Biology and Biomedical Sciences: Molecular Microbiology and Microbial Pathogenesis
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
The apicomplexan parasite, Plasmodium falciparum, is the causative agent of the most severe form of malaria, resulting in nearly 1 million deaths each year. The parasite establishes its replicative niche within human erythrocytes, where it degrades massive amounts of host cell hemoglobin, salvaging the released amino acids for its own use. However, human hemoglobin does not contain the amino acid isoleucine, which is one of the most prevalent amino acids in the parasite's proteome. Since P. falciparum cannot synthesize isoleucine, it must acquire this amino acid from human serum. Optimal growth and, ultimately, the survival of P. falciparum depend on the availability of circulating essential nutrients such as isoleucine, which is often scarce in undernourished malaria patients. To understand how P. falciparum responds to isoleucine starvation, we monitored parasite growth in isoleucine-limiting conditions. We observed that in vitro parasite growth is notably slower in medium containing low concentrations of isoleucine, but completion of the life cycle, consisting of steady progression through the ring, trophozoite, and schizont stages, followed by subsequent rounds of re-invasion and gradual expansion of the culture, continues at a reduced rate. However, when subjected to isoleucine starvation, parasites progress only through the trophozoite stage. Interestingly, supplementation with isoleucine restores normal asexual growth, suggesting the involvement of sensory/response elements in the growth control mechanism of the parasite. The focus of this thesis was to characterize the dynamic metabolic properties of this remarkable starvation-induced state in P. falciparum and uncover the molecular basis behind this response. In this work, it was found that isoleucine starvation effectively slows down the metabolic growth of P. falciparum, resulting in cell cycle inhibition, reduced protein translation, and delayed gene expression. Although appreciable parasite growth could be recovered upon isoleucine repletion even after several days of starvation, active proteolysis during extended starvation was required to maintain viability. The canonical amino acid-starvation responsive GCN2/ eIF2α signaling pair is functionally conserved in P. falciparum, exhibiting remarkable specificity in detecting isoleucine availability, however, its activity was not essential to preserving the parasite in a growth-competent state during starvation. These data indicate that the starvation response of P. falciparum is unique: although the parasite maintains an active remnant of a conventional eukaryotic amino acid-stress response pathway, its regulatory role is inconsequential. We conclude that isoleucine starvation induces a hibernating state in P. falciparum, an effective default pathway suitable for its parasitic lifestyle.
Ledbetter, Shalon, "The Unconventional Amino Acid Starvation Response of the Malaria Parasite, Plasmodium falciparum" (2012). All Theses and Dissertations (ETDs). 708.