Genome-wide analysis of Staufen-associated mRNAs identifies secondary structures that confer target specificity

The RNA-binding protein Staufen1 impairs myogenic differentiation via a c-myc-dependent mechanism

The expression pattern of Staufen1 during mouse skeletal muscle development is described. Sustained expression of Staufen1 in myoblasts prevents normal differentiation by causing decreases in the expression of key myogenic markers by an SMD-independent mechanism and by promoting the translational regulation of c-myc.

Recent work has shown that Staufen1 plays key roles in skeletal muscle, yet little is known about its pattern of expression during embryonic and postnatal development. Here we first show that Staufen1 levels are abundant in mouse embryonic muscles and that its expression decreases thereafter, reaching low levels in mature muscles. A similar pattern of expression is seen as cultured myoblasts differentiate into myotubes. Muscle degeneration/regeneration experiments revealed that Staufen1 increases after cardiotoxin injection before returning to the low levels seen in mature muscles. We next prevented the decrease in Staufen1 during differentiation by generating stable C2C12 muscle cell lines overexpressing Staufen1. Cells overexpressing Staufen1 differentiated poorly, as evidenced by reductions in the differentiation and fusion indices and decreases in MyoD, myogenin, MEF2A, and MEF2C, independently of Staufen-mediated mRNA decay. However, levels of c-myc, a factor known to inhibit differentiation, were increased in C2C12 cells overexpressing Staufen1 through enhanced translation. By contrast, the knockdown of Staufen1 decreased c-myc levels in myoblasts. Collectively our results show that Staufen1 is highly expressed during early stages of differentiation/development and that it can impair differentiation by regulating c-myc, thereby highlighting the multifunctional role of Staufen1 in skeletal muscle cells.

Staufen targets coracle mRNA to Drosophila neuromuscular junctions and regulates GluRIIA synaptic accumulation and bouton number

The post-synaptic translation of localised mRNAs has been postulated to underlie several forms of plasticity at vertebrate synapses, but the mechanisms that target mRNAs to these postsynaptic sites are not well understood. Here we show that the evolutionary conserved dsRNA binding protein, Staufen, localises to the postsynaptic side of the Drosophila neuromuscular junction (NMJ), where it is required for the localisation of coracle mRNA and protein. Staufen plays a well-characterised role in the localisation of oskar mRNA to the oocyte posterior, where Staufen dsRNA-binding domain 5 is specifically required for its translation. Removal of Staufen dsRNA-binding domain 5, disrupts the postsynaptic accumulation of Coracle protein without affecting the localisation of cora mRNA, suggesting that Staufen similarly regulates Coracle translation. Tropomyosin II, which functions with Staufen in oskar mRNA localisation, is also required for coracle mRNA localisation, suggesting that similar mechanisms target mRNAs to the NMJ and the oocyte posterior. Coracle, the orthologue of vertebrate band 4.1, functions in the anchoring of the glutamate receptor IIA subunit (GluRIIA) at the synapse. Consistent with this, staufen mutant larvae show reduced accumulation of GluRIIA at synapses. The NMJs of staufen mutant larvae have also a reduced number of synaptic boutons. Altogether, this suggests that this novel Staufen-dependent mRNA localisation and local translation pathway may play a role in the developmentally regulated growth of the NMJ.

Alternative polyadenylation in the nervous system: to what lengths will 3' UTR extensions take us?

Alternative cleavage and polyadenylation (APA) can diversify coding and non-coding regions, but has particular impact on increasing 3' UTR diversity. Through the gain or loss of regulatory elements such as RNA binding protein and microRNA sites, APA can influence transcript stability, localization, and translational efficiency. Strikingly, the central nervous systems of invertebrate and vertebrate species express a broad range of transcript isoforms bearing extended 3' UTRs. The molecular mechanism that permits proximal 3' end bypass in neurons is mysterious, and only beginning to be elucidated. This landscape of neural 3' UTR extensions, many reaching unprecedented lengths, may help service the unique post-transcriptional regulatory needs of neurons. A combination of approaches, including transcriptome-wide profiling, genetic screening to identify APA factors, biochemical dissection of alternative 3' end formation, and manipulation of individual neural APA targets, will be necessary to gain fuller perspectives on the mechanism and biology of neural-specific 3' UTR lengthening.

The multifunctional Staufen proteins: conserved roles from neurogenesis to synaptic plasticity

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Staufen (Stau) proteins have evolutionarily conserved functions in the brain.

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Stau proteins asymmetrically segregate mRNAs during mouse and fly neurogenesis.

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Stau proteins regulate synaptic plasticity and memory formation in flies and mammals.

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Stau proteins have roles in translation, localisation, and ribonucleoprotein formation.

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New data indicate that mammalian Stau1 and Stau2 can both stabilise and destabilise target mRNAs.

Staufen (Stau) proteins belong to a family of RNA-binding proteins (RBPs) that are important for RNA localisation in many organisms. In this review we discuss recent findings on the conserved role played by Stau during both the early differentiation of neurons and in the synaptic plasticity of mature neurons. Recent molecular data suggest mechanisms for how Stau2 regulates mRNA localisation, mRNA stability, translation, and ribonucleoprotein (RNP) assembly. We offer a perspective on how this multifunctional RBP has been adopted to regulate mRNA localisation under several different cellular and developmental conditions.

Cell cycle-dependent regulation of the RNA-binding protein Staufen1

Staufen1 (Stau1) is a ribonucleic acid (RNA)-binding protein involved in the post-transcriptional regulation of gene expression. Recent studies indicate that Stau1-bound messenger RNAs (mRNAs) mainly code for proteins involved in transcription and cell cycle control. Consistently, we report here that Stau1 abundance fluctuates through the cell cycle in HCT116 and U2OS cells: it is high from the S phase to the onset of mitosis and rapidly decreases as cells transit through mitosis. Stau1 down-regulation is mediated by the ubiquitin-proteasome system and the E3 ubiquitin ligase anaphase promoting complex/cyclosome (APC/C). Stau1 interacts with the APC/C co-activators Cdh1 and Cdc20 via its first 88 N-terminal amino acids. The importance of controlling Stau1⁵⁵ levels is underscored by the observation that its overexpression affects mitosis entry and impairs proliferation of transformed cells. Microarray analyses identified 275 Stau1⁵⁵-bound mRNAs in prometaphase cells, an early mitotic step that just precedes Stau1 degradation. Interestingly, several of these mRNAs are more abundant in Stau1⁵⁵-containing complexes in cells arrested in prometaphase than in asynchronous cells. Our results point out for the first time to the possibility that Stau1 participates in a mechanism of post-transcriptional regulation of gene expression that is linked to cell cycle progression in cancer cells.

Staufen1-mediated mRNA decay induces Requiem mRNA decay through binding of Staufen1 to the Requiem 3'UTR

Requiem (REQ/DPF2) was originally identified as an apoptosis-inducing protein in mouse myeloid cells and belongs to the novel Krüppel-type zinc finger d4-protein family of proteins, which includes neuro-d4 (DPF1) and cer-d4 (DPF3). Interestingly, when a portion of the REQ messenger ribonucleic acid (mRNA) 3′ untranslated region (3′UTR), referred to as G8, was overexpressed in K562 cells, β-globin expression was induced, suggesting that the 3′UTR of REQ mRNA plays a physiological role. Here, we present evidence that the REQ mRNA 3′UTR, along with its trans-acting factor, Staufen1 (STAU1), is able to reduce the level of REQ mRNA via STAU1-mediated mRNA decay (SMD). By screening a complementary deoxyribonucleic acid (cDNA) expression library with an RNA–ligand binding assay, we identified STAU1 as an interactor of the REQ mRNA 3′UTR. Specifically, we provide evidence that STAU1 binds to putative 30-nucleotide stem–loop-structured RNA sequences within the G8 region, which we term the protein binding site core; this binding triggers the degradation of REQ mRNA and thus regulates translation. Furthermore, we demonstrate that siRNA-mediated silencing of either STAU1 or UPF1 increases the abundance of cellular REQ mRNA and, consequently, the REQ protein, indicating that REQ mRNA is a target of SMD.

Functional signature for the recognition of specific target mRNAs by human Staufen1 protein

Cellular messenger RNAs (mRNAs) are associated to proteins in the form of ribonucleoprotein particles. The double-stranded RNA-binding (DRB) proteins play important roles in mRNA synthesis, modification, activity and decay. Staufen is a DRB protein involved in the localized translation of specific mRNAs during Drosophila early development. The human Staufen1 (hStau1) forms RNA granules that contain translation regulation proteins as well as cytoskeleton and motor proteins to allow the movement of the granule on microtubules, but the mechanisms of hStau1-RNA recognition are still unclear. Here we used a combination of affinity chromatography, RNAse-protection, deep-sequencing and bioinformatic analyses to identify mRNAs differentially associated to hStau1 or a mutant protein unable to bind RNA and, in this way, defined a collection of mRNAs specifically associated to wt hStau1. A common sequence signature consisting of two opposite-polarity Alu motifs was present in the hStau1-associated mRNAs and was shown to be sufficient for binding to hStau1 and hStau1-dependent stimulation of protein expression. Our results unravel how hStau1 identifies a wide spectrum of cellular target mRNAs to control their localization, expression and fate.

GraphProt: modeling binding preferences of RNA-binding proteins

We present GraphProt, a computational framework for learning sequence- and structure-binding preferences of RNA-binding proteins (RBPs) from high-throughput experimental data. We benchmark GraphProt, demonstrating that the modeled binding preferences conform to the literature, and showcase the biological relevance and two applications of GraphProt models. First, estimated binding affinities correlate with experimental measurements. Second, predicted Ago2 targets display higher levels of expression upon Ago2 knockdown, whereas control targets do not. Computational binding models, such as those provided by GraphProt, are essential for predicting RBP binding sites and affinities in all tissues. GraphProt is freely available at http://www.bioinf.uni-freiburg.de/Software/GraphProt.

Global regulation of mRNA translation and stability in the early Drosophila embryo by the Smaug RNA-binding protein

Background

Smaug is an RNA-binding protein that induces the degradation and represses the translation of mRNAs in the early Drosophila embryo. Smaug has two identified direct target mRNAs that it differentially regulates: nanos and Hsp83. Smaug represses the translation of nanos mRNA but has only a modest effect on its stability, whereas it destabilizes Hsp83 mRNA but has no detectable effect on Hsp83 translation. Smaug is required to destabilize more than one thousand mRNAs in the early embryo, but whether these transcripts represent direct targets of Smaug is unclear and the extent of Smaug-mediated translational repression is unknown.

Results

To gain a panoramic view of Smaug function in the early embryo, we identified mRNAs that are bound to Smaug using RNA co-immunoprecipitation followed by hybridization to DNA microarrays. We also identified mRNAs that are translationally repressed by Smaug using polysome gradients and microarrays. Comparison of the bound mRNAs to those that are translationally repressed by Smaug and those that require Smaug for their degradation suggests that a large fraction of Smaug’s target mRNAs are both translationally repressed and degraded by Smaug. Smaug directly regulates components of the TRiC/CCT chaperonin, the proteasome regulatory particle and lipid droplets, as well as many metabolic enzymes, including several glycolytic enzymes.

Conclusions

Smaug plays a direct and global role in regulating the translation and stability of a large fraction of the mRNAs in the early Drosophila embryo, and has unanticipated functions in control of protein folding and degradation, lipid droplet function and metabolism.

Staufen2 regulates neuronal target RNAs

RNA-binding proteins play crucial roles in directing RNA translation to neuronal synapses. Staufen2 (Stau2) has been implicated in both dendritic RNA localization and synaptic plasticity in mammalian neurons. Here, we report the identification of functionally relevant Stau2 target mRNAs in neurons. The majority of Stau2-copurifying mRNAs expressed in the hippocampus are present in neuronal processes, further implicating Stau2 in dendritic mRNA regulation. Stau2 targets are enriched for secondary structures similar to those identified in the 3' UTRs of Drosophila Staufen targets. Next, we show that Stau2 regulates steady-state levels of many neuronal RNAs and that its targets are predominantly downregulated in Stau2-deficient neurons. Detailed analysis confirms that Stau2 stabilizes the expression of one synaptic signaling component, the regulator of G protein signaling 4 (Rgs4) mRNA, via its 3' UTR. This study defines the global impact of Stau2 on mRNAs in neurons, revealing a role in stabilization of the levels of synaptic targets.

Staufen1 senses overall transcript secondary structure to regulate translation

Human Staufen1 (Stau1) is a double-stranded RNA (dsRNA)-binding protein implicated in multiple post-transcriptional gene-regulatory processes. Here we combined RNA immunoprecipitation in tandem (RIPiT) with RNase footprinting, formaldehyde cross-linking, sonication-mediated RNA fragmentation and deep sequencing to map Staufen1-binding sites transcriptome wide. We find that Stau1 binds complex secondary structures containing multiple short helices, many of which are formed by inverted Alu elements in annotated 3' untranslated regions (UTRs) or in 'strongly distal' 3' UTRs. Stau1 also interacts with actively translating ribosomes and with mRNA coding sequences (CDSs) and 3' UTRs in proportion to their GC content and propensity to form internal secondary structure. On mRNAs with high CDS GC content, higher Stau1 levels lead to greater ribosome densities, thus suggesting a general role for Stau1 in modulating translation elongation through structured CDS regions. Our results also indicate that Stau1 regulates translation of transcription-regulatory proteins.

Finding the target sites of RNA-binding proteins

RNA–protein interactions differ from DNA–protein interactions because of the central role of RNA secondary structure. Some RNA-binding domains (RBDs) recognize their target sites mainly by their shape and geometry and others are sequence-specific but are sensitive to secondary structure context. A number of small- and large-scale experimental approaches have been developed to measure RNAs associated in vitro and in vivo with RNA-binding proteins (RBPs). Generalizing outside of the experimental conditions tested by these assays requires computational motif finding. Often RBP motif finding is done by adapting DNA motif finding methods; but modeling secondary structure context leads to better recovery of RBP-binding preferences. Genome-wide assessment of mRNA secondary structure has recently become possible, but these data must be combined with computational predictions of secondary structure before they add value in predicting in vivo binding. There are two main approaches to incorporating structural information into motif models: supplementing primary sequence motif models with preferred secondary structure contexts (e.g., MEMERIS and RNAcontext) and directly modeling secondary structure recognized by the RBP using stochastic context-free grammars (e.g., CMfinder and RNApromo). The former better reconstruct known binding preferences for sequence-specific RBPs but are not suitable for modeling RBPs that recognize shape and geometry of RNAs. Future work in RBP motif finding should incorporate interactions between multiple RBDs and multiple RBPs in binding to RNA. WIREs RNA 2014, 5:111–130. doi: 10.1002/wrna.1201