mTORC1 hyperactivation arrests bone growth in lysosomal storage disorders by suppressing autophagy

The mammalian target of rapamycin complex 1 (mTORC1) kinase promotes cell growth by activating biosynthetic pathwaysand suppressing catabolic pathways, particularly that of macroautophagy. A prerequisite for mTORC1 activation is itstranslocation to the lysosomal surface. Deregulation of mTORC1 has been associated with the pathogenesis of severaldiseases, but its role in skeletal disorders is largely unknown. Here, we show that enhanced mTORC1 signaling arrests bonegrowth in lysosomal storage disorders (LSDs). We found that lysosomal dysfunction induces a constitutive lysosomalassociation and consequent activation of mTORC1 in chondrocytes, the cells devoted to bone elongation. mTORC1hyperphosphorylates the protein UV radiation resistance–associated gene (UVRAG), reducing the activity of the associatedBeclin 1–Vps34 complex and thereby inhibiting phosphoinositide production. Limiting phosphoinositide production leads toa blockage of the autophagy flux in LSD chondrocytes. As a consequence, LSD chondrocytes fail to properly secrete collagens,the main components of the cartilage extracellular matrix. In mouse models of LSD, normalization of mTORC1 signaling orstimulation of the Beclin 1–Vps34–UVRAG complex rescued the autophagy flux, restored collagen levels in cartilage, andameliorated the bone phenotype. Taken together, these data unveil a role for mTORC1 and autophagy in the pathogenesis ofskeletal disorders and suggest potential therapeutic approaches for the treatment of LSDs.