Role Of HB-EGF In Bone Development And Bone Metabolism

Heparin-binding EGF-like growth factor(HB-EGF)is a membraneanchored glycoprotein.It belongs to the EGF superfamily.It can form soluble HB-EGF under the action of proteases.In vitro studies have found that HB-EGF-EGFR signaling can promote MSC proliferation and inhibit its differentiation.It have also been reported that HB-EGF levels are elevated in osteoarthritic chondrocytes in arthritic patients and in ACLT or DMM-induced osteoarthritis,and in which EGFR signaling pathway is activated.HB-EGF can also promote the formation of osteoclasts by regulating the expression of genes,such as RANKL,OPG,M-CSF in osteoblasts.In this thesis,based on the previous studies.I used Cre/lox P technology to generate mutant mice that specifically over-express HB-EGF in MSCs,chondrocytes and osteoclasts.We study the role of HB-EGF and underlying pathological mechanisms in the skeleton systematically.We found that when HB-EGF was specifically knocked out in Dermo1-labeled mesenchymal cells,there was no abnormality in bone size and articular cartilage,but there was a moderate increase in bone mass and bone density.In contrast,overexpression of HB-EGF in Dermo1-labeled mesenchymal cells significantly decreased bone mass and bone density in mice,markedly decreased bone formation and bone mineralization.This shows that HBEGF expressed in stromal cells plays an important role in bone remodeling in mice.By H/E staining of the bone sections,we found that the mutant mice had distorted knee joints,increased femur width,degraded articular cartilage.The mutant mice also developed chondroma and fibrosis-like phenotypes,with the later stained positive for FSP1,a fibroblast marker.Ki67 staining revealed that HB-EGF overexpression led to an increase in the number of proliferating chondrocytes in the growth plate and at the articular cartilage surface,suggesting that HB-EGF promoted expansion of chondrocytes at these two locations.We also found chondroma in the spinal growth plate.At the same time,we found that overexpressing HB-EGF in MSC preduced no difference in osteoclastogenesis.To more fully demonstrate the effect of HB-EGF on bone resorption,we constructed mice that overexpress HB-EGF in Lyz-labeled osteoclasts.There was no abnormality in bone size and growth plate cartilage,no change in bone mass and bone density,and no difference in osteoclast differentiation ability.Further mechanism analysis revealed that HB-EGF promotes MSC proliferation by activating the Erk and Akt signaling pathways downstream of EGFR signaling.In addition,HB-EGF inhibits MSC differentiation by inhibiting the Smad1/5/8 signal downstream of the BMP signaling pathway.AG1478,an inhibitor of EGFR signaling pathway in mice,can well rescue the abnormal phenotype of cartilage.In summary,this study first analyzed in details the role of HB-EGF in bone development and bone metabolism in mice.The results of the study indicate that HB-EGF overexpression in stromal cells can influence bone growths,whereas overexpression of HB-EGF in osteoclasts has no effect on bone.Knocking out HB-EGF in stromal cells does not affect the normal development of cartilage.On the contrary,overexpression of HB-EGF may have a destructive effect on the development of joint and growth plate chondrocytes.These findings expand our understanding of the occurrence of arthritis and the production of endogenous chondromas,and provide new possible therapeutic strategies for clinical treatment.