Hindlimb unloading: rodent analog for microgravity

A Surgical Approach to Hindlimb Suspension: A Mouse Model of Disuse-Induced Atrophy

Hindlimb suspension is a well-established rodent model of disuse-induced atrophy and is commonly used to simulate the effects of bed rest and space flight on humans. Over the decades, this method has undergone many changes to reduce the stress response on the animals and improve the reliability of the data. Here, we detail our method of performing hindlimb suspension in mice that minimizes stress, maximizes the replicability of the data, and uses space efficiently.

Fresh Gastrodia elata Blume alleviates simulated weightlessness-induced cognitive impairment by regulating inflammatory and apoptosis-related pathways

In aerospace medicine, the influence of microgravity on cognition has always been a risk factor threatening astronauts' health. The traditional medicinal plant and food material Gastrodia elata Blume has been used as a therapeutic drug for neurological diseases for a long time due to its unique neuroprotective effect. To study the effect of fresh Gastrodia elata Blume (FG) on cognitive impairment caused by microgravity, hindlimb unloading (HU) was used to stimulate weightlessness in mice. The fresh Gastrodia elata Blume (0.5 g/kg or 1.0 g/kg) was intragastrically administered daily to mice exposed to HU and behavioral tests were conducted after four weeks to detect the cognitive status of animals. The behavioral tests results showed that fresh Gastrodia elata Blume therapy significantly improved the performance of mice in the object location recognition test, Step-Down test, and Morris Water Maze test, including short-term and long-term spatial memory. According to the biochemical test results, fresh Gastrodia elata Blume administration not only reduced serum factor levels of oxidative stress but also maintained the balance of pro-inflammatory and anti-inflammatory factors in the hippocampus, reversing the abnormal increase of NLRP3 and NF-κB. The apoptosis-related proteins were downregulated which may be related to the activation of the PI3K/AKT/mTOR pathway by fresh Gastrodia elata Blume therapy, and the abnormal changes of synapse-related protein and glutamate neurotransmitter were corrected. These results identify the improvement effect of fresh Gastrodia elata Blume as a new application form of Gastrodia elata Blume on cognitive impairment caused by simulated weightlessness and advance our understanding of the mechanism of fresh Gastrodia elata Blume on the neuroprotective effect.

Protective effects of ginsenosides Rg1 and Rb1 against cognitive impairment induced by simulated microgravity in rats

Microgravity experienced during space flight is known to exert several negative effects on the learning ability and memory of astronauts. Few effective strategies are currently available to counteract these effects. Rg1 and Rb1, the major steroidal components of ginseng, have shown potent neuroprotective effects with a high safety profile. The present study aimed to investigate the effects of Rg1 and Rb1 on simulated microgravity-induced learning and memory dysfunction and its underlying mechanism in the hindlimb suspension (HLS) rat model. Administration of Rg1 (30 and 60 μmol/kg) and Rb1 (30 and 60 μmol/kg) for 2 weeks resulted in a significant amelioration of impaired spatial and associative learning and memory caused by 4-week HLS exposure, measured using the Morris water maze and Reward operating conditioning reflex (ROCR) tests, respectively. Furthermore, Rg1 and Rb1 administration alleviated reactive oxygen species production and enhanced antioxidant enzyme activities in the prefrontal cortex (PFC). Rg1 and Rb1 also assisted in the recovery of mitochondrial complex I (NADH dehydrogenase) activities, increased the expression of Mfn2 and decreased the fission marker dynamin-related protein (Drp)-1expression. Additionally, Rg1 and Rb1 treatment increased the SYN, and PSD95 protein expressions and decreased the ratio of Bax:Bcl-2 and reduced the expression of cleaved caspase-3 and cytochrome C. Besides these, the BDNF-TrkB/PI3K-Akt pathway was also activated by Rg1 and Rb1 treatment. Altogether, Rg1 and Rb1 treatment attenuated cognitive deficits induced by HLS, mitigated mitochondrial dysfunction, attenuated oxidative stress, inhibited apoptosis, increased synaptic plasticity, and restored BDNF-TrkB/PI3K-Akt signaling.

Global cannabinoid receptor 1 deficiency affects disuse-induced bone loss in a site-specific and sex-dependent manner

Bone loss during mechanical unloading increases fracture risk and is a major concern for the general population and astronauts during spaceflight. The endocannabinoid system (ECS) plays an important role in bone metabolism. One of the main ECS receptors, cannabinoid receptor 1 (CB1), has been studied in regards to basic bone metabolism; however, little is known as to how CB1 and the ECS affect bone in different mechanical environments. In this study, we analyzed the influence of global CB1 deficiency and sex on mice during disuse caused by single limb immobilization. Female mice were more sensitive to disuse-induced BV/TV loss than males in both the femoral metaphysis and tibial epiphysis. Genotype also affected bone loss in a sex-dependent manner, with male mice deficient in CB1 receptors (CB1KO) and female wildtype (WT) mice experiencing increased bone loss in both the tibial metaphysis and femoral epiphysis. Genotype affected the response to disuse as CB1KO mice displayed greater changes in femoral ultimate force, along with lower tibial ultimate stress, compared to WT mice. Female mice had a significantly higher femoral, and lower tibial ultimate force compared to male mice. These results reveal that disuse-induced bone loss due to CB1 deficiency is sex-dependent. CB1 deficiency in male mice exacerbated bone loss, while in females CB1 deficiency appeared to protect against disuse-induced bone loss. Regardless of genotype, female mice were more sensitive than males to disuse. These results suggest that CB1 receptors may represent a potential therapeutic target for mitigation of disuse-induced bone loss.

Long-term simulated microgravity alters gut microbiota and metabolome in mice

Spaceflight and microgravity has a significant impact on the immune, central nervous, bone, and muscle support and cardiovascular systems. However, limited studies are available on the adverse effects of long-term microgravity on the intestinal microbiota, metabolism, and its relationships. In this study, a ground-based simulated microgravity (SMG) mouse model was established to evaluate the impact of long-term microgravity on gut microbiota and metabolome. After 8 weeks of SMG, alterations of the intestinal microbiota and metabolites were detected using 16S rRNA sequencing and untargeted metabolomics. Compared to the control, no significant differences in α-diversity were observed at weeks 2, 4 and 8. Nevertheless, there were clear differences in community structures at different time points. The phylum Verrucomicrobia significantly declined from 2 to 8 weeks of SMG, yet the relative abundance of Actinobacteria and Deferribacteres expanded remarkably at weeks 8. SMG decreased the genus of Allobaculum and increased Bacteroides significantly throughout the period of 8 weeks. Besides, Genus Akkermansia, Gracilibacter, Prevotella, Odoribacter, Rothia, Sporosarcina, Gracilibacter, Clostridium, and Mucispirillum were identified as biomarkers for SMG group. Desulfovibrio_c21_c20, Akkermansia_muciniphila, and Ruminococcus_gnavus dropped at week 2, which tend to recover at week 4, except for Akkermansia_muciniphila. Bacteroides_uniformis and Faecalibacterium_prausnitzii declined significantly, while Ruminococcus_flavefaciens and Mucispirillum_schaedleri elevated at week 8. Furthermore, intestinal metabolome analysis showed that 129 were upregulated and 146 metabolites were downregulated in SMG. Long-term SMG most affected steroid hormone biosynthesis, tryptophan, cysteine, methionine, arginine, proline metabolism, and histidine metabolism. Correlated analysis suggested that the potential beneficial taxa Allobaculum, Akkermansia, and Faecalibacterium were negatively associated with tryptophan, histidine, arginine, and proline metabolism, but positively with steroid hormone biosynthesis. Yet Bacteroides, Lachnospiraceae_Clostridium, Rothia, Bilophila, and Coprococcus were positively correlated with arginine, proline, tryptophan, and histidine metabolism, while negatively associated with steroid hormone biosynthesis. These results suggest that Long-term SMG altered the community of intestinal microbiota, and then further disturbed intestinal metabolites and metabolic pathways, which have great potential to help understand and provide clues for revealing the mechanisms of long-term SMG involved diseases.

Evidence of the static magnetic field effects on bone-related diseases and bone cells

Static magnetic fields (SMFs), magnetic fields with constant intensity and orientation, have been extensively studied in the field of bone biology both fundamentally and clinically as a non-invasive physical factor. A large number of animal experiments and clinical studies have shown that SMFs have effective therapeutic effects on bone-related diseases such as non-healing fractures, bone non-union of bone implants, osteoporosis and osteoarthritis. The maintenance of bone health in adults depends on the basic functions of bone cells, such as bone formation by osteoblasts and bone resorption by osteoclasts. Numerous studies have revealed that SMFs can regulate the proliferation, differentiation, and function of bone tissue cells, including bone marrow mesenchymal stem cells (BMSCs), osteoblasts, bone marrow monocytes (BMMs), osteoclasts, and osteocytes. In this paper, the effects of SMFs on bone-related diseases and bone tissue cells are reviewed from both in vivo studies and in vitro studies, and the possible mechanisms are analyzed. In addition, some challenges that need to be further addressed in the research of SMF and bone are also discussed.

Cold stress during room temperature housing alters skeletal response to simulated microgravity (hindlimb unloading) in growing female C57BL6 mice

Laboratory mice are typically housed at temperatures below the thermoneutral zone for the species, resulting in cold stress and premature cancellous bone loss. Furthermore, mice are more dependent upon non-shivering thermogenesis to maintain body temperature during spaceflight, suggesting that microgravity-induced bone loss may be due, in part, to altered thermogenesis. Consequently, we assessed whether housing mice at room temperature modifies the skeletal response to simulated microgravity. This possibility was tested using the hindlimb unloading (HLU) model to mechanically unload femora. Humeri were also assessed as they remain weight bearing during HLU. Six-week-old female C57BL6 (B6) mice were housed at room temperature (22 °C) or near thermoneutral (32 °C) and HLU for 2 weeks. Compared to baseline, HLU resulted in cortical bone loss in femur, but the magnitude of reduction was greater in mice housed at 22 °C. Cancellous osteopenia in distal femur (metaphysis and epiphysis) was noted in HLU mice housed at both temperatures. However, bone loss occurred at 22 °C, whereas the bone deficit at 32 °C was due to failure to accrue bone. HLU resulted in cortical and cancellous bone deficits (compared to baseline) in humeri of mice housed at 22 °C. In contrast, fewer osteopenic changes were detected in mice housed at 32 °C. These findings support the hypothesis that environmental temperature alters the skeletal response to HLU in growing female mice in a bone compartment-specific manner. Taken together, species differences in thermoregulation should be taken into consideration when interpreting the skeletal response to simulated microgravity.

Zebrafish Models for Skeletal Muscle Senescence: Lessons from Cell Cultures and Rodent Models

Human life expectancy has markedly increased over the past hundred years. Consequently, the percentage of elderly people is increasing. Aging and sarcopenic changes in skeletal muscles not only reduce locomotor activities in elderly people but also increase the chance of trauma, such as bone fractures, and the incidence of other diseases, such as metabolic syndrome, due to reduced physical activity. Exercise therapy is currently the only treatment and prevention approach for skeletal muscle aging. In this review, we aimed to summarize the strategies for modeling skeletal muscle senescence in cell cultures and rodents and provide future perspectives based on zebrafish models. In cell cultures, in addition to myoblast proliferation and myotube differentiation, senescence induction into differentiated myotubes is also promising. In rodents, several models have been reported that reflect the skeletal muscle aging phenotype or parts of it, including the accelerated aging models. Although there are fewer models of skeletal muscle aging in zebrafish than in mice, various models have been reported in recent years with the development of CRISPR/Cas9 technology, and further advancements in the field using zebrafish models are expected in the future.

Hepatology in space: Effects of spaceflight and simulated microgravity on the liver

Microgravity as experienced during spaceflight affects a number of physiological processes in various organs. However, effects on the liver have yet been poorly documented. Nevertheless, the liver is a metabolically highly active organ involved in carbohydrate metabolism, lipid metabolism and xenobiotic biotransformation. The present paper provides an overview of the effects of microgravity on the liver observed in experimental animals during actual spaceflight and upon simulation of microgravity on Earth. These include (i) induction of liver injury and inflammation associated with apoptosis and oxidative stress, (ii) changes in liver carbohydrate metabolism resulting in the onset of a diabetogenic phenotype, (iii) modifications in hepatic lipid metabolism leading to early non-alcoholic fatty liver disease and (iv) alterations of the hepatic xenobiotic biotransformation machinery. Although most of these observations remain to be fully validated in humans, appropriate measures to counteract liver pathogenesis should be considered, especially in view of long-term space missions.

Effect of Microgravity on the Gut Microbiota Bacterial Composition in a Hindlimb Unloading Model

We utilised a ground-based microgravity hindlimb unloading (HU) mouse model to elucidate the gut microbiota bacterial changes in mice under a simulated microgravity environment. Four-month-old, male C57/Bl6 mice were randomly divided into ground-based controls and the HU groups and kept under controlled environmental conditions. For the microgravity environment, the mice were suspended in special cages individually for 20 days. At the end of the suspension, the mice were sacrificed; gut dissections were performed, followed by a metagenomic analysis of bacterial species, which was carried out by extracting DNA and 16S rRNA analysis. The results revealed that the gut bacterial communities of mice under gravity and microgravity were different. Notably, our findings revealed differences in the bacterial community structure. Around 449 bacterial OTUs were specific to mice kept under normal gravity versus 443 bacterial OTUs under microgravity conditions. In contrast, 694 bacterial OTUs were common to both groups. When the relative abundance of taxa was analyzed, Bacteroidetes dominated the gut (64.7%) of normal mice. Conversely, mice in the microgravity environment were dominated by Firmicutes (42.7%), and the relative abundance of Bacteroidetes differed significantly between the two groups (p < 0.05). The distribution of Muribaculaceae between normal mice versus microgravity mice was significantly different, at 62% and 36.4%, respectively (p < 0.05). Furthermore, a significant decrease in 11 bacteria was observed in mice under simulated microgravity, including Akkermansia muciniphila, Eubacterium coprostanoligenes, Bacteroides acidifaciens, Clostridium leptum, Methylorubrum extorquens, Comamonas testosterone, Desulfovibrio fairfieldensis, Bacteroides coprocola, Aerococcus urinaeequi, Helicobacter hepaticus, and Burkholderiales. Further studies are needed to elucidate gut bacterial metabolites of these identified bacterial species in microgravity conditions and normal environment. Notably, the influence of these metabolites on obesity, neuroprotection, musculoskeletal and cardiovascular dysfunction, longevity, inflammation, health, and disease in astronauts ought to be investigated and will be important in developing procedures against adverse effects in astronauts following space travel.

Mitochondria-Targeted Human Catalase in the Mouse Longevity MCAT Model Mitigates Head-Tilt Bedrest-Induced Neuro-Inflammation in the Hippocampus

Microgravity (modeled by head-tilt bedrest and hind-limb unloading), experienced during prolonged spaceflight, results in neurological consequences, central nervous system (CNS) dysfunction, and potentially impairment during the performance of critical tasks. Similar pathologies are observed in bedrest, sedentary lifestyle, and muscle disuse on Earth. In our previous study, we saw that head-tilt bedrest together with social isolation upregulated the milieu of pro-inflammatory cytokines in the hippocampus and plasma. These changes were mitigated in a MCAT mouse model overexpressing human catalase in the mitochondria, pointing out the importance of ROS signaling in this stress response. Here, we used a head-tilt model in socially housed mice to tease out the effects of head-tilt bedrest without isolation. In order to find the underlying molecular mechanisms that provoked the cytokine response, we measured CD68, an indicator of microglial activation in the hippocampus, as well as changes in normal in-cage behavior. We hypothesized that hindlimb unloading (HU) will elicit microglial hippocampal activations, which will be mitigated in the MCAT ROS-quenching mice model. Indeed, we saw an elevation of the activated microglia CD68 marker following HU in the hippocampus, and this pathology was mitigated in MCAT mice. Additionally, we identified cytokines in the hippocampus, which had significant positive correlations with CD68 and negative correlations with exploratory behaviors, indicating a link between neuroinflammation and behavioral consequences. Unveiling a correlation between molecular and behavioral changes could reveal a biomarker indicative of these responses and could also result in a potential target for the treatment and prevention of cognitive changes following long space missions and/or muscle disuse on Earth.

Hindlimb unloading induces time-dependent disruption of testicular histology in mice

Mechanical unloading of the body in the hindlimb unloaded (HU) mice induces pathology in multiple organs, but the effects on testes are poorly characterized. We investigated the histology and Raman spectroscopy of the mouse testes following HU condition. We divided male, c57BL/6j mice into ground-based controls or experimental groups for two and four weeks of HU. The testes tissues were dissected after euthanasia to investigate histological and Raman spectroscopic analysis. We found an HU-induced atrophy of testes irrespective of the time duration (p < 0.05). Histological analysis revealed that the HU induced epithelial thinning, luminal widening, and spermatozoa decline in the seminiferous tubules of the mouse testes. In addition, we found a thickening of the epididymal epithelia and tunica albuginea. These changes were accompanied by a generalized depression in the Raman spectra, indicating an altered concentration and/or orientation of several molecules. We also report reduced signal intensities of hydroxyproline and tryptophan, potentially contributing to testicular pathology during HU. Taken together, our findings indicate that the two or four weeks of HU induce disruption of testicular architecture and molecular phenotypes. Our results may have implications for understanding and/or treating male infertility associated with prolonged bed rest and spaceflight.

Long-duration head-down tilt bed rest confirms the relevance of the neutrophil to lymphocyte ratio and suggests coupling it with the platelet to lymphocyte ratio to monitor the immune health of astronauts

The identification of safe and easily-determined-inflight biomarkers to monitor the immune system of astronauts is mandatory to ensure their well-being and the success of the missions. In this report, we evaluated the relevance of two biomarkers whose determination could be easily implemented in a spacecraft in the near future by using bedridden volunteers as a ground-based model of the microgravity of spaceflight. Our data confirm the relevance of the neutrophil to lymphocyte ratio (NLR) and suggest platelet to lymphocyte ratio (PLR) monitoring to assess long-lasting immune diseases. We recommend coupling these ratios to other biomarkers, such as the quantification of cytokines and viral load measurements, to efficiently detect immune dysfunction, determine when countermeasures should be applied to promote immune recovery, prevent the development of disease, and track responses to treatment.

Perioperative Oral β-Hydroxy-β-Methylbutyrate Supplementation Ameliorates Sarcopenia in Rats Undergoing Major Hepatectomy

β-Hydroxy-β-methylbutyrate (HMB), a metabolite of leucine, is known to increase muscle mass and strength. However, the effect of perioperative HMB supplementation in liver surgery is unclear. Moreover, the impact of HMB on the skeletal muscle fiber type also remains unclear. We investigated the impact of HMB on the body composition and skeletal muscle fiber type in sarcopenic rats undergoing major hepatectomy. Nine-week-old male F344/NSlc rats were maintained in hindlimb suspension (HLS) and were forcedly supplemented with HMB calcium salt (HMB-Ca, 0.58 g/kg×2 times) or distilled water in addition to free feeding. After 2 wk of HLS, the rats underwent 70% hepatectomy and were sacrificed 3 d after surgery. Body composition factors and the proportion of slow-twitch fibers in hindlimb muscles were evaluated. HMB maintained the body composition and hindlimb force and acted against their deterioration in sarcopenic rats, exerting a particular effect on lean mass weight, which was significant. In the histological study, HMB significantly increased the proportion of slow-twitch fibers in the soleus (p=0.044) and plantaris (p=0.001) of sarcopenic rats. HMB ameliorated deterioration of the body composition and increased the proportion of slow-twitch fibers in sarcopenic rats undergoing major hepatectomy.

A new type of simulated partial gravity apparatus for rats based on a pully-spring system

The return to the Moon and the landing on Mars has emphasized the need for greater attention to the effects of partial gravity on human health. Here, we sought to devise a new type of simulated partial gravity apparatus that could more efficiently and accurately provide a partial gravity environment for rat hindlimbs. The new apparatus uses a pulley system and tail suspension to create the simulated partial gravity of the rat’s hind limbs by varying the weight in a balance container attached to the pulley system. An experiment was designed to verify the reliability and stability of the new apparatus. In this experiment, 25 seven-week-old male Wistar Hannover rats were randomly divided into five groups (n = 5 per group): hindlimb full weight-bearing control (1G), sham (1G), and the simulated gravity groups including Mars (3/8G), Moon (1/6G), and interplanetary space (microgravity: µG). The levels of partial gravity experienced by rat hindlimbs in the Mars and Moon groups were provided by a novel simulated partial gravity device. Changes in bone parameters [overall bone mineral density (BMD), trabecular BMD, cortical BMD, cortical bone thickness, minimum moment of area (MMA), and polar moment of area (PMA)] were evaluated using computed tomography in all rats at the proximal, middle, and distal regions of femur and tibia. Reduced gravity led to decreases in bone parameters (overall BMD, trabecular BMD, cortical BMD, MMA, and PMA) in the simulated gravity groups, mainly in distal femur and proximal tibia. The proximal tibia, MMA, and PMA findings indicated greater weakness in the µG group than in the Mars group. The sham group design also excluded the decrease in lower limb bone parameters caused by the suspension attachment of the rat’s tail. The new simulated partial gravity apparatus can provide a continuous and stable level of partial gravity. It offers a reliable and valuable model for studying the effects of extraterrestrial gravity environments on humans.

Real and Simulated Microgravity: Focus on Mammalian Extracellular Matrix

The lack of gravitational loading is a pivotal risk factor during space flights. Biomedical studies indicate that because of the prolonged effect of microgravity, humans experience bone mass loss, muscle atrophy, cardiovascular insufficiency, and sensory motor coordination disorders. These findings demonstrate the essential role of gravity in human health quality. The physiological and pathophysiological mechanisms of an acute response to microgravity at various levels (molecular, cellular, tissue, and physiological) and subsequent adaptation are intensively studied. Under the permanent gravity of the Earth, multicellular organisms have developed a multi-component tissue mechanosensitive system which includes cellular (nucleo- and cytoskeleton) and extracellular (extracellular matrix, ECM) “mechanosensory” elements. These compartments are coordinated due to specialized integrin-based protein complexes, forming a distinctive mechanosensitive unit. Under the lack of continuous gravitational loading, this unit becomes a substrate for adaptation processes, acting as a gravisensitive unit. Since the space flight conditions limit large-scale research in space, simulation models on Earth are of particular importance for elucidating the mechanisms that provide a response to microgravity. This review describes current state of art concerning mammalian ECM as a gravisensitive unit component under real and simulated microgravity and discusses the directions of further research in this field.

Skeletal muscle deconditioning during partial weight-bearing in rodents - A systematic review and meta-analysis

Space agencies are planning to send humans back to the Lunar surface, in preparation for crewed exploration of Mars. However, the effect of hypogravity on human skeletal muscle is largely unknown. A recently established rodent partial weight-bearing model has been employed to mimic various levels of hypogravity loading and may provide valuable insights to better understanding how human muscle might respond to this environment. The aim of this study was to perform a systematic review regarding the effects of partial weight-bearing on the morphology and function of rodent skeletal muscle. Five online databases were searched with the following inclusion criteria: population (rodents), intervention (partial weight-bearing for ≥1 week), control (full weight-bearing), outcome(s) (skeletal muscle morphology/function), and study design (animal intervention). Of the 2,993 studies identified, eight were included. Partial weight-bearing at 20%, 40%, and 70% of full loading caused rapid deconditioning of skeletal muscle morphology and function within the first one to two weeks of exposure. Calf circumference, hindlimb wet muscle mass, myofiber cross-sectional area, front/rear paw grip force, and nerve-stimulated plantarflexion force were reduced typically by medium to very large effects. Higher levels of partial weight-bearing often attenuated deconditioning but failed to entirely prevent it. Species and sex mediated the deconditioning response. Risk of bias was low/unclear for most studies. These findings suggest that there is insufficient stimulus to mitigate muscular deconditioning in hypogravity settings highlighting the need to develop countermeasures for maintaining astronaut/cosmonaut muscular health on the Moon and Mars.

Similarities Between Disuse and Age-Induced Bone Loss

Disuse and aging are known risk factors associated with low bone mass and quality deterioration, resulting in increased fracture risk. Indeed, current and emerging evidence implicate a large number of shared skeletal manifestations between disuse and aging scenarios. This review provides a detailed overview of current preclinical models of musculoskeletal disuse and the clinical scenarios they seek to recapitulate. We also explore and summarize the major similarities between bone loss after extreme disuse and advanced aging at multiple length scales, including at the organ/tissue, cellular, and molecular level. Specifically, shared structural and material alterations of bone loss are presented between disuse and aging, including preferential loss of bone at cancellous sites, cortical thinning, and loss of bone strength due to enhanced fragility. At the cellular level bone loss is accompanied, during disuse and aging, by increased bone resorption, decreased formation, and enhanced adipogenesis due to altered gap junction intercellular communication, WNT/β-catenin and RANKL/OPG signaling. Major differences between extreme short-term disuse and aging are discussed, including anatomical specificity, differences in bone turnover rates, periosteal modeling, and the influence of subject sex and genetic variability. The examination also identifies potential shared mechanisms underlying bone loss in aging and disuse that warrant further study such as collagen cross-linking, advanced glycation end products/receptor for advanced glycation end products (AGE-RAGE) signaling, reactive oxygen species (ROS) and nuclear factor κB (NF-κB) signaling, cellular senescence, and altered lacunar-canalicular connectivity (mechanosensation). Understanding the shared structural alterations, changes in bone cell function, and molecular mechanisms common to both extreme disuse and aging are paramount to discovering therapies to combat both age-related and disuse-induced osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Bone deconditioning during partial weight-bearing in rodents - A systematic review and meta-analysis

Space agencies are preparing to send humans to the Moon (16% Earth's gravity) and Mars (38% Earth's gravity), however, there is limited evidence regarding the effects of hypogravity on the skeletal system. A novel rodent partial weight-bearing (PWB) model may provide insight into how human bone responds to hypogravity. The aim of this study was to perform a systematic review investigating the effect of PWB on the structure and function of rodent bone. Five online databases were searched with the following inclusion criteria: population (rodents), intervention (PWB for ≥1-week), control (full weight-bearing), outcomes (bone structure/function), and study design (animal intervention). Of the 2,993 studies identified, eight were included. The main findings were that partial weight-bearing exposure for 21-28 days at 20%, 40%, and 70% of full loading causes: (1) loss of bone mineral density, (2) loss of trabecular bone volume, thickness, number, and increased separation, (3) loss of cortical area and thickness, and 4) reduced bone stiffness and strength. These findings predominately relate the tibia/femur of young/mature female mice, however, their deconditioning response appeared similar, but not identical, to male rats. A dose-response trend was frequently observed between the magnitude of deconditioning and PWB level. The deconditioning patterns in PWB resembled those in rodents and humans exposed to microgravity and microgravity analogs. The present findings suggest that countermeasures against bone deconditioning may be required for humans exploring the Lunar and Martian surfaces.

Suppression of endoplasmic reticulum stress prevents disuse muscle atrophy in a mouse model of microgravity

BACKGROUND

Hind-limb unloaded (HLU) mouse model exhibits skeletal muscle atrophy and weakness mimicking the conditions such as prolonged spaceflight. However, the molecular mechanisms and interventions of muscle loss during muscle unloading remain elusive. Dysfunction of protein folding by ednoplasmic reticulum (ER), a condition called ER stress, is implicated in diseases of various cell types, but its contribution to skeletal muscle detriment remains elusive. In this study, we investigated the contribution of ER stress to muscle atrophy.

METHODS

Sixteen-week-old c57BL/6j male mice were grouped into ground-based controls and HLU group, which was subsequently injected with injected saline (HLU-sal.) or pan-ER stress inhibitor 4-PBA (100mg/kg/d; HLU- 4PBA) via intraperitoneal injections for three weeks.

RESULTS

Three weeks of HLU resulted in reduction in muscle mass and strength, which were restored with 4PBA injections. We also report myofibers atrophy, myonuclear apoptosis, and aterations in the expressions of genes associated with ER stress, apoptosis, and calcium dysregulation. These findings were reversed by 4-PBA treatment.

CONCLUSION

Altogether, our results indicate that ER stress contributes to muscle atrophy in HLU conditions. We suggest that blocking ER stress may be an effective pharmacological therapy to prevent muscle weakness and atrophy during prolonged muscle unloading.

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.

B-Cell Homeostasis Is Maintained During Two Months of Head-Down Tilt Bed Rest With or Without Antioxidant Supplementation

Alterations of the immune system could seriously impair the ability to combat infections during future long-duration space missions. However, little is known about the effects of spaceflight on the B-cell compartment. Given the limited access to astronaut samples, we addressed this question using blood samples collected from 20 healthy male volunteers subjected to long-duration bed rest, an Earth-based analog of spaceflight. Hematopoietic progenitors, white blood cells, total lymphocytes and B-cells, four B-cell subsets, immunoglobulin isotypes, six cytokines involved in inflammation, cortisone and cortisol were quantified at five time points. Tibia microarchitecture was also studied. Moreover, we investigated the efficiency of antioxidant supplementation with a cocktail including polyphenols, omega 3, vitamin E and selenium. Our results show that circulating hematopoietic progenitors, white blood cells, total lymphocytes and B-cells, and B-cell subsets were not affected by bed rest. Cytokine quantification suggested a lower systemic inflammatory status, supported by an increase in serum cortisone, during bed rest. These data confirm the in vivo hormonal dysregulation of immunity observed in astronauts and show that bed rest does not alter B-cell homeostasis. This lack of an impact of long-term bed rest on B-cell homeostasis can, at least partially, be explained by limited bone remodeling. None of the evaluated parameters were affected by the administration of the antioxidant supplement. The non-effectiveness of the supplement may be because the diet provided to the non-supplemented and supplemented volunteers already contained sufficient antioxidants. Given the limitations of this model, further studies will be required to determine whether B-cell homeostasis is affected, especially during future deep-space exploration missions that will be of unprecedented durations.

Muscle Atrophy Evaluation via Radiomics Analysis using Ultrasound Images: A Cohort Data Study

OBJECTIVE

Existing methods for muscle atrophy evaluation based on muscle size measures from ultrasound images are inadequate in precision. Radiomics has been widely used in various medical studies, but its validity for the evaluation of muscle atrophy has not been fully explored.

METHODS

This study presents a radiomics analysis for muscle atrophy evaluation using ultrasound images. The hindlimb unloading rat model was developed to simulate weightlessness muscle atrophy and ultrasound images of the hind limbs were acquired for both the hindlimb unloaded (HU) and control groups during a 21-day HU period. A total of 368 radiomics features were extracted and the stable and informative features were selected through a two-stage feature selection procedure. The feature change trajectory of the stable features was analyzed using the hierarchical clustering method. Finally, an adaptive longitudinal feature selection and grading network, ALNet, was developed to evaluate muscle atrophy.

RESULTS

The clustering trajectories of ultrasound image features showed similar trends to the changes in muscle atrophy at the molecular level. The best grading accuracy achieved by the ALNet was 79.5% for the Soleus (Sol) muscle and 82.6% for the Gastrocnemius (Gas) muscle.

CONCLUSION

The test-retest is essential in performing radiomics analysis on ultrasound images. The longitudinal feature selection is important for muscle atrophy grading. The ultrasound image features of the Gas muscle have better discrimination ability than that of the Sol muscle. This study proves for the first time the capability of ultrasound image features for muscle atrophy evaluation.

The Epidermis in Microgravity and Unloading Conditions and Their Effects on Wound Healing

The future objectives of human space flight are changing from low-term permanence in the International Space Station to missions beyond low Earth orbit to explore other planets. This implies that astronauts would remain exposed for long time to a micro-gravity environment with limited medical support available. This has sparkled medical research to investigate how tissues may adapt to such conditions and how wound repair may be influenced. This mini-review is focused on the effects of microgravity and unloading conditions on the epidermis and its keratinocytes. Previous studies, originally aimed at improving the in vitro protocols to generate skin substitutes for plastic surgery purposes, showed that epidermal stem cells cultured in simulated microgravity underwent enhanced proliferation and viability and reduced terminal differentiation than under normal gravity. In the meantime, microgravity also triggered epithelial-mesenchymal transition of keratinocytes, promoting a migratory behavior. The molecular mechanisms, only partially understood, involve mechano-trasduction signals and pathways whereby specific target genes are activated, i.e., those presiding to circadian rhythms, migration, and immune suppression, or inhibited, i.e., those involved in stress responses. However, despite the above in vitro studies suggest that microgravity would accelerate keratinocyte growth rate and migration, in vivo findings on animals in experimental set-ups to simulate low gravity rather suggest that prolonged mechanical unloading contributes to delayed and impaired epidermal repair. This is in keeping with the finding that microgravity interferes at multiple levels with the regulatory signals which coordinate the different cell types involved in the repair process, thereby negatively influencing skin wound healing.

Effects of Mechanical Stress Stimulation on Function and Expression Mechanism of Osteoblasts

Osteoclasts and osteoblasts play a major role in bone tissue homeostasis. The homeostasis and integrity of bone tissue are maintained by ensuring a balance between osteoclastic and osteogenic activities. The remodeling of bone tissue is a continuous ongoing process. Osteoclasts mainly play a role in bone resorption, whereas osteoblasts are mainly involved in bone remodeling processes, such as bone cell formation, mineralization, and secretion. These cell types balance and restrict each other to maintain bone tissue metabolism. Bone tissue is very sensitive to mechanical stress stimulation. Unloading and loading of mechanical stress are closely related to the differentiation and formation of osteoclasts and bone resorption function as well as the differentiation and formation of osteoblasts and bone formation function. Consequently, mechanical stress exerts an important influence on the bone microenvironment and bone metabolism. This review focuses on the effects of different forms of mechanical stress stimulation (including gravity, continuously compressive pressure, tensile strain, and fluid shear stress) on osteoclast and osteoblast function and expression mechanism. This article highlights the involvement of osteoclasts and osteoblasts in activating different mechanical transduction pathways and reports changings in their differentiation, formation, and functional mechanism induced by the application of different types of mechanical stress to bone tissue. This review could provide new ideas for further microscopic studies of bone health, disease, and tissue damage reconstruction.

Mouse Digit Tip Regeneration Is Mechanical Load Dependent

Amputation of the mouse digit tip results in blastema-mediated regeneration. In this model, new bone regenerates de novo to lengthen the amputated stump bone, resulting in a functional replacement of the terminal phalangeal element along with associated non-skeletal tissues. Physiological examples of bone repair, such as distraction osteogenesis and fracture repair, are well known to require mechanical loading. However, the role of mechanical loading during mammalian digit tip regeneration is unknown. In this study, we demonstrate that reducing mechanical loading inhibits blastema formation by attenuating bone resorption and wound closure, resulting in the complete inhibition of digit regeneration. Mechanical unloading effects on wound healing and regeneration are completely reversible when mechanical loading is restored. Mechanical unloading after blastema formation results in a reduced rate of de novo bone formation, demonstrating mechanical load dependence of the bone regenerative response. Moreover, enhancing the wound-healing response of mechanically unloaded digits with the cyanoacrylate tissue adhesive Dermabond improves wound closure and partially rescues digit tip regeneration. Taken together, these results demonstrate that mammalian digit tip regeneration is mechanical load-dependent. Given that human fingertip regeneration shares many characteristics with the mouse digit tip, these results identify mechanical load as a previously unappreciated requirement for de novo bone regeneration in humans. © 2021 American Society for Bone and Mineral Research (ASBMR).

Short-Term Reloading After Prolonged Unloading Ensures Restoration of Stromal but Not Hematopoietic Precursor Activity in Tibia Bone Marrow of C57Bl/6N Mice

Bone and muscle tissues are mostly susceptible to different kinds of hypodynamia, including real and simulated microgravity (sμg). To evaluate the effect of sμg on bone marrow (BM), male C57Bl/6N mice were divided into three groups: vivarium control (VC), 30-day hindlimb suspension (HS), and subsequent 12-h short-term support reloading (RL). The effects on BM total mononucleated cells (MNCs) as well as stromal and hematopoietic progenitors from murine tibia were studied. The number of BM MNCs, immunophenotype, proliferation, colony-forming units (CFUs), differentiation and secretory activity of hematopoietic and stromal BM cells were determined. HS led to a twofold decrease in MNCs, alteration of surface molecule expression profiles, suppression of proliferative activity of BM cells, and change of soluble mediators' levels. The stromal compartment was characterized by a decrease of CFU of fibroblasts and suppression of spontaneous osteo-commitment after HS. Among the hematopoietic precursors, a decrease in the total number of CFUs was found mainly at the expense of suppression of CFU-GM and CFU-GEMM. After RL, restoration of the stromal precursor's functional activity to control levels and overabundance of paracrine mediator's production were detected, whereas the complete recovery of hematopoietic precursor's activity did not occur. These data demonstrate the fast functional reaction of the stromal compartment on restoration of loading support.

Simulated microgravity accelerates aging of human skeletal muscle myoblasts at the single cell level

Earth's gravity is essential for maintaining skeletal muscle mass and function in the body. The role of gravity in the myogenic mechanism has been studied with animal experiments in the International Space Station. Recently, gravity-control devices allow to study the effects of gravity on cultured cells on the ground. This study demonstrated that simulated microgravity accelerated aging of human skeletal muscle myoblasts in an in-vitro culture. The microgravity culture induced a significant decrease in cell proliferation and an enlargement of the cytoskeleton and nucleus of cells. Similar changes are often observed in aged myoblasts following several passages. In fact, by the microgravity culture the expression of senescence associated β-Gal was significantly enhanced, and some muscle-specific proteins decreased in the enlarged cells. Importantly, these microgravity effects remained with the cells even after a return to normal gravity conditions. Consequently, the microgravity-affected myoblasts demonstrated a reduced capability of differentiation into myotubes. In the body, it is difficult to interpret the disability of microgravity-affected myoblasts, since muscle regeneration is linked to the supply of new myogenic cells. Therefore, our in-vitro cell culture study will be advantageous to better understand the role of each type of myogenic cell in human muscle without gravitational stress at the single cell level.

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.

The Skeletal Cellular and Molecular Underpinning of the Murine Hindlimb Unloading Model

Bone adaptation to spaceflight results in bone loss at weight bearing sites following the absence of the stimulus represented by ground force. The rodent hindlimb unloading model was designed to mimic the loss of mechanical loading experienced by astronauts in spaceflight to better understand the mechanisms causing this disuse-induced bone loss. The model has also been largely adopted to study disuse osteopenia and therefore to test drugs for its treatment. Loss of trabecular and cortical bone is observed in long bones of hindlimbs in tail-suspended rodents. Over the years, osteocytes have been shown to play a key role in sensing mechanical stress/stimulus via the ECM-integrin-cytoskeletal axis and to respond to it by regulating different cytokines such as SOST and RANKL. Colder experimental environments (~20-22°C) below thermoneutral temperatures (~28-32°C) exacerbate bone loss. Hence, it is important to consider the role of environmental temperatures on the experimental outcomes. We provide insights into the cellular and molecular pathways that have been shown to play a role in the hindlimb unloading and recommendations to minimize the effects of conditions that we refer to as confounding factors.

Simulated Microgravity Subtlety Changes Monoamine Function across the Rat Brain

Microgravity, one of the conditions faced by astronauts during spaceflights, triggers brain adaptive responses that could have noxious consequences on behaviors. Although monoaminergic systems, which include noradrenaline (NA), dopamine (DA), and serotonin (5-HT), are widespread neuromodulatory systems involved in adaptive behaviors, the influence of microgravity on these systems is poorly documented. Using a model of simulated microgravity (SMG) during a short period in Long Evans male rats, we studied the distribution of monoamines in thirty brain regions belonging to vegetative, mood, motor, and cognitive networks. SMG modified NA and/or DA tissue contents along some brain regions belonging to the vestibular/motor systems (inferior olive, red nucleus, cerebellum, somatosensorily cortex, substantia nigra, and shell of the nucleus accumbens). DA and 5-HT contents were reduced in the prelimbic cortex, the only brain area exhibiting changes for 5-HT content. However, the number of correlations of one index of the 5-HT metabolism (ratio of metabolite and 5-HT) alone or in interaction with the DA metabolism was dramatically increased between brain regions. It is suggested that SMG, by mobilizing vestibular/motor systems, promotes in these systems early, restricted changes of NA and DA functions that are associated with a high reorganization of monoaminergic systems, notably 5-HT.

Otoconia Structure After Short- and Long-Duration Exposure to Altered Gravity

Vertebrates use weight-lending otoconia in the inner ear otolith organs to enable detection of their translation during self or imposed movements and a change in their orientation with respect to gravity. In spaceflight, otoconia are near weightless. It has been hypothesized that otoconia undergo structural remodeling after exposure to weightlessness to restore normal sensation. A structural remodeling is reasoned to occur for hypergravity but in the opposite sense. We explored these hypotheses in several strains of mice within a Biospecimen Sharing Program in separate space- and ground-based projects. Mice were housed 90 days on the International Space Station, 13 days on two Shuttle Orbiter missions, or exposed to 90 days of hindlimb unloading or net 2.38 g via centrifugation. Corresponding flight habitat and standard cage vivarium controls were used. Utricular otoliths were visually analyzed using scanning electron microscopy and in selected samples before and after focused ion beam (FIB) milling. Results suggest a possible mass addition to the otoconia outer shell might occur after exposure to longer-duration spaceflight, but not short ones or hindlimb unloading. A destructive process is clearly seen after centrifugation: an ablation or thinning of the outer shell and cavitation of the inner core. This study provides a purely descriptive account of otoconia remodeling after exposures to altered gravity. The mechanism(s) underlying these processes must be identified and quantitatively validated to develop countermeasures to altered gravity levels during exploration missions.

An Analysis of the Effects of Spaceflight and Vaccination on Antibody Repertoire Diversity

Ab repertoire diversity plays a critical role in the host's ability to fight pathogens. CDR3 is partially responsible for Ab-Ag binding and is a significant source of diversity in the repertoire. CDR3 diversity is generated during VDJ rearrangement because of gene segment selection, gene segment trimming and splicing, and the addition of nucleotides. We analyzed the Ab repertoire diversity across multiple experiments examining the effects of spaceflight on the Ab repertoire after vaccination. Five datasets from four experiments were analyzed using rank-abundance curves and Shannon indices as measures of diversity. We discovered a trend toward lower diversity as a result of spaceflight but did not find the same decrease in our physiological model of microgravity in either the spleen or bone marrow. However, the bone marrow repertoire showed a reduction in diversity after vaccination. We also detected differences in Shannon indices between experiments and tissues. We did not detect a pattern of CDR3 usage across the experiments. Overall, we were able to find differences in the Ab repertoire diversity across experimental groups and tissues.

Meta-Analysis-Assisted Detection of Gravity-Sensitive Genes in Human Vascular Endothelial Cells

Gravity affects the function and maintenance of organs, such as bones, muscles, and the heart. Several studies have used DNA microarrays to identify genes with altered expressions in response to gravity. However, it is technically challenging to combine the results from various microarray datasets because of their different data structures. We hypothesized that it is possible to identify common changes in gene expression from the DNA microarray datasets obtained under various conditions and methods. In this study, we grouped homologous genes to perform a meta-analysis of multiple vascular endothelial cell and skeletal muscle datasets. According to the t-distributed stochastic neighbor embedding (t-SNE) analysis, the changes in the gene expression pattern in vascular endothelial cells formed specific clusters. We also identified candidate genes in endothelial cells that responded to gravity. Further, we exposed human umbilical vein endothelial cells (HUVEC) to simulated microgravity (SMG) using a clinostat and measured the expression levels of the candidate genes. Gene expression analysis using qRT-PCR revealed that the expression level of the prostaglandin (PG) transporter gene SLCO2A1 decreased in response to microgravity, consistent with the meta-analysis of microarray datasets. Furthermore, the direction of gravity affected the expression level of SLCO2A1, buttressing the finding that its expression was affected by gravity. These results suggest that a meta-analysis of DNA microarray datasets may help identify new target genes previously overlooked in individual microarray analyses.

Tail suspension delays ectopic ossification in proteoglycan-induced ankylosing spondylitis in mice via miR-103/DKK1

Ankylosing spondylitis (AS), characterized by inflammatory lesions and osteophyte formation, is a common immune rheumatic disease affecting the sacroiliac and axial joints. A high-intensity mechanical load is known to accelerate the heterotopic ossification associated with enthesitis in AS. Thus, the present study explored whether decreased mechanical load could delay the heterotopic ossification in AS. First, 24-week-old female BALB/c mice were induced with proteoglycan (PG) to establish an AS model. The AS-induced pathological and bone morphological changes of the sacroiliac joint were confirmed by hematoxylin and eosin staining and microCT analysis, respectively. Subsequently, the mice were treated with interventions of different mechanical loads. Using reverse transcription-quantitative PCR, it was revealed that expression levels of the osteogenesis-related genes bone morphogenetic protein-2, runt-related transcription factor 2 and osteocalcin were significantly reduced in sacroiliac bone tissue after intervention with a reduced mechanical load. The level of mechanosensory microRNA (miR)-103 increased in response to reduced mechanical loads. Consistently, in groups with reduced mechanical load, proteins with mechanical functions, including ρ-associated coiled-coil-containing protein kinase 1 (ROCK1), phosphorylated (p)-Erk1/2 and β-catenin, were reduced compared with the PG control. A dual-luciferase assay verified that miR-103 binds to the 3'-untranslated region end of Rock1 mRNA, thus negatively regulating the activity of Rock1 and affecting pathological ossification during AS. However, immunohistochemical staining indicated that the expression of dickkopf Wnt signaling pathway inhibitor 1, an inhibitor of the Wnt/β-catenin pathway, was increased in sacroiliac tissues. The results indicated that tail suspension decreased the mechanical load, thus reducing the bone formation in AS mice. Furthermore, tail suspension could inhibit the activation of mechanical kinase ROCK1 and p-Erk1/2 in the MAPK signaling pathway by upregulating miR-103, thereby inhibiting the classical osteogenesis-related Wnt/β-catenin pathway in AS. In summary, the present study uncovered the ameliorative effect of suspension on AS and its therapeutic potential for AS.

Metabolic Dynamics in Short- and Long-Term Microgravity in Human Primary Macrophages

Microgravity acts on cellular systems on several levels. Cells of the immune system especially react rapidly to changes in gravity. In this study, we performed a correlative metabolomics analysis on short-term and long-term microgravity effects on primary human macrophages. We could detect an increased amino acid concentration after five minutes of altered gravity, that was inverted after 11 days of microgravity. The amino acids that reacted the most to changes in gravity were tightly clustered. The observed effects indicated protein degradation processes in microgravity. Further, glucogenic and ketogenic amino acids were further degraded to Glucose and Ketoleucine. The latter is robustly accumulated in short-term and long-term microgravity but not in hypergravity. We detected highly dynamic and also robust adaptative metabolic changes in altered gravity. Metabolomic studies could contribute significantly to the understanding of gravity-induced integrative effects in human cells.

Genome-Wide Atlas of Promoter Expression Reveals Contribution of Transcribed Regulatory Elements to Genetic Control of Disuse-Mediated Atrophy of Skeletal Muscle

Simple Summary

The genetic process underlying the control of skeletal muscle homeostasis is a key factor in methods that develop technologies to prevent age and immobility-driven atrophy. In the current paper, using advanced methods for the whole-genome profiling of transcription starting sites in fast and slow muscle in rats, we developed an integrative database of transcribed regulatory elements. Employing methods of comparative transcriptomics, we demonstrate that cis-regulatory elements are actively involved in the control of atrophy and recovery, and that the differential use of promoters and enhancers is the one of the key mechanisms that distinguishes between specific processes in slow and fast skeletal muscles.

Abstract

The prevention of muscle atrophy carries with it clinical significance for the control of increased morbidity and mortality following physical inactivity. While major transcriptional events associated with muscle atrophy-recovery processes are the subject of active research on the gene level, the contribution of non-coding regulatory elements and alternative promoter usage is a major source for both the production of alternative protein products and new insights into the activity of transcription factors. We used the cap-analysis of gene expression (CAGE) to create a genome-wide atlas of promoter-level transcription in fast (m. EDL) and slow (m. soleus) muscles in rats that were subjected to hindlimb unloading and subsequent recovery. We found that the genetic regulation of the atrophy-recovery cycle in two types of muscle is mediated by different pathways, including a unique set of non-coding transcribed regulatory elements. We showed that the activation of “shadow” enhancers is tightly linked to specific stages of atrophy and recovery dynamics, with the largest number of specific regulatory elements being transcriptionally active in the muscles on the first day of recovery after a week of disuse. The developed comprehensive database of transcription of regulatory elements will further stimulate research on the gene regulation of muscle homeostasis in mammals.

The effects of combined environmental factors on the intestinal flora of mice based on ground simulation experiments

The composition and function of intestinal microbial communities are important for human health. However, these intestinal floras are sensitive to changes in the environment. Adverse changes to intestinal flora can affect the health of astronauts, resulting in difficulties in implementing space missions. We randomly divided mice into three groups and placed each group in either a normal environment, simulated microgravity environment or a combined effects environment, which included simulated microgravity, low pressure and noise. Fecal samples of the mice were collected for follow-up analysis based on metagenomics technology. With the influence of different space environmental factors, the species composition at the phylum and genus levels were significantly affected by the combined effects environment, especially the abundance of the Firmicutes and Bacteroidetes. Furthermore, screening was conducted to identify biomarkers that could be regarded as environmental markers. And there have also been some noticeable changes in the function of intestinal floras. Moreover, the abundance of antibiotic resistance genes (ARGs) was also found to be changed under different environmental conditions, such as bacitracin and vancomycin. The combined effects environment could significantly affect the species composition, function, and the expression of ARGs of intestinal flora of mice which may provide a theoretical basis for space medical supervision and healthcare.

The effects of biophysical stimulation on osteogenic differentiation and the mechanisms from ncRNAs

Ample proof showed that non-coding RNAs (ncRNAs) play a crucial role in proliferation and differentiation of osteoblasts and bone marrow stromal cells (BMSCs). Varied forms of biophysical stimuli like mechanical strain, fluid shear stress (FSS), microgravity and vibration are verified to regulate ncRNAs expression in osteogenic differentiation and influence the expression of target genes associated with osteogenic differentiation and ultimately regulate bone formation. The consequences of biophysical stimulation on osteogenic differentiation validate the prospect of exercise for the prevention and treatment of osteoporosis. In this review, we tend to summarize the studies on regulation of osteogenic differentiation by ncRNAs beneath biophysical stimulation and facilitate to reveal the regulatory mechanism of biophysical stimulation on ncRNAs, and provide an update for the prevention of bone metabolism diseases by exercise.

Simulated weightlessness induces cognitive changes in rats illustrated by performance in operant conditioning tasks

The weightless environment encountered in space flight can cause cognitive changes, affecting mission execution and endanger mission safety. Ground simulations provide the means to evaluate these effects and the resulting risks. In this study, rats were used as model animals and subjected to simulated weightlessness by suspending them from their tails. Tail-suspension and operant task learning experiments were conducted to study the effects of simulated weightlessness on the complex operant conditioning abilities of the subject's acquisition, maintenance, and signal discrimination skills. The results showed that simulated weightlessness did not affect the animals' acquisition abilities but did affect their ability to maintain learned reflexes and recognize signals. This study may have general potential to research the effects of weightlessness on cognition in the space environment.

Placenta-Expanded Stromal Cell Therapy in a Rodent Model of Simulated Weightlessness

Long duration spaceflight poses potential health risks to astronauts during flight and re-adaptation after return to Earth. There is an emerging need for NASA to provide successful and reliable therapeutics for long duration missions when capability for medical intervention will be limited. Clinically relevant, human placenta-derived therapeutic stromal cells (PLX-PAD) are a promising therapeutic alternative. We found that treatment of adult female mice with PLX-PAD near the onset of simulated weightlessness by hindlimb unloading (HU, 30 d) was well-tolerated and partially mitigated decrements caused by HU. Specifically, PLX-PAD treatment rescued HU-induced thymic atrophy, and mitigated HU-induced changes in percentages of circulating neutrophils, but did not rescue changes in the percentages of lymphocytes, monocytes, natural killer (NK) cells, T-cells and splenic atrophy. Further, PLX-PAD partially mitigated HU effects on the expression of select cytokines in the hippocampus. In contrast, PLX-PAD failed to protect bone and muscle from HU-induced effects, suggesting that the mechanisms which regulate the structure of these mechanosensitive tissues in response to disuse are discrete from those that regulate the immune- and central nervous system (CNS). These findings support the therapeutic potential of placenta-derived stromal cells for select physiological deficits during simulated spaceflight. Multiple countermeasures are likely needed for comprehensive protection from the deleterious effects of prolonged spaceflight.

Analysis of the effects of spaceflight and local administration of thrombopoietin to a femoral defect injury on distal skeletal sites

With increased human presence in space, bone loss and fractures will occur. Thrombopoietin (TPO) is a recently patented bone healing agent. Here, we investigated the systemic effects of TPO on mice subjected to spaceflight and sustaining a bone fracture. Forty, 9-week-old, male, C57BL/6 J were divided into 4 groups: (1) Saline+Earth; (2) TPO + Earth; (3) Saline+Flight; and (4) TPO + Flight (n = 10/group). Saline- and TPO-treated mice underwent a femoral defect surgery, and 20 mice were housed in space ("Flight") and 20 mice on Earth for approximately 4 weeks. With the exception of the calvarium and incisor, positive changes were observed in TPO-treated, spaceflight bones, suggesting TPO may improve osteogenesis in the absence of mechanical loading. Thus, TPO, may serve as a new bone healing agent, and may also improve some skeletal properties of astronauts, which might be extrapolated for patients on Earth with restraint mobilization and/or are incapable of bearing weight on their bones.

Gravitational Experimental Platform for Animal Models, a New Platform at ESA's Terrestrial Facilities to Study the Effects of Micro- and Hypergravity on Aquatic and Rodent Animal Models

Using rotors to expose animals to different levels of hypergravity is an efficient means of understanding how altered gravity affects physiological functions, interactions between physiological systems and animal development. Furthermore, rotors can be used to prepare space experiments, e.g., conducting hypergravity experiments to demonstrate the feasibility of a study before its implementation and to complement inflight experiments by comparing the effects of micro- and hypergravity. In this paper, we present a new platform called the Gravitational Experimental Platform for Animal Models (GEPAM), which has been part of European Space Agency (ESA)'s portfolio of ground-based facilities since 2020, to study the effects of altered gravity on aquatic animal models (amphibian embryos/tadpoles) and mice. This platform comprises rotors for hypergravity exposure (three aquatic rotors and one rodent rotor) and models to simulate microgravity (cages for mouse hindlimb unloading and a random positioning machine (RPM)). Four species of amphibians can be used at present. All murine strains can be used and are maintained in a specific pathogen-free area. This platform is surrounded by numerous facilities for sample preparation and analysis using state-of-the-art techniques. Finally, we illustrate how GEPAM can contribute to the understanding of molecular and cellular mechanisms and the identification of countermeasures.

Hindlimb unloading-induced reproductive suppression via Downregulation of hypothalamic Kiss-1 expression in adult male rats

BACKGROUND

Spaceflights-induced microgravity can alter various physiological processes in human's body including the functional status of the reproductive system. Rodent model of tail-suspension hindlimb unloading is extensively used to stimulate the organs responses to the microgravity condition. This study explores the potential effects of hindlimb unloading on testicular functions and spermatogenesis in adult male rats and the underlying mechanism/s.

METHODS

Twenty Sprague-Dawley rats were allotted into two groups: normally loaded group (control; all arms were in touch with the grid floor) and hindlimb unloaded group (HU; only the forearms were in contact with the grid floor).

RESULTS

Following 30 days of exposure, the HU group saw a decline in body weight, testicular and epidydimal weights, and all semen parameters. The circulating concentrations of gonadotropin-releasing hormone (GnRH), follicle stimulating hormone (FSH), luteinizing hormone (LH) and testosterone significantly decreased, while levels of kisspeptin, corticosterone, inhibin, prolactin and estradiol (E2) increased in the HU group. Intratesticular levels of 5α-reductase enzyme and dihydrotestosterone (DHT) were suppressed, while the levels of aromatase and kisspeptin were significantly elevated in the HU group. Hypothalamic kisspeptin (Kiss1) mRNA expression levels were downregulated while its receptors (Kiss1R) were upregulated in the HU group. On the contrary, the mRNA expression levels of testicular Kiss1 were upregulated while Kiss1R were downregulated. The pituitary mRNA expression levels of FSHβ and LHβ decreased in the HU group. The levels of the antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and nitric oxide (NO) concentrations, and total antioxidant capacity (TAC) were elevated while malondialdehyde (MDA) concentrations declined in the testes of HU group. The testes of the HU rats showed positive immunostaining of caspase-3, heat shock protein 70 (HSP70) and Bcl2.

CONCLUSIONS

Altogether, these results revealed an inhibitory effect of hindlimb unloading on kisspeptin signaling in the hypothalamic-pituitary-testicular axis with impaired spermatogenesis and steroidogenesis.

Therapeutic potential of iron chelators on osteoporosis and their cellular mechanisms

Iron is an essential trace element in the metabolism of almost all living organisms. Iron overload can disrupt bone homeostasis by significant inhibition of osteogenic differentiation and stimulation of osteoclastogenesis, consequently leading to osteoporosis. Iron accumulation is also involved in the osteoporosis induced by multiple factors, such as estrogen deficiency, ionizing radiation, and mechanical unloading. Iron chelators are first developed for treating iron overloaded disorders. However, growing evidence suggests that iron chelators can be potentially used for the treatment of bone loss. In this review, we focus on the therapeutic effects of iron chelators on bone loss. Iron chelators have therapeutic effects not only on iron overload induced osteoporosis, but also on osteoporosis induced by estrogen deficiency, ionizing radiation, and mechanical unloading, and in Alzheimer's disease-associated osteoporotic deficits. Iron chelators differently affect the cellular behaviors of bone cells. For osteoblast lineage cells (bone mesenchymal stem cells and osteoblasts), iron chelation stimulates osteogenic differentiation. Conversely, iron chelation significantly inhibits osteoclast differentiation. These different responses may be associated with the different needs of iron during differentiation. Fibroblast growth factor 23, angiogenesis, and antioxidant capability are also involved in the osteoprotective effects of iron chelators.

Simultaneous exposure to chronic irradiation and simulated microgravity differentially alters immune cell phenotype in mouse thymus and spleen

Deep-space missions may alter immune cell phenotype in the primary (e.g., thymus) and secondary (e.g., spleen) lymphoid organs contributing to the progression of a variety of diseases. In deep space missions, astronauts will be exposed to chronic low doses of HZE radiation while being in microgravity. Ground-based models of long-term uninterrupted exposures to HZE radiation are not yet available. To obtain insight in the effects of concurrent exposure to microgravity and chronic irradiation (CIR), mice received a cumulative dose of chronic 0.5 Gy gamma rays over one month ± simulated microgravity (SMG). To obtain insight in a dose rate effect, additional mice were exposed to single acute irradiation (AIR) at 0.5 Gy gamma rays. We measured proportions of immune cells relative to total number of live cells in the thymus and spleen, stress level markers in plasma, and change in body weight, food consumption, and water intake. CIR affected thymic CD3+/CD335+ natural killer T (NK-T) cells, CD25+ regulatory T (Treg) cells, CD27+/CD335- natural killer (NK1) cells and CD11c+/CD11b- dendritic cells (DCs) differently in mice subjected to SMG than in mice with normal loading. No such effects of CIR on SMG as compared to normal loading were observed in cell types from the spleen. Differences between CIR and AIR groups (both under normal loading) were found in thymic Treg and DCs. Food consumption, water intake, and body weight were less after coexposure than singular or no exposure. Compared to sham, all treatment groups exhibited elevated plasma levels of the stress marker catecholamines. These data suggest that microgravity and chronic irradiation may interact with each other to alter immune cell phenotypes in an organ-specific manner and appropriate strategies are required to reduce the health risk of crewmembers.

RNA sequencing (RNA-seq) analysis of gene expression provides new insights into hindlimb unloading-induced skeletal muscle atrophy

Background

Weightlessness-induced skeletal muscle atrophy, accompanied by complex biochemical and physiological changes, has potentially damaged consequences. However, there is still an insufficient effective strategy to treat skeletal muscle atrophy. Therefore, exploring the molecular mechanisms regulating skeletal muscle atrophy and effective protection is necessary.

Methods

RNA sequencing (RNA-seq) analysis was used to detect differentially expressed genes (DEGs) in the soleus muscle at 12, 24, 36 hours, three days, and seven days after hindlimb unloading in rats. Pearson correlation heatmaps and principal component analysis (PCA) were applied to analyze DEGs’ expression profiles. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were used for cluster analysis of DEGs. Ingenuity pathway analysis (IPA) was used to analyze specific biological processes further.

Results

At different time points (12, 24, 36 hours, three days, seven days) after hindlimb unloading, the expression levels of 712, 1,109, 1,433, 1,162, and 1,182 genes in rat soleus muscle were upregulated, respectively, whereas the expression levels of 1,186, 1,324, 1,632, 1,446, and 1,596 genes were downregulated, respectively. PCA revealed that rat soleus muscle showed three different transcriptional phases within seven days after hindlimb unloading. KEGG and GO annotation indicated that the first transcriptional phase primarily involved the activation of stress responses, including oxidative stress, and the inhibition of cell proliferation and angiogenesis; the second transcriptional phase primarily involved the activation of proteolytic systems and, to a certain degree, inflammatory responses; and the third transcriptional phase primarily involved extensive activation of the proteolytic system, significant inhibition of energy metabolism, and activation of the aging process and slow-to-fast muscle conversion.

Conclusions

Different physiological processes in rat skeletal muscles were activated sequentially after unloading. From these activated biological processes, the three transcriptional phases after skeletal muscle unloading can be successively defined as the stress response phase, the atrophic initiation phase, and the atrophic phase. Our study not only helps in the understanding of the molecular mechanisms underlying weightlessness-induced muscle atrophy but may also provide an important time window for the treatment and prevention of weightlessness-induced muscle atrophy.

Tail suspension is useful as a sarcopenia model in rats

Background

Sarcopenia promotes skeletal muscle atrophy and exhibits a high mortality rate. Its elucidation is of the highest clinical importance, but an animal experimental model remains controversial. In this study, we investigated a simple method for studying sarcopenia in rats.

Results

Muscle atrophy was investigated in 24-week-old, male, tail-suspended (TS), Sprague Dawley and spontaneously hypertensive rats (SHR). Age-matched SD rats were used as a control group. The skeletal muscle mass weight, muscle contraction, whole body tension (WBT), cross-sectional area (CSA), and Muscle RING finger-1 (MuRF-1) were assessed. Enzyme-linked immunosorbent assay was used to evaluate the MuRF-1 levels. Two muscles, the extensor digitorum longus and soleus muscles, were selected for representing fast and slow muscles, respectively. All data, except CSA, were analyzed by a one-way analysis of variance, whereas CSA was analyzed using the Kruskal-Wallis test.

Muscle mass weight, muscle contraction, WBT, and CSA were significantly lower in the SHR (n = 7) and TS (n = 7) groups than in the control group, whereas MuRF-1 expression was dominant.

Conclusions

TS and SHR presented sarcopenic phenotypes in terms of muscle mass, muscle contraction and CSA. TS is a useful technique for providing muscle mass atrophy and weakness in an experimental model of sarcopenia in rats.

Nox2 signaling and muscle fiber remodeling are attenuated by losartan administration during skeletal muscle unloading

Reduced mechanical loading results in atrophy of skeletal muscle fibers. Increased reactive oxygen species (ROS) are causal in sarcolemmal dislocation of nNOS and FoxO3a activation. The Nox2 isoform of NADPH oxidase and mitochondria release ROS during disuse in skeletal muscle. Activation of the angiotensin II type 1 receptor (AT1R) can elicit Nox2 complex formation. The AT1R blocker losartan was used to test the hypothesis that AT1R activation drives Nox2 assembly, nNOS dislocation, FoxO3a activation, and thus alterations in morphology in the unloaded rat soleus. Male Fischer 344 rats were divided into four groups: ambulatory control (CON), ambulatory + losartan (40 mg kg^-1  day^-1 ) (CONL), 7 days of tail-traction hindlimb unloading (HU), and HU + losartan (HUL). Losartan attenuated unloading-induced loss of muscle fiber cross-sectional area (CSA) and fiber-type shift. Losartan mitigated unloading-induced elevation of ROS levels and upregulation of Nox2. Furthermore, AT1R blockade abrogated nNOS dislocation away from the sarcolemma and elevation of nuclear FoxO3a. We conclude that AT1R blockade attenuates disuse remodeling by inhibiting Nox2, thereby lessening nNOS dislocation and activation of FoxO3a.