Emerging roles of circular RNAs in neuropathic pain

Abstract Neuropathic pain is a major type of chronic pain caused by the disease or injury of the somatosensory nervous system. It afflicts about 10% of the general population with a significant proportion of patients’ refractory to conventional medical treatment. This highlights the importance of a better understanding of the molecular pathogenesis of neuropathic pain so as to drive the development of novel mechanism‐driven therapy. Circular RNAs (circRNAs) are a type of non‐coding, regulatory RNAs that exhibit tissue‐ and disease‐specific expression. An increasing number of studies reported that circRNAs may play pivotal roles in the development of neuropathic pain. In this review, we first summarize circRNA expression profiling studies on neuropathic pain. We also highlight the molecular mechanisms of specific circRNAs (circHIPK3, circAnks1a, ciRS‐7, cZRANB1, circZNF609 and circ_0005075) that play key functional roles in the pathogenesis of neuropathic pain and discuss their potential diagnostic, prognostic, and therapeutic utilization in the clinical management of neuropathic pain.


| INTRODUC TI ON
Neuropathic pain is a chronic pain condition caused by disease or injury of the somatosensory nervous system. [1][2][3][4][5] It encompasses aetiologically distinct yet mechanistically similar disease entities, including postherpetic neuralgia, trigeminal neuralgia, painful diabetic neuropathy, cancer-related and chemotherapy-induced neuropathic pain, and neural injury or impingement, such as spinal cord injury and nerve root compression. [6][7][8][9][10][11] Although the exact prevalence of neuropathic pain varies from country to country, it has been estimated that up to 10% of the general population is afflicted with this potentially disabling condition. [12][13][14][15][16] Different medical treatments (eg tricyclic antidepressants, selective serotonin noradrenaline reuptake inhibitors, opioids, lidocaine) have been used clinically for the treatment of neuropathic pain, but up to half of the patients with neuropathic pain are refectory to these drugs. [17][18][19][20][21][22] The ineffectiveness of medical interventions arises partly due to the poorly understood molecular mechanism of neuropathic pain. Both peripheral and central sensitization are known to be implicated in the pathogenesis of neuropathic pain. [23][24][25] Whereas the anomalous excitability of the primary sensory neurons during peripheral sensitization may be due to maladaptive changes in the gene transcription and translation of enzymes, receptors, and voltage-dependent ion channels in the dorsal root ganglion, neuroinflammation caused by pathological microglia activation takes a major part in the process of central sensitization. [26][27][28][29] However, how the deranged molecular pathways underlying peripheral and central sensitization could be targeted therapeutically is still an active area of investigation. Circular RNAs (circRNAs) are a type of non-coding, regulatory RNAs evolutionarily conserved across mammalian species. [30][31][32][33][34] CircRNAs exhibit brain region-specific expression, 35,36 and the abundance of circRNAs in the brains of various species are largely similar. [37][38][39][40] CircRNAs exert their biological functions principally by acting as competing endogenous RNAs (ceRNAs) to regulate gene expression post-transcriptionally by sponging microRNAs (miR-NAs). [41][42][43][44] CircRNAs have been shown to be deregulated in different human diseases, including neurological disorders. 45,46 Recently, studies found that circRNAs may play important roles in the development of neuropathic pain. 47,48 In our review, we firstly summarize circRNA expression profiling studies on neuropathic pain so as to provide the scientific community with a comprehensive collection of data sets for subsequent integrative analysis. The biological functions and molecular mechanisms of specific circRNAs involved in the pathogenesis of neuropathic pain are also discussed in relation to their diagnostic, prognostic and therapeutic potentials in clinical settings.

| CIRCRNA E XPRE SS ION PROFILING AND INTEG R ATIVE ANALYS IS IN NEUROPATHIC PAIN
Microarray is an efficient tool for circRNA profiling. Cao and colleagues inflicted chronic constriction injury (CCI) to the sciatic nerve of rats to induce neuropathic pain. 49  mice. 52 They found that there were 135 circRNAs were differentially expressed and 71 circRNAs were downregulated and 64 circRNAs were overexpressed in spinal cord between control group and DNP group. Amongst these, circ_0010794, circ_0006623, circ_0006175, circ_0007095, circ_0005297, circ_0012840 and circ_0001580 was overexpressed and circ_0016083, circ_0006471, circ_0008757, circ_0004843 and circ_0013996 was downregulated ( Figure 1 and Table 1).

| CircHIPK3
CircHIPK3 is a circRNA that has been shown to function as a tumour suppressor gene or oncogene in a context-dependent manner to modulate tumour progression through sponging miRNAs. [53][54][55] Wang and colleagues investigated the potential regulatory role of circHIPK3 in the development of diabetic neuropathic pain. 56 Their data showed that circHIPK3 abundance was increased in the dorsal root ganglion from streptococci-induced diabetic rats and serum from patients with diabetic neuropathic pain. Higher expression of circHIPK3 was positively correlated with the grade neuropathic pain in cases with type 2 diabetes. Functionally, knockdown of circHIPK3 alleviated neuropathic pain in the diabetic rats and suppressed interleukin (IL)-12, tumour necrosis factor (TNF)α, IL-1β and IL-6.
Moreover, they showed that circHIPK3 was found to sponge miR-124 to contribute to neuroinflammation and exacerbate neuropathic pain in the diabetic rats. Therefore, circHIPK3 may be a potential therapeutic target for the treatment of neuropathic pain.

| CircAnks1a
Zhang and colleagues identified the aberrant upregulation of cir-

| ciRS-7
The circRNA ciRS-7 has been found to take part in the development of different diseases. [57][58][59][60] For example, Han and colleagues demonstrated that ciRS-7 induced migration and growth through modulating the miR-7-EGFR axis in papillary thyroid cancer. 61 Zhang and colleagues also showed that ciRS-7 enhanced epithelial-mesenchymal transition through sponging miR-641 to derepress MDM2 and ZEB1 expression. 62 In the CCI model of neuropathic pain, Cai et al.
found that ciRS-7 expression in the rat spinal cord dorsal horn was positively correlated with development of neuropathic pain partly through promoting inflammation, in which knockdown of ciRS-7 attenuated microglia activation and expression of pro-inflammatory cytokines IL-6, IL-12 and TNFα. 63 Mechanistically, ciRS-7 bound to and increased the availability of miR-135a-5p, whose inhibition also mitigated neuroinflammation and neuropathic pain. Their data indicated that either targeting ciRS-7 or miR-135a-5p could alleviate neuropathic pain through suppressing neuroinflammation.

| cZRANB1
Wei and colleagues studied the expression and functional role of miR-24-3p in the development of neuropathic pain in the CCI rat models. 64 They found that miR-24-3p expression was upregulated in the dorsal spinal cords of CCI rats, in which ablation of miR-24-3p significantly alleviated thermal hyperalgesia and mechanical allodynia. Moreover, miR-24-3p upregulated Wnt5aβ-catenin signalling pathway to induce neuropathic pain by inhibiting LPAR3 expression.
As the upstream modulator, the circRNA cZRANB1 was found to sponge miR-24-3p as predicted by bioinformatics analysis and confirmed by luciferase reporter assay and biotinylated RNA pull-down.
Importantly, cZRANB1 expression was decreased in CCI rats, in which enforced expression of cZRANB1 alleviated thermal hyperalgesia and mechanical allodynia. The regulation of miR-24-3p-LPAR3 axis by cZRANB1 was also confirmed in the CCI model.

| CircZNF609
Li  Table 2). and (3)  However, more in-depth functional and mechanistic investigations on pain-related circRNAs are warranted. It is hopeful that, with more translational studies, circRNAs will one day be utilized for the clinical management of patients with neuropathic pain.

CO N FLI C T O F I NTE R E S T S
The authors declare that they have no competing interests.