Proteases are fundamental to successful parasitism, including that of the schistosome flatworm parasite, which causes the disease
schistosomiasis in 200 million people worldwide. The
proteasome is receiving attention as a potential
drug target for treatment of a variety of infectious
parasitic diseases, but it has been understudied in the schistosome. Adult Schistosoma mansoni were incubated with 1 μM concentrations of the
proteasome inhibitors bortezomib,
carfilzomib, and
MG132. After 24 h,
bortezomib and
carfilzomib decreased worm motility by more than 85% and endogenous
proteasome activity by >75%, and after 72 h, they increased
caspase activity by >4.5-fold. The association between the engagement of the
proteasome target and the phenotypic and biochemical effects recorded encouraged the chromatographic enrichment of the S. mansoni
proteasome (Sm20S). Activity assays with fluorogenic
proteasome substrates revealed that Sm20S contains
caspase-type (β1),
trypsin-type (β2), and
chymotrypsin-type (β5) activities. Sm20S was screened with 11
peptide epoxyketone inhibitors derived from the marine
natural product carmaphycin B. Analogue 17 was 27.4-fold less cytotoxic to HepG2 cells than
carmaphycin B and showed equal potency for the β5 subunits of Sm20S, human constitutive
proteasome, and human immunoproteasome. However, this analogue was 13.2-fold more potent at targeting Sm20S β2 than it was at targeting the equivalent subunits of the human
enzymes. Furthermore, 1 μM 17 decreased both worm motility and endogenous Sm20S activity by more than 90% after 24 h. We provide direct evidence of the
proteasome's importance to schistosome viability and identify a lead for which future studies will aim to improve the potency, selectivity, and safety.