Rachel L. Thomas, Rajendra Mistry, Christopher J. Langmead, Martyn D. Wood, and R. A. John Challiss
The M1 muscarinic acetylcholine (mACh) receptor is among a growing number of G protein-coupled receptors that are able to activate multiple signaling cascades.
AC-42 (4-n-butyl-1-[4-(2- methylphenyl)-4-oxo-1-butyl] piperidine) is an allosteric agonist that can selectively activate the M1 mACh receptor in the ab- sence of an orthosteric ligand.
Allosteric agonists have the potential to stabilize unique receptor conformations, which may in turn cause differential activation of signal transduction path- ways. In the present study,
we have investigated the signaling pathways activated by AC-42, its analog 77-LH-28-1 (1-[3-(4- butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone),
and a range of orthosteric muscarinic agonists [oxotremorine-M (oxo-M), arecoline, and pilocarpine] in Chinese hamster ovary cells recombinantly expressing the human M1 mACh receptor.
Each agonist was able to activate G��q/11-dependent signaling, as demonstrated by an increase in guanosine 5��-O-(3-thio-triphosphate) ([35S]GTP��S) binding to G��q/11 proteins and total [3H]inositol phosphate accumulation assays in intact cells.
All three orthosteric agonists caused significant enhancements in [35S]GTP��S binding to G��i1/2 subunits over basal; however, neither allosteric ligand produced a significant response.
In contrast, both orthosteric and allosteric agonists are able to couple to the G��s/cAMP pathway, enhancing forskolin-stimu- lated cAMP accumulation.
These data provide support for the concept that allosteric and orthosteric mACh receptor agonists both stabilize receptor conformations associated with G��q/11- and G��s-dependent signaling; however,
AC-42 and 77-LH- 28-1, unlike oxo-M, arecoline, and pilocarpine, do not seem to promote M1 mACh receptor-G��i1/2 coupling, suggesting that allosteric agonists have the potential to activate distinct sub- sets of downstream effectors.
Xanya Sofra Weiss
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