Microgravity and Space Medicine 2.0

This Special Issue (SI), "Microgravity and Space Medicine 2 [...].

This Special Issue (SI), "Microgravity and Space Medicine 2.0", comprises research articles on the research areas of gravitational biology, space medicine and molecular oncology. It covers studies investigating the effects of altered gravity conditions on poorly differentiated follicular thyroid cancer cells and bacteria (Escherichia coli) during real microgravity (r-µg) on the International Space Station (ISS) [1,2]. Furthermore, it addresses the impact of simulated microgravity (s-µg) on human cells and of hypergravity (hyper-g) on C57BL/6 mice [3]. In addition, two research articles [4,5] investigated Euglena gracilis, a photosynthetic flagellate, which had been studied earlier on board of the American Space Shuttle Columbia [6]. The two novel studies proposed Euglena gracilis as a working model of gravitaxis [4], and the applied molecular toolkits and methods can be used to bioengineer E. gracilis for fundamental studies in space or on Earth [5].
Space or µg-research is a novel and unusual method applied to study wound healing, tissue engineering or to fight cancer, among others. Cancer researchers and space biologists decided to apply the power of µg to advance translational regenerative medicine, tissue engineering or to find future cancer therapies [7][8][9].
Space travel has always been a dream of humankind. Humans live under the constant force of gravity on Earth. Life in space has enormous well-described effects on our health [10]. To counteract these health problems, a large number of studies focusing on cardiovascular changes, bone loss or the immune system have been performed in recent years [11][12][13]. A spaceflight negatively influences astronauts' bone health, similar to mechanical unloading on Earth. Without countermeasures, bone formation and mineral deposition are decreased during a long-term stay on the ISS [14]. One contribution of this SI addresses this health problem. The authors showed that the nutraceuticals curcumin, carnosic acid and zinc synergistically promoted the process of osteogenesis in cultured 7F2 osteoblasts exposed to a random positioning machine (RPM) and mitigated inhibition of differentiation and maturation [15]. Such intermixes of phytonutrients might be tested in future space missions to investigate whether they are effective in humans against bone loss.
Differences in cell growth, gene and protein expression changes have been described in various models on Earth and in space [7,8]. Microgravity impacts survival, apoptosis, proliferation, migration, adhesion, the cytoskeleton, the extracellular matrix, focal adhesion and growth factors in human cells [7].
µg-research studies as performed and published in this SI had been realized in space on the ISS or using special devices designed to create µg on Earth. These ground-based facilities are acknowledged by the European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) and used worldwide for µg-experiments on Earth in our laboratories. Examples are the NASA-developed Rotating Wall Vessel, the two-and three-dimensional (3D) clinostat or the RPM. These machines were extensively reviewed in [7]. This SI presents nine research articles investigating the impact of r-µg on cells [1] and bacteria [2] as well as of s-µg on human cells [15][16][17][18]. The effects of hyper-g were tested using a small animal centrifuge on mice [3].
These nine excellent papers were published as detailed in Table 1.

Research
Article [3] This SI covered two publications investigating follicular thyroid cancer cells and bacteria on the ISS [1,2]. Moreover, four publications are listed which investigated changes of human cells exposed to s-µg using the RPM [15][16][17][18]. The effect of hyper-g using a small animal centrifuge was studied in mice [3]. Additionally, two research papers investigated Euglena gracilis with modern molecular biological methods [4,5].
The CellBox-1 experiment studied human thyroid cancer cells (FTC-133 cell line) in an automatic hardware on the ISS (SpaceX CRS-3 cargo mission) [19,20]. Wise et al. investigated the supernatants of the space-flown FTC-133 follicular thyroid cancer cells and static controls and analyzed the exosomal microRNA composition [1]. The use of cuttingedge technologies delivered further information about the cellular changes of thyroid cancer cells in space even several years after the space mission. Furthermore, it was shown how adaptable tumor cells react to changes in the surrounding environment [1].
The second ISS experiment studied bacteria in space [2]. The authors measured changes in bacterial physiology caused by MF and r-µg in E. coli grown in a specially developed device aboard the ISS. The detected effects of MF on E. coli cultivated on the ISS might be helpful for industrial purposes. The potential of MF to modify bacterial behavior in space may be useful for future space missions [2].
Osteoblastic cells were exposed to the RPM to test the effects of antioxidant nutraceuticals under s-µg [15]. The authors showed that curcumin and carnosic acid and the trace element zinc promoted cellular growth in the absence of traditional osteogenic media. These three nutraceuticals stimulated osteogenic differentiation. The osteogenic effects of the plant-derived nutraceuticals were synergistic. The cells treated with the nutraceuticals could counteract RPM-based inhibition. Therefore, intermixes of phytonutrients may be interesting countermeasures against bone loss in space [15].
Another study investigated fibroblasts on the RPM and reported that fibroblast differentiation is severely impaired after RPM exposure for short periods [16]. Furthermore, fibroblasts' conversion into myofibroblasts was inhibited. The authors demonstrated that the interplay between fibroblasts and keratinocytes in 3D co-culture experiments was remarkably altered, resulting in abnormalities in organoid-like structures. These findings may be caused by oxidative damage enacted by the stress associated with s-µg [16]. In addition, a second study focused on fibroblast differentiation during microgravity [17]. The results revealed an impairment of fibroblast differentiation and a decreased matrix remodeling and production under RPM conditions. Furthermore, RNA seq data showed that RPM exposure had less effect on fibroblast transcriptomes, while s-µg triggered changes in the transcriptome of myofibroblasts [17].
Zhivoderniko et al. [18] demonstrated that a 10-day RPM-exposure of MSCs induced a reduction in the collagenous components of the extracellular matrix (ECM). This result may be caused by the decrease in collagen synthesis and activation of proteases [18]. The authors showed that ECM-associated molecules of both native and osteocommitted MSCs may be involved in bone matrix reorganization during a space mission.
Little is known about the joint effect of hyper-g and medications on organ functions under outer space environment conditions. A further contribution to this SI investigated whether single and multiple loads of hyper-g stress affect APAP nephrotoxicity and hepatotoxicity in mice [3]. The authors showed that kidney function was affected when APAP was coupled with hyper-g stimulation. Furthermore, multiple hyper-g loads could ameliorate APAP toxicity via adaptation and enabled the mice to overcome kidney injury (KI). These are novel mechanistic insights explaining how hyper-g stress plus APAP medication might induce KI, which may be overcome by repeated hyper-g exposure of mice [3].
In summary, the excellent papers included in this SI report novel findings in the field of "Microgravity and Space Medicine".
I would like to thank all the authors who supported this SI. I am convinced that the application of space research using the ISS as well as devices for s-µg in combination with novel molecular biological technologies will be useful for the health protection of future astronauts, cosmonauts, taikonauts or space tourists who conquer the universe during deep space exploration missions and will also be applicable in translational regenerative medicine on Earth. Acknowledgments: I would like to thank Markus Wehland, Otto von Guericke University Magdeburg, Germany for his help with EndNote and his important suggestions.

Conflicts of Interest:
The author declares no conflict of interest.