Revealing the Sequence and Resulting Cellular Morphology of Receptor-Ligand Interactions during Plasmodium falciparum Invasion of Erythrocytes

Abstract

During blood stage Plasmodium falciparum infection, merozoites invade uninfected erythrocytes via a complex, multistep process involving a series of distinct receptor-ligand binding events. Understanding each element in this process increases the potential to block the parasite’s life cycle via drugs or vaccines. To investigate specific receptor-ligand interactions, they were systematically blocked using a combination of genetic deletion, enzymatic receptor cleavage and inhibition of binding via antibodies, peptides and small molecules, and the resulting temporal changes in invasion and morphological effects on erythrocytes were filmed using live cell imaging. Analysis of the videos have shown receptor-ligand interactions occur in the following sequence with the following cellular morphologies; 1) an early heparin-blockable interaction which weakly deforms the erythrocyte, 2) EBA and PfRh ligands which strongly deform the erythrocyte, a process dependant on the merozoite’s actin-myosin motor, 3) a PfRh5-basigin binding step which results in a pore or opening between parasite and host through which it appears small molecules and possibly invasion components can flow and 4) an AMA1–RON2 interaction that mediates tight junction formation, which acts as an anchor point for internalization. In addition to enhancing general knowledge of apicomplexan biology, this work provides a rational basis to combine sequentially acting merozoite vaccine candidates in a single multi-receptor-blocking vaccine.

Author Summary

The development of an effective malaria vaccine is a world health priority and would be a critical step toward the control and eventual elimination of this disease. In addition, new pharmacological solutions are necessary as Plasmodium falciparum, the deadliest of the malaria-causing parasites, has developed resistance to every drug currently approved for treatment. Understanding the interactions required for the parasite to invade its erythrocyte host, as well as being valuable to our basic knowledge of parasite biology, is important for the development of drug-based therapies and vaccines. In this study we have, for the first time, filmed P. falciparum parasites invading erythrocytes while systematically blocking several specific interactions between the parasite and the erythrocyte. We have shown there is a sequential progression of specific interactions that occur in at least four distinct steps leading up to invasion. Previous vaccine attempts have targeted one or two of these steps, however, if a single vaccine were designed to block interactions at all four steps, the combined effect might so reduce invasion that parasite growth and disease progression would be arrested. A better understanding of each interaction during invasion, their role and order, can also inform the development of new anti-malarial drugs.