Orthogonal Activation of the Reengineered A3Adenosine Receptor (Neoceptor) Using Tailored Nucleoside Agonists

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Abstract
An alternative approach to overcome the inherent lack of specificity of conventional agonist therapy can be the reengineering of the GPCRs and their agonists. A reengineered receptor (neoceptor) could be selectively activated by a modified agonist, but not by the endogenous agonist. Assisted by rhodopsin-based molecular modeling, we pinpointed mutations of the A3 adenosine receptor (AR) for selective affinity enhancement following complementary modifications of adenosine. Ribose modifications examined included, at 3‘: amino, aminomethyl, azido, guanidino, ureido; and at 5‘: uronamido, azidodeoxy. N6-Variations included 3-iodobenzyl, 5-chloro-2-methyloxybenzyl, and methyl. An N6-3-iodobenzyl-3‘-ureido adenosine derivative 10 activated phospholipase C in COS-7 cells (EC50 = 0.18 μM) or phospholipase D in chick primary cardiomyocytes, both mediated by a mutant (H272E), but not the wild-type, A3AR. The affinity enhancements for 10 and the corresponding 3‘-acetamidomethyl analogue 6 were >100-fold and >20-fold, respectively. 10 concentration-dependently protected cardiomyocytes transfected with the neoceptor against hypoxia. Unlike 10, adenosine activated the wild-type A3AR (EC50 of 1.0 μM), but had no effect on the H272E mutant A3AR (100 μM). Compound 10 was inactive at human A1, A2A, and A2BARs. The orthogonal pair comprising an engineered receptor and a modified agonist should be useful for elucidating signaling pathways and could be therapeutically applied to diseases following organ-targeted delivery of the neoceptor gene.