227 Frizzled7 Mediates Lysine-Activated β-Catenin to Promote Satellite Cells in Manipulating Skeletal Muscle Growth

Abstract Wnt/β-catenin pathways have an important role in porcine skeletal muscle growth. Nevertheless, the mechanism for how it respond to environmental stimulations, especially nutrients, has not been well-investigated. In this study, piglets and skeletal muscle satellite cells (MuSCs) were used to investigate the work process of the lysine (Lys) signal transmitted to β-catenin in governing skeletal muscle growth. Briefly, the piglets and MuSCs were divided into the control group, the Lys deficiency group, and the Lys rescue group, taken together with the specific inhibitor and target gene knockdown for further study. We found that the transmembrane frizzled7 (FZD7) receptor displayed the same changes with the Wnt/β-catenin pathway and positively correlated with Lys levels in skeletal muscle and MuSCs. Meanwhile, the molecular docking analyses showed that FZD7 and Lys form connections at Gly17, Phe18, Cys19, and Asp84. In contrast, FZD7 specific inhibitor Fz7-21-TFA suppressed Lys rescued MuSC viability (P< 0.05). The distribution of Ki67 and active β-catenin in the Lys rescue group were also decreased by FZD7 inhibitor (P< 0.05). Furthermore, the decreased FZD7 level caused Lys re-activated TCF4/LEF activity and the Wnt/β-catenin pathway suppression (P< 0.05). Additionally, FZD7 knockdown restricted C2C12 viability and proliferation ability (P< 0.05). Besides, FZD7 knockdown inhibited the Wnt/β-catenin pathway and Lys deficiency caused much more serious inhibition of the Wnt/β-catenin pathway (P< 0.05), and these restrictions fail to be rescued by re-supplemented Lys or Wnt3a (P< 0.05). Moreover, we titrated Lys solution into recombinant pig FZD7 (rpFZD7) protein solution by using isothermal titration calorimetry, and the heat release during titration demonstrated there was an interaction between Lys and rpFZD7. Collectively, these results indicated that Lys is not only a molecular block for protein synthesis, but is also a ligand that binds to FZD7, directly activating the Wnt/β-catenin pathway to stimulate MuSCs to promote skeletal muscle growth.