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

Abstract

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).

Autophagy-dependent survival is controlled with a unique regulatory network upon various cellular stress events

“…The phase plane diagram of ULK1/2 and AMPK with one unstable intersection shows a limit cycle oscillation, where grey arrows indicate the direction of motion among the limit cycle. Corresponding to our previous experimental results 55 , time course of sustained oscillation of ULK1/2 and AMPK has also depicted and AUEX gets periodically active (Fig. 2C ) too, suggesting that autophagy has an ON and OFF characteristic under starvation.…”

The Mixed Blessing of AMPK Signaling in Cancer Treatments

“…Model parameters are chosen so that the model predicts oscillatory time-profiles of mTORC1, ULK1 and AMPK consistent with experimental observations under energetic stress (27) (see materials and methods). The predicted time-courses are shown in Fig.…”

“…The predicted time-courses are shown in Fig. 1(A-C) together with experimental data (27). The period of oscillation is approximately 24 hours.…”

“…A-C , oscillatory model solution under energetic stress. Predicted time profiles are shown for pmTORC1, pAMPK, and pULK1, together with the corresponding experimental data points (27). D , predicted phosphorylated-to-unphosphorylated mTORC1 ratio as a function of AMPK activation, compared with data from (28).…”

“…Specifically, model parameters are calibrated to satisfy the following criteria simultaneously: (i) the model predicts oscillations in mTORC1, ULK1 and AMPK under energetic stress that approximate experimental data in Ref. (27), (ii) the feedback response of mTORC1 to variations in AMPK approximate experimental data in Ref. (28), and (iii) the model predicts changes in AMPK and AKT activation in response to treatment of AICAR, an AMPK activator, consistent with experimental data in Ref.…”