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_2BRs). 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_2BR orthosteric site, using both Ca²⁺ 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 Ca²⁺ transients within the smooth muscle cells of the vessel wall. PZQ also triggered cytoplasmic Ca²⁺ 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.