Exposure of Plasmodium sporozoites to the intracellular concentration of potassium enhances infectivity and reduces cell passage activity
Introduction
Plasmodium sporozoites traverse the cytosol of cells prior to transformation into exoerythrocytic (EEF) stages [1]. The ability of sporozoites to traverse host cells is required for the completion of the Plasmodium life cycle. When infected mosquitoes feed on the mammalian hosts, the sporozoites are deposited in a pool of blood formed by the rupture of skin capillaries [2]. As shown by real time studies, the parasites pass through skin cells prior to entering the blood or lymph [2], [3]. Once in the blood circulation, sporozoites are retained in the liver sinusoids by the interaction of the circumsporozoite protein (CS) with heparan sulfate proteoglycans (HSPGs) [4], [5]. Then the parasites glide onto the endothelium, and traverse Kupffer cells [6], to enter the liver parenchyma. Only after navigating rapidly through the cytoplasm of several hepatocytes [7], they turn on a program for a “productive” invasion that leads to the formation of a parasitophorous vacuole (PV) whereupon sporozoites transform into EEFs. The sporozoite “activation” is associated with the exocytosis of microneme products including TRAP (thrombospondin related anonymous protein), [8] an essential component of the molecular motor that drives the active invasion process [9], [10]. While sporozoites enter and exit the cytoplasm of host cells, the parasites are exposed to profound changes in K+ concentration. Here we explore the possibility that these shifts in [K+] trigger the profound phenotypic changes that sporozoites undergo when they travel from the skin to the liver.
Section snippets
Sporozoites
P. berghei and P. yoelii cycles were maintained by allowing Anopheles stephensi mosquitoes to feed on SW mice (Jackson Laboratory) that had been infected with sporozoites. Midgut infection level was assessed on day 12 for P. berghei and day 8 for P. yoelii by checking for the presence of the midgut oocyst stages. The infected salivary glands were dissected on days 18–20 post blood meal for P. berghei and on days 14–16 for P. yoelli in DMEM (Gibco) containing 2 mM l-glutamine and 4.5 g/l glucose
Results and discussion
P. berghei sporozoites were incubated for 35 min at room temperature in medium containing [142 mM] K+ (“intracellular” medium), or with the same medium but containing instead [142 mM] Na+ (“intracellular control” medium), or with “control” medium (DMEM containing 4.5 g/l glucose supplemented with 10% FCS). Following incubation in different media, the sporozoites were added to hepatoma cultures maintained in control medium. In this paper the sporozoites exposed to [142 mM] K+ are named “treated”. In
Acknowledgements
The authors thank Dr. Ruth Nussenzweig for valuable comments on the manuscript. We also thank Dr. Photini Sinnis and Dr. Alida Coppi for sharing the protocol and assistance with performing calcein green migration assay.
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