Reduced ossification caused by 3D simulated microgravity exposure is short-term in larval zebrafish

Understanding how skeletal tissues respond to microgravity is ever more important with the increased interest in human space travel. Here, we exposed larval Danio rerio at 3.5 dpf to simulated microgravity (SMG) using a 3D mode of rotation in a ground-based experiment and then studied different cellular, molecular, and morphological bone responses both immediately after exposure and one week later. Our results indicate an overall decrease in ossification in several developing skeletal elements immediately after SMG exposure with the exception of the otoliths, however ossification returns to normal levels seven days after exposure. Coincident with the reduction in overall ossification tnfsf11 (RANKL) expression is highly elevated after 24 h of SMG exposure and also returns to normal levels seven days after exposure. We also show that genes associated with osteoblasts are unaffected immediately after SMG exposure. Thus, the observed reduction in ossification is primarily the result of a high level of bone resorption. This study sheds insight into the nuances of how osteoblasts and osteoclasts in the skeleton of a vertebrate organism respond to an external environmental disturbance, in this case simulated microgravity.

Pre-proenkephalin 1 is Downregulated Under Unloading and is Involved in Osteoblast Biology

Pre-proenkephalin 1 (Penk1) is a pro-neuropeptide that belongs to the typical opioid peptide's family, having analgesic properties. We previously found Penk1 to be the most downregulated gene in a whole gene profiling analysis performed in osteoblasts subjected to microgravity as a model of mechanical unloading. In this work, Penk1 downregulation was confirmed in the bones of two in vivo models of mechanical unloading: tail-suspended and botulinum toxin A (botox)-injected mice. Consistently, in the sera from healthy volunteers subjected to bed rest, we observed an inverse correlation between PENK1 and bed rest duration. These results prompted us to investigate a role for this factor in bone. Penk1 was highly expressed in mouse bone, but its global deletion failed to impact bone metabolism in vivo. Indeed, Penk1 knock out (Penk1-/-) mice did not show an overt bone phenotype compared to the WT littermates. Conversely, in vitro Penk1 gene expression progressively increased during osteoblast differentiation and its transient silencing in mature osteoblasts by siRNAs upregulated the transcription of the Sost1 gene encoding sclerostin, and decreased Wnt3a and Col1a1 mRNAs, suggesting an altered osteoblast activity due to an impairment of the Wnt pathway. In line with this, osteoblasts treated with the Penk1 encoded peptide, Met-enkephalin, showed an increase of Osx and Col1a1 mRNAs and enhanced nodule mineralization. Interestingly, primary osteoblasts isolated from Penk1-/- mice showed lower metabolic activity, ALP activity, and nodule mineralization, as well as a lower number of CFU-F compared to osteoblasts isolated from WT mice, suggesting that, unlike the transient inhibition, the chronic Penk1 deletion affects both osteoblast differentiation and activity. Taken together, these results highlight a role for Penk1 in the regulation of the response of the bone to mechanical unloading, potentially acting on osteoblast differentiation and activity in a cell-autonomous manner.

The role of adipokines in osteoporosis management: a mini review

The prevalence of osteoporosis has been on the rise globally. With ageing populations, research has sought therapeutic solutions in novel areas. One such area is that of the adipokines. Current literature points to an important role for these chemical mediators in relation to bone metabolism. Well-established adipokines have been broadly reported upon. These include adiponectin and leptin. However, other novel adipokines such as visfatin, nesfatin-1, meteorin-like protein (Metrnl), apelin and lipocalin-2 are starting to be addressed pre-clinically and clinically. Adipokines hold pro-inflammatory and anti-inflammatory properties that influence the pathophysiology of various bone diseases. Omentin-1 and vaspin, two novel adipokines, share cardioprotective effects and play essential roles in bone metabolism. Studies have reported bone-protective effects of omentin-1, whilst others report negative associations between omentin-1 and bone mineral density. Lipocalin-2 is linked to poor bone microarchitecture in mice and is even suggested to mediate osteoporosis development from prolonged disuse. Nesfatin-1, an anorexigenic adipokine, has been known to preserve bone density. Animal studies have demonstrated that nesfatin-1 treatment limits bone loss and increases bone strength, suggesting exogenous use as a potential treatment for osteopenic disorders. Pre-clinical studies have shown adipokine apelin to have a role in bone metabolism, mediated by the enhancement of osteoblast genesis and the inhibition of programmed cell death. Although many investigations have reported conflicting findings, sufficient literature supports the notion that adipokines have a significant influence on the metabolism of bone. This review aims at highlighting the role of novel adipokines in osteoporosis while also discussing their potential for treating osteoporosis.

Lipocalin 2-A bone-derived anorexigenic and β-cell promoting signal: From mice to humans

The skeleton is traditionally known for its structural support, organ protection, movement, and maintenance of mineral homeostasis. Over the last 10 years, bone has emerged as an endocrine organ with diverse physiological functions. The two key molecules in this context are fibroblast growth factor 23 (FGF23), secreted by osteocytes, and osteocalcin, a hormone produced by osteoblasts. FGF23 affects mineral homeostasis through its actions on the kidneys, and osteocalcin has beneficial effects in improving glucose homeostasis, muscle function, brain development, cognition, and male fertility. In addition, another osteoblast-derived hormone, lipocalin 2 (LCN2) has emerged into the researchers' field of vision. In this review, we mainly focus on LCN2's role in appetite regulation and glucose metabolism and also briefly introduce its effects in other pathophysiological conditions, such as nonalcoholic fatty liver disease, sarcopenic obesity, and cancer-induced cachexia.

Effects of simulated microgravity and vibration on osteoblast and osteoclast activity in cultured zebrafish scales

Zebrafish cultured scales have been used effectively to study cellular and molecular responses of bone cells. In order to expose zebrafish scales to simulated microgravity (SMG) and/or vibration, we first determined via apoptosis staining whether cells of the scale survive in culture for two days and hence, we restricted our analyses to two-day durations. Next, we measured the effects of SMG and vibration on cell death, osteoclast tartrate-resistant acid phosphatase, and osteoblast alkaline phosphatase activity and on the number of Runx2a positive cells. We found that during the SMG treatment, osteoclast tartrate-resistant acid phosphatase activity increased on average, while the number of Runx2a positive cells decreased significantly. In contrast, SMG exposure caused a decrease in osteoblast activity. The vibration treatment showed an increase, on average, in the osteoblast alkaline phosphatase activity. This study demonstrates the effect of SMG and vibration on zebrafish scales and the effects of SMG on bone cells. We also show that zebrafish scales can be used to examine the effects of SMG on bone maintenance.

Sex-dependent expression of neutrophil gelatinase-associated lipocalin in aortic stenosis

BACKGROUND

Accumulating evidence suggest the existence of sex-related differences in the pathogenesis of aortic stenosis (AS) with inflammation, oxidative stress, fibrosis and calcification being over-represented in men. Neutrophil gelatinase-associated lipocalin (NGAL) is expressed in a myriad of tissues and cell types, and it is associated with acute and chronic pathological processes comprising inflammation, fibrosis or calcification. Sex-dependent signatures have been evidenced for NGAL which expression has been associated predominantly in males to metabolic and cardiovascular disorders. We aimed to analyse sex-related differences of NGAL in AS and its role in the inflammatory and fibrocalcific progression of AS.

METHODS AND RESULTS

220 (60.45% men) patients with severe AS elective for surgical aortic valve (AV) replacement were recruited. Immunohistochemistry revealed higher expression of NGAL in calcific areas of AVs and that was validated by qPCR in in 65 (60% men) donors. Valve interstitial cells (VICs) were a source of NGAL in these samples. Proteome profiler analyses evidenced higher expression of NGAL in men compared to women, and that was further validated by ELISA. NGAL expression in the AV was correlated with inflammation, oxidative stress, and osteogenic markers, as well as calcium score. The expression of NGAL, both intracellular and secreted (sNGAL), was significantly deregulated only in calcifying male-derived VICs. Depletion of intracellular NGAL in calcifying male-derived VICs was associated with pro-inflammatory profiles, dysbalanced matrix remodelling and pro-osteogenic profiles. Conversely, exogenous NGAL mediated inflammatory and dysbalanced matrix remodelling in calcifying VICs, and all that was prevented by the pharmacological blockade of NGAL.

CONCLUSIONS

Owing to the over-expression of NGAL, the AV from men may be endowed with higher expression of inflammatory, oxidative stress, matrix remodelling and osteogenic markers supporting the progression of calcific AS phenotypes. The expression of NGAL in the VIC emerges as a potential therapeutic checkpoint, with its effects being potentially reverted by the pharmacological blockade of extracellular NGAL.

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

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

Osteomodulin attenuates smooth muscle cell osteogenic transition in vascular calcification

Rationale

Vascular calcification is a prominent feature of late‐stage diabetes, renal and cardiovascular disease (CVD), and has been linked to adverse events. Recent studies in patients reported that plasma levels of osteomodulin (OMD), a proteoglycan involved in bone mineralisation, associate with diabetes and CVD. We hypothesised that OMD could be implicated in these diseases via vascular calcification as a common underlying factor and aimed to investigate its role in this context.

Methods and results

In patients with chronic kidney disease, plasma OMD levels correlated with markers of inflammation and bone turnover, with the protein present in calcified arterial media. Plasma OMD also associated with cardiac calcification and the protein was detected in calcified valve leaflets by immunohistochemistry. In patients with carotid atherosclerosis, circulating OMD was increased in association with plaque calcification as assessed by computed tomography. Transcriptomic and proteomic data showed that OMD was upregulated in atherosclerotic compared to control arteries, particularly in calcified plaques, where OMD expression correlated positively with markers of smooth muscle cells (SMCs), osteoblasts and glycoproteins. Immunostaining confirmed that OMD was abundantly present in calcified plaques, localised to extracellular matrix and regions rich in α‐SMA⁺ cells. In vivo, OMD was enriched in SMCs around calcified nodules in aortic media of nephrectomised rats and in plaques from ApoE^−/− mice on warfarin. In vitro experiments revealed that OMD mRNA was upregulated in SMCs stimulated with IFNγ, BMP2, TGFβ1, phosphate and β‐glycerophosphate, and by administration of recombinant human OMD protein (rhOMD). Mechanistically, addition of rhOMD repressed the calcification process of SMCs treated with phosphate by maintaining their contractile phenotype along with enriched matrix organisation, thereby attenuating SMC osteoblastic transformation. Mechanistically, the role of OMD is exerted likely through its link with SMAD3 and TGFB1 signalling, and interplay with BMP2 in vascular tissues.

Conclusion

We report a consistent association of both circulating and tissue OMD levels with cardiovascular calcification, highlighting the potential of OMD as a clinical biomarker. OMD was localised in medial and intimal α‐SMA⁺ regions of calcified cardiovascular tissues, induced by pro‐inflammatory and pro‐osteogenic stimuli, while the presence of OMD in extracellular environment attenuated SMC calcification.

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.

Lipocalin 2 Influences Bone and Muscle Phenotype in the MDX Mouse Model of Duchenne Muscular Dystrophy

Lipocalin 2 (Lcn2) is an adipokine involved in bone and energy metabolism. Its serum levels correlate with bone mechanical unloading and inflammation, two conditions representing hallmarks of Duchenne Muscular Dystrophy (DMD). Therefore, we investigated the role of Lcn2 in bone loss induced by muscle failure in the MDX mouse model of DMD. We found increased Lcn2 serum levels in MDX mice at 1, 3, 6, and 12 months of age. Consistently, Lcn2 mRNA was higher in MDX versus WT muscles. Immunohistochemistry showed Lcn2 expression in mononuclear cells between muscle fibres and in muscle fibres, thus confirming the gene expression results. We then ablated Lcn2 in MDX mice, breeding them with Lcn2^−/− mice (MDXxLcn2^−/−), resulting in a higher percentage of trabecular volume/total tissue volume compared to MDX mice, likely due to reduced bone resorption. Moreover, MDXxLcn2^−/− mice presented with higher grip strength, increased intact muscle fibres, and reduced serum creatine kinase levels compared to MDX. Consistently, blocking Lcn2 by treating 2-month-old MDX mice with an anti-Lcn2 monoclonal antibody (Lcn2Ab) increased trabecular volume, while reducing osteoclast surface/bone surface compared to MDX mice treated with irrelevant IgG. Grip force was also increased, and diaphragm fibrosis was reduced by the Lcn2Ab. These results suggest that Lcn2 could be a possible therapeutic target to treat DMD-induced bone loss.

Acute Effects of Strength and Endurance Training on Bone Turnover Markers in Young Adults and Elderly Men

CONTEXT

Exercise is recognized as an important strategy to prevent bone loss, but its acute effects on bone turnover markers (BTMs) and related markers remain uncertain.

OBJECTIVE

To assess the acute effects of two different exercise modes on BTMs and related markers in young adults of both sexes and elderly men.

DESIGN SETTING PARTICIPANTS

This was a three-group crossover within-subjects design study with a total of 53 participants-19 young women (aged 22-30), 20 young men (aged 21-30 years), and 14 elderly men (aged 63-74 years)-performing two different exercise sessions [strength training (ST) and high-intensity interval training (HIIT)] separated by 2 weeks, in a supervised laboratory setting.

MAIN OUTCOME MEASURES

Plasma volume-corrected serum measurements of the BTMs C-terminal telopeptide of type 1 collagen (CTX-I) and procollagen of type 1 N-terminal propeptide (P1NP), total osteocalcin (OC), sclerostin, and lipocalin-2 (LCN2) at baseline, immediately after, and 3 and 24 h after each of the two exercise modes were performed.

RESULTS AND CONCLUSION

Analyses revealed sex- and age-dependent differences in BTMs and related bone markers at baseline and time-, sex-, and age-dependent differences in response to exercise. No differences between exercise modes were observed for BTM response except for sclerostin in young men and LCN2 in elderly men. An acute, transient, and uniform increase in P1NP/CTX-1 ratio was found in young participants, demonstrating that beneficial skeletal effects on bone metabolism can be attained through both aerobic endurance and resistance exercise, although this effect seems to be attenuated with age. The acute effects of exercise on bone-related biomarkers were generally blunted after 24 h, suggesting that persistent alterations following prolonged exercise interventions should be assessed at later time points.

Single-cell RNA-sequencing Reveals Thoracolumbar Vertebra Heterogeneity and Rib-genesis in Pigs

Thoracolumbar vertebra (TLV) and rib primordium (RP) development is a common evolutionary feature across vertebrates, although whole-organism analysis of TLV and RP gene expression dynamics has been lacking. Here we investigated the single-cell transcriptomic landscape of thoracic vertebra (TV), lumbar vertebra (LV), and RP cells from a pig embryo at 27 days post-fertilization (dpf) and identified six cell types with distinct gene-expression signatures. In-depth dissection of the gene-expression dynamics and RNA velocity revealed a coupled process of osteogenesis and angiogenesis during TLV and rib development. Further analysis of cell-type-specific and strand-specific expression uncovered the extremely high levels of Hoxa10 3'-untranslated region (UTR) sequence specific to osteoblast of LV cells, which may function as anti-Hoxa10-antisense by counteracting the Hoxa10-antisense effect to determine TLV transition. Thus, this work provides a valuable resource for understanding embryonic osteogenesis and angiogenesis underlying vertebrate TLV and RP development at the cell-type-specific resolution, which serves as a comprehensive view on the transcriptional profile of animal embryo development.

Association of plasma neutrophil gelatinase-associated lipocalin with parameters of CKD-MBD in maintenance hemodialysis patients

INTRODUCTION

Neutrophil gelatinase-associated lipocalin (NGAL) is not only a biomarker of kidney injury but also a bone-derived factor involved in metabolism. We aimed to explore relationships between plasma NGAL and chronic kidney disease-mineral bone disorder (CKD-MBD) parameters in maintenance hemodialysis (MHD) patients.

MATERIALS AND METHODS

First, a cross sectional observational study, including 105 MHD patients, was conducted to explore relationships between plasma NGAL levels and CKD-MBD parameters. Second, impact of parathyroidectomy and auto-transplantation (PTX + AT) on plasma NGAL was investigated in 12 MHD patients with severe secondary hyperparathyroidism (SHPT).

RESULTS

According to Spearman correlation analysis, plasma NGAL levels were positively correlated with female (r = 0.243, P = 0.012), vintage (r = 0.290, P = 0.003), Klotho (r = 0.234, P = 0.016), calcium(Ca) (r = 0.332, P = 0.001), alkaline phosphatase (ALP) (r = 0.401, P < 0.001) and intact parathyroid hormone (iPTH) (r = 0.256, P = 0.008); while inversely correlated with albumin(Alb) (r = - 0.201, P = 0.039). After adjusting for age, sex, vintage, Alb and all parameters of CKD-MBD(Ca, P, lg(ALP), lg(iPTH), Klotho and fibroblast growth factor 23(FGF23)), lg(NGAL) were positively correlated with Ca (r = 0.481, P < 0.001), P (r = 0.336, P = 0.037), lg(ALP) (r = 0.646, P < 0.001) in Partial correlation analysis; further multiple linear regression analysis showed same positive associations between lg(NGAL) and Ca (β = 0.330, P = 0.002), P (β = 0.218, P = 0.037), lg(ALP) (β = 0.671, P < 0.001). During the 4-7 days after PTX + AT, plasma NGAL decreased from 715.84 (578.73, 988.14) to 688.42 (660.00, 760.26) ng/mL (P = 0.071), Klotho increased from 496.45 (341.73, 848.30) to 1138.25 (593.87, 2009.27) pg/mL (P = 0.099).

CONCLUSION

Plasma NGAL levels were positively associated with ALP in MHD patients; and downtrends were shown after PTX + AT in patients with severe SHPT. These findings suggest that NGAL is a participant in CKD-MBD under MHD condition.

Lipocalin 2 as a link between ageing, risk factor conditions and age-related brain diseases

Chronic (neuro)inflammation plays an important role in many age-related central nervous system (CNS) diseases, including Alzheimer's disease, Parkinson's disease and vascular dementia. Inflammation also characterizes many conditions that form a risk factor for these CNS disorders, such as physical inactivity, obesity and cardiovascular disease. Lipocalin 2 (Lcn2) is an inflammatory protein shown to be involved in different age-related CNS diseases, as well as risk factor conditions thereof. Lcn2 expression is increased in the periphery and the brain in different age-related CNS diseases and also their risk factor conditions. Experimental studies indicate that Lcn2 contributes to various neuropathophysiological processes of age-related CNS diseases, including exacerbated neuroinflammation, cell death and iron dysregulation, which may negatively impact cognitive function. We hypothesize that increased Lcn2 levels as a result of age-related risk factor conditions may sensitize the brain and increase the risk to develop age-related CNS diseases. In this review we first provide a comprehensive overview of the known functions of Lcn2, and its effects in the CNS. Subsequently, this review explores Lcn2 as a potential (neuro)inflammatory link between different risk factor conditions and the development of age-related CNS disorders. Altogether, evidence convincingly indicates Lcn2 as a key constituent in ageing and age-related brain diseases.

Lipocalin 2 increases after high-intensity exercise in humans and influences muscle gene expression and differentiation in mice

Lipocalin 2 (LCN2) is an adipokine that accomplishes several functions in diverse organs. However, its importance in muscle and physical exercise is currently unknown. We observed that following acute high‐intensity exercise (“Gran Sasso d'Italia” vertical run), LCN2 serum levels were increased. The Wnt pathway antagonist, DKK1, was also increased after the run, positively correlating with LCN2, and the same was found for the cytokine Interleukin 6. We, therefore, investigated the involvement of LCN2 in muscle physiology employing an Lcn2 global knockout (Lcn2^−/−) mouse model. Lcn2^−/− mice presented with smaller muscle fibres but normal muscle performance (grip strength metre) and muscle weight. At variance with wild type (WT) mice, the inflammatory cytokine Interleukin 6 was undetectable in Lcn2^−/− mice at all ages. Intriguingly, Lcn2^−/− mice did not lose gastrocnemius and quadriceps muscle mass and muscle performance following hindlimb suspension, while at variance with WT, they lose soleus muscle mass. In vitro, LCN2 treatment reduced the myogenic differentiation of C2C12 and primary mouse myoblasts and influenced their gene expression. Treating myoblasts with LCN2 reduced myogenesis, suggesting that LCN2 may negatively affect muscle physiology when upregulated following high‐intensity exercise.

Lipocalin 2 serum levels correlate with age and bone turnover biomarkers in healthy subjects but not in postmenopausal osteoporotic women

Purpose

Lipocalin 2 (LCN2) is an adipokine involved in many physiological functions, including bone metabolism. We previously demonstrated its implication in mouse models of mechanical unloading-induced osteoporosis and in a cohort of bed rest volunteers. We therefore aimed at studying its involvement in postmenopausal osteoporosis.

Methods

We measured serum LCN2 and correlated its levels to Dickkopf WNT Signaling Pathway Inhibitor 1 (DKK1), Tartrate Resistant Acid Phosphatase 5B (TRAcP5B), sclerostin, urinary N-terminal telopeptide of type I collagen (NTX), serum C-terminal telopeptide of type I collagen (CTX), parathyroid hormone and vitamin K by ELISA performed in a cohort of younger (50–65 years) and older (66–90 years) osteoporotic women in comparison to healthy subjects. A cohort of male healthy and osteoarthritic patients was also included. Sobel mediation analysis was used to test indirect associations among age, LCN2 and DKK1 or NTX.

Results

LCN2 levels were unchanged in osteoporotic and in osteoarthritis patients when compared to healthy subjects and did not correlate with BMD. However, serum LCN2 correlated with age in healthy women (R = 0.44; P = 0.003) and men (R = 0.5; P = 0.001) and serum concentrations of DKK1 (R = 0.47; P = 0.003) and urinary NTX (R = 0.34; P = 0.04). Sobel mediation analysis showed that LCN2 mediates an indirect relationship between age and DKK1 (P = 0.02), but not with NTX, in healthy subjects.

Conclusions

Taken together, the results suggest a hitherto unknown association between LCN2, DKK1 and age in healthy individuals, but not in postmenopausal osteoporotic women.

The combined effects of simulated microgravity and X-ray radiation on MC3T3-E1 cells and rat femurs

Microgravity is well-known to induce Osteopenia. However, the combined effects of microgravity and radiation that commonly exist in space have not been broadly elucidated. This research investigates the combined effects on MC3T3-E1 cells and rat femurs. In MC3T3-E1 cells, simulated microgravity and X-ray radiation, alone or combination, show decreased cell activity, increased apoptosis rates by flow cytometric analysis, and decreased Runx2 and increased Caspase-3 mRNA and protein expressions. In rat femurs, simulated microgravity and X-ray radiation, alone or combination, show increased bone loss by micro-CT test and Masson staining, decreased serum BALP levels and Runx2 mRNA expressions, and increased serum CTX-1 levels and Caspase-3 mRNA expressions. The strongest effect is observed in the combined group in MC3T3-E1 cells and rat femurs. These findings suggest that the combination of microgravity and radiation exacerbates the effects of either treatment alone on MC3T3-E1 cells and rat femurs.

Serum Metabolomics Associating With Circulating MicroRNA Profiles Reveal the Role of miR-383-5p in Rat Hippocampus Under Simulated Microgravity

Microgravity impacts various aspects of human health. Yet the mechanisms of spaceflight-induced health problems are not elucidated. Here, we mapped the fusion systemic analysis of the serum metabolome and the circulating microRNAome in a hindlimb unloading rat model to simulate microgravity. The response of serum metabolites and microRNAs to simulated microgravity was striking. Integrated pathway analysis of altered serum metabolites and target genes of the significantly altered circulating miRNAs with Integrated Molecular Pathway-Level Analysis (IMPaLA) software was mainly suggestive of modulation of neurofunctional signaling pathways. Particularly, we revealed significantly increased miR-383-5p and decreased aquaporin 4 (AQP4) in the hippocampus. Using rabies virus glycoprotein-modified exosomes, delivery of miR-383-5p inhibited the expression of AQP4 not only in rat C6 glioma cells in vitro but also in the hippocampus in vivo. Using bioinformatics to map the crosstalk between the circulating metabolome and miRNAome could offer opportunities to understand complex biological systems under microgravity. Our present results suggested that the change of miR-383-5p level and its regulation of target gene AQP4 was one of the potential molecular mechanisms of microgravity-induced cognitive impairment in the hippocampus.

Phenotypic transitions enacted by simulated microgravity do not alter coherence in gene transcription profile

Cells in simulated microgravity undergo a reversible morphology switch, causing the appearance of two distinct phenotypes. Despite the dramatic splitting into an adherent-fusiform and a floating-spherical population, when looking at the gene-expression phase space, cell transition ends up in a largely invariant gene transcription profile characterized by only mild modifications in the respective Pearson's correlation coefficients. Functional changes among the different phenotypes emerging in simulated microgravity using random positioning machine are adaptive modifications-as cells promptly recover their native phenotype when placed again into normal gravity-and do not alter the internal gene coherence. However, biophysical constraints are required to drive phenotypic commitment in an appropriate way, compatible with physiological requirements, given that absence of gravity foster cells to oscillate between different attractor states, thus preventing them to acquire a exclusive phenotype. This is a proof-of-concept of the adaptive properties of gene-expression networks supporting very different phenotypes by coordinated 'profile preserving' modifications.

Gene Expression in Osteoblasts and Osteoclasts Under Microgravity Conditions: A Systematic Review

Background

Microgravity (μG) negatively influences bone metabolism by affecting normal osteoblast and osteoclast function. μG effects on bone metabolism has been an extensive field of study in recent years, due to the challenges presented by space flight.

Methods

We systematically reviewed research data from genomic studies performed in real or simulat-ed μG, on osteoblast and osteoclast cells. Our search yielded 50 studies, of which 39 concerned cells of the osteoblast family and 11 osteoclast precursors.

Results

Osteoblastic cells under μG show a decreased differentiation phenotype, proved by diminished expression levels of Alkaline Phosphatase (ALP) and Osteocalcin (OCN) but no apoptosis. Receptor Activator of NF-κB Ligand (RANKL)/ Osteoprotegerine (OPG) ratio is elevated in favor of RANKL in a time-dependent manner, and further RANKL production is caused by upregulation of Interleukin-6 (IL-6) and the inflammation pathway. Extracellular signals and changes in the gravitational environment are perceived by mechanosensitive proteins of the cytoskeleton and converted to intracellular signals through the Mitogen Activated Protein Kinase pathway (MAPK). This is followed by changes in the ex-pression of nuclear transcription factors of the Activator Protein-1 (AP-1) family and in turn of the NF-κB, thus affecting osteoblast differentiation, cell cycle, proliferation and maturation. Pre-osteoclastic cells show increased expression of the marker proteins such as Tryptophan Regulated Attenuation Protein (TRAP), cathepsin K, Matrix Metalloproteinase-9 (MMP-9) under μG conditions and become sensitized to RANKL.

Conclusion

Suppressing the expression of fusion genes such as syncytine-A which acts independently of RANKL, could be possible future therapeutic targets for microgravity side effects.

Osteoblasts Regulate Angiogenesis in Response to Mechanical Unloading

During mechanical unloading, endothelial cells reduce osteogenesis and increase bone resorption. Here we describe the feedback response of endothelial cells to unloaded osteoblasts. Primary endothelial cells, ex vivo mouse aortic rings and chicken egg yolk membranes were incubated with conditioned medium from mouse primary osteoblasts (OB-CM) subjected to unit gravity or simulated microgravity, to assess its effect on angiogenesis. In vivo injection of botulin toxin A (Botox) in the quadriceps and calf muscles of C57BL/6J mice was performed to mimic disuse osteoporosis. Unloaded osteoblasts showed strong upregulation of the pro-angiogenic factor, VEGF, and their conditioned medium increased in vitro endothelial cell viability, Cyclin D1 expression, migration and tube formation, ex vivo endothelial cell sprouting from aortic rings, and in ovo angiogenesis. Treatment with the VEGF blocker, avastin, prevented unloaded OB-CM-mediated in vitro and ex vivo enhancement of angiogenesis. Bone mechanical unloading by Botox treatment, known to reduce bone mass, prompted the overexpression of VEGF in osteoblasts. The cross talk between osteoblasts and endothelial cells plays a pathophysiologic role in the response of the endothelium to unloading during disuse osteoporosis. In this context, VEGF represents a prominent osteoblast factor stimulating angiogenesis.

Updates on Osteoimmunology: What's New on the Cross-Talk Between Bone and Immune System

The term osteoimmunology was coined many years ago to describe the research field that deals with the cross-regulation between bone cells and the immune system. As a matter of fact, many factors that are classically considered immune-related, such as InterLeukins (i.e., IL-6, -11, -17, and -23), Tumor Necrosis Factor (TNF)-α, Receptor-Activator of Nuclear factor Kappa B (RANK), and its Ligand (RANKL), Nuclear Factor of Activated T-cell, cytoplasmatic-1 (NFATc1), and others have all been found to be crucial in osteoclast and osteoblast biology. Conversely, bone cells, which we used to think would only regulate each other and take care of remodeling bone, actually regulate immune cells, by creating the so-called "endosteal niche." Both osteoblasts and osteoclasts participate to this niche, either by favoring engraftment, or mobilization of Hematopoietic Stem Cells (HSCs). In this review, we will describe the main milestones at the base of the osteoimmunology and present the key cellular players of the bone-immune system cross-talk, including HSCs, osteoblasts, osteoclasts, bone marrow macrophages, osteomacs, T- and B-lymphocytes, dendritic cells, and neutrophils. We will also briefly describe some pathological conditions in which the bone-immune system cross-talk plays a crucial role, with the final aim to portray the state of the art in the mechanisms regulating the bone-immune system interplay, and some of the latest molecular players in the field. This is important to encourage investigation in this field, to identify new targets in the treatment of bone and immune diseases.

Bone: Another potential target to treat, prevent and predict diabetes

Type 2 diabetes mellitus is now a worldwide health problem with increasing prevalence. Mounting efforts have been made to treat, prevent and predict this chronic disease. In recent years, increasing evidence from mice and clinical studies suggests that bone-derived molecules modulate glucose metabolism. This review aims to summarize our current understanding of the interplay between bone and glucose metabolism and to highlight potential new means of therapeutic intervention. The first molecule recognized as a link between bone and glucose metabolism is osteocalcin (OCN), which functions in its active form, that is, undercarboxylated OCN (ucOC). ucOC acts in promoting insulin expression and secretion, facilitating insulin sensitivity, and favouring glucose and fatty acid uptake and utilization. A second bone-derived molecule, lipocalin2, functions in suppressing appetite in mice through its action on the hypothalamus. Osteocytes, the most abundant cells in bone matrix, are suggested to act on the browning of white adipose tissue and energy expenditure through secretion of bone morphogenetic protein 7 and sclerostin. The involvement of bone resorption in glucose homeostasis has also been examined. However, there is evidence indicating the implication of the receptor activator of nuclear factor κ-B ligand, neuropeptide Y, and other known and unidentified bone-derived factors that function in glucose homeostasis. We summarize recent advances and the rationale for treating, preventing and predicting diabetes by skeleton intervention.

A Complex Role for Lipocalin 2 in Bone Metabolism: Global Ablation in Mice Induces Osteopenia Caused by an Altered Energy Metabolism

Lipocalin 2 (Lcn2) is an adipokine that carries out a variety of functions in diverse organs. We investigated the bone phenotype and the energy metabolism of Lcn2 globally deleted mice (Lcn2^-/- ) at different ages. Lcn2^-/- mice were largely osteopenic, exhibiting lower trabecular bone volume, lesser trabecular number, and higher trabecular separation when compared to wild-type (WT) mice. Lcn2^-/- mice showed a lower osteoblast number and surface over bone surface, and subsequently a significantly lower bone formation rate, while osteoclast variables were unremarkable. Surprisingly, we found no difference in alkaline phosphatase (ALP) activity or in nodule mineralization in Lcn2^-/- calvaria osteoblast cultures, while less ALP-positive colonies were obtained from freshly isolated Lcn2^-/- bone marrow stromal cells, suggesting a nonautonomous osteoblast response to Lcn2 ablation. Given that Lcn2^-/- mice showed higher body weight and hyperphagia, we investigated whether their osteoblast impairment could be due to altered energy metabolism. Lcn2^-/- mice showed lower fasted glycemia and hyperinsulinemia. Consistently, glucose tolerance was significantly higher in Lcn2^-/- compared to WT mice, while insulin tolerance was similar. Lcn2^-/- mice also exhibited polyuria, glycosuria, proteinuria, and renal cortex vacuolization, suggesting a kidney contribution to their phenotype. Interestingly, the expression of the glucose transporter protein type 1, that conveys glucose into the osteoblasts and is essential for osteogenesis, was significantly lower in the Lcn2^-/- bone, possibly explaining the in vivo osteoblast impairment induced by the global Lcn2 ablation. Taken together, these results unveil an important role of Lcn2 in bone metabolism, highlighting a link with glucose metabolism that is more complex than expected from the current knowledge. © 2018 American Society for Bone and Mineral Research.

The relationship among serum lipocalin 2, bone turnover markers, and bone mineral density in outpatient women

PURPOSE

We aimed to investigate associations among serum levels of LCN2, bone resorption marker carboxy-terminal cross-linking telopeptide of type-1 collagen (CTx), bone formation marker osteocalcin (OCN), and bone mineral densities (BMDs) in ambulatory healthy women.

METHODS

This cross-sectional study analyzed 1012 previously enrolled outpatient Han Chinese women. BMDs of the lumbar spine and femoral neck were measured using dual energy X-ray absorptiometry. Serum levels of LCN2, CTx, OCN, and creatinine (Scr) were measured.

RESULTS

Circulating LCN2 was inversely correlated with BMDs at the lumbar spine and femoral neck (Spearman's r = -0.08, P = 0.010 and r = -0.14, P < 0.001; respectively). A significant positive correlation between LCN2 and CTx (r = 0.11, P < 0.001), OCN (r = 0.06, P = 0.047), age (r = 0.21, P < 0.001), and Scr (r = 0.24, P < 0.001) was also observed. After adjusting for age and Scr, the correlation among LCN2, BMDs and OCN disappeared, but LCN2 was still positively associated with CTx (r = 0.08, P = 0.010). The circulating concentration of LCN2 showed no significant difference between subjects with and without osteoporotic fractures (43.63 (35.29, 53.66) vs. 42.25 (34.43, 51.46) ng/ml, respectively, P = 0.111). Serum CTx concentrations rose with serum LCN2 increasing from the lowest to the highest quartile (P for trend = 0.005), even after adjusting for age and Scr (P for trend = 0.040). In multivariate regression analysis, LCN2 was one of the main determinants for changes in serum CTx (standard β = 0.061, P = 0.005).

CONCLUSIONS

In ambulatory healthy women, the relationships among serum LCN2 level, BMDs, and OCN were confounded by age and Scr. Although LCN2 was positively related with CTx, the correlation was very weak and may not be physiologically relevant.

Genome‑wide analysis and prediction of functional long noncoding RNAs in osteoblast differentiation under simulated microgravity

Long noncoding RNAs (lncRNAs) have been regarded as important regulators in numerous biological processes during cell development. However, the holistic lncRNA expression pattern and potential functions during osteoblast differentiation under simulated microgravity remain unknown. In the present study, a high throughput microarray assay was performed to detect lncRNA and mRNA expression profiles during MC3TC-E1 pre-osteoblast cell osteo-differentiation under simulated microgravity. The expression of 857 lncRNAs and 2,264 mRNAs was significantly altered when MC3T3-E1 cells were exposed to simulated microgravity. A relatively consistent distribution pattern on the chromosome and a co-expression network were observed between the differentially-expressed lncRNAs and mRNAs. Genomic context analysis further identified 132 differentially-expressed lncRNAs and nearby coding gene pairs. Subsequently, 3 lncRNAs were screened out for their possible function in osteoblast differentiation, based on their co-expression association and potential cis-acting regulatory pattern with the deregulated mRNAs. The present study aimed to provide a comprehensive understanding of and a foundation for future studies into lncRNA function in mechanical signal-mediated osteoblast differentiation.

Effects of short-term dry immersion on bone remodeling markers, insulin and adipokines

Background

Dry immersion (DI), a ground-based model of microgravity previously used in Russia, has been recently implemented in France. The aim of this study was to analyze early events in a short-term DI model in which all conditions are met to investigate who is first challenged from osteo- or adipo-kines and to what extent they are associated to insulin-regulating hormones.

Methods

Twelve healthy men were submitted to a 3-day DI. Fasting blood was collected during pre-immersion phase for the determination of the baseline data collection (BDC), daily during DI (DI_24h, DI_48H and DI_72h), then after recovery (R_+3h and R_+24h). Markers of bone turnover, phosphocalcic metabolism, adipokines and associated factors were measured.

Results

Bone resorption as assessed by tartrate-resistant acid phosphatase isoform 5b and N-terminal crosslinked telopeptide of type I collagen levels increased as early as DI_24h. At the same time, total procollagen type I N- and C-terminal propeptides and osteoprotegerin, representing bone formation markers, decreased. Total osteocalcin [OC] was unaffected, but its undercarboxylated form [Glu-OC] increased from DI_24h to R_+3h. The early and progressive increase in bone alkaline phosphatase activities suggested an increased mineralization. Dickkopf-1 and sclerostin, as negative regulators of the Wnt-β catenin pathway, were unaltered. No change was observed either in phosphocalcic homeostasis (calcium and phosphate serum levels, 25-hydroxyvitamin D, fibroblast growth factor 23 [FGF23]) or in inflammatory response. Adiponectemia was unchanged, whereas circulating leptin concentrations increased. Neutrophil gelatinase-associated lipocalin [lipocalin-2], a potential regulator of bone homeostasis, was found elevated by 16% at R_+3h compared to DI_24h. The secretory form of nicotinamide phosphoribosyl-transferase [visfatin] concentrations almost doubled after one day of DI and remained elevated. Serum insulin-like growth factor 1 levels progressively increased. Fasting insulin concentrations increased during the entire DI, whereas fasting glucose levels tended to be higher only at DI_24h and then returned to BDC values. Changes in bone resorption parameters negatively correlated with changes in bone formation parameters. Percent changes of ultra-sensitive C-reactive protein positively correlated with changes in osteopontin, lipocalin-2 and fasting glucose. Furthermore, a positive correlation was found between changes in FGF23 and Glu-OC, the two main osteoblast-/osteocyte-derived hormones.

Conclusion

Our results demonstrated that DI induced an unbalanced remodeling activity and the onset of insulin resistance. This metabolic adaptation was concomitant with higher levels of Glu-OC. This finding confirms the role of bone as an endocrine organ in humans. Furthermore, visfatin for which a great responsiveness was observed could represent an early and sensitive marker of unloading in humans.

Osteoblast Differentiation at a Glance

Ossification is a tightly regulated process, performed by specialized cells called osteoblasts. Dysregulation of this process may cause inadequate or excessive mineralization of bones or ectopic calcification, all of which have grave consequences for human health. Understanding osteoblast biology may help to treat diseases such as osteogenesis imperfecta, calcific heart valve disease, osteoporosis, and many others. Osteoblasts are bone-building cells of mesenchymal origin; they differentiate from mesenchymal progenitors, either directly or via an osteochondroprogenitor. The direct pathway is typical for intramembranous ossification of the skull and clavicles, while the latter is a hallmark of endochondral ossification of the axial skeleton and limbs. The pathways merge at the level of preosteoblasts, which progress through 3 stages: proliferation, matrix maturation, and mineralization. Osteoblasts can also differentiate into osteocytes, which are stellate cells populating narrow interconnecting passages within the bone matrix.

The key molecular switch in the commitment of mesenchymal progenitors to osteoblast lineage is the transcription factor cbfa/runx2, which has multiple upstream regulators and a wide variety of targets. Upstream is the Wnt/Notch system, Sox9, Msx2, and hedgehog signaling. Cofactors of Runx2 include Osx, Atf4, and others. A few paracrine and endocrine factors serve as coactivators, in particular, bone morphogenetic proteins and parathyroid hormone. The process is further fine-tuned by vitamin D and histone deacetylases. Osteoblast differentiation is subject to regulation by physical stimuli to ensure the formation of bone adequate for structural and dynamic support of the body. Here, we provide a brief description of the various stimuli that influence osteogenesis: shear stress, compression, stretch, micro- and macrogravity, and ultrasound. A complex understanding of factors necessary for osteoblast differentiation paves a way to introduction of artificial bone matrices.

Interleukin-1β, lipocalin 2 and nitric oxide synthase 2 are mechano-responsive mediators of mouse and human endothelial cell-osteoblast crosstalk

Endothelial cells are spatially close to osteoblasts and regulate osteogenesis. Moreover, they are sensitive to mechanical stimuli, therefore we hypothesized that they are implicated in the regulation of bone metabolism during unloading. Conditioned media from endothelial cells (EC-CM) subjected to simulated microgravity (0.08g and 0.008g) increased osteoblast proliferation and decreased their differentiation compared to unit gravity (1g) EC-CM. Microgravity-EC-CM increased the expression of osteoblast Rankl and subsequent osteoclastogenesis, and induced the osteoblast de-differentiating factor, Lipocalin 2 (Lcn2), whose downregulation recovered osteoblast activity, decreased Rankl expression and reduced osteoclastogenesis. Microgravity-EC-CM enhanced osteoblast NO-Synthase2 (NOS2) and CycloOXygenase2 (COX2) expression. Inhibition of NOS2 or NO signaling reduced osteoblast proliferation and rescued their differentiation. Nuclear translocation of the Lcn2/NOS2 transcription factor, NF-κB, occurred in microgravity-EC-CM-treated osteoblasts and in microgravity-treated endothelial cells, alongside high expression of the NF-κB activator, IL-1β. IL-1β depletion and NF-κB inhibition reduced osteoblast proliferation and rescued differentiation. Lcn2 and NOS2 were incremented in ex vivo calvarias cultured in microgravity-EC-CM, and in vivo tibias and calvarias injected with microgravity-EC-CM. Furthermore, tibias of botulin A toxin-treated and tail-suspended mice, which featured unloading and decreased bone mass, showed higher expression of IL-1β, Lcn2 and Nos2, suggesting their pathophysiologic involvement in endothelial cell-osteoblast crosstalk.

The adipokine lipocalin-2 in the context of the osteoarthritic osteochondral junction

Obesity and osteoarthritis (OA) form a vicious circle in which obesity contributes to cartilage destruction in OA, and OA-associated sedentary behaviour promotes weight gain. Lipocalin-2 (LCN2), a novel adipokine with catabolic activities in OA joints, contributes to the obesity and OA pathologies and is associated with other OA risk factors. LCN2 is highly induced in osteoblasts in the absence of mechanical loading, but its role in osteoblast metabolism is unclear. Therefore, because osteochondral junctions play a major role in OA development, we investigated the expression and role of LCN2 in osteoblasts and chondrocytes in the OA osteochondral junction environment. Our results showed that LCN2 expression in human osteoblasts and chondrocytes decreased throughout osteoblast differentiation and was induced by catabolic and inflammatory factors; however, TGF-β1 and IGF-1 reversed this induction. LCN2 reduced osteoblast viability in the presence of iron and enhanced the activity of MMP-9 released by osteoblasts. Moreover, pre-stimulated human osteoblasts induced LCN2 expression in human chondrocytes, but the inverse was not observed. Thus, LCN2 is an important catabolic adipokine in osteoblast and chondrocyte metabolism that is regulated by differentiation, inflammation and catabolic and anabolic stimuli, and LCN2 expression in chondrocytes is regulated in a paracrine manner after osteoblast stimulation.

miRNA-132-3p inhibits osteoblast differentiation by targeting Ep300 in simulated microgravity

Recent studies have demonstrated that miRNAs can play important roles in osteoblast differentiation and bone formation. However, the function of miRNAs in bone loss induced by microgravity remains unclear. In this study, we investigated the differentially expressed miRNAs in both the femur tissues of hindlimb unloading rats and primary rat osteoblasts (prOB) exposed to simulated microgravity. Specifically, miR-132-3p was found up-regulated and negatively correlated with osteoblast differentiation. Overexpression of miR-132-3p significantly inhibited prOB differentiation, whereas inhibition of miR-132-3p function yielded an opposite effect. Furthermore, silencing of miR-132-3p expression effectively attenuated the negative effects of simulated microgravity on prOB differentiation. Further experiments confirmed that E1A binding protein p300 (Ep300), a type of histone acetyltransferase important for Runx2 activity and stability, was a direct target of miR-132-3p. Up-regulation of miR-132-3p by simulated microgravity could inhibit osteoblast differentiation in part by decreasing Ep300 protein expression, which, in turn, resulted in suppression of the activity and acetylation of Runx2, a key regulatory factor of osteoblast differentiation. Taken together, our findings are the first to demonstrate that miR-132-3p can inhibit osteoblast differentiation and participate in the regulation of bone loss induced by simulated microgravity, suggesting a potential target for counteracting decreases in bone formation.

Circulating Lipocalin 2 Levels Predict Fracture-Related Hospitalizations in Elderly Women: A Prospective Cohort Study

Lipocalin 2 (LCN2) or neutrophil gelatinase-associated lipocalin (NGAL) is expressed in a wide range of cells and pathological states. Mounting evidence suggests lipocalin 2 may be an important regulator of bone homeostasis. Recently it has been suggested LCN2 is a novel mechanoresponsive gene central to the pathological response to low mechanical force. We undertook a prospective study of 1009 elderly women over 70 years of age to study the association between circulating LCN2 and potential associated variables, including estimated glomerular filtration rate, physical activity, and baseline measures of hip bone density and heel bone quality. Osteoporotic fractures requiring hospitalizations were identified from the Western Australian Data Linkage System. Over 14.5 years, 272 (27%) of women sustained an osteoporotic fracture-related hospitalization; of these, 101 were hip fractures. Circulating LCN2 levels were correlated with body mass index and estimated glomerular filtration rate (r = 0.249, p < 0.001 and r = -0.481, p < 0.001, respectively) that modified the association with hip and heel bone measures. Per standard deviation increase in LCN2, there was a 30% multivariable-adjusted increase in the risk of any osteoporotic fracture (hazard ratio [HR] = 1.30, 95% confidence interval [CI] 1.13-1.50, p < 0.001). In participants with elevated LCN2 levels above the median (76.6 ng/mL), there was an 80% to 81% increase in the risk of any osteoporotic or hip fracture (HR = 1.81, 95% CI 1.38-2.36, p < 0.001 and HR = 1.80, 95% CI 1.16-2.78, p = 0.008, respectively). These associations remained significant after adjustment for total hip bone mineral density (p < 0.05). In conclusion, we have demonstrated that circulating LCN2 levels predict future risk of osteoporotic fractures requiring hospitalization. Measurement of LCN2 levels may improve fracture prediction in addition to current fracture risk factors in the elderly, particularly in those with impaired renal function.

Histological and Transcriptomic Analysis of Adult Japanese Medaka Sampled Onboard the International Space Station

To understand how humans adapt to the space environment, many experiments can be conducted on astronauts as they work aboard the Space Shuttle or the International Space Station (ISS). We also need animal experiments that can apply to human models and help prevent or solve the health issues we face in space travel. The Japanese medaka (Oryzias latipes) is a suitable model fish for studying space adaptation as evidenced by adults of the species having mated successfully in space during 15 days of flight during the second International Microgravity Laboratory mission in 1994. The eggs laid by the fish developed normally and hatched as juveniles in space. In 2012, another space experiment (“Medaka Osteoclast”) was conducted. Six-week-old male and female Japanese medaka (Cab strain osteoblast transgenic fish) were maintained in the Aquatic Habitat system for two months in the ISS. Fish of the same strain and age were used as the ground controls. Six fish were fixed with paraformaldehyde or kept in RNA stabilization reagent (n = 4) and dissected for tissue sampling after being returned to the ground, so that several principal investigators working on the project could share samples. Histology indicated no significant changes except in the ovary. However, the RNA-seq analysis of 5345 genes from six tissues revealed highly tissue-specific space responsiveness after a two-month stay in the ISS. Similar responsiveness was observed among the brain and eye, ovary and testis, and the liver and intestine. Among these six tissues, the intestine showed the highest space response with 10 genes categorized as oxidation–reduction processes (gene ontogeny term GO:0055114), and the expression levels of choriogenin precursor genes were suppressed in the ovary. Eleven genes including klf9, klf13, odc1, hsp70 and hif3a were upregulated in more than four of the tissues examined, thus suggesting common immunoregulatory and stress responses during space adaptation.

The Wnt Inhibitor Sclerostin Is Up-regulated by Mechanical Unloading in Osteocytes in Vitro

Although bone responds to its mechanical environment, the cellular and molecular mechanisms underlying the response of the skeleton to mechanical unloading are not completely understood. Osteocytes are the most abundant but least understood cells in bones and are thought to be responsible for sensing stresses and strains in bone. Sclerostin, a product of the SOST gene, is produced postnatally primarily by osteocytes and is a negative regulator of bone formation. Recent studies show that SOST is mechanically regulated at both the mRNA and protein levels. During prolonged bed rest and immobilization, circulating sclerostin increases both in humans and in animal models, and its increase is associated with a decrease in parathyroid hormone. To investigate whether SOST/sclerostin up-regulation in mechanical unloading is a cell-autonomous response or a hormonal response to decreased parathyroid hormone levels, we subjected osteocytes to an in vitro unloading environment achieved by the NASA rotating wall vessel system. To perform these studies, we generated a novel osteocytic cell line (Ocy454) that produces high levels of SOST/sclerostin at early time points and in the absence of differentiation factors. Importantly, these osteocytes recapitulated the in vivo response to mechanical unloading with increased expression of SOST (3.4 ± 1.9-fold, p < 0.001), sclerostin (4.7 ± 0.1-fold, p < 0.001), and the receptor activator of nuclear factor κΒ ligand (RANKL)/osteoprotegerin (OPG) (2.5 ± 0.7-fold, p < 0.001) ratio. These data demonstrate for the first time a cell-autonomous increase in SOST/sclerostin and RANKL/OPG ratio in the setting of unloading. Thus, targeted osteocyte therapies could hold promise as novel osteoporosis and disuse-induced bone loss treatments by directly modulating the mechanosensing cells in bone.

Lipocalin 2: a new mechanoresponding gene regulating bone homeostasis

Mechanical loading represents a crucial factor in the regulation of skeletal homeostasis. Its reduction causes loss of bone mass, eventually leading to osteoporosis. In a previous global transcriptome analysis performed in mouse calvarial osteoblasts subjected to simulated microgravity, the most upregulated gene compared to unit gravity condition was Lcn2, encoding the adipokine Lipocalin 2 (LCN2), whose function in bone metabolism is poorly known. To investigate the mechanoresponding properties of LCN2, we evaluated LCN2 levels in sera of healthy volunteers subjected to bed rest, and found a significant time-dependent increase of this adipokine compared to time 0. We then evaluated the in vivo LCN2 regulation in mice subjected to experimentally-induced mechanical unloading by (1) tail suspension, (2) muscle paralysis by botulin toxin A (Botox), or (3) genetically-induced muscular dystrophy (MDX mice), and observed that Lcn2 expression was upregulated in the long bones of all of them, whereas physical exercise counteracted this increase. Mechanistically, in primary osteoblasts transfected with LCN2-expression-vector (OBs-Lcn2) we observed that Runx2 and its downstream genes, Osterix and Alp, were transcriptionally downregulated, and alkaline phosphatase (ALP) activity was less prominent versus empty-vector transduced osteoblasts (OBs-empty). OBs-Lcn2 also exhibited an increase of the Rankl/Opg ratio and IL-6 mRNA, suggesting that LCN2 could link poor differentiation of osteoblasts to enhanced osteoclast stimulation. In fact, incubation of purified mouse bone marrow mononuclear cells with conditioned media from OBs-Lcn2 cultures, or their coculture with OBs-Lcn2, improved osteoclastogenesis compared to OBs-empty, whereas treatment with recombinant LCN2 had no effect. In conclusion, our data indicate that LCN2 is a novel osteoblast mechanoresponding gene and that its regulation could be central to the pathological response of the bone tissue to low mechanical forces.

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.

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.

Altered bone development and turnover in transgenic mice over-expressing lipocalin-2 in bone

Lipocalin-2 (LCN2) is a protein largely expressed in many tissues, associated with different biological phenomena such as cellular differentiation, inflammation and cancer acting as a survival/apoptotic signal. We found that LCN2 was expressed during osteoblast differentiation and we generated transgenic (Tg) mice over-expressing LCN2 in bone. Tg mice were smaller and presented bone microarchitectural changes in both endochondral and intramembranous bones. In particular, Tg bones displayed a thinner layer of cortical bone and a decreased trabecular number. Osteoblast bone matrix deposition was reduced and osteoblast differentiation was slowed-down. Differences were also observed in the growth plate of young transgenic mice where chondrocyte displayed a more immature phenotype and a lower proliferation rate. In bone marrow cell cultures from transgenic mice, the number of osteoclast progenitors was increased whereas in vivo it was increased the number of mature osteoclasts expressing tartrate-resistant acid phosphatase (TRAP). Finally, while osteoprotegerin (OPG) levels remained unchanged, the expression of the conventional receptor activator of nuclear factor-κB ligand (RANKL) and of the IL-6 was enhanced in Tg mice. In conclusion, we found that LCN2 plays a role in bone development and turnover having both a negative effect on bone formation, by affecting growth plate development and interfering with osteoblast differentiation, and a positive effect on bone resorption by enhancing osteoclast compartment.

Genetic loci that control the loss and regain of trabecular bone during unloading and reambulation

Changes in trabecular morphology during unloading and reloading are marked by large variations between individuals, implying that there is a strong genetic influence on the magnitude of the response. Here, we subjected more than 350 second-generation (BALBxC3H) 4-month-old adult female mice to 3 weeks of hindlimb unloading followed by 3 weeks of reambulation to identify the quantitative trait loci (QTLs) that define an individual's propensity to either lose trabecular bone when weight bearing is removed or to gain trabecular bone when weight bearing is reintroduced. Longitudinal in vivo micro-computed tomography (µCT) scans demonstrated that individual mice lost between 15% and 71% in trabecular bone volume fraction (BV/TV) in the distal femur during unloading (average: -43%). Changes in trabecular BV/TV during the 3-week reambulation period ranged from a continuation of bone loss (-18%) to large additions (56%) of tissue (average: +10%). During unloading, six QTLs accounted for 21% of the total variability in changes in BV/TV whereas one QTL accounted for 6% of the variability in changes in BV/TV during reambulation. QTLs were also identified for changes in trabecular architecture. Most of the QTLs defining morphologic changes during unloading or reambulation did not overlap with those QTLs identified at baseline, suggesting that these QTLs harbor genes that are specific for sensing changes in the levels of weight bearing. The lack of overlap in QTLs between unloading and reambulation also emphasizes that the genes modulating the trabecular response to unloading are distinct from those regulating tissue recovery during reloading. The identified QTLs contain the regulatory genes underlying the strong genetic regulation of trabecular bone's sensitivity to weight bearing and may help to identify individuals that are most susceptible to unloading-induced bone loss and/or the least capable of recovering.

Increased expression of a set of genes enriched in oxygen binding function discloses a predisposition of breast cancer bone metastases to generate metastasis spread in multiple organs

Bone is the preferential site of distant metastasis in breast carcinoma (BrCa). Patients with metastasis restricted to bone (BO) usually show a longer overall survival compared to patients who rapidly develop multiple metastases also involving liver and lung. Hence, molecular predisposition to generate bone and visceral metastases (BV) represents a clear indication of poor clinical outcome. We performed microarray analysis with two different chip platforms, Affymetrix and Agilent, on bone metastasis samples from BO and BV patients. The unsupervised hierarchical clustering of the resulting transcriptomes correlated with the clinical progression, segregating the BO from the BV profiles. Matching the twofold significantly regulated genes from Affymetrix and Agilent chips resulted in a 15-gene signature with 13 upregulated and two downregulated genes in BV versus BO bone metastasis samples. In order to validate the resulting signature, we isolated different MDA-MB-231 clonal subpopulations that metastasize only in the bone (MDA-BO) or in bone and visceral tissues (MDA-BV). Six of the signature genes were also significantly upregulated in MDA-BV compared to MDA-BO clones. A group of upregulated genes, including Hemoglobin B (HBB), were involved in oxygen metabolism, and in vitro functional analysis of HBB revealed that its expression in the MDA subpopulations was associated with a reduced production of hydrogen peroxide. Expression of HBB was detected in primary BrCa tissue but not in normal breast epithelial cells. Metastatic lymph nodes were frequently more positive for HBB compared to the corresponding primary tumors, whereas BO metastases had a lower expression than BV metastases, suggesting a positive correlation between HBB and ability of bone metastasis to rapidly spread to other organs. We propose that HBB, along with other genes involved in oxygen metabolism, confers a more aggressive metastatic phenotype in BrCa cells disseminated to bone.

IMPACT OF OSTEOCLAST PRECURSORS SUBJECTED TO RANDOM POSITIONING MACHINE ON OSTEOBLASTS

Osteoblast-osteoclast interaction plays an important role in the bone remodeling. During long duration space flight, astronauts undergo serious bone loss mainly due to the disruption of equivalence between bone formation and bone resorption. Osteoclast precursors often operate under the control of osteoblasts. However, here we show that the osteoclast precursors could in turn influence osteoblasts. RAW264.7 cells, the murine osteoclast precursors, were treated in the simulated weightlessness produced by a Random Positioning Machine (RPM). After 72 h, conditioned mediums (CM) by the RAW264.7 cells from RPM (RCM) or static control (CCM) were collected and were used to culture osteoblastic-like MC3T3-E1 cells. The results showed that the RCM culture inhibited cell viability and slightly altered cell cycle, but the morphology of the MC3T3-E1 cells was not changed by RCM compared to that of CCM. Furthermore, the intracellular ALP level, NO release and expression of osteoblastic marker genes were all down-regulated by RCM culture. These results suggest that osteoclast precursors subjected to RPM exert negative regulation on osteoblasts.

BMP2 and mechanical loading cooperatively regulate immediate early signalling events in the BMP pathway

Background

Efficient osteogenic differentiation is highly dependent on coordinated signals arising from growth factor signalling and mechanical forces. Bone morphogenetic proteins (BMPs) are secreted proteins that trigger Smad and non-Smad pathways and thereby influence transcriptional and non-transcriptional differentiation cues. Crosstalk at multiple levels allows for promotion or attenuation of signalling intensity and specificity. Similar to BMPs, mechanical stimulation enhances bone formation. However, the molecular mechanism by which mechanical forces crosstalk to biochemical signals is still unclear.

Results

Here, we use a three-dimensional bioreactor system to describe how mechanical forces are integrated into the BMP pathway. Time-dependent phosphorylation of Smad, mitogen-activated protein kinases and Akt in human fetal osteoblasts was investigated under loading and/or BMP2 stimulation conditions. The phosphorylation of R-Smads is increased both in intensity and duration under BMP2 stimulation with concurrent mechanical loading. Interestingly, the synergistic effect of both stimuli on immediate early Smad phosphorylation is reflected in the transcription of only a subset of BMP target genes, while others are differently affected. Together this results in a cooperative regulation of osteogenesis that is guided by both signalling pathways.

Conclusions

Mechanical signals are integrated into the BMP signalling pathway by enhancing immediate early steps within the Smad pathway, independent of autocrine ligand secretion. This suggests a direct crosstalk of both mechanotransduction and BMP signalling, most likely at the level of the cell surface receptors. Furthermore, the crosstalk of both pathways over longer time periods might occur on several signalling levels.

Selection of suitable reference genes from bone cells in large gradient high magnetic field based on GeNorm algorithm

Studies of animals and humans subjected to spaceflight demonstrate that weightlessness negatively affects the mass and mechanical properties of bone tissue. Bone cells could sense and respond to the gravity unloading, and genes sensitive to gravity change were considered to play a critical role in the mechanotransduction of bone cells. To evaluate the fold-change of gene expression, appropriate reference genes should be identified because there is no housekeeping gene having stable expression in all experimental conditions. Consequently, expression stability of ten candidate housekeeping genes were examined in osteoblast-like MC3T3-E1, osteocyte-like MLO-Y4, and preosteoclast-like FLG29.1 cells under different apparent gravities (μg, 1 g, and 2 g) in the high-intensity gradient magnetic field produced by a superconducting magnet. The results showed that the relative expression of these ten candidate housekeeping genes was different in different bone cells; Moreover, the most suitable reference genes of the same cells in altered gravity conditions were also different from that in strong magnetic field. It demonstrated the importance of selecting suitable reference genes in experimental set-ups. Furthermore, it provides an alternative choice to the traditionally accepted housekeeping genes used so far about studies of gravitational biology and magneto biology.

Using cell and organ culture models to analyze responses of bone cells to mechanical stimulation

Bone cells of the osteoblastic lineage are responsive to the local mechanical environment. Through integration of a number of possible loading-induced regulatory stimuli, osteocyte, osteoblast, and osteoclast behaviour is organized to fashion a skeletal element of sufficient strength and toughness to resist fracture and crack propagation. Early pre-osteogenic responses had been determined in vivo and this led to the development of bone organ culture models to elucidate other pre-osteogenic responses where osteocytes and osteoblasts retain the natural orientation, connections and attachments to their native extracellular matrix. The application of physiological mechanical loads to bone in these organ culture models generates the regulatory stimuli. As a consequence, these experiments can be used to illustrate the distinctive mechanisms by which osteocytes and osteoblasts respond to mechanical loads and also differences in these responses, suggesting co-ordinated and cooperatively between cell populations. Organ explant cultures are awkward to maintain, and have a limited life, but length of culture times are improving. Monolayer cultures are much easier to maintain and permit the application of a particular mechanical stimulation to be studied in isolation; mainly direct mechanical strain or fluid shear strains. These allow for the response of a single cell type to the applied mechanical stimulation to be monitored precisely.The techniques that can be used to apply mechanical strain to bone and bone cells have not advanced greatly since the first edition. The output from such experiments has, however, increased substantially and their importance is now more broadly accepted. This suggests a growing use of these approaches and an increasing awareness of the importance of the mechanical environment in controlling normal bone cell behaviour. We expand the text to include additions and modifications made to the straining apparatus and update the research cited to support this growing role of cell and organ culture models to analyze responses of bone cells to mechanical stimulation.

Microarray profile of gene expression during osteoclast differentiation in modelled microgravity

Microgravity (µXg) leads to a 10-15% loss of bone mass in astronauts during space flight. Osteoclast (OCL) is the multinucleated bone-resorbing cell. In this study, we used the NASA developed ground-based rotating wall vessel bioreactor (RWV), rotary cell culture system (RCCS) to simulate µXg conditions and demonstrated a significant increase (2-fold) in osteoclastogenesis compared to normal gravity control (Xg). Gene expression profiling of RAW 264.7 OCL progenitor cells in modelled µXg by Agilent microarray analysis revealed significantly increased expression of critical molecules such as cytokines/growth factors, proteases and signalling proteins, which play an important role in enhanced OCL differentiation/function. Transcription factors such as c-Jun, MITF and CREB implicated in OCL differentiation are upregulated; however no significant change in the levels of NFATc1 expression in preosteoclast cells subjected to modelled µXg. We also identified high-level expression of calcium-binding protein, S100A8 (calcium-binding protein molecule A8/calgranulin A) in preosteoclast cells under µXg. Furthermore, modelled µXg stimulated RAW 264.7 cells showed elevated cytosolic calcium (Ca(2+)) levels/oscillations compared to Xg cells. siRNA knock-down of S100A8 expression in RAW 264.7 cells resulted in a significant decrease in modelled µXg stimulated OCL differentiation. We also identified elevated levels of phospho-CREB in preosteoclast cells subjected to modelled µXg compared to Xg. Thus, modelled µXg regulated gene expression profiling in preosteoclast cells provide new insights into molecular mechanisms and therapeutic targets of enhanced OCL differentiation/activation to prevent bone loss and fracture risk in astronauts during space flight missions.

Intracellular signal transduction as a factor in the development of "smart" biomaterials for bone tissue engineering

Signal transduction involves studying the intracellular mechanisms that govern cellular responses to external stimuli such as hormones, cytokines, and also cell adhesion to biomaterials surfaces. Several events have been shown to be responsible for cellular adhesion and adaptation onto different surfaces. For instance, cytoskeletal rearrangements during cell adhesion require the recruitment of specific protein tyrosine kinases into focal adhesion structures that promote transient focal adhesion kinase and Src phosphorylations, initially modulating cell behavior. In addition, the phosphorylation of tyrosine (Y) residues have been generally accepted as a critical regulator of a wide range of cell-related processes, including cell proliferation, migration, differentiation, survival signalling, and energy metabolism. The understanding of the signaling involved on the mechanisms of osteoblast adhesion, proliferation, and differentiation on implant surfaces is fundamental for the successful design of novel "smart" materials, potentially decreasing the repair time, thereby allowing for faster patient rehabilitation.

Activation of nervous system development genes in bone marrow derived mesenchymal stem cells following spaceflight exposure

Stalled cell division in precursor bone cells and reduced osteoblast function are considered responsible for the microgravity-induced bone loss observed during spaceflight. However, underlying molecular mechanisms remain unraveled. Having overcome technological difficulties associated with flying cells in a space mission, we present the first report on the behavior of the potentially osteogenic murine bone marrow stromal cells (BMSC) in a 3D culture system, flown inside the KUBIK aboard space mission ISS 12S (Soyuz TMA-8 + Increment 13) from March 30 to April 8, 2006 (experiment "Stroma-2"). Flight 1g control cultures were performed in a centrifuge located within the payload. Ground controls were maintained on Earth in another KUBIK payload and in Petri dishes. Half of the cultures were stimulated with osteo-inductive medium. Differences in total RNA extracted suggested that cell proliferation was inhibited in flight samples. Affymetrix technology revealed that 1,599 genes changed expression after spaceflight exposure. A decreased expression of cell-cycle genes confirmed the inhibition of cell proliferation in space. Unexpectedly, most of the modulated expression was found in genes related to various processes of neural development, neuron morphogenesis, transmission of nerve impulse and synapse, raising the question on the lineage restriction in BMSC.

Microarray analysis of spaceflown murine thymus tissue reveals changes in gene expression regulating stress and glucocorticoid receptors

The detrimental effects of spaceflight and simulated microgravity on the immune system have been extensively documented. We report here microarray gene expression analysis, in concert with quantitative RT-PCR, in young adult C57BL/6NTac mice at 8 weeks of age after exposure to spaceflight aboard the space shuttle (STS-118) for a period of 13 days. Upon conclusion of the mission, thymus lobes were extracted from space flown mice (FLT) as well as age- and sex-matched ground control mice similarly housed in animal enclosure modules (AEM). mRNA was extracted and an automated array analysis for gene expression was performed. Examination of the microarray data revealed 970 individual probes that had a 1.5-fold or greater change. When these data were averaged (n = 4), we identified 12 genes that were significantly up- or down-regulated by at least 1.5-fold after spaceflight (P < or = 0.05). The genes that significantly differed from the AEM controls and that were also confirmed via QRT-PCR were as follows: Rbm3 (up-regulated) and Hsph110, Hsp90aa1, Cxcl10, Stip1, Fkbp4 (down-regulated). QRT-PCR confirmed the microarray results and demonstrated additional gene expression alteration in other T cell related genes, including: Ctla-4, IFN-alpha2a (up-regulated) and CD44 (down-regulated). Together, these data demonstrate that spaceflight induces significant changes in the thymic mRNA expression of genes that regulate stress, glucocorticoid receptor metabolism, and T cell signaling activity. These data explain, in part, the reported systemic compromise of the immune system after exposure to the microgravity of space.

Molecular genetic studies of gene identification for osteoporosis: the 2009 update

Osteoporosis is a complex human disease that results in increased susceptibility to fragility fractures. It can be phenotypically characterized using several traits, including bone mineral density, bone size, bone strength, and bone turnover markers. The identification of gene variants that contribute to osteoporosis phenotypes, or responses to therapy, can eventually help individualize the prognosis, treatment, and prevention of fractures and their adverse outcomes. Our previously published reviews have comprehensively summarized the progress of molecular genetic studies of gene identification for osteoporosis and have covered the data available to the end of September 2007. This review represents our continuing efforts to summarize the important and representative findings published between October 2007 and November 2009. The topics covered include genetic association and linkage studies in humans, transgenic and knockout mouse models, as well as gene-expression microarray and proteomics studies. Major results are tabulated for comparison and ease of reference. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis.