Identification of the molecular mechanism by which mTORC1 regulates endolysosomal nutrient acquisition

• Applicant: Dr. Wilhelm Palm
• Subject Area: Cell Biology
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445249866

Nutrient uptake supplies energetic and biosynthetic pathways, which underlie all cellular functions. Thus, it is critical to understand how cellular nutrient acquisition is regulated to sustain metabolic homeostasis during growth and starvation, and how its dysregulation drives metabolic alterations in human disease. While it was assumed that cells rely on free amino acid uptake to survive and grow, we discovered that endocytosis and lysosomal catabolism of extracellular proteins constitutes an alternative amino acid source. We further identified a critical function for the mTORC1 signaling pathway in blocking endolysosomal generation of amino acids from extracellular proteins. As a surprising corollary, mTORC1 inhibition, while suppressing growth in amino acid-replete conditions, effectively enhances growth of nutrient-deprived cells and vascularly compromised tumors. Here, I propose to determine the cellular process and molecular mechanism by which mTORC1 regulates the endolysosomal system to suppress the nutritional use of extracellular proteins and thereby prevents cell proliferation under amino acid deprivation. Specifically, I propose three independent and complementary objectives:Objective 1 Define the subcellular process through which mTORC1 blocks lysosomal catabolism of endocytosed proteins: We will apply imaging approaches to determine whether mTORC1 regulates endosomal trafficking or lysosomal proteolytic activity.Objective 2 Identify endolysosomal mTORC1 kinase targets: Combining subcellular fractionation and proteomics, we will determine the mTORC1-regulated phosphoproteome of endosomes and lysosomes. Based on this, we will identify mTORC1 kinase targets that are associated with the endolysosomal system.Objective 3 Functionally investigate how endolysosomal mTORC1 effectors regulate the nutritional use of extracellular proteins: We will dissect the mechanism through which mTORC1 suppresses catabolism of extracellular proteins. To establish pathophysiological relevance, we will investigate how endolysosomal mTORC1 effectors regulate proliferation of cancer cells that feed on extracellular proteins. The results will identify the subcellular step and molecular mechanism through which mTORC1 blocks lysosomal catabolism of endocytosed proteins. The results will help understand how cells proliferate in the presence of different amino acid sources, with important implications for the ambivalent role of mTORC1 in cancer.

• DFG Programme: Research Grants