UCSF

Malaria is caused by protozoan parasites of the genus Plasmodium. Among the five species that infect humans, P. falciparum is most lethal, though its range is restricted to the tropics. P. vivax, responsible for up to 40% of disease episodes, can be found in temperate zones as well and is a serious cause of morbidity and reduced economic well-being. Cumulatively, parasite species precipitate in excess of half a billion malaria episodes each year, a million fatal.

The lifecycle of Plasmodium parasites is complex, encompassing multiple sexual and asexual stages specialized to inhabit a preferred tissue-type in host or vector. Disease symptoms and pathology, however, stem from the asexual intraerythrocytic cycle, during which parasites repeatedly invade and proliferate within red blood cells; these blood-stage parasites are the target of antimalarial chemotherapy, and the development during the 20th century of novel antimalarial drugs reduced the proportion of the world's population at risk of malaria from over 70% in 1900 to 40% today. However, acquisition of resistance to control measures has led to a recent increase in disease incidence and nearly complete loss of efficacy of the former frontline antimalarial chloroquine.

While pathogenic resistance to chemotherapeutics is often conferred by the mutation of their target proteins, chloroquine's toxic effect to Plasmodium parasites stems from its impairment of an essential but immutable process, that of the crystallization of host-derived heme liberated from hemoglobin during amino acid scavenging. In P. falciparum, resistance to the drug arises from selection of mutations in the P. falciparum chloroquine resistance transporter (PFCRT), but other Plasmodium parasites exhibit independent modes of chloroquine resistance acquisition. Plasmodium's genetic intractability precludes comprehensive in vivo examination of the mechanics of chloroquine resistance. We have expressed PFCRT in S. cerevisiae for chemical-genetic profiling in the presence and absence of chloroquine and PfCRT; we have also investigated yeast's short-term transcriptional response to chloroquine treatment. We find that chloroquine impairs yeast vacuolar homeostasis, protein trafficking, and ability to sense nutritional repletion, and that expression of chloroquine-resistant PFCRT can partially suppress these defects.