Interaction between protein kinase Cmu and the vanilloid receptor type 1

Getting smart - Deciphering the neuronal functions of protein kinase D

Protein kinase D (PKD) is a family of serine/threonine kinases that play important roles in various signalling pathways in cells, including neuronal cells. In the nervous system, PKD has been shown to be involved in learning and memory formation by regulating neurotransmitter release, neurite outgrowth and dendrite development, synapse formation and synaptic plasticity. In addition, PKD has been implicated in pain perception or neuroprotection during oxidative stress. Dysregulation of PKD expression and activity has been linked to several neurological disorders, including autism and epilepsy. In this review, we summarize the current knowledge on the function of the PKD family members in neuronal cells, including the spatial regulation of their downstream signalling pathways. We will further discuss the potential role of PKD in the pathogenesis of neurological disorders.

The dual role of TRPV1 in peripheral neuropathic pain: pain switches caused by its sensitization or desensitization

The transient receptor potential vanilloid 1 (TRPV1) channel plays a dual role in peripheral neuropathic pain (NeuP) by acting as a "pain switch" through its sensitization and desensitization. Hyperalgesia, commonly resulting from tissue injury or inflammation, involves the sensitization of TRPV1 channels, which modulates sensory transmission from primary afferent nociceptors to spinal dorsal horn neurons. In chemotherapy-induced peripheral neuropathy (CIPN), TRPV1 is implicated in neuropathic pain mechanisms due to its interaction with ion channels, neurotransmitter signaling, and oxidative stress. Sensitization of TRPV1 in dorsal root ganglion neurons contributes to CIPN development, and inhibition of TRPV1 channels can reduce chemotherapy-induced mechanical hypersensitivity. In diabetic peripheral neuropathy (DPN), TRPV1 is involved in pain modulation through pathways including reactive oxygen species and cytokine production. TRPV1's interaction with TRPA1 channels further influences chronic pain onset and progression. Therapeutically, capsaicin, a TRPV1 agonist, can induce analgesia through receptor desensitization, while TRPV1 antagonists and siRNA targeting TRPV1 show promise in preclinical studies. Cannabinoid modulation of TRPV1 provides another potential pathway for alleviating neuropathic pain. This review summarizes recent preclinical research on TRPV1 in association with peripheral NeuP.

Lipin1 mediates cognitive impairment in fld mice via PKD-ERK pathway

Lipin1 is important in lipid synthesis because of its phosphatidate phosphatase activity, and it also functions as transcriptional coactivators to regulate the expression of genes involved in lipid metabolism. We found that fld mice exhibit cognitive impairment, and it is related to the DAG-PKD-ERK pathway. We used fld mice to explore the relationship between lipin1 and cognitive function. Our results confirmed the presence of cognitive impairment in the hippocampus of lipin1-deficient mice. As shown in behavioral test, the spatial learning and memory ability of fld mice was much worse than that of wild-type mice. Electron microscopy results showed that the number of synapses in hippocampus of fld mice was significantly reduced. BDNF,SYP, PSD95 were significantly reduced. These results suggest that lipin1 impairs synaptic plasticity. Hence,a deficiency of lipin1 leads to decreased DAG levels and inhibits PKD activation, thereby affecting the phosphorylation of ERK and the CREB.

CBD Effects on TRPV1 Signaling Pathways in Cultured DRG Neurons

INTRODUCTION

Cannabidiol (CBD) is reported to produce pain relief, but the clinically relevant cellular and molecular mechanisms remain uncertain. The TRPV1 receptor integrates noxious stimuli and plays a key role in pain signaling. Hence, we conducted in vitro studies, to elucidate the efficacy and mechanisms of CBD for inhibiting neuronal hypersensitivity in cultured rat sensory neurons, following activation of TRPV1.

METHODS

Adult rat dorsal root ganglion (DRG) neurons were cultured and supplemented with the neurotrophic factors NGF and GDNF, in an established model of neuronal hypersensitivity. Neurons were stimulated with CBD (Adven 150, EMMAC Life Sciences) at 1, 10, 100 nMol/L and 1, 10 and 50 µMol/L, 48 h after plating. In separate experiments, DRG neurons were also stimulated with capsaicin with or without CBD (1 nMol/L to10 µMol/L), in a functional calcium imaging assay. The effects of the adenylyl cyclase activator forskolin and the calcineurin inhibitor cyclosporin were determined. We also measured forskolin-stimulated cAMP levels, without and after treatment with CBD, using a homogenous time-resolved fluorescence (HTRF) assay. The results were analysed using Mann-Whitney test.

RESULTS

DRG neurons treated with 10 and 50 µMol/L CBD showed calcium influx, but not at lower doses. Neurons treated with capsaicin demonstrated robust calcium influx, which was dose-dependently reduced in the presence of low dose CBD (IC₅₀ = 100 nMol/L). The inhibition or desensitization by CBD was reversed in the presence of forskolin and cyclosporin. Forskolin-stimulated cAMP levels were significantly reduced in CBD treated neurons.

CONCLUSION

CBD at low doses corresponding to plasma concentrations observed physiologically inhibits or desensitizes neuronal TRPV1 signalling by inhibiting the adenylyl cyclase - cAMP pathway, which is essential for maintaining TRPV1 phosphorylation and sensitization. CBD also facilitated calcineurin-mediated TRPV1 inhibition. These mechanisms may underlie nociceptor desensitization and the therapeutic effect of CBD in animal models and patients with acute and chronic pain.

CDK5 inhibits the clathrin-dependent internalization of TRPV1 by phosphorylating the clathrin adaptor protein AP2μ2

Transient receptor potential vanilloid 1 (TRPV1), a nonselective, ligand-gated cation channel, responds to multiple noxious stimuli and is targeted by many kinases that influence its trafficking and activity. Studies on the internalization of TRPV1 have mainly focused on that induced by capsaicin or other agonists. Here, we report that constitutive internalization of TRPV1 occurred in a manner dependent on clathrin, dynamin, and adaptor protein complex 2 (AP2). The μ2 subunit of AP2 (AP2μ2) interacted directly with TRPV1 and was required for its constitutive internalization. Cyclin-dependent kinase 5 (CDK5) phosphorylated AP2μ2 at Ser⁴⁵, which reduced the interaction between TRPV1 and AP2μ2, leading to decreased TRPV1 internalization. Intrathecal delivery of a cell-penetrating fusion peptide corresponding to the Cdk5 phosphorylation site in AP2μ2, which competed with AP2μ2 for phosphorylation by Cdk5, increased the abundance of TRPV1 on the surface of dorsal root ganglion neurons and reduced complete Freund's adjuvant (CFA)-induced inflammatory thermal hyperalgesia in rats. In addition to describing a mechanism of TRPV1 constitutive internalization and its inhibition by CDK5, these findings demonstrate that CDK5 promotes inflammatory thermal hyperalgesia by reducing TRPV1 internalization, providing previously unidentified insights into the search for drug targets to treat pain.

KChIP3 N-Terminal 31-50 Fragment Mediates Its Association with TRPV1 and Alleviates Inflammatory Hyperalgesia in Rats

Potassium voltage-gated channel interacting protein 3 (KChIP3), also termed downstream regulatory element antagonist modulator (DREAM) and calsenilin, is a multifunctional protein belonging to the neuronal calcium sensor (NCS) family. Recent studies revealed the expression of KChIP3 in dorsal root ganglion (DRG) neurons, suggesting the potential role of KChIP3 in peripheral sensory processing. Herein, we show that KChIP3 colocalizes with transient receptor potential ion channel V1 (TRPV1), a critical molecule involved in peripheral sensitization during inflammatory pain. Furthermore, the N-terminal 31-50 fragment of KChIP3 is capable of binding both the intracellular N and C termini of TRPV1, which substantially decreases the surface localization of TRPV1 and the subsequent Ca²⁺ influx through the channel. Importantly, intrathecal administration of the transmembrane peptide transactivator of transcription (TAT)-31-50 remarkably reduces Ca²⁺ influx via TRPV1 in DRG neurons and alleviates thermal hyperalgesia and gait alterations in a complete Freund's adjuvant-induced inflammatory pain model in male rats. Moreover, intraplantar injection of TAT-31-50 attenuated the capsaicin-evoked spontaneous pain behavior and thermal hyperalgesia, which further strengthened the regulatory role of TAT-31-50 on TRPV1 channel. In addition, TAT-31-50 could also alleviate inflammatory thermal hyperalgesia in kcnip3^-/- rats generated in our study, suggesting that the analgesic effect mediated by TAT-31-50 is independent of endogenous KChIP3. Our study reveals a novel peripheral mechanism for the analgesic function of KChIP3 and provides a potential analgesic agent, TAT-31-50, for the treatment of inflammatory pain.SIGNIFICANCE STATEMENT Inflammatory pain arising from inflamed or injured tissues significantly compromises the quality of life in patients. This study aims to elucidate the role of peripheral potassium channel interacting protein 3 (KChIP3) in inflammatory pain. Direct interaction of the KChIP3 N-terminal 31-50 fragment with transient receptor potential ion channel V1 (TRPV1) was demonstrated. The KChIP3-TRPV1 interaction reduces the surface localization of TRPV1 and thus alleviates heat hyperalgesia and gait alterations induced by peripheral inflammation. Furthermore, the transmembrane transactivator of transcription (TAT)-31-50 peptide showed analgesic effects on inflammatory hyperalgesia independently of endogenous KChIP3. This work reveals a novel mechanism of peripheral KChIP3 in inflammatory hyperalgesia that is distinct from its classical role as a transcriptional repressor in pain modulation.

PKD1 Promotes Functional Synapse Formation Coordinated with N-Cadherin in Hippocampus

Functional synapse formation is critical for the wiring of neural circuits in the developing brain. The cell adhesion molecule N-cadherin plays important roles in target recognition and synaptogenesis. However, the molecular mechanisms that regulate the localization of N-cadherin and the subsequent effects remain poorly understood. Here, we show that protein kinase D1 (PKD1) directly binds to N-cadherin at amino acid residues 836-871 and phosphorylates it at Ser 869, 871, and 872, thereby increasing the surface localization of N-cadherin and promoting functional synapse formation in primary cultured hippocampal neurons obtained from embryonic day 18 rat embryos of either sex. Intriguingly, neuronal activity enhances the interactions between N-cadherin and PKD1, which are critical for the activity-dependent growth of dendritic spines. Accordingly, either disruption the binding between N-cadherin and PKD1 or preventing the phosphorylation of N-cadherin by PKD1 in the hippocampal CA1 region of male rat leads to the reduction in synapse number and impairment of LTP. Together, this study demonstrates a novel mechanism of PKD1 regulating the surface localization of N-cadherin and suggests that the PKD1-N-cadherin interaction is critical for synapse formation and function.SIGNIFICANCE STATEMENT Defects in synapse formation and function lead to various neurological diseases, although the mechanisms underlying the regulation of synapse development are far from clear. Our results suggest that protein kinase D1 (PKD1) functions upstream of N-cadherin, a classical synaptic adhesion molecule, to promote functional synapse formation. Notably, we identified a crucial binding fragment to PKD1 at C terminus of N-cadherin, and this fragment also contains PKD1 phosphorylation sites. Through this interaction, PKD1 enhances the stability of N-cadherin on cell membrane and promotes synapse morphogenesis and synaptic plasticity in an activity-dependent manner. Our study reveals the role of PKD1 and the potential downstream mechanism in synapse development, and contributes to the research for neurodevelopment and the therapy for neurological diseases.

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

Protease-activated receptors (PARs) belong to the G-protein-coupled receptor family, that are expressed in many body tissues especially in different epithelial cells, mast cells and also in neurons and astrocytes. PARs play different physiological roles according to the location of their expression. Increased evidence supports the importance of PARs activation during nociceptive signaling and in the development of chronic pain states. This short review focuses on the role of PAR2 receptors in nociceptive transmission with the emphasis on the modulation at the spinal cord level. PAR2 are cleaved and subsequently activated by endogenous proteases such as tryptase and trypsin. In vivo, peripheral and intrathecal administration of PAR2 agonists induces thermal and mechanical hypersensitivity that is thought to be mediated by PAR2-induced release of pronociceptive neuropeptides and modulation of different receptors. PAR2 activation leads also to sensitization of transient receptor potential channels (TRP) that are crucial for nociceptive signaling and modulation. PAR2 receptors may play an important modulatory role in the development and maintenance of different pathological pain states and could represent a potential target for new analgesic treatments.

Protein kinase D1-dependent phosphorylation of dopamine D1 receptor regulates cocaine-induced behavioral responses

The dopamine (DA) D1 receptor (D1R) is critically involved in reward and drug addiction. Phosphorylation-mediated desensitization or internalization of D1R has been extensively investigated. However, the potential for upregulation of D1R function through phosphorylation remains to be determined. Here we report that acute cocaine exposure induces protein kinase D1 (PKD1) activation in the rat striatum, and knockdown of PKD1 in the rat dorsal striatum attenuates cocaine-induced locomotor hyperactivity. Moreover, PKD1-mediated phosphorylation of serine 421 (S421) of D1R promotes surface localization of D1R and enhances downstream extracellular signal-regulated kinase signaling in D1R-transfected HEK 293 cells. Importantly, injection of the peptide Tat-S421, an engineered Tat fusion-peptide targeting S421 (Tat-S421), into the rat dorsal striatum inhibits cocaine-induced locomotor hyperactivity and injection of Tat-S421 into the rat hippocampus or the shell of the nucleus accumbens (NAc) also inhibits cocaine-induced conditioned place preference (CPP). However, injection of Tat-S421 into the rat NAc shell does not establish CPP by itself and injection of Tat-S421 into the hippocampus does not influence spatial learning and memory. Thus, targeting S421 of D1R represents a promising strategy for the development of pharmacotherapeutic treatments for drug addiction and other disorders that result from DA imbalances.

Protein kinase D: a new player among the signaling proteins that regulate functions in the nervous system

Protein kinase D (PKD) is an evolutionarily-conserved family of protein kinases. It has structural, regulatory, and enzymatic properties quite different from the PKC family. Many stimuli induce PKD signaling, including G-protein-coupled receptor agonists and growth factors. PKD1 is the most studied member of the family. It functions during cell proliferation, differentiation, secretion, cardiac hypertrophy, immune regulation, angiogenesis, and cancer. Previously, we found that PKD1 is also critically involved in pain modulation. Since then, a series of studies performed in our lab and by other groups have shown that PKDs also participate in other processes in the nervous system including neuronal polarity establishment, neuroprotection, and learning. Here, we discuss the connections between PKD structure, enzyme function, and localization, and summarize the recent findings on the roles of PKD-mediated signaling in the nervous system.

Protease-activated receptor 2 signalling pathways: a role in pain processing

INTRODUCTION

Pain is a complex biological phenomenon that includes intricate neurophysiological, behavioural, psychosocial and affective components. Despite decades of pain research, many patients continue suffering from chronic pain that may be refractory to current medical regimens. Accumulating evidence has indicated an important role of protease-activated receptor 2 (PAR2) in the pathogenesis of pain, including inflammation, neuropathic and cancer pain.

AREAS COVERED

In this review, the role of the PAR2 signalling pathway in pain processes, basic mechanism of PAR2 activation and expression of PAR2 in the nervous system is covered. Furthermore, intracellular signalling pathways that are activated by PAR2 are also described.

EXPERT OPINION

The role of PAR2 in pain processing is becoming increasingly clear, and although causal implication remains to be established, PAR2 activation has been observed in several disease model systems. Since PAR2 is activated after nerve injury as well as by trypsin and related serine proteases, and PAR2 plays an important role in pain development and maintenance, exploring PAR2 and its corresponding signalling pathways will provide unfathomable knowledge in understanding the molecular basis of pain. This will also help to identify new targets for pharmacological intervention; however, in the context of potential PAR2-directed therapies, several aspects should be clarified.

Functionally important amino acid residues in the transient receptor potential vanilloid 1 (TRPV1) ion channel--an overview of the current mutational data

This review aims to create an overview of the currently available results of site-directed mutagenesis studies on transient receptor potential vanilloid type 1 (TRPV1) receptor. Systematization of the vast number of data on the functionally important amino acid mutations of TRPV1 may provide a clearer picture of this field, and may promote a better understanding of the relationship between the structure and function of TRPV1. The review summarizes information on 112 unique mutated sites along the TRPV1, exchanged to multiple different residues in many cases. These mutations influence the effect or binding of different agonists, antagonists, and channel blockers, alter the responsiveness to heat, acid, and voltage dependence, affect the channel pore characteristics, and influence the regulation of the receptor function by phosphorylation, glycosylation, calmodulin, PIP2, ATP, and lipid binding. The main goal of this paper is to publish the above mentioned data in a form that facilitates in silico molecular modelling of the receptor by promoting easier establishment of boundary conditions. The better understanding of the structure-function relationship of TRPV1 may promote discovery of new, promising, more effective and safe drugs for treatment of neurogenic inflammation and pain-related diseases and may offer new opportunities for therapeutic interventions.

Cyclin-dependent kinase 5 controls TRPV1 membrane trafficking and the heat sensitivity of nociceptors through KIF13B

The number of functional transient receptor potential vanilloid 1 (TRPV1) channels at the surface, especially at the peripheral terminals of primary sensory neurons, regulates heat sensitivity, and increased surface localization of TRPV1s contributes to heat hyperalgesia. However, the mechanisms for regulating TRPV1 surface localization are essentially unknown. Here, we show that cyclin-dependent kinase 5 (Cdk5), a new player in thermal pain sensation, positively regulates TRPV1 surface localization. Active Cdk5 was found to promote TRPV1 anterograde transport in vivo, suggesting a regulatory role of Cdk5 in TRPV1 membrane trafficking. TRPV1-containing vesicles bind to the forkhead-associated (FHA) domain of the KIF13B (kinesin-3 family member 13B) and are thus delivered to the cell surface. Overexpression of Cdk5 or its activator p35 promoted and inhibition of Cdk5 activity prevented the KIF13B-TRPV1 association, indicating that Cdk5 promotes TRPV1 anterograde transport by mediating the motor-cargo association. Cdk5 phosphorylates KIF13B at Thr-506, a residue located in the FHA domain. T506A mutation reduced the motor-cargo interaction and the cell-permeable TAT-T506 peptide, targeting to the Thr-506, decreased TRPV1 surface localization, demonstrating the essential role of Thr-506 phosphorylation in TRPV1 transport. Moreover, complete Freund's adjuvant (CFA) injection-induced activation of Cdk5 increased the anterograde transport of TRPV1s, contributing to the development and possibly the maintenance of heat hyperalgesia, whereas intrathecal delivery of the TAT-T506 peptide alleviated CFA-induced heat hyperalgesia in rats. Thus, Cdk5 regulation of TRPV1 membrane trafficking is a fundamental mechanism controlling the heat sensitivity of nociceptors, and moderate inhibition of Thr-506 phosphorylation during inflammation might be helpful for the treatment of inflammatory thermal pain.

Glucose transporter 2 expression is down regulated following P2X7 activation in enterocytes

With the diabetes epidemic affecting the world population, there is an increasing demand for means to regulate glycemia. Dietary glucose is first absorbed by the intestine before entering the blood stream. Thus, the regulation of glucose absorption by intestinal epithelial cells (IECs) could represent a way to regulate glycemia. Among the molecules involved in glycemia homeostasis, extracellular ATP, a paracrine signaling molecule, was reported to induce insulin secretion from pancreatic β cells by activating P2Y and P2X receptors. In rat's jejunum, P2X7 expression was previously immunolocalized to the apex of villi, where it has been suspected to play a role in apoptosis. However, using an antibody recognizing the receptor extracellular domain and thus most of the P2X7 isoforms, we showed that expression of this receptor is apparent in the top two-thirds of villi. These data suggest a different role for this receptor in IECs. Using the non-cancerous IEC-6 cells and differentiated Caco-2 cells, glucose transport was reduced by more than 30% following P2X7 stimulation. This effect on glucose transport was not due to P2X7-induced cell apoptosis, but rather was the consequence of glucose transporter 2 (Glut2)'s internalization. The signaling pathway leading to P2X7-dependent Glut2 internalization involved the calcium-independent activation of phospholipase Cγ1 (PLCγ1), PKCδ, and PKD1. Although the complete mechanism regulating Glut2 internalization following P2X7 activation is not fully understood, modulation of P2X7 receptor activation could represent an interesting approach to regulate intestinal glucose absorption.

Physiology and pharmacology of the vanilloid receptor

The identification and cloning of the vanilloid receptor 1 (TRPV1) represented a significant step for the understanding of the molecular mechanisms underlying the transduction of noxious chemical and thermal stimuli by peripheral nociceptors. TRPV1 is a non-selective cation channel gated by noxious heat, vanilloids and extracellular protons. TRPV1 channel activity is remarkably potentiated by pro-inflammatory agents, a phenomenon that is thought to underlie the peripheral sensitisation of nociceptors that leads to thermal hyperalgesia. Cumulative evidence is building a strong case for the involvement of this receptor in the etiology of both peripheral and visceral inflammatory pain, such as inflammatory bowel disease, bladder inflammation and cancer pain. The validation of TRPV1 receptor as a key therapeutic target for pain management has thrust intensive drug discovery programs aimed at developing orally active antagonists of the receptor protein. Nonetheless, the real challenge of these drug discovery platforms is to develop antagonists that preserve the physiological activity of TRPV1 receptors while correcting over-active channels. This is a condition to ensure normal pro-prioceptive and nociceptive responses that represent a safety mechanism to prevent tissue injury. Recent and exciting advances in the function, dysfunction and modulation of this receptor will be the focus of this review.

The substrates and binding partners of protein kinase Cepsilon

The epsilon isoform of protein kinase C (PKCepsilon) has important roles in the function of the cardiac, immune and nervous systems. As a result of its diverse actions, PKCepsilon is the target of active drug-discovery programmes. A major research focus is to identify signalling cascades that include PKCepsilon and the substrates that PKCepsilon regulates. In the present review, we identify and discuss those proteins that have been conclusively shown to be direct substrates of PKCepsilon by the best currently available means. We will also describe binding partners that anchor PKCepsilon near its substrates. We review the consequences of substrate phosphorylation and discuss cellular mechanisms by which target specificity is achieved. We begin with a brief overview of the biology of PKCepsilon and methods for substrate identification, and proceed with a discussion of substrate categories to identify common themes that emerge and how these may be used to guide future studies.

Protein kinase D isoforms are expressed in rat and mouse primary sensory neurons and are activated by agonists of protease-activated receptor 2

Serine proteases generated during injury and inflammation cleave protease-activated receptor 2 (PAR(2)) on primary sensory neurons to induce neurogenic inflammation and hyperalgesia. Hyperalgesia requires sensitization of transient receptor potential vanilloid (TRPV) ion channels by mechanisms involving phospholipase C and protein kinase C (PKC). The protein kinase D (PKD) serine/threonine kinases are activated by diacylglycerol and PKCs and can phosphorylate TRPV1. Thus, PKDs may participate in novel signal transduction pathways triggered by serine proteases during inflammation and pain. However, it is not known whether PAR(2) activates PKD, and the expression of PKD isoforms by nociceptive neurons is poorly characterized. By using HEK293 cells transfected with PKDs, we found that PAR(2) stimulation promoted plasma membrane translocation and phosphorylation of PKD1, PKD2, and PKD3, indicating activation. This effect was partially dependent on PKCepsilon. By immunofluorescence and confocal microscopy, with antibodies against PKD1/PKD2 and PKD3 and neuronal markers, we found that PKDs were expressed in rat and mouse dorsal root ganglia (DRG) neurons, including nociceptive neurons that expressed TRPV1, PAR(2), and neuropeptides. PAR(2) agonist induced phosphorylation of PKD in cultured DRG neurons, indicating PKD activation. Intraplantar injection of PAR(2) agonist also caused phosphorylation of PKD in neurons of lumbar DRG, confirming activation in vivo. Thus, PKD1, PKD2, and PKD3 are expressed in primary sensory neurons that mediate neurogenic inflammation and pain transmission, and PAR(2) agonists activate PKDs in HEK293 cells and DRG neurons in culture and in intact animals. PKD may be a novel component of a signal transduction pathway for protease-induced activation of nociceptive neurons and an important new target for antiinflammatory and analgesic therapies.

Characterization of EVL-I as a protein kinase D substrate

EVL-I is a splice variant of EVL (Ena/VASP like protein), whose in vivo function and regulation are still poorly understood. We found that Protein Kinase D (PKD) interacts in vitro and in vivo with EVL-I and phosphorylates EVL-I in a 21 amino acid alternately-included insert in the EVH2 domain. Following knockdown of the capping protein CPbeta and spreading on laminin, phosphorylated EVL-I can support filopodia formation and the phosphorylated EVL-I is localized at filopodial tips. Furthermore, we found that the lamellipodial localization of EVL-I is unaffected by phosphorylation, but that impairment of EVL-I phosphorylation is associated with ruffling of lamellipodia upon PDBu stimulation. Besides the lamellipodial and filopodial localization of phosphorylated EVL-I in fibroblasts, we determined that EVL-I is hyperphosphorylated and localized in the cell-cell contacts of certain breast cancer cells and mouse embryo keratinocytes. Taken together, our results show that phosphorylated EVL-I is present in lamellipodia, filopodia and cell-cell contacts and suggest the existence of signaling pathways that may affect EVL-I via phosphorylation of its EVH2 domain.

Influence of capsaicin on visceral hyperalgesia and its mechanism

Capsaicin (CAP) has multiple pharmacological actions, and researches have been centered on its effect on visceral hyperalgesia (VHL). Relevant studies have shown that low doses of CAP may cause VHL, while high doses can inhibit VHL. This kind of mechanism may be associated with vanilloid receptor subtype 1 (VR1) phosphorylation and dephosphorylation, substance P (SP), calcitonin-gene-related peptide (CGRP) and protease-activated receptor 2 (PAR2). CAP may be promising as a new drug for VHL treatment.

Intracellular signaling in primary sensory neurons and persistent pain

During evolution, living organisms develop a specialized apparatus called nociceptors to sense their environment and avoid hazardous situations. Intense stimulation of high threshold C- and Adelta-fibers of nociceptive primary sensory neurons will elicit pain, which is acute and protective under normal conditions. A further evolution of the early pain system results in the development of nociceptor sensitization under injury or disease conditions, leading to enhanced pain states. This sensitization in the peripheral nervous system is also called peripheral sensitization, as compared to its counterpart, central sensitization. Inflammatory mediators such as proinflammatory cytokines (TNF-alpha, IL-1beta), PGE(2), bradykinin, and NGF increase the sensitivity and excitability of nociceptors by enhancing the activity of pronociceptive receptors and ion channels (e.g., TRPV1 and Na(v)1.8). We will review the evidence demonstrating that activation of multiple intracellular signal pathways such as MAPK pathways in primary sensory neurons results in the induction and maintenance of peripheral sensitization and produces persistent pain. Targeting the critical signaling pathways in the periphery will tackle pain at the source.

Capsaicin inhibits the in vitro binding of peptides selective for mu- and kappa-opioid, and nociceptin-receptors

Capsaicin inhibited the equilibrium specific binding of endogenous opioid-like peptide ligands such as endomorphin-1, nociceptin, and dynorphin((1-17)) in rat brain membrane preparations. We studied the in vitro effect of capsaicin (1-10 microM) on homologous and heterologous competitive binding of opioid ligands, using unlabeled synthetic peptides and the following tritiated compounds: [(3)H]endomorphin-1, [(3)H]endomorphin-2, [(3)H]nociceptin((1-17)) and [(3)H]dynorphin((1-17)). Capsaicin-dependent inhibition was also observed in [(35)S]GTPgammaS stimulation assays in the presence of certain opioid peptides. The inhibition of opioid binding was further investigated using other synthetic and natural mu-opioid ligands such as [D-Ala(2),(NMe)Phe(4),Gly(5)-ol]enkephalin (DAMGO), morphine and naloxone. The decrease in opioid ligand affinity upon capsaicin treatments was most apparent with endomorphin-1, followed by nociceptin and dynorphin. The binding of other investigated opioids were not affected in the presence of capsaicin. In [(3)H]endomorphin-1 binding assays, capsazepine antagonized the inhibitory effect of capsaicin in rat brain membranes suggesting the involvement of TRPV1 receptors. In Chinese hamster ovary (CHO) cells stably expressing mu-opioid receptors, but lacking vanilloid receptors, the inhibition by capsaicin on the binding of [(3)H]endomorphin-1 was not present. It is concluded that the inhibitory effect of capsaicin on the receptor binding affinity of endogenous opioid peptides in brain membrane preparations seems not to be a direct effect, it is rather a negative feedback interaction with opioid receptors.

Both the establishment and maintenance of neuronal polarity require the activity of protein kinase D in the Golgi apparatus

Neuronal polarization requires coordinated regulation of membrane trafficking and cytoskeletal dynamics. Several signaling proteins are involved in neuronal polarization via modulation of cytoskeletal dynamics in neurites. However, very little is known about signaling proteins in the neuronal soma, which regulate polarized membrane trafficking and neuronal polarization. Protein kinase D (PKD) constitutes a family of serine/threonine-specific protein kinases and is important in regulating Golgi dynamics and membrane trafficking. Here, we show that two members of the PKD family, PKD1 and PKD2, are essential for the establishment and maintenance of neuronal polarity. Loss of function of PKD with inhibitor, dominant negative, and short interfering RNA disrupts polarized membrane trafficking and induces multiple axon formation. Gain of function of PKD can rescue the disruption of polarized membrane trafficking and neuronal polarity caused by cytochalasin D, which results in F-actin depolymerization. PKD1 and PKD2 are also found to be involved in the maintenance of neuronal polarity, as evidenced by the conversion of preexisting dendrites into axons on PKD inhibition. Unlike other polarity proteins, PKD does not interact with the cytoskeleton in neurites. Instead, PKD regulates neuronal polarity through its activity in the Golgi apparatus. These data reveal a novel mechanism regulating neuronal polarity in the Golgi apparatus.

The functional regulation of TRPV1 and its role in pain sensitization

Transient receptor potential V1 (TRPV1) is specifically expressed in the nociceptive receptors and can detect a variety of noxious stimuli, thus potentiating pain sensitization. While peripheral delivery of capsaicin causes the desensitization of sensory neurons, thus alleviating pain. Therefore capsaicin is used in the clinical treatment of various types of pain; however, these treatments will bring many side effects, such as a strong burning pain in the early stages of treatment which hampers the further use of capsaicin. Thus, the studies of the functional regulation of TRPV1 are mainly focused on two aspects: to develop more potent analogues of capsaicin with less side effects; or to elucidate the mechanisms of TRPV1 in pain sensitivity, especially of that TRPV1 as a target of various protein kinases such as PKD1 and Cdk5 is involved pain hypersensitivity. Thus we would summarize the progress of these two aspects in this mini review.

Neurokinin 2 receptor-mediated activation of protein kinase C modulates capsaicin responses in DRG neurons from adult rats

Patch-clamp techniques and Ca2+ imaging were used to examine the interaction between neurokinins (NK) and the capsaicin(CAPS)-evoked transient receptor potential vanilloid receptor 1 (TRPV1) responses in rat dorsal root ganglia neurons. Substance P (SP; 0.2-0.5 microM) prevented the reduction of Ca2+ transients (tachyphylaxis) evoked by repeated brief applications of CAPS (0.5 microM). Currents elicited by CAPS were increased in amplitude and desensitized more slowly after administration of SP or a selective NK2 agonist, [Ala8]-neurokinin A (4-10) (NKA). Neither an NK1-selective agonist, [Sar9, Met11]-SP, nor an NK3-selective agonist, [MePhe7]-NKB, altered the CAPS currents. The effects of SP on CAPS currents were inhibited by a selective NK2 antagonist, MEN10,376, but were unaffected by the NK3 antagonist, SB 235,375. Phorbol 12,13-dibutyrate (PDBu), an activator of protein kinase C(PKC), also increased the amplitude and slowed the desensitization of CAPS responses. Phosphatase inhibitors, decamethrin and alpha-naphthyl acid phosphate (NAcPh), also enhanced the currents and slowed desensitization of CAPS currents. Facilitatory effects of SP, NKA and PDBu were reversed by bisindolylmaleimide, a PKC inhibitor, and gradually decreased in magnitude when the agents were administered at increasing intervals after CAPS application. The decrease was partially prevented by prior application of NAcPh. These data suggest that activation of NK2 receptors in afferent neurons leads to PKC-induced phosphorylation of TRPV1, resulting in sensitization of CAPS-evoked currents and slower desensitization. Thus, activation of NK2 autoreceptors by NKs released from the peripheral afferent terminals or by mast cells during inflammatory responses may be a mechanism that sensitizes TRPV1 channels and enhances afferent excitability.

Stimulation of the neurokinin 3 receptor activates protein kinase C epsilon and protein kinase D in enteric neurons

Tachykinins, acting through NK(3) receptors (NK(3)R), contribute to excitatory transmission to intrinsic primary afferent neurons (IPANs) of the small intestine. Although this transmission is dependent on protein kinase C (PKC), its maintenance could depend on protein kinase D (PKD), a downstream target of PKC. Here we show that PKD1/2-immunoreactivity occurred exclusively in IPANs of the guinea pig ileum, demonstrated by double staining with the IPAN marker NeuN. PKCepsilon was also colocalized with PKD1/2 in IPANs. PKCepsilon and PKD1/2 trafficking was studied in enteric neurons within whole mounts of the ileal wall. In untreated preparations, PKCepsilon and PKD1/2 were cytosolic and no signal for activated (phosphorylated) PKD was detected. The NK(3)R agonist senktide evoked a transient translocation of PKCepsilon and PKD1/2 from the cytosol to the plasma membrane and induced PKD1/2 phosphorylation at the plasma membrane. PKCepsilon translocation was maximal at 10 s and returned to the cytosol within 2 min. Phosphorylated-PKD1/2 was detected at the plasma membrane within 15 s and translocated to the cytosol by 2 min, where it remained active up to 30 min after NK(3)R stimulation. PKD1/2 activation was reduced by a PKCepsilon inhibitor and prevented by NK(3)R inhibition. NK(3)R-mediated PKCepsilon and PKD activation was confirmed in HEK293 cells transiently expressing NK(3)R and green fluorescent protein-tagged PKCepsilon, PKD1, PKD2, or PKD3. Senktide caused membrane translocation and activation of kinases within 30 s. After 15 min, phosphorylated PKD had returned to the cytosol. PKD activation was confirmed through Western blotting. Thus stimulation of NK(3)R activates PKCepsilon and PKD in sequence, and sequential activation of these kinases may account for rapid and prolonged modulation of IPAN function.

Differential modulation of agonist and antagonist structure activity relations for rat TRPV1 by cyclosporin A and other protein phosphatase inhibitors

The transient receptor potential V1 channel (vanilloid receptor, TRPV1) represents a promising therapeutic target for inflammatory pain and other conditions involving C-fiber sensory afferent neurons. Sensitivity of TRPV1 is known to be subject to modulation by numerous signaling pathways, in particular by phosphorylation, and we wished to determine whether TRPV1 structure activity relations could be differentially affected. We demonstrate here that the structure activity relations of TRPV1, as determined by (45)Ca(2) uptake, were substantially altered by treatment of the cells with cyclosporin A, an inhibitor of protein phosphatase 2B. Whereas the potency of resiniferatoxin for stimulation of (45)Ca(2) was not altered by cyclosporin A treatment, the potencies of some other agonists were increased up to 8-fold. Among the antagonists examined, potencies were reduced to a lesser extent, ranging from 1- to 2.5-fold. Finally, the efficacy of partial agonists was increased. In contrast to cyclosporin A, okadaic acid, an inhibitor of protein phosphatases 1 and 2A, had little effect on agonist potencies, and calyculin A, an inhibitor of protein phosphatases 1 and 2A but with somewhat different selectivity from that of okadaic acid, caused changes in structure activity relations distinct from those induced by cyclosporin A. Because phosphatase activity differentially modulates the structure activity relations of TRPV1 agonists and antagonists, our findings predict that it may be possible to design agonists and antagonists selective for TRPV1 in a specific regulatory environment. A further implication is that it may be desirable to tailor screening approaches for drug discovery to reflect the desired regulatory state of the targeted TRPV1.

Protein kinase d in the cardiovascular system: emerging roles in health and disease

The protein kinase D (PKD) family is a recent addition to the calcium/calmodulin-dependent protein kinase group of serine/threonine kinases, within the protein kinase complement of the mammalian genome. Relative to their alphabetically superior cousins in the AGC group of kinases, namely the various isoforms of protein kinase A, protein kinase B/Akt, and protein kinase C, PKD family members have to date received limited attention from cardiovascular investigators. Nevertheless, increasing evidence now points toward important roles for PKD-mediated signaling pathways in the cardiovascular system, particularly in the regulation of myocardial contraction, hypertrophy and remodeling. This review provides a primer on PKD signaling, using information gained from studies in multiple cell types, and discusses recent data that suggest novel functions for PKD-mediated pathways in the heart and the circulation.

The Neuroscience Research Institute at Peking University: a place for the solution of pain and drug abuse

Neuroscience research in China has undergone rapid expansion since 1980. The Neuroscience Research Institute of Peking University, one of the most active neuroscience research groups in China, was founded in 1987. Currently, the institute is overseeing four research areas, i.e., (1) pain and analgesia, (2) drug abuse and acupuncture treatment for drug addiction, (3) the mechanism of neurological degenerative disorders, and (4) the role of neuroglia in central nervous system injury. The institute is simultaneously investigating both theoretical and clinical studies. Acupuncture remains the core of research, while pain and drug abuse form the two disciplines.

Interaction between protein kinase D1 and transient receptor potential V1 in primary sensory neurons is involved in heat hypersensitivity

In previous studies we demonstrated that protein kinase D1 (PKD1/PKCmu) could directly phosphorylate the transient receptor potential V1 (TRPV1) at its N-terminal region and enhance the function of TRPV1 in CHO cells stably transfected with TRPV1. In the current study we assessed the involvement of PKD1 in pain modulation and explored the possible interaction between PKD1 and TRPV1 in rat inflammatory heat hypersensitivity. PKD1 was translocated to cytoplasmic membrane fraction and was trans-phosphorylated only in membrane fraction but not in cytoplasmic fraction of dorsal root ganglia (DRG) at 2 and 6h after Complete Freund's Adjuvant (CFA) treatment. Pre i.t. injection of PKD1 antisense for 4 d or post-i.t. injection for 4 d both alleviated CFA-induced thermal hypersensitivity. Likewise, overexpression of PKD1 in DRG significantly enhanced, while dominant negative PKD1 (DN-PKD1) partly attenuated, heat hypersensitivity. Both PKD1 and TRPV1 were translocated to the cytoplasmic membrane in DRG 6 h after CFA treatment and, at that time, PKD1 interacted with TRPV1 by co-immunoprecipitation in DRG. Electrophysiological measurements indicated that DRG with overexpression of PKD1 were more sensitive to low dose capsaicin than those expressing DN-PKD1. The average magnitude of the peak inward current evoked by capsaicin was greater in the DRG overexpressing PKD1 than in those expressing DN-PKD1. Furthermore, overexpressed PKD1 could up regulate, whereas PKD1 antisense could knock down TRPV1 content in DRG through posttranscriptional regulation manner. We concluded that PKD1 in DRG, through interaction with TRPV1, is involved in developing and maintaining inflammatory heat hypersensitivity.

TRPV1 expression-dependent initiation and regulation of filopodia

Transient receptor potential vanilloid subtype 1 (TRPV1), a non-selective cation channel, is present endogenously in dorsal root ganglia (DRG) neurons. It is involved in the recognition of various pain producing physical and chemical stimuli. In this work, we demonstrate that expression of TRPV1 induces neurite-like structures and filopodia and that the expressed protein is localized at the filopodial tips. Exogenous expression of TRPV1 induces filopodia both in DRG neuron-derived F11 cells and in non-neuronal cells, such as HeLa and human embryonic kidney (HEK) cells. We find that some of the TRPV1 expression-induced filopodia contain microtubules and microtubule-associated components, and establish cell-to-cell extensions. Using live cell microscopy, we demonstrate that the filopodia are responsive to TRPV1-specific ligands. But both, initiation and subsequent cell-to-cell extension formation, is independent of TRPV1 channel activity. The N-terminal intracellular domain of TRPV1 is sufficient for filopodial structure initiation while the C-terminal cytoplasmic domain is involved in the stabilization of microtubules within these structures. In addition, exogenous expression of TRPV1 results in altered cellular distribution and in enhanced endogenous expression of non-conventional myosin motors, namely myosin IIA and myosin IIIA. These data indicate a novel role of TRPV1 in the regulation of cellular morphology and cellular contact formation.

Protein Kinase D Induces Transcription through Direct Phosphorylation of the cAMP-response Element-binding Protein

Protein kinase D (PKD), a family of serine/threonine kinases, can be activated by a multitude of stimuli in a protein kinase C-dependent or -independent manner. PKD is involved in signal transduction pathways controlling cell proliferation, apoptosis, motility, and protein trafficking. Despite its versatile functions, few genuine in vivo substrates for PKD have been identified. In this study we demonstrate that the transcription factor cAMP-response element-binding protein (CREB) is a direct substrate for PKD. PKD1 and CREB interact in cells, and activated PKD1 provokes CREB phosphorylation at Ser-133 both in vitro and in vivo. A constitutive active mutant of PKD1 stimulates GAL4-CREB-mediated transcription in a Ser-133-dependent manner, activates CRE-responsive promoters, and increases the expression of CREB target genes. PKD1 also enhances transcription mediated by two other members of the CREB family, ATF-1 and CREM. Our results describe a novel mechanism for PKD-induced signaling through activation of the transcription factor CREB and suggest that stimulus-induced phosphorylation of CREB, reported to be mediated by protein kinase C, may involve downstream activated PKD.

Lipopolysaccharide-induced protein kinase D activation mediated by interleukin-1beta and protein kinase C

Protein kinase D (PKD), a newly described serine/threonine kinase, has been implicated in many signal transduction pathways. The present study was designed to determine whether and how PKD is activated in inflammation. The results demonstrated that lipopolysaccharide (LPS, 30 microg/ml) stimulated PKD and protein kinase C (PKC) phosphorylation in spinal neurons within 0.5 h, and the activation reached a maximum at 3 or 8 h and declined at 12 h. The phosphorylation could be inhibited by the selective inhibitors for PKC (100 nM), mainly for PKCalpha and PKCbeta, suggesting the involvement of the PKC pathway. Particularly, PKCalpha might be critical for LPS-induced PKD activation since the PKCbeta inhibitor (100 nM) observed no effect on the phosphorylation of PKD. Furthermore, the expression of interleukin-1beta (IL-1beta) was significantly induced by LPS within 0.5 h, and reached a maximum at 8 h. IL-1 receptor antagonist inhibited PKD and PKCs activation induced by LPS at a concentration of 50 nM and achieved maximum at 1000 nM. These results demonstrated for the first time that PKD could be activated by LPS in spinal neurons, might via the IL-1beta/PKCalpha pathway. Additionally, immunostaining showed an increase in number of phosphorylated PKD-immunoreactive cells of adult spinal dorsal horn induced by intraplantar injected carrageenan (2 microg/100 microl), and antisense oligodeoxynucleotide to IL-1 receptor type I (50 microg/10 microl, intrathecal injected) inhibited the PKD activation, suggesting an involvement of IL-1beta/PKD pathway in inflammation in adult spinal cord.

Derepression of pathological cardiac genes by members of the CaM kinase superfamily

In response to pathologic stresses such as hypertension or myocardial infarction, the heart undergoes a remodeling process that is characterized by myocyte hypertrophy, myocyte death and fibrosis, resulting in impaired cardiac function and heart failure. Cardiac remodeling is associated with derepression of genes that contribute to disease progression. This review focuses on evidence linking members of the Ca(2+)/calmodulin-dependent protein kinase (CaMK) superfamily, specifically CaMKII, protein kinase D (PKD) and microtubule associated kinase (MARK), to stress-induced derepression of pathological cardiac gene expression through their effects on class IIa histone deacetylases (HDACs).

Protease-activated receptor 2 sensitizes the transient receptor potential vanilloid 4 ion channel to cause mechanical hyperalgesia in mice

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR2) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR2-mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). Immunoreactive TRPV4 was coexpressed by rat dorsal root ganglia (DRG) neurons with PAR2, substance P (SP) and calcitonin gene-related peptide (CGRP), mediators of pain transmission. In PAR2-expressing cell lines that either naturally expressed TRPV4 (bronchial epithelial cells) or that were transfected to express TRPV4 (HEK cells), pretreatment with a PAR2 agonist enhanced Ca2+ and current responses to the TRPV4 agonists phorbol ester 4alpha-phorbol 12,13-didecanoate (4alphaPDD) and hypotonic solutions. PAR2-agonist similarly sensitized TRPV4 Ca2+ signals and currents in DRG neurons. Antagonists of phospholipase Cbeta and protein kinases A, C and D inhibited PAR2-induced sensitization of TRPV4 Ca2+ signals and currents. 4alphaPDD and hypotonic solutions stimulated SP and CGRP release from dorsal horn of rat spinal cord, and pretreatment with PAR2 agonist sensitized TRPV4-dependent peptide release. Intraplantar injection of PAR2 agonist caused mechanical hyperalgesia in mice and sensitized pain responses to the TRPV4 agonists 4alphaPDD and hypotonic solutions. Deletion of TRPV4 prevented PAR2 agonist-induced mechanical hyperalgesia and sensitization. This novel mechanism, by which PAR2 activates a second messenger to sensitize TRPV4-dependent release of nociceptive peptides and induce mechanical hyperalgesia, may underlie inflammatory hyperalgesia in diseases where proteases are activated and released.

Potentiation of transient receptor potential V1 functions by the activation of metabotropic 5-HT receptors in rat primary sensory neurons

5-Hydroxytryptamine (5-HT) is one of the major chemical mediators released in injured and inflamed tissue and is capable of inducing hyperalgesia in vivo. However, the cellular mechanisms of 5-HT-induced hyperalgesia remain unclear. Transient receptor potential V1 (TRPV1) plays a pivotal role in nociceptive receptors. In the present study, we determined whether 5-HT changes TRPV1 functions in cultured dorsal root ganglion (DRG) neurons isolated from neonatal rats, using Ca(2+) imaging and whole-cell patch-clamp techniques. In more than 70% of DRG neurons, 5-HT potentiated the increases of [Ca(2+)](i) induced by capsaicin, protons and noxious heat. Capsaicin-induced current and depolarizing responses, and proton-induced currents were also augmented by 5-HT. RT-PCR analysis revealed the expression of 5-HT(2A) and 5-HT(7) receptors in rat DRG neurons. Agonists for 5-HT(2A) and 5-HT(7) receptors mimicked the potentiating effect of 5-HT, and their antagonists decreased it. In DRG ipsilateral to the complete Freund's adjuvant-injected inflammation side, expression levels of 5-HT(2A) and 5-HT(7) mRNAs increased, and the potentiating effect of 5-HT was more prominent than in the contralateral control side. These results suggest that the PKC- and PKA-mediated signalling pathways are involved in the potentiating effect of 5-HT on TRPV1 functions through the activation of 5-HT(2A) and 5-HT(7) receptors, respectively. Under inflammatory conditions, the increases of the biosynthesis of these 5-HT receptors may lead to further potentiation of TRPV1 functions, resulting in the generation of inflammatory hyperalgesia in vivo.

Protease-activated receptor 2 sensitizes TRPV1 by protein kinase Cepsilon- and A-dependent mechanisms in rats and mice

Proteases that are released during inflammation and injury cleave protease-activated receptor 2 (PAR2) on primary afferent neurons to cause neurogenic inflammation and hyperalgesia. PAR2-induced thermal hyperalgesia depends on sensitization of transient receptor potential vanilloid receptor 1 (TRPV1), which is gated by capsaicin, protons and noxious heat. However, the signalling mechanisms by which PAR2 sensitizes TRPV1 are not fully characterized. Using immunofluorescence and confocal microscopy, we observed that PAR2 was colocalized with protein kinase (PK) Cepsilon and PKA in a subset of dorsal root ganglia neurons in rats, and that PAR2 agonists promoted translocation of PKCepsilon and PKA catalytic subunits from the cytosol to the plasma membrane of cultured neurons and HEK 293 cells. Subcellular fractionation and Western blotting confirmed this redistribution of kinases, which is indicative of activation. Although PAR2 couples to phospholipase Cbeta, leading to stimulation of PKC, we also observed that PAR2 agonists increased cAMP generation in neurons and HEK 293 cells, which would activate PKA. PAR2 agonists enhanced capsaicin-stimulated increases in [Ca2+]i and whole-cell currents in HEK 293 cells, indicating TRPV1 sensitization. The combined intraplantar injection of non-algesic doses of PAR2 agonist and capsaicin decreased the latency of paw withdrawal to radiant heat in mice, indicative of thermal hyperalgesia. Antagonists of PKCepsilon and PKA prevented sensitization of TRPV1 Ca2+ signals and currents in HEK 293 cells, and suppressed thermal hyperalgesia in mice. Thus, PAR2 activates PKCepsilon and PKA in sensory neurons, and thereby sensitizes TRPV1 to cause thermal hyperalgesia. These mechanisms may underlie inflammatory pain, where multiple proteases are generated and released.

The mu opioid agonist morphine modulates potentiation of capsaicin-evoked TRPV1 responses through a cyclic AMP-dependent protein kinase A pathway

Background

The vanilloid receptor 1 (TRPV1) is critical in the development of inflammatory hyperalgesia. Several receptors including G-protein coupled prostaglandin receptors have been reported to functionally interact with the TRPV1 through a cAMP-dependent protein kinase A (PKA) pathway to potentiate TRPV1-mediated capsaicin responses. Such regulation may have significance in inflammatory pain. However, few functional receptor interactions that inhibit PKA-mediated potentiation of TRPV1 responses have been described.

Results

In the present studies we investigated the hypothesis that the μ opioid receptor (MOP) agonist morphine can modulate forskolin-potentiated capsaicin responses through a cAMP-dependent PKA pathway. HEK293 cells were stably transfected with TRPV1 and MOP, and calcium (Ca²⁺) responses to injection of the TRPV1 agonist capsaicin were monitored in Fluo-3-loaded cells. Pre-treatment with morphine did not inhibit unpotentiated capsaicin-induced Ca²⁺ responses but significantly altered capsaicin responses potentiated by forskolin. TRPV1-mediated Ca²⁺ responses potentiated by the direct PKA activator 8-Br-cAMP and the PKC activator Phorbol-12-myristate-13-acetatewere not modulated by morphine.

Immunohistochemical studies confirmed that the TRPV1 and MOP are co-expressed on cultured Dorsal Root Ganglion neurones, pointing towards the existence of a functional relationship between the G-protein coupled MOP and nociceptive TRPV1.

Conclusion

The results presented here indicate that the opioid receptor agonist morphine acts via inhibition of adenylate cyclase to inhibit PKA-potentiated TRPV1 responses. Targeting of peripheral opioid receptors may therefore have therapeutic potential as an intervention to prevent potentiation of TRPV1 responses through the PKA pathway in inflammation.

Protease-activated receptor-2 activation exaggerates TRPV1-mediated cough in guinea pigs

A lowered threshold to the cough response frequently accompanies chronic airway inflammatory conditions. However, the mechanism(s) that from chronic inflammation results in a lowered cough threshold is poorly understood. Irritant agents, including capsaicin, resiniferatoxin, and citric acid, elicit cough in humans and in experimental animals through the activation of the transient receptor potential vanilloid 1 (TRPV1). Protease-activated receptor-2 (PAR2) activation plays a role in inflammation and sensitizes TRPV1 in cultured sensory neurons by a PKC-dependent pathway. Here, we have investigated whether PAR2 activation exaggerates TRPV1-dependent cough in guinea pigs and whether protein kinases are involved in the PAR2-induced cough modulation. Aerosolized PAR2 agonists (PAR2-activating peptide and trypsin) did not produce any cough per se. However, they potentiated citric acid- and resiniferatoxin-induced cough, an effect that was completely prevented by the TRPV1 receptor antagonist capsazepine. In contrast, cough induced by hypertonic saline, a stimulus that provokes cough in a TRPV1-independent manner, was not modified by aerosolized PAR2 agonists. The PKC inhibitor GF-109203X, the PKA inhibitor H-89, and the cyclooxygenase inhibitor indomethacin did not affect cough induced by TRPV1 agonists, but abated the exaggeration of this response produced by PAR2 agonists. In conclusion, PAR2 stimulation exaggerates TRPV1-dependent cough by activation of diverse mechanism(s), including PKC, PKA, and prostanoid release. PAR2 activation, by sensitizing TRPV1 in primary sensory neurons, may play a role in the exaggerated cough observed in certain airways inflammatory diseases such as asthma and chronic obstructive pulmonary disease.

Increased sensitivity of desensitized TRPV1 by PMA occurs through PKCepsilon-mediated phosphorylation at S800

Important mechanisms that regulate inhibitory and facilitatory effects on TRPV1-mediated nociception are desensitization and phosphorylation, respectively. Using Ca2+-imaging, we have previously shown that desensitization of TRPV1 upon successive capsaicin applications was reversed by protein kinase C activation in dorsal root ganglion neurons and CHO cells. Here, using both Ca2+-imaging and patch-clamp methods, we show that PMA-induced activation of PKCepsilon is essential for increased sensitivity of desensitized TRPV1. TRPV1 has two putative substrates S502 and S800 for PKCepsilon-mediated phosphorylation. Patch-clamp analysis showed that contribution of single mutant S502A or S800A towards increased sensitivity of desensitized TRPV1 is indistinguishable from that observed in a double mutant S502A/S800A. Since S502 is a non-specific substrate for TRPV1 phosphorylation by kinases like PKC, PKA or CAMKII, evidence for a role of PKC specific substrate S800 was investigated. Evidence for in vivo phosphorylation of TRPV1 at S800 was demonstrated for the first time. We also show that the expression level of PKCepsilon paralleled the amount of phosphorylated TRPV1 protein using an antibody specific for phosphorylated TRPV1 at S800. Furthermore, the anti-phosphoTRPV1 antibody detected phosphorylation of TRPV1 in mouse and rat DRG neurons and may be useful for research regarding nociception in native tissues. This study, therefore, identifies PKCepsilon and S800 as important therapeutic targets that may help regulate inhibitory effects on TRPV1 and hence its desensitization.

Downregulation of transient receptor potential melastatin 8 by protein kinase C-mediated dephosphorylation

Transient receptor potential melastatin 8 (TRPM8) and transient receptor potential vanilloid 1 (TRPV1) are ion channels that detect cold and hot sensations, respectively. Their activation depolarizes the peripheral nerve terminals resulting in action potentials that propagate to brain via the spinal cord. These receptors also play a significant role in synaptic transmission between dorsal root ganglion (DRG) and dorsal horn (DH) neurons. Here, we show that TRPM8 is functionally downregulated by activation of protein kinase C (PKC) resulting in inhibition of membrane currents and increases in intracellular Ca2+ compared with upregulation of TRPV1 in cloned and native receptors. Bradykinin significantly downregulates TRPM8 via activation of PKC in DRG neurons. Activation of TRPM8 or TRPV1 at first sensory synapse between DRG and DH neurons leads to a robust increase in frequency of spontaneous/miniature EPSCs. PKC activation blunts TRPM8- and facilitates TRPV1-mediated synaptic transmission. Significantly, downregulation is attributable to PKC-mediated dephosphorylation of TRPM8 that could be reversed by phosphatase inhibitors. These findings suggest that inflammatory thermal hyperalgesia mediated by TRPV1 may be further aggravated by downregulation of TRPM8, because the latter could mediate the much needed cool/soothing sensation.

Protein kinase D directly phosphorylates histone deacetylase 5 via a random sequential kinetic mechanism

Class II histone deacetylases (HDACs) are signal-responsive repressors of gene transcription. In the heart, class II HDAC5 suppresses expression of genes that govern stress-induced cardiomyocyte growth. Signaling via pro-growth G protein coupled receptors triggers phosphorylation of HDAC5 on two serine residues (Ser(259) and Ser(498)), resulting in nuclear export of HDAC5 and de-repression downstream target genes. Although prior studies established a role for protein kinase D (PKD) in the regulation of HDAC5 phosphorylation, it remained unclear whether PKD functions directly or indirectly to control the phosphorylation status of this transcriptional repressor. Here, we demonstrate that PKD catalyzes direct phosphoryl-group transfer to Ser(498) of HDAC5. Each of the three PKD family members, PKD1, PKD2, and PKD3, is capable of phosphorylating HDAC5 (K(m) for substrate=2.07, 3.12, and 1.43microM, respectively), although PKD2 exhibits highest catalytic efficiency (k(cat)/K(m)=6.77min(-1)microM(-1)). Kinetic studies revealed that the three PKD isozymes phosphorylate HDAC5 through a random sequential mechanism, and that ATP has no effect on association of kinase with peptide substrate. In addition, we demonstrate that ADP competitively inhibits phosphorylation of HDAC5 (K(i)=8.50, 17.54, and 11.98microM for PKD1, PKD2, and PKD3, respectively). These findings define PKD as an HDAC kinase and thus suggest key roles for PKD family members in the control of chromatin structure and gene expression.