ATG13 dynamics in nonselective autophagy and mitophagy: insights from live imaging studies and mathematical modeling.

ABSTRACT

During macroautophagy/autophagy, the ULK complex nucleates autophagic precursors, which give rise to autophagosomes. We analyzed, by live imaging and mathematical modeling, the translocation of ATG13 (part of the ULK complex) to the autophagic puncta in starvation-induced autophagy and ivermectin-induced mitophagy. In nonselective autophagy, the intensity and duration of ATG13 translocation approximated a normal distribution, whereas wortmannin reduced this effect and shifted to a log-normal distribution. During mitophagy, multiple translocations of ATG13 with increasing time between peaks were observed. We hypothesized that these multiple translocations arise because the engulfment of mitochondrial fragments required successive nucleation of phagophores on the same target, and a mathematical model based on this idea reproduced the oscillatory behavior. Significantly, model and experimental data were also in agreement that the number of ATG13 translocations is directly proportional to the diameter of the targeted mitochondrial fragments. Thus, our data provide novel insights into the early dynamics of selective and nonselective autophagy.Abbreviations ATG autophagy related 13; CFP cyan fluorescent protein; dsRED Discosoma red fluorescent protein; GABARAP GABA type A receptor-associated protein; GFP green fluorescent protein; IVM ivermectin; MAP1LC3/LC3 microtubule-associated protein 1 light chain 3; MTORC1 mechanistic target of rapamycin kinase complex 1; PIK3C3/VPS34 phosphatidylinositol 3-kinase catalytic subunit type 3; PtdIns3P PtdIns-3-phosphate; ULK unc-51 like autophagy activating kinase.