FGF signalling regulates bone growth through autophagy

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Skeletal growth relies on both biosynthetic and catabolic processes1,2. While the role of the former is clearly established, how the latter contributes to growth-promoting pathways is less understood. Macroautophagy, hereafter referred to as autophagy, is a catabolic process that plays a fundamental part in tissue homeostasis3. We investigated the role of autophagy during bone growth, which is mediated by chondrocyte rate of proliferation, hypertrophic differentiation and extracellular matrix (ECM) deposition in growth plates4. Here we show that autophagy is induced in growth-plate chondrocytes during post-natal development and regulates the secretion of type II collagen (Col2), the major component of cartilage ECM. Mice lacking the autophagy related gene 7 (Atg7) in chondrocytes experience endoplasmic reticulum storage of type II procollagen (PC2) and defective formation of the Col2 fibrillary network in the ECM. Surprisingly, post-natal induction of chondrocyte autophagy is mediated by the growth factor FGF18 through FGFR4 and JNK-dependent activation of the autophagy initiation complex VPS34-beclin-1. Autophagy is completely suppressed in growth plates fromFgf18−/−embryos, whileFgf18+/− heterozygous andFgfr4−/−mice fail to induce autophagy during post-natal development and show decreased Col2 levels in the growth plate. Strikingly, theFgf18+/− andFgfr4−/−phenotypes can be rescuedin vivoby pharmacological activation of autophagy, pointing to autophagy as a novel effector of FGF signalling in bone. These data demonstrate that autophagy is a developmentally regulated process necessary for bone growth, and identify FGF signalling as a crucial regulator of autophagy in chondrocytes.

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