Hearts in Space: Lessons Learned from Long-Duration Research on the ISS (RR3 Mission)

Objectives: The microgravity environment in space is a model to study aging and used by our group to understand the pathomechanism of vascular aging. Rodent Research-3 was a life science research mission that launched on April 8, 2016 as part of NASA's eighth cargo resupply flight using the Space Exploration Technologies (SpaceX) Dragon spacecraft.

Methods: Ten BALB/c mice were flown to the International Space Station and housed in NASA's rodent habitat until on-orbit dissections were performed 5 weeks later. Carotid arteries were stored in RNAlater or 4%PFA for subsequent RNA sequencing and histopathology, respectively, and results were compared with ground controls.

Results: Comparisons between spaceflight mice (SF) and ground control mice (CG) showed significantly increased vessel media area (SF: 0.031 ± 0.004 mm2 vs CG: 0.024 ± 0.001 mm2; p < 0.05) and maximal media thickness (SF: 0.047 ± 0.005 mm vs CG: 0.034 ± 0.003 mm; p < 0.05). To reveal the basis for vessel wall thickening, we compared the numbers of cells in histological carotid artery cross-sections. Based on DAPI-counts, very similar cell numbers were observed (SF: 132.3 ± 19.7 vs CG: 141.0 ± 12.5). Immunofluorescence stainings for the cell proliferation marker K_i-67 showed no differences between the groups. To evaluate whether cell hypertrophy might have been the cause for increased vessel wall thickness, immunofluorescence stainings for smooth muscle cell (SMC) contractile proteins were performed. Specifically, significantly higher fluorescence intensities were found for smooth muscle actin (SMA), SM-22, and myocardin. These results were confirmed by qRT-PCR. RNA sequencing was performed and pathway analyses obtained. We found that spaceflight induced a significant upregulation of hypertrophy and contraction-related pathways. Major networks influencing calcium homeostasis and mechanotransduction are upregulated.

Conclusion: Collectively, these data suggest that spaceflight induces structural and bio-molecular changes in carotid arteries. These changes may lead to impaired cerebral circulation and blood flow control, causing post-flight orthostatic intolerance. These results are even more important since microgravity is a model for aging and therefore this mechanism is disease-relevant in bed-rest and aged patients on Earth.