Rab7 knockout unveils regulated autolysosome maturation induced by glutamine starvation

Macroautophagy (simply called autophagy hereafter) is an intracellular degradation mechanism that is activated by nutrient starvation. Although it is well known that starvation induces autophagosome formation in an mTORC1-dependent manner, whether starvation also regulates autophagosome or autolysosome maturation was unclear. In the present study, we succeeded in demonstrating that starvation activates autolysosome maturation in mammalian cells. We found that knockout (KO) of Rab7 (herein referring to the Rab7a isoform) caused an accumulation of a massive number of LC3-positive autolysosomes under nutrient-rich conditions, indicating that Rab7 is dispensable for autophagosome-lysosome fusion. Intriguingly, the autolysosomes that had accumulated in Rab7-KO cells matured and disappeared after starvation for a brief period (similar to 10 min), and we identified glutamine as an essential nutrient for autolysosome maturation. In contrast, forced inactivation of mTORC1 through treatment with its inhibitor Torin2 failed to induce autolysosome maturation, suggesting that the process is controlled by an mTORC1-independent mechanism. Since starvation-induced autolysosome maturation was also observed in wild-type cells, the nutrient-starvation-induced maturation of autolysosomes is likely to be a generalized mechanism in the same manner as starvation-induced autophagosome formation. Such multistep regulatory mechanisms would enable efficient autophagic flux during starvation.