One of the biggest problems to solve for long-duration spaceflights is how to stop astronauts’ bones weakening in microgravity. But a series of experiments on live medaka fish aboard the International Space Station (ISS) might hold the key. A new study looking at how microgravity affects these fish suggests that the mitochondria in the cells responsible for the breakdown of bone might have a role to play.
It has already been suggested that microgravity somehow affects the activation of osteoclasts, the cells found in pits on the bone that regulate its resorption, where the tissue is broken down before being replaced with new bone. The experiments with the medaka fish are intended to look into this relationship, and so far findings of this recent study seem to be supportive of this idea. The researchers found that the fish which had experienced microgravity showed increased osteoclast activity and a significant reduction in bone mineral density.
Some of the little medaka fish in microgravity on the ISS. JAXA
But when looking at the osteoclast cells under an electron microscope, the researchers found out something else interesting. The energy-generating mitochondria of the cells seemed abnormal, and genetic analysis of the DNA contained within these structures revealed significantly increased activity of two genes. This, the researchers claim, suggests that osteoclast activation could be linked to how the mitochondria react to the microgravity.
“If this is also true for astronauts, medicines that target mitochondria dysfunction may restore the bone loss in space,” explains Akira Kudo, the lead researcher for the study published in Scientific Reports. “It has been difficult to understand the mechanism of age-related bone loss on Earth. The Medaka experiment finding of new genes affected by microgravity provides a good animal model to clarify this mechanism.”
JAXA astronaut Akihiko Hoshide at the Aquatic Habitat in the Kibo module of the International Space Station. NASA
The ISS actually got its aquarium over three years ago, and rather than just providing the astronauts with a serene and relaxing pastime, it has been helping answer questions on the density and break down of bone tissue. Called the Aquatic Habitat (AQH), it is a closed system filled with small fish called medaka (Oryzias latipes), also known as Japanese killifish. Chosen because of its extensive use as a model organism in biology, the little fish is ideal to use due to their short generation time and the fact that they’re transparent, allowing researchers to track bone and tissue growth.
The astronauts on the ISS are able to keep the fish alive for up to 90 days at a go, which is long enough for them to pass through three generations, since they’ve been able to mate and breed successfully in microgravity. The AQH maintains all the necessary conditions for the fish, including a special air-water interface, allowing the medaka to “peck” air from the surface. It’s thought that this development could one day allow astronauts to keep amphibians, such as frogs, on the ISS. The system is sponsored by the Japan Aerospace eXploration Agency (JAXA), and as such is housed on the Japanese Experimental Module, known as Kibo.