The homeostasis of malaria-infected red blood cells

Plasmodium falciparum causes the most severe form of malaria in humans, with over two million deaths per year worldwide, mostly children. There is a desperate struggle between the rate at which this parasite develops resistance to the drugs available to treat malaria and the rate at which new effective strategies and treatments can be developed. A profound understanding of all the aspects of P. falciparum biology is therefore of major interest and potential therapeutic relevance. The recurrent asexual reproduction cycle of the parasite takes place in human red blood cells. A parasite invades a red cell, develops and replicates, and after about 48 hours the cell bursts releasing 20 or more parasites ready to infect new red cells and continue the cycle. During this cycle the red cell undergoes dramatic changes. The red cell becomes very leaky, to allow the entrance of nutrients the parasite needs and the exit of waste products the parasite generates. In addition, the parasite invests a lot of energy ingesting and digesting most of the haemoglobin of the hosting cell. Perplexingly, only a tiny fraction of this meal is used; over 80% of the digested products are discarded as waste. Two major scientific puzzles arise from these observations: how do red blood cells rendered so leaky by the parasite avoid swelling and bursting much earlier that the 48 hours the parasite needs to replicate, and why do the parasites ingest and digest so much more haemoglobin than needed. A mathematical model of an infected red cell provided a unifying explanation to both puzzles (the 'colloidosmotic hypothesis'): the clever parasite needs to digest haemoglobin in large excess to reduce the pressure that drives water into the cells (colloidosmotic pressure) thus allowing the leaky cell to survive intact until the end of the parasite's reproduction cycle. Although initial tests of the hypothesis proved supportive, critical model predictions remain to be tested. The purpose of this project is to test these predictions using a battery of state-of-the-art analytical methodologies. It is hoped that an improved understanding of this aspect of the biology of the infected red blood cell, besides its intrinsic scientific interest, may help develop new therapeutic strategies. These will be aimed at inducing the premature destruction of the infected red cells, with the release of immature malaria parasites unable to invade new cells. The project brings together a team of researchers with highly complementary expertise in mathematical modelling of cellular homeostasis, malaria research, X-ray microanalysis, and advanced optical imaging.

Plasmodium falciparum parasites cause the most severe form of malaria in humans. During their ~ 48 h asexual reproduction cycle within red blood cells (RBCs) the parasites cause major changes to host cell homeostasis: they ingest and digest up to 80% of the host haemoglobin but use only a small fraction of the aminoacids produced; they also permeabilize the host cell plasma membrane to a variety of ions, nutrients and waste products (including the excess aminoacids released) by the activation or induction of new permeation pathways. If uninfected RBCs were permeabilized as much as infected RBCs (IRBCs), the uninfected RBCs would swell and burst 'prematurely', before the 48 h span of the asexual reproduction cycle of the parasite. How then is the integrity of the infected cell sustained for ~ 48 h in the face of such drastic changes, considering that, in addition, there is a growing parasite inside? Predictions from a mathematical model of the homeostasis of a malaria-infected red blood cell (the IRBC model), developed by two of the co-investigators (VLL and TT) suggested a novel 'colloidosmotic hypothesis' as an evolutionary strategy to ensure the sustained integrity of IRBCs: excess haemoglobin consumption reduces the colloidosmotic pressure within the host thus reducing its rate of swelling towards premature haemolysis. The IRBC model predicted specific, stage-dependent changes in the volume and haemoglobin concentration of IRBCs which have not been tested yet and which pose formidable technical challenges. We propose here to investigate these predictions using a battery of state of the art methodologies such as fluorescent confocal deconvolution microscopy, electron-tomography and X-ray microanalysis of Fe in cytoplasmic domains of stage-separated IRBCs. It is hoped that precise knowledge of the changing homeostasis of IRBCs, besides its intrinsic scientific interest, may help develop new therapeutic strategies in this disease.