The Digestive Food Vacuole of the Malaria Parasite Is a Dynamic Intracellular Ca2+ Store

Abstract

The acidic food vacuole of Plasmodium falciparum has been the subject of intense scientific investigation in the 40 years since its role in the digestion of host hemoglobin was first suggested. This proposed role has important implications for the complex host-parasite inter-relationship and also for the mode of action of several of the most effective antimalarial drugs. In addition, adaptive changes in the physiology of this organelle are implicated in drug resistance. Here we show that in addition to these functions, the digestive food vacuole of the malaria parasite is a dynamic internal store for free Ca 2؉ , a role hitherto unsuspected. With the aid of live-cell laser scanning confocal imaging, spatiotemporal studies revealed that maintenance of elevated free Ca 2؉ in the digestive food vacuole (relative to cytosolic levels) is achieved by a thapsigargin (and cyclopiazonic acid)-sensitive Ca 2؉ -pump in cooperation with a H ؉ -dependent Ca 2؉ transporter. Redistribution of free cytosolic and vacuolar Ca 2؉ during parasite growth also suggests that vacuolar Ca 2؉ plays an essential role in parasite morphogenesis. These data imply that the digestive food vacuole of the malaria parasite is functionally akin to the vacuole of plants (tonoplast) and the small electron-dense granules of some parasites (acidocalcisomes) whereby H ؉ -coupled Ca 2؉ transport is involved in ion transport, Ca 2؉ homeostasis, and signal transduction. These findings have significant implications for parasite development, antimalarial drug action, and mechanisms of drug resistance.Malaria remains one of the largest global health (and economic) problems, resulting in more than a million deaths, mostly in young African children (1). With this in mind, it is astonishing how little is known about the basic physiology of the intraerythrocytic malaria parasite and its organelles. The food vacuole is a major digestive organelle of the malaria parasite and a proven chemotherapeutic target. This organelle has a role in degradation of host-derived hemoglobin, is the site of action of important classes of antimalarials, and harbors transporters associated with drug resistance (2-4). Hydrolysis of hemoglobin in the malaria parasite digestive food vacuole occurs by the integrated action of aspartic, cysteine, and metalloproteases (5), resulting in the production of hemozoin (malaria pigment), a biocrystal of the toxic precursor ferriprotoporphyrin IX (FPIX) 1 (6). The internal pH of the digestive food vacuole is lower than the parasite cytosol, although the precise pH value is under debate (7). In collaboration with Prof. Kiaran Kirk's laboratory (8), we have recently described the function of two H ϩ -pumping mechanisms in the digestive food vacuole; one is a bafilomycin-sensitive V-type H ϩ -ATPase, and the other is a NaF-sensitive H ϩ -pyrophosphatase showing a partial dependence on K ϩ . The combination of a H ϩ -ATPase and a H ϩ -pyrophosphatase acting in the digestive vacuole is analogous to the situation in acidic tonoplasts of plant cells (9)...

Chloroquine-resistant isoforms of the Plasmodium falciparum chloroquine resistance transporter acidify lysosomal pH in HEK293 cells more than chloroquine-sensitive isoforms

“…Ca 2+ ions have also been suggested as a potential modifier of parasite drug resistance in the P. falciparum DV [34]. Therefore, we measured the steady-state Ca 2+ content of lysosomes in HEK293 cells expressing PfCRT isoforms (Fig.…”

Quantitative Calcium Measurements in Subcellular Compartments of Plasmodium falciparum-infected Erythrocytes

“…However, a continuous net uptake of Ca 2ϩ occurs as the parasite matures (10), which has been attributed to an increased Ca 2ϩ permeability of the host erythrocyte plasma membrane (11,12). The parasite's cytoplasm is thought to contain a low free [Ca 2ϩ ] i of ϳ100 nM (13), similar to that of other eukaryotic cells (14), although quantitative determinations are controversial, ranging from 50 to 700 nM (15,16). Early during intraerythrocytic development, high concentrations of Ca 2ϩ appear to accumulate transiently in the parasitophorous vacuole (17), which separates the parasite from the erythrocyte cytosol.…”

“…Early during intraerythrocytic development, high concentrations of Ca 2ϩ appear to accumulate transiently in the parasitophorous vacuole (17), which separates the parasite from the erythrocyte cytosol. Other subcellular organelles considered to serve as intracellular Ca 2ϩ stores are the parasite's endoplasmic reticulum (ER), 1 the food vacuole (16,18), and the acidocalcisomes (15, 18 -22).…”

Antimalarial drugs disrupt ion homeostasis in malarial parasites

“…It has been reported that P. falciparum and P. chabaudi extrude H + to the extracellular medium by a Na + independent mechanism (Saliba & Kirk 1999). Other mechanisms for pH regulation described in malaria parasites may involve the V-H + -ATPase and the V-H + -PPase in acidocalcisomes (Marchesini et al 2000, McIntosh & Vaidya 2002, Saliba et al 2003, Biagini et al 2003. We have reported the existence of Ca 2+ pools in the acidic compartment of rodent as well as human malaria parasites as a possible mechanism for the regulation of calcium concentration in these parasites , Passos & Garcia 1998.…”