Trabecular bone turnover and bone marrow cell development in tail-suspended mice

Predicting altered bone biomechanics in juvenile mice: insights from microgravity simulation, loading interventions, and Raman Spectroscopy

BACKGROUND

Microgravity, a condition experienced in a spatial environment, poses unique challenges to the skeletal system, particularly in juvenile organisms. This study aimed to investigate alterations in bone biomechanics of juvenile mice due to unloading - that simulates microgravity in the laboratory-and the effects of a bone-loading intervention. We compared bone compositional and mechanical properties between 21-six-week-old C57Bl/6 from a control group (wild type) and a group that underwent a tail-suspension unloading protocol to mimic microgravity (MG). The second group (MG) experienced additional in vivo loading protocol (MG + LDG) on the right hind leg, where dynamic compressive loading was applied to the right knee using a custom-built loading device.

RESULTS

Our results show that after two weeks, we successfully induced bone alterations by (i) decreasing the energy dissipated before fracture and (ii) decreasing the yield and maximum stress. In addition, we showed that Mineral to matrix component [ν1PO4/Amide I], Carbonate to Amide [CO3/Amide I], and Crystallinity [1/FWHM(ν1PO4)] are strongly linked in physiological bone but not in microgravity even after loading intervention. While Crystallinity is very sensitive to bone deformation (strain) alterations coming from simulated microgravity, we show that Carbonate to Amide [CO3/Amide I] - a common marker of turnover rate/remodeling activity-is a specific predictor of bone deformation for bone after simulated microgravity. Our results also invalidate the current parameters of the loading intervention to prevent bone alterations entirely in juvenile mice.

CONCLUSIONS

Our study successfully induced bone alterations in juvenile mice by using an unloading protocol to simulate microgravity, and we provided a new Raman Spectroscopy (RS) dataset of juvenile mice that contributes to the prediction of cortical bone mechanical properties, where the degree of interrelationship for RS data for physiological bone is improved compared to the most recent evidence.

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.

Trabecular bone volume fraction in Holocene and Late Pleistocene humans

Research suggests that recent modern humans have gracile skeletons in having low trabecular bone volume fraction (BV/TV) and that gracilization of the skeleton occurred in the last 10,000 years. This has been attributed to a reduction in physical activity in the Holocene. However, there has been no thorough sampling of BV/TV in Pleistocene humans due to limited access to high resolution images of fossil specimens. Therefore, our study investigates the gracilization of BV/TV in Late Pleistocene humans and recent (Holocene) modern humans to improve our understanding of the emergence of gracility. We used microcomputed tomography to measure BV/TV in the femora, humeri and metacarpals of a sample of Late Pleistocene humans from Dolní Věstonice (Czech Republic, ∼26 ka, n = 6) and Ohalo II (Israel, ∼19 ka, n = 1), and a sample of recent humans including farming groups (n = 39) and hunter-gatherers (n = 6). We predicted that 1) Late Pleistocene humans would exhibit greater femoral and humeral head BV/TV compared with recent humans and 2) among recent humans, metacarpal head BV/TV would be greater in hunter-gatherers compared with farmers. Late Pleistocene humans had higher BV/TV compared with recent humans in both the femur and humerus, supporting our first prediction, and consistent with previous findings that Late Pleistocene humans are robust as compared to recent humans. However, among recent humans, there was no significant difference in BV/TV in the metacarpals between the two subsistence groups. The results highlight the similarity in BV/TV in the hand of two human groups from different geographic locales and subsistence patterns and raise questions about assumptions of activity levels in archaeological populations and their relationships to trabecular BV/TV.

Mouse models of accelerated aging in musculoskeletal research for assessing frailty, sarcopenia, and osteoporosis - A review

Musculoskeletal aging encompasses the decline in bone and muscle function, leading to conditions such as frailty, osteoporosis, and sarcopenia. Unraveling the underlying molecular mechanisms and developing effective treatments are crucial for improving the quality of life for those affected. In this context, accelerated aging models offer valuable insights into these conditions by displaying the hallmarks of human aging. Herein, this review focuses on relevant mouse models of musculoskeletal aging with particular emphasis on frailty, osteoporosis, and sarcopenia. Among the discussed models, PolgA mice in particular exhibit hallmarks of musculoskeletal aging, presenting early-onset frailty, as well as reduced bone and muscle mass that closely resemble human musculoskeletal aging. Ultimately, findings from these models hold promise for advancing interventions targeted at age-related musculoskeletal disorders, effectively addressing the challenges posed by musculoskeletal aging and associated conditions in humans.

Tibial bone strength is negatively affected by volumetric muscle loss injury to the adjacent muscle in male mice

This study's objective was to investigate how contractile strength loss associated with a volumetric muscle loss (VML) injury affects the adjacent tibial bone structural and functional properties in male C57BL/6J mice. Mice were randomized into one of two experimental groups: VML-injured mice that were injured at age 12 weeks and aged to 20 weeks (8 weeks postinjury, VML) and 20-week-old age-matched uninjured mice (Uninjured-20). Tibial bone strength, mid-diaphysis cortical geometry, intrinsic material properties, and metaphyseal trabecular bone structure were assessed by three-point bending and microcomputed tomography (µCT). The plantar flexor muscle group (gastrocnemius, soleus, plantaris) was analyzed for its functional capacities, that is, peak-isometric torque and peak-isokinetic power. VML-injured limbs had 25% less peak-isometric torque and 31% less peak-isokinetic power compared to those of Uninjured-20 mice (p < 0.001). Ultimate load, but not stiffness, was significantly less (10%) in tibias of VML-injured limbs compared to those from Uninjured-20 (p = 0.014). µCT analyses showed cortical bone thickness was 6% less in tibias of VML-injured limbs compared to Uninjured-20 (p = 0.001). Importantly, tibial bone cross-section moment of inertia, the primary determinant of bone ultimate load, was 16% smaller in bones of VML-injured limbs compared to bones from Uninjured-20 (p = 0.046). Metaphyseal trabecular bone structure was also altered up to 23% in tibias of VML-injured limbs (p < 0.010). These changes in tibial bone structure and function after a VML injury occur during a natural maturation phase between the age of 12 and 20 weeks, as evidenced by Uninjured-20 mice having greater tibial bone size and strength compared to uninjured-aged 12-week mice.

Circulating Extracellular Vesicles Express Receptor Activator of Nuclear Factor κB Ligand and Other Molecules Informative of the Bone Metabolic Status of Mouse Models of Experimentally Induced Osteoporosis

Extracellular vesicles (EVs) are potent means of cell-to-cell communication. They are released in biological fluids, including blood, urine, and saliva, and can be exploited to identify new biomarkers of diseases. We hypothesized that EVs contain molecular cargos involved in bone metabolism, possibly mirroring biological differences between postmenopausal and disuse osteoporosis. We tested this hypothesis in primary murine osteoblasts subjected to steroid depletion or to unloading, and in the serum of animal models of osteoporosis induced by ovariectomy or hindlimb tail suspension. EVs were isolated by ultracentrifugation and analysed by transmission electron microscopy, cytofluorimetry, immunoblotting and RT-PCR. Large-scale analyses were performed by Real-Time arrays and Proteome Profiler™ Antibody arrays. Finally, precise titration of analytes was carried out by ELISA assay. In vitro, we confirmed an increased release of EVs enriched in surface RANKL by primary mouse osteoblasts subjected to steroid depletion or simulated microgravity compared to controls. In vivo, circulating EVs isolated from the sera of control female mice expressed RANKL along with other genes associated with bone metabolism. Serum EVs from ovariectomized or hindlimb tail-suspended mice showed distinct molecular profiles. They expressed RANKL with different kinetics, while transcriptomic and proteomic profiles uncovered unique molecular signatures that discriminated the two conditions, unveiling exclusive molecules expressed in time- and osteoporosis type-dependent manner. These results suggest that circulating EVs could represent a new tool for monitoring the onset and the progression of diverse types of the disease in mice, paving the way for their exploitation to diagnose human osteoporosis in liquid biopsies.

Effects of reduced mobility on trabecular bone density in captive big cats

Bone responds to elevated mechanical loading by increasing in mass and density. Therefore, wild animals should exhibit greater skeletal mass and density than captive conspecifics. This expectation is pertinent to testing bone functional adaptation theories and to comparative studies, which commonly use skeletal remains that combine zoo and wild-caught specimens. Conservationists are also interested in the effects of captivity on bone morphology as it may influence rewilding success. We compared trabecular bone volume fraction (BVF) between wild and captive mountain lions, cheetahs, leopards and jaguars. We found significantly greater BVF in wild than in captive felids. Effects of captivity were more marked in the humerus than in the femur. A ratio of humeral/femoral BVF was also lower in captive animals and showed a positive relationship to home range size in wild animals. Results are consistent with greater forelimb than hindlimb loading during terrestrial travel, and possibly reduced loading of the forelimb associated with lack of predatory behaviour in captive animals. Thus, captivity among felids has general effects on BVF in the postcranial skeleton and location-specific effects related to limb use. Caution should be exercised when identifying skeletal specimens for use in comparative studies and when rearing animals for conservation purposes.

Punicalin Attenuates Breast Cancer-Associated Osteolysis by Inhibiting the NF-κB Signaling Pathway of Osteoclasts

Background: Breast cancer bone metastasis and osteoporosis are both severe diseases that seriously threaten human health. These diseases are closely associated with osteolytic lesions. And osteoclasts are the key targets of this pathological process. Given the lack of effective preventive or treatment options against these diseases, the exploitation of new pharmacological agents is critically required.

Method: We assessed the efficacy of punicalin on receptor activator of nuclear factor-κB ligand (RANKL)-mediated osteoclast formation, F-actin ring formation, gene expression, bone resorption, nuclear factor-κB (NF-κB) as well as on mitogen-activated protein kinase (MAPK) signaling pathways and molecular docking in vitro. The impact of punicalin on breast cancer-induced osteoclastogenesis, breast cancer cell proliferation, and apoptosis were examined. Transwell assays were also performed. Moreover, we evaluated in vivo effects of punicalin in postmenopausal osteoporosis models and breast cancer bone metastasis model by micro-CT scanning and histomorphometry.

Results: Punicalin inhibited osteoclast formation, F-actin ring formation, bone resorption, as well as osteoclast-related gene expression by suppressing the NF-κB signaling pathway. In vitro, punicalin also suppressed the breast cancer-induced osteoclastogenesis, and proliferation, migration as well as invasion of MDA-MB-231 cells and dose-dependently promoted their apoptosis. In vivo, punicalin significantly suppressed breast cancer-induced osteolysis, breast cancer-associated bone metastasis, and ovariectomized (OVX)-mediated osteoporosis by repressing osteoclast and breast cancer cell.

Conclusion: Punicalin is expected to offer a novel treatment for the prevention of osteolysis diseases, including osteoporosis and breast cancer-associated osteolysis.

The Osteocyte: From "Prisoner" to "Orchestrator"

Osteocytes are the most abundant bone cells, entrapped inside the mineralized bone matrix. They derive from osteoblasts through a complex series of morpho-functional modifications; such modifications not only concern the cell shape (from prismatic to dendritic) and location (along the vascular bone surfaces or enclosed inside the lacuno-canalicular cavities, respectively) but also their role in bone processes (secretion/mineralization of preosseous matrix and/or regulation of bone remodeling). Osteocytes are connected with each other by means of different types of junctions, among which the gap junctions enable osteocytes inside the matrix to act in a neuronal-like manner, as a functional syncytium together with the cells placed on the vascular bone surfaces (osteoblasts or bone lining cells), the stromal cells and the endothelial cells, i.e., the bone basic cellular system (BBCS). Within the BBCS, osteocytes can communicate in two ways: by means of volume transmission and wiring transmission, depending on the type of signals (metabolic or mechanical, respectively) received and/or to be forwarded. The capability of osteocytes in maintaining skeletal and mineral homeostasis is due to the fact that it acts as a mechano-sensor, able to transduce mechanical strains into biological signals and to trigger/modulate the bone remodeling, also because of the relevant role of sclerostin secreted by osteocytes, thus regulating different bone cell signaling pathways. The authors want to emphasize that the present review is centered on the morphological aspects of the osteocytes that clearly explain their functional implications and their role as bone orchestrators.

Prevention of bone loss and improvement of pain-related behavior in hind limb-unloaded mice by administration of teriparatide and bisphosphonate

OBJECTIVES

There are few reports on the comparison between teriparatide (PTH) and bisphosphonate (BP) in terms of osteoporosis pain-related behavior and immunohistochemical findings. The aims of this study were to evaluate skeletal pain associated with osteoporosis and to examine the inhibitory effect of PTH and BP on pain and bone loss in hind limb-unloaded (HU) mice. The mechanism of osteoporotic pain in HU mice was evaluated by examining pain-related behavior and immunohistochemical findings. The effects of PTH and alendronate (ALN), a potent osteoclast inhibitor, on these parameters were also assessed.

METHODS

Eight-week-old male ddY mice were tail-suspended for 2 weeks and assigned to four groups: hind limb-loaded (HL) mice with only tail suspension treated with vehicle; HU mice with tail suspension treated with vehicle; HU mice treated with PTH; and HU mice treated with ALN. Starting immediately after reloading, vehicle, PTH, or ALN was injected subcutaneously. After a 2-week treatment, mechanical sensitivity was examined using von Frey filaments. Bilateral hind limbs were removed for micro-computed tomography, immunohistochemical analysis, and messenger RNA (mRNA) expression analysis.

RESULTS

HU mice with tail suspension developed bone loss and mechanical hyperalgesia in the hind limbs. The HU mice showed an increased osteoclasts and sclerostin-positive cells in the hind limb bone. Furthermore, PTH and ALN both prevented HU-induced bone loss and mechanical hyperalgesia in the osteoporotic animal models. Histological examination of the hind limb bone revealed that, similar to ALN, PTH inhibited the osteoclasts and sclerostin-positive cells. The mRNA levels of TNFα and IL-6 tended to decrease with ALN or PTH treatment compared with those without any treatment.

CONCLUSIONS

Treatment with PTH as well as BP prevented bone loss, mechanical hyperalgesia, osteoclast increase, and osteocyte increase. Similar to BP, the inhibitory effect of PTH on osteoclasts might contribute to the improvement of skeletal pain.

Transient receptor potential vanilloid 1 and 4 double knockout leads to increased bone mass in mice

Calcium balance is important in bone homeostasis. The transient receptor potential vanilloid (TRPV) channel is a nonselective cation channel permeable to calcium and is activated by various physiological and pharmacological stimuli. TRPV1 and TRPV4, in particular, have important roles in intracellular Ca²⁺ signaling and extracellular calcium homeostasis in bone cells. TRPV1 and TRPV4 separately mediate osteoclast and osteoblast differentiation, and deficiency in any of these channels leads to increased bone mass. However, it remains unknown whether bone mass increases in the absence of both TRPV1 and TRPV4. In this study, we used TRPV1 and TRPV4 double knockout (DKO) mice to evaluate their bone mass in vivo, and osteoclast and osteoblast differentiation in vitro. Our results showed that DKO mice and wild type (WT) mice had no significant difference in body weight and femur length. However, the results of dual-energy X-ray absorption, microcomputed tomography, and bone histomorphometry clearly showed that DKO mice had higher bone mass than WT mice. Furthermore, DKO mice had less multinucleated osteoclasts and had lower bone resorption. In addition, the results of cell culture using flushed bone marrow from mouse femurs and tibias showed that osteoclast differentiation was suppressed, whereas osteoblast differentiation was promoted in DKO mice. In conclusion, our results suggest that the increase in bone mass in DKO mice was induced not only by the suppression of osteoclast differentiation and activity but also by the augmentation of osteoblast differentiation and activity. Our findings reveal that both the single deficiency of TRPVs and the concurrent deficiency of TRPVs result in an increase in bone mass. Furthermore, our data showed that DKO mice and single KO mice had varying approaches to osteoclast and osteoblast differentiation in vitro, and therefore, it is important to conduct further studies on TRPVs regarding the increase in bone mass to explore not only individual but also a combination of TRPVs.

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Knockout of either TRPV1 or TRPV4 results in increased bone mass in mice.

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This study evaluates the effects of TRPV1 and TRPV4 double knockout (DKO) in mice.

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Concurrent TRPV1 and TRPV4 deficiency increases mouse bone mass.

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TRPV1 and TRPV4 DKO suppresses osteoclast differentiation and activity.

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TRPV1 and TRPV4 DKO enhances osteoblast differentiation and activity.

Skeletal unloading reduces cluster of differentiation (CD) 38 expression in the bone marrow and osteoblasts of mice

BACKGROUND

Mechanical unloading induces bone loss in human weight-loaded bones. The findings of recent studies have revealed that cluster of differentiation 38 knockout mice display bone loss similar to that observed in osteoporosis. This study aimed to determine whether the expression of cluster of differentiation 38 is implicated in skeletal unloading and reloading.

METHODS

Eight-week-old male C57BL/6J mice were assigned to control, tail-suspension, or reloading after tail-suspension groups. In the tail-suspension group, tail suspension elevated the hind limbs for 1 week. The bilateral femurs and tibias from the groups were evaluated for cluster of differentiation 38 immunocytochemistry, and the cluster of differentiation 38 messenger ribonucleic acid levels and the expression of cluster of differentiation 38 and other cell-surface antigens were evaluated using quantitative real-time polymerase chain reaction and flow cytometric analyses.

RESULTS

In the tail-suspension group, the alkaline phosphatase reactivity, cluster of differentiation 38 immunoreactivity in the bone marrow and osteoblasts, and the expression of cluster of differentiation 38 messenger ribonucleic acid and that of other cell-surface antigens were significantly lower than those in the control group. In the reloading after tail-suspension group, the level of cluster of differentiation 38 expression was restored to the same level as that in the control group.

CONCLUSIONS

Cluster of differentiation 38 expression declined after skeletal unloading and recovered to normal levels after reloading. In the bone marrow, cluster of differentiation 38 expression plays a crucial role in bone formation in response to mechanical stress.

Separate and combined effects of hypobaric hypoxia and hindlimb suspension on skeletal homeostasis and hematopoiesis in mice

Purpose

Bone marrow response to an organismal stress is made by orchestrating the interplay between hematopoietic stem/progenitor cells (HSPCs) and mesenchymal stromal cells (MSCs). Neither the cellular nor the molecular factors that regulate this process are fully understood, especially since this mechanism probably varies depending on the type of stress. Herein, we explored the differentiation and fate of MSCs and HSPCs in mice challenged with a hematopoietic stress or a mechanical stress applied separately or in combination.

Methods

Mice were subjected to 4 days of hypobaric hypoxia (hematopoietic challenge) and/or 7 days of hindlimb suspension (stromal challenge) and then sacrificed for blood and bone collection. Using hematological measurements, colony-forming unit assays, bone histomorphometry and array-based multiplex ELISA analysis, we evaluated challenge influences on both MSC and HSPC mobilization, differentiation (osteoblasts, osteoclasts, and mature blood cells) and fate.

Results

We found that hypoxia leads to HSPC mobilization and that an imbalance between bone formation and bone resorption accounts for this mobilization. Whilst suspension is also associated with an imbalance between bone formation and bone resorption, it does not induce HSPC mobilization. Then, we revealed cellular interactions by combining hematopoietic and stromal challenges together in mice. We showed that the hypoxia-driven HSPC mobilization is moderated by suspension. Moreover, when applied in a hypoxic environment, suspension offsets bone imbalance. We identified stroma cell-derived factors MIP-1α, HGF and SDF-1 as potent molecular key players sustaining interactions between hindlimb suspension and hypobaric hypoxia.

Conclusion

Taken together, our data highlight the benefit of combining different types of stress to better understand the interplay between MSCs and HSPCs.

The effects on the mandibular condyle of Botox injection into the masseter are not transient

OBJECTIVES

To evaluate whether the effects on the mandibular condylar cartilage (MCC) and subchondral bone are transient of botulinum neurotoxin (Botox) injection into the masseter muscle.

METHODS

Botox (0.3 U) was injected into the right masseter of 6-week-old female mice (C57BL/6; n = 16). In addition, 16 mice were used as control and received no injections. Experimental and matching control mice were killed 4 or 8 weeks after the single Botox injection. Mandibles and mandibular condyles were analyzed by means of microscopic computed tomography (microCT) and histology. Sagittal sections of condyles were stained for tartrate-resistant acid phosphatase (TRAP), toluidine blue, 5-ethynyl-2'-deoxyuridine (EdU), and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling.

RESULTS

Bone volume fraction was significantly decreased on the subchondral bone of the Botox-injected side, compared with the control side and control mice, 4 and 8 weeks after injection. Furthermore, histologic analysis revealed decrease in mineralization, cartilage thickness, TRAP activity, and EdU-positive cells in the MCC of the Botox-injected side 4 and 8 weeks after injection.

CONCLUSIONS

The effects on the MCC and subchondral bone of Botox injection into the masseter muscle persisted for 8 weeks after injection and were not considered to be transient.

Findings as a starting point to unravel the underlying mechanisms of in vivo interactions involving Wnt10a in bone, fat and muscle

Wnt10a is a member of the WNT family. Although deficiency of this gene causes symptoms related to teeth, hair, nails, and skin, we recently demonstrated a new phenotype of Wnt10a knockout (KO) mice involving bone and fat. The in vivo effect of the Wnt10a gene on bone and fat is unclear, and the relationship between bone/fat and muscle in Wnt10a signaling is also interesting. We aimed to evaluate the tissue changes in Wnt10a KO mice compared to wild-type mice and show the findings as a starting point to unravel the underlying mechanisms of in vivo interactions involving Wnt10a in bone, fat and muscle. Trabecular bone loss in the lower limbs of Wnt10a mice and decreased bone mineralization were observed. The adipose tissue in bone marrow was also decreased, and adipocyte differentiation was reduced. The body fat mass in Wnt10a KO mice was decreased, and white adipocytes in subcutaneous fat were converted to beige adipocytes. The muscle weight of the lower limbs was not decreased despite trabecular bone loss, but Gdf8/myostatin expression was reduced in the subcutaneous fat and gastrocnemius muscles of Wnt10a KO mice. Thus, in vivo deletion of Wnt10a inhibited osteogenic activity, promoted beige adipogenesis of white adipocytes and maintained muscle mass. These results suggest that regulation of Gdf8 by Wnt10a may help maintain the muscle mass of Wnt10a KO mice. This study was the first to histologically evaluate the bone, fat and muscle phenotypes of Wnt10a KO mice. The results of this study, which were obtained by investigating the three tissues together, could influence the understanding of in vivo interactions involving the Wnt10a gene.

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.

Osteocytic oxygen sensing controls bone mass through epigenetic regulation of sclerostin

Preservation of bone mass is crucial for healthy ageing and largely depends on adequate responses of matrix-embedded osteocytes. These cells control bone formation and resorption concurrently by secreting the WNT/β-catenin antagonist sclerostin (SOST). Osteocytes reside within a low oxygen microenvironment, but whether and how oxygen sensing regulates their function remains elusive. Here, we show that conditional deletion of the oxygen sensor prolyl hydroxylase (PHD) 2 in osteocytes results in a high bone mass phenotype, which is caused by increased bone formation and decreased resorption. Mechanistically, enhanced HIF-1α signalling increases Sirtuin 1-dependent deacetylation of the Sost promoter, resulting in decreased sclerostin expression and enhanced WNT/β-catenin signalling. Additionally, genetic ablation of PHD2 in osteocytes blunts osteoporotic bone loss induced by oestrogen deficiency or mechanical unloading. Thus, oxygen sensing by PHD2 in osteocytes negatively regulates bone mass through epigenetic regulation of sclerostin and targeting PHD2 elicits an osteo-anabolic response in osteoporotic models.

Osteocytes reside in a low oxygen environment, but it is not clear if oxygen sensing regulates their function. Here, the authors show that deletion of the oxygen sensor prolyl hydroxylase 2 in osteocytes leads to increased bone mass via regulation of sclerostin, and reduces bone loss in mouse models of osteoporosis.

Intensity‑dependent effect of treadmill running on differentiation of rat bone marrow stromal cells

The effect of running on bone mass depends on its intensity. However, the underlying molecular mechanism that associates running intensity with bone mass is unclear. The current study examined the effects of treadmill running at different intensities on bone mass and osteogenic differentiation of bone marrow stromal cells (BMSCs) in a rat model. A total of 24 male Wistar rats were randomly divided into groups and subjected to no running (Con group), low‑intensity running (LIR group), moderate‑intensity running (MIR group), and high‑intensity running (HIR group). Histological, immunohistochemistry and micro‑CT examinations were performed on the femora harvested after 8 weeks of treadmill running. The study demonstrated that treadmill running affected trabecular bone mass in an intensity‑dependent manner. In addition, such an intensity‑dependent effect was also demonstrated on the osteogenic and adipogenic differentiation and proliferation of BMSCs. Furthermore, the Wnt/β‑catenin signaling pathway may be involved in the running‑induced increase in bone mass in rats in the MIR group. There appears to be a biomechanical 'window', in which running‑induced strain signals can increase the number of BMSCs and progenitor cells (specific to the osteoblast lineage) causing upregulation of osteogenesis and downregulation of adipogenesis of BMSCs. This finding may provide insight into the molecular and cellular mechanisms responsible for bone homeostasis.

Cortical bone loss due to skeletal unloading in aldehyde dehydrogenase 2 gene knockout mice is associated with decreased PTH receptor expression in osteocytes

Aldehyde dehydrogenase 2 (ALDH2) is the enzyme that degrades and detoxifies the acetaldehyde produced by alcohol metabolism. In our previous study, we found that compared with wild-type mice (WT), climbing exercises did not increase trabecular bone mass in Aldh2 knockout mice (KO). The purpose of this study was to clarify the effect of the Aldh2 gene on cortical bone structure and on the dynamics of skeletal unloading. Eight-week-old male KO and WT were divided into ground control (GC) or tail suspension (TS) groups for one week (i.e., the KOGC, KOTS, WTGC and WTTS groups). We measured the bone mineral density (BMD) of the femur using dual-energy X-ray absorptiometry. We assessed the femoral morphometry using peripheral quantitative computed tomography (pQCT) and evaluated the femoral cortex histomorphometry, and cortical mRNA using quantitative RT-PCR and cortical bone immunohistostaining. No significant differences were found between the femoral BMD of WTGC and that of WTTS, but the BMD in KOTS was significantly lower than that of KOGC. The pQCT results revealed that the cortical BMD of the femoral diaphysis in KOTS was significantly lower than that of KOGC. Furthermore, the cortical bone area and cortical thickness were significantly lower in KOTS than in the other three groups. Cortical histomorphometric analysis revealed that the endosteal and periosteal bone formation parameters were significantly lower in KOTS than in KOGC. Bone formation signals such as parathyroid hormone receptor (PTHR) were significantly decreased in KOTS compared with the levels in KOGC. Cortical bone immunohistostaining revealed a significantly decreased expression of PTHR in the osteocytes of KOTS compared with the expression level in KOGC. Thus, we concluded that when the Aldh2 gene is disrupted, skeletal unloading suppresses bone formation to decrease cortical bone mass, which may be mediated by a decreased expression of PTH receptors in osteocytes.

Disruption of the Aldehyde Dehydrogenase 2 Gene Results in No Increase in Trabecular Bone Mass Due to Skeletal Loading in Association with Impaired Cell Cycle Regulation Through p21 Expression in the Bone Marrow Cells of Mice

Approximately 45% of people of East Asian descent have the inactive aldehyde dehydrogenase 2 (ALDH2) phenotype. The enzyme defect of ALDH2 has been found to adversely influence the risk of osteoporosis. The aim of this study was to clarify the effect of skeletal loading on trabecular bone structure and dynamics in Aldh2-disrupted mice in the absence of alcohol consumption. Four-week-old male Aldh2^-/- (KO) and Aldh2^+/+ (WT) mice were divided into a ground control (GC) group and a climbing exercise (CE) group in each genotype. The trabecular bone mineral density of the distal femur measured by peripheral quantitative computed tomography in the wild-type CE (WTCE) group was significantly higher than that in the wild-type GC (WTGC) group; however, there was no significant difference between the knockout CE (KOCE) and knockout GC (KOGC) groups. Bone histomorphometry revealed that osteogenic parameters were significantly increased in the WTCE group compared with the WTGC group, but not increased in the KOCE group compared with the KOGC group. Quantitative reverse transcriptase polymerase chain reaction and flow cytometry revealed that mRNA and protein expression levels of p21 were significantly decreased in the WTCE group compared with those in the WTGC group, while these differences were not observed between the KOGC and KOCE groups. This study provides the first in vivo evidence that p21 expression in the bone marrow is not decreased after skeletal loading and osteoblast differentiation is impaired in the absence of Aldh2 gene.

Regional osteoporosis due to osteoclast activation as a trigger for the pain-like behaviors in tail-suspended mice

Pathological conditions with refractory skeletal pain are often characterized by regional osteoporotic changes such as transient osteoporosis of the hip, regional migratory osteoporosis, or complex regional pain syndrome (CRPS). Our previous study demonstrated that the acidic microenvironment created by osteoclast activation under high bone turnover conditions induced pain-like behaviors in ovariectomized mice through the stimulation of acid-sensing nociceptors. The aim of the present study was to examine whether regional transient osteoporotic changes are related to pain-like behaviors in the hind limb using tail-suspended model mice. The hind limbs of tail-suspended mice were unloaded for 2 weeks, during which time the mice revealed significant regional osteoporotic changes in their hind limbs accompanied by osteoclast activation. In addition, these changes were significantly recovered by the resumption of weight bearing on the hind limbs for 4 weeks. Consistent with the pathological changes in the hind limbs, pain-like behaviors in the mice were induced by tail suspension and recovered by the resumption of weight bearing. Moreover, treatment with bisphosphonate significantly prevented the triggering of the regional osteoporosis and pain-like behaviors, and antagonists of the acid-sensing nociceptors, such as transient receptor potential channel vanilloid subfamily member 1 and acid-sensing ion channels, significantly improved the pain-like behaviors in the tail-suspended mice. We, therefore, believe that regional transient osteoporosis due to osteoclast activation might be a trigger for the pain-like behaviors in tail-suspended model mice. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1226-1236, 2017.

LncRNA-H19 Modulates Wnt/β-catenin Signaling by Targeting Dkk4 in Hindlimb Unloaded Rat

OBJECTIVE

To investigate the biological functions of long noncoding RNA-H19 (H19) in the pathogenesis of disuse osteoporosis (DOP).

METHODS

Fifty-four male Sprague Dawley (SD) rats were randomly divided into three groups: baseline control (BC, 6), age-matched control (AC, 24), and hindlimb unloading (HLU, 24). The rats in the BC group were sacrificed at the beginning of the experiment, while the AC and HLU rats were sacrificed at different times (7, 14, 21 and 28 days after HLU). The DOP model was verified by micro-CT scan, and quantitative real-time polymerase chain reaction (qRT-PCR) was used to quantify the expression of osteogenic genes (OPG, RunX2 and OPG). Gene sequencing and bioinformatic analysis were performed to find H19 target genes and the associated signaling pathway, which were first verified on tissue samples. Further verification was performed by knocking down the H19 and related gene in rat osteoblast cell line (UMR106 cell). Then, the changes of associated signaling pathway and osteogenic function were examined to confirm the prediction of the bioinformatic analysis.

RESULTS

Micro-CT scans and quantitative real-time polymerase chain reaction (qRT-PCR) tests showed progressively deteriorated trabecular bone and decreased level of osteogenic genes in the metaphysis of distal femur during HLU, indicating the successful establishment of a DOP model. According to RNA sequencing, 1351 mRNA and 464 lncRNA were abnormally expressed in response to mechanical unloading, in which the H19 decreased 2.86 fold in HLU rats. There were 1426 mRNA predicted to be the target genes of H19, and KEGG pathway analysis suggested that Wnt signaling pathway (Wnt signaling) was the top pathway responsible for these target genes. In the Wnt-associated genes targeted by H19, 11 were differentially expressed between HLU and AC rats, among which Dkk4 increased 2.44 fold in HLU rats when compared to normal controls. These results of sequencing and bioinformatic analysis were confirmed by the low expression of H19, overexpression of Dkk4 and inhibited Wnt signaling observed in DOP rats. Subsequent in vitro cell assay further demonstrated that knockdown of H19 led to upregulation of Dkk4, and inhibition of Wnt signaling and osteogenic function in UMR106 cell. These effects can be greatly reversed after application of knocking down Dkk4.

CONCLUSION

Our findings demonstrated that low expression of H19, induced by mechanical unloading, leads to development of DOP through inhibition of Wnt signaling by promoting Dkk4 expression.

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.

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.

Elcatonin prevents bone loss caused by skeletal unloading by inhibiting preosteoclast fusion through the unloading-induced high expression of calcitonin receptors in bone marrow cells

This study aimed to clarify whether elcatonin (EL) has a preventive action on bone dynamics in skeletal unloading. Seven-week-old male C57BL/6J mice with either ground control (GC) or tail suspension (TS) were administered EL 20U/kg or a vehicle (veh) three times per week and assigned to one of the following four groups: GCEL, GCveh, TSEL, and TSveh. Blood samples and bilateral femurs and tibias of the mice were obtained for analysis. After 7days of unloading, the trabecular bone mineral density in the distal femur obtained via peripheral quantitative computed tomography and the trabecular bone volume were significantly higher in the TSEL group than in the TSveh group. The bone resorption histomorphometric parameters, such as the osteoclast surface and osteoclast number, were significantly suppressed in the TSEL mice, whereas the number of preosteoclasts was significantly increased. The plasma level of tartrate-resistant acid phosphatase-5b (TRACP-5b) was significantly lower in the TSEL group than in all other groups. In the bone marrow cell culture, the number of TRACP-positive (TRACP(+)) multinucleated cells was significantly lower in the TSEL mice than in the TSveh mice, whereas the number of TRACP(+) mononucleated cells was higher in the TSEL mice. On day 4, the expression of nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1), cathepsin K and d2 isoform of vacuolar ATPase V0 domain (ATP6V0D2) mRNA in the bone marrow cells in the TSEL mice was suppressed, and the expression of calcitonin receptor (Calcr) mRNA on day 1 and Calcr antigen on day 4 were significantly higher in the TSveh mice than in the GCveh mice. EL prevented the unloading-induced bone loss associated with the high expression of Calcr in the bone marrow cells of mouse hindlimbs after tail suspension, and it suppressed osteoclast development from preosteoclasts to mature osteoclasts through bone-resorbing activity. This study of EL-treated unloaded mice provides the first in vivo evidence of a physiological role of EL in the inhibition of the differentiation process from preosteoclasts to osteoclasts.

Effect of vitamin K2 on the development of stress-induced osteopenia in a growing senescence-accelerated mouse prone 6 strain

Vitamin K2 (VK2) has been used as a therapeutic agent for osteoporosis, since it has been suggested to be able to reduce the frequency of fractures by improving bone quality; however, bone turnover is strictly regulated by various cytokines and hormones. In the present study, the effect of menaquinone-4 (MK-4) on bone turnover was investigated using the senescence-accelerated mouse prone 6 (SAMP6) strain. Since water-immersion restraint stress (WRS) causes a significant decrease in bone mineral density (BMD), WRS was used as the bone resorption model in the SAMP6 strain. Six-week-old SAMP6 male mice were divided into the following three groups: Control, WRS and WRS + MK-4. WRS was performed for 6 h per day, 5 times a week, for 4 weeks. Following WRS, MK-4 (30 mg/kg) was injected subcutaneously 3 times a week for 4 weeks. No growth retardation was observed in the WRS groups as compared with the control group. In the WRS groups, the BMD was significantly lower than that in the control group. The levels of bone formation and resorption markers were increased in the WRS groups, indicating that WRS reduced the BMD by promoting high bone turnover. A bone histomorphometrical examination showed that the trabecular (Tb) bone mass in the secondary spongiosa at the distal femur was significantly reduced in the WRS mice, and this reduction was abrogated by MK-4 treatment. Specifically, the Tb bone reduction was caused by the activation of osteoclasts (Ocs), and Oc activity was suppressed by MK-4. The number of osteoblasts and the mineral apposition rate were significantly increased in the WRS and WRS + MK-4 mice, suggesting that WRS triggered a significantly higher mineral apposition rate. These results indicate that MK-4 can induce recovery from the bone mineral loss caused by WRS treatment. Further studies are required to clarify the association between bone quality and MK-4.

Immobilization induced osteopenia is strain specific in mice

Immobilization causes rapid and massive bone loss. By comparing Botulinum Toxin A (BTX)-induced bone loss in mouse strains with different genetic backgrounds we investigated whether the genetic background had an influence on the severity of the osteopenia. Secondly, we investigated whether BTX had systemic effects on bone. Female mice from four inbred mouse strains (BALB/cJ, C57BL/6 J, DBA/2 J, and C3H/HeN) were injected unilaterally with BTX (n = 10/group) or unilaterally with saline (n = 10/group). Mice were euthanized after 21 days, and the bone properties evaluated using μCT, DXA, bone histomorphometry, and mechanical testing. BTX resulted in substantially lower trabecular bone volume fraction (BV/TV) and trabecular thickness in all mouse strains. The deterioration of BV/TV was significantly greater in C57BL/6 J (− 57%) and DBA/2 J (− 60%) than in BALB/cJ (− 45%) and C3H/HeN (− 34%) mice. The loss of femoral neck fracture strength was significantly greater in C57BL/6 J (− 47%) and DBA/2 J (− 45%) than in C3H (− 25%) mice and likewise the loss of mid-femoral fracture strength was greater in C57BL/6 J (− 17%), DBA/2 J (− 12%), and BALB/cJ (− 9%) than in C3H/HeN (− 1%) mice, which were unaffected. Using high resolution μCT we found no evidence of a systemic effect on any of the microstructural parameters of the contralateral limb. Likewise, there was no evidence of a systemic effect on the bone strength in any mouse strain. We did, however, find a small systemic effect on aBMD in DBA/2 J and C3H/HeN mice. The present study shows that BTX-induced immobilization causes the greatest loss of cortical and trabecular bone in C57BL/6 J and DBA/2 J mice. A smaller loss of bone microstructure and fracture strength was seen in BALB/cJ mice, while the bone microstructure and fracture strength of C3H/HeN mice were markedly less affected. This indicates that BTX-induced loss of bone is mouse strain dependent. We found only minimal systemic effects of BTX.

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Botulinum Toxin A (Botox) causes only minimal systemic effects in mice.

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The degree of immobilization induced osteopenia is highly strain specific in mice.

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The greatest degree of bone loss was observed with C57BL/6 J and DBA/2 J mice followed by BALB/cJ mice after Botox-injection.

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C3H/HeN mice had the smallest bone loss following Botox-injection.

Animal models for osteoporosis

The major types of osteoporosis in humans are postmenopausal osteoporosis, disuse osteoporosis, and glucocorticoid-induced osteoporosis. Animal models for postmenopausal osteoporosis are generated by ovariectomy. Bone loss occurs in estrogen deficiency due to enhanced bone resorption and impaired osteoblast function. Estrogen receptor α induces osteoclast apoptosis, but the mechanism for impaired osteoblast function remains to be clarified. Animal models for unloading are generated by tail suspension or hind limb immobilization by sciatic neurectomy, tenotomy, or using plaster cast. Unloading inhibits bone formation and enhances bone resorption, and the involvement of the sympathetic nervous system in it needs to be further investigated. The osteocyte network regulates bone mass by responding to mechanical stress. Osteoblast-specific BCL2 transgenic mice, in which the osteocyte network is completely disrupted, can be a mouse model for the evaluation of osteocyte functions. Glucocorticoid treatment inhibits bone formation and enhances bone resorption, and markedly reduces cancellous bone in humans and large animals, but not consistently in rodents.

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.

A comparative study of trabecular bone mass distribution in cursorial and non-cursorial limb joints

Skeletal design among cursorial animals is a compromise between a stable body that can withstand locomotor stress and a light design that is energetically inexpensive to grow, maintain, and move. Cursors have been hypothesized to reduce distal musculoskeletal mass to maintain a balance between safety and energetic cost due to an exponential increase in energetic demand observed during the oscillation of the distal limb. Additionally, experimental research shows that the cortical bone in distal limbs experiences higher strains and remodeling rates, apparently maintaining lower mass at the expense of a smaller safety factor. This study tests the hypothesis that the trabecular bone mass in the distal limb epiphyses of cursors is relatively lower than that in the proximal limb epiphyses to minimize the energetic cost of moving the limb. This study utilized peripheral quantitative computed tomography scanning to measure the trabecular mass in the lower and upper limb epiphyses of hominids, cercopithecines, and felids that are considered cursorial and non-cursorial. One-way ANOVA with Tukey post hoc corrections was used to test for significant differences in trabecular mass across limb epiphyses. The results indicate that overall, both cursors and non-cursors exhibit varied trabecular mass in limb epiphyses and, in certain instances, conform to a proximal-distal decrease in mass irrespective of cursoriality. Specifically, hominid and cercopithecine hind limb epiphyses exhibit a proximal-distal decrease in mass irrespective of cursorial adaptations. These results suggest that cursorial mammals employ other energy saving mechanisms to minimize energy costs during running.

Hind limb unloading, a model of spaceflight conditions, leads to decreased B lymphopoiesis similar to aging

Within the bone marrow, the endosteal niche plays a crucial role in B-cell differentiation. Because spaceflight is associated with osteoporosis, we investigated whether changes in bone microstructure induced by a ground-based model of spaceflight, hind limb unloading (HU), could affect B lymphopoiesis. To this end, we analyzed both bone parameters and the frequency of early hematopoietic precursors and cells of the B lineage after 3, 6, 13, and 21 d of HU. We found that limb disuse leads to a decrease in both bone microstructure and the frequency of B-cell progenitors in the bone marrow. Although multipotent hematopoietic progenitors were not affected by HU, a decrease in B lymphopoiesis was observed as of the common lymphoid progenitor (CLP) stage with a major block at the progenitor B (pro-B) to precursor B (pre-B) cell transition (5- to 10-fold decrease). The modifications in B lymphopoiesis were similar to those observed in aged mice and, as with aging, decreased B-cell generation in HU mice was associated with reduced expression of B-cell transcription factors, early B-cell factor (EBF) and Pax5, and an alteration in STAT5-mediated IL-7 signaling. These findings demonstrate that mechanical unloading of hind limbs results in a decrease in early B-cell differentiation resembling age-related modifications in B lymphopoiesis.

In vivo hypobaric hypoxia performed during the remodeling process accelerates bone healing in mice

We investigated the effects of respiratory hypobaric hypoxia on femoral bone-defect repair in mice because hypoxia is believed to influence both mesenchymal stromal cell (MSC) and hematopoietic stem cell mobilization, a process involved in the bone-healing mechanism. To mimic conditions of non-weight-bearing limb immobilization in patients suffering from bone trauma, our hypoxic mouse model was further subjected to hind-limb unloading. A hole was drilled in the right femur of adult male C57/BL6J mice. Four days after surgery, mice were subjected to hind-limb unloading for 1 week. Seven days after surgery, mice were either housed for 4 days in a hypobaric room (FiO2 at 10%) or kept under normoxic conditions. Unsuspended control mice were housed in either hypobaric or normoxic conditions. Animals were sacrificed on postsurgery day 11 to allow for collection of both contralateral and lesioned femurs, blood, and spleen. As assessed by microtomography, delayed hypoxia enhanced bone-healing efficiency by increasing the closing of the cortical defect and the newly synthesized bone volume in the cavity by +55% and +35%, respectively. Proteome analysis and histomorphometric data suggested that bone-repair improvement likely results from the acceleration of the natural bone-healing process rather than from extended mobilization of MSC-derived osteoprogenitors. Hind-limb unloading had hardly any effect beyond delayed hypoxia-enhanced bone-healing efficiency.

Disruption of aldehyde dehydrogenase 2 gene results in altered cortical bone structure and increased cortical bone mineral density in the femoral diaphysis of mice

INTRODUCTION

Aldehyde dehydrogenase 2 (ALDH2) degrades acetaldehyde produced by the metabolism of alcohol. The inactive ALDH2 phenotype is prevalent in East Asians, and an association between this ALDH2 polymorphism and osteoporosis has been reported. In our previous study, we found that alcohol consumption results in decreased trabecular bone volume in aldh2 knockout (aldh2(-/-)) mice compared with the volume in wild-type (aldh2(+/+)) mice. However, the effect of aldh2 gene on the skeletal phenotype in the absence of alcohol consumption remains unknown. The aim of this study was to clarify the effect of aldh2 disruption on femoral bone structure and dynamics in aldh2-disrupted mice in the absence of alcohol consumption.

MATERIALS AND METHODS

We examined aldh2(-/-) and aldh2(+/+) mice at the ages of 4, 8 and 12weeks. The femoral bone length and bone mineral density (BMD) were measured using peripheral quantitative computed tomography. The mechanical strength was assessed by the three-point bending test at 12weeks, and cortical bone histomorphometry at the femur diaphysis was performed at all three time points. Osteogenic activities in aldh2(-/-) and aldh2(+/+) mice were assessed by osteoblast culture from calvariae of the neonatal mice. Bilateral femoral and tibial bones containing no bone marrow cells of 8-week-old mice were used for analysis of mRNA expression. In addition, mRNA expression in aldh2(-/-) and aldh2(+/+) mice after tail suspension or climbing exercise for 7days from 8weeks was analyzed to clarify the response to mechanical loading.

RESULTS

At 12weeks, there were no significant differences in femoral bone length, trabecular BMD in the distal metaphyses of the femurs, or mechanical strength between aldh2(-/-) and aldh2(+/)(+) mice, whereas cortical BMD and cortical thickness were significantly increased and cross-sectional area and bone marrow area were significantly decreased in the femoral diaphysis of aldh2(-/-) mice relative to the corresponding values in aldh2(+/+) mice. At 8weeks, bone formation rate and mineral apposition rate on the periosteal and endocortical surfaces were significantly increased in aldh2(-/-) mice relative to the rates in aldh(+/+) mice. Calvarial osteoblast culture study revealed that the percentage of alkaline phosphatase stained cells was significantly higher in aldh2(-/-) mice compared to that in aldh(+/+) mice. Quantitative real-time RT-PCR revealed a significant increase in the expressions of bmp2, osterix, runx2, and col1a1 mRNA in aldh2(-/-) mice, along with an increase in the expression of wnt5a mRNA and the lrp5/sost mRNA ratio. The mRNA expressions of bmp2, osterix, runx2 and pthr in aldh2(-/-) mice were significantly decreased after climbing exercise compared to those in the control, although the mRNA expressions of bmp2, osterix, runx2 were not significantly decreased and pthr mRNA expression was increased in aldh(+/+) mice after climbing exercise.

CONCLUSION

Our results show that disruption of aldh2 gene resulted in altered cortical bone structure and dynamics in mice. Cross-sectional area was decreased. Cortical BMD was increased owing to the promotion of cortical bone formation on the periosteal and endocortical surfaces of the femoral diaphysis. The possible mechanisms underlying altered cortical bone structure in aldh2(-/-) mice were gene-related higher osteogenic activity of osteoblasts and weakened osteogenice response to mechanical loading in growth period.

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.

Differential effects of jump versus running exercise on trabecular architecture during remobilization after suspension-induced osteopenia in growing rats

High-impact exercise is considered to be very beneficial for bones. We investigated the ability of jump exercise to restore bone mass and structure after the deterioration induced by tail suspension in growing rats and made comparisons with treadmill running exercise. Five-week-old male Wistar rats (n = 28) were randomly assigned to four body weight-matched groups: a spontaneous recovery group after tail suspension (n = 7), a jump exercise group after tail suspension (n = 7), a treadmill running group after tail suspension (n = 7), and age-matched controls without tail suspension or exercise (n = 7). Treadmill running was performed at 25 m/min, 1 h/day, 5 days/wk. The jump exercise protocol consisted of 10 jumps/day, 5 days/wk, with a jump height of 40 cm. Bone mineral density (BMD) of the total right femur was measured by dual-energy X-ray absorptiometry. Three-dimensional trabecular bone architecture at the distal femoral metaphysis was evaluated using microcomputed tomography. After 5 wk of free remobilization, right femoral BMD, right hindlimb muscle weight, and body weight returned to age-matched control levels, but trabeculae remained thinner and less connected. Although both jump and running exercises during the remobilization period increased trabecular bone mass, jump exercise increased trabecular thickness, whereas running exercise increased trabecular number. These results indicate that restoration of trabecular bone architecture induced by jump exercise during remobilization is predominantly attributable to increased trabecular thickness, whereas running adds trabecular bone mass through increasing trabecular number, and suggest that jumping and running exercises have different mechanisms of action on structural characteristics of trabecular bone.

Effects of microgravity on osteoclast bone resorption and osteoblast cytoskeletal organization and adhesion

Exposure to microgravity has been associated with several physiological changes in astronauts, including an osteoporosis-like loss in bone mass. Despite many in vivo and in vitro studies in both microgravity and simulated microgravity conditions, the mechanism for bone loss is still not clear. The lack of weight-bearing forces makes microgravity an ideal physical stimulus to assess bone cell responses. In this work, we conduct a unique investigation of the effects of microgravity on bone-producing osteoblasts and, in parallel, on bone-resorbing osteoclasts. An increase in total number of discrete resorption pits is observed in osteoclasts that experienced microgravity versus ground controls. We further show that osteoblasts exposed to 5 days of microgravity have shorter and wavier microtubules (MTs), smaller and fewer focal adhesions, and thinner cortical actin and stress fibers. Space-flown osteoblasts present extended cell shapes as well as significantly more disrupted and often fragmented or condensed nuclei. The absence of gravitational forces therefore causes both an increase in bone resorption by osteoclasts, and a decrease in osteoblast cellular integrity. The observed effects on both major bone cell types likely accelerate bone loss in microgravity environments, and additionally offer a potential explanation to the development of disuse osteoporosis on Earth.

Metacarpal head biomechanics: a comparative backscattered electron image analysis of trabecular bone mineral density in Pan troglodytes, Pongo pygmaeus, and Homo sapiens

Great apes and humans use their hands in fundamentally different ways, but little is known about joint biomechanics and internal bone variation. This study examines the distribution of mineral density in the third metacarpal heads in three hominoid species that differ in their habitual joint postures and loading histories. We test the hypothesis that micro-architectural properties relating to bone mineral density reflect habitual joint use. The third metacarpal heads of Pan troglodytes, Pongo pygmaeus, and Homo sapiens were sectioned in a sagittal plane and imaged using backscattered electron microscopy (BSE-SEM). For each individual, 72 areas of subarticular cortical (subchondral) and trabecular bone were sampled from within 12 consecutive regions of the BSE-SEM images. In each area, gray levels (representing relative mineralization density) were quantified. Results show that chimpanzee, orangutan, and human metacarpal III heads have different gray level distributions. Weighted mean gray levels (WMGLs) in the chimpanzee showed a distinct pattern in which the 'knuckle-walking' regions (dorsal) and 'climbing' regions (palmar) are less mineralized, interpreted to reflect elevated remodeling rates, than the distal regions. Pongo pygmaeus exhibited the lowest WMGLs in the distal region, suggesting elevated remodeling rates in this region, which is loaded during hook grip hand postures associated with suspension and climbing. Differences among regions within metacarpal heads of the chimpanzee and orangutan specimens are significant (Kruskal-Wallis, p < 0.001). In humans, whose hands are used for manipulation as opposed to locomotion, mineralization density is much more uniform throughout the metacarpal head. WMGLs were significantly (p < 0.05) lower in subchondral compared to trabecular regions in all samples except humans. This micro-architectural approach offers a means of investigating joint loading patterns in primates and shows significant differences in metacarpal joint biomechanics among great apes and humans.

Modeled microgravity and hindlimb unloading sensitize osteoclast precursors to RANKL-mediated osteoclastogenesis

Mechanical forces are essential to maintain skeletal integrity, and microgravity exposure leads to bone loss. The underlying molecular mechanisms leading to the changes in osteoblasts and osteoclast differentiation and function remain to be fully elucidated. Because of the infrequency of spaceflights and payload constraints, establishing in vitro and in vivo systems that mimic microgravity conditions becomes necessary. We have established a simulated microgravity (modeled microgravity, MMG) system to study the changes induced in osteoclast precursors. We observed that MMG, on its own, was unable to induce osteoclastogenesis of osteoclast precursors; however, 24 h of MMG activates osteoclastogenesis-related signaling molecules ERK, p38, PLCγ2, and NFATc1. Receptor activator of NFkB ligand (RANKL) (with or without M-CSF) stimulation for 3-4 days in gravity of cells that had been exposed to MMG for 24 h enhanced the formation of very large tartrate-resistant acid phosphatase (TRAP)-positive multinucleated (>30 nuclei) osteoclasts accompanied by an upregulation of the osteoclast marker genes TRAP and cathepsin K. To validate the in vitro system, we studied the hindlimb unloading (HLU) system using BALB/c mice and observed a decrease in BMD of femurs and a loss of 3D microstructure of both cortical and trabecular bone as determined by micro-CT. There was a marked stimulation of osteoclastogenesis as determined by the total number of TRAP-positive multinucleated osteoclasts formed and also an increase in RANKL-stimulated osteoclastogenesis from precursors removed from the tibias of mice after 28 days of HLU. In contrast to earlier reported findings, we did not observe any histomorphometric changes in the bone formation parameters. Thus, the foregoing observations indicate that microgravity sensitizes osteoclast precursors for increased differentiation. The in vitro model system described here is potentially a valid system for testing drugs for preventing microgravity-induced bone loss by targeting the molecular events occurring in microgravity-induced enhanced osteoclastogenesis.

Heavy ion irradiation and unloading effects on mouse lumbar vertebral microarchitecture, mechanical properties and tissue stresses

Astronauts are exposed to both musculoskeletal disuse and heavy ion radiation in space. Disuse alters the magnitude and direction of forces placed upon the skeleton causing bone remodeling, while energy deposited by ionizing radiation causes free radical formation and can lead to DNA strand breaks and oxidative damage to tissues. Radiation and disuse each result in a net loss of mineralized tissue in the adult, although the combined effects, subsequent consequences for mechanical properties and potential for recovery may differ. First, we examined how a high dose (2 Gy) of heavy ion radiation ((56)Fe) causes loss of mineralized tissue in the lumbar vertebrae of skeletally mature (4 months old), male, C57BL/6 mice using microcomputed tomography and determined the influence of structural changes on mechanical properties using whole bone compression tests and finite element analyses. Next, we tested if a low dose (0.5 Gy) of heavy particle radiation prevents skeletal recovery from a 14-day period of hindlimb unloading. Irradiation with a high dose of (56)Fe (2 Gy) caused bone loss (-14%) in the cancellous-rich centrum of the fourth lumbar vertebra (L4) 1 month later, increased trabecular stresses (+27%), increased the propensity for trabecular buckling and shifted stresses to the cortex. As expected, hindlimb unloading (14 days) alone adversely affected microarchitectural and mechanical stiffness of lumbar vertebrae, although the reduction in yield force was not statistically significant (-17%). Irradiation with a low dose of (56)Fe (0.5 Gy) did not affect vertebrae in normally loaded mice, but significantly reduced compressive yield force in vertebrae of unloaded mice relative to sham-irradiated controls (-24%). Irradiation did not impair the recovery of trabecular bone volume fraction that occurs after hindlimb unloaded mice are released to ambulate normally, although microarchitectural differences persisted 28 days later (96% increase in ratio of rod- to plate-like trabeculae). In summary, (56)Fe irradiation (0.5 Gy) of unloaded mice contributed to a reduction in compressive strength and partially prevented recovery of cancellous microarchitecture from adaptive responses of lumbar vertebrae to skeletal unloading. Thus, irradiation with heavy ions may accelerate or worsen the loss of skeletal integrity triggered by musculoskeletal disuse.

Ultrasound evaluation of site-specific effect of simulated microgravity on articular cartilage

Space flight induces acute changes in normal physiology in response to the microgravity environment. Articular cartilage is subjected to high loads under a ground reaction force on Earth. The objectives of this study were to investigate the site dependence of morphological and ultrasonic parameters of articular cartilage and to examine the site-specific responses of articular cartilage to simulated microgravity using ultrasound biomicroscopy (UBM). Six rats underwent tail suspension (simulated microgravity) for four weeks and six other rats were kept under normal Earth gravity as controls. Cartilage thickness, ultrasound roughness index (URI), integrated reflection coefficient (IRC) and integrated backscatter coefficient (IBC) of cartilage tissues, as well as histological degeneration were measured at the femoral head (FH), medial femoral condyle (MFC), lateral femoral condyle (LFC), patello-femoral groove (PFG) and patella (PAT). The results showed site dependence not significant in all UBM parameters except cartilage thickness (p < 0.01) in the control specimens. Only minor changes in articular cartilage were induced by 4-week tail suspension, although there were significant decreases in cartilage thickness at the MFC and PAT (p < 0.05) and a significant increase in URI at the PAT (p < 0.01). This study suggested that the 4-week simulated microgravity had only mild effects on femoral articular cartilage in the rat model. This information is useful for human spaceflight and clinical medicine in improving understanding of the effect of microgravity on articular cartilage. However, the effects of longer duration microgravity experience on articular cartilage need further investigation.

Tail-suspended mice lacking calponin H1 experience decreased bone loss

Calponin h1 (CNh1) is an actin-binding protein originally isolated from vascular smooth muscle and has been reported to suppress bone formation. We are therefore curious how CNh1 is involved in bone loss that is caused by space flight in microgravity. We assessed the effects of tail suspension (TS) in C57BL/6J wild (CN+/+) and CNh1-deleted (CN-/-) mice to elucidate the role of CNh1 in bone loss under weightless conditions. Bone mineral density (BMD) of tibiae was measured by single energy X-ray absorptiometry, and bone volume fraction (BV/TV), mineral apposition rate (MAR), and bone formation rate (BFR/BS) were measured by bone histomorphometry. BMD, BV/TV, MAR, and BFR/BS were lower in CN+/+ mice with TS than in those without. In the CN-/- group, however, the decrease in each of these parameters by TS was ameliorated. Decreases in serum osteocalcin levels by TS in CN+/+ mice were attenuated in CN-/- mice. Furthermore, urinary deoxypyridinolin (DPD), an indicator of bone resorption, was increased in CN+/+ mice following TS, but not in CN-/- mice. In transfection experiments, the degree of induction of bone formation markers, alkaline phosphatase (ALP) activity and bone morphogenetic protein (BMP)-4 mRNA expression, under stimulation with BMP-2, was lower in MC3T3-E1 mouse osteoblast-like cells expressing CNh1 than that in mock transfected cells. Notably, the BMP-2-induced ALP activity was decreased by CNh1 expression, which was partially rescued by treatment with the Rho kinase inhibitor Y27632. Taken together, these results indicate that CNh1 is responsible for weightlessness-induced bone loss in part through Rho signaling pathway.

Short-term effects of whole-body exposure to (56)fe ions in combination with musculoskeletal disuse on bone cells

Space travel and prolonged bed rest cause bone loss due to musculoskeletal disuse. In space, radiation fields may also have detrimental consequences because charged particles traversing the tissues of the body can elicit a wide range of cytotoxic and genotoxic lesions. The effects of heavy-ion radiation exposure in combination with musculoskeletal disuse on bone cells and tissue are not known. To explore this, normally loaded 16-week-old male C57BL/6 mice were exposed to (56)Fe ions (1 GeV/nucleon) at doses of 0 cGy (sham), 10 cGy, 50 cGy or 2 Gy 3 days before tissue harvest. Additional mice were hindlimb unloaded by tail traction continuously for 1 week to simulate weightlessness and exposed to (56)Fe-ion radiation (0 cGy, 50 cGy, 2 Gy) 3 days before tissue harvest. Despite the short duration of this study, low-dose (10, 50 cGy) irradiation of normally loaded mice reduced trabecular volume fraction (BV/TV) in the proximal tibiae by 18% relative to sham-irradiated controls. Hindlimb unloading together with 50 cGy radiation caused a 126% increase in the number of TRAP(+) osteoclasts on cancellous bone surfaces relative to normally loaded, sham-irradiated controls. Together, radiation and hindlimb unloading had a greater effect on suppressing osteoblastogenesis ex vivo than either treatment alone. In sum, low-dose exposure to heavy ions (50 cGy) caused rapid cancellous bone loss in normally loaded mice and increased osteoclast numbers in hindlimb unloaded mice. In vitro irradiation also was more detrimental to osteoblastogenesis in bone marrow cells that were recovered from hindlimb unloaded mice compared to cells from normally loaded mice. Furthermore, irradiation in vitro stimulated osteoclast formation in a macrophage cell line (RAW264.7) in the presence of RANKL (25 ng/ml), showing that heavy-ion radiation can stimulate osteoclast differentiation even in the absence of osteoblasts. Thus heavy-ion radiation can acutely increase osteoclast numbers in cancellous tissue and, under conditions of musculoskeletal disuse, can enhance the sensitivity of bone cells, in particular osteoprogenitors, to the effects of radiation.

Unipedal standing exercise and hip bone mineral density in postmenopausal women: a randomized controlled trial

The aim of this study was to test the effect of unipedal standing exercise on bone mineral density (BMD) of the hip in postmenopausal women. Japanese postmenopausal women (n = 94) were assigned at random to an exercise or control group (no exercise). The 6-month exercise program consisted of standing on a single foot for 1 min per leg 3 times per day. BMD of the hip was measured by dual-energy X-ray absorptiometry. There was no significant difference in age and baseline hip BMD between the exercise group (n = 49) and control group (n = 45). Exercise did not improve hip BMD compared with the control group. Stepwise regression analysis identified old age as a significant determinant (p = 0.034) of increased hip total BMD at 6 months after exercise. In 31 participants aged >/=70 years, the exercise group (n = 20) showed significant increase in the values of hip BMD at the areas of total (p = 0.008), intertrochanteric (p = 0.023), and Ward's triangle (p = 0.032). The same parameters were decreased in the control group (n = 11). The percent changes in hip BMD of the exercise group were not significantly different from those of the control group either in the participants with low baseline hip total BMD (<80% of the young adult mean) or high baseline hip total BMD (> or =80% of the young adult mean). In conclusion, unipedal standing exercise for 6 months did not improve hip BMD in Japanese postmenopausal women. Effect of exercise on hip total BMD was age dependent. In participants aged > or =70 years, the exercise significantly increased hip total BMD.

Influence of ovariectomy and masticatory hypofunction on mandibular bone remodeling

INTRODUCTION

This study was designed to examine the effect of masticatory hypofunction and estrogen deficiency on mandible bone mass and compare this site with spine and femoral bone.

METHODS

Twenty-four rats were ovariectomized (OVX) or Sham-operated (Sham) and analyzed after feeding with hard diet (Hard) or soft diet (Soft). They were divided into four groups: (GI)Sham-Hard; (GII)OVX-Hard; (GIII)Sham-Soft and (GIV)OVX-Soft. Bone mineral density (BMD) was measured in the spine and femur in the baseline and at the end of the study, and DeltaBMD (final BMD - baseline BMD) was calculated. In mandible bone, BMD and histomorphometry were analyzed at the end of the experiment.

RESULTS

Sham rats showed higher spine (GI: 13.5%vs GII: 0.74%, P < 0.01; GIII: 10.67%vs GIV: -4.36%, P < 0.001) and femur DeltaBMD (GI: 14.43%vs GII: 4.42%, P < 0.01; GIII: 10.58%vs GIV: 0.49%, P < 0.001) than OVX, but no difference was observed in mandible BMD among these groups (P > 0.05). Soft-diet groups showed decreased mandible BMD compared with hard-diet groups (GIV vs GII, P < 0.01; GIII vs GI, P < 0.01). Similarly, mandibular condyle histomorphometry showed that soft-diet groups presented a significant decrease in trabecular thickness and volume (GIV vs GII, P < 0.05; GIII vs GI, P < 0.01) compared with hard diet.

CONCLUSION

Our results suggest that mandibular bone loss resulted from decreased of mechanical loading during mastication, and was not affect by estrogen depletion.

Association of unipedal standing time and bone mineral density in community-dwelling Japanese women

Bone mineral density (BMD) and physical performance of the lower extremities decrease with age. In community-dwelling Japanese women, unipedal standing time, timed up and go test, and age are associated with BMD while in women aged 70 years and over, unipedal standing time is associated with BMD.

INTRODUCTION

The aim of this study was to clarify whether unipedal standing time is significantly associated with BMD in community-dwelling women.

METHODS

The subjects were 90 community-dwelling Japanese women aged 54.7 years. BMD of the second metacarpal bone was measured by computed X-ray densitometry. We measured unipedal standing time as well as timed up and go test to assess physical performance of the lower extremities.

RESULTS

Unipedal standing time decreased with increased age. Timed up and go test significantly correlated with age. Low BMD was significantly associated with old age, short unipedal standing time, and long timed up and go test. Stepwise regression analysis revealed that age, unipedal standing time, and timed up and go test were significant factors associated with BMD. In 21 participants aged 70 years and over, body weight and unipedal standing time, but not age, were significantly associated with BMD.

CONCLUSION

BMD and physical performance of the lower extremities decrease with older age. Unipedal standing time, timed up and go test, and age are associated with BMD in community-dwelling Japanese women. In women aged 70 years and over, unipedal standing time is significantly associated with BMD.

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.

Jump exercise during remobilization restores integrity of the trabecular architecture after tail suspension in young rats

Three-dimensional trabecular architecture was investigated in the femora of tail-suspended young growing rats, and the effects of jump exercise during remobilization were examined. Five-week-old male Wistar rats (n = 35) were randomly assigned to five body weight-matched groups: tail-suspended group (SUS; n = 7); sedentary control group for SUS (S(CON); n = 7); spontaneous recovery group after tail suspension (S+R(CON), n = 7); jump exercise group after tail suspension (S+R(JUM); n = 7); and age-matched control group for S+R(CON) and S+R(JUM) without tail suspension and exercise (S(CON)+R(CON); n = 7). Rats in SUS and S(CON) were killed immediately after tail suspension for 14 days. The jump exercise protocol consisted of 10 jumps/day, 5 days/wk, and jump height was 40 cm. Bone mineral density (BMD) of the femur and three-dimensional trabecular bone architecture at the distal femoral metaphysis were measured. Tail suspension induced a 13.6% decrease in total femoral BMD (P < 0.001) and marked deterioration of trabecular architecture. After 5 wk of free remobilization, femoral BMD, calf muscle weight, and body weight returned to age-matched control levels, but trabeculae remained thinner and less connected. On the other hand, S+R(JUM) rats showed significant increases in trabecular thickness, number, and connectivity compared with S+R(CON) rats (62.8, 31.6, and 24.7%, respectively; P < 0.05), and these parameters of trabecular architecture returned to the levels of S(CON)+R(CON). These results indicate that suspension-induced trabecular deterioration persists after remobilization, but jump exercise during remobilization can restore the integrity of trabecular architecture and bone mass in the femur in young growing rats.

Blockade of beta-adrenergic signaling does not influence the bone mechano-adaptive response in mice

The involvement of the sympathetic nervous system (SNS) in the modulation of bone adaptation to its load-bearing demand remains controversial. This study tested the involvement of SNS in the adaptive response of trabecular and cortical bone to either external loading or disuse. External loading consisted of cyclic strain (40 cycles, peak 1500 microstrain) applied for 7 min, 3 days/week, while disuse was induced by unilateral sciatic neurectomy (SN). C57Bl/J6 mice, female, 9 weeks old, were subjected to loading or disuse for 2 weeks. Half of the loaded and SN mice were injected with the beta-adrenergic antagonist, propranolol (PRO, 20 mug/g) 1 week before the start of loading or disuse and during all the duration of the experiment. MicroCT analysis of the tibiae showed that the applied load induced significant changes on both trabecular architecture and cortical geometry compared to the contralateral controls, indicating increased bone mass. In contrast, disuse markedly reduced trabecular and cortical indexes. However, these adaptive responses were not altered by PRO treatment. We further tested whether the lack of protective effect of PRO against disuse-induced bone loss was due to the very short duration of treatment by blocking SNS signaling for 8 weeks with either PRO (0.5 mg/ml in drinking water) or guanethidine sulfate (GS, 40 mug/g, injected). At the end of fourth week of treatment, mice underwent SN surgery so that disuse was induced for the remaining 4 weeks. Again, neither PRO nor GS treatments altered the disuse-induced bone loss in the neurectomized tibia. In addition, blockade of SNS signaling for either 3 or 8 weeks did not affect the basal trabecular bone architecture in control tibiae and in L4 vertebrae. This study shows that the mechano-adaptive response occurring in trabecular and cortical bone upon loading or disuse is not altered by inactivation of beta-adrenergic signaling. Furthermore, sympathectomy had no effect on trabecular bone at different skeletal sites. This suggests that the osteo-regulatory action of beta-adrenergic signaling is not involved in the bone mechano-adaptive response and must therefore affect other bone regulatory pathways.