RhoA and cytoskeletal disruption mediate reduced osteoblastogenesis and enhanced adipogenesis of human mesenchymal stem cells in modeled microgravity

Further genetic evidence suggesting a role for the RhoGTPase-RhoGEF pathway in osteoporosis

Osteoporosis is a highly heritable trait that appears to be influenced by multiple genes. Genome-wide linkage studies have highlighted the chromosomal region 3p14-p21 as a quantitative trait locus for BMD. We have previously published evidence suggesting that the ARHGEF3 gene from this region is associated with BMD in women. The product of this gene activates the RHOA GTPase, the gene for which is also located within this region. The aim of this study was to evaluate the influence of genetic polymorphism in RHOA on bone density in women. Sequence variation within the RHOA gene region was determined using 9 single nucleotide polymorphisms (SNPs) in a discovery cohort of 769 female sibs. Of the 9 SNPs, one was found to be monomorphic with the others representing 3 distinct linkage disequilibrium (LD) blocks. Using FBAT software, significant associations were found between two of these LD blocks and BMD Z-score of the spine and hip (P=0.001-0.036). The LD block tagged by the SNP rs17595772 showed maximal association, with the more common G allele at rs17595772 associated with decreased BMD Z-score. Genotyping for rs17595772 in a replication cohort of 780 postmenopausal women confirmed an association with BMD Z-score (P=0.002-0.036). Again, the G allele was found to be associated with a reduced hip and spine BMD Z-score. These results support the implication of the RhoGTPase-RhoGEF pathway in osteoporosis, and suggest that one or more genes in this pathway may be responsible for the linkage observed between 3p14-p21 and BMD.

PLGA doping of PCL affects the plastic potential of human mesenchymal stem cells, both in the presence and absence of biological stimuli

A range of poly epsilon-caprolactone (PCL) films mixed/doped with poly(lactide-co-glycolide) (PLGA) (65:35) in 0, 10, 20, and 30 wt % were produced, sterilized using ethylene oxide, and analyzed using FTIR. Characterized human mesenchymal stem cells (hMSCs) were cultured in contact with the materials in basal, chondrogenic, and osteogenic medium for time periods up to 28 days, to determine if the materials could induce differentiation of MSC both in the presence and absence of biological stimuli. Viable cell adhesion was analyzed under all conditions. Collagen I, collagen II, sox-9, osteocalcin, osteopontin, osteonectin, and CBFA1 were evaluated at both the mRNA (real-time PCR) and protein production levels (fluorescent immunohistochemistry) and used to identify cell differentiation. Pure PCL and PCL mixed with PLGA demonstrated a chondrogenic potential. Only PCL 8 (80 wt % PCL, 20 wt % PLGA) facilitated osteogenic differentiation of MSCs under osteogenic conditions. This was attributed to the increased hydrophilic nature of the surface allowing sufficient homogeneous cell attachment and the formation of filamentous F-actin in the cells, allowing osteogenic differentiation. Of all materials tested, PCL 7 (70 wt % PCL, 30 wt % PLGA) demonstrated the greatest chondrogenic differentiation potential under basal and stimulated conditions at both the mRNA and protein production level.

Degenerative muscle fiber accelerates adipogenesis of intramuscular cells via RhoA signaling pathway

In some pathological conditions such as Duchenne muscular dystrophy, it has been known that a fatty infiltration in skeletal muscle is often observed and that is also one of primary factors to induce marked decline of muscular strength. However, the mechanism of fatty infiltration, cellular origin of accumulated adipocytes and its significance are not fully understood. The fact that persistent degenerative muscle fibers are present on dystrophic muscle leads us to hypothesize that muscle fiber condition affects fatty infiltration in skeletal muscle. We employed a single fiber culture system to determine whether fiber condition affects an appearance of adipocytes on the fibers. Artificially hyper-contracted muscle fibers (HCF), generated from isolated intact fibers (IF) of rat extensor digitrum longus muscle, were maintained as non-adherent cultures for 5-7 days. Interestingly, there appeared to be considerable numbers of mature adipocytes on HCF, whereas no adipocytes were seen on IF, indicating that cells on HCF spontaneously differentiated into mature adipocytes. Activation of RhoA signaling by the addition of thrombin decreased the number of adipocytes on HCF in a dose-dependent manner, whereas the number of MyoD-positive myoblasts increased. In contrast, Y-27632, a specific inhibitor of Rho kinases (ROCK), induced adipogenic differentiation of cells derived from IF. In addition, administration of Y-27632 into mouse regenerating muscle resulted in fat accumulation in the muscle. Taken together, the present studies clearly demonstrated that muscle fiber condition affects fat accumulation in skeletal muscle and that is possibly mediated by the RhoA signaling pathway.

Activation of AMP kinase and inhibition of Rho kinase induce the mineralization of osteoblastic MC3T3-E1 cells through endothelial NOS and BMP-2 expression

AMP-activated protein kinase (AMPK) and Rho kinase (ROK) are known to modulate the mevalonate pathway. Activation of AMPK suppresses 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase. ROK acts downstream of HMG-CoA reductase, and its inhibition exerts antiatherosclerosis effects. However, whether or not these enzymes are involved in bone metabolism is unclear. The present study was undertaken to investigate the effects of an AMPK activator, 5-aminoimidazole-4-carboxamide1-beta-d-ribonucleoside (AICAR), and a ROK inhibitor, fasudil hydrochrolide, on the mineralization of osteoblastic MC3T3-E1 cells. Real-time PCR and mineralization stainings revealed that both AICAR and fasudil significantly stimulated endothelial nitric oxide synthase (eNOS), bone morphogenetic protein-2 (BMP-2), and osteocalcin mRNA expression as well as mineralization in the cells. Supplementation of either mevalonate or geranyl-geranyl pyrophosphate, the downstream molecules of HMG-CoA reductase, or coincubation with either a nitric oxide synthase inhibitor, N(G)-nitro-l-arginine methyl ester, or a BMP-2 antagonist, noggin, significantly reversed these AICAR-induced reactions. Western blot analysis showed that AICAR activated protein kinase B and extracellular signal-regulated kinase (ERK). ERK inhibitor significantly reversed the AICAR-induced increase in eNOS and BMP-2 mRNA expression. Measurement of ROK activities by enzyme-linked immunosorbent assay revealed that both AICAR and fasudil significantly suppressed the phosphorylation of the myosin-binding subunit of myosin phosphate, a ROK substrate. These findings suggest that the AMPK activator and the ROK inhibitor are able to stimulate the mineralization of osteoblasts through modulating the mevalonate pathway. These agents could be candidate drugs that promote bone formation for the treatment of osteoporosis.

Signal transduction in cells of the immune system in microgravity

Life on Earth developed in the presence and under the constant influence of gravity. Gravity has been present during the entire evolution, from the first organic molecule to mammals and humans. Modern research revealed clearly that gravity is important, probably indispensable for the function of living systems, from unicellular organisms to men. Thus, gravity research is no more or less a fundamental question about the conditions of life on Earth. Since the first space missions and supported thereafter by a multitude of space and ground-based experiments, it is well known that immune cell function is severely suppressed in microgravity, which renders the cells of the immune system an ideal model organism to investigate the influence of gravity on the cellular and molecular level. Here we review the current knowledge about the question, if and how cellular signal transduction depends on the existence of gravity, with special focus on cells of the immune system. Since immune cell function is fundamental to keep the organism under imnological surveillance during the defence against pathogens, to investigate the effects and possible molecular mechanisms of altered gravity is indispensable for long-term space flights to Earth Moon or Mars. Thus, understanding the impact of gravity on cellular functions on Earth will provide not only important informations about the development of life on Earth, but also for therapeutic and preventive strategies to cope successfully with medical problems during space exploration.

Integrins, insulin like growth factors, and the skeletal response to load

Bone loss during skeletal unloading, whether due to neurotrauma resulting in paralysis or prolonged immobilization due to a variety of medical illnesses, accelerates bone loss. In this review the evidence that skeletal unloading leads to bone loss, at least in part, due to disrupted insulin like growth factor (IGF) signaling, resulting in reduced osteoblast proliferation and differentiation, will be examined. The mechanism underlying this disruption in IGF signaling appears to involve integrins, the expression of which is reduced during skeletal unloading. Integrins play an important, albeit not well defined, role in facilitating signaling not only by IGF but also by other growth factors. However, the interaction between selected integrins such as alphaupsilonbeta3 and beta1 integrins and the IGF receptor are of especial importance with respect to the ability of bone to respond to mechanical load. Disruption of this interaction blocks IGF signaling and results in bone loss.

Identification of a role for the ARHGEF3 gene in postmenopausal osteoporosis

Osteoporosis is a common and debilitating bone disease characterized by low bone mineral density (BMD), a highly heritable and polygenic trait. Genome-wide linkage studies have identified 3p14-p21 as a quantitative trait locus for BMD. The ARHGEF3 gene is situated within this region and was identified as a strong positional candidate. The aim of this study was to evaluate the role of variation in ARHGEF3 on bone density in women. Sequence variation within ARHGEF3 was analyzed with 17 single-nucleotide polymorphisms (SNPs) in a discovery cohort of 769 female sibs. Significant associations were found with family-based association tests between five SNPs and various measures of age-adjusted BMD (p = 0.0007-0.041) with rs7646054 showing maximal association. Analysis of the data with QPDTPHASE suggested that the more common G allele at rs7646054 is associated with decreased age-adjusted BMD. Significant associations were also demonstrated between 3-SNP haplotypes and age-adjusted spine and femoral-neck BMD (p = 0.002 and 0.003, respectively). rs7646054 was then genotyped in a replication cohort, and significant associations with hip and spine BMD were confirmed (p = 0.003-0.038), as well as an association with fracture rate (p = 0.02). Again, the G allele was associated with a decrease in age-adjusted BMD at each site studied. In conclusion, genetic variation in ARHGEF3 plays a role in the determination of bone density in Caucasian women. This data implicates the RhoGTPase-RhoGEF pathway in osteoporosis.

Modeled gravitational unloading induced downregulation of endothelin-1 in human endothelial cells

Many space missions have shown that prolonged space flights may increase the risk of cardiovascular problems. Using a three-dimensional clinostat, we investigated human endothelial EA.hy926 cells up to 10 days under conditions of simulated microgravity (microg) to distinguish transient from long-term effects of microg and 1g. Maximum expression of all selected genes occurred after 10 min of clinorotation. Gene expression (osteopontin, Fas, TGF-beta(1)) declined to slightly upregulated levels or rose again (caspase-3) after the fourth day of clinorotation. Caspase-3, Bax, and Bcl-2 protein content was enhanced for 10 days of microgravity. In addition, long-term accumulation of collagen type I and III and alterations of the cytoskeletal alpha- and beta-tubulins and F-actin were detectable. A significantly reduced release of soluble factors in simulated microgravity was measured for brain-derived neurotrophic factor, tissue factor, vascular endothelial growth factor (VEGF), and interestingly for endothelin-1, which is important in keeping cardiovascular balances. The gene expression of endothelin-1 was suppressed under microg conditions at days 7 and 10. Alterations of the vascular endothelium together with a decreased release of endothelin-1 may entail post-flight health hazards for astronauts.

Cyclic AMP (cAMP)-mediated stimulation of adipocyte differentiation requires the synergistic action of Epac- and cAMP-dependent protein kinase-dependent processes

Cyclic AMP (cAMP)-dependent processes are pivotal during the early stages of adipocyte differentiation. We show that exchange protein directly activated by cAMP (Epac), which functions as a guanine nucleotide exchange factor for the Ras-like GTPases Rap1 and Rap2, was required for cAMP-dependent stimulation of adipocyte differentiation. Epac, working via Rap, acted synergistically with cAMP-dependent protein kinase (protein kinase A [PKA]) to promote adipogenesis. The major role of PKA was to down-regulate Rho and Rho-kinase activity, rather than to enhance CREB phosphorylation. Suppression of Rho-kinase impaired proadipogenic insulin/insulin-like growth factor 1 signaling, which was restored by activation of Epac. This interplay between PKA and Epac-mediated processes not only provides novel insight into the initiation and tuning of adipocyte differentiation, but also demonstrates a new mechanism of cAMP signaling whereby cAMP uses both PKA and Epac to achieve an appropriate cellular response.

Simulated microgravity inhibits the proliferation and osteogenesis of rat bone marrow mesenchymal stem cells

OBJECTIVES

Microgravity is known to affect the differentiation of bone marrow mesenchymal stem cells (BMSCs). However, a few controversial findings have recently been reported with respect to the effects of microgravity on BMSC proliferation. Thus, we investigated the effects of simulated microgravity on rat BMSC (rBMSC) proliferation and their osteogeneic potential.

MATERIALS AND METHODS

rBMSCs isolated from marrow using our established effective method, based on erythrocyte lysis, were identified by their surface markers and their proliferation characteristics under normal conditions. Then, they were cultured in a clinostat to simulate microgravity, with or without growth factors, and in osteogenic medium. Subsequently, proliferation and cell cycle parameters were assessed using methylene blue staining and flow cytometry, respectively; gene expression was determined using Western blotting and microarray analysis.

RESULTS

Simulated microgravity inhibited population growth of the rBMSCs, cells being arrested in the G(0)/G(1) phase of cell cycle. Growth factors, such as insulin-like growth factor-I, epidermal growth factor and basic fibroblastic growth factor, markedly stimulated rBMSC proliferation in normal gravity, but had only a slight effect in simulated microgravity. Akt and extracellular signal-related kinase 1/2 phosphorylation levels and the expression of core-binding factor alpha1 decreased after 3 days of clinorotation culture. Microarray and gene ontology analyses further confirmed that rBMSC proliferation and osteogenesis decreased under simulated microgravity.

CONCLUSIONS

The above data suggest that simulated microgravity inhibits population growth of rBMSCs and their differentiation towards osteoblasts. These changes may be responsible for some of the physiological changes noted during spaceflight.

Simulated microgravity inhibits the proliferation and osteogenesis of rat bone marrow mesenchymal stem cells

OBJECTIVES

Microgravity is known to affect the differentiation of bone marrow mesenchymal stem cells (BMSCs). However, a few controversial findings have recently been reported with respect to the effects of microgravity on BMSC proliferation. Thus, we investigated the effects of simulated microgravity on rat BMSC (rBMSC) proliferation and their osteogeneic potential.

MATERIALS AND METHODS

rBMSCs isolated from marrow using our established effective method, based on erythrocyte lysis, were identified by their surface markers and their proliferation characteristics under normal conditions. Then, they were cultured in a clinostat to simulate microgravity, with or without growth factors, and in osteogenic medium. Subsequently, proliferation and cell cycle parameters were assessed using methylene blue staining and flow cytometry, respectively; gene expression was determined using Western blotting and microarray analysis.

RESULTS

Simulated microgravity inhibited population growth of the rBMSCs, cells being arrested in the G(0)/G(1) phase of cell cycle. Growth factors, such as insulin-like growth factor-I, epidermal growth factor and basic fibroblastic growth factor, markedly stimulated rBMSC proliferation in normal gravity, but had only a slight effect in simulated microgravity. Akt and extracellular signal-related kinase 1/2 phosphorylation levels and the expression of core-binding factor alpha1 decreased after 3 days of clinorotation culture. Microarray and gene ontology analyses further confirmed that rBMSC proliferation and osteogenesis decreased under simulated microgravity.

CONCLUSIONS

The above data suggest that simulated microgravity inhibits population growth of rBMSCs and their differentiation towards osteoblasts. These changes may be responsible for some of the physiological changes noted during spaceflight.

Modulation of cellular mechanics during osteogenic differentiation of human mesenchymal stem cells

Recognition of the growing role of human mesenchymal stem cells (hMSC) in tissue engineering and regenerative medicine requires a thorough understanding of intracellular biochemical and biophysical processes that may direct the cell's commitment to a particular lineage. In this study, we characterized the distinct biomechanical properties of hMSCs, including the average Young's modulus determined by atomic force microscopy (3.2 +/- 1.4 kPa for hMSC vs. 1.7 +/- 1.0 kPa for fully differentiated osteoblasts), and the average membrane tether length measured with laser optical tweezers (10.6 +/- 1.1 microm for stem cells, and 4.0 +/- 1.1 microm for osteoblasts). These differences in cell elasticity and membrane mechanics result primarily from differential actin cytoskeleton organization in these two cell types, whereas microtubules did not appear to affect the cellular mechanics. The membrane-cytoskeleton linker proteins may contribute to a stronger interaction of the plasma membrane with F-actins and shorter membrane tether length in osteoblasts than in stem cells. Actin depolymerization or ATP depletion caused a two- to threefold increase in the membrane tether length in osteoblasts, but had essentially no effect on the stem-cell membrane tethers. Actin remodeling in the course of a 10-day osteogenic differentiation of hMSC mediates the temporally correlated dynamical changes in cell elasticity and membrane mechanics. For example, after a 10-day culture in osteogenic medium, hMSC mechanical characteristics were comparable to those of mature bone cells. Based on quantitative characterization of the actin cytoskeleton remodeling during osteodifferentiation, we postulate that the actin cytoskeleton plays a pivotal role in determining the hMSC mechanical properties and modulation of cellular mechanics at the early stage of stem-cell osteodifferentiation.

Alterations of the actin cytoskeleton and increased nitric oxide synthesis are common features in human primary endothelial cell response to changes in gravity

Because endothelial cells are fundamental to the maintenance of the functional integrity of the vascular wall, endothelial modifications in altered gravity conditions might offer some insights into the mechanisms leading to circulatory impairment in astronauts. We cultured human endothelial cells in a dedicated centrifuge (MidiCAR) to generate hypergravity and in two different devices, namely the Rotating Wall Vessel and the Random Positioning Machine, to generate hypogravity. Hypogravity stimulated endothelial growth, did not affect migration, and enhanced nitric oxide production. It also remodeled the actin cytoskeleton and reduced the total amounts of actin. Hypergravity did not affect endothelial growth, markedly stimulated migration, and enhanced nitric oxide synthesis. In addition, hypergravity altered the distribution of actin fibers without, however, affecting the total amounts of actin. A short exposure to hypergravity (8 min) abolished the hypogravity induced growth advantage. Our results indicate that cytoskeletal alterations and increased nitric oxide production represent common denominators in endothelial responses to both hypogravity and hypergravity.

Changes in mRNA gene expression during growth in the femoral head of the young rat

The rate of physeal growth slows as an animal matures with changes in mRNA gene expression due to the altered cellular activity. To measure the change in gene expression during the juvenile growth period, the femoral head, enclosing the proximal femoral physis, primary spongiosa, and articular cartilage, was collected from both femora of 16 female Sprague-Dawley rats between 4 and 10 weeks of age. One femur of each rat had had a mid-diaphyseal femoral fracture at 4 weeks of age. RNA was extracted and hybridized to 16 Affymetrix Rat Genomic 230 2.0 GeneChip microarrays with probe sets for 31,000 genes of which 18,200 were expressed. Of these, 8002 genes had a significant change in gene expression during growth, about half increasing and half decreasing. These changes included up-regulation with time of genes related to cartilage, blood vessels, osteoprotegerin, osteomodulin, and most ribosomal proteins. There was down-regulation with maturity of genes related to bone, growth-promoting cytokines, G proteins, GTPase-mediated signal transduction factors, cytokine receptors, mitosis, integrin-linked kinase, and the cytoskeleton. In summary, the slowing of growth with maturity was associated with changes in mRNA gene expression in the femoral head for a large number of genes. These changes in gene expression between young and mature rats suggest factors which are important for the support of the rapid linear growth during early life.

Conversion of red bone marrow into yellow – Cause and mechanisms

Marrow cavities in all the bones of newborn mammals contain active hematopoietic tissue, known as red bone marrow. From the early postnatal period onwards, the hematopoietic tissue, mainly in the bones of the extremities, is gradually replaced by non-hematopoietic mesenchymal cells that accumulate lipid drops, known as yellow or fatty bone marrow. For its maintenance, hematopoietic tissue depends on the support of special mesenchymal cells in the bone marrow cavity, known as hematopoietic microenvironment. Both bone-forming cells and hematopoietic microenvironment cells have common progenitors - mesenchymal stem cells (MSCs). We hypothesize that: (1) Hematopoietic microenvironment cells advance along a three stage differentiation/maturation pathway. In the first stage, they support hematopoiesis and contain no fat. In the second stage, cells accumulate fat and no longer support steady state hematopoiesis; however, under conditions of increased hematopoietic requirement, they lose fat and regain their ability to support hematopoiesis. In the last stage, hematopoietic microenvironment cells retain the appearance of yellow bone marrow and do not support hematopoiesis regardless of the state of hematopoietic requirement.(2) Since MSCs are bound to endosteal and trabecular surfaces, in tubular bones their number is relatively small, compared to cancellous bones that have much larger areas of internal bone surface. MSCs are exposed to proliferative and differentiative pressures, leading to gradual reduction of their number. Consequently, the MSC population in tubular bones becomes exhausted rather early, and the post-maturation compartment of mesenchymal cells finally consists of unipotential bone precursors maintaining bone tissue and hematopoietic microenvironment advancing towards the last (fatty) stage of differentiation. In contrast, in cancellous bones the relatively large number of MSCs does not suffer exhaustion and continues to provide newly differentiated hematopoietic microenvironment, thus maintaining red bone marrow throughout the organism's life.(3) Osteogenic and hematopoietic microenvironment differentiation pathways compete with each other for their common precursor. During the organism's growth period osteogenic stimuli prevail, while in the post-maturation period, MSC differentiation into hematopoietic microenvironment increases at the expense of differentiation into bone. This results in the reduction of bone volume and expansion of marrow cavities in hematopoietically active cancellous bones, but not in tubular bones already depleted of MSCs and not participating in hematopoiesis. Experimental and clinical data supporting these hypotheses are discussed.

Mechanisms of osteoporosis in spinal cord injury

Osteoporosis is a known complication of spinal cord injury (SCI), but its mechanism remains unknown. The pathogenesis of osteoporosis after SCI is generally considered disuse. However, although unloading is an important factor in the pathogenesis of osteoporosis after SCI, neural lesion and hormonal changes also seem to be involved in this process. Innervation and neuropeptides play an important role in normal bone remodelling. SCI results in denervation of the sublesional bones and the neural lesion itself may play a pivotal role in the development of osteoporosis after SCI. Although upper limbs are normally loaded and innervated, bone loss also occurs in the upper extremities in patients with paraplegia, indicating that hormonal changes may be associated with osteoporosis after SCI. SCI-mediated hormonal changes may contribute to osteoporosis after SCI by different mechanisms: (1) increased renal elimination and reduced intestinal absorption of calcium leading to a negative calcium balance; (2) vitamin D deficiency plays a role in the pathogenesis of SCI-induced osteoporosis; (3) SCI antagonizes gonadal function and inhibits the osteoanabolic action of sex steroids; (4) hyperleptinaemia after SCI may contribute to the development of osteoporosis; (5) pituitary suppression of TSH may be another contributory factor to bone loss after SCI; and (6) bone loss after SCI may be caused directly, at least in part, by insulin resistance and IGFs. Thus, oversupply of osteoclasts relative to the requirement for bone resorption and/or undersupply of osteoblasts relative to the requirement for cavity repair results in bone loss after SCI. Mechanisms for the osteoporosis following SCI include a range of systems, and osteoporosis after SCI should not be simply considered as disuse osteoporosis. Unloading, neural lesion and hormonal changes after SCI result in severe bone loss. The aim of this review is to improve understanding with regard to the mechanisms of osteoporosis after SCI. The understanding of the pathogenesis of osteoporosis after SCI can help in the consideration of new treatment strategies. Because bone resorption after SCI is very high, intravenous bisphosphonates and denosumab should be considered for the treatment of osteoporosis after SCI.

Proteomics and genomics of microgravity

Many serious adverse physiological changes occur during spaceflight. In the search for underlying mechanisms and possible new countermeasures, many experimental tools and methods have been developed to study microgravity caused physiological changes, ranging from in vitro bioreactor studies to spaceflight investigations. Recently, genomic and proteomic approaches have gained a lot of attention; after major scientific breakthroughs in the fields of genomics and proteomics, they are now widely accepted and used to understand biological processes. Understanding gene and/or protein expression is the key to unfolding the mechanisms behind microgravity-induced problems and, ultimately, finding effective countermeasures to spaceflight-induced alterations. Significant progress has been made in identifying the genes/proteins responsible for these changes. Although many of these genes and/or proteins were observed to be either upregulated or downregulated, however, no large-scale genomics and proteomics studies have been published so far. This review aims at summarizing the current status of microgravity-related genomics and proteomics studies and stimulating large-scale proteomics and genomics research activities.

Microcarriers in the engineering of cartilage and bone

A major problem in tissue engineering is the availability of a sufficient number of cells with the appropriate phenotype for delivery to damaged or diseased cartilage and bone; the challenge is to amplify cell numbers and maintain the appropriate phenotype for tissue repair and restoration of function. The microcarrier bioreactor culture system offers an attractive method for cell amplification and enhancement of phenotype expression. Besides serving as substrates for the propagation of anchorage-dependent cells, microcarriers can also be used to deliver the expanded undifferentiated or differentiated cells to the site of the defect. The present article provides an overview of the microcarrier culture system, its utility as an in vitro research tool and its potential applications in tissue engineering, particularly in the repair of cartilage and bone.

Microgravity: the immune response and bone

Exposure to microgravity during space flight affects almost all human physiological systems. The affected systems that are of key importance to human space exploration are the musculoskeletal, neurovestibular, and cardiovascular systems. However, alterations in the immune and endocrine functions have also been described. Bone loss has been shown to be site specific, predominantly in the weight-bearing regions of the legs and lumbar spine. This phenomenon has been attributed to a reduction in bone formation resulting from a decrease in osteoblastic function and an increase in osteoclastic resorption. In order to examine the effects of microgravity on cellular function here on earth, several ground-based studies have been performed using different systems to model microgravity. Our studies have shown that modeled microgravity (MMG) inhibits the osteoblastic differentiation of human mesenchymal stem cells (hMSCs) while increasing their adipogenic differentiation. Here, we discuss the potential molecular mechanisms that could be altered in microgravity. In particular, we examine the role of RhoA kinase in maintaining the formation of actin stress fibers and the expression of nitric oxide synthase under MMG conditions. These proposed mechanisms, although only examined in hMSCs, could be part of a global response to microgravity that ultimately alters human physiology.