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Background A critical step in uncovering rules of RNA processing is to study the in vivo regulatory networks of RNA binding proteins (RBPs). Crosslinking and immunoprecipitation (CLIP) methods enable mapping RBP targets transcriptome-wide, but methodological differences present challenges to large-scale analysis across datasets. The development of enhanced CLIP (eCLIP) enabled the mapping of targets for 150 RBPs in K562 and HepG2, creating a unique resource of RBP interactomes profiled with a standardized methodology in the same cell types. Results Our analysis of 223 eCLIP datasets reveals a range of binding modalities, including highly resolved positioning around splicing signals and mRNA untranslated regions that associate with distinct RBP functions. Quantification of enrichment for repetitive and abundant multicopy elements reveals 70% of RBPs have enrichment for non-mRNA element classes, enables identification of novel ribosomal RNA processing factors and sites, and suggests that association with retrotransposable elements reflects multiple RBP mechanisms of action. Analysis of spliceosomal RBPs indicates that eCLIP resolves AQR association after intronic lariat formation, enabling identification of branch points with single-nucleotide resolution, and provides genome-wide validation for a branch point-based scanning model for 3′ splice site recognition. Finally, we show that eCLIP peak co-occurrences across RBPs enable the discovery of novel co-interacting RBPs. Conclusions This work reveals novel insights into RNA biology by integrated analysis of eCLIP profiling of 150 RBPs with distinct functions. Further, our quantification of both mRNA and other element association will enable further research to identify novel roles of RBPs in regulating RNA processing.

The Rbfox family of developmentally important splicing factors controls alternative splicing in a tissue-specific manner. Genome-wide analyses now show that more than half of Rbfox-binding sites are located distally from exons, that these distal sites are conserved and functionally important, and that long-range RNA-RNA secondary structures mediate distal splicing regulation by Rbfox. Alternative splicing (AS) enables programmed diversity of gene expression across tissues and development. We show here that binding in distal intronic regions (>500 nucleotides (nt) from any exon) by Rbfox splicing factors important in development is extensive and is an active mode of splicing regulation. Similarly to exon-proximal sites, distal sites contain evolutionarily conserved GCATG sequences and are associated with AS activation and repression upon modulation of Rbfox abundance in human and mouse experimental systems. As a proof of principle, we validated the activity of two specific Rbfox enhancers in KIF21A and ENAH distal introns and showed that a conserved long-range RNA-RNA base-pairing interaction (an RNA bridge) is necessary for Rbfox-mediated exon inclusion in the ENAH gene. Thus we demonstrate a previously unknown RNA-mediated mechanism for AS control by distally bound RNA-binding proteins.

The RNA-binding protein (RBP) TAF15 is implicated in amyotrophic lateral sclerosis (ALS). To compare TAF15 function to that of two ALS-associated RBPs, FUS and TDP-43, we integrate CLIP-seq and RNA Bind-N-Seq technologies, and show that TAF15 binds to ∼4,900 RNAs enriched for GGUA motifs in adult mouse brains. TAF15 and FUS exhibit similar binding patterns in introns, are enriched in 3′ untranslated regions and alter genes distinct from TDP-43. However, unlike FUS and TDP-43, TAF15 has a minimal role in alternative splicing. In human neural progenitors, TAF15 and FUS affect turnover of their RNA targets. In human stem cell-derived motor neurons, the RNA profile associated with concomitant loss of both TAF15 and FUS resembles that observed in the presence of the ALS-associated mutation FUS R521G, but contrasts with late-stage sporadic ALS patients. Taken together, our findings reveal convergent and divergent roles for FUS, TAF15 and TDP-43 in RNA metabolism. Abnormal functions of RNA-binding proteins TAF15, FUS and TDP43 are associated with amyotrophic lateral sclerosis. Here, Kapeli et al . characterize the RNA targets of TAF15 and identify points of convergence and divergence between the targets of TAF15, FUS and TDP43 in several neuronal systems.

Allele-specific protein-RNA binding is an essential aspect that may reveal functional genetic variants (GVs) mediating post-transcriptional regulation. Recently, genome-wide detection of in vivo binding of RNA-binding proteins is greatly facilitated by the enhanced crosslinking and immunoprecipitation (eCLIP) method. We developed a new computational approach, called BEAPR, to identify allele-specific binding (ASB) events in eCLIP-Seq data. BEAPR takes into account crosslinking-induced sequence propensity and variations between replicated experiments. Using simulated and actual data, we show that BEAPR largely outperforms often-used count analysis methods. Importantly, BEAPR overcomes the inherent overdispersion problem of these methods. Complemented by experimental validations, we demonstrate that the application of BEAPR to ENCODE eCLIP-Seq data of 154 proteins helps to predict functional GVs that alter splicing or mRNA abundance. Moreover, many GVs with ASB patterns have known disease relevance. Overall, BEAPR is an effective method that helps to address the outstanding challenge of functional interpretation of GVs. Differential binding of RNA-binding proteins mediated by genetic variants (GVs) can influence posttranscriptional regulation. Here, the authors develop BEAPR, a computational approach to identify allele-specific binding events in eCLIP-Seq data.

Adenosine-to-inosine (A-to-I) editing, mediated by the ADAR enzymes, diversifies the transcriptome by altering RNA sequences. Recent studies reported global changes in RNA editing in disease and development. Such widespread editing variations necessitate an improved understanding of the regulatory mechanisms of RNA editing. Here, we study the roles of >200 RNA-binding proteins (RBPs) in mediating RNA editing in two human cell lines. Using RNA-sequencing and global protein-RNA binding data, we identify a number of RBPs as key regulators of A-to-I editing. These RBPs, such as TDP-43, DROSHA, NF45/90 and Ro60, mediate editing through various mechanisms including regulation of ADAR1 expression, interaction with ADAR1, and binding to Alu elements. We highlight that editing regulation by Ro60 is consistent with the global up-regulation of RNA editing in systemic lupus erythematosus. Additionally, most key editing regulators act in a cell type-specific manner. Together, our work provides insights for the regulatory mechanisms of RNA editing. Giovanni Quinones-Valdez et al. examined the role of over 200 RNA-binding proteins in mediating A-to-I RNA editing. They identified several RNA-binding proteins that regulate ADAR1 expression, interaction, or binding with Alu elements in a cell type-specific manner.

Enhanced CLIP yields complex libraries of RNA components of ribonucleoprotein complexes and maintains single-nucleotide resolution of binding sites. eCLIP enables large scale profiling, as demonstrated with the binding profiles of 73 RBPs in two human cancer cell lines. As RNA-binding proteins (RBPs) play essential roles in cellular physiology by interacting with target RNA molecules, binding site identification by UV crosslinking and immunoprecipitation (CLIP) of ribonucleoprotein complexes is critical to understanding RBP function. However, current CLIP protocols are technically demanding and yield low-complexity libraries with high experimental failure rates. We have developed an enhanced CLIP (eCLIP) protocol that decreases requisite amplification by ∼1,000-fold, decreasing discarded PCR duplicate reads by ∼60% while maintaining single-nucleotide binding resolution. By simplifying the generation of paired IgG and size-matched input controls, eCLIP improves specificity in the discovery of authentic binding sites. We generated 102 eCLIP experiments for 73 diverse RBPs in HepG2 and K562 cells (available at https://www.encodeproject.org ), demonstrating that eCLIP enables large-scale and robust profiling, with amplification and sample requirements similar to those of ChIP-seq. eCLIP enables integrative analysis of diverse RBPs to reveal factor-specific profiles, common artifacts for CLIP and RNA-centric perspectives on RBP activity.

Discovery of interaction sites between RNA-binding proteins (RBPs) and their RNA targets plays a critical role in enabling our understanding of how these RBPs control RNA processing and regulation. Cross-linking and immunoprecipitation (CLIP) provides a generalizable, transcriptome-wide method by which RBP/RNA complexes are purified and sequenced to identify sites of intermolecular contact. By simplifying technical challenges in prior CLIP methods and incorporating the generation of and quantitative comparison against size-matched input controls, the single-end enhanced CLIP (seCLIP) protocol allows for the profiling of these interactions with high resolution, efficiency and scalability. Here, we present a step-by-step guide to the seCLIP method, detailing critical steps and offering insights regarding troubleshooting and expected results while carrying out the ~4-d protocol. Furthermore, we describe a comprehensive bioinformatics pipeline that offers users the tools necessary to process two replicate datasets and identify reproducible and significant peaks for an RBP of interest in ~2 d. This protocol describes a streamlined cross-linking and immunoprecipitation sequencing approach for transcriptome-wide detection of RNA-binding protein localization that is complemented by a comprehensive bioinformatics pipeline.

The RNA-binding protein Musashi-2 increases the self-renewing abilities of human haematopoietic stem cells, which have the potential to be used for regenerative therapies. Musashi-2 boots stem cell renewal Only a limited number of haematopoietic stem cells can be recovered from umbilical cord blood, limiting their therapeutic use. Little is known about the post-transcriptional mechanisms regulating self-renewal and fate decision in human haematopoietic stem cells. Kristin Hope and colleagues find that the RNA-binding protein Musashi-2 increases the self-renewal properties of human haematopoietic stem cells, including the ability to expand the long-term haematopoietic stem cells ex vivo . The authors use a global approach to identify the RNAs interacting with Musashi-2 and they identify the aryl hydrocarbon receptor (AHR) signalling pathway as a critical downstream component of the effects of Musashi-2 on the regenerative potential of cord blood-derived haematopoietic stem cells. Umbilical cord blood-derived haematopoietic stem cells (HSCs) are essential for many life-saving regenerative therapies. However, despite their advantages for transplantation, their clinical use is restricted because HSCs in cord blood are found only in small numbers 1 . Small molecules that enhance haematopoietic stem and progenitor cell (HSPC) expansion in culture have been identified 2 , 3 , but in many cases their mechanisms of action or the nature of the pathways they impinge on are poorly understood. A greater understanding of the molecular circuitry that underpins the self-renewal of human HSCs will facilitate the development of targeted strategies that expand HSCs for regenerative therapies. Whereas transcription factor networks have been shown to influence the self-renewal and lineage decisions of human HSCs 4 , 5 , the post-transcriptional mechanisms that guide HSC fate have not been closely investigated. Here we show that overexpression of the RNA-binding protein Musashi-2 (MSI2) induces multiple pro-self-renewal phenotypes, including a 17-fold increase in short-term repopulating cells and a net 23-fold ex vivo expansion of long-term repopulating HSCs. By performing a global analysis of MSI2–RNA interactions, we show that MSI2 directly attenuates aryl hydrocarbon receptor (AHR) signalling through post-transcriptional downregulation of canonical AHR pathway components in cord blood HSPCs. Our study gives mechanistic insight into RNA networks controlled by RNA-binding proteins that underlie self-renewal and provides evidence that manipulating such networks ex vivo can enhance the regenerative potential of human HSCs.

A surprising portion of both mammalian and Drosophila genomes are transcriptionally paused, undergoing initiation without elongation. We tested the hypothesis that transcriptional pausing is an obligate transition state between definitive activation and silencing as human embryonic stem cells (hESCs) change state from pluripotency to mesoderm. Chromatin immunoprecipitation for trimethyl lysine 4 on histone H3 (ChIP-Chip) was used to analyze transcriptional initiation, and 3' transcript arrays were used to determine transcript elongation. Pluripotent and mesodermal cells had equivalent fractions of the genome in active and paused transcriptional states (∼48% each), with ∼4% definitively silenced (neither initiation nor elongation). Differentiation to mesoderm changed the transcriptional state of 12% of the genome, with roughly equal numbers of genes moving toward activation or silencing. Interestingly, almost all loci (98-99%) changing transcriptional state do so either by entering or exiting the paused state. A majority of these transitions involve either loss of initiation, as genes specifying alternate lineages are archived, or gain of initiation, in anticipation of future full-length expression. The addition of chromatin dynamics permitted much earlier predictions of final cell fate compared to sole use of conventional transcript arrays. These findings indicate that the paused state may be the major transition state for genes changing expression during differentiation, and implicate control of transcriptional elongation as a key checkpoint in lineage specification.

An overall downregulation of RNA editing was observed in postmortem brains from people with autism, which was consistent across brain regions and genetic disorder subtypes. These changes were regulated by the RNA-binding proteins FMRP and FXR1P.