The injury of adult skeletal muscle initiates series of well-coordinated events that lead to the efficient repair of the damaged tissue. Any disturbances during muscle myolysis or reconstruction may result in the unsuccessful regeneration, characterised by strong inflammatory response and formation of connective tissue, that is, fibrosis. The switch between proper regeneration of skeletal muscle and development of fibrosis is controlled by various factors. Amongst them are those belonging to the transforming growth factor b family. One of the TGF-b family members is TGF-b1, a multifunctional cytokine involved in the regulation of muscle repair via satellite cells activation, connective tissue formation, as well as regulation of the immune response intensity. Here, we present the role of TGF-b1 in myogenic differentiation and muscle repair. The understanding of the mechanisms controlling these processes can contribute to the better understanding of skeletal muscle atrophy and diseases which consequence is fibrosis disrupting muscle function.

Skeletal muscles exhibit great plasticity and an ability to reconstruct in response to injury. However, the repair process is often inefficient and hindered by the development of fibrosis. We explored the possibility that during muscle repair, the different regeneration ability of the fast (extensor digitorum longus; EDL) and slow twitch (Soleus) muscles depends on the differential expression of metalloproteinases (MMP-9 and MMP-2) involved in the remodeling of the extracellular matrix. Our results show that MMP-9 and MMP-2 are present in the intact muscle and are up-regulated after crush-induced muscle injury. The expression and the activity of these two enzymes depend on the type of muscle and the phase of muscle regeneration. In the regenerating Soleus muscle, elevated levels of MMP-9 occurred during the myolysis and reconstruction phase. In contrast, regenerating EDL muscles exhibited decreased MMP-9 levels during myolysis and increased MMP-2 activity at the reconstruction phase. Moreover, satellite cells (mononuclear myoblasts) derived from Soleus and EDL muscles showed no differences in localization or activity of MMP-9 and MMP-2 during proliferation and differentiation in vitro. MMP-9 activity was present during all stages of myoblast differentiation, whereas MMP-2 activity reached its highest level during myoblast fusion. We conclude that MMPs are involved in muscle repair, and that fast and slow twitch muscles exhibit different patterns of MMP-9 and MMP-2 activity.

Thus, positive impact of Sdf-1 on muscle regeneration is related to the mobilisation of endogenous cells, that is satellite cells and myoblasts, as well as non-muscle stem cells, expressing Cxcr4 and CD34.

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When injured by crushing, the repair of the slow-twitch soleus rat muscle, unlike the fast-twitch EDL, is associated with fibrosis. As TGFβ1, whose activity can be controlled by glycosaminoglycans (GAG), plays a major role in fibrosis, we hypothesized that levels of TGFβ1 and GAG contents could account for this differential quality of regeneration. Here we show that the regeneration of the soleus was accompanied by elevated and more sustained TGFβ1 level than in the EDL. Neutralization of TGFβ1 effects by antibodies to TGFβ1 or its receptor TGFβ-R1 improved muscle repair, especially of the soleus muscle, increased in vitro growth of myoblasts, and accelerated their differentiation. These processes were accompanied by alterations of GAG contents. These results indicate that the control of TGFβ1 activity is important to improve regeneration of injured muscle and accelerate myoblast differentiation, in part through changes in GAG composition of muscle cell environment.