Variation in apoptosis mechanisms employed by malaria parasites: the roles of inducers, dose dependence and parasite stages

Abstract

BackgroundPlasmodium berghei ookinetes exhibit an apoptotic phenotype when developing within the mosquito midgut lumen or when cultured in vitro. Markers of apoptosis increase when they are exposed to nitric oxide or reactive oxygen species but high concentrations of hydrogen peroxide cause death without observable signs of apoptosis. Chloroquine and other drugs have been used to induce apoptosis in erythrocytic stages of Plasmodium falciparum and to formulate a putative pathway involving cysteine protease activation and mitochondrial membrane permeabilization; initiated, at least in the case of chloroquine, after its accumulation in the digestive vacuole causes leakage of the vacuole contents. The lack of a digestive vacuole in ookinetes prompted the investigation of the effect of chloroquine and staurosporine on this stage of the life cycle. Finally, the suggestion that apoptosis may have evolved as a strategy employed by ookinetes to increase the fitness of surviving parasites was explored by determining whether increasing the ecological triggers parasite density and nutrient depletion induced apoptosis.MethodsOokinetes were grown in culture then either exposed to hydrogen peroxide, chloroquine or staurosporine, or incubated at different densities and in different media. The proportion of ookinetes displaying positive markers for apoptosis in treated samples was compared with controls and results were analyzed using analysis of variance followed by a Turkey’s test, or a Kruskal-Wallis test as appropriate.ResultsHydrogen peroxide below 50 μM triggered apoptosis but cell membranes were rapidly compromised by higher concentrations, and the mode of death could not be defined. Both chloroquine and staurosporine cause a significant increase in ookinetes with condensed chromatin, caspase-like activity and, in the case of chloroquine, phosphatidylserine translocation and DNA fragmentation (not investigated for staurosporine). However, mitochondrial membrane potential remained intact. No relationship between ookinete density and apoptosis was detected but nutrient depletion significantly increased the proportion of ookinetes with chromatin condensation in four hours.ConclusionsIt is proposed that both a mitochondrial and an amitochondrial apoptotic pathway may be involved, dependent upon the trigger that induces apoptosis, and that pathways may differ between erythrocytic stages and ookinetes, or between rodent and human malaria parasites.

Hexane fraction of Globimetula braunii induces mitochondria-mediated apoptosis in Plasmodium berghei-infected mice

“…Indeed, antimalarial drugs such as chloroquine act as apoptotic inducer in the ookinete stage of the parasite. Our study further shows that apoptosis is involved in parasite clearance in the host as proposed by previous authors [24] , [29] , [35] . However, the reasons for further cell death in uninfected cells in attempt to clear malarial parasite remains a paradox; cell death for host survival, decrease in parasite burden and unfortunately, tissue wastage via death of uninfected cells.…”

Injury and immune response: applying the danger theory to mosquitoes

“…It is possible that the immunological response mounted by a mosquito during an ookinete invasion is not the only mechanism limiting the density of parasites. The malaria parasite itself might contribute to the regulation of its density in mosquitoes by means of programmed cell death (PCD; Pollitt et al, 2010 ; Matthews et al, 2012 ). This possibility together with the apoptosis of midgut cells that are invaded by ookinetes ( Zieler and Dvorak, 2000 ), which are extruded from the epithelium into the lumen of the midgut ( Han et al, 2000 ), suggests a certain limit for the number of ookinetes than can pass through the midgut before the physiology of the intestinal tissue is compromised.…”

The first suicides: a legacy inherited by parasitic protozoans from prokaryote ancestors

“…Over time, an increasing number of ookinetes, developing in cultured gametocytaemic mouse blood, showed loss of ΔΨ m and exhibited markers of apoptosis [98,99]. However, exposure to nitric oxide donors or chloroquine significantly increased the proportion of parasites showing signs of apoptosis, whilst not causing loss of ΔΨ m [100,101]. This suggests the existence of alternative pathways that are dependent upon the method of induction of cell death, one of which bypasses the mitochondrion.…”