Determinants of Subtype-Selectivity of the Anthelmintic Paraherquamide A on Caenorhabditis elegans Nicotinic Acetylcholine ReceptorsS

The anthelmintic paraherquamide A acts selectively on the nematode L-type nicotinic acetylcholine receptors (nAChRs), but the mechanism of its selectivity is unknown. This study tar-geted the basis of paraherquamide A selectivity by determining an X-ray crystal structure of the acetylcholine binding protein (AChBP), a surrogate nAChR ligand-binding domain, com-plexed with the compound and by measuring its actions on wild-type and mutant Caenorhabditis elegans nematodes and functionally expressed C. elegans nAChRs. Paraherquamide A showed a higher efficacy for the levamisole-sensitive [L-type (UNC-38/UNC-29/UNC-63/LEV-1/LEV-8)] nAChR than the nicotine-sensitive [N-type (ACR-16)] nAChR, a result consistent with in vivo studies on wild-type worms and worms with mutations in subunits of these two classes of receptors. The X-ray crystal structure of the Ls-AChBP-paraherquamide A complex and site -directed amino acid mutation studies showed for the first time that loop C, loop E, and loop F of the orthosteric receptor binding site play critical roles in the observed L-type nAChR selective actions of paraherquamide A. SIGNIFICANCE STATEMENT Paraherquamide A, an oxindole alkaloid, has been shown to act selectively on the L-type over N-type nAChRs in nematodes, but the mechanism of selectivity is unknown. We have co -crystallized paraherquamide A with the acetylcholine binding protein, a surrogate of nAChRs, and found that structural features of loop C, loop E, and loop F contribute to the L-type nAChR selectivity of the alkaloid. The results create a new platform for the design of anthelmintic drugs targeting cholinergic neurotrans-mission in parasitic nematodes.

Automated summary

This study aimed to determine the selectivity mechanism of paraherquamide A on nematode L-type nicotinic acetylcholine receptors (nAChRs). The researchers utilized the acetylcholine binding protein (AChBP) to analyze the compound's actions and co-crystallized it with the AChBP. The results revealed the critical role of loop C, loop E, and loop F in the observed L-type nAChR selective actions of paraherquamide A, providing insights for the design of anthelmintic drugs.