Autolysosome Formation Requires Clearance Of Phosphatidylinositol-3-phosphate By Myotubularin Phosphatase MTM-3 In C. Elegans

Autophagy is a lysosome-mediated degradation process that removes portions of the cytoplasm or selectively clears mis-folded or aggregate-prone proteins and damaged organelles. The formation and maturation of the autophagosome are very important regulatory events for autophagy activity.PI3P and its effectors play very important roles in many aspects of autophagy, including the control of autophagosome formation and maturation. Production of PI3P on the forming autophagic structures has been proposed to facilitate the expansion of membrane structures through recruiting several PI3P effectors. And it is reported that PI3P level is also tightly regulated by myotubularin phosphatases including MTMR14 and MTMR3, which play negatively regulatory role during the formation of autophagosomes. However, Ymrl, the only myotubularin phosphatase in yeast, is recently reported to be essential for normal progression of autophagy. It is unclear whether the distinct effect of human and yeast myotubularin phosphatases on autophagy is due to the intrinsic differences between mammalian and yeast autophagy, or because PtdIns3P turnover has opposing effects if it occurs at different stages of autophagy.Genetic screens performed in the nematode C. elegans identified both conserved Atg homologs and metazoan-specific autophagy regulators, establishing C. elegans as a useful genetic model to study autophagy. The hierarchical order of autophagy proteins in the aggrephagy pathway (which degrades protein aggregates) has also been established in worms, providing a genetic platform for mechanistic studies of autophagosome formation and maturation in higher eukaryotes.We found that loss of MTM-3, but not other myotubularin phosphatases in C. elegans causes defects in degradation of various autophagic substrates. Our epistasis analyses indicate that MTM-3 acts at the step downstream of the ATG-2/EPG-6 complex and upstream of EPG-5 in the aggrephagy pathway to promote autophagosome maturation into autolysosomes. MTM-3 can be recruited to autophagosomes by PtdIns3P and loss of MTM-3 causes increased autophagic association of ATG-18 in a PtdIns3P-dependent manner.Our data reveal critical roles of PtdIns3P turnover in autophagosome maturation and/or autolysosome formation.