Regulation of mTORC1-Dependent Growth Signaling by Lysosomal Cholesterol
Mechanistic target of rapamycin complex 1 (mTORC1) is an evolutionarily conserved serine/threonine protein kinase that acts as a master regulator of growth and proliferation in eukaryotic cells. mTORC1 integrates a variety of signals including cellular stress, nutrients and growth factors, and, in response, phosphorylates key substrates involved in coordinating cellular metabolism. When nutrients are abundant, mTORC1 fuels the biosynthesis of cellular building blocks including nucleotides and lipids, and enhances mRNA translation and ribosomal biogenesis. In addition, mTORC1 inhibits catabolic pathways such as autophagy, which consume cell mass.
A heterodimer of small guanosine triphosphatases, known as the Rag GTPases, communicates cellular nutrient status to mTORC1, such that the active nucleotide state (RagAGTP/RagCGDP) promotes translocation of mTORC1 from the cytosol to the surface of the lysosome. At the lysosome, another small GTPase, Rheb, binds to mTORC1 and unlocks the autoinhibited kinase domain in response to growth factor signals. Thus, growth regulation occurs via an elegant coincidence detection mechanism, whereby intracellular nutrients direct mTORC1 to a physical location, and growth factors provide long-range signals from distant tissues to activate mTORC1 kinase via Rheb.
Despite major advances in our understanding of mTORC1 regulation, it remains unclear how mTORC1 senses and integrates chemically diverse molecules. Here, I identify cholesterol, an essential lipid for cellular growth, as a nutrient input that drives mTORC1 recruitment and activation at the lysosomal surface. I found that the lysosomal transmembrane protein SLC38A9 is required for mTORC1 activation by cholesterol through conserved cholesterol-responsive motifs. Moreover, SLC38A9 enables cholesterol-mediated mTORC1 activation independently from its arginine-sensing function. Conversely, the Niemann-Pick C1 (NPC1) protein, which regulates cholesterol export from the lysosome, binds to SLC38A9 and inhibits mTORC1 signaling through its sterol transport function. Thus, lysosomal cholesterol drives mTORC1 activation and growth signaling through the SLC38A9-NPC1 complex. Importantly, this suggests that aberrant mTORC1 activity may contribute to the metabolic derangement observed in Niemann-Pick Type C (NPC) disease in humans.
The mechanism of mTORC1 recruitment by the lysosomal scaffolding complex is not fully understood. To address this, I purified the full-length human Ragulator:RagA/C complex in the active nucleotide loading state and, in collaboration with the Hurley Lab, determined the cryo-EM structure. This is the first structure to reveal the nucleotide binding G domains of the Rag GTPases in the context of their lysosomal scaffold, Ragulator. Overall, this work elucidates a new nutrient input to the mTORC1 signaling network and provides structural insights into the regulation of mTORC1 docking at the lysosome, suggesting new strategies for therapeutically targeting metabolic diseases.