Loss of autophagy in chondrocytes causes severe growth retardation

Chondrogenesis is accompanied by not only cellular renovation, but also metabolic stress. Therefore, macroautophagy/autophagy is postulated to be involved in this process. Previous reports have shown that suppression of autophagy during chondrogenesis causes mild growth retardation. However, the role of autophagy in chondrocyte differentiation still largely remains unclear. Here, we show the important role of autophagy on chondrogenesis. The transition of mesenchymal cells to chondrocytes was severely impaired by ablation of Atg7, a gene essential for autophagy. Mice lacking Atg7 after the transition exhibited phenotypes severer than mutant mice in which Atg7 was removed before the transition. Atg7-deficient chondrocytes accumulated large numbers of glycogen granules, hardly proliferate and died specifically in the proliferative zone without any ER-stress signal. Our results suggest that the suppression of autophagy in prechondrogenic cells drives compensatory mechanism(s) that mitigate defective chondrogenesis, and that autophagy participates in glycogenolysis to supply glucose in avascular growth plates. Abbreviations: DDIT3/CHOP: DNA damage inducible transcript 3; ER: endoplasmic reticulum; NFE2L2/NRF2: nuclear factor, erythroid derived 2, like 2; SQSTM1/p62: sequestosome 1; STBD1: starch-binding domain-containing protein 1