Abstract
BackgroundDuring its intraerythrocytic asexual reproduction cycle Plasmodium falciparum consumes up to 80% of the host cell hemoglobin, in large excess over its metabolic needs. A model of the homeostasis of falciparum-infected red blood cells suggested an explanation based on the need to reduce the colloid-osmotic pressure within the host cell to prevent its premature lysis. Critical for this hypothesis was that the hemoglobin concentration within the host cell be progressively reduced from the trophozoite stage onwards.Methodology/Principal FindingsThe experiments reported here were designed to test this hypothesis by direct measurements of the hemoglobin concentration in live, infected red cells. We developed a novel, non-invasive method to quantify the hemoglobin concentration in single cells, based on Förster resonance energy transfer between hemoglobin molecules and the fluorophore calcein. Fluorescence lifetime imaging allowed the quantitative mapping of the hemoglobin concentration within the cells. The average fluorescence lifetimes of uninfected cohorts was 270±30 ps (mean±SD; N = 45). In the cytoplasm of infected cells the fluorescence lifetime of calcein ranged from 290±20 ps for cells with ring stage parasites to 590±13 ps and 1050±60 ps for cells with young trophozoites and late stage trophozoite/ early schizonts, respectively. This was equivalent to reductions in hemoglobin concentration spanning the range from 7.3 to 2.3 mM, in line with the model predictions. An unexpected ancillary finding was the existence of a microdomain under the host cell membrane with reduced calcein quenching by hemoglobin in cells with mature trophozoite stage parasites.Conclusions/SignificanceThe results support the predictions of the colloid-osmotic hypothesis and provide a better understanding of the homeostasis of malaria-infected red cells. In addition, they revealed the existence of a distinct peripheral microdomain in the host cell with limited access to hemoglobin molecules indicating the concentration of substantial amounts of parasite-exported material.
Highlights
Plasmodium falciparum (Pf) causes the most severe form of malaria
We report here direct measurements of Hb concentration within the cytosol of live infected red blood cells (IRBCs) using a novel technique based on fluorescence lifetime imaging (FLIM), to test the validity of the colloid-osmotic hypothesis
The vertical bar singles out a period of mature parasite growth with a high probability of detecting substantial reductions in Hb concentration within the host cell cytoplasm (Fig. 1A), from normal values of about 7 mM in uninfected or ring-stage infected red cells down to,4 mM in cells with mid-age trophozoites and 2–3 mM in schizontcontaining cells, regardless of the IRBC volume (Fig. 1B)
Summary
Plasmodium falciparum (Pf) causes the most severe form of malaria. Over one third of the world population is estimated to be at risk [1]. A previous analysis of the homeostasis of the infected red blood cells (IRBCs) using a mathematical model developed ad hoc, combined with experimental results that showed that the osmotic fragility of IRBCs becomes progressively increased throughout the asexual cycle, suggested that IRBCs are prone to premature haemolysis, and that the parasite may prevent lysis by reducing the colloid osmotic pressure within the host cell [4]. A model of the homeostasis of falciparum-infected red blood cells suggested an explanation based on the need to reduce the colloid-osmotic pressure within the host cell to prevent its premature lysis. Critical for this hypothesis was that the hemoglobin concentration within the host cell be progressively reduced from the trophozoite stage onwards
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