Apoptosis stalks Plasmodium falciparum maintained in continuous culture condition

Abstract

BackgroundGrowth kinetic of Plasmodium falciparum in culture or in the host fall short of expected growth rate considering that there are 4 x 106/µL red blood cell (RBCs) available for invasion and about 16 merozoites growing in each infected RBC. This study determined whether apoptotic machinery is operable to keep the parasite population under check.MethodsA synchronized culture of P. falciparum (Dd2 strain) was initiated at 0.5% ring stage parasitaemia and kept under conditions not limiting for RBCs and nutrient by adjusting hematocrit to 5% at each schizogony and changing growth media daily. Parasite growth pattern and morphology was evaluated by blood smear microscopy and flow-cytometry using SYBR green. The apoptotic processes were evaluated for evidence of: DNA fragmentation by TUNEL, collapse of mitochondria membrane potential (ΔΨm) by TMRE, expression of metacaspse gene by RT-qPCR and by probing parasite proteins with anti-caspase antibodies.ResultsFrom the seeding parasitaemia of 0.5%, the parasites doubled every 48 hours to a parasitaemia of 4%. Thereafter, the growth stagnated and the culture consistently crashed at about 6% parasitaemia. ΔΨm potential collapsed as the parasite density increased and DNA fragmentation increased steadily from 0.2% to ~6%. The expression of metacaspase gene and protein was observed in all stages, but their abundance was variable among the stages.ConclusionThese findings suggest existence of P. falciparum quorum sensing that keep the parasite population under check.

A high parasite density environment induces transcriptional changes and cell death in Plasmodium falciparum blood stages

“…The death phenotype as defined above is different from the cell death modalities described in P. falciparum blood stages treated with antimalarial drugs , and is also morphologically distinct from autophagic‐like cell death, crisis forms, and glucose‐starved parasites, which rapidly shrink and die within the erythrocyte . While aberrant parasites in high‐density cultures shared similarities with apoptotic cells, they did not entirely conform to the conventional mammalian cell paradigm for programmed cell death or to other reports detailing programmed cell death in Plasmodium . Interestingly, DNA fragmentation was never observed in the high‐density parasites.…”

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

“…Loss of ΔΨ m has also been associated with drug-induced apoptosis in erythrocytic stages of P. falciparum [71,80,96,97]. However, the situation was found to be more complex when the ookinete stage of P. berghei was examined.…”

“…Disease progression and survival of L. major is dependent upon a proportion of the promastigote population undergoing apoptosis [111]. There is also evidence that blood stage P. falciparum parasites maintain low parasite density in culture, as a proportion of them undergo apoptosis in response to a form of quorum sensing [97]. The use of apoptosis to control population density in a dividing, clonal population of unicellular parasites would certainly be advantageous if the host were likely to be overwhelmed before parasite transmission occurred.…”

A stochastic microscopic model for the dynamics of antigenic variation

“…This density-dependent cell-cycle arrest (which is very similar to quorum sensing in bacteria) is well characterized from the biochemical and genetic point of view in Trypanosoma brucei (Mony et al, 2014). Other parasites that undergo antigenic variation and parasitemia waves, such as Plasmodium falciparum, also appear to exhibit a similar mechanism of population growth control (Mutai and Waitumbi, 2010;Pollitt et al, 2010). This population behavior has been modeled before by Savill and Seed (see Savill and Seed, 2004) using a set of deterministic differential equations.…”