Effects of intermittent or continuous gravitational stresses on cell-matrix adhesion: quantitative analysis of focal contacts in osteoblastic ROS 17/2.8 cells

Exploring the influence of microgravity on chemotherapeutic drug response in cancer: Unveiling new perspectives

Microgravity, an altered gravity condition prevailing in space, has been reported to have a profound impact on human health. Researchers are very keen to comprehensively investigate the impact of microgravity and its intricate involvement in inducing physiological changes. Evidenced transformations were observed in the internal architecture including cytoskeletal organization and cell membrane morphology. These alterations can significantly influence cellular function, signalling pathways and overall cellular behaviour. Further, microgravity has been reported to alter in the expression profile of genes and metabolic pathways related to cellular processes, signalling cascades and structural proteins in cancer cells contributing to the overall changes in the cellular architecture. To investigate the effect of microgravity on cellular and molecular levels numerous ground-based simulation systems employing both in vitro and in vivo models are used. Recently, researchers have explored the possibility of leveraging microgravity to potentially modulate cancer cells against chemotherapy. These findings hold promise for both understanding fundamental processes and could potentially lead to the development of more effective, personalized and innovative approaches in therapeutic advancements against cancer.

Evaluation of the Effects of Microgravity on Activated Primary Human Hepatic Stellate Cells

In recent years, research has been conducted to develop new medical treatments by simulating environments existing in space, such as zero-gravity. In this study, we evaluated the cell proliferation and gene expression of activated primary human hepatic stellate cells (HHSteCs) under simulated microgravity (SMG). Under SMG, cell proliferation was slower than in 1 G, and the evaluation of gene expression changes on day 1 of SMG by serial analysis of gene expression revealed the presence of Sirtuin, EIF2 signaling, hippo signaling, and epithelial adherence junction signaling. Moreover, reactive oxygen species were upregulated under SMG, and when N-acetyl-cystein was added, no difference in proliferation between SMG and 1 G was observed, suggesting that the oxidative stress generated by mitochondrial dysfunction caused a decrease in proliferation. Upstream regulators such as smad3, NFkB, and FN were activated, and cell-permeable inhibitors such as Ly294002 and U0126 were inhibited. Immunohistochemistry performed to evaluate cytoskeletal changes showed that more β-actin was localized in the cortical layer under SMG.

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.

The effect of macropore size of hydroxyapatite scaffold on the osteogenic differentiation of bone mesenchymal stem cells under perfusion culture

Previous studies have proved that dynamic culture could facilitate nutrients transport and apply mechanical stimulation to the cells within three-dimensional scaffolds, thus enhancing the differentiation of stem cells towards the osteogenic phenotype. However, the effects of macropore size on osteogenic differentiation of stem cells under dynamic condition are still unclear. Therefore, the objective of this study was to investigate the effects of macropore size of hydroxyapatite (HAp) scaffolds on osteogenic differentiation of bone mesenchymal stem cells under static and perfusion culture conditions. In vitro cell culture results showed that cell proliferation, alkaline phosphate (ALP) activity, mRNA expression of ALP, collagen-I (Col-I), osteocalcin (OCN) and osteopontin (OPN) were enhanced when cultured under perfusion condition in comparison to static culture. Under perfusion culture condition, the ALP activity and the gene expression of ALP, Col-I, OCN and OPN were enhanced with the macropore size decreasing from 1300 to 800 µm. However, with the further decrease in macropore size from 800 to 500 µm, the osteogenic related gene expression and protein secretion were reduced. Computational fluid dynamics analysis showed that the distribution areas of medium- and high-speed flow increased with the decrease in macropore size, accompanied by the increase of the fluid shear stress within the scaffolds. These results confirm the effects of macropore size on fluid flow stimuli and cell differentiation, and also help optimize the macropore size of HAp scaffolds for bone tissue engineering.

The effects of locomotion on bone marrow mesenchymal stem cell fate: insight into mechanical regulation and bone formation

Bone marrow mesenchymal stem cells (BMSCs) refer to a heterogeneous population of cells with the capacity for self-renewal. BMSCs have multi-directional differentiation potential and can differentiate into chondrocytes, osteoblasts, and adipocytes under specific microenvironment or mechanical regulation. The activities of BMSCs are closely related to bone quality. Previous studies have shown that BMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. Thus, a goal of this review is to discuss how these ubiquious signals arising from mechanical stimulation are perceived by BMSCs and then how the cells respond to them. Studies in recent years reported a significant effect of locomotion on the migration, proliferation and differentiation of BMSCs, thus, contributing to our bone mass. This regulation is realized by the various intersecting signaling pathways including RhoA/Rock, IFG, BMP and Wnt signalling. The mechanoresponse of BMSCs also provides guidance for maintaining bone health by taking appropriate exercises. This review will summarize the regulatory effects of locomotion/mechanical loading on BMSCs activities. Besides, a number of signalling pathways govern MSC fate towards osteogenic or adipocytic differentiation will be discussed. The understanding of mechanoresponse of BMSCs makes the foundation for translational medicine.

Rapid Morphological and Cytoskeletal Response to Microgravity in Human Primary Macrophages

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

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.

Mechanical remodeling of normally sized mammalian cells under a gravity vector

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

Mechanical forces induce changes in VEGF and VEGFR-1/sFlt-1 expression in human chondrocytes

Expression of the pro-angiogenic vascular endothelial growth factor (VEGF) stimulates angiogenesis and correlates with the progression of osteoarthritis. Mechanical joint loading seems to contribute to this cartilage pathology. Cyclic equibiaxial strains of 1% to 16% for 12 h, respectively, induced expression of VEGF in human chondrocytes dose- and frequency-dependently. Stretch-mediated VEGF induction was more prominent in the human chondrocyte cell line C-28/I2 than in primary articular chondrocytes. Twelve hours of 8% stretch induced VEGF expression to 175% of unstrained controls for at least 24 h post stretching, in promoter reporter and enzyme-linked immunosorbent assay (ELISA) studies. High affinity soluble VEGF-receptor, sVEGFR-1/sFlt-1 was less stretch-inducible than its ligand, VEGF-A, in these cells. ELISA assays demonstrated, for the first time, a stretch-mediated suppression of sVEGFR-1 secretion 24 h after stretching. Overall, strained chondrocytes activate their VEGF expression, but in contrast, strain appears to suppress the secretion of the major VEGF decoy receptor (sVEGFR-1/sFlt-1). The latter may deplete a biologically relevant feedback regulation to inhibit destructive angiogenesis in articular cartilage. Our data suggest that mechanical stretch can induce morphological changes in human chondrocytes in vitro. More importantly, it induces disturbed VEGF signaling, providing a molecular mechanism for a stress-induced increase in angiogenesis in cartilage pathologies.

Osteoblast-osteocyte transformation. A SEM densitometric analysis of endosteal apposition in rabbit femur

Transformation of osteoblasts into osteocytes is marked by changes in volume and cell shape. The reduction of volume and the entrapment process are correlated with the synthesis activity of the cell which decreases consequently. This transformation process has been extensively investigated by transmission electron microscopy (TEM) but no data have yet been published regarding osteoblast-osteocyte dynamic histomorphometry. Scanning electron microscope (SEM) densitometric analysis was carried out to determine the osteoblast and open osteocyte lacunae density in corresponding areas of a rabbit femur endosteal surface. The lining cell density was 4900.1 ± 30.03 n mm(-2), the one of open osteocyte lacunae 72.89 ± 22.55 n mm(-2). This corresponds to an index of entrapment of one cell every 67.23 osteoblasts (approximated by defect). The entrapment sequence begins with flattening of the osteoblast and spreading of equatorial processes. At first these are covered by the new apposed matrix and then also the whole cellular body of the osteocyte undergoing entrapment. The dorsal aspect of the cell membrane suggests that closure of the osteocyte lacuna may be partially carried out by the same osteoblast-osteocyte which developed a dorsal secretory territory. A significant proportion of the endosteal surface was analysed by SEM, without observing any evidence of osteoblast mitotic figures. This indicates that recruitment of the pool of osteogenic cells in cortical bone lamellar systems occurs prior to the entrapment process. No further additions occurred once osteoblasts were positioned on the bone surface and began lamellar apposition. The number of active osteoblasts on the endosteal surface exceeded that of the cells which become incorporated as osteocytes (whose number was indicated by the number of osteocyte lacunae). Therefore such a balance must be equilibrated by the osteoblasts' transformation in resting lining cells or by apoptosis. The current work characterised osteoblast shape changes throughout the entrapment process, allowing approximate calculation of an osteoblast entrapment index in the rabbit endosteal cortex.

Mechano-regulation of alternative splicing

Alternative splicing contributes to the complexity of proteome by producing multiple mRNAs from a single gene. Affymetrix exon arrays and experiments in vivo or in vitro demonstrated that alternative splicing was regulated by mechanical stress. Expression of mechano-growth factor (MGF) which is the splicing isoform of insulin-like growth factor 1(IGF-1) and vascular endothelial growth factor (VEGF) splicing variants such as VEGF₁₂₁, VEGF_165, VEGF₂₀₆, VEGF₁₈₉, VEGF₁₆₅ and VEGF₁₄₅ are regulated by mechanical stress. However, the mechanism of this process is not yet clear. Increasing evidences showed that the possible mechanism is related to Ca²⁺ signal pathway and phosphorylation signal pathway. This review proposes possible mechanisms of mechanical splicing regulation. This will contribute to the biomechanical study of alternative splicing.

Bone turnover in wild type and pleiotrophin-transgenic mice housed for three months in the International Space Station (ISS)

Bone is a complex dynamic tissue undergoing a continuous remodeling process. Gravity is a physical force playing a role in the remodeling and contributing to the maintenance of bone integrity. This article reports an investigation on the alterations of the bone microarchitecture that occurred in wild type (Wt) and pleiotrophin-transgenic (PTN-Tg) mice exposed to a near-zero gravity on the International Space Station (ISS) during the Mice Drawer System (MDS) mission, to date, the longest mice permanence (91 days) in space. The transgenic mouse strain over-expressing pleiotrophin (PTN) in bone was selected because of the PTN positive effects on bone turnover. Wt and PTN-Tg control animals were maintained on Earth either in a MDS payload or in a standard vivarium cage. This study revealed a bone loss during spaceflight in the weight-bearing bones of both strains. For both Tg and Wt a decrease of the trabecular number as well as an increase of the mean trabecular separation was observed after flight, whereas trabecular thickness did not show any significant change. Non weight-bearing bones were not affected. The PTN-Tg mice exposed to normal gravity presented a poorer trabecular organization than Wt mice, but interestingly, the expression of the PTN transgene during the flight resulted in some protection against microgravity’s negative effects. Moreover, osteocytes of the Wt mice, but not of Tg mice, acquired a round shape, thus showing for the first time osteocyte space-related morphological alterations in vivo. The analysis of specific bone formation and resorption marker expression suggested that the microgravity-induced bone loss was due to both an increased bone resorption and a decreased bone deposition. Apparently, the PTN transgene protection was the result of a higher osteoblast activity in the flight mice.

Long-term stent use can prevent postoperative canal stenosis in patients with congenital aural atresia

OBJECTIVE

Postoperative external auditory canal (EAC) stenosis is the most common complication after congenital aural atresia (CAA) surgery. The authors applied an ear mold or hearing aid as an EAC stent following surgery and analyzed the impact on postoperative EAC stenosis.

STUDY DESIGN

Historical cohort study.

SETTING

A tertiary hospital.

SUBJECTS AND METHODS

Ninety-six patients who underwent canaloplasty between 1996 and 2010 were included in this study. To discover factors contributing to postoperative EAC stenosis, clinical parameters, including age, sex, Marx grading, Schuknecht classification, Jahrsdoerfer score, surgical approach, triamcinolone injection, and use of stenting with an ear mold or hearing aid, were reviewed and analyzed. Each stent was used for at least 6 months postoperatively. Pure-tone audiometry was performed preoperatively and 3, 6, and 12 months after canaloplasty.

RESULTS

Postoperative EAC stenosis was the most common postoperative complication, occurring in 8 (8.2%) cases with a mean time interval of 4.1 months. For patients who did not use an ear mold or a hearing aid during the postoperative follow-up period, the relative risk for the development of postoperative EAC stenosis was 5.125 (95% confidence interval, 1.428-18.400; P = .023). Other factors did not show an association with postoperative stenosis. Preoperative air-bone gap (ABG) was 49.00 dB, and closure of the ABG within 30 dB was obtained in 56.9%, 58.1%, and 48.7% of patients at the 3-, 6-, and 12-month follow-up, respectively.

CONCLUSION

Stenting with an ear mold or hearing aid might be a useful method for preventing postoperative EAC stenosis in CAA patients.

Differential gene regulation under altered gravity conditions in follicular thyroid cancer cells: relationship between the extracellular matrix and the cytoskeleton

Extracellular matrix proteins, adhesion molecules, and cytoskeletal proteins form a dynamic network interacting with signalling molecules as an adaptive response to altered gravity. An important issue is the exact differentiation between real microgravity responses of the cells or cellular reactions to hypergravity and/or vibrations. To determine the effects of real microgravity on human cells, we used four DLR parabolic flight campaigns and focused on the effects of short-term microgravity (22 s), hypergravity (1.8 g), and vibrations on ML-1 thyroid cancer cells. No signs of apoptosis or necrosis were detectable. Gene array analysis revealed 2,430 significantly changed transcripts. After 22 s microgravity, the F-actin and cytokeratin cytoskeleton was altered, and ACTB and KRT80 mRNAs were significantly upregulated after the first and thirty-first parabolas. The COL4A5 mRNA was downregulated under microgravity, whereas OPN and FN were significantly upregulated. Hypergravity and vibrations did not change ACTB, KRT-80 or COL4A5 mRNA. MTSS1 and LIMA1 mRNAs were downregulated/slightly upregulated under microgravity, upregulated in hypergravity and unchanged by vibrations. These data indicate that the graviresponse of ML-1 cells occurred very early, within the first few seconds. Downregulated MTSS1 and upregulated LIMA1 may be an adaptive mechanism of human cells for stabilizing the cytoskeleton under microgravity conditions.

Different effects of intermittent and continuous fluid shear stresses on osteogenic differentiation of human mesenchymal stem cells

A reasonable mechanical microenvironment similar to the bone microenvironment in vivo is critical to the formation of engineering bone tissues. As fluid shear stress (FSS) produced by perfusion culture system can lead to the osteogenic differentiation of human mesenchymal stem cells (hMSCs), it is widely used in studies of bone tissue engineering. However, effects of FSS on the differentiation of hMSCs largely depend on the FSS application manner. It is interesting how different FSS application manners influence the differentiation of hMSCs. In this study, we examined the effects of intermittent FSS and continuous FSS on the osteogenic differentiation of hMSCs. The phosphorylation level of ERK1/2 and FAK is measured to investigate the effects of different FSS application manners on the activation of signaling molecules. The results showed that intermittent FSS could promote the osteogenic differentiation of hMSCs. The expression level of osteogenic genes and the alkaline phosphatase (ALP) activity in cells under intermittent FSS application were significantly higher than those in cells under continuous FSS application. Moreover, intermittent FSS up-regulated the activity of ERK1/2 and FAK. Our study demonstrated that intermittent FSS is more effective to induce the osteogenic differentiation of hMSCs than continuous FSS.

Diamagnetic levitation causes changes in the morphology, cytoskeleton, and focal adhesion proteins expression in osteocytes

Diamagnetic levitation technology is a novel simulated weightless technique and has recently been applied in life-science research. We have developed a superconducting magnet platform with large gradient high magnetic field (LG-HMF), which can provide three apparent gravity levels, namely, μg (diamagnetic levitation), 1g, and 2g for diamagnetic materials. In this study, the effects of LG-HMF on the activity, morphology, and cytoskeleton (actin filament, microtubules, and vimentin intermediate filaments) in osteocyte - like cell line MLO-Y4 were detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) methods, hematoxylin-eosin (HE) staining, and laser scanning confocal microscopy (LSCM), respectively. The changes induced by LG-HMF in distribution and expression of focal adhesion (FA) proteins, including vinculin, paxillin, and talin in MLO-Y4 were determined by LSCM and Western blotting. The results showed that LG-HMF produced by superconducting magnet had no lethal effects on MLO-Y4. Compared to control, diamagnetic levitation (μg) affected MLO-Y4 morphology, nucleus size, cytoskeleton architecture, and FA proteins distribution and expression. The study indicates that osteocytes are sensitive to altered gravity and FA proteins (vinculin, paxillin, and talin) may be involved in osteocyte mechanosensation. The diamagnetic levitation may be a novel ground-based space-gravity simulator and can be used for biological experiment at cellular level.

Effects of oriented substrates on cell morphology, the cell cycle, and the cytoskeleton in Ros 17/2.8 cells

Absence of gravity or microgravity influences the cellular functions of bone forming osteoblasts. The underlying mechanism, however, of cellular sensing and responding to the gravity vector is poorly understood. This work quantified the impact of vector-directional gravity on the biological responses of Ros 17/2.8 cells grown on upward-, downward- or edge-on-oriented substrates. Cell morphology and nuclear translocation, cell proliferation and the cell cycle, and cytoskeletal reorganization were found to vary significantly in the three orientations. All of the responses were duration-dependent. These results provide a new insight into understanding how osteoblasts respond to static vector-directional gravity.

Mechanical tensile stress effects on the expression of bone sialoprotein in bovine cementoblasts

OBJECTIVE

To develop a new cementoblast culture method and to detect bone sialoprotein (BSP) expression in response to high and low mechanical tensile stress in cementoblast in vitro.

MATERIALS AND METHODS

Cementoblasts were collected from the roots of newborn bovine teeth and were identified with cementum-derived attachment protein (CAP) antibody 3G9. Cell proliferation was evaluated by MTT [3-(4,5-dimethylthazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay, and mineralization was confirmed by von Kossa staining. Mechanical tensile stress was applied in vitro to the cementoblast with the use of a uniaxial four-point bending system with 2000 or 4000 microstrains, at a frequency of 0.5 Hz for 3, 6, 12, 24, or 36 hours. BSP mRNA level was quantified by real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR).

RESULTS

A large amount of cementoblast was observed to be expressing CAP. Cementoblasts had a proliferation tendency similar to that of osteoblasts but different from that of periodontal ligament (PDL) cells. Cementoblasts had the ability to become mineralized between osteoblasts and PDL cells. The mechanical tensile stress significantly up-regulated BSP mRNA expression, which reached a peak at 24 hours in both 2000 and 4000 microstrain groups (P < .01) and was tenfold and sixfold higher than that of controls, respectively. BSP expression dropped toward baseline levels at 36 hours in both groups.

CONCLUSIONS

Mechanical tensile stress up-regulated the expression of BSP. Low mechanical tensile stress induced earlier and more intensive up-regulation of BSP mRNA; this might represent the optimal stimuli for cementoblast activity.

Mechanical signals modulated vascular endothelial growth factor-A (VEGF-A) alternative splicing in osteoblastic cells through actin polymerisation

Since VEGF-A is involved in mechanically induced bone gain and because vegf exists under 6 isoforms exerting various biological effects, we studied vegf isoform expression and VEGF protein production in osteoblastic cells (rat Ros17/2.8 and human osteoblasts) submitted to 4 mechanical regimens. Mechanical regimens (1% stretch deformation) were designed with a fixed number of cycles (450) delivered at various frequencies (0.05 to 5 Hz). We found a negative correlation (R(2)=0.76, p<0.0001) between production of soluble VEGF and mechanical stretch frequency and a positive correlation (R(2)=0.99, p<0.0001) between production of matrix-bound VEGF and mechanical stretch frequency. mRNA expressions of soluble VEGF isoforms (121, 165) were specifically expressed under low frequency while matrix-bound VEGF isoforms (206, 189, 165, 145) were specifically expressed under high frequency in human osteoblasts. As f-actin stress fiber formation was significantly increased selectively in high frequency conditions, we disrupted actin fibers in Ros17/2.8 and found that immobilisation of VEGF was abolished. Conversely, Jasplakinolide treatment which increases stress fiber formation was able to mimic high frequency stretch-induced immobilisation of VEGF. Thus, we speculate that the stretch-induced increase in cell tension is responsible for matrix-bound vegf isoform production. Mechanically induced selection of soluble or matrix-bound VEGF production may modify osteoblast and endothelial cell crosstalk crucial during osteogenesis and fracture healing.

The effect of simulated microgravity on osteoblasts is independent of the induction of apoptosis

Bone loss during spaceflight has been attributed, in part, to a reduction in osteoblast number, altered gene expression, and an increase in cell death. To test the hypothesis that microgravity induces osteoblast apoptosis and suppresses the mature phenotype, we created a novel system to simulate spaceflight microgravity combining control and experimental cells within the same in vitro environment. Cells were encapsulated into two types of alginate carriers: non-rotationally stabilized (simulated microgravity) and rotationally stabilized (normal gravity). Using these specialized carriers, we were able to culture MC3T3-E1 osteoblast-like cells for 1-14 days in simulated microgravity and normal gravity in the same rotating wall vessel (RWV). The viability of cells was not affected by simulated microgravity, nor was the reductive reserve. To determine if simulated microgravity sensitized the osteoblasts to apoptogens, cells were challenged with staurosporine or sodium nitroprusside and the cell death was measured. Simulated microgravity did not alter the sensitivity of C3H10T-1/2 stem cells, MC3T3-E1 osteoblast-like cells, or MLO-A5 osteocyte-like cells to the action of these agents. RT-PCR analysis indicated that MC3T3-E1 osteoblasts maintained expression of RUNX2, osteocalcin, and collagen type I, but alkaline phosphatase expression was decreased in cells subjected to simulated microgravity for 5 days. We conclude that osteoblast apoptosis is not induced by vector-averaged gravity, thus suggesting that microgravity does not directly induce osteoblast death.

Experiments with osteoblasts cultured under hypergravity conditions

To understand further the role of gravity in osteoblast attachment, osteoblasts were subjected to hypergravity conditions in vitro. Scanning electron microscopy of all confluent coverslips from FPA units show that the number of attached osteoblasts was similar among gravitational levels and growth durations (~90 cells/microscopic field). Specifically, confluent 1.0 G control cultures contained an average of 91 +/- 8 cells/field, 3.3 G samples had 88 +/- 8 cells/field, and 4.0 G cultures averaged 90 +/- 7 cells/field. The sparsely plated cultures assessed by immunohistochemistry also had similar numbers of cells at each time point (l.0 G was similar to 3.3 and 4.0 G), but cell number changed from one time point to the next as those cells proliferated. Immunohistochemistry of centrifuged samples showed an increase in number (up to 160% increase) and thickness (up to 49% increase) of actin fibers, a decrease in intensity of fibronectin fluorescence (18-23% decrease) and an increase in number of vinculin bulbs (202-374% increase in number of vinculin bulbs/area). While hypergravity exposure did not alter the number of attached osteoblasts, it did result in altered actin, fibronectin, and vinculin elements, changing some aspects of osteoblast- substrate adhesion.

Effect of hyper- and microgravity on collagen post-translational controls of MC3T3-E1 osteoblasts

We attempted to study the effects of microgravity (by clinostat) and hypergravity (using centrifugation) on collagen metabolism using murine MC3T3-E1 osteoblasts, especially focusing on collagen cross-link formation. We found that altered gravitational load affected the post-translational modification of collagen, particularly the collagen maturation pathway, through altered expression of enzymes involved in cross-link formation.

INTRODUCTION

Gravitational loading plays important roles in the stimulation of differentiated osteoblast function and in the maintenance of skeletal tissues, whereas microgravity seems to result in osteopenia caused by impaired osteoblast differentiation. The aim of our study was to clarify the effects of altered gravitational environments on collagen metabolism, particularly the relationship between post-translational collagen quality and enzymes involved in cross-link formation, using murine osteoblastic MC3T3-E1 cells.

MATERIALS AND METHODS

Cells were cultured under vector-averaged microgravity (1 x 10(-3) g) using a clinostat or under conventional centrifugation techniques to generate hypergravity (20 g and 40 g) for 72 h. We then examined the expression patterns of lysyl oxidase and the two lysyl hydroxylase isoforms telopeptidyl lysyl hydroxylase (TLH; procollagen-lysine, 2-oxyglutarate, 5-dioxigenase 2 [PLOD2]) and helical lysyl hydroxylase (HLH; [PLOD1]) by quantitative real time polymerase chain reaction (PCR) analysis. Quantitative analysis of reducible immature (dihydroxylysinonorleucine, hydroxylysinonorleucine, and lysinonorleucine) and nonreducible mature (pyridinoline and deoxypyridinoline) cross-links, and maturation rate analysis of immature to mature cross-links by conventional metabolic labeling using tritium lysine were also performed.

RESULTS

Hypergravity upregulated both TLH mRNA expression and enzyme activity compared with stationary cultures, whereas microgravity stimulated both HLH mRNA expression and enzyme activity. These results were consistent with increased relative occupancy rates of telopeptidyl hydroxylysine-derived cross-links and helical hydroxylysine-derived forms observed under hypergravity and microgravity, respectively. Hypergravity stimulated not only lysyl oxidase mRNA expression but also increased enzyme activity and the sum of immature and mature cross-links. Furthermore, the conversion rate of immature cross-links to mature compounds was markedly increased under hypergravity but decreased under microgravity.

CONCLUSION

Altered gravitational loading may affect the post-translational modification of collagen through altered expression of enzymes involved in cross-link formation. These observations may be important in elucidating the mechanisms of osteopenia during space flight.

Function of the cytoskeleton in gravisensing during spaceflight

Since astronauts and cosmonauts have significant bone loss in microgravity we hypothesized that there would be physiological changes in cellular bone growth and cytoskeleton in the absence of gravity. Investigators from around the world have studied a multitude of bone cells in microgravity including Ros 17/2.8, Mc3T3-E1, MG-63, hFOB and primary chicken calvaria. Changes in cytoskeleton and extracellular matrix (ECM) have been noted in many of these studies. Investigators have noted changes in shape of cells exposed to as little as 20 seconds of microgravity in parabolic flight. Our laboratory reported that quiescent osteoblasts activated by sera under microgravity conditions had a significant 60% reduction in growth (p<0.001) but a paradoxical 2-fold increase in release of the osteoblast autocrine factor PGE2 when compared to ground controls. In addition, a collapse of the osteoblast actin cytoskeleton and loss of focal adhesions has been noted after 4 days in microgravity. Later studies in Biorack on STS-76, 81 and 84 confirmed the increased release of PGE2 and collapse of the actin cytoskeleton in cells grown in microgravity conditions, however flown cells under 1 g conditions maintained normal actin cytoskeleton and fibronectin matrix. The changes seen in the cytoskeleton are probably not due to alterations in fibronectin message or protein synthesis since no differences have been noted in microgravity. Multiple investigators have observed actin and microtubule cytoskeletal modifications in microgravity, suggesting a common root cause for the change in cell architecture. The inability of the 0 g grown osteoblast to respond to sera activation suggests that there is a major alteration in anabolic signal transduction under microgravity conditions, most probably through the growth factor receptors and/or the associated kinase pathways that are connected to the cytoskeleton. Cell cycle is dependent on the cytoskeleton. Alterations in cytoskeletal structure can block cell growth either in G1 (F-actin microfilament collapse), or in G2/M (inhibition of microtubule polymerization during G2/M-phase). We therefore hypothesize that microgravity would inhibit growth in either G1, or G2/M.

The structure and function of normally mineralizing avian tendons

The leg tendons of certain avian species normally calcify. The gastrocnemius, or Achilles, tendon of the domestic turkey, Meleagris gallopavo, is one such example. Its structure and biomechanical properties have been studied to model the adaptive nature of this tendon to external forces, including the means by which mineral deposition occurs and the functional role mineralization may play in this tissue. Structurally, the distal rounded, thick gastrocnemius bifurcates into two smaller proximal segments that mineralize with time. Mineral deposition occurs at or near the bifurcation, proceeding in a distal-to-proximal direction along the segments toward caudal and medial muscle insertions of the bird hip. Mineral formation appears mediated first by extracellular matrix vesicles and later by type I collagen fibrils. Biomechanical analyses indicate lower tensile strength and moduli for the thick distal gastrocnemius compared to narrow, fan-shaped proximal segments. Tendon mineralization here appears to be strain-induced, the muscle forces causing matrix deformation leading conceptually to calcium binding through the exposure of charged groups on collagen, release of sequestered calcium by proteoglycans, and increased diffusion. Functionally, the mineralized tendons limit further tendon deformation, reduce tendon strain at a given stress, and provide greater load-bearing capacity to the tissue. They also serve as important and efficient elastic energy storage reservoirs, increasing the amount of stored elastic energy by preventing flexible type I collagen regions from stretching and preserving muscle energy during locomotion of the animals.

MAPK and SRC-kinases control EGR-1 and NF-kappa B inductions by changes in mechanical environment in osteoblasts

Bone loss occurs in microgravity whereas an increase in bone mass is observed after skeletal loading. This tissue adaptation involves changes in osteoblastic proliferation and differentiation whose mechanisms remain largely unknown. In this context, we investigated the expression and the nuclear translocation of Egr-1 and NF-kappa B, in a simulated microgravity model (clinostat) and in a model of mechanical strain (Flexcell). We performed RT-PCR and immunocytochemistry analyses at baseline and up to 2 h after stimulation (a mitogenic regimen, 1% stretch, 0.05 Hz, 10 min, or clinorotation 50 rpm, 10 min) in osteoblastic ROS17/2.8 cells. Egr-1 induction as well as NF-kappa B nuclear translocation were activated by mechanical changes. PKC downregulation and COX1/2 inhibition did not alter these inductions. In contrast, ERK1/2, p38(MAPK) and src-kinases pathways were differentially involved in both models. Thus, we demonstrated that changes in the mechanical environment induced an activation of Egr-1 and NF-kappa B with specific kinetics and involved various transduction pathways including MAPKs and src-kinases. These could partially explain the later alterations of proliferation observed.

Temporal changes in cytoskeletal organisation within isolated chondrocytes quantified using a novel image analysis technique

This paper examines temporal changes in the organisation of the cytoskeleton within isolated articular chondrocytes cultured for up to 7 days in agarose constructs. Fluorescent labelling and confocal microscopy were employed to visualise microtubules (MT), vimentin intermediate filaments (VIF) and actin microfilaments (AMF). To quantify the degree of cytoskeletal organisation within populations of cells, a novel image analysis technique has been developed and fully characterised. Organisation was quantified in terms of an Edge Index, which reflects the density of 'edges' present within the confocal images as defined by a Sobel digital filter. This parameter was shown to be independent of image intensity and, for all three cytoskeletal components, was validated statistically against a visual assessment of organisation. Both MT and VIF exhibited fibrous networks extending throughout the cytoplasm, while AMF appeared as punctate units associated with the cell membrane. The use of the Edge Index parameter revealed statistical significant temporal variation, in particular associated with VIF and AMF. These findings indicate the possibility of cytoskeletal mediated temporal variation in many aspects of cell behaviour following isolation from the intact tissue. Furthermore, the image analysis techniques are likely to be useful for future studies aiming to quantify changes in cytoskeletal organisation.

Microgravity and hypergravity effects on fertilization of the salamander Pleurodeles waltl (urodele amphibian)

Effects of microgravity (microG) on fertilization were studied in the urodele amphibian Pleurodeles waltl on board the MIR space station. Genetic and cytomorphologic analyses ruled out parthenogenesis or gynogenesis and proved that fertilization did occur in microG. Actual fertilization was demonstrated by the analysis of the distribution of peptidase-1 genes, a polymorphic sex-linked enzyme, in progenies obtained in microG. Further evidence of fertilization was provided by the presence of spermatozoa in the perivitelline space and in the fertilization layer of the microG eggs and by the presence of a female pronucleus and male pronuclei in the egg cytoplasm. Experiments in microG and in 1.4G, 2G, and 3G hypergravity showed for the first time that, compared to eggs in 1G, several characteristics of the fertilization process including the cortical reaction and the microvillus transformations were altered depending on the gravitational force applied to the eggs. Microvillus elevation, the most evident feature, was reduced on microG-eggs and amplified on eggs submitted to 2G and 3G. No lethal consequences of these alterations on the early development of microG-eggs were observed.

Molecular events caused by mechanical stress in bone

The shape of bone changes as a result of bone remodeling corresponding to physical circumstances such as mechanical stress. The tissue which receives the loaded mechanical stress most efficiently is bone matrix. Recent studies revealed the function of osteocytes as mechanosensors in the early stage of bone remodeling. Loaded mechanical stress is converted to a series of biochemical reactions, and finally activates osteoclasts and osteoblasts to cause bone resorption and formation. Biochemical and molecular biological studies have recently resulted in the identification of the gene of which expression level is changed by mechanical stress. Nitric oxide (NO) and cAMP is secreted in response to mechanical stress in the immediate early stage. Genes encoding enzymes such as glutamate/aspartate transporter (GLAST), nitric oxide synthetase (NOS) and prostaglandin G/H synthetase (PGHS-2) are identified as mechanical stress-responsive. The expression level of IGF-I is enhanced under the control of PTH/PTHrP. The expression of c-fos is increased by loading of mechanical stress. AP1, a heterodimer of c-FOS/c-JUN, functions as a transcription factor of downstream gene(s). Elements including AP1 sites, cyclic AMP response elements (CRE) and shear stress response elements (SSRE) are found in the promoter region of mechanical stress-response genes. The enhanced expression of osteopontin (OPN) in the osteocytes of bone resorption sites was demonstrated by in situ hybridization and immunohistochemistry and transdifferentiation of chondrocytes with the abundant expression of BMP-2 and -4 in the process of distraction osteogenesis was observed.

Osteoblast adhesion on biomaterials

The development of tissue engineering in the field of orthopaedic surgery is now booming. Two fields of research in particular are emerging: the association of osteo-inductive factors with implantable materials; and the association of osteogenic stem cells with these materials (hybrid materials). In both cases, an understanding of the phenomena of cell adhesion and, in particular, understanding of the proteins involved in osteoblast adhesion on contact with the materials is of crucial importance. The proteins involved in osteoblast adhesion are described in this review (extracellular matrix proteins, cytoskeletal proteins, integrins, cadherins, etc.). During osteoblast/material interactions, their expression is modified according to the surface characteristics of materials. Their involvement in osteoblastic response to mechanical stimulation highlights the significance of taking them into consideration during development of future biomaterials. Finally, an understanding of the proteins involved in osteoblast adhesion opens up new possibilities for the grafting of these proteins (or synthesized peptide) onto vector materials, to increase their in vivo bioactivity or to promote cell integration within the vector material during the development of hybrid materials.

Culture in vector-averaged gravity under clinostat rotation results in apoptosis of osteoblastic ROS 17/2.8 cells

Space flight experiments and studies carried out in altered gravity environments have revealed that exposure to altered gravity conditions results in (mal)adaptation of cellular function. In the present study, we used a clinostat to generate a vector-averaged gravity environment. We then evaluated the responses of osteoblast-like ROS 17/2.8 cells subsequent to rotation at 50 revolutions per minute (rpm) for 6-24 h. We found that the cells started to detach from the substrate between 12 h and 24 h of rotation in clinostat but not in stationary cultures or after horizontal rotation (the latter serving as a motion control for turbulence, shear forces, and vibrations). At 24 h, 35% of clinorotated cells had detached and the cells underwent apoptotic death as evidenced by DNA fragmentation analysis, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) staining, and flow cytometry with Annexin V staining. The apoptotic death was associated with perinuclear distribution of cell-surface integrin beta1 and disorganization of actin cytoskeleton. These results suggest that vector-averaged gravity causes apoptosis of osteoblasts by altering the organization of the cytoskeleton. We hypothesize that apoptotic death of osteoblasts might play an important role in the pathogenesis of osteoporotic bone loss as observed in actual space flights.

The effect of microgravity on morphology and gene expression of osteoblasts in vitro

The mass and architecture of the skeletal system adapt, to some extent, to their mechanical environment. A site-specific bone loss of 1-2% is observed in astronauts and in-flight animals after 1 month of spaceflight. Biochemical data of astronauts and histomorphometric analysis of rat bones show that the change in bone mass is a result of decreased bone formation in association with normal (or increased) bone resorption. The changes in bone formation appear to be due in part to decreased osteoblast differentiation, matrix maturation, and mineralization. Recent data show that spaceflight alters the mRNA level for several bone-specific proteins in rat bone, suggesting that the characteristics of osteoblasts are altered during spaceflight. A possible underlying mechanism is that osteoblasts themselves are sensitive to altered gravity levels as suggested by several studies investigating the effect of microgravity on osteoblasts in vitro. Changes in cell and nuclear morphology were observed as well as alterations in the expression of growth factors (interleukin-6 and insulin-like growth factor binding proteins) and matrix proteins (collagen type I and osteocalcin). Taken together, this altered cellular function in combination with differences in local or systemic factors may mediate the effects of spaceflight on bone physiology.

The syndecans, tuners of transmembrane signaling

Syndecans, a family of transmembrane proteoglycans, are putative integrators of extracellular signals. The interaction of syndecans with extracellular ligands via particular motifs in their heparan sulfate chains, their clustering, association with particular cytoskeletal structures, binding to cytoplasmic effectors, and intracellular phosphorylation represent as many means to bring this role to a successful conclusion. In this review, we will briefly address the characteristics of syndecans as heparan sulfate proteoglycans (HSPGs) and focus mainly on the properties, binding interactions, and potential signaling functions of the cytoplasmic domains of these molecules.

Effects of gravitational changes on the bone system in vitro and in vivo

Spaceflight data obtained on bone cells, rodents, and humans are beginning to shed light on the importance of gravitational loading on the skeletal system. The space environment is a relevant model to explore the bone cell response to minimal strains. However, whether there is a direct effect of gravity on the cell rather than changes related to lack of convection forces in cell cultures performed in microgravity is unknown. In vitro studies carried out using osteoblastic cell cultures in space show changes in cell shape, suggesting that cell attachment structures as well as cytoskeleton reorganization might be involved. Valuable information is expected from in vitro models of an increase or decrease in mechanical stress in order to identify the different pathways of mechanoreception and mechanotransduction in the osteoblastic lineage. Results obtained from both humans and rodents after spaceflights indicated that bone mass changes are site specific rather than evenly distributed throughout the skeleton, thus emphasizing the need to perform measurements at different bone sites: weight- and non-weight-bearing bones, and cancellous and cortical envelopes. Bone mass measurements and biochemical parameters of bone remodeling are currently under evaluation in cosmonauts. Histomorphometric studies of bones from rats after space missions of various periods provided the time course of the cancellous bone cellular events: transient increase in resorption and sustained decrease in bone formation. The underlying bone loss occurred first in weight-bearing bones and later in less weight-bearing bones. During the postflight period, time required to recover the lost bone was greater than the mission length. Thus, the postflight period deserves more attention than it is currently receiving. On earth, the rat tail-suspension model is currently used to mimic spaceflight-induced bone loss. Data from the model confirmed the impairment of osteoblastic activity and showed an alteration in osteoblast recruitment with skeletal unloading. However, this model needs to be further validated.