Pathological nucleation of hemozoin in malaria parasites

Sergey Kapishnakov from Helmholtz Zentrum Berlin

Malaria is a widespread and severe disease caused by parasites of the genus Plasmodium and transmitted by Anopheles mosquitoes. The emerging parasitic resistance to current antimalarial treatments underscores the need to design efficient new drugs or modify existing drugs to improve their efficacy. One promising drug target is the process of hemozoin crystal formation.  Hemozoin is a byproduct of the parasite’s digestion of its food source, hemoglobin. The free heme that results from this digestion is toxic to the parasite, but normally this heme is detoxified by its dimerization and crystallization into inert hemozoin. Once hemozoin crystals are nucleated, additional free heme binds to the crystal faces thereby detoxifying the parasitic digestive vacuole. Thus, if crystal nucleation could be prevented, or at least retarded, free heme would be left to react and destroy the parasitic membranes. Unfortunately, little is understood about the dynamics of heme detoxification. Two key processes must be investigated: nucleation of hemozoin crystals and the pathway of heme transfer from hemoglobin to hemozoin. We have used X-ray diffraction and quantitative X-ray microscopy to study nucleation of hemozoin in-vivo, and have found evidence for templated nucleation of hemozoin via its {100} face. Blocking this face should be most efficient in retardation of hemozoin nucleation and growth. We have also made an estimate of not-yet-crystallized heme content within the digestive vacuole leading to a better understanding of the heme detoxification process. Our results suggest an assembly line process of heme detoxification involving a histidine rich heme detoxification protein as the main highway towards hemozoin formation. Blocking hemozoin growth sabotages this process leading to build up of the toxic heme, which in turn destroys the parasite. The large hemozoin {100} face, the heme detoxification protein and the rate of catabolization of hemoglobin all play a key role in the delicate balance of heme detoxification process and as such are an attractive target for an antimalarial drug treatment.

This research strategy, based on combination of quantitative in-vivo measurements and computational modelling, has a strong potential to be expanded to other drugs and treatment pathways in malaria, encompassing a wider range of fundamental questions in chemistry and biophysics. Finally, this research platform can be transferred onto other biological systems.