Endosome maturation links PI3Kα signaling to lysosome repopulation during basal autophagy

Abstract

Autophagy depends on the repopulation of lysosomes to degrade intracellular components and recycle nutrients. How cells coordinate lysosome repopulation during basal autophagy, which occurs constitutively under nutrient-rich conditions, is unknown. Here, we identify an endosome-dependent phosphoinositide pathway that links PI3Ka signaling to lysosome repopulation during basal autophagy. We show that PI3Ka-derived PI(3)P generated by INPP4B on late endosomes was required for basal but not starvationinduced 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)P 2 . The SNX-BAR protein, SNX2, was recruited to endolysosomes by PI(3,5)P 2 and promoted lysosome reformation. Inhibition of INPP4B/PIKfyvedependent lysosome reformation reduced autophagic clearance of protein aggregates during proteotoxic stress leading to increased cytotoxicity. Therefore under nutrient-rich conditions, PI3Ka, INPP4B, and PIKfyve sequentially contribute to basal autophagic degradation and protection from proteotoxic stress via PI(3,5)P 2 -dependent lysosome reformation from endolysosomes. These findings reveal that endosome maturation couples PI3Ka signaling to lysosome reformation during basal autophagy.

Autophagic lysosome reformation in health and disease

“…However, PIKFYVE also promotes TFEB-dependent lysosomal gene expression [ 180 ]. During autophagy PIKFYVE is required for lysosome reformation from endolysosomes [ 181 , 182 ] and its generation of phosphatidylinositol-5-phosphate (PtdIns5P) promotes autophagosome formation [ 183 , 184 ]. Therefore, dissection of the role ALR dysfunction plays in PIKFYVE complex-associated disorders is challenging.…”

Reversible assembly and disassembly of V-ATPase during the lysosome regeneration cycle

“…Analysis of events occurring during ELR has proved challenging because budding, tubulation and scission can occur rapidly and are not synchronised throughout the cell (Bright et al ., 2005; Bissig et al ., 2017). Recently, an approach to circumventing these problems has been developed using spinning disc microscopy and a rapid imaging/analysis workflow to quantitate events(Rodgers et al ., 2022). Our approach was different, taking advantage of our previous observation that the generation of sucrosomes in NRK cells does not activate autophagy or result in TFEB translocation to the nucleus(Bright et al ., 2016) unlike in many other cell types (Sardiello et al ., 2009; Higuchi et al ., 2015).…”

“…Given that we did not see loss of RAB7a and disassembly of the V-ATPase until after scission of lysosome reformation tubules, except when scission was inhibited, other important questions include the exact timing of TBC1D15 recruitment, RAB7a loss and V-ATPase disassembly. A further task for future work is to explore the mechanisms of tubule formation and fission during lysosome reformation in mTORC1-independent ELR, which may have both similarities(Boutry et al ., 2023) and differences from that in mTORC1-regulated ALR and has been suggested to require the sorting nexin SNX2(Rodgers et al ., 2022). Based on our findings to date, a model of V-ATPase reversible assembly/disassembly throughout the lysosome fusion and regeneration cycle in continuously fed cells is presented in Fig 7.…”

Revving the engine: PKB/AKT as a key regulator of cellular glucose metabolism

“…PI3K is embedded in the lipid layer to regulate the formation, translocation, and anchoring of the GLUT4 vesicle ( Chen et al, 2019 ; Liu et al, 2020 ). Phosphatidylinositol 3-phosphate 5-kinase (PIKfyve) is mainly responsible for catalyzing the phosphorylation of PI(3)P to PI(5)P and PI(3,5)P2 ( Shisheva, 2008b ; Rodgers et al, 2022 ). PI(3,5)P2 is present in GLUT4 vesicles and may regulate the transport of GLUT4 vesicles ( Shisheva, 2008a ).…”