Date of Award
Doctor of Philosophy (PhD)
Biochemistry and Molecular Biology
Entamoeba histolytica is a parasitic protozoan that causes amebic dysentery, which affects approximately 90 million people annually, killing between 50-100,000. E. histolytica is transmitted via the oral-fecal route, through ingestion of contaminated food and water. The infectious form of the disease is the quiescent cyst, which after ingestion, undergoes excystation in the small intestine to the proliferative trophozoite. The trophozoite is the disease-causing form of the parasite, which colonizes the large intestine. Some trophozoites encyst in the large intestine and are passed out of the host, propagating the infectious cycle.
Despite the global burden of this disease, study of the infectious cycle has been impeded by the lack of an E. histolytica model for encystation and excystation. The reptilian pathogen Entamoeba invadens (E. invadens) has been used as a model; however, the extensive evolutionary differences between these species, and the different environments they inhabit, render E. invadens an imperfect model. We present here the first model for E. histolytica encystation in axenic culture. The generated cysts displayed all four hallmarks: tetranucleation, a chitinous cell wall, small round shape, and detergent resistance. High cell density and glucose starvation were shown to be essential signals for encystation. Additionally, the presence of physiologically relevant concentrations of short chain fatty acids was shown to accelerate encystation in the parasite.
E. histolytica relies heavily on glycolysis for production of ATP. However, E. histolytica colonizes the human gut, where glucose is extremely scarce. Glycogen is used in Entamoeba as a storage molecule, providing glucose as a precursor to either glycolysis or chitin synthesis during encystation. Therefore, glycogen metabolism in E. histolytica is thought to be essential for both the growth and proliferation of the parasite. Here, we examine glycogen metabolism in E. histolytica, and its role in cell growth and encystation. We examine how trophozoites grow in conditions of nutrient stress and show that cellular glycogen consumption allows for proper cell growth even absent glucose. We also show generate two knockdown strains, inhibiting the function of glycogen synthase and two glycogen phosphorylases, each responsible for half of the glycogen cycle. These knockdowns are prevented from accumulating cellular glycogen, growing, and encysting in a comparable fashion to wild type cells. This demonstrates that glycogen synthesis and degradation are vital to the growth and propagation of the parasite in a low glucose environment.
Wesel, Jordan Lynne, "Encystation and Glycogen Metabolism in Entamoeba histolytica" (2020). All Dissertations. 2752.