Novel Positive Allosteric Modulators Bias Acetylcholine Signaling at Human M4 Muscarinic Receptors

Abstract

M4 muscarinic acetylcholine receptors (mAChRs) are heavily expressed in the CNS, and activation of these receptors is thought to mediate the antipsychotic effects of the M1/M4‐preferring agonist xanomeline. Clinical trials have shown xanomeline to be highly effective in treating the positive symptoms, negative symptoms, and cognitive deficits of schizophrenia as well as Alzheimer's type dementia. As a consequence, much interest has been placed on developing M4‐selective drugs for the treatment of various forms of psychosis. To achieve robust selectivity, our group has identified M4‐selective positive allosteric modulators (PAMs); these compounds have shown robust efficacy in animal models used to predict antipsychotic efficacy. In this study, we examined the ability of M4‐selective PAMs to differentially modulate several intracellular signaling pathways (ERK phosphorylation, GIRK channel activation, arrestin recruitment) and differentially desensitize G‐protein‐mediated responses. Interestingly, multiple PAMs have the ability to bias the signaling of acetylcholine towards specific responses and have profound and differential effects on M4 receptor β‐arrestin recruitment. These data indicate that M4 allosteric ligands engender receptor conformational changes and signaling outcomes not previously described for GPCR orthosteric ligand interactions. Identification of signal bias and differential desensitization of M4 receptors may be important for therapeutic actions of these compounds and will guide future M4 receptor drug development efforts. Future efforts will concentrate on signaling properties that correlate with robust antipsychotic efficacy in vivo.