Rat hindlimb unloading by tail suspension reduces osteoblast differentiation, induces IL-6 secretion, and increases bone resorption in ex vivo cultures

A Surgical Approach to Hindlimb Suspension: A Mouse Model of Disuse-Induced Atrophy

Hindlimb suspension is a well-established rodent model of disuse-induced atrophy and is commonly used to simulate the effects of bed rest and space flight on humans. Over the decades, this method has undergone many changes to reduce the stress response on the animals and improve the reliability of the data. Here, we detail our method of performing hindlimb suspension in mice that minimizes stress, maximizes the replicability of the data, and uses space efficiently.

Phorbol-12-myristate 13-acetate inhibits Nephronectin gene expression via Protein kinase C alpha and c-Jun/c-Fos transcription factors

Nephronectin (Npnt) is an extracellular matrix protein and ligand of integrin α₈β₁ known to promote differentiation of osteoblasts. A search for factors that regulate Npnt gene expression in osteoblasts revealed that phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC), had a strong effect to suppress that expression. Research was then conducted to elucidate the signaling pathway responsible for regulation of Npnt gene expression by PMA in osteoblasts. Treatment of MC3T3-E1 cells with PMA suppressed cell differentiation and Npnt gene expression. Effects were noted at a low concentration of PMA, and were time- and dose-dependent. Furthermore, treatment with the PKC signal inhibitor Gö6983 inhibited down-regulation of Npnt expression, while transfection with small interfering RNA (siRNA) of PKCα, c-Jun, and c-Fos suppressed that down-regulation. The present results suggest regulation of Npnt gene expression via the PKCα and c-Jun/c-Fos pathway.

Transgenic inhibition of interleukin-6 trans-signaling does not prevent skeletal pathologies in mucolipidosis type II mice

Severe skeletal alterations are common symptoms in patients with mucolipidosis type II (MLII), a rare lysosomal storage disorder of childhood. We have previously reported that progressive bone loss in a mouse model for MLII is caused by an increased number of bone-resorbing osteoclasts, which is accompanied by elevated expression of the cytokine interleukin-6 (IL-6) in the bone microenvironment. In the present study we addressed the question, if pharmacological blockade of IL-6 can prevent the low bone mass phenotype of MLII mice. Since the cellular IL-6 response can be mediated by either the membrane-bound (classic signaling) or the soluble IL-6 receptor (trans-signaling), we first performed cell culture assays and found that both pathways can increase osteoclastogenesis. We then crossed MLII mice with transgenic mice expressing the recombinant soluble fusion protein sgp130Fc, which represents a natural inhibitor of IL-6 trans-signaling. By undecalcified histology and bone-specific histomorphometry we found that high circulating sgp130Fc levels do not affect skeletal growth or remodeling in wild-type mice. Most importantly, blockade of IL-6 trans-signaling did neither reduce osteoclastogenesis, nor increase bone mass in MLII mice. Therefore, our data clearly demonstrate that the bone phenotype of MLII mice cannot be corrected by blocking the IL-6 trans-signaling.

Functional Block of Interleukin-6 Reduces a Bone Pain Marker but Not Bone Loss in Hindlimb-Unloaded Mice

Interleukin-6 (IL-6) is widely accepted to stimulate osteoclasts. Our aim in this study was to examine whether the inhibitory effect of IL-6 on bone loss and skeletal pain associated with osteoporosis in hindlimb-unloaded (HU) mice in comparison with bisphosphonate. Eight-week-old male ddY mice were tail suspended for 2 weeks. Starting immediately after reload, vehicle (HU group), alendronate (HU-ALN group), or anti-IL-6 receptor antibody (HU-IL-6i group) was injected subcutaneously. After a 2-week treatment, pain-related behavior was examined using von Frey filaments. The bilateral distal femoral and proximal tibial metaphyses were analyzed three-dimensionally with micro-computed tomography. Calcitonin gene-related peptide (CGRP) expressions in dorsal root ganglion (DRG) neurons innervating the hindlimbs were examined using immunohistochemistry. HU mice with tail suspension developed bone loss. The HU mice showed mechanical hyperalgesia in the hindlimbs and increased CGRP immunoreactive neurons in the L3-5 DRG. Treatment with IL-6i and ALN prevented HU-induced mechanical hyperalgesia and upregulation of CGRP expressions in DRG neurons. Furthermore, ALN but not IL-6i prevented HU-induced bone loss. In summary, treatment with IL-6i prevented mechanical hyperalgesia in hindlimbs and suppressed CGRP expressions in DRG neurons of osteoporotic models. The novelty of this research suggests that IL-6 is one of the causes of immobility-induced osteoporotic pain regardless improvement of bone loss.

Dietary countermeasure mitigates simulated spaceflight-induced osteopenia in mice

Spaceflight is a unique environment that includes at least two factors which can negatively impact skeletal health: microgravity and ionizing radiation. We have previously shown that a diet supplemented with dried plum powder (DP) prevented radiation-induced bone loss in mice. In this study, we investigated the capacity of the DP diet to prevent bone loss in mice following exposure to simulated spaceflight, combining microgravity (by hindlimb unloading) and radiation exposure. The DP diet was effective at preventing most decrements in bone micro-architectural and mechanical properties due to hindlimb unloading alone and simulated spaceflight. Furthermore, we show that the DP diet can protect osteoprogenitors from impairments resulting from simulated microgravity. Based on our findings, a dietary supplementation with DP could be an effective countermeasure against the skeletal deficits observed in astronauts during spaceflight.

AMP-Activated Protein Kinase as a Key Trigger for the Disuse-Induced Skeletal Muscle Remodeling

Molecular mechanisms that trigger disuse-induced postural muscle atrophy as well as myosin phenotype transformations are poorly studied. This review will summarize the impact of 5′ adenosine monophosphate -activated protein kinase (AMPK) activity on mammalian target of rapamycin complex 1 (mTORC1)-signaling, nuclear-cytoplasmic traffic of class IIa histone deacetylases (HDAC), and myosin heavy chain gene expression in mammalian postural muscles (mainly, soleus muscle) under disuse conditions, i.e., withdrawal of weight-bearing from ankle extensors. Based on the current literature and the authors’ own experimental data, the present review points out that AMPK plays a key role in the regulation of signaling pathways that determine metabolic, structural, and functional alternations in skeletal muscle fibers under disuse.

The Effects of Progressive Resistance Exercise on Recovery Rate of Bone and Muscle in a Rodent Model of Hindlimb Suspension

Purpose: This study aimed to examine the exercise-mediated musculoskeletal recovery following hindlimb suspension (HS) in order to identify whether bone modeling and muscle hypertrophy would eventuate in a synchronized manner during recovery stage.

Methods: To identify whether 2-week HS would be sufficient to induce a significant reduction of physiological indices in both tibia and adjacent hindlimb muscles, a total of 20 rats was randomized into 2-week HS (n = 10) and age-matched control group (n = 10, CON). Another batch of rats were randomly assigned to three different groups to identify recovery intervention effects following suspension: (1) 2-week HS followed by 4-week spontaneous reloading recovery (HRE, n = 7). (2) 2-week HS followed by 4-week progressive resistance ladder climbing exercise (HEX, n = 7). (3) Age-matched control (CON, n = 7). DXA, micro-CT, and ¹⁸F-sodium fluoride (NaF) imaging, and EIA analysis were utilized to measure tibia bone indices. Hindlimb muscle wet weight and grip strength were measured to evaluate muscle mass and strength, respectively.

Results: In study 1, bone quality values [bone volume/total volume (BV/TV): -27%, areal bone mineral density (aBMD): -23%, mineral contents: -7.9%, mineral density: –4.1%, and bone density: -38.9%] and skeletal muscle weight (soleus: -46.8%, gastrocnemius: -19.6%, plantaris: -20.8%, TA: -22.8%, and EDL: -9.9%) were significantly lower in HS group compared to CON group. In study 2, micro-CT and DXA-based bone morphology (bone density, BT/TV, and aBMD) were fully recovered in HRE or HEX group. However, suspension-induced dysregulation of bone mineral metabolism was returned to age-matched control group in only HEX group, but not in HRE group. A greater level of biomarkers of bone formation (P1NF) and resorption (CTX-1) was observed in only HRE group compared to CON. The hindlimb skeletal muscle mass was significantly lower in both HRE and HEX groups compared to CON group. Hindlimb grip strength was the greatest in HEX group, followed by CON and HRE groups.

Conclusion: Following HS, progressive resistance exercise promotes recovery rates of bone and skeletal muscle strength without a significant increase in muscular mass, suggesting that exercise-induced reacquisition of bone and muscle strength is independent of muscle hypertrophy during early recovery stage.

Osteophytes and fracture calluses share developmental milestones and are diminished by unloading

Osteophytes are a typical radiographic finding during osteoarthritis (OA), but the mechanisms leading to their formation are not well known. Comparatively, fracture calluses have been studied extensively; therefore, drawing comparisons between osteophytes and fracture calluses may lead to a deeper understanding of osteophyte formation. In this study, we compared the time courses of osteophyte and fracture callus formation, and investigated mechanisms contributing to development of these structure. Additionally, we investigated the effect of mechanical unloading on the formation of both fracture calluses and osteophytes. Mice underwent either transverse femoral fracture or non-invasive anterior cruciate ligament rupture. Fracture callus and osteophyte size and ossification were evaluated after 3, 5, 7, 14, 21, or 28 days. Additional mice were subjected to hindlimb unloading after injury for 3, 7, or 14 days. Protease activity and gene expression profiles after injury were evaluated after 3 or 7 days of normal ambulation or hindlimb unloading using in vivo fluorescence reflectance imaging (FRI) and quantitative PCR. We found that fracture callus and osteophyte growth achieved similar developmental milestones, but fracture calluses formed and ossified at earlier time points. Hindlimb unloading ultimately led to a threefold decrease in chondro/osteophyte area, and a twofold decrease in fracture callus area. Unloading was also associated with decreased inflammation and protease activity in injured limbs detected with FRI, particularly following ACL rupture. qPCR analysis revealed disparate cellular responses in fractured femurs and injured joints, suggesting that fracture calluses and osteophytes may form via different inflammatory, anabolic, and catabolic pathways. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:699-710, 2018.

Inhibition of interleukin-6 decreases atrogene expression and ameliorates tail suspension-induced skeletal muscle atrophy

BACKGROUND

Interleukin-6 (IL-6) is an inflammatory cytokine. Whether systemic IL-6 affects atrogene expression and disuse-induced skeletal muscle atrophy is unclear.

METHODS

Tail-suspended mice were used as a disuse-induced muscle atrophy model. We administered anti-mouse IL-6 receptor antibody, beta-hydroxy-beta-methylbutyrate (HMB) and vitamin D to the mice and examined the effects on atrogene expression and muscle atrophy.

RESULTS

Serum IL-6 levels were elevated in the mice. Inhibition of IL-6 receptor suppressed muscle RING finger 1 (MuRF1) expression and prevented muscle atrophy. HMB and vitamin D inhibited the serum IL-6 surge, downregulated the expression of MuRF1 and atrogin-1 in the soleus muscle, and ameliorated atrophy in the mice.

CONCLUSION

Systemic IL-6 affects MuRF1 expression and disuse-induced muscle atrophy.

Pleiotropic Functions of High Fat Diet in the Etiology of Osteoarthritis

Obesity is a risk factor for osteoarthritis (OA). To investigate the roles of increased mechanical loading in the onset of obesity-induced OA, knee joints were histologically analyzed after applying a tail suspension (TS) model to a high-fat diet (HFD)-induced OA model. Mice were divided into four groups: normal diet (ND) with normal loading (NL) group; HFD with NL group; ND with TS group; and HFD with TS group. Whole knee joints were evaluated by immunohistological analysis. The infrapatellar fat pad (IPFP) was excised and mRNA expression profiles were compared by qPCR analysis. After twelve weeks of the diet, body weight was increased by HFD in both the NL group and TS group. Upon histological analysis, the irregularity of the surface layer of articular cartilage was observed only in the NL+HFD group. Osteophyte area increased as a result of HFD in both the NL and TS groups, although osteophyte area in the TS+HFD group was smaller than that of the NL+HFD group. In the evaluation of the IPFP by qPCR, adipokines and inflammatory cytokines also increased as a result of HFD. While TGF-β increased as a result of HFD, the trend was slightly lower in the TS group, in parallel with osteophyte area. To detect apoptosis of articular chondrocytes, TUNEL staining was employed. TUNEL-positive cells were abundantly observed in the articular cartilage in the HFD mice regardless of mechanical loading. IPFP inflammation, enhanced chondrocyte apoptosis, and osteophyte formation were seen even in the TS group as a result of a HFD. In all, these data demonstrate that HFD contributed to osteophyte formation through mechanical loading dependent and independent mechanisms.

Trabecular and Cortical Bone of Growing C3H Mice Is Highly Responsive to the Removal of Weightbearing

Genetic make-up strongly influences the skeleton’s susceptibility to the loss of weight bearing with some inbred mouse strains experiencing great amounts of bone loss while others lose bone at much smaller rates. At young adulthood, female inbred C3H/HeJ (C3H) mice are largely resistant to catabolic pressure induced by unloading. Here, we tested whether the depressed responsivity to unloading is inherent to the C3H genetic make-up or whether a younger age facilitates a robust skeletal response to unloading. Nine-week-old, skeletally immature, female C3H mice were subjected to 3wk of hindlimb unloading (HLU, n = 12) or served as normal baseline controls (BC, n = 10) or age-matched controls (AC, n = 12). In all mice, cortical and trabecular architecture of the femur, as well as levels of bone formation and resorption, were assessed with μCT, histomorphometry, and histology. Changes in bone marrow progenitor cell populations were determined with flow cytometry. Following 21d of unloading, HLU mice had 52% less trabecular bone in the distal femur than normal age-matched controls. Reflecting a loss of trabecular tissue compared to baseline controls, trabecular bone formation rates (BFR/BS) in HLU mice were 40% lower than in age-matched controls. Surfaces undergoing osteoclastic resorption were not significantly different between groups. In the mid-diaphysis, HLU inhibited cortical bone growth leading to 14% less bone area compared to age-matched controls. Compared to AC, BFR/BS of HLU mice were 53% lower at the endo-cortical surface and 49% lower at the periosteal surface of the mid-diaphysis. The enriched osteoprogenitor cell population (OPC) comprised 2% of the bone marrow stem cells in HLU mice, significantly different from 3% OPC in the AC group. These data show that bone tissue in actively growing C3H mice is lost rapidly, or fails to grow, during the removal of functional weight bearing—in contrast to the insignificant response previously demonstrated in female young adult C3H mice. Thus, the attributed low sensitivity of the C3H mouse strain to the loss of mechanical signals is not apparent at a young age and this trait therefore does not reflect a genetic regulation throughout the life span of this strain. These results highlight the significance of age in modulating the contribution of genetics in orchestrating bone’s response to unloading and that the skeletal unresponsiveness of young adult C3H mice to the loss of weight bearing is not genetically hard-wired.

Metaphyseal and diaphyseal bone loss in the tibia following transient muscle paralysis are spatiotemporally distinct resorption events

When the skeleton is catabolically challenged, there is great variability in the timing and extent of bone resorption observed at cancellous and cortical bone sites. It remains unclear whether this resorptive heterogeneity, which is often evident within a single bone, arises from increased permissiveness of specific sites to bone resorption or localized resorptive events of varied robustness. To explore this question, we used the mouse model of calf paralysis induced bone loss, which results in metaphyseal and diaphyseal bone resorption of different timing and magnitude. Given this phenotypic pattern of resorption, we hypothesized that bone loss in the proximal tibia metaphysis and diaphysis occurs through resorption events that are spatially and temporally distinct. To test this hypothesis, we undertook three complimentary in vivo/μCT imaging studies. Specifically, we defined spatiotemporal variations in endocortical bone resorption during the 3weeks following calf paralysis, applied a novel image registration approach to determine the location where bone resorption initiates within the proximal tibia metaphysis, and explored the role of varied basal osteoclast activity on the magnitude of bone loss initiation in the metaphysis using μCT based bone resorption parameters. A differential response of metaphyseal and diaphyseal bone resorption was observed throughout each study. Acute endocortical bone loss following muscle paralysis occurred almost exclusively within the metaphyseal compartment (96.5% of total endocortical bone loss within 6days). Using our trabecular image registration approach, we further resolved the initiation of metaphyseal bone loss to a focused region of significant basal osteoclast function (0.03mm(3)) adjacent to the growth plate. This correlative observation of paralysis induced bone loss mediated by basal growth plate cell dynamics was supported by the acute metaphyseal osteoclastic response of 5-week vs. 13-month-old mice. Specifically, μCT based bone resorption rates normalized to initial trabecular surface (BRRBS) were 3.7-fold greater in young vs. aged mice (2.27±0.27μm(3)/μm(2)/day vs. 0.60±0.44μm(3)/μm(2)/day). In contrast to the focused bone loss initiation in the metaphysis, diaphyseal bone loss initiated homogeneously throughout the long axis of the tibia predominantly in the second week following paralysis (81.3% of diaphyseal endocortical expansion between days 6 and 13). The timing and homogenous nature are consistent with de novo osteoclastogenesis mediating the diaphyseal resorption. Taken together, our data suggests that tibial metaphyseal and diaphyseal bone loss induced by transient calf paralysis are spatially and temporally discrete events. In a broader context, these findings are an essential first step toward clarifying the timing and origins of multiple resorptive events that would require targeting to fully inhibit bone loss following neuromuscular trauma.

BONE CELLS UNDER MICROGRAVITY

Weightlessness environment (also microgravity) during the exploration of space is the major condition which must be faced by astronauts. One of the most serious adverse effects on astronauts is the weightlessness-induced bone loss due to the unbalanced bone remodeling. Bone remodeling of human beings has evolved during billions of years to make bone tissue adapt to the gravitational field of Earth (1g) and maintain skeleton structure to meet mechanical loading on Earth. However, under weightlessness environment the skeleton system no longer functions against the pull of gravity, so there is no necessity to keep bone strong enough to support the body's weight. Therefore, the balance of bone remodeling is disrupted and bone loss occurs, which is extremely deleterious to an astronaut's health during long-term spaceflight. Bone remodeling is mainly orchestrated by bone mesenchymal stem cells, osteoblasts, osteocytes, and osteoclasts. Here, we review how these bone cells respond to microgravity environment.

Simulated spaceflight produces a rapid and sustained loss of osteoprogenitors and an acute but transitory rise of osteoclast precursors in two genetic strains of mice

Loss of skeletal weight bearing or skeletal unloading as occurs during spaceflight inhibits bone formation and stimulates bone resorption. These are associated with a decline in the osteoblast (Ob.S/BS) and an increase in the osteoclast (Oc.S/BS) bone surfaces. To determine the temporal relationship between changes in the bone cells and their marrow precursor pools during sustained unloading, and whether genetic background influences these relationships, we used the hindlimb unloading model to induce bone loss in two strains of mice known to respond to load and having significantly different cancellous bone volumes (C57BL/6 and DBA/2 male mice). Skeletal unloading caused a progressive decline in bone volume that was accompanied by strain-specific changes in Ob.S/BS and Oc.S/BS. These were associated with a sustained reduction in the osteoprogenitor population and a dramatic but transient increase in the osteoclast precursor pool size in both strains. The results reveal that bone adaptation to skeletal unloading involves similar rapid changes in the osteoblast and osteoclast progenitor populations in both strains of mice but striking differences in Oc.S/BS dynamics, BFR, and cancellous bone structure. These strain-specific differences suggest that genetics plays an important role in determining the osteoblast and osteoclast populations on the bone surface and the dynamics of bone loss in response to skeletal unloading.

Early response of bone marrow osteoprogenitors to skeletal unloading and sclerostin antibody

Sclerostin functions as an antagonist to Wnt signaling and inhibits bone-forming activity. We studied the effects of skeletal unloading and treatment with sclerostin antibody (Scl-Ab) on mesenchymal stem cell, osteoprogenitor and osteoclast precursor pools, and their relationship to bone formation and resorption. Male C57BL/6 mice (5-months-old) were hind limb unloaded for 1 week or allowed normal ambulation and treated with Scl-Ab (25 mg/kg, s.c. injections on days 1 and 4) or placebo. Unloading decreased the serum concentration of bone formation marker P1NP (-35 %), number of colony-forming units (CFU) (-38 %), alkaline phosphatase-positive CFUs (CFU-AP+) (-51 %), and calcified nodules (-35 %); and resulted in a fourfold increase in the number of osteoclast precursors. The effects of Scl-Ab treatment on unloaded and normally loaded mice were nearly identical; Scl-Ab increased serum P1NP and the number of CFU, CFU-AP+, and calcified nodules in ex vivo cultures; and increased osteoblast and bone mineralizing surfaces in vivo. Although the marrow-derived osteoclast precursor population increased with Scl-Ab, the bone osteoclast surface did not change, and the serum concentration of osteoclast activity marker TRACP5b decreased. Our data suggest that short-term Scl-Ab treatment can prevent the decrease in osteoprogenitor population associated with skeletal unloading and increase osteoblast surface and bone mineralizing surface in unloaded animals. The anabolic effects of Scl-Ab treatment on bone are preserved during skeletal unloading. These findings suggest that Scl-Ab treatment can both increase bone formation and decrease bone resorption, and provide a new means for prevention and treatment of disuse osteoporosis.

Large gradient high magnetic field affects FLG29.1 cells differentiation to form osteoclast-like cells

PURPOSE

We aimed to investigate the effects of different apparent gravities (μ g, 1 g and 2 g) produced by large gradient high magnetic field (LGHMF) on human preosteoclast FLG29.1 cells.

MATERIALS AND METHODS

FLG29.1 cells were cultured in Roswell Park Memorial Institute (RPMI)-1640 medium. Cells were exposed to LGHMF for 72 h. On culture day 1, 2, 3, cell proliferation was detected by 3-(4,5)-dimethylthiahi-azo (-z-y1)-3,5-di-phenytetrazoliumromide (MTT) method. On day 3, cell apoptosis and necrosis were assayed by Hoechst and propidium iodide (PI) staining. After cells were exposed to LGHMF for 72 h with the induction of 12-o-tetradecanoylphorbol 13-acetate (TPA), Tartrate-Resistant Acid Phosphatase (TRAP) positive cells and nitric oxide (NO) release were detected by TRAP staining and Griess method, respectively. Intracellular TRAP activity was measured using nitrophenylphosphate (pNPP) as the substrate.

RESULTS

MTT detection revealed that compared to control, FLG 29.1 cell proliferation in the μ g and 2 g groups were promoted. However, there is no obvious difference between the 1 g and control groups. Hoechst-PI staining showed that LGHMF promoted cell apoptosis and necrosis, especially in the 2 g group. Exposure to LGHMF inhibited the NO concentration of supernatant. Both the TRAP activity and the number of TRAP positive cells were higher in cells of μ g group than those in 2 g group. In the 1 g group, they were decreased significantly compared to control.

CONCLUSIONS

These findings indicate that LGHMF could directly affect human preosteoclast FLG29.1 cells survival and differentiation. High magnetic flux inhibited osteoclasts formation and differentiation while reduced apparent gravity enhanced osteoclastogenesis.

Ischemic stroke in rats enhances bone resorption in vitro

We hypothesized that the formation and differentialtion of osteoclasts are accelerated and the potential of bone resorption is increased in the hemiplegic bone marrow in the early stage of stroke. We randomly divided white female Sprague-Dawley (SD) rats (n = 30) into two groups, stroke (n = 15) and sham group (n = 15). On the 7th day after stroke, after cutting away the epiphyses of the femurs and tibias, diaphyseal channels were flushed using α-minimum essential medium (α-MEM) and bone marrow cells were collected. Bone marrow stem cells, which were extracted from the femur and tibia, were cultured on the 7th day after middle cerebral artery occlusion. We then estimated the ratio of non-adherent cells to total bone marrow cells that included osteoclast precursor cells. After culturing these cells separately, cells that tested positive on the tartrate resistant acid phosphatase (TRAP) were counted and bone resorption was evaluated by using the OAAS™ plate. In comparison to the control group, the stroke group showed a higher increase of non-adherent cells in the hemiplegic side bone marrow. In addition, after the primary culture, the stroke group showed an increased number of TRAP positive cells and a higher degree of bone resorption estimated by OAAS™ plate. As a result, osteoclastogenesis and osteoclast differentiation are accelerated and the potential of bone resorption is increased in the hemiplegic bone marrow and these changes are detected as early as within the first week after middle cerebral artery occlusion in SD rats.

ERRalpha regulates osteoblastic and adipogenic differentiation of mouse bone marrow mesenchymal stem cells

The orphan nuclear receptor estrogen-related receptor-alpha (ERRalpha) has been reported to have both a positive and a negative regulatory role in osteoblastic and adipocytic differentiation. We have studied the role of ERRalpha in osteoblastic and adipogenic differentiation of mesenchymal stem cells. Bone marrow mesenchymal stem cells were isolated from ERRalpha deficient mice and their differentiation capacities were compared to that of the wild-type cells. ERRalpha deficient cultures displayed reduced cellular proliferation, osteoblastic differentiation, and mineralization. In the complementary experiment, overexpression of ERRalpha in MC3T3-E1 cells increased the expression of osteoblastic markers and mineralization. Alterations in the expression of bone sialoprotein (BSP) may at least partially explain the effects on mineralization as BSP expression was reduced in ERRalpha deficient MSCs and enhanced upon ERRalpha overexpression in MC3T3-E1 cells. Furthermore, a luciferase reporter construct driven by the BSP promoter was efficiently transactivated by ERRalpha. Under adipogenic conditions, ERRalpha deficient cultures displayed reduced adipocytic differentiation. Our data thus propose a positive role for ERRalpha in osteoblastic and adipocytic differentiation. The variability in the results yielded in the different studies implies that ERRalpha may play different roles in bone under different physiological conditions.

Oxidative stress and gamma radiation-induced cancellous bone loss with musculoskeletal disuse

Exposure of astronauts in space to radiation during weightlessness may contribute to subsequent bone loss. Gamma irradiation of postpubertal mice rapidly increases the number of bone-resorbing osteoclasts and causes bone loss in cancellous tissue; similar changes occur in skeletal diseases associated with oxidative stress. Therefore, we hypothesized that increased oxidative stress mediates radiation-induced bone loss and that musculoskeletal disuse changes the sensitivity of cancellous tissue to radiation exposure. Musculoskeletal disuse by hindlimb unloading (1 or 2 wk) or total body gamma irradiation (1 or 2 Gy of (137)Cs) of 4-mo-old, male C57BL/6 mice each decreased cancellous bone volume fraction in the proximal tibiae and lumbar vertebrae. The extent of radiation-induced acute cancellous bone loss in tibiae and lumbar vertebrae was similar in normally loaded and hindlimb-unloaded mice. Similarly, osteoclast surface in the tibiae increased 46% as a result of irradiation, 47% as a result of hindlimb unloading, and 64% as a result of irradiation + hindlimb unloading compared with normally loaded mice. Irradiation, but not hindlimb unloading, reduced viability and increased apoptosis of marrow cells and caused oxidative damage to lipids within mineralized tissue. Irradiation also stimulated generation of reactive oxygen species in marrow cells. Furthermore, injection of alpha-lipoic acid, an antioxidant, mitigated the acute bone loss caused by irradiation. Together, these results showed that disuse and gamma irradiation, alone or in combination, caused a similar degree of acute cancellous bone loss and shared a common cellular mechanism of increased bone resorption. Furthermore, irradiation, but not disuse, may increase the number of osteoclasts and the extent of acute bone loss via increased reactive oxygen species production and ensuing oxidative damage, implying different molecular mechanisms. The finding that alpha-lipoic acid protected cancellous tissue from the detrimental effects of irradiation has potential relevance to astronauts and radiotherapy patients.

From streaming-potentials to shear stress: 25 years of bone cell mechanotransduction

Mechanical loads are vital regulators of skeletal mass and architecture as evidenced by the increase in bone formation following the addition of exogenous loads and loss of bone mass following their removal. While our understanding of the molecular mechanisms by which bone cells perceive changes in their mechanical environment has increased rapidly in recent years, much remains to be learned. Here, we outline the effects of interstitial fluid flow, a potent biophysical signal induced by the deformation of skeletal tissue in response to applied loads, on bone cell behavior. We focus on the molecular mechanisms by which bone cells are hypothesized to perceive interstitial fluid flow, the cell signaling cascades activated by fluid flow, and the use of this signal in tissue engineering protocols.

A brief review of bone adaptation to unloading

Weight-bearing bone is constantly adapting its structure and function to mechanical environments. Loading through routine exercises stimulates bone formation and prevents bone loss, but unloading through bed rest and cast immobilization as well as exposure to weightlessness during spaceflight reduces its mass and strength. In order to elucidate the mechanism underlying unloading-driven bone adaptation, ground-based in vitro and in vivo analyses have been conducted using rotating cell culturing and hindlimb suspension. Focusing on gene expression studies in osteoblasts and hindlimb suspension studies, this minireview introduces our recent understanding on bone homeostasis under weightlessness in space. Most of the existing data indicate that unloading has the opposite effects to loading through common signaling pathways. However, a question remains as to whether any pathway unique to unloading (and not to loading) may exist.

Effects of hindlimb unloading on ex vivo growth and osteogenic/adipogenic potentials of bone marrow-derived mesenchymal stem cells in rats

The goal of this study was to determine the effects of hindlimb unloading (HU) on the ex vivo growth and the osteogenic potential of mesenchymal stem cells (MSCs) from the femurs of rats. Microgravity was simulated by 28-day HU in male Sprague-Dawley (SD) rats, and the bone marrow (BM) was collected from hindlimb femurs of HU or control (CTL) rats. MSCs were isolated from BM and cultured for eight passages. Then MSCs at passages 2, 4, and 8 were induced for osteogenesis or adipogenesis. The results revealed that HU decreased the osteogenic potential of MSCs and also decreased the expression of osteoblast gene marker mRNAs in cells induced by osteogenic conditions. Meanwhile, the expression of Runx2 mRNA and the phosphorylation of ERK were also decreased. There were no significant differences of osteoblast gene marker and Runx2 mRNA expression between cells induced from different passages of MSCs in UH rats. Under adipogenic conditions, HU increased both the adipogenic potential of MSCs and the expression of adipocytic gene marker mRNAs in induced cells. HU also increased the expression of PPAR gamma 2 mRNA, but with no effect on the phosphorylation of p38MAPK. The adipogenic potential of MSCs and the expression of adipocytic gene marker mRNAs in induced cells decreased along with cell cultures under normal gravity. This suggests that the normal gravity during in vitro MSC culture and the centrifugal force produced during cell harvest after each passage could decrease the adipogenic potential of MSCs, but could not reverse the effect of HU on the osteogenic potential of MSCs.

Transforming growth factor beta derived from bone matrix promotes cell proliferation of prostate cancer and osteoclast activation-associated osteolysis in the bone microenvironment

Metastatic prostate tumors in the bone microenvironment stimulate bone resorption, resulting in release of growth factors from the bone matrix that play important roles in tumor growth and osteoclast induction. Transforming growth factor beta (TGFbeta) is one of the most abundantly stored cytokines in bone matrix, regulating diverse biological activities. Here we evaluate its involvement in prostate tumor growth in the bone microenvironment, comparing with tumor growth in the subcutaneous microenvironment as a control. Rat prostate tumors were transplanted onto the cranial bone and into the subcutis of F344 male rats. Tumor cell proliferation, apoptosis, and TGFbeta signal transduction were compared between the tumor-bone interface and the tumor-subcutaneous interface. Effects of TGFbeta on osteoclast differentiation were also evaluated in vitro. Inhibitory effects of TGFbeta receptor 1 antisense oligonucleotide on TGFbeta signaling, osteolysis, osteoblasts, and tumor growth were examined in vivo. Osteolytic changes were extensively observed at the tumor-bone interface, where the TGFbeta level, TGFbeta signal transduction, and tumor cell proliferation were higher than at the tumor-subcutaneous interface. In vitro treatment with receptor activator of nuclear factor-kappaB ligand induced osteoclast differentiation of bone marrow stromal cells, and additional exposure to TGFbeta exerted promotive effects on osteoclast induction. Intratumoral injection of TGFbeta receptor 1 antisense oligonucleotide significantly reduced TGFbeta signal transduction, osteolysis, induction of osteoclast and osteoblast, and tumor cell proliferation. Thus, we experimentally show that TGFbeta derived from bone matrix promotes cell proliferation of rat prostate cancer and osteoclast activation-associated osteolysis in the bone microenvironment.

Transforming growth factor-beta prevents osteoblast apoptosis induced by skeletal unloading via PI3K/Akt, Bcl-2, and phospho-Bad signaling

Loss of mechanical loading induces rapid bone loss resulting from reduced osteoblastogenesis and decreased bone formation. The signaling mechanisms involved in this deleterious effect on skeletal metabolism remain poorly understood. We have previously shown that hindlimb suspension in rats increases osteoblast apoptosis associated with decreased phosphatidylinositol 3-kinase (PI3K) signaling. In this study, we investigated whether transforming growth factor (TGF)-beta2 may prevent the altered signaling and osteoblast apoptosis induced by skeletal unloading in vivo. Hindlimb suspension-induced decreased bone volume was associated with reduced alpha(5)beta(1)-integrin protein levels and PI3K/Akt signaling in unloaded bone. Continuous administration of TGF-beta2 using osmotic minipumps prevented the decreased alpha(5)beta(1)-integrin expression and the reduced PI3K/Akt signaling in unloaded bone, resulting in the prevention of osteoblast apoptosis. We also show that TGF-beta2 prevented the decreased Bcl-2 levels induced by unloading, which suggests that TGF-beta2 targets Bcl-2 via PI3K/Akt to prevent osteoblast apoptosis in unloaded bone. Furthermore, we show that TGF-beta2 prevented the decrease in phosphorylated Bad, the inactive form of the proapoptotic protein Bad, induced by unloading. These results identify a protective role for TGF-beta2 in osteoblast apoptosis induced by mechanical unloading via the alpha(5)beta(1)/PI3K/Akt signaling cascade and downstream Bcl-2 and phospho-Bad survival proteins. We thus propose a novel role for TGF-beta2 in protection from unloading-induced apoptosis in vivo.

Receptor activator of nuclear factor kappaB ligand and osteoprotegerin regulation of bone remodeling in health and disease

Osteoclasts and osteoblasts dictate skeletal mass, structure, and strength via their respective roles in resorbing and forming bone. Bone remodeling is a spatially coordinated lifelong process whereby old bone is removed by osteoclasts and replaced by bone-forming osteoblasts. The refilling of resorption cavities is incomplete in many pathological states, which leads to a net loss of bone mass with each remodeling cycle. Postmenopausal osteoporosis and other conditions are associated with an increased rate of bone remodeling, which leads to accelerated bone loss and increased risk of fracture. Bone resorption is dependent on a cytokine known as RANKL (receptor activator of nuclear factor kappaB ligand), a TNF family member that is essential for osteoclast formation, activity, and survival in normal and pathological states of bone remodeling. The catabolic effects of RANKL are prevented by osteoprotegerin (OPG), a TNF receptor family member that binds RANKL and thereby prevents activation of its single cognate receptor called RANK. Osteoclast activity is likely to depend, at least in part, on the relative balance of RANKL and OPG. Studies in numerous animal models of bone disease show that RANKL inhibition leads to marked suppression of bone resorption and increases in cortical and cancellous bone volume, density, and strength. RANKL inhibitors also prevent focal bone loss that occurs in animal models of rheumatoid arthritis and bone metastasis. Clinical trials are exploring the effects of denosumab, a fully human anti-RANKL antibody, on bone loss in patients with osteoporosis, bone metastasis, myeloma, and rheumatoid arthritis.

Mechanical stimulation of bone in vivo reduces osteocyte expression of Sost/sclerostin

Sclerostin, the protein product of the Sost gene, is a potent inhibitor of bone formation. Among bone cells, sclerostin is found nearly exclusively in the osteocytes, the cell type that historically has been implicated in sensing and initiating mechanical signaling. The recent discovery of the antagonistic effects of sclerostin on Lrp5 receptor signaling, a crucial mediator of skeletal mechanotransduction, provides a potential mechanism for the osteocytes to control mechanotransduction, by adjusting their sclerostin (Wnt inhibitory) signal output to modulate Wnt signaling in the effector cell population. We investigated the mechanoregulation of Sost and sclerostin under enhanced (ulnar loading) and reduced (hindlimb unloading) loading conditions. Sost transcripts and sclerostin protein levels were dramatically reduced by ulnar loading. Portions of the ulnar cortex receiving a greater strain stimulus were associated with a greater reduction in Sost staining intensity and sclerostin-positive osteocytes (revealed via in situ hybridization and immunohistochemistry, respectively) than were lower strain portions of the tissue. Hindlimb unloading yielded a significant increase in Sost expression in the tibia. Modulation of sclerostin levels appears to be a finely tuned mechanism by which osteocytes coordinate regional and local osteogenesis in response to increased mechanical stimulation, perhaps via releasing the local inhibition of Wnt/Lrp5 signaling.

A new titanium biofunctionalized interface based on poly(pyrrole-3-acetic acid) coating: proliferation of osteoblast-like cells and future perspectives

In recent years, many procedures based on surface modification have been suggested to improve the biocompatibility and biofunctionality of orthopedic titanium-based implants. In this contest, the development of a new titanium-based biomaterial that could be covalently modified with biologically active molecules (i.e., RGD-peptides, growth factors, etc.) able to improve osteoblasts response was investigated. The strategy followed was based on a preliminary coating of the implant material by an adherent thin polymer film to which bioactive molecules could be grafted exploiting the polymer surface chemical reactivity. In this work, we focused our attention on pyrrole-3-acetic acid (Py-3-acetic), a pyrrole with carboxylic acid substituent, whose electrosynthesis and characterization on titanium substrates were already accomplished and whose potentialities in the design of new biocompatible surfaces are well evident. As first step, the biocompatibility of the electrochemically grown PPy-3-acetic films was investigated performing in vitro tests (adhesion and proliferation) with mouse bone marrow cells. Successively, the availability and reactivity of surface carboxylic groups were tested through the grafting of an aminoacidic residue to PPy-3-acetic films.

Absence of mechanical loading in utero influences bone mass and architecture but not innervation in Myod-Myf5-deficient mice

Although the responses of bone to increased loading or exercise have been studied in detail, our understanding of the effects of decreased usage of the skeleton has been limited by the scarcity of suitable models. Such models should ideally not affect bone innervation, which appears to be a mediator of physiological responses of bone to unloading. MyoD-/-/Myf5-/- (dd/ff) mice lack skeletal muscle, so the fetuses develop without any active movement in utero and die soon after birth. We used micro-compter tomography and histology to analyse their bone development and structure during endochondral ossification in parallel with the establishment of bone innervation. Long bones from mutant mice were found to be profoundly different from controls, with shorter mineralized zones and less mineralization. They lacked many characteristics of adult bones - curvatures, changes in shaft diameter and traction epiphyses where muscles originate or insert - that were evident in the controls. Histologically, dd/ff mice showed the same degree of endochondral development as wild-type animals, but presented many more osteoclasts in the newly layed bone. Innervation and the expression pattern of semaphorin-3A signalling molecules were not disturbed in the mutants. Overall, we have found no evidence for a major defect of development in dd/ff mice, and specifically no alteration or delay in endochondral ossification and bone innervation. The altered morphological features of dd/ff mice and the increased bone resorption show the role of muscle activity in bone shaping and the consequences of bone unloading.

Loss of myostatin (GDF8) function increases osteogenic differentiation of bone marrow-derived mesenchymal stem cells but the osteogenic effect is ablated with unloading

Myostatin (GDF8) is a negative regulator of skeletal muscle growth and mice lacking myostatin show a significant increase in muscle mass and bone density compared to normal mice. In order to further define the role of myostatin in regulating bone mass we sought to determine if loss of myostatin function significantly altered the potential for osteogenic differentiation in bone marrow-derived mesenchymal stem cells in vitro and in vivo. We first examined expression of the myostatin receptor, the type IIB activin receptor (AcvrIIB), in bone marrow-derived mesenchymal stem cells (BMSCs) isolated from mouse long bones. This receptor was found to be expressed at high levels in BMSCs, and we were also able to detect AcvrIIB protein in BMSCs in situ using immunofluorescence. BMSCs isolated from myostatin-deficient mice showed increased osteogenic differentiation compared to wild-type mice; however, treatment of BMSCs from myostatin-deficient mice with recombinant myostatin did not attenuate the osteogenic differentiation of these cells. Loading of BMSCs in vitro increased the expression of osteogenic factors such as BMP-2 and IGF-1, but treatment of BMSCs with recombinant myostatin was found to decrease the expression of these factors. We investigated the effects of myostatin loss-of-function on the differentiation of BMSCs in vivo using hindlimb unloading (7-day tail suspension). Unloading caused a greater increase in marrow adipocyte number, and a greater decrease in osteoblast number, in myostatin-deficient mice than in normal mice. These data suggest that the increased osteogenic differentiation of BMSCs from mice lacking myostatin is load-dependent, and that myostatin may alter the mechanosensitivity of BMSCs by suppressing the expression of osteogenic factors during mechanical stimulation. Furthermore, although myostatin deficiency increases muscle mass and bone strength, it does not prevent muscle and bone catabolism with unloading.

The influence of masseter activity on rat mandibular growth

Many studies have shown that mandibular and condylar growth is affected by compressive forces on mandibular bone and the condyle. It has been reported that chondroblastic differentiation and proliferation in chondrocytes play important roles in condylar growth. However, the influence of reduced compressive force on chondroblastic proliferation and mandibular bone formation is not fully understood. Thirty-six 3-week-old male Wistar rats were used in this study. In the experimental group, the masseter muscles were bilaterally resected to evaluate the influence of masticatory force on mandibular and condylar bone morphology. Six weeks after the operation, while the rats were in the pubertal growth stage, lateral X-rays were taken to analyze the skeletal pattern of the mandible. The form of the condyle and the thickness of the chondroblastic layers were evaluated by toluidine blue staining. Chondroblastic proliferation was identified by insulin-like growth factor-1 receptor (IGF-1r) immunostaining and bone resorption of the condyle was assessed by measuring tartrate-resistant acid phosphatase (TRAP) activity. Lateral X-rays of the mandible showed that rats in the experimental group tended to have large mandibular plane angles. The chondroblastic layer in the condyles of the experimental group rats was thinner than in the control group. The expression of IGF-1r immunopositive cells in the experimental group was significantly lower than in the control chondrocytes, and the number of TRAP-positive cells was significantly higher in the condylar bone of the experimental group. We conclude that masseter muscle activity is closely related to mandibular morphology during growth.

Aging alters the skeletal response to disuse in the rat

Disuse has been shown to cause a rapid and dramatic loss of skeletal mass and strength in the load-bearing bones of young and mature animals and humans. However, little is known about the skeletal effects of disuse in aged mammals. The present study was designed to determine whether the skeletal effects of disuse are maintained with extreme age. Fischer 344/Brown Norway male rats (6 and 32 mo old) were hindlimb suspended (HS) or housed individually for 2 wk. Trabecular volume and microarchitecture in the proximal tibia were significantly decreased by HS only in young rats. HS significantly reduced cortical bone mineral density and increased cortical porosity only in old rats by inducing new pore formation. Cortical pore diameter was also increased in old rats, regardless of loading condition. Ex vivo osteogenic and adipogenic cultures established from each group demonstrated that age and HS decreased osteoblastogenesis. Age, but not HS, decreased sensitivity to endogenous bone morphogenetic protein stimulation, as measured by treatment with exogenous Noggin. Adipocyte development increased with age, whereas HS suppressed sensitivity to peroxisome proliferator-activated receptor-gamma-induced differentiation. Serum insulin-like growth factor I levels were reduced with HS in young rats and with age in control and HS rats. These results suggest that the site of bone loss due to disuse is altered with age and that the loss of osteogenic potential with disuse in the old rats may be due to the combined effects of decreased insulin-like growth factor I levels and sensitivity, as well as diminished bone morphogenetic protein production.

The Effect of Varying Magnitudes of Whole-Body Vibration on Several Skeletal Sites in Mice

It has been reported that whole-body vibration (WBV) is anabolic to trabecular bone in animal models and humans. It is likely that this anabolic response does not occur uniformly throughout the entire body. Two factors that may affect the observed anabolic response are vibration magnitude and skeletal site of interest. In this study, mice were loaded with WBV of varying magnitudes. After five weeks of loading, bone marrow was flushed from tibias in order to quantify osteoprogenitor cells. Staining with alizarin red (an indicator of mineralization) showed a significant decrease in percent stained area in the 0.3 g loaded group compared to the control group and the 1.0 g group. MicroCT analysis was performed at five skeletal sites: the proximal tibial metaphysis, femoral condyles, distal femoral metaphysis, proximal femur, and L5 vertebral body. Increasing magnitudes of WBV were associated with a non-dose-dependent increase in trabecular bone volume (BV/TV) at the proximal tibial metaphysis, although other sites were unresponsive. There were statistically significant increases in BV/TV in the 0.1 g group (32% increase) and 1.0 g group (43% increase) compared to control (p < 0.05). The 0.1 g and 1.0 g groups also had higher BV/TV than the 0.3 g loaded group. If this non-dose-dependent phenomenon is verified by future studies, it suggests that a range of magnitudes should be examined for each application of WBV.

Effects of hind limb unloading and reloading on nitric oxide synthase expression and apoptosis of osteocytes and chondrocytes

In rat bone, the absence of mechanical load results in a reduction in bone formation, inhibition of longitudinal growth, and a decrease in the number of osteoblasts and osteoprogenitors in cancellous bone. Unloading has also been linked to an increase in apoptosis of osteocytes and chondrocytes through production of nitric oxide (NO) and increased expression of NO synthases (NOS). Reloading results in recovery of bone volume within 14 days, although osteoblast and osteoclast numbers remain below control values, suggesting decreased bone turnover. This study was designed to evaluate the effects of hind limb unloading and subsequent reloading on apoptosis, NOS expression, and histomorphometric parameters in trabecular and cortical bone, articular cartilage, and growth plate cartilage of the proximal tibia of the hind limbs. Compared to ambulatory controls, 2 weeks of unloading resulted in a 66% increase in the percentage of apoptotic osteocytes in the trabecular metaphysis, a 14% increase in osteoclast number and a 48% decrease in bone volume. The percentage of eNOS- or iNOS-positive osteocytes was unchanged. Upon reloading, the percentage of apoptotic osteocytes and bone volume returned to baseline whereas the percentage of iNOS-positive osteocytes increased by 50% and osteoclast number decreased by 30% compared to ambulatory controls. More striking changes were observed in articular and growth plate cartilage. Unloading resulted in a 230% increase in apoptotic chondrocytes, a 400% increase in iNOS-positive chondrocytes, and a 17% reduction in width in articular cartilage. Reloading for 2 weeks resulted in partial recovery. Chondrocytes in the proliferative and hypertrophic zones of the growth plate responded similarly to those in the articular cartilage. In summary, we observed that 14 days of unloading increased apoptosis of osteocytes and chondrocytes. This was associated with an increase in the proportion of iNOS-positive chondrocytes whereas the proportion of iNOS-positive osteocytes remained unchanged. Reloading for 14 days restored osteocyte apoptosis to control levels but the percentage of iNOS- and eNOS-positive osteocytes increased in reloaded bone compared to controls. This was associated with a decrease in osteoclast number. In cartilage, reloading for 2 weeks did not result in a return to baseline in any of the parameters measured, suggesting that the effects of unloading on articular cartilage and the growth plate last longer than those in bone and may have prolonged effects on joint biomechanics and longitudinal bone growth.

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.

Down-regulation of N-methyl D-aspartate receptor in rat-modeled disuse osteopenia

Lack of mechanical stress may result in osteoporosis; however, the underlying mechanisms of disuse osteoporosis remain unclear. It has been indicated that mechanical loading causes extracellular glutamate accumulation in osteoblasts. We hypothesized that the glutamate receptor mediation on bone cells might also be involved in mechanically stimulated osteogenesis. In this study, we investigated the changes of bone formation and the expressions of osteogenic genes and N-methyl D-aspartate (NMDA) receptors, the major glutamate receptors, in disused bones. Rat modeled disuse osteopenia in hind limbs was induced by a 3-week tail suspension in Sprague-Dawley rats. Bone mineral density and trabecular bone volume of distal femurs were measured to verify the osteopenia of disused bones. The mRNA expressions of cbfa1/Runx2, type I collagen, alkaline phosphatase (ALP) and osteocalcin (OC) in bones were measured as osteogenic markers. The influences of mechanical unloading on the expressions of NMDA receptors (NR1 and NR2D) in bones were also examined. The effects of NMDA mediation on osteogenesis were tested by a treatment of MK-801, a non-competitive NMDA receptor antagonist, in cultured osteoblasts and bone marrow stroma cells. Our result showed that mRNA expressions of cbfa1/Runx2, type I collagen, ALP and OC were significantly decreased in disused bones. The mRNA and protein expressions of NR1 and NR2D were significantly decreased in disused bones; furthermore, immunolocalization of both receptors showed decreases in osteoblasts, but not in osteoclasts. The results from the in vitro study showed that MK-801 inhibited mRNA expression of cbfa1/Runx2 in bone marrow stroma cells and also inhibited those of collagen type I, ALP and OC of osteoblasts in a dose-dependent manner. These results suggest that NMDA receptor mediation may play an important role in transmitting mechanical loading in bones, and decreases of the expressions of NMDA receptors in disused bones, especially in osteoblasts, may contribute to the decrease of osteogenesis.

The effect of reloading on bone volume, osteoblast number, and osteoprogenitor characteristics: studies in hind limb unloaded rats

Skeletal unloading during space flight results in bone loss. In astronauts the extent to which bone is lost varies greatly between different bones of the skeleton as well as between different individuals. Following return to earth, recovery of bone mass during reloading also varies between different bones and different individuals. Due to this variability and the limited number of subjects it is difficult to study the effects of unloading/reloading on bone in humans. A viable alternative is to use the rat model of hind limb unloading developed at NASA. We have previously demonstrated that, in 6-week-old male rats, 14 days of unloading result in a decrease in osteoprogenitor number in cell populations isolated from the proximal femur. The goal of the present study was to determine the number of osteoprogenitor cells present in cell populations derived from the proximal femur of young rats after 14 days of unloading followed by 14 days of reloading and to characterize their proliferative capacity. To do this, we determined the number of alkaline phosphatase-positive colony forming units (CFU-AP) and osteoblast CFU (CFU-O). To establish whether the effects of unloading and reloading were specific for cells of the osteoblast lineage, we also determined the number of fibroblastic CFU (CFU-F). Effects on proliferation were evaluated by measuring the size of CFU-O. Unloading resulted in a 66% reduction in CFU-AP. CFU-O numbers were decreased by 76% and mean colony size was 33% less than controls. The decrease in osteogenic and osteoprogenitor cells in vitro paralleled the decrease in bone volume (- 50%), osteoblast number (- 35%), and bone formation rate (- 46%) observed in the proximal tibial metaphysis of unloaded rats. Unloading had no effect on osteoclast number or surface. Subsequent reloading for 14 days restored CFU-AP. CFU-O numbers were only partially restored at 14 days (83% of controls) but nodule size was 1.2-fold greater than controls. Neither unloading nor reloading had an effect on the total number of progenitors (CFU-F). Reloading restored bone volume back to control values, but osteoblast number and bone formation rate were still lower than those in corresponding controls. Both osteoclast number and surface were lower in reloaded animals than in age-matched controls. Our results indicate that 14 days of unloading result in a decrease in osteoprogenitor number and that reloading for 14 subsequent days completely restores CFU-AP and bone volume to control levels, while the number of CFU-O in vitro and osteoblasts in vivo were partially recovered but still lower than corresponding controls. Strikingly, osteoclastic bone resorption after 14 days of reloading was greatly reduced compared to controls, suggesting a significant contribution of this to the recovery process.

Leptin modulates both resorption and formation while preventing disuse-induced bone loss in tail-suspended female rats

In vitro studies have demonstrated leptin-positive effects on the osteoblast lineage and negative effects on osteoclastogenesis. Therefore, we tested the hypothesis that leptin may prevent tail-suspension-induced bone loss characterized by an uncoupling pattern of bone remodeling, through both mechanisms. Female rats were randomly tail-suspended or not and treated either with ip administration of leptin or vehicle for 3, 7, and 14 d. As measured by dual energy x-ray absorptiometry, tail-suspension induced a progressive decrease in tibia-metaphysis bone mineral density, which was prevented by leptin. Histomorphometry showed that this was related to the prevention of the transient increase in osteoclast number observed with suspension at d 7. These effects could be mediated by the receptor activator of nuclear factor kappaB-ligand (RANKL)/osteoprotegerin (OPG) pathway since we observed using direct RT-PCR, a suspension-induced increase in RANKL gene expression in proximal tibia at d 3, which was counterbalanced by leptin administration with a similar 3-fold increase in OPG expression and a RANKL to OPG ratio close to nonsuspended conditions. In addition, leptin prevented the decrease in bone formation rate induced by tail-suspension at d 14. The latter could be related to the role of leptin in mediating the reciprocal differentiation between adipocytes and osteoblasts, because leptin concurrently blunted the disuse-induced increase in bone marrow adipogenesis. In summary, these data suggest that peripheral administration of leptin could prevent disuse-induced bone loss through, first, a major inhibitory effect on bone resorption and, second, a delayed effect preventing the decrease in bone formation.

Effect of simulated weightlessness on osteoprogenitor cell number and proliferation in young and adult rats

Experiments with rats flown in space or hind limb unloaded (HU) indicate that bone loss in both conditions is associated with a decrease in bone volume and osteoblast surface in cancellous and cortical bone. We hypothesize that the decrease in osteoblastic bone formation and osteoblast surface is related to a decrease in the number of osteoprogenitors and/or decreased proliferation of their progeny. We tested this hypothesis by evaluating the effect of 14 days of HU on the number of osteoprogenitors (osteoblast colony forming units; CFU-O), fibroblastic colony forming units (CFU-F), and alkaline phosphatase-positive CFU (CFU-AP) in cell populations derived from the proximal femur (unloaded) and the proximal humerus (normally loaded) in 6-week-old and 6-month-old rats. To confirm the effect of unloading on bone volume and structure, static histomorphometric parameters were measured in the proximal tibial metaphysis. Effects of HU on proliferation of osteoprogenitors were evaluated by measuring the size of CFU-O. HU did not affect the total number of progenitors (CFU-F) in young or adult rats in any of the cell populations. In femoral populations of young rats, HU decreased CFU-O by 71.0% and mean colony size was reduced by 20%. HU decreased CFU-AP by 31.3%. As expected, no changes in CFU-O or CFU-AP were seen in cell populations from the humerus. In femoral cell populations of adult rats, HU decreased CFU-O and CFU-AP by 16.6% and 36.6%, respectively. Again, no effects were seen in cell populations from the humerus. In 6-week-old rats, there was a greater decrease in bone volume, osteoblast number, and osteoblast surface in the proximal tibial metaphysis than that observed in adult rats. Both trabecular thickness and trabecular number were decreased in young rats but remained unaffected in adults. Neither osteoclast number nor surface was affected by unloading. Our results show that the HU-induced decrease in the number of osteoprogenitors observed in vitro parallels the effects of HU on bone volume and osteoblast number in young and old rats in vivo, suggesting that the two may be interdependent. HU also reduced CFU-O colony size in femoral populations indicating a diminished proliferative capacity of osteoblastic colonies.

Skeletal muscle atrophy leads to loss and dysfunction of muscle precursor cells

Atrophy of skeletal muscle leads to decreases in myofiber size and nuclear number; however, the effects of atrophic conditions on muscle precursor cells (MPC) are largely unknown. MPC lie outside myofibers and represent the main source of additional myonuclei necessary for muscle growth and repair. In the present study, we examined the properties of MPC after hindlimb suspension (HS)-induced atrophy and subsequent recovery of the mouse hindlimb muscles. We demonstrated that the number of MPC in atrophied muscles was decreased. RT-PCR analysis of cells isolated from atrophied muscles indicated that several mRNA characteristic of the myogenic program in MPC were absent. Cells isolated from atrophied muscles failed to properly proliferate and undergo differentiation into multinucleated myotubes. Thus atrophy led to a decrease in MPC and caused dysfunction in those MPC that remained. Upon regrowth of the atrophied muscles, these deleterious effects were reversed. Our data suggest that preventing loss or dysfunction of MPC may be a new pharmacological target during muscle atrophy.

Quantification of tartrate resistant acid phosphatase distribution in mouse tibiae using image analysis

Tartrate resistant acid phosphatase (TRAP) activity of bone is a suitable biochemical marker for osteoclastic bone resorption. Qualitatively, the histochemical distribution of TRAP has been used to identify osteoclasts responsible for bone resorption; however, there have been few attempts to quantify TRAP localization. We describe a method for evaluating bone resorption by quantifying area percentages of positive TRAP localization using image analysis. Mouse tibiae were paraffin embedded following demineralization in disodium ethylenediamine tetraacetic acid. Longitudinal sections of tibia were cut from 15 levels in the left and the right limbs of six mice (180 sections total) and stained for TRAP distribution. Positive TRAP localization was quantified by pixel area count and reported as a percentage of the total tissue area specified. The 1.85 mm2 region of interest was placed at the midpoint of the epiphyseal growth plate containing the provisional calcification layer and the primary spongiosa, while excluding cortical bone of each mouse tibia. The percentage of TRAP localization ranged from 0.95 to 1.31% and was not significantly different from level to level or limb to limb in each mouse (p > 0.100). Within the same region of interest, an osteoclast count along the bone perimeter also was performed. We demonstrated a strong correlation (r2 = 0.903) between the conventional histomorphometric osteoclast index and positive TRAP localization, validating the latter as an alternative method to assess bone resorption. Quantitative analysis of TRAP is significant because it allows statistical comparisons between treatment groups, promotes precise pathological diagnoses and facilitates a reference data base that may aid the study of bone related diseases involving increased bone resorption.

The teleost fish medaka (Oryzias latipes) as genetic model to study gravity dependent bone homeostasis in vivo

Long-term space flight and microgravity result in bone loss that can be explained by reduced activity of bone-forming cells (osteoblasts) and/or an increase in activity of bone resorbing cells (osteoclasts). Osteoprotegerin (OPG) has been shown to regulate the balance between osteoblast and osteoclast cell numbers and is involved in maintaining constant bone mass under normal gravitational conditions. The small bony fish medaka (Oryzias latipes) has attracted increasing attention as a genetic model system to study normal embryonic developmental and pathological processes. To analyze the molecular mechanisms of bone formation in this small vertebrate, we have isolated two opg genes, opgl and opg2, from medaka. Our phylogenetic analysis reveals that both genes originated from a common ancestor by fish-specific gene duplication and represent the orthologs of the mammalian opg gene. Both opg genes are differentially expressed during embryonic and larval development, in adult tissues and in cultured primary osteoblast-like cells. Furthermore, we have characterized the opg2 promoter region and identified consensus binding sites for the transcription factor core-binding-factor-1A (CBFA1). In mammals, CBFA1 has been shown to be a regulator of opg expression and to be essential for several steps during osteoblast differentiation. Here we show that sequence and expression domains of opg, cbfal and a member of the dlx gene family are highly conserved between medaka and higher vertebrates. This suggests that not only single genes but entire genetic networks for bone formation are conserved between teleosts and mammals. These findings will open medaka fish as a genetic model to monitor bone formation under different gravity conditions in a living whole animal allowing the identification of novel factors involved in bone homeostasis.