The travels of mRNAs through all cells large and small

lncRNAs: the unexpected link between protein synthesis and cancer adaptation

Cancer progression relies on the ability of cells to adapt to challenging environments overcoming stresses and growth constraints. Such adaptation is a multifactorial process that depends on the rapid reorganization of many basic cellular mechanisms. Protein synthesis is often dysregulated in cancer, and translational reprogramming is emerging as a driving force of cancer adaptive plasticity. Long non-coding RNAs (lncRNAs) represent the main product of genome transcription. They outnumber mRNAs by an order of magnitude and their expression is regulated in an extremely specific manner depending on context, space and time. This heterogeneity is functional and allows lncRNAs to act as context-specific, fine-tuning controllers of gene expression. Multiple recent evidence underlines how, besides their consolidated role in transcription, lncRNAs are major players in translation control. Their capacity to establish multiple and highly dynamic interactions with proteins and other transcripts makes these molecules able to play a central role across all phases of protein synthesis. Even if through a myriad of different mechanisms, the action of these transcripts is dual. On one hand, by modulating the overall translation speed, lncRNAs participate in the process of metabolic adaptation of cancer cells under stress conditions. On the other hand, by prioritizing the synthesis of specific transcripts they help cancer cells to maintain high levels of essential oncogenes. In this review, we aim to discuss the most relevant evidence regarding the involvement of lncRNAs in translation regulation and to discuss how this specific function may affect cancer plasticity and resistance to stress. We also expect to provide one of the first collective perspectives on the way these transcripts modulate gene expression beyond transcription.

Premature termination codon readthrough in human cells occurs in novel cytoplasmic foci and requires UPF proteins

Nonsense-mutation-containing messenger ribonucleoprotein particles (mRNPs) transit through cytoplasmic foci called P-bodies before undergoing nonsense-mediated mRNA decay (NMD), a cytoplasmic mRNA surveillance mechanism. This study shows that the cytoskeleton modulates transport of nonsense-mutation-containing mRNPs to and from P-bodies. Impairing the integrity of cytoskeleton causes inhibition of NMD. The cytoskeleton thus plays a crucial role in NMD. Interestingly, disruption of actin filaments results in both inhibition of NMD and activation of premature termination codon (PTC) readthrough, while disruption of microtubules causes only NMD inhibition. Activation of PTC readthrough occurs concomitantly with the appearance of cytoplasmic foci containing UPF proteins and mRNAs with nonsense mutations but lacking the P-body marker DCP1a. These findings demonstrate that in human cells, PTC readthrough occurs in novel 'readthrough bodies' and requires the presence of UPF proteins.

IMPs: an RNA-binding protein family that provides a link between stem cell maintenance in normal development and cancer

This review by Degrauwe et al. summarizes our current understanding of the functions of IMPs during normal development and focuses on a series of recent observations that have provided new insight into how their physiological functions enable IMPs to play a potentially key role in cancer stem cell maintenance and tumor growth.

IMPs, also known as insulin-like growth factor 2 (IGF2) messenger RNA (mRNA)-binding proteins (IGF2BPs), are highly conserved oncofetal RNA-binding proteins (RBPs) that regulate RNA processing at several levels, including localization, translation, and stability. Three mammalian IMP paralogs (IMP1–3) have been identified that are expressed in most organs during embryogenesis, where they are believed to play an important role in cell migration, metabolism, and stem cell renewal. Whereas some IMP2 expression is retained in several adult mouse organs, IMP1 and IMP3 are either absent or expressed at very low levels in most tissues after birth. However, all three paralogs can be re-expressed upon malignant transformation and are found in a broad range of cancer types where their expression often correlates with poor prognosis. IMPs appear to resume their physiological functions in malignant cells, which not only contribute to tumor progression but participate in the establishment and maintenance of tumor cell hierarchies. This review summarizes our current understanding of the functions of IMPs during normal development and focuses on a series of recent observations that have provided new insight into how their physiological functions enable IMPs to play a potentially key role in cancer stem cell maintenance and tumor growth.

Integration of mRNP formation and export

Expression of protein-coding genes in eukaryotes relies on the coordinated action of many sophisticated molecular machineries. Transcription produces precursor mRNAs (pre-mRNAs) and the active gene provides an environment in which the pre-mRNAs are processed, folded, and assembled into RNA–protein (RNP) complexes. The dynamic pre-mRNPs incorporate the growing transcript, proteins, and the processing machineries, as well as the specific protein marks left after processing that are essential for export and the cytoplasmic fate of the mRNPs. After release from the gene, the mRNPs move by diffusion within the interchromatin compartment, making up pools of mRNPs. Here, splicing and polyadenylation can be completed and the mRNPs recruit the major export receptor NXF1. Export competent mRNPs interact with the nuclear pore complex, leading to export, concomitant with compositional and conformational changes of the mRNPs. We summarize the integrated nuclear processes involved in the formation and export of mRNPs.

Molecular dynamics of FMRP and other RNA-binding proteins in MEG-01 differentiation: the role of mRNP complexes in non-neuronal development

Asymmetrically differentiating cells are formed with the aid of RNA-binding proteins (RBPs), which can bind, stabilize, regulate, and transport target mRNAs. The loss of RBPs in neurons may lead to severe neurodevelopmental diseases such as the Fragile X Syndrome with the absence of the Fragile X Mental Retardation Protein (FMRP). Because the latter is ubiquitous and shares many similarities with other RBPs involved in the development of peripheral cells, we suggest that FMRP would have a role in the differentiation of all tissues where it is expressed. A MEG-01 differentiation model was, therefore, established to study the global developmental functions of FMRP. PMA induction of MEG-01 cells causes important morphological changes driven by cytoskeletal dynamics. Cytoskeleton change and colocalization analyses were performed by confocal microscopy and sucrose gradient fractionation. Total cellular protein content and de novo synthesis were also analyzed. Microtubular transport mediates the displacement of FMRP and other RBP-containing mRNP complexes towards regions of the cell in development. De novo protein synthesis decreases significantly upon differentiation and total protein content composition is altered. Because those results are comparable with those obtained in neurons, the absence of FMRP would have significant consequences in cells everywhere in the body. The latter should be further investigated to give a better understanding of the systemic implications of imbalances of FMRP and other functionally similar RBPs.

Human Staufen1 associates to miRNAs involved in neuronal cell differentiation and is required for correct dendritic formation

Double-stranded RNA-binding proteins are key elements in the intracellular localization of mRNA and its local translation. Staufen is a double-stranded RNA binding protein involved in the localised translation of specific mRNAs during Drosophila early development and neuronal cell fate. The human homologue Staufen1 forms RNA-containing complexes that include proteins involved in translation and motor proteins to allow their movement within the cell, but the mechanism underlying translation repression in these complexes is poorly understood. Here we show that human Staufen1-containing complexes contain essential elements of the gene silencing apparatus, like Ago1-3 proteins, and we describe a set of miRNAs specifically associated to complexes containing human Staufen1. Among these, miR-124 stands out as particularly relevant because it appears enriched in human Staufen1 complexes and is over-expressed upon differentiation of human neuroblastoma cells in vitro. In agreement with these findings, we show that expression of human Staufen1 is essential for proper dendritic arborisation during neuroblastoma cell differentiation, yet it is not necessary for maintenance of the differentiated state, and suggest potential human Staufen1 mRNA targets involved in this process.

Myosin-Va-dependent cell-to-cell transfer of RNA from Schwann cells to axons

To better understand the role of protein synthesis in axons, we have identified the source of a portion of axonal RNA. We show that proximal segments of transected sciatic nerves accumulate newly-synthesized RNA in axons. This RNA is synthesized in Schwann cells because the RNA was labeled in the complete absence of neuronal cell bodies both in vitro and in vivo. We also demonstrate that the transfer is prevented by disruption of actin and that it fails to occur in the absence of myosin-Va. Our results demonstrate cell-to-cell transfer of RNA and identify part of the mechanism required for transfer. The induction of cell-to-cell RNA transfer by injury suggests that interventions following injury or degeneration, particularly gene therapy, may be accomplished by applying them to nearby glial cells (or implanted stem cells) at the site of injury to promote regeneration.

Mapping molecular memory: navigating the cellular pathways of learning

A consolidated map of the signalling pathways that function in the formation of short- and long-term cellular memory could be considered the ultimate means of defining the molecular basis of learning. Research has established that experience-dependent activation of these complex cellular cascades leads to many changes in the composition and functioning of a neuron's proteome, resulting in the modulation of its synaptic strength and structure. However, although generally accepted that synaptic plasticity is the mechanism whereby memories are stored in the brain, there is much controversy over whether the site of this neuronal memory expression is predominantly pre- or postsynaptic. Much of the early research into the neuromolecular mechanisms of memory performed using the model organism, the marine snail Aplysia, has focused on the associated presynaptic events. Recently however, postsynaptic mechanisms have been shown to contribute definitively to long term memory processes, and are in fact critical for persistent learning-induced synaptic changes. In this review, in which we aimed to integrate many of the early and recent advances concerning coordinated neuronal signaling in both the pre- and postsynaptic neurons, we have provided a detailed account of the diverse cellular events that lead to modifications in synaptic strength. Thus, a comprehensive synaptic model is presented that could explain a few of the shortcomings that arise when the presynaptic and postsynaptic changes are considered separately. Although it is clear that there is still much to be learnt and that the exact nature of many of the signalling cascades and their components are yet to be fully understood, this still incomplete but integrated illustrative map of the cellular pathways involved provides an overview which expands understanding of the neuromolecular mechanisms of learning and memory.

Transgene mobilization and regulatory uncertainty for non-GE fruit products of transgenic rootstocks

Genetically engineered (GE) rootstocks may offer some advantages for biotechnology applications especially in woody perennial crops such as grape or walnut. Transgrafting combines horticultural grafting practices with modern GE methods for crop improvement. Here, a non-GE conventional scion (upper stem portion) is grafted onto a transgenic GE rootstock. Thus, the scion does not contain the genetic modification present in the rootstock genome. We examined transgene presence in walnut and tomato GE rootstocks and non-GE fruit-bearing scions. Mobilization of transgene DNA, protein, and mRNA across the graft was not detected. Though transgenic siRNA mobilization was not observed in grafted tomatoes or walnut scions, transgenic siRNA signal was detected in walnut kernels. Prospective benefits from transgrafted plants include minimized risk of GE pollen flow (Lev-Yadun and Sederoff, 2001), possible use of more than one scion per approved GE rootstock which could help curb the estimated US$136 million (CropLife International, 2011) cost to bring a GE crop to international markets, as well as potential for improved consumer and market acceptance since the consumable product is not itself GE. Thus, transgrafting provides an alternative option for agricultural industries wishing to expand their biotechnology portfolio.

Molecular composition of staufen2-containing ribonucleoproteins in embryonic rat brain

Messenger ribonucleoprotein particles (mRNPs) are used to transport mRNAs along neuronal dendrites to their site of translation. Numerous mRNA-binding and regulatory proteins within mRNPs finely regulate the fate of bound-mRNAs. Their specific combination defines different types of mRNPs that in turn are related to specific synaptic functions. One of these mRNA-binding proteins, Staufen2 (Stau2), was shown to transport dendritic mRNAs along microtubules. Its knockdown expression in neurons was shown to change spine morphology and synaptic functions. To further understand the molecular mechanisms by which Stau2 modulates synaptic function in neurons, it is important to identify and characterize protein co-factors that regulate the fate of Stau2-containing mRNPs. To this end, a proteomic approach was used to identify co-immunoprecipitated proteins in Staufen2-containing mRNPs isolated from embryonic rat brains. The proteomic approach identified mRNA-binding proteins (PABPC1, hnRNP H1, YB1 and hsc70), proteins of the cytoskeleton (α- and β-tubulin) and RUFY3 a poorly characterized protein. While PABPC1 and YB1 associate with Stau2-containing mRNPs through RNAs, hsc70 is directly bound to Stau2 and this interaction is regulated by ATP. PABPC1 and YB1 proteins formed puncta in dendrites of embryonic rat hippocampal neurons. However, they poorly co-localized with Stau2 in the large dendritic complexes suggesting that they are rather components of Stau2-containing mRNA particles. All together, these results represent a further step in the characterization of Stau2-containing mRNPs in neurons and provide new tools to study and understand how Stau2-containing mRNPs are transported, translationally silenced during transport and/or locally expressed according to cell needs.

Reaction-diffusion systems in intracellular molecular transport and control

Chemical reactions make cells work only if the participating chemicals are delivered to desired locations in a timely and precise fashion. Most research to date has focused on active-transport mechanisms, although passive diffusion is often equally rapid and energetically less costly. Capitalizing on these advantages, cells have developed sophisticated reaction-diffusion (RD) systems that control a wide range of cellular functions-from chemotaxis and cell division, through signaling cascades and oscillations, to cell motility. These apparently diverse systems share many common features and are "wired" according to "generic" motifs such as nonlinear kinetics, autocatalysis, and feedback loops. Understanding the operation of these complex (bio)chemical systems requires the analysis of pertinent transport-kinetic equations or, at least on a qualitative level, of the characteristic times of the constituent subprocesses. Therefore, in reviewing the manifestations of cellular RD, we also describe basic theory of reaction-diffusion phenomena.

The life of an mRNA in space and time

Nuclear transcribed genes produce mRNA transcripts destined to travel from the site of transcription to the cytoplasm for protein translation. Certain transcripts can be further localized to specific cytoplasmic regions. We examined the life cycle of a transcribed beta-actin mRNA throughout gene expression and localization, in a cell system that allows the in vivo detection of the gene locus, the transcribed mRNAs and the cytoplasmic beta-actin protein that integrates into the actin cytoskeleton. Quantification showed that RNA polymerase II elongation progressed at a rate of 3.3 kb/minute and that transactivator binding to the promoter was transient (40 seconds), and demonstrated the unique spatial structure of the coding and non-coding regions of the integrated gene within the transcription site. The rates of gene induction were measured during interphase and after mitosis, demonstrating that daughter cells were not synchronized in respect to transcription initiation of the studied gene. Comparison of the spatial and temporal kinetics of nucleoplasmic and cytoplasmic mRNA transport showed that the beta-actin-localization response initiates from the existing cytoplasmic mRNA pool and not from the newly synthesized transcripts arising after gene induction. It was also demonstrated that mechanisms of random movement were predominant in mediating the efficient translocation of mRNA in the eukaryotic cell.

Effect of endotoxin on the expression of GnRH and GnRHR genes in the hypothalamus and anterior pituitary gland of anestrous ewes

An immune/inflammatory challenge can affect reproduction at the level of the hypothalamus, pituitary gland, or gonads. Nonetheless, the major impact is thought to occur within the brain or the pituitary gland. The present study was designed to examine the effect of intravenous (i.v.) lipopolysaccharide (LPS) injection on the expression of gonadotropin-releasing hormone (GnRH) and the gonadotropin-releasing hormone receptor (GnRHR) genes in the hypothalamic structures where GnRH neurons are located as well as in the anterior pituitary gland (AP) of anestrous ewes. We also determined the effect of LPS on luteinizing hormone (LH) release. It was found that i.v. LPS injection significantly decreased GnRH and GnRHR mRNAs levels in the preoptic area (40%, p<or=0.05 and 60%, p<or=0.01 respectively) and median eminence (50% and 50% respectively; p<or=0.01). Endotoxin injection decreased plasma LH concentration (25%; p<or=0.05) and GnRHR gene expression (80%, p<or=0.01) in the AP. Together, these observations indicate that inflammatory challenge can affect the reproductive system at the hypothalamic level through modulation of the activity of GnRH-ergic neurons as well as at the level of the AP via inhibition of LH secretion or/and through reduction of the sensitivity of GnRH reactive pituitary cells to GnRH stimulation. The presence of GnRH mRNA in the median eminence, the hypothalamic structure where GnRH-ergic neurons' terminals are located, suggests that the axonal transport of GnRH mRNA may occur in these neurons. This phenomenon could play an important role in the physiology of GnRH neurons. Our data demonstrate that immune stress could be important inhibitor of this process.

In situ visualization of gene expression using polymer-coated quantum-dot-DNA conjugates

Imaging of specific mRNA targets in cells is of great importance in understanding gene expression and cell signaling processes. Subcellular localization of mRNA is known as a universal mechanism for cells to sequester specific mRNA for high production of required proteins. Various gene expressions in Drosophila cells are studied using quantum dots (QDs) and the fluorescence in situ hybridization (FISH) method. The excellent photostability and highly luminescent properties of QDs compared to conventional fluorophores allows reproducible obtainment of quantifiable mRNA gene expression imaging. Amine-modified oligonucleotide probes are designed and covalently attached to the carboxyl-terminated polymer-coated QDs via EDC chemistry. The resulting QD-DNA conjugates show sequence-specific hybridization with target mRNAs. Quantitative analysis of FISH on the Diptericin gene after lipopolysaccharide (LPS) treatment shows that the intensity and number of FISH signals per cell depends on the concentration of LPS and correlates well with quantitative real-time PCR results. In addition, our QD-DNA probes exhibit excellent sensitivity to detect the low-expressing Dorsal-related immunity factor gene. Importantly, multiplex FISH of Ribosomal protein 49 and Actin 5C using green and red QD-DNA conjugates allows the observation of cellular distribution of the two independent genes simultaneously. These results demonstrate that highly fluorescent and stable QD-DNA probes can be a powerful tool for direct localization and quantification of gene expression in situ.

Identification of a putative network of actin-associated cytoskeletal proteins in glomerular podocytes defined by co-purified mRNAs

The glomerular podocyte is a highly specialized and polarized kidney cell type that contains major processes and foot processes that extend from the cell body. Foot processes from adjacent podocytes form interdigitations with those of adjacent cells, thereby creating an essential intercellular junctional domain of the renal filtration barrier known as the slit diaphragm. Interesting parallels have been drawn between the slit diaphragm and other sites of cell-cell contact by polarized cells. Notably mutations in several genes encoding proteins localized to the foot processes can lead to proteinuria and kidney failure. Mutations in the Wilm's tumor gene (WT1) can also lead to kidney disease and one isoform of WT1, WT1(+KTS), has been proposed to regulate gene expression post-transcriptionally. We originally sought to identify mRNAs associated with WT1(+KTS) through an RNA immunoprecipitation and microarray approach, hypothesizing that the proteins encoded by these mRNAs might be important for podocyte morphology and function. We identified a subset of mRNAs that were remarkably enriched for transcripts encoding actin-binding proteins and other cytoskeletal proteins including several that are localized at or near the slit diaphragm. Interestingly, these mRNAs included those of α-actinin-4 and non-muscle myosin IIA that are mutated in genetic forms of kidney disease. However, isolation of the mRNAs occurred independently of the expression of WT1, suggesting that the identified mRNAs were serendipitously co-purified on the basis of co-association in a common subcellular fraction. Mass spectroscopy revealed that other components of the actin cytoskeleton co-purified with these mRNAs, namely actin, tubulin, and elongation factor 1α. We propose that these mRNAs encode a number of proteins that comprise a highly specialized protein interactome underlying the slit diaphragm. Collectively, these gene products and their interactions may prove to be important for the structural integrity of the actin cytoskeleton in podocytes as well as other polarized cell types.

FMR1: a gene with three faces

The FMR1 gene is involved in three different syndromes, the fragile X syndrome (FXS), premature ovarian insufficiency (POI) and the fragile X-associated tremor/ataxia syndrome (FXTAS) at older age. Fragile X syndrome is caused by an expansion of a CGG repeat above 200 units in the FMR1 gene resulting in the absence of the FMR1 mRNA and protein. The FMR1 protein is proposed to act as a regulator of mRNA transport and of translation of target mRNAs at the synapse. FXS is seen as a loss of function disorder. POI and FXTAS are found in individuals with an expanded repeat between 50 and 200 CGGs and are associated with increased FMR1 mRNA levels. The presence of elevated FMR1 mRNA in FXTAS suggests that FXTAS may represent a toxic RNA gain-of-function effect. The molecular basis of POI is yet unknown. The role of the FMR1 gene in these disorders is discussed.

The 5' cap of tobacco mosaic virus (TMV) is required for virion attachment to the actin/endoplasmic reticulum network during early infection

Almost nothing is known of the earliest stages of plant virus infections. To address this, we microinjected Cy3 (UTP)-labelled tobacco mosaic virus (TMV) into living tobacco trichome cells. The Cy3-virions were infectious, and the viral genome trafficked from cell to cell. However, neither the fluorescent vRNA pool nor the co-injected green fluorescent protein (GFP) left the injected trichome, indicating that the synthesis of (unlabelled) progeny viral (v)RNA is required to initiate cell-to-cell movement, and that virus movement is not accompanied by passive plasmodesmatal gating. Cy3-vRNA formed granules that became anchored to the motile cortical actin/endoplasmic reticulum (ER) network within minutes of injection. Granule movement on actin/ER was arrested by actin inhibitors indicating actin-dependent RNA movement. The 5' methylguanosine cap was shown to be required for vRNA anchoring to the actin/ER. TMV vRNA lacking the 5' cap failed to form granules and was degraded in the cytoplasm. Removal of the 3' untranslated region or replicase both inhibited replication but did not prevent granule formation and movement. Dual-labelled TMV virions in which the vRNA and the coat protein were highlighted with different fluorophores showed that both fluorescent signals were initially located on the same ER-bound granules, indicating that TMV virions may become attached to the ER prior to uncoating of the viral genome.

Frameworks for programming biological function through RNA parts and devices

One of the long-term goals of synthetic biology is to reliably engineer biological systems that perform human-defined functions. Currently, researchers face several scientific and technical challenges in designing and building biological systems, one of which is associated with our limited ability to access, transmit, and control molecular information through the design of functional biomolecules exhibiting novel properties. The fields of RNA biology and nucleic acid engineering, along with the tremendous interdisciplinary growth of synthetic biology, are fueling advances in the emerging field of RNA programming in living systems. Researchers are designing functional RNA molecules that exhibit increasingly complex functions and integrating these molecules into cellular circuits to program higher-level biological functions. The continued integration and growth of RNA design and synthetic biology presents exciting potential to transform how we interact with and program biology.

Import of nuclear-encoded mitochondrial proteins: a cotranslational perspective

A growing amount of evidence suggests that the cytosolic translation of nuclear-encoded mitochondrial proteins and their subsequent import into mitochondria are tightly coupled in a process termed cotranslational import. In addition to the original posttranslational view of mitochondrial protein import, early literature also provides both in vitro and in vivo experimental evidence supporting the simultaneous existence of a cotranslational protein-import mechanism in mitochondria. Recent investigations have started to reveal the cotranslational import mechanism which is initiated by transporting either a translation complex or a translationally competent mRNA encoding a mitochondrial protein to the mitochondrial surface. The intracellular localization of mRNA to the mitochondrial surface has emerged as the latest addition to our understanding of mitochondrial biogenesis. It is mediated by targeting elements within the mRNA molecule in association with potential mRNA-binding proteins.

cis-acting sequences and trans-acting factors in the localization of mRNA for mitochondrial ribosomal proteins

mRNA localization is a conserved post-transcriptional process crucial for a variety of systems. Although several mechanisms have been identified, emerging evidence suggests that most transcripts reach the protein functional site by moving along cytoskeleton elements. We demonstrated previously that mRNA for mitochondrial ribosomal proteins are asymmetrically distributed in the cytoplasm, and that localization in the proximity of mitochondria is mediated by the 3'-UTR. Here we show by biochemical analysis that these mRNA transcripts are associated with the cytoskeleton through the microtubule network. Cytoskeleton association is functional for their intracellular localization near the mitochondrion, and the 3'-UTR is involved in this cytoskeleton-dependent localization. To identify the minimal elements required for localization, we generated DNA constructs containing, downstream from the GFP gene, deletion mutants of mitochondrial ribosomal protein S12 3'-UTR, and expressed them in HeLa cells. RT-PCR analysis showed that the localization signals responsible for mRNA localization are located in the first 154 nucleotides. RNA pull-down assays, mass spectrometry, and RNP immunoprecipitation assay experiments, demonstrated that mitochondrial ribosomal protein S12 3'-UTR interacts specifically with TRAP1 (tumor necrosis factor receptor-associated protein1), hnRNPM4 (heterogeneous nuclear ribonucleoprotein M4), Hsp70 and Hsp60 (heat shock proteins 70 and 60), and alpha-tubulin in vitro and in vivo.

Microtubule-dependent distribution of mRNA in adult cardiocytes

Synthesis of myofibrillar proteins in the diffusion-restricted adult cardiocyte requires microtubule-based active transport of mRNAs as part of messenger ribonucleoprotein particles (mRNPs) to translation sites adjacent to nascent myofibrils. This is especially important for compensatory hypertrophy in response to hemodynamic overloading. The hypothesis tested here is that excessive microtubule decoration by microtubule-associated protein 4 (MAP4) after cardiac pressure overloading could disrupt mRNP transport and thus hypertrophic growth. MAP4-overexpressing and pressure-overload hypertrophied adult feline cardiocytes were infected with an adenovirus encoding zipcode-binding protein 1-enhanced yellow fluorescent protein fusion protein, which is incorporated into mRNPs, to allow imaging of these particles. Speed and distance of particle movement were measured via time-lapse microscopy. Microtubule depolymerization was used to study microtubule-based transport and distribution of mRNPs. Protein synthesis was assessed as radioautographic incorporation of [3H]phenylalanine. After microtubule depolymerization, mRNPs persist only perinuclearly and apparent mRNP production and protein synthesis decrease. Reestablishing microtubules restores mRNP production and transport as well as protein synthesis. MAP4 overdecoration of microtubules via adenovirus infection in vitro or following pressure overloading in vivo reduces the speed and average distance of mRNP movement. Thus cardiocyte microtubules are required for mRNP transport and structural protein synthesis, and MAP4 decoration of microtubules, whether directly imposed or accompanying pressure-overload hypertrophy, causes disruption of mRNP transport and protein synthesis. The dense, highly MAP4-decorated microtubule network seen in severe pressure-overload hypertrophy both may cause contractile dysfunction and, perhaps even more importantly, may prevent a fully compensatory growth response to hemodynamic overloading.

RNA transport and localized protein synthesis in neurological disorders and neural repair

Neural cells are able to finely tune gene expression through post-transcriptional mechanisms. Localization of mRNAs to subcellular regions has been detected in neurons, oligodendrocytes, and astrocytes providing these domains with a locally renewable source of proteins. Protein synthesis in dendrites has most frequently been associated with synaptic plasticity, while axonally synthesized proteins appear to facilitate pathfinding and injury responses. For oligodendrocytes, mRNAs encoding several proteins for myelin formation are locally generated suggesting that this mechanism assists in myelination. Astrocytic processes have not been well studied but localization of GFAP mRNA has been demonstrated. Both RNA transport and localized translation are regulated processes. RNA transport appears to be highly selective and, at least in part, the destiny of individual mRNAs is determined in the nucleus. RNA-protein and protein-protein interactions determine which mRNAs are targeted to subcellular regions. Several RNA binding proteins that drive mRNA localization have also been shown to repress translation during transport. Activity of the translational machinery is also regulated in distal neural cell processes. Clinically, disruption of mRNA localization and/or localized mRNA translation may contribute to pathophysiology of fragile X mental retardation and spinal muscular atrophy. Axonal injury has been shown to activate localized protein synthesis, providing both a means to initiate regeneration and retrogradely signal injury to the cell body. Decreased capacity to transport mRNAs and translational machinery into distal processes could jeopardize the ability to respond to injury or local stimuli within axons and dendrites.

RNA trafficking in axons

A substantial number of studies over a period of four decades have indicated that axons contain mRNAs and ribosomes, and are metabolically active in synthesizing proteins locally. For the most part, little attention has been paid to these findings until recently when the concept of targeting of specific mRNAs and translation in subcellular domains in polarized cells emerged to contribute to the likelihood and acceptance of mRNA targeting to axons as well. Trans-acting factor proteins bind to cis-acting sequences in the untranslated region of mRNAs integrated in ribonucleoprotein (RNPs) complexes determine its targeting in neurons. In vitro studies in immature axons have shown that molecular motors proteins (kinesins and myosins) associate to RNPs suggesting they would participate in its transport to growth cones. Tau and actin mRNAs are transported as RNPs, and targeted to axons as well as ribosomes. Periaxoplasmic ribosomal plaques (PARPs), which are systematically distributed discrete peripheral ribosome-containing, actin-rich formations in myelinated axons, also are enriched with actin and myosin Va mRNAs and additional regulatory proteins. The localization of mRNAs in PARPs probably means that PARPs are local centers of translational activity, and that these domains are the final destination in the axon compartment for targeted macromolecular traffic originating in the cell body. The role of glial cells as a potentially complementary source of axonal mRNAs and ribosomes is discussed in light of early reports and recent ultrastructural observations related to the possibility of glial-axon trans-endocytosis.

mRNA transport to and translation in neuronal dendrites

Transport of mRNA is an important biological process in all cells that sets up gradients of translated protein from the site of mRNA docking and translation. Neurons are highly polarized cells where the targeted movement of RNAs and local translation at that site have been shown to be integral to the proper functioning of the neuron. Indeed, this specialized biological function for localized RNAs in particular neurons may in part confer a selective advantage on these cells such that they "out-compete" others in the race to establish synaptic connectivity. In this mini-review we highlight some of the salient features of RNA targeting and translation in neurons.

The RNA-binding protein HuD: a regulator of neuronal differentiation, maintenance and plasticity

mRNA stability is increasingly recognized as being essential for controlling the expression of a wide variety of transcripts during neuronal development and synaptic plasticity. In this context, the role of AU-rich elements (ARE) contained within the 3' untranslated region (UTR) of transcripts has now emerged as key because of their high incidence in a large number of cellular mRNAs. This important regulatory element is known to significantly modulate the longevity of mRNAs by interacting with available stabilizing or destabilizing RNA-binding proteins (RBP). Thus, in parallel with the emergence of ARE, RBP are also gaining recognition for their pivotal role in regulating expression of a variety of mRNAs. In the nervous system, the member of the Hu family of ARE-binding proteins known as HuD, has recently been implicated in multiple aspects of neuronal function including the commitment and differentiation of neuronal precursors as well as synaptic remodeling in mature neurons. Through its ability to interact with ARE and stabilize multiple transcripts, HuD has now emerged as an important regulator of mRNA expression in neurons. The present review is designed to provide a comprehensive and updated view of HuD as an RBP in the nervous system. Additionally, we highlight the role of HuD in multiple aspects of a neuron's life from early differentiation to changes in mature neurons during learning paradigms and in response to injury and regeneration. Finally, we describe the current state of knowledge concerning the molecular and cellular events regulating the expression and activity of HuD in neurons.

mRNA specific subcellular localization represents a crucial step for fine-tuning of gene expression in mammalian cells

mRNA subcellular distribution and translational control are key player mechanisms for post-transcriptional gene expression regulation. In the last decade it has become increasingly clear that these processes are associated with various human diseases. Understanding the interconnected multistep process of mRNA localization and its involvement in organelle biogenesis and in the overall spatial structure of eukaryotic cells will be an important step towards the long-term goal of curing individual molecular defects. In a recent issue, Russo et al. [The 3'-untranslated region directs ribosomal protein-encoding mRNAs to specific cytoplasmic regions, Biochim. Biophys. Acta, Mol. Cell Res. 1763 (8) (2006) 833-843] reported interesting findings on the mechanisms that direct mRNAs encoding different ribosomal proteins to specific cytoplasmic regions in human cells.

Chronic exposure to Delta9-tetrahydrocannabinol downregulates oxytocin and oxytocin-associated neurophysin in specific brain areas

Cannabinoids are widely abused drugs. Our goal was to identify genes modulated by Delta9-tetrahydrocannabinol (Delta9-THC) treatment. We found that chronic administration of Delta9-THC (1.5 mg/kg/day, i.p.; 7 days) to rats, downregulates the expression of oxytocin-neurophysin (OT-NP) mRNA and of OT and oxytocin-associated NP (NPOT) immunoreactivity in nucleus accumbens (NAc) and ventral tegmental area (VTA), brain areas involved in reward and addiction. Real-time PCR revealed a 60% and 53% reduction of OT-NP mRNA in NAc and VTA, respectively, under chronic treatment, while no changes were observed in NAc after 24 h. Immunohistochemistry showed a large decrease in number of OT and NPOT-stained fibers in NAc (by 59% and 52%, respectively) and VTA (by 50% and 56%, respectively). No changes in cell staining were observed in the paraventricular nucleus and supraoptic nucleus. As OT is known to inhibit development of drug tolerance and attenuate withdrawal symptoms, we suggest that OT downregulation could play a role during the establishment of the chronic effects of Delta9-THC.

Characterization of an RNA granule from developing brain

In brain, mRNAs are transported from the cell body to the processes, allowing for local protein translation at sites distant from the nucleus. Using subcellular fractionation, we isolated a fraction from rat embryonic day 18 brains enriched for structures that resemble amorphous collections of ribosomes. This fraction was enriched for the mRNA encoding beta-actin, an mRNA that is transported in dendrites and axons of developing neurons. Abundant protein components of this fraction, determined by tandem mass spectrometry, include ribosomal proteins, RNA-binding proteins, microtubule-associated proteins (including the motor protein dynein), and several proteins described only as potential open reading frames. The conjunction of RNA-binding proteins, transported mRNA, ribosomal machinery, and transporting motor proteins defines these structures as RNA granules. Expression of a subset of the identified proteins in cultured hippocampal neurons confirmed that proteins identified in the proteomics were present in neurites associated with ribosomes and mRNAs. Moreover many of the expressed proteins co-localized together. Time lapse video microscopy indicated that complexes containing one of these proteins, the DEAD box 3 helicase, migrated in dendrites of hippocampal neurons at the same speed as that reported for RNA granules. Although the speed of the granules was unchanged by activity or the neurotrophin brain-derived neurotrophic factor, brain-derived neurotrophic factor, but not activity, increased the proportion of moving granules. These studies define the isolation and composition of RNA granules expressed in developing brain.

Localization of all seven messenger RNAs for the actin-polymerization nucleator Arp2/3 complex in the protrusions of fibroblasts

The actin-related protein 2/3 (Arp2/3) complex is a crucial actin polymerization nucleator and is localized to the leading protrusions of migrating cells. However, how the multiprotein complex is targeted to the protrusions remains unknown. Here, we demonstrate that mRNAs for the seven subunits of the Arp2/3 complex are localized to the protrusions in fibroblasts, supporting a hypothesis that the Arp2/3 complex is targeted to its site of function by mRNA localization. Depletion of serum from culture medium inhibits Arp2/3-complex mRNA localization to the protrusion, whereas serum stimulation leads to significant mRNA localization within 30 minutes. The effect of serum suggests that Arp2/3-complex mRNA localization is a cellular response to extracellular stimuli. The localization of the Arp2/3 complex mRNAs is dependent on both actin filaments and microtubules, because disruption of either cytoskeletal system (with cytochalasin D and colchicine, respectively) inhibited the localization of all seven subunit mRNAs. In addition, myosin inhibitors significantly inhibit Arp2 mRNA localization in chicken embryo fibroblasts, suggesting a myosin motor dependent mechanism for Arp2/3-complex mRNA localization.

Effect of posttranscriptional regulatory elements on transgene expression and virus production in the context of retrovirus vectors

Ineffective transgene expression in a sufficient amount of target cells is still a limitation in retroviral vector mediated gene therapy. Thus, we systematically evaluated four genetic modulators, (i) the woodchuck posttranscriptional regulatory element (WPRE), (ii) the mouse RNA transport element (RTE), (iii) the constitutive transport element (CTE) of the simian retrovirus type 1 (SRV-1), and (iv) the 5' untranslated region of the human heat shock protein 70 (Hsp70 5'UTR), all of them involved in the posttranscriptional control of mRNA nucleo/cytoplasmatic transport, RNA stability, and translation efficiency, in an MLV-based retrovirus vector context. Insertion of the WPRE into the retrovirus vector resulted in enhancement of transgene expression (EGFP) both in transfected virus producing cells as well as in infected recipient cells irrespective of the location in the vector. The best effect was observed with two copies of the WPRE, 3' of the transgene and in the 3' untranslated region of the vector backbone. However, oligomerization of this element does not further increase transgene expression. Presence of the WPRE resulted also in an increase in virus production. Introduction of the CTE and/or RTE in the retroviral vector did not alter transgene expression and infectious particle production. Positive effects were observed only in vectors harboring the CTE and/or RTE in combination with the WPRE. The activity of the Hsp70 5'UTR as a translational enhancer was found to be negligible in the context of the retroviral vector. However, interference of the Hsp70 5'UTR strong secondary structure with the packaging sequence of the viral RNA was experimentally excluded as being the cause of this. These data suggest that only the WPRE is a suitable element for the improvement of transgene expression and oncoretroviral vector production.

Polarised distribution of the RNA-binding protein Staufen in differentiated intestinal epithelial cells

mRNA localisation, as a mechanism for directing localised protein synthesis, plays a vital role in the functioning of certain cells, such as neurons and oocytes. Potentially this mechanism may also occur in polarised intestinal epithelial cells. Here we show that Staufen1(55), a protein involved in mRNA localisation and transport, is asymmetrically distributed in differentiated Caco-2 intestinal epithelial cells and partly co-localised with calnexin, a marker of the endoplasmic reticulum. The localisation to the apical region of the cell indicates that Staufen may be involved in localisation of transcripts to this domain.

Moving molecules: mRNA trafficking in Mammalian oligodendrocytes and neurons

Numerous mRNA molecules are localized in regions of the dendrites of neurons, some moving along dendrites in response to synaptic activity. The proteins encoded by these RNAs have diverse functions, including participation in memory formation and long-term potentiation. Recent experiments have shown that a cytoplasmic RNA trafficking pathway described for oligodendrocytes also operates in neurons. Transported RNAs possess a cis-acting element that directs them to granules, which are transported along microtubules by the motor proteins kinesin and dynein. These RNA molecules are recruited to the cytoplasmic transport granules by cooperative interaction with a cognate trans-acting factor. mRNAs containing the 11-nucleotide A2RE11 or 21-nucleotide A2RE sequences bind heterogeneous nuclear ribonucleoproteins A2 and A3, which are abundant in the brain. Mutations in this cis-acting element that weaken its interaction with hnRNP A2 also interfere with RNA trafficking. Several dendritically localized mRNAs, including those encoding calcium-calmodulin-dependent protein kinase II alpha subunit and neurogranin, possess A2RE-like sequences, suggesting that they may be localized by interaction with these heterogeneous nuclear ribonucleoproteins. Calcium-calmodulin-dependent protein kinase II alpha subunit is of particular interest: Its RNA is transported in depolarized neurons, and the protein it encodes is essential for establishing long-term memory. Several other cis-acting sequences and trans-acting factors that participate in neuronal RNA localization have been discovered.

A novel actin isoform is expressed in the ovotestis of Aplysia californica

The actin family encodes a large number of protein isoforms with quasi-identical primary structure but distinct function and localization. In oocytes, actin is known to play important roles in different processes such as those leading to fertilization or to mRNA localization during oogenesis. In this paper, we report the characterization of a novel actin isoform (apACTov) in Aplysia californica that is specifically expressed in ovotestis. The apACTov cDNA codes for a putative protein of 376 amino acids that shows 96% and 94% sequence identity with two other actin isoforms previously characterized in Aplysia. In situ hybridization experiments showed that the apACTov transcript is not uniformly distributed but is found in crescent or filipodia-like structures at the surface of the oocyte. Our results suggest that apACTov may contribute to the differential distribution of critical material during egg division and/or cell differentiation.

Translational control at the synapse

The strength of synaptic connections can undergo long-lasting changes, and such long-term plasticity is thought to underlie higher brain functions such as learning and memory. De novo synthesis of proteins is required for such plastic changes. This model is now supported by several lines of experimental data. Components of translational machinery have been identified in dendrites, including ribosomes, translation-al factors, numerous RNAs, and components of posttranslational secretory pathways. Various RNAs have been shown to be actively and rapidly transported to dendrites. Dendritic RNAs typically contain transport-specifying elements (dendritic targeting elements). Such dendritic targeting elements associate with trans-acting factors to form transport-competent ribonucleoprotein particles. It is assumed that molecular motors mediate transport of such particles along dendritic cytoskeletal elements. Once an mRNA has arrived at its dendritic destination site, appropriate spatiotemporal control of its translation, for example, in response to transsynaptic activity, becomes vital. Such local translational control, recent evidence indicates, is implemented at different levels and through various pathways. In the default state, translation is assumed to be repressed, and several mechanisms, some including small untranslated RNAs, have been proposed to contribute to such repression. Translational control at the synapse thus provides a molecular basis for the long-term, input-specific modulation of synaptic strength.

Myocyte differentiation generates nuclear invaginations traversed by myofibrils associating with sarcomeric protein mRNAs

Certain types of cell both in vivo and in vitro contain invaginated or convoluted nuclei. However, the mechanisms and functional significance of the deformation of the nuclear shape remain enigmatic. Recent studies have suggested that three types of cytoskeleton, microfilaments, microtubules and intermediate filaments, are involved in the formation of nuclear invaginations, depending upon cell type or conditions. Here, we show that undifferentiated mouse C2C12 skeletal muscle myoblasts had smoothsurfaced spherical or ellipsoidal nuclei, whereas prominent nuclear grooves and invaginations were formed in multinucleated myotubes during terminal differentiation. Conversion of mouse fibroblasts to myocytes by the transfection of MyoD also resulted in the formation of nuclear invaginations after differentiation. C2C12 cells prevented from differentiation did not have nuclear invaginations, but biochemically differentiated cells without cell fusion exhibited nuclear invaginations. Thus, biochemical differentiation is sufficient for the nuclear deformation. Although vimentin markedly decreased both in the biochemically and in the terminally differentiated cells, exogenous expression of vimentin in myotubes did not rescue nuclei from the deformation. On the other hand, non-striated premyofibrils consisting of sarcomeric actinmyosin filament bundles and cross-striated myofibrils traversed the grooves and invaginations. Time-lapse microscopy showed that the preformed myofibrillar structures cut horizontally into the nuclei. Prevention of myofibril formation retarded the generation of nuclear invaginations. These results indicate that the myofibrillar structures are, at least in part, responsible for the formation of nuclear grooves and invaginations in these myocytes. mRNA of sarcomeric proteins including myosin heavy chain and alpha-actin were frequently associated with the myofibrillar structures running along the nuclear grooves and invaginations. Consequently, the grooves and invaginations might function in efficient sarcomeric protein mRNA transport from the nucleus along the traversing myofibrillar structures for active myofibril formation.

The role of 3' UTRs in the mechanisms for segregating mitochondrial proteins

The mitochondrial proteome, meant as the collection of proteins localized in the mitochondrion, whose fundamental function is the energy production for the life of the cell, is estimated to be constituted by at least 1000 proteins. Since the mitochondrial genome codes only for 13 polypeptides, in practice all of them are coded by nuclear genes and are imported in the mitochondrion by means of a sophisticated mechanism of sorting that is not completely known. Most classified mitochondrial proteins are synthesized as precursors with an amino-terminal extension (leader peptide), provided with peculiar physical and structural characteristics that are responsible for the recognition and import system. However, there is experimental evidence that complementary or alternative import systems exist. In particular, the 3' UnTranslated Regions (UTRs) of the mRNAs may be involved, through their secondary structure, in the mechanisms of recognition and localization. In this paper, the information content of different human mRNA sequences has been analyzed, and the results show that a significative short-term correlation exists in the 3' UTR sequences of the mRNA's coding for the proteins provided with leader peptides, probably representing the structural constraints that are necessary to the sorting machinery.

Sequential dimerization of human zipcode-binding protein IMP1 on RNA: a cooperative mechanism providing RNP stability

Active cytoplasmic RNA localization depends on the attachment of RNA-binding proteins that dictate the destination of the RNA molecule. In this study, we used an electrophoretic mobility-shift assay in combination with equilibrium and kinetic analyses to characterize the assembly of the human zipcode-binding protein IMP1 on targets in the 3'-UTR from Igf-II mRNA and in H19 RNA. In both cases, two molecules of IMP1 bound to RNA by a sequential, cooperative mechanism, characterized by an initial fast step, followed by a slow second step. The first step created an obligatory assembly intermediate of low stability, whereas the second step was the discriminatory event that converted a putative RNA target into a 'locked' stable RNP. The ability to dimerize was also observed between members of the IMP family of zipcode-binding proteins, providing a multitude of further interaction possibilities within RNP granules and with the localization apparatus.

Neuronal MAP2 mRNA: Species-dependent Differential Dendritic Targeting Competence

Providing the basis for local protein synthesis in dendritic microdomains, RNA transport in dendrites is thought to be underlying long-term neuronal plasticity. Dendritic RNA targeting mechanisms can therefore be expected to confer selective advantages in the evolution of complex neural systems. The question thus arises as to when and how dendritically targeted transcripts first acquired their targeting competence. To address this question, the dendritic targeting competence of MAP2 transcripts was examined in chicken, mouse and rat. In one approach, we established the somato-dendritic distribution of MAP2 transcripts in vivo. We found that in contrast to rodent MAP2 mRNAs, which are highly enriched in dendritic regions of the retina, chicken MAP2 transcripts are virtually absent from such areas and are rather confined to neuronal somata. In an independent line of investigation, we determined that a dendritic targeting element (DTE) corresponding to the mammalian MAP2 DTE is not contained in the 3' untranslated region (UTR) of avian MAP2 mRNA. The combined results indicate that in contrast to mammalian MAP2 transcripts, avian MAP2 mRNA is lacking dendritic targeting competence. The data thus suggest that the acquisition of such competence has likely been a relatively recent event in evolution.

The fragile X syndrome: from molecular genetics to neurobiology

Since the identification of the FMR1 gene basic research has been focused on the molecular characterization of the FMR1 gene product, the fragile X mental retardation protein (FMRP). Recent developments in fragile X research have provided new insights and knowledge about the physiological function of FMRP in the cell and the nerve cell in particular. Currently, compelling evidence suggests a role for FMRP in the transport/translation of dendritically localized mRNAs. In addition, the identification of some of the target mRNAs of FMRP have led to an increased interest in the neurobiology of the syndrome. This review highlights the role of FMRP in dendritic mRNA transport/translation in relation to synaptic plasticity, a molecular mechanism implicated in learning and memory.

A new yeast PUF family protein, Puf6p, represses ASH1 mRNA translation and is required for its localization

In yeast Saccharomyces cerevisiae, Ash1p, a protein determinant for mating-type switching, is segregated within the daughter cell nucleus to establish asymmetry of HO expression. The accumulation of Ash1p results from ASH1 mRNA that is sorted as a ribonucleoprotein particle (mRNP or locasome) to the distal tip of the bud where translation occurs. To study the mechanism regulating ASH1 mRNA translation, we isolated the ASH1 locasome and characterized the associated proteins by MALDI-TOF. One of these proteins was Puf6p, a new member of the PUF family of highly conserved RNA-binding proteins such as Pumilio in Drosophila, responsible for translational repression, usually to effect asymmetric expression. Puf6p-bound PUF consensus sequences in the 3'UTR of ASH1 mRNA and repressed the translation of ASH1 mRNA both in vivo and in vitro. In the puf6 Delta strain, asymmetric localization of both Ash1p and ASH1 mRNA were significantly reduced. We propose that Puf6p is a protein that functions in the translational control of ASH1 mRNA, and this translational inhibition is necessary before localization can proceed.

The insulin-like growth factor mRNA binding-protein IMP-1 and the Ras-regulatory protein G3BP associate with tau mRNA and HuD protein in differentiated P19 neuronal cells

Tau mRNA is axonally localized mRNA that is found in developing neurons and targeted by an axonal localization signal (ALS) that is located in the 3'UTR of the message. The tau mRNA is trafficked in an RNA-protein complex (RNP) from the neuronal cell body to the distal parts of the axon, reaching as far as the growth cone. This movement is microtubule-dependent and is observed as granules that contain tau mRNA and additional proteins. A major protein contained in the granule is HuD, an Elav protein family member, which has an identified mRNA binding site on the tau 3'UTR and stabilizes the tau message and several axonally targeted mRNAs. Using GST-HuD fusion protein as bait, we have identified four proteins contained within the tau RNP, in differentiated P19 neuronal cells. In this work, we studied two of the identified proteins, i.e. IGF-II mRNA binding protein 1 (IMP-1), the orthologue of chick beta-actin binding protein-ZBP1, and RAS-GAP SH3 domain binding protein (G3BP). We show that IMP-1 associates with HuD and G3BP-1 proteins in an RNA-dependent manner and binds directly to tau mRNA. We also show an RNA-dependent association between G3BP-1 and HuD proteins. These associations are investigated in relation to the neuronal differentiation of P19 cells.

Expression of fragments of translation initiation factor eIF4GI reveals a nuclear localisation signal within the N-terminal apoptotic cleavage fragment N-FAG

The eukaryotic initiation factor eIF4GI plays a central role in the assembly of a competent initiation complex at the 5′ end of an mRNA. Five isoforms of eIF4G exist in cells, arising from alternative translation initiation. During picornaviral infection or apoptosis, eIF4GI is cleaved proteolytically to yield distinct fragments. Using HeLa cells, we have examined the fate of these proteins in the cell. We have found that while endogenous eIF4GI is predominantly cytoplasmic, a population can also be visualised in the nucleus. Furthermore, eIF4GI is localised primarily at the nuclear periphery in the vicinity of eIF4E and PABP1. Transient transfection of HeLa cells with different myc-tagged isoforms of eIF4GI did not result in any obvious differences in their localisation. However, expression of discrete fragments of eIF4GI corresponding to those generated after apoptosis or picornaviral infection generated a distinctive, but intricate localisation pattern. Our work shows that the N-terminal apoptotic cleavage fragment N-FAG contains a sequence of basic amino acids that can act as a nuclear localisation signal. In addition, the presence or absence of the sequence flanking and including the eIF4E binding site (residues 533-682) confers a distinct cellular distribution pattern for the central domain of eIF4GI.

The Drosophila SDE3 homolog armitage is required for oskar mRNA silencing and embryonic axis specification

Polarization of the microtubule cytoskeleton during early oogenesis is required to specify the posterior of the Drosophila oocyte, which is essential for asymmetric mRNA localization during mid-oogenesis and for embryonic axis specification. The posterior determinant oskar mRNA is translationally silent until mid-oogenesis. We show that mutations in armitage and three components of the RNAi pathway disrupt oskar mRNA translational silencing, polarization of the microtubule cytoskeleton, and posterior localization of oskar mRNA. armitage encodes a homolog of SDE3, a presumptive RNA helicase involved in posttranscriptional gene silencing (RNAi) in Arabidopsis, and is required for RNAi in Drosophila ovaries. Armitage forms an asymmetric network associated with the polarized microtubule cytoskeleton and is concentrated with translationally silent oskar mRNA in the oocyte. We conclude that RNA silencing is essential for establishment of the cytoskeletal polarity that initiates embryonic axis specification and for translational control of oskar mRNA.