Myostatin levels in regenerating rat muscles and in myogenic cell cultures

Myostatin is a newly described member of the TGF-beta superfamily acting as a secreted negative regulator of skeletal muscle mass in several species, but whose mode of action remains largely unknown. In the present work, we f ollowed the myostatin mRNA and protein levels in rat soleus and extensor di gitorum longus (EDL) muscles regenerating in vivo from notexin-induced necr osis, and the myostatin transcript levels in two different in vitro myogeni c differentiation models: i.e. in mouse BC(3)H1 and C2C12 cultured cells. T he in vivo regenerating rat skeletal muscles showed a characteristic time-d ependent expression of myostatin mRNA. After notexin injection, the transcr ipt levels dropped below the detection limit on day 1 in soleus and close t o the detection limit on day 3 in EDL, then increased to a maximum on day 7 in soleus and after 28 days finally reached the control values in both typ es of muscles. In contrast, the myostatin protein levels increased dramatic ally on the first days of regeneration in both muscles, i.e. at the time wh en its transcript level was low. Later on the myostatin protein level gradu ally declined to normal in soleus while in EDL it stayed high some days lon ger and decreased to normal on days 21-28. In vitro proliferating myoblasts produced low level of myostatin mRNA, which increased upon induction of di fferentiation suggesting that functional innervation is no prerequisite for myostatin expression. Myostatin production in vitro seems not to be depend ent on myocyte fusion either, since it is observed in differentiated BC(3)H 1 cells, which are defective in myofiber formation.