Effects of hypergravity on gene levels in anti-gravity muscle and bone through the vestibular system in mice

Abstract

We recently reported that hypergravity with 3 g for 4 weeks affects muscle and bone through the vestibular system in mice. The purpose of this study was to investigate the effects of hypergravity with 2 g, which had no influence on circulating glucocorticoid level, on the gene levels in muscle and bone, as well as the roles of the vestibular system in those changes using vestibular lesioned (VL) mice. Hypergravity for 2 and 8 weeks or VL exerted little effects on the mRNA levels of muscle differentiation factors and myokines in the soleus muscle. Although hypergravity for 2 weeks significantly elevated alkaline phosphatase (ALP) and type I collagen mRNA levels in the tibia, VL significantly attenuated the levels of ALP mRNA enhanced by hypergravity. In conclusion, the present study suggests that a 2-g load for 2 weeks enhances osteoblast differentiation partly through the vestibular system in mice.

Keywords: Bone; Gravity change; Muscle; Vestibular system.

Fig. 1

Effects of hypergravity and vestibular lesion (VL) on serum corticosterone levels in mice. Blood samples were collected from mice treated with VL or sham surgery after exposure to 1 g or 2 g for 2 weeks (a) and 8 weeks (b). Then, serum corticosterone levels were measured. Data represent the mean ± SEM of six mice in each group (a) as well as 8 (1 g-sham, 2 g-sham) and 7 (1 g-VL, 2 g-VL) mice (b). Open bar and filled bar represent 1 and 2 g, respectively

Fig. 2

Effects of hypergravity and VL on the expression of myogenic differentiation genes in the soleus muscle of mice. Total RNA was extracted from the soleus muscle of mice treated with VL or sham surgery after exposure to 1 or 2 g for 2 weeks (a) and 8 weeks (b). Then, the level of MyoD, Myf5, myogenin, myosin heavy chain-I (MHC-I), atrogin-1, MuRF1 or glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was assessed by real-time PCR analysis. Data are expressed relative to the levels of GAPDH. The data represent the mean ± SEM of 6 (a) and 4 (b) tissues in each group. **P < 0.01

Fig. 3

Effects of hypergravity and VL on the expression of osteogenic differentiation genes in the tibia of mice. Total RNA was extracted from the tibia of mice treated with VL or sham surgery after exposure to 1 or 2 g for 2 (a) and 8 weeks (b). Then, the mRNA level of Runx2, Osterix, alkaline phosphatase (ALP), type I collagen (Col-1), osteocalcin (OCN) or GAPDH was assessed by real-time PCR analysis. Data are expressed relative to the levels of GAPDH. The data represent the mean ± SEM of 6 (a) and 4 (b) tissues in each group. **P < 0.01, *P < 0.05

Fig. 4

Effects of hypergravity and VL on the expression of receptor activator of nuclear factor-κB ligand (RANKL), osteoprotegerin (OPG) and receptor activator of nuclear factor-κB (RANK) in the tibia of mice. Total RNA was extracted from the tibia of mice treated with VL or sham surgery after exposure to 1 g or 2 g for 2 (a) and 8 weeks (b). Then, the mRNA level of RANKL, OPG, RANK and GAPDH was assessed by real-time PCR analysis. Data are expressed relative to the levels of GAPDH and ratio of RANKL to OPG mRNA levels. The data represent the mean ± SEM of 6 (a) and 4 (b) tissues in each group