteins regulate every aspect of RNA metabolism, including pre-mRNA splicing, mRNA trafficking, stability, and translation. This review summarizes the available information on molecular mechanisms of translational repression by RNA-binding proteins. By using a specific set of well-defined examples, we also describe how regulation can be reversed.

Functional domains of Drosophila UNR in translational control

Translational repression of male-specific-lethal 2 (msl-2) mRNA by Sex-lethal (SXL) is an essential regulatory step of X chromosome dosage compensation in Drosophila. Translation inhibition requires that SXL recruits the protein upstream of N-ras (UNR) to the 3' UTR of msl-2 mRNA. UNR is a conserved, ubiquitous protein that contains five cold-shock domains (CSDs). Here, we dissect the domains of UNR required for translational repression and complex formation with SXL and msl-2 mRNA. Using gel-mobility shift assays, the domain involved in interactions with SXL and msl-2 was mapped specifically to the first CSD (CSD1). Indeed, excess of a peptide containing this domain derepressed msl-2 translation in vitro. The CSD1 of human UNR can also form a complex with SXL and msl-2. Comparative analyses of the CSDs of the Drosophila and human proteins together with site-directed mutagenesis experiments revealed that three exposed residues within CSD1 are required for complex formation. Tethering assays showed that CSD1 is not sufficient for translational repression, indicating that UNR binding to SXL and msl-2 can be distinguished from translation inhibition. Repression by tethered UNR requires residues from both the amino-terminal Q-rich stretch and the two first CSDs, indicating that the translational effector domain of UNR resides within the first 397 amino acids of the protein. Our results identify domains and residues required for UNR function in translational control.

Identification and gene expression profiling of the Pum1 and Pum2 members of the Pumilio family in the chicken

Members of the Pumilio (Pum) family of RNA-binding proteins act as translational repressors and are required for germ cell development and asymmetric division. We identified the chicken Pum1 and Pum2 genes and analyzed their expression patterns in various tissues. Comparative sequence analysis of the Pum1 and Pum2 proteins from the drosophila, chicken, mouse, and human revealed a high degree of evolutionary conservation in terms of the levels of homology of the peptide sequences and the structure of Pumilio homology domain (PUM-HD), C-terminal RNA-binding domain, with similar spacing between the adjacent Pum eight tandem repeats. In addition, phylogenetic patterns of pumilio family showed that Pum 1 and 2 of chicken are more closely related to those of mouse and human than other species and Pum1 is more conserved than Pum2. Using real-time RT-PCR, the expression levels of the Pum1 and Pum2 genes were found to be highest in hatched female gonads, and high-level expression of Pum2 was detected in 12-day and hatched gonads among the various chicken embryonic tissues tested. In adult tissues, the expression levels of Pum1 and Pum2 were expressed at higher levels in the testis and muscle than in any other tissue. The characteristics of the tissue-specific expression of Pum genes suggest that Pum1 and Pum2 have effects crucially in particular stage during development of chicken gonads depending on sexual maturation.

Dynein regulates epithelial polarity and the apical localization of stardust A mRNA

Intense investigation has identified an elaborate protein network controlling epithelial polarity. Although precise subcellular targeting of apical and basolateral determinants is required for epithelial architecture, little is known about how the individual determinant proteins become localized within the cell. Through a genetic screen for epithelial defects in the Drosophila follicle cells, we have found that the cytoplasmic Dynein motor is an essential regulator of apico–basal polarity. Our data suggest that Dynein acts through the cytoplasmic scaffolding protein Stardust (Sdt) to localize the transmembrane protein Crumbs, in part through the apical targeting of specific sdt mRNA isoforms. We have mapped the sdt mRNA localization signal to an alternatively spliced coding exon. Intriguingly, the presence or absence of this exon corresponds to a developmental switch in sdt mRNA localization in which apical transcripts are only found during early stages of epithelial development, while unlocalized transcripts predominate in mature epithelia. This work represents the first demonstration that Dynein is required for epithelial polarity and suggests that mRNA localization may have a functional role in the regulation of apico–basal organization. Moreover, we introduce a unique mechanism in which alternative splicing of a coding exon is used to control mRNA localization during development.

Cells within epithelial sheets are highly polarized with distinct apical and basolateral membrane domains. This cellular organization is critical to both epithelial form and function, and a failure to maintain epithelial polarity is often linked to tumor progression. The protein network that establishes and maintains the two membrane domains relies on the precise subcellular localization of its molecular components, but little is known about how these proteins are targeted to their sites of action. We have shown that the localization of the apical determinant protein Stardust depends on the microtubule motor Dynein. While investigating the relationship between Dynein and Stardust, we also made two unexpected observations about stardust mRNA regulation. First, the mechanism by which Dynein localizes Stardust may depend, in part, on the apical targeting of the stardust mRNA. Second, some stardust mRNA is apically localized during early stages of epithelial development, but the selective removal of the apical localization signal leads to the sole production of uniformly localized transcripts in mature epithelial cells. Together, these results introduce roles for Dynein in apico–basal polarity regulation and raise important questions about the role of mRNA localization in the targeting of polarity determinant proteins and epithelial maturation.

Bicaudal-C recruits CCR4-NOT deadenylase to target mRNAs and regulates oogenesis, cytoskeletal organization, and its own expression

Bicaudal-C (Bic-C) encodes an RNA-binding protein required maternally for patterning the Drosophila embryo. We identified a set of mRNAs that associate with Bic-C in ovarian ribonucleoprotein complexes. These mRNAs are enriched for mRNAs that function in oogenesis and in cytoskeletal regulation, and include Bic-C RNA itself. Bic-C binds specific segments of the Bic-C 5' untranslated region and negatively regulates its own expression by binding directly to NOT3/5, a component of the CCR4 core deadenylase complex, thereby promoting deadenylation. Bic-C overexpression induces premature cytoplasmic-streaming, a posterior-group phenotype, defects in Oskar and Kinesin heavy chain:betaGal localization as well as dorsal-appendage defects. These phenotypes are largely reciprocal to those of Bic-C mutants, and they affect cellular processes that Bic-C-associated mRNAs are known, or predicted, to regulate. We conclude that Bic-C regulates expression of specific germline mRNAs by controlling their poly(A)-tail length.

The endocytic pathway acts downstream of Oskar in Drosophila germ plasm assembly

Cell fate is often determined by the intracellular localization of RNAs and proteins. In Drosophila oocytes, oskar (osk) RNA localization and the subsequent Osk synthesis at the posterior pole direct the assembly of the pole plasm, where factors for the germline and abdomen formation accumulate. osk RNA produces two isoforms, long and short Osk, which have distinct functions in pole plasm assembly. Short Osk recruits downstream components of the pole plasm, whose anchoring to the posterior cortex requires long Osk. The anchoring of pole plasm components also requires actin cytoskeleton, and Osk promotes long F-actin projections in the oocyte posterior cytoplasm. However, the mechanism by which Osk mediates F-actin reorganization remains elusive. Furthermore, although long Osk is known to associate with endosomes under immuno-electron microscopy, it was not known whether this association is functionally significant. Here we show that Rabenosyn-5 (Rbsn-5), a Rab5 effector protein required for the early endocytic pathway, is crucial for pole plasm assembly. rbsn-5(-) oocytes fail to maintain microtubule polarity, which secondarily disrupts osk RNA localization. Nevertheless, anteriorly misexpressed Osk, particularly long Osk, recruits endosomal proteins, including Rbsn-5, and stimulates endocytosis. In oocytes lacking rbsn-5, the ectopic Osk induces aberrant F-actin aggregates, which diffuse into the cytoplasm along with pole plasm components. We propose that Osk stimulates endosomal cycling, which in turn promotes F-actin reorganization to anchor the pole plasm components to the oocyte cortex.

Squid, Cup, and PABP55B function together to regulate gurken translation in Drosophila

During Drosophila melanogaster oogenesis, the proper localization of gurken (grk) mRNA and protein is required for the establishment of the dorsal-ventral axis of the egg and future embryo. Squid (Sqd) is an RNA-binding protein that is required for the correct localization and translational regulation of the grk message. We show that Cup and polyA-binding protein (PABP) interact physically with Sqd and with each other in ovaries. We show that cup mutants lay dorsalized eggs, enhance dorsalization of weak sqd alleles, and display defects in grk mRNA localization and Grk protein accumulation. In contrast, pAbp mutants lay ventralized eggs and enhance grk haploinsufficiency. PABP also interacts genetically and biochemically with Encore. These data predict a model in which Cup and Sqd mediate translational repression of unlocalized grk mRNA, and PABP and Enc facilitate translational activation of the message once it is fully localized to the dorsal-anterior region of the oocyte. These data also provide the first evidence of a link between the complex of commonly used trans-acting factors and Enc, a factor that is required for grk translation.

A neuropeptide hormone cascade controls the precise onset of post-eclosion cuticular tanning in Drosophila melanogaster

A neuropeptide hormone-signalling pathway controls events surrounding eclosion in Drosophila melanogaster. Ecdysis-triggering hormone, eclosion hormone and crustacean cardioactive peptide (CCAP) together control pre-eclosion and eclosion events, whereas bursicon, through its receptor rickets (RK), controls post-eclosion development. Cuticular tanning is a convenient visible marker of the temporally precise post-eclosion developmental progression, and we investigated how it is controlled by the ecdysis neuropeptide cascade. Together, two enzymes, tyrosine hydroxylase (TH, encoded by ple) and dopa decarboxylase (DDC, encoded by Ddc), produce the dopamine that is required for tanning. Levels of both the ple and Ddc transcripts begin to accumulate before eclosion, coincident with the onset of pigmentation of the pharate adult bristles and epidermis. Since DDC activity is high before the post-eclosion onset of tanning, a different factor must be regulated to switch on tanning. Transcriptional control of ple does not regulate the onset of tanning because ple transcript levels remain unchanged from 24 hours before to 12 hours after eclosion. TH protein present before eclosion is degraded, and no TH activity can be detected at eclosion. However, TH protein rapidly accumulates within an hour of eclosion and we provide evidence that CCAP controls this process. Furthermore, we show that TH is transiently activated during tanning by phosphorylation at Ser32, as a result of bursicon signalling. We conclude that the ecdysis hormone cascade acts as a regulatory switch to control the precise onset of tanning by both translational and activational control of TH.

K-ras oncogene silencing strategy reduces tumor growth and enhances gemcitabine chemotherapy efficacy for pancreatic cancer treatment

Pancreatic adenocarcinoma remains a fatal disease characterized by rapid tumor progression, high metastatic potential and profound chemoresistance. Gemcitabine is the current standard chemotherapy for advanced pancreatic cancer, but it is still far from optimal and novel therapeutic strategies are needed urgently. Mutations in the k-ras gene have been found in more than 90% of pancreatic cancers and are believed to play a key role in this malignancy. Thus, the goal of this study was to investigate the impact of k-ras oncogene silencing on pancreatic tumor growth. Additionally, we examined whether combining k-ras small interfering RNA (siRNA) with gemcitabine has therapeutic potential for pancreatic cancer. The treatment of tumor cell cultures with the corresponding k-ras siRNA resulted in a significant inhibition of k-ras endogenous expression and cell proliferation. In vivo, tumor xenografts were significantly reduced with k-ras siRNA(GAT) delivered by electroporation. Moreover, combined treatment with pSsik-ras(GAT) plus gemcitabine resulted in strong growth inhibition of orthotopic pancreatic tumors. Survival rate was significantly prolonged and the mean tumor volume was dramatically reduced in mice receiving the combined treatment compared with single agents. Collectively, these findings show that targeting mutant k-ras through specific siRNA might be effective for k-ras oncogene silencing and tumor growth inhibition. The improvement of gemcitabine-based chemotherapy suggests that this strategy might be used therapeutically against human pancreatic cancer to potentiate the effects of conventional therapy.

AU-rich-element-mediated upregulation of translation by FXR1 and Argonaute 2

AU-rich elements (AREs), present in mRNA 3'-UTRs, are potent posttranscriptional regulatory signals that can rapidly effect changes in mRNA stability and translation, thereby dramatically altering gene expression with clinical and developmental consequences. In human cell lines, the TNFalpha ARE enhances translation relative to mRNA levels upon serum starvation, which induces cell-cycle arrest. An in vivo crosslinking-coupled affinity purification method was developed to isolate ARE-associated complexes from activated versus basal translation conditions. We surprisingly found two microRNP-related proteins, fragile-X-mental-retardation-related protein 1 (FXR1) and Argonaute 2 (AGO2), that associate with the ARE exclusively during translation activation. Through tethering and shRNA-knockdown experiments, we provide direct evidence for the translation activation function of both FXR1 and AGO2 and demonstrate their interdependence for upregulation. This novel cell-growth-dependent translation activation role for FXR1 and AGO2 allows new insights into ARE-mediated signaling and connects two important posttranscriptional regulatory systems in an unexpected way.

Properties of the various Botmar1 transcripts in imagoes of the bumble bee, Bombus terrestris (Hymenoptera: Apidae)

Botmar1 elements are mariner-like elements (MLEs), class II transposable elements that occur in the genome of the bumble bee, Bombus terrestris. Each haploid B. terrestris genome contains about 230 Botmar1, consisting entirely of 1.3-kb and 0.85-kb elements. During their evolution in the B. terrestris genome, two Botmar1 lineages have been differentiated in terms of their nucleic acid sequences and the differences found in their 5' untranslated regions suggest that they could be transcribed differently in B. terrestris. Here, we show that small amounts of Botmar1 mRNA occur in RNA extracts purified from B. terrestris imagoes. This indicates that the Botmar1 transcription is either weak in imagoes, or is restricted to very few cells. The cloning of several mRNAs reveals that only lineage-2 Botmar1 elements are transcribed. This transcription is specific, and uses cardinal initiators and terminators of eukaryotic elements in the Botmar1 elements. The intrastrand stem-loop folds in the mRNA theoretically synthesized by elements of the first lineage suggest that mRNA maintenance in cells might be self-regulated by RNA interference.