Real-time imaging and transcriptome analysis of medaka fish aboard space station

Space travel in a reduced gravity environment can have lasting effects on the body. For example, studies clearly show that astronauts undergo a significant drop in bone mineral density during space missions, but the precise molecular mechanisms responsible for such changes in bone structure are unclear.

Now, Akira Kudo at Tokyo Tech, together with scientists in Japan in support of other countries, have performed remote, real-time, live imaging for fluorescent signals derived from osteoblasts and osteoclasts of medaka fish after only one day of exposure to microgravity aboard the International Space Station (ISS). They found increases in both osteoblast- and osteoclast-specific, promoter-driven GFP and DsRed signals one day after launch, which continued for up to eight days.

In their experiments, the team used four different double medaka transgenic lines focusing on up-regulation of fluorescent signals of osteoblasts and osteoclasts to clarify the effect of gravity on the interaction of osteoblast-osteoclast. They also studied changes in the gene expression in the transgenic fish by so-called transcriptome analysis.

These findings suggest that exposure to microgravity induced an immediate "dynamic alteration of gene expressions in osteoblasts and osteoclasts." Namely, these experiments based on real-time imaging of medaka from Earth and transcriptome analysis could be the prelude to the establishment of a new scientific research field of "gravitational biology."

Methodology

The live imaging of fluorescence microscopy signals from the fish aboard the ISS were monitored remotely from Tsukuba Space Center in Japan.

Live imaging of osteoblasts showed an increase in the intensity of osterix- and osteocalcin-DsRed in pharyngeal bones one day after launch. This increased effect continued for eight days for osterix- and five days for osteocalcin.

In the case of osteoclasts, the fluorescent signals observed from TRAP-GFP and MMP9-DsRed increased significantly on the fourth and sixth days after launch.

The fluorescent analysis was complimented by using transcriptome analysis to measure gene expression in the transgenic fish. The researchers state that, "HiSeq from pharyngeal bones of juvenile fish at day two after launch showed up-regulation of two osteoblast- and three osteoclast- related genes".

Also, transcription of the "nucleus" was found to be significantly enhanced based on whole-body gene ontology analysis of RNA-Seq, with the researchers observing transcription-regulators to be more up-regulated at day two compared with during day six.

Finally, Kudo and the team identified five genes: (c-fos and jun-b, pai-1 and ddit4, and tsc22d3) that were all up-regulated in the whole-body on days two and six, and in the pharyngeal bone on day two.

Life in so-called 'microgravity' environments—where the force of gravity is considerably less than on Earth—can cause significant problems for the human body. Astronauts who spend a number of months in space have been shown to suffer from reduced bone mineral density, leading to skeletal problems. Surprisingly, the loss of calcium starts at least 10 days after launch in astronauts in Skylab Flights.

In the next space experiment, Kudo and colleagues will clarify the role of glucocorticoid receptor (GR) on cells in microgravity.