Metamorphosis and development of Plasmodium within liver cells

Coppens, Isabelle

Abstract

During their lifecycle, Plasmodium parasites encounter diverse conditions as they cycle between their vertebrate host and mosquito vector. Adaptation to these distinct environments requires the malaria parasite to drastically change its morphology and metabolism. For example, in the hepatocyte the motile sporozoite converts into a trophozoite, which prepares the parasite for the generation of thousands of infective erythrocyte-invasive forms. Our research goal is to decipher the molecular mechanisms involved in the metamorphosis of sporozoites into trophozoites. We observed that converting parasites undergo spectacular shape change and major interior remodeling, including the elimination of organelles unnecessary for the liver stage. Our central hypothesis is that the phenotypic changes associated with sporozoite differentiation are the result of multiple cellular processes. Our preliminary data enlighten the contribution of two biological systems, exocytosis and autophagy, to organelle elimination. We demonstrated that the parasite compartmentalizes organelles before their expulsion and trigger an autophagic process to selectively sequester cytosolic constituents destined for degradation. Plasmodium contains a rudimentary set of autophagy components, which includes the ubiquitin-like protein conjugation system of Atg8. We characterized this system and present evidence that it may play a role in sporozoite conversion. In this proposal, we will pursue our functional studies on the Atg8 conjugation system in Plasmodium liver stage and determine whether autophagy takes place alongside vesicular trafficking pathways as a degradative mechanism that controls parasite structure and differentiation.
Specific Aim I will focus on the formation of Atg8-containing structures including the characterization of the site of assembly of Atg proteins, and the role of the phosphatidylinositol 3-kinase complex in the docking of Atg proteins to the phagophore.
Specific Aim 2 will investigate the function of the Plasmodium Atg8 conjugation system in sporozoite metamorphosis and explore the itinerary of Atg8- containing structures, i.e. fusion with lysosomes for digestion or the plasma membrane for organelle expulsion. Upon completion of these aims, we expect to present on overview of the molecular machinery of autophagy-related proteins of Plasmodium, determine the interconnectivity of Atg8-containing compartments with parasite vesicular pathways and reveal the significant differences in the autophagy process between the parasite and mammalian cells. While the development of new therapeutics targeting the liver stage will have 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. In the long-term, analysis of the multifaceted aspects of sporozoite metamorphosis in the liver may provide new approaches to prevent malaria infections, based on chemotherapeutic interference with the Plasmodium differentiation machineries.

Public Health Relevance

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 parasite metamorphosis and adaptation to the liver in order to reveal unusual mechanisms that would be amenable to therapeutic intervention.