Inflammatory Pain Signaling Research Articles

Unraveling the Mechanism of Action of Ubiquitin-Specific Protease 5 and Its Inhibitors in Tumors.

Ubiquitin-specific protease 5 (USP5), a member of the ubiquitin-specific proteases (USPs) family, functions by specifically removing ubiquitin chains from target proteins for stabilization and degrading unbound polyubiquitin chains to maintain a steady-state monoubiquitin pool. Ubiquitin-specific protease 5 regulates various cellular activities, including DNA double-strand break repair, transmission of neuropathic and inflammatory pain signals, immune response, and tumor cell proliferation. Furthermore, USP5 is involved in the development of multiple tumors such as liver, lung, pancreatic, and breast cancers as well as melanoma. Downstream regulatory mechanisms associated with USP5 are complex and diverse. Ubiquitin-specific protease 5 has been revealed as an emerging target for tumor treatment. This study has introduced some molecules upstream to control the expression of USP5 at the levels of transcription, translation, and post-translation. Furthermore, the study incorporated inhibitors known to be associated with USP5, including partially selective deubiquitinase (DUB) inhibitors such as WP1130, EOAI3402143, vialinin A, and chalcone derivatives. It also included the ubiquitin-activating enzyme E1 inhibitor, PYR-41. These small molecule inhibitors impact the occurrence and development of various tumors. Therefore, this article comprehensively reviews the pivotal role of USP5 in different signaling pathways during tumor progression and resumes the progress made in developing USP5 inhibitors, providing a theoretical foundation for their clinical translation.

Investigation of Neuropathology after Nerve Release in Chronic Constriction Injury of Rat Sciatic Nerve.

Peripheral compressive neuropathy causes significant neuropathic pain, muscle weakness and prolong neuroinflammation. Surgical decompression remains the gold standard of treatment but the outcome is suboptimal with a high recurrence rate. From mechanical compression to chemical propagation of the local inflammatory signals, little is known about the distinct neuropathologic patterns and the genetic signatures after nerve decompression. In this study, controllable mechanical constriction forces over rat sciatic nerve induces irreversible sensorimotor dysfunction with sustained local neuroinflammation, even 4 weeks after nerve release. Significant gene upregulations are found in the dorsal root ganglia, regarding inflammatory, proapoptotic and neuropathic pain signals. Genetic profiling of neuroinflammation at the local injured nerve reveals persistent upregulation of multiple genes involving oxysterol metabolism, neuronal apoptosis, and proliferation after nerve release. Further validation of the independent roles of each signal pathway will contribute to molecular therapies for compressive neuropathy in the future.

TRPV1 Activation Promotes β-arrestin2 Interaction with the Ribosomal Biogenesis Machinery in the Nucleolus:Implications for p53 Regulation and Neurite Outgrowth.

Transient receptor potential vanilloids (TRPV1) are non-selective cation channels that sense and transduce inflammatory pain signals. We previously reported that activation of TRPV1 induced the translocation of β-arrestin2 (ARRB2) from the cytoplasm to the nucleus, raising questions about the functional role of ARRB2 in the nucleus. Here, we determined the ARRB2 nuclear signalosome by conducting a quantitative proteomic analysis of the nucleus-sequestered L395Q ARRB2 mutant, compared to the cytosolic wild-type ARRB2 (WT ARRB2), in a heterologous expression system. We identified clusters of proteins that localize to the nucleolus and are involved in ribosomal biogenesis. Accordingly, L395Q ARRB2 or WT ARRB2 after capsaicin treatment were found to co-localize and interact with the nucleolar marker nucleophosmin (NPM1), treacle protein (TCOF1) and RNA polymerase I (POL I). We further investigated the role of nuclear ARRB2 signaling in regulating neuroplasticity. Using neuroblastoma (neuro2a) cells and dorsal root ganglia (DRG) neurons, we found that L395Q ARRB2 mutant increased POL I activity, inhibited the tumor suppressorp53 (p53) level and caused a decrease in the outgrowth of neurites. Together, our results suggest that the activation of TRPV1 promotes the ARRB2-mediated regulation of ribosomal biogenesis in the nucleolus. The ARRB2-TCOF1-p53 checkpoint signaling pathway might be involved in regulating neurite outgrowth associated with pathological pain conditions.

CaMKII binding to GluN2B at S1303 has no role in acute or inflammatory pain

Activation of Ca2+/calmodulin kinase II (CaMKII) and the N-Methyl D-aspartate receptor (NMDAR), particularly its GluN2B subunit, contribute to the central sensitization of nociceptive pathways and persistent pain. Using mutant mice wherein the activity-driven binding of CaMKII to S1303 in GluN2B is abrogated (GluN2BKI), this study investigated the importance of this interaction for acute and persistent inflammatory nociception. GluN2BKI, wild type and heterozygote mice did not differ in responses to acute noxious heat stimuli as measured with tail flick, paw flick, or hot plate assays, nor did they differ in their responses to mechanical stimulation with von Frey filaments. Surprisingly, the three genotypes exhibited similar spontaneous pain behaviors and hypersensitivity to heat or mechanical stimuli induced by intraplantar injection of capsaicin; however, GluN2BKI mice did not immediately attend to the paw. WT and GluN2BKI mice also did not differ in the nociceptive behaviors elicited by intraplantar injection of formalin, even though MK801 greatly reduced these behaviors in both genotypes concordant with NMDAR dependence. CaMKII binding to GluN2B at S1303 therefore does not appear to be critical for the development of inflammatory nociception. Finally, intrathecal KN93 reduced formalin-induced nociceptive behaviors in GluN2BKI mice. KN93 does not inhibit CaKMII, but rather binds Ca2+/calmodulin. It has multiple other targets including Ca2+-, Na+- and K+-channels, as well as various kinases. Therefore, the use of GluN2BKI mice provided genetic specificity in assessing the role of CaMKII in inflammatory pain signaling cascades. These results challenge current thinking on the involvement of the CaMKII-NMDAR interaction in inflammatory pain.

Regulation of TRPM8 channel activity by Src-mediated tyrosine phosphorylation.

The transient receptor potential melastatin type 8 (TRPM8) receptor channel is expressed in primary afferent neurons where it is the main transducer of innocuous cold temperatures and also in a variety of tumors, where it is involved in progression and metastasis. Modulation of this channel by intracellular signaling pathways has therefore important clinical implications. We investigated the modulation of recombinant and natively expressed TRPM8 by the Src kinase, which is known to be involved in cancer pathophysiology and inflammation. Human TRPM8 expressed in HEK293T cells is constitutively tyrosine phosphorylated by Src which is expressed either heterologously or endogenously. Src action on TRPM8 potentiates its activity, as treatment with PP2, a selective Src kinase inhibitor, reduces both TRPM8 tyrosine phosphorylation and cold-induced channel activation. RNA interference directed against the Src kinase diminished the extent of PP2-induced functional downregulation of TRPM8, confirming that PP2 acts mainly through Src inhibition. Finally, the effect of PP2 on TRPM8 cold activation was reproduced in cultured rat dorsal root ganglion neurons, and this action was antagonized by the protein tyrosine phosphatase inhibitor pervanadate, confirming that TRPM8 activity is sensitive to the cellular balance between tyrosine kinases and phosphatases. This positive modulation of TRPM8 by Src kinase may be relevant for inflammatory pain and cancer signaling.

α7 nicotinic acetylcholine receptor in tumor-associated macrophages inhibits colorectal cancer metastasis through the JAK2/STAT3 signaling pathway.

Considerable evidence has implied that α7 nicotinic receptor subtypes play an important role in chronic inflammatory and neuropathic pain signaling. The aim of the present study was to determine the role of endogenous α7nAChR signaling in tumor-associated macrophages (TAMs) in human colorectal cancer (CRC) metastasis and prognosis. α7nAChR expression in primary tumor cells and adjacent stroma cells especially in TAMs in 51 CRC patients was observed. Using a human monocyte THP-derived macrophages (TMs) with α7nAChR-siRNA knockdown (TMα7−/−) and a CRC cell Transwell co-culture model, the migration and invasion of two CRC cells, LoVo and SW620, were determined. Western blotting was carried out to investigate the expression of multiple molecules involved in the NF-κB, STAT3, PI3K signaling pathways in mimic TAMs, i.e., TMs exposed to in-direct LoVo cell stimulation. A nicotinic α7 receptor antagonist [α-bungarotoxin (α-Btx)] and three pharmaceutical inhibitors: AG490 (JAK2/STAT3 inhibitor), LY294002 (PI3K inhibitor) and Bay 11–7082 (NF-κB inhibitor) were applied to evaluate whether these signaling pathways were associated with the enhanced migration of CRC cells when co-cultured with α7nAChR knockdown TMs. The results revealed that the expression of α7nAChR in TAMs differed in patients. However, CRC patients who had a high incidence of hepatic metastasis showed no or low expression of α7nAChR in TAMs. TMs with α7nAChR-siRNA knockdown (TMα7−/−) significantly enhanced the migration and invasion of the two CRC cell lines LoVo and SW620. α7nAChR knockdown in TMs significantly downregulated phosphorylation of STAT3, PI3K p85 and NF-κB p65 after co-culturing with LoVo cells. Inhibition of JAK2/STAT3 prevented the TMα7−/−-enhanced migration of LoVo cells. α7nAChR expressed in TAMs in human CRC patients plays an important role in preventing metastasis and could be a prognostic marker in CRCs, which may be regulated by the JAK2/STAT3 signaling pathway.

Pellitorine, an extract of Tetradium daniellii, is an antagonist of the ion channel TRPV1

BackgroundTransient Receptor Potential Vanilloid 1 (TRPV1) confers noxious heat and inflammatory pain signals in the peripheral nervous system. Clinical trial of resiniferatoxin from Euphorbia species is successfully aimed at TRPV1 in cancer pain management and heading toward new selective painkiller status that further validates this target for drug discovery efforts. Evodia species, used in traditional medicine for hundreds of years, are a recognised source of different TRPV1 agonists, but no antagonist has yet been reported. Hypothesis/PurposeIn a search for painkiller leads, we noted for the first time a TRPV1 antagonist activity in the fresh fruits of Tetradium daniellii (Benn.) T.G. Hartley (syn. Evodia hupehensis Dode). MethodsThrough a combination of extraction and purification methods with functional TRPV1-specific Ca2+ uptake assays (bioactivity-guided fractionation/isolation/purification); we isolated a new painkiller candidate that is a distant structural homologue of capsiate exovanilloids and endovanilloids such as anandamide, but a putative competitive inhibitor of the TRPV1. Four additional inactive compounds (N-isobutyl-4,5-epoxy-2E-decadienamide, geranylpsoralen, 8-(7′,8′-epoxygeranyloxy)psoralen, and xanthotoxol) were also co-purified with pellitorine. Their structures were established by extensive 1D- and 2D-NMR spectroscopic analysis. Results1H- and 13C NMR determination of the chemical structure revealed it to be pellitorine, (2E,4E)-N-(2-methylpropyl)deca-2,4-dienamide, which can compete structurally with algesics released in inflammation. In contrast to previous isolates from Evodia species, pellitorine blocked capsaicin-evoked Ca2+ uptake with an IC50 of 154 µg/ml (0.69 mM/l). N-Isobutyl-4,5-epoxy-2E-decadienamide and geranylpsoralen, 8-(7′,8′-epoxygeranyloxy)psoralen, and xanthotoxol did not affect the TRPV1. ConclusionThis is the first evidence that pellitorine, an aliphatic alkylamide analogue of capsaicin, can serve as an antagonist of the TRPV1 and may inhibit exovanilloid-induced pain.

Structural Insight into the Molecular Mechanism of TRPA1 Inhibition and Activation

Pain, while serving the beneficial function of provoking our attention to dangerous situations, is an unpleasant sensory and emotional experience. Transient Receptor Potential Ankyrin 1 (TRPA1) is a member of the transient receptor potential (TRP) cation channel family and is localized in nerve fibers called “nociceptors” where it plays a key role in the transduction of chemical, inflammatory and neuropathic pain signals from the periphery to the brain.TRPA1 is a non-selective ion channel, which is activated by a variety of structurally diverse compounds. TRPA1 agonists are exogenous electrophilic compounds and endogenous thiol-reactive inflammatory mediators, all of which target and activate the channel via a cluster of cysteine residues at the N-terminus. Activation is clearly dependent on a thiol-reactive moiety, thus explaining the structural diversity of this group. Although near-atomic resolution structures of TRPA1 were resolved recently by cryo-EM, both in the presence of agonist and antagonists, detailed mechanisms of channel activation and inhibition by these modulators could not be determined.To determine the structural details of TRPA1 conformation upon agonist and antagonist binding, we used limited proteolysis and mass spectrometry. Our study indicates that conformational rearrangements of the N-terminus ankyrin repeats and the linker region between C-terminus and the transmembrane domain contribute to the channel modulation.

Probing the Effects and Mechanisms of Electroacupuncture at Ipsilateral or Contralateral ST36-ST37 Acupoints on CFA-induced Inflammatory Pain.

Transient receptor potential vanilloid 1 (TRPV1) and associated signaling pathways have been reported to be increased in inflammatory pain signaling. There are accumulating evidences surrounding the therapeutic effect of electroacupuncture (EA). EA can reliably attenuate the increase of TRPV1 in mouse inflammatory pain models with unclear signaling mechanisms. Moreover, the difference in the clinical therapeutic effects between using the contralateral and ipsilateral acupoints has been rarely studied. We found that inflammatory pain, which was induced by injecting the complete Freund’s adjuvant (CFA), (2.14 ± 0.1, p < 0.05, n = 8) can be alleviated after EA treatment at either ipsilateral (3.91 ± 0.21, p < 0.05, n = 8) or contralateral acupoints (3.79 ± 0.25, p < 0.05, n = 8). EA may also reduce nociceptive Nav sodium currents in dorsal root ganglion (DRG) neurons. The expression of TRPV1 and associated signaling pathways notably increased after the CFA injection; this expression can be further attenuated significantly in EA treatment. TRPV1 and associated signaling pathways can be prevented in TRPV1 knockout mice, suggesting that TRPV1 knockout mice are resistant to inflammatory pain. Through this study, we have increased the understanding of the mechanism that both ipsilateral and contralateral EA might alter TRPV1 and associated signaling pathways to reduce inflammatory pain.

CHRNA7 inhibits cell invasion and metastasis of LoVo human colorectal cancer cells through PI3K/Akt signaling.

The α7 neuronal nicotinic receptor gene (CHRNA7) is widely expressed in both the brain and periphery whereas its encoding protein of α7 neuronal acetylcholine receptor (α7nAChR) belongs to the nicotinic acetylcholine receptor family. Considerable evidence suggests that α7nAChR plays an important role in chronic inflammatory and neuropathic pain signaling and thus has been proposed as a potential target for treating cognitive deficits in patients with schizophrenia, attention deficit hyperactivity disorder (ADHD) and Alzheimer's disease. The aim of the present study was to determine the role of endogenous α7nAChR signaling in human colorectal cancer growth and metastasis. pLVX‑CHRNA7 encoding the full length of CHRNA7 was constructed and transfected into LoVo human colorectal cancer cells. Cell proliferation was measured by Cell Counting Kit‑8 (CCK‑8), and cell migration and invasion were detected by Transwell chamber assays. Expression and activity of metastasis‑related metalloproteinases (MMPs) were analyzed by western blotting and gelatin zymography, respectively. Activation of metastasis-related signaling molecules was detected by western blotting. LY294002 was used to specifically block the phosphatidylinositol 3‑kinase/v‑akt murine thymoma viral oncogene homologue (PI3K/Akt) pathway. We showed that concomitantly with an increase in α7nAChR expression after transfection, LoVo cells presented reduced abilities for migration and invasion, which was accompanied by reduced expression levels of MMP‑1 and MMP‑9 as well as activation of the PI3K/Akt signaling pathway. The application of LY294002 restored the migration and invasion abilities of the LoVo cells bearing CHRNA7. Collectively, we conclude that overexpression of CHRNA7 negatively controls colorectal cancer LoVo cell invasion and metastasis via PI3K/Akt pathway activation and may serve as either a diagnostic marker or a therapeutic target for colorectal cancer metastasis.

Z944: a first in class T-type calcium channel modulator for the treatment of pain.

Zalicus, Inc. is a small, publicly-traded biotech company in Cambridge, Massachusetts. The potential target for neuropathic pain lies in the T-type calcium channels, members of the low-voltage activated channel family. By opening at relatively negative membrane depolarizations, T-type channels can act as modulators of the threshold of the action potential by increasing the calcium current, leading to further depolarization of the cell and activation of other calcium channels, and ultimately, sodium and potassium channels. This is a target that is widely expressed in both the peripheral as well as the central nervous system (Zamponi et al., 2009). It is known that T-type calcium channel blockade can modulate pain behavior and disrupting this mechanism can reduce pain in neuropathic, inflammatory, and visceral pain models. There is evidence that the calcium channel protein itself as well as the current that is delivered by this channel is increased in certain pain models (Marger et al., 2011). Another important feature of this target is its central action. There are three subtypes of the T-type calcium channel, denoted Cav3.1, 3.2, and 3.3. They are expressed in different places in the thalamus, the cerebellum, and other parts of the brain. It is thought that they participate in the regulation of connectivity or crosstalk between the sensory nervous system and the cortex by promoting thalamic neuron burst firing and network oscillations important for relay and interpretation of signals to and by the cortex. A number of central nervous system (CNS) diseases have been linked to aberrant thalamocortical oscillations (thalamocortical dysrhythmia), and there is evidence that T-type calcium channel mutants can play a role in hyperexcitability disorders, for example, epilepsy syndromes. Some approved anticonvulsants also hit the T-type channel as part of their polypharmacology. Zalicus' approach has been to discover and develop a small molecule blocker of T-type calcium channels as a potential therapeutic for pain. The medication, Z944, is a novel, oral, state-dependent, selective T-type calcium channel blocker. This blocker has >150-fold selectivity vs. non-T-type voltage-gated ion channels and has been shown to inhibit native neuronal T-type currents at high potency. The blockade of Cav3.2 by Z944 is more pronounced during high-frequency firing and may provide some advantage to the molecule by selectively targeting a pathological state of the channel (Short et al., 2013). A phase 1, double-blind, placebo-controlled study to determine the safety, tolerability, and pharmacokinetics of single and multiple ascending doses of oral Z944 in healthy male subjects in the fed and fasted states using an immediate release formulation showed that the medication was safe and well tolerated. Mild and expected CNS side effects were observed; no serious adverse events were reported. A phase 1b clinical study utilized a state-of-the-art experimental clinical model measuring Laser-Evoked Potentials (LEP) from skin irritated following topical application of capsaicin (modeling neuropathic pain signaling and central sensitization) or exposure to UV light (modeling inflammatory pain signaling). Healthy volunteers received 20, 40, and 80 mg doses split in four administrations every 2 h vs. placebo. The data showed that Z944 reduced peak-to-peak amplitudes of LEPs from both capsaicin and UV irritated skin models and reduced subjective Visual Analog Scale (VAS) pain scores in both skin types. The noted adverse events were generally mild to moderate and primarily central nervous system related. No serious adverse events occurred in this study. On the basis of these findings, several modified release formulations of Z944 are currently being advanced in clinical development for pain indications.

Anti-hyperalgesic and anti-allodynic activities of capillarisin via suppression of inflammatory signaling in animal model

Ethnopharmacological relevanceArtemisia capillaris has widespread traditional and pharmacological applications such as analgesic, anti-inflammatory, anti-pyretic, enhance immunity and anti-tumor activity properties. To evaluate the pharmacological activities of this plant, capillarisin, one of the potent constituent of Artemisia capillaris was studied based on anti-hyperalgesic and anti-allodynic effects with detailed mechanism. It can be assumed that measurement of anti-nociceptive effects of capillarisin is one of the parameter for the evaluation of this herb. Capillarisin has extensive pharmacological properties and has been considered to have promising ant-inflammatory and anti-nociceptive activities. The aim of the current study is to investigate the effect of capillarisin and underlying molecular mechanisms of action in preventing acute and subchronic inflammatory pain. Materials and methodsThe inflammatory pain was induced after 40min or 1h of administration of vehicle, 70% EtOH extract of Artemisia capillaris (100mg/kg) or capillarisin (20 and 80mg/kg) by intraplantar (i.p.l.) injections of CFA and carrageenan in ICR mice, respectively. Mechanical hyperalgesia and allodynia were evaluated in both acute and subchronic models. Further analysis was performed in CFA-induced mice exploring various molecular and signaling pathways such as NF-κB, AP-1, and ERK-CREB involved in the persistent pain sensations. ResultsIn acute model, mechanical hyperalgesia and allodynia were evaluated after every 2h until 6h of CFA and after 4h of carrageenan injections. Whereas, in subchronic inflammatory pain model, mechanical hyperalgesia and paw edema were measured after 4h of CFA injection and every day after 4h of daily treatment until 5 days with interval of day four in order to assess the tolerance effect of capillarisin. Further analysis was performed in CFA-induced mice exploring various molecular and signaling pathways such as NF-κB, AP-1 and ERK-CREB involved in the persistent of pain sensations. Pre-treatment of capillarisin strongly inhibited NF-κB mediated genes (iNOS, COX-2), involved in pain. The plasma leading nitrite production was significantly reduced by capillarisin. Moreover, i.p. administration of capillarisin markedly suppressed the adenosine 5׳-triphosphate (ATP) in plasma and substance P in CFA-induced paw tissue. ConclusionsThe present study indicates that capillarisin possessed promising anti-hyperalgesic and anti-allodynic effects through the inhibition of various inflammatory pain signaling, suggesting that capillarisin constitutes a significant component for the treatment of inflammatory pain.

Functional role of alpha7 nicotinic receptor in chronic neuropathic and inflammatory pain: Studies in transgenic mice

A growing body of evidence indicates that α7 nicotinic receptor subtypes play an important role in chronic inflammatory and neuropathic pain signaling. In the present study, we investigated the role of the endogenous α7 nicotinic receptors (nAChRs) signaling in pain and inflammation using transgenic mice. For that we evaluated pain-related behaviors in the α7 mutant mice (KO) and its complementary α7 hypersensitive mice (KI) expressing the L250T α7 nAChRs and their respective WT mice in acute, chronic inflammatory and neuropathic mouse models. α7 KO and KI mice showed no significant changes in pain responses evoked by acute noxious thermal and mechanical stimuli as compared with WT littermates. While α7 KO mice showed no alterations in thermal and mechanical allodynia compared to WT mice after chronic nerve injury in the CCI test, α7 KI mice showed a significant reduction in these pain-related responses. However, marked increase in edema, hyperalgesia, and allodynia associated with intraplantar CFA injection was observed in the α7 KO mice compared with the WT littermates. In contrast, α7 KI mice displayed lesser degree of hyperalgesia and allodynia after CFA injection. Finally, the ability of systemic nicotine to reverse already-developed mechanical allodynia produced by intraplantar CFA seen in WT mice was lost in the α7 KO animals. Overall, our results demonstrate that endogenous α7 nAChRs mechanisms play an important role in chronic inflammatory and neuropathic pain models. This provides an additional rationale for the utility of α7 nAChR agonists in the treatment of inflammatory and chronic pain.

Stimulation of human TRPA1 channels by clinical concentrations of the antirheumatic drug auranofin

Gold compounds, which were widely used to treat rheumatoid arthritis, have been recently used as experimental agents for tumor treatment. Transient receptor potential (TRP) ankyrin repeat 1 (TRPA1) is a Ca(2+)-permeable ion channel that senses acute and inflammatory pain signals. Electrophilic compounds such as mustard oil and cinnamaldehyde activate TRPA1 by interacting with TRPA1 cysteine residues. Here we investigate the effects of the gold compound auranofin (AUR) on TRPA1 channels. Intracellular Ca(2+) and whole cell patch-clamp recordings were performed on human embryonic kidney cells transiently expressed with TRPA1, TRP melastatin 8 (TRPM8), and vanilloid type TRP (TRPV1-4) channels. AUR stimulated TRPA1 in a concentration-dependent manner with a half-maximum potency of around 1.0 μM. The AUR-induced response was effectively blocked by HC030031, a TRPA1 antagonist. On the other hand, AUR failed to activate TRPM8 and TRPV1-4 channels, which are highly expressed in sensory neurons as nociceptors. The stimulatory effect on TRPA1 channels depended on the C414, C421, C621, and C633 cysteine residues and not on the inhibition of thioredoxin reductase by AUR. Moreover, AUR effectively activated TRPA1 channels expressed in human differentiated neuroblastoma cell lines. The study shows that AUR is a potent stimulator of TRPA1 channels.

Transforming Growth Factor-β1 Regulates Cdk5 Activity in Primary Sensory Neurons

In addition to many important roles for Cdk5 in brain development and synaptic function, we reported previously that Cdk5 regulates inflammatory pain signaling, partly through phosphorylation of transient receptor potential vanilloid 1 (TRPV1), an important Na(+)/Ca(2+) channel expressed in primary nociceptive afferent nerves. Because TGF-β regulates inflammatory processes and its receptor is expressed in TRPV1-positive afferents, we studied the cross-talk between these two pathways in sensory neurons during experimental peripheral inflammation. We demonstrate that TGF-β1 increases transcription and protein levels of the Cdk5 co-activator p35 through ERK1/2, resulting in an increase in Cdk5 activity in rat B104 neuroblastoma cells. Additionally, TGF-β1 enhances the capsaicin-induced Ca(2+) influx in cultured primary neurons from dorsal root ganglia (DRG). Importantly, Cdk5 activity was reduced in the trigeminal ganglia and DRG of 14-day-old TGF-β1 knock-out mice, resulting in reduced Cdk5-dependent phosphorylation of TRPV1. The decreased Cdk5 activity is associated with attenuated thermal hyperalgesia in TGF-β1 receptor conditional knock-out mice, where TGF-β signaling is significantly reduced in trigeminal ganglia and DRG. Collectively, our results indicate that active cross-talk between the TGF-β and Cdk5 pathways contributes to inflammatory pain signaling.

Molecular Architecture and Subunit Organization of TRPA1 Ion Channel Revealed by Electron Microscopy

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective ion channel, which is expressed in nociceptor sensory neurons and transduces chemical, inflammatory, and neuropathic pain signals. Numerous non-reactive compounds and electrophilic compounds, such as endogenous inflammatory mediators and exogenous pungent chemicals, can activate TRPA1. Here we report a 16-Å resolution structure of purified, functional, amphipol-stabilized TRPA1 analyzed by single-particle EM. Molecular models of the N and C termini of the channel were generated using the I-TASSER protein structure prediction server and docked into the EM density to provide insight into the TRPA1 subunit organization. This structural analysis suggests a location for critical N-terminal cysteine residues involved in electrophilic activation at the interface between neighboring subunits. Our results indicate that covalent modifications within this pocket may alter interactions between subunits and promote conformational changes that lead to channel activation.

Inflammatory Pain Signals an Increase in Functional Expression of Organic Anion Transporting Polypeptide 1a4 at the Blood-Brain Barrier

Pain is a dominant symptom associated with inflammatory conditions. Pharmacotherapy with opioids may be limited by poor blood-brain barrier (BBB) permeability. One approach that may improve central nervous system (CNS) delivery is to target endogenous BBB transporters such as organic anion-transporting polypeptide 1a4 (Oatp1a4). It is critical to identify and characterize biological mechanisms that enable peripheral pain/inflammation to "transmit" upstream signals and alter CNS drug transport processes. Our goal was to investigate, in vivo, BBB functional expression of Oatp1a4 in animals subjected to peripheral inflammatory pain. Inflammatory pain was induced in female Sprague-Dawley rats (200-250 g) by subcutaneous injection of 3% λ-carrageenan into the right hind paw; control animals were injected with 0.9% saline. In rat brain microvessels, Oatp1a4 expression was increased during acute pain/inflammation. Uptake of taurocholate and [d-penicillamine(2,5)]-enkephalin, two established Oatp substrates, was increased in animals subjected to peripheral pain, suggesting increased Oatp1a4-mediated transport. Inhibition of inflammatory pain with the anti-inflammatory drug diclofenac attenuated these changes in Oatp1a4 functional expression, suggesting that inflammation in the periphery can modulate BBB transporters. In addition, diclofenac prevented changes in the peripheral signaling cytokine transforming growth factor-β1 (TGF-β1) levels and brain microvascular TGF-β receptor expression induced by inflammatory pain. Pretreatment with the pharmacological TGF-β receptor inhibitor 4-[4-(1,3-benzodioxol-5-yl)-5-(2-pyridinyl)-1H-imidazol-2-yl]benzamide (SB431542) increased Oatp1a4 functional expression in λ-carrageenan-treated animals and saline controls, suggesting that TGF-β signaling is involved in Oatp1a4 regulation at the BBB. Our findings indicate that BBB transporters (i.e., Oatp1a4) can be targeted during drug development to improve CNS delivery of highly promising therapeutics.

Activation of peroxisome proliferator-activated receptor γ in brain inhibits inflammatory pain, dorsal horn expression of Fos, and local edema

Systemic administration of thiazolidinediones reduces peripheral inflammation in vivo, presumably by acting at peroxisome proliferator-activated receptor gamma (PPARgamma) in peripheral tissues. Based on a rapidly growing body of literature indicating the CNS as a functional target of PPARgamma actions, we postulated that brain PPARgamma modulates peripheral edema and the processing of inflammatory pain signals in the dorsal horn of the spinal cord. To test this in the plantar carrageenan model of inflammatory pain, we measured paw edema, heat hyperalgesia, and dorsal horn expression of the immediate-early gene c-fos after intracerebroventricular (ICV) administration of PPARgamma ligands or vehicle. We found that ICV rosiglitazone (0.5-50 microg) or 15d-PGJ(2) (50-200 microg), but not vehicle, dose-dependently reduced paw thickness, paw volume and behavioral withdrawal responses to noxious heat. These anti-inflammatory and anti-hyperalgesia effects result from direct actions in the brain and not diffusion to other sites, because intraperitoneal and intrathecal administration of rosiglitazone (50 microg) and 15d-PGJ(2) (200 microg) had no effect. PPARgamma agonists changed neither overt behavior nor motor coordination, indicating that non-specific behavioral effects do not contribute to PPAR ligand-induced anti-hyperalgesia. ICV administration of structurally dissimilar PPARgamma antagonists (either GW9662 or BADGE) reversed the anti-inflammatory and anti-hyperalgesic actions of both rosiglitazone and 15d-PGJ(2). To evaluate the effects of PPARgamma agonists on a classic marker of noxious stimulus-evoked gene expression, we quantified Fos protein expression in the dorsal horn. The number of carrageenan-induced Fos-like immunoreactive profiles was less in rosiglitazone-treated rats as compared to vehicle controls. We conclude that pharmacological activation of PPARgamma in the brain rapidly inhibits local edema and the spinal transmission of noxious inflammatory signals.

TRPA1 Mediates the Noxious Effects of Natural Sesquiterpene Deterrents

Plants, fungi, and animals generate a diverse array of deterrent natural products that induce avoidance behavior in biological adversaries. The largest known chemical family of deterrents are terpenes characterized by reactive alpha,beta-unsaturated dialdehyde moieties, including the drimane sesquiterpenes and other terpene species. Deterrent sesquiterpenes are potent activators of mammalian peripheral chemosensory neurons, causing pain and neurogenic inflammation. Despite their wide-spread synthesis and medicinal use as desensitizing analgesics, their molecular targets remain unknown. Here we show that isovelleral, a noxious fungal sesquiterpene, excites sensory neurons through activation of TPRA1, an ion channel involved in inflammatory pain signaling. TRPA1 is also activated by polygodial, a drimane sesquiterpene synthesized by plants and animals. TRPA1-deficient mice show greatly reduced nocifensive behavior in response to isovelleral, indicating that TRPA1 is the major receptor for deterrent sesquiterpenes in vivo. Isovelleral and polygodial represent the first fungal and animal small molecule agonists of nociceptive transient receptor potential channels.

(137) Cellular localization of inflammatory pain signaling in spinal cord dorsal horn

(137) Cellular localization of inflammatory pain signaling in spinal cord dorsal horn