Malaria ookinetes exhibit multiple markers for apoptosis-like programmed cell death in vitro

Background: A wide range of unicellular eukaryotes have now been shown to undergo a form of programmed cell death (PCD) that resembles apoptosis; exhibiting morphological and, in some cases, biochemical markers typical of metazoans. However, reports that sexual and asexual stages of malaria parasites exhibit these markers have been challenged. Here we use a rodent malaria model, Plasmodium berghei, to determine whether, and what proportion of cultured ookinetes show signs of apoptosis-like death and extend the study to examine ookinetes of Plasmodium falciparum in vivo. Results: Ookinetes displayed the following markers of PCD: loss of mitochondrial membrane potential, nuclear chromatin condensation, DNA fragmentation, translocation of phosphatidylserine to the outer surface of the cell membrane and caspase-like activity. The proportion of parasites expressing apoptosis markers rose with time, particularly when cultured in phosphate buffered saline. Some ookinetes positive for apoptosis markers also had compromised membranes, which could represent a late stage in the process. When these are included a similar proportion of ookinetes display each marker. Over 50% of P. falciparum ookinetes, removed from the mosquito midgut lumen 24 h post-infection, had nuclei containing fragmented DNA. Conclusion: We have confirmed previous reports that Plasmodium ookinetes display multiple signs that suggest they die by a mechanism resembling apoptosis. This occurs in vivo and in vitro without experimental application of triggers. Our findings support the hypothesis that non-necrotic mechanisms of cell death evolved before the advent of multicellular organisms.