Protein kinase C (PKC) modulates the function of the capsaicin receptor transient receptor potential vanilloid 1 (TRPV1). This modulation manifests as increased current when the channel is activated by capsaicin. In addition, studies have suggested that phosphorylation by PKC might directly gate the channel, because PKC-activating phorbol esters induce TRPV1 currents in the absence of applied ligands. To test whether PKC both modulates and gates the TRPV1 function by direct phosphorylation, we used direct sequencing to determine the major sites of PKC phosphorylation on TRPV1 intracellular domains. We then tested the ability of the PKC-activating phorbol 12-myristate 13-acetate (PMA) to potentiate capsaicin-induced currents and to directly gate TRPV1. We found that mutation of S800 to alanine significantly reduced the PMA-induced enhancement of capsaicin-evoked currents and the direct activation of TRPV1 by PMA. Mutation of S502 to alanine reduced PMA enhancement of capsaicin-evoked currents, but had no effect on direct activation of TRPV1 by PMA. Conversely, mutation of T704 to alanine had no effect on PMA enhancement of capsaicin-evoked currents but dramatically reduced direct activation of TRPV1 by PMA. These results, combined with pharmacological studies showing that inactive phorbol esters also weakly activate TRPV1, suggest that PKC-mediated phosphorylation modulates TRPV1 but does not directly gate the channel. Rather, currents induced by phorbol esters result from the combination of a weak direct ligand-like activation of TRPV1 and the phosphorylation-induced enhancement of the TRPV1 function. Furthermore, modulation of the TRPV1 function by PKC appears to involve distinct phosphorylation sites depending on the mechanism of channel activation. P rotein kinase C (PKC) in peripheral sensory afferents plays a prominent role in hypersensitivity to thermal and mechanical stimuli after tissue injury. PKC sensitizes heat responses and potentiates peptide release from cultured dorsal root ganglion neurons (1, 2) and sensitizes nociceptive afferent neurons to thermal and mechanical stimuli in intact peripheral nerve preparations (3, 4). Diabetic neuropathic hyperalgesia and epinephrine-induced hyperalgesia are attenuated by PKC inhibitors in vivo (5, 6). Recently, several studies have focused on the role of the PKC isoform. Specific blockade of PKC diminishes PKCmediated enhancement of heat currents in sensory neurons and epinephrine-induced hypersensitivity in vivo (7,8). PKC knockout mice exhibit reduced hyperalgesia after intracutaneous injection of epinephrine and nerve growth factor (8). Whereas a role of PKC in peripheral sensitization is well established, PKC-mediated phosphorylation and modulation of specific substrates during peripheral sensitization is not fully understood.Transient receptor potential vanilloid 1 [TRPV1; formerly known as vanilloid receptor 1 (VR1)] is an attractive PKC effector in peripheral nociceptors. TRPV1 was cloned as a capsaicin receptor and is a ligand-gated ion channel, which ...
The transient outward potassium currents (also known as A-type currents or IA) are important determinants of neuronal excitability. In the brain, IA is modulated by protein kinase C (PKC), protein kinase A (PKA), and extracellular signal-related kinase (ERK), three kinases that have been shown to be critical modulators of nociception. We wanted to determine the effects of these kinases on IA in superficial dorsal horn neurons. Using whole cell recordings from cultured mouse spinal cord superficial dorsal horn neurons, we found that PKC and PKA both inhibit IA in these cells, and that PKC has a tonic inhibitory action on IA. Further, we provide evidence supporting the hypothesis that PKC and PKA do not modulate IA directly, but rather act as upstream activators of ERKs, which modulate IA. These results suggest that ERKs serve as signal integrators in modulation of IA in dorsal horn neurons and that modulation of A-type potassium currents may underlie aspects of central sensitization mediated by PKC, PKA, and ERKs.
The metabotropic glutamate receptors (mGluRs) have been predicted to have a classical seven transmembrane domain structure similar to that seen for members of the G-protein-coupled receptor (GPCR) superfamily. However, the mGluRs (and other members of the family C GPCRs) show no sequence homology to the rhodopsinlike GPCRs, for which this seven transmembrane domain structure has been experimentally confirmed. Furthermore, several transmembrane domain prediction algorithms suggest that the mGluRs have a topology that is distinct from these receptors. In the present study, we set out to test whether mGluR5 has seven true transmembrane domains. Using a variety of approaches in both prokaryotic and eukaryotic systems, our data provide stong support for the proposed seven transmembrane domain model of mGluR5. We propose that this membrane topology can be extended to all members of the family C GPCRs.Glutamate is the primary excitatory neurotransmitter of the central nervous system and activates ligand-gated ion channels or ionotropic glutamate receptors (iGluRs) and G-protein-coupled metabotropic glutamate receptors (mGluRs). 1The mGluRs modulate synaptic transmission and neuronal excitability throughout the peripheral and central nervous system. They have been classified into three groups based on sequence homology, pharmacology, and signal transduction coupling. Group I mGluRs, consisting of mGluR1 and mGluR5, uniformly couple to phospholipase C, while group II and III mGluRs, composed of mGluR2, mGluR3, mGluR4, mGluR6, mGluR7, and mGluR8, inhibit adenylyl cyclase activity in heterologous expression systems (1, 2).Since their cloning, the structure-function relationships of mGluRs have garnered extensive interest. Relatively recent classification of G-protein-coupled receptors (GPCRs) has categorized mGluRs as family C GPCRs characterized structurally by a large N-terminal extracellular domain and consisting of mGluRs, GABA B receptors, Ca 2ϩ -sensing receptors, and certain pheromone receptors. The large N-terminal extracellular domain of mGluRs exhibits distant homology to bacterial amino acid periplasmic binding proteins, suggesting the domain is responsible for ligand binding (3). Structural modeling of mGluRs based on this homology accurately predicts important residues for ligand binding (4). Exchange of this domain among mGluRs also endows specific ligand binding properties (5-7). Recently, crystal structure determination of the N-terminal domain with bound ligand has definitively confirmed its role in ligand binding and conformational changes of the domain with binding have made several predictions for potential mechanisms for receptor activation (8).While the structure of the ligand binding N-terminal domain has been clearly resolved, little data exists on the structure and membrane topology of the subsequent transmembrane domains. The complete structure of the canonical GPCR, rhodopsin, has recently been solved and confirms a long line of studies delineating seven membrane spanning ␣-helices (9 -12). Based...
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