Structure-Mediated Degradation of CircRNAs

Unraveling the crosstalk: circRNAs and the wnt signaling pathway in cancers of the digestive system

Circular RNA (circRNA) is a unique type of noncoding RNA molecule characterized by its closed-loop structure. Functionally versatile, circRNAs play pivotal roles in gene expression regulation, protein activity modulation, and participation in cell signaling processes. In the context of cancers of the digestive system, the Wnt signaling pathway holds particular significance. Anomalous activation of the Wnt pathway serves as a primary catalyst for the development of colorectal cancer. Extensive research underscores the notable participation of circRNAs associated with the Wnt pathway in the progression of digestive system tumors. These circRNAs exhibit pronounced dysregulation across esophageal cancer, gastric cancer, liver cancer, colorectal cancer, pancreatic cancer, and cholangiocarcinoma. Furthermore, the altered expression of circRNAs linked to the Wnt pathway correlates with prognostic factors in digestive system tumors. Additionally, circRNAs related to the Wnt pathway showcase potential as diagnostic, therapeutic, and prognostic markers within the realm of digestive system tumors. This comprehensive review outlines the interplay between circRNAs and the Wnt signaling pathway in cancers of the digestive system. It seeks to provide a comprehensive perspective on their association while delving into ongoing research that explores the clinical applications of circRNAs associated with the Wnt pathway.

YTHDC1/CRM1 Facilitates m6A-Modified circRNA388 Nuclear Export to Induce Coelomocyte Autophagy via the miR-2008/ULK Axis in Apostichopus japonicus

N 6-methyladenosine (m6A), the most prevalent internal modification in eukaryotic RNA, was able to mediate circular RNA (circRNA) function in many immune processes. Nevertheless, the functional role of m6A-modified circRNAs in innate immunity of invertebrates remained unclear. In this study, we identified m6A-modified circRNA388 from cultured sea cucumber (Apostichopus japonicus) coelomocytes, which was mainly detected in cytoplasm after Vibrio splendidus infection. A knockdown assay indicated that cytoplasm circRNA388 promoted coelomocyte autophagy and decreased the number of intracellular V. splendidus. Mechanistically, the circRNA388 in the cytoplasm directly sponged miR-2008 to block its interaction with Unc-51-like kinase 1 from A. japonicus (AjULK) and further promoted autophagy to resist V. splendidus infection. More importantly, we found that m6A modification was vital to circRNA388 nuclear export with YTH domain-containing protein 1 from A. japonicus (AjYTHDC1) as the reader. AjYTHDC1 facilitated the nuclear export of m6A-modified circRNA388 via interaction with exportin-1 (chromosomal maintenance 1) from A. japonicus (AjCRM1). Knockdown of AjCRM1 could significantly decrease the content of cytoplasm circRNA388. Overall, our results provide the first evidence that nuclear export of m6A-modified circRNA388 is dependent on the novel AjCRM1 to our knowledge, which was further promoted coelomocyte autophagy by miR-2008/AjULK axis to clear intracellular V. splendidus.

Circular RNAs: Emerging roles and new insights in human cancers

Circular RNAs (circRNAs) are single-stranded, covalently closed RNA molecules formed by mRNA exon back-splicing. Although the circRNA functions remain largely unknown, their currently known biological activities include: acting as competing endogenous RNA (ceRNA) to adsorb microRNA (miRNA), binding proteins, regulating transcription or splicing, and ability to be translated into proteins or peptides. A growing number of studies have found that many circRNAs are abnormally expressed in various cancers, and their dysregulation is highly correlated with tumor progression. Therefore, diagnosis and treatment using circRNAs as biomarkers and therapeutic targets, respectively, has gradually become an attractive research topic. In this review, we introduced the canonical biogenesis pathways and degradation mechanisms of circRNAs. In addition, we examined the biological functions of circRNAs in vivo. Finally, we discussed the current clinical applications and challenges faced by circRNA, and proposed future directions for this promising research field.

Comparative Transcriptomics Identify Key Pituitary Circular RNAs That Participate in Sheep (Ovis aries) Reproduction

CircRNAs have been found to play key roles in many biological processes and have diverse biological functions. There have been studies on circRNAs in sheep pituitary, and some important circRNAs have been found. But there are still few studies on circRNAs in sheep pituitary with different fecundity. In this study, we obtained the circRNAs expression profiles in the pituitary of FecB ++ genotype Small Tail Han sheep with different fecundity and estrous phases. A total of 34,878 circRNAs were identified in 12 pituitary samples, 300 differentially expressed circRNAs (DE circRNAs) (down: 104; up: 196) were identified in polytocous sheep in the follicular phase (PF) and monotocous sheep in the follicular phase (MF) (PF vs. MF), and 347 DE circRNAs (down: 162; up: 185) were identified in polytocous sheep in the luteal phase (PL) and monotocous sheep in the luteal phase (ML) (PL vs. ML). Cortisol synthesis and secretion pathway (follicular phase) and estrogen signaling pathway (luteal phase) were obtained by functional enrichment analysis of circRNAs source genes. Competing endogenous RNA (ceRNA) network analysis of key DE circRNAs revealed that oar-circ-0022776 (source gene ITPR2, follicular phase) targeted oar-miR-432, oar-circ-0009003 (source gene ITPR1, luteal phase) and oar-circ-0003113 (source gene PLCB1, luteal phase) targeted oar-miR-370-3p. We also explored the coding ability of DE circRNAs. In conclusion, our study shows that changes in the pituitary circRNAs may be related to the response of the pituitary to steroid hormones and regulate the reproductive process of sheep by affecting the pituitary function. Results of this study provide some new information for understanding the functions of circRNAs and the fecundity of FecB ++ genotype sheep.

ATF4-mediated circTDRD3 promotes gastric cancer cell proliferation and metastasis by regulating the miR-891b/ITGA2 axis and AKT signaling pathway

BACKGROUND

Gastric cancer (GC) is a cancer of the gastrointestinal tract that is highly malignant and has poor prognosis. Circular RNAs are a class of nonclassical RNA molecules that have been determined to be involved in GC malignancy in various ways. However, the underlying function and mechanism of circTDRD3 in gastric cancer remain largely unknown.

METHODS

We analyzed circTDRD3 expression in databases and verified the findings in GC cell lines and tissue specimens. A series of functional gene overexpression and knockdown assays in vivo and in vitro were carried out to investigate the role of circTDRD3 in proliferation and metastasis. Here, we revealed the role of the miR-891b/ITGA2 axis by analyzing bioinformatics datasets. Furthermore, we performed dual-luciferase, fluorescence in situ hybridization, RNA pull-down, and functional rescue experiments to examine the relationships between circTDRD3 and its interacting molecules. Western blot confirmed the positive regulatory role of circTDRD3 in the AKT signaling pathway. A promoting effect of ATF4 on circTDRD3 was determined through chromatin immunoprecipitation.

RESULTS

CircTDRD3 was significantly overexpressed in GC tissues compared with adjacent benign tissue, and its expression level was positively correlated with tumor volume and lymph node metastasis. CircTDRD3 promoted GC cell proliferation and migration in vitro and in vivo. Mechanistically, circTDRD3 exerted a tumor-promoting effect by regulating the miR-891b/ITGA2 axis and AKT signaling pathway in a positive feedback manner mediated by the transcription factor ATF4.

CONCLUSIONS

ATF4-mediated circTDRD3 overexpression modulates the proliferation and metastasis of GC cells through the miR-891b/ITGA2 axis in a positive feedback manner.

Non-Coding RNAs Modulating Estrogen Signaling and Response to Endocrine Therapy in Breast Cancer

The largest part of human DNA is transcribed into RNA that does not code for proteins. These non-coding RNAs (ncRNAs) are key regulators of protein-coding gene expression and have been shown to play important roles in health, disease and therapy response. Today, endocrine therapy of ERα-positive breast cancer (BC) is a successful treatment approach, but resistance to this therapy is a major clinical problem. Therefore, a deeper understanding of resistance mechanisms is important to overcome this resistance. An increasing amount of evidence demonstrate that ncRNAs affect the response to endocrine therapy. Thus, ncRNAs are considered versatile biomarkers to predict or monitor therapy response. In this review article, we intend to give a summary and update on the effects of microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs) on estrogen signaling in BC cells, this pathway being the target of endocrine therapy, and their role in therapy resistance. For this purpose, we reviewed articles on these topics listed in the PubMed database. Finally, we provide an assessment regarding the clinical use of these ncRNA types, particularly their circulating forms, as predictive BC biomarkers and their potential role as therapy targets to overcome endocrine resistance.

UPF1-From mRNA Degradation to Human Disorders

Up-frameshift protein 1 (UPF1) plays the role of a vital controller for transcripts, ready to react in the event of an incorrect translation mechanism. It is well known as one of the key elements involved in mRNA decay pathways and participates in transcript and protein quality control in several different aspects. Firstly, UPF1 specifically degrades premature termination codon (PTC)-containing products in a nonsense-mediated mRNA decay (NMD)-coupled manner. Additionally, UPF1 can potentially act as an E3 ligase and degrade target proteins independently from mRNA decay pathways. Thus, UPF1 protects cells against the accumulation of misfolded polypeptides. However, this multitasking protein may still hide many of its functions and abilities. In this article, we summarize important discoveries in the context of UPF1, its involvement in various cellular pathways, as well as its structural importance and mutational changes related to the emergence of various pathologies and disease states. Even though the state of knowledge about this protein has significantly increased over the years, there are still many intriguing aspects that remain unresolved.

CircAMOTL1 RNA and AMOTL1 Protein: Complex Functions of AMOTL1 Gene Products

The complexity of the cellular proteome facilitates the control of a wide range of cellular processes. Non-coding RNAs, including microRNAs and long non-coding RNAs, greatly contribute to the repertoire of tools used by cells to orchestrate various functions. Circular RNAs (circRNAs) constitute a specific class of non-coding RNAs that have recently emerged as a widely generated class of molecules produced from many eukaryotic genes that play essential roles in regulating cellular processes in health and disease. This review summarizes current knowledge about circRNAs and focuses on the functions of AMOTL1 circRNAs and AMOTL1 protein. Both products from the AMOTL1 gene have well-known functions in physiology, cancer, and other disorders. Using AMOTL1 as an example, we illustrate how focusing on both circRNAs and proteins produced from the same gene contributes to a better understanding of gene functions.

The function and clinical implication of circular RNAs in lung cancer

Lung cancer is the leading cause of cancer-related deaths worldwide. Despite the recent advent of promising new targeted therapies, lung cancer diagnostic strategies still have difficulty in identifying the disease at an early stage. Therefore, the characterizations of more sensible and specific cancer biomarkers have become an important goal for clinicians. Circular RNAs are covalently close, endogenous RNAs without 5' end caps or 3'poly (A) tails and have been characterized by high stability, abundance, and conservation as well as display cell/tissue/developmental stage-specific expressions. Numerous studies have confirmed that circRNAs act as microRNA (miRNA) sponges, RNA-binding protein, and transcriptional regulators; some circRNAs even act as translation templates that participate in multiple pathophysiological processes. Growing evidence have confirmed that circRNAs are involved in the pathogenesis of lung cancers through the regulation of proliferation and invasion, cell cycle, autophagy, apoptosis, stemness, tumor microenvironment, and chemotherapy resistance. Moreover, circRNAs have emerged as potential biomarkers for lung cancer diagnosis and prognosis and targets for developing new treatments. In this review, we will summarize recent progresses in identifying the biogenesis, biological functions, potential mechanisms, and clinical applications of these molecules for lung cancer diagnosis, prognosis, and targeted therapy.

Function and Clinical Significance of Circular RNAs in Thyroid Cancer

Thyroid cancer (TC) is the leading cause and mortality of endocrine malignancies worldwide. Tumourigenesis involves multiple molecules including circular RNAs (circRNAs). circRNAs with covalently closed single-stranded structures have been identified as a type of regulatory RNA because of their high stability, abundance, and tissue/developmental stage-specific expression. Accumulating evidence has demonstrated that various circRNAs are aberrantly expressed in thyroid tissues, cells, exosomes, and body fluids in patients with TC. CircRNAs have been identified as either oncogenic or tumour suppressor roles in regulating tumourigenesis, tumour metabolism, metastasis, ferroptosis, and chemoradiation resistance in TC. Importantly, circRNAs exert pivotal effects on TC through various mechanisms, including acting as miRNA sponges or decoys, interacting with RNA-binding proteins, and translating functional peptides. Recent studies have suggested that many different circRNAs are associated with certain clinicopathological features, implying that the altered expression of circRNAs may be characteristic of TC. The purpose of this review is to provide an overview of recent advances on the dysregulation, functions, molecular mechanisms and potential clinical applications of circRNAs in TC. This review also aimes to improve our understanding of the functions of circRNAs in the initiation and progression of cancer, and to discuss the future perspectives on strategies targeting circRNAs in TC.

Factors influencing degradation kinetics of mRNAs and half-lives of microRNAs, circRNAs, lncRNAs in blood in vitro using quantitative PCR

RNAs are rapidly degraded in samples and during collection, processing and testing. In this study, we used the same method to explore the half-lives of different RNAs and the influencing factors, and compared the degradation kinetics and characteristics of different RNAs in whole blood and experimental samples. Fresh anticoagulant blood samples were incubated at room temperature for different durations, RNAs were extracted, and genes, including internal references, were amplified by real-time quantitative PCR. A linear half-life model was established according to cycle threshold (Ct) values. The effects of experimental operations on RNA degradation before and after RNA extraction were explored. Quantitative analysis of mRNA degradation in samples and during experimental processes were explored using an orthogonal experimental design. The storage duration of blood samples at room temperature had the greatest influence on RNA degradation. The half-lives of messenger RNAs (mRNAs) was 16.4 h. The half-lives of circular RNAs (circRNAs), long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) were 24.56 ± 5.2 h, 17.46 ± 3.0 h and 16.42 ± 4.2 h, respectively. RNA degradation occurred mainly in blood samples. The half-life of mRNAs was the shortest among the four kinds of RNAs. Quantitative experiments related to mRNAs should be completed within 2 h. The half-lives of circRNAs and lncRNAs were longer than those of the former two.

Circular RNAs Modulate Cancer Hallmark and Molecular Pathways to Support Cancer Progression and Metastasis

Simple Summary

Circular RNAs (circRNA) are a type of RNA molecule of circular shape that are now being extensively studied due to the important roles they play in different biological processes. In addition, they were also shown to be implicated in disease such as cancer. Cancer is a complex process which is often defined by a combination of specific processes called cancer hallmarks. In this review, we summarize the literature on circRNAs in cancer and classify them as being implicated in specific cancer hallmarks.

Abstract

Circular RNAs (circRNAs) are noncoding products of backsplicing of pre-mRNAs which have been established to possess potent biological functions. Dysregulated circRNA expression has been linked to diseases including different types of cancer. Cancer progression is known to result from the dysregulation of several molecular mechanisms responsible for the maintenance of cellular and tissue homeostasis. The dysregulation of these processes is defined as cancer hallmarks, and the molecular pathways implicated in them are regarded as the targets of therapeutic interference. In this review, we summarize the literature on the investigation of circRNAs implicated in cancer hallmark molecular signaling. First, we present general information on the properties of circRNAs, such as their biogenesis and degradation mechanisms, as well as their basic molecular functions. Subsequently, we summarize the roles of circRNAs in the framework of each cancer hallmark and finally discuss the potential as therapeutic targets.

N6-methyladenosine-dependent modification of circGARS acts as a new player that promotes SLE progression through the NF-κB/A20 axis

Background

Certain circRNAs could be used as biomarkers to determine the risk of development and/or severity of systemic lupus erythematosus, and their new function in the regulation of gene expression has motivated us to investigate their role in SLE

Methods

Experimental methods including qRT-PCR, RNA immunoprecipitation (RIP), pulldown, dual luciferase reporter assay, RNA interference and cell transfection, RNA fluorescence in situ hybridization, western blotting, and mass spectrometry were used to assessed circGARS (hsa_circRNA_0009000) for immune functions and defined mechanisms by which circGARS promotes the progression in SLE.

Results

Our results demonstrated that the levels of circGARS was remarkably upregulated in SLE and correlated with clinicopathological features. CircGARS directly combined with microRNA-19a (miR-19a). Functionally, circGARS downregulated the expression of TNFAIP3 (A20, tumor necrosis factor alpha-induced protein 3) to mediate the activation of immune responses that were regulated by the nuclear factor-κB (NF-κB) pathway as a negative feedback mechanism. In addition, miR-19a regulated A20 (TNFAIP3) degradation by downregulating the expression of YTH N6-methyladenosine RNA-binding protein 2 (YTHDF2).

Conclusions

The circGARS sponges miR-19a to regulate YTHDF2 expression to promote SLE progression through the A20/NF-κB axis and may act as an independent biomarker to help the treatment of SLE patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-022-02732-x.

Translation and emerging functions of non-coding RNAs in inflammation and immunity

Regulatory non‐coding RNAs (ncRNAs) including small non‐coding RNAs (sRNAs), long non‐coding RNAs (lncRNAs), and circular RNAs (circRNAs) have gained considerable attention in the last few years. This is mainly due to their condition‐ and tissue‐specific expression and their various modes of action, which suggests them as promising biomarkers and therapeutic targets. One important mechanism of ncRNAs to regulate gene expression is through translation of short open reading frames (sORFs). These sORFs can be located in lncRNAs, in non‐translated regions of mRNAs where upstream ORFs (uORFs) represent the majority, or in circRNAs. Regulation of their translation can function as a quick way to adapt protein production to changing cellular or environmental cues, and can either depend solely on the initiation and elongation of translation, or on the roles of the produced functional peptides. Due to the experimental challenges to pinpoint translation events and to detect the produced peptides, translational regulation through regulatory RNAs is not well studied yet. In the case of circRNAs, they have only recently started to be recognized as regulatory molecules instead of mere artifacts of RNA biosynthesis. Of the many roles described for regulatory ncRNAs, we will focus here on their regulation during inflammation and in immunity.

Circular RNAs in cell cycle regulation: Mechanisms to clinical significance

Circular RNAs (circRNAs), a type of non‐coding RNA, are single‐stranded circularized molecules characterized by high abundance, evolutionary conservation and cell development‐ and tissue‐specific expression. A large body of studies has found that circRNAs exert a wide variety of functions in diverse biological processes, including cell cycle. The cell cycle is controlled by the coordinated activation and deactivation of cell cycle regulators. CircRNAs exert mutifunctional roles by regulating gene expression via various mechanisms. However, the functional relevance of circRNAs and cell cycle regulation largely remains to be elucidated. Herein, we briefly describe the biogenesis and mechanistic models of circRNAs and summarize their functions and mechanisms in the regulation of critical cell cycle modulators, including cyclins, cyclin‐dependent kinases and cyclin‐dependent kinase inhibitors. Moreover, we highlight the participation of circRNAs in cell cycle‐related signalling pathways and the clinical value of circRNAs as promising biomarkers or therapeutic targets in diseases related to cell cycle disorder.

Circular RNAs in cardiovascular diseases

In eukaryotes, precursor mRNAs (pre-mRNAs) produce a unique class of biologically active molecules namely circular RNAs (circRNAs) with a covalently closed-loop structure via back-splicing. Because of this unconventional circular form, circRNAs exhibit much higher stability than linear RNAs due to the resistance to exonuclease degradation and thereby play exclusive cellular regulatory roles. Recent studies have shown that circRNAs are widely expressed in eukaryotes and display tissue- and disease-specific expression patterns, including in the cardiovascular system. Although numerous circRNAs are discovered by in silico methods, a limited number of circRNAs have been studied. This review intends to summarize the current understanding of the characteristics, biogenesis, and functions of circRNAs and delineate the practical approaches for circRNAs investigation. Moreover, we discuss the emerging roles of circRNAs in cardiovascular diseases.

Mobius strip in pancreatic cancer: biogenesis, function and clinical significance of circular RNAs

Pancreatic cancer (PC) is a kind of common digestive system cancer with the worst prognosis for its insidious symptoms and high invasiveness. Circular RNAs (circRNAs) are endogenous non-coding RNAs with covalently closed circular structure, which are more stable and conservative than linear RNAs and process major functions of microRNA (miRNA) sponge, RNA binding protein (RBP) sponge and polypeptide translation template. Incremental researches have proved that circRNAs express aberrantly and play a vital role in various types of cancer. Hence, we reviewed the biogenesis, degradation, characteristics, and biological functions of circRNAs and summarized the roles circRNAs played in the proliferation, invasion, metastasis, chemoresistance and exosome-mediated intercellular communication of PC. We then summed up a workflow regarding circRNA research in cancer and relative specific databases and experimental methods. In the future, more efforts ought to be put into circRNAs research in PC, including basic research of discovering and testifying circRNAs centered ceRNA networks, and clinical research of exploiting exosomal or circulating circRNAs as a diagnostic biomarker, chemotherapy sensitivity predictor and prognostic predictor.

The Regulation Network and Clinical Significance of Circular RNAs in Breast Cancer

Breast cancer is one of the most common malignant tumors in women worldwide. Circular RNA (circRNA) is a class of structurally stable non-coding RNA with a covalently closed circular structure. In recent years, with the development of high-throughput RNA sequencing, many circRNAs have been discovered and have proven to be clinically significant in the development and progression of breast cancer. Importantly, several regulators of circRNA biogenesis have been discovered. Here, we systematically summarize recent progress regarding the network of regulation governing the biogenesis, degradation, and distribution of circRNAs, and we comprehensively analyze the functions, mechanisms, and clinical significance of circRNA in breast cancer.

Reduced expression of circRNA novel_circ_0005280 and its clinical value in the diagnosis of non-small cell lung cancer

Background

Circular RNAs (circRNAs) are a class of novel RNAs with important biologic functions. The aberrant expression of circRNAs has been implicated in human diseases; however, the clinical significance of circRNAs in non-small cell lung cancer (NSCLC) is still unclear. The aim of the present study was to evaluate the expression and clinical implications of novel_circ_0005280 in patients with NSCLC.

Methods

We evaluated differential circRNA expression in cancer and adjacent normal tissues from 3 patients with NSCLC via RNA sequencing. Among these circRNAs, 17 and 64 circRNAs showed higher and lower expressions, respectively. Novel_circ_0005280 expression in cancer tissues (n=41) was examined using quantitative real-time polymerase chain reaction, and the results are presented in the form of paired graph and scatter graph and its correlation with clinicopathological features and patient prognosis was analyzed by drawing receiver-operating characteristic (ROC) curve and Kaplan-Meier survival analysis.

Results

Novel_circ_0005280 expression was significantly decreased in NSCLC tumor tissues (n=41, obtained via biopsies), compared with adjacent normal tissues (n=27). Novel_circ_0005280 expression was correlated with tumor diameter and age. The area under the receiver-operating characteristic curve, cutoff, sensitivity, and specificity of novel_circ_0005280 were 0.944, 10.23, 85.2%, and 95.1%, respectively. Low novel_circ_0005280 expression was associated with a worse prognosis.

Conclusions

Novel_circ_0005280 may be a useful biomarker for the diagnosis and prognosis of NSCLC.

The Role of Circular RNAs in Pancreatic Ductal Adenocarcinoma and Biliary-Tract Cancers

Simple Summary

Pancreatic and biliary tract cancers often present with non-specific symptoms, resulting in diagnosis at a late stage. This may be too late for curative surgery. Earlier detection and characterisation may guide treatment options and increase survival. Natural “circles” of RNA (circRNAs) are shown to regulate cancer-related genes, and act as cancer “biomarkers”. Recent research has shown that circRNAs are both abundant and stable, both of which are desirable characteristics for clinically useful biomarkers. In this systematic review, we describe the roles of circRNAs in pancreatic and biliary tract cancers, summarise the current published research and explore their utility as a biomarker. A total of 32 articles were included: 22 considering Pancreatic Cancer, 7 for Bile Duct Cancer and 3 for Gallbladder Cancer. CircRNA proved an exciting prospect as a biomarker for these cancers and future work should continue to develop and expand this field of research.

Abstract

Pancreatic Ductal Adenocarcinoma (PDAC) and biliary-tract cancers (BTC) often present at a late stage, and consequently patients have poor survival-outcomes. Circular RNAs (circRNAs) are non-coding RNA molecules whose role in tumourigenesis has recently been realised. They are stable, conserved and abundant, with tissue-specific expression profiles. Therefore, significant interest has arisen in their use as potential biomarkers for PDAC and BTC. High-throughput methods and more advanced bioinformatic techniques have enabled better profiling and progressed our understanding of how circRNAs may function in the competing endogenous RNA (ceRNA) network to influence the transcriptome in these cancers. Therefore, the aim of this systematic review was to describe the roles of circRNAs in PDAC and BTC, their potential as biomarkers, and their function in the wider ceRNA network in regulating microRNAs and the transcriptome. Medline, Embase, Scopus and PubMed were systematically reviewed to identify all the studies addressing circRNAs in PDAC and BTC. A total of 32 articles were included: 22 considering PDAC, 7 for Cholangiocarcinoma (CCA) and 3 for Gallbladder Cancer (GBC). There were no studies investigating Ampullary Cancer. Dysregulated circRNA expression was associated with features of malignancy in vitro, in vivo, and ex vivo. Overall, there have been very few PDAC and BTC tissues profiled for circRNA signatures. Therefore, whilst the current studies have demonstrated some of their functions in these cancers, further work is required to elucidate their potential role as cancer biomarkers in tissue, biofluids and biopsies.

Circular RNAs in liver diseases: Mechanisms and therapeutic targets

Circular RNAs (circRNAs) are formed from the genome through diverse back splicing and feature the closed loop. circRNAs are widely available in a variety of cells and characterized by conservation, structural stability, high abundance and tissue-specific or developmental-specific expression. Recent studies have shown that circRNAs are closely related to liver diseases, such as metabolic-associated fatty liver disease, hepatitis, liver cirrhosis and hepatocellular carcinoma. circRNAs play an important role in the progression of liver diseases, are potential diagnostic and prognostic markers, and have translational value in therapy. This article reviews the research on circRNAs in liver diseases, with a view to providing a theoretical basis and new ideas for future research and treatment of liver diseases.

Circular RNAs in the Brain: A Possible Role in Memory?

Higher-order organisms possess information processing capabilities that are only made possible by their biological complexity. Emerging evidence indicates a critical role for regulatory RNAs in coordinating many aspects of cellular function that are directly involved in experience-dependent neural plasticity. Here, we focus on a structurally distinct class of RNAs known as circular RNAs. These closed loop, single-stranded RNA molecules are highly stable, enriched in the brain, and functionally active in both healthy and disease conditions. Current evidence implicating this ancient class of RNA as a contributor toward higher-order functions such as cognition and memory is discussed.

Circular RNAs in cancer: limitations in functional studies and diagnostic potential

Circular RNAs (circRNAs) are a large class of noncoding RNAs, generated from a process called back-splicing, that possess critical regulatory functions in many cellular events. A large body of literature has reported various circRNA functions and their underlying mechanisms, including sponging miRNA, exerting transcriptional and translational regulation, interacting with proteins, and translating into peptides and proteins. CircRNA dysregulation has been implicated in many cancers, including lung, breast, liver, gastric, colorectal, and ovarian cancer. They are detectable in bodily fluids and relatively stable, making them potential cancer biomarker candidates. Furthermore, targeting circRNA expression levels is a potential therapeutic approach for treating cancers. In this review, we describe the functional mechanisms of circRNAs and discuss limitations of current mechanism studies. Following this, we outline the potential of circRNAs to be effective biomarkers in various cancers and present circRNA-based therapeutic approaches. Finally, we discuss challenges in using circRNAs as diagnostic and therapeutic tools and propose future research directions.

UPF1-Mediated RNA Decay-Danse Macabre in a Cloud

Nonsense-mediated RNA decay (NMD) is the prototype example of a whole family of RNA decay pathways that unfold around a common central effector protein called UPF1. While NMD in yeast appears to be a linear pathway, NMD in higher eukaryotes is a multifaceted phenomenon with high variability with respect to substrate RNAs, degradation efficiency, effector proteins and decay-triggering RNA features. Despite increasing knowledge of the mechanistic details, it seems ever more difficult to define NMD and to clearly distinguish it from a growing list of other UPF1-mediated RNA decay pathways (UMDs). With a focus on mammalian, we here critically examine the prevailing NMD models and the gaps and inconsistencies in these models. By exploring the minimal requirements for NMD and other UMDs, we try to elucidate whether they are separate and definable pathways, or rather variations of the same phenomenon. Finally, we suggest that the operating principle of the UPF1-mediated decay family could be considered similar to that of a computing cloud providing a flexible infrastructure with rapid elasticity and dynamic access according to specific user needs.