Circular RNAs: biogenesis, expression and their potential roles in reproduction

Circular RNAs in childhood-related diseases and cancers: A review

Research into the diagnosis, treatment and prevention of childhood-related diseases is the key to reducing their morbidity and mortality. Circular RNAs (circRNAs) play critical roles, both in physiology and pathology, and there is ample evidence to show that they play varying roles in tissue development and gene regulation. Studies on circRNAs in different childhood-related diseases have confirmed their great potential for disease prevention and treatment. These breakthroughs highlight the pathological role of circRNAs in cancers, as well as cardiovascular and hereditary childhood illnesses. In this review, we summarize the role of circRNAs in childhood-related diseases and cancer, and provide an update of the possible diagnostic and therapeutic application of circRNAs.

Circ_0001438 participates in the pathogenesis of preeclampsia via the circ_0001438/miR-942/NLRP3 regulatory network

INTRODUCTION

Preeclampsia (PE) is a common pregnancy disorder with multisystem complications. The growing data suggest that circular RNAs (circRNAs) involve in the development of PE. This study proposed to investigate the function and potential mechanisms of circ_0001438 in PE.

METHODS

The expression of circ_0001438, miR-942 and NOD-like receptor pyrin domain-containing protein 3 (NLRP3) was measured by quantitative real-time polymerase chain reaction (qRT-PCR). The expression at the protein level of NLRP3, interleukin 1 beta (IL-1β), interleukin 10 (IL-10), B-cell lymphoma 2 (Bcl-2), Cleaved-caspase-3 (Cleaved-casp-3), N-cadherin and E-cadherin was detected by Western blot. Cell proliferation was assessed by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay and colony formation assay. Cell apoptosis was determined by flow cytometry assay. Cell migration and invasion were monitored by transwell assay. The target genes were obtained and verified by the online bioinformatics tool and dual-luciferase reporter assay.

RESULTS

The expression of circ_0001438 and NLRP3 was enhanced in PE placenta tissues. Circ_0001438 knockdown promoted cell proliferation, migration and invasion but inhibited apoptosis and inflammatory responses in HTR-8/Svneo cells, and these effects were reversed by the inhibition of miR-942, a target of circ_0001438. Moreover, NLRP3 was bounded by miR-942. The enrichment of miR-942 accelerated cell proliferation, migration and invasion but depleted apoptosis and inflammatory responses, while these impacts were partly abolished by NLRP3 overexpression.

DISCUSSION

Circ_0001438 sponged miR-942 to regulate the expression of NLRP3, and circ_0001438 aggravated the dysfunctions of human villous trophoblasts by mediating the miR-942/NLRP3 axis at least in part.

Biogenesis and functions of circular RNAs and their role in diseases of the female reproductive system

A member of the newly discovered RNA family, circular RNA (circRNA) is considered as the intermediate product of by-product splicing or abnormal RNA splicing. With the development of RNA sequencing, circRNA has recently drawn research interest. CircRNA exhibits stability, species conservatism, and tissue cell specificity. It acts as a miRNA sponge in the circRNA-microRNA (miRNA-mRNA axis, which can regulate gene transcription and protein translation. Studies have confirmed that circRNA is ubiquitous in eukaryotic cells, which play an important role in the regulation of human gene expression and participate in the occurrence and development of various human diseases. CircRNA may be closely related to the occurrence and development of female reproductive system diseases. By analyzing the biological functions and mechanism of circRNA, we find that circRNA has certain development prospects as biomarkers of the female reproductive system diseases. The production and degradation of circRNA, biological functions, and their association with the occurrence of diseases of female reproductive system are reviewed in this article.

Circ_0084927 Facilitates Cervical Cancer Development via Sponging miR-142-3p and Upregulating ARL2

Background

Cervical cancer is a fatal burden for women. Circular RNAs (circRNAs) are important regulators in cancer development. Our study aimed to investigate the function and action mechanism of a novel circRNA, circ_0084927, in cervical cancer.

Methods

The expression of circ_0084927, miR-142-3p and ADP-ribosylation factor-like protein 2 (ARL2) mRNA was measured by quantitative real-time polymerase chain reaction (qRT-PCR). For functional analyses, cell proliferation was assessed using cell counting kit-8 (CCK-8) assay. Cell cycle distribution was monitored by flow cytometry assay. Cell migration and cell invasion were evaluated by transwell assay. The interaction between miR-142-3p and circ_0084927 or ARL2 was predicted by the bioinformatics analysis and validated by dual-luciferase reporter assay and RNA immunoprecipitation assay (RIP) assay. The expression of ARL2 at the protein level was detected by Western blot. Animal tumor formation assay was performed to monitor the tumorigenicity of circ_0084927 in vivo.

Results

The expression of circ_0084927 and ARL2 was enhanced in cervical cancer tissues and cells, while the expression of miR-142-3p was opposite to them. Circ_0084927 knockdown significantly blocked cervical cancer cell proliferation, migration and invasion and induced cell cycle arrest. MiR-142-3p was targeted by circ_0084927, and miR-142-3p inhibition reversed the effects of circ_0084927 knockdown. Besides, miR-142-3p bound to ARL2, and the inhibitory effects of miR-142-3p restoration on cell proliferation, cycle, migration and invasion were counteracted by ARL2 overexpression. More importantly, circ_0084927 upregulated ARL2 expression by sponging miR-142-3p. Circ_0084927 knockdown retarded tumor growth in vivo by regulating miR-142-3p and ARL2.

Conclusion

Circ_0084927 accelerated the progression of cervical cancer partly by mediating the miR-142-3p/ARL2 axis.

Circ_0000885 Enhances Osteosarcoma Progression by Increasing FGFR1 Expression via Sponging MiR-1294

Background

As a malignant tumor, the progression of osteosarcoma (OS) is mediated by multiple regulators, including circular RNAs (circRNAs). However, the role of circ_0000885 in OS is unclear.

Materials and Methods

Quantitative real-time polymerase chain reaction (qRT-PCR) was applied to detect the expression of circ_0000885, miR-1294 and fibroblast growth factor receptor 1 (FGFR1). Cell proliferation was evaluated using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay and colony formation assay. Flow cytometry and transwell assay were employed to determine the cell cycle distribution, cell migration and invasion, respectively. Moreover, the relationship between miR-1294 and circ_0000885 or FGFR1 was confirmed by dual-luciferase reporter assay. The protein level of FGFR1 was assessed via Western blot (WB) analysis. Animal experiments were used to verify the effect of circ_0000885 silencing on OS tumor growth in vivo.

Results

Circ_0000885 level was increased in OS tissues and cells. Knockdown of circ_0000885 repressed the proliferation, migration, invasion and induced cell cycle arrest in OS cells. There was a binding relationship between miR-1294 and circ_0000885, and miR-1294 inhibitor could reverse the inhibitory effect of silenced circ_0000885 on OS progression. MiR-1294 could target FGFR1, and overexpressed FGFR1 could invert the suppression effect of miR-1294 mimic on OS progression. Silencing of circ_0000885 hindered FGFR1 expression, while this effect could be recovered by miR-1294 inhibitor. In addition, circ_0000885 knockdown reduced OS tumor growth via regulating the FGFR1 expression by sponging miR-1294 in vivo.

Conclusion

Circ_0000885 played an active role in OS progression, indicating that it might be a potential target for OS therapy.

Circular RNAs in Sepsis: Biogenesis, Function, and Clinical Significance

Sepsis is a life-threatening condition that occurs when the body responds to an infection that damages it is own tissues. The major problem in sepsis is rapid, vital status deterioration in patients, which can progress to septic shock with multiple organ failure if not properly treated. As there are no specific treatments, early diagnosis is mandatory to reduce high mortality. Despite more than 170 different biomarkers being postulated, early sepsis diagnosis and prognosis remain a challenge for clinicians. Recent findings propose that circular RNAs (circRNAs) may play a prominent role in regulating the patients’ immune system against different pathogens, including bacteria and viruses. Mounting evidence also suggests that the misregulation of circRNAs is an early event in a wide range of diseases, including sepsis. Despite circRNA levels being altered in sepsis, the specific mechanisms controlling the dysregulation of these noncoding RNAs are not completely elucidated, although many factors are known to affect circRNA biogenesis. Therefore, there is a need to explore the molecular pathways that lead to this disorder. This review describes the role of this new class of regulatory RNAs in sepsis and the feasibility of using circRNAs as diagnostic biomarkers for sepsis, opening up new avenues for circRNA-based medicine.

circ_0067934 increases bladder cancer cell proliferation, migration and invasion through suppressing miR-1304 expression and increasing Myc expression levels

circRNAs have been demonstrated to be key regulators of bladder cancer progression. The present study aimed to investigate the effects of circular RNA (circ)_0067934 in bladder cancer progression. A total of 54 patients with primary bladder cancer were enrolled, and their tumor tissues and adjacent normal bladder tissues were collected. For in vitro functional assays, T24 cells were transfected with sicirc_0067934, and Cell Counting Kit-8 was used to analyze the proliferative capacity of T24 cells. In addition, Transwell and Matrigel assays were used to assess the cell migration and invasion abilities, and a dual-luciferase reporter assay was used to investigate the relationship between miR-1304 and circ_0067934. Finally, reverse transcription-quantitative PCR and western blotting were performed to analyze gene and protein expression levels, respectively. circ_0067934 expression levels were significantly increased in bladder cancer tissues (P<0.001), which was associated with metastasis and a significantly decreased 5-year overall (P<0.05) and disease-free survival (P<0.05). In vitro, T24 cells in the small interfering RNA (si)circ_0067934 group demonstrated significantly reduced proliferation, migration and invasion abilities compared with the si negative control (siNC) group (P<0.01). In addition, the knockdown of circ_0067934 directly increased microRNA (miR)-1304 expression levels in T24 cells. Myc was subsequently discovered to be directly inhibited by miR-1304 and circ_0067934 was observed to increase Myc expression levels in T24 cells through inhibiting miR-1304 expression levels (P<0.01). Compared with the siNC group and sicirc_0067934 + Myc overexpression group, T24 cells in the sicirc_0067934 group exhibited significantly decreased proliferative, migratory and invasive abilities (P<0.01). In conclusion, circ_0067934 was demonstrated to increase bladder cancer cell proliferation, migration and invasion through promoting Myc expression levels via the suppression of miR-1304 expression.

Circular RNAs in Embryogenesis and Cell Differentiation With a Focus on Cancer Development

In the recent years thousands of non-coding RNAs have been identified, also thanks to highthroughput sequencing technologies. Among them, circular RNAs (circRNAs) are a well-represented class characterized by the high sequence conservation and cell type specific expression in eukaryotes. They are covalently closed loops formed through back-splicing. Recently, circRNAs were shown to regulate a variety of cellular processes functioning as miRNA sponges, RBP binding molecules, transcriptional regulators, scaffold for protein translation, as well as immune regulators. A growing number of studies are showing that deregulated expression of circRNAs plays important and decisive actions during the development of several human diseases, including cancer. The research on their biogenesis and on the various molecular mechanisms in which they are involved is going very fast, however, there are still few studies that address their involvement in embryogenesis and eukaryotic development. This review has the intent to describe the most recent progress in the study of the biogenesis and molecular activities of circRNAs providing insightful information in the field of embryogenesis and cell differentiation. In addition, we describe the latest research on circRNAs as novel promising biomarkers in diverse types of tumors.

Transcriptomic Analysis of circRNAs and mRNAs Reveals a Complex Regulatory Network That Participate in Follicular Development in Chickens

Follicular development plays a key role in poultry reproduction, affecting clutch traits and thus egg production. Follicular growth is determined by granulosa cells (GCs), theca cells (TCs), and oocyte at the transcription, translation, and secretion levels. With the development of bioinformatic and experimental techniques, non-coding RNAs have been shown to participate in many life events. In this study, we investigated the transcriptomes of GCs and TCs in three different physiological stages: small yellow follicle (SYF), smallest hierarchical follicle (F6), and largest hierarchical follicle (F1) stages. A differential expression (DE) analysis, weighted gene co-expression network analysis (WGCNA), and bioinformatic analyses were performed. A total of 18,016 novel circular RNAs (circRNAs) were detected in GCs and TCs, 8127 of which were abundantly expressed in both cell types. and more circRNAs were differentially expressed between GCs and TCs than mRNAs. Enrichment analysis showed that the DE transcripts were mainly involved in cell growth, proliferation, differentiation, and apoptosis. In the WGCNA analysis, we identified six specific modules that were related to the different cell types in different stages of development. A series of central hub genes, including MAPK1, CITED4, SOD2, STC1, MOS, GDF9, MDH1, CAPN2, and novel_circ0004730, were incorporated into a Cytoscape network. Notably, using both DE analysis and WGCNA, ESR1 was identified as a key gene during follicular development. Our results provide valuable information on the circRNAs involved in follicle development and identify potential genes for further research to determine their roles in the regulation of different biological processes during follicle growth.

Beyond MicroRNAs: Emerging Role of Other Non-Coding RNAs in HPV-Driven Cancers

Persistent infection with high-risk Human Papilloma Virus (HPV) leads to the development of several tumors, including cervical, oropharyngeal, and anogenital squamous cell carcinoma. In the last years, the use of high-throughput sequencing technologies has revealed a number of non-coding RNA (ncRNAs), distinct from micro RNAs (miRNAs), that are deregulated in HPV-driven cancers, thus suggesting that HPV infection may affect their expression. However, since the knowledge of ncRNAs is still limited, a better understanding of ncRNAs biology, biogenesis, and function may be challenging for improving the diagnosis of HPV infection or progression, and for monitoring the response to therapy of patients affected by HPV-driven tumors. In addition, to establish a ncRNAs expression profile may be instrumental for developing more effective therapeutic strategies for the treatment of HPV-associated lesions and cancers. Therefore, this review will address novel classes of ncRNAs that have recently started to draw increasing attention in HPV-driven tumors, with a particular focus on ncRNAs that have been identified as a direct target of HPV oncoproteins.

Identification of circular RNAs in porcine sperm and evaluation of their relation to sperm motility

Circular RNAs (circRNAs) are emerging as a novel class of noncoding RNAs which potential role as gene regulators is quickly gaining interest. circRNAs have been studied in different tissues and cell types across several animal species. However, a thorough characterization of the circRNAome in ejaculated sperm remains unexplored. In this study, we profiled the sperm circRNA catalogue using 40 porcine ejaculates. A complex population of 1,598 circRNAs was shared in at least 30 of the 40 samples. Generally speaking, the predicted circRNAs presented low abundances and were tissue-specific. Around 80% of the circRNAs identified in the boar sperm were reported as novel. Results from abundance correlation between circRNAs and miRNAs together with the prediction of microRNA (miRNA) target sites in circRNAs suggested that circRNAs may act as miRNA sponges. Moreover, we found significant correlations between the abundance of 148 exonic circRNAs and sperm motility parameters. Two of these correlations, involving ssc_circ_1458 and ssc_circ_1321, were confirmed by RT-qPCR using 36 additional samples with extreme and opposite sperm motility values. Our study provides a thorough characterization of circRNAs in sperm and suggests that circRNAs hold potential as noninvasive biomarkers for sperm quality and male fertility.

VirusCircBase: a database of virus circular RNAs

Circular RNAs (circRNAs) are covalently closed long noncoding RNAs critical in diverse cellular activities and multiple human diseases. Several cancer-related viral circRNAs have been identified in double-stranded DNA viruses (dsDNA), yet no systematic study about the viral circRNAs has been reported. Herein, we have performed a systematic survey of 11 924 circRNAs from 23 viral species by computational prediction of viral circRNAs from viral-infection-related RNA sequencing data. Besides the dsDNA viruses, our study has also revealed lots of circRNAs in single-stranded RNA viruses and retro-transcribing viruses, such as the Zika virus, the Influenza A virus, the Zaire ebolavirus, and the Human immunodeficiency virus 1. Most viral circRNAs had reverse complementary sequences or repeated sequences at the flanking sequences of the back-splice sites. Most viral circRNAs only expressed in a specific cell line or tissue in a specific species. Functional enrichment analysis indicated that the viral circRNAs from dsDNA viruses were involved in KEGG pathways associated with cancer. All viral circRNAs presented in the current study were stored and organized in VirusCircBase, which is freely available at http://www.computationalbiology.cn/ViruscircBase/home.html and is the first virus circRNA database. VirusCircBase forms the fundamental atlas for the further exploration and investigation of viral circRNAs in the context of public health.

Prospective Advances in Non-coding RNAs Investigation

Non-coding RNAs (ncRNAs) play significant roles in numerous physiological cellular processes and molecular alterations during pathological conditions including heart diseases, cancer, immunological disorders and neurological diseases. This chapter is focusing on the basis of ncRNA relation with their functions and prospective advances in non-coding RNAs particularly miRNAs investigation in the cardiovascular disease management.The field of ncRNAs therapeutics is a very fascinating and challenging too. Scientists have opportunity to develop more advanced therapeutics as well as diagnostic approaches for cardiovascular conditions. Advanced studies are critically needed to deepen the understanding of the molecular biology, mechanism and modulation of ncRNAs and chemical formulations for managing CVDs.

The roles of cirRNA in the development of germ cells

Circular RNA (CircRNA), a type of endogenous non-coding RNAs (ncRNAs), is generally generated from precursor mRNA (pre-mRNA) by canonical splicing and head-to-tail back splicing. The structure without a polyA tail renders circRNA highly insensitive to ribonuclease. Simultaneously, the distribution of circRNAs is tissue and developmental stage-specific. There are five potential biological functions of circRNAs: 1) promote transcription of their parental genes; 2) function as a miRNA sponge; 3) RNA binding protein (RBP) sponge; 4) encode protein; 5) act as an mRNA trap. Recently, circRNA has attracted attention because studies have shown that circRNAs are associated with follicular development, ovarian senescence, spermatogenesis, and germ cell development process, suggesting that circRNAs may function in germ cells regulation. The investigation of circRNAs in germ cells will provide an excellent opportunity to understand its potential molecular basis, and potentially improving reproduction status in human. In this article, the relationship between circRNA and germ cell development will be discussed.

Comprehensive Analysis of Differentially Expressed Profiles of mRNA, lncRNA, and circRNA in the Uterus of Seasonal Reproduction Sheep

Photoperiod is one of the important factors leading to seasonal reproduction of sheep. However, the molecular mechanisms underlying the photoperiod regulation of seasonal reproduction remain poorly understood. In this study, we compared the expression profiles of mRNAs, lncRNAs, and circRNAs in uterine tissues from Sunite sheep during three different photoperiods, namely, the short photoperiod (SP), short transfer to long photoperiod (SLP), and long photoperiod (LP). The results showed that 298, 403, and 378 differentially expressed (DE) mRNAs, 171, 491, and 499 DE lncRNAs, and 124, 270, and 400 DE circRNAs were identified between SP and LP, between SP and SLP, and between LP and SLP, respectively. Furthermore, functional enrichment analysis showed that the differentially expressed RNAs were mainly involved in the GnRH signaling pathway, thyroid hormone synthesis, and thyroid hormone signaling pathway. In addition, co-expression networks of lncRNA–mRNA were constructed based on the correlation analysis between the differentially expressed RNAs. Our study provides new insights into the expression changes of RNAs in different photoperiods, which might contribute to understanding the molecular mechanisms of seasonal reproduction in sheep.

Genomewide analysis of circular RNA in pituitaries of normal and heat-stressed sows

BACKGROUND

As a newly characterized type of noncoding RNA, circular RNA (circRNA) has been shown to have functions in diverse biological processes of animals. It has been reported that several noncoding RNAs may regulate animals' response to heat stress which can be easily induced by hyperthermia in summer. However, the expression and functions of circRNAs in the pituitary of sows and whether they participate in heat stress adaption are still unclear.

RESULTS

In this study, we found that high temperature over the thermoneutral zone of sows during the summer increased the serum heat shock protein 70 (HSP70) level, decreased the superoxide dismutase (SOD) vitality and prolactin (PRL) concentration, and induced heat stress in sows. Then, we explored circRNA in the pituitary of heat-stressed and normal sows using RNA sequencing and bioinformatics analysis. In total, 12,035 circRNAs were detected, with 59 circRNAs differentially expressed, including 42 up-regulated and 17 down-regulated circRNAs in pituitaries of the heat-stressed sows. Six randomly selected circRNAs were identified through reverse transcription PCR followed by DNA sequencing and other 7 randomly selected differentially expressed circRNAs were verified by quantitative real-time PCR analysis. The predicted target genes regulated by circRNAs through sponging microRNAs (miRNAs) were enriched in metabolic pathway. Furthermore, the predicted circRNA-miRNA-mRNA interactions showed that some circRNAs might sponge miRNAs to regulate pituitary-specific genes and heat shock protein family members, indicating circRNA's roles in pituitary hormone secretion and heat stress response.

CONCLUSIONS

Our results provided a meaningful reference to understand the functions of circRNA in the porcine pituitary and the mechanisms by which circRNA may participate in animals' response to heat stress.

Non-coding RNAs in bone remodelling and bone metastasis: Mechanisms of action and translational relevance

Bone metastases are frequent complications in patients with advanced cancer, which can be fatal or may rapidly impede the quality of life of patients. Current treatments for patients with bone metastases are palliative. Therefore, a better understanding of the molecular mechanisms that precede the overt development of skeletal lesions could lead to better therapeutic interventions. In this review, we present evidence that non-coding RNAs (ncRNAs) such as long ncRNAs, microRNAs, and circular RNAs are emerging as master regulators of bone metastasis formation. We highlight potential opportunities for the therapeutic targeting of ncRNAs. Furthermore, we discuss the possibility that ncRNAs may be used as biomarkers in the context of bone metastases, which might provide insight for improving the response to current bone-targeting therapies. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Genome-Wide Differential Expression Profiling of Ovarian circRNAs Associated With Litter Size in Pigs

Circular RNAs (circRNAs) have been emerging as an important regulator in mammalian reproduction via acting as miRNA sponges. However, the circRNAs in porcine ovaries related with litter size remains largely unknown. In this study, porcine ovaries with smaller or larger litter size (LLS) were subjected to high-throughput RNA sequencing. In total, 38,722 circRNAs were identified, of which 1,291 circRNAs were commonly expressed in all samples. There were 56 circRNAs significantly down-regulated and 54 circRNAs up-regulated in LLS pig (|log2 (fold change) | > 1, FDR < 0.05). Bioinformatics predicted that most of circRNAs harbored miRNA binding sites, and the expression patterns of circRNAs and their putative binding miRNAs were validated by qPCR. Moreover, the expression of circ-TCP11/miR-183 was significantly reversely correlated and their direct interaction was confirmed by dual-luciferase assay. Our study indicates that circRNAs may play potential effects on modulating porcine litter size.

Evaluation of circ_100219 and miR-135b in serum and exosomes of healthy pregnant women

Aim: Circular RNAs (circRNAs) are recently discovered and highly stable noncoding RNAs acting as gene regulators. These circRNAs can function as miRNA sponges, thereby upregulating or downregulating miRNA target gene expression. MiR-135b is expressed in placenta tissue and can be found in maternal circulation, thus playing a functional role in pregnancy. This miR is a target of circ_100219. This preliminary study was aimed to evaluate circ_100219 and miR-135b expression in pregnant and nonpregnant women, and explore the relationship between circ_100219 and miR-135b in serum and exosomes.Methods: Total RNA was isolated from serum and exosomes of 30 healthy pregnant women (32.9 ± 5.1 years) between 23-27 gestational weeks and 30 healthy nonpregnant women (31.3 ± 5.4 years). Quantitative real-time polymerase chain reaction (qRT-PCR) was used to quantify circ_100219 and miR-135b expression. GAPDH and U6 snRNA were chosen as reference for normalizing expression levels. The differences between pregnant and nonpregnant women were assessed with Mann-Whitney test and correlation with Spearman's test.Results: The circ_100219 expression levels were significantly lower both in serum and exosomes of second trimester pregnant women compared to the control group (p < .0001), whilst Mir-135b expression levels were significantly higher in pregnant than in the control group (p < .0001). A significant negative correlation was observed between circ_100219 and miR-135b expression levels in both serum and exosomes (r = -0.34 and p = .009; r = -0.31 and p = .01, respectively). The circ_100219:miR-135b ratio was significantly increased in nonpregnant women compared to the pregnant group, in both serum and exosomes (49.0 versus 1.1, p < .0001 and 2042.4 versus 28.5, p < .0001, respectively).Conclusions: Our results confirm a role for circ_100219 and miR-135b in physiological pregnancy. Further studies are needed to investigate the circ_100219:miR-135b ratio in pregnancy complications.

CircRNAs: From anonymity to novel regulators of gene expression in cancer (Review)

Circular RNAs (circRNAs) are a group of non‑coding RNAs, formed mostly through a unique backsplicing mechanism. Originally proposed to be a by‑product from errors in splicing, recent studies have shown they exhibit a range of roles in regulating gene expression, including sponging of microRNAs (miRNAs), interactions with RNA‑binding proteins and regulation of transcription. Though research is still in its infancy, evidence suggests circRNA levels are tightly regulated in the cell, reinforced by dysregulated circRNAs levels being implicated in a range of diseases, including cancer and viral infection. There is growing interest in circRNAs playing specific roles in cancers, either oncogenic or as tumour suppressors, with particular focus on their potential as novel biomarkers. This review will provide an overview of circRNA biogenesis and regulation, and their potential roles in the cell, with a focus on their dysregulation in cancer.

Differential Expression of Circular RNAs in Polytocous and Monotocous Uterus during the Reproductive Cycle of Sheep

Simple Summary

The uterus is an important reproductive organ that provides nutrition and place for embryonic development. In this study, we identified circular RNAs by deep sequencing and analyzed their expression in the uteri of polytocous and monotocous sheep (FecB++) during follicular and luteal phases. Gene Ontology (GO) and KEGG enrichment analyses revealed that the source genes of these differential circular RNAs (circRNAs) were mainly enriched in reproductive hormone- and energy metabolism-related pathways. These results provide information on the molecular mechanisms of sheep prolificacy.

Abstract

CircRNA plays important roles in cell proliferation, differentiation, autophagy and apoptosis during development. However, there are few reports on circRNAs related to livestock reproduction. In this study, we identified circRNAs by deep sequencing and analyzed their expression in the uteri of polytocous and monotocous sheep (FecB++) during follicular and luteal phases. There were 147 and 364 circRNAs with differential expression in the follicular and luteal phases, respectively. GO and KEGG enrichment analysis was performed for the host genes of the circRNAs to predict the functions of differentially expressed circRNAs. These source genes were mainly involved in the estrogen signaling pathway, TGFβ signaling pathway, GnRH signaling pathway, oxytocin signaling pathway, pentose phosphate pathway, and starch and sucrose metabolism related to reproduction and energy metabolism. CircRNA expression patterns were validated by RT-qPCR. Our findings provide a solid foundation for the identification and characterization of key important circRNAs involved in reproduction.

Mechanisms regulating mammalian spermatogenesis and fertility recovery following germ cell depletion

Mammalian spermatogenesis is a highly complex multi-step process sustained by a population of mitotic germ cells with self-renewal potential known as spermatogonial stem cells (SSCs). The maintenance and regulation of SSC function are strictly dependent on a supportive niche that is composed of multiple cell types. A detailed appreciation of the molecular mechanisms underpinning SSC activity and fate is of fundamental importance for spermatogenesis and male fertility. However, different models of SSC identity and spermatogonial hierarchy have been proposed and recent studies indicate that cell populations supporting steady-state germline maintenance and regeneration following damage are distinct. Importantly, dynamic changes in niche properties may underlie the fate plasticity of spermatogonia evident during testis regeneration. While formation of spermatogenic colonies in germ-cell-depleted testis upon transplantation is a standard assay for SSCs, differentiation-primed spermatogonial fractions have transplantation potential and this assay provides readout of regenerative rather than steady-state stem cell capacity. The characterisation of spermatogonial populations with regenerative capacity is essential for the development of clinical applications aimed at restoring fertility in individuals following germline depletion by genotoxic treatments. This review will discuss regulatory mechanisms of SSCs in homeostatic and regenerative testis and the conservation of these mechanisms between rodent models and man.

Circular PVT1: an oncogenic non-coding RNA with emerging clinical importance

The importance of circular RNAs (circRNAs) in pathological processes like cancer is evident. Among the circRNAs, recent studies have brought circPVT1 under focus as the most potent oncogenic non-coding RNA. Recent studies on various aspects of circPVT1, including its biogenesis, molecular alteration and its probable role in oncogenesis, have been conducted for research and clinical interest. In this review, a first attempt has been made to summarise the available data on circPVT1 from PubMed and other relevant databases with special emphasis on its role in development, progression and prognosis of various malignant conditions. CircPVT1 is derived from the same genetic locus encoding for long non-coding RNA lncPVT1; however, existing literature suggested circPVT1 and lncPVT1 are transcripted independently by different promoters. The interaction between circRNA and microRNA has been highlighted in majority of the few malignancies in which circPVT1 was studied. Besides its importance in diagnostic and prognostic procedures, circPVT1 seemed to have huge therapeutic potential as evident from differential drug response of cancer cell line as well as primary tumors depending on expression level of the candidate. circPVT1 in cancer therapeutics might be promising as a biomarker to make the existing treatment protocol more effective and also as potential target for designing novel therapeutic intervention.

Identification of circular RNAs in the immature and mature rat anterior pituitary

Circular RNAs (circRNAs) are a new class of RNA that have a stable structure characterized by covalently closed circular molecules and are involved in invasive pituitary adenomas and recurrent clinically nonfunctioning pituitary adenomas. However, information on circRNAs in the normal pituitary, especially in rats, is limited. In this study, we identified 4123 circRNAs in the immature (D15) and mature (D120) rat anterior pituitary using the Illumina platform, and 32 differentially expressed circRNAs were found. A total of 150 Gene Ontology terms were significantly enriched, and 16 KEGG pathways were found to contain differentially expressed genes. Moreover, we randomly selected eight highly expressed circRNAs and detected their relative expression levels in the mature and immature rat pituitary by qPCR. In addition, we predicted 90 interactions between 53 circRNAs and 57 miRNAs using miRanda. Notably, circ_0000964 and circ_0001303 are potential miRNA sponges that may regulate the Fshb gene. The expression profile of circRNAs in the immature and mature rat anterior pituitary may provide more information about the roles of circRNAs in the development and reproduction in mammals.

Deregulation of Circular RNAs in Cancer From the Perspectives of Aberrant Biogenesis, Transport and Removal

CircRNAs (circular RNAs) are a class of RNAs generated from circularization with multiple novel functions. Recent studies have revealed the aberrant expression and aberrant functions of circRNAs in various tumors; thus, circRNAs have been recognized as promising cancer biomarkers. However, the underlying mechanisms behind their aberrant expression and functions remain unclear. In this review, we discuss at length the cancer-specific deregulation of circRNAs and the potential underlying aberrant events in circRNA biogenesis, localization and removal in cancer cells.

Dynamic expression and functional analysis of circRNA in granulosa cells during follicular development in chicken

Background

Circular RNA (circRNA) is a type of noncoding RNA involved in a variety of biological processes, especially in post-transcriptional regulation. The granulosa cells of follicles play a determining role in ovarian development. However, the function of circRNA in chicken follicles is unclear. To better understand the molecular mechanism underlying follicular development and granulosa cell function, we performed a strategy of second-generation sequencing and linear RNA depletion for granulosa cells from small yellow follicles (SYF, 5–8 mm), the smallest hierarchal follicles (F6, 9–12 mm), and the largest hierarchal follicles (F1, ~ 40 mm).

Results

We predicted a total of 11,642 circRNAs that distributed on almost all chromosomes. The majority of the splice lengths of circRNAs were 200–500 nt and mainly produced from intron and CDS regions. During follicle growth, differentially expressed (DE) circRNAs showed dynamic changes which were tissue- and stage-specific. The host genes of DE circRNAs were functionally enriched in GTPase activity and several pathways involved in reproduction. Moreover, bioinformatic prediction analysis for circRalGPS2 demonstrated that circRNAs from the same genes may share common miRNA to act as a sponge. The predicted target genes were enriched in various biological processes including cognition, cell communication, and regulation of signaling, and several pathways related to reproduction such as tight junction, oocyte meiosis, progesterone-mediated oocyte maturation, and GnRH signaling.

Conclusions

This study provides a starting point for further experimental investigations into chicken circRNAs and casts a light on the understanding of follicle development.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5462-2) contains supplementary material, which is available to authorized users.

Biological functions of circular RNAs and their roles in occurrence of reproduction and gynecological diseases

Circular RNAs (circRNAs) are a large class of non coding endogenous RNAs in eukaryotic that are formed through 3'-5' ligation of a single RNA molecule. According to the different sources of the sequences, circRNA can be divided into three types: exon circRNA (ecRNA), intron circRNA (ciRNA), and exon-intron circRNA. Accumulating studies have shown that circRNAs are abundant, diverse, stable, and cell or tissue specific expression, etc. CircRNA plays a regulating role in gene expression, and an essential role in the process of biological development, such as miRNA sponges, endogenous RNAs and biomarkers, as well as critical role in the diagnosis of diseases. Studies have verified the interplay between circRNAs and the development of embryos, sperms, ovarian epithelial tumors, endometrial cancer and preeclampsia, suggesting the potential of circRNAs to become biomarkers or therapeutical targets for human diseases. In this paper, we reviewed the researches on circRNAs' characteristics, databases of circRNA, high-throughput sequencing of circRNA, and effect on reproductive and gynecological diseases.

Circ_0056618 and CXCR4 act as competing endogenous in gastric cancer by regulating miR-206

Circular RNAs (circRNAs) have proved to play an important role in gastric cancer. In this study, we found that circ_0056618 took part in gastric cancer cell proliferation and survival. The real-time polymerase chain reaction result showed that circ_0056618 was overexpressed in tumor tissues or cells compared with adjacent normal tissues or normal cells and had a negative relationship to gastric cancer patients' survival time. Meanwhile, the inhibition of circ_0056618 suppressed the proliferation and metastasis of gastric cancer cells. Further bioinformatical analysis indicated that circ_0056618 sponged miR-206 to regulate CXCR4 according to the prediction of TargetScan and miRanda. Dual-luciferase reporter assay and Western blot analysis revealed the underlying relationship of circ_0056618, miR-206, and CXCR4. Hence, circ_0056618 negatively regulated miR-206 expression promoting CXCR4 expression. Altogether, circ_0056618 is a potential gastric cancer prognostic marker, as well as a potential therapeutic target to inhibit gastric cancer metastasis.

A potential risk factor of essential hypertension in case-control study: Circular RNA hsa_circ_0037911

BACKGROUND

Essential hypertension (EH) is a high prevalence with multifactorial diseases. Human studies on the impact of genes on this disease are just in the initial stage, the mechanism of gene regulation is still remains unclear. Circular RNAs (circRNAs) as a continuous cycle of covalent closure, RNA molecules added to the 3'-5' end covalently bound by the formation of incidental event. CircRNAs may be an important biomolecule in revealing the molecule regulate mechanisms of EH.

METHODS

The circRNAs were selected and validated with qRT-PCR followed. Our experiment was conducted with case-control studies among 200 EH participants. The t-test was used to evaluate the different expression of circRNAs and miRNAs, the significance of which was set as p < 0.05.

RESULTS

The hsa_circ_0037911 expression level in EH cases were significantly higher than healthy controls (p = 0.005). There was still important significance when adjusted by logistic regression (adjusted p = 0.026). We also found that hsa_circ_0037911 was an effective marker of EH (area under curve = 0.627; p = 0.002). The levels of hsa_circ_0037911 were significantly differences in gender, BMI, smoking and drinking among EH cases. There was a positive correlation between Serum creatinine (Scr) and hsa_circ_0037911.

CONCLUSIONS

Our findings suggested that higher expression hsa_circ_0037911 may be key circRNAs for EH development by changing the concentration of Scr and could be a stable biomarker for early diagnosis of EH.

Circular RNA in Liver: Health and Diseases

Circular RNA (circRNA) is an important class of noncoding RNA characterized by covalently closed continuous loop structures. In recent years, the various functions of circRNAs have been continuously documented, including effects on cell proliferation and apoptosis and nutrient metabolism. The liver is the largest solid organ in mammals, and it also performs many functions in the body, which is considered to be the busiest organ in the body. At the same time, the liver is vulnerable to multiple pathogenic factors, causing various acute and chronic liver diseases. The pathogenesis of liver disease is still not fully understood. As a rising star for the past few years, circRNAs have been proven involved in the regulation of liver homeostasis and disease. This chapter will explain the role of circRNAs in liver health and diseases and sort out the confusion in the present study.