Macrophages decide between regeneration and fibrosis in muscle. Muñoz-Cánoves Pura,Serrano Antonio L Trends in endocrinology and metabolism: TEM Defective muscle regeneration is usually accompanied by excessive matrix accumulation (fibrosis). A recent study reveals how this fibrotic process is prevented to allow successful regeneration, through a timely interplay between macrophages and fibro/adipogenic progenitor cells. This has biomedical relevance for fibrotic muscular dystrophies. 10.1016/j.tem.2015.07.005

Mechanisms of muscle injury, repair, and regeneration. Tidball James G Comprehensive Physiology Skeletal muscle continuously adapts to changes in its mechanical environment through modifications in gene expression and protein stability that affect its physiological function and mass. However, mechanical stresses commonly exceed the parameters that induce adaptations, producing instead acute injury. Furthermore, the relatively superficial location of many muscles in the body leaves them further vulnerable to acute injuries by exposure to extreme temperatures, contusions, lacerations or toxins. In this article, the molecular, cellular, and mechanical factors that underlie muscle injury and the capacity of muscle to repair and regenerate are presented. Evidence shows that muscle injuries that are caused by eccentric contractions result from direct mechanical damage to myofibrils. However, muscle pathology following other acute injuries is largely attributable to damage to the muscle cell membrane. Many feaures in the injury-repair-regeneration cascade relate to the unregulated influx of calcium through membrane lesions, including: (i) activation of proteases and hydrolases that contribute muscle damage, (ii) activation of enzymes that drive the production of mitogens and motogens for muscle and immune cells involved in injury and repair, and (iii) enabling protein-protein interactions that promote membrane repair. Evidence is also presented to show that the myogenic program that is activated by acute muscle injury and the inflammatory process that follows are highly coordinated, with myeloid cells playing a central role in modulating repair and regeneration. The early-invading, proinflammatory M1 macrophages remove debris caused by injury and express Th1 cytokines that play key roles in regulating the proliferation, migration, and differentiation of satellite cells. The subsequent invasion by anti-inflammatory, M2 macrophages promotes tissue repair and attenuates inflammation. Although this system provides an effective mechanism for muscle repair and regeneration following acute injury, it is dysregulated in chronic injuries. In this article, the process of muscle injury, repair and regeneration that occurs in muscular dystrophy is used as an example of chronic muscle injury, to highlight similarities and differences between the injury and repair processes that occur in acutely and chronically injured muscle. 10.1002/cphy.c100092

Stem cell-derived exosomes prevent pyroptosis and repair ischemic muscle injury through a novel exosome/circHIPK3/ FOXO3a pathway. Yan Bing,Zhang Yu,Liang Chun,Liu Bin,Ding Fengzhi,Wang Yanli,Zhu Bao,Zhao Ranzun,Yu Xi-Yong,Li Yangxin Theranostics Ischemic injury of the skeletal muscle remains a serious clinical problem and currently there is no effective therapy. The aim of the present study is to determine whether human umbilical cord mesenchymal stem cells- derived exosomes (UMSC-Exo) could repair ischemic injury by releasing circular RNA. To create hindlimb ischemia, we surgically ligated the left femoral artery in C57BL/6 mice. Using circRNA-seq analyses of total RNA from ischemic and control muscles, we found reduced expression of circHIPK3 in the ischemic muscle. To explore the role of circHIPK3 in ischemic injury, the mice were randomly assigned into three groups after surgery: 1) vehicle; 2) UMSC-Exo; 3) UMSC-Exo and siRNA targeting circHIPK3 (UMSC-Exo /si-circHIPK3). UMSC-Exo treatment significantly increased expression of circHIPK3 and improved blood perfusion, running distance and muscle force, which were reversed by injection of UMSC-Exo /si-circHIPK3, suggesting that UMSC-Exo improve muscle function by releasing circHIPK3. UMSC-Exo treatment also inhibited ischemia induced pyroptosis - cell death caused by inflammasome as evidenced by activation of NLRP3, cleaved caspase-1, and subsequent increase of IL-1β and IL-18, and the effects were reversed by injection UMSC-Exo /si-circHIPK3. Bioinformatic analysis identified miR-421/FOXO3a as a potential target for circHIPK3, which was confirmed by luciferase reporter assay. Knockdown of circHIPK3 in C2C12 cells resulted in increased expression of miR-421. We established an model of pyroptosis by stimulating C2C12 cells with LPS and ATP. LPS and ATP treatment resulted in reduced expression of circHIPK3 and increased expression of miR-421, which was prevented by UMSC-Exo. Western blot analysis showed reduced levels of NLRP3 and cleaved caspase-1 when cells were treated by UMSC-Exo. The expression of FOXO3a in C2C12 cells was increased in the presence of miR-421 inhibitor, and the expression was reduced when cells were treated by LPS and ATP. Importantly, the expression of FOXO3a was upregulated by UMSC-Exo but was reduced when si-circHIPK3 was present. Using loss/gain-of function method, we demonstrated that miR-421/FOXO3a is the direct target of circHIPK3, and UMSC-Exo prevent ischemic injury by releasing circHIPK3, which in turn down regulate miR-421, resulting in increased expression of FOXO3a, leading to inhibition of pyroptosis and release of IL-1β and IL-18. 10.7150/thno.42259