hnRNP H and hnRNP F complex with Fox2 to silence fibroblast growth factor receptor 2 exon IIIc. Mol Cell Biol. 2008;28:5403-5419. [PMID: 18573884 DOI: 10.1128/mcb.00739-08]

Alternative splicing regulation and its therapeutic potential in bladder cancer

Bladder cancer is one of the leading causes of mortality globally. The development of bladder cancer is closely associated with alternative splicing, which regulates human gene expression and enhances the diversity of functional proteins. Alternative splicing is a distinctive feature of bladder cancer, and as such, it may hold promise as a therapeutic target. This review aims to comprehensively discuss the current knowledge of alternative splicing in the context of bladder cancer. We review the process of alternative splicing and its regulation in bladder cancer. Moreover, we emphasize the significance of abnormal alternative splicing and splicing factor irregularities during bladder cancer progression. Finally, we explore the impact of alternative splicing on bladder cancer drug resistance and the potential of alternative splicing as a therapeutic target.

Alternative splicing in EMT and TGF-β signaling during cancer progression

Epithelial to mesenchymal transition (EMT) is a physiological process during development where epithelial cells transform to acquire mesenchymal characteristics, which allows them to migrate and colonize secondary tissues. Many cellular signaling pathways and master transcriptional factors exert a myriad of controls to fine tune this vital process to meet various developmental and physiological needs. Adding to the complexity of this network are post-transcriptional and post-translational regulations. Among them, alternative splicing has been shown to play important roles to drive EMT-associated phenotypic changes, including actin cytoskeleton remodeling, cell-cell junction changes, cell motility and invasiveness. In advanced cancers, transforming growth factor-β (TGF-β) is a major inducer of EMT and is associated with tumor cell metastasis, cancer stem cell self-renewal, and drug resistance. This review aims to provide an overview of recent discoveries regarding alternative splicing events and the involvement of splicing factors in the EMT and TGF-β signaling. It will emphasize the importance of various splicing factors involved in EMT and explore their regulatory mechanisms.

ePRINT: exonuclease assisted mapping of protein-RNA interactions

RNA-binding proteins (RBPs) regulate key aspects of RNA processing including alternative splicing, mRNA degradation and localization by physically binding RNA molecules. Current methods to map these interactions, such as CLIP, rely on purifying single proteins at a time. Our new method, ePRINT, maps RBP-RNA interaction networks on a global scale without purifying individual RBPs. ePRINT uses exoribonuclease XRN1 to precisely map the 5' end of the RBP binding site and uncovers direct and indirect targets of an RBP of interest. Importantly, ePRINT can also uncover RBPs that are differentially activated between cell fate transitions, including neural progenitor differentiation into neurons.

The Repurposing of Cellular Proteins during Enterovirus A71 Infection

Viruses pose a great threat to people's lives. Enterovirus A71 (EV-A71) infects children and infants all over the world with no FDA-approved treatment to date. Understanding the basic mechanisms of viral processes aids in selecting more efficient drug targets and designing more effective antivirals to thwart this virus. The 5'-untranslated region (5'-UTR) of the viral RNA genome is composed of a cloverleaf structure and an internal ribosome entry site (IRES). Cellular proteins that bind to the cloverleaf structure regulate viral RNA synthesis, while those that bind to the IRES also known as IRES trans-acting factors (ITAFs) regulate viral translation. In this review, we survey the cellular proteins currently known to bind the 5'-UTR and influence viral gene expression with emphasis on comparing proteins' functions and localizations pre- and post-(EV-A71) infection. A comprehensive understanding of how the host cell's machinery is hijacked and reprogrammed by the virus to facilitate its replication is crucial for developing effective antivirals.

Hypoplastic Left Heart Syndrome: Signaling & Molecular Perspectives, and the Road Ahead

Hypoplastic left heart syndrome (HLHS) is a lethal congenital heart disease (CHD) affecting 8-25 per 100,000 neonates globally. Clinical interventions, primarily surgical, have improved the life expectancy of the affected subjects substantially over the years. However, the etiological basis of HLHS remains fundamentally unclear to this day. Based upon the existing paradigm of studies, HLHS exhibits a multifactorial mode of etiology mediated by a complicated course of genetic and signaling cascade. This review presents a detailed outline of the HLHS phenotype, the prenatal and postnatal risks, and the signaling and molecular mechanisms driving HLHS pathogenesis. The review discusses the potential limitations and future perspectives of studies that can be undertaken to address the existing scientific gap. Mechanistic studies to explain HLHS etiology will potentially elucidate novel druggable targets and empower the development of therapeutic regimens against HLHS in the future.

The HNRNPF/H RNA binding proteins and disease

The members of the HNRNPF/H family of heterogeneous nuclear RNA proteins-HNRNPF, HNRNPH1, HNRNPH2, HNRNPH3, and GRSF1, are critical regulators of RNA maturation. Documented functions of these proteins include regulating splicing, particularly alternative splicing, 5' capping and 3' polyadenylation of RNAs, and RNA export. The assignment of these proteins to the HNRNPF/H protein family members relates to differences in the amino acid composition of their RNA recognition motifs, which differ from those of other RNA binding proteins (RBPs). HNRNPF/H proteins typically bind RNA sequences enriched with guanine (G) residues, including sequences that, in the presence of a cation, have the potential to form higher-order G-quadruplex structures. The need to further investigate members of the HNRNPF/H family of RBPs has intensified with the recent descriptions of their involvement in several disease states, including the pediatric tumor Ewing sarcoma and the hematological malignancy mantle cell lymphoma; newly described groups of developmental syndromes; and neuronal-related disorders, including addictive behavior. Here, to foster the study of the HNRNPF/H family of RBPs, we discuss features of the genes encoding these proteins, their structures and functions, and emerging contributions to disease. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

How Driver Oncogenes Shape and Are Shaped by Alternative Splicing Mechanisms in Tumors

The development of RNA sequencing methods has allowed us to study and better understand the landscape of aberrant pre-mRNA splicing in tumors. Altered splicing patterns are observed in many different tumors and affect all hallmarks of cancer: growth signal independence, avoidance of apoptosis, unlimited proliferation, invasiveness, angiogenesis, and metabolism. In this review, we focus on the interplay between driver oncogenes and alternative splicing in cancer. On one hand, oncogenic proteins-mutant p53, CMYC, KRAS, or PI3K-modify the alternative splicing landscape by regulating expression, phosphorylation, and interaction of splicing factors with spliceosome components. Some splicing factors-SRSF1 and hnRNPA1-are also driver oncogenes. At the same time, aberrant splicing activates key oncogenes and oncogenic pathways: p53 oncogenic isoforms, the RAS-RAF-MAPK pathway, the PI3K-mTOR pathway, the EGF and FGF receptor families, and SRSF1 splicing factor. The ultimate goal of cancer research is a better diagnosis and treatment of cancer patients. In the final part of this review, we discuss present therapeutic opportunities and possible directions of further studies aiming to design therapies targeting alternative splicing mechanisms in the context of driver oncogenes.

The RNA-binding protein hnRNP F is required for the germinal center B cell response

The T cell-dependent (TD) antibody response involves the generation of high affinity, immunoglobulin heavy chain class-switched antibodies that are generated through germinal center (GC) response. This process is controlled by coordinated transcriptional and post-transcriptional gene regulatory mechanisms. RNA-binding proteins (RBPs) have emerged as critical players in post-transcriptional gene regulation. Here we demonstrate that B cell-specific deletion of RBP hnRNP F leads to diminished production of class-switched antibodies with high affinities in response to a TD antigen challenge. B cells deficient in hnRNP F are characterized by defective proliferation and c-Myc upregulation upon antigenic stimulation. Mechanistically, hnRNP F directly binds to the G-tracts of Cd40 pre-mRNA to promote the inclusion of Cd40 exon 6 that encodes its transmembrane domain, thus enabling appropriate CD40 cell surface expression. Furthermore, we find that hnRNP A1 and A2B1 can bind to the same region of Cd40 pre-mRNA but suppress exon 6 inclusion, suggesting that these hnRNPs and hnRNP F might antagonize each-other's effects on Cd40 splicing. In summary, our study uncovers an important posttranscriptional mechanism regulating the GC response.

A splicing silencer in SMN2 intron 6 is critical in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a fatal neuromuscular disease caused by homozygous deletions or mutations of the SMN1 gene. SMN2 is a paralogous gene of SMN1 and a modifying gene of SMA. A better understanding of how SMN2 exon 7 splicing is regulated helps discover new therapeutic targets for SMA therapy. Based on an antisense walk method to map exonic and intronic splicing silencers (ESSs and ISSs) in SMN2 exon 7 and the proximal regions of its flanking introns, we identified one ISS (ISS6-KH) at upstream of the branch point site in intron 6. By using mutagenesis-coupled RT-PCR with SMN1/2 minigenes, immunochromatography, overexpression and siRNA-knockdown, we found this ISS consists of a bipartite hnRNP A1 binding cis-element and a poly-U sequence located between the proximal hnRNP A1 binding site (UAGCUA) and the branch site. Both HuR and hnRNP C1 proteins promote exon 7 skipping through the poly-U stretch. Mutations or deletions of these motifs lead to efficient SMN2 exon 7 inclusion comparable to SMN1 gene. Furthermore, we identified an optimal antisense oligonucleotide that binds the intron six ISS and causes striking exon 7 inclusion in the SMN2 gene in patient fibroblasts and SMA mouse model. Our findings demonstrate that this novel ISS plays an important role in SMN2 exon 7 skipping and highlight a new therapeutic target for SMA therapy.

N-nitrosamines-mediated downregulation of LncRNA-UCA1 induces carcinogenesis of esophageal squamous by regulating the alternative splicing of FGFR2

Concerns are raised over the risk to digestive system's tumors from the N-nitrosamines (NAs) exposure in drinking water. Albeit considerable studies are conducted to explore the underlying mechanism responsible for NAs-induced esophageal squamous cell carcinoma (ESCC), the exact molecular mechanisms remain largely unknown, especially at the epigenetic regulation level. In this study, it is revealed that the urinary concentration of N-Nitrosodiethylamine is higher in high incidence area of ESCC, and the lncRNA-UCA1(UCA1) is significantly decreased in ESCC tissues. In vitro and in vivo experiments further show that UCA1 is involved in the malignant transformation of Het-1A cells and precancerous lesions of the rat esophagus induced by N-nitrosomethylbenzylamine (NMBzA). Functional gain and loss experiments verify UCA1 can affect the proliferation, migration, and invasion of ESCC cells in vitro and in vivo. Mechanically, through binding to heterogeneous nuclear ribonucleoprotein F (hnRNP F) protein, UCA1 regulates alternative splicing of fibroblast growth factor receptor 2 (FGFR2), which promotes the FGFR2IIIb isoform switching to FGFR2 IIIc isoform, and the latter activates epithelial-mesenchymal transition via PI3K-AKT signaling pathways impacting tumorigenesis. Therefore, NAs-mediated downregulation of UCA1 promotes ESCC progression through targeting hnRNP F/FGFR2/PI3k-AKT axis, which provides a new chemical carcinogenic target and establishes a previously unknown mechanism for NAs-induced ESCC.

The global Protein-RNA interaction map of ESRP1 defines a post-transcriptional program that is essential for epithelial cell function

The epithelial splicing regulatory proteins, ESRP1 and ESRP2, are essential for mammalian development through the regulation of a global program of alternative splicing of genes involved in the maintenance of epithelial cell function. To further inform our understanding of the molecular functions of ESRP1, we performed enhanced crosslinking immunoprecipitation coupled with high-throughput sequencing (eCLIP) in epithelial cells of mouse epidermis. The genome-wide binding sites of ESRP1 were integrated with RNA-Seq analysis of alterations in splicing and total gene expression that result from epidermal ablation of Esrp1 and Esrp2. These studies demonstrated that ESRP1 functions in splicing regulation occur primarily through direct binding in a position-dependent manner to promote either exon inclusion or skipping. In addition, we also identified widespread binding of ESRP1 in 3' and 5' untranslated regions (UTRs) of genes involved in epithelial cell function, suggesting that its post-transcriptional functions extend beyond splicing regulation.

HiFENS: high-throughput FISH detection of endogenous pre-mRNA splicing isoforms

Splicing factors play an essential role in regulation of alternative pre-mRNA splicing. While much progress has been made in delineating the mechanisms of the splicing machinery, the identity of signal transduction pathways and upstream factors that regulate splicing factor activity is largely unknown. A major challenge in the discovery of upstream regulatory factors of pre-mRNA splicing is the scarcity of functional genomics screening methods to monitor splicing outcomes of endogenous genes. Here, we have developed HiFENS (high throughput FISH detection of endogenous splicing isoforms), a high-throughput imaging assay based on hybridization chain reaction (HCR) and used HiFENS to screen for cellular factors that regulate alternative splicing of endogenous genes. We demonstrate optimized detection with high specificity of endogenous splicing isoforms and multiplexing of probes for accurate detection of splicing outcomes with single cell resolution. As proof-of-principle, we perform an RNAi screen of 702 human kinases and identify potential candidate upstream splicing regulators of the FGFR2 gene. HiFENS should be a useful tool for the unbiased delineation of cellular pathways involved in alternative splicing regulation.

Selective C9orf72 G-Quadruplex-Binding Small Molecules Ameliorate Pathological Signatures of ALS/FTD Models

The G-quadruplex (G4) forming C9orf72 GGGGCC (G4C2) expanded hexanucleotide repeat (EHR) is the predominant genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Developing selective G4-binding ligands is challenging due to the conformational polymorphism and similarity of G4 structures. We identified three first-in-class marine natural products, chrexanthomycin A (cA), chrexanthomycin B (cB), and chrexanthomycin C (cC), with remarkable bioactivities. Thereinto, cA shows the highest permeability and lowest cytotoxicity to live cells. NMR titration experiments and in silico analysis demonstrate that cA, cB, and cC selectively bind to DNA and RNA G4C2 G4s. Notably, cA and cC dramatically reduce G4C2 EHR-caused cell death, diminish G4C2 RNA foci in (G4C2)₂₉-expressing Neuro2a cells, and significantly eliminate ROS in HT22 cells. In (G4C2)₂₉-expressing Drosophila, cA and cC significantly rescue eye degeneration and improve locomotor deficits. Overall, our findings reveal that cA and cC are potential therapeutic agents deserving further clinical study.

PKC Regulates YAP Expression through Alternative Splicing of YAP 3'UTR Pre-mRNA by hnRNP F

The Yes-associated protein (YAP) is a transcriptional co-activator that plays critical roles in organ development and tumorigenesis, and is verified to be inhibited by the Hippo signaling pathway. In the present study, we show that the YAP 3′UTR is alternatively spliced to generate a novel 950 bp 3′UTR mRNA from the full length 3′UTR region (3483 bp) in human cancer cells. The ratio of full length 3′UTR YAP mRNA to alternatively spliced 3′UTR YAP mRNA is up-regulated by exposure of the cells to PKC inhibitor chelerythrine chloride. Further study using luciferase reporter assay showed that the expression of the alternatively spliced 3′UTR mRNA is much lower compared with the full length 3′UTR mRNA, suggesting that alternatively spliced 3′UTR YAP mRNA may have a shorter half-life than full length 3′UTR mRNA. Interestingly, PKC represses YAP 3′UTR–mediated mRNA stability is dependent on a splicing factor, hnRNP F. Activation of PKC induces nuclear translocation of cytosolic hnRNP F. Ectopic expression of hnRNP F enhances YAP 3′UTR splicing. Our results suggest that hnRNP F regulates YAP 3′UTR-mediated mRNA stability in an alternative splicing-dependent manner, and PKC regulated YAP expression is dependent on nuclear translocation of hnRNP F in human cancer cell lines.

hnRNP H/F drive RNA G-quadruplex-mediated translation linked to genomic instability and therapy resistance in glioblastoma

RNA G-quadruplexes (RG4s) are four-stranded structures known to control mRNA translation of cancer relevant genes. RG4 formation is pervasive in vitro but not in cellulo, indicating the existence of poorly characterized molecular machinery that remodels RG4s and maintains them unfolded. Here, we performed a quantitative proteomic screen to identify cytosolic proteins that interact with a canonical RG4 in its folded and unfolded conformation. Our results identified hnRNP H/F as important components of the cytoplasmic machinery modulating the structural integrity of RG4s, revealed their function in RG4-mediated translation and uncovered the underlying molecular mechanism impacting the cellular stress response linked to the outcome of glioblastoma.

RNA G-quadruplexes (RG4s) have been functionally linked to cancer gene expression. Here, Herviou, Le Bras et al. have identified the protein machinery modulating RG4s and reveal the role and mechanism of hnRNP H/F and DHX36 in RG4-mediated translational regulation affecting cancer treatment in glioblastoma.

Cellular stress orchestrates the localization of hnRNP H to stress granules

Heterogeneous nuclear ribonucleoprotein (hnRNP) H is a member of hnRNP H/F protein subfamily of hnRNPs that regulate the maturation and post-transcriptional processing of pre-mRNA. As a component of an mRNA export complex, hnRNP H shuttles mature mRNA from the nucleus to the cytoplasm. Although hnRNP H is primarily a nuclear protein, it can accumulate in the cytoplasm in certain tissues and cell types; however, the physiological relevance of hnRNP H cytoplasmic accumulation is unknown. Here we show that under cellular stress hnRNP H accumulates in the cytoplasm and is required for efficient recovery from cellular stress. Moreover, we find that cytoplasmic hnRNP H localizes to stress granules and that the RRM3 domain of hnRNP H is necessary for this localization. Together, our results demonstrate that hnRNP H accumulates in the cytoplasm under cellular stress and is recruited to stress granules.

Genetic diversity in alveolar soft part sarcoma: A subset contain variant fusion genes, highlighting broader molecular kinship with other MiT family tumors

Alveolar soft part sarcoma (ASPS) is a rare malignancy that, since its initial description, remains a neoplasm of uncertain histogenesis. The disease-defining molecular event characterizing the diagnosis of ASPS is the ASPSCR1-TFE3 fusion gene. Following identification of an index case of ASPS with a novel TFE3 fusion partner, we performed a retrospective review to determine whether this represents an isolated event. We identified two additional cases, for a total of three cases lacking ASPSCR1 partners. The average patient age was 46 years (range, 17-65); two patients were female. The sites of origin included the transverse colon, foot, and dura. Each case exhibited a histomorphology typical of ASPS, and immunohistochemistry was positive for TFE3 in all cases. Routine molecular testing of the index patient demonstrated a HNRNPH3-TFE3 gene fusion; the remaining cases were found to have DVL2-TFE3 or PRCC-TFE3 fusion products. The latter two fusions have previously been identified in renal cell carcinoma; to our knowledge, this is the first report of a HNRNPH3-TFE3 gene fusion. These findings highlight a heretofore underrecognized genetic diversity in ASPS, which appears to more broadly molecularly overlap with that of translocation-associated renal cell carcinoma and PEComa. These results have immediate implications in the diagnosis of ASPS since assays reliant upon ASPSCR1 may yield a false negative result. While these findings further understanding of the molecular pathogenesis of ASPS, issues related to the histogenesis of this unusual neoplasm remain unresolved.

Alternative Splicing Enhances the Transcriptome Complexity of Liriodendron chinense

Alternative splicing (AS) plays pivotal roles in regulating plant growth and development, flowering, biological rhythms, signal transduction, and stress responses. However, no studies on AS have been performed in Liriodendron chinense, a deciduous tree species that has high economic and ecological value. In this study, we used multiple tools and algorithms to analyze transcriptome data derived from seven tissues via hybrid sequencing. Although only 17.56% (8,503/48,408) of genes in L. chinense were alternatively spliced, these AS genes occurred in 37,844 AS events. Among these events, intron retention was the most frequent AS event, producing 1,656 PTC-containing and 3,310 non-PTC-containing transcripts. Moreover, 183 long noncoding RNAs (lncRNAs) also underwent AS events. Furthermore, weighted gene coexpression network analysis (WGCNA) revealed that there were great differences in the activities of transcription and post-transcriptional regulation between pistils and leaves, and AS had an impact on many physiological and biochemical processes in L. chinense, such as photosynthesis, sphingolipid metabolism, fatty acid biosynthesis and metabolism. Moreover, our analysis showed that the features of genes may affect AS, as AS genes and non-AS genes had differences in the exon/intron length, transcript length, and number of exons/introns. In addition, the structure of AS genes may impact the frequencies and types of AS because AS genes with more exons or introns tended to exhibit more AS events, and shorter introns tended to be retained, whereas shorter exons tended to be skipped. Furthermore, eight AS genes were verified, and the results were consistent with our analysis. Overall, this study reveals that AS and gene interaction are mutual-on one hand, AS can affect gene expression and translation, while on the other hand, the structural characteristics of the gene can also affect AS. This work is the first to comprehensively report on AS in L. chinense, and it can provide a reference for further research on AS in L. chinense.

HnRNP F/H associate with hTERC and telomerase holoenzyme to modulate telomerase function and promote cell proliferation

Human telomerase RNA component hTERC comprises multiple motifs that contribute to hTERC biogenesis, holoenzyme activity, and enzyme recruitment to telomeres. hTERC contains several guanine tracts (G-tracts) at its 5′-end, but its associated proteins and potential roles in telomerase function are still poorly understood. The heterogeneous nuclear ribonucleoproteins F, H1, and H2 (hnRNP F/H) are splicing factors that preferentially bind to poly(G)-rich sequences RNA. Here, we demonstrate that hnRNP F/H associate with both hTERC and telomerase holoenzyme to regulate telomerase activity. We reveal hnRNP F/H bind to the 5′-end region of hTERC in vitro and in vivo, and identify the first three G-tracts of hTERC and qRRM1 domain of hnRNP F/H are required for their interaction. Furthermore, hnRNP F/H also directly interact with telomerase holoenzyme. Functionally, we show that hnRNP F/H plays important roles in modulating telomerase activity and telomere length. Moreover, hnRNP F/H deletion greatly impair cancer and stem cell proliferation, and induce stem cell senescence, while hnRNP F/H overexpression delay stem cell senescence. Collectively, our findings unveil a novel role of hnRNP F/H as the binding partners of hTERC and telomerase holoenzyme to regulate telomerase function.

C9-ALS/FTD-linked proline-arginine dipeptide repeat protein associates with paraspeckle components and increases paraspeckle formation

A GGGGCC hexanucleotide repeat expansion in the C9ORF72 gene has been identified as the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. The repeat expansion undergoes unconventional translation to produce five dipeptide repeat proteins (DPRs). Although DPRs are thought to be neurotoxic, the molecular mechanism underlying the DPR-caused neurotoxicity has not been fully elucidated. The current study shows that poly-proline-arginine (poly-PR), the most toxic DPR in vitro, binds to and up-regulates nuclear paraspeckle assembly transcript 1 (NEAT1) that plays an essential role as a scaffold non-coding RNA during the paraspeckle formation. The CRISPR-assisted up-regulation of endogenous NEAT1 causes neurotoxicity. We also show that the poly-PR modulates the function of several paraspeckle-localizing heterogeneous nuclear ribonucleoproteins. Furthermore, dysregulated expression of TAR DNA-binding protein 43 (TDP-43) up-regulates NEAT1 expression and induces neurotoxicity. These results suggest that the increase in the paraspeckle formation may be involved in the poly-PR- and TDP-43-mediated neurotoxicity.

Repeat-associated RNA structure and aberrant splicing

Over 30 hereditary disorders attributed to the expansion of microsatellite repeats have been identified. Despite variant nucleotide content, number of consecutive repeats, and different locations in the genome, many of these diseases have pathogenic RNA gain-of-function mechanisms. The repeat-containing RNAs can form structures in vitro predicted to contribute to the disease through assembly of intracellular RNA aggregates termed foci. The expanded repeat RNAs within these foci sequester RNA binding proteins (RBPs) with important roles in the regulation of RNA metabolism, most notably alternative splicing (AS). These deleterious interactions lead to downstream alterations in transcriptome-wide AS directly linked with disease symptoms. This review summarizes existing knowledge about the association between the repeat RNA structures and RBPs as well as the resulting aberrant AS patterns, specifically in the context of myotonic dystrophy. The connection between toxic, structured RNAs and dysregulation of AS in other repeat expansion diseases is also discussed. This article is part of a Special Issue entitled: RNA structure and splicing regulation edited by Francisco Baralle, Ravindra Singh and Stefan Stamm.

PCBP1 inhibits the expression of oncogenic STAT3 isoform by targeting alternative splicing of STAT3 exon 23

STAT3 plays very important roles in the initiation and development of tumors. Despite of extensive studies in repressing its activation and function via multiple ways, so far, there are few effective therapeutic methods to inhibit STAT3 in the clinic. STAT3 has two isoforms generated by alternative splicing of exon 23. STAT3α is the longer isoform and encodes the full-length oncogenic STAT3α protein. STAT3β is shorter and encodes the truncated and tumor-suppressive STAT3β protein. It remains unknown how the alternative splicing of STAT3 exon 23 is regulated. Here, we discovered that there is an exonic splicing suppressor (ESS) in exon 23. Importantly, splicing factor PCBP1 binds to this ESS. Overexpression of PCBP1 significantly reduced the proportion of STAT3α /STAT3β isoforms and the expression of STAT3α protein. Moreover, increased PCBP1 inhibited the growth of oral squamous cell carcinoma and breast cancer cells, and the expression of STAT3 target genes. Our results demonstrated that PCBP1 is the key splicing factor that promotes the switch from oncogenic isoform STAT3α to tumor-suppressive isoform STAT3β. Our results pave the way for finding new anti-STAT3 methods for cancer treatment.

Predicting gene structure changes resulting from genetic variants via exon definition features

Motivation

Genetic variation that disrupts gene function by altering gene splicing between individuals can substantially influence traits and disease. In those cases, accurately predicting the effects of genetic variation on splicing can be highly valuable for investigating the mechanisms underlying those traits and diseases. While methods have been developed to generate high quality computational predictions of gene structures in reference genomes, the same methods perform poorly when used to predict the potentially deleterious effects of genetic changes that alter gene splicing between individuals. Underlying that discrepancy in predictive ability are the common assumptions by reference gene finding algorithms that genes are conserved, well-formed and produce functional proteins.

Results

We describe a probabilistic approach for predicting recent changes to gene structure that may or may not conserve function. The model is applicable to both coding and non-coding genes, and can be trained on existing gene annotations without requiring curated examples of aberrant splicing. We apply this model to the problem of predicting altered splicing patterns in the genomes of individual humans, and we demonstrate that performing gene-structure prediction without relying on conserved coding features is feasible. The model predicts an unexpected abundance of variants that create de novo splice sites, an observation supported by both simulations and empirical data from RNA-seq experiments. While these de novo splice variants are commonly misinterpreted by other tools as coding or non-coding variants of little or no effect, we find that in some cases they can have large effects on splicing activity and protein products and we propose that they may commonly act as cryptic factors in disease.

Availability and implementation

The software is available from geneprediction.org/SGRF.

Supplementary information

Supplementary information is available at Bioinformatics online.

Heterogeneous ribonucleoprotein F regulates YAP expression via a G-tract in 3'UTR

The Yes-associated protein (YAP) is a transcription coactivator that plays crucial roles in organ size control and tumorigenesis, and was demonstrated to be inhibited by the Hippo signaling pathway. To date, the molecular mechanisms regulating the expression of YAP in human cells remain unknown. In the present study, we found that hnRNP F and hnRNP U negatively regulate YAP expression. We also showed that downregulation of YAP expression by hnRNP F and hnRNP U was not at the transcriptional level. Knockdown of hnRNP F or hnRNP U increased YAP mRNA stability, suggesting the downregulation of YAP expression was by a post-transcriptional mechanism. A putative hnRNP F binding site was identified in the YAP 3'UTR at 685 to 698, and deletion of this putative hnRNP F element abolished the down-regulation effect of YAP mRNA stability by hnRNP F. Binding of the hnRNP F to the YAP 3'UTR was demonstrated by Cross-linked RNA Immunoprecipitation. mRNA stability is a possible secondary effect of alternative splicing or other nuclear process. Understanding the regulation of YAP expression would provide insights into the mechanisms underlying the maintenance of tissue size homeostasis and tumorigenesis.

Alternative-splicing defects in cancer: Splicing regulators and their downstream targets, guiding the way to novel cancer therapeutics

Defects in alternative splicing are frequently found in human tumors and result either from mutations in splicing-regulatory elements of specific cancer genes or from changes in the regulatory splicing machinery. RNA splicing regulators have emerged as a new class of oncoproteins and tumor suppressors, and contribute to disease progression by modulating RNA isoforms involved in the hallmark cancer pathways. Thus, dysregulation of alternative RNA splicing is fundamental to cancer and provides a potentially rich source of novel therapeutic targets. Here, we review the alterations in splicing regulatory factors detected in human tumors, as well as the resulting alternatively spliced isoforms that impact cancer hallmarks, and discuss how they contribute to disease pathogenesis. RNA splicing is a highly regulated process and, as such, the regulators are themselves tightly regulated. Differential transcriptional and posttranscriptional regulation of splicing factors modulates their levels and activities in tumor cells. Furthermore, the composition of the tumor microenvironment can also influence which isoforms are expressed in a given cell type and impact drug responses. Finally, we summarize current efforts in targeting alternative splicing, including global splicing inhibition using small molecules blocking the spliceosome or splicing-factor-modifying enzymes, as well as splice-switching RNA-based therapeutics to modulate cancer-specific splicing isoforms. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing.

FOXP3 interacts with hnRNPF to modulate pre-mRNA alternative splicing

FOXP3 promotes the development and function of regulatory T cells mainly through regulating the transcription of target genes. RNA alternative splicing has been implicated in a wide range of physiological and pathophysiological processes. We report here that FOXP3 associates with heterogeneous nuclear ribonucleoprotein (hnRNP) F through the exon 2-encoded region of FOXP3 and the second quasi-RNA recognition motif (qRRM) of hnRNPF. FOXP3 represses the ability of hnRNPF to bind to its target pre-mRNA and thus modulates RNA alternative splicing. Furthermore, overexpression of mouse hnRNPF in in vitro-differentiated regulatory T cells (Tregs) reduced their suppressive function. Thus, our studies identify a novel mechanism by which FOXP3 regulates mRNA alternative splicing to modulate the function of regulatory T cells.

Human IL12RB1 expression is allele-biased and produces a novel IL12 response regulator

Human IL12RB1 is an autosomal gene that is essential for mycobacterial disease resistance and T cell differentiation. Using primary human tissue and PBMCs, we demonstrate that lung and T cell IL12RB1 expression is allele-biased, and the extent to which cells express one IL12RB1 allele is unaffected by activation. Furthermore, following its expression the IL12RB1 pre-mRNA is processed into either IL12RB1 Isoform 1 (IL12Rβ1, a positive regulator of IL12-responsiveness) or IL12RB1 Isoform 2 (a protein of heretofore unknown function). T cells’ choice to process pre-mRNA into Isoform 1 or Isoform 2 is controlled by intragenic competition of IL12RB1 exon 9-10 splicing with IL12RB1 exon 9b splicing, as well as an IL12RB1 exon 9b-associated polyadenylation site. Heterogeneous nuclear ribonucleoprotein H (hnRNP H) binds near the regulated polyadenylation site, but is not required for exon 9b polyadenylation. Finally, microRNA-mediated knockdown experiments demonstrated that IL12RB1 Isoform 2 promotes T cell IL12 responses. Collectively, our data support a model wherein tissue expression of human IL12RB1 is allele-biased and produces an hnRNP H bound pre-mRNA, the processing of which generates a novel IL12 response regulator.

Emerging roles of RNA-binding proteins in diabetes and their therapeutic potential in diabetic complications

Diabetes is a debilitating health care problem affecting 422 million people around the world. Diabetic patients suffer from multisystemic complications that can cause mortality and morbidity. Recent advancements in high-throughput next-generation RNA-sequencing and computational algorithms led to the discovery of aberrant posttranscriptional gene regulatory programs in diabetes. However, very little is known about how these regulatory programs are mis-regulated in diabetes. RNA-binding proteins (RBPs) are important regulators of posttranscriptional RNA networks, which are also dysregulated in diabetes. Human genetic studies provide new evidence that polymorphisms and mutations in RBPs are linked to diabetes. Therefore, we will discuss the emerging roles of RBPs in abnormal posttranscriptional gene expression in diabetes. Questions that will be addressed are: Which posttranscriptional mechanisms are disrupted in diabetes? Which RBPs are responsible for such changes under diabetic conditions? How are RBPs altered in diabetes? How does dysregulation of RBPs contribute to diabetes? Can we target RBPs using RNA-based methods to restore gene expression profiles in diabetic patients? Studying the evolving roles of RBPs in diabetes is critical not only for a comprehensive understanding of diabetes pathogenesis but also to design RNA-based therapeutic approaches for diabetic complications. WIREs RNA 2018, 9:e1459. doi: 10.1002/wrna.1459 This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing Translation > Translation Regulation.

The splicing code

This issue dedicated to the code of life tackles very challenging and open questions in Biology. The genetic code, brilliantly uncovered over 50 years ago is an example of a univocal biological code. In fact, except for very few and marginal variations, it is the same from bacteria to man, the RNA stretch: 5' GUGUUC 3' reads as the dipeptide: Val-Phe in bacteria, in yeast, in Arabidopsis, in zebra fish, in mouse and in human. A degree of ambiguity is possible if mutations are introduced in the tRNAs in a way that the anticodon reads one amino acid but the aminoacyl-transferase attaches a different one onto the tRNA. These were the very useful suppressor genes that aided greatly the study of bacterial genetics. Other biological codes however, are more akin to social codes and are less amenable to an unambiguous deciphering. Legal and ethical codes, weather we like it or not, are flexible and depend on the structure and history of the society that has produced them, as well as a specific point in time. The codes that govern RNA splicing have similar characteristics. In fact, the splicing code depends on a myriad of different factors that in part are influenced by the background in which they are read such as different cells, tissues or developmental stages. Given the complexity of the splicing process, the construction of an algorithm that can define exons or their fate with certainty has not yet been achieved. However a substantial amount of information towards the deciphering of the splicing code has been gathered and in this manuscript we summarize the point reached.

Competitive regulation of alternative splicing and alternative polyadenylation by hnRNP H and CstF64 determines acetylcholinesterase isoforms

Acetylcholinesterase (AChE), encoded by the ACHE gene, hydrolyzes the neurotransmitter acetylcholine to terminate synaptic transmission. Alternative splicing close to the 3΄ end generates three distinct isoforms of AChE_T, AChE_H and AChE_R. We found that hnRNP H binds to two specific G-runs in exon 5a of human ACHE and activates the distal alternative 3΄ splice site (ss) between exons 5a and 5b to generate AChE_T. Specific effect of hnRNP H was corroborated by siRNA-mediated knockdown and artificial tethering of hnRNP H. Furthermore, hnRNP H competes for binding of CstF64 to the overlapping binding sites in exon 5a, and suppresses the selection of a cryptic polyadenylation site (PAS), which additionally ensures transcription of the distal 3΄ ss required for the generation of AChE_T. Expression levels of hnRNP H were positively correlated with the proportions of the AChE_T isoform in three different cell lines. HnRNP H thus critically generates AChE_T by enhancing the distal 3΄ ss and by suppressing the cryptic PAS. Global analysis of CLIP-seq and RNA-seq also revealed that hnRNP H competitively regulates alternative 3΄ ss and alternative PAS in other genes. We propose that hnRNP H is an essential factor that competitively regulates alternative splicing and alternative polyadenylation.

Novel Rbfox2 isoforms associated with alternative exon usage in rat cortex and suprachiasmatic nucleus

Transcriptome diversity in adult neurons is partly mediated by RNA binding proteins (RBPs), including the RBFOX factors. RBFOX3/NeuN, a neuronal maturity marker, is strangely depleted in suprachiasmatic nucleus (SCN) neurons, and may be compensated by a change in Rbfox2 expression. In this study, we found no superficial changes in Rbfox2 expression in the SCN, but mRNA population analysis revealed a distinct SCN transcript profile that includes multiple novel Rbfox2 isoforms. Of eleven isoforms in SCN and cerebral cortex that exhibit exon variation across two protein domains, we found a 3-fold higher abundance of a novel (‘−12–40’) C-terminal domain (CTD)-variant in the SCN. This isoform embraces an alternative reading frame that imparts a 50% change in CTD protein sequence, and functional impairment of exon 7 exclusion activity in a RBFOX2-target, the L-type calcium channel gene, Cacna1c. We have also demonstrated functional correlates in SCN gene transcripts; inclusion of Cacna1c exon 7, and also exclusion of both NMDA receptor gene Grin1 exon 4, and Enah exon 12, all consistent with a change in SCN RBFOX activity. The demonstrated regional diversity of Rbfox2 in adult brain highlights the functional adaptability of this RBP, enabling neuronal specialization, and potentially responding to disease-related neuronal dysfunction.

Regulation of splicing and circularisation of RNA in epithelial mesenchymal plasticity

Interconversions between epithelial and mesenchymal states, often referred to as epithelial mesenchymal transition (EMT) and its reverse MET, play important roles in embryonic development and are recapitulated in various adult pathologies including cancer progression. These conversions are regulated by complex transcriptional and post-transcriptional mechanisms including programs of alternative splicing which are orchestrated by specific splicing factors. This review will focus on the latest developments in our understanding of the splicing factors regulating epithelial mesenchymal plasticity associated with cancer progression and the induction of pluripotency, including potential roles for circular RNAs (circRNAs) which have been recently implicated in these processes.

A summary of relationships between alternative splicing and breast cancer

Alternative splicing (AS) is the process of combinatorial rearrangement of parts of exons, and/or parts of introns into mature RNA to result in a multitude of transcripts. AS is a biological process through which organisms produce as many protein variants as possible by a limited genetic resource. It plays an important role in growth and development of the organisms. Over the past few years, alternative splicing has been discovered to be critical for genesis and development of malignant tumors, including breast cancer. If the relationships between AS and breast cancer can be discussed more deeply, it will be helpful for better diagnosis, judging prognosis and intervening with breast cancer. In this paper, the relationships between AS and breast cancer are elaborated from different angles, in hope that this summary is beneficial for readers to understand the roles of AS and breast cancer.

EMT and stemness: flexible processes tuned by alternative splicing in development and cancer progression

Epithelial-to-mesenchymal transition (EMT) is associated with metastasis formation as well as with generation and maintenance of cancer stem cells. In this way, EMT contributes to tumor invasion, heterogeneity and chemoresistance. Morphological and functional changes involved in these processes require robust reprogramming of gene expression, which is only partially accomplished at the transcriptional level. Alternative splicing is another essential layer of gene expression regulation that expands the cell proteome. This step in post-transcriptional regulation of gene expression tightly controls cell identity between epithelial and mesenchymal states and during stem cell differentiation. Importantly, dysregulation of splicing factor function and cancer-specific splicing isoform expression frequently occurs in human tumors, suggesting the importance of alternative splicing regulation for cancer biology.

In this review, we briefly discuss the role of EMT programs in development, stem cell differentiation and cancer progression. Next, we focus on selected examples of key factors involved in EMT and stem cell differentiation that are regulated post-transcriptionally through alternative splicing mechanisms. Lastly, we describe relevant oncogenic splice-variants that directly orchestrate cancer stem cell biology and tumor EMT, which may be envisioned as novel targets for therapeutic intervention.

Rbfox Proteins Regulate Splicing as Part of a Large Multiprotein Complex LASR

Rbfox proteins control alternative splicing and posttranscriptional regulation in mammalian brain and are implicated in neurological disease. These proteins recognize the RNA sequence (U)GCAUG, but their structures and diverse roles imply a variety of protein-protein interactions. We find that nuclear Rbfox proteins are bound within a large assembly of splicing regulators (LASR), a multimeric complex containing the proteins hnRNP M, hnRNP H, hnRNP C, Matrin3, NF110/NFAR-2, NF45, and DDX5, all approximately equimolar to Rbfox. We show that splicing repression mediated by hnRNP M is stimulated by Rbfox. Virtually all the intron-bound Rbfox is associated with LASR, and hnRNP M motifs are enriched adjacent to Rbfox crosslinking sites in vivo. These findings demonstrate that Rbfox proteins bind RNA with a defined set of cofactors and affect a broader set of exons than previously recognized. The function of this multimeric LASR complex has implications for deciphering the regulatory codes controlling splicing networks.

The prevalent deep intronic c. 639+919 G>A GLA mutation causes pseudoexon activation and Fabry disease by abolishing the binding of hnRNPA1 and hnRNP A2/B1 to a splicing silencer

Fabry disease is an X-linked recessive inborn disorder of the glycosphingolipid metabolism, caused by total or partial deficiency of the lysosomal α-galactosidase A enzyme due to mutations in the GLA gene. The prevalent c.639+919 G>A mutation in GLA leads to pathogenic insertion of a 57bp pseudoexon sequence from intron 4, which is responsible for the cardiac variant phenotype. In this study we investigate the splicing regulatory mechanism leading to GLA pseudoexon activation. Splicing analysis of GLA minigenes revealed that pseudoexon activation is influenced by cell-type. We demonstrate that the wild-type sequence harbors an hnRNP A1 and hnRNP A2/B1-binding exonic splicing silencer (ESS) overlapping the 5'splice site (5'ss) that prevents pseudoexon inclusion. The c.639+919 G>A mutation disrupts this ESS allowing U1 snRNP recognition of the 5'ss. We show that the wild-type GLA 5'ss motif with the ESS is also able to inhibit inclusion of an unrelated pseudoexon in the FGB gene, and that also in the FGB context inactivation of the ESS by the c.639+919 G>A mutation causes pseudoexon activation, underscoring the universal nature of the ESS. Finally, we demonstrate that splice switching oligonucleotide (SSO) mediated blocking of the pseudoexon 3'ss and 5'ss effectively restores normal GLA splicing. This indicates that SSO based splicing correction may be a therapeutic alternative in the treatment of Fabry disease.

RNA-binding proteins, neural development and the addictions

Transcriptional and post-transcriptional regulation of gene expression defines the neurobiological mechanisms that bridge genetic and environmental risk factors with neurobehavioral dysfunction underlying the addictions. More than 1000 genes in the eukaryotic genome code for multifunctional RNA-binding proteins (RBPs) that can regulate all levels of RNA biogenesis. More than 50% of these RBPs are expressed in the brain where they regulate alternative splicing, transport, localization, stability and translation of RNAs during development and adulthood. Dysfunction of RBPs can exert global effects on their targetomes that underlie neurodegenerative disorders such as Alzheimer's and Parkinson's diseases as well as neurodevelopmental disorders, including autism and schizophrenia. Here, we consider the evidence that RBPs influence key molecular targets, neurodevelopment, synaptic plasticity and neurobehavioral dysfunction underlying the addictions. Increasingly well-powered genome-wide association studies in humans and mammalian model organisms combined with ever more precise transcriptomic and proteomic approaches will continue to uncover novel and possibly selective roles for RBPs in the addictions. Key challenges include identifying the biological functions of the dynamic RBP targetomes from specific cell types throughout subcellular space (e.g. the nuclear spliceome vs. the synaptic translatome) and time and manipulating RBP programs through post-transcriptional modifications to prevent or reverse aberrant neurodevelopment and plasticity underlying the addictions.

Developmental regulation of RNA processing by Rbfox proteins

The Rbfox genes encode an ancient family of sequence-specific RNA binding proteins (RBPs) that are critical developmental regulators in multiple tissues including skeletal muscle, cardiac muscle, and brain. The hallmark of Rbfox proteins is a single high-affinity RRM domain, highly conserved from insects to humans, that binds preferentially to UGCAUG motifs at diverse regulatory sites in pre-mRNA introns, mRNA 3'UTRs, and pre-miRNAs hairpin structures. Versatile regulatory circuits operate on Rbfox pre-mRNA and mRNA to ensure proper expression of Rbfox1 protein isoforms, which then act on the broader transcriptome to regulate alternative splicing networks, mRNA stability and translation, and microRNA processing. Complex Rbfox expression is encoded in large genes encompassing multiple promoters and alternative splicing options that govern spatiotemporal expression of structurally distinct and tissue-specific protein isoforms with different classes of RNA targets. Nuclear Rbfox1 is a candidate master regulator that binds intronic UGCAUG elements to impact splicing efficiency of target alternative exons, many in transcripts for other splicing regulators. Tissue-specificity of Rbfox-mediated alternative splicing is executed by combinatorial regulation through the integrated activity of Rbfox proteins and synergistic or antagonistic splicing factors. Studies in animal models show that Rbfox1-related genes are critical for diverse developmental processes including germ cell differentiation and memory in Drosophila, neuronal migration and function in mouse brain, myoblast fusion and skeletal muscle function, and normal heart function. Finally, genetic and biochemical evidence suggest that aberrations in Rbfox-regulated circuitry are risk factors for multiple human disorders, especially neurodevelopmental disorders including epilepsy and autism, and cardiac hypertrophy. WIREs RNA 2017, 8:e1398. doi: 10.1002/wrna.1398 For further resources related to this article, please visit the WIREs website.

Protein and gene expression characteristics of heterogeneous nuclear ribonucleoprotein H1 in esophageal squamous cell carcinoma

AIM

To investigate the expression characteristics of heterogeneous nuclear ribonucleoprotein H1 (HNRNPH1) mRNA and protein in cell lines and tissues of esophageal squamous cell carcinoma (ESCC).

METHODS

Western blotting was used to assess the expression of HNRNPH1 protein in seven ESCC cell lines and 30 paired fresh tissue specimens. The subcellular localization of HNRNPH1 was determined by immunofluorescence in ESCC cells. The RNA sequencing data from 87 patients with ESCC were obtained from the cancer genome atlas (TCGA), and the expression and clinical characteristics analysis of different transcript variants of HNRNPH1 were evaluated in this dataset. In addition, immunohistochemistry was carried out to detect the expression of HNRNPH1 protein in 125 patients.

RESULTS

The expression of HNRNPH1 protein varied across different ESCC cell lines. It was exclusively restricted to the nucleus of the ESCC cells. There are two transcript variants of the HNRNPH1 gene. Variant 1 was constitutively expressed, and its expression did not change during tumorigenesis. In contrast, levels of variant 2 were low in non-tumorous tissues and were dramatically increased in ESCC (P = 0.0026). The high levels of variant 2 were associated with poorer differentiated tumors (P = 0.0287). Furthermore, in paired fresh tissue specimens, HNRNPH1 protein was overexpressed in 73.3% (22/30) of neoplastic tissues. HNRNPH1 was significantly upregulated in ESCC, with strong staining in 43.2% (54/125) of tumor tissues and 22.4% (28/125) of matched non-cancerous tissues (P = 0.0005). Positive HNRNPH1 expression was significantly associated with poor tumor differentiation degree (P = 0.0337).

CONCLUSION

The different alternative transcript variants of HNRNPH1 exhibited different expression changes during tumorigenesis. Its mRNA and protein were overexpressed in ESCC and associated with poorer differentiation of tumor cells. These findings highlight the potential of HNRNPH1 in the therapy and diagnosis of ESCC.

Rbfox2 function in RNA metabolism is impaired in hypoplastic left heart syndrome patient hearts

Hypoplastic left heart syndrome (HLHS) is a fatal congenital heart disease in which the left side of the heart is underdeveloped, impairing the systemic circulation. Underdeveloped left ventricle exerts biomechanical stress on the right ventricle that can progress into heart failure. Genome-wide transcriptome changes have been identified at early stages in the right ventricle (RV) of infants with HLHS, although the molecular mechanisms remain unknown. Here, we demonstrate that the RNA binding protein Rbfox2, which is mutated in HLHS patients, is a contributor to transcriptome changes in HLHS patient RVs. Our results indicate that majority of transcripts differentially expressed in HLHS patient hearts have validated Rbfox2 binding sites. We show that Rbfox2 regulates mRNA levels of targets with 3’UTR binding sites contributing to aberrant gene expression in HLHS patients. Strikingly, the Rbfox2 nonsense mutation identified in HLHS patients truncates the protein, impairs its subcellular distribution and adversely affects its function in RNA metabolism. Overall, our findings uncover a novel role for Rbfox2 in controlling transcriptome in HLHS.

High-throughput analyses of hnRNP H1 dissects its multi-functional aspect

hnRNPs are polyvalent RNA binding proteins that have been implicated in a range of regulatory roles including splicing, mRNA decay, translation, and miRNA metabolism. A variety of genome wide studies have taken advantage of methods like CLIP and RIP to identify the targets and binding sites of RNA binding proteins. However, due to the complex nature of RNA-binding proteins, these studies are incomplete without assays that characterize the impact of RBP binding on mRNA target expression. Here we used a suite of high-throughput approaches (RIP-Seq, iCLIP, RNA-Seq and shotgun proteomics) to provide a comprehensive view of hnRNP H1s ensemble of targets and its role in splicing, mRNA decay, and translation. The combination of RIP-Seq and iCLIP allowed us to identify a set of 1,086 high confidence target transcripts. Binding site motif analysis of these targets suggests the TGGG tetramer as a prevalent component of hnRNP H1 binding motif, with particular enrichment around intronic hnRNP H1 sites. Our analysis of the target transcripts and binding sites indicates that hnRNP H1s involvement in splicing is 2-fold: it directly affects a substantial number of splicing events, but also regulates the expression of major components of the splicing machinery and other RBPs with known roles in splicing regulation. The identified mRNA targets displayed function enrichment in MAPK signaling and ubiquitin mediated proteolysis, which might be main routes by which hnRNP H1 promotes tumorigenesis.

FGF10: A multifunctional mesenchymal-epithelial signaling growth factor in development, health, and disease

The FGF family comprises 22 members with diverse functions in development and health. FGF10 specifically activates FGFR2b in a paracrine manner with heparan sulfate as a co-factor. FGF10and FGFR2b are preferentially expressed in the mesenchyme and epithelium, respectively. FGF10 is a mesenchymal signaling molecule in the epithelium. FGF10 knockout mice die shortly after birth due to the complete absence of lungs as well as fore- and hindlimbs. FGF10 is also essential for the development of multiple organs. The phenotypes of Fgf10 knockout mice are very similar to those of FGFR2b knockout mice, indicating that FGF10 acts as a ligand that is specific to FGFR2b in mouse multi-organ development. FGF10 also plays roles in epithelial-mesenchymal transition, the repair of tissue injury, and embryonic stem cell differentiation. In humans, FGF10 loss-of-function mutations result in inherited diseases including aplasia of lacrimal and salivary gland, lacrimo-auriculo-dento-digital syndrome, and chronic obstructive pulmonary disease. FGF10 is also involved in the oncogenicity of pancreatic and breast cancers. Single nucleotide polymorphisms in FGF10 are also potential risk factors for limb deficiencies, cleft lip and palate, and extreme myopia. These findings indicate that FGF10 is a crucial paracrine signal from the mesenchyme to epithelium for development, health, and disease.

The splicing regulators Esrp1 and Esrp2 direct an epithelial splicing program essential for mammalian development

Tissue- and cell-type-specific regulators of alternative splicing (AS) are essential components of posttranscriptional gene regulation, necessary for normal cellular function, patterning, and development. Mice with ablation of Epithelial splicing regulatory protein (Esrp1) develop cleft lip and palate. Loss of both Esrp1 and its paralog Esrp2 results in widespread developmental defects with broad implications to human disease. Deletion of the Esrps in the epidermis revealed their requirement for establishing a proper skin barrier, a primary function of epithelial cells comprising the epidermis. We profiled the global Esrp-mediated splicing regulatory program in epidermis, which revealed large-scale programs of epithelial cell-type-specific splicing required for epithelial cell functions. These mice represent a valuable model for evaluating the essential role for AS in development and function of epithelial cells, which play essential roles in tissue homeostasis in numerous organs, and provide a genetic tool to evaluate important functional properties of epithelial-specific splice variants in vivo.

DOI:http://dx.doi.org/10.7554/eLife.08954.001

Genes are turned into their protein products via two steps. The first, transcription, produces an intermediate RNA molecule or ‘transcript’; the second step, translation, turns the transcript into a protein. Most genes in mammals contain stretches of DNA called exons, which code for protein, interspersed with sequences called introns that do not. Therefore, a transcript must be ‘spliced’ before translation—the introns are removed and the exons joined.

In some genes, certain exons can be optionally included or excluded from a transcript to produce different versions of the same protein that can often have very different functions. This is known as alternative splicing, and is essential for normal development. A large number of regulatory proteins control this process, many of which are only made in specific types of cells or tissues.

Esrp1 and Esrp2 are two proteins that regulate alternative splicing in epithelial cells. These specialized cells form sheets that line most organs in the body and are found in the epidermis, the outermost layer of the skin. Although Esrp1 and Esrp2 have previously been studied in the laboratory using cultured cell lines, their roles have not been investigated in living animals.

Bebee, Park et al. have now examined mice that are unable to produce one or both of these proteins. Mice that only lacked Esrp1 developed a cleft lip and palate. In mice that lacked both proteins, many organs failed to develop correctly and in some cases did not form at all. In the epidermis, the loss of Esrp1 and Esrp2 disrupted the splicing of the transcripts from genes that give epithelial cells many of their specialized characteristics, such as the ability to form sheets of cells with well formed junctions between them. This meant that epidermis that lacked Esrp1 and Esrp2 could not form a proper barrier layer, which is a crucial role of epithelia in skin as well as in other organs.

In future, the mutant mice will be valuable for exploring how alternative splicing affects the development of epithelial cells and their properties.

DOI:http://dx.doi.org/10.7554/eLife.08954.002

Evolution of the Antisense Overlap between Genes for Thyroid Hormone Receptor and Rev-erbα and Characterization of an Exonic G-Rich Element That Regulates Splicing of TRα2 mRNA

The α-thyroid hormone receptor gene (TRα) codes for two functionally distinct proteins: TRα1, the α-thyroid hormone receptor; and TRα2, a non-hormone-binding variant. The final exon of TRα2 mRNA overlaps the 3' end of Rev-erbα mRNA, which encodes another nuclear receptor on the opposite strand of DNA. To understand the evolution of this antisense overlap, we sequenced these genes and mRNAs in the platypus Orthorhynchus anatinus. Despite its strong homology with other mammals, the platypus TRα/Rev-erbα locus lacks elements essential for expression of TRα2. Comparative analysis suggests that alternative splicing of TRα2 mRNA expression evolved in a stepwise fashion before the divergence of eutherian and marsupial mammals. A short G-rich element (G30) located downstream of the alternative 3'splice site of TRα2 mRNA and antisense to the 3'UTR of Rev-erbα plays an important role in regulating TRα2 splicing. G30 is tightly conserved in eutherian mammals, but is absent in marsupials and monotremes. Systematic deletions and substitutions within G30 have dramatically different effects on TRα2 splicing, leading to either its inhibition or its enhancement. Mutations that disrupt one or more clusters of G residues enhance splicing two- to three-fold. These results suggest the G30 sequence can adopt a highly structured conformation, possibly a G-quadruplex, and that it is part of a complex splicing regulatory element which exerts both positive and negative effects on TRα2 expression. Since mutations that strongly enhance splicing in vivo have no effect on splicing in vitro, it is likely that the regulatory role of G30 is mediated through linkage of transcription and splicing.

RBFOX2 protein domains and cellular activities

RBFOX2 (RNA-binding protein, Fox-1 homologue 2)/RBM9 (RNA-binding-motif protein 9)/RTA (repressor of tamoxifen action)/HNRBP2 (hexaribonucleotide-binding protein 2) encodes an RNA-binding protein involved in tissue specific alternative splicing regulation and steroid receptors transcriptional activity. Its ability to regulate specific splicing profiles depending on context has been related to different expression levels of the RBFOX2 protein itself and that of other splicing regulatory proteins involved in the shared modulation of specific genes splicing. However, this cannot be the sole explanation as to why RBFOX2 plays a widespread role in numerous cellular mechanisms from development to cell survival dependent on cell/tissue type. RBFOX2 isoforms with altered protein domains exist. In the present article, we describe the main RBFOX2 protein domains, their importance in the context of splicing and transcriptional regulation and we propose that RBFOX2 isoform distribution may play a fundamental role in RBFOX2-specific cellular effects.

Deregulation of splicing factors and breast cancer development

It is well known that many genes implicated in the development and progression of breast cancer undergo aberrant alternative splicing events to produce proteins with pro-cancer properties. These changes in alternative splicing can arise from mutations or single-nucleotide polymorphisms (SNPs) within the DNA sequences of cancer-related genes, which can strongly affect the activity of splicing factors and influence the splice site choice. However, it is important to note that absence of mutations is not sufficient to prevent misleading choices in splice site selection. There is now increasing evidence to demonstrate that the expression profile of ten splicing factors (including SRs and hnRNPs) and eight RNA-binding proteins changes in breast cancer cells compared with normal cells. These modifications strongly influence the alternative splicing pattern of many cancer-related genes despite the absence of any detrimental mutations within their DNA sequences. Thus, a comprehensive assessment of the splicing factor status in breast cancer is important to provide insights into the mechanisms that lead to breast cancer development and metastasis. Whilst most studies focus on mutations that affect alternative splicing in cancer-related genes, this review focuses on splicing factors and RNA-binding proteins that are themselves deregulated in breast cancer and implicated in cancer-related alternative splicing events.

A heroin addiction severity-associated intronic single nucleotide polymorphism modulates alternative pre-mRNA splicing of the μ opioid receptor gene OPRM1 via hnRNPH interactions

Single nucleotide polymorphisms (SNPs) in the OPRM1 gene have been associated with vulnerability to opioid dependence. The current study identifies an association of an intronic SNP (rs9479757) with the severity of heroin addiction among Han-Chinese male heroin addicts. Individual SNP analysis and haplotype-based analysis with additional SNPs in the OPRM1 locus showed that mild heroin addiction was associated with the AG genotype, whereas severe heroin addiction was associated with the GG genotype. In vitro studies such as electrophoretic mobility shift assay, minigene, siRNA, and antisense morpholino oligonucleotide studies have identified heterogeneous nuclear ribonucleoprotein H (hnRNPH) as the major binding partner for the G-containing SNP site. The G-to-A transition weakens hnRNPH binding and facilitates exon 2 skipping, leading to altered expressions of OPRM1 splice-variant mRNAs and hMOR-1 proteins. Similar changes in splicing and hMOR-1 proteins were observed in human postmortem prefrontal cortex with the AG genotype of this SNP when compared with the GG genotype. Interestingly, the altered splicing led to an increase in hMOR-1 protein levels despite decreased hMOR-1 mRNA levels, which is likely contributed by a concurrent increase in single transmembrane domain variants that have a chaperone-like function on MOR-1 protein stability. Our studies delineate the role of this SNP as a modifier of OPRM1 alternative splicing via hnRNPH interactions, and suggest a functional link between an SNP-containing splicing modifier and the severity of heroin addiction.

Context-dependent control of alternative splicing by RNA-binding proteins

Sequence-specific RNA-binding proteins (RBPs) bind to pre-mRNA to control alternative splicing, but it is not yet possible to read the 'splicing code' that dictates splicing regulation on the basis of genome sequence. Each alternative splicing event is controlled by multiple RBPs, the combined action of which creates a distribution of alternatively spliced products in a given cell type. As each cell type expresses a distinct array of RBPs, the interpretation of regulatory information on a given RNA target is exceedingly dependent on the cell type. RBPs also control each other's functions at many levels, including by mutual modulation of their binding activities on specific regulatory RNA elements. In this Review, we describe some of the emerging rules that govern the highly context-dependent and combinatorial nature of alternative splicing regulation.

A targeted oligonucleotide enhancer of SMN2 exon 7 splicing forms competing quadruplex and protein complexes in functional conditions

The use of oligonucleotides to activate the splicing of selected exons is limited by a poor understanding of the mechanisms affected. A targeted bifunctional oligonucleotide enhancer of splicing (TOES) anneals to SMN2 exon 7 and carries an exonic splicing enhancer (ESE) sequence. We show that it stimulates splicing specifically of intron 6 in the presence of repressing sequences in intron 7. Complementarity to the 5' end of exon 7 increases U2AF65 binding, but the ESE sequence is required for efficient recruitment of U2 snRNP. The ESE forms at least three coexisting discrete states: a quadruplex, a complex containing only hnRNP F/H, and a complex enriched in the activator SRSF1. Neither hnRNP H nor quadruplex formation contributes to ESE activity. The results suggest that splicing limited by weak signals can be rescued by rapid exchange of TOES oligonucleotides in various complexes and raise the possibility that SR proteins associate transiently with ESEs.