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

CircPRKD3/miR-6783-3p responds to mechanical force to facilitate the osteogenesis of stretched periodontal ligament stem cells

BACKGROUND

The mechanotransduction mechanisms by which cells regulate tissue remodeling are not fully deciphered. Circular RNAs (circRNAs) are crucial to various physiological processes, including cell cycle, differentiation, and polarization. However, the effects of mechanical force on circRNAs and the role of circRNAs in the mechanobiology of differentiation and remodeling in stretched periodontal ligament stem cells (PDLSCs) remain unclear. This article aims to explore the osteogenic function of mechanically sensitive circular RNA protein kinase D3 (circPRKD3) and elucidate its underlying mechanotransduction mechanism.

MATERIALS AND METHODS

PDLSCs were elongated with 8% stretch at 0.5 Hz for 24 h using the Flexcell® FX-6000™ Tension System. CircPRKD3 was knockdown or overexpressed with lentiviral constructs or plasmids. The downstream molecules of circPRKD3 were predicted by bioinformatics analysis. The osteogenic effect of related molecules was evaluated by quantitative real-time PCR (qRT-PCR) and western blot.

RESULTS

Mechanical force enhanced the osteogenesis of PDLSCs and increased the expression of circPRKD3. Knockdown of circPRKD3 hindered PDLSCs from osteogenesis under mechanical force, while overexpression of circPRKD3 promoted the early osteogenesis process of PDLSCs. With bioinformatics analysis and multiple software predictions, we identified hsa-miR-6783-3p could act as the sponge of circPRKD3 to indirectly regulate osteogenic differentiation of mechanically stimulated PDLSCs.

CONCLUSIONS

Our results first suggested that both circPRKD3 and hsa-miR-6783-3p could enhance osteogenesis of stretched PDLSCs. Furthermore, hsa-miR-6783-3p could sponge circPRKD3 to indirectly regulate RUNX2 during the periodontal tissue remodeling process in orthodontic treatment.

Circular RNA-Mediated Regulation of Oral Tissue-Derived Stem Cell Differentiation: Implications for Oral Medicine and Orthodontic Applications

Circular RNAs (circRNAs) are a novel class of endogenous non-coding RNAs (ncRNAs) which unlike linear RNAs, have a covalently closed continuous loop structure. circRNAs are found abundantly in human cells and their biology is complex. They feature unique expression to different types of cells, tissues, and developmental stages. To the present, the functional roles of circular RNAs are not fully understood. They reportedly act as microRNA (miRNA) sponges, therefore having key regulatory functions in diverse physiological and pathological processes. As for dentistry field, lines of evidence indicate that circRNAs play vital roles in the odontogenic and osteogenic differentiation of dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs). Abnormal expression of circRNAs have been found in other areas of pathology frequently reflected also in the oral environment, such as inflammation or bone and soft tissue loss. Therefore, circRNAs could be of significant importance in various fields in dentistry, especially in bone and soft tissue engineering and regeneration. Understanding the molecular mechanisms occurring during the regulation of oral biological and tissue remodeling processes could augment the discovery of novel diagnostic biomarkers and therapeutic strategies that will improve orthodontic and other oral therapeutic protocols.

Regulatory mechanisms of circular RNAs during human mesenchymal stem cell osteogenic differentiation

Human osteogenic differentiation is a complex and well-orchestrated process which involves a plethora of molecular players and cellular processes. A growing number of studies have underlined that circular RNAs (circRNAs) play an important regulatory role during human osteogenic differentiation. CircRNAs are single-stranded, covalently closed non-coding RNA molecules that are acquiring increased attention as epigenetic regulators of gene expression. Given their intrinsic high conformational stability, abundance, and specificity, circRNAs can undertake various biological activities in order to regulate multiple cellular processes, including osteogenic differentiation. The most recent evidence indicates that circRNAs control human osteogenesis by preventing the inhibitory activity of miRNAs on their downstream target genes, using a competitive endogenous RNA mechanism. The aim of this review is to draw attention to the currently known regulatory mechanisms of circRNAs during human osteogenic differentiation. Specifically, we provide an understanding of recent advances in research conducted on various human mesenchymal stem cell types that underlined the importance of circRNAs in regulating osteogenesis. A comprehensive understanding of the underlying regulatory mechanisms of circRNA in osteogenesis will improve knowledge on the molecular processes of bone growth, resulting in the potential development of novel preclinical and clinical studies and the discovery of novel diagnostic and therapeutic tools for bone disorders.

Virus-Encoded Circular RNAs: Role and Significance in Viral Infections

Circular RNAs (circRNAs) have been the focus of intense scientific research to understand their biogenesis, mechanisms of action and regulatory functions. CircRNAs are single stranded, covalently closed RNA molecules lacking the 5'-terminal cap and the 3'-terminal polyadenine chain, characteristics that make them very stable and resistant. Synthesised by both cells and viruses, in the past circRNAs were considered to have no precise function. Today, increasing evidence shows that circRNAs are ubiquitous, some of them are tissue- and cell-specific, and critical in multiple regulatory processes (i.e., infections, inflammation, oncogenesis, gene expression). Moreover, circRNAs are emerging as important biomarkers of viral infection and disease progression. In this review, we provided an updated overview of current understanding of virus-encoded and cellular-encoded circRNAs and their involvement in cellular pathways during viral infection.

Anti-apoptosis and anti-inflammation activity of circ_0097010 downregulation in lipopolysaccharide-stimulated periodontal ligament cells by miR-769-5p/Krüppel like factor 6 axis

Background/purpose

Periodontitis is a prevalent infectious inflammatory disease. Growing evidence has revealed important roles for circular RNAs (circRNAs) and circRNA sponge activity in periodontitis. Here, we elucidated the precise part of circ_0097010 in periodontitis pathogenesis.

Materials and methods

Human periodontal ligament cells (hPDLCs) were exposed to lipopolysaccharide (LPS). Cell viability, proliferation and apoptosis were evaluated by CCK-8 assay, EdU incorporation assay and flow cytometry, respectively. Circ_0097010, microRNA (miR)-769-5p and Krüppel like factor 6 (KLF6) were quantified by qRT-PCR and Western blot. Interleukin 6 (IL-6) level, tumor necrosis factor-α (TNF-α) secretion, superoxide dismutase (SOD) activity and malondialdehyde (MDA) level were detected by enzyme-linked immunosorbent assay (ELISA). Dual-luciferase reporter, RNA immunoprecipitation (RIP) and RNA pull-down assays were used to confirm the direct relationship between miR-769-5p and circ_0097010 or KLF6.

Results

Our data showed that LPS repressed cell proliferation and induced cell apoptosis and inflammation in hPDLCs. Circ_0097010 was upregulated in periodontitis samples and LPS-exposed hPDLCs. Downregulation of circ_0097010 exerted anti-apoptosis and anti-inflammation functions in LPS-exposed hPDLCs. Mechanistically, circ_0097010 acted as a miR-769-5p sponge, and reduced abundance of miR-769-5p reversed the anti-apoptosis and anti-inflammation effects of circ_0097010 suppression. KLF6 was a direct miR-769-5p target, and miR-769-5p-mediated inhibition of KLF6 possessed anti-apoptosis and anti-inflammation functions in LPS-induced hPDLCs. Moreover, circ_0097010 controlled KLF6 expression by miR-769-5p.

Conclusion

These data identify circ_0097010 as a key regulator of LPS-induced inflammation and apoptosis in hPDLCs and highlight a novel mechanism of circ_0097010 regulation through miR-769-5p/KLF6 axis.

Correlation Analysis of miR-1246 Expression in Saliva of Patients with Chronic Periodontitis and Periodontal Indexes, Inflammatory Cytokines, and Protease Molecules

Objective. The study aimed to investigate the correlation of miR-1246 in saliva with periodontal indicators, inflammatory cytokines, and protease molecules in patients with chronic periodontitis. Methods. Thirty-five patients with chronic periodontitis were included as the chronic periodontitis group, and 35 healthy individuals were selected as the healthy control group during the same period. The miR-1246 levels, inflammatory cytokine interleukin (IL)-1β, IL-6, IL-17, tumor necrosis factor-α (TNF-α), matrix metalloproteinase (MMP-1), MMP-8, and MMP tissue inhibitor (TIMP-1) in saliva were determined, and periodontal indexes, including the plaque index (PLI), bleeding index (BI), periodontal probing depth (PD), and attachment loss (AL) were examined. Results. The salivary levels of miR-1246, IL-1β, IL-6, IL-17, TNF-α, MMP-1, MMP-8, and TIMP-1 and the periodontal indexes PLI, GI, PD, and AL in the chronic periodontitis group were significantly higher than those in the healthy control ( $P<0.05$ ). Salivary levels of miR-1246 in patients with chronic periodontitis were positively correlated with the levels of IL-1β, IL-6, IL-17, TNF-α, MMP-1, MMP-8, TIMP-1, PLI, GI, PD, and AL ( $P<0.05$ ). Conclusion. Abnormally elevated levels of miR-1246 in saliva of patients with chronic periodontitis correlate with levels of periodontal indices, inflammatory cytokines, and protease molecules.

Characterization of circulating molecules and activities in plasma of patients after allogeneic and autologous intraoral bone grafting procedures: a prospective randomized controlled clinical trial in humans

Background

The objective was to assess whether intraoral bone augmentation procedures have an impact on the patient’s plasma levels of circulating nucleic acids, exosomes, miRNA levels and caspase activities. The null hypothesis was tested, that no significant differences between the two groups will be found.

Methods

In this prospective randomized controlled clinical trial 35 systemically healthy non-smoking participants were randomly allocated using sealed envelopes by a blinded clinician not involved in the clinical setting. Plasma samples were collected preoperatively and 3 times postoperatively (immediately, 5 weeks and 4 months postoperatively). The test group consisted of twenty-five patients who received allogeneic bone grafting material and the control group of ten patients who received autologous bone grafts. Levels of cell-free DNA (cfDNA) and microRNAs (miR-21, miR-27a, miR-218) were quantified by real-time PCR, caspase activities and exosome concentrations were determined by ELISA.

Results

Statistical evaluation reveled a significantly higher exosome level before surgery (p = 0.013) and the first postsurgical sample (p = 0.017) in the control group compared to the test group. The levels of miR-27a and miR-218 significantly differed between the plasma samples before surgery and after surgery in both groups. The levels of miR-21 only significantly differed between the pre- and postsurgical plasma samples in the test group, but not in the control group. All patients completed the study, no adverse events were recorded.

Conclusions

Our data show the diagnostic potential of the plasma levels of miR-27a, miR-218 and miR-21 in detecting changes in bone metabolism after alveolar bone augmentation. Our very promising results indicate that there might be a high diagnostic potential in evaluating the plasma levels of the before mentioned miRNAs in order to detect bone resorption activities before they become clinically relevant.

Trial registration Ethical commission of the Ärztekammer Hamburg, Germany (PV5211) on 11/03/2016 as well as by the German Registry of Clinical Studies (DRKS 00,013,010) on 30/07/2018 (http://apps.who.int/trialsearch/).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-02036-7.

Emerging important roles of circRNAs in human cancer and other diseases

CircRNAs are a large class of endogenous single-stranded RNA that is different from other linear RNA, which are produced by back-splicing and fusion of either exons, introns, or both exon-intron into covalently closed loops. CircRNAs are found in almost all living organisms and have emerged as potentially important players effecting on all life activities. It was characterized by stable structure, resistant to RNA degradation, highly abundance and conservation and tissue-specific expression. Early circRNAs were ignored as a by-product of meaningless abnormally cut RNA and had little biological function. Currently, circRNAs have become a research hotspot due to its special characteristics. CircRNAs could function as miRNA sponges, interfere with splicing and bind to protein to regulate the expression of parental genes and so on. In recent years, an increasing number of studies have revealed that circRNAs are closely related to a series of physiological and pathological processes. Additionally, circRNAs play an important role in the occurrence and development of a variety of diseases, suggesting circRNAs may be as novel indicators or biomarkers for cancer and other diseases with which they are associated. In this article, we review the biogenesis, biological functions of circRNAs and recent advances in circRNAs research in human diseases. Results will provide new insights on the roles and new ideas of circRNAs for the diagnosis and treatment of diseases and possible directions and approach for future circRNA applications.

Circular RNAs: New Epigenetic Signatures in Viral Infections

Covalent closed circular RNAs (circRNAs) can act as a bridge between non-coding RNAs and coding messenger RNAs. CircRNAs are generated by a back-splicing mechanism during post-transcriptional processing and are abundantly expressed in eukaryotic cells. CircRNAs can act via the modulation of RNA transcription and protein production, and by the sponging of microRNAs (miRNAs). CircRNAs are now thought to be involved in many different biological and pathological processes. Some studies have suggested that the expression of host circRNAs is dysregulated in several types of virus-infected cells, compared to control cells. It is highly likely that viruses can use these molecules for their own purposes. In addition, some viral genes are able to produce viral circRNAs (VcircRNA) by a back-splicing mechanism. However, the viral genes that encode VcircRNAs, and their functions, are poorly studied. In this review, we highlight some new findings about the interaction of host circRNAs and viral infection. Moreover, the potential of VcircRNAs derived from the virus itself, to act as biomarkers and therapeutic targets is summarized.