The obligate intracellular protozoan Toxoplasma gondii is the etiological agent of toxoplasmic encephalitis that remains a major cause of death in AIDS patients. The currently available treatment for toxoplasmosis shows no efficacy against Toxoplasma long-term chronic infections, resulting in the recrudescence of active infections. New drugs are imperatively needed. During mammalian cell invasion, the parasite creates a specialized parasitophorous vacuole (PV) that is demarcated from the host cytoplasm by a unique membrane containing parasite proteins. Toxoplasma is notorious for extensively modifying the host cell and rerouting host organelles to its PV, largely for purposes of nutrient acquisition. The parasite usually establishes its 'nest'near the host peri-Golgi microtubule-organizing center. This region is at the intersection of the endocytic and biosynthetic pathways, and PV positioning in this area of the cell could facilitate the interception of vesicular traffic and satisfy parasite requirements for nutrients and lipid membranes. In support of this hypothesis, we demonstrated that T. gondii has a marked requirement for host cell lipids derived both from lysosomes (e.g. cholesterol) and the Golgi (e.g. sphingolipids). The PV does not fuse with host organelles, and the mechanisms by which the parasite retrieves lipids from endocytic and exocytic organelles are still poorly understood. The overall goal of our proposal is to decipher the molecular details of host lysosome and Golgi exploitation by Toxoplasma. We previously showed that Toxoplasma uses the host microtubular network to redirect host lysosomes to its vacuole and sequester these organelles within PV membrane invaginations formed by microtubules. This strategy allows the parasite to have access to nutrients provided by organelles of the endocytic cascade. However, extensive gaps remain in our current model.
Specific Aim 1 proposes experiments to better understand the intersection of T. gondii with the host endocytic pathway. We will clarify the nature and contribution of the endocytic structures intercepted by Toxoplasma to parasite development. We will examine the molecular mechanism of implicated in host microtubule-PV interaction and PV membrane transformations for lysosome sequestration. Almost nothing is known about the role of host Golgi during Toxoplasma infection. Our preliminary data show that the parasite associates with the Golgi, fragments this organelle into Golgi ministacks that align along the PV and intercepts the Golgi vesicular trafficking by engulfing secretory vesicles into the PV. This strategy may allow the parasite to retrieve Golgi lipids from sequestered Golgi vesicles.
Specific Aim 2 combines experiments to verify our hypothetic model on host- Golgi-PV interaction. We will study the mechanisms leading to host Golgi breakdown and the delivery of Golgi vesicles to the PV by identifying the host Golgi proteins that are targeted by the parasite. We will explore the connection between Golgi remodeling and lipid salvage by the parasite. Our results may raise the provocative notion of novel treatments against Toxoplasma based on interference with host organelle functions.
The human parasite Toxoplasma is the most common cause of brain lesions in HIV/AIDS patients. This pathogen invades mammalian cells and develops within a specialized niche that serves as a platform for modulation of host cell functions. Our proposal focuses on the identification of unique features of lipid uptake from host organelles developed by the parasite in order to uncover targets for new therapeutics.
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