Differential expression of paralog RNA binding proteins establishes a dynamic splicing program required for normal cerebral cortex development
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ABSTRACT: The paralog RNA binding proteins (RBPs) Sam68 and SLM2 are co-expressed in the cerebral cortex and display very similar splicing activity. However, their relative function(s) in this context is unknown. By performing a time-course analysis, we found that these RBPs exhibit an opposite expression pattern during development. Sam68 expression declines postnatally while SLM2 increases after birth, and this developmental pattern is reinforced by hierarchical control of Sam68 expression by SLM2. Analysis of Sam68:Slm2 double knockout (Sam68:Slm2dko) mice revealed hundreds of exons that are sensitive to concomitant ablation of these proteins. Moreover, parallel analysis of single and double knockout cortices indicated that exons regulated mainly by SLM2 are characterized by a dynamic splicing pattern during development, whereas Sam68-dependent exons are spliced at relatively constant rates. Dynamic splicing of SLM2-sensitive exons is completely suppressed in the Sam68:Slm2dko developing cortex. Sam68:Slm2dko mice die perinatally with defects in neurogenesis and in neuronal differentiation, and the development of a hydrocephalus, consistent with splicing alterations in genes related to these biological processes. Thus, our study reveals that maintenance of the Sam68 and Slm2 paralog genes encoding homologous RBPs enables the orchestration of a dynamic splicing program while ensuring a robust redundant mechanism that supports proper cortical development.
Project description:Neuronal alternative splicing is dynamically regulated in a spatiotemporal fashion. We previously found that STAR family proteins (SAM68, SLM1, SLM2) regulate spatiotemporal alternative splicing in the nervous system. However, the whole aspect of alternative splicing programs governed by STARs remains unclear. We deciphered the alternative splicing programs of SAM68 and SLM1 proteins using transcriptomics. We reveal that SAM68 and SLM1 encode distinct alternative splicing programs; SAM68 preferentially controls alternative last exon (ALE) splicing. Interleukin 1-receptor accessory protein (Il1rap) is a novel target for SAM68. The usage of Il1rap ALEs results in mainly two variants encoding two functionally different isoforms, a membrane-bound (mIL1RAcP) and a soluble (sIL1RAcP) type. The brain exclusively expresses mIL1RAcP. SAM68 knockout results in remarkable conversion into sIL1RAcP in the brain, which significantly disturbs IL1RAcP neuronal function. Thus, we uncover the critical role of proper neuronal isoform selection through ALE choice by the SAM68-specific splicing program.
Project description:Alternative splicing—the production of multiple mRNA isoforms from a single gene—is regulated in part by RNA-binding proteins (RBPs). While the RBPs Tra2α and Tra2β have both been implicated in the regulation of alternative splicing, their relative contribution to this process are not well understood. Here we use iCLIP to identify Tra2β target exons in MDA-MB-231 cells. We find that simultaneous—but not individual—depletion of Tra2α and Tra2β induces substantial shifts in the splicing pattern of endogenous Tra2β target exons identified by iCLIP. We next use RNA-seq following joint Tra2 protein depletion to comprehensively identify Tra2 protein-dependent exons in MDA-MB-231 cells.
Project description:Alternative splicing—the production of multiple mRNA isoforms from a single gene—is regulated in part by RNA-binding proteins (RBPs). While the RBPs Tra2? and Tra2? have both been implicated in the regulation of alternative splicing, their relative contribution to this process are not well understood. Here we use iCLIP to identify Tra2? target exons in MDA-MB-231 cells. We find that simultaneous—but not individual—depletion of Tra2? and Tra2? induces substantial shifts in the splicing pattern of endogenous Tra2? target exons identified by iCLIP. We next use RNA-seq following joint Tra2 protein depletion to comprehensively identify Tra2 protein-dependent exons in MDA-MB-231 cells. Endogenous Tra2? binding sites were mapped across the MDA-MB-231 cell transcriptome in biological triplicate iCLIP experiments. RNA-seq was performed using three biological replicates of negative control siRNA treated MDA-MB-231 cells and three biological replicates of TRA2A and TRA2B siRNA treated MDA-MB-231 cells.
Project description:RNA binding protein-RNA interactions mediate a variety of processes including pre-mRNA splicing, translation, decay, polyadenylation and many others. Previous high -throughput studies have characterized general sequence features associated with increased and decreased splicing of certain exons, but these studies are limited by not knowing the mechanisms underlying these associations. Here we utilize ENCODE data from diverse data modalities to identify functional splicing regulatory elements and their associated RNA binding proteins (RBPs). We identify features which make splicing events more sensitive to depletion of RBPs, as well as which RBPs act as splicing regulators sensitive tupon RBP depletion. To analyze the sequence determinants underlying RBP-RNA interactions impacting splicing, we assay tens of thousands of sequence variants in a high-throughput splicing reporter called Vex-seq and confirm a small subset in their endogenous loci using CRISPR base editors. Finally, we leverage other large transcriptomic datasets to confirm the importance of RBPs which we designed experiments around and identify additional RBPs which may act as additional splicing regulators of the exons studied.
Project description:Differential expression of paralog RNA binding proteins establishes a dynamic splicing program required for normal cerebral cortex development
Project description:Purpose: The goal of this study was to identify the RBPs that regulate the inclusion of the exons in the muscular isoform of the DMD gene (Dp427m). We compared the splicing pattern of the Dp427m from C25Cl48 treated with a control siRNA or siRNA targeting 16 different RBPs. Method: The C25Cl48 were treated by RNAi in duplicates (or n=4 for the control condition) and submitted to 3 days of myogenic differentiation. The RBPs tested were CELF1, DAZAP1, DDX5/DDX17, hnRNPA1, hnRNPA2, MBNL1, PTBP1, QKI, RBFOX1, RBFOX2, RBM4, SRSF1, SRSF2, TDP-43 and Tra2b. The silencing efficiency was controlled by RT-qPCR and Western blotting analysis. RNA was reverse transcribed using oligodT and the Dp427m (11.3kb) coding sequence was amplified by Long-Range PCR and sequenced on a 454 GS junior platform. Reads that passed quality filters were mapped to the human X chromosome with STAR. The Percent-Spliced-In (PSI) was calculated for each splicing event using intron-centric metrics (Pervouchine et al., 2013), with the SJPIPE pipeline, from the ipsa package. Junctions were filtered to keep only those covered by a minimum of 5 reads in at least one out of the two biological replicates or 2/4 in the control condition. Results: Using this DMD-targeted RNA-seq approach, we obtained an average read depth of 1638X, allowing reliable detection of alternative splicing events. Based on a cutoff of |[delta]PSI|≥0.05, we identified 8 exons that were deregulated in at least one of the RBPs tested, and 15 RNAi conditions induced splicing changes. Conclusion: The 8 deregulated exons identified here are mainly exons that were previously found alternatively spliced in other dystrophin isoforms.
Project description:Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by the absence of a functional Survival of Motor Neuron 1 gene (SMN1). The nearly identical paralog, SMN2, cannot compensate for the loss of SMN1 because exon 7 is aberrantly skipped from most SMN2 transcripts, a process mediated by synergistic activities of Sam68/KHDR1 and hnRNP A1. This results in the production of a truncated, non-functional protein that is rapidly degraded. Here we present several crystal structures of Sam68 RNA-binding domain (RBD). Sam68-RBD forms stable symmetric homodimers by antiparallel association of helices α3 from two monomers. However, the details of domain organization and the dimerization interface differ significantly from previously characterized homologs. We demonstrate that Sam68 and hnRNP A1 bind proximal but distinct motifs within the central region of SMN2(ex7). Our findings have important implications for the etiology of SMA and open new avenues for the design of novel therapeutics to treat splicing diseases.
Project description:The studies of spliceosomal interactions are challenging due to their dynamic nature. Here we developed spliceosome iCLIP, which immunoprecipitates SmB along with snRNPs and auxiliary RNA binding proteins (RBPs) to simultaneously map the spliceosomal binding to human snRNAs and pre-mRNAs. This identified 9 distinct regions on pre-mRNAs, which overlap with position-dependent binding patterns of 15 RBPs. Using spliceosome iCLIP, we additionally identified >50,000 branchpoints (BPs) that have canonical features, unlike those identified by RNA-seq. The iCLIP BPs generally overlap with the computationally predicted BPs, and alternative BPs are associated with extended regions of structurally accessible RNA. We find that the position and strength of BPs defines the binding patterns of SF3 and U2AF complexes, whereas the RNA structure around BPs affects the sensitivity of exons to perturbation of these complexes. Our findings introduce spliceosome iCLIP as a new method for transcriptomic studies of BPs and splicing mechanisms.
Project description:The studies of spliceosomal interactions are challenging due to their dynamic nature. Here we developed spliceosome iCLIP, which immunoprecipitates SmB along with snRNPs and auxiliary RNA binding proteins (RBPs) to simultaneously map the spliceosomal binding to human snRNAs and pre-mRNAs. This identified 9 distinct regions on pre-mRNAs, which overlap with position-dependent binding patterns of 15 RBPs. Using spliceosome iCLIP, we additionally identified >50,000 branchpoints (BPs) that have canonical features, unlike those identified by RNA-seq. The iCLIP BPs generally overlap with the computationally predicted BPs, and alternative BPs are associated with extended regions of structurally accessible RNA. We find that the position and strength of BPs defines the binding patterns of SF3 and U2AF complexes, whereas the RNA structure around BPs affects the sensitivity of exons to perturbation of these complexes. Our findings introduce spliceosome iCLIP as a new method for transcriptomic studies of BPs and splicing mechanisms.
Project description:We uncover a Sam68-dependent splicing program during cerebellar development. These events direct proper isoform expression of the genes required to guarantee the establishment of the correct spatial/temporal neural circuitry. The dysregulation in Sam68 null mice leads to functional defects in adult neurons