Roles of noncoding RNAs in neuropathic pain

This paper reviews the specific mechanisms involved in the regulation of neuropathic pain by 3 noncoding RNAs (ncRNAs) and explores the feasibility of using ncRNAs as therapeutic targets for neuropathic pain.

Abstract

Neuropathic pain is caused by a lesion or disease of the somatosensory (peripheral or central) nervous system. Most patients are associated with persistent or intermittent spontaneous pain. Intercellular interactions and molecular signaling underlie injury sensitization. These include abnormal expression of ion channels, epigenetic modulation, etc. Noncoding RNAs (ncRNAs) mainly include microRNA (miRNA), long noncoding RNA, and circular RNA. Recent studies have increasingly demonstrated the involvement of ncRNAs in the development and progression of neuropathic pain. This involves multiple regulatory mechanisms, including direct targeting of ncRNAs to downstream target genes, internal “sponge” regulation of ncRNAs, and a complex regulatory network between multiple ncRNAs and target genes. Within this comprehensive paper, a thorough analysis is conducted on the intricate biological functions of 3 distinct ncRNAs in relation to chronic neuropathic pain. Furthermore, the study investigates the utilization of animal models to better comprehend this condition and explores the promising possibilities of ncRNAs for diagnosing and treating neuropathic pain. The ultimate objective of this review is to offer fresh perspectives on the management and prevention of neuropathic pain.

1. Introduction

Neuropathic pain (NP) progression encompasses multiple organs and systems, encompassing the dorsal root ganglia, spinal cord, and brain.^102,109,118 Various forms of neuropathy can cause damage to the entire nervous system or target specific components such as axons or myelin. Within the realm of medical care, a significant concern revolves around effectively managing patients afflicted with persistent pain. An estimated 7% to 10% of the global population endures the burden of chronic neuropathic pain, with its prevalence being particularly pronounced among individuals aged 50 and above. As we delve into the academic discourse, it is necessary to elaborate further on the points above to provide a more comprehensive understanding. The global impact of chronic neuropathic pain on both the economy and public health remains a pressing concern, as the underlying complexities of its pathogenesis are yet to be fully unraveled.

Noncoding RNAs (NcRNAs), a distinct cluster of transcripts,¹³⁰ do not encompass directives for protein production; nevertheless, their involvement in pivotal biological activities at the RNA level is imperative. Consequently, the prominence of ncRNAs as regulators transcend the realm of protein-encoding, thereby underscoring their inherent significance in the regulatory landscape. In general, microRNA (miRNA), long noncoding RNAs (lncRNA), and circular RNA (circRNA) are used.

1.1. The basic biological functions of noncoding RNAs

For miRNA, we have described its biogenesis in detail in published papers,⁸⁴ with the help of a series of enzymes and proteins, it undergoes transcription, processing, and translocation, and finally forms a mature miRNA in the cytoplasm. miRNAs can bind to Argonaute proteins to form RNA-induced silencing complexes (RISCs). When miRNA entirely complements the 3'untranslated region (3'UTR) of the target mRNA, RISC can be directed to cleave the mRNA, leading to its degradation.¹²³ When miRNA is partially complementary to the target mRNA, RISC inhibits the translation of the mRNA and reduces protein synthesis.³⁴ MicroRNAs can also indirectly regulate gene expression by epigenetic modification or affecting mRNA stability.⁵⁵ For example, Xu et al.¹⁵³ found that miR-143 expression was significantly downregulated after spinal nerve ligation (SNL) injury, leading to an increase in Dnmt3a mRNA and protein expression in the dorsal root ganglia (DRG), which promotes neuropathic pain by silencing pain-related genes (the opioid receptor gene, Oprm1) through methylation. miR-143-Dnmt3a-pain gene methylation is an epigenetic regulatory axis, providing new therapeutic targets (development of miR-143 agonists or Dnmt3a inhibitors) for neuropathic pain. The essential biological functions of lncRNA can be divided into the following aspects: by recruiting chromatin-modifying complexes to specific genomic sites, they regulate histone modification or DNA methylation, thus affecting gene expression¹⁰³; lncRNAs can act as competing endogenous RNAs (ceRNAs) to regulate the expression of target genes by adsorption of miRNAs¹¹³; lncRNAs can bind to specific proteins binding to regulate the activity, cellular localization, or stability of the corresponding proteins, thus affecting protein function and biological processes.⁵⁹ The circRNA molecule has a closed loop structure, which is not affected by RNA exonuclease, and its expression is more stable and does not easily degrade. This stability enables circRNA to exist for a long time in the cell and play its biological function. circRNA molecules are rich in miRNA binding sites, which can act as miRNA sponges,⁴⁰ adsorb, and seal miRNAs, thus lifting the inhibitory effect of miRNAs on their target genes and elevating the expression level of target genes. This mechanism of action is the same as that of lncRNA. Circular RNA can bind to RNA-binding proteins (RBPs) to form RNA–protein complexes,⁵ which enhance the function of specific proteins and regulate gene transcription and translation. In addition, circRNA with internal ribosome entry site (IRES) structures can be used as translational templates to directly recruit ribosomes and initiate translation to produce biologically functional peptides.⁶²

As research progressed, scientists realized that ncRNAs, previously regarded as insignificant transcripts, play a crucial role in regulating critical biological processes within cells. These include altering chromatin, controlling transcription, facilitating post-transcriptional modifications, and aiding in signaling transduction.² An increasing body of scientific research has demonstrated ncRNAs' critical regulatory role in various diseases, such as tumors⁴³ and neuralgia.⁵⁶ These findings^{143,154,169,188} have underscored the significance of ncRNAs as key players in the pathogenesis and progression of these conditions. In light of these emerging discoveries, it becomes increasingly evident that a comprehensive understanding of the intricate mechanisms by which ncRNAs influence NP processes is of paramount importance.^69,94 Consequently, further exploration of ncRNAs' intricate involvement in NP etiology and elucidation of their underlying molecular mechanisms warrant considerable attention and investigation within the academic community.

2. Noncoding RNA and neuropathic pain

2.1. Unraveling the complexity of neuralgia: pathogenesis, classification, and animal model systems

Neuropathic pain, encompassing pain resulting from injury or affliction of the somatosensory nervous system, is explicitly defined as such. The intricate amalgamation of contributing elements that lead to neurological impairment, coupled with the diverse range of diseases responsible for pain manifestation, substantially contributes to the elaborate nature of the classification above encompassing neuropathic pain.³⁵ The perception of abnormal neuropathic pain injury is rooted in the complex interplay between cellular interactions and neuronal hyperexcitability resulting from aberrant signaling. Notably, this encompassing phenomenon involves the activation of immune cells, the release of mediators by glial cells, and the modulation of gene expression through epigenetic mechanisms. Furthermore, the dysregulation of signaling pathways leads to abnormal activation, further exacerbating neuralgia.²³ In addition, neuralgia is also attributed to alterations in receptors and ion channels, primarily attributed to specific gene mutations. It is important to emphasize that these multifaceted processes collectively contribute to initiating and progressing neuralgia, necessitating a comprehensive understanding to develop practical therapeutic approaches (Fig. 1).

Figure 1.

The pathogenesis of neuralgia: the peripheral or central nervous system is damaged, neurons are overexcited, and nerves are compressed.

Chronic pain can be categorized into 3 primary groups: neuropathic pain, which is caused by disease or injury to the somatosensory system; injurious pain, which is caused by disease or injury to the tissues; and mixed pain, which is a combination of both harmful and neuropathic pain.⁸ One specific type of chronic pain, known as chronic neuralgia, affects approximately 7% to 10% of the global population, predominantly individuals over the age of 50. Neuralgia exhibits distinct characteristics that set it apart from nociceptive pain, including spontaneous pain, pain triggered by external stimuli, postictal sensation, heightened pain sensitivity, and associated pain.⁹⁶ Chronic neuralgia encompasses pain originating from both the peripheral and central nervous systems. Peripheral neuropathic pain is further categorized into trigeminal neuralgia, neuropathic pain resulting from peripheral nerve injury, postherpetic neuralgia, painful polyneuropathy, and painful radiculopathy. Chronic central neuropathic pain includes conditions such as chronic central neuropathic pain associated with spinal cord injury, chronic central neuropathic pain associated with brain injury, chronic central poststroke pain, and chronic central neuropathic pain caused by multiple sclerosis. The neuralgia diagnosis does not currently have a universally accepted standard, specific method, or biomarker. Diagnostic challenges may arise, particularly with certain neuropathic pain conditions, such as postherpetic neuralgia, painful diabetic neuropathy, and pain after a central stroke.³³ In some cases of mixed pain, distinguishing between neuropathic and nonneuropathic pain may be more complex.

Animal models play an essential role in biology, serving as a crucial instrument of inquiry and bridging the gap between the laboratory and clinical applications. Within the domain of neuropathic pain, various animal models have been extensively employed, encompassing inflammatory foot pain RO models, cutaneous melanoma pain models, postoperative pain models, inertial ischemia–reperfusion pain models, arthritic pain models, bilateral sciatic nerve chronic constriction injury (bCCI) pain models, chronic constriction injury (CCI) of the sciatic nerve pain models, and spinal cord injury (SCI) pain models.^18,46,60 Moreover, other notable models include the STZ-induced type Ⅰ and type Ⅱ diabetic neuralgia models,⁷⁶ sporadic virus infection models, as well as intraperitoneal injection of resin toxin models for PHN (postherpetic neuralgia).³⁸ This comprehensive array of animal models significantly contributes to the advancement of knowledge in this field.

2.2. MicroRNA and neuropathic pain

MicroRNA refers to a subclass of noncoding single-stranded RNA molecules, typically consisting of approximately 22 nucleotides(nt) encoded by endogenous genes. These molecules actively participate in the intricate process of post-transcriptional gene expression regulation, effectively governing the physiological mechanisms inherent in both animals and plants.¹¹⁶ The scientific community has increasingly conducted research in recent years, unveiling an expanding body of evidence that underscores the count role played by miRNAs in orchestrating the onset and progression of neuralgia.^25,65,101 For example, hsa-miR-19a-3p and hsa-miR-19b-3p related to neurological pain induced by spinal cord injury were found in the expression spectrum screening of miRNA whole genomes.¹⁶⁰ Electroacupuncture (EA) relieves neurological pain, and its mechanism is related to miR-206-3p. The electroacupuncture can adjust miR-206-3p,¹¹⁹ while miR-206-3p targets BDNF (brain-derived neurotrophic factor) to relieve nerve pain. EP300 (E-cadherin transcriptional activator) adjusts BDNF through histones on the promoter of acetylation of BDNF,¹⁰⁵ and miR-30a-3p targets EP300 to relieve neuralgia symptoms.

One significant mechanism through which miRNAs exert their biological functions is assembling into the RNA-induced silencing complex (RISCs).^45,123 Within this complex, miRNAs facilitate the degradation of downstream target genes or impede the translation process of mRNAs. This intricate regulatory function is observed not only in various biological processes but also in the initiation and progression of neuropathic pain, thereby highlighting the vital role of miRNAs in modulating this pathological condition. For example, MiR-124-3p relieved neuralgia-related symptoms by targeting EGR1 (early growth response 1), because the latter's overexpression was involved in the occurrence and development of neuralgia.⁵⁰ Like the above, there are many examples of miRNA relieving neuralgia symptoms by targeting a molecule involved in the occurrence and development of neuralgia downstream, such as miR-142-3p targeted HMGB1 (high mobility group box 1) to lower its expression to relieve neuralgia symptoms¹⁸¹; there are also miR-129-5p¹¹⁴ and miR-381¹⁶⁴ targeting HMGB1; in CCI-induced neuropathic pain mice, the expression of miR-144 was reduced, and overexpression of miR-144 through intrathecal injection could relieve the symptoms of neuralgia, and RSA1 (RAS P21 Protein Activator 1) has proved to be the downstream target of miR-144¹⁷⁷; pain injury leads to downregulation of HDAC9 (histone deacetylase 9) and increased histone acetylation, which in turn promotes upregulation of miR-203a-3p expression. Upregulated miR-203a-3p is involved in the onset and development of neuropathic pain by targeting PC1 (proprotein convertase 1) to inhibit the synthesis of β-endorphin with analgesic effects¹¹¹; Zhong et al.¹⁹⁰ found that exogenous injection of miR-21-5p significantly relieved neuralgia symptoms in CCI rat, and CCL1 (C‐C motif ligand 1) and TIMP3 (tissue inhibitor of metalloproteinase‐3) were identified as downstream target genes of miR-21-5p. In addition, miR-122-5p, miR-124-3p, miR-134-5p, miR-194, miR-206-3p, miR-202, targeted PDK4¹²⁶ (pyruvate dehydrogenase kinase 4), Twist1,⁴⁸ FOXA1¹⁷⁶ (Forkhead box protein A1), HDAC4¹³⁸ (histone deacetylase 4), RAP1A³² (Ras-related protein Rap-1A), and respectively to inhibit or relieve neuralgia. In diabetic neuralgia, miR-590-3p alleviated DNP by targeting RAP1A and inhibiting T-cell infiltration.¹⁴⁵

2.2.1. MicroRNA regulates neuralgia by mediating signaling pathways

The occurrence of neuralgia is inherently intertwined with the process of signal conduction within the nervous system. This intricate relationship between neuralgia and signal conduction necessitates thoroughly exploring and comprehending the underlying mechanisms involved. It is imperative to delve deeper into the complexity of this association, as it holds significant implications for understanding the etiology and management of neuralgia. A study¹⁵⁸ has shown that miR-7a alleviated neurological pain by inhibiting neuromercerized polypeptides to block the signal transducers and activators of transcription signal pathways. After sciatic nerve injury in rats, the expression of miR-30c-5p was upregulated in the spinal cord, and the expression of miR-30c-5p was positively correlated with the severity of ectopic pain. Its specific mechanism was related to endogenous opioid systems.¹¹⁸ Wang et al.¹³⁵ found that intracerebroventricular administration of miRNA-195 could significantly aggravate facial pain development in CCI-IoN (infraorbital nerve chronic constriction injury) rats by targeting Patched1 in the SHH (Sonic hedgehog) signaling pathway. MiR-130a-3p participated in the occurrence and development of neuralgia by targeting the IGF-1/IGF-1R signaling pathway.¹⁵⁹ The knockdown of miR-130a-3p could alleviate neuropathic pain caused by spinal cord injury. Similarly, a study¹³ by Cai et al. has shown that overexpression of miR-150 could significantly inhibit AKT3 levels, while alleviating mechanical pain allergy and thermal pain allergy in CCI rat models. In a similar vein, miR-101 contributed to the alleviation of neuralgia through its ability to target various signaling pathways, including mTOR¹⁵⁰ and NF-κB⁷² signaling pathways. In CCI rat models, overexpression of miR‐98 attenuated neuropathic pain development via targeting STAT3¹⁸⁹ (signal transducer and activator of transcription 3). TRPV1 (transient receptor potential vanilloid 1), targeted by miR-338-3p,⁸⁰ induced neuropathic pain by interacting with NECAB2. MiR‐183 was a part of the negative regulator that could relieve neuropathic pain by targeting MAP3K4⁴⁴ (mitogen-activated protein kinase kinase kinase 4). MiR-216a-5p alleviated neuropathic pain in rats by targeting KDM3A¹³⁴ (lysine demethylase 3A) and inactivating the Wnt/β-catenin signaling pathway.

Peter et al.³⁶ found that miRNAs targeting toll-like receptor signals, such as miR-124 and miR-146a, were effective in relieving nerve pain, emphasizing the clinical potential of these noncoding RNAs. In the field of neuralgia research, the application of intrathecal injection and intracerebroventricular administration has become prevalent as indispensable technical methods for studying animal models. These techniques involve the targeted delivery of substances directly into the spinal canal or cerebral ventricles, respectively, allowing for precise investigation of the underlying mechanisms and pathophysiology associated with neuralgia. By employing these methodologies, researchers can gain valuable insights into the intricate neural processes involved in developing and progressing this debilitating condition, thus facilitating the development of novel therapeutic interventions. Jin et al.⁵³ found that intrathecal injection of miRNA-138 could significantly relieve neuralgia symptoms in SNL rats by targeting TLR4 (toll-like receptor 4) and MIP-1α/CCR1 signaling pathways. In bCCI rats, TLR5 (toll‐like receptor 5) was upregulated, and miR-217 could relieve neuropathic pain by targeting TLR5⁵¹; TLR5 was also a target gene of miR-150.⁴⁷ In SNL model mice, the expression of TLR8 and miR-21 in DRG neurons was upgraded, and intrathecal injection of TLR8 (Toll-like receptor 8) agonists or miR-21 could cause the occurrence and development of neuralgia, while inhibiting the expression of miR-21 could relieve neuralgia.¹⁸⁵

2.2.2. MicroRNA regulates ion channels in neuropathic pain

Receptor proteins and ion channels are fundamental to the functioning of the nervous system. Receptor proteins, located on the surface of nerve cells, are responsible for receiving and transmitting signals, often in response to external stimuli.¹⁰⁰ On the other hand, ion channels regulate ions' flow across the neuronal membrane, crucial for generating and propagating nerve impulses. Together, these components facilitate the complex interplay of signals within the nervous system. Dysfunctions in these processes, due to genetic, environmental, or other factors, can lead to the development of neuropathic pain, characterized by abnormal or heightened sensitivity to pain.⁴ In CCI model rats, Sirt1 (silent information regulator 1) activation may inhibit the expression of Nav1.7 by upgrading the expression of miR-182 in DRG, thus relieving neuralgia.⁴⁹ Overexpression of miR-182 by micro-injection could significantly reduce the expression level of Nav1.7 (sodium channel 1.7) and significantly reduce the pain hypersensitivity of SNI rats.¹⁵ MiR-30b-5p assumed a significant role in alleviating neuralgia during the onset and progression of NP. The underlying mechanism behind the action of miR-30b-5p was characterized by a relatively intricate interplay of factors. Liu et al.⁷⁴ found that miR-30b-5p could target CYP24A1, and Liu et al.⁷⁵ found that miR-30b-5p could target voltage-gated sodium channel Nav1.6. Therefore, miR-30b-5p can target different downstream molecules, such as molecules in a signaling pathway or a particular ionic channel protein molecule. In addition, miR-183-5P participated in the regulation of CCI-induced neuropathic pain by inhibiting the expression of TREK-1⁹⁸ (a K+ channel). Consequently, an in-depth exploration of these intricate biological pathways is warranted to gain a comprehensive understanding of the therapeutic potential and implications of miR-30b-5p in mitigating neuralgia.

2.2.3. MicroRNA governs neuroinflammation

Neural inflammation and neuralgia are interconnected conditions that affect the nervous system. Neural inflammation, an immune response within the nervous system, can lead to neuralgia. Common causes include infections, autoimmune disorders, and physical injuries. Understanding these conditions' intricacies is crucial for effective management and patient care.

The mesenchymal stem cells of the exocrine were expected to be used to treat neuralgia. Research¹⁸⁴ showed that miR181c-5p in the exocrine relieved CCI-induced NP by inhibiting neuroinflammation. In SNL model mice, miR-21-5p inhibitors can significantly relieve neuralgia. Further study has shown that CCL5 (CC-chemokine ligand 5) was the downstream target of the miR-21-5p.⁵⁸ A significant decrease in the expression of miR-26a-5p was detected in the spinal cord tissue of CCI rats, and overexpression of miR-26a-5p significantly inhibited neurological pain and neuroinflammation in CCI rats.¹⁸² Conditional knockdown of miR-21-5p in DRG neurons inhibited the upregulation of the chemokine CCL2 and the accumulation of CCR2+ macrophages after nerve injury, while promoting the shift of macrophages to an M2-like analgesic phenotype and activation of TGF-β–related pathways.¹⁶³ It was found that miR-421 was able to target lncRNA ZFAS1 directly. miR-421 expression was significantly downregulated in NP, which may lead to the upregulation of the expression of lncRNA ZFAS1, increasing the expression of proinflammatory factors and decreasing the expression of anti-inflammatory factors, which may then exacerbate the pathological process of NP.²² Dual-luciferase reporter assay experiments identified MAPK6 (mitogen-activated protein kinase 6) as a downstream target gene of miR-26a-5p. ZEB1 was significantly increased in microglia of CCI rats, and miR-128-3p relieved nerve pain by targeting ZEB1 to inhibit neuroinflammation¹⁷⁸; ZEB1 was still the target gene of miR-28-5p.⁷ Zhou et al.¹⁹¹ found that intrathecal infusion of agomir-miR-547-5p blocked the CCI-induced increases in the IL-33 (Interleukin-33) and ST2 (suppressor of tumorigenicity 2), and pain sensitivity.¹⁹¹ The expression of miR-15a in the spinal cord tissue of CCI rats decreased significantly, and neuroinflammation increased. Overexpression miR-15a could reduce the expression of AKT3 and induce the expression of autophagy-associated proteins, thus relieving neuralgia symptoms.¹² The gene AKT3 remained the focus of attention for miR-20b-5p¹⁶¹ and miR-145⁹⁹ as their prime target. miR-34c targeted NLRP1 (Nod-like receptor protein 3) to reduce its expression and inhibit NLRP1-mediated neuroinflammation, thus relieving neuralgia development.¹⁵⁵ It has the exact mechanism as miR-34c, and miR-223¹⁹³ and miR-23a⁹⁰ were also targeted at NLRP1 to inhibit neuroinflammation and relieve neuralgia. Yan et al.¹⁵⁷ constructed the SNL model of rats and found that miR-32-5p showed a high expression trend. Lowering miR-32-5p significantly relieved the pain hypersensitivity of rats and reduced the expression of cell inflammatory factors. Dusp5 (dual-specificity phosphatase 5) was further confirmed as the target of miR-32-5p. MiR-22 recruited RNA polymerase Ⅱ to enhance the expression level of Mtf1 (metal-regulatory transcription factor 1) by combining it in the promoter region of Mtf1.⁴¹ The high expression of Mtf1 then enhanced the sensitivity of the spinal cord center and induced inflammatory pain. Contrary to that, miR‐124‐3p was able to attenuate neuroinflammation and neuropathic pain by targeting EZH2 (Enhancer of Zeste Homolog-2).¹⁸⁰

At present, there is an imperative requirement to foster novel pharmaceuticals to address the persisting predicament of chronic neuralgia, a condition characterized by intricate pathogenesis and a dearth of efficacious preventive and therapeutic interventions. Given the inherent restrictions and insufficiencies of current therapeutic agents, our interest lies in exploring fresh target drugs that exhibit heightened effectiveness, minimal adverse effects, and a diminished tendency for resistance development. Furthermore, during the drug development process, careful consideration must be given to the pharmacological properties, targeting efficiency, and binding capacity of these drugs. Hence, directing our focus toward the dysregulated miRNA molecules implicated in chronic neuropathic pain undeniably represents a promising avenue for further investigation and advancement. Moreover, preclinical research endeavors have introduced miRNA mimics and miRNA inhibitors as novel therapeutic agents,⁷⁷ showcasing promising capabilities. These molecular entities are currently under investigation and have exhibited substantial potential, instilling hope for their future application in therapeutic interventions. The miRNA molecules mentioned above are summarized in Table 1.

Table 1.

Relevant examples of microRNAs implication in neuropathic pain

MiRNA	Downstream target(s)	Model(s)	Disease background	Biological function in the occurrence and development of neuralgia
miR-21-5p	CCL5 and YWHAE	SNL mice	Neuropathic pain	↑
miR-182	Nav1.7	CCI rat	Neuropathic pain	↓
miR-138	TLR4	SNL rat	Neuropathic pain	↓
miR-26-5p	MAPK6	CCI rat	Neuropathic pain	↓
miR-150	AKT3	CCI rat	Neuropathic pain	↓
miR-206-3p	BDNF	CCI adult male	Neuropathic pain	↑
miR-30a-3p	EP300	CCI rat	Neuropathic pain	↓
miR-195	Patched1	CCI-IoN rat	Trigeminal neuralgia	↑
miR-130a-3p	IGF-1	SCI SD rat	Neuropathic pain	↓
miR-124-3p	EGR1	SNL SD rat	Nerve injury–induced neuropathic pain	↑
miR-128-3p	ZEB1	CCI rat	Nerve injury–induced neuropathic pain	↓
miR-142-3p	HMGB1	SNL murine	Neuropathic pain	↓
miR-144	RSA1	CCI mice	Neuropathic pain	↓
miR-182	Nav1.7	SNI rat	Neuropathic pain	↓
miR-217	TLR5	bCCI rat	Neuropathic pain	↑
miR-547-5p	IL-33/ST2	CCI rat	Neuropathic pain	↑
miR-15a	AKT3	CCI rat	Neuropathic pain	↑
miR-20b-5p	AKT3	CCI rat	Neuropathic pain	↓
miR-21-5p	TIMP3 and CCL1	CCI rat	Neuropathic pain	↑
miR-28-5p	ZEB1	CCI rat	Neuropathic pain	↓
miR-30b-5p	CYP24A1	CCI rat	Neuropathic pain	↓
miR-30b-5p	Nav1.6	Rat	Oxaliplatin-induced peripheral neuropathic pain	↑
miR-101	mTOR	CCI rat	Neuropathic pain	↓
miR-101	KPNB1	None	Neuropathic pain	↓
miR-34c	NLRP3	CCI mice	Neuropathic pain	↓
miR-129-5p	HMGB1	CCI rat	Neuropathic pain	↑
miR-145	AKT3	CCI rat	Neuropathic pain	↓
miR-150	TLR5	CCI rat	Neuropathic pain	↓
miR-223	NLRP3	CCI mice	Neuropathic pain	↑
miR-381	HMGB1	CCI rat	Neuropathic pain	↓
miR-122-5p	PDK4	CCI rat	Neuropathic pain	↓
miR-124-3p	EZH2	CCI rat	Neuropathic pain	↓
miR-134-5p	Twist1	CCI rat	Neuropathic pain	↓
miR-183-5p	TREK-1	CCI rat	Neuropathic pain	↓
miR-194	FOXA1	CCI rat	Neuropathic pain	↓
miR-206-3p	HDAC4	CCI rat	Neuropathic pain	↓
miR-216a-5p	KDM3A	CCI rat	Neuropathic pain	↓
miR-98	STAT3	CCI rat	Neuropathic pain	↓
miR-183	MAP3K4	CCI rat	Neuropathic pain	↓
MiR-202	RAP1A	bCCI rat	Neuropathic pain	↑
miR-23a	NLRP3	pSNL mice	Neuropathic pain	↓
miR-338-3p	TRPV1	CCI rat	Neuropathic pain	↓
miR-32-5p	Dusp5	SNL rat	Neuropathic pain	↑
miR-22	Mtf1	CFA CCI mice	Inflammatory pain	↑
miR-155	None	miR-155 KO mice	SCI elicits chronic pain	↑
miR-203a-3p	PC1	Rodent pain model	Neuropathic pain	↑
miR-21-5p	CCL2	None	Neuropathic pain	↑
miR-421	lncRNA ZFAS1	CCI rat	Neuropathic pain	↓

↑Signifies the facilitation of the onset and progression of neuralgia.

↓Signifies the alleviation of neuralgia symptoms.

2.3. Long noncoding RNA and neuropathic pain

Long noncoding RNA represents a class of ncRNAs that surpasses a length of 200 nt. Unlike protein-coding RNAs, lncRNAs lack a significant open reading frame and are incapable of encoding proteins. The production of lncRNA is intricately regulated by specific stimuli that are unique to individual cell types and developmental stages, resulting in a cell type– and stage-specific biogenesis process.²⁴ As such, the generation of lncRNA is tightly controlled, ensuring their precise expression and functionality in various cellular contexts. In the aftermath of peripheral nerve injury, numerous lncRNA display disrupted expression patterns within regions associated with pain sensation.¹⁴³ Moreover, given the momentous role of lncRNA in modulating gene transcription and the exclusive presence of 40% of lncRNA solely in the nervous system,¹⁴⁴ it becomes imperative to direct our focus toward comprehending the intricate interplay between lncRNA and nerve pain. For the first time, Zhao et al.¹⁸⁶ systematically resolved the specific molecular mechanism of lncRNA in neuropathic pain. Specifically, after peripheral nerve injury, the transcription factor MZF1 is activated and binds to the promoter of the Kcna2 antisense RNA gene to upregulate its expression. kcna2 antisense RNA reduces potassium channel protein synthesis by silencing the voltage-gated potassium channel gene, Kcna2, resulting in elevated excitability of DRG neurons and triggering nociceptive hypersensitivity. Blocking the expression of this lncRNA restored potassium channel function and significantly alleviated neuropathic pain. This study established the potential of lncRNA as a new target for pain therapy, which is a milestone for understanding the pathological mechanisms of neuropathic pain and developing innovative therapies. Consequently, a more extensive analysis and exploration of this association are warranted to gain deeper insights into the underlying mechanisms and potential therapeutic interventions for pain management.

2.3.1. Long noncoding RNA regulates neuralgia by mediating opioid receptors and neuroinflammation

Neuroinflammation holds substantial significance within the realm of neuropathic pain research, warranting thorough investigation and analysis. Its multifaceted nature necessitates a comprehensive exploration to comprehensively address its underlying mechanisms and potential therapeutic interventions. Wen et al.¹³⁹ found that lncRNA FIRRE expression was significantly upregulated in the spinal cord tissue of CCI mice and microglia of LPS-induced. Silencing lncRNA FIRRE followed not only suppressed the expression of inflammatory-related factors in microglia but also alleviated pain symptoms in mice. Du et al. identified and named NIS-lncRNA (nerve injury–specific lncRNA). Upregulation of NIS-lncRNA can increase CCL2 (C–C chemokine ligand 2) expression and ultimately produce symptoms associated with neuralgia by recruiting the RNA-interacting protein FUS to bind to the promoter of CCL2.³¹ In addition, lncRNA FTX attenuates NP and neuroinflammation by modulating the miR-320a/RUNX2 axis.⁷⁸ It was shown that TGF-β1–stimulated lncRNA UCA1 in exosomes derived from UCMSCs attenuated LPS-induced microglial cell proliferation and inflammation in primary microglial cells via the lncRNA UCA1/miR-96-5p/FOXO3a axis.⁸⁶ Long noncoding RNA Neat1 organizes and regulates proinflammatory gene expression in DRG, which is upregulated after nerve injury, and its upregulation was suppressed by Neat1 knockdown.⁸¹ The involvement of opioid receptor activation and ion channel regulation in the initiation and progression of neuropathic pain holds significant importance within the realm of pain physiology and clinical practice. Pan et al.⁸⁸ reported downregulating DS-lncRNA (DRG-specifically enriched lncRNA) expression after peripheral nerve injury. Downregulation of DS-lncRNA increased rally (a transcriptional cofactor)-triggered Ehmt2 (euchromatic histone lysine methyltransferase 2) expression and correspondingly decreased opioid receptor and Kcna2 expression in DRG, leading to neuropathic pain symptoms in male mice in the absence of nerve injury. Sensory neuron–specific lncRNA (SS-lncRNA)¹²⁸ was able to bind to the promoter region of KCNN1(calcium-activated potassium channel subfamily N member 1) and affect its transcriptional activity, thereby restoring the expected expression level of KCNN1 after nerve injury and alleviating neuropathic pain. Furthermore, it was revealed that SS-lncRNA specific downregulation in small nonpeptidergic sensory neurons is required for nerve injury–induced mechanical hypersensitivity, which is likely achieved by silencing of KCNN1 expression, which is mediated by KDM6B-mediated increase in the enrichment of H3K27me3 in Kcnn1 promoter.¹²⁷ In the CCI-induced neuropathic pain model, lncRNA SNHG5 (small nucleolar RNA host gene 5)¹²⁹ was significantly upregulated in DRG, and SNHG5 enhanced SCN9A-encoded Nav1.7 expression by recruiting CDK9, which promotes neuronal apoptosis and injury and exacerbates neuropathic pain.

2.3.2. Interaction between long noncoding RNA, RNA-binding protein, and signaling in neuropathic pain

The interaction between LncRNA, RBP (RNA-binding protein), and signaling in NP, a critical aspect of cellular regulation, has recently garnered significant interest in the context of neuropathological conditions. We aim to explore the intricacies of lncRNA RBP interactions and their implications in NP. By focusing on the molecular mechanisms and signaling pathways, we seek to deepen our understanding of neurodegenerative diseases and potentially unearth novel therapeutic targets. This will offer a unique window into the complex landscape of neural health and disease, providing insights that are crucial for advancing medical research and treatment strategies in the realm of neurology. Zhang et al.¹⁸³ found that Lncenc1 (lncRNA embryonic stem cells expressed 1) expression was significantly upregulated in pSNL mice and interacted with RBP EZH2 (enhancer of zest homolog 2) to downregulate BAI1 (brain-specific angiogenesis inhibitor 1) expression, thereby promoting neuropathic pain. In addition, in microglia, overexpression of Lncenc1 led to high expression of inflammatory factors triggering neuroinflammation. Similarly, LINC01119 also could interact with RBP: ELAVL1 (ELAV-like protein 1), and the LINC01119-ELAVL1 complex can bind to and act as a stabilizer of BDNF (brain-derived neurotrophic factor) mRNA, thereby enhancing BDNF expression and ultimately promoting the development of neuralgia.¹⁷⁰ Diabetic neuropathic pain (DNP) represents a prevalent form of chronic pain, characterized by a multifaceted etiology that remains inadequately understood, and the quest for efficacious therapeutic interventions has yet to yield definitive results.¹⁰⁴ Liu et al.⁷¹ found that intrathecal injection of siRNA for lncRNA BC168687 significantly alleviated neuralgia symptoms and reduced P2X7 (Purinoceptors 2) expression in DRG of DNP rats. Kcnq1ot1 reduces the protein level of MyD88, a key molecule in the Toll-like receptor signaling pathway, by binding to MyD88 (myeloid differentiation factor 88), promoting its ubiquitination and degradation, thereby reducing the inflammatory response and ultimately alleviating neuropathic pain.⁶³ Long noncoding RNA Anxa10 to 203 enhances Mc1r mRNA stability by recruiting DHX30 in the trigeminal ganglion to facilitate neuropathic pain.⁷⁴ In a sciatic nerve injury model, upregulation of SIX1 (Sine Oculis Homeobox 1) gene expression increased the lncRNA HULC (upregulated in Liver Cancer). This upregulation of HULC, in turn, affected the progression of neuropathic pain.¹⁶⁶ Long noncoding RNA Gm14376 promoted the development of neuropathic pain by upregulating Fgf3 (fibroblast growth factor 3) expression and activating the PI3K/Akt pathway.¹⁰⁶ Long noncoding RNA RMST (rhabdomyosarcoma 2-associated transcripts) promoted neuropathic pain by recruiting HuR to stabilize DNMT3A (DNA methyltransferase 3 alpha) mRNA expression in dorsal root ganglion neurons.³⁷ LOC100911498¹¹⁰ was involved in the process of pain onset and development by regulating the P2X4R/p-p38/BDNF signaling pathway. In addition, mechanical nociceptive hypersensitivity in rats could be effectively attenuated by interfering with the expression of LOC100911498. The expression of lncRNA DILC (downregulated in liver cancer stem cells) was significantly upregulated in bCCI model rats. Its expression was downregulated by intrathecal injection, which could induce the expression of SOCS3 (suppressor of cytokine signaling) and thus inhibit p-STAT3 (signal transducer and activator of transcription 3) signaling pathway,⁷³ and finally significantly alleviate the mechanical and thermal nociceptive hypersensitivity in rats.

2.3.3. Long noncoding RNA function as sponges of microRNAs in neuropathic pain

In the pathogenesis and advancement of numerous diseases, lncRNA frequently functions as a molecular decoy for miRNA, thus facilitating the modulation of downstream target genes of miRNA and ultimately governing the disease mechanisms. The lncRNA–miRNA–target gene regulatory axis undoubtedly represents a classical regulatory pathway, and its significance remains unaltered in the context of the development and progression of neuropathic pain. It is imperative to emphasize that the interplay between lncRNA, miRNA, and target genes plays a considerable role in shaping the intricate landscape of disease pathophysiology, underscoring the indispensability of comprehending these regulatory mechanisms for therapeutic interventions targeting neuropathic pain. Zhang et al.¹⁷¹ found that downregulation of lncRNA PVT1 significantly reduced pain hypersensitivity symptoms in a SCI rat model, and further studies showed that lncRNA PVT1, as a ceRNA for miR-186-5p, upregulated the expression of CXCL13 (chemokine ligand 13)/CXCR5 (chemokine receptor 5) and thus triggered neuralgia. Similarly, inhibition of lncRNA DLEU1 (deleted in lymphocytic leukemia 1) expression alleviated neuroinflammation and neuropathic pain behavior in rats.⁶⁸ Mechanistically, lncRNA DLEU1 acted as a ceRNA of miR-133a-3p and upregulated the expression of SRPK1, thereby regulating the onset and progression of neuropathic pain. LINC00657 expression was significantly upregulated in DRG and spinal cord tissues of CCI model rats and increased ZEB1 expression by targeting miR-136 to cause abnormal pain hypersensitivity in rats. Knockdown of LINC00657 expression reduced the expression of inflammatory factors in rat microglia and alleviated pain symptoms in rats.⁹⁷ Similar to the regulatory mechanism of LINC00657, DGCR5 also targeted downstream miRNA molecules (miR-330-3p) to mediate the expression of PDCD4 (programmed cell death protein 4), thus playing a negative regulatory role in the onset and development of neuropathic pain.⁹² It was also found that UCA1 may be involved in regulating neuropathic pain by targeting and regulating miR-135a-5p.¹⁴¹ Inhibition of lncRNA TUG1 attenuates neuropathic pain in chronically contraction-injured rats by modulating the miR-29b-3p/HMGB1 axis.²⁸ Long noncoding RNA PCat19 promotes KDM3A-mediated demethylation of BDNF by interacting with miR-378a-3p as a ceRNA, which in turn promotes microglia activation and neuroinflammatory responses and accelerates the development of chronic neuropathic pain.¹⁸⁷ There was also lncRNA NEAT1 in CCI model rats by targeting miR-381 to upregulate HMGB1 (high-mobility group protein B1) expression to promote neuropathic pain symptoms in rats and downregulation of lncRNA NEAT1 also alleviated neuroinflammation in rats.¹⁴⁶ Regarding lncRNA NEAT1, another study¹⁴⁷ reported that it acts as a ceRNA for miR-128-3p to regulate the expression of AQP4 (aquaporin-4), thus participating in the onset and development of neuropathic pain. Similarly, lncRNA MALAT1 could target miR-129-5p, miR-154-5p, miR-20, respectively, to upregulate HMGB1,⁷⁹ AQP9 (aquaporin-9),¹⁴² ZEB1²⁰ and ultimately promote neuropathic pain. In SNL rats, Linc00052 expression was significantly elevated. Knockdown of Linc00052 significantly alleviated neuroinflammation. RNA immunoprecipitation experiments verified the targeting relationship between Linc00052 and miR-448. JAK1 (Janus kinase 1) was a downstream target of miR-448.¹³³ The lncRNA CRNDE (colorectal neoplasia differentially expressed gene) ultimately promoted the development and progression of neuralgia by competitively binding to miR-146-5p and thereby upregulating WNT5A expression in CCI rats.¹⁷² Also, in CCI model rats, lncRNA H19⁸² expression was significantly upregulated and inhibition of LncRNA H19 expression significantly alleviated nociceptive hypersensitivity and inhibited the expression and secretion of inflammatory factors. Dong et al.²⁹ found that lncRNA MEG3 as ceRNA of miR-130-5p upregulated CXCL12 (CXC motif chemokine receptor 12) as a way to promote inflammation in glial cells and induce pain symptoms in rats. Many other lncRNAs exert regulation of neuropathic pain processes through such mechanisms of action, such as lncRNA MIAT,¹⁷⁵ lncRNA p21,⁷⁵ lncRNA GAS5,¹¹⁵ which also competitively targeted binding to different miRNAs molecules to regulate the expression of relevant pain-related genes and proteins, as detailed in Table 2. Numerous small nucleolar RNA host genes (SNHG) significantly contribute to the initiation and progression of neuropathic pain, a condition of immense clinical significance. The integral involvement of these SNHG genes in the pathogenesis of neuropathic pain underscores their significant role in shaping the complex molecular landscape governing this debilitating disorder. In SCI rat models, expression of lncRNA SNHG12 was significantly increased, and downregulation of lncRNA SNHG12 expression reduced SCI-induced pain symptoms and decreased the expression of related cytokines.¹⁶⁷ Further study showed that lncRNA SNHG12 targets miR-494-3p to regulate RAD23B (nucleotide excision repair protein) expression, thus participating in the onset and development of neuropathic pain. Moreover, SNHG5 acted as a ceRNA for miR-142-5p to upregulate the expression of CAMK2A (calcium/calmodulin-dependent protein kinase II α).⁵² In another paper, SNHG5 also sponged miR-154-5p¹⁹ to upregulate CXCL13 expression; SNHG16⁶⁴ sponged miR-124-3p and miR-141-3p to upregulate the expression of JAG1 (a protein associated with Alagille syndrome). SNHG4⁸⁷ sponged miR-423-5p and SNHG1¹⁶⁸ directly targeted CDK4. Although the downstream target molecules and specific mechanisms of action of the 5 lncRNAs are different, all of them play biological functions to promote the occurrence and development of neuropathic pain. lncRNA CCAT1³⁰ (colon cancer associated transcript-1) enhances serum and glucocorticoid-regulated protein kinase 3 (SGK3) expression via sponge adsorption of miR-155 and thus regulates neuropathic pain progression. lncXIST²¹ acts as a molecular sponge for miR-211-5p, which binds to and downregulates the expression of miR-211-5p in neuronal cells, thereby deregulating its inhibitory effect on glial cell line–derived neurotrophic factor (GDNF) and increasing the level of GDNF. Glial cell line–derived neurotrophic factor further activates the receptor RET in pancreatic cancer cells and promotes PNI (perineural invasion) in pancreatic cancer, while PNI is an important pathological feature of pancreatic cancer and is closely related to cancer recurrence and pain.

Table 2.

Relevant examples of long noncoding RNA implication in neuropathic pain

lncRNA	Downstream target(s)	Model(s)	Disease background	Biological function in the occurrence and development of neuralgia
NIS-lncRNA	CCL2	SNL/CCI mice	Neuropathic pain	↑
lncRNA PVT1	miR-186-5p	SCI rat models	Neuropathic pain	↑
DS-lncRNA	raly	CCI/SNL mice	Neuropathic pain	↑
lncRNA FIRRE	HMGB1	CCI mice LPS-induced microglia	Neuropathic pain	↑
LncRNA DLEU1	miR-133a-3p	CCI rats	Neuropathic pain	↓
LncRNA BC168687	P2X7	DNP rats	DNP	↑
lncRNA SNHG12	miR-494-3p	SCI rats	Neuropathic pain	↑
lncRNA DILC	SOCS3	bCCI rats/primary microglia	Neuropathic pain	↑
LINC00657	miR-136	CCI rats/rat microglia	Neuropathic pain	↑
DGCR5	miR-330-3p	CCI rat	Neuropathic pain	↓
lncRNA NEAT1	miR-381	CCI rat	Neuropathic pain	↑
Linc00052	miR‐448	SNL rats	Neuropathic pain	↓
lncRNA NEAT1	miR-128-3p	SNL rats	Neuropathic pain	↑
lncRNA MALAT1	miR-129-5p	CCI rats	Neuropathic pain	↑
lncRNA MALAT1	miR-154-5p	CCI rats	Neuropathic pain	↑
SNHG5	miR-142-5p	CCI mice	Neuropathic pain	↑
SNHG16	miR-124-3p and miR-141-3p	CCI rats	Neuropathic pain	↑
SNHG1	CDK4	SCI rats	Neuropathic pain	↑
SNHG5	miR-154-5p	SNL mice	Neuropathic pain	↑
lncRNA MALAT1	miR-206	CCI rats	Neuropathic pain	↑
lncRNA CRNDE	miR-146a-5p	CCI rats	Neuropathic pain	↑
LncRNA H19	miR-141	CCI rats	Neuropathic pain	↑
lncRNA MEG3	miR-130-5p	LPS-induced astrocyte/CCI rats	Neuropathic pain	↑
LncRNA Miat	miR-362-3p	CCI mice	Neuropathic pain	↑
LncRNA P21	miR-181b	LPS-induced BV-2 microglia/SNL rats	Neuropathic pain	↑
LncRNA GAS5	miR-452-5p	CCI rats	Neuropathic pain	↓
LncRNA SNHG4	miR-423-5p	SNL rats	Neuropathic pain	↑
Lncenc1	Bai1	pSNL mice	Neuropathic pain	↑
LINC01119	BDNF	SNL rats	Neuropathic pain	↑
LncRNA FTX	miR-320a	CCI rats	Chronic constriction injury	↓
lncRNA UCA1	miR-96-5p	CCI rats and LPS-induced microglia cell model	Neuropathic pain	↑
Neat1 lncRNA	Proinflammatory genes	None	Neuropathic pain	↑
SS-lncRNA	KCNN1	None	Neuropathic pain	↓
SS-lncRNA	KCNN1	MrgprdCreERT2/+ lines with Rosa26SS-lncRNA knock-in mice and SS-lncRNAfl/fl mice	Neuropathic pain	↓
SNHG5	SCN9A	CCI rats	Neuropathic pain	↑
LncRNA Kcnq1ot1re	Myd88	CCI mice	Neuropathic pain	↓
LncRNA Anxa10-203	MC1R	CCI-ION mice	Trigeminal nerve injury	↑
HULC	None mention	SNI rats	Neuropathic pain	↑
Gm14376	Fgf3	SNI mice	Neuropathic pain	↑
Rmst	Hur	SNL and CCI mice	Neuropathic pain	↓
LOC100911498	P2X4R	SNI rats	Neuropathic pain	↑
UCA1	miR-135a-5p	CCI rats	Neuropathic pain	↓
TUG1	miR-29b-3p	CCI rats	Neuropathic pain	↓
lncRNA PCAT19	BDNF	CCI mice	Neuropathic pain	↑
CCAT1	miR-155	bCCI rats	Neuropathic pain	↓
XIST	miR-211-5p	None	Perineural invasion	↑

↑Signifies the facilitation of the onset and progression of neuralgia.

↓Signifies the alleviation of neuralgia symptoms.

Zhang et al.¹⁷⁴ proposed a sequence-derived linear neighborhood propagation method (SLNPM) to predict lncRNA–miRNA interactions. miRNAs can affect the half-life of lncRNA by promoting its degradation, and lncRNA can act as a “sponge” for miRNA, reducing their regulatory effects on target mRNA. The findings, as mentioned above, indicate that the interplay between lncRNA and miRNA exerts an influence on the progression of neuropathic pain, thereby contributing to an enhanced comprehension of the functional role of lncRNA. By shifting the focus from individual components to a comprehensive regulatory network, encompassing multiple levels of organization, it is possible to adopt a broader perspective that may unveil novel insights into the management of chronic neuropathic pain. This wider perspective may provide a valuable framework for exploring potential therapeutic interventions. The lncRNA molecules mentioned above are summarized in Table 2.

2.4. Circular RNA and neuropathic pain

The discovery of circRNA molecules, which do not have a 3′ polyadenylated tail or a terminal 5' cap,⁹¹ was first reported in 1796 in virus-like organisms.⁹⁵ With the continual progress in RNA sequencing technology, a growing number of circRNA have been successfully recognized and distinguished. Circular RNA can be effectively categorized into 4 primary classes: intergenic circRNA, exon–intron circRNA, circRNA originating from introns, and exonic circRNA.⁶⁷

One key mechanism through which circRNAs contribute to the initiation and progression of neuropathic pain is by acting as molecular sponges that adsorb specific miRNAs.¹⁸⁸ This adsorption event leads to the regulation of downstream cognate mRNAs and proteins involved in the neuropathic pain pathway.¹⁸⁸ For example, Li et al.⁶⁶ showed that circZNF609 upregulated the expression of ENO1 (Enolase 1) through sponge adsorption of miR-22-3p, thereby promoting inflammatory factor expression, ultimately leading to neuropathic pain. Peripheral nerve injury resulted in downregulation of ciRNA-Kat6b¹⁴⁹ in the dorsal horn of the spinal cord, which decreased miRNA-26a binding to ciRNA-Kat6b and increased miRNA-26a binding to Kcnk1 (potassium channel) mRNA and degradation of Kcnk1 mRNA, whereas reversal of ciRNA-Kat6b downregulation attenuated CCI-induced pain hypersensitivity. CircAnks1a¹⁷³ acted as a ceRNA for miR-324-3p, thereby upregulating VEGFB (a member of the VEGF family) expression, ultimately promoting the onset and development of neuropathic pain. Zhang et al.¹⁷⁹ demonstrated that miR-151a-3p is a potential target of circ_0005075 and in CCI model rats, overexpression of circ_0005075 significantly reversed the alleviating effect of miR-151a-3p on neuropathic pain symptoms. Again, in CCI pain model rats, ciRs-7 upregulated the level of autophagy and inflammation through sponge adsorption of miR-135a-3p thus promoting neuropathic pain¹⁴; circular RNA zRANB1¹³⁶ as ceRNA of miR-24-3p upregulated the expression of LPAR3, hence regulating Wnt5a/β-Catenin signaling pathway and ultimately alleviated neuropathic pain symptoms; circular RNA SMEK1¹⁵² competitively bound miR-216-5p to upregulate TXNIP (thioredoxin interacting protein) expression, thereby promoting neuroinflammation and microglia polarization. Zhou et al.¹⁹² found that CircRNA.2837, as a sponge of the miR-34 family, regulated neuronal autophagy, and the silencing of circRNA.2837 could protect neurons from damage. The expression of circHIPK3¹³² was significantly upregulated in the serum of patients with DNP as well as in a diabetic rat model induced by intraperitoneal injection of STZ. Silencing of circHIPK3 by intrathecal infusion of shRNA significantly alleviated neuralgia and neuroinflammation in rats. Another mode through which circRNA molecules exert their biological regulatory functions involves their interaction with RNA-binding proteins (RBPs), thereby influencing the epigenetic modification, expression, or stability of downstream molecules. This pathway represents an additional mechanism by which circRNA can modulate cellular processes and highlights their potential as essential regulators in gene expression and protein synthesis. For example, circularRNA-Filip1l contributed to neuroinflammation by binding and promoting the expression of Ubr5 (ubiquitin protein ligase E3 component n-recognin 5).⁸⁹ Interestingly, the maturation process of circularRNA-Filip1l molecule is regulated by miRNA-1224, which bound and sheared the precursor of circularRNA-Filip1l in an Ago2-dependent manner, thereby negatively regulating the expression of circularRNA-Filip1l. The study¹³¹ confirmed the interaction between circGRIN2B and NF-κB and found that circGRIN2B promotes the transcription of SLICK (sodium-dependent potassium current flux) gene by binding to NF-κB to relieve neuropathic pain. The expression of circ_lrrc49 was downregulated in the trigeminal ganglion,¹⁰⁷ and the downregulation of circ_lrrc49 was accompanied by a significant downregulation of the expression of Ist1, a protein involved in the regulation of autophagy, as well as by impaired autophagic activity, which exacerbated the neuropathic pain in mice. CircFhit¹⁵⁶ affects the synthesis and release of GABA transmitters by modulating the expression of its parental gene Fhit, which in turn regulates synaptic transmission in GABAergic neurons in the dorsal horn of the spinal cord and ultimately mediates neuropathic pain.

The dysregulation of circRNAs has been associated with various diseases, including cancer.¹²⁵ Notably, due to their distinctive circular structure as opposed to linear RNA molecules, circRNA expression offers increased stability and holds great potential as a novel biomarker for disease detection.⁸³ While the precise mechanisms and functions of circRNAs in the context of neuropathic pain remain incompletely understood, a mounting body of evidence from multiple investigations^16,148 highlights the biological role circRNAs play in the initiation and progression of neuropathic pain. Further exploration and elucidation of these mechanisms (Fig. 2) are warranted to enhance our understanding of circRNA involvement in neuropathic pain and potentially pave the way for innovative therapeutic strategies. The circRNA molecules mentioned above are summarized in Table 3.

Figure 2.

Interactions between ncRNAs and proteins affect neuroinflammation and abnormal excitation of ion channels in NP. ncRNAs, noncoding RNAs; NP, neuropathic pain.

Table 3.

Relevant examples of circular RNAs implication in neuropathic pain

Circular RNA	Downstream target(s)	Model(s)	Disease background	Biological function in the occurrence and development of neuralgia
CircZNF609	miR-22-3p	CCI rats	Neuropathic pain	↑
CircAnks1a	miR-324-3p	SNL rats	Neuropathic pain	↑
Circ_0005075	miR-151a-3p	CCI rats	Neuropathic pain	↑
ciRs-7	miR-135a-3p	CCI rats	Neuropathic pain	↑
zRANB1	miR-24-3p	CCI rats	Neuropathic pain	↓
CircularRNA-Filip1l	Ubr5	CFA mice	Neuroinflammation	↑
circHIPK3	miR-124	DNP rats	DNP	↑
SMEK1	miR-216-5p	CCI rats	Neuropathic pain and neuroinflammation	↑
circRNA.2837	miR-34 family	SNI rats	Neuropathic pain	↑
ciRNA-Kat6b	miRNA-26a	CCI mice	Neuropathic pain	↓
Circular RNA-GRIN2B	NF-κB/SLICK pathway	CCI rats	Neuropathic pain	↓
circ_lrrc49	Ist1	CCI-ION mice	Neuropathic pain	↑

↑Signifies the facilitation of the onset and progression of neuralgia.

↓Signifies the alleviation of neuralgia symptoms.

3. Conclusion and discussion

3.1. Recent progress in treatment of neuropathic pain

Based on the complex pathogenesis and the current lack of effective diagnostic methods and preventive medications, neuropathic pain has become a complex disease to treat.¹⁷ The currently recommended first-line treatment medications are antidepressants and antiepileptic drugs. Opioids have too many side effects, often with adverse related effects, and are frequently used in second and third-line treatment, which includes the FDA-approved tapentadol and tramadol.¹²² More potent opioids, such as oxycodone, and morphine, tend to be used in third-line therapy.⁶ In addition, data from experimental studies suggest that cannabinoid-derived compounds have potential efficacy in some posterior neuralgia, which is generally caused by diabetes, chemotherapy, etc. Antagonists of glutamate receptors associated with ion channels have also been used in the treatment of neuralgia. However, further studies and evaluations of the efficacy of these drugs are needed before a conclusion can be reached. The recent rapid development of nanomaterials as carriers of targeted drugs has also demonstrated great potential and clinical benefits in treating neuropathic pain, and certain breakthroughs have been achieved in animal models of neuropathic pain.¹⁰ Neuroinflammation is also a nonnegligible point in the development of neuropathic pain. Some anti-inflammatory or proinflammatory mediators regulate the neuroinflammatory and cellular activation state, and the abnormal activation of these cells promotes the onset and development of neuropathic pain.^1,124 Therefore, targeting neuroinflammation as a potential target to treat neuropathic pain is also very promising. Indeed, several anti-inflammatory natural compounds^3,57 have been widely used in therapeutic studies on neuropathic pain, and receptors⁷⁰ expressed by abnormally activated microglia are coming into the public eye as potential targets for the treatment of chronic pain. Even so, the identification of new diagnostic markers and potential drug targets, as well as the development of new drug formulations, appears to be critical.

3.2. Potential of ncRNAs as biomarkers in neuropathic pain

Given the relatively high stability of ncRNAs in body fluids, their specific expression in certain tissues, and their possession of a unique expression profile,^85,120,121 this gives them more tremendous potential as candidate diagnostic markers. In cancer, it has been shown that ncRNAs in extracellular vesicles released from tumor cells by somatic fluid biopsy are ideal for screening and treatment, and that the degree of dysregulation of ncRNA expression can contribute to the development of ncRNAs-base therapies.⁶¹ Similarly, in chronic neuropathic pain, there is an aberrant expression of ncRNAs, and these differentially expressed ncRNAs are considered as potential biomarkers for NP diagnosis, assessment, treatment, prediction, and prognosis. For example, lncRNA NONRATT021972 expression was upregulated in the blood of patients with diabetic neuropathic pain,¹⁶² and miR-124a⁴² expression was upregulated in patients with neuropathic pain. These studies suggest that ncRNAs may serve as potential diagnostic biomarkers and therapeutic targets for clinical applications in patients with NP. However, most existing studies have focused on animal neuropathic pain models, and there is still a long way to go before these findings can be translated to clinical patients. Looking in a better direction, a bio-classification system consisting of a set of biomarkers is desirable to achieve high sensitivity and specificity in the diagnosis of neuropathic pain.

3.3. Prospects and challenges of ncRNA clinical applications

With advances in high-throughput sequencing technologies such as microarrays and RNA sequencing, a large number of ncRNAs have been identified, thereby elucidating the complexity of the transcriptome and demonstrating the dysregulated expression of many ncRNAs in NP. These ncRNAs are involved in the regulation of multiple signaling pathways and regulate the expression of a large number of downstream genes, which in turn are involved in the expression of ion channels,¹⁵¹ transcription factors, glucose metabolism, amino acid metabolism, and related molecules that affect neuronal excitation³¹ related to the occurrence and development of NP. It has been shown that glucose metabolism and amino acid metabolism³⁹ can affect peripheral neuropathy. In addition, several downstream molecules such as HMGB1, AKT3, and ZEB1 deserve our attention as common target genes of several ncRNAs in the development of neuropathic pain, and an in-depth understanding of these genes, which are highly correlated with neuropathic pain, may help us better understand the pathogenesis of neuropathic pain. Long noncoding RNA and circRNA act as miRNA sponges to regulate miRNA expression in NP. This regulatory axis of lncRNA/circRNA–miRNA–mRNA needs to be studied in depth, and we look forward to seeing the complete picture of the substantial regulatory network of ncRNAs, downstream target mRNA, and proteins soon. This will undoubtedly greatly benefit our research and alternative ways of thinking about the treatment of a range of diseases, including neuropathic pain. Therefore, designing targeted drugs for these ncRNAs or their downstream target genes to treat neuropathic pain is a good therapeutic strategy. It was found⁹ that the expression level of NIS-lncRNA in vivo could be effectively reduced by systemic administration of antisense oligonucleotides of NIS-lncRNA. This strategy of reducing expression, in turn, alleviated neuropathic pain symptoms in experimental animal models. In addition, by intrathecal injection of antisense oligonucleotides of NIS-lncRNA,¹³⁷ the expression level of NIS-lncRNA in the spinal cord can be significantly reduced, which in turn alleviates the symptoms of neuropathic pain caused by various reasons such as neurotrauma, chemotherapeutic drugs, or diabetes. shRNA treatment against lncRNA MSTRG.81401 attenuates pain and depression-like behaviors in DNP and MDD comorbidities in type 2 diabetic rats by inhibiting P2X7 receptor-mediated pyroptosis pathways and inflammatory responses in the hippocampus.¹⁶⁵ In addition, lncRNA PCat19 shRNA injection also could reduce neuropathic pain.¹⁸⁷

Growing evidence¹⁸⁶ shows that miRNA-based therapies, either restoring or suppressing miRNA expression and activity, hold great promise. In fact, ncRNAs show great potential in the field of biomedicine, especially in disease diagnosis, prognostic evaluation, and treatment. Its clinical application still faces many challenges but also contains broad prospects.¹⁴⁰ A variety of treatments for ncRNAs have been developed, among which small interfering RNAs (siRNAs) and miRNA-based therapies^27,108 have been extensively studied. However, the characteristics and interaction mechanisms of ncRNAs limit their clinical translation. For example, miRNA is easily degraded by enzymes, which affects its stability as a biomarker or therapeutic agent. Studies have shown that carriers such as liposomes, polymers, and extracellular vesicles (EVs) can bind to nucleic acids or package them to efficiently deliver miRNA into cells to play a role while avoiding RNA degradation.^11,26,54 The regulatory effect of ncRNA often depends on sequence specificity, but its targeting may be affected by cell type, tissue environment, and other factors. Due to sequence similarity or higher-than-expected endogenous levels, ncRNA drugs may incorrectly target nontarget RNAs,¹⁴⁰ leading to off-target effects. Such nonspecific targeting not only reduces therapeutic efficacy but also may trigger unwanted side effects. In addition, cells other than the target cells may uptake the ncRNA drug, leading to uneven drug distribution and affecting the therapeutic effect. This problem can be addressed by improving the specificity and targeting of ncRNAs by using nanoscale vectors, artificial expression of viral transduction, or by introducing chemical modifications or adding biomolecular conjugators to customize therapeutic oligonucleotides to improve the cellular uptake efficiency of ncRNAs.^26,93 Prolonged use of ncRNA drugs may lead to patient tolerance, making the drugs progressively less effective or ineffective.¹¹² Although some chemical modifications¹¹⁷ have been developed to reduce the immunogenicity of ncRNA drugs, this problem has not been completely solved. Endogenous miRNAs and others may also act as TLR agonists, causing NF-κB activation and inflammatory factor production. Designing ncRNA drugs with high potency, specificity, and low immunogenicity is a technical challenge. Moving ncRNA drugs from the laboratory to the clinic requires a rigorous clinical trial and regulatory approval process, which can require a significant investment of time and money.

In conclusion, although some progress has been made in addressing the stability and specificity of RNA therapy, ncRNA therapy involves gene regulation, can cause unpredictable long-term effects, and its safety still has much to explore.¹⁴⁰

Currently, there is a lack of homogeneity and standardization in the study of ncRNAs, and there is no set of formulas that are as truthful in nature as physics or mathematics. Clinical sequencing of patients with neuropathic pain can determine which genes or ncRNAs are dysregulated in expression, which is a more accurate diagnostic and prognostic tool, and collecting more clinical data can better guide and manage prognosis. The establishment of transcriptomics and proteomics on neuropathic pain has helped us to understand better the pathogenesis of neuropathic pain at the molecular level. This review primarily presents a comprehensive overview of the significant contributions made by ncRNAs in the pathogenesis and progression of neuropathic pain over recent years. In addition, it sheds light on the various animal models employed in studying this phenomenon. The insights garnered from this analysis underscore the potential of ncRNAs as valuable diagnostic biomarkers and promising therapeutic targets for managing neuropathic pain. An in-depth examination of the available literature supports these assertions. By elucidating the intricate involvement of ncRNAs in the underlying mechanisms of neuropathic pain, this review contributes to the existing body of knowledge in this field. It provides a foundation for future research endeavors. The multifaceted roles played by ncRNAs in this context indicate their significance not only in the diagnosis of neuropathic pain but also in the development of novel therapies targeting this debilitating condition. Therefore, further exploration of ncRNAs in neuropathic pain holds immense promise and can potentially revolutionize the diagnosis and treatment landscape in the field of pain management.

Disclosures

No conflict of interest between authors.