Opioids are chemical substances similar to opiates, the active substances found in opium (morphine, codeine etc.). Opioid action is mediated by the receptors for endogenous opioids; peptides such as the enkephalins, the endorphins or the dynorphins. Opioids possess powerful analgesic and sedative effects, and are widely used as pain-killers. Their main side-effect is the rapid establishment of a strong addiction. Opioids receptors are G-protein coupled receptors (GPCR). There are four classes of receptors: mu (MOR), kappa (KOR) and delta (DOR), and the nociceptin receptor (NOP).
Bilbao A, Parkitna JR, Engblom D, Perreau-Lenz S, Sanchis-Segura C, Schneider M, Konopka W, Westphal M, Breen G, Desrivieres S, Klugmann M, Guindalini C, Vallada H, Laranjeira R, de Fonseca FR, Schumann G, Schütz G, Spanagel R.; ''Loss of the Ca2+/calmodulin-dependent protein kinase type IV in dopaminoceptive neurons enhances behavioral effects of cocaine.''; PubMedEurope PMCScholia
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Oldham WM, Van Eps N, Preininger AM, Hubbell WL, Hamm HE.; ''Mechanism of the receptor-catalyzed activation of heterotrimeric G proteins.''; PubMedEurope PMCScholia
Gerhardt MA, Neubig RR.; ''Multiple Gi protein subtypes regulate a single effector mechanism.''; PubMedEurope PMCScholia
Bibb JA, Snyder GL, Nishi A, Yan Z, Meijer L, Fienberg AA, Tsai LH, Kwon YT, Girault JA, Czernik AJ, Huganir RL, Hemmings HC, Nairn AC, Greengard P.; ''Phosphorylation of DARPP-32 by Cdk5 modulates dopamine signalling in neurons.''; PubMedEurope PMCScholia
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Goraya TA, Masada N, Ciruela A, Cooper DM.; ''Sustained entry of Ca2+ is required to activate Ca2+-calmodulin-dependent phosphodiesterase 1A.''; PubMedEurope PMCScholia
Krasel C, Dammeier S, Winstel R, Brockmann J, Mischak H, Lohse MJ.; ''Phosphorylation of GRK2 by protein kinase C abolishes its inhibition by calmodulin.''; PubMedEurope PMCScholia
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Reed KA, Tucker DE, Aloulou A, Adler D, Ghomashchi F, Gelb MH, Leslie CC, Oates JA, Boutaud O.; ''Functional characterization of mutations in inherited human cPLA₂ deficiency.''; PubMedEurope PMCScholia
Ahn JH, Sung JY, McAvoy T, Nishi A, Janssens V, Goris J, Greengard P, Nairn AC.; ''The B''/PR72 subunit mediates Ca2+-dependent dephosphorylation of DARPP-32 by protein phosphatase 2A.''; PubMedEurope PMCScholia
Taussig R, Tang WJ, Hepler JR, Gilman AG.; ''Distinct patterns of bidirectional regulation of mammalian adenylyl cyclases.''; PubMedEurope PMCScholia
Li X, Li HP, Amsler K, Hyink D, Wilson PD, Burrow CR.; ''PRKX, a phylogenetically and functionally distinct cAMP-dependent protein kinase, activates renal epithelial cell migration and morphogenesis.''; PubMedEurope PMCScholia
Wall MA, Coleman DE, Lee E, Iñiguez-Lluhi JA, Posner BA, Gilman AG, Sprang SR.; ''The structure of the G protein heterotrimer Gi alpha 1 beta 1 gamma 2.''; PubMedEurope PMCScholia
Kleuss C, Raw AS, Lee E, Sprang SR, Gilman AG.; ''Mechanism of GTP hydrolysis by G-protein alpha subunits.''; PubMedEurope PMCScholia
Nishi A, Bibb JA, Matsuyama S, Hamada M, Higashi H, Nairn AC, Greengard P.; ''Regulation of DARPP-32 dephosphorylation at PKA- and Cdk5-sites by NMDA and AMPA receptors: distinct roles of calcineurin and protein phosphatase-2A.''; PubMedEurope PMCScholia
Taussig R, Iñiguez-Lluhi JA, Gilman AG.; ''Inhibition of adenylyl cyclase by Gi alpha.''; PubMedEurope PMCScholia
Wang JB, Johnson PS, Persico AM, Hawkins AL, Griffin CA, Uhl GR.; ''Human mu opiate receptor. cDNA and genomic clones, pharmacologic characterization and chromosomal assignment.''; PubMedEurope PMCScholia
Zigman JM, Westermark GT, LaMendola J, Boel E, Steiner DF.; ''Human G(olf) alpha: complementary deoxyribonucleic acid structure and expression in pancreatic islets and other tissues outside the olfactory neuroepithelium and central nervous system.''; PubMedEurope PMCScholia
Liang Z, Liu F, Grundke-Iqbal I, Iqbal K, Gong CX.; ''Down-regulation of cAMP-dependent protein kinase by over-activated calpain in Alzheimer disease brain.''; PubMedEurope PMCScholia
Francken BJ, Jurzak M, Vanhauwe JF, Luyten WH, Leysen JE.; ''The human 5-ht5A receptor couples to Gi/Go proteins and inhibits adenylate cyclase in HEK 293 cells.''; PubMedEurope PMCScholia
James MA, Lu Y, Liu Y, Vikis HG, You M.; ''RGS17, an overexpressed gene in human lung and prostate cancer, induces tumor cell proliferation through the cyclic AMP-PKA-CREB pathway.''; PubMedEurope PMCScholia
Lambright DG, Noel JP, Hamm HE, Sigler PB.; ''Structural determinants for activation of the alpha-subunit of a heterotrimeric G protein.''; PubMedEurope PMCScholia
Sette C, Conti M.; ''Phosphorylation and activation of a cAMP-specific phosphodiesterase by the cAMP-dependent protein kinase. Involvement of serine 54 in the enzyme activation.''; PubMedEurope PMCScholia
Gullingsrud J, Kim C, Taylor SS, McCammon JA.; ''Dynamic binding of PKA regulatory subunit RI alpha.''; PubMedEurope PMCScholia
Di Pasquale G, Stacey SN.; ''Adeno-associated virus Rep78 protein interacts with protein kinase A and its homolog PRKX and inhibits CREB-dependent transcriptional activation.''; PubMedEurope PMCScholia
Saidak Z, Blake-Palmer K, Hay DL, Northup JK, Glass M.; ''Differential activation of G-proteins by mu-opioid receptor agonists.''; PubMedEurope PMCScholia
Walaas SI, Aswad DW, Greengard P.; ''A dopamine- and cyclic AMP-regulated phosphoprotein enriched in dopamine-innervated brain regions.''; PubMedEurope PMCScholia
Alt A, Clark MJ, Woods JH, Traynor JR.; ''Mu and Delta opioid receptors activate the same G proteins in human neuroblastoma SH-SY5Y cells.''; PubMedEurope PMCScholia
Hemmings HC, Williams KR, Konigsberg WH, Greengard P.; ''DARPP-32, a dopamine- and adenosine 3':5'-monophosphate-regulated neuronal phosphoprotein. I. Amino acid sequence around the phosphorylated threonine.''; PubMedEurope PMCScholia
Murakami M, Taketomi Y, Sato H, Yamamoto K.; ''Secreted phospholipase A2 revisited.''; PubMedEurope PMCScholia
Hemmings HC, Greengard P, Tung HY, Cohen P.; ''DARPP-32, a dopamine-regulated neuronal phosphoprotein, is a potent inhibitor of protein phosphatase-1.''; PubMedEurope PMCScholia
Lin LL, Wartmann M, Lin AY, Knopf JL, Seth A, Davis RJ.; ''cPLA2 is phosphorylated and activated by MAP kinase.''; PubMedEurope PMCScholia
Chuang TT, Paolucci L, De Blasi A.; ''Inhibition of G protein-coupled receptor kinase subtypes by Ca2+/calmodulin.''; PubMedEurope PMCScholia
Levay K, Satpaev DK, Pronin AN, Benovic JL, Slepak VZ.; ''Localization of the sites for Ca2+-binding proteins on G protein-coupled receptor kinases.''; PubMedEurope PMCScholia
Berstein G, Blank JL, Jhon DY, Exton JH, Rhee SG, Ross EM.; ''Phospholipase C-beta 1 is a GTPase-activating protein for Gq/11, its physiologic regulator.''; PubMedEurope PMCScholia
Schulz S, Mayer D, Pfeiffer M, Stumm R, Koch T, Höllt V.; ''Morphine induces terminal micro-opioid receptor desensitization by sustained phosphorylation of serine-375.''; PubMedEurope PMCScholia
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Régnauld KL, Leteurtre E, Gutkind SJ, Gespach CP, Emami S.; ''Activation of adenylyl cyclases, regulation of insulin status, and cell survival by G(alpha)olf in pancreatic beta-cells.''; PubMedEurope PMCScholia
Clark JD, Lin LL, Kriz RW, Ramesha CS, Sultzman LA, Lin AY, Milona N, Knopf JL.; ''A novel arachidonic acid-selective cytosolic PLA2 contains a Ca(2+)-dependent translocation domain with homology to PKC and GAP.''; PubMedEurope PMCScholia
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Caricasole A, Sala C, Roncarati R, Formenti E, Terstappen GC.; ''Cloning and characterization of the human phosphoinositide-specific phospholipase C-beta 1 (PLC beta 1).''; PubMedEurope PMCScholia
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Chen TY, Illing M, Molday LL, Hsu YT, Yau KW, Molday RS.; ''Subunit 2 (or beta) of retinal rod cGMP-gated cation channel is a component of the 240-kDa channel-associated protein and mediates Ca(2+)-calmodulin modulation.''; PubMedEurope PMCScholia
Ross D, Joyner WL.; ''Resting distribution and stimulated translocation of protein kinase C isoforms alpha, epsilon and zeta in response to bradykinin and TNF in human endothelial cells.''; PubMedEurope PMCScholia
Coleman DE, Berghuis AM, Lee E, Linder ME, Gilman AG, Sprang SR.; ''Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis.''; PubMedEurope PMCScholia
Johnson EA, Oldfield S, Braksator E, Gonzalez-Cuello A, Couch D, Hall KJ, Mundell SJ, Bailey CP, Kelly E, Henderson G.; ''Agonist-selective mechanisms of mu-opioid receptor desensitization in human embryonic kidney 293 cells.''; PubMedEurope PMCScholia
Burford NT, Tolbert LM, Sadee W.; ''Specific G protein activation and mu-opioid receptor internalization caused by morphine, DAMGO and endomorphin I.''; PubMedEurope PMCScholia
Simpson RE, Ciruela A, Cooper DM.; ''The role of calmodulin recruitment in Ca2+ stimulation of adenylyl cyclase type 8.''; PubMedEurope PMCScholia
Clark MJ, Traynor JR.; ''Mediation of adenylyl cyclase sensitization by PTX-insensitive GalphaoA, Galphai1, Galphai2 or Galphai3.''; PubMedEurope PMCScholia
Gaibelet G, Meilhoc E, Riond J, Saves I, Exner T, Liaubet L, Nürnberg B, Masson JM, Emorine LJ.; ''Nonselective coupling of the human mu-opioid receptor to multiple inhibitory G-protein isoforms.''; PubMedEurope PMCScholia
When cAMP level rises, the PKA catalytic subunit (C subunit) released from the holoenzyme enters the nucleus by passive diffusion whereas termination of signaling to the nucleus involves an active mechanism. In the nucleus, the C subunit binds to the heat-stable protein kinase inhibitor (PKI), and this binding not only inactivates the C subunit but also by conformational change unveils a nuclear export signal in PKI which leads to export of the C-PKI complex from the nucleus.
The IP3 receptor (IP3R) is an IP3-gated calcium channel. It is a large, homotetrameric protein, similar to other calcium channel proteins such as ryanodine. The four subunits form a 'four-leafed clover' structure arranged around the central calcium channel. Binding of ligands such as IP3 results in conformational changes in the receptor's structure that leads to channel opening.
The high affinity complex beta-endorphin:mu opioid receptor binds to the heterotrimeric G-protein. This binding stabilises a conformation of the G-protein alpha i subunit presenting a low affinity for GDP, but a high affinity for GTP
PP2B (calcineurin) is a calcium-dependent, calmodulin-stimulated protein phosphatase. It comprises of two components; a catalytic subunit and a regulatory subunit which confers calcium sensitivity to the complex. PP2B is in equilibrium between active and inactive forms. Because the affinity of calmodulin for the active form is higher than for the inactive form, it stabilises PP2B.
PLC-beta1 is a GTPase-activating protein (GAP) for Gq-alpha, exchanging GTP for GDP and releasing the alpha subunit to cycle back to the membrane and reassociate with the beta-gamma subunits. Between itself and the receptor, they regulate the amplitude of the PLC signal and the rates of signal initiation and termination.
The 85kDa cytosolic phospholipase A2 (cPLA2 - PLA2G4A) is involved in cell signalling processes and inflammatory response and is regulated by phosphorylation and calcium concentrations. cPLA2 is phosphorylated at Ser727 and by a MAPK at Ser505. When phosphorylation is coupled with an influx of calcium ions, PLA2 becomes stimulated and translocates to the membrane where it releases arachidonic acid (AA) from membrane phospholipids. Calcium does not itself activate cPLA2. cPLA2 contains an N-terminal calcium-dependent phospholipid binding domain (CaLB) which shares homology with C2 domains (plays roles in signal transduction and membrane trafficking) and binds it to the membrane. Arachidonic acid is both a signalling molecule and the precursor for other signalling molecules termed eicosanoids (e.g., prostaglandins, leukotrienes and platelet-activating factor). A strict regulation of the activity of phospholipase enzyme is essential.
Autophosphorylation of the N-terminus Ser12-Ser13 is required for full activation after Ca2+/calmodulin binding and phosphorylation of the Ca2+/calmodulin-bound enzyme on Thr200 by a Ca2+/calmodulin-dependent protein kinase kinase.
The four protein kinase A (PKA) regulatory subunit isoforms differ in their tissue specificity and functional characteristics. The specific isoform activated in response to glucagon signalling is not known. The PKA kinase is a tetramer of two regulatory and two catalytic. The regulatory subunits block the catalytic subunits. Binding of cAMP to the regulatory subunit leads to the dissociation of the tetramer into two active dimers made up of a regulatory and a catalytic subunit.
Once bound to the membrane, cPLA2 hydrolyzes phosphatidylcholine to produce arachidonic acid (AA), a precursor to inflammatory mediators. While several phospholipases can catalyze this reaction in cells overexpressing the enzymes, PLA2G4A is the major enzyme that catalyzes this reaction in vivo (Reed et al. 2011). At the same time, possible physiological roles have been described for soluble phospholipases (sPLA) in the mobilization of arachidonic acid in some cell types or under some physiological conditions (Murakami et al. 2011). Here, the major role of PLA2G4A has been annotated.
G proteins can deactivate themselves via their intrinsic GTPase activity, which hydrolyzes GTP to GDP. Effectors such as adenylate cyclase can increase the G protein GTPase rate, acting like GTPase-activating proteins (GAPs).
The ternary complex neurotransmitter:receptor:G-protein dissociates. Both the alpha-i subunit and beta:gamma complex become active, by conformational transition and surface exposure, and both are free to activate downstream effectors.
CaMKIV becomes fully activated after a three-step mechanism: Upon a transient increase in intracellular calcium, calcium-bound calmodulin (Ca2+/CaM) binds to its autoregulatory domain, which relieves intersteric inhibition. An activating protein kinase, calcium/calmodulin-dependent protein kinase kinase (CaMKK), binds to the Ca2+/CaM:CaMKIV complex and phosphorylates CaMKIV on a threonine residue in the activation loop. After full activation by the three-step mechanism mentioned above, the activity of CaMKIV becomes autonomous and no longer requires bound Ca2+/CaM. This activity is required for CaMKIV-mediated transcriptional regulation. The CaMKIV-associated PP2A then dephosphorylates CaMKIV, thereby terminating autonomous activity and CaMKIV-mediated gene transcription.
The phospholipase C (PLC) family of enzymes is both diverse and complex. The isoforms beta, gamma and delta (each have subtypes) make up the members of this family. One type, PLC-beta1, hydrolyzes phosphatidylinositol bisphosphate (PIP2) into two second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 mobilizes intracellular calcium stores while DAG activates protein kinase C isoforms which are involved in regulatory functions.
The amino-acid sequence of DARPP-32 contains consensus phosphorylation sites for proline-directed kinases, including Cdk5, a cyclin-dependent kinase family member which is present in post-mitotic neurons expressing high levels of DARPP-32.
cAMP-dependent protein kinase A (PKA) phosphorylates in vitro CREB at a specific residue, serine 133; Phosphorylation of Ser133 is required for signal-induced transcription in vivo.
Once the intrinsic GTPase hydrolyzes GTP to GDP, Galpha-i dissociates from adenylate cyclase, allowing it to re-associate with G-beta-gamma and starting a new cycle.
G alpha-olf:GTP binds to inactive adenylate cyclase, causing a conformational transition in adenylate cyclase exposing the catalytic site and activating it.
Different ligands of the MOR receptor can promote MOR phosphorylation, uncoupling, endocytosis or inactivation. For example, the endogenous peptide ligands at the MOR induce rapid desensitization, endocytosis and rapid receptor recycling. By contrast, morphine induces little to no endocytosis, tolerance and dependence. The agonist-dependent phosphorylation of opioid receptors changes the receptor conformation and increases the affinity of the receptors for cytosolic beta-arrestin proteins. This results in an uncoupling of G protein signalling and recruitment of the endocytotic machinery leading to receptor internalization and rapid resensitization. By contrast, PKC phosphorylation by non internalizing opioid ligands (e.g., morphine) cause receptors to remain inactivated in the plasma membrane, leading to signaling desensitization and opioid tolerance. In this case receptors appear to require activity of a phosphatase to be resensitized.
The active form of G protein alpha subunit q (Gq-alpha) was found to activate phospholipase C beta-1 (PLC-beta1), in investigations using bovine membranes. Subsequently, all 4 human isoforms have been shown to be activated by Gq, though activation of PLCbeta-4 is limited. In recombinant assays, several activated rat G alpha q family members were found to stimulate human PLC-beta isoforms with the same rank order of decreasing potency. PLC-beta1 stimulation was slightly more than for PLC-beta3; PLC-beta3 stimulation was 10-fold greater than for beta-2. PLC-beta2 is expressed specifically in hematopoietic cells. PLC-beta acts directly on Gq to accelerate hydrolysis of bound GTP, thus PLC-betas are GTPase activating proteins (GAPs). The crystal structure of the C-terminal region from Turkey PLC-beta, revealed a novel fold composed almost entirely of three long helices forming a coiled-coil that dimerizes along its long axis in an antiparallel orientation. The extent of the dimer interface and gel exclusion chromatography data suggest that PLC-betas are functionally dimeric.
GRK2 is a Serine/Threonine kinase. G-protein-coupled receptor kinases (GRKs) are important regulators of G-protein-coupled receptor function. Binding of calmodulin to GRK2 results in inhibition of the kinase activity. This inhibition is almost completely abolished when GRK2 is phosphorylated by PKC.
The cAMP-responsive element binding protein (CREB), a key regulator of gene expression, is activated by phosphorylation on Ser-133. Several different protein kinases possess the capability of driving this phosphorylation, making it a point of potential convergence for multiple intracellular signaling cascades. Work in neurons has indicated that physiologic synaptic stimulation recruits a fast calmodulin kinase IV (CaMKIV)-dependent pathway that dominates early signaling to CREB.
PP2A is ubiquitously expressed in eukaryotic cells, existing as a heterotrimeric enzyme composed of a 36-kDa catalytic C subunit, a 64-kDa scaffolding A subunit, and multiple regulatory B subunits. The B subunits are thought to influence enzyme activity, substrate specificity, and subcellular localization. PKA phosphorylates PP2A thereby activating the enzyme and is responsible for dopamine/cAMP-dependent dephosphorylation of Thr-75 of DARPP-32.
G proteins can deactivate themselves via their intrinsic GTPase activity, which hydrolyzes GTP to GDP. Effectors such as adenylate cyclase can increase the G protein GTPase rate, acting like GTPase-activating proteins (GAPs).
The binding of an opiate peptide to the mu opiate receptor stabilises the receptor conformation in a state of high affinity, both for the ligand itself, and for the G-protein.
Calcineurin has been identified as a Ca2+- and calmodulin-dependent phosphoprotein phosphatase. The concentration of the enzyme is relatively high in mammalian brain.
G proteins can deactivate themselves via their intrinsic GTPase activity, which hydrolyzes GTP to GDP. Effectors such as adenylate cyclase can increase the G protein GTPase rate, acting like GTPase-activating proteins (GAPs).
DARPP-32 is phosphorylated by cAMP-dependent protein kinase (PKA) on a single threonine residue, Thr34, resulting in its conversion into a potent inhibitor of protein phosphatase-1.
DARPP-32 is phosphorylated by cAMP-dependent protein kinase (PKA) on a single threonine residue, Thr34, resulting in its conversion into a potent inhibitor of protein phosphatase-1.
GRK2 is phosphorylated at serine 29 in vitro and in vivo by the alpha, gamma and delta isoforms of PKC. PKC-mediated phosphorylation at Ser29 increases GRK2 kinase activity towards GPCR substrates, contributing to GPCR desensitization. Phosphorylation at Ser29, which falls within the calmodulin-binding region of GRK2, abolishes the inhibitory effect of calmodulin on GRK2 kinase activity.
DARPP-32 is converted into an inhibitor of protein kinase A (PKA) when phosphorylated at threonine 75 by cyclin-dependent kinase 5 (Cdk5) in brain cells.
Once the intrinsic GTPase hydrolyzes GTP to GDP, Galpha-olf dissociates from adenylate cyclase, allowing it to re-associate with G-beta-gamma and starting a new cycle.
G-proteins in the Gi class inhibit adenylate cyclase activity, decreasing the production of cAMP from ATP, which has many consequences but classically results in decreased activity of Protein Kinase A (PKA). cAMP also activates the cyclic nucleotide-gated ion channels, a process that is particularly important in olfactory cells.
Adenylate cyclase is responsive to calcium and calmodulin and produces cAMP. One important physiological role for Calmodulin is the regulation of adenylylcyclases. Four of the ten known adenylylcyclases are calcium sensitive, in particular type 8 (AC8).
ERK2 phosphorylates cPLA2, increasing enzymatic activity. The site of cPLA2 phosphorylation by ERK2 is Ser-505, the major site of cPLA2 phosphorylation observed in phorbol ester-treated cells.
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GDP
Adenylate cyclaseGTP
Adenylate cyclaseMOR
G protein-GDP complexMOR
G protein-GTP complexMOR
G-protein complexAnnotated Interactions
GDP
Adenylate cyclaseGTP
Adenylate cyclaseGTP
Adenylate cyclaseMOR
G-protein complexMOR
G-protein complex