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Stahl E, Ellis J. Novel allosteric effects of amiodarone at the muscarinic M5 receptor. J Pharmacol Exp Ther 2010; 334:214-22. [PMID: 20348203 DOI: 10.1124/jpet.109.165316] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Allosteric sites on muscarinic receptors may present superior therapeutic targets for several central nervous system disorders, due to the potential of allosteric ligands to provide more selective modulation and to preserve the spatiotemporal patterning that is characteristic of synaptic transmission. We have found that the antiarrhythmic drug amiodarone interacts allosterically with M(1) and M(5) muscarinic receptors. At both M(1) and M(5), amiodarone was only able to partially inhibit the binding of the orthosteric antagonist [(3)H]N-methylscopolamine (NMS). In addition, amiodarone was able to alter the rate of dissociation of [(3)H]NMS from M(1) and M(5) receptors. These findings suggest that NMS and amiodarone are able to bind to the receptor simultaneously. The pharmacology of the effect on NMS dissociation demonstrated that amiodarone was not interacting at the "common" site at which gallamine, obidoxime, and many other muscarinic allosteric ligands are known to bind. In functional studies, amiodarone enhanced the ability of acetylcholine (at EC(20)) to activate the M(5) receptor; however, under the same conditions, amiodarone did not enhance M(1) activation. More detailed studies at M(5) found that the effect of amiodarone was to enhance the efficacy of acetylcholine, without increasing its potency. This report describes the first demonstration of allosteric enhancement of efficacy at the M(5) receptor, and the first demonstration of enhancement of efficacy but not potency at any muscarinic receptor. In summary, amiodarone has been shown to be a novel positive allosteric modulator of muscarinic receptors that is selective for the M(5) subtype, relative to M(1).
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Affiliation(s)
- Edward Stahl
- Department of Psychiatry, Penn State University College of Medicine, Hershey, PA 17033, USA
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102
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A subpopulation of neuronal M4 muscarinic acetylcholine receptors plays a critical role in modulating dopamine-dependent behaviors. J Neurosci 2010; 30:2396-405. [PMID: 20147565 DOI: 10.1523/jneurosci.3843-09.2010] [Citation(s) in RCA: 120] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Acetylcholine (ACh) regulates many key functions of the CNS by activating cell surface receptors referred to as muscarinic ACh receptors (M(1)-M(5) mAChRs). Like other mAChR subtypes, the M(4) mAChR is widely expressed in different regions of the forebrain. Interestingly, M(4) mAChRs are coexpressed with D(1) dopamine receptors in a specific subset of striatal projection neurons. To investigate the physiological relevance of this M(4) mAChR subpopulation in modulating dopamine-dependent behaviors, we used Cre/loxP technology to generate mutant mice that lack M(4) mAChRs only in D(1) dopamine receptor-expressing cells. The newly generated mutant mice displayed several striking behavioral phenotypes, including enhanced hyperlocomotor activity and increased behavioral sensitization following treatment with psychostimulants. These behavioral changes were accompanied by a lack of muscarinic inhibition of D(1) dopamine receptor-mediated cAMP stimulation in the striatum and an increase in dopamine efflux in the nucleus accumbens. These novel findings demonstrate that a distinct subpopulation of neuronal M(4) mAChRs plays a critical role in modulating several important dopamine-dependent behaviors. Since enhanced central dopaminergic neurotransmission is a hallmark of several severe disorders of the CNS, including schizophrenia and drug addiction, our findings have substantial clinical relevance.
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103
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Molecular mechanisms of action and in vivo validation of an M4 muscarinic acetylcholine receptor allosteric modulator with potential antipsychotic properties. Neuropsychopharmacology 2010; 35:855-69. [PMID: 19940843 PMCID: PMC3055367 DOI: 10.1038/npp.2009.194] [Citation(s) in RCA: 132] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
We recently identified LY2033298 as a novel allosteric potentiator of acetylcholine (ACh) at the M(4) muscarinic acetylcholine receptor (mAChR). This study characterized the molecular mode of action of this modulator in both recombinant and native systems. Radioligand-binding studies revealed that LY2033298 displayed a preference for the active state of the M(4) mAChR, manifested as a potentiation in the binding affinity of ACh (but not antagonists) and an increase in the proportion of high-affinity agonist-receptor complexes. This property accounted for the robust allosteric agonism displayed by the modulator in recombinant cells in assays of [(35)S]GTPgammaS binding, extracellular regulated kinase 1/2 phosphorylation, glycogen synthase kinase 3beta phosphorylation, and receptor internalization. We also found that the extent of modulation by LY2033298 differed depending on the signaling pathway, indicating that LY2033298 engenders functional selectivity in the actions of ACh. This property was retained in NG108-15 cells, which natively express rodent M(4) mAChRs. Functional interaction studies between LY2033298 and various orthosteric and allosteric ligands revealed that its site of action overlaps with the allosteric site used by prototypical mAChR modulators. Importantly, LY2033298 reduced [(3)H]ACh release from rat striatal slices, indicating retention of its ability to allosterically potentiate endogenous ACh in situ. Moreover, its ability to potentiate oxotremorine-mediated inhibition of condition avoidance responding in rodents was significantly attenuated in M(4) mAChR knockout mice, validating the M(4) mAChR as a key target of action of this novel allosteric ligand.
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104
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Modulation of prepulse inhibition through both M(1) and M (4) muscarinic receptors in mice. Psychopharmacology (Berl) 2010; 208:401-16. [PMID: 20013114 PMCID: PMC3895331 DOI: 10.1007/s00213-009-1740-z] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/07/2009] [Accepted: 11/17/2009] [Indexed: 12/11/2022]
Abstract
RATIONALE Muscarinic cholinergic M(1) and M(4) receptors may participate in schizophrenia's etiology and have been proposed as targets for antipsychotic medications. OBJECTIVE Here, we investigated the involvement of these receptors in behavioral measures pertinent to schizophrenia using knockout mice lacking M(1) receptors (M(1)-/-), M(4) receptors (M(4)-/-), or both (M(1)-/-M(4)-/-). METHODS We measured prepulse inhibition (PPI) of startle without drugs and after treatment with scopolamine (0.32-1.8 mg/kg), xanomeline (3.2 mg/kg), oxotremorine (0.032-0.1 mg/kg), clozapine (1.0-5.6 mg/kg), or haloperidol (0.32-3.2 mg/kg). RESULTS In female (but not male) mice, combined deletion of both M(1) and M(4) receptors decreased PPI relative to wild-type mice, while knockout of either receptor alone had no significant effect. Scopolamine disrupted PPI in wild-type and M(4)-/- mice, but not in female M(1)-/-M(4)-/- or female M(1)-/- mice. When administered before scopolamine, xanomeline restored PPI in wild-type mice and M(1)-/- mice, but not in M(4)-/- mice. In contrast, pretreatment with oxotremorine increased PPI regardless of genotype. Effects of clozapine and haloperidol on PPI were not hindered by either mutation. CONCLUSIONS Deletion of both M(1) and M(4) receptors can disrupt PPI, suggesting that (at least partially redundant) M(1) and M(4) receptor-dependent functions are involved in sensorimotor gating mechanisms. PPI-disrupting effects of muscarinic antagonists appeared dependent upon M(1) receptor blockade. Our data also suggest that xanomeline exerts antipsychotic-like effects mainly through M(4) receptor stimulation, while stimulation of non-M(1)/M(4) subtypes may also have antipsychotic potential. Finally, our results do not support a role of M(1)/M(4) receptors in mediating antipsychotic-like effects of clozapine.
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105
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Shirey JK, Brady AE, Jones PJ, Davis AA, Bridges TM, Kennedy JP, Jadhav SB, Menon UN, Xiang Z, Watson ML, Christian EP, Doherty JJ, Quirk MC, Snyder DH, Lah JJ, Levey AI, Nicolle MM, Lindsley CW, Conn PJ. A selective allosteric potentiator of the M1 muscarinic acetylcholine receptor increases activity of medial prefrontal cortical neurons and restores impairments in reversal learning. J Neurosci 2009; 29:14271-86. [PMID: 19906975 PMCID: PMC2811323 DOI: 10.1523/jneurosci.3930-09.2009] [Citation(s) in RCA: 194] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2009] [Accepted: 09/08/2009] [Indexed: 01/10/2023] Open
Abstract
M(1) muscarinic acetylcholine receptors (mAChRs) may represent a viable target for treatment of disorders involving impaired cognitive function. However, a major limitation to testing this hypothesis has been a lack of highly selective ligands for individual mAChR subtypes. We now report the rigorous molecular characterization of a novel compound, benzylquinolone carboxylic acid (BQCA), which acts as a potent, highly selective positive allosteric modulator (PAM) of the rat M(1) receptor. This compound does not directly activate the receptor, but acts at an allosteric site to increase functional responses to orthosteric agonists. Radioligand binding studies revealed that BQCA increases M(1) receptor affinity for acetylcholine. We found that activation of the M(1) receptor by BQCA induces a robust inward current and increases spontaneous EPSCs in medial prefrontal cortex (mPFC) pyramidal cells, effects which are absent in acute slices from M(1) receptor knock-out mice. Furthermore, to determine the effect of BQCA on intact and functioning brain circuits, multiple single-unit recordings were obtained from the mPFC of rats that showed BQCA increases firing of mPFC pyramidal cells in vivo. BQCA also restored discrimination reversal learning in a transgenic mouse model of Alzheimer's disease and was found to regulate non-amyloidogenic APP processing in vitro, suggesting that M(1) receptor PAMs have the potential to provide both symptomatic and disease modifying effects in Alzheimer's disease patients. Together, these studies provide compelling evidence that M(1) receptor activation induces a dramatic excitation of PFC neurons and suggest that selectively activating the M(1) mAChR subtype may ameliorate impairments in cognitive function.
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Affiliation(s)
| | | | | | - Albert A. Davis
- Center for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, Georgia 30322
| | | | | | | | | | | | - Mona L. Watson
- Departments of Internal Medicine/Gerontology and
- Physiology and Pharmacology, Wake Forest University School of Medicine, Winston Salem, North Carolina 27157
| | - Edward P. Christian
- Department of Neuroscience, AstraZeneca Pharmaceuticals, Wilmington, Delaware, 19850-5437, and
| | - James J. Doherty
- Department of Neuroscience, AstraZeneca Pharmaceuticals, Wilmington, Delaware, 19850-5437, and
| | - Michael C. Quirk
- Department of Neuroscience, AstraZeneca Pharmaceuticals, Wilmington, Delaware, 19850-5437, and
| | - Dean H. Snyder
- Department of Neuroscience, AstraZeneca Pharmaceuticals, Wilmington, Delaware, 19850-5437, and
| | - James J. Lah
- Center for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, Georgia 30322
| | - Allan I. Levey
- Center for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, Georgia 30322
| | - Michelle M. Nicolle
- Departments of Internal Medicine/Gerontology and
- Physiology and Pharmacology, Wake Forest University School of Medicine, Winston Salem, North Carolina 27157
| | - Craig W. Lindsley
- Departments of Pharmacology and
- Chemistry
- Vanderbilt Program in Drug Discovery, Vanderbilt University Medical Center, Nashville, Tennessee 37232-6600
| | - P. Jeffrey Conn
- Departments of Pharmacology and
- Vanderbilt Program in Drug Discovery, Vanderbilt University Medical Center, Nashville, Tennessee 37232-6600
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106
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107
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Acetylcholine (muscarinic). Br J Pharmacol 2009. [DOI: 10.1111/j.1476-5381.2009.00501_4.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
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108
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Kennedy JP, Bridges TM, Gentry PR, Brogan JT, Kane AS, Jones CK, Brady AE, Shirey JK, Conn PJ, Lindsley CW. Synthesis and structure-activity relationships of allosteric potentiators of the m(4) muscarinic acetylcholine receptor. ChemMedChem 2009; 4:1600-7. [PMID: 19705385 PMCID: PMC2887613 DOI: 10.1002/cmdc.200900231] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2009] [Indexed: 11/08/2022]
Affiliation(s)
- J. Phillip Kennedy
- Department of Chemistry, Vanderbilt University 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - Thomas M. Bridges
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - Patrick R. Gentry
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - John T. Brogan
- Department of Chemistry, Vanderbilt University 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - Alexander S. Kane
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - Carrie K. Jones
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
- U.S. Department of Veterans Affairs, Tennessee Valley Healthcare System (TVHS), 1310 24th Avenue South, Nashville, TN 37212 (USA)
| | - Ashley E. Brady
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - Jana K. Shirey
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - P. Jeffrey Conn
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
| | - Craig W. Lindsley
- Department of Chemistry, Vanderbilt University 12475 MRB4, Nashville, TN 37232-6600 (USA)
- Vanderbilt Program in Drug Discovery, Department of Pharmacology Vanderbilt University Medical Center 12475 MRB4, Nashville, TN 37232-6600 (USA)
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109
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Lebois EP, Bridges TM, Lewis LM, Dawson ES, Kane AS, Xiang Z, Jadhav SB, Yin H, Kennedy JP, Meiler J, Niswender CM, Jones CK, Conn PJ, Weaver CD, Lindsley CW. Discovery and characterization of novel subtype-selective allosteric agonists for the investigation of M(1) receptor function in the central nervous system. ACS Chem Neurosci 2009; 1:104-121. [PMID: 21961051 DOI: 10.1021/cn900003h] [Citation(s) in RCA: 84] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Cholinergic transmission in the forebrain is mediated primarily by five subtypes of muscarinic acetylcholine receptors (mAChRs), termed M(1)-M(5). Of the mAChR subtypes, M(1) is among the most heavily expressed in regions that are critical for learning and memory, and has been viewed as the most critical mAChR subtype for memory and attention mechanisms. Unfortunately, it has been difficult to develop selective activators of M(1) and other individual mAChR subtypes, which has prevented detailed studies of the functional roles of selective activation of M(1). Using a functional HTS screen and subsequent diversity-oriented synthesis approach we have discovered a novel series of highly selective M(1) allosteric agonists. These compounds activate M(1) with EC(50) values in the 150 nM to 500 nM range and have unprecedented, clean ancillary pharmacology (no substantial activity at 10μM across a large panel of targets). Targeted mutagenesis revealed a potentially novel allosteric binding site in the third extracellular loop of the M(1) receptor for these allosteric agonists. Optimized compounds, such as VU0357017, provide excellent brain exposure after systemic dosing and have robust in vivo efficacy in reversing scopolamine-induced deficits in a rodent model of contextual fear conditioning. This series of selective M(1) allosteric agonists provides critical research tools to allow dissection of M(1)-mediated effects in the CNS and potential leads for novel treatments for Alzheimer's disease and schizophrenia.
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Affiliation(s)
| | | | - L. Michelle Lewis
- Department of Pharmacology
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
| | - Eric S Dawson
- Department of Chemistry
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
- Vanderbilt Program in Drug Discovery
- Vanderbilt Center for Structural Biology
| | | | - Zixiu Xiang
- Department of Pharmacology
- Vanderbilt Program in Drug Discovery
| | | | - Huiyong Yin
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
| | | | - Jens Meiler
- Department of Chemistry
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
- Vanderbilt Program in Drug Discovery
- Vanderbilt Center for Structural Biology
| | | | - Carrie K Jones
- Department of Pharmacology
- Vanderbilt Program in Drug Discovery
| | - P Jeffrey Conn
- Department of Pharmacology
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
- Vanderbilt Program in Drug Discovery
| | - C David Weaver
- Department of Pharmacology
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
- Vanderbilt Program in Drug Discovery
| | - Craig W Lindsley
- Department of Pharmacology
- Department of Chemistry
- Vanderbilt Specialized Chemistry Center for Accelerated Probe Development (MLPCN)
- Vanderbilt Program in Drug Discovery
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110
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Thomas RL, Langmead CJ, Wood MD, Challiss RAJ. Contrasting effects of allosteric and orthosteric agonists on m1 muscarinic acetylcholine receptor internalization and down-regulation. J Pharmacol Exp Ther 2009; 331:1086-95. [PMID: 19767446 DOI: 10.1124/jpet.109.160242] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
A new class of subtype-selective muscarinic acetylcholine (mACh) receptor agonist that activates the receptor through interaction at a site distinct from the orthosteric acetylcholine binding site has been reported recently. Here, we have compared the effects of orthosteric (oxotremorine-M, arecoline, pilocarpine) and allosteric [4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine (AC-42); 1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone (77-LH-28-1)] agonists on M(1) mACh receptor internalization and down-regulation, as well as functional coupling in a Chinese hamster ovary (CHO) cell line. In contrast to full and partial orthosteric agonists, which cause significant receptor internalization and down-regulation, prolonged exposure to AC-42 did not significantly alter either cell-surface or total cellular M(1) mACh receptor expression. 77-LH-28-1, an AC-42 homolog, did cause some receptor internalization, but not down-regulation. The presence of atropine completely prevented the orthosteric agonist-induced adaptive changes in receptor populations; however, in contrast, the copresence of atropine and AC-42 significantly increased both cell-surface receptor and total M(1) mACh receptor expression. Maximal phosphoinositide hydrolysis responses to the partial agonist arecoline were similar in CHO-M(1) cells pretreated for 24 h with either AC-42 or vehicle; in contrast, these responses were markedly reduced when cells were pretreated with oxotremorine-M or pilocarpine. These data indicate that, whereas AC-42 binding to the M(1) mACh receptor can initiate signal transduction, the AC-42-liganded receptor is resistant to the usual mechanisms regulating receptor internalization and down-regulation. In addition, our data suggest unusual interactions between allosteric agonists and orthosteric antagonists to regulate cell-surface and total cellular receptor expression.
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Affiliation(s)
- Rachel L Thomas
- Department of Cell Physiology and Pharmacology, University of Leicester, Room 4/04, Henry Wellcome Building, Lancaster Road, Leicester, LE1 9HN, United Kingdom
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111
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Wang L, Martin B, Brenneman R, Luttrell LM, Maudsley S. Allosteric modulators of g protein-coupled receptors: future therapeutics for complex physiological disorders. J Pharmacol Exp Ther 2009; 331:340-8. [PMID: 19667132 DOI: 10.1124/jpet.109.156380] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
G protein-coupled receptors (GPCRs) are one of the most important classes of proteins in the genome, not only because of their tremendous molecular diversity but because they are the targets of nearly 50% of current pharmacotherapeutics. The majority of these drugs affect GPCR activity by binding to a similar molecular site as the endogenous cognate ligand for the receptor. These "orthosterically" targeted drugs currently dominate the existing pharmacopeia. Over the past two decades, novel opportunities for drug discovery have risen from a greater understanding of the complexity of GPCR signaling. A striking example of this is the appreciation that many GPCRs possess functional allosteric binding sites. Allosteric modulator ligands bind receptor domains topographically distinct from the orthosteric site, altering the biological activity of the orthosteric ligand by changing its binding affinity, functional efficacy, or both. This additional receptor signaling complexity can be embraced and exploited for the next generation of GPCR-targeted therapies. Despite the challenges associated with detecting and quantifying the myriad of possible allosteric effects on GPCR activity, allosteric ligands offer the prospect of engendering a facile stimulus-bias in orthosteric ligand signaling, paving the way for not only receptor-selective but also signaling pathway-selective therapies. Allosteric modulators possess specific advantages when considering the treatment of multifactorial syndromes, such as metabolic diseases or age-related cognitive impairment, because they may not greatly affect neurotransmitter or hormone release patterns, thus maintaining the integrity of complex signaling networks that underlie perception, memory patterns, or neuroendocrinological axes while introducing therapeutically beneficial signal bias.
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Affiliation(s)
- Liyun Wang
- Receptor Pharmacology Unit, and Metabolism Unit, the National Institutes of Health, National Institute on Aging, Baltimore, Maryland 21224, USA
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112
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Sheffler DJ, Williams R, Bridges TM, Xiang Z, Kane AS, Byun NE, Jadhav S, Mock MM, Zheng F, Lewis LM, Jones CK, Niswender CM, Weaver CD, Lindsley CW, Conn PJ. A novel selective muscarinic acetylcholine receptor subtype 1 antagonist reduces seizures without impairing hippocampus-dependent learning. Mol Pharmacol 2009; 76:356-68. [PMID: 19407080 DOI: 10.1124/mol.109.056531] [Citation(s) in RCA: 103] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Previous studies suggest that selective antagonists of specific subtypes of muscarinic acetylcholine receptors (mAChRs) may provide a novel approach for the treatment of certain central nervous system (CNS) disorders, including epileptic disorders, Parkinson's disease, and dystonia. Unfortunately, previously reported antagonists are not highly selective for specific mAChR subtypes, making it difficult to definitively establish the functional roles and therapeutic potential for individual subtypes of this receptor subfamily. The M(1) mAChR is of particular interest as a potential target for treatment of CNS disorders. We now report the discovery of a novel selective antagonist of M(1) mAChRs, termed VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide]. Equilibrium radioligand binding and functional studies demonstrate a greater than 75-fold selectivity of VU0255035 for M(1) mAChRs relative to M(2)-M(5). Molecular pharmacology and mutagenesis studies indicate that VU0255035 is a competitive orthosteric antagonist of M(1) mAChRs, a surprising finding given the high level of M(1) mAChR selectivity relative to other orthosteric antagonists. Whole-cell patch-clamp recordings demonstrate that VU0255035 inhibits potentiation of N-methyl-D-aspartate receptor currents by the muscarinic agonist carbachol in hippocampal pyramidal cells. VU0255035 has excellent brain penetration in vivo and is efficacious in reducing pilocarpine-induced seizures in mice. We were surprised to find that doses of VU0255035 that reduce pilocarpine-induced seizures do not induce deficits in contextual freezing, a measure of hippocampus-dependent learning that is disrupted by nonselective mAChR antagonists. Taken together, these data suggest that selective antagonists of M(1) mAChRs do not induce the severe cognitive deficits seen with nonselective mAChR antagonists and could provide a novel approach for the treatment certain of CNS disorders.
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Affiliation(s)
- Douglas J Sheffler
- Department of Pharmacology, Vanderbilt University Medical Center, Nashville, TN 37232, USA
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113
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Servent D, Fruchart-Gaillard C. Muscarinic toxins: tools for the study of the pharmacological and functional properties of muscarinic receptors. J Neurochem 2009; 109:1193-202. [PMID: 19457160 DOI: 10.1111/j.1471-4159.2009.06092.x] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Muscarinic receptors mediate metabotropic actions of acetylcholine in the CNS and PNS and autocrine functions of acetylcholine in non-neuronal systems. Because of the lack of highly selective muscarinic ligands, the precise location, functional role, and roles in various diseases of the five muscarinic receptor subtypes remain unclear. Muscarinic toxins isolated from the venom of Dendroaspis snakes have a natural high affinity and selectivity, associated with roles as competitive antagonists, allosteric modulators, and potential agonists. These toxins may therefore be invaluable tools for studying muscarinic receptors. We review data on the structural and pharmacological characterization of the muscarinic toxins, focusing on recent structure-function studies on toxin-receptor interactions. We discuss the potential benefits of using these toxins for investigating muscarinic function in vivo.
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Affiliation(s)
- Denis Servent
- CEA, iBiTecS, Service d'Ingénierie Moléculaire des Protéines (SIMOPRO), Gif sur Yvette, France.
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114
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Marlo JE, Niswender CM, Days EL, Bridges TM, Xiang Y, Rodriguez AL, Shirey JK, Brady AE, Nalywajko T, Luo Q, Austin CA, Williams MB, Kim K, Williams R, Orton D, Brown HA, Lindsley CW, Weaver CD, Conn PJ. Discovery and characterization of novel allosteric potentiators of M1 muscarinic receptors reveals multiple modes of activity. Mol Pharmacol 2009; 75:577-88. [PMID: 19047481 PMCID: PMC2684909 DOI: 10.1124/mol.108.052886] [Citation(s) in RCA: 115] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2008] [Accepted: 12/01/2008] [Indexed: 01/17/2023] Open
Abstract
Activators of M(1) muscarinic acetylcholine receptors (mAChRs) may provide novel treatments for schizophrenia and Alzheimer's disease. Unfortunately, the development of M(1)-active compounds has resulted in nonselective activation of the highly related M(2) to M(5) mAChR subtypes, which results in dose-limiting side effects. Using a functional screening approach, we identified several novel ligands that potentiated agonist activation of M(1) with low micromolar potencies and induced 5-fold or greater leftward shifts of the acetylcholine (ACh) concentration-response curve. These ligands did not compete for binding at the ACh binding site, indicating that they modulate receptor activity by binding to allosteric sites. The two most selective compounds, cyclopentyl 1,6-dimethyl-4-(6-nitrobenzo[d][1,3]-dioxol-5-yl)-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (VU0090157) and (E)-2-(4-ethoxyphenylamino)-N'-((2-hydroxynaphthalen-1-yl)methylene)acetohydrazide (VU0029767), induced progressive shifts in ACh affinity at M(1) that were consistent with their effects in a functional assay, suggesting that the mechanism for enhancement of M(1) activity by these compounds is by increasing agonist affinity. These compounds were strikingly different, however, in their ability to potentiate responses at a mutant M(1) receptor with decreased affinity for ACh and in their ability to affect responses of the allosteric M(1) agonist, 1-[1'-(2-tolyl)-1,4'-bipiperidin-4-yl]-1,3-dihydro-2H-benzimidazol-2-one. Furthermore, these two compounds were distinct in their abilities to potentiate M(1)-mediated activation of phosphoinositide hydrolysis and phospholipase D. The discovery of multiple structurally distinct positive allosteric modulators of M(1) is an exciting advance in establishing the potential of allosteric modulators for selective activation of this receptor. These data also suggest that structurally diverse M(1) potentiators may act by distinct mechanisms and differentially regulate receptor coupling to downstream signaling pathways.
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Affiliation(s)
- Joy E Marlo
- Department of Pharmacology, Vanderbilt University, Nashville, Tennessee 37232, USA
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115
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Sánchez G, de Oliveira Alvares L, Oberholzer MV, Genro B, Quillfeldt J, da Costa JC, Cerveñansky C, Jerusalinsky D, Kornisiuk E. M4muscarinic receptors are involved in modulation of neurotransmission at synapses of Schaffer collaterals on CA1 hippocampal neurons in rats. J Neurosci Res 2009; 87:691-700. [DOI: 10.1002/jnr.21876] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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116
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Subtype-selective allosteric modulators of muscarinic receptors for the treatment of CNS disorders. Trends Pharmacol Sci 2009; 30:148-55. [PMID: 19201489 DOI: 10.1016/j.tips.2008.12.002] [Citation(s) in RCA: 226] [Impact Index Per Article: 15.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2008] [Revised: 12/01/2008] [Accepted: 12/03/2008] [Indexed: 01/21/2023]
Abstract
Muscarinic acetylcholine receptors (mAChRs) have long been viewed as viable targets for novel therapeutic agents for the treatment of Alzheimer's disease (AD) and other disorders involving impaired cognitive function. More recent evidence indicates that mAChR activators might also have utility in treating psychosis and other symptoms associated with schizophrenia and other central nervous system (CNS) disorders. Efforts to develop mAChR subtype-selective agonists have been hampered by difficulty in achieving high selectivity for individual mAChR subtypes important for CNS function (M(1) and M(4)) and adverse effects due to activation of peripheral mAChRs (especially M(2) and M(3)). Major advances have now been achieved in the discovery of allosteric agonists and positive allosteric modulators of M(1) and M(4) that show greater selectivity for individual mAChR subtypes than do previous mAChR agonists. Early studies indicate that these allosteric mAChR activators have properties needed for optimization as potential clinical candidates and have robust effects in animal models that predict efficacy in the treatment of AD, schizophrenia and related disorders.
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117
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Conn PJ, Christopoulos A, Lindsley CW. Allosteric modulators of GPCRs: a novel approach for the treatment of CNS disorders. Nat Rev Drug Discov 2009; 8:41-54. [PMID: 19116626 PMCID: PMC2907734 DOI: 10.1038/nrd2760] [Citation(s) in RCA: 825] [Impact Index Per Article: 55.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
Despite G-protein-coupled receptors (GPCRs) being among the most fruitful targets for marketed drugs, intense discovery efforts for several GPCR subtypes have failed to deliver selective drug candidates. Historically, drug discovery programmes for GPCR ligands have been dominated by efforts to develop agonists and antagonists that act at orthosteric sites for endogenous ligands. However, in recent years, there have been tremendous advances in the discovery of novel ligands for GPCRs that act at allosteric sites to regulate receptor function. These compounds provide high selectivity, novel modes of efficacy and may lead to novel therapeutic agents for the treatment of multiple psychiatric and neurological human disorders.
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Affiliation(s)
- P Jeffrey Conn
- Department of Pharmacology, Vanderbilt Program in Drug Discovery, Vanderbilt Medical Center, 1215 Light Hall, Nashville, Tennessee 37232, USA.
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118
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Brady AE, Jones CK, Bridges TM, Kennedy JP, Thompson AD, Heiman JU, Breininger ML, Gentry PR, Yin H, Jadhav SB, Shirey JK, Conn PJ, Lindsley CW. Centrally active allosteric potentiators of the M4 muscarinic acetylcholine receptor reverse amphetamine-induced hyperlocomotor activity in rats. J Pharmacol Exp Ther 2008; 327:941-53. [PMID: 18772318 PMCID: PMC2745822 DOI: 10.1124/jpet.108.140350] [Citation(s) in RCA: 163] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Previous clinical and animal studies suggest that selective activators of M(1) and/or M(4) muscarinic acetylcholine receptors (mAChRs) have potential as novel therapeutic agents for treatment of schizophrenia and Alzheimer's disease. However, highly selective centrally penetrant activators of either M(1) or M(4) have not been available, making it impossible to determine the in vivo effects of selective activation of these receptors. We previously identified VU10010 [3-amino-N-(4-chlorobenzyl)-4, 6-dimethylthieno[2,3-b]pyridine-2-carboxamide] as a potent and selective allosteric potentiator of M(4) mAChRs. However, unfavorable physiochemical properties prevented use of this compound for in vivo studies. We now report that chemical optimization of VU10010 has afforded two centrally penetrant analogs, VU0152099 [3-amino-N-(benzo[d][1,3]dioxol-5-ylmethyl)-4,6-dimethylthieno[2,3-b]pyridine carboxamide] and VU0152100 [3-amino-N-(4-methoxybenzyl)-4,6-dimethylthieno[2,3-b]pyridine carboxamide], that are potent and selective positive allosteric modulators of M(4). VU0152099 and VU0152100 had no agonist activity but potentiated responses of M(4) to acetylcholine. Both compounds were devoid of activity at other mAChR subtypes or at a panel of other GPCRs. The improved physiochemical properties of VU0152099 and VU0152100 allowed in vivo dosing and evaluation of behavioral effects in rats. Interestingly, these selective allosteric potentiators of M(4) reverse amphetamine-induced hyperlocomotion in rats, a model that is sensitive to known antipsychotic agents and to nonselective mAChR agonists. This is consistent with the hypothesis that M(4) plays an important role in regulating midbrain dopaminergic activity and raises the possibility that positive allosteric modulation of M(4) may mimic some of the antipsychotic-like effects of less selective mAChR agonists.
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Affiliation(s)
- Ashley E Brady
- Department of Pharmacology, Vanderbilt Program in Drug Discovery, Vanderbilt University Medical Center, Nashville, TN 37232-6600, USA
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119
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Jones CK, Brady AE, Davis AA, Xiang Z, Bubser M, Tantawy MN, Kane AS, Bridges TM, Kennedy JP, Bradley SR, Peterson TE, Ansari MS, Baldwin RM, Kessler RM, Deutch AY, Lah JJ, Levey AI, Lindsley CW, Conn PJ. Novel selective allosteric activator of the M1 muscarinic acetylcholine receptor regulates amyloid processing and produces antipsychotic-like activity in rats. J Neurosci 2008; 28:10422-33. [PMID: 18842902 PMCID: PMC2577155 DOI: 10.1523/jneurosci.1850-08.2008] [Citation(s) in RCA: 194] [Impact Index Per Article: 12.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2008] [Revised: 07/20/2008] [Accepted: 07/30/2008] [Indexed: 11/21/2022] Open
Abstract
Recent studies suggest that subtype-selective activators of M(1)/M(4) muscarinic acetylcholine receptors (mAChRs) may offer a novel approach for the treatment of psychotic symptoms associated with schizophrenia and Alzheimer's disease. Previously developed muscarinic agonists have provided clinical data in support of this hypothesis, but failed in clinical development because of a lack of true subtype specificity and adverse effects associated with activation of other mAChR subtypes. We now report characterization of a novel highly selective agonist for the M(1) receptor with no agonist activity at any of the other mAChR subtypes, termed TBPB [1-(1'-2-methylbenzyl)-1,4'-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one]. Mutagenesis and molecular pharmacology studies revealed that TBPB activates M(1) through an allosteric site rather than the orthosteric acetylcholine binding site, which is likely critical for its unprecedented selectivity. Whole-cell patch-clamp recordings demonstrated that activation of M(1) by TBPB potentiates NMDA receptor currents in hippocampal pyramidal cells but does not alter excitatory or inhibitory synaptic transmission, responses thought to be mediated by M(2) and M(4). TBPB was efficacious in models predictive of antipsychotic-like activity in rats at doses that did not produce catalepsy or peripheral adverse effects of other mAChR agonists. Finally, TBPB had effects on the processing of the amyloid precursor protein toward the non-amyloidogenic pathway and decreased Abeta production in vitro. Together, these data suggest that selective activation of M(1) may provide a novel approach for the treatment of symptoms associated with schizophrenia and Alzheimer's disease.
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Affiliation(s)
- Carrie K. Jones
- Departments of Pharmacology
- Vanderbilt Program in Drug Discovery, and
| | - Ashley E. Brady
- Departments of Pharmacology
- Vanderbilt Program in Drug Discovery, and
| | - Albert A. Davis
- Center for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, Georgia 30322
| | | | | | - Mohammed Noor Tantawy
- Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, Tennessee 37232
| | | | | | | | | | - Todd E. Peterson
- Radiology and Radiological Sciences/PET Chemistry
- Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, Tennessee 37232
| | - M. Sib Ansari
- Radiology and Radiological Sciences/PET Chemistry
- Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, Tennessee 37232
| | - Ronald M. Baldwin
- Radiology and Radiological Sciences/PET Chemistry
- Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, Tennessee 37232
| | | | | | - James J. Lah
- Center for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, Georgia 30322
| | - Allan I. Levey
- Center for Neurodegenerative Disease and Department of Neurology, Emory University, Atlanta, Georgia 30322
| | - Craig W. Lindsley
- Departments of Pharmacology
- Chemistry
- Vanderbilt Program in Drug Discovery, and
| | - P. Jeffrey Conn
- Departments of Pharmacology
- Vanderbilt Program in Drug Discovery, and
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120
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Antony J, Kellershohn K, Mohr-Andrä M, Kebig A, Prilla S, Muth M, Heller E, Disingrini T, Dallanoce C, Bertoni S, Schrobang J, Tränkle C, Kostenis E, Christopoulos A, Höltje HD, Barocelli E, De Amici M, Holzgrabe U, Mohr K. Dualsteric GPCR targeting: a novel route to binding and signaling pathway selectivity. FASEB J 2008; 23:442-50. [PMID: 18842964 DOI: 10.1096/fj.08-114751] [Citation(s) in RCA: 116] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Selective modulation of cell function by G protein-coupled receptor (GPCR) activation is highly desirable for basic research and therapy but difficult to achieve. We present a novel strategy toward this goal using muscarinic acetylcholine receptors as a model. The five subtypes bind their physiological transmitter in the highly conserved orthosteric site within the transmembrane domains of the receptors. Orthosteric muscarinic activators have no binding selectivity and poor signaling specificity. There is a less well conserved allosteric site at the extracellular entrance of the binding pocket. To gain subtype-selective receptor activation, we synthesized two hybrids fusing a highly potent oxotremorine-like orthosteric activator with M(2)-selective bis(ammonio)alkane-type allosteric fragments. Radioligand binding in wild-type and mutant receptors supplemented by receptor docking simulations proved M(2) selective and true allosteric/orthosteric binding. G protein activation measurements using orthosteric and allosteric blockers identified the orthosteric part of the hybrid to engender receptor activation. Hybrid-induced dynamic mass redistribution in CHO-hM(2) cells disclosed pathway-specific signaling. Selective receptor activation (M(2)>M(1)>M(3)) was verified in living tissue preparations. As allosteric sites are increasingly recognized on GPCRs, the dualsteric concept of GPCR targeting represents a new avenue toward potent agonists for selective receptor and signaling pathway activation.
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Affiliation(s)
- Johannes Antony
- Pharmacology and Toxicology Section, Institute of Pharmacy, Rheinische Friedrich-Wilhelms-University, Bonn, Germany
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121
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Bridges TM, Lindsley CW. G-protein-coupled receptors: from classical modes of modulation to allosteric mechanisms. ACS Chem Biol 2008; 3:530-41. [PMID: 18652471 DOI: 10.1021/cb800116f] [Citation(s) in RCA: 135] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Heterotrimeric G-protein-coupled receptors (GPCRs) represent a large protein family responsible for mediating extracellular to intracellular signaling within a broad range of physiological contexts. Various conventional models have been used to describe their interactions with ligands and G-proteins. In recent years, however, numerous novel ligand-receptor interactions not adequately addressed by classical receptor theory have been recognized. In addition to traditional orthosteric ligands, many GPCRs can bind allosteric ligands that modulate receptor activity by interacting with distinct or overlapping receptor sites. Such ligands include positive allosteric modulators, which have become the focus of pharmaceutical drug discovery programs and have gained the attention of a growing body of basic and translational researchers within the academic community. Here, we review the fundamental aspects of allosteric GPCR modulation by small-molecule ligands, with particular focus on the emerging position of positive allosteric modulators in modern drug discovery.
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Affiliation(s)
- Thomas M. Bridges
- Vanderbilt Program in Drug Discovery, Department of Pharmacology, Vanderbilt Medical Center, Nashville, Tennessee 37232
| | - Craig W. Lindsley
- Vanderbilt Program in Drug Discovery, Department of Pharmacology, Vanderbilt Medical Center, Nashville, Tennessee 37232
- Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
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122
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Nawaratne V, Leach K, Suratman N, Loiacono RE, Felder CC, Armbruster BN, Roth BL, Sexton PM, Christopoulos A. New Insights into the Function of M4 Muscarinic Acetylcholine Receptors Gained Using a Novel Allosteric Modulator and a DREADD (Designer Receptor Exclusively Activated by a Designer Drug). Mol Pharmacol 2008; 74:1119-31. [DOI: 10.1124/mol.108.049353] [Citation(s) in RCA: 93] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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123
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Lewis JA, Lebois EP, Lindsley CW. Allosteric modulation of kinases and GPCRs: design principles and structural diversity. Curr Opin Chem Biol 2008; 12:269-80. [PMID: 18342020 DOI: 10.1016/j.cbpa.2008.02.014] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2008] [Revised: 02/05/2008] [Accepted: 02/07/2008] [Indexed: 12/01/2022]
Abstract
Allosteric binding sites, as opposed to traditional orthosteric binding sites, offer unparalleled opportunities for drug discovery by providing high levels of selectivity, mimicking physiological conditions, affording fewer side effects because of desensitization/downregulation, and engendering ligands with chemotypes divergent from orthosteric ligands. For kinases, allosteric mechanisms described to date include alteration of protein kinase conformation blocking productive ATP binding which appear 'ATP competitive' or blocking kinase activation by conformational changes that are 'ATP non-competitive'. For GPCRs, allosteric mechanisms impart multiple modes of target modulation (positive allosteric modulation (PAM), negative allosteric modulation (NAM), neutral cooperativity, partial antagonism (PA), allosteric agonism and allosteric antagonism). Here, we review recent developments in the design principles and structural diversity of allosteric ligands for kinases and GPCRs.
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Affiliation(s)
- Jana A Lewis
- Department of Pharmacology and Chemistry, Vanderbilt Program in Drug Discovery, Vanderbilt Institute of Chemical Biology, Vanderbilt University Medical Center, Nashville, TN 37232-6600, USA.
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124
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Niswender CM, Johnson KA, Luo Q, Ayala JE, Kim C, Conn PJ, Weaver CD. A novel assay of Gi/o-linked G protein-coupled receptor coupling to potassium channels provides new insights into the pharmacology of the group III metabotropic glutamate receptors. Mol Pharmacol 2008; 73:1213-24. [PMID: 18171729 DOI: 10.1124/mol.107.041053] [Citation(s) in RCA: 93] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The group III metabotropic glutamate receptors (mGluRs) represent a family of presynaptically expressed G-protein-coupled receptors (GPCRs) with enormous therapeutic potential; however, robust cellular assays to study their function have been difficult to develop. We present here a new assay, compatible with traditional high-throughput screening platforms, to detect activity of pharmacological ligands interacting with G(i/o)-coupled GPCRs, including the group III mGluRs 4, 7, and 8. The assay takes advantage of the ability of the Gbetagamma subunits of G(i) and G(o) heterotrimers to interact with G-protein regulated inwardly rectifying potassium channels (GIRKs), and we show here that we are able to detect the activity of multiple types of pharmacophores including agonists, antagonists, and allosteric modulators of several distinct GPCRs. Using GIRK-mediated thallium flux, we perform a side-by-side comparison of the activity of a number of commercially available compounds, some of which have not been extensively evaluated because of the previous lack of robust assays at each of the three major group III mGluRs. It is noteworthy that several compounds previously considered to be general group III mGluR antagonists have very weak activity using this assay, suggesting the possibility that these compounds may not effectively inhibit these receptors in native systems. We anticipate that the GIRK-mediated thallium flux strategy will provide a novel tool to advance the study of G(i/o)-coupled GPCR biology and promote ligand discovery and characterization.
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Affiliation(s)
- Colleen M Niswender
- 1215 MRB IV, Department of Pharmacology, Vanderbilt University, Nashville, TN 37232, USA.
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