Fusion with Promiscuous Gα16 Subunit Reveals Signaling Bias at Muscarinic Receptors.

A complex evaluation of agonist bias at G-protein coupled receptors at the level of G-protein classes and isoforms including non-preferential ones is essential for advanced agonist screening and drug development. Molecular crosstalk in downstream signaling and a lack of sufficiently sensitive and selective methods to study direct coupling with G-protein of interest complicates this analysis. We performed binding and functional analysis of 11 structurally different agonists on prepared fusion proteins of individual subtypes of muscarinic receptors and non-canonical promiscuous α-subunit of G16 protein to study agonist bias. We have demonstrated that fusion of muscarinic receptors with Gα16 limits access of other competitive Gα subunits to the receptor, and thus enables us to study activation of Gα16 mediated pathway more specifically. Our data demonstrated agonist-specific activation of G16 pathway among individual subtypes of muscarinic receptors and revealed signaling bias of oxotremorine towards Gα16 pathway at the M2 receptor and at the same time impaired Gα16 signaling of iperoxo at M5 receptors. Our data have shown that fusion proteins of muscarinic receptors with α-subunit of G-proteins can serve as a suitable tool for studying agonist bias, especially at non-preferential pathways.

Oxotremorine-M potentiates NMDA receptors by muscarinic receptor dependent and independent mechanisms.

Muscarinic acetylcholine M1 receptors play an important role in synaptic plasticity in the hippocampus and cortex. Potentiation of NMDA receptors as a consequence of muscarinic acetylcholine M1 receptor activation is a crucial event mediating the cholinergic modulation of synaptic plasticity, which is a cellular mechanism for learning and memory. In Alzheimer's disease, the cholinergic input to the hippocampus and cortex is severely degenerated, and agonists or positive allosteric modulators of M1 receptors are therefore thought to be of potential use to treat the deficits in cognitive functions in Alzheimer's disease. In this study we developed a simple system in which muscarinic modulation of NMDA receptors can be studied in vitro. Human M1 receptors and NR1/2B NMDA receptors were co-expressed in Xenopus oocytes and various muscarinic agonists were assessed for their modulatory effects on NMDA receptor-mediated responses. As expected, NMDA receptor-mediated responses were potentiated by oxotremorine-M, oxotremorine or xanomeline when the drugs were applied between subsequent NMDA responses, an effect which was fully blocked by the muscarinic receptor antagonist atropine. However, in oocytes expressing NR1/2B NMDA receptors but not muscarinic M1 receptors, oxotremorine-M co-applied with NMDA also resulted in a potentiation of NMDA currents and this effect was not blocked by atropine, demonstrating that oxotremorine-M is able to directly potentiate NMDA receptors. Oxotremorine, which is a close analogue of oxotremorine-M, and xanomeline, a chemically distinct muscarinic agonist, did not potentiate NMDA receptors by this direct mechanism. Comparing the chemical structures of the three different muscarinic agonists used in this study suggests that the tri-methyl ammonium moiety present in oxotremorine-M is important for the compound's interaction with NMDA receptors.

A simple method for quantifying functional selectivity and agonist bias.

Activation of seven-transmembrane (7TM) receptors by agonists does not always lead to uniform activation of all signaling pathways mediated by a given receptor. Relative to other ligands, many agonists are "biased" toward producing subsets of receptor behaviors. A hallmark of such "functional selectivity" is cell type dependence; this poses a particular problem for the profiling of agonists in whole cell test systems removed from the therapeutic one(s). Such response-specific cell-based variability makes it difficult to guide medicinal chemistry efforts aimed at identifying and optimizing therapeutically meaningful agonist bias. For this reason, we present a scale, based on the Black and Leff operational model, that contains the key elements required to describe 7TM agonism, namely, affinity (K(A) (-1)) for the receptor and efficacy (τ) in activating a particular signaling pathway. Utilizing a "transduction coefficient" term, log(τ/K(A)), this scale can statistically evaluate selective agonist effects in a manner that can theoretically inform structure-activity studies and/or drug candidate selection matrices. The bias of four chemokines for CCR5-mediated inositol phosphate production versus internalization is quantified to illustrate the practical application of this method. The independence of this method with respect to receptor density and the calculation of statistical estimates of confidence of differences are specifically discussed.

Novel oxotremorine-related heterocyclic derivatives: Synthesis and in vitro pharmacology at the muscarinic receptor subtypes.

A set of novel heterocyclic ligands (6-27) structurally related to Oxotremorine 2 was designed, synthesized and tested at muscarinic receptor subtypes (mAChRs). In the binding experiments at cloned human receptors (hm1-5), compounds 7 and 15 evidenced a remarkable affinity and selectivity for the hm2 subtype. The in vitro functional assays, performed on a selected group of derivatives at M(1), M(2), and M(3) tissue preparations, singled out the 3-butynyloxy-5-methylisoxazole trimethylammonium salt 7 as a potent unselective muscarinic agonist [pEC(50): 7.40 (M(1)), 8.18 (M(2)), and 8.14 (M(3))], whereas its 5-phenyl analogue 12 behaved as a muscarinic antagonist, slightly selective for the M(1) subtype [pK(B): 6.88 (M(1)), 5.95 (M(2)), 5.53 (M(3))]. Moreover, the functional data put in evidence that the presence of the piperidine ring may generate a functional selectivity, e.g., an M(1) antagonist/M(2) partial agonist/M(3) full agonist profile (compound 21), at variance with the corresponding quaternary ammonium salt (compound 22) which behaved as a muscarinic agonist at all M(1-3) receptors, with an appreciable selectivity for the cardiac M(2) receptors.

Recovery of oligomers and cooperativity when monomers of the M2 muscarinic cholinergic receptor are reconstituted into phospholipid vesicles.

FLAG- and HA-tagged M2 muscarinic receptors from coinfected Sf9 cells have been purified in digitonin-cholate and reconstituted into phospholipid vesicles. The purified receptor was predominantly monomeric: it showed no detectable coimmunoprecipitation; it migrated as a monomer during electrophoresis before or after cross-linking with bis(sulfosuccinimidyl)suberate; and it bound agonists and antagonists in a manner indicative of identical and mutually independent sites. Receptor cross-linked after reconstitution or after reconstitution and subsequent solubilization in digitonin-cholate migrated almost exclusively as a tetramer. The binding properties of the reconstituted receptor mimicked those reported previously for cardiac muscarinic receptors. The apparent capacity for N-[3H]methylscopolamine (NMS) was only 60% of that for [3H]quinuclidinylbenzilate (QNB), yet binding at saturating concentrations of [3H]QNB was inhibited fully and in a noncompetitive manner at comparatively low concentrations of unlabeled NMS. Reconstitution of the receptor with a saturating quantity of functional G proteins led to the appearance of three classes of sites for the agonist oxotremorine-M in assays with [3H]QNB; GMP-PNP caused an apparent interconversion from highest to lowest affinity and the concomitant emergence of a fourth class of intermediate affinity. All of the data can be described quantitatively in terms of cooperativity among four interacting sites, presumably within a tetramer; the effect of GMP-PNP can be accommodated as a shift in the distribution of tetramers between two states that differ in their cooperative properties. Monomers of the M2 receptor therefore can be assembled into tetramers with binding properties that closely resemble those of the muscarinic receptor in myocardial preparations.

Synthesis and in vitro pharmacology of novel heterocyclic muscarinic ligands.

A set of novel heterocyclic ligands (7a-9a, 7b-9b, and 9c) structurally related to oxotremorine 2 was designed, synthesized, and tested at muscarinic receptor subtypes. In the binding experiments at cloned hm1-5, the presence of the 2-methylimidazole/2-methyl-3-alkylimidazolium moiety in place of the pyrrolidine ring revealed, in derivatives 8a, 8b, and 9c, a moderate selectivity for some receptor subtypes. The functional in vitro assays yielded results that correlated closely to binding data. In general, on passing from agonists bearing the pyrrolidine moiety to their analogues carrying the 2-methylimidazole function, the overall pharmacological efficacy profile is shifted from agonism toward partial agonism. The insertion of the 2-methyl-3-alkylimidazolium moiety advances the effect such that the compounds are pure antagonists. Quite similarly, chiral 3-oxo-Delta(2)-isoxazoline (+/-)-10 behaved as a weak antagonist unable to discriminate the different muscarinic receptor subtypes.

Evaluation of an agonist index: affinity ratio for compounds active on muscarinic cholinergic M2 receptors.

A protocol for predicting full agonist, partial agonist, and antagonist profiles of compounds with M2 muscarinic cholinergic receptor activity was developed using radioligand binding assay techniques with [3H]-N-methyl scopolamine (NMS) and [3H]-Oxotremorine-M (Oxo-M) as radioligands. Full muscarinic cholinergic receptor agonists such as muscarine and oxotremorine-M expressed a high agonist index (> 3000 for M1 muscarinic cholinergic receptors and > 900 for M2 muscarinic cholinergic receptor), whereas muscarinic receptor antagonists (selective or non-selective) for different receptor subtypes gave a low (0.5-10) agonist index. Functional studies performed on preparations of guinea-pig ileum and heart were consistent with radioligand binding assay experiments. The above results suggest that similarly as already established for the M1 muscarinic cholinergic receptor subtype, evaluation of the [3H]-NMS/[3H]-Oxo-M ratio may provide useful information on the profile of compounds acting at the M2 muscarinic cholinergic receptor subtype. The availability of simple and predictive techniques for the characterization of muscarinic M2 cholinergic receptor agonists, may help the identification of new compounds in therapeutic areas in which stimulation or inhibition of this receptor is desirable.

Molecular probes for muscarinic receptors: functionalized congeners of selective muscarinic antagonists.

The muscarinic agonist oxotremorine and the tricyclic muscarinic antagonists pirenzepine and telenzepine have been derivatized using a functionalized congener approach for the purpose of synthesizing high affinity ligand probes that are suitable for conjugation with prosthetic groups, for receptor cross-linking, fluorescent and radioactive detection, etc. A novel fluorescent conjugate of TAC (telenzepine amine congener), an n-decylamino derivative of the m1-selective antagonist, with the fluorescent trisulfonated pyrene dye Cascade Blue may be useful for assaying the receptor as an alternative to radiotracers. In a rat m3 receptor mutant containing a single amino acid substitution in the sixth transmembrane domain (Asn507 to Ala) the parent telenzepine lost 636-fold in affinity, while TAC lost only 27-fold. Thus, the decylamino group of TAC stabilizes the bound state and thus enhances potency by acting as a distal anchor in the receptor binding site. We have built a computer-assisted molecular model of the transmembrane regions of muscarinic receptors based on homology with the G-protein coupled receptor rhodopsin, for which a low resolution structure is known. We have coordinated the antagonist pharmacophore (tricyclic and piperazine moieties) with residues of the third and seventh helices of the rat m3 receptor. Although the decylamino chain of TAC is likely to be highly flexible and may adopt many conformations, we located one possible site for a salt bridge formation with the positively charged -NH3+ group, i.e. Asp113 in helix II.

The synthesis and biochemical pharmacology of enantiomerically pure methylated oxotremorine derivatives.

Previous pharmacological studies of methylated oxotremorine derivatives bearing substituents at the 3-, 4-, and 5-positions of the pyrrolidinone ring have been conducted using racemic mixtures, and not with optically active compounds. The synthesis and radioligand binding data of optically active, methylated oxotremorine derivatives at the 3- and 4-positions are described. There are significant pharmacological differences between the 3- and 4-position derivatives. The 4-position enantiomers have weak, approximately equal affinity and antagonist-like profiles, whereas the 3-position enantiomers have significantly different affinities and partial agonist-like profiles.

Cholinergic activity of acetylenic imidazoles and related compounds.

A series of acetylenic imidazoles related to oxotremorine (1a) were prepared and evaluated as cholinergic agents with in vitro binding assays and in vivo pharmacological tests in mice. 1-[4-(1H-Imidazol-1-yl)-2-butynyl]-2-pyrrolidinone (1b) was a cholinergic agonist with one-half the potency of oxotremorine. Analogues of 1b with a 5- or 2-methyl substituent in the imidazole ring (compounds 1c and 1g) were cholinergic partial agonists. Analogues of 1b with a methyl substituent at the 5-position in the pyrrolidinone ring (7b) or at the alpha-position in the acetylenic chain (8b) were antagonists. Various analogues of these imidazole acetylenes where the pyrrolidinone ring was replaced by an amide, carbamate, or urea residue were prepared. Several compounds which contained 5-methylimidazole as the amine substituent were partial agonists. The activities of the imidazole compounds are compared with those of the related pyrrolidine and dimethylamine analogues. Agonist and antagonist conformations for these compounds at muscarinic receptors are proposed.