Lysosomal control of cellular nutrient biology

To sustain life, cells must constantly take up elements in various chemical forms from their environments. Often, these elements are taken up as simple nutrients, such as glucose or amino acids. Cells can either extract energy from these nutrients to use as fuel, or they can use them as the raw materials needed to build and maintain cellular structures. Elements can also be taken up in more complex forms, such as whole proteins, in which case they must first be broken down into simple nutrients before they can be used. The process of breaking down complex molecules into simple nutrients occurs inside a cellular organelle called the lysosome, which acts like the cell's digestive system, breaking down cellular ‘food’ into nutrients. Regardless of where the nutrients originate from - whether directly from the environment or from breaking down complex molecules in the lysosome -they must first be sensed by the cell and then metabolized into something useful. Both of these processes are controlled by mTOR, a cellular protein that orchestrates nutrient-dependent cell growth. However, despite nutrients from different sources being able to activate mTOR, exactly how they activate it differs depending on where the nutrients came from. In my lab we want to understand why this is the case. Specifically, we are asking the following questions. How do nutrients that were derived inside lysosomes activate mTOR? And how is this process different from mTOR activation by nutrients that came from the cellular environment? To begin to answer these questions, I am proposing two short-term objectives. Objective 1 - Investigate lysosome protein dynamics under differential cellular nutrient conditions Objective 2 - Characterize how a key lysosome regulatory network - the Rab7-HOPS-Retromer network - enables mTOR activation by lysosome-derived nutrients To answer these questions we are using a powerful instrument called a mass spectrometer, which is essentially an extremely accurate molecular scale. If you can precisely weigh a molecule, you can identify it. We are using mass spectrometers to identify the cellular proteins that lysosomes - and lysosome-derived nutrients - use to activate mTOR. Successful completion of the objectives proposed in this grant will shed further light on how lysosomes - once thought of purely as the cell's digestive system - are able to respond to the complex mixtures of nutrients and other macromolecules in a cell's environment to control cell growth.