Essay: Skeletal muscle atrophy

Important insights have been made concerning the molecular and genetic bases of skeletal muscle atrophy and aging in cell culture and animal models, but little is known about the underlying molecular mechanisms of skeletal muscle atrophy with aging and disuse in humans. Certain controversy exists in the literature regarding whether muscle atrophy in human skeletal muscle is regulated primarily via an increase in protein degradation or a decrease in protein synthesis. In the murine model, solid evidence has pointed at protein degradation as the driving factor, with the ubiquitin-dependant proteolytic system being rapidly activated (Gomes et al., 2001b;Bodine et al., 2001c;Lecker et al., 2004c) in relation to unloading and various disease states (Bodine et al., 2001b;Lecker et al., 2004b;Sacheck et al., 2007e). In contrast, data from human in vivo studies have been less consistent (de Boer et al., 2007c;Jones et al., 2004;Chen et al., 2007a;Abadi et al., 2009;Leger et al., 2006). Our data revealed a 1-2 fold up-regulation of MuRF-1 and Atrogin-1 in both young and old muscle during the initial days of immobility (~2-4 days), supporting a role for the ubiquitin-proteasome pathway in the initiation of human skeletal muscle atrophy. The fact that we observed more modest changes compared to previous animal reports may reflect that more systemic wasting models were used in these animal studies (Lecker et al., 2004a;Gomes et al., 2001c;Bodine et al., 2001a) compared to human immobilisation models. Notably, the present data revealed that the expression levels of both Atrogin-1 and MuRF-1 returned to basal levels after 14 days of immobility, indicating that in human skeletal muscle the ubiquitin-proteasome pathway may not be important to maintain a more chronic atrophy response. A biphasic time-course in mRNA expression profiles of atrogenes has previously been shown to exist in the rodent model (Sacheck et al., 2007f), which may explain that early transcriptional changes have been overlooked in previous human studies as later time points mainly have been studied. A coordinate regulation of the ubiquitin-proteasome and the autophagy-lysosome pathways has been shown to exist in the murine model (Mammucari et al., 2007c;Sandri, 2008;Zhao et al., 2007), but somewhat surprisingly we did not observe any change in the mRNA expression profiles of ATG4, GABARAPL or FoxO3. This lack of autophagy activation could indicate that cross-talk between the ubiquitin-proteasome and the autophagy-lysosome pathways mainly occurs in more systemic atrophy models, although species-specific differences between the rodent and human model has been suggested to exist (Welle et al., 2001b).
In addition to being a central regulator of muscle protein synthesis and muscle hypertrophy the IGF-1/Akt signaling pathway has been proposed to be a potent suppressor of myofibrillar proteolysis and atrophy related ubiquitin ligases, respectively (Sacheck et al., 2004). Importantly, our findings of an age-specific decrease in P-Akt protein indicate that immobility leads to a rapid (2-4 days) and sustained (14 days) decrease in protein synthesis exclusively in young muscle, which at least in part explains the observation of a larger muscle loss in young compared old individuals (study VII). In support of these data a diminshed phosphorylation of Akt pathway components has been reported following 48h of immobility in young human subjects (Abadi et al., 2009). In contrast, the present data point toward a muscle specific upregulation of the IGF1-pathway exclusively in old subjects, which in combination with a lack of changes in the Akt pathway may explain the attenuated atrophy response in old muscle. However, our current knowledge regarding the age-related differences in the regulation of this pathway is very limited, and more studies clearly are needed to reveal the mechanisms underlying the apparent age-specific disparity that was observed in the present study.
Further, a marked down-regulation of genes involved in mitochondrial metabolism was observed (study VII), consistent with recent human gene array studies (Abadi et al., 2009;Chen et al., 2007b). Specifically, our results demonstrated age-specific differences in the modulation of PGC-1?? and PGC-1??, with a later and smaller response in old muscle compared to that of young (study VII). These findings support the hypothesis that the down-regulation of PGC-1?? and PGC-1?? are important determinants for the initiation of human skeletal muscle atrophy, as also observed in rodents (Sandri et al., 2006c), although no indications of FoxO3-dependant transcriptional changes were noted in the present study in contrast to previous animal data obtained using systemic muscle wasting (Sandri et al., 2004c;Sandri et al., 2006d). It could be speculated, that one reason for the slower and/or attenuated atrophy response to immobility in aged compared to young human muscle could be a consequence of the general decrease in oxidative metabolism observed with aging, which paradoxically in the case of muscle inactivity may protect aging muscle from a rapid loss in myofibrillar proteins.
Another topic of debate has been the role of the apoptotic pathway in human skeletal muscle atrophy and sarcopenia. Using animal models there are a significant amount of data indicating an important role for apoptosis in the development of muscle atrophy observed with aging (Dirks & Leeuwenburgh, 2002a;Marzetti et al., 2008;Marzetti et al., 2009;Phillips & Leeuwenburgh, 2005;Pistilli et al., 2006;Siu et al., 2005), whereas human data have been more inconsistent (Malmgren et al., 2001;Strasser et al., 2000;Whitman et al., 2009). In essence, our data showed a marked and rapid increase in the expression of apoptotic markers with immobilization, with an indication of a more pronounced response in old muscle cells (study VII). Notably, despite that an increase in TUNEL-positive nuclei was observed primarily in muscle biopsies from old individuals after immobilisation, there was no sign of specific cellular TUNEL-positive myonuclei in neither young nor old muscle, in contrast to previous findings in the murine model (Dupont-Versteegden et al., 2006). Thus, in the present study TUNEL-positive nuclei mainly were localized in the interstitiel space, indicating that myofiber apoptosis may not play a key role for the mediation of human disuse-muscle atrophy.