Changes in skeletal muscle and tendon structure and function following genetic inactivation of myostatin in rats

Key points Myostatin is an important regulator of muscle mass and a potential therapeutic target for the treatment of diseases and injuries that result in muscle atrophy. Targeted genetic mutations of myostatin have been generated in mice, and spontaneous loss‐of‐function mutations have been reported in several species. The impact of myostatin deficiency on the structure and function of muscles has been well described for mice, but not for other species. We report the creation of a genetic model of myostatin deficiency in rats using zinc finger nuclease technology. The main findings of the study are that genetic inactivation of myostatin in rats results in increases in muscle mass without a deleterious impact on the specific force production and tendon mechanical properties. The increases in mass occur through a combination of fibre hypertrophy, hyperplasia and activation of the insulin‐like growth factor‐1 pathway, with no substantial changes in atrophy‐related pathways. This large rodent model has enabled us to identify that the chronic loss of myostatin is void of the negative consequences to muscle fibres and extracellular matrix observed in mouse models. Furthermore, the greatest impact of myostatin in the regulation of muscle mass may not be to induce atrophy directly, but rather to block hypertrophy signalling. Myostatin is a negative regulator of skeletal muscle and tendon mass. Myostatin deficiency has been well studied in mice, but limited data are available on how myostatin regulates the structure and function of muscles and tendons of larger animals. We hypothesized that, in comparison to wild‐type (MSTN+/+) rats, rats in which zinc finger nucleases were used to genetically inactivate myostatin (MSTNΔ/Δ) would exhibit an increase in muscle mass and total force production, a reduction in specific force, an accumulation of type II fibres and a decrease and stiffening of connective tissue. Overall, the muscle and tendon phenotype of myostatin‐deficient rats was markedly different from that of myostatin‐deficient mice, which have impaired contractility and pathological changes to fibres and their extracellular matrix. Extensor digitorum longus and soleus muscles of MSTNΔ/Δ rats demonstrated 20–33% increases in mass, 35–45% increases in fibre number, 20–57% increases in isometric force and no differences in specific force. The insulin‐like growth factor‐1 pathway was activated to a greater extent in MSTNΔ/Δ muscles, but no substantial differences in