Structural insights into the agonist selectivity and structure-based engineering of the adenosine A3 receptor
Adenosine receptors, expressed across various tissues, play pivotal roles in physiological processes and are implicated in diverse diseases, including neurological disorders and inflammation, highlighting the therapeutic potential of receptor-selective agents. The Adenosine A3 receptor (A3R), the last identified adenosine receptor, is also activated by breakdown products of post-transcriptionally modified tRNA and exhibits dual roles in neuron, heart, and immune cells, and is often overexpressed in tumors, making it a target for anticancer therapy. Despite extensive studies on the other adenosine receptors, the structure and activation mechanism of A3R, especially by selective agonists like N6-methyladenosine (m6A) and namodenoson, remained elusive. Here, we identified N6-isopentenyl adenosine (i6A), a novel A3R-selective ligand, via comprehensive modified adenosine library screening. Cryo-EM analyses of A3R-Gi signaling complexes with two nonselective and three selective agonists revealed the structural basis for A3R activation. We further conducted structure-guided engineering of m6A-insensitive A3R, which would greatly facilitate future discoveries of the physiological functions of the selective activation of A3R by modified adenosines. Our results clarify the selective activation of adenosine receptors, providing the basis for future drug discovery.