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

Abstract

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 (XLMTM), a severe myopathy caused by mutations in the MTM1 gene. MTM1 acts as a phosphoinositide 3-phosphatase, targeting PI3P and PI(3,5)P₂, crucial for endolysosomal trafficking. Studies in XLMTM animal models have demonstrated that loss of MTM1 results in PI3P accumulation in muscle. Moreover, inactivating the class II phosphoinositide 3-kinase beta (PI3KC2β) 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 knockout 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-positive endosomes, leading to impaired Rab4 recycling vesicle biogenesis in the absence of MTM1. Furthermore, depletion of PI3KC2β rescued Mtm1 knockout 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 vesicles trafficking.