Ephrin B2 (EFNB2) potentially protects against intervertebral disc degeneration through inhibiting nucleus pulposus cell apoptosis

Nucleus pulposus (NP) cell apoptosis is a significant indication of accelerated intervertebral disc degeneration; however, the precise mechanism is unelucidated as of yet. Ephrin B2 (EFNB2), the only gene down-regulated in the three degraded intervertebral disc tissue microarray groups (GSE70362, GSE147383 and GSE56081), was screened for examination in this study. Subsequently, EFNB2 was verified to be down-regulated in degraded NP tissue samples. Interleukin-1 (IL-1β) treatment of NP cells to simulate the IDD environment indicated that IL-1β treatment decreased EFNB2 expression. In degenerative NP cells stimulated by IL-1β, EFNB2 knockdown significantly increased the rate of apoptosis as well as the apoptosis-related molecules cleaved-caspase-3 and the Bax to Bcl-2 ratio. EFNB2 was found to promote AKT, PI3K, and mTOR phosphorylation; the PI3K/AKT signaling role was investigated using the PI3K inhibitor LY294002. EFNB2 overexpression significantly increased PI3K/AKT pathway activity in IL-1β-stimulated NP cells than the normal control. Moreover, EFNB2 partially alleviated NP cell apoptosis induced by IL-1β, reduced the cleaved-cas3 level, and decreased the Bax/Bcl-2 ratio after the addition of the inhibitor LY294002. Additionally, EFNB2 overexpression inhibited the ERK1/2 phosphorylation; the effects of EFNB2 overexpression on ERK1/2 phosphorylation, degenerative NP cell viability, and cell apoptosis were partially reversed by ERK signaling activator Ceramide C6. EFNB2 comprehensively inhibited the apoptosis of NP cells by activating the PI3K/AKT signaling and inhibiting the ERK signaling, obviating the exacerbation of IDD. EFNB2 could be a potential target to protect against degenerative disc changes.

Imbalanced EphB4/EphrinB2 Signaling Modulates Bone Resorption in Periodontitis Induced by Porphyromonas gingivalis

Periodontitis, a chronic infectious disease in periodontal tissues, is characterized by an imbalance of alveolar bone resorption and remodeling, which eventually results in tooth loosening and even tooth loss. The etiology of periodontitis is polymicrobial synergy and dysbiosis, in which Porphyromonas gingivalis (P. gingivalis) is one of the primary pathogens responsible for periodontitis progression. The interplay of EphrinB2/EphB4 is crucial for osteoblast-osteoclast communication during bone remodeling and healing. This study investigates the mechanism of EphB4/EphrinB2 transduction modulating osteogenesis inhibition and bone resorption in periodontitis induced by P. gingivalis. An in vivo model of chronic periodontitis provoked by P. gingivalis was constructed, the inflammation and bone resorption were evaluated. The expression of EphB4 and EphrinB2 proteins in periodontal tissues was detected, which was also evaluated, respectively, in osteoblasts and osteoclasts infected with P. gingivalis in vitro. Then, a simulated coculture model of osteoblasts and osteoclasts was established to activate the forward and reverse pathways of EphB4/EphrinB2 with P. gingivalis infection. This study showed that P. gingivalis infection promoted alveolar bone resorption in rats and enhanced EphB4 and EphrinB2 expression in periodontal tissues. EphB4 and molecules associated with osteogenesis in osteoblasts infected with P. gingivalis were inhibited, while EphrinB2 and osteoclast differentiation-related markers in osteoclasts were activated. In conclusion, this study suggested that EphB4/EphrinB2 proteins were involved in alveolar bone remodeling in the process of periodontitis induced by P. gingivalis infection. Moreover, attenuated EphB4/EphrinB2 with P. gingivalis infection weakened osteoblast activity and enhanced osteoclast activity.

The Therapeutic Effects of EFNB2-Fc in a Cell Model of Kawasaki Disease

The EphrinB2/EphB4 signaling pathway involves the regulation of vascular morphogenesis and angiogenesis. However, little is known about EphrinB2/EphB4 in the pathogenesis of Kawasaki disease (KD) and coronary artery aneurysm formation. Hence, this study aimed to explore the role of EphrinB2/EphB4 and the potential therapeutic effect of EphrinB2-Fc in the coronary arterial endothelial injury of KD. The levels of EphB4 were compared between KD patients and healthy children. Human coronary artery endothelial cells (HCAECs) were stimulated with sera from acute KD patients to establish the KD cell model. The overexpression of EphB4 or treatment with EphrinB2-Fc was found to intervene in the cell model. The cell migration, angiogenesis, and proliferation ability were assessed, and the expression of inflammation-related factors was measured. Our study showed that EphB4 showed low expression in both KD patients and the cell model of KD. The EphB4 protein levels in the CECs of CAA+ KD patients were much lower than those in healthy children. EphrinB2-Fc treatment of KD sera-activated HCAECs suppressed cell proliferation, reduced the expression of inflammation-related factors (such as IL-6 and P-selectin), and elevated cell angiogenesis ability. The results reveal that EphrinB2-Fc has a protective function in endothelial cells and has promising clinical applications for protecting vascular endothelium in patients with KD.

Early zoledronate treatment inhibits subchondral bone microstructural changes in skeletally-mature, ACL-transected canine knees

Anterior cruciate ligament (ACL) tear leads to post-traumatic osteoarthritis (PTOA), a significant clinical burden worldwide that currently has no cure. Recent studies suggest a role of subchondral bone adaptations in the development of PTOA. Particularly, microstructural changes in the rod-and-plate microstructure of subchondral bone may precede and contribute to OA progression. In this study, we quantified microstructural changes in subchondral trabecular rods and plates after ACL-transection for the first time in the well-established preclinical canine model of PTOA and investigated the therapeutic potentials of a bisphosphonate (zoledronate) and NSAID treatment (meloxicam). Unilateral hindlimb ACL transection was performed on skeletally-mature (2-year-old, N = 20) and juvenile (10-month-old, N = 20) male beagles. Animals were assigned to 4 groups (N = 5): ACLT, un-operated control, ACLT with zoledronate, and ACLT with meloxicam treatment. Subchondral bone microstructure was evaluated by micro-computed tomography and cartilage integrity was evaluated histologically. We found that ACL-induced subchondral bone changes depended on skeletal maturity of animals. In mature animals, significant loss of trabecular plates that resulted in reduced PR ratio occurred at Month 1 and persisted until Month 8. Zoledronate treatment prevented trabecular plate loss while meloxicam treatment did not. Whether cartilage degeneration is also attenuated warrants further investigation. In juvenile animals that have not reached skeletal maturity, transient changes in trabecular plate and rod microstructure occurred at Month 3 but not Month 9. Neither zoledronate nor meloxicam treatment attenuated bone microstructural changes or cartilage damages. Findings from this study suggest that early inhibition of bone resorption by bisphosphonate after injury may be a promising therapeutic approach to prevent alterations in subchondral bone microstructure associated with PTOA. Our results further demonstrate that pathogenesis of PTOA may differ between adolescent and adult patients and therefore require distinct management strategies.

Insight into the potential pathogenesis of human osteoarthritis via single-cell RNA sequencing data on osteoblasts

Osteoarthritis (OA) is the most common degenerative joint disease caused by osteoblastic lineage cells. However, a comprehensive molecular program for osteoblasts in human OA remains underdeveloped. The single-cell gene expression of osteoblasts and microRNA array data were from human. After processing the single-cell RNA sequencing (scRNA-seq) data, it was subjected to principal component analysis (PCA) and T-Stochastic neighbor embedding analysis (TSNE). Differential expression analysis was aimed to find marker genes. Gene-ontology (GO) enrichment, Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analysis and Gene set enrichment analysis (GSEA) were applied to characterize the molecular function of osteoblasts with marker genes. Protein-protein interaction (PPI) networks and core module were established for marker genes by using the STRING database and Cytoscape software. All nodes in the core module were considered to be hub genes. Subsequently, we predicted the potential miRNA of hub genes through the miRWalk, miRDB and TargetScan database and experimentally verified the miRNA by GSE105027. Finally, miRNA-mRNA regulatory network was constructed using the Cytoscape software. We characterized the single-cell expression profiling of 4387 osteoblasts from normal and OA sample. The proportion of osteoblasts subpopulations changed dramatically in the OA, with 70.42% of the pre-osteoblasts. 117 marker genes were included and the results of GO analysis show that up-regulated marker genes enriched in collagen-containing extracellular matrix were highly expressed in the pre-osteoblasts cluster. Both KEGG and GSEA analyses results indicated that IL-17 and NOD-like receptor signaling pathways were enriched in down-regulated marker genes. We visualize the weight of marker genes and constructed the core module in PPI network. In potential mRNA-miRNA regulatory network, hsa-miR-449a and hsa-miR-218-5p may be involved in the development of OA. Our study found that alterations in osteoblasts state and cellular molecular function in the subchondral bone region may be involved in the pathogenesis of osteoarthritis.

A Novel EphA4 Signaling-Based Therapeutic Strategy for Osteoarthritis in Mice

This study took advantage of the recent discovery that the EphA4 signaling has anti-catabolic effects on osteoclasts/macrophages/synoviocytes but pro-anabolic effects on articular chondrocytes and sought to develop an EphA4 signaling-based therapeutic strategy for osteoarthritis (OA) using a mouse model of OA/posttraumatic OA (PTOA). The injured joint of C57BL/6J mice received biweekly intraarticular injections of a soluble EphA4-binding ligand (EfnA4-fc) at 1 day after the tibial plateau injury or at 5 weeks post-injury. The animals were euthanized 5 weeks later. The injured right and contralateral uninjured left joints were analyzed for hallmarks of OA by histology. Relative severity was determined by a modified Mankin OA scoring system and serum COMP and CTX-II levels. Tibial plateau injury caused more severe OA in Epha4 null mice than in wild-type (WT) littermates, suggesting a protective role of EphA4 signaling in OA. A prototype strategy of an EphA4 signaling-based strategy involving biweekly injections of EfnA4-fc into injured joints was developed and was shown to be highly effective in preventing OA/PTOA when it was administered at 1 day post-injury and in treating OA/PTOA when it was applied after OA has been established. The efficacy of this prototype was dose- and time-dependent. The effects were not caused by the Fc moiety of EfnA4-fc. Other soluble EfnA ligands of EphA4, ie, EfnA1-fc and EfnA2-fc, were also effective. A prototype of a novel EphA4 signaling-based therapy was developed for OA/PTOA that not only reduces the progressive destruction of articular cartilage but may also promote regeneration of the damaged cartilage. © 2022 American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

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

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

The protective role of Ephrin-B2/EphB4 signaling in osteogenic differentiation under inflammatory environment

Inflammation and alveolar bone destruction constitute the main pathological process of periodontitis. However, the molecular mechanisms of bone destruction under the inflammation environment remain unclear. This study aims to explore the role of Ephrin-B2/EphB4 signaling in osteogenic differentiation under the inflammation environment. Mouse pre-osteoblasts MC3T3-E1 were pretreated with lipopolysaccharide of Porphyromonas gingivalis (Pg-LPS). The Ephrin-B2/EphB4 signaling was activated, and the osteogenic differentiation of cells was examined. The results showed that activation of Ephrin-B2/EphB4 signaling promoted the expression levels of osteogenic differentiation-related genes, and also relieved the inhibitory effect of Pg-LPS on osteogenesis. Noticeably, the effect of Ephrin-B2/EphB4 signaling might be related to the mitogen-activated protein kinase (MAPK) pathway. While applying Ephrin-B2-Fc and EphB4-Fc to periodontitis mice, we observed the reduction of alveolar crest destruction. The current study revealed the possible role of Ephrin-B2/EphB4 signaling in reducing bone destruction in periodontitis and suggested its potential values for further research.

The EphA4 Signaling is Anti-catabolic in Synoviocytes but Pro-anabolic in Articular Chondrocytes

The expression and activation of EphA4 in the various cell types in a knee joint was upregulated upon an intraarticular injury. To determine if EphA4 signaling plays a role in osteoarthritis, we determined whether deficient EphA4 expression (in EphA4 knockout mice) or upregulation of the EphA4 signaling (with the EfnA4-fc treatment) would alter cellular functions of synoviocytes and articular chondrocytes. In synoviocytes, deficient EphA4 expression enhanced, whereas activation of the EphA4 signaling reduced, expression and secretion of key inflammatory cytokines and matrix metalloproteases. Conversely, in articular chondrocytes, activation of the EphA4 signaling upregulated, while deficient EphA4 expression reduced, expression levels of chondrogenic genes (e.g., aggrecan, lubricin, type-2 collagen, and Sox9). EfnA4-fc treatment in wildtype, but not EphA4-deficient, articular chondrocytes promoted the formation and activity of acidic proteoglycan-producing colonies. Activation of the EphA4 signaling in articular chondrocytes upregulated Rac1/2 and downregulated RhoA via enhancing Vav1 and reducing Ephexin1 activation, respectively. However, activation of the EphA4 signaling in synoviocytes suppressed the Vav/Rac signaling while upregulated the Ephexin/Rho signaling. In summary, the EphA4 signaling in synoviocytes is largely of anti-catabolic nature through suppression of the expression of inflammatory cytokines and matrix proteases, but in articular chondrocytes the signaling is pro-anabolic in that it promotes the biosynthesis of articular cartilage. The contrasting action of the EphA4 signaling in synoviocytes as opposing to articular chondrocytes may in part be mediated through the opposite differential effects of the EphA4 signaling on the Vav/Rac signaling and Ephexin/Rho signaling in the two skeletal cell types.

Ablation of Ephrin B2 in Col2 Expressing Cells Delays Fracture Repair

Ephrin B2 is critical for endochondral bone development. In this study, we investigated its role in fracture repair by deleting ephrin B2 in type II collagen (Col.2) expressing cells. We used a nonstable tibia fracture model to evaluate fracture repair at 3 sites: intramembranous bone formation, endochondral bone formation, and intramedullary bone formation. We observed that during fracture repair, deletion of ephrin B2 impaired periosteal stem cell activation, inhibited their proliferation, decreased their survival, and blocked their differentiation into osteoblasts and chondrocytes. In addition, deletion of ephrin B2 decreased vascular endothelial growth factor production as well as vascular invasion into the fracture site. These changes led to reduced cartilage to bone conversion in the callus with decreased new bone formation, resulting in impaired fracture repair. Our data indicate that ephrin B2 in Col2-expressing cells is a critical regulator of fracture repair, pointing to a new and potentially targetable mechanism to enhance fracture repair.

Icariin Alleviates Glucocorticoid-Induced Osteoporosis through EphB4/Ephrin-B2 Axis

PURPOSE

Glucocorticoid (GC) is the most important risk factor for osteoporosis (OP); in the present study, we examined the potential mechanism of icariin, a natural bioactive compound isolated from the traditional Chinese herbal Epimedium, for GC-induced OP to explore its potential therapeutic effect.

METHODS

We used a GC-induced OP mice model and treated with icariin. Pathological changes were measured by H&E staining, and the effects of icariin on osteoblasts and osteoclasts were measured by immunohistochemistry (IHC) staining and western blot (WB) analyses, while trabecular bone parameters were detected by micro-CT imaging in vivo.

RESULTS

The results showed that in GC-induced OP symptoms, icariin treatment significantly increased the density of the trabecular bone when exposed to GC, revealed by H&E staining and micro-CT imaging. IHC staining showed that GC-induced OP had a lower EphB4 expression and higher Ephrin-B2 expression, but icariin could promote EphB4 while suppressing Ephrin-B2 expression. The WB results also provided evidence of the same protein expression trend, showing that the osteoblast marker OCN and the EphB4 downstream factor RhoA in the GC group were decreased, while both OCN and RhoA expression were significantly increased and the Ephrin-B2 downstream factor Grb4 in in GC group was increased after icariin treatment.

CONCLUSION

Icariin could improve the characteristics of OP through regulating the balance of the EphB4/Ephrin-B2 pathway. Further preclinical trial is needed to provide certainty of clinical benefits for OP patients.

MMAE Delivery Using the Bicycle Toxin Conjugate BT5528

The EphA2 receptor is found at high levels in tumors and low levels in normal tissue and high EphA2 expression in biopsies is a predictor of poor outcome in patients. Drug discovery groups have therefore sought to develop EphA2-based therapies using small molecule, peptide, and nanoparticle-based approaches (1-3). However, until now only EphA2-targeting antibody-drug conjugates (ADC) have entered clinical development. For example, MEDI-547 is an EphA2-targeting ADC that displayed encouraging antitumor activity in preclinical models and progressed to phase I clinical testing in man. Here we describe the development of BT5528, a bicyclic peptide ("Bicycle") conjugated to the auristatin derivative maleimidocaproyl-monomethyl auristatin E to generate the Bicycle toxin conjugate BT5528. The report compares and contrasts the Pharmacokinetics (PK) characteristics of antibody and Bicycle-based targeting systems and discusses how the PK and payload characteristics of different delivery systems impact the efficacy-toxicity trade off which is key to the development of successful cancer therapies. We show that BT5528 gives rise to rapid update into tumors and fast renal elimination followed by persistent toxin levels in tumors without prolonged exposure of parent drug in the vasculature. This fast in, fast out kinetics gave rise to more favorable toxicology findings in rats and monkeys than were observed with MEDI-547 in preclinical and clinical studies.Graphical Abstract: http://mct.aacrjournals.org/content/molcanther/19/7/1385/F1.large.jpg.

The synergistic effects of IL-6/IL-17A promote osteogenic differentiation by improving OPG/RANKL ratio and adhesion of MC3T3-E1 cells on hydroxyapatite

OBJECTIVE

In this study, we evaluated the potential role of IL-6 and/or IL-17A in regulating the OPG/RANKL (osteoprotegerin/receptor activator of nuclear factor kappa b ligand) system of murine osteoblast cell line (MC3T3-E1) cultured on hydroxyapatite (HA).

METHODS

MC3T3-E1 cells were seeded on HA and treated with recombinant IL-6 or rIL-17A or combination of the two cytokines. Cell proliferation and differentiation activity were measured by MTS and alkaline phosphatase assays respectively. Observation of cell adhesion and proliferation was examined by scanning electron microscopy. Gene and protein expressions were performed on RANKL and OPG using qPCR, Western blot and ELISA.

RESULTS

We demonstrated that treatment with recombinant IL-17A (rIL-17A) and the combination rIL-6/rIL-17A promoted better adhesion and higher proliferation of cells on HA. Cells treated with rIL-17A and the combination cytokines showed a significant increase in differentiation activity on day 7, 10 and 14 as indicated by ALP activity (p < 0.001). Gene and protein expressions showed significant up-regulation of OPG and ALP (p < 0.001) and down-regulation of RANKL (p < 0.001) expression by all the treated groups. Interestingly, the combination of the two cytokines resulted in a significant increase of OPG/RANKL ratio (p < 0.001).

CONCLUSION

These findings indicated that treatment with the combination of the two cytokines (IL-6/IL-17A) has synergistic effects to promote osteoblastic differentiation but suppress osteoclastogenesis by altering the OPG/RANKL ratio.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

Osteoblast Role in Rheumatic Diseases

Alterations in osteoblast growth, differentiation and activity play a role in the pathogenesis of several rheumatic diseases, such as rheumatoid arthritis, spondyloarthritides, osteoarthritis, and osteoporosis. In fact, in these rheumatic diseases, abnormal activity of Wnt signaling, receptor activator of nuclear factor-κB (RANK)-RANK ligand (RANKL)-osteoprotegerin (OPG) signaling, bone morphogenetic proteins (BMPs) pathway and other mechanisms have been described in osteoblasts. This review article is focused on current knowledge on the role of osteoblast dysregulation occurring in rheumatic diseases.

Osteoblast role in osteoarthritis pathogenesis

Even if osteoarthritis pathogenesis is still poorly understood, numerous evidences suggest that osteoblasts dysregulation plays a key role in osteoarthritis pathogenesis. An abnormal expression of OPG and RANKL has been described in osteoarthritis osteoblasts, which is responsible for abnormal bone remodeling and decreased mineralization. Alterations in genes expression are involved in dysregulation of osteoblast function, bone remodeling, and mineralization, leading to osteoarthritis development. Moreover, osteoblasts produce numerous transcription factors, growth factors, and other proteic molecules which are involved in osteoarthritis pathogenesis.

Systematic Identification, Characterization and Target Gene Analysis of microRNAs Involved in Osteoarthritis Subchondral Bone Pathogenesis

This study aimed to identify the microRNAs associated with sclerotic status of subchondral bone in the pathogenesis of osteoarthritis (OA). Total RNA was extracted from non-sclerotic and sclerotic OA subchondral bone from patients undergoing knee replacement surgeries. miRCURY™ LNA miRNA chip and qRT-PCR were used to profile and validate differential microRNA expression. In addition, we further confirmed profiles of altered miRNAs in an OA rat meniscectomy animal model and their putative targets of the miRNAs were predicted using ingenuity (IPA) software. Finally, five short-listed miRNAs were reactivated by transient in vitro overexpression (miRNA mimics) in subchondral bone osteoblasts and their phenotypes were assessed. Functional screening identified 30 differentiated miRNAs in sclerotic subchondral bone compared to non-sclerotic bone of OA patients. Data integration resulted in confirmation of the eight miRNAs, with aberrant expression in independent human OA bone sample set. In silico analysis (IPA) identified 732 mRNA transcripts as putative targets of the eight altered miRNAs, of which twenty genes were validated to be differentially expressed in sclerotic compared to non-sclerotic bone samples. Out of eight dysregulated miRNA's, five of them showed consistent time-dependent downregulation in a rat OA model. Furthermore, synthetic miR-199a-3p, miR-199a-5p, miR-590-5p, and miR-211-5p mimics rescued the abnormal osteoarthritic subchondral bone osteoblast gene expression and mineralization. We have identified four novel miRNAs that play important roles in subchondral bone pathogenesis in OA. Additional studies are required to develop these miRNAs into therapeutic modalities for OA.

Bidirectional ephrin signaling in bone

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

Membrane-mediated regulation of vascular identity

Vascular diseases span diverse pathology, but frequently arise from aberrant signaling attributed to specific membrane-associated molecules, particularly the Eph-ephrin family. Originally recognized as markers of embryonic vessel identity, Eph receptors and their membrane-associated ligands, ephrins, are now known to have a range of vital functions in vascular physiology. Interactions of Ephs with ephrins at cell-to-cell interfaces promote a variety of cellular responses such as repulsion, adhesion, attraction, and migration, and frequently occur during organ development, including vessel formation. Elaborate coordination of Eph- and ephrin-related signaling among different cell populations is required for proper formation of the embryonic vessel network. There is growing evidence supporting the idea that Eph and ephrin proteins also have postnatal interactions with a number of other membrane-associated signal transduction pathways, coordinating translation of environmental signals into cells. This article provides an overview of membrane-bound signaling mechanisms that define vascular identity in both the embryo and the adult, focusing on Eph- and ephrin-related signaling. We also discuss the role and clinical significance of this signaling system in normal organ development, neoplasms, and vascular pathologies.

Pregnancy-Induced Changes in Systemic Gene Expression among Healthy Women and Women with Rheumatoid Arthritis

Background

Pregnancy induces drastic biological changes systemically, and has a beneficial effect on some autoimmune conditions such as rheumatoid arthritis (RA). However, specific systemic changes that occur as a result of pregnancy have not been thoroughly examined in healthy women or women with RA. The goal of this study was to identify genes with expression patterns associated with pregnancy, compared to pre-pregnancy as baseline and determine whether those associations are modified by presence of RA.

Results

In our RNA sequencing (RNA-seq) dataset from 5 healthy women and 20 women with RA, normalized expression levels of 4,710 genes were significantly associated with pregnancy status (pre-pregnancy, first, second and third trimesters) over time, irrespective of presence of RA (False Discovery Rate (FDR)-adjusted p value<0.05). These genes were enriched in pathways spanning multiple systems, as would be expected during pregnancy. A subset of these genes (n = 256) showed greater than two-fold change in expression during pregnancy compared to baseline levels, with distinct temporal trends through pregnancy. Another 98 genes involved in various biological processes including immune regulation exhibited expression patterns that were differentially associated with pregnancy in the presence or absence of RA.

Conclusions

Our findings support the hypothesis that the maternal immune system plays an active role during pregnancy, and also provide insight into other systemic changes that occur in the maternal transcriptome during pregnancy compared to the pre-pregnancy state. Only a small proportion of genes modulated by pregnancy were influenced by presence of RA in our data.

The in vivo effect of prophylactic subchondral bone protection of osteoarthritic synovial membrane in bone-specific Ephb4-overexpressing mice

Osteoarthritis (OA) is characterized by progressive joint destruction, including synovial membrane alteration. EphB4 and its ligand ephrin-B2 were found in vitro to positively affect OA subchondral bone and cartilage. In vivo in an experimental mouse model overexpressing bone-specific Ephb4 (TgEphB4), a protective effect was found on both the subchondral bone and cartilage during OA. We investigated in the TgEphB4 mouse model the in vivo effect on synovial membrane during OA. Knee OA was surgically induced by destabilization of the medial meniscus (DMM). Synovial membrane was evaluated using histology, histomorphometry, IHC, and real-time PCR. Compared to DMM-wild-type (WT) mice, DMM-TgEphB4 mice had a significant decrease in synovial membrane thickness, vascular endothelial growth factor, and the profibrotic markers fibrin, type 1 procollagen, type 3 collagen, connective tissue growth factor, smooth muscle actin-α, cartilage oligomeric matrix protein, and procollagen-lysine, and 2-oxoglutarate 5-dioxygenase 2. Moreover, factors known to modulate transforming growth factor-β signaling, transforming growth factor receptor 1/ALK1, phosphorylated Smad-1, and heat shock protein 90β were significantly decreased in DMM-TgEphB4 compared with DMM-WT mice. Ephb4 overexpression also exhibited a protective effect on synovial membrane thickness of aged (24-month-old) mice. Overexpression of bone-specific Ephb4 clearly demonstrated prevention of the development and/or progression of fibrosis in OA synovial membrane, reinforcing the hypothesis that protecting the subchondral bone prophylactically and during OA reduces the pathologic changes in other articular tissues.

The coupling of bone and cartilage turnover in osteoarthritis: opportunities for bone antiresorptives and anabolics as potential treatments?

Osteoarthritis (OA) is the most common form of arthritic disease, and a major cause of disability and impaired quality of life in the elderly. OA is a complex disease of the entire joint, affecting bone, cartilage and synovium that thereby presents multiple targets for treatment. This manuscript will summarise emerging observations from cell biology, preclinical and preliminary clinical trials that elucidate interactions between the bone and cartilage components in particular. Bone and cartilage health are tightly associated. Ample evidence has been found for bone changes during progression of OA including, but not limited to, increased turnover in the subchondral bone, undermineralisation of the trabecular structure, osteophyte formation, bone marrow lesions and sclerosis of the subchondral plate. Meanwhile, a range of investigations has shown positive effects on cartilage health when bone resorption is suppressed, or deterioration of the cartilage when resorption is increased. Known bone therapies, namely oestrogens, selective oestrogen receptor modifiers (SERMs), bisphosphonates, strontium ranelate, calcitonin and parathyroid hormone, might prove useful for treating two critical tissue components of the OA joint, the bone and the cartilage. An optimal treatment for OA likely targets at least these two tissue components. The patient subgroups for whom these therapies are most appropriate have yet to be fully defined but would likely include, at a minimum, those with high bone turnover.

EphB4 enhances the process of endochondral ossification and inhibits remodeling during bone fracture repair

Previous reports have identified a role for the tyrosine kinase receptor EphB4 and its ligand, ephrinB2, as potential mediators of both bone formation by osteoblasts and bone resorption by osteoclasts. In the present study, we examined the role of EphB4 during bone repair after traumatic injury. We performed femoral fractures with internal fixation in transgenic mice that overexpress EphB4 under the collagen type 1 promoter (Col1-EphB4) and investigated the bone repair process up to 12 weeks postfracture. The data indicated that Col1-EphB4 mice exhibited stiffer and stronger bones after fracture compared with wild-type mice. The fractured bones of Col1-EphB4 transgenic mice displayed significantly greater tissue and bone volume 2 weeks postfracture compared with that of wild-type mice. These findings correlated with increased chondrogenesis and mineral formation within the callus site at 2 weeks postfracture, as demonstrated by increased safranin O and von Kossa staining, respectively. Interestingly, Col1-EphB4 mice were found to possess significantly greater numbers of clonogenic mesenchymal stromal progenitor cells (CFU-F), with an increased capacity to form mineralized nodules in vitro under osteogenic conditions, when compared with those of the wild-type control mice. Furthermore, Col1-EphB4 mice had significantly lower numbers of TRAP-positive multinucleated osteoclasts within the callus site. Taken together, these observations suggest that EphB4 promotes endochondral ossification while inhibiting osteoclast development during callus formation and may represent a novel drug target for the repair of fractured bones.

Eph/Ephrin signaling in injury and inflammation

The Eph/ephrin receptor-ligand system plays an important role in embryogenesis and adult life, principally by influencing cell behavior through signaling pathways, resulting in modification of the cell cytoskeleton and cell adhesion. There are 10 EphA receptors, and six EphB receptors, distinguished on sequence difference and binding preferences, that interact with the six glycosylphosphatidylinositol-linked ephrin-A ligands and the three transmembrane ephrin-B ligands, respectively. The Eph/ephrin proteins, originally described as developmental regulators that are expressed at low levels postembryonically, are re-expressed after injury to the optic nerve, spinal cord, and brain in fish, amphibians, rodents, and humans. In rodent spinal cord injury, the up-regulation of EphA4 prevents recovery by inhibiting axons from crossing the injury site. Eph/ephrin proteins may be partly responsible for the phenotypic changes to the vascular endothelium in inflammation, which allows fluid and inflammatory cells to pass from the vascular space into the interstitial tissues. Specifically, EphA2/ephrin-A1 signaling in the lung may be responsible for pulmonary inflammation in acute lung injury. A role in T-cell maturation and chronic inflammation (heart failure, inflammatory bowel disease, and rheumatoid arthritis) is also reported. Although there remains much to learn about Eph/ephrin signaling in human disease, and specifically in injury and inflammation, this area of research raises the exciting prospect that novel therapies will be developed that precisely target these pathways.

Bone cell communication factors and Semaphorins

Bone tissue is continuously renewed throughout adult life by a process called 'remodeling', which involves a dynamic interplay among bone cells including osteoclasts, osteoblasts and osteocytes. For example, a tight coupling between bone resorption and formation is essential for the homeostasis of the skeletal system. Studies on the coupling mechanism in physiological and pathological settings have revealed that osteoclasts or osteoclastic bone resorption promote bone formation through the production of diverse coupling factors. The classical coupling factors are the molecules that promote bone formation after resorption, but there may be distinct mechanisms at work in various phases of bone remodeling. A recent study revealed that the Semaphorin 4D expressed by osteoclasts inhibits bone formation, which represents a mechanism by which coupling is dissociated. Furthermore, it has been demonstrated that osteoblastic expression of Semaphorin 3A exerts an osteoprotective effect by both suppressing bone resorption and increasing bone formation. Thus, recent advances have made it increasingly clear that bone remodeling is regulated by not only classical coupling factors, but also molecules that mediate cell-cell communication among bone cells. We propose that such factors be called bone cell communication factors, which control the delicate balance of the interaction of bone cells so as to maintain bone homeostasis.

The osteoarthritic niche and modulation of skeletal stem cell function for regenerative medicine

Osteoarthritis (OA) is the most common cause of arthritis worldwide and represents a significant healthcare burden, particularly in the context of an ageing population. Traditionally, painkillers, injections and physiotherapy have been the mainstay of treatment, with patients being referred for joint replacement surgery (arthroplasty) when these options fail. Whilst effective in reducing pain and improving joint function, these approaches are not without potential complications. With the development of tissue-engineering techniques over recent years there has been considerable interest in applying these strategies to provide new, innovative, alternative effective means of treating OA. This review explores the unique microenvironment present within an osteoarthritic joint, highlighting the features that comprise the osteoarthritic niche and could be modulated in the development of novel treatments for OA. Existing tissue-engineering strategies for repairing bone and cartilage defects are discussed, with particular reference to how these might be modified, both to improve existing treatments, such as impaction bone grafting, as well as in the development of future treatments for OA.

Bone cell interactions through Eph/ephrin: bone modeling, remodeling and associated diseases

Bones cannot properly form or be maintained without cell-cell interactions through ephrin ligands and Eph receptors. Cell culture analysis and evaluation of genetic mouse models and human diseases reveal various ephrins and Eph functions in the skeletal system. Migration, attachment and spreading of mesenchymal stem cells are regulated by ephrinB ligands and EphB receptors. ephrinB1 loss-of-function is associated with craniofrontonasal syndrome (CFNS) in humans and mice. In bone remodeling, ephrinB2 is postulated to act as a “coupling stimulator.” In that case, bidirectional signaling between osteoclastic ephrinB2 and osteoblastic EphB4 suppresses osteoclastic bone resorption and enhances osteoblastic bone formation, facilitating the transition between these two states. Parathyroid hormone (PTH) induces ephrinB2 in osteoblasts and enhances osteoblastic bone formation. In contrast to ephrinB2, ephrinA2 acts as a “coupling inhibitor,” since ephrinA2 reverse signaling into osteoclasts enhances osteoclastogenesis and EphA2 forward signaling into osteoblasts suppresses osteoblastic bone formation and mineralization. Furthermore, ephrins and Ephs likely modulate pathological conditions such as osteoarthritis, rheumatoid arthritis, multiple myeloma and osteosarcoma. This review focuses on ephrin/Eph-mediated cell-cell interactions in bone biology.

EphB/ephrin-B interactions mediate human MSC attachment, migration and osteochondral differentiation

Bone marrow derived mesenchymal stem/stromal cells (MSC) contribute to skeletal tissue formation and the regulation of haematopoiesis. The Eph/ephrin family of receptor tyrosine kinases is potentially important in the maintenance of the stem cell niche within neural, intestinal and dental tissues and has recently been shown to play a role in regulating bone homeostasis. However, the contribution of EphB/ephrin-B molecules in human MSC function remains to be determined. In the present study, EphB and ephrin-B molecules were expressed by ex vivo expanded human MSC populations and within human bone marrow trephine samples. To elucidate the contribution of EphB/ephrin-B molecules in MSC recruitment, we performed functional spreading and migration assays and showed that reverse ephrin-B signalling inhibited MSC attachment and spreading by activating Src-, PI3Kinase- and JNK-dependent signalling pathways. In contrast, forward EphB2 signalling promoted MSC migration by activating the Src kinase- and Abl-dependent signalling pathways. Furthermore, activation of ephrin-B1 and/or ephrin-B2 molecules expressed by MSC was found to increase osteogenic differentiation, while ephrin-B1 activation promoted chondrogenic differentiation. These observations suggest that EphB/ephrin-B interactions may mediate the recruitment, migration and differentiation of MSC during bone repair.

Osteoblast physiology in normal and pathological conditions

Osteoblasts are mononucleated cells that are derived from mesenchymal stem cells and that are responsible for the synthesis and mineralization of bone during initial bone formation and later bone remodelling. Osteoblasts also have a role in the regulation of osteoclast activity through the receptor activator of nuclear factor κ-B ligand and osteoprotegerin. Abnormalities in osteoblast differentiation and activity occur in some common human diseases such as osteoporosis and osteoarthritis. Recent studies also suggest that osteoblast functions are compromised at sites of focal bone erosion in rheumatoid arthritis.

Targeting subchondral bone for treating osteoarthritis: what is the evidence?

Over the past few decades, significant progress has been made with respect to new concepts about the pathogenesis of osteoarthritis (OA). This article summarises some of the knowledge we have today on the involvement of the subchondral bone in OA. It provides substantial evidence that changes in the metabolism of the subchondral bone are an integral part of the OA disease process and that these alterations are not merely secondary manifestations, but are part of a more active component of the disease. Thus, a strong rationale exists for therapeutic approaches that target subchondral bone resorption and/or formation, and data evaluating the drugs targeting bone remodelling raise the hope that new treatment options for OA may become available.

New molecular targets for the treatment of osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disorder characterized by destruction of the articular cartilage, subchondral bone alterations and synovitis. Current treatments are focused on symptomatic relief but they lack efficacy to control the progression of this disease which is a leading cause of disability. Therefore, the development of effective disease-modifying drugs is urgently needed. Different initiatives are in progress to define the molecular mechanisms involved in the initiation and progression of OA. These studies support the therapeutic potential of pathways relevant in joint metabolism such as Wnt/beta-catenin, discoidin domain receptor 2 or proteinase-activated receptor 2. The dysregulation in cartilage catabolism and subchondral bone remodeling could be improved by selective inhibitors of matrix metalloproteinases, aggrecanases and other proteases. Another approach would favor the activity of anabolic processes by using growth factors or regulatory molecules. Recent studies have also revealed the role of oxidative stress and synovitis in the progression of this disease, supporting the development of a number of inhibitory strategies. Novel targets in OA are represented by genes involved in OA pathophysiology discovered using gene network, epigenetic and microRNA approaches. Further insights into the molecular mechanisms involved in OA initiation and progression may lead to the development of new therapies able to control joint destruction and repair.

Cross-talk among bone cells

PURPOSE OF REVIEW

Bone cells such as osteoclasts, osteoblasts, and osteocytes 'talk' to each other throughout adult life to maintain bone integrity. This review highlights frontier areas of research on intercellular communication among bone cells.

RECENT FINDINGS

Bone cells communicate to regulate the balance between bone resorption and bone formation. Recent findings have suggested that bone remodeling compartment is critical for osteoclast-osteoblast communication during bone remodeling. New molecules and mechanisms for bone cell communication, including 'coupling' of bone formation to resorption, have been revealed. Osteoclastic regulation of pH within the bone remodeling compartment is a mechanism that has been posited to rapidly activate osteoblastic bone formation, whereas osteocytes in the bone matrix regulate osteoclasts and osteoblasts on the bone surface through the lacuno-canaliculi network.

SUMMARY

Differentiation, activation, and apoptosis of bone cells are often dependent on the status of other types of bone cells. Bone cells in different lineages achieve intercellular communication not only by ligand-receptor interactions but also by molecules and ions traveling in the extracellular space or across gap junctions, processes that depend profoundly on the four-dimensional (space and time) architecture of bone tissue.

Treatment with ephrin B2 positively impacts the abnormal metabolism of human osteoarthritic chondrocytes

Introduction

Members of the ephrin system, the ephrin receptor erythropoietin-producing hepatocellular B4 (EphB4) and its specific ligand, ephrin B2, appear to be involved in the bone remodelling process. We recently showed that their interaction inhibits the resorptive activity of human osteoarthritic (OA) subchondral bone osteoblasts. Hence, we further investigated the possible implication of these ephrin members on the catabolic/anabolic activities of human OA chondrocytes.

Methods

EphB4 receptor and ephrin B2 levels were determined by quantitative PCR and immunohistochemistry, and the effects of ephrin B2 on the expression/production of factors involved in the OA process.

Results

EphB4 receptors and ephrin B2 ligands are expressed and produced by human normal and OA chondrocytes. Ephrin B2 protein was found at similar levels in both cartilage types, whereas EphB4 receptor expression (P < 0.0001) and production (P < 0.01) levels were significantly increased in OA chondrocytes/cartilage. Ephrin B2 treatment significantly inhibited the interleukin (IL)-1beta, IL-6, matrix metalloproteinase-1 (MMP-1), MMP-9, MMP-13, and proteinase-activated receptor-2 (PAR-2) gene expression levels, whereas MMP-2 was unaffected, and significantly increased collagen type II, a cartilage specific macromolecule. It also inhibited the IL-1beta stimulated protein production of IL-6, MMP-1 and MMP-13.

Conclusions

Our study is the first to provide data on the presence and role of ephrin B2/EphB4 receptors in human chondrocytes/cartilage. Data showed that ephrin B2 treatment positively impacts the abnormal metabolism of OA cartilage by inhibiting important catabolic factors involved in this disease at the same time as increasing anabolic activity.

Eph and ephrin interactions in bone

Bone cells communicate with each other using various cell surface molecules. Membrane-bound ephrin ligands and Eph tyrosine kinase receptors have been characterized in diverse biological processes, including angiogenesis and neuronal development. Several ephrins and Ephs are expressed in osteoclasts and osteoblasts and regulate bone mineral metabolism through bidirectional signaling into not only receptor-expressing cells but also into ligand-expressing cells. We propose that interaction between ephrinB2-expressing osteoclasts and EphB4-expressing osteoblasts facilitates the transition from bone resorption to bone formation during bone remodeling. Other groups have reported the regulation of ephrinB2 by PTH or PTHrP and the possible involvement of EphB4 in osteoarthritis. It is likely that various ephrins and Ephs mediate interaction among bone cells.