A deep analysis of the proteomic and phosphoproteomic alterations that occur in skeletal muscle after the onset of immobilization.

KEY POINTS: A decrease in protein synthesis plays a major role in the loss of muscle mass that occurs in response to immobilization. In mice, immobilization leads to a rapid (within 6 h) and progressive decrease in the rate of protein synthesis and this effect is mediated by a decrease in translational efficiency. Deep proteomic and phosphoproteomic analyses of mouse skeletal muscles revealed that the rapid immobilization-induced decrease in protein synthesis cannot be explained by changes in the abundance or phosphorylation state of proteins that have been implicated in the regulation of translation. ABSTRACT: The disuse of skeletal muscle, such as that which occurs during immobilization, can lead to the rapid loss of muscle mass, and a decrease in the rate of protein synthesis plays a major role in this process. Indeed, current dogma contends that the decrease in protein synthesis is mediated by changes in the activity of protein kinases (e.g. mTOR); however, the validity of this model has not been established. Therefore, to address this, we first subjected mice to 6, 24 or 72 h of unilateral immobilization and then used the SUnSET technique to measure changes in the relative rate of protein synthesis. The result of our initial experiments revealed that immobilization leads to a rapid (within 6 h) and progressive decrease in the rate of protein synthesis and that this effect is mediated by a decrease in translational efficiency. We then performed a deep mass spectrometry-based analysis to determine whether this effect could be explained by changes in the expression and/or phosphorylation state of proteins that regulate translation. From this analysis, we were able to quantify 4320 proteins and 15,020 unique phosphorylation sites, and surprisingly, the outcomes revealed that the rapid immobilization-induced decrease in protein synthesis could not be explained by changes in either the abundance, or phosphorylation state, of proteins. The results of our work not only challenge the current dogma in the field, but also provide an expansive resource of information for future studies that are aimed at defining how disuse leads to loss of muscle mass.