UC San Diego

All eukaryotic cells must not only be able to interpret their energy, or ATP, status but also enact downstream effectors to regain homeostasis upon energy crisis. One of the ways that eukaryotic cells achieve this regulation is through the highly conserved serine/threonine kinase AMP- Activated Protein Kinase (AMPK). When cellular energy levels decrease, AMPK becomes activated and can turn off many anabolic processes while at the same time turning on catabolic ones through acute phosphorylation of its downstream substrates. The net result is that a cell undergoing energy stress stops what it is doing until homeostasis is restored. AMPK has been implicated in regulating many disparate processes in efforts to regain homeostasis, however few known bone fide in vivo substrates of AMPK could explain this diverse and wide- ranging regulation. In order to better understand AMPK and processes downstream, our lab conducted a screen for novel substrates of this energy sensing kinase. Using a bioinformatics and proteomic approach, we have identified about 50 potential substrates of AMPK, which are also conserved across evolution and contain a consensus site for AMPK. One substrate in particular that I chose to focus on was a poorly studied autophagy initiating kinase, ULK1. AMPK directly phosphorylates ULK1 in vivo and in vitro, and cells that are unable to engage in this regulation are defective in engaging the cytoprotective mechanism of autophagy. Additionally, we show that this regulation is an important node for mitochondrial homeostasis because these mutant cells accumulated damaged mitochondria which should be otherwise degraded by autophagy. Finally, in order to better understand ULK1 function and to manipulate it pharmacologically in vivo, we have developed small molecule ATP-competitive inhibitors of ULK1/2. These molecules will be useful tools to not only dissect ULK1/2 function, but to also to inhibit ULK1/2 in various oncogenic settings where autophagy could be enabling tumor survival. Taken together, these findings contribute to our understanding of how cellular energy crisis can trigger the cytoprotective process, autophagy. This AMPK-dependent regulation of the ULK1 complex represents one of the first signals triggered by energy crisis to activate the cytoprotective autophagy cascade