Resolvins: Potent Pain Inhibiting Lipid Mediators via Transient Receptor Potential Regulation

Biosynthesis of resolvin D1, resolvin D2, and RCTR1 from 7,8(S,S)-epoxytetraene in human neutrophils and macrophages

While the acute inflammatory response to harmful stimuli is protective, unrestrained neutrophil swarming drives collateral tissue damage and inflammation. Biosynthesized from omega-3 essential polyunsaturated fatty acids, resolvins are a family of signaling molecules produced by immune cells within the resolution phase to orchestrate return to homeostasis. Understanding the mechanisms that govern biosynthesis of these potent molecules gives insight into stimulating endogenous resolution and offers fresh opportunities for preventing and treating excessive inflammation. In this report, using materials prepared by total synthesis and liquid chromatography and tandem mass spectrometry-based matching studies, we established the role of 7,8(S,S)-epoxytetraene intermediate in the biosynthesis of resolvin D1, resolvin D2, and the resolvin conjugate in tissue regeneration (RCTR1) by human phagocytes. We demonstrated that this 7,8(S,S)-epoxy-containing intermediate is directly converted to resolvin D2 by human M2-like macrophages and to resolvin D1 and RCTR1 by human macrophages, neutrophils, and peripheral blood mononuclear cells. In addition, both human recombinant soluble epoxide hydrolase (sEH) and the glutathione S-transferase leukotriene C4 synthase (LTC4S) each catalyze conversion of this epoxide to resolvin D1 and RCTR1, respectively. MS3 ion-trap scans and isotope incorporation of 18O from H218O with sEH indicated that the oxygen atom at C-8 in resolvin D1 is derived from water. Results from molecular docking simulations with biosynthetic precursor 17S-hydroperoxy-4,7,10,13,19-cis-15-trans-docosahexaenoic acid and the epoxy intermediate were consistent with 5-lipoxygenase production of resolvin D1. Together, these results give direct evidence for the role of resolvin 7,8(S,S)-epoxytetraene intermediate in the endogenous formation of resolution-phase mediators resolvin D1, resolvin D2, and RCTR1 by human phagocytes.

Translational control in the spinal cord regulates gene expression and pain hypersensitivity in the chronic phase of neuropathic pain

Sensitization of spinal nociceptive circuits plays a crucial role in neuropathic pain. This sensitization depends on new gene expression that is primarily regulated via transcriptional and translational control mechanisms. The relative roles of these mechanisms in regulating gene expression in the clinically relevant chronic phase of neuropathic pain are not well understood. Here, we show that changes in gene expression in the spinal cord during the chronic phase of neuropathic pain are substantially regulated at the translational level. Downregulating spinal translation at the chronic phase alleviated pain hypersensitivity. Cell-type-specific profiling revealed that spinal inhibitory neurons exhibited greater changes in translation after peripheral nerve injury compared to excitatory neurons. Notably, increasing translation selectively in all inhibitory neurons or parvalbumin-positive (PV+) interneurons, but not excitatory neurons, promoted mechanical pain hypersensitivity. Furthermore, increasing translation in PV+ neurons decreased their intrinsic excitability and spiking activity, whereas reducing translation in spinal PV+ neurons prevented the nerve injury-induced decrease in excitability. Thus, translational control mechanisms in the spinal cord, particularly in inhibitory neurons, play a role in mediating neuropathic pain hypersensitivity.

Evaluation of the Metabolite Profile of Fish Oil Omega-3 Fatty Acids (n-3 FAs) in Micellar and Enteric-Coated Forms-A Randomized, Cross-Over Human Study

This study evaluated the differences in the metabolite profile of three n-3 FA fish oil formulations in 12 healthy participants: (1) standard softgels (STD) providing 600 mg n-3 FA; (2) enteric-coated softgels (ENT) providing 600 mg n-3 FA; (3) a new micellar formulation (LMF) providing 374 mg n-3 FA. The pharmacokinetics (PKs), such as the area under the plot of plasma concentration (AUC), and the peak blood concentration (Cmax) of the different FA metabolites including HDHAs, HETEs, HEPEs, RvD1, RvD5, RvE1, and RvE2, were determined over a total period of 24 h. Blood concentrations of EPA (26,920.0 ± 10,021.0 ng/mL·h) were significantly higher with respect to AUC0-24 following LMF treatment vs STD and ENT; when measured incrementally, blood concentrations of total n-3 FAs (EPA/DHA/DPA3) up to 11 times higher were observed for LMF vs STD (iAUC 0-24: 16,150.0 ± 5454.0 vs 1498.9 ± 443.0; p ≤ 0.0001). Significant differences in n-3 metabolites including oxylipins were found between STD and LMF with respect to 12-HEPE, 9-HEPE, 12-HETE, and RvD1; 9-HEPE levels were significantly higher following the STD vs. ENT treatment. Furthermore, within the scope of this study, changes in blood lipid levels (i.e., cholesterol, triglycerides, LDL, and HDL) were monitored in participants for up to 120 h post-treatment; a significant decrease in serum triglycerides was detected in participants (~20%) following the LMF treatment; no significant deviations from the baseline were detected for all the other lipid biomarkers in any of the treatment groups. Despite a lower administered dose, LMF provided higher blood concentrations of n-3 FAs and certain anti-inflammatory n-3 metabolites in human participants-potentially leading to better health outcomes.

The role of neuroinflammation in the transition of acute to chronic pain and the opioid-induced hyperalgesia and tolerance

Current evidence suggests that activation of glial and immune cells leads to increased production of proinflammatory mediators, creating a neuroinflammatory state. Neuroinflammation has been proven to be a fundamental mechanism in the genesis of acute pain and its transition to neuropathic and chronic pain. A noxious event that stimulates peripheral afferent nerve fibers may also activate pronociceptive receptors situated at the dorsal root ganglion and dorsal horn of the spinal cord, as well as peripheral glial cells, setting off the so-called peripheral sensitization and spreading neuroinflammation to the brain. Once activated, microglia produce cytokines, chemokines, and neuropeptides that can increase the sensitivity and firing properties of second-order neurons, upregulating the signaling of nociceptive information to the cerebral cortex. This process, known as central sensitization, is crucial for chronification of acute pain. Immune-neuronal interactions are also implicated in the lesser-known complex regulatory relationship between pain and opioids. Current evidence suggests that activated immune and glial cells can alter neuronal function, induce, and maintain pathological pain, and disrupt the analgesic effects of opioid drugs by contributing to the development of tolerance and dependence, even causing paradoxical hyperalgesia. Such alterations may occur when the neuronal environment is impacted by trauma, inflammation, and immune-derived molecules, or when opioids induce proinflammatory glial activation. Hence, understanding these intricate interactions may help in managing pain signaling and opioid efficacy beyond the classical pharmacological approach.

The therapeutic potential of resolvins in pulmonary diseases

Uncontrolled inflammation leads to nonspecific destruction and remodeling of tissues and can contribute to many human pathologies, including pulmonary diseases. Stimulation of inflammatory resolution is considered an important process that protects against the progression of chronic inflammatory diseases. Resolvins generated from essential omega-3 polyunsaturated fatty acids have been demonstrated to be signaling molecules in inflammation with important pro-resolving and anti-inflammatory capabilities. By binding to specific receptors, resolvins can modulate inflammatory processes such as neutrophil migration, macrophage phagocytosis and the presence of pro-inflammatory mediators to reduce inflammatory pathologies. The discovery of these pro-resolving mediators has led to a shift in drug research from suppressing pro-inflammatory molecules to investigating compounds that promote resolution to treat inflammation. The exploration of inflammatory resolution also provided the opportunity to further understand the pathophysiology of pulmonary diseases. Alterations of resolution are now linked to both the development and exacerbation of diseases such as asthma, chronic obstructive pulmonary disease, cystic fibrosis, acute respiratory distress syndrome, cancer and COVID-19. These findings have resulted in the rise of novel design and testing of innovative resolution-based therapeutics to treat diseases. Hence, this paper reviews the generation and mechanistic actions of resolvins and investigates their role and therapeutic potential in several pulmonary diseases that may benefit from resolution-based pharmaceuticals.

Role of Resolvins in Inflammatory and Neuropathic Pain

Chronic pain is an unpleasant experience associated with actual or potential tissue damage. Inflammatory pain alerts the body to inflammation and promotes healing; however, unresolved inflammation can lead to chronic pain. Conversely, neuropathic pain, due to somatosensory damage, can be a disease in itself. However, inflammation plays a considerable role in the progression of both types of pain. Resolvins, derived from omega-3 fatty acids, actively suppress pro-inflammatory mediators and aid in the resolution of inflammation. Resolvins alleviate various inflammatory and neuropathic pain models by reducing hypersensitivity and regulating inflammatory cytokines and glial activation in the spinal cord and dorsal root ganglia. Thus, resolvins are a promising alternative for pain management with the potential to reduce the side effects associated with conventional medications. Continued research is crucial to unlock the therapeutic potential of resolvins and integrate them into effective clinical pain management strategies. This review aimed to evaluate the literature surrounding the resolvins in inflammatory and neuropathic pain.

Damage-mediated macrophage polarization in sterile inflammation

Most of the leading causes of death, such as cardiovascular diseases, cancer, dementia, neurodegenerative diseases, and many more, are associated with sterile inflammation, either as a cause or a consequence of these conditions. The ability to control the progression of inflammation toward tissue resolution before it becomes chronic holds significant clinical potential. During sterile inflammation, the initiation of inflammation occurs through damage-associated molecular patterns (DAMPs) in the absence of pathogen-associated molecules. Macrophages, which are primarily localized in the tissue, play a pivotal role in sensing DAMPs. Furthermore, macrophages can also detect and respond to resolution-associated molecular patterns (RAMPs) and specific pro-resolving mediators (SPMs) during sterile inflammation. Macrophages, being highly adaptable cells, are particularly influenced by changes in the microenvironment. In response to the tissue environment, monocytes, pro-inflammatory macrophages, and pro-resolution macrophages can modulate their differentiation state. Ultimately, DAMP and RAMP-primed macrophages, depending on the predominant subpopulation, regulate the balance between inflammatory and resolving processes. While sterile injury and pathogen-induced reactions may have distinct effects on macrophages, most studies have focused on macrophage responses induced by pathogens. In this review, which emphasizes available human data, we illustrate how macrophages sense these mediators by examining the expression of receptors for DAMPs, RAMPs, and SPMs. We also delve into the signaling pathways induced by DAMPs, RAMPs, and SPMs, which primarily contribute to the regulation of macrophage differentiation from a pro-inflammatory to a pro-resolution phenotype. Understanding the regulatory mechanisms behind the transition between macrophage subtypes can offer insights into manipulating the transition from inflammation to resolution in sterile inflammatory diseases.

Targeting dorsal root ganglia and primary sensory neurons for the treatment of chronic pain: an update

INTRODUCTION

Current treatments for chronic pain are inadequate. Here, we provide an update on the new therapeutic strategies that target dorsal root ganglia (DRGs) in the peripheral nervous system for a better and safer treatment of chronic pain.

AREAS COVERED

Despite the complex nature of chronic pain and its underlying mechanisms, we do know that changes in the plasticity and modality of neurons in DRGs play a pivotal role. DRG neurons are heterogenous and offer potential pain targets for different therapeutic interventions. We discuss the last advancements of these interventions, which include the use of systemic and local administrations, selective nerve drug delivery, and gene therapy. In particular, we provide updates and further details on the molecular characterization of primary sensory neurons, new analgesics entering the market, and future gene therapy approaches.

EXPERT OPINION

DRGs and primary sensory neurons are promising targets for chronic pain treatment due to their key role in pain signaling, unique anatomical location, and the potential for different targeted therapeutic interventions.

Randomized, double-blind, placebo-controlled study to evaluate the effect of treatment with an SPMs-enriched oil on chronic pain and inflammation, functionality, and quality of life in patients with symptomatic knee osteoarthritis: GAUDI study

BACKGROUND

Specialized pro-resolving mediators (SPMs), including 18-HEPE, 17-HDHA, and 14-HDHA are recognized as potentially therapeutic in inflammatory diseases because SPMs regulate the inflammation process, which leads to, for example; swelling and the sensation of pain. In osteoarthritis (OA), chronic pain is described as the symptom that reduces patients´ quality of life (QoL). The GAUDI study evaluated the efficacy of SPMs supplementation in reducing pain in the symptomatic knee of OA patients.

METHODS

This randomized, multicenter, double-blind, and placebo-controlled parallel-group pilot study was performed in Spain and conducted on adults 18-68 years old diagnosed with symptomatic knee OA. Patients were enrolled in the study for up to 24 weeks, which included a 12-week intervention period and a follow-up visit on week 24. The primary endpoint was pain change measured through a Visual Analog Scale (VAS). Secondary endpoints included: Pain change evaluation, stiffness, and function according to the WOMAC index; assessment of constant, intermittent, and total pain according to the OMERACT-OARSI score; evaluation of changes in health-related QoL parameters; the use or not of concomitant, rescue, and anti-inflammatory medication; and safety and tolerability assessments.

RESULTS

Patients were enrolled in the study from May 2018 to September 2021. VAS pain score was evaluated in the per protocol population (n = 51 patients), in which we observed a statistically significant reduction after 8 weeks (p = 0.039) and 12 weeks (p = 0.031) of treatment in patients consuming SPMs (n = 23 subjects) vs. placebo (n = 28 subjects). In line with the OMERACT-OARSI score, intermittent pain was reduced after 12 weeks with statistical significance (p = 0.019) in patients treated with SPMs (n = 23 subjects) vs. placebo (n = 28 subjects). Functional status as WOMAC score did not significantly change after SPMs or placebo consumption. Notably, patients consuming SPMs showed improvements in all five aspects of the EUROQoL-5, including a significant improvement in the usual-activities dimension. None of the patients required rescue medication, nor were any adverse events reported.

CONCLUSIONS

These findings suggest that sustained SPMs consumption reduces pain in OA patients while also improving their Quality of Life. These results also support the safety profile of SPMs supplementation. Trial registration NCT05633849. Registered 1 December 1 2022. Retrospectively registered, https://clinicaltrials.gov/ct2/show/study/NCT05633849.

Targeting Nociceptive Neurons and Transient Receptor Potential Channels for the Treatment of Migraine

Migraine is a neurovascular disorder that affects approximately 12% of the global population. While its exact causes are still being studied, researchers believe that nociceptive neurons in the trigeminal ganglia play a key role in the pain signals of migraine. These nociceptive neurons innervate the intracranial meninges and convey pain signals from the meninges to the thalamus. Targeting nociceptive neurons is considered promising due to their accessibility and distinct molecular profile, which includes the expression of several transient receptor potential (TRP) channels. These channels have been linked to various pain conditions, including migraine. This review discusses the role and mechanisms of nociceptive neurons in migraine, the challenges of current anti-migraine drugs, and the evidence for well-studied and emerging TRP channels, particularly TRPC4, as novel targets for migraine prevention and treatment.

Pain-resolving immune mechanisms in neuropathic pain

Interactions between the immune and nervous systems are of central importance in neuropathic pain, a common and debilitating form of chronic pain caused by a lesion or disease affecting the somatosensory system. Our understanding of neuroimmune interactions in pain research has advanced considerably. Initially considered as passive bystanders, then as culprits in the pathogenesis of neuropathic pain, immune responses in the nervous system are now established to underpin not only the initiation and progression of pain but also its resolution. Indeed, immune cells and their mediators are well-established promoters of neuroinflammation at each level of the neural pain pathway that contributes to pain hypersensitivity. However, emerging evidence indicates that specific subtypes of immune cells (including antinociceptive macrophages, pain-resolving microglia and T regulatory cells) as well as immunoresolvent molecules and modulators of the gut microbiota-immune system axis can reduce the pain experience and contribute to the resolution of neuropathic pain. This Review provides an overview of the immune mechanisms responsible for the resolution of neuropathic pain, including those involved in innate, adaptive and meningeal immunity as well as interactions with the gut microbiome. Specialized pro-resolving mediators and therapeutic approaches that target these neuroimmune mechanisms are also discussed.

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.

Future applications of exosomes delivering resolvins and cytokines in facilitating diabetic foot ulcer healing

Type 2 diabetes mellitus (T2DM) increases the risk of many lethal and debilitating conditions. Among them, foot ulceration due to neuropathy, vascular disease, or trauma affects the quality of life of millions in the United States and around the world. Physiological wound healing is stalled in the inflammatory phase by the chronicity of inflammation without proceeding to the resolution phase. Despite advanced treatment, diabetic foot ulcers (DFUs) are associated with a risk of amputation. Thus, there is a need for novel therapies to address chronic inflammation, decreased angiogenesis, and impaired granulation tissue formation contributing to the non-healing of DFUs. Studies have shown promising results with resolvins (Rv) and anti-inflammatory therapies that resolve inflammation and enhance tissue healing. But many of these studies have encountered difficulty in the delivery of Rv in terms of efficiency, tissue targetability, and immunogenicity. This review summarized the perspective of optimizing the therapeutic application of Rv and cytokines by pairing them with exosomes as a novel strategy for targeted tissue delivery to treat non-healing chronic DFUs. The articles discussing the T2DM disease state, current research on Rv for treating inflammation, the role of Rv in enhancing wound healing, and exosomes as a delivery vehicle were critically reviewed to find support for the proposition of using Rv and exosomes in combination for DFUs therapy. The literature reviewed suggests the beneficial role of Rv and exosomes and exosomes loaded with anti-inflammatory agents as promising therapeutic agents in ulcer healing.

Diosgenin Exerts Analgesic Effects by Antagonizing the Selective Inhibition of Transient Receptor Potential Vanilloid 1 in a Mouse Model of Neuropathic Pain

Diosgenin is a botanical steroidal saponin with immunomodulatory, anti-inflammatory, anti-oxidative, anti-thrombotic, anti-apoptotic, anti-depressant, and anti-nociceptive effects. However, the effects of diosgenin on anti-nociception are unclear. Transient receptor potential vanilloid 1 (TRPV1) plays an important role in nociception. Therefore, we investigated whether TRPV1 antagonism mediates the anti-nociceptive effects of diosgenin. In vivo mouse experiments were performed to examine nociception-related behavior, while in vitro experiments were performed to examine calcium currents in dorsal root ganglion (DRG) and Chinese hamster ovary (CHO) cells. The duration of capsaicin-induced licking (pain behavior) was significantly reduced following oral and intraplantar administration of diosgenin, approaching levels observed in mice treated with the TRPV1 antagonist N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropyrazine-1(2H)-carbox-amide. Additionally, oral administration of diosgenin blocked capsaicin-induced thermal hyperalgesia. Further, diosgenin reduced capsaicin-induced Ca²⁺ currents in a dose-dependent manner in both DRG and CHO cells. Oral administration of diosgenin also improved thermal and mechanical hyperalgesia in the sciatic nerve constriction injury-induced chronic pain model by reducing the expression of TRPV1 and inflammatory cytokines in DRG cells. Collectively, our results suggest that diosgenin exerts analgesic effects via antagonism of TRPV1 and suppression of inflammation in the DRG in a mouse model of neuropathic pain.

Ratio of Omega-6/Omega-3 Polyunsaturated Fatty Acids Associated With Somatic and Depressive Symptoms in People With Painful Temporomandibular Disorder and Irritable Bowel Syndrome

Somatic symptom disturbance is among the strongest predictors of painful temporomandibular disorder (TMD). Related psychological constructs, such as anxiety and depression, respond therapeutically to omega-3 polyunsaturated fatty acids (PUFAs) in clinical trials. This cross-sectional study investigated associations between the omega-6/omega-3 PUFA ratio and somatic symptom disturbance and depressive symptoms in a community-based sample of 501 adults and determined whether these associations differed between adults with and without TMD or irritable bowel syndrome (IBS). Liquid chromatography tandem mass spectrometry quantified PUFAs in circulating erythrocytes. Somatic symptoms and depression were quantified using Symptom Checklist-90-Revised subscales. Presence or absence of TMD and IBS, respectively, were determined by clinical examination and Rome III screening questions. The standardized beta coefficient for the omega-6/omega-3 long-chain PUFA ratio was 0.26 (95% confidence limits (CL): 0.08, 0.43) in a multivariable linear regression model in which somatic symptom disturbance was the dependent variable. When modelling depressive symptoms as the dependent variable, the standardized beta coefficient was 0.17 (95% CL:0.01, 0.34). Both associations were stronger among TMD cases and IBS cases than among non-cases. Future randomized control trials that lower the omega-6/omega-3 PUFA ratio could consider somatic or depressive symptoms as a therapeutic target for TMD or IBS pain. PERSPECTIVE: In people with TMD or IBS, a high n-6/n-3 PUFA ratio was positively associated with somatic symptom disturbance and depressive symptoms. Both measures of psychological distress were elevated in people with painful TMD and IBS. Future randomized clinical trials will determine whether lowering the n-6/n-3 ratio is therapeutic for pain.

Dhanabalan K, Agarwal S, Agarwal R. Resolvin D1-loaded nanoliposomes promote M2 macrophage polarization and are effective in the treatment of osteoarthritis

Current treatments for osteoarthritis (OA) offer symptomatic relief but do not prevent or halt the disease progression. Chronic low-grade inflammation is considered a significant driver of OA. Specialized proresolution mediators are powerful agents of resolution but have a short in vivo half-life. In this study, we have engineered a Resolvin D1 (RvD1)-loaded nanoliposomal formulation (Lipo-RvD1) that targets and resolves the OA-associated inflammation. This formulation creates a depot of the RvD1 molecules that allows the controlled release of the molecule for up to 11 days in vitro. In surgically induced mice model of OA, only controlled-release formulation of Lipo-RvD1 was able to treat the progressing cartilage damage when administered a month after the surgery, while the free drug was unable to prevent cartilage damage. We found that Lipo-RvD1 functions by damping the proinflammatory activity of synovial macrophages and recruiting a higher number of M2 macrophages at the site of inflammation. Our Lipo-RvD1 formulation was able to target and suppress the formation of the osteophytes and showed analgesic effect, thus emphasizing its ability to treat clinical symptoms of OA. Such controlled-release formulation of RvD1 could represent a patient-compliant treatment for OA.