Differential Effects of Allosteric M₁ Muscarinic Acetylcholine Receptor Agonists on Receptor Activation, Arrestin 3 Recruitment, and Receptor Downregulation

Abstract

Muscarinic acetylcholine receptors (mAChRs) are drug targets for multiple neurodegenerative and neuropsychiatric disorders, but the full therapeutic potential of mAChR-targeted drugs has not been realized, mainly because of a lack of subtype-selective agonists. Recent advances have allowed the development of highly selective agonists that bind to an allosteric site on the M1 mAChR that is spatially distinct from the orthosteric acetylcholine binding site, but less is known about the profile of intracellular signals activated by orthosteric versus allosteric M1 mAChR agonists. We investigated the activation and regulatory mechanisms of two structurally distinct allosteric M1 mAChR agonists, AC260584 and TBPB. We show that allosteric agonists potently activate multiple signal transduction pathways linked to the M1 mAChR receptor but, compared to orthosteric agonists, much less efficiently recruit arrestin 3, a protein involved in regulation of G-protein coupled receptor signaling. Consistent with decreased arrestin recruitment, both allosteric agonists showed blunted responses in measurements of receptor desensitization, internalization, and downregulation. These results advance the understanding of mAChR biology and may shed light on unanticipated differences in the pharmacology of orthosteric vs. allosteric agonists that might be capitalized upon for drug development for the treatment of CNS diseases.

Distinct Roles of β-Arrestin 1 and β-Arrestin 2 in ORG27569-induced Biased Signaling and Internalization of the Cannabinoid Receptor 1 (CB1)

“…Interestingly, unlike CP55940-induced ERK1/2 phosphorylation, ORG27569-induced ERK1/2 phosphorylation is G i protein-independent. Thus, it possesses some agonistic properties as an "ago-allosteric modulator" (33)(34)(35)(36)(37)(38).…”

AF710B, a Novel M1/σ1 Agonist with Therapeutic Efficacy in Animal Models of Alzheimer's Disease

“…Both M1 orthosteric and allosteric agonists increase phosphorylation of extracellular signal regulated kinase (ERK 1/2) [24,25]. The activated ERK/ mitogen activated protein kinase (MAPK) cascade has multiple targets, including cAMP response element binding protein (CREB), which mediates its ability to induce memory consolidation and long-term memory formation [26,27].…”

Allosteric Modulator ORG27569 Induces CB1 Cannabinoid Receptor High Affinity Agonist Binding State, Receptor Internalization, and Gi Protein-independent ERK1/2 Kinase Activation

“…More recently, it has become evident that some allosteric modulators can also activate the receptor for signal transduction on their own (termed "allosteric agonist") (63,64). In addition to modulating the binding and efficacy of an orthosteric ligand, these allosteric agonists were shown to directly affect receptor activation, cellular localization, or down-regulation of various GPCRs including the M1 muscarinic acetylcholine receptor, mGluR7, and adenosine A1 receptor (65)(66)(67)(68). Here, we show that ORG27569 can activate CB1 and promote downstream effects in the absence of the orthosteric agonist.…”