Conditional disruption of IGF-I gene in type 1α collagen-expressing cells shows an essential role of IGF-I in skeletal anabolic response to loading

Delayed skeletal development and IGF-1 deficiency in a mouse model of lysinuric protein intolerance

SLC7A7 deficiency, or lysinuric protein intolerance (LPI), causes loss of function of the y+LAT1 transporter critical for efflux of arginine, lysine and ornithine in certain cells. LPI is characterized by urea cycle dysfunction, renal disease, immune dysregulation, growth failure, delayed bone age and osteoporosis. We previously reported that Slc7a7 knockout mice (C57BL/6×129/SvEv F2) recapitulate LPI phenotypes, including growth failure. Our main objective in this study was to characterize the skeletal phenotype in these mice. Compared to wild-type littermates, juvenile Slc7a7 knockout mice demonstrated 70% lower body weights, 87% lower plasma IGF-1 concentrations and delayed skeletal development. Because poor survival prevents evaluation of mature knockout mice, we generated a conditional Slc7a7 deletion in mature osteoblasts or mesenchymal cells of the osteo-chondroprogenitor lineage, but no differences in bone architecture were observed. Overall, global Slc7a7 deficiency caused growth failure with low plasma IGF-1 concentrations and delayed skeletal development, but Slc7a7 deficiency in the osteoblastic lineage was not a major contributor to these phenotypes. Future studies utilizing additional tissue-specific Slc7a7 knockout models may help dissect cell-autonomous and non-cell-autonomous mechanisms underlying phenotypes in LPI.

T-cell factor 7L2 is a novel regulator of osteoblast functions that acts in part by modulation of hypoxia signaling

T-cell-like factor (TCF)7l2, a key effector of canonical Wnt signaling, is highly expressed in bone but nothing is known about its role in regulating osteoblast function. To test this, we generated mice with conditional disruption of Tcf7l2 gene in osteoblast lineages using Tcf7l2 floxed and Col1α2-Cre mice. Skeletal parameters were evaluated using heterozygous conditional knockdown (HCKD) mice since homozygous conditional knockout died during pregnancy or immediately after birth. At 5 wk of age, trabecular bone mass of long bones was reduced by 35% as measured by microcomputed tomography (μCT). Histology data showed a 42% reduction in femur trabecular bone mass caused by reduced bone formation. Knockdown of Tcf7l2 expression in osteoblasts decreased proliferation and differentiation by 20%-40%. Expression levels of genes (Hif1α, Vegf, and β-catenin) targeted by TCF7L2 were decreased by 50% in Tcf7l2-deficient osteoblasts and bones of HCKD mice. We found that the Hif1α gene promoter contained multiple putative TCF7L2 motifs and stabilization of HIF1α protein levels rescued expression of TCF7L2 target genes and alkaline phosphatase (ALP) activity in Tcf7l2-deficient osteoblasts. Furthermore, Tcf7l2 overexpression increased proliferation in the presence of canonical Wnt3a that was not affected by β-catenin inhibitor providing evidence for a noncanonical signaling in mediating TCF7L2 effects. Tcf7l2 expression was increased in response to mechanical strain (MS) in vitro and in vivo, and disruption of Tcf7l2 expression in osteoblasts reduced MS-induced ALP activity by 35%. We conclude that Tcf7l2, a mechanoresponsive gene, is an important regulator of osteoblast function acting, in part, via hypoxia signaling.NEW & NOTEWORTHY TCF7L2 is expressed by bone but it was not known whether TCF7L2 expression influenced bone development. By using a mouse model with conditional disruption of Tcf7l2 in osteoblast lineage cells, we have demonstrated for the first time, that TCF7L2 plays an important role in regulating osteoblasts via a noncanonical pathway.

Low Intensity Pulsed Ultrasound for Bone Tissue Engineering

As explained by Wolff's law and the mechanostat hypothesis, mechanical stimulation can be used to promote bone formation. Low intensity pulsed ultrasound (LIPUS) is a source of mechanical stimulation that can activate the integrin/phosphatidylinositol 3-OH kinase/Akt pathway and upregulate osteogenic proteins through the production of cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). This paper analyzes the results of in vitro and in vivo studies that have evaluated the effects of LIPUS on cell behavior within three-dimensional (3D) titanium, ceramic, and hydrogel scaffolds. We focus specifically on cell morphology and attachment, cell proliferation and viability, osteogenic differentiation, mineralization, bone volume, and osseointegration. As shown by upregulated levels of alkaline phosphatase and osteocalcin, increased mineral deposition, improved cell ingrowth, greater scaffold pore occupancy by bone tissue, and superior vascularization, LIPUS generally has a positive effect and promotes bone formation within engineered scaffolds. Additionally, LIPUS can have synergistic effects by producing the piezoelectric effect and enhancing the benefits of 3D hydrogel encapsulation, growth factor delivery, and scaffold modification. Additional research should be conducted to optimize the ultrasound parameters and evaluate the effects of LIPUS with other types of scaffold materials and cell types.

The effects of locomotion on bone marrow mesenchymal stem cell fate: insight into mechanical regulation and bone formation

Bone marrow mesenchymal stem cells (BMSCs) refer to a heterogeneous population of cells with the capacity for self-renewal. BMSCs have multi-directional differentiation potential and can differentiate into chondrocytes, osteoblasts, and adipocytes under specific microenvironment or mechanical regulation. The activities of BMSCs are closely related to bone quality. Previous studies have shown that BMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. Thus, a goal of this review is to discuss how these ubiquious signals arising from mechanical stimulation are perceived by BMSCs and then how the cells respond to them. Studies in recent years reported a significant effect of locomotion on the migration, proliferation and differentiation of BMSCs, thus, contributing to our bone mass. This regulation is realized by the various intersecting signaling pathways including RhoA/Rock, IFG, BMP and Wnt signalling. The mechanoresponse of BMSCs also provides guidance for maintaining bone health by taking appropriate exercises. This review will summarize the regulatory effects of locomotion/mechanical loading on BMSCs activities. Besides, a number of signalling pathways govern MSC fate towards osteogenic or adipocytic differentiation will be discussed. The understanding of mechanoresponse of BMSCs makes the foundation for translational medicine.

Eph-Ephrin Signaling Mediates Cross-Talk Within the Bone Microenvironment

Skeletal integrity is maintained through the tightly regulated bone remodeling process that occurs continuously throughout postnatal life to replace old bone and to repair skeletal damage. This is maintained primarily through complex interactions between bone resorbing osteoclasts and bone forming osteoblasts. Other elements within the bone microenvironment, including stromal, osteogenic, hematopoietic, endothelial and neural cells, also contribute to maintaining skeletal integrity. Disruption of the dynamic interactions between these diverse cellular systems can lead to poor bone health and an increased susceptibility to skeletal diseases including osteopenia, osteoporosis, osteoarthritis, osteomalacia, and major fractures. Recent reports have implicated a direct role for the Eph tyrosine kinase receptors and their ephrin ligands during bone development, homeostasis and skeletal repair. These membrane-bound molecules mediate contact-dependent signaling through both the Eph receptors, termed forward signaling, and through the ephrin ligands, referred to as reverse signaling. This review will focus on Eph/ ephrin cross-talk as mediators of hematopoietic and stromal cell communication, and how these interactions contribute to blood/ bone marrow function and skeletal integrity during normal steady state or pathological conditions.

Potential applications for rhIGF-I: Bone disease and IGFI

Growth hormone (GH) and insulin like growth factor-I (IGFI) are key bone trophic hormones, whose rising levels during puberty are critical for pubertal bone accrual. Conditions of GH deficiency and genetic resistance impact cortical and trabecular bone deleteriously with reduced estimates of bone strength. In humans, conditions of undernutrition (as in anorexia nervosa (AN), or subsequent to chronic illnesses) are associated with low IGF-I levels, which correlate with disease severity, and also with lower bone mineral density (BMD), impaired bone structure and lower strength estimates. In adolescents and adults with AN, studies have demonstrated a nutritionally acquired GH resistance with low IGF-I levels despite high concentrations of GH. IGF-I levels go up with increasing body weight, and are associated with rising levels of bone turnover markers. In short-term studies lasting 6-10 days, recombinant human IGF-I (rhIGF-I) administration in physiologic replacement doses normalized IGF-I levels and increased levels of bone formation markers in both adults and adolescents with AN. In a randomized controlled trial in adults with AN in which participants were randomized to one of four arms: (i) rhIGF-I with oral estrogen-progesterone (EP), (ii) rhIGF-I alone, (iii) EP alone, or (iv) neither for 9 months, a significant increase in bone formation markers was noted in the groups that received rhIGF-I, and a significant decrease in bone resorption markers in the groups that received EP. The group that received both rhIGF-I and EP had a significant increase in bone density at the spine and hip compared to the group that received neither. Side effects were minimal, with no documented fingerstick glucose of <50 mg/dl. These data thus suggest a potential role for rhIGF-I administration in optimizing bone accrual in states of undernutrition associated with low IGF-I.

Conditional knockout of ephrinB1 in osteogenic progenitors delays the process of endochondral ossification during fracture repair

The Eph receptor tyrosine kinase ligand, ephrinB1 (EfnB1) is important for correct skeletal and cartilage development, however, the role of EfnB1 in fracture repair is unknown. This study investigated the role of EfnB1 during fracture repair where EfnB1 expression increased significantly at 1 and 2 weeks post fracture in C57Bl/6 wildtype mice, coinciding with the haematoma, soft callus formation/remodelling stages, respectively. To investigate the specific role of EfnB1 within the osteogenic lineage during fracture repair, male mice with a conditional deletion of EfnB1 in the osteogenic lineage (EfnB1_OB^fl/O), driven by the Osterix (Osx) promoter, and their male Osx:Cre counterparts were subject to a femoral fracture with internal fixation. Two weeks post fracture micro computed tomography (μCT) analysis revealed that EfnB1_OB^fl/O mice displayed a significant decrease in bone volume relative to tissue volume within the fracture callus. This was attributed to an alteration in the distribution of osteoclasts within the fracture site, a significant elevation in cartilaginous tissue and reduction in the osteoprogenitor population and calcein labelled bone within the fracture site of EfnB1_OB^fl/O mice. Supportive in vitro studies demonstrated that under osteogenic conditions, cultured EfnB1_OB^fl/O stromal cells derived from the 2 week fracture site exhibited a reduced capacity to produce mineral and decreased expression of the osteogenic gene, Osterix, when compared to Osx:Cre controls. These findings suggest that the loss of EfnB1 delays the fracture repair process. The present study confirmed that EFNB1 activation in human BMSC, following stimulation with soluble-EphB2 resulted in de-phosphorylation of TAZ, demonstrating similarities in EfnB1 signalling between human and mouse stromal populations. Overall, the present study provides evidence that loss of EfnB1 in the osteo/chondrogenic lineages delays the soft callus formation/remodelling stages of the fracture repair process.

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.

The Osteocyte as the New Discovery of Therapeutic Options in Rare Bone Diseases

Osteocytes are the most abundant (~95%) cells in bone with the longest half-life (~25 years) in humans. In the past osteocytes have been regarded as vestigial cells in bone, since they are buried inside the tough bone matrix. However, during the last 30 years it has become clear that osteocytes are as important as bone forming osteoblasts and bone resorbing osteoclasts in maintaining bone homeostasis. The osteocyte cell body and dendritic processes reside in bone in a complex lacuno-canalicular system, which allows the direct networking of osteocytes to their neighboring osteocytes, osteoblasts, osteoclasts, bone marrow, blood vessels, and nerves. Mechanosensing of osteocytes translates the applied mechanical force on bone to cellular signaling and regulation of bone adaptation. The osteocyte lacuno-canalicular system is highly efficient in transferring external mechanical force on bone to the osteocyte cell body and dendritic processes via displacement of fluid in the lacuno-canalicular space. Osteocyte mechanotransduction regulates the formation and function of the osteoblasts and osteoclasts to maintain bone homeostasis. Osteocytes produce a variety of proteins and signaling molecules such as sclerostin, cathepsin K, Wnts, DKK1, DMP1, IGF1, and RANKL/OPG to regulate osteoblast and osteoclast activity. Various genetic abnormality-associated rare bone diseases are related to disrupted osteocyte functions, including sclerosteosis, van Buchem disease, hypophosphatemic rickets, and WNT1 and plastin3 mutation-related disorders. Meticulous studies during the last 15 years on disrupted osteocyte function in rare bone diseases guided for the development of various novel therapeutic agents to treat bone diseases. Studies on genetic, molecular, and cellular mechanisms of sclerosteosis and van Buchem disease revealed a role for sclerostin in bone homeostasis, which led to the development of the sclerostin antibody to treat osteoporosis and other bone degenerative diseases. The mechanism of many other rare bone diseases and the role of the osteocyte in the development of such conditions still needs to be investigated. In this review, we mainly discuss the knowledge obtained during the last 30 years on the role of the osteocyte in rare bone diseases. We speculate about future research directions to develop novel therapeutic drugs targeting osteocyte functions to treat both common and rare bone diseases.

Growth Hormone Effects on Bone Loss-Induced by Mild Traumatic Brain Injury and/or Hind Limb Unloading

Growth hormone (GH) deficiency and loss of physical activity are common features in traumatic brain injury (TBI) patients that may contribute to bone loss. Therefore, we tested the hypothesis that GH treatment will rescue the hind limb unloading (UL)-induced skeletal deficit in TBI mice. Mild TBI was induced once per day for four consecutive days. UL (right hind limb) and treatment (3 mg/day GH or vehicle) began two weeks after the first TBI episode and lasted for four weeks. GH treatment increased femur BMD and lean body mass but decreased the % fat measured by DXA in the Control group. Micro-CT analysis revealed that the TBI, UL and TBI-UL groups showed reduced tibia trabecular (Tb) bone mass by 15%, 70%, and 75%, respectively compared to Control mice and that GH treatment significantly increased Tb. bone mass in all four groups. Vertebra also showed reduced Tb. bone mass in TBI, UL and TBI-UL groups. GH treatment increased vertebral Tb. bone mass in Control and UL groups but not in the TBI or TBI-UL group. GH treatment increased serum IGF-I levels similarly in TBI, UL and TBI-UL groups at day 14, suggesting the GH effect on liver IGF-I production was unaffected by skeletal UL. In contrast, GH effect on expression of ALP, IGFBP5 and axin2 in bone were compromised by UL. In conclusion, skeletal UL caused a greater Tb. bone deficit than mild TBI alone and that GH anabolic effects in the TBI and UL groups vary depending on the skeletal site.

Whole-methylome analysis of circulating monocytes in acute diabetic Charcot foot reveals differentially methylated genes involved in the formation of osteoclasts

AIM

To assess whether DNA methylation of monocytes play a role in the development of acute diabetic Charcot foot (CF).

PATIENTS & METHODS

We studied the whole methylome (WM) of circulating monocytes in 18 patients with Type 2 diabetes (T2D) and acute CF, 18 T2D patients with equivalent neuropathy and 18 T2D patients without neuropathy, using the enhanced reduced representation bisulfite sequencing technique.

RESULTS & CONCLUSION

WM analysis demonstrated that CF monocytes are differentially methylated compared with non-CF monocytes, in both CpG-site and gene-mapped analysis approaches. Among the methylated genes, several are involved in the migration process during monocyte differentiation into osteoclasts or are indirectly involved through the regulation of inflammatory pathways. Finally, we demonstrated an association between methylation and gene expression in cis- and trans-association.

The osteoprogenitor-specific loss of ephrinB1 results in an osteoporotic phenotype affecting the balance between bone formation and resorption

The present study investigated the effects of conditional deletion of ephrinB1 in osteoprogenitor cells driven by the Osterix (Osx) promoter, on skeletal integrity in a murine model of ovariectomy-induced (OVX) osteoporosis. Histomorphometric and μCT analyses revealed that loss of ephrinB1 in sham Osx:cre-ephrinB1^fl/fl mice caused a reduction in trabecular bone comparable to OVX Osx:Cre mice, which was associated with a significant reduction in bone formation rates and decrease in osteoblast numbers. Interestingly, these observations were not exacerbated in OVX Osx:cre-ephrinB1^fl/fl mice. Furthermore, sham Osx:cre-ephrinB1^fl/fl mice displayed significantly higher osteoclast numbers and circulating degraded collagen type 1 compared to OVX Osx:Cre mice. Confirmation studies found that cultured monocytes expressing EphB2 formed fewer TRAP⁺ multinucleated osteoclasts and exhibited lower resorption activity in the presence of soluble ephrinB1-Fc compared to IgG control. This inhibition of osteoclast formation and function induced by ephrinB1-Fc was reversed in the presence of an EphB2 chemical inhibitor. Collectively, these observations suggest that ephrinB1, expressed by osteoprogenitors, influences bone loss during the development of osteoporosis, by regulating both osteoblast and osteoclast formation and function, leading to a loss of skeletal integrity.

Role of IGF1 and EFN-EPH signaling in skeletal metabolism

Insulin-like growth factor 1(IGF1) and ephrin ligand (EFN)-receptor (EPH) signaling are both crucial for bone cell function and skeletal development and maintenance. IGF1 signaling is the major mediator of growth hormone-induced bone growth, but a host of different signals and factors regulate IGF1 signaling at the systemic and local levels. Disruption of the Igf1 gene results in reduced peak bone mass in both experimental animal models and humans. Additionally, EFN-EPH signaling is a complex system which, particularly through cell-cell interactions, contributes to the development and differentiation of many bone cell types. Recent evidence has demonstrated several ways in which the IGF1 and EFN-EPH signaling pathways interact with and depend upon each other to regulate bone cell function. While much remains to be elucidated, the interaction between these two signaling pathways opens a vast array of new opportunities for investigation into the mechanisms of and potential therapies for skeletal conditions such as osteoporosis and fracture repair.

IGF-1 signaling mediated cell-specific skeletal mechano-transduction

Mechanical loading preserves bone mass and stimulates bone formation, whereas skeletal unloading leads to bone loss. In addition to osteocytes, which are considered the primary sensor of mechanical load, osteoblasts, and bone specific mesenchymal stem cells also are involved. The skeletal response to mechanical signals is a complex process regulated by multiple signaling pathways including that of insulin-like growth factor-1 (IGF-1). Conditional osteocyte deletion of IGF-1 ablates the osteogenic response to mechanical loading. Similarly, osteocyte IGF-1 receptor (IGF-1R) expression is necessary for reloading-induced periosteal bone formation. Transgenic overexpression of IGF-1 in osteoblasts results in enhanced responsiveness to in vivo mechanical loading in mice, a response which is eliminated by osteoblastic conditional disruption of IGF-1 in vivo. Bone marrow derived stem cells (BMSC) from unloaded bone fail to respond to IGF-1 in vitro. IGF-1R is required for the transduction of a mechanical stimulus to downstream effectors, transduction which is lost when the IGF-1R is deleted. Although the molecular mechanisms are not yet fully elucidated, the IGF signaling pathway and its interactions with potentially interlinked signaling cascades involving integrins, the estrogen receptor, and wnt/β-catenin play an important role in regulating adaptive response of cancer bone cells to mechanical stimuli. In this review, we discuss recent advances investigating how IGF-1 and other interlinked molecules and signaling pathways regulate skeletal mechano-transduction involving different bone cells, providing an overview of the IGF-1 signaling mediated cell-specific response to mechanical stimuli. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:576-583, 2018.

The RhoGAP Myo9b Promotes Bone Growth by Mediating Osteoblastic Responsiveness to IGF-1

The Ras homolog A (RhoA) subfamily of Rho guanosine triphosphatases (GTPases) regulates actin-based cellular functions in bone such as differentiation, migration, and mechanotransduction. Polymorphisms or genetic ablation of RHOA and some of its regulatory guanine exchange factors (GEFs) have been linked to poor bone health in humans and mice, but the effects of RhoA-specific GTPase-activating proteins (GAPs) on bone quality have not yet been identified. Therefore, we examined the consequences of RhoGAP Myo9b gene knockout on bone growth, phenotype, and cellular activity. Male and female mice lacking both alleles demonstrated growth retardation and decreased bone formation rates during early puberty. These mice had smaller, weaker bones by 4 weeks of age, but only female KOs had altered cellular numbers, with fewer osteoblasts and more osteoclasts. By 12 weeks of age, bone quality in KOs worsened. In contrast, 4-week-old heterozygotes demonstrated bone defects that resolved by 12 weeks of age. Throughout, Myo9b ablation affected females more than males. Osteoclast activity appeared unaffected. In primary osteogenic cells, Myo9b was distributed in stress fibers and focal adhesions, and its absence resulted in poor spreading and eventual detachment from culture dishes. Similarly, MC3T3-E1 preosteoblasts with transiently suppressed Myo9b levels spread poorly and contained decreased numbers of focal adhesions. These cells also demonstrated reduced ability to undergo IGF-1-induced spreading or chemotaxis toward IGF-1, though responses to PDGF and BMP-2 were unaffected. IGF-1 receptor (IGF1R) activation was normal in cells with diminished Myo9b levels, but the activated receptor was redistributed from stress fibers and focal adhesions into nuclei, potentially affecting receptor accessibility and gene expression. These results demonstrate that Myo9b regulates a subset of RhoA-activated processes necessary for IGF-1 responsiveness in osteogenic cells, and is critical for normal bone formation in growing mice. © 2017 American Society for Bone and Mineral Research.

Serum sclerostin decreases following 12months of resistance- or jump-training in men with low bone mass

PURPOSE

We previously reported that 12months of resistance training (RT, 2×/wk, N=19) or jump training (JUMP, 3×/wk, N=19) increased whole body and lumbar spine BMD and increased serum bone formation markers relative to resorption in physically active (≥4h/wk) men (mean age: 44±2y; median: 44y) with osteopenia of the hip or spine. The purpose of this secondary analysis was to examine the effects of the RT or JUMP intervention on potential endocrine mediators of the exercise effects on bone, specifically IGF-I, PTH and sclerostin.

METHODS

Fasting blood samples were collected after a 24-h period of no exercise at baseline and after 12months of RT or JUMP. IGF-I, PTH and sclerostin were measured in serum by ELISA. The effects of RT or JUMP on IGF-I, PTH and sclerostin were evaluated using 2×2 repeated measures ANOVA (time, group). This study was conducted in accordance with the Declaration of Helsinki and was approved by the University of Missouri IRB.

RESULTS

Sclerostin concentrations in serum significantly decreased and IGF-I significantly increased after 12months of RT or JUMP; while PTH remained unchanged.

CONCLUSION

The beneficial effects of long-term, progressive-intensity RT or JUMP on BMD in moderately active men with low bone mass are associated with decreased sclerostin and increased IGF-I.

Stress and Alterations in Bones: An Interdisciplinary Perspective

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

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

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.

[Bone and adipose tissue formation]

Leptin has been described to have a crucial role in bone homeostasis by systemic as well as local action. Systemically, leptin seems to inhibit bone formation controlled by a feedback loop including osteocalcin and insulin. Even though the action seems to be bone site specific, as well as gender- and time-dependent, the results showing the interaction of these three factors are in part still inconsistent. In this article the complex effects of leptin, insulin, and osteocalcin on bone and fat metabolism are summarized.

Liver-derived IGF-I regulates cortical bone mass but is dispensable for the osteogenic response to mechanical loading in female mice

Low circulating IGF-I is associated with increased fracture risk. Conditional depletion of IGF-I produced in osteoblasts or osteocytes inhibits the bone anabolic effect of mechanical loading. Here, we determined the role of endocrine IGF-I for the osteogenic response to mechanical loading in young adult and old female mice with adult, liver-specific IGF-I inactivation (LI-IGF-I(-/-) mice, serum IGF-I reduced by ≈70%) and control mice. The right tibia was subjected to short periods of axial cyclic compressive loading three times/wk for 2 wk, and measurements were performed using microcomputed tomography and mechanical testing by three-point bending. In the nonloaded left tibia, the LI-IGF-I(-/-) mice had lower cortical bone area and increased cortical porosity, resulting in reduced bone mechanical strength compared with the controls. Mechanical loading induced a similar response in LI-IGF-I(-/-) and control mice in terms of cortical bone area and trabecular bone volume fraction. In fact, mechanical loading produced a more marked increase in cortical bone mechanical strength, which was associated with a less marked increase in cortical porosity, in the LI-IGF-I(-/-) mice compared with the control mice. In conclusion, liver-derived IGF-I regulates cortical bone mass, cortical porosity, and mechanical strength under normal (nonloaded) conditions. However, despite an ∼70% reduction in circulating IGF-I, the osteogenic response to mechanical loading was not attenuated in the LI-IGF-I(-/-) mice.

Bidirectional ephrin signaling in bone

The interaction between ephrin ligands (efn) and their receptors (Eph) is capable of inducing forward signaling, from ligand to receptor, as well as reverse signaling, from receptor to ligand. The ephrins are widely expressed in many tissues, where they mediate cell migration and adherence, properties that make the efn-Eph signaling critically important in establishing and maintaining tissue boundaries. The efn-Eph system has also received considerable attention in skeletal tissues, as ligand and receptor combinations are predicted to mediate interactions between the different types of cells that regulate bone development and homeostasis. This review summarizes our current understanding of efn-Eph signaling with a particular focus on the expression and functions of ephrins and their receptors in bone.

Regulation of skeletal growth and mineral acquisition by the GH/IGF-1 axis: Lessons from mouse models

The growth hormone (GH) and its downstream mediator, the insulin-like growth factor-1 (IGF-1), construct a pleotropic axis affecting growth, metabolism, and organ function. Serum levels of GH/IGF-1 rise during pubertal growth and associate with peak bone acquisition, while during aging their levels decline and associate with bone loss. The GH/IGF-1 axis was extensively studied in numerous biological systems including rodent models and cell cultures. Both hormones act in an endocrine and autocrine/paracrine fashion and understanding their distinct and overlapping contributions to skeletal acquisition is still a matter of debate. GH and IGF-1 exert their effects on osteogenic cells via binding to their cognate receptor, leading to activation of an array of genes that mediate cellular differentiation and function. Both hormones interact with other skeletal regulators, such as sex-steroids, thyroid hormone, and parathyroid hormone, to facilitate skeletal growth and metabolism. In this review we summarized several rodent models of the GH/IGF-1 axis and described key experiments that shed new light on the regulation of skeletal growth by the GH/IGF-1 axis.

Does the GH/IGF-1 axis contribute to skeletal sexual dimorphism? Evidence from mouse studies

The contribution of the gonadotropic axis to skeletal sexual dimorphism (SSD) was clarified in recent years. Studies with animal models of estrogen receptor (ER) or androgen receptor (AR) null mice, as well as mice with bone cell-specific ablation of ER or AR, revealed that both hormones play major roles in skeletal acquisition, and that estrogen regulates skeletal accrual in both sexes. The growth hormone (GH) and its downstream effector, the insulin-like growth factor-1 (IGF-1) are also major determinants of peak bone mass during puberty and young adulthood, and play important roles in maintaining bone integrity during aging. A few studies in both humans and animal models suggest that in addition to the differences in sex steroid actions on bone, sex-specific effects of GH and IGF-1 play essential roles in SSD. However, the contributions of the somatotropic (GH/IGF-1) axis to SSD are controversial and data is difficult to interpret. GH/IGF-1 are pleotropic hormones that act in an endocrine and autocrine/paracrine fashion on multiple tissues, affecting body composition as well as metabolism. Thus, understanding the contribution of the somatotropic axis to SSD requires the use of mouse models that will differentiate between these two modes of action. Elucidation of the relative contribution of GH/IGF-1 axis to SSD is significant because GH is approved for the treatment of normal children with short stature and children with congenital growth disorders. Thus, if the GH/IGF-1 axis determines SSD, treatment with GH may be tailored according to sex. In the following review, we give an overview of the roles of sex steroids in determining SSD and how they may interact with the GH/IGF-1 axis in bone. We summarize several mouse models with impaired somatotropic axis and speculate on the possible contribution of that axis to SSD.

Bone and skeletal muscle: Key players in mechanotransduction and potential overlapping mechanisms

The development and maintenance of skeletal muscle and bone mass is critical for movement, health and issues associated with the quality of life. Skeletal muscle and bone mass are regulated by a variety of factors that include changes in mechanical loading. Moreover, bone mass is, in large part, regulated by muscle-derived mechanical forces and thus by changes in muscle mass/strength. A thorough understanding of the cellular mechanism(s) responsible for mechanotransduction in bone and skeletal muscle is essential for the development of effective exercise and pharmaceutical strategies aimed at increasing, and/or preventing the loss of, mass in these tissues. Thus, in this review we will attempt to summarize the current evidence for the major molecular mechanisms involved in mechanotransduction in skeletal muscle and bone. By examining the differences and similarities in mechanotransduction between these two tissues, it is hoped that this review will stimulate new insights and ideas for future research and promote collaboration between bone and muscle biologists.(1).

Role of WNT16 in the regulation of periosteal bone formation in female mice

In this study, we evaluated the role of WNT16 in regulating bone size, an important determinant of bone strength. Mice with targeted disruption of the Wnt16 gene exhibited a 24% reduction in tibia cross-sectional area at 12 weeks of age compared with that of littermate wild-type (WT) mice. Histomorphometric studies revealed that the periosteal bone formation rate and mineral apposition rate were reduced (P < .05) by 55% and 32%, respectively, in Wnt16 knockout (KO) vs WT mice at 12 weeks of age. In contrast, the periosteal tartrate resistant acid phosphatase-labeled surface was increased by 20% in the KO mice. Because mechanical strain is an important physiological regulator of periosteal bone formation (BF), we determined whether mechanical loading-induced periosteal BF is compromised in Wnt16 KO mice. Application of 4800-μe strain to the right tibia using a 4-point bending loading method for 2 weeks (2-Hz frequency, 36 cycles per day, 6 days/wk) produced a significant increase in cross-sectional area (11% above that of the unloaded left tibia, P < .05, n = 6) in the WT but not in the KO mice (-0.2% change). Histomorphometric analyses revealed increases in the periosteal bone formation rate and mineral apposition rate in the loaded bones of WT but not KO mice. Wnt16 KO mice showed significant (20%-70%) reductions in the expression levels of markers of canonical (β-catenin and Axin2) but not noncanonical (Nfatc1 and Tnnt2) WNT signaling in the periosteum at 5 weeks of age. Our findings suggest that WNT16 acting via canonical WNT signaling regulates mechanical strain-induced periosteal BF and bone size.

Conditional disruption of miR17-92 cluster in collagen type I-producing osteoblasts results in reduced periosteal bone formation and bone anabolic response to exercise

In this study, we evaluated the role of the microRNA (miR)17-92 cluster in osteoblast lineage cells using a Cre-loxP approach in which Cre expression is driven by the entire regulatory region of the type I collagen α2 gene. Conditional knockout (cKO) mice showed a 13-34% reduction in total body bone mineral content and area with little or no change in bone mineral density (BMD) by DXA at 2, 4, and 8 wk in both sexes. Micro-CT analyses of the femur revealed an 8% reduction in length and 25-27% reduction in total volume at the diaphyseal and metaphyseal sites. Neither cortical nor trabecular volumetric BMD was different in the cKO mice. Bone strength (maximum load) was reduced by 10% with no change in bone toughness. Quantitative histomorphometric analyses revealed a 28% reduction in the periosteal bone formation rate and in the mineral apposition rate but with no change in the resorbing surface. Expression levels of periostin, Elk3, Runx2 genes that are targeted by miRs from the cluster were decreased by 25-30% in the bones of cKO mice. To determine the contribution of the miR17-92 cluster to the mechanical strain effect on periosteal bone formation, we subjected cKO and control mice to 2 wk of mechanical loading by four-point bending. We found that the periosteal bone response to mechanical strain was significantly reduced in the cKO mice. We conclude that the miR17-92 cluster expressed in type I collagen-producing cells is a key regulator of periosteal bone formation in mice.

Combined treatment with GH and IGF-I: additive effect on cortical bone mass but not on linear bone growth in female rats

The growth-promoting effect of combined therapy with GH and IGF-I in normal rats is not known. We therefore investigated the efficacy of treatment with recombinant human (rh)GH and/or rhIGF-I on longitudinal bone growth and bone mass in intact, prepubertal, female Sprague-Dawley rats. rhGH was injected twice daily sc (5 mg/kg·d) and rhIGF-I continuously infused sc (2.2 or 4.4 mg/kg·d) for 28 days. Longitudinal bone growth was monitored by weekly x-rays of tibiae and nose-anus length measurements, and tibial growth plate histomorphology was analyzed. Bone mass was evaluated by peripheral quantitative computed tomography. In addition, serum levels of IGF-I, rat GH, acid labile subunit, IGF binding protein-3, 150-kDa ternary complex formation, and markers of bone formation and degradation were measured. Monotherapy with rhGH was more effective than rhIGF-I (4.4 mg/kg·d) to increase tibia and nose-anus length, whereas combined therapy did not further increase tibia, or nose-anus, lengths or growth plate height. In contrast, combined rhGH and rhIGF-I (4.4 mg/kg·d) therapy had an additive stimulatory effect on cortical bone mass vs rhGH alone. Combined treatment with rhGH and rhIGF-I resulted in markedly higher serum IGF-I concentrations vs rhGH alone but did not compromise the endogenous secretion of GH. We conclude that rhIGF-I treatment augments cortical bone mass but does not further improve bone growth in rhGH-treated young, intact, female rats.

IGFBP-2 directly stimulates osteoblast differentiation

Insulin-like growth factor binding protein 2 (IGFBP-2) is important for acquisition of normal bone mass in mice; however, the mechanism by which IGFBP-2 functions is not defined. These studies investigated the role of IGFBP-2 in stimulating osteoblast differentiation. MC-3T3 preosteoblasts expressed IGFBP-2, and IGFBP-2 knockdown resulted in a substantial delay in osteoblast differentiation, reduced osteocalcin expression and Alizarin red staining. These findings were replicated in primary calvarial osteoblasts obtained from IGFBP-2(-/-) mice, and addition of IGFBP-2 rescued the differentiation program. In contrast, overexpression of IGFBP-2 accelerated the time course of differentiation as well as increasing the total number of differentiating cells. By day 6, IGFBP-2-overexpressing cells expressed twice as much osteocalcin as control cultures and this difference persisted. To determine the mechanism by which IGFBP-2 functions, the interaction between IGFBP-2 and receptor tyrosine phosphatase β (RPTPβ) was examined. Disruption of this interaction inhibited the ability of IGFBP-2 to stimulate AKT activation and osteoblast differentiation. Knockdown of RPTPβ enhanced osteoblast differentiation, whereas overexpression of RPTPβ was inhibitory. Adding back IGFBP-2 to RPTPβ-overexpressing cells was able to rescue cell differentiation via enhancement of AKT activation. To determine the region of IGFBP-2 that mediated this effect, an IGFBP-2 mutant that contained substitutions of key amino acids in the heparin-binding domain-1 (HBD-1) was prepared. This mutant had a major reduction in its ability to stimulate differentiation of calvarial osteoblasts from IGFBP-2(-/-) mice. Addition of a synthetic peptide that contained the HBD-1 sequence to calvarial osteoblasts from IGFBP-2(-/-) mice rescued differentiation and osteocalcin expression. In summary, the results clearly demonstrate that IGFBP-2 stimulates osteoblast differentiation and that this effect is mediated through its heparin-binding domain-1 interacting with RPTPβ. The results suggest that stimulation of differentiation is an important mechanism by which IGFBP-2 regulates the acquisition of normal bone mass in mice.

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

Sclerostin, encoded by the Sost gene, is an important negative regulator of bone formation that has been proposed to have a key role in regulating the response to mechanical loading. To investigate the effect of long-term Sclerostin deficiency on mechanotransduction in bone, we performed experiments on unloaded or loaded tibiae of 10 week old female Sost-/- and wild type mice. Unloading was induced via 0.5U botulinum toxin (BTX) injections into the right quadriceps and calf muscles, causing muscle paralysis and limb disuse. On a separate group of mice, increased loading was performed on the left tibiae through unilateral cyclic axial compression of equivalent strains (+1200 µe) at 1200 cycles/day, 5 days/week. Another cohort of mice receiving equivalent loads (-9.0 N) also were assessed. Contralateral tibiae served as normal load controls. Loaded/unloaded and normal load tibiae were assessed at day 14 for bone volume (BV) and formation changes. Loss of BV was seen in the unloaded tibiae of wild type mice, but BV was not different between normal load and unloaded Sost-/- tibiae. An increase in BV was seen in the loaded tibiae of wild type and Sost-/- mice over their normal load controls. The increased BV was associated with significantly increased mid-shaft periosteal mineralizing surface/bone surface (MS/BS), mineral apposition rate (MAR), and bone formation rate/bone surface (BFR/BS), and endosteal MAR and BFR/BS. Notably, loading induced a greater increase in periosteal MAR and BFR/BS in Sost-/- mice than in wild type controls. Thus, long-term Sclerostin deficiency inhibits the bone loss normally induced with decreased mechanical load, but it can augment the increase in bone formation with increased load.

In vivo evidence of IGF-I-estrogen crosstalk in mediating the cortical bone response to mechanical strain

Although insulin-like growth factor-I (IGF-I) and estrogen signaling pathways have been shown to be involved in mediating the bone anabolic response to mechanical loading, it is not known whether these two signaling pathways crosstalk with each other in producing a skeletal response to mechanical loading. To test this, at 5 weeks of age, partial ovariectomy (pOVX) or a sham operation was performed on heterozygous IGF-I conditional knockout (H IGF-I KO) and control mice generated using a Cre-loxP approach. At 10 weeks of age, a 10 N axial load was applied on the right tibia of these mice for a period of 2 weeks and the left tibia was used as an internal non-non-loaded control. At the cortical site, partial estrogen loss reduced total volumetric bone mineral density (BMD) by 5% in control pOVX mice (P=0.05, one-way ANOVA), but not in the H IGF-I KO pOVX mice. At the trabecular site, bone volume/total volume (BV/TV) was reduced by 5%–6% in both control pOVX (P<0.05) and H IGF-I KO pOVX (P=0.05) mice. Two weeks of mechanical loading caused a 7%–8% and an 11%–13% (P<0.05 vs. non-loaded bones) increase in cortical BMD and cortical thickness (Ct.Th), respectively, in the control sham, control pOVX and H IGF-I KO sham groups. By contrast, the magnitude of cortical BMD (4%, P=0.13) and Ct.Th (6%, P<0.05) responses were reduced by 50% in the H IGF-I KO pOVX mice compared to the other three groups. The interaction between genotype and estrogen deficiency on the mechanical loading-induced cortical bone response was significant (P<0.05) by two-way ANOVA. Two weeks of axial loading caused similar increases in trabecular BV/TV (13%–17%) and thickness (17%–23%) in all four groups of mice. In conclusion, partial loss of both estrogen and IGF-I significantly reduced cortical but not the trabecular bone response to mechanical loading, providing in vivo evidence of the above crosstalk in mediating the bone response to loading.

Role of Osteocyte-derived Insulin-Like Growth Factor I in Developmental Growth, Modeling, Remodeling, and Regeneration of the Bone

The osteocyte has long been considered to be the primary mechanosensory cell in the bone. Recent evidence has emerged that the osteocyte is also a key regulator of various bone and mineral metabolism and that its regulatory effects are in part mediated through locally produced osteocyte-derived factors, such as sclerostin, receptor activator of nuclear factor-kappa B ligand (RANKL), and fibroblast growth factor (FGF)-23. Osteocytes secrete large amounts of insulin-like growth factor (IGF)-I in bone. Although IGF-I produced locally by other bone cells, such as osteoblasts and chondrocytes, has been shown to play important regulatory roles in bone turnover and developmental bone growth, the functional role of osteocyte-derived IGF-I in the bone and mineral metabolism has not been investigated and remains unclear. However, results of recent studies in osteocyte Igf1 conditional knockout transgenic mice have suggested potential regulatory roles of osteocyte-derived IGF-I in various aspects of bone and mineral metabolism. In this review, evidence supporting a regulatory role for osteocyte-derived IGF-I in the osteogenic response to mechanical loading, the developmental bone growth, the bone response to dietary calcium depletion and repletion, and in fracture repair is discussed. A potential coordinated regulatory relationship between the effect of osteocyte-derived IGF-I on bone size and the internal organ size is also proposed.

ENU mutation mapped to a distal region of chromosome 11 is a major determinant of bone size

Using a phenotype driven n-ethyl-nitrosourea (ENU) screen in growth hormone-deficient mice, we have identified a mutant (named 14104) that exhibited a smaller bone size. Phenotype measurements by microcomputed tomography revealed that mutant mice exhibited a 43 and 34% reduction in tissue area and bone area, respectively at the femur middiaphysis. Dynamic histomorphometry revealed a 30 and 15% lower bone formation rate at the periosteal and endosteal surface, respectively. Breaking strength of the femur was reduced by 30% in the mutant mice. To determine if the 14104 locus is involved in a mechanical loading signaling pathway, the skeletal anabolic response to tibia axial loading was evaluated. The increase in trabecular response in the loaded region was severely compromised by the 14014 mutation. To identify the location of mutation, we performed linkage analysis using 62 polymorphic markers in the B6-DBA/2J F2 mice. The genome-wide linkage analysis identified the location of the mutation to a 72 to 83 cM region on chromosome 11 with peak logarithm of the odds scores of 15 for periosteal circumference at marker D11mit338. Sequence analysis revealed no mutation in the coding region of 11 potential candidate genes. Based on these data and published data on the skeletal phenotype of genes in this region, we concluded that the 109-119 Mb region of chromosome 11 harbors a bone size gene that regulates periosteal bone formation. The mutant strain developed in this study provides an important tool to identify a novel mechanosensitive gene that determines bone size during postnatal development.

External mechanical microstimuli modulate the osseointegration of titanium implants in rat tibiae

Purpose. To assess the effect of external mechanical microstimuli of controlled magnitude on the microarchitecture of the peri-implant bone beds in rat tibiae. Materials and Methods. Tibiae of forty rats were fitted with two transcutaneous titanium cylinders. After healing, the implants were loaded to 1 to 3 N, five days/week for four weeks. These force levels translated into intraosseous strains of 700 ± 200 με, 1400 ± 400 με, and 2100 ± 600 με. After sacrifice, the implants' pullout strength was assessed. Second, the bone's microarchitecture was analyzed by microcomputed tomography (μCT) in three discrete regions of interest (ROIs). Third, the effect of loading on bone material properties was determined by nanoindentation. Results. The trabecular BV/TV significantly increased in an ROI of 0.98 mm away from the test implant in the 1 N versus the 3 N group with an opposite trend for cortical thickness. Pull-out strength significantly increased in the 2 N relatively to the nonstimulated group. Higher values of E-modulus and hardness were observed in the trabecular bone of the 2 N group. Conclusion. The in vivo mechanical loading of implants induces load-dependent modifications in bone microarchitecture and bone material properties in rat tibiae. In pull-out strength measurements, implant osseointegration was maximized at 2 N (1400 ± 400 με).

Osteocyte-derived insulin-like growth factor I is essential for determining bone mechanosensitivity

This study sought to determine whether deficient Igf1 expression in osteocytes would affect loading-induced osteogenic response. Tibias of osteocyte Igf1 conditional knockout (KO) mice (generated by cross-breeding Igf1 floxed mice with Dmp1-Cre transgenic mice) and wild-type (WT) littermates were subjected to four-point bending for 2 wk. Microcomputed tomography confirmed that the size of tibias of conditional mutants was smaller. Loading with an equivalent loading strain increased periosteal woven bone and endosteal lamellar bone formation in WT mice but not in conditional KO mice. Consistent with the lack of an osteogenic response, the loading failed to upregulate expression of early mechanoresponsive genes (Igf1, Cox-2, c-fos) or osteogenic genes (Cbfa-1, and osteocalcin) in conditional KO bones. The lack of osteogenic response was not due to reduced osteocyte density or insufficient loading strain. Deficient osteocyte Igf1 expression reduced the loading-induced upregulation of expression of canonical Wnt signaling genes (Wnt10b, Lrp5, Dkk1, sFrp2). The loading also reduced (by 40%) Sost expression in WT mice, but the loading not only did not reduce but upregulated (~1.5-fold) Sost expression in conditional KO mice. Conditional disruption of Igf1 in osteocytes also abolished the loading-induced increase in the bone β-catenin protein level. These findings suggest an impaired response in the loading-induced upregulation of the Wnt signaling in conditional KO mice. In summary, conditional disruption of Igf1 in osteocytes abolished the loading-induced activation of the Wnt signaling and the corresponding osteogenic response. In conclusion, osteocyte-derived IGF-I plays a key determining role in bone mechanosensitivity.

Transgenic overexpression of ephrin b1 in bone cells promotes bone formation and an anabolic response to mechanical loading in mice

To test if ephrin B1 overexpression enhances bone mass, we generated transgenic mice overexpressing ephrin B1 under the control of a 3.6 kb rat collagen 1A1 promoter (Col3.6-Tg^efnb1). Col3.6-Tg^efnb1 mice express 6-, 12- and 14-fold greater levels of full-length ephrin B1 protein in bone marrow stromal cells, calvarial osteoblasts, and osteoclasts, respectively. The long bones of both genders of Col3.6-Tg^efnb1 mice have increased trabecular bone volume, trabecular number, and trabecular thickness and decreased trabecular separation. Enhanced bone formation and decreased bone resorption contributed to this increase in trabecular bone mass in Col3.6-Tg^efnb1 mice. Consistent with these findings, our in vitro studies showed that overexpression of ephrin B1 increased osteoblast differentiation and mineralization, osterix and collagen 1A1 expression in bone marrow stromal cells. Interaction of ephrin B1 with soluble clustered EphB2-Fc decreased osteoclast precursor differentiation into multinucleated cells. Furthermore, we demonstrated that the mechanical loading-induced increase in EphB2 expression and newly formed bone were significantly greater in the Col3.6-Tg^efnb1 mice than in WT littermate controls. Our findings that overexpression of ephrin B1 in bone cells enhances bone mass and promotes a skeletal anabolic response to mechanical loading suggest that manipulation of ephrin B1 actions in bone may provide a means to sensitize the skeleton to mechanical strain to stimulate new bone formation.

Anabolic effects of IGF-1 signaling on the skeleton

This review focuses on the anabolic effects of IGF-1 signaling on the skeleton, emphasizing the requirement for IGF-1 signaling in normal bone formation and remodeling. We first discuss the genomic context, splicing variants, and species conservation of the IGF-1 locus. The modulation of IGF-1 action by growth hormone (GH) is then reviewed while also discussing the current model which takes into account the GH-independent actions of IGF-1. Next, the skeletal phenotypes of IGF-1-deficient animals are described in both embryonic and postnatal stages of development, which include severe dwarfism and an undermineralized skeleton. We then highlight two mechanisms by which IGF-1 exerts its anabolic action on the skeleton. Firstly, the role of IGF-1 signaling in the modulation of anabolic effects of parathyroid hormone (PTH) on bone will be discussed, presenting in vitro and in vivo studies that establish this concept and the proposed underlying molecular mechanisms involving Indian hedgehog (Ihh) and the ephrins. Secondly, the crosstalk of IGF-1 signaling with mechanosensing pathways will be discussed, beginning with the observation that animals subjected to skeletal unloading by hindlimb elevation are unable to mitigate cessation of bone growth despite infusion with IGF-1 and the failure of IGF-1 to activate its receptor in bone marrow stromal cell cultures from unloaded bone. Disrupted crosstalk between IGF-1 signaling and the integrin mechanotransduction pathways is discussed as one of the potential mechanisms for this IGF-1 resistance. Next, emerging paradigms on bone-muscle crosstalk are examined, focusing on the potential role of IGF-1 signaling in modulating such interactions. Finally, we present a future outlook on IGF research.

Disruption of the insulin-like growth factor-1 gene in osteocytes impairs developmental bone growth in mice

This study evaluated the role of osteocyte-derived insulin-like growth factor 1 (IGF-1) in developmental bone growth by assessing the bone phenotype of osteocyte Igf1 conditional knockout (KO) mice, generated by crossing the Dmp1-driven Cre-expressing transgenic mice with Igf1 floxed mice containing loxP sites that flank exon 4 of the Igf1 gene. The periosteal diameter of femurs of homozygous conditional KO mutants was 8-12% smaller than wild-type (WT) littermates. The conditional mutants had 14-20%, 10-21%, and 15-31% reduction in total, trabecular, and cortical bone mineral contents, respectively. However, there were no differences in the total, trabecular, or cortical bone mineral densities, or in trabecular bone volume, thickness, number, and separation at secondary spongiosa between the mutants and WT littermates. The conditional KO mutants showed reduction in dynamic bone formation parameters at both periosteal and endosteal surfaces at the mid-diaphysis and in trabecular bone formation rate and resorption parameters at secondary spongiosa. The lower plasma levels of PINP and CTx in conditional KO mice support a regulatory role of osteocyte-derived IGF-1 in the bone turnover. The femur length of conditional KO mutants was 4-7% shorter due to significant reduction in the length of growth plate and hypertropic zone. The effect on periosteal expansion appeared to be bigger than that on longitudinal bone growth. The conditional KO mice had 14% thinner calvaria than WT littermates, suggesting that deficient osteocyte IGF-1 production also impairs developmental growth of intramembraneous bone. Conditional disruption of Igf1 in osteocytes did not alter plasma levels of IGF-1, calcium, or phosphorus. In summary, this study shows for the first time that osteocyte-derived IGF-1 plays an essential role in regulating bone turnover during developmental bone growth.

Systemic administration of an antagomir designed to inhibit miR-92, a regulator of angiogenesis, failed to modulate skeletal anabolic response to mechanical loading

The goal of this study is to evaluate if promotion of angiogenesis by systemic treatment with an antagomir against miR-92a, a well established inhibitor of angiogenesis, will maximize the benefits of exercise on bone. Ten week old female C57BL6/J mice were subjected to two weeks of external load by four point bending. During the first week of mechanical loading (ML), mice were injected (2.7 mg/kg of bodyweight) with antagomir against miR-92 or control antagomir (3 alternate days via retro-orbital). No difference in tissues weights (heart, kidney, liver) were found in mice treated with miR-92 vs. control antagomir suggesting no side effects. Two weeks of ML increased tibia TV, BV/TV and density by 6-15 %, as expected, in the control antagomir treated mice. Similar increases in the above parameters (7-16 %) were also seen in mice treated miR-92 antagomir. Administration of miR-92 antagomir was effective in reducing levels of mir-92 in heart, liver and skeletal muscle and in contrast, expression levels of two other microRNA's miR-93 and miR-20a remain constant, thus suggesting specificity of the antagomir used. Surprisingly, we failed to detect significant changes in the expression levels of vascular genes (VEGF, CD31 and Tie2) in heart, liver or skeletal muscle. Based on these findings, we conclude that systemic administration of antagomir against miR-92 while reduced expression levels of miR-92 in the tissues; it did not significantly alter either angiogenic or osteogenic response, thus suggesting possible redundancy in miR-92 regulation of angiogenesis.

Signaling pathways affecting skeletal health

Skeletal health is dependent on the balance between bone resorption and formation during bone remodeling. Multiple signaling pathways play essential roles in the maintenance of skeletal integrity by positively or negatively regulating bone cells. During the last years, significant advances have been made in our understanding of the essential signaling pathways that regulate bone cell commitment, differentiation and survival. New signaling anabolic pathways triggered by parathyroid hormone, local growth factors, Wnt signaling, and calcium sensing receptor have been identified. Novel signals induced by interactions between bone cells-matrix (integrins), osteoblasts/osteocytes (cadherins, connexins), and osteoblasts/osteoclast (ephrins, Wnt-RhoA, semaphorins) have been discovered. Recent studies revealed the key pathways (MAPK, PI3K/Akt) that critically control bone cells and skeletal mass. This review summarizes the most recent knowledge on the major signaling pathways that control bone cells, and their potential impact on the development of therapeutic strategies to improve human bone health.

Role of insulin-like growth factor-1 in the regulation of skeletal growth

The importance of the insulin-like growth factor (IGF)-I axis in the regulation of bone size and bone mineral density, two important determinants of bone strength, has been well established from clinical studies involving patients with growth hormone deficiency and IGF-I gene disruption. Data from transgenic animal studies involving disruption and overexpression of components of the IGF-I axis also provide support for a key role for IGF-I in bone metabolism. IGF-I actions in bone are subject to regulation by systemic hormones, local growth factors, as well as mechanical stress. In this review we describe findings from various genetic mouse models that pertain to the role of endocrine and local sources of IGF-I in the regulation of skeletal growth.