Molecular characterization and functional analysis of the Schistosoma mekongi Ca²⁺-dependent cysteine protease (calpain)

Abstract

© 2019 The Author(s). Background: Schistosoma mekongi, which causes schistosomiasis in humans, is an important public health issue in Southeast Asia. Treatment with praziquantel is the primary method of control but emergence of praziquantel resistance requires the development of alternative drugs and vaccines. Calcium-dependent cysteine protease (calpain) is a novel vaccine candidate that has been studied in S. mansoni, S. japonicum, and protozoans including malaria, leishmania and trypanosomes. However, limited information is available on the properties and functions of calpain in other Schistosoma spp., including S. mekongi. In this study, we functionally characterized calpain 1 of S. mekongi (SmeCalp1). Results: Calpain 1 of S. mekongi was obtained from transcriptomic analysis of S. mekongi; it had the highest expression level of all isoforms tested and was predominantly expressed in the adult male. SmeCalp1 cDNA is 2274 bp long and encodes 758 amino acids, with 85% to 90% homology with calpains in other Schistosoma species. Recombinant SmeCalp1 (rSmeCalp1), with a molecular weight of approximately 86.7 kDa, was expressed in bacteria and stimulated a marked antibody response in mice. Native SmeCalp1 was detected in crude worm extract and excretory-secretory product, and it was mainly localized in the tegument of the adult male; less signal was detected in the adult female worm. Thus, SmeCalp1 may play a role in surface membrane synthesis or host-parasite interaction. We assessed the protease activity of rSmeCalp1 and demonstrated that rSmeCalp1 could cleave the calpain substrate N-succinyl-Leu-Leu-Val-Tyr-7-amino-4-methylcoumarin, that was inhibited by calpain inhibitors (MDL28170 and E64c). Additionally, rSmeCalp1 could degrade the biological substrates fibronectin (blood clotting protein) and human complement C3, indicating important roles in the intravascular system and in host immune evasion. Conclusions: SmeCalp1 is expressed on the tegumental surface of the parasite and can cleave host defense molecules; thus, it might participate in growth, development and survival during the entire life-cycle of S. mekongi. Information on the properties and functions of SmeCalp1 reported herein will be advantageous in the development of effective drugs and vaccines against S. mekongi and other schistosomes.