Emerging roles of microRNAs in chronic pain

BDNF in Neuropathic Pain; the Culprit that Cannot be Apprehended

In males but not in females, brain derived neurotrophic factor (BDNF) plays an obligatory role in the onset and maintenance of neuropathic pain. Afferent terminals of injured peripheral nerves release colony stimulating factor (CSF-1) and other mediators into the dorsal horn. These transform the phenotype of dorsal horn microglia such that they express P2X4 purinoceptors. Activation of these receptors by neuron-derived ATP promotes BDNF release. This microglial-derived BDNF increases synaptic activation of excitatory dorsal horn neurons and decreases that of inhibitory neurons. It also alters the neuronal chloride gradient such the normal inhibitory effect of GABA is converted to excitation. By as yet undefined processes, this attenuated inhibition increases NMDA receptor function. BDNF also promotes the release of pro-inflammatory cytokines from astrocytes. All of these actions culminate in the increase dorsal horn excitability that underlies many forms of neuropathic pain. Peripheral nerve injury also alters excitability of structures in the thalamus, cortex and mesolimbic system that are responsible for pain perception and for the generation of co-morbidities such as anxiety and depression. The weight of evidence from male rodents suggests that this preferential modulation of excitably of supra-spinal pain processing structures also involves the action of microglial-derived BDNF. Possible mechanisms promoting the preferential release of BDNF in pain signaling structures are discussed. In females, invading T-lymphocytes increase dorsal horn excitability but it remains to be determined whether similar processes operate in supra-spinal structures. Despite its ubiquitous role in pain aetiology neither BDNF nor TrkB receptors represent potential therapeutic targets.

miRNA contributes to neuropathic pains

Neuropathic pain (NP) is a kind of chronic pain caused by direct injury to the peripheral or central nervous system (CNS). microRNAs (miRNAs) are small noncoding RNAs that mostly interact with the 3 untranslated region of messenger RNAs (mRNAs) to regulate the expression of multiple genes. NP is characterized by changes in the expression of receptors and mediators, and there is evidence that miRNAs may contribute to some of these alterations. In this review, we aimed to fully comprehend the connection between NP and miRNA; and also, to establish a link between neurology, biology, and dentistry. Studies have shown that targeting miRNAs may be an effective therapeutic strategy for the treatment of chronic pain and potential target for the prevention of NP.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Non-Coding RNAs Regulate Spinal Cord Injury-Related Neuropathic Pain via Neuroinflammation

Secondary chronic neuropathic pain (NP) in addition to sensory, motor, or autonomic dysfunction can significantly reduce quality of life after spinal cord injury (SCI). The mechanisms of SCI-related NP have been studied in clinical trials and with the use of experimental models. However, in developing new treatment strategies for SCI patients, NP poses new challenges. The inflammatory response following SCI promotes the development of NP. Previous studies suggest that reducing neuroinflammation following SCI can improve NP-related behaviors. Intensive studies of the roles of non-coding RNAs in SCI have discovered that ncRNAs bind target mRNA, act between activated glia, neuronal cells, or other immunocytes, regulate gene expression, inhibit inflammation, and influence the prognosis of NP.

Tuina for peripherally-induced neuropathic pain: A review of analgesic mechanism

Peripherally-induced neuropathic pain (pNP) is a kind of NP that is common, frequent, and difficult to treat. Tuina, also known as massage and manual therapy, has been used to treat pain in China for thousands of years. It has been clinically proven to be effective in the treatment of pNP caused by cervical spondylosis, lumbar disc herniation, etc. However, its analgesic mechanism is still not clear and has been the focus of research. In this review, we summarize the existing research progress, so as to provide guidance for clinical and basic studies. The analgesic mechanism of tuina is mainly manifested in suppressing peripheral inflammation by regulating the TLR4 pathway and miRNA, modulating ion channels (such as P2X3 and piezo), inhibiting the activation of glial cells, and adjusting the brain functional alterations. Overall, tuina has an analgesic effect by acting on different levels of targets, and it is an effective therapy for the treatment of pNP. It is necessary to continue to study the mechanism of tuina analgesia.

Mediators of Neuropathic Pain; Focus on Spinal Microglia, CSF-1, BDNF, CCL21, TNF-α, Wnt Ligands, and Interleukin 1β

Intractable neuropathic pain is a frequent consequence of nerve injury or disease. When peripheral nerves are injured, damaged axons undergo Wallerian degeneration. Schwann cells, mast cells, fibroblasts, keratinocytes and epithelial cells are activated leading to the generation of an “inflammatory soup” containing cytokines, chemokines and growth factors. These primary mediators sensitize sensory nerve endings, attract macrophages, neutrophils and lymphocytes, alter gene expression, promote post-translational modification of proteins, and alter ion channel function in primary afferent neurons. This leads to increased excitability and spontaneous activity and the generation of secondary mediators including colony stimulating factor 1 (CSF-1), chemokine C-C motif ligand 21 (CCL-21), Wnt3a, and Wnt5a. Release of these mediators from primary afferent neurons alters the properties of spinal microglial cells causing them to release tertiary mediators, in many situations via ATP-dependent mechanisms. Tertiary mediators such as BDNF, tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and other Wnt ligands facilitate the generation and transmission of nociceptive information by increasing excitatory glutamatergic transmission and attenuating inhibitory GABA and glycinergic transmission in the spinal dorsal horn. This review focusses on activation of microglia by secondary mediators, release of tertiary mediators from microglia and a description of their actions in the spinal dorsal horn. Attention is drawn to the substantial differences in the precise roles of various mediators in males compared to females. At least 25 different mediators have been identified but the similarity of their actions at sensory nerve endings, in the dorsal root ganglia and in the spinal cord means there is considerable redundancy in the available mechanisms. Despite this, behavioral studies show that interruption of the actions of any single mediator can relieve signs of pain in experimental animals. We draw attention this paradox. It is difficult to explain how inactivation of one mediator can relieve pain when so many parallel pathways are available.

Dorsal Root Injury-A Model for Exploring Pathophysiology and Therapeutic Strategies in Spinal Cord Injury

Unraveling the cellular and molecular mechanisms of spinal cord injury is fundamental for our possibility to develop successful therapeutic approaches. These approaches need to address the issues of the emergence of a non-permissive environment for axonal growth in the spinal cord, in combination with a failure of injured neurons to mount an effective regeneration program. Experimental in vivo models are of critical importance for exploring the potential clinical relevance of mechanistic findings and therapeutic innovations. However, the highly complex organization of the spinal cord, comprising multiple types of neurons, which form local neural networks, as well as short and long-ranging ascending or descending pathways, complicates detailed dissection of mechanistic processes, as well as identification/verification of therapeutic targets. Inducing different types of dorsal root injury at specific proximo-distal locations provide opportunities to distinguish key components underlying spinal cord regeneration failure. Crushing or cutting the dorsal root allows detailed analysis of the regeneration program of the sensory neurons, as well as of the glial response at the dorsal root-spinal cord interface without direct trauma to the spinal cord. At the same time, a lesion at this interface creates a localized injury of the spinal cord itself, but with an initial neuronal injury affecting only the axons of dorsal root ganglion neurons, and still a glial cell response closely resembling the one seen after direct spinal cord injury. In this review, we provide examples of previous research on dorsal root injury models and how these models can help future exploration of mechanisms and potential therapies for spinal cord injury repair.

Correlation of miRNA expression with intensity of neuropathic pain in man

Background

Peripheral nerve injury causes changes in expression of multiple receptors and mediators that participate in pain processing. We investigated the expression of microRNAs (miRNAs) – a class of post-transcriptional regulators involved in many physiological and pathophysiological processes – and their potential role in the development or maintenance of chronic neuropathic pain following lingual nerve injury in human and rat.

Methods

We profiled miRNA expression in Sprague-Dawley rat and human lingual nerve neuromas using TaqMan® low-density array cards. Expression of miRNAs of interest was validated via specific probes and correlated with nerve injury-related behavioural change in rat (time spent drinking) and clinical pain (visual analogue scale (VAS) score). Target prediction was performed using publicly available algorithms; gene enrichment and pathway analysis were conducted with MetaCore. Networks of miRNAs and putative target genes were created with Cytoscape; interaction of miRNAs and target genomes in rat and human was displayed graphically using CircosPlot.

Results

rno-miR-138 was upregulated in lingual nerve of injured rats versus sham controls. rno-miR-138 and rno-miR-667 expression correlated with behavioural change at day 3 post-injury (with negative (rno-miR-138) and positive (rno-miR-667) correlations between expression and time spent drinking). In human, hsa-miR-29a was downregulated in lingual nerve neuromas of patients with higher pain VAS scores (painful group) versus patients with lower pain VAS scores (non-painful). A statistically significant negative correlation was observed between expression of both hsa-miR-29a and hsa-miR-500a, and pain VAS score.

Conclusions

Our results show that following lingual nerve injury, there are highly significant correlations between abundance of specific miRNAs, altered behaviour and pain scores. This study provides the first demonstration of correlations between human miRNA levels and VAS scores for neuropathic pain and suggests a potential contribution of specific miRNAs to the development of chronic pain following lingual nerve injury. Putative targets for candidate miRNAs include genes related to interleukin and chemokine receptors and potassium channels.

Epigenetic reduction of miR-214-3p upregulates astrocytic colony-stimulating factor-1 and contributes to neuropathic pain induced by nerve injury

Emerging evidence has indicated that colony-stimulating factor-1 (CSF1) modulates neuroinflammation in the central nervous system and the development of neuropathic pain, while the underlying mechanism remains unknown. Here, we identified the increased expression of CSF1 derived from activated astrocytes in the ipsilateral dorsal horn in rats with spinal nerve ligation (SNL). Suppression of CSF1 expression alleviated neuroinflammation, neuronal hyperexcitability, and glutamatergic receptor subunit upregulation in the dorsal horn and improved SNL-induced pain behavior. We also found reduced miR-214-3p expression in the ipsilateral dorsal horn following an SNL procedure; miR-214-3p directly bound to the 3'-UTR of CSF1 mRNA and negatively regulated CSF1 expression. Intrathecal delivery of miR-214-3p mimic reversed the enhanced expression of CSF1 and astrocyte overactivity and alleviated the IL-6 upregulation and pain behavior induced by SNL. Moreover, suppression of spinal miR-214-3p increased astrocyte reactivity, promoted CSF1 and IL-6 production, and induced pain hypersensitivity in naive animals. Furthermore, SNL induced the expression of DNA methyltransferase 3a (DNMT3a) that was associated with the hypermethylation of the miR-214-3p promoter, leading to reduced miR-214-3p expression in the model rodents. Treatment with the DNMT inhibitor zebularine significantly reduced cytosine methylation in the miR-214-3p promoter; this reduced methylation consequently increased the expression of miR-214-3p and decreased the content of CSF1 in the ipsilateral dorsal horn and, further, attenuated IL-6 production and pain behavior in rats with SNL. Together, our data indicate that the DNMT3a-mediated epigenetic suppression of miR-214-3p enhanced CSF1 production in astrocytes, which subsequently induced neuroinflammation and pain behavior in SNL model rats.

miR-200a-3p modulates gene expression in comorbid pain and depression: Molecular implication for central sensitization

Chronic pain and depression are often comorbid exhibiting common clinical presentations and biological connections related to central nervous system sensitization. Epigenetic regulation of gene expression in the brain plays a crucial role in response to long-lasting stress and chronic pain, and microRNA imbalance in the prefrontal cortex (PFC) might be involved in central sensitization. Male Sprague Dawley rats were subjected to unpredictable chronic mild stress (UCMS) and spared nerve injury (SNI) to initiate depressive-like behavior and chronic pain behavior, respectively. The next-generation sequencing technique was employed to analyze PFC microRNAs in both the UCMS and SNI models. Rats exposed to either UCMS or SNI exhibited both depressive-like and chronic pain behaviors. Five specific microRNAs (miR-10a-5p, miR-182, miR-200a-3p, miR-200b-3p, and miR-429) were simultaneously down-regulated in the depressive-like and chronic pain models after 4 weeks of short-term stress. Gene ontology revealed that the 4-week period of stress enhanced neurogenesis. Only the miR-200a-3p level was continuously elevated under prolonged stress, suggesting roles of reduced neurogenesis, inflammatory activation, disturbed circadian rhythm, lipid metabolism, and insulin secretion in the co-existence of pain and depression. Thus we conclude that miR-200a-3p might be a specific biomarker of central sensitization in chronic pain and depression.

Inhibition of MicroRNA-15a/16 Expression Alleviates Neuropathic Pain Development through Upregulation of G Protein-Coupled Receptor Kinase 2

There is accumulating evidence that microRNAs are emerging as pivotal regulators in the development and progression of neuropathic pain. MicroRNA-15a/16 (miR-15a/16) have been reported to play an important role in various diseases and inflammation response processes. However, whether miR-15a/16 participates in the regulation of neuroinflammation and neuropathic pain development remains unknown. In this study, we established a mouse model of neuropathic pain by chronic constriction injury (CCI) of the sciatic nerves. Our results showed that both miR-15a and miR-16 expression was significantly upregulated in the spinal cord of CCI rats. Downregulation of the expression of miR-15a and miR-16 by intrathecal injection of a specific inhibitor significantly attenuated the mechanical allodynia and thermal hyperalgesia of CCI rats. Furthermore, inhibition of miR-15a and miR-16 downregulated the expression of interleukin-1β and tumor-necrosis factor-α in the spinal cord of CCI rats. Bioinformatic analysis predicted that G protein-coupled receptor kinase 2 (GRK2), an important regulator in neuropathic pain and inflammation, was a potential target gene of miR-15a and miR-16. Inhibition of miR-15a and miR-16 markedly increased the expression of GRK2 while downregulating the activation of p38 mitogen-activated protein kinase and NF-κB in CCI rats. Notably, the silencing of GRK2 significantly reversed the inhibitory effects of miR-15a/16 inhibition in neuropathic pain. In conclusion, our results suggest that inhibition of miR-15a/16 expression alleviates neuropathic pain development by targeting GRK2. These findings provide novel insights into the molecular pathogenesis of neuropathic pain and suggest potential therapeutic targets for preventing neuropathic pain development.

Functional roles of lncRNAs and its potential mechanisms in neuropathic pain

Neuropathic pain (NP) is ranked as one of the major forms of chronic pain and emerges as a direct consequence of a lesion or disease affecting the somatosensory nervous system. Despite great advances into the mechanisms of NP, clinical practice is still not satisfactory. Fortunately, progress in elucidating unique features and multiple molecular mechanisms of long non-coding RNAs (lncRNAs) in NP has emerged in the past 10 years, suggesting that novel therapeutic strategies for pain treatment may be proposed. In this review, we will concentrate on recent studies associated with lncRNAs in NP. First, we will describe the alterations of lncRNA expression after spinal cord injury (SCI) and peripheral nerve injury (PNI), and then we illustrate the role of some specific lncRNAs in detail, which may offer new insights into our understanding of the etiology and pathophysiology of NP. Finally, we put special emphasis on the altered expression of lncRNAs in the diverse biological process of NP. Recent advances we summarized above in the development of NP may facilitate translation of these findings from bench to bedside in the future.

MicroRNAs in contusion spinal cord injury: pathophysiology and clinical utility

Spinal cord injury (SCI) in humans is a common central nervous system trauma. Pathophysiologically, SCI involves both primary and secondary damages. Therapeutically, targeting secondary damage including inflammation, neuropathic pain, apoptosis, demyelination, and glial reaction to promote functional benefits for SCI patients has long been considered a potential treatment strategy by neuroscientists and clinicians. As a type of small non-coding RNA, microRNAs (miRNAs) have been shown to play essential roles in the regulation of pathophysiologic processes of SCI and are considered to be an effective treatment method for SCI. Dysregulated expression of miRNAs is observed in SCI patients and animal models of SCI. Furthermore, miRNAs might also be used as biomarkers for diagnostic and prognostic purposes in SCI. Given contusion injury is the most clinically relevant type of SCI, this review mainly focuses on the role of miRNAs in the pathophysiology of contusion SCI and the putative utilization of miRNAs as diagnostic biomarkers and therapeutic targets for contusion SCI.

Precision Health: Use of Omics to Optimize Self-Management of Chronic Pain in Aging

Chronic pain has become a public health epidemic based on the number of Americans affected and its associated health care costs. Unfortunately, there are few efficacious treatments to manage chronic pain and as the population of older adults and centenarians who are at high risk for chronic pain continues to grow, the chronic pain epidemic will continue to worsen unless new therapeutic strategies are discovered. In the current era of precision medicine, there is a major emphasis being placed on the use of self-management and omics to discover new therapeutic targets and design treatment strategies that are tailored to the individual patient. This commentary discusses the current state of the science related to omics and self-management of chronic pain in older adults, the role of gerontological nurses in this process, and future directions. [Res Gerontol Nurs. 2018; 11(1):7-13.].

MicroRNA-124 and microRNA-146a both attenuate persistent neuropathic pain induced by morphine in male rats

We have recently reported that a short course of morphine, starting 10 days after sciatic chronic constriction injury (CCI), prolonged the duration of mechanical allodynia for months after morphine ceased. Maintenance of this morphine-induced persistent sensitization was dependent on microglial reactivity and Toll-like receptor 4 signaling. Given that microRNAs (miRNAs) such as miR-124 and miR-146a possess the ability to modulate such signaling, we directly compared their function in this model. We found that both miRNAs reversed established allodynia in our model of morphine-induced persistent sensitization. The efficacy of miR-124 and miR-146a were comparable, and in both cases allodynia returned within hours to days of miRNA dosing conclusion. Our findings demonstrate that miRNAs targeting Toll-like receptor signaling are effective in reversing neuropathic pain, which underscores the clinical potential of these non-coding RNAs.

RNA-binding proteins as targets for pain therapeutics

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Post-transcriptional regulation is linked to the development of pain hypersensitivity.

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A growing body of evidence indicates that RNA-binding proteins are involved in pain.

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RNA-based inhibitors are a potential new source of pain therapeutics.

RNA-protein interactions permeate biology. Transcription, translation, processing, and mRNA decay all hinge on widespread use of regulatory information decoded by RNA-binding proteins. The final committed step of protein synthesis, translation, is intimately linked to nociceptor excitability. Understanding the factors that control translation is essential as nociceptor plasticity is a hallmark of persistent pain. Here, we review the growing body of evidence for widespread involvement of RNA-binding proteins in pain. Many of the relevant factors have been implicated in post-transcriptional and translational mechanisms of mRNA control. We propose that recent advances in the development of RNA-based therapeutics provide a potential means to exploit our current understanding of liaisons between RNAs and proteins for therapeutic purposes.

Role of MicroRNA-143 in Nerve Injury-Induced Upregulation of Dnmt3a Expression in Primary Sensory Neurons

Peripheral nerve injury increased the expression of the DNA methyltransferase 3A (Dnmt3a) mRNA and its encoding Dnmt3a protein in injured dorsal root ganglia (DRG). This increase is considered as an endogenous instigator in neuropathic pain genesis through epigenetic silencing of pain-associated genes (such as Oprm1) in injured DRG. However, how DRG DNMT3a is increased following peripheral nerve injury is still elusive. We reported here that peripheral nerve injury caused by the fifth spinal nerve ligation (SNL) downregulated microRNA (miR)-143 expression in injured DRG. This downregulation was required for SNL-induced DRG Dnmt3a increase as rescuing miR-143 downregulation through microinjection of miR-143 mimics into injured DRG blocked the SNL-induced increase in Dnmt3a and restored the SNL-induced decreases in Oprm1 mRNA and its encoding mu opioid receptor (MOR) in injured DRG, impaired spinal cord central sensitization and neuropathic pain, and improved morphine analgesic effects following SNL. Mimicking SNL-induced DRG miR-143 downregulation through DRG microinjection of miR143 inhibitors in naive rats increased the expression of Dnmt3a and reduced the expression of Oprm1 mRNA and MOR in injected DRG and produced neuropathic pain-like symptoms. These findings suggest that miR-143 is a negative regulator in Dnmt3a expression in the DRG under neuropathic pain conditions and may be a potential target for therapeutic management of neuropathic pain.

MicroRNA-142-3p relieves neuropathic pain by targeting high mobility group box 1

MicroRNA (miRNA) are emerging as critical regulators of neuropathic pain development. Neuroinflammation contributes to the development of neuropathic pain. miR‑142‑3p has been characterized as an inflammation‑related miRNA in various pathological processes. However, little is known about the role of miR‑142‑3p in neuroinflammation and neuropathic pain. The present study aimed to investigate the function of miR‑142‑3p in neuropathic pain by creating a murine model using spinal nerve ligation (SNL). A significant reduction in miR‑142‑3p expression was observed in the dorsal root ganglion of mice with SNL (P<0.05) compared with control mice. Overexpression of miR‑142‑3p significantly inhibited neuropathic pain and neuroinflammation in mice with SNL (P<0.05). High mobility group box 1 (HMGB1) was identified as a direct target gene of miR‑142‑3p by bioinformatic analysis and dual‑luciferase reporter assays. Overexpression of miR‑142‑3p significantly reduced the mRNA and protein expression levels of HMGB1 in vitro and in vivo (P<0.05). In addition, HMGB1 mRNA expression and miR‑142‑3p expression were inversely correlated in mice with SNL. Furthermore, overexpression of HMGB1 significantly reversed the inhibitory effect of miR‑142‑3p on neuroinflammation and neuropathic pain development (P<0.05). Overall, these results suggest that miR‑142‑3p functions as a negative regulator of neuropathic pain development through the downregulation of HMGB1, indicating that miR‑142‑3p may serve as a potential therapeutic target for neuropathic pain.

MicroRNA cluster miR-17-92 regulates multiple functionally related voltage-gated potassium channels in chronic neuropathic pain

miR-17-92 is a microRNA cluster with six distinct members. Here, we show that the miR-17-92 cluster and its individual members modulate chronic neuropathic pain. All cluster members are persistently upregulated in primary sensory neurons after nerve injury. Overexpression of miR-18a, miR-19a, miR-19b and miR-92a cluster members elicits mechanical allodynia in rats, while their blockade alleviates mechanical allodynia in a rat model of neuropathic pain. Plausible targets for the miR-17-92 cluster include genes encoding numerous voltage-gated potassium channels and their modulatory subunits. Single-cell analysis reveals extensive co-expression of miR-17-92 cluster and its predicted targets in primary sensory neurons. miR-17-92 downregulates the expression of potassium channels, and reduced outward potassium currents, in particular A-type currents. Combined application of potassium channel modulators synergistically alleviates mechanical allodynia induced by nerve injury or miR-17-92 overexpression. miR-17-92 cluster appears to cooperatively regulate the function of multiple voltage-gated potassium channel subunits, perpetuating mechanical allodynia.

Dysregulation of voltage gated potassium channels is a feature of neuropathic pain. Here in a rat model the authors identify the microRNA cluster miR-17-92 as a regulator of voltage gated potassium channels in the dorsal root ganglion neurons.

MicroRNA-93 alleviates neuropathic pain through targeting signal transducer and activator of transcription 3

Emerging evidence suggests that microRNAs (miRNAs) play a critical role in the pathogenesis of neuropathic pain. However, the exact role of miRNAs in regulating neuropathic pain remains largely unknown. In this study, we aimed to investigate the potential role of miR-93 in a rat model of neuropathic pain induced by chronic constriction sciatic nerve injury (CCI). We found a significant decrease of miR-93 in the spinal cord of CCI rats compared with sham rats. Overexpression of miR-93 significantly alleviated neuropathic pain development and reduced inflammatory cytokine expression, including interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and IL-6 in CCI rats. By bioinformatic analysis and dual-luciferase reporter assay, we found that miR-93 directly targeted the 3'-untranslated region (UTR) of signal transducer and activator of transcription 3 (STAT3), an important regulator of inflammation. Overexpression of miR-93 markedly suppressed the expression of STAT3 in vitro and in vivo. Furthermore, overexpression of STAT3 significantly reversed the miR-93 overexpression-induced suppressive effects on neuropathic pain development and neuroinflammation. Taken together, our study suggests that miR-93 inhibits neuropathic pain development of CCI rats possibly through inhibiting STAT3-mediated neuroinflammation. Our findings indicate that miR-93 may serve as a novel therapeutic target for neuropathic pain intervention.

miR-15b mediates oxaliplatin-induced chronic neuropathic pain through BACE1 down-regulation

BACKGROUND AND PURPOSE

Although oxaliplatin is an effective anti-cancer platinum compound, it can cause painful chronic neuropathy, and its molecular mechanisms are poorly understood. MicroRNAs (miRNAs) are small non-coding RNAs that negatively regulate gene expression in a sequence-specific manner. Although miRNAs have been increasingly recognized as important modulators in a variety of pain conditions, their involvement in chemotherapy-induced neuropathic pain is unknown.

EXPERIMENTAL APPROACH

Oxaliplatin-induced chronic neuropathic pain was induced in rats by i.p. injections of oxaliplatin (2 mg·kg^-1 ) for five consecutive days. The expression levels of miR-15b and β-site amyloid precursor protein-cleaving enzyme 1 (BACE1 also known as β-secretase 1) were examined in the dorsal root ganglion (DRG). To examine the function of miR-15b, an adeno-associated viral vector encoding miR-15b was injected into the DRG in vivo.

KEY RESULTS

Among the miRNAs examined in the DRG in the late phase of oxaliplatin-induced neuropathic pain, miR-15b was most robustly increased. Our in vitro assay results determined that BACE1 was a target of miR-15b. BACE1 and miR-15b were co-expressed in putative myelinated and unmyelinated DRG neurons. Overexpression of miR-15b in DRG neurons caused mechanical allodynia in association with reduced expression of BACE1. Consistent with these results, a BACE1 inhibitor dose-dependently induced significant mechanical allodynia.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that miR-15b contributes to oxaliplatin-induced chronic neuropathic pain at least in part through the down-regulation of BACE1.

Recent advances in understanding neuropathic pain: glia, sex differences, and epigenetics

Neuropathic pain results from diseases or trauma affecting the nervous system. This pain can be devastating and is poorly controlled. The pathophysiology is complex, and it is essential to understand the underlying mechanisms in order to identify the relevant targets for therapeutic intervention. In this article, we focus on the recent research investigating neuro-immune communication and epigenetic processes, which gain particular attention in the context of neuropathic pain. Specifically, we analyze the role of glial cells, including microglia, astrocytes, and oligodendrocytes, in the modulation of the central nervous system inflammation triggered by neuropathy. Considering epigenetics, we address DNA methylation, histone modifications, and the non-coding RNAs in the regulation of ion channels, G-protein-coupled receptors, and transmitters following neuronal damage. The goal was not only to highlight the emerging concepts but also to discuss controversies, methodological complications, and intriguing opinions.

The Role of microRNA Markers in the Diagnosis, Treatment, and Outcome Prediction of Spinal Cord Injury

Spinal cord injury (SCI) is a devastating condition that affects many people worldwide. Treatment focuses on controlling secondary injury cascade and improving regeneration. It has recently been suggested that both the secondary injury cascade and the regenerative process are heavily regulated by microRNAs (miRNAs). The measurement of specific biomarkers could improve our understanding of the disease processes, and thereby provide clinicians with the opportunity to guide treatment and predict clinical outcomes after SCI. A variety of miRNAs exhibit important roles in processes of inflammation, cell death, and regeneration. These miRNAs can be used as diagnostic tools for predicting outcome after SCI. In addition, miRNAs can be used in the treatment of SCI and its symptoms. Significant laboratory and clinical evidence exist to show that miRNAs could be used as robust diagnostic and therapeutic tools for the treatment of patients with SCI. Further clinical studies are warranted to clarify the importance of each subtype of miRNA in SCI management.

Pain in an era of armed conflicts: Prevention and treatment for warfighters and civilian casualties

Chronic pain is a common squealae of military- and terror-related injuries. While its pathophysiology has not yet been fully elucidated, it may be potentially related to premorbid neuropsychobiological status, as well as to the type of injury and to the neural alterations that it may evoke. Accordingly, optimized approaches for wounded individuals should integrate primary, secondary and tertiary prevention in the form of thorough evaluation of risk factors along with specific interventions to contravene and mitigate the ensuing chronicity. Thus, Premorbid Events phase may encompass assessments of psychological and neurobiological vulnerability factors in conjunction with fostering preparedness and resilience in both military and civilian populations at risk. Injuries per se phase calls for immediate treatment of acute pain in the field by pharmacological agents that spare and even enhance coping and adaptive capabilities. The key objective of the Post Injury Events is to prevent and/or reverse maladaptive peripheral- and central neural system's processes that mediate transformation of acute to chronic pain and to incorporate timely interventions for concomitant mental health problems including post-traumatic stress disorder and addiction We suggest that the proposed continuum of care may avert more disability and suffering than the currently employed less integrated strategies. While the requirements of the armed forces present a pressing need for this integrated continuum and a framework in which it can be most readily implemented, this approach may be also instrumental for the care of civilian casualties.

MicroRNA-Based Biomarkers in Pain

Biomarkers are measurable characteristics reflective of the physiological or diseased state and a crucial feature in rendering personalized medicine more precise. Dysregulated expression of circulating microRNAs (miRNAs) in bodily fluids is being explored as noninvasive clinical biomarker for a variety of disorders including chronic pain. High-precision qPCR-based signal amplification of these miRNAs enables the detection of small changes making them ideal biomarker candidates. Presence of circulating miRNAs in exosomes, small vesicles that mediate intercellular communication, opens up novel avenues for target intervention and biomarker discovery. miRNA signatures specific to different pain conditions, and their reversal on treatment in patients and animal models can be beneficial in patient stratification, prognosis, and in bridging preclinical and clinical results. Identification of multiple miRNAs as opposed to reliance on one specific molecule as a biomarker could improve treatment efficacies in an extremely heterogeneous pain patient population. Additionally, owing to the stability of miRNAs, retrospective studies could be performed using banked samples from completed clinical trials. Irrespective of the phase and outcome, these studies can provide insights on molecular underpinnings influencing treatment outcome, or specific therapeutic intervention. Identification of miRNAs altered in chronic pain states will have a significant impact on the identification of right leads, targets, doses, and patients. Effective implementation of miRNA-based biomarkers would provide treatment guidance for clinicians, better clinical trial designs for pharmaceutical companies, all leading to individualized care and better treatment outcome for chronic pain patients.

Downregulation of microRNA-218 relieves neuropathic pain by regulating suppressor of cytokine signaling 3

Neuropathic pain is an incapacitating disease that affects a large number of people worldwide, but effective therapies have not yet been established. microRNAs (miRs) are short non-coding RNAs that participate in several biological processes and states, including neuropathic pain. Nevertheless, the precise role of miRs in regulating neuropathic pain remains largely unknown. In the present study, we investigated the role of miR-218 in neuropathic pain using a rat model of chronic constriction injury (CCI). miR-218 expression was induced and studied in the spinal cord and microglial cells of rats with CCI. We noted that downregulation of miR-218 by a specific miR-218 inhibitor significantly attenuated mechanical allodynia, thermal hyperalgesia, and proinflammatory cytokine release in CCI rats. A dual-luciferase reporter assay, RT-qPCR, and western blot analysis results demonstrated that miR-218 directly targeted the 3'-UTR of the suppressor of cytokine signaling 3 (SOCS3) and regulated mRNA and protein expression of SOCS3. Treatment with miR-218 inhibitors inactivated Janus kinase/signal transducer and activator of transcription 3 (STAT3) signaling in rats with CCI in vivo. Moreover, miR-218 inhibitors significantly inhibited the activation of microglial cell STAT3 signaling and downstream proinflammatory genes in microglial cells. These results suggest that miR-218 regulated neuropathic pain and neuroinflammation by regulating SOCS3 expression, which negatively mediated STAT3 signaling. Thus, we propose that silencing of miR-218 may be a promising and novel treatment for neuropathic pain.

MicroRNA circulating in the early aftermath of motor vehicle collision predict persistent pain development and suggest a role for microRNA in sex-specific pain differences

BACKGROUND

Molecular mediators influencing the transition from acute to persistent musculoskeletal pain following common stress exposures such as motor vehicle collision (MVC) remain poorly understood. In this exploratory, proof of concept study, we compared circulating microRNA (miRNA) expression profiles in the early aftermath of MVC among individuals who did and did not subsequently develop persistent pain. Blood RNA samples were obtained from African American individuals (n = 53) who presented to the emergency department after MVC and were discharged to home after evaluation. The presence or absence of severe pain in the axial region, the most common and morbid region in which post-MVC pain occurs, was assessed 6 weeks following MVC via standardized questionnaire. miRNA expression was determined using miRNA-sequencing; nonparametric analyses were used to compare miRNA expression levels among individuals with and without persistent pain.

RESULTS

Thirty-two mature miRNA were differentially expressed (p < 0.05) in those with and without severe axial pain at 6 weeks. miR-135a-5p, a regulator of the serotonin receptor that is known to be stress-responsive, differed most significantly between groups (p = 3 × 10(-4)). This miRNA, and miR-3613-3p (p = 0.001) survived correction for multiple testing (FDR = 0.15) in this small sample. Interestingly, differentially expressed miRNA were enriched for X chromosome location. In secondary analyses, the eight X chromosome miRNA were (a) more significantly associated with axial pain in women than men, (b) expressed more highly in the peripheral blood of women than men, and (c) predicted in pathway analyses (DIANA miRPath v 2.0) to regulate neuronal and neuroendocrine pathways previously implicated in various pain pathologies.

CONCLUSIONS

These results show that circulating miRNA predict persistent severe axial pain after MVC and suggest that they may be involved in the pathogenesis of post-traumatic musculoskeletal pain. However, further studies are needed to determine if these miRNA play a direct causal role.

Expression changes of microRNA-1 and its targets Connexin 43 and brain-derived neurotrophic factor in the peripheral nervous system of chronic neuropathic rats

Background

MicroRNAs (miRNAs) are involved in the neuroplastic changes which induce and maintain neuropathic pain. However, it is unknown whether nerve injury leads to altered miRNA expression and modulation of pain relevant target gene expression within peripheral nerves. In the present study, expression profiles of miR-1 and the pain-relevant targets, brain derived neurotrophic factor (BDNF) and Connexin 43 (Cx43), were studied in peripheral neuropathic pain, which was induced by chronic constriction injury (CCI) of the sciatic nerve in rats. The expression of miR-1 was investigated in the sciatic nerve, dorsal root ganglion (DRG) and the ipsilateral spinal cord by qPCR. Changes of BDNF and Cx43 expression patterns were studied using qPCR, Western blot analysis, ELISA and immunohistochemistry.

Results

In sciatic nerves of naïve rats, expression levels of miR-1 were more than twice as high as in DRG and spinal cord. In neuropathic rats, CCI lead to a time-dependent downregulation of miR-1 in the sciatic nerve but not in DRG and spinal cord. Likewise, protein expression of the miR-1 targets BDNF and Cx43 was upregulated in the sciatic nerve and DRG after CCI. Immunohistochemical staining revealed an endoneural abundancy of Cx43 in injured sciatic nerves which was absent after Sham operation.

Conclusions

This study demonstrates that CCI leads to a regulation of miRNAs (miR-1) in the peripheral nervous system. This regulation is associated with alterations in the expression and localization of the miR-1 dependent pain-relevant proteins BDNF and Cx43. Further studies will have to explore the function of miRNAs in the context of neuropathic pain in the peripheral nervous system.

MicroRNA biology and pain

Endogenously expressed small noncoding microRNAs (miRNAs) play an important role in posttranscriptionally regulating gene expression by binding to mRNAs with complementary sequences. miRNA-mRNA interactions allow for cellular flexibility to fine-tune gene expression by controlling translation in response to a multitude of signaling events. Disease states or perturbations in cellular homeostasis can lead to aberrant miRNA expression. The discovery of stable miRNAs in circulation generated enormous interest in exploring their utility as potential noninvasive biomarkers. Additionally, selectively inhibiting or supplementing an miRNA contributing to pathogenesis is being pursued as a therapeutic strategy for a variety of disorders. Studies from rodent models of pain and patients have now implicated a role for miRNAs in mediating various aspects of pain processing. These noncoding RNAs can provide mechanistic insights into the pathways modulated and could serve as therapeutic targets. Here, we discuss the challenges associated with miRNA research and the promises ahead in this vastly unexplored avenue in pain biology.

The pain of pain: challenges of animal behavior models

Berend Olivier has had a long-standing interest in the utility of animal models for a wide variety of therapeutic indications. His work has spanned multiple types of models, blending ethological, or species typical and naturalistic behaviors, along with methodologies based on learned behavior. He has consistently done so, from an analytical as well as predictive perspective, and has made multiple contributions while working in both the pharmaceutical industry and within an academic institution. Although focused primarily on psychiatric disorders, Berend has conducted research in the area of pain in humans and in animals, demonstrating an expansive appreciation for the breadth, scope and significance of the science and applications of the discipline of pharmacology to these diverse areas. This review focuses on the use of animal models in pain research from the perspective of the long-standing deficiencies in the development of therapeutics in this area and from a preclinical perspective where the translational weaknesses have been quite problematic. The challenges confronting animal models of pain, however, are not unique to this area of research, as they cut across several therapeutic areas. Despite the deficiencies, failures and concerns, existing animal models of pain continue to be of widespread use and are essential to progress in pain research as well as in other areas. Although not focusing on specific animal models of pain, this paper seeks to examine general issues facing the use of these models. It does so by exploring alternative approaches which capture recent developments, which build upon principles and concepts we have learned from Berend's contributions, and which provide the prospect of helping to address the absence of novel therapeutics in this area.

microRNA and Pain

Pain is an important protective system that alerts organisms to actual or possible tissue damage. However, a variety of pathologies can lead to chronic pain that is no longer beneficial. Lesions or diseases of the somatosensory nervous system cause intractable neuropathic pain that occasionally lasts even after the original pathology subsides. Chronic inflammatory diseases like arthritis are also associated with severe pain. Because conventional analgesics such as non-steroidal anti-inflammatory drugs and opioids have limited efficacy and/or severe adverse events associated with long-term use, chronic pain remains a major problem in clinical practice. Recently, causal roles of microRNAs in chronic pain and their therapeutic potential have been emerging. microRNA expressions are altered not only at the primary origin of pain, but also along the somatosensory pathways. Notably, microRNA expressions are differentially affected depending on the causes of chronic pain. This chapter summarizes current insights into the roles of microRNAs in pain based on the underlying pathologies.

Suppression of microRNA-155 attenuates neuropathic pain by regulating SOCS1 signalling pathway

Chronic neuropathic pain is an unfavourable pathological pain characterised by allodynia and hyperalgesia which has brought considerable trouble to people's physical and mental health, but effective therapeutics are still lacking. MicroRNAs (miRNAs) have been widely studied in the development of neuropathic pain and neuronal inflammation. Among various miRNAs, miR-155 has been widely studied. It is intensively involved in regulating inflammation-associated diseases. However, the role of miR-155 in regulating neuropathic pain development is poorly understood. In the present study, we aimed to investigate whether miR-155 is associated with neuropathic pain and delineate the underlying mechanism. Using a neuropathic pain model of chronic constriction injury (CCI), miR-155 expression levels were markedly increased in the spinal cord. Inhibition of miR-155 significantly attenuated mechanical allodynia, thermal hyperalgesia and proinflammatory cytokine expression. We also demonstrated that miR-155 directly bound with the 3'-untranslated region of the suppressor of cytokine signalling 1 (SOCS1). The expression of SOCS1 significantly decreased in the CCI rat model, but this effect could be reversed by miR-155 inhibition. Furthermore, knockdown of SOCS1 abrogated the inhibitory effects of miR-155 inhibition on neuropathic development and neuronal inflammation. Finally, we demonstrated that inhibition of miR-155 resulted in the suppression of nuclear factor-κB and p38 mitogen-activated protein kinase activation by mediating SOCS1. Our data demonstrate the critical role of miR-155 in regulating neuropathic pain through SOCS1, and suggest that miR-155 may be an important and potential target in preventing neuropathic pain development.

An approach to identify microRNAs involved in neuropathic pain following a peripheral nerve injury

Peripheral nerve injury alters the expression of hundreds of proteins in dorsal root ganglia (DRG). Targeting some of these proteins has led to successful treatments for acute pain, but not for sustained post-operative neuropathic pain. The latter may require targeting multiple proteins. Since a single microRNA (miR) can affect the expression of multiple proteins, here, we describe an approach to identify chronic neuropathic pain-relevant miRs. We used two variants of the spared nerve injury (SNI): Sural-SNI and Tibial-SNI and found distinct pain phenotypes between the two. Both models induced strong mechanical allodynia, but only Sural-SNI rats maintained strong mechanical and cold allodynia, as previously reported. In contrast, we found that Tibial-SNI rats recovered from mechanical allodynia and never developed cold allodynia. Since both models involve nerve injury, we increased the probability of identifying differentially regulated miRs that correlated with the quality and magnitude of neuropathic pain and decreased the probability of detecting miRs that are solely involved in neuronal regeneration. We found seven such miRs in L3-L5 DRG. The expression of these miRs increased in Tibial-SNI. These miRs displayed a lower level of expression in Sural-SNI, with four having levels lower than those in sham animals. Bioinformatic analysis of how these miRs could affect the expression of some ion channels supports the view that, following a peripheral nerve injury, the increase of the seven miRs may contribute to the recovery from neuropathic pain while the decrease of four of them may contribute to the development of chronic neuropathic pain. The approach used resulted in the identification of a small number of potentially neuropathic pain relevant miRs. Additional studies are required to investigate whether manipulating the expression of the identified miRs in primary sensory neurons can prevent or ameliorate chronic neuropathic pain following peripheral nerve injuries.