STING controls nociception via type I interferon signalling in sensory neurons

The role of STING signaling in central nervous system infection and neuroinflammatory disease

The cyclic guanosine monophosphate-adenosine monophosphate (GMP-AMP) synthase-Stimulator of Interferon Genes (cGAS-STING) pathway is a critical innate immune mechanism for detecting the presence of double-stranded DNA (dsDNA) and prompting a robust immune response. Canonical cGAS-STING activation occurs when cGAS, a predominantly cytosolic pattern recognition receptor, binds microbial DNA to promote STING activation. Upon STING activation, transcription factors enter the nucleus to cause the production of Type I interferons, inflammatory cytokines whose primary function is to prime the host for viral infection by producing a number of antiviral interferon-stimulated genes. While the pathway was originally described in viral infection, more recent studies have implicated cGAS-STING signaling in a number of different contexts, including autoimmune disease, cancer, injury, and neuroinflammatory disease. This review focuses on how our understanding of the cGAS-STING pathway has evolved over time with an emphasis on the role of STING-mediated neuroinflammation and infection in the nervous system. We discuss recent findings on how STING signaling contributes to the pathology of pain, traumatic brain injury, and stroke, as well as how mitochondrial DNA may promote STING activation in common neurodegenerative diseases. We conclude by commenting on the current knowledge gaps that should be filled before STING can be an effective therapeutic target in neuroinflammatory disease. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Infectious Diseases > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology.

STING controls opioid-induced itch and chronic itch via spinal tank-binding kinase 1-dependent type I interferon response in mice

BACKGROUND

Patients receiving epidural or intrathecal opioids administration for neuraxial analgesia frequently suffer from an irritating itch. STING (stimulator of interferon genes), an innate immune modulator, is strongly implicated in pain pathogenesis via neuron-immune modulation. Given that pain and itch share some common neurocircuits, we evaluate the therapeutic potential of STING agonists in opioid-induced itch and chronic itch.

METHODS

Opioids (morphine, fentanyl and sufentanil) were intrathecally injected to induce acute itch. Chronic itch was induced by dry skin and contact dermatitis. Opioids analgesic effect, itch-induced scratching behavior, spinal expression of STING, phosphorylation of TBK1 (tank-binding kinase 1), IRF3 (interferon regulatory factor-3) and ERK (extracellular signal-regulated kinase), as well as production of IFN-α and IFN-β were examined. STING agonists (DMXAA and ADU-S100), TBK1 inhibitor, recombinant IFN-α and IFN-β elucidated the mechanism and treatment of itch. Whole-brain functional connectivity was evaluated using resting-state fMRI.

RESULTS

We report the primary expression of STING protein by the spinal dorsal horn neurons. Intraperitoneal injection of DMXAA dose-dependently reduces morphine-induced scratch bouts, without impairing morphine antinociception. Simultaneously, DMXAA alleviates fentanyl- and sufentanil-induced itching-like behavior, and chronic scratching behavior caused by dry skin and contact dermatitis. Furthermore, DMXAA drastically increases spinal phosphorylation of TBK1 and IRF3 following morphine exposure, dry skin and contact dermatitis. DMXAA-induced anti-pruritus effects and spinal productions of IFN-α and IFN-β are compensated by intrathecal delivery of the TBK1 inhibitor. Also, ADU-S100, recombinant IFN-α and IFN-β exhibits remarkable attenuation in scratching behaviors after morphine injection and dermatitis. Recombinant IFN-α inhibits morphine-induced spinal phosphorylation of ERK. Finally, DMXAA prevents dermatitis-induced the increase of cerebral functional connectivity between regions of interests such as primary somatosensory cortex, piriform cortex, retrosplenial cortex, colliculus and ventral thalamus.

CONCLUSIONS

STING activation confers protection against opioid-induced itch and chronic itch through spinal up-regulation of TBK1-IRF3-type I interferon cascades in mice, suggesting that STING agonists are promising candidates in translational development for pruritus relief.

Glabridin Therapy Reduces Chronic Allodynia, Spinal Microgliosis, and Dendritic Spine Generation by Inhibiting Fractalkine-CX3CR1 Signaling in a Mouse Model of Tibial Fractures

Patients undergoing bone fractures frequently suffer from irritating chronic pain after orthopedic repairs. Chemokine-mediated interactions between neurons and microglia are important steps for neuroinflammation and excitatory synaptic plasticity during the spinal transmission of pathological pain. Recently, glabridin, the main bioactive component of licorice, has been shown to exhibit anti-nociceptive and neuroprotective properties for inflammatory pain. This present study evaluated the therapeutic potential of glabridin and its analgesic mechanisms using a mouse model of tibial fracture-associated chronic pain. Repetitive injections of glabridin were delivered spinally daily for 4 continuous days from days 3 to 6 after the fractures. Herein, we discovered that repeated administrations of glabridin (10 and 50 μg, but not 1 μg) could prevent prolonged cold allodynia and mechanical allodynia following bone fractures. A single intrathecal intervention with glabridin (50 μg) relieved an existing chronic allodynia two weeks following the fracture surgeries. Systemic therapies with glabridin (intraperitoneal; 50 mg kg^-1) were protective against long-lasting allodynia caused by fractures. Furthermore, glabridin restricted the fracture-caused spinal overexpressions of the chemokine fractalkine and its receptor CX3CR1, as well as the elevated number of microglial cells and dendritic spines. Strikingly, glabridin induced the inhibition of pain behaviors, microgliosis, and spine generation, which were abolished with the co-administration of exogenous fractalkine. Meanwhile, the exogenous fractalkine-evoked acute pain was compensated after microglia inhibition. Additionally, spinal neutralization of fractalkine/CX3CR1 signaling alleviated the intensity of postoperative allodynia after tibial fractures. These key findings identify that glabridin therapies confer protection against inducing and sustaining fracture-elicited chronic allodynia by suppressing fractalkine/CX3CR1-dependent spinal microgliosis and spine morphogenesis, suggesting that glabridin is a promising candidate in the translational development of chronic fracture pain control.

Microbiological and Physiological Effects of Pain

Pain is an important innate defense mechanism that can dramatically alter a person's quality of life. Understanding the microbiological and physiological effects of pain may be important in the pursuit of novel pain interventions. The three descriptors of pain recognized by the International Association for the Study of Pain are nociceptive, neuropathic, and nociplastic pain. Our review examined the current understanding of all three pain types, focusing on the key molecules involved in the manifestation of each type as well as physiological effects. Additionally, we compared the differences in painful and painless neuropathies and discussed the neuroimmune interaction involved in the manifestation of pain.

IL-33/ST2 antagonizes STING signal transduction via autophagy in response to acetaminophen-mediated toxicological immunity

BACKGROUND

Interleukin-33 (IL-33), defined as "alarming", exert diverse functions through signaling via the suppression of tumorigenicity 2 (ST2). However, the physiological roles of IL-33/ST2 signaling during acetaminophen (APAP)-induced liver injury are still poorly understood by modern medicine (AILI). This research aims to explore the relationship between IL-33/ST2 and stimulator of interferon (IFN) response cGAMP interactor 1 (STING)-mediated signal transduction.

METHODS

C57BL/6N mice (WT) and IL-33-deficient mice (KO) were intraperitoneally injected with APAP (250 mg/kg). Recombinant IL-33 (500 ng/mouse) and the cGAS/STING inhibitor RU.521 (200 g/kg) were combined to treat AILI. For mechanistic research in vitro, CRISPR-mediated KD technology, immunoprecipitation, mass spectrometry, and immunofluorescence were utilized.

RESULTS

We discovered that IL-33 deficient mice had increased APAP-induced hepatotoxicity, DNA accumulation, and type 1 IFN production. Mechanistic analysis revealed that IL-33/ST2 enhanced the interaction between Beclin-1 and STING, disrupting STING dimerization, IRF3 phosphorylation, nuclear transport, and IFN-1 gene transcription in HepaRG and Huh7 cells. Beclin-1 interacted with the C-terminus of STING, causing Lys338 acetylation and autophagy degradation of STING. ST2 depletion increased STING signal transduction and IFN-1 promoter activity. Surprisingly, the cGAS/STING inhibitor RU.521 and recombinant IL-33 together improved AILI in vivo.

CONCLUSIONS

These results shed insight on the potential of inhibiting cGAS/STING as a therapy for AILI and emphasize the crucial role of IL-33/ST2 signaling in the regulation of APAP-induced STING signaling. Video Abstract.

Potential Therapeutic Value of the STING Inhibitors

The stimulator of interferon genes (STING) is a critical protein in the activation of the immune system in response to DNA. It can participate the inflammatory response process by modulating the inflammation-preferred translation program through the STING-PKR-like endoplasmic reticulum kinase (PERK)-eIF2α pathway or by inducing the secretion of type I interferons (IFNs) and a variety of proinflammatory factors through the recruitment of TANK-binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3) or the regulation of the nuclear factor kappa-B (NF-κB) pathway. Based on the structure, location, function, genotype, and regulatory mechanism of STING, this review summarizes the potential value of STING inhibitors in the prevention and treatment of infectious diseases, psoriasis, systemic lupus erythematosus, non-alcoholic fatty liver disease, and other inflammatory and autoimmune diseases.

Electrically-evoked oscillating calcium transients in mono- and co-cultures of iPSC glia and sensory neurons

Activated glia are known to exhibit either neuroprotective or neurodegenerative effects, depending on their phenotype, while participating in chronic pain regulation. Until recently, it has been believed that satellite glial cells and astrocytes are electrically slight and process stimuli only through intracellular calcium flux that triggers downstream signaling mechanisms. Though glia do not exhibit action potentials, they do express both voltage- and ligand-gated ion channels that facilitate measurable calcium transients, a measure of their own phenotypic excitability, and support and modulate sensory neuron excitability through ion buffering and secretion of excitatory or inhibitory neuropeptides (i.e., paracrine signaling). We recently developed a model of acute and chronic nociception using co-cultures of iPSC sensory neurons (SN) and spinal astrocytes on microelectrode arrays (MEAs). Until recently, only neuronal extracellular activity has been recorded using MEAs with a high signal-to-noise ratio and in a non-invasive manner. Unfortunately, this method has limited compatibility with simultaneous calcium transient imaging techniques, which is the most common method for monitoring the phenotypic activity of astrocytes. Moreover, both dye-based and genetically encoded calcium indicator imaging rely on calcium chelation, affecting the culture’s long-term physiology. Therefore, it would be ideal to allow continuous and simultaneous direct phenotypic monitoring of both SNs and astrocytes in a high-to-moderate throughput non-invasive manner and would significantly advance the field of electrophysiology. Here, we characterize astrocytic oscillating calcium transients (OCa2+Ts) in mono- and co-cultures of iPSC astrocytes as well as iPSC SN-astrocyte co-cultures on 48 well plate MEAs. We demonstrate that astrocytes exhibit OCa2+Ts in an electrical stimulus amplitude- and duration-dependent manner. We show that OCa2+Ts can be pharmacologically inhibited with the gap junction antagonist, carbenoxolone (100 μM). Most importantly, we demonstrate that both neurons and glia can be phenotypically characterized in real time, repeatedly, over the duration of the culture. In total, our findings suggest that calcium transients in glial populations may serve as a stand-alone or supplemental screening technique for identifying potential analgesics or compounds targeting other glia-mediated pathologies.

USP48 alleviates bone cancer pain and regulates MrgC stabilization in spinal cord neurons of male mice

BACKGROUND

Ubiquitin-mediated degradation of the Mas-related G protein-coupled receptor C (MrgC) reduces the number of receptors. However, the specific deubiquitinating enzyme antagonize this process has not been reported. In this study, we investigated the effect of ubiquitin-specific protease-48 (USP48) on bone cancer pain (BCP) and its effect on MrgC.

METHODS

A mouse model of BCP was established. BCP behaviours of mice were assessed after intrathecal injection of adeno-associated virus (AAV)-USP48. USP48 and MrgC interactions were studied by immunoprecipitation. Overexpression and knockdown of USP48 were conducted in N2a cells to investigate the effect of USP48 on MrgC receptor number and ubiquitination.

RESULTS

Spinal cord level USP48 expression was reduced in mice with BCP. Intrathecal injection of AAV-USP48 increased paw withdrawal mechanical threshold and reduced spontaneous flinching in mice. In N2a cells, there were increased number of MrgC receptors after overexpression of USP48 and decreased number of MrgC receptors after knockdown of USP48. USP48 interacted with MrgC and overexpression of USP48 altered the level of ubiquitination of MrgC.

CONCLUSION

USP48 antagonizes ubiquitin-mediated autophagic degradation of MrgC and alleviates BCP in a murine animal model. Our findings may provide a new perspective for the treatment of BCP.

SIGNIFICANCE

Our finding may provide an important theoretical basis as well as an intervention target for clinical development of drugs for BCP.

A TRPV4-dependent neuroimmune axis in the spinal cord promotes neuropathic pain

Microglia, resident macrophages of the CNS, are essential to brain development, homeostasis, and disease. Microglial activation and proliferation are hallmarks of many CNS diseases, including neuropathic pain. However, molecular mechanisms that govern the spinal neuroimmune axis in the setting of neuropathic pain remain incompletely understood. Here, we show that genetic ablation or pharmacological blockade of transient receptor potential vanilloid type 4 (TRPV4) markedly attenuated neuropathic pain-like behaviors in a mouse model of spared nerve injury. Mechanistically, microglia-expressed TRPV4 mediated microglial activation and proliferation and promoted functional and structural plasticity of excitatory spinal neurons through release of lipocalin-2. Our results suggest that microglial TRPV4 channels reside at the center of the neuroimmune axis in the spinal cord, which transforms peripheral nerve injury into central sensitization and neuropathic pain, thereby identifying TRPV4 as a potential new target for the treatment of chronic pain.

RNA profiling of human dorsal root ganglia reveals sex differences in mechanisms promoting neuropathic pain

Neuropathic pain is a leading cause of high-impact pain, is often disabling and is poorly managed by current therapeutics. Here we focused on a unique group of neuropathic pain patients undergoing thoracic vertebrectomy where the dorsal root ganglia is removed as part of the surgery allowing for molecular characterization and identification of mechanistic drivers of neuropathic pain independently of preclinical models. Our goal was to quantify whole transcriptome RNA abundances using RNA-seq in pain-associated human dorsal root ganglia from these patients, allowing comprehensive identification of molecular changes in these samples by contrasting them with non-pain-associated dorsal root ganglia. We sequenced 70 human dorsal root ganglia, and among these 50 met inclusion criteria for sufficient neuronal mRNA signal for downstream analysis. Our expression analysis revealed profound sex differences in differentially expressed genes including increase of IL1B, TNF, CXCL14 and OSM in male and CCL1, CCL21, PENK and TLR3 in female dorsal root ganglia associated with neuropathic pain. Coexpression modules revealed enrichment in members of JUN-FOS signalling in males and centromere protein coding genes in females. Neuro-immune signalling pathways revealed distinct cytokine signalling pathways associated with neuropathic pain in males (OSM, LIF, SOCS1) and females (CCL1, CCL19, CCL21). We validated cellular expression profiles of a subset of these findings using RNAscope in situ hybridization. Our findings give direct support for sex differences in underlying mechanisms of neuropathic pain in patient populations.

Up-regulation of oxytocin receptors on peripheral sensory neurons mediates analgesia in chemotherapy-induced neuropathic pain

BACKGROUND AND PURPOSE

Chemotherapy-induced neuropathic pain (CINP) currently has limited effective treatment. Although the roles of oxytocin (OXT) and the oxytocin receptor (OXTR) in central analgesia have been well documented, the expression and function of OXTR in the peripheral nervous system remain unclear. Here, we evaluated the peripheral antinociceptive profiles of OXTR in CINP.

EXPERIMENTAL APPROACH

Paclitaxel (PTX) was used to establish CINP. Quantitative real-time polymerase chain reaction (qRT-PCR), in situ hybridization, and immunohistochemistry were used to observe OXTR expression in dorsal root ganglia (DRG). The antinociceptive effects of OXT were assessed by hot-plate and von Frey tests. Whole-cell patch clamp was performed to record sodium currents, excitability of DRG neurons, and excitatory synapse transmission.

KEY RESULTS

Expression of OXTR in DRG neurons was enhanced significantly after PTX treatment. Activation of OXTR exhibited antinociceptive effects, by decreasing the hyperexcitability of DRG neurons in PTX-treated mice. Additionally, OXTR activation up-regulated the phosphorylation of protein kinase C (pPKC) and, in turn, impaired voltage-gated sodium currents, particularly the voltage-gated sodium channel 1.7 (Na_V 1.7) current, that plays an indispensable role in PTX-induced neuropathic pain. OXT suppressed excitatory transmission in the spinal dorsal horn as well as excitatory inputs from primary afferents in PTX-treated mice.

CONCLUSION AND IMPLICATIONS

The OXTR in small-sized DRG neurons is up-regulated in CINP and its activation relieved CINP by inhibiting the neural excitability by impairment of Na_V 1.7 currents via pPKC. Our results suggest that OXTR on peripheral sensory neurons is a potential therapeutic target to relieve CINP.

Specialized Pro-Resolving Mediators as Resolution Pharmacology for the Control of Pain and Itch

Specialized pro-resolving mediators (SPMs), including resolvins, protectins, and maresins, are endogenous lipid mediators that are synthesized from omega-3 polyunsaturated fatty acids during the acute phase or resolution phase of inflammation. Synthetic SPMs possess broad safety profiles and exhibit potent actions in resolving inflammation in preclinical models. Accumulating evidence in the past decade has demonstrated powerful analgesia of exogenous SPMs in rodent models of inflammatory, neuropathic, and cancer pain. Furthermore, endogenous SPMs are produced by sham surgery and neuromodulation (e.g., vagus nerve stimulation). SPMs produce their beneficial actions through multiple G protein-coupled receptors, expressed by immune cells, glial cells, and neurons. Notably, loss of SPM receptors impairs the resolution of pain. I also highlight the emerging role of SPMs in the control of itch. Pharmacological targeting of SPMs or SPM receptors has the potential to lead to novel therapeutics for pain and itch as emerging approaches in resolution pharmacology.

Neuropathic Pain: Mechanisms, Sex Differences, and Potential Therapies for a Global Problem

The study of chronic pain continues to generate ever-increasing numbers of publications, but safe and efficacious treatments for chronic pain remain elusive. Recognition of sex-specific mechanisms underlying chronic pain has resulted in a surge of studies that include both sexes. A predominant focus has been on identifying sex differences, yet many newly identified cellular mechanisms and alterations in gene expression are conserved between the sexes. Here we review sex differences and similarities in cellular and molecular signals that drive the generation and resolution of neuropathic pain. The mix of differences and similarities reflects degeneracy in peripheral and central signaling processes by which neurons, immune cells, and glia codependently drive pain hypersensitivity. Recent findings identifying critical signaling nodes foreshadow the development of rationally designed, broadly applicable analgesic strategies. However, the paucity of effective, safe pain treatments compels targeted therapies as well to increase therapeutic options that help reduce the global burden of suffering.

Driving axon regeneration by orchestrating neuronal and non-neuronal innate immune responses via the IFNγ-cGAS-STING axis

The coordination mechanism of neural innate immune responses for axon regeneration is not well understood. Here, we showed that neuronal deletion of protein tyrosine phosphatase non-receptor type 2 sustains the IFNγ-STAT1 activity in retinal ganglion cells (RGCs) to promote axon regeneration after injury, independent of mTOR or STAT3. DNA-damage-induced cGAMP synthase (cGAS)-stimulator of interferon genes (STINGs) activation is the functional downstream signaling. Directly activating neuronal STING by cGAMP promotes axon regeneration. In contrast to the central axons, IFNγ is locally translated in the injured peripheral axons and upregulates cGAS expression in Schwann cells and infiltrating blood cells to produce cGAMP, which promotes spontaneous axon regeneration as an immunotransmitter. Our study demonstrates that injured peripheral nervous system (PNS) axons can direct the environmental innate immune response for self-repair and that the neural antiviral mechanism can be harnessed to promote axon regeneration in the central nervous system (CNS).

Targeting STING: From antiviral immunity to treat osteoporosis

The cGAS-STING signaling pathway can trigger innate immune responses by detecting dsDNA from outside or within the host. In addition, the cGAS-STING signaling pathway has emerged as a critical mediator of the inflammatory response and a new target for inflammatory diseases. STING activation leads to dimerization and translocation to the endoplasmic reticulum Golgi intermediate compartment or Golgi apparatus catalyzed by TBK1, triggers the production of IRF3 and NF-κB and translocates to the nucleus to induce a subsequent interferon response and pro-inflammatory factor production. Osteoporosis is a degenerative bone metabolic disease accompanied by chronic sterile inflammation. Activating the STING/IFN-β signaling pathway can reduce bone resorption by inhibiting osteoclast differentiation. Conversely, activation of STING/NF-κB leads to the formation of osteoporosis by increasing bone resorption and decreasing bone formation. In addition, activation of STING inhibits the generation of type H vessels with the capacity to osteogenesis, thereby inhibiting bone formation. Here, we outline the mechanism of action of STING and its downstream in osteoporosis and discuss the role of targeting STING in the treatment of osteoporosis, thus providing new ideas for the treatment of osteoporosis.

Antiviral immunity rewired for axon regeneration

In Neuron's January 18^th issue, Wang et al.¹ report that activating the immune signaling IFNγ-cGAS-STING axis promotes axon regeneration in both the peripheral and central nervous systems. Their findings uncover the coordination mechanism of neural innate immune responses for axon regeneration.

Resolvin D2 Reduces Chronic Neuropathic Pain and Bone Cancer Pain via Spinal Inhibition of IL-17 Secretion, CXCL1 Release and Astrocyte Activation in Mice

Chronic pain burdens patients and healthcare systems worldwide. Pain control remains urgently required. IL-17 (interleukin-17)-mediated neuroinflammation is of unique importance in spinal nociceptive transduction in pathological pain development. Recently, resolvin D2 (RvD2), as a bioactive, specialized pro-resolving mediator derived from docosahexaenoic acid, exhibits potent resolution of inflammation in several neurological disorders. This preclinical study evaluates the therapeutic potential and underlying targets of RvD2 in two mouse models of chronic pain, including sciatic nerve ligation-caused neuropathic pain and sarcoma-caused bone cancer pain. Herein, we report that repetitive injections of RvD2 (intrathecal, 500 ng) reduce the initiation of mechanical allodynia and heat hyperalgesia following sciatic nerve damage and bone cancer. Single exposure to RvD2 (intrathecal, 500 ng) attenuates the established neuropathic pain and bone cancer pain. Furthermore, systemic RvD2 (intravenous, 5 μg) therapy is effective in attenuating chronic pain behaviors. Strikingly, RvD2 treatment suppresses spinal IL-17 overexpression, chemokine CXCL1 release and astrocyte activation in mice undergoing sciatic nerve trauma and bone cancer. Pharmacological neutralization of IL-17 ameliorates chronic neuropathic pain and persistent bone cancer pain, as well as reducing spinal CXCL1 release. Recombinant IL-17-evoked acute pain behaviors and spinal CXCL1 release are mitigated after RvD2 administration. In addition, RvD2 treatment dampens exogenous CXCL1-caused transient pain phenotypes. Overall, these current findings identify that RvD2 therapy is effective against the initiation and persistence of long-lasting neuropathic pain and bone cancer pain, which may be through spinal down-modulation of IL-17 secretion, CXCL1 release and astrocyte activation.

Mechanism and effects of STING-IFN-I pathway on nociception: A narrative review

Since the discovery of STING in 2008, numerous studies have investigated its functions in immunity, inflammation, and cancer. STING activates downstream molecules including IFN-I, NLRP3, and NF-κB. The STING-IFN-I pathway plays a vital role in nociception. After receiving the upstream signal, STING is activated and induces the expression of IFN-I, and after paracrine and autocrine signaling, IFN-I binds to IFN receptors. Subsequently, the activity of ion channels is inhibited by TYK2, which induces an acute antinociceptive effect. JAK activates PIK3 and MAPK-MNK-eIF4E pathways, which sensitize nociceptors in the peripheral nervous system. In the mid-late stage, the STING-IFN-I pathway activates STAT, increases pro-inflammatory and anti-inflammatory cytokines, inhibits ER-phagy, and promotes microglial M1-polarization in the central nervous system, leading to central sensitization. Thus, the STING-IFN-I pathway may exert complex effects on nociception at various stages, and these effects require further comprehensive elucidation. Therefore, in this review, we systematically summarized the mechanisms of the STING-IFN-I pathway and discussed its function in nociception.

Transcription factor Nrf2 as a potential therapeutic target for COVID-19

The coronavirus disease 2019 (COVID-19) is caused by a novel severe acute respiratory syndrome (SARS)-like coronavirus (SARS-CoV-2). Critically ill patients with SARS-COV-2 infection frequently exhibit signs of high oxidative stress and systemic inflammation, which accounts for most of the mortality. Antiviral strategies to inhibit the pathogenic consequences of COVID-19 are urgently required. The nuclear factor erythroid 2-related transcription factor (Nrf2) is a transcription factor that is involved in antioxidant and anti-inflammatory defense in several tissues and cells. This review tries to present an overview of the role of Nrf2 in the treatment of COVID-19.

Activation of the STING pathway induces peripheral sensitization via neuroinflammation in a rat model of bone cancer pain

BACKGROUND

Neuroinflammation in the peripheral nervous system has been linked to cancer metastasis-induced bone pain. The stimulator of interferon genes (STING), an innate immune sensor for cytosolic DNA, plays an important role in inflammation and cancer metastasis and is reported to be a critical regulator of nociception. Here, we examined the role of STING in primary nociceptive neurons and chronic pain to determine if it could be a new target for treating bone cancer pain (BCP).

METHODS

Walker 256 cancer cells were injected intratibially to induce bone cancer pain in rats. STING and its downstream inflammatory factors in dorsal root ganglia (DRG) were detected using western blotting and immunofluorescent staining. Transmission electron microscopy and the BCL2-associated X (Bax) expression were used to detect the mitochondrial stress in DRG neurons. C-176, a specific inhibitor of STING, was used to block STING activation and to test the pain behavior.

RESULTS

Mechanical hyperalgesia and spontaneous pain were observed in BCP rats, accompanied by the upregulation of the STING expression in the ipsilateral L4-5 DRG neurons which showed significant mitochondrion stress. The STING/TANK-binding kinase 1 (TBK1)/nuclear factor-kappa B (NF-κB) pathway activation was observed in the DRGs of BCP rats as well as increased IL-1β, IL-6, and TNF-α expression. C-176 alleviated bone cancer pain and reduced the STING and its downstream inflammatory pathway.

CONCLUSION

We provide evidence that STING pathway activation leads to neuroinflammation and peripheral sensitization. Pharmacological blockade of STING may be a promising novel strategy for preventing BCP.

Multifaceted functions of STING in human health and disease: from molecular mechanism to targeted strategy

Since the discovery of Stimulator of Interferon Genes (STING) as an important pivot for cytosolic DNA sensation and interferon (IFN) induction, intensive efforts have been endeavored to clarify the molecular mechanism of its activation, its physiological function as a ubiquitously expressed protein, and to explore its potential as a therapeutic target in a wide range of immune-related diseases. With its orthodox ligand 2'3'-cyclic GMP-AMP (2'3'-cGAMP) and the upstream sensor 2'3'-cGAMP synthase (cGAS) to be found, STING acquires its central functionality in the best-studied signaling cascade, namely the cGAS-STING-IFN pathway. However, recently updated research through structural research, genetic screening, and biochemical assay greatly extends the current knowledge of STING biology. A second ligand pocket was recently discovered in the transmembrane domain for a synthetic agonist. On its downstream outputs, accumulating studies sketch primordial and multifaceted roles of STING beyond its cytokine-inducing function, such as autophagy, cell death, metabolic modulation, endoplasmic reticulum (ER) stress, and RNA virus restriction. Furthermore, with the expansion of the STING interactome, the details of STING trafficking also get clearer. After retrospecting the brief history of viral interference and the milestone events since the discovery of STING, we present a vivid panorama of STING biology taking into account the details of the biochemical assay and structural information, especially its versatile outputs and functions beyond IFN induction. We also summarize the roles of STING in the pathogenesis of various diseases and highlight the development of small-molecular compounds targeting STING for disease treatment in combination with the latest research. Finally, we discuss the open questions imperative to answer.

Model for predicting age-dependent safety and immunomodulatory effects of STING ligands in non-human primates

Stimulator of interferon genes (STING) is a cytoplasmic dinucleotide sensor used as an immunomodulatory agent for cancer treatment. The efficacy of the STING ligand (STING-L) against various tumors has been evaluated in mouse models; however, its safety and efficacy in non-human primates have not been reported. We examined the effects of escalating doses of cyclic-di-adenosine monophosphate (c-di-AMP) or cyclic [G (3',5')pA (3',5'p] (3'-3'-cGAMP) administered intramuscularly or intravenously to cynomolgus macaques. Both ligands induced transient local and systemic inflammatory responses and systemic immunomodulatory responses, including the upregulation of interferon-α (IFN-α) and IFN-γ expression and the activation of multiple immunocompetent cell subsets. Better immunological responses were observed in animals that received c-di-AMP compared with those that received 3'-3'-cGAMP. Multi-parameter analysis using a dataset obtained before administering the ligands predicted the efficacy outcome partially. Importantly, the efficacy of these ligands was reduced in older macaques. We propose that 0.5 mg/kg c-di-AMP via intramuscular administration should be the optimal starting point for clinical studies. Our study is the first to demonstrate the age-dependent safety and efficacy of STING-L in non-human primates and supports the potential of STING-L use as a direct immunomodulator in vivo.

Revisiting the immune landscape post spinal cord injury: More than black and white

Spinal cord injury (SCI) induced catastrophic neurological disability is currently incurable, especially in elderly patients. Due to the limited axon regeneration capacity and hostile microenvironment in the lesion site, essential neural network reconstruction remains challenging. Owing to the blood-spinal cord barrier (BSCB) created immune cells and cytokines isolation, the immune elements were incorrectly recognized as innocent bystanders during the SCI pathological process traditionally. Emerging evidence demonstrated that the central nervous system (CNS) is an "immunological quiescent" rather than "immune privileged" area, and the CNS-associated immune response played mixed roles which dedicate beneficial and detrimental contributions throughout the SCI process. Consequently, coordinating double-edged immunomodulation is vital to promote tissue repair and neurological recovery post-SCI. The comprehensive exploration and understanding of the immune landscape post-SCI are essential in establishing new avenues for further basic and clinical studies. In this context, this review summarizes the recent significant breakthroughs in key aspects of SCI-related immunomodulation, including innate and adaptive immune response, immune organ changes, and holistic immune status modification. Moreover, the currently existing immune-oriented therapies for SCI will be outlined.

MPYS Modulates Fatty Acid Metabolism and Immune Tolerance at Homeostasis Independent of Type I IFNs

MPYS/STING (stimulator of IFN genes) senses cyclic dinucleotides (CDNs), generates type I IFNs, and plays a critical role in infection, inflammation, and cancer. In this study, analyzing genotype and haplotype data from the 1000 Genomes Project, we found that the R71H-G230A-R293Q (HAQ) MPYS allele frequency increased 57-fold in East Asians compared with sub-Saharan Africans. Meanwhile, the G230A-R293Q (AQ) allele frequency decreased by 98% in East Asians compared with sub-Saharan Africans. We propose that the HAQ and AQ alleles underwent a natural selection during the out-of-Africa migration. We used mouse models of HAQ and AQ to investigate the underlying mechanism. We found that the mice carrying the AQ allele, which disappeared in East Asians, had normal CDN-type I IFN responses. Adult AQ mice, however, had less fat mass than did HAQ or wild-type mice on a chow diet. AQ epididymal adipose tissue had increased regulatory T cells and M2 macrophages with protein expression associated with enhanced fatty acid oxidation. Conditional knockout mice and adoptive cell transfer indicate a macrophage and regulatory T cell-intrinsic role of MPYS in fatty acid metabolism. Mechanistically, AQ/IFNAR1-/- mice had a similar lean phenotype as for the AQ mice. MPYS intrinsic tryptophan fluorescence revealed that the R71H change increased MPYS hydrophilicity. Lastly, we found that the second transmembrane (TM) and the TM2-TM3 linker region of MPYS interact with activated fatty acid, fatty acyl-CoA. In summary, studying the evolution of the human MPYS gene revealed an MPYS function in modulating fatty acid metabolism that may be critical during the out-of-Africa migration.

Alternative pathways driven by STING: From innate immunity to lipid metabolism

The Stimulator of Interferon Genes (STING) is a major adaptor protein that is central to the initiation of type I interferon responses and proinflammatory signalling. STING-dependent signalling is triggered by the presence of cytosolic nucleic acids that are generated following pathogen infection or cellular stress. Beyond this central role in controlling immune responses through the production of cytokines and chemokines, recent reports have uncovered inflammation-independent STING functions. Amongst these, a rapidly growing body of evidence demonstrates a key role of STING in controlling metabolic pathways at several levels. Since immunity and metabolic homeostasis are tightly interconnected, these findings deepen our understanding of the involvement of STING in human pathologies. Here, we discuss these findings and reflect on their impact on our current understanding of how nucleic acid immunity controls homeostasis and promotes pathological outcomes.

STING/NF-κB/IL-6-Mediated Inflammation in Microglia Contributes to Spared Nerve Injury (SNI)-Induced Pain Initiation

Innate immune response acts as the first line of host defense against damage and is initiated following the recognition of pathogen-associated molecular patterns (PAMPs). For double-stranded DNA (dsDNA) sensing, interferon gene stimulator (STING) was discovered to be an integral sensor and could mediate the immune and inflammatory response. Selective STING antagonist C-176 was administered and pain behaviors were assessed following spared nerve injury (SNI)-induced neuropathic pain. The level of serum dsDNA following neuropathic pain was assessed using Elisa analysis. STING signaling pathway, microglia activation, and proinflammatory cytokines were assessed by qPCR, western blots, Elisa, and immunofluorescence staining. STING agonist DMXAA was introduced into BV-2 cells to assess the inflammatory response in microglial cells. dsDNA was significantly increased following SNI and STING/TANK-binding kinase 1 (TBK1)/nuclear factor-kappa B (NF-κB) pathway was activated in vivo and vitro. Early but not the late intrathecal injection of C-176 attenuated SNI-induced pain hypersensitivity, microglia activation, proinflammatory factors, and phosphorylated JAK2/STAT3 in the spinal cord dorsal horn, and the analgesic effect of C-176 was greatly abolished by recombinant IL-6 following SNI. We provided evidence clarifying dsDNA mediated activation of microglia STING signaling pathway, after which promoting expression of proinflammatory cytokines that are required for hyperalgesia initiation in the spinal cord dorsal horn of SNI model. Further analysis showed that microglial STING/TBK1/NF-κB may contribute to pain initiation via IL-6 signaling. Pharmacological blockade of STING may be a promising target in the treatment of initiation of neuropathic pain.

Hydrogen-Rich Saline Attenuates Chronic Allodynia after Bone Fractures via Reducing Spinal CXCL1/CXCR2-Mediated Iron Accumulation in Mice

PURPOSE

Neuroinflammation often initiates iron overload in the pathogenesis of neurological disorders. Chemokine-driven neuroinflammation is required for central sensitization and chronic allodynia following fractures, but specific molecular modulations are elusive. This present study explored whether hydrogen-rich saline, as one potent anti-inflammatory pharmaceutical, could alleviate fracture-caused allodynia by suppressing chemokine CXCL1 expression and iron overload.

METHODS

A mouse model of tibial fracture with intramedullary pinning was employed for establishing chronic allodynia. Three applications of hydrogen-rich saline (1, 5 or 10 mL/kg) were administrated intraperitoneally on a daily basis from days 4 to 6 following fractures. Spinal CXCL1 and its receptor CXCR2 levels, transferrin receptor 1 (TfR1) expression and iron concentration were examined. Recombinant CXCL1, a selective CXCR2 antagonist and an iron chelator were used for verification of mechanisms.

RESULTS

Repetitive injections of hydrogen-rich saline (5 and 10 mL/kg but not 1 mL/kg) prevent fracture-caused mechanical allodynia and cold allodynia in a dose-dependent manner. Single exposure to hydrogen-rich saline (10 mL/kg) on day 14 after orthopedic surgeries controls the established persistent fracture allodynia. Furthermore, hydrogen-rich saline therapy reduces spinal CXCL1/CXCR2 over-expression and TfR1-mediated iron accumulation in fracture mice. Spinal CXCR2 antagonism impairs allodynia and iron overload following fracture surgery. Intrathecal delivery of recombinant CXCL1 induces acute allodynia and spinal iron overload, which is reversed by hydrogen-rich saline. Moreover, iron chelation alleviates exogenous CXCL1-induced acute pain behaviors.

CONCLUSIONS

These findings identify that hydrogen-rich saline confers protection against fracture-caused chronic allodynia via spinal down-modulation of CXCL1-dependent TfR1-mediated iron accumulation in mice.

Emerging Roles of Type-I Interferons in Neuroinflammation, Neurological Diseases, and Long-Haul COVID

Interferons (IFNs) are pleiotropic cytokines originally identified for their antiviral activity. IFN-α and IFN-β are both type I IFNs that have been used to treat neurological diseases such as multiple sclerosis. Microglia, astrocytes, as well as neurons in the central and peripheral nervous systems, including spinal cord neurons and dorsal root ganglion neurons, express type I IFN receptors (IFNARs). Type I IFNs play an active role in regulating cognition, aging, depression, and neurodegenerative diseases. Notably, by suppressing neuronal activity and synaptic transmission, IFN-α and IFN-β produced potent analgesia. In this article, we discuss the role of type I IFNs in cognition, neurodegenerative diseases, and pain with a focus on neuroinflammation and neuro-glial interactions and their effects on cognition, neurodegenerative diseases, and pain. The role of type I IFNs in long-haul COVID-associated neurological disorders is also discussed. Insights into type I IFN signaling in neurons and non-neuronal cells will improve our treatments of neurological disorders in various disease conditions.

Low-dose ganciclovir ameliorates dextran sulfate sodium-induced ulcerative colitis through inhibiting macrophage STING activation in mice

Ganciclovir (GCV) is a prodrug nucleoside analogue and is clinically used as antiviral drug for the treatment of cytomegalovirus (CMV) and other infections. Based on the potential anti-inflammatory activity of GCV, this study aimed to investigate the therapeutic effects of ganciclovir on dextran sulfate sodium (DSS)-induced ulcerative colitis (UC), which may involve cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathways. Our results demonstrated that incubation of GCV (50 μM) inhibited cGAS-STING pathway in macrophage RAW264.7 cells. Then, it was found that intestinal cGAS-STING pathways were upregulated in UC patients, Crohn's disease colitis (CD) patients, and DSS-induced colitis mice. Intraperitoneal injection of low-dose GCV (10 mg/kg/day) attenuated DSS-induced colitis and abdominal pain in mice. GCV treatment significantly inhibited the upregulation of cGAS-STING pathway in DSS-induced colitis mice. Moreover, DSS-induced colitis and gut dysbiosis was markedly attenuated in STING deficient mice compared with that of wild-type (WT) mice. Finally, there was lacking therapeutic effect of GCV on DSS-induced colitis in STING deficient mice. Together, our results indicated that low-dose GCV ameliorated DSS-induced UC in mice, possibly through inhibiting STING signaling in colonic macrophages, indicating that GCV may be useful for the treatment of UC.

Nociceptor neurons affect cancer immunosurveillance

Solid tumours are innervated by nerve fibres that arise from the autonomic and sensory peripheral nervous systems1-5. Whether the neo-innervation of tumours by pain-initiating sensory neurons affects cancer immunosurveillance remains unclear. Here we show that melanoma cells interact with nociceptor neurons, leading to increases in their neurite outgrowth, responsiveness to noxious ligands and neuropeptide release. Calcitonin gene-related peptide (CGRP)-one such nociceptor-produced neuropeptide-directly increases the exhaustion of cytotoxic CD8+ T cells, which limits their capacity to eliminate melanoma. Genetic ablation of the TRPV1 lineage, local pharmacological silencing of nociceptors and antagonism of the CGRP receptor RAMP1 all reduced the exhaustion of tumour-infiltrating leukocytes and decreased the growth of tumours, nearly tripling the survival rate of mice that were inoculated with B16F10 melanoma cells. Conversely, CD8+ T cell exhaustion was rescued in sensory-neuron-depleted mice that were treated with local recombinant CGRP. As compared with wild-type CD8+ T cells, Ramp1-/- CD8+ T cells were protected against exhaustion when co-transplanted into tumour-bearing Rag1-deficient mice. Single-cell RNA sequencing of biopsies from patients with melanoma revealed that intratumoral RAMP1-expressing CD8+ T cells were more exhausted than their RAMP1-negative counterparts, whereas overexpression of RAMP1 correlated with a poorer clinical prognosis. Overall, our results suggest that reducing the release of CGRP from tumour-innervating nociceptors could be a strategy to improve anti-tumour immunity by eliminating the immunomodulatory effects of CGRP on cytotoxic CD8+ T cells.

Activation of GPR183 by 7α,25-Dihydroxycholesterol Induces Behavioral Hypersensitivity through Mitogen-Activated Protein Kinase and Nuclear Factor-κB

Emerging evidence implicates the G-protein coupled receptor (GPCR) GPR183 in the development of neuropathic pain. Further investigation of the signaling pathways downstream of GPR183 is needed to support the development of GPR183 antagonists as analgesics. In rodents, intrathecal injection of its ligand, 7α,25-dihydroxycholesterol (7α,25-OHC), causes time-dependent development of mechano-and cold- allodynia (behavioral hypersensitivity). These effects are blocked by the selective small molecule GPR183 antagonist, SAE-14. However, the molecular mechanisms engaged downstream of GPR183 in the spinal cord are not known. Here, we show that 7α,25-OHC-induced behavioral hypersensitivity is Gα i dependent, but not β-arrestin 2-dependent. Non-biased transcriptomic analyses of dorsal-horn spinal cord (DH-SC) tissues harvested at the time of peak hypersensitivity implicate potential contributions of mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB). In support, we found that the development of 7α,25-OHC/GPR183-induced mechano-allodynia was associated with significant activation of MAPKs (extracellular signal-regulated kinase [ERK], p38) and redox-sensitive transcription factors (NF-κB) and increased formation of inflammatory and neuroexcitatory cytokines. SAE-14 blocked these effects and behavioral hypersensitivity. Our findings provide novel mechanistic insight into how GPR183 signaling in the spinal cord produces hypersensitivity through MAPK and NF-κB activation. SIGNIFICANCE STATEMENT: Using a multi-disciplinary approach, we have characterized the molecular mechanisms underpinning 7α,25-OHC/GPR183-induced hypersensitivity in mice. Intrathecal injections of the GPR183 agonist 7α,25-OHC induce behavioral hypersensitivity, and these effects are blocked by the selective GPR183 antagonist SAE-14. We found that 7α,25-OHC-induced allodynia is dependent on MAPK and NF-κB signaling pathways and results in an increase in pro-inflammatory cytokine expression. This study provides a first insight into how GPR183 signaling in the spinal cord is pronociceptive.

STING Contributes to Cancer-Induced Bone Pain by Promoting M1 Polarization of Microglia in the Medial Prefrontal Cortex

The medial prefrontal cortex (mPFC) is the main cortical area for processing both sensory and affective aspects of pain. Recently, mPFC was reported to participate in cancer-induced bone pain (CIBP) via the mechanism of central inflammation. STING is a key component of neuroinflammation in the central neuron system by activating downstream TBK1 and NF-κB signaling pathways. We aimed to investigate whether STING regulated neuroinflammation in the mPFC in rat models of CIBP. It is worth noting that we found a significant upregulation of STING in the mPFC after CIBP, accompanied by activation of TBK1 and NF-κB signaling pathways. In addition, pain and anxiety-like behaviors were alleviated by intraperitoneal injection of the STING inhibitor C-176. Furthermore, in microglia GMI-R1 cells, C-176 reversed LPS-induced M1 polarization. Collectively, this evidence indicated that STING may contribute to cancer-induced bone pain by activating TBK1 and NF-κB, and by promoting M1 polarization of microglia in the mPFC.

Pharmacological inhibition of the cGAS-STING signaling pathway suppresses microglial M1-polarization in the spinal cord and attenuates neuropathic pain

Neuroinflammation plays a vital role in the development of neuropathic pain and is mediated mainly by microglia. Suppressing microglial M1-polarization attenuates neuropathic pain. Recently, the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway has emerged as a key mediator of inflammation and shows potential in modulating microglial polarization. In this study, we evaluated whether cGAS-STING is a potential therapeutic target. Spared nerve injury (SNI) surgery was conducted in adult male rats to establish a neuropathic pain model. We showed that SNI promoted microglial M1-polarization and induced cGAS-STING pathway activation in the spinal cord. Double-label immunofluorescence assays showed that cGAS-STING activation mainly occurred in neurons and microglia but not astrocytes. We further conducted in vitro experiments using BV-2 microglial cells. The results showed that LPS-induced microglial M1-polarization was accompanied by cGAS-STING pathway activation, but cGAS-STING inhibition by antagonists suppressed LPS-induced microglial M1-polarization. In vivo, we also showed that a cGAS antagonist and a STING antagonist suppressed the microglial M1-polarization and ameliorated the mechanical allodynia induced by SNI. These findings suggested that the cGAS-STING pathway might be a potential therapeutic target for treating neuropathic pain. However, further research is warranted to verify our findings in female rodents.

Neurotrophin Pathway Receptors NGFR and TrkA Control Perineural Invasion, Metastasis, and Pain in Oral Cancer

Oral squamous cell carcinoma (OSCC) patients suffer from poor survival due to metastasis or locoregional recurrence, processes that are both facilitated by perineural invasion (PNI). OSCC has higher rates of PNI than other cancer subtypes, with PNI present in 80% of tumors. Despite the impact of PNI on oral cancer prognosis and pain, little is known about the genes that drive PNI, which in turn drive pain, invasion, and metastasis. In this study, clinical data, preclinical, and in vitro models are leveraged to elucidate the role of neurotrophins in OSCC metastasis, PNI, and pain. The expression data in OSCC patients with metastasis, PNI, or pain demonstrate dysregulation of neurotrophin genes. TrkA and nerve growth factor receptor (NGFR) are focused, two receptors that are activated by NGF, a neurotrophin expressed at high levels in OSCC. It is demonstrated that targeted knockdown of these two receptors inhibits proliferation and invasion in an in vitro and preclinical model of OSCC, and metastasis, PNI, and pain. It is further determined that TrkA knockdown alone inhibits thermal hyperalgesia, whereas NGFR knockdown alone inhibits mechanical allodynia. Collectively the results highlight the ability of OSCC to co-opt different components of the neurotrophin pathway in metastasis, PNI, and pain.

Chronic Pain after Bone Fracture: Current Insights into Molecular Mechanisms and Therapeutic Strategies

Bone fracture following traumatic injury or due to osteoporosis is characterized by severe pain and motor impairment and is a major cause of global mortality and disability. Fracture pain often originates from mechanical distortion of somatosensory nerve terminals innervating bones and muscles and is maintained by central sensitization. Chronic fracture pain (CFP) after orthopedic repairs is considered one of the most critical contributors to interference with the physical rehabilitation and musculoskeletal functional recovery. Analgesics available for CFP in clinics not only have poor curative potency but also have considerable side effects; therefore, it is important to further explore the pathogenesis of CFP and identify safe and effective therapies. The typical physiopathological characteristics of CFP are a neuroinflammatory response and excitatory synaptic plasticity, but the specific molecular mechanisms involved remain poorly elucidated. Recent progress has deepened our understanding of the emerging properties of chemokine production, proinflammatory mediator secretion, caspase activation, neurotransmitter release, and neuron-glia interaction in initiating and sustaining synaptogenesis, synaptic strength, and signal transduction in central pain sensitization, indicating the possibility of targeting neuroinflammation to prevent and treat CFP. This review summarizes current literature on the excitatory synaptic plasticity, microgliosis, and microglial activation-associated signaling molecules and discusses the unconventional modulation of caspases and stimulator of interferon genes (STING) in the pathophysiology of CFP. We also review the mechanisms of action of analgesics in the clinic and their side effects as well as promising therapeutic candidates (e.g., specialized pro-resolving mediators, a caspase-6 inhibitor, and a STING agonist) for pain relief by the attenuation of neuroinflammation with the aim of better managing patients undergoing CFP in the clinical setting.

What role of the cGAS-STING pathway plays in chronic pain?

Chronic pain interferes with daily functioning and is frequently accompanied by depression. Currently, traditional clinic treatments do not produce satisfactory analgesic effects and frequently result in various adverse effects. Pathogen recognition receptors (PRRs) serve as innate cellular sensors of danger signals, sense invading microorganisms, and initiate innate and adaptive immune responses. Among them, cGAS-STING alerts on the presence of both exogenous and endogenous DNA in the cytoplasm, and this pathway has been closely linked to multiple diseases, including auto-inflammation, virus infection, and cancer. An increasing numbers of evidence suggest that cGAS-STING pathway involves in the chronic pain process; however, its role remains controversial. In this narrative review, we summarize the recent findings on the involvement of the cGAS-STING pathway in chronic pain, as well as several possible mechanisms underlying its activation. As a new area of research, this review is unique in considering the cGAS-STING pathway in sensory neurons and glial cells as a part of a broader understanding of pain, including potential mechanisms of inflammation, immunity, apoptosis, and autophagy. It will provide new insight into the treatment of pain in the future.

Immune-Triggered Forms of Plasticity Across Brain Regions

Immune cells play numerous roles in the host defense against the invasion of microorganisms and pathogens, which induces the release of inflammatory mediators (e.g., cytokines and chemokines). In the CNS, microglia is the major resident immune cell. Recent efforts have revealed the diversity of the cell types and the heterogeneity of their functions. The refinement of the synapse structure was a hallmark feature of the microglia, while they are also involved in the myelination and capillary dynamics. Another promising feature is the modulation of the synaptic transmission as synaptic plasticity and the intrinsic excitability of neurons as non-synaptic plasticity. Those modulations of physiological properties of neurons are considered induced by both transient and chronic exposures to inflammatory mediators, which cause behavioral disorders seen in mental illness. It is plausible for astrocytes and pericytes other than microglia and macrophage to induce the immune-triggered plasticity of neurons. However, current understanding has yet achieved to unveil what inflammatory mediators from what immune cells or glia induce a form of plasticity modulating pre-, post-synaptic functions and intrinsic excitability of neurons. It is still unclear what ion channels and intracellular signaling of what types of neurons in which brain regions of the CNS are involved. In this review, we introduce the ubiquitous modulation of the synaptic efficacy and the intrinsic excitability across the brain by immune cells and related inflammatory cytokines with the mechanism for induction. Specifically, we compare neuro-modulation mechanisms by microglia of the intrinsic excitability of cerebellar Purkinje neurons with cerebral pyramidal neurons, stressing the inverted directionality of the plasticity. We also discuss the suppression and augmentation of the extent of plasticity by inflammatory mediators, as the meta-plasticity by immunity. Lastly, we sum up forms of immune-triggered plasticity in the different brain regions with disease relevance. Together, brain immunity influences our cognition, sense, memory, and behavior via immune-triggered plasticity.

Activation of STING in the pancreatic tumor microenvironment: A novel therapeutic opportunity

Pancreatic ductal adenocarcinoma (PDAC) is a cancer of poor prognosis that presents with a dense desmoplastic stroma that contributes to therapeutic failure. PDAC patients are mostly unresponsive to immunotherapy. However, hopes to elicit response to immunotherapy have emerged with novel strategies targeting the Stimulator of Interferon Genes (STING) protein, which is a major regulator of tumor-associated inflammation. Combination of STING agonists with conventional immunotherapy approaches has proven to potentiate therapeutic benefits in several cancers. However, recent data underscore that the output of STING activation varies depending on the cellular and tissue context. This suggests that tumor heterogeneity, and in particular the heterogeneity of the tumor microenvironment (TME), is a key factor determining whether STING activation would bear benefits for patients. In this review, we discuss the potential benefits of STING activation in PDAC. To this aim, we describe the major components of the PDAC TME, and the expected consequences of STING activation.

Artesunate Therapy Alleviates Fracture-Associated Chronic Pain After Orthopedic Surgery by Suppressing CCL21-Dependent TREM2/DAP12 Inflammatory Signaling in Mice

Chronic pain after bone fracture and orthopedic surgery is often refractory to most analgesics currently in use, thus emphasizing the urgent need for improved therapeutic medications. Chemokine-dependent neuroinflammation is critical for excitatory synaptic plasticity and central nociception sensitization. Recent studies have focused on the inhibition of inflammatory responses by artesunate, the first anti-malaria drug extracted from artemisinin. The present study investigated the analgesic effects and potential targets of artesunate in a mouse model of chronic pain induced by tibial fracture and orthopedic surgery. Three injections of artesunate were intrathecally administered on a daily basis from days 4 to 6 after fracture. We reported that repetitive exposure to artesunate (10 and 100 μg but not 1 μg) dose-dependently prevented fracture-induced mechanical and cold allodynia. Moreover, single intrathecal injection of artesunate (100 μg) alleviated the established chronic pain on day 14 after fracture surgery. Intraperitoneal artesunate (10 and 50 mg kg^-1) therapy was effective against chronic fracture pain. Intriguingly, artesunate inhibited the upregulation of spinal chemokine CCL21, triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12) expressions and microglia activation in fracture mice. Furthermore, spinal CCL21 neutralization attenuated the severity of fracture-associated post-surgical pain. Exogenous CCL21-induced acute inflammatory pain was impaired by artesunate therapy. Additionally, the pharmacological blockage of TREM2 reduced recombinant CCL21-elicited behavioral hypernociception. The present findings demonstrate that artesunate therapy reduces the initiation and maintenance of fracture-associated chronic postoperative pain by inhibiting CCL21-dependent TREM2/DAP12 inflammatory signaling and microglia activation, thus suggesting that artesunate could emerge as a therapeutic strategy for fracture pain management.

Dexmedetomidine and Ketamine Attenuated Neuropathic Pain Related Behaviors via STING Pathway to Induce ER-Phagy

Our previous work indicated that ER-phagy level had altered in spinal nerve ligation (SNL) rats. In this study, we investigated whether dexmedetomidine or ketamine exhibits anti-anxiety or anti-nociceptive effects via modulation of the spinal STING/TBK pathway to alter ER-phagy in SNL rats. We evaluated the analgesic and anti-anxiety effects of ketamine and dexmedetomidine in SNL rats. 2’3’-cGAMP (a STING pathway agonist) was administrated to investigate whether enhanced spinal STING pathway activation could inhibit dexmedetomidine or ketamine treatment effects in SNL rats. Analgesic effects were assessed with the mechanical withdrawal threshold (MWT) and anti-anxiety effects were measured via an open field test (OFT). Protein expression levels were evaluated by immunoblotting. Distribution and cellular localization of Grp78 (ER stress marker) were evaluated by confocal immunofluorescence. SNL induced mechanical hypersensitivity and anxiety in rats; dexmedetomidine and ketamine both provided analgesia and anti-anxiety effects in SNL rats. Furthermore, the STING pathway was involved in the modulation of ER stress and ER-phagy in SNL rats and dexmedetomidine and ketamine alleviated ER stress by inhibiting STING pathway to enhance ER-phagy. Thus, both ketamine and dexmedetomidine provided anti-anxiety and anti-nociceptive effects by alleviating ER stress through the inhibition of the STING/TBK pathway to modulate spinal ER-phagy in SNL rats.

A novel spinal neuron connection for heat sensation

Heat perception enables acute avoidance responses to prevent tissue damage and maintain body thermal homeostasis. Unlike other modalities, how heat signals are processed in the spinal cord remains unclear. By single-cell gene profiling, we identified ErbB4, a transmembrane tyrosine kinase, as a novel marker of heat-sensitive spinal neurons in mice. Ablating spinal ErbB4+ neurons attenuates heat sensation. These neurons receive monosynaptic inputs from TRPV1+ nociceptors and form excitatory synapses onto target neurons. Activation of ErbB4+ neurons enhances the heat response, while inhibition reduces the heat response. We showed that heat sensation is regulated by NRG1, an activator of ErbB4, and it involves dynamic activity of the tyrosine kinase that promotes glutamatergic transmission. Evidence indicates that the NRG1-ErbB4 signaling is also engaged in hypersensitivity of pathological pain. Together, these results identify a spinal neuron connection consisting of ErbB4+ neurons for heat sensation and reveal a regulatory mechanism by the NRG1-ErbB4 signaling.

Protein Tyrosine Phosphatase Receptor Type D Regulates Neuropathic Pain After Nerve Injury via the STING-IFN-I Pathway

Neuropathic pain is usually caused by injury or dysfunction of the somatosensory system, and medicine is a common way of treatment. Currently, there are still no satisfactory drugs, like opioids and lidocaine, which carry a high risk of addiction. Protein tyrosine phosphatase receptor type D (PTPRD) is a known therapeutic target in addiction pathways and small molecule inhibitors targeting it, such as 7-butoxy illudalic acid analog (7-BIA), have recently been developed to tackle addition. PTPRD is also upregulated in the dorsal root ganglion (DRG) in a rat model of neuropathic pain, but is not yet clear whether PTPRD contributes to the development of neuropathic pain. Here, we established a chronic constriction injury (CCI) and evaluated PTPRD expression and its association with neuropathic pain. PTPRD expression was found to gradually increase after CCI in DRGs, and its expression was concomitant with the progressive development of hypersensitivity as assessed by both mechanical and thermal stimuli. Both PTPRD knockdown and administration of PTPRD inhibitor 7-BIA alleviated CCI-induced neuropathic pain while upregulating STING and IFN-α in the DRG. Treatment with H-151, a STING inhibitor, abolished the analgesic effects of PTPRD knockdown. Taken together, our study suggests that increased levels of PTPRD in the DRG following CCI are involved in the development of neuropathic pain via the STING-IFN-I pathway. 7-BIA, a small molecule inhibitor of PTPRD with anti-addiction effects, may represent a novel and safe therapeutic strategy for the clinical management of neuropathic pain without the risk of addiction.

Inhibiting Hv1 channel in peripheral sensory neurons attenuates chronic inflammatory pain and opioid side effects

Both opioids and nonsteroidal anti-inflammatory drugs (NSAIDS) produce deleterious side effects and fail to provide sustained relief in patients with chronic inflammatory pain. Peripheral neuroinflammation (PN) is critical for initiation and development of inflammatory pain. A better understanding of molecular mechanisms underlying PN would facilitate the discovery of new analgesic targets and the development of new therapeutics. Emerging evidence suggests that peripheral sensory neurons are not only responders to painful stimuli, but are also actively engaged in inflammation and immunity, whereas the intrinsic regulatory mechanism is poorly understood. Here we report the expression of proton-selective ion channel Hv1 in peripheral sensory neurons in rodents and humans, which was previously shown as selectively expressed in microglia in mammalian central nervous system. Neuronal Hv1 was up-regulated by PN or depolarizing stimulation, which in turn aggravates inflammation and nociception. Inhibiting neuronal Hv1 genetically or by a newly discovered selective inhibitor YHV98-4 reduced intracellular alkalization and ROS production in inflammatory pain, mitigated the imbalance in downstream SHP-1-pAKT signaling, and also diminished pro-inflammatory chemokine release to alleviate nociception and morphine-induced hyperalgesia and tolerance. Thus, our data reveal neuronal Hv1 as a novel target in analgesia strategy and managing opioids-related side effects.

Transcriptomic Profiling in Mice With CB1 receptor Deletion in Primary Sensory Neurons Suggests New Analgesic Targets for Neuropathic Pain

Type 1 and type 2 cannabinoid receptors (CB1 and CB2, respectively) mediate cannabinoid-induced analgesia. Loss of endogenous CB1 is associated with hyperalgesia. However, the downstream targets affected by ablation of CB1 in primary sensory neurons remain unknown. In the present study, we hypothesized that conditional knockout of CB1 in primary sensory neurons (CB1cKO) alters downstream gene expression in the dorsal root ganglion (DRG) and that targeting these pathways alleviates neuropathic pain. We found that CB1cKO in primary sensory neurons induced by tamoxifen in adult Advillin-Cre:CB1-floxed mice showed persistent hyperalgesia. Transcriptome/RNA sequencing analysis of the DRG indicated that differentially expressed genes were enriched in energy regulation and complement and coagulation cascades at the early phase of CB1cKO, whereas pain regulation and nerve conduction pathways were affected at the late phase of CB1cKO. Chronic constriction injury in mice induced neuropathic pain and changed transcriptome expression in the DRG of CB1cKO mice, and differentially expressed genes were mainly associated with inflammatory and immune-related pathways. Nerve injury caused a much larger increase in CB2 expression in the DRG in CB1cKO than in wildtype mice. Interfering with downstream target genes of CB1, such as antagonizing CB2, inhibited activation of astrocytes, reduced neuroinflammation, and alleviated neuropathic pain. Our results demonstrate that CB1 in primary sensory neurons functions as an endogenous analgesic mediator. CB2 expression is regulated by CB1 and may be targeted for the treatment of neuropathic pain.

Innate Receptors Expression by Lung Nociceptors: Impact on COVID-19 and Aging

The lungs are constantly exposed to non-sterile air which carries harmful threats, such as particles and pathogens. Nonetheless, this organ is equipped with fast and efficient mechanisms to eliminate these threats from the airways as well as prevent pathogen invasion. The respiratory tract is densely innervated by sensory neurons, also known as nociceptors, which are responsible for the detection of external stimuli and initiation of physiological and immunological responses. Furthermore, expression of functional innate receptors by nociceptors have been reported; however, the influence of these receptors to the lung function and local immune response is poorly described. The COVID-19 pandemic has shown the importance of coordinated and competent pulmonary immunity for the prevention of pathogen spread as well as prevention of excessive tissue injury. New findings suggest that lung nociceptors can be a target of SARS-CoV-2 infection; what remains unclear is whether innate receptor trigger sensory neuron activation during SARS-CoV-2 infection and what is the relevance for the outcomes. Moreover, elderly individuals often present with respiratory, neurological and immunological dysfunction. Whether aging in the context of sensory nerve function and innate receptors contributes to the disorders of these systems is currently unknown. Here we discuss the expression of innate receptors by nociceptors, particularly in the lungs, and the possible impact of their activation on pulmonary immunity. We then demonstrate recent evidence that suggests lung sensory neurons as reservoirs for SARS-CoV-2 and possible viral recognition via innate receptors. Lastly, we explore the mechanisms by which lung nociceptors might contribute to disturbance in respiratory and immunological responses during the aging process.

Neuro-Immunity and Gut Dysbiosis Drive Parkinson's Disease-Induced Pain

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1-2% of the population aged 65 and over. Additionally, non-motor symptoms such as pain and gastrointestinal dysregulation are also common in PD. These impairments might stem from a dysregulation within the gut-brain axis that alters immunity and the inflammatory state and subsequently drives neurodegeneration. There is increasing evidence linking gut dysbiosis to the severity of PD's motor symptoms as well as to somatosensory hypersensitivities. Altogether, these interdependent features highlight the urgency of reviewing the links between the onset of PD's non-motor symptoms and gut immunity and whether such interplays drive the progression of PD. This review will shed light on maladaptive neuro-immune crosstalk in the context of gut dysbiosis and will posit that such deleterious interplays lead to PD-induced pain hypersensitivity.

IL-23 Enhances C-Fiber-Mediated and Blue Light-Induced Spontaneous Pain in Female Mice

The incidence of chronic pain is especially high in women, but the underlying mechanisms remain poorly understood. Interleukin-23 (IL-23) is a pro-inflammatory cytokine and contributes to inflammatory diseases (e.g., arthritis and psoriasis) through dendritic/T cell signaling. Here we examined the IL-23 involvement in sexual dimorphism of pain, using an optogenetic approach in transgenic mice expressing channelrhodopsin-2 (ChR2) in TRPV1-positive nociceptive neurons. In situ hybridization revealed that compared to males, females had a significantly larger portion of small-sized (100-200 μm2) Trpv1+ neurons in dorsal root ganglion (DRG). Blue light stimulation of a hindpaw of transgenic mice induced intensity-dependent spontaneous pain. At the highest intensity, females showed more intense spontaneous pain than males. Intraplantar injection of IL-23 (100 ng) induced mechanical allodynia in females only but had no effects on paw edema. Furthermore, intraplantar IL-23 only potentiated blue light-induced pain in females, and intrathecal injection of IL-23 also potentiated low-dose capsaicin (500 ng) induced spontaneous pain in females but not males. IL-23 expresses in DRG macrophages of both sexes. Intrathecal injection of IL-23 induced significantly greater p38 phosphorylation (p-p38), a marker of nociceptor activation, in DRGs of female mice than male mice. In THP-1 human macrophages estrogen and chemotherapy co-application increased IL-23 secretion, and furthermore, estrogen and IL-23 co-application, but not estrogen and IL-23 alone, significantly increased IL-17A release. These findings suggest a novel role of IL-23 in macrophage signaling and female-dominant pain, including C-fiber-mediated spontaneous pain. Our study has also provided new insight into cytokine-mediated macrophage-nociceptor interactions, in a sex-dependent manner.

Neuropsychiatric disorders: An immunological perspective

Neuropsychiatric diseases have traditionally been studied from brain, and mind-centric perspectives. However, mounting epidemiological and clinical evidence shows a strong correlation of neuropsychiatric manifestations with immune system activation, suggesting a likely mechanistic interaction between the immune and nervous systems in mediating neuropsychiatric disease. Indeed, immune mediators such as cytokines, antibodies, and complement proteins have been shown to affect various cellular members of the central nervous system in multitudinous ways, such as by modulating neuronal firing rates, inducing cellular apoptosis, or triggering synaptic pruning. These observations have in turn led to the exciting development of clinical therapies aiming to harness this neuro-immune interaction for the treatment of neuropsychiatric disease and symptoms. Besides the clinic, important theoretical fundamentals can be drawn from the immune system and applied to our understanding of the brain and neuropsychiatric disease. These new frameworks could lead to novel insights in the field and further potentiate the development of future therapies to treat neuropsychiatric disease.

African swine fever virus MGF505-11R inhibits type I interferon production by negatively regulating the cGAS-STING-mediated signaling pathway

African swine fever (ASF) is an acute, hemorrhagic, and highly contact infectious disease caused by African swine fever virus (ASFV) infecting domestic pigs or wild boars, the mortality rate up to 100 %. Evasion of host innate immunity plays a vital role in the pathogenesis of ASFV. Studies have showed that the MGF505 genes involve in regulating the IFN-I response, but its mechanism of action remains poorly understood. In our present study, ASFV MGF505-11R inhibited IFN-β and ISRE activation induced by cGAS, IRF7, IRF3-5D, STING, IKKε and TBK1 accompanied by decreases of IFN-β, ISG15 and ISG56 mRNA transcription. ASFV MGF505-11R interacted with STING, degrading STING expression by the lysosomal, ubiquitin-proteasome and autophagy pathways. Moreover, ASFV MGF505-11R could inhibit the phosphorylation of TBK1 and IRF3 stimulated by cGAS/STING overexpression. Finally, the truncation mutation analysis indicated that the 1-191 aa and 182-360 aa of ASFV MGF505-11R could inhibit cGAS-STING-mediated activation of IFN-β promoters. In short, these results demonstrated that ASFV MGF505-11R involved in regulating the IFN-I response by negatively regulating the cGAS signaling pathway. In summary, this study preliminarily clarified the molecular mechanism of ASFV MGF505-11R gene antagonizing IFN-I-mediated antiviral, which will helpfully provide new strategies for treatment and prevention of ASF.