Fine-tuning of AMPK-ULK1-mTORC1 regulatory triangle is crucial for autophagy oscillation.

Autophagy is an intracellular digestive process, which has a crucial role in maintaining cellular homeostasis by self-eating the unnecessary and/or damaged components of the cell at various stress events. ULK1, one of the key elements of autophagy activator complex, together with the two sensors of nutrient and energy conditions, called mTORC1 and AMPK kinases, guarantee the precise function of cell response mechanism. We claim that the feedback loops of AMPK-mTORC1-ULK1 regulatory triangle determine an accurate dynamical characteristic of autophagic process upon cellular stress. By using both molecular and theoretical biological techniques, here we reveal that a delayed negative feedback loop between active AMPK and ULK1 is essential to manage a proper cellular answer after prolonged starvation or rapamycin addition. AMPK kinase quickly gets induced followed by AMPK-P-dependent ULK1 activation, whereas active ULK1 has a rapid negative effect on AMPK-P resulting in a delayed inhibition of ULK1. The AMPK-P → ULK1 ˧ AMPK-P negative feedback loop results in a periodic repeat of their activation and inactivation and an oscillatory activation of autophagy, as well. We demonstrate that the periodic induction of self-cannibalism is necessary for the proper dynamical behaviour of the control network when mTORC1 is inhibited with respect to various stress events. By computational simulations we also suggest various scenario to introduce "delay" on AMPK-P-dependent ULK1 activation (i.e. extra regulatory element in the wiring diagram or multi-phosphorylation of ULK1).