Protein expression and modifications in denervated atrophic and hypertrophic skeletal muscle

Abstract: Skeletal muscle comprises about 45% of the total body weight. It is the largest tissue in the body, vital for maintaining posture, produce locomotion, breathing but also functions as a large reservoir of proteins. Regulation and maintenance of muscle mass is a delicate process and is determined by the balance between protein synthesis and degradation. Following denervation skeletal muscles change their functional and structural properties. Most muscles start to lose weight due to the inactivity caused by denervation and become atrophic, whereas the hemidiaphragm initially increases in weight the first 6-10 days and becomes hypertrophic, followed by a decrease in weight. The aim of this thesis was to examine the expression and phosphorylation of factors potentially involved in the regulation of skeletal muscle mass in denervated atrophic hind-limb muscles and in denervated hypertrophic hemidiaphragm muscles in mice. Another aim was to identify markers that could be linked to lysosomal and autophagic activities in denervated muscle.Factors of the Akt/mTOR pathway were studied and results indicative of increased protein synthesis were obtained in both denervated atrophic and hypertrophic muscles. This suggests that skeletal muscle atrophy following denervation is more likely to depend on increased protein degradation rather than an overall decrease in protein synthesis (paper I). A differential response of MK2 Thr317 phosphorylation in denervated atrophic and hypertrophic muscles was confirmed, without corresponding changes in phosphorylation of p38, indicating that other factors than p38 are responsible for this differential response, possibly Hsp70. Factors other than MK2 may be responsible for the phosphorylation of Hsp25, since increased levels of phosphorylated Hsp25 were seen in all denervated muscles studied (paper II). Results also suggested FoxO1 and MuRF1 to have potential roles in tissue remodeling that occurs after denervation (paper III). Immunoreactivity for the lysosomal marker Lamp1 and for the autophagic marker protein Lc3 was observed in characteristic ring-like structures in transverse sections of denervated muscle fibers (paper IV).