MTORC1/AMPK responses define a core gene set for developmental cell fate switching

Pundrik Jaiswal,Alan R Kimmel

doi:10.1186/s12915-019-0673-1

Abstract

BackgroundKinases mTORC1 and AMPK act as energy sensors, controlling nutrient responses and cellular growth. Changes in nutrient levels affect diverse transcriptional networks, making it challenging to identify downstream paths that regulate cellular growth or a switch to development via nutrient variation. The life cycle of Dictyostelium presents an excellent model to study the mTORC1 signaling function for growth and development. Dictyostelium grow as single cells in nutrient-rich media, but, upon nutrient withdrawal, growth ceases and cells enter a program for multi-cell development. While nearly half the genome shows gene expression changes upon nutrient removal, we hypothesized that not all of these genes are required for the switch to program development. Through manipulation of mTORC1 activity alone, without nutrient removal, we focused on a core network of genes that are required for switching between growth and development for regulation of cell fate decisions.ResultsTo identify developmentally essential genes, we sought ways to promote development in the absence of nutrient loss. We first examined the activities of mTORC1 and AMPK in Dictyostelium during phases of rapid growth and starvation-induced development and showed they exhibited reciprocal patterns of regulation under various conditions. Using these as initial readouts, we identified rich media conditions that promoted rapid cell growth but, upon mTORC1 inactivation by rapamycin, led to a growth/development switch. Examination of gene expression during cell fate switching showed that changes in expression of most starvation-regulated genes were not required for developmental induction. Approximately 1000 genes which become downregulated upon rapamycin treatment comprise a cellular growth network involving ribosome biogenesis, protein synthesis, and cell cycle processes. Conversely, the upregulation of ~ 500 genes by rapamycin treatment defines essential signaling pathways for developmental induction, and ~ 135 of their protein products intersect through the well-defined cAMP/PKA network. Many of the rapamycin-induced genes we found are currently unclassified, and mutation analyses of 5 such genes suggest a novel gene class essential for developmental regulation.ConclusionsWe show that manipulating activities of mTORC1/AMPK in the absence of nutrient withdrawal is sufficient for a growth-to-developmental fate switch in Dictyostelium, providing a means to identify transcriptional networks and signaling pathways essential for early development.

Highlights

IntroductionThe mTOR (mechanistic target of rapamycin) kinase is primarily associated with two functionally distinct protein complexes, mTORC1 and mTORC2 [1, 2]
The mTOR kinase is primarily associated with two functionally distinct protein complexes, mTORC1 and mTORC2 [1, 2]
For developmental induction, we show that 5 unclassified genes that were randomly selected from the rapamycin-induced set were essential for early multi-cell development, whereas, in full contrast, none of 10 randomly selected non-rapamycin regulated genes had a significant role for early multi-cell formation or developmental gene expression

Summary

Introduction

The mTOR (mechanistic target of rapamycin) kinase is primarily associated with two functionally distinct protein complexes, mTORC1 and mTORC2 [1, 2]. These, in turn, are often suggested to involve separate cellular functions, with mTORC1 being a nutrient sensor and growth regulator and mTORC2 a component of developmental processes [1,2,3,4]. Where mTORC1 phosphorylation of S6K and 4EBP1 is essential for protein synthesis and cell growth, the AMPdependent kinase AMPK serves as a reciprocal nutrient/ energy sensor to adjust growth to reducing environmental sustenance [5,6,7]. Rich media with high concentrations of amino acids and glucose support respectively activation of mTORC1 or inhibition of AMPK. Through manipulation of mTORC1 activity alone, without nutrient removal, we focused on a core network of genes that are required for switching between growth and development for regulation of cell fate decisions

Methods

Results

Discussion

Conclusion