Blasting Off: How Spaceflight Affects Muscle Strength and Function

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Research finds that astronauts experience a loss of muscle mass, decreased strength, and develop insulin resistance while in space, introducing strong implications for future space flight.

As technology continues to rapidly advance and evolve, the possibility of spaceflight becoming a common occurrence becomes more likely. In 2020, SpaceX became the first company to successfully launch humans to the International Space Station (ISS) from the United States. Companies continue to compete to make space travel more of a possibility, both for research and tourism.¹ However, scientists state health concerns associated with space travel, including risks of microgravity, radiation and contamination by exo-life forms.² Writing in Nexus, Murgia et al. focuses on the health risks of microgravity on muscular protein expression, or muscle proteomics.³ With human missions to deep space becoming more likely, improved knowledge of muscle unloading is essential to develop countermeasures and improve human health when traveling in space. 

Murgia et al. studied how muscle unloading affects muscle proteomics in astronauts and in a group of healthy volunteers on bed rest. Ten healthy volunteers spent 15 days at Izola General Hospital, including three days of familiarization to the study’s environment and diet. Muscle biopsies were taken from the vastus lateralis muscle immediately before bed rest, at day 5 and at day 10. Muscle fibers were dissected under stereomicroscopes from fresh biopsies that were kept in an ice cold solution. Two astronaut crew members of equal sex and comparable age had biopsies obtained pre-flight, one day after landing and 14 days after landing. Murgia et al. created and analyzed two peptide libraries with these results. 

Murgia et al. found over 7,500 proteins in muscle fibers, with over 93% of proteins discovered at all three time points of bed rest and 65% in all ten donors. 7,100 proteins were found in whole muscle tissues, with 74% discovered at all three time points and 82% in both astronauts. Humans have three different muscle types: slow-1, fast-2A and fast-2X, all of which have different specialized functions.⁴ The study found 82 muscle fibers as slow-1, 56 as fast-2A, and 25 as fast-2X. The authors found 470 proteins that had significantly different expression between slow-1 and fast-2A fiber, and 481 significantly different proteins between slow-1 and fast-2X. Myosin and troponin, which have a role in muscle contraction and movement, had the largest difference between fiber types.

Proteins that were downregulated, or had decreased expression, during bed rest were often in pathways that led to muscle contraction and movement. Expression of actomyosin, the Z-disk and calcium channels decreased during bed rest. Furthermore, proteins involved in fiber-matrix adhesion were also downregulated during bedrest, as well as fiber expressed proteins like XIRP1, XIRP2 and UTN. These downregulations suggest that a loss of muscle mass is correlated with changes in muscle structure. Comparatively, enzymes involved in glycolysis increased during bed rest, as well as proteasomal proteins and antioxidant enzymes, suggesting alterations in metabolism and insulin regulation. 

Murgia et al. found a common link between astronauts and bedrest regarding muscle loading and matrix composition, furthering support of the connection between muscle loading and muscle structure. Proteins involved in metabolic and physiological processes like FABP5, CHP1 and DSG1 were downregulated in both bedrest and spaceflight. 17 proteins were upregulated in both bedrest and spaceflight, indicating that their expression increases under muscle unloading. In astronauts, mitochondrial protein expression was also compared, with Murgia et al. finding over 90% of mitochondrial proteins were expressed at lower levels pre-mission than on the day of landing. Only 50% of mitochondrial DNA had significant downregulation in spaceflight. While ten days of bedrest caused small changes in the mitochondrial proteome, six months of spaceflight caused a steep decrease in expression of the mitochondrial proteome.

The study’s findings provide new insights into how muscle unloading affects protein expression, but further research is needed to determine how microgravity will truly influence human health. Firstly, the study indicated that Earth-bound studies like bedrest only provide a small view into how muscle unloading affects the body, showing smaller effects compared to spaceflight studies. Additional steps should be taken to understand how spaceflight molecularly affects protein expression and muscular structure. Furthermore, this study did not include diverse representation, and this is essential towards understanding how spaceflight affects various human bodies before advancing space travel further. By advancing understanding of how spaceflight affects the protein expression of a diverse population, additional steps can be taken to protect human health for future space travel. 

References

1. Drake, N. The future of spaceflight—from orbital vacations to humans on Mars. Science https://www.nationalgeographic.com/science/article/future-spaceflight (2020).
2. Cohen, E. Outer space mobilities and human health. Tourism Geographies 24, 1123–1133 (2022).
3. Murgia, M. et al. Signatures of muscle disuse in spaceflight and bed rest revealed by single muscle fiber proteomics. PNAS Nexus 1, pgac086 (2022).
4. Murgia, M. et al. Single Muscle Fiber Proteomics Reveals Fiber-Type-Specific Features of Human Muscle Aging. Cell Reports 19, 2396–2409 (2017).

Written by Maddie Marquardt. Contact the author at mamarquardt@davidson.edu.

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