Spaceflight alters human heart cells but they return to (mostly) normal on Earth

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Past studies have shown that spaceflight can reduce the heart rate, lower arterial pressure and increase cardiac output. But new research examines how microgravity — or zero gravity — influences the human heart at the cellular level.

“We’ve been able to check astronauts’ health while they’re in space and when they’re back down but we’ve had no way to understand the molecular cell changes,” said Joseph C. Wu of Stanford University’s School of Medicine, who is also a senior author on the study.

In an attempt to answer this, the researchers from Stanford University examined cardiac function and gene expression in human heart cells from three individuals. Instead of coming from biopsies — which is a very invasive procedure — Wu said these heart cells were made by “reprogramming” a small sample of blood into human stem cells. The heart cells were then cultured aboard the International Space Station for five-and-a-half weeks — the first study of its kind.

They found that exposure to microgravity altered the expression of 2,635 genes — a temporary change in the RNA, which is made from DNA — but most returned to normal patterns of gene expression within 10 days after returning to Earth.

“If you see DNA as a permanent cookbook, RNA is a temporary, handwritten copy of a recipe in that cookbook,” Wu explained. “Gene expression … was thus temporarily changed by the environment, which was microgravity in this case.”

Wu said the changes observed were “subtle” but statistically significant. He said it was hard to say whether the changes would affect how the heart functioned.

“Keep in mind our study was only five weeks — it’s a short time. I don’t know what gene changes would be if it was six months,” Wu continued. “I’m sure if you examined astronauts after a longer period, you would see more changes.”

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Humans have been living in space for 19 years, with astronauts spending an average of six months at a time on the International Space Station.

Wu said he was surprised about how quickly human heart muscle cells were able to adapt to microgravity.

“These studies may provide insight into cellular mechanisms that could benefit astronaut health during long-duration spaceflight, or potentially lay the foundation for new insights into improving heart health on Earth,” he said.

The stem cell-derived heart cells were flown to the International Space Station aboard a SpaceX spacecraft as part of a commercial re-supply mission in 2017. Once there, the cells were cultured by astronaut Kate Rubins, who coincidentally had been trained in molecular biology. Simultaneously, the same kind of cells were cultured on Earth for comparison purposes. RNA sequencing was performed on the cells at 4.5 weeks on board the ISS and 10 days after returning to Earth.

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“Logistically, it was a very difficult study to deal with,” Wu said. For example, the samples, which were protected by an incubator, had to be recovered by helicopter once the re-entry vessel landed in the Pacific ocean.

Wu and his team are currently working on a more sophisticated study, which involves sending a chunk of engineered 3D human heart tissue, which will look more similar to a human heart, into space next year.

As to whether the human body is up to years-long space journeys — such as a trip to Mars — Wu isn’t sure.

“I don’t know,” he said. “I think it’s going to be a lot of stress to the human body, and that’s why we’re interested in doing research on this very timely topic.”

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