Plasmodium parasites encounter diverse conditions as they cycle between their vertebrate host and the mosquito vector. Adaptation to these distinct environments requires the parasite to drastically change its morphology and metabolism. A key to the parasite's successful intracellular development in the liver is the conversion of the elongated sporozoite to the round trophozoite, a process that prepares the parasite for the replication in hepatocytes. Very little is known about the biological systems involved in this phenotypic transformation. Our published work has outlined that converting sporozoites expel into their environment organelles such as micronemes that are needed for host cell invasion but useless for parasite replication. We found that the ATG8-conjugation system of the parasite's autophagy machinery is upregulated during sporozoite differentiation, suggesting that an autophagy-like process is activated during conversion. Our preliminary data reveal that the parasites sequester micronemes into autophagosomal compartments defined by the presence of parasite ATG8. We provide morphological evidence that PbATG8-labeled structures associate with the Golgi protein PbGRASP and combine with endosomal multivesicular bodies to form PbVPS4- and PbGRASP-positive autophagic compartments, or amphisomes. In eukaryotic cells, GRASP promotes the fusion of amphisomes with the plasma membrane to release the amphisomal content extracellularly. The overall goal of this proposal is to provide a detailed picture of the mechanisms underlying autophagy-related pathways in sporozoites during their conversion into liver forms. Our hypothetical model highlights a cooperation between the endocytic-exocytic and autophagic systems in intrahepatic Plasmodium to promote microneme exocytosis.
Specific Aim 1 will obtain a real-time view of autophagosome maturation and itinerary for extracellular microneme disposal.
Specific Aim 2 will analyze the phenotypic traits of Plasmodium Atg8 conditional knockout parasites to evaluate the importance of the parasite ATG8-conjugation system for microneme exocytosis.
Specific Aim 3 will investigate the contribution of Plasmodium GRASP to the process of secretory autophagy by generating parasites lacking GRASP expression. While the development of new therapeutics targeting the liver stage has the potential to arrest the onset of a malaria infection, extensive efforts must be deployed to better understand the events that take place in the infected liver. The results of the proposed studies are likely to identify unique and essential components of the organelle remodeling pathways that can serve as potent targets for pharmacologic therapy.
Malaria is responsible for more mortality than any parasitic disease. Malaria parasite species that infect humans, must first take up residence in liver cells before invading red blood cells which instigates the pathology associated with malaria. We are studying the molecular events leading to the parasite metamorphosis in the liver in order to reveal unusual mechanisms that would be amenable to therapeutic intervention.
| Voss, Christiane; Ehrenman, Karen; Mlambo, Godfree et al. (2016) Overexpression of Plasmodium berghei ATG8 by Liver Forms Leads to Cumulative Defects in Organelle Dynamics and to Generation of Noninfectious Merozoites. MBio 7: |
