title: Chondrocytic ephrin B2 promotes cartilage destruction by osteoclasts in endochondral ossification.
abstract: The majority of the skeleton arises by endochondral ossification, whereby cartilaginous templates expand and are resorbed by osteoclasts then replaced by osteoblastic bone formation. Ephrin B2 is a receptor tyrosine kinase expressed by osteoblasts and growth plate chondrocytes that promotes osteoblast differentiation and inhibits osteoclast formation. We investigated the role of ephrin B2 in endochondral ossification using Osx1Cre-targeted gene deletion. Neonatal Osx1Cre.Efnb2(Δ/Δ) mice exhibited a transient osteopetrosis demonstrated by increased trabecular bone volume with a high content of growth plate cartilage remnants and increased cortical thickness, but normal osteoclast numbers within the primary spongiosa. Osteoclasts at the growth plate had an abnormal morphology and expressed low levels of tartrate-resistant acid phosphatase; this was not observed in more mature bone. Electron microscopy revealed a lack of sealing zones and poor attachment of Osx1Cre.Efnb2(Δ/Δ) osteoclasts to growth plate cartilage. Osteoblasts at the growth plate were also poorly attached and impaired in their ability to deposit osteoid. By 6 months of age, trabecular bone mass, osteoclast morphology and osteoid deposition by Osx1Cre.Efnb2(Δ/Δ) osteoblasts were normal. Cultured chondrocytes from Osx1Cre.Efnb2(Δ/Δ) neonates showed impaired support of osteoclastogenesis but no significant change in Rankl (Tnfsf11) levels, whereas Adamts4 levels were significantly reduced. A population of ADAMTS4(+) early hypertrophic chondrocytes seen in controls was absent from Osx1Cre.Efnb2(Δ/Δ) neonates. This suggests that Osx1Cre-expressing cells, including hypertrophic chondrocytes, are dependent on ephrin B2 for their production of cartilage-degrading enzymes, including ADAMTS4, and this might be required for attachment of osteoclasts and osteoblasts to the cartilage surface during endochondral ossification.
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia The University of Melbourne, Department of Medicine at St Vincent's Hospital, Fitzroy, Victoria 3065, Australia.
authors.forename: Stephen
authors.initials: S
authors.lastname: Tonna
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
authors.forename: Ingrid J
authors.initials: IJ
authors.lastname: Poulton
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
authors.forename: Farzin
authors.initials: F
authors.lastname: Taykar
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
authors.forename: Patricia W M
authors.initials: PW
authors.lastname: Ho
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
authors.forename: Brett
authors.initials: B
authors.lastname: Tonkin
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
authors.forename: Blessing
authors.initials: B
authors.lastname: Crimeen-Irwin
authors.affiliation: The University of Melbourne, Faculty of Veterinary and Agricultural Sciences, Parkville 3010, Australia.
authors.forename: Liliana
authors.initials: L
authors.lastname: Tatarczuch
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
authors.forename: Narelle E
authors.initials: NE
authors.lastname: McGregor
authors.affiliation: The University of Melbourne, Faculty of Veterinary and Agricultural Sciences, Parkville 3010, Australia.
authors.forename: Eleanor J
authors.initials: EJ
authors.lastname: Mackie
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia The University of Melbourne, Department of Medicine at St Vincent's Hospital, Fitzroy, Victoria 3065, Australia.
authors.forename: T John
authors.initials: TJ
authors.lastname: Martin
authors.affiliation: St Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia The University of Melbourne, Department of Medicine at St Vincent's Hospital, Fitzroy, Victoria 3065, Australia nsims@svi.edu.au.
authors.forename: Natalie A
authors.initials: NA
authors.lastname: Sims
mesh_terms: D051722:ADAM Proteins / Q000378:metabolism; D000071121:ADAMTS4 Protein; D000818:Animals; D000831:Animals, Newborn; D002356:Cartilage / Q000378:metabolism / Q000473:pathology*; D002448:Cell Adhesion; D002454:Cell Differentiation; D019902:Chondrocytes / Q000378:metabolism* / Q000473:pathology; D036388:Ephrin-B2 / Q000378:metabolism*; D005260:Female; D005786:Gene Expression Regulation; D007150:Immunohistochemistry; D019426:Integrases / Q000378:metabolism; D008810:Mice, Inbred C57BL; D008954:Models, Biological; D009929:Organ Size; D010006:Osteoblasts / Q000473:pathology; D010010:Osteoclasts / Q000378:metabolism / Q000473:pathology* / Q000648:ultrastructure; D010012:Osteogenesis / Q000235:genetics; D010022:Osteopetrosis / Q000235:genetics / Q000473:pathology; D010641:Phenotype; D011348:Procollagen N-Endopeptidase / Q000378:metabolism; D013977:Tibia / Q000378:metabolism / Q000473:pathology
chemical_list: D036388:Ephrin-B2; C045073:Cre recombinase; D019426:Integrases; D051722:ADAM Proteins; D011348:Procollagen N-Endopeptidase; D000071121:ADAMTS4 Protein; C000603919:Adamts4 protein, mouse
keywords: Chondrocyte; Endochondral ossification; Ephrin B2; Mouse; Osteoblast; Osteoclast; Sp7