Drexel University College of Medicine

Malaria is one of the world’s most intractable human afflictions. Despite intensive campaigns against the parasitic disease, an estimated 300 to 500 million cases still occur each year, and the situation may be worsening due, in large part, to the emergence and spread of drug resistant parasites. As a member of the Center for Molecular Parasitology, under the direction of Dr. Akhil Vaidya, I am involved in studies on the parasite’s mitochondrial function, the integration of the mitochondrion into cellular physiology, and the function of proton pumps in the parasite’s cellular membrane, with an eye toward the identification and characterization of potential drug targets.

Following the completion of the recent genomic sequencing projects for the human malaria parasite Plasmodium falciparum and several other malarial and apicomplexan species, many research studies have been initiated to uncover new targets for drug development. With the increasing occurrence in the field of resistance to commonly used antimalarial drugs, such new (and ideally inexpensive) drugs are urgently needed.

One relatively new antimalarial drug is atovaquone, which targets the ubiquinol-cytochrome c oxidoreductase in the mitochondrial membrane. Our laboratory has been involved in studies to elucidate the mechanism of action of the drug, the cause of the relatively facile development of resistance by the parasite, and the mechanism behind the synergistic action of the prodrug proguanil when administered together with atovaquone. Recently, additional compounds from multiple chemical classes have been discovered that target the same complex and hold out the promise of drugs that are less susceptible to the development of resistance and less costly to manufacture.

The functional roles of many additional enzymes potentially involved in central metabolism, energy conversion and mitochondrial physiology, and their potential as drug targets, are under investigation in our laboratory. These include the ATP synthase, dihyroorotate dehydrogenase and several other ubiquinone-dependent oxidoreductases, TCA cycle enzymes, ADP-ATP translocators, and proton-pumping pyrophosphatases. Exciting results have recently emerged from work on the “TCA cycle” pathway. In Plasmodium, the “cycle” turns out to be a non-cyclical, bifurcated pathway, in contrast to the metabolic arrangements in most other eukaryotic organisms. Furthermore, we have generated genomic knockouts of the genes encoding four of the enzymes, representing both pathway branches, establishing that they are not essential for the growth of the blood stage of the parasite. In contrast, two other TCA enzymes proved recalcitrant to disruption and thus may be required for growth, making them potential drug targets. Collaborative studies are ongoing to work out the details of the metabolic pathways and any alternatives that may come into play when specific enzymes are knocked out.

The malarial ATP synthase may also be unusual, since no genes for the critical a and b subunits that are required for energy coupling could be found in the genome. We recently initiated studies of the ATP synthase in the ciliate Tetrahymena thermophila, a distant relative from which mitochondria are more easily prepared in quantity. Structural EM and proteomics studies revealed a ciliate ATP synthase that indeed has novel features, as well as revealing candidate protein subunits that may serve as divergent a and b subunits. One Tetrahymena b subunit candidate has a possible homolog in apicomplexan species, which we are investigating. We are now applying techniques learned in the Tetrahymena project to initiate studies of the Plasmodium ATP synthase.

Dr. Mather is a research assistant professor in the Department of Microbiology and Immunology at Drexel University College of Medicine.