Deformability of Plasmodium falciparum parasitized red blood cells

Abstract

The biophysical properties of the human red blood cell (RBC) permit large deformations required for passage through narrow capillaries and spleen sinusoids. Several pathologic conditions alter RBC deformability that can result in abnormal circulation behavior. In the present work, altered RBC deformability caused by invading Plasmodium falciparum parasites, which are responsible for the disease malaria, is evaluated. P. falciparum parasitized RBCs (pRBCs) display decreased deformability and novel cytoadherence properties, and sequester in the microcirculation. Parasite-exported proteins that interact with the pRBC membrane are identified as the cause for these alterations. It is postulated that sequestration of pRBCs is responsible for severe cases malaria. Previous studies of pRBC deformability could not characterize deformability over all parasite intra-erythrocytic developmental stages due to experimental limitations. In the present work, a technique based on optical tweezers is developed to permit testing of pRBC deformability over all intra-erythrocytic stages. Optical tweezers can measure the force versus displacement response of a single RBC in uniaxial tension. The membrane shear modulus, which is a major factor in determining overall RBC deformability, can be interpreted based on the single RBC force versus displacement response.

(cont.) Initial tests with optical tweezers conducted on healthy RBCs demonstrate that shear modulus values were consistent with accepted values from standard techniques. Next, deformability of P. falciparum pRBCs was measured at all intra-erythrocytic parasite developmental stages. Deformability is found to decrease ten-fold during parasitization, which is three to four times greater than previously estimated. Finally, optical tweezers experiments are combined with targeted gene disruption techniques to measure the effect of a single parasite-exported protein on pRBC deformability. It is shown that Ring-infected Erythrocyte Surface Antigen (RESA), a membrane binding parasite-exported protein, plays a major role in reducing deformability of pRBCs at the early stages of intra-erythrocytic parasite development. Furthermore, the effect of RESA ondeformability is more pronounced at febrile temperature, which early stage pRBCs can be exposed to during a malaria attack, than at normal body temperature.