The persistent release of HMGB1 contributes to tactile hyperalgesia in a rodent model of neuropathic pain

The influence of sex on neuroimmune communication, pain, and physiology

With the National Institutes of Health's mandate to consider sex as a biological variable (SABV), there has been a significant increase of studies utilizing both sexes. Historically, we have known that biological sex and hormones influence immunological processes and now studies focusing on interactions between the immune, endocrine, and nervous systems are revealing sex differences that influence pain behavior and various molecular and biochemical processes. Neuroendocrine-immune interactions represent a key integrative discipline that will reveal critical processes in each field as it pertains to novel mechanisms in sex differences and necessary therapeutics. Here we appraise preclinical and clinical literature to discuss these interactions and key pathways that drive cell- and sex-specific differences in immunity, pain, and physiology.

Pathogenic mechanisms in genetically defined Ehlers-Danlos syndromes

The Ehlers-Danlos syndromes (EDS) are a group of rare heritable connective tissue disorders, common hallmarks of which are skin hyperextensibility, joint hypermobility, and generalized connective tissue fragility. Currently, 13 EDS types are recognized, caused by defects in 20 genes which consequently alter biosynthesis, organization, and/or supramolecular assembly of collagen fibrils in the extracellular matrix (ECM). Molecular analyses on patient samples (mostly dermal fibroblast cultures), combined with studies on animal models, have highlighted that part of EDS pathogenesis can be attributed to impaired cellular dynamics. Although our understanding of the full extent of (extra)cellular consequences is still limited, this narrative review aims to provide a comprehensive overview of our current knowledge on the extracellular, pericellular, and intracellular alterations implicated in EDS pathogenesis.

Histone deacetylase as emerging pharmacological therapeutic target for neuropathic pain: From epigenetic to selective drugs

BACKGROUND

Neuropathic pain remains a formidable challenge for modern medicine. The first-line pharmacological therapies exhibit limited efficacy and unfavorable side effect profiles, highlighting an unmet need for effective therapeutic medications. The past decades have witnessed an explosion in efforts to translate epigenetic concepts into pain therapy and shed light on epigenetics as a promising avenue for pain research. Recently, the aberrant activity of histone deacetylase (HDAC) has emerged as a key mechanism contributing to the development and maintenance of neuropathic pain.

AIMS

In this review, we highlight the distinctive role of specific HDAC subtypes in a cell-specific manner in pain nociception, and outline the recent experimental evidence supporting the therapeutic potential of HDACi in neuropathic pain.

METHODS

We have summarized studies of HDAC in neuropathic pain in Pubmed.

RESULTS

HDACs, widely distributed in the neuronal and non-neuronal cells of the dorsal root ganglion and spinal cord, regulate gene expression by deacetylation of histone or non-histone proteins and involving in increased neuronal excitability and neuroinflammation, thus promoting peripheral and central sensitization. Importantly, pharmacological manipulation of aberrant acetylation using HDAC-targeted inhibitors (HDACi) has shown promising pain-relieving properties in various preclinical models of neuropathic pain. Yet, many of which exhibit low-specificity that may induce off-target toxicities, underscoring the necessity for the development of isoform-selective HDACi in pain management.

CONCLUSIONS

Abnormally elevated HDACs promote neuronal excitability and neuroinflammation by epigenetically modulating pivotal gene expression in neuronal and immune cells, contributing to peripheral and central sensitization in the progression of neuropathic pain, and HDACi showed significant efficacy and great potential for alleviating neuropathic pain.

High mobility group box-1: A therapeutic target for analgesia and associated symptoms in chronic pain

The number of patients with chronic pain continues to increase against the background of an ageing society and a high incidence of various epidemics and disasters. One factor contributing to this situation is the absence of truly effective analgesics. Chronic pain is a persistent stress for the organism and can trigger a variety of neuropsychiatric symptoms. Hence, the search for useful analgesic targets is currently being intensified worldwide, and it is anticipated that the key to success may be molecules involved in emotional as well as sensory systems. High mobility group box-1 (HMGB1) has attracted attention as a therapeutic target for a variety of diseases. It is a very unique molecule having a dual role as a nuclear protein while also functioning as an inflammatory agent outside the cell. In recent years, numerous studies have shown that HMGB1 acts as a pain inducer in primary sensory nerves and the spinal dorsal horn. In addition, HMGB1 can function in the brain, and is involved in the symptoms of depression, anxiety and cognitive dysfunction that accompany chronic pain. In this review, we will summarize recent research and discuss the potential of HMGB1 as a useful drug target for chronic pain.

Role of pattern recognition receptors in chemotherapy-induced neuropathic pain

Progress in the development of effective chemotherapy is producing a growing population of patients with acute and chronic painful chemotherapy-induced peripheral neuropathy (CIPN), a serious treatment-limiting side effect for which there is currently no US Food and Drug Administration-approved treatment. CIPNs induced by diverse classes of chemotherapy drugs have remarkably similar clinical presentations, leading to the suggestion they share underlying mechanisms. Sensory neurons share with immune cells the ability to detect damage associated molecular patterns (DAMPs), molecules produced by diverse cell types in response to cellular stress and injury, including by chemotherapy drugs. DAMPs, in turn, are ligands for pattern recognition receptors (PRRs), several of which are found on sensory neurons, as well as satellite cells, and cells of the immune system. In the present experiments, we evaluated the role of two PRRs, TLR4 and RAGE, present in dorsal root ganglion (DRG), in CIPN. Antisense (AS)-oligodeoxynucleotides (ODN) against TLR4 and RAGE mRNA were administered intrathecally before ('prevention protocol') or 3 days after ('reversal protocol') the last administration of each of three chemotherapy drugs that treat cancer by different mechanisms (oxaliplatin, paclitaxel and bortezomib). TLR4 and RAGE AS-ODN prevented the development of CIPN induced by all three chemotherapy drugs. In the reversal protocol, however, while TLR4 AS-ODN completely reversed oxaliplatin- and paclitaxel-induced CIPN, in rats with bortezomib-induced CIPN it only produced a temporary attenuation. RAGE AS-ODN, in contrast, reversed CIPN induced by all three chemotherapy drugs. When a TLR4 antagonist was administered intradermally to the peripheral nociceptor terminal, it did not affect CIPN induced by any of the chemotherapy drugs. However, when administered intrathecally, to the central terminal, it attenuated hyperalgesia induced by all three chemotherapy drugs, compatible with a role of TLR4 in neurotransmission at the central terminal but not sensory transduction at the peripheral terminal. Finally, since it has been established that cultured DRG neurons can be used to study direct effects of chemotherapy on nociceptors, we also evaluated the role of TLR4 in CIPN at the cellular level, using patch-clamp electrophysiology in DRG neurons cultured from control and chemotherapy-treated rats. We found that increased excitability of small-diameter DRG neurons induced by in vivo and in vitro exposure to oxaliplatin is TLR4-dependent. Our findings suggest that in addition to the established contribution of PRR-dependent neuroimmune mechanisms, PRRs in DRG cells also have an important role in CIPN.

Neuroimmune Mechanisms Underlying Post-acute Sequelae of SARS-CoV-2 (PASC) Pain, Predictions from a Ligand-Receptor Interactome

PURPOSE OF REVIEW

Individuals with post-acute sequelae of SARS-CoV-2 (PASC) complain of persistent musculoskeletal pain. Determining how COVID-19 infection produces persistent pain would be valuable for the development of therapeutics aimed at alleviating these symptoms.

RECENT FINDINGS

To generate hypotheses regarding neuroimmune interactions in PASC, we used a ligand-receptor interactome to make predictions about how ligands from PBMCs in individuals with COVID-19 communicate with dorsal root ganglia (DRG) neurons to induce persistent pain. In a structured literature review of -omics COVID-19 studies, we identified ligands capable of binding to receptors on DRG neurons, which stimulate signaling pathways including immune cell activation and chemotaxis, the complement system, and type I interferon signaling. The most consistent finding across immune cell types was an upregulation of genes encoding the alarmins S100A8/9 and MHC-I. This ligand-receptor interactome, from our hypothesis-generating literature review, can be used to guide future research surrounding mechanisms of PASC-induced pain.

Hmgb1 Silencing in the Amygdala Inhibits Pain-Related Behaviors in a Rat Model of Neuropathic Pain

Chronic pain presents a therapeutic challenge due to the highly complex interplay of sensory, emotional-affective and cognitive factors. The mechanisms of the transition from acute to chronic pain are not well understood. We hypothesized that neuroimmune mechanisms in the amygdala, a brain region involved in the emotional-affective component of pain and pain modulation, play an important role through high motility group box 1 (Hmgb1), a pro-inflammatory molecule that has been linked to neuroimmune signaling in spinal nociception. Transcriptomic analysis revealed an upregulation of Hmgb1 mRNA in the right but not left central nucleus of the amygdala (CeA) at the chronic stage of a spinal nerve ligation (SNL) rat model of neuropathic pain. Hmgb1 silencing with a stereotaxic injection of siRNA for Hmgb1 into the right CeA of adult male and female rats 1 week after (post-treatment), but not 2 weeks before (pre-treatment) SNL induction decreased mechanical hypersensitivity and emotional-affective responses, but not anxiety-like behaviors, measured 4 weeks after SNL. Immunohistochemical data suggest that neurons are a major source of Hmgb1 in the CeA. Therefore, Hmgb1 in the amygdala may contribute to the transition from acute to chronic neuropathic pain, and the inhibition of Hmgb1 at a subacute time point can mitigate neuropathic pain.

Spinal alarmin HMGB1 and the activation of TLR4 lead to chronic stress-induced nociceptive hypersensitivity in rodents

Chronic stress affects millions of people around the world, and it can trigger different behavioral disorders like nociceptive hypersensitivity and anxiety, among others. However, the mechanisms underlaying these chronic stress-induced behavioral disorders have not been yet elucidated. This study was designed to understand the role of high-mobility group box-1 (HMGB1) and toll-like receptor 4 (TLR4) in chronic stress-induced nociceptive hypersensitivity. Chronic restraint stress induced bilateral tactile allodynia, anxiety-like behaviors, phosphorylation of ERK and p38MAPK and activation of spinal microglia. Moreover, chronic stress enhanced HMGB1 and TLR4 protein expression at the dorsal root ganglion, but not at the spinal cord. Intrathecal injection of HMGB1 or TLR4 antagonists reduced tactile allodynia and anxiety-like behaviors induced by chronic stress. Additionally, deletion of TLR4 diminished the establishment of chronic stress-induced tactile allodynia in male and female mice. Lastly, the antiallodynic effect of HMGB1 and TLR4 antagonists were similar in stressed male and female rats and mice. Our results suggest that chronic restraint stress induces nociceptive hypersensitivity, anxiety-like behaviors, and up-regulation of spinal HMGB1 and TLR4 expression. Blockade of HMGB1 and TLR4 reverses chronic restraint stress-induced nociceptive hypersensitivity and anxiety-like behaviors and restores altered HMGB1 and TLR4 expression. The antiallodynic effects of HMGB1 and TLR4 blockers in this model are sex independent. TLR4 could be a potential pharmacological target for the treatment of the nociceptive hypersensitivity associated with widespread chronic pain.

The expanding impact of methylglyoxal on behavior-related disorders

Methylglyoxal (MGO) is a reactive dicarbonyl compound formed as a byproduct of glycolysis. MGO is a major cell-permeant precursor of advanced glycation end products (AGEs), since it readily reacts with basic phospholipids and nucleotides, as well as amino acid residues of proteins, such as arginine, cysteine, and lysine. The AGEs production induced by MGO are widely associated with several pathologies, including neurodegenerative diseases. However, the impact of MGO metabolism and AGEs formation in the central nervous system (particularly in neurons, astrocytes and oligodendrocytes) on behavior and psychiatric diseases is not fully understood. Here, we briefly present background information on the biological activity of MGO in the central nervous system. It was gathered the available information on the role of MGO metabolism at the physiological processes, as well as at the neurobiology of psychiatry diseases, especially pain-related experiences, anxiety, depression, and cognition impairment-associated diseases. To clarify the role of MGO on behavior and associated diseases, we reviewed primarily the main findings at preclinical studies focusing on genetic and pharmacological approaches. Since monoamine neurotransmitter systems are implicated as pivotal targets on the pathophysiology and treatment of psychiatry and cognitive-related diseases, we also reviewed how MGO affects these neurotransmission systems and the implications of this phenomenon for nociception and pain; learning and cognition; and mood. In summary, this review highlights the pivotal role of glyoxalase 1 (Glo1) and MGO levels in modulating behavioral phenotypes, as well as related cellular and molecular signaling. Conclusively, this review signals dopamine as a new neurochemical MGO target, as well as highlights how MGO metabolism can modulate the pathophysiology and treatment of pain, psychiatric and cognitive-related diseases.

The Role of Microglia in Neuroinflammation of the Spinal Cord after Peripheral Nerve Injury

Peripheral nerve injuries induce a pronounced immune reaction within the spinal cord, largely governed by microglia activation in both the dorsal and ventral horns. The mechanisms of activation and response of microglia are diverse depending on the location within the spinal cord, type, severity, and proximity of injury, as well as the age and species of the organism. Thanks to recent advancements in neuro-immune research techniques, such as single-cell transcriptomics, novel genetic mouse models, and live imaging, a vast amount of literature has come to light regarding the mechanisms of microglial activation and alluding to the function of microgliosis around injured motoneurons and sensory afferents. Herein, we provide a comparative analysis of the dorsal and ventral horns in relation to mechanisms of microglia activation (CSF1, DAP12, CCR2, Fractalkine signaling, Toll-like receptors, and purinergic signaling), and functionality in neuroprotection, degeneration, regeneration, synaptic plasticity, and spinal circuit reorganization following peripheral nerve injury. This review aims to shed new light on unsettled controversies regarding the diversity of spinal microglial-neuronal interactions following injury.

Pain in the Ehlers-Danlos syndromes: Mechanisms, models, and challenges

Chronic pain is one of the most common, yet poorly studied, complaints in people suffering from Ehlers-Danlos syndromes (EDS). This heterogeneous group of heritable connective tissue disorders is typically characterized by skin hyperextensibility, joint hypermobility, and generalized connective tissue fragility. Most EDS types are caused by genetic defects that affect connective tissue biosynthesis, thereby compromising collagen biosynthesis or fibrillogenesis and resulting in a disorganized extracellular matrix. Even though chronic pain is a major source of disability, functional impairment, and psychosocial suffering in EDS, currently used analgesics and other treatment strategies provide inadequate pain relief and thus represents an important unmet medical need. An important contributor to this is the lack of knowledge about the underlying mechanisms. In this narrative review, we summarize the current understanding of pain and the associated mechanisms in EDS based on clinical studies focusing on questionnaires and experimental pain testing as well as studies in animal models of EDS. In addition, we highlight the challenges, gaps, and opportunities in EDS-pain research.

Neurons Are a Primary Driver of Inflammation via Release of HMGB1

Recent data show that activation of nociceptive (sensory) nerves turns on localized inflammation within the innervated area in a retrograde manner (antidromically), even in the absence of tissue injury or molecular markers of foreign invaders. This neuroinflammatory process is activated and sustained by the release of neuronal products, such as neuropeptides, with the subsequent amplification via recruitment of immunocompetent cells, including macrophages and lymphocytes. High mobility group box 1 protein (HMGB1) is a highly conserved, well characterized damage-associated molecular pattern molecule expressed by many cells, including nociceptors and is a marker of inflammatory diseases. In this review, we summarize recent evidence showing that neuronal HMGB1 is required for the development of neuroinflammation, as knock out limited to neurons or its neutralization via antibodies ameliorate injury in models of nerve injury and of arthritis. Further, the results of study show that HMGB1 is actively released during neuronal depolarization and thus plays a previously unrecognized key etiologic role in the initiation and amplification of neuroinflammation. Direct targeting of HMGB1 is a promising approach for novel anti-inflammatory therapy.

Modulation of Vagal Sensory Neurons via High Mobility Group Box-1 and Receptor for Advanced Glycation End Products: Implications for Respiratory Viral Infections

Vagal sensory neurons contribute to the symptoms and pathogenesis of inflammatory pulmonary diseases through processes that involve changes to their morphological and functional characteristics. The alarmin high mobility group box-1 (HMGB1) is an early mediator of pulmonary inflammation and can have actions on neurons in a range of inflammatory settings. We hypothesized that HMGB1 can regulate the growth and function of vagal sensory neurons and we set out to investigate this and the mechanisms involved. Culturing primary vagal sensory neurons from wildtype mice in the presence of HMGB1 significantly increased neurite outgrowth, while acute application of HMGB1 to isolated neurons under patch clamp electrophysiological investigation produced inward currents and enhanced action potential firing. Transcriptional analyses revealed the expression of the cognate HMGB1 receptors, Receptor for Advanced Glycation End products (RAGE) and Toll-like Receptor 4 (TLR4), in subsets of vagal sensory neurons. HMGB1-evoked growth and electrophysiological responses were significantly reduced in primary vagal sensory neurons harvested from RAGE deficient mice and completely absent in neurons from RAGE/TLR4 double deficient mice. Immunohistochemical analysis of vagal sensory neurons collected from mice after intranasal infection with murine pneumovirus or influenza A virus (IAV), or after intratracheal administration with the viral mimetic PolyI:C, revealed a significant increase in nuclear-to-cytoplasm translocation of HMGB1 compared to mock-inoculated mice. Neurons cultured from virus infected wildtype mice displayed a significant increase in neurite outgrowth, which was not observed for neurons from virus infected RAGE or RAGE/TLR4 deficient mice. These data suggest that HMGB1 can enhance vagal sensory neuron growth and excitability, acting primarily via sensory neuron RAGE. Activation of the HMGB1-RAGE axis in vagal sensory neurons could be an important mechanism leading to vagal hyperinnervation and hypersensitivity in chronic pulmonary disease.

Sensory Neuron TLR4 mediates the development of nerve-injury induced mechanical hypersensitivity in female mice

Recent studies have brought to light the necessity to discern sex-specific differences in various pain states and different cell-types that mediate these differences. These studies have uncovered the role of neuroimmune interactions to mediate pain states in a sex-specific fashion. While investigating immune function in pain development, we discovered that females utilize immune components of sensory neurons to mediate neuropathic pain development. We utilized two novel transgenic mouse models that eitherrestore expression of toll-like receptor (TLR) 4 inNa_v1.8 nociceptors on a TLR4-null background (TLR4^LoxTB) or remove TLR4 specifically from Na_v1.8 nociceptors (TLR4^fl/fl). After spared nerve injury (SNI), a model of neuropathic injury, we observed a robust female-specific onset of mechanical hypersensitivity in our transgenic animals. Female Na_v1.8-TLR4^fl/fl knockout animals were less mechanically sensitive than cre-negative TLR4^fl/fl littermates. Conversely, female Na_v1.8-TLR4^LoxTB reactivated animals were as mechanically sensitive as their wild-type counterparts. These sex and cell-specific effects were not recapitulated in male animals of either strain. Additionally, we find the danger associated molecular pattern, high mobility group box-1 (HGMB1), a potent TLR4 agonist, localization and ATF3 expression in females is dependent on TLR4 expression in dorsal root ganglia (DRG) populations following SNI. These experiments provide novel evidence toward sensory neuron specific modulation of pain in a sex-dependent manner.

HMGB1 released from nociceptors mediates inflammation

Inflammation, the body's primary defensive response system to injury and infection, is triggered by molecular signatures of microbes and tissue injury. These molecules also stimulate specialized sensory neurons, termed nociceptors. Activation of nociceptors mediates inflammation through antidromic release of neuropeptides into infected or injured tissue, producing neurogenic inflammation. Because HMGB1 is an important inflammatory mediator that is synthesized by neurons, we reasoned nociceptor release of HMGB1 might be a component of the neuroinflammatory response. In support of this possibility, we show here that transgenic nociceptors expressing channelrhodopsin-2 (ChR2) directly release HMGB1 in response to light stimulation. Additionally, HMGB1 expression in neurons was silenced by crossing synapsin-Cre (Syn-Cre) mice with floxed HMGB1 mice (HMGB1f/f). When these mice undergo sciatic nerve injury to activate neurogenic inflammation, they are protected from the development of cutaneous inflammation and allodynia as compared to wild-type controls. Syn-Cre/HMGB1fl/fl mice subjected to experimental collagen antibody-induced arthritis, a disease model in which nociceptor-dependent inflammation plays a significant pathological role, are protected from the development of allodynia and joint inflammation. Thus, nociceptor HMGB1 is required to mediate pain and inflammation during sciatic nerve injury and collagen antibody-induced arthritis.

HMGB1, NLRP3, IL-6 and ACE2 levels are elevated in COVID-19 with headache: a window to the infection-related headache mechanism

Background and aim

Pathogenesis of COVID-19 -related headache is unknown, though the induction of the trigeminal neurons through inflammation is proposed. We aimed to investigate key systemic circulating inflammatory molecules and their clinical relations in COVID-19 patients with headache.

Methods

This cross-sectional study enrolled 88 COVID-19 patients, hospitalized on a regular ward during the second wave of the pandemic. Clinical characteristics of COVID-19 patients were recorded, and laboratory tests were studied.

Results

The mean ages of 48 COVID-19 patients with headache (47.71 ± 10.8) and 40 COVID-19 patients without headache (45.70 ± 12.72) were comparable. COVID-19 patients suffered from headache had significantly higher serum levels of HMGB1, NLRP3, ACE2, and IL-6 than COVID-19 patients without headache, whereas CGRP and IL-10 levels were similar in the groups. Angiotensin II level was significantly decreased in the headache group. COVID-19 patients with headache showed an increased frequency of pulmonary involvement and increased D- dimer levels. Furthermore, COVID-19 was more frequently associated with weight loss, nausea, and diarrhea in patients with headache. Serum NLRP3 levels were correlated with headache duration and hospital stay, while headache response to paracetamol was negatively correlated with HMGB1 and positively associated with IL-10 levels.

Conclusion

Stronger inflammatory response is associated with headache in hospitalized COVID-19 patients with moderate disease severity. Increased levels of the circulating inflammatory and/or nociceptive molecules like HMGB1, NLRP3, and IL-6 may play a role in the potential induction of the trigeminal system and manifestation of headache secondary to SARS-CoV-2 infection.

LPS Primes Brain Responsiveness to High Mobility Group Box-1 Protein

High mobility group box (HMGB)1 action contributes to late phases of sepsis, but the effects of increased endogenous plasma HMGB1 levels on brain cells during inflammation are unclear. Here, we aimed to further investigate the role of HMGB1 in the brain during septic-like lipopolysaccharide-induced inflammation in rats (LPS, 10 mg/kg, i.p.). HMGB-1 mRNA expression and release were measured in the periphery/brain by RT-PCR, immunohistochemistry and ELISA. In vitro experiments with disulfide-HMGB1 in primary neuro-glial cell cultures of the area postrema (AP), a circumventricular organ with a leaky blood–brain barrier and direct access to circulating mediators like HMGB1 and LPS, were performed to determine the direct influence of HMGB1 on this pivotal brain structure for immune-to-brain communication. Indeed, HMGB1 plasma levels stayed elevated after LPS injection. Immunohistochemistry of brains and AP cultures confirmed LPS-stimulated cytoplasmatic translocation of HMGB1 indicative of local HMGB1 release. Moreover, disulfide-HMGB1 stimulation induced nuclear factor (NF)-κB activation and a significant release of interleukin-6, but not tumor necrosis factor α, into AP culture supernatants. However, only a few AP cells directly responded to HMGB1 with increased intracellular calcium concentration. Interestingly, priming with LPS induced a seven-fold higher percentage of responsive cells to HMGB1. We conclude that, as a humoral and local mediator, HMGB1 enhances brain inflammatory responses, after LPS priming, linked to sustained sepsis symptoms.

Ammonium Glycyrrhizinate Prevents Apoptosis and Mitochondrial Dysfunction Induced by High Glucose in SH-SY5Y Cell Line and Counteracts Neuropathic Pain in Streptozotocin-Induced Diabetic Mice

Glycyrrhiza glabra, commonly known as liquorice, contains several bioactive compounds such as flavonoids, sterols, triterpene, and saponins; among which, glycyrrhizic acid, an oleanane-type saponin, is the most abundant component in liquorice root. Diabetic peripheral neuropathy is one of the major complications of diabetes mellitus, leading to painful condition as neuropathic pain. The pathogenetic mechanism of diabetic peripheral neuropathy is very complex, and its understanding could lead to a more suitable therapeutic strategy. In this work, we analyzed the effects of ammonium glycyrrhizinate, a derivate salt of glycyrrhizic acid, on an in vitro system, neuroblastoma cells line SH-SY5Y, and we observed that ammonium glycyrrhizinate was able to prevent cytotoxic effect and mitochondrial fragmentation after high-glucose administration. In an in vivo experiment, we found that a short-repeated treatment with ammonium glycyrrhizinate was able to attenuate neuropathic hyperalgesia in streptozotocin-induced diabetic mice. In conclusion, our results showed that ammonium glycyrrhizinate could ameliorate diabetic peripheral neuropathy, counteracting both in vitro and in vivo effects induced by high glucose, and might represent a complementary medicine for the clinical management of diabetic peripheral neuropathy.

Pretreatment with High Mobility Group Box-1 Monoclonal Antibody Prevents the Onset of Trigeminal Neuropathy in Mice with a Distal Infraorbital Nerve Chronic Constriction Injury

Persistent pain following orofacial surgery is not uncommon. High mobility group box 1 (HMGB1), an alarmin, is released by peripheral immune cells following nerve injury and could be related to pain associated with trigeminal nerve injury. Distal infraorbital nerve chronic constriction injury (dIoN-CCI) evokes pain-related behaviors including increased facial grooming and hyper-responsiveness to acetone (cutaneous cooling) after dIoN-CCI surgery in mice. In addition, dIoN-CCI mice developed conditioned place preference to mirogabalin, suggesting increased neuropathic pain-related aversion. Treatment of the infraorbital nerve with neutralizing antibody HMGB1 (anti-HMGB1 nAb) before dIoN-CCI prevented both facial grooming and hyper-responsiveness to cooling. Pretreatment with anti-HMGB1 nAb also blocked immune cell activation associated with trigeminal nerve injury including the accumulation of macrophage around the injured IoN and increased microglia activation in the ipsilateral spinal trigeminal nucleus caudalis. The current findings demonstrated that blocking of HMGB1 prior to nerve injury prevents the onset of pain-related behaviors, possibly through blocking the activation of immune cells associated with the nerve injury, both within the CNS and on peripheral nerves. The current findings further suggest that blocking HMGB1 before tissue injury could be a novel strategy to prevent the induction of chronic pain following orofacial surgeries.

Perineural high-mobility group box 1 induces mechanical hypersensitivity through activation of spinal microglia: Involvement of glutamate-NMDA receptor dependent mechanism in spinal dorsal horn

High mobility box 1 (HMGB1), a damage-associated molecular pattern, has crucial roles in induction of neuropathic pain. Upregulation of HMGB1 around the injured sciatic nerve contributes to mechanical hypersensitivity following partial sciatic nerve ligation (PSNL) of mice. However, central mechanisms mediating perineural HMGB1-induced nociceptive hypersensitivity, especially within the spinal dorsal horn, have not been determined. The current study shows that perineural treatment of naïve mice with recombinant HMGB1, which mimics increased HMGB1 around the injured sciatic nerve of PSNL mice, significantly induced activation of microglia, but not astrocytes, in the spinal dorsal horn. Intraperitoneal injection of minocycline, a microglial inhibitor, ameliorated perineural rHMGB1-induced mechanical hypersensitivity. In addition, blockade of spinal N-methyl-D-aspartate (NMDA) receptors significantly prevented perineural rHMGB1-induced mechanical hypersensitivity and microglial activation. In contrast, non-NMDA receptors, neurokinin 1 receptor, colony-stimulating factor 1 receptor and P2Y₁₂ receptor were not involved in perineural rHMGB1-induced mechanical hypersensitivity. Furthermore, repeated perineural treatment with an anti-HMGB1 antibody blocked activation of spinal microglia in PSNL mice. Collectively, the current findings demonstrate that increased HMGB1 around injured sciatic nerve might induce nociceptive hypersensitivity through activation of spinal microglia. Thus, HMGB1-dependent mechanisms between the injured sciatic nerve and spinal dorsal horn could be crucial in induction of neuropathic pain.

Sex- and cell-dependent contribution of peripheral high mobility group box 1 and TLR4 in arthritis-induced pain

ABSTRACT

Spinal high mobility group box 1 protein (HMGB1) plays crucial roles in arthritis-induced pain; however, the involvement of peripheral HMGB1 has not been examined previously. In this study, we addressed the role of peripheral HMGB1 and explored if sex contributes differentially to nociception in arthritis. We found Hmgb1 expression to be elevated in the ankle joints of male and female mice subjected to collagen antibody-induced arthritis. Blocking the action of peripheral HMGB1, however, only reversed collagen antibody-induced arthritis-mediated hypersensitivity in males. Intra-articular injection of the toll-like receptor (TLR)4-activating, partially reduced disulfide, but not the fully reduced all-thiol, HMGB1 evoked mechanical hypersensitivity in both sexes. A sex-dependent temporal profile in expression of inflammatory factors in the ankle joint was observed in response to intra-articular injection of disulfide HMGB1, with male mice showing a delayed, yet longer-lasting increase in mRNA levels for several of the investigated factors. Intra-articular HMGB1 did not induce cellular infiltration in the ankle joint suggesting its action on tissue resident cells. To further explore possible sex differences in cellular involvement, we used the macrophage inhibitor, minocycline, and mice with specific TLR4 depletion in myeloid cells or nociceptors. We found that inhibition of resident macrophages attenuated HMGB1-induced pain-like behavior only in male mice. Interestingly, although the contribution of TLR4 on myeloid cells to nociception was minimal in females compared to males, TLR4 on nociceptors are important for HMGB1-induced pain in both sexes. Collectively, our work highlights sex- and cellular location-dependent roles of HMGB1 and TLR4 in peripheral pain mechanisms.

Liquorice for pain?

Liquorice has a long history of use in traditional Chinese, Ayurvedic and herbal medicine. The liquorice plant contains numerous bioactive compounds, including triterpenes, flavonoids and secondary metabolites, with glycyrrhizin being the main active compound. Liquorice constituents have been found to have anti-inflammatory, antioxidant, antiviral, anticancer, hepatoprotective and neuroprotective properties. In addition, they appear to have antidepressant actions and effects on morphine tolerance. Glycyrrhizin, its metabolite glycyrrhetic (glycyrrhetinic) acid and other liquorice-derived compounds such as isoflavonoids and trans-chalcones, exert potent anti-inflammatory effects via a wide range of mechanisms including high mobility group box 1 protein (HMGB1) inhibition, gap junction blockade and α_2A-adrenoceptor antagonism. These properties, together with an increasing body of preclinical studies and a long history of use in herbal medicine, suggest that liquorice constituents may be useful for pain management. Glycyrrhizin is used widely in the confectionary, food and tobacco industries, but has documented adverse effects that may limit clinical use. Whether liquorice plant-derived compounds represent a novel class of analgesics is yet to be established. Having a host of bioactive compounds with a broad range of mechanisms of effect, liquorice is a plant that, in the future, may give rise to new therapies for pain.

Concentrations of HMGB1 and Hsp70 of healthy subjects in upper and lower airway: Literature Review and Meta-analysis

Although high-mobility group box 1 and heat-shock protein 70 are implicated in airway diseases and suggested as relevant diagnostic biomarkers, their control concentrations in the airways have not yet been determined. This study aimed to evaluate concentration of healthy subjects for both these proteins in the upper and lower airways via meta-analysis. We searched MEDLINE, EMBASE, and Google Scholar for articles describing concentration of healthy subjects for these proteins. Data from healthy populations were combined using a random-effects model, and subgroup and sensitivity analyses were performed to determine between-study heterogeneity. We analyzed 22 studies involving 485 patients. Concentration of healthy subjects of high-mobility group box 1 and heat-shock protein 70 varied from "not detected" to 326.13 ng/mL and from 0.20 pg/mL to 9240.00 pg/mL, respectively, with the values showing significant heterogeneity. Subgroup analysis for high-mobility group box 1 revealed 13.63 ng/mL (95% CI 12.13-15.14), 100.31 ng/mL (95% CI -31.28-231.91), 9.54 ng/mL (95% CI 8.91-10.17), and 65.82 ng/mL (95% CI 55.51-76.14) for the lower airway, upper airway, pediatric populations, and adults, respectively, whereas that for heat-shock protein 70 revealed 20.58 pg/mL (95% CI 7.87-33.29) for the lower airway and 9240.00 ±11820 pg/mL for the upper airway. Although concentrations of healthy subjects of these proteins varied in the upper and lower airways, the levels of both these proteins were higher in the upper airway than in the lower airway, and these concentrations differed according to the age and sampling procedure. Our findings support the further evaluation of these proteins as biomarkers for airway-related diseases.

An Investigation of the Molecular Mechanisms Underlying the Analgesic Effect of Jakyak-Gamcho Decoction: A Network Pharmacology Study

Herbal drugs have drawn substantial interest as effective analgesic agents; however, their therapeutic mechanisms remain to be fully understood. To address this question, we performed a network pharmacology study to explore the system-level mechanisms that underlie the analgesic activity of Jakyak-Gamcho decoction (JGd; Shaoyao-Gancao-Tang in Chinese and Shakuyaku-Kanzo-To in Japanese), an herbal prescription consisting of Paeonia lactiflora Pallas and Glycyrrhiza uralensis Fischer. Based on comprehensive information regarding the pharmacological and chemical properties of the herbal constituents of JGd, we identified 57 active chemical compounds and their 70 pain-associated targets. The JGd targets were determined to be involved in the regulation of diverse biological activities as follows: calcium- and cytokine-mediated signalings, calcium ion concentration and homeostasis, cellular behaviors of muscle and neuronal cells, inflammatory response, and response to chemical, cytokine, drug, and oxidative stress. The targets were further enriched in various pain-associated signalings, including the PI3K-Akt, estrogen, ErbB, neurotrophin, neuroactive ligand-receptor interaction, HIF-1, serotonergic synapse, JAK-STAT, and cAMP pathways. Thus, these data provide a systematic basis to understand the molecular mechanisms underlying the analgesic activity of herbal drugs.

Cystitis-Related Bladder Pain Involves ATP-Dependent HMGB1 Release from Macrophages and Its Downstream H2S/Cav3.2 Signaling in Mice

Cystitis-related bladder pain involves RAGE activation by HMGB1, and increased Ca_v3.2 T-type Ca²⁺ channel activity by H₂S, generated by upregulated cystathionine-γ-lyase (CSE) in mice treated with cyclophosphamide (CPA). We, thus, investigated possible crosstalk between the HMGB1/RAGE and CSE/H₂S/Ca_v3.2 pathways in the bladder pain development. Bladder pain (nociceptive behavior/referred hyperalgesia) and immuno-reactive CSE expression in the bladder were determined in CPA-treated female mice. Cell signaling was analyzed in urothelial T24 and macrophage-like RAW264.7 cells. The CPA-induced bladder pain was abolished by pharmacological inhibition of T-type Ca²⁺ channels or CSE, and genetic deletion of Ca_v3.2. The CPA-induced CSE upregulation, as well as bladder pain was prevented by HMGB1 inactivation, inhibition of HMGB1 release from macrophages, antagonists of RAGE or P2X₄/P2X₇ receptors, and N-acetylcysteine, an antioxidant. Acrolein, a metabolite of CPA, triggered ATP release from T24 cells. Adenosine triphosphate (ATP) stimulated cell migration via P2X₇/P2X₄, and caused HMGB1 release via P2X₇ in RAW264.7 cells, which was dependent on p38MAPK/NF-κB signaling and reactive oxygen species (ROS) accumulation. Together, our data suggest that CPA, once metabolized to acrolein, causes urothelial ATP-mediated, redox-dependent HMGB1 release from macrophages, which in turn causes RAGE-mediated CSE upregulation and subsequent H₂S-targeted Ca_v3.2-dependent nociceptor excitation, resulting in bladder pain.

Role of high-mobility group box 1 and its modulation by thrombomodulin/thrombin axis in neuropathic and inflammatory pain

High-mobility group box 1 (HMGB1), a nuclear protein, once released to the extracellular space, facilitates pain signals as well as inflammation. Intraplantar or intraspinal application of HMGB1 elicits hyperalgesia/allodynia in rodents by activating the advanced glycosylation end-product specific receptor (receptor for advanced glycation end-products; RAGE) or Toll-like receptor 4 (TLR4). Endogenous HMGB1 derived from neurons, perineuronal cells or immune cells accumulating in the dorsal root ganglion or sensory nerves participates in somatic and visceral pain consisting of neuropathic and/or inflammatory components. Endothelial thrombomodulin (TM) and recombinant human soluble TM, TMα, markedly increase thrombin-dependent degradation of HMGB1, and systemic administration of TMα prevents and reverses various HMGB1-dependent pathological pain. Low MW compounds that directly inactivate HMGB1 or antagonize HMGB1-targeted receptors would be useful to reduce various forms of intractable pain. Thus, HMGB1 and its receptors are considered to serve as promising targets in developing novel agents to prevent or treat pathological pain. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc.

Targeting Inflammation Driven by HMGB1

High mobility group box 1 (HMGB1) is a highly conserved, nuclear protein present in all cell types. It is a multi-facet protein exerting functions both inside and outside of cells. Extracellular HMGB1 has been extensively studied for its prototypical alarmin functions activating innate immunity, after being actively released from cells or passively released upon cell death. TLR4 and RAGE operate as the main HMGB1 receptors. Disulfide HMGB1 activates the TLR4 complex by binding to MD-2. The binding site is separate from that of LPS and it is now feasible to specifically interrupt HMGB1/TLR4 activation without compromising protective LPS/TLR4-dependent functions. Another important therapeutic strategy is established on the administration of HMGB1 antagonists precluding RAGE-mediated endocytosis of HMGB1 and HMGB1-bound molecules capable of activating intracellular cognate receptors. Here we summarize the role of HMGB1 in inflammation, with a focus on recent findings on its mission as a damage-associated molecular pattern molecule and as a therapeutic target in inflammatory diseases. Recently generated HMGB1-specific inhibitors for treatment of inflammatory conditions are discussed.

lncRNA MALAT1 contributes to neuropathic pain development through regulating miR-129-5p/HMGB1 axis in a rat model of chronic constriction injury

Backgrounds: Mounting studies pay attention to the functional roles of long non-coding RNAs (lncRNAs) in many human diseases including neuropathic pain. LncRNA MALAT1 has been indicated to serve as a critical mediator in neuropathic pain with unclear mechanisms. The present study aims to explore the functional roles of MALAT1 in neuropathic pain progression and the related mechanisms.Methods: Bilateral sciatic nerves were ligated to induce chronic constriction injury (CCI) in order to establish the neuropathic pain rat model followed by behavioral tests, RT-qPCR, Western blotting, and ELISA. Dual luciferase activity assay was performed to determine the binding effect between MALAT1 or HMGB1 and miR-129-5p.Results: The mRNA levels of MALAT1 were significantly enhanced in CCI rats. MALAT1 inhibition repressed the development of neuropathic pain and neuroinflammation. Additionally, miR-129-5p was decreased and HMGB1 was increased, both of which could be rectified by MALAT1 inhibition. Meanwhile, MALAT1 targeted miR-129-5p/HMGB1 axis. Finally, miR-129-5p suppression attenuated the inhibitory effect of MALAT1 inhibition on neuropathic pain and neuroinflammation development in CCI rats.Conclusion: The present study demonstrates that MALAT1 might modulate neuropathic pain via targeting miR-129-5p/HMGB1 axis. These findings may lead to a promising and efficacious clinical approach for the treatment of neuropathic pain.

Insufficient efferocytosis by M2-like macrophages as a possible mechanism of neuropathic pain induced by nerve injury

Peripheral nerve injury typically leads to chronic inflammation through recruitment of immune cells, which may induce neuropathic pain. We previously reported that M1-like macrophages at sites of peripheral nerve injury induced neuropathic pain; however, the involvement of other immune cells (e.g. M2-like macrophages) in the progression of neuropathic pain remains unclear. In addition, the immune responses that occur at sites of nerve injury have not been well characterized. In this study, we show that M2-like macrophages accumulate in injured nerves to participate in the clearance of dead or dying cells (i.e., efferocytosis). Because MerTK (a receptor of dead or dying cells) levels on the surface of macrophages are limited, it seems to induce the insufficient of efferocytosis, such that the levels of dead or dying cells cannot be controlled in injured nerves. Given that efferocytosis is pivotal for resolution of inflammation, our data suggest that insufficient efferocytosis is a contributing factor in the development of chronic inflammation in injured nerves.

Role of Melatonin in the Regulation of Pain

Melatonin is a pleiotropic hormone synthesized and secreted mainly by the pineal gland in vertebrates. Melatonin is an endogenous regulator of circadian and seasonal rhythms. Melatonin is involved in many physiological and pathophysiological processes demonstrating antioxidant, antineoplastic, anti-inflammatory, and immunomodulatory properties. Accumulating evidence has revealed that melatonin plays an important role in pain modulation through multiple mechanisms. In this review, we examine recent evidence for melatonin on pain regulation in various animal models and patients with pain syndromes, and the potential cellular mechanisms.

High-Mobility Group Box 1 Protein Signaling in Painful Diabetic Neuropathy

Diabetes is a global epidemic and more than 50% diabetic patients are also diagnosed with neuropathy, which greatly affects the quality of life of the patients. Available treatments are not always successful due to the limited efficacy and complications, such as addiction and dependency. Studies have implicated that high mobility group box1 (HMGB1) protein plays a crucial role in neuroinflammation and the development of neuropathic conditions. HMGB1 is a proinflammatory cytokine that can be released from necrotic cells in passive form or in response to inflammatory signals as an active form. HMGB1 is the ligand for the receptor for advanced glycation end products (RAGE), and toll-like receptors, (TLR)-2 and TLR4, which also indirectly activates C-X-C chemokine receptor type 4 (CXCR4). We investigated whether blocking of HMGB1 can reduce pain and inflammation in diabetic neuropathic animals to further understand the role of HMGB1 in diabetic neuropathy. Type 2 diabetic rats and mice were treated with natural inhibitor of HMGB1, glycyrrhizin (GLC) for five days/week for four weeks at a dose of 50 mg/kg per day by intraperitoneal injection. The animals were divided into three categories: naïve control, diabetic alone, diabetic with GLC treatment. All of the behavioral analyses were conducted before and after the treatment. The expression of inflammatory markers and changes in histone acetylation in the peripheral nervous system were measured by immunohistochemistry and Western blot analysis after the completion of the treatment. Our study revealed that TLR4, HMGB1, CXCR4, and Nod-like receptor protein 3 (NLRP3) levels were increased in the spinal and dorsal root ganglia (DRG) neurons of Type 2 diabetic mice and rats with painful neuropathy. GLC treatment inhibited the increases in TLR4, NLRP3, and CXCR4 expressions and improved the mechanical and thermal pain threshold in these animals. Immunohistochemical studies revealed that hyperglycemia mediated inflammation influenced HMGB1 acetylation and its release from the neurons. It also altered histone 3 acetylation in the microglial cells. The inhibition of HMGB1 by GLC prevented the release of HMGB1 as well as H3K9 acetylation. These findings indicate that the interruption of HMGB1 mediated inflammation could ameliorate diabetic neuropathy and might exhibit a unique target for the treatment.

Inhibition of Neuroinflammation by AIBP: Spinal Effects upon Facilitated Pain States

Apolipoprotein A-I binding protein (AIBP) reduces lipid raft abundance by augmenting the removal of excess cholesterol from the plasma membrane. Here, we report that AIBP prevents and reverses processes associated with neuroinflammatory-mediated spinal nociceptive processing. The mechanism involves AIBP binding to Toll-like receptor-4 (TLR4) and increased binding of AIBP to activated microglia, which mediates selective regulation of lipid rafts in inflammatory cells. AIBP-mediated lipid raft reductions downregulate LPS-induced TLR4 dimerization, inflammatory signaling, and expression of cytokines in microglia. In mice, intrathecal injections of AIBP reduce spinal myeloid cell lipid rafts, TLR4 dimerization, neuroinflammation, and glial activation. Intrathecal AIBP reverses established allodynia in mice in which pain states were induced by the chemotherapeutic cisplatin, intraplantar formalin, or intrathecal LPS, all of which are pro-nociceptive interventions known to be regulated by TLR4 signaling. These findings demonstrate a mechanism by which AIBP regulates neuroinflammation and suggest the therapeutic potential of AIBP in treating preexisting pain states.

HMGB1/TLR4 promotes apoptosis and reduces autophagy of hippocampal neurons in diabetes combined with OSA

AIMS

Obstructive sleep apnea (OSA) combined with type 2 diabetes (T2DM) may lead to cognitive dysfunction. We previously reported that cognitive impairment is exacerbated in KKAy mice exposed to intermittent hypoxia (IH), during which the DNA binding protein HMGB1 mediates hippocampal neuronal apoptosis by maintaining microglia-associated neuroinflammation, but the underlying mechanism remains largely unknown.

MATERIALS AND METHODS

We performed immunofluorescence, Western blotting, and immunohistochemistry experiments in mouse hippocampal tissues and HT22 cells. KKAy type 2 diabetes model mice and normal C57BL/6J mice were exposed to IH or intermittent normoxia. HT22 cells were cultured in high glucose medium and exposed to IH or intermittent normoxia. We transfected HMGB1 siRNA into HT22 cells and then treated them with high glucose combined with intermittent hypoxia.

KEY FINDINGS

In conclusion, IH aggravated apoptosis and autophagy defects in T2DM mice, and increased the protein expression of HMGB1 and TLR4. This was also confirmed in HG + IH-treated hippocampal HT22 cells. HMGB1 siRNA can significantly reduce the protein expression of HMGB1 and TLR4, reverse neuronal apoptosis and enhance autophagy.

SIGNIFICANCE

We believe that HMGB1 is a key factor in the regulation of hippocampal neuronal apoptosis and autophagy defects in T2DM combined with OSA. Targeting HMGB1/TLR4 signaling as a novel approach may delay or prevent the increased apoptosis and decreased autophagy induced by T2DM combined with OSA, and may ultimately improve cognitive dysfunction.

Profiling of how nociceptor neurons detect danger - new and old foes

The host evolves redundant mechanisms to preserve physiological processing and homeostasis. These functions range from sensing internal and external threats, creating a memory of the insult and generating reflexes, which aim to resolve inflammation. Impairment in such functioning leads to chronic inflammatory diseases. By interacting through a common language of ligands and receptors, the immune and sensory nervous systems work in concert to accomplish such protective functions. Whilst this bidirectional communication helps to protect from danger, it can contribute to disease pathophysiology. Thus, the somatosensory nervous system is anatomically positioned within primary and secondary lymphoid tissues and mucosa to modulate immunity directly. Upstream of this interplay, neurons detect danger, which prompts the release of neuropeptides initiating (i) defensive reflexes (ranging from withdrawal response to coughing) and (ii) chemotaxis, adhesion and local infiltration of immune cells. The resulting outcome of such neuro-immune interplay is still ill-defined, but consensual findings start to emerge and support neuropeptides not only as blockers of T_H 1-mediated immunity but also as drivers of T_H 2 immune responses. However, the modalities detected by nociceptors revealed broader than mechanical pressure and temperature sensing and include signals as various as cytokines and pathogens to immunoglobulins and even microRNAs. Along these lines, we aggregated various dorsal root ganglion sensory neuron expression profiling datasets supporting such wide-ranging sensing capabilities to help identifying new danger detection modalities of these cells. Thus, revealing unexpected aspects of nociceptor neuron biology might prompt the identification of novel drivers of immunity, means to resolve inflammation and strategies to safeguard homeostasis.

Melatonin MT2 receptor agonist IIK-7 produces antinociception by modulation of ROS and suppression of spinal microglial activation in neuropathic pain rats

Background

In recent years, several melatonin (MLT) receptor agonists have been approved by FDA for the treatment of sleep disorders and depression. Very few studies have shed light on their efficacy against neuropathic pain (NP). IIK-7 is an MT-2 agonist known to promote sleep. Whether IIK-7 suppresses NP has not been reported, and the signaling profile is unknown.

Objective

To investigate the effect of melatonin type 2 receptor agonist IIK-7 on partial sciatic nerve transection-induced NP in rats and elucidate the underlying molecular mechanisms.

Methods

NP was induced by the PSNT in the left leg of adult male Wistar rats. On post-transection day 7, rats were implanted with intrathecal (i.t) catheter connected to an infusion pump and divided in to four groups: sham-operated/vehicle, PSNT/vehicle, PSNT/0.5 μg/hr IIK-7 and PSNT/0.5 μg IIK-7/1 μg 4-p/hr. To test the MT-2 dependence on IIK-7 activity, the animals were implanted with a single i.t catheter and injected MT-2 antagonist 4-Phenyl-2-propionamidotetralin (4-p) 20 mins prior to IIK-7 injection on day 7 after PSNT. The antinociceptive response was measured using a mechanical paw withdrawal threshold. Activation of microglial cells and the expression of NP-associated proteins in the spinal cord dorsal horn was assessed by immunofluorescence assay (IFA) and Western blotting (WB). Reactive oxygen species (ROS) scavenging ability of IIK-7 was evaluated by using bone marrow-derived macrophages (BMDM).

Results

Treatment with the MT-2 agonist IIK-7 significantly alleviated PSNT-induced mechanical allodynia and glial activation along with the inhibition of P44/42 MAPK, HMGB-1, STAT3, iNOS and casp-3 proteins.

Conclusion

IIK-7 attenuates NP through the suppression of glial activation and suppression of proteins involved in inflammation and apoptosis. MT-2 receptor agonists may establish a promising and unique therapeutic approach for the treatment of NP.

Anti-HMGB1 monoclonal antibody therapy for a wide range of CNS and PNS diseases

High mobility group box-1 (HMGB1), a representative damage associated-molecular pattern (DAMP), has been reported to be involved in many inflammatory diseases. Several drugs are thought to have potential to control the translocation and secretion of HMGB1, or to neutralize extracellular HMGB1 by binding to it. One of these drugs, anti-HMGB1 monoclonal antibody (mAb), is highly specific for HMGB1 and has been shown to be effective for the treatment of a wide range of CNS diseases when modeled in animals, including stroke, traumatic brain injury, Parkinson's disease, epilepsy and Alzheimer's disease. Thus, anti-HMGB1 mAb not only is useful for target validation but also has extensive potential for the treatment of the above-mentioned diseases. In this review, we summarize existing knowledge on the effects of anti-HMGB1 mAb on CNS and PNS diseases, the common features of translocation and secretion of HMGB1 and the functional roles of HMGB1 in these diseases. The existing literature suggests that anti-HMGB1 mAb therapy would be effective for a wide range of CNS and PNS diseases.

Small-molecule CaVα1⋅CaVβ antagonist suppresses neuronal voltage-gated calcium-channel trafficking

Extracellular calcium flow through neuronal voltage-gated Ca_V2.2 calcium channels converts action potential-encoded information to the release of pronociceptive neurotransmitters in the dorsal horn of the spinal cord, culminating in excitation of the postsynaptic central nociceptive neurons. The Ca_V2.2 channel is composed of a pore-forming α₁ subunit (Ca_Vα₁) that is engaged in protein-protein interactions with auxiliary α₂/δ and β subunits. The high-affinity Ca_V2.2α₁⋅Ca_Vβ₃ protein-protein interaction is essential for proper trafficking of Ca_V2.2 channels to the plasma membrane. Here, structure-based computational screening led to small molecules that disrupt the Ca_V2.2α₁⋅Ca_Vβ₃ protein-protein interaction. The binding mode of these compounds reveals that three substituents closely mimic the side chains of hot-spot residues located on the α-helix of Ca_V2.2α₁ Site-directed mutagenesis confirmed the critical nature of a salt-bridge interaction between the compounds and Ca_Vβ₃ Arg-307. In cells, compounds decreased trafficking of Ca_V2.2 channels to the plasma membrane and modulated the functions of the channel. In a rodent neuropathic pain model, the compounds suppressed pain responses. Small-molecule α-helical mimetics targeting ion channel protein-protein interactions may represent a strategy for developing nonopioid analgesia and for treatment of other neurological disorders associated with calcium-channel trafficking.

Paclitaxel-induced HMGB1 release from macrophages and its implication for peripheral neuropathy in mice: Evidence for a neuroimmune crosstalk

Given our recent evidence for the role of high mobility group box 1 (HMGB1) in chemotherapy-induced peripheral neuropathy (CIPN) in rats, we examined the origin of HMGB1 and the upstream and downstream mechanisms of HMGB1 release involved in paclitaxel-induced neuropathy in mice. Paclitaxel treatment developed mechanical allodynia in mice, as assessed by von Frey test, which was prevented by an anti-HMGB1-neutralizing antibody or thrombomodulin alfa capable of inactivating HMGB1. RAGE or CXCR4 antagonists, ethyl pyruvate or minocycline, known to inhibit HMGB1 release from macrophages, and liposomal clodronate, a macrophage depletor, prevented the paclitaxel-induced allodynia. Paclitaxel caused upregulation of RAGE and CXCR4 in the dorsal root ganglia and macrophage accumulation in the sciatic nerve. In macrophage-like RAW264.7 cells, paclitaxel evoked cytoplasmic translocation of nuclear HMGB1 followed by its extracellular release, and overexpression of CBP and PCAF, histone acetyltransferases (HATs), known to cause acetylation and cytoplasmic translocation of HMGB1, which were suppressed by ethyl pyruvate, N-acetyl-l-cysteine, an anti-oxidant, and SB203580 and PDTC, inhibitors of p38 MAP kinase (p38MAPK) and NF-κB, respectively. Paclitaxel increased accumulation of reactive oxygen species (ROS) and phosphorylation of p38MAPK, NF-κB p65 and I-κB in RAW264.7 cells. In mice, N-acetyl-l-cysteine or PDTC prevented the paclitaxel-induced allodynia. Co-culture of neuron-like NG108-15 cells or stimulation with their conditioned medium promoted paclitaxel-induced HMGB1 release from RAW264.7 cells. Our data indicate that HMGB1 released from macrophages through the ROS/p38MAPK/NF-κB/HAT pathway participates in the paclitaxel-induced peripheral neuropathy in mice, and unveils an emerging therapeutic avenue targeting a neuroimmune crosstalk in CIPN.

Lupus antibodies induce behavioral changes mediated by microglia and blocked by ACE inhibitors

Cognitive impairment occurs in 40-90% of patients with systemic lupus erythematosus (SLE), which is characterized by autoantibodies to nuclear antigens, especially DNA. We discovered that a subset of anti-DNA antibodies, termed DNRAbs, cross reacts with the N-methyl-d-aspartate receptor (NMDAR) and enhances NMDAR signaling. In patients, DNRAb presence associates with spatial memory impairment. In a mouse model, DNRAb-mediated brain pathology proceeds through an acute phase of excitotoxic neuron loss, followed by persistent alteration in neuronal integrity and spatial memory impairment. The latter pathology becomes evident only after DNRAbs are no longer detectable in the brain. Here we investigate the mechanism of long-term neuronal dysfunction mediated by transient exposure to antibody. We show that activated microglia and C1q are critical mediators of neuronal damage. We further show that centrally acting inhibitors of angiotensin-converting enzyme (ACE) can prevent microglial activation and preserve neuronal function and cognitive performance. Thus, ACE inhibition represents a strong candidate for clinical trials aimed at mitigating cognitive dysfunction.

Sensory neuronal sensitisation occurs through HMGB-1-RAGE and TRPV1 in high-glucose conditions

Many potential causes for painful diabetic neuropathy have been proposed including actions of cytokines and growth factors. High mobility group protein B1 (HMGB1) is a RAGE (also known as AGER) agonist whose levels are increased in diabetes and that contributes to pain by modulating peripheral inflammatory responses. HMGB1 enhances nociceptive behaviour in naïve animals through an unknown mechanism. We tested the hypothesis that HMGB1 causes pain through direct neuronal activation of RAGE and alteration of nociceptive neuronal responsiveness. HMGB1 and RAGE expression were increased in skin and primary sensory (dorsal root ganglion, DRG) neurons of diabetic rats at times when pain behaviour was enhanced. Agonist-evoked TRPV1-mediated Ca2+ responses increased in cultured DRG neurons from diabetic rats and in neurons from naïve rats exposed to high glucose concentrations. HMGB1-mediated increases in TRPV1-evoked Ca2+ responses in DRG neurons were RAGE- and PKC-dependent, and this was blocked by co-administration of the growth factor splice variant VEGF-A165b. Pain behaviour and the DRG RAGE expression increases were blocked by VEGF-A165b treatment of diabetic rats in vivo Hence, we conclude that HMGB1-RAGE activation sensitises DRG neurons in vitro, and that VEGF-A165b blocks HMGB-1-RAGE DRG activation, which may contribute to its analgesic properties in vivo.

Traumatic Brain Injury-Induced Acute Lung Injury: Evidence for Activation and Inhibition of a Neural-Respiratory-Inflammasome Axis

Approximately 20-25% of traumatic brain injury (TBI) subjects develop acute lung injury (ALI), but the pathomechanisms of TBI-induced ALI remain poorly defined. Our previous work has shown that the inflammasome plays a critical role in TBI-induced secondary pathophysiology and that inflammasome proteins are released in extracellular vesicles (EV) after TBI. Here we investigated whether EV-mediated inflammasome signaling contributed to the etiology of TBI-induced ALI. C57/BL6 male mice were subjected to controlled cortical impact (CCI), and the brains and lungs were examined for inflammasome activation and ALI at 4 and 24 h after TBI. We show that TBI releases EV containing inflammasome proteins into serum that target the lung to cause ALI, supporting activation of a neural-respiratory-inflammasome axis. Administration of a low-molecular-weight heparin (enoxaparin, a blocker of EV uptake) or treatment with a monoclonal antibody against apoptosis speck-like staining protein containing a caspase recruitment domain (anti-ASC) after adoptive transfer of EV isolated from TBI-injured mice significantly inhibited inflammasome activation in the lungs of recipient mice resulting in improved ALI scores.This axis constitutes an important arm of the innate inflammatory response in lung pathology after TBI and targeting this axis represents a novel therapeutic treatment for TBI-induced ALI.

Peripheral Mechanisms Contributing to Osteoarthritis Pain

PURPOSE OF REVIEW

Osteoarthritis (OA) is the most common form of arthritis and a major source of pain and disability worldwide. OA-associated pain is usually refractory to classically used analgesics, and disease-modifying therapies are still lacking. Therefore, a better understanding of mechanisms and mediators contributing to the generation and maintenance of OA pain is critical for the development of efficient and safe pain-relieving therapies.

RECENT FINDINGS

Both peripheral and central mechanisms contribute to OA pain. Clinical evidence suggests that a strong peripheral nociceptive drive from the affected joint maintains pain and central sensitization associated with OA. Mediators present in the OA joint, including nerve growth factor, chemokines, cytokines, and inflammatory cells can contribute to sensitization. Furthermore, structural alterations in joint innervation and nerve damage occur in the course of OA. Several interrelated pathological processes, including joint damage, structural reorganization of joint afferents, low-grade inflammation, neuroplasticity, and nerve damage all contribute to the pain observed in OA. It can be anticipated that elucidating exactly how these mechanisms are operational in the course of progressive OA may lead to the identification of novel targets for intervention.

Extracellular HMGB1 as a therapeutic target in inflammatory diseases

INTRODUCTION

High-mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that promotes inflammation when released extracellularly after cellular activation, stress, damage or death. HMGB1 operates as one of the most intriguing molecules in inflammatory disorders via recently elucidated signal and molecular transport mechanisms. Treatments based on antagonists specifically targeting extracellular HMGB1 have generated encouraging results in a wide number of experimental models of infectious and sterile inflammation. Clinical studies are still to come. Areas covered: We here summarize recent advances regarding pathways for extracellular HMGB1 release, receptor usage, and functional consequences of post-translational modifications. The review also addresses results of preclinical HMGB1-targeted therapy studies in multiple inflammatory conditions and outlines the current status of emerging clinical HMGB1-specific antagonists. Expert opinion: Blocking excessive amounts of extracellular HMGB1, particularly the disulfide isoform, offers an attractive clinical opportunity to ameliorate systemic inflammatory diseases. Therapeutic interventions to regulate intracellular HMGB1 biology must still await a deeper understanding of intracellular HMGB1 functions. Future work is needed to create more robust assays to evaluate functional bioactivity of HMGB1 antagonists. Forthcoming clinical studies would also greatly benefit from a development of antibody-based assays to quantify HMGB1 redox isoforms, presently assessed by mass spectrometry methods.

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.

Toll-like receptors and their role in persistent pain

One of the fundamental mechanisms whereby the innate immune system coordinates inflammatory signal transduction is through Toll-like receptors (TLRs), which function to protect and defend the host organism by initiating inflammatory signaling cascades in response to tissue damage or injury. TLRs are positioned at the neuroimmune interface, and accumulating evidence suggests that the inflammatory consequences of TLR activation on glia (including microglia and astrocytes), sensory neurons, and other cell types can influence nociceptive processing and lead to states of exaggerated and unresolved pain. In this review, we summarize our current understanding of how different TLRs and their accessory or adaptor molecules can contribute to the development and maintenance of persistent pain. The challenges and opportunities of targeting TLRs for new treatment strategies against chronic pain are discussed, including the therapeutic context of TLR-mediated signaling in opioid analgesia and chemotherapy-induced pain. Considering the prevalence of persistent pain and the insufficient efficacy and safety of current treatment options, a deeper understanding of Toll-like receptors holds the promise of novel therapies for managing pathological pain.

An overview of pathways encoding nociception

The nervous system detects and interprets a variety of chemical, mechanical, and thermal stimuli. In the face of tissue injury, local inflammatory products perpetuate ongoing activity and sensitisation of the peripheral nerve termini. This ongoing activity evokes a state of robust spinal facilitation mediated by a number of local circuits, the net effect yielding an enhanced message of nociception to higher centres. This messaging typically wanes with the resolution of inflammation or wound healing. However, there are situations in which peripheral and central components of the pain transmission pathway extend and enhance the pain state, leading to a persistent hypersensitivity, e.g., an acute to chronic pain transition. Current work points to the contribution of innate and adaptive immunity in creating these enduring conditions. We briefly describe the underlying biological components of both physiological pain processing and pathological pain processing, as well as the acute to chronic pain transition and the role of innate and adaptive immunity in this transition.