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

Abstract

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. ß

mTORC1 Plays an Important Role in Skeletal Development by Controlling Preosteoblast Differentiation

“…Three-point bending analyses to failure were conducted using femurs isolated from 4-week-old Osx:cre, Rptor ob Ϫ/ϩ , and Rptor ob Ϫ/Ϫ mice as previously described (70). Isolation of eYFP ؉ cells and real-time PCR.…”

The role of Eph/ephrin molecules in stromal–hematopoietic interactions

“…3), where Eph/ephrin interactions are known to regulate bone remodelling between stromal cells and osteoclasts [7,8,18,[57][58][59][60][61]. Forward signalling through EphB4 promotes BMSC osteogenic differentiation, whereas reverse signalling via ephrin-B2 suppresses osteoclast differentiation by inhibiting the c-Fos-NFATc1 signalling cascade.…”

“…Enhanced bone fracture repair [7] EphB4 Enhanced stromal support of hematopoiesis [44] EphB4 Protected subchondral bone and cartilage during osteoarthritis [82] Ephrin-B1 Loss of ephrin-B1 reduced bone formation [60] Ephrin-B2 Interactions with PTH and PTHrP in bone formation process [58,67] Fig. 3 Human BMSC expression of EphB and ephrin-B molecules.…”

“…The cytoplasmic region of the Eph receptor contains SRC homology 2 binding sites, a juxtamembrane region, a tyrosine kinase domain, a sterile alpha motif (SAM) protein-protein interaction domain, and a post-synaptic density, disc large, zona occludens-1 (PDZ) binding motif [5]. Each region is important in activating various signalling cascades and mediating a number of downstream events including cell shape, attachment, migration, proliferation, differentiation [6][7][8][9]. The Eph receptors signal and mediate these responses through tyrosine phosphorylation and protein-protein interactions.…”

“…2). Eph/ephrin molecules function through cell to cell interactions [2] where their signalling is crucial in developmental cell positioning [15], tissue and organ patterning [16], post-natal stem cell niche maintenance within the brain, intestine, skin, skeletal muscle, tooth [9,14,17], bone [7,8,18] and hematopoietic tissues [19][20][21][22][23][24][25][26][27][28].…”