Circular RNAs: Novel Regulators of Neuronal Development

Circular RNAs in early brain development and their influence and clinical significance in neuropsychiatric disorders

Neuropsychiatric disorders represent a set of severe and complex mental illnesses, and the exact etiologies of which are unknown. It has been well documented that impairments in the early development of the brain contribute to the pathogenesis of many neuropsychiatric disorders. Currently, the diagnosis of neuropsychiatric disorders largely relies on subjective cognitive assessment, because there are no widely accepted biochemical or genetic biomarkers for diagnosing mental illness. Circular RNAs (circRNAs) are a novel class of endogenous non-coding RNA (ncRNA) with a closed-loop structure. In recent years, there have been tremendous advances in our understanding of the expression profiles and biological roles of circRNAs. In the brain, circRNAs are particularly enriched and are expressed more abundantly in contrast to linear counterpart transcripts. They are highly active at neuronal synapses. These features make circRNAs uniquely crucial for understanding brain health, disease, and neuropsychiatric disorders. This review focuses on the role of circRNAs in early brain development and other brain-related processes that have been associated with the development of neuropsychiatric disorders. In addition, we discuss the potential for blood or cerebrospinal fluid circRNAs to be used as novel biomarkers in the early diagnosis of neuropsychiatric disorders. The findings reviewed here may provide new insight into the pathological mechanisms underlying the onset and progression of neuropsychiatric disorders.

Transcriptomic Profiling of Circular RNA in Different Brain Regions of Parkinson's Disease in a Mouse Model

Parkinson's disease (PD) is the second most common neurodegenerative disease and although many studies have been done on this disease, the underlying mechanisms are still poorly understood and further studies are warranted. Therefore, this study identified circRNA expression profiles in the cerebral cortex (CC), hippocampus (HP), striatum (ST), and cerebellum (CB) regions of the 1-methyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mouse model using RNA sequencing (RNA-seq), and differentially expressed circRNA were validated using reverse transcription quantitative real-time PCR (qRT-PCR). Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and competing endogenous RNA (ceRNA) network analyses were also performed to explore the potential function of circRNAs. The results show that, compared with the control group, 24, 66, 71, and 121 differentially expressed circRNAs (DE-circRNAs) were found in the CC, HP, ST, and CB, respectively. PDST vs. PDCB, PDST vs. PDHP, and PDCB vs. PDHP groups have 578, 110, and 749 DE-circRNAs, respectively. Then, seven DE-cirRNAs were selected for qRT-PCR verification, where the expressions were consistent with the sequencing analysis. The GO and KEGG pathway analyses revealed that these DE-circRNAs participate in several biological functions and signaling pathways, including glutamic synapse, neuron to neuron synapse, cell morphogenesis involved in neuron differentiation, Parkinson's disease, axon guidance, cGMP-PKG signaling pathway, and PI3K-Akt signaling pathway. Furthermore, the KEGG analysis of the target genes predicted by DE-circRNAs indicated that the target genes predicted by mmu_circRNA_0003292, mmu_circRNA_0001320, mmu_circRNA_0005976, and mmu_circRNA_0005388 were involved in the PD-related pathway. Overall, this is the first study on the expression profile of circRNAs in the different brain regions of PD mouse model. These results might facilitate our understanding of the potential roles of circRNAs in the pathogenesis of PD. Moreover, the results also indicate that the mmu_circRNA_0003292-miRNA-132-Nr4a2 pathway might be involved in the regulation of the molecular mechanism of Parkinson's disease.

Circulating RNAs as Potential Biomarkers in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a complex multi-system neurodegenerative disorder with currently limited diagnostic and no therapeutic options. Despite the intense efforts no clinically applicable biomarkers for ALS are yet established. Most current research is thus focused, in particular, in identifying potential non-invasive circulating biomarkers for more rapid and accurate diagnosis and monitoring of the disease. In this review, we have focused on messenger RNA (mRNA), non-coding RNAs (lncRNAs), micro RNAs (miRNAs) and circular RNA (circRNAs) as potential biomarkers for ALS in peripheral blood serum, plasma and cells. The most promising miRNAs include miR-206, miR-133b, miR-27a, mi-338-3p, miR-183, miR-451, let-7 and miR-125b. To test clinical potential of this miRNA panel, a useful approach may be to perform such analysis on larger multi-center scale using similar experimental design. However, other types of RNAs (lncRNAs, circRNAs and mRNAs) that, together with miRNAs, represent RNA networks, have not been yet extensively studied in blood samples of patients with ALS. Additional research has to be done in order to find robust circulating biomarkers and therapeutic targets that will distinguish key RNA interactions in specific ALS-types to facilitate diagnosis, predict progression and design therapy.

Role of circular RNAs in brain development and CNS diseases

In mammals, many classes of noncoding RNAs (ncRNAs) are expressed at a much higher level in the brain than in other organs. Recent studies have identified a new class of ncRNAs called circular RNAs (circRNAs), which are produced by back-splicing and fusion of either exons, introns, or both exon-intron into covalently closed loops. The circRNAs are also highly enriched in the brain and increase continuously from the embryonic to the adult stage. Although the functional significance and mechanism of action of circRNAs are still being actively explored, they are thought to regulate the transcription of their host genes and sequestration of miRNAs and RNA binding proteins. Some circRNAs are also shown to have translation potential to form peptides. The expression and abundance of circRNAs seem to be spatiotemporally maintained in a normal brain. Altered expression of circRNAs is also thought to mediate several disorders, including brain-tumor growth, and acute and chronic neurodegenerative disorders by affecting mechanisms such as angiogenesis, neuronal plasticity, autophagy, apoptosis, and inflammation. This review discusses the involvement of various circRNAs in brain development and CNS diseases. A better understanding of the circRNA function will help to develop novel therapeutic strategies to treat CNS complications.

Silencing of circular RNA HIPK2 in neural stem cells enhances functional recovery following ischaemic stroke

Background

Circular RNAs (circRNAs) have been reported to be involved in central nervous system (CNS) diseases and to have a close connection with neuronal development. However, the role of circRNAs in neural stem cell (NSC) differentiation and the treatment of ischaemic stroke remains unknown.

Methods

Ischaemic stroke was induced in mice using transient middle cerebral artery occlusion (tMCAO). NSCs were transducted with circHIPK2 siRNA (si-circHIPK2-NSCs) or vehicle control (si-circCon-NSCs) and microinjected into lateral ventricle of brain at 7 d post-tMCAO. Magnetic resonance imaging (MRI) was used to detect brain damage, and functional deficits were evaluated with sensorimotor behavioural tests. The distribution of the transplanted NSCs was investigated by near-infrared fluorescence imaging (NIF) and immunofluorescence. The neural plasticity of si-circHIPK2-NSCs was verified by western blot and immunofluorescence in vivo and in vitro.

Findings

We investigated the role of circHIPK2 in NCS differentiation. In vitro, silencing of circHIPK2 facilitated NSCs directionally differentiated to neurons but had no effect on the differentiation to astrocytes. In vivo, microinjected NSCs could migrate to the ischaemic hemisphere after stroke induction. Si-circHIPK2-NSCs increased neuronal plasticity in the ischaemic brain, conferred long-lasting neuroprotection, and significantly reduced functional deficits.

Interpretations

Si-circHIPK2 regulates NSC differentiation, and microinjection of si-circHIPK2-NSCs exhibits a promising therapeutic strategy to neuroprotection and functional recovery after stroke.

Funding

The National Key Research and Development Program of China; the International Cooperation and Exchange of the National Natural Science Foundation of China; the National Natural Science Foundation of China; the Jiangsu Innovation & Entrepreneurship Team Program.

Circular RNA MAPK4 (circ-MAPK4) inhibits cell apoptosis via MAPK signaling pathway by sponging miR-125a-3p in gliomas

Background

Recent evidences have shown that circular RNAs (circRNAs) are frequently dysregulated and play paramount roles in various cancers. circRNAs are abundant in central nervous system (CNS); however, few studies describe the clinical significance and role of circRNAs in gliomas, which is the most common and aggressive primary malignant tumor in the CNS.

Methods

A bioinformatics analysis was performed to profile and screen the dyregulated circRNAs during early neural development. Quantitative real-time PCR was used to detect the expression of circ-MAPK4 and target miRNAs. Glioma cells were transfected with circ-MAPK4 siRNAs, then cell proliferation, apoptosis, transwell assays, as well as tumorigenesis and TUNEL assays, were performed to examine effect of circ-MAPK4 in vitro and vivo. Biotinylated-circ-MAPK4 probe based pull-down assay was conducted to confirm the relationship between circ-MAPK4 and miR-125-3p.

Results

In this study, we identified a circRNA, circ-MAPK4 (has_circ_0047688), which was downregulated during early neural differentiation. In gliomas, circ-MAPK4 acted as an oncogene, was inversely upregulated and linked to clinical pathological stage of gliomas (P < 0.05). Next, we verified that circ-MAPK4 promoted the survival and inhibited the apoptosis of glioma cells in vitro and in vivo. Furthermore, we proved that circ-MAPK4 was involved in regulating p38/MAPK pathway, which affected glioma proliferation and apoptosis. Finally, miR-125a-3p, a miRNA exhibited tumor-suppressive function through impairing p38/MAPK pathway, which was increased by inhibiting circ-MAPK4 and could be pulled down by circ-MAPK4. Inhibition of miR-125a-3p could partly rescue the increased phosphorylation levels of p38/MAPK and the elevated amount of apoptosis inducing by knockdown of circ-MAPK4.

Conclusions

Our findings suggest that circ-MAPK4 is a critical player in glioma cell survival and apoptosis via p38/MAPK signaling pathway through modulation of miR-125a-3p, which can serve as a new therapeutic target for treatment of gliomas.

The Role of Circular RNAs in Brain Injury

Circular RNAs are an increasingly important topic in non-coding RNA biology, drawing considerable attention in recent years. Accumulating evidence suggests a critical role for circular RNAs in both early and latent stages of disease pathogenesis. Circular RNAs are abundantly expressed in brain tissue, with significant implications for neural development and disease progression. Disruption of these processes, including those seen in response to brain injury, can have serious consequences such as hemiplegia, aphasia, coma, and death. In this review, we describe the role of circular RNAs in the context of brain injury and explore the potential connection between circular RNAs, brain hypoxic ischemic injury, ischemia-reperfusion injury, and traumatic injury.

Abnormal expression of rno_circRNA_014900 and rno_circRNA_005442 induced by ketamine in the rat hippocampus

BACKGROUND

Recent studies have shown that circular RNA (circRNA) is rich in microRNA (miRNA) binding sites. We have previously demonstrated that the antidepressant effect of ketamine is related to the abnormal expression of various miRNAs in the brain. This study determined the expression profile of circRNAs in the hippocampus of rats treated with ketamine.

METHODS

The aberrantly expressed circRNAs in rat hippocampus after ketamine injection were analyzed by microarray chip, and we further validated these circRNAs by quantitative reverse-transcription PCR (qRT-PCR). The target genes of the different circRNAs were predicted using bioinformatic analyses, and the functions and signal pathways of these target genes were investigated by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses.

RESULTS

Microarray analysis showed that five circRNAs were aberrantly expressed in rat hippocampus after ketamine injection (fold change > 2.0, p < 0.05). The results from the qRT-PCR showed that one of the circRNAs was significantly increased (rno_circRNA_014900; fold change = 2.37; p = 0.03), while one was significantly reduced (rno_circRNA_005442; fold change = 0.37; p = 0.01). We discovered a significant enrichment in several GO terms and pathways associated with depression.

CONCLUSION

Our findings showed the abnormal expression of ketamine-induced hippocampal circRNAs in rats.

Circular RNAs as Potential Blood Biomarkers in Amyotrophic Lateral Sclerosis

Circular RNAs (circRNAs) are emerging as a novel, yet powerful player in many human diseases. They are involved in several cellular processes and are becoming a noteworthy type of biomarkers. Among other functions, circRNAs can serve as RNA sponges or as scaffolds for RNA-binding proteins. Here, we investigated a microarray expression profile of circRNAs in leukocyte samples from ALS patients and age- and sex-matched healthy controls to identify differentially expressed circRNAs. We selected 10 of them for a qPCR validation of expression on a larger set of samples, identification of their associations with clinical parameters, and evaluation of their diagnostic potential. In total, expression of 7/10 circRNAs was significant in a larger cohort of ALS patients, compared with age- and sex-matched healthy controls. Three of them (hsa_circ_0023919, hsa_circ_0063411, and hsa_circ_0088036) showed the same regulation as in microarray results. These three circRNAs also had AUC > 0.95, and sensitivity and specificity for the optimal threshold point > 90%, showing their potential for using them as diagnostic biomarkers.

Fine-tuning of microRNAs in Type 2 Diabetes Mellitus

Type 2 diabetes mellitus is a metabolic disease widely spread across industrialized countries. Sedentary lifestyle and unhealthy alimentary habits lead to obesity, boosting both glucose and fatty acid in the bloodstream and eventually, insulin resistance, pancreas inflammation and faulty insulin production or secretion, all of them very well-defined hallmarks of type 2 diabetes mellitus. miRNAs are small sequences of non-coding RNA that may regulate several processes within the cells, fine-tuning protein expression, with an unexpected and subtle precision and in time-frames ranging from minutes to days. Since the discovery of miRNA and their possible implication in pathologies, several groups aimed to find a relationship between type 2 diabetes mellitus and miRNAs. Here we discuss the pattern of expression of different miRNAs in cultured cells, animal models and diabetic patients. We summarize the role of the most important miRNAs involved in pancreas growth and development, insulin secretion and liver, skeletal muscle or adipocyte insulin resistance in the context of type 2 diabetes mellitus.

The biogenesis, biology and characterization of circular RNAs

Circular RNAs (circRNAs) are covalently closed, endogenous biomolecules in eukaryotes with tissue-specific and cell-specific expression patterns, whose biogenesis is regulated by specific cis-acting elements and trans-acting factors. Some circRNAs are abundant and evolutionarily conserved, and many circRNAs exert important biological functions by acting as microRNA or protein inhibitors ('sponges'), by regulating protein function or by being translated themselves. Furthermore, circRNAs have been implicated in diseases such as diabetes mellitus, neurological disorders, cardiovascular diseases and cancer. Although the circular nature of these transcripts makes their detection, quantification and functional characterization challenging, recent advances in high-throughput RNA sequencing and circRNA-specific computational tools have driven the development of state-of-the-art approaches for their identification, and novel approaches to functional characterization are emerging.

Silencing circular RNA hsa_circ_0004491 promotes metastasis of oral squamous cell carcinoma

AIMS

Oral squamous cell carcinoma (OSCC) is an oral cavity malignancy which is the eighth most common cancer. However, the molecular mechanisms underlying the pathogenesis of OSCC remain largely unknown, and effective methods for the prognosis and diagnosis of this disease are lacking. Circular RNAs (circRNAs) are widely expressed among mammals. A growing number of studies have shown that circRNA expression is altered in cancers. In this study, we investigated the role of hsa_circ_0004491 in OSCC cell migration and invasion and examined the clinical characteristics associated with its expression.

MAIN METHODS

Hsa_circ_0004491 expression was examined in 40 paired OSCC and normal tissue samples using quantitative real-time PCR (qRT-PCR). Wound healing, transwell and western blotting assays were conducted in OSCC cells up- or down-regulation of hsa_circ_0004491 to evaluate changes in cell migration, invasion and protein expression. KEYFINDINGS: qRT-PCR analysis revealed that hsa_circ_0004491 was significantly downregulated in OSCC tissues compared with paired normal tissues (P < 0.001), and this low expression was associated with lymph node metastasis (P = 0.0398; area under ROC curve = 0.7510). Hsa_circ_000449 silencing or overexpression significantly affected the invasion and migration abilities of OSCC cells. Western blotting analysis showed that EMT-related proteins expression was significantly changed after hsa_circ_000449 silencing or overexpression.

SIGNIFICANCE

The expression level of hsa_circ_0004491 affects the migration and invasion of OSCC cells. Hsa_circ_0004491 may therefore play a role in the progression of oral squamous cell carcinoma.

Expression profile of circular RNA s in TMJ osteoarthritis synovial tissues and potential functions of hsa_circ_0000448 with specific back-spliced junction

OBJECTIVE

As essential players in the occurrence and development of osteoarthritis, circular RNAs (circRNAs) have gradually received attention in recent years. However, their roles in temporomandibular joint osteoarthritis (TMJOA) featured with pain, restricted mouth opening and joint movement dysfunction, remains elusive.

METHODS

The expression profile of circRNAs in TMJ synovial tissues was analyzed by RNA-Seq. The differentially expressed circRNAs in the TMJOA were identified. The potential biological functions of these circRNAs were evaluated.

RESULTS

The expression profile of circRNAs in TMJ synovial tissues was stable and abundant, and most of which were newly discovered. A total of 58 differentially expressed circRNAs were identified in TMJOA, and four of which were identified by in vitro experiments. Among them, the up-regulated hsa_circ_0000448 with specific back-splice junction was involved in the TNF-α signaling pathway through CeRNA mechanism by targeting related microRNAs. Additionally, it was also predicted to bind several RNA binding proteins (RBPs), but almost had no protein-coding ability.

CONCLUSION

circRNAs in TMJ synovial tissue participate in the progression of TMJOA and may become a potential therapeutic target. The highly up-regulated has_circ_0000448 probably promotes TNF-α secretion of synovium through CeRNA mechanism in TMJOA.

CircPOMT1 and circMCM3AP inhibit osteogenic differentiation of human adipose-derived stem cells by targeting miR-6881-3p

Circular RNAs (circRNAs), novel endogenous non-coding RNAs with the special circular structure, have been found to play critical roles in various development of tissues and diseases. However, few studies have focused on the functions and mechanisms of circRNAs in the osteogenesis of human adipose-derived stem cells (hASCs). Here, we performed the circRNAs sequencing and bioinformatic analysis to investigate the expression profiles of hASCs during osteogenic differentiation. There were 150 upregulated circRNAs and 60 downregulated circRNAs expressed differentially. Among them, the expression of circPOMT1 and circMCM3AP were downregulated during the osteogenesis of hASCs. hsa-miR-6881-3p could promote the osteogenic differentiation of hASCs, while the expression of circPOMT1 and circMCM3AP were negatively correlated with it. Smad6 and Chordin, critical inhibitors of the BMPs signaling pathway, were predicted to be the targets of hsa-miR-6881-3p. Therefore, circPOMT1 and circMCM3AP might influence the osteogenic differentiation of hASCs by targeting hsa-miR-6881-3p via BMPs signaling pathway. CircPOMT1 and circMCM3AP are potential novel targets for the repairment of bone defects.

Abnormal expression of circRNA_089763 in the plasma exosomes of patients with post‑operative cognitive dysfunction after coronary artery bypass grafting

Post-operative cognitive dysfunction (POCD) is a complication of the central nervous system characterized by mental disorders, anxiety, personality changes and impaired memory. POCD occurs frequently after coronary artery bypass grafting (CABG) and can severely affect quality of life for patients. To date, the development of POCD biomarkers remains a challenge. Alterations in the expression of non-coding RNAs from brain tissue and peripheral blood have been linked to POCD. The present study aimed to detect the differential circular RNAs (circRNAs) in plasma exosomes of patients with POCD after CABG. The relative expression levels of circRNAs were analyzed using circRNA microarray analysis in the plasma exosomes of patients with POCD. Differentially altered circRNAs (P<0.05, fold change >1.5) were validated by reverse transcription-quantitative PCR in the plasma exosomes of patients with POCD. The target genes of the microRNAs were predicted using bioinformatics analysis. The functions and signaling pathways of these target genes were investigated by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes analyses. The microarray results indicated that the levels of nine circRNAs in patients with POCD were higher than those in the control subjects; and six circRNAs were at a lower level than those in control subjects. The RT-qPCR results from patients with POCD showed that only circRNA_089763 of the 15 circRNAs identified was significantly increased compared with control subjects. circRNA target gene prediction and functional annotation analysis showed significant enrichment in several GO terms and pathways associated with POCD. The present study provides evidence for the abnormal expression of POCD-induced circRNA_089763 in human plasma exosomes, as well as the involvement of POCD.

Epigenetics in Neurodevelopment: Emerging Role of Circular RNA

Canonical epigenetic modifications, including DNA methylation, histone modification and chromatin remodeling, play a role in numerous life processes, particularly neurodevelopment. Epigenetics explains the development of cells in an organism with the same DNA sequence into different cell types with various functions. However, previous studies on epigenetics have only focused on the chromatin level. Recently, epigenetic modifications of RNA, which mainly include 6-methyladenosine (m⁶A), pseudouridine, 5-methylcytidine (m⁵C), inosine (I), 2′-O-ribosemethylation, and 1-methyladenosine (m¹A), have gained increasing attention. Circular RNAs (circRNAs), which are a type of non-coding RNA without a 5′ cap or 3′ poly (A) tail, are abundantly found in the brain and might respond to and regulate synaptic function. Also, circRNAs have various functions, such as microRNA sponge, regulation of gene transcription and interaction with RNA binding protein. In addition, circRNAs are methylated by N⁶-methyladenosine (m⁶A). In this review, we discuss the crucial roles of epigenetic modifications of circRNAs, such as m⁶A, in the genesis and development of neurons and in synaptic function and plasticity. Thus, this type of changes in circRNAs might be a therapeutic target in central nervous system (CNS) disorders and could aid the diagnosis and treatment of these disorders.

Exosomal circRNAs: biogenesis, effect and application in human diseases

Exosomes have emerged as critical mediators of intercellular communication, both locally and systemically, by regulating a diverse range of biological processes between cells. Circular RNA (circRNA) is a novel member of endogenous noncoding RNAs with widespread distribution and diverse cellular functions. Recently, circular RNAs have been identified for their enrichment and stability in exosomes. In this review, we outline the origin, biogenesis and function of exosomal circRNAs as well as their roles in various diseases. Although their precise roles and mechanisms of gene regulation remain largely elusive, exosomal circRNAs have potential applications as disease biomarkers and novel therapeutic targets.

High-Throughput Data Reveals Novel Circular RNAs via Competitive Endogenous RNA Networks Associated with Human Intracranial Aneurysms

Background

Little is known about epigenetic regulation of intracranial aneurysms (IAs). Circular non-coding RNAs (circRNAs) play crucial roles in cardiovascular diseases, but they have received scant research attention regarding their relationship with IAs. This study aimed to explore new pathological mechanisms of IA through circRNA expression profiles and to provide novel therapeutic strategies.

Material/Methods

The comprehensive circRNA and mRNA expression profiles were detected by RNA-Seq in human IA walls and superficial temporal arteries (STAs). The RNA-Seq findings were validated by qRT-PCR. GO and KEGG analyses indicated the functions of these circRNAs. A competing endogenous RNA (ceRNA) network was constructed to reveal the circRNA-miRNA-mRNA relationship. Two newly discovered circRNAs were further detected in peripheral blood of IA patients and healthy people to clarify their expression patterns in the periphery.

Results

Many differentially expressed circRNAs are closely involved in immune/inflammatory response and cell adhesion/adherens junction. The novel circRNAs (hsa_circ_0072309 and hsa_circ_0008433) regulate DDR2 and MMP2, respectively, which are associated with SMC dysfunction and vascular injury through ceRNA. Moreover, we found differential expression of these 2 circRNAs in the peripheral blood of IA patients, and the expression pattern of hsa_circ_0072309 had central and peripheral consistency.

Conclusions

To the best of our knowledge, this is the first study to perform circRNA sequencing analysis of IAs. hsa_circ_0072309 and hsa_circ_0008433 are novel and pivotal circRNAs related to IAs. This study provides new insights into therapeutic targets and biomarkers for IA patients.

Circular RNAs constitute an inherent gene regulatory axis in the mammalian eye and brain

Circular RNAs (circRNAs) are being hailed as a newly rediscovered class of covalently closed transcripts that are produced via alternative, noncanonical pre-mRNA back-splicing events. These single-stranded RNA molecules have been identified in organisms ranging from the worm (Cortés-López et al. 2018. BMC Genomics, 19: 8; Ivanov et al. 2015. Cell Rep. 10: 170–177) to higher eukaryotes (Yang et al. 2017. Cell Res. 27: 626–641) to plants (Li et al. 2017. Biochem. Biophys. Res. Commun. 488: 382–386). At present, research on circRNAs is an active area because of their diverse roles in development, health, and diseases. Partly because their circularity makes them resistant to degradation, they hold great promise as unique biomarkers for ocular and central nervous system (CNS) disorders. We believe that further work on their applications could help in developing them as “first-in-class” diagnostics, therapeutics, and prognostic targets for numerous eye conditions. Interestingly, many circRNAs play key roles in transcriptional regulation by acting as miRNAs sponges, meaning that they serve as master regulators of RNA and protein expression. Since the retina is an extension of the brain and is part of the CNS, we highlight the current state of circRNA biogenesis, properties, and function and we review the crucial roles that they play in the eye and the brain. We also discuss their regulatory roles as miRNA sponges, regulation of their parental genes or linear mRNAs, translation into micropeptides or proteins, and responses to cellular stress. We posit that future advances will provide newer insights into the fields of RNA metabolism in general and diseases of the aging eye and brain in particular. Furthermore, in keeping pace with the rapidly evolving discipline of RNA“omics”-centered metabolism and to achieve uniformity among researchers, we recently introduced the term “cromics” (circular ribonucleic acids based omics) (Singh et al. 2018. Exp. Eye Res. 174: 80–92).

A comprehensive profile of the tilapia (Oreochromis niloticus) circular RNA and circRNA-miRNA network in the pathogenesis of meningoencephalitis of teleosts

The pathogenesis of tilapia meningoencephalitis is still unclear, where the involvement of circRNA is considered for its active role as a "miRNA sponge". Therefore, we aimed to investigate the profile of circRNA in tilapia meningoencephalitis further by constructing the circRNA-miRNA network for in-depth mechanism exploration. Briefly, a nile tilapia model of meningitis was established by injecting Streptococcus agalactiae (1.0 × 10⁷ cfu per mL) and we evaluated the infected tilapia brain for the expression profile of circRNAs, their potential functions and their correlation with genes involved in inflammatory pathways. A total of 11 263 circRNAs were identified from RNA sequencing (RNA-seq) data in nile tilapia (Oreochromis niloticus), a commercially important fish in China and East Asia. GO and KEGG analyses revealed that the biological functions of genes hosting the circRNAs were enriched in the progression of metabolism and binding. Notably, we found that 99% circRNAs in tilapia had abundant miRNA-binding sites and a total of 2136 of the identified circRNAs had two to six miRNA-binding sites. Six circRNAs were validated by qRT-PCR and the final circRNA-miRNA network was constructed. This is the first report of comprehensive identification of O. niloticus circRNAs being differentially regulated in the brain in normal conditions relating to S. agalactiae infection. This work will shed novel light on gene expression mechanisms under disease conditions and may identify circRNAs as new biomarkers for meningoencephalitis and neurodegenerative disorders.

Role of Circular RNAs in Preeclampsia

Circular RNAs (circRNAs) are noncoding RNAs characterized by circular covalently closed structures, which are generated by back-splicing. circRNA is more stable and conserved than linear RNA and exists in various organisms. Preeclampsia (PE), a common hypertensive disorder of pregnancy, has a profound impact on maternal and neonatal mortality and morbidity. Recent studies demonstrated that circRNAs were differentially expressed in PE maternal-fetal interface compared with those in the control and might mediate pathological processes in pregnancy complications. However, the mechanisms of action of circRNAs in PE are still unclear. Here, we provide a comprehensive review on the current state of knowledge on circRNAs associated with PE. We summarize the known expression profiles of circRNAs and discuss their potential application as biomarkers of PE. The possible mechanisms underlying circRNA dysregulation in the etiology of PE are also explored.

Circular RNAs: Diversity of Functions and a Regulatory Nova in Oral Medicine: A Pilot Review

Oral diseases, such as cancers, inflammation, loss of bone/tooth/soft tissues, are serious threats to human health since some can cause systemic disease and effective treatments are limited. Thus, discovering promising biomarkers for physiological and pathological processes in oral medicine, and identifying novel targets for therapy have become a most critical issue. Recently, circular RNAs (circRNAs), which were once thought to be a class of non-coding RNAs (ncRNAs), are found to be of coding potential. CircRNAs are highly present in the cytoplasm of eukaryotic cells and are key elements in the physiological and biological processes of various pathological conditions, and are also reflected in oral development and progress. Previous studies have indicated that circRNAs are involved in the initiation and development of different types of diseases and tissues (e.g., cancers, cardiovascular diseases, neural development, growth and development, wood healing, liver regeneration). Moreover, growing evidence demonstrates that circRNAs play vital roles in oral cancers and osteogenic differentiation of periodontal ligament stem cells (PDLSCs). Here, we focus on the biological characteristics of circRNAs, beginning with an overview of previous studies on the functional roles of circRNAs as diagnostic biomarkers and therapeutic targets in oral medicine. We hope this will give us a promising new comprehension of the underlying mechanisms occurring during related biological and pathological progress, and contribute to the development of effective diagnostic biomarkers and therapeutic targets for oral diseases.

Circular RNA expression and function in the brain

Within the last decade, active research on circular RNAs (circRNAs) has dramatically improved our understanding of the expression and function of these non-coding RNAs. While several mechanisms for circRNA function have been proposed, including sequestration of microRNAs and regulation of cellular proteins, studies provide evidence that circRNAs can regulate transcription and may also serve as biomarkers. Due to the heterogeneous nature of the brain, and the dynamic transcriptional mechanisms that support neurobiological pathways, the influence of circRNAs is potentially extensive. Understanding how circRNAs contribute to key neurological pathways will fill gaps in our understanding of brain function and provide valuable insight into novel therapeutic approaches to treat neurological diseases. Here, we review recent research on circRNA expression in the brain, describe the proposed functions of circRNAs, and evaluate the role of circRNAs in neurological diseases.

Insights into the biogenesis and potential functions of exonic circular RNA

Circular RNAs (circRNAs) exhibit unique properties due to their covalently closed nature. Models of circRNAs synthesis and function are emerging but much remains undefined about this surprisingly prevalent class of RNA. Here, we identified exonic circRNAs from human and mouse RNA-sequencing datasets, documenting multiple new examples. Addressing function, we found that many circRNAs co-sediment with ribosomes, indicative of their translation potential. By contrast, circRNAs with potential to act as microRNA sponges were scarce, with some support for a collective sponge function by groups of circRNAs. Addressing circRNA biogenesis, we delineated several features commonly associated with circRNA occurrence. CircRNA-producing genes tend to be longer and to contain more exons than average. Back-splice acceptor exons are strongly enriched at ordinal position 2 within genes, and circRNAs typically have a short exon span with two exons being the most prevalent. The flanking introns either side of circRNA loci are exceptionally long. Of note also, single-exon circRNAs derive from unusually long exons while multi-exon circRNAs are mostly generated from exons of regular length. These findings independently validate and extend similar observations made in a number of prior studies. Furthermore, we analysed high-resolution RNA polymerase II occupancy data from two separate human cell lines to reveal distinctive transcription dynamics at circRNA-producing genes. Specifically, RNA polymerase II traverses the introns of these genes at above average speed concomitant with an accentuated slow-down at exons. Collectively, these features indicate how a perturbed balance between transcription and linear splicing creates important preconditions for circRNA production. We speculate that these preconditions need to be in place so that looping interactions between flanking introns can promote back-splicing to raise circRNA production to appreciable levels.

Decoding epigenetic cell signaling in neuronal differentiation

Neurogenesis is the process by which new neurons are generated in the brain. Neural stem cells (NSCs) are differentiated into neurons, which are integrated into the neural network. Nowadays, pluripotent stem cells, multipotent stem cells, and induced pluripotent stem cells can be artificially differentiated into neurons utilizing several techniques. Specific transcriptional profiles from NSCs during differentiation are frequently used to approach and observe phenotype alteration and functional determination of neurons. In this context, the role of non-coding RNA, transcription factors and epigenetic changes in neuronal development and differentiation has gained importance. Epigenetic elucidation has become a field of intense research due to distinct patterns of normal conditions and different neurodegenerative disorders, which can be explored to develop new diagnostic methods or gene therapies. In this review, we discuss the complexity of transcription factors, non-coding RNAs, and extracellular vesicles that are responsible for guiding and coordinating neural development.

Circular RNA expression profiles during the differentiation of mouse neural stem cells

Background

Circular RNAs (circRNAs) have recently been found to be expressed in human brain tissue, and many lines ofevidence indicate that circRNAs play regulatory roles in neurodevelopment. Proliferation and differentiation of neural stem cells (NSCs) are critical parts during development of central nervous system (CNS).To date, there have been no reports ofcircRNA expression profiles during the differentiation of mouse NSCs. We hypothesizethat circRNAs mayregulate gene expression in the proliferation anddifferentiation of NSCs.

Results

In this study, we obtained NSCs from the wild-type C57BL/6 J mouse fetal cerebral cortex. We extracted total RNA from NSCs in different differentiation stagesand then performed RNA-seq. By analyzing the RNA-Seq data, we found 37circRNAs and 4182 mRNAs differentially expressedduringthe NSC differentiation. Gene Ontology (GO) enrichment analysis of thecognate linear genes of these circRNAsrevealed that some enriched GO terms were related to neural activity. Furthermore, we performed a co-expression network analysis of these differentially expressed circRNAs and mRNAs. The result suggested a stronger GO enrichmentin neural features for both the cognate linear genes of circRNAs and differentially expressed mRNAs.

Conclusion

We performed the first circRNA investigation during the differentiation of mouse NSCs. Wefound that12 circRNAs might have regulatory roles duringthe NSC differentiation, indicating that circRNAs might be modulated during NSC differentiation.Our network analysis suggested the possible complex circRNA-mRNA mechanisms during differentiation, and future experimental workis need to validate these possible mechanisms.

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Long noncoding RNA (lncRNA): a target in neuropathic pain

INTRODUCTION

Current treatments for neuropathic pain are limited in part due to the incomplete understanding of its underlying mechanisms. Recent evidence reveals the dysregulated expression of long non-coding RNAs (lncRNAs) in the damaged nerve, dorsal root ganglion (DRG), and spinal cord dorsal horn following peripheral nerve injury. However, the role of the majority of lncRNAs in neuropathic pain genesis is still elusive. Unveiling the mechanisms of how lncRNAs participate in neuropathic pain may develop new strategies to prevent and/or treat this disorder. Areas covered: This review focuses on the dysregulation of lncRNAs in the DRG, dorsal horn, and the injured nerves from preclinical models of neuropathic pain. We provide evidence of how peripheral nerve injury causes the dysregulation of lncRNAs in these pain-related regions. The potential mechanisms of how dysregulated lncRNAs contribute to the pathogenesis of neuropathic pain are discussed. Expert opinion: The investigation on the role of the dysregulated lncRNAs in neuropathic pain might open up a novel avenue for therapeutic treatment of this disorder. However, current investigation is at the infancy stage, which challenges the translation of preclinical findings. More intensive studies on lncRNAs are required before the preclinical findings are translated into therapeutic management for neuropathic pain.

Circular RNAs: Functions and Prospects in Glioma

Improving the survival rate of patients with glioma, a malignant tumor of the human brain has become increasingly important. In recent years, the function of circular RNAs (circRNAs) in different diseases and the pathophysiological mechanisms involved have been elucidated. In the pathophysiological mechanism, the primary function of circRNAs is to act as microRNA sponges. An increasing number of studies have found that circRNAs are differentially expressed in gliomas and regulate the occurrence, proliferation, and invasion of glioma and thus may be potential markers for the diagnosis of gliomas. Additionally, some circRNAs have been associated with glioma staging and may be useful in determining prognosis. Based on the stability and high conservation of circRNAs, we believe that circRNAs may have molecular targets that are useful for the treatment of glioma. In this review, we summarize the current research regarding the role of circRNAs in gliomas, discuss the potential value and role of circRNAs in gliomas, and provide new perspectives for future research.

Expression profiling and in situ screening of circular RNAs in human tissues

Circular RNAs (circRNAs) were recently discovered as a class of widely expressed noncoding RNA and have been implicated in regulation of gene expression. However, the function of the majority of circRNAs remains unknown. Studies of circRNAs have been hampered by a lack of essential approaches for detection, quantification and visualization. We therefore developed a target-enrichment sequencing method suitable for screening of circRNAs and their linear counterparts in large number of samples. We also applied padlock probes and in situ sequencing to visualize and determine circRNA localization in human brain tissue at subcellular levels. We measured circRNA abundance across different human samples and tissues. Our results highlight the potential of this RNA class to act as a specific diagnostic marker in blood and serum, by detection of circRNAs from genes exclusively expressed in the brain. The powerful and scalable tools we present will enable studies of circRNA function and facilitate screening of circRNA as diagnostic biomarkers.

Post-transcriptional Processing of mRNA in Neurons: The Vestiges of the RNA World Drive Transcriptome Diversity

Neurons are morphologically complex cells that rely on the compartmentalization of protein expression to develop and maintain their extraordinary cytoarchitecture. This formidable task is achieved, at least in part, by targeting mRNA to subcellular compartments where they are rapidly translated. mRNA transcripts are the conveyor of genetic information from DNA to the translational machinery, however, they are also endowed with additional functions linked to both the coding sequence (open reading frame, or ORF) and the flanking 5′ and 3′ untranslated regions (UTRs), that may harbor coding-independent functions. In this review, we will highlight recent evidences supporting new coding-dependent and -independent functions of mRNA and discuss how nuclear and cytoplasmic post-transcriptional modifications of mRNA contribute to localization and translation in mammalian cells with specific emphasis on neurons. We also describe recently developed techniques that can be employed to study RNA dynamics at subcellular level in eukaryotic cells in developing and regenerating neurons.

Epigenetic Alterations in Anesthesia-Induced Neurotoxicity in the Developing Brain

Before birth and early in life, the developing brain is particularly sensitive to environmental and pharmacological influences. Increasing experimental evidence suggests that an association exists between exposure to anesthesia during a vulnerable period of brain development and subsequent poor neurodevelopmental outcomes. However, the mechanisms underlying this association are not fully understood. Epigenetics, broadly defined as the regulation of gene expression without alterations of DNA sequence, has become a field of tremendous interest in neuroscience. In recent years, a growing body of literature suggests that anesthesia-induced long-term changes in gene transcription and functional deficits in learning and behavior later in life are mediated via epigenetic modifications. This brief review provides an overview of epigenetic mechanisms and highlights the emerging roles played by epigenetic dysfunctions in the processes of anesthesia-induced neurotoxicity in the developing brain. Epigenetic targeting of DNA methyltransferases and/or histone deacetylases may have some therapeutic value. Epigenetics may lead to the identification of novel markers that contribute toward considerable translational significance in the field of neuroprotection.

Circular RNA expression profiles in hippocampus from mice with perinatal glyphosate exposure

Glyphosate is the active ingredient in numerous herbicide formulations. The roles of glyphosate in embryo-toxicity and neurotoxicity have been reported in human and animal models. Recently, several studies have reported evidence linking neurodevelopmental disorders (NDDs) with gestational glyphosate exposure. However, the role of glyphosate in neuronal development is still not fully understood. Our previous study found that perinatal glyphosate exposure resulted in differential microRNA expression in the prefrontal cortex of mouse offspring. However, the mechanism of glyphosate-induced neurotoxicity in the developing brain is still not fully understood. Considering the pivotal role of Circular RNAs (circRNAs) in the regulation of gene expression, a circRNA microarray method was used in this study to investigate circRNA expression changes in the hippocampus of mice with perinatal glyphosate exposure. The circRNA microarrays revealed that 663 circRNAs were significantly altered in the perinatal glyphosate exposure group compared with the control group. Among them, 330 were significantly upregulated, and the other 333 were downregulated. Furthermore, the relative expression levels of mmu-circRNA-014015, mmu-circRNA-28128 and mmu-circRNA-29837 were verified using quantitative real-time polymerase chain reaction (qRT-PCR). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses demonstrated that stress-associated steroid metabolism pathways, such as aldosterone synthesis and secretion pathways, may be involved in the neurotoxicity of glyphosate. These results showed that circRNAs are aberrantly expressed in the hippocampus of mice with perinatal glyphosate exposure and play potential roles in glyphosate-induced neurotoxicity.

Role of non-coding RNAs in cardiotoxicity of chemotherapy

The long-time paradoxical situation of non-coding RNAs (ncRNAs) has been terminated for they emerge as executive at full spectrum of gene expression and translation. More recently, it has been demonstrated that some ncRNAs apparently are associated with chemotherapy, causing cardiotoxicity, which taint long-term recovery of patients in growing body of evidence. The current review focused on up-to-date knowledge on regulation change and molecular signaling of ncRNAs, at mean time evaluate their potentials as diagnostic biomarkers or therapeutic targets to monitor and protect cardio function.

An Emerging Role for Circular RNAs in Osteoarthritis

Circular RNAs (circRNAs) are currently classed as non-coding RNAs that, unlike the better known canonical linear RNAs, form a covalently closed continuous loop without 5' or 3' polarities. With the development of high throughput sequencing technology, a large number of circRNAs have been discovered in many species. More importantly, growing evidence suggests that circRNAs are abundant, evolutionally conserved, and relatively stable in cells and tissues. Strikingly, recent studies have discovered that circRNAs can serve as microRNA sponges, interact with RNA-binding protein, and regulate gene transcription, as well as protein translation. Osteoarthritis (OA) is the most common chronic degenerative joint disease. CircRNAs are differentially expressed in OA cartilage. Moreover, some circRNAs are involved in multiple pathological processes during OA, mainly extracellular matrix degradation, inflammation, and apoptosis. In this review, we briefly delineate the biogenesis, characteristics, and biofunctions of circRNAs, and then, focus on the role of circRNAs in the occurrence and progression OA.

The Emerging Field of Epitranscriptomics in Neurodevelopmental and Neuronal Disorders

Analogous to DNA methylation and histone modifications, RNA modifications represent a novel layer of regulation of gene expression. The dynamic nature and increasing number of RNA modifications offer new possibilities to rapidly alter gene expression upon specific environmental changes. Recent lines of evidence indicate that modified RNA molecules and associated complexes regulating and “reading” RNA modifications play key roles in the nervous system of several organisms, controlling both, its development and function. Mutations in several human genes that modify transfer RNA (tRNA) have been linked to neurological disorders, in particular to intellectual disability. Loss of RNA modifications alters the stability of tRNA, resulting in reduced translation efficiency and generation of tRNA fragments, which can interfere with neuronal functions. Modifications present on messenger RNAs (mRNAs) also play important roles during brain development. They contribute to neuronal growth and regeneration as well as to the local regulation of synaptic functions. Hence, potential combinatorial effects of RNA modifications on different classes of RNA may represent a novel code to dynamically fine tune gene expression during brain function. Here we discuss the recent findings demonstrating the impact of modified RNAs on neuronal processes and disorders.

Alternative Splicing in Neurogenesis and Brain Development

Alternative splicing of precursor mRNA is an important mechanism that increases transcriptomic and proteomic diversity and also post-transcriptionally regulates mRNA levels. Alternative splicing occurs at high frequency in brain tissues and contributes to every step of nervous system development, including cell-fate decisions, neuronal migration, axon guidance, and synaptogenesis. Genetic manipulation and RNA sequencing have provided insights into the molecular mechanisms underlying the effects of alternative splicing in stem cell self-renewal and neuronal fate specification. Timely expression and perhaps post-translational modification of neuron-specific splicing regulators play important roles in neuronal development. Alternative splicing of many key transcription regulators or epigenetic factors reprograms the transcriptome and hence contributes to stem cell fate determination. During neuronal differentiation, alternative splicing also modulates signaling activity, centriolar dynamics, and metabolic pathways. Moreover, alternative splicing impacts cortical lamination and neuronal development and function. In this review, we focus on recent progress toward understanding the contributions of alternative splicing to neurogenesis and brain development, which has shed light on how splicing defects may cause brain disorders and diseases.

Hsa_circ_0001649: A circular RNA and potential novel biomarker for colorectal cancer

The circRNAs are differentially expressed in a wide range of cancers in regulating their initiation and progression, and could be used to make a diagnosis for some diseases like tumor as a new biomarker. However, the correlation and the mechanism of action between circRNAs and colorectal cancer (CRC) are still unclear. In this study, by using qRT-PCRs, we detected the expression level of hsa_circ_0001649 in tissue and serum samples from CRC patients, and the cultured cell has been detected. We found that the hsa_circ_0001649 in CRC is significantly lower than the expression level of correspondent nontumorous tissues (n = 64, P < 0.01). We also tested the HCT116 cell lines, and the similar result is observed (n = 15, P < 0.01). Moreover, we detected the serum samples obtained before and after surgery, showing significantly the expression level of hsa_circ_0001649 in the same patient is up-regulated after surgery (n = 18, P < 0.01). Besides, we analyzed the correlation between clinicopathological date and the expression level of hsa_circ_0001649, we found that hsa_circ_0001649 expression level is closely associated with pathological differentiation (P = 0.037), and the result also illustrated that the expression level of hsa_circ_0001649 is no direct correlation with age, gender, TMN stage, lymphatic metastasis, CEA, CA19-9, and CA-724 levels. The area under the receiver operating characteristic (ROC) curve was 0.857. In conclusion, this study showed that the expression level of hsa_circ_0001649 was down-regulated in CRC and could use it as a new biomarker for specific and sensitive inspection of CRC.

Mechanisms regulating dendritic arbor patterning

The nervous system is populated by diverse types of neurons, each of which has dendritic trees with strikingly different morphologies. These neuron-specific morphologies determine how dendritic trees integrate thousands of synaptic inputs to generate different firing properties. To ensure proper neuronal function and connectivity, it is necessary that dendrite patterns are precisely controlled and coordinated with synaptic activity. Here, we summarize the molecular and cellular mechanisms that regulate the formation of cell type-specific dendrite patterns during development. We focus on different aspects of vertebrate dendrite patterning that are particularly important in determining the neuronal function; such as the shape, branching, orientation and size of the arbors as well as the development of dendritic spine protrusions that receive excitatory inputs and compartmentalize postsynaptic responses. Additionally, we briefly comment on the implications of aberrant dendritic morphology for nervous system disease.

Circular RNAs (circRNAs) in Health and Disease

Splicing events do not always produce a linear transcript. Circular RNAs (circRNAs) are a class of RNA that are emerging as key new members of the gene regulatory milieu, which are produced by back-splicing events within genes. In circRNA formation, rather than being spliced in a linear fashion, exons can be circularised by use of the 3′ acceptor splice site of an upstream exon, leading to the formation of a circular RNA species. circRNAs have been demonstrated across species and have the potential to present genetic information in new orientations distinct from their parent transcript. The importance of these RNA players in gene regulation and normal cellular homeostasis is now beginning to be recognised. They have several potential modes of action, from serving as sponges for micro RNAs and RNA binding proteins, to acting as transcriptional regulators. In accordance with an important role in the normal biology of the cell, perturbations of circRNA expression are now being reported in association with disease. Furthermore, the inherent stability of circRNAs conferred by their circular structure and exonuclease resistance, and their expression in blood and other peripheral tissues in association with endosomes and microvesicles, renders them excellent candidates as disease biomarkers. In this review, we explore the state of knowledge on this exciting class of transcripts in regulating gene expression and discuss their emerging role in health and disease.

Circular RNAs: A Novel Player in Development and Disease of the Central Nervous System

Circular RNAs (circRNAs) own unique capabilities to communicate with nucleic acids and ribonucleoproteins and are emerging as indispensable compositions of the regulatory messages encoded in the genome. Due to lack of 3′ termini, circRNAs are more resistant to degradation by exonuclease RNase R and possess greater stability than linear RNAs. Moreover, circRNAs can act as microRNA (miRNA) sponge and affect messenger RNA (mRNA) splicing and transcription. By virtue of their great stability and elaborate regulatory mechanisms of gene expression, circRNAs play important roles in certain physiological activities. The development, homeostasis and stress response of the central nervous system (CNS) depend upon precise temporal and spatial regulation of gene networks. Moreover, emerging evidence has revealed that circRNAs are spatiotemporally regulated and dynamically expressed during brain development; therefore, they can exert significant influences on CNS development and diseases. In this review, we highlight the biogenesis of circRNAs and their central roles in regulation of CNS development and diseases.

Circular RNAs are abundant and dynamically expressed during embryonic muscle development in chickens

The growth and development of skeletal muscle is regulated by proteins as well as non-coding RNAs. Circular RNAs (circRNAs) are universally expressed in various tissues and cell types, and regulate gene expression in eukaryotes. To identify the circRNAs during chicken embryonic skeletal muscle development, leg muscles of female Xinghua (XH) chicken at three developmental time points 11 embryo age (E11), 16 embryo age (E16) and 1 day post hatch (P1) were performed RNA sequencing. We identified 13,377 circRNAs with 3,036 abundantly expressed and most were derived from coding exons. A total of 462 differentially expressed circRNAs were identified (fold change > 2; q-value < 0.05). Parental genes of differentially expressed circRNAs were related to muscle biological processes. There were 946 exonic circRNAs have been found that harbored one or more miRNA-binding site for 150 known miRNAs. We validated that circRBFOX2s promoted cell proliferation through interacted with miR-206. These data collectively indicate that circRNAs are abundant and dynamically expressed during embryonic muscle development and could play key roles through sequestering miRNAs as well as other functions.

Screening circular RNA expression patterns following focal cerebral ischemia in mice

Circular RNAs (circRNAs) have been demonstrated to act as microRNA (miRNA) sponges and they play important roles in regulating gene expression through a circRNA-miRNA-gene pathway. The specific roles of circRNAs in the pathogenesis of cerebral ischemia, however, are still unclear. Thus, the aim of this study is to determine circRNA expression profiles in the ischemic brain after stroke, which was induced by 45 min of transient middle cerebral artery occlusion (MCAO). The results from the circRNA microarrays revealed that 1027 circRNAs were significantly altered 48 hours after reperfusion in the ischemic brain compared with the sham group. Among them, 914 circRNAs were significantly upregulated, and the remaining 113 were significantly downregulated. In addition, the expressions of the three selected circRNAs, mmu_circRNA_40001, mmu_circRNA_013120, and mmu_circRNA_40806, were verified using quantitative real-time polymerase chain reaction (qRT-PCR). After predicting their target genes, the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analyses were further used to predict the associated significant cell signaling pathways and functions. The results show that the most enriched pathways are associated with the Rap1 signaling pathway and the Hippo signaling pathway, which regulate cell survival and death. Finally, we constructed an interaction network of circRNA-miRNA-target genes, including 13 miRNAs and their corresponding genes, indicating that changes in circRNA are associated with genes related with brain injury and recovery. In conclusion, circRNAs are complicated in the pathological development of brain injury after stroke, suggesting novel diagnostic and therapeutic targets for stroke therapy.

Circular RNAs: Regulators of Cancer-Related Signaling Pathways and Potential Diagnostic Biomarkers for Human Cancers

Circular RNAs (circRNAs) are newly discovered endogenous non-coding RNAs featuring structural stability, high abundance, and tissue-specific expression. CircRNAs are prevalent and conserved in mammalian cells. They are involved in cellular processes and regulate gene expression at the transcriptional or post-transcriptional level by interacting with microRNAs (miRNAs) and other molecules. Recent studies have shown that circRNAs play an important role in the progression of various human diseases including atherosclerosis, nervous system disorders, diabetes, and cancer. In this review, we summarize the advances on endogenous circRNAs in eukaryotic cells and elucidate their diagnostic and prognostic significance in human cancers. Especially, we highlight the involvement of circRNAs in signal transduction pathways as well as their clinical potential to serve as biomarkers.

Increased circular RNA UBAP2 acts as a sponge of miR-143 to promote osteosarcoma progression

Deregulated expression of circular RNA (circRNA) has been determined to be important in carcinogenesis and progression; however, in the most common type of primary malignant bone tumor osteosarcoma, the roles of circRNA in cancer development still remain to be elucidated. Here, we found that circRNA UBAP2 (circUBAP2) expression is significantly increased in human osteosarcoma tissues as compared to those in matched controls. Increased circUBAP2 expression was significantly correlated with human osteosarcoma progression and prognosis. Furthermore, increased circUBAP2 could promote osteosarcoma growth and inhibit apoptosis both in vitro and in vivo. Mechanistically, circUBAP2 was found to inhibit the expression of microRNA-143 (miR-143), thus enhancing the expression and function of anti-apoptotic Bcl-2, which is a direct target of miR-143. Together, our results suggest the roles of circUBAP2 in osteosarcoma development and implicate its potential in prognosis prediction and cancer therapy.