During its life cycle within human erythrocytes, Plasmodium falciparum undergoes major developmental and metabolic changes and multiplies to produce up to 36 new daughter parasites. This rapid multiplication requires an active synthesis of structural and signaling lipids important for many essential parasite functions such as the production of new membranes following parasite multiplication, and the synthesis of diacylglycerol for activation of parasite kinases, to name only a few. The metabolic machineries that govern the synthesis of these macromolecules are fueled by precursors such as serine, ethanolamine and fatty acids scavenged from the host. These machineries have long been regarded as excellent targets for the development of novel antimalarial drugs. To date only a few of these machineries have been thoroughly characterized in Plasmodium parasites and pharmacological studies targeting some of them have successfully resulted in the production of highly potent antimalarial drugs. Serine obtained from the host serves as the primary precursor for the synthesis of the major phospholipids phosphatidylcholine and phosphatidylethanolamine. Serine is decarboxylated by a parasite specific serine decarboxylase (PfSD) to form ethanolamine, which is subsequently used as a precursor for the synthesis of both phosphatidylcholine and phosphatidylethanolamine. Serine is also incorporated into phosphatidylserine, which serves as an alternate precursor for the synthesis of phosphatidylethanolamine, via a reaction catalyzed by a parasite phosphatidylserine decarboxylase (PfPSD). Because of their predicted essential functions, PfSD and PfPSD are regarded as potential targets for the development of new antimalarial drugs. Moreover, human cells do not contain SD enzymes thereby making SD a species-specific vulnerability of the parasite. Using a Plasmodium cDNA library constructed in a yeast expression vector we have successfully complemented a yeast mutant lacking PSD activity and identified the malarial PSD gene. Available data suggest that the malarial PSD plays an essential role in the intraerythrocytic life cycle of the parasite and is an excellent target for the development of nove antimalarial drugs. The malarial SD gene, however, remains to be identified. The overall objectives of this grant application are to complete the biochemical and genetic characterization of the PfPSD gene in P. falciparum (Aim 1); to take advantage of the newly developed and successful functional complementation assay using yeast as a surrogate system to screen a library of antimalarial active compounds to search for inhibitors of PfPSD activity (Aim 2); and employ genetic and biochemical analyses to identify the malarial serine decarboxylase gene and characterize its importance in P. falciparum intraerythrocytic development and survival (Aim 3). These studies hold the potential for elucidating the importance of PfSD and PfPSD specifically, and phospholipid metabolism in general during P. falciparum development as well as fostering the design of specific inhibitors. This work will provide new therapeutic insights for combating a disease that affects 250 million people worldwide and causes 1 million deaths each year.
The main objectives of the proposed research are to identify and characterize two steps in the biosynthesis of the major phospholipids of P. falciparum, which are predicted to play an essential role in parasite development and survival within human erythrocytes, and search for inhibitors that block these steps and kill the parasite.
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