mTORC1 hyperactivation arrests bone growth in lysosomal storage disorders by suppressing autophagy
Rosa Bartolomeo, Laura Cinque, Chiara De Leonibus, Alison Forrester, Anna Chiara Salzano, Jlenia Monfregola, Emanuela De Gennaro, Edoardo Nusco, Isabella Azario, Carmela Lanzara, Marta Serafini, Beth Levine, Andrea Ballabio, Carmine Settembre
Rosa Bartolomeo, Laura Cinque, Chiara De Leonibus, Alison Forrester, Anna Chiara Salzano, Jlenia Monfregola, Emanuela De Gennaro, Edoardo Nusco, Isabella Azario, Carmela Lanzara, Marta Serafini, Beth Levine, Andrea Ballabio, Carmine Settembre
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Concise Communication Bone biology

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

Abstract

The mammalian target of rapamycin complex 1 (mTORC1) kinase promotes cell growth by activating biosynthetic pathways and suppressing catabolic pathways, particularly that of macroautophagy. A prerequisite for mTORC1 activation is its translocation to the lysosomal surface. Deregulation of mTORC1 has been associated with the pathogenesis of several diseases, but its role in skeletal disorders is largely unknown. Here, we show that enhanced mTORC1 signaling arrests bone growth in lysosomal storage disorders (LSDs). We found that lysosomal dysfunction induces a constitutive lysosomal association and consequent activation of mTORC1 in chondrocytes, the cells devoted to bone elongation. mTORC1 hyperphosphorylates the protein UV radiation resistance–associated gene (UVRAG), reducing the activity of the associated Beclin 1–Vps34 complex and thereby inhibiting phosphoinositide production. Limiting phosphoinositide production leads to a 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 or stimulation of the Beclin 1–Vps34–UVRAG complex rescued the autophagy flux, restored collagen levels in cartilage, and ameliorated the bone phenotype. Taken together, these data unveil a role for mTORC1 and autophagy in the pathogenesis of skeletal disorders and suggest potential therapeutic approaches for the treatment of LSDs.

Authors

Rosa Bartolomeo, Laura Cinque, Chiara De Leonibus, Alison Forrester, Anna Chiara Salzano, Jlenia Monfregola, Emanuela De Gennaro, Edoardo Nusco, Isabella Azario, Carmela Lanzara, Marta Serafini, Beth Levine, Andrea Ballabio, Carmine Settembre

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Figure 4

Normalization of mTORC1 signaling restores collagen trafficking and rescues bone phenotype in MPS VII mice.

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Normalization of mTORC1 signaling restores collagen trafficking and resc...
(A) Representative images of femoral growth plate sections isolated from Gusb+/+, Gusb–/–, and Gusb–/– Rpt+/– mice at P15. P-S6 immunostaining (arrows, brown, top row), intracellular PC2 (green, second row), COL X (third row), and COL II (brown, bottom row). Nuclei were counterstained with hematoxylin (pink, top row and bottom row) or DAPI (blue, second row). Scale bars: 100 μm. Insets show magnification ×3. (B) Bar graphs show the length of the HZ (COL X+ area). n = 5 mice per genotype. (C) Quantification of collagen isolated from femoral and tibia cartilages from mice of the indicated genotypes and expressed as the percentage of Gusb+/+ mice. n ≥ 5 mice per genotype. (D) Representative images of SQSTM1/p62 and Lamp-1 immunofluorescence in femoral growth plates isolated from Gusb–/– and Gusb–/– Rpt+/– mice at P15. n = 3 mice per group. Scale bars: 10 μm; zoom, ×5. (E) Quantification of P62–Lamp-1 colocalization. (F) Representative images of Alcian blue and alizarin red staining of femurs and tibiae from P15 Gusb+/+, Gusb–/–, and Gusb–/– Rpt+/– mice. (G) Femur and tibia lengths for mice of the indicated genotypes. n = 6 mice per genotype. (H) Representative images of Alcian blue and alizarin red staining of femurs and tibiae from P30 Gusb+/+, Gusb–/–, and Gusb–/– Rpt+/– mice. (I) Femur and tibia lengths for mice of the indicated genotypes. n = 6 mice per genotype. Data represent the mean values derived from the indicated number of mice. Error bars indicate the SEM. *P ≤ 0.05, **P ≤ 0.005, and ***P ≤ 0.0005, by ANOVA followed by Tukey’s post-hoc test (B, C, G, and I) and unpaired Student’s t test (E).