Mechanical load increases in bone formation via a sclerostin-independent pathway

Effects of acute- and long-term aerobic exercises at different intensities on bone in mice

INTRODUCTION

Exercise intensity determines the benefits of aerobic exercise. Our objectives were, in aerobic exercise at different intensities, to determine (1) changes in bone metabolism-related genes after acute exercise and (2) changes in bone mass, strength, remodeling, and bone formation-related proteins after long-term exercise.

MATERIALS AND METHODS

Total 36 male C57BL/6J mice were divided into a control group and exercise groups at 3 different intensities: low, moderate, or high group. Each exercise group was assigned to acute- or long-term exercise groups. Tibias after acute exercise were evaluated by real-time PCR analysis. Furthermore, hindlimbs of long-term exercise were assessed by micro-CT, biomechanical, histological, and immunohistochemical analyses.

RESULTS

Acute moderate-intensity exercise decreased RANKL level as bone resorption marker, whereas low- and high-intensity exercise did not alter it. Additionally, only long-term exercise at moderate intensity increased bone mass and strength. Moderate-intensity exercise promoted osteoblast activity and suppressed osteoclast activity. After low- and high-intensity exercise, osteoblast and osteoclast activity were unchanged. An increase in the number of β-catenin-positive cells and a decrease in sclerostin-positive cells were observed in the only moderate group.

CONCLUSION

These results showed that moderate-intensity exercise can inhibit bone resorption earlier, and long-term exercise can increase bone mass and strength through promoted bone formation via the Wnt/β-catenin activation. High-intensity exercise, traditionally considered better for bone, may fail to stimulate bone remodeling, leading to no change in bone mass and strength. Our findings suggest that moderate-intensity exercise, neither too low nor high, can maintain bone health.

The Emerging Role of Interleukin-(IL)-11/IL-11R in Bone Metabolism and Homeostasis: From Cytokine to Osteokine

Interleukin-(IL)-11 is a cytokine involved in hematopoiesis, cancer metastasis, and inflammation. IL-11 belongs to the IL-6 cytokine family, binding to the complex of receptors glycoprotein gp130 and the ligand-specific-receptor subunits (IL-11Rα or their soluble counterpart sIL-11R). IL-11/IL-11R signaling enhances osteoblast differentiation and bone formation and mitigates osteoclast-induced bone resorption and cancer bone metastasis. Recent studies have shown that systemic and osteoblast/osteocyte-specific IL-11 deficiency leads to reduced bone mass and formation, but also adiposity, glucose intolerance, and insulin resistance. In humans, mutations of IL-11 and the receptor IL-11RA genes are associated with height reduction, osteoarthritis, and craniosynostosis. In this review, we describe the emerging role of IL-11/IL-11R signaling in bone metabolism by targeting osteoblasts, osteoclasts, osteocytes, and bone mineralization. Furthermore, IL-11 promotes osteogenesis and suppresses adipogenesis, thereby influencing the fate of osteoblast/adipocyte differentiation derived from pluripotent mesenchymal stem cells. We have newly identified IL-11 as a bone-derived cytokine that regulates bone metabolism and the link between bone and other organs. Thus, IL-11 is vital in bone homeostasis and could be considered a potential therapeutic strategy.

Osteoimmunology in Periodontitis and Orthodontic Tooth Movement

PURPOSE OF REVIEW

To review the role of the immune cells and their interaction with cells found in gingiva, periodontal ligament, and bone that leads to net bone loss in periodontitis or bone remodeling in orthodontic tooth movement.

RECENT FINDINGS

Periodontal disease is one of the most common oral diseases causing inflammation in the soft and hard tissues of the periodontium and is initiated by bacteria that induce a host response. Although the innate and adaptive immune response function cooperatively to prevent bacterial dissemination, they also play a major role in gingival inflammation and destruction of the connective tissue, periodontal ligament, and alveolar bone characteristic of periodontitis. The inflammatory response is triggered by bacteria or their products that bind to pattern recognition receptors that induce transcription factor activity to stimulate cytokine and chemokine expression. Epithelial, fibroblast/stromal, and resident leukocytes play a key role in initiating the host response and contribute to periodontal disease. Single-cell RNA-seq (scRNA-seq) experiments have added new insight into the roles of various cell types in the response to bacterial challenge. This response is modified by systemic conditions such as diabetes and smoking. In contrast to periodontitis, orthodontic tooth movement (OTM) is a sterile inflammatory response induced by mechanical force. Orthodontic force application stimulates acute inflammatory responses in the periodontal ligament and alveolar bone stimulated by cytokines and chemokines that produce bone resorption on the compression side. On the tension side, orthodontic forces induce the production of osteogenic factors, stimulating new bone formation. A number of different cell types, cytokines, and signaling/pathways are involved in this complex process. Inflammatory and mechanical force-induced bone remodeling involves bone resorption and bone formation. The interaction of leukocytes with host stromal cells and osteoblastic cells plays a key role in both initiating the inflammatory events as well as inducing a cellular cascade that results in remodeling in orthodontic tooth movement or in tissue destruction in periodontitis.

Sclerostin antibody promotes bone formation through the Wnt/β-catenin signaling pathway in femoral trochlear after patellar instability

PURPOSE

The molecular mechanism of patellar instability (PI) remains unknown. The purpose of this study was to explore the function of SOST/sclerostin in PI and examine the effect of sclerostin antibody (Scl-Ab).

MATERIALS AND METHODS

We randomly divided 60 male 3-week-old C57Bl/6 mice into four groups: sham, PI, Scl-Ab intraperitoneal injection (Scl-Ab IP), Scl-Ab intraarticular injection (Scl-Ab IA). PI was established in the latter three groups. The Scl-Ab IP/IA groups were administered with an intraperitoneal/intraarticular Scl-Ab injection (100 mg/kg, 20 µl), respectively, at 5-day intervals. Distal femurs were collected 30 days after the surgery. The SOST/sclerostin, β-catenin, ALP, OPG and RANKL expression in distal femur were determined. Trochlear morphology and structural parameters of the trabecular and cortical bone compartments were determined by micro-CT. Further sub-regional analysis was performed. HE staining and Masson's trichrome staining were performed to evaluate cartilage changes.

RESULTS

PI increased the expression of SOST/sclerostin and RANKL, and decreased β-catenin, ALP and OPG levels, while Scl-Ab IP reversed these changes. Scl-Ab IP brought trochlear morphology closer to normality. Additionally, Scl-Ab IP significantly improved most of the bone parameters. Importantly, both PI and Scl-Ab IP acted mainly on trabecular bone. Histological analysis showed that Scl-Ab IP protected cartilage from degeneration. However, Scl-Ab IA did not protect against bone loss or cartilage degradation.

CONCLUSIONS

SOST/sclerostin plays an important role in PI and systemic Scl-Ab use promotes bone formation through the Wnt/β-catenin signaling pathway in the femoral trochlear after PI.

Constitutive bone marrow adipocytes suppress local bone formation

BM adipocytes (BMAd) are a unique cell population derived from BM mesenchymal progenitors and marrow adipogenic lineage precursors. Although they have long been considered to be a space filler within bone cavities, recent studies have revealed important physiological roles in hematopoiesis and bone metabolism. To date, the approaches used to study BMAd function have been confounded by contributions by nonmarrow adipocytes or by BM stromal cells. To address this gap in the field, we have developed a BMAd-specific Cre mouse model to deplete BMAds by expression of diphtheria toxin A (DTA) or by deletion of peroxisome proliferator-activated receptor gamma (Pparg). We found that DTA-induced loss of BMAds results in decreased hematopoietic stem and progenitor cell numbers and increased bone mass in BMAd-enriched locations, including the distal tibiae and caudal vertebrae. Elevated bone mass appears to be secondary to enhanced endosteal bone formation, suggesting a local effect caused by depletion of BMAd. Augmented bone formation with BMAd depletion protects mice from bone loss induced by caloric restriction or ovariectomy, and it facilitates the bone-healing process after fracture. Finally, ablation of Pparg also reduces BMAd numbers and largely recapitulates high-bone mass phenotypes observed with DTA-induced BMAd depletion.

Osteocytes regulate bone anabolic response to mechanical loading in male mice via activation of integrin α5

Physical mechanical stimulation can maintain and even increase bone mass. Here, we report an important role of osteocytic integrin α5 in regulating the anabolic response of bone to mechanical loading using an Itga5 conditional gene knockout (cKO) mouse model. Integrin α5 gene deletion increased apoptotic osteocytes and reduced cortical anabolic responses to tibial compression including decreased endosteal osteoblasts and bone formation, and increased endosteal osteoclasts and bone resorption, contributing to the decreased bone area fraction and biomechanical properties, leading to an enlarged bone marrow area in cKO mice. Similar disruption of anabolic responses to mechanical loading was also detected in cKO trabecular bone. Moreover, integrin α5 deficiency impeded load-induced Cx43 hemichannel opening, and production and release of PGE2, an anabolic factor, resulting in attenuated effects of the loading on catabolic sclerostin (SOST) reduction and anabolic β-catenin increase. Together, this study shows an indispensable role of integrin α5 in osteocytes in the anabolic action of mechanical loading on skeletal tissue through activation of hemichannels and PGE2-evoked gene expression. Integrin α5 could act as a potential new therapeutic target for bone loss, especially in the elderly population with impeded mechanical sensitivity.

Lipolysis of bone marrow adipocytes is required to fuel bone and the marrow niche during energy deficits

To investigate roles for bone marrow adipocyte (BMAd) lipolysis in bone homeostasis, we created a BMAd-specific Cre mouse model in which we knocked out adipose triglyceride lipase (ATGL, Pnpla2 gene). BMAd-Pnpla2-/- mice have impaired BMAd lipolysis, and increased size and number of BMAds at baseline. Although energy from BMAd lipid stores is largely dispensable when mice are fed ad libitum, BMAd lipolysis is necessary to maintain myelopoiesis and bone mass under caloric restriction. BMAd-specific Pnpla2 deficiency compounds the effects of caloric restriction on loss of trabecular bone in male mice, likely due to impaired osteoblast expression of collagen genes and reduced osteoid synthesis. RNA sequencing analysis of bone marrow adipose tissue reveals that caloric restriction induces dramatic elevations in extracellular matrix organization and skeletal development genes, and energy from BMAd is required for these adaptations. BMAd-derived energy supply is also required for bone regeneration upon injury, and maintenance of bone mass with cold exposure.

The Importance of on Sclerostin as Bone-Muscle Mediator Crosstalk

Loss of bone and muscle mass is a frequent aging condition and has become a growing public health problem. The term “osteosarcopenia” denotes close links between bone and muscle. Mechanical exercise was once thought to be the only mechanism of crosstalk between muscle and bone. Sclerostin is an important player in the process of unloading-induced bone loss and plays an important role in mechanotransduction in the bone. Furthermore, bones and muscles are categorized as endocrine organs because they produce hormone-like substances, resulting in “bone-muscle crosstalk.” Sclerostin, an inhibitor of bone development, has recently been shown to play a role in myogenesis. This review discusses the importance of sclerostin in bone-muscle crosstalk.

Dissociation of Bone Resorption and Formation in Spaceflight and Simulated Microgravity: Potential Role of Myokines and Osteokines?

The dissociation of bone formation and resorption is an important physiological process during spaceflight. It also occurs during local skeletal unloading or immobilization, such as in people with neuromuscular disorders or those who are on bed rest. Under these conditions, the physiological systems of the human body are perturbed down to the cellular level. Through the absence of mechanical stimuli, the musculoskeletal system and, predominantly, the postural skeletal muscles are largely affected. Despite in-flight exercise countermeasures, muscle wasting and bone loss occur, which are associated with spaceflight duration. Nevertheless, countermeasures can be effective, especially by preventing muscle wasting to rescue both postural and dynamic as well as muscle performance. Thus far, it is largely unknown how changes in bone microarchitecture evolve over the long term in the absence of a gravity vector and whether bone loss incurred in space or following the return to the Earth fully recovers or partly persists. In this review, we highlight the different mechanisms and factors that regulate the humoral crosstalk between the muscle and the bone. Further we focus on the interplay between currently known myokines and osteokines and their mutual regulation.

Sclerostin and Its Involvement in the Pathogenesis of Idiopathic Scoliosis

Idiopathic scoliosis is a disorder of unknown etiology. Bone biopsies from idiopathic scoliosis patients revealed changes at cellular and molecular level. Osteocytic sclerostin is downregulated, and serum level of sclerostin is decreased. Osteocytes in idiopathic scoliosis appear to be less active with abnormal canaliculi network. Differentiation of osteoblasts to osteocytes is decelerated, while Wnt/β-catenin signaling pathway is overactivated and affects normal bone mineralization that leads to inferior mechanical properties of the bone, which becomes susceptible to asymmetrical forces and causes deformity of the spinal column. Targeting bone metabolism during growth by stimulating sclerostin secretion from osteocytes and restoring normal function of Wnt/β-catenin signaling pathway could, in theory, increase bone strength and prevent deterioration of the scoliotic deformity.

Gene expression of intracortical bone demonstrates loading-induced increases in Wnt1 and Ngf and inhibition of bone remodeling processes

Osteocytes are the primary mechanosensitive cells in bone. However, their location in mineralized matrix has limited the in vivo study of osteocytic genes induced by mechanical loading. Laser Capture Microdissection (LCM) allows isolation of intracortical bone (Intra-CB), enriched for osteocytes, from bone tissue for gene expression analysis. We used microarray to analyze gene expression from mouse tibial Intra-CB dissected using LCM 4 h after a single loading bout or after 5 days of loading. Osteocyte enrichment was supported by greater expression of Sost, Dmp1, Dkk1, and Mepe in Intra-CB regions vs. Mixed regions containing periosteum and muscle (fold-change (FC) = 3.4, 2.2, 5.1, 3.0, respectively). Over 150 differentially expressed genes (DEGs) due to loading (loaded vs. contralateral control) in Intra-CB were found on Day 1 and Day 5, but only 10 genes were differentially expressed on both days, including Ngf (Day 1 FC = 13.5, Day 5 FC = 11.1) and Wnt1 (Day 1 FC = 1.5, Day 5 FC = 5.1). The expression of Ngf and Wnt1 within Intra-CB was confirmed by in situ hybridization, and a significant increase in number of Wnt1 mRNA molecules occurred on day 1. We also found changes in extracellular matrix remodeling with Timp1 (FC = 3.1) increased on day 1 and MMP13 (FC = 0.3) decreased on day 5. Supporting this result, IHC for osteocytic MMP13 demonstrated a marginal decrease due to loading on day 5. Gene Ontology (GO) biological processes for loading DEGs indicated regulation of vasculature, neuronal and immune processes while cell-type specific gene lists suggested regulation of osteoclast, osteoblast, and endothelial related genes. In summary, microarray analysis of microdissected Intra-CB revealed differential regulation of Ngf, Wnt1, and MMP13 due to loading in osteocytes.

Mechanical load regulates bone growth via periosteal Osteocrin

Mechanical stimuli including loading after birth promote bone growth. However, little is known about how mechanical force triggers biochemical signals to regulate bone growth. Here, we identified a periosteal-osteoblast-derived secretory peptide, Osteocrin (OSTN), as a mechanotransducer involved in load-induced long bone growth. OSTN produced by periosteal osteoblasts regulates growth plate growth by enhancing C-type natriuretic peptide (CNP)-dependent proliferation and maturation of chondrocytes, leading to elongation of long bones. Additionally, OSTN cooperates with CNP to regulate bone formation. CNP stimulates osteogenic differentiation of periosteal osteoprogenitors to induce bone formation. OSTN binds to natriuretic peptide receptor 3 (NPR3) in periosteal osteoprogenitors, thereby preventing NPR3-mediated clearance of CNP and consequently facilitating CNP-signal-mediated bone growth. Importantly, physiological loading induces Ostn expression in periosteal osteoblasts by suppressing Forkhead box protein O1 (FoxO1) transcription factor. Thus, this study reveals a crucial role of OSTN as a mechanotransducer converting mechanical loading to CNP-dependent bone formation.

Does obesity mediate the relationship between diabetes and osteoporosis in Chinese elderly population?

OBJECTIVES

Diabetes mellitus (DM), osteoporosis (OP), and obesity (OB) are three complex diseases. OB is associated with both DM and OP, but it is unclear whether OB mediates association between DM and OP. The study aimed to investigate the potential mediation effects of OB on association between DM and bone mineral density (BMD) by the causal inference tests (CIT).

METHODS

A total of 5682 Chinese aged over 65 years were enrolled in an ongoing cohort: Osteoporosis Preventive Project (OPP). Obesity-related indexes, including body mass index (BMI), waist circumference, and waist circumference-hip circumference-ratio (WHR), and BMD at total hip (TH) and femur neck (FN) were measured.

RESULTS

Subjects with DM had significant greater values of age, weight, BMI, waist circumference, WHR, and BMD than non-DM subjects. BMD at TH and FN was significantly associated with DM (p < 0.05) with adjustment of age both in males and females. Further CIT showed that OB-related indexes (BMI, waist circumference, and WHR) are significantly mediators in the associations between DM and BMD in females, but not in males. Furthermore, the mediation effects of waist circumference were detected on DM and TH BMD in the females of normal-weight group.

CONCLUSIONS

Obesity-related indexes, especially waist circumference, serve as significant mediator(s) between DM and OP in Chinese female elderly. Diabetes increases BMD by increasing obesity-related indexes. The findings established the intermediate role of OB underlying the association between DM and OP in human population.

Bone health in spacefaring rodents and primates: systematic review and meta-analysis

Animals in space exploration studies serve both as a model for human physiology and as a means to understand the physiological effects of microgravity. To quantify the microgravity-induced changes to bone health in animals, we systematically searched Medline, Embase, Web of Science, BIOSIS, and NASA Technical reports. We selected 40 papers focusing on the bone health of 95 rats, 61 mice, and 9 rhesus monkeys from 22 space missions. The percentage difference from ground control in rodents was -24.1% [Confidence interval: -43.4, -4.9] for trabecular bone volume fraction and -5.9% [-8.0, -3.8] for the cortical area. In primates, trabecular bone volume fraction was lower by -25.2% [-35.6, -14.7] in spaceflight animals compared to GC. Bone formation indices in rodent trabecular and cortical bone were significantly lower in microgravity. In contrast, osteoclast numbers were not affected in rats and were variably affected in mice. Thus, microgravity induces bone deficits in rodents and primates likely through the suppression of bone formation.

Amelioration of bone fragility by pulsed electromagnetic fields in type 2 diabetic KK-Ay mice involving Wnt/β-catenin signaling

Type 2 diabetes mellitus (T2DM) results in compromised bone microstructure and quality, and subsequently increased risks of fractures. However, it still lacks safe and effective approaches resisting T2DM bone fragility. Pulsed electromagnetic fields (PEMFs) exposure has proven to be effective in accelerating fracture healing and attenuating osteopenia/osteoporosis induced by estrogen deficiency. Nevertheless, whether and how PEMFs resist T2DM-associated bone deterioration remain not fully identified. The KK-Ay mouse was used as the T2DM model. We found that PEMF stimulation with 2 h/day for 8 wk remarkably improved trabecular bone microarchitecture, decreased cortical bone porosity, and promoted trabecular and cortical bone material properties in KK-Ay mice. PEMF stimulated bone formation in KK-Ay mice, as evidenced by increased serum levels of bone formation (osteocalcin and P1NP), enhanced bone formation rate, and increased osteoblast number. PEMF significantly suppressed osteocytic apoptosis and sclerostin expression in KK-Ay mice. PEMF exerted beneficial effects on osteoblast- and osteocyte-related gene expression in the skeleton of KK-Ay mice. Nevertheless, PEMF exerted no effect on serum biomarkers of bone resorption (TRAcP5b and CTX-1), osteoclast number, or osteoclast-specific gene expression (TRAP and cathepsin K). PEMF upregulated gene expression of canonical Wnt ligands (including Wnt1, Wnt3a, and Wnt10b), but not noncanonical Wnt5a. PEMF also upregulated skeletal protein expression of downstream p-GSK-3β and β-catenin in KK-Ay mice. Moreover, PEMF-induced improvement in bone microstructure, mechanical strength, and bone formation in KK-Ay mice was abolished after intragastric administration with the Wnt antagonist ETC-159. Together, our results suggest that PEMF can improve bone microarchitecture and quality by enhancing the biological activities of osteoblasts and osteocytes, which are associated with the activation of the Wnt/β-catenin signaling pathway. PEMF might become an effective countermeasure against T2DM-induced bone deterioration.NEW & NOTEWORTHY PEMF improved trabecular bone microarchitecture and suppressed cortical bone porosity in T2DM KK-Ay mice. It attenuated T2DM-induced detrimental consequence on trabecular and cortical bone material properties. PEMF resisted bone deterioration in KK-Ay mice by enhancing osteoblast-mediated bone formation. PEMF also significantly suppressed osteocytic apoptosis and sclerostin expression in KK-Ay mice. The therapeutic potential of PEMF on T2DM-induced bone deterioration was associated with the activation of Wnt/ß-catenin signaling.

Mechanistic Insight into Orthodontic Tooth Movement Based on Animal Studies: A Critical Review

Alveolar bone remodeling in orthodontic tooth movement (OTM) is a highly regulated process that coordinates bone resorption by osteoclasts and new bone formation by osteoblasts. Mechanisms involved in OTM include mechano-sensing, sterile inflammation-mediated osteoclastogenesis on the compression side and tensile force-induced osteogenesis on the tension side. Several intracellular signaling pathways and mechanosensors including the cilia and ion channels transduce mechanical force into biochemical signals that stimulate formation of osteoclasts or osteoblasts. To date, many studies were performed in vitro or using human gingival crevicular fluid samples. Thus, the use of transgenic animals is very helpful in examining a cause and effect relationship. Key cell types that participate in mediating the response to OTM include periodontal ligament fibroblasts, mesenchymal stem cells, osteoblasts, osteocytes, and osteoclasts. Intercellular signals that stimulate cellular processes needed for orthodontic tooth movement include receptor activator of nuclear factor-κB ligand (RANKL), tumor necrosis factor-α (TNF-α), dickkopf Wnt signaling pathway inhibitor 1 (DKK1), sclerostin, transforming growth factor beta (TGF-β), and bone morphogenetic proteins (BMPs). In this review, we critically summarize the current OTM studies using transgenic animal models in order to provide mechanistic insight into the cellular events and the molecular regulation of OTM.

Botulinum Toxin A and Osteosarcopenia in Experimental Animals: A Scoping Review

We conducted a scoping review to investigate the effects of intramuscular injection of Botulinum Toxin A (BoNT-A) on bone morphology. We investigated if the muscle atrophy associated with Injection of BoNT-A had effects on the neighboring bone. We used the search terms: osteopenia, bone atrophy, Botulinum Toxin A, Micro-CT, mice or rat. The following databases were searched: Medline, Embase, PubMed and the Cochrane Library, between 1990 and 2020. After removal of duplicates, 228 abstracts were identified of which 49 studies satisfied our inclusion and exclusion criteria. The majority of studies (41/49) reported a quantitative reduction in at least one measure of bone architecture based on Micro-CT. The reduction in the ratio of bone volume to tissue volume varied from 11% to 81% (mean 43%) according to the experimental set up and study time points. While longer term studies showed muscle recovery, no study showed complete recovery of all bone properties at the termination of the study. In experimental animals, intramuscular injection of BoNT-A resulted in acute muscle atrophy and acute degradation of the neighboring bone segment. These findings may have implications for clinical protocols in the use of Botulinum Toxin in children with cerebral palsy, with restraint recommended in injection protocols and consideration for monitoring bone density. Clinical studies in children with cerebral palsy receiving injections of Botulinum are indicated.

Mechanically Driven Counter-Regulation of Cortical Bone Formation in Response to Sclerostin-Neutralizing Antibodies

Sclerostin (Scl) antibodies (Scl-Ab) potently stimulate bone formation, but these effects are transient. Whether the rapid inhibition of Scl-Ab anabolic effects is due to a loss of bone cells' capacity to form new bone or to a mechanostatic downregulation of Wnt signaling once bone strength exceeds stress remains unclear. We hypothesized that bone formation under Scl-Ab could be reactivated by increasing the dose of Scl-Ab and/or by adding mechanical stimuli, and investigated the molecular mechanisms involved in this response, in particular the role of periostin (Postn), a co-activator of the Wnt pathway in bone. For this purpose, C57Bl/6, Postn^-/- and Postn^+/+ mice were treated with vehicle or Scl-Ab (50 to 100 mg/kg/wk) for various durations and subsequently subjected to tibia axial compressive loading. In wild-type (WT) mice, Scl-Ab anabolic effects peaked between 2 and 4 weeks and declined thereafter, with no further increase in bone volume and strength between 7 and 10 weeks. Doubling the dose of Scl-Ab did not rescue the decline in bone formation. In contrast, mechanical stimulation was able to restore cortical bone formation concomitantly to Scl-Ab treatment at both doses. Several Wnt inhibitors, including Dkk1, Sost, and Twist1, were upregulated, whereas Postn was markedly downregulated by 2 to 4 weeks of Scl-Ab. Mechanical loading specifically upregulated Postn gene expression. In turn, Scl-Ab effects on cortical bone were more rapidly downregulated in Postn^-/- mice. These results indicate that bone formation is not exhausted by Scl-Ab but inhibited by a mechanically driven downregulation of Wnt signaling. Hence, increasing mechanical loads restores bone formation on cortical surfaces, in parallel with Postn upregulation. © 2020 American Society for Bone and Mineral Research (ASBMR).

New Advances in Osteocyte Mechanotransduction

PURPOSE OF REVIEW

Skeletal adaptation to mechanical loading plays a critical role in bone growth and the maintenance of bone homeostasis. Osteocytes are postulated to serve as a hub orchestrating bone remodeling. The recent findings on the molecular mechanisms by which osteocytes sense mechanical loads and the downstream bone-forming factors are reviewed.

RECENT FINDINGS

Calcium channels have been implicated in mechanotransduction in bone cells for a long time. Efforts have been made to identify a specific calcium channel mediating the skeletal response to mechanical loads. Recent studies have revealed that Piezo1, a mechanosensitive ion channel, is critical for normal bone growth and is essential for the skeletal response to mechanical loading. Identification of mechanosensors and their downstream effectors in mechanosensing bone cells is essential for new strategies to modulate regenerative responses and develop therapies to treat the bone loss related to disuse or advanced age.

Mechanical loading prevents bone destruction and exerts anti-tumor effects in the MOPC315.BM.Luc model of myeloma bone disease

Bone continually adapts to changing external loading conditions via (re)modeling (modeling and remodeling) processes. While physical activity is known to beneficially enhance bone mass in healthy individuals, little is known in how physical stimuli affect osteolytic bone destruction in patients suffering from multiple myeloma bone disease. Multiple myeloma (MM) is caused by malignant plasma cells in the bone marrow, shifting the balance in bone remodeling towards massive resorption. We hypothesized that in vivo tibial mechanical loading has anabolic effects in mice with locally injected MOPC315.BM.Luc cells. Conventional microCT analysis revealed enhanced cortical bone mass and microstructure in loaded compared to nonloaded mice. State-of-the-art time-lapse microCT based image analysis demonstrated bone (re)modeling processes at the endosteal and periosteal surfaces as the underlying causes of increased bone mass. Loading prevented the progression and development of osteolytic destruction. Physical stimuli also diminished local MM cell growth and dissemination evidenced by quantification of MM cell-specific immunoglobulin A levels in the serum of mice and by bioluminescence analysis. These data indicate that mechanical loading not only rescues the bone phenotype, but also exerts cell-extrinsic anti-myeloma effects in the MOPC315.BM.Luc model. In conclusion, the use of physical stimuli should be further investigated as an anabolic treatment for osteolytic bone destruction in patients with MM.

Emerging insights into the comparative effectiveness of anabolic therapies for osteoporosis

Over the past three decades, the mainstay of treatment for osteoporosis has been antiresorptive agents (such as bisphosphonates), which have been effective with continued administration in lowering fracture risk. However, the clinical landscape has changed as adherence to these medications has declined due to perceived adverse effects. As a result, decreases in hip fracture rates that followed the introduction of bisphosphonates have now levelled off, which is coincident with a decline in the use of the antiresorptive agents. In the past two decades, two types of anabolic agents (including three new drugs), which represent a novel approach to improving bone quality by increasing bone formation, have been approved. These therapies are expected to lead to a new clinical paradigm in which anabolic agents will be used either alone or in combination with antiresorptive agents to build new bone and reduce fracture risk. This Review examines the mechanisms of action for these anabolic agents by detailing their receptor-activating properties for key cell types in the bone and marrow niches. Using these advances in bone biology as context, the comparative effectiveness of these anabolic agents is discussed in relation to other therapeutic options for osteoporosis to better guide their clinical application in the future.

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.

The interactions between mTOR and NF-κB: A novel mechanism mediating mechanical stretch-stimulated osteoblast differentiation

Mechanical stretch is known to promote osteoblast differentiation in vitro and accelerate bone regeneration in vivo, whereas the relevant mechanism remains unclear. Recent studies have shown the importance of reciprocal interactions between mammalian target of rapamycin (mTOR) and nuclear factor kappa B (NF-κB; two downstream molecules of Akt) in the regulation of tumor cells. Thus, we hypothesize that mTOR and NF-κB as well as their interconnection play a critical role in mediating stretch-induced osteogenic differentiation in osteoblasts. We herein found that mechanical stretch (10% elongation at six cycles/min) significantly promoted the expression of osteoblast differentiation-related markers (including ALP, BMP2, Col1α, OCN, and Runx2) in osteoblast-like MG-63 cells, accompanied by increased mTOR phosphorylation and NF-κB p65 phosphorylation and nuclear translocation. Blockade of mTOR by antagonist or small interfering RNA suppressed osteogenesis-related gene expression in response to mechanical stretch, whereas inhibition of NF-κB further increased stretch-induced osteoblast differentiation. Moreover, inhibition of mTOR decreased the phosphorylation of NF-κB, and blockade of NF-κB reduced the mTOR activation in MG63 cells under mechanical stretch. Coinhibition of mTOR and NF-κB abolishes the alteration of osteogenic differentiation induced by single mTOR or NF-κB inhibition under mechanical stretch, which is equivalent to the noninhibition level for osteoblasts under mechanical stretch. The expression levels of osteogenic differentiation in osteoblasts after inhibition of Akt were similar to those after co-inhibition of mTOR and NF-κB under mechanical stretch. This study for the first time reveals the reciprocal interconnection between mTOR and NF-κB in osteoblasts under mechanical stretch and indicates that mTOR and NF-κB as well as their interactions play a key role in the regulation of cellular homeostasis of osteoblasts in response to mechanical stretch. These findings are helpful for enriching our basic knowledge of the molecular mechanisms of osteoblast mechanotransduction, and also providing insight into the clinical therapeutic modality associated with mechanical stretch (e.g., distraction osteogenesis).

Sports and Metabolic Bone Disease

Physical activity (PA) increases bone mass and bone strength through different mechanisms and also reduces the risk of falls in the elderly, through proprioception and balance training. The benefits seen in adolescence continue into adulthood. Exercise delays and attenuates the effects of osteoporosis, and particular sports activities may be recommended to improve bone mineral density (BMD) of the spine or regional BMD, improve balance, and prevent falls. Stress injuries related to exercise are more common in osteopenic and osteoporotic individuals.Sports activity may in some cases be detrimental for bone health, with nutrition restriction a frequent cause for negative effects of the practice of PA on bone. The examples are the so-called female athlete triad of menstrual dysfunction resulting in reduced estrogen levels, low energy due to malnutrition, and decreased BMD. A similar triad is described in male athletes. This review analyzes the effects of sport on bone metabolism and in particular its relationship with metabolic bone disease.

Bone turnover biomarkers, disease activity, and MRI changes of sacroiliac joints in patients with spondyloarthritis

The lack of valid biomarkers in patients with spondyloarthritis (SpA) requires searching for additional options to increase sacroiliac joint (SIJ) evaluation effectiveness. We assessed the serum levels of bone turnover markers and their relationships with active and chronic changes in SIJs using magnetic resonance imaging (MRI), indices, and laboratory parameters of disease activity in SpA patients. 102 patients with SpA and 15 healthy subjects were included. Testing of serum levels of transforming growth factor-beta (TGF-β1), Wnt3, sclerostin, and Dickkopf-1 (Dkk-1) was conducted. Active inflammatory lesions in SIJs were evaluated using Spondyloarthritis Research Consortium of Canada (SPARCC) MRI SIJ score, and chronic changes using the Danish scoring method. Bath Ankylosing Spondylitis Disease Activity Index, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), Ankylosing Spondylitis Disease Activity Scores with CRP, and ESR were used to assess disease activity. Serum levels of Dkk-1, TGF-β1, and sclerostin were significantly lower in SpA patients compared to healthy controls. The serum levels of Dkk-1 positively correlated with CRP. Dkk-1 had a significant negative correlation with Danish score. The sclerostin serum level had a weak negative correlation with the active inflammatory MRI SIJ lesions. There were positive correlations between TGF-β1 and sclerostin with Dkk-1, and negative correlation between Wnt3 and sclerostin. Dkk-1 positively correlated with CRP and negatively with chronic SIJ changes by Danish score. Sclerostin negatively correlated with the active SIJ lesions by SPARCC. This suggests that Dkk-1 and sclerostin are the most promising candidates to reveal inflammation and bone turnover in patients with SpA.

Osteosarcopenia in Reproductive-Aged Women with Polycystic Ovary Syndrome: A Multicenter Case-Control Study

CONTEXT

Osteosarcopenia (loss of skeletal muscle and bone mass and/or function usually associated with aging) shares pathophysiological mechanisms with polycystic ovary syndrome (PCOS). However, the relationship between osteosarcopenia and PCOS remains unclear.

OBJECTIVE

We evaluated skeletal muscle index% (SMI% = [appendicular muscle mass/weight (kg)] × 100) and bone mineral density (BMD) in PCOS (hyperandrogenism + oligoamenorrhea), and contrasted these musculoskeletal markers against 3 reproductive phenotypes (i): HA (hyperandrogenism + eumenorrhea) (ii); OA (normoandrogenic + oligoamenorrhea) and (iii), controls (normoandrogenic + eumenorrhea). Endocrine predictors of SMI% and BMD were evaluated across the groups.

DESIGN, SETTING, AND PARTICIPANTS

Multicenter case-control study of 203 women (18-48 years old) in New York State.

RESULTS

PCOS group exhibited reduced SMI% (mean [95% confidence interval (CI)]; 26.2% [25.1,27.3] vs 28.8% [27.7,29.8]), lower-extremity SMI% (57.6% [56.7,60.0] vs 62.5% [60.3,64.6]), and BMD (1.11 [1.08,1.14] vs 1.17 [1.14,1.20] g/cm2) compared to controls. PCOS group also had decreased upper (0.72 [0.70,0.74] vs 0.77 [0.75,0.79] g/cm2) and lower (1.13 [1.10,1.16] vs 1.19 [1.16,1.22] g/cm2) limb BMD compared to HA. Matsuda index was lower in PCOS vs controls and positively associated with SMI% in all groups (all Ps ≤ 0.05). Only controls showed associations between insulin-like growth factor (IGF) 1 and upper (r = 0.84) and lower (r = 0.72) limb BMD (all Ps < 0.01). Unlike in PCOS, IGF-binding protein 2 was associated with SMI% in controls (r = 0.45) and HA (r = 0.67), and with upper limb BMD (r = 0.98) in HA (all Ps < 0.05).

CONCLUSIONS

Women with PCOS exhibit early signs of osteosarcopenia when compared to controls likely attributed to disrupted insulin function. Understanding the degree of musculoskeletal deterioration in PCOS is critical for implementing targeted interventions that prevent and delay osteosarcopenia in this clinical population.

A FAK/HDAC5 signaling axis controls osteocyte mechanotransduction

Osteocytes, cells ensconced within mineralized bone matrix, are the primary skeletal mechanosensors. Osteocytes sense mechanical cues by changes in fluid flow shear stress (FFSS) across their dendritic projections. Loading-induced reductions of osteocytic Sclerostin (encoded by Sost) expression stimulates new bone formation. However, the molecular steps linking mechanotransduction and Sost suppression remain unknown. Here, we report that class IIa histone deacetylases (HDAC4 and HDAC5) are required for loading-induced Sost suppression and bone formation. FFSS signaling drives class IIa HDAC nuclear translocation through a signaling pathway involving direct HDAC5 tyrosine 642 phosphorylation by focal adhesion kinase (FAK), a HDAC5 post-translational modification that controls its subcellular localization. Osteocyte cell adhesion supports FAK tyrosine phosphorylation, and FFSS triggers FAK dephosphorylation. Pharmacologic FAK catalytic inhibition reduces Sost mRNA expression in vitro and in vivo. These studies demonstrate a role for HDAC5 as a transducer of matrix-derived cues to regulate cell type-specific gene expression.

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.

Piezo1/2 mediate mechanotransduction essential for bone formation through concerted activation of NFAT-YAP1-ß-catenin

Mechanical forces are fundamental regulators of cell behaviors. However, molecular regulation of mechanotransduction remain poorly understood. Here, we identified the mechanosensitive channels Piezo1 and Piezo2 as key force sensors required for bone development and osteoblast differentiation. Loss of Piezo1, or more severely Piezo1/2, in mesenchymal or osteoblast progenitor cells, led to multiple spontaneous bone fractures in newborn mice due to inhibition of osteoblast differentiation and increased bone resorption. In addition, loss of Piezo1/2 rendered resistant to further bone loss caused by unloading in both bone development and homeostasis. Mechanistically, Piezo1/2 relayed fluid shear stress and extracellular matrix stiffness signals to activate Ca²⁺ influx to stimulate Calcineurin, which promotes concerted activation of NFATc1, YAP1 and ß-catenin transcription factors by inducing their dephosphorylation as well as NFAT/YAP1/ß-catenin complex formation. Yap1 and ß-catenin activities were reduced in the Piezo1 and Piezo1/2 mutant bones and such defects were partially rescued by enhanced ß-catenin activities.

Osteocyte-Mediated Translation of Mechanical Stimuli to Cellular Signaling and Its Role in Bone and Non-bone-Related Clinical Complications

PURPOSE OF REVIEW

Osteocytes comprise > 95% of the cellular component in bone tissue and produce a wide range of cytokines and cellular signaling molecules in response to mechanical stimuli. In this review, we aimed to summarize the molecular mechanisms involved in the osteocyte-mediated translation of mechanical stimuli to cellular signaling, and discuss their role in skeletal (bone) diseases and extra-skeletal (non-bone) clinical complications.

RECENT FINDINGS

Two decades before, osteocytes were assumed as a dormant cells buried in bone matrix. In recent years, emerging evidences have shown that osteocytes are pivotal not only for bone homeostasis but also for vital organ functions such as muscle, kidney, and heart. Osteocyte mechanotransduction regulates osteoblast and osteoclast function and maintains bone homeostasis. Mechanical stimuli modulate the release of osteocyte-derived cytokines, signaling molecules, and extracellular cellular vesicles that regulate not only the surrounding bone cell function and bone homeostasis but also the distant organ function in a paracrine and endocrine fashion. Mechanical loading and unloading modulate the osteocytic release of NO, PGE₂, and ATPs that regulates multiple cellular signaling such as Wnt/β-catenin, RANKL/OPG, BMPs, PTH, IGF1, VEGF, sclerostin, and others. Therefore, the in-depth study of the molecular mechanism of osteocyte mechanotransduction could unravel therapeutic targets for various bone and non-bone-related clinical complications such as osteoporosis, sarcopenia, and cancer metastasis to bone.

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

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

CYLD, a mechanosensitive deubiquitinase, regulates TGFβ signaling in load-induced bone formation

Many signaling pathways involved in bone homeostasis also participate in the anabolic response of bone to mechanical loading. For example, TGFβ signaling coordinates the maintenance of bone mass and bone quality through its effects on osteoblasts, osteoclasts, and osteocytes. TGFβ signaling is also essential for the mechanosensitive formation of new bone. However, the mechanosensitive mechanisms controlling TGFβ signaling in osteocytes remain to be determined, particularly those that integrate TGFβ signaling with other early responses to mechanical stimulation. Here, we used an in vivo mouse hindlimb loading model to identify mechanosensitive molecules in the TGFβ pathway, and MLO-Y4 cells to evaluate their interactions with the prostaglandin E2 (PGE2) pathway, which is well-known for its rapid response to mechanical stimulation and its role in bone anabolism. Although mRNA levels for several TGFβ ligands, receptors, and effectors were unchanged, the level of phosphorylated Smad2/3 (pSmad2/3) was reduced in tibial bone as early as 3 h after early mechanical stimulation. We found that PGE2 and its receptor, EP2, repress pSmad2/3 levels and transactivation of Serpine1 in osteocytes. PGE2 and EP2 control the level of pSmad2/3 through a proteasome-dependent mechanism that relies on the deubiquitinase CYLD. CYLD protein levels were also reduced in the tibiae within 3 h of mechanical loading. Using CYLD-deficient mice, we found that CYLD is required for the rapid load-mediated repression of pSmad2/3 and for load-induced bone formation. These data introduce CYLD as a mechanosensitive deubiquitinase that participates in the prostaglandin-dependent repression of TGFβ signaling in osteocytes.

Increased anabolic bone response in Dkk1 KO mice following tibial compressive loading

A viable Dkk1 knockout (KO) mouse strain in which embryonic lethality is rescued by developmental Wnt3 heterozygosity (Dkk1^-/-:Wnt3^+/-) exhibits increased bone formation and a high bone mass phenotype. We hypothesized that in vivo mechanical loading would further augment the bone formation response in Dkk1 KO mice, comparable to results from Sost KO mice. A cyclic loading protocol was applied to Dkk1 KO mice, wild type mice (WT; Dkk1^+/+:Wnt3^+/+), and Wnt3 heterozygote (Wnt3^+/-; Dkk1^+/+:Wnt3^+/-) controls. The left tibiae of 10-week-old female mice were dynamically loaded in vivo with 7N maximum compressive force 5 days/week for 2 weeks. Dkk1 KO bones were significantly stiffer, and so an additional group of Dkk1 KO received 12N maximum compressive force to achieve an equivalent +1200με strain at the mid-diaphysis. MicroCT and bone histomorphometry analyses were subsequently performed. All groups responded to tibial loading with increased mid-diaphyseal bone volume. The largest effect size was in the Dkk1 KO -12N group. Thus, Dkk1 KO animals had enhanced sensitivity to mechanical loading. Increases in cortical bone volume reflected increased periosteal bone formation. Bone volume and formation were not altered between WT and Wnt3^+/- controls. These data support the concept that agonists of Wnt/β-catenin signaling can act synergistically with load-bearing exercise. Notably, Sost expression decreased with loading in Dkk1 KO and WT mice, independent of genotype. These data suggest that a compensatory downregulation of Sost in Dkk1 KO mice is not likely the primary mechanism for the augmented response to mechanical load.

High shear stress amplitude in combination with prolonged stimulus duration determine induction of osteoclast formation by hematopoietic progenitor cells

To date, it is unclear how fluid dynamics stimulate mechanosensory cells to induce an osteoprotective or osteodestructive response. We investigated how murine hematopoietic progenitor cells respond to 2 minutes of dynamic fluid flow stimulation with a precisely controlled sequence of fluid shear stresses. The response was quantified by measuring extracellular adenosine triphosphate (ATP), immunocytochemistry of Piezo1, and sarcoplasmic/endoplasmic Ca²⁺ reticulum ATPase 2 (SERCA2), and by the ability of soluble factors produced by mechanically stimulated cells to modulate osteoclast differentiation. We rejected our initial hypothesis that peak wall shear stress rate determines the response of hematopoietic progenitor cells to dynamic fluid shear stress, as it had only a minor correlation with the abovementioned parameters. Low stimulus amplitudes corresponded to activation of Piezo1, SERCA2, low concentrations of extracellular ATP, and inhibition of osteoclastogenesis and resorption area, while high amplitudes generally corresponded to osteodestructive responses. At a given amplitude (3 Pa) and waveform (square), the duration of individual stimuli (duty cycle) showed a strong correlation with the release of ATP and osteoclast number and resorption area. Collectively, our data suggest that hematopoietic progenitor cells respond in a viscoelastic manner to loading, since a combination of high shear stress amplitude and prolonged duty cycle is needed to trigger an osteodestructive response. PLAIN LANGUAGE SUMMARY: In case of painful joints or missing teeth, the current intervention is to replace them with an implant to keep a high-quality lifestyle. When exercising or chewing, the cells in the bone around the implant experience mechanical loading. This loading generally supports bone formation to strengthen the bone and prevent breaking, but can also stimulate bone loss when the mechanical loading becomes too high around orthopedic and dental implants. We still do not fully understand how cells in the bone can distinguish between mechanical loading that strengthens or weakens the bone. We cultured cells derived from the bone marrow in the laboratory to test whether the bone loss response depends on (i) how fast a mechanical load is applied (rate), (ii) how intense the mechanical load is (amplitude), or (iii) how long each individual loading stimulus is applied (duration). We mimicked mechanical loading as it occurs in the body, by applying very precisely controlled flow of fluid over the cells. We found that a mechanosensitive receptor Piezo1 was activated by a low amplitude stimulus, which usually strengthens the bone. The potential inhibitor of Piezo1, namely SERCA2, was only activated by a low amplitude stimulus. This happened regardless of the rate of application. At a constant high amplitude, a longer duration of the stimulus enhanced the bone-weakening response. Based on these results we deduce that a high loading amplitude tends to be bone weakening, and the longer this high amplitude persists, the worse it is for the bone.

Conditional Activation of NF-κB Inducing Kinase (NIK) in the Osteolineage Enhances Both Basal and Loading-Induced Bone Formation

Studies from global loss-of-function mutants suggest that alternative NF-κB downstream of NF-κB inducing kinase (NIK) is a cell-intrinsic negative regulator of osteogenesis. However, the interpretation of the osteoblast and/or osteocyte contribution to the bone phenotype is complicated by simultaneous osteoclast defects in these models. Therefore, we turned to a transgenic mouse model to investigate the direct role of NIK in the osteolineage. Osx-Cre;NT3 animals (NT3-Cre +), which bear a constitutively active NIK allele (NT3) driven by Osx-Cre, were compared with their Cre-negative, Control (Ctrl) littermates. NT3-Cre + mice had elevated serum P1NP and CTX levels. Despite this high turnover state, µCT showed that constitutive activation of NIK resulted in a net increase in basal bone mass in both cortical and cancellous compartments. Furthermore, NT3-Cre + mice exhibited a greater anabolic response following mechanical loading compared with controls. We next performed RNA-Seq on nonloaded and loaded tibias to elucidate possible mechanisms underlying the increased bone anabolism seen in NT3-Cre + mice. Hierarchical clustering revealed two main transcriptional programs: one loading-responsive and the other NT3 transgene-driven. Gene ontology (GO) analysis indicated a distinct upregulation of receptor, kinase, and growth factor activities including Wnts, as well as a calcium-response signature in NT3-Cre + limbs. The promoters of these GO-term associated genes, including many known to be bone-anabolic, were highly enriched for multiple κB recognition elements (κB-RE) relative to the background frequency in the genome. The loading response in NT3-Cre + mice substantially overlapped (>90%) with Ctrl. Surprisingly, control animals had 10-fold more DEGs in response to loading. However, most top DEGs shared between genotypes had a high incidence of multiple κB-RE in their promoters. Therefore, both transcriptional programs (loading-responsive and NT3 transgene-driven) are modulated by NF-κB. Our studies uncover a previously unrecognized role for NF-κB in the promotion of both basal and mechanically stimulated bone formation. © 2019 American Society for Bone and Mineral Research.

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/β-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.

Reptiles in Space Missions: Results and Perspectives

Reptiles are a rare model object for space research. However, some reptile species demonstrate effective adaptation to spaceflight conditions. The main scope of this review is a comparative analysis of reptile experimental exposure in weightlessness, demonstrating the advantages and shortcomings of this model. The description of the known reptile experiments using turtles and geckos in the space and parabolic flight experiments is provided. Behavior, skeletal bones (morphology, histology, and X-ray microtomography), internal organs, and the nervous system (morphology, histology, and immunohistochemistry) are studied in the spaceflight experiments to date, while molecular and physiological results are restricted. Therefore, the results are discussed in the scope of molecular data collected from mammalian (mainly rodents) specimens and cell cultures in the parabolic and orbital flights and simulated microgravity. The published data are compared with the results of the gecko model studies after the 12–44.5-day spaceflights with special reference to the unique peculiarities of the gecko model for the orbital experiments. The complex study of thick-toed geckos after three spaceflights, in which all geckos survived and demonstrated effective adaptation to spaceflight conditions, was performed. However, future investigations are needed to study molecular mechanisms of gecko adaptation in space.

Association of serum bone- and muscle-derived factors with age, sex, body composition, and physical function in community-dwelling middle-aged and elderly adults: a cross-sectional study

BACKGROUND

Understanding interactions between bone and muscle based on endocrine factors may help elucidate the relationship between osteoporosis and sarcopenia. However, whether the abundance or activity of these endocrine factors is affected by age and sex or whether these factors play a causal role in bone and muscle formation and function is unclear. We aimed to evaluate the association of serum bone- and muscle-derived factors with age, sex, body composition, and physical function in community-dwelling middle-aged and elderly adults.

METHODS

In all, 254 residents (97 men, 157 women) participated in this cross-sectional study conducted in Japan. The calcaneal speed of sound (SOS) was evaluated by quantitative ultrasound examination. Skeletal muscle mass index (SMI) was calculated by bioelectrical impedance analysis. Grip strength was measured using a dynamometer. Gait speed was measured by optical-sensitive gait analysis. Serum sclerostin, osteocalcin (OC), insulin-like growth factor-1 (IGF-1), myostatin, and tartrate-resistant acid phosphatase-5b (TRACP-5b) concentrations were measured simultaneously. The difference by sex was determined using t test. Correlations between serum bone- and muscle-derived factors and age, BMI, SOS, SMI, grip strength, gait speed, and TRACP-5b in men and women were determined based on Pearson's correlation coefficients. Multiple regression analysis was performed using the stepwise method.

RESULTS

There was no significant difference with regard to age between men (75.0 ± 8.9 years) and women (73.6 ± 8.1 years). Sclerostin was significantly higher in men than in women and tended to increase with age in men; it was significantly associated with SOS and TRACP-5b levels. OC was significantly higher in women than in men and was significantly associated with TRACP-5b levels and age. IGF-1 tended to decrease with age in both sexes and was significantly associated with SOS and body mass index. Myostatin did not correlate with any assessed variables.

CONCLUSIONS

Sclerostin was significantly associated with sex, age, and bone metabolism, although there was no discernable relationship between serum sclerostin levels and muscle function. There was no obvious relationship between OC and muscle parameters. This study suggests that IGF-1 is an important modulator of muscle mass and function and bone metabolism in community-dwelling middle-aged and elderly adults.

G-CSF partially mediates effects of sleeve gastrectomy on the bone marrow niche

Bariatric surgeries are integral to the management of obesity and its metabolic complications. However, these surgeries cause bone loss and increase fracture risk through poorly understood mechanisms. In a mouse model, vertical sleeve gastrectomy (VSG) caused trabecular and cortical bone loss that was independent of sex, body weight, and diet, and this loss was characterized by impaired osteoid mineralization and bone formation. VSG had a profound effect on the bone marrow niche, with rapid loss of marrow adipose tissue, and expansion of myeloid cellularity, leading to increased circulating neutrophils. Following VSG, circulating granulocyte-colony stimulating factor (G-CSF) was increased in mice, and was transiently elevated in a longitudinal study of humans. Elevation of G-CSF was found to recapitulate many effects of VSG on bone and the marrow niche. In addition to stimulatory effects of G-CSF on myelopoiesis, endogenous G-CSF suppressed development of marrow adipocytes and hindered accrual of peak cortical and trabecular bone. Effects of VSG on induction of neutrophils and depletion of marrow adiposity were reduced in mice deficient for G-CSF; however, bone mass was not influenced. Although not a primary mechanism for bone loss with VSG, G-CSF plays an intermediary role for effects of VSG on the bone marrow niche.

Differential changes in bone strength of two inbred mouse strains following administration of a sclerostin-neutralizing antibody during growth

Administration of sclerostin-neutralizing antibody (Scl-Ab) treatment has been shown to elicit an anabolic bone response in growing and adult mice. Prior work characterized the response of individual mouse strains but did not establish whether the impact of Scl-Ab on whole bone strength would vary across different inbred mouse strains. Herein, we tested the hypothesis that two inbred mouse strains (A/J and C57BL/6J (B6)) will show different whole bone strength outcomes following sclerostin-neutralizing antibody (Scl-Ab) treatment during growth (4.5–8.5 weeks of age). Treated B6 femurs showed a significantly greater stiffness (S) (68.8% vs. 46.0%) and maximum load (ML) (84.7% vs. 44.8%) compared to A/J. Although treated A/J and B6 femurs showed greater cortical area (Ct.Ar) similarly relative to their controls (37.7% in A/J and 41.1% in B6), the location of new bone deposition responsible for the greater mass differed between strains and may explain the greater whole bone strength observed in treated B6 mice. A/J femurs showed periosteal expansion and endocortical infilling, while B6 femurs showed periosteal expansion. Post-yield displacement (PYD) was smaller in treated A/J femurs (-61.2%, p < 0.001) resulting in greater brittleness compared to controls; an effect not present in B6 mice. Inter-strain differences in S, ML, and PYD led to divergent changes in work-to-fracture (Work). Work was 27.2% (p = 0.366) lower in treated A/J mice and 66.2% (p < 0.001) greater in treated B6 mice relative to controls. Our data confirmed the anabolic response to Scl-Ab shown by others, and provided evidence suggesting the mechanical benefits of Scl-Ab administration may be modulated by genetic background, with intrinsic growth patterns of these mice guiding the location of new bone deposition. Whether these differential outcomes will persist in adult and elderly mice remains to be determined.

Physical Activity and Bone Accretion: Isotemporal Modeling and Genetic Interactions

PURPOSE

This study aimed to determine if replacing time spent in high- and low-impact physical activity (PA) predicts changes in pediatric bone mineral density (BMD) and content (BMC).

METHODS

We analyzed data from the longitudinal Bone Mineral Density in Childhood Study (N = 2337 with up to seven visits). The participants were age 5-19 yr at baseline, 51.2% were female, and 80.6% were nonblack. Spine, total hip, and femoral neck areal BMD and total body less head (TBLH) BMC Z-scores were calculated. Hours per day spent in high- and low-impact PA were self-reported. Standard covariate-adjusted (partition model) and time allocation-sensitive isotemporal substitution modeling frameworks were applied to linear mixed models. Statistical interactions with sex, self-reported ancestry, age, and bone fragility genetic scores (percentage of areal BMD-lowering alleles carried) were tested.

RESULTS

In standard models, high-impact PA was positively associated with bone Z-score at all four skeletal sites (e.g., TBLH-BMC Z-score: beta = 0.05, P = 2.0 × 10), whereas low-impact PA was not associated with any of the bone Z-scores. In isotemporal substitution models, replacing 1 h·d of low- for high-impact PA was associated with higher bone Z-scores (e.g., TBLH-BMC Z-score: beta = 0.06, P = 2.9 × 10). Conversely, replacing 1 h·d of high- for low-impact PA was associated with lower bone Z-scores (e.g., TBLH-BMC Z-score: beta = -0.06, P = 2.9 × 10). The substitution associations were similar for each sex and ancestry group, and for those with higher and lower genetic scores for bone fragility (P-interactions > 0.05), but increased in strength among the older adolescents (P-age interactions < 0.05).

CONCLUSIONS

Time-sensitive models suggest that replacing low-impact PA for high-impact PA would be beneficial for the growing skeleton in the majority of children.

Body Fat Percentage, Body Mass Index, Fat Mass Index and the Ageing Bone: Their Singular and Combined Roles Linked to Physical Activity and Diet

This study took a multi-analytical approach including group differences, correlations and unit-weighed directional z-score comparisons to identify the key mediators of bone health. A total of 190 participants (18⁻80 years) were categorized by body fat%, body mass index (BMI) and fat mass index (FMI) to examine the effect of differing obesity criteria on bone characteristics. A subset of 50 healthy-eating middle-to-older aged adults (44⁻80 years) was randomly selected to examine any added impact of lifestyle and inflammatory profiles. Diet was assessed using a 3-day food diary, bone mineral density (BMD) and content (BMC) by dual energy x-ray absorptiometry in the lumbar, thoracic, (upper and lower) appendicular and pelvic areas. Physical activity was assessed using the Baecke questionnaire, and endocrine profiling was assessed using multiplex luminometry. Obesity, classed via BMI, positively affected 20 out of 22 BMC- and BMD-related outcome measures, whereas FMI was associated with 14 outcome measures and adiposity only modulated nine out of 22 BMC- and BMD-related outcome measures. Whilst bivariate correlations only linked vitamin A and relative protein intake with BMD, the Z-score composite summary presented a significantly different overall dietary quality between healthy and osteopenic individuals. In addition, bivariate correlations from the subset revealed daily energy intake, sport-based physical activity and BMI positive mediators of seven out of 10 BMD sites with age and body fat% shown to be negative mediators of bone characteristics. In conclusion, whilst BMI is a good indicator of bone characteristics, high body fat% should also be the focus of osteoporosis risk with ageing. Interestingly, high BMI in conjunction with moderate to vigorous activity supplemented with an optimal diet (quality and quantity) are identified as positive modulators of bone heath.

Treadmill running and targeted tibial loading differentially improve bone mass in mice

Treadmill running and tibial loading are two common modalities used to assess the role of mechanical stimulation on the skeleton preclinically. The primary advantage of treadmill running is its physiological relevance. However, the applied load is complex and multiaxial, with observed results influenced by cardiovascular and musculoskeletal effects. In contrast, with tibial loading, a direct uniaxial load is applied to a single bone, providing the advantage of greater control but with less physiological relevance. Despite the importance and wide-spread use of both modalities, direct comparisons are lacking. In this study, we compared effects of targeted tibial loading, treadmill running, and their combination on cancellous and cortical architecture in a murine model. We show that tibial loading and treadmill running differentially improve bone mass, with tibial loading resulting in thicker trabeculae and increased cortical mass, and exercise resulting in greater number of trabeculae and no cortical mass-based effects. Combination of the modalities resulted in an additive response. These data suggest that tibial loading and exercise may improve mass differentially.

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Tibial loading increased trabecular thickness while exercise increased number.

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Combined effects of loading and exercise were additive in cancellous bone.

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In cortical bone, loading increased cross-sectional area.

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No mass-based effects were noted due to exercise.

How is mechanobiology involved in mesenchymal stem cell differentiation toward the osteoblastic or adipogenic fate?

Mechanobiology plays a major role in transducing physical cues from the dynamic cellular environment into biochemical modifications that promote cell-specific differentiation paths. Mesenchymal stem cells in the bone marrow or in other mesenchymal tissues will differentiate according to the expression of transcription factors (TFs) that govern their lineage commitment. The favoring of either osteogenic or adipogenic differentiation relies on TF expression as well as mechanical properties of the cells' niche that are translated into the activation of certain signaling pathways. Physical factors can induce significant shifts in bipotential lineage commitment between osteogenesis and adipogenesis. The stiffness of the extracellular matrix (ECM) surrounding a cell, varying greatly from rigid environments close to the bone surface to softer regions in the bone marrow, can influence the path of differentiation. Additionally, mechanical loading through exercise appears to favor osteogenesis whereas disuse conditions seem to promote adipogenesis.

In Vivo Osteocyte Mechanotransduction: Recent Developments and Future Directions

PURPOSE OF REVIEW

Mechanical loading is an essential stimulus for skeletal tissues. Osteocytes are primarily responsible for sensing mechanical stimuli in bone and for orchestrating subsequent responses. This is critical for maintaining homeostasis, and responding to injury/disease. The osteocyte mechanotransduction pathway, and the downstream effects it mediates, is highly complex. In vivo models have proved invaluable in understanding this process. This review summarizes the commonly used models, as well as more recently developed ones, and describes how they are used to address emerging questions in the field.

RECENT FINDINGS

Minimally invasive animal models can be used to determine mechanisms of osteocyte mechanotransduction, at the cell and molecular level, while simultaneously reducing potentially confounding responses such as inflammation/wound-healing. The details of osteocyte mechanotransduction in bone are gradually becoming clearer. In vivo model systems are a key tool in pursing this question. Advances in this field are explored and discussed in this review.

The Bromodomain Inhibitor N-Methyl pyrrolidone Prevents Osteoporosis and BMP-Triggered Sclerostin Expression in Osteocytes

(1) Background: In an adult skeleton, bone is constantly renewed in a cycle of bone resorption, followed by bone formation. This coupling process, called bone remodeling, adjusts the quality and quantity of bone to the local needs. It is generally accepted that osteoporosis develops when bone resorption surpasses bone formation. Osteoclasts and osteoblasts, bone resorbing and bone forming cells respectively, are the major target in osteoporosis treatment. Inside bone and forming a complex network, the third and most abundant cells, the osteocytes, have long remained a mystery. Osteocytes are responsible for mechano-sensation and -transduction. Increased expression of the osteocyte-derived bone inhibitor sclerostin has been linked to estrogen deficiency-induced osteoporosis and is therefore a promising target for osteoporosis management. (2) Methods: Recently we showed in vitro and in vivo that NMP (N-Methyl-2-pyrrolidone) is a bioactive drug enhancing the BMP-2 (Bone Morphogenetic Protein 2) induced effect on bone formation while blocking bone resorption. Here we tested the effect of NMP on the expression of osteocyte-derived sclerostin. (3) Results: We found that NMP significantly decreased sclerostin mRNA and protein levels. In an animal model of osteoporosis, NMP prevented the estrogen deficiency-induced increased expression of sclerostin. (4) Conclusions: These results support the potential of NMP as a novel therapeutic compound for osteoporosis management, since it preserves bone by a direct interference with osteoblasts and osteoclasts and an indirect one via a decrease in sclerostin expression by osteocytes.

Role of Osteocytes in Myeloma Bone Disease: Anti-sclerostin Antibody as New Therapeutic Strategy

Osteocytes are terminally differentiated cells of the osteoblast lineage. They are involved in the regulation of bone remodeling by increasing osteoclast formation or decreasing bone formation by the secretion of the osteoblast inhibitor sclerostin. Monoclonal antibody anti-sclerostin, Romosozumab, has been developed and tested in clinical trials in patients with osteoporosis. In the last years, the role of osteocytes in the development of osteolytic bone lesions that occurs in multiple myeloma, have been underlined. Myeloma cells increase osteocyte death through the up-regulation of both apoptosis and autophagy that, in turn, triggers osteoclast formation, and activity. When compared to healthy controls, myeloma patients with bone disease have higher osteocyte cell death, but the treatment with proteasome inhibitor bortezomib has been shown to maintain osteocyte viability. In preclinical mouse models of multiple myeloma, treatment with blocking anti-sclerostin antibody increased osteoblast numbers and bone formation rate reducing osteolytic bone lesions. Moreover, the combination of anti-sclerostin antibody and the osteoclast inhibitor zoledronic acid increased bone mass and fracture resistance synergistically. However, anti-sclerostin antibody did not affect tumor burden in vivo or the efficacy of anti-myeloma drugs in vitro. Nevertheless, the combination therapy of anti-sclerostin antibody and the proteasome inhibitor carfilzomib, displayed potent anti-myeloma activity as well as positive effects on bone disease in vivo. In conclusion, all these data suggest that osteocytes are involved in myeloma bone disease and may be considered a novel target for the use of antibody-mediated anti-sclerostin therapy also in multiple myeloma patients.

Dkk1 KO Mice Treated with Sclerostin Antibody Have Additional Increases in Bone Volume

Dickkopf-1 (DKK1) and sclerostin are antagonists of the Wnt/β-catenin pathway and decreased expression of either results in increased bone formation and mass. As both affect the same signaling pathway, we aimed to elucidate the redundancy and/or compensation of sclerostin and DKK1. Weekly sclerostin antibody (Scl-Ab) was used to treat 9-week-old female Dkk1 KO (Dkk1^-/-:Wnt3^+/-) mice and compared to Scl-Ab-treated wild-type mice as well as vehicle-treated Dkk1 KO and wild-type animals. While Wnt3 heterozygote (Wnt3^+/-) mice show no bone phenotype, Scl-Ab and vehicle-treated control groups of this genotype were included. Specimens were harvested after 3 weeks for microCT, bone histomorphometry, anti-sclerostin immunohistochemistry, and biomechanical testing. Scl-Ab enhanced bone anabolism in all treatment groups, but with synergistic enhancement seen in the cancellous compartment of Dkk1 KO mice (bone volume + 55% Dkk1 KO p < 0.01; + 22% wild type p < 0.05). Scl-Ab treatment produced less marked increases in cortical bone of the tibiae, with anabolic effects similar across genotypes. Mechanical testing confirmed that Scl-Ab improved strength across all genotypes; however, no enhancement was seen within Dkk1 KO mice. Dynamic bone labeling showed that Scl-Ab treatment was associated with increased bone formation, regardless of genotype. Immunohistochemical staining for sclerostin protein indicated no differences in the Dkk1 KO mice, indicating that the increased Wnt signaling associated with DKK1 deficiency was not compensated by upregulation of sclerostin protein. These data suggest complex interactions between Wnt signaling factors in bone, but critically illustrate synergy between DKK1 deficiency and Scl-Ab treatment. These data support the application of dual-targeted therapeutics in the modulation of bone anabolism.

Sclerostin Neutralizing Antibody Treatment Enhances Bone Formation but Does Not Rescue Mechanically Induced Delayed Healing

During bone healing, tissue formation processes are governed by mechanical strain. Sost/sclerostin, a key Wnt signaling inhibitor and mechano-sensitive pathway, is downregulated in response to mechanical loading. Sclerostin neutralizing antibody (SclAb) increases bone formation. Nevertheless, it remains unclear whether sclerostin inhibition can rescue bone healing in situations of mechanical instability, which otherwise delay healing. We investigated SclAb's influence on tissue formation in a mouse femoral osteotomy, stabilized with rigid or semirigid external fixation. The different fixations allowed different magnitudes of interfragmentary movement during weight bearing, thereby influencing healing outcome. SclAb or vehicle (veh) was administeredand bone healing was assessed at multiple time points up to day 21 postoperatively by in vivo micro-computed tomography, histomorphometry, biomechanical testing, immunohistochemistry, and gene expression. Our results show that SclAb treatment caused a greater bone volume than veh. However, SclAb could not overcome the characteristic delayed healing of semirigid fixation. Indeed, semirigid fixation resulted in delayed healing with a prolonged endochondral ossification phase characterized by increased cartilage, lower bone volume fraction, and less bony bridging across the osteotomy gap than rigid fixation. In a control setting, SclAb negatively affected later stages of healing under rigid fixation, evidenced by the high degree of endosteal bridging at 21 days in the rigid-SclAb group compared with rigid-veh, indicating delayed fracture callus remodeling and bone marrow reconstitution. Under rigid fixation, Sost and sclerostin expression at the gene and protein level, respectively, were increased in SclAb compared with veh-treated bones, suggesting a negative feedback mechanism. Our results suggest that SclAb could be used to enhance overall bone mass but should be carefully considered in bone healing. SclAb may help to increase bone formation early in the healing process but not during advanced stages of fracture callus remodeling and not to overcome delayed healing in semirigid fixation. © 2018 American Society for Bone and Mineral Research.

Sclerostin Antibody Augments the Anabolic Bone Formation Response in a Mouse Model of Mechanical Tibial Loading

Decreased activity or expression of sclerostin, an endogenous inhibitor of Wnt/β-catenin signaling, results in increased bone formation and mass. Antibodies targeting and neutralizing sclerostin (Scl-Ab) have been shown to increase bone mass and reduce fracture risk. Sclerostin is also important in modulating the response of bone to changes in its biomechanical environment. However, the effects of Scl-Ab on mechanotransduction are unclear, and it was speculated that the loading response may be altered for individuals receiving Scl-Ab therapy. To address this, we carried out a 2-week study of tibial cyclic compressive loading on C57Bl/6 mice treated with vehicle or 100 mg/kg/wk Scl-Ab. Increases in bone volume, density, and dynamic bone formation were found with loading, and the anabolic response was further increased by the combination of load and Scl-Ab. To investigate the underlying mechanism, gene profiling by RNA sequencing (RNAseq) was performed on tibias isolated from mice from all four experimental groups. Major alterations in Wnt/β-catenin gene expression were found with tibial loading, however not with Scl-Ab treatment alone. Notably, the combination of load and Scl-Ab elicited a synergistic response from a number of specific Wnt-related and mechanotransduction factors. An unexpected finding was significant upregulation of factors in the Rho GTPase signaling pathway with combination treatment. In summary, combination therapy had a more profound anabolic response than either Scl-Ab or loading treatment alone. The Wnt/β-catenin and Rho GTPase pathways were implicated within bone mechanotransduction and support the concept that bone mechanotransduction is likely to encompass a number of interconnected signaling pathways. © 2017 American Society for Bone and Mineral Research.