Cell Injury , Repair , Aging and Apoptosis Macrophages Protect against Muscle Atrophy and Promote Muscle Recovery in Vivo and in Vitro A Mechanism Partly Dependent on the Insulin-Like Growth Factor-1 Signaling Molecule

Hindlimb unloading and reloading are characterized by a major loss of muscle force and are associated with classic leukocyte infiltration during recovery from muscle atrophy. Macrophages act as a cellular cornerstone by playing both proand anti-inflammatory roles during muscle recovery from atrophy. In the present study, we investigated the role of macrophages in muscle atrophy and regrowth using in vivo and in vitro models. Mice depleted in monocytes/ macrophages and submitted to a hindlimb unloading and reloading protocol experienced a significant delay in muscle force recovery compared with matched placebo mice at 7 and 14 days after reloading. Furthermore, an in vitro myotube/macrophage coculture showed that anti-inflammatory macrophages, which contain apoptotic neutrophils and express low levels of cyclooxygenase-2, completely prevented the loss of protein content and the myotube atrophy observed after 2 days in low serum medium. The presence of macrophages also protected against the decrease in myosin heavy chain content in myotubes exposed to low serum medium for 1 day. Interestingly , the addition of an anti-IGF-1 antibody to the coculture significantly decreased the ability of macrophages to protect against myotube atrophy and myosin heavy chain loss after 2 days in low serum medium. These results clearly indicate that macrophages and, more precisely , the release of IGF-1 by macrophages , play an important role in recovery from muscle atrophy. (Am J Pathol 2010, 176:2228–2235; DOI: 10.2353/ajpath.2010.090884) Monocytes are large mononuclear cells that circulate in the blood and differentiate into macrophages in invaded tissues in response to various stimuli. Macrophages have a strong phagocytic capacity and can orchestrate the inflammatory process via the release of a wide variety of cytokines and chemokines such as interleukin (IL)-1, tumor necrosis factor, and macrophage inflammatory protein-2. Numerous studies have also demonstrated that macrophages play a direct role in tissue recovery through the release of the anti-inflammatory molecules and anabolic growth factors IL-10, basic fibroblast growth factor, and insulin-like growth factor-1 (IGF-1). The regulation of the multiple and occasionally opposing functions of macrophages is very complex and poorly understood. To add to this complexity, the diversity of experimental models can also lead to different conclusions regarding the roles of each macrophage phenotype in muscle recovery from atrophy or damage. For example, in a model of eccentric contractions, in which injured muscle is basically devoid of neutrophils, macrophage invasion contributes to secondary damage to the skeletal muscle. On the other hand, the phagocytosis of apoptotic neutrophils and necrotic cells by macrophages induces a change in macrophage phenotype from proto anti-inflammatory, which has a strong modulatory effect on cytokine profiles and is essential for dampening environmental inflammatory signals. Regarding the different subsets of macrophages, coculture experiments have shown that pro-inflammatory macrophages (Ly6C) stimulate myogenic cell proliferation, whereas anti-inflammatory macrophages (Ly-6C) exert a strong differentiating influence on myogenic cells. Nonethe-