Effects of gravitational loading levels on protein expression related to metabolic and/or morphologic properties of mouse neck muscles

Abstract

The effects of 3 months of spaceflight (SF), hindlimb suspension, or exposure to 2G on the characteristics of neck muscle in mice were studied. Three 8-week-old male C57BL/10J wild-type mice were exposed to microgravity on the International Space Station in mouse drawer system (MDS) project, although only one mouse returned to the Earth alive. Housing of mice in a small MDS cage (11.6 × 9.8-cm and 8.4-cm height) and/or in a regular vivarium cage was also performed as the ground controls. Furthermore, ground-based hindlimb suspension and 2G exposure by using animal centrifuge (n = 5 each group) were performed. SF-related shift of fiber phenotype from type I to II and atrophy of type I fibers were noted. Shift of fiber phenotype was related to downregulation of mitochondrial proteins and upregulation of glycolytic proteins, suggesting a shift from oxidative to glycolytic metabolism. The responses of proteins related to calcium handling, myofibrillar structure, and heat stress were also closely related to the shift of muscular properties toward fast-twitch type. Surprisingly, responses of proteins to 2G exposure and hindlimb suspension were similar to SF, although the shift of fiber types and atrophy were not statistically significant. These phenomena may be related to the behavior of mice that the relaxed posture without lifting their head up was maintained after about 2 weeks. It was suggested that inhibition of normal muscular activities associated with gravitational unloading causes significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Keywords: gravitational unloading; mouse neck muscle; protein expression.

Figure 1.

Responses of fiber type distribution in mouse neck muscles. Mean ± SEM. I and II, fibers expressing pure type I and II myosin heavy chain (MHC); I+II, fibers expressing both type I and II MHC; Pre, preexperimental control; C, cage control at the 3rd month in Osaka, Japan; LC, 3‐month laboratory control housed in regular vivarium cage in Genova, Italy; GC, 3‐month control housed in mouse drawer system (MDS) in Genova; SF, spaceflight; HS, hindlimb suspended for 3 months in Osaka; 2G, exposed to 2G for 3 months in Osaka; RHS, recovered from HS on the floor for 3 months; RC, age‐matched cage control housed on the floor for 3 months. (n) = number of mice in each group. The age of mice is also shown.

Figure 2.

Responses of fiber type–specific cross‐sectional area of neck muscle fibers of mice. Mean ± SEM. See Figure 1 for the abbreviations.

Figure 3.

Numbers of up‐ and downregulated proteins in response to spaceflight versus the age‐matched ground‐based vivarium laboratory control. Mito, mitochondria; Glyc, glycolysis; O₂ transp, oxygen transport; Ca²⁺, calcium metabolism; Struct, myofibrillar structure; HSP, heat‐shock proteins; Proteol, proteolysis.

Figure 4.

Numbers of up‐ and downregulated proteins in response to hindlimb suspension versus the age‐matched cage control. Mito, mitochondria; Glyc, glycolysis; O₂ transp, oxygen transport; Ca²⁺, calcium metabolism; Struct, myofibrillar structure; HSP, heat‐shock proteins; Proteol, proteolysis.

Figure 5.

Numbers of up‐ and downregulated proteins in response to 2G exposure versus the age‐matched cage control. Mito, mitochondria; Glyc, glycolysis; O₂ transp, oxygen transport; Ca²⁺, calcium metabolism; Struct, myofibrillar structure; HSP, heat‐shock proteins; Proteol, proteolysis.