Physiological and pathological functions of circular RNAs in the nervous system

Abstract

Circular RNAs (circRNAs) are a class of covalently closed single-stranded RNAs that are expressed during the development of specific cells and tissues. CircRNAs play crucial roles in physiological and pathological processes by sponging microRNAs, modulating gene transcription, controlling the activity of certain RNA-binding proteins, and producing functional peptides. A key focus of research at present is the functionality of circRNAs in the nervous system and several advances have emerged over the last 2 years. However, the precise role of circRNAs in the nervous system has yet to be comprehensively reviewed. In this review, we first summarize the recently described roles of circRNAs in brain development, maturity, and aging. Then, we focus on the involvement of circRNAs in various diseases of the central nervous system, such as brain cancer, chronic neurodegenerative diseases, acute injuries of the nervous system, and neuropathic pain. A better understanding of the functionality of circRNAs will help us to develop potential diagnostic, prognostic, and therapeutic strategies to treat diseases of the nervous system.

Introduction

Circular RNAs (circRNAs), generated from genes with protein-encoding function by back-splicing, are an important class of single-stranded RNAs that are expressed in cells and tissues at specific stages of development (Wilusz, 2018; Xiao et al., 2020; Chen et al., 2022f). Many circRNAs have been identified in the protein-coding genes of eukaryotic cells by virtue of the advancement of RNA sequencing and novel bioinformatic algorithms (Li et al., 2020a). According to their specific characteristics, circRNAs can be divided into four types: EcircRNAs (from one or more exons), EIciRNAs (from exon-intron regions), ciRNAs (from intron lariats) (Figure 1A), and mecciRNA (from the mitochondria genome) (Figure 1B; Jeck et al., 2013; Chen et al., 2015; Wilusz, 2018; Liu et al., 2020; Zhou et al., 2022). EIciRNAs and ciRNAs have been reported to be predominantly localized to the nuclei (Zhang et al., 2013; Li et al., 2015; Song et al., 2021b), while the vast majority of circRNAs (particularly EcircRNAs) are known to accumulate in the cytoplasm (Jia et al., 2019; Li et al., 2019; Zhou et al., 2021c). Once synthesized in the nucleus, most circRNAs are transported to the cytoplasm to perform their functional roles or for degradation (Zhou et al., 2021c). For example, GW182, a canonical RNA-binding protein that can function in both the nucleus (Jia et al., 2021, 2022) and the cytoplasm (Niaz and Hussain, 2018), can only influence circRNA stability in the cytoplasm (Jia et al., 2019). Furthermore, the efficient nuclear export of circRNAs is essential for their appropriate functionality under normal physiological conditions, and abnormalities in this export function cause physiological problems (Chen et al., 2022d). Recent studies have indicated that circRNAs mainly apply four mechanisms for nuclear export to the cytoplasm, including length-dependent (Figure 1C), m6A-dependent (Figure 1D), NXF1-NXT1-dependent (Figure 1E), and exportin 4 (XPO4)-dependent mechanisms (Figure 1F; Huang et al., 2018; Chen et al., 2019b, 2022d; Li et al., 2019; Wang et al., 2021a; Zhou et al., 2021c). Furthermore, many functional circRNAs have been demonstrated to exert crucial roles in various molecular and cellular events by sequestering microRNAs away from targets (i.e., the suppression of microRNA functionality), thus exerting influence on transcription initiation and/or elongation, controlling the activity of certain RNA-binding proteins, and synthesizing functional peptides (Li et al., 2015; Kristensen et al., 2019; Chen et al., 2021; Song et al., 2022b).

Figure 1.

Types and nuclear export mechanisms of circRNAs.

CircRNAs are divided into four classes, including EcircRNAs, EIciRNAs, ciRNAs (A), and mecciRNA (B). (C) Length-dependent nuclear export of circRNAs: Drosophila Hel25E and its human homolog UAP56 modulate the transport of long circRNA, while Hel25E and another human homolog, URH49, regulate the transport of short circRNA. (D) The m⁶A reader YTHDC1 is involved in the nuclear export of m⁶A-circRNAs. (E) NXF1-NXT1 contributes to the nuclear export of GC-rich ciRNA. (F) XPO4 is required for the nuclear export of EcircRNAs with lower GC content, a larger length, and higher expression levels. Created with Adobe Illustrator CS5. CircRNAs: Circular RNAs; ciRNAs: intronic circRNAs; EcircRNAs: exonic circRNAs; EIciRNAs: exon-intron circRNAs; Hel25E: Helicase at 25E; mecciRNA: mitochondria-encoded circRNAs; NXF1: nuclear RNA export factor 1; NXT1: nuclear transport factor 2 like export factor 1; UAP56: ATP-dependent RNA helicase Uap56; XPO4: exportin 4; YTHDC1: YTH N6-methyladenosine RNA binding protein C1.

CircRNAs are known to play a regulatory role in the nervous system (Memczak et al., 2013; D’Ambra et al., 2019; Jiang et al., 2022a; Bai et al., 2023). Importantly, an increasing body of data suggests that certain brain-enriched circRNAs are spatiotemporally modulated in a development-dependent manner (Memczak et al., 2013; Westholm et al., 2014; Rybak-Wolf et al., 2015). Consequently, these special RNA molecules appear to be crucial for normal physiology but may also result in multiple neural diseases if their expression profiles undergo significant changes in the brain. Although the exact functionality of many circRNAs generated in the brain remain uncertain, it is becoming clear as to whether brain circRNAs are required for dynamic regulation during development and neuronal activity (Memczak et al., 2013; Liu et al., 2022b; Xiong et al., 2022). In this review, we focus on the recently described roles of circRNAs in the development, maturity, and aging of the brain, and the involvement of circRNAs in various diseases of the central nervous system, such as brain cancer, chronic neurodegenerative diseases, acute injuries in the nervous system, and neuropathic pain.

Search Strategy

The articles described in this review were identified by searching the Web of Science and PubMed databases updated until April 2023 with the following keywords: circRNA, brain development, maturity and aging, brain cancer, glioma, chronic neurodegenerative diseases, Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, spinal muscular atrophy, acute nervous system injuries, stroke, traumatic brain injury, spinal cord injury, and neuropathic pain.

Circular RNAs in Brain Development, Maturity, and Aging

Various circRNAs are known to serve as vital modulators of normal physiological processed in the nervous systems of both animals and humans (Table 1). The functional roles of some circRNAs have been proven in development, maturity and aging, including cellular, genetic, and molecular functions (Xu et al., 2021; Chen et al., 2022c; Pamudurti et al., 2022). In the next section, we discuss recent research studies that have delineated the cerebral expression of circRNAs along with recent progress relating to the functionality of circRNAs in the healthy brain.

Table 1.

Reported circRNAs with a known function in healthy brains and nervous system diseases

CircRNAs	Physiology or diseases	Biological functions	Molecular mechanisms	References
circAcbd6	Development: Neurogenesis	Promotes differentiation of neural stem cells into cholinergic neurons	Sponges miR-320-5p to regulate Osbpl2	Li et al., 2022b
circZNF827	Development: Neurogenesis	Balances neuronal differentiation	Functions as a scaffold for a transcription inhibitory complex including ZNF827, hnRNP L, and hnRNP K	Hollensen et al., 2020
circSLC45A4	Development: Neurogenesis	Facilitates spontaneous neuronal differentiation and impairs the cell-fate switch from apical progenitor to basal progenitor	Not determined	Suenkel et al., 2020
circEdis	Neuronal development	Regulates neuronal development, locomotor activity, and lifespan of adult flies	Regulates transcription factor Relish and castor	Liu et al., 2022b
CDR1as	Brain development	Decreases midbrain sizes to impair brain development	Sponging of miR-7	Memczak et al., 2013
circSfl	Aging	Extends the lifespan of fruit flies	Translated into a protein	Weigelt et al., 2020
circGRIA1	Aging	Modulates age-related improvement of synaptogenesis	Regulates Gria1 transcription	Xu et al., 2020
circTOP2A	Glioma	Promotes glioma proliferation and invasion through miR-422a/RPN2 axis	Regulates miR-422a/RPN2 axis	Sun et al., 2023
circBFAR	Glioma	Regulates the proliferative and invasive abilities of glioma in vitro	Sponges miR-548b to regulate its targeted gene FoxM1	Li et al., 2022a
circ_0001367	Glioma	Inhibits the proliferation, migration, and invasion of glioma cells in vitro and suppresses glioma growth in vivo	Sponges miR-431 to regulate its target NRXN3	Liu et al., 2021a
circKIF18A	Glioblastoma	Accelerates angiogenesis in vitro	Binds to FOXC2	Jiang et al., 2022b
circ-FBXW7	Glioma	Suppresses proliferation and cell cycle acceleration of glioma cells	Translated into FBXW7-185aa protein	Yang et al., 2018
circ-SHPRH	Glioblastoma	Decreases malignant behavior and tumorigenicity in vitro and in vivo	Translated into SHPRH-146aa protein	Zhang et al., 2018a
circ-AKT3	Glioblastoma	Suppresses the cell proliferation, and tumorigenicity of glioblastoma in vivo	Translated into AKT3-174aa protein	Xia et al., 2019
circ-E-Cad	Glioblastoma	Maintains self-renewal and tumorigenicity of glioma stem cells	Translated into C-E-Cad-254aa protein	Gao et al., 2021
circ-SMO	Glioblastoma	Regulates self-renewal and tumorigenicity of cancer stem cells	Translated into SM0-193aa protein	Wu et al., 2021
circHDAC9	Alzheimer’s disease	Modulates APP processing and synaptic function	Regulates the miR-138/Sirt1 axis	Lu et al., 2019
circCwc27	Alzheimer’s disease	Improves AD-related pathological traits, such as Aβ deposition, neuroinflammatory, and neurodegenerative effects	Binds RNA-binding protein Pur-α	Song et al., 2022a
circ-Pank1	Parkinson’s disease	Relieves dopaminergic neuron injury and locomotor dysfunction	Regulates the miR-7a-5p/α-synuclein signaling axis	Liu et al., 2022a
circ_0070441	Parkinson’s disease	Mitigates MPP+-induced neuronal damage by modulating cell apoptosis and inflammation	Regulates the miR-626/IRS2 axis	Cao et al., 2022
circSV2b	Parkinson’s disease	Resists oxidative stress	Regulates the miR-5107-5p/Foxk1/Akt1 axis	Cheng et al., 2022a
circHIPK3	Stroke	Decreases BBP, brain water content, and neurological deficit scores	Regulates the miR-148b-3p/CDK5R1/SIRT1 axis	Chen et al., 2022b
circOGDH	Stroke	Increases neuronal cell viability	Sponges miR-5112 to further enhance the expression of COL4A4	Liu et al., 2022c
circUSP36	Stroke	Relieves brain injury and neurological deficit	Regulates the miR-139-3p/SMAD3/Bcl2 signal axis	Yang et al., 2022a
circ-FoxO3	Stroke	Relieves BBB damage mainly via autophagy activation	Suppresses mTORC1 activity principally by sequestering mTOR and E2F1	Yang et al., 2022b
circSCMH1	Stroke	Increases the neuronal plasticity and suppresses glial activation and peripheral immune cell infiltration	Binds to the transcription factor MeCP2	Yang et al., 2020
circLphn3	TBI	Reduces the hemin-induced high permeability of the bEnd.3 cells model of the blood-brain barrier	Sponges miR-185-5p to regulate the expression of ZO-1	Cheng et al., 2022b
circHtra1	TBI	Promotes neuronal loss and immune deficiency	Regulates the miR-3960/GRB10 axis	Zheng et al., 2022
circlgfbp2	TBI	Relieves mitochondrial dysfunction and oxidative stress-induced synapse dysfunction	Sponges miR-370-3p to regulate BACH1 and HO-1	Du et al., 2022
circ-Usp10	SCI	Facilitates microglial activation and led to neuronal death	Regulates the miR-152-5p/CD84 axis	Tong et al., 2021
ciRS-7	Neuropathic pain	Regulates the progress of neuropathic pain	Sponges miR-135a-5	Cai et al., 2020
circSMEK1	Neuropathic pain	Promotes inflammation of neuropathic pain and microglia M1 polarization	Regulates the miR-216a-5p/TXNIP axis	Xin et al., 2021
circAnks1a	Neuropathic pain	Relieves the pain-like hypersensitivity	Suppresses miR-324-3p to enhance the translation of VEGFB mRNAs and binds to the promoter of VEGFB and facilitates YBX recruitment to promote VEGFB transcription	Zhang et al., 2019a

AD: Alzheimer’s disease; Akt1: AKT serine/threonine kinase 1; BACH1: BTB domain and CNC homolog 1; Bcl2: BCL2 apoptosis regulator; CDK5R1: cyclin dependent kinase 5 regulatory subunit 1; CircRNAs: circular RNAs; COL4A4: collagen type IV alpha 4 chain; E2F1: E2F transcription factor 1; FOXC2: forkhead box C2; Foxk1: forkhead box K1; FoxM1: forkhead box M1; GRB10: growth factor receptor bound protein 10; hnRNP K: heterogeneous nuclear ribonucleoprotein K; hnRNP L: heterogeneous nuclear ribonucleoprotein L; IRS2: insulin receptor substrate 2; MeCP2: methyl-CpG binding protein 2; NRXN3: neurexin 3; Osbpl2: oxysterol binding protein like 2; RPN2: ribophorin II; SCI: spinal cord injury; Sirt1: sirtuin 1; SMAD3: SMAD family member 3; TBI: traumatic brain injury; TXNIP: thioredoxin interacting protein; VEGFB: vascular endothelial growth factor B; ZNF827: zinc finger protein 827.

Approximately 20–21% of all eukaryotic protein-coding genes have been identified as circRNA-encoded genes in the mammalian brain (You et al., 2015). Studies have revealed that the expression levels of many circRNAs in brain are much higher than that in other tissues, as demonstrated in a wide range of species, including humans (Rybak-Wolf et al., 2015), mice (You et al., 2015), rats (Mahmoudi and Cairns, 2019), pigs (Veno et al., 2015), and fruit flies (Westholm et al., 2014). These studies demonstrate that circRNAs may play essential roles in the brain due to their abundance. In fact, it has been confirmed that certain circRNAs function in several stages of brain development (Memczak et al., 2013; Table 1). Many circRNAs have been found to accumulate in the brain and play functional roles in neuronal development, maturation, and loss, as well as the development of oligodendroglia (Table 1). For example, circAcbd6 facilitates the transition process during which neural stem cells differentiate into cholinergic neurons by inhibiting the function of miR-320-5p in Osbpl2 expression. This provided a crucial viewpoint into the mechanism by which circRNAs promote or inhibit neurogenesis (Li et al., 2022b; Figure 2A). Another study showed that circZNF827 acts as a scaffold for a transcription inhibitory complex including ZNF827, hnRNP L and hnRNP K, to contribute to a vital balance of neuronal differentiation and self-renewal/proliferation (Hollensen et al., 2020; Figure 2A). Furthermore, circSLC45A4 is known to be a conserved circRNA in the human embryonic frontal (22 weeks) cortex (Suenkel et al., 2020). In SH-SY5Y cells, the knockdown of circSLC45A4 by small interfering RNAs led to spontaneous neuronal differentiation. However, in vivo, in the developing mouse cortex, the knockdown of circSLC45A4 specifically impairs cell-fate, which leads to a switch from an apical progenitor to a basal progenitor phenotype (Suenkel et al., 2020); however, the molecular mechanism responsible has yet to be determined. Moreover, circEdis has been shown to predominantly accumulate in the brain of Drosophila melanogaster. The depletion of circEdis leads to injuries in axonal projection models of cerebral mushroom body neurons, impairs locomotor activity, and shortens the longevity of adult flies (Liu et al., 2022b). Mechanistically, circEdis functions via the transcription factors Relish and castor to regulate neuronal development (Liu et al., 2022b; Figure 2A). Furthermore, it has been reported that the depletion of XPO4 results in the accumulation of EcircRNAs in the nucleus. Furthermore, the number of neurons was significantly reduced in the hippocampus of XPO4^+/− mice when compared to XPO4^+/+ mice (Chen et al., 2022d; Figure 2B). These results demonstrate that insufficient XPO4 dosage and a deficiency of EcircRNA export can lead to neuronal loss (Chen et al., 2022d; Figure 2B). In addition, many new circRNAs undergoing dynamic modulation during the early differentiation of oligodendroglia have been identified by A-tailing RNase R techniques and pseudoreference alignment; these results indicate that the circRNA-miRNA-mRNA axis plays a key role in the development of human oligodendroglia (Li et al., 2022c). Together, these data suggest that circRNAs exhibit a dynamic expression profile in the brain; thus, brain-enriched circRNAs may represent fundamental modulators during brain development.

Figure 2.

The physiological roles of circRNAs in the nervous system.

(A) CircAcbd6 facilitates the differentiation of neural stem cells into cholinergic neurons via the miR-320-5p/Osbpl2 axis. CircZNF827 serves as a scaffold for a transcription inhibitory complex including ZNF827, hnRNP L, and hnRNP K, to contribute to a vital balance of neuronal differentiation in L-AN-5 cells. CircEdis functions via the transcription factor Relish and castor to regulate neuronal development in Drosophila. (B) Insufficient XPO4 dosage leads to the nuclear accumulation of EcircRNA, which then causes neuronal loss in XPO4^+/− mice. (C) The overexpression of circSfl can extend the lifespan of fruit flies. Created with Adobe Illustrator CS5. hnRNP K: Heterogeneous nuclear ribonucleoprotein K; hnRNP L: heterogeneous nuclear ribonucleoprotein L; Osbpl2: oxysterol binding protein like 2; XPO4: exportin 4; ZNF827: zinc finger protein 827.

There is increasing evidence that circRNAs also participate in aging, as identified in Drosophila (Westholm et al., 2014; Weigelt et al., 2020), mice (Gruner et al., 2016), rats (Mahmoudi and Cairns, 2019), Caenorhabditis elegans (Cortes-Lopez et al., 2018), pigs (Chen et al., 2019a), and the rhesus macaque (Xu et al., 2018, 2020; Table 1). For example, an increase in circRNA expression with age has also been reported in the brains of Drosophila (Westholm et al., 2014), mice (Gruner et al., 2016), rats (Mahmoudi and Cairns, 2019), pigs (Chen et al., 2019a), and the rhesus macaque (Xu et al., 2018, 2020). Similarly, many circRNAs have been shown to be enriched across the whole lifespan of Caenorhabditis elegans (Cortes-Lopez et al., 2018). However, we know little about the functions of circRNAs during aging. For example, circSfl is steadily upregulated in the brains of long-lived insulin mutant flies (Weigelt et al., 2020). The overexpression of circSfl can extend lifetime, thus revealing the vital impact of circSfl during aging (Weigelt et al., 2020; Figure 2C). Another study showed that circGRIA1 exhibits age-associated and male-specific enhanced expression in the hippocampus and prefrontal cortex of the rhesus macaque (Xu et al., 2020). The knockdown of circGRIA1 led to an age-associated improvement of synaptogenesis and enhanced GluR1 activity-related synaptic plasticity in the hippocampal neurons of male but not female rhesus macaques (Xu et al., 2020). Although these findings demonstrated that a subgroup of circRNAs are significantly enriched with age in different species, the functional roles of these special circRNAs during aging have yet to be elucidated. It is likely that other circRNAs with regulatory functions will be identified in future due to the abundance of cerebral circRNAs.

Circular RNAs in the Pathogenesis of Nervous System Diseases

CircRNAs play vital roles in different stages of nervous system development. CircRNAs have been investigated in several neurological diseases and exhibit functionality by serving as miRNA sponges, regulating RNA-binding proteins, and encoding proteins. CircRNAs exert vital effects on the occurrence and progression of a diverse range of nervous system diseases, including brain cancer (Sun et al., 2020b), chronic neurodegenerative diseases (Chen et al., 2022c), acute insults of the nervous system (Mehta et al., 2020) and neuropathic pain (Chen et al., 2022f). In this section, we describe the involvement of circRNAs in these nervous system diseases.

Brain cancer

Glioma is the most common primary tumor in the central nervous system (Zhang et al., 2020b). The progression of glioma is regulated by cell proliferation, migration, apoptosis and invasion (Mehta et al., 2020); circRNAs are known to play a vital role in these processes, as evidenced by many studies that have revealed a powerful association between the expression of circRNAs and the progression of glioma (Li and Diao, 2019; Zhang et al., 2019b; Zheng et al., 2019; Liu et al., 2021a; Wu et al., 2022a). Several circRNAs, including circNFIX, circKIF4A, circ_0001162, circCDK14, circFAM53B, circABCC1, circTTBK2, circRNA_0067934, circTOP2A, and circBFAR, are known to facilitate the progression of glioma by acting as microRNAs sponges, thereby regulating their targeted genes (Zheng et al., 2017; Ding et al., 2019; Huo et al., 2020; Zhou et al., 2021b; Chen et al., 2022e; Li et al., 2022a; Pei et al., 2022a, b; Wang et al., 2022; Sun et al., 2023). For instance, circTOP2A, which is highly expressed in glioma, has been shown to promote glioma proliferation and invasion via the miR-422a/RPN2 axis (Sun et al., 2023; Table 1 and Figure 3A). The miR-422a/RPN2 axis plays important roles in glioma tumorigenesis (Sun et al., 2020a). Similarly, circBFAR, which is produced from exon2 of the BFAR gene, appears to regulate the proliferative and invasive abilities of glioma in vitro by sponging miR-548b, thereby regulating the expression of its targeted gene FoxM1 (Li et al., 2022a; Table 1 and Figure 3A). FoxM1 is known to participate in glioma tumorigenesis (Zhang et al., 2011). Another study indicated that circ_0001367, which is downregulated in glioma tissues, can inhibit the invasion, migration, and proliferation of glioma cells in vitro and suppress the growth of glioma in vivo by sponging miR-431, thereby regulating its target neurexin 3 (NRXN3) (Liu et al., 2021a; Table 1 and Figure 3A). NRXN3, a member of the NRXN gene family, plays an important role in the progression of glioma (Sun et al., 2013).

Figure 3.

The roles of circRNAs in brain cancer.

(A) CircTOP2A, circBFAR, and circ_0001367 facilitate the progression of glioma by acting as miRNA sponges. (B) CircKIF18A reduces cell viability, proliferation, migration, invasion, the number of branches, and the tubule length of human brain microvessel endothelial cells by directly binding to FOXC2. (C) Circ-FBXW7 and circAKT3 are translated into FBXW7-185aa and AKT3-174aa to regulate glioma tumorigenesis, respectively. Circ-E-Cad and circ-SMO are translated into C-E-Cad-254aa and SMO-193aa to modulate the self-renewal of cancer stem cells. Created with Adobe Illustrator CS5. circRNAs: Circular RNAs; FOXC2: forkhead box C2; RBP: RNA binding protein.

In addition to acting as microRNA sponges, circRNAs can also exert direct regulatory effects on proteins. It has been recently reported that the knockdown of circKIF18A (from glioblastoma-associated microglia-derived exosomes) reduces cell proliferation, viability, migration, and invasion of human brain microvessel endothelial cells. Mechanistically, circKIF18A binds directly to FOXC2 to perform its functional role (Jiang et al., 2022b; Figure 3B and Table 1). These results indicated that circKIF18A could accelerate angiogenesis in vitro by binding directly to FOXC2. FOXC2 is known to be involved in tumorigenesis and angiogenesis via different angiogenic pathways (Kume, 2008; Wang et al., 2018b; Hargadon et al., 2022).

In addition, recent investigations have shown that some circRNAs, including circ-FBXW7, circ-SHPRH, circ-LINCPINT, circ-AKT3, circ-E-Cad, circ-EGFR, circ-SMO, and circEZH2, can be translated and have been shown to play vital roles in glioma (Yang et al., 2018; Zhang et al., 2018a, b; Xia et al., 2019; Gao et al., 2021; Liu et al., 2021b; Wu et al., 2021; Zhong et al., 2022). It has been reported that circ-FBXW7 can be translated to FBXW7-185aa. The overexpression of FBXW7-185aa in glioma is known to suppress cell proliferation and can block the cell cycle (Yang et al., 2018; Table 1 and Figure 3C). Mechanistically, FBXW7-185aa competitively interacts with USP28 to inhibit the binding of USP18 and FBXW7α and then facilitates the ubiquitination and degradation of c-Myc. C-Myc is known to be a crucial regulator of tumorigenesis. Similarly, circ-SHPRH, encoded by the SHPRH gene, produces a short protein known as SHPRH-146aa (Zhang et al., 2018a). The overexpression of this short protein in glioblastoma cells is known to reduce malignant behavior and tumorigenicity by protecting SHPRH from degradation (Zhang et al., 2018a; Table 1). Similarly, AKT3-174aa is synthesized from circ-AKT3 and suppresses cell proliferation and tumorigenicity in glioblastoma in vivo by competitively interacting with phosphorylated PDK1 (Xia et al., 2019; Table 1 and Figure 3C). In addition to regulating glioma tumorigenesis, two circRNAs have been shown to be involved in the self-renewal of cancer stem cells by serving as a translation template. For example, circ-E-Cad is encoded by the E-Cad gene and generates C-E-Cad-254aa protein. This protein maintains self-renewal and the tumorigenicity of glioma stem cells both in vitro and in vivo (Gao et al., 2021; Table 1 and Figure 3C). Another study showed that SMO-193aa, encoded by circ-SMO, modulates the self-renewal ability of cancer stem cells (Wu et al., 2021; Table 1 and Figure 3C).

Chronic neurodegenerative diseases

CircRNAs have been shown undergo differential changes in chronic neurodegenerative diseases (Huang et al., 2020a; D’Anca et al., 2022; Doxakis, 2022; Wu et al., 2023). In this section, we discuss the role of circRNAs in several neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and spinal muscular atrophy (SMA).

The accumulation of β-amyloid protein (Aβ), neuroinflammation, neuronal oxidative stress, autophagy, and alterations of synaptic plasticity, are all known to contribute to the pathogenesis of AD (Huang et al., 2020a; Cai et al., 2022). Researchers have previously reviewed the role of circRNAs in the accumulation of β-amyloid protein (Aβ), neuroinflammation, neuronal oxidative stress, autophagy, and the alteration of synaptic plasticity in AD (Huang et al., 2020a). Several circRNAs are known to play a role in AD by modulating Aβ expression. For instance, circAβ-a, synthesized by the amyloid precursor protein (APP) gene, generates Aβ₁₇₅ polypeptide, thus indicating that circAβ-a and its translation product might represent new therapeutic targets for AD (Mo et al., 2020). Another study showed that circHDAC9 regulates APP processing and synaptic function by acting as a sponge for miR-138, thereby modulating the expression of sirtuin 1 (Sirt 1) (Lu et al., 2019; Figure 4A). In addition, a recent study showed that circCwc27 is highly expressed in AD mice and patients (Song et al., 2022a). The knockdown of circCwc27 markedly ameliorates AD-related pathological traits, including Aβ deposition and neuroinflammatory and neurodegenerative effects, and improves cognitive dysfunction by directly binding to RNA-binding protein Pur-α (Song et al., 2022a; Figure 4A).

Figure 4.

The functions of circRNAs in chronic neurodegenerative diseases, ANSI, and neuropathic pain.

(A) CircRNAs are involved in AD-related pathological characteristics by acting as miRNA sponges (e.g., circHDAC9) and by regulating RNA bind proteins (e.g., circCwc27). (B) The depletion of circ-Pank1 relieves dopaminergic neuron injury and the overexpression of circSV2b resists oxidative stress by serving as a miRNA sponge in PD mice. (C) CircRNAs relieve blood-brain permeability (BBP) or blood-brain barrier (BBB) damage by acting as a miRNA sponge (e.g., circHIPK3) or by regulating RNA-binding protein (e.g., circ-FoxO3). (D) The overexpression of circLphn3 reduces the hemin-induced high permeability of the bEnd.3 cell model of the BBB in vitro. CircHtra1 promotes neuronal loss by acting as a miRNA sponge in TBI. (E) Circ-Usp10 promotes microglial activation and causes neuronal death via miR-152-5p/CD84 in microglia. (F) CiRS-7 relieves neuroinflammation and neuropathic pain in rats with chronic constriction injury. Created with Adobe Illustrator CS5. AD: Alzheimer’s disease; ANSI: acute nervous system insults; circRNAs: Circular RNAs; PD: Parkinson’s disease; SCI: spinal cord injury; TBI: traumatic brain injury.

An increasing body of evidence suggests that circRNAs play a role in PD-related processes, including α-synuclein dysregulation, neuroinflammation, and oxidative stress, by acting as microRNA sponges (Kumar et al., 2018; Hanan et al., 2020; Cao et al., 2022; Cheng et al., 2022a; Doxakis, 2022; Liu et al., 2022a). For instance, circ-Pank1 is significantly upregulated in the substantia nigra of PD mice (Liu et al., 2022a). The knockdown of circ-Pank1 relieves dopaminergic neuron injury and locomotor dysfunction via the miR-7a-5p/α-synuclein axis (Liu et al., 2022a; Figure 4B). Furthermore, the knockdown of circ_0070441 mitigates MPP⁺-induced neuronal damage and modulates cell inflammation and apoptosis by sequestering miR-626 from IRS2 mRNAs in an in vitro cellular model of PD (Cao et al., 2022). Another study showed that circSV2b was downregulated in the striatum of MPTP-induced PD mice (Cheng et al., 2022a). The overexpression of circSV2b resists oxidative stress by reducing the increases levels of oxidation product led by MPTP, and by increasing the activity of antioxidant enzymes via the miR-5107-5p/Foxk1/Akt1 axis in PD mice (Cheng et al., 2022a; Figure 4B). In summary, we know little about PD-related circRNAs and further research is required to fully investigate the functional roles of circRNAs in PD.

Some studies have investigated the relationship between circRNAs and ALS. For example, the expression levels of circRNAs underwent alterations in spinal cord regions, leukocyte samples, and skeletal muscle biopsies of ALS patients when compared to a normal control group (Dolinar et al., 2019; Aquilina-Reid et al., 2022; Tsitsipatis et al., 2022). Recently, another study showed that RNA-binding protein FUS (mutation P525L) can alter the metabolism of circRNAs, including formation and nuclear/cytoplasmic partitioning in human motor neurons, thus indicating the function of circRNAs in the pathogenesis of ALS (Colantoni et al., 2023). Studies relating to ALS-related circRNA are limited; thus, research is needed to investigate the functions of circRNAs in ALS.

Low levels of survival motor neuron (SMN) protein leads to SMA. It has been reported that SMN genes produce lots of circRNAs due to the high presence of inverted Alu repeats (Ottesen et al., 2017, 2019; Pagliarini et al., 2020). Recently, Luo et al. (2022) found that SMN circRNAs are localized in the cytoplasm. However, the precise function of circRNAs in the pathogenesis of SMA remains unclear.

Acute nervous system insults

Acute insults of the nervous system, including stroke, traumatic brain injury (TBI), and spinal cord injury (SCI), are primary causes of long-term disability and death in humans. Acute insults in the nervous system can damage cognitive and motor functions in injured individuals. Many studies have found that acute injuries to the nervous system can lead to dramatic alterations in circRNA expression levels and functions (Mehta et al., 2017; Chen et al., 2019c; Jiang et al., 2019; Wu et al., 2019; Dong et al., 2020; Ostolaza et al., 2020; Yuan et al., 2020; Ma et al., 2022). CircRNAs exhibit diverse roles in stroke, TBI, and SCI, thus implying their potential for biomedical applications (Table 1).

Stroke, including either ischemic stroke or hemorrhagic stroke, is the primary cause of adult-acquired disability in many regions (O’Donnell et al., 2010; Yan et al., 2015; Ma et al., 2018). Of these, ischemic stroke is the most common form, accounting for over 80% of all strokes (Hankey, 2017; Wang et al., 2018a; Campbell et al., 2019). Several circRNAs, including circTLK1, circ_0000811, circHIPK3, circCTNNB1, circOGDH, circUSP36, circ-CDR1as, circCDC14A, and circ_0025984, are known to ameliorate ischemic stroke-induced brain injury or cerebral ischemia/reperfusion (I/R) injury by serving as sponges for microRNAs (Wang and Wang, 2021; Zhou et al., 2021a; Zuo et al., 2021; Chen et al., 2022a, b; Huang et al., 2022; Liu et al., 2022c; Wu et al., 2022b; Yang et al., 2022a). For example, circHIPK3, produced by the HIPK3 gene, is known to be upregulated in ischemic brain tissues in a mouse model of stroke that is triggered by middle cerebral artery occlusion (MCAO) (Chen et al., 2022b). The knockdown of circHIPK3 reduces blood-brain permeability (BBP) and neurological deficit scores, and improves mitochondrial dysfunction in transient MCAO mice by regulating the miR-148b-3p/CDK5R1/SIRT1 axis (Chen et al., 2022b; Figure 4C). Similarly, circOGDH, derived from the oxoglutarate dehydrogenase (OGDH) gene, is highly expressed in the penumbra tissues of MCAO mice (Liu et al., 2022c). The depletion of circOGDH significantly increased neuronal cell viability via the miR-5112/COL4A4 axis (Liu et al., 2022c). In addition, another study showed that the overexpression of circUSP36 relieves brain damage and neurological injury and accelerates the recovery of motor function of transient MCAO mice by regulating the miR-139-3p/SMAD3/Bcl2 axis (Yang et al., 2022a). In addition to acting as microRNA sponges, some circRNAs can also directly regulate proteins to exert their roles in ischemic stroke. For instance, circ-FoxO3, synthesized from the FoxO3 gene, is upregulated in brain tissues after I/R in mice, particularly in brain microvascular endothelial cells and astrocytes (Yang et al., 2022b). Circ-FoxO3 relieves blood-brain barrier (BBB) injury mainly via the activation of autophagy. Mechanistically, circ-FoxO3 suppresses mTORC1 activity by inhibiting mTOR and E2F1, thereby promoting autophagy during cerebral I/R (Yang et al., 2022b; Figure 4C). Another study showed that, when compared to healthy controls, the plasma levels of the stroke-related circRNA circSCMH1 were reduced in patients with stroke. CircSCMH1 has been shown to enhance neuronal plasticity, suppress glial activation and peripheral immune cell infiltration, and improve motor recovery after stroke in models of ischemic stroke (Yang et al., 2020). Mechanistically, circSCM1 binds to the transcription factor MeCP2, thereby relieving the repressive impact of MeCP2 on the transcription of its target gene (Yang et al., 2020).

Previous studies revealed that TBI causes significant alterations in the expression profiles of circRNAs in the cerebral cortex, hippocampus, and even exosomes present in the extracellular space of the central nervous system in mice or rats (Xie et al., 2018; Zhao et al., 2018; Chen et al., 2019c; Jiang et al., 2019). Recently, some studies explored the roles of circRNAs acting as microRNA sponges in TBI. For example, circLphn3 was downregulated in the brain tissues of mice after TBI (Cheng et al., 2022b). The overexpression of circLphn3 reduced the hemin-induced high permeability of the bEnd.3 cell model of BBB in vitro by sponging miR-185-5p to regulate ZO-1 expression (Cheng et al., 2022b; Figure 4D). Another study showed that circHtra1 was highly expressed in the plasma of patients with TBI (Zheng et al., 2022). The knockdown of circHtra1 facilitated immune deficiency and neuronal loss by regulating the miR-3960/GRB10 axis (Zheng et al., 2022; Figure 4D). Similarly, after TBI, the knockdown of circlgfbp2 relieved mitochondrial and synapse dysfunction by sponging miR-370-3p, thereby regulating BACH1 and HO-1 (Du et al., 2022). Changes in the expression levels of circRNAs also occur after SCI (Qin et al., 2019; Zhou et al., 2019; Wang et al., 2021b). Inflammation and neuronal cell death are closely associated with the pathophysiology of SCI. Glial cells modulate neuronal apoptosis and inflammation, during which circRNAs primarily function as microRNA sponges (Tong et al., 2021; Zhang et al., 2022). For example, circ-Usp10 was highly expressed in damaged spinal cord tissue of mice with SCI when compared with a sham group (Tong et al., 2021). Circ-Usp10 facilitated the activation of microglia and led to neuronal death by sponging miR-152-5p, thereby regulating CD84 in microglia (Tong et al., 2021; Figure 4E).

Neuropathic pain

Many studies have shown that neuropathic pain leads to dramatic alterations in circRNA profiles and functionality (Cao et al., 2017; Zhou et al., 2017; Pan et al., 2019; Zhang et al., 2019a, 2020a; Mao et al., 2022). Several circRNAs, including cirRS-7, circSMEK1, circAnks1a, circRNA-Filip1l, circRNA.2837, circ_0005075, and CircZNF609 are known to be involved in neuropathic pain progression by sponging microRNAs, thereby regulating their target genes (Zhou et al., 2018; Pan et al., 2019; Zhang et al., 2019a, 2021; Cai et al., 2020; Li et al., 2020b; Xin et al., 2021). For example, ciRS-7 has been shown to regulate the progress of neuropathic pain by sponging miR-135a-5; the suppression of ciRS-7 has also been shown to relieve neuroinflammation and neuropathic pain in rats with chronic constriction injury (CCI) (Cai et al., 2020; Figure 4F). Similarly, circSMEK1 has been shown to promote the inflammation of neuropathic pain and microglia M1 polarization in CCI rats by regulating the miR-216a-5p/TXNIP axis (Xin et al., 2021). In addition, another study indicated that the knockdown of circAnks1a relieved the pain-like hypersensitivity caused by spinal nerve ligation in rats (Zhang et al., 2019a). Mechanistically, cytoplasmic circAnks1a has been shown to suppress miR-324-3p to promote the translation of VEGFB mRNAs, whereas in the nucleus, circAnks1 bound directly to the promoter of VEGFB and facilitated YBX recruitment to promote VEGFB transcription (Zhang et al., 2019a).

Limitations

In this review, we mainly focused on investigations relating to circRNAs in the nervous system. Some topics, such as the regulation of circRNA metabolism and the functional relevance of circRNAs in metabolic disease (Lee and Olefsky, 2021; Ren et al., 2023), cancer (Li et al., 2021) are reviewed elsewhere (Chen and Shan, 2021; Chen et al., 2022c, f).

Conclusions and Perspectives

CircRNAs are dynamically expressed and/or controlled in the peripheral nervous system and brain in a manner specific to the developmental stage. Although studies have indicated that circRNAs are enriched with age in various species, thus implying that some circRNAs might be directly involved in the regulation of these processes, their unique functions in aging have yet to be elucidated. It is likely that other circRNAs with regulatory roles will be identified in the future due to large number of cerebral circRNAs.

The appropriate regulation of circRNA metabolism is closely associated with a variety of physiological processes. For example, it has been reported that the depletion of XPO4 results in the accumulation of EcircRNAs in cell nuclei. Compared to XPO4^+/+ mice, XPO4^+/− mice were shown to exhibit neuronal loss in the hippocampus. This supports the fact that sufficient XPO4 and effective EcircRNA export are important for the appropriate functionality of hippocampal neurons (Chen et al., 2022d). However, few research studies have investigated the transport of circRNAs within the entire nervous system. The association between XPO4 and patients with nervous system diseases remains unclear, reminiscent of the role of Hel25E in circRNA export.

The translation ability of circRNAs is receiving increasing levels of attention. For example, it has been reported that eIF3j could regulate the translation of circSfl by directly interacting with an RNA regulon (Song et al., 2022b). This research offers a vital view into the field of cap-independent translation initiation. In theory, exogenous circRNA is a perfect template to produce functional proteins because of its resistance to exonuclease-mediated RNA degradation. However, the relatively low translation activity of translatable circRNAs fundamentally limits their applications in the clinic and even basic scientific research. Recently, Chen et al. (2023) discovered a highly efficient strategy to increase the yields of circRNA by several hundred-fold by improving five functional elements regulating the translation of circRNA including 5′ and 3′ untranslated regions, internal ribosome entry sites, vector topology, and synthetic aptamers recruiting translation initiation machinery. These results provide hope to obtain powerful and long-lasting protein production by translatable circRNA. Furthermore, translatable circRNAs have also been explored in glioma and AD. However, translatable circRNAs in other diseases of the nervous system have yet to be investigated comprehensively. Whether translatable circRNAs can be used to treat diseases of the nervous system requires further research. In addition, an increasing body of research is investigating the functionality of circRNAs by applying small interfering RNAs (siRNAs) and antisense oligonucleotides (ASO) specifically targeting circular junctions (Huang et al., 2020b; Song et al., 2021a; You et al., 2021). However, siRNAs and ASO targeting functional circRNAs in diseases of the nervous system need to be investigated further.

In conclusion, although there have been significant advances in the physiological and pathological functions of circRNAs in the nervous system, future research still needs to identify the downstream effectors of circRNAs. It is also extremely important for researchers to investigate the molecular mechanisms of fate determination in circRNAs and explore the clinical application of circRNAs in the nervous system.

Additional file: Open peer review reports 1 ^{(90.8KB, pdf)} and 2 ^{(98.5KB, pdf)} .