Comparing histological, vascular and molecular responses associated with woven and lamellar bone formation induced by mechanical loading in the rat ulna

Effect of locally delivered protein complex-loaded nanoparticles on bone remodelling of atrophic alveolar ridge in beagles

OBJECTIVE

To investigate the effect of local injection of mineralized hybrid nanoparticles loading dentin matrix protein-1 (DMP-1) and matrix metalloproteinase-13 (MMP-13) complex (P-NPs) on the bone remodelling on atrophic alveolar ridges (AAR) ahead of orthodontic tooth movement (OTM).

SETTINGS AND SAMPLE POPULATION

Four beagles were randomly allocated into Group C (OTM only) and Group NP (OTM with P-NPs injection). Experimental model of AAR was prepared in 8 mandibular quadrants after extraction of the third premolars (n = 4 per Group).

MATERIALS AND METHODS

Reciprocal traction of the second and fourth premolars was performed towards AAR for 8 weeks. P-NPs were prepared by loading recombinant DMP-1 and MMP-13 complex into calcium carbonate (CaCO3 )-mineralized hybrid nanoparticles and injected at 0, 3 and 6 weeks. The rate of OTM and the bone remodelling characteristics were compared between Groups using fluorescent microscopic analysis and microstructural histomorphometric analysis.

RESULTS

Group NP revealed higher bone volume fraction and higher trabecular ratio with lower bone mineral density than Group C on AAR area. Meanwhile, the root movement towards AAR was facilitated in Group NP representing more bodily movement than Group C.

CONCLUSION

Non-invasive intervention of P-NPs injection suggested a clinical potential to facilitate translational movement into the AAR with sustaining woven bone-like microstructural environment.

Fracture Healing Research-Shift towards In Vitro Modeling?

Fractures are one of the most frequently occurring traumatic events worldwide. Approximately 10% of fractures lead to bone healing disorders, resulting in strain for affected patients and enormous costs for society. In order to shed light into underlying mechanisms of bone regeneration (habitual or disturbed), and to develop new therapeutic strategies, various in vivo, ex vivo and in vitro models can be applied. Undeniably, in vivo models include the systemic and biological situation. However, transferability towards the human patient along with ethical concerns regarding in vivo models have to be considered. Fostered by enormous technical improvements, such as bioreactors, on-a-chip-technologies and bone tissue engineering, sophisticated in vitro models are of rising interest. These models offer the possibility to use human cells from individual donors, complex cell systems and 3D models, therefore bridging the transferability gap, providing a platform for the introduction of personalized precision medicine and finally sparing animals. Facing diverse processes during fracture healing and thus various scientific opportunities, the reliability of results oftentimes depends on the choice of an appropriate model. Hence, we here focus on categorizing available models with respect to the requirements of the scientific approach.

Cortical Thickness Adaptive Response to Mechanical Loading Depends on Periosteal Position and Varies Linearly With Loading Magnitude

The aim of the current study was to quantify the local effect of mechanical loading on cortical bone formation response at the periosteal surface using previously obtained μCT data from a mouse tibia mechanical loading study. A novel image analysis algorithm was developed to quantify local cortical thickness changes (ΔCt.Th) along the periosteal surface due to different peak loads (0N ≤ F ≤ 12N) applied to right-neurectomised mature female C57BL/6 mice. Furthermore, beam analysis was performed to analyse the local strain distribution including regions of tensile, compressive, and low strain magnitudes. Student's paired t-test showed that ΔCt.Th in the proximal (25%), proximal/middle (37%), and middle (50%) cross-sections (along the z-axis of tibia) is strongly associated with the peak applied loads. These changes are significant in a majority of periosteal positions, in particular those experiencing high compressive or tensile strains. No association between F and ΔCt.Th was found in regions around the neutral axis. For the most distal cross-section (75%), the association of loading magnitude and ΔCt.Th was not as pronounced as the more proximal cross-sections. Also, bone formation responses along the periosteum did not occur in regions of highest compressive and tensile strains predicted by beam theory. This could be due to complex experimental loading conditions which were not explicitly accounted for in the mechanical analysis. Our results show that the bone formation response depends on the load magnitude and the periosteal position. Bone resorption due to the neurectomy of the loaded tibia occurs throughout the entire cross-sectional region for all investigated cortical sections 25, 37, 50, and 75%. For peak applied loads higher than 4 N, compressive and tensile regions show bone formation; however, regions around the neutral axis show constant resorption. The 50% cross-section showed the most regular ΔCt.Th response with increased loading when compared to 25 and 37% cross-sections. Relative thickness gains of approximately 70, 60, and 55% were observed for F = 12 N in the 25, 37, and 50% cross-sections. ΔCt.Th at selected points of the periosteum follow a linear response with increased peak load; no lazy zone was observed at these positions.

Effect of fatigue loading and rest on impact strength of rat ulna

Stress fracture is a common injury among athletes and military personnel and is associated with fatigue-initiated damage and impact loading. The recovery of bending strength has been shown to be a function of the rest days allowed after fatigue loading in rodents and the aim of this study was to investigate if similar results would occur under impact conditions. In this study, cyclic axial compression load was applied in vivo on the right forelimbs while left forelimbs served as controls. Two rest groups were used: one day of rest and seven days of rest. Afterwards, all ulnae were scanned using micro-Computed Tomography followed by impact testing. The micro-CT scan confirmed the formation of woven bone on loaded ulnae after seven days rest. The peak impact force was 37.5% higher in the control (mean = 174.96 ± 33.25 N) specimens compared to the loaded bones (mean = 130.34 ± 22.37 N). Fourier-transformed infrared spectroscopy analyses suggested no significant change of chemical composition in the cortical region between the loaded and control ulnae, but woven bone region had lower carbonate and amide I content than contralateral controls (p < 0.05). We find that cyclic fatigue loading had a negative effect on bone's impact response. Bones that experienced fatigue loading became less stiff, weaker, and more prone to fracture when subjected to impact. The formation of woven bone after seven days of rest did not restore the stiffness upon impact and confirm that rest time is crucial to the recovery of fatigue damage.

Proliferating osteoblasts are necessary for maximal bone anabolic response to loading in mice

Following mechanical loading, osteoblasts may arise via activation, differentiation, or proliferation to form bone. Our objective was to ablate proliferating osteoblast lineage cells in order to investigate the importance of these cells as a source for loading-induced bone formation. We utilized 3.6Col1a1-tk mice in which replicating osteoblast lineage cells can be ablated in an inducible manner using ganciclovir (GCV). Male and female mice were aged to 5- and 12-months and subjected to 5 days of tibial compression. "Experimental" mice were tk-positive, treated with GCV; "control" mice were either tk-negative treated with GCV, or tk-positive treated with PBS. We confirmed that experimental mice had a decrease in tk-positive cells that arose from proliferation. Next, we assessed bone formation after loading to low (7N) and high (11N) forces and observed that periosteal bone formation rate in experimental mice was reduced by approximately 70% for both forces. Remarkably, woven bone formation induced by high-force loading was blocked in experimental mice. Loading-induced lamellar bone formation was diminished but not prevented in experimental mice. We conclude that osteoblast proliferation induced by mechanical loading is a critical source of bone forming osteoblasts for maximal lamellar formation and is essential for woven bone formation.

Indian Hedgehog regulates senescence in bone marrow-derived mesenchymal stem cell through modulation of ROS/mTOR/4EBP1, p70S6K1/2 pathway

Premature senescence of bone marrow-derived mesenchymal stem cells (BMSC) remains a major concern for their application clinically. Hedgehog signaling has been reported to regulate aging-associated markers and MSC skewed differentiation. Indian Hedgehog (IHH) is a ligand of Hedgehog intracellular pathway considered as an inducer in chondrogenesis of human BMSC. However, the role of IHH in the aging of BMSC is still unclear. This study explored the role IHH in the senescence of BMSC obtained from human samples and senescent mice. Isolated BMSC were transfected with IHH siRNA or incubated with exogenous IHH protein and the mechanisms of aging and differentiation investigated. Moreover, the interactions between IHH, and mammalian target of rapamycin (mTOR) and reactive oxygen species (ROS) were evaluated using the corresponding inhibitors and antioxidants. BMSC transfected with IHH siRNA showed characteristics of senescence-associated features including increased senescence-associated β-galactosidase activity (SA-β-gal), induction of cell cycle inhibitors (p53/p16), development of senescence-associated secretory phenotype (SASP), activation of ROS and mTOR pathways as well as the promotion of skewed differentiation. Interestingly, BMSC treatment with IHH protein reversed the senescence markers and corrected biased differentiation. Moreover, IHH shortage-induced senescence signs were compromised after mTOR and ROS inhibition. Our findings presented anti-aging activity for IHH in BMSC through down-regulation of ROS/mTOR pathways. This discovery might contribute to increasing the therapeutic, immunomodulatory and regenerative potency of BMSC and introduce a novel remedy in the management of aging-related diseases.

Murine Axial Compression Tibial Loading Model to Study Bone Mechanobiology: Implementing the Model and Reporting Results

In vivo, tibial loading in mice is increasingly used to study bone adaptation and mechanotransduction. To achieve standardized and defined experimental conditions, loading parameters and animal-related factors must be considered when performing in vivo loading studies. In this review, we discuss these loading and animal-related experimental conditions, present methods to assess bone adaptation, and suggest reporting guidelines. This review originated from presentations by each of the authors at the workshop "Developing Best Practices for Mouse Models of In Vivo Loading" during the Preclinical Models Section at the Orthopaedic Research Society Annual Meeting, San Diego, CA, March 2017. Following the meeting, the authors engaged in detailed discussions with consideration of relevant literature. The guidelines and recommendations in this review are provided to help researchers perform in vivo loading experiments in mice, and thus further our knowledge of bone adaptation and the mechanisms involved in mechanotransduction. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:233-252, 2020.

Rat bone properties and their relationship to gait during growth

Allometric relationships have been studied over different Orders of mammals to understand how bone accommodates the mechanical demands associated with increasing mass. However, less attention has been given to the scaling of bone within a single lifetime. We aimed to determine how bone morphology and tissue density are related to (1) bending and compressive strength, and (2) gait dynamics. Longitudinal in vivo computed tomography of the hindlimbs and gait data were collected from female rats (n=5, age 8-20 weeks). Cross-sectional properties and tissue density were measured at the diaphysis, distal and proximal regions of the tibia and scaling exponents were calculated. Finite element models of the tibia were used to simulate loading during walking using joint forces from inverse dynamics calculation to determine the strain energy density and longitudinal strain at the midshaft. Second moment of area at the diaphysis followed strain similarity-based allometry, while bone area trended towards positive allometry. Strain energy in the diaphysis under transverse loading was lower than axial loading throughout growth. While both axial and transverse loading resulted in bending, tensile strains were mitigated by a change in the neutral axis and resulted in overall lower longitudinal tensile strains. The tissue density and cross-sectional properties initially increased and converged by 11 weeks of age and were correlated with changes in ground reaction forces. The scaling analyses imply that rodent tibia is (re)modeled in order to sustain bending at the midshaft during growth. The finite element results and relatively constant density after 10 weeks of age indicate that structural parameters may be the primary determinant of bone strength in the growing rodent tibia. The correlations between bone properties and joint angles imply that the changes in posture may affect bone growth in specific regions.

Individual response variations in scaffold-guided bone regeneration are determined by independent strain- and injury-induced mechanisms

This study explored the regenerative osteogenic response in the distal femur of sheep using scaffolds having stiffness values within, and above and below, the range of trabecular bone apparent modulus. Scaffolds 3D-printed from stiff titanium and compliant polyamide were implanted into a cylindrical metaphyseal defect 15 × 15 mm. After six weeks, bone ingrowth varied between 7 and 21% of the scaffold pore volume and this was generally inversely proportional to scaffold stiffness. The individual reparative response considerably varied among the animals, which could be divided into weak and strong responders. Notably, bone regeneration specifically within the interior of the scaffold was inversely proportional to scaffold stiffness and was strain-driven in strongly-responding animals. Conversely, bone regeneration at the periphery of the defect was injury-driven and equal in all scaffolds and in all strongly- and weakly-responding animals. The observation of the strain-driven response in some, but not all, animals highlights that scaffold compliance is desirable for triggering host bone regeneration, but scaffold permanence is important for the load-bearing, structural role of the bone-replacing device. Indeed, scaffolds may benefit from being nonresorbable and mechanically reliable for those unforeseeable cases of weakly responding recipients.

The Interaction of BMP2-Induced Defect Healing in Rat and Fixator Stiffness Modulates Matrix Alignment and Contraction

Successful fracture healing requires a tight interplay between mechanical and biological cues. In vitro studies illustrated that mechanical loading modulates bone morphogenetic protein (BMP) signaling. However, in the early phases of large bone defect regeneration in vivo, the underlying mechanisms leading to this mechanosensation remained unknown. We investigated the interaction of BMP2 stimulation and mechanical boundary conditions in a rat critical‐sized femoral defect model (5 mm) stabilized with three distinctly different external fixator stiffness. Defects were treated with 5 μg rhBMP2 loaded on an absorbable collagen sponge. Early matrix alignment was monitored by second‐harmonic generation imaging. Bony bridging of defects and successive healing was monitored by histology at day 7 and day 14 as well as in vivo microCT at days 10, 21, and 42 post‐operation. Femora harvested at day 42 were characterized mechanically assessing torsional load to failure ex vivo. At tissue level, differences between groups were visible at day 14 with manifest bone formation in the microCT. Histologically, we observed prolonged chondrogenesis upon flexible fixation, whereas osteogenesis started earlier after rigid and semirigid fixation. At later time points, there was a boost of bone tissue formation upon flexible fixation, whereas other groups already displayed signs of tissue maturation. Based on gene expression profiling, we analyzed the mechanobiological interplay. Already at day 3, these analyses revealed differences in expression pattern, specifically of genes involved in extracellular matrix formation. Gene regulation correlating with fixator stiffness was pronounced at day 7 comprising genes related to immunological processes and cellular contraction. The influence of loading on matrix contraction was further investigated and confirmed in a 3D bioreactor. Taken together, we demonstrate an early onset of mechanical conditions influencing BMP2‐induced defect healing and shed light on gene regulatory networks associated with extracellular matrix organization and contraction that seemed to directly impact healing outcomes. © 2018 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Temporal mechanically-induced signaling events in bone and dorsal root ganglion neurons after in vivo bone loading

Mechanical signals play an integral role in the regulation of bone mass and functional adaptation to bone loading. The osteocyte has long been considered the principle mechanosensory cell type in bone, although recent evidence suggests the sensory nervous system may play a role in mechanosensing. The specific signaling pathways responsible for functional adaptation of the skeleton through modeling and remodeling are not clearly defined. In vitro studies suggest involvement of intracellular signaling through mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), and mammalian target of rapamycin (mTOR). However, anabolic signaling responses to bone loading using a whole animal in vivo model have not been studied in detail. Therefore, we examined mechanically-induced signaling events at five time points from 0 to 24 hours after loading using the rat in vivo ulna end-loading model. Western blot analysis of bone for MAPK's, PI3K/Akt, and mTOR signaling, and quantitative reverse transcription polymerase chain reaction (qRT-PCR) to estimate gene expression of calcitonin gene-related protein alpha (CGRP-α), brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), c-jun, and c-fos in dorsal root ganglion (DRG) of the brachial intumescence were performed. There was a significant increase in signaling through MAPK's including extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase (JNK) in loaded limbs at 15 minutes after mechanical loading. Ulna loading did not significantly influence expression of the genes of interest in DRG neurons. Bone signaling and DRG gene expression from the loaded and contralateral limbs was correlated (SR>0.40, P<0.05). However, bone signaling did not correlate with expression of the genes of interest in DRG neurons. These results suggest that signaling through the MAPK pathway may be involved in load-induced bone formation in vivo. Further characterization of the molecular events involved in regulation of bone adaptation is needed to understand the timing and impact of loading events, and the contribution of the neuronal signaling to functional adaptation of bone.

Exogenous hedgehog antagonist delays but does not prevent fracture healing in young mice

Fracture healing recapitulates many aspects of developmental osteogenesis. The hedgehog (Hh) signaling pathway, essential to skeletal development, is upregulated during fracture healing, although its importance is unclear. Our goal was to assess the functional importance of Hh signaling in endochondral fracture healing. We created closed, transverse diaphyseal femur fractures in mice, stabilized with an intramedullary pin, and administered a systemic Hh inhibitor or vehicle. Because Hh pathway activation is mediated by the receptor Smoothened (Smo), we used the Smo antagonist GDC-0449 (GDC, 50mg/kg, twice daily) to target the pathway. First, in vehicle-treated 10-wk. female C57BL/6 mice we confirmed that Hh signaling was increased in fracture callus compared to intact bone, with >5-fold upregulation of target genes Ptch1 and Gli1. Additionally, using 10-wk. male and female Gli1 reporter mice, we saw a strong activation of the reporter in the osseous regions of the fracture callus 7-10days after fracture. GDC treatment significantly blunted these responses, indicating effective inhibition of fracture-induced Hh signaling in bone. Moreover, microCT analysis revealed that GDC treatment significantly reduced cancellous and cortical bone volume at non-fracture sites (tibial metaphysis and diaphysis), suggesting that the drug inhibited normal bone formation. GDC treatment had a modest effect on fracture healing, with evidence of delayed callus mineralization radiographically (significantly lower Goldberg score at day 14) and by microCT (reduced callus vBMD at 14days), and a delay in the recovery of torsional rotation to normal (elevated rotation-at-peak torque at 21days). On the other hand, GDC treatment did not inhibit qPCR or morphological measures of chondrogenesis or angiogenesis, and did not impair the recovery of failure torque (at day 14 or 21), a measure of biomechanical competence. In summary, GDC treatment inhibited Hh signaling, which delayed but did not prevent fracture healing in young mice. We conclude that Hh signaling is strongly induced after fracture and may play a role in early callus mineralization, although it does not appear to be required for eventual healing.

Parathyroid hormone's enhancement of bones' osteogenic response to loading is affected by ageing in a dose- and time-dependent manner

Decreased effectiveness of bones' adaptive response to mechanical loading contributes to age-related bone loss. In young mice, intermittent administration of parathyroid hormone (iPTH) at 20-80μg/kg/day interacts synergistically with artificially applied loading to increase bone mass. Here we report investigations on the effect of different doses and duration of iPTH treatment on mice whose osteogenic response to artificial loading is impaired by age. One group of aged, 19-month-old female C57BL/6 mice was given 0, 25, 50 or 100μg/kg/day iPTH for 4weeks. Histological and μCT analysis of their tibiae revealed potent iPTH dose-related increases in periosteally-enclosed area, cortical area and porosity with decreased cortical thickness. There was practically no effect on trabecular bone. Another group was given a submaximal dose of 50μg/kg/day iPTH or vehicle for 2 or 6weeks with loading of their right tibia three times per week for the final 2weeks. In the trabecular bone of these mice the loading-related increase in BV/TV was abrogated by iPTH primarily by reduction of the increase in trabecular number. In their cortical bone, iPTH treatment time-dependently increased cortical porosity. Loading partially reduced this effect. The osteogenic effects of iPTH and loading on periosteally-enclosed area and cortical area were additive but not synergistic. Thus in aged, unlike young mice, iPTH and loading appear to have separate effects. iPTH alone causes a marked increase in cortical porosity which loading reduces. Both iPTH and loading have positive effects on cortical periosteal bone formation but these are additive rather than synergistic.

Post-Traumatic Osteoarthritis in Mice Following Mechanical Injury to the Synovial Joint

We investigated the spectrum of lesions characteristic of post-traumatic osteoarthritis (PTOA) across the knee joint in response to mechanical injury. We hypothesized that alteration in knee joint stability in mice reproduces molecular and structural features of PTOA that would suggest potential therapeutic targets in humans. The right knees of eight-week old male mice from two recombinant inbred lines (LGXSM-6 and LGXSM-33) were subjected to axial tibial compression. Three separate loading magnitudes were applied: 6N, 9N, and 12N. Left knees served as non-loaded controls. Mice were sacrificed at 5, 9, 14, 28, and 56 days post-loading and whole knee joint changes were assessed by histology, immunostaining, micro-CT, and magnetic resonance imaging. We observed that tibial compression disrupted joint stability by rupturing the anterior cruciate ligament (except for 6N) and instigated a cascade of temporal and topographical features of PTOA. These features included cartilage extracellular matrix loss without proteoglycan replacement, chondrocyte apoptosis at day 5, synovitis present at day 14, osteophytes, ectopic calcification, and meniscus pathology. These findings provide a plausible model and a whole-joint approach for how joint injury in humans leads to PTOA. Chondrocyte apoptosis, synovitis, and ectopic calcification appear to be targets for potential therapeutic intervention.

A comparison of different compressive forces on graft materials during alveolar ridge preservation

Purpose

Following tooth extraction, alveolar ridge preservation (ARP) can maintain the dimensions of ridge height and width. Although previous studies have demonstrated the effects of ARP, few if any studies have investigated the compressive force applied during grafting. The aim of this study was to determine the effects of different compressive forces on the graft materials during ARP.

Methods

After tooth extraction, sockets were filled with deproteinized bovine bone mineral with 10% porcine collagen and covered by a resorbable collagen membrane in a double-layered fashion. The graft materials were compressed using a force of 5 N in the test group (n=12) and a force of 30 N in the control group (n=12). A hidden X suture was performed to secure the graft without primary closure. Cone-beam computed tomography (CBCT) was performed immediately after grafting and 4 months later, just before implant surgery. Tissue samples were retrieved using a trephine bur from the grafted sites during implant surgery for histologic and histomorphometric evaluations. Periotest values (PTVs) were measured to assess the initial stability of the dental implants.

Results

Four patients dropped out from the control group and 20 patients finished the study. Both groups healed without any complications. The CBCT measurements showed that the ridge volume was comparably preserved vertically and horizontally in both groups (P>0.05). Histomorphometric analysis demonstrated that the ratio of new bone formation was significantly greater in the test group (P<0.05). The PTVs showed no significant differences between the 2 groups (P>0.05).

Conclusions

The application of a greater compressive force on biomaterials during ARP significantly enhanced new bone formation while preserving the horizontal and vertical dimensions of the alveolar ridge. Further studies are required to identity the optimal compressive force for ARP.

Calvarial Defect Healing Induced by Small Molecule Smoothened Agonist

Hedgehog (Hh) signaling positively regulates both endochondral and intramembranous ossification. Use of small molecules for tissue engineering applications poses several advantages. In this study, we examined whether use of an acellular scaffold treated with the small molecule Smoothened agonist (SAG) could aid in critical-size mouse calvarial defect repair. First, we verified the pro-osteogenic effect of SAG in vitro, using primary neonatal mouse calvarial cells (NMCCs). Next, a 4 mm nonhealing defect was created in the mid-parietal bone of 10-week-old CD-1 mice. The scaffold consisted of a custom-fabricated poly(lactic-co-glycolic acid) disc with hydroxyapatite coating (measuring 4 mm diameter × 0.5 mm thickness). Treatment groups included dimethylsulfoxide control (n = 6), 0.5 mM SAG (n = 7) or 1.0 mM SAG (n = 7). Evaluation was performed at 4 and 8 weeks postoperative, by a combination of high-resolution microcomputed tomography, histology (H & E, Masson's Trichrome), histomorphometry, and immunohistochemistry (BSP, OCN, VEGF). In vivo results showed that SAG treatment induced a significant and dose-dependent increase in calvarial bone healing by all radiographic parameters. Histomorphometric analysis showed an increase in all parameters of bone formation with SAG treatment, but also an increase in blood vessel number and density. In summary, SAG is a pro-osteogenic, provasculogenic stimulus when applied locally in a bone defect environment.

miRNA-29b improves bone healing in mouse fracture model

A number of miRNAs regulates bone remodeling and their levels in circulation were associated with bone fracture, however no miRNAs have yet been shown to improve fracture healing directly. This study aimed to investigate the effect of miR-29b-3p on mice femoral fracture healing through site-specific delivery with microbubble-ultrasound system. miR-29b-3p promoted osteogenesis of mouse bone marrow-derived mesenchymal stem cells as indicated with quantitative real-time polymerase chain reaction (qPCR) and Alizarin red S staining. Animal study showed that single injection of miR-29b-3p at week 2 post fracture improved healing outcome as indicated by significant decrease of callus width and area with radiographic analysis without causing significant weight loss. Static bone histomorphometry analysis showed that miR-29b-3p increased bone volume fraction (BV/TV), and micro-computed tomography (micro-CT) measurement showed increased BV/TV of high density bone and bone mineral density (BMD) of the callus. 3 point bending mechanical test showed improved relative stiffness. However, repeated injection of miR-29b-3p at weeks 2 and 3 did not result in additive therapeutic outcome, and caused increased total tissue volume and reduced BMD of the callus. This is the first report showing significant therapeutic effect of miR-29b-3p on femoral fracture healing through site-specific delivery with microbubble-ultrasound system. Further studies are warranted to investigate the underlying mechanisms and to refine the treatment protocol.

Exercise does not enhance aged bone's impaired response to artificial loading in C57Bl/6 mice

Bones adapt their structure to their loading environment and so ensure that they become, and are maintained, sufficiently strong to withstand the loads to which they are habituated. The effectiveness of this process declines with age and bones become fragile fracturing with less force. This effect in humans also occurs in mice which experience age-related bone loss and reduced adaptation to loading. Exercise engenders many systemic and local muscular physiological responses as well as engendering local bone strain. To investigate whether these physiological responses influence bones' adaptive responses to mechanical strain we examined whether a period of treadmill exercise influenced the adaptive response to an associated period of artificial loading in young adult (17-week) and old (19-month) mice. After treadmill acclimatization, mice were exercised for 30 min three times per week for two weeks. Three hours after each exercise period, right tibiae were subjected to 40 cycles of non-invasive axial loading engendering peak strain of 2250 με. In both young and aged mice exercise increased cross-sectional muscle area and serum sclerostin concentration. In young mice it also increased serum IGF1. Exercise did not affect bone's adaptation to loading in any measured parameter in young or aged bone. These data demonstrate that a level of exercise sufficient to cause systemic changes in serum, and adaptive changes in local musculature, has no effect on bone's response to loading 3h later. This study provides no support for the beneficial effects of exercise on bone in the elderly being mediated by systemic or local muscle-derived effects rather than local adaptation to altered mechanical strain.

Hedgehog signaling mediates woven bone formation and vascularization during stress fracture healing

Hedgehog (Hh) signaling is critical in developmental osteogenesis, and recent studies suggest it may also play a role in regulating osteogenic gene expression in the post-natal setting. However, there is a void of studies directly assessing the effect of Hh inhibition on post-natal osteogenesis. This study utilized a cyclic loading-induced ulnar stress fracture model to evaluate the hypothesis that Hh signaling contributes to osteogenesis and angiogenesis during stress fracture healing. Immediately prior to loading, adult rats were given GDC-0449 (Vismodegib - a selective Hh pathway inhibitor; 50mg/kg orally twice daily), or vehicle. Hh signaling was upregulated in response to stress fracture at 3 days (Ptch1, Gli1 expression), and was markedly inhibited by GDC-0449 at 1 day and 3 days in the loaded and non-loaded ulnae. GDC-0449 did not affect Hh ligand expression (Shh, Ihh, Dhh) at 1 day, but decreased Shh expression by 37% at 3 days. GDC-0449 decreased woven bone volume (-37%) and mineral density (-17%) at 7 days. Dynamic histomorphometry revealed that the 7 day callus was composed predominantly of woven bone in both groups. The observed reduction in woven bone occurred concomitantly with decreased expression of Alpl and Ibsp, but was not associated with differences in early cellular proliferation (as determined by callus PCNA staining at 3 days), osteoblastic differentiation (Osx expression at 1 day and 3 days), chondrogenic gene expression (Acan, Sox9, and Col2α1 expression at 1 day and 3 days), or bone resorption metrics (callus TRAP staining at 3 days, Rankl and Opg expression at 1 day and 3 days). To evaluate angiogenesis, vWF immunohistochemistry showed that GDC-0449 reduced fracture callus blood vessel density by 55% at 3 days, which was associated with increased Hif1α gene expression (+30%). Dynamic histomorphometric analysis demonstrated that GDC-0449 also inhibited lamellar bone formation. Lamellar bone analysis of the loaded limb (directly adjacent to the woven bone callus) showed that GDC-0449 significantly decreased mineral apposition rate (MAR) and bone formation rate (BFR/BS) (-17% and -20%, respectively). Lamellar BFR/BS in the non-loaded ulna was also significantly decreased (-37%), indicating that Hh signaling was required for normal bone modeling. In conclusion, Hh signaling plays an important role in post-natal osteogenesis in the setting of stress fracture healing, mediating its effects directly through regulation of bone formation and angiogenesis.

Bmp2 conditional knockout in osteoblasts and endothelial cells does not impair bone formation after injury or mechanical loading in adult mice

Post-natal osteogenesis after mechanical trauma or stimulus occurs through either endochondral healing, intramembranous healing or lamellar bone formation. Bone morphogenetic protein 2 (BMP2) is up-regulated in each of these osteogenic processes and is expressed by a variety of cells including osteoblasts and vascular cells. It is known that genetic knockout of Bmp2 in all cells or in osteo-chondroprogenitor cells completely abrogates endochondral healing after full fracture. However, the importance of BMP2 from differentiated osteoblasts and endothelial cells is not known. Moreover, the importance of BMP2 in non-endochondral bone formation such as intramembranous healing or lamellar bone formation is not known. Using inducible and tissue-specific Cre-lox mediated targeting of Bmp2 in adult (10-24 week old) mice, we assessed the role of BMP2 expression globally, by osteoblasts, and by vascular endothelial cells in endochondral healing, intramembranous healing and lamellar bone formation. These three osteogenic processes were modeled using full femur fracture, ulnar stress fracture, and ulnar non-damaging cyclic loading, respectively. Our results confirmed the requirement of BMP2 for endochondral fracture healing, as mice in which Bmp2 was knocked out in all cells prior to fracture failed to form a callus. Targeted deletion of Bmp2 in osteoblasts (osterix-expressing) or vascular endothelial cells (vascular endothelial cadherin-expressing) did not impact fracture healing in any way. Regarding non-endochondral bone formation, we found that BMP2 is largely dispensable for intramembranous bone formation after stress fracture and also not required for lamellar bone formation induced by mechanical loading. Taken together our results indicate that osteoblasts and endothelial cells are not a critical source of BMP2 in endochondral fracture healing, and that non-endochondral bone formation in the adult mouse is not as critically dependent on BMP2.

HIF-1α regulates bone formation after osteogenic mechanical loading

HIF-1 is a transcription factor typically associated with angiogenic gene transcription under hypoxic conditions. In this study, mice with HIF-1α deleted in the osteoblast lineage (ΔHIF-1α) were subjected to damaging or non-damaging mechanical loading known to produce woven or lamellar bone, respectively, at the ulnar diaphysis. By microCT, ΔHIF-1α mice produced significantly less woven bone than wild type (WT) mice 7days after damaging loading. This decrease in woven bone volume and extent was accompanied by a significant decrease in vascularity measured by immunohistochemistry against vWF. Additionally, osteocytes, rather than osteoblasts, appear to be the main bone cell expressing HIF-1α following damaging loading. In contrast, 10days after non-damaging mechanical loading, dynamic histomorphometry measurements demonstrated no impairment in loading-induced lamellar bone formation in ΔHIF-1α mice. In fact, both non-loaded and loaded ulnae from ΔHIF-1α mice had increased bone formation compared with WT ulnae. When comparing the relative increase in periosteal bone formation in loaded vs. non-loaded ulnae, it was not different between ΔHIF-1α mice and controls. There were no significant differences observed between WT and ΔHIF-1α mice in endosteal bone formation parameters. The increases in periosteal lamellar bone formation in ΔHIF-1α mice are attributed to non-angiogenic effects of the knockout. In conclusion, these results demonstrate that HIF-1α is a pro-osteogenic factor for woven bone formation after damaging loading, but an anti-osteogenic factor for lamellar bone formation under basal conditions and after non-damaging loading.

Periosteum: characteristic imaging findings with emphasis on radiologic-pathologic comparisons

The periosteum covers most bone structures. It has an outer fibrous layer and an inner cambial layer that exhibits osteogenic activity. The periosteum is a dynamic structure that plays a major role in bone modeling and remodeling under normal conditions. In several disorders such as infections, benign and malignant tumors, and systemic diseases, the osteogenic potential of the periosteum is stimulated and new bone is produced. The newly formed bone added onto the surface of the cortex adopts various configurations depending on the modalities and pace of bone production. Our aim here is to describe the anatomy, histology, and physiology of the periosteum and to review the various patterns of periosteal reaction with emphasis on relations between radiological and histopathological findings. A careful evaluation of the periosteal reaction and appearance of the underlying cortex, in combination with the MRI, clinical, and laboratory data, provides valuable information on lesion duration and aggressiveness, thereby assisting in the etiological diagnosis and optimizing patient management. A solid reaction strongly suggests a benign and slow-growing process that gives the bone enough time to wall off the lesion. Single lamellar reactions occur in acute and usually benign diseases. Multilamellar reactions are associated with intermediate aggressiveness and a growth rate close to the limit of the walling-off capabilities of the bone. Spiculated, interrupted, and complex combined reactions carry the worst prognosis, as they occur in the most aggressive and fast-growing diseases: the periosteum attempts to create new bone but is overwhelmed and may be breached.

Antagonizing the αv β3 integrin inhibits angiogenesis and impairs woven but not lamellar bone formation induced by mechanical loading

Angiogenesis and osteogenesis are critically linked, although the role of angiogenesis is not well understood in osteogenic mechanical loading. In this study, either damaging or non-damaging cyclic axial compression was used to generate woven bone formation (WBF) or lamellar bone formation (LBF), respectively, at the mid-diaphysis of the adult rat forelimb. αv β3 integrin-targeted nanoparticles or vehicle was injected intravenously after mechanical loading. β3 integrin subunit expression on vasculature was maximal 7 days after damaging mechanical loading, but was still robustly expressed 14 days after loading. Accordingly, targeted nanoparticle delivery in WBF-loaded limbs was increased compared with non-loaded limbs. Vascularity was dramatically increased after WBF loading (+700% on day 14) and modestly increased after LBF loading (+50% on day 14). This increase in vascularity was inhibited by nanoparticle treatment in both WBF- and LBF-loaded limbs at days 7 and 14 after loading. Decreased vascularity led to diminished woven, but not lamellar, bone formation. Decreased woven bone formation resulted in impaired structural properties of the skeletal repair, particularly in post-yield behavior. These results demonstrate that αv β3 integrin-mediated angiogenesis is critical for recovering fracture resistance after bone injury but is not required for bone modeling after modest mechanical strain. © 2014 American Society for Bone and Mineral Research.

Age-related impairment of bones' adaptive response to loading in mice is associated with sex-related deficiencies in osteoblasts but no change in osteocytes

Bones adjust their mass and architecture to be sufficiently robust to withstand functional loading by adapting to their strain environment. This mechanism appears less effective with age, resulting in low bone mass. In male and female young adult (17-week-old) and old (19-month-old) mice, we investigated the effect of age in vivo on bones' adaptive response to loading and in vitro in primary cultures of osteoblast-like cells derived from bone cortices. Right tibias were axially loaded on alternate days for 2 weeks. Left tibias were non-loaded controls. In a separate group, the number of sclerostin-positive osteocytes and the number of periosteal osteoblasts were analyzed 24 hours after a single loading episode. The responses to strain of the primary osteoblast-like cells derived from these mice were assessed by EGR2 expression, change in cell number and Ki67 immunofluorescence. In young male and female mice, loading increased trabecular thickness and the number of trabecular connections. Increase in the number of trabecular connections was impaired with age but trabecular thickness was not. In old mice, the loading-related increase in periosteal apposition of the cortex was less than in young ones. Age was associated with a lesser loading-related increase in osteoblast number on the periosteal surface but had no effect on loading-related reduction in the number of sclerostin-positive osteocytes. In vitro, strain-related proliferation of osteoblast-like cells was lower in cells from old than young mice. Cells from aged female mice demonstrated normal entry into the cell cycle but subsequently arrested in G2 phase, reducing strain-related increases in cell number. Thus, in both male and female mice, loading-related adaptive responses are impaired with age. This impairment is different in females and males. The deficit appears to occur in osteoblasts' proliferative responses to strain rather than earlier strain-related responses in the osteocytes.

Long bone structure and strength depend on BMP2 from osteoblasts and osteocytes, but not vascular endothelial cells

The importance of bone morphogenetic protein 2 (BMP2) in the skeleton is well known. BMP2 is expressed in a variety of tissues during development, growth and healing. In this study we sought to better identify the role of tissue-specific BMP2 during post-natal growth and to determine if BMP2 knockout affects the ability of terminally differentiated cells to create high quality bone material. We targeted BMP2 knockout to two differentiated cell types known to express BMP2 during growth and healing, early-stage osteoblasts and their progeny (osterix promoted Cre) and vascular endothelial cells (vascular-endothelial-cadherin promoted Cre). Our objectives were to assess post-natal bone growth, structure and strength. We hypothesized that removal of BMP2 from osteogenic and vascular cells (separately) would result in smaller skeletons with inferior bone material properties. At 12 and 24 weeks of age the osteoblast knockout of BMP2 reduced body weight by 20%, but the vascular knockout had no effect. Analysis of bone in the tibia revealed reductions in cortical and cancellous bone size and volume in the osteoblast knockout, but not in the vascular endothelial knockout. Furthermore, forelimb strength testing revealed a 30% reduction in ultimate force at both 12 and 24 weeks in the osteoblast knockout of BMP2, but no change in the vascular endothelial knockout. Moreover, mechanical strength testing of femurs from osteoblast knockout mice demonstrated an increased Young's modulus (greater than 35%) but decreased post-yield displacement (greater than 50%) at both 12 and 24 weeks of age. In summary, the osteoblast knockout of BMP2 reduced bone size and altered mechanical properties at the whole-bone and material levels. Osteoblast-derived BMP2 has an important role in post-natal skeletal growth, structure and strength, while vascular endothelial-derived BMP2 does not.

Reduced trabecular bone mass and strength in mice overexpressing Gα11 protein in cells of the osteoblast lineage

G protein-coupled receptors (GPCRs) require G proteins for intracellular signaling to regulate a variety of growth and maintenance processes, including osteogenesis and bone turnover. Bone maintenance events may be altered by changes in the activity or level of G proteins, which then modify signaling in bone cells such as osteoblasts. We have previously reported increased levels of Gα11 protein and signaling to phospholipase C/protein kinase C pathways in response to dexamethasone in osteoblastic UMR 106-01 cells. Here we generated pOBCol3.6-GNA11 transgenic mice that overexpress Gα11 protein in cells of the osteoblast lineage (G11-Tg mice). G11-Tg mice exhibit an osteopenic phenotype characterized by significant reductions in trabecular bone mineral density, thickness, number and strength. The numbers of osteoblasts and osteocytes were unchanged in G11-Tg bone, but early markers of osteoblast differentiation, Alp and Bsp, were increased while the late stage differentiation marker Ocn was not changed suggesting reduced osteoblast maturation in G11-Tg trabecular bone which was accompanied by a decreased bone formation rate. Furthermore, in vitro cultures of G11-Tg primary osteoblasts show delayed osteoblast differentiation and mineralization. Histological analyses also revealed increased osteoclast parameters, accompanied by elevated mRNA expression of Trap and Ctsk. mRNA levels of Rankl and M-csf were elevated in vitro in bone marrow stromal cells undergoing osteogenesis and in trabecular bone in vivo. Together, these findings demonstrate that increasing Gα11 protein expression in osteoblasts can alter gene expression and result in a dual mechanism of trabecular bone loss.

The Contribution of Experimental in vivo Models to Understanding the Mechanisms of Adaptation to Mechanical Loading in Bone

Changing loading regimens by natural means such as exercise, with or without interference such as osteotomy, has provided useful information on the structure:function relationship in bone tissue. However, the greatest precision in defining those aspects of the overall strain environment that influence modeling and remodeling behavior has been achieved by relating quantified changes in bone architecture to quantified changes in bones' strain environment produced by direct, controlled artificial bone loading. Jiri Hert introduced the technique of artificial loading of bones in vivo with external devices in the 1960s using an electromechanical device to load rabbit tibiae through transfixing stainless steel pins. Quantifying natural bone strains during locomotion by attaching electrical resistance strain gages to bone surfaces was introduced by Lanyon, also in the 1960s. These studies in a variety of bones in a number of species demonstrated remarkable uniformity in the peak strains and maximum strain rates experienced. Experiments combining strain gage instrumentation with artificial loading in sheep, pigs, roosters, turkeys, rats, and mice has yielded significant insight into the control of strain-related adaptive (re)modeling. This diversity of approach has been largely superseded by non-invasive transcutaneous loading in rats and mice, which is now the model of choice for many studies. Together such studies have demonstrated that over the physiological strain range, bone's mechanically adaptive processes are responsive to dynamic but not static strains; the size and nature of the adaptive response controlling bone mass is linearly related to the peak loads encountered; the strain-related response is preferentially sensitive to high strain rates and unresponsive to static ones; is most responsive to unusual strain distributions; is maximized by remarkably few strain cycles, and that these are most effective when interrupted by short periods of rest between them.

Skeletal Blood Flow in Bone Repair and Maintenance

Bone is a highly vascularized tissue, although this aspect of bone is often overlooked. In this article, the importance of blood flow in bone repair and regeneration will be reviewed. First, the skeletal vascular anatomy, with an emphasis on long bones, the distinct mechanisms for vascularizing bone tissue, and methods for remodeling existing vasculature are discussed. Next, techniques for quantifying bone blood flow are briefly summarized. Finally, the body of experimental work that demonstrates the role of bone blood flow in fracture healing, distraction osteogenesis, osteoporosis, disuse osteopenia, and bone grafting is examined. These results illustrate that adequate bone blood flow is an important clinical consideration, particularly during bone regeneration and in at-risk patient groups.

Nitric oxide-mediated vasodilation increases blood flow during the early stages of stress fracture healing

Despite the strong connection between angiogenesis and osteogenesis in skeletal repair conditions such as fracture and distraction osteogenesis, little is known about the vascular requirements for bone formation after repetitive mechanical loading. Here, established protocols of damaging (stress fracture) and nondamaging (physiological) forelimb loading in the adult rat were used to stimulate either woven or lamellar bone formation, respectively. Positron emission tomography was used to evaluate blood flow and fluoride kinetics at the site of bone formation. In the group that received damaging mechanical loading leading to woven bone formation (WBF), (15)O water (blood) flow rate was significantly increased on day 0 and remained elevated 14 days after loading, whereas (18)F fluoride uptake peaked 7 days after loading. In the group that received nondamaging mechanical loading leading to lamellar bone formation (LBF), (15)O water and (18)F fluoride flow rates in loaded limbs were not significantly different from nonloaded limbs at any time point. The early increase in blood flow rate after WBF loading was associated with local vasodilation. In addition, Nos2 expression in mast cells was increased in WBF-, but not LBF-, loaded limbs. The nitric oxide (NO) synthase inhibitor N(ω)-nitro-l-arginine methyl ester was used to suppress NO generation, resulting in significant decreases in early blood flow rate and bone formation after WBF loading. These results demonstrate that NO-mediated vasodilation is a key feature of the normal response to stress fracture and precedes woven bone formation. Therefore, patients with impaired vascular function may heal stress fractures more slowly than expected.

Male mice housed in groups engage in frequent fighting and show a lower response to additional bone loading than females or individually housed males that do not fight

Experiments to investigate bone's physiological adaptation to mechanical loading frequently employ models that apply dynamic loads to bones in vivo and assess the changes in mass and architecture that result. It is axiomatic that bones will only show an adaptive response if the applied artificial loading environment differs in a significant way from that to which the bones have been habituated by normal functional loading. It is generally assumed that this normal loading is similar between experimental groups. In the study reported here we found that this was not always the case. Male and female 17-week-old C57BL/6 mice were housed in groups of six, and a single episode (40 cycles) of non-invasive axial loading, engendering 2,200 με on the medial surface of the proximal tibiae in sample mice, was applied to right tibiae on alternate days for two weeks. This engendered an adaptive increase in bone mass in females, but not males. Observation revealed the main difference in behaviour between males and females was that males were involved in fights 1.3 times per hour, whereas the females never fought. We therefore housed all mice individually. In females, there was a similar significant osteogenic response to loading in cortical and trabecular bone of both grouped and individual mice. In contrast, in males, adaptive increases in the loaded compared with non-loaded control bones was only apparent in animals housed individually. Our interpretation of these findings is that the frequent vigorous fighting that occurs between young adult males housed in groups could be sufficient to engender peak strains and strain rates that equal or exceed the stimulus derived from artificial loading. This indicates the importance of ensuring that physical activity is consistent between groups. Reducing the background level of the naturally engendered strain environment allows adaptive responses to artificial loading to be demonstrated at lower loads.

The cyclooxygenase-2 selective inhibitor NS-398 does not influence trabecular or cortical bone gain resulting from repeated mechanical loading in female mice

A single injection of the cyclooxygenase-2 (COX-2) selective inhibitor NS-398 reduces bone's osteogenic response to a single period of mechanical loading in female rats, while women taking COX-2 selective inhibitors do not have lower bone mass. We show that daily NS-398 injection does not influence bone gain from repeated loading in female mice.

INTRODUCTION

Prostaglandins are mediators of bone cells' early response to mechanical stimulation. COX-2 expression is up-regulated by exposure of these cells to mechanical strain or fluid flow, and the osteogenic response to a single loading period is reduced by COX-2 inhibition. This study determined, in female mice in vivo, the effect of longer term COX-2 inhibition on adaptive (re)modelling of cortical and trabecular bone in response to repeated loading.

METHODS

Nineteen-week-old female C57BL/6 mice were injected with vehicle or NS-398 (5 mg/kg/day) 5 days a week for 2 weeks. On three alternate days each week, the right tibiae/fibulae were axially loaded [40 cycles (7 min)/day] three hours after injection. Left limbs acted as internal controls. Changes in three-dimensional bone architecture were analysed by high-resolution micro-computed tomography.

RESULTS

In control limbs NS-398 was associated with reduced trabecular number but had no influence on cortical bone. In loaded limbs trabecular thickness and cortical periosteally enclosed volume increased. NS-398 showed no effect on this response.

CONCLUSION

Pharmacological inhibition of COX-2 by NS-398 does not affect trabecular or cortical bone's response to repeated mechanical loading in female mice and thus would not be expected to impair the functional adaptation of bone to physical activity in women.

Angiogenesis is required for stress fracture healing in rats

Although angiogenesis and osteogenesis are critically linked, the importance of angiogenesis for stress fracture healing is unknown. In this study, mechanical loading was used to create a non-displaced stress fracture in the adult rat forelimb. Fumagillin, an anti-angiogenic agent, was used as the water soluble analogue TNP-470 (25mg/kg) as well as incorporated into lipid-encapsulated α(v)β(3) integrin targeted nanoparticles (0.25mg/kg). In the first experiment, TNP-470 was administered daily for 5 days following mechanical loading, and changes in gene expression, vascularity, and woven bone formation were quantified. Although no changes in vascularity were detected 3 days after loading, treatment-related downregulation of angiogenic (Pecam1) and osteogenic (Bsp, Osx) genes was observed at this early time point. On day 7, microCT imaging of loaded limbs revealed diminished woven bone formation in treated limbs compared to vehicle treated limbs. In the second experiment, α(v)β(3) integrin targeted fumagillin nanoparticles were administered as before, albeit with a 100-fold lower dose, and changes in vascularity and woven bone formation were determined. There were no treatment-related changes in vessel count or volume 3 days after loading, although fewer angiogenic (CD105 positive) blood vessels were present in treated limbs compared to vehicle treated limbs. This result manifested on day 7 as a reduction in total vascularity, as measured by histology (vessel count) and microCT (vessel volume). Similar to the first experiment, treated limbs had diminished woven bone formation on day 7 compared to vehicle treated limbs. These results indicate that angiogenesis is required for stress fracture healing, and may have implications for inducing rapid repair of stress fractures.

Adaptive and Injury Response of Bone to Mechanical Loading

Bone responds to supraphysiological mechanical loads by increasing bone formation. Depending on the applied strain magnitude (and other loading parameters) the response can be either adaptive (mostly lamellar bone) or injury (mostly woven bone). Seminal studies of Hert, Lanyon, and Rubin originally established the basic "rules" of bone mechanosensitivity. These were reinforced by subsequent studies using non-invasive rodent loading models, most notably by Turner et al. More recent work with these models have been able to explore the structural, transcriptional, and molecular mechanisms which distinguish the two responses (lamellar vs. woven). Wnt/Lrp signaling has emerged as a key mechanoresponsive pathway for lamellar bone. However, there is still much to study with regard to effects of ageing, osteocytes, other signaling pathways, and the molecular regulation that modulates lamellar vs. woven bone formation. This review summarizes not only the historical findings but also the current data for these topics.

Bones' adaptive response to mechanical loading is essentially linear between the low strains associated with disuse and the high strains associated with the lamellar/woven bone transition

There is a widely held view that the relationship between mechanical loading history and adult bone mass/strength includes an adapted state or "lazy zone" where the bone mass/strength remains constant over a wide range of strain magnitudes. Evidence to support this theory is circumstantial. We investigated the possibility that the "lazy zone" is an artifact and that, across the range of normal strain experience, features of bone architecture associated with strength are linearly related in size to their strain experience. Skeletally mature female C57BL/6 mice were right sciatic neurectomized to minimize natural loading in their right tibiae. From the fifth day, these tibiae were subjected to a single period of external axial loading (40, 10-second rest interrupted cycles) on alternate days for 2 weeks, with a peak dynamic load magnitude ranging from 0 to 14 N (peak strain magnitude: 0-5000 µε) and a constant loading rate of 500 N/s (maximum strain rate: 75,000 µε/s). The left tibiae were used as internal controls. Multilevel regression analyses suggest no evidence of any discontinuity in the progression of the relationships between peak dynamic load and three-dimensional measures of bone mass/strength in both cortical and cancellous regions. These are essentially linear between the low-peak locomotor strains associated with disuse (∼300 µε) and the high-peak strains derived from artificial loading and associated with the lamellar/woven bone transition (∼5000 µε). The strain:response relationship and minimum effective strain are site-specific, probably related to differences in the mismatch in strain distribution between normal and artificial loading at the locations investigated.

Development of silk-based scaffolds for tissue engineering of bone from human adipose-derived stem cells

Silk fibroin is a potent alternative to other biodegradable biopolymers for bone tissue engineering (TE), because of its tunable architecture and mechanical properties, and its demonstrated ability to support bone formation both in vitro and in vivo. In this study, we investigated a range of silk scaffolds for bone TE using human adipose-derived stem cells (hASCs), an attractive cell source for engineering autologous bone grafts. Our goal was to understand the effects of scaffold architecture and biomechanics and use this information to optimize silk scaffolds for bone TE applications. Silk scaffolds were fabricated using different solvents (aqueous vs. hexafluoro-2-propanol (HFIP)), pore sizes (250-500 μm vs. 500-1000 μm) and structures (lamellar vs. spherical pores). Four types of silk scaffolds combining the properties of interest were systematically compared with respect to bone tissue outcomes, with decellularized trabecular bone (DCB) included as a "gold standard". The scaffolds were seeded with hASCs and cultured for 7 weeks in osteogenic medium. Bone formation was evaluated by cell proliferation and differentiation, matrix production, calcification and mechanical properties. We observed that 400-600 μm porous HFIP-derived silk fibroin scaffold demonstrated the best bone tissue formation outcomes, as evidenced by increased bone protein production (osteopontin, collagen type I, bone sialoprotein), enhanced calcium deposition and total bone volume. On a direct comparison basis, alkaline phosphatase activity (AP) at week 2 and new calcium deposition at week 7 were comparable to the cells cultured in DCB. Yet, among the aqueous-based structures, the lamellar architecture induced increased AP activity and demonstrated higher equilibrium modulus than the spherical-pore scaffolds. Based on the collected data, we propose a conceptual model describing the effects of silk scaffold design on bone tissue formation.

Tibial loading increases osteogenic gene expression and cortical bone volume in mature and middle-aged mice

There are conflicting data on whether age reduces the response of the skeleton to mechanical stimuli. We examined this question in female BALB/c mice of different ages, ranging from young to middle-aged (2, 4, 7, 12 months). We first assessed markers of bone turnover in control (non-loaded) mice. Serum osteocalcin and CTX declined significantly from 2 to 4 months (p<0.001). There were similar age-related declines in tibial mRNA expression of osteoblast- and osteoclast-related genes, most notably in late osteoblast/matrix genes. For example, Col1a1 expression declined 90% from 2 to 7 months (p<0.001). We then assessed tibial responses to mechanical loading using age-specific forces to produce similar peak strains (−1300 µε endocortical; −2350 µε periosteal). Axial tibial compression was applied to the right leg for 60 cycles/day on alternate days for 1 or 6 weeks. qPCR after 1 week revealed no effect of loading in young (2-month) mice, but significant increases in osteoblast/matrix genes in older mice. For example, in 12-month old mice Col1a1 was increased 6-fold in loaded tibias vs. controls (p = 0.001). In vivo microCT after 6 weeks revealed that loaded tibias in each age group had greater cortical bone volume (BV) than contralateral control tibias (p<0.05), due to relative periosteal expansion. The loading-induced increase in cortical BV was greatest in 4-month old mice (+13%; p<0.05 vs. other ages). In summary, non-loaded female BALB/c mice exhibit an age-related decline in measures related to bone formation. Yet when subjected to tibial compression, mice from 2–12 months have an increase in cortical bone volume. Older mice respond with an upregulation of osteoblast/matrix genes, which increase to levels comparable to young mice. We conclude that mechanical loading of the tibia is anabolic for cortical bone in young and middle-aged female BALB/c mice.

Mechanical loading-related changes in osteocyte sclerostin expression in mice are more closely associated with the subsequent osteogenic response than the peak strains engendered

Osteocyte sclerostin is regulated by loading and disuse in mouse tibiae but is more closely related to subsequent local osteogenesis than the peak strains engendered.

INTRODUCTION

The purpose of this study was to assess the relationship between loading-related change in osteocyte sclerostin expression, local strain magnitude, and local bone modeling/remodeling.

METHODS

The right tibiae of 19-week-old female C57BL/6 mice were subjected to non-invasive, dynamic axial loading and/or to sciatic neurectomy-induced disuse. The sclerostin status of osteocytes was evaluated immunohistochemically, changes in bone mass by micro-computed tomography, new bone formation by histomorphometry, and loading-induced strain by strain gauges and finite element analysis.

RESULTS

In cortical bone of the tibial shaft, loading engendered strains of similar peak magnitude proximally and distally. Proximally, sclerostin-positive osteocytes decreased and new bone formation increased. Distally, there was neither decrease in sclerostin-positive osteocytes nor increased osteogenesis. In trabecular bone of the proximal secondary spongiosa, loading decreased sclerostin-positive osteocytes and increased bone volume. Neither occurred in the primary spongiosa. Disuse increased sclerostin-positive osteocytes and decreased bone volume at all four sites. Loading reversed this sclerostin upregulation to a level below baseline in the proximal cortex and secondary spongiosa.

CONCLUSION

Loading-related sclerostin downregulation in osteocytes of the mouse tibia is associated preferentially with regions where new bone formation is stimulated rather than where high peak strains are engendered. The mechanisms involved remain unclear, but could relate to peak surface strains not accurately reflecting the strain-related osteogenic stimulus or that sclerostin regulation occurs after sufficient signal processing to distinguish between local osteogenic and non-osteogenic responses.

Skeletal biology over the life span: a view from the surfaces

The biocultural interpretation of skeletal remains is based upon the foundation of skeletal biology. In this review we examine the current state of skeletal biology research outside of the mainstream anthropology literature. The focus is on the structural changes of bone development and growth, and modeling and repair in the four bone surfaces: periosteal, Haversian, endosteal, and trabecular. The pattern of skeletal changes is placed within the framework of the human life span. New perspectives and direction of research on the environmental, biological, and genetic influences on modeling and remodeling processes are discussed chronologically at each bone surface. Implications for biological anthropologists are considered. This approach emphasizes variation in skeletal biology as a dynamic record of development, maturity, and aging.

Differential gene expression from microarray analysis distinguishes woven and lamellar bone formation in the rat ulna following mechanical loading

Formation of woven and lamellar bone in the adult skeleton can be induced through mechanical loading. Although much is known about the morphological appearance and structural properties of the newly formed bone, the molecular responses to loading are still not well understood. The objective of our study was to use a microarray to distinguish the molecular responses between woven and lamellar bone formation induced through mechanical loading. Rat forelimb loading was completed in a single bout to induce the formation of woven bone (WBF loading) or lamellar bone (LBF loading). A set of normal (non-loaded) rats were used as controls. Microarrays were performed at three timepoints after loading: 1 hr, 1 day and 3 days. Confirmation of microarray results was done for a select group of genes using quantitative real-time PCR (qRT-PCR). The micorarray identified numerous genes and pathways that were differentially regulated for woven, but not lamellar bone formation. Few changes in gene expression were evident comparing lamellar bone formation to normal controls. A total of 395 genes were differentially expressed between formation of woven and lamellar bone 1 hr after loading, while 5883 and 5974 genes were differentially expressed on days 1 and 3, respectively. Results suggest that not only are the levels of expression different for each type of bone formation, but that distinct pathways are activated only for woven bone formation. A strong early inflammatory response preceded an increase in angiogenic and osteogenic gene expression for woven bone formation. Furthermore, at later timepoints there was evidence of bone resorption after WBF loading. In summary, the vast coverage of the microarray offers a comprehensive characterization of the early differences in expression between woven and lamellar bone formation.

Hypoxia-inducible factor-1α protein negatively regulates load-induced bone formation

Mechanical loads induce profound anabolic effects in the skeleton, but the molecular mechanisms that transduce such signals are still poorly understood. In this study, we demonstrate that the hypoxia-inducible factor-1α (Hif-1α) is acutely up-regulated in response to exogenous mechanical stimuli secondary to prostanoid signaling and Akt/mTOR (mammalian target of rapamycin) activation. In this context, Hif-1α associates with β-catenin to inhibit Wnt target genes associated with bone anabolic activity. Mice lacking Hif-1α in osteoblasts and osteocytes form more bone when subjected to tibia loading as a result of increased osteoblast activity. Taken together, these studies indicate that Hif-1α serves as a negative regulator of skeletal mechanotransduction to suppress load-induced bone formation by altering the sensitivity of osteoblasts and osteocytes to mechanical signals.

Risedronate does not reduce mechanical loading-related increases in cortical and trabecular bone mass in mice

To establish whether the combination of anti-resorptive therapy with mechanical loading has a negative, additive or synergistic effect on bone structure, we assessed the separate and combined effects of risedronate and non-invasive dynamic loading on trabecular and cortical bone. Seventeen-week-old female C57BL/6 mice were given daily subcutaneous injections of vehicle (n=20) or risedronate at a dose of 0.15, 1.5, 15 or 150 μg/kg/day (n=10 in each) for 17 days. From the fourth day of treatment, the right tibiae were subjected to a single period of axial loading (40 cycles/day) for three alternate days per week for two weeks. The left tibiae were used as internal controls. Trabecular and cortical sites in the tibiae were analyzed by high-resolution micro-computed tomography and imaging of fluorochrome labels. In the non-loaded tibiae, treatment with the higher doses of risedronate at 15 or 150 μg/kg/day resulted in higher trabecular bone volume and trabecular number than in vehicle-treated controls, whereas such treatment was associated with no differences in cortical bone volume at any dose. In the loaded tibiae, loading induced increases in trabecular and cortical bone volume compared with contra-lateral controls primarily through increased trabecular thickness and periosteal expansion, respectively, independently of risedronate treatment. In conclusion, the response to mechanical loading in both trabecular and cortical bone in mice is therefore not impaired by short-term treatment with risedronate, even over a 1000-fold dose range. In considering the optimization of treatments for osteoporosis, it is reassuring that anti-resorptive therapy and mechanical loading can exert independent beneficial effects.