Downregulated Myostatin Signaling Does Not Enhance Protein Kinase B/Akt Phosphorylation in Skeletal Muscle of Low Birthweight Neonatal Pigs

Low birthweight (LBWT) neonatal pigs are at significant risk for morbidity and mortality, and experience restricted postnatal muscle growth. Myostatin (MSTN), a member of transforming growth factor-β (TGF-β) family, is expressed and secreted predominantly in skeletal muscle. MSTN negatively regulates skeletal muscle development and growth by inhibiting protein synthesis. Our previous data suggest that LBWT neonatal pigs have reduced protein synthesis but unaltered protein degradation rates, which result in lower protein deposition in skeletal muscle. We hypothesized that decreased protein synthesis in skeletal muscle of LBWT pigs is due to higher abundance of MSTN and upregulated MSTN signaling through protein kinase B/Akt dependent process. Twelve pairs of newborn male LBWT (816 ± 55 g) and normal birth weight (NBWT; 1,642 ± 55 g) littermates were euthanized to collect longissimus dorsi (LD) muscle. Muscle mRNA expression was measured by real-time PCR, and protein abundance and phosphorylation were measured by western blot. mRNA and protein abundance of MSTN were lower (P < 0.05) in LD muscle of LBWT pigs compared with their NBWT littermates. There was no difference in mRNA expression of MSTN receptors in muscle between LBWT and NBWT pigs, including activin A receptor type IIB and activin receptor-like kinase 4. In addition, muscle mRNA expression of follistatin, a physiological inhibitor of MSTN, did not differ between two groups. However, mRNA expression of decorin was higher (P < 0.05) in muscle of LBWT than NBWT pigs, suggesting more MSTN could be sequestered thus lowering its inhibitory action on muscle growth. There was no difference in protein abundance of smad2 in LD muscle between LBWT and NBWT pigs. However, abundance and phosphorylation of smad3 were lower (P < 0.05) in LBWT pigs compared with their NBWT siblings. Despite this reduction, there was no difference in protein abundance and phosphorylation of protein kinase B/Akt. Our data suggest that although abundance of MSTN was lower and MSTN signaling was downregulated, these changes do not explain the diminished protein synthesis rates previously observed in skeletal muscle of LBWT pigs. Thus it is likely that downregulated MSTN signaling may be a compensatory mechanism rather than the cause for the impaired muscle growth in LBWT neonatal pigs.