Philippou A, Minozzo FC, Spinazzola JM, Smith LR, Lei H, Rassier DE, Barton ER. Masticatory muscles of mouse do not undergo atrophy in space. FASEB: Federation of American Societies for Experimental Biology Journal. 201;29(7):2769-2779.
Muscle loading is important for maintaining muscle mass; when load is removed, atrophy is inevitable. However, in clinical situations such as critical care myopathy, masticatory muscles do not lose mass. Thus, their properties may be harnessed to preserve mass. We compared masticatory and appendicular muscles responses to microgravity, using mice aboard the space shuttle Space Transportation System-135. Age- and sex-matched controls remained on the ground. After 13 days of space flight, 1 masseter (MA) and tibialis anterior (TA) were frozen rapidly for biochemical and functional measurements, and the contralateral MA was processed for morphologic measurements. Flight TA muscles exhibited 20 ± 3% decreased muscle mass, 2-fold decreased phosphorylated (P)-Akt, and 4- to 12-fold increased atrogene expression. In contrast, MAs had no significant change in mass but a 3-fold increase in P-focal adhesion kinase, 1.5-fold increase in P-Akt, and 50–90% lower atrogene expression compared with limb muscles, which were unaltered in microgravity. Myofibril force measurements revealed that microgravity caused a 3-fold decrease in specific force and maximal shortening velocity in TA muscles. It is surprising that myofibril-specific force from both control and flight MAs were similar to flight TA muscles, yet power was compromised by 40% following flight. Continued loading in microgravity prevents atrophy, but masticatory muscles have a different set point that mimics disuse atrophy in the appendicular muscle.—Philippou, A., Minozzo, F. C., Spinazzola, J. M., Smith, L. R., Lei, H., Rassier, D. E., Barton, E. R. Masticatory muscles of mouse do not undergo atrophy in space.
Skeletal muscle has the remarkable ability to adapt to changes in workload. Numerous muscle properties can be modulated, including muscle mass, contractile properties, and metabolism. Changes in patterns of gene expression and shifts in the balance between protein synthesis and degradation are required for adaptational responses. How well the existing properties meet the demands on the tissue is coordinated by mechanical, chemical, and metabolic information to instigate the process of muscle adaptation. Identification of major pathways that directly regulate gene expression and protein synthesis/degradation demonstrate that multiple inputs can converge on final common pathways for muscle adaptation. Understanding the contribution of the wide variety of inputs on muscle adaptation has been challenging. Skeletal muscle mass generally is regulated by a dynamic balance between protein synthesis and degradation and a vital equilibrium between the signals driving these processes (1, 2).
In skeletal muscle, sensors of mechanical loading are situated in the sarcolemma tethering the intracellular cytoskeleton to the extracellular matrix. Specifically, two major protein complexes—the focal adhesion complex and the dystrophin glycoprotein complex—are important for sensing mechanical stress at the membrane and are thought to coordinate the balance between muscle growth and atrophy (3–5). Both complexes transmit mechanical information to the cell nucleus via their association with specific nonreceptor protein tyrosine kinases such as focal adhesion kinase (FAK) (6). Phosphorylation of FAK affects its association with other signaling proteins, leading to the activation of the Ras-Raf-MEK-ERK pathway, as well as the phosphatidylinositol 3-kinase-Akt pathway, through which FAK mediates its signaling to promote muscle cell survival and muscle mass maintenance (7).
In response to reduction of external mechanical loading, including disuse and microgravity, the dynamic balance is shifted in favor of protein degradation over synthesis (2, 8–10). Systematic muscle protein degradation occurs by the activation of muscle-specific ubiquitin ligases, most prominently Atrogin-1 (MaFbx) and muscle ringer finger-1 (MuRF-1) (11, 12). The expression of progrowth genes is down-regulated simultaneously (13–15).
In the microgravity environment of space flight, absence of weight bearing has detrimental effects on skeletal muscle, including reprogramming of the expression pattern of various genes related to muscle growth/atrophy, transformation of muscle fiber types, and mass reduction (16–18). Most of these previous studies on mice subjected to microgravity have focused on limb muscles, where much has been revealed regarding adaptational responses of appendicular muscle to lack of external load. A differential response may occur in masticatory muscles, which has not been addressed. We have reported previously clear differences in terms of loading signals between the masseters (MAs) and limb muscles (19). Further, in clinical situations where there is severe muscle wasting, as seen in patients with acute quadriplegic myopathy in the intensive care unit, the masticatory muscles are spared. This suggests these muscles are equipped with a different load sensing program than limb muscles (20, 21). Animal models for acute quadriplegic myopathy recapitulate the protection against atrophy in MA muscles in stark contrast to the muscle atrophy in the rest of the body (22, 23). These studies raise the possibility that masticatory muscles have a unique loading set point and that they do not respond to unloading in the same manner as appendicular muscles.
In the current study, we compared the signaling, expression, and functional responses of appendicular versus masticatory muscles to the microgravity environment of space flight. We obtained tibialis anterior (TA) and MA muscles from mice subjected to microgravity and age- and sex-matched ground controls on the last space shuttle mission, Space Transportation System (STS)-135, of the National Aeronautics and Space Administration (NASA). To evaluate the loading response thoroughly, we also compared the responses of the masticatory muscles in mice subjected to a liquid diet, which eliminates the loading from normal chewing but still affords muscle movement and activity. We hypothesized that the loading of MA muscles comes in part from normal chewing activity, and therefore the mouse MAs may be spared from atrophy in the weightlessness environment, yet they would still succumb to atrophy on a liquid diet.
