PI3KC2β depletion rescues endosomal trafficking defects in Mtm1 knockout skeletal muscle cells

Phosphoinositides constitute a class of seven phospholipids found in cell membranes, regulating various cellular processes like trafficking and signaling. Mutations in their metabolizing enzymes are implicated in several pathologies, including X-linked myotubular myopathy, a severe myopathy caused by mutations in the MTM1 gene. MTM1 (myotubularin 1) acts as a phosphoinositide 3-phosphatase, targeting PI3P (phosphatidylinositol 3-phosphate) and phosphatidylinositol 3,5-bisphosphate, crucial for endolysosomal trafficking. Studies in X-linked myotubular myopathy animal models have demonstrated that loss of MTM1 results in PI3P accumulation in muscle. Moreover, inactivating the class II phosphoinositide 3-kinase beta rescues the pathological phenotype and decreases PI3P levels, suggesting that the normalization of PI3P levels could be responsible for that rescue mechanism. In this study, using an Mtm1-KO skeletal muscle cell line, we investigated the localization of the PI3P pool metabolized by MTM1 in endosomal compartments. Our findings reveal that MTM1 metabolizes a pool of PI3P on EEA1 (early endosome antigen 1)-positive endosomes, leading to impaired Rab4 recycling vesicle biogenesis in the absence of MTM1. Furthermore, depletion of class II phosphoinositide 3-kinase beta rescued Mtm1-KO cell phenotype, normalized PI3P level on EEA1-positive endosomes, and restored Rab4-positive vesicle biogenesis. These results indicate that MTM1 is critical for the homeostasis of endosomal trafficking, and that depletion of MTM1 potentially alters cargo recycling through Rab4-positive vesicle trafficking.