Space flight affects motility and cytoskeletal structures in human monocyte cell line J-111

Cellular and Molecular Effects of Microgravity on the Immune System: A Focus on Bioactive Lipids

Microgravity is one of the main stressors that astronauts are exposed to during space missions. This condition has been linked to many disorders, including those that feature dysfunctional immune homeostasis and inflammatory damage. Over the past 30 years, a significant body of work has been gathered connecting weightlessness-either authentic or simulated-to an inefficient reaction to pathogens, dysfunctional production of cytokines and impaired survival of immune cells. These processes are also orchestrated by a plethora of bioactive lipids, produced by virtually all cells involved in immune events, which control the induction, magnitude, outcome, compartmentalization and trafficking of immunocytes during the response to injury. Despite their crucial importance in inflammation and its modulation, however, data concerning the role of bioactive lipids in microgravity-induced immune dysfunctions are surprisingly scarce, both in quantity and in variety, and the vast majority of it focuses on two lipid classes, namely eicosanoids and endocannabinoids. The present review aims to outline the accumulated knowledge addressing the effects elicited by microgravity-both simulated and authentic-on the metabolism and signaling of these two prominent lipid groups in the context of immune and inflammatory homeostasis.

Managing Hemostasis in Space

Human space travel requires exposure to weightlessness, ionizing radiation, isolation, and austerity. A recent report of internal jugular vein thrombosis in astronauts in low Earth orbit confirms that these exposures also affect vascular biology to influence diseases of thrombosis and hemostasis. This brief review summarizes the known influences of space travel on inflammation, blood coagulation, and the cardiovascular system and conceptualizes how they might combine to affect thrombosis and hemostasis. In the event of a major thrombotic or bleeding emergency, it is anticipated that the unique physiological influences of the space environment and logistical limitations of providing medical care in space would require a response that is unique from our current experience. We also look towards the future to discuss lessons learned from our current experiences on Earth and in space.

Microgravity and immune cells

The microgravity environment experienced during spaceflight severely impaired immune system, making astronauts vulnerable to various diseases that seriously threaten the health of astronauts. Immune cells are exceptionally sensitive to changes in gravity and the microgravity environment can affect multiple aspects of immune cells through different mechanisms. Previous reports have mainly summarized the role of microgravity in the classification of innate and adaptive immune cells, lacking an overall grasp of the laws that microgravity effects on immune cells at different stages of their entire developmental process, such as differentiation, activation, metabolism, as well as function, which are discussed and concluded in this review. The possible molecular mechanisms are also analysed to provide a clear understanding of the specific role of microgravity in the whole development process of immune cells. Furthermore, the existing methods by which to reverse the damage of immune cells caused by microgravity, such as the use of polysaccharides, flavonoids, other natural immune cell activators etc. to target cell proliferation, apoptosis and impaired function are summarized. This review will provide not only new directions and ideas for the study of immune cell function in the microgravity environment, but also an important theoretical basis for the development of immunosuppression prevention and treatment drugs for spaceflight.

MRTF may be the missing link in a multiscale mechanobiology approach toward macrophage dysfunction in space

Macrophages exhibit impaired phagocytosis, adhesion, migration, and cytokine production in space, hindering their ability to elicit immune responses. Considering that the combined effect of spaceflight microgravity and radiation is multiscale and multifactorial in nature, it is expected that contradictory findings are common in the field. This theory paper reanalyzes research on the macrophage spaceflight response across multiple timescales from seconds to weeks, and spatial scales from the molecular, intracellular, extracellular, to the physiological. Key findings include time-dependence of both pro-inflammatory activation and integrin expression. Here, we introduce the time-dependent, intracellular localization of MRTF-A as a hypothetical confounder of macrophage activation. We discuss the mechanosensitive MRTF-A/SRF pathway dependence on the actin cytoskeleton/nucleoskeleton, microtubules, membrane mechanoreceptors, hypoxia, oxidative stress, and intracellular/extracellular crosstalk. By adopting a multiscale perspective, this paper provides the first mechanistic answer for a three-decade-old question regarding impaired cytokine secretion in microgravity—and strengthens the connection between the recent advances in mechanobiology, microgravity, and the spaceflight immune response. Finally, we hypothesize MRTF involvement and complications in treating spaceflight-induced cardiovascular, skeletal, and immune disease.

Bearing Extremes: Impacts from Simulated Outer Space Conditions and Effective Ultraviolet Radiation Shielding Materials on Tardigrade Life History

ABSTRACT

Questions about astrobiological resilience, whether entities with evolutionary histories on Earth would survive in outer space or on distant planets, for instance, no longer reside exclusively in the science fiction realm. In this study, we assess life history characteristics for individuals in the tardigrade species Grevenius annulatus post exposure to simulated outer space conditions with nonionizing radiation as a proxy for anticipated oxidative stress and damage incurred from exposure to full-spectrum environmental radiation. Using a planetary environment simulator, we exposed unshielded and shielded specimens to UVB and UVC radiation for 30 min and monitored and recorded subsequent life history characteristics. Survivorship was lower in an unshielded group relative to control as well as Kevlar and polyethylene shielded groups, demonstrating that Kevlar and polyethylene reduce impact from these types of nonionizing radiation, which are not expected to fully penetrate the shields. Cumulative egg production was lowest in the unshielded group, but egg viability and average egg production rate was highest. Due to insignificant differences, additional research to determine the relative effectiveness for Kevlar and polyethylene as shielding materials from survivorship and reproduction perspectives is warranted. This work provides a progressive step from which important conditions excluded in the current study, like vibrations, temperatures, debris-impacts, and ionizing radiation, can be included in future studies.

Chronic Hypergravity Induces a Modification of Histone H3 Lysine 27 Trimethylation at TCRβ Locus in Murine Thymocytes

Gravity changes are major stressors encountered during spaceflight that affect the immune system. We previously evidenced that hypergravity exposure during gestation affects the TCRβ repertoire of newborn pups. To identify the mechanisms underlying this observation, we studied post-translational histone modifications. We first showed that among the four studied post-translational histone H3 modifications, only lysine 27 trimethylation (H3K27me3) is downregulated in the thymus of mice exposed to 2× g for 21 days. We then asked whether the TCRβ locus chromatin structure is altered by hypergravity exposure. ChIP studies performed on four Vβ segments of the murine double-negative SCIET27 thymic cell line, which corresponds to the last maturation stage before V(D)J recombination, revealed increases in H3K27me3 after 2× g exposure. Finally, we evaluated the implication for the EZH2 methyltransferase in the regulation of the H3K27me3 level at these Vβ segments by treating SCIET27 cells with the GSK126-specific inhibitor. These experiments showed that the downregulation of H3K27me3 contributes to the regulation of the Vβ germline transcript expression that precedes V(D)J recombination. These data show that modifications of H3K27me3 at the TCRβ locus likely contribute to an explanation of why the TCR repertoire is affected by gravity changes and imply, for the first time, EZH2 in the regulation of the TCRβ locus chromatin structure.

Frequency-specific sensitivity of 3T3-L1 preadipocytes to low-intensity vibratory stimulus during adipogenesis

Adipocyte accumulation in the bone marrow is a severe complication leading to bone defects and reduced regenerative capacity. Application of external mechanical signals to bone marrow cellular niche is a non-invasive and non-pharmaceutical methodology to improve osteogenesis and suppress adipogenesis. However, in the literature, the specific parameters related to the nature of low-intensity vibratory (LIV) signals appear to be arbitrarily selected for amplitude, bouts, and applied frequency. In this study, we performed a LIV frequency sweep ranging from 30 to 120 Hz with increments of 15 Hz applied onto preadipocytes during adipogenesis for 10 d. We addressed the effect of LIV with different frequencies on single-cell density, adipogenic gene expression, lipid morphology, and triglycerides content. Results showed that LIV signals with 75-Hz frequency had the most significant suppressive effect during adipogenesis. Our results support the premise that mechanical-based interventions for suppressing adipogenesis may benefit from optimizing input parameters.

Effects of Titanium Dioxide Nanoparticles on Cell Growth and Migration of A549 Cells under Simulated Microgravity

With the increasing application of nanomaterials in aerospace technology, the long-term space exposure to nanomaterials especially in the space full of radiation coupled with microgravity condition has aroused great health concerns of the astronauts. However, few studies have been conducted to assess these effects, which are crucial for seeking the possible intervention strategy. Herein, using a random positioning machine (RPM) to simulate microgravity, we investigated the behaviors of cells under simulated microgravity and also evaluated the possible toxicity of titanium dioxide nanoparticles (TiO₂ NPs), a multifunctional nanomaterial with potential application in aerospace. Pulmonary epithelial cells A549 were exposed to normal gravity (1 g) and simulated gravity (~10⁻³ g), respectively. The results showed that simulated microgravity had no significant effect on the viability of A549 cells as compared with normal gravity within 48 h. The effects of TiO₂ NPs exposure on cell viability and apoptosis were marginal with only a slightly decrease in cell viability and a subtle increase in apoptosis rate observed at a high concentration of TiO₂ NPs (100 μg/mL). However, it was observed that the exposure to simulated microgravity could obviously reduce A549 cell migration compared with normal gravity. The disruption of F-actin network and the deactivation of FAK (Tyr397) might be responsible for the impaired mobility of simulated microgravity-exposed A549 cells. TiO₂ NPs exposure inhibited cell migration under two different gravity conditions, but to different degrees, with a milder inhibition under simulated microgravity. Meanwhile, it was found that A549 cells internalized more TiO₂ NPs under normal gravity than simulated microgravity, which may account for the lower cytotoxicity and the lighter inhibition of cell migration induced by the same exposure concentration of TiO₂ NPs under simulated microgravity at least partially. Our study has provided some tentative information on the effects of TiO₂ NPs exposure on cell behaviors under simulated microgravity.

Optical coherence tomography evaluation of the spatiotemporal effects of 3D bone marrow stromal cell culture using a bioreactor

Performing cell culture in a three-dimensional (3D) environment has various advantages. In cartilage tissue engineering, 3D in vitro cultures utilizing biomaterials and bioreactors can mimic the biological environment. However, the biggest drawback of these 3D culture systems is a limited ability to evaluate 3D cell distribution. Optical coherence tomography (OCT) has recently been used to evaluate 3D cellular morphology and structure in a timely manner. Here, we showed that OCT could be used to visually assess the distribution and the morphology of bone marrow stromal cells under chondrogenic 3D cultivation using alginate gels and rotary culture. In particular, OCT was able to visualize living cells embedded in alginate gels in a non-destructive and 3D manner, as well as quantitatively evaluate cell distribution and spheroid volume. We also found that cells were centralized in rotary culture but peripherally distributed in static culture, while rotary culture enhanced the hypertrophy of marrow stromal cells (MSCs) embedded in alginate gels. Together, our findings demonstrate that OCT can be used to evaluate the spatiotemporal effects of 3D cultivation using alginate gels and rotary culture. Therefore, this method may allow the observation of pre-cultured tissue over time and the optimization of culture conditions for regenerative tissue engineering.

Gravity-Vector Induces Mechanical Remodeling of rMSCs via Combined Substrate Stiffness and Orientation

Distinct physical factors originating from the cellular microenvironment are crucial to the biological homeostasis of stem cells. While substrate stiffness and orientation are known to regulate the mechanical remodeling and fate decision of mesenchymal stem cells (MSCs) separately, it remains unclear how the two factors are combined to manipulate their mechanical stability under gravity vector. Here we quantified these combined effects by placing rat MSCs onto stiffness-varied poly-dimethylsiloxane (PDMS) substrates in upward (180°), downward (0°), or edge-on (90°) orientation. Compared with those values onto glass coverslip, the nuclear longitudinal translocation, due to the density difference between the nucleus and the cytosol, was found to be lower at 0° for 24 h and higher at 90° for 24 and 72 h onto 2.5 MPa PDMS substrate. At 0°, the cell was mechanically supported by remarkably reduced actin and dramatically enhanced vimentin expression. At 90°, both enhanced actin and vimentin expression worked cooperatively to maintain cell stability. Specifically, perinuclear actin stress fibers with a large number, low anisotropy, and visible perinuclear vimentin cords were formed onto 2.5 MPa PDMS at 90° for 72 h, supporting the orientation difference in nuclear translocation and global cytoskeleton expression. This orientation dependence tended to disappear onto softer PDMS, presenting distinctive features in nuclear translocation and cytoskeletal structures. Moreover, cellular morphology and focal adhesion were mainly affected by substrate stiffness, yielding a time course of increased spreading area at 24 h but decreased area at 72 h with a decrease of stiffness. Mechanistically, the cell tended to be stabilized onto these PDMS substrates via β1 integrin–focal adhesion complexes–actin mechanosensitive axis. These results provided an insight in understanding the combination of substrate stiffness and orientation in defining the mechanical stability of rMSCs.

Cells respond to space microgravity through cytoskeleton reorganization

Decades of spaceflight studies have provided abundant evidence that individual cells in vitro are capable of sensing space microgravity and responding with cellular changes both structurally and functionally. However, how microgravity is perceived, transmitted, and converted to biochemical signals by single cells remains unrevealed. Here in this review, over 40 cellular biology studies of real space fights were summarized. Studies on cells of the musculoskeletal system, cardiovascular system, and immune system were covered. Among all the reported cellular changes in response to space microgravity, cytoskeleton (CSK) reorganization emerges as a key indicator. Based on the evidence of CSK reorganization from space flight research, a possible mechanism from the standpoint of "cellular mechanical equilibrium" is proposed for the explanation of cellular response to space microgravity. Cytoskeletal equilibrium is broken by the gravitational change from ground to space and is followed by cellular morphological changes, cell mechanical properties changes, extracellular matrix reorganization, as well as signaling pathway activation/inactivation, all of which ultimately lead to the cell functional changes in space microgravity.

Cytoskeletal Tensegrity in Microgravity

In order for Man to venture further into Space he will have to adapt to its conditions, including microgravity. Life as we know it has evolved on Earth with a substantial gravitational field. If they spend considerable time away from Earth, astronauts experience physiological, mental, and anatomical changes. It is not clear if these are pathological or adaptations. However, it is true that they experience difficulties on their return to stronger gravity. The cytoskeleton is a key site for the detection of gravitational force within the body, due to its tensegrity architecture. In order to understand what happens to living beings in space, we will need to unravel the role cytoskeletal tensegrity architecture plays in the building and function of cells, organs, the body, and mind.

The Effects of Simulated Microgravity on Macrophage Phenotype

The effects of spaceflight, including prolonged exposure to microgravity, can have significant effects on the immune system and human health. Altered immune cell function can lead to adverse health events, though precisely how and to what extent a microgravity environment impacts these cells remains uncertain. Macrophages, a key immune cell, effect the inflammatory response as well as tissue remodeling and repair. Specifically, macrophage function can be dictated by phenotype that can exist between spectrums of M0 macrophage: the classically activated, pro-inflammatory M1, and the alternatively activated, pro-healing M2 phenotypes. This work assesses the effects of simulated microgravity via clinorotation on M0, M1, and M2 macrophage phenotypes. We focus on phenotypic, inflammatory, and angiogenic gene and protein expression. Our results show that across all three phenotypes, microgravity results in a decrease in TNF-α expression and an increase in IL-12 and VEGF expression. IL-10 was also significantly increased in M1 and M2, but not M0 macrophages. The phenotypic cytokine expression profiles observed may be related to specific gravisensitive signal transduction pathways previously implicated in microgravity regulation of macrophage gene and protein expression. Our results highlight the far-reaching effects that simulated microgravity has on macrophage function and provides insight into macrophage phenotypic function in microgravity.

May the Force Be with You (Or Not): The Immune System under Microgravity

All terrestrial organisms have evolved and adapted to thrive under Earth's gravitational force. Due to the increase of crewed space flights in recent years, it is vital to understand how the lack of gravitational forces affects organisms. It is known that astronauts who have been exposed to microgravity suffer from an array of pathological conditions including an impaired immune system, which is one of the most negatively affected by microgravity. However, at the cellular level a gap in knowledge exists, limiting our ability to understand immune impairment in space. This review highlights the most significant work done over the past 10 years detailing the effects of microgravity on cellular aspects of the immune system.

Gravitational Influence on Human Living Systems and the Evolution of Species on Earth

Gravity constituted the only constant environmental parameter, during the evolutionary period of living matter on Earth. However, whether gravity has affected the evolution of species, and its impact is still ongoing. The topic has not been investigated in depth, as this would require frequent and long-term experimentations in space or an environment of altered gravity. In addition, each organism should be studied throughout numerous generations to determine the profound biological changes in evolution. Here, we review the significant abnormalities presented in the cardiovascular, immune, vestibular and musculoskeletal systems, due to altered gravity conditions. We also review the impact that gravity played in the anatomy of snakes and amphibians, during their evolution. Overall, it appears that gravity does not only curve the space–time continuum but the biological continuum, as well.

Macrophages in microgravity: the impact of space on immune cells

The effects of a microgravity environment on the myriad types of immune cells present within the human body have been assessed both by bench-scale simulation and suborbital methods, as well as in true spaceflight. Macrophages have garnered increased research interest in this context in recent years. Their functionality in both immune response and tissue remodeling makes them a unique cell to investigate in regards to gravisensitive effects as well as parameters of interest that could impact astronaut health. Here, we review and summarize the literature investigating the effects of microgravity on macrophages and monocytes regarding the microgravity environment simulation/generation methods, cell sources, experiment durations, and parameters of interest utilized within the field. We discuss reported findings on the impacts of microgravity on macrophage/monocyte structure, adhesion and migration, proliferation, genetic expression, cytokine secretion, and reactive oxygen species production, as well as polarization. Based on this body of data, we make recommendations to the field for careful consideration of experimental design to complement existing reports, as the multitude of disparate study methods previously published can make drawing direct comparisons difficult. However, the breadth of different testing methodologies can also lend itself to attempting to identify the most robust and consistent responses to microgravity across various testing conditions.

Implications of microgravity-induced cell signaling alterations upon cancer cell growth, invasiveness, metastatic potential, and control by host immunity

The human endeavor to venture beyond the orbit of Earth is challenged by both continuous space radiation and microgravity-induced immune dysfunction. If cancers were to develop in astronauts, it is unclear how these abnormal cells would grow and progress in the microgravity environment. It is unknown if the astronaut's immune response would be able to control or eradicate cancer. A better molecular understanding of how the mechanical force of gravity affects the cell as well as the aggressiveness of cancers and the functionality of host immunity is needed. This review will summarize findings related to microgravity-mediated alterations in the cell cytoskeleton, cell-cell, and cell-extracellular matrix interactions including cadherins, immunoglobulin superfamily of adhesion molecules, selectins, and integrins and related cell signaling. The effects of spaceflight and simulated microgravity on cell viability, cancer cell growth, invasiveness, angiogenesis, metastasis as well as immune cell functions and the subsequent signaling pathways involved will be discussed. Microgravity-induced alterations in function and signaling of the major anti-cancer immune populations will be examined including natural killer cells, dendritic cells, CD4⁺ T cells, and CD8⁺ T cells. Further studies regarding the molecular events impacted by microgravity in both cancer and immune cells will greatly increase the development of therapies to restrict tumor growth and enhance cancer-specific responses for both astronauts and patients on Earth.

Primary cilia act as microgravity sensors by depolymerizing microtubules to inhibit osteoblastic differentiation and mineralization

Microgravity-induced bone deterioration is a major challenge in long-term spaceflights since the underlying mechanisms remain elusive. Previously, we reported that primary cilia of osteoblasts gradually disappeared in microgravity conditions, and cilia abrogation was necessary for the inhibition of osteogenesis induced by microgravity. However, the precise roles of primary cilia have not been fully elucidated. Here, we report that microgravity depolymerizes the microtubule network of rat calvarial osteoblasts (ROBs) reversibly but has no effect on the architecture of actin filaments. Preventing primary ciliogenesis by chloral hydrate or a small interfering RNA sequence (siRNA) targeting intraflagellar transport protein 88 (IFT88) effectively relieves microgravity-induced microtubule depolymerization, whereas the stabilization of microtubules using pharmacological approaches cannot prevent the disappearance of primary cilia in microgravity conditions. Furthermore, quantification of the number of microtubules emerging from the ciliary base body shows that microgravity significantly decreases the number of basal microtubules, which is dependent on the existence of primary cilia. Finally, microgravity-induced repression of the differentiation, maturation, and mineralization of ROBs is abrogated by the stabilization of cytoplasmic microtubules. Taken together, these data suggest that primary cilia-dependent depolymerization of microtubules is responsible for the inhibition of osteogenesis induced by microgravity. Our study provides a new perspective regarding the mechanism of microgravity-induced bone loss, supporting the previously established role of primary cilia as a sensor in bone metabolism.

A Simulated Microgravity Environment Causes a Sustained Defect in Epithelial Barrier Function

Intestinal epithelial cell (IEC) junctions constitute a robust barrier to invasion by viruses, bacteria and exposure to ingested agents. Previous studies showed that microgravity compromises the human immune system and increases enteropathogen virulence. However, the effects of microgravity on epithelial barrier function are poorly understood. The aims of this study were to identify if simulated microgravity alters intestinal epithelial barrier function (permeability), and susceptibility to barrier-disrupting agents. IECs (HT-29.cl19a) were cultured on microcarrier beads in simulated microgravity using a rotating wall vessel (RWV) for 18 days prior to seeding on semipermeable supports to measure ion flux (transepithelial electrical resistance (TER)) and FITC-dextran (FD4) permeability over 14 days. RWV cells showed delayed apical junction localization of the tight junction proteins, occludin and ZO-1. The alcohol metabolite, acetaldehyde, significantly decreased TER and reduced junctional ZO-1 localization, while increasing FD4 permeability in RWV cells compared with static, motion and flask control cells. In conclusion, simulated microgravity induced an underlying and sustained susceptibility to epithelial barrier disruption upon removal from the microgravity environment. This has implications for gastrointestinal homeostasis of astronauts in space, as well as their capability to withstand the effects of agents that compromise intestinal epithelial barrier function following return to Earth.

Wheat Space Odyssey: "From Seed to Seed". Kernel Morphology

The long-term autonomous existence of man in extraterrestrial conditions is associated with the need to cultivate plants-the only affordable and effective means for both providing oxygen and CO2 utilization, and providing one of the most habitual and energetically valuable products: plant food. In this study, we analyzed the results of the space odyssey of wheat and compared the morphological features of parental grains harvested from soil grown wheat plants, the grains obtained from plants grown in a specialized device for plant cultivation-the "Lada" space greenhouses during space flight in the ISS, and the grains obtained from plants in the same device on Earth. The seeds obtained under various conditions were studied using scanning electron microscopy. We studied the mutual location of the surface layers of the kernel cover tissues, the structural features of the tube and cross cells of the fruit coat (pericarp), and the birsh hairs of the kernels. It was found that the grains obtained under wheat plants cultivation on board of the ISS in near space had some specific differences from the parental, original grains, and the grains obtained from plants grown in the "Lada" greenhouse in ground conditions. These changes were manifested in a shortening of the birsh hairs, and a change in the size and relative arrangement of the cells of the kernel coat. We suggest that such changes are a manifestation of the sensitivity of the cytoskeleton reorganization systems and water exchange to the influence of particular physical conditions of space flight (microgravity, increased doses of radiation, etc.). Thus, the revealed changes did not hinder the wheat grains production "from seed to seed", which allows the cultivation of this crop in stable life support systems in near earth orbit.

The Effects of Spaceflight Factors on the Human Plasma Proteome, Including Both Real Space Missions and Ground-Based Experiments

The aim of the study was to compare proteomic data on the effects of spaceflight factors on the human body, including both real space missions and ground-based experiments. LC-MS/MS-based proteomic analysis of blood plasma samples obtained from 13 cosmonauts before and after long-duration (169-199 days) missions on the International Space Station (ISS) and for five healthy men included in 21-day-long head-down bed rest (HDBR) and dry immersion experiments were performed. The semi-quantitative label-free analysis revealed significantly changed proteins: 19 proteins were significantly different on the first (+1) day after landing with respect to background levels; 44 proteins significantly changed during HDBR and 31 changed in the dry immersion experiment. Comparative analysis revealed nine common proteins (A1BG, A2M, SERPINA1, SERPINA3, SERPING1, SERPINC1, HP, CFB, TF), which changed their levels after landing, as well as in both ground-based experiments. Common processes, such as platelet degranulation, hemostasis, post-translational protein phosphorylation and processes of protein metabolism, indicate common pathogenesis in ground experiments and during spaceflight. Dissimilarity in the lists of significantly changed proteins could be explained by the differences in the dynamics of effective development in the ground-based experiments. Data are available via ProteomeXchange using the identifier PXD013305.

Rapid Morphological and Cytoskeletal Response to Microgravity in Human Primary Macrophages

The FLUMIAS (Fluorescence-Microscopic Analyses System for Life-Cell-Imaging in Space) confocal laser spinning disk fluorescence microscope represents a new imaging capability for live cell imaging experiments on suborbital ballistic rocket missions. During the second pioneer mission of this microscope system on the TEXUS-54 suborbital rocket flight, we developed and performed a live imaging experiment with primary human macrophages. We simultaneously imaged four different cellular structures (nucleus, cytoplasm, lysosomes, actin cytoskeleton) by using four different live cell dyes (Nuclear Violet, Calcein, LysoBrite, SiR-actin) and laser wavelengths (405, 488, 561, and 642 nm), and investigated the cellular morphology in microgravity (10⁻⁴ to 10⁻⁵ g) over a period of about six minutes compared to 1 g controls. For live imaging of the cytoskeleton during spaceflight, we combined confocal laser microscopy with the SiR-actin probe, a fluorogenic silicon-rhodamine (SiR) conjugated jasplakinolide probe that binds to F-actin and displays minimal toxicity. We determined changes in 3D cell volume and surface, nuclear volume and in the actin cytoskeleton, which responded rapidly to the microgravity environment with a significant reduction of SiR-actin fluorescence after 4–19 s microgravity, and adapted subsequently until 126–151 s microgravity. We conclude that microgravity induces geometric cellular changes and rapid response and adaptation of the potential gravity-transducing cytoskeleton in primary human macrophages.

"Microgravity" as a unique and useful stem cell culture environment for cell-based therapy

Cell-based therapy using mesenchymal stem cells or pluripotent stem cells such as induced pluripotent stem cells has seen dramatic progress in recent years. Part of cell-based therapy are already covered by public medical insurance. Recently, researchers have attempted to improve therapeutic effects toward various diseases by cell transplantation. Culture environment is considered to be one of the most important factors affecting therapeutic effects, in particular factors such as physical stimuli, because cells have the potential to adapt to their surrounding environment. In this review, we provide an overview of the research on the effects of gravity alteration on cell kinetics such as proliferation or differentiation and on potential therapeutic effects, and we also summarize the remarkable possibilities of the use of microgravity culture in cell-based therapy for various diseases.

Biological effects of space environmental factors: A possible interaction between space radiation and microgravity

In the mid-1980s, space experiments began to examine if microgravity could alter the biological effects of space radiation. In the late 1990s, repair of DNA strand breaks was reported to not be influenced by microgravity using the pre-irradiated cells, because the exposure doses of space radiation were few due to the short spaceflight. There were, however, conflicting reports depending on the biological endpoints used in various systems. While almost no attempts were made to assess the possibility that the microgravity effects could be altered by space radiation. This was probably due to the general understanding that microgravity plays a major role in space and works independently from space radiation. Recent ground-based simulation studies focusing on DNA oxidative damage and signal transduction suggested that combined effects of microgravity and space radiation might exist. These studies also implicated the importance of research focusing not only on chromosomal DNA but also on cytoplasm, especially mitochondria. Therefore, we propose a new model which accounts for the combined-effects through the window of cellular responses. In this model, the interactions between microgravity and space radiation might occur during the following cellular-responses; (A) damaging and signaling by ROS, (B) damage responses on DNA (repair, replication, transcription, etc.), and (C) expression of gene and protein (regulation by chromatin, epigenetic control, etc.).

A Model of Chronic Exposure to Unpredictable Mild Socio-Environmental Stressors Replicates Some Spaceflight-Induced Immunological Changes

During spaceflight, astronauts face radiations, mechanical, and socio-environmental stressors. To determine the impact of chronic socio-environmental stressors on immunity, we exposed adult male mice to chronic unpredictable mild psychosocial and environmental stressors (CUMS model) for 3 weeks. This duration was chosen to simulate a long flight at the human scale. Our data show that this combination of stressors induces an increase of serum IgA, a reduction of normalized splenic mass and tends to reduce the production of pro-inflammatory cytokines, as previously reported during or after space missions. However, CUMS did not modify major splenic lymphocyte sub-populations and the proliferative responses of splenocytes suggesting that these changes could be due to other factors such as gravity changes. Thus, CUMS, which is an easy to implement model, could contribute to deepen our understanding of some spaceflight-associated immune alterations and could be useful to test countermeasures.

The influence of simulated microgravity on proliferation and apoptosis in U251 glioma cells

Several studies have indicated that microgravity can influence cellular progression, proliferation, and apoptosis in tumor cell lines. In this study, we observed that simulated microgravity inhibited proliferation and induced apoptosis in U251 malignant glioma (U251MG) cells. Furthermore, expression of the apoptosis-associated proteins, p21 and insulin-like growth factor binding protein-2 (IGFBP-2), was upregulated and downregulated, respectively, following exposure to simulated microgravity. These findings indicate that simulated microgravity inhibits proliferation while inducing apoptosis of U251MG cells. The associated effects appear to be mediated by inhibition of IGFBP-2 expression and stimulation of p21 expression. This suggests that simulated microgravity might represent a promising method to discover new targets for glioma therapeutic strategy.

Gravireceptors in eukaryotes-a comparison of case studies on the cellular level

We have selected five evolutionary very different biological systems ranging from unicellular protists via algae and higher plants to human cells showing responses to the gravity vector of the Earth in order to compare their graviperception mechanisms. All these systems use a mass, which may either by a heavy statolith or the whole content of the cell heavier than the surrounding medium to operate on a gravireceptor either by exerting pressure or by pulling on a cytoskeletal element. In many cases the receptor seems to be a mechanosensitive ion channel activated by the gravitational force which allows a gated ion flux across the membrane when activated. This has been identified in many systems to be a calcium current, which in turn activates subsequent elements of the sensory transduction chain, such as calmodulin, which in turn results in the activation of ubiquitous enzymes, gene expression activation or silencing. Naturally, the subsequent responses to the gravity stimulus differ widely between the systems ranging from orientational movement and directed growth to physiological reactions and adaptation to the environmental conditions.

Cytoskeletal stability and metabolic alterations in primary human macrophages in long-term microgravity

The immune system is one of the most affected systems of the human body during space flight. The cells of the immune system are exceptionally sensitive to microgravity. Thus, serious concerns arise, whether space flight associated weakening of the immune system ultimately precludes the expansion of human presence beyond the Earth's orbit. For human space flight, it is an urgent need to understand the cellular and molecular mechanisms by which altered gravity influences and changes the functions of immune cells. The CELLBOX-PRIME (= CellBox-Primary Human Macrophages in Microgravity Environment) experiment investigated for the first time microgravity-associated long-term alterations in primary human macrophages, one of the most important effector cells of the immune system. The experiment was conducted in the U.S. National Laboratory on board of the International Space Station ISS using the NanoRacks laboratory and Biorack type I standard CELLBOX EUE type IV containers. Upload and download were performed with the SpaceX CRS-3 and the Dragon spaceship on April 18th, 2014 / May 18th, 2014. Surprisingly, primary human macrophages exhibited neither quantitative nor structural changes of the actin and vimentin cytoskeleton after 11 days in microgravity when compared to 1g controls. Neither CD18 or CD14 surface expression were altered in microgravity, however ICAM-1 expression was reduced. The analysis of 74 metabolites in the cell culture supernatant by GC-TOF-MS, revealed eight metabolites with significantly different quantities when compared to 1g controls. In particular, the significant increase of free fucose in the cell culture supernatant was associated with a significant decrease of cell surface-bound fucose. The reduced ICAM-1 expression and the loss of cell surface-bound fucose may contribute to functional impairments, e.g. the activation of T cells, migration and activation of the innate immune response. We assume that the surprisingly small and non-significant cytoskeletal alterations represent a stable "steady state" after adaptive processes are initiated in the new microgravity environment. Due to the utmost importance of the human macrophage system for the elimination of pathogens and the clearance of apoptotic cells, its apparent robustness to a low gravity environment is crucial for human health and performance during long-term space missions.

Rapid adaptation to microgravity in mammalian macrophage cells

Despite the observed severe effects of microgravity on mammalian cells, many astronauts have completed long term stays in space without suffering from severe health problems. This raises questions about the cellular capacity for adaptation to a new gravitational environment. The International Space Station (ISS) experiment TRIPLE LUX A, performed in the BIOLAB laboratory of the ISS COLUMBUS module, allowed for the first time the direct measurement of a cellular function in real time and on orbit. We measured the oxidative burst reaction in mammalian macrophages (NR8383 rat alveolar macrophages) exposed to a centrifuge regime of internal 0 g and 1 g controls and step-wise increase or decrease of the gravitational force in four independent experiments. Surprisingly, we found that these macrophages adapted to microgravity in an ultra-fast manner within seconds, after an immediate inhibitory effect on the oxidative burst reaction. For the first time, we provided direct evidence of cellular sensitivity to gravity, through real-time on orbit measurements and by using an experimental system, in which all factors except gravity were constant. The surprisingly ultra-fast adaptation to microgravity indicates that mammalian macrophages are equipped with a highly efficient adaptation potential to a low gravity environment. This opens new avenues for the exploration of adaptation of mammalian cells to gravitational changes.

Mechanotransduction as an Adaptation to Gravity

Gravity has played a critical role in the development of terrestrial life. A key event in evolution has been the development of mechanisms to sense and transduce gravitational force into biological signals. The objective of this manuscript is to review how living organisms on Earth use mechanotransduction as an adaptation to gravity. Certain cells have evolved specialized structures, such as otoliths in hair cells of the inner ear and statoliths in plants, to respond directly to the force of gravity. By conducting studies in the reduced gravity of spaceflight (microgravity) or simulating microgravity in the laboratory, we have gained insights into how gravity might have changed life on Earth. We review how microgravity affects prokaryotic and eukaryotic cells at the cellular and molecular levels. Genomic studies in yeast have identified changes in genes involved in budding, cell polarity, and cell separation regulated by Ras, PI3K, and TOR signaling pathways. Moreover, transcriptomic analysis of late pregnant rats have revealed that microgravity affects genes that regulate circadian clocks, activate mechanotransduction pathways, and induce changes in immune response, metabolism, and cells proliferation. Importantly, these studies identified genes that modify chromatin structure and methylation, suggesting that long-term adaptation to gravity may be mediated by epigenetic modifications. Given that gravity represents a modification in mechanical stresses encounter by the cells, the tensegrity model of cytoskeletal architecture provides an excellent paradigm to explain how changes in the balance of forces, which are transmitted across transmembrane receptors and cytoskeleton, can influence intracellular signaling pathways and gene expression.

Towards human exploration of space: The THESEUS review series on immunology research priorities

Dysregulation of the immune system occurs during spaceflight and may represent a crew health risk during exploration missions because astronauts are challenged by many stressors. Therefore, it is crucial to understand the biology of immune modulation under spaceflight conditions in order to be able to maintain immune homeostasis under such challenges. In the framework of the THESEUS project whose aim was to develop an integrated life sciences research roadmap regarding human space exploration, experts working in the field of space immunology, and related disciplines, established a questionnaire sent to scientists around the world. From the review of collected answers, they deduced a list of key issues and provided several recommendations such as a maximal exploitation of currently available resources on Earth and in space, and to increase increments duration for some ISS crew members to 12 months or longer. These recommendations should contribute to improve our knowledge about spaceflight effects on the immune system and the development of countermeasures that, beyond astronauts, could have a societal impact.

Spaceflight induced changes in the human proteome

INTRODUCTION

Spaceflight is one of the most extreme conditions encountered by humans: Individuals are exposed to radiation, microgravity, hypodynamia, and will experience isolation. A better understanding of the molecular processes induced by these factors may allow us to develop personalized countermeasures to minimize risks to astronauts. Areas covered: This review is a summary of literature searches from PubMed, NASA, Roskosmos and the authors' research experiences and opinions. The review covers the available proteomic data on the effects of spaceflight factors on the human body, including both real space missions and ground-based model experiments. Expert commentary: Overall, the authors believe that the present background, methodology and equipment improvements will enhance spaceflight safety and support accumulation of new knowledge on how organisms adapt to extreme conditions.

Mechanical remodeling of normally sized mammalian cells under a gravity vector

Translocation of the dense nucleus along a gravity vector initiates mechanical remodeling of a cell, but the underlying mechanisms of cytoskeletal network and focal adhesion complex (FAC) reorganization in a mammalian cell remain unclear. We quantified the remodeling of an MC3T3-E1 cell placed in upward-, downward-, or edge-on-orientated substrate. Nucleus longitudinal translocation presents a high value in downward orientation at 24 h or in edge-on orientation at 72 h, which is consistent with orientation-dependent distribution of perinuclear actin stress fibers and vimentin cords. Redistribution of total FAC area and fractionized super mature adhesion number coordinates this dependence at short duration. This orientation-dependent remodeling is associated with nucleus flattering and lamin A/C phosphorylation. Actin depolymerization or Rho-associated protein kinase signaling inhibition abolishes the orientation dependence of nucleus translocation, whereas tubulin polymerization inhibition or vimentin disruption reserves the dependence. A biomechanical model is therefore proposed for integrating the mechanosensing of nucleus translocation with cytoskeletal remodeling and FAC reorganization induced by a gravity vector.-Zhang, C., Zhou, L., Zhang, F., Lü, D., Li, N., Zheng, L., Xu, Y., Li, Z., Sun, S., Long, M. Mechanical remodeling of normally sized mammalian cells under a gravity vector.

Simulated microgravity inhibits the migration of mesenchymal stem cells by remodeling actin cytoskeleton and increasing cell stiffness

Exposure to microgravity during space flight affects almost all human physiological systems. Migration, proliferation, and differentiation of stem cells are crucial for tissues repair and regeneration. However, the effect of microgravity on the migration potentials of bone marrow mesenchymal stem cells (BMSCs) is unclear, which are important progenitor and supporting cells. Here, we utilized a clinostat to model simulated microgravity (SMG) and found that SMG obviously inhibited migration of rat BMSCs. We detected significant reorganization of F-actin filaments and increased Young's modulus of BMSCs after exposure to SMG. Moreover, Y-27632 (a specific inhibitor of ROCK) abrogated the inhibited migration capacity and polymerized F-actin filament of BMSCs under SMG. Interestingly, we found that transferring BMSCs to normal gravity also attenuated the polymerized F-actin filament and Young's modulus of BMSCs induced by SMG, but could not recover migration capacity of BMSCs inhibited by SMG. Taken together, we propose that SMG inhibits migration of BMSCs through remodeling F-actin and increasing cell stiffness.

Simulated Microgravity Disrupts Cytoskeleton Organization and Increases Apoptosis of Rat Neural Crest Stem Cells Via Upregulating CXCR4 Expression and RhoA-ROCK1-p38 MAPK-p53 Signaling

Neural crest stem cells (NCSCs) are a population of multipotent stem cells that are distributed broadly in many tissues and organs and are capable of differentiating into a variety of cell types that are dispersed throughout three germ layers. We are interested in studying the effects of simulated microgravity on the survival and self-renewal of NCSCs. NCSCs extracted from the hair follicle bulge region of the rat whisker pad were cultured in vitro, respectively, in a 2D adherent environment and a 3D suspension environment using the rotatory cell culture system (RCCS) to simulate microgravity. We found that rat NCSCs (rNCSCs) cultured in the RCCS for 24 h showed disrupted organization of filamentous actin, increased globular actin level, formation of plasma membrane blebbing and neurite-like artifact, as well as decreased levels of cortactin and vimentin. Interestingly, ∼70% of RCCS-cultured rNCSCs co-expressed cleaved (active) caspase-3 and neuronal markers microtubule-associated protein 2 (MAP2) and Tuj1 instead of NCSC markers, suggesting stress-induced formation of neurite-like artifact in rNCSCs. In addition, rNCSCs showed increased C-X-C chemokine receptor 4 (CXCR4) expression, RhoA GTPase activation, Rho-associated kinase 1 (ROCK1) and p38 mitogen-activated protein kinase (MAPK) phosphorylation, and p53 expression in the nucleus. Incubation of rNCSCs with the Gα protein inhibitor pertussis toxin or CXCR4 siRNA during RCCS-culturing prevented cytoskeleton disorganization and plasma membrane blebbing, and it suppressed apoptosis of rNCSCs. Taken together, we demonstrate for the first time that simulated microgravity disrupts cytoskeleton organization and increases apoptosis of rNCSCs via upregulating CXCR4 expression and the RhoA-ROCK1-p38 MAPK-p53 signaling pathway.

Combined Exposure to Simulated Microgravity and Acute or Chronic Radiation Reduces Neuronal Network Integrity and Survival

During orbital or interplanetary space flights, astronauts are exposed to cosmic radiations and microgravity. However, most earth-based studies on the potential health risks of space conditions have investigated the effects of these two conditions separately. This study aimed at assessing the combined effect of radiation exposure and microgravity on neuronal morphology and survival in vitro. In particular, we investigated the effects of simulated microgravity after acute (X-rays) or during chronic (Californium-252) exposure to ionizing radiation using mouse mature neuron cultures. Acute exposure to low (0.1 Gy) doses of X-rays caused a delay in neurite outgrowth and a reduction in soma size, while only the high dose impaired neuronal survival. Of interest, the strongest effect on neuronal morphology and survival was evident in cells exposed to microgravity and in particular in cells exposed to both microgravity and radiation. Removal of neurons from simulated microgravity for a period of 24 h was not sufficient to recover neurite length, whereas the soma size showed a clear re-adaptation to normal ground conditions. Genome-wide gene expression analysis confirmed a modulation of genes involved in neurite extension, cell survival and synaptic communication, suggesting that these changes might be responsible for the observed morphological effects. In general, the observed synergistic changes in neuronal network integrity and cell survival induced by simulated space conditions might help to better evaluate the astronaut's health risks and underline the importance of investigating the central nervous system and long-term cognition during and after a space flight.

Alterations of the cytoskeleton in human cells in space proved by life-cell imaging

Microgravity induces changes in the cytoskeleton. This might have an impact on cells and organs of humans in space. Unfortunately, studies of cytoskeletal changes in microgravity reported so far are obligatorily based on the analysis of fixed cells exposed to microgravity during a parabolic flight campaign (PFC). This study focuses on the development of a compact fluorescence microscope (FLUMIAS) for fast live-cell imaging under real microgravity. It demonstrates the application of the instrument for on-board analysis of cytoskeletal changes in FTC-133 cancer cells expressing the Lifeact-GFP marker protein for the visualization of F-actin during the 24^th DLR PFC and TEXUS 52 rocket mission. Although vibration is an inevitable part of parabolic flight maneuvers, we successfully for the first time report life-cell cytoskeleton imaging during microgravity, and gene expression analysis after the 31^st parabola showing a clear up-regulation of cytoskeletal genes. Notably, during the rocket flight the FLUMIAS microscope reveals significant alterations of the cytoskeleton related to microgravity. Our findings clearly demonstrate the applicability of the FLUMIAS microscope for life-cell imaging during microgravity, rendering it an important technological advance in live-cell imaging when dissecting protein localization.

Microgravity-driven remodeling of the proteome reveals insights into molecular mechanisms and signal networks involved in response to the space flight environment

Space is a hostile environment characterized by high vacuum, extreme temperatures, meteoroids, space debris, ionospheric plasma, microgravity and space radiation, which all represent risks for human health. A deep understanding of the biological consequences of exposure to the space environment is required to design efficient countermeasures to minimize their negative impact on human health. Recently, proteomic approaches have received a significant amount of attention in the effort to further study microgravity-induced physiological changes. In this review, we summarize the current knowledge about the effects of microgravity on microorganisms (in particular Cupriavidus metallidurans CH34, Bacillus cereus and Rhodospirillum rubrum S1H), plants (whole plants, organs, and cell cultures), mammalian cells (endothelial cells, bone cells, chondrocytes, muscle cells, thyroid cancer cells, immune system cells) and animals (invertebrates, vertebrates and mammals). Herein, we describe their proteome's response to microgravity, focusing on proteomic discoveries and their future potential applications in space research.

BIOLOGICAL SIGNIFICANCE

Space experiments and operational flight experience have identified detrimental effects on human health and performance because of exposure to weightlessness, even when currently available countermeasures are implemented. Many experimental tools and methods have been developed to study microgravity induced physiological changes. Recently, genomic and proteomic approaches have received a significant amount of attention. This review summarizes the recent research studies of the proteome response to microgravity inmicroorganisms, plants, mammalians cells and animals. Current proteomic tools allow large-scale, high-throughput analyses for the detection, identification, and functional investigation of all proteomes. Understanding gene and/or protein expression is the key to unlocking the mechanisms behind microgravity-induced problems and to finding effective countermeasures to spaceflight-induced alterations but also for the study of diseases on earth. Future perspectives are also highlighted.

The influence of simulated microgravity on the proteome of Daphnia magna

Background:

The waterflea Daphnia is an interesting candidate for bioregenerative life support systems (BLSS). These animals are particularly promising because of their central role in the limnic food web and its mode of reproduction. However, the response of Daphnia to altered gravity conditions has to be investigated, especially on the molecular level, to evaluate the suitability of Daphnia for BLSS in space.

Methods:

In this study, we applied a proteomic approach to identify key proteins and pathways involved in the response of Daphnia to simulated microgravity generated by a two-dimensional (2D) clinostat. We analyzed five biological replicates using 2D-difference gel electrophoresis proteomic analysis.

Results:

We identified 109 protein spots differing in intensity (P<0.05). Substantial fractions of these proteins are involved in actin microfilament organization, indicating the disruption of cytoskeletal structures during clinorotation. Furthermore, proteins involved in protein folding were identified, suggesting altered gravity induced breakdown of protein structures in general. In addition, simulated microgravity increased the abundance of energy metabolism-related proteins, indicating an enhanced energy demand of Daphnia.

Conclusions:

The affected biological processes were also described in other studies using different organisms and systems either aiming to simulate microgravity conditions or providing real microgravity conditions. Moreover, most of the Daphnia protein sequences are well-conserved throughout taxa, indicating that the response to altered gravity conditions in Daphnia follows a general concept. Data are available via ProteomeXchange with identifier PXD002096.

Preparing normal tissue cells for space flight experiments

Deterioration of health is a problem in modern space flight business. In order to develop countermeasures, research has been done on human bodies and also on single cells. Relevant experiments on human cells in vitro are feasible when microgravity is simulated by devices such as the Random Positioning Machine or generated for a short time during parabolic flights. However, they become difficult in regard to performance and interpretation when long-term experiments are designed that need a prolonged stay on the International Space Station (ISS). One huge problem is the transport of living cells from a laboratory on Earth to the ISS. For this reason, mainly rapidly growing, rather robust human cells such as cancer cells, embryonic cells, or progenitor cells have been investigated on the ISS up to now. Moreover, better knowledge on the behavior of normal mature cells, which mimic the in vivo situation, is strongly desirable. One solution to the problem could be the use of redifferentiable cells, which grow rapidly and behave like cancer cells in plain medium, but are reprogrammed to normal cells when substances like retinoic acid are added. A list of cells capable of redifferentiation is provided, together with names of suitable drugs, in this review.

RhoGTPases as key players in mammalian cell adaptation to microgravity

A growing number of studies are revealing that cells reorganize their cytoskeleton when exposed to conditions of microgravity. Most, if not all, of the structural changes observed on flown cells can be explained by modulation of RhoGTPases, which are mechanosensitive switches responsible for cytoskeletal dynamics control. This review identifies general principles defining cell sensitivity to gravitational stresses. We discuss what is known about changes in cell shape, nucleus, and focal adhesions and try to establish the relationship with specific RhoGTPase activities. We conclude by considering the potential relevance of live imaging of RhoGTPase activity or cytoskeletal structures in order to enhance our understanding of cell adaptation to microgravity-related conditions.

Simulated microgravity: critical review on the use of random positioning machines for mammalian cell culture

Random Positioning Machines (RPMs) have been used since many years as a ground-based model to simulate microgravity. In this review we discuss several aspects of the RPM. Recent technological development has expanded the operative range of the RPM substantially. New possibilities of live cell imaging and partial gravity simulations, for example, are of particular interest. For obtaining valuable and reliable results from RPM experiments, the appropriate use of the RPM is of utmost importance. The simulation of microgravity requires that the RPM's rotation is faster than the biological process under study, but not so fast that undesired side effects appear. It remains a legitimate question, however, whether the RPM can accurately and reliably simulate microgravity conditions comparable to real microgravity in space. We attempt to answer this question by mathematically analyzing the forces working on the samples while they are mounted on the operating RPM and by comparing data obtained under real microgravity in space and simulated microgravity on the RPM. In conclusion and after taking the mentioned constraints into consideration, we are convinced that simulated microgravity experiments on the RPM are a valid alternative for conducting examinations on the influence of the force of gravity in a fast and straightforward approach.

Identification of reference genes in human myelomonocytic cells for gene expression studies in altered gravity

Gene expression studies are indispensable for investigation and elucidation of molecular mechanisms. For the process of normalization, reference genes (“housekeeping genes”) are essential to verify gene expression analysis. Thus, it is assumed that these reference genes demonstrate similar expression levels over all experimental conditions. However, common recommendations about reference genes were established during 1 g conditions and therefore their applicability in studies with altered gravity has not been demonstrated yet. The microarray technology is frequently used to generate expression profiles under defined conditions and to determine the relative difference in expression levels between two or more different states. In our study, we searched for potential reference genes with stable expression during different gravitational conditions (microgravity, normogravity, and hypergravity) which are additionally not altered in different hardware systems. We were able to identify eight genes (ALB, B4GALT6, GAPDH, HMBS, YWHAZ, ABCA5, ABCA9, and ABCC1) which demonstrated no altered gene expression levels in all tested conditions and therefore represent good candidates for the standardization of gene expression studies in altered gravity.

The role of the cytoskeleton in sensing changes in gravity by nonspecialized cells

A large body of evidence indicates that single cells in vitro respond to changes in gravity, and that this response might play an important role for physiological changes at the organism level during spaceflight. Gravity can lead to changes in cell proliferation, differentiation, signaling, and gene expression. At first glance, gravitational forces seem too small to affect bodies with the size of a cell. Thus, the initial response to gravity is both puzzling and important for understanding physiological changes in space. This also offers a unique environment to study the mechanical response of cells. In the past 2 decades, important steps have been made in the field of mechanobiology, and we use these advances to reevaluate the response of single cells to changes in gravity. Recent studies have focused on the cytoskeleton as initial gravity sensor. Thus, we review the observed changes in the cytoskeleton in a microgravity environment, both during spaceflight and in ground-based simulation techniques. We also evaluate to what degree the current experimental evidence supports the cytoskeleton as primary gravity sensor. Finally, we consider how the cytoskeleton itself could be affected by changed gravity. To make the next step toward understanding the response of cells to altered gravity, the challenge will be to track changes quantitatively and on short timescales.

Morphological and physiological changes in mature in vitro neuronal networks towards exposure to short-, middle- or long-term simulated microgravity

One of the objectives of the current international space programmes is to investigate the possible effects of the space environment on the crew health. The aim of this work was to assess the particular effects of simulated microgravity on mature primary neuronal networks and specially their plasticity and connectivity. For this purpose, primary mouse neurons were first grown for 10 days as a dense network before being placed in the Random Positioning Machine (RPM), simulating microgravity. These cultures were then used to investigate the impact of short- (1 h), middle- (24 h) and long-term (10 days) exposure to microgravity at the level of neurite network density, cell morphology and motility as well as cytoskeleton properties in established two-dimensional mature neuronal networks. Image processing analysis of dense neuronal networks exposed to simulated microgravity and their subsequent recovery under ground conditions revealed different neuronal responses depending on the duration period of exposure. After short- and middle-term exposures to simulated microgravity, changes in neurite network, neuron morphology and viability were observed with significant alterations followed by fast recovery processes. Long exposure to simulated microgravity revealed a high adaptation of single neurons to the new gravity conditions as well as a partial adaptation of neuronal networks. This latter was concomitant to an increase of apoptosis. However, neurons and neuronal networks exposed for long-term to simulated microgravity required longer recovery time to re-adapt to the ground gravity. In conclusion, a clear modulation in neuronal plasticity was evidenced through morphological and physiological changes in primary neuronal cultures during and after simulated microgravity exposure. These changes were dependent on the duration of exposure to microgravity.

Spaceflight influences both mucosal and peripheral cytokine production in PTN-Tg and wild type mice

Spaceflight is associated with several health issues including diminished immune efficiency. Effects of long-term spaceflight on selected immune parameters of wild type (Wt) and transgenic mice over-expressing pleiotrophin under the human bone-specific osteocalcin promoter (PTN-Tg) were examined using the novel Mouse Drawer System (MDS) aboard the International Space Station (ISS) over a 91 day period. Effects of this long duration flight on PTN-Tg and Wt mice were determined in comparison to ground controls and vivarium-housed PTN-Tg and Wt mice. Levels of interleukin-2 (IL-2) and transforming growth factor-beta1 (TGF-β1) were measured in mucosal and systemic tissues of Wt and PTN-Tg mice. Colonic contents were also analyzed to assess potential effects on the gut microbiota, although no firm conclusions could be made due to constraints imposed by the MDS payload and the time of sampling. Spaceflight-associated differences were observed in colonic tissue and systemic lymph node levels of IL-2 and TGF-β1 relative to ground controls. Total colonic TGF-β1 levels were lower in Wt and PTN-Tg flight mice in comparison to ground controls. The Wt flight mouse had lower levels of IL-2 and TGF-β1 compared to the Wt ground control in both the inguinal and brachial lymph nodes, however this pattern was not consistently observed in PTN-Tg mice. Vivarium-housed Wt controls had higher levels of active TGF-β1 and IL-2 in inguinal lymph nodes relative to PTN-Tg mice. The results of this study suggest compartmentalized effects of spaceflight and on immune parameters in mice.