Microgravity Induces Pelvic Bone Loss through Osteoclastic Activity, Osteocytic Osteolysis, and Osteoblastic Cell Cycle Inhibition by CDKN1a/p21

Abstract

Bone is a dynamically remodeled tissue that requires gravity-mediated mechanical stimulation for maintenance of mineral content and structure. Homeostasis in bone occurs through a balance in the activities and signaling of osteoclasts, osteoblasts, and osteocytes, as well as proliferation and differentiation of their stem cell progenitors. Microgravity and unloading are known to cause osteoclast-mediated bone resorption; however, we hypothesize that osteocytic osteolysis, and cell cycle arrest during osteogenesis may also contribute to bone loss in space. To test this possibility, we exposed 16-week-old female C57BL/6J mice (n = 8) to microgravity for 15-days on the STS-131 space shuttle mission. Analysis of the pelvis by µCT shows decreases in bone volume fraction (BV/TV) of 6.29%, and bone thickness of 11.91%. TRAP-positive osteoclast-covered trabecular bone surfaces also increased in microgravity by 170% (p = 0.004), indicating osteoclastic bone degeneration. High-resolution X-ray nanoCT studies revealed signs of lacunar osteolysis, including increases in cross-sectional area (+17%, p = 0.022), perimeter (+14%, p = 0.008), and canalicular diameter (+6%, p = 0.037). Expression of matrix metalloproteinases (MMP) 1, 3, and 10 in bone, as measured by RT-qPCR, was also up-regulated in microgravity (+12.94, +2.98 and +16.85 fold respectively, p<0.01), with MMP10 localized to osteocytes, and consistent with induction of osteocytic osteolysis. Furthermore, expression of CDKN1a/p21 in bone increased 3.31 fold (p<0.01), and was localized to osteoblasts, possibly inhibiting the cell cycle during tissue regeneration as well as conferring apoptosis resistance to these cells. Finally the apoptosis inducer Trp53 was down-regulated by −1.54 fold (p<0.01), possibly associated with the quiescent survival-promoting function of CDKN1a/p21. In conclusion, our findings identify the pelvic and femoral region of the mouse skeleton as an active site of rapid bone loss in microgravity, and indicate that this loss is not limited to osteoclastic degradation. Therefore, this study offers new evidence for microgravity-induced osteocytic osteolysis, and CDKN1a/p21-mediated osteogenic cell cycle arrest.

Stem Cell Health and Tissue Regeneration in Microgravity

“…Similar results were seen in studies of somatic bone marrow stem cells isolated from mouse bones fl own on the STS -131 mission. In these studies, microgravity caused an inhibition of differentiation in mouse bone marrow osteoprogenitors required for normal bone regeneration, accompanied by p21 -mediated cell cycle arrest [43,49]. Upon reloading following microgravity, the regenerative potential of bone marrow stromal cells to differentiate was greatly increased refl ecting an accumulation of undifferentiated stem cell precursors in bone marrow.…”

Effects of simulated weightlessness on cellular morphology and biological characteristics of cell lines SGC-7901 and HFE-145

“…Regarding the biological characteristics of normal cells, microgravity could block the cell cycle in human osteoblast cells (Blaber et al, 2013) and potentiate stem cell proliferation, while sustaining the capacity for differentiation (Yuge et al, 2006). Cell cycle control proteins in human T lymphocytes showed altered microgravity (Thiel et al, 2012).…”

Ergonomic Challenges for Astronauts during Space Travel and the Need for Space Medicine

“…The use of model systems such as meduka fish studies on the ISS [119] demonstrate a rapid alteration in gene expression in osteoblasts and osteoclasts which suggest involvement of genes associated with growth and differentiation which have been shown to drive the production of bone [120]. Recently, a new osteo-inductive protein called NELL-1 has been shown promotes bone growth in non-clinical animal models [121] where bone loss is artificially induced by ovariectomy, which is a standard methodology for the short term assessment of potential new agents with anabolic and anti-osteoclastic activity [122].…”