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Developmental regulation of bone formation in the jaw skeleton is essential to species-specific adaptation. The jaws are derived from neural crest mesenchyme (NCM), a progenitor population that directs skeletal patterning by exerting temporal and spatial control over molecular and cellular programs for osteogenesis. One important NCM-mediated gene isRunx2, which is a transcription factor required for osteoblast differentiation. RUNX2 protein binds many target genes involved in the deposition and resorption of bone. To determine the extent to which changes inRunx2structure, function, and expression underlie the evolution of the jaw skeleton, we compareRunx2across vertebrates and within birds.Runx2contains two alternative promoters, tandem repeats of glutamine and alanine with variable lengths in different species, a conserved DNA-binding domain, an exon that is alternatively spliced, as well as two possible C-termini. Such alternative splicing produces eight potential isoforms that show distinct stage- and species-specific patterns in the jaw primordia of chick, quail and duck embryos. We also find that certain isoforms are strongly induced by TGFβ signaling whereas others are not. OverexpressingRunx2isoforms in NCM reveals that some are transcriptionally activating, while others are repressive. But context appears to be relevant since species-specific polymorphisms in the promoter of target genes likeMmp13, can modulate the effects of different isoforms. Overall, our study indicates that the structure and species-specific deployment ofRunx2isoforms affect the transcriptional activity of target genes in ways that may have played a generative and regulatory role in the evolution of the avian jaw skeleton. |
