Antagonist binding profile of the split chimeric muscarinic m2-trunc/m3-tail receptor

Variants of G protein-coupled receptors: a reappraisal of their role in receptor regulation

Truncated or shorter forms of G protein-coupled receptors (GPCRs), originating by alternative splicing, have been considered physiologically irrelevant for a rather long time. Nevertheless, it is now recognized that alternative splicing variants of GPCRs greatly increase the total number of receptor isoforms and can regulate receptor trafficking and signalling. Furthermore, dimerization of these truncated variants with other receptors concurs to expand receptor diversity. Highly truncated variants of GPCRs, typically, are retained in the endoplasmic reticulum (ER) and by heteromerization prevent the wild-type receptor to reach the plasma membrane, exerting a dominant-negative effect on its function. This can be responsible for some pathological conditions but in some other cases, it can offer protection from a disease because the expression of the receptor, that is necessary for binding an infectious agent, is attenuated. Here, we propose a possible new mechanism of creation of truncated GPCR variants through an internal ribosome entry site (IRES), a nucleotide sequence that allows cap independent translation of proteins by recruiting the ribosome in proximity of an internal initiation codon. We suggest that an IRES, situated in the third cytoplasmic loop, could be responsible for the translation of the last two transmembrane (TM) regions of the muscarinic M2receptor. IRES driven expression of this C-terminal part of the muscarinic M2receptor could represent a novel and additional mechanism of receptor regulation.

Comparison of the pharmacological antagonism of M2 and M3 muscarinic receptors expressed in isolation and in combination

We compared the binding properties of selective muscarinic antagonists with their potencies for antagonizing muscarinic responses in Chinese hamster ovary (CHO) cells expressing M(2) and M(3) muscarinic receptors in combination and in isolation. When measured by the competitive displacement of [3H]N-methylscopolamine binding to CHO cells expressing both M(2) and M(3) muscarinic receptors (CHO M(2)+M(3) cells), the competition curves of the subtype-selective muscarinic antagonists were consistent with a two-site model. One site exhibited binding properties identical to those of CHO M(2) cells, whereas the other site exhibited properties like those of CHO M(3) cells. Oxotremorine-M, a muscarinic agonist, elicited a robust, pertussis toxin-insensitive stimulation of phosphoinositide hydrolysis in both CHO M(3) and CHO M(2)+M(3) cells, but not in CHO M(2) cells. The pharmacological antagonism of the phosphoinositide response exhibited similar properties in both CHO M(3) and CHO M(2)+M(3) cells. Oxotremorine-M elicited a pertussis toxin-sensitive, robust inhibition of forskolin-stimulated cyclic AMP (cAMP) accumulation in both CHO M(2) and CHO M(2)+M(3) cells and a less robust inhibition in CHO M(3) cells. At higher concentrations, oxotremorine-M elicited an increase in cAMP accumulation over the maximal inhibition noted at lower concentrations in both CHO M(3) and CHO M(2)+M(3) cells. Following pertussis toxin treatment, only the stimulatory phase of the cAMP response to oxotremorine-M was observed in CHO M(2), CHO M(3), and CHO M(2)+M(3) cells. The pharmacological antagonism of the cAMP response in CHO M(2)+M(3) cells resembled that expected for a response mediated independently by both M(2) and M(3) receptors.

Polymethylene tetraamine backbone as template for the development of biologically active polyamines

The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of ligands for different biological targets. As a matter of fact, the insertion of different pharmacophores onto the polymethylene tetraamine backbone can tune both affinity and selectivity for any given receptor. The application of this approach provided a prospect of modifying benextramine (1). structure to achieve specific recognition of muscarinic receptors that led to the discovery of methoctramine (2). which is widely used as a pharmacological tool for muscarinic receptor characterization. In turn, appropriate structural modifications performed on the structure of methoctramine led to the discovery of new polyamines endowed with high affinity and selectivity for (a). muscarinic receptor subtypes, (b). G(i) proteins, and (c). muscle-type nicotinic receptors. Thus, polyamines tripitramine (9) and spirotramine (33), among others, were designed, which were shown to be highly selective for muscarinic M(2) and M(1) receptors, respectively. Several polyamines have been discovered, which inhibit noncompetitively a closed state of the nicotinic receptor. These ligands, such as 66, resulted in important tools for elucidating the mode and site of interaction of polyamines with the ion channel. It was discovered that reducing the flexibility of the diaminohexane spacer of methoctramine led to polyamines, such as 70, which are endowed with a biological profile significantly different from that of the prototype. Most likely, tetraamine (70) is a potent activator of G(i) proteins. Finally, the universal template approach formed the basis for modifying benextramine (1) structure to the design of ligands, which display affinity for acetylcholinesterase and muscarinic M(2) receptors. Thus, these polyamines, such as caproctamine (78), could have potential in the investigation of Alzheimer disease.

Oligomerisation of G-protein-coupled receptors

A range of approaches have recently provided evidence that G-protein-coupled receptors can exist as oligomeric complexes. Both homo-oligomers, comprising multiple copies of the same gene product, and hetero-oligomers containing more than one receptor have been detected. In several, but not all, examples, the extent of oligomerisation is regulated by the presence of agonist ligands, and emerging evidence indicates that receptor hetero-oligomers can display distinct pharmacological characteristics. A chaperonin-like role for receptor oligomerisation in effective delivery of newly synthesised receptors to the cell surface is a developing concept, and recent studies have employed a series of energy-transfer techniques to explore the presence and regulation of receptor oligomerisation in living cells. However, the majority of studies have relied largely on co-immunoprecipitation techniques, and there is still little direct information on the fraction of receptors existing as oligomers in intact cells.

Analysis of the muscarinic receptor subtype mediating inhibition of the neurogenic contractions in rabbit isolated vas deferens by a series of polymethylene tetra-amines

The pharmacological characteristics of the presynaptic muscarinic receptor subtype, which mediates inhibition of the neurogenic contractions in the prostatic portion of rabbit vas deferens, have been investigated by using a series of polymethylene tetra-amines, which were selected for their ability to differentiate among muscarinic receptor subtypes. It was found that all tetra-amines antagonized McN-A-343-induced inhibition in electrically stimulated rabbit vas deferens in a competitive manner and with affinity values (pA:(2)) ranging between 6.27+/-0.09 (spirotramine) and 8.51+/-0.02 (AM170). Competition radioligand binding studies, using native muscarinic receptors from rat tissues (M(1), cortex; M(2), heart; M(3), submaxillary gland) or from NG 108-15 cells (M(4)) and human cloned muscarinic M(1)-M(4) receptors expressed in CHO-K1 cells, were undertaken with the same tetra-amines employed in functional assays. All antagonists indicated a one-site fit. The affinity estimates (pK:(i)) of tetra-amines calculated in binding assays using native receptors were similar to those obtained using cloned receptors. Among these compounds some displayed selectivity between muscarinic receptor subtypes, indicating that they may be valuable tools in receptor characterization. Spirotramine was selective for M(1) receptors versus all other subtypes (pK:(i) native: M(1), 7.32+/-0.10; M(2), 6.50+/-0.11; M(3), 6.02+/-0.13; M(4), 6.28+/-0.16; pK:(i) cloned: M(1), 7.69+/-0.08; M(2), 6.22+/-0.14; M(3), 6.11+/-0.16; 6.35+/-0.11) whereas CC8 is highly selective for M(2) receptors versus the other subtypes (pK:(i) native: M(1), 7.50+/-0.04; M(2), 9.01+/-0.12; M(3), 6.70+/-0.08; M(4), 7.56+/-0.04; pK:(i) cloned: M(1), 7.90+/-0.20; M(2), 9.04+/-0.08; M(3), 6.40+/-0.07; M(4), 7.40+/-0.04). Furthermore, particularly relevant for this investigation were tetra-amines dipitramine and AM172 for their ability to significantly differentiate M(1) and M(4) receptors. The apparent affinity values (pA:(2)) obtained for tetra-amines in functional studies using the prostatic portion of rabbit vas deferens correlated most closely with the values (pK:(i)) obtained at either native or human recombinant muscarinic M(4) receptors. This supports the view that the muscarinic receptor mediating inhibition of neurogenic contractions of rabbit vas deferens may not belong to the M(1) type but rather appears to be of the M(4) subtype.