Different roles of Runx2 during early neural crest-derived bone and tooth development.
We compared gene expression profiles between Runx2 null mutant mice and their wildtype littermates. Most Runx2-dependent genes in bones were different from those in teeth, implying that the target genes of Runx2 are tissue-dependent. In vitro experiments determined that Runx2 is a part of the FGF and BMP signaling pathways in tooth and bone development, respectively.


BACKGROUND
Runx2 (Cbfa1) is expressed in the neural crest-derived mesenchyme of developing bone and tooth. Runx2 homozygous null mice lack bone through a failure in osteoblast differentiation and have arrested tooth development at the late bud stage. The aim of this study was to discover and compare the identities and the roles of Runx2 target genes in bone and tooth development.


METHODS
Wildtype and Runx2-/- tissue was collected from mouse embryos, and gene expression was compared by Affymetrix microarray analysis and radioactive in situ hybridization of embryonic tissue sections (E12-E14). Induction of target genes by growth factors in bone and tooth tissue was studied using in vitro experiments, including a novel method involving hanging-drop cultures and RT-PCR.


RESULTS
Thirteen bone and four tooth genes were identified that are Runx2-dependent. The identities of these genes do not significantly overlap between bone and tooth, indicating tissue specificity of several genes regulated by Runx2. Genes downregulated in bone development in Runx2 null mutants were Bambi, Bmp4, Bono1, Dkk1, Fgf receptor1, Gli1, Lef1, Patched, Prostaglandin F receptor1, Tcf1, Tgfbeta1, Wnt10a, and Wnt10b. Several of these genes were induced by BMPs in bone tissue in a Runx2-independent manner. Genes downregulated in tooth development were Dkk1, Dusp6, Enpp1, and Igfbp3. These genes were all induced by fibroblast growth factors (FGFs) in dental tissue. FGF-induction of Dkk1 was completely dependent on Runx2 function.


CONCLUSIONS
The contrasting identities and distinctive mechanisms that stimulate the expression of Runx2-dependent genes in bone and tooth development imply that the developmental roles of Runx2 in these separate tissues are different. In tooth development, Dkk1 may be a direct transcriptional target of Runx2. Bone genes were stimulated by BMP4 before the formation of the ossification center, suggesting that BMPs may mediate the early epithelial-mesenchymal interactions involved in bone formation.
James
Martyn J
MJ
Institute of Biotechnology, Viikki Biocenter, University of Helsinki, Finland. martyn.james@helsinki.fi
Järvinen
Elina
E
Wang
Xiu-Ping
XP
Thesleff
Irma
I
D000818:Animals; D001846:Bone Development / Q000502:physiology*; D002450:Cell Communication / Q000502:physiology; D050659:Core Binding Factor Alpha 1 Subunit / Q000172:deficiency / Q000378:metabolism*; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism; D020869:Gene Expression Profiling; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D008648:Mesoderm / Q000166:cytology / Q000378:metabolism; D051379:Mice; D008817:Mice, Mutant Strains; D009432:Neural Crest / Q000196:embryology*; D020411:Oligonucleotide Array Sequence Analysis; D014070:Tooth / Q000196:embryology*
D050659:Core Binding Factor Alpha 1 Subunit; C493716:Runx2 protein, mouse