Endosome maturation links PI3Kα signaling to lysosome repopulation during basal autophagy

Autophagy depends on the repopulation of lysosomes to degrade intracellular components and recycle nutrients. How cells co-ordinate lysosome repopulation during basal autophagy, which occurs constitutively under nutrient-rich conditions, is unknown. Here, we identify an endosome-dependent phosphoinositide pathway that links PI3Kα signaling to lysosome repopulation during basal autophagy. We show that PI3Kα-derived PI(3)P generated by INPP4B on late endosomes was required for basal but not starvation-induced autophagic degradation. PI(3)P signals were maintained as late endosomes matured into endolysosomes, and served as the substrate for the 5-kinase, PIKfyve, to generate PI(3,5)P2. The SNX-BAR protein, SNX2, was recruited to endolysosomes by PI(3,5)P2 and promoted lysosome reformation. Inhibition of INPP4B/PIKfyve-dependent lysosome reformation reduced autophagic clearance of protein aggregates during proteotoxic stress leading to increased cytotoxicity. Therefore under nutrient-rich conditions, PI3Kα, INPP4B and PIKfyve sequentially contribute to basal autophagic degradation and protection from proteotoxic stress via PI(3,5)P2-dependent lysosome reformation from endolysosomes. These findings reveal that endosome maturation couples PI3Kα signaling to lysosome reformation during basal autophagy.

细胞自噬（autophagy）依赖溶酶体（lysosome）的再生，以降解细胞内组分并循环利用营养物质。在营养充足条件下持续稳态发生的基础自噬过程中，细胞如何协调调控溶酶体的再生，目前仍未明确。本研究鉴定出一条内体依赖的磷酸肌醇通路，该通路可将PI3Kα信号通路与基础自噬过程中的溶酶体再生相耦联。研究表明，由肌醇多磷酸4-磷酸酶2B（INPP4B）在晚期内体（late endosome）上催化生成的PI3Kα依赖性磷脂酰肌醇3-磷酸（PI(3)P），是介导基础自噬降解的必需因子，而不参与饥饿诱导的自噬降解过程。随着晚期内体成熟为内体溶酶体（endolysosome），PI(3)P信号得以维持，并作为5-激酶PIKfyve的底物生成磷脂酰肌醇3,5-二磷酸（PI(3,5)P2）。分选连接蛋白2（SNX2）作为SNX-BAR蛋白家族成员，可通过PI(3,5)P2被招募至内体溶酶体，进而促进溶酶体重塑。抑制INPP4B/PIKfyve依赖的溶酶体重塑过程，会降低蛋白毒性应激（proteotoxic stress）状态下自噬对蛋白聚集体的清除效率，最终加剧细胞毒性（cytotoxicity）。因此，在营养充足条件下，PI3Kα、INPP4B与PIKfyve依次发挥作用，通过内体溶酶体来源的PI(3,5)P2依赖性溶酶体重塑，参与基础自噬降解并对抗蛋白毒性应激。本研究结果揭示，在基础自噬过程中，内体成熟可将PI3Kα信号通路与溶酶体重塑相耦联。