Molecular Mechanism of the ß3AR Agonist Activity of a ß-Blocker.

ABSTRACT

Development of subtype-selective drugs for G protein-coupled receptors poses a significant challenge due to high similarity between subtypes, as exemplified by the three ß-adrenergic receptors (ßARs). The ß3AR agonists show promise for treating the overactive bladder or preterm birth, but their potential is hindered by off-target activation of ß1AR and ß2AR. Interestingly, several ß-blockers, which are antagonists of the ß1ARs and ß2ARs, have been reported to exhibit agonist activity at the ß3AR. However, the molecular mechanism remains elusive. Understanding the underlying mechanism should facilitate the development of ß3AR agonist drugs with improved selectivity and reduced off-target effects. In this work, we determined the structures of human ß3AR in complex with the endogenous agonist epinephrine or with a synthetic ß3AR agonist carazolol, which is also a high-affinity ß-blocker. Structure comparison, mutagenesis studies and molecular dynamics simulations revealed that the differences on the flexibility of D3.32 directly contribute to carazolol's distinct activities as an antagonist for the ß2AR and an agonist for the ß3AR. The process is also indirectly influenced by the extracellular loops (ECL), especially ECL1. Taken together, these results provide key guidance for development of selective ß3AR agonists, paving the way for new therapeutic opportunities.