IL-6 induced upregulation of T-type Ca2+ currents and sensitization of DRG nociceptors is attenuated by MNK inhibition

Research progress on pathogenesis and treatment of febrile seizures

Febrile seizures (FSs) are the most common pediatric neurological disorder, affecting approximately 5 % of children aged 6 months to 5 years. While most FSs are self-limiting and benign, about 20-30 % present as complex FSs (CFSs), which pose a risk of acute brain injury and the development of temporal lobe epilepsy. Various factors, including age, geographical distribution, and type of infection influence the occurrence of FS. Infection is the primary external trigger for FS, while the underlying intrinsic factors are linked to the immature and incomplete myelination of the brain during specific developmental stages. Although the precise pathogenesis of FS is not yet fully understood, it is likely caused by the interaction of immature brain development, fever, neuroinflammation, and genetic susceptibility. This review discussed the pathogenesis of febrile seizures, focusing on factors such as age, fever, neuroinflammation, genetics, and intestinal microbiota, and summarized existing therapeutic approaches. Our review may facilitate the identification of new targets for mechanistic studies and clinical treatment of febrile seizures.

Profiling Human iPSC-Derived Sensory Neurons for Analgesic Drug Screening Using a Multi-Electrode Array

Chronic pain is a major global health issue, yet effective treatments are limited by poor translation from preclinical studies to humans. To address this, we developed a high-content screening (HCS) platform for analgesic discovery using hiPSC-derived nociceptors. These cells were cultured on multi-well micro-electrode arrays to monitor activity, achieving nearly 100% active electrodes by week two, maintaining stable activity for at least two weeks. After maturation (28 days), we exposed the nociceptors to various drugs, assessing their effects on neuronal activity, with excellent assay performance (Z' values >0.5). Pharmacological tests showed responses to analgesic targets, including ion channels (Nav, Cav, Kv, TRPV1), neurotransmitter receptors (AMPAR, GABA-R), and kinase inhibitors (tyrosine, JAK1/2). Transcriptomic analysis confirmed the presence of these drug targets, although expression levels varied compared to primary human dorsal root ganglion cells. This HCS platform facilitates the rapid discovery of novel analgesics, reducing the risk of preclinical-to-human translation failure.

Motivation

Chronic pain affects approximately 1.5 billion people worldwide, yet effective treatments remain elusive. A significant barrier to progress in analgesic drug discovery is the limited translation of preclinical findings to human clinical outcomes. Traditional rodent models, although widely used, often fail to accurately predict human responses, while human primary tissues are limited by scarcity, technical difficulties, and ethical concerns. Recent advancements have identified human induced pluripotent stem cell (hiPSC)-derived nociceptors as promising alternatives; however, current differentiation protocols produce cells with inconsistent and physiologically questionable phenotypes.To address these challenges, our study introduces a novel high-content screening (HCS) platform using hiPSC-derived nociceptors cultured on multi-well micro-electrode arrays (MEAs). The "Anatomic" protocol, used to generate these nociceptors, ensures cells with transcriptomic profiles closely matching human primary sensory neurons. Our platform achieves nearly 100% active electrode yield within two weeks and demonstrates sustained, stable activity over time. Additionally, robust Z' factor analysis (exceeding 0.5) confirms the platform's reliability, while pharmacological validation establishes the functional expression of critical analgesic targets. This innovative approach improves both the efficiency and clinical relevance of analgesic drug screening, potentially bridging the translational gap between preclinical studies and human clinical trials, and offering new hope for effective pain management.

Peripheral CaV2.2 Channels in the Skin Regulate Prolonged Heat Hypersensitivity during Neuroinflammation

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuroimmune cell signaling, but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal (i.d.) capsaicin via IL-1ɑ cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the i.d. capsaicin animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by i.d. complete Freund adjuvant (CFA). i.d. CFA, a model of chronic neuroinflammation, involves ongoing cytokine signaling for days leading to pronounced edema and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in the skin was required for the full development and week-long time course of heat hypersensitivity induced by i.d. CFA, but paw edema and mechanical hypersensitivity were independent of CaV2.2 channel activity. CFA induced increases in several cytokines in hindpaw fluid including IL-6 which was also dependent on CaV2.2 channel activity. Using IL-6-specific neutralizing antibodies in vivo, we show that IL-6 contributes to heat hypersensitivity and that neutralizing both IL-1ɑ and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in the skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in the skin.

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

ephrin-B2 promotes nociceptive plasticity and hyperalgesic priming through EphB2-MNK-eIF4E signaling in both mice and humans

Ephrin-B-EphB signaling can promote pain through ligand-receptor interactions between peripheral cells, like immune cells expressing ephrin-Bs, and EphB receptors expressed by DRG neurons. Previous studies have shown increased ephrin-B2 expression in peripheral tissues like synovium of rheumatoid and osteoarthritis patients, indicating the clinical significance of this signaling. The primary goal of this study was to understand how ephrin-B2 acts on mouse and human DRG neurons, which express EphB receptors, to promote pain and nociceptor plasticity. We hypothesized that ephrin-B2 would promote nociceptor plasticity and hyperalgesic priming through MNK-eIF4E signaling, a critical mechanism for nociceptive plasticity induced by growth factors, cytokines and nerve injury. Both male and female mice developed dose-dependent mechanical hypersensitivity in response to ephrin-B2, and both sexes showed hyperalgesic priming when challenged with PGE2 injection either to the paw or the cranial dura. Acute nociceptive behaviors and hyperalgesic priming were blocked in mice lacking MNK1 (Mknk1 knockout mice) and by eFT508, a specific MNK inhibitor. Sensory neuron-specific knockout of EphB2 using Pirt-Cre demonstrated that ephrin-B2 actions require this receptor. In Ca2+-imaging experiments on cultured DRG neurons, ephrin-B2 treatment enhanced Ca2+ transients in response to PGE2 and these effects were absent in DRG neurons from MNK1-/- and EphB2-PirtCre mice. In experiments on human DRG neurons, ephrin-B2 increased eIF4E phosphorylation and enhanced Ca2+ responses to PGE2 treatment, both blocked by eFT508. We conclude that ephrin-B2 acts directly on mouse and human sensory neurons to induce nociceptor plasticity via MNK-eIF4E signaling, offering new insight into how ephrin-B signaling promotes pain.

Peripheral CaV2.2 channels in skin regulate prolonged heat hypersensitivity during neuroinflammation

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuro-immune cell signaling but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal capsaicin via IL-1α cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the same animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by the intradermal (id) Complete Freund's Adjuvant (CFA) model of chronic neuroinflammation that involves ongoing cytokine signaling for days. Ongoing CFA-induced cytokine signaling cascades in skin lead to pronounced edema, and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in skin is required for the full development and week-long time course of heat hypersensitivity induced by id CFA. CaV2.2 channels, by contrast, are not involved in paw edema and mechanical hypersensitivity. CFA induced increases in cytokines in hind paws including IL-6 which was dependent on CaV2.2 channel activity. Using IL-6 specific neutralizing antibodies, we show that IL-6 contributes to heat hypersensitivity and, neutralizing both IL-1α and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in skin.

Interleukin-6 induces nascent protein synthesis in human dorsal root ganglion nociceptors primarily via MNK-eIF4E signaling

Plasticity of dorsal root ganglion (DRG) nociceptors in the peripheral nervous system requires new protein synthesis. This plasticity is believed to be responsible for the physiological changes seen in DRG nociceptors in animal models of chronic pain. Experiments in human DRG (hDRG) neurons also support this hypothesis, but a direct observation of nascent protein synthesis in response to a pain promoting substance, like interleukin-6 (IL-6), has not been measured in these neurons. To fill this gap in knowledge, we used acutely prepared human DRG explants from organ donors. These explants provide a physiologically relevant microenvironment, closer to in vivo conditions, allowing for the examination of functional alterations in DRG neurons reflective of human neuropathophysiology. Using this newly developed assay, we demonstrate upregulation of the target of the MNK1/2 kinases, phosphorylated eIF4E (p-eIF4E), and nascently synthesized proteins in a substantial subset of hDRG neurons following exposure to IL-6. To pinpoint the specific molecular mechanisms driving this IL-6-driven increase in nascent proteins, we used the specific MNK1/2 inhibitor eFT508. Treatment with eFT508 resulted in the inhibition of IL-6-induced increases in p-eIF4E and nascent proteins. Additionally, using TRPV1 as a marker for nociceptors, we found that these effects occurred in a large number of human nociceptors. Our findings provide clear evidence that IL-6 drives nascent protein synthesis in human TRPV1+ nociceptors primarily via MNK1/2-eIF4E signaling. The work links animal findings to human nociception, creates a framework for additional hDRG signaling experiments, and substantiates the continued development of MNK inhibitors for pain.

A mouse model of chronic primary pain that integrates clinically relevant genetic vulnerability, stress, and minor injury

Chronic primary pain conditions (CPPCs) affect over 100 million Americans, predominantly women. They remain ineffectively treated, in large part because of a lack of valid animal models with translational relevance. Here, we characterized a CPPC mouse model that integrated clinically relevant genetic (catechol-O-methyltransferase; COMT knockdown) and environmental (stress and injury) factors. Compared with wild-type mice, Comt+/- mice undergoing repeated swim stress and molar extraction surgery intervention exhibited pronounced multisite body pain and depressive-like behavior lasting >3 months. Comt+/- mice undergoing the intervention also exhibited enhanced activity of primary afferent nociceptors innervating hindpaw and low back sites and increased plasma concentrations of norepinephrine and pro-inflammatory cytokines interleukin-6 (IL-6) and IL-17A. The pain and depressive-like behavior were of greater magnitude and longer duration (≥12 months) in females versus males. Furthermore, increases in anxiety-like behavior and IL-6 were female-specific. The effect of COMT genotype × stress interactions on pain, IL-6, and IL-17A was validated in a cohort of 549 patients with CPPCs, demonstrating clinical relevance. Last, we assessed the predictive validity of the model for analgesic screening and found that it successfully predicted the lack of efficacy of minocycline and the CB2 agonist GW842166X, which were effective in spared nerve injury and complete Freund's adjuvant models, respectively, but failed in clinical trials. Yet, pain in the CPPC model was alleviated by the beta-3 adrenergic antagonist SR59230A. Thus, the CPPC mouse model reliably recapitulates clinically and biologically relevant features of CPPCs and may be implemented to test underlying mechanisms and find new therapeutics.

Readiness of nociceptor cell bodies to generate spontaneous activity results from background activity of diverse ion channels and high input resistance

ABSTRACT

Nociceptor cell bodies generate "spontaneous" discharge that can promote ongoing pain in persistent pain conditions. Little is known about the underlying mechanisms. Recordings from nociceptor cell bodies (somata) dissociated from rodent and human dorsal root ganglia have shown that previous pain in vivo is associated with low-frequency discharge controlled by irregular depolarizing spontaneous fluctuations of membrane potential (DSFs), likely produced by transient inward currents across the somal input resistance. Using mouse nociceptors, we show that DSFs are associated with high somal input resistance over a wide range of membrane potentials, including depolarized levels where DSFs approach action potential (AP) threshold. Input resistance and both the amplitude and frequency of DSFs were increased in neurons exhibiting spontaneous activity. Ion substitution experiments indicated that the depolarizing phase of DSFs is generated by spontaneous opening of channels permeable to Na + or Ca 2+ and that Ca 2+ -permeable channels are especially important for larger DSFs. Partial reduction of the amplitude or frequency of DSFs by perfusion of pharmacological inhibitors indicated small but significant contributions from Nav1.7, Nav1.8, TRPV1, TRPA1, TRPM4, and N-type Ca 2+ channels. Less specific blockers suggested a contribution from NALCN channels, and global knockout suggested a role for Nav1.9. The combination of high somal input resistance plus background activity of diverse ion channels permeable to Na + or Ca 2+ produces DSFs that are poised to reach AP threshold if resting membrane potential depolarizes, AP threshold decreases, or DSFs become enhanced-all of which can occur under painful neuropathic and inflammatory conditions.

Acute ketamine induces neuronal hyperexcitability and deficits in prepulse inhibition by upregulating IL-6

Acute ketamine administration results in psychotic symptoms similar to those observed in schizophrenia and is regarded as a pharmacological model of schizophrenia. Accumulating evidence suggests that patients with schizophrenia show increased IL-6 levels in the blood and cerebrospinal fluid and that IL-6 levels are associated with the severity of psychotic symptoms. In the present study, we found that a single ketamine exposure led to increased expression of IL-6 and IL-6Rα, decreased dendritic spine density, increased expression and currents of T-type calcium channels, and increased neuron excitability in the hippocampal CA1 area 12 h after exposure. Acute ketamine administration also led to impaired prepulse inhibition (PPI) 12 h after administration. Additionally, we found that the expression of signaling molecules IKKα/β, NF-κB, JAK2, and STAT3 was upregulated 12 h after a single ketamine injection. The decreases in dendritic spine density, the increases in calcium currents and neuron excitability, and the impairments in PPI were ameliorated by blocking IL-6 or IL-6Rα. Our findings show that blocking IL-6 or its receptor may protect hippocampal neurons from hyperexcitability, thereby ameliorating ketamine-induced psychotic effects. Our study provides additional evidence that targeting IL-6 and its receptor is a potential strategy for treating psychotic symptoms in acute ketamine-induced psychosis and schizophrenia.

Discovery of D25, a Potent and Selective MNK Inhibitor for Sepsis-Associated Acute Spleen Injury

Mitogen-activated protein kinase-interacting protein kinases (MNKs) and phosphorylate eukaryotic initiation factor 4E (p-eIF4E) play a critical role in regulating mRNA translation and protein synthesis associated with the development of cancer, metabolism, and inflammation. This study undertakes the modification of a 4-(3-(piperidin-4-yl)-1H-pyrazol-5-yl)pyridine structure, leading to the discovery of 4-(3-(piperidin-4-yl)-1H-pyrazol-5-yl)-1H-pyrrolo[2,3-b]pyridine (D25) as a potent and selective MNK inhibitor. D25 demonstrated inhibitory activity, with IC50 values of 120.6 nM for MNK1 and 134.7 nM for MNK2, showing exceptional selectivity. D25 inhibited the expression of pro-inflammation cytokines in RAW264.7 cells, such as inducible NO synthase, cyclooxygenase-2, and interleukin-6 (IL-6). In the lipopolysaccharide-induced sepsis mouse model, D25 significantly reduced p-eIF4E in spleen tissue and decreased the expression of tumor necrosis factor α, interleukin-1β, and IL-6, and it also reduced the production of reactive oxygen species, resulting in improved organ injury caused by inflammation. This suggests that D25 may provide a potential treatment for sepsis and sepsis-associated acute spleen injury.

Aging and injury drive neuronal senescence in the dorsal root ganglia

Aging negatively impacts central nervous system function; however, the cellular impact of aging in the peripheral nervous system remains poorly understood. Aged individuals are more likely to experience increased pain and slower recovery after trauma. Such injury can damage vulnerable peripheral axons of dorsal root ganglion (DRG) neurons resulting in somatosensory dysfunction. One cellular mechanism common to both aging and injury is cellular senescence, a complex cell state that can contribute to the aged pro-inflammatory environment. We uncovered, for the first time, DRG neuron senescence in the context of aging and pain-inducing peripheral nerve injury in young and aged mice. Aged DRG neurons displayed multiple markers of senescence (SA-β-gal, p21, p16, IL6) when compared to young DRG neurons. Peripheral nerve injury triggered a further accumulation of senescent DRG neurons over time post-injury in young and aged DRG. These senescent neurons were dynamic and heterogeneous in their expression of senescence markers, p16, p21, and senescence-associated secretory phenotype (SASP) expression of IL6, which was influenced by age. An electrophysiological characterization of senescence marker-expressing neurons revealed high-firing and nociceptor-like phenotypes within these populations. In addition, we observed improvement in nociceptive behaviors in young and aged nerve-injured mice after treatment with a senolytic agent that eliminates senescent cells. Finally, we confirmed in human post-mortem DRG samples that neuronal senescence is present and increases with age. Overall, we describe a susceptibility of the peripheral nervous system to neuronal senescence with age or injury that may be a targetable mechanism to treat sensory dysfunction, such as chronic pain, particularly in aged populations.

Secretion of Interleukin 6 in Human Skeletal Muscle Cultures Depends on Ca2+ Signalling

The systemic effects of physical activity are mediated by the release of IL-6 and other myokines from contracting muscle. Although the release of IL-6 from muscle has been extensively studied, the information on the cellular mechanisms is fragmentary and scarce, especially regarding the role of Ca²⁺ signals. The aim of this study was to characterize the role of the main components of Ca²⁺ signals in human skeletal muscle cells during IL-6 secretion stimulated by the Ca²⁺ mobilizing agonist ATP. Primary cultures were prepared from surgical samples, fluorescence microscopy was used to evaluate the Ca²⁺ signals and the stimulated release of IL-6 into the medium was determined using ELISA. Intracellular calcium chelator Bapta, low extracellular calcium and the Ca²⁺ channels blocker La³⁺ reduced the ATP-stimulated, but not the basal secretion. Secretion was inhibited by blockers of L-type (nifedipine, verapamil), T-type (NNC55-0396) and Orai1 (Synta66) Ca²⁺ channels and by silencing Orai1 expression. The same effect was achieved with inhibitors of ryanodine receptors (ryanodine, dantrolene) and IP3 receptors (xestospongin C, 2-APB, caffeine). Inhibitors of calmodulin (calmidazolium) and calcineurin (FK506) also decreased secretion. IL-6 transcription in response to ATP was not affected by Bapta or by the T channel blocker. Our results prove that ATP-stimulated IL-6 secretion is mediated at the post-transcriptional level by Ca²⁺ signals, including the mobilization of calcium stores, the activation of store-operated Ca²⁺ entry, and the subsequent activation of voltage-operated Ca²⁺ channels and calmodulin/calcineurin pathways.

MNK1 and MNK2 Expression in the Human Dorsal Root and Trigeminal Ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5'-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG (N = 3 ganglia for both DRG and TG) for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors of males and females and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

Persistent nociceptor hyperactivity as a painful evolutionary adaptation

Chronic pain caused by injury or disease of the nervous system (neuropathic pain) has been linked to persistent electrical hyperactivity of the sensory neurons (nociceptors) specialized to detect damaging stimuli and/or inflammation. This pain and hyperactivity are considered maladaptive because both can persist long after injured tissues have healed and inflammation has resolved. While the assumption of maladaptiveness is appropriate in many diseases, accumulating evidence from diverse species, including humans, challenges the assumption that neuropathic pain and persistent nociceptor hyperactivity are always maladaptive. We review studies indicating that persistent nociceptor hyperactivity has undergone evolutionary selection in widespread, albeit selected, animal groups as a physiological response that can increase survival long after bodily injury, using both highly conserved and divergent underlying mechanisms.

The Role of IL-6 and TMEM100 in Lumbar Discogenic Pain and the Mechanism of the Glycine-Serine-Threonine Metabolic Axis: A Metabolomic and Molecular Biology Study

Background

It is well established that discogenic low back pain (DLBP) is often caused by the inflammatory response during intervertebral disc degeneration (IDD). However, it remains unclear how inflammatory mediators such as Interleukin-6 (IL-6) are involved in discogenic pain caused by degeneration and intervertebral disc (IVD) metabolism. The purpose of this study is to study the relationship between IL-6 and Transmembrane protein 100 (TMEM100), and to analyze the different metabolites and metabolic pathways in various rat intervertebral discs by metabonomics.

Methods

We established a rat model of IDD-DLBP by disc punctures and PBS infusion to examine the rat pain behaviors. Intervertebral disc tissues were harvested for molecular biology experiments to explore the relationship between IL-6 and TMEM100. High-resolution mass spectrometry (HRMS) was performed for untargeted metabolomics, and nuclear magnetic resonance spectroscopy metabolomics (MRS) for differential metabolites and metabolic pathways.

Results

The results showed a significant decrease in vonFrey test, hot plate test and acetone test (P < 0.05). The expression of IL-6 and TMEM100 in DLBP model was significantly higher than that in sham control group and IDD discs without PBS infusion group (P < 0.05). There were 15 major contributing metabolites identified in the DLBP intervertebral discs through metabolomic studies, involving 6 major metabolic pathways. The main differential metabolites included nitric oxide (NO), ammonia, and lactic acid as the metabolic endpoints; and the differential metabolic pathways included the glycine-serine-threonine (Gly-Ser-Thr), which is gradually weakened with the progression of inflammation.

Conclusion

The change of TMEM100 expression mediated by il-6 is related to the Gly-Ser-Thr metabolic axis and plays an important role in the relief of discogenic pain.

MNK1/2 contributes to periorbital hypersensitivity and hyperalgesic priming in preclinical migraine models

Migraine is thought to involve sensitization of the trigeminal nociceptive system. In preclinical pain models, activation of MNK-eIF4E signalling contributes to nociceptor sensitization and the development of persistent pain. Despite these observations, the role of MNK signalling in migraine remains unclear. Here, we investigate whether activation of MNK contributes to hypersensitivity in two rodent models of migraine. Female and male wild-type (WT) and MNK1 knock-out mice were subjected to repeated restraint stress or a dural injection of interleukin-6 (IL-6) and tested for periorbital hypersensitivity and grimacing. Upon returning to baseline thresholds, stressed mice were administered a low dose of the nitric oxide donor sodium nitroprusside and mice previously injected with IL-6 were given a second dural injection of pH 7.0 to test for hyperalgesic priming. MNK1 knock-out mice were significantly less hypersensitive than the WT following dural IL-6 and did not prime to pH 7.0 or sodium nitroprusside. Furthermore, treatment with the selective MNK inhibitor, eFT508, in WT mice prevented hypersensitivity caused by dural IL-6 or pH 7.0. Together, these results implicate MNK-eIF4E signalling in the development of pain originating from the dura and strongly suggest that targeting MNK inhibition may have significant therapeutic potential as a treatment for migraine.

MNK1 and MNK2 expression in the human dorsal root and trigeminal ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5’-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

The Role of Neuro-Immune Interactions in Chronic Pain: Implications for Clinical Practice

Chronic pain remains a public health problem and contributes to the ongoing opioid epidemic. Current pain management therapies still leave many patients with poorly controlled pain, thus new or improved treatments are desperately needed. One major challenge in pain research is the translation of preclinical findings into effective clinical practice. The local neuroimmune interface plays an important role in the initiation and maintenance of chronic pain and is therefore a promising target for novel therapeutic development. Neurons interface with immune and immunocompetent cells in many distinct microenvironments along the nociceptive circuitry. The local neuroimmune interface can modulate the activity and property of the neurons to affect peripheral and central sensitization. In this review, we highlight a specific subset of many neuroimmune interfaces. In the central nervous system, we examine the interface between neurons and microglia, astrocytes, and T lymphocytes. In the periphery, we profile the interface between neurons in the dorsal root ganglion with T lymphocytes, satellite glial cells, and macrophages. To bridge the gap between preclinical research and clinical practice, we review the preclinical studies of each neuroimmune interface, discuss current clinical treatments in pain medicine that may exert its action at the neuroimmune interface, and highlight opportunities for future clinical research efforts.

MiR-223-3p alleviates trigeminal neuropathic pain in the male mouse by targeting MKNK2 and MAPK/ERK signaling

BACKGROUND

Trigeminal neuralgia (TN) is a neuropathic pain that occurs in branches of the trigeminal nerve. MicroRNAs (miRNAs) have been considered key mediators of neuropathic pain. This study was aimed to elucidate the pathophysiological function and mechanisms of miR-223-3p in mouse models of TN.

METHODS

Infraorbital nerve chronic constriction injury (CCI-ION) was applied in male C57BL/6J mice to establish mouse models of TN. Pain responses were assessed utilizing Von Frey method. The expression of miR-223-3p, MKNK2, and MAPK/ERK pathway protein in trigeminal ganglions (TGs) of CCI-ION mice was measured using RT-qPCR and Western blotting. The concentrations of inflammatory cytokines were evaluated using Western blotting. The relationship between miR-223-3p and MKNK2 was tested by a luciferase reporter assay.

RESULTS

We found that miR-223-3p was downregulated, while MKNK2 was upregulated in TGs of CCI-ION mice. MiR-223-3p overexpression by an intracerebroventricular injection of Lv-miR-223-3p attenuated trigeminal neuropathic pain in CCI-ION mice, as well as reduced the protein levels of pro-inflammatory cytokines in TGs of CCI-ION mice. MKNK2 was verified to be targeted by miR-223-3p. Additionally, miR-223-3p overexpression decreased the phosphorylation levels of ERK1/2, JNK, and p38 protein in TGs of CCI-ION mice to inhibit MAPK/ERK signaling.

CONCLUSIONS

Overall, miR-223-3p attenuates the development of TN by targeting MKNK2 to suppress MAPK/ERK signaling.

Update on the Development of MNK Inhibitors as Therapeutic Agents

Mitogen-activated protein kinase-interacting kinases 1 and 2 (MNK1/2) represent a central class of enzymes that are activated by extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinases. MNK1 and MNK2 coordinate cellular signaling, control production of inflammatory chemokines, and regulate cell proliferation and survival. MNK1/2 are referred to as serine/threonine kinases as they phosphorylate serine or threonine residues on their substrates. Upon activation, MNK1/2 phosphorylate eukaryotic translation initiation factor 4E (eIF4E) at Ser209, which in turn initiates ribosome assembly and protein translation. Deleterious overexpression of MNK1/2 and/or eIF4E have been reported in several diseases including cancers, neurological disorders, autism, and inflammation. Recently, there have been intense efforts toward the development of potent and selective inhibitors of MNK1/2 in both academia and industry. Herein, we review the current understanding of the structural and biological aspects of MNK1/2 and provide an update of pharmacological inhibitors of MNK1/2 including candidates in clinical trials.

A Role for Translational Regulation by S6 Kinase and a Downstream Target in Inflammatory Pain

BACKGROUND AND PURPOSE

Translational controls pervade neurobiology. Nociceptors play an integral role in the detection and propagation of pain signals. Nociceptors can undergo persistent changes in their intrinsic excitability. Pharmacologic disruption of nascent protein synthesis diminishes acute and chronic forms of pain-associated behaviors. Yet, the targets of translational controls that facilitate plasticity in nociceptors are unclear.

EXPERIMENTAL APPROACH

We used ribosome profiling to probe the translational landscape in DRG neurons after treatment of the inflammatory mediators NGF and IL-6. We validated the expression dynamics of c-Fos using immunoblotting and immunohistochemistry. Given that inflammation is known to stimulate mTOR signaling, we reasoned that downstream factors (e.g., ribosomal protein S6 kinase 1, S6K1) might control c-Fos levels. We utilized small-molecule inhibitors of S6K1 (DG2) or c-Fos (T-5224) to probe their effects on nociceptor activity in vitro using multi-electrode arrays (MEAs) and pain behavior in vivo using a hyperalgesic priming model.

KEY RESULTS

We demonstrate that c-Fos is expressed in sensory neurons. Inflammatory mediators that promote pain in both humans and rodents promote c-Fos translation. We demonstrate that the mTOR effector S6K1 is essential for c-Fos biosynthesis. Inhibition of S6K1 or c-Fos with small molecules diminish mechanical and thermal hypersensitivity in response to inflammatory cues. Additionally, both inhibitors reduce evoked nociceptor activity.

CONCLUSION

Our data reveal a novel role of S6K1 in modulating rapid response to inflammatory mediators, with c-Fos being one key downstream target. Targeting the S6 kinase pathway or c-Fos is an exciting new avenue for pain-modulating compounds.

Role of IL-6 in the regulation of neuronal development, survival and function

The pleiotropic cytokine interleukin-6 (IL-6) is emerging as a molecule with both beneficial and destructive potentials. It can exert opposing actions triggering either neuron survival after injury or causing neurodegeneration and cell death in neurodegenerative or neuropathic disorders. Importantly, neurons respond differently to IL-6 and this critically depends on their environment and whether they are located in the peripheral or the central nervous system. In addition to its hub regulator role in inflammation, IL-6 is recently emerging as an important regulator of neuron function in health and disease, offering exciting possibilities for more mechanistic insight into the pathogenesis of mental, neurodegenerative and pain disorders and for developing novel therapies for diseases with neuroimmune and neurogenic pathogenic components.