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How do human heart cells react to microgravity?

06 Dec 2019
Astronaut Kathleen Rubins
Astronaut Kathleen Rubins examines human heart cells cultured aboard the International Space Station. (Courtesy: NASA)

Human heart cells exposed to microgravity show surprisingly quick changes in function and gene expression, but largely return to normal when back on Earth, say researchers in the US. The team compared the RNA, morphology and behaviour of two sets of cardiac muscle cells in vitro, one of which spent more than five weeks onboard the International Space Station (ISS). Further experiments on 3D cultures and more complex tissue structures could eventually lead to treatments for a range of conditions suffered by astronauts during long space missions.

National space agencies and commercial companies share the goal of sending humans to Mars in the next decade or two. Barring a revolution in propulsion technologies, this will almost certainly involve astronauts spending many months in weightless conditions, causing physiological changes such as bone-density loss, muscular atrophy and decreased heart function. Despite decades of experience gained on the ISS and its predecessors, how these changes play out at the cellular level is still relatively unknown.

To help fill this knowledge gap, Alexa Wnorowski and Arun Sharma, at Stanford University School of Medicine, and colleagues used stem cells from three individuals to generate human cardiomyocytes – heart muscle cells that contract to give the organ its beat. They split each of the three cell lines into two 2D cultures, cultivating one set on the ground while the other was launched into space (Stem Cell Reports 10.1016/j.stemcr.2019.10.006).

During the samples’ 5.5 weeks on the ISS, astronaut and co-author Kathleen Rubins, of NASA Johnson Space Center, observed the cells’ contraction dynamics using video microscopy. After the cells were returned to Earth, the researchers used phase-contrast microscopy and immunofluorescence microscopy to measure the cell morphology. Equivalent observations on the ground-based cells revealed no significant differences in shape and structure between the two sets of cultures. However, those in orbit beat less regularly and contracted and relaxed more slowly, which the researchers attribute to changes in the way calcium was cycled within the cells.

The researchers also harvested cells during and after the mission so that their transcriptome could be sequenced. The transcriptome is the cell’s set of RNA molecules, and indicates which genes are active at a given time. More than 3000 genes showed differences in their level of expression between the ground and flight cultures, with changes clustered mainly around those that regulate cellular metabolism.

These changes in the transcriptome did not include genes related to calcium cycling, so the researchers think that the altered contraction dynamics seen in the spaceborne cells were a non-genetic response to the change in their environment.

“There are known pressure- and tension-sensing proteins that enable cells to sense and respond to their environment. This may be responsible for some of the cellular responses that we observed,” says Joseph Wu, the team’s leader, and director of Stanford Cardiovascular Institute.

Whether it was just microgravity that the cells were responding to is not certain, however, as the researchers note that the ISS samples experienced additional forces during launch and re-entry, as well as a possible increase in radiation during their time in orbit. They say that future experiments will need to include a spaceborne control group that is cultured in a centrifuge to simulate surface gravity.

Ten days after the cultures returned to Earth, the researchers assessed the cells’ behaviour again, and took further samples for RNA analysis. By this time, most of the differences in gene expression between the two groups had faded away, but about 1000 remained – as did the functional changes related to calcium cycling. Wnorowski, Sharma and colleagues cannot say whether the cells would have returned fully to their pre-flight state given a longer recovery period.

As above, so below

The study represents only the first investigation into the effects of microgravity on human cardiac cells, and will be followed by experiments involving engineered human heart tissues consisting of multiple cell types. Wu thinks that the insights that these studies yield could find use in space and back on Earth.

“The ‘easy’ way to keep the heart and other muscles of the body as healthy as possible is for astronauts to exercise regularly and intensely, but perhaps there could be therapeutic means to prevent this cardiac remodelling that occurs in orbit,” says Wu. “Such a potential ‘heart strengthening’ therapy could also have applications for individuals experiencing heart failure on the ground.”

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