Regulation of Long Bone Morphogenesis by ERG

Introduction and Objectives: Long bone development requires strict topographical mechanisms. It starts with formation of cartilaginous primodium. The diaphysis of cartilage rudiments then organize growth-plates and becomes surrounded by a bone collar while the epiphyseal ends remain cartilaginous and compose permanent articular chondrocytes. It is not well known how this topographical feat is controlled. In this study, we investigated the role of an Ets-related transcription factor (ERG), which is preferentially expressed in articular cartilage and prevents maturation of chondrocytes, in articular cartilage and bone collar formation.

Methods: Expression pattern of ERG and cartilage markers was analyzed by in situ hybridization. Relative expression levels of those genes were estimated by realtime RT-PCR. Gain and loss of function experiments were carried out by infection of ERG or DN-ERG encoding retrovirus vectors.

Results: In mouse, ERG was specifically expressed in incipient limb joint at E15.5 and remained in articular surface. The expression pattern of ERG was closely matched to that of GDF-5. GDF-5 induced ERG in chondrocytes within 6 h. ERG expression was initially found in entire perichondrium, but became limited to the epiphyseal perichondrium while the diaphyseal cartilage is well organized. Interestingly, disappearance of ERG transcripts from diaphyseal perichondrium coincided with up-regulation of Runx2 expression. To ask the physiological meanings of ERG and Runx2 in perichondrium, we examined the effects of ERG and Runx2 on osteoprogenitor cells. Over-expression of ERG inhibited osteoblastic differentiation whereas Runx2 over-expression did enhance it.

Conclusion: These findings suggest that ERG and Runx2 harmoniously regulates osteogenic differentiation in perichondrium during long bone development. Runx2 gene expression is directing bone formation around and within the incipient diaphysis, while ERG expression at the epiphyses would block it and favor the development of the epiphyses into joint (and bone-free) structures.

Support: NIH grants AR051007 and AR46000, and YMUSA Foundation