Regulation of protein metabolism in skeletal muscle of low-birth-weight neonatal pigs

The neonatal period in mammals is characterized by high rates of growth, attributed to rapid myonuclear accretion and protein deposition in muscle. Low-birth-weight (LBWT) neonates experience restricted muscle development, which leads to impaired postnatal growth and metabolic disorders later in life. The overall hypothesis of this dissertation was that dysfunction of myogenic satellite cells and aberrant regulation of protein synthesis and degradation signaling predispose LBWT neonatal pigs to slower postnatal growth. We sought to determine the proliferation and differentiation of satellite cells (SCs) derived from skeletal muscle of LBWT neonatal pigs and to elucidate the cellular mechanisms that regulate protein synthesis and degradation in LBWT pig muscles. Newborn pigs were considered as normal-birth-weight (NBWT) or LBWT when weight at birth was within 0.5 SD and below 2 SD of litter average respectively. SCs isolated from longissimus dorsi (LD) muscle of NBWT and LBWT neonatal pigs displayed similar proliferation rates. Fusion was modestly diminished in SCs from muscle of LBWT pigs compared with their NBWT siblings, suggesting SCs were not intrinsically different between the two groups and were unlikely a major contributor to the impaired muscle growth of LBWT pigs. Plasma and muscle insulin-like growth factor (IGF)-I was diminished in LBWT compared with NBWT pigs. In addition, reduced activation of key components of IGF-I downstream signaling pathway in LBWT pigs muscle may lead to diminished translation initiation signaling and thus decreased protein synthesis in these animals. However, IGF-I receptor expression and myostatin signaling inversely correlated to LBWT, indicating they may participate in compensatory responses for the reduction in protein synthesis signaling. Expression of eukaryotic initiation factor (eIF) 4F complex subunits, eIF4E, eIF4G, and eIF4A was reduced in LBWT compared with NBWT pigs. This would suggest that diminished translation initiation signaling in skeletal muscle of LBWT pigs is the main factor that predisposes LBWT pigs to slower growth rates in the neonatal period. In contrast, changes in protein degradation signaling do not appear to affect protein turnover in LBWT neonatal pigs.