The Structural Basis for GTS-21 Selectivity between Human and Rat Nicotinic α7 Receptors

The 7 nAChR-selective partial agonist 3-(2,4-dimethoxybenzylidene)anabaseine (GTS-21) is more efficacious and potent for rat receptors than for human 7 receptors. Four single amino acid differences exist between human and rat 7i n the agonist binding site, two in the C loop, and one each in the E and F loops. Reciprocal mutations were made in these three domains and evaluated in Xenopus laevis oocytes. Mutations in the C and F loops significantly increased the efficacy of GTS-21 for the human receptor mutants but not to the level of the wild-type, and reciprocal mutations in rat 7 did not decrease responses to GTS-21. Whereas mutations in the E loop alone were without effect, the E- and F-loop mutations together increased GTS-21 efficacy and potency for human receptors, but the EF mutations in the rat receptors decreased the GTS-21 potency without changing the efficacy. The only mutants that showed a full reversal of the efficacy differences between human and rat 7 contained complete exchange of all four sites in the C, E, and F loops or just the sites in the C and F loops. However, the reversal of the potency ratio seen with the EF mutants was not evident in the CEF mutants. Our data therefore indicate that the pharmacological differences between rat and human 7 receptors are caused by reciprocal differences in sites within and around the binding site. Specific features in the agonist molecule itself are also identified that interact with these structural features of the receptor agonist binding site. A crucial assumption for the translation of preclinical research from animal studies to human therapeutics is that receptor pharmacology will be consistent between species. That is, drugs shown to be useful based on their ability to work in animal (rodent) models would also have similar activity on human forms of the receptors. The neuronal 7type nicotinic acetylcholine receptor (nAChR) has been identified as a potential target for the treatment of Alzheimer’s disease (Lindstrom, 1997), and 3-(2,4-dimethoxybenzylidene) anabaseine (GTS-21; also called DMXBA), which selectively targets this receptor, has been shown to improve learning and memory in animal models of cholinergic hypofunction (Kem, 2000). This 7-selective partial agonist has also been shown to prevent the death of differentiated PC-12 cells that occurs after nerve growth factor removal and the death of cultured primary neurons that occurs after high levels of NMDA receptor activation (Martin et al., 1994; Shimohama