MicroRNAs are a varied family of 19–24 nucleotide short noncoding single-stranded RNAs that have fundamental biological activities in posttranscriptional gene silencing. The ribonuclease II enzymes, Drosha and Dicer sequentially cleave precursor RNA transcripts to generate miRNAs (Hutvágner et al., 2001; Gregory et al., 2004; Han et al., 2004). The miRNA-induced silencing complex (miRISC) is then formed by loading miRNAs onto the effector protein Argonaute (Ago) (Schwarz et al., 2003). By using translational repression and/or mRNA instability, miRISC silences mRNA targets by binding to them via sequence complementarity (Schwarz et al., 2003; Macfarlane and Murphy, 2010; Figure 1).

MiRNAs, which regulate approximately one-third of human genes, are widely present in mammalian cells. More than 1800 miRNAs, which target 60% of human mRNAs, are found in the human genome (Olive et al., 2015). Numerous human disorders, including neuropathic pain, are linked to mutations in miRNA genes and miRNA pathway genes. Furthermore, the peripheral and central nervous systems, which contain pain-related components, are widely dispersed with microRNAs. Despite not being directly engaged in peptide synthesis, miRNAs are a type of noncoding RNAs that have a significant impact on miRNA stability and protein translation (Saliminejad et al., 2019).

By binding to the 3’UTR of target mRNAs, miRNAs control posttranscriptional gene expression by lowering the majority (84%) of protein products (Lutz et al., 2014). One miRNA can target more than one mRNA transcript, and multiple miRNAs can simultaneously act on one mRNA strand (Lutz et al., 2014; Li et al., 2015). There is a general consensus that miRNAs play pivotal roles in regulating important biological processes, including early development, cell proliferation, cell death and apoptosis, fat metabolism, cell differentiation, and disease progression (Lima et al., 2011). Various disorders, including cardiovascular, cerebrovascular, and neurodegenerative diseases, may be impacted by changed miRNA expression profiles, according to the results of experimental models (Barwari et al., 2016; Wang et al., 2020).

MiRNAs have been demonstrated to be involved in almost all known biological processes and many pathophysiological conditions, including neuropathic pain. The activation of thermoreceptors and mechanoreceptors in sensory neurons, which takes place in chronic inflammatory pain conditions, leads to the pathophysiology of neuropathic pain, which includes polyneuropathies, fibromyalgia syndrome, complex regional pain syndrome (CRPS), and postherpetic neuralgia (PHN) (Sayed and Abdellatif, 2011). Numerous miRNAs are becoming more widely recognized as master switches in the pathophysiology condition and as regulators of various neuroinflammation and neuronal gene expression (Figure 2). Differential expression of various miRNAs has been directly reported in the dorsal root ganglion (DRG) after the induction of inflammatory and neuropathic pain (Fang et al., 2022). Therefore, it is necessary to discuss how miRNAs regulate the infiltration of immune cells and neuroinflammation after nerve injury.

After peripheral nerve injury, the survival of neurons is an essential prerequisite for neural regeneration and functional recovery. According to earlier research, prolonged pain causes spinal cord and peripheral nerve cell body damage or possibly cell death (Cohen et al., 2021).

Recent studies have identified that injured peripheral neurons can activate the secretion of intrinsic neurotrophic factors that promote neuronal survival and axon regeneration, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) (Keefe et al., 2017). In this regard, miRNAs can also play a role in nerve regeneration (Figure 2). Coincidentally, it was discovered that the DRG has aberrant expression of a number of miRNAs that target genes involved in nerve regeneration (Bai et al., 2007). Moreover, it was suggested that, by increasing the expression of the X-linked inhibitor of apoptosis protein (XIAP), downregulation of miR-192-5p can decrease the apoptosis of nerve cells and aid in the regeneration process following sciatic nerve injury (SNI) (Liu X. et al., 2020). Additionally, the overexpression of miR-210 contributed to neuronal survival by preventing apoptosis through targeting ephrin-A3 (EFNA3), thereby promoting the regeneration of axons (Hu et al., 2016). Kruppel-like factor 4 (KLF4), a really well intrinsic inhibitor of axonal outgrowth and regeneration, can be efficiently silenced by miR-135a and miR-135b to stimulate axonal outgrowth and cortical neuron migration (van Battum et al., 2018).

In spinal cord injury (SCI) rats, direct 3D fiber hydrogel scaffold implantation combined with continuous supply of a cocktail of axon miRNAs (miR-132, miR-222, and miR-431) dramatically improved axon regeneration (Zhang et al., 2021). In earlier research, miR-125b overexpression supported axon regeneration after spinal cord injury through controlling the JAK/STAT pathway. Furthermore, through lowering neuronal apoptosis and the inflammatory reaction, miR-125b demonstrated a neuronal protective effect (Dai et al., 2018). In vivo, miR-155 deletion enhanced injury-induced expression of SPRR1A, a regeneration-related gene, in neurons and reduced inflammatory signalling in macrophages, thereby enhancing axon regeneration (Gaudet et al., 2016). Mice with an optic nerve compression greatly increased their ability to regenerate their axons in vivo by enhancing the levels of miR-19a in their retinal ganglion cells (Mak et al., 2020). Additionally, injury-induced changes in the expression of miR-21 and miR-199a-3p changed the ability of axons to develop by altering both systemic and intra-axonal protein synthesis through control of the PTEN/mTOR pathway (Kar et al., 2021). The PTEN/mTOR pathway is a major factor in determining axonal regeneration. The tumor suppressor phosphatase and tensin homologue (PTEN) is a PIP3 phosphatase, and an inhibitor of mTOR signaling. The serine/threonine protein kinase known as the mechanistic target of rapamycin (mTOR), whose activation improved protein synthesis and mRNA translation (Tang, 2018). They synergistically regulate axon growth. MiRNA-21 controls the expression of TGF-I, TIMP3, and EPHA4 target genes, which is crucial for increasing Schwann cell (SC) proliferation and axon regeneration during the healing of damaged peripheral nerves (Ning et al., 2020). Moreover, endogenous miR-26a in sensory neurons enhanced the regeneration of sensory axons after spinal cord injury (SNI) by promoting the induced activation of Smad1 and inhibiting the expression of glycogen synthase kinase 3β (GSK3β) (Jiang et al., 2015).

In contrast, many miRNAs can also negatively regulate neuronal regeneration, thereby supporting neuropathic progression. By controlling the epidermal growth factor receptor (EGFR) pathway, miR-21, for instance, promotes the hyperactivation of astrocytes and the development of glial scar tissue, preventing the regeneration of axons (Li et al., 2018). Additionally, miR-455-5p inhibition suppressed axonal growth and regeneration and downregulated activation of the GSK3β/Tau protein pathway in murine sensory neurons (Su et al., 2020). miR let-7 and miR-9 hindered axonal regeneration through inhibition of the protein levels of Ntn1 and Dcc (Wang et al., 2019), respectively. Likewise, researchers found that axon regeneration is also driven by miR-9 through regulation of FoxP1 triggered by injury. However, sensory neurons with high endogenous miR-9 levels were unable to regenerate their axons (Jiang et al., 2017). Additionally, a novel mechanism for regulating the capacity for intrinsic axon regeneration is provided by the mutual negative response regulatory loop formed by miR-138 and SIRT1 (Liu et al., 2013).

These findings present a novel idea for the future study of axon regeneration in neuropathic pain. There are still many obstacles to be overcome in order to create therapies that achieve complete regeneration and functional recovery of neurons, even though substantial advances have been made in comprehending the fundamental mechanisms of peripheral nerve regeneration and how these pathways can be effectively utilized to promote regeneration after peripheral nerve injury (PNI).

After the nerve fiber is injured, the structure and function of ion channels in the nerve endings and DRG of the spinal cord may change, leading to the ectopic discharge of neurons, and neuropathic pain (Jiang et al., 2022). Thus, ion channels play a key role in neuronal excitability and may be targets of miRNAs under pain conditions (Figure 2). Notably, voltage-gated channels involved in the pain pathway have become the main targets of neuropathic pain treatment interventions.

Exosomes (Exos), a class of nanosized EVs with sizes from 40 to 200 nm, are released from all cell types and participate in paracrine interactions between various cells, including neurons, glial cells, mesenchymal stem cells, endothelial cells, and leukocytes (Mathivanan et al., 2010). As another type of secreted factor, these biological nanocarriers, which are rich in a variety of genetic materials, including miRNAs, long noncoding RNA, proteins, and lipids, can be easily distributed in biofluids and modulate biochemical responses and cell viability during physiological and pathological conditions in neurodegenerative or inflammatory diseases (Cata et al., 2022). Blood, saliva, breast milk, urine, and other bodily fluids all contain significant amounts of exosomal miRNAs (Zhang et al., 2015; Ding et al., 2019).

Pain is frequently caused by inflammation. A variety of cytokines, chemokines, and additional elements contribute to the emergence of acute inflammatory pain. Chronic inflammation can start the processes that lead to cerebral and peripheral sensitization (Jiang et al., 2020). Exosomal miRNAs can be transported to different sites after autocrine production, acting on macrophages, microglia, neurons or other tissue cells, and regulate the process of neuropathic pain by participating in the secretion of inflammatory factors, and oxidative stress, and regulating neural remodelling or nerve regeneration (Figure 3). Exosomes have the ability to regulate the release from cells of nociceptive mediators, which are involved in neuroinflammation and are recognized to sensitize sensory terminals (Groot and Lee, 2020). For instance, immunological cells such as T lymphocytes and antigen-presenting dendritic cells (DCs) can release and absorb exosomal miRNAs, indicating that exosomal transfer of miRNAs may constitute a novel method of intercellular communication (Torralba et al., 2018). As a result, it is believed that exosomal miRNA transmission is significant for a number of systems and processes, such as the immune reaction and neuron–glia communication (Kalani et al., 2014).

Interestingly, these exosomal miRNAs can have both proinflammatory and anti-inflammatory effects. As mentioned above, some exosomal miRNAs can release cytokines or other proinflammatory mediators that directly act on target organs (Zhang et al., 2014; Console et al., 2019). For example, exosomes from chondrocytes, neutrophils, and synovial fibroblasts encouraged macrophages to produce IL-1 and metalloproteinases. Furthermore, the regulation of the synthesis of these inflammatory chemicals has been linked to the transfer of miR-206 and miR-449a-5p in the cargo of exosomes (Liu et al., 2018; Ni et al., 2019). MiR-449a-5p in particular mediates ATG4B inhibition, which in turn controls macrophage autophagy, encourages inflammosome activation, and exacerbates the inflammatory response.

Evidence suggests that miR-21 was increased in serum exosomes from neuropathic mice. Further research revealed that miR-21-5p-containing macrophage-derived exosomes promoted pyroptosis via A20, encouraging a proinflammatory phenotype and exacerbating podocyte damage in diabetic nephropathy mice (Ding et al., 2021). Notably, the DRG’s hyperalgesia and macrophage recruitment were reduced by either intrathecal miR-21-5p antagomir addition or miR-21 conditional deletion in sensory neurons. A20 is an inhibitor of the NF-κB signaling pathway. Similar research showed that after nerve damage, DRG sensory neurons released EVs that were miR-23a-enriched and were then ingested by macrophages to improve M1 polarization in vitro. Additionally, by blocking A20 to promote NF-κB signaling, an EV-miR-23a antagomir delivered intrathecally reduced M1 macrophages and improved neuropathic hyperalgesia (Zhang et al., 2021).

Conversely, other exosomal miRNAs exert anti-inflammatory and analgesic effects in chronic pain models via the transfer of therapeutic factors to injured neurons in the central nervous system (CNS) and peripheral nervous system (PNS) (Ren J. et al., 2019). The promotion of inflammation is thought to be the leading cause of pain. Exosomal miRNAs can also suppress the production of proinflammatory cytokines such as IL-1β, IL-6, TNF-α, and PGE2 in injured areas and stimulate the release of IL-10, leading to antinociceptive effects (Cata et al., 2022). These miRNAs have the ability to modify nociception, and intrathecal administration of miR-124, miR-103, miR-23b, miR-25, and miR-544, for example, reduced inflammatory and neuropathic pain by modifying intracellular neuronal, astrocytic, and microglial activities (Favereaux et al., 2011; Willemen et al., 2012; Wang et al., 2018; Zhao et al., 2019; Li et al., 2020). MiR-124 inhibits GRK2 expression, thereby regulating the M1/M2 phenotypic balance of the spinal cord.

Similar to the effect of some neurotrophic factors, including GDNF, IGF-1, BDNF, NGF, and FGF-1, exosomal miRNAs can enhance axonal growth and neuronal viability and intensify therapeutic effects (Lv et al., 2021). To the best of our knowledge, the targeted and modular EV loading (TAMEL) method has not been implemented in experimental pain studies. Despite the fact that many studies have tried to provide new and more effective mechanistic insights into the function of exosomal miRNAs in NP, the cellular and molecular functions of exosomal miRNAs and their downstream targets remain to be elucidated.