Cortical bone adaptation to a moderate level of mechanical loading in male Sost deficient mice.

Loss-of-function mutations in the Sost gene lead to high bone mass phenotypes. Pharmacological inhibition of Sost/sclerostin provides a new drug strategy for treating osteoporosis. Questions remain as to how physical activity may affect bone mass under sclerostin inhibition and if that effect differs between males and females. We previously observed in female Sost knockout (KO) mice an enhanced cortical bone formation response to a moderate level of applied loading (900 µÎµ at the tibial midshaft). The purpose of the present study was to examine cortical bone adaptation to the same strain level applied to male Sost KO mice. Strain-matched in vivo compressive loading was applied to the tibiae of 10-, 26- and 52-week-old male Sost KO and littermate control (LC) mice. The effect of tibial loading on bone (re)modeling was measured by microCT, 3D time-lapse in vivo morphometry, 2D histomorphometry and gene expression analyses. As expected, Sost deficiency led to high cortical bone mass in 10- and 26-week-old male mice as a result of increased bone formation. However, the enhanced bone formation associated with Sost deficiency did not appear to diminish with skeletal maturation. An increase in bone resorption was observed with skeletal maturation in male LC and Sost KO mice. Two weeks of in vivo loading (900 µÎµ at the tibial midshaft) induced only a mild anabolic response in 10- and 26-week-old male mice, independent of Sost deficiency. A decrease in the Wnt inhibitor Dkk1 expression was observed 3 h after loading in 52-week-old Sost KO and LC mice, and an increase in Lef1 expression was observed 8 h after loading in 10-week-old Sost KO mice. The current results suggest that long-term inhibition of sclerostin in male mice does not influence the adaptive response of cortical bone to moderate levels of loading. In contrast with our previous strain-matched study in females showing enhanced bone responses with Sost ablation, these results in males indicate that the influence of Sost deficiency on the cortical bone formation response to a moderate level of loading differs between males and females. Clinical studies examining antibodies to inhibit sclerostin may need to consider that the efficacy of additional physical activity regimens may be sex dependent.

Effects of Long-Term Sclerostin Deficiency on Trabecular Bone Mass and Adaption to Limb Loading Differ in Male and Female Mice.

A new therapeutic option to treat osteoporosis is focused on Wnt signaling and its inhibitor sclerostin, a product of the Sost gene. In this work, we study the effect of sclerostin deficiency on trabecular bone formation and resorption in male and female mice and whether it affects mechano-responsiveness. Male and female 10- and 26-week-old Sost knockout (KO) and littermate controls (LCs) were subjected to in vivo mechanical loading of the left tibia for 2 weeks. The right tibia served as internal control. The mice were imaged using in vivo micro-computed tomography at days 0, 5, 10, and 15 and tibiae were collected for histomorphometric analyses after euthanasia. Histomorphometry and micro-CT-based 3D time-lapse morphometry revealed an anabolic and anti-catabolic effect of Sost deficiency although increased trabecular bone resorption accompanied by diminished trabecular bone formation occurred with age. Loading led to diminished resorption in adult female but not in male mice. A net gain in bone volume could be achieved with mechanical loading in Sost KO adult female mice, which occurred through a further reduction in resorbed bone volume. Our data show that sclerostin deficiency has a particularly positive effect in adult female mice. Sclerostin antibodies are approved to treat postmenopausal women with high risk of osteoporotic fractures. Further studies are required to clarify whether both sexes benefit equally from sclerostin inhibition.

Sost deficiency leads to reduced mechanical strains at the tibia midshaft in strain-matched in vivo loading experiments in mice.

Sclerostin, a product of the Sost gene, is a Wnt-inhibitor and thus negatively regulates bone accrual. Canonical Wnt/ß-catenin signalling is also known to be activated in mechanotransduction. Sclerostin neutralizing antibodies are being tested in ongoing clinical trials to target osteoporosis and osteogenesis imperfecta but their interaction with mechanical stimuli on bone formation remains unclear. Sost knockout (KO) mice were examined to gain insight into how long-term Sost deficiency alters the local mechanical environment within the bone. This knowledge is crucial as the strain environment regulates bone adaptation. We characterized the bone geometry at the tibial midshaft of young and adult Sost KO and age-matched littermate control (LC) mice using microcomputed tomography imaging. The cortical area and the minimal and maximal moment of inertia were higher in Sost KO than in LC mice, whereas no difference was detected in either the anterior-posterior or medio-lateral bone curvature. Differences observed between age-matched genotypes were greater in adult mice. We analysed the local mechanical environment in the bone using finite-element models (FEMs), which showed that strains in the tibiae of Sost KO mice are lower than in age-matched LC mice at the diaphyseal midshaft, a region commonly used to assess cortical bone formation and resorption. Our FEMs also suggested that tissue mineral density is only a minor contributor to the strain distribution in tibial cortical bone from Sost KO mice compared to bone geometry. Furthermore, they indicated that although strain gauging experiments matched strains at the gauge site, strains along the tibial length were not comparable between age-matched Sost KO and LC mice or between young and adult animals within the same genotype.

Examining tissue composition, whole-bone morphology and mechanical behavior of GorabPrx1 mice tibiae: A mouse model of premature aging.

Gerodermia osteodysplastica (GO) is a segmental progeroid disorder caused by loss-of-function mutations in the GORAB gene, associated with early onset osteoporosis and bone fragility. A conditional mouse model of GO (GorabPrx1) was generated in which the Gorab gene was deleted in long bones. We examined the biomechanical/functional relevance of the GorabPrx1 mutants as a premature aging model by characterizing bone composition, tissue-level strains, and whole-bone morphology and mechanical properties of the tibia. MicroCT imaging showed that GorabPrx1 tibiae had an increased anterior convex curvature and decreased cortical cross-sectional area, cortical thickness and moments of inertia, compared to littermate control (LC) tibiae. Fourier transform infrared (FTIR) imaging indicated a 34% decrease in mineral/matrix ratio and a 27% increase in acid phosphate content in the posterior metaphyseal cortex of the GorabPrx1 tibiae (p < .05), suggesting delayed mineralization. In vivo strain gauge measurement and finite element analysis showed ∼two times higher tissue-level strains within the GorabPrx1 tibiae relative to LC tibiae when subjected to axial compressive loads of the same magnitude. Three-point bending tests suggested that GorabPrx1 tibiae were weaker and more brittle, as indicated by decreasing whole-bone strength (46%), stiffness (55%), work-to-fracture (61%) and post-yield displacement (47%). Many of these morphological and biomechanical characteristics of the GorabPrx1 tibia recapitulated changes in other animal models of skeletal aging. Future studies are necessary to confirm how our observations might guide the way to a better understanding and treatment of GO.

Sost deficiency led to a greater cortical bone formation response to mechanical loading and altered gene expression.

Bone adaptation optimizes mass and structure, but the mechano-response is already reduced at maturation. Downregulation of sclerostin was believed to be a mandatory step in mechano-adaptation, but in young mice it was shown that load-induced formation can occur independent of sclerostin, a product of the Sost gene. We hypothesized that the bone formation and resorption response to loading is not affected by Sost deficiency, but is age-specific. Our findings indicate that the anabolic response to in vivo tibial loading was reduced at maturation in Sost Knockout (KO) and littermate control (LC) mice. Age affected all anabolic and catabolic parameters and altered Sost and Wnt target gene expression. While load-induced cortical resorption was similar between genotypes, loading-induced gains in mineralizing surface was enhanced in Sost KO compared to LC mice. Loading led to a downregulation in expression of the Wnt inhibitor Dkk1. Expression of Dkk1 was greater in both control and loaded limbs of Sost KO compared to LC mice suggesting a compensatory role in the absence of Sost. These data suggest physical activity could enhance bone mass concurrently with sclerostin-neutralizing antibodies, but treatment strategies should consider the influence of age on ultimate load-induced bone mass gains.

In vivo implant forces acting on a vertebral body replacement during upper body flexion.

Knowledge about in vivo spinal loads is required for the identification of risk factors for low back pain and for realistic preclinical testing of spinal implants. Therefore, the aim of the present study was to measure the in vivo forces on a vertebral body replacement (VBR) during trunk flexion and to analyze in detail the typical relationship between trunk inclination and spinal load. Telemeterized VBRs were implanted in five patients. In vivo loads were measured 135 times during flexion while standing or sitting. The trunk inclination was simultaneously recorded. To reveal elementary differences between flexion while standing and sitting, the force increases at the maximal inclination, as compared to the upright position, were also determined. Approximately 90% of all standing trials showed a characteristic inclination-load relationship, with an initial increase of the resultant force followed by a plateau or even a decrease of the force at an inclination of approximately 33°. Further flexion to the average maximal inclination angle of 53° only marginally affected the implant loads (~450N). Maximal forces were measured during the return to the initial standing position (~565N). Flexion during standing led to a greater force increase (~330N) than during sitting (~200N) when compared to the respective upright positions. The force plateau at greater inclination angles might be explained by abdominal load support, complex stabilization of active and passive spinal structures or intricate load sharing within the implant complex. The data presented here aid in understanding the loads acting on an instrumented lumbar spine.

Age-related loss of lumbar spinal lordosis and mobility--a study of 323 asymptomatic volunteers.

BACKGROUND: The understanding of the individual shape and mobility of the lumbar spine are key factors for the prevention and treatment of low back pain. The influence of age and sex on the total lumbar lordosis and the range of motion as well as on different lumbar sub-regions (lower, middle and upper lordosis) in asymptomatic subjects still merits discussion, since it is essential for patient-specific treatment and evidence-based distinction between painful degenerative pathologies and asymptomatic aging. METHODS AND FINDINGS: A novel non-invasive measuring system was used to assess the total and local lumbar shape and its mobility of 323 asymptomatic volunteers (age: 20-75 yrs; BMI <26.0 kg/m2; males/females: 139/184). The lumbar lordosis for standing and the range of motion for maximal upper body flexion (RoF) and extension (RoE) were determined. The total lordosis was significantly reduced by approximately 20%, the RoF by 12% and the RoE by 31% in the oldest (>50 yrs) compared to the youngest age cohort (20-29 yrs). Locally, these decreases mostly occurred in the middle part of the lordosis and less towards the lumbo-sacral and thoraco-lumbar transitions. The sex only affected the RoE. CONCLUSIONS: During aging, the lower lumbar spine retains its lordosis and mobility, whereas the middle part flattens and becomes less mobile. These findings lay the ground for a better understanding of the incidence of level- and age-dependent spinal disorders, and may have important implications for the clinical long-term success of different surgical interventions.