Sodium regulation of opioid agonist binding is potentiated by pertussis toxin

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

Increased agonist affinity at the μ-opioid receptor induced by prolonged agonist exposure

Prolonged exposure to high-efficacy agonists results in desensitization of the μ-opioid receptor (MOR). Desensitized receptors are thought to be unable to couple to G-proteins, preventing downstream signaling; however, the changes to the receptor itself are not well characterized. In the current study, confocal imaging was used to determine whether desensitizing conditions cause a change in agonist-receptor interactions. Using rapid solution exchange, the binding kinetics of fluorescently labeled opioid agonist, dermorphin Alexa594 (derm A594), to MORs was measured in live cells. The affinity of derm A594 binding increased after prolonged treatment of cells with multiple agonists that are known to cause receptor desensitization. In contrast, binding of a fluorescent antagonist, naltrexamine Alexa594, was unaffected by similar agonist pretreatment. The increased affinity of derm A594 for the receptor was long-lived and partially reversed after a 45 min wash. Treatment of the cells with pertussis toxin did not alter the increase in affinity of the derm A594 for MOR. Likewise, the affinity of derm A594 for MORs expressed in mouse embryonic fibroblasts derived from arrestin 1 and 2 knock-out animals increased after treatment of the cells with the desensitization protocol. Thus, opioid receptors were "imprinted" with a memory of prior agonist exposure that was independent of G-protein activation or arrestin binding that altered subsequent agonist-receptor interactions. The increased affinity suggests that acute desensitization results in a long-lasting but reversible conformational change in the receptor.

Differential binding properties of oripavines at cloned mu- and delta-opioid receptors

This study examines the possibility that oripavine opioid receptor agonists bind equally to both high and low affinity states of the mu-opioid receptor. Studies were performed in C6 cells expressing mu- or delta-opioid receptors; high and low agonist affinity states of the receptors were defined by the absence and presence, respectively of Na+ ions and the GTP analog Gpp(NH)p. At the mu-opioid receptor dihydroetorphine and etorphine were full agonists, buprenorphine had moderate efficacy while diprenorphine was an antagonist. At the delta-opioid receptor, dihydroetorphine, etorphine, and diprenorphine had moderate efficacy while buprenorphine was an antagonist. The binding affinities of the oripavines at the mu-opioid receptor decreased only one to 2-fold in the presence of NaCl and Gpp(NH)p. In contrast, decreases in oripavine affinity at the delta-opioid receptor correlated with delta-opioid receptor efficacy. The ability of oripavine agonists to bind with high affinity to the low agonist affinity state of the nu-opioid receptor may explain the high potencies of these compounds in vivo.

Opioid receptor-coupled second messenger systems

Although pharmacological data provide strong evidence for different types of opioid receptors (e.g., mu, delta, and kappa), they share many common properties in their ability to couple to second messenger systems. All opioid receptor types are coupled to G-proteins, since agonist binding is diminished by guanine nucleotides and agonist-stimulated GTPase activity has been identified in several preparations. Moreover, all three types inhibit adenylyl cyclase. This second messenger system has been identified for opioid receptors in both isolated brain membranes and in transformed cell culture. Studies with chronic treatment with opioid agonists suggest that the coupling of receptors with G-proteins and second messenger effectors may play important roles in development of opioid tolerance.

Guanine nucleotide regulation of [125I]beta-endorphin binding to NG108-15 and SK-N-SH cell membranes: specific cation requirements

Regulation of [125I]beta h-endorphin binding by guanine nucleotides was investigated in membrane preparations from two opioid receptor-containing cell lines: NG108-15, which contains only delta opioid receptors, and SK-N-SH, which contains predominantly mu opioid receptors. In contrast to the binding of the delta-selective agonist [3H][D-penicillamine2,D-penicillamine5]enkephalin to NG108-15 cell membranes, and of the mu-selective agonist [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin to SK-N-SH cell membranes, [125I]beta h-endorphin binding to NG108-15 and SK-N-SH cell membranes was not altered by guanosine triphosphate (GTP) or guanylyl-5'-imidodiphosphate (Gpp(NH)p) in the absence of cations. However, in the presence of NaCl, [125I]beta h-endorphin binding to both cell lines was inhibited by GTP and Gpp(NH)p in a concentration-dependent manner. In SK-N-SH cell membranes, the ability of sodium to promote regulation of [125I]beta h-endorphin binding by GTP was mimicked by the monovalent cations lithium and potassium, but not by the divalent cations magnesium, calcium, or manganese. In NG108-15 cell membranes, only sodium was effective in promoting inhibition of [125I]beta h-endorphin binding by GTP. The effect of GTP or Gpp(NH)p in the presence of sodium was also observed with guanosine diphosphate, but not guanosine monophosphate or any of the non-guanine nucleotides tested. These results indicate that the presence of monovalent cations is required for regulation of [125I]beta h-endorphin binding by guanine nucleotides, and that the specificity of this cation requirement differs between the mu and delta receptor-containing cell lines.

Opioid receptors in magnesium-digitonin-solubilized rat brain membranes are tightly coupled to a pertussis toxin-sensitive guanine nucleotide-binding protein

Opioid receptors solubilized in Mg2+-digitonin (2%, wt/vol) from Mg2+-pretreated rat brain membranes maintain, in addition to high-affinity opioid agonist binding, the modulation by guanine nucleotides. One of the modes of expression of the latter property is an attenuation of agonist binding by guanine nucleotides in the presence of Na+. To investigate the molecular basis of this modulation and to identify the G protein(s) involved, the soluble receptors were [32P]ADP-ribosylated by means of Bordetella pertussis toxin and subjected to molecular size exclusion chromatography. In addition, soluble extracts were chromatographed on lectin and hydrophobic affinity columns. The binding of 35S- and 3H-labelled analogues of GTP was also monitored in the species separated. The oligomeric G protein-coupled opioid receptors and the guanine nucleotide/pertussis toxin-sensitive species showed similar chromatographic properties in all three systems. This indicates that the biochemically functional G protein-opioid receptor complex formed in Mg2+-pretreated membranes in the absence of an agonist is stable in digitonin solution and to chromatographic separation. Further analysis showed that the guanine nucleotide modulation of opioid receptors is via the pertussis toxin substrates with Mr of 41,000 and 39,000, which are identified as Gi and Go alpha subunits, respectively.

ADP-ribosylation with pertussis toxin modulates the GTP-sensitive opioid ligand binding in digitonin-soluble extracts of rat brain membranes

Pertussis toxin-catalyzed ADP-ribosylation of the guanine nucleotide-binding proteins Gi and Go is shown to proceed in Mg2+-digitonin extracts from rat brain; the Mr 41,000 and Mr 39,000 peptides are labelled there as in the membranes. The ADP-ribosylation in detergent solution retains the differential sensitivity to guanine nucleotide analogues. This reaction also removes the partial inhibition by the guanine nucleotides of the binding of opioid agonists, as does the same treatment in the membranes. The partial inhibition of agonist binding by Na+, however, is left unchanged. The binding of the antagonist naloxone is little affected by Na+ or by guanine nucleotides in the treated membranes, but the treated soluble receptors show an enhanced binding in high-Na+ medium, although still guanine nucleotide insensitive. The data suggest that the toxin reaction in the absence of guanine nucleotides and agonist stabilizes the opioid receptor in a receptor-G-protein coupled state which is no longer sensitive to guanine nucleotides but retains its sensitivity to the Na+ ions.

Guanine nucleotide regulation of [125I]beta-endorphin binding to rat brain membranes: monovalent cation requirement

The binding of [125I]beta h-endorphin to rat brain membranes was investigated in the presence of GTP and guanylyl-5'-imidodiphosphate. In contrast to the binding of the mu-selective opioid agonist, [3H][D-Ala2,MePhe4,Glyol5]enkephalin, and the delta-selective opioid agonist, [3H][D-penicillamine2, D-penicillamine5]enkephalin, [125I]beta h-endorphin binding was not affected by GTP or guanylyl-5'-imidodiphosphate in a concentration-dependent manner in the absence of cations. However, in the presence of NaCl, the inclusion of either GTP or guanylyl-5'-imidodiphosphate resulted in a concentration-dependent inhibition of [125I]beta h-endorphin binding. This inhibition was significantly greater than the decrease in [125I]beta h-endorphin binding observed in the presence of sodium alone. Although GTP most potently inhibited [125I]beta h-endorphin binding in the presence of sodium, inhibition of [125I]beta h-endorphin binding by GTP was also observed in the presence of the monovalent cations lithium and potassium, but not the divalent cations magnesium, calcium, or manganese. The effect produced by GTP in the presence of NaCl was mimicked by GDP, but not by GMP or other nucleotides. Unlike [125I]beta h-endorphin, the binding of the putative sigma receptor agonist, (+)-[3H]SKF 10,047, was not significantly altered by GTP or guanylyl-5'-imidodiphosphate in the absence or presence of sodium.

Coupling of multiple opioid receptors to GTPase following selective receptor alkylation in brain membranes

Opioid agonists of the mu, kappa and delta types stimulated low-Km guanosine triphosphatase (GTPase) in membranes, from the brain of the rat by up to 34%, with potencies the rank order of which corresponded to the respective binding affinities to opioid receptor. In general, kappa ligands stimulated GTPase to a lesser degree than mu or delta opiates. The coupling of a given type of opioid receptor to GTPase was resolved by direct or protective alkylation of the other receptors. Treatment of the membranes with beta-funaltrexamine abolished the stimulation of GTPase by sufentanil and levorphanol (mu), but not by bremazocine (kappa) or DSLET (delta). On the other hand, prior incubation with Superfit, an alkylating agent with selectivity for the delta opioid receptor, specifically eliminated the effect of DSLET. Partial alkylation by increasing concentrations of Superfit gradually reduced the extent of stimulation of GTPase by DSLET. The successive treatment of membranes with Superfit and beta-funaltrexamine blocked the actions of DSLET, sufentanil and levorphanol, but had no effect on the stimulation of the GTPase by bremazocine. Selective coupling of an opioid receptor to GTPase was also obtained after incubation of membranes with beta-chlornaltrexamine in the presence of protective concentrations of mu, kappa or delta opioid ligands. Alkylation resolved the coupling of the non-selective opiate etorphine: the sum of stimulation of GTPase in the receptor-selective membranes equalled maximal stimulation of enzyme in untreated membranes. Naloxone blocked the stimulation of GTPase by mu, kappa or delta agonists, but ICI-174,864 specifically inhibited the effect of DSLET.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of sodium-induced increase in opiate antagonist binding sites in rat brain membranes

Sodium decreases agonist binding and increases antagonist binding to opiate receptor sites in brain membranes. This study characterizes in detail the 20-40% increase in [3H]naloxone binding caused by sodium. This increase in binding was specific to sodium (not mimicked by potassium or lithium) and was maximal at 10 mM NaCl. The sodium effect was reversible: washing membranes free of sodium restored binding to normal. Sodium increased Bmax, not KD, of [3H]naloxone binding. The sodium-induced increased binding was not inhibited by either N-ethylmaleimide (NEM) or phospholipase A2 at concentrations which inhibit 50-85% of normal [3H]naloxone binding. In NEM-treated membranes, the effect of sodium on increasing naloxone binding was actually increased. The regional distribution of these sodium-dependent sites were different from normal naloxone sites. These results suggest that physiological concentrations of sodium expose naloxone sites which differ in biochemical properties from naloxone sites assayed in the absence of sodium.

Sodium regulation of agonist and antagonist binding to beta-adrenoceptors in intact and Ns-deficient membranes

Agonist binding to various hormone receptors mediating adenylate cyclase inhibition is decreased by sodium ions. We studied the influence of Na+ on agonist and antagonist binding to beta-adrenoceptors in membrane preparations of guinea pig lung, S49 lymphoma wild-type cells (WT) and their Ns-deficient cyc- variants by measuring binding of the antagonist, [125I]iodocyanopindolol [( 125I]CYP). At 37 degrees C, sodium decreased the receptor affinity for the agonist, isoproterenol, in all three membrane preparations. In lung and WT membranes, Na+ steepened the shallow agonist competition curves in a manner similar to and synergistic with guanine nucleotides. When binding was performed at 4 degrees C, sodium regulation but not guanine nucleotide regulation of agonist binding was preserved. At the low temperature, [125I]CYP affinity was reduced, and sodium increased [125I]CYP binding in both Ns-containing and Ns-deficient membranes by increasing the antagonist affinity without significant change in total receptor number. Compared to Na+, Li+ and K+ were much less potent and efficient in decreasing agonist and increasing antagonist binding. Na+ and Mg2+ had opposite effects on agonist binding in the Ns-containing lung and WT membranes but not in the Ns-deficient cyc- membranes. The data indicate that sodium not only regulates binding of inhibitory hormone receptors but also agonist and antagonist binding to the adenylate cyclase stimulatory beta-adrenoceptor. The finding that sodium regulation of beta-adrenoceptor binding is also observed in the Ns (alpha s)-deficient cyc- membranes, furthermore, indicates that the target of sodium is not the alpha-subunit of Ns but possibly a component common to both types of receptor systems regulating adenylate cyclase activity.

Bacterial toxins and the role of ADP-ribosylation

Studies on the pathogenesis of "Whooping Cough" and cholera have resulted in the discovery of important pathways in the regulation of cellular metabolism leading to the realization of a complex family of proteins that appear to play central roles in the regulation of hormonal responses and which utilize guanine nucleotides in their mechanism of action. The fact that these bacterial toxins interfere so precisely with the complex regulation of eukaryotic cellular metabolism and the discovery of analogous enzymes within the cytosol of eukaryotic cells suggests that ADP-ribosylation may be an important pathway through which the cell can establish its responsiveness to its environment. Clearly, future work directed towards the role of ADP-ribosylation and towards the mechanisms of the regulation of these endogenous ADP-ribosyltransferases and lyases may provide great insights into the mechanisms of hormone action.