Depletion of Macrophage Impairs Skeletal Muscle Regeneration by Inhibited the Expression of Muscle Regeneration Regulatory Factors and Akt/mTOR Protein Synthesis Signaling Pathway

Objective: The purpose of this study is to explore the relationship between macrophage and muscle regeneration regulatory factors and Akt/mTOR signaling pathway, in order to investigate the role and mechanism of macrophages in the regeneration of injured skeletal muscle. Methods: Eighty C57 BL/6 mice were randomly divided into muscle contusion (S, n=32) , muscle contusion+ macrophages depleted (T, n=32) , control (SCon, n=8) , and macrophages depleted control groups (TCon, n=8) . Their outer layer of the gastrocnemius muscles were harvested at the time points of 1, 3, 7 and 14 d post-injury. The changes of skeletal muscle morphology were assessed by hematoxylin and eosin (HE) stains. The gene and protein expression was analyzed by real-time polymerase chain reaction (RT-PCR) and western blotting (WB) . Results: 1) The HE results showed that skeletal muscle fibers were significantly impaired and the injured fibers had almost degenerated at 1 d and 3 d postinjury in both S and T groups. At 7 d after injury, the damaged muscle area in the group S had been replaced mostly by newly formed muscle fibers, whereas numerous necrotic myofibers and inflammatory cells dominated the injured muscle regions of group T. In addition, a large number of regenerated muscle fibers were observed at 14 d after injury in the group T. 2) The Myogenic Differentiation Antigen (Myo D) and myogenin m RNA increased significantly post-injury (p<0.01) . As compared to the S group, macrophage depletion significantly inhibited Myo D m RNA level and increased myogenin m RNA level in the group T post-injury (p<0.05) . 3) The regulatory factors of muscle regeneration (except mechano growth factor, MGF) m RNA increased significantly in the group S post-injury. However, compared with group S, the expression of regulatory factors of muscle regeneration of T group was significantly down regulated. 4) Hypoxia-inducible factor-1α (HIF-1α) and angiopoietin1 (Angpt1) m RNA increased significantly post-injury in group S. Compared with group S, HIF-1 and Angpt1 in T group were significantly up-regulated in the later stage of muscle regeneration. 5) WB analysis showed that p-Akt/Akt, p-mTOR/mTOR, p-p70 S6 K/p70 S6 k and p-4 EBP1/4 EBP1 increased significantly post-injury in group S. However, there were no significantly change in the expression of p-Akt/Akt, p-mTOR/mTOR, p-p70 S6 K/p70 S6 k and p-4 EBP1/4 EBP1 in the group T after injury. Conclusion: Macrophages play important roles in muscle regeneration after injury. Macrophage depletion impairs muscle regeneration and that the inhibition of muscle regeneration regulatory factors and Akt/mTOR pathway may involve in the process.