Unloading-Induced Cortical Bone Loss is Exacerbated by Low-Dose Irradiation During a Simulated Deep Space Exploration Mission.

Spaceflight-induced bone losses have been reliably reproduced in Hind-Limb-Unloading (HLU) rodent models. However, a considerable knowledge gap exists regarding the effects of low-dose radiation and microgravity together. Ten-week-old male C57BL/6J mice, randomly allocated to Control (CONT), Hind-Limb Unloading (HLU), and Hind-Limb Unloading plus Irradiation (HLUIR), were acclimatized at 28 °C, close to thermoneutral temperature, for 28 days prior to the 14-day HLU protocol. HLUIR mice received a 25 mGy dose of X-ray irradiation, simulating 14 days of exposure to the deep space radiation environment, on day 7 of the HLU protocol. Trabecular bone mass was similarly reduced in HLU and HLUIR mice when compared to CONT, with losses driven by osteoclastic bone resorption rather than changes to osteoblastic bone formation. Femoral cortical thickness was reduced only in the HLUIR mice (102 µm, 97.5-107) as compared to CONT (108.5 µm, 102.5-120.5). Bone surface area was also reduced only in the HLUIR group, with no difference between HLU and CONT. Cortical losses were driven by osteoclastic resorption on the posterior endosteal surface of the distal femoral diaphysis, with no increase in the numbers of dead osteocytes. In conclusion, we show that low-dose radiation exposure negatively influences bone physiology beyond that induced by microgravity alone.

Erratum: One-month spaceflight compromises the bone microstructure, tissue-level mechanical properties, osteocyte survival and lacunae volume in mature mice skeletons.

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.

One-month spaceflight compromises the bone microstructure, tissue-level mechanical properties, osteocyte survival and lacunae volume in mature mice skeletons.

The weightless environment during spaceflight induces site-specific bone loss. The 30-day Bion-M1 mission offered a unique opportunity to characterize the skeletal changes after spaceflight and an 8-day recovery period in mature male C57/BL6 mice. In the femur metaphysis, spaceflight decreased the trabecular bone volume (-64% vs. Habitat Control), dramatically increased the bone resorption (+140% vs. Habitat Control) and induced marrow adiposity invasion. At the diaphysis, cortical thinning associated with periosteal resorption was observed. In the Flight animal group, the osteocyte lacunae displayed a reduced volume and a more spherical shape (synchrotron radiation analyses), and empty lacunae were highly increased (+344% vs. Habitat Control). Tissue-level mechanical cortical properties (i.e., hardness and modulus) were locally decreased by spaceflight, whereas the mineral characteristics and collagen maturity were unaffected. In the vertebrae, spaceflight decreased the overall bone volume and altered the modulus in the periphery of the trabecular struts. Despite normalized osteoclastic activity and an increased osteoblast number, bone recovery was not observed 8 days after landing. In conclusion, spaceflight induces osteocyte death, which may trigger bone resorption and result in bone mass and microstructural deterioration. Moreover, osteocyte cell death, lacunae mineralization and fatty marrow, which are hallmarks of ageing, may impede tissue maintenance and repair.

Divergent Anabolic Signalling responses of Murine Soleus and Tibialis Anterior Muscles to Chronic 2G Hypergravity.

The purpose of the study was to assess the rate of protein synthesis (PS) and elucidate signalling pathways regulating PS in mouse soleus (Sol) and tibialis anterior (TA) muscles following chronic hypergravity (30-day centrifugation at 2G). The content of the key signalling proteins of the various anabolic signalling pathways was determined by Western-blotting. The rate of PS was assessed using in-vivo SUnSET technique. An exposure to 2G centrifugation did not induce any significant changes in the rate of PS as well as phosphorylation status of the key anabolic markers (AKT, p70s6k, 4E-BP1, GSK-3beta, eEF2) in Sol. On the contrary, a significant 55% increase in PS (p < 0.05) was found in TA. The cause of such a rise in PS could be associated with an increase in AKT (+72%, p < 0.05), GSK-3beta (+60%, p < 0.05) and p70s6k (+40%, p < 0.05) phosphorylation, as well as a decrease in eEF2 phosphorylation (-46%, p < 0.05) as compared to control values. Thus, the results of our study indicate that 30-day 2G centrifugation induces a distinct anabolic response in mouse Sol and TA muscles. The activation of the PS rate in TA could be linked to an up-regulation of both mTORC1-dependent and mTORC1-independent signalling pathways.

High-acceleration whole body vibration stimulates cortical bone accrual and increases bone mineral content in growing mice.

Whole body vibration (WBV) is a promising tool for counteracting bone loss. Most WBV studies on animals have been performed at acceleration <1g and frequency between 30 and 90Hz. Such WBV conditions trigger bone growth in osteopenia models, but not in healthy animals. In order to test the ability of WBV to promote osteogenesis in young animals, we exposed seven-week-old male mice to vibration at 90Hz and 2g peak acceleration for 15min/day, 5 days/week. We examined the effects on skeletal tissues with micro-computed tomography and histology. We also quantified bone vascularization and mechanosensitive osteocyte proteins, sclerostin and DMP1. Three weeks of WBV resulted in an increase of femur cortical thickness (+5%) and area (+6%), associated with a 25% decrease of sclerostin expression, and 35% increase of DMP1 expression in cortical osteocytes. Mass-structural parameters of trabecular bone were unaltered in femur or vertebra, while osteoclastic parameters and bone formation rate were increased at both sites. Three weeks of WBV resulted in higher blood vessel numbers (+23%) in the distal femoral metaphysis. After 9-week WBV, we have not observed the difference in structural cortical or trabecular parameters. However, the tissue mineral density of cortical bone was increased by 2.5%. Three or nine weeks of 2g/90Hz WBV treatment did not affect longitudinal growth rate or body weight increase under our experimental conditions, indicating that these are safe to use. These results validate a potential of 2g/90Hz WBV to stimulate trabecular bone cellular activity, accelerate cortical bone growth, and increase bone mineral density.

Effects of chronic hypergravity: from adaptive to deleterious responses in growing mouse skeleton.

One of the most important but least studied environmental factors playing a major role in bone physiology is gravity. While the knowledge of deleterious effects of microgravity on the skeleton is expanding, little is known about hypergravity and its osteogenic potential. Centrifugation was used to assess effects of 21-day continuous 2- or 3-g acceleration on femur and L2-vertebra of 7-wk-old male C57BL/6 mice. Under 3 g, body mass growth slowed down, and deleterious skeletal effects were found (P < 0.05 compared with control): cortical thinning, osteoclasts surface increase (+41% in femur, +20% in vertebra), and bone formation rate decrease (-34% in femur, -38% in vertebra). A 2-g centrifugation did not reduce body mass and improved trabecular volume (+18% in femur, +13% in vertebra) and microarchitecture (+32% connectivity density in femur, +9% trabecular thickness in vertebra, P < 0.05 compared with control). Centrifugation at 2 g also decreased osteoclast surfaces (-36% in femur, -16% in vertebra) and increased the extent of mineralized surfaces (+31% in femur, +48% in vertebra, P < 0.05 compare to control). Quantitative immunohistochemistry revealed an increase of dentin matrix acidic phosphoprotein 1 (DMP1) and decrease of sclerostin (+60% and -35% respectively, P < 0.001 compared with control) in the femur cortex of 2-g mice. In the distal femur metaphysis, the number and volume of blood vessels increased by 22 and 44%, respectively (P < 0.05 compared with control). In conclusion, the effects of continuous hypergravity were bone compartment-specific and depended on the gravity level, with a threshold between beneficial 2-g and deleterious 3-g effects.

Skeletal site-specific effects of whole body vibration in mature rats: from deleterious to beneficial frequency-dependent effects.

Whole body vibration (WBV) is receiving increasing interest as an anti-osteoporotic prevention strategy. In this context, selective effects of different frequency and acceleration magnitude modalities on musculoskeletal responses need to be better defined. Our aim was to investigate the bone effects of different vibration frequencies at constant g level. Vertical WBV was delivered at 0.7 g (peak acceleration) and 8, 52 or 90 Hz sinusoidal vibration to mature male rats 10 min daily for 5 days/week for 4 weeks. Peak accelerations measured by skin or bone-mounted accelerometers at L2 vertebral and tibia crest levels revealed similar values between adjacent skin and bone sites. Local accelerations were greater at 8 Hz compared with 52 and 90 Hz and were greater in vertebra than tibia for all the frequencies tested. At 52 Hz, bone responses were mainly seen in L2 vertebral body and were characterized by trabecular reorganization and stimulated mineral apposition rate (MAR) without any bone volume alteration. At 90 Hz, axial and appendicular skeletons were affected as were the cortical and trabecular compartments. Cortical thickness increased in femur diaphysis (17%) along with decreased porosity; trabecular bone volume increased at distal femur metaphysis (23%) and even more at L2 vertebral body (32%), along with decreased SMI and increased trabecular connectivity. Trabecular thickness increased at the tibia proximal metaphysis. Bone cellular activities indicated a greater bone formation rate, which was more pronounced at vertebra (300%) than at long bone (33%). Active bone resorption surfaces were unaffected. At 8 Hz, however, hyperosteoidosis with reduced MAR along with increased resorption surfaces occurred in the tibia; hyperosteoidosis and trend towards decreased MAR was also seen in L2 vertebra. Trabecular bone mineral density was decreased at femur and tibia. Thus the most favorable regimen is 90 Hz, while deleterious effects were seen at 8 Hz. We concluded that the skeleton is frequency-scalable, thus highlighting the importance of WBV regimen conditions and suggesting that cautions are required for frequencies less than 10 Hz, at least in rats.