Androgen receptor disruption increases the osteogenic response to mechanical loading in male mice

Relationships between longitudinal changes in body composition and bone mineral density in middle-to-older aged Australians

There are limited longitudinal data regarding relationships between changes in body composition and bone mineral density (BMD). In 3671 participants aged 46-70 years at baseline, ∆lean mass was a stronger determinant than ∆fat mass of ∆BMD over 6 years. Maintained or increased lean mass may slow down age-related bone loss.

PURPOSE

There are limited longitudinal data regarding relationships between changes in body composition and bone mineral density (BMD) with ageing. We examined these in the Busselton Healthy Ageing Study.

METHODS

We studied 3671 participants (2019 females) aged 46-70 years at baseline with body composition and BMD assessments by dual-energy x-ray absorptiometry at baseline and after ~6 years. Relationships between changes in total body mass (∆TM), lean mass (∆LM) and fat mass (∆FM) with ∆BMD at total hip, femoral neck and lumbar spine were evaluated using restricted cubic spline modelling (accounting for baseline covariates) and mid-quartile least square means were compared.

RESULTS

∆TM was positively associated with ∆BMD of total hip and femoral neck in both sexes, and spine in females; in females but not males, associations plateaued at ∆TM above ~5kg for all sites. In females, ∆LM was positively associated with ∆BMD of all three sites with plateauing of the relationship at ∆LM above ~1kg. Women in the highest quartile of ∆LM (Q4, mid-quartile value +1.6 kg) had 0.019-0.028 g/cm2 less reduction in BMD than those in the lowest quartile (Q1, -2.1 kg). In males, ∆LM was positively associated with ∆BMD of total hip and femoral neck; men in Q4 (+1.6 kg) had 0.015 and 0.011 g/cm2 less bone loss, respectively, compared with Q1 (-2.7 kg). ∆FM was positively associated with ∆BMD of total hip only in both sexes.

CONCLUSION

∆LM is a stronger determinant than ∆FM of ∆BMD. Maintained or increased LM is associated with less age-related bone loss.

Osteocyte Estrogen Receptor β (Ot-ERβ) Regulates Bone Turnover and Skeletal Adaptive Response to Mechanical Loading Differently in Male and Female Growing and Adult Mice

Age-related bone loss is a failure of balanced bone turnover and diminished skeletal mechanoadaptation. Estrogen receptors, ERα and ERβ, play critical roles in osteoprotective regulation activated by estrogen and mechanical signals. Previous studies mainly focused on ERα and showed that osteocyte-ERα (Ot-ERα) regulated trabecular, but not cortical bone, and played a minor role in load-induced cortical adaptation. However, the role of Ot-ERβ in bone mass regulation remains unrevealed. To address this issue, we characterized bone (re)modeling and gene expression in male and female mice with Ot-ERβ deletion (ERβ-dOT) and littermate control (LC) at 10 weeks (young) or 28 weeks (adult) of age, as well as their responses to in vivo tibial compressive loading. Increased cancellous bone mass appeared in the L4 vertebral body of young male ERβ-dOT mice. At the same time, femoral cortical bone gene expression showed signs consistent with elevated osteoblast and osteoclast activities (type-I collagen, Cat K, RANKL). Upregulated androgen receptor (AR) expression was observed in young male ERβ-dOT mice relative to LC, suggesting a compensatory effect of testosterone on male bone protection. In contrast, bone mass in L4 decreased in adult male ERβ-dOT mice, attributed to potentially increased bone resorption activity (Cat K) with no change in bone formation. There was no effect of ERβ-dOT on bone mass or gene expression in female mice. Sex-dependent regulation of Ot-ERβ also appeared in load-induced cortical responsiveness. Young female ERβ-dOT mice showed an enhanced tibial cortical anabolic adaptation compared with LC. In contrast, an attenuated cortical anabolic response presented at the proximal tibia in male ERβ-dOT mice at both ages. For the first time, our findings suggest that Ot-ERβ regulates bone (re)modeling and the response to mechanical signals through different mechanisms in males and females. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Protein Kinase G2 Is Essential for Skeletal Homeostasis and Adaptation to Mechanical Loading in Male but Not Female Mice

We previously showed that the NO/cGMP/protein kinase G (PKG) signaling pathway positively regulates osteoblast proliferation, differentiation, and survival in vitro, and that cGMP-elevating agents have bone-anabolic effects in mice. Here, we generated mice with an osteoblast-specific (OB) knockout (KO) of type 2 PKG (gene name Prkg2) using a Col1a1(2.3 kb)-Cre driver. Compared to wild type (WT) littermates, 8-week-old male OB Prkg2-KO mice had fewer osteoblasts, reduced bone formation rates, and lower trabecular and cortical bone volumes. Female OB Prkg2-KO littermates showed no bone abnormalities, despite the same degree of PKG2 deficiency in bone. Expression of osteoblast differentiation- and Wnt/β-catenin-related genes was lower in primary osteoblasts and bones of male KO but not female KO mice compared to WT littermates. Osteoclast parameters were unaffected in both sexes. Since PKG2 is part of a mechano-sensitive complex in osteoblast membranes, we examined its role during mechanical loading. Cyclical compression of the tibia increased cortical thickness and induced mechanosensitive and Wnt/β-catenin-related genes to a similar extent in male and female WT mice and female OB Prkg2-KO mice, but loading had a minimal effect in male KO mice. We conclude that PKG2 drives bone acquisition and adaptation to mechanical loading via the Wnt/β-catenin pathway in male mice. The striking sexual dimorphism of OB Prkg2-KO mice suggests that current U.S. Food and Drug Administration-approved cGMP-elevating agents may represent novel effective treatment options for male osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Inactivation of AR or ERα in Extrahypothalamic Neurons Does not Affect Osteogenic Response to Loading in Male Mice

Failure of bone mass maintenance in spite of functional loading is an important contributor to osteoporosis and related fractures. While the link between sex steroids and the osteogenic response to loading is well established, the underlying mechanisms are unknown, hampering clinical relevance. Androgens inhibit mechanoresponsiveness in male mice, but the cell type mediating this effect remains unidentified. To evaluate the role of neuronal sex steroid receptor signaling in the male bone's adaptive capacity, we subjected adult male mice with an extrahypothalamic neuron-specific knockout of the androgen receptor (N-ARKO) or the estrogen receptor alpha (N-ERαKO) to in vivo mechanical stimulation of the tibia. Loading increased cortical thickness in the control animals mainly through periosteal expansion, as total cross-sectional tissue area and cortical bone area but not medullary area were higher in the loaded than the unloaded tibia. Trabecular bone volume fraction also increased upon loading in the control group, mostly due to trabecular thickening. N-ARKO and N-ERαKO males displayed a loading response at both the cortical and trabecular bone compartments that was not different from their control littermates. In conclusion, we show that the presence of androgen receptor or estrogen receptor alpha in extrahypothalamic neurons is dispensable for the osteogenic response to mechanical loading in male mice.

Effects of Extracellular Osteoanabolic Agents on the Endogenous Response of Osteoblastic Cells

The complex multidimensional skeletal organization can adapt its structure in accordance with external contexts, demonstrating excellent self-renewal capacity. Thus, optimal extracellular environmental properties are critical for bone regeneration and inextricably linked to the mechanical and biological states of bone. It is interesting to note that the microstructure of bone depends not only on genetic determinants (which control the bone remodeling loop through autocrine and paracrine signals) but also, more importantly, on the continuous response of cells to external mechanical cues. In particular, bone cells sense mechanical signals such as shear, tensile, loading and vibration, and once activated, they react by regulating bone anabolism. Although several specific surrounding conditions needed for osteoblast cells to specifically augment bone formation have been empirically discovered, most of the underlying biomechanical cellular processes underneath remain largely unknown. Nevertheless, exogenous stimuli of endogenous osteogenesis can be applied to promote the mineral apposition rate, bone formation, bone mass and bone strength, as well as expediting fracture repair and bone regeneration. The following review summarizes the latest studies related to the proliferation and differentiation of osteoblastic cells, enhanced by mechanical forces or supplemental signaling factors (such as trace metals, nutraceuticals, vitamins and exosomes), providing a thorough overview of the exogenous osteogenic agents which can be exploited to modulate and influence the mechanically induced anabolism of bone. Furthermore, this review aims to discuss the emerging role of extracellular stimuli in skeletal metabolism as well as their potential roles and provide new perspectives for the treatment of bone disorders.

NLRP3 is involved in long bone edification and the maturation of osteogenic cells

Overexpression of the nucleotide-binding leucine-rich repeat protein 3 (NLRP3) inflammasome in chronic auto-immune diseases leads to skeletal anomalies, with severe osteopenia due to the activation of osteoclasts. Reproducing this phenotype in Nlrp3 knock-in mice has provided insights into the role of NLRP3 in bone metabolism. We studied the role of NLRP3 in physiological bone development using a complete Nlrp3 knock-out mouse model. We found impaired skeletal development in Nlrp3-/- mice, resulting in a shorter stature than that of Nlrp3+/+  mice. These growth defects were associated with altered femur bone growth, characterized by a deficient growth plate and an osteopenic profile of the trabeculae. No differences in osteoclast recruitment or activity were observed. Instead, Nlrp3-/- femurs showed a less mineralized matrix in the trabeculae than those of Nlrp3+/+  mice, as well as less bone sialoprotein (BSP) expressing hypertrophic chondrocytes. In vitro, primary osteoblasts lacking NLRP3 expression showed defective mineralization, together with the downregulation of BSP expression. Finally, follow-up by micro-CT highlighted the role of NLPR3 in bone growth, occurring early in living mice, as the osteopenic phenotype diminishes over time. Overall, our data suggest that NLRP3 is involved in bone edification via the regulation of hypertrophic chondrocyte maturation and osteoblast activity. Furthermore, the defect appeared to be transitory, as the skeleton recovered with aging.

Disparate bone anabolic cues activate bone formation by regulating the rapid lysosomal degradation of sclerostin protein

The downregulation of sclerostin in osteocytes mediates bone formation in response to mechanical cues and parathyroid hormone (PTH). To date, the regulation of sclerostin has been attributed exclusively to the transcriptional downregulation of the Sost gene hours after stimulation. Using mouse models and rodent cell lines, we describe the rapid, minute-scale post-translational degradation of sclerostin protein by the lysosome following mechanical load and PTH. We present a model, integrating both new and established mechanically and hormonally activated effectors into the regulated degradation of sclerostin by lysosomes. Using a mouse forelimb mechanical loading model, we find transient inhibition of lysosomal degradation or the upstream mechano-signaling pathway controlling sclerostin abundance impairs subsequent load-induced bone formation by preventing sclerostin degradation. We also link dysfunctional lysosomes to aberrant sclerostin regulation using human Gaucher disease iPSCs. These results reveal how bone anabolic cues post-translationally regulate sclerostin abundance in osteocytes to regulate bone formation.

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 με 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 με 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.

Scl-Ab reverts pro-osteoclastogenic signalling and resorption in estrogen deficient osteocytes

Background

Neutralising antibodies to sclerostin (Scl-Ab) have shown significant potential to induce bone formation and decrease bone resorption, increase strength and substantially reduce fracture risk in animal studies and clinical trials. Mechanical loading negatively regulates sclerostin expression, and sclerostin has been shown to induce RANKL synthesis in osteocytes. However, how Scl-Ab governs osteocyte regulation of osteoclast differentiation and function is not fully understood. We have recently discovered that osteoblasts and osteocytes alter osteoclastogenic signalling (RANKL/OPG) during estrogen-deficiency, and that osteoblast-induced osteoclastogenesis and resorption are exacerbated. However, it is not known whether estrogen deficient osteocytes exacerbate osteoclastogenesis. The aims of this study were to (1) establish whether osteocytes induce osteoclastogenesis and bone resorption during estrogen deficiency in vitro (2) investigate whether the sclerostin antibody can revert osteocyte-mediated osteoclastogenesis and resorption by attenuating RANKL/OPG expression.

Results

Using conditioned media and co-culture experiments we found increased osteocyte-induced osteoclastogenesis and bone resorption in estrogen deficient conditions. This is the first study to report that administration of Scl-Ab has the ability to revert osteocyte-mediated osteoclastogenesis and resorption by decreasing RANKL/OPG ratio expression and increasing WISP1 expression in estrogen deficient osteocytes.

Conclusions

This study provides an enhanced understanding of the biological changes underpinning decreases in bone resorption following Scl-Ab treatment observed in vivo by revealing that Scl-Ab can reduce pro-osteoclastogenic cell signalling between osteocytes and osteoclasts.

The loss of STAT3 in mature osteoclasts has detrimental effects on bone structure

Signal Transducer and Activator of Transcription 3 (STAT3) has recently been shown to be involved in bone development and has been implicated in bone diseases, such as Job’s Syndrome. Bone growth and changes have been known for many years to differ between sexes with male bones tending to have higher bone mass than female bones and older females tending to lose bone mass at faster rates than older males. Previous studies using conditional knock mice with Stat3 specifically deleted from the osteoblasts showed both sexes exhibited decreased bone mineral density (BMD) and strength. Using the Cre-Lox system with Cathepsin K promotor driving Cre to target the deletion of the Stat3 gene in mature osteoclasts (STAT3-cKO mice), we observed that 8-week old STAT3-cKO female femurs exhibited significantly lower BMD and bone mineral content (BMC) compared to littermate control (CN) females. There were no differences in BMD and BMC observed between male knock-out and male CN femurs. However, micro-computed tomography (μCT) analysis showed that both male and female STAT3-cKO mice had significant decreases in bone volume/tissue volume (BV/TV). Bone histomorphometry analysis of the distal femur, further revealed a decrease in bone formation rate and mineralizing surface/bone surface (MS/BS) with a significant decrease in osteoclast surface in female, but not male, STAT3-cKO mice. Profiling gene expression in an osteoclastic cell line with a knockdown of STAT3 showed an upregulation of a number of genes that are directly regulated by estrogen receptors. These data collectively suggest that regulation of STAT3 differs in male and female osteoclasts and that inactivation of STAT3 in osteoclasts affects bone turnover more in females than males, demonstrating the complicated nature of STAT3 signaling pathways in osteoclastogenesis. Drugs targeting the STAT3 pathway may be used for treatment of diseases such as Job’s Syndrome and osteoporosis.

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.

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.

Locomotor training with adjuvant testosterone preserves cancellous bone and promotes muscle plasticity in male rats after severe spinal cord injury

Loading and testosterone may influence musculoskeletal recovery after spinal cord injury (SCI). Our objectives were to determine (a) the acute effects of bodyweight-supported treadmill training (TM) on hindlimb cancellous bone microstructure and muscle mass in adult rats after severe contusion SCI and (b) whether longer-term TM with adjuvant testosterone enanthate (TE) delivers musculoskeletal benefit. In Study 1, TM (40 min/day, 5 days/week, beginning 1 week postsurgery) did not prevent SCI-induced hindlimb cancellous bone loss after 3 weeks. In Study 2, TM did not attenuate SCI-induced plantar flexor muscles atrophy nor improve locomotor recovery after 4 weeks. In our main study, SCI produced extensive distal femur and proximal tibia cancellous bone deficits, a deleterious slow-to-fast fiber-type transition in soleus, lower muscle fiber cross-sectional area (fCSA), impaired muscle force production, and levator ani/bulbocavernosus (LABC) muscle atrophy after 8 weeks. TE alone (7.0 mg/week) suppressed bone resorption, attenuated cancellous bone loss, constrained the soleus fiber-type transition, and prevented LABC atrophy. In comparison, TE+TM concomitantly suppressed bone resorption and stimulated bone formation after SCI, produced near-complete cancellous bone preservation, prevented the soleus fiber-type transition, attenuated soleus fCSA atrophy, maintained soleus force production, and increased LABC mass. 75% of SCI+TE+TM animals recovered voluntary over-ground hindlimb stepping, while no SCI and only 20% of SCI+TE animals regained stepping ability. Positive associations between testosterone and locomotor function suggest that TE influenced locomotor recovery. In conclusion, short-term TM alone did not improve bone, muscle, or locomotor recovery in adult rats after severe SCI, while longer-term TE+TM provided more comprehensive musculoskeletal benefit than TE alone.

Mechanical Loading Differentially Affects Osteocytes in Fibulae from Lactating Mice Compared to Osteocytes in Virgin Mice: Possible Role for Lacuna Size

Hormonal changes during lactation are associated with profound changes in bone cell biology, such as osteocytic osteolysis, resulting in larger lacunae. Larger lacuna shape theoretically enhances the transmission of mechanical signals to osteocytes. We aimed to provide experimental evidence supporting this theory by comparing the mechanoresponse of osteocytes in the bone of lactating mice, which have enlarged lacunae due to osteocytic osteolysis, with the response of osteocytes in bone from age-matched virgin mice. The osteocyte mechanoresponse was measured in excised fibulae that were cultured in hormone-free medium for 24 h and cyclically loaded for 10 min (sinusoidal compressive load, 3000 µε, 5 Hz) by quantifying loading-related changes in Sost mRNA expression (qPCR) and sclerostin and β-catenin protein expression (immunohistochemistry). Loading decreased Sost expression by ~ threefold in fibulae of lactating mice. The loading-induced decrease in sclerostin protein expression by osteocytes was larger in lactating mice (55% decrease ± 14 (± SD), n = 8) than virgin mice (33% decrease ± 15, n = 7). Mechanical loading upregulated β-catenin expression in osteocytes in lactating mice by 3.5-fold (± 0.2, n = 6) which is significantly (p < 0.01) higher than the 1.6-fold increase in β-catenin expression by osteocytes in fibulae from virgin mice (± 0.12, n = 4). These results suggest that osteocytes in fibulae from lactating mice with large lacunae may respond stronger to mechanical loading than those from virgin mice. This could indicate that osteocytes residing in larger lacuna show a stronger response to mechanical loading.

Defective Wnt3 expression by testicular Sertoli cells compromise male fertility

Testicular Sertoli cells make a niche for the division and differentiation of germ cells. Sertoli cells respond to increased follicle-stimulating hormone (FSH) and testosterone (T) levels at the onset of puberty by producing paracrine factors which affect germ cells and trigger robust onset of spermatogenesis. Such paracrine support to germ cells is absent during infancy, despite Sertoli cells being exposed to high FSH and T within the infant testis. This situation is similar to certain cases of male idiopathic infertility where post-pubertal Sertoli cells fail to support germ cell division and differentiation in spite of endogenous or exogenous hormonal support. Defective Sertoli cells in such individuals may fail to express the full complement of their paracrine repertoire. Identification and supplementation with such factors may overcome Sertoli cells deficiencies and help trigger quantitatively and qualitatively normal differentiation of germ cells. To this end, we compared the transcriptome of FSH- and T-treated infant and pubertal monkey Sertoli cells by DNA microarray. Expression of Wnt3, a morphogen of the Wnt/β-catenin pathway, was higher in pubertal Sertoli cells relative to infant Sertoli cells. Transgenic mice were generated by us in which Wnt3 expression was curtailed specifically in post-pubertal Sertoli cells by shRNA. Subfertility and oligozoospermia were noticed in such animals with low Wnt3 expression in post-pubertal Sertoli cells along with diminished expression of Connexin43, a gap-junctional molecule essential for germ cell development. We report that the FSH- and T-targetedf Wnt3 governs Sertoli cell-mediated regulation of spermatogenesis and hence is crucial for fertility.

Sclerostin's role in bone's adaptive response to mechanical loading

Mechanical loading is the primary functional determinant of bone mass and architecture, and osteocytes play a key role in translating mechanical signals into (re)modelling responses. Although the precise mechanisms remain unclear, Wnt signalling pathway components, and the anti-osteogenic canonical Wnt inhibitor Sost/sclerostin in particular, play an important role in regulating bone's adaptive response to loading. Increases in loading-engendered strains down-regulate osteocyte sclerostin expression, whereas reduced strains, as in disuse, are associated with increased sclerostin production and bone loss. However, while sclerostin up-regulation appears to be necessary for the loss of bone with disuse, the role of sclerostin in the osteogenic response to loading is more complex. While mice unable to down-regulate sclerostin do not gain bone with loading, Sost knockout mice have an enhanced osteogenic response to loading. The molecular mechanisms by which osteocytes sense and transduce loading-related stimuli into changes in sclerostin expression remain unclear but include several, potentially interlinked, signalling cascades involving periostin/integrin, prostaglandin, estrogen receptor, calcium/NO and Igf signalling. Deciphering the mechanisms by which changes in the mechanical environment regulate sclerostin production may lead to the development of therapeutic strategies that can reverse the skeletal structural deterioration characteristic of disuse and age-related osteoporosis and enhance bones' functional adaptation to loading. By enhancing the osteogenic potential of the context in which individual therapies such as sclerostin antibodies act it may become possible to both prevent and reverse the age-related skeletal structural deterioration characteristic of osteoporosis.

Hormonal and systemic regulation of sclerostin

The Wnt/β-catenin signaling pathway plays an essential role in osteoblast biology. Sclerostin is a soluble antagonist of Wnt/β-catenin signaling secreted primarily by osteocytes. Current evidence indicates that sclerostin likely functions as a local/paracrine regulator of bone metabolism rather than as an endocrine hormone. Nonetheless, circulating sclerostin levels in humans often reflect changes in the bone microenvironment, although there may be exceptions to this observation. Using existing assays, circulating sclerostin levels have been shown to be altered in response to both hormonal stimuli and across a variety of normal physiological and pathophysiological conditions. In both rodents and humans, parathyroid hormone provided either intermittently or continuously suppresses sclerostin levels. Likewise, most evidence from both human and animal studies supports a suppressive effect of estrogen on sclerostin levels. Efforts to examine non-hormonal/systemic regulation of sclerostin have in general shown less consistent findings or have provided associations rather than direct interventional information, with the exception of mechanosensory studies which have consistently demonstrated increased sclerostin levels with skeletal unloading, and conversely decreases in sclerostin with enhanced skeletal loading. Herein, we will review the existent literature on both hormonal and non-hormonal/systemic factors which have been studied for their impact on sclerostin regulation.

Transcriptional control of Sost in bone

Sclerostin is an osteocyte derived negative regulator of bone formation. A highly specific expression pattern and the exclusive bone phenotype have made Sclerostin an attractive target for therapeutic intervention in treating metabolic bone diseases such as osteoporosis and in facilitating fracture repair. Understanding the molecular mechanisms that regulate Sclerostin transcription is of great interest as it may unveil new avenues for therapeutic approaches. Such studies may also elucidate how various signaling pathways intersect to modulate bone metabolism. Here we review the current understanding of the upstream molecular mechanisms that regulate Sost/SOST transcription, in bone.

Estrogens and Androgens in Skeletal Physiology and Pathophysiology

Estrogens and androgens influence the growth and maintenance of the mammalian skeleton and are responsible for its sexual dimorphism. Estrogen deficiency at menopause or loss of both estrogens and androgens in elderly men contribute to the development of osteoporosis, one of the most common and impactful metabolic diseases of old age. In the last 20 years, basic and clinical research advances, genetic insights from humans and rodents, and newer imaging technologies have changed considerably the landscape of our understanding of bone biology as well as the relationship between sex steroids and the physiology and pathophysiology of bone metabolism. Together with the appreciation of the side effects of estrogen-related therapies on breast cancer and cardiovascular diseases, these advances have also drastically altered the treatment of osteoporosis. In this article, we provide a comprehensive review of the molecular and cellular mechanisms of action of estrogens and androgens on bone, their influences on skeletal homeostasis during growth and adulthood, the pathogenetic mechanisms of the adverse effects of their deficiency on the female and male skeleton, as well as the role of natural and synthetic estrogenic or androgenic compounds in the pharmacotherapy of osteoporosis. We highlight latest advances on the crosstalk between hormonal and mechanical signals, the relevance of the antioxidant properties of estrogens and androgens, the difference of their cellular targets in different bone envelopes, the role of estrogen deficiency in male osteoporosis, and the contribution of estrogen or androgen deficiency to the monomorphic effects of aging on skeletal involution.

Androgens have antiresorptive effects on trabecular disuse osteopenia independent from muscle atrophy

Aging hypogonadal men are at increased risk of osteoporosis and sarcopenia. Testosterone is a potentially appealing strategy to prevent simultaneous bone and muscle loss. The androgen receptor (AR) mediates antiresorptive effects on trabecular bone via osteoblast-lineage cells, as well as muscle-anabolic actions. Sex steroids also modify the skeletal response to mechanical loading. However, it is unclear whether the effects of androgens on bone remain effective independent of mechanical stimulation or rather require indirect androgen effects via muscle. This study aims to characterize the effects and underlying mechanisms of androgens on disuse osteosarcopenia. Adult male mice received a unilateral botulinum toxin (BTx) injection, and underwent sham surgery or orchidectomy (ORX) without or with testosterone (ORX+T) or dihydrotestosterone (ORX+DHT) replacement. Compared to the contralateral internal control hindlimb, acute trabecular number and bone volume loss was increased by ORX and partially prevented DHT. T was more efficient and increased BV/TV in both hindlimbs over sham values, although it did not reduce the detrimental effect of BTx. Both androgens and BTx regulated trabecular osteoclast surface as well as tartrate-resistant acid phosphatase expression. Androgens also prevented BTx-induced body weight loss but did not significantly influence paralysis or muscle atrophy. BTx and ORX both reduced cortical thickness via endosteal expansion, which was prevented by T but not DHT. In long-term follow-up, the residual trabecular bone volume deficit in sham-BTx hindlimbs was prevented by DHT but T restored it more efficiently to pre-treatment levels. Conditional AR deletion in late osteoblasts and osteocytes or in the satellite cell lineage increased age-related trabecular bone loss in both hindlimbs without influencing the effect of BTx on trabecular osteopenia. We conclude that androgens have antiresorptive effects on trabecular disuse osteopenia which do not require AR actions on bone via muscle or via osteocytes.

Muscle-bone interactions: From experimental models to the clinic? A critical update

Bone is a biomechanical tissue shaped by forces from muscles and gravitation. Simultaneous bone and muscle decay and dysfunction (osteosarcopenia or sarco-osteoporosis) is seen in ageing, numerous clinical situations including after stroke or paralysis, in neuromuscular dystrophies, glucocorticoid excess, or in association with vitamin D, growth hormone/insulin like growth factor or sex steroid deficiency, as well as in spaceflight. Physical exercise may be beneficial in these situations, but further work is still needed to translate acceptable and effective biomechanical interventions like vibration therapy from animal models to humans. Novel antiresorptive and anabolic therapies are emerging for osteoporosis as well as drugs for sarcopenia, cancer cachexia or muscle wasting disorders, including antibodies against myostatin or activin receptor type IIA and IIB (e.g. bimagrumab). Ideally, increasing muscle mass would increase muscle strength and restore bone loss from disuse. However, the classical view that muscle is unidirectionally dominant over bone via mechanical loading is overly simplistic. Indeed, recent studies indicate a role for neuronal regulation of not only muscle but also bone metabolism, bone signaling pathways like receptor activator of nuclear factor kappa-B ligand (RANKL) implicated in muscle biology, myokines affecting bone and possible bone-to-muscle communication. Moreover, pharmacological strategies inducing isolated myocyte hypertrophy may not translate into increased muscle power because tendons, connective tissue, neurons and energy metabolism need to adapt as well. We aim here to critically review key musculoskeletal molecular pathways involved in mechanoregulation and their effect on the bone-muscle unit as a whole, as well as preclinical and emerging clinical evidence regarding the effects of sarcopenia therapies on osteoporosis and vice versa.

Effects of sex steroids on bones and muscles: Similarities, parallels, and putative interactions in health and disease

Estrogens and androgens influence the growth and maintenance of bones and muscles and are responsible for their sexual dimorphism. A decline in their circulating levels leads to loss of mass and functional integrity in both tissues. In the article, we highlight the similarities of the molecular and cellular mechanisms of action of sex steroids in the two tissues; the commonality of a critical role of mechanical forces on tissue mass and function; emerging evidence for an interplay between mechanical forces and hormonal and growth factor signals in both bones and muscles; as well as the current state of evidence for or against a cross-talk between muscles and bone. In addition, we review evidence for the parallels in the development of osteoporosis and sarcopenia with advancing age and the potential common mechanisms responsible for the age-dependent involution of these two tissues. Lastly, we discuss the striking difference in the availability of several drug therapies for the prevention and treatment of osteoporosis, as compared to none for sarcopenia. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Wnt16 Is Associated with Age-Related Bone Loss and Estrogen Withdrawal in Murine Bone

Genome Wide Association Studies suggest that Wnt16 is an important contributor to the mechanisms controlling bone mineral density, cortical thickness, bone strength and ultimately fracture risk. Wnt16 acts on osteoblasts and osteoclasts and, in cortical bone, is predominantly derived from osteoblasts. This led us to hypothesize that low bone mass would be associated with low levels of Wnt16 expression and that Wnt16 expression would be increased by anabolic factors, including mechanical loading. We therefore investigated Wnt16 expression in the context of ageing, mechanical loading and unloading, estrogen deficiency and replacement, and estrogen receptor α (ERα) depletion. Quantitative real time PCR showed that Wnt16 mRNA expression was lower in cortical bone and marrow of aged compared to young female mice. Neither increased nor decreased (by disuse) mechanical loading altered Wnt16 expression in young female mice, although Wnt16 expression was decreased following ovariectomy. Both 17β-estradiol and the Selective Estrogen Receptor Modulator Tamoxifen increased Wnt16 expression relative to ovariectomy. Wnt16 and ERβ expression were increased in female ERα-/- mice when compared to Wild Type. We also addressed potential effects of gender on Wnt16 expression and while the expression was lower in the cortical bone of aged males as in females, it was higher in male bone marrow of aged mice compared to young. In the kidney, which we used as a non-bone reference tissue, Wnt16 expression was unaffected by age in either males or females. In summary, age, and its associated bone loss, is associated with low levels of Wnt16 expression whereas bone loss associated with disuse has no effect on Wnt16 expression. In the artificially loaded mouse tibia we observed no loading-related up-regulation of Wnt16 expression but provide evidence that its expression is influenced by estrogen receptor signaling. These findings suggest that while Wnt16 is not an obligatory contributor to regulation of bone mass per se, it potentially plays a role in influencing pathways associated with regulation of bone mass during ageing and estrogen withdrawal.

Associations between body mass index, lean and fat body mass and bone mineral density in middle-aged Australians: The Busselton Healthy Ageing Study

Low BMI is a risk factor for osteoporosis, but it is not clear if relationships between BMI, lean mass (LM), fat mass (FM) and BMD are consistent across different levels of BMI. We studied 1929 Caucasian participants (1014 females) aged 45-66years in the Busselton Healthy Ageing Study in Western Australia. Body composition and BMD of total body, lumbar spine, total hip and femoral neck were measured using DXA. From generalized additive models, the positive relationships between BMI and BMD were weaker at high BMI, particularly at the spine and in males. In the entire cohort, adjusting for relevant covariates, LM and FM were significant predictors of all BMD measures in both genders. In men, analysis by tertiles of BMI showed that LM and FM (in kg) were positively associated with BMD (in mg/cm(2)) in tertile 1 except for LM and spine BMD (LM β: 5.18-6.80, FM β: 3.38-9.24, all P<0.05), but not in the middle or upper tertiles (LM β: -3.12-3.07, FM β: -4.75-1.82, P>0.05). In women, LM was positively associated with BMD in each tertile of BMI, except for spine BMD in the upper tertile, with regression coefficients lower in the upper tertile (β: 5.16-9.95, 5.76-9.56 and 2.80-5.78, respectively, all P<0.05). FM was positively associated with total body, spine and total hip BMD in women in BMI tertile 1 (β: 2.86-6.68, P<0.05); these associations were weaker or absent in the middle and upper tertiles. In conclusion, in middle-aged adults the positive relationships between lean or fat mass with BMD among those with higher BMI are absent in males and weaker in females.

Sex steroid actions in male bone

Sex steroids are chief regulators of gender differences in the skeleton, and male gender is one of the strongest protective factors against osteoporotic fractures. This advantage in bone strength relies mainly on greater cortical bone expansion during pubertal peak bone mass acquisition and superior skeletal maintenance during aging. During both these phases, estrogens acting via estrogen receptor-α in osteoblast lineage cells are crucial for male cortical and trabecular bone, as evident from conditional genetic mouse models, epidemiological studies, rare genetic conditions, genome-wide meta-analyses, and recent interventional trials. Genetic mouse models have also demonstrated a direct role for androgens independent of aromatization on trabecular bone via the androgen receptor in osteoblasts and osteocytes, although the target cell for their key effects on periosteal bone formation remains elusive. Low serum estradiol predicts incident fractures, but the highest risk occurs in men with additionally low T and high SHBG. Still, the possible clinical utility of serum sex steroids for fracture prediction is unknown. It is likely that sex steroid actions on male bone metabolism rely also on extraskeletal mechanisms and cross talk with other signaling pathways. We propose that estrogens influence fracture risk in aging men via direct effects on bone, whereas androgens exert an additional antifracture effect mainly via extraskeletal parameters such as muscle mass and propensity to fall. Given the demographic trends of increased longevity and consequent rise of osteoporosis, an increased understanding of how sex steroids influence male bone health remains a high research priority.

Comparison of three methods of calculating strain in the mouse ulna in exogenous loading studies

Axial compression of mouse limbs is commonly used to induce bone formation in a controlled, non-invasive manner. Determination of peak strains caused by loading is central to interpreting results. Load-strain calibration is typically performed using uniaxial strain gauges attached to the diaphyseal, periosteal surface of a small number of sacrificed animals. Strain is measured as the limb is loaded to a range of physiological loads known to be anabolic to bone. The load-strain relationship determined by this subgroup is then extrapolated to a larger group of experimental mice. This method of strain calculation requires the challenging process of strain gauging very small bones which is subject to variability in placement of the strain gauge. We previously developed a method to estimate animal-specific periosteal strain during axial ulnar loading using an image-based computational approach that does not require strain gauges. The purpose of this study was to compare the relationship between load-induced bone formation rates and periosteal strain at ulnar midshaft using three different methods to estimate strain: (A) Nominal strain values based solely on load-strain calibration; (B) Strains calculated from load-strain calibration, but scaled for differences in mid-shaft cross-sectional geometry among animals; and (C) An alternative image-based computational method for calculating strains based on beam theory and animal-specific bone geometry. Our results show that the alternative method (C) provides comparable correlation between strain and bone formation rates in the mouse ulna relative to the strain gauge-dependent methods (A and B), while avoiding the need to use strain gauges.

Androgens and estrogens in skeletal sexual dimorphism

Bone is an endocrine tissue expressing androgen and estrogen receptors as well as steroid metabolizing enzymes. The bioactivity of circulating sex steroids is modulated by sex hormone-binding globulin and local conversion in bone tissue, for example, from testosterone (T) to estradiol (E2) by aromatase, or to dihydrotestosterone by 5α-reductase enzymes. Our understanding of the structural basis for gender differences in bone strength has advanced considerably over recent years due to increasing use of (high resolution) peripheral computed tomography. These microarchitectural insights form the basis to understand sex steroid influences on male peak bone mass and turnover in cortical vs trabecular bone. Recent studies using Cre/LoxP technology have further refined our mechanistic insights from global knockout mice into the direct contributions of sex steroids and their respective nuclear receptors in osteoblasts, osteoclasts, osteocytes, and other cells to male osteoporosis. At the same time, these studies have reinforced the notion that androgen and estrogen deficiency have both direct and pleiotropic effects via interaction with, for example, insulin-like growth factor 1, inflammation, oxidative stress, central nervous system control of bone metabolism, adaptation to mechanical loading, etc., This review will summarize recent advances on these issues in the field of sex steroid actions in male bone homeostasis.

Protein kinase Cα (PKCα) regulates bone architecture and osteoblast activity

Bones' strength is achieved and maintained through adaptation to load bearing. The role of the protein kinase PKCα in this process has not been previously reported. However, we observed a phenotype in the long bones of Prkca^−/− female but not male mice, in which bone tissue progressively invades the medullary cavity in the mid-diaphysis. This bone deposition progresses with age and is prevented by disuse but unaffected by ovariectomy. Castration of male Prkca^−/− but not WT mice results in the formation of small amounts of intramedullary bone. Osteoblast differentiation markers and Wnt target gene expression were up-regulated in osteoblast-like cells derived from cortical bone of female Prkca^−/− mice compared with WT. Additionally, although osteoblastic cells derived from WT proliferate following exposure to estradiol or mechanical strain, those from Prkca^−/− mice do not. Female Prkca^−/− mice develop splenomegaly and reduced marrow GBA1 expression reminiscent of Gaucher disease, in which PKC involvement has been suggested previously. From these data, we infer that in female mice, PKCα normally serves to prevent endosteal bone formation stimulated by load bearing. This phenotype appears to be suppressed by testicular hormones in male Prkca^−/− mice. Within osteoblastic cells, PKCα enhances proliferation and suppresses differentiation, and this regulation involves the Wnt pathway. These findings implicate PKCα as a target gene for therapeutic approaches in low bone mass conditions.

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

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

The role of estrogen and androgen receptors in bone health and disease

Mouse models with cell-specific deletion of the estrogen receptor (ER) α, the androgen receptor (AR) or the receptor activator of nuclear factor κB ligand (RANKL), as well as cascade-selective estrogenic compounds have provided novel insights into the function and signalling of ERα and AR. The studies reveal that the effects of estrogens on trabecular versus cortical bone mass are mediated by direct effects on osteoclasts and osteoblasts, respectively. The protection of cortical bone mass by estrogens is mediated via ERα, using a non-nucleus-initiated mechanism. By contrast, the AR of mature osteoblasts is indispensable for the maintenance of trabecular bone mass in male mammals, but not required for the anabolic effects of androgens on cortical bone. Most unexpectedly, and independently of estrogens, ERα in osteoblast progenitors stimulates Wnt signalling and periosteal bone accrual in response to mechanical strain. RANKL expression in B lymphocytes, but not T lymphocytes, contributes to the loss of trabecular bone caused by estrogen deficiency. In this Review, we summarize this evidence and discuss its implications for understanding the regulation of trabecular and cortical bone mass; the integration of hormonal and mechanical signals; the relative importance of estrogens versus androgens in the male skeleton; and, finally, the pathogenesis and treatment of osteoporosis.

Androgen receptor (AR) pathophysiological roles in androgen-related diseases in skin, bone/muscle, metabolic syndrome and neuron/immune systems: lessons learned from mice lacking AR in specific cells

The androgen receptor (AR) is expressed ubiquitously and plays a variety of roles in a vast number of physiological and pathophysiological processes. Recent studies of AR knockout (ARKO) mouse models, particularly the cell type- or tissue-specific ARKO models, have uncovered many AR cell type- or tissue-specific pathophysiological roles in mice, which otherwise would not be delineated from conventional castration and androgen insensitivity syndrome studies. Thus, the AR in various specific cell types plays pivotal roles in production and maturation of immune cells, bone mineralization, and muscle growth. In metabolism, the ARs in brain, particularly in the hypothalamus, and the liver appear to participate in regulation of insulin sensitivity and glucose homeostasis. The AR also plays key roles in cutaneous wound healing and cardiovascular diseases, including atherosclerosis and abdominal aortic aneurysm. This article will discuss the results obtained from the total, cell type-, or tissue-specific ARKO models. The understanding of AR cell type- or tissue-specific physiological and pathophysiological roles using these in vivo mouse models will provide useful information in uncovering AR roles in humans and eventually help us to develop better therapies via targeting the AR or its downstream signaling molecules to combat androgen/AR-related diseases.

Correlates of bone microarchitectural parameters and serum sclerostin levels in men: the STRAMBO study

Sclerostin is predominantly expressed by osteocytes. Serum sclerostin levels are positively correlated with areal bone mineral density (aBMD) measured by dual-energy X-ray absorptiometry (DXA) and bone microarchitecture assessed by high-resolution peripheral quantitative computed tomography (HR-pQCT) in small studies. We assessed the relation of serum sclerostin levels with aBMD and microarchitectural parameters based on HR-pQCT in 1134 men aged 20 to 87 years using multivariable models adjusted for confounders (age, body size, lifestyle, comorbidities, hormones regulating bone metabolism, muscle mass and strength). The apparent age-related increase in serum sclerostin levels was faster before the age of 63 years than afterward (0.43 SD versus 0.20 SD per decade). In 446 men aged ≤63 years, aBMD (spine, hip, whole body), trabecular volumetric BMD (Tb.vBMD), and trabecular number (Tb.N) at the distal radius and tibia were higher in the highest sclerostin quartile versus the three lower quartiles combined. After adjustment for aBMD, men in the highest sclerostin quartile had higher Tb.vBMD (mainly in the central compartment) and Tb.N at both skeletal sites (p < 0.05 to 0.001). In 688 men aged >63 years, aBMD was positively associated with serum sclerostin levels at all skeletal sites. Cortical vBMD (Ct.vBMD) and cortical thickness (Ct.Th) were lower in the first sclerostin quartile versus the three higher quartiles combined. Tb.vBMD increased across the sclerostin quartiles, and was associated with lower Tb.N and more heterogeneous trabecular distribution (higher Tb.Sp.SD) in men in the lowest sclerostin quartile. After adjustment for aBMD, men in the lowest sclerostin quartile had lower Tb.vBMD and Tb.N, but higher Tb.Sp.SD (p < 0.05 to 0.001) at both the skeletal sites. In conclusion, serum sclerostin levels in men are strongly positively associated with better bone microarchitectural parameters, mainly trabecular architecture, regardless of the potential confounders.

Novel insights in the regulation and mechanism of androgen action on bone

PURPOSE OF REVIEW

This review provides an update on the associations of testosterone, estrogens, sex hormone binding globulin, GH-IGF-I, osteocalcin and mechanical loading with relevance to skeletal health.

RECENT FINDINGS

The simple concept of a dual model of action of androgens, i.e. either directly via the androgen receptor or indirectly by estrogens, is proving more complicated because of novel interactions of these hormones and their receptors with other hormonal as well as mechanical signals.

SUMMARY

Testosterone - in contrast with estrogen - is not uniformly associated with fracture risk in men. However, androgen receptor mediated action is clearly important for trabecular bone maintenance in male mice whereas both estrogens and androgens regulate cortical bone growth. The osteoblast and osteocyte appear to be involved in such androgen receptor mediated action on bone in male mice. Studies in mice also showed an unexpected interaction between osteocalcin and testosterone production in males and, vice versa, between ovarian production of follicle-stimulating hormone with testosterone and potentially bone formation.

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

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

Effects of sex steroids on serum sclerostin levels during the menstrual cycle

BACKGROUND/AIMS

Previous studies on the effects of estrogen on sclerostin regulation were conducted in postmenopausal women in humans or animals following estrogen treatment or induced estrogen deficiency. The aim of this study was to evaluate the effects of sex hormones on serum sclerostin levels in premenopausal women with normal menstrual cycles.

METHODS

A prospective observational clinical study. 80 voluntary premenopausal women were recruited for the study. Data from 31 patients were available for the statistical analysis. Serum sclerostin, free estradiol, free testosterone, and progesterone levels were measured during the menstruation, late follicular and mid-luteal phases. The unique protocol IDs were BEAH FTR-4 and NCT01418924 at ClinicalTrials.gov ID.

RESULTS

Serum sclerostin values were 1.03 ± 0.58 ng/ml during the menstruation phase, 1.0 ± 0.36 ng/ml during the late follicular phase, and 1.18 ± 0.67 ng/ml during the mid-luteal phase (p = 0.543). There was no significant relationship between serum levels of sex steroids and sclerostin.

CONCLUSIONS

Previous studies have not investigated the impact of sex hormone fluctuations on serum sclerostin levels during the menstrual cycle. The present study shows that serum sclerostin levels were not affected by sex steroids in premenopausal women with normal menstrual cycles.

Estrogen receptor-α is required for the osteogenic response to mechanical loading in a ligand-independent manner involving its activation function 1 but not 2

Estrogen receptor-α (ERα) is crucial for the adaptive response of bone to loading but the role of endogenous estradiol (E2) for this response is unclear. To determine in vivo the ligand dependency and relative roles of different ERα domains for the osteogenic response to mechanical loading, gene-targeted mouse models with (1) a complete ERα inactivation (ERα(-/-) ), (2) specific inactivation of activation function 1 (AF-1) in ERα (ERαAF-1(0) ), or (3) specific inactivation of ERαAF-2 (ERαAF-2(0) ) were subjected to axial loading of tibia, in the presence or absence (ovariectomy [ovx]) of endogenous E2. Loading increased the cortical bone area in the tibia mainly as a result of an increased periosteal bone formation rate (BFR) and this osteogenic response was similar in gonadal intact and ovx mice, demonstrating that E2 (ligand) is not required for this response. Female ERα(-/-) mice displayed a severely reduced osteogenic response to loading with changes in cortical area (-78% ± 15%, p < 0.01) and periosteal BFR (-81% ± 9%, p < 0.01) being significantly lower than in wild-type (WT) mice. ERαAF-1(0) mice also displayed a reduced response to mechanical loading compared with WT mice (cortical area -40% ± 11%, p < 0.05 and periosteal BFR -41% ± 8%, p < 0.01), whereas the periosteal osteogenic response to loading was unaffected in ERαAF-2(0) mice. Mechanical loading of transgenic estrogen response element (ERE)-luciferase reporter mice did not increase luciferase expression in cortical bone, suggesting that the loading response does not involve classical genomic ERE-mediated pathways. In conclusion, ERα is required for the osteogenic response to mechanical loading in a ligand-independent manner involving AF-1 but not AF-2.

Estrogen receptor α mediates proliferation of osteoblastic cells stimulated by estrogen and mechanical strain, but their acute down-regulation of the Wnt antagonist Sost is mediated by estrogen receptor β

Mechanical strain and estrogens both stimulate osteoblast proliferation through estrogen receptor (ER)-mediated effects, and both down-regulate the Wnt antagonist Sost/sclerostin. Here, we investigate the differential effects of ERα and -β in these processes in mouse long bone-derived osteoblastic cells and human Saos-2 cells. Recruitment to the cell cycle following strain or 17β-estradiol occurs within 30 min, as determined by Ki-67 staining, and is prevented by the ERα antagonist 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride. ERβ inhibition with 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-β]pyrimidin-3-yl] phenol (PTHPP) increases basal proliferation similarly to strain or estradiol. Both strain and estradiol down-regulate Sost expression, as does in vitro inhibition or in vivo deletion of ERα. The ERβ agonists 2,3-bis(4-hydroxyphenyl)-propionitrile and ERB041 also down-regulated Sost expression in vitro, whereas the ERα agonist 4,4′,4″-[4-propyl-(1H)-pyrazol-1,3,5-triyl]tris-phenol or the ERβ antagonist PTHPP has no effect. Tamoxifen, a nongenomic ERβ agonist, down-regulates Sost expression in vitro and in bones in vivo. Inhibition of both ERs with fulvestrant or selective antagonism of ERβ, but not ERα, prevents Sost down-regulation by strain or estradiol. Sost down-regulation by strain or ERβ activation is prevented by MEK/ERK blockade. Exogenous sclerostin has no effect on estradiol-induced proliferation but prevents that following strain. Thus, in osteoblastic cells the acute proliferative effects of both estradiol and strain are ERα-mediated. Basal Sost down-regulation follows decreased activity of ERα and increased activity of ERβ. Sost down-regulation by strain or increased estrogens is mediated by ERβ, not ERα. ER-targeting therapy may facilitate structurally appropriate bone formation by enhancing the distinct ligand-independent, strain-related contributions to proliferation of both ERα and ERβ.

Estrogen receptor-α in osteocytes is important for trabecular bone formation in male mice

The bone-sparing effect of estrogen in both males and females is primarily mediated via estrogen receptor-α (ERα), encoded by the Esr1 gene. ERα in osteoclasts is crucial for the trabecular bone-sparing effect of estrogen in females, but it is dispensable for trabecular bone in male mice and for cortical bone in both genders. We hypothesized that ERα in osteocytes is important for trabecular bone in male mice and for cortical bone in both males and females. Dmp1-Cre mice were crossed with ERα(flox/flox) mice to generate mice lacking ERα protein expression specifically in osteocytes (Dmp1-ERα(-/-)). Male Dmp1-ERα(-/-) mice displayed a substantial reduction in trabecular bone volume (-20%, P < 0.01) compared with controls. Dynamic histomorphometry revealed reduced bone formation rate (-45%, P < 0.01) but the number of osteoclasts per bone surface was unaffected in the male Dmp1-ERα(-/-) mice. The male Dmp1-ERα(-/-) mice had reduced expression of several osteoblast/osteocyte markers in bone, including Runx2, Sp7, and Dmp1 (P < 0.05). Gonadal intact Dmp1-ERα(-/-) female mice had no significant reduction in trabecular bone volume but ovariectomized Dmp1-ERα(-/-) female mice displayed an attenuated trabecular bone response to supraphysiological E2 treatment. Dmp1-ERα(-/-) mice of both genders had unaffected cortical bone. In conclusion, ERα in osteocytes regulates trabecular bone formation and thereby trabecular bone volume in male mice but it is dispensable for the trabecular bone in female mice and the cortical bone in both genders. We propose that the physiological trabecular bone-sparing effect of estrogen is mediated via ERα in osteocytes in males, but via ERα in osteoclasts in females.

Mechanical regulation of signaling pathways in bone

A wide range of cell types depend on mechanically induced signals to enable appropriate physiological responses. The skeleton is particularly dependent on mechanical information to guide the resident cell population towards adaptation, maintenance and repair. Research at the organ, tissue, cell and molecular levels has improved our understanding of how the skeleton can recognize the functional environment, and how these challenges are translated into cellular information that can site-specifically alter phenotype. This review first considers those cells within the skeleton that are responsive to mechanical signals, including osteoblasts, osteoclasts, osteocytes and osteoprogenitors. This is discussed in light of a range of experimental approaches that can vary parameters such as strain, fluid shear stress, and pressure. The identity of mechanoreceptor candidates is approached, with consideration of integrins, pericellular tethers, focal adhesions, ion channels, cadherins, connexins, and the plasma membrane including caveolar and non-caveolar lipid rafts and their influence on integral signaling protein interactions. Several mechanically regulated intracellular signaling cascades are detailed including activation of kinases (Akt, MAPK, FAK), β-catenin, GTPases, and calcium signaling events. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of diseases such as osteoporosis, and to the development of interventions to improve bone strength.

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

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSION

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

Estrogen, exercise, and the skeleton

Patterns of variation in bone size and shape provide crucial data for reconstructing hominin paleobiology, including ecogeographic adaptation, life history, and functional morphology. Measures of bone strength, including robusticity (diaphyseal thickness relative to length) and cross-sectional geometric properties such as moments of area, are particularly useful for inferring behavior because bone tissue adapts to its mechanical environment. Particularly during skeletal growth, exercise-induced strains can stimulate periosteal modeling so that, to some extent, bone thickness reflects individual behavior. Thus, patterns of skeletal robusticity have been used to identify gender-based activity differences, temporal shifts in mobility, and changing subsistence strategies. Although there is no doubt that mechanical loading leaves its mark on the skeleton, less is known about whether individuals differ in their skeletal responses to exercise. For example, the potential effects of hormones or growth factors on bone-strain interactions are largely unexplored. If the hormonal background can increase or decrease the effects of exercise on skeletal robusticity, then the same mechanical loads might cause different degrees of bone response in different individuals. Here I focus on the role of the hormone estrogen in modulating exercise-induced changes in human bone thickness.

VDR dependent and independent effects of 1,25-dihydroxyvitamin D3 on nitric oxide production by osteoblasts

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) strongly mediates bone mass. Mechanical stimulation also affects bone mass, partly via enhancing nitric oxide (NO) production by osteoblasts. We aimed to determine whether 1,25(OH)(2)D(3) affects NO production by osteoblasts in the presence or absence of mechanical stimulation. We hypothesised that 1,25(OH)(2)D(3) stimulates NO production via nuclear actions of the vitamin D receptor (VDR), which requires hours of incubation with 1,25(OH)(2)D(3) to occur. MC3T3-E1 osteoblasts and long-bone osteoblasts of adult wildtype and VDR(-/-) mice were pre-incubated for 24h with or without 1,25(OH)(2)D(3) (10(-13)-10(-9)M), followed by 30 min pulsating fluid flow (PFF; 0.7±0.3 Pa, 5 Hz) or static culture with or without 1,25(OH)(2)D(3). NO production and NO synthase (NOS) expression were quantified. 10(-11)M 1,25(OH)(2)D(3) for 24h, but not 30 min, stimulated NO production by MC3T3-E1 osteoblasts (eightfold). 1,25(OH)(2)D(3) for 24h increased inducible-NOS gene-expression (twofold), suggesting that 1,25(OH)(2)D(3) stimulated NO production via activation of NOS gene transcription. PFF rapidly increased NO production by MC3T3-E1 osteoblasts, wildtype osteoblasts, and VDR(-/-) osteoblasts. This PFF effect was abolished after incubation with 1,25(OH)(2)D(3) for 24h, or during PFF only. Our results suggest that 1,25(OH)(2)D(3) stimulates inducible-NOS expression and NO production by osteoblasts in the absence of mechanical stimulation, likely via genomic VDR action. In contrast, 1,25(OH)(2)D(3) may affect mechanical loading-induced NO production independent of genomic VDR action, since 1,25(OH)(2)D(3) diminished PFF-induced NO production in VDR(-/-) bone cells. In conclusion, 1,25(OH)(2)D(3) and mechanical loading interact at the level of mechanotransduction, whereby 1,25(OH)(2)D(3) seems to act independently of VDR genomic mechanism.

Bone vs. fat: embryonic origin of progenitors determines response to androgen in adipocytes and osteoblasts

Although androgen is considered an anabolic hormone, the consequences of androgen receptor (AR) overexpression in skeletally-targeted AR-transgenic lines highlight the detrimental effect of enhanced androgen sensitivity on cortical bone quality. A compartment-specific anabolic response is observed only in male and not in female AR3.6-transgenic (tg) mice, with increased periosteal bone formation and calvarial thickening. To identify anabolic signaling cascades that have the potential to increase bone formation, qPCR array analysis was employed to define expression differences between AR3.6-tg and wild-type (WT) periosteal tissue. Notably, categories that were significantly different between the two genotypes included axonal guidance, CNS development and negative regulation of Wnt signaling with a node centered on stem cell pathways. Further, fine mapping of AR3.6-tg calvaria revealed that anabolic thickening in vivo is not uniform across the calvaria, occurring only in frontal and in not parietal bones. Multipotent fraction 1 progenitor populations from both genotypes were cultured separately as frontal bone neural crest stem-like cells (fNCSC) and parietal bone mesenchymal stem-like cells (pMSC). Both osteoblastic and adipogenic differentiation in these progenitor populations was influenced by embryonic lineage and by genotype. Adipogenesis was enhanced in WT fNCSC compared to pMSC, but transgenic cultures showed strong suppression of lipid accumulation only in fNCSC cells. Osteoblastogenesis was significantly increased in transgenic fNCSC cultures compared to WT, with elevated alkaline phosphatase (ALP) activity and induction of mineralization and nodule formation assessed by alizarin red and von Kossa staining. Osteocalcin (OC) and ALP mRNA levels were also increased in fNCSC cultures from AR3.6-tg vs. WT, but in pMSC cultures ALP mRNA levels, mineralization and nodule formation were decreased in AR3.6-tg cells. Expression differences identified by array in long bone periosteal tissue from AR3.6-tg vs. WT were recapitulated in the fNCSC samples while pMSC profiles reflected cortical expression. These observations reveal the opposing effects of androgen signaling on lineage commitment and osteoblast differentiation that is enhanced in cells derived from a neural crest origin but inhibited in cells derived from a mesodermal origin, consistent with in vivo compartment-specific responses to androgen. Combined, these results highlight the complex action of androgen in the body that is dependent on the embryonic lineage and developmental origin of the cell. Further, these data these data suggest that the periosteum surrounding long bone is derived from neural crest.

Sost down-regulation by mechanical strain in human osteoblastic cells involves PGE2 signaling via EP4

Sclerostin is a potent inhibitor of bone formation which is down-regulated by mechanical loading. To investigate the mechanisms involved we subjected Saos2 human osteoblastic cells to short periods of dynamic strain and used quantitative reverse transcriptase polymerase chain reaction to compare their responses to unstrained controls. Strain-induced Sost down-regulation was recapitulated by cyclo-oxygenase-2-mediated PGE2, acting through the EP4 receptor, whereas strain-related up-regulation of osteocalcin was mediated by the EP2 receptor. Strain-related Sost regulation required extracellular signal-regulated kinase signaling, whereas osteocalcin required protein kinase C. These findings indicate early divergence in the signaling pathways stimulated by strain and establish PGE2/EP4 as the pathway used by strain to regulate Sost expression.

Role of endocrine and paracrine factors in the adaptation of bone to mechanical loading

There appears to be no unique mechanically sensitive pathway by which changes in bone loading regulate bone mass and architecture to ensure adequate structural strength. Rather, strain-derived changes in bone cells activate a number of nonspecific strain-sensitive pathways (including calcium fluxes, prostanoids, nitric oxide, extracellular signal-regulated kinase, and sclerostin), the activities of which are modified by a number of factors (including estrogen receptors) for which this contribution is subsidiary to other purposes. The strain-sensitive pathways modified by these factors interact with a number of other pathways, some of which appear to have specific osteoregulatory potential (eg, the parathyroid hormone pathway), whereas others such as the Wnt pathway appear to be associated primarily with the response mechanisms of proliferation, differentiation, and apoptosis. The outcome of these multiple interactions are stimuli for local bone formation, resorption, or maintenance of the status quo, to maintain existing bone architecture or adapt it to a new mechanical regimen.

Mechanical loading-related bone gain is enhanced by tamoxifen but unaffected by fulvestrant in female mice

Accumulating evidence indicates that estrogen receptors (ERs) are involved in the mechano-adaptive mechanisms by which loading influences the mass and architecture of bones to establish and maintain their structural load-bearing competence. In the present study, we assessed the effects of the ER modulators tamoxifen and fulvestrant (ICI 182,780) on loading-related changes in the volume and structure of trabecular and cortical bone in the tibiae of female mice. Ten days after actual or sham ovariectomy, 17-wk-old female C57BL/6 mice were treated with vehicle (peanut oil), tamoxifen (0.02, 0.2, or 2 mg/kg · d), fulvestrant (4 mg/kg · d), or their combination and the right tibiae subjected to a short period of noninvasive axial loading (40 cycles/d) on 5 d during the subsequent 2 wk. In the left control tibiae, ovariectomy, tamoxifen, or fulvestrant did not have any significant effect on cortical bone volume, whereas trabecular bone volume was decreased by ovariectomy, increased by tamoxifen, and unaffected by fulvestrant. In the right tibiae, loading was associated with increases in both trabecular and cortical bone volume. Notably, the medium dose of tamoxifen synergistically enhanced loading-related gain in trabecular bone volume through an increase in trabecular thickness. Fulvestrant had no influence on the effects of loading but abrogated the enhancement of loading-related bone gain by tamoxifen. These data demonstrate that, at least in female mice, the adaptive response to mechanical loading of trabecular bone can be enhanced by ER modulators, in this case by tamoxifen.