Novel modes of mTORC1 regulation by amino acids

Abstract

In order to maintain homeostasis, cells must be able to sense, interpret, and coordinate responses to environmental factors including energy status, amino acid availability, and the presence of growth factors. One important pathway that coordinates these signals and dictates the appropriate response is the mechanistic target of Rapamycin complex 1 (mTORC1) pathway. In conditions that favor growth, mTORC1 activates pathways involved in protein, lipid, and nucleotide synthesis, and inhibits breakdown of biomolecules through lysosomal degradative pathways. Mutations in the pathways that regulate mTORC1 lead to constitutively activated mTORC1, which contributes to disease states such as cancer. My thesis attempts to elucidate the pathways involved in coordinating amino acid availability with mTORC1 activity so we may better understand how healthy cells maintain homeostasis given a constantly changing environment, and how diseased cells with mutations in these pathways will respond to various treatments. My thesis will show that cells respond to withdrawal of individual amino acids in a manner very distinct from the response to a lack of all amino acids. Cells sense the removal of amino acids and downregulate mTORC1 activity. I discovered that withdrawal of the individual amino acids arginine or leucine allows for mTORC1 reactivation for a period of several hours. This reactivation occurs after a brief decrease in mTORC1 activity, and is not observed following withdrawal of total amino acids. I demonstrate that this response requires an upstream activator of mTORC1, Akt, and is not due to amino acids being regenerated via autophagy or other means. I also observe an increase in Akt activity during the time frame of mTORC1 recovery, suggesting that longer term amino acid deprivation stimulates mTORC1 activity through activation of Akt. My thesis also describes a phenomenon in which proteins involved in mTORC1 regulation alter their location in response to amino acids, and I summarize my work to investigate the mechanism and functions of this movement. I discuss the implications of these findings, and propose alternate interpretations to the current models of mTORC1 activation downstream of amino acids.