Sequential conversion of PtdIns3P to PtdIns(3,5)P2 via endosome maturation couples nutrient signaling to lysosome reformation and basal autophagy

Abstract

Macroautophagy/autophagy occurs basally under nutrient-rich conditions in most mammalian cells, contributing to protein and organelle quality control, and protection against aging and neurodegeneration. During autophagy, lysosomes are heavily utilized via their fusion with autophagosomes and must be repopulated to maintain autophagic degradative capacity. During starvation-induced autophagy, lysosomes are generated via de novo biogenesis under the control of TFEB (transcription factor EB), or by the recycling of autolysosome membranes via autophagic lysosome reformation (ALR). However, these lysosome repopulation processes do not operate under nutrient-rich conditions. In our recent study, we identify a sequential phosphoinositide conversion pathway that enables lysosome repopulation under nutrient-rich conditions to facilitate basal autophagy. Phosphatidylinositol-3,4-bisphosphate (PtdIns[3,4]P₂) signals generated downstream of phosphoinositide 3-kinase alpha (PI3Kα) during growth factor stimulation are converted to phosphatidylinositol-3-phosphate (PtdIns3P) on endosomes by INPP4B (inositol polyphosphate-4-phosphatase type II B). We show that PtdIns3P is retained as endosomes mature into endolysosomes, and serves as a substrate for PIKFYVE (phosphoinositide kinase, FYVE-type zinc finger containing) to generate phosphatidylinositol-3,5-bisphosphate (PtdIns[3,5]P₂) to promote SNX2-dependent lysosome reformation, basal autophagic flux and protein aggregate degradation. Therefore, endosome maturation couples nutrient signaling to lysosome repopulation during basal autophagy by delivering PI3Kα-derived PtdIns3P to endolysosomes for PtdIns(3,5)P₂-dependent lysosome reformation.Abbreviations: ALR: autophagic lysosome reformation; INPP4B: inositol polyphosphate-4-phosphatase type II B; PI3Kα: phosphoinositide 3-kinase alpha; PIKFYVE: phosphoinositide kinase FYVE-type zinc finger containing; PtdIns3P: phosphatidylinositol-3-phosphate; PtdIns(3,4)P₂: phosphatidylinositol-3,4-bisphosphate; PtdIns(3,5)P₂ phosphatidylinositol-3,5-bisphosphate; SNX2 sorting nexin 2; PIK3C3/VPS34 phosphatidylinositol 3-kinase catalytic subunit type 3.

Keywords: Endosome; INPP4B; PI3Kα; PIKFYVE; lysosome; phosphoinositides.

Figure 1.

Under nutrient-rich conditions, sequential phosphoinositide conversion via endosomes delivers PtdIns3P to endolysosomes for conversion to PtdIns(3,5)P₂ to regulate lysosome reformation, basal autophagy and proteostasis. Basal autophagy enables protein quality control by degrading protein aggregates, thereby protecting against cellular stress. PtdIns(4,5)P₂ is converted to PtdIns(3,4,5)P₃ by PI3Kα at the plasma membrane in response to growth factor stimulation, which is subsequently dephosphorylated by INPP5 (inositol polyphosphate-5-phosphatase) enzymes to form PtdIns(3,4)P₂. A pool of PtdIns(3,4)P₂ is internalized during endocytosis and is maintained on endosomes where it is dephosphorylated by INPP4B to form PtdIns3P. Following endosome maturation to endolysosomes, PtdIns3P is phosphorylated by PIKFYVE to generate PtdIns(3,5)P₂, which recruits SNX2. SNX2 promotes lysosome reformation to facilitate basal autophagic degradation and protein aggregate clearance. When INPP4B is inactivated, PtdIns(3,4)P₂ is not converted to PtdIns3P at endosomes resulting in inhibition of endosome maturation and lysosome reformation. PIKFYVE inactivation prevents SNX2 recruitment to endolysosomes, suppressing lysosome reformation. As a consequence, inactivation of INPP4B or PIKFYVE decreases lysosomes thereby reducing autophagy function and leading to the accumulation of protein aggregates and increased proteotoxic stress.