Evaluation of gene, protein and neurotrophin expression in the brain of mice exposed to space environment for 91 days

Behavioral and multiomics analysis of 3D clinostat simulated microgravity effect in mice focusing on the central nervous system

A study was conducted to evaluate the three-dimensional clinostat simulated microgravity effect on mouse models, focusing on the central nervous system. Eighteen mice were divided into three groups: control, survival box, and clinostat + survival box. Behavioral tests, femur micro-CT, brain transcriptomics, serum metabolomics, and fecal microbiomics were performed. Results showed decreased activity, altered gait, enhanced fear memory, bone loss, immune/endocrine changes in brain transcriptome, and altered metabolic pathways in serum and gut microbiota in clinostat-treated mice. The model closely mimics spaceflight-induced transcriptome changes, suggesting its value in studying microgravity-related neurological alterations and highlighting the need for attention to emotional changes in space.

Formaldehyde initiates memory and motor impairments under weightlessness condition

During space flight, prolonged weightlessness stress exerts a range of detrimental impacts on the physiology and psychology of astronauts. These manifestations encompass depressive symptoms, anxiety, and impairments in both short-term memory and motor functions, albeit the precise underlying mechanisms remain elusive. Recent studies have revealed that hindlimb unloading (HU) animal models, which simulate space weightlessness, exhibited a disorder in memory and motor function associated with endogenous formaldehyde (FA) accumulation in the hippocampus and cerebellum, disruption of brain extracellular space (ECS), and blockage of interstitial fluid (ISF) drainage. Notably, the impairment of the blood-brain barrier (BBB) caused by space weightlessness elicits the infiltration of albumin and hemoglobin from the blood vessels into the brain ECS. However, excessive FA has the potential to form cross-links between these two proteins and amyloid-beta (Aβ), thereby obstructing ECS and inducing neuron death. Moreover, FA can inhibit N-methyl-D-aspartate (NMDA) currents by crosslinking NR1 and NR2B subunits, thus impairing memory. Additionally, FA has the ability to modulate the levels of certain microRNAs (miRNAs) such as miRNA-29b, which can affect the expression of aquaporin-4 (AQP4) so as to regulate ECS structure and ISF drainage. Especially, the accumulation of FA may inactivate the ataxia telangiectasia-mutated (ATM) protein kinase by forming cross-linking, a process that is associated with ataxia. Hence, this review presents that weightlessness stress-derived FA may potentially serve as a crucial catalyst in the deterioration of memory and motor abilities in the context of microgravity.

Whole-transcriptome RNA sequencing reveals CeRNA regulatory network under long-term space composite stress in Rats

To systematically evaluate the effect of simulated long-term spaceflight composite stress (LSCS) in hippocampus and gain more insights into the transcriptomic landscape and molecular mechanism, we performed whole-transcriptome sequencing based on the control group (Ctrl) and the simulated long-term spaceflight composite stress group (LSCS) from six hippocampus of rats. Subsequently, differential expression analysis was performed on the Ctrl and LSCS groups, followed by enrichment analysis and functional interaction prediction analysis to investigate gene-regulatory circuits in LSCS. In addition, competitive endogenous RNA (ceRNA) network was constructed to gain insights into genetic interaction. The result showed that 276 differentially expressed messenger RNAs (DEmRNAs), 139 differentially expressed long non-coding RNAs (DElncRNAs), 103 differentially expressed circular RNAs (DEcircRNAs), and 52 differentially expressed microRNAs (DEmiRNAs) were found in LSCS samples compared with the controls, which were then subjected to enrichment analysis of Gene Ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways to find potential functions. PI3K-Akt signaling pathway and MAPK signaling pathway may play fundamental roles in the pathogenesis of LSCS. A ceRNA network was constructed with the predicted 340 DE pairs, which revealed the interaction roles of 220 DEmiRNA-DEmRNA pairs, 76 DEmiRNA-DElncRNA pairs, and 44 DEmiRNA-DEcircRNA pairs. Further, Thrombospondins2 was found to be a key target among those ceRNAs. Overall, we conducted for the first time a full transcriptomic analysis of the response of hippocampus to the LSCS that involved a potential ceRNA network, thus providing a basis to study the underlying mechanism of the LSCS.

NASA GeneLab derived microarray studies of Mus musculus and Homo sapiens organisms in altered gravitational conditions

One of the greatest challenges of humanity for deep space exploration is to fully understand how altered gravitational conditions affect human physiology. It is evident that the spaceflight environment causes multiple alterations to musculoskeletal, cardiovascular, immune and central nervous systems, to name a few known effects. To better characterize these biological effects, we compare gene expression datasets from microarray studies found in NASA GeneLab, part of the NASA Open Science Data Repository. In this review, we summarize these archived results for various tissues, emphasizing key genes which are highly reproducible in different mice or human experiments. Such exhaustive mining shows the potential of NASA Open Science data to identify and validate mechanisms taking place when mammalian organisms are exposed to microgravity or other spaceflight conditions. Our comparative meta-analysis findings highlight certain degrees of overlap and reproducibility in genes identified as differentially expressed within musculoskeletal tissues in each species across a variety of altered gravity conditions. However, the level of overlap between species was found to be significantly limited, partly attributed to the limited availability of human samples.

Microglial activation in spaceflight and microgravity: potential risk of cognitive dysfunction and poor neural health

Due to the increased crewed spaceflights in recent years, it is vital to understand how the space environment affects human health. A lack of gravitational force is known to risk multiple physiological functions of astronauts, particularly damage to the central nervous system (CNS). As innate immune cells of the CNS, microglia can transition from a quiescent state to a pathological state, releasing pro-inflammatory cytokines that contribute to neuroinflammation. There are reports indicating that microglia can be activated by simulating microgravity or exposure to galactic cosmic rays (GCR). Consequently, microglia may play a role in the development of neuroinflammation during spaceflight. Prolonged spaceflight sessions raise concerns about the chronic activation of microglia, which could give rise to various neurological disorders, posing concealed risks to the neural health of astronauts. This review summarizes the risks associated with neural health owing to microglial activation and explores the stressors that trigger microglial activation in the space environment. These stressors include GCR, microgravity, and exposure to isolation and stress. Of particular focus is the activation of microglia under microgravity conditions, along with the proposal of a potential mechanism.

Long-term spaceflight composite stress induces depression and cognitive impairment in astronauts-insights from neuroplasticity

The environment on the space station is quite unique compared to Earth, which is a composite of multiple stressors, such as microgravity, isolation, confinement, noise, circadian rhythm disturbance, and so on. During prolonged space missions, astronauts have to stay in such extreme environments for long periods, which could induce adverse effects on both their physical and mental health. In some circumstances, this kind of long-term spaceflight composite stress (LSCS) could also induce depression and cognitive impairment in various ways, including dysregulating the neuroplasticity of the brains of astronauts, which should be attached to great importance. Here, we have comprehensively reviewed the impact of individual and combined stressors on depression and cognitive function during long-term spaceflight, explained the underlying mechanisms of those effects from the perspective of neuroplasticity, and current countermeasures for mitigating these challenges. This review provides insights into LSCS and potential neuroplasticity mechanisms, current with potentially great impact for understanding and mitigating the mental health risks and traumas of career astronauts and space tourists.

The Study of the Caudal Vertebrae of Thick-Toed Geckos after a Prolonged Space Flight by X-ray Phase-Contrast Micro-CT

The proximal caudal vertebrae and notochord in thick-toed geckos (TG) (Chondrodactylus turneri, Gray, 1864) were investigated after a 30-day space flight onboard the biosatellite Bion-M1. This region has not been explored in previous studies. Our research focused on finding sites most affected by demineralization caused by microgravity (G0). We used X-ray phase-contrast tomography to study TG samples without invasive prior preparation to clarify our previous findings on the resistance of TG's bones to demineralization in G0. The results of the present study confirmed that geckos are capable of preserving bone mass after flight, as neither cortical nor trabecular bone volume fraction showed statistically significant changes after flight. On the other hand, we observed a clear decrease in the mineralization of the notochordal septum and a substantial rise in intercentrum volume following the flight. To monitor TG's mineral metabolism in G0, we propose to measure the volume of mineralized tissue in the notochordal septum. This technique holds promise as a sensitive approach to track the demineralization process in G0, given that the volume of calcification within the septum is limited, making it easy to detect even slight changes in mineral content.

Spaceflight-Induced Gene Expression Profiles in the Mouse Brain Are Attenuated by Treatment with the Antioxidant BuOE

The demands of deep space pose a health risk to the central nervous system that has long been a concern when sending humans to space. While little is known about how spaceflight affects transcription spatially in the brain, a greater understanding of this process has the potential to aid strategies that mitigate the effects of spaceflight on the brain. Therefore, we performed GeoMx Digital Spatial Profiling of mouse brains subjected to either spaceflight or grounded controls. Four brain regions were selected: Cortex, Frontal Cortex, Corunu Ammonis I, and Dentate Gyrus. Antioxidants have emerged as a potential means of attenuating the effects of spaceflight, so we treated a subset of the mice with a superoxide dismutase mimic, MnTnBuOE-2-PyP 5+ (BuOE). Our analysis revealed hundreds of differentially expressed genes due to spaceflight in each of the four brain regions. Both common and region-specific transcriptomic responses were observed. Metabolic pathways and pathways sensitive to oxidative stress were enriched in the four brain regions due to spaceflight. These findings enhance our understanding of brain regional variation in susceptibility to spaceflight conditions. BuOE reduced the transcriptomic effects of spaceflight at a large number of genes, suggesting that this compound may attenuate oxidative stress-induced brain damage caused by the spaceflight environment.

Simulated Microgravity Modulates Focal Adhesion Gene Expression in Human Neural Stem Progenitor Cells

We analyzed the morphology and the transcriptomic changes of human neural stem progenitor cells (hNSPCs) grown on laminin in adherent culture conditions and subjected to simulated microgravity for different times in a random positioning machine apparatus. Low-cell-density cultures exposed to simulated microgravity for 24 h showed cell aggregate formation and significant modulation of several genes involved in focal adhesion, cytoskeleton regulation, and cell cycle control. These effects were much more limited in hNSPCs cultured at high density in the same conditions. We also found that some of the genes modulated upon exposure to simulated microgravity showed similar changes in hNSPCs grown without laminin in non-adherent culture conditions under normal gravity. These results suggest that reduced gravity counteracts the interactions of cells with the extracellular matrix, inducing morphological and transcriptional changes that can be observed in low-density cultures.

Characterization of gene expression profiles in the mouse brain after 35 days of spaceflight mission

It has been proposed that neuroinflammatory response plays an important role in the neurovascular remodeling in the brain after stress. The goal of the present study was to characterize changes in the gene expression profiles associated with neuroinflammation, neuronal function, metabolism and stress in mouse brain tissue. Ten-week old male C57BL/6 mice were launched to the International Space Station (ISS) on SpaceX-12 for a 35-day mission. Within 38 ± 4 h of splashdown, mice were returned to Earth alive. Brain tissues were collected for analysis. A novel digital color-coded barcode counting technology (NanoString^TM) was used to evaluate gene expression profiles in the spaceflight mouse brain. A set of 54 differently expressed genes (p < 0.05) significantly segregates the habitat ground control (GC) group from flight (FLT) group. Many pathways associated with cellular stress, inflammation, apoptosis, and metabolism were significantly altered by flight conditions. A decrease in the expression of genes important for oligodendrocyte differentiation and myelin sheath maintenance was observed. Moreover, mRNA expression of many genes related to anti-viral signaling, reactive oxygen species (ROS) generation, and bacterial immune response were significantly downregulated. Here we report that significantly altered immune reactions may be closely associated with spaceflight-induced stress responses and have an impact on the neuronal function.

The Protective Role of Neurogenetic Components in Reducing Stress-Related Effects during Spaceflights: Evidence from the Age-Related Positive Memory Approach

Fighting stress-related effects during spaceflight is crucial for a successful mission. Emotional, motivational, and cognitive mechanisms have already been shown to be involved in the decrease of negative emotions. However, emerging evidence is pointing to a neurogenetic profile that may render some individuals more prone than others to focusing on positive information in memory and increasing affective health. The relevance for adaptation to the space environment and the interaction with other stressors such as ionizing radiations is discussed. In particular, to clarify this approach better, we will draw from the psychology and aging literature data. Subsequently, we report on studies on candidate genes for sensitivity to positive memories. We review work on the following candidate genes that may be crucial in adaptation mechanisms: ADRA2B, COMT, 5HTTLPR, CB1, and TOMM40. The final aim is to show how the study of genetics and cell biology of positive memory can help us to reveal the underlying bottom-up pathways to also increasing positive effects during a space mission.

Comparing effects of microgravity and amyotrophic lateral sclerosis in the mouse ventral lumbar spinal cord

Microgravity (MG) exposure and motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), lead to motor deficits, including muscle atrophy and loss of neuronal activity. Abnormalities in motor neurons and muscles caused by MG exposure can be recovered by subsequent ground exercise. In contrast, the degeneration that occurs in ALS is irreversible. A common phenotype between MG exposure and ALS pathology is motor system abnormality, but the causes may be different. In this study, to elucidate the motor system that is affected by each condition, we investigated the effects of MG and the human SOD1 ALS mutation on gene expression in various cell types of the mouse ventral lumbar spinal cord, which is rich in motor neurons innervating the lower limb. To identify cell types affected by MG or ALS pathogenesis, we analyzed differentially expressed genes with known cell-type markers, which were determined from previous single-cell studies of the spinal cord in MG-exposed and SOD1^G93A mice, an ALS mouse model. Differentially expressed genes were observed in MG mice in various spinal cord cell types, including neurons, microglia, astrocytes, oligodendrocytes, oligodendrocyte precursor cells, meningeal cells/Schwann cells, and vascular cells. We also examined neuronal populations in the spinal cord. Gene expression in putative excitatory and inhibitory neurons changed more than that in cholinergic motor neurons of the spinal cord in both MG and SOD1^G93A mice. Many putative neuron types, especially visceral motor neurons, and axon initial segments (AIS) were affected in MG mice. In contrast, the effect on neurons and AIS in SOD1^G93A mice was slight at P30 but progressed with aging. Interestingly, changes in dopaminergic system-related genes were specifically altered in the spinal cord of MG mice. These results indicate that MG and ALS pathology in various cell types contribute to motor neuron degeneration. Furthermore, there were more alterations in neurons in MG-exposed mice than in SOD1^G93A mice. A large number of differentially expressed genes (DEGs) in MG mice represent more than SOD1^G93A mice with ALS pathology. Elucidation of MG pathogenesis may provide more insight into the pathophysiology of neurodegenerative diseases.

The basic leucine zipper transcription factor OsbZIP83 and the glutaredoxins OsGRX6 and OsGRX9 facilitate rice iron utilization under the control of OsHRZ ubiquitin ligases

Under low iron availability, plants induce the expression of various genes for iron uptake and translocation. The rice (Oryza sativa) ubiquitin ligases OsHRZ1 and OsHRZ2 cause overall repression of these iron-related genes at the transcript level, but their protein-level regulation is unclear. We conducted a proteome analysis to identify key regulators whose abundance was regulated by OsHRZs at the protein level. In response to iron deficiency or OsHRZ knockdown, many genes showed differential regulation between the transcript and protein levels, including the TGA-type basic leucine zipper transcription factor OsbZIP83. We also identified two glutaredoxins, OsGRX6 and OsGRX9, as OsHRZ-interacting proteins in yeast and plant cells. OsGRX6 also interacted with OsbZIP83. Our in vitro degradation assay suggested that OsbZIP83, OsGRX6 and OsGRX9 proteins are subjected to 26S proteasome- and OsHRZ-dependent degradation. Proteome analysis and our in vitro degradation assay also suggested that OsbZIP83 protein was preferentially degraded under iron-deficient conditions in rice roots. Transgenic rice lines overexpressing OsGRX9 and OsbZIP83 showed improved tolerance to iron deficiency. Expression of iron-related genes was affected in the OsGRX9 and OsGRX6 knockdown lines, suggesting disturbed iron utilization and signaling. OsbZIP83 overexpression lines showed enhanced expression of OsYSL2 and OsNAS3, which are involved in internal iron translocation, in addition to OsGRX9 and genes related to phytoalexin biosynthesis and the salicylic acid pathway. The results suggest that OsbZIP83, OsGRX6 and OsGRX9 facilitate iron utilization downstream of the OsHRZ pathway.

Mice display learning and behavioral deficits after a 30-day spaceflight on Bion-M1 satellite

Profound effects of spaceflight on the physiology of humans and non-human animals are well-documented but incompletely explored. Current goals to undertake interplanetary missions increase the urgency to learn more about adaptation to prolonged spaceflight and readaptation to Earth-normal conditions, especially with the inclusion of radiation exposures greater than those confronted in traditional, orbital flights. The 30-day-long Bion M-1 biosatellite flight was conducted at a relatively high orbit, exposing the mice to greater doses of radiation in addition to microgravity, a combination of factors relevant to Mars missions. Results of the present studies with mice provide insights into the consequences on brain function of long-duration spaceflight. After landing, mice showed profound deficits in vestibular responses during aerial drop tests. Spaceflown mice displayed reduced grip strength, rotarod performance, and voluntary wheel running, each, which improved gradually but incompletely over the 7-days of post-flight testing. Continuous monitoring in the animals' home cage activity, in combination with open-field and other tests of motor performance, revealed indices of altered affect, expressed as hyperactivity, potentiated thigmotaxis, and avoidance of open areas which, together, presented a syndrome of persistent anxiety-like behavior. A learned, operant response acquired before spaceflight was retained, whereas the acquisition of a new task was impaired after the flight. We integrate these observations with other results from Bion-M1's program, identifying deficits in musculoskeletal and cardiovascular systems, as well as in the brain and spinal cord, including altered gene expression patterns and the accompanying neurochemical changes that could underlie our behavioral findings.

Micro-CT Study of Mongolian Gerbil Humeral Bone After Prolonged Spaceflight Based on a New Algorithm for Delimitation of Long-Bone Regions

The Mongolian gerbil displays unique physiological and anatomical features that make this species an attractive object for biological experiments in space. However, until recently, the Mongolian gerbil has remained a novel, mostly unstudied animal model in investigating bone loss in weightlessness (G₀). After 12 days of orbital Foton-M3 mission, the humerus of Mongolian gerbils has been studied here via micro-computed tomography (micro-CT) to quantify bone morphometric parameters. The samples from the flight group, delayed synchronous ground-control group, and basal control group were investigated, and main morphometric parameters were reported in the article. The accurate selection of a region of interest is an essential step for a correct assessment of bone parameters. We proposed a new, easy and efficient method for delimiting the bone’s basic regions in the humerus. It is based on quantitative estimation of X-ray attenuation in the cortical bone as a function of humerus bone length. The micro-CT analysis of the basic bone regions revealed a difference in bone morphometric parameters between the flight and control gerbils. The most significant bone loss was observed in the cortical part of the proximal humeral zone in the flight group. No statistically significant changes of volume fraction in the cancellous tissue of proximal and distal epiphyses and metaphyses were observed. A statistically significant increase in both cancellous bone volume and bone X-ray attenuation in the flight group was detected in the proximal part of the diaphyses. We assume that enhanced calcium deposition in the diaphyseal cancellous tissue occurred due to a bone response to G₀ conditions.

Neuro-consequences of the spaceflight environment

As human space exploration advances to establish a permanent presence beyond the Low Earth Orbit (LEO) with NASA's Artemis mission, researchers are striving to understand and address the health challenges of living and working in the spaceflight environment. Exposure to ionizing radiation, microgravity, isolation and other spaceflight hazards pose significant risks to astronauts. Determining neurobiological and neurobehavioral responses, understanding physiological responses under Central Nervous System (CNS) control, and identifying putative mechanisms to inform countermeasure development are critically important to ensuring brain and behavioral health of crew on long duration missions. Here we provide a detailed and comprehensive review of the effects of spaceflight and of ground-based spaceflight analogs, including simulated weightlessness, social isolation, and ionizing radiation on humans and animals. Further, we discuss dietary and non-dietary countermeasures including artificial gravity and antioxidants, among others. Significant future work is needed to ensure that neural, sensorimotor, cognitive and other physiological functions are maintained during extended deep space missions to avoid potentially catastrophic health and safety outcomes.

Dynamic Foot Stimulations During Short-Term Hindlimb Unloading Prevent Dysregulation of the Neurotransmission in the Hippocampus of Rats

Spaceflight and simulated microgravity both affect learning and memory, which are mostly controlled by the hippocampus. However, data about molecular alterations in the hippocampus in real or simulated microgravity conditions are limited. Adult Wistar rats were recruited in the experiments. Here we analyzed whether short-term simulated microgravity caused by 3-day hindlimb unloading (HU) will affect the glutamatergic and GABAergic systems of the hippocampus and how dynamic foot stimulation (DFS) to the plantar surface applied during HU can contribute in the regulation of hippocampus functioning. The results demonstrated a decreased expression of vesicular glutamate transporters 1 and 2 (VGLUT1/2) in the hippocampus after 3 days of HU, while glutamate decarboxylase 67 (GAD67) expression was not affected. HU also significantly induced Akt signaling and transcriptional factor CREB that are supposed to activate the neuroprotective mechanisms. On the other hand, DFS led to normalization of VGLUT1/2 expression and activity of Akt and CREB. Analysis of exocytosis proteins revealed the inhibition of SNAP-25, VAMP-2, and syntaxin 1 expression in DFS group proposing attenuation of excitatory neurotransmission. Thus, we revealed that short-term HU causes dysregulation of glutamatergic system of the hippocampus, but, at the same time, stimulates neuroprotective Akt-dependent mechanism. In addition, most importantly, we demonstrated positive effect of DFS on the hippocampus functioning that probably depends on the regulation of neurotransmitter exocytosis.

Spaceflight affects neuronal morphology and alters transcellular degradation of neuronal debris in adult Caenorhabditis elegans

Extended space travel is a goal of government space agencies and private companies. However, spaceflight poses risks to human health, and the effects on the nervous system have to be better characterized. Here, we exploited the unique experimental advantages of the nematode Caenorhabditis elegans to explore how spaceflight affects adult neurons in vivo. We found that animals that lived 5 days of adulthood on the International Space Station exhibited hyperbranching in PVD and touch receptor neurons. We also found that, in the presence of a neuronal proteotoxic stress, spaceflight promotes a remarkable accumulation of neuronal-derived waste in the surrounding tissues, suggesting an impaired transcellular degradation of debris released from neurons. Our data reveal that spaceflight can significantly affect adult neuronal morphology and clearance of neuronal trash, highlighting the need to carefully assess the risks of long-duration spaceflight on the nervous system and to develop adequate countermeasures for safe space exploration.

Fundamental Biological Features of Spaceflight: Advancing the Field to Enable Deep-Space Exploration

Research on astronaut health and model organisms have revealed six features of spaceflight biology that guide our current understanding of fundamental molecular changes that occur during space travel. The features include oxidative stress, DNA damage, mitochondrial dysregulation, epigenetic changes (including gene regulation), telomere length alterations, and microbiome shifts. Here we review the known hazards of human spaceflight, how spaceflight affects living systems through these six fundamental features, and the associated health risks of space exploration. We also discuss the essential issues related to the health and safety of astronauts involved in future missions, especially planned long-duration and Martian missions.

Spaceflight and brain plasticity: Spaceflight effects on regional expression of neurotransmitter systems and neurotrophic factors encoding genes

The critical problem of space exploration is the effect of long-term space travel on brain functioning. Current information concerning the effects of actual spaceflight on the brain was obtained on rats and mice flown on five missions of Soviet/Russian biosatellites, NASA Neurolab Mission STS90, and International Space Station (ISS). The review provides converging lines of evidence that: 1) long-term spaceflight affects both principle regulators of brain neuroplasticity - neurotransmitters (5-HT and DA) and neurotrophic factors (CDNF, GDNF but not BDNF); 2) 5-HT- (5-HT_2A receptor and MAO A) and especially DA-related genes (TH, MAO A, COMT, D1 receptor, CDNF and GDNF) belong to the risk neurogenes; 3) brain response to spaceflight is region-specific. Substantia nigra, striatum and hypothalamus are highly sensitive to the long-term spaceflight: in these brain areas spaceflight decreased the expression of both DA-related and neurotrophic factors genes. Since DA system is involved in the regulation of movement and cognition the data discussed in the review could explain dysfunction of locomotion and behavior of astronauts and direct further investigations to the DA system.

Integrated RNA-seq Analysis Indicates Asynchrony in Clock Genes between Tissues under Spaceflight

Rodent models have been widely used as analogs for estimating spaceflight-relevant molecular mechanisms in human tissues. NASA GeneLab provides access to numerous spaceflight omics datasets that can potentially generate novel insights and hypotheses about fundamental space biology when analyzed in new and integrated fashions. Here, we performed a pilot study to elucidate space biological mechanisms across tissues by reanalyzing mouse RNA-sequencing spaceflight data archived on NASA GeneLab. Our results showed that clock gene expressions in spaceflight mice were altered compared with those in ground control mice. Furthermore, the results suggested that spaceflight promotes asynchrony of clock gene expressions between peripheral tissues. Abnormal circadian rhythms are associated not only with jet lag and sleep disorders but also with cancer, lifestyle-related diseases, and mental disorders. Overall, our findings highlight the importance of elucidating the causes of circadian rhythm disruptions using the unique approach of space biology research to one day potentially develop countermeasures that benefit humans on Earth and in space.

Challenges to the central nervous system during human spaceflight missions to Mars

Space travel presents a number of environmental challenges to the central nervous system, including changes in gravitational acceleration that alter the terrestrial synergies between perception and action, galactic cosmic radiation that can damage sensitive neurons and structures, and multiple factors (isolation, confinement, altered atmosphere, and mission parameters, including distance from Earth) that can affect cognition and behavior. Travelers to Mars will be exposed to these environmental challenges for up to 3 years, and space-faring nations continue to direct vigorous research investments to help elucidate and mitigate the consequences of these long-duration exposures. This article reviews the findings of more than 50 years of space-related neuroscience research on humans and animals exposed to spaceflight or analogs of spaceflight environments, and projects the implications and the forward work necessary to ensure successful Mars missions. It also reviews fundamental neurophysiology responses that will help us understand and maintain human health and performance on Earth.

Reptiles in Space Missions: Results and Perspectives

Reptiles are a rare model object for space research. However, some reptile species demonstrate effective adaptation to spaceflight conditions. The main scope of this review is a comparative analysis of reptile experimental exposure in weightlessness, demonstrating the advantages and shortcomings of this model. The description of the known reptile experiments using turtles and geckos in the space and parabolic flight experiments is provided. Behavior, skeletal bones (morphology, histology, and X-ray microtomography), internal organs, and the nervous system (morphology, histology, and immunohistochemistry) are studied in the spaceflight experiments to date, while molecular and physiological results are restricted. Therefore, the results are discussed in the scope of molecular data collected from mammalian (mainly rodents) specimens and cell cultures in the parabolic and orbital flights and simulated microgravity. The published data are compared with the results of the gecko model studies after the 12–44.5-day spaceflights with special reference to the unique peculiarities of the gecko model for the orbital experiments. The complex study of thick-toed geckos after three spaceflights, in which all geckos survived and demonstrated effective adaptation to spaceflight conditions, was performed. However, future investigations are needed to study molecular mechanisms of gecko adaptation in space.

Proteomic Analysis of Mouse Brain Subjected to Spaceflight

There is evidence that spaceflight poses acute and late risks to the central nervous system. To explore possible mechanisms, the proteomic changes following spaceflight in mouse brain were characterized. Space Shuttle Atlantis (STS-135) was launched from the Kennedy Space Center (KSC) on a 13-day mission. Within 3–5 h after landing, brain tissue was collected to evaluate protein expression profiles using quantitative proteomic analysis. Our results showed that there were 26 proteins that were significantly altered after spaceflight in the gray and/or white matter. While there was no overlap between the white and gray matter in terms of individual proteins, there was overlap in terms of function, synaptic plasticity, vesical activity, protein/organelle transport, and metabolism. Our data demonstrate that exposure to the spaceflight environment induces significant changes in protein expression related to neuronal structure and metabolic function. This might lead to a significant impact on brain structural and functional integrity that could affect the outcome of space missions.

Central Nervous System Responses to Simulated Galactic Cosmic Rays

In preparation for lunar and Mars missions it is essential to consider the challenges to human health that are posed by long-duration deep space habitation via multiple stressors, including ionizing radiation, gravitational changes during flight and in orbit, other aspects of the space environment such as high level of carbon dioxide, and psychological stress from confined environment and social isolation. It remains unclear how these stressors individually or in combination impact the central nervous system (CNS), presenting potential obstacles for astronauts engaged in deep space travel. Although human spaceflight research only within the last decade has started to include the effects of radiation transmitted by galactic cosmic rays to the CNS, radiation is currently considered to be one of the main stressors for prolonged spaceflight and deep space exploration. Here we will review the current knowledge of CNS damage caused by simulated space radiation with an emphasis on neuronal and glial responses along with cognitive functions. Furthermore, we will present novel experimental approaches to integrate the knowledge into more comprehensive studies, including multiple stressors at once and potential translation to human functions. Finally, we will discuss the need for developing biomarkers as predictors for cognitive decline and therapeutic countermeasures to prevent CNS damage and the loss of cognitive abilities.

Generation of Neural Organoids from Human Embryonic Stem Cells Using the Rotary Cell Culture System: Effects of Microgravity on Neural Progenitor Cell Fate

Progress in aeronautics and spaceflight technologies requires in parallel further research on how microgravity may affect human tissue. To date, little is known about the effects of microgravity on human development. In this study we used the rotary cell culture system to investigate whether microgravity supports the generation and maintenance of neural organoids derived from human embryonic stem cells (hESCs) as a model of human brain development. Our results show that although neural organoids could be generated and maintained in microgravity conditions, there were changes in expression of rostral–caudal neural patterning genes and cortical markers compared to organoids generated in standard conditions. This phenomenon was also observed in hESC-derived cortical organoids exposed to microgravity for relatively shorter periods. These results are one of the first for analyzing human neurogenesis in a microgravity environment.

Effects of gravity changes on gene expression of BDNF and serotonin receptors in the mouse brain

Spaceflight entails various stressful environmental factors including microgravity. The effects of gravity changes have been studied extensively on skeletal, muscular, cardiovascular, immune and vestibular systems, but those on the nervous system are not well studied. The alteration of gravity in ground-based animal experiments is one of the approaches taken to address this issue. Here we investigated the effects of centrifugation-induced gravity changes on gene expression of brain-derived neurotrophic factor (BDNF) and serotonin receptors (5-HTRs) in the mouse brain. Exposure to 2g hypergravity for 14 days showed differential modulation of gene expression depending on regions of the brain. BDNF expression was decreased in the ventral hippocampus and hypothalamus, whereas increased in the cerebellum. 5-HT1BR expression was decreased in the cerebellum, whereas increased in the ventral hippocampus and caudate putamen. In contrast, hypergravity did not affect gene expression of 5-HT1AR, 5-HT2AR, 5-HT2CR, 5-HT4R and 5-HT7R. In addition to hypergravity, decelerating gravity change from 2g hypergravity to 1g normal gravity affected gene expression of BDNF, 5-HT1AR, 5-HT1BR, and 5-HT2AR in various regions of the brain. We also examined involvement of the vestibular organ in the effects of hypergravity. Surgical lesions of the inner ear's vestibular organ removed the effects induced by hypergravity on gene expression, which suggests that the effects of hypergravity are mediated through the vestibular organ. In summary, we showed that gravity changes induced differential modulation of gene expression of BDNF and 5-HTRs (5-HT1AR, 5-HT1BR and 5-HT2AR) in some brain regions. The modulation of gene expression may constitute molecular bases that underlie behavioral alteration induced by gravity changes.

iTRAQ-based proteomics analysis of hippocampus in spatial memory deficiency rats induced by simulated microgravity

It has been demonstrated that simulated microgravity (SM) may lead to cognitive dysfunction. However, the underlying mechanism remains unclear. In present study, tail-suspension (30°) rat was employed to explore the effects of 28 days of SM on hippocampus-dependent learning and memory capability and the underlying mechanisms. We found that 28-day tail-suspension rats displayed decline of learning and memory ability in Morris water maze (MWM) test. Using iTRAQ-based proteomics analysis, a total of 4774 proteins were quantified in hippocampus. Of these identified proteins, 147 proteins were differentially expressed between tail-suspension and control group. Further analysis showed these differentially expressed proteins (DEPs) involved in different molecular function categories, and participated in many biological processes. Based on the results of PANTHER pathway analysis and further western blot verification, we observed the expression of glutamate receptor 1 (GluR1) and glutamate receptor 4 (GluR4) which involved in metabotropic glutamate receptor group III pathway and ionotropic glutamate receptor pathway were significantly induced by SM. Moreover, an increased concentration of glutamic acid (Glu) was also found in hippocampus while the concentrations of 5-hydroxytryptamine (5-HT), dopamine (DA), γ-amino acid butyric acid (GABA) and epinephrine (E) were decreased. Our finding confirms that 28-day SM exposure can cause degrading of the spatial learning and memory capability and the possible mechanisms might be related with glutamate excitotoxicity and imbalances in specific neurotransmitters.

BIOLOGICAL SIGNIFICANCE

The goal of sending astronauts farther into space and extending the duration of spaceflight missions from months to years will challenge the current capabilities of bioastronautics. The investigation of the physiological and pathological changes induced by spaceflight will be critical in developing countermeasures to ensure astronauts to complete spaceflight mission accurately and effectively and return to earth safely. It has been demonstrated that spaceflight may lead to impairments in cognitive function which is crucial for mission success. Here we show that long-term simulated microgravity, the most potent environment risk factor during spaceflight, impairs the spatial learning and memory of rats and the underlying mechanism may be involved in glutamate excitotoxicity and imbalances in specific neurotransmitters release in hippocampus, which may provide new insight for the countermeasures of cognitive impairment during spaceflight.

Negative rebound in hippocampal neurogenesis following exercise cessation

Physical exercise can improve brain function, but the effects of exercise cessation are largely unknown. This study examined the time-course profile of hippocampal neurogenesis following exercise cessation. Male C57BL/6 mice were randomly assigned to either a control (Con) or an exercise cessation (ExC) group. Mice in the ExC group were reared in a cage with a running wheel for 8 wk and subsequently placed in a standard cage to cease the exercise. Exercise resulted in a significant increase in the density of doublecortin (DCX)-positive immature neurons in the dentate gyrus (at week 0). Following exercise cessation, the density of DCX-positive neurons gradually decreased and was significantly lower than that in the Con group at 5 and 8 wk after cessation, indicating that exercise cessation leads to a negative rebound in hippocampal neurogenesis. Immunohistochemistry analysis suggests that the negative rebound in neurogenesis is caused by diminished cell survival, not by suppression of cell proliferation and neural maturation. Neither elevated expression of ΔFosB, a transcription factor involved in neurogenesis regulation, nor increased plasma corticosterone, were involved in the negative neurogenesis rebound. Importantly, exercise cessation suppressed ambulatory activity, and a significant correlation between change in activity and DCX-positive neuron density suggested that the decrease in activity is involved in neurogenesis impairment. Forced treadmill running following exercise cessation failed to prevent the negative neurogenesis rebound. This study indicates that cessation of exercise or a decrease in physical activity is associated with an increased risk for impaired hippocampal function, which might increase vulnerability to stress-induced mood disorders.

Activation of HIF-1α and its downstream targets in rat hippocampus after long-term simulated microgravity exposure

Microgravity has many detrimental impact on brain functions, however the underlying mechanism remain unclear. In present study, 28 days of tail-suspension (30°) was used to simulate microgravity in rats. We showed that oxidative stress in hippocampus was increased after 28 days of simulated microgravity in consideration of the decreased expression of NF-E2-related factor 2 (Nrf2) and the declined activities of total superoxide dismutase (T-SOD), CuZn-SOD, glutathione peroxidase (GSH-PX) and total antioxidant capacity (T-AOC). Using RNA-seq, we further investigated the effect of simulated microgravity on the expression of genes in hippocampus, and 849 genes were found to be differentially expressed. According to pathway analysis, the differentially expressed genes involved in cytoskeleton, metabolism, immunity, transcription regulation, etc. It is interesting to note that the differentially expressed genes were involved in hypoxia-associated pathway. In agreement with this, the expression of hypoxia induced factor-1α (HIF-1α), the master regulator of oxygen homeostasis, was significantly increased. Meanwhile, HIF-2α, a HIF-1α paralog, was elevated compared with the control group. The expression of pyruvate dehydrogenase kinase 1 (PDK1), lactate dehydrogenase A (LDHA) and vascular endothelial growth factor (VEGF), three well-defined downstream targets of HIF-1α, were up-regulated in hippocampus after 28 days of simulated microgravity exposure. Additionally, brain oxygen saturation (SO₂) and blood flow analyzed by the tissue oxygen analysis system were also significantly reduced. These findings indicate that simulated microgravity might cause an alteration in oxygen homeostasis, providing novel insight into better understanding of how simulated microgravity affects the function of hippocampus and a new direction to the development of countermeasure for brain dysfunction during spaceflight (actual microgravity).

Intramuscular injection of mechano growth factor E domain peptide regulated expression of memory-related sod, miR-134 and miR-125b-3p in rat hippocampus under simulated weightlessness

OBJECTIVE

To investigate the expression of memory-related antioxidant genes and miRNAs under simulated weightlessness and the regulation of mechano growth factor (MGF) E domain, the peptide preventing nerve damage.

RESULTS

Igf-iea and mgf mRNA levels, expression of antioxidant genes sod1 and sod2 and levels of miR-134 and miR-125b-3p increased in rat hippocampus after 14 days tail suspension to simulate weightlessness which was inhibited with intramuscular injection of E domain peptide. Therefore, administration of MGF E domain peptide could reverse increased expressions of memory-related igf-iea, mgf, sod1, sod2, miR-134 and miR-125b-3p in rat hippocampus under simulated weightlessness.

CONCLUSIONS

MGF may regulate the redox state and miRNA-targeted NR-CREB signaling, and intramuscular injection may be the alternative administration because of its safety, convenience and ability to pass through the blood brain barrier.

Combined effects of antiorthostatic suspension and ionizing radiation on the behaviour and neurotransmitters changes in different brain structures of rats

Space flight factors (SFF) significantly affect the operating activity of astronauts during deep space missions. In contrast to an orbital flight, leaving the Earth's magnetic field is fraught with the dangers of exposure to ionizing radiation and more specifically, the high-energy nuclei component of galactic cosmic rays. Microgravity, just another critical non-radiation factor, significantly affects the normal functioning of the CNS. Some morphological structures of the brain, such as the prefrontal cortex and the hippocampus, that are rich in monoaminergic and acetylcholinergic neurones, are the most sensitive to the effects of ionizing radiation and non-radiation spaceflight factors (SFF). In this work we have studied the combined effects of microgravity (in antiorthostatic suspension model, AS) and irradiation (γ-ray and protons in spread-out Bragg peak) on the behaviour, cognitive abilities, and metabolism of monoamines and acetylcholine in the key structures of the rat's brain. Irradiation (as independently as combined with AS) resulted in the decrease of thigmotaxis in rats. Learning problems, caused by the malfunctioning of the working memory but not the spatial memory, were observed in response to AS as well as to the SFF in combination. Analysis of monoamines metabolism showed that the serotoninergic system was the most affected by the SFF. Concentration of acetylcholine in the hippocampus significantly increased in the groups of irradiated rats, and in the groups which were exposed to the SFF in combination, compared to the rats exposed only to AS.

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.

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.

Histological and Transcriptomic Analysis of Adult Japanese Medaka Sampled Onboard the International Space Station

To understand how humans adapt to the space environment, many experiments can be conducted on astronauts as they work aboard the Space Shuttle or the International Space Station (ISS). We also need animal experiments that can apply to human models and help prevent or solve the health issues we face in space travel. The Japanese medaka (Oryzias latipes) is a suitable model fish for studying space adaptation as evidenced by adults of the species having mated successfully in space during 15 days of flight during the second International Microgravity Laboratory mission in 1994. The eggs laid by the fish developed normally and hatched as juveniles in space. In 2012, another space experiment (“Medaka Osteoclast”) was conducted. Six-week-old male and female Japanese medaka (Cab strain osteoblast transgenic fish) were maintained in the Aquatic Habitat system for two months in the ISS. Fish of the same strain and age were used as the ground controls. Six fish were fixed with paraformaldehyde or kept in RNA stabilization reagent (n = 4) and dissected for tissue sampling after being returned to the ground, so that several principal investigators working on the project could share samples. Histology indicated no significant changes except in the ovary. However, the RNA-seq analysis of 5345 genes from six tissues revealed highly tissue-specific space responsiveness after a two-month stay in the ISS. Similar responsiveness was observed among the brain and eye, ovary and testis, and the liver and intestine. Among these six tissues, the intestine showed the highest space response with 10 genes categorized as oxidation–reduction processes (gene ontogeny term GO:0055114), and the expression levels of choriogenin precursor genes were suppressed in the ovary. Eleven genes including klf9, klf13, odc1, hsp70 and hif3a were upregulated in more than four of the tissues examined, thus suggesting common immunoregulatory and stress responses during space adaptation.

Skin physiology in microgravity: a 3-month stay aboard ISS induces dermal atrophy and affects cutaneous muscle and hair follicles cycling in mice

AIMS

The Mice Drawer System (MDS) Tissue Sharing program was the longest rodent space mission ever performed. It provided 20 research teams with organs and tissues collected from mice having spent 3 months on the International Space Station (ISS). Our participation to this experiment aimed at investigating the impact of such prolonged exposure to extreme space conditions on mouse skin physiology.

METHODS

Mice were maintained in the MDS for 91 days aboard ISS (space group (S)). Skin specimens were collected shortly after landing for morphometric, biochemical, and transcriptomic analyses. An exact replicate of the experiment in the MDS was performed on ground (ground group (G)).

RESULTS

A significant reduction of dermal thickness (-15%, P=0.05) was observed in S mice accompanied by an increased newly synthetized procollagen (+42%, P=0.03), likely reflecting an increased collagen turnover. Transcriptomic data suggested that the dermal atrophy might be related to an early degradation of defective newly formed procollagen molecules. Interestingly, numerous hair follicles in growing anagen phase were observed in the three S mice, validated by a high expression of specific hair follicles genes, while only one mouse in the G controls showed growing hairs. By microarray analysis of whole thickness skin, we observed a significant modulation of 434 genes in S versus G mice. A large proportion of the upregulated transcripts encoded proteins related to striated muscle homeostasis.

CONCLUSIONS

These data suggest that a prolonged exposure to space conditions may induce skin atrophy, deregulate hair follicle cycle, and markedly affect the transcriptomic repertoire of the cutaneous striated muscle panniculus carnosus.

Effect of actual long-term spaceflight on BDNF, TrkB, p75, BAX and BCL-XL genes expression in mouse brain regions

Mice of C57BL/6J strain were exposed to 1-month spaceflight on Russian biosatellite Bion-M1 to determine the effect of long-term actual spaceflight on the expression of genes involved in the processes of neurogenesis and apoptosis. Specifically, we focused on the genes encoding proapoptotic factor BAX, antiapoptotic factor BCL-XL, brain-derived neurotrophic factor (BDNF) and BDNF receptors TrkB and p75. Spaceflight reduced the expression of the antiapoptotic BCL-XL gene in the striatum and hypothalamus, but increased it in the hippocampus. To estimate environmental stress contribution into spaceflight effects we analyzed spaceflight-responsive genes in mice housed for 1 month on Earth in the same shuttle cabins that were used for spaceflight, and in mice of the laboratory control group. It was shown that 1-month shuttle cabin housing decreased BCL-XL gene expression in the striatum but failed to alter BCL-XL mRNA levels in the hippocampus or hypothalamus. Spaceflight failed to alter the expression of the proapoptotic BAX gene in all investigated brain structures, although the insignificant increase of the BAX mRNA level in the hippocampus of spaceflight mice was found. At the same time, shuttle cabin housing produced insignificant decrease in BAX gene expression in the hippocampus. In contrast to the BCL-XL gene, genes encoding BAX, BDNF as well as TrkB and p75 receptors did not respond to 30-day spaceflight. Thus, long-term spaceflight (1) did not affect the expression of genes encoding BDNF as well as TrkB and p75 receptors, (2) produced dysregulation in genetic control of the neuronal apoptosis, (3) implicated BCL-XL as the risk factor for spaceflight-induced behavioral abnormalities.

The development of vestibular system and related functions in mammals: impact of gravity

This chapter reviews the knowledge about the adaptation to Earth gravity during the development of mammals. The impact of early exposure to altered gravity is evaluated at the level of the functions related to the vestibular system, including postural control, homeostatic regulation, and spatial memory. The hypothesis of critical periods in the adaptation to gravity is discussed. Demonstrating a critical period requires removing the gravity stimulus during delimited time windows, what is impossible to do on Earth surface. The surgical destruction of the vestibular apparatus, and the use of mice strains with defective graviceptors have provided useful information on the consequences of missing gravity perception, and the possible compensatory mechanisms, but transitory suppression of the stimulus can only be operated during spatial flight. The rare studies on rat pups housed on board of space shuttle significantly contributed to this problem, but the use of hypergravity environment, produced by means of chronic centrifugation, is the only available tool when repeated experiments must be carried out on Earth. Even though hypergravity is sometimes considered as a mirror situation to microgravity, the two situations cannot be confused because a gravitational force is still present. The theoretical considerations that validate the paradigm of hypergravity to evaluate critical periods are discussed. The question of adaption of graviceptor is questioned from an evolutionary point of view. It is possible that graviception is hardwired, because life on Earth has evolved under the constant pressure of gravity. The rapid acquisition of motor programming by precocial mammals in minutes after birth is consistent with this hypothesis, but the slow development of motor skills in altricial species and the plasticity of vestibular perception in adults suggest that gravity experience is required for the tuning of graviceptors. The possible reasons for this dichotomy are discussed.

Effects of gravitational loading levels on protein expression related to metabolic and/or morphologic properties of mouse neck muscles

The effects of 3 months of spaceflight (SF), hindlimb suspension, or exposure to 2G on the characteristics of neck muscle in mice were studied. Three 8‐week‐old male C57BL/10J wild‐type mice were exposed to microgravity on the International Space Station in mouse drawer system (MDS) project, although only one mouse returned to the Earth alive. Housing of mice in a small MDS cage (11.6 × 9.8‐cm and 8.4‐cm height) and/or in a regular vivarium cage was also performed as the ground controls. Furthermore, ground‐based hindlimb suspension and 2G exposure by using animal centrifuge (n = 5 each group) were performed. SF‐related shift of fiber phenotype from type I to II and atrophy of type I fibers were noted. Shift of fiber phenotype was related to downregulation of mitochondrial proteins and upregulation of glycolytic proteins, suggesting a shift from oxidative to glycolytic metabolism. The responses of proteins related to calcium handling, myofibrillar structure, and heat stress were also closely related to the shift of muscular properties toward fast‐twitch type. Surprisingly, responses of proteins to 2G exposure and hindlimb suspension were similar to SF, although the shift of fiber types and atrophy were not statistically significant. These phenomena may be related to the behavior of mice that the relaxed posture without lifting their head up was maintained after about 2 weeks. It was suggested that inhibition of normal muscular activities associated with gravitational unloading causes significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Inhibition of gravitational loading in space and on the Earth for 3 months caused similar responses of protein expression in mouse neck muscle. Downregulation of mitochondrial proteins and upregulation of glycolytic proteins were induced, suggesting a shift from oxidative to glycolytic metabolism. Furthermore, the responses of proteins, involved in calcium handling, myofibrillar structure, and heat stress, related to the shift of muscular properties toward fast‐twitch type were also noted. It was suggested that inhibition of normal muscular activities associated with gravitational unloading caused significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

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.