Effect of a selective agonist for prostaglandin E receptor subtype EP4 (ONO-4819) on the cortical bone response to mechanical loading

Connexin 43 hemichannels and prostaglandin E2 release in anabolic function of the skeletal tissue to mechanical stimulation

Bone adapts to changes in the physical environment by modulating remodeling through bone resorption and formation to maintain optimal bone mass. As the most abundant connexin subtype in bone tissue, connexin 43 (Cx43)-forming hemichannels are highly responsive to mechanical stimulation by permitting the exchange of small molecules (<1.2 kDa) between bone cells and the extracellular environment. Upon mechanical stimulation, Cx43 hemichannels facilitate the release of prostaglandins E2 (PGE2), a vital bone anabolic factor from osteocytes. Although most bone cells are involved in mechanosensing, osteocytes are the principal mechanosensitive cells, and PGE2 biosynthesis is greatly enhanced by mechanical stimulation. Mechanical stimulation-induced PGE2 released from osteocytic Cx43 hemichannels acts as autocrine effects that promote β-catenin nuclear accumulation, Cx43 expression, gap junction function, and protects osteocytes against glucocorticoid-induced osteoporosis in cultured osteocytes. In vivo, Cx43 hemichannels with PGE2 release promote bone formation and anabolism in response to mechanical loading. This review summarizes current in vitro and in vivo understanding of Cx43 hemichannels and extracellular PGE2 release, and their roles in bone function and mechanical responses. Cx43 hemichannels could be a significant potential new therapeutic target for treating bone loss and osteoporosis.

KMN-159, a novel EP4 receptor selective agonist, stimulates osteoblastic differentiation in cultured whole rat bone marrow

KMN-159 is the lead compound from a series of novel difluorolactam prostanoid EP₄ receptor agonists aimed at inducing local bone formation while avoiding the inherent side effects of systemic EP₄ activation. KMN-159 is a potent, selective small molecule possessing pharmacokinetic properties amenable to local administration. Unfractionated rat bone marrow cells (BMCs) were treated once at plating with escalating doses of KMN-159 (1 pM to 10 μM). The resulting elevated alkaline phosphatase (ALP) levels measured 9 days post-dose are consistent with increased osteoblastic differentiation and exposure to KMN-159 at low nanomolar concentrations for as little as 30 min was sufficient to induce complete osteoblast differentiation of the BMCs from both sexes and regardless of age. ALP induction was blocked by an EP₄ receptor antagonist but not by EP₁ or EP₂ receptor antagonists and was not induced by EP₂ or EP₃ receptor agonists. Addition of BMCs to plates coated with KMN-159 24 days earlier resulted in ALP activation, highlighting the chemical stability of the compound. The expression of phenotype markers such as ALP, type I collagen, and osteocalcin was significantly elevated throughout the osteoblastic differentiation timecourse initiated by KMN-159 stimulation. An increased number of tartrate-resistant acid phosphatase-positive cells was observed KMN-159 or PGE₂ treated BMCs but only in the presence of exogenous receptor activator of nuclear factor kappa-Β ligand (RANKL). No change in the number of adipocytes was observed. KMN-159 also increased bone healing in a rat calvarial defect model with a healing rate equivalent to recombinant human bone morphogenetic protein-2. Our studies show that KMN-159 is able to stimulate osteoblastic differentiation with a very short time of exposure, supporting its potential as a therapeutic candidate for augmenting bone mass.

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.

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

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

A sustained-release drug-delivery system of synthetic prostacyclin agonist, ONO-1301SR: a new reagent to enhance cardiac tissue salvage and/or regeneration in the damaged heart

Cardiac failure is a major cause of mortality and morbidity worldwide, since the standard treatment for cardiac failure in the clinical practice is chiefly to focus on removal of insults against the heart or minimisation of additional factors to exacerbate cardiac failure, but not on regeneration of the damaged cardiac tissue. A synthetic prostacyclin agonist, ONO-1301, has been developed as a long-acting drug for acute and chronic pathologies related to regional ischaemia, inflammation and/or interstitial fibrosis by pre-clinical studies. In addition, poly-lactic co-glycolic acid-polymerised form of ONO-1301, ONO-1301SR, was generated to achieve a further sustained release of this drug into the targeted region. This unique reagent has been shown to act on fibroblasts, vascular smooth muscle cells and endothelial cells in the tissue via the prostaglandin IP receptor to exert paracrinal release of multiple protective factors, such as hepatocyte growth factor, vascular endothelial growth factor or stromal cell-derived factor-1, into the adjacent damaged tissue, which is salvaged and/or regenerated as a result. Our laboratory developed a new surgical approach to treat acute and chronic cardiac failure using a variety of animal models, in which ONO-1301SR is directly placed over the cardiac surface to maximise the therapeutic effects and minimise the systemic complications. This review summarises basic and pre-clinical information of ONO-1301 and ONO-1301SR as a new reagent to enhance tissue salvage and/or regeneration, with a particular focus on the therapeutic effects on acute and chronic cardiac failure and underlying mechanisms, to explore a potential in launching the clinical study.

Effects of eldecalcitol on cortical bone response to mechanical loading in rats

Background

Mechanical loading of bones activates modeling and suppresses remodeling by promoting bone formation. Eldecalcitol is approved for the treatment of osteoporosis in Japan and is often used in patients undergoing exercise therapy. However, the effects of eldecalcitol on bone formation during mechanical loading are unknown. The aim of this study was to clarify the influence of eldecalcitol administration on bone response to mechanical loading using a four-point bending device.

Methods

Forty six-month-old female Wistar rats were randomized into four groups based on eldecalcitol dose (vehicle administration (VEH), low dose (ED-L), medium dose (ED-M), and high dose (ED-H)). Loads of 38 N were applied in vivo to the right tibia for 36 cycles at 2 Hz, by four-point bending, 3 days per week for 3 weeks. After calcein double-labeling, rats were sacrificed and tibial cross sections were prepared from the region with maximal bending at the central diaphysis. Histomorphometry was performed on the entire periosteal and endocortical surface of the tibiae, dividing the periosteum into lateral and medial surfaces.

Results

The effects of external loading on bone formation parameters were significant at all three surfaces. Bone formation parameters were highest in the ED-H group, and the effects of eldecalcitol on bone formation rate were significant at the endocortical surface. In addition, the interaction between loading and eldecalcitol dose significantly affected bone formation rate at the endocortical surface.

Conclusions

Eldecalcitol enhanced the cortical bone response to mechanical loading and a synergistic effect was observed in a rat model.

The prostanoid EP4 receptor and its signaling pathway

The EP4 prostanoid receptor is one of four receptor subtypes for prostaglandin E2. It belongs to the family of G protein-coupled receptors. It was originally identified, similar to the EP2 receptor as a G(s)α-coupled, adenylyl cyclase-stimulating receptor. EP4 signaling plays a variety of roles through cAMP effectors, i.e., protein kinase A and exchange protein activated by cAMP. However, emerging evidence from studies using pharmacological approaches and genetically modified mice suggests that EP4, unlike EP2, can also be coupled to G(i)α, phosphatidylinositol 3-kinase, β-arrestin, or β-catenin. These signaling pathways constitute unique roles for the EP4 receptor. EP4 is widely distributed in the body and thus plays various physiologic and pathophysiologic roles. In particular, EP4 signaling is closely related to carcinogenesis, cardiac hypertrophy, vasodilation, vascular remodeling, bone remodeling, gastrointestinal homeostasis, renal function, and female reproductive function. In addition to the classic anti-inflammatory action of EP4 on mononuclear cells and T cells, recent evidence has shown that EP4 signaling contributes to proinflammatory action as well. The aim of this review is to present current findings on the biologic functions of the EP4 receptor. In particular, we will discuss its diversity from the standpoint of EP4-mediated signaling.

Effects of minodronate on cortical bone response to mechanical loading in rats

The effects of BPs on bone formation during mechanical loading are still unknown. In this study, we evaluated the effect of minodronate on the cortical bone response to mechanical loading applied using a 4-point bending device. We used six-month old female Wistar rats and randomized into five groups (N=10/group): Vehicle administration (VEH), low dose minodronate administration (MIN-L, 0.01 mg/kg BW), middle dose minodronate administration (MIN-M, 0.1mg/kg BW), high-dose minodronate administration (MIN-H 1mg/kg BW), and very high-dose minodronate administration (MIN-VH, 10mg/kg BW). Minodronate or vehicle was administered orally using the feeding needle at a dosage 3 times/week for 3 weeks. Loads on the right tibia at 38 N for 36 cycles at 2 Hz were applied in vivo by 4-point bending on the same day for 3 weeks. After calcein double labeling the rats were sacrificed and tibial cross sections were prepared from the region with maximal bending at the central diaphysis. Histomorphometry was performed at the entire periosteal and endocortical surface of the tibiae, dividing the periosteum into lateral and medial surfaces. The formation surface was reduced significantly in MIN-H and MIN-VH groups at the medial surface, and in MIN-VH group at the endocortical surface of the loaded tibia (p<0.01 vs. VEH). The mineral appositional rate was reduced significantly in MIN-H and MIN-VH groups at the endocortical surface of the loaded tibia (p<0.01 vs. VEH). The bone formation rate was significantly reduced in MIN-H group at the medial surface, and in MIN-H and MIN-VH groups at the endocortical surface of the loaded tibia (p<0.01 vs. VEH). However, no significant differences were observed in any parameters between the VEH group and either the MIN-L or MIN-M groups for both the loaded and non-loaded tibiae. Based on previous preventive studies in OVX rats, the optimal dose of minodronate for the treatment of osteoporosis would be 0.03 mg/kg (0.21 mg/kg/week). Therefore, we used 0.1mg/kg of minodronate 3 times/week (0.30 mg/kg/week) that was close to 0.21 mg/kg/week. In conclusion, minodronate does not reduce the cortical bone response to mechanical loading at the optimal dose for the treatment of osteoporosis in rat model.

Differential load-regulated global gene expression in mouse trabecular osteocytes

Osteocytes are considered the skeletal mechanosensors. However, because osteocytes, particularly trabecular, are barely accessible to in vivo molecular analyses, very little is known on the signals transmitted by these cells to the extra-trabecular milieu. To investigate so called "osteocytic genes" involved in extracellular signaling, we have used a recently developed model whereby a single caudal mouse vertebra (C5) is subjected to controlled compression loading and further devised a method for the isolation of high quality RNA from trabecular osteocytes. RNA samples from loaded and sham-loaded individual vertebrae where then subjected to gene array analysis following the administration of a single or repetitive loading doses (thrice weekly for 4 weeks). Focusing on extracellular genes potentially involved in mediating osteocyte-derived signals to the trabecular surface, we identified sets of genes differentially regulated by either single or multiple loading bouts as well as genes affected by both loading protocols. A comparison with published studies on load-regulated genes in cortical osteocytes revealed that the majority of these genes are specifically activated/silenced in the trabecular bone. Many of these genes could be clustered according to processes directly relevant to the life cycle and activity of osteoblasts and osteoclasts and their progenitors. The present findings are consistent with an osteocytic role in the control of trabecular bone remodeling and mass and provide a comprehensive database of load-regulated genes in trabecular osteocytes that is potentially useful in further mouse genetic studies and identification of drug targets to combat osteoporosis.

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

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

Mechanical stress-activated integrin α5β1 induces opening of connexin 43 hemichannels

The connexin 43 (Cx43) hemichannel (HC) in the mechanosensory osteocytes is a major portal for the release of factors responsible for the anabolic effects of mechanical loading on bone formation and remodeling. However, little is known about how the Cx43 molecule responds to mechanical stimulation leading to the opening of the HC. Here, we demonstrate that integrin α5β1 interacts directly with Cx43 and that this interaction is required for mechanical stimulation-induced opening of the Cx43 HC. Direct mechanical perturbation via magnetic beads or conformational activation of integrin α5β1 leads to the opening of the Cx43 HC, and this role of the integrin is independent of its association with an extracellular fibronectin substrate. PI3K signaling is responsible for the shear stress-induced conformational activation of integrin α5β1 leading to the opening of the HC. These results identify an unconventional function of integrin that acts as a mechanical tether to induce opening of the HC and provide a mechanism connecting the effect of mechanical forces directly to anabolic function of the bone.

Mouse tail vertebrae adapt to cyclic mechanical loading by increasing bone formation rate and decreasing bone resorption rate as shown by time-lapsed in vivo imaging of dynamic bone morphometry

It is known that mechanical loading leads to an increase in bone mass through a positive shift in the balance between bone formation and bone resorption. How the remodeling sites change over time as an effect of loading remains, however, to be clarified. The purpose of this paper was to investigate how bone formation and resorption sites are modulated by mechanical loading over time by using a new imaging technique that extracts three dimensional formation and resorption parameters from time-lapsed in vivo micro-computed tomography images. To induce load adaptation, the sixth caudal vertebra of C57BL/6 mice was cyclically loaded through pins in the adjacent vertebrae at either 8 N or 0 N (control) three times a week for 5 min (3000 cycles) over a total of 4 weeks. The results showed that mechanical loading significantly increased trabecular bone volume fraction by 20% (p<0.001) and cortical area fraction by 6% (p<0.001). The bone formation rate was on average 23% greater (p<0.001) and the bone resorption rate was on average 25% smaller (p<0.001) for the 8 N group than for the 0 N group. The increase in bone formation rate for the 8 N group was mostly an effect of a significantly increased surface of bone formation sites (on average 16%, p<0.001), while the thickness of bone formation packages was less affected (on average 5% greater, p<0.05). At the same time the surface of bone resorption sites was significantly reduced (on average 15%, p<0.001), while the depth of resorption pits remained the same. For the 8 N group, the strength of the whole bone increased significantly by 24% (p<0.001) over the loading period, while the strain energy density in the trabecular bone decreased significantly by 24% (p<0.001). In conclusion, mouse tail vertebrae adapt to mechanical loading by increasing the surface of formation sites and decreasing the surface of resorption sites, leading to an overall increase in bone strength. This new imaging technique will provide opportunities to investigate in vivo bone remodeling in the context of disease and treatment options, with the added value that both bone formation and bone resorption parameters can be nondestructively calculated over time.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

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

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

Biomimetic bone mechanotransduction modeling in neonatal rat femur organ cultures: structural verification of proof of concept

The goal of this work was to develop and validate a whole bone organ culture model to be utilized in biomimetic mechanotransduction research. Femurs harvested from 2-day-old neonatal rat pups were maintained in culture for 1 week post-harvest and assessed for growth and viability. For stimulation studies, femurs were physiologically stimulated for 350 cycles 24 h post-harvest then maintained in culture for 1 week at which time structural tests were conducted. Comparing 1 and 8 days in culture, bones grew significantly in size over the 7-day culture period. In addition, histology supported adequate diffusion and organ viability at 2 weeks in culture. For stimulation studies, 350 cycles of physiologic loading 24 h post-harvest resulted in increased bone strength over the 7-day culture period. In this work, structural proof of concept was established for the use of whole bone organ cultures as mechanotransduction models. Specifically, this work established that these cultures grow and remain viable in culture, are adequately nourished via diffusion and are capable of responding to a brief bout of mechanical stimulation with an increase in strength.

Small molecules for bone diseases

IMPORTANCE OF THE FIELD

Bones play many roles in the body, providing structure, protecting organs, anchoring muscles and storing calcium. Over 100 million people worldwide suffer from bone diseases, mainly osteoporosis, cancer-related bone loss, osteoarthritis and inflammatory arthritis. Osteoporosis itself has no specific symptoms, and the main consequence is the increased risk of bone fractures. Therefore, the prevention of bone diseases is important to maintain the quality of life in the human society. However, treatment options are still insufficient.

AREAS COVERED IN THIS REVIEW

This review article gives a summary of the low molecular mass modulators of bone diseases targets disclosed in patent applications and articles, mainly during the last 5 years.

WHAT THE READER WILL GAIN

Readers will rapidly gain an overview of these modulators not only for historical targets, but also of emerging and re-visited targets. Readers will also be able to see the current research trend and the main players in this field.

TAKE HOME MESSAGE

Drug discovery for bone diseases has made progress in the last years. The research area has dynamically shifted from historical targets (bisphosphonate, parathyroid hormone and calcitonin) to newly confirmed targets or targets re-visited which were biologically validated in the past. Cathepsin K inhibitors should be very close to launching in the market.

Functional adaptation to mechanical loading in both cortical and cancellous bone is controlled locally and is confined to the loaded bones

In order to validate whether bones' functional adaptation to mechanical loading is a local phenomenon, we randomly assigned 21 female C57BL/6 mice at 19 weeks of age to one of three equal numbered groups. All groups were treated with isoflurane anesthesia three times a week for 2 weeks (approximately 7 min/day). During each anaesthetic period, the right tibiae/fibulae in the DYNAMIC+STATIC group were subjected to a peak dynamic load of 11.5 N (40 cycles with 10-s intervals between cycles) superimposed upon a static "pre-load" of 2.0 N. This total load of 13.5 N engendered peak longitudinal strains of approximately 1400 microstrain on the medial surface of the tibia at a middle/proximal site. The right tibiae/fibulae in the STATIC group received the static "pre-load" alone while the NOLOAD group received no artificial loading. After 2 weeks, the animals were sacrificed and both tibiae, fibulae, femora, ulnae and radii analyzed by three-dimensional high-resolution (5 mum) micro-computed tomography (microCT). In the DYNAMIC+STATIC group, the proximal trabecular percent bone volume and cortical bone volume at the proximal and middle levels of the right tibiae as well as the cortical bone volume at the middle level of the right fibulae were markedly greater than the left. In contrast, the left bones in the DYNAMIC+STATIC group showed no differences compared to the left or right bones in the NOLOAD or STATIC group. These microCT data were confirmed by two-dimensional examination of fluorochrome labels in bone sections which showed the predominantly woven nature of the new bone formed in the loaded bones. We conclude that the adaptive response in both cortical and trabecular regions of bones subjected to short periods of dynamic loading, even when this response is sufficiently vigorous to stimulate woven bone formation, is confined to the loaded bones and does not involve changes in other bones that are adjacent, contra-lateral or remote to them.

Effects of global or targeted deletion of the EP4 receptor on the response of osteoblasts to prostaglandin in vitro and on bone histomorphometry in aged mice

Because global deletion of the prostaglandin EP4 receptor results in neonatal lethality, we generated a mouse with targeted EP4 receptor deletion using Cre-LoxP methodology and a 2.3 kb collagen I a1 promoter driving Cre recombinase that is selective for osteoblastic cells. We compared wild type (WT), global heterozygote (G-HET), targeted heterozygote (T-HET) and knockout (KO) mice. KO mice had one targeted and one global deletion of the EP4 receptor. All mice were in a mixed background of C57BL/6 and CD-1. Although there were one third fewer G-HET or KO mice at weaning compared to WT and T-HET mice, G-HET and KO mice appeared healthy. In cultures of calvarial osteoblasts, prostaglandin E(2) (PGE(2)) increased alkaline phosphatase (ALP) activity in cells from WT mice, and this effect was significantly decreased in cells from either G-HET or T-HET mice and further decreased in cells from KO mice. A selective agonist for EP4 receptor increased ALP activity and osteocalcin mRNA levels in cells from WT but not KO mice. A selective COX-2 inhibitor, NS-398, decreased osteoblast differentiation in WT but not KO cells. At 15 to 18 months of age there were no differences in serum creatinine, calcium, PTH, body weight or bone mineral density among the different genotypes. Static and dynamic histomorphometry showed no consistent changes in bone volume or bone formation. We conclude that expression of the EP4 receptor in osteoblasts is critical for anabolic responses to PGE(2) in cell culture but may not be essential for maintenance of bone remodeling in vivo.

Joint loading-driven bone formation and signaling pathways predicted from genome-wide expression profiles

Joint loading is a recently developed loading modality that induces anabolic responses by lateral loads applied to a synovial joint such as an elbow and a knee. The present study extended this loading modality to an ankle and addressed a question: does ankle loading promote bone formation in the tibia? If so, what signaling pathways are involved in the anabolic responses? Using C57BL/6 female mice as a model system, lateral loads of 0.5 N were applied to the ankle at 5 Hz for 3 min/day for 3 consecutive days and load-driven bone formation was evaluated at three tibial cross-sections (the proximal, middle, and distal diaphysis). Furthermore, total RNA was isolated for 3 pairs of microarray experiments as well as quantitative real-time PCR analyses. The histomorphometric results revealed that in all cross-sections ankle loading elevated the cortical area and thickness as well as the calcein-labeled surface. Signaling pathway analysis from microarray-derived whole-genome mRNA expression profiles and quantitative real-time PCR predicted that molecules in phosphoinositide 3-kinase (PI3K), ECM-receptor interactions, TGFbeta signaling, and Wnt signaling were involved in the joint-loading driven responses. Since ankle loading stimulates bone formation throughout the tibia both in the endosteum and the periosteum, it may provide a non-pharmacological approach to effectively activate molecular signaling necessary for preventing bone loss.

Reduced COX-2 expression in aged mice is associated with impaired fracture healing

The cellular and molecular events responsible for reduced fracture healing with aging are unknown. Cyclooxygenase 2 (COX-2), the inducible regulator of prostaglandin E(2) (PGE(2)) synthesis, is critical for normal bone repair. A femoral fracture repair model was used in mice at either 7-9 or 52-56 wk of age, and healing was evaluated by imaging, histology, and gene expression studies. Aging was associated with a decreased rate of chondrogenesis, decreased bone formation, reduced callus vascularization, delayed remodeling, and altered expression of genes involved in repair and remodeling. COX-2 expression in young mice peaked at 5 days, coinciding with the transition of mesenchymal progenitors to cartilage and the onset of expression of early cartilage markers. In situ hybridization and immunohistochemistry showed that COX-2 is expressed primarily in early cartilage precursors that co-express col-2. COX-2 expression was reduced by 75% and 65% in fractures from aged mice compared with young mice on days 5 and 7, respectively. Local administration of an EP4 agonist to the fracture repair site in aged mice enhanced the rate of chondrogenesis and bone formation to levels observed in young mice, suggesting that the expression of COX-2 during the early inflammatory phase of repair regulates critical subsequent events including chondrogenesis, bone formation, and remodeling. The findings suggest that COX-2/EP4 agonists may compensate for deficient molecular signals that result in the reduced fracture healing associated with aging.

Influence of glucocorticoid on bone in 3-, 6-, and 12-month-old rats as determined by bone mass and histomorphometry

The influence of glucocorticoid (GC) on bone in rats at different ages was investigated in order to provide insight into human glucocorticoid induced osteoporosis (GCOP). Three-, 6-, and 12-month-old female Wistar rats were divided into four groups: Zero-time control (ZT), vehicle (Cont), prednisolone (PSL) 2 mg/kg (P-L), PSL 20 mg/kg (P-H). PSL was subcutaneously administered every day for 4 weeks. Bone mineral density (BMD) at the proximal metaphysis and diaphysis of the tibia was measured by peripheral quantitative computed tomography. Histomorphometry of the tibia was performed for 3- and 6-month-old rats. GC increased trabecular and cortical BMD at the metaphysis in all 3-month-old rats with time. Trabecular BMD at the metaphysis in the P-L and P-H groups was significantly higher than in the control group. Histomorphometric parameters for both bone formation and resorption were also increased by GC treatment. In the 6-month-old rats, the metaphyseal trabecular BMD did not significantly change in any group, but the diaphyseal trabecular BMD significantly increased in the control group with time. The trabecular BMD of the metaphysis and diaphysis was significantly lower in the P-L and P-H groups than in the control group at week 4. Histomorphometric parameters for bone formation and resorption were both reduced by GC treatment. The BMD remained unchanged in all 12-month-old rats. Six-month-old rats treated with 20 mg/kg GC are suitable models for GC-induced osteoporosis with dominant cancellous bone decrease and reduced bone turnover. The pathology induced by 20 mg/kg prednisolone in 6-month-old female rats seems to be most similar to glucocorticoid-induced osteoporosis in humans.

Joint loading modality: its application to bone formation and fracture healing

Sports-related injuries such as impact and stress fractures often require a rehabilitation programme to stimulate bone formation and accelerate fracture healing. This review introduces a recently developed joint loading modality and evaluates its potential applications to bone formation and fracture healing in post-injury rehabilitation. Bone is a dynamic tissue whose structure is constantly altered in response to its mechanical environments. Indeed, many loading modalities can influence the bone remodelling process. The joint loading modality is, however, able to enhance anabolic responses and accelerate wound healing without inducing significant in situ strain at the site of bone formation or fracture healing. This review highlights the unique features of this loading modality and discusses its potential underlying mechanisms as well as possible clinical applications.

Mechanisms of tooth eruption and orthodontic tooth movement

Teeth move through alveolar bone, whether through the normal process of tooth eruption or by strains generated by orthodontic appliances. Both eruption and orthodontics accomplish this feat through similar fundamental biological processes, osteoclastogenesis and osteogenesis, but there are differences that make their mechanisms unique. A better appreciation of the molecular and cellular events that regulate osteoclastogenesis and osteogenesis in eruption and orthodontics is not only central to our understanding of how these processes occur, but also is needed for ultimate development of the means to control them. Possible future studies in these areas are also discussed, with particular emphasis on translation of fundamental knowledge to improve dental treatments.

Bone formation and tooth movement are synergistically enhanced by administration of EP4 agonist

INTRODUCTION

Stimulation of the prostaglandin receptor EP4 induces bone anabolic responses to external loading. The aim of this study was to determine the possibility of synergistic enhancement of tooth movement along with bone formation in vivo through the stimulation of the prostaglandin receptor EP4.

METHODS

The sample consisted of 25 male rats, 6 to 7 weeks old. Experimental tooth movement was induced in the maxillary right molars with a split-mouth design. During the 7-day experimental period, some rats were injected periodically with a specific EP4 agonist (ONO-AE1-329), a drug that binds to the EP4 receptor to mimic the actions of ligand binding; the injection site was on the tension side in the interproximal region between the first and second molars. Other rats received vehicle injection (saline solution only). For control, EP4 agonist or vehicle was injected into some rats, but tooth movement was not initiated. The amounts of tooth movement and bone volume of the paradental region were evaluated by using soft x-ray and micro-computed tomography.

RESULTS

The injection of EP4 agonist alone did not induce notable changes in body weight, macro-structures, or bone volume compared with the vehicle-treated group or the untouched control side. EP4 agonist injection in conjunction with tooth movement significantly enhanced the amounts of tooth movement and regional bone volume in the tension side compared with the vehicle-treated group (P <.05).

CONCLUSIONS

Our preliminary study suggests the possibilities of synergistic enhancement of the rates of tooth movement and bone formation during tooth movement through the local injection of a specific EP4 receptor agonist. However, more study on the synergistic effect on tooth movement and bone formation in response to an EP4 agonist is needed to determine its mechanisms.

A comparative study of the bone-restorative efficacy of anabolic agents in aged ovariectomized rats

INTRODUCTION

The study was designed to compare the bone anabolic effects of basic fibroblast growth factor (bFGF), a selective agonist for prostaglandin E receptor subtype EP4, and parathyroid hormone (PTH) in aged ovariectomized (OVX) rats with severe cancellous osteopenia.

METHODS

Groups of aged OVX rats were maintained untreated for 1 year postovariectomy (15 months of age) to develop severe tibial cancellous osteopenia. These animals were then treated with bFGF or the EP4 agonist (EP4) for 3 weeks. Other groups of aged OVX rats were treated with EP4 or PTH alone for 11 weeks, or sequentially with bFGF or EP4 for 3 weeks followed by PTH for 8 weeks. Cancellous and cortical bone histomorphometry were performed in the right proximal tibial metaphysis and tibial diaphysis respectively.

RESULTS

Treatment with bFGF for 3 weeks markedly increased serum osteocalcin, osteoid volume, and osteoblast and osteoid surfaces to a greater extent than EP4. Basic FGF, but not EP4 or PTH, induced formation of osteoid islands within bone marrow. EP4 stimulated cancellous bone turnover, but failed to restore lost cancellous bone in the severely osteopenic proximal tibia after 11 weeks of treatment. In contrast, EP4, much like PTH, increased cortical bone mass in the tibial diaphysis by stimulating both periosteal and endocortical bone formation. Treatment of aged OVX rats with PTH alone tended to partially reverse the severe tibial cancellous osteopenia, whereas sequential treatment with bFGF and PTH increased tibial cancellous bone mass to near the level of vehicle-treated control rats. These findings indicate that bFGF had the strongest stimulatory effect on cancellous bone formation, and was the only anabolic agent to induce formation of osteoid islands within the bone marrow of the severely osteopenic proximal tibia. Therefore, bFGF may be more effective for the reversal of severe cancellous osteopenia. PTH and EP4 increased cortical bone mass to nearly the same extent, but cancellous bone mass was greater by two-fold in PTH-treated OVX rats than in EP4-treated OVX rats.

CONCLUSION

These findings in aged OVX rats suggest that PTH is more efficacious than EP4 for augmentation of cancellous bone in the severely osteopenic, estrogen-deplete skeleton.

Anabolic agents and the bone morphogenetic protein pathway

A major unmet need in the medical field today is the availability of suitable treatments for the ever-increasing incidence of osteoporosis and the treatment of bone deficit conditions. Although therapies exist which prevent bone loss, the options are extremely limited for patients once a substantial loss of skeletal bone mass has occurred. Patients who have reduced bone mass are predisposed to fractures and further morbidity. The FDA recently approved PTH (1-34) (Teriparatide) for the treatment of postmenopausal osteoporosis after both preclinical animal and clinical human studies indicated it induces bone formation. This is the only approved bone anabolic agent available but unfortunately it has limited use, it is relatively expensive and difficult to administer. Consequently, the discovery of low cost orally available bone anabolic agents is critical for the future treatment of bone loss conditions. The intricate process of bone formation is co-ordinated by the action of many different bone growth factors, some stored in bone matrix and others released into the bone microenvironment from surrounding cells. Although all these factors play important roles, the bone morphogenetic proteins (BMPs) clearly play a central role in both bone cartilage formation and repair. Recent research into the regulation of the BMP pathway has led to the discovery of a number of small molecular weight compounds as candidate bone anabolic agents. These agents may usher in a new wave of more innovative and versatile treatments for osteoporosis as well as orthopedic and dental indications.

Effects of selective prostaglandins E2 receptor agonists on cultured calvarial murine osteoblastic cells

We compared the direct effects of selective EP4 and EP2 receptor agonists (EP4A and EP2A) with prostaglandin E(2) (PGE(2)) on the differentiation of cultured murine calvarial osteoblastic cells. EP4A increased alkaline phosphatase activity and osteocalcin mRNA levels in these cultures similar to PGE(2). This effect was seen with both "direct plating" immediately after isolating the cells, or "indirect plating" in which the cells were grown to confluence and replated. EP2A had a smaller effect, significant only in "indirect plating" experiments. All three agents decreased the DNA and protein content in indirect plating experiments, but not in direct plating experiments. We conclude that the anabolic effect of PGE(2) in calvarial osteoblastic cell cultures is largely mediated by activation of the EP4 receptor, while activation of the EP2 receptor is less effective.

Mechanosensation and Transduction in Osteocytes

There may be no single mechanoreceptor in osteocytes, but instead a combination of events that has to be triggered for mechanosensation and transduction of signal to occur. Possibilities include shear stress along dendritic processes and/or the cell body, cell deformation in response to strain, and primary cilia. These events could occur independently or simultaneously to activate mechanotransduction. Signal initiators include calcium channel activation and ATP, nitric oxide, and prostaglandin release. Means of signal transfer include gap junctions and hemichannels, and the release of signaling molecules into the bone fluid. Questions remain regarding the magnitude of strain necessary to induce an osteocyte response, how the response propagates within the osteocyte network, and the timing involved in the initiation of bone resorption and/or formation on the bone surface. Mechanical loading in the form of shear stress is clearly involved not only in mechanosensation and transduction, but also in osteocyte viability. It remains to be determined if mechanical loading can also affect mineral homeostasis and mineralization, which are newly recognized functions of osteocytes.

Drug delivery strategies for cathepsin inhibitors in joint diseases

Cathepsins play important roles in the development of joint and bone diseases such as osteoporosis, rheumatoid arthritis (RA) and osteoarthritis (OA). Cathepsin inhibitors are presently in development and clinical testing for use as novel disease-modifying drugs for the improved treatment of osteoporosis. They may also be applicable for the treatment of joint diseases. However, some barriers still hamper their clinical applications in these indications. Based on pathophysiological features of RA and OA, the authors discuss six potential drug delivery strategies for the effective delivery of cathepsin inhibitors or other antiarthritic drugs to the arthritic joint tissue. Successful application of these strategies may significantly contribute to a more effective and safe treatment of RA and OA.

Prostaglandin E2 (PGE2) increases the number of rat bone marrow osteogenic stromal cells (BMSC) via binding the EP4 receptor, activating sphingosine kinase and inhibiting caspase activity

Prostaglandin E(2) (PGE(2)) is bone-anabolic, i.e. stimulates bone formation and increases bone mass. In this study, we explored possible intracellular mechanisms of its increase of osteogenic cells in rat bone marrow. Adherent rat bone marrow cells were counted after 12-48 h or cultured for 21 days and mineralized nodules were counted. Also, apoptosis of marrow cells was measured after in vivo PGE(2) injection. PGE(2) (100 nM) increased 2-3 fold the number of adherent BMSC, an effect which was mediated via binding the EP(4) receptor since it was mimicked by forskolin and 11-deoxy-prostaglandin E(1) (PGE(1)) and was blocked by DDA and L-161982 (EP(4) antagonist). PGE(2) stimulated sphingosine kinase (SPK) activity since its effects were blocked by DMS (SPK inhibitor) and mimicked by SPP (SPK product). PGE(2) reduced the activity of caspase-3 and -8 in BMSC and their inhibitors increased BMSC number and nodule formation. In vivo, PGE(2) prevented the increase in the apoptosis of bone marrow cells caused by indomethacin. We propose that PGE(2) exerts an anti-apoptotic effect on BMSC, thereby increasing their number and subsequent osteoblastic differentiation. Such an effect could explain how PGE(2) stimulates bone formation in vivo.

Early changes in prostaglandins precede bone formation in a rabbit model of heterotopic ossification

We have tested the hypothesis that the formation of heterotopic ossification (HO) in a rabbit model is correlated with a local increase in specific prostaglandins that may modulate mechanisms of ossification. Rabbits were sacrificed at 1 to 21 days following the daily forcible flexion of immobilized knees. The extraction and analysis of prostaglandins (PG) E2, F2alpha, D2, 6-keto-F1alpha, and thromboxane B2 in vastus intermedius muscles of manipulated legs revealed increases compared to control hindlimbs for all five prostaglandins, albeit of differing magnitude. The earliest increase was observed for PGF2alpha after 24 h (to 2.6-fold of control) with peak levels observed at day ten (185-fold of control). PGE2 was increased above control from 2 to 21 days following manipulation, with a peak level of 33-fold of control after 10 days. In a separate arm of the study, the role of PGE2 was investigated through the use of pharmacological antagonist of the PGE2 receptors and one of its second messengers, cAMP. Rabbits were preadministered the PGE2/PGD receptor antagonist AH 6809 or the cAMP antagonist Rp-cAMP prior to undergoing the regimen of limb immobilization and passive exercise. Both AH 6809 and Rp-cAMP were found to prevent the later development of radiographically documented heterotopic ossification in 15 out of 16 animals, thus identifying prostaglandins as being required for the development of ectopic bone. In this latter group, all but one pharmacologically treated animal showed an absence of HO at 3, 4, 5, or 6 weeks. These findings suggest an obligate cascade of prostaglandins for HO that offers the potential for novel prophylactic therapies, including those that target receptors for specific prostaglandins.