Structure‐activity relationships of a distinct family of selective agonists of α7‐nAChRs

Abstract

Nicotinic acetylcholine receptors (nAChRs) are lead members of a pentameric ligand‐gated ion channel family; they are excitatory receptors, activated by the endogenous neurotransmitter, acetylcholine. nAChRs are present in the peripheral and central nervous system, where they mediate and modulate neuromuscular, autonomic and CNS functions. Receptor‐selective ligands that cross the blood‐brain barriers may form useful therapies for disorders of aging, such as Alzheimers disease and disorders of development, such as schizophrenia. In previous work, we developed a small sub‐group of 4,6 di‐substituted 2‐aminopyrimidines that interact with the acetylcholine binding protein (AChBP) and activate a7‐nAChRs. These agents show Kd and EC50 values for both AChBP binding and a7‐nAChR activation between 10 and 70nM. Moreover, they are structurally distinct from classical nicotinic agonists that are quaternary amines or typically contain a strong base separated from a weaker base or hydrogen bonding acceptor, often on separate ring systems. Selectivity for the a7‐AChR and high affinity for AChBP was achieved by substituting a dipicolyl group at the 4 position of the pyrimidine. In contrast to classical nicotinic agonists, the nitrogens on the pyrimidine and pyridine rings are weakly basic. Moreover, X‐ray crystallographic studies show that these ligands bind to the classical agonist‐antagonist site, but with a distinctive orientation of the core pyrimidine ring and the associated picolyl moiety. We have expanded our structural landscape of these AChBP binding and a7‐selective ligands, based on our previous leads by combining the pyrimidine system with the di‐picolyl substitution at the 4‐position. To examine receptor interactions of this family of compounds on nAChRs and related pentameric ligand‐gated ion channels, we employ HEK cells transfected with cDNA's encoding one of three requisite receptor subtypes: α7‐nAChR, α4β2‐nAChR and 5HT3AR, along with a fluorescent reporter. To assess rapid activation of lead candidates we use an oocyte system, revealing an agonist behavior distinguishing them from silent agonists and ago‐PAM's. To examine the molecular determinants and ligand poses involved, we have crystallized the higher affinity complexes with AChBP. Accordingly, we have a structure‐guided approach to novel ligand design for nAChRs. In conclusion, we find: 1) a di‐picolyl moiety in the 4‐position is fundamental for the activation of the a7‐nAChR, 2) the 2‐amino group tolerates only certain structural changes controlled by steric and bonding constraints, 3) substitutions at the 6‐position affect both the potency and receptor subtype selectivity of the ligands.Support or Funding InformationCONACYT Mexico graduate fellowship to GACHThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.