The end stages of the cellular engulfment mechanisms phagocytosis and entosis, which mediate the uptake of exogenous substrates, and autophagy, which mediates the engulfment of intracellular substrates, involve lysosomal digestion of engulfed cargo and export of catabolites to the cytosol for use in biosynthesis. Despite the critical role of these pathways for the removal of targeted substrates, and the importance lysosome- mediated digestion for the actual clearance and recycling of engulfed material, little is known about how the lysosomal processing of engulfed cargo and export of degraded components is regulated. We have found that the mTORC1 protein kinase, a regulator of mRNA translation and autophagy, and the lipid kinase PIKfyve, are required for a program of phagosome fission that shrinks large lysosomal vacuoles as internalized cargo is degraded, which reminiscent of the recently described autophagic lysosome reformation (ALR) program that functions similarly during autophagy. Vacuole fission is associated with nutrient recovery that rescues engulfing cells from the effects of amino acid or glucose/pyruvate starvation, and reactivates mTORC1. Reactivated mTOR recruits specifically to large lysosomal vacuoles harboring degrading cells and controls their fission. The proposed research will identify amino acid and sugar transporters acting at lysosomes that mediate nutrient recovery and mTORC1 activation, and will identify mTOR and PIKfyve-regulated proteins that control the fission of lysosomal vacuoles.
The mTOR kinase is a promising therapeutic target for the treatment of a variety of cancers such as breast carcinoma, lung and colon carcinoma, leukemias, and others, primarily due to its control over mRNA translation. But the mechanism of upstream activation of mTOR, as well as the consequences of mTOR inhibition, is only partially understood. The proposed studies will identify mechanisms that control the activity of mTOR by amino acid and glucose signaling from lysosomes, and examine a novel downstream function of mTOR in controlling lysosome membrane dynamics.