Disrupted Osteogenic Gene expression of Osteoblasts in vivo under simulated microgravity: Vibration exposure as a...

While the effects of microgravity on bone loss are well-documented, far less is known about how reduced gravity influences bone during bone growth. Using zebrafish larvae, we investigated how simulated microgravity (SMG) affects osteoblast and osteoclast-related gene expression. Zebrafish larvae were exposed to SMG continuously for 24 h similar to our previous studies. We assessed the impact on bone tissue with and without a targeted osteoblast ablation prior to SMG exposure in order to determine the response to bone cells. We also assessed the impact on bone-related genes after a 24 h vibration exposure in order to assess whether vibration could be utilised as a mechanical countermeasure to SMG. RT-qPCR analysis of osteoblast and osteoclast-related genes was conducted and supplemented with an assessment of operculum morphology, and expression of runx2a and osterix using transgenic reporter lines. We found that SMG exposure significantly reduced the amount of runx2a along the growing edge of the operculum and reduced the number of osterix expressing cells. These results indicate delayed or reduced osteoblast differentiation. Gene expression analyses also revealed increased expression of osteoclast-associated markers, suggesting a shift toward bone resorptive signaling. Combining osteoblast ablation with SMG exposure did not produce an additive suppression of osteoblast markers, suggesting shared regulatory pathways and/or a biological limit to osteoblast reduction. Interestingly, vibration exposure after SMG exposure restored osterix-positive cell density, demonstrating its potential effectiveness in mitigating the effects of mechanical unloading. Together, these findings show that short-term SMG exposure disrupts osteoblast differentiation and that mechanical vibration may be an appropriate countermeasure to SMG.