A kinetic model for positive allosteric modulator (PAM)‐antagonists for the type 1 cannabinoid (CB₁) receptor

Abstract

Background and PurposeThe cannabinoid (CB1) receptor is among the most abundant G protein‐coupled receptors in brain. Allosteric ligands bind to a different site on receptors than the orthosteric ligand can have effects that are unique to the allosteric ligand and modulate orthosteric ligand activity. We propose a unified mathematical model describing the interaction effects of the allosteric ligand Org27569 and the orthosteric agonist CP55940 on CB1 receptor.Experimental ApproachA ternary complex model was constructed, which incorporated kinetic properties to describe the time course of effects of Org27569 and CP55940 reported in the literature: (i) enhanced receptor binding of CP55940, (ii) reduced internalisation and (iii), time‐dependent modulation of cAMP. Underlying mechanisms of time‐dependent modulation by Org27569 were evaluated by simulation.Key ResultsA hypothetical transitional state of CP55940–CB1–Org27569, which can internalise but cannot inhibit cAMP, was shown to be necessary and was sufficient to describe the allosteric modulation by Org27569, prior to receptors adopting an inactive conformation. The model indicated that the formation of this transitional CP55940–CB1–Org27569 state and final inactive CP55940–CB1–Org27569 state contributes to the enhanced CP55940 binding. The inactive CP55940–CB1–Org27569 cannot internalise or inhibit cAMP, leading to reduced internalisation and cessation of cAMP inhibition.Conclusions and ImplicationsIn conclusion, a kinetic mathematical model for CB1 receptor allosteric modulation was developed. However, a standard ternary complex model was not sufficient to capture the data and a hypothetical transitional state was required to describe the allosteric modulation properties of Org27569.

Insight into the mechanism of action of ORG27569 at the cannabinoid type one receptor utilising a unified mathematical model

“…This study concluded that the existence of a transitional state ( ARB T ) and an inactive state ARB , when both ORG and CP are bound to CB 1 , are imperative to describe all signalling data (Fig. 1 ; Yang et al 2023 ). This is a unique feature of this model, as classical allosteric ternary complex models do not have the existence of such a transitional state.…”

“…Recently, we published a unified mathematical model describing the effect of ORG on the signalling of the CB 1 orthosteric agonist CP55940 (Fig. 1 ; Yang et al 2023 ). This model describes previous in vitro data and comprises a two-part model explaining receptor binding and internalisation and downstream cAMP signalling (Cawston et al 2013 ; Yang et al 2023 ).…”

“…This model describes previous in vitro data and comprises a two-part model explaining receptor binding and internalisation and downstream cAMP signalling (Cawston et al 2013 ; Yang et al 2023 ). Inhibition of cAMP is a complex process involving G proteins, adenylate cyclase, and other modulators and molecules, but in this model, constitutively active receptor ( R ) and agonist bound receptor ( AR ) directly inhibit cAMP (Yang et al 2023 ). This was done to simplify the model; however, the model could be expanded in the future to include signalling molecules and more accurately represent the biological system.…”

“…Within the model, A fers to CP55940, B refers to ORG, and R refers to CB 1 (Fig. 1 ; Yang et al 2023 ). This study concluded that the existence of a transitional state ( ARB T ) and an inactive state ARB , when both ORG and CP are bound to CB 1 , are imperative to describe all signalling data (Fig.…”

“…This is a unique feature of this model, as classical allosteric ternary complex models do not have the existence of such a transitional state. The transitional state proposed cannot signal through G proteins but is able to undergo internalisation (Yang et al 2023 ). Importantly, this model proposes an empirically testable mechanism of action for ORG at CB 1 .…”