Trends in Parasitology
Volume 23, Issue 10, October 2007, Pages 481-484
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Is invasion efficiency in malaria controlled by pre-invasion events?

https://doi.org/10.1016/j.pt.2007.08.001Get rights and content

The invasion efficiency of Plasmodium falciparum merozoites was found to decrease with increasing red blood cell density, a finding relevant to protection strategies against falciparum malaria. The mechanism of this ‘density effect’ remained unexplained. Searching for possible explanations, we studied selected video recordings of the dynamic events during merozoite invasion and identified a pre-invasion stage as a crucial mediator of invasion efficiency. We suggest that the role of the pre-invasion stage is to induce the apical alignment of the merozoite, and propose a working hypothesis on its mechanism, with a crucial role for elevated intracellular Ca2+.

Section snippets

The ‘density effect’ and the search for possible mechanisms

Recent observations [1] demonstrated that the efficiency of Plasmodium falciparum merozoites to invade red blood cells (RBCs) decreases inversely with RBC density. The presence of abnormally dense RBCs in the circulation, by reducing the fraction of RBCs available for invasion, would prevent the development of high parasitaemias, and hence reduce the incidence of cerebral malaria, which is the form of the disease that causes mortality [2]. This represents a protection strategy additional to any

Examining the invasion process

The video recording by Dvorak et al. [7], with fixed focus at ×1000 magnification, shows the invasion events in real time in a variety of cells. In one instance, a merozoite, reportedly attached to the glass, is seen in edge contact with a typical biconcave-shaped RBC. While the merozoite remained steady, the RBC is seen to move spontaneously towards and away from the merozoite with its membrane enfolding substantial extents of the merozoite surface in each approach, reminiscent of phagocytic

Analysis of the merozoite invasion sequence

The observations just described identify two distinct dynamic morphological stages during merozoite invasion. In the first stage, the pre-invasion stage, merozoite attachment triggers immediate and rapid shape changes of the targeted RBC. These enfold the merozoite, radiating from the point of merozoite attachment, and vary continuously in shape and direction. After a variable time of ∼10–30 s, all dynamic motion ceases, and the RBC returns to its relaxed biconcave shape while the second stage,

Function of the pre-invasion stage

Because penetration starts immediately after the first wave of deformation, it follows that the merozoite must be apically aligned at the end of the pre-invasion stage. Hence, the processes taking place during the pre-invasion stage represent the actual mechanism of apical reorientation, a function also foreseen by Hermentin [15] and by Cowman and Crabb [16]. The duration of the pre-invasion stage would be determined by the time it takes to position the merozoite in apical contact with the RBC,

A working hypothesis on the pre-invasion stage

Of the various mechanisms that could possibly explain the pre-invasion events we wish to consider one which might have become amenable to experimental scrutiny with current methodologies. Invasion requires the presence of Ca2+ in the medium 19, 20, 21, 22. It is prevented in intact RBCs loaded non-disruptively with Ca2+ chelators and suspended in Ca2+-containing media, indicating that external Ca2+ is required for elevating the intracellular free calcium concentration ([Ca2+]i) 23, 24.

Significance and relevance of the working hypothesis

The pre-invasion process tests the ability of RBCs to provide an adequate motile response to merozoite contacts. A poor motile response reduces the probability of a successful apical alignment and hence the frequency of invasion. Thus invasion efficiency will be reduced in any condition in which abnormalities of RBC hydration, haemoglobin, cytoskeleton, membrane lipid composition, metabolism, or membrane transport generate a deficient motile response during the pre-invasion stage 43, 44, 45.

Acknowledgements

We thank the Wellcome Trust and the Biotechnology and Biological Sciences Research Council for funds. We are grateful to Hagai Ginsburg and to the anonymous Reviewers for helpful comments and criticisms.

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