Bidirectional interconversion between PtdIns4P and PtdIns(4,5)P2 is required for autophagic lysosome reformation and protection from skeletal muscle disease

Abstract

Autophagic lysosome reformation (ALR) recycles autolysosome membranes formed during autophagy, to make lysosomes and is essential for continued autophagy function. Localized membrane remodeling on autolysosomes leads to the extension of reformation tubules, which undergo scission to form new lysosomes. The phosphoinositides phosphatidylinositol-4-phosphate (PtdIns4P) and phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P₂) induce this remodeling by recruiting protein effectors to membranes. We identified the inositol polyphosphate 5-phosphatase INPP5K, which converts PtdIns(4,5)P₂ to PtdIns4P is essential for ALR in skeletal muscle. INPP5K mutations that reduce its 5-phosphatase activity are known to cause muscular dystrophy, via an undefined mechanism. We generated skeletal muscle-specific inpp5k knockout mice which exhibited severe muscle disease, with lysosome depletion and marked autophagy inhibition. This was due to decreased PtdIns4P and increased PtdIns(4,5)P₂ on autolysosomes, causing reduced scission of reformation tubules. ALR was restored in cells with loss of INPP5K by expression of wild-type INPP5K, but not muscle-disease causing mutants. Therefore on autolysosomes, both PtdIns(4,5)P₂ generation and its removal by INPP5K is required for completion of ALR. Furthermore, skeletal muscle shows a dependence on the membrane recycling ALR pathway to maintain lysosome homeostasis and ensure the protective role of autophagy against disease.

Keywords: Autophagic lysosome reformation; INPP5K; PtdIns(4, 5)P2; PtdIns4P; autophagy; inositol polyphosphate 5-phosphatase; lysosome; muscular dystrophy; phosphoinositide; skeletal muscle.

Figure 1.

Autophagic lysosome reformation relies on the bidirectional interconversion between phosphoinositides PtdIns4P and PtdIns(4,5)P₂, to ensure lysosome homeostasis, autophagy function and protection from disease. During ALR, a hierarchical succession of phosphoinositide kinase and phosphatase enzymes is required for dynamic PtdIns4P and PtdIns(4,5)P₂ signaling on autolysosomes. First, PtdIns4P is made from PI by either PI4KB (phosphatidylinositol 4-kinase beta) or PI4K2A (phosphatidylinositol 4-kinase, type 2 alpha). In turn, PtdIns4P is used by two different PtdIns4P 5-kinases, PIP5K1A or PIP5K1B, to make PtdIns(4,5)P₂. This signaling is opposed by the PtdIns(4,5)P₂ 5-phosphatase INPP5K, which converts PtdIns(4,5)P₂ back to PtdIns4P. The generation of PtdIns(4,5)P₂ microdomains on the autolysosome membrane recruits effector proteins which drive specialized changes to membrane ultrastructure. The AP2-clathrin complex induces membrane budding, then the microtubule motor protein KIF5B and the actin remodeling protein WHAMM, exert forces to extrude the bud to form a membrane tubule. DNM2 is thought to facilitate tubule scission to generate lysosomes. When INPP5K function is lost, either by reduced expression or catalytically inactivating disease mutants, the dynamic association of PtdIns(4,5)P₂ effectors is dysregulated leading to the inability of reformation tubules to undergo scission to generate new lysosomes. This results in the enlargement of autolysosomes and the hyperextension of reformation tubules. The depletion of lysosomes when INPP5K function is lost, causes autophagy inhibition and muscle disease