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

Combining PTH(1-34) and mechanical loading has increased benefit to tibia bone mechanics in ovariectomised mice

Combined treatment with PTH(1-34) and mechanical loading confers increased structural benefits to bone than monotherapies. However, it remains unclear how this longitudinal adaptation affects the bone mechanics. This study quantified the individual and combined longitudinal effects of PTH(1-34) and mechanical loading on the bone stiffness and strength evaluated in vivo with validated micro-finite element (microFE) models. C57BL/6 mice were ovariectomised at 14-week-old and treated either with injections of PTH(1-34), compressive tibia loading or both interventions concurrently. Right tibiae were in vivo microCT-scanned every 2 weeks from 14 until 24-week-old. MicroCT images were rigidly registered to reference tibia and the cortical organ level (whole bone) and tissue level (midshaft) morphometric properties and bone mineral content were quantified. MicroCT images were converted into voxel-based homogeneous, linear elastic microFE models to estimate the bone stiffness and strength. This approach allowed us for the first time to quantify the longitudinal changes in mechanical properties induced by combined treatments in a model of accelerated bone resorption. Both changes of stiffness and strength were higher with co-treatment than with individual therapies, consistent with increased benefits with the tibia bone mineral content and cortical area, properties strongly associated with the tibia mechanics. The longitudinal data shows that the two bone anabolics, both individually and combined, had persistent benefit on estimated mechanical properties, and that benefits (increased stiffness and strength) remained after treatment was withdrawn.

The loading direction dramatically affects the mechanical properties of the mouse tibia

Introduction: The in vivo tibial loading mouse model has been extensively used to evaluate bone adaptation in the tibia after mechanical loading treatment. However, there is a prevailing assumption that the load is applied axially to the tibia. The aim of this in silico study was to evaluate how much the apparent mechanical properties of the mouse tibia are affected by the loading direction, by using a validated micro-finite element (micro-FE) model of mice which have been ovariectomized and exposed to external mechanical loading over a two-week period. Methods: Longitudinal micro-computed tomography (micro-CT) images were taken of the tibiae of eleven ovariectomized mice at ages 18 and 20 weeks. Six of the mice underwent a mechanical loading treatment at age 19 weeks. Micro-FE models were generated, based on the segmented micro-CT images. Three models using unitary loads were linearly combined to simulate a range of loading directions, generated as a function of the angle from the inferior-superior axis (θ, 0°-30° range, 5° steps) and the angle from the anterior-posterior axis (ϕ, 0°: anterior axis, positive anticlockwise, 0°-355° range, 5° steps). The minimum principal strain was calculated and used to estimate the failure load, by linearly scaling the strain until 10% of the nodes reached the critical strain level of -14,420 με. The apparent bone stiffness was calculated as the ratio between the axial applied force and the average displacement along the longitudinal direction, for the loaded nodes. Results: The results demonstrated a high sensitivity of the mouse tibia to the loading direction across all groups and time points. Higher failure loads were found for several loading directions (θ = 10°, ϕ 205°-210°) than for the nominal axial case (θ = 0°, ϕ = 0°), highlighting adaptation of the bone for loading directions far from the nominal axial one. Conclusion: These results suggest that in studies which use mouse tibia, the loading direction can significantly impact the failure load. Thus, the magnitude and direction of the applied load should be well controlled during the experiments.

Site specific differences in vBMD and geometry in postmenopausal women with primary hyperparathyroidism

PURPOSE

Primary Hyperparathyroidism (PHPT) is associated with catabolic effects at both trabecular and cortical bone. Mechanical loading is one of the most important natural anabolic stimuli for bone at all ages. The present study was designed to assess the impact of PHPT on vBMD and bone geometry using peripheral quantitative computed tomography (pQCT) at the radius and tibia, sites with similar structural characteristics, but subject to different loading conditions.

METHODS

We evaluated the impact of PHPT on bone, by comparing the z-scores of volumetric Bone Mineral Density (vBMD) and indices of bone geometry simultaneously at the tibia and the radius by pQCT, skeletal sites with similar structure, but subject to different loading conditions. Forty-one postmenopausal women with PHPT and 79 controls, comprised the study group.

RESULTS

At both trabecular and cortical sites, vBMD and bone geometry indices were significantly lower in patients compared with controls. In patients with PHPT, apart from a lower z-score for total vBMD (p = 0.01) at the radius, there was no other difference between the radius and the tibia at the trabecular sites. On the contrary, at cortical sites, the z-scores of cortical bone mineral content (p = 0.02), cortical vBMD (p = 0.01) and cortical cross-sectional area (p = 0.05) were significantly lower at the radius compared with the tibia, indicating that cortical bone at the weight bearing tibia might be less affected by the catabolic actions of continuous parathyroid hormone (PTH) exposure. PTH levels were positively associated with the difference in z-scores of cort BMD (r = 0.439, p < 0.01) indicating that in more severe cases, as expressed by higher PTH levels, the deleterious effects at the non-weight bearing radius might be accentuated.

CONCLUSION

We found that in postmenopausal women with PHPT, both trabecular and cortical bone are adversely affected. However, at the weight bearing tibia as compared with the radius, the deleterious effects, especially on cortical bone, seem to be attenuated.

TRIAL REGISTRATION NUMBER

NCT05426512, 21/06/2022, "retrospectively registered".

Early protection against bone stress injuries by mobilization of endogenous targeted bone remodeling

Bone stress injuries are common overuse injuries, especially in soldiers, athletes, and performers. In contrast to various post-injury treatments, early protection against bone stress injuries can provide greater benefit. This study explored the early protection strategies against bone stress injuries by mobilization of endogenous targeted bone remodeling. The effects of various pharmaceutical/biophysical approaches, individual or combinational, were investigated by giving intervention before fatigue loading. We optimized the dosage and administration parameters and found that early intervention with pulsed electromagnetic field and parathyroid hormone (i.e., PEMF+PTH) resulted in the most pronounced protective effects among all the approaches against the bone stress injuries. In addition, the mechanisms by which the strategy mobilizes targeted bone remodeling and enhances the self-repair capacity of bone were systematically investigated. This study proposes strategies to reduce the incidence of bone stress injuries in high-risk populations (e.g., soldiers and athletes), particularly for those before sudden increased physical training.

Regional modular responses in different bone compartments to the anabolic effect of PTH (1-34) and axial loading in mice

Beneficial effects of intermittent parathyroid hormone (PTH) on bone mass and architecture are described to either simply add to, or to synergise with those of mechanical loading. We evaluate whether interaction with in vivo loading is reinforced by PTH dosing regimen and exhibits compartment-specific sensitivities. Female 12-week-old C57Bl6 mice received daily (7/7) or interrupted 5 day/week (5/7) PTH for 3 weeks (two vehicle groups). All mice had six loading episodes (12N) applied to right tibia (left, non-loaded) for the last 2 weeks. Micro-CT scans were used to evaluate mass and architecture in almost the entire cortical and proximal trabecular regions. Epiphyseal cortical, trabecular and marrow space volumes, and bony growth-plate bridge incidence were evaluated. Statistical analyses employed a linear mixed-effects model at each percentile and 2-way ANOVA with post-hoc test for epiphyses and bridging. We found that daily PTH enhances cortical mass and modifies shape along almost the entire tibia and that these effects are partly mitigated by brief interruption in treatment. Mechanical loading alone augments cortical mass and modifies shape but only in a region proximal to the tibiofibular junction. The effect of combining load and daily PTH dosing is solely additive for cortical bone mass with no significant load: PTH interaction, but exhibits clear synergy with interrupted PTH treatment. Daily, not interrupted PTH stimulates trabecular bone gains, yet load:PTH interaction is present at limited regions with both daily and interrupted treatment. PTH treatment, but not loading, modifies epiphyseal bone but, in contrast, only loading modifies bridge number and areal density. Our findings demonstrate impressive local effects on tibial mass and shape of combined loading and PTH that are sensitive to dosing regimen and exert their effects modularly. These findings emphasise a need to clarify PTH dosing regimens and that advantages could be accrued by aligning treatment accordingly to patient requirements and life-style.

Extramedullary Osseointegration-A Novel Design of Percutaneous Osseointegration Prosthesis for Amputees

The percutaneous osseointegrated (OI) prostheses have greatly improved the overall quality of life for amputees. However, the long-term maintenance of the OI prostheses is still challenging. A major problem is bone resorption around the bone-implant-skin interface, which might cause implant loosening or osteomyelitis. Another problem is the breakage of connecting components between the intramedullary implant and external prosthesis due to excessive stress. We designed a novel osseointegration implant by changing the bone-implant contact from the inner cortex to the outer surface of cortical bone. In the current study, we compared the extramedullary cap-shaped implants with the intramedullary screw-type implants in rabbits. Osteointegration was confirmed at the interface of bone to implant contact (BIC) in both implant types. The external implant induced intramedullary bone regeneration in the medullary canal and increased the cortical bone density at the end of the stump. This study provides a new perspective on the design of osseointegration implants which might prevent the currently reported complications of the intramedullary OI systems.

Positive interactions of mechanical loading and PTH treatments on spatio-temporal bone remodelling

Osteoporosis is one of the most common skeletal diseases, but current therapies are limited to generalized antiresorptive or anabolic interventions, which do not target regions that would benefit from improvements to skeletal health. To improve the evaluation of treatment plans, we used a spatio-temporal multiscale approach that combines longitudinal in vivo micro-computed tomography (micro-CT) and in silico subject-specific finite element modeling to quantitatively map bone adaptation changes due to disease and treatment at high resolution. Our findings show time and region-dependent modifications in bone remodelling following one and two sets of mechanical loading and/or pharmacological interventions. The multiscale results highlighted that the distal section was unaffected by mechanical loading alone but the proximal tibia had the greatest gain from positive interactions of combined therapies. Mechanical loading abated the catabolic effect of PTH, but the main benefit of combined treatments occurred from the additive interactions of the two therapies in periosteal apposition. These results provide detailed insight into the efficacy of combined treatments, facilitating the optimisation of dosage and treatment duration in preclinical mouse studies, and the development of novel interventions for skeletal diseases. STATEMENT OF SIGNIFICANCE: Combined mechanical loading and pharmacotherapy have the potential to slow osteoporosis-induced bone loss but current therapies do not target the regions in need of strengthening. We show for the first time spatial region-dependant interactions between PTH and mechanical loading treatment in OVX mouse tibiae, highlighting local regions in the tibia that benefitted from separate and combined treatments. Combined experimental-computational analysis also detailed the lasting period of each treatment per location in the tibia, the extent of positive (or negative) interactions of the combined therapies, and the impact of each treatment on the regulation of bone adaptation spatio-temporally. This approach can be used to create hypothesis about the interactions of different treatments to optimise the design of biomaterials and medical interventions.

Combination of parathyroid hormone pretreatment and mechanical stretch promotes osteogenesis of periodontal ligament fibroblasts

INTRODUCTION

Parathyroid hormone (PTH) potentiates the mechanical loading induced bone formation in fracture healing and orthodontics. This study aimed to gain insight into the underlying mechanisms in periodontal ligament fibroblasts (PDLFs).

METHODS

Human PDLFs were cultured and subjected to uniaxial cyclic stretch at 0.5 Hz and 2000μ for 0, 6, 12, and 24 hours, respectively. 10 nM PTH was preadministered for 30 minutes before loading. The expression of PTH1R and osteogenic biomarkers Runx2, osteopontin, collagen type 1, alkaline phosphatase was assessed via immunofluorescence staining, quantitative polymerase chain reaction, or Western blot. Transfection of siPTH1R was applied, and alterations of osteogenic biomarkers were examined by Western blot. The expression of essential Wnt signal components Wnt3a, β-catenin, low-density lipoprotein receptor-related protein 5, Wnt5a, receptor tyrosine kinase-like orphan receptor 2 were examined, and the influence of dickkopf-related protein 1 on osteogenic biomarkers was evaluated.

RESULTS

The expression of PTH1R was instantaneously upregulated with PTH pretreatment and maintained a gradual increase until 24 hours. PTH synergistically enhanced the increase of Runx2, osteopontin, collagen type 1, and alkaline phosphatase under cyclic stretch, which was substantially attenuated by siPTH1R transfection. As for Wnt signal components, synergistic upregulation was detected on Wnt3a, β-catenin, and low-density lipoprotein receptor-related protein 5, whereas Wnt5a and receptor tyrosine kinase-like orphan receptor 2 increased relatively mildly. Blockage of the canonical Wnt/β-catenin pathway by dickkopf-related protein 1 impaired the boost of osteogenic biomarkers under the combined action of PTH and cyclic stretch.

CONCLUSIONS

The combination of PTH pretreatment and cyclic stretch promotes osteogenesis of PDLFs synergistically, and the canonical Wnt/β-catenin pathway is crucially involved in the underlying mechanism.

The Role of the Loading Condition in Predictions of Bone Adaptation in a Mouse Tibial Loading Model

The in vivo mouse tibial loading model is used to evaluate the effectiveness of mechanical loading treatment against skeletal diseases. Although studies have correlated bone adaptation with the induced mechanical stimulus, predictions of bone remodeling remained poor, and the interaction between external and physiological loading in engendering bone changes have not been determined. The aim of this study was to determine the effect of passive mechanical loading on the strain distribution in the mouse tibia and its predictions of bone adaptation. Longitudinal micro-computed tomography (micro-CT) imaging was performed over 2 weeks of cyclic loading from weeks 18 to 22 of age, to quantify the shape change, remodeling, and changes in densitometric properties. Micro-CT based finite element analysis coupled with an optimization algorithm for bone remodeling was used to predict bone adaptation under physiological loads, nominal 12N axial load and combined nominal 12N axial load superimposed to the physiological load. The results showed that despite large differences in the strain energy density magnitudes and distributions across the tibial length, the overall accuracy of the model and the spatial match were similar for all evaluated loading conditions. Predictions of densitometric properties were most similar to the experimental data for combined loading, followed closely by physiological loading conditions, despite no significant difference between these two predicted groups. However, all predicted densitometric properties were significantly different for the 12N and the combined loading conditions. The results suggest that computational modeling of bone's adaptive response to passive mechanical loading should include the contribution of daily physiological load.

Teriparatide and Abaloparatide Have a Similar Effect on Bone in Mice

Three bone anabolic pharmaceuticals are currently approved for treatment of osteoporosis, teriparatide (PTH (1-34)), the parathyroid hormone-related protein analog abaloparatide (ABL), and romosozumab. The present study compared the effect of intermittent PTH (1-34) and ABL on bone tissue directly mole-to-mole in female mice. Forty-seven C57BL/6 mice were randomly allocated to the following groups: Baseline (n = 11), Control (Ctrl) (n = 12), PTH (n = 12), and ABL (n = 12). The mice were injected s.c. with PTH (100 µg/kg), ABL (96 µg/kg), or saline (Ctrl) five days a week for three weeks. To assess the effect of PTH and ABL, the hindlimb bones were analyzed with DXA, µCT, mechanical testing, dynamic bone histomorphometry, and histological quantification of bone cells. In addition, serum calcium concentration was determined. PTH and ABL significantly increased femoral areal bone mineral density (aBMD) (borderline significant p = 0.06 for PTH), femoral mid-diaphyseal bone strength, femoral metaphyseal and epiphyseal and vertebral bone volume fraction (BV/TV), connectivity density, volumetric bone mineral density (vBMD), and bone formation rate (BFR/BS) compared to Ctrl. In addition, ABL also significantly increased mid-diaphyseal cortical thickness and bone area compared to Ctrl. Neither PTH nor ABL significantly increased bone strength at the femoral neck. In conclusion, abaloparatide and PTH have similar bone anabolic properties when compared directly mole-to-mole in mice.

Mapping Regional Cortical Bone Responses to Local Changes in Loading and Systemic Stimuli

Quantification of cortical bone mass and architecture using μCT is commonplace in osteoporosis and osteoarthritis research. Different groups often report substantially divergent mouse cortical bone responses to nominally comparable interventions. In the case of studies assessing bones' responses to externally applied loading, these differences are commonly associated with methodological differences in the loading regime. This chapter describes a widely published, standardized method of in vivo mouse tibia axial loading to produce lamellar bone formation. Despite uniform application of axial loading, changes in bone mass are highly site-specific within individual bones. For example, the mouse proximal tibia rapidly accrues new bone following axial loading, but this osteogenic response tapers to produce undetectable differences distally. Consequently, the bone sites selected for comparisons substantially influence the magnitude of differences observed. Application of the freely available Site Specificity software allows site-specific responses to be identified by rapidly quantifying cortical bone mass at each 1% site along the bone's length. This high-content screening tool has been informatively applied to study the local effects of changes in loading as well as systemic interventions including hormonal treatment and aging. Automated multisite analyses of cortical mass is increasingly identifying site-specific effects of "systemic" interventions such as global gene deletions. Biological mechanisms underlying this apparent regionalization of cortical responses are largely unknown but may start to be elucidated by increasingly widespread application of Site Specificity methods.

Activin type IIA decoy receptor and intermittent parathyroid hormone in combination overturns the bone loss in disuse-osteopenic mice

Damage of the lower motor neuron cell bodies or their axons results in reduced or abolished voluntary movement accompanied by a substantial loss of bone and muscle mass. Intermittent parathyroid hormone 1-34 (PTH) (teriparatide) is one of the most potent bone-anabolic treatment regimens. ActRIIA-mFc is an activin type IIA decoy receptor that increases bone mass mediated by inhibition of the activin receptor signaling pathway. We investigated whether PTH or ActRIIA-mFc alone or in combination could prevent loss of bone and muscle mass induced by injecting botulinum toxin A (BTX) into the right hind limb in mice. Seventy-two 16-week-old female C57BL/6 mice were allocated to the following groups: Baseline, Control, BTX, BTX + ActRIIA-mFc (10 mg/kg), BTX + PTH (100 μg/kg), and BTX + ActRIIA-mFc + PTH. The mice were sacrificed after three weeks of disuse and treatment. In contrast to monotherapy with PTH, ActRIIA-mFc alone or in combination with PTH was able partly or completely to prevent disuse-induced loss of whole femoral bone mass, trabecular thickness, and bone strength. Moreover, an additive effect of ActRIIA-mFc and PTH on areal bone mineral density and trabecular bone volume was found. In summary, ActRIIA-mFc and PTH in combination were more effective in preventing disuse-induced bone loss and deterioration of trabecular micro-architecture than either treatment alone.

Intermittent Parathyroid Hormone Accelerates Stress Fracture Healing More Effectively Following Cessation of Bisphosphonate Treatment

Parathyroid hormone (PTH) and bisphosphonates (BPs), including alendronate (ALN), have opposing effects on bone dynamics. The extent to which PTH remains effective in the treatment of stress fracture (SFx) in the presence of an ongoing BP treatment has not been tested. SFx was induced in 150 female Wistar rats, divided into five equal groups (n = 30). All rats were pretreated with ALN (1 μg/kg^-1/day^-1) for 14 days prior to SFx induction, followed by ALN cessation or continuation for the duration of the experiment; this was combined with daily PTH (8 μg/100 g^-1/day^-1) on SFx induction for 14 days, followed by cessation or continuation of ALN after SFx induction or an equivalent vehicle as a control. Ulnas were examined 2 weeks or 6 weeks following SFx. Two toluidine blue- and two tartrate-resistant acid phosphatase-stained sections were examined for histomorphometric analysis using Osteomeasure software. There was a significant interaction between the effects of time and treatment type on the woven bone width and apposition rate, as well as an improvement in the woven bone architecture. However, woven bone variables remained unaffected by the cessation or continuation of ALN. Cessation of ALN increased osteoclast number when compared with the ALN-PTH continuation group (p = 0.006), and vehicle (p = 0.024) after 2 weeks. There was a significant interaction between the effects of time and treatment type on the number of osteoclasts per unit BMU area and length. The number of osteoclasts per unit BMU area and length was significantly greater in ALN cessation groups. It was concluded that intermittent short-duration iPTH treatment effectively increased remodeling of SFx with a concurrent BP treatment, provided that BP was ceased at the time of SFx. Our results could help develop shorter iPTH treatment protocols for the clinical management of SFxs and guide clinical decision-making to cease BP treatment in cases of SFx. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

PTH(1-34) treatment and/or mechanical loading have different osteogenic effects on the trabecular and cortical bone in the ovariectomized C57BL/6 mouse

In preclinical mouse models, a synergistic anabolic response to PTH(1–34) and tibia loading was shown. Whether combined treatment improves bone properties with oestrogen deficiency, a cardinal feature of osteoporosis, remains unknown. This study quantified the individual and combined longitudinal effects of PTH(1–34) and loading on the bone morphometric and densitometric properties in ovariectomised mice. C57BL/6 mice were ovariectomised at 14-weeks-old and treated either with injections of PTH(1–34); compressive loading of the right tibia; both interventions concurrently; or both interventions on alternating weeks. Right tibiae were microCT-scanned from 14 until 24-weeks-old. Trabecular metaphyseal and cortical midshaft morphometric properties, and bone mineral content (BMC) in 40 different regions of the tibia were measured. Mice treated only with loading showed the highest trabecular bone volume fraction at week 22. Cortical thickness was higher with co-treatment than in the mice treated with PTH alone. In the mid-diaphysis, increases in BMC were significantly higher with loading than PTH. In ovariectomised mice, the osteogenic benefits of co-treatment on the trabecular bone were lower than loading alone. However, combined interventions had increased, albeit regionally-dependent, benefits to cortical bone. Increased benefits were largest in the mid-diaphysis and postero-laterally, regions subjected to higher strains under compressive loads.

Bone gain following loading is site-specifically enhanced by prior and concurrent disuse in aged male mice

The primary aim of osteoanabolic therapies is to strategically increase bone mass in skeletal regions likely to experience high strains. In the young healthy skeleton, this is primarily achieved by bone's adaptation to loading. This adaptation appears to fail with age, resulting in osteoporosis and fractures. We previously demonstrated that prior and concurrent disuse enhances bone gain following loading in old female mice. Here, we applied site specificity micro-computed tomography analysis to map regional differences in bone anabolic responses to axial loading of the tibia between young (19-week-old) and aged (19-month-old), male and female mice. Loading increased bone mass specifically in the proximal tibia in both sexes and ages. Young female mice gained more cortical bone than young males in specific regions of the tibia. However, these site-specific sex differences were lost with age such that bone gain following loading was not significantly different between old males and females. To test whether disuse enhances functional adaption in old male mice as it does in females, old males were subjected to sciatic neurectomy or sham surgery, and loading was initiated four days after surgery. Disuse augmented tibial cortical bone gain in response to loading in old males, but only in regions which were load-responsive in the young. Prior and concurrent disuse also increased loading-induced trabecular thickening in the proximal tibia of old males. Understanding how diminished background loading rejuvenates the osteogenic loading response in the old may improve osteogenic exercise regimes and lead to novel osteoanabolic therapies.

Aging and Mechanoadaptive Responsiveness of Bone

PURPOSE OF REVIEW

Osteoporosis is an age-related disorder characterized by bone loss and increased fracture susceptibility. Whether this is due to reduced loading in less active elderly individuals or inherent modifications in bone cells is uncertain. We suppose that osteoporosis is nonetheless prima facie evidence for impaired mechanoadaptation; either capacity to accrue new bone declines, or the stimulus for such accrual is absent/can no longer be triggered in the aged. Herein, we provide only sufficient background to enable a focus on recent advances which seek to address such dilemmas.

RECENT FINDINGS

Recent advances from innovative high-impact loading regimes emphasize the priming of mechanoadaptation in the aged, such that low-to-moderate intensity loading becomes beneficial. These new findings lead us to speculate that aged bone mechanoadaptation is not driven solely by strain magnitude but is instead sensitive to high strain gradients. Impaired mechanoadaptation is a feature of the aged skeleton. Recent advances indicate that novel interventional loading regimes can restore mechanoadaptive capacity, enabling new approaches for retaining bone health in the aged. Innovative exercise paradigms appear to be capable of "hacking" into the osteogenic signal produced by exercise such that low-to-moderate intensity activities may also become more beneficial. Deciphering the underpinning mechanism(s) will also enable new pharmacological intervention for retaining bone health in the aged.

The longitudinal effects of ovariectomy on the morphometric, densitometric and mechanical properties in the murine tibia: A comparison between two mouse strains

Oestrogen deficiency-related bone loss in the ovariectomized (OVX) mouse is a common model for osteoporosis. However, a comprehensive in vivo assessment of intervention-related changes in multiple bone properties, and in multiple mouse strains, is required in order to identify an appropriate model for future evaluation of novel anti-osteoporotic therapies. The aim of this study was to evaluate the effect of OVX on the morphometric and densitometric properties measured in the microCT images and the mechanical properties estimated with finite element models of the tibia in two mouse strains, C57BL/6 and BALB/c. 14-weeks-old female C57BL/6 and BALB/c mice were divided into two groups per strain: (1) ovariectomized, (2) non-operated control. The right tibia was scanned at baseline (14 weeks) and then every two weeks thereafter, until 24-weeks-old, using in vivo microCT. Changes in trabecular and cortical bone morphometry, spatiotemporal changes in densitometric properties and in mechanical properties (from micro-finite element (μFE) analysis) were computed. Differences between OVX and non-operated controls were evaluated by ANCOVA, adjusted for 14-weeks baseline. In morphometry, trabecular bone mass was significantly reduced in both C57BL/6 and BALB/c from four weeks following surgery. Though the OVX-effect was transient in BALB/c as bone mass reached skeletal homeostasis. OVX inhibited the age-related thickening of cortical bone only in C57BL/6. In both strains, increments in bone mineral content were significantly lower with OVX only in the proximal tibia, with intervention-related differences increasing with time. OVX had no effect on μFE estimates of stiffness nor failure load in either strain. The results of this study show strain-, time- and region-(trabecular or cortical) dependent changes in morphometric and densitometric properties. These findings highlight the importance of choosing an appropriate mouse model and time points for research of treatments against accelerated bone resorption.

Increased Cellular Presence After Sciatic Neurectomy Improves the Bone Mechano-adaptive Response in Aged Mice

The mechano-adaptive response of bone to loading in the murine uniaxial tibial loading model is impaired in aged animals. Previous studies have shown that in aged mice, the amount of bone formed in response to loading is augmented when loads are applied following sciatic neurectomy. The synergistic effect of neurectomy and loading remains to be elucidated. We hypothesize that sciatic neurectomy increases cellular presence, thereby augmenting the response to load in aged mice. We examined bone adaptation in four groups of female C57BL/6J mice, 20-22 months old: (1) sham surgery + 9N loading; (2) sciatic neurectomy, sacrificed after 5 days; (3) sciatic neurectomy, sacrificed after 19 days; (4) sciatic neurectomy + 9N loading. We examined changes in bone cross sectional properties with micro-CT images, and static and dynamic histomorphometry with histological sections taken at the midpoint between tibiofibular junctions. The response to loading at 9N was not detectable with quantitative micro-CT data, but surface-specific histomorphometry captured an increase in bone formation in specific regions. 5 days following sciatic neurectomy, the amount of bone in the neurectomized leg was the same as the contralateral leg, but 19 days following sciatic neurectomy, there was significant bone loss in the neurectomized leg, and both osteoclasts and osteoblasts were recruited to the endosteal surfaces. When sciatic neurectomy and loading at 9N were combined, 3 out of 4 bone quadrants had increased bone formation, on the endosteal and periosteal surfaces (increased osteoid surface and mineralizing surface respectively). These data demonstrate that sciatic neurectomy increases cellular presence on the endosteal surface. With long-term sciatic-neurectomy, both osteoclasts and osteoblasts were recruited to the endosteal surface, which resulted in increased bone formation when combined with a sufficient mechanical stimulus. Controlled and localized recruitment of both osteoblasts and osteoclasts combined with appropriate mechanical loading could inform therapies for mechanically-directed bone formation.

Examining the influence of PTH(1-34) on tissue strength and composition

The lacunar-canaliculi system is a network of channels that is created and maintained by osteocytes as they are embedded throughout cortical bone. As osteocytes modify their lacuna space, the local tissue composition and tissue strength are subject to change. Although continual exposure to parathyroid hormone (PTH) can induce adaptation at the lacunar wall, the impact of intermittent PTH treatment on perilacunar adaptation remains unclear. Therefore, the primary objective of this study was to establish how intermittent PTH(1-34) treatment influences perilacunar adaptation with respect to changes in tissue composition. We hypothesized that local changes in tissue composition following PTH(1-34) are associated with corresponding gains in tissue strength and resistance to microdamage at the whole bone level. Adult male C57BL/6J mice were treated daily with PTH(1-34) or vehicle for 3 weeks. In response to PTH(1-34), Raman spectroscopy revealed a significant decrease in the carbonate-to-phosphate ratio and crystallinity across the entire tissue, while the mineral-to-matrix ratio demonstrated a significant decrease in just the perilacunar region. The shift in perilacunar composition largely explained the corresponding increase in tissue strength, while the degree of new tissue added at the endosteum and periosteum did not produce any significant changes in cortical area or moment of inertia that would explain the increase in tissue strength. Furthermore, fatigue testing revealed a greater resistance to crack formation within the existing tissue following PTH(1-34) treatment. As a result, the shift in perilacunar composition presents a unique mechanism by which PTH(1-34) produces localized differences in tissue quality that allow more energy to be dissipated under loading, thereby increasing tissue strength and resistance to microdamage. In addition, our findings demonstrate the potential for PTH(1-34) to amplify osteocytes' mechanotransduction by producing a more compliant tissue. Overall, the present study demonstrates that changes in tissue composition localized at the lacuna wall contribute to the strength and fatigue resistance of cortical bone gained in response to intermittent PTH(1-34) treatment.

Stress and Alterations in Bones: An Interdisciplinary Perspective

Decades of research have demonstrated that physical stress (PS) stimulates bone remodeling and affects bone structure and function through complex mechanotransduction mechanisms. Recent research has laid ground to the hypothesis that mental stress (MS) also influences bone biology, eventually leading to osteoporosis and increased bone fracture risk. These effects are likely exerted by modulation of hypothalamic-pituitary-adrenal axis activity, resulting in an altered release of growth hormones, glucocorticoids and cytokines, as demonstrated in human and animal studies. Furthermore, molecular cross talk between mental and PS is thought to exist, with either synergistic or preventative effects on bone disease progression depending on the characteristics of the applied stressor. This mini review will explain the emerging concept of MS as an important player in bone adaptation and its potential cross talk with PS by summarizing the current state of knowledge, highlighting newly evolving notions (such as intergenerational transmission of stress and its epigenetic modifications affecting bone) and proposing new research directions.