Na+ Regulation in the Malaria Parasite Plasmodiumfalciparum Involves the Cation ATPase PfATP4 and Is a Target of the Spiroindolone Antimalarials
Chloroquine resistance in Plasmodium vivax threatens the use of this drug as first-line treatment for millions of people infected each year worldwide. Unlike Plasmodium falciparum, in which chloroquine resistance is associated with mutations in the pfcrt gene encoding a digestive vacuole transmembrane protein, no point mutations have been associated with chloroquine resistance in the P. vivax ortholog gene, pvcrt-o (also called pvcg10). However, the question remains whether pvcrt-o can affect chloroquine response independent of mutations. Since P. vivax cannot be cultured in vitro, we used two heterologous expression systems to address this question. Results from the first system, in which chloroquine sensitive P. falciparum parasites were transformed with pvcrt-o, showed a 2.2-fold increase in chloroquine tolerance with pvcrt-o expression under a strong promoter; this effect was reversed by verapamil. In the second system, wild type pvcrt-o or a mutated form of the gene was expressed in Dictyostelium discoideum. Forms of PvCRT-o engineered to express either lysine or threonine at position 76 produced a verapamil-reversible reduction of chloroquine accumulation in this system to approximately 60% of that in control cells. Our data support an effect of PvCRT-o on chloroquine transport and/or accumulation by P. vivax, independent of the K76T amino acid substitution.