Molecular mechanism of the wake-promoting agent TAK-925

The OX(2) orexin receptor (OX(2)R) is a highly expressed G protein-coupled receptor (GPCR) in the brain that regulates wakefulness and circadian rhythms in humans. Antagonism of OX(2)R is a proven therapeutic strategy for insomnia drugs, and agonism of OX(2)R is a potentially powerful approach for narcolepsy type 1, which is characterized by the death of orexinergic neurons. Until recently, agonism of OX(2)R had been considered ‘undruggable.’ We harness cryo-electron microscopy of OX(2)R-G protein complexes to determine how the first clinically tested OX(2)R agonist TAK-925 can activate OX(2)R in a highly selective manner. Two structures of TAK-925-bound OX(2)R 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 OX(1) and OX(2) receptor subtypes at the orthosteric pocket. Finally, differences in the polarity of interactions at the G protein binding interfaces help to rationalize OX(2)R’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 OX(2)R agonists for narcolepsy and other circadian disorders.