Structural insights into the molecular effects of the anthelmintics monepantel and betaine on the Caenorhabditis elegans acetylcholine receptor ACR-23.

Anthelmintics are drugs used for controlling pathogenic helminths in animals and plants. The natural compound betaine and the recently developed synthetic compound monepantel are both anthelmintics that target the acetylcholine receptor ACR-23 and its homologs in nematodes. Here, we present cryo-electron microscopy structures of ACR-23 in apo, betaine-bound, and betaine- and monepantel-bound states. We show that ACR-23 forms a homo-pentameric channel, similar to some other pentameric ligand-gated ion channels (pLGICs). While betaine molecules are bound to the classical neurotransmitter sites in the inter-subunit interfaces in the extracellular domain, monepantel molecules are bound to allosteric sites formed in the inter-subunit interfaces in the transmembrane domain of the receptor. Although the pore remains closed in betaine-bound state, monepantel binding results in an open channel by wedging into the cleft between the transmembrane domains of two neighboring subunits, which causes dilation of the ion conduction pore. By combining structural analyses with site-directed mutagenesis, electrophysiology and in vivo locomotion assays, we provide insights into the mechanism of action of the anthelmintics monepantel and betaine. Synopsis: The anthelmintics monepantel and betaine target acetylcholine receptor like-23 (ACR-23) of the nematode Caenorhabditis elegans. This study provides cryo-electron microscopy structures of ACR-23 in various states to characterize how these compounds exert their anthelmintic functions. ACR-23 forms a homopentameric channel. One betaine molecule is bound to each of the five inter-subunit interfaces in the extracellular domain of ACR-23. One monepantel molecule is bound to each of the five inter-subunit interfaces in the transmembrane domain of ACR-23. The pore remains closed in the betaine-bound state, whereas monepantel binding induces pore dilation. Binding of both compounds together leads to outward movement and rotation of the pore-lining M2 helix, and eventually to channel opening. Structural and functional analyses elucidate the distinct binding modes of the anthelmintics monepantel and betaine on their receptor ACR-23. [ABSTRACT FROM AUTHOR]