Alternative polyadenylation dependent function of splicing factor SRSF3 contributes to cellular senescence

SLC30A4-AS1 Mediates the Senescence of Periodontal Ligament Stem Cells in Inflammatory Environments via the Alternative Splicing of TP53BP1

Periodontal ligament stem cells (PDLSCs) are key cells that suppress periodontal damage during both the progression and recovery stages of periodontitis. Although substantial evidence has demonstrated that incubation under an inflammatory condition may accelerate senescence of PDLSCs, whether cellular senescence in response to inflammatory incubation contributes to cell dysfunction remain unexplored. In this study, we first observed inflammation-caused PDLSC senescence in periodontitis based on comparisons of matched patients, and this cellular senescence was demonstrated in healthy cells that were subjected to inflammatory conditions. We subsequently designed further experiments to investigate the possible mechanism underlying inflammation-induced PDLSC senescence with a particular focus on the role of long noncoding RNAs (lncRNAs). LncRNA microarray analysis and functional gain/loss studies revealed SLC30A4-AS1 as a regulator of inflammation-mediated PDLSC senescence. By full-length transcriptome sequencing, we found that SLC30A4-AS1 interacted with SRSF3 to affect the alternative splicing (AS) of TP53BP1 and alter the expression of TP53BP1-204. Further functional studies showed that decreased expression of TP53BP1-204 reversed PDLSC senescence, and SLC30A4-AS1 overexpression-induced PDLSC senescence was abolished by TP53BP1-204 knockdown. Our data suggest for the first time that SLC30A4-AS1 plays a key role in regulating PDLSC senescence in inflammatory environments by modulating the AS of TP53BP1.

Analysis of post-transcriptional regulatory signatures and immune cell subsets in premature ovarian insufficiency based on full-length transcriptome

Premature ovarian insufficiency (POI) is a reproductive endocrine disorder characterized by infertility and the perimenopausal syndrome. Its genetic etiology is highly heterogeneous and not yet fully understood. Limited by short-read sequencing, the profile and structural variation of the full-length transcript for POI have remained elusive. Therefore, this study included peripheral blood samples from 5 POI patients and 5 controls, characterizing full-length transcripts of POI using Oxford Nanopore sequencing firstly. Ultimately, we identified 26,122 transcripts, including 7,724 novel gene loci and 13,593 novel transcripts. A total of 382 differentially expressed transcripts were identified, including 366 down-regulated and 16 up-regulated transcripts. Based on transcript structure variant analysis, 8,834 alternative splicing events, 65,254 alternative polyadenylation sites and 32 motifs were further identified, revealing the diversity sources of transcript isoforms, proteins and genetic complexity. Enrichment analysis of differentially AS genes suggested that the ferroptosis pathway may play an important role in the pathogenesis of POI.Additionally, 494 high-confidence lncRNAs, 1,768 transcription factors, and novel gene-coding regions were predicted based on full-length transcript sequence. Analysis of immune cell subtypes revealed the expression of CD8 + T cells and monocytes were down-regulated in POI, which was significantly positively correlated with AMH, suggesting that CD8 + T cells and monocytes could serve as potential diagnostic markers and immunotherapy targets for POI. Conclusively, this study provides new perspectives on the pathogenesis, post-transcriptional regulation mechanisms, and immune targets of POI.

Coupling of alternative splicing and alternative polyadenylation

RNA splicing and 3'-cleavage and polyadenylation (CPA) are essential processes for the maturation of RNA. There have been extensive independent studies of these regulated processing events, including alternative splicing (AS) and alternative polyadenylation (APA). However, growing evidence suggests potential crosstalk between splicing and 3'-end processing in regulating AS or APA. Here, we first provide a brief overview of the molecular machines involved in splicing and 3'-end processing events, and then review recent studies on the functions and mechanisms of the crosstalk between the two processes. On the one hand, 3'-end processing can affect splicing, as 3'-end processing factors and CPA-generated polyA tail promote the splicing of the last intron. Beyond that, 3'-end processing factors can also influence the splicing of internal and terminal exons. Those 3'-end processing factors can also interact with different RNA-binding proteins (RBPs) to exert their effects on AS. The length of 3' untranslated region (3' UTR) can affect the splicing of upstream exons. On the other hand, splicing and CPA may compete within introns in generating different products. Furthermore, splicing within the 3' UTR is a significant factor contributing to 3' UTR diversity. Splicing also influences 3'-end processing through the actions of certain splicing factors. Interestingly, some classical RBPs play dual roles in both splicing and 3'-end processing. Finally, we discuss how long-read sequencing technologies aid in understanding the coordination of AS-APA events and envision that these findings may potentially promote the development of new strategies for disease diagnosis and treatment.

Nanoparticle-Protein Corona-Based Tissue Proteomics for the Aging Mouse Proteome Atlas

Highly abundant proteins present in biological fluids and tissues significantly interfere with low-abundance protein identification by mass spectrometry (MS), limiting proteomic depth and hindering protein biomarker discovery. Herein, to enhance the coverage of tissue proteomics, we developed a nanoparticle-protein corona (NP-PC)-based method for the aging mouse proteome atlas. Based on this method, we investigated the complexity of life process of 5 major organs, including the heart, liver, spleen, lungs, and kidneys, from 4 groups of mice at different ages. Compared with the conventional strategy, NP-PC-based proteomics significantly increased the number of identified protein groups in the heart (from 3007 to 3927; increase of 30.6%), liver (from 2982 to 4610; increase of 54.6%), spleen (from 5047 to 7351; increase of 45.7%), lungs (from 4984 to 6903; increase of 38.5%), and kidneys (from 3550 to 5739; increase of 61.7%), and we identified a total of 10 104 protein groups. The overall data indicated that 3-week-old mice showed more differences compared with the other three age groups. The proteins of amino acid-related metabolism were increased in aged mice compared with those in the 3-week-old mice. Protein-related infections were increased in the spleen of the aged mice. Interestingly, the spliceosome-related pathway significantly changed from youth to elders in the liver, spleen, and lungs, indicating the vital role of the spliceosome during the aging process. Our established aging mouse organ proteome atlas provides comprehensive insights into understanding the aging process, and it may help in prevention and treatment of age-related diseases.

Protein interaction networks in the vasculature prioritize genes and pathways underlying coronary artery disease

Population-based association studies have identified many genetic risk loci for coronary artery disease (CAD), but it is often unclear how genes within these loci are linked to CAD. Here, we perform interaction proteomics for 11 CAD-risk genes to map their protein-protein interactions (PPIs) in human vascular cells and elucidate their roles in CAD. The resulting PPI networks contain interactions that are outside of known biology in the vasculature and are enriched for genes involved in immunity-related and arterial-wall-specific mechanisms. Several PPI networks derived from smooth muscle cells are significantly enriched for genetic variants associated with CAD and related vascular phenotypes. Furthermore, the networks identify 61 genes that are found in genetic loci associated with risk of CAD, prioritizing them as the causal candidates within these loci. These findings indicate that the PPI networks we have generated are a rich resource for guiding future research into the molecular pathogenesis of CAD.

The rosetta stone of successful ageing: does oral health have a role?

Ageing is an inevitable aspect of life and thus successful ageing is an important focus of recent scientific efforts. The biological process of ageing is mediated through the interaction of genes with environmental factors, increasing the body's susceptibility to insults. Elucidating this process will increase our ability to prevent and treat age-related disease and consequently extend life expectancy. Notably, centenarians offer a unique perspective on the phenomenon of ageing. Current research highlights several age-associated alterations on the genetic, epigenetic and proteomic level. Consequently, nutrient sensing and mitochondrial function are altered, resulting in inflammation and exhaustion of regenerative ability.Oral health, an important contributor to overall health, remains underexplored in the context of extreme longevity. Good masticatory function ensures sufficient nutrient uptake, reducing morbidity and mortality in old age. The relationship between periodontal disease and systemic inflammatory pathologies is well established. Diabetes, rheumatoid arthritis and cardiovascular disease are among the most significant disease burdens influenced by inflammatory oral health conditions. Evidence suggests that the interaction is bi-directional, impacting progression, severity and mortality. Current models of ageing and longevity neglect an important factor in overall health and well-being, a gap that this review intends to illustrate and inspire avenues for future research.

Downregulation of apoptotic repressor AVEN exacerbates cardiac injury after myocardial infarction

Myocardial infarction (MI) is a leading cause of heart failure (HF), associated with morbidity and mortality worldwide. As an essential part of gene expression regulation, the role of alternative polyadenylation (APA) in post-MI HF remains elusive. Here, we revealed a global, APA-mediated, 3' untranslated region (3' UTR)-lengthening pattern in both human and murine post-MI HF samples. Furthermore, the 3' UTR of apoptotic repressor gene, AVEN, is lengthened after MI, contributing to its downregulation. AVEN knockdown increased cardiomyocyte apoptosis, whereas restoration of AVEN expression substantially improved cardiac function. Mechanistically, AVEN 3' UTR lengthening provides additional binding sites for miR-30b-5p and miR-30c-5p, thus reducing AVEN expression. Additionally, PABPN1 (poly(A)-binding protein 1) was identified as a potential regulator of AVEN 3' UTR lengthening after MI. Altogether, our findings revealed APA as a unique mechanism regulating cardiac injury in response to MI and also indicated that the APA-regulated gene, AVEN, holds great potential as a critical therapeutic target for treating post-MI HF.

Role of the long non-coding RNAs in regulation of Gemcitabine response in tumor cells

Chemotherapy is widely used as one of the first line therapeutic methods in cancer patients. However, chemotherapeutic resistance is one of the most common problems in cancer patients, which leads to the therapeutic failure and tumor relapse. Considering the side effects of chemotherapy drugs in normal tissues, it is required to investigate the molecular mechanisms involved in drug resistance to improve the therapeutic strategies in cancer patients. Long non-coding RNAs (lncRNAs) have pivotal roles in regulation of cellular processes associated with drug resistance. LncRNAs deregulations have been frequently reported in a wide range of chemo-resistant tumors. Gemcitabine (GEM) as a nucleoside analog has a wide therapeutic application in different cancers. However, GEM resistance is considered as a therapeutic challenge. Considering the role of lncRNAs in the occurrence of GEM resistance, in the present review we discussed the molecular mechanisms of lncRNAs in regulation of GEM response among cancer patients. It has been reported that lncRNAs have mainly an oncogenic role as the inducers of GEM resistance through direct or indirect regulation of transcription factors, autophagy, polycomb complex, and signaling pathways such as PI3K/AKT, MAPK, WNT, JAK/STAT, and TGF-β. This review paves the way to present the lncRNAs as non-invasive markers to predict GEM response in cancer patients. Therefore, lncRNAs can be introduced as the efficient markers to reduce the possible chemotherapeutic side effects in GEM resistant cancer patients and define a suitable therapeutic strategy among these patients.

Oncogenic SRSF3 in health and diseases

Serine/arginine rich splicing factor 3 (SRSF3) is an important multi-functional splicing factor, and has attracted increasing attentions in the past thirty years. The importance of SRSF3 is evidenced by its impressively conserved protein sequences in all animals and alternative exon 4 which represents an autoregulatory mechanism to maintain its proper cellular expression level. New functions of SRSF3 have been continuously discovered recently, especially its oncogenic function. SRSF3 plays essential roles in many cellular processes by regulating almost all aspects of RNA biogenesis and processing of many target genes, and thus, contributes to tumorigenesis when overexpressed or disregulated. This review updates and highlights the gene, mRNA, and protein structure of SRSF3, the regulatory mechanisms of SRSF3 expression, and the characteristics of SRSF3 targets and binding sequences that contribute to SRSF3's diverse molecular and cellular functions in tumorigenesis and human diseases.

CAFuncAPA: a knowledgebase for systematic functional annotations of APA events in human cancers

Alternative polyadenylation (APA) is a widespread posttranscriptional regulation process. APA generates diverse mRNA isoforms with different 3' UTR lengths, affecting mRNA expression, miRNA binding regulation and alternative splicing events. Previous studies have demonstrated the important roles of APA in tumorigenesis and cancer progression through diverse aspects. Thus, a comprehensive functional landscape of diverse APA events would aid in a better understanding of the underlying mechanisms related to APA in human cancers. Here, we built CAFuncAPA (https://relab.xidian.edu.cn/CAFuncAPA/) to systematically annotate the functions of 15478 APA events in human pan-cancers. Specifically, we first identified APA events associated with cancer survival and tumor progression. We annotated the potential downstream effects of APA on genes/isoforms expression, regulation of miRNAs, RNA binding proteins (RBPs) and alternative splicing events. Moreover, we also identified up-regulators of APA events, including the effects of genetic variants on poly(A) sites and RBPs, as well as the effect of methylation phenotypes on APA events. These findings suggested that CAFuncAPA can be a helpful resource for a better understanding of APA regulators and potential functions in cancer biology.

Non-canonical functions of spliceosome components in cancer progression

Dysregulation of pre-mRNA splicing is a common hallmark of cancer cells and it is associated with altered expression, localization, and mutations of the components of the splicing machinery. In the last few years, it has been elucidated that spliceosome components can also influence cellular processes in a splicing-independent manner. Here, we analyze open source data to understand the effect of the knockdown of splicing factors in human cells on the expression and splicing of genes relevant to cell proliferation, migration, cell cycle regulation, DNA repair, and cell death. We supplement this information with a comprehensive literature review of non-canonical functions of splicing factors linked to cancer progression. We also specifically discuss the involvement of splicing factors in intercellular communication and known autoregulatory mechanisms in restoring their levels in cells. Finally, we discuss strategies to target components of the spliceosome machinery that are promising for anticancer therapy. Altogether, this review greatly expands understanding of the role of spliceosome proteins in cancer progression.

Integrative analysis of Iso-Seq and RNA-seq reveals dynamic changes of alternative promoter, alternative splicing and alternative polyadenylation during Angiotensin II-induced senescence in rat primary aortic endothelial cells

In eukaryotes, alternative promoter (AP), alternative splicing (AS), and alternative polyadenylation (APA) are three crucial regulatory mechanisms that modulate message RNA (mRNA) diversity. Although AP, AS and APA are involved in diverse biological processess, whether they have dynamic changes in Angiotensin II (Ang II) induced senescence in rat primary aortic endothelial cells (RAECs), an important cellular model for studying cardiovascular disease, remains unclear. Here we integrated both PacBio single-molecule long-read isoform sequencing (Iso-Seq) and Illumina short-read RNA sequencing (RNA-seq) to analyze the changes of AP, AS and APA in Ang II-induced senescent RAECs. Iso-Seq generated 36,278 isoforms from 10,145 gene loci and 65.81% of these isoforms are novel, which were further cross-validated by public data obtained by other techonologies such as CAGE, PolyA-Seq and 3'READS. APA contributed most to novel isoforms, followed by AS and AP. Further investigation showed that AP, AS and APA could all contribute to the regulation of isoform, but AS has more dynamic changes compared to AP and APA upon Ang II stimulation. Genes undergoing AP, AS and APA in Ang II-treated cells are enriched in various pathways related to aging or senescence, suggesting that these molecular changes are involved in functional alterations during Ang II-induced senescence. Together, the present study largely improved the annotation of rat genome and revealed gene expression changes at isoform level, extending the understanding of the complexity of gene regulation in Ang II-treated RAECs, and also provided novel clues for discovering the regulatory mechanism undelying Ang II caused vascular senescence and diseases.

Comprehensive mapping of alternative polyadenylation site usage and its dynamics at single-cell resolution

Alternative polyadenylation (APA) plays an important role in posttranscriptional gene regulation such as transcript stability and translation efficiency. However, our knowledge about APA dynamics at the single-cell level is largely unexplored. Here, we developed single-cell polyadenylation sequencing, a strand-specific approach for sequencing the 3' end of transcripts, to investigate the landscape of APA at the single-cell level. By analyzing several cell lines, we found many genes using multiple polyA sites in bulk data are prone to use only one polyA site in each single cell. Interestingly, cell cycle genes were significantly enriched in genes with high variation in polyA site usages. Furthermore, the 414 genes showing a polyA site usage switch after cell synchronization enriched cell cycle genes, while the differentially expressed genes after cell synchronization did not enrich cell cycle genes. We further identified 812 genes showing polyA site usage changes between neighboring cell cycles, which were grouped into six clusters, with cell phase-specific functional categories enriched in each cluster. Deletion of one polyA site in MSL1 and SCCPDH results in slower and faster cell cycle progression, respectively, supporting polyA site usage switch played an important role in cell cycle. These results indicate that APA is an important layer for cell cycle regulation.

Context-specific regulation and function of mRNA alternative polyadenylation

Alternative cleavage and polyadenylation (APA) is a widespread mechanism to generate mRNA isoforms with alternative 3' untranslated regions (UTRs). The expression of alternative 3' UTR isoforms is highly cell type specific and is further controlled in a gene-specific manner by environmental cues. In this Review, we discuss how the dynamic, fine-grained regulation of APA is accomplished by several mechanisms, including cis-regulatory elements in RNA and DNA and factors that control transcription, pre-mRNA cleavage and post-transcriptional processes. Furthermore, signalling pathways modulate the activity of these factors and integrate APA into gene regulatory programmes. Dysregulation of APA can reprogramme the outcome of signalling pathways and thus can control cellular responses to environmental changes. In addition to the regulation of protein abundance, APA has emerged as a major regulator of mRNA localization and the spatial organization of protein synthesis. This role enables the regulation of protein function through the addition of post-translational modifications or the formation of protein-protein interactions. We further discuss recent transformative advances in single-cell RNA sequencing and CRISPR-Cas technologies, which enable the mapping and functional characterization of alternative 3' UTRs in any biological context. Finally, we discuss new APA-based RNA therapeutics, including compounds that target APA in cancer and therapeutic genome editing of degenerative diseases.

miR-486 improves fibrotic activity in myocardial infarction by targeting SRSF3/p21-Mediated cardiac myofibroblast senescence

The regulation of fibrotic activities is key to improving pathological remodelling post‐myocardial infarction (MI). Currently, in the clinic, safe and curative therapies for cardiac fibrosis and improvement of the pathological fibrotic environment, scar formation and pathological remodelling post‐MI are lacking. Previous studies have shown that miR‐486 is involved in the regulation of fibrosis. However, it is still unclear how miR‐486 functions in post‐MI regeneration. Here, we first demonstrated that miR‐486 targeting SRSF3/p21 mediates the senescence of cardiac myofibroblasts to improve their fibrotic activity, which benefits the regeneration of MI by limiting scar size and post‐MI remodelling. miR‐486‐targeted silencing has high potential as a novel target to improve fibrotic activity, cardiac fibrosis and pathological remodelling.

New hallmarks of ageing: a 2022 Copenhagen ageing meeting summary

Genomic instability, telomere attrition, epigenetic alterations, mitochondrial dysfunction, loss of proteostasis, deregulated nutrient-sensing, cellular senescence, stem cell exhaustion, and altered intercellular communication were the original nine hallmarks of ageing proposed by López-Otín and colleagues in 2013. The proposal of these hallmarks of ageing has been instrumental in guiding and pushing forward research on the biology of ageing. In the nearly past 10 years, our in-depth exploration on ageing research has enabled us to formulate new hallmarks of ageing which are compromised autophagy, microbiome disturbance, altered mechanical properties, splicing dysregulation, and inflammation, among other emerging ones. Amalgamation of the 'old' and 'new' hallmarks of ageing may provide a more comprehensive explanation of ageing and age-related diseases, shedding light on interventional and therapeutic studies to achieve healthy, happy, and productive lives in the elderly.

SRSF3-mediated regulation of N6-methyladenosine modification-related lncRNA ANRIL splicing promotes resistance of pancreatic cancer to gemcitabine

Serine/arginine-rich splicing factor 3 (SRSF3) regulates mRNA alternative splicing of more than 90% of protein-coding genes, providing an essential source for biological versatility. This study finds that SRSF3 expression is associated with drug resistance and poor prognosis in pancreatic cancer. We also find that SRSF3 regulates ANRIL splicing and m6A modification of ANRIL in pancreatic cancer cells. More importantly, we demonstrate that m6A methylation on lncRNA ANRIL is essential for the splicing. Moreover, our results show that SRSF3 promotes gemcitabine resistance by regulating ANRIL's splicing and ANRIL-208 (one of the ANRIL spliceosomes) can enhance DNA homologous recombination repair (HR) capacity by forming a complex with Ring1b and EZH2. In conclusion, this study establishes a link between SRSF3, m6A modification, lncRNA splicing, and DNA HR in pancreatic cancer and demonstrates that abnormal alternative splicing and m6A modification are closely related to chemotherapy resistance in pancreatic cancer.

TDP43 promotes stemness of breast cancer stem cells through CD44 variant splicing isoforms

Alternative splicing (AS) is a promising clinical target for cancer treatment at the post-transcriptional level. We previously identified a unique AS profile in triple-negative breast cancer (TNBC), which is regulated by the splicing regulator TAR DNA-binding protein-43 (TDP43), thus indicating the crucial role of TDP43 in heterogeneous TNBC. Cluster of differentiation 44 (CD44), a widely recognized marker for breast cancer stem cells (BCSCs), is extensively spliced into CD44 variant AS isoforms (CD44v) during the development of breast cancer. At present, however, the regulatory mechanism of CD44v is not fully understood. In the current study, we found that loss of TDP43 inhibits BCSC stemness by reducing the abundance of CD44v. In addition, serine-arginine-rich splicing factor 3 (SRSF3), another splicing factor and partner of TDP43, acts as an upstream regulator of TDP43 to maintain CD44v isoforms and thereafter BCSC stemness. Mechanistically, SRSF3 stabilizes the mRNA of TDP43 by inhibiting nonsense-mediated decay (NMD). These findings illustrate the important role of complicated regulatory networks formed by splicing factors in TNBC progression, thus providing potential therapeutic targets from an AS perspective.

SRSF3 Expression Serves as a Potential Biomarker for Prognostic and Immune Response in Pan-Cancer

Serine-rich splicing factor3 (SRSF3) plays an essential role in cell proliferation and inducing and maintaining of cancers as a proto-oncogene. However, the mechanisms of SRSF3 in pan-cancers are still unknown. In our study, a visualized prognostic landscape of SRSF3 in pan-cancer was investigated and the relationship between SRSF3 expression and immune infiltration was also investigated. The expression pattern and prognostic worth of SRSF3 among pan-cancers were explored through different databases, namely, the TCGA and Kaplan–Meier Plotter. Moreover, the survival analysis including Kaplan-Meier method for evaluating between groups was conducted. Further analyses including the correlation between expression SRSF expression and immune infiltration including tumor mutation burden (TMB), microsatellite instability (MSI) was investigated using Spearman test. In ACC, KIRP and UCEC cancer, upregulated expression of SRSF3 was associated with worse disease-free interval (DFI), representing a mechanism in promoting progression of tumor. Our results showed that SRSF3 expression was positively correlated immune cell infiltration, TMB, MSI in certain cancer types, indicating SRSF3 expression to potential value of therapy response. Additionally, we explored the functional characteristics of SRSF in vitro through western blot detecting the expression level of the apoptosis-related proteins in SW480 and 786-O cells. SRSF3 expression was upregulated in pan-cancer tissue compared with normal tissue, which confirmed by immunohistochemistry and its expression indicated poor overall survival and death-specific survival. Therefore, SRSF3 was found to be a possible biomarker for prognostic and therapeutic assessment through bioinformatic analysis. SRSF3 is expressed in various cancers and its high expression correlated to poor survival and disease progression. In summary, SRSF3 expression can be considered as a prognostic biomarker in pan-cancer and therapeutic evaluation.

Biological function and molecular mechanism of SRSF3 in cancer and beyond

Serine/arginine-rich splicing factor 3 (SRSF3; also known as SRp20), an important member of the family of SRSFs, is abnormally expressed in tumors, resulting in aberrant splicing of hub genes, such as CD44, HER2, MDM4, Rac family small GTPase 1 and tumor protein p53. Under normal conditions, the splicing and expression of SRSF3 are strictly regulated. However, the splicing, expression and phosphorylation of SRSF3 are abnormal in tumors. SRSF3 plays important roles in the occurrence and development of tumors, including the promotion of tumorigenesis, cellular proliferation, the cell cycle and metastasis, as well as inhibition of cell senescence, apoptosis and autophagy. SRSF3-knockdown significantly inhibits the proliferation and metastatic characteristics of tumor cells. Therefore, SRSF3 may be suggested as a novel anti-tumor target. The other biological functions of SRSF3 and its regulatory mechanisms are also summarized in the current review.

Dynamic Variations of 3'UTR Length Reprogram the mRNA Regulatory Landscape

This paper concerns 3′-untranslated regions (3′UTRs) of mRNAs, which are non-coding regulatory platforms that control stability, fate and the correct spatiotemporal translation of mRNAs. Many mRNAs have polymorphic 3′UTR regions. Controlling 3′UTR length and sequence facilitates the regulation of the accessibility of functional effectors (RNA binding proteins, miRNAs or other ncRNAs) to 3′UTR functional boxes and motifs and the establishment of different regulatory landscapes for mRNA function. In this context, shortening of 3′UTRs would loosen miRNA or protein-based mechanisms of mRNA degradation, while 3′UTR lengthening would strengthen accessibility to these effectors. Alterations in the mechanisms regulating 3′UTR length would result in widespread deregulation of gene expression that could eventually lead to diseases likely linked to the loss (or acquisition) of specific miRNA binding sites. Here, we will review the mechanisms that control 3′UTR length dynamics and their alterations in human disorders. We will discuss, from a mechanistic point of view centered on the molecular machineries involved, the generation of 3′UTR variability by the use of alternative polyadenylation and cleavage sites, of mutually exclusive terminal alternative exons (exon skipping) as well as by the process of exonization of Alu cassettes to generate new 3′UTRs with differential functional features.

Prognostic Signatures of Alternative Splicing Events in Esophageal Carcinoma Based on TCGA Splice-Seq Data

An alternative splicing (AS) event is a highly complex process that plays an essential role in post-transcriptional gene expression. Several studies have suggested that abnormal AS events were the primary element in the pathological process of cancer. However, few works are dedicated to the study of AS events in esophageal carcinoma (EC). In the present study, clinical information and RNA-seq data of EC patients were downloaded from The Cancer Genome Atlas (TCGA) database. The percent spliced in (PSI) values of AS events were acquired from the TCGA Splice-seq. A total of 183 EC patients were enrolled in this study, and 2,212 AS events were found significantly associated with the overall survival of these patients by univariate Cox regression analysis. The prognostic signatures based on AS events were built by multivariate Cox analysis. Receiver operating characteristic (ROC) curves displayed that the area under the curve (AUC) of the following prognostic signatures, including exon skip (ES), alternate terminator (AT), alternate acceptor site (AA), alternate promoter (AP), alternate donor site (AD), retained intron (RI), and total events, was greater than 0.8, suggesting that these seven signatures had valuable prognosis prediction capacity. Finally, the risk score of prognostic signatures was indicated as an independent risk factor of survival. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed to explore the function of splicing factors (SFs) that were associated with AS events. Also, the interactive network between AS events and SFs identified several hub genes and AS events which need further study. This was a comprehensive study that explored prognosis-related AS events and established valuable prognosis signatures in EC patients. The network of interactions between AS events and SFs might offer novel insights into the fundamental mechanisms of tumorigenesis and progression of EC.

Alternative polyadenylation: An enigma of transcript length variation in health and disease

Alternative polyadenylation (APA) is a molecular mechanism during a pre-mRNA processing that involves usage of more than one polyadenylation site (PA-site) generating transcripts of varying length from a single gene. The location of a PA-site affects transcript length and coding potential of an mRNA contributing to both mRNA and protein diversification. This variation in the transcript length affects mRNA stability and translation, mRNA subcellular and tissue localization, and protein function. APA is now considered as an important regulatory mechanism in the pathophysiology of human diseases. An important consequence of the changes in the length of 3'-untranslated region (UTR) from disease-induced APA is altered protein expression. Yet, the relationship between 3'-UTR length and protein expression remains a paradox in a majority of diseases. Here, we review occurrence of APA, mechanism of PA-site selection, and consequences of transcript length variation in different diseases. Emerging evidence reveals coordinated involvement of core RNA processing factors including poly(A) polymerases in the PA-site selection in diseases-associated APAs. Targeting such APA regulators will be therapeutically significant in combating drug resistance in cancer and other complex diseases. This article is categorized under: RNA Processing > 3' End Processing RNA in Disease and Development > RNA in Disease Translation > Regulation.

QuantifyPoly(A): reshaping alternative polyadenylation landscapes of eukaryotes with weighted density peak clustering

The dynamic choice of different polyadenylation sites in a gene is referred to as alternative polyadenylation, which functions in many important biological processes. Large-scale messenger RNA 3' end sequencing has revealed that cleavage sites for polyadenylation are presented with microheterogeneity. To date, the conventional determination of polyadenylation site clusters is subjective and arbitrary, leading to inaccurate annotations. Here, we present a weighted density peak clustering method, QuantifyPoly(A), to accurately quantify genome-wide polyadenylation choices. Applying QuantifyPoly(A) on published 3' end sequencing datasets from both animals and plants, their polyadenylation profiles are reshaped into myriads of novel polyadenylation site clusters. Most of these novel polyadenylation site clusters show significantly dynamic usage across different biological samples or associate with binding sites of trans-acting factors. Upstream sequences of these clusters are enriched with polyadenylation signals UGUA, UAAA and/or AAUAAA in a species-dependent manner. Polyadenylation site clusters also exhibit species specificity, while plants ones generally show higher microheterogeneity than that of animals. QuantifyPoly(A) is broadly applicable to any types of 3' end sequencing data and species for accurate quantification and construction of the complex and dynamic polyadenylation landscape and enables us to decode alternative polyadenylation events invisible to conventional methods at a much higher resolution.

A computational pipeline to infer alternative poly-adenylation from 3' sequencing data

An increasing number of investigations have established alternative polyadenylation (APA) as a key mechanism of gene regulation through altering the length of 3' untranslated region (UTR) and generating distinct mRNA termini. Further, appreciation for the significance of APA in disease contexts propelled the development of several 3' sequencing techniques. While these RNA sequencing technologies have advanced APA analysis, the intrinsic limitation of 3' read coverage and lack of appropriate computational tools constrain precise mapping and quantification of polyadenylation sites. Notably, Poly(A)-ClickSeq (PAC-seq) overcomes limiting factors such as poly(A) enrichment and 3' linker ligation steps using click-chemistry. Here we provide an updated PolyA-miner protocol, a computational approach to analyze PAC-seq or other 3'-Seq datasets. As a key practical constraint, we also provide a detailed account on the impact of sequencing depth on the number of detected polyadenylation sites and APA changes. This protocol is also updated to handle unique molecular identifiers used to address PCR duplication potentially observed in PAC-seq.

HMGB1 coordinates SASP-related chromatin folding and RNA homeostasis on the path to senescence

Spatial organization and gene expression of mammalian chromosomes are maintained and regulated in conjunction with cell cycle progression. This is perturbed once cells enter senescence and the highly abundant HMGB1 protein is depleted from nuclei to act as an extracellular proinflammatory stimulus. Despite its physiological importance, we know little about the positioning of HMGB1 on chromatin and its nuclear roles. To address this, we mapped HMGB1 binding genome-wide in two primary cell lines. We integrated ChIP-seq and Hi-C with graph theory to uncover clustering of HMGB1-marked topological domains that harbor genes involved in paracrine senescence. Using simplified Cross-Linking and Immuno-Precipitation and functional tests, we show that HMGB1 is also a bona fide RNA-binding protein (RBP) binding hundreds of mRNAs. It presents an interactome rich in RBPs implicated in senescence regulation. The mRNAs of many of these RBPs are directly bound by HMGB1 and regulate availability of SASP-relevant transcripts. Our findings reveal a broader than hitherto assumed role for HMGB1 in coordinating chromatin folding and RNA homeostasis as part of a regulatory loop controlling cell-autonomous and paracrine senescence.

Systematic evaluation of the effect of polyadenylation signal variants on the expression of disease-associated genes

Single nucleotide variants (SNVs) within polyadenylation signals (PASs), a specific six-nucleotide sequence required for mRNA maturation, can impair RNA-level gene expression and cause human diseases. However, there is a lack of genome-wide investigation and systematic confirmation tools for identifying PAS variants. Here, we present a computational strategy to integrate the most reliable resources for discovering distinct genomic features of PAS variants and also develop a credible and convenient experimental tool to validate the effect of PAS variants on expression of disease-associated genes. This approach will greatly accelerate the deciphering of PAS variation-related human diseases.

SRp20: A potential therapeutic target for human tumors

As an important member of SR protein family, SRp20 plays a crucial role in alternative splicing. It not only participates in cell cycle regulation, export of mRNA, cleaving of primary microRNAs, homologous recombination-mediated DNA repair, cellular senescence and apoptosis, but also gets involved in the integrity and pluripotency of genome. Alternative splicing maintains a strict balance in the body to ensure the normal physiological function of cells. Once the balance is broken, diseases, even tumors, will follow. Through the analysis of SRp20-related articles, we found that Alzheimer's disease, glaucoma, bipolar disorder and other diseases have a certain relationship with SRp20. More importantly, SRp20 is closely related to the occurrence, proliferation, invasion and metastasis of various tumors, as well as chemotherapy resistance. Some SRp20 inhibitors have shown significant anticancer efficacy, suggesting a potential therapeutic strategy for tumors.

On the function and relevance of alternative 3'-UTRs in gene expression regulation

Messanger RNA (mRNA) isoforms with alternative 3'-untranslated regions (3'-UTRs) are produced by alternative polyadenylation (APA), which occurs during transcription in most eukaryotic genes. APA fine-tunes gene expression in a cell-type- and cellular state-dependent manner. Selection of an APA site entails the binding of core cleavage and polyadenylation factors to a particular polyadenylation site localized in the pre-mRNA and is controlled by multiple regulatory determinants, including transcription, pre-mRNA cis-regulatory sequences, and protein factors. Alternative 3'-UTRs serve as platforms for specific RNA binding proteins and microRNAs, which regulate gene expression in a coordinated manner by controlling mRNA fate and function in the cell. Genome-wide studies illustrated the full extent of APA prevalence and revealed that specific 3'-UTR profiles are associated with particular cellular states and diseases. Generally, short 3'-UTRs are associated with proliferative and cancer cells, and long 3'-UTRs are mostly found in polarized and differentiated cells. Fundamental new insights on the physiological consequences of this widespread event and the molecular mechanisms involved have been revealed through single-cell studies. Publicly available comprehensive databases that cover all APA mRNA isoforms identified in many cellular states and diseases reveal specific APA signatures. Therapies tackling APA mRNA isoforms or APA regulators may be regarded as innovative and attractive tools for diagnostics or treatment of several pathologies. We highlight the function of APA and alternative 3'-UTRs in gene expression regulation, the control of these mechanisms, their physiological consequences, and their potential use as new biomarkers and therapeutic tools. This article is categorized under: RNA Processing > 3' End Processing RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA in Disease and Development > RNA in Disease.

SRSF3 and SRSF7 modulate 3'UTR length through suppression or activation of proximal polyadenylation sites and regulation of CFIm levels

Background

Alternative polyadenylation (APA) refers to the regulated selection of polyadenylation sites (PASs) in transcripts, which determines the length of their 3′ untranslated regions (3′UTRs). We have recently shown that SRSF3 and SRSF7, two closely related SR proteins, connect APA with mRNA export. The mechanism underlying APA regulation by SRSF3 and SRSF7 remained unknown.

Results

Here we combine iCLIP and 3′-end sequencing and find that SRSF3 and SRSF7 bind upstream of proximal PASs (pPASs), but they exert opposite effects on 3′UTR length. SRSF7 enhances pPAS usage in a concentration-dependent but splicing-independent manner by recruiting the cleavage factor FIP1, generating short 3′UTRs. Protein domains unique to SRSF7, which are absent from SRSF3, contribute to FIP1 recruitment. In contrast, SRSF3 promotes distal PAS (dPAS) usage and hence long 3′UTRs directly by counteracting SRSF7, but also indirectly by maintaining high levels of cleavage factor Im (CFIm) via alternative splicing. Upon SRSF3 depletion, CFIm levels decrease and 3′UTRs are shortened. The indirect SRSF3 targets are particularly sensitive to low CFIm levels, because here CFIm serves a dual function; it enhances dPAS and inhibits pPAS usage by binding immediately downstream and assembling unproductive cleavage complexes, which together promotes long 3′UTRs.

Conclusions

We demonstrate that SRSF3 and SRSF7 are direct modulators of pPAS usage and show how small differences in the domain architecture of SR proteins can confer opposite effects on pPAS regulation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-021-02298-y.

Inhibition of SRSF3 Alleviates Proliferation and Migration of Gastric Cancer Cells by Regulating the PI3K/AKT/mTOR Signalling Pathway

This study was aimed to investigate the impact of serine/arginine-rich splicing factor 3 (SRSF3) on the proliferation and migration of gastric cancer (GC) cells. SRSF3 levels in GC tissues and cell lines were measured by Western blotting. Functional assays were used for evaluation of GC cell proliferation, migration and invasion. The PI3K/AKT/mTOR pathway was then examined by Western blotting. SRSF3 exhibits abnormal expression for the significantly increased levels in GC. SRSF3 knockdown significantly suppressed GC progression. SRSF3 knockdown significantly inhibited activation of PI3K/AKT/mTOR signalling. Inhibition of SRSF3 alleviates proliferation and migration of GC cells, and this process is mediated by inactivation of PI3K/ AKT/mTOR signalling. Targeting SRSF3 may be a promising strategy to combat GC.

Alternative Polyadenylation: a new frontier in post transcriptional regulation

Polyadenylation of pre-messenger RNA (pre-mRNA) specific sites and termination of their downstream transcriptions are signaled by unique sequence motif structures such as AAUAAA and its auxiliary elements. Alternative polyadenylation (APA) is an important post-transcriptional regulatory mechanism that processes RNA products depending on its 3'-untranslated region (3'-UTR) specific sequence signal. APA processing can generate several mRNA isoforms from a single gene, which may have different biological functions on their target gene. As a result, cellular genomic stability, proliferation capability, and transformation feasibility could all be affected. Furthermore, APA modulation regulates disease initiation and progression. APA status could potentially act as a biomarker for disease diagnosis, severity stratification, and prognosis forecast. While the advance of modern throughout technologies, such as next generation-sequencing (NGS) and single-cell sequencing techniques, have enriched our knowledge about APA, much of APA biological process is unknown and pending for further investigation. Herein, we review the current knowledge on APA and how its regulatory complex factors (CFI/IIm, CPSF, CSTF, and RBPs) work together to determine RNA splicing location, cell cycle velocity, microRNA processing, and oncogenesis regulation. We also discuss various APA experiment strategies and the future direction of APA research.

PolyA-miner: accurate assessment of differential alternative poly-adenylation from 3'Seq data using vector projections and non-negative matrix factorization

Almost 70% of human genes undergo alternative polyadenylation (APA) and generate mRNA transcripts with varying lengths, typically of the 3′ untranslated regions (UTR). APA plays an important role in development and cellular differentiation, and its dysregulation can cause neuropsychiatric diseases and increase cancer severity. Increasing awareness of APA’s role in human health and disease has propelled the development of several 3′ sequencing (3′Seq) techniques that allow for precise identification of APA sites. However, despite the recent data explosion, there are no robust computational tools that are precisely designed to analyze 3′Seq data. Analytical approaches that have been used to analyze these data predominantly use proximal to distal usage. With about 50% of human genes having more than two APA isoforms, current methods fail to capture the entirety of APA changes and do not account for non-proximal to non-distal changes. Addressing these key challenges, this study demonstrates PolyA-miner, an algorithm to accurately detect and assess differential alternative polyadenylation specifically from 3′Seq data. Genes are abstracted as APA matrices, and differential APA usage is inferred using iterative consensus non-negative matrix factorization (NMF) based clustering. PolyA-miner accounts for all non-proximal to non-distal APA switches using vector projections and reflects precise gene-level 3′UTR changes. It can also effectively identify novel APA sites that are otherwise undetected when using reference-based approaches. Evaluation on multiple datasets—first-generation MicroArray Quality Control (MAQC) brain and Universal Human Reference (UHR) PolyA-seq data, recent glioblastoma cell line NUDT21 knockdown Poly(A)-ClickSeq (PAC-seq) data, and our own mouse hippocampal and human stem cell-derived neuron PAC-seq data—strongly supports the value and protocol-independent applicability of PolyA-miner. Strikingly, in the glioblastoma cell line data, PolyA-miner identified more than twice the number of genes with APA changes than initially reported. With the emerging importance of APA in human development and disease, PolyA-miner can significantly improve data analysis and help decode the underlying APA dynamics.

Dynamic mRNP Remodeling in Response to Internal and External Stimuli

Signal transduction and the regulation of gene expression are fundamental processes in every cell. RNA-binding proteins (RBPs) play a key role in the post-transcriptional modulation of gene expression in response to both internal and external stimuli. However, how signaling pathways regulate the assembly of RBPs with mRNAs remains largely unknown. Here, we summarize observations showing that the formation and composition of messenger ribonucleoprotein particles (mRNPs) is dynamically remodeled in space and time by specific signaling cascades and the resulting post-translational modifications. The integration of signaling events with gene expression is key to the rapid adaptation of cells to environmental changes and stress. Only a combined approach analyzing the signal transduction pathways and the changes in post-transcriptional gene expression they cause will unravel the mechanisms coordinating these important cellular processes.

Regulation of alternative polyadenylation in the yeast Saccharomyces cerevisiae by histone H3K4 and H3K36 methyltransferases

Adjusting DNA structure via epigenetic modifications, and altering polyadenylation (pA) sites at which precursor mRNA is cleaved and polyadenylated, allows cells to quickly respond to environmental stress. Since polyadenylation occurs co-transcriptionally, and specific patterns of nucleosome positioning and chromatin modifications correlate with pA site usage, epigenetic factors potentially affect alternative polyadenylation (APA). We report that the histone H3K4 methyltransferase Set1, and the histone H3K36 methyltransferase Set2, control choice of pA site in Saccharomyces cerevisiae, a powerful model for studying evolutionarily conserved eukaryotic processes. Deletion of SET1 or SET2 causes an increase in serine-2 phosphorylation within the C-terminal domain of RNA polymerase II (RNAP II) and in the recruitment of the cleavage/polyadenylation complex, both of which could cause the observed switch in pA site usage. Chemical inhibition of TOR signaling, which causes nutritional stress, results in Set1- and Set2-dependent APA. In addition, Set1 and Set2 decrease efficiency of using single pA sites, and control nucleosome occupancy around pA sites. Overall, our study suggests that the methyltransferases Set1 and Set2 regulate APA induced by nutritional stress, affect the RNAP II C-terminal domain phosphorylation at Ser2, and control recruitment of the 3′ end processing machinery to the vicinity of pA sites.

SRSF3: Newly discovered functions and roles in human health and diseases

The serine/arginine rich proteins (SR proteins) are members of a family of RNA binding proteins involved in regulating various features of RNA metabolism, including pre-mRNA constitutive and alternative splicing. In humans, a total of 12 SR splicing factors (SRSFs) namely SRSF1-SRSF12 have been reported. SRSF3, the smallest member of the SR family and the focus of this review, regulates critical steps in mRNA metabolism and has been shown to have mRNA-independent functions as well. Recent studies on SRSF3 have uncovered its role in a wide array of complex biological processes. We have also reviewed the involvement of SRSF3 in disease conditions like cancer, ageing, neurological and cardiac disorders. Finally, we have discussed in detail the autoregulation of SRSF3 and its implications in cancer and commented on the potential of SRSF3 as a therapeutic target, especially in the context of cancer.

Tempo-spatial alternative polyadenylation analysis reveals that 3' UTR lengthening of Mdm2 regulates p53 expression and cellular senescence in aged rat testis

Although tissue aging is accompanied with cellular senescence, it is much complicated than senescence given both types and number of cells change with age. Alternative polyadenylation (APA) had shown tissue specificity and APA-mediated 3' untranslated region (3' UTR) lengthening could regulate senescence-associated phenotypes. However, whether tissue aging shows similar trends remains unknown. Here, we performed a comprehensive analysis on RNA-seq datasets derived from multiple cells and rat tissues of young and old age. Although APA-mediated 3' UTR lengthening in various senescent cells reinforced the previous discovery, tissue aging showed much more complexity in APA. Interestingly, testis was the only tissue displaying dramatic 3' UTR lengthening and decreased expression trend of corresponding genes in aged rat. Genes with longer 3' UTR in aged testis were enriched in senescence-associated pathways, among which, Mdm2, encoding an E3 ligase of p53, favored distal poly(A) site resulting in lengthened 3' UTR and decreased expression. Longer 3' UTR of Mdm2 generated less protein, and decreased Mdm2 expression led to senescence-associated phenotypes along with increased p53 and p21 protein abundance, which could all be reversed by Mdm2 overexpression. Our work revealed complicated APA changes during tissue aging and discovered APA-mediated 3' UTR lengthening of Mdm2 is a hidden layer in regulating the well-known senescence-related p53-p21 signal axis during testis aging, and also has potential implications regarding declined male fertility along aging.