The effect of weightlessness on the human skeletal system is one of the greatest concerns in safely extending space missions [1–11]. The ability to understand and counteract weightlessness-induced bone mineral loss will be vital to crew health and safety during and after extended-duration space sta- tion and exploration missions [1–7]. Research on bone mineral loss during space flight has gone on for more than half a century, and recent studies have shown significant progress in developing coun- termeasures that have proved to be effective, including good nutrition and exercise. We review the history of this research here and provide a summary of recent and ongoing studies, including efforts to counteract bone and calcium loss resulting from weightlessness. Unfortunately, the most obvious nutritional countermeasure—providing excess calcium—does not protect against bone loss . This result is likely related to the decreased calcium absorption observed in space flight and in ground-based models [13–16]. Phosphate supplementation was also ineffective at reducing calcium excretion . Combination therapy with calcium and phosphorus was also unsuccessful at mitigating bone loss and hypercalciuria . Other nutrients, specifically sodium, protein, potassium, vitamin K, and omega-3 fatty acids, have also been proposed and/or tested as bone loss countermeasures , and are discussed in more detail below.
Research Containing: Unloading
Understanding the effects of spaceflight on head–trunk coordination during walking and obstacle avoidance
Prolonged exposure to spaceflight conditions results in a battery of physiological changes, some of which contribute to sensorimotor and neurovestibular deficits. Upon return to Earth, functional performance changes are tested using the Functional Task Test (FTT), which includes an obstacle course to observe post-flight balance and postural stability, specifically during turning. The goal of this study was to quantify changes in movement strategies during turning events by observing the latency between head-and-trunk coordinated movements. It was hypothesized that subjects experiencing neurovestibular adaptations would exhibit head-to-trunk locking (‘en bloc’ movement) during turning, exhibited by a decrease in latency between head and trunk movement. FTT data samples were collected from 13 ISS astronauts and 26 male 70-day head down tilt bed rest subjects, including bed rest controls (10 BRC) and bed rest exercisers (16 BRE). Samples were analyzed three times pre-exposure, immediately post-exposure (0 or 1-day post) and 2–3 times during recovery from the unloading environment. Two 3D inertial measurements units (XSens MTx) were attached to subjects, one on the head and one on the upper back. This study focused primarily on the yaw movements about the subject’s center of rotation. Time differences (latency) between head and trunk movement were averaged across a slalom obstacle portion, consisting of three turns (approximately three 601 turns). All participants were grouped as ‘decreaser’ or ‘increaser’, relating to their change in head-to-trunk movement latency between pre- and post-environmental adaptation measures. Spaceflight unloading (ISS) showed a bimodal response between the ‘increaser’ and ‘decreaser’ group, while both bed rest control (BRC) and bed rest exercise (BRE) populations showed increased preference towards a ‘decreaser’ categorization, displaying greater head–trunk locking. It is clear that changes in movement strategies are adopted during exposure to an unloading environment. These results further the under- standing of vestibular–somatosensory convergence and support the use of bed rest as an exclusionary model to better understand sensorimotor changes in spaceflight.
Osteoporosis is a disease characterized by low bone mass and structural deterioration of bone tissue, leading to bone fragility and increased susceptibility to fractures. The microgravity of space creates an extreme environment that provides a model for osteoporosis in humans. This greatly accelerated form of osteopenia results in a 0.5-2% loss of bone mass per month. Rat models for this osteoporosis have been examined on many occasions, but STS-108 was the first Space Shuttle flight to use mice. Data reported to date indicate that spaceflight experiments with mice hold promise in predicting some spaceflight effects on humans. Due to the cost and infrequency of flights, ground-based models have been developed to mimic the deleterious effects of the microgravity environment. Hindlimb suspension is one such localized model. This model removes gravitational loading from the hindlimbs by suspending the animal by its tail to a guy wire that runs lengthwise across the cage. Because mice had not flown before STS-108, a direct comparison of this model’s ability to predict spaceflight results has not been examined. The objective of this research is to closely repeat the STS- 108 profile, with hindlimb suspension replacing spaceflight. This includes examining the ability of the protein osteoprotegerin, an osteoclast-inhibiting therapeutic, to mitigate the deleterious effects of skeletal unloading. It is expected that the results will lead to better understanding of the mechanisms of mineralization and bone remodeling to aid in development of countermeasures to prevent spaceflight induced osteoporosis and aid the treatment of osteoporosis here on earth.
The Japan Aerospace Exploration Agency developed the mouse Habitat Cage Unit (HCU) for installation in the Cell Biology Experiment Facility (CBEF) onboard the Japanese Experimental Module (“Kibo”) on the International Space Station. The CBEF provides “space-based controls” by generating artificial gravity in the HCU through a centrifuge, enabling a comparison of the biological consequences of microgravity and artificial gravity of 1 g on mice housed in space. Therefore, prior to the space experiment, a ground-based study to validate the habitability of the HCU is necessary to conduct space experiments using the HCU in the CBEF. Here, we investigated the ground-based effect of a 32-day housing period in the HCU breadboard model on male mice in comparison with the control cage mice. Morphology of skeletal muscle, the thymus, heart, and kidney, and the sperm function showed no critical abnormalities between the control mice and HCU mice. Slight but significant changes caused by the HCU itself were observed, including decreased body weight, increased weights of the thymus and gastrocnemius, reduced thickness of cortical bone of the femur, and several gene expressions from 11 tissues. Results suggest that the HCU provides acceptable conditions for mouse phenotypic analysis using CBEF in space, as long as its characteristic features are considered. Thus, the HCU is a feasible device for future space experiments.
The bone mineral density (BMD) of astronauts decreases specifically in the weight-bearing sites during spaceflight. It seems that osteoclasts would be affected by a change in gravity; however, the molecular mechanism involved remains unclear. Here, we show that the mineral density of the pharyngeal bone and teeth region of TRAP-GFP/Osterix-DsRed double transgenic medaka fish was decreased and that osteoclasts were activated when the fish were reared for 56 days at the international space station. In addition, electron microscopy observation revealed a low degree of roundness of mitochondria in osteoclasts. In the whole transcriptome analysis, fkbp5 and ddit4 genes were strongly up-regulated in the flight group. The fish were filmed for abnormal behavior; and, interestingly, the medaka tended to become motionless in the late stage of exposure. These results reveal impaired physiological function with a change in mechanical force under microgravity, which impairment was accompanied by osteoclast activation.
Mechanical unloading in microgravity is thought to induce tissue degeneration by various mechanisms, including inhibition of regenerative stem cell differentiation. To address this hypothesis, we investigated the effects of microgravity on early lineage commitment of mouse embryonic stem cells (mESCs) using the embryoid body (EB) model of tissue differentiation. We found that exposure to microgravity for 15 days inhibits mESC differentiation and expression of terminal germ layer lineage markers in EBs. Additionally, microgravity-unloaded EBs retained stem cell self-renewal markers, suggesting that mechanical loading at Earth’s gravity is required for normal differentiation of mESCs. Finally, cells recovered from microgravity-unloaded EBs and then cultured at Earth’s gravity showed greater stemness, differentiating more readily into contractile cardiomyocyte colonies. These results indicate that mechanical unloading of stem cells in microgravity inhibits their differentiation and preserves stemness, possibly providing a cellular mechanistic basis for the inhibition of tissue regeneration in space and in disuse conditions on earth.
Various environmental stresses elevate the phosphorylation level of eukaryotic translation initiation factor 2 alpha (eIF2α) and induce transcriptional activation of a set of stress responsive genes such as activating transcription factors 3 and 6 (ATF3 and ATF6), CCAAT/enhancer-binding protein homologous protein (CHOP), and Xbp1 (X-box binding protein 1). These stress sources include radiation, oxidation, and stress to the endoplasmic reticulum, and it is recently reported that unloading by hindlimb unloading is such a stress source. No studies, however, have examined the phosphorylation level of eIF2α (eIF2α-p) using skeletal samples that have experienced microgravity in space. In this study we addressed a question: Does a mouse tibia flown in space show altered levels of eIF2α-p? To address this question, we obtained STS-135 flown samples that were harvested 4–7 h after landing. The tibia and femur isolated from hindlimb unloaded mice were employed as non-flight controls. The effects of loading were also investigated in non- flight controls. Results indicate that the level of eIF2α-p of the non-flight controls was elevated during hindlimb unloading and reduced after being released from unloading. Second, the eIF2α-p level of space-flown samples was decreased, and mechanical loading to the tibia caused the reduction of the eIF2α-p level. Third, the mRNA levels of ATF3, ATF6, and CHOP were lowered in space-flown samples as well as in the non-flight samples 4–7 h after being released from unloading. Collectively, the results herein indicated that a release from hindlimb unloading and a return to normal weight environment from space provided a suppressive effect to eIF2α-linked stress responses and that a period of 2–4 h is sufficient to induce this suppressive outcome.
Prolonged microgravity exposure alters human skeletal muscle by markedly reducing size, function, and metabolic capacity. Preserving skeletal muscle health presents a major challenge to space exploration beyond low Earth orbit. Humans express three distinct pure myosin heavy chain (MHC) muscle fiber types (slow Æ fast: MHC I, IIa, and IIx), along with hybrids (MHC I/IIa, IIa/IIx, and I/IIa/IIx). After reviewing current research, this paper presents evidence for a “slow to fast” microgravity-induced skeletal muscle fiber type shift in humans. Spaceflight and bed rest induce decreased MHC I fiber proportion while increasing fast hybrid types (particularly MHC IIa/IIx fibers). This alteration in muscle cell phenotype negatively impacts performance and induces undesirable metabolic adaptations. While exercise has been postulated to minimize the negative effects of microgravity on human muscle, past spaceflight countermeasures have insufficiently prevented fiber type shifts in humans. However, a new high-intensity, low volume resistance and aerobic exercise regimen has recently been implemented aboard the International Space Station (ISS). This paper aims to reveal that 1) a slow to fast microgravity-induced fiber type shift occurs in humans and 2) the new high-intensity, low volume exercise countermeasures program onboard the ISS has promise to mitigate this fiber type transition and preserve skeletal muscle health.
Prolonged exposure to microgravity during spaceflight is thought to adversely affect the human spine because of reports that disc herniation risk is increased post-spaceflight. The increased herniation risk is highest during the first post-spaceflight year, and gradually subsides thereafter. Consequently, we hypothesized that the biomechanical properties of the intervertebral disc (IVD) deteriorate during spaceflight but then recover after acclimation to normal gravity. To test this hypothesis, we compared the compressive creep properties of caudal IVDs of murine subjects that had returned from a 13-day Shuttle mission (STS-133) to those of ground-based control mice. Spaceflight (n=6) and control (n=10) groups consisted of 13-week-old, BALB/c mice (11 weeks at launch). Mice were sacrificed +1 day, +5 days, or +7 days after the landing of STS-133. Disc height was measured in situ, and compressive creep rate was fit to a fluid transport model to determine disc biomechanical properties. Compared to controls, spaceflight mice had 12.6% lower disc height and 23.1% lower straindependence on swelling pressure. Biomechanical properties did not recover significantly over the 7-day post-flight period. Biomechanical properties of the murine caudal IVD were diminished by spaceflight, consistent with observations that prolonged exposure to microgravity increases disc herniation risk. These properties did not recover after short-term reacclimation to 1g loading.