Function of the cytoskeleton in gravisensing during spaceflight

Influence of gut microbiome on metabolic diseases: a new perspective based on microgravity

Purpose

Microgravity, characterized by gravity levels of 10-3-10-6g, has been found to significantly impair various physiological systems in astronauts, including cardiovascular function, bone density, and metabolism. With the recent surge in human spaceflight, understanding the impact of microgravity on biological health has become paramount.

Methods

A comprehensive literature search was performed using the PubMed database to identify relevant publications pertaining to the interplay between gut microbiome, microgravity, space environment, and metabolic diseases.

Results

This comprehensive review primarily focuses on the progress made in investigating the gut microbiome and its association with metabolic diseases under microgravity conditions. Microgravity induces notable alterations in the composition, diversity, and functionality of the gut microbiome. These changes hold direct implications for metabolic disorders such as cardiovascular disease (CVD), bone metabolism disorders, energy metabolism dysregulation, liver dysfunction, and complications during pregnancy.

Conclusion

This novel perspective is crucial for preparing for deep space exploration and interstellar migration, where understanding the complex interplay between the gut microbiome and metabolic health becomes indispensable.

Morphological Changes of 3T3 Cells under Simulated Microgravity

BACKGROUND

Cells are sensitive to changes in gravity, especially the cytoskeletal structures that determine cell morphology. The aim of this study was to assess the effects of simulated microgravity (SMG) on 3T3 cell morphology, as demonstrated by a characterization of the morphology of cells and nuclei, alterations of microfilaments and microtubules, and changes in cycle progression.

METHODS

3T3 cells underwent induced SMG for 72 h with Gravite®, while the control group was under 1G. Fluorescent staining was applied to estimate the morphology of cells and nuclei and the cytoskeleton distribution of 3T3 cells. Cell cycle progression was assessed by using the cell cycle app of the Cytell microscope, and Western blot was conducted to determine the expression of the major structural proteins and main cell cycle regulators.

RESULTS

The results show that SMG led to decreased nuclear intensity, nuclear area, and nuclear shape and increased cell diameter in 3T3 cells. The 3T3 cells in the SMG group appeared to have a flat form and diminished microvillus formation, while cells in the control group displayed an apical shape and abundant microvilli. The 3T3 cells under SMG exhibited microtubule distribution surrounding the nucleus, compared to the perinuclear accumulation in control cells. Irregular forms of the contractile ring and polar spindle were observed in 3T3 cells under SMG. The changes in cytoskeleton structure were caused by alterations in the expression of major cytoskeletal proteins, including β-actin and α-tubulin 3. Moreover, SMG induced 3T3 cells into the arrest phase by reducing main cell cycle related genes, which also affected the formation of cytoskeleton structures such as microfilaments and microtubules.

CONCLUSIONS

These results reveal that SMG generated morphological changes in 3T3 cells by remodeling the cytoskeleton structure and downregulating major structural proteins and cell cycle regulators.

Next generation of astronauts or ESA astronaut 2.0 concept and spotlight on immunity

Although we have sent humans into space for more than 50 years, crucial questions regarding immune response in space conditions remain unanswered. There are many complex interactions between the immune system and other physiological systems in the human body. This makes it difficult to study the combined long-term effects of space stressors such as radiation and microgravity. In particular, exposure to microgravity and cosmic radiation may produce changes in the performance of the immune system at the cellular and molecular levels and in the major physiological systems of the body. Consequently, abnormal immune responses induced in the space environment may have serious health consequences, especially in future long-term space missions. In particular, radiation-induced immune effects pose significant health challenges for long-duration space exploration missions with potential risks to reduce the organism's ability to respond to injuries, infections, and vaccines, and predispose astronauts to the onset of chronic diseases (e.g., immunosuppression, cardiovascular and metabolic diseases, gut dysbiosis). Other deleterious effects encountered by radiation may include cancer and premature aging, induced by dysregulated redox and metabolic processes, microbiota, immune cell function, endotoxin, and pro-inflammatory signal production^1,2. In this review, we summarize and highlight the current understanding of the effects of microgravity and radiation on the immune system and discuss knowledge gaps that future studies should address.

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.

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.

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 and space radiation inhibit autophagy in human capillary endothelial cells, through either opposite or synergistic effects on specific molecular pathways

Microgravity and space radiation (SR) are two highly influential factors affecting humans in space flight (SF). Many health problems reported by astronauts derive from endothelial dysfunction and impaired homeostasis. Here, we describe the adaptive response of human, capillary endothelial cells to SF. Reference samples on the ground and at 1g onboard permitted discrimination between the contribution of microgravity and SR within the combined responses to SF. Cell softening and reduced motility occurred in SF cells, with a loss of actin stress fibers and a broader distribution of microtubules and intermediate filaments within the cytoplasm than in control cells. Furthermore, in space the number of primary cilia per cell increased and DNA repair mechanisms were found to be activated. Transcriptomics revealed the opposing effects of microgravity from SR for specific molecular pathways: SR, unlike microgravity, stimulated pathways for endothelial activation, such as hypoxia and inflammation, DNA repair and apoptosis, inhibiting autophagic flux and promoting an aged-like phenotype. Conversely, microgravity, unlike SR, activated pathways for metabolism and a pro-proliferative phenotype. Therefore, we suggest microgravity and SR should be considered separately to tailor effective countermeasures to protect astronauts' health.

Cytoskeletal Tensegrity in Microgravity

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

Mechano-Immunomodulation in Space: Mechanisms Involving Microgravity-Induced Changes in T Cells

Of the most prevalent issues surrounding long-term spaceflight, the sustainability of human life and the maintenance of homeostasis in an extreme environment are of utmost concern. It has been observed that the human immune system is dysregulated in space as a result of gravitational unloading at the cellular level, leading to potential complications in astronaut health. A plethora of studies demonstrate intracellular changes that occur due to microgravity; however, these ultimately fall short of identifying the underlying mechanisms and dysfunctions that cause such changes. This comprehensive review covers the changes in human adaptive immunity due to microgravity. Specifically, there is a focus on uncovering the gravisensitive steps in T cell signaling pathways. Changes in gravitational force may lead to interrupted immune signaling cascades at specific junctions, particularly membrane and surface receptor-proximal molecules. Holistically studying the interplay of signaling with morphological changes in cytoskeleton and other cell components may yield answers to what in the T cell specifically experiences the consequences of microgravity. Fully understanding the nature of this problem is essential in order to develop proper countermeasures before long-term space flight is conducted.

Alterations of Growth and Focal Adhesion Molecules in Human Breast Cancer Cells Exposed to the Random Positioning Machine

In this study, we evaluated changes in focal adhesions (FAs) in two types of breast cancer cell (BCC) lines (differentiated MCF-7 and the triple-negative MDA-MB-231 cell line) exposed to simulated microgravity (s-μg) created by a random positioning machine (RPM) for 24 h. After exposure, the BCC changed their growth behavior and exhibited two phenotypes in RPM samples: one portion of the cells grew as a normal two-dimensional monolayer [adherent (AD) BCC], while the other portion formed three-dimensional (3D) multicellular spheroids (MCS). After 1 h and 30 min (MDA-MB-231) and 1 h 40 min (MCF-7), the MCS adhered completely to the slide flask bottom. After 2 h, MDA-MB-231 MCS cells started to migrate, and after 6 h, a large number of the cells had left the MCS and continued to grow in a scattered pattern, whereas MCF-7 cells were growing as a confluent monolayer after 6 h and 24 h. We investigated the genes associated with the cytoskeleton, the extracellular matrix and FAs. ACTB, TUBB, FN1, FAK1, and PXN gene expression patterns were not significantly changed in MDA-MB-231 cells, but we observed a down-regulation of LAMA3, ITGB1 mRNAs in AD cells and of ITGB1, TLN1 and VCL mRNAs in MDA-MB-231 MCS. RPM-exposed MCF-7 cells revealed a down-regulation in the gene expression of FAK1, PXN, TLN1, VCL and CDH1 in AD cells and PXN, TLN and CDH1 in MCS. An interaction analysis of the examined genes involved in 3D growth and adhesion indicated a central role of fibronectin, vinculin, and E-cadherin. Live cell imaging of eGFP-vinculin in MCF-7 cells confirmed these findings. β-catenin-transfected MCF-7 cells revealed a nuclear expression in 1g and RPM-AD cells. The target genes BCL9, MYC and JUN of the Wnt/β-catenin signaling pathway were differentially expressed in RPM-exposed MCF-7 cells. These findings suggest that vinculin and β-catenin are key mediators of BCC to form MCS during 24 h of RPM-exposure.

Latrunculin B facilitates gravitropic curvature of Arabidopsis root by inhibiting cell elongation, especially the cells in the lower flanks of the transition and elongation zones

Gravitropism plays a critical role in the growth and development of plants. Previous reports proposed that the disruption of the actin cytoskeleton resulted in enhanced gravitropism; however, the mechanism underlying these phenomena is still unclear. In the present study, real-time observation on the effect of Latrunculin B (Lat B), a depolymerizing agent of microfilament cytoskeleton, on gravitropism of the primary root of Arabidopsis was undertaken using a vertical stage microscope. The results indicated that Lat B treatment prevented the growth of root, and the growth rates of upper and lower flanks of the horizontally placed root were asymmetrically inhibited. The growth of the lower flank was influenced by Lat B more seriously, resulting in an increased differential growth rate between the upper and lower flanks of the root. Further analysis indicated that Lat B affected cell growth mainly in the transition and elongation zones. Briefly, the current data revealed that Lat B treatment inhibited cell elongation, especially the cells in the lower flanks of the transition and elongation zones, which finally manifested as the facilitation of gravitropic curvature of the primary root.

Investigating into anti-cancer potential of lycopene: Molecular targets

Lycopene, the main pigment of tomatoes, possess the strongest antioxidant activity among carotenoids. Lycopene has unique structure and chemical properties. We searched the literature, via PubMed, Embase, Web of Science and Google database so on to screen citations from inception to Oct 2020 for inclusion in this study. We found that as a common phytochemical, it did not attract much attention in the past few years. However, recent studies have indicated that, in addition to antioxidant activity and the second stage of detoxification, the anticancer of lycopene is also considered to be an important determinant of tumor development including the inhibition of cell proliferation, inhibition of cell cycle progression, induction of apoptosis, inhibition of cell invasion, angiogenesis and metastasis. The effect mechanisms of lycopene are related to the regulation of several signal transduction pathways, such as PI3K/Akt pathway, modulation of insulin-like growth factors system, the suppression of activity of sex steroid hormones, the modification of relevant gene expression, and the alteration of mitochondrial function. These novel findings have suggested that lycopene acts as a promising functional natural pigment, and may be associated with a decreased risk of different types of cancer. This review presents the latest knowledge with respect to its molecular mechanisms and its molecular targets of the inhibitory effects on carcinogenesis.

Evaluating the effect of spaceflight on the host-pathogen interaction between human intestinal epithelial cells and Salmonella Typhimurium

Spaceflight uniquely alters the physiology of both human cells and microbial pathogens, stimulating cellular and molecular changes directly relevant to infectious disease. However, the influence of this environment on host-pathogen interactions remains poorly understood. Here we report our results from the STL-IMMUNE study flown aboard Space Shuttle mission STS-131, which investigated multi-omic responses (transcriptomic, proteomic) of human intestinal epithelial cells to infection with Salmonella Typhimurium when both host and pathogen were simultaneously exposed to spaceflight. To our knowledge, this was the first in-flight infection and dual RNA-seq analysis using human cells.

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.

Sperm Motility of Mice under Simulated Microgravity and Hypergravity

For deep space exploration, reproductive health must be maintained to preserve the species. However, the mechanisms underlying the effect of changes in gravity on male germ cells remain poorly understood. The aim of this study was to determine the effect of simulated micro- and hypergravity on mouse sperm motility and the mechanisms of this change. For 1, 3 and 6 h, mouse sperm samples isolated from the caudal epididymis were subjected to simulated microgravity using a random position machine and 2g hypergravity using a centrifuge. The experimental samples were compared with static and dynamic controls. The sperm motility and the percentage of motile sperm were determined using microscopy and video analysis, cell respiration was determined by polarography, the protein content was assessed by Western blotting and the mRNA levels were determined using qRT-PCR. The results indicated that hypergravity conditions led to more significant changes than simulated microgravity conditions: after 1 h, the speed of sperm movement decreased, and after 3 h, the number of motile cells began to decrease. Under the microgravity model, the speed of movement did not change, but the motile spermatozoa decreased after 6 h of exposure. These changes are likely associated with a change in the structure of the microtubule cytoskeleton, and changes in the energy supply are an adaptive reaction to changes in sperm motility.

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.

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.

Real Microgravity Influences the Cytoskeleton and Focal Adhesions in Human Breast Cancer Cells

With the increasing number of spaceflights, it is crucial to understand the changes occurring in human cells exposed to real microgravity (r-µg) conditions. We tested the effect of r-µg on MCF-7 breast cancer cells with the objective to investigate cytoskeletal alterations and early changes in the gene expression of factors belonging to the cytoskeleton, extracellular matrix, focal adhesion, and cytokines. In the Technische Experimente unter Schwerelosigkeit (TEXUS) 54 rocket mission, we had the opportunity to conduct our experiment during 6 min of r-µg and focused on cytoskeletal alterations of MCF-7 breast cancer cells expressing the Lifeact-GFP marker protein for the visualization of F-actin as well as the mCherry-tubulin fusion protein using the Fluorescence Microscopy Analysis System (FLUMIAS) for fast live-cell imaging under r-µg. Moreover, in a second mission we investigated changes in RNA transcription and morphology in breast cancer cells exposed to parabolic flight (PF) maneuvers (31st Deutsches Zentrum für Luft- und Raumfahrt (DLR) PF campaign). The MCF-7 cells showed a rearrangement of the F-actin and tubulin with holes, accumulations in the tubulin network, and the appearance of filopodia- and lamellipodia-like structures in the F-actin cytoskeleton shortly after the beginning of the r-µg period. PF maneuvers induced an early up-regulation of KRT8, RDX, TIMP1, CXCL8 mRNAs, and a down-regulation of VCL after the first parabola. E-cadherin protein was significantly reduced and is involved in cell adhesion processes, and plays a significant role in tumorigenesis. Changes in the E-cadherin protein synthesis can lead to tumor progression. Pathway analyses indicate that VCL protein has an activating effect on CDH1. In conclusion, live-cell imaging visualized similar changes as those occurring in thyroid cancer cells in r-µg. This result indicates the presence of a common mechanism of gravity perception and sensation.

Microgravity Affects Thyroid Cancer Cells during the TEXUS-53 Mission Stronger than Hypergravity

Thyroid cancer is the most abundant tumor of the endocrine organs. Poorly differentiated thyroid cancer is still difficult to treat. Human cells exposed to long-term real (r-) and simulated (s-) microgravity (µg) revealed morphological alterations and changes in the expression profile of genes involved in several biological processes. The objective of this study was to examine the effects of short-term µg on poorly differentiated follicular thyroid cancer cells (FTC-133 cell line) resulting from 6 min of exposure to µg on a sounding rocket flight. As sounding rocket flights consist of several flight phases with different acceleration forces, rigorous control experiments are mandatory. Hypergravity (hyper-g) experiments were performed at 18g on a centrifuge in simulation of the rocket launch and s-µg was simulated by a random positioning machine (RPM). qPCR analyses of selected genes revealed no remarkable expression changes in controls as well as in hyper-g samples taken at the end of the first minute of launch. Using a centrifuge initiating 18g for 1 min, however, presented moderate gene expression changes, which were significant for COL1A1, VCL, CFL1, PTK2, IL6, CXCL8 and MMP14. We also identified a network of mutual interactions of the investigated genes and proteins by employing in-silico analyses. Lastly, µg-samples indicated that microgravity is a stronger regulator of gene expression than hyper-g.

Effects of simulated microgravity on the expression profiles of RNA during osteogenic differentiation of human bone marrow mesenchymal stem cells

Objectives

Exposure to microgravity induces many adaptive and pathological changes in human bone marrow mesenchymal stem cells (hBMSCs). However, the underlying mechanisms of these changes are poorly understood. We revealed the gene expression patterns of hBMSCs under normal ground (NG) and simulated microgravity (SMG), which showed an interpretation for these changes by gene regulation and signal pathways analysis.

Materials and methods

In this study, hBMSCs were osteogenically induced for 0, 2, 7 and 14 days under normal ground gravity and simulated microgravity, followed by analysis of the differences in transcriptome expression during osteogenic differentiation by RNA sequencing and some experimental verification for these results.

Results

The results indicated that 837, 399 and 894 differentially expressed genes (DEGs) were identified in 2, 7 and 14 days samples, respectively, out of which 13 genes were selected for qRT‐PCR analysis to confirm the RNA‐sequencing results. After analysis, we found that proliferation was inhibited in the early stage of induction. In the middle stage, osteogenic differentiation was inhibited, whereas adipogenic differentiation benefited from SMG. Moreover, SMG resulted in the up‐regulation of genes specific for tumorigenesis in the later stage.

Conclusion

Our data revealed that SMG inhibits the proliferation and inhibits the differentiation towards osteoblasts but promotes adipogenesis. SMG also selects highly tumorigenic cells for survival under prolonged SMG.

Effectiveness of endothelial progenitor cell culture under microgravity for improved angiogenic potential

Endothelial progenitor cell (EPC) transplantation is beneficial for ischemic diseases such as critical limb ischemia and ischemic heart disease. The scarcity of functional EPCs in adults is a limiting factor for EPC transplantation therapy. The quality and quantity culture (QQc) system is an effective ex vivo method for enhancing the number and angiogenic potential of EPCs. Further, microgravity environments have been shown to enhance the functional potential of stem cells. We therefore hypothesized that cells cultured with QQc under microgravity may have enhanced functionality. We cultured human peripheral blood mononuclear cells using QQc under normal (E), microgravity (MG), or microgravity followed by normal (ME) conditions and found that ME resulted in the most significant increase in CD34+ and double positive Dil-Ac-LDL-FITC-Ulex-Lectin cells, both EPC markers. Furthermore, angiogenic potential was determined by an EPC-colony forming assay. While numbers of primitive EPC-colony forming units (pEPC-CFU) did not change, numbers of definitive EPC-CFU colonies increased most under ME conditions. Gene-expression profiling also identified increases in angiogenic factors, including vascular endothelial growth factor, under MG and ME conditions. Thus, QQc along with ME conditions could be an efficient system for significantly enhancing the number and angiogenic potential of EPCs.

Simulated Microgravity Reduces Focal Adhesions and Alters Cytoskeleton and Nuclear Positioning Leading to Enhanced Apoptosis via Suppressing FAK/RhoA-Mediated mTORC1/NF-κB and ERK1/2 Pathways

Simulated-microgravity (SMG) promotes cell-apoptosis. We demonstrated that SMG inhibited cell proliferation/metastasis via FAK/RhoA-regulated mTORC1 pathway. Since mTORC1, NF-κB, and ERK1/2 signaling are important in cell apoptosis, we examined whether SMG-enhanced apoptosis is regulated via these signals controlled by FAK/RhoA in BL6-10 melanoma cells under clinostat-modelled SMG-condition. We show that SMG promotes cell-apoptosis, alters cytoskeleton, reduces focal adhesions (FAs), and suppresses FAK/RhoA signaling. SMG down-regulates expression of mTORC1-related Raptor, pS6K, pEIF4E, pNF-κB, and pNF-κB-regulated Bcl2, and induces relocalization of pNF-κB from the nucleus to the cytoplasm. In addition, SMG also inhibits expression of nuclear envelope proteins (NEPs) lamin-A, emerin, sun1, and nesprin-3, which control nuclear positioning, and suppresses nuclear positioning-regulated pERK1/2 signaling. Moreover, rapamycin, the mTORC1 inhibitor, also enhances apoptosis in cells under 1 g condition via suppressing the mTORC1/NF-κB pathway. Furthermore, the FAK/RhoA activator, toxin cytotoxic necrotizing factor-1 (CNF1), reduces cell apoptosis, restores the cytoskeleton, FAs, NEPs, and nuclear positioning, and converts all of the above SMG-induced changes in molecular signaling in cells under SMG. Therefore, our data demonstrate that SMG reduces FAs and alters the cytoskeleton and nuclear positioning, leading to enhanced cell apoptosis via suppressing the FAK/RhoA-regulated mTORC1/NF-κB and ERK1/2 pathways. The FAK/RhoA regulatory network may, thus, become a new target for the development of novel therapeutics for humans under spaceflight conditions with stressed physiological challenges, and for other human diseases.

Simulated microgravity inhibits cell focal adhesions leading to reduced melanoma cell proliferation and metastasis via FAK/RhoA-regulated mTORC1 and AMPK pathways

Simulated microgravity (SMG) was reported to affect tumor cell proliferation and metastasis. However, the underlying mechanism is elusive. In this study, we demonstrate that clinostat-modelled SMG reduces BL6-10 melanoma cell proliferation, adhesion and invasiveness in vitro and decreases tumor lung metastasis in vivo. It down-regulates metastasis-related integrin α6β4, MMP9 and Met72 molecules. SMG significantly reduces formation of focal adhesions and activation of focal adhesion kinase (FAK) and Rho family proteins (RhoA, Rac1 and Cdc42) and of mTORC1 kinase, but activates AMPK and ULK1 kinases. We demonstrate that SMG inhibits NADH induction and glycolysis, but induces mitochondrial biogenesis. Interestingly, administration of a RhoA activator, the cytotoxic necrotizing factor-1 (CNF1) effectively converts SMG-triggered alterations and effects on mitochondria biogenesis or glycolysis. CNF1 also converts the SMG-altered cell proliferation and tumor metastasis. In contrast, mTORC inhibitor, rapamycin, produces opposite responses and mimics SMG-induced effects in cells at normal gravity. Taken together, our observations indicate that SMG inhibits focal adhesions, leading to inhibition of signaling FAK and RhoA, and the mTORC1 pathway, which results in activation of the AMPK pathway and reduced melanoma cell proliferation and metastasis. Overall, our findings shed a new light on effects of microgravity on cell biology and human health.

Actin cytoskeleton mediates BMP2-Smad signaling via calponin 1 in preosteoblast under simulated microgravity

Microgravity influences the activity of osteoblast, induces actin microfilament disruption and leads to bone loss during spaceflight. Mechanical stress such as gravity, regulates cell function, response and differentiation through dynamic cytoskeleton changes, but the mechanotransduction mechanism remains to be fully elucidated. Previous, we demonstrated actin microfilament mediated osteoblast Cbfa1 responsiveness to BMP2 under simulated microgravity (SMG). Here, we explored a potential molecular and its detailed mechanism of actin cytoskeleton functioning on BMP2-Smad signaling in MC3T3-E1 under SMG. Results showed that the actin microfilament-disrupting agent, cytochalasin B (CB), reduced BMP2-induced activation, translocation of Smad1/5/8 and Runx2 expression. SMG also inhibited BMP2-Smad signaling, which was rescued by actin cytoskeleton stabilizing agent, Jasplakinolide (JAS). Furthermore, we found that siRNA mediated knockdown of calponin 1 (CNN1), an actin binding protein, markedly promoted BMP2-Smad signaling and abolished both inhibition of CB, SMG on BMP2-Smad signaling and the rescue action of JAS. Overexpression of CNN1 inhibited the p-Smad induced by BMP2. Bidirectional Co-IP experiments demonstrated CNN1 could interacted with Smad or p-Smad protein. Furthermore, CB or SMG decreased the phosphorylated CNN1 and increased its interaction with Smad or p-Smad. Combined with the phosphorylation of CNN1 inhibites its actin binding activity, these results indicate that actin cytoskeleton depolymerization inhibites BMP2 signaling via blocking of Smad by dephosphorylated CNN1 in osteoblast cells. Thus, we provide new important insights into the mechanism of mechanotransduction under SMG condition, which probably contribute to bone formation decrease induced by SMG.

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

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

RhoGTPase stimulation is associated with strontium chloride treatment to counter simulated microgravity-induced changes in multipotent cell commitment

Microgravity-related cytoskeletal disorganization is associated with an altered balance between osteoblastogenesis and adipogenesis of multipotent cells. Strontium chloride is known to increase osteoblastogenesis and repress adipogenesis, but its effects in microgravity-related conditions have not been established. Our goal was to investigate early events in this process, focusing on RhoGTPases as controllers of cytoskeletal organization leading to stem cell commitment. We cultivated C3H10T1/2 on microspheres using a rotating wall vessel bioreactor (NASA) in order to simulate microgravity-related conditions in adipogenesis and osteoblastogenesis conditions independently. We observed that rotating wall vessel cultures presented increased adipogenesis, while osteoblastogenesis was reduced. Strontium-treated multipotent cells presented a significant repression in adipogenesis (−90 %, p < 0.001 PPARyD8) and an activation of osteoblastogenesis (+95 %, p < 0.001 bone sialoprotein and osteopontin D8), even in gravity altered conditions. We established that concomitant RhoA/Rac1 activations were associated with osteoblastogenesis enhancement and adipogenesis limitation in uncommitted cells. As vascular endothelial growth factor splicing is mechanosensitive and its signaling is central to stem cell commitment, we investigated vascular endothelial growth factor production, isoforms and receptors expressions in our conditions. We observed that vascular endothelial growth factor and receptors expressions were not significantly affected, but we found that presence of soluble vascular endothelial growth factor was associated with RhoA/Rac1 activations, whereas sequestration of vascular endothelial growth factor by cells was associated with RhoA/Rac1 inhibitions. We propose that strontium triggers secretion of vascular endothelial growth factor and the subsequent Rac1 and RhoA activations leading to repression of adipogenesis and osteogenesis stimulation validating strontium as a counter measure for microgravity-induced alteration of cell commitment.

A chemical element naturally found for instance in seafood or grains, could counter bone loss from long-term spaceflight. Alain Guignandon and colleagues from the Université de Lyon à St-Etienne in France exposed multipotent embryonic fibroblasts to microgravity conditions similar to those found in space. They found the balance shifted in these stem cells from differentiating to bone-forming cells (osteoblasts) to differentiating to fatty-tissue forming cells (adipocytes). When the cells were treated with strontium, the shift toward osteoblastogenesis was regained. Strontium achieves this by sustaining the activity of two proteins that play a role in bone development but are suppressed in space. Strontium’s effect on the proteins could happen via release of vascular endothelial growth factor, which, under normal gravity conditions, plays a role in committing the cell to differentiation into osteoblasts rather than adipoyctes.

Mechanotransduction as an Adaptation to Gravity

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

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

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

Simulated Microgravity Using a Rotary Culture System Compromises the In Vitro Development of Mouse Preantral Follicles

Background

Growing cells in simulated weightlessness condition might be a highly promising new technique to maintain or generate tissue constructs in a scaffold-free manner. There is limited evidence that microgravity condition may affect development of ovarian follicles. The objective of the present study was to investigate the effects of simulated microgravity on the in vitro development of mouse preantral follicles.

Methods and Results

Ovarian tissue from 14-day-old mice, or preantral follicles mechanically isolated from 14-day-old mouse ovaries were cultured at a simulated microgravity condition generated using a rotating wall vessel apparatus. Follicle survival was assessed quantitatively using H&E staining. Follicle diameter and oocyte diameter were measured under an inverted microscope. Ultrastructure of oocytes was evaluated using transmission electron microscopy. We observed that simulated microgravity compromised follicle survival in vitro, downregulated PCNA and GDF-9 expressions, and caused ultrastructural abnormalities in oocytes.

Conclusion

This study showed for the first time that three-dimensional culture condition generated by simulated microgravity is detrimental to the initial stage development of mouse preantral follicles in vitro. The experimental setup provides a model to further investigate the mechanisms involved in the in vitro developmental processes of oocytes/granulosa cells under the microgravity condition.

Simulated Microgravity Promotes Cell Apoptosis Through Suppressing Uev1A/TICAM/TRAF/NF-κB-Regulated Anti-Apoptosis and p53/PCNA- and ATM/ATR-Chk1/2-Controlled DNA-Damage Response Pathways

Microgravity has been known to induce cell death. However, its underlying mechanism is less studied. In this study, BL6-10 melanoma cells were cultured in flasks under simulated microgravity (SMG). We examined cell apoptosis, and assessed expression of genes associated with apoptosis and genes regulating apoptosis in cells under SMG. We demonstrate that SMG induces cell morphological changes and microtubule alterations by confocal microscopy, and enhances apoptosis by flow cytometry, which was associated with up- and down-regulation of pro-apoptotic and anti-apoptotic genes, respectively. Moreover, up- and down-regulation of pro-apoptotic (Caspases 3, 7, 8) and anti-apoptotic (Bcl2 and Bnip3) molecules was confirmed by Western blotting analysis. Western blot analysis also indicates that SMG causes inhibition of an apoptosis suppressor, pNF-κB-p65, which is complemented by the predominant localization of NF-κB-p65 in the cytoplasm. SMG also reduces expression of molecules regulating the NF-κB pathway including Uev1A, TICAM, TRAF2, and TRAF6. Interestingly, 10 DNA repair genes are down-regulated in cells exposed to SMG, among which down-regulation of Parp, Ercc8, Rad23, Rad51, and Ku70 was confirmed by Western blotting analysis. In addition, we demonstrate a significant inhibition of molecules involved in the DNA-damage response, such as p53, PCNA, ATM/ATR, and Chk1/2. Taken together, our work reveals that SMG promotes the apoptotic response through a combined modulation of the Uev1A/TICAM/TRAF/NF-κB-regulated apoptosis and the p53/PCNA- and ATM/ATR-Chk1/2-controlled DNA-damage response pathways. Thus, our investigation provides novel information, which may help us to determine the cause of negative alterations in human physiology occurring at spaceflight environment. J. Cell. Biochem. 117: 2138-2148, 2016. © 2016 Wiley Periodicals, Inc.

Transient gene and microRNA expression profile changes of confluent human fibroblast cells in spaceflight

Microgravity, or an altered gravity environment different from the 1 g of the Earth, has been shown to influence global gene expression patterns and protein levels in cultured cells. However, most of the reported studies that have been conducted in space or by using simulated microgravity on the ground have focused on the growth or differentiation of these cells. It has not been specifically addressed whether nonproliferating cultured cells will sense the presence of microgravity in space. In an experiment conducted onboard the International Space Station, confluent human fibroblast cells were fixed after being cultured in space for 3 and 14 d, respectively, to investigate changes in gene and microRNA (miRNA) expression profiles in these cells. Results of the experiment showed that on d 3, both the flown and ground cells were still proliferating slowly, as measured by the percentage of Ki-67(+) cells. Gene and miRNA expression data indicated activation of NF-κB and other growth-related pathways that involve hepatocyte growth factor and VEGF as well as the down-regulation of the Let-7 miRNA family. On d 14, when the cells were mostly nonproliferating, the gene and miRNA expression profile of the flight sample was indistinguishable from that of the ground sample. Comparison of gene and miRNA expressions in the d 3 samples, with respect to d 14, revealed that most of the changes observed on d 3 were related to cell growth for both the flown and ground cells. Analysis of cytoskeletal changes via immunohistochemistry staining of the cells with antibodies for α-tubulin and fibronectin showed no difference between the flown and ground samples. Taken together, our study suggests that in true nondividing human fibroblast cells in culture, microgravity experienced in space has little effect on gene and miRNA expression profiles.-Zhang, Y., Lu, T., Wong, M., Wang, X., Stodieck, L., Karouia, F., Story, M., Wu, H. Transient gene and microRNA expression profile changes of confluent human fibroblast cells in spaceflight.

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

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

Mechanisms of three-dimensional growth of thyroid cells during long-term simulated microgravity

Three-dimensional multicellular spheroids (MCS) of human cells are important in cancer research. We investigated possible mechanisms of MCS formation of thyroid cells. Both, normal Nthy-ori 3–1 thyroid cells and the poorly differentiated follicular thyroid cancer cells FTC-133 formed MCS within 7 and 14 days of culturing on a Random Positioning Machine (RPM), while a part of the cells continued to grow adherently in each culture. The FTC-133 cancer cells formed larger and numerous MCS than the normal cells. In order to explain the different behaviour, we analyzed the gene expression of IL6, IL7, IL8, IL17, OPN, NGAL, VEGFA and enzymes associated cytoskeletal or membrane proteins (ACTB, TUBB, PFN1, CPNE1, TGM2, CD44, FLT1, FLK1, PKB, PKC, ERK1/2, Casp9, Col1A1) as well as the amount of secreted proteins (IL-6, IL-7, IL-8, IL-17, OPN, NGAL, VEGFA). Several of these components changed during RPM-exposure in each cell line. Striking differences between normal and malignant cells were observed in regards to the expression of genes of NGAL, VEGFA, OPN, IL6 and IL17 and to the secretion of VEGFA, IL-17, and IL-6. These results suggest several gravi-sensitive growth or angiogenesis factors being involved in 3D formation of thyroid cells cultured under simulated microgravity.

Stem cell health and tissue regeneration in microgravity

Exposure to microgravity causes significant mechanical unloading of mammalian tissues, resulting in rapid alterations of their physiology, which poses a significant risk for long-duration manned spaceflight. The immediate degenerative effects of spaceflight we understand best are those studied during short-term low-Earth-orbit experiments, and include rapid microgravity-adaptive bone and muscle loss, loss of cardiovascular capacity, defects in wound and bone fracture healing, and impaired immune function. Over the long-term, exposure to microgravity may cause severe deficits in mammalian stem cell-based tissue regenerative health, including, osteogenesis, hematopoiesis, and lymphopoeisis, as well as cause significant stem cell-based tissue degeneration in amphibian tail and lens regeneration. To address the needs for stem cell and other cell science research on the International Space Station (ISS), NASA has developed the new Bioculture System that will allow investigators to initiate and conduct on-orbit experiments that astronauts will be able to monitor and interact with during the course of cell cultures. This cell culture capability combined with advanced technologies for molecular biology and on-orbit measurement of gene expression (WetLab2) and other tools that are now coming online bring the ISS National Laboratory a step closer to becoming a fully functional space laboratory for advancing space biological sciences.

Moderate alterations of the cytoskeleton in human chondrocytes after short-term microgravity produced by parabolic flight maneuvers could be prevented by up-regulation of BMP-2 and SOX-9

Real and simulated microgravity induce a variety of changes in human cells. Most importantly, changes in the cytoskeleton have been noted, and studies on microtubules have shown that they are gravisensitive. This study focuses on the effects of short-term real microgravity on gene expression, protein content, and cytoskeletal structure of human chondrocytes. We cultivated human chondrocytes, took them along a parabolic flight during the 24th Deutsches Zentrum für Luft- und Raumfahrt Parabolic (DLR) Flight Campaign, and fixed them after the 1st and the 31st parabola. Immunofluorescence microscopy revealed no changes after the 1st parabola, but disruptions of β-tubulin, vimentin, and cytokeratin networks after the 31st parabola. No F-actin stress fibers were detected even after 31 parabolas. Furthermore, mRNA and protein quantifications after the 31st parabola showed a clear up-regulation of cytoskeletal genes and proteins. The mRNAs were significantly up-regulated as follows: TUBB, 2-fold; VIM, 1.3-fold; KRT8, 1.8-fold; ACTB, 1.9-fold; ICAM1, 4.8-fold; OPN, 7-fold; ITGA10, 1.5-fold; ITGB1, 1.2-fold; TGFB1, 1.5-fold; CAV1, 2.6-fold; SOX9, 1.7-fold; BMP-2, 5.3-fold. However, SOX5 (-25%) and SOX6 (-28%) gene expression was decreased. Contrary, no significant changes in gene expression levels were observed during vibration and hypergravity experiments. These data suggest that short-term microgravity affects the gene expression of distinct proteins. In contrast to poorly differentiated follicular thyroid cancer cells or human endothelial cells, chondrocytes only exert moderate cytoskeletal alterations. The up-regulation of BMP-2, TGF-β1, and SOX9 in chondrocytes may play a key role in preventing cytoskeletal alterations.

RhoGTPases as key players in mammalian cell adaptation to microgravity

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

Regulation of ICAM-1 in cells of the monocyte/macrophage system in microgravity

Cells of the immune system are highly sensitive to altered gravity, and the monocyte as well as the macrophage function is proven to be impaired under microgravity conditions. In our study, we investigated the surface expression of ICAM-1 protein and expression of ICAM-1 mRNA in cells of the monocyte/macrophage system in microgravity during clinostat, parabolic flight, sounding rocket, and orbital experiments. In murine BV-2 microglial cells, we detected a downregulation of ICAM-1 expression in clinorotation experiments and a rapid and reversible downregulation in the microgravity phase of parabolic flight experiments. In contrast, ICAM-1 expression increased in macrophage-like differentiated human U937 cells during the microgravity phase of parabolic flights and in long-term microgravity provided by a 2D clinostat or during the orbital SIMBOX/Shenzhou-8 mission. In nondifferentiated U937 cells, no effect of microgravity on ICAM-1 expression could be observed during parabolic flight experiments. We conclude that disturbed immune function in microgravity could be a consequence of ICAM-1 modulation in the monocyte/macrophage system, which in turn could have a strong impact on the interaction with T lymphocytes and cell migration. Thus, ICAM-1 can be considered as a rapid-reacting and sustained gravity-regulated molecule in mammalian cells.

Active transmembrane drug transport in microgravity: a validation study using an ABC transporter model

Microgravity has been shown to influence the expression of ABC (ATP-Binding Cassette) transporters in bacteria, fungi and mammals, but also to modify the activity of certain cellular components with structural and functional similarities to ABC transporters. Changes in activity of ABC transporters could lead to important metabolic disorders and undesired pharmacological effects during spaceflights. However, no current means exist to study the functionality of these transporters in microgravity. To this end, a Vesicular Transport Assay ^® (Solvo Biotechnology, Hungary) was adapted to evaluate multi-drug resistance-associated protein 2 (MRP2) trans-membrane estradiol-17-β-glucuronide (E17βG) transport activity, when activated by adenosine-tri-phosphate (ATP) during parabolic flights. Simple diffusion, ATP-independent transport and benzbromarone inhibition were also evaluated. A high accuracy engineering system was designed to perform, monitor and synchronize all procedures. Samples were analysed using a validated high sensitivity drug detection protocol. Experiments were performed in microgravity during parabolic flights, and compared to 1g on ground results using identical equipment and procedures in all cases. Our results revealed that sufficient equipment accuracy and analytical sensitivity were reached to detect transport activity in both gravitational conditions. Additionally, transport activity levels of on ground samples were within commercial transport standards, proving the validity of the methods and equipment used. MRP2 net transport activity was significantly reduced in microgravity, so was signal detected in simple diffusion samples. Ultra-structural changes induced by gravitational stress upon vesicle membranes or transporters could explain the current results, although alternative explanations are possible. Further research is needed to provide a conclusive answer in this regard. Nevertheless, the present validated technology opens new and interesting research lines in biology and human physiology with the potential for significant benefits for both space and terrestrial medicine.

The impact of simulated and real microgravity on bone cells and mesenchymal stem cells

How microgravity affects the biology of human cells and the formation of 3D cell cultures in real and simulated microgravity (r- and s-µg) is currently a hot topic in biomedicine. In r- and s-µg, various cell types were found to form 3D structures. This review will focus on the current knowledge of tissue engineering in space and on Earth using systems such as the random positioning machine (RPM), the 2D-clinostat, or the NASA-developed rotating wall vessel bioreactor (RWV) to create tissue from bone, tumor, and mesenchymal stem cells. To understand the development of 3D structures, in vitro experiments using s-µg devices can provide valuable information about modulations in signal-transduction, cell adhesion, or extracellular matrix induced by altered gravity conditions. These systems also facilitate the analysis of the impact of growth factors, hormones, or drugs on these tissue-like constructs. Progress has been made in bone tissue engineering using the RWV, and multicellular tumor spheroids (MCTS), formed in both r- and s-µg, have been reported and were analyzed in depth. Currently, these MCTS are available for drug testing and proteomic investigations. This review provides an overview of the influence of µg on the aforementioned cells and an outlook for future perspectives in tissue engineering.

Terrestrial stress analogs for spaceflight associated immune system dysregulation

Recent data indicates that dysregulation of the immune system occurs and persists during spaceflight. Impairment of immunity, especially in conjunction with elevated radiation exposure and limited clinical care, may increase certain health risks during exploration-class deep space missions (i.e. to an asteroid or Mars). Research must thoroughly characterize immune dysregulation in astronauts to enable development of a monitoring strategy and validate any necessary countermeasures. Although the International Space Station affords an excellent platform for on-orbit research, access may be constrained by technical, logistical vehicle or funding limitations. Therefore, terrestrial spaceflight analogs will continue to serve as lower cost, easier access platforms to enable basic human physiology studies. Analog work can triage potential in-flight experiments and thus result in more focused on-orbit studies, enhancing overall research efficiency. Terrestrial space analogs generally replicate some of the physiological or psychological stress responses associated with spaceflight. These include the use of human test subjects in a laboratory setting (i.e. exercise, bed rest, confinement, circadian misalignment) and human remote deployment analogs (Antarctica winterover, undersea, etc.) that incorporate confinement, isolation, extreme environment, physiological mission stress and disrupted circadian rhythms. While bed rest has been used to examine the effects of physical deconditioning, radiation and microgravity may only be simulated in animal or microgravity cell culture (clinorotation) analogs. This article will characterize the array of terrestrial analogs for spaceflight immune dysregulation, the current evidence base for each, and interpret the analog catalog in the context of acute and chronic stress.

Rac1 GTPase silencing counteracts microgravity-induced effects on osteoblastic cells

Bone cells exposed to real microgravity display alterations of their cytoskeleton and focal adhesions, two major mechanosensitive structures. These structures are controlled by small GTPases of the Ras homology (Rho) family. We investigated the effects of RhoA, Rac1, and Cdc42 modulation of osteoblastic cells under microgravity conditions. Human MG-63 osteoblast-like cells silenced for RhoGTPases were cultured in the automated Biobox bioreactor (European Space Agency) aboard the Foton M3 satellite and compared to replicate ground-based controls. The cells were fixed after 69 h of microgravity exposure for postflight analysis of focal contacts, F-actin polymerization, vascular endothelial growth factor (VEGF) expression, and matrix targeting. We found that RhoA silencing did not affect sensitivity to microgravity but that Rac1 and, to a lesser extent, Cdc42 abrogation was particularly efficient in counteracting the spaceflight-related reduction of the number of focal contacts [-50% in silenced, scrambled (SiScr) controls vs. -15% for SiRac1], the number of F-actin fibers (-60% in SiScr controls vs. -10% for SiRac1), and the depletion of matrix-bound VEGF (-40% in SiScr controls vs. -8% for SiRac1). Collectively, these data point out the role of the VEGF/Rho GTPase axis in mechanosensing and validate Rac1-mediated signaling pathways as potential targets for counteracting microgravity effects.

Physiological effects of microgravity on bone cells

Life on Earth developed under the influence of normal gravity (1g). With evidence from previous studies, scientists have suggested that normal physiological processes, such as the functional integrity of muscles and bone mass, can be affected by microgravity during spaceflight. During the life span, bone not only develops as a structure designed specifically for mechanical tasks but also adapts for efficiency. The lack of weight-bearing forces makes microgravity an ideal physical stimulus to evaluate bone cell responses. One of the most serious problems induced by long-term weightlessness is bone mineral loss. Results from in vitro studies that entailed the use of bone cells in spaceflights showed modification in cell attachment structures and cytoskeletal reorganization, which may be involved in bone loss. Humans exposed to microgravity conditions experience various physiological changes, including loss of bone mass, muscle deterioration, and immunodeficiency. In vitro models can be used to extract valuable information about changes in mechanical stress to ultimately identify the different pathways of mechanotransduction in bone cells. Despite many in vivo and in vitro studies under both real microgravity and simulated conditions, the mechanism of bone loss is still not well defined. The objective of this review is to summarize the recent research on bone cells under microgravity conditions based on advances in the field.

Stem Cells toward the Future: The Space Challenge

Astronauts experience weightlessness-induced bone loss due to an unbalanced process of bone remodeling that involves bone mesenchymal stem cells (bMSCs), as well as osteoblasts, osteocytes, and osteoclasts. The effects of microgravity on osteo-cells have been extensively studied, but it is only recently that consideration has been given to the role of bone MSCs. These live in adult bone marrow niches, are characterized by their self-renewal and multipotent differentiation capacities, and the published data indicate that they may lead to interesting returns in the biomedical/bioengineering fields. This review describes the published findings concerning bMSCs exposed to simulated/real microgravity, mainly concentrating on how mechanosignaling, mechanotransduction and oxygen influence their proliferation, senescence and differentiation. A comprehensive understanding of bMSC behavior in microgravity and their role in preventing bone loss will be essential for entering the future age of long-lasting, manned space exploration.

A firmer understanding of the effect of hypergravity on thyroid tissue: cholesterol and thyrotropin receptor

Maintaining a good health requires the maintenance of a body homeostasis which largely depends on correct functioning of thyroid gland. The cells of the thyroid tissue are strongly sensitive to hypogravity, as already proven in mice after returning to the earth from long-term space missions. Here we studied whether hypergravity may be used to counteract the physiological deconditioning of long-duration spaceflight. We investigated the influence of hypergravity on key lipids and proteins involved in thyroid tissue function. We quantified cholesterol (CHO) and different species of sphingomyelin (SM) and ceramide, analysed thyrotropin (TSH) related molecules such as thyrotropin-receptor (TSHR), cAMP, Caveolin-1 and molecule signalling such as Signal transducer and activator of transcription-3 (STAT3). The hypergravity treatment resulted in the upregulation of the TSHR and Caveolin-1 and downregulation of STAT3 without changes of cAMP. TSHR lost its specific localization and spread throughout the cell membrane; TSH treatment facilitated the shedding of α subunit of TSHR and its releasing into the extracellular space. No specific variations were observed for each species of SM and ceramide. Importantly, the level of CHO was strongly reduced. In conclusion, hypergravity conditions induce change in CHO and TSHR of thyroid gland. The possibility that lipid rafts are strongly perturbed by hypergravity-induced CHO depletion by influencing TSH-TSHR interaction was discussed.

Growing tissues in real and simulated microgravity: new methods for tissue engineering

Tissue engineering in simulated (s-) and real microgravity (r-μg) is currently a topic in Space medicine contributing to biomedical sciences and their applications on Earth. The principal aim of this review is to highlight the advances and accomplishments in the field of tissue engineering that could be achieved by culturing cells in Space or by devices created to simulate microgravity on Earth. Understanding the biology of three-dimensional (3D) multicellular structures is very important for a more complete appreciation of in vivo tissue function and advancing in vitro tissue engineering efforts. Various cells exposed to r-μg in Space or to s-μg created by a random positioning machine, a 2D-clinostat, or a rotating wall vessel bioreactor grew in the form of 3D tissues. Hence, these methods represent a new strategy for tissue engineering of a variety of tissues, such as regenerated cartilage, artificial vessel constructs, and other organ tissues as well as multicellular cancer spheroids. These aggregates are used to study molecular mechanisms involved in angiogenesis, cancer development, and biology and for pharmacological testing of, for example, chemotherapeutic drugs or inhibitors of neoangiogenesis. Moreover, they are useful for studying multicellular responses in toxicology and radiation biology, or for performing coculture experiments. The future will show whether these tissue-engineered constructs can be used for medical transplantations. Unveiling the mechanisms of microgravity-dependent molecular and cellular changes is an up-to-date requirement for improving Space medicine and developing new treatment strategies that can be translated to in vivo models while reducing the use of laboratory animals.