Résumé :
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Macrophages are important for skeletal muscle regeneration and may exert beneficial effects on myogenic cell growth through mitogenic and anti-apoptotic activities. However, macrophages are highly versatile and may exert various, and even opposite, functions depending on their activation state. We have characterized the phenotype of monocyte/macrophages, studied their role during skeletal muscle regeneration and analyzed their effect on in vitro myogenesis. After injury, skeletal muscle recruited CX3CR1lo/Gr-1+ monocytes from blood that exhibited a non-dividing proinflammatory profile. Then, within muscle, these cells switched their phenotype to become proliferating anti-inflammatory CX3CR1hi/Gr-1- cells that further differentiated into F4/80hi macrophages. We have shown that in vitro, phagocytosis of necrotic muscle cells induced a switch of proinflammatory macrophages toward an anti-inflammatory phenotype releasing TGFbeta1 suggesting that phenotype transition may occur in vivo upon phagocytosis of muscle debris. Depletion of circulating monocytes in CD11b-DTR mouse at time of injury totally prevented muscle regeneration, myofibres remaining in a necrotic state, indicating that macrophages are necessary for muscle repair. Depletion of the intramuscular F4/80hi macrophages at later stages after injury reduced the diameter of regenerating fibres suggesting that they are involved in myofibre growth. In vitro, we showed that pro-inflammatory macrophages stimulated human myogenic precursor cell (mpc) proliferation and prevented their differentiation whereas anti-inflammatory macrophages exhibited differentiating activity, assessed by myogenin expression and fusion into myotubes. We are investigating each step of in vitro myogenesis: migration, differentiation (expression of the myogenic programme) and fusion itself. Our first data showed that pro-inflammatory macrophages stimulated mpc migration and inhibited their differentiation and fusion. Inversely, anti-inflammatory macrophages slowed down mpc migration and stimulated their fusion into multinucleated myotubes. Finally, skeletal muscle regeneration would be sequentially associated with two main types of macrophages: first, inflammatory macrophages that sustain mpc proliferation, then anti-inflammatory macrophages that stimulate myogenesis and fiber growth.
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