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.

Expression of the rat m4 muscarinic acetylcholine receptor gene is regulated by the neuron-restrictive silencer element/repressor element 1

Neuronal cell-specific expression of the rat m4 muscarinic acetylcholine receptor (mAChR) is regulated by a silencer element. A likely mediator of this silencing is the neuron-restrictive silencer element/repressor element 1 (NRSE/RE1), which is present 837 base pairs (bp) upstream from the transcription initiation site of the m4 mAChR gene (Wood, I. C., Roopra, A., Harrington, C., and Buckley, N. J. (1995) J. Biol. Chem. 270, 30933-30940; Mieda, M., Haga, T., and Saffen, D. W. (1996) J. Biol. Chem. 271, 5177-5182). In the present study, we examined whether this putative NRSE/RE1 functions as a silencer. Transient expression assays using m4 mAChR promoter/luciferase expression vectors showed that the m4 NRSE/RE1 is necessary and sufficient to repress m4 promoter activity in non-neuronal L6 cells. m4 promoter activity was only partially repressed, however, in neuronal NG108-15 cells exogenously expressing the neuronal-restrictive silencer factor/RE1-silencing transcription factor (NRSF/REST). By contrast, the promoter activity of the type II sodium channel (NaII) gene was nearly completely repressed in NRSF/REST-expressing NG108-15 cells. Experiments with expression vectors containing chimeric promoters revealed that the NRSE/RE1 elements derived from both the m4 and NaII genes are independently sufficient to silence NaII gene promoter activity, but only partially repress m4 mAChR gene promoter activity in NRSF/REST-expressing NG108-15 cells. Thus, the repression activity of NRSF/REST depends upon the species of promoter to which it is linked. Gel-shift assays showed that the NRSF/REST is the only protein that binds to a 92-bp segment from the m4 mAChR promoter containing NRSE/RE1. This and the fact that m4 promoter activity was completely repressed in L6 cells suggest that the proteins that bind to the m4 constitutive promoter may be different from those in NG108-15 cells. Deletion analysis of the m4 constitutive promoter revealed that a 90-bp segment immediately upstream from the transcription initiation site contains significant promoter activity. Gel-shift assays revealed that several proteins in nuclear extracts prepared from L6 and NG108-15 cells bind to this 90-bp segment and that some of these proteins are L6 or NG108-15 cell-specific. These data support the idea that the repression activity of NRSF/REST depends upon the species of promoter to which it is linked and upon the proteins that bind to those promoters.

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.

Purification and functional reconstitution with GTP-binding regulatory proteins of hexahistidine-tagged muscarinic acetylcholine receptors (m2 subtype)

We have expressed human m2 muscarinic acetylcholine receptors tagged with six histidine residues at the carboxy-terminal region in insect cells (Sf9) and purified them using metal-immobilized Chelating Sepharose gels. Co(2+)-immobilized gels were found to be much more efficient for purification of m2 receptors than gels containing Ni2+ or other metal ions. Twenty-fold purification was attained by a simple, single-step procedure, and approximately 40% of solubilized receptors were recovered as a partially purified preparation with a specific activity of 1.6 nmol/mg of protein. Purified receptors were functionally active in that carbamylcholine stimulated binding of [35S]GTP gamma S to the G-protein G12 reconstituted in lipid vesicles with purified m2 receptors. The extent of stimulation of [35S] GTP gamma S binding to G12 by hexahistidine-tagged m2 receptors was essentially the same as that observed for m2 receptors that lack histidine tags. In addition, palmitoylation at the carboxy-terminal region was not impaired by the hexahistidine-tag fusion. The method described in this study should be applicable to the purification of other G-protein-coupled receptors in functionally active form.

Interactions between wild-type and mutant prion proteins modulate neurodegeneration in transgenic mice

Transgenic mice overexpressing approximately eightfold the mouse (Mo) prion protein (PrP) gene carrying the P102L mutation of GSS developed neurodegeneration between 150 and 300 days of age, while controls expressing the wild-type MoPrP-A transgene at the same level remained healthy. Mice overexpressing the wild-type MoPrP-A transgene were highly susceptible to inoculated mouse prions, exhibiting abbreviated scrapie incubation times of 45 days. After crossing the mutant transgene onto a null (Prnp 0/0) background, the resulting Tg(MoPrP-P101L)Prnp 0/0 mice displayed a highly synchronous onset of illness at 145 days of age, which was shortened to 85 days upon breeding to homozygosity for the transgene array. Besides occasional PrP plaques and modest spongiform degeneration, Tg(MoPrP-P101L) mice suffered from a myopathy and a peripheral neuropathy. Disruption of the wild-type MoPrP gene increased the number of PrP plaques and the severity of spongiform degeneration. Brain extracts prepared from spontaneously ill transgenic mice transmitted disease to Tg196/Prnp 0/0 mice, expressing low levels of the mutant transgene. Our results demonstrate that the presence of wild-type PrP genes, the level of PrP transgene expression, and the sequence of the transgene can profoundly modify experimental prion disease.

Calcium-bound recoverin targets rhodopsin kinase to membranes to inhibit rhodopsin phosphorylation

In rod photoreceptor cells, Ca2+-bound recoverin associates with disk membranes and inhibits light-dependent phosphorylation of rhodopsin. However, the functional significance of Ca2+-induced membrane association of recoverin has not been fully evaluated. We found that Ca2+-bound recoverin forms a complex with rhodopsin kinase preferentially at the membrane surface. Addition of increasing amounts of membranes promoted the membrane association of recoverin, and remarkably suppressed rhodopsin kinase activity. It was concluded that the Ca2+-recoverin-rhodopsin kinase complex is stabilized by membrane association, leading to effective suppression of the kinase activity.

Sequestration of the short and long isoforms of dopamine D2 receptors expressed in Chinese hamster ovary cells

The short (D2S) and long (D2L) isoforms of dopamine D2 receptors were stably expressed in Chinese hamster ovary cells, and dopamine-induced sequestration was examined by measuring the loss of binding of the hydrophilic ligand [3H]sulpiride from the cell surface. Dopamine treatment of Chinese hamster ovary cells expressing D2S for 30 min at 37 degrees caused a 43.8 +/- 3.4% decrease in [3H]sulpiride binding activity measured by incubation of the treated cells with [3H]sulpiride at 4 degrees for 4 hr after the dopamine was washed out. The half-life of the decrease in binding was estimated to be 18.7 +/- 1.6 min, and the concentration of dopamine giving a half-maximal effect (EC50) was estimated to be 180 +/- 90 nM. The decrease was reversible, and the binding activity was recovered by washing out the dopamine and incubating the cells at 37 degrees for 30 min but was not reversible when the cells were incubated at 4 degrees. The binding activity of [3H]spiperone, a hydrophobic ligand, was not affected by the dopamine treatment under the same experimental conditions. These results indicate that approximately one half of the D2S receptors undergo agonist-induced sequestration, probably endocytosis, in a reversible and temperature-dependent manner. Sequestration of D2L receptors was not as apparent as that of D2S receptors; the decrease in [3H]sulpiride binding activity was 21.6 +/- 0.9% and the rate of the decrease was delayed, with a half-life of 33.2 +/- 7.8 min, although effective concentrations of dopamine were similar, with EC50 = 170 +/- 50 nM. A D2S receptor variant containing a missense mutation changing Ser311 in the third intracellular loop to cysteine was found to be sequestered to a significantly lesser extent than with wild-type D2S receptors. This finding was discussed with respect to the report that this variant gene is found more frequently in schizophrenic patients than in control subjects.

Promoter region of the rat m4 muscarinic acetylcholine receptor gene contains a cell type-specific silencer element

We describe here the characterization of the rat m4 muscarinic acetylcholine receptor gene and the identification of its regulatory region. Two 5'-noncoding exons are located approximately 5 kilobases upstream from the coding exon, and at least two alternatively spliced variants of m4 mRNA are expressed in the neuronal cell line PC12D. There are two transcription initiation sites. The promoter region is GC-rich, contains no TATA-box, but has two potential CAAT boxes and several putative binding sites for transcription factors Sp1 and AP-2. We assessed the m4 promoter activity functionally in transient expression assays using luciferase as a reporter. The proximal 435-base pair (bp) sequence of the 5'-flanking region produced luciferase activity in both m4-expressing neuronal cell lines (PC12D and NG108-15) and non-neuronal cell lines (L6 and 3Y1B). A longer fragment containing an additional 638-bp sequence produced luciferase activity only in m4-expressing neuronal cell lines. These data suggest that the proximal 435-bp sequence contains a constitutive promoter and that a 638-bp sequence farther upstream contains a cell type-specific silencer element. A consensus sequence for the neural-restrictive silencer element is found within this 638-bp segment.

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.

Molecular cloning and characterization of a new member of the RAC protein kinase family: association of the pleckstrin homology domain of three types of RAC protein kinase with protein kinase C subspecies and beta gamma subunits of G proteins

cDNA clones encoding the third member of the RAC protein kinase family, termed RAC-PK gamma, were isolated from a rat brain cDNA library. The deduced amino acid sequence of RAC-PK gamma was highly related to those of previously identified family members, RAC-PK alpha and beta, that have a pleckstrin homology domain and a protein-serine/threonine kinase catalytic domain at the amino- and carboxyl-terminal regions, respectively. Northern blot analysis indicated that RAC-PK gamma was expressed abundantly in brain and testis. Specific activities of RAC-PK alpha, beta, and gamma purified from transfected COS-7 cells were similar when measured by using myelin basic protein as a phosphate acceptor. Analysis using fusion proteins of glutathione S-transferase revealed that the pleckstrin homology domain of the three subtypes of RAC-PK associate with both protein kinase C subspecies and beta gamma subunits of G proteins. These results suggest that the pleckstrin homology domains of RAC protein kinase family could associate more than one protein to regulate the activity and/or intracellular distribution of this enzyme family by different ways.

Synthesis and antipancreatitis activities of novel N-(2-sulfonylamino-5-trifluoromethyl-3-pyridyl)carboxamide derivatives as phospholipase A2 inhibitors

Novel N-(2-sulfonylamino-5-trifluoromethyl-3-pyridyl)carboxamide derivatives have been prepared and evaluated as phospholipase A2 (PLA2) inhibitors. Among these compounds, IS-741 (sodium salt of 1j), which showed the highest and the most stable therapeutic effect on acute hemorrhagic pancreatitis induced by the closed duodenal loop method in rats, was selected as a candidate for further development.

Direct x-ray imaging system using an amplified metal-oxide-semiconductor imager in the 4-13-nm wavelength region

We describe a direct x-ray imaging system that uses an amplified metal-oxide-semiconductor imager to detect soft x rays directly for real-time imaging. From the absolute sensitivity of this system as measured through the use of a monochromatic synchrotron radiation beam and a GaAsP Schottky-type photodiode, the minimum sensitivity at a wavelength of 13 nm was estimated to be greater than 10(8)photons mm(-2). This is sufficient to detect soft x rays directly for real-time imaging. Onion cell observations at wavelengths of 4.3 and 4.6 nm indicate that x-ray absorption by the carbon in the cells was detected. This is a promising imaging system for the soft x-ray region in which conventional CCD's are difficult to use.

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.

[Cystic duct anatomy on DIC-helical CT]

Laparoscopic cholecystectomy (LC) is becoming increasingly common. When performing LC, the surgeon may face the risk of ductal injury because of the narrow visual field. Thus, detailed anatomical information on the biliary tree is necessary. DIC-helical CT was carried out in 39 patients suspected of having biliary diseases. The cystic duct was visualized in all cases. Posterior (30.8%) and posterolateral (17.9%) cysticohepatic junction were more common than in previous reports. The cystic duct took the anterior course of the bile duct in a few cases (12.8%). DIC-helical CT is useful for the noninvasive detection of biliary anatomy.

Ligand binding properties of muscarinic acetylcholine receptor subtypes (m1-m5) expressed in baculovirus-infected insect cells

Five subtypes of muscarinic acetylcholine receptors (m1-m5) have been expressed in insect cells (Spodoptera frugiperda, Sf9) using the baculovirus system. Up to 6 nmol of muscarinic acetylcholine receptors were produced by 1 liter culture; 0.3 to 0.6 (human m1), 3 to 6 (human m2), 2 to 4 (rat m3), 1 to 2 (rat m4) and 0.5 to 1 (human m5) nmol. Pirenzepine, AF-DX116 and hexahidrosiladifenidol showed the highest affinity for the m1, m2 and m3 subtype, respectively, indicating that these receptors expressed in Sf9 cells retain the same substrate specificity as those in mammalian tissues or cultured cells. Among 32 kinds of muscarinic ligands examined in the present studies, prifinium was found to have the highest affinity for the m4 subtype, and pilocarpine, oxotremorine, McN-A343 and promethazine the highest affinity for the m5 subtype, although the differences in the affinities among the five subtypes were less than 10-fold. Alcuronium increased the binding of [3H]N-methylscopalamine to the m2 subtype, but not the m1, m4 and m5 subtypes and only slightly to the m3 subtype. Similar but smaller effects of fangchinoline and tetrandrine were found for [3H]N-methylscopalamine binding to only the m3 subtype. These effects may also be useful for the discrimination of individual subtypes.

Characterization of Gq family G proteins GL1 alpha (G14 alpha), GL2 alpha (G11 alpha), and Gq alpha expressed in the baculovirus-insect cell system

The alpha subunits of Gq family G proteins, GL1 alpha (G14 alpha), GL2 alpha(G11 alpha), and Gq alpha were expressed with G protein beta 1 and gamma 2 subunits in insect cells using a baculovirus system. The trimeric forms of G proteins, GL1 (GL1 alpha beta gamma), GL2 (GL2 alpha beta gamma), and Gq (Gq alpha beta gamma), were solubilized by 1% sodium cholate and purified by sequential chromatography on three kinds of columns. GL1, GL2, and Gq activated phospholipase C-beta purified from bovine brain in the presence of aluminum fluoride to the same extent. Muscarinic acetylcholine receptor m1 subtype stimulated the guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding to GL1, GL2, and Gq in the presence of similar concentrations of carbamylcholine. When m1 receptor, G protein, and phospholipase C-beta were reconstituted in lipid vesicles, each subtype of Gq family G proteins mediated the activation of phospholipase C-beta by carbamylcholine in the presence of either 1 microM GTP gamma S or 1 mM GTP. Phospholipase C-beta stimulated the GTPase activity of GL1, GL2, and Gq in the presence of m1 receptor and carbamylcholine but did not stimulate the GTPase activity of GO. Protein kinase C phosphorylated m1 receptor and phospholipase C-beta, but the phosphorylation did not significantly affect the ability of the m1 receptor to stimulate phospholipase C-beta in the reconstitution system of purified proteins.

Sequestration of muscarinic acetylcholine receptor m2 subtypes. Facilitation by G protein-coupled receptor kinase (GRK2) and attenuation by a dominant-negative mutant of GRK2

Sequestration of m2 receptors (muscarinic acetylcholine receptor m2 subtypes), which was assessed as loss of N-[3H]methylscopolamine ([3H]NMS) binding activity from the cell surface, was examined in COS 7 and BHK-21 cells that had been transfected with expression vectors encoding the m2 receptor and, independently, vectors encoding a G protein-coupled receptor kinase (GRK2) (beta-adrenergic receptor kinase 1) or a GRK2 dominant-negative mutant (DN-GRK2). The sequestration of m2 receptors became apparent when the cells were treated with 10(-5) M or higher concentrations of carbamylcholine. In this case, approximately 40% or 20-25% of the [3H]NMS binding sites on COS 7 or BHK-21 cells, respectively, were sequestered with a half-life of 15-25 min. In cells in which GRK2 was also expressed, the sequestration became apparent in the presence of 10(-7) M carbamylcholine. Approximately 40% of the [3H]NMS binding sites on both COS 7 and BHK-21 cells were sequestered in the presence of 10(-6) M or higher concentrations of carbamylcholine. When DN-GRK2 was expressed in COS 7 cells, the proportion of [3H]NMS binding sites sequestered in the presence of 10(-5) M or higher concentrations of carbamylcholine was reduced to 20-30%. These results indicate that the phosphorylation of m2 receptors by GRK2 facilitates their sequestration. These results are in contrast with the absence of a correlation between sequestration and the phosphorylation of beta-adrenergic receptors by the GRK2 and suggests that the consequences of phosphorylation by GRK2 are different for different receptors.