Regeneration of skeletal muscle depends on a population of adult stem cells (satellite cells) that remain quiescent throughout life. Satellite cell regenerative functions decline with ageing. Here we report that geriatric satellite cells are incapable of maintaining their normal quiescent state in muscle homeostatic conditions, and that this irreversibly affects their intrinsic regenerative and self-renewal capacities. In geriatric mice, resting satellite cells lose reversible quiescence by switching to an irreversible pre-senescence state, caused by derepression of p16(INK4a) (also called Cdkn2a). On injury, these cells fail to activate and expand, undergoing accelerated entry into a full senescence state (geroconversion), even in a youthful environment. p16(INK4a) silencing in geriatric satellite cells restores quiescence and muscle regenerative functions. Our results demonstrate that maintenance of quiescence in adult life depends on the active repression of senescence pathways. As p16(INK4a) is dysregulated in human geriatric satellite cells, these findings provide the basis for stem-cell rejuvenation in sarcopenic muscles.

Skeletal muscles adapt to increasing workload by augmenting their fiber size, through mechanisms that are poorly understood. This study identifies the cytokine interleukin-6 (IL-6) as an essential regulator of satellite cell (muscle stem cell)-mediated hypertrophic muscle growth. IL-6 is locally and transiently produced by growing myofibers and associated satellite cells, and genetic loss of IL-6 blunted muscle hypertrophy in vivo. IL-6 deficiency abrogated satellite cell proliferation and myonuclear accretion in the preexisting myofiber by impairing STAT3 activation and expression of its target gene cyclin D1. The growth defect was indeed muscle cell intrinsic, since IL-6 loss also affected satellite cell behavior in vitro, in a STAT3-dependent manner. Myotube-produced IL-6 further stimulated cell proliferation in a paracrine fashion. These findings unveil a role for IL-6 in hypertrophic muscle growth and provide mechanistic evidence for the contribution of satellite cells to this process.

In the fatal degenerative Duchenne muscular dystrophy (DMD), skeletal muscle is progressively replaced by fibrotic tissue. Here, we show that fibrinogen accumulates in dystrophic muscles of DMD patients and mdx mice. Genetic loss or pharmacological depletion of fibrinogen in these mice reduced fibrosis and dystrophy progression. Our results demonstrate that fibrinogen-Mac-1 receptor binding, through induction of IL-1␤, drives the synthesis of transforming growth factor-␤ (TGF␤) by mdx macrophages, which in turn induces collagen production in mdx fibroblasts. Fibrinogen-produced TGF␤ further amplifies collagen accumulation through activation of profibrotic alternatively activated macrophages. Fibrinogen, by engaging its ␣v␤3 receptor on fibroblasts, also directly promotes collagen synthesis. These data unveil a profibrotic role of fibrinogen deposition in muscle dystrophy.Supplemental material is available at http://www.genesdev.org.Received November 30, 2007; revised version accepted April 28, 2008. Duchenne muscular dystrophy (DMD) results from mutations in the gene coding for the protein dystrophin, which localizes at the inner face of the sarcolemma (Campbell 1995). Besides progressive muscle degeneration and inflammation, fibrotic transition of muscle tissue is critical in DMD as it progressively deteriorates locomotor capacity, posture maintenance, and the vital function of cardiac and respiratory muscles. Indeed, DMD individuals have a high degree of fibrosis increasing with age, which is reproduced in the diaphragm muscle of mdx mice (the mouse model of DMD) (Stedman et al. 1991). Importantly, the underlying mechanisms of fibrosis development within dystrophic muscle remain largely unknown.Fibrinogen is a soluble acute phase protein, which is released into the blood in response to stress. Apart from its key role in controlling blood loss following vascular injury, fibrinogen also extravasates at sites of inflammation or increased vascular permeability where it is immobilized and/or converted to fibrin (Rybarczyk et al. 2003) (from hereon we refer to both by the term "fibrin/ ogen"). We showed previously that mice with defective fibrinolysis exhibited impaired muscle regeneration after experimental injury (Suelves et al. 2002). In this study, we investigated the role of fibrin/ogen deposition in the development of fibrosis in dystrophic muscle. Results and DiscussionWe first analyzed fibrin/ogen deposition in muscles of DMD patients and its correlation with disease course. Compared with muscles of healthy individuals or of fibromyalgia patients, DMD muscles showed significant fibrin/ogen accumulation (Fig. 1A). Similarly, in mdx mice muscles, fibrin/ogen deposits were readily detectable after disease onset, while absent before disease onset (Fig. 1B,C). Thus, fibrin/ogen deposition is associated with muscle dystrophinopathy.Collagen deposition (fibrosis) was prominent in DMD muscles and particularly found in the same areas occupied by fibrin/ogen (Fig. 1D). To investigate the relationship between the e...

The p38 mitogen-activated protein kinase (MAPK) pathway plays a critical role in skeletal muscle differentiation. However, the relative contribution of the four p38 MAPKs (p38a, p38b, p38c and p38d) to this process is unknown. Here we show that myoblasts lacking p38a, but not those lacking p38b or p38d, are unable to differentiate and form multinucleated myotubes, whereas p38c-deficient myoblasts exhibit an attenuated fusion capacity. The defective myogenesis in the absence of p38a is caused by delayed cell-cycle exit and continuous proliferation in differentiation-promoting conditions. Indeed, activation of JNK/cJun was enhanced in p38a-deficient myoblasts leading to increased cyclin D1 transcription, whereas inhibition of JNK activity rescued the proliferation phenotype. Thus, p38a controls myogenesis by antagonizing the activation of the JNK proliferation-promoting pathway, before its direct effect on muscle differentiation-specific gene transcription. More importantly, in agreement with the defective myogenesis of cultured p38a D/D myoblasts, neonatal muscle deficient in p38a shows cellular hyperproliferation and delayed maturation. This study provides novel evidence of a fundamental role of p38a in muscle formation in vitro and in vivo.