Molecular mechanism of the wake-promoting agent TAK-925.

Abstract

The OX₂ orexin receptor (OX₂R) is a highly expressed G protein-coupled receptor (GPCR) in the brain that regulates wakefulness and circadian rhythms in humans. Antagonism of OX₂R is a proven therapeutic strategy for insomnia drugs, and agonism of OX₂R is a potentially powerful approach for narcolepsy type 1, which is characterized by the death of orexinergic neurons. Until recently, agonism of OX₂R had been considered 'undruggable.' We harness cryo-electron microscopy of OX₂R-G protein complexes to determine how the first clinically tested OX₂R agonist TAK-925 can activate OX₂R in a highly selective manner. Two structures of TAK-925-bound OX₂R with either a G_q mimetic or G_i reveal that TAK-925 binds at the same site occupied by antagonists, yet interacts with the transmembrane helices to trigger activating microswitches. Our structural and mutagenesis data show that TAK-925's selectivity is mediated by subtle differences between OX₁ and OX₂ receptor subtypes at the orthosteric pocket. Finally, differences in the polarity of interactions at the G protein binding interfaces help to rationalize OX₂R's coupling selectivity for G_q signaling. The mechanisms of TAK-925's binding, activation, and selectivity presented herein will aid in understanding the efficacy of small molecule OX₂R agonists for narcolepsy and other circadian disorders.