Activation of a GTP-binding protein and a GTP-binding-protein-coupled receptor kinase (beta-adrenergic-receptor kinase-1) by a muscarinic receptor m2 mutant lacking phosphorylation sites

Proteoliposome-based selection of a recombinant antibody fragment against the human M2 muscarinic acetylcholine receptor

The development of antibodies against human G-protein-coupled receptors (GPCRs) has achieved limited success, which has mainly been attributed to their low stability in a detergent-solubilized state. We herein describe a method that can generally be applied to the selection of phage display libraries with human GPCRs reconstituted in liposomes. A key feature of this approach is the production of biotinylated proteoliposomes that can be immobilized on the surface of streptavidin-coupled microplates or paramagnetic beads and used as a binding target for antibodies. As an example, we isolated a single chain Fv fragment from an immune phage library that specifically binds to the human M2 muscarinic acetylcholine receptor with nanomolar affinity. The selected antibody fragment recognized the GPCR in both detergent-solubilized and membrane-embedded forms, which suggests that it may be a potentially valuable tool for structural and functional studies of the GPCR. The use of proteoliposomes as immunogens and screening bait will facilitate the application of phage display to this difficult class of membrane proteins.

Role of the third intracellular loop in the subtype-specific internalization and recycling of muscarinic M2 and M4 receptors

Muscarinic M2, M4, and M2-M4 chimera receptors were transiently expressed in HEK-293 tsA201 cells, and agonist-dependent internalization of these receptors and recycling of internalized receptors were examined by measuring the amount of cell-surface receptors as [3H]N-methylscopolamine (NMS) binding activity. Coexpression of a dominant negative form of dynamin (DN-dynamin,dynamin K44A) greatly reduced the agonist-dependent internalization of M4 receptors but not of M2 receptors, as was reported by Vögler et al. (J Biol Chem 273, 12155-12160, 1998).The agonist-dependent internalization of M2/M4-i3/M2 chimera receptors (M2 receptors with the i3 loop replaced by that of M4 receptors) was greatly reduced by co-expression of DN-dynamin as was the case for M4 receptors, whereas the agonist-dependent internalization of M4/M2-i3/M4 chimera receptors was hardly affected by co-expression of DN-dynamin as was the case for M2 receptors.Internalized M2/M4-i3/M2 receptors as well as internalized M4 receptors were shown to be recycled back to the cell surface after removal of agonists, whereas no recycling was observed for M4/M2-i3/M4 receptors as well as M2 receptors. These results indicate that the i3 loops of M2 and M4 receptors take a major role in their agonist-dependent internalization and recycling.

Interaction of the muscarinic acetylcholine receptor M₂ subtype with G protein Gα(i/o) isotypes and Gβγ subunits as studied with the maltose-binding protein-M₂-Gα(i/o) fusion proteins expressed in Escherichia coli

We expressed the fusion proteins of the muscarinic acetylcholine receptor M2 subtype (M2 receptor) with a maltose-binding protein (MBP) and various G protein α subunits (Gα(i1-i3/o)) at its N- and C-terminals, respectively (MBP-M2-Gα(i/o)), in Escherichia coli, and examined the effect of G protein βγ subunits (Gβγ) on the receptor-Gα interaction as assessed by agonist- and GDP-dependent [(35)S]GTPγS binding of the fusion proteins. We found that (i) Gβγ promoted both the agonist-dependent and -independent [(35)S]GTPγS binding with little effect on the guanine nucleotide-sensitive high-affinity agonist binding, (ii) the specific [(35)S]GTPγS binding activity was much greater for MBP-M2-Gα(oA) than for MBP-M2-Gα(i1-i3) in the absence of Gβγ, whereas Gβγ preferentially promoted the agonist-dependent decrease in the affinity for GDP of MBP-M2-Gα(i1-i3) rather than of MBP-M2-Gα(oA), and (iii) the proportion of agonist-dependent [(35)S]GTPγS binding was roughly 50% irrespective of species of Gα and the presence or absence of Gβγ. These results demonstrate that receptor-Gα fusion proteins expressed in E. coli could be useful for studies of receptor-G interaction.

Molecular properties of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors, which comprise five subtypes (M1-M5 receptors), are expressed in both the CNS and PNS (particularly the target organs of parasympathetic neurons). M1-M5 receptors are integral membrane proteins with seven transmembrane segments, bind with acetylcholine (ACh) in the extracellular phase, and thereafter interact with and activate GTP-binding regulatory proteins (G proteins) in the intracellular phase: M1, M3, and M5 receptors interact with Gq-type G proteins, and M2 and M4 receptors with Gi/Go-type G proteins. Activated G proteins initiate a number of intracellular signal transduction systems. Agonist-bound muscarinic receptors are phosphorylated by G protein-coupled receptor kinases, which initiate their desensitization through uncoupling from G proteins, receptor internalization, and receptor breakdown (down regulation). Recently the crystal structures of M2 and M3 receptors were determined and are expected to contribute to the development of drugs targeted to muscarinic receptors. This paper summarizes the molecular properties of muscarinic receptors with reference to the historical background and bias to studies performed in our laboratories.

Structure of the human M2 muscarinic acetylcholine receptor bound to an antagonist

The parasympathetic limb of the autonomic nervous system regulates the activity of multiple organ systems. Muscarinic receptors are G protein coupled receptors (GPCRs) that mediate the response to acetylcholine released from parasympathetic nerves.^1–5 Their role in the unconscious regulation of organ and central nervous system function makes them potential therapeutic targets for a broad spectrum of diseases. The M₂ muscarinic acetylcholine receptor (M₂ receptor) is essential for the physiologic control of cardiovascular function through activation of G protein-coupled inwardly-rectifying potassium channels, and is of particular interest because of its extensive pharmacological characterization with both orthosteric and allosteric ligands. Here we report the structure of antagonist-bound M₂ receptor, the first human acetylcholine receptor to be characterized structurally. The antagonist QNB binds in the middle of a long aqueous channel extending approximately two-thirds through the membrane. The orthosteric binding pocket is formed by amino acids that are identical in all 5 muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species. A layer of tyrosine residues forms an aromatic cap restricting dissociation of the bound ligand. A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap. The M₂ receptor structure provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.

Identification of an Ascaris G protein-coupled acetylcholine receptor with atypical muscarinic pharmacology

Acetylcholine (ACh) is a neurotransmitter/neuromodulator in the nematode nervous system and induces its effects through interaction with both ligand-gated ion channels (LGICs) and G protein-coupled receptors (GPCRs). The structure, pharmacology and physiological importance of LGICs have been appreciably elucidated in model nematodes, including parasitic species where they are targets for anthelmintic drugs. Significantly less, however, is understood about nematode ACh GPCRs, termed GARs (G protein-linked ACh receptors). What is known comes from the free-living Caenorhabditis elegans as no GARs have been characterized from parasitic species. Here we clone a putative GAR from the pig gastrointestinal nematode Ascaris suum with high structural homology to the C. elegans receptor GAR-1. Our GPCR, dubbed AsGAR-1, is alternatively spliced and expressed in the head and tail of adult worms but not in dorsal or ventral body wall muscle, or the ovijector. ACh activated AsGAR-1 in a concentration-dependent manner but the receptor was not activated by other small neurotransmitters. The classical muscarinic agonists carbachol, arecoline, oxotremorine M and bethanechol were also AsGAR-1 agonists but pilocarpine was ineffective. AsGAR-1 activation by ACh was partially antagonized by the muscarinic blocker atropine but pirenzepine and scopolamine were largely ineffective. Certain biogenic amine GPCR antagonists were also found to block AsGAR-1. Our conclusion is that Ascaris possesses G protein-coupled ACh receptors that are homologous in structure to those present in C. elegans, and that although they have some sequence homology to vertebrate muscarinic receptors, their pharmacology is atypically muscarinic.

Muscarinic M4 receptor recycling requires a motif in the third intracellular loop

The present study was performed to identify sequence(s) in the third intracellular loop (i3) of the muscarinic acetylcholine receptor M4 subtype (M4 receptor) involved in its internalization and recycling. In transiently transfected human embryonic kidney 293-tsA201 cells, 40 to 50% of cell-surface M4 receptors are internalized in an agonist-dependent manner, and approximately 65% of internalized receptors are recycled back to the cell surface after removal of the agonist. We examined the internalization and recycling of M4 receptor mutants with partial deletion in i3 and found that various mutants (M4del-K(235)-K(240), M4del-T(241)-K(271), and M4del-W(339)-N(372)) showed internalization and cell-surface recycling in a similar manner to the M4 receptor. We also found that the mutant M4del-L(272)-R(338) was internalized to only half the extent of the M4 receptor and was recycled after agonist removal, and the mutant M4del-V(373)-A(393) was also internalized to half the extent of the wild type but was not recycled back to the cell surface after agonist removal. When the sequence corresponding to Val(373)-Ala(393) was grafted onto the i3 portion of a recycling-negative mutant of muscarinic M2 receptor with deletion of almost the whole of the i3 sequence, approximately 40% of the chimeric receptor on the cell surface was internalized, and more than 65% of the internalized receptors were recycled back to the cell surface. These results indicate that the regions including Leu(272)-Arg(338) and Val(373)-Ala(393) are involved in internalization of the M4 receptor, and the region including Val(373)-Ala(393) is indispensable for its recycling, whereas the other regions of i3 are dispensable for internalization and recycling.

Activation of nematode G protein GOA-1 by the human muscarinic acetylcholine receptor M2 subtype. Functional coupling of G-protein-coupled receptor and G protein originated from evolutionarily distant animals

Signal transduction mediated by heterotrimeric G proteins regulates a wide variety of physiological functions. We are interested in the manipulation of G-protein-mediating signal transduction using G-protein-coupled receptors, which are derived from evolutionarily distant organisms and recognize unique ligands. As a model, we tested the functionally coupling GOA-1, G alpha(i/o) ortholog in the nematode Caenorhabditis elegans, with the human muscarinic acetylcholine receptor M2 subtype (M2), which is one of the mammalian G alpha(i/o)-coupled receptors. GOA-1 and M2 were prepared as a fusion protein using a baculovirus expression system. The affinity of the fusion protein for GDP was decreased by addition of a muscarinic agonist, carbamylcholine and the guanosine 5'-[3-O-thio]triphosphate ([35S]GTPgammaS) binding was increased with an increase in the carbamylcholine concentrations in a dose-dependent manner. These effects evoked by carbamylcholine were completely abolished by a full antagonist, atropine. In addition, the affinity for carbamylcholine decreased under the presence of GTP as reported for M2-G alpha(i/o) coupling. These results indicate that the M2 activates GOA-1 as well as G alpha(i/o).

The structure of the third intracellular loop of the muscarinic acetylcholine receptor M2 subtype

We have examined whether the long third intracellular loop (i3) of the muscarinic acetylcholine receptor M2 subtype has a rigid structure. Circular dichroism (CD) and nuclear magnetic resonance spectra of M2i3 expressed in and purified from Escherichia coli indicated that M2i3 consists mostly of random coil. In addition, the differential CD spectrum between the M2 and M2deltai3 receptors, the latter of which lacks most of i3 except N- and C-terminal ends, gave no indication of secondary structure. These results suggest that the central part of i3 of the M2 receptor has a flexible structure.

Analysis of differential modulatory activities of GRK2 and GRK4 on Galphaq-coupled receptor signaling

G-protein-coupled receptor kinases (GRK) contain a regulator of G-protein signaling (RGS)-like domain located at the N terminus (GRK-Nter) of their sequence. This domain is present in all the GRK subtypes, but the RGS-like domain of GRK2 was documented to be functionally active, as it is able to interact selectively with Galphaq (both in vitro and in cells) and to inhibit Galphaq-dependent signaling. In contrast GRK4, GRK5, and GRK6 are unable to interact with Galphaq. This article describes the methodology to investigate the modulatory activity of GRK2 and GRK4 on GPCR-stimulated Galphaq signaling. This analysis is essentially based on three types of experiments: (a) study of the effect of the GRK-Nter on GPCR-dependent signaling; (b) analysis of the binding of GRK-Nter to Galphaq in vitro; and (c) analysis of the interaction of GRK with Galphaq in cells.

Regulation of muscarinic acetylcholine receptor signaling

Multiple mechanisms regulate the signaling of the five members of the family of the guanine nucleotide binding protein (G protein)-coupled muscarinic acetylcholine (ACh) receptors (mAChRs). Following activation by classical or allosteric agonists, mAChRs can be phosphorylated by a variety of receptor kinases and second messenger-regulated kinases. The phosphorylated mAChR subtypes can interact with beta-arrestin and presumably other adaptor proteins as well. As a result, the various mAChR signaling pathways may be differentially altered, leading to short-term or long-term desensitization of a particular signaling pathway, receptor-mediated activation of the mitogen-activated protein kinase pathway downstream of mAChR phosphorylation, as well as long-term potentiation of mAChR-mediated phospholipase C stimulation. Agonist activation of mAChRs may also induce receptor internalization and down-regulation, which proceed in a highly regulated manner, depending on receptor subtype and cell type. In this review, our current understanding of the complex regulatory processes that underlie signaling of mAChR is summarized.

Identification of sites of phosphorylation by G-protein-coupled receptor kinase 2 in beta-tubulin

G-protein-coupled receptor kinase 2 (GRK2) is known to specifically phosphorylate the agonist-bound forms of G-protein-coupled receptors (GPCRs). This strict specificity is due at least partly to activation of GRK2 by agonist-bound GPCRs, in which basic residues in intracellular regions adjacent to transmembrane segments are thought to be involved. Tubulin was found to be phosphorylated by GRK2, but it remains unknown if tubulin can also serve as both a substrate and an activator for GRK2. Purified tubulin, phosphorylated by GRK2, was subjected to biochemical analysis, and the phosphorylation sites in beta-tubulin were determined to be Thr409 and Ser420. In addition, the Ser444 in beta III-tubulin was also indicated to be phosphorylated by GRK2. The phosphorylation sites in tubulin for GRK2 reside in the C-terminal domain of beta-tubulin, which is on the outer surface of microtubules. Pretreatment of tubulin with protein phosphatase type-2A (PP2A) resulted in a twofold increase in the phosphorylation of tubulin by GRK2. These results suggest that tubulin is phosphorylated in situ probably by GRK2 and that the phosphorylation may affect the interaction of microtubules with microtubule-associated proteins. A GST fusion protein of a C-terminal region of beta I-tubulin (393-445 residues), containing 19 acidic residues but only one basic residue, was found to be a good substrate for GRK2, like full-length beta-tubulin. These results, together with the finding that GRK2 may phosphorylate synuclein and phosducin in their acidic domains, indicate that some proteins with very acidic regions but without basic activation domains could serve as substrates for GRK2.

Role of receptor kinase in long-term desensitization of the cardiac muscarinic receptor-K+ channel system

Desensitization of the cardiac muscarinic K+ channel was studied in cultured neonatal rat atrial cells and in Chinese hamster ovary (CHO) cells transfected with muscarinic receptor (HM(2)), G protein-coupled inward rectifying K+ channels 1 and 4, and G protein-coupled receptor kinase 2. In atrial cells incubated in 10 microM carbachol for 24 h, channel activity in cell-attached patches was substantially reduced as a result of long-term desensitization. The long-term desensitization was also observed in CHO cells transfected with the wild-type receptor and receptor kinase (as well as the channel). However, long-term desensitization was greatly reduced or abolished if the cells were 1) not transfected with the receptor kinase, 2) transfected with a mutant receptor lacking phosphorylation sites (rather than the wild-type receptor), or 3) transfected with a mutant receptor kinase lacking kinase activity (rather than the wild-type receptor kinase). We suggest that long-term desensitization of the cardiac muscarinic receptor-K+ channel system to muscarinic agonist may involve phosphorylation of the receptor by receptor kinase.

Effects of short-term toluene exposure on ligand binding to muscarinic acetylcholine receptors in the rat frontal cortex and hippocampus

Changes in the binding affinity of the muscarinic acetylcholine receptor agonist carbamylcholine were determined in membranes isolated from the brains of rats exposed to toluene at concentrations of 500-2,000 ppm for 6 h. Membrane fractions of the frontal cortex and hippocampus were prepared and agonist-binding affinities were determined by measuring the displacement of [3H]N-methyl scopolamine-binding activity by carbamylcholine. In the frontal cortex, the affinity of high-affinity carbamylcholine binding was reduced following exposure to toluene at a concentration of 1000 ppm or higher. However, in the hippocampus, the affinity of high-affinity binding of carbamylcholine was increased following exposure to toluene. These observations suggest that toluene exposure affects binding affinity of carbamylcholine, and the effect differs by brain region.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Role of receptor kinase in short-term desensitization of cardiac muscarinic K+ channels expressed in Chinese hamster ovary cells

1. The cardiac muscarinic receptor-K+ channel system was reconstructed in Chinese hamster ovary (CHO) cells by transfecting the cells with the various components of the system. The activity of the muscarinic K+ channel was measured with the cell-attached configuration of the patch clamp technique. 2. In CHO cells transfected with the channel (Kir3.1/Kir3.4), receptor (hm2) and receptor kinase (GRK2), on exposure to agonist, there was a decline in channel activity as a result of desensitization, similar to that in atrial cells. 3. Whereas the desensitization was almost abolished by not transfecting with the receptor kinase or by transfecting with a mutant receptor lacking phosphorylation sites, it was only reduced (by approximately 39%) by transfecting with a mutant receptor kinase with little/kinase activity. 4. These results suggest that the receptor kinase is responsible for desensitization of the muscarinic K+ channel and that this involves phosphorylation-dependent and -independent mechanisms.

Internalization and down-regulation of human muscarinic acetylcholine receptor m2 subtypes. Role of third intracellular m2 loop and G protein-coupled receptor kinase 2

Internalization and down-regulation of human muscarinic acetylcholine m2 receptors (hm2 receptors) and a hm2 receptor mutant lacking a central part of the third intracellular loop (I3-del m2 receptor) were examined in Chinese hamster ovary (CHO-K1) cells stably expressing these receptors and G protein-coupled receptor kinase 2 (GRK2). Agonist-induced internalization of up to 80-90% of hm2 receptors was demonstrated by measuring loss of [3H]N-methylscopolamine binding sites from the cell surface, and transfer of [3H]quinuclidinyl benzilate binding sites from the plasma membrane into the light-vesicle fractions separated by sucrose density gradient centrifugation. Additionally, translocation of hm2 receptors with endocytic vesicles were visualized by immunofluorescence confocal microscopy. Agonist-induced down-regulation of up to 60-70% of hm2 receptors was demonstrated by determining the loss of [3H]quinuclidinyl benzilate binding sites in the cells. The half-time (t1/2) of internalization and down-regulation in the presence of 10(-4) M carbamylcholine was estimated to be 9.5 min and 2.3 h, respectively. The rates of both internalization and down-regulation of hm2 receptors in the presence of 10(-6) M or lower concentrations of carbamylcholine were markedly increased by coexpression of GRK2. Agonist-induced internalization of I3-del m2 receptors was barely detectable upon incubation of cells for 1 h, but agonist-induced down-regulation of up to 40-50% of I3-del m2 receptors occurred upon incubation with 10(-4) M carbamylcholine for 16 h. However, the rate of down-regulation was lower compared with wild type receptors (t1/2 = 9.9 versus 2.3 h). These results indicate that rapid internalization of hm2 receptors is facilitated by their phosphorylation with GRK2 and does not occur in the absence of the third intracellular loop, but down-regulation of hm2 receptors may occur through both GRK2-facilitating pathway and third intracellular loop-independent pathways.

GTP-binding-protein-coupled receptor kinases--two mechanistic models

Six vertebrate protein kinases (G-protein-coupled receptor kinases; GRKs) that regulate the function of G-protein-coupled receptors (GPCRs) were recently cloned; several distinct properties set them apart from conventional second-messenger regulated protein kinases. It appears that GRKs bind GPCR* through two separate sites: a high-affinity site, which involves intracellular loops of the activated receptor, and the lower-affinity site, encompassing the phosphorylation region. The high-affinity interaction may involve complementary structural elements of GRKs and GPCRs* rather than precise amino acid alignment, thus allowing broad and overlapping specificities of these kinases, in spite of differences in the sequences of GPCRs. In addition, GRK structures are modified by several posttranslational modifications, including phosphorylation, autophosphorylation, prenylation, carboxymethylation, and palmitoylation, probably affecting properties of these enzymes. While GRKs phosphorylate and inactivate receptor molecules which are engaged in G-protein activation, controversy surrounds whether GRKs might be activated and phosphorylate unstimulated GPCRs, leading to a desensitization of a larger population of the receptors. In this review, mechanistic aspects of GPCR* phosphorylation related to the distinct properties, regulation and modes of action of GRKs are described.

Pharmacological and structural integrity of muscarinic M2 acetylcholine receptors produced in Sf9 insect cells

Muscarinic acetylcholine receptors (human m2 subtype), expressed in Sf9 cells, using the baculovirus system, were purified and found to display the expected ligand binding properties, whether membrane-bound or affinity-purified. The purified recombinant receptors were specifically photolabelled with p-N,N-[3H]dimethylamino and p-N,N-[3H]dibutylamino benzene diazonium derivatives. Electrophoretic patterns for covalent radioactive incorporation of the probes were essentially similar to those for [3H]propylbenzilylcholine mustard-labelled receptor sites but were dependent on the infection time of Sf9 cells. Pharmacological properties of the recombinant receptors being unaltered did not reflect structural integrity of the protein as substantial proteolytic fragmentation was detected at a prolonged infection time, i.e., at the highest level of expression. Selection of overexpression conditions, as illustrated here for muscarinic receptors, thus requires not only pharmacological controls, but also analysis of the covalently labelled protein under strongly dissociating conditions.

Synergistic regulation of m2 muscarinic acetylcholine receptor desensitization and sequestration by G protein-coupled receptor kinase-2 and beta-arrestin-1

The m2 muscarinic acetylcholine receptor (m2 mAChR) belongs to the superfamily of G protein-coupled receptors and is regulated by many processes that attenuate signaling following prolonged stimulation by agonist. We used a heterologous expression system to examine the ability of G protein-coupled receptor kinase-2 (GRK2) and beta-arrestin-1 to regulate the phosphorylation state and to promote desensitization and sequestration of the m2 mAChR. Treatment of JEG-3 cells transiently expressing the m2 mAChR with a muscarinic agonist induced an approximately 4- or 8-fold increase in receptor phosphorylation in the absence or presence of cotransfected GRK2, respectively, compared with untreated cells transfected with receptor alone. Using the expression of a cAMP-regulated reporter gene to measure receptor function, we found that transiently transfected m2 mAChRs underwent functional desensitization following exposure to agonist. Transfected GRK2 enhanced agonist-induced functional desensitization in a manner that was synergistically enhanced by cotransfection of beta-arrestin-1, which had no effect on m2 mAChR function when coexpressed in the absence of GRK2. Finally, GRK2 and beta-arrestin-1 synergistically enhanced both the rate and extent of agonist-induced m2 mAChR sequestration. These results are the first to demonstrate that agonist-induced desensitization and sequestration of the m2 mAChR in the intact cell can be enhanced by the presence of GRK2 and beta-arrestin-1 and show that these molecules have multiple actions on the m2 mAChR.

Inhibition of M3 muscarinic acetylcholine receptor-mediated Ca2+ influx and intracellular Ca2+ mobilization in neuroblastoma cells by the Ca2+/calmodulin-dependent protein kinase inhibitor 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-trosyl]-4-phenylpiperazin e (KN-62)

The role of Ca2+/calmodulin-dependent protein kinase (CaM kinase; EC 2.7.1.123) in the generation of Ca2+ signals by muscarinic acetylcholine receptors (mAChR) was studied. Changes in intracellular Ca2+ concentrations ([Ca2+]i) induced by mAChR activation were monitored in SK-N-SH human neuroblastoma cells using the dye Fura-2. SK-N-SH cells express M3 mAChR, as well as CaM kinase types II and IV, which are specifically inhibited by the CaM kinase antagonist KN-62 (1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazi ne). Carbamylcholine (100 microM) elicited an initial transient peak in [Ca2+]i due to mobilization of Ca2+ from internal stores, followed by a sustained elevation in [Ca2+]i that depended on the influx of extracellular Ca2+ and which was inhibited by EGTA and Ni2+. These mAChR-induced Ca2+ signals were diminished to an equal extent by preincubating the cells with 0.01 to 100 microM KN-62. KN-62 inhibited mAChR-induced Ca2+ influx and mobilization from internal stores by about 25-30%, producing a half-maximal effect at approximately 1 microM. In contrast, KN-62 (25 microM) almost completely abolished carbamylcholine-stimulated entry of divalent cations through Mn2+-permeant channels, as revealed by Mn2+ quenching of Fura-2 fluorescence. KN-62 also almost completely abolished Ca2+ influx induced by depolarization of the cells with 25 mM K+ (IC50 = 3 microM). These results suggest that CaM kinases regulate both the mobilization of intracellular Ca2+ and the stimulation of Ca2+ influx that are induced by mAChR activation, and indicate that the mAChR-induced influx of Ca2+ occurs through Ca2+ channels other than, or in addition to, the voltage-gated calcium channels or Mn2+-permeant channels which are inhibited by KN-62.

Palmitoylation of muscarinic acetylcholine receptor m2 subtypes: reduction in their ability to activate G proteins by mutation of a putative palmitoylation site, cysteine 457, in the carboxyl-terminal tail

A putative palmitoylation site, Cys457, of muscarinic acetylcholine receptor m2 subtype (m2 receptor) was eliminated by conversion to alanine or stop codon by site-directed mutagenesis. The mutant m2 receptor C457A was not metabolically labeled with [3H] palmitic acid when expressed in Sf9 cells, whereas the wild-type m2 receptor was labeled under the same conditions. These results confirm that the Cys457 is the palmitoylation site. The rate of palmitoylation was markedly accelerated by addition of agonist, indicating that the palmitoylation reaction is affected by conformational changes of the receptor induced by agonist binding. The m2 receptor mutants without palmitoylation were purified and reconstituted with G proteins into phospholipid vesicles. Both mutants were good substrates of G protein-coupled receptor kinase 2 and the phosphorylation was stimulated by agonist and G protein beta gamma subunits, as was the case for wild-type receptors. The mutant receptors interacted with and activate Gi2 and G(o). However, the rate of [35S] GTP gamma S binding to Gi2 was half as much for the mutants as that for the wild type, and the proportion of guanine nucleotide-sensitive high-affinity agonist binding sites was significantly less for mutants (42-42%) compared to wild type (62%). These results indicate that the palmitoylation of m2 receptors is not an absolute requirement for their interaction with G proteins but enhances the ability of the receptors to interact with G proteins.

Ca2+-dependent inhibition of G protein-coupled receptor kinase 2 by calmodulin

Agonist- or light-dependent phosphorylation of muscarinic acetylcholine receptor m2 subtypes (m2 receptors) or rhodopsin by G protein-coupled receptor kinase 2 (GRK2) was found to be inhibited by calmodulin in a Ca2+-dependent manner. The phosphorylation was fully inhibited in the absence of G protein betagamma subunits and partially inhibited in the presence of betagamma subunits. The dose-response curve for stimulation by betagamma subunits of the m2 and rhodopsin phosphorylation was shifted to the higher concentration of betagamma subunits by addition of Ca2+-calmodulin. The phosphorylation by GRK2 of a glutathione S-transferase fusion protein containing a peptide corresponding to the central part of the third intracellular loop of m2 receptors (I3-GST) was not affected by Ca2+-calmodulin in the presence or absence of betagamma subunits, but the agonist-dependent stimulation of I3-GST phosphorylation by an I3-deleted m2 receptor mutant in the presence of betagamma subunits was suppressed by Ca2+-calmodulin. These results indicate that Ca2+-calmodulin does not directly interact with the catalytic site of GRK2 but inhibits the kinase activity of GRK2 by interfering with the activation of GRK2 by agonist-bound m2 receptors and G protein betagamma subunits. In agreement with the assumption that GRK2 activity is suppressed by the increase in intracellular Ca2+, the sequestration of m2 receptors expressed in Chinese hamster ovary cells was found to be attenuated by the treatment with a Ca2+ ionophore, A23187.

G protein-coupled receptor kinase GRK2 is a phospholipid-dependent enzyme that can be conditionally activated by G protein betagamma subunits

G protein-coupled receptor kinases (GRKs) mediate agonist-dependent phosphorylation of G protein-coupled receptors (GPRs) and initiate homologous receptor desensitization. Previously, we reported that charged phospholipids directly interacted with the two GRK isoforms, GRK2 and GKR3, via a pleckstrin homology (PH) domain to regulate GRK activity (DebBurman, S. K., Ptasienski, J., Boetticher, E., Lomasney, J. W., Benovic, J. L., and Hosey, M. M. (1995) J. Biol. Chem. 270: 5742-5747). Here, evidence is provided to support the hypothesis that charged phospholipids are required for agonist-dependent phosphorylation of receptors by GRK2. In the absence of charged phospholipids, the purified human m2 muscarinic acetylcholine receptor (hm2mAChR) reconstituted in pure phosphatidylcholine vesicles or in a noninhibitory detergent was not a substrate for GRK2. However, these receptor preparations were stoichiometrically phosphorylated in an agonist-dependent manner upon addition of charged phospholipids. The known ability of G protein betagamma subunits to stimulate mAChR phosphorylation also was found to be absolutely dependent on the presence of charged phospholipids, including phosphatidylinositol 4,5-bisphosphate (PIP2). Phospholipids also regulated GRK-mediated phosphorylation of casein, a nonreceptor-soluble substrate. Among lipids tested, lipid inositol phosphates, PIP2 and phosphatidylinositol 4-monophosphate, were found to be the most potent activators of GRK2 and were the only lipids that regulated GRK2 in a complex biphasic manner. At low micro concentrations, PIP2 activated GRK2 via an interaction with the GRK pleckstrin homology domain; however, at high micro concentrations, PIP2 inhibited GRK2, apparently via another mechanism. PIP2-mediated inhibition could be partly relieved by increasing ATP. The results support the hypothesis that GRK2 is a lipid-dependent protein kinase that requires charged phospholipids for enzyme activation, for regulation by Gbetagamma subunits, and potentially for membrane association.

Basal phosphorylation of mu opioid receptor is agonist modulated and Ca2+-dependent

The mu opioid receptor was shown to be phosphorylated at a basal rate in the absence of agonist, measured in permeabilized HEK293 cells transfected with an epitope tagged mu receptor (EE-mu) [Arden, J., Segredo, V., Wang, Z., Lameh, J. and Sadee, W. (1995) J. Neurochem. 65, 1636-1645]. In the present study, basal phosphorylation was found to be Ca2+ dependent; however, several inhibitors of protein kinase C and Ca2+-calmodulin dependent kinases failed to affect basal mu receptor phosphorylation. Thus, the basal mu receptor phosphorylating activity differed from the main kinases involved in receptor regulation. The general kinase inhibitor H7 (100 microM) suppressed basal mu receptor phosphorylation. Pretreatment with the agonist morphine, followed by drug removal, resulted in a sustained increase of basal mu receptor phosphorylation. The gradual agonist dependent modulation of basal mu receptor phosphorylation suggests a novel regulatory mechanism which may play a role in narcotic tolerance and dependence.

Functional and quantitative measures of receptor-coupled G proteins in human mononuclear leukocytes: no change with age

Aging has been associated with alterations in signal transduction for a number of neurotransmitter receptors in human tissues. Heterotrimeric G proteins play a pivotal role in postreceptor information transduction, by coupling a variety of hormone and neurotransmitter receptors to several intracellular effector functions. Developmental and age-related changes in the abundance of specific G alpha subunits have been shown in the human brain. In the present study, functional and quantitative measures of G proteins were conducted in human mononuclear leukocytes obtained from 19 healthy subjects of increasing age. Gs protein function, assessed through cholera toxin-sensitive beta-adrenergic and dopaminergic agonists induced increases in 3H-Gpp(NH)p binding capacities to membranes of mononuclear leukocytes, and Gi protein function, assessed through pertussis toxin-sensitive muscarinic agonist induced increase in guanine nucleotide binding capacity, were found to be unaltered by increasing age. Immunobloting analyses with specific polyclonal antibodies against G alpha s, G alpha i, and G alpha q subunit proteins in mononuclear leukocyte membranes obtained from the same subjects showed that the quantities of these proteins in mononuclear leukocytes were as well independent of age. Insofar as age-related alterations in cellular information transduction mechanisms in peripheral tissues are important from the etiological, diagnostic, and pharmacological aspects of age-related disorders, it is important to know that both the coupling of receptors to G proteins, the function of these proteins, and their abundance in human peripheral mononuclear leukocytes stays unaltered by the aging process.

Phosphorylation of human m1 muscarinic acetylcholine receptors by G protein-coupled receptor kinase 2 and protein kinase C

Human muscarinic acetylcholine receptor m1 subtypes (m1 receptors) were expressed in and purified from insect Sf9 cells and then subjected to phosphorylation by G protein-coupled receptor kinase 2 (GRK2) expressed in and purified from Sf9 cells and by protein kinase C purified from rat brain (a mixture of alpha, beta, and gamma types, PKC). The m1 receptor was phosphorylated by either GRK2 or PKC in an agonist-dependent or independent manner, respectively. G protein beta gamma subunits stimulated the phosphorylation by GRK2 but did not affect the phosphorylation by PKC. The number of incorporated phosphates was 4.6 and 2.8 mol/mol of receptor for phoshorylation by GRK2 and PKC, respectively. The number of incorporated phosphates was 7.5 mol/mol receptor for phosphorylation by GRK2 followed by PKC, but was 5.8 mol/mol of receptor for the phosphorylation by PKC followed by GRK2. Major sites phosphorylated by GRK2 and PKC were located in the third intracellular loop and the carboxyl-terminal tail, respectively. These results indicate that GRK2 and PKC phosphorylate different sites of m1 receptors and that the phosphorylation by PKC partially inhibits the phosphorylation by GRK2, probably by affecting activation of GRK2 by agonist-bound receptors.

Desensitization and internalization of the m2 muscarinic acetylcholine receptor are directed by independent mechanisms

The phenomenon of acute desensitization of G-protein-coupled receptors has been associated with several events, including receptor phosphorylation, loss of high affinity agonist binding, receptor:G-protein uncoupling, and receptor internalization. However, the biochemical events underlying these processes are not fully understood, and their contributions to the loss of signaling remain correlative. In addition, the nature of the kinases and the receptor domains which are involved in modulation of activity have only begun to be investigated. In order to directly measure the role of G-protein-coupled receptor kinases (GRKs) in the desensitization of the m2 muscarinic acetylcholine receptor (m2 mAChR), a dominant-negative allele of GRK2 was used to inhibit receptor phosphorylation by endogenous GRK activity in a human embryonic kidney cell line. The dominant-negative GRK2K220R reduced agonist-dependent phosphorylation of the m2 mAChR by approximately 50% and prevented acute desensitization of the receptor as measured by the ability of the m2 mAChR to attenuate adenylyl cyclase activity. In contrast, the agonist-induced internalization of the m2 mAChR was unaffected by the GRK2K220R construct. Further evidence linking receptor phosphorylation to acute receptor desensitization was obtained when two deletions of the third intracellular loop were made which created m2 mAChRs that did not become phosphorylated in an agonist-dependent manner and did not desensitize. However, the mutant mAChRs retained the ability to internalize. These data provide the first direct evidence that GRK-mediated receptor phosphorylation is necessary for m2 mAChR desensitization; the likely sites of in vivo phosphorylation are in the central portion of the third intracellular loop (amino acids 282-323). These results also indicate that internalization of the m2 receptor is not a key event in desensitization and is mediated by mechanisms distinct from GRK phosphorylation of the receptor.

Receptor kinase-dependent desensitization of the muscarinic K+ current in rat atrial cells

1. Activity of rat atrial muscarinic K+ channels has been measured in five configurations of the patch clamp technique. 2. In configurations in which the normal intracellular solution was lost, the slow phase of desensitization (a slow decline of channel activity during an exposure to ACh) was much reduced (or absent) and deactivation (on wash-off of ACh) was slowed as compared with desensitization and deactivation in configurations in which normal intracellular solution was retained. This suggests that soluble intracellular regulators are involved in these processes. 3. When a G protein-coupled receptor kinase (GRK2) was applied to the cytoplasmic surface of conventional outside-out patches in the presence of ATP, the slow phase of desensitization was restored. In the absence of ATP, GRK2 failed to restore the slow phase. 4. It is concluded that (i) G protein-coupled receptor kinase dependent phosphorylation of the muscarinic receptor is responsible for the slow phase of desensitization and (ii) a soluble factor (such as a GTPase activating protein or 'GAP') is responsible for normal rapid deactivation.