Molecular and cellular basis of praziquantel action in the cardiovascular system.

The anthelmintic drug praziquantel (PZQ) causes contraction of parasitic schistosomes as well as constriction of blood vessels within the mesenteric vasculature of the host where the adult blood flukes reside. The contractile action of PZQ on the vasculature is mediated by the activation of host serotonergic 5-HT 2B receptors (5-HT 2B Rs). However, the molecular basis for PZQ interaction with these targets and the location of these 5-HT 2B receptors in the vessel wall have not been experimentally defined. Evaluation of a PZQ docking pose within the 5-HT 2B R orthosteric site, using both Ca 2+ reporter and bioluminescence resonance energy transfer (BRET) assays, identified residues F340 6.51 and F341 6.52 (transmembrane helix 6, TM6) as well as L209 EL2 (extracellular loop 2) as critical for PZQ-mediated agonist activity. A key determinant of PZQ selectivity for the 5-HT 2B receptor over the 5-HT 2A / 2C receptors was determined by M218 5.39 in transmembrane helix 5 (TM5) of the orthosteric site. Mutation of this residue to valine (M218V), as found in 5-HT 2A and 5-HT 2C , decreased PZQ agonist activity, whereas the reciprocal mutation (V215M) in 5-HT 2C increased PZQ activity. Two-photon imaging in intact mesenteric arterial strips visualized PZQ-evoked Ca 2+ transients within the smooth muscle cells of the vessel wall. PZQ also triggered cytoplasmic Ca 2+ signals in arterial smooth muscle cells in primary culture that were isolated from mesenteric blood vessels. These data define the molecular basis for PZQ action on 5-HT 2B receptors localized in vascular smooth muscle.