Lunar life drives jawbone formation.

Background/purpose: The Japan Aerospace Exploration Agency bred mice in the "Kibo" module on the International Space Station to clarify the effects of gravity changes on the body. We determined the effects of lunar gravity (partial gravity, 1/6G) on the jawbone and elucidated its relationship with the oral cavity. Materials and methods: Mandibular bones of C57BL/6J male mice from the Kibo group (1/6G) and the Earth group (1G) were obtained. Ribonucleic acid was extracted from the samples; microarray and gene set enrichment analysis (GSEA) were performed; bone structure and histology were evaluated via micro-computed tomography, immunohistochemistry, and specific staining. Results: Microarray analysis revealed a significant increase in dentin matrix protein 1 (DMP1) expression in the 1/6G group. GSEA revealed enhanced Hedgehog signaling in this group. The weight and density of the cancellous bone were significantly greater in the 1/6G group than in the 1G group, as confirmed by histological analysis. Furthermore, the expression of insulin-like growth factor 1 (IGF-1), which is associated with mastication-induced bone formation, was increased in the 1/6G group. Although secreted phosphoprotein 1 (SPP1), which is associated with bone resorption under low mechanical stress, increased in the 1/6G group, receptor activator of nuclear factor kappa-B ligand (RANKL)-RANK signaling did not increase. No significant differences in the number or activity of osteoclasts were observed between the 1G and 1/6G groups. Conclusion: Bone formation, but not bone resorption, is promoted by lunar gravity and is associated with mastication. Proper oral management is necessary for extended lunar habitation.