Endocrine Abstracts

The transcription factors Runx2 and osterix (Osx), which are known as downstream genes in BMP2 pathway, are essential for osteoblast differentiation and bone formation. Runx2 expression is normal in Osx null mice, but Osx is not expressed in Runx2 null mice, indicating that Osx acts as a downstream of Runx2. Here, we questioned whether or not all bones could be formed by BMP2–Runx2–Osx pathway or unknown their own gene axis. To answer this question, we first analyzed bone phenotype of Runx2;Osx double heterozygotes which were generated by Runx2 and Osx heterozygous mice. Compared to each Runx2 and Osx heterozygous embryos, Runx2;Osx double heterozygotes showed a reduction of the length of long bones including humerus and femur. Maxillary and palatine shelf, presphenoid bone, zygomatic bone, and tympanic ring were hypoplastic or missing. Severe inward bendings were observed in ribs and humerus. In histological analysis, the region of hypertrophic chondrocytes was expanded and the area of mineralized bones was reduced in Runx2;Osx double heterozygotes. To further elucidate the difference of Runx2 and Osx downstream target genes in skeletal development, DNA microarray analysis was conducted in calvaria of Runx2, Osx, and Runx2;Osx double heterozygotes. Many genes involved in cell cycle regulation, growth and apoptosis, immune response, extracellular matrix structure, skeletal development and morphogenesis were upregulated or downreguated in Runx2;Osx double heterozygotes. Especially, among skeletal development related genes, matrilin 1 and 4, hyaluronan and proteoglycan link protein 1, aggrecan, epiphycan, Col2a1, Col9a1, and Sox9 were decreased, while JunD, peptidase inhibitor 16, elastin microfibril interfacer 2, mast cell protease, dynein, and angiotensin II receptor were remarkably increased in Runx2;Osx double heterozygotes. Taken together, differential gene expression profiles in Runx2;Osx double heterozygotes provide the understanding of the correlation between Runx2 and Osx in skeletal development (Grant support: RTI04-01-01, KRF 2008-331-E00039).