Effects of exercise at different ages on bone density and mechanical properties of femoral bone of aged mice

Effects of more natural housing conditions on the muscular and skeletal characteristics of female C57BL/6J mice

BACKGROUND

Enrichment of home cages in laboratory experiments offers clear advantages, but has been criticized in some respects. First, there is a lack of definition, which makes methodological uniformity difficult. Second, there is concern that the enrichment of home cages may increase the variance of results in experiments. Here, the influence of more natural housing conditions on physiological parameters of female C57BL/6J mice was investigated from an animal welfare point of view. For this purpose, the animals were kept in three different housing conditions: conventional cage housing, enriched housing and the semi naturalistic environment. The focus was on musculoskeletal changes after long-term environmental enrichment.

RESULTS

The housing conditions had a long-term effect on the body weight of the test animals. The more complex and natural the home cage, the heavier the animals. This was associated with increased adipose deposits in the animals. There were no significant changes in muscle and bone characteristics except for single clues (femur diameter, bone resorption marker CTX-1). Additionally, the animals in the semi naturalistic environment (SNE) were found to have the fewest bone anomalies. Housing in the SNE appears to have the least effect on stress hormone concentrations. The lowest oxygen uptake was observed in enriched cage housing.

CONCLUSIONS

Despite increasing values, observed body weights were in the normal and strain-typical range. Overall, musculoskeletal parameters were slightly improved and age-related effects appear to have been attenuated. The variances in the results were not increased by more natural housing. This confirms the suitability of the applied housing conditions to ensure and increase animal welfare in laboratory experiments.

Age-Related Changes in the Mechanical Regulation of Bone Healing Are Explained by Altered Cellular Mechanoresponse

Increasing age is associated with a reduced bone regeneration potential and increased risk of morbidities and mortality. A reduced bone formation response to mechanical loading has been shown with aging, and it remains unknown if the interplay between aging and mechanical stimuli during regeneration is similar to adaptation. We used a combined in vivo/in silico approach to investigate age-related alterations in the mechanical regulation of bone healing and identified the relative impact of altered cellular function on tissue patterns during the regenerative cascade. To modulate the mechanical environment, femoral osteotomies in adult and elderly mice were stabilized using either a rigid or a semirigid external fixator, and the course of healing was evaluated using histomorphometric and micro-CT analyses at 7, 14, and 21 days post-surgery. Computer models were developed to investigate the influence of the local mechanical environment within the callus on tissue formation patterns. The models aimed to identify the key processes at the cellular level that alter the mechanical regulation of healing with aging. Fifteen age-related biological alterations were investigated on two levels (adult and elderly) with a design of experiments setup. We show a reduced response to changes in fixation stability with age, which could be explained by reduced cellular mechanoresponse, simulated as alteration of the ranges of mechanical stimuli driving mesenchymal stem cell differentiation. Cellular mechanoresponse has been so far widely ignored as a therapeutic target in aged patients. Our data hint to mechanotherapeutics as a potential treatment to enhance bone healing in the elderly. © 2019 American Society for Bone and Mineral Research.

Age Dependence of Systemic Bone Loss and Recovery Following Femur Fracture in Mice

The most reliable predictor of future fracture risk is a previous fracture of any kind. The etiology of this increased fracture risk is not fully known, but it is possible that fracture initiates systemic bone loss, leading to greater fracture risk at all skeletal sites. In this study, we investigated systemic bone loss and recovery after femoral fracture in young (3-month-old) and middle-aged (12-month-old) mice. Transverse femur fractures were created using a controlled impact, and whole-body bone mineral density (BMD), trabecular and cortical microstructure, bone mechanical properties, bone formation and resorption rates, mouse voluntary movement, and systemic inflammation were quantified at multiple time points post-fracture. We found that fracture led to decreased whole-body BMD in both young and middle-aged mice 2 weeks post-fracture; this bone loss was recovered by 6 weeks in young but not middle-aged mice. Similarly, trabecular bone volume fraction (BV/TV) of the L5 vertebral body was significantly reduced in fractured mice relative to control mice 2 weeks post-fracture (-11% for young mice, -18% for middle-aged mice); no significant differences were observed 6 weeks post-fracture. At 3 days post-fracture, we observed significant increases in serum levels of interleukin-6 and significant decreases in voluntary movement in fractured mice compared with control mice, with considerably greater changes in middle-aged mice than in young mice. At this time point, we also observed increased osteoclast number on L5 vertebral body trabecular bone of fractured mice compared with control mice. These data show that systemic bone loss occurs after fracture in both young and middle-aged mice, and recovery from this bone loss may vary with age. This systemic response could contribute to increased future fracture risk after fracture; these data may inform clinical treatment of fractures with respect to improving long-term skeletal health. © 2018 American Society for Bone and Mineral Research.

Variability in strain distribution in the mice tibia loading model: A preliminary study using digital volume correlation

It is well known that bone has an enormous adaptive capacity to mechanical loadings, and to this extent, several in vivo studies on mouse tibia use established cyclic compressive loading protocols to investigate the effects of mechanical stimuli. In these experiments, the applied axial load is well controlled but the positioning of the hind-limb between the loading endcaps may dramatically affect the strain distribution induced on the tibia. In this study, the full field strain distribution induced by a typical in vivo setup on mouse tibiae was investigated through a combination of in situ compressive testing, µCT scanning and a global digital volume correlation (DVC) approach. The precision of the DVC method and the effect of repositioning on the strain distributions were evaluated. Acceptable uncertainties of the DVC approach for the analysis of loaded tibiae (411 ± 58µɛ) were found for nodal spacing of approximately 50 voxels (520 µm). When pairs of in situ preloaded and loaded images were registered, low variability of the strain distributions within the tibia were seen (range of mean differences in principal strains: 585-1800µɛ). On contrary, larger differences were seen after repositioning (range of mean differences in principal strains: 2500-5500µɛ). To conclude, these preliminary results on thee specimens showed that the DVC approach applied to the mouse tibia can be precise enough to evaluate local strain distributions under loads, and that repositioning of the hind-limb within the testing machine can induce large differences in the strain distributions that should be accounted for when modelling this system.

Increased intrinsic stiffness and mineralization in femurs of adult rats after high intensity treadmill running training

BACKGROUND

Physical activity plays a tremendous role in determining bone mechanical behavior, which is superimposed to gravidity.

OBJECTIVE

Compare the geometric and material responses of the rat femur to a high intensity treadmill running training of a relatively short duration, as assessed by 3-point mechanical test.

METHODS

Mature male rats (180.0 ± 30 g) were assigned (7 rats/group) to no exercise (NE) or treadmill exercise (EX). After a preconditioning period, the running speed was set at 45 cm.seg-1 during 2 wks, frequency 5 d/wk, 2-hour sessions/day. Body weight and weight of the crural quadriceps were registered at euthanasia. The right femur was mechanically tested through 3-point bending. The left femur was ashed to estimate bone mineral content. Geometric and material bone properties were estimated directly or calculated by appropriate equations.

RESULTS

1) Final body weight was 14% reduced in EX rats, while the crural quadriceps was 47% increased. Yield and fracture loads, and structural stiffness were significantly higher in the EX rats, as were the apparent elastic modulus, the bone mineral content and the degree of mineralization. Geometric properties were not affected.

CONCLUSIONS

High intensity treadmill running training increases bone strength and stiffness by increasing material stiffness and mineralization, without affecting geometric bone parameters.

Revisiting the Debate: Does Exercise Build Strong Bones in the Mature and Senescent Skeleton?

Traditional exercise programs seem to be less osteogenic in the mature and post-mature skeleton compared to the young skeleton. This is likely because of the decline in sensitivity of bone to mechanical loading that occurs with advancing age. Another factor contributing to the apparently diminished benefit of exercise in older adults is failure of widely used measurement techniques (i.e., DXA) to identify changes in 3-dimensional bone structure, which are important determinants of bone strength. Moreover, although hormonal contributors to bone loss in the elderly are well-recognized, the influence of age-related increases in sympathetic nervous system activity, which impacts bone metabolism, is rarely considered. In this Perspective, we cite evidence from animal and human studies demonstrating anabolic effects of exercise on bone across the lifespan and we discuss theoretical considerations for designing exercise regimens to optimize bone health. We conclude with suggestions for future research that should help define the osteogenic potential of exercise in older individuals.

PTH signaling mediates perilacunar remodeling during exercise

Mechanical loading and release of endogenous parathyroid hormone (PTH) during exercise facilitate the adaptation of bone. However, it remains unclear how exercise and PTH influence the composition of bone and how exercise and PTH-mediated compositional changes influence the mechanical properties of bone. Thus, the primary purpose of this study was to establish compositional changes within osteocytes' perilacunar region of cortical bone following exercise, and evaluate the influence of endogenous PTH signaling on this perilacunar adaptation. Raman spectroscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to evaluate tissue composition surrounding individual lacuna within the tibia of 19week old male mice exposed to treadmill running for 3weeks. As a result of exercise, tissue within the perilacunar region (within 0-5μm of the lacuna wall) had a lower mineral-to-matrix ratio (MMR) compared to sedentary controls. In addition, exercise also increased the carbonate-to-phosphate ratio (CPR) across both perilacunar and non-perilacunar regions (5-10μm and 10-15μm from the lacuna walls). Tibial post-yield work had a significant negative correlation with perilacunar MMR. Inhibition of PTH activity with PTH(7-34) demonstrated that perilacunar remodeling during exercise was dependent on the cellular response to endogenous PTH. The osteocytes' response to endogenous PTH during exercise was characterized by a significant reduction in SOST expression and significant increase in FGF-23 expression. The potential reduction in phosphate levels due to FGF-23 expression may explain the increase in carbonate substitution. Overall, this is the first study to demonstrate that adaptation in tissue composition is localized around individual osteocytes, may contribute to the changes in whole bone mechanics during exercise, and that PTH signaling during exercise contributes to these adaptations.

Aging Leads to a Dysregulation in Mechanically Driven Bone Formation and Resorption

Physical activity is essential to maintain skeletal mass and structure, but its effect seems to diminish with age. To test the hypothesis that bone becomes less sensitive to mechanical strain with age, we used a combined in vivo/in silico approach. We investigated how maturation and aging influence the mechanical regulation of bone formation and resorption to 2 weeks of noninvasive in vivo controlled loading in mice. Using 3D in vivo morphometrical assessment of longitudinal microcomputed tomography images, we quantified sites in the mouse tibia where bone was deposited or resorbed in response to controlled in vivo loading. We compared the (re)modeling events (formation/resorption/quiescent) to the mechanical strains induced at these sites (predicted using finite element analysis). Mice of all age groups (young, adult, and elderly) responded to loading with increased formation and decreased resorption, preferentially at high strains. Low strains were associated with no anabolic response in adult and elderly mice, whereas young animals showed a strong response. Adult animals showed a clear separation between strain ranges where formation and resorption occurred but without an intermediate quiescent "lazy zone". This strain threshold disappeared in elderly mice, as mechanically induced (re)modeling became dysregulated, apparent in an inability to inhibit resorption or initiate formation. Contrary to what is generally believed until now, aging does not shift the mechanical threshold required to initiate formation or resorption, but rather blurs its specificity. These data suggest that pharmaceutical strategies augmenting physical exercise should consider this dysfunction in the mechanical regulation of bone (re)modeling to more effectively combat age-related bone loss.

PTH Signaling During Exercise Contributes to Bone Adaptation

Improving the structural integrity of bone reduces fracture risk and development of osteoporosis later in life. Exercise can increase the mechanical properties of bone, and this increase is often attributed to the dynamic loading created during exercise. However, the increase in systemic parathyroid hormone (PTH) levels during exercise gives reason to hypothesize that PTH signaling also regulates bone adaptation in response to exercise. Therefore, the first aim of this study was to establish the impact PTH signaling has on bone adaptation during exercise by inhibiting PTH signaling with PTH(7-34); the second aim was to determine whether increasing PTH levels during exercise with PTH(1-34) can augment bone adaptation. Thirty minutes after a single bout of running on a treadmill, mice exhibited a twofold increase in systemic PTH levels. Under the same exercise regimen, the influence of PTH signaling on bone adaptation during exercise was then evaluated in mice after 21 consecutive days of exercise and treatment with PTH(7-34), PTH(1-34), or vehicle. Exercise alone caused a significant increase in trabecular bone volume with adaptation to a more platelike structure, which was inhibited with PTH(7-34) during exercise. Changes in structural-level and tissue-level mechanical properties during exercise occurred in the absence of significant changes to cortical bone geometry. Inhibition of PTH signaling during exercise attenuated the changes in structural-level mechanical properties, but not tissue-level properties. Enhanced PTH signaling during exercise with PTH(1-34) increased trabecular and cortical bone volume, but had little effect on the structural-level and tissue-level mechanical properties compared to exercise alone. Our study is the first to demonstrate that bone adaptation during exercise is not only a function of dynamic loading, but also PTH release, and that PTH signaling contributes differently at the structural and tissue levels.

Skeletal maturity leads to a reduction in the strain magnitudes induced within the bone: a murine tibia study

Bone adapts to changes in the local mechanical environment (e.g. strains) through formation and resorption processes. However, the bone adaptation response is significantly reduced with increasing age. The mechanical strains induced within the bone by external loading are determined by bone morphology and tissue material properties. Although it is known that changes in bone mass, architecture and bone tissue quality occur with age, to what extent they contribute to the altered bone adaptation response remains to be determined. This study investigated alterations in strains induced in the tibia of different aged female C57Bl/6J mice (young, 10-week-old; adult, 26-week-old; and elderly, 78-week-old) subjected to in vivo compressive loading. Using a combined in vivo/in silico approach, the strains in the bones were assessed by both strain gauging and finite element modeling experiments. In cortical bone, strain magnitudes induced at the mid-diaphysis decreased by 20% from young to adult mice and by 15% from adult to elderly mice. In the cancellous bone (at the proximal metaphysis), induced strains were 70% higher in young compared with adult and elderly mice. Taking into account previous studies showing a reduced bone adaptation response to mechanical loading in adulthood, these results suggest that the diminished adaptive response is in part due to a reduction in the strains induced within the bone.

Increased endoplasmic reticulum stress in mouse osteocytes with aging alters Cox-2 response to mechanical stimuli

Aging reduces bone mass as well as the anabolic response of bone to mechanical stimuli, resulting in osteopenia. Endoplasmic reticulum (ER) stress impairs the response of myogenic cells to anabolic stimuli, and is involved in sarcopenia, but whether ER stress also contributes to osteopenia is unknown. Therefore, we tested whether ER stress exists in bones of aged mice, and whether this impairs the osteocyte response to mechanical stimulation. Primary osteocytes were obtained from long bones of adult (8 months) and old (24-26 months) mice, treated with or without the pharmacological ER stress inducer tunicamycin, and either or not subjected to mechanical loading by pulsating fluid flow (PFF). The osteocyte response to PFF was assessed by measuring cyclooxygenase-2 (Cox-2) mRNA levels and nitric oxide (NO) production. mRNA levels of ER stress markers were higher in old versus adult osteocytes (+40% for activating transcription factor-4, +120% for C/EBP homologous protein, and +120% for spliced X-box binding protein-1, p < 0.05). The Cox-2 response to PFF was fourfold decreased in cells from old bones (p < 0.001), while tunicamycin decreased PFF-induced Cox-2 expression by threefold in cells from adult bones (p < 0.01). PFF increased NO production by 50% at 60 min in osteocytes from old versus adult bones (p < 0.01). In conclusion, our data indicate that the expression of several ER stress markers was higher in osteocytes from bones of old compared to adult mice. Since ER stress altered the response of osteocytes to mechanical loading, it could be a novel factor contributing to osteopenia.

The influence of age on adaptive bone formation and bone resorption

Bone is a tissue with enormous adaptive capacity, balancing resorption and formation processes. It is known that mechanical loading shifts this balance towards an increased formation, leading to enhanced bone mass and mechanical performance. What is not known is how this adaptive response to mechanical loading changes with age. Using dynamic micro-tomography, we show that structural adaptive changes of trabecular bone within the tibia of living mice subjected to two weeks of in vivo cyclic loading are altered by aging. Comparisons of 10, 26 and 78 weeks old animals reveal that the adaptive capacity diminishes. Strikingly, adaptation was asymmetric in that loading increases formation more than it reduces resorption. This asymmetry further shifts the (re)modeling balance towards a net bone loss with age. Loading results in a major increase in the surface area of mineralizing bone. Interestingly, the resorption thickness is independent of loading in trabecular bone in all age groups. This data suggests that during youth, mechanical stimulation induces the recruitment of bone modeling cells whereas in old age, only bone forming cells are affected. These findings provide mechanistic insights into the processes that guide skeletal aging in mice as well as in other mammals.

Short-term exercise-induced improvements in bone properties are for the most part not maintained during aging in hamsters

Physical exercise during growth affects composition, structure and mechanical properties of bone. In this study we investigated whether the beneficial effects of exercise during the early growth phase have long-lasting effects or not. Female Syrian golden hamsters (total n=152) were used in this study. Half of the hamsters had access to running wheels during their rapid growth phase (from 1 to 3months of age). The hamsters were sacrificed at the ages of 1, 3, 12, and 15months. The diaphysis of the mineralized humerus was analyzed with microCT and subjected to three-point-bending mechanical testing. The trabecular bone in the tibial metaphysis was also analyzed with microCT. The collagen matrix of the humerus bone was studied by tensile testing after decalcification. The weight of the hamsters as well as the length of the bone and the volumetric bone mineral density (BMDvol) of the humerus was higher in the running group at the early age (3months). Moreover, the mineralized bone showed improved mechanical properties in humerus and had greater trabecular thickness in the subchondral bone of tibia in the runners. However, by the age of 12 and 15months, these differences were equalized with the sedentary group. The tensile strength and Young's modulus of decalcified humerus were higher in the runners at early stage, indicating a stronger collagen network. In tibial metaphysis, trabecular thickness was significantly higher for the runners in the old age groups (12 and 15months). Our study demonstrates that physical exercise during growth improves either directly or indirectly through weight gain bone properties of the hamsters. However, the beneficial effects were for the most part not maintained during aging.

Connexin43 modulates post-natal cortical bone modeling and mechano-responsiveness

Recent advances have established connexin43 (Cx43) as a key regulator of osteoblast function and of bone response to mechanical stimuli. Work by independent laboratories has consistently demonstrated postnatal development of larger than normal cross-section of long bones after conditional ablation of the Cx43 gene, Gja1, selectively in osteoblasts and/or osteocytes. This phenotype is caused by excessive endocortical bone resorption associated with periosteal expansion and cortical thinning. Review of published data suggests that the earlier in the osteogenic lineage is Gja1 deleted, the more severe is the cortical phenotype, implying functional roles of Cx43 at different stages of the osteoblast differentiation program. Such cortical modeling abnormalities resemble the changes occurring in the cortex upon disuse or aging. Indeed, Cx43 deficiency desensitizes endocortical osteoclasts from activation induced by removal of mechanical load, thus preventing medullary area expansion. The action of Cx43 on cancellous bone is controversial. Furthermore, the absence of Cx43 in osteoblasts and osteocytes results in activation of periosteal bone formation at lower strains than in wild-type bones, suggesting that Cx43 deficiency increased cortical sensitivity to mechanical load. Thus, Cx43 modulates cortical bone modeling in homeostatic conditions and in response to mechanical load by restraining both endocortical bone resorption and periosteal bone formation. Cx43 may represent a novel pharmacologic target for improving cortical bone strength through modulation of mechano-responsiveness.

Diminished response to in vivo mechanical loading in trabecular and not cortical bone in adulthood of female C57Bl/6 mice coincides with a reduction in deformation to load

Bone loss occurs during adulthood in both women and men and affects trabecular bone more than cortical bone. The mechanism responsible for trabecular bone loss during adulthood remains unexplained, but may be due at least in part to a reduced mechanoresponsiveness. We hypothesized that trabecular and cortical bone would respond anabolically to loading and that the bone response to mechanical loading would be reduced and the onset delayed in adult compared to postpubescent mice. We evaluated the longitudinal adaptive response of trabecular and cortical bone in postpubescent, young (10 week old) and adult (26 week old) female C57Bl/6J mice to axial tibial compression using in vivo microCT (days 0, 5, 10, and 15) and dynamic histomorphometry (day 15). Loading elicited an anabolic response in both trabecular and cortical bone in young and adult mice. As hypothesized, trabecular bone in adult mice exhibited a reduced and delayed response to loading compared to the young mice, apparent in trabecular bone volume fraction and architecture after 10 days. No difference in mechanoresponsiveness of the cortical bone was observed between young and adult mice. Finite element analysis showed that load-induced strain was reduced with age. Our results suggest that trabecular bone loss that occurs in adulthood may in part be due to a reduced mechanoresponsiveness in this tissue and/or a reduction in the induced tissue deformation which occurs during habitual loading. Therapeutic approaches that address the mechanoresponsiveness of the bone tissue may be a promising and alternate strategy to maintain trabecular bone mass during aging.

A model of poorly controlled type 1 Diabetes Mellitus and its treatment with aerobic exercise training

BACKGROUND

Modern exogenous insulin therapy can improve the quality of life of Type 1 Diabetic Mellitus (T1DM) patients, although maintenance of normal glycaemic levels is often a challenge given the variety of factors that alter it. A number of studies have examined the effect of exercise in T1DM; however, the majority of experimental studies have utilized diabetic rodents with severe hyperglycaemia. Given that T1DM patients are likely to refrain from hyperglycaemia, studies examining the effects of regular exercise in which blood glucose is poorly controlled would better represent the T1DM population.

METHODS

The current study examined the ability of a ten-week aerobic exercise training program to modify markers of cardiovascular function and bone health in STZ-induced diabetic rodents maintained in the 9-15 mM glycaemic range through insulin therapy.

RESULTS

Moderate hyperglycaemia, when prolonged, leads to significant changes in cardiac structure, bone health, and glucose handling capacity. Ten weeks of exercise was able to alleviate many of these deleterious events as no significant cardiovascular functional alterations were evident except a reduction in resting heart rate and an increase in stroke volume index. Further, despite changes in cardiac dimensions, exercise was able to elevate cardiac output index and increase the E/A ratio of exercising diabetic animals which would be indicative of improvements of cardiac function.

CONCLUSIONS

Together, this study demonstrates that despite moderate hyperglycaemia, the combined role of a ten-week exercise training program coupled with insulin therapy is able to alleviate many of the well-known complications associated with diabetes progression.

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.

Collagen and mineral deposition in rabbit cortical bone during maturation and growth: effects on tissue properties

We characterized the composition and mechanical properties of cortical bone during maturation and growth and in adult life in the rabbit. We hypothesized that the collagen network develops earlier than the mineralized matrix. Growth was monitored, and the rabbits were euthanized at birth (newborn), and at 1, 3, 6, 9, and 18 months of age. The collagen network was assessed biochemically (collagen content, enzymatic and non-enzymatic cross-links) in specimens from the mid-diaphysis of the tibia and femur and biomechanically (tensile testing) from decalcified whole tibia specimens. The mineralized matrix was analyzed using pQCT and 3-point bend tests from intact femur specimens. The collagen content and the Young's modulus of the collagen matrix increased significantly until the rabbits were 3 months old, and thereafter remained stable. The amount of HP and LP collagen cross-links increased continuously from newborn to 18 months of age, whereas PEN cross-links increased after 6 months of age. Bone mineral density and the Young's modulus of the mineralized bone increased until the rabbits were at least 6 months old. We concluded that substantial changes take place during the normal process of development in both the biochemical and biomechanical properties of rabbit cortical bone. In cortical bone, the collagen network reaches its mature composition and mechanical strength prior to the mineralized matrix.

Aged mice have enhanced endocortical response and normal periosteal response compared with young-adult mice following 1 week of axial tibial compression

With aging, the skeleton may lose its ability to respond to positive mechanical stimuli. We hypothesized that aged mice are less responsive to loading than young-adult mice. We subjected aged (22 months) and young-adult (7 months) BALB/c male mice to daily bouts of axial tibial compression for 1 week and evaluated cortical and trabecular responses using micro-computed tomography (µCT) and dynamic histomorphometry. The right legs of 95 mice were loaded for 60 rest-inserted cycles per day to 8, 10, or 12 N peak force (generating mid-diaphyseal strains of 900 to 1900 µε endocortically and 1400 to 3100 µε periosteally). At the mid-diaphysis, mice from both age groups showed a strong anabolic response on the endocortex (Ec) and periosteum (Ps) [Ec.MS/BS and Ps.MS/BS: loaded (right) versus control (left), p < .05]. Generally, bone formation increased with increasing peak force. At the endocortical surface, contrary to our hypothesis, aged mice had a significantly greater response to loading than young-adult mice (Ec.MS/BS and Ec.BFR/BS: 22 months versus 7 months, p < .001). Responses at the periosteal surface did not differ between age groups (p > .05). The loading-induced increase in bone formation resulted in increased cortical area in both age groups (loaded versus control, p < .05). In contrast to the strong cortical response, loading only weakly stimulated trabecular bone formation. Serial (in vivo) µCT examinations at the proximal metaphysis revealed that loading caused a loss of trabecular bone in 7-month-old mice, whereas it appeared to prevent bone loss in 22-month-old mice. In summary, 1 week of daily tibial compression stimulated a robust endocortical and periosteal bone-formation response at the mid-diaphysis in both young-adult and aged male BALB/c mice. We conclude that aging does not limit the short-term anabolic response of cortical bone to mechanical stimulation in our animal model.

Physical exercise improves properties of bone and its collagen network in growing and maturing mice

This study characterized bone structure, composition, and mechanical properties in growing male mice. The development of the collagen network during maturation was monitored, and the effect of voluntary physical exercise was investigated. We hypothesized that increased bone loading from exercise would increase the amount and improve the properties of the collagen network during growth and maturation. Half of the mice (total n = 168) had access to running wheels, while half were kept sedentary. Weight and running activity were recorded, and groups of mice were killed at 1, 2, 4, and 6 months of age. The collagen network was assessed by biochemical evaluation of collagen content and cross-links and by tensile testing of decalcified bone. Mineralized femur was analyzed with pQCT and three-point-bending and femoral neck-strength tests. After 6 months, the exercising mice had 10% lower body weight than the sedentary group. There was no difference in the amount of collagen or collagen cross-links, while tensile testing had higher breaking force and stiffness of the collagen network in runners after 4 months but not after 6 months. The bone mineral density and cross-sectional area were higher in the running group after 6 months. Runners also showed higher breaking force and stiffness of the diaphysis and the femoral neck at 2 and 6 months. The significant modulation of mechanical properties of the collagen network without any change in collagen content indicates that physical exercise improves properties of the collagen network in maturing bone. The improvement after exercise of the properties of mineralized bone appears to be more pronounced and long-lasting compared to the early improved properties of the collagen network.

Long-term voluntary exercise of male mice induces more beneficial effects on cancellous and cortical bone than on the collagenous matrix

The effects of lifelong physical exercise on the composition, structure and mechanical properties of bone are not well understood. Earlier, we found that voluntary physical exercise improved various properties of bone in maturing male mice up to 6 months of age. In this study, we extended the previous study to 18 months. Half of the mice (total N=144) had access to running wheels while half were kept sedentary. The collagen network was assessed biochemically and by tensile testing of decalcified bone. The mineralized femur was analyzed with pQCT and three-point-bending of the diaphysis and neck-strength-test. The proximal tibia was analyzed with microCT. The bone collagen revealed inferior tensional properties with aging and the mineralized femur demonstrated decreased stiffness with age. In the running mice, tensile properties and the BMD were reduced at 18 months of age compared to the sedentary mice. In contrast, the stiffness of both the diaphysis and femoral neck was higher, and trabecular architecture and structure were improved in the running mice. In summary, the results suggest that lifelong exercise training of male mice results in more beneficial effects on intact mineralized bone in both the diaphysis and epiphysis than on bone collagenous matrix.

Systemic first-line phenotyping

With the completion of the mouse genome sequence an essential task for biomedical sciences in the twenty-first century will be the generation and functional analysis of mouse models for every gene in the mammalian genome. More than 30,000 mutations in ES cells will be engineered and thousands of mouse disease models will become available over the coming years by the collaborative effort of the International Mouse Knockout Consortium. In order to realize the full value of the mouse models proper characterization, archiving and dissemination of mouse disease models to the research community have to be performed. Phenotyping centers (mouse clinics) provide the necessary capacity, broad expertise, equipment, and infrastructure to carry out large-scale systemic first-line phenotyping. Using the example of the German Mouse Clinic (GMC) we will introduce the reader to the different aspects of the organization of a mouse clinic and present selected methods used in first-line phenotyping.

Pathogenesis of age-related osteoporosis: impaired mechano-responsiveness of bone is not the culprit

Background

According to prevailing understanding, skeletal mechano-responsiveness declines with age and this apparent failure of the mechano-sensory feedback system has been attributed to the gradual bone loss with aging (age-related osteoporosis). The objective of this study was to evaluate whether the capacity of senescent skeleton to respond to increased loading is indeed reduced as compared to young mature skeleton.

Methods and Findings

108 male and 101 female rats were randomly assigned into Exercise and Control groups. Exercise groups were subjected to treadmill training either at peak bone mass between 47–61 weeks of age (Mature) or at senescence between 75–102 weeks of age (Senescent). After the training intervention, femoral necks and diaphysis were evaluated with peripheral quantitative computed tomography (pQCT) and mechanical testing; the proximal tibia was assessed with microcomputed tomography (μCT). The μCT analysis revealed that the senescent bone tissue was structurally deteriorated compared to the mature bone tissue, confirming the existence of age-related osteoporosis. As regards the mechano-responsiveness, the used loading resulted in only marginal increases in the bones of the mature animals, while significant exercise-induced increases were observed virtually in all bone traits among the senescent rats.

Conclusion

The bones of senescent rats displayed a clear ability to respond to an exercise regimen that failed to initiate an adaptive response in mature animals. Thus, our observations suggest that the pathogenesis of age-related osteoporosis is not attributable to impaired mechano-responsiveness of aging skeleton. It also seems that strengthening of even senescent bones is possible – naturally provided that safe and efficient training methods can be developed for the oldest old.

Exercise-induced changes in the cortical bone of growing mice are bone- and gender-specific

Fracture risk and mechanical competence of bone are functions of bone mass and tissue quality, which in turn are dependent on the bone's mechanical environment. Male mice have a greater response to non-weight-bearing exercise than females, resulting in larger, stronger bones compared with control animals. The aim of this study was to test the hypothesis that short-term weight-bearing running during growth (21 days starting at 8 weeks of age; 30 min/day; 12 m/min; 5 degrees incline; 7 days/week) would similarly have a greater impact on cross-sectional geometry and mechanical competence in the femora and tibiae of male mice versus females. Based on the orientation of the legs during running and the proximity of the tibia to the point of impact, this response was hypothesized to be greatest in the tibia. Exercise-related changes relative to controls were assayed by four-point bending tests, while volumetric bone mineral density and cross-sectional geometry were also assessed. The response to running was bone- and gender-specific, with male tibiae demonstrating the greatest effects. In male tibiae, periosteal perimeter, endocortical perimeter, cortical area, medial-lateral width and bending moment of inertia increased versus control mice suggesting that while growth is occurring in these mice between 8 and 11 weeks of age, exercise accelerated this growth resulting in a greater increase in bone tissue over the 3 weeks of the study. Exercise increased tissue-level strain-to-failure and structural post-yield deformation in the male tibiae, but these post-yield benefits came at the expense of decreased yield deformation, structural and tissue-level yield strength and tissue-level ultimate strength. These results suggest that exercise superimposed upon growth accelerated growth-related increases in tibial cross-sectional dimensions. Exercise also influenced the quality of this forming bone, significantly impacting structural and tissue-level mechanical properties.

Ação da atividade física sobre as propriedades mecânicas dos fêmures e tíbias de ratas osteopênicas

Avaliamos as propriedades mecânicas, obtidas através de ensaios de flexo-compressão de fêmures e de flexão em 3 pontos de tíbias, de ratas adultas, ovariectomizadas e submetidas à atividade física. Foram utilizadas 30 ratas divididas em 3 grupos: G1: Controle. G2: Animais ovariectomizados e não submetidos a exercícios. G3: Foram treinados em uma gaiola giratória por cinco dias consecutivos, em seguida submetidos à ovariectomia, permanecendo em repouso por 24 horas. Exercitaram por 30 minutos, 5 dias por semana durante 9 semanas com velocidade de aproximadamente 0,31 m/s. Os valores obtidos de carga e de deformação evidenciaram que os fêmures das ratas ovariectomizadas apresentaram redução estatisticamente significativa nas propriedades mecânicas de carga e de deformação no limite máximo. Os fêmures do grupo ovariectomizado e submetido à prática de exercícios apresentaram valores de carga e de deformação no limite máximo superiores aos do grupo apenas ovariectomizado, no entanto, sem significância estatística. As tíbias não apresentaram alterações significativas em nenhuma das propriedades mecânicas estudadas. A atividade física aplicada por 30 minutos, 5 dias por semana durante 9 semanas a 0,31 m/s não foi suficiente para corrigir as alterações biomecânicas do tecido ósseo provocadas pela ovariectomia.

Finite element analysis of the mouse tibia: estimating endocortical strain during three-point bending in SAMP6 osteoporotic mice

To support future studies of tibial bending in a murine model of senile osteoporosis (SAMP6), we sought to determine the relationship between applied external bending force and peak endocortical strain in the tibiae of SAMP6 and control SAMR1 mice. The lower hindlimbs of mice were loaded by three-point bending in the lateral-medial plane with a support length of 10 mm. Force-periosteal strain relations were first determined using standard strain gauge methods. Finite-element analysis (FEA) models of the tibia-fibula were generated based on microcomputed tomography images. After choosing appropriate boundary conditions, FEA predictions of periosteal strains were within 15% of measured values. FEA revealed a narrow (3-4 mm) region of the central tibia with well-developed bending strains (tension medially, compression laterally); outside this region, we observed high shear strains. Both the strain gauge data and the finite-element simulations indicated that the tibia of the SAMP6 mouse was 20-25% stiffer than the SAMR1 tibia, consistent with a larger moment of inertia and higher cortical bone modulus. Thus, higher levels of force are required to produce the same target values of strain in the SAMP6 tibia. The ratio of periosteal to endocortical strain in the region of interest was similar for the two mouse strains (1.5-1.6). Based on these ratios, we scaled the strain gauge data to estimate the force-endocortical strain relations for the two mouse strains. In conclusion, FEA, with supporting strain gauge measurements, has provided unique insight regarding the strain environment throughout the tibia during three-point bending in mice.

Age does not influence the bone response to treadmill exercise in female rats

PURPOSE

Because it is believed that bone may respond to exercise differently at different ages, we compared bone responses in immature and mature rats after 12 wk of treadmill running.

METHODS

Twenty-two immature (5-wk-old) and 21 mature (17-wk-old) female Sprague Dawley rats were randomized into a running (trained, P = 10 immature, 9 mature) or a control group (controls, P = 12 immature, 12 mature) before sacrifice 12 wk later. Rats ran on a treadmill five times per week for 60-70 min at speeds up to 26 m.min. Both at baseline and after intervention, we measured total body, lumbar spine, and proximal femoral bone mineral, as well as total body soft tissue composition using dual-energy x-ray absorptiometry (DXA). After sacrificing the animals, we measured dynamic and static histomorphometry and three-point bending strength of the tibia.

RESULTS

Running training was associated with greater differences in tibial subperiosteal area, cortical cross-sectional area, peak load, stiffness, and moment of inertia in immature and mature rats (P < 0.05). The trained rats had greater periosteal bone formation rates (P < 0.01) than controls, but there was no difference in tibial trabecular bone histomorphometry. Similar running-related gains were seen in DXA lumbar spine area (P = 0.04) and bone mineral content (BMC; P = 0.03) at both ages. For total body bone area and BMC, the immature trained group increased significantly compared with controls (P < 0.05), whereas the mature trained group gained less than did controls (P < 0.01).

CONCLUSION

In this model, where a similar physical training program was performed by immature and mature female rats, we demonstrated that both age groups were sensitive to loading and that bone strength gains appeared to result more from changes in bone geometry than from improved material properties.

Long bones from the senescence accelerated mouse SAMP6 have increased size but reduced whole-bone strength and resistance to fracture

The senescence accelerated mouse strain P6 (SAMP6) has emerged as a useful model of senile osteoporosis because it has many features of the disease, including low trabecular bone formation and low areal bone density. We further characterized the SAMP6 model of senile osteoporosis by comparing morphological, mechanical, and densitometric properties of femurs and tibias from SAMP6 mice to those of the control strain (SAMR1) at 4 months and 12 months of age. SAMP6 long bones had increased periosteal width and endosteal area (p < 0.05), resulting in an average increase of 30% in moments of inertia (p < 0.05), but no difference in bone area (p > 0.05) compared with control. Despite their increased moments of inertia, long bones from SAMP6 mice were relatively weak and brittle. Ultimate bending moment was reduced by 25%, and both postyield displacement and energy-to-fracture were reduced by 60% compared with SAMR1 controls (p < 0.001). Average cortical ash fraction was increased slightly from 0.74 in SAMR1 to 0.76 in SAMP6 bones (p < 0.05), indicating that increased mineralization may have contributed to the brittleness of SAMP6 bones. The relative differences we observed--increased endosteal and periosteal dimensions, reduced bending strength, increased brittleness, and increased mineralization--are analogous to changes that occur in the aging human skeleton. Moreover, these features were consistently observed in young (4-month) and old (12-month) animals. These findings extend the previous descriptions of the SAMP6 mouse and identify key mechanical features that further validate its relevance as a unique and functionally relevant model of senile osteoporosis.