In two new reports, STAT3 signaling is shown to be increased in adult muscle satellite cells, and its inhibition improves muscle regeneration. Diminished regenerative capacity of skeletal muscle occurs during adulthood. We identified a reduction in the intrinsic capacity of mouse adult satellite cells to contribute to muscle regeneration and repopulation of the niche. Gene expression analysis identified higher expression of JAK-STAT signaling targets in 18-month-old relative to 3-week-old mice. Knockdown of Jak2 or Stat3 significantly stimulated symmetric satellite stem cell divisions on cultured myofibers. Genetic knockdown of Jak2 or Stat3 expression in prospectively isolated satellite cells markedly enhanced their ability to repopulate the satellite cell niche after transplantation into regenerating tibialis anterior muscle. Pharmacological inhibition of Jak2 and Stat3 activity similarly stimulated symmetric expansion of satellite cells in vitro and their engraftment in vivo. Intramuscular injection of these drugs resulted in a marked enhancement of muscle repair and force generation after cardiotoxin injury. Together these results reveal age-related intrinsic properties that functionally distinguish satellite cells and suggest a promising therapeutic avenue for the treatment of muscle-wasting diseases.

Wnt7a/Fzd7 signaling stimulates skeletal muscle growth and repair by inducing the symmetric expansion of satellite stem cells through the planar cell polarity pathway and by activating the Akt/mTOR growth pathway in muscle fibers. Here we describe a third level of activity where Wnt7a/Fzd7 increases the polarity and directional migration of mouse satellite cells and human myogenic progenitors through activation of Dvl2 and the small GTPase Rac1. Importantly, these effects can be exploited to potentiate the outcome of myogenic cell transplantation into dystrophic muscles. We observed that a short Wnt7a treatment markedly stimulated tissue dispersal and engraftment, leading to significantly improved muscle function. Moreover, myofibers at distal sites that fused with Wnt7a-treated cells were hypertrophic, suggesting that the transplanted cells deliver activated Wnt7a/Fzd7 signaling complexes to recipient myofibers. Taken together, we describe a viable and effective ex vivo cell modulation process that profoundly enhances the efficacy of stem cell therapy for skeletal muscle.

The hypothesis that mechanical loading regulates talin expression in developing and adult muscle was tested using in vitro and in vivo models. Talin was selected for study because it is a key structural link between the cytoskeleton and cell membrane. In the in vitro model, C₂C₁₂myotubes were subjected to cyclic strains for 48 h. In the in vivo model, rat hindlimb muscles were unloaded for 10 days, then reloaded for 2 days. Cyclic loading of myotubes resulted in significant increases in the quantity of talin (68%) and its 190-kDa proteolytic fragment (70%), as well as talin mRNA (180%), relative to unloaded myotube cultures. Similarly, talin concentration and its mRNA increased by 68 and 136%, respectively, in soleus muscles reloaded for 2 days relative to ambulatory controls. Immunohistochemistry and in situ RT-PCR showed that talin and its mRNA are concentrated and colocalized at myotendinous junctions. Thus these findings indicate that increased mechanical loading promotes talin synthesis, which occurs principally at myotendinous junctions, according to talin mRNA distribution.

Previous observations have shown that neutrophil invasion precedes macrophage invasion during muscle inflammation and that peak muscle injury is observed at the peak of ED1⁺ macrophage invasion. We tested the hypothesis that neutrophil invasion causes subsequent invasion by ED1⁺ macrophages and that ED1⁺ macrophages then contribute significantly to muscle membrane injury during modified muscle use. Rat hindlimbs were unloaded for 10 days followed by reloading by normal ambulation to induce inflammation. Membrane injury was measured by assaying Evans blue-bound serum protein influx through membrane lesions. Muscle neutrophil populations increased significantly during the first 2 h of reloading but ED1⁺ macrophages did not increase until 24 h. Neutrophil invasion was uncoupled from subsequent macrophage invasion by reloading rat hindlimbs for 2hto cause neutrophil invasion, followed by resuspension for hours 2–24. This produced similar increases in neutrophil concentration as measured in muscles continuously reloaded for 24 h without causing an increase in macrophages. However, resuspension did not reduce the extent of muscle damage compared with that occurring in muscles that were reloaded continuously for 24 h. Thus, muscle invasion by neutrophils is not sufficient to cause invasion by ED1⁺ macrophages. In addition, muscle membrane injury that occurs during reloading is independent of invasion by ED1⁺ macrophages. J. Leukoc. Biol. 65: 492–498; 1999.