EP1(-/-) mice have enhanced osteoblast differentiation and accelerated fracture repair

G. AR, Al Kawas S, Samsudin AR, A. C. SA. Impact of Bone Morphogenetic Protein 7 and Prostaglandin receptors on osteoblast healing and organization of collagen

PURPOSE

This study seeks to investigate the impact of co-administering either a Prostaglandin EP2 receptor agonist or an EP1 receptor antagonist alone with a low dose BMP7 on in vitro healing process, collagen content and maturation of human osteoblasts.

METHODOLOGY

Human osteoblast cells were used in this study. These cells were cultured and subjected to different concentrations of Prostaglandin EP2 receptor agonist, EP1 receptor antagonist, BMP7, Control (Ct) (Vehicle alone), and various combinations treatments. Cell viability at 24, 48 and 72 hours (h) was evaluated using the XTT assay. A wound healing assay was conducted to observe the migration ability of human osteoblast cells. Additionally, Sirius red staining and Fourier-Transform Infrared Spectroscopy Imaging (FT-IR) was employed to analyze various parameters, including total protein concentration, collagen production, mature collagen concentration, and mineral content.

RESULTS

The combination of low dose BMP7 and Prostaglandin EP2 receptor agonist resulted to the lowest cell viability when compared to both the Ct and individual treatments. In contrast, the Prostaglandin EP1 receptor antagonist alone showed the highest cellular viability at 72 h. In the wound healing assay, the combined treatment of low dose BMP7 with the Prostaglandin EP2 receptor agonist and EP1 receptor antagonist showed a decrease in human osteoblast healing after 24 h. Analysis of FT-IR data indicated a reduction in total protein content, collagen maturity, collagen concentration and mineral content in combination treatment compared to the single or Ct treatments.

CONCLUSION

The combination of a Prostaglandin EP2 receptor agonist or an EP1 receptor antagonist when combined with low dose BMP7 significantly hinders both human osteoblast healing and collagen maturity/concentration in comparison to low dose BMP7 treatment alone.

MAGP2 promotes osteogenic differentiation during fracture healing through its crosstalk with the β-catenin pathway

Osteogenic differentiation is important for fracture healing. Microfibrial-associated glycoprotein 2 (MAGP2) is found to function as a proangiogenic regulator in bone formation; however, its role in osteogenic differentiation during bone repair is not clear. Here, a mouse model of critical-sized femur fracture was constructed, and the adenovirus expressing MAGP2 was delivered into the fracture site. Mice with MAGP2 overexpression exhibited increased bone mineral density and bone volume fraction (BV/TV) at Day 14 postfracture. Within 7 days postfracture, overexpression of MAGP2 increased collagen I and II expression at the fracture callus, with increasing chondrogenesis. MAGP2 inhibited collagen II level but elevated collagen I by 14 days following fracture, accompanied by increased endochondral bone formation. In mouse osteoblast precursor MC3T3-E1 cells, MAGP2 treatment elevated the expression of osteoblastic factors (osterix, BGLAP and collagen I) and enhanced ALP activity and mineralization through activating β-catenin signaling after osteogenic induction. Besides, MAGP2 could interact with lipoprotein receptor-related protein 5 (LRP5) and upregulated its expression. Promotion of osteogenic differentiation and β-catenin activation mediated by MAGP2 was partially reversed by LRP5 knockdown. Interestingly, β-catenin/transcription factor 4 (TCF4) increased MAGP2 expression probably by binding to MAGP2 promoter. These findings suggest that MAGP2 may interact with β-catenin/TCF4 to enhance β-catenin/TCF4's function and activate LRP5-activated β-catenin signaling pathway, thus promoting osteogenic differentiation for fracture repair. mRNA sequencing identified the potential targets of MAGP2, providing novel insights into MAGP2 function and the directions for future research.

Identification of key immune genes of osteoporosis based on bioinformatics and machine learning

Introduction

Immunity is involved in a variety of bone metabolic processes, especially osteoporosis. The aim of this study is to explore new bone immune-related markers by bioinformatics method and evaluate their ability to predict osteoporosis.

Methods

The mRNA expression profiles were obtained from GSE7158 in Gene expression Omnibus (GEO), and immune-related genes were obtained from ImmPort database (https://www.immport.org/shared/). immune genes related to bone mineral density(BMD) were screened out for differential analysis. protein-protein interaction (PPIs) networks were used to analyze the interrelationships between different immune-related genes (DIRGs). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of DIRGs function were performed. A least absolute shrinkage and selection operation (LASSO) regression model and multiple Support Vector Machine-Recursive Feature Elimination (mSVM-RFE) model were constructed to identify the candidate genes for osteoporosis prediction The receiver operator characteristic (ROC) curves were used to validate the performances of predictive models and candidate genes in GEO database (GSE7158,GSE13850).Through the RT - qPCR verify the key genes differentially expressed in peripheral blood mononuclear cells Finally, we constructed a nomogram model for predicting osteoporosis based on five immune-related genes. CIBERSORT algorithm was used to calculate the relative proportion of 22 immune cells.

Results

A total of 1158 DEGs and 66 DIRGs were identified between high-BMD and low-BMD women. These DIRGs were mainly enriched in cytokine-mediated signaling pathway, positive regulation of response to external stimulus and the cellular components of genes are mostly localized to external side of plasma membrane. And the KEGG enrichment analysis were mainly involved in Cytokine-cytokine receptor interaction, PI3K-Akt signaling pathway, Neuroactive ligand-receptor interaction,Natural killer cell mediated cytotoxicity. Then five key genes (CCR5, IAPP, IFNA4, IGHV3-73 and PTGER1) were identified and used as features to construct a predictive prognostic model for osteoporosis using the GSE7158 dataset.

Conclusion

Immunity plays an important role in the development of osteoporosis.CCR5, IAPP, IFNA4, IGHV3-73 and PTGER1were play an important role in the occurrences and diagnosis of OP.

The effects of prostaglandin E2 on gene expression of IDG-SW3-derived osteocytes in 2D and 3D culture

Prostaglandin E2 (PGE2) is a key signaling molecule produced by osteocytes in response to mechanical loading, but its effect on osteocytes is less understood. This work examined the effect of PGE2 on IDG-SW3-derived osteocytes in standard 2D culture (collagen-coated tissue culture polystyrene) and in a 3D degradable poly(ethylene glycol) hydrogel. IDG-SW3 cells were differentiated for 35 days into osteocytes in 2D and 3D cultures. 3D culture led to a more mature osteocyte phenotype with 100-fold higher Sost expression. IDG-SW3-derived osteocytes were treated with PGE2 and assessed for expression of genes involved in PGE2, anabolic, and catabolic signaling. In 2D, PGE2 had a rapid (1 h) and sustained (24 h) effect on many PGE2 signaling genes, a rapid stimulatory effect on Il6, and a sustained inhibitory effect on Tnfrsf11b and Bglap. Comparing culture environment without PGE2, osteocytes had higher expression of all four EP receptors and Sost but lower expression of Tnfrsf11b, Bglap, and Gja1 in 3D. Osteocytes were more responsive to PGE2 in 3D. With increasing PGE2, 3D led to increased Gja1 and decreased Sost expressions and a higher Tnfrsf11b/Tnfsf11 ratio, indicating an anabolic response. Further analysis in 3D revealed that EP4, the receptor implicated in PGE2 signaling in bone, was not responsible for the PGE2-induced gene expression changes in osteocytes. In summary, osteocytes are highly responsive to PGE2 when cultured in an in vitro 3D hydrogel model suggesting that autocrine and paracrine PGE2 signaling in osteocytes may play a role in bone homeostasis.

Irisin promotes fracture healing by improving osteogenesis and angiogenesis

Background

Osteogenesis and angiogenesis are important for bone fracture healing. Irisin is a muscle-derived monokine that is associated with bone formation.

Methods

To demonstrate the effect of irisin on bone fracture healing, closed mid-diaphyseal femur fractures were produced in 8-week-old C57BL/6 mice. Irisin was administrated intraperitoneally every other day after surgery, fracture healing was assessed by using X-rays. Bone morphometry of the fracture callus were assessed by using micro-computed tomography. Femurs of mice from each group were assessed by the three-point bending testing. Effect of irisin on osteogenic differentiation in mesenchymal stem cells in vitro was evaluated by quantitative real-time polymerase chain reaction (qRT-PCR), alkaline phosphatase staining and alizarin red staining. Angiogenesis of human umbilical vein endothelial cells (HUVECs) were evaluated by qRT-PCR, migration tests, and tube formation assays.

Results

Increased callus formation, mineralization and tougher fracture healing were observed in the irisin-treated group than in the control group, indicating the better fracture callus healing due to Irisin treatment. The vessel surface and vessel volume fraction of the callus also increased in the irisin-treated group. The expression of BMP2, CD31, and VEGF in callus were enhanced in the irisin-treated group. In mouse bone mesenchymal stem cells, irisin promoted ALP expression and mineralization, and increased the expression of osteogenic genes, including OSX, Runx2, OPG, ALP, OCN and BMP2. Irisin also promoted HUVEC migration and tube formation. Expression of angiogenic genes, including ANGPT1, ANGPT2, VEGFb, CD31, FGF2, and PDGFRB in HUVECs were increased by irisin.

Conclusion

All the results indicate irisin can promote fracture healing through osteogenesis and angiogenesis. These findings help in the understanding of muscle–bone interactions during fracture healing.

The Translational Potential of this Article

Irisin was one of the most important monokine secreted by skeletal muscle. Studies have found that irisin have anabolic effect one bone remodeling through affecting osteocyte and osteoblast. Based on our study, irisin could promote bone fracture healing by increasing bone mass and vascularization, which provide a potential usage of irisin to promote fracture healing and improve clinical outcomes.

Prostaglandin E2 receptors and their role in gastrointestinal motility - Potential therapeutic targets

Prostaglandin E2 (PGE2) is found throughout the gastrointestinal tract in a diverse variety of functions and roles. The recent discovery of four PGE2 receptor subtypes in intestinal muscle layers as well as in the enteric plexus has led to much interest in the study of their roles in gut motility. Gut dysmotility has been implicated in functional disease processes including irritable bowel syndrome (IBS) and slow transit constipation, and lubiprostone, a PGE2 derivative, has recently been licensed to treat both conditions. The diversity of actions of PGE2 in the intestinal tract is attributed to its differing effects on its downstream receptor types, as well as their varied distribution in the gut, in both health and disease. This review aims to identify the role and distribution of PGE2 receptors in the intestinal tract, and aims to elucidate their distinct role in gut motor function, with a specific focus on functional intestinal pathologies.

SCUBE3 loss-of-function causes a recognizable recessive developmental disorder due to defective bone morphogenetic protein signaling

Signal peptide-CUB-EGF domain-containing protein 3 (SCUBE3) is a member of a small family of multifunctional cell surface-anchored glycoproteins functioning as co-receptors for a variety of growth factors. Here we report that bi-allelic inactivating variants in SCUBE3 have pleiotropic consequences on development and cause a previously unrecognized syndromic disorder. Eighteen affected individuals from nine unrelated families showed a consistent phenotype characterized by reduced growth, skeletal features, distinctive craniofacial appearance, and dental anomalies. In vitro functional validation studies demonstrated a variable impact of disease-causing variants on transcript processing, protein secretion and function, and their dysregulating effect on bone morphogenetic protein (BMP) signaling. We show that SCUBE3 acts as a BMP2/BMP4 co-receptor, recruits the BMP receptor complexes into raft microdomains, and positively modulates signaling possibly by augmenting the specific interactions between BMPs and BMP type I receptors. Scube3-/- mice showed craniofacial and dental defects, reduced body size, and defective endochondral bone growth due to impaired BMP-mediated chondrogenesis and osteogenesis, recapitulating the human disorder. Our findings identify a human disease caused by defective function of a member of the SCUBE family, and link SCUBE3 to processes controlling growth, morphogenesis, and bone and teeth development through modulation of BMP signaling.

Chronotherapy of Non-Steroidal Anti-Inflammatory Drugs May Enhance Postoperative Recovery

Postoperative pain relief is crucial for full recovery. With the ongoing opioid epidemic and the insufficient effect of acetaminophen on severe pain; non-steroidal anti-inflammatory drugs (NSAIDs) are heavily used to alleviate this pain. However, NSAIDs are known to inhibit postoperative healing of connective tissues by inhibiting prostaglandin signaling. Pain intensity, inflammatory mediators associated with wound healing and the pharmacological action of NSAIDs vary throughout the day due to the circadian rhythm regulated by the clock genes. According to this rhythm, most of wound healing mediators and connective tissue formation occurs during the resting phase, while pain, inflammation and tissue resorption occur during the active period of the day. Here we show, in a murine tibia fracture surgical model, that NSAIDs are most effective in managing postoperative pain, healing and recovery when drug administration is limited to the active phase of the circadian rhythm. Limiting NSAID treatment to the active phase of the circadian rhythm resulted in overexpression of circadian clock genes, such as Period 2 (Per2) at the healing callus, and increased serum levels of anti-inflammatory cytokines interleukin-13 (IL-13), interleukin-4 (IL-4) and vascular endothelial growth factor. By contrast, NSAID administration during the resting phase resulted in severe bone healing impairment.

Prostaglandins and Bone

Prostaglandins (PGs) are highly bioactive fatty acids. PGs, especially prostaglandin E₂ (PGE₂), are abundantly produced by cells of both the bone-forming (osteoblast) lineage and the bone-resorbing (osteoclast) lineage. The inducible cyclooxygenase, COX-2, is largely responsible for most PGE₂ production in bone, and once released, PGE₂ is rapidly degraded in vivo. COX-2 is induced by multiple agonists - hormones, growth factors, and proinflammatory factors - and the resulting PGE₂ may mediate, amplify, or, as we have recently shown for parathyroid hormone (PTH), inhibit responses to these agonists. In vitro, PGE₂ can directly stimulate osteoblast differentiation and, indirectly via stimulation of RANKL in osteoblastic cells, stimulate the differentiation of osteoclasts. The net balance of these two effects of PGE₂ in vivo on bone formation and bone resorption has been hard to predict and, as expected for such a widespread local factor, hard to study. Some of the complexity of PGE₂ actions on bone can be explained by the fact that there are four receptors for PGE₂ (EP1-4). Some of the major actions of PGE₂ in vitro occur via EP2 and EP4, both of which can stimulate cAMP signaling, but there are other distinct signaling pathways, important in other tissues, which have not yet been fully elucidated in bone cells. Giving PGE₂ or agonists of EP2 and EP4 to accelerate bone repair has been examined with positive results. Further studies to clarify the pathways of PGE₂ action in bone may allow us to identify new and more effective ways to deliver the therapeutic benefits of PGE₂ in skeletal disorders.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Cyclooxygenase 2 augments osteoblastic but suppresses chondrocytic differentiation of CD90+ skeletal stem cells in fracture sites

Cyclooxygenase 2 (COX-2) is essential for normal tissue repair. Although COX-2 is known to enhance the differentiation of mesenchymal stem cells (MSCs), how COX-2 regulates MSC differentiation into different tissue-specific progenitors to promote tissue repair remains unknown. Because it has been shown that COX-2 is critical for normal bone repair and local COX-2 overexpression in fracture sites accelerates fracture repair, this study aimed to determine the MSC subsets that are targeted by COX-2. We showed that CD90⁺ mouse skeletal stem cells (mSSCs; i.e., CD45^-Tie2^-AlphaV⁺ MSCs) were selectively recruited by macrophage/monocyte chemoattractant protein 1 into fracture sites following local COX-2 overexpression. In addition, local COX-2 overexpression augmented osteoblast differentiation and suppressed chondrocyte differentiation in CD90⁺ mSSCs, which depended on canonical WNT signaling. CD90 depletion data demonstrated that local COX-2 overexpression targeted CD90⁺ mSSCs to accelerate fracture repair. In conclusion, CD90⁺ mSSCs are promising targets for the acceleration of bone repair.

Inhibition of the Prostaglandin EP-1 Receptor in Periosteum Progenitor Cells Enhances Osteoblast Differentiation and Fracture Repair

Fracture healing is a complex and integrated process that involves mesenchymal progenitor cell (MPC) recruitment, proliferation and differentiation that eventually results in bone regeneration. Prostaglandin E2 (PGE2) is an important regulator of bone metabolism and has an anabolic effect on fracture healing. Prior work from our laboratory showed EP1^-/- mice have enhanced fracture healing, stronger cortical bones, higher trabecular bone volume and increased in vivo bone formation. We also showed that bone marrow MSCs from EP1^-/- mice exhibit increased osteoblastic differentiation in vitro. In this study we investigate the changes in the periosteal derived MPCs (PDMPCs), which are crucial for fracture repair, upon EP1 deletion. EP1^-/- PDMPCs exhibit increased numbers of total (CFU-F) and osteoblastic colonies (CFU-O) as well as enhanced osteoblastic and chondrogenic differentiation. Moreover, we tested the possible therapeutic application of a specific EP1 receptor antagonist to accelerate fracture repair. Our findings showed that EP1 antagonist administration to wild type mice in the early stages of repair similarly resulted in enhanced CFU-F, CFU-O, and osteoblast differentiation in PDMPCs and resulted in enhanced fracture callus formation at 10 days post fracture and increased bone volume and improved biomechanical healing of femur fractures at 21 days post fracture.

Early Fracture Healing is Delayed in the Col1a2+/G610C Osteogenesis Imperfecta Murine Model

Osteogenesis imperfecta (OI) is a rare heritable skeletal dysplasia mainly caused by type I collagen abnormalities and characterized by bone fragility and susceptibility to fracture. Over 85% of the patients carry dominant mutations in the genes encoding for the collagen type I α1 and α2 chains. Failure of bone union and/or presence of hyperplastic callus formation after fracture were described in OI patients. Here we used the Col1a2^+/G610C mouse, carrying in heterozygosis the α2(I)-G610C substitution, to investigate the healing process of an OI bone. Tibiae of 2-month-old Col1a2^+/G610C and wild-type littermates were fractured and the healing process was followed at 2, 3, and 5 weeks after injury from fibrous cartilaginous tissue formation to its bone replacement by radiography, micro-computed tomography (µCT), histological and biochemical approaches. In presence of similar fracture types, in Col1a2^+/G610C mice an impairment in the early phase of bone repair was detected compared to wild-type littermates. Smaller callus area, callus bone surface, and bone volume associated to higher percentage of cartilage and lower percentage of bone were evident in Col1a2^+/G610C at 2 weeks post fracture (wpf) and no change by 3 wpf. Furthermore, the biochemical analysis of collagen extracted from callus 2 wpf revealed in mutants an increased amount of type II collagen, typical of cartilage, with respect to type I, characteristic of bone. This is the first report of a delay in OI bone fracture repair at the modeling phase.

Nonsteroidal Anti-Inflammatory Drugs and Bone-Healing: A Systematic Review of Research Quality

BACKGROUND

Nonsteroidal anti-inflammatory drugs (NSAIDs) are often avoided by orthopaedic surgeons because of their possible influence on bone-healing. This belief stems from multiple studies, in particular animal studies, that show delayed bone-healing or nonunions associated with NSAID exposure. The purpose of this review was to critically analyze the quality of published literature that evaluates the impact of NSAIDs on clinical bone-healing.

METHODS

A MEDLINE and Embase search was conducted to identify all articles relating to bone and fracture-healing and the utilization of NSAIDs. All human studies, including review articles, were identified for further analysis. Non-English-language manuscripts and in vitro and animal studies were excluded. A total of twelve clinical articles and twenty-four literature reviews were selected for analysis. The quality of the clinical studies was assessed with a modified Coleman Methodology Score with emphasis on the NSAID utilization. Review articles were analyzed with regard to variability in the cited literature and final conclusions.

RESULTS

The mean modified Coleman Methodology Score (and standard deviation) was significantly lower (p = 0.032) in clinical studies that demonstrated a negative effect of NSAIDs on bone-healing (40.0 ± 14.3 points) compared with those that concluded that NSAIDs were safe (58.8 ± 10.3 points). Review articles also demonstrated substantial variability in the number of cited clinical studies and overall conclusions. There were only two meta-analyses and twenty-two narrative reviews. The mean number (and standard deviation) of clinical studies cited was significantly greater (p = 0.008) for reviews that concluded that NSAIDs were safe (8.0 ± 4.8) compared with those that recommended avoiding them (2.1 ± 2.1). Unanimously, all reviews admitted to the need for prospective randomized controlled trials to help clarify the effects of NSAIDs on bone-healing.

CONCLUSIONS

This systematic literature review highlights the great variability in the interpretation of the literature addressing the impact of NSAIDs on bone-healing. Unfortunately, there is no consensus regarding the safety of NSAIDs following orthopaedic procedures, and future studies should aim for appropriate methodological designs to help to clarify existing discrepancies to improve the quality of care for orthopaedic patients.

CLINICAL RELEVANCE

This systematic review highlights the limitations in the current understanding of the effects of NSAIDs on bone healing. Thus, withholding these medications does not have any proven scientific benefit to patients and may even cause harm by increasing narcotic requirements in cases in which they could be beneficial for pain management. This review should encourage further basic-science and clinical studies to clarify the risks and benefits of anti-inflammatory medications in the postoperative period, with the aim of improving patient outcomes.

Osteoimmunology in Bone Fracture Healing

PURPOSE OF REVIEW

In the process of bone fracture healing, inflammation is thought to be an essential process that precedes bone formation and remodeling. We review recent studies on bone fracture healing from an osteoimmunological point of view.

RECENT FINDINGS

Based on previous observations that many types of immune cells infiltrate into the bone injury site and release a variety of molecules, recent studies have addressed the roles of specific immune cell subsets. Macrophages and interleukin (IL)-17-producing γδ T cells enhance bone healing, whereas CD8⁺ T cells impair bone repair. Additionally, IL-10-producing B cells may contribute to bone healing by suppressing excessive and/or prolonged inflammation. Although the involvement of other cells and molecules has been suggested, the precise underlying mechanisms remain elusive. Accumulating evidence has begun to reveal the deeper picture of bone fracture healing. Further studies are required for the development of novel therapeutic strategies for bone fracture.

PGE2 Receptor Subtype 1 (EP1) Regulates Mesenchymal Stromal Cell Osteogenic Differentiation by Modulating Cellular Energy Metabolism

Mesenchymal stromal cells (MSCs) are multipotent progenitors capable of differentiation into osteoblasts and can potentially serve as a source for cell-based therapies for bone repair. Many factors have been shown to regulate MSC differentiation into the osteogenic lineage such as the Cyclooxygenase-2 (COX2)/Prostaglandin E2 (PGE2) signaling pathway that is critical for bone repair. PGE2 binds four different receptors EP1-4. While most studies focus on the role PGE2 receptors EP2 and EP4 in MSC differentiation, our study focuses on the less studied, receptor subtype 1 (EP1) in MSC function. Recent work from our laboratory showed that EP1^-/- mice have enhanced fracture healing, stronger cortical bones, higher trabecular bone volume and increased in vivo bone formation, suggesting that EP1 is a negative regulator of bone formation. In this study, the regulation of MSC osteogenic differentiation by EP1 receptor was investigated using EP1 genetic deletion in EP1^-/- mice. The data suggest that EP1 receptor functions to maintain MSCs in an undifferentiated state. Loss of the EP1 receptor changes MSC characteristics and permits stem cells to undergo more rapid osteogenic differentiation. Notably, our studies suggest that EP1 receptor regulates MSC differentiation by modulating MSC bioenergetics, preventing the shift to mitochondrial oxidative phosphorylation by maintaining high Hif1α activity. Loss of EP1 results in inactivation of Hif1α, increased oxygen consumption rate and thus increased osteoblast differentiation. J. Cell. Biochem. 118: 4383-4393, 2017. © 2017 Wiley Periodicals, Inc.

Understanding the local actions of lipids in bone physiology

The adult skeleton is a metabolically active organ system that undergoes continuous remodeling to remove old and/or stressed bone (resorption) and replace it with new bone (formation) in order to maintain a constant bone mass and preserve bone strength from micro-damage accumulation. In that remodeling process, cellular balances--adipocytogenesis/osteoblastogenesis and osteoblastogenesis/osteoclastogenesis--are critical and tightly controlled by many factors, including lipids as discussed in the present review. Interest in the bone lipid area has increased as a result of in vivo evidences indicating a reciprocal relationship between bone mass and marrow adiposity. Lipids in bones are usually assumed to be present only in the bone marrow. However, the mineralized bone tissue itself also contains small amounts of lipids which might play an important role in bone physiology. Fatty acids, cholesterol, phospholipids and several endogenous metabolites (i.e., prostaglandins, oxysterols) have been purported to act on bone cell survival and functions, the bone mineralization process, and critical signaling pathways. Thus, they can be regarded as regulatory molecules important in bone health. Recently, several specific lipids derived from membrane phospholipids (i.e., sphingosine-1-phosphate, lysophosphatidic acid and different fatty acid amides) have emerged as important mediators in bone physiology and the number of such molecules will probably increase in the near future. The present paper reviews the current knowledge about: (1°) bone lipid composition in both bone marrow and mineralized tissue compartments, and (2°) local actions of lipids on bone physiology in relation to their metabolism. Understanding the roles of lipids in bone is essential to knowing how an imbalance in their signaling pathways might contribute to bone pathologies, such as osteoporosis.

Loss of the PGE2 receptor EP1 enhances bone acquisition, which protects against age and ovariectomy-induced impairments in bone strength

PGE2 exerts anabolic and catabolic effects on bone through the discrete actions of four prostanoid receptors (EP1-4). We have previously demonstrated that loss EP1 accelerates fracture repair by enhancing bone formation. In the present study we defined the role of EP1 in bone maintenance and homeostasis during aging and in response to ovariectomy. The femur and L4 vertebrae of wild type (WT) and EP1(-/-) mice were examined at 2-months, 6-months, and 1-year of age, and in WT and EP1(-/-) mice following ovariectomy (OVX) or sham surgery. Bone volume fraction, trabecular architecture and mechanical properties were maintained during aging in EP1(-/-) mice to a greater degree than age-matched WT mice. Moreover, significant increases in bone formation rate (BFR) (+60%) and mineral apposition rate (MAR) (+50%) were observed in EP1(-/-), relative to WT, while no change in osteoclast number and osteoclast surface were observed. Following OVX, loss of EP1 was protective against bone loss in both femur and L4 vertebrae, with increased bone volume/total volume (BV/TV) (+32% in femur) and max load at failure (+10% in femur) relative to WT OVX, likely resulting from the increased bone formation rate that was observed in these mice. Taken together these studies identify inhibition of EP1 as a potential therapeutic approach to suppress bone loss in aged or post-menopausal patients.

The convergence of fracture repair and stem cells: interplay of genes, aging, environmental factors and disease

The complexity of fracture repair makes it an ideal process for studying the interplay between the molecular, cellular, tissue, and organ level events involved in tissue regeneration. Additionally, as fracture repair recapitulates many of the processes that occur during embryonic development, investigations of fracture repair provide insights regarding skeletal embryogenesis. Specifically, inflammation, signaling, gene expression, cellular proliferation and differentiation, osteogenesis, chondrogenesis, angiogenesis, and remodeling represent the complex array of interdependent biological events that occur during fracture repair. Here we review studies of bone regeneration in genetically modified mouse models, during aging, following environmental exposure, and in the setting of disease that provide insights regarding the role of multipotent cells and their regulation during fracture repair. Complementary animal models and ongoing scientific discoveries define an increasing number of molecular and cellular targets to reduce the morbidity and complications associated with fracture repair. Last, some new and exciting areas of stem cell research such as the contribution of mitochondria function, limb regeneration signaling, and microRNA (miRNA) posttranscriptional regulation are all likely to further contribute to our understanding of fracture repair as an active branch of regenerative medicine.

Low-dose X-ray irradiation promotes osteoblast proliferation, differentiation and fracture healing

Great controversy exists regarding the biologic responses of osteoblasts to X-ray irradiation, and the mechanisms are poorly understood. In this study, the biological effects of low-dose radiation on stimulating osteoblast proliferation, differentiation and fracture healing were identified using in vitro cell culture and in vivo animal studies. First, low-dose (0.5 Gy) X-ray irradiation induced the cell viability and proliferation of MC3T3-E1 cells. However, high-dose (5 Gy) X-ray irradiation inhibited the viability and proliferation of osteoblasts. In addition, dynamic variations in osteoblast differentiation markers, including type I collagen, alkaline phosphatase, Runx2, Osterix and osteocalcin, were observed after both low-dose and high-dose irradiation by Western blot analysis. Second, fracture healing was evaluated via histology and gene expression after single-dose X-ray irradiation, and low-dose X-ray irradiation accelerates fracture healing of closed femoral fractures in rats. In low-dose X-ray irradiated fractures, an increase in proliferating cell nuclear antigen (PCNA)-positive cells, cartilage formation and fracture calluses was observed. In addition, we observed more rapid completion of endochondral and intramembranous ossification, which was accompanied by altered expression of genes involved in bone remodeling and fracture callus mineralization. Although the expression level of several osteoblast differentiation genes was increased in the fracture calluses of high-dose irradiated rats, the callus formation and fracture union were delayed compared with the control and low-dose irradiated fractures. These results reveal beneficial effects of low-dose irradiation, including the stimulation of osteoblast proliferation, differentiation and fracture healing, and highlight its potential translational application in novel therapies against bone-related diseases.

Microarray Analysis of Gene Expression Reveals that Cyclo-oxygenase-2 Gene Therapy Up-regulates Hematopoiesis and Down-regulates Inflammation During Endochondral Bone Fracture Healing

BACKGROUND

Cyclo-oxygenase-2 (Cox-2) is an inflammatory mediator that is necessary for the tissue repair, including bone fracture healing. Although the application of Cox-2 gene therapy to a murine closed femoral fracture has accelerated bony union, but the beneficial effect was not observed until the endochondral stage of bone repair that is well after the inflammatory stage normally subsides.

METHODS

To identify the molecular pathways through which Cox-2 regulates fracture healing, we examined gene expression profile in fracture tissues in response to Cox-2 gene therapy during the endochondral bone repair phase. Cox-2 gene therapy was applied to the closed murine femur fracture model. Microarray analysis was performed at 10 days post-fracture to examine global gene expression profile in the fracture tissues during the endochondral bone repair phase. The entire repertoire of significantly expressed genes was examined by gene set enrichment analysis, and the most up-regulated individual genes were evaluated further.

RESULTS

The genes that normally promote inflammation were under-represented in the microarray analysis, and the expression of several inflammatory chemokines was significantly down-regulated. There was an up-regulation of two key transcription factor genes that regulate hematopoiesis and erythropoiesis. More surprisingly, there was no significant up-regulation in the genes that are normally involved in angiogenesis or bone formation. However, the expression of two tissue remodeling genes was up-regulated.

CONCLUSIONS

The down-regulation of the inflammatory genes in response to Cox-2 gene therapy was unexpected, given the pro-inflammatory role of prostaglandins. Cox-2 gene therapy could promote bony union through hematopoietic precursor proliferation during endochondral bone repair and thereby enhances subsequently fracture callus remodeling that leads to bony union of the fracture gap.

Cellular and molecular bases of skeletal regeneration: what can we learn from genetic mouse models?

Although bone repairs through a very efficient regenerative process in 90% of the patients, many factors can cause delayed or impaired healing. To date, there are no reliable biological parameters to predict or diagnose bone repair defects. Orthopedic surgeons mostly base their diagnoses on radiographic analyses. With the recent progress in our understanding of the bone repair process, new methods may be envisioned. Animal models have allowed us to define the key steps of bone regeneration and the biological and mechanical factors that may influence bone healing in positive or negative ways. Most importantly, small animal models such as mice have provided powerful tools to apprehend the genetic bases of normal and impaired bone healing. The current review presents a state of the art of the genetically modified mouse models that have advanced our understanding of the cellular and molecular components of bone regeneration and repair. The review illustrates the use of these models to define the role of inflammation, skeletal cell lineages, signaling pathways, the extracellular matrix, osteoclasts and angiogenesis. These genetic mouse models promise to change the field of orthopedic surgery to help establish genetic predispositions for delayed repair, develop models of non-union that mimic the human conditions and elaborate new therapeutic approaches to enhance bone regeneration.

Fracture healing and lipid mediators

Lipid mediators regulate bone regeneration during fracture healing. Prostaglandins and leukotrienes are well-known lipid mediators that regulate inflammation and are synthesized from the Ω-6 fatty acid, arachidonic acid. Cyclooxygenase (COX-1 or COX-2) and 5-lipoxygenase (5-LO) catalyze the initial enzymatic steps in the synthesis of prostaglandins and leukotrienes, respectively. Inhibition or genetic ablation of COX-2 activity impairs fracture healing in animal models. Genetic ablation of COX-1 does not affect the fracture callus strength in mice, suggesting that COX-2 activity is primarily responsible for regulating fracture healing. Inhibition of cyclooxygenase activity with nonsteroidal anti-inflammatory drugs (NSAIDs) is performed clinically to reduce heterotopic ossification, although clinical evidence that NSAID treatment impairs fracture healing remains controversial. In contrast, inhibition or genetic ablation of 5-LO activity accelerates fracture healing in animal models. Even though prostaglandins and leukotrienes regulate inflammation, loss of COX-2 or 5-LO activity appears to primarily affect chondrogenesis during fracture healing. Prostaglandin or prostaglandin analog treatment, prostaglandin-specific synthase inhibition and prostaglandin or leukotriene receptor antagonism also affect callus chondrogenesis. Unlike the Ω-6-derived lipid mediators, lipid mediators derived from Ω-3 fatty acids, such as resolvin E1 (RvE1), have anti-inflammatory activity. In vivo, RvE1 can inhibit osteoclastogenesis and limit bone resorption. Although Ω-6 and Ω-3 lipid mediators have clear-cut effects on inflammation, the role of these lipid mediators in bone regeneration is more complex, with apparent effects on callus chondrogenesis and bone remodeling.

BMP-2 induces ATF4 phosphorylation in chondrocytes through a COX-2/PGE2 dependent signaling pathway

OBJECTIVE

Bone morphogenic protein (BMP)-2 is approved for fracture non-union and spine fusion. We aimed to further dissect its downstream signaling events in chondrocytes with the ultimate goal to develop novel therapeutics that can mimic BMP-2 effect but have less complications.

METHODS

BMP-2 effect on cyclooxygenase (COX)-2 expression was examined using Real time quantitative PCR (RT-PCR) and Western blot analysis. Genetic approach was used to identify the signaling pathway mediating the BMP-2 effect. Similarly, the pathway transducing the PGE2 effect on ATF4 was investigated. Immunoprecipitation (IP) was performed to assess the complex formation after PGE2 binding.

RESULTS

BMP-2 increased COX-2 expression in primary mouse costosternal chondrocytes (PMCSC). The results from the C9 Tet-off system demonstrated that endogenous BMP-2 also upregulated COX-2 expression. Genetic approaches using PMCSC from ALK2(fx/fx), ALK3(fx/fx), ALK6(-/-), and Smad1(fx/fx) mice established that BMP-2 regulated COX-2 through activation of ALK3-Smad1 signaling. PGE-2 EIA showed that BMP-2 increased PGE2 production in PMCSC. ATF4 is a transcription factor that regulates bone formation. While PGE2 did not have significant effect on ATF4 expression, it induced ATF4 phosphorylation. In addition to stimulating COX-2 expression, BMP-2 also induced phosphorylation of ATF4. Using COX-2 deficient chondrocytes, we demonstrated that the BMP-2 effect on ATF4 was COX-2-dependent. Tibial fracture samples from COX-2(-/-) mice showed reduced phospho-ATF4 immunoreactivity compared to wild type (WT) ones. PGE2 mediated ATF4 phosphorylation involved signaling primarily through the EP2 and EP4 receptors and PGE2 induced an EP4-ERK1/2-RSK2 complex formation.

CONCLUSIONS

BMP-2 regulates COX-2 expression through ALK3-Smad1 signaling, and PGE2 induces ATF4 phosphorylation via EP4-ERK1/2-RSK2 axis.

PGE2 and BMP-2 in bone and cartilage metabolism: 2 intertwining pathways

Osteoarthritis and lesions to cartilage tissue are diseases that frequently result in impaired joint function and patient disability. The treatment of osteoarthritis, along with local bone defects and systemic skeletal diseases, remains a significant clinical challenge for orthopaedic surgeons. Several bone morphogenetic proteins (BMPs) are known to have osteoinductive effects, whereof BMP-2 and BMP-7 are already approved for clinical applications. There is growing evidence that the metabolism of bone as well as the cartilage damage associated with the above disease processes are strongly inter-related with the interactions of the inflammation-related pathways (in particular prostaglandin E2 (PGE2)) and osteogenesis (in particular bone morphogenetic protein-2 (BMP-2)). There is strong evidence that the pathways of prostaglandins and bone morphogenetic proteins are intertwined, and they have recently come into focus in several experimental and clinical studies. This paper focuses on PGE2 and BMP-2 intertwining pathways in bone and cartilage metabolism, and summarizes the recent experimental and clinical data.

Do nonsteroidal anti-inflammatory drugs affect bone healing? A critical analysis

Nonsteroidal anti-inflammatory drugs (NSAIDs) play an essential part in our approach to control pain in the posttraumatic setting. Over the last decades, several studies suggested that NSAIDs interfere with bone healing while others contradict these findings. Although their analgesic potency is well proven, clinicians remain puzzled over the potential safety issues. We have systematically reviewed the available literature, analyzing and presenting the available in vitro animal and clinical studies on this field. Our comprehensive review reveals the great diversity of the presented data in all groups of studies. Animal and in vitro studies present so conflicting data that even studies with identical parameters have opposing results. Basic science research defining the exact mechanism with which NSAIDs could interfere with bone cells and also the conduction of well-randomized prospective clinical trials are warranted. In the absence of robust clinical or scientific evidence, clinicians should treat NSAIDs as a risk factor for bone healing impairment, and their administration should be avoided in high-risk patients.

Harnessing the parathyroid hormone, Wnt, and bone morphogenetic protein signaling cascades for successful bone tissue engineering

Tissue engineering holds great promise as a way of enhancing the normal regenerative potential of bone. By deconstructing the skeleton into its components and examining how each component influences the reparative response, it is clear that cells resident in bone, bioactive molecules produced by these cells and those brought into bone via the circulation and the unique extracellular matrix that makes up the bone itself are involved in a continuous and ever-changing set of reciprocal interactions during regeneration. Reviewed here is current information regarding the efficacy of 3 prominent signaling cascades that orchestrate bone formation, parathyroid hormone, Wnt and bone morphogenetic proteins, in enhancing bone repair. I suggest how we might successfully generate new bone in increasingly complex clinical situations by modulating the availability of these signals to cells already present within bone tissue.

Harnessing and modulating inflammation in strategies for bone regeneration

Inflammation is an immediate response that plays a critical role in healing after fracture or injury to bone. However, in certain clinical contexts, such as in inflammatory diseases or in response to the implantation of a biomedical device, the inflammatory response may become chronic and result in destructive catabolic effects on the bone tissue. Since our previous review 3 years ago, which identified inflammatory signals critical for bone regeneration and described the inhibitory effects of anti-inflammatory agents on bone healing, a multitude of studies have been published exploring various aspects of this emerging field. In this review, we distinguish between regenerative and damaging inflammatory processes in bone, update our discussion of the effects of anti-inflammatory agents on bone healing, summarize recent in vitro and in vivo studies demonstrating how inflammation can be modulated to stimulate bone regeneration, and identify key future directions in the field.