The role of protease-activated receptor type 2 in nociceptive signaling and pain

Single-Nucleotide Polymorphisms of the PAR2 and IL-17A Genes Are Significantly Associated with Chronic Pain

Patients with chronic pain are affected psychologically and socially. There are also individual differences in treatment efficacy. Insufficient research has been conducted on genetic polymorphisms that are related to individual differences in the susceptibility to chronic pain. Autoimmune disorders can lead to inflammation and chronic pain; therefore, we focused on the autoimmune-related protease-activated receptor 2 (PAR2/F2RL1) and interleukin 17A (IL-17A/IL17A) genes. PAR2 and IL-17A are associated with autoimmune diseases that lead to chronic pain, and PAR2 regulates T-helper (Th) cell activation and differentiation. We hypothesized that the PAR2 and IL-17A genes are associated with chronic pain. The present study used a case-control design to statistically examine associations between genetic polymorphisms and the vulnerability to chronic pain. The rs2243057 polymorphism of the PAR2 gene and rs3819025 polymorphism of the IL-17A gene were previously reported to be associated with pain- or autoimmune-related phenotypes. Thus, these polymorphisms were investigated in the present study. We found that both rs2243057 and rs3819025 were significantly associated with a susceptibility to chronic pain. The present findings revealed autoimmune-related genetic factors that are involved in individual differences in chronic pain, further aiding understanding of the pathomechanism that underlies chronic pain and possibly contributing to future personalized medicine.

Protease-Activated Receptor 2 (PAR2) Expressed in Sensory Neurons Contributes to Signs of Pain and Neuropathy in Paclitaxel Treated Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose-limiting side effect of cancer therapy. Protease-activated receptor 2 (PAR2) is implicated in a variety of pathologies, including CIPN. In this study, we demonstrate the role of PAR2 expressed in sensory neurons in a paclitaxel (PTX)-induced model of CIPN in mice. PAR2 knockout/wildtype (WT) mice and mice with PAR2 ablated in sensory neurons were treated with PTX administered via intraperitoneal injection. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. We then examined immunohistochemical staining of dorsal root ganglion (DRG) and hind paw skin samples from CIPN mice to measure satellite cell gliosis and intra-epidermal nerve fiber (IENF) density. The pharmacological reversal of CIPN pain was tested with the PAR2 antagonist C781. Mechanical allodynia caused by PTX treatment was alleviated in PAR2 knockout mice of both sexes. In the PAR2 sensory neuronal conditional knockout (cKO) mice, both mechanical allodynia and facial grimacing were attenuated in mice of both sexes. In the DRG of the PTX-treated PAR2 cKO mice, satellite glial cell activation was reduced compared to control mice. IENF density analysis of the skin showed that the PTX-treated control mice had a reduction in nerve fiber density while the PAR2 cKO mice had a comparable skin innervation as the vehicle-treated animals. Similar results were seen with satellite cell gliosis in the DRG, where gliosis induced by PTX was absent in PAR cKO mice. Finally, C781 was able to transiently reverse established PTX-evoked mechanical allodynia. PERSPECTIVE: Our work demonstrates that PAR2 expressed in sensory neurons plays a key role in PTX-induced mechanical allodynia, spontaneous pain, and signs of neuropathy, suggesting PAR2 as a possible therapeutic target in multiple aspects of PTX CIPN.

PAR2 activation in the dura causes acute behavioral responses and priming to glyceryl trinitrate in a mouse migraine model

BACKGROUND

Migraine is a severely debilitating disorder that affects millions of people worldwide. Studies have indicated that activation of protease-activated receptor-2 (PAR2) in the dura mater causes headache responses in preclinical models. It is also well known that vasodilators such as nitric oxide (NO) donors can trigger migraine attacks in migraine patients but not controls. In the current study we examined whether activation of PAR2 in the dura causes priming to the NO donor glyceryl trinitrate (GTN).

METHODS

A preclinical behavioral model of migraine was used where stimuli (PAR2 agonists: 2at-LIGRL-NH₂ (2AT) or neutrophil elastase (NE); and IL-6) were applied to the mouse dura through an injection made at the intersection of the lamdoidal and sagittal sutures on the skull. Following dural injection, periorbital von Frey thresholds and facial grimace responses were measured until their return to baseline. GTN was then given by intraperitoneal injection and periorbital hypersensitivity and facial grimace responses observed until they returned to baseline.

RESULTS

We found that application of the selective PAR2 agonist 2at-LIGRL-NH₂ (2AT) onto the dura causes headache-related behavioral responses in WT but not PAR2^-/- mice with no differences between sexes. Additionally, dural PAR2 activation with 2AT caused priming to GTN (1 mg/kg) at 14 days after primary dural stimulation. PAR2^-/- mice showed no priming to GTN. We also tested behavioral responses to the endogenous protease neutrophil elastase, which can cleave and activate PAR2. Dural neutrophil elastase caused both acute responses and priming to GTN in WT but not PAR2^-/- mice. Finally, we show that dural IL-6 causes acute responses and priming to GTN that is identical in WT and PAR2^-/- mice, indicating that IL-6 does not act through PAR2 in this model.

CONCLUSIONS

These results indicate that PAR2 activation in the meninges can cause acute headache behavioral responses and priming to an NO donor, and support further exploration of PAR2 as a novel therapeutic target for migraine.

C781, a β-Arrestin Biased Antagonist at Protease-Activated Receptor-2 (PAR2), Displays in vivo Efficacy Against Protease-Induced Pain in Mice

Given the limited options and often harmful side effects of current analgesics and the suffering caused by the opioid crisis, new classes of pain therapeutics are needed. Protease-activated receptors (PARs), particularly PAR2, are implicated in a variety of pathologies, including pain. Since the discovery of the role of PAR2 in pain, development of potent and specific antagonists has been slow. In this study, we describe the in vivo characterization of a novel small molecule/peptidomimetic hybrid compound, C781, as a β-arrestin-biased PAR2 antagonist. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. Pharmacokinetic studies were done to assess pharmacokinetic/pharmacodynamic relationship in vivo. We used both prevention and reversal paradigms with protease treatment to determine whether C781 could attenuate protease-evoked pain. C781 effectively prevented and reversed mechanical and spontaneous nociceptive behaviors in response to small molecule PAR2 agonists, mast cell activators, and neutrophil elastase. The ED50 of C781 (intraperitoneal dosing) for inhibition of PAR2 agonist (20.9 ng 2-AT)-evoked nociception was 6.3 mg/kg. C781 was not efficacious in the carrageenan inflammation model. Pharmacokinetic studies indicated limited long-term systemic bioavailability for C781 suggesting that optimizing pharmacokinetic properties could improve in vivo efficacy. Our work demonstrates in vivo efficacy of a biased PAR2 antagonist that selectively inhibits β-arrestin/MAPK signaling downstream of PAR2. Given the importance of this signaling pathway in PAR2-evoked nociception, C781 exemplifies a key pharmacophore for PAR2 that can be optimized for clinical development. PERSPECTIVE: Our work provides evidence that PAR2 antagonists that only block certain aspects of signaling by the receptor can be effective for blocking protease-evoked pain in mice. This is important because it creates a rationale for developing safer PAR2-targeting approaches for pain treatment.

Involvement of Mast-Cell-Tryptase- and Protease-Activated Receptor 2-Mediated Signaling and Urothelial Barrier Dysfunction with Reduced Uroplakin II Expression in Bladder Hyperactivity Induced by Chronic Bladder Ischemia in the Rat

We aimed to investigate the relationship between mast cell (MC) infiltration into the bladder with urothelial barrier dysfunction and bladder hyperactivity in a chronic bladder ischemia (CBI) rat model. We compared CBI rats (CBI group; n = 10) with normal rats (control group; n = 10). We measured the expression of mast cell tryptase (MCT) and protease-activated receptor 2 (PAR2), which are correlated with C fiber activation via MCT, and Uroplakins (UP Ia, Ib, II and III), which are critical to urothelial barrier function, via Western blotting. The effects of FSLLRY-NH2, a PAR2 antagonist, administered intravenously, on the bladder function of CBI rats were evaluated with a cystometrogram. In the CBI group, the MC number in the bladder was significantly greater (p = 0.03), and the expression of MCT (p = 0.02) and PAR2 (p = 0.02) was significantly increased compared to that of the control group. The 10 μg/kg FSLLRY-NH2 injection significantly increased the micturition interval of CBI rats (p = 0.03). The percentage of UP-II-positive cells on the urothelium with immunohistochemical staining was significantly lower in the CBI group than in the control group (p < 0.01). Chronic ischemia induces urothelial barrier dysfunction via impairing UP II, consequently inducing MC infiltration into the bladder wall and increased PAR2 expression. PAR2 activation by MCT may contribute to bladder hyperactivity.

Electroacupuncture Regulates TRPV1 through PAR2/PKC Pathway to Alleviate Visceral Hypersensitivity in FD Rats

Visceral hypersensitivity (VH) is the predominant pathogenesis of functional dyspepsia (FD). Duodenal hypersensitivity along with nausea further reduces the comfort level in gastric balloon dilatation and inhibits gastric receptive relaxation. The potential mechanism behind electroacupuncture- (EA-) mediated alleviation of VH has not been elucidated. In an FD rat model with tail clamping stress, iodine acetamide (IA) induced VH. The rats were treated with EA with or without PAR2 antagonist FSLLRY-NH2, and the body weight, gastric sensitivity, compliance, and gastrointestinal motility were determined. Mast cells and activated degranulation were stained with toluidine blue (TB) staining and visualized under a transmission electron microscope (TEM). Immunofluorescence was used to detect the expression of PAR2, PKC, and TRPV1 in the duodenum and dorsal root ganglion (DRG) and that of CGRP, SP in DRG, and c-fos in the spinal cord. EA alone and EA + antagonist enhanced the gastrointestinal motility but diminished the expression of TRPV1, CGRP, SP, and c-fos-downstream of PAR2/PKC pathway and alleviated VH in FD rats. However, there was no obvious superposition effect between the antagonists and EA + antagonists. The effect of EA alone was better than that of antagonists and EA + antagonists 2 alone. EA-induced amelioration of VH in FD rats was mediated by TRPV1 regulation through PAR2/PKC pathway. This protective mechanism involved several pathways and included several targets.

Subsynaptic positioning of AMPARs by LRRTM2 controls synaptic strength

Recent evidence suggests that nano-organization of proteins within synapses may control the strength of communication between neurons in the brain. The unique subsynaptic distribution of glutamate receptors, which cluster in nanoalignment with presynaptic sites of glutamate release, supports this hypothesis. However, testing it has been difficult because mechanisms controlling subsynaptic organization remain unknown. Reasoning that transcellular interactions could position AMPA receptors (AMPARs), we targeted a key transsynaptic adhesion molecule implicated in controlling AMPAR number, LRRTM2, using engineered, rapid proteolysis. Severing the LRRTM2 extracellular domain led quickly to nanoscale declustering of AMPARs away from release sites, not prompting their escape from synapses until much later. This rapid remodeling of AMPAR position produced significant deficits in evoked, but not spontaneous, postsynaptic receptor activation. These results dissociate receptor numbers from their nanopositioning in determination of synaptic function and support the novel concept that adhesion molecules acutely position receptors to dynamically control synaptic strength.

Bone marrow derived mast cells injected into the osteoarthritic knee joints of mice induced by sodium monoiodoacetate enhanced spontaneous pain through activation of PAR2 and action of extracellular ATP

Conditions that resemble osteoarthritis (OA) were produced by injection of sodium monoiodoacetate (MIA) into the knee joints of mice. Bone marrow derived mast cells (BMMCs) injected into the OA knee joints enhanced spontaneous pain. Since no spontaneous pain was observed when BMMCs were injected into the knee joints of control mice that had not been treated with MIA, BMMCs should be activated within the OA knee joints and release some pain-inducible factors. Protease activated receptor-2 (PAR2) antagonist (FSLLRY-NH₂) almost abolished the pain-enhancing effects of BMMCs injected into the OA knee joints, suggesting that tryptase, a mast cell protease that is capable of activating PAR2, should be released from the injected BMMCs and enhance pain through activation of PAR2. When PAR2 agonist (SLIGKV-NH₂) instead of BMMCs was injected into the OA knee joints, it was also enhanced pain. Apyrase, an ATP degrading enzyme, injected into the OA knee joints before BMMCs suppressed the pain enhanced by BMMCs. We showed that purinoceptors (P2X4 and P2X7) were expressed in BMMCs and that extracellular ATP stimulated the release of tryptase from BMMCs. These observations suggest that ATP may stimulate degranulation of BMMCs and thereby enhanced pain. BMMCs injected into the OA knee joints stimulated expression of IL-1β, IL-6, TNF-α, CCL2, and MMP9 genes in the infrapatellar fat pads, and PAR2 antagonist suppressed the stimulatory effects of BMMCs. Our study suggests that intermittent pain frequently observed in OA knee joints may be due, at least partly, to mast cells through activation of PAR2 and action of ATP, and that intraarticular injection of BMMCs into the OA knee joints may provide a useful experimental system for investigating molecular mechanisms by which pain is induced in OA knee joints.

Cold pain sensitivity is associated with single-nucleotide polymorphisms of PAR2/F2RL1 and TRPM8

Pain sensitivity differs individually, but the mechanisms and genetic factors that underlie these differences are not fully understood. To investigate genetic factors that are involved in sensing cold pain, we applied a cold-induced pain test and evaluated protease-activated receptor 2 (PAR2/F2RL1) and transient receptor potential melastatin 8 (TRPM8), which are related to pain. We statistically investigated the associations between genetic polymorphisms and cold pain sensitivity in 461 healthy patients who were scheduled to undergo cosmetic orthognathic surgery for mandibular prognathism. We found an association between cold pain sensitivity and the rs2243057 polymorphism of the PAR2 gene. We also found a significant association between cold pain sensitivity and the rs12992084 polymorphism of the TRPM8 gene. Carriers of the minor A allele of the rs2243057 polymorphism of PAR2 and minor C allele of the rs12992084 polymorphism of TRPM8 exhibited a longer latency to pain perception in the cold-induced pain test, reflecting a decrease in cold pain sensitivity. These results suggest that genetic polymorphisms of both PAR2 and TRPM8 are involved in individual differences in cold pain sensitivity.

Nociceptive Sensitization by Activation of Protease-Activated Receptor 2 in a Rat Model of Incisional Pain

Postoperative pain and consequent inflammatory responses after tissue incision adversely affects many surgical patients due to complicated mechanisms. In this study, we examined whether activation of protease-activated receptor 2 (PAR-2), which is stimulated by tryptase from mast cells, elicits nociception and whether the PAR-2 antagonist could reduce incisional nociceptive responses in vivo and in vitro. The effects of a selective PAR-2 antagonist, N3-methylbutyryl-N-6-aminohexanoyl-piperazine (ENMD-1068), pretreatment on pain behaviors were assessed after plantar incision in rats. The effects of a PAR-2 agonist, SLIGRL-NH₂, on nociception was assessed after the injection into the hind paw. Furthermore, the responses of C-mechanosensitive nociceptors to the PAR-2 agonist were observed using an in vitro skin–nerve preparation as well. Intraplantar injection of SLIGRL-NH₂ elicited spontaneous nociceptive behavior and hyperalgesia. Local administration of ENMD-1068 suppressed guarding behaviors, mechanical and heat hyperalgesia only within the first few hours after incision. SLIGRL-NH₂ caused ongoing activity in 47% of C-mechanonociceptors in vitro. This study suggests that PAR-2 may support early nociception after incision by direct or indirect sensitization of C-fibers in rats. Moreover, PAR-2 may play a regulatory role in the early period of postoperative pain together with other co-factors to that contribute to postoperative pain.

Spinal PAR2 Activation Contributes to Hypersensitivity Induced by Peripheral Inflammation in Rats

The mechanisms of inflammatory pain need to be identified in order to find new superior treatments. Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) are highly co-expressed in dorsal root ganglion neurons and implicated in pain development. Here, we examined the role of spinal PAR2 in hyperalgesia and the modulation of synaptic transmission in carrageenan-induced peripheral inflammation, using intrathecal (i.t.) treatment in the behavioral experiments and recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs and eEPSCs) in spinal cord slices. Intrathecal PAR2-activating peptide (AP) administration aggravated the carrageenan-induced thermal hyperalgesia, and this was prevented by a TRPV1 antagonist (SB 366791) and staurosporine i.t. pretreatment. Additionally, the frequency of the mEPSC and sEPSC and the amplitude of the eEPSC recorded from the superficial dorsal horn neurons were enhanced after acute PAR2 AP application, while prevented with SB 366791 or staurosporine pretreatment. PAR2 antagonist application reduced the thermal hyperalgesia and decreased the frequency of mEPSC and sEPSC and the amplitude of eEPSC. Our findings highlight the contribution of spinal PAR2 activation to carrageenan-induced hyperalgesia and the importance of dorsal horn PAR2 and TRPV1 receptor interactions in the modulation of nociceptive synaptic transmission.

Characterization and preclinical evaluation of a protease activated receptor 2 (PAR2) monoclonal antibody as a preventive therapy for migraine

AIM

Migraine pain is thought to result from activation of meningeal nociceptors that might involve dural mast cell degranulation and release of proteases and pronociceptive mediators. Tryptase, the most abundant dural mast cell protease, has been demonstrated to stimulate dural mast cells, as well as trigeminal nociceptors by activating the protease activated receptor 2. Mast cell or neuronal protease activated receptors 2 may therefore represent a novel target for migraine treatment. In this study, we characterized and evaluated a novel protease activated receptor 2 monoclonal antibody as a preventive anti-migraine pain therapy in preclinical models.

METHODS

Flow cytometry, immunocytochemistry, calcium imaging, Homogeneous Time Resolved Technology (HTRF) epitope competition assay and serum pharmacokinetic (PK) assay in rats were performed to confirm the activity, specificity and in vivo stability of PAR650097, a novel anti- protease activated receptor 2 monoclonal antibody. In vivo assessment was performed in female C57BL/6J mice by evaluation of PAR650097 in preventing cutaneous allodynia elicited by (a) supradural injection of the protease activated receptor 2 agonist, Ser-Leu-Ile-Gly-Arg-Leu-amide trifluoroacetate (SLIGRL), or calcitonin gene-related (CGRP) peptide, and (b) induction of latent sensitization by priming with three daily episodes of restraint stress followed by challenge with a subthreshold inhalational exposure to umbellulone (UMB), a transient receptor potential ankyrin 1 (TRPA1) agonist. PAR650097 was administered as a pretreatment prior to the first restraint stress, umbellulone exposure, SLIGRL or calcitonin gene-related peptide injection. Additionally, fremanezumab, a calcitonin gene-related peptide antibody was administered as pre-treatment prior to supradural administration of calcitonin gene-related peptide or SLIGRL.

RESULTS

In vitro, PAR650097 demonstrated rapid interaction with protease activated receptor 2, enabling it to fully inhibit protease-induced protease activated receptor 2 activation, in human and mouse cells, with high potency. Furthermore, PAR650097 was highly selective for protease activated receptor 2, demonstrating no affinity for protease activated receptor 1 protein and no functional effect on the activation of cellular protease activated receptor 1 with thrombin. In addition, PAR650097 had an acceptable PK profile, compatible with testing the effects of selective protease activated receptor 2 inhibition in vivo. In vivo, PAR650097 blocked cutaneous allodynia induced by either supradural SLIGRL or calcitonin gene-related peptide. Fremanezumab abolished cutaneous allodynia induced by supradural CGRP, and partially attenuated cutaneous allodynia induced by SLIGRL. Administration of PAR650097, before the first restraint stress episode, did not prevent the acute stress-induced cutaneous allodynia or restraint stress priming revealed by cutaneous allodynia induced by inhalational umbellulone. In contrast, PAR650097 prevented expression of cutaneous allodynia when given before the umbellulone challenge in restraint stress-primed animals.

CONCLUSION

PAR650097 specifically inhibits endogenously expressed protease activated receptor 2 in human and mouse cells with high potency. This antibody has an acceptable PK profile in rodents and effectively blocked SLIGR-induced cutaneous allodynia. PAR650097 additionally prevented cutaneous allodynia induced by supradural calcitonin gene-related peptide, indicating that the protease activated receptor 2 receptor is a downstream consequence of calcitonin gene-related peptide actions. Fremanezumab effectively blocked calcitonin gene-related peptide-induced cutaneous allodynia and only partially reduced cutaneous allodynia induced by a protease activated receptor 2 activator, suggesting both calcitonin gene-related peptide-dependent and -independent mechanisms in promoting migraine pain. While PAR650097 did not prevent stress-induced cutaneous allodynia or priming, it effectively prevented cutaneous allodynia induced by a TRPA1 agonist in animals with latent sensitization. Activation of protease activated receptor 2, therefore, contributes to both calcitonin gene-related peptide-dependent and -independent mechanisms in promoting migraine-like pain. Therapeutic targeting of protease activated receptor 2 receptors may represent an anti-migraine pain strategy with a potentially broad efficacy profile.

Mast Cell Activation, Neuroinflammation, and Tight Junction Protein Derangement in Acute Traumatic Brain Injury

Traumatic brain injury (TBI) is one of the major health problems worldwide that causes death or permanent disability through primary and secondary damages in the brain. TBI causes primary brain damage and activates glial cells and immune and inflammatory cells, including mast cells in the brain associated with neuroinflammatory responses that cause secondary brain damage. Though the survival rate and the neurological deficiencies have shown significant improvement in many TBI patients with newer therapeutic options, the underlying pathophysiology of TBI-mediated neuroinflammation, neurodegeneration, and cognitive dysfunctions is understudied. In this study, we analyzed mast cells and neuroinflammation in weight drop-induced TBI. We analyzed mast cell activation by toluidine blue staining, serum chemokine C-C motif ligand 2 (CCL2) level by enzyme-linked immunosorbent assay (ELISA), and proteinase-activated receptor-2 (PAR-2), a mast cell and inflammation-associated protein, vascular endothelial growth factor receptor 2 (VEGFR2), and blood-brain barrier tight junction-associated claudin 5 and Zonula occludens-1 (ZO-1) protein expression in the brains of TBI mice. Mast cell activation and its numbers increased in the brains of 24 h and 72 h TBI when compared with sham control brains without TBI. Mouse brains after TBI show increased CCL2, PAR-2, and VEGFR2 expression and derangement of claudin 5 and ZO-1 expression as compared with sham control brains. TBI can cause mast cell activation, neuroinflammation, and derangement of tight junction proteins associated with increased BBB permeability. We suggest that inhibition of mast cell activation can suppress neuroimmune responses and glial cell activation-associated neuroinflammation and neurodegeneration in TBI.

Serine proteases at the cutting edge of IBD: Focus on gastrointestinal inflammation

Serine proteases have been long recognized to coordinate many physiological processes and play key roles in regulating the inflammatory response. Accordingly, their dysregulation has been regularly associated with several inflammatory disorders and suggested as a central mechanism in the pathophysiology of digestive inflammation. So far, studies addressing the proteolytic homeostasis in the gut have mainly focused on host serine proteases as candidates of interest, while largely ignoring the potential contribution of their bacterial counterparts. The human gut microbiota comprises a complex ecosystem that contributes to host health and disease. Yet, our understanding of microbially produced serine proteases and investigation of whether they are causally linked to IBD is still in its infancy. In this review, we highlight recent advances in the emerging roles of host and bacterial serine proteases in digestive inflammation. We also discuss the application of available tools in the gut to monitor disease-related serine proteases. An exhaustive representation and understanding of such functional potential would help in closing existing gaps in mechanistic knowledge.

N-acetylcysteine (NAC) alleviates the peripheral neuropathy associated with liver cirrhosis via modulation of neural MEG3/PAR2/ NF-ҡB axis

BACKGROUND AND AIM

Oxidative stress (OS) is accused in pathogenesis of many diseases, including liver cirrhosis by many mechanisms. One of them is the disturbance of long non coding maternally expressed 3 (MEG3)/protease activated receptor 2 (PAR2) downstream pathway. We aimed to investigate the role of this axis in cirrhotic neuropathy and whether an antioxidant compound such as N-acetylcysteine (NAC) could improve the peripheral nerve function through repression of MEG3/PAR2.

METHODS

Thirty Wistar rats were used and divided into 5 groups; naive, thiacetamide (TAA) (200 mg/kg 3 times/week. i.p. for 8 weeks) and TAA+NAC (50 or 100 or 200 mg/kg/day) groups. Von Frey (VF) test for mechanical nociceptive responses, hepatic& neural MEG3, NF-ҡB and neural PAR2 expression by PCR, histological studies for liver and sciatic nerve together with the dorsopedal skin thickness were done.

RESULTS

TAA induced significant decrease in liver function, negative VF test, an increase in the expression of hepatic& neural MEG3, NF-ҡB and neural PAR2. The histological studies showed cirrhotic changes with atrophy of the sciatic nerve and the dorsal skin. NAC improved the liver function together with reversal of the neural: functional, biochemical and histological changes in a dose dependent manner.

CONCLUSIONS

NAC could improve the peripheral neuropathy in cirrhotic rat through suppression of MEG3/PAR2 expression.

Role of the Bone Microenvironment in the Development of Painful Complications of Skeletal Metastases

Cancer-induced bone pain (CIBP) is the most common and painful complication in patients with bone metastases. It causes a significant reduction in patient quality of life. Available analgesic treatments for CIBP, such as opioids that target the central nervous system, come with severe side effects as well as the risk of abuse and addiction. Therefore, alternative treatments for CIBP are desperately needed. Although the exact mechanisms of CIBP have not been fully elucidated, recent studies using preclinical models have demonstrated the role of the bone marrow microenvironment (e.g., osteoclasts, osteoblasts, macrophages, mast cells, mesenchymal stem cells, and fibroblasts) in CIBP development. Several clinical trials have been performed based on these findings. CIBP is a complex and challenging condition that currently has no standard effective treatments other than opioids. Further studies are clearly warranted to better understand this painful condition and develop more effective and safer targeted therapies.

Transcriptional Profiling of Somatostatin Interneurons in the Spinal Dorsal Horn

The spinal dorsal horn (SDH) is comprised of distinct neuronal populations that process different somatosensory modalities. Somatostatin (SST)-expressing interneurons in the SDH have been implicated specifically in mediating mechanical pain. Identifying the transcriptomic profile of SST neurons could elucidate the unique genetic features of this population and enable selective analgesic targeting. To that end, we combined the Isolation of Nuclei Tagged in Specific Cell Types (INTACT) method and Fluorescence Activated Nuclei Sorting (FANS) to capture tagged SST nuclei in the SDH of adult male mice. Using RNA-sequencing (RNA-seq), we uncovered more than 13,000 genes. Differential gene expression analysis revealed more than 900 genes with at least 2-fold enrichment. In addition to many known dorsal horn genes, we identified and validated several novel transcripts from pharmacologically tractable functional classes: Carbonic Anhydrase 12 (Car12), Phosphodiesterase 11 A (Pde11a), and Protease-Activated Receptor 3 (F2rl2). In situ hybridization of these novel genes showed differential expression patterns in the SDH, demonstrating the presence of transcriptionally distinct subpopulations within the SST population. Overall, our findings provide new insights into the gene repertoire of SST dorsal horn neurons and reveal several novel targets for pharmacological modulation of this pain-mediating population and treatment of pathological pain.

Protease-Mediated Suppression of DRG Neuron Excitability by Commensal Bacteria

Peripheral pain signaling reflects a balance of pronociceptive and antinociceptive influences; the contribution by the gastrointestinal microbiota to this balance has received little attention. Disorders, such as inflammatory bowel disease and irritable bowel syndrome, are associated with exaggerated visceral nociceptive actions that may involve altered microbial signaling, particularly given the evidence for bacterial dysbiosis. Thus, we tested whether a community of commensal gastrointestinal bacteria derived from a healthy human donor (microbial ecosystem therapeutics; MET-1) can affect the excitability of male mouse DRG neurons. MET-1 reduced the excitability of DRG neurons by significantly increasing rheobase, decreasing responses to capsaicin (2 μm) and reducing action potential discharge from colonic afferent nerves. The increase in rheobase was accompanied by an increase in the amplitude of voltage-gated K⁺ currents. A mixture of bacterial protease inhibitors abrogated the effect of MET-1 effects on DRG neuron rheobase. A serine protease inhibitor but not inhibitors of cysteine proteases, acid proteases, metalloproteases, or aminopeptidases abolished the effects of MET-1. The serine protease cathepsin G recapitulated the effects of MET-1 on DRG neurons. Inhibition of protease-activated receptor-4 (PAR-4), but not PAR-2, blocked the effects of MET-1. Furthermore, Faecalibacterium prausnitzii recapitulated the effects of MET-1 on excitability of DRG neurons. We conclude that serine proteases derived from commensal bacteria can directly impact the excitability of DRG neurons, through PAR-4 activation. The ability of microbiota-neuronal interactions to modulate afferent signaling suggests that therapies that induce or correct microbial dysbiosis may impact visceral pain.SIGNIFICANCE STATEMENT Commercially available probiotics have the potential to modify visceral pain. Here we show that secretory products from gastrointestinal microbiota derived from a human donor signal to DRG neurons. Their secretory products contain serine proteases that suppress excitability via activation of protease-activated receptor-4. Moreover, from this community of commensal microbes, Faecalibacterium prausnitzii strain 16-6-I 40 fastidious anaerobe agar had the greatest effect. Our study suggests that therapies that induce or correct microbial dysbiosis may affect the excitability of primary afferent neurons, many of which are nociceptive. Furthermore, identification of the bacterial strains capable of suppressing sensory neuron excitability, and their mechanisms of action, may allow therapeutic relief for patients with gastrointestinal diseases associated with pain.

Sensitization of ASIC3 by proteinase-activated receptor 2 signaling contributes to acidosis-induced nociception

Background

Tissue acidosis and inflammatory mediators play critical roles in pain. Pro-inflammatory agents trypsin and tryptase cleave and activate proteinase-activated receptor 2 (PAR₂) expressed on sensory nerves, which is involved in peripheral mechanisms of inflammation and pain. Extracellular acidosis activates acid-sensing ion channel 3 (ASIC3) to trigger pain sensation. Here, we show that a functional interaction of PAR₂ and ASIC3 could contribute to acidosis-induced nociception.

Methods

Electrophysiological experiments were performed on both rat DRG neurons and Chinese hamster ovary (CHO) cells expressing ASIC3 and PAR₂. Nociceptive behavior was induced by acetic acid in rats.

Results

PAR₂-AP, PAR₂-activating peptide, concentration-dependently increased the ASIC3 currents in CHO cells transfected with ASIC3 and PAR₂. The proton concentration–response relationship was not changed, but that the maximal response increased 58.7 ± 3.8% after pretreatment of PAR₂-AP. PAR₂ mediated the potentiation of ASIC3 currents via an intracellular cascade. PAR₂-AP potentiation of ASIC3 currents disappeared after inhibition of intracellular G protein, PLC, PKC, or PKA signaling. Moreover, PAR₂ activation increased proton-evoked currents and spikes mediated by ASIC3 in rat dorsal root ganglion neurons. Finally, peripheral administration of PAR₂-AP dose-dependently exacerbated acidosis-induced nocifensive behaviors in rats.

Conclusions

These results indicated that PAR₂ signaling sensitized ASIC3, which may contribute to acidosis-induced nociception. These represent a novel peripheral mechanism underlying PAR₂ involvement in hyperalgesia by sensitizing ASIC3 in primary sensory neurons.

Homeostatic effects of coagulation protease-dependent signaling and protease activated receptors

A homeostatic function of the coagulation system in regard to hemostasis is well established. Homeostasis of blood coagulation depends partially on protease activated receptor (PAR)-signaling. Beyond coagulation proteases, numerous other soluble and cell-bound proteases convey cellular effects via PAR signaling. As we learn more about the mechanisms underlying cell-, tissue-, and context-specific PAR signaling, we concurrently gain new insights into physiological and pathophysiological functions of PARs. In this regard, regulation of cell and tissue homeostasis by PAR signaling is an evolving scheme. Akin to the control of blood clotting per se (the fibrin-platelet interaction) coagulation proteases coordinately regulate cell- and tissue-specific functions. This review summarizes recent insights into homeostatic regulation through PAR signaling, focusing on blood coagulation proteases. Considering the common use of drugs altering coagulation protease activity through either broad or targeted inhibitory activities, and the advent of PAR modulating drugs, an in-depth understanding of the mechanisms through which coagulation proteases and PAR signaling regulate not only hemostasis, but also cell and tissue homeostasis is required.