Translational signalling, atrogenic and myogenic gene expression during unloading and reloading of skeletal muscle in myostatin-deficient mice.

Skeletal muscles of myostatin null (Mstn(-/-)) mice are more susceptible to atrophy during hind limb suspension (HS) than are muscles of wild-type mice. Here we sought to elucidate the mechanism for this susceptibility and to determine if Mstn(-/-) mice can regain muscle mass after HS. Male Mstn(-/-) and wild-type mice were subjected to 0, 2 or 7 days of HS or 7 days of HS followed by 1, 3 or 7 days of reloading (n = 6 per group). Mstn(-/-) mice lost more mass from muscles expressing the fast type IIb myofibres during HS and muscle mass was recovered in both genotypes after reloading for 7 days. Concentrations of MAFbx and MuRF1 mRNA, crucial ligases regulating the ubiquitin-proteasome system, but not MUSA1, a BMP-regulated ubiquitin ligase, were increased more in muscles of Mstn(-/-) mice, compared with wild-type mice, during HS and concentrations decreased in both genotypes during reloading. Similarly, concentrations of LC3b, Gabarapl1 and Atg4b, key effectors of the autophagy-lysosomal system, were increased further in muscles of Mstn(-/-) mice, compared with wild-type mice, during HS and decreased in both genotypes during reloading. There was a greater abundance of 4E-BP1 and more bound to eIF4E in muscles of Mstn(-/-) compared with wild-type mice (P<0.001). The ratio of phosphorylated to total eIF2α increased during HS and decreased during reloading, while the opposite pattern was observed for rpS6. Concentrations of myogenic regulatory factors (MyoD, Myf5 and myogenin) mRNA were increased during HS in muscles of Mstn(-/-) mice compared with controls (P<0.001). We attribute the susceptibility of skeletal muscles of Mstn(-/-) mice to atrophy during HS to an up- and downregulation, respectively, of the mechanisms regulating atrophy of myofibres and translation of mRNA. These processes are reversed during reloading to aid a faster rate of recovery of muscle mass in Mstn(-/-) mice.

The decrease in mature myostatin protein in male skeletal muscle is developmentally regulated by growth hormone.

Myostatin inhibits myogenesis and there is reduced abundance of the mature protein in skeletal muscles of adult male compared with female mice. This reduction probably occurs after translation, which suggests that it is a regulated mechanism to reduce the availability of myostatin in males. Reduced myostatin may, thereby, contribute to the development of sexually dimorphic growth of skeletal muscle. Our first objective was to determine if the decrease in mature myostatin protein occurs before the linear growth phase to aid growth, or afterwards to maintain the mass of adult muscle. Mice were killed from 2 to 32 weeks and the gastrocnemius muscle was excised. Myostatin mRNA increased from 2 to 32 weeks and was higher in males than females (P < 0.001). In contrast, mature protein decreased in males after 6 weeks (P < 0.001). Our second objective was to determine if growth hormone (GH) induces the decrease in mature myostatin protein. GH increased myostatin mRNA and decreased the abundance of mature protein in hypophysectomised mice (P < 0.05). Our final objective was to determine if the decrease in mature protein occurs in skeletal muscles of male Stat5b(-/-) mice (Stat5b mediates the actions of GH). As expected, mature myostatin protein was not reduced in Stat5b(-/-) males compared with females. However, myostatin mRNA remained higher in males than females irrespective of genotype. These data suggest that: (1) the decrease in mature myostatin protein is developmentally regulated, (2) GH acting via Stat5b regulates the abundance of mature myostatin and (3) GH acts via a non-Stat5b pathway to regulate myostatin mRNA.

Intrapericardial IGF-I improves cardiac function in an ovine model of chronic heart failure.

BACKGROUND: Following myocardial infarction, progressive deterioration of left ventricular function often follows, leading eventually to overt heart failure. In the myocardium, there is increased expression of insulin-like growth factor I (IGF-I) mRNA, protein and receptor levels, particularly in the peri-infarct zone, suggesting that IGF-I has a role to play in post-infarct cardiac structure and function. In this study, we examine the effects of exogenous IGF-I on cardiac function. METHODS: Intrapericardial IGF-I (15 microg/kg/d, n=3) or vehicle (sterile saline, n=3) was administered to sheep in chronic heart failure and the results of intrapericardial delivery compared with those of subcutaneous delivery. Left ventricular ejection fraction (EF) was measured to assess cardiac performance. Concentrations of plasma IGF-I were quantified by radioimmunoassay. RESULTS: Intrapericardial delivery of IGF-I resulted in a rapid and sustained increase (P<0.001) in EF, which remained elevated 14 days after cessation of treatment. Subcutaneous IGF-I treatment did not affect EF. Both subcutaneous and intrapericardial IGF-I administration increased concentrations of plasma IGF-I, although concentrations declined prior to the cessation of treatment. CONCLUSIONS: We conclude that the higher concentration of IGF-I in the myocardium, which results from intrapericardial delivery significantly increases EF in chronic heart failure but that subcutaneous delivery of IGF-I does not.

Myostatin-deficient mice lose more skeletal muscle mass than wild-type controls during hindlimb suspension.

Myostatin inhibits myogenesis. Therefore, we sought to determine if mice lacking the myostatin gene [Mstn(-/-)] would lose less muscle mass than wild-type mice during 7 days of hindlimb suspension (HS). Male Mstn(-/-) and wild-type (C57) mice were subjected to HS or served as ground-based controls (n = 6/group). Wild-type mice lost 8% of body mass and approximately 13% of wet mass from biceps femoris, quadriceps femoris, and soleus, whereas the mass of extensor digitorum longus (EDL) was unchanged after HS. Unexpectedly, Mstn(-/-) mice lost more body (13%, P < 0.05) and quadriceps femoris (17%, P < 0.05) mass than wild-type mice and lost 33% of EDL mass (P < 0.01) after HS. Protein expression of myostatin in biceps femoris and quadriceps femoris was not altered, whereas expression of MyoD, Myf-5, and myogenin increased in wild-type mice and tended to decrease in muscles of Mstn(-/-) mice. These data suggest that HS induced myogenesis in wild-type mice to counter atrophy, whereas myogenesis was not induced in Mstn(-/-) mice, thereby resulting in a greater loss of muscle mass.