Gene Expression in Osteoblasts and Osteoclasts Under Microgravity Conditions: A Systematic Review

Background: Microgravity (mu G) negatively influences bone metabolism by affecting normal osteoblast and osteoclast function. mu G effects on bone metabolism has been an extensive field of study in recent years, due to the challenges presented by space flight. Methods: We systematically reviewed research data from genomic studies performed in real or simulated mu G, on osteoblast and osteoclast cells. Our search yielded 50 studies, of which 39 concerned cells of the osteoblast family and 11 osteoclast precursors. Results: Osteoblastic cells under mu G show a decreased differentiation phenotype, proved by diminished expression levels of Alkaline Phosphatase (ALP) and Osteocalcin (OCN) but no apoptosis. Receptor Activator of NF-.B Ligand (RANKL)/Osteoprotegerine (OPG) ratio is elevated in favor of RANKL in a time-dependent manner, and further RANKL production is caused by upregulation of Interleukin- 6 (IL-6) and the inflammation pathway. Extracellular signals and changes in the gravitational environment are perceived by mechanosensitive proteins of the cytoskeleton and converted to intracellular signals through the Mitogen Activated Protein Kinase pathway (MAPK). This is followed by changes in the expression of nuclear transcription factors of the Activator Protein-1 (AP-1) family and in turn of the NF-.B, thus affecting osteoblast differentiation, cell cycle, proliferation and maturation. Pre-osteoclastic cells show increased expression of the marker proteins such as Tryptophan Regulated Attenuation Protein (TRAP), cathepsin K, Matrix Metalloproteinase-9 (MMP-9) under mu G conditions and become sensitized to RANKL. Conclusion: Suppressing the expression of fusion genes such as syncytine-A which acts independently of RANKL, could be possible future therapeutic targets for microgravity side effects.