Attenuation of inflammatory and neuropathic pain behaviors in mice through activation of free fatty acid receptor GPR40

Metabolite-Sensing Receptors: Emerging Targets for Modulating Chronic Pain Pathways

Chronic pain is a debilitating condition affecting millions worldwide, often resulting from complex interactions between the nervous and immune systems. Recent advances highlight the critical role of metabolite-sensing G protein-coupled receptors (GPCRs) in various chronic pain types. These receptors link metabolic changes with cellular responses, influencing inflammatory and degenerative processes. Receptors such as free fatty acid receptor 1 (FFAR1/GPR40), free fatty acid receptor 4 (FFAR4/GPR120), free fatty acid receptor 2 (FFAR2/GPR43), and Takeda G protein-coupled receptor 5 (TGR5/GPR131/GPBAR1) are key modulators of nociceptive signaling. GPR40, activated by long-chain fatty acids, exhibits strong anti-inflammatory effects by reducing cytokine expression. Butyrate-activated GPR43 inhibits inflammatory mediators like nitric oxide synthase-2 and cyclooxygenase-2, mitigating inflammation. TGR5, activated by bile acids, regulates inflammation and cellular senescence through pathways like NF-κB and p38. These receptors are promising therapeutic targets in chronic pain, addressing the metabolic and inflammatory factors underlying nociceptive sensitization and tissue degeneration. This review explores the molecular mechanisms of metabolite-sensing receptors in chronic pain, their therapeutic potential, and challenges in clinical application. By uncovering these mechanisms, metabolite-sensing receptors could lead to safer, more effective pain management strategies.

Anti-Inflammatory and Antinociceptive Properties of the Quercetin-3-Oleate AV2, a Novel FFAR1 Partial Agonist

Free fatty acid receptor 1 (FFAR1) has emerged as the most targeted isoform of the free fatty acid receptors because of its involvement in the modulation of energy balance and its potential role in the control of inflammatory and pain conditions. Quercetin-3-oleate (AV2), recognized as a new FFAR1 partial agonist, was investigated for its ability to modulate inflammation and nociception. Human immortal neuroblastoma SH and the murine macrophagic RAW 264.7 cells were used to evaluate cell viability, the potential cytoprotective activity, and the anti-inflammatory properties of AV2 in vitro. Paw edema, caused by zymosan-A, and the formalin test were used to assess the in vivo anti-inflammatory and antinociceptive effects in CD-1 mice. In vitro, AV2 was devoid of cytotoxicity, significantly reduced ROS in both cell types, and protected RAW 264.7 cells from lipopolysaccharide damage by reducing tumor necrosis factor-α production. Interestingly, AV2 induced a transient elevation of intracellular calcium that was reduced in cells, pre-incubated with the FFAR1 antagonist DC260126. In vivo, AV2 reduced formalin-induced nociception and zymosan A-induced paw edema, and both effects were reversed by the FFAR1 antagonist GW1100. In conclusion, these data strongly support the AV2-mediated antioxidant, anti-inflammatory, and antinociceptive activity. AV2 represents a promising molecule for the clinical management of inflammatory-related pain conditions.

Critical role of G protein-coupled receptor 40 in B cell response and the pathogenesis of rheumatoid arthritis in mice and patients

Rheumatoid arthritis (RA) is marked by joint damage and inflammation, with B cells playing a key role by generating autoantibodies. This study shows that G protein-coupled receptor 40 (GPR40) deficiency in B cells leads to increased activation, proliferation, antibody production, germinal center formation, and class switch recombination. GPR40 regulates Plcγ2 phosphorylation and intracellular calcium flux downstream of the B cell receptor by binding to the Gαq protein. In GPR40-deficient mice, susceptibility to collagen-induced arthritis was higher. GPR40 agonists showed potential as therapeutic agents, and their reduced expression in patients with RA correlated with disease onset, suggesting GPR40 as a potential therapeutic target and diagnostic marker.

Metabolite-sensing GPCRs in rheumatoid arthritis

Persistent inflammation in damaged joints results in metabolic dysregulation of the synovial microenvironment, causing pathogenic alteration of cell activity in rheumatoid arthritis (RA). Recently, the role of metabolite and metabolite-sensing G protein-coupled receptors (GPCRs) in the RA-related inflammatory immune response (IIR) has become a focus of research attention. These GPCRs participate in the progression of RA by modulating immune cell activation, migration, and inflammatory responses. Here, we discuss recent evidence implicating metabolic dysregulation in RA pathogenesis, focusing on the connection between RA-related IIR and GPCR signals originating from the synovial joint and gut. Furthermore, we discuss future directions for targeting metabolite-sensing GPCRs for therapeutic benefit, emphasizing the importance of identifying endogenous ligands and investigating the various transduction mechanisms involved.

[A long chain fatty acid receptor signaling as a new therapeutic target for stress-induced chronic pain]

Psychological and social stresses are known to be risk factors for psychiatric disorders, including depression and anxiety. On the other hand, exposure to these stresses can also cause prolonged and severe pain. However, the pathological mechanism for stress-induced chronic pain is complex, and there are many unresolved aspects, and no effective therapeutic drugs have been established. Since the discovery of the long-chain fatty acid receptor GPR40/FFAR1 about 20 years ago, research on the mechanism that promotes insulin secretion in the pancreas has progressed. Previously, we have worked to elucidate the physiological effects of GPR40/FFAR1 in the central nervous system and has found that it is involved in the regulation of pain and emotion. Based on these findings, they are now investigating the involvement of fatty acid receptors signaling in the development of stress-related chronic pain. In this review, we discuss the status of psychological stress-related chronic pain and the GPR40/FFAR1-mediated and -striking regulatory mechanisms of stress-induced chronic pain, based on our findings using a mouse model of chronic pain created by loading postoperative pain to a social defeat stress model mouse that mimics psychosocial stress. We summarized about the involvement of fatty acid receptor signaling as a new therapeutic candidate for chronic pain in this review.

Vincamine as an agonist of G-protein-coupled receptor 40 effectively ameliorates diabetic peripheral neuropathy in mice

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes, which has yet no curable medication. Neuroinflammation and mitochondrial dysfunction are tightly linked to DPN pathology. G-protein-coupled receptor 40 (GPR40) is predominantly expressed in pancreatic β-cells, but also in spinal dorsal horn and dorsal root ganglion (DRG) neurons, regulating neuropathic pain. We previously have reported that vincamine (Vin), a monoterpenoid indole alkaloid extracted from Madagascar periwinkle, is a GPR40 agonist. In this study, we evaluated the therapeutic potential of Vin in ameliorating the DPN-like pathology in diabetic mice. Both STZ-induced type 1 (T1DM) and db/db type 2 diabetic (T2DM) mice were used to establish late-stage DPN model (DPN mice), which were administered Vin (30 mg·kg-1·d-1, i.p.) for 4 weeks. We showed that Vin administration did not lower blood glucose levels, but significantly ameliorated neurological dysfunctions in DPN mice. Vin administration improved the blood flow velocities and blood perfusion areas of foot pads and sciatic nerve tissues in DPN mice. We demonstrated that Vin administration protected against sciatic nerve myelin sheath injury and ameliorated foot skin intraepidermal nerve fiber (IENF) density impairment in DPN mice. Moreover, Vin suppressed NLRP3 inflammasome activation through either β-Arrestin2 or β-Arrestin2/IκBα/NF-κB signaling, improved mitochondrial dysfunction through CaMKKβ/AMPK/SIRT1/PGC-1α signaling and alleviated oxidative stress through Nrf2 signaling in the sciatic nerve tissues of DPN mice and LPS/ATP-treated RSC96 cells. All the above-mentioned beneficial effects of Vin were abolished by GPR40-specific knockdown in dorsal root ganglia and sciatic nerve tissues. Together, these results support that pharmacological activation of GPR40 as a promising therapeutic strategy for DPN and highlight the potential of Vin in the treatment of this disease.

[Possible involvement of FFAR1 signaling in mouse emotional behaviors through the regulation of brain monoamine releases]

The free fatty acid receptor 1 (FFAR1) is suggested to function as a G protein-coupled receptor for medium- to long-chain free fatty acids. We have previously shown that FFAR1 signaling pathway plays an important suppressive role in spinal nociceptive processing after peripheral inflammation and nerve injury, and that FFAR1 agonists might serve as a new class of analgesics for treating inflammatory and neuropathic pain. To further pursue the functional significance of central FFAR1 signaling, we investigated the possible involvement of FFAR1 in endogenous pain modulation, depressive-like behavior, and aberrant behavior induced by addictive drugs using FFAR1 agonist (GW9508), FFAR1 antagonist (GW1100), and FFAR1 gene-deficient mice. As a result, FFAR1-deficient mice were found to exhibit stronger inflammatory and peripheral neuropathic pain-like behavior as well as depressive-like behavior. In particular, we noticed that peripheral nerve injury-induced depressive-like behavior was insensitive to imipramine. Next, we employed in vivo microdialysis to investigate whether FFAR1 is actually involved in the regulation of brain monoamines (dopamine and serotonin) releases. Our findings suggest that FFAR1 indirectly regulates dopamine release by promoting serotonin release. Thus, we are currently investigating how FFAR1 is involved in behavioral changes induced by addictive drugs such as cocaine and morphine. In this review, we briefly discuss about the possible involvement of FFAR1 in cocaine-induced locomotor hyperactivity.

Omega-3 polyunsaturated fatty acids and corneal nerve health: Current evidence and future directions

Corneal nerves play a key role in maintaining ocular surface integrity. Corneal nerve damage, from local or systemic conditions, can lead to ocular discomfort, pain, and, if poorly managed, neurotrophic keratopathy. Omega-3 polyunsaturated fatty acids (PUFAs) are essential dietary components that play a key role in neural development, maintenance, and function. Their potential application in modulating ocular and systemic inflammation has been widely reported. Omega-3 PUFAs and their metabolites also have neuroprotective properties and can confer benefit in neurodegenerative disease. Several preclinical studies have shown that topical administration of omega-3 PUFA-derived lipid mediators promote corneal nerve recovery following corneal surgery. Dietary omega-3 PUFA supplementation can also reduce corneal epithelial nerve loss and promote corneal nerve regeneration in diabetes. Omega-3 PUFAs and their lipid mediators thus show promise as therapeutic approaches to modulate corneal nerve health in ocular and systemic disease. This review discusses the role of dietary omega-3 PUFAs in maintaining ocular surface health and summarizes the possible applications of omega-3 PUFAs in the management of ocular and systemic conditions that cause corneal nerve damage. In examining the current evidence, this review also highlights relatively underexplored applications of omega-3 PUFAs in conferring neuroprotection and addresses their therapeutic potential in mediating corneal nerve regeneration.

The role of free fatty acid receptor pathways in a selective regulation of TRPA1 and TRPV1 by resolvins in primary sensory neurons

Transient receptor potential ankyrin 1 and vanilloid 1 (TRPA1 and TRPV1, respectively) channels contribute to inflammatory and neuropathic pain, indicating that their pharmacological inhibition could be a novel strategy for treating painful diseases. However, the mechanisms of TRPA1/V1 channel modulation have been mostly characterized to be upregulation and sensitization via variety of exogenous stimuli, endogenous inflammatory mediators, and metabolites of oxidative stress. Here we used calcium imaging of dorsal root ganglion neurons to identify an inhibitor signaling pathway for TRPA1 and TRPV1 regulated by resolvins (RvD1 and RvE1), which are endogenous anti‐inflammatory lipid mediators. TRPA1 and TRPV1 channel activations were evoked by the TRPA1 agonist allyl isothiocyanate and the TRPV1 agonist capsaicin. Our results show that RvD1‐induced selective inhibition of TRPA1 activity was mediated by free fatty acid receptor 4 (FFAR4)‐protein kinase C (PKC) signaling. Experiments assessing RvE1‐induced TRPV1 inhibition showed that RvE1 actions required both FFAR1 and FFAR4. Combined stimulation of FFAR1/FFAR4 or FFAR1/PKC mimicked TRPV1 inhibition by RvE1, and these effects were blocked by a protein kinase D (PKD) inhibitor, implying that PKD is an effector of the FFAR/PKC signaling axis in RvE1‐induced TRPV1 inhibition. Despite selective inhibition of TRPV1 in the nanomolar range of RvE1, higher concentrations of RvE1 also inhibited TRPA1, possibly through PKC. Collectively, our findings reveal FFAR1 and FFAR4 as key signaling pathways mediating the selective targeting of resolvins to regulate TRPA1 and TRPV1, elucidating endogenous analgesic mechanisms that could be exploited as potential therapeutic targets.

GPR40 Activation Abolishes Diabetes-Induced Painful Neuropathy by Suppressing VEGF-A Expression

G-protein-coupled receptor 40 (GPR40) is a promising target to support glucose-induced insulin release in patients with type 2 diabetes. We studied the role of GPR40 in the regulation of blood-nerve barrier integrity and its involvement in diabetes-induced neuropathies. Because GPR40 modulates insulin release, we used the streptozotocin model for type 1 diabetes, in which GPR40 functions can be investigated independently of its effects on insulin release. Diabetic wild-type mice exhibited increased vascular endothelial permeability and showed epineural microlesions in sciatic nerves, which were also observed in naïve GPR40-/- mice. Fittingly, expression of vascular endothelial growth factor-A (VEGF-A), an inducer of vascular permeability, was increased in diabetic wild-type and naïve GPR40-/- mice. GPR40 antagonists increased VEGF-A expression in murine and human endothelial cells as well as permeability of transendothelial barriers. In contrast, GPR40 agonists suppressed VEGF-A release and mRNA expression. The VEGF receptor inhibitor axitinib prevented diabetes-induced hypersensitivities and reduced endothelial and epineural permeability. Importantly, the GPR40 agonist GW9508 reverted established diabetes-induced hypersensitivity, an effect that was blocked by VEGF-A administration. Thus, GPR40 activation suppresses VEGF-A expression, thereby reducing diabetes-induced blood-nerve barrier permeability and reverting diabetes-induced hypersensitivities.

Genetic Inactivation of Free Fatty Acid Receptor 3 Impedes Behavioral Deficits and Pathological Hallmarks in the APPswe Alzheimer's Disease Mouse Model

The free fatty acid FFA3 receptor (FFA3R) belongs to the superfamily of G-protein-coupled receptors (GPCRs). In the intestine and adipose tissue, it is involved in the regulation of energy metabolism, but its function in the brain is unknown. We aimed, first, to investigate the expression of the receptor in the hippocampus of Alzheimer disease (AD) patients at different stages of the disease and, second, to assess whether genetic inactivation of the Ffar3 gene could affect the phenotypic features of the APP_swe mouse model. The expression of transcripts for FFA receptors in postmortem human hippocampal samples and in the hippocampus of wild-type and transgenic mice was analyzed by RT-qPCR. We generated a double transgenic mouse, FFA3R^−/−/APP_swe, to perform cognition studies and to assess, by immunoblotting Aβ and tau pathologies and the differential expression of synaptic plasticity-related proteins. For the first time, the occurrence of the FFA3R in the human hippocampus and its overexpression, even in the first stages of AD, was demonstrated. Remarkably, FFA3R^−/−/APP_swe mice do not have the characteristic memory impairment of 12-month-old APP_swe mice. Additionally, this newly generated transgenic line does not develop the most important Alzheimer’s disease (AD)-related features, such as amyloid beta (Aβ) brain accumulations and tau hyperphosphorylation. These findings are accompanied by increased levels of the insulin-degrading enzyme (IDE) and lower activity of the tau kinases GSK3β and Cdk5. We conclude that the brain FFA3R is involved in cognitive processes and that its inactivation prevents AD-like cognitive decline and pathological hallmarks.

Glycosides for Peripheral Neuropathic Pain: A Potential Medicinal Components

Neuropathic pain is a refractory disease that occurs across the world and pharmacotherapy has limited efficacy and/or safety. This disease imposes a significant burden on both the somatic and mental health of patients; indeed, some patients have referred to neuropathic pain as being 'worse than death'. The pharmacological agents that are used to treat neuropathic pain at present can produce mild effects in certain patients, and induce many adverse reactions, such as sedation, dizziness, vomiting, and peripheral oedema. Therefore, there is an urgent need to discover novel drugs that are safer and more effective. Natural compounds from medical plants have become potential sources of analgesics, and evidence has shown that glycosides alleviated neuropathic pain via regulating oxidative stress, transcriptional regulation, ion channels, membrane receptors and so on. In this review, we summarize the epidemiology of neuropathic pain and the existing therapeutic drugs used for disease prevention and treatment. We also demonstrate how glycosides exhibit an antinociceptive effect on neuropathic pain in laboratory research and describe the antinociceptive mechanisms involved to facilitate the discovery of new drugs to improve the quality of life of patients experiencing neuropathic pain.

FFAR1/GPR40 Contributes to the Regulation of Striatal Monoamine Releases and Facilitation of Cocaine-Induced Locomotor Activity in Mice

The free fatty acid receptor 1 (FFAR1) is suggested to function as a G protein-coupled receptor (GPR40) for medium-to-long-chain free fatty acids. Previous studies on the expression of FFAR1 revealed that the nigrostriatal region is one of the areas which express abundant FFAR1 mRNA/protein in the central nervous system (CNS). However, the role of FFAR1 in the CNS has been still largely unclarified. Here, we examined a possible functional role of FFAR1 in the control of extracellular concentrations of striatal monoamines and cocaine-induced locomotor activity. Microdialysis analysis revealed that the basal level of extracellular dopamine (DA) was significantly elevated, while the basal serotonin (5-HT) level tended to be reduced in the striatum of FFAR1 knockout (-/-) mice. Interestingly, local application of a FFAR1 agonist, GW9508, markedly augmented the striatal 5-HT release in FFAR1 wild-type (+/+) mice, whereas topical application of a FFAR1 antagonist, GW1100, significantly reduced the 5-HT release. However, the enhanced 5-HT release was completely lost in -/- mice. Although acute administration of cocaine enhanced the locomotor activity in both +/+ and -/- mice, the magnitude of the enhancement was significantly reduced in -/- mice. In addition, intraperitoneal injection of GW1100 significantly decreased the cocaine-induced locomotor enhancement. These results suggest that FFAR1 has a facilitatory role in striatal 5-HT release, and the evoked 5-HT release might contribute to enhance cocaine-induced locomotor activity.

GW9508 ameliorates cognitive dysfunction via the external treatment of encephalopathy in Aβ1-42 induced mouse model of Alzheimer's disease

The functions and mechanisms of GPR40 receptor to ameliorating the Alzheimer's disease (AD) by external treatment of encephalopathy remain unknown. In present study, the typical Aβ_1-42 induced mice model was applied to explore the functions and mechanisms of GPR40 receptor by external treatment of encephalopathy in AD. GPR40 agonist GW9508 and antagonist GW1100 were given by i.g injection to activate/inhibit the GPR40 receptor respectively in the gut of AD mouse which illustrated the function and mechanism of GPR40 receptor in ameliorating AD symptoms by external treatment of encephalopathy. A series of behavioral experiments were used to investigate the cognitive function and memory ability of mice, while molecular biology experiments such as Western blot, ELISA, flow cytometry were used to detect the corresponding changes of signaling pathways. The results revealed that intragastric administrated GW9508 could significantly ameliorate cognitive deficits of AD mouse, up-regulate the expression levels of gut-brain peptides both in blood circulation and hypothalamus thus up-regulate the expression levels of α-MSH in hypothalamus, while the negative autophagy-related proteins and inflammation-related proteins were down-regulated correspondingly. Meanwhile, GW9508 could also inhibit the pathological process of neuroinflammation in microglia. GW1100 reversed the effects of GW9508 significantly. These results suggested that GPR40 was an underlying therapeutic target for the external treatment of encephalopathy related to AD and GPR40 agonist could be explored as the emerging AD therapeutic drug.

Recent Updates on Free Fatty Acid Receptor 1 (GPR-40) Agonists for the Treatment of Type 2 Diabetes Mellitus

BACKGROUND

The global incidence of type 2 diabetes mellitus (T2DM) has enthused the development of new antidiabetic targets with low toxicity and long-term stability. In this respect, free fatty acid receptor 1 (FFAR1), which is also recognized as a G protein-coupled receptor 40 (GPR40), is a novel target for the treatment of T2DM. FFAR1/GPR40 has a high level of expression in β-cells of the pancreas, and the requirement of glucose for stimulating insulin release results in immense stimulation to utilise this target in the medication of T2DM.

METHODS

The data used for this review is based on the search of several scienctific databases as well as various patent databases. The main search terms used were free fatty acid receptor 1, FFAR1, FFAR1 agonists, diabetes mellitus, G protein-coupled receptor 40 (GPR40), GPR40 agonists, GPR40 ligands, type 2 diabetes mellitus and T2DM.

RESULTS

The present review article gives a brief overview of FFAR1, its role in T2DM, recent developments in small molecule FFAR1 (GPR40) agonists reported till now, compounds of natural/plant origin, recent patents published in the last few years, mechanism of FFAR1 activation by the agonists, and clinical status of the FFAR1/GPR40 agonists.

CONCLUSION

The agonists of FFAR1/GRP40 showed considerable potential for the therapeutic control of T2DM. Most of the small molecule FFAR1/GPR40 agonists developed were aryl alkanoic acid derivatives (such as phenylpropionic acids, phenylacetic acids, phenoxyacetic acids, and benzofuran acetic acid derivatives) and thiazolidinediones. Some natural/plant-derived compounds, including fatty acids, sesquiterpenes, phenolic compounds, anthocyanins, isoquinoline, and indole alkaloids, were also reported as potent FFAR1 agonists. The clinical investigations of the FFAR1 agonists demonstrated their probable role in the improvement of glucose control. Though, there are some problems still to be resolved in this field as some FFAR1 agonists terminated in the late phase of clinical studies due to "hepatotoxicity." Currently, PBI-4050 is under clinical investigation by Prometic. Further investigation of pharmacophore scaffolds for FFAR1 full agonists as well as multitargeted modulators and corresponding clinical investigations will be anticipated, which can open up new directions in this area.

Nonesterified Fatty Acids and Depression in Cancer Patients and Caregivers

BACKGROUND

Nonesterified fatty acids (NEFAs) are known to have inflammatory effects. The inflammatory hypothesis of depression suggests that omega-3 (ω-3) and omega-6 (ω-6) fatty acids might be negatively and positively correlated with depression, respectively.

OBJECTIVE

An exploratory study was conducted to determine the association between dietary free fatty acids and depressive symptoms in cancer patients and caregivers.

METHODS

Associations between depression and the NEFA pool were investigated in 56 cancer patients and 23 caregivers using a combination of nonparametric tests and regularized regression. Plasma NEFAs were measured using thin layer and gas chromatography with flame ionization detection. Depression was characterized both as a continuous severity score using the GRID-Hamilton Depression Rating Scale (GRID Ham-D), and as a categorical diagnosis of major depression by structured clinical interview.

RESULTS

Initial hypotheses regarding the relation between depression and omega-3 or omega-6 fatty acids were not well supported. However, elaidic acid, a trans-unsaturated fatty acid found in hydrogenated vegetable oils, was found to be negatively correlated with continuous depression scores in cancer patients. No significant associations were found in caregivers.

CONCLUSIONS

An unexpected negative association between elaidic acid and depression was identified, supporting recent literature on the role of G protein-coupled receptors in depression. Further research is needed to confirm this result and to evaluate the potential role of G protein agonists as therapeutic agents for depression in cancer patients.

Aging-Related Phenotypic Conversion of Medullary Microglia Enhances Intraoral Incisional Pain Sensitivity

Activated microglia involved in the development of orofacial pain hypersensitivity have two major polarization states. The aim of this study was to assess the involvement of the aging-related phenotypic conversion of medullary microglia in the enhancement of intraoral pain sensitivity using senescence-accelerated mice (SAM)-prone/8 (SAMP8) and SAM-resistant/1 (SAMR1) mice. Mechanical head-withdrawal threshold (MHWT) was measured for 21 days post palatal mucosal incision. The number of CD11c-immunoreactive (IR) cells [affective microglia (M1)] and CD163-IR cells [protective microglia (M2)], and tumor-necrosis-factor-α (TNF-α)-IR M1 and interleukin (IL)-10-IR M2 were analyzed via immunohistochemistry on days 3 and 11 following incision. The decrease in MHWT observed following incision was enhanced in SAMP8 mice. M1 levels and the number of TNF-α-IR M1 were increased on day 3 in SAMP8 mice compared with those in SAMR1 mice. On day 11, M1 and M2 activation was observed in both groups, whereas IL-10-IR M2 levels were attenuated in SAMP8 mice, and the number of TNF-α-IR M1 cells increased, compared to those in SAMR1 mice. These results suggest that the mechanical allodynia observed following intraoral injury is potentiated and sustained in SAMP8 mice due to enhancement of TNF-α signaling, M1 activation, and an attenuation of M2 activation accompanying IL-10 release.

Oxidized Lipids in Persistent Pain States

Chemotherapy, nerve injuries, or diseases like multiple sclerosis can cause pathophysiological processes of persistent and neuropathic pain. Thereby, the activation threshold of ion channels is reduced in peripheral sensory neurons to normally noxious stimuli like heat, cold, acid, or mechanical due to sensitization processes. This leads to enhanced neuronal activity, which can result in mechanical allodynia, cold allodynia, thermal hyperalgesia, spontaneous pain, and may initiate persistent and neuropathic pain. The treatment options for persistent and neuropathic pain patients are limited; for about 50% of them, current medication is not efficient due to severe side effects or low response to the treatment. Therefore, it is of special interest to find additional treatment strategies. One approach is the control of neuronal sensitization processes. Herein, signaling lipids are crucial mediators and play an important role during the onset and maintenance of pain. As preclinical studies demonstrate, lipids may act as endogenous ligands or may sensitize transient receptor potential (TRP)-channels. Likewise, they can cause enhanced activity of sensory neurons by mechanisms involving G-protein coupled receptors and activation of intracellular protein kinases. In this regard, oxidized metabolites of the essential fatty acid linoleic acid, 9- and 13-hydroxyoctadecadienoic acid (HODE), their dihydroxy-metabolites (DiHOMEs), as well as epoxides of linoleic acid (EpOMEs) and of arachidonic acid (EETs), as well as lysophospholipids, sphingolipids, and specialized pro-resolving mediators (SPMs) have been reported to play distinct roles in pain transmission or inhibition. Here, we discuss the underlying molecular mechanisms of the oxidized linoleic acid metabolites and eicosanoids. Furthermore, we critically evaluate their role as potential targets for the development of novel analgesics and for the treatment of persistent or neuropathic pain.

Integrated Analysis of DNA Methylation and Biochemical/Metabolic Parameter During the Long-Term Isolation Environment

Numerous studies have shown that changes in the epigenome are an important cause of human biochemical or metabolic parameter changes. Biochemical/metabolic parameter disorders of the human body are usually closely related to the occurrence of disease. Therefore, constructing credible DNA methylation site-biochemical/metabolic parameter associations are key in interpreting the pathogenesis of diseases. However, there is a lack of research on systematic integration analysis of DNA methylation with biochemical/metabolic parameter and diseases. In this study, we attempted to use the four-people, multiple time point detected data from the long-term isolation experiment to conduct a correlation analysis. We used the biclustering algorithm FABIA to cluster the DNA methylation site-parameter correlation matrixes into 28 biclusters. The results of the biological function analysis for these biclusters were consistent with the biochemical/metabolic parameter change characteristics of the human body during long-term isolation, demonstrating the reliability of the biclusters identified by our method. In addition, from these biclusters, we obtained highly credible biochemical/metabolic parameter-disease associations, which is supported by several studies. Our results indicate that there is an overlap of biochemical/metabolic parameter-disease associations derived from a small sample, multiple time point data in healthy populations and the associations obtained from a large sample data in patients during disease development. These findings provide insights into understanding the role of the epigenome in biochemical/metabolic parameter change and disease development and has potential applications in biology and medicine research.

Fatty Acid Signaling Mechanisms in Neural Cells: Fatty Acid Receptors

Fatty acids (FAs) are typically associated with structural and metabolic roles, as they can be stored as triglycerides, degraded by β-oxidation or used in phospholipids’ synthesis, the main components of biological membranes. It has been shown that these lipids exhibit also regulatory functions in different cell types. FAs can serve as secondary messengers, as well as modulators of enzymatic activities and substrates for cytokines synthesis. More recently, it has been documented a direct activity of free FAs as ligands of membrane, cytosolic, and nuclear receptors, and cumulative evidence has emerged, demonstrating its participation in a wide range of physiological and pathological conditions. It has been long known that the central nervous system is enriched with poly-unsaturated FAs, such as arachidonic (C20:4ω-6) or docosohexaenoic (C22:6ω-3) acids. These lipids participate in the regulation of membrane fluidity, axonal growth, development, memory, and inflammatory response. Furthermore, a whole family of low molecular weight compounds derived from FAs has also gained special attention as the natural ligands for cannabinoid receptors or key cytokines involved in inflammation, largely expanding the role of FAs as precursors of signaling molecules. Nutritional deficiencies, and alterations in lipid metabolism and lipid signaling have been associated with developmental and cognitive problems, as well as with neurodegenerative diseases. The molecular mechanism behind these effects still remains elusive. But in the last two decades, different families of proteins have been characterized as receptors mediating FAs signaling. This review focuses on different receptors sensing and transducing free FAs signals in neural cells: (1) membrane receptors of the family of G Protein Coupled Receptors known as Free Fatty Acid Receptors (FFARs); (2) cytosolic transport Fatty Acid-Binding Proteins (FABPs); and (3) transcription factors Peroxisome Proliferator-Activated Receptors (PPARs). We discuss how these proteins modulate and mediate direct regulatory functions of free FAs in neural cells. Finally, we briefly discuss the advantages of evaluating them as potential targets for drug design in order to manipulate lipid signaling. A thorough characterization of lipid receptors of the nervous system could provide a framework for a better understanding of their roles in neurophysiology and, potentially, help for the development of novel drugs against aging and neurodegenerative processes.

Activation of GPR40 produces mechanical antiallodynia via the spinal glial interleukin-10/β-endorphin pathway

Background

The G protein-coupled receptor 40 (GPR40), broadly expressed in various tissues such as the spinal cord, exerts multiple physiological functions including pain regulation. This study aimed to elucidate the mechanisms underlying GPR40 activation-induced antinociception in neuropathic pain, particularly related to the spinal glial expression of IL-10 and subsequent β-endorphin.

Methods

Spinal nerve ligation-induced neuropathic pain model was used in this study. β-Endorphin and IL-10 levels were measured in the spinal cord and cultured primary microglia, astrocytes, and neurons. Double immunofluorescence staining of β-endorphin with glial and neuronal cellular biomarkers was also detected in the spinal cord and cultured primary microglia, astrocytes, and neurons.

Results

GPR40 was expressed on microglia, astrocytes, and neurons in the spinal cords and upregulated by spinal nerve ligation. Intrathecal injection of the GPR40 agonist GW9508 dose-dependently attenuated mechanical allodynia and thermal hyperalgesia in neuropathic rats, with E_max values of 80% and 100% MPE and ED₅₀ values of 6.7 and 5.4 μg, respectively. Its mechanical antiallodynia was blocked by the selective GPR40 antagonist GW1100 but not GPR120 antagonist AH7614. Intrathecal GW9508 significantly enhanced IL-10 and β-endorphin immunostaining in spinal microglia and astrocytes but not in neurons. GW9508 also markedly stimulated gene and protein expression of IL-10 and β-endorphin in cultured primary spinal microglia and astrocytes but not in neurons, originated from 1-day-old neonatal rats. The IL-10 antibody inhibited GW9508-stimulated gene expression of the β-endorphin precursor proopiomelanocortin (POMC) but not IL-10, whereas the β-endorphin antibody did not affect GW9508-stimulated IL-10 or POMC gene expression. GW9508 increased phosphorylation of mitogen-activated protein kinases (MAPKs) including p38, extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK), and its stimulatory effects on IL-10 and POMC expression were blocked by each MAPK isoform inhibitor. Spinal GW9508-induced mechanical antiallodynia was completely blocked by intrathecal minocycline, IL-10 neutralizing antibody, β-endorphin antiserum, and μ-opioid receptor-preferred antagonist naloxone.

Conclusions

Our results illustrate that GPR40 activation produces antinociception via the spinal glial IL-10/β-endorphin antinociceptive pathway.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1457-9) contains supplementary material, which is available to authorized users.

TAK-875 Mitigates β-Cell Lipotoxicity-Induced Metaflammation Damage through Inhibiting the TLR4-NF-κB Pathway

Metabolic inflammatory damage, characterized by Toll-like receptor 4 (TLR4) signaling activation, is a major mechanism underlying lipotoxicity-induced β-cell damage. The present study is aimed at determining whether G protein-coupled receptor 4 (GPR40) agonist can improve β-cell lipotoxicity-induced damage by inhibiting the TLR4-NF-κB pathway. Lipotoxicity, inflammation-damaged β-cells, obese SD, and TLR4^KO rat models were used in the study. In vitro, TAK-875 inhibited the lipotoxicity- and LPS-induced β-cell apoptosis in a concentration-dependent manner, improved the insulin secretion, and inhibited the expression of TLR4 and NF-κB subunit P65. Besides, silencing of TLR4 expression enhanced the protective effects of TAK-875, while TLR4 overexpression attenuated this protective effect. Activation of TLR4 or NF-κB attenuated the antagonism of TAK-875 on PA-induced damage. Moreover, the above process of TAK-875 was partially independent of GPR40 expression. TAK-875 reduced the body weight and inflammatory factors, rebalanced the number and distribution of α or β-cells, inhibited the apoptosis of islet cells, and inhibited the expression of TLR4 and NF-κB subunit P65 in obese rats. Further knockout of the rat TLR4 gene delayed the damage induced by the high-fat diet and synergy with the action of TAK-875. These data suggest that GPR40 agonists antagonized the lipotoxicity β-cell damage by inhibiting the TLR4-NF-κB pathway.

GPR40 modulates epileptic seizure and NMDA receptor function

Epilepsy is a common neurological disease, and approximately 30% of patients do not respond adequately to antiepileptic drug treatment. Recent studies suggest that G protein-coupled receptor 40 (GPR40) is expressed in the central nervous system and is involved in the regulation of neurological function. However, the impact of GPR40 on epileptic seizures remains unclear. In this study, we first reported that GPR40 expression was increased in epileptic brains. In the kainic acid-induced epilepsy model, GPR40 activation after status epilepticus alleviated epileptic activity, whereas GPR40 inhibition showed the opposite effect. In the pentylenetetrazole-induced kindling model, susceptibility to epilepsy was reduced with GPR40 activation and increased with GPR40 inhibition. Whole-cell patch-clamp recordings demonstrated that GPR40 affected N-methyl-d-aspartate (NMDA) receptor-mediated synaptic transmission. Moreover, GPR40 regulated NR2A and NR2B expression on the surface of neurons. In addition, endocytosis of NMDA receptors and binding of GPR40 with NR2A and NR2B can be regulated by GPR40. Together, our findings indicate that GPR40 modulates epileptic seizures, providing a novel antiepileptic target.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

The Role of Metabolite-Sensing G Protein-Coupled Receptors in Inflammation and Metabolic Disease

SIGNIFICANCE

Great attention has been placed on the link between metabolism and immune function giving rise to the term "immunometabolism." It is widely accepted that inflammation and oxidative stress are key processes that underlie metabolic complications during obesity, diabetes, and atherosclerosis. Therefore, identifying the mechanisms and mediators that are involved in the regulation of both inflammation and metabolic homeostasis is of high scientific and therapeutic interest. Recent Advances: G protein-coupled receptors (GPCRs) that signal in response to metabolites have emerged as attractive therapeutic targets in inflammatory disease. Critical Issues and Future Directions: In this review, we discuss recent findings about the physiological role of the main metabolite-sensing GPCRs, their implication in immunometabolic disorders, their principal endogenous and synthetic ligands, and their potential as drug targets in inflammation and metabolic disease. Antioxid. Redox Signal. 29, 237-256.

Dyslipidemia impairs mitochondrial trafficking and function in sensory neurons

Mitochondrial trafficking plays a central role in dorsal root ganglion (DRG) neuronal cell survival and neurotransmission by transporting mitochondria from the neuronal cell body throughout the bundles of DRG axons. In type 2 diabetes (T2DM), dyslipidemia and hyperglycemia damage DRG neurons and induce mitochondrial dysfunction; however, the impact of free fatty acids and glucose on mitochondrial trafficking in DRG neurons remains unknown. To evaluate the impact of free fatty acids compared to hyperglycemia on mitochondrial transport, primary adult mouse DRG neuron cultures were treated with physiologic concentrations of palmitate and glucose and assessed for alterations in mitochondrial trafficking, mitochondrial membrane potential, and mitochondrial bioenergetics. Palmitate treatment significantly reduced the number of motile mitochondria in DRG axons, but physiologic concentrations of glucose did not impair mitochondrial trafficking dynamics. Palmitate-treated DRG neurons also exhibited a reduction in mitochondrial velocity, and impaired mitochondrial trafficking correlated with mitochondrial depolarization in palmitate-treated DRG neurons. Finally, we found differential bioenergetic effects of palmitate and glucose on resting and energetically challenged mitochondria in DRG neurons. Together, these results suggest that palmitate induces DRG neuron mitochondrial depolarization, inhibiting axonal mitochondrial trafficking and altering mitochondrial bioenergetic capacity.-Rumora, A. E., Lentz, S. I., Hinder, L. M., Jackson, S. W., Valesano, A., Levinson, G. E., Feldman, E. L. Dyslipidemia impairs mitochondrial trafficking and function in sensory neurons.

Dysfunctional GPR40/FFAR1 signaling exacerbates pain behavior in mice

We previously showed that activation of G protein-coupled receptor 40/free fatty acid receptor 1 (GPR40/FFAR1) signaling modulates descending inhibition of pain. In this study, we investigated the involvement of fatty acid-GPR40/FFAR1 signaling in the transition from acute to chronic pain. We used GPR40/FFAR1-knockout (GPR40KO) mice and wild-type (WT) mice. A plantar incision was performed, and mechanical allodynia and thermal hyperalgesia were evaluated with a von Frey filament test and plantar test, respectively. Immunohistochemistry was used to localize GPR40/FFAR1, and the levels of free fatty acids in the hypothalamus were analyzed with liquid chromatography-tandem mass spectrometry. The repeated administration of GW1100, a GPR40/FFAR1 antagonist, exacerbated the incision-induced mechanical allodynia and significantly increased the levels of phosphorylated extracellular signal-regulated kinase in the spinal cord after low-threshold touch stimulation in the mice compared to vehicle-treated mice. The levels of long-chain free fatty acids, such as docosahexaenoic acid, oleic acid, and palmitate, which are GPR40/FFAR1 agonists, were significantly increased in the hypothalamus two days after the surgery compared to levels in the sham group. Furthermore, the incision-induced mechanical allodynia was exacerbated in the GPR40KO mice compared to the WT mice, while the response in the plantar test was not changed. These findings suggested that dysfunction of the GPR40/FFAR1 signaling pathway altered the endogenous pain control system and that this dysfunction might be associated with the development of chronic pain.

Complex Pharmacology of Free Fatty Acid Receptors

G protein-coupled receptors (GPCRs) are historically the most successful family of drug targets. In recent times it has become clear that the pharmacology of these receptors is far more complex than previously imagined. Understanding of the pharmacological regulation of GPCRs now extends beyond simple competitive agonism or antagonism by ligands interacting with the orthosteric binding site of the receptor to incorporate concepts of allosteric agonism, allosteric modulation, signaling bias, constitutive activity, and inverse agonism. Herein, we consider how evolving concepts of GPCR pharmacology have shaped understanding of the complex pharmacology of receptors that recognize and are activated by nonesterified or "free" fatty acids (FFAs). The FFA family of receptors is a recently deorphanized set of GPCRs, the members of which are now receiving substantial interest as novel targets for the treatment of metabolic and inflammatory diseases. Further understanding of the complex pharmacology of these receptors will be critical to unlocking their ultimate therapeutic potential.

Spinal astrocytic activation contributes to both induction and maintenance of pituitary adenylate cyclase-activating polypeptide type 1 receptor-induced long-lasting mechanical allodynia in mice

Background

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are present in the spinal dorsal horn and dorsal root ganglia, suggesting an important role of PACAP–PACAP receptors signaling system in the modulation of spinal nociceptive transmission. We have previously reported that a single intrathecal injection of PACAP or a PACAP specific (PAC1) receptor selective agonist, maxadilan, in mice induced dose-dependent aversive behaviors, which lasted more than 30 min, and suggested that the maintenance of the nociceptive behaviors was associated with the spinal astrocytic activation.

Results

We found that a single intrathecal administration of PACAP or maxadilan also produced long-lasting hind paw mechanical allodynia, which persisted at least 84 days without affecting thermal nociceptive threshold. In contrast, intrathecal application of vasoactive intestinal polypeptide did not change mechanical threshold, and substance P, calcitonin gene-related peptide, or N-methyl-D-aspartate induced only transient mechanical allodynia, which disappeared within 21 days. Western blot and immunohistochemical analyses with an astrocytic marker, glial fibrillary acidic protein, revealed that the spinal PAC1 receptor stimulation caused sustained astrocytic activation, which also lasted more than 84 days. Intrathecal co-administration of L-α-aminoadipate, an astroglial toxin, with PACAP or maxadilan almost completely prevented the induction of the mechanical allodynia. Furthermore, intrathecal treatment of L-α-aminoadipate at 84 days after the PAC1 stimulation transiently reversed the mechanical allodynia accompanied by the reduction of glial fibrillary acidic protein expression level.

Conclusion

Our data suggest that spinal astrocytic activation triggered by the PAC1 receptor stimulation contributes to both induction and maintenance of the long-term mechanical allodynia.

Pituitary adenylate cyclase-activating polypeptide type 1 receptor signaling evokes long-lasting nociceptive behaviors through the activation of spinal astrocytes in mice

Intrathecal (i.t.) administration of pituitary adenylate cyclase-activating polypeptide (PACAP) induces long-lasting nociceptive behaviors for more than 60 min in mice, while the involvement of PACAP type1 receptor (PAC1-R) has not been clarified yet. The present study investigated signaling mechanisms of the PACAP-induced prolonged nociceptive behaviors. Single i.t. injection of a selective PAC1-R agonist, maxadilan (Max), mimicked nociceptive behaviors in a dose-dependent manner similar to PACAP. Pre- or post-treatment of a selective PAC1-R antagonist, max.d.4, significantly inhibited the nociceptive behaviors by PACAP or Max. Coadministration of a protein kinase A inhibitor, Rp-8-Br-cAMPS, a mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase inhibitor, PD98059 or a c-Jun N-terminal kinase (JNK) inhibitor, SP600125, significantly inhibited the nociceptive behaviors by Max. Immunohistochemistry and immunoblotting analysis revealed that spinal administration of Max-induced ERK phosphorylation and JNK phosphorylation, and also augmented an astrocyte marker, glial fibrillary acidic protein in mouse spinal cord. Furthermore, an astroglial toxin, l-α-aminoadipate, significantly attenuated the development of the nociceptive behaviors and ERK phosphorylation by Max. These results suggest that the activation of spinal PAC1-R induces long-lasting nociception through the interaction of neurons and astrocytes.

New paradigms in GPCR drug discovery

G protein-coupled receptors (GPCRs) remain a major domain of pharmaceutical discovery. The identification of GPCR lead compounds and their optimization are now structure-based, thanks to advances in X-ray crystallography, molecular modeling, protein engineering and biophysical techniques. In silico screening provides useful hit molecules. New pharmacological approaches to tuning the pleotropic action of GPCRs include: allosteric modulators, biased ligands, GPCR heterodimer-targeted compounds, manipulation of polypharmacology, receptor antibodies and tailoring of drug molecules to fit GPCR pharmacogenomics. Measurements of kinetics and drug efficacy are factors influencing clinical success. With the exception of inhibitors of GPCR kinases, targeting of intracellular GPCR signaling or receptor cycling for therapeutic purposes remains a futuristic concept. New assay approaches are more efficient and multidimensional: cell-based, label-free, fluorescence-based assays, and biosensors. Tailoring GPCR drugs to a patient's genetic background is now being considered. Chemoinformatic tools can predict ADME-tox properties. New imaging technology visualizes drug action in vivo. Thus, there is reason to be optimistic that new technology for GPCR ligand discovery will help reverse the current narrowing of the pharmaceutical pipeline.