The human heart is truly amazing. Each day, this small muscular organ beats approximately 100,000 times and pumps around 2,000 gallons of blood, bringing life-sustaining oxygen and nutrients to all parts of the body.
The heart continuously pumps blood—unless a coronary artery, which supplies blood to the heart, suddenly becomes blocked and the blood flow is severely restricted or stops. Without oxygen, the heart tissue rapidly begins to die. Even after blood flow is restored, the damaged tissue is unable to pump blood as well as healthy tissue. According to the Centers for Disease Control and Prevention, about 735,000 Americans have a heart attack and around 610,000 die from heart disease yearly, accounting for one out of every four deaths in the United States.
But what if there were a therapy that could regenerate heart tissue and help restore cardiac function? The microgravity environment of the ISS National Lab allows scientists to study cells in ways not possible on the ground, and research being conducted on the orbiting laboratory could help lead to the development of cell-based regenerative therapies for people with heart disease back on Earth.
One promising area of research into cell-based regenerative therapies is focused on human cardiovascular progenitor cells (CPCs). These cells are immature but in a beneficial way. CPCs are at a very early developmental stage along the path to becoming cardiovascular cells. This means they can differentiate into several different types of cells, such as heart muscle cells or the endothelial cells that line blood vessels.
On Earth, scientists are studying how CPCs might play a role in regenerative treatments for heart disease—and as unexpected as it may seem, knowledge gained from studying CPCs in space could accelerate their development as a therapeutic tool.
To examine the effects of spaceflight on CPCs, Loma Linda University researcher Mary Kearns-Jonker and her team sent cultures of CPCs to the ISS National Lab. The team looked at the effects of microgravity on both neonatal CPCs as well as adult CPCs, as the age of the person from whom the cells are derived affects how well the cells function.
“Our goal was to identify the functional effects of the CPCs after they’ve been flown in the spaceflight environment,” Kearns-Jonker said. “We found that microgravity does cause some very distinct changes that are unique to neonatal cells when comparing them to adult cells.”
Taking Cells to Space
Previous research using ground-based simulated microgravity—achieved by placing the CPCs in a 2D rotating culture device—yielded several indicators suggesting microgravity may hold promise for adaptation of CPCs for human therapies on Earth, said Jonathan Baio, who worked as a doctoral student in Kearns-Jonker’s lab at the time of the team’s ISS National Lab investigation.
“We looked at both neonatal and adult cells in microgravity to understand the differences that occur between the two and whether this information could ultimately be leveraged for regenerative therapies or to better understand how the heart develops,” Baio said.
The team worked with ISS National Lab commercial service provider BioServe Space Technologies and optimized conditions for the cells to grow in BioServe’s BioCell cell culture system. Preflight optimization was challenging because Kearns-Jonker and her team had to find the ideal concentration of cells to put into the BioCell hardware. The team had to make sure the cell population was not too sparse initially but also did not become too crowded as the cells reproduced. Another challenge was to optimize the feeding schedules so that the neonatal and adult CPCs could be fed on the same day at the same time even though each type of cell grows at a slightly different rate.
Last-minute changes to the launch schedule also posed challenges, said Aida Martinez, a medical student at Frank H. Netter MD School of Medicine at Quinnipiac University who worked as a research assistant in Kearns-Jonker’s lab during the team’s ISS National Lab investigation. The first launch date was cancelled due to bad weather, bumping the launch back by two days.
“This heightened the anticipation and excitement, but we had to go back and rethink part of it too, because our experiment had very specific timepoints,” Martinez said. “The cells had to be under specific conditions for a certain amount of time with little wiggle room, so we had to brainstorm beforehand and in real time how we would deal with a change in the schedule.”
In preparing an investigation for launch, preflight validation studies and activities involve contingency planning driven by the ISS National Lab Operations team and the commercial service provider to help prepare researchers for circumstances such as launch slips that could lead to setbacks. This allows the research team to think ahead and develop a detailed mitigation plan to help ensure the success of their investigation.
The weather cooperated on the second launch date, and the team’s investigation was launched on SpaceX’s 11th commercial resupply services mission to the ISS, while the control cells, which were also grown in BioCell hardware, remained on the ground. After 12 days in space, some samples were placed in a fixative that stabilizes RNA, and the rest of the live cells were returned to Earth after 30 days in orbit.
“We took the samples from splashdown, drove them straight to the lab, and were very happy to find that the cells had excellent viability,” said Kearns-Jonker. “We didn’t let the cells recover in the lab and did all of the functional studies right then to minimize the recovery time.”
In the lab, Kearns-Jonker and her team looked at the functional characteristics of the live cells—their ability to divide, move, and communicate via signaling. Such analyses had to be done immediately, before the cells had a chance to readapt to Earth’s gravity. The team also analyzed the cells’ gene expression and microRNA (noncoding RNA molecules that help regulate gene expression) profiles. The team compared results from the cells grown on Earth with those grown in microgravity, while also comparing differences between the neonatal and adult CPCs.
Examining Changes From Microgravity
Kearns-Jonker and her team found that microgravity induced changes in the CPCs when compared with Earth-grown cells. Some changes were seen in both the neonatal and adult CPCs, while others were unique to the neonatal cells alone.
In the spaceflight samples, the team measured several markers of cardiac development. The neonatal CPCs were found to exhibit markers characteristic of a slightly earlier stage of development. This slight de-differentiation is associated with enhanced “stemness”—making the CPCs behave more like stem cells and enhancing their potential to develop into different types of cardiovascular cells. Interestingly, these changes were not found in the adult CPCs.
In the neonatal CPCs, calcium signaling and AKT signaling were both activated in response to spaceflight. This is significant because calcium signaling plays a prominent role in the early stages of heart development, Baio said. “Additionally, AKT is an important molecule in promoting pluripotency and stemness and the ability of a stem cell to continue to divide and expand and retain its stem-like state,” he said.
The neonatal CPCs grown in microgravity were also found to have enhanced proliferation, meaning they were able to divide and increase in number more rapidly. In addition, both the neonatal and adult CPCs exposed to microgravity exhibited an enhanced ability to migrate. Migration is important, because once therapeutic cells are injected into the heart, you want them to be able to migrate and move to injured or damaged areas, Baio said.
These spaceflight results are significant because researchers could use this knowledge to recapitulate the effects on the ground in the context of advancing cell-based regenerative therapies. “There are multiple examples in the literature where a slight de-differentiation and activation of the specific transcription factors that we see elevated here have been associated with improved outcomes in a cell transplant setting,” said Kearns-Jonker.
A transcription factor is a molecule that helps regulate gene expression by controlling whether a gene’s DNA is transcribed into RNA
Bringing Benefits Back to Earth
Cells with markers of early stages of development and enhanced stemness could enable a more effective integration of therapeutic cells into heart tissue and improve tissue regeneration after an injury such as a heart attack, Baio said. Thus, the microgravity-induced changes in the neonatal CPCs may be helpful in developing cell-based therapies on Earth that can improve outcomes for patients with heart disease.
The next step would be to explore whether the microgravity-induced changes observed in neonatal CPCs produce beneficial effects in vivo, that is, in living organisms, Kearns-Jonker said. “What we don’t know is: How do the gene expression and microRNA changes noted after exposure to microgravity translate into effects that we can see in vivo? We actually need to test that to see whether or not what appears to be something that could be very good for regeneration is, in fact, beneficial.”
It is also important to continue to study CPCs to more fully understand the mechanisms behind the microgravity-induced changes. Although ground-based simulated microgravity is not a perfect model for true microgravity, scientists may be able to use simulated microgravity as a method to adapt CPCs for use in cell-based therapies.
“If one finds that microgravity-induced benefits can be recapitulated in simulated microgravity models on Earth, it’s very reasonable to envision pretreating the cells with simulated microgravity for prospective therapeutic applications,” Kearns-Jonker said.
Looking to the future, there is great potential in how microgravity or simulated microgravity could be used to directly affect cardiac repair on Earth, Baio said. “Being able to do space-based research is critical to being able to provide a unique perspective into cellular physiology and what the impacts could be for human health and, potentially, new therapeutics that we would never otherwise consider.”