Embryonic overexpression of Galgt2 inhibits skeletal muscle growth via activation of myostatin signaling.

Many proteins that affect skeletal muscle growth are secreted glycoproteins, yet the nature of how glycosylation regulates the expression and growth-promoting properties of such factors is not well understood. One type of glycosylation that affects muscle growth is that controlled by the CT GalNAc transferase (Galgt2), the enzyme responsible for the synthesis of the beta1,4GalNAc linkage on the CT carbohydrate antigen (GalNAcbeta1,4[Neu5Ac(or Gc)alpha2,3]Galbeta1,4GlcNAcbeta-R). In the mouse, both Galgt2 protein and the CT carbohydrate become confined to the neuromuscular synapse in skeletal muscle by the second postnatal week. Galgt2 transgenic mice that overexpress the CT carbohydrate from embryonic time-points onward in skeletal muscle had profoundly impaired muscle growth that was maintained throughout adulthood. Transgenic overexpression of Galgt2 increased myostatin protein expression and stimulated myostatin signaling, whereas expression of follistatin protein, a myostatin inhibitor, was decreased. Changed myostatin and follistatin protein levels were controlled at a posttranslational level, and inhibition of muscle growth was overcome if serum follistatin levels were normalized to wild-type levels. In contrast to embryonic Galgt2 overexpression, postnatal overexpression of Galgt2 had no effect on either myostatin or follistatin expression or muscle growth. These experiments demonstrate that Galgt2 can control growth by modulating the expression of myostatin and myostatin inhibitors during particular periods of muscle development.