Nucleoside Transport as a Potential Target for Chemotherapy in Malaria

Abstract

Malaria constitutes an enormous drain on the health and economies of many countries and causes more than a million deaths annually. Moreover, resistance to existing antimalarial drugs is a growing problem, rendering the search for new targets urgent. Protozoan parasites of the genus Plasmodium that cause malaria lack the ability to synthesise the purine ring de novo and so are reliant upon salvage of purines, including hypoxanthine, inosine and adenosine, from the host. The transport systems responsible for uptake of these precursors are therefore promising targets for novel antimalarial drugs. In humans, purine uptake into many cell types is mediated by members of the Equilibrative Nucleoside Transporter (ENT) family, in particular hENT1 and hENT2. Genome sequencing has revealed that P. falciparum and P. vivax, the species responsible for the majority of malaria cases, each also possesses four members of this family, and in P. falciparum transcripts of each are expressed in the erythrocytic stages of the parasite responsible for clinical disease. One of the proteins, PfENT1, is known to be present in the parasite plasma membrane, and the kinetic properties of the heterologously expressed transporter are consistent with its representing the major purine uptake system in the trophozoite. Importantly, its inhibitor specificity and permeant selectivity differ from those of the host. In this review we discuss the possibility of exploiting these differences to develop novel antimalarial drugs that either selectively inhibit purine uptake into the pararasite or are selectively delivered by the transporter to the parasite cytoplasm.

Transmembrane Segment 11 Appears to Line the Purine Permeation Pathway of the Plasmodium falciparum Equilibrative Nucleoside Transporter 1 (PfENT1)

“…There have been several attempts to generate structural models of ENT family proteins. One generated a homology model of the PfENT1 transporter based on the structure of the prokaryotic glycerol-3-phosphate transporter, a member of the major facilitator superfamily (32,17,33), and the other used the Rosetta protein structure prediction software to generate an ab initio model of LdNT1.1 (34). The organization of the transmembrane segments was similar in both models supporting the hypothesis that the ENTs may be members of the major facilitator superfamily (34).…”

“…PfENT1 knock-out parasites are not viable during in vitro culture at physiologic purine concentrations but can be rescued by growth at supraphysiologic purine levels (13,15). Thus, a PfENT1 inhibitor might be an effective antimalarial drug (17). The ENTs might also serve to transport antimalarial drugs that target purine salvage pathway enzymes into the parasite cytoplasm so that they can reach their target proteins (18).…”

Identification of the Intracellular Gate for a Member of the Equilibrative Nucleoside Transporter (ENT) Family

“…Support for structural commonality between ENTs and the major facilitator superfamily (MFS) of transporters was recently provided by site-directed mutagenesis studies of mammalian and protozoa ENTs, suggesting a common evolutionary origin (10) and similar packing of transmembrane (TM) helices around a solvent-accessible permeant binding site (11). These observations allowed the construction, by computational approaches, of putative tertiary structures of several protozoan ENTs, including Plasmodium falciparum PfENT1 (12), Trypanosoma brucei TbNBT1 (13), and Leishmania donovani LdNT2 (14). and LdNT1.1 (15).…”

Purine Salvage Pathways in the Intraerythrocytic Malaria Parasite Plasmodium falciparum

“…We provide a short description of this process below; more-thorough reviews of purine transport in Plasmodium spp. have been provided elsewhere (2,29).…”