Abstract
This study examined fiber type-dependent differences in the regulation of protein synthesis in individual muscle fibers found within the same whole muscle. Specifically, the in vivo SUrface SEnsing of Translation (SUnSET) methodology was used to measure protein synthesis in type 1, 2A, 2X and 2B fibers of the mouse plantaris muscle, in response to food deprivation (FD), and mechanical overload induced by synergist ablation (SA). The results show that 48 h of FD induced a greater decrease in protein synthesis in type 2X and 2B fibers compared to type 1 and 2A fibers. Type 2X and 2B fibers also had the largest FD-induced decrease in total S6 protein and Ser240/244 S6 phosphorylation, respectively. Moreover, only type 2X and 2B fibers displayed a FD-induced decrease in cross-sectional area (CSA). Ten days of SA also induced fiber type-dependent responses, with type 2B fibers having the smallest SA-induced increases in protein synthesis, CSA and Ser240/244 S6 phosphorylation, but the largest increase in total S6 protein. Embryonic myosin heavy chain (MHCEmb) positive fibers were also found in SA muscles and the protein synthesis rates, levels of S6 Ser240/244 phosphorylation, and total S6 protein content, were 3.6-, 6.1- and 2.9-fold greater than that found in fibers from control muscles, respectively. Overall, these results reveal differential responses in the regulation of protein synthesis and fiber size between fiber types found within the same whole muscle. Moreover, these findings demonstrate that changes found at the whole muscle level do not necessarily reflect changes in individual fiber types.
Highlights
Mammalian skeletal muscles are heterogeneous tissues composed of different fiber types that are identified by their expression of specific myosin heavy chain (MHC) isoforms
Consistent with the larger decreases in protein synthesis, it was determined that food deprivation (FD) only induced a significant decrease in the cross-sectional area (CSA) of the type 2X and 2B fibers (Fig. 1G and Fig. S1)
We have reported that 30– 40% of the total fibers in 14 day synergist ablation (SA) muscles are MHCEmb positive [16], these fibers are likely to account for a significant proportion of the increase in total S6 protein, S6 Ser240/244 phosphorylation and protein synthesis that is observed at the whole muscle level
Summary
Mammalian skeletal muscles are heterogeneous tissues composed of different fiber types that are identified by their expression of specific myosin heavy chain (MHC) isoforms. We developed a non-radioactive method for measuring in vivo rates of protein synthesis at the single muscle fiber level. This method is based on the principles of the SUrface SEnsing of Translation (SUnSET) technique and it involves injecting animals with a low dose of puromycin (a structural analog of tyrosyl-tRNA) [7,8]. We have successfully used the SUnSET approach to demonstrate that basal rates of protein synthesis in the mouse plantaris (PLT) muscle vary in a fiber-type dependent manner (2B , 2X , 2A < 1) [8]
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