Praziquantel affects the regulatory myosin light chain of Schistosoma mansoni.

Praziquantel (PZQ) is the drug of choice for schistosomiasis and probably is the only highly effective drug currently available for treating schistosomiasis-infected individuals. The mode of action of PZQ involves increasing the calcium uptake of the parasite, resulting in tegumental damage and death of the parasite. Despite its remarkable function, the target of PZQ has not been identified yet. To begin to understand where PZQ acts, in this study we expressed the cDNA library of Schistosoma mansoni on the surface of T7 bacteriophages and screened this library with labeled PZQ. This procedure identified a clone that strongly bound to PZQ. Subsequent DNA analysis of inserts showed that the clone coded for regulatory myosin light chain protein. The gene was then cloned, and recombinant S. mansoni myosin light chain (SmMLC) was expressed. Immunoblot analysis using antibodies raised to recombinant SmMLC (rSmMLC) showed that SmMLC is abundantly expressed in schistosomula and adult stages compared to the amount in cercarial stages. In vitro analyses also confirmed that PZQ strongly binds to rSmMLC. Further, peptide mapping studies showed that PZQ binds to amino acids 46 to 76 of SmMLC. Immunoprecipitation analysis confirmed that SmMLC is phosphorylated in vivo upon exposure to PZQ. Interestingly, significant levels of anti-SmMLC antibodies were present in vaccinated mice compared to the amount in infected mice, suggesting that SmMLC may be a potential target for protective immunity in schistosomiasis. These findings suggest that PZQ affects SmMLC function, and this may have a role in PZQ action.