Insulin-like Growth Factor-1 Gene Transfer Augments Muscle Protein Synthesis Rate And Size In Adult Mice

Abstract

Insulin-like growth factor I (IGF-I) is a potent myogenic factor that has been shown to play a critical role in muscle regeneration and muscle hypertrophy. Viral-mediated gene transfer of IGF-I upregulates muscle size and increases muscle strength. It is not clear whether IGF-I-induced muscle hypertrophy is mediated by an increase in protein synthesis and/or a decrease in proteolysis rates in existing myofibers and satellite cells. PURPOSE The objective of this study was to investigate the effect of IGF-I overexpression on in vivo muscle protein synthesis rate. METHODS The left hindlimb of young adult C57BL6 mice was injected with a recombinant adeno-associated virus vector for IGF-I (rAAV-1). The construct consisted of the entire rat IGF-I cDNA which encodes for IGF-I, a Myosin Light Chain (MLC) 1/3 promoter and enhancer, and SV40 polyadenlyation sequence. Six mice (age=3 weeks) were injected with 80ul of 10% glycerol/PBS containing approximately 1010 rAAV particles into the interstitial space of the anterior compartment of one hindlimb, targeting the extensor digitorum longus (EDL). Three months after transfection, in vivo mixed muscle protein synthesis rates were measured using the incorporation of intravenously administered L- [13C]-phe into EDL muscle proteins. [13C]-phe abundance in the structural proteins was quantitated using gas chromatography-combustion-isotope ratio mass spectrometry. Paired samples t-tests were used for comparisons between transfected and contralateral hindlimbs. RESULTS rAAV-IGF1 transfection resulted in a significant increase (mean ± SD) (12.1 ± 8.5%) in muscle wet weight and cross-sectional area (9.9 ± 9.1%)(p<0.05). In vivo mixed muscle protein synthesis rate was 26.9 ± 15.6% higher in the rAAV-IGF1 injected limbs compared to the contralateral control limb (p<0.05). Average EDL muscle protein synthesis rate was 10.5 ± 2.8%/day in the rAAV-IGF1 injected limb and 8.2 ± 1.9%/day in the contralateral limb. CONCLUSIONS These results demonstrate that muscle specific adeno-associated virus overexpression of IGF-I increased mixed muscle protein synthesis rate and this contributed to muscle hypertrophy. Further research will determine EDL contractile properties, and the components of the IGF-I signaling pathway that are activated in satellite cells and mature myofibers. Supported by R01-HD042955, NCRR-000954.