Role of the M3 muscarinic acetylcholine receptor in β-cell function and glucose homeostasis

The release of insufficient amounts of insulin in the presence of elevated blood glucose levels is one of the key features of type 2 diabetes. Various lines of evidence indicate that acetylcholine (ACh), the major neurotransmitter of the parasympathetic nervous system, can enhance glucose-stimulated insulin secretion from pancreatic β-cells. Studies with isolated islets prepared from whole body M₃ muscarinic ACh receptor knockout mice showed that cholinergic amplification of glucose-dependent insulin secretion is exclusively mediated by the M₃ muscarinic receptor subtype. To investigate the physiological relevance of this muscarinic pathway, we used Cre/loxP technology to generate mutant mice that lack M₃ receptors only in pancreatic β-cells. These mutant mice displayed impaired glucose tolerance and significantly reduced insulin secretion. In contrast, transgenic mice overexpressing M₃ receptors in pancreatic β-cells showed a pronounced increase in glucose tolerance and insulin secretion and were resistant to diet-induced glucose intolerance and hyperglycaemia. These findings indicate that β-cell M₃ muscarinic receptors are essential for maintaining proper insulin secretion and glucose homeostasis. Moreover, our data suggest that enhancing signalling through β-cell M₃ muscarinic receptors may represent a new avenue in the treatment of glucose intolerance and type 2 diabetes.     

Pronounced pharmacologic deficits in M2 muscarinic acetylcholine receptor knockout mice

Members of the muscarinic acetylcholine receptor family (M1–M5) are known to be involved in a great number of important central and peripheral physiological and pathophysiological processes. Because of the overlapping expression patterns of the M1–M5 muscarinic receptor subtypes and the lack of ligands endowed with sufficient subtype selectivity, the precise physiological functions of the individual receptor subtypes remain to be elucidated. To explore the physiological roles of the M2 muscarinic receptor, we have generated mice lacking functional M2 receptors by using targeted mutagenesis in mouse embryonic stem cells. The resulting mutant mice were analyzed in several behavioral and pharmacologic tests. These studies showed that the M2 muscarinic receptor subtype, besides its well documented involvement in the regulation of heart rate, plays a key role in mediating muscarinic receptor-dependent movement and temperature control as well as antinociceptive responses, three of the most prominent central muscarinic effects. These results offer a rational basis for the development of novel muscarinic drugs.     

Sj?gren autoantibodies modify neonatal cardiac function via M1 muscarinic acetylcholine receptor activation.

Isolated congenital heart block may be associated with primary Sj?gren syndrome. In this work we describe circulating antibodies in the sera of primary Sj?gren syndrome patients that are able to interact with neonatal myocardium by activating muscarinic acetylcholine receptors of M1 subtype. We report on the presence of autoantibodies against the second extracellular loop of human M1 muscarinic acetylcholine receptors in primary Sj?gren syndrome mothers whose children have congenital heart block using a synthetic peptide in indirect immunofluorescence technique. Autoantibodies from primary Sj?gren syndrome patients gave positive image on neonatal atria but not on adult atria slices. The synthetic M1 peptide selectively abrogated indirect immunofluorescence recognition. The primary Sj?gren syndrome-immunoglobulin G also displayed an 'agonist like' activity modifying the intracellular events associated with muscarinic acetylcholine receptor activation. The mechanism appears to occur secondarily to stimulation of phosphoinositides turnover via phospholipase C activation. This, in turn, triggers cascade reactions involving calcium/calmodulin and leads to activation of nitric oxide synthase and soluble guanylate cyclase. All of these effects were selectively blunted by pirenzepine and neutralized by M1 synthetic peptide. These biological effects were not obtained using adult instead of neonatal rat atria and neither occurred with the sera of normal healthy women of childbearing age. It could be concluded that antibodies against neonatal M1 muscarinic acetylcholine receptor may be another serum factor to be considered in the pathophysiology of the development of congenital heart block associated with primary Sj?gren syndrome mothers.     

Selective activation of the M1 muscarinic acetylcholine receptor achieved by allosteric potentiation

The forebrain cholinergic system promotes higher brain function in part by signaling through the M₁ muscarinic acetylcholine receptor (mAChR). During Alzheimer''s disease (AD), these cholinergic neurons degenerate, therefore selectively activating M₁ receptors could improve cognitive function in these patients while avoiding unwanted peripheral responses associated with non-selective muscarinic agonists. We describe here benzyl quinolone carboxylic acid (BQCA), a highly selective allosteric potentiator of the M₁ mAChR. BQCA reduces the concentration of ACh required to activate M₁ up to 129-fold with an inflection point value of 845 nM. No potentiation, agonism, or antagonism activity on other mAChRs is observed up to 100 μM. Furthermore studies in M₁^−/− mice demonstrates that BQCA requires M₁ to promote inositol phosphate turnover in primary neurons and to increase c-fos and arc RNA expression and ERK phosphorylation in the brain. Radioligand-binding assays, molecular modeling, and site-directed mutagenesis experiments indicate that BQCA acts at an allosteric site involving residues Y179 and W400. BQCA reverses scopolamine-induced memory deficits in contextual fear conditioning, increases blood flow to the cerebral cortex, and increases wakefulness while reducing delta sleep. In contrast to M₁ allosteric agonists, which do not improve memory in scopolamine-challenged mice in contextual fear conditioning, BQCA induces β-arrestin recruitment to M₁, suggesting a role for this signal transduction mechanism in the cholinergic modulation of memory. In summary, BQCA exploits an allosteric potentiation mechanism to provide selectivity for the M₁ receptor and represents a promising therapeutic strategy for cognitive disorders.Basal forebrain cholinergic neurons innervate information processing centers in the hippocampus and cortex to promote attention and memory. During AD, these neurons profoundly degenerate, contributing to cognitive impairment (1). While cholinesterase inhibitors demonstrate the therapeutic potential for boosting cholinergic function in AD, they are limited by tolerability and provide modest benefit, thus there remains a tremendous need for improved therapies (2). Selectively targeting the ACh receptors involved in memory, while sparing receptors involved in other physiological processes, could provide additional efficacy, a widely pursued approach that has yet to lead to new medicines.ACh signals by activating ligand-gated ion channels (nicotinic receptors) and metabotropic (muscarinic) G protein-coupled receptors (GPCRs) designated M₁–M₅. Among the mAChRs, M₁ is most abundantly expressed in the hippocampus, cortex, and striatum, and localizes to postsynaptic membranes (3), where it signals via G_q/G₁₁ G-proteins to phospholipase C and through other G-proteins to additional signaling systems (4, 5). M₁ regulates several ion channels including KCNQ inwardly rectifying K⁺ currents, voltage-gated calcium channels, and NMDA receptors (4–9). Thus M₁ could mediate much of the cognitive effects of ACh. Supporting this hypothesis xanomeline, an M₁/M₄ preferring agonist, improved cognition and behavior in AD patients but was not tolerated due to unwanted cholinergic effects (10). Additional studies suggest that M₁ activation could slow AD progression by reducing Aβ42 peptides (11). Thus a drug that activates M₁ could potentially improve cognition while over time slowing the progression of the disease. Unfortunately, conservation of the ACh binding site has precluded the discovery of selective agonists.Many GPCRs, including mAChRs (12), have allosteric binding sites bound by small molecules that activate the receptor in the absence of ligand (allosteric agonist) or enhance the response to native ligand (positive allosteric modulator) (13). As allosteric sites are theoretically under less evolutionary constraint, targeting them affords opportunities for selectivity. This concept was demonstrated by the M₁ allosteric agonist TBPB (14) and a collection of relatively selective positive allosteric modulators (15). Here we describe BQCA [1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid], an orally available drug-like molecule that regulates memory and brain function by potentiating M₁.     

Generation and functional analysis of monoclonal antibodies against the second extracellular loop of human M3 muscarinic acetylcholine receptor

The M3 muscarinic acetylcholine receptor (M3R) plays a crucial role in the activation of salivary and lachrymal glands. The M3R contains four extracellular domains (the N-terminal, and the first, second, and third extracellular loops), and we recently detected antibodies against each of these four domains in a subgroup of patients with Sj?gren’s syndrome (SS). Functional analysis indicated that the influence of such anti-M3R antibodies on salivary secretion might differ based on the epitopes to which they bind. To clarify the relationship between B-cell epitopes on the M3R and its function, we generated two hybridomas producing anti-M3R monoclonal antibodies against the second extracellular loop of M3R (anti-M3R^2nd mAbs) and analyzed their function by Ca²⁺-influx assays, using a human salivary gland (HSG) cell line. These two anti-M3R^2nd mAbs suppressed Ca²⁺-influx in the HSG cells induced by cevimeline stimulation, suggesting that autoantibodies against the second extracellular loop of M3R could be involved in salivary dysfunction in patients with SS.     

Multiple functional defects in peripheral autonomic organs in mice lacking muscarinic acetylcholine receptor gene for the M3 subtype

Muscarinic acetylcholine receptors consist of five distinct subtypes and have been important targets for drug development. In the periphery, muscarinic acetylcholine receptors mediate cholinergic signals to autonomic organs, but specific physiological functions of each subtype remain poorly elucidated. Here, we have constructed and analyzed mutant mice lacking the M(3) receptor and have demonstrated that this subtype plays key roles in salivary secretion, pupillary constriction, and bladder detrusor contractions. However, M(3)-mediated signals in digestive and reproductive organs are dispensable, likely because of redundant mechanisms through other muscarinic acetylcholine receptor subtypes or other mediators. In addition, we have found prominent urinary retention only in the male, which indicates a considerable sex difference in the micturition mechanism. Accordingly, this mutant mouse should provide a useful animal model for investigation of human diseases that are affected in the peripheral cholinergic functions.     

Deletion of the M5 muscarinic acetylcholine receptor attenuates morphine reinforcement and withdrawal but not morphine analgesia

Little is known about the physiological roles of the M5 muscarinic receptor, the last member of the muscarinic receptor family (M1-M5) to be cloned. In the brain, the M5 receptor subtype is preferentially expressed by dopaminergic neurons of the substantia nigra and the ventral tegmental area. Dopaminergic neurons located in the ventral tegmental area are known to play important roles in mediating both the rewarding effects of opiates and other drugs of abuse and the manifestations of opiate/drug withdrawal symptoms. We therefore speculated that acetylcholine-dependent activation of M5 receptors might modulate the manifestations of opiate reward and withdrawal. This hypothesis was tested in a series of behavioral, biochemical, and neurochemical studies using M5 receptor-deficient mice (M5-/- mice) as novel experimental tools. We found that the rewarding effects of morphine, as measured in the conditioned place preference paradigm, were substantially reduced in M5-/- mice. Furthermore, both the somatic and affective components of naloxone-induced morphine withdrawal symptoms were significantly attenuated in M5-/- mice. In contrast, the analgesic efficacy of morphine and the development of tolerance to the analgesic effects of morphine remained unaltered by the lack of M5 receptors. The finding that M5 receptor activity modulates both morphine reward and withdrawal processes suggests that M5 receptors may represent a novel target for the treatment of opiate addiction.     

Cholinergic dilation of cerebral blood vessels is abolished in M5 muscarinic acetylcholine receptor knockout mice

The M(5) muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M(1)-M(5)) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M(5) receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M(5) receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M(5) receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M(5) receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M(5) receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M(5) muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M(5) receptors may represent an attractive therapeutic target.     

Purification of the muscarinic acetylcholine receptor from porcine atria.

The muscarinic acetylcholine receptor from porcine atria has been purified 100,000-fold to homogeneity by solubilization in digitonin/cholate and sequential chromatography on wheat germ agglutinin-agarose, diethylaminoethylagarose, hydroxylapatite, and 3-(2'-aminobenzhydryloxy)tropane-agarose. The yield of purified receptor was 4.3% of that found in the membrane fraction, and the purified receptor bound 11.1-12.8 nmol of L-[3H]quinuclidinyl benzilate per mg of protein, corresponding to a binding component Mr of 78,400-90,000. The purified receptor preparation consisted of two polypeptides in approximately equimolar amounts when examined on silver-stained sodium dodecyl sulfate/polyacrylamide gels. The larger polypeptide (Mr 78,000 on 8% polyacrylamide gels) was specifically alkylated with [3H]propylbenzilylcholine mustard, whereas the smaller polypeptide (Mr 14,800) was not labeled. The possibility that the small polypeptide is a contaminant fortuitously appearing in equimolar amounts with the large polypeptide cannot be ruled out at this time. The purified preparation was highly stable, with no measurable change in the number of ligand binding sites or the gel pattern after 1 month's storage on ice. Scatchard analysis showed a single class of binding sites for the antagonist L-[3H]quinuclidinyl benzilate with a dissociation constant of 61 +/- 4 pM. Equilibrium titration experiments demonstrated that the antagonist L-hyoscyamine displaced L-[3H]quinuclidinyl benzilate from a single class of sites (Kd = 475 +/- 30 pM), whereas the agonist carbamoylcholine interacted at two populations of sites (53% +/- 3% high affinity, Kd = 1.1 +/- 0.3 microM; 47% +/- 3% low affinity, Kd = 67 +/- 14 microM). The ligand binding data were very similar to that for the membrane-bound receptor, suggesting that the receptor has not been altered radically during purification.     

下丘脑对血糖的调节

下丘脑腹内侧核(VMH)中存在的血糖感受神经元参与全身血糖稳定状态的调节。其中分为两种不同功能的血糖感受神经元:葡萄糖兴奋神经元——当细胞外周环境中葡萄糖浓度升高时能降低其代谢率;葡萄糖抑制神经元——当外周葡萄糖浓度升高时能提高其代谢率。除VMH外,脑中还有更高级的血糖调节中枢,能和VMH部位通过促肾上腺皮质激素释放因子(CRF)、尿皮质醇及促肾上腺皮质激素释放因子受体(CRFRs)相互作用。1型糖尿病患者对低血糖的负调节反应(CRRs)存在缺陷,导致在胰岛素治疗的过程中发生低血糖事件。研究VMH部位对血糖调节的机制能为其治疗提供新的线索。     

Effect of a muscarinic M3 receptor agonist on gastric motility

Muscarinic M3 receptors exist in the gastrointestinal wall in humans and the muscarinic M3 agonist cevimeline hydrochloride (Evoxac) is a candidate therapeutic agent for the treatment of xerostomia in Sjögren’s syndrome. However, M3 receptor agonists are not known to show efficacy for diseases associated with abnormal gastrointestinal motility. Herein the effects are reported of cevimeline on gastric motility in two patients with non‐ulcer dyspepsia. The patients both received long‐term proton pump inhibitor therapy for 6 months, but their symptoms persisted. Then cevimeline was administered orally for 8 weeks at 30 mg three times daily (90 mg/day) and their dyspepsia symptoms improved. Electrogastrography was performed to examine gastric motility before and after administration of the M3 agonist. The fasting or nocturnal wave rate was significantly increased after administration compared with before administration, but no significant postprandial changes were seen. No adverse effects of cevimeline were observed. This drug might be a candidate therapeutic agent for non‐ulcer dyspepsia. Because its postprandial effects on gastrointestinal motility are unclear, a dose‐finding clinical study should be performed in the future.     

Neuronal M3 muscarinic acetylcholine receptors are essential for somatotroph proliferation and normal somatic growth

The molecular pathways that promote the proliferation and maintenance of pituitary somatotrophs and other cell types of the anterior pituitary gland are not well understood at present. However, such knowledge is likely to lead to the development of novel drugs useful for the treatment of various human growth disorders. Although muscarinic cholinergic pathways have been implicated in regulating somatotroph function, the physiological relevance of this effect and the localization and nature of the receptor subtypes involved in this activity remain unclear. We report the surprising observation that mutant mice that selectively lack the M₃ muscarinic acetylcholine receptor subtype in the brain (neurons and glial cells; Br-M3-KO mice) showed a dwarf phenotype associated with a pronounced hypoplasia of the anterior pituitary gland and a marked decrease in pituitary and serum growth hormone (GH) and prolactin. Remarkably, treatment of Br-M3-KO mice with CJC-1295, a synthetic GH-releasing hormone (GHRH) analog, rescued the growth deficit displayed by Br-M3-KO mice by restoring normal pituitary size and normal serum GH and IGF-1 levels. These findings, together with results from M₃ receptor/GHRH colocalization studies and hypothalamic hormone measurements, support a model in which central (hypothalamic) M₃ receptors are required for the proper function of hypothalamic GHRH neurons. Our data reveal an unexpected and critical role for central M₃ receptors in regulating longitudinal growth by promoting the proliferation of pituitary somatotroph cells.     

Enhancement of D1 dopamine receptor-mediated locomotor stimulation in M(4) muscarinic acetylcholine receptor knockout mice

Muscarinic acetylcholine receptors (M(1)-M(5)) regulate many key functions of the central and peripheral nervous system. Primarily because of the lack of receptor subtype-selective ligands, the precise physiological roles of the individual muscarinic receptor subtypes remain to be elucidated. Interestingly, the M(4) receptor subtype is expressed abundantly in the striatum and various other forebrain regions. To study its potential role in the regulation of locomotor activity and other central functions, we used gene-targeting technology to create mice that lack functional M(4) receptors. Pharmacologic analysis of M(4) receptor-deficient mice indicated that M(4) receptors are not required for muscarinic receptor-mediated analgesia, tremor, hypothermia, and salivation. Strikingly, M(4) receptor-deficient mice showed an increase in basal locomotor activity and greatly enhanced locomotor responses (as compared with their wild-type littermates) after activation of D1 dopamine receptors. These results indicate that M(4) receptors exert inhibitory control on D1 receptor-mediated locomotor stimulation, probably at the level of striatal projection neurons where the two receptors are coexpressed at high levels. Our findings offer new perspectives for the treatment of Parkinson's disease and other movement disorders that are characterized by an imbalance between muscarinic cholinergic and dopaminergic neurotransmission.     

Activation and dynamic network of the M2 muscarinic receptor

G-protein-coupled receptors (GPCRs) mediate cellular responses to various hormones and neurotransmitters and are important targets for treating a wide spectrum of diseases. Although significant advances have been made in structural studies of GPCRs, details of their activation mechanism remain unclear. The X-ray crystal structure of the M2 muscarinic receptor, a key GPCR that regulates human heart rate and contractile forces of cardiomyocytes, was determined recently in an inactive antagonist-bound state. Here, activation of the M2 receptor is directly observed via accelerated molecular dynamics simulation, in contrast to previous microsecond-timescale conventional molecular dynamics simulations in which the receptor remained inactive. Receptor activation is characterized by formation of a Tyr206^5.58–Tyr440^7.53 hydrogen bond and ∼6-Å outward tilting of the cytoplasmic end of transmembrane α-helix 6, preceded by relocation of Trp400^6.48 toward Phe195^5.47 and Val199^5.51 and flipping of Tyr430^7.43 away from the ligand-binding cavity. Network analysis reveals that communication in the intracellular domains is greatly weakened during activation of the receptor. Together with the finding that residue motions in the ligand-binding and G-protein-coupling sites of the apo receptor are correlated, this result highlights a dynamic network for allosteric regulation of the M2 receptor activation.