Nutrient/Starvation sensing for Reciprocal mTORC1/AMPK response in Dictyostelium, at the junction between Growth and Development

Abstract

The TOR kinase functions in two protein complexes, mTORC1 and mTORC2. mTORC1 is a key regulator for cell growth, sensing signals from nutrients and energy levels. While mTORC1 activity is inhibited by nutrient removal, it is also inactivated by the immunosuppressive drug rapamycin. In contrast, mTORC2 does not regulate cell growth, is insensitive to the direct action of rapamycin, but is crucial for developmental pathways. mTOR kinase complexes are critical to the life cycle of Dictyostelium where the transition from growth‐to‐development is initiated upon nutrient depletion. Thus, withdrawal of nutrients from or addition of rapamycin to growing cells leads to a rapid (<5 min) de‐phosphorylation of the mTORC1 mediated growth regulators pS6K and p4EBP1. Even in the presence of nutrients, rapamycin suppression of mTORC1 promotes mRNA unloading from polysomes and inhibition of protein synthesis, similar to that of nutrient‐depleted cells. In parallel, and following inactivation of mTORC1 in nutrient‐starved or rapamycin‐treated cells, AMP Kinase (AMPK), an energy status regulator, becomes phosphorylated/activated. As active mTORC1 may inhibit AMPK, AMPK similarly appears to reciprocally regulate mTORC1. Activation of AMPK in growth media by phospho‐AICAR, an AMP mimic, (or 2‐deoxy glucose) will inhibit mTORC1, while the AMPK‐inhibitor dorsomorphin blocks suppression of mTORC1, under conditions of starvation. We have identified other key regulators involving cross‐talk in the AMPK‐mTORC1 pathway, which ultimately integrates with mTORC2 for development progression and have further developed conditions, where treatment of cells in growth media with rapamycin alone (in absence of nutrient depletion) is sufficient to drive the growth‐to‐development transition (GDT) and developmentally regulated gene expression. Thus, de‐activated mTORC1 but activated AMPK are placed at the junction of GDT. To identify metabolic pathways and cell signalling involved in GDT that are specifically sensitive to directed mTORC1 inhibition independent of nutrient withdrawal, we compared cells in growth media, cells starved for nutrients, and cells in growth media treated with rapamycin, using quantitative mass spectrometry of small molecules and nascent RNA sequencing analyses. We observe alterations in metabolic pathways, including glycolysis, lipolysis, and protein synthesis and degradation, upon mTORC1 inactivation and relate these to the energy deficit state in control of GDT. Nascent RNA sequencing analysis following GDT showed up‐regulation of genes previously associated with development, and down‐regulation of growth‐essential genes, as well as classes of uncharacterized novel gene sets.Support or Funding InformationThis work was supported by the Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.