Abstract
We established a differentiation method for homogeneous α7 integrin-positive human skeletal muscle stem cell (α7(+)hSMSC)-derived osteoblast-like cells with bone morphogenetic protein (BMP)-2. To explore the early signaling cascade for osteoblastic differentiation, we examined the upregulation of autophagy-related gene (Atg) and wingless/int1 (Wnt) signaling during BMP-2-mediated human osteoblastic differentiation. In a screening experiment, BMP-2 increased the mRNA and protein levels of Atg7, Wnt16, and Lrp5/Fzd2 (a Wnt receptor), but not microtubule-associated protein 1 light chain (LC3; a mammalian homolog of yeast Atg8), TFE3, Beclin1, Atg5, Atg12, Wnt3a, or Wnt5, together with the amounts of autophagosomes and autophagy fluxes. Treatment with siRNAs against Atg7 and Wnt16 individually suppressed the BMP-2-induced increase in osteoblastic differentiation. The osteoblastic phenotype, involving osteocalcin (BGLAP), osteopontin (SPP1), and osterix (SP7) expression, decreased when autophagy was inhibited by chloroquine (an autophagy inhibitor), but increased after treatment with rapamycin (an autophagy enhancer). Taken together with our previous findings, we have revealed a unique sequential cascade of BMP-2→Atg7→Wnt16→Lrp5/Fzd2→matrix metalloproteinase-13→osteoblastic differentiation. This cascade results in a potent increase in osteoblastic cell differentiation, indicating the unique involvement of Atg7, autophagy, and Wnt16 signaling in BMP-2-induced differentiation of α7(+)hSMSCs into osteoblast-like cells at a relatively early stage.