Sterol-dependent regulation of mTORC1 activation at the lysosomal surface

The mechanistic target of rapamycin complex 1 (mTORC1) is a multi-subunit protein kinase that regulates growth and metabolism in response to nutrients and environmental signals. Since its discovery in 1991, intensive studies have shown that these signals regulate mTORC1 in experimental systems ranging from yeast to fruit flies, mice and human cells. Only recently has research identified molecules responsible for the mechanisms involved in mTORC1 regulation, especially by amino acids. I led a collaboration with others in my research group, by which I identified cholesterol as a novel positive input for mTORC1 activation and characterized components that the cell uses to sense and convey the cholesterol signal. I showed that sterol levels correlate with mTORC1 activity in vivo, and that the lysosome has the machinery necessary for sterol-dependent regulation of mTORC1 in vitro. I found that a putative lysosomal cholesterol transporter, Niemann-Pick C1 (NPC1), is necessary for modulating mTORC1 activity in response to changing cholesterol levels. In cells lacking NPC1, or expressing NPC1 mutants incapable of transporting cholesterol, mTORC1 activity is elevated and no longer sensitive to changes in sterol levels, suggesting that NPC1 functions as a negative regulator. Conversely, an apparent amino acid transporter, SLC38A9, was identified as necessary for sterol-dependent activation of mTORC1. SLC38A9 contains conserved cholesterol responsive motifs that I showed are necessary to convey cholesterol responsiveness to mTORC1 because mutagenesis of these motifs decoupled the effect of amino acids and cholesterol in regulation mTORC1 activity. Overall, this work elucidated a novel mechanism by which cells integrate cholesterol- and amino acid-dependent signals to precisely regulate mTORC1 function for optimal cell growth.