During asexual intraerythrocytic growth, Plasmodium falciparum utilizes
hemoglobin obtained from the host red blood cell (RBC) as a nutrient source.
Papain-like
cysteine proteases, falcipains 2 and 3, have been reported to be involved in
hemoglobin digestion and are targets of current
antimalarial drug development efforts. However, their expression during gametocytogenesis, which is required for
malaria parasite transmission, has not been studied. Many of the available
antimalarials do not inhibit development of sexual stage parasites, and therefore, the persistence of gametocytes after
drug treatment allows continued transmission of the disease. In the work reported here, incubation of stage V gametocytes with membrane-permeant
cysteine protease inhibitor E64d significantly inhibited oocyst production (80 to 100%). The same conditions inhibited processing of gametocyte-
surface antigen Pfs230 during gametogenesis but did not alter the morphology of the food vacuole in gametocytes, inhibit emergence, or block male exflagellation. E64d reduced the level of oocyst production more effectively than that reported previously for
falcipain 1-knockout parasites, suggesting that falcipains 2 and 3 may also be involved in
malaria parasite transmission. However, in this study only
falcipain 3 and not
falcipain 2 was found to be expressed in stage V gametocytes. Interestingly, during gametocytogenesis
falcipain 3 was transported into the red blood cell and by stage V was localized in vesicles along the RBC surface, consistent with a role during gamete emergence. The ability of a membrane-permeant
cysteine protease inhibitor to significantly reduce
malaria parasite transmission suggests that future
drug design should include evaluation of gametogenesis and sporogonic development.