Myostatin, a member of the TGF-beta family, negatively regulates skeletal muscle development. Depression of myostatin activity leads to increased muscle growth and carcass lean yield. In an attempt to down-regulate myostatin, transgenic mice were produced with a ribozyme-based construct or a myostatin pro domain construct. Though the expression of the ribozyme was detected, muscle development was not altered by the ribozyme transgene. However, a dramatic muscling phenotype was observed in transgenic mice carrying the myostatin pro domain gene. Expression of the pro domain transgene at 5% of beta-actin mRNA levels resulted in a 17-30% increase in body weight (P < 0.001). The carcass weight of the transgenic mice showed a 22-44% increase compared with nontransgenic littermates at 9 weeks of age (16.05 +/- 0.67 vs. 11.16 +/- 0.28 g in males; 9.99 +/- 0.38 vs. 8.19 +/- 0.19 g in females, P < 0.001). Extreme muscling was present throughout the whole carcass of transgenic mice as hind and fore limbs and trunk weights, all increased significantly (P < 0.001). Epididymal fat pad weight, an indicator of body fat, was significantly decreased in pro domain transgenic mice (P < 0.001). Analysis of muscle morphology indicated that cross-sectional areas of fast-glycolytic fibers (gastrocnemius) and fast-oxidative glycolytic fibers (tibialis) were larger in pro domain transgenic mice than in their controls (P < 0.01), whereas fiber number (gastrocnemius) was not different (P > 0.05). Thus, the muscular phenotype is attributable to myofiber hypertrophy rather than hyperplasia. The results of this study suggest that the over-expression of myostatin pro domain may provide an alternative to myostatin knockouts as a means of increasing muscle mass in other mammals.
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