Effects of microgravity on osteoclast bone resorption and osteoblast cytoskeletal organization and adhesion

Icariin promotes cell adhesion for osteogenesis in bone marrow stromal cells via binding to integrin α5β1

BACKGROUND AND PURPOSE

Icariin, an 8-prenylated flavonoid glycoside, is an anabolic agent that could exert rapid estrogenic actions via ligand-independent activation of estrogen receptor alpha (ERα) in osteoblastic cells to promote osteogenesis. However, relatively little is known about its direct cellular target, its protective effects, and cell adhesion activities in bone marrow stromal cells (BMSCs) against microgravity. In the present study, the effects of icariin on osteogenesis and cell adhesion under microgravity were examined with the involvement of integrin receptor α5β1, connexin 43, and CAMs.

STUDY DESIGN AND METHODS

Icariin was orally administered to 6-month-old ovariectomized (OVX) Sprague-Dawley (SD) rats for 3 months through daily intake of phytoestrogen-free rodent diets containing icariin at 2 different dosages (50 and 500 ppm). BMSCs were harvested for experiments and RNA-sequencing analysis to examine the mechanism of action of icariin and its direct cellular target in stimulating osteogenesis.

RESULTS

The results revealed that icariin induced the expression of cell adhesion molecules (CAMs) and protected against microgravity-induced disruption of actin cytoskeleton and the loss of osteogenic activities in BMSCs through the activation of connexin-43 (Cx43) and Ras homolog family member A (RhoA) and Rac family small GTPase 1 (Rac1)-mediated signaling pathways. Computerized molecular docking techniques and the competitive solid-phase binding ELISA assay confirmed that icariin could be a direct ligand of integrin alpha 5 beta 1 (α5β1), and it could also increase the protein expression of integrin α5β1 for mechanosensing.

CONCLUSION

Our findings suggest that icariin could directly activate cell adhesion signaling by binding to integrin α5β1, which opens up new avenues for the development of integrin α5β1 ligand as an agent to protect against unloading-induced bone loss.

Mechanical protein polycystin-1 directly regulates osteoclastogenesis and bone resorption

Mechanical loading is required for bone homeostasis, but the underlying mechanism is still unclear. Our previous studies revealed that the mechanical protein polycystin-1 (PC1, encoded by Pkd1) is critical for bone formation. However, the role of PC1 in bone resorption is unknown. Here, we found that PC1 directly regulates osteoclastogenesis and bone resorption. The conditional deletion of Pkd1 in the osteoclast lineage resulted in a reduced number of osteoclasts, decreased bone resorption, and increased bone mass. A cohort study of 32,500 patients further revealed that autosomal dominant polycystic kidney disease, which is mainly caused by loss-of-function mutation of the PKD1 gene, is associated with a lower risk of hip fracture than those with other chronic kidney diseases. Moreover, mice with osteoclast-specific knockout of Pkd1 showed complete resistance to unloading-induced bone loss. A mechanistic study revealed that PC1 facilitated TAZ nuclear translocation via the C-terminal tail-TAZ complex and that conditional deletion of Taz in the osteoclast lineage resulted in reduced osteoclastogenesis and increased bone mass. Pharmacological regulation of the PC1-TAZ axis alleviated unloading- and estrogen deficiency- induced bone loss. Thus, the PC1-TAZ axis may be a potential therapeutic target for osteoclast-related osteoporosis.

Space research to explore novel biochemical insights on Earth

Travel to space has overcome unprecedent technological challenges and this has resulted in transfer of these technological results on Earth to better our lives. Health technology, medical devices, and research advancements in human biology are the first beneficiaries of this transfer. The real breakthrough came with the International Space Station, which endorsed multidisciplinary international scientific collaborations and boosted the research on pathophysiological adaptation of astronauts to life on space. These studies evidenced that life in space appeared to have exposed the astronauts to an accelerated aging-related pathophysiological dysregulation across multiple systems. In this review we emphasize the interaction between several biomarkers and their alteration in concentrations/expression/function by space stress factors. These altered interactions, suggest that different biochemical and hormonal factors, and cell signals, contribute to a complex network of pathophysiological mechanisms, orchestrating the homeostatic dysregulation of various organs/metabolic pathways. The main effects of space travel on altering cell organelles biology, ultrastructure, and cross-talk, have been observed in cell aging as well as in the disruption of metabolic pathways, which are also the causal factor of rare inherited metabolic disorders, one of the major pediatric health issue. The pathophysiologic breakthrough from space research could allow the development of precision health both on Earth and Space by promoting the validation of improved biomarker-based risk scores and the exploration of new pathophysiologic hypotheses and therapeutic targets. Nonstandard abbreviations: International Space Station (ISS), Artificial Intelligence (AI), European Space Agency (ESA), National Aeronautics and Space Agency (NASA), Low Earth Orbit (LEO), high sensitive troponin (hs-cTn), high sensitive troponin I (hs-cTn I), high sensitive troponin T, Brain Natriuretic Peptide (BNP), N terminal Brain Natriuretic Peptide (NT-BNP), cardiovascular disease (CVD), parathyroid hormone (PTH), urinary hydroxyproline (uHP), urinary C- and N-terminal telopeptides (uCTX and uNTX), pyridinoline (PYD), deoxypyridinoline (DPD), half-time (HF), serum Bone Alkaline Phosphatase (sBSAP), serum Alkaline Phosphatase (sAP), Carboxy-terminal Propeptide of Type 1 Procollagen (P1CP), serum Osteocalcin (sOC)), advanced glycation end products (AGEs), glycated hemoglobin A1c (HbA1c), Insulin-like growth factor 1 (IGF1), Growth Hormone (GH), amino acid (AA), β-hydroxy-β methyl butyrate (HMB), maple syrup urine disease (MSUD), non-communicable diseases (NCDs).

Mechanically strained osteocyte-derived exosomes contained miR-3110-5p and miR-3058-3p and promoted osteoblastic differentiation

BACKGROUND

Osteocytes are critical mechanosensory cells in bone, and mechanically stimulated osteocytes produce exosomes that can induce osteogenesis. MicroRNAs (miRNAs) are important constituents of exosomes, and some miRNAs in osteocytes regulate osteogenic differentiation; previous studies have indicated that some differentially expressed miRNAs in mechanically strained osteocytes likely influence osteoblastic differentiation. Therefore, screening and selection of miRNAs that regulate osteogenic differentiation in exosomes of mechanically stimulated osteocytes are important.

RESULTS

A mechanical tensile strain of 2500 με at 0.5 Hz 1 h per day for 3 days, elevated prostaglandin E2 (PGE2) and insulin-like growth factor-1 (IGF-1) levels and nitric oxide synthase (NOS) activity of MLO-Y4 osteocytes, and promoted osteogenic differentiation of MC3T3-E1 osteoblasts. Fourteen miRNAs differentially expressed only in MLO-Y4 osteocytes which were stimulated with mechanical tensile strain, were screened, and the miRNAs related to osteogenesis were identified. Four differentially expressed miRNAs (miR-1930-3p, miR-3110-5p, miR-3090-3p, and miR-3058-3p) were found only in mechanically strained osteocytes, and the four miRNAs, eight targeted mRNAs which were differentially expressed only in mechanically strained osteoblasts, were also identified. In addition, the mechanically strained osteocyte-derived exosomes promoted the osteoblastic differentiation of MC3T3-E1 cells in vitro, the exosomes were internalized by osteoblasts, and the up-regulated miR-3110-5p and miR-3058-3p in mechanically strained osteocytes, were both increased in the exosomes, which was verified via reverse transcription quantitative polymerase chain reaction (RT-qPCR).

CONCLUSIONS

In osteocytes, a mechanical tensile strain of 2500 με at 0.5 Hz induced the fourteen differentially expressed miRNAs which probably were in exosomes of osteocytes and involved in osteogenesis. The mechanically strained osteocyte-derived exosomes which contained increased miR-3110-5p and miR-3058-3p (two of the 14 miRNAs), promoted osteoblastic differentiation.

Biomechanical Effects of Mechanical Stress on Cells Involved in Fracture Healing

Fracture healing is a complex staged repair process in which the mechanical environment plays a key role. Bone tissue is very sensitive to mechanical stress stimuli, and the literature suggests that appropriate stress can promote fracture healing by altering cellular function. However, fracture healing is a coupled process involving multiple cell types that balance and limit each other to ensure proper fracture healing. The main cells that function during different stages of fracture healing are different, and the types and molecular mechanisms of stress required are also different. Most previous studies have used a single mechanical stimulus on individual mechanosensitive cells, and there is no relatively uniform standard for the size and frequency of the mechanical stress. Analyzing the mechanisms underlying the effects of mechanical stimulation on the metabolic regulation of signaling pathways in cells such as in bone marrow mesenchymal stem cells (BMSCs), osteoblasts, chondrocytes, and osteoclasts is currently a challenging research hotspot. Grasping how stress affects the function of different cells at the molecular biology level can contribute to the refined management of fracture healing. Therefore, in this review, we summarize the relevant literature and describe the effects of mechanical stress on cells associated with fracture healing, and their possible signaling pathways, for the treatment of fractures and the further development of regenerative medicine.

Cellular mechanotransduction of human osteoblasts in microgravity

Astronauts experience significant and rapid bone loss as a result of an extended stay in space, making the International Space Station (ISS) the perfect laboratory for studying osteoporosis due to the accelerated nature of bone loss on the ISS. This prompts the question, how does the lack of load due to zero-gravity propagate to bone-forming cells, human fetal osteoblasts (hFOBs), altering their maturation to mineralization? Here, we aim to study the mechanotransduction mechanisms by which bone loss occurs in microgravity. Two automated experiments, microfluidic chips capable of measuring single-cell mechanics via aspiration and cell spheroids incubated in pressure-controlled chambers, were each integrated into a CubeLab deployed to the ISS National Laboratory. For the first experiment, we report protrusion measurements of aspirated cells after exposure to microgravity at the ISS and compare these results to ground control conducted inside the CubeLab. We found slightly elongated protrusions for space samples compared to ground samples indicating softening of hFOB cells in microgravity. In the second experiment, we encapsulated osteoblast spheroids in collagen gel and incubated the samples in pressure-controlled chambers. We found that microgravity significantly reduced filamentous actin levels in the hFOB spheroids. When subjected to pressure, the spheroids exhibited increased pSMAD1/5/9 expression, regardless of the microgravity condition. Moreover, microgravity reduced YAP expression, while pressure increased YAP levels, thus restoring YAP expression for spheroids in microgravity. Our study provides insights into the influence of microgravity on the mechanical properties of bone cells and the impact of compressive pressure on cell signaling in space.

Cellular mechanotransduction of human osteoblasts in microgravity

Astronauts experience significant and rapid bone loss as a result of an extended stay in space, making the International Space Station (ISS) the perfect laboratory for studying osteoporosis due to the accelerated nature of bone loss on the ISS. This prompts the question, how does the lack of load due to zero-gravity propagate to bone-forming cells, human fetal osteoblasts (hFOBs), altering their maturation to mineralization? Here, we aim to study the mechanotransduction mechanisms by which bone loss occurs in microgravity. Two automated experiments, 4 microfluidic chips capable of measuring single-cell mechanics of hFOBs via aspiration and cell spheroids incubated in pressure-controlled chambers, were each integrated into a CubeLab deployed to the ISS National Laboratory. For the first experiment, we report protrusion measurements of aspirated cells after exposure to microgravity at the ISS and compare these results to ground control conducted inside the CubeLab. Our analysis revealed slightly elongated protrusions for space samples compared to ground samples indicating softening of hFOB cells in microgravity. In the second experiment, we encapsulated osteoblast spheroids in collagen gel and incubated the samples in pressure-controlled chambers. We found that microgravity significantly reduced filamentous actin levels in the hFOB spheroids. When subjected to pressure, the spheroids exhibited increased pSMAD1/5/9 expression, regardless of the microgravity condition. Moreover, microgravity reduced YAP expression, while pressure increased YAP levels, thus restoring YAP expression for spheroids in microgravity. Our study provides insights into the influence of microgravity on the mechanical properties of bone cells and the impact of compressive pressure on cell behavior and signaling in space.

Biomanufacturing of 3D Tissue Constructs in Microgravity and their Applications in Human Pathophysiological Studies

The growing interest in bioengineering in-vivo-like 3D functional tissues has led to novel approaches to the biomanufacturing process as well as expanded applications for these unique tissue constructs. Microgravity, as seen in spaceflight, is a unique environment that may be beneficial to the tissue-engineering process but cannot be completely replicated on Earth. Additionally, the expense and practical challenges of conducting human and animal research in space make bioengineered microphysiological systems an attractive research model. In this review, published research that exploits real and simulated microgravity to improve the biomanufacturing of a wide range of tissue types as well as those studies that use microphysiological systems, such as organ/tissue chips and multicellular organoids, for modeling human diseases in space are summarized. This review discusses real and simulated microgravity platforms and applications in tissue-engineered microphysiological systems across three topics: 1) application of microgravity to improve the biomanufacturing of tissue constructs, 2) use of tissue constructs fabricated in microgravity as models for human diseases on Earth, and 3) investigating the effects of microgravity on human tissues using biofabricated in vitro models. These current achievements represent important progress in understanding the physiological effects of microgravity and exploiting their advantages for tissue biomanufacturing.

Microgravity and Musculoskeletal Health: What Strategies Should Be Used for a Great Challenge?

Space colonization represents the most insidious challenge for mankind, as numerous obstacles affect the success of space missions. Specifically, the absence of gravitational forces leads to systemic physiological alterations, with particular emphasis on the musculoskeletal system. Indeed, astronauts exposed to spaceflight are known to report a significant impairment of bone microarchitecture and muscle mass, conditions clinically defined as osteoporosis and sarcopenia. In this context, space medicine assumes a crucial position, as the development of strategies to prevent and/or counteract weightlessness-induced alterations appears to be necessary. Furthermore, the opportunity to study the biological effects induced by weightlessness could provide valuable information regarding adaptations to spaceflight and suggest potential treatments that can preserve musculoskeletal health under microgravity conditions. Noteworthy, improving knowledge about the latest scientific findings in this field of research is crucial, as is thoroughly investigating the mechanisms underlying biological adaptations to microgravity and searching for innovative solutions to counter spaceflight-induced damage. Therefore, this narrative study review, performed using the MEDLINE and Google Scholar databases, aims to summarize the most recent evidence regarding the effects of real and simulated microgravity on the musculoskeletal system and to discuss the effectiveness of the main defence strategies used in both real and experimental settings.

Hemodynamic Characteristics of Cardiovascular System in Simulated Zero and Partial Gravities Based on CFD Modeling and Simulation

Zero and partial gravities (ZPG) increase cardiovascular risk, while the corresponding theoretical foundation remains uncertain. In the article, the ZPG were generated through a rotating frame with two degrees of freedom in combination with the random walk algorithm. A precise 3D geometric configuration of the cardiovascular system was developed, and the Navier-Stokes laminar flow and solid mechanics were used as governing equations for blood flow and the surrounding tissue in the cardiovascular system. The ZPG were designed into governing equations through the volume force term. The computational fluid dynamics' (CFD) simulations in combination with proper boundary conditions were carried out to investigate the influences of ZPG on the distribution of blood flow velocity, pressure, and shear stress in the cardiovascular system. The findings show that as simulated gravity gradually decreases from 0.7 g to 0.5 g to 0.3 g to 0 g, as opposed to normal gravity of 1 g, the maximum values of blood flow velocity, pressure, and shear stress on the walls of the aorta and its ramification significantly increase, which would lead to cardiovascular diseases. The research will lay a theoretical foundation for the comprehension of the ZPG effect on cardiovascular risk and the development of effective prevention and control measures under the circumstance of ZPG.

Potential Roles of YAP/TAZ Mechanotransduction in Spaceflight-Induced Liver Dysfunction

Microgravity exposure during spaceflight causes the disordered regulation of liver function, presenting a specialized mechano-biological coupling process. While YAP/TAZ serves as a typical mechanosensitive pathway involved in hepatocyte metabolism, it remains unclear whether and how it is correlated with microgravity-induced liver dysfunction. Here, we discussed liver function alterations induced by spaceflight or simulated effects of microgravity on Earth. The roles of YAP/TAZ serving as a potential bridge in connecting liver metabolism with microgravity were specifically summarized. Existing evidence indicated that YAP/TAZ target gene expressions were affected by mechanotransductive pathways and phase separation, reasonably speculating that microgravity might regulate YAP/TAZ activation by disrupting these pathways via cytoskeletal remodeling or nuclear deformation, or disturbing condensates formation via diffusion limit, and then breaking liver homeostasis.

The actin cytoskeleton: Morphological changes in pre- and fully developed lung cancer

Actin, a primary component of the cell cytoskeleton can have multiple isoforms, each of which can have specific properties uniquely suited for their purpose. These monomers are then bound together to form polymeric filaments utilizing adenosine triphosphate hydrolysis as a source of energy. Proteins, such as Arp2/3, VASP, formin, profilin, and cofilin, serve important roles in the polymerization process. These filaments can further be linked to form stress fibers by proteins called actin-binding proteins, such as α-actinin, myosin, fascin, filamin, zyxin, and epsin. These stress fibers are responsible for mechanotransduction, maintaining cell shape, cell motility, and intracellular cargo transport. Cancer metastasis, specifically epithelial mesenchymal transition (EMT), which is one of the key steps of the process, is accompanied by the formation of thick stress fibers through the Rho-associated protein kinase, MAPK/ERK, and Wnt pathways. Recently, with the advent of "field cancerization," pre-malignant cells have also been demonstrated to possess stress fibers and related cytoskeletal features. Analytical methods ranging from western blot and RNA-sequencing to cryo-EM and fluorescent imaging have been employed to understand the structure and dynamics of actin and related proteins including polymerization/depolymerization. More recent methods involve quantifying properties of the actin cytoskeleton from fluorescent images and utilizing them to study biological processes, such as EMT. These image analysis approaches exploit the fact that filaments have a unique structure (curvilinear) compared to the noise or other artifacts to separate them. Line segments are extracted from these filament images that have assigned lengths and orientations. Coupling such methods with statistical analysis has resulted in development of a new reporter for EMT in lung cancer cells as well as their drug responses.

Changes in interstitial fluid flow, mass transport and the bone cell response in microgravity and normogravity

In recent years, our scientific interest in spaceflight has grown exponentially and resulted in a thriving area of research, with hundreds of astronauts spending months of their time in space. A recent shift toward pursuing territories farther afield, aiming at near-Earth asteroids, the Moon, and Mars combined with the anticipated availability of commercial flights to space in the near future, warrants continued understanding of the human physiological processes and response mechanisms when in this extreme environment. Acute skeletal loss, more severe than any bone loss seen on Earth, has significant implications for deep space exploration, and it remains elusive as to why there is such a magnitude of difference between bone loss on Earth and loss in microgravity. The removal of gravity eliminates a critical primary mechano-stimulus, and when combined with exposure to both galactic and solar cosmic radiation, healthy human tissue function can be negatively affected. An additional effect found in microgravity, and one with limited insight, involves changes in dynamic fluid flow. Fluids provide the most fundamental way to transport chemical and biochemical elements within our bodies and apply an essential mechano-stimulus to cells. Furthermore, the cell cytoplasm is not a simple liquid, and fluid transport phenomena together with viscoelastic deformation of the cytoskeleton play key roles in cell function. In microgravity, flow behavior changes drastically, and the impact on cells within the porous system of bone and the influence of an expanding level of adiposity are not well understood. This review explores the role of interstitial fluid motion and solute transport in porous bone under two different conditions: normogravity and microgravity.

Bioinformatic analysis of the RNA expression patterns in microgravity-induced bone loss

Researchers have linked microgravity in space to the significant imbalance between bone formation and bone resorption that induces persistent bone loss in load-bearing bones. However, the underlying molecular mechanisms are still unclear, which hinders the development of therapeutic measures. The aim of this study was to identify hub genes and explore novel molecular mechanisms underlying microgravity-induced bone loss using transcriptome datasets obtained from the GEO and SRA databases. In summary, comparative RNA expression pattern studies that differ in species (Homo or Mus), models (in vitro or in vivo), microgravity conditions (real microgravity or ground-based simulators) and microgravity duration showed that it is difficult to reach a consistent conclusion about the pathogenesis of microgravity-induced bone loss across these studies. Even so, we identified 11 hub genes and some miRNA-mRNA interactions mainly based on the GSE100930 dataset. Also, the expression of CCL2, ICAM1, IGF1, miR-101-3p and miR-451a markedly changed under clinorotation-microgravity condition. Remarkedly, ICAM1 and miR-451a were key mediators of the osteogenesis of hMSCs under clinorotation-microgravity condition. These findings provide novel insights into the molecular mechanisms of bone loss during microgravity and could indicate potential targets for further countermeasures against this condition.

Scaffold-based bone tissue engineering in microgravity: potential, concerns and implications

One of humanity's greatest challenges is space exploration, which requires an in-depth analysis of the data continuously collected as a necessary input to fill technological gaps and move forward in several research sectors. Focusing on space crew healthcare, a critical issue to be addressed is tissue regeneration in extreme conditions. In general, it represents one of the hottest and most compelling goals of the scientific community and the development of suitable therapeutic strategies for the space environment is an urgent need for the safe planning of future long-term manned space missions. Osteopenia is a commonly diagnosed disease in astronauts due to the physiological adaptation to altered gravity conditions. In order to find specific solutions to bone damage in a reduced gravity environment, bone tissue engineering is gaining a growing interest. With the aim to critically investigate this topic, the here presented review reports and discusses bone tissue engineering scenarios in microgravity, from scaffolding to bioreactors. The literature analysis allowed to underline several key points, such as the need for (i) biomimetic composite scaffolds to better mimic the natural microarchitecture of bone tissue, (ii) uniform simulated microgravity levels for standardized experimental protocols to expose biological materials to the same testing conditions, and (iii) improved access to real microgravity for scientific research projects, supported by the so-called democratization of space.

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

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

Space flight and central nervous system: Friends or enemies? Challenges and opportunities for neuroscience and neuro-oncology

Space environment provides many challenges to pilots, astronauts, and space scientists, which are constantly subjected to unique conditions, including microgravity, radiations, hypoxic condition, absence of the day and night cycle, etc. These stressful stimuli have the potential to affect many human physiological systems, triggering physical and biological adaptive changes to re‐establish the homeostatic state. A particular concern regards the risks for the effects of spaceflight on the central nervous system (CNS), as several lines of evidence reported a great impact on neuroplasticity, cognitive functions, neurovestibular system, short‐term memory, cephalic fluid shift, reduction in motor function, and psychological disturbances, especially during long‐term missions. Aside these potential detrimental effects, the other side of the coin reflects the potential benefit of applicating space‐related conditions on Earth‐based life sciences, as cancer research. Here, we focused on examining the effect of real and simulated microgravity on CNS functions, both in humans and in cellular models, browsing the different techniques to experience or mime microgravity on‐ground. Increasing evidence demonstrate that cancer cells, and brain cancer cells in particular, are negatively affected by microgravity, in terms of alteration in cell morphology, proliferation, invasion, migration, and apoptosis, representing an advancing novel side of space‐based investigations. Overall, deeper understandings about the mechanisms by which space environment influences CNS and tumor biology may be promisingly translated into many clinical fields, ranging from aerospace medicine to neuroscience and oncology, representing an enormous pool of knowledge for the implementation of countermeasures and therapeutic applications.

Estrogen Receptor 1 (ESR1) and the Wnt/β-Catenin Pathway Mediate the Effect of the Coumarin Derivative Umbelliferon on Bone Mineralization

Bone physiology is regulated by osteoblast and osteoclast activities, both involved in the bone remodeling process, through deposition and resorption mechanisms, respectively. The imbalance between these two phenomena contributes to the onset of bone diseases. Among these, osteoporosis is the most common metabolic bone disorder. The therapies currently used for its treatment include antiresorptive and anabolic agents associated with side effects. Therefore, alternative therapeutic approaches, including natural molecules such as coumarin and their derivatives, have recently shown positive results. Thus, our proposal was to investigate the effect of the coumarin derivative umbelliferon (UF) using an interesting model of human osteoblasts (hOBs) isolated from osteoporotic patients. UF significantly improved the activity of osteoporotic-patient-derived hOBs via estrogen receptor 1 (ESR1) and the downstream activation of β-catenin pathway. Additionally, hOBs were co-cultured in microgravity with human osteoclasts (hOCs) using a 3D system bioreactor, able to reproduce the bone remodeling unit in bone loss conditions in vitro. Notably, UF exerted its anabolic role by reducing the multinucleated cells. Overall, our study confirms the potential efficacy of UF in bone health, and identified, for the first time, a prospective alternative natural compound useful to prevent/treat bone loss diseases such as osteoporosis.

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.

Impairment of 7F2 osteoblast function by simulated partial gravity in a Random Positioning Machine

The multifaceted adverse effects of reduced gravity pose a significant challenge to human spaceflight. Previous studies have shown that bone formation by osteoblasts decreases under microgravity conditions, both real and simulated. However, the effects of partial gravity on osteoblasts’ function are less well understood. Utilizing the software-driven newer version of the Random Positioning Machine (RPM^SW), we simulated levels of partial gravity relevant to future manned space missions: Mars (0.38 G), Moon (0.16 G), and microgravity (Micro, ~10⁻³ G). Short-term (6 days) culture yielded a dose-dependent reduction in proliferation and the enzymatic activity of alkaline phosphatase (ALP), while long-term studies (21 days) showed a distinct dose-dependent inhibition of mineralization. By contrast, expression levels of key osteogenic genes (Alkaline phosphatase, Runt-related Transcription Factor 2, Sparc/osteonectin) exhibited a threshold behavior: gene expression was significantly inhibited when the cells were exposed to Mars-simulating partial gravity, and this was not reduced further when the cells were cultured under simulated Moon or microgravity conditions. Our data suggest that impairment of cell function with decreasing simulated gravity levels is graded and that the threshold profile observed for reduced gene expression is distinct from the dose dependence observed for cell proliferation, ALP activity, and mineral deposition. Our study is of relevance, given the dearth of research into the effects of Lunar and Martian gravity for forthcoming space exploration.

The effects of microgravity on bone structure and function

Humans are spending an increasing amount of time in space, where exposure to conditions of microgravity causes 1–2% bone loss per month in astronauts. Through data collected from astronauts, as well as animal and cellular experiments conducted in space, it is evident that microgravity induces skeletal deconditioning in weight-bearing bones. This review identifies contentions in current literature describing the effect of microgravity on non-weight-bearing bones, different bone compartments, as well as the skeletal recovery process in human and animal spaceflight data. Experiments in space are not readily available, and experimental designs are often limited due to logistical and technical reasons. This review introduces a plethora of on-ground research that elucidate the intricate process of bone loss, utilising technology that simulates microgravity. Observations from these studies are largely congruent to data obtained from spaceflight experiments, while offering more insights behind the molecular mechanisms leading to microgravity-induced bone loss. These insights are discussed herein, as well as how that knowledge has contributed to studies of current therapeutic agents. This review also points out discrepancies in existing data, highlighting knowledge gaps in our current understanding. Further dissection of the exact mechanisms of microgravity-induced bone loss will enable the development of more effective preventative and therapeutic measures to protect against bone loss, both in space and possibly on ground.

Role of Myostatin in Muscle Degeneration by Random Positioning Machine Exposure: An in vitro Study for the Treatment of Sarcopenia

Several scientific evidence have shown that exposure to microgravity has a significant impact on the health of the musculoskeletal system by altering the expression of proteins and molecules involved in bone–muscle crosstalk, which is also observed in the research of microgravity effect simulation. Among these, the expression pattern of myostatin appears to play a key role in both load-free muscle damage and the progression of age-related musculoskeletal disorders, such as osteoporosis and sarcopenia. Based on this evidence, we here investigated the efficacy of treatment with anti-myostatin (anti-MSTN) antibodies on primary cultures of human satellite cells exposed to 72 h of random positioning machine (RPM). Cell cultures were obtained from muscle biopsies taken from a total of 30 patients (controls, osteoarthritic, and osteoporotic) during hip arthroplasty. The Pax7 expression by immunofluorescence was carried out for the characterization of satellite cells. We then performed morphological evaluation by light microscopy and immunocytochemical analysis to assess myostatin expression. Our results showed that prolonged RPM exposure not only caused satellite cell death, but also induced changes in myostatin expression levels with group-dependent variations. Surprisingly, we observed that the use of anti-MSTN antibodies induced a significant increase in cell survival after RPM exposure under all experimental conditions. Noteworthy, we found that the negative effect of RPM exposure was counteracted by treatment with anti-MSTN antibodies, which allowed the formation of numerous myotubes. Our results highlight the role of myostatin as a major effector of the cellular degeneration observed with RPM exposure, suggesting it as a potential therapeutic target to slow the muscle mass loss that occurs in the absence of loading.

Dissociation of Bone Resorption and Formation in Spaceflight and Simulated Microgravity: Potential Role of Myokines and Osteokines?

The dissociation of bone formation and resorption is an important physiological process during spaceflight. It also occurs during local skeletal unloading or immobilization, such as in people with neuromuscular disorders or those who are on bed rest. Under these conditions, the physiological systems of the human body are perturbed down to the cellular level. Through the absence of mechanical stimuli, the musculoskeletal system and, predominantly, the postural skeletal muscles are largely affected. Despite in-flight exercise countermeasures, muscle wasting and bone loss occur, which are associated with spaceflight duration. Nevertheless, countermeasures can be effective, especially by preventing muscle wasting to rescue both postural and dynamic as well as muscle performance. Thus far, it is largely unknown how changes in bone microarchitecture evolve over the long term in the absence of a gravity vector and whether bone loss incurred in space or following the return to the Earth fully recovers or partly persists. In this review, we highlight the different mechanisms and factors that regulate the humoral crosstalk between the muscle and the bone. Further we focus on the interplay between currently known myokines and osteokines and their mutual regulation.

Cells respond to space microgravity through cytoskeleton reorganization

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

Microgravity-induced alterations of mouse bones are compartment- and site-specific and vary with age

The age at which astronauts experience microgravity is a critical consideration for skeletal health and similarly has clinical relevance for musculoskeletal disuse on Earth. While astronauts are extensively studied for bone and other physiological changes, rodent studies enable direct evaluation of skeletal changes with microgravity. Yet, mouse spaceflight studies have predominately evaluated tissues from young, growing mice. We evaluated bone microarchitecture in tibiae and femurs from Young (9-week-old) and Mature (32-weeks-old) female, C57BL/6N mice flown in microgravity for ~2 and ~3 weeks, respectively. Microgravity-induced changes were both compartment- and site-specific. Changes were greater in trabecular versus cortical bone in Mature mice exposed to microgravity (-40.0% Tb. BV/TV vs -4.4% Ct. BV/TV), and bone loss was greater in the proximal tibia as compared to the distal femur. Trabecular thickness in Young mice increased by +25.0% on Earth and no significant difference following microgravity. In Mature mice exposed to microgravity, trabecular thickness rapidly decreased (-24.5%) while no change was detected in age-matched mice that were maintained on Earth. Mature mice exposed to microgravity experienced greater bone loss than Young mice with net skeletal growth. Moreover, machine learning classification models confirmed that microgravity exposure-driven decrements in trabecular microarchitecture and cortical structure occurred disproportionately in Mature than in Young mice. Our results suggest that age of disuse onset may have clinical implications in osteoporotic or other at-risk populations on Earth and may contribute to understanding bone loss patterns in astronauts.

Update on the effects of microgravity on the musculoskeletal system

With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

Exosomes derived from cyclic mechanical stretch-exposed bone marrow mesenchymal stem cells inhibit RANKL-induced osteoclastogenesis through the NF-κB signaling pathway

Background

Skeletal unloading usually induces severe disuse osteoporosis (DOP), which often occurs in patients subjected to prolonged immobility or in spaceflight astronauts. Increasing evidence suggests that exosomes are important mediators in maintaining the balance between bone formation and resorption. We hypothesized that exosomes play an important role in the maintenance of bone homeostasis through intercellular communication between bone marrow mesenchymal stem cells (BMSCs) and osteoclasts under mechanical loading.

Methods

Cells were divided into cyclic mechanical stretch (CMS)-treated BMSCs and normal static-cultured BMSCs, and exosomes were extracted by ultracentrifugation. After incubation with CMS-treated BMSC-derived exosomes (CMS_Exos) or static-cultured BMSC-derived exosomes (static_Exos), the apoptosis rates of bone marrow macrophages (BMMs) were determined by flow cytometry, and cell viability was detected with a Cell Counting Kit-8 (CCK-8) assay. Osteoclast differentiation was determined with an in vitro osteoclastogenesis assay. Signaling pathway activation was evaluated by western blotting and immunofluorescence staining. Hindlimb unloading (HU)-induced DOP mouse models were prepared to evaluate the function of exosomes in DOP.

Results

Both CMS_Exos and static_Exos could be internalized by BMMs, and CMS_Exos did not affect BMM viability or increase apoptosis. The CMS_Exos effectively suppressed receptor activator of nuclear factor kappa-B ligand (RANKL)-mediated osteoclastogenesis and F-actin ring formation. Further molecular investigation demonstrated that CMS_Exos impaired osteoclast differentiation via inhibition of the RANKL-induced nuclear factor kappa-B (NF-κB) signaling pathway. Both CMS_Exos and static_Exos partly rescued the osteoporosis caused by mechanical unloading; however, the CMS_Exo group showed more obvious rescue. Treatment with CMS_Exos significantly decreased the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts. Exosomes derived from CMS-treated BMSCs strongly inhibited osteoclast differentiation by attenuating the NF-κB signaling pathway in vitro and rescued osteoporosis caused by mechanical unloading in an HU mouse model in vivo.

Conclusions

In this research, we demonstrated that Exosomes derived from CMS-treated BMSCs inhibited osteoclastogenesis by attenuating NF-κB signaling pathway activity in vitro and ameliorated bone loss caused by mechanical unloading in an HU mouse model, providing new insights into intercellular communication between osteoblasts and osteoclasts under mechanical loading.

The Effect of Space Travel on Bone Metabolism: Considerations on Today's Major Challenges and Advances in Pharmacology

Microgravity-induced bone loss is currently a significant and unresolved health risk for space travelers, as it raises the likelihood for irreversible changes that weaken skeletal integrity and the incremental onset of fracture injuries and renal stone formation. Another issue related to bone tissue homeostasis in microgravity is its capacity to regenerate following fractures due to weakening of the tissue and accidental events during the accomplishment of particularly dangerous tasks. Today, several pharmacological and non-pharmacological countermeasures to this problem have been proposed, including physical exercise, diet supplements and administration of antiresorptive or anabolic drugs. However, each class of pharmacological agents presents several limitations as their prolonged and repeated employment is not exempt from the onset of serious side effects, which limit their use within a well-defined range of time. In this review, we will focus on the various countermeasures currently in place or proposed to address bone loss in conditions of microgravity, analyzing in detail the advantages and disadvantages of each option from a pharmacological point of view. Finally, we take stock of the situation in the currently available literature concerning bone loss and fracture healing processes. We try to understand which are the critical points and challenges that need to be addressed to reach innovative and targeted therapies to be used both in space missions and on Earth.

Osteogenic differentiation system based on biopolymer nanoparticles for stem cells in simulated microgravity

An efficient long-term intracellular growth factor release system in simulated microgravity for osteogenic differentiation was prepared based on polylactic acid (PLA) and polyhydroxyalkanoate (PHA) nanoparticles for loading of bone morphogenetic protein 2 (BMP2) and bone morphogenetic protein 7 (BMP7) (defined as sB2-PLA-NP and sB7-PHA-NP), respectively, associated with osteogenic differentiation of human adipose derived stem cells (hADSCs). On account of soybean lecithin (SL) as biosurfactants, sB2-PLA-NPs and sB7-PHA-NPs had a high encapsulation efficiency (>80%) of BMPs and uniform small size (<100 nm), and showed different slow-release to provide BMP2 in early stage and BMP7 in late stages of osteogenic differentiation within 20 days, due to degradation rate of PLA and PHA in cells. After uptake into hADSCs, by comparison with single sB2-PLA-NP or sB7-PHA-NP, the Mixture NPs, compound of sB2-PLA-NP and sB7-PHA-NP with a mass ratio of 1:1, can well-promote ALP activity, expression of OPN and upregulated related osteo-genes. Directed osteo-differentiation of Mixture NPs was similar to result of sustained free-BMP2 and BMP7-supplying (sFree-B2&B7) in simulated microgravity, which demonstrated the reliability and stability of Mixture NPs as a long-term osteogenic differentiation system in space medicine and biology in future.

Fast transport of HCl across a hydrophobic layer over macroscopic distances by using a Pt(II) compound as the transporter: micro- and nanometric aggregates as effective transporters

Bis-(diethyl-dithioxamidate)platinum(ii) is able to transport HCl from the donor aqueous phase to the receiving one over a mean distance of 12 cm in about 3 minutes across an organic membrane in the bulk, without stirring of the organic phase, i.e. at a rate far exceeding the unidirectional macroscopic diffusion coefficient. The way in which this surprising phenomenon can happen is linked to the behaviour of HCl which, because of dynamic interactions with [Pt(HEt2C2N2S2)2] (in which HCl is hosted as a tight ion pair [Pt(H2Et2C2N2S2)2][Cl]2) and chloroform molecules, gives rise to observable nanometric and micrometric domains, more dense than the surrounding bulk, whose formation and disaggregation processes accelerate the unidirectional macroscopic diffusion of HCl. Thermodynamic parameters obtained from the study of acid-base behaviour of the system Pt(ii) species/HCl/CHCl3 also agree with the proposed mechanism of HCl transport.

Effect of Mechanical Strain on Cells Involved in Fracture Healing

Secondary fracture healing is a complex multi‐stage process in which the mechanical environment plays a key role. The use of an appropriate mechanical stimulation such as strain is conducive to tissue formation between fracture ends, thus aiding the healing process. However, if the strain is too large or too small, the biological behavior of the cells involved in bone healing will be affected, resulting in non‐union or delayed healing. In this review, we summarize the current state of knowledge regarding the effect of strain on cells that play a role in the fracture‐healing process. Overall, the related literature suggests that selection of an adequate strain promotes fracture healing through the stimulation of angiogenesis and osteogenesis, along with inhibition of osteoclast differentiation and bone resorption. However, standardized methods for the application of mechanical stimulation are lacking, and a unified consensus on the mechanism by which strain promotes cell differentiation has not yet been reached. These issues, therefore, deserve further investigation.

Implications of Altered Endosome and Lysosome Biology in Space Environments

Space exploration poses multiple challenges for mankind, not only on a technical level but also to the entire physiology of the space traveller. The human system must adapt to several environmental stressors, microgravity being one of them. Lysosomes are ubiquitous to every cell and essential for their homeostasis, playing significant roles in the regulation of autophagy, immunity, and adaptation of the organism to changes in their environment, to name a few. Dysfunction of the lysosomal system leads to age-related diseases, for example bone loss, reduced immune response or cancer. As these conditions have been shown to be accelerated following exposure to microgravity, this review elucidates the lysosomal response to real and simulated microgravity. Microgravity activates the endo-lysosomal system, with resulting impacts on bone loss, muscle atrophy and stem cell differentiation. The investigation of lysosomal adaptation to microgravity can be beneficial in the search for new biomarkers or therapeutic approaches to several disease pathologies on earth as well as the potential to mitigate pathophysiology during spaceflight.

4-Acetylantroquinonol B Inhibits Osteoclastogenesis by Inhibiting the Autophagy Pathway in a Simulated Microgravity Model

Astronauts suffer from 1–2% bone loss per month during space missions. Targeting osteoclast differentiation has been regarded as a promising strategy to prevent osteoporosis in microgravity (μXg). 4-acetylantroquinonol B (4-AAQB), a ubiquinone from Antrodia cinnamomea, has shown anti-inflammatory and anti-hepatoma activities. However, the effect of 4-AAQB on μXg-induced osteoclastogenesis remains unclear. In this study, we aimed to explore the mechanistic impact of 4-AAQB on osteoclast formation under μXg conditions. The monocyte/macrophage-like cell line RAW264.7 was exposed to simulated μXg (Rotary Cell Culture System; Synthecon, Houston, TX, USA) for 24 h and then treated with 4-AAQB or alendronate (ALN) and osteoclast differentiation factor receptor activator of nuclear factor kappa-B ligand (RANKL). Osteoclastogenesis, bone resorption activity, and osteoclast differentiation-related signaling pathways were analyzed using tartrate-resistant acid phosphatase (TRAP) staining, actin ring fluorescent staining, bone resorption, and western blotting assays. Based on the results of TRAP staining, actin ring staining, and bone resorption assays, we found that 4-AAQB significantly inhibited μXg-induced osteoclast differentiation. The critical regulators of osteoclast differentiation, including nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), c-Fos, and dendritic cell-specific transmembrane protein (DC-STAMP), were consistently decreased. Meanwhile, osteoclast apoptosis and cell cycle arrest were also observed along with autophagy suppression. Interestingly, the autophagy inhibitors 3-methyladenine (3-MA) and chloroquine (CQ) showed similar effects to 4-AAQB. In conclusion, we suggest that 4-AAQB may serve as a potential agent against μXg-induced osteoclast formation.

Osteoclasts and Microgravity

Astronauts are at risk of losing 1.0% to 1.5% of their bone mass for every month they spend in space despite their adherence to diets and exercise regimens designed to protect their musculoskeletal systems. This loss is the result of microgravity-related impairment of osteocyte and osteoblast function and the consequent upregulation of osteoclast-mediated bone resorption. This review describes the ontogeny of osteoclast hematopoietic stem cells and the contributions macrophage colony stimulating factor, receptor activator of the nuclear factor-kappa B ligand, and the calcineurin pathways make in osteoclast differentiation and provides details of bone formation, the osteoclast cytoskeleton, the immune regulation of osteoclasts, and osteoclast mechanotransduction on Earth, in space, and under conditions of simulated microgravity. The article discusses the need to better understand how osteoclasts are able to function in zero gravity and reviews current and prospective therapies that may be used to treat osteoclast-mediated bone disease.

Blockade of IL-6 alleviates bone loss induced by modeled microgravity in mice

This study investigated the effects of blockade of IL-6 on bone loss induced by modeled microgravity (MG). Adult male mice were exposed to hind-limb suspension (HLS) and treated with IL-6-neutralizing antibody (IL-6 nAb) for 4 weeks. HLS in mice led to upregulation of IL-6 expression in both sera and femurs. IL-6 nAb treatment in HLS mice significantly alleviated bone loss, evidenced by increased bone mineral density of whole tibia, trabecular thickness and number, bone volume fraction of proximal tibiae, and ultimate load and stiffness of femoral diaphysis. IL-6 nAb treatment in HLS mice significantly enhanced levels of osteocalcin in sera and reduced levels of deoxypyridinoline. In MC3T3-E1 cells exposed to MG in vitro, IL-6 nAb treatment increased mRNA expression and activity of alkaline phosphatase, mRNA expression of osteopontin and runt-related transcription factor 2, and protein levels of osteoprotegerin and decreased protein levels of receptor activator of the NF-κB ligand. In RAW254.7 cells exposed to MG, IL-6 nAb treatment downregulated mRNA expression of cathepsin K and tartrate-resistant acid phosphatase (TRAP) and reduced numbers of TRAP-positive multinucleated osteoclasts. In conclusion, blockade of IL-6 alleviated the bone loss induced by MG.

Osteoclast Multinucleation: Review of Current Literature

Multinucleation is a hallmark of osteoclast maturation. The unique and dynamic multinucleation process not only increases cell size but causes functional alterations through reconstruction of the cytoskeleton, creating the actin ring and ruffled border that enable bone resorption. Our understanding of the molecular mechanisms underlying osteoclast multinucleation has advanced considerably in this century, especially since the identification of DC-STAMP and OC-STAMP as “master fusogens”. Regarding the molecules and pathways surrounding these STAMPs, however, only limited progress has been made due to the absence of their ligands. Various molecules and mechanisms other than the STAMPs are involved in osteoclast multinucleation. In addition, several preclinical studies have explored chemicals that may be able to target osteoclast multinucleation, which could enable us to control pathogenic bone metabolism more precisely. In this review, we will focus on recent discoveries regarding the STAMPs and other molecules involved in osteoclast multinucleation.

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

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

Irisin prevents microgravity-induced impairment of osteoblast differentiation in vitro during the space flight CRS-14 mission

Understanding molecular mechanisms responsible for bone cells unbalance in microgravity would allow the development of better countermeasures for astronauts, and eventually advancing terrestrial osteoporosis treatments. We conduct a unique investigation by using a controlled 3D in vitro cell model to mimic the bone microenvironment in microgravity aboard the SpaceX Dragon cargo ferry to the ISS. Osteoblasts (OBs), osteoclasts (OCs), and endothelial cells (ECs), seeded on Skelite discs, were cultured w/ or w/o rec-Irisin and exposed to 14 days of microgravity in the eOSTEO hardware. Gene expression analysis was assessed, and results were compared to ground controls treated within identical payloads. Our results show that the microgravity-induced downregulation of mRNA levels of genes encoding for OB key transcription factors (Atf4 -75%, P < .01; RunX2 -87%, P < .001, Osterix -95%, P < .05 vs ground) and proteins (Collagen I -84%, P < .05; Osteoprotegerin -94%, P < .05) were prevented by irisin. Despite it was not effective in preventing Trap and Cathepsin K mRNA increase, irisin induced a 2.8-fold increase of Osteoprotegerin (P < .05) that might act for reducing osteoclastogenesis in microgravity. Our results provide evidence that irisin supports OB differentiation and activity in microgravity and it might represent a countermeasure to prevent bone loss in astronauts.

Antioxidant Strategy to Prevent Simulated Microgravity-Induced Effects on Bone Osteoblasts

The effects induced by microgravity on human body functions have been widely described, in particular those on skeletal muscle and bone tissues. This study aims to implement information on the possible countermeasures necessary to neutralize the oxidative imbalance induced by microgravity on osteoblastic cells. Using the model of murine MC3T3-E1 osteoblast cells, cellular morphology, proliferation, and metabolism were investigated during exposure to simulated microgravity on a random positioning machine in the absence or presence of an antioxidant—the 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox). Our results confirm that simulated microgravity-induced morphological and metabolic alterations characterized by increased levels of reactive oxygen species and a slowdown of the proliferative rate. Interestingly, the use of Trolox inhibited the simulated microgravity-induced effects. Indeed, the antioxidant-neutralizing oxidants preserved cell cytoskeletal architecture and restored cell proliferation rate and metabolism. The use of appropriate antioxidant countermeasures could prevent the modifications and damage induced by microgravity on osteoblastic cells and consequently on bone homeostasis.

The Impact of Spaceflight and Simulated Microgravity on Cell Adhesion

Microgravity induces a number of significant physiological changes in the cardiovascular, nervous, immune systems, as well as the bone tissue of astronauts. Changes in cell adhesion properties are one aspect affected during long-term spaceflights in mammalian cells. Cellular adhesion behaviors can be divided into cell-cell and cell-matrix adhesion. These behaviors trigger cell-cell recognition, conjugation, migration, cytoskeletal rearrangement, and signal transduction. Cellular adhesion molecule (CAM) is a general term for macromolecules that mediate the contact and binding between cells or between cells and the extracellular matrix (ECM). In this review, we summarize the four major classes of adhesion molecules that regulate cell adhesion, including integrins, immunoglobulin superfamily (Ig-SF), cadherins, and selectin. Moreover, we discuss the effects of spaceflight and simulated microgravity on the adhesion of endothelial cells, immune cells, tumor cells, stem cells, osteoblasts, muscle cells, and other types of cells. Further studies on the effects of microgravity on cell adhesion and the corresponding physiological behaviors may help increase the safety and improve the health of astronauts in space.

Optimal Human Functioning Requires Exercise Across the Lifespan: Mobility in a 1g Environment Is Intrinsic to the Integrity of Multiple Biological Systems

It is widely acknowledged that achieving and maintaining a healthier lifestyle can be enhanced through regular participation in sport and physical activity. Coevally, a growing number of health professionals regard exercise as a legitimate intervention strategy for those who have lost their health. Exercise has been shown to be effective for overweight or obese individuals, who are at risk to lose their health due to development of type II diabetes, cardiovascular disease, as well as, infiltration of muscles, bone and other organs with fat, so it can be considered medicine. However, exercise and associated mobility likely also have a strong prevention component that can effectively contribute to the maintenance of the integrity of multiple biological systems for those who do not have overt risk factors or ongoing disease. While prevention is preferred over intervention in the context of disease, it is clear that exercise and associated mobility, generally, can be an effective influence, although overtraining and excessive loading can be deleterious to health. The basis for the generally positive influence of exercise likely lies in the fact that many of our physiological systems are designed to function in the mechanically dynamic and active 1g environment of Earth (e.g., muscles, cartilage, ligaments, tendons, bones, and cardiovascular system, and neuro-cognitive function), and nearly all these systems subscribe to the "use it or lose it" paradigm. This conclusion is supported by the changes observed over the more than 50 years of space flight and exposure to microgravity conditions. Therefore, the premise advanced is: "exercise is preventative for loss of health due to age-related decline in the integrity of several physiological systems via constant reinforcement of those systems, and thus, optimal levels of exercise and physical activity are endemic to, essential for, and intrinsic to optimal health and wellbeing."

Modeling the Impact of Microgravity at the Cellular Level: Implications for Human Disease

A lack of gravity experienced during space flight has been shown to have profound effects on human physiology including muscle atrophy, reductions in bone density and immune function, and endocrine disorders. At present, these physiological changes present major obstacles to long-term space missions. What is not clear is which pathophysiological disruptions reflect changes at the cellular level versus changes that occur due to the impact of weightlessness on the entire body. This review focuses on current research investigating the impact of microgravity at the cellular level including cellular morphology, proliferation, and adhesion. As direct research in space is currently cost prohibitive, we describe here the use of microgravity simulators for studies at the cellular level. Such instruments provide valuable tools for cost-effective research to better discern the impact of weightlessness on cellular function. Despite recent advances in understanding the relationship between extracellular forces and cell behavior, very little is understood about cellular biology and mechanotransduction under microgravity conditions. This review will examine recent insights into the impact of simulated microgravity on cell biology and how this technology may provide new insight into advancing our understanding of mechanically driven biology and disease.

Simulated Microgravity Influences VEGF, MAPK, and PAM Signaling in Prostate Cancer Cells

Prostate cancer is one of the leading causes of cancer mortality in men worldwide. An unusual but unique environment for studying tumor cell processes is provided by microgravity, either in space or simulated by ground-based devices like a random positioning machine (RPM). In this study, prostate adenocarcinoma-derived PC-3 cells were cultivated on an RPM for time periods of 3 and 5 days. We investigated the genes associated with the cytoskeleton, focal adhesions, extracellular matrix, growth, survival, angiogenesis, and metastasis. The gene expression of signaling factors of the vascular endothelial growth factor (VEGF), mitogen-activated protein kinase (MAPK), and PI3K/AKT/mTOR (PAM) pathways was investigated using qPCR. We performed immunofluorescence to study the cytoskeleton, histological staining to examine the morphology, and a time-resolved immunofluorometric assay to analyze the cell culture supernatants. When PC-3 cells were exposed to simulated microgravity (s-µg), some cells remained growing as adherent cells (AD), while most cells detached from the cell culture flask bottom and formed multicellular spheroids (MCS). After 3-day RPM exposure, PC-3 cells revealed significant downregulation of the VEGF, SRC1, AKT, MTOR, and COL1A1 gene expression in MCS, whereas FLT1, RAF1, MEK1, ERK1, FAK1, RICTOR, ACTB, TUBB, and TLN1 mRNAs were not significantly changed. ERK2 and TLN1 were elevated in AD, and FLK1, LAMA3, COL4A5, FN1, VCL, CDH1, and NGAL mRNAs were significantly upregulated in AD and MCS after 3 days. After a 5-day culture in s-µg, the PC-3 cells showed significant downregulations of VEGF mRNA in AD and MCS, and FN1, CDH1, and LAMA3 in AD and SCR1 in MCS. In addition, we measured significant upregulations in FLT1, AKT, ERK1, ERK2, LCN2, COL1A1, TUBB, and VCL mRNAs in AD and MCS, and increases in FLK1, FN1, and COL4A5 in MCS as well as LAMB2, CDH1, RAF1, MEK1, SRC1, and MTOR mRNAs in AD. FAK1 and RICTOR were not altered by s-µg. In parallel, the secretion rate of VEGFA and NGAL proteins decreased. Cytoskeletal alterations (F-actin) were visible, as well as a deposition of collagen in the MCS. In conclusion, RPM-exposure of PC-3 cells induced changes in their morphology, cytoskeleton, and extracellular matrix protein synthesis, as well as in their focal adhesion complex and growth behavior. The significant upregulation of genes belonging to the PAM pathway indicated their involvement in the cellular changes occurring in microgravity.

Spaceflight-Induced Bone Tissue Changes that Affect Bone Quality and Increase Fracture Risk

PURPOSE OF REVIEW

Bone mineral density and systemic factors are used to assess skeletal health in astronauts. Yet, even in a general population, these measures fail to accurately predict when any individual will fracture. This review considers how long-duration human spaceflight requires evaluation of additional bone structural and material quality measures that contribute to microgravity-induced skeletal fragility.

RECENT FINDINGS

In both humans and small animal models following spaceflight, bone mass is compromised via reduced bone formation and elevated resorption levels. Concurrently, bone structural quality (e.g., trabecular microarchitecture) is diminished and the quality of bone material is reduced via impaired tissue mineralization, maturation, and maintenance (e.g., mediated by osteocytes). Bone structural and material quality are both affected by microgravity and may, together, jeopardize astronaut operational readiness and lead to increased fracture risk upon return to gravitational loading. Future studies need to directly evaluate how bone quality combines with diminished bone mass to influence bone strength and toughness (e.g., resistance to fracture). Bone quality assessment promises to identify novel biomarkers and therapeutic targets.

Stem Cell Culture Under Simulated Microgravity

Challenging environment of space causes several pivotal alterations in living systems, especially due to microgravity. The possibility of simulating microgravity by ground-based systems provides research opportunities that may lead to the understanding of in vitro biological effects of microgravity by eliminating the challenges inherent to spaceflight experiments. Stem cells are one of the most prominent cell types, due to their self-renewal and differentiation capabilities. Research on stem cells under simulated microgravity has generated many important findings, enlightening the impact of microgravity on molecular and cellular processes of stem cells with varying potencies. Simulation techniques including clinostat, random positioning machine, rotating wall vessel and magnetic levitation-based systems have improved our knowledge on the effects of microgravity on morphology, migration, proliferation and differentiation of stem cells. Clarification of the mechanisms underlying such changes offers exciting potential for various applications such as identification of putative therapeutic targets to modulate stem cell function and stem cell based regenerative medicine.

A Novel, Tumor-Induced Osteoclastogenesis Pathway Insensitive to Denosumab but Interfered by Cannabidiol

Bone metabolism is strictly regulated, and impaired regulation caused by hormonal imbalances induces systemic bone loss. Local bone loss caused by tumor invasion into bone is suggested to be induced by the generation of cytokines, which affect bone metabolism, by tumor cells. The major cause of systemic and local bone losses is excess bone resorption by osteoclasts, which differentiate from macrophages by receptor activator of nuclear factor kappa-B ligand (RANKL) or tumor necrosis factor-alpha (TNF-α). We previously found a novel pathway for tumor-induced osteoclastogenesis targeting osteoclast precursor cells (OPCs). Tumor-induced osteoclastogenesis was resistant to RANKL and TNF-α inhibitors. In the present study, we confirmed that exosomes derived from oral squamous cell carcinoma (OSCC) cells induced osteoclasts from OPCs. We also showed that the depletion of exosomes from culture supernatants of OSCC cells partially interfered with osteoclastogenesis, and cannabidiol, an innoxious cannabinoid without psychotropic effects, almost completely suppressed tumor-induced osteoclastogenesis. Osteoclastogenesis and its interference by cannabidiol were independent of the expression of nuclear factor of T cell c1 (NFATc1). These results show that osteoclastogenesis induced by OSCC cells targeting OPCs is a novel osteoclastogenic pathway independent of NFATc1 expression that is partially caused by tumor-derived exosomes and suppressed by cannabidiol.

Translating current biomedical therapies for long duration, deep space missions

It is been shown that spaceflight-induced molecular, cellular, and physiologic changes cause alterations across many modalities of the human body, including cardiovascular, musculoskeletal, hematological, immunological, ocular, and neurological systems. The Twin Study, a multi-year, multi-omic study of human response to spaceflight, provided detailed and comprehensive molecular and cellular maps of the human response to radiation, microgravity, isolation, and stress. These rich data identified epigenetic, gene expression, inflammatory, and metabolic responses to spaceflight, facilitating a better biomedical roadmap of features that should be monitored and safe-guarded in upcoming missions. Further, by exploring new developments in pre-clinical models and clinical trials, we can begin to design potential cellular interventions for exploration-class missions to Mars and potentially farther. This paper will discuss the overall risks astronauts face during spaceflight, what is currently known about human response to these risks, what pharmaceutical interventions exist for use in space, and which tools of precision medicine and cellular engineering could be applied to aerospace and astronaut medicine.

The effects of simulated +Gz and microgravity on intervertebral disc degeneration in rabbits

The overall objective of this study was to test the hypothesis that +Gz (hypergravity/positive acceleration) and microgravity can both aggravate intervertebral disc degeneration (IVDD). Due to +Gz and microgravity, many pilots develop IVDD. However, the lack of animal models of IVDD under conditions of simulated +Gz and microgravity has hampered research on the onset and prevention of IVDD. Rabbits were randomly allotted to a control group, microgravity group, +Gz group, or mixed (+Gz + microgravity) group. A tail-suspension model was utilized to simulate a microgravity environment and an animal centrifuge to mimic +Gz conditions. After exposure to the above conditions for 4, 8, and 24 weeks, the body weights (BW) of animals in the control group gradually increased over time, while those of animals in the microgravity and mixed groups both decreased (p < 0.001). As compared with the control group, the proteoglycan content of animals in the other three groups was significantly reduced (F = 192.83, p < 0.001). The imageological, histopathological, and immunohistochemical changes to the L6-S1 intervertebral disc samples suggests that the effects of +Gz and microgravity can aggravate IVDD over time. The mixed effects of +Gz and microgravity had the greatest effect on degeneration and +Gz had a particularly greater effect than microgravity.

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.