As individuals age, they often experience persistent, unresolved pain, impacting their quality of life. Aging as a process is accompanied by "inflammaging," a state of chronic, low-grade systemic inflammation contributing to various diseases. Understanding the functional link between inflammaging and age-related development of pain is crucial for identifying novel therapeutic targets. We hypothesized that the circulatory milieu plays a role in regulating pain and that inflammaging contributes to changes in pain behavior with age. To test these hypotheses, we monitored nociception and postsurgical pain in male and female mice aged 3 and 24 months and analyzed their serum proteome, including cytokine/chemokine profiles. Our results demonstrated that compared with young mice, aging mice were hyposensitive to mechanical stimulation, yet their pain response to incision was aggravated and prolonged. Serum proteomic analysis revealed sex-specific inflammaging patterns. To explore the link between inflammaging and age-related alteration in pain behavior, we applied a rejuvenation strategy by transferring serum from 3-month-old mice to 19- to 21-month-old mice. Young serum normalized mechanical sensitivity in aged mice, alleviated postsurgical mechanical pain, and promoted recovery. Alongside the improvements in pain behavior phenotype, young serum recalibrated the aging serum profile. It reduced age-associated increases of cytokine/chemokine levels in male mice and rescued age-related, female-selective downregulation of inflammatory pathways such as liver X receptor/retinoid X receptor activation, D24-dehydrocholesterol reductase, and complement signaling. Our findings suggest that the circulatory environment, notably inflammaging, plays a significant role in altered pain behavior of aging mice. The sex-specific signature of age-dependent systemic inflammation highlights the importance of investigating inflammaging through the lens of sexual dimorphism.

Although nerve injury-associated neuroinflammation contributes to neuropathic pain, the long-term impact of such injury on systemic homeostasis and its potential role in pain remains elusive. In this study, we aim to understand the systemic changes that are present alongside chronic pain in nerve-injured male and female mice across their lifespan. We monitored mechanical and cold sensitivity in male and female mice starting at the age of 3-4 months old when they received spared nerve injury (SNI), up to 20-month post-injury. Alongside, we collected blood samples to track changes in immune cells with flow cytometry, and to assess inflammation-related serum proteome using a 111-target Proteome Profiler. We also transferred serum from sham/SNI mice to naïve mice to determine the potential of systemic contribution to pain. While nerve injury did not affect immune cell composition in the blood, it triggered a long-lasting disturbance of molecular profile in the serum of sham/SNI mice, in a sex-dependent manner. Compared to sham surgery, nerve injury amplified regulation of inflammatory proteins in males, but slightly reduced it in females. These changes in the serum occurred in parallel with long-lasting mechanical and cold hypersensitivity in the nerve-injured mice. Both male and female SNI serum induced hypersensitivity when transferred to naïve mice, regardless of a sex-matched or sex-mismatched transfer. Our results highlight that a local nerve injury can have persistent systemic impact. Injury-associated systemic inflammation could contribute to neuropathic pain, but the underlying mechanisms may be sexually dimorphic.

Fibromyalgia is a prevalent syndrome characterized by widespread pain in the absence of evident tissue injury or pathology, making it one of the most mysterious chronic pain conditions. The composition of the gut microbiota in individuals with fibromyalgia differs from that of healthy controls, but its functional role in the syndrome is unknown. Here, we show that fecal microbiota transplantation from fibromyalgia patients, but not from healthy controls, into germ-free mice induces pain and numerous molecular phenotypes that parallel known changes in fibromyalgia patients, including immune activation and metabolomic profile alterations. Replacing the fibromyalgia microbiota with a healthy microbiota substantially alleviated pain in mice. An open-label trial in women with fibromyalgia (Registry MOH_2021-11-04_010374) showed that transplantation of a healthy microbiota is associated with reduced pain and improved quality of life. We conclude that altered gut microbiota has a role in fibromyalgia pain, highlighting it as a promising target for therapeutic interventions.

A significant public health burden is peripheral nerve damage (PNI), which is frequently brought on by trauma. Macrophages were essential to the effective regeneration of nerves and restoration of function. It is still not entirely understood how macrophages and Schwann cells interact after damage during remyelination. Here, we established an inflammatory model in bone marrow-derived macrophages (BMDMs) and a rat sciatic nerve damage model to investigate the possible relationship between lipopolysaccharides (LPS)-induced exosomes derived from Schwann cells (LPS SCs-Exos) and peripheral nerve repair. The pro-inflammatory macrophage was changed into a pro -regeneration macrophage by LPS SC-Exos. Notably, it was discovered that SC-Exos had a substantial enrichment of OTULIN. OTULIN was a key mediator in the regulatory effects of LPS SC-Exos by deubiquitinating ERBB2 and preventing its degradation. The local injection of SC-Exos into the nerve damage site led in a faster functional recovery, axon regeneration and remyelination, and an increased M2 macrophage polarization, whereas OTULIN knockdown reversed these effects in vivo . Our results indicate that LPS SC-Exos may offer a therapeutic avenue for peripheral nerve regeneration by promoting macrophage polarization toward an M2 phenotype through the shuttling of OTULIN and deubiquitination of ERBB2. Significance statement: OTULIN protein from SC-Exos mediated the macrophages polarization and axonal growth in BMDMs through promoting ubiquitination of ERBB2 and triggering the degradation of ERBB2. The findings offered prospective therapeutic hints for PNI therapy approaches that target axonal regrowth.

Fibromyalgia is a chronic syndrome characterized by widespread pain in the absence of evident tissue injury or pathology, making it one of the most mysterious chronic pain conditions. Despite affecting 2–4% of the population, primarily women 1 , the cause and underlying mechanisms of fibromyalgia remain elusive, and effective targeted treatments are currently unavailable. The gut microbiota of women with fibromyalgia differs from healthy controls 2,3 . However, it is unknown whether changes in gut microbiota have a causal role in mediating pain and other symptoms of fibromyalgia. Here, we show that fecal microbiota transplantation (FMT) from individuals with fibromyalgia, but not from healthy controls, into germ-free mice induces persistent pain hypersensitivity. FMT from fibromyalgia patients led to a reduction in intraepidermal nerve fiber density and alterations in the peripheral immune profile, and induced activation of spinal microglia, which contributed to the development of pain in mice. Notably, the pain hypersensitivity in mice that were administered microbiota from fibromyalgia patients resolved after FMT from healthy controls. Consistent with these findings, an open-label pilot study showed that transplanting microbiota from healthy individuals to humans with fibromyalgia alleviated pain and reduced overall symptom severity. Thus, altered gut microbiota has a causal role in fibromyalgia pain, highlighting it as a promising target for therapeutic interventions.

Neuropathic pain is a complex, debilitating disease that results from injury to the somatosensory nervous system. The presence of systemic chronic inflammation has been observed in chronic pain patients, but whether it plays a causative role remains unclear. This study aims to determine the perturbation of systemic homeostasis by an injury to peripheral nerve and its involvement in neuropathic pain. We assessed the proteomic profile in the serum of mice at 1-day and 1-month following partial sciatic nerve injury (PSNL) or sham surgery. We also assessed mouse mechanical and cold sensitivity in naïve mice after receiving intravenous administration of serum from PSNL or sham mice. Mass spectrometry-based proteomic analysis revealed that PSNL resulted in a long-lasting alteration of serum proteome, where the majority of differentially expressed proteins were in inflammation related pathways, involving cytokines/chemokines, autoantibodies and complement factors. While transferring sham serum to naïve mice did not change their pain sensitivity, PSNL serum significantly lowered mechanical thresholds and induced cold hypersensitivity in naïve mice. With broad anti-inflammatory properties, bone marrow cell extracts (BMCE) not only partially restored serum proteomic homeostasis, but also significantly ameliorated PSNL-induced mechanical allodynia, and serum from BMCE-treated PSNL mice no longer induced hypersensitivity in naïve mice. These findings clearly demonstrate that nerve injury has a long-lasting impact on systemic homeostasis, and nerve injury associated systemic inflammation contributes to the development of neuropathic pain.

Activation of microglia in the spinal cord dorsal horn after peripheral nerve injury contributes to the development of pain hypersensitivity. How activated microglia selectively enhance the activity of spinal nociceptive circuits is not well understood. We discovered that after peripheral nerve injury, microglia degrade extracellular matrix structures, perineuronal nets (PNNs), in lamina I of the spinal cord dorsal horn. Lamina I PNNs selectively enwrap spinoparabrachial projection neurons, which integrate nociceptive information in the spinal cord and convey it to supraspinal brain regions to induce pain sensation. Degradation of PNNs by microglia enhances the activity of projection neurons and induces pain-related behaviors. Thus, nerve injury-induced degradation of PNNs is a mechanism by which microglia selectively augment the output of spinal nociceptive circuits and cause pain hypersensitivity.

AIMS:We determined the ability of the multi-chemokine receptor (CCR2/CCR5/CCR8) antagonist RAP-103 to modulate pain behaviors in an acute model of surgical pain, with and without an added opioid (morphine), and by itself in a chronic model of Streptozotocin (STZ)-induced diabetic peripheral neuropathy (DPN). MATERIALS AND METHODS:Pain behaviors were assessed by mechanical and thermal tests in rats. Cytokine and chemokine biomarkers in sciatic nerve and spinal cord were assessed by in situ qPCR. KEY FINDINGS:In the incisional pain assay, RAP-103 (0.01-1 mg/kg, i.p.) alone had no antiallodynic effect post-surgery. RAP-103 (0.5 mg/kg) when co-administered with morphine (0.5-5 mg/kg), reduced the ED50 of morphine from 3.19 mg/kg to 1.42 mg/kg. In a DPN model, rats exhibited persistent mechanical and cold allodynia. Oral administration of RAP-103 (0.5-0.02 mg/kg/day) resulted in a complete reversal of established hypersensitivity in DPN rats (P < .001), which gradually returned to pain hypersensitivity after the cessation of the treatment. The mRNA expression of cytokines, IL-1β, TNFα; chemokines CCL2, CCL3; and chemokine receptors CCR2 and CCR5 in DPN rat sciatic nerve, but not spinal cord, were significantly increased. RAP-103 resulted in significant reductions in sciatic nerve expression of IL-1β, TNFα and CCL3 in STZ-induced diabetic rats with trends toward lower levels for CCL2 and CCR5, while CCR2 was unchanged. SIGNIFICANCE:In acute pain, co-administration of RAP-103 with morphine provided the same antinociceptive effect with a reduced dose of morphine, reducing opioid side-effects and risks. RAP-103 by itself is an effective non-opioid antinociceptive treatment for diabetic neuropathic pain.

Pain in elderly persons is often not adequately treated, and current treatments may lead to poor outcomes. Therefore, new treatment strategies need to be developed based on a better understanding of the mechanisms underlying the development of chronic pain. Recent studies have shown that Tac1 -/- mice display a significant decrease in nociceptive pain responses to moderate or intense stimuli but present no phenotypic changes following light or nonpainful stimuli. Moreover, the deletion of the Tac1 gene led to a deficit of opioid peptides, which are essential to endogenous pain control mechanisms. Thus, we investigated whether Tac1 -/- mice show defective pain modulatory pathways by specifically profiling protein kinases in mice spinal cord using phosphoproteomics and bioinformatics. Protein phosphorylation is a key feature of the cellular regulatory mechanism, and phosphorylation status is related to the regulation and modulation of protein–protein binding. Bioinformatics analysis revealed that MAPK, tyrosine kinase, senescence, interleukin signaling, and TCR signaling are modulated in Tac1 -/- mice. Interestingly, these processes are intimately linked with inflammatory responses leading to the release of cytokines and chemokines implicated in the interactions and communications between cells. They are key players involved in the initiation and persistence of pathologic pain. The absence of the Tac1 gene products may trigger a much wider cell response to compensate for the lack of important components of the nociceptive pain transmission system.

[This corrects the article DOI: 10.3389/fimmu.2021.720733.].

Background While the etiology remains elusive, macrophages and T cells in peripheral nerves are considered as effector cells mediating autoimmune peripheral neuropathy (APN), such as Guillain-Barre syndrome. By recognizing both pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) signals, TLRs play a central role in the initiation of both innate and adaptive immune responses. In this study, we aimed to understand the involvement of TLR4 in the pathogenesis of APN and explore the potential of TLR4 as a drug target for therapeutic use. Methods APN was induced by a partial ligation on one of the sciatic nerves in B7.2 (L31) transgenic mice which possess a predisposed inflammatory background. APN pathology and neurological function were evaluated on the other non-injured sciatic nerve. Results TLR4 and its endogenous ligand HMGB1 were highly expressed in L31 mice, in circulating immune cells and in peripheral nerves. Enhanced TLR4 signaling was blocked with TAK 242, a selective TLR4 inhibitor, before and after disease onset. Intraperitoneal administration of TAK 242 not only inhibited monocyte, macrophage and CD8 + T cell activation, but also reduced the release of pro-inflammatory cytokines. TAK 242 protected mice from severe myelin and axonal loss, resulting in a remarkable improvement in mouse motor and sensory functions. TAK 242 was effective in alleviating the disease in both preventive and reversal paradigms. Conclusion The study identified the critical contribution of TLR4-mediated macrophage activation in disease course and provided strong evidence to support TLR4 as a useful drug target for treating inflammatory autoimmune neuropathy.

One of the major clinical manifestations of peripheral neuropathy, either resulting from trauma or diseases, is chronic pain. While it significantly impacts patients’ quality of life, the underlying mechanisms remain elusive, and treatment is not satisfactory. Systemic chronic inflammation (SCI) that we are referring to in this perspective is a state of low-grade, persistent, non-infective inflammation, being found in many physiological and pathological conditions. Distinct from acute inflammation, which is a protective process fighting against intruders, SCI might have harmful effects. It has been associated with many chronic non-communicable diseases. We hypothesize that SCI could be a predisposing and/or precipitating factor in the development of chronic pain, as well as associated comorbidities. We reviewed evidence from human clinical studies indicating the coexistence of SCI with various types of chronic pain. We also collated existing data about the sources of SCI and who could have it, showing that those individuals or patients having SCI usually have higher prevalence of chronic pain and psychological comorbidities. We thus elaborate on the need for further research in the connection between SCI and chronic pain. Several hypotheses have been proposed to explain these complex interactions.

One hallmark of Guillain-Barre syndrome (GBS), a prototypic autoimmune peripheral neuropathy (APN) is infiltration of leukocytes (macrophages and T cells) into peripheral nerves, where chemokines and their receptors play major roles. In this study, we aimed to understand the potential contribution of chemokine receptors CCR2 and CX3CR1 in APN by using a well-established mouse model, B7.2 transgenic (L31) mice, which possesses a predisposed inflammatory background. We crossbred respectively CCR2KO and CX3CR1KO mice with L31 mice. The disease was initiated by partial ligation on one of the sciatic nerves. APN pathology and neurological function were evaluated on the other non-ligated sciatic nerve/limb. Our results revealed that L31/CX3CR1KO but not L31/CCR2KO mice were resistant to APN. CX3CR1 is needed for maintaining circulating monocyte and CD8+ T cell survival. While migration of a significant number of activated CD8+ T cells to peripheral nerves is essential in autoimmune response in nerve, recruitment of monocytes into PNS seems optional. Disease onset is independent of CCR2 mediated blood-derived macrophage recruitment, which can be replaced by compensatory proliferation of resident macrophages in peripheral nerve. CX3CR1 could also contribute to APN via its critical involvement in maintaining nerve macrophage phagocytic ability. We conclude that blockade of CX3CR1 signaling may represent an interesting anti-inflammatory strategy to improve therapeutic management for GBS patients.

Background Though it is well-known that a high-salt diet (HSD) is associated with many chronic diseases, the effects of long-term high-salt intake on physiological functions and homeostasis remain elusive. In this study, we investigated whether and how an HSD affects mouse nociceptive thresholds, and myeloid cell trafficking and activation. Methods Healthy C57BL/6 male and female mice were fed an HSD (containing 4% NaCl in chow and 1% NaCl in water) from the time of weaning for 3 to 4 months. Circulating monocytes, nerve macrophages, spinal microglia, and associated inflammatory responses were scrutinized using flow cytometry, immunohistochemistry, and quantitative real-time polymerase chain reaction (qPCR) approaches. Mouse pain sensitivity to mechanical stimuli was monitored with von Frey tests along the experimental duration. Results Mice on an HSD have reduced mechanical thresholds. They feel more pain than those on a normal diet (ND), e.g., regular laboratory chow (0.3% NaCl in chow). An HSD induced not only a remarkable expansion of circulating monocytes, CCR2 + Ly6C hi inflammatory monocytes in particular, but also an accumulation of CD11b + F4/80 + macrophages in the peripheral nerves and an activation of Iba-1 + spinal microglia. Replacing an HSD with a ND was unable to reverse the HSD-induced mechanical hypersensitivity or rescue the altered immune responses. However, treating HSD-fed mice with a chemokine receptor CCR2 antagonist effectively normalized the pain thresholds and immune cell profile in the periphery and spinal cord. An HSD failed to alter pain thresholds and myeloid cell activation in CCR2-deficient mice. Spinal microglial activation is required for HSD-induced mechanical hypersensitivity in male, but not in female mice. Conclusion Overall, this study provides evidence that an HSD has a long-term impact on physiological function. CCR2-mediated cellular response, including myeloid cell trafficking and associated inflammation, plays pivotal roles in salt-dietary modulation of pain sensitivity.

Regulatory T (Treg) cells integrate diverse environmental signals to modulate their function for optimal suppression. Translational regulation represents a favorable mechanism for Treg cell environmental sensing and adaptation. In this study, we carry out an unbiased screen of the Treg cell translatome and identify serum/glucocorticoid-regulated kinase 1 (SGK1), a known salt sensor in T cells, as being preferentially translated in activated Treg cells. We show that high salt (HS) drives thymic Treg cells to adopt a T helper type 17 (Th17)-like phenotype and enhances generation of Th17-like induced Treg cells in a SGK1 -dependent manner, all the while maintaining suppressive function. Salt-mediated Th17-like differentiation of Treg cells was evident in mice fed with HS diet or injected with HS-preconditioned T cells. Overall, SGK1 enables Treg cells to adapt their function in response to environmental cues. By understanding these environmental-sensing mechanisms, we envision targeted approaches to fine-tune Treg cell function for better control of inflammation.

Older individuals have an elevated risk for chronic pain as half of all individuals over 65 years old have at least one chronic pain condition. Unfortunately, relevant assessment tools and recommendations for chronic pain management targeting older adults are lacking. This study explores changes in response to pain between young (2-3 months old) and geriatric (20-24 months old) ages using mice. Although cutaneous thresholds to brisk stimuli (von Frey and radiant heat assays) were not affected, behavioral responses to tonic stimuli (acetone and capsaicin assays) were more pronounced in geriatric animals. After nerve injury, geriatric mice present an altered neuropathic pain profile with hypersensitivity to mechanical stimuli but not acetone and an impairment in conditioned noxious stimuli avoidance. This altered behavioral response pattern was associated with an abnormal monoaminergic signature in the medial prefrontal cortex, suggesting decreased COMT function. We conclude that young and geriatric mice exhibit different behavioral and physiological responses to the experience of pain, suggesting that knowledge and practices must be adjusted for geriatric populations. (C) 2020 The Authors. Published by Elsevier Inc.