Down-regulation and desensitization of the muscarinic M1 and M2 receptors in transfected fibroblast B82 cells

Regulation of KCNQ2/KCNQ3 current by G protein cycling: the kinetics of receptor-mediated signaling by Gq

Receptor-mediated modulation of KCNQ channels regulates neuronal excitability. This study concerns the kinetics and mechanism of M₁ muscarinic receptor–mediated regulation of the cloned neuronal M channel, KCNQ2/KCNQ3 (Kv7.2/Kv7.3). Receptors, channels, various mutated G-protein subunits, and an optical probe for phosphatidylinositol 4,5-bisphosphate (PIP₂) were coexpressed by transfection in tsA-201 cells, and the cells were studied by whole-cell patch clamp and by confocal microscopy. Constitutively active forms of Gα_q and Gα₁₁, but not Gα₁₃, caused a loss of the plasma membrane PIP₂ and a total tonic inhibition of the KCNQ current. There were no further changes upon addition of the muscarinic agonist oxotremorine-M (oxo-M). Expression of the regulator of G-protein signaling, RGS2, blocked PIP₂ hydrolysis and current suppression by muscarinic stimulation, confirming that the G_q family of G-proteins is necessary. Dialysis with the competitive inhibitor GDPβS (1 mM) lengthened the time constant of inhibition sixfold, decreased the suppression of current, and decreased agonist sensitivity. Removal of intracellular Mg²⁺ slowed both the development and the recovery from muscarinic suppression. When combined with GDPβS, low intracellular Mg²⁺ nearly eliminated muscarinic inhibition. With nonhydrolyzable GTP analogs, current suppression developed spontaneously and muscarinic inhibition was enhanced. Such spontaneous suppression was antagonized by GDPβS or GTP or by expression of RGS2. These observations were successfully described by a kinetic model representing biochemical steps of the signaling cascade using published rate constants where available. The model supports the following sequence of events for this G_q-coupled signaling: A classical G-protein cycle, including competition for nucleotide-free G-protein by all nucleotide forms and an activation step requiring Mg²⁺, followed by G-protein–stimulated phospholipase C and hydrolysis of PIP₂, and finally PIP₂ dissociation from binding sites for inositol lipid on the channels so that KCNQ current was suppressed. Further experiments will be needed to refine some untested assumptions.

A pool of Y2 neuropeptide Y receptors activated by modifiers of membrane sulfhydryl or cholesterol balance

The cloned guinea-pig Y2 neuropeptide Y (NPY) receptors expressed in Chinese hamster ovary (CHO) cells, as well as the Y2 receptors natively expressed in rat forebrain, are distributed in two populations. A smaller population that is readily accessed by agonist peptides on the surface of intact cells constitutes less than 30% of Y2 receptors detected in particulates after cell homogenization. A much larger fraction of cell surface Y2 sites can be activated by sulfhydryl modifiers. A fast and large activation of these masked or cryptic sites could be obtained with membrane-permeating, vicinal cysteine-bridging arsenical phenylarsine oxide. A lower activation is effected by N-ethylmaleimide, an alkylator that slowly penetrates lipid bilayers. The restricted-access alkylator, 2-[(trimethylammonium)ethyl]methanethiosulfonate, was not effective in unmasking these sites. Some of the hidden cell surface Y2 sites could be activated by polyene filipin III through complexing of membrane cholesterol. The results are consistent with the presence of a large Y2 reserve in a compartment that can be accessed by alteration of sulfhydryl balance or fluidity of the cell membrane, and by treatments that affect the anchoring and aggregation of membrane proteins.

A predicted cortical serotonergic/cholinergic/GABAergic interface as a site of pathology in schizophrenia

1. The pathological process that precipitates schizophrenia has yet to be identified. However, many lines of evidence suggest that a change in the functioning of the frontal cortex is an important abnormality that underlies schizophrenia. 2. Studies in Brodmann's area 9, obtained post-mortem, have shown changes in 5-hydroxytryptamine 5-HT2A, muscarinic M1 and GABA(A) receptors in tissue from subjects with schizophrenia. 3. Animal studies suggest a site in the cortex where there would be an interaction between serotonergic and cholinergic innervation and that this interaction would involve the 5-HT2A and the M1 receptor. This site, in turn, would be a potent modulator of GABA activity and, hence, levels of GABA(A) receptors. 4. From combining these data, a theoretical site is proposed that, if proven to exist in human cortex, is likely to be central to the pathology of that illness.

Effect of continuous administration of a cholinergic agonist on [3H]4-DAMP binding and m3 mRNA expression in cultured human ciliary muscle cells

It is well known that chronic topical administration of cholinergic agonists results in a subsensitization in ciliary muscle-mediated increases in outflow facility and accommodation in monkey eyes in vivo. These physiologic changes are apparently mediated by the M3 subtype receptor. However, the nature of this subsensitization remains unclear. This study investigated the effect of the continuous presence of carbachol, a muscarinic agonist, on the expression of the muscarinic receptor subtype m3 and the binding of [3H]4-DAMP in cultured human ciliary muscle cells (H7CM). The H7CM cell line, derived from the ciliary muscle of a one-day-old human infant, was used in this study. Confluent monolayers were treated individually with 1 mM carbachol for 2, 6, 24 and 48 hours. The level of mRNA encoding muscarinic receptor subtype m3 was measured by RNase protection. For confirmation, a receptor binding assay was done using [3H]4-DAMP, a radioligand selective for M3 subtype receptors. At each timepoint, results were compared with untreated controls. Treatment with carbachol resulted in a down regulation ranging from 23.4% to 34.8% of m3 mRNA expression at all time points. All [3H]4-DAMP binding assay results also decreased, ranging from 24.5% to 31.0%.

Desensitization of G-protein-coupled receptors in the cardiovascular system

Multiple mechanisms exist to control the signaling and density of G-protein-coupled receptors (GPRs). Upon agonist binding and receptor activation, a series of reactions participate in the turn off or desensitization of GPRs. Many GPRs are phosphorylated by protein kinases and consequently uncoupled from G proteins. In addition, many GPRs are sequestered from the cell surface and become inaccessible to their activating ligands. Both receptor:G protein uncoupling and receptor sequestration may involve the participation of arrestins or other proteins. A model for receptor regulation has been developed from studies of the beta-adrenergic receptor. However, recent studies suggest that other GPRs important in the cardiovascular system, such as the muscarinic cholinergic receptors that regulate heart rate, might be regulated by mechanisms other than those that regulate the beta-adrenergic receptors. This review summarizes our current understanding of the processes involved in the desensitization of GPRs.

Structural correlates for down-regulation of m1 and m2 muscarinic receptor subtypes

Three chimeric receptors stably expressed in murine fibroblast (B82) cells were used to examine how different parts of the rat muscarinic m1 and m2 receptors contribute to the down-regulation process. The MCH7 chimeric m2 receptor contained a fragment between VIth TM and C-terminal end derived from the m1 receptor. The MCH3 and MCH5 receptors have exchanged N-terminal and third intracellular loop regions of the MCH7 receptor. Fibroblast cells stably expressing individual muscarinic wild type (m1, m2) or chimeric (MCH3, MCH5, or MCH7) receptors were treated with plain medium (control) or medium containing carbachol for 24 h. Receptor density changes were measured by [3H](-)1-N-methyl-3-quinuclidinyl benzilate ([3H](-)MQNB) saturation binding studies. There was a significant loss of receptor density, different for each receptor studied, following carbachol treatment relative to control cells. We related this loss of [3H](-)MQNB binding to the number of amino acids derived from m1 or m2 receptors for each constructed chimera and to the affinity of carbachol to the receptors studied. We demonstrate that: 1) the region from the VIth TMD to the end of C-terminal controls the extent of m1 and m2 receptor down-regulation; 2) the overall receptor conformation and the interaction between intracellular portions of the receptor influence the extent of receptor down-regulation; and 3) resistance to down-regulation by carbachol correlates with the affinity of carbachol to the muscarinic receptor construct.

Identification of adenylyl cyclase isoenzymes in CHO and B82 cells

The identification of adenylyl cyclase isoenzymes in mammalian host cells is important for the interpretation of data obtained from cell lines heterologously expressing G-protein coupled receptors. Reverse transcriptase polymerase chain reaction (RT-PCR) was used to amplify adenylyl cyclase cDNAs from Chinese hamster ovary (CHO) and mouse fibroblast (B82) cells. The isolated fragments were identified by restriction analyses and by sequencing. We found mRNAs for adenylyl cyclases VI and VII in CHO and adenylyl cyclases IX and VII in B82 cells.

Increased myocardial muscarinic receptor density in idiopathic dilated cardiomyopathy: an in vivo PET study

BACKGROUND

Congestive heart failure is associated with decreased stimulated myocardial adenylate cyclase activity, increased Gi-binding protein, attenuated parasympathetic tone, and increased modulation of beta-adrenergic inotropic left ventricular stimulation by parasympathetic agonists. Despite these abnormalities, changes in the density or affinity of ventricular muscarinic receptors have not been demonstrated in patients.

METHODS AND RESULTS

The density and affinity constants of myocardial muscarinic receptors were evaluated noninvasively by means of positron emission tomography with 11C-MQNB (methylquinuclidinyl benzilate), a specific hydrophilic antagonist, in 20 patients with congestive heart failure due to idiopathic dilated cardiomyopathy (mean left ventricular ejection fraction, 22+/-9%) and compared with values in 12 normal subjects. The mean receptor concentration was significantly higher in patients than in control subjects (B'max, 34.5+/-8.9 versus 25+/-7.7 pmol/mL, P<.005), with no changes in affinity constants. The change in heart rate after injection of 0.6 mg of cold MQNB was lower in patients than in control subjects (34+/-20% versus 55+/-36%, P<.05), and receptor density correlated negatively with maximal heart rate in the patients (r=.45, P<.05).

CONCLUSIONS

Congestive heart failure is associated with an upregulation of myocardial muscarinic receptors. This may be an adaptive mechanism to beta-agonist stimulation and should increase the number of potential targets for pharmacological intervention.

Downregulation of muscarinic M2 receptors linked to K+ current in cultured guinea-pig atrial myocytes

1. Desensitization of muscarinic K+ current (IK(ACh)) was studied in cultured atrial myocytes from guinea-pig hearts using whole-cell voltage clamp. 2. Three different types of desensitization could be identified. A fast component which upon rapid superfusion with ACh resulted in a partial relaxation of IK(ACh) within a few seconds to a plateau which was maintained in the presence of ACh. Recovery from this type of desensitization paralleled the decay of IK(ACh) after washout of the agonist. A second type of desensitization was observed within minutes. This was reversed around 10 min after washout of ACh. Both types were heterologous with regard to the A1 receptor and the novel phospholipid (Pl) receptor, both of which activate IK(ACh) via the same signalling pathway. 3. A third type of desensitization (downregulation) occurred upon exposure of the cultures for 24-48 h to the muscarinic agonist carbachol (CCh). The level of downregulation depended on the concentration of CCh (0.1 microM < or = [CCh] < or = 10 microM). No recovery was observed within 5 h after washout of CCh. Thereafter sensitivity to ACh slowly returned (half-time (t1/2), approximately 20 h). 4. Downregulation by CCh (0.1-5 microM) was characterized by an increase in EC50 for ACh with no reduction in maximum IK(ACh). With 5 microM CCh, EC50 was increased from 0.1 to 3.7 microM. At 10 microM CCh EC50 was increased to 15 microM and maximal current that could be evoked by ACh was reduced to 15%. 5. Downregulation by CCh was homologous with regard to A1 and Pl receptors. Maximum IK(ACh), assayed by a saturating concentration of Pl, was not reduced in downregulated cells, suggesting a mechanism localized at the M2 receptor. 6. The changes in the concentration-response curves can be accounted for by assuming an excess of M2 receptors relative to the subsequent component of the signalling pathway. 7. As the intact heart is under tonic vagal control, downregulation is likely to contribute to controlling the sensitivity of the heart to vagal activity in situ.

Regulation of GABAA receptor structure and function by chronic drug treatments in vivo and with stably transfected cells

In this article, we review the use of stably transfected cells to study the regulation of receptor structure and function by chronic drug treatments and compare results from these cells to results obtained from other systems, including neuronal cultures and intact animals. We focus on the gamma-aminobutyric acid type A (GABAA) receptor complex. Sedative/hypnotic drugs such as benzodiazepines, barbiturates and alcohol that potentiate GABAA receptor function produce tolerance and dependence. Chronic treatment of GABAA receptor preparations from brain and neuronal cultures with GABAA agonists, as well as these other three classes of drugs, results in regulation of several properties of the receptor. Drug treatments may regulate levels of binding sites, allosteric binding interactions, receptor function, levels of receptor subunit mRNA and levels of receptor subunit protein. Some or all of these effects may comprise the molecular mechanisms of tolerance to these GABAA-modulatory drugs. The use of cells stably transfected with neurotransmitter receptors provides a homogeneous population that can be cultured under controlled conditions. As most preparations contain mixed populations of GABAA receptor subunits, stably transfected cells offer the advantage of the expression of receptors with a defined subunit composition. We conclude that chronic drug treatments regulate allosteric coupling and function of GABAA receptors in stably transfected cells. This regulation does not appear to be due to decreases in the expression of alpha 1- or beta 1-receptor subunits or to expression of subunits other than alpha 1, beta 1, gamma 2L. Therefore, it is unlikely to be due to changes in receptor subunit composition and probably represents post-translational changes. The rapid regulation of allosteric coupling and function by drug treatment of the stably transfected cells should provide insights to the mechanisms of coupling between GABAA and benzodiazepine receptors as well as tolerance and dependence of benzodiazepines and ethanol.

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.

Long-term carbachol treatment-induced down-regulation of muscarinic M2-receptors but not m2 receptor mRNA in a human lung cell line

1. The molecular mechanisms involved in the regulation of muscarinic receptor gene expression are poorly understood. We have investigated the effect of homologous stimulation on the regulation of M2 muscarinic receptor protein and gene in human embryonic lung fibroblasts (HEL 299 cells). 2. Saturation studies performed with the non-selective hydrophilic ([3H]-N-methyl-scopolamine, [3H]-NMS) and lipophilic (3H]-quinuclidinyl benzilate, [3H]-QNB) muscarinic antagonists revealed a single class of high affinity binding sites. 3. Carbachol (1 mM) induced a rapid down-regulation of [3H]-NMS binding sites. Within 12 h, the process had approached steady state with 40 to 60% loss of receptors at 12 and 24 h. 4. The loss of [3h]-QNB binding sites (40% reduction at 24 h) occurred at a slower rate than did loss of [3H]-NMS binding sites as a result of receptor sequestration. 5. Carbachol treatment was accompanied by a functional desensitization of the receptor after 24 h of agonist treatment. In untreated cells, forskolin induced a large increase in cyclic AMP accumulation which was inhibited significantly by carbachol. The inhibitory effect of carbachol on forskolin-induced cyclic AMP accumulation was lost following 24 h carbachol stimulation. 6. The steady state level of muscarinic m2 mRNA measured by Northern blot analysis was not affected by carbachol had no effect on the stability of m2 mRNA. 7. The rate of transcription of m2 muscarinic receptor gene as measured by nuclear RNA run-on assay was unaltered by carbachol stimulation. 8. These results suggest that homologous sequestration, desensitization, and down-regulation of M2 modifications of m2 muscarinic receptor mRNAs.