Differential regulation by agonist and phorbol ester of cloned m1 and m2 muscarinic acetylcholine receptors in mouse Y1 adrenal cells and in Y1 cells deficient in cAMP-dependent protein kinase

The M2 muscarinic receptor, in association to M1 , regulates the neuromuscular PKA molecular dynamics

Muscarinic acetylcholine receptor 1 subtype (M₁ ) and muscarinic acetylcholine receptor 2 subtype (M₂ ) presynaptic muscarinic receptor subtypes increase and decrease, respectively, neurotransmitter release at neuromuscular junctions. M₂ involves protein kinase A (PKA), although the muscarinic regulation to form and inactivate the PKA holoenzyme is unknown. Here, we show that M₂ signaling inhibits PKA by downregulating Cβ subunit, upregulating RIIα/β and liberating RIβ and RIIα to the cytosol. This promotes PKA holoenzyme formation and reduces the phosphorylation of the transmitter release target synaptosome-associated protein 25 and the gene regulator cAMP response element binding. Instead, M₁ signaling, which is downregulated by M₂ , opposes to M₂ by recruiting R subunits to the membrane. The M₁ and M₂ reciprocal actions are performed through the anchoring protein A kinase anchor protein 150 as a common node. Interestingly, M₂ modulation on protein expression needs M₁ signaling. Altogether, these results describe the dynamics of PKA subunits upon M₂ muscarinic signaling in basal and under presynaptic nerve activity, uncover a specific involvement of the M₁ receptor and reveal the M₁ /M₂ balance to activate PKA to regulate neurotransmission. This provides a molecular mechanism to the PKA holoenzyme formation and inactivation which could be general to other synapses and cellular models.

Muscarinic receptor trafficking

Knowledge of the mechanisms responsible for the trafficking of neurotransmitter receptors away from the cell surface is of obvious importance in understanding what regulates their expression and function. This chapter will focus on the mechanisms responsible for the internalization and degradation of muscarinic receptors. There are both receptor subtype-specific and cell-type specific differences in muscarinic receptor trafficking. Studies on muscarinic receptor trafficking both in cells in culture and in vivo will be described, and the potential physiological consequences of this trafficking will be discussed.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

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.

Altered cardiovascular responses in mice lacking the M(1) muscarinic acetylcholine receptor

Although the M(2) muscarinic acetylcholine receptor (mAChR) is the predominant functional mAChR subtype in the heart, some responses of the cardiovascular system to acetylcholine (ACh) may be mediated by other mAChR subtypes. The potential effect of M(1) mAChR on heart function was investigated using M(1) knockout (M(1)-KO) mice. In vivo cardiodynamic analysis showed that basal values of heart rate (HR), developed left ventricular pressure (DLVP), left ventricular dP/dt(max) (LV dP/dt(max)), and mean blood pressure (MBP) were similar between wild-type (WT) and M(1)-KO mice. Injection of the putative M(1)-selective agonist 4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium (McN-A-343) produced an increase in LV dP/dt(max), DLVP, HR, and MBP in WT mice but did not affect hemodynamic function in the M(1)-KO mice. The stimulatory effect of McN-A-343 in WT mice was blocked by pretreatment with propranolol, indicating that stimulation of the M(1) mAChRs on sympathetic postganglionic neurons evoked release of catecholamines. Intravenous injection of ACh in both WT and M(1)-KO mice caused atrioventricular conduction block, without a significant change in the frequency of atrial depolarization, or atrial fibrillation. Immunoprecipitation and reverse transcriptase-polymerase chain reaction failed to detect the expression of M(1) mAChR in cardiac tissue from WT mice. The carbachol-induced increase of phospholipase C activity in cardiac tissues was not different between WT and M(1)-KO mice. These results demonstrate that 1) activation of M(1) mAChR subtype on sympathetic postganglionic cells results in catecholamine-mediated cardiac stimulation, 2) M(1) mAChR is not expressed in mouse heart, and 3) administration of ACh to mice induces arrhythmia.

Multiple topological domains mediate subtype-specific internalization of the M2 muscarinic acetylcholine receptor

Endocytosis of agonist-activated G protein-coupled receptors (GPCRs) is required for both resensitization and recycling to the cell surface as well as lysosomal degradation. Thus, this process is crucial for regulation of receptor signaling and cellular responsiveness. Although many GPCRs internalize into clathrin-coated vesicles in a dynamin-dependent manner, some receptors, including the M(2) muscarinic acetylcholine receptor (mAChR), can also exhibit dynamin-independent internalization. We have identified five amino acids, located in the sixth and seventh transmembrane domains and the third intracellular loop, that are essential for agonist-induced M(2) mAChR internalization via a dynamin-independent mechanism in JEG-3 choriocarcinoma cells. Substitution of these residues into the M(1) mAChR, which does not internalize in these cells, is sufficient for conversion to the internalization-competent M(2) mAChR phenotype, whereas removal of these residues from the M(2) mAChR blocks internalization. Cotransfection of a dominant-negative isoform of dynamin has no effect on M(2) mAChR internalization. An internalization-incompetent M(2) mutant that lacks a subset of the necessary residues can still internalize via a G protein-coupled receptor kinase-2 and beta-arrestin-dependent pathway. Furthermore, internalization is independent of the signal transduction pathway that is activated. These results identify a novel motif that specifies structural requirements for subtype-specific dynamin-independent internalization of a GPCR.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Receptor subtype-specific regulation of muscarinic acetylcholine receptor sequestration by dynamin. Distinct sequestration of m2 receptors

Sustained stimulation of muscarinic acetylcholine receptors (mAChRs) and other G protein-coupled receptors usually leads to a loss of receptor binding sites from the plasma membrane, referred to as receptor sequestration. Receptor sequestration can occur via endocytosis of clathrin-coated vesicles that bud from the plasma membrane into the cell but may also be accomplished by other, as yet ill-defined, mechanisms. Previous work has indicated that the monomeric GTPase dynamin controls the endocytosis of plasma membrane receptors via clathrin-coated vesicles. To investigate whether mAChRs sequester in a receptor subtype-specific manner via dynamin-dependent clathrin-coated vesicles, we tested the effect of overexpressing the dominant-negative dynamin mutant K44A on m1, m2, m3, and m4 mAChR sequestration in HEK-293 cells. The m1, m2, m3, and m4 mAChRs sequestered rapidly in HEK-293 cells following agonist exposure but displayed dissimilar sequestration pathways. Overexpression of dynamin K44A mutant fully blocked m1 and m3 mAChR sequestration, whereas m2 mAChR sequestration was not affected. Also, m4 mAChRs, which like m2 mAChRs preferentially couple to pertussis toxin-sensitive G proteins, sequestered in a completely dynamin-dependent manner. Following agonist removal, sequestered m1 mAChRs fully reappeared on the cell surface, whereas sequestered m2 mAChRs did not. The distinct sequestration of m2 mAChRs was also apparent in COS-7 and Chinese hamster ovary cells. We conclude that the m2 mAChR displays unique subtype-specific sequestration that distinguishes this receptor from the m1, m3, and m4 subtypes. These results are the first to demonstrate that receptor sequestration represents a new type of receptor subtype-specific regulation within the family of mAChRs.

Protein kinase C regulation of ATP-induced phosphoinositide hydrolysis in bovine aorta endothelial cells

This study investigated the mechanism of protein kinase C-mediated inhibition of ATP-induced phospholipase C activation in cultured bovine aorta endothelial cells (BAEC). In BAEC labeled with 3H-inositol, phorbol myristate acetate (PMA) prevented ATP-induced inositol bisphosphate and inositol trisphosphate formation. In membranes prepared from these PMA-treated cells, Ca(2+)-, sodium fluoride-, GTP gamma S-, and ATP plus GTP gamma S-stimulated inositol bisphosphate, but not inositol trisphosphate, formation was inhibited. Inositol trisphosphate phosphatase activity was not altered in membranes from PMA-treated BAEC. These results suggest that 1) protein kinase C inhibits ATP-induced phospholipase C activation in BAEC through interference with the coupling of phospholipase C with a G-protein and through an effect on phospholipase C itself, and 2) different mechanisms are responsible for the inhibition by protein kinase C of the phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate and phosphatidyl-inositol phosphate.

Internalization of the m2 muscarinic acetylcholine receptor. Arrestin-independent and -dependent pathways

Recent studies have identified agonist-dependent phosphorylation as a critical event in the rapid uncoupling of the m2 muscarinic cholinergic receptors (mAChR) from G-proteins and sequestration of the receptors away from the cell surface. However, mutant m2 mAChRs were identified that were phosphorylated but unable to desensitize in adenylyl cyclase assays, while they internalized like wild type (WT) mAChRs. We have tested whether these properties might stem from differences in the abilities of the WT and mutant mAChR to bind arrestins, proteins implicated in both receptor/G-protein uncoupling and internalization. We have determined that arrestin binding requires phosphorylation at a cluster of Ser/Thr residues in amino acids 307-311 in the m2 mAChR. A strong correlation was found between the ability of WT and mutant receptors to bind arrestins in vitro or in vivo and to desensitize in adenylyl cyclase assays. However, the phosphorylation-dependent internalization of the m2 mAChR in HEK-tsA201 cells did not require arrestins and did not proceed via clathrin-mediated endocytosis. While the m2 mAChR was able to enter a clathrin- and arrestin-dependent pathway when arrestin 2 or arrestin 3 was significantly overexpressed, the preferred pathway of internalization of WT and certain mutant m2 mAChR in HEK-tsA201 cells did not involve participation of arrestins. The results suggest that the phosphorylation-mediated regulation of the m2 mAChR may involve arrestin-dependent and -independent events.

Identification of a region required for subtype-specific agonist-induced sequestration of the m2 muscarinic acetylcholine receptor

When the m1 and m2 muscarinic acetylcholine receptors are transiently expressed in JEG-3 cells, the m2, but not the m1, receptor undergoes agonist-induced sequestration. Both receptors exhibit internalization when expressed in Y1 cells. These results suggest that the m1 and m2 receptors use distinct cellular mechanisms or pathways for agonist-induced internalization and that JEG-3 cells are deficient in the mechanism or pathway used by the m1 receptor. Transfection experiments with chimeric receptors indicate that the specificity for agonist-induced internalization for the m2 receptor lies in the carboxyl-terminal fifth of the receptor. The intracellular carboxyl-terminal tail of the m2 receptor is neither sufficient nor required for the m2-specific sequestration. Site-directed mutagenesis demonstrates that two amino acids in the carboxyl-terminal end of the third cytoplasmic loop of the m2 receptor are required for sequestration in JEG-3 cells. In addition, the sixth transmembrane domain, which is adjacent to this cytoplasmic domain, is also required. Thus, m2-specific agonist-induced sequestration requires sequences both in the carboxyl-terminal end of the third cytoplasmic loop and the adjacent transmembrane domain.

The role of membrane proximal threonine residues conserved among guanine-nucleotide-binding-protein-coupled receptors in internalization of the m4 muscarinic acetylcholine receptor

Many guanine-nucleotide-binding-protein-coupled receptors contain consensus sequences for phosphorylation by cAMP-dependent protein kinase (PKA), often located in the membrane proximal regions critically important for receptor signalling. In the present study, we have evaluated by site-directed mutagenesis the role of the putative PKA phosphorylation sites in the m4 muscarinic acetylcholine receptor (mAChR), i.e. Thr145 in the second cytoplasmic loop and Thr399 in the third cytoplasmic loop, and the influence of PKA on m4 mAChR function and internalization. Antagonist binding was unaltered by any of the mutations studied, while the agonist-binding affinity was either not affected (Thr145 alanine), increased (Thr399 alanine) or decreased (Thr399 serine or aspartic acid). m4 mAChR-mediated inhibition of adenylyl cyclase was unaltered by the mutations, except for an approximately tenfold reduced agonist potency of the Thr399 aspartic acid mutated receptor. Agonist-induced receptor internalization was unaltered with Thr399 serine or aspartic acid mutations of the receptors, but was strongly decreased in its rate and extent upon replacement of Thr399, Thr145 or both of these residues with alanine. These mutational effects could not be reproduced by treatment of wild-type receptor-expressing cells with the PKA inhibitor H-8. Furthermore, maximal stimulation of cellular PKA neither affected receptor internalization nor signalling measured as receptor-mediated Ca2+ mobilization. We conclude that the membrane proximal threonine residues of the m4 mAChR are not required for receptor signalling, but replacement by alanine residues can significantly affect receptor internalization, independently of PKA phosphorylation. Sequence comparisons suggest that threonine residues at corresponding positions may be relevant to internalization of other guanine-nucleotide-binding-protein-coupled 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.

Transcriptional down-regulation of m2 muscarinic receptor gene expression in human embryonic lung (HEL 299) cells by protein kinase C

m2 muscarinic receptor gene expression was investigated following stimulation of protein kinase C (PKC) with the phorbol ester 4 beta-phorbol dibutyrate (PDBu) in HEL 299 cells. PDBu (100 nM) caused a time-dependent decrease in the steady-state levels of m2 receptor mRNA and in specific [3H]N-methyl-scopolamine binding. Preincubation with the PKC inhibitor GF-109203X inhibited the reduction in M2 receptor and mRNA levels induced by PDBu, confirming the involvement of PKC. Chronic PDBu treatment also caused desensitization of the receptor as forskolin-stimulated cAMP accumulation, inhibited by carbachol in control cells, was lost upon treatment with PDBu for 24 h. Co-incubation with PDBu and the protein synthesis inhibitor cycloheximide, inhibited PDBu-mediated reduction of m2 receptor mRNA, indicating new protein synthesis is required for down-regulation. Half-life studies using the transcriptional inhibitor actinomycin D suggested that the stability of the m2 receptor mRNA was not altered by PDBu treatment (t1/2 = 2 h). Nuclear run-on assays showed a 50% reduction in the rate of m2 receptor gene transcription after treatment with PDBu for 12 h. In conclusion we have provided evidence for heterologous regulation of m2 receptor gene expression through changes in gene transcription resulting in uncoupling of M2 receptors. Furthermore, the synthesis of an unidentified factor is required for the down-regulation process.

Protein kinase C activity does not mediate the inhibitory effect of carbachol on chloride secretion by T84 cells

Carbachol induces calcium-dependent chloride secretion and activates protein kinase C in T84 cells. However, prolonged stimulation with carbachol or direct activation of protein kinase C inhibits subsequent calcium-dependent chloride secretion. Furthermore, the ability of carbachol to elevate inositol tetrakisphosphate levels may be linked to inhibition of chloride secretion. Here we demonstrate that protein kinase C activation increases levels of inositol tetrakisphosphates (1,3,4,6- and 3,4,5,6-isomers) in T84 colonic epithelia. Furthermore, this corresponds to an inhibition of chloride secretion. However, protein kinase C is unlikely to mediate the analogous effects of carbachol. Neither the ability of carbachol to inhibit calcium-dependent chloride secretion nor its effects on inositol 3,4,5,6-tetrakisphosphate levels were reversed by staurosporine. Carbachol also has quantitatively and qualitatively different effects on inositol tetrakisphosphate isomers than protein kinase C activators. Thus protein kinase C activity can increase levels of various inositol tetrakisphosphate isomers within T84 cells but does not mediate carbachol-induced increases in these putative messengers. These data further support the hypothesis that inositol 3,4,5,6-tetrakisphosphate is a negative second messenger, uncoupling epithelial chloride secretion from changes in intracellular calcium.

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

Murine fibroblast cell lines stably transfected with the rat muscarinic m1 or m2 receptor genes were used to study the homologous regulation of the muscarinic M1 or M2 receptors. The cells were pretreated with the muscarinic receptor agonists, (+)-cismethyl-dioxolane, carbachol, 2,8-dimethyl-3-methylene-1-oxa-8-aza-spiro- [4,5]decane ((+) or (-)-YM796) or the muscarinic receptor antagonist atropine for up to 24 h. Our study has demonstrated that the muscarinic receptor nonselective full agonist, (+)-cismethyl-dioxolane, induced the down-regulation of both the muscarinic M1 and the M2 receptors in association with desensitization of the receptor-mediated functions. The muscarinic M1 receptors are down-regulated without significant receptor internalization while the muscarinic M2 receptors are more sensitive to down-regulation than the muscarinic M1 receptors because of significant internalization of the muscarinic M2 receptors in our system. The muscarinic M1 receptor partial agonist, (-)-YM796 induced less down-regulation and no significant desensitization of the muscarinic M1 receptors with no substantial effect on the muscarinic M2 receptor density or function.

5-Hydroxytryptamine receptor-mediated phosphoinositide hydrolysis in canine cultured tracheal smooth muscle cells

1. 5-Hydroxytryptamine (5-HT) has been shown to induce contraction of tracheal smooth muscle. However, the mechanisms of action of 5-HT are not known. We therefore investigated the effects of 5-HT on phospholipase C (PLC)-mediated phosphoinositide (PI) hydrolysis and its regulation in canine cultured tracheal smooth muscle cells (TSMCs) labelled with [3H]-inositol. 5-HT-induced inositol phosphates (IPs) accumulation was time- and dose-dependent with a half-maximal response (EC50) and a maximal response at 0.38 +/- 0.05 and 10 microM, respectively. 2. Ketanserin and mianserin (10 and 100 nM), 5-HT2 receptor antagonists, were equipotent in blocking the 5-HT-induced IPs accumulation with pKB values of 8.46 and 8.21, respectively. In contrast, the dose-response curves of 5-HT-induced IPs accumulation were not shifted until the concentrations of NAN-190 and metoclopramide (5-HT1A and 5-HT3 receptor antagonists, respectively) were increased up to 10 microM. 3. Pretreatment of TSMCs with pertussis toxin or cholera toxin did not inhibit the 5-HT-induced IPs accumulation, but partially inhibited the AlF(4-)-induced IPs response. 4. Stimulation of IPs accumulation by 5-HT required the presence of external Ca2+ and was blocked by EGTA. The addition of Ca2+ (3-620 nM) to digitonin-permeabilized TSMCs directly stimulated IPs accumulation. A further Ca(2+)-dependent increase in IPs accumulation was obtained by inclusion of either guanosine 5'-O-(3-thiotriphoshate) (GTP gamma S) or 5-HT. The combination of GTP gamma S and 5-HT elicited an additive effect on IPs accumulation. 5. Treatment with phorbol 12-myristate 13-acetate (PMA, 1 microM, 30 min) abolished the 5-HT-induced IPs accumulation. The concentrations of PMA that gave a half-maximal and maximal inhibition of 5-HT-induced IPs accumulation were 2.2 +/- 0.4 nM and 1 microM, n = 3, respectively. The protein kinase C (PKC) activator, 4 alpha-phorbol 12,13-didecanoate, at 1 microM, did not influence this response. The inhibitory effect of PMA was reversed by staurosporine, a PKC inhibitor, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. 6. The site of this inhibition was further investigated by examining the effect of PMA on AlF(4-)-induced IPs accumulation in canine TSMCs. AlF(4-)-stimulated IPs accumulation was inhibited by PMA treatment, suggesting that the effect of PMA is distal to the 5-HT receptor. 7. Acetylcholine-induced IPs accumulation was completely inhibited by atropine, but not affected by ketanserin or mianserin, suggesting that 5-HT-induced IPs accumulation is not due to release of acetylcholine.8. These results demonstrate that 5-HT directly stimulates PLC-mediated PI hydrolysis via a pertussis toxin- and cholera toxin-insensitive GTP binding protein in canine TSMCs and that this coupling process is negatively regulated by PKC. 5-HT2 receptors may be predominantly mediating IPs accumulation and presumably IP-induced Ca2+ release may function as the transducing mechanism for 5-HT stimulated contraction of tracheal smooth muscle.

Glutamate-stimulated phosphatidylinositol metabolism in the avian cochlear nucleus

This study examined the ability of the excitatory amino acid glutamate and its analogs to stimulate phosphatidylinositol metabolism in isolated cochlear nucleus tissue from young chicks. In the presence of lithium chloride, glutamate and (+/-)-1-aminocyclopentyl-trans-1,3-dicarboxylate (ACPD) stimulated the formation of inositol phosphates to levels significantly above unstimulated control levels. Unexpectedly, quisqualate did not stimulate inositol phosphates formation. The N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV), the ionotropic kainate/quisqualate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the putative metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionate (AP3) had no effect on the glutamate stimulated formation of inositol phosphates. We conclude that a metabotropic glutamate receptor is present on cochlear nucleus neurons of posthatch chicks and is able to stimulate formation of inositol phosphates.

Protein kinase regulation of muscarinic receptor signalling in colonic smooth muscle

1. We have previously demonstrated that M2 and M3 muscarinic receptors coexist in the circular smooth muscle of canine proximal colon. Activation of receptors of the M2 subtype leads to inhibition of adenylyl cyclase activity through the GTP-binding protein, Gi, while M3 receptors are coupled to a pertussis toxin-insensitive GTP-binding protein and mediate phosphoinositide hydrolysis. 2. In the present study, the interactions between these second messenger systems were examined. Activation of either protein kinase C or adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase attenuated carbachol-stimulated phosphoinositide hydrolysis without affecting basal activity. Activation of both protein kinases produced greater attenuation of inositol 1,4,5-trisphosphate formation than activation of either kinase alone. 3. In contrast to its inhibitory effect on phosphoinositide hydrolysis, activation of protein kinase C had no effect on adenylyl cyclase activity. 4. Activation of protein kinase C by phorbol ester treatment resulted in the sequestration of M3 muscarinic receptors from the cell surface without effecting the M2 muscarinic receptor population. Sequestered M3 muscarinic receptors were not rapidly degraded. 5. In contrast, elevation of cellular cyclic AMP decreased the affinity of cell surface muscarinic receptors for an antagonist radioligand without affecting their density. 6. Muscarinic agonist binding was not affected by either activation of protein kinase C or elevation of cellular cyclic AMP. 7. These data support the notion of negative feedback by protein kinase C and cyclic AMP-dependent protein kinase on phosphoinositide hydrolysis. In canine colonic circular smooth muscle this negative feedback regulation of inositol phosphate generation by muscarinic receptor stimulation does not appear to involve the guanine nucleotide binding protein:receptor interaction.

Regulation of expression and function of muscarinic receptors

The regulation of expression and function of the muscarinic acetylcholine receptor has been studied using several different systems. The role of glycosylation of the m2 receptor was examined by removal of glycosylation sites using site-directed mutagenesis followed by expression in stably transfected cells. The results demonstrated that glycosylation was not required for the synthesis and appearance of the receptors on the cell surface or for the coupling of the receptors to inhibition of adenylyl cyclase activity. Site-directed mutagenesis also was used to demonstrate that the single cysteine in the carboxy terminal domain of the m2 receptor was not required for receptor function, thus rendering unlikely a model suggesting a requirement for palmitoylation of this cysteine in receptor function. The muscarinic receptors expressed in embryonic chick heart were identified by molecular cloning. Two genes were initially identified which are expressed in chick heart and correspond to the chick m2 and m4 receptors. Experiments using the polymerase chain reaction to identify low abundance mRNAs indicate that at least one addition receptor gene is expressed in chick heart. In cell culture, activation of the muscarinic receptors decreases the levels of mRNA encoding the cm2 and cm4 receptors. This probably results from decreased gene transcription due to both mAChR-mediated inhibition of adenylyl cyclase and mAChR-mediated stimulation of phospholipase C. The elucidation of the factors which regulate the expression and function of muscarinic acetylcholine receptors (mAChR) is of obvious importance in understanding the mechanisms underlying cholinergic transmission. In this chapter, we will describe studies on the expression and function of wild type and mutant muscarinic receptors, the molecular characterization of mAChR expressed in chick heart, and the regulation of mAChR gene expression in response to muscarinic receptor activation.