Microgravity promotes osteoclast activity in medaka fish reared at the international space station

Abstract

The bone mineral density (BMD) of astronauts decreases specifically in the weight-bearing sites during spaceflight. It seems that osteoclasts would be affected by a change in gravity; however, the molecular mechanism involved remains unclear. Here, we show that the mineral density of the pharyngeal bone and teeth region of TRAP-GFP/Osterix-DsRed double transgenic medaka fish was decreased and that osteoclasts were activated when the fish were reared for 56 days at the international space station. In addition, electron microscopy observation revealed a low degree of roundness of mitochondria in osteoclasts. In the whole transcriptome analysis, fkbp5 and ddit4 genes were strongly up-regulated in the flight group. The fish were filmed for abnormal behavior; and, interestingly, the medaka tended to become motionless in the late stage of exposure. These results reveal impaired physiological function with a change in mechanical force under microgravity, which impairment was accompanied by osteoclast activation.

Acute transcriptional up-regulation specific to osteoblasts/osteoclasts in medaka fish immediately after exposure to microgravity

“…In our results, expressions of these genes were altered under microgravity, which thus suggests that NO-GR signaling is related to the “microgravitational stress”. Taken together, available data indicate that the GR is the main molecule involved in the effect of microgravity on cells, consistent with our observations of activation of GRs in medaka fish reared long-term in the ISS21. Finally, to clarify the GR action under microgravity, further analyses are required.…”

“…It has been reported that there are many osteoblasts and osteoclasts in pharyngeal bones21, and osteoblasts are important for osteoclastogenesis in these bones15. To study the co-localization of osteoblasts and osteoclasts, we observed osterix -DsRed/ TRAP -GFP merged images (Fig.…”

“…2g,h). Then, we focused on the pharyngeal bone region, in which bone turnover is high and sensitive to microgravity21, and observed ground and flight samples at high magnification with a 20x lens (Fig. 3a,b) to examine the details of fluorescent signals in osteoblasts.…”

“…To identify osteoblasts and osteoclasts in-vivo , we previously developed a medaka osterix promoter-DsRed transgenic line for the visualization of osteoblasts20 and a medaka TRAP promoter-GFP transgenic line for that of osteoclasts14. Finally, we established a double transgenic line of osterix -DsRed and TRAP -GFP21 to examine the cooperation between osteoblasts and osteoclasts in the same animal. In the analysis of pharyngeal bones, by using this double transgenic line we showed the important role of osteoblasts for controlling osteoclasts to modify the attachment bone during tooth replacement in medaka pharyngeal teeth15.…”

“…When medaka fish at 6 weeks after hatching were launched to the ISS in 2012 and reared there for 2 months, activation of osteoclasts together with bone loss occurred in the flight fish21. In that study, to examine the alteration of gene expression early in orbit, we preserved specimens with RNAlater at days 2 and 6 after launch.…”